_s_- ”3” W‘ “i fiai'la‘ii, Chlga'fi 32'” I '3“ u t a.’ ' mumsty “v. m, NHW I, "" *- \_' 'V' v-fi "T“ "V" I .. “v __.__ MSU RETURNING MATERIALS: Place 1n booE drop to remove this checkout from w your record. FINES wil] be charged if book is returned after the date stamped below. '3'"fo 1 1 , x 1493' . Mm“ AUG 1 9 a. SEP 1 51992 W?) s 1913 55" 2 7191:. :flGS" ;‘ CONCURRENTFLOW VERSUS CONVENTIONAL DRYING OF RICE BY Carlos Fontana A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Agricultural Engineering 1983 [J /-3( ac; / .m_‘ ABSTRACT CONCURRENTFLOW VERSUS CONVENTIONAL DRYING OF RICE BY Carlos Fontana Rice is the basic source of food for 70 percent of the world population. Large increases have been reached in rice yields in the last decade but post-harvest technology has not kept pace with such improvements. In the USA alone approximately 15 million dollars are lost annually due to improper drying of the crop. A multistage steady state concurrentflow/counterflow drying model was modified to simulate rice drying. The model was utilized to analyze the drying process and to develop recommendations for the design and operation of concurrentflow rice dryers. —_.——.— __ Carlos Fontana Concurrently dried medium- and long-grain rice maintained a better head yield, color, and seed viability than crossflow treated rice during the drying process even at inlet air temperatures of 150 C. The fuel efficiency of concurrentflow dryers was approximately 3,500 kJ/kg of water removed for long-grain rough rice and about 4,200 kJ/kg for medium-grain rough rice. Medium- and long-grain rough rice can be dried in a three-stage concurrentflow dryer from 20.0 - 13.7 percent moisture content in one drying pass at capacities of about 2.2 tons of dry matter per hour per square meter of bed area. The depth of the drying beds and tempering zones in a multistage concurrentflow rice dryer should be less than in equivalent shelled corn dryers. A substantial increase in dryer capacity can be obtained by limiting the bed depths to 0.6 meters. Crossflow dryers operating at inlet air temperatures between 70 and 80 C and high grain velocities (50 m/hr) present fuel efficiency of about 5,000 kJ/kg of water removed and acceptable head yields. Concurrentflow dryers present the potential of producing the best quality rice due to the ability of controlling the moisture removal and rice temperature of individual kernels in each drying stage. The i‘l [I‘ll I l ‘1 l \II‘IIIIII‘ Ill-Ill? l|l|\‘[ ,l\1l\|. |\I I\ l Carlos Fontana concurrentflow drying/tempering/counterflow cooling process improves the head yield of rice by gently sealing kernel fissures. QW/gw Chairman, Agricultural Engineering Department Major Professor \§;¢%i;19:§ ‘4': [till III I I‘ll lll‘ l’ll'llllll I‘lill‘lllIII-llllln‘l‘l‘llll‘ fl —— — —— ‘——-‘——qe/—¥ —— _.__. _ H 040-. (p 0 ~ ' 'f" ’1 (D (n U) ACKNOWLEDGEMENTS I am deeply indebted to my major professor Dr. F. W. Bakker-Arkema, who provided not only technical and moral support, but also friendship and examples that will positively influence my way of life. I am thankful to the other members of my guidance commettee, Dr. L. Copeland, Dr. J. F. Steffe, Dr. G. D. Schwab, and Dr. S. Harsh, for their assistance during this work. Special thanks is given to Dr. I. P. Schisler and Mr. M. Harding, for their help in the development of the computer programs. Special thanks also to Mr. C. M. Westelaken and D. Rastin of Blount, Inc., Montgomery, AL for their help with the experimental work. Special thanks and appreciation is sincerely expressed to Blount, Inc. and the Brazilian Government for financial support. The author thanks Mr. J. Molnar, general manager of DePue Warehouse Company, Williams, CA and Mr. L. Cranek, vice president of Rovi Farms, Inc., Edna, TX for their __-.._.___ n_—\,.__-.—_.p ,—_ . _— ——__ _- :ccperat For students Dr. J. C ration. For 5. Migan: thazks, _-.~:x"=—5- - cooperation during testing of concurrentflow dryers. For their moral support, the fellow graduate. students: Mr. E. N. Mwaura, Mr. S. J. Kalchik, and Dr. J. C. Rodriguez and their families deserve special mention. For their words of encouragements, Mr. and Mrs. E. Migandi and Mr. and Mrs. R. Chaffin deserve special thanks. A very special thanks is given to Sr. D. J. Reinert for his moral support and friendship. I am indebted to the faculty, staff, and graduate and undergraduate students of the Department of Agricultural Engineering, at Michigan State University for their friendship. The moral support of Dr. M. Correa and Mr. W. Coon is greatly appreciated. Deepest appreciation goes to my wife, Elena, for her patience and tolerance. To her, I can only offer myself and my love. A...‘ m ’F _—‘ 2m- 0? 1m op an o; LIST OF LIST OF LIST OF Chapter 1. 2. 3. TABLE OF CONTENTS Page TABLES I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I x F I GURES I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I O x vi i i SYMBOLS Ill-OIIOOIIIIIOOOOIOII.III...00.0.0... XXiV INTRODUCTION ..IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII l OBJECTIVES IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 4 LITERATURE REVIEW .....IIIIIIIIIIIIIIIIIIIIIII. 6 3.1 Rice in the United States, Brazil, and in the world IIIIIIIIIIIIIIIIIIIIIIIIIIII 6 3.2 Physical and Thermal Properties of Rough Rice IIIIIIIIIIIIIIIIIIIIIIIIIIIIII 9 3.2.1 Physical Properties of Rough Rice 10 3.2.1.1 Physical Dimensions ..... 12 3.2.1.2 wet Bulk Density IIIIIIII 14 3.2.1.3 Porosity IIIIIIIIIIIIIIII 16 3.2.1.4 Specific Surface Area ... 16 3.2.1.5 Diffusion Coefficient ... 17 3.2.1.6 Head Yield IIIIIIIIIIIIII 22 3.2.1.7 Pressure Drop ........... 23 3.2.1.8 Variation in Initial Moisture Content ........ 27 - Chapter 3.3 3.2.2 Drying 3.3.1 3.3.2 3.3.4 3.3.5 Page Thermal Properties of Rough Rice . 28 3I2I2.1 specific Heat IIIIIIIIIII 28 3.2.2.2 Convective Heat Transfer Coefficient IIIIIIIICIIII 30 3.2.2.3 Latent Heat of Vaporization ............ 31 3.2.2.4 Thermal Conductivity and Thermal Diffusivity ..... 32 of Rough Rice IIIIIIIIIIIIIIIIIIII 36 On-Farm versus Commercial Drying . 38 Types of Rice Dryers ............. 40 3.3.2.1 Crossflow Dryers ........ 40 3.3.2.2 Cascade Dryers .......... 44 3.3.2.3 Rotary Dryers ........... 47 3.3.2.4 Fixed-Bed (In-Bin) Dryers 50 3.3.2.5 Counterflow Dryers ...... 53 3.3.2.6 Concurrentflow Dryers ... 53 Rice Drying Models ............... 59 3.3.3.1 Single Kernel Drying .... 59 3.3.3.1.1 Empirical Models ........ 61 3.3.3.1.2 Diffusion Models ........ 65 3.3.3.2 Deep-Bed Drying Models .. 68 3.3.3.3 Equilibrium Moisture Content Models .......... 75 3.3.3.4 Tempering Models ........ 82 Drying of Cereal Grains Other than Rice oeeo'oooooooococoa-coo... 88 Quality Aspects of Rice .......... 92 vi ”VA 0 O O O O 0 l ..“l’ \ ‘. Chapter Page 4. ANALYSIS OF RICE DRYING SIMULATION ........... 98 4.1l Single Kernel Drying Simulation ......... 98 4.1.1 Non-Diffusion Thin-Layer Equations 99 4.1.2 Diffusion Thin-Layer Equations ... 105 4.1.2.1 Whole Kernel Model ...... 105 4.1.2.2 Composite Kernel Model .. 106 4.1.3 Comparing Drying Models and Drying of Rice Types ............. 123 4.2 Deep-Bed Drying Simulation .............. 130 4.2.1 The Fixed-Bed Model .............. 130 4.2.2 The Crossflow Model .............. 132 4.2.3 The Concurrentflow Model ......... 132 4.2.4 The Counterflow Model ............ 133 4.2.5 Solution of Deep-Bed Simulation Models ........................... 134 5. EXPERIMENTATION ...IIIIIIII.IIIIIIIIIIIIIIIII. 135 5.1 Concurrentflow Dryer .................... 135 5.1.1 Dryer Design IIIIIIIIIIIIIIIIIIIII 136 5.1.2 Procedure and Instrumentation .... 138 5.2 crOSSflow Dryer I.IIIIOIIOOIIOCIIIIOIIIII 141 5.2.1 Dryer Design IIIIIIIIIIIIIIIIIIIII 141 5.2.2, Procedure and Instrumentation .... 141 Fuel Efficiency Calculation ............. 144 Head Yield Determination ................ 144 Seed Viability Determination ............ 145 Color Determination ..................... 146 0101010! I I I I 0‘01le :2 rum-r- Chapter RESU 6.1 6.2 Chapter Page 6. RESULTS AND DISCUSSION ....................... 147 6.1 Experimental Results .................... 147 6.1.1 Concurrentflow Drying ............ 148 6.1.1.1 Standard Bed Depth ...... 148 6.1.1.1.1 Medium-Grain Rice ......... 148 6.1.1.1.2 Long-grain RiCe IIIIIIIII 158 6.1.1.2 Discussion of the Standard Bed Results .... 164 6.1.1.3 Shortenned Bed Depth .... 167 6.1.1.4 Discussion of the Shortenned Bed Depth Results IIOIII'IIIIIIIIOI 170 2 Crossflow Drying . I I I I I I I I I I I I I I I O 171 3 Discussion of Crossflow Drying Results .IIIIIIIIIIIIIIIIIIIIIIIII 175 6.1. 6.1. 6.2 Simulation Results IOIOIIIOIIIIIOIIIOOOOI 176 6.2.1 Concurrentflow Drying ............ 177 6.2.2 Crossflow Drying IIIIIIIDIIIIIIIII 193 6.2.3 Fixed-Bed Drying ooooouooooo-ooo-o 198 6.3 Comparison of Rice Drying Methods ....... 205 f viii V — 7 ms 7.7 7.: 7.: 7.4 7.! 7.7 7.. 7. 3 5:: a. a. 8. 9 cc 10 m L: APPENDICW 7, 3« a. 7 C. Chapter Page 7. DESIGN ANALYSIS OF CONCURRENTFLOW RICE DRYING 211 7.1 Standard Conditions for Dryer Simulation 211 7.2 Effect of Bed Depth ..................... 213 7.3 Effect of Tempering ..................... 230 7.4 Effect of Number of Stages .............. 236 7.5 Effect of Multipassing .................. 241 7.6 Effect of Initial Moisture Content ...... 243 7.7 Effect of Inlet Air Temperature and Moisture Content ........................ 244 7.8 Operation of Concurrentflow Rice Dryers . 248 8. SWARY IOIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 253 8.1 Concurrentflow Rice Drying .............. 253 8.1.]- Simulation Model IIIIIIIIOIIIIIIII 253 8.1.2 Quality Aspects of Dried Rice .... 254 8.1.3 Recommended Design Changes ....... 256 8.1.4 Operation of Concurrentflow Rice Dryers IIIIIIOICIIIIIIIIII.0...... 258 8.2 Crossflow and Fixed-bed Drying Models ... 259 8.3 Comparison of Rice Drying Methods ....... 260 9. CONCLUSIONS .................................. 262 10. RECOMMENDATIONS FOR FURTHER RESEARCH ......... 265 LIST OF REFERENCES ........................... 267 APPENDI ces ‘ A. Standards for Rough, Broyn, and Milled Rice .. 277 B. Table of Conversion Factors .................. 281 C- Typical Input/Output of the Concurrentflow Rice Drying Model .IIIIIIIIIIIIIIIIIIIIIIIIII. 282 Table 3.1 3.8 3.9 3.10 3.11 LIST OF TABLES Page United States rough rice production and yield (Rice Journal, 1981) IIIIIIIIIIIIIIOIIIIIIIIIIIII‘I 7 United States, Brazil, and World rough rice exports IIIIIIIIIIIIIIIIIII'IIIIIIIIIIIIIIIIIIIIII 7 Range of average grain size, shape, and weight measurements of 0.5. commercial long-, medium-, and short-grain types at 13.0 % moisture content (Webb, 1980) ..................................... 11 Range of average test weight of typical U.S. commercial long-, medium, and short-grain types at 13.0 % moisture content (Webb, 1980) .......... 11 Volume of long-, and medium-grain rough rice kernels (Wratten et a1., 1969) ................... 11 Wet bulk density of long-, medium-, and short- grain rough rice IIQIIIIIIIIIIIIIIIIIIIIIIIIII'III 15 Porosity of long- and medium-grain rough rice (Wratten et a1., 1969) ........................... 15 Diffusion coefficient for rough rice kernel and rough rice components assuming spherical kernels IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIOOOI 19 Diffusivity of rough rice and rough rice components as a function of temperature assuming spherical kernels IIIIII.IIIIIIIIIII'IOI'IIIIQ.IIIIIIIIIIOIII 19 Pressure drop across a bed of long-, medium-, and short-grain rough rice (Steffe et a1., 1980) ..... 24 Bulk specific heat of rough rice as a function of mOiSture Content IIIIIIOIIII...IOIIIIIOIIOOOOIOCOO 29 Table 3.12 3.13 3.155 3.16; 3.17 3.20 3.21 Page Convective heat transfer coefficient of medium- grain rough rice as a function of airflow rate at 45 C and 30 % relative humidity at atmos- pheric pressure (Wang et a1., 1979) .............. 29 Latent heat of vaporization of rough rice as a function of temperature at 25 % moisture content dry basis 0.0.000.000.0000......CCOUOOOIUOCOOIDOOO 35 Latent heat of vaporization of rough rice as a function of moisture content at 50 C temperature ....IDIOIOOOI...IOOODIOOOOOOIOOOOOOOOO 35 Bulk thermal conducivity and bulk thermal diffusivity of rough rice as a function of moisture content COOOIIIOOIOOIOOOOOOO...IIOOOIQOI. 37 Thin-layer drying equations for short-grain rice (Bakshi and Singh, 1979) 0.0.0.0...0.000.000.0000. 63 Moisture content distribution in a long-grain rough rice kernel during drying at 121 C in a concurrentflow dryer (Walker, 1978) .............. 84 Moisture content distribution in a long-grain rough rice kernel during tempering at 32 C in a concurrentflow dryer (Walker, 1978) .............. 84 Time required to achieve 95 percent and complete tempering when short-grain rough rice is dried at different temperatures and 50 % relative humidity and subsequently tempered (Steffe and Singh, 1980) ..................................... 87 Thin-layer drying of cereal grains at 49 C temperature and 60 % relative humidity ........... 87 Range of average milling yields of typical U.S. commercial long-, medium-, and short-grain types (Webb, 1980) ...-I0....OOI.........OIOIOOIIOOOIOI. Thin-layer drying of rough rice using empirical equations II...0.0.0...........II...I.O.I...IIO.CO 104 xi Table Page 4.2 Thin-layer drying of rough rice using diffusion theory and considering the rice kernel as a Sphere 00.00....0......IOOIIOI...IOIOIOIOOOCOOOIOO 111 4.3 Equilibrium moisture content for rough rice using the Henderson-Thompson, Chung-Pfost, and Zuritz models for 10 - 100 C temperatures and 1 - 99 % relative humidities .............................. 114 4.4 Simulated moisture content distribution (% w.b.) ‘ within a long-grain rice kernel during drying from 25 % MC at 45 C and 30 % relative humidity using a spherical diffusion equation (Steffe and Singh, 1980) ..................................... 116 4.5 Simulated moisture content distribution (% w.b.) within a long-grain rice kernel during tempering at 45 C and after being dried for 12, 36 and 60 min. at 45 C and 30 % relative humidity .......... 117 4.6 Time required to achieve 50, 75, and 95 percent tempering when long-grain rice is tempered at 45 C after being dried at 45 C and 30 % relative humidity for 12, 36, and 60 min. . ...... .......... 116 4.7 Simulated moisture content of long-grain rice dried at 45 C and 30 % relative humidity for 12 and 36 min. after being dried to 17.2 % w.b. at 45 C and 30 % relative humidity for 36 min. and 0, 50, 75, and 95 % tempering .................... 117 4.8 Simulated moisture content distribution (% w.b.) within a medium-grain rice kernel during drying from 25 % MC at 45 C and 30 % relative humidity using a spherical diffusion equation (Steffe and Singh, 1980) ..................................... 119 4-9 Simulated moisture content distribution (% w.b.) within a medium-grain rice kernel during tempering at 45 C and after being dried for 12, 36, and 60 min. at 45 C and 30% relative humidity ......................................... 120 4'10 TWJne required to achieve 50, 75, and 95 % tempering ‘Vhen medium-grain rice is tempered at 45 C after lacing dried at 45 C and 30 % relative humidity f<3r 12, 36, and 60 min. .......................... 119 ' Table Page 4.11 Simulated moisture content of medium-grain rice dried at 45 C and 30 % relative humidity for 12 and 36 min. after being dried to 20.5 % w.b. at 45 C and 30 % relative humidity for 36 min. and 95 % tempering ................................... 120 4.12 Simulated moisture content distribution (% w.b.) within a short-grain rice kernel during drying from 25 % MC at 45 C and 30 % relative humidity using a spherical diffusion equation (Steffe and Singh, 1980) ..................................... 121 4.13 Simulated moisture content distribution (% w.b.) within a short-grain rice kernel during tempering at 45 C and after being dried for 12, 36, and 60 min. at 45 C and 30 % relative humidity .......... 122 4.14 Time required to achieve 50, 75, and 95 percent tempering when short-grain rice is tempered at 45 C after being dried at 45 C and 30 % relative humidity for 12, 36, and 60 min. ................. 121 4.15 Simulated moisture content of short-grain rice dried at 45 C and 30 % relative humidity for 12 and 36 min. after being dried to 21.1 % w.b. at 45 C and 30 % relative humidity for 36 min. and 0, 50, 75, and 95 % tempering .................... 122 4.16 Physical and thermal property constants of long-, medium-, and short-grain rice used in the computer simulation models ....................... 131 6.1 Average ambient conditions and experimental air and rice temperatures for the successful tests conducted at Williams, CA with the three-stage CCF dryer and with medium-grain rough rice ....... 149 6-2 Average ambient conditions and experimental air and rice temperatures for the unsuccessful tests conducted at Williams, CA with the three-stage CCF dryer and with medium-grain rough rice ....... 151 6'3 Tqiree-stage CCF dryer performance for the successful tests conducted at Williams, CA with medium—grain rough rice 0.0.0....IOIOIIOOIUOOOIOCO 153 xiii Table ' Page 6.4 Three-stage CCF dryer performance for the unsuccessful tests conducted at Williams, CA with medium-grain rough rice ..................... 153 6.5 Bulk density and milling yield for the successful tests conducted with the three-stage CCF rice dryer at Williams, CA, and with medium-grain rough rice ....................................... 155 6.6 Bulk density and milling yield for the unsuccessful tests conducted with the three-stage CCF rice dryer at Williams, CA, and with medium-grain rough rice ....................................... 157 6.7 Ambient conditions and experimental air and long- grain rough rice temperatures for the 1981 and 1982 tests with the two-stage CCF dryer at Edna, TX ......................................... 159 6.8 Two-stage CCF dryer performance for the 1981 and 1982 tests conducted at Edna, TX with long-grain rough rice ..OOOOOOOOOI.........IOOCOIOOOIIOOIOOO. 159 6.9 Long-grain rough rice milling yield results for the 1981 and 1982 tests conducted at Edna, TX with the two-stage CCF rice dryer ................ 162 6.10 Seed viability (germination) of long-grain rough rice dried in a two-stage CCF dryer during the 1982 drying tests at Edna, TX .....IOOIIOOOCOOOOOO 163 6.11 Average ambient conditions and experimental air and rice temperatures for the tests conducted at _ Louise, TX, with the three-stage CCF dryer and long-grain parboiled rice ........................ 163 6.12 Three-stage CCF dryer performance for the tests conducted at Louise, TX with long-grain parboiled rice ................................... 169 6-13 Milling yield of concurrentflow dried long-grain parboiled rice ......OOIOIOOOOICOOOI...0.0.00.0... 169 6'14 HUnterlab Color Difference Meter results for long- grain parboiled and long-grain white rice ........ 169 Table 6.15 6.19 Page Average ambient conditions and experimental air and rice temperatures in a crossflow rice dryer at Edna, TX COCO-......Q0........IOOOOOOODOCOIOOOO 172 Experimental crossflow dryer performance for long- grain rough rice at Edna, TX ..................... 174 Milling yield of crossflow dried long-grain rough rice at Edna, TX, 1982 ........................... 174 Typical computer output of the simulation results of the first stage of a three-stage concurrent- flow dryer (Test 2, Table 6.3) drying medium- grain rice at 30 C and 36.5 m3/min/m2 ............ 179 Internal moisture content distribution (% w.b.) in a medium-grain rice kernel after leaving the first stage of the three-stage concurrentflow dryer simulated in Table 6.18 and after tempering in the 5.2 m tempering zone for 1.4 hours aSSuming spherical kernel ................................. 179 Auxiliary values of the simulation in Tables 6.18 and 60 9IOIIODIOOCOOIOIQO......OOOIOOOOOOIOIOOOIO 182 Simulated results of the counterflow cooler of the three-stage concurrentflow dryer (Test 2, Table 6.3) cooling medium-grain rough rice at 14 C and 10.6 m3/min/m2 ................................... 182 Comparison between experimental and simulated drying of medium-grain rough rice in a three- stage concurrentflow dryer in Williams, CA, 1981 (Test No. 2, Table 6.3) .......................... 183 Comparison between experimental and simulated drying of medium-grain rough rice in a three- stage concurrentflow dryer in Williams, CA, 1981 (Test No. 13, Table 6.3) ......................... 185 Comparison between experimental and simulated drying of medium-grain rough rice in a three- stage concurrentflow dryer in Williams, CA, 1981 (Test No. 23, Table 6.4) ......................... 189 XV Table Page 6.25 Comparison between experimental and simulated drying of long-grain rough rice in a two-stage concurrentflow dryer in Edna, Tx, 1981 (Test No. 1, Table 6.8) .......................... 192 6.26 Simulation results of a crossflow dryer with grain mixing device (turn-flow) drying long- grain rice II.0.0.0...I.I.......OOOIOOIOIOIIOIOOOC 194 6.27 Comparison between experimental and simulated drying of long- grain rice in a crossflow dryer in Edna, TX, 1982 00......0.00....-OOOOIIOOICCOOOOCOO 197 6.28 Simulation results of a fixed-bed dryer with medium- grain rough rice at 25 C and 0.08 m3/min/m3 ...... 199 7.1 Standard conditions for concurrentflow dryer simulation 00.00.0000...I.........IOOIIOCIIOIOOIOO 212 7.2 Standard inputs for the simulation runs to investigate the effect of bed depth on concurrent- flow dryer performance ....... ...... .............. 212 7.3 Theoretical power requirement as a function of air- flow rates and static pressure for four bed depths of medium-grain rough rice ....................... 215 7.4 Theoretical power requirements as a function of air- flow rates and static pressure for four bed depths of long-grain rough rice ......................... 216 7-5 Simulations for the three-stage concurrentflow dryer with medium-grain rough rice investigating the effect of bed depths on dryer performance ........ 219 7-5 Simulations for the three-stage concurrentflow dryer with long-grain rough rice investigating the effect of bed depths on dryer performance ............... 227 7-7 Simulations for the three-stage concurrentflow dryer with medium-grain rough rice investigating the effect of tempering on dryer performance ......... 231 Table Page 7.8 Simulations for the three-stage concurrentflow dryer with long-grain rough rice investigating the effect of tempering on dryer performance ......... 233 7.9 Simulations for the two- and three-stage concurrent- flow dryers with medium-grain rough rice ......... 237 7.10 Simulations for the two- and three-stage concurrent- flow dryers with long-grain rough rice ........... 240 7.11 Dryer settings for a three-stage concurrentflow rice dryer with medium-grain rough rice using a double pass System ...-...IIOIDOI......CIOIOO0.00.0000... 249 7.12 Dryer settings for two- and three-stage concurrent- flow rice dryers with medium-grain rough rice using a single pass system ....................... 251 xvii LI ST OF FIGURES Figure Page 3.1 Diffusivity of rough rice and rough rice components (Table, 3.9) ....................... 20 3.2 Resistance of rough rice to airflow (Steffe et a1., 1980' Table 3.10) ......CODOOOCOOOOOIIOOOO 25 3.3 Latent heat of vaporization of rough rice as a function of temperature at 25 % moisture content, dry basis OI.......IIIOIOOOOOOOOOOOOOI 33 3.4 Latent heat of vaporization of rough rice as a function of moisture content at 50 C temper- ature ...-.....OOOIOIOOOIII.....OIIOIOOOIOIO... 34 3.5 A non-mixing-type rice dryer (Steffe et a1., 1980) ......OIIOOOOOIIO......IIOODOOOUCCOOIOOOC 41 3.6 Schematic of a non-mixing-type rice dryer (Wasserman and Calderwood, 1972) .............. 41 3-7 A mixing-type (baffle) rice dryer (Steffe et aloyla ....IODOIOOOODOOOOO....ICIDOIOOCOOCI 43 3-8 Schematic of a mixing-type (baffle) rice dryer (Wasserman and Calderwood, 1972) .............. 43 3-9 Conventional continuous crossflow dryer (Brooke: et a1., 1974) ......C....-............ 45 3'10 Cross-section of a columnar batch dryer (Brooker et a1., 1974) ........................ 45 3'11- Continuous crossflow dryer with reversedflow cooling (Brooker et a1., 1974) ................ 46 3'12 Continuousflow-type dryer of the Louisiana State University design (Steffe et a1., 1980) . 46 xviii W‘_——————' . Figure Page 3.13 Schematic of a cascade grain dryer (Bakker- Arkema et a1., 1978) .......................... 48 3.14 Schematic of a triple pass rotary dryer ....... 49 3.15 A circular rice drying/storage bin with a stirring device (Steffe et a1., 1980) ......... 51 3.16 A rectangular structure for drying rice (Steffe etal.'1980) ......OIO.......IOCOOICIICOOCIICI 51 3.17 Concurrentflow and counterflow grain drying systems (Steffe et a1., 1980) ................. 54 3.18 Schematic of an on-farm concurrentflow dryer . (Brookereta10’1974) ......IIOIOOOIOOIOOIIOI. 54 3.19 Schematic of a commercial two-stage concurrent- flow dryer with counterflow cooler and air recirculation (Blount, Inc.) .................. 56 3.20 Block diagram of a two-stage concurrentflow rice dryer with counterflow cooler and air recirculation 0.0.0..........OIOOOOOOIOOOCIOOOO 57 3-21 Schematic of the drying floor of the Blount concurrentflow dryer .........IIOOOOOIIOOOOOOO. 58 3°22 Rough rice equilibrium moisture content for the Henderson-Thompson equation ................... 77 3'23 Rough rice equilibrium moisture content for the Chung—Pfost equationI.O...-IOCOIOODIOOIOCCUOOOO 78 3'24 Rough rice equilibrium moisture content at low temperatures for the Zuritz equation .......... 80 3'25 Rough rice equilibrium moisture content at high temperatures for the Zuritz equation .......... 81 3‘26 Thin-layer drying of six cereal grains at 49 C and 60 % relative humidity .................... 91 3‘27 Rice milling process, approximate product pro- portions and main uses (Willson, 1979) ........ 93 Figure 4.1 4.2 4.10 4.11 Drying of short-grain rough rice using the thin-layer equation by Agrawal and Singh (1977) Drying of medium-grain rough rice using the quadratic equation by Wang and Singh (1978) ... Drying of medium-grain rough rice using the single term approximate form of diffusion equation by Wang and Singh (1978) ............. Drying of medium-grain rough rice using the Page type equation by Wang and Singh (1978) ... Long-grain rough rice moisture content versus time when dried in a thin layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Steffe and Singh (1982) ........................................ Long-grain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Wang and Singh (1978) ........................................ Medium-grain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Steffe and Singh (1982) ........................................ Medium-grain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Wang and Singh (1978) ........................................ Comparison of equilibrium moisture content models for rough rice at 40 c ................. Thin-layer drying of rough and parboiled short- grain rice (Bakshi and Singh, 1979) ........... Comparison of thin-layer drying equations for rough rice when dried at 30 C and 20 % relative humidity 00......IIIOOIIOOOOOOOO.ICOOIIOOOCOOIII XX Page 100 101 102 103 107 108 109 110 113 124 126 Figure 4.12 4.13 6.2 6.4 Comparison of thin-layer drying equations for rough rice when dried at 40 C and 20 % relative humidity ...II'IIOOIOIOOOOOC......IIICOOCOIOIOO Comparison of thin-layer drying equations for rough rice when dried at 50 C and 20 % relative humidity ......OOICIICIOOICI...I.......IOIIICOO Comparison of thin-layer drying equations for rough rice when dried at 60 C and 20 % relative humidity 0.00.0.0...0.0.0..........OOIOOOOOI... Schematic of the crossflow dryer and location of the thermocouples (Table 6.15) ............. Simulated air and rice temperatures versus time and dryer length in the first 10 cm of the drying bed of a three-stage CCF dryer (Test No. 2, Table 6.3) ............................. Simulated temperature (air and rice) and moisture content versus dryer length and time (Test No. 2, Table 6.3) in the three-stage CCF rice dryer .................................... Simulated temperature (air and rice) and moisture content versus dryer length and time (Test No. 13, Table 6.3) in the three-stage CCF rifle dryer .................................... Simulated temperature (air and rice) and moisture content versus dryer length and time (Test No. 23, Table 6.4) in the three-stage concurrentflow rice dryer ..................... Simulated temperature (air and rice) and moisture content versus dryer length and time (Test No. 1, Table 6.8) in the two-stage CCF rice dryer .................................... Simulated rice temperature and moisture content versus time and dryer length in a crossflow dryer ............................... Drying time at different inlet air tempera- tures and airflow rates for a 3 m fixed-bed rough rice dryer IOOOOOOOIOOOOOOIOIOOIOO...-... xxi Page 127 128 129 142 178 181 185 188 191 195 202 Figure 6.8 7.5 Page Energy efficiency at different inlet air temperatures and airflow rates for a 3 m fixed-bed rough rice dryer .................... 203 Moisture content gradient at different inlet air temperatures and airflow rates for a 3 m fixed-bed rough rice dryer .................... 207 Direction of flow and equipment required for a multipass crossflow rice drying system (De Padua' 1976) ......OIOOOOOOOOOOOIOOOOOIOIII 208 Power requirement versus airflow rate for medium-grain rough rice (Table 7.3) ........... 217 Power requirement versus airflow rate for long-grain rough rice (Table 7.4 .............. 218 Temperature (air and medium-grain rice) and moisture content versus dryer length and time of the standard three-stage CCF dryer design (Run No. 1, Table 7.5) ........................ 221 Temperature (air and long-grain rice) and moisture content versus dryer length and time of the standard three-stage CCF dryer design (Run No. 1, Table 7.6) ........................ 222 Temperature (air and medium-grain rice) and moisture content versus dryer length and time of the three-stage CCF dryer with 0.61 m bed depths (Run No. 7, Table 7.5) ................. 225 Temperature (air and long-grain rice) and moisture content versus dryer length and time of the three-stage CCF dryer with 0.61 m bed depths (Run No. 7, Table 7.6) ................. 226 Moisture removal from medium-grain rough rice at different inlet air temperatures and airflow rates in the top stage of a three- stage CCF rice dryer .......................... 246 Maximum medium-grain rough rice temperature at different inlet air temperatures and airflow rates in the top stage of a three- stage CCF rice dryer .......................... 246 xxii Figure 7.9 Page Outlet medium-grain rough rice temperature at different inlet air temperatures and airflow rates in the top stage of a three-stage CCF rice dryer .................................... 247 Dryer energy efficiency with medium-grain rough rice at different inlet air tempera- tures and airflow rates in the top stage of a three-stage CCF rice dryer .................... 247 xxiii y X m CF Ca Ce Co CD Cv Cw cm cm3 LIST OF SYMBOLS constant specific surface area, m2/m3 thermal diffusivity, m2/hr constant constant constant local moisture concentration, kg/m3 concurrentflow crossflow specific heat of air, kJ/kg/C equilibrium moisture concentration, kg/m3 initial moisture concentration, kg/m3 specific heat of product, kJ/kg/C specific heat of vapor, kJ/kg/C specific heat of water, kJ/kg/C centimeter cubic centimeter diffusion coefficient, m2/hr xxiv DELM DTIME D1 D2 D3 db EMC Ga Gp gm hfg hr *9 RJ kPa kW lb moisture reduction, decimal dry basis elapsed time, hr diffusivity of starchy endosperm, m2/hr diffusivity of bran, mZ/hr diffusivity of hull, m2/hr dry basis equilibrium moisture content degrees farenheit constant dry weight flow rate of air, kg/hr/m2 dry weight flow rate of product, kg/hr/mz gram humidity ratio, kg/kg convective heat transfer coefficient, kJ/kg/hr/C latent heat of vaporization, kJ/kg hour degrees kelvin thermal conductivity, W/m/C kilogram kiloJoule kilopascal kilowatt bed depth of the dryer, m Pound: 1°Cal moisture content, decimal dry basis 2‘ MC Me MO MP Mw Pa 2 R2 R3 rpm SP average moisture content, decimal dry basis moisture content moisture ratio equilibrium moisture content, decimal dry basis initial moisture content, decimal dry basis moisture content, percent dry basis moisture content, percent wet basis meter minutes millimeter square meter cubic meter cubic millimeter porosity, percent pascal airflow rate, m3/s/m2 rough rice equivalent radius, m relative humidity, decimal white rice equivalent radius, m brown rice equivalent radius, m rough rice equivalent radius, m kEI'nel radial coordinate, m revolution per minute bulk, density, kg/m3 Static pressure, Pa Second xxvi TF Ta Tabs tm ton ts tt wb air temperature, C air temperature, F absolute temperature, K absolute temperature, R drying time, hr drying time, min metric ton, 1,000 kg drying time, s tempering time, hr product temperature, C velocity, m/hr wet basis constant bed depth coordinate, m constant bed width coordinate, m xxvii CHAPTER 1 INTRODUCTION Rice has beén the major food of millions of people around the world. It is today the basic source of food for more than 70 percent of the world population (Willson, 1979). Rice was grown in China around 3,000 B. C., and introduced in India around 2,800 B. C. Attempts were made to grow rice in North America as early as 1609; rice became firmly established in South Carolina around 1690 (Adair, 1972) . Rice was introduced in California in 1909 and had beeome a commercial crop by 1912 (Walker, 1978). The UnitEG States is one of the largest rice exporting countries in the world. The main U. S. producing areas are located in Arkansas, California, Texas, and Louisiana. Three types of rice are grown in the United States: long‘grain, medium-grain, and short-grain rice. Most of th . e rlce produced is of the long-grain and medium-grain is no longer type; the production of short-grain rice significant. Rice is grown around the world under four different ecosystems: irrigated, rainfed, deep water, and upland rice. The rice moisture content (grain) should be between 23 and 25 percent at harvest to minimize grain breakage during milling. Consequently, drying after harvest is required (Willson, 1979). Much rice around the world is dried in the field using the sun's energy. In the United States most of the rice was dried under field conditions until the 1930's. During the 40's and 50's the sack dryer method of rice drying was POFHJlar. The non-mixing columnar crossflow-type dryer be(tame the first continuous flow rice dryer to be used in the IJnited States in the 40's along with the baffle mixing Cl'OSSflow dryer (Willson, 1979). The cascade or rack dryer (als¢> known as the Louisiana State University or LSU dryer) has laeen used since 1943. Rotary-type dryers are still in Use t°day, especially to dry high moisture parboiled rice. The “Se of rotary dryers has been questioned because of the high e1"Iergy consumption per kg of water removed. A new dryer on the market is the concurrentflow-type dryer which emp1°Ys high drying air temperatures resulting in high fine er’jf ‘Eefficiency and uniform product quality. The primary market for rice is for whole kernels of milled or "head" rice. Therefore, a successful rice culture requires proper drying after the grain is harvested. The correct temperature, time of exposure, and moisture removal rate are the main factors affecting rice quality. The concurrentflow dryer, especially the multistage model, poses more challenges than the crossflow dryer in establishing the optimum operating conditions with respect to drying air temperatures, airflow and grainflow rates, and bed depths. Using computer simulation, the task of producing optimum quality (e. 9. highest head yield) rice at the lowest cost can itself be optimized. 1 CHAPTER 2 O B J E C T I V E S To model multistage steady state concurrentflow rice drying using the best available deep bed models along with the equations for equilibrium moisture content, convective heat transfer coefficient, latent heat of vaporization, and thin-layer drying (or diffusion). To determine the quality aspects of concurrent and crossflow dried rice with respect to head yield, color and seed viability, and to establish the critical conditions for preventing grain damage. To develop a set of design recommendations for concurrentflow rice drying with respect to bed depths, airflows, tempering-zone lengths, inlet air temperatures and grainflow rates. To develop a set of recommendations for concurrentflow dryer operation (e. g. the best setting of the drying air temperatures, airflows, and grainflow rates in order to obtain a target final moisture content) including the number of stages to be in operation for drying low moisture content rice. To experimentally test concurrentflow and crossflow rice drying with respect to fuel efficiency, rice quality, and capacity. To model crossflow and in-bin rice drying using current versions of Michigan State University (MSU) drying models. To compare the performance (capacity, fuel efficiency, and grain quality) of concurrentflow, crossflow, and in-bin rice dryers. CHAPTER 3 LITERATURE REVIEW 3.1 Rice in the United States, in Brazil, and in the World Table 3.1 presents the United States rice production and yield. In 1980, rice production in the United States was 6.5 million metric tons as compared to 5.1 in 1974. The rough rice yield has remained about the same at 5,000 kg per hectare. The State of Arkansas produces more than 30 percent of the rough rice and the State of California has the highest average yield (about 6,000 kg per hectare). The United States rice production is relatively small compared to the world production (400 million metric tons). since most of the United States production is exported (Table 3.2), the world market is very important to U.S. farmers. r7 . we, Table 3.1 United States rough rice production and yield (Rice Journal, 1981). Year State ’ Production Percentage Yield (metric tons) of U.S. (kg/ha) Arkansas 1,482,296 29 5,083 California 1,138,972 22 6,030 Louisiana 1,092,705 21 4,091 1974 Mississippi 204,706 4 4,685 Missouri 24,675 l 4,355 Texas 1,145,485 23 5,040 U.S. total 45,098,112 100 4J967 Arkansas 1,830,792 35 5,346 California 998,676 19 6,187 Louisiana 1,007,112 19 4,382 1976 Mississippi 274,333 5 4,707 Missouri 26,671 1 4,707 Texas 1,108,127 21 5,391 U.S. total 5,245,711 100 5,226 Arkansas 2,200,152 36 4,842 California 1,160,200 19 5,851 Louisiana 1,017,182 17 4,281 1978 Mississippi 414,493 7 4,763 Missouri 58,876 1 4,853 Texas 1,189,593 20 5,268 U.S. total 6,040,497 100 5,026 Arkansas 2,369,796 37 4,539 California 1,576,145 24 6,893 Louisiana 963,658 15 3,923 1980 Mississippi 419,573 7 u,147 Missouri 81,647 1 4,483 Texas 1,066,851 16 4,483 U.S. total 6.477.670 100 4,785 Table 3.2 United States, Brazil, and World rough rice exports. Rice exports (metric tons) Year United States (1) Brazil (2) World (2) 1974 2,014,519 42,093 13,020,672 1975 2.563.521 3.799 12,662,257 1976 2,976,406 71,678 13,224,438 1977 3.303.085 564-323 15,533,529 1978 3,489,111 171,289 14,038,088 7979 - 3.833.938 418 18,181,765 1980 4,083,485 - - (1) Rice Journal (1981) (2) Lavoura Arrozeira (1982) The price of medium-grain milled rice in May 1982 was just over half the price of May 1981 (17 dollars per hundred weight versus 30 dollars per hundred weight, f.o.b. Arkansas). This resulted from the overalll world recession and the low purchasing power of the rice importing countries, especially the oil producing countries (USDA, 1982). In 1980, 63 percent of the United States rough rice production was exported (Tables 3.1 and 3.2). In 1979, the United States rough rice exports represented 27 percent of the world exports- The World rice crop in 1982/1983 is about equal to the 1981/1982 record of 276 million tons of milled rice (411 million tons of rough rice; USDA, 1982). In 1980, an estimated 46 million tons of cereal grains (corn, soybeans, rice, and wheat) were produced in Brazil (Veja, 1980). The rice production is variable and varies from seven to nine million tons (Dorfman and Rosa, 1980). The irregular production is mainly due to the lack of irrigation in about 90 percent of the rice fields. The irrigated fields, with only ten percent of the area, account for 30 percent of the Brazilian rice production and have an average yield of 3,750 kg per hectare. The overalll average rough rice yield in Brazil has remained unchanged (1,500 kg per hectare) since 1960 (Lavoura grtozeira, 1982) . n. 'V-i I“ The per capita consumption of rice in Brazil is about 45 kg compared to five kg in the United States, 100 kg in Japan and 58 kg worldwide (Lu and Chang, 1980). In Brazil, the irrigated rice is combine harvested at moisture contents between 15 and 20 percent and dried in continuous flow (cascade or crossflow) and batch type (crossflow and in-bin) dryers at temperatures of about 30 to 40 degrees C. The upland rice is harvested at lower moisture contents (13 to 18 percent) and sundried or heated air dried if artificial drying is needed. Most of the Brazilian rice is handled and stored in bags. 3.2 Physical and Thermal Properties of Rough Rice The knowledge of physical and thermal properties of rough rice is essential when investigating heat and mass transfer phenomena in rice drying (Morita and Singh, 1977). A limited amount of information is available on these properties. Properties like physical dimensions, bulk density, specific surface area, specific heat, latent heat of vaporization, convective heat transfer coefficient and diffusion coefficient are required in the computer simulation models. In this dissertation, moisture content is referred as wet basis, unless otherwise specified. Rough rice (paddy rice) can refer to long-, medium-, or short-grain rice (the type of rice is often not mentioned in the literature). 3.2.1 Physical Properties of Rough Rice The physical properties of rice required in dryer Simulation models are: the physical dimensions, the bulk density, the specific surface area and the diffusion coefficient. The physical dimensions and bulk density vary With rice variety, type, and moisture content. If the rice to be dried is of a known variety, direct measurements can be made of the kernel dimensions and bulk density. If the rice to be dried is a mixture of varieties, the values listed in Tables 3.3 and 3.4 can be used. The dry matter density (dry bulk density) is an input in the dryer simulation models. From the moisture content and Wet bulk density, the dry matter density of rice can be directly calculated. The specific surface area of a bed of ”tee is calculated from the average individual kernel surface area, the weight of the kernel, and the wet bulk densi ty' 11 Table 3.3 Range of average grain size, shape, and weight measurements of U.S. commercial long-, medium-. and short-grain types at 13.0 2 moisture content (Webb, 1980). Grain Grain Average Average Average Ratio Average 1000 type form length width thickness (length/ grain weight (M) (m) (M) wi d t h) (9!!!) long 6.7-7.0 1.9-2.0 1.5-1. 3.h-3.6 15-18 medium milled 5.5-5.8 2.h-2.7 1.7-1.8 2.1-2.3 17-21 short 5.2-5.h 2.7-3.1 1.9-2 0 1.7-2.0 20-23 long 7.0-7.5 2.0-2.1 1.6-1.8 3.h-3 6 16-20 medium brown 5.9-6.1 2.5-2.8 1.8-2.0 2.2-2 h l8-22 short 5.h-5.5 2.8-3.0 2.0-2.1 1.8-2 0 22-2h long 8.9-9.6 ' 2.3-2 5 1.8—] 9 3.8-3.9 21-2h medium rough 7.9-8.2 3.0-3 2 1.9-2 l 2.5-2.6 23-25 short 7.h-7.5 3.1-3 6 2.1-2.3 2.1-2. 26-30 Table 3.h Range of average test weight of typical U.S. commercial long-, medium-. and short-grain types at 13.0 X moisture content (Webb, 1980). Grain Average bushel Average bulk type weight (lb) . density (kg/cubic meter) long . ‘42-‘45 5““579 medium Ali-A7 566-605 slwort hS-hs 579-618 Table 3.5 Volume of long-, and medium-grain rough rice kernels (Wratten et al., 1969). "0 i sture long-grain medium-grain COI‘Itent (* w.b.) Volume No. kernels per Volume No. kernels per (mm3/kernel) unit volume(cm3) (mm3/kernellunit volume(cm3) (bulk) (bulk 18 26 15 2h 19 27 17 26 20 28 20 28 21 30 22 29 12 Diffusion coefficients are experimentally determined. 3.2.1.1 Physical Dimensions The physical dimensions of rough rice have been reported by Wratten et a1. (1969), Morita and Singh (1977), and Webb (1975, 1980). Physical dimensions vary with rice type and rice form. The classification of rice types in the United States is based on the dry brown rice dimensions as follows (Webb and Stermer, 1972): a) long-grain rice has an average length of 6.61 to 7..50 mm and an average length to width ratio over 3.00. b) medium-grain rice has an average length of 5.51 to 6.60 mm and an average length to width ratio between 2.10 and mom c) short-grain rice has an average length of 5.50 mm c>r less and an average length to width ratio of up to 2.10. The forms of rice are the following: a) rough rice are kernels of rice with hull, bran, germ, and starchy endosperm. b) brown rice are kernels of rice from which the hulls haVe have been removed. c) milled rice are kernels of brown rice from which the bran and germ have been removed. Table 3.3 presents the range of average grain size, shape, and weight among U.S. commercial long-, medium-, and short-grain types . The volume of single rough rice kernels (medium- and long-grain) was determined by Wratten et a1. (1969) as a function of moisture content. For long-grain rough rice the kernel volume is: V = 14.40 + 0.229 * MW (3.1) and for medium-grain rough rice: V = 9.73 + 0.508 * Mw (3.2) Where V is the volume of a single kernel (mm3) and Mw is the moisture content (percent, wet basis). Table 3.5 presents the volume of long- and medium-grain rough rice kernels at different moisture contents. The short-grain rough rice estimated volumes at 30.0 and 15.0 percent moisture content were determined by Steffe and Singh (1980c) as 24.4 and 21.8 mm3, respectively. This represents a volumetric reduction of 10.7 percent during the drying process. L 1‘ 7 14 3.2.1.2 Wet Bulk Density Bulk density of rough rice has been determined by Lorenzen (1958), by Wratten et al. (1969), by Morita and Singh (1977), and by Webb (1980). Table 3.4 presents the test weight of typical U.S. commercial long-, medium-, and Short-grain types at storage moisture content (13 percent). bulk densities as a function of moisture content have (1969) for long- and Viet been reported by Wratten et a1. inedium-grain rice (loose fill) and by Morita and Singh (21977) for short-grain rough rice (packed fill). long-grain rice f = 519.4 + 5.29 * Mw (3.3) medium-grain rice f = 499.7 + 8.33 * Mw (3.4) short-grain rice 9 = 583.6 + 4.27 * Mw (3.5) where f is the wet bulk density (kg/m3) and Mw is the moisture content (percent, wet basis). long-, Table 3.6 presents wet bulk density values for mEdium-v, and short-grain rice types. 15 Table 3.6 Wet bulk density of long-. medium-. and short-grain rough rice. Moisture Bulk density (kg/cubic meter) content (Zwb) Wratten et al., l969 Morita and Singh, 1977 long-grain medium-grain short-grain 0 519 500 58h 5 5H6 5k] 605 10 572 583 626 15 599 625 6h8 20 625 666 669 25 652 708 690 30 678 750 712 Table 3.7 Porosity of long- and medium-grain rough rice (Wratten et al., 1969). Moisture Porosity (percent) content (*wb) long-grain medium-grain 10 61 60 15 58 56 20 56 51 25 51. #7 16 3.2.1.3 Porosity The porosity of a bed of rough rice (long- and medium-grain) has been determined by Wratten et a1. (1969) as_a function of moisture content: long-grain rice P = 65.55 - 0.475 * Mw (3.6) medium-grain rice P a 69.05 - 0.885 * Mw (3.7) Where P is the porosity (percent) and Mw is the moisture content (percent, wet basis). Table 3.7 presents the porosity of a bed of long- and medium-grain rough rice . 3.2.1.4 Specific Surface Area The surface area of rough rice kernels has been studied by Wratten et a1. (1969), by Hosokawa and Motohashi (1975) , and by Morita and Singh (1977) . Different methods were used yielding different results. The method used by Morita and Singh (1977), that considers the small ridges found on the surface of the rough rice kErnels, is more accurate. This method produces results as much as 30 to 100 percent higher than the methods used by g 17 other researchers. A surface area of 0.00008723 square meters per kernel was obtained by Morita and Singh (1977) for short-grain rough rice at 18 percent moisture content. Wratten et a1. (1969) reported a value of 0.00004245 square meters per kernel of medium-grain rough rice. The number of kernels per unit volume (Table 5.3) can be computed from the weight of each kernel (Table 3.3) and the wet bulk density (Table 3.4 or 3.6). The specific surfade area can be calculated by multiplying the surface area per kernel by the number of kernels per unit volume. The calculated values (by the author) for specific surface area for long-, medium-, and Short-grain rough rice are: 2437 (m2/m3), 2361 (m2/m3), and 2054 (m2/m3), respectively. These values are used in this dissertation. If the surface area by Wratten et a1. (1969) is used (about half the value presented by Morita and Singh, 1977) the MSU concurrentflow model predicts a 20 percent decrease in the drying rate. 3.2- 1.5 Diffusion Coefficient Diffusion coefficients for rough rice have been determined by Wang and Singh (1978), by Steffe and Singh (19803 , 1982), by Hussain et al. (1973), and by Bakshi and 18 Singh (1979). Tables 3.8 and 3.9 present the temperature dependency of the diffusion coefficient for medium- and short-grain rough rice. Hussein et a1. (1973) were the only researchers to propose a moisture dependent diffusion coefficient for rough rice (Bluebelle variety): D=A*exp(B*Mp) (3.8) with A = 9.48787E-B*exp(-7730.65/1.8*Ta) B - 8.8333-4*(l.8*Ta)-0.3788 D is the diffusion coefficient (m2/hr), Mp is the moisture Content (percent, dry basis), and Ta is the absolute temperature (K). The rice kernel was considered a cylinder during the development of Eq. (3.8). The experimental data fias obtained at air temperatures of 48.9, 60.0, and 71.1 C, at relative humidities of 20 and 60 percent, and at initial moisture contents of 20.6 and 24.2 percent wet basis. The diffusivity for rough rice (whole kernel) and for rough rice components is shown in Fig. 3.1. The endosperm offers the least resistance and the hulls the most resistance to moisture flow. There appears to be no logical explanation for the large difference in the values 015 the diffusion coefficients for whole kernels proposed by Steffe and Singh (1982) and by Wang and Singh (1978). Moisture dependent diffusion coefficients for corn were reported by Chu and Hustrulid (1968) and for soybeans by sabbah et a1. (1979). The diffusion coefficients for L 19 Table 3.8 Diffusion coefficient for rough rice kernel and rough rice components assuming spherical kernels. _.__‘_ I D - A exp (B/Ta) 1 Diffusivity (square meter/hr) A 8 Rough rice starchy endosperm (1) 0.00257 -2880. Rough rice bran (1) 0.79700 -5110. Rough rice hull (l) &8&.00000 -7380. \Jhole rough rice kernel (2) 0.00983 -&151. \Jhole rough rice kernel (3) 33.60000 -6&20. tlhole rough rice kernel (&) 3,3029.00000 -862&. Whole brown rice kernel (&) 0.79790 -&933. \Vhole parboiled rough kernel (&) &11.86000 -6978. \dhole parboiled brown kernel (&) &01.62000 -67&&. (1) Steffe and Singh (1980a): short-grain smooth hulled variety $6. 35.& - 5&.8 C, equivalent radius 0.177 cm (2) Wang and Singh (1978): medium-grain smooth hulled variety CSHS, 30.0 - 55.0 C, equivalent radius 0.18& cm (3) Steffe and Singh (1982): short-grain smooth hulled variety S6. 35.& - 5&.8 C, equivalent radius 0.177 cm (&) Bakshi and Singh (1979): short-grain smooth hulled variety S6, &0. 0 - 55. 0 C, equivalent radius 0.177 cm Ta Absolute temperature (K) D Diffusion doefficient (square meter/hour) Table 3.9 Diffusivity of rough rice and rough rice components .as a function of temperature assuming spherical kernels. Diffusivity (square meter/hour) x 1,000,000,000 ‘ Temperature Short-grain Medium-grain (Steffe, 1979) Steffe and Wang and Singh (C) Singh (1982 (1978) Endosperm Bran Hull Kernel Kernel 5‘ 0 68 6 l 2 2 10 199 12 2 5 & 20 1&0 22 6 10 7 30 . 193 38 13 22 11 &o 262 66 28 &2 17 50 3&8 109 59 80 26 60 &55 175 117 1&5 38 70 585 27& 22& 25& 55 80 7&8 &17 &11 &32 78 90 938 621 730 712 107 L.‘__19° 1152 906 1258 ll&3 1&6 0.14 ~12 :10“ 0-10 1 I.oa 0 0.06 IVITY (HZ/HR l DIFFUS 0.04 0.02 l avian Fig. 20 ROUGH RICE DIFFUSIVITY Equations - Table 3.8 1 - saucepan - narration) - short-grain t - IRAN - OTEFFEC 1079) - short-grain 3 - HULL - 81EFFE‘19793 - short-grain ‘ " “MEL " ”EFF! HID 811101“ 18823 - short—grai j l I l 40.00 60.00 80.00 100.00 TEMPERHTURE (C) 3.1 Diffusivity of rough rice and rough rice components (Table 3.9). ~00 20.00 21 kernels of rough rice, corn, and soybeans at 20.0 percent moisture content and 40 C are 49E-9, 2808-9, and 3703-9, mZ/hr respectively. The small equivalent radius of rice is partially offset by a low diffusion coefficient. The diffusion coefficient is a product property and should be the same regardless of the product shape. The rough rice kernel should have the same diffusion coefficient when considered either as a cylinder or as a sphere. The diffusion coefficients determined by Steffe (1979) for rough rice components are used in this dissertation in the diffusion equation (drying and tempering of rice in the Concurrentflow model) because they appear to be the most accurate available in the literature in the range of temperatures for which the model is utilized (35.4 to 54.8 c). The diffusion coefficient by Steffe and Singh (1982) f‘31'.a.whole kernel of. rough rice (one component) is used to c<>mpare diffusion equations with empirical equations on a tliin-layer basis (Chapter Four). 22 3.2.1.6 Head Yield Milling of rough rice is done to remove the hulls, bran and germ with a minimum breakage of endosperm (Webb, 1980). Milling results are reported in terms of total milled rice and whole kernel (head) rice. The total milling yield refers to the percentage by weight of milled kernels (whole and brokens) and the head yield refers to the percentage of whole (head) kernels of rice. High head Yield is a desirable characteristic because whole kernels have the greatest economic value. Head yield varies with rice variety, grain type, growing conditions, drying, Storing, and milling conditions. On the average, short- and medium-grain varieties have higher head yields (63 - 68 Pel'cent) compared to long-grain (56 - 61 percent) varieties (Webb, 1980). The average head rice milling yield in California (medium-grain rice) was 58.8 percent in 1970 COmpared to 51.9 percent in 1964 (Steffe et a1., 1980). The determination of rice quality and head yield is discussed in sections 5.4, 5.5, and 5.6. 23 3.2.1.7 Pressure Drop Rice provides a resistance to airflow which must be overcome by applying positive pressure at the air inlet to the grain or negative pressure at the air outlet. Resistance to airflow is evidenced as a drop in pressure as the air travels through the rice bed. Experimental data were used by Shedd (1953), by Cervinka (1971), by Calderwood (1973), and by Henderson and Parsons (1974) to determine the pressure necessary to achieve a particular airflow rate. The resistance to airflow is affected by the Presence of foreign matter, degree of packing, type of kelt'nel surfaces, shape of the kernels, and the moisture cOntent of the rice. The equations relating airflow rate and pressure drop per unit bed depth of rough rice are given in Table 3.10. Straight lines are obtained when plotting airflow versus pressure drop on log-log paper (Fig. 3.2). Medium-grain riCe offers more resistance to airflow than long-grain rice (Calderwood, 1973). The method of filling a COMercial-size bin affects the packing and may increase or decrease the resistance to airflow. 24 Table 3.10 Pressure drop across a bed of long-, medium-, and short-grain rough rice (Steffe et al., 1980). sp . a a Q as b Grain Moisture Kernel Foreign Range of type content surface matter airflows a b CurveiSource (two) (2) tested Fig. (M3/s/m2) 3.2 short dry - clean 0.08-0.&l 7319. 1.5006 0 (1) medium 2&.& rough 0.66 0.08-0.&l &&87. 1.&715 A (2) Medium 12.7 rough 0.66 0.08-0.&1 5309. 1.&853 B (2) medium 27.6 smooth 0.88 0.08-0.41 7319. 1.5006 c (2) medium 12.7 smooth 0.88 0.08-0.&1 7&12. 1.&631 E (2) medium dry smooth clean 0.08-0.&1 9261. 1.&628 G (1) medium 16.5 - clean 0.01-0.13 &873. 1.1605 I (3) long 13.& - clean 0.01-0.15 &832. 1.1671 F (&) long 15.2 - clean 0.01-0.13 3938. l.1&67 H (3) SP - pressure drop (Pascals/meter) Q - airflow rate (m3/s/m2) (1) I Cervinka (1971) (2) - Henderson and Parsons (197&) (3) - Calderwood (1973) (&) - Shedd (1953) 25 .moH.m pome .ommm ..Hm no ommoumu onmnHm ou moan swoop mo mocmumfimom ma .moht whammoum coca ooa O H C 2:. m/s/zm ‘9181 motgxtv Z 26 Calderwood (1973) concluded that the data presented by Shedd (1953) can be used although the calculated pressure drop could be up to 100 percent higher depending on the degree of packing. The higher the moisture content the less is the resistance to airflow, but the effect is small (Calderwood, 1973). Dust, small weed seeds, and green material such as straw may increase the resistance 'to airflow. The equation, SP=a*Q**b, with coefficients (a and b, Table 3.10) by Henderson and Parsons (1974) for medium-grain rough hulled rice. at 24.4 percent moisture cOntent, is used in the concurrentflow model in this dissertation. This is a suitable equation when considering rice type, moisture content, and range of airflows under "hich the equation was developed. The data had to be extrapolated from 25.0 to 50.0 m3/min/m2 due to the lack of 3’1 equation for high airflow rates. A 17 percent increase it! pressure drop was added due to packing and presence of £5J‘tes and straw; such an increase was suggested by calderwood (1973). For long-grain rice the coefficients (a and b, Table 3-10) are from Calderwood (1973). 27 3.2.1.8 Variation in Initial Moisture Content Rice plants produce tillers during the vegetative period; subsequent flowering takes place over a period of time. The panicles that flower late will mature late and are at a higher moisture content during harvest than the earlier maturing panicles. Chau and Kunze (1982) determined the moisture content variation among harvested grains of a medium-grain rice variety. The moisture difference ranged from 21 to 29 percent for groups of grains harvested from the tops of ten of the most mature Panicles and from the bottoms of the ten least mature Particles. 'As the field moisture content at harvest d'i‘tzreased (from 29 to 18 percent), the head yield decreased (from 63 to 53 percent), and the total yield increased slightly (from 70 to 72 percent). Morse et a1. (1967) found that the maximum head yield £01? a short-grain variety (Caloro) occurred at 26 percent In‘Pisture content. Once the harvested rice kernels are mixed (wagon, trfuck, or bin) the low moisture kernels absorb moisture from the high moisture kernels. Kunze and Prasad (1978) showed that the moisture migration process causes fissures at the surface of kernels, which result in lower head 28 yields during the milling process. 3.2.2 Thermal Properties of Rough Rice The following are the thermal properties relevant in the rice drying models: specific heat, convective heat transfer coefficient, latent heat of vaporization, thermal conductivity, and thermal diffusivity. 3.2. 2.1 Specific Heat The specific heat of rough rice has been studied by Hasemen (1954), by Wratten et al. (1969), and by Morita and Singh (1977). Regression lines were obtained for bulk specific heat as a function of moisture content. R°Ugh rice Cp - 1.10953 + 0.04480 * Mw (3.9) medium-grain rice Cp .. 0.92145 + 0.05447 * Mw (3.10) shoft-grain rice Cp - 1.26947 + 0.03488 * Mw (3.11) whet’e Cp is the bulk specific heat (kJ/kg/C) and Mw is the m - . olsture content (percent, wet ba51s). Table 3.11 present v all-les for specific heat at different moisture contents. The specific heat of rough rice was found to be 1'. (”dependent of temperature in the range of 20 to 70C F- L on tana, 1982). 29 Table 3.11 Bulk specific heat of rough rice as a function of moisture content. Moisture Specific heat (kJ/kg/C) content (2; Nb) Haswell Wratten et al. Morita and Singh (1959) (1969) (1977) Rough rice Medium-grain Short-grain 10 1.56 1.&7 1.62 15 1.78 1.7& 1.79 20 2.01 2.01 1.97 .25 2.23 2.28 2.1& 'Table 3.12 Convective heat transfer coefficient of medium-grain rough rice as a function of airflow rate at &5 C and 30 2 relative humidity at atmospheric pressure (Wang et a1., 1979). Airflow rate Convective heat transfer coefficient (m3/s/m2) (kg dry air/s/mZ) (kJ/m3/s/c) (w/mz/c) \ 0.2 0.185 9.690 9.335 0-3 0.277- 16-377 15.777 0.& 0.370 23.861 22.987 0.5 0.&62 31.8&6 30.680 0.6 0.555 &0.&20 38.9&0 \ 30 3.2.2.2 Convective Heat Transfer Coefficient Wang et al. (1979) proposed an equation to predict the convective heat transfer coefficient in a packed bed of medium-grain rough rice. The equation, developed by comparing experimental and theoretically predicted heating Curves, has the following' form: 11 = 86900. * Ga ** 1.30 (3.12) "hei'e h is the volumetric heat transfer coefficient (J/s/m3/K) and Ga rice (kg dry air/s/m2). is the airflow rate through the bed of Table 3.12 presents the convective of medium-grain rough rice for 30 heat transfer coefficient airflow rates from 0.2 to 0.6 m3/s/m2 at 45 degrees C, percent relative humidity and atmospheric pressure. Walker (1978) developed (and used in his simulation In°dell a convective heat transfer coefficient equation for a bed of long-grain rough rice. No equation for h for S . . . . hoI‘t-grain rough rice was found in the literature. hfg 8 2.323 *(1090. - 1.026*(9 + 17.78)) 31 3.2.2.3 Latent Heat of Vaporization Wang and Singh (1978a) developed an equation for the latent heat of vaporization of medium-grain rough rice as a function of grain temperature and moisture content: hfg = 4.1818 *(415.6734 - 0.6495*9)*(M**-0.308) (3.13) Where hfg is the latent heat of vaporization (kJ/kg), e is the grain' temperature (C), and M is the moisture content (decimal, dry basis). Eq. (3.13) was formulated from the equilibrium moisture content data determined by Zuritz et a1- (1979) for a medium-grain smooth hulled variety (C8145). Another equation for the latent heat of vaporization of rough rice was developed by Brook and Foster (1979). The equation utilizes the equilibrium moisture content data of Most et al. (1976). The Brook-Foster equation, which Incorporates data from different varieties (rice type not sI>ecified) , is: (1. + 2.9462*exp (-21.733*M)) (3.14) w here hfg, 6, and M are defined in Eq. (3.13).' L 32 Figs. 3.3 and 3.4 show the latent heat of vaporization of rough rice as a function of temperature and moisture content, respectively. Tables 3.13 and 3.14 Present the tabulated values from Figs. 3.3 and 3.4. Eq. (3.14) for hfg is used in this dissertation. The difference between the hfg values (four to ten percent) in 395. (3.13) and (3.14).was found to have no effect on the conc urrentflow model results . 3-2..2.4 Thermal Conductivity and Thermal Diffusivity Bulk thermal conductivity and bulk thermal diffusivity ‘3f rough rice have been reported by Wratten et al. (1969) and by Morita and Singh (1977). Regression equations were obtained for medium- and short-grain rough rice as a fur‘Iction of moisture content. Bulk thermal conductivity: medium-grain rice x = 0.08656 + 0.001327 * Mw (3.15) short-grain rice x = 0.09999 + 0.001107 * Mw (3.16) "}‘¢313e K is the thermal conductivity (W/m/C) and Mw is the moiSture content (percent, wet basis). Bulk thermal diffusivity: in - edlhm-grain rice :4. .. 0.000456 - 0.00000896 * Mw (3.17) k 2800 .00 l 1101 0 2840.00 2720 .00 2560.0 1 (J/ 1 L31 HEHT VFlP (K 2400.00 2400.00 1 1 2320.00 4240.00 Fig. 33 ROUGH RICE HFG (Table 3.13) 1 - m awn um (1978a) (medium-grain) 2 - m 010 FOSTER (1979) (rough rice) fi 0.00 40.00 60.00 50.00 100.00 TEHPERHTURE (C) 3.3 Latent heat of vaporization of rough rice as a function of temperature at 25 % moisture content, dry basis. 34 o o 8 ROUGH RICE HFG “1 0 (Table 3.14) 9 3- 1 - wnwo nun anion (1978a) w (medium-grain) o 2 - skunk awn roars): (1979) c.) (rough rice) “ 8 53...- X 0 HO 6 O xflgq \w- fl K We a}? a a: >2— 0.. G: uJE3 Is. .‘m‘ G: ~J o c? s_ 2 N \ Q 1 c? O (D I l '13.00 40.00 60.00 10.00 20.00 30.00 MOISTURE CONTENT (OB) Fig. 3.4 Latent heat of vaporization of rough rice as a function of moisture content at 50°C temperature. 35 Table 3.13 Latent heat of vaporization of rough rice as a function of temperature at 25 2 moisture content, dry basis. Temperature Latent heat of vaporization (kJ/kg) (C) Wang and Singh (1978a) Brook and Foster (1979) (medium-grain) (rough rice)(*) 0 266& 2522 10 2622 2&98 20 2581 2&73 30 2539 2&&9 to 2&98 . 2&25 50 2&56 2&01 60 251& 2377 70 2373 2353 80 2331 2329 90 2289 230& 100 22&8 2280 * - rice type not specified ‘Table 3.1& Latent heat of vaporization of rough rice as a function of moisture content at 50 C temperature. '1c>isture Latent heat of vaporization (kJ/kg) content . (3 db) Wang and Singh (1978a) Brook and Foster (1979) J (medium-grain) (rough rice)(*) 10 3257 3165 12 3079 2885 1& 2936 270& 16 2818 2586 18 2717 2510 20 2631 2&61 22 2555 2&29 2& 2&87 2&08 26 2&26 2395 28 2372 2386 30 2322 2381 32 2276 2377 (‘34 2231. 2375 * - rice type not specified 36 short-grain rice o< = 0.000451 - 0.00000585 * Mw (3.18) where 06 is the thermal diffusivity (m2/hr) and Mw is the moisture content (percent, wet basis). Table 3.15 lists the values for the bulk thermal conductivity and the bulk thermal diffusivity of medium- and short-grain rough rice. No. literature values are available for long-grain rough rice. The thermal properties of rough rice discussed above vary not only with moisture content but also with temperature. The variations with temperature, in the drying range (30 to 60 C), are small (less than 10 Percent). 3.3 Drying of Rough Rice Rice is frequently harvested at moisture contents above those considered safe for storage. Artificial drying is needed to decrease the moisture content to 12.0 - 13.0 petcent. Much of the rice grown in the United States is dried with heated air in batch or continuousflow dryers. The design and operation of such dryers depends on the physical, thermal and chemical properties, and the drying characteristics of the product. 37 Table 3.15 Bulk thermal conductivity and bulk thermal diffusivity of rough rice as a function of moisture content. Moisture content (2 wb) Thermal conductivity (W/m/C) Thermal diffusivity (m2/hr) Wratten et a1. (1969) (medium-grain) Morita - Singh (1977) (short-grain) Wratten et al. (1969) (medium-grain) Morita - Singh (1977) (short-grain) 10 15 20 25 0.100 0.106 0.113 0.120 0.111 0.117 0.122 0.128 0.000366 0.000322 0.000277 0.000232 0.000393 0.000363 0.00033& 0.000305 38 Rice is more susceptible to cracking (fissuring) than Other cereal grains during the drying operation. Rice cracks easily when stressed by excessive temperature/moisture gradients and high moisture removal hates. Cracked rice breaks during the milling operation; the price of broken kernels is approximately half of the Prfiice for unbroken kernels. The equipment designed for drying corn is generally suitable for drying rice, but methods of operation must be changed to accommodate the lfliique requirements of rice (Kunze and Calderwood, 1980). Because of variations in climatic conditions and of time physical properties of rice, the procedures recommended f<>r drying rice difer in different areas. 0f the three tYpes of rice grown in the United States, the long-grain Varieties dry the fastest, and the short-grain types the slowest (Run ze and Calderwood, 1980). 3 . 3.1 On-Farm versus Commercial Rice Drying Much of the rice grown in the United States is dried <>" a commercial basis. On-farm drying is done with di‘ieep-bed (in-bin) grain dryers where the rice is dried in a £31:Orage bin (Kunze and Calderwood, 1980). Portable grain drYers are not widely employed. In-bin drying systems use ¥ 39 low temperatures and low airflow rates. Supplemental heat may be needed to decrease the relative humidity of the drying air to below 60 percent. In-bin rice drying bed depths are usually less than three meters. Long periods of time (up to 30 days) are required for in-bin drying especially if weather conditions are unfavorable. Sorenson and Crane (1960) studied in-bin drying of rice from 19.0 to 12.5 percent moisture content with an airflow rate of 0.0582 m3/s/ton. Unheated air and supplemental heat (six degrees C temperature rise) were compared. The operation required 31 days with unheated air and only 17 days with supplemental heat'. Solar heat can be used with in-bin drying. Calderwood (1977) conducted tests in Texas using a solar collector giving an average increase in temperature of six degrees C, with an airflow rate of 0«0582 m3/s/ton during a ten hour period on clear days. Comparable lots took 75 percent less time for solar heated air than for unheated air, when drying from 19.6 to 12.0 percent moisture content. It took 52 percent less time for s<>lar heated air than for unheated air during another test when drying from 16.0 to 12.0 percent moisture content. There was little difference in the percentage of head rice bétween rice dried with unheated air and with solar heat. Portable grain dryers, either continuous-flow or be tch, are not widely used for drying (Kunze and caZLderwood, 1980). Calderwood (1970) tested a batch ¥ 40 crossflow-type rice dryer. The drying time varied from six hours with 49 C air to three hours with 66 C air. The head yield was reduced by eight percentage points at 66 C when compared to rice dried at ambient conditions. Commercial drying of rice involves (a) farm dryers, (b) cooperative dryers, and (c) rice mill dryers. Most of the commercial dryers used in the United States are of the continuousflow crossflow-type. Crossflow dryers can be of the non-mixing- or mixing-type. The LSU dryer (cascade dryer) is a mixing-type dryer. Rotary dryers are presently used to dry pa rboiled rice . 3.3.2 Types of Rice Dryers (Batch and continuous-flow rice dryers employed on farms and commercial installations are described in the isOllowing sections. 3 - 3.2.1 Crossflow Dryers Two common crossflow dryers are the non-mixing- and mixing-type. Fig. 3.5 shows the non-mixing crossflow atriver. The rice flows by gravity in a straight path k type rice dryer (Wasserman 0323 go. wmmcfigwfiegfi $0.923! 3.5 A non-mixing-type rice dryer (Steffe et a1., 1980). Rice m 0am W Dacha-90M! 3.6 Schematic of a non-mixing and Calderwood, 1972). Fig. Fig. 42 between two screens, usually 15 to 30 cm apart. The dryer may be up to 30 meters high and five meters wide. The grainflow rate is controlled by variable speed discharge rolls. The air is forced to flow perpendicular to the moving bed of rice. Relatively high airflows and low temperatures are characteristics for this dryer. The grain side of the screen is exposed to the of the on the air inlet hottest temperatures; the grain on the outlet side screen is exposed to the coldest air (Fig. 3.6). According to Wasserman and Calderwood (1972) the typical airflow rate is 0.01 m3/s/ton with the air temperatures up to 55 C. Fig. 3.7 shows a mixing-type (baffle) rice dryer. In terms of quality, the mixing-type dryer has several advantages over the non-mixing-type. In.the baffle dryer, l‘ice takes a downward, zigzag path. With this type of movement, individual kernels are not continuously exposed to the hottest drying air. This can be seen in Fig. 3.8. According to Wasserman and Calderwood (1972). the typical airflow rate for the mixing-type dryer is 0.073 - 0.161 n"13/s/ton, and the air temperature is up to 82 C. Calderwood and Webb (1971) presented data showing that a mixing-type dryer can use inlet air temperatures of 82 C without decreasing the rice head yield due to the high 9t‘ain velocity through the dryer. ¥ 1 I | 4 I 1111". .1 1.1. l U HE“ Fig. 3.7 A mixing-type (baffle) rice dryer (Steffe et al., 1980). Fig. 3.8 Schematic of a mixing-type (baffle) rice dryer (Wasserman and Calderwood, 1972). 44 Both the non-mixing- and mixing-type dryers operate on a multipass basis. The moisture content of the rice is reduced by one to three percentage points each time it passes through the dryer. Between passes, rice is held for a period of time to allow the kernel moisture gradients, developed during drying, to be reduced (Steffe et al., 1980). More is discussed about moisture equalization in a Special section on tempering models. Conventional crossflow dryers designed for products such as corn, can be used for the drying of rice. Fig. 3.9 shows a crossflow dryer with a cooling fan. Other crossflow-type dryers such as the batch type, have been used for drying rice, especially on the farm level. Fig. 3.10 shows a cross section of a columnar batch dryer. Fig. 3.11 shows a crossflow dryer with forced air drying and reversed flow cooling. The grain dried in the dryers shown in Figs. 3.9, 3.10, and 3.11 is overdried on the plenum Side of the column, and underdried on the exhaust side. 3.3.2.2 Cascade Dryers The cascade (or rack) dryer is a continous-flow l"lixing-type dryer originally designed at Lousiana State University (Figs. 3.12 and 3.13). In this type of dryer, ‘1‘ ice 'flows downward over inverted V-shaped air channels. Fig. 3.9 Conventional continuous crossflow dryer (Brooker et a1., 1974). NET DRAIN SUPPLY DRAIN SLIDE NEATED RIR CHANGER DRYING DOLDNNS WN RERFDR ATED 'ALLD CONVEYOR R DR RENDVINC DRIED DRAIN Fig. 3.10 Cross-section of a columnar batch dryer (Brooker et al., 1974). gll g5 -\ CDOLIIO Al Fig. 3.11 Continous crossflow dryer with reversedflow cooling (Brooker et a1., 1974). VVET RICE IN .s-Air Inlet JAir Outlet .3- Inlet .3- 001101 ..5-Inhl ' .s- Outlet DRY REE OUT Fig. 3.12 Continuousflow-type dryer of the Louisiana State University design (Steffe et a1., 1980). ii, PIC. :11: ... 47 Air flows in and out on alternate rows of channels. Mixing is accomplished because the inlet and outlet air ducts are offset from one another (Wasserman and Calderwood, 1972). The grain mixing results in more uniform drying of rice. Higher temperatures and airflow rates can be used in cascade dryers than in mixing-type crossflow dryers. The grain is exposed to heated air for 15 to 30 minutes during each pass. Fig. 3.13 shows a schematic of a cascade grain dryer. Cascade dryers require air pollution equipment in the USA; as a result, the employment of this dryer type has decreased in the last ten years. 3.3.2.3 Rotary Dryers Rotary dryers consist of a drum slightly inclined to the horizontal, typically one to two meters in diameter, 15 to 30 meters of length, and sloping two to four degrees; the drum rotates at four to eight rpm (Thorne and Kelly, 1980). Fig. 3.14 shows the schematic of a triple pass rotary dryer. In a rotary dryer the product is introduced at one end, with mass or heat transfer taking place between air and solids until the dried product is discharged at the other end. 48 3.13 Schematic of a cascade grain dryer 1978). I (Bakker-Arkema et al. Fig. 49 .aoxap xemuoe mmmm mamaep a mo oflumsocom «H.m .mam nosed . Annu new no mcw>moa oowm Aunnnu AHHHHu AHHHHU AU . 2 9 "V “V \ “V «9 note OHU AUHU 8 95.808 00am 0930 House more oamapospoucH ops» noncou wcfipzmwam Homeoch 50 A rotary dryer can be concurrent, countercurrent, or have a crossflow arrangement. Solids advance through a rotary dryer in a series of cascades. The air-product contact takes place with the use of longitudinal lifting flights attached inside the drum wall. Another purpose of the lifting flights (Fig. 3.14) is to slow down the material through the air stream. The variables of rotary dryers are: (1) residence time, (2) dryer slope, (3) speed of rotation (4) air velocity, and (5) length and diameter of dryer. Rotary dryers are frequently used 'in the drying of high moisture rice in the production of parboiled rice (air temperatures of 150 to 350 C). Rotary dryers have also been used for the drying of rough rice in a limited basis (Willson, 1979). They have a low energy efficiency (7,000 - 9,000 kJ/kg of water removed). 3.3.2.4 Fixed-Bed (In-Bin) Dryers In-bin rice drying is widely practiced on farms as the final drying step (Wasserman and Calderwood, 1972); it is also used at some commercial installations. Figs. 3.15 and 3.16 show two common types of fixed-bed dryers. Fixed-bed dryers are designed for both drying and storing. 51 Fig. 3.15 A circular rice drying/storage bin with a stirring device (Steffe et a1., 1980). Fig. 3.16 A rectangular structure for drying rice (Steffe et al., 1980).. Air perfc later suite the It 62 to; dryil qual The avai Qrai Pars air 35 c rate the regL Piss eque 52 Air distribution systems commonly used consist of a perforated floor, or different arrangements of ducts and laterals. Both axial flow and centrifugal fans are suitable for bin drying. The stirring device in Fig. 3.15 moves the grain from the bottom of the drying bed.and deposits it on the top. It also loosens the bulk of grain reducing the resistance to airflow. Properly managed stirrer bins offer increased drying capacity, reduced drying risk, and improved rice quality (Dealson, 1980).' Airflow direction in bin dryers is usually upward. The last rice to dry is at the top where it is readily available for sample and visual examination. The depth of grain is usually between four and six meters (Henderson and Parsons, 1974). The'air may, or may not, be heated and the air temperature may be increased to 30 C in California, and 35 C in Texas. Airflow rates of 0.058 m3/s/ton are recommended. The pressure drop per unit depth depends on the airflow rate through the rice bed. The static pressure requirements to achieve various airflow rates were presented by Steffe et a1., 1980; the static pressure equations are presented in Table 3.10. 3.3. opp: pot! bed a1. dry C017 and te: In~ Co: lo' 31 0f C0 53 3.3.2.5 Counterflow Dryers In a counterflow dryer, the grain and the air flow in opposite directions. The counterflow dryer has the potential for removing more moisture per meter of drying bed than concurrentflow and crossflow dryers (Steffe et al., 1980). Limited research has been conducted on rice drying in counterflow dryers. Thompson et a1. (1969) made a comparison of crossflow, concurrentflow, and counterflow dryers. The counterflow and concurrentflow patterns are shown in Fig. 3.17. An in-bin counterflow dryer (Shivvers system) has been tested with corn in the State of Michigan (Silva, 1980). In-bin counterflow drying presented the lowest operating costs as compared to in-bin dryeration, natural-air, and - low temperature combination drying. 3.3.2.6 Concurrentflow Dryers In a concurrentflow dryer, the product and the drying aix' flow in the same direction. A cross-sectional drawing <>f an on-farm type concurrentflow dryer with counterflow Cooler is shown in Fig. 3.18. The rice is cooled ina 54 CONCURRENT NTERFLOW FLOW CCU “an >( )0. Ah GRIHN Fig. 3.17 Concurrentflow and counterflow grain drying systems (Steffe et a1., 1980). Fig. 3.18 Schematic of an on-farm concurrentflow dryer (Brooker et a1., 1974). 55 counterflow cooler, where the air and rice flow in opposite directions. The grain flows through the dryer by gravity, and the rate is regulated by varying the rotation speed of the metering rolls. Fig. 3.19 shows a schematic of a continuousflow two-stage concurrentflow dryer; Fig. 3.20 shows the corresponding block diagram. The three-stage model has three drying stages and two tempering zones. The inlet air temperature in the drying beds can be varied independently for each drying stage. ' The airflows in the different drying/cooling stages of a concurrentflow dryer can be changed by valving the inlet air ducts. High static pressures (2.0 to 4.0 kPa) are common because of the depths of the drying beds (0.75 to 1.50 m). Lower bed depths are preferred for rice. The drying floor shown in Fig. 3.21 allows the heated air and wet grain to mix homogeneously for uniform drying. Since the hottest inlet air contacts only the wettest grain, efficient, higher temperatures can be employed without damaging the rice quality. The concurrentflow drying process ensures that each kernel receives the same drying treatment resulting in uniform drying and quality. The dried product temperature and moisture content are uniform. This is not the case in crossflow dryers where a wide variation in drying (over- and under-drying) occurs. Fig. OWMUOGJ? 3.19 56 ...}... ‘;~..'I. .,‘.. : ..ICT..- a. ."- '~" ?. 0“. -. 'E -o o '. I”... ..v'Vp'. . 4.434112." . .0 "0'. ' O ..."".‘..O. . 'l , .- 0 I' a n0 . . . _, 0 -,~ 0 4’ o o .’.:'. 'g‘..°‘.."}‘f' ‘3 ““Y.',"‘:_" . 3‘ I- o «in. ',."-'I ".:.'l'..'i.' I ‘-° .'...,'.".. ' 5,?o-r.._..0~"- o. 1. ;--, {.1 4.51“":4 . A .V., 2 ' 9.. ... ."'V.-'n. ., T . \ ". .:‘-~'-' '1'.-‘.'--'- “:.' .- 'r '." 0.0 4. . .'..I‘ 0 fl, ....' ." ... I ..r. ._“ I... I . .... f, . '2 . 0321 . ‘_."a.. 0.. '8'. 0' ..- .....1. f ' f . .'.' ”... . D. ." (..11" -" ‘.‘4 I . .'-' . .. '.o’ . ' I ' . ' '4" U ' F '1'}. 3%. . ' ' 0 I . 0‘ . ' o O ‘ I [3b '0' 0. ‘ -._ O . "3' .u -- .0 -_‘ N ..‘.. ' c .. :cw.¢.-'.'V‘. : .'... ig'ufl . ' § .. ...“.u." .3 ‘ - ' . 00". , :0: ’09:. . ..‘ tag-t: £9. 03' o ",“9- 1 0.1. . a .3" - Wet rice - Drying floor - Exhaust air - Tempering section - Air recirculation - Counterflow cooler - Metering rolls J ... _‘ ,. \“'*I"Mvv lo. .. ..I, ..I- I... .z. . 0‘. ‘ .x.‘fifidstvwmw -o'.' ...“ 5' f|flif£,}tl. ‘4’”; “w ,‘fi 'I" . v Ambient air Hot air - First stage Hot air - Second stage Ambient air - Cooler Dry rice Schematic of a commercial two-stage concurrentflow dryer w1th counterflow cooler and air recirculation (Blount, Inc.). .I..111 II II- '1" 57 RICE AMBIENT AIR FIRST STAGE CONCURRENT DRYING LTEMPERINGT AMBIENT SECOND STAGE CONCURRENT DRYING EXHAUST AIR RECYCLING COUNTER FLOW COOLING L FAN AMBIENT AIR RICE (NJT Fig. 3.20 Block diagram of a two-stage concurrentflow rice dryer with counterflow cooler and air recirculation. 58 Patented CQFADMDQ Floor Wet gram“ Fig. 3.21 Schematic of the drying floor of the Blount concurrentflow dryer. 59 The concurrentflow dryer has many desirable features which are important to ensure high head yield of milled rice. Much of the research reported in this dissertation is conducted with concurrentflow dryers. The design and operational characteristics of the concurrentflow rice dryer will be discussed in detail' in the following chapters. 3.3.3 Rice Drying Models The drying of a solid material involves simultaneous mass and heat transfer. Heat is required to evaporate the moisture which is removed from the product's surface by the external drying medium, usually air. The basic drying theory for grain products is reported in the literature in two categories: (a) the single kernel or thin-layer drying, and (b) deep-bed drying. 3.3.3.1 Single Kernel Drying Cereal grains, including rice, dry solely during the falling-rate period (Brooker et al., 1974 and Bakshi and Singh, 1979). This implies that the drying rate decreases continuously during the course of drying. Not only d0° 60 external transfer mechanisms (convective heat and mass transfer) have to be considered in the analysis, but the transfer mechanisms within the product (heat and mass diffusion) must be included. Semitheoretical and empirical relationships have been developed to predict the drying behavior of cereal grains. Some of the proposed mechanisms for describing the transfer of moisture in individual kernels are (Brooker et al., 1954): 1) liquid movement due to surface forces (capillary flow); 2) liquid movement due to moisture concentration (liquid diffusion); 3) liquid movement due to diffusion of moisture on the pore surface (surface diffusion); 4) vapor movement due to moisture concentration differences (vapor diffusion); 5) vapor movement due to temperature differences (thermal diffusion): and 6) water and vapor movement due to total pressure differences (hydrodynamic flow). The word "single kernel" or "thin-layer" refers to a layer of grain that is approximately one kernel deep. The following sections are a review of single kernel empirical and diffusion type equations. Single kernel equations are used in the modeling of different deep-bed drying configurations. The empirical and diffusion type single 61 kernel equations are compared in section 4.1.3. 3.3.3.1.1 Empirical Models Many empirical studies have been conducted using thin-layer of grain to examine the drying behavior of the particular product in question. When thin layers of grain are dried with air at different conditions (temperature and relative humidity), the resulting data can be treated to yield an equation which specifies moisture removal as a function of time and drying air temperature and relative humidity. Henderson and Henderson (1968) proposed an empirical equation to describe the drying behavior of short-grain rough rice (Colusa 1600) at 42 C: M - Me . 0.65 ( exp (-0.22*t) - 0.22*t ) (3.19) Mo- Me where M is the moisture content (decimal, dry basis), Me is the equilibrium moisture content (decimal, dry basis), M0 is the initial moisture content (decimal, dry basis) and t is the drying time (hours). The left hand side of Eq. (3.19), (M-Me)/(Mo-Me), is called the moisture ratio (MR). 62 Chancellor (1968), using rice data from various references, developed the following thin-layer equation for short-grain rough rice (temperature range, 38 to 71 C): MR = 0.75*(exp(-G*t) + 0.llll*exp(-G*t)) (3.20) where G=8860*exp(-6l47/Tabs) and Tabs is the absolute temperature (R): the moisture ratio (MR) and t are defined in Eq. (3.19). Eqs. (3.19) and (3.20) were developed for very specific conditions. They are of little value when used to predict the drying rate of rice over a wide temperature and relative humidity range. Agrawal and Singh (1977) presented an equation for short-grain rough rice in a form originally developed by Page (1949). The temperature range during the experimentation was 32 to 51 C: MR = exp (-X*t**Y) (3.21) where X 8 0.002958 - 0.44565*RH + 0.01215*T Y 8 0.13365 + 1.93653*RH - 1.77431*RH**2 + 0.009468*T RH is the relative humidity (decimal), T is the air temperature (C): t and the moisture ratio (MR) have been defined in Eq. (3.19). Table 3.16 presents the x and Y coefficients for Eq. 3.21 for rough, brown, parboiled rough, and parboiled brown short-grain rice (Bakshi and Singh, 1979). The values of X and Y for rough rice were calculated from data of Steffe 63 Table 3.l6 Thin-layer drying equations for short-grain rice (Bakshi and Singh, l979). (M - Me)/(Mo - Me) - Exp ( - X * t ** Y ) Rice form X and Y Rough X - 0.7hh982 + 0.00026l5*T - 0.0003335*RH Y - 0.09%070 + 0.00598h0*T - 0.0ll3h65*RH Brown X - 0.038957 + 0.00027h8*T - 0.0006705*RH Y - 0.093302 + 0.0072720*T - 0.0l55l8h*RH Parboiled X - 0.503265 + 0.000273h*T - 0.000l760*RH rough Y - 0.06hhh5 + 0.00h6369*T - 0.0lh719h*RH Parboiled X - 0.0l6538 + 0.000l737*T - 0.006h722*RH brown Y --0.776610 + 0.00lhl73tT - 0.0736700*RH T - air temperature (C), range h0.0 RH - relative humidity (decimal) t - drying time (hours) 55.0 C 64 (1979). Wang and Singh (1978) presented three equations for the drying of medium-grain rough rice. The constants were developed from data obtained for temperatures between 30 and 55 C and relative humidities of the drying air between 25 and 95 percent. The first Wang-Singh (1978) empirical drying equation is based on an approximate form of the solution to the diffusion equation: MR = X exp(-Y*tm) (3.22) where X = 0.96 - 0.0008826*T + 0.02324 *RH Y = 0.002814 + 0.0001267*T - 0.003620*RH' and tm is the drying time (minutes). The moisture ratio (MR), T, and RH are defined in Eq. (3.21). The second thin-layer drying equation for medium-grain rough rice presented by Wang and Singh (1978) has the same form as the equation developed by Page (1949): NR = exp (-X*tm**Y) (3.23) Where X = 0.01579 + 0.0001746*T - 0.01413*RH Y a 0.6545 + 0.002425*T + 0.07867*RH tm, T, RH, and the moisture ratio (MR) are defined in Eq. (3.22). The third equation presented by Wang and Singh (1978) for medium-grain rough rice, the quadratic equation, has the same form as an equation developed by Thompson et al. 65 (1968) for corn: MR = 1.0 + X*tm + Y*tm**2 (3.24) where X = 0.001308*T**0.4687*RH**‘0.3187 Y 8 0.00006625*T**0.03408*RH**-0.4842 tm, T, RH, and the moisture ratio (MR) are defined in Eq. (3.22). Eq. (3.23) produced the best results when modeling crossflow and fixed-bed drying of rice; thus, it is used in this dissertation. 3.3.3.1.2 Diffusion Models The solution of the diffusion equation for a sphere can be found in Crank (1974): 5’ 2 2 MR = 1152;: exp ( flfii ) (3.25) n=1 n where n is the diffusion. coefficient (m2/hr), R is the mid kernel equivalent radius (m), and t and the moisture ratio (MR) have been defined in Eq. (3.19). Table 3.8 presents diffusion doefficients (D) for whole kernels of short-grain rough, brown, parboiled rough, and parboiled brown rice developed by several researchers; it also presents the diffusion coefficient for whole kernels of medium-grain rough rice. 66 Steffe (1979) developed three diffusion-type models for simulating the moisture concentration of white, brown, and rough rice. The rice kernels were considered to be a sphere (starchy endosperm) surrounded by two concentric shells (bran and hull). The spherical diffusion equation with the appropriate boundary conditions for the endosperm material is: 2 EH31 Lg+2§£ ‘ (3.26) at 3" rar 29‘0”“0’13’0 (3.27) 3r C ‘ Ce: " = R1: t> 0 (3.23) C=Co,0éréRl,t=0 (3.29) where r is the kernel coordinate (m), D1 is the diffusivity of starchy endosperm (m2/hr), R1 is the white rice equivalent radius (m), C is the_ variable moisture concentration (kg/m3), Ce is the equilibrium moisture concentration (kg/m3), and t is the time (hr). The solution is (Crank, 1974): 67 c - Ce _ 2R1ig-1)" . nzr -I)1n2"2 t ---- - -:— s1n(-———)exp( " ) C0 - Ce urn-:1 n R1 R12 (3.30) and the integration over the radius yields _ 6 °° 1 -01 "2".2 t (3.31) MR - exp ( 72;; :17 R12 ) The model developed by Steffe (1979) for brown rice consists of a spherical core (starchy endosperm) surrounded by a single shell (bran). The differential equation with appropriate initial and boundary conditions is: 30 - 0 32C + Z 29 (3.32) " ' m "2 ' at 3r r'ar m = 1,2 E = 0’ r = 0, t > o (3.33) 3r ’ c = Ce, r = R2, t> 0 (3-34) C=Co.OéréR1,t=0 (3-35) C=Co.R14réR2.t=0 (3.36) where D2 is the bran diffusivity (m2/hr) and R2 is the brown rice equivalent radius (m). The other variables are defined in Eqs. (3.25) through (3.29). The Crank-Nicolson finite difference method was used to solve the brown rice model. 68 The differential equation and initial conditions describing the rough rice drying model (Steffe, 1979) consist of Eqs. (3.32), (3.33), (3.35), and (3.36) plus Eqs. (3.37) and (3.38) with m = l, 2, and 3 plus C = C9, Y‘ = R3: t > 0 (3.37) n I ‘ (20. R2 4 r é R3. 11 = 0 (3.38) where R3 is the equivalent radius of rough rice (m) and D3 is the hull diffusivity (m2/hr): the other variables are defined in Eqs. (3.25) through (3.36). The Crank-Nicolson method is used to solve the rough rice model. The diffusivity equations and values for diffusion coefficients of rough rice and rough rice components are presented in Tables 3.8 and 3.9, respectively. The rough rice model of Steffe (1979) is used in this dissertation for the modeling of concurentflow drying of rice. 3.3.3.2 Deep-Bed Simulation Models Semitheoretical drying models have been developed and used successfully by Thompson et al., (1969). 69 Deep-bed dryer simulation models have been developed at Michigan State University (MSU) by Bakker-Arkema et al. (1974) and are subjected to the following assumptions: 1) the volume shrinkage of the bed is negligible during the drying process; 2) the temperature gradient within an individual particle is negligible: 3) the particle to particle conduction is negligible; 4) the airflow and grainflow are plug-type (no wall effects): ' 5) BT/at and 3H/at are negligible compared. to aT/ax and bH/ax; 6) the bin or dryer walls are adiabatic, with negligible heat capacity; 7) the heat capacity of moist air and of grain are constant during short time periods; and 8) accurate thin-layer, moisture equilibrium isothermm and latent heat of vaporization equations are known. The models developed by Bakker-Arkema et a1. (1974) are based on the laws of heat and mass transfer. The models use thin-layer equations to predict the drying rate of a given product. The solution of the models allows the calculation of grain temperature, the air temperature, the air absolute humidity, air relative humidity, and grain moisture content as a function of time and position in the 70 drying/cooling bed. The models are general and can handle all particulate products. The accuracy of the simulated results depends greatly on the accuracy of the equations for the physical and thermal properties, and for the drying characteristics of a particular product. For each drying model, four balances are made resulting in four equations (Brooker et al., 1974): A) the fixed-bed model 3T - h*a - 8 *(T-e) (3.39) 3x Ga*Ca + Ga*Cv*H ae h*a hfg + Cv*(T-6) ah -— 3 *(T-e) - _ *Ga*- (3.40) at f*Cp + 11*ch! f*Cp*Cw*M 3.. ah ah _ — . - L- (3.41) ax Ga at an? a— = an appropriate thin-layer equation (3.42) t where T is the air temperature (C), x is the position in the drying bed in the direction of airflow (m), h is the convective heat transfer coefficient (kJ/hr/mZ/C), a is the specific surface area of the drying bed (m2/m3), 6 is the product temperature (C), Ga is the airflow rate (kg of dry air/hr/mZ), Ca is the specific heat of air (kJ/kg/C), H is the humidity ratio (kg water/kg dry air), t is the time (hr), ‘19 is the dry bulk density of the product (kg/m3), 71 Cp is the specific heat of product (kJ/kg/C), Cw is the specific heat of water (kJ/kg/C), M’is the average moisture content of the product (decimal, dry basis), and hfg is the latent heat of vaporization of product (kJ/kg). The boundary and initial conditions are: (a) T(0,t)=T(in1et), (b) 6(x,0)=6(initia1), (c) H(O,t)=H(inlet), and (d) M(x,0)=M(initial). Chan (1976) used the MSU fixed-bed corn drying model ' to simulate rough rice drying. Appropriate changes were made to account for the differences in physical and thermal properties and in the drying rate between rice and corn. More experimental data are needed to validate the model presented by Chan (1976). B) the crossflow drying model 3T - h*a - = *(T-e) (3.43) ax Ga*Ca + Ga*Cv*H 36 h*a hfg + Cv*(T-6) 3H .. g . *(T-G) - _ *Ga*- (3.44) By Gp*Cp + Gp*Cw*R Gp*Cp*Cw*M ax 3H Gp M — = - -*’- (3.45) ax Ga at . ar‘i - s an appropriate thin-layer equation (3.46) at where y is the dryer length in the perpendicular direction of the airflow (m), and Gp is the grainflow rate (kg of dry product/hr/mZ): the other variables are defined in Eqs. 72 (3.39) through (3.42). The boundary and initial conditions are: (a) T(0,y)=T(inlet), (b) 6(x,0)=6(in1et), (c) H(O,y)=H(inlet), and (d) M(x,0)=fi(in1et). Wang (1978) used the MSU crossflow model to simulate rough rice drying; a thin-layer equation for medium-grain rough rice was taken from Wang and Singh (1978). The diffusion model was found unsuitable for short interval drying time of less than 40 minutes. The 'model used for the drying rate equation, the so-called quadratic equation (Eq. 3.24), compared well with the experimental data. Further testing of the MSU crossflow model was done by Bakshi and Singh (1979) and by Singh et al. (1980). BakShi and Singh (1979) used the modified MSU crossflow model developed by Wang (1978) to simulate the drying of short-grain rough and of parboiled rice. Higher drying rates and better energy efficiencies were obtained for parboiled rice since the hulls offer no resistance to the moisture 'flow. Also, Singh et a1. (1980) compared experimental data from a pilot scale crossflow dryer with’ simulated resuslts from the modified MSU crossflow model: 9 empirical data of a short-grain variety (S6) showed good agreement with the simulated figures. 73 C) the concurrentflow model dT - h*a - = *(T-e) (3.47) dx Ga*Ca + Ga*Cv*H de h*a hfg + Cv*(T-e) dH - = *(T-e) - *Ga*- (3.48) dx Gp*Cp + Gp*Cw*M Gp*Cp*Cw*M dx dH Gp dfl - = - -*— (3.49) dx Ga dt -- s an appropriate thin-layer equation (3.50) dt The boundary and initial conditions are: (a) T(0)=T(inlet), (b) 9(0)=G(inlet), (c) H(0)=H(inlet), and (d) R(O)=fi(in1et). ‘The MSU concurrentflow (CCF) drying model was used by Walker (1978) to investigate the physical feasibility of drying long-grain rough rice in a multistage concurrentflow dryer. From pilot scale experiments with a one-stage CCF model and computer simulations of multistage dryers, Walker (1978) recommended certain operating conditions and design configurations. Long-grain rough rice was successfully dried in the laboratory one-stage concurrentflow unit at 121 C. The effects of tempering on rice quality and drying rates were established. __..__.._._ —. inv mod) to Roi: d'l‘ dx - d6 d: - dH dx II glg‘. (197: drYil 74 The drying of soybeans in a concurrentflow dryer was investigated by Dalpasquale (1981). The MSU concurrentflow model was also used by Brook and Bakker-Arkema (1978, 1980) to investigate the drying of corn and peabeans, and by Rodriguez (1982) to analyze the drying of corn. D) the counterflow drying model dT - h*a ‘- = *(T-e) (3.51) dx Ga*Ca + Ga*Cv*H as h*a hfg + Cv*(T-6) an - = *(T-G) + *Ga*- (3.52) dx Gp*Cp + Gp*Cw*M Gp*Cp*Cw*M dx dH Gp dfi - *3 -*- (3.53) dx Ga dt dfi - = an appropriate thin-layer equation (3.54) dt The boundary and initial conditions are: (a) T(x)=T(inlet), (b) 6(0)=6(inlet), (c) H(x)=H(inlet), and (d) M(0)=M(inlet). The counterflow drying model was solved by Evans (1970) and modified by Schisler (1983) to simulate the drying of rice. 75 3.3.3.3 Equilibrium Moisture Content Models The equilibrium moisture content (EMC) refers to the quantity of moisture in the product when it is at equilibrium with the surrounding environment. The EMC of rice depends on the air temperature and relative humidity, the grain variety, and previous history (Steffe et al., 1980). The EMC depends on whether the rice absorbs or desorbs moisture in reaching equilibrium. The EMC achieved by desorption is higher than that obtained by adsorption. This phenomenon is referred to as the hystheresis effect. The concept of EMC is important in the study of grain drying because EMC determines the minimum moisture content (boundary condition) to which the grain can be dried under a given set of drying conditions (Brooker et al., 1974). EMC equations for rough rice have been determined by several researchers. Henderson (1952) presented an EMC equation which was modified by Thompson (1972). The Henderson-Thompson equation for rough rice as presented by Pfost et al. (1976) is: 76 Me = 0.01 * (A/B) ** 0.409 (3.55) where A = 1n (l-RH) ' B = - 0.000019187 * (T+51.161) Me is the moisture content (decimal, dry basis), T is the product temperature (C), and RH is the relative humidity (decimal). Fig. 3.22 shows the Henderson-Thompson EMC curves for rough rice at different temperatures. Chung and Pfost (1967) presented and EMC equation which was modified by Pfost et a1. (1976). The Chung-Pfost equation for rough rice as presented by Pfost et al. (1976) is: Me = 0.325535 - 0.046015 * in (A) (3.56) where A = - 1.987 * (T + 35.703) * ln (RH) Me, T, and RH are defined in Eq. (3.55). Eq. (3.56) is a more general equation, since it was developed with data from many sources and includes both desorption and adsorption information. Fig. 3.23 shows the Chung-Pfost EMC curves for rough rice temperatures from 10 to 90 C. Steffe (1979) used the EMC models presented by Zuritz et al. (1979) in analyzing drying and tempering of short-grain rough rice. The Zuritz EMC equation was developed from desorption isotherms using static equilibrium moisture content data for medium-grain rough rice. Eq. (3.57) gives the EMC of rough rice for dry bulb ICE ENC (08) R 6 77 D O é. ROUGH RICE E N C (Eq. 3.55) 8.!” o z-aoc C? S- 800 N. "'m (D s-ooc l S 2 E? i 5 O 9 9 er D 9 cb.00 23.00 45.00 60.00 66.00 Tho.oo RELRTIVE HUMIDITY Fig. 3.22 Rough rice equilibrium moisture content for the Henderson-Thompson equation. 78 ROUGH RICE E H C (Eq. 3.56) 40.00 :00 000 8- 600 700 000 32.00 I 2.4 .00 CE ENC (DB) ROUGH RI .00 19.00 I .00 20.00 40.00 60.00 00.00 {00.00 RELRTIVE HUMIDITY cp.00 Fig. 3.23 Rough rice equilibrium moisture content for the Chung-Pfost equation. 79 temperatures below 42.5 C: Me = 0.001 * ((A/B) ** C) (3.57) where A s - In (1.0 - RH) *Ta B = (1.0 - Ta/64l.7) ** -23.438 * 2.667E-7 C = 1.0/(4.0ES * Ta ** - 2.1166) Ta is the absolute product temperature (K): Me and RH are defined in Eq. (3.55). Fig. 3.24 shows the Zuritz EMC for medium-grain rough rice at low temperatures. For dry bulb temperatures greater than 42.5 C Eq. (3.58) is recommended: Me a (A - B)/C (3.58) where A = 1n (-1n(RH)) B = 1n (2.38789 * (Ta ** -3.444)) C I - 0.02118 * (Ta ** 1.1852) Ta, Me, and RH have been defined in Eq. (3.57). Fig. 3.25 shows the Zuritz EMC for medium-grain rough rice at high temperatures. Vemuganti et a1. (1980) presented coefficients for the Henderson-Thompson and Chung-Pfost equations for equilibrium relative humidity and equilibrium moisture content of 20 food grains including medium-grain rough rice. Kachru and Matthes (1976) reported desorption/adsorption equilibrium moisture content for long-grain rough rice at different temperatures (low range, 19 to 38 C) and different relative humidities (5 to 95 80 . ROUGH RICE E H C (medium-grain) 40.00 (Eq. 3.57) §§§§ I 2 S 4 Air temperatures less than 42.5°C 32.00 1 ICE ENC (DB) 24.00 R 6.00 1 ROUGH1 8.00 .00 20.00 40.00 60.00 00.00 {00.00 RELRTIVE HUMIDITY cp.00 Fig. 3.24 Rough rice equilibrium moisture content at low temperatures for the Zuritz equation. (08) ENG ROUGH RICE 81 O O g1 ROUGH RICE E n c 1 (medium-grain) 2 1- soc E , . 3 o 2-ooc (q 358) 4 C3 8«- 100 . 5 .j._ o-eoc on 8- 000 Air temperatures greater or equal to g; 42.s°c 9;. O 9 0‘ O 9 :3. c: 9 (I? I I 1 I I 0.00 20.00 40.00 80.00 80.00 100.00 RELRTIVE HUMIDITY Fig. 3.25 Rough rice equilibrium moisture content at high temperatures for the Zuritz equation. 82 percent). A comparison of equilibrium moisture content models is presented in section 4.1.2.2. 3.3.3.4 Tempering Models Little experimental work has been conducted on the tempering process of rice. During the tempering process moisture migrates inside the kernel which equalizes the moisture concentration throughout the kernel (Steffe, 1979). During the drying process a moisture gradient is set up because the moisture is first removed from the surface of the kernel. Radial vapor and liquid diffusion is the primary mechanism of moisture migration inside a rice kernel (Steffe, 1979). Brook (1977) studied the tempering of corn and peabeans; the tempering of soybeans between drying stages in a~ concurrentflow dryer was studied by Dalpasquale (1981). Walker (1978) defined the tempering process by using Eqs. (3.47), through (3.50) with the following boundary conditions: 83 M e T H :3- = 2- :2- 3— = o.o (3.59) 3r (surface) 3x 3x 3x M(r,0) = M(r,L) (3.60) where L is the bed depth of the dryer (m). The boundary conditions imply that: (1) no moisture is lost from the the kernel surface (0M/ar is equal to zero), (2) there is no .product temperature change (as/5x is equal to zero), (3) the' tempering process takes place in a constant air temperature/relative humidity environment. The kernel moisture distribution from the previous stage of the dryer is the initial moisture distribution for the tempering stage. Ozeike and Otten (1981) concluded that redistribution of moisture within the kernels during tempering results in a kernel temperature decrease (change in the enthalpy of the grain): this was also observed in_ practice. Bakker-Arkema (1983) has observed the opposite during the tempering of rough rice. Table 3.17 shows the moisture distribution inside a longegrain rough rice kernel (considered as a cylinder) during the drying at 121 C in the first stage of a concurrentflow dryer. Table 3.18 shows the moisture distribution during the subsequent tempering process at 32 C. The moisture gradient (center minus surface node) l"! Ill-ll I 84 Table 3.17 Moisture content distribution in a long-grain rough rice kernel during drying at 121 C in a concurrentflow dryer (Walker, 1978). Depth Time Moisture content distribution (percent, dry basis) (m) (hr) center node surface node 0.00000 m 0.00098 m l 2 3 4 5 6 7 8 9 10 0.00 0.00 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 0.02 0.01 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 23.8 0.15 0.07 25.0 25.0 25.0 25.0 25.0 25.0 24.8 24.2 22.0 16.3 0.31 0.14 25.0 25.0 25.0 25.0 24.9 24.8 24.3 22.9 20.1 16.0 0.46 0.20 25.0 25.0 25.0 24.9 24.8 24.5 23.7 22.0 19.5 16.7 0.62 0.27 25.0 25.0 25.0 24.9 24.7 24.2 23.2 21.5 19.4 17.4 0.76 0.34 25.0 25.0 24.9 24.8 24.5 23.8 22.8 21.2 19.4 17.9 0.92 0.40 24.9 24.9 24.8 24.7 24.3 23.6 22.5 21.0 19.4 18. Cylindrical kernel equivalent radius of 0.00098 meters Table 3.18 Moisture content distribution in a Long-grain rough rice kernel during tempering at 32 C in a concurrentflow dryer (Walker, 1978). Depth Time Moisture content distribution (percent, dry basis) (m) (hr) center node surface node 0.00000 m 0.00098 m I 2 3 4 5 6 7 8 9 10 0.00 0.00 24.9 24.9 24.8 24.7 24.3 23.6 22.5 21.0 19.4 18.1 0.46 0.20 24.8 24.7 24.5 24.2 23.7 22.9 22.0 20.9 20.0 19.7 0.92 0.40 24.4 24.4 24.2 23.6 23.2 22.5 21.8 21.0 20.5 20.3 1.37 0.60 24.1 24.0 23.8 23.4 22.9 22.3 21.7 21.2 20.8 20.7 1.83 0.81 23.7 23.6 23.4 23.1 22.6 22.2 21.7 21.3 21.1 21.0 2.29 1.01 23.3 23.3 23.1 22.8 22.5 22.1 21.7 21.4 21.2 21.2 2.75 1.21 23.0 23.0 22.8 22.6 22.3 22.0 21.7 21.5 21.4 21.3 3.05 1.34 22.9 22.8 22.7 22.5 22.3 22.0 21.8 21.6 21.4 21.4 Cylindrical kernel equivalent radius of 0.00098 meters 85 after 24 minutes of drying is 6.8 percentage points. This gradient is reduced to 1.5 percentage points after 80 minutes of tempering. Walker (1978) concluded that tempering does not significantly increase the drying rate of long-grain rice in a concurrentflow dryer. This is due to the relatively small equivalent radius of long-grain rough rice kernels (0.00098 meters) and uniformity of moisture distribution (individual kernels) when leaving the drying stage. Steffe (1979) simulated the drying and tempering of single kernels of short-grain rough rice; the tempering models are basically an extension of his drying model (see section 3.3.3.1.2). Similar assumptions as those made by Walker (1978) were used by Steffe (1979): ac - - 0.0, r = R3, t?,0.0 ‘ (3.61) ar C = C(r), 0.0 £1“ £123, 1: = 0.0 (3.62) The results of the drying and tempering model by Steffe (1979) are discussed in Chapter Four. The time required to temper short-grain rough rice is presented by Steffe and Singh (1980b). Eqs. (3.63) through (3.66) calculate the time required to achieve 95 percent and complete tempering (moisture content at the center is equal to the moisture content at the surface of the kernel), when either moisture reduction during drying 86 or drying time is specified: complete tempering - moisture reduction specified: tt = 24.145-5.344*ln(T)+0.253*ln(DELM)-0.287*RH+1.096*Mo 95 percent tempering - moisture reduction specified: (3.63) tt 2 11.194-2.365*ln(T)+0.259*ln(DBLM)-0.223*RH+1.146*M0 complete tempering - drying time specified: (3.64) tt s 21.537-4.899*ln(T)+0.131*ln(DTIME)-0.491*RH+2.132*Mo 95 percent tempering - drying time specified: (3.65) tt = 8.509-1.905*1n(T)+0.135*1n(DTIME)-0.012*RH-0.083*Mo (3.66) where tt is the time required to temper the grain (hr), T is the drying temperature (C), RH is the relative humidity (decimal), Mo is the initial moisture content (decimal, dry basis), DELM is the moisture reduction during one drying pass ‘(decimal, dry basis), and DTIME is the time elapsed during one drying pass (hr). During the drying experiments, the temperature range was 34.8 - 55.5 C; the moisture 'content range was 19.3 - 24.1 percent, the relative humidity was 8 and 16 percent, and the drying time was 0.5 and 1.0 hours. Eqs. (3.63) through (3.66) were developed from simulation data. Table 3.19 shows the times required to achieve 95 percent and complete tempering after short-grain rough rice is dried at 30, 40, and 50 C of temperature and 50 percent relative humidity. The temperature has the greatest effect Table 3.19 87 Time required to achieve 95 percent and complete tempering when short-grain rough rice is dried at different temperatures and 50 2 relative humidity and subsequently tempered (Steffe and Singh, 1980b). Temperature (C) Tempering time (hr) when moisture removal is known Tempering time (hr) when drying time is known 95 percent tempering complete tempering 95 percent tempering complete tempering Moisture removal (2 db) Drying time (hr) 2 4 2 4 0.5 1.0 0.5 1.0 30 2.41 2.57 5.20 5.38 1.96 .05 5.25 5.34 40 1.73 1.91 3.66 3.89 1.41 1.50 3.84 3.94 50 1.20 1.38 2.47 2.65 0.98 1.08 2.75 2.84 Table 3.20 Thin-layer drying of cereal grains at 49 C temperature and 60 percent relative humidity. Time Moisture content (percent, wet basis) (hr) Corn(1) Rice(2) BarleY(3) Nheat(4) Sorghum(5) Soybeans(6) 0.0 22.0 22.0 22.0 22.0 22.0 22.0 0.2 20.8 20.9 21.0 20.3 19.8 20.6 0.4 20.3 20.1 20.1 18.5 18.7 19.8 0.6 19.9 19.4 19.3 17.6 17.8 19.3 0.8 19.6 18.5 18.5 16.5 17.2 18.8 1.0 19.3 18.3 17.7 15.7 16.6 18.3 2.0 18.3 16.3 15.2 13.0 14.6 16.7 3.0 17.6 15.0 13.6 11.9 13.2 15.6 4.0 17.0 14.0 12.6 11.5 12.2 14.7 5.0 16.6 13.3 12.0 11.3 11.5 14.0 (1) - Misra and Brooker (1978) (2) - Wang and Singh (1978) (3) - O'Callaghan et al. (1971) (4) - O'Callaghan et a1. (1971) (5) - Paulsen and Thompson (1973) (6) - Overhultz et a1. (1973) with coefficients from White et al. (1973) 88 on the time required to achieve different tempering levels; this is. due to the increase in the diffusion coefficient (Table 3.9). The moisture removal per pass, or drying time, has a lesser effect on the time required to achieve 95 percent and complete tempering. Table 3.19 shows that tempering times of more than two hours are seldom required for short-grain rough rice. 3.3.4 Drying of Cereal Grains Other than Rice The drying rates of five cereal grains (corn, barley, wheat, sorghum, and soybeans) are compared to the drying rates of medium-grain rough rice. Experimental thin-layer equations were used to analyze the drying rates of cereal grains. The Henderson-Thompson EMC equation (Eq. 3.55) with constants for corn, barley, wheat, sorghum, and soybeans as presented by Pfost et al. (1976) was used to calculate the appropriate moisture equilibrium values. For medium-grain rice Eq. (3.23) is used. The thin-layer equation for corn of Misra and Brooker (1978) is: 89 M - Me = MR = exp (-X * t ** Y) (3.67) Mo - Me where X 8 “0.0347 + 0.00287*TF Y I 0.54 + 0.00324*RH TF is the drying air temperature (F), t is the drying time (hr), RH is the relative humidity (decimal) and the moisture ratio (M - Me)/(Mo - Me) is the moisture ratio (MR) defined in Eq. (3.19). The thin-layer equation for barley and wheat of O'Callaghan et a1. (1971) is: MR = exp (-R*’ts) (3.68) where for barley K x 193.3*exp(-7976/(TF+460)) for wheat K e 2000.0*exp(-9l79/(TF+460)) ts is the drying time (seconds) and TF is the drying air temperature (F). The thin-layer for sorghum of Paulsen and Thompson (1973) is: MR = exp ((-A-SQTR(A**2+4*B*t))/(2*B)) (3.69) where A 8 '25.87 + 0.34*TF - 0.00108*TF**2 for TF .9160 F A a 0.54 - 0.0017*TF for 160‘TF 5240 F B - 30.35 * exp(-0.018*TF) t is the drying time (hr) and SQRT is the square root. The other variables are defined in Eq. (3.68). 90 The thin-layer equation for soybeans of 0verhultz et a1. (1973) with coefficients by White et a1. (1978) is: MR= exp (-(X*t)**Y) ' (3.70) where X = -0.207 + 0.00357*T + 0.216*Mo + 0.261*RH +0.0003202*M0*T Y = 0.33 + 0.25*RH + 0.003*T T is the temperature (C), t is the drying time (hr), RH is the relative humidity (decimal) and M0 is the initial moisture content (decimal, dry basis). The moisture ratio (MR) is defined in Eq. (3.19). Table 3.20 presents the moisture content of the six cereal grains when dried in a thin-layer at 60 percent relative humidity and at a drying temperature of 49 C. The drying rates are also shown in Fig. 3.26. Medium-grain rough rice dries slower than wheat, sorghum, and barley but it dries faster than corn and soybeans. Medium-grain rough rice has a smaller equivalent radius and a larger surface area when compared to corn and soybeans. The drying rates of medium-grain rough rice are lower (due to the presence of hulls) than the drying rates of wheat, sorghum, and barley. 91 c3 THIN-LAYER DRYING OF CEREAL GRAINS c3 4 TEMPERATURE = 49°C , N RELATIVE HUMIDITY = 60 9. HENDERSON-THOMPSON EMC 0 c: (“a N c: c: 0- ,‘00 DO I! 77:: 1—9 IZODq “J"! I— II CD 63:5 corn LIJCO. mil-0 :3 p. 03 HO 199 2::5g soybeans medium- grain rice 0 9 cqfi barley H sorghum ““ wheat O c: C) " I I I I I "0.00 1.00 2.00 3.00 4.00 6.00 TIME (HOURS) Fig. 3.26 Thin-layer drying of six cereal grains at 49°C and 60 % relative humidity. 92 3.3.5 Quality Aspects of Rice The quality characteristics of the rice grain are related to a complex of physical and chemical properties (Chang and Li, 1980). According to Webb (1980) the quality of rice may be categorized into four broad areas: (1) milling quality, (2) cooking, eating and processing qualities, (3) nutritive quality, and (4) cleaniless, soundness and purity.’ Since rice has many different uses, the desired quality characteristics vary considerably. Chacarteristics that influence rice quality are a function of harvesting, drying, handling, milling, and storage. The milling yield and color of the milled rice and seed viability of the rough rice are the main quality criteria used in the rice industry. The United States grade and grade requirements for rough, brwon, and milled rice are presented in Appendix A (Tables A.1, A.2, A.3, and A.4). Fig. 3.27 shows the approximate proportions and main uses during the rice milling process. Boiled rice consumed in the home constitutes the greatest useage. The milling yield of rice depends on the rice variety, growing conditions (environmental effect), the harvesting technique, the handling of rice, the drying method, the 93 mm ooH . daeoe poem mama comm xooumm>fia poem xooumo>aa ox m ... mmHAom uuomxo och comm xuoumo>fla oHumoEoo comm ms ca meoaoommwm xooumm>HH @x b ... dem 6x m .. mmmmemg mocman uuomxw muOmmmoowm oaummaoo muusoowm mx ma moan poncho 600m womb zmxom muuomxo Hmoumo noon 66666... . .....wmmw 04mm ozoumm oeummEOp ms oH ... memes- mHOmmmuoum anew gash mahmmooohm Hmmuou we on mon meHmz ommmmn uuomxw pmmmxumm Hmeomcoo uuomxw oommxomm umEsmcou oauwwEOU mm mm .. mon 04mm cmmnu a moon @003 ox H wads: ox ma poem amass me on .. mass: .82.; .6825 mom: name use wwucsoo mcauuomfifl >9 owumeEoo mcwHHHE xaso owuuomxo mm ow mon 230mm mCOHpuomouQ poopoum oumEonummm .mmmoonm msflHHfiE woflm nm.m .mflm mm ooa NUHK $090M 94 degree of tempering, and the type and adjustment of the milling machine. Long-grain varieties are more susceptible to breakage (Chen, 1980). Table 3.21 presents the average milling yields in terms of head rice and total milled rice for the three commercial rice types grown in the United States. On the average short-, and medium-grain types have higher head yields than the long-grain varieties. The environmental effects on the fissuring of rice have been studied by Kunze (1964), by Kunze and Prasad (1978), and by Chen (1980). Rice is a hygroscopic product. Depending on the temperature and relative humidity of the air and on the maturity stage (moisture content of the grain), may absorb or desorb moisture. Rough rice may fissure when exposed to an adsorbing environment for a significant period of time. Fissured rice reduces the head yield after milling. The harvesting operation may cause severe cracking of rice kernels, especially if the rice is harvested at low moisture contents. The moisture content of rice kernels at harvest varies from as low as ten percent to as high as 40 percent (Chau and Runze, 1982). Once the rice is harvested and mixed, the low moisture kernels may adsorb moisture and develop fissures. Mechanical (auger) damage can be caused by excessive handling of low moisture rice and can result in lower head yields. 95 Table 3.21 Range of average milling yields of typical U.S. commercial 1ong-, medium-, and short-grain types (Webb, 1980). Grain Average milling yield (percent) type Whole kernel (head yield) Total milled rice long 56 - 61 68 - 71 medium 65 - 68 71 - 72 short 63 - 68 73 - 7h 96 The drying and tempering have effect on the head yield of rice (Walker, 1978 and Steffe et a1., 1978). Drying air temperature and relative humidity affect the moisture removal rate and the kernel moisture gradient (the difference between the moisture at the center and the moisture at the surface of the kernel). The loss of seed viability during heated air drying was studied by Nellist (1981). High temperatures for a prolonged period of time cause damage to the embryo and loss of viability. According to Roberts (1981) the recommendations for hot-air drying of seeds are still based on crude practical observations. Contradictory data have been published about the effect of drying conditions on head yield and seed viability of rice. Most of the data indicate that rice kernels (especially at high moisture contents) when exposed to high temperatures and low relative humidity environments for a prolonged period of time, develop fissures. Walker (1978) showed tempering to be more important for maintaining head yield than for aiding drying. The results reported by Steffe et a1. (1979) also indicate that higher drying rates occur after tempering when drying short-grain rough rice. 97 The breakage of rice during milling has been investigated by Spadaro et a1. (1980). Breakage is affected by the properties of rice, by the weather during maturation of the panicles, by the drying conditions, and by the conditions under which the grain is milled. .111: Ill: 1! Illllllll' ill I‘ CHAPTER 4 ANALYSIS OF RICE DRYING SIMULATION The physical and thermal properties of rough rice, the EMC, and 'the thin-layer drying equations have been discussed in the literature review. The following sections present results and discussions for single kernel drying of rough rice. The simulation results for concurrentflow, crossflow, and in-bin drying of rice are presented in Chapters Six and Seven. 4.1 Single Kernel Drying Simulation The results for single kernel drying (empirical and diffusion-type equations) are compared and discussed. 98 99 4.1.1 Non-Diffusion Thin-Layer Equations Thin-layer drying of short-grain rough rice was simulated using the Agrawal-Singh equation (Eq. 3.21) utilizing the Henderson-Thompson EMC model (Eq. 3.55). Fig. 4.1 shows the drying rate of short-grain rough rice, initially at 25 percent moisture content, at 20 percent relative humidity and air temperatures from 10 to 50 C. Thin-layer of medium-grain rough rice was simulated using the Wang-Singh equations (Eqs. 3.22, 3.23, and 3.24) in conjunction with the Henderson-Thompson model (Eq. 3.55). The drying conditions used are: 20 percent relative humidity and 10 to 50 C air temperature. Eq. (3.24) can only be used for short interval drying time (Fig. 4.2); it was never intended to model long drying periods. Fig. 4.3 shows the drying rate when using the single term approximate form of the Wang-Singh diffusion equation (Eq. 3.22). Fig. 4.4 shows the drying rate when using the Page-type Wang-Singh equation (Eq. 3.23). The Page-type equation of Wang and Singh (1978) simulates the drying of medium-grain rough rice well and thus, Eq. (3.23) is used in this dissertation to model crossflow and in-bin drying. 100 RELIITIYE HUMIDITY 18 20 PERCENT 65 Henderson-Thompson EMC ‘1' (Eq. 3.21) N1 :3 9 00‘ 1() C as09 ... DD 2! Tic: 1—9 2:01)q 0 C mod I— I: CDC: Us DJum. 0 C “H :3 I— 0) HO CDC? ‘(1 C 2:...q ... 50c :3 D ‘1' I I I I I 0.00 1.00 2.00 3.00 4.00 5.00 TIME (HOURS) Fig. 4.1 Drying of short-grain rough rice using the thin-layer equation by Agrawal and Singh (1977). MOISTURE CONTENT (N8) 101 OURDRRTIC EOUHTION (Eq. 3.24) Henderson-Thompson EMC RELIITIVE HUMIDITY I8 20 PERCENT 25.00 V II) C l 24.00 20 l 23.00 31 l 22.00 40 21.00 1 I 1 1 j 0.20 0.40 0.80 1.00 0.80 TIME (HOURS) c20.00 a D Fig. 4.2 Drying of medium-grain rough rice using the quadratic equation by Wang and Singh (1978). 28.00 A 24.00 l 20.00 16.00 I MOISTURE CONTENT (NB) 12.00 102 SINGLE TERM RPPROXIMRTE RELIITIVE HUMIDITY ID 20 PERCENT Henderson-Thompson EMC (m.323 1() C 2() C 3() C 4() C 50 c13.00 I 1 1 l 1.00 2.00 4.00 5.00 3.00 TIME (HOURS) Drying of mediumjgrain rough rice using the single term approximate form of diffusion equation by Wang and Singh (1978). I M0 STURE CONTENT (N81 103 PRGE’S EOURTION RELIITIVE HUMIDITY I8 20 PERCENT g; Henderson-Thompson EMC ' E. . 81 (Cl 323) c: 9 :51 c: 9 C3. N 210 C E; 20 C: (D "1 30c c: ‘0 C 9 $1 0c 0 D I I 1 I “0.00 1.00 3'.00 4.00 5.00 TIME (HOURS) Fig. 4.4 Drying of medium-grain rough rice using the Page type equation by Wang and Singh (1978). 104 Table 4.1 Thin-layer drying of rough rice using empirical equations(a). Moisture content (percent, wet basis) Time Temperature (30 C) Temperature (40 C) (hr) Relative humidity (50 2) Relative humidity (20 2) (1) (2) (3) (4) (1) (2) -(3) (4) 0.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 0.2 24.9 24.0 24.0 23.9 23.7 23.1 23.2 23.1 0.4 24.6 23.4 23.3 23.2 22.6 21.9 22.1 21.9 0.6 24.2 23.1 22.7 22.6 21.5 21.4 21.0 20.8 0.8 23.7 23.2 22.1 22.0 20.5 21.8 20.0 20.0 1.0 23.1 23.8 21.6 21.6 19.5 22.7 19.1 19.2 2.0 19.1 - 19.1 19.6 15.7 - 15.2 16.1 3.0 15.3 - 17.2 18.1 12.9 - 12.4 13.9 4.0 12.7 - 15.6 17.0 11.0 - 10.5 12.4 5.0 11.4 - 14.5. 16.1 9.6 - 9.2 11.2 (a) 3 The Henderson-Thompson EMC model (Eq. 3.55) was used. (1) - Page type equation (Eq. 3.21) for short-grain rough rice (2) - Quadratic equation (Eq. 3.24) for medium-grain rough rice (3) - Single term approximate (Eq. 3.22) for medium-grain rough rice (4) Page type equation (Eq. 3.23) for medium-grain rough rice. 105 Table 4.1 presents moisture content versus time when short- and medium-grain rice are dried at two temperatures and two relative humidities. Short-grain rough rice dries slower than medium-grain rough rice. The moisture contents obtained when using the quadratic equation (Eq. 3.24) compare well with those for the single term approximate (Eq. 3.22) and Page's (Eq. 3.23) equations for drying time less than 30 minutes. 4.1.2 Diffusion Thin-Layer Equations 4.1.2.1 Whole Kernel Model Diffusion-type models, such as those presented by Wang and Singh (1978) and by Steffe (1979) were tested with different diffusion coefficients. The whole kernel diffusion model (Eq. 3.25) was used to simulate the drying of long- and medium-grain rice kernels. The equivalent radius (defined as the average of kernel width and thickness divided by two) for long- and medium-grain rice kernels are: 0.00100 and 0.00156 meters, respectively. 106 The equivalent radius based on the kernel width and thickness is used in this dissertation as a curve-fitting device. The equivalent radius based on the kernel volume (Table 3.5) can be used; this results in similar drying rates for medium-, and long-grain rice (similar kernel volumes). Walker (1978) used an equivalent radius of 0.00098 meters for long-grain rough rice (comparable to 0.00100 meters used in this dissertation). Figs. 4.5 and 4.6 show the drying rate of long-grain rough rice (considered as a sphere) when using the diffusion coefficients by Wang and Singh (1978) and by Steffe and Singh (1982), respectively. Similar curves are shown in Figs. 4.7 and 4.8 for medium-grain rough rice. Table 4.2 presents the values for the moisture content during the drying of long- and medium-grain rough rice. Long-grain rough rice dries faster than medium-grain rough rice. Higher drying rates are observed when the diffusion coefficient by Steffe and Singh (1982) is used. 4.1.2.2 Composite Kernel Model The drying and tempering of single kernels of long-, medium-, and short-grain rough rice was simulated in order to predict the moisture distribution within individual kernels. The rate of drying and tempering of individual 107 DIFFUSION EOURTION RELRTIYE HUMIDITY I8 20 PERCENT Henderson-Thompson EMC Kernel equivalent radius = 0.00100 m (Eq. 3.25) I: 10 ('1 20 C ES 3 0 C c: c? 4 0 C a: *—-50c 4-00 0.00 1300 2. 00 3.00 4100 5100 TIME (HOURS) Fig. 4.5 Long-grain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Steffe and Singh (1982). CONTENT (NB) I 0 108 DIFFUSION EDURTION RELRTIVE HUMIDITY I8 20 PERCENT STURE 16 H0 0 65 Henderson~Thompson EMC 2&1 Kernel equivalent radius = 0.00100 m (Eq. 3.25) D 9 * on D 9 8‘ 10c 2() C c: O ' 3() C fl 4() C &. soc F. D D . T j fl “b.00 1.00 2.00 3.00 4100 6400 TIME (HOURS) Fig. 4.6 Long-grain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Wang and Singh (1978). 28.00 J 24.00 20.00 J MOISTURE CONTENT (MB) 16.00 12.00 J 109 DIFFUSION EDURTION RELRTIVE HUMIDITY IS 20 PERCENT Henderson-Thompson EMC Kernel equivalent radius = 0.00156 m (Eq. 3.25) 10 (I 20 (I 30 C 4() C cp.00 1100 2100 3100 4100 6100 TIME (HOURS) Mediumegrain rough rice moisture content versus time when dried in a thin-layer; assuming spherical geometry and using the diffusion coefficient (Table 3.8) of Steffe and Singh (1982). 26.00 24.00 Fig. 1 1.430 2 (30 and using 110 DIFFUSION EOURTION RELRTIVE HUMIDITY I8 20 PERCENT Henderson-Thompson EMC Kernel equivalent radius (Eq. 3.25) TIME (HOURS) 1 4.00 = 0.00156 m 1.0 20 .30 (40 5() 1 5.00 4. 8 Mediumigrain rough rice moisture content versus time when dried in a thin- layer; assuming spherical geometry the diffusion coefficient (Table 3.8) of Wang and Singh (1978). 111 Table 4.2 Thin-layer drying of rough rice using diffusion theory and considering the rice kernel as a sphere (a). Moisture content (percent, wet basis) Time Long-grain rough rice Medium-grain rough rice (hr) TEMP (30 C) TEMP (40 C) TEMP (30 C) TEMP (40 C) RH (50 2) RH (20 2) RH (50 8) RH (20 2) (l) (2) (1) (2) (l) (2) (1) (2) 0.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 . 25.0 0.2 22.4 23.1 20.6 22.1 23.3 23.8 22.1 23.1 0.4 21.4 22.4 18.8 21.0 22.7 23.3 21.0 22.4 0.6 20.6 21.8 17.6 20.1 22.1 22.9 20.1 21.8 0.8 20.0 21.3 16.5 19.4 21.7 22.6 19.4 21.3 1.0 19.5 20.9 16.6 18.7 21.4 22.3 18.7 20.9 2.0 17.5 19.4 12.5 16.4 20.0 21.2 16.4 19.3 3.0 16.1 18.3 10.6 14.8 18.9 20.5 14.7 18.1 4.0 15.1 17.4 9.3 13.5 18.1 19.9 13.4 17.1 5.0 14.3 16.6 8.5 12.4 17.4 19.3 12.4 16.3 (a) - The Henderson-Thompson EMC model (Eq. 3.55) was used. The kernel equivalent radius for long-grain rough rice is 0.001 meters and 0.00156 meters for medium-grain rough rice. (1) I Diffusion coefficient by Steffe and Singh (1982) - Table 3.3 (2) - Diffusion coefficient by Wang and Singh (1978) - Table 3.8. 112 kernels is determined by the value of the diffusion coefficients in Eq. (3.32) and by the size of the rice kernel component radii. The diffusion model developed by Steffe (1979) as described in the literature review (section 3.3.3.1.2) considers rough rice as a sphere surrounded by two concentric shells (bran and hull). The diffusion coefficients for rough rice components presented by Steffe and Singh (1980a) are used (Table 3.8). The equivalent component radii for short-grain rice are (Steffe, 1979): 0.00158, 0.00166, and 0.00177 meters for the endosperm, bran, and hull, respectively. The calculated values from measurements of kernel width and thickness for medium-grain rice are: 0.00138, 0.00146, 0.00156 meters for the endosperm, bran, and hull, respectively. For long-grain rice the calculated values are: 0.00088, 0.00093, and 0.00100‘ meters for the endosperm, bran, and hull, respectively. Fig. 4.9 shows a comparison of the four EMC models presented in the literature review (Eqs. 3.55, 3.56, 3.57, and 3.58). Table 4.3 presents the value of the EMC of rough rice for the Henderson-Thompson, Chung-Pfost, and Zuritz models. The results compare well, except at low (less than ten percent) and at high (over 90 percent) relative humidities. 113 D I; ROUGH RICE E M C HT 40 C - Zuritz — Low temperature - Zuritz - High temperature 2 Henderson-Thompson - Chung-Pfost 4 1 2 3- 4 0:1 53:. c? 3 L1"- - Eq. (3.57) E“ - Eq. (3.58) . (3.55) 11.1 - Eq. (3.56) bCNNI—A I [TI ..0 O D ‘11.00 20.00 40.00 611.00 30.00 100.00 RELRTIVE HUMIDITY Fig. 4.9 Comparison of equilibrium moisture content models for rough rice at 40°C. Table 4.3 114 Equilibrium moisture content for rough rice using the Henderson-Thompson, Chung-Pfost, and Zuritz models for 10 - 100 C temperatures and 1 - 99 2 relative humidities. Equilibrium moisture content (2 d.b.) TEMP MODEL 1 10 20 30 40 50 60 70 80 90 99 (C) Henderson- Thompson 2.4 6.3 8.5 10.4 12.0 13.6 15.2 17.0 19.2 22.2 29.5 10 Chung-Pfost 4.8 8.0 9.6 10.9 12.2 13.5 14.9 16.5 18.7 22.2 33.0 Zuritz 2.8 7.0 9.4 11.3 13.0 14.6 16.3 18.1 20.3 23.3 30.4 Henderson- Thompson 2.2 5.9 8.0 9.7 11.3 12.8 14.3 16.0 18.0 20.9 27.7 20 Chung-Pfost 3.4 7.1 8.7 10.0 11.3 12.6 14.0 15.6 17.8 21.2 32.1 Zuritz 2.4 6.3 8.6 10.5 12.2 13.9 15.6 17.4 19.7 22.9 30.5 Henderson- Thompson 2.1 5.6 7.6 9.2 10.7 12.1 13.6 15.2 17.1 19.8 26.3 30 Chung-Pfost 3.1 6.3 7.9 9.3 10.5 11.8 13.3 14.9 17.0 20.5 31.3 Zuritz 1.9 5.4 7.6 9.4 11.0 12.6 14.3 16.2 18.4 21.6 29. Henderson- Thompson 2.0 5.3 7.3 8.8 10.2 11.5 12.9 14.5 16.3 18.9 25.1 40 Chung-Pfost 2.5 5.6 7.3 8.6 9.9 11.2 12.6 14.2 16.4 19.8 30.6 Zuritz 1.4 4.5 6.4 8.0 9.5 11.0 12.6 14.4 16.5 19.6 27.3 Henderson- Thompson 2.0 5.1 7.0 8.4 9.8 11.1 12.4 13.9 15.6 18.1 24.0 50 Chung-Pfost 2.0 5.1 6.7 8.1 9.3 10.6 12.0 13.7 15.8 19.3 30.1 Zuritz 0.8 4.3 6.1 7.5 8.9 10.3 11.8 13.6 16.0 19.7 31.5 Henderson- Thompson 2.0 4.9 6.7 8.1 9.4 10.6 11.9 13.3 15.0 17.4 23.1 60 Chung-Pfost 1.4 4.6 6.2 7.5 8.8 10.1 11.5 13.1 15.3 18.8 29.6 Zuritz 0.3 3.6. 5.4 6.8 8.1 9.4 10.9 12.6 14.9 18.5 29.9 Henderson- Thompson 1.8 4.8 6.5 7.8 9.1 10.3 11.5 12.9 14.5 16.8 22.3 70 Chung-Pfost 1.9 4.1 5.7 7.1 8.3 9.6 11.0 12.7 14.8 18.3 29.1 Zuritz -0.2 3.0 4.7 6.1 7.3 8.6 10.1 11.7 13.9 17.3 28.4 Henderson- l Thompson 1.8 4.6 6.3 7.6 8.8 9.9 11.2 12.5 14.0 16.3 21.6 80 Chung-Pfost 0.5 3.7 5.3 6.7 7.9 9.2 10.6 12.3 14.4 17.9 28.7 Zuritz -0.6 2.5 4.1 5.4 6.6 7.9 9.3 10.9 13.0 16.4 27.0 Henderson- Thompson 1.7 4.6 6.1 7.4 8.5 9.7 10.8 12.1 13.6 15.9 21.0 90 Chung-Pfost 0.1 3.3 5.0 6.3 7.5 8.8 10.2 11.9 14.0 17.1 28.3 Zuritz -l.0 2.0 3.5 4.8 6.0 7.2 8.7 10.1 12.2 15.4 25.7 Henderson- . Thompson 1.7 4.3 5.9 7.2 8.3 9.4 10.5 11.8 13.3 15.3 20.4 100 Chung-Pfost -0.2 3.0 4.6 5.9 7.2 8.5' 9.9 11.5 13.7 17.1 28.0 Zuritz -1.4 1.5 3.0 4. 5.4 6.6 7.9 9.4 11.4 14.6 24.5 115 The Zuritz EMC models are used in this dissertation to simulate the drying and tempering of long-, medium-, and short-grain rough rice. Eqs. (3.57) and (3.58) appear to be the best available in the literature in the range of temperatures of which the model is utilized. In the tempering process it is assumed that rice does not.change in average moisture content or temperature as it passes through the tempering zone. The moisture content distribution in fourteen nodes from the center to the surface of the kernel was simulated to evaluate the effect of tempering on the concurrentflow dryer designs. Table 4.4 present the moisture content distribution within a 25.0 % moisture content long-grain rice kernel during drying at 45 C and 30 % relative humidity. The center of the kernel dries in one hour to 17.5 % w.b., the surface of the kernel to 7.4 % (the convective mass transfer coefficient at the surface is assumed to be infinite). The moisture content gradient after 12, 36, and '60 minutes of drying is 17.4, 13.9, and 10.1 percentage points, respectively. Table 4.5 presents the moisture content distribution in the partially dried long-grain rice kernel during subsequent tempering at 45 C. The tempering is simulated for rice dried for 12, 36, and 60 minutes. The moisture content gradients are reduced from 14.4, 13.9, and 10.1 116 Table 4.4 Simulated moisture content distribution (2 w.b.) within a long-grain rice kernel during drying from 25 2 MC at 45 C and 30 2 relative humidity Using a spherical diffusion equation (Steffe and Singh, 1980). Time Distance from center of kernel, m Average Ifénter surface moisture (min) node node content 0.00000 0.00044 0.00088 0.00093 0.00096 0.00100 (2 w.b.) 0.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 0.6 25.0 25.0 25.0 24.9 22.6 7.4 24 2 1.5 25.0 25.0 24.9 24.2 19.3 7.4 23.7 3.0 25.0 25.0 24.5 22.8 16.9 7.4 23.2 6.0 25.0 24.9 23.3 21.0 14.9 7.4 22.4 12.0 24.8 24.4 21.5 19.1 13.6 7.4 21.1 24.0 23.4 22.4 19.0 17.0 12.4 7.4 19.0 36.0 21.3 20.3 17.2 15.4 11.5 7.4 17.2 48.0 19.3 18.3 15.6 14.1 10.8 7.4 15.6 60.0 17.5 16.6 14.3 13.0 10.2 7.4 14.3 Table 4.6 Time required to achieve 50, 75, and 95 percent tempering when long-grain rice is tempered at 45 C after being dried at 45 C and 30 2 relative humidity for 12, 36, and 60 min. Tempering Time Required (min) Level (2) Dried for Dried for Dried for 12 min 36 min 60 min 50 3.2 3.5 3.6 75 3-9 9.5 9.6 95 23.4 24.5 24.8 Tempering Level the surface at time t minus the moisture content at the surface at time zero divided by the moisture content at the surface at time infinity minus the moisture content at the is defined as the moisture content at surface at time zero, expressed as a percentage. Table 4.5 117 Simulated moisture content distribution (2 w.b.) within a long-grain rice kernel during tempering at 45 C and after being dried for 12, 36 and 60 min at 45 C and 30 2 relative humidity. Drying Tempering Distance from center of kernel, m Average time time Center Surface moisture (min) (min) .ngde_i node ,content 0.00000 0.00044 0.00088 0.00093 0.00096 0.00100 (2 w.b.) 0 24.8 24.4 21.5 19.1 13.6 7.4 21.1 3 24.6 23.9 20.9 19.0 15.9 14.6 21.1 6 24.3 23.5 20.7 19.5 17.6 16.9 21.1 12 12 23.4 22.6 20.8 20.2 19.4 19.1 21.1 18 22.7 22.0 20.9 20.6 20.2 20.1 21.1 30 21.7 21.4 21.0 21.0 20.8 20.8 21.1 0 21.3 20.3 17.2 15.4 11.5 7.4 17.2 3 20.8 19.7 16.8 15.4 13.2 12.2 17.2 6 20.3 19.3 16.8 15.8 14.4 13.9 17.2 36 12 19.3 18.5 16.9 16.4 15.8 15.6 17.2 18 18.5 18.0 17.0 16.7 16.4 16.3 17.2 30 17.8 17.4 17.1 17.0 16.9 16.9 17.2 0 17.5 16.6 14.3 13.0 10.2 7.4 14.3 3 17.0 16.2 14.1 13.1 11.4 10.8 14.3 6 16.6 15.9 14.0 13.3 12.3 11.9 14.3 60 12 15.9 15.3 14.1 13.8 13.3 13.1 14.3 18 15.3 14.9 14.2 14.0 13.8 13.7 14.3 30 14.7 14.5 14.2 14.2 14.1 14.1 14.3 Table 4.7 Simulated moisture content of long-grain rice dried at 45 C and 30 2 relative humidity for 12 and 36 min after being dried to 17.2 2 w.b. at 45 C and 30 2 relative humidity for 36 min and 0, 50, 75, and 95 2 tempering. Tempering Moisture Content, 2 w.b. Level (2) Dried for 12 min Dried for 36 min 0 15.6 13.2 50 15.4 13.0 75 15.1 12.8 95 14.9 12.6 118 percentage points to 0.9, 0.9, and 0.6 percentage points, respectively, when rice is tempered for 30 minutes. The time required to achieve 50, 75, and 95 percent tempering is presented in Table 4.6. Approximately 10 and 25 minutes are required for long-grain rice to achieve 75 and 95 percent tempering at 45 C (the approximate temperature maintained in the concurrentflow dryer tempering zones, regardless of the length of time the rice is dried). Table 4.7 presents drying rates for the second pass. Rice with an initial moisture content of 17.2 %; after being dried for 36 minutes and tempered to achieve 0, 50, 75, and 95 percent tempering, is dried for 12 and 36 minutes at 45 C and 30 % relative humidity. It can be seen that the tempering level affects the moisture removal in the subsequent drying pass (or stage). The 95 percent tempering level increases the drying rate from 1.6 percentage points (no tempering) to 2.3 percentage points when long-grain rice is dried for 12 minutes in the second pass. The drying rate is increased from 4.0 to 4.6 percentage points when rice is dried for 36 minutes. Tables 4.8 through 4.11 present the drying of medium-grain rough rice at 45 C and 30 percent relative humidity with subsequent tempering at 45 C. 119 Table 4.8 Simulated moisture content distribution (2 w.b.) within a medium-grain rice kernel during drying from 25 2 MC - at 45 C and 30 2 relative humidity using a spherical diffusion equation (Steffe and Singh, 1980). Time Distance from center of kernel, m Average Center surfacenmoisture (min) node node content 0.00000 0.00069 0.00138 0.00146 0.00151 0.00156 (2 w.b.) 0.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 0.6 25.0 25.0 25.0 25.0 24.9 7.4 24.5 1.5 25.0 25.0 25.0 24.8 22.0 7.4 24.2 3.0 25.0 25.0 24.9 24.2 19.4 7.4 23.8 6.0 25.0 25.0 24.6 22.8 16.9 7.4 23.4 12.0 25.0 25.0 23.5 21.0 15.0 7.4 22.6 24.0 24.9 24.6 21.8 19.2 13.7 7.4 21.5 36.0 24.6 23.9 20.5 18.1 13.0 7.4 20.5 48.0 24.0 23.0 19.5 17.2 12.5 7.4 19.5 60.0 23.2 22.1 18.6 16.4 12.0 7.4 18.7 Table 4.10 Time required to achieve 50, 75, and 95 percent tempering when medium-grain rice is tempered at 45 C after being dried at 45 C and 30 2 relative humidity for 12. 36, and 60 min. Tempering Time Required (min) Level (2) Dried for Dried for Dried for 12 min _36 min ' 60 min 50 5-7 7-2 7-5 75 l5.9 19.5 20.4 95 49.2 55.8 56.4 Tempering Level is defined as the moisture content at the surface at time t minus the moisture content at the surface at time zero divided by the moisture content at the surface at time infinity minus the moisture content at the surface at time zero, expressed as a percentage. Table 4.9 120 Simulated moisture content distribution (2 w.b.) within a medium-grain rice kernel during tempering at 45 C and after being dried for 12. 36 and 60 min at 45 C and 30 2 relative humidity. Drying Temperingl Distance from center of kernel, m Average time time Center Surface moisture (min) (min) node node content .00000 0.00069 0.00138 0.00146 0.00151 0.00156 (2 w.b.) 0 25.0 25.0 23.5 21.0 15.0 7.4 22.6 3 25.0 ' 24.9 23.0 20.6 15.9 13.8 22.6 6 25.0 24.8 22.6 20.6 17.3 15.9 22.6 12 12 24.9 24.6 22.3 20.9 19.0 18.3 22.6 18 24.8 24.3 22.3 21.3 20.1 19.7 22.6 30 24.4 23.8 22.3 21.9 21.3 21.1 22.6 0 24.6 23.9 20.5 18.1 13.0 7.4 20.5 3 24.5 23.7 20.2 17.9 13.9 12.1 20.5 6 24.3 23.5 20.0 18.1 15.1 13.9 20.5 36 12 24.0 23.0 19.9 18.6 16.7 16.1 20.5 18 23.6 22.6 19.9 19.0 17.8 17.4 20.5 30 22.8 21.9 20.1 19.6 19.0 18.8 20.5 0 23.2 22.1 18.6 16.4 12.0 7.4 18.7 3 23.0 21.9 18.4 16.3 12.9 11.3 18.7 6 22.7 21.6 18.2 16.5 13.9 12.8 18.7 60 12 22.3 21.2 18.1 16.9 15.3 14.7 18.7 18 21.9 20.8 18.2 17.3 16.2 15.8 18.7 30 21.0 20.1 18.3 17.9 17.3 17.1 18.7 Table 4.11 Simulated moisture content of medium-grain rice dried at 45 C and 30 2 relative humidity for 12 and 36 min after being dried to 20.5 2 w.b. at 45 C and 30 2 relative humidity for 36 min and 0, 50, 75, and 95 2 tempering. Tempering Moisture Content, 2 w.b. Level (2) Dried for 12 min Dried for 36 min 0 19.6 17.9 50 19.2 17.6 75 13-9 17-3 95 18.7 17.1 121 Table 4.12 Simulated moisture content distribution (2 w.b.) within a short-grain rice kernel during drying from 25 2 MC at 45 C and 30 2 relative humidity using a spherical diffusion equation (Steffe and Singh, 1980). Time Distance from center of kernel, m Average ' Center surface moisture (min) node node content 0.00000 0.00079 0.00l58 0.00l66 0.00l7l 0.00l77 (2 W.b.) 0.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 0.6 25.0 25.0 25.0 25.0 24.3 7.4 24.5 1.5 25.0 25.0 25.0 24.9 22.6 7.4 24.3 3.0 25.0 25.0 25.0 24.5 20.2 7.4 24.0 6.0 25.0 25.0 24.7 23.3 l7.5 7.4 23.6 l2.0 25.0 25.0 23.8 2l.6 l5.4 7.4 22.9 24.0 25.0 24.8 22.2 l9.8 l4.l 7.4 2l.9 36.0 24.8 24.3 2l.l l8.7 l3.4 7.4 21.] 48.0 24.5 23.7 20.l l7.9 l2.9 7.4 20.3 60.0 24.0 23.0 l9.3 l7.2 l2.5 7.4 l9.6 Table 4.14 Time required to achieve 50, 75, and 95 percent tempering when short-grain rice is tempered at 45 C after being dried at 45 C and 30 2 relative humidity for 12, 36, and 60 min. Tempering Time Required (min) Level ‘ (2) Dried for Dried for Dried for 12 min 36 min 60 min 50 5.8 8.1 9.0 75 l8.6 22.5 - 24.0 95 57.9 63.6 67-5 Tempering Level the surface at time t minus the moisture content at the surface at time zero divided by the moisture content at the surface at time infinity minus the moisture content at the surface at time zero. expressed as a percentage. is defined as the moisture content at 122 Table 4.13 Simulated moisture content distribution (2 w.b.) within a short-grain rice kernel during tempering at 45 C and after being dried for 12; 36 and 60 min at 45 C and 30 2 relative humidity. Drying Tempering Distance from center of kernel, m Average time time Center Surface moisture (min) (min) node node content 0.00000 0.00079 0.0015 0.00166 0.00171 0.00177 (2 w.b.) 0 25.0 25.0 23.8 21.6 15.4 7.4 22.9 3 25.0 25.0 23.3 21.1 16.1 13.6 22.9 6 25.0 24.9 23.0 21.0 17.3 15.7 22.9 12 12 25.0 24.8 22.6 21.2 19.0 18.1 22.9 18 24.9 24.6 22.5 21.5 20.0 19.5 22.9 30 24.7 24.2 22.5 22.0 21.3 21.0 22.9 0 24.8 24.3 21.1 18.7 13.4 7.4 21.1 3 24.8 24.2 20.8 18.5 14.1 12.0 21.1 6 24.7 24.0 20.6 18.6 15.2 13.8 21.1 36 12 24.5 23.7 20.4 19.0 19.0 16.8 21.1 18 24.3 23.4 20.4 19.4 17.9 17.3 21.1 30 23.7 22.8 20.6 20.0 19.2 18.9 21.1 0 24.0 23.0 19.3 17.2 12.5 7.4 19.6 3 23.9 22.8 19.1 17.1 13.2 11.3 19.6 6 23.7 22.6 19.0 17.2 14.1 12.9 19.6 60 12 23.4 22.3 .18.9 17.5 15.5 14.9 19.6 18 23.1 21.9 18.9 17.9 16.5 16.0 19.6 30 22.4 21.3 19.0 18.5 17.7 17.5 19.6 Table 4.15 Simulated moisture content of short-grain rice dried at 45 C and 30 2 relative humidity for 12 and 36 min after being dried to 21.1 2 w.b. at 45 C and 30 2 relative humidity for 36 min and 0, 50, 75, and 95 2 tempering. Tempering Moisture Content, 2 w.b. Level (2) Dried for 12 min Dried for 36 min 0 20.3 l8.9 50 20.0 18.6 75 19-7 13-3 95 l9.5 l8.0 123 Tables 4.12 through 4.15 present the drying of short-grain rough rice under similar conditions. Medium- and short-grain rice have lower drying rates and take longer periods of time to achieve a desired tempering level than for long-grain rice. The drying rates in the second pass (or stage) are increased substantially for medium- and short-grain rice. The tempering is of special advantage when rice is dried for a short period of time in the subsequent pass (or stage). For medium-grain rice the drying rate results in a two-fold moisture removal rate increase (from 0.9 to 1.8 percentage points); this also occurs with short-grain rice (from 0.8 to 1.6 percentage points). 4.1.3 Comparing Drying Models and Drying of Rice Types Fig. 4.10 compares the drying rate of short-grain rough and parboiled rice using the diffusion coefficients (Table 3.8) by Bakshi and Singh (1979). Parboiled rice dries about 25 percent faster than non-parboiled rice, under similar drying conditions. The drying rate of non-parboiled rice kernels is lower due to the presence of unsplitted hulls. 25 24 23 22 21 4520 W. o 6 Moisture content, 19 18 17 16 15 14 13 124 Thin~1ayer drying of short-grain rice Spherical diffusion equation (Eq. 3.25) Diffusion coefficient from Bakshi and Singh (1979) Henderson-Thompson equilibrium moisture content Drying conditions: Temperature = 50°C Relative humidity = 35 2 Kernel equivalent radius = 0.00177 m rough rice parboiled rice I U 1 I I O 0.2 0.4 0.6 0.8 1.0 Drying time, hr Fig. 4.10 Thin-layer drying of rough and parboiled short-grain rice (Bakshi and Singh, 1979). 125 Figs. 4.11 through 4.14 show the drying rates of rough rice when using four non-diffusion type thin-layer equations and one diffusion type equation at temperatures from 30 to 60 C. The Henderson-Thompson EMC model was used. The non-diffusion equations compare well with the diffusion equation at low temperatures. The non-diffusion equation (Eq. 3.20) by Chancellor (1968) shows rice drying at a nearly constant rate at all temperatures. The equation (Eq. 3.23) by Page (1949) with coefficients by Wang and Singh (1978) approximates the diffusion equation. Higher drying rates are obtained with the diffusion coefficient by Steffe (1979) and Steffe and Singh (1982) compared to those obtained when using the diffusion coefficient by Wang and Singh (1978). Long-grain rice dries faster than medium-grain rough rice (Tables 4.4 and 4.8). Medium-grain rough rice dries faster than short-grain rough rice (Tables 4.8 and 4.12). Short- and medium-grain rough rice (Tables 4.6 and 4.10) require longer periods of time to temper than long-grain rough rice (Table 4.14). Higher drying rates in the second drying pass are obtained with short- and medium-grain rough rice (Tables 4.11 and 4.15) than with long-grain rough rice (Table 4.7). l 126 THIN LRYER DRYING - ROUGH RICE TENPERRTURE IS 30 C RELHTIVE HUMIDITY IS 20 PERCENT 1 - flDRflllflL nun mm mm - short-grain o 2 - eunucrtton um) - short-gram . C: s - ”IDLE TERM nrruoxmr: - medium-grain D- 4 - PflOE’B EOURUON - medium-grain N a - otrruaxou caustics . _ 0 av orrrrt nun mm (1902) - medlum-graln Henderson-Thompson EMC 0 . 9 3:. 1 - Eq. (3.21) " 2 - Eq. (3.20) g: (x 3 - Eq. (3.22) c. *~ 4 - Eq. (3.23) ‘3 s - Eq. (3.25) 20.. . “JN ..— I: DD “9 uJaL. Mod :3 .— 03 o—co <99 ZN‘ ... 3 o 9 1 1 fl 0:0.00 1.00 4.00 5.00 2100 3100 TIME (HOURS) Fig. 4.11 Comparison of thin-layer drying equations for rough rice when dried at 30°C and 20 2 relative humidity. 127 THIN LRYER DRYING - ROUGH RICE TEMPERRTURE IS 40 C RELRTIVE HUMIDITY IS 20 PERCENT I - BDRRHRL MD 8111011 (1077) - short-grain o 2 - WCELLOR (19801- short-grain o s - stunt: mm nrrnoxmntt .. medium-grain a; 4 - PAOE’B EQUATION - medium-grain on 5 - DIFFUSION EOUBTIOM 0 BY DTEFFE MD SIMON (1982) - medium-grain Henderson-Thompson EMC 0 9. 1-Eq.OJU :5- 2 - Eq. (3.20) F“ 3 - Eq. (3.22) g 4 - Eq. (3.23) U S — Eq. ((3.25) 0 1—9 20.1 “JN I- I: CDC) ”0. LLko- med .2 U) 2 HO CDC? 5 N. CI" 4 l c: 3 D . I I I I I “b.00 1.00 2.00 3.00 4.00 5.00 TIME (HOURS) Fig. 4.12 Comparison of thin-layer drying equations for rough rice when dried at 40°C and 20 2 relative humidity. 128 THIN LHYER DRYING - ROUGH RICE TEMPERRTURE IS 50 C RELRTIVE HUMIDITY IS 20 PERCENT I - fiORfllIll-IL 3M0 ”MOM (1077) - short-grain D l - CHRNCELLOR (1930) - short-grain b 4 - PROF/8 COURT!!!" - medium-grain N 5 - DIFFUMDM £01.18le 0 BY "EFF! 9ND SIMON (1082) - medium-grain c: Henderson-Thompson EMC 9 .. \ AN \ g9 \\\ 1 — Eq. (3.21) fig 2 - Eq. (3.20) 53 \\ 3 - Eq. (3.22) F— . 4 - Eq. (3.23) 55524 S - Eq. (3.25) p— I: CDC: Us: uJua tr~T :3 .— U) hac: 2 ‘99 II...q fl 4 5 3 g; 1 g I j I I I 0.00 1.00 2.00 3.00 4.00 5.00 TIME (HOURS) Fig. 4.13 Comparison of thin-layer drying equations for rough rice when dried at 50°C and 20 2 relative humidity. 129 THIN LRYER DRYING - ROUGH RICE TEMPERRTURE IS 60 C . RELHTIVE HUMIDITY IS 20 PERCENT I - annual. MD BIND" (1071) - short-grain O 2 - WNCELLOR (10081 - short—grain :3. a - stunt: mm arraoxmr: - medium-grain " 4 - mac's COURT"!!! - medium-grain N s - otrruaxou swarm 0 07 8TB": BNO DINO“ “082) - medium-grain D \ Henderson-Thompson EMC O a. nac' . (3.21) m . (3.20) E: . (3.22) we . (3.23) F_c? . (3.25) ZIO- “JG-l *- I: 00 “=2 1.1.quq mull. :3 5 2 HO CD :29. 4 GD 3 5 l O 9 *0 .00 100 2'.oo 3100 4100 '3-00 TIME (HOURS) Fig. 4.14 Comparison of thin-layer drying equations for rough rice when dried at 60 C and 20 2 relative humidity. 130 Page's equation (Eq. 3.23) gives comparable results to the diffusion equation (diffusion coefficient by Steffe and Singh, 1982) and can be used to simulate deep-bed drying of medium-grain rough rice. When tempering of rice is investigated a diffusion type model such as the one developed by Steffe (1979) should be used. 4.2 Deep-Bed Drying Simulation The latest version of the MSU fixed-bed, crossflow, and concurrentflow drying models as presented by Dalpasquale (1981) and by Rodriguez (1982) was modified to simulate the drying of rice. A counterflow version modified by Schisler (1983) was used. 4.2.1 The Fixed-Bed Model The 1982 version of the MSU fixed-bed drying model (developed for corn and soybeans) was modified to simulate rice drying. Eqs. (3.39), (3.40), (3.41), and (3.23) constitute the fixed-bed model. The Zuritz EMC model was used. Eq. (3.14) was chosen for the latent heat of vaporization of rough rice. Table 4.16 presents the values for the physical and thermal properties of long-, medium-, 131 Table 4.16 Physical and thermal property constants of long-, medium-, and short-grain rice used in the computer simulation models. Rice type Property Units long medium short Equivalent radius rough rice 0.100 (1) 0.156 (1) 0.177 (2) cm brown rice 0.093 (1) 0.146 (1) 0.166 (2) cm white rice 0.088 (1) 0.138 (1) 0.158 (2) cm Dry bulk density 519.4 (3) 553.8 (1) 583.6 (4) kg/m3 Specific surface area 2437. (5) 2361. (5) 2051. (5) m2/m3 Specific heat 2.01 (3) 2.01 (3) 1.79 (4) kJ/kg/C (1) measured ~ (2) Steffe (1979) (3) Wratten et a1. (1969) (4) Morita and Singh (1977) (5) Calculated from surface area (Morita and Singh, 1977), weight of kernels (Webb, 1980), and wet bulk density (Wratten et a1., 1969 and Morita and Singh, 1977). 132 and short-grain rough rice. Pressure drop through the rice bed was calculated as presented in Table 3.10 for short-, medium-, and long-grain rough rice. The specific heat of dry air and water were obtained from Holman, 1981: 1.01 and 1.88 kJ/kg/C, respectively. 4.2.2 The Crossflow Model A 1982 version of the MSU crossflow drying model for corn presented by Rodriguez (1982) was modified to simulate rice drying.) Eqs. (3.43), (3.44), (3.45), and (3.23) constitute the crossflow model. The equations for EMC and latent heat of vaporization are the same as those in the fixed-bed model. The physical and thermal properties of rice types are given in Table 4.16. 4.2.3 The Concurrentflow Model A 1981 version of the MSU concurrentflow model, as presented by Dalpasquale (1981) .was used to model rice drying. Eqs. (3.47), (3.48), (3.49), (3.32), and (4.1) constitute the concurrentflow model. The average kernel moisture content (M) is calculated from: 133 1 R R = - f M dr (4.1) R o where M is the average kernel moisture content (decimal, dry basis), M is the moisture content at different positions inside the rice kernel (decimal, dry basis), R is the rough rice equivalent radius (m), and r is the kernel coordinate (m). Eqs. (3.57) and (3.58) are used for the EMC and Eq. (3.14) for the latent heat of vaporization. The composite model developed by Steffe (1979), as described in section 3.3.3.1.2, is used for the drying and tempering. Diffusion coefficients for rough rice components are given in Table 3.8. The equivalent radius for white, brown, and milled rice are listed in Table 4.16. 4.2.4 The Counterflow Model The counterflow drying model is composed of Eqs. (3.51), (3.52), (3.53), and (3.23). The counterflow drying model as presented by Evans (1970) and modified by Schisler (1983) was used to simulate the counterflow cooling of rice. 134 4.2.5 Solution of the Deep-Bed Simulation Models The crossflow model is solved using backward differences (Dalpasquale,~ 1981 and Rodriguez, 1982). A data subroutine is supplied for the properties of long-, medium-, and short-grain rough rice. The solution of the fixed-bed model is similar to the crossflow model. The set of differential equations describing steady-state concurrentflow drying consists of a number of first-order differental equations along with a second-order differential equation. The solution of the first order differential equations is given by Dalpasquale (1981). The solution for the second order differental equation is given by Steffe (1979). A solution of the counterflow model is presented by Evans (1970) and Schisler (1983). CHAPTER 5 EXPERIMENTATION In order to validate the computer models, several drying tests were conducted in 1981, 1982, and 1983. Experimental drying data were obtained for long-grain rice in Texas, and for medium-grain rice in California. Concurrentflow and crossflow rice dryers were tested for capacity, fuel efficiency, and rice quality. 5.1 Concurrentflow dryer Experiments were conducted with three multistage concurrentflow dryers to verify the technical and economical feasibility of drying rough rice at temperatures well above those normally used by conventional crossflow dryers. A three-stage concurrentflow dryer was tested during the drying of parboiled rice. 135 136 5.1.1 Dryer Design Three concurrentflow dryers manufactured by Blount, Inc., Montgomery, Alabama, were experimentally tested in 1981, 1982, and 1983: (1) a two-stage 2.44 m x 2.44 m model with long-grain rice in Edna, TX, (2) a three-stage 3.66 m x 3.66 m unit with medium-grain rice in Williams, CA, and (3) a three-stage 2.44 m x 2.44 m model with long-grain parboiled rice in Louise, TX. Except for their size and number of drying stages, the three Blount concurrentflow (CCF) dryers are identical. A block diagram of the two-stage model is illustrated in Fig. 3.20. In a concurrentflow dryer, the rice and the drying air flow in the same direction. The outlet air from the first stage is exhausted to the atmosphere. The partially dried rice flows from the first drying stage by gravity to the tempering zone. After 60 - 100 minutes (depending on the grainflow rate through the dryer), the tempered, partially dried rice flows into the second drying stage. In a three-stage dryer a second tempering zone and a third drying stage are present. From the last drying stage, the warm, dried rice flows to a counterflow cooler in which the cooling air flows in the opposite direction of the rice. 137 Outlet air from the cooler and from the last drying stage can be recirculated to the inlet fan of the first stage (Fig. 3.18) for increasing the inlet air humidity (thereby maintaining the grain quality, Ban (1971)) and the temperature (thereby improving the dryer energy efficiency). No special air pollution equipment is required on the CCF dryers because of the relatively low velocity of the exhaust air. The Blount CCF three-stage concurrentflow dryer has a total height (up to the top of the garner bin) of about 35 m; the two-stage model scales approximately 27 m. The lengths of the drying stages are 1.1 m (top), 1.2 m (middle), and 1.4 m (bottom). The length of the tempering zone(s) is 5.2 m. The cooler length is 1.7 m. The CCF dryer tested in Louise, TX, is similar to the one described above with respect to the length of the drying stages, tempering zones, and cooling stage. The lengths of the second and third drying stages were decreased from 1.2 and 1.4 m to 0.8 m. The airflow in each stage of a CCF dryer can be independently controlled by valving the air in the inlet ducts to the centrifugal fans. The three-stage 3.66 x 3.66 m model with regular depths is supplided with motors of 93 kw (first stage), 93 kw (second stage), 75 kW (third stage); and 37 kW (cooling stage). The motor sizes 138 on the three-stage dryer with shortened bed depths are: 45 kW (top stage), 37 kw (middle stage), 37 kW (bottom stage), and 15 kw (cooling stage). The motor sizes on the two-stage 2.44 m x 2.44 m dryer are 45 kw (first stage), 37 kw (second stage), and 19 kW (cooling stage). During the experimental tests the airflows in the two three-stage dryers were 40, 37, 30, and 15 (all plus or minus five cubic meter per minute per square meter of bed area) in the first, second, and third drying stages, and in the cooling stage, respectively; and, 40, 30, and 15 (all plus or minus three) m3/min/m2 in the two-stage model. The airflow in each stage was continuously monitored from static pressure readings. Natural gas was used as the fuel to heat the inlet drying air in all tested dryers. Rice temperatures were monitored after each drying stage in the tempering zone(s) and after the cooling stage to prevent overheating of the rice and quality deterioration. 5.1.2 Procedure and Instrumentation The duration of each test varied from six to 14 hours.. After an hour of' steady state operation, a test was started. A mixture of medium-grain varieties was dried in California; in Texas the long-grain variety LaBelle was 139 used. The) following parameters were utilized in the performance evaluation of the CCF dryers: 1) the moisture content before and after drying, 2) the initial and final rice temperature, 3) the initial and final test weight, 4) the initial and final rice quality as determined by milling yield, germination, and color (parboiled rice), 5) the drying capacity in tons of dry rice per hour, 6) the ambient and drying air temperatures, 7) the airflow rates, and 8) the energy (natural gas) consumption. The rice moisture contents were measured with a resistance/conductance type moisture meter. Samples were collected every half-hour. The temperatures were measured with copper-COnstantan thermocouples in conjuction with a potentiometer. The rice head yield was determined for each inlet and outlet sample according to established standard methods (USDA, 1976). Head yield determinations were made by Comet, Inc. (Houston, TX). The inlet samples were dried at laboratory conditions at a maximum temperature of 32 C in shallow beds. Details about milling yield, germination, and color determination are presented in sections 5.4, 5.5, and 5.6. 140 The drying capacity was determined by weighing the grain during a pre-determined period at 25 revolutions per minute (rpm) of the discharge auger. The capacity at other rpm values was obtained by assuming linearity between rpm and capacity, for the 1981 tests. In 1982, dryer capacities were calculated from the rpm values of the discharge auger. In 1983 (parboiled rice tests), the dryer capacity was fixed at 11 tons per hour; the capacities were weighted directly in terms of metric tons of dried rice (14.5 percent moisture content, (wet basis) per hour and converted to grain velocities. Static pressures within the inlet air ducts were measured with pressure gauges. The airflows were calculated from standard static pressure-airflow data (Steffe et al., 1980). The natural gas usage was obtained from recently calibrated gas meters. The gas companies were consulted for average caloric content of fuel and the appropriate correction factors. The electricity consumed by the electric motors was not accounted for in the energy efficiency calculations (section 5.3) since it accounts for less than five percent of the total energy consumption (Bakker-Arkema et al., 1981). 141 5.2 Crossflow Dryer A conventional crossflow dryer without air recirculation, manufactured by Berico Industries, Overland Park, KS was experimentally tested with long-grain rice (variety LaBelle) in Edna, TX in the fall of 1982. 5.2.1 Dryer Design The significant dimensions of the crossflow dryer are: (1) column thickness, 0.30 m; (2) column width, 3.66 m; and (3) column length, 15.24 m. A turn-flow (grain mixing) device is located 6.10 m from the top of the drying column. The gas burner is located after the fan. The motor size is 75 kW. The airflow tests was 26 m3/s at a static pressure of 498 Pa. A diagram ofthe crossflow dryer is given in Fig. 5.1. 5.2.2 Procedure and Instrumentation The crossflow drying required multipassing. Two or three passes were required for each of the six lots tested. Between passes the rice was tempered for approximately 24 142 EXIHUJST? ’ ‘F—- ._d <— RICE ~—- HOT _, RICE ._> EXHAUST AIRi Y V AIR AIR .8 7 o 3 O V Y i / \ +—— TURNFLOW \ / DEVICE DRYING F— COLUMN PLENUM EXHAUSé L_. — ‘ D AIR RICE ~ HOT -——~ RICE —> ExHAUST mthpwhomEmu moan cam new woumassfiw H.o .mwm mccooom .oefih om ow on co om ov on om OH o d i! . I! I I d I 1 I So «camcoq 3 o ‘oIniexadmal 179 Table 6.18 Typical computer output of the simulation results of the first stage of a three-stage concurrentflow dryer (Test 2, Table 6.3) drying medium-grain rice at 30 C and 36.5 m3/min/m2. DEPTH TIME AIR ABS REL GRAIN MOISTURE TEMP HUM HUM TEMP CONTENT (m) (hr) (C) (kg/kg) (2) (C) (2 wb) 0.0000 0.0000 135.0 0.0090 00.4 24.0 25.5 0.1007 0.0267 47.4 0.0198 28.2 47.1 24.4 0.2071 0.0544 43.7 0.0255 43.4 43.6 24.2 0.3015 0.0799 41.1 0.0291 55.6 41.3 24.0 0.4079 0.1081 40.1 0.0311 63.7 40.0 23.9 0.5009 0.1327 39.2 0.0326 69.7 39.1 23.9 0.6015 0.1594 38.5 0.0335 74.1 38.5 23.8 0.7023 0.1861 37.9 0.0345 78.6 37.9 23.8 0.8007 0.2122 37.3 0.0355 83.8 37.2 23.8 0.9015 0.2389 36.9 0.0360 86.6 36.9 23.7 1.0015 0.2654 36.5 0.0367 90.2 36.5 23.7 1.1000 0.2915 36.3 0.0369 91.5 36.3 23.7 Table 6.19 Internal moisture content distribution (2 wb) in a medium-grain rice kernel after leaving the first stage of the three-stage concurrentflow dryer simulated in Table 6.18 and after tempering in the 5.2 m tempering zone for 1.4 hours assuming spherical kernel. Distance from the center of kernel, m Average Time moisture (hr) center surface content _ggge node (2 wb) 0.00000 0.00069 0.00138 0.00146 0.00151 0.00156 0.0 25.5 24.1 22.8 21.5 18.8 17.4 23.7 0.1 25.5 23.8 22.8 21.8 20.0 19.3 23.7 0.2 25.4 23.6 22.8 22.1 20.9 20.4 23.7 0.3 25.3 23.6 22.9 22.4 21.5 21.2 23.7 0.4 25.2 23.5 23.0 22.6 21.9 21.7 23.7 0.5 25.1 23.5 23.1 22.8 22.3 22.1 23.7 0.6 25.0 23.5 23.2 22.9 22.5 22.4 23.7 0.7 24.8 23.5 23.3 23.1 22.7 22.6 23.7 0.8 24.7 23.6 23.3 23.2 22.9 22.8 23.7 0.9 24.6 .23.6 23.4 23.2 23.0 23.0 23.7 1.0 24.5 23.6 23.4 23.3 23.2 23.1 23.7 1.1 24.4 23.6 23.5 23.4 23.2 23.2 23.7 1.2 24.3 23.6 23.5 23.4 23.3 23.3 23.7 1.3 24.2 23.6 23.5 23.5 23.4 23.2 23.7 1.4 24.2 23.6 23.6 23.5 23.4 23.4 23.7 180 average moisture content of the kernel remains unchanged at 23.7 percent during the tempering process. Table 6.20 presents auxiliary values of the simulation in Tables 6.18 and 6.19. The moisture content and air/rice temperatures in the three-stage dryer (Test 2, Table 6.3) are illustrated in Fig. 6.2. Table 6.21 presents the simulation results of the counterflow cooler of the three-stage CCF dryer (Test 2, Table 6.3) cooling medium-grain rough rice at 14 C and 10.6 m3/min/m2. Table 6.22 presents the comparison between experimental and simulated drying of medium-grain rough rice (Test 2, Table 6.3) in a three-stage CCF dryer with counterflow cooler. The experimental dryer fuel efficiency is 4,012 kJ/kg as compared to 3,653 kJ/kg from the simulation. The difference (nine percent) in fuel efficiency is due to heat losses in the air ducts and inaccuracies in predicting the drying behavior through mathematical simulation. The moisture removal is 5.6 percentage points for the experimental and simulated results. Of special interest to the design engineer are the moisture content decrease per stage, the degree of saturation of the exhaust air, the maximum rice temperature, the time of rice exposure to a certain temperature, the energy to heat the drying air, and the fan power requirement at a particular inlet air temperature-airflow rate-grain velocity combination. 181 °q'M % ‘iuaiuoo aInisIow guwcofi wmxap momno> pcmucou Unaumfios use “moan pom Remy owsumnomaou woumfiosflm N.o .wam Ho.m A: .oEfiH mo.H m h E dumcoq m_o mH om 5H wfi ma om am NN MN «N mm 0N “ .uoxwp gown mum ommpmloounp on» cw hm.o oanmh .N .02 umohv mafia paw wh.m Hv.m mama mm.o H 1H Iw4W|_w . . («mm 1 . . ._ . E H = T‘ . Mm -.. _...n N ... H m . ~ m .. _... m; ..-. - ........- IN T- 1%.. A - --_ w;;;::hl.i. :sxn.Hlv-ez. M ii; 11 u . u _ . U _ M . g:;-w ij -3..43 u -m azMP 4-? 1 l; ..g i W T muHm _ 11.7% W . I I . _ : . z . . #4!- i fi*f r - 9 <_ -p F d— —1 - _ I _ a . _ (III. 0.5 9.1:. W . , . _ .-l.I..I. 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L111 ..r;%-;. ezwezoo ..M -4 . mmaemuoz ., 1.1.... 4.1.4: EZON. mw acoucoo Unaumwoe paw aoofle cam pfiwv weapmuomEou popmasawm m.o .me hm.N w©.N mm.H H: «OEMH MM.H MN.O I H.VH .3 _ . n _ o d 1. 4 Id m.e e..auweog m.e H.H o _. I .4 .4 . e fi. :1m . ..2 ..2:13. . _ -2+l+;,a2me r1. ... A H Elf. MUHK “ .__-III..-4:.:II..l-_l-l... ' l .....4 .— q»— .. ......I I- 1 0 “ ‘ ‘1 ‘ i} I V l ”-_-_...4,_. - ‘ -—I ._ c—c—ur-o —..4 -v l l , . . , .. __. ,_._.4,4‘._+..4_..--T-~‘_- . I . ..... i I I i I; lb- m I o I H T . 1‘ ‘T‘ ov 9 1 I I I I . . . I . ..fi-..4_.._.-.~‘ :——{ '7 n . . .. ' I v -r«s—4»—4«~ . --...- I — I .... 44..-.1 . o ._.._.. -; ezmezoe F. , ovH .W mm2~m~.oz. s .: . 4 -- . , 1.2.1" I . ......l: ..-- _ ...... .. a _ -. .u . .,a\ ”II. 1"- 1;...141. -mhflmw M w .Ea l. . . ._ . ....... w: .. . . . e2me ‘4. a ezme it em -1- 24-:r. maq .+ 14;- - in .. maa- . --w . "M gsliil. M .. . A. . .w .7. 4 ......l- .. ea“ 4. -.-.e 1+.e..- -azh.:»4- lain» . «is.mfi.. .- _. _ q . r w. . . ONH -. _w ..H, _. .. H H ezmezou... as ,M_.» mmsemaoz rJ.. I .»..—...__._... M .. FZMHzou_ 4 mmzhmmoz .. . 5111114111 02 owH ‘1“: « OIIRVIII 141110 thyria 4 al. .II . om. com : mzoN . wszmezme .. mu¢o . GZHKLEZUA _ mumo.e. .1 e : .1 .... .m _ _ u 9 .. _ 2: . . - ... , , , . . . , , _ d-‘- —,_ — - ’ 0 w <1 I l ovm Q o ‘axniexadmal 186 Table 6.23 Comparison between experimental and simulated drying of medium-grain rough rice in a three-stage concurrentflow dryer in Williams, CA, 198l. Input/output variables and parameters Test number: l3, Table 6.3 Experimental Simulated ‘ Grainflow rate (tons/hr/m2) Grainflow speed (m/hr) Inlet moisture content (2 w.b.) Ambient temperature (C) Ambient relative humidity (3) Inlet rice temperature (C) NU’I—ON HOWNbN COO-'CDO‘ £LB§I_§IA§£ Bed depth (m) Tempering length (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature (C) Outlet rice temperature (C) Outlet air temperature (C) Outlet relative humidity (2) Static pressure (kPa) Outlet moisture content (2 w. b. ) Stage fuel efficiency (kJ/kg H20) r“ NV“—‘ 0 - .5.h86. o a. NON-i 62. hi. hi. 73- 19- OGWOOOO SECOND STAGE Bed depth (m) Tempering length (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature (C) Outlet rice temperature (C) Outlet air temperature (C) Outlet relative humidity (2) Static pressure (kPa) Outlet moisture content (2 w.b.) . Stage fuel efficiency (kJ/kg H20) ud \DO‘U‘Id O‘ONN Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature (C) Outlet rice temperature (C) Outlet air temperature (C) Outlet relative humidity (2) Static pressure (kPa) Outlet moisture content (2 w.b.) Sta e fuel efficiency (kJ/kg H20) O‘WWU" -'O‘\»U'ICDCDN OO‘U’IOOOO o oo “—0 COOLER Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Outlet moisture content (2 w.b.) Percentage points removed (3 w.b.) Dryer fuel efficiency (kJ/kg H20) a—l—l J-‘w—o O O 0 row w :' o r. d FO‘U‘I o o 0 ONE) 187 The length of the tempering zone in a multistage concurrentflow dryer can be designed by the engineer from simulated internal moisture content distribution data such as presented in Table 6.19. Tempering levels of 94 percent are achieved in the first and second tempering zones of the CCF dryer (Test 2, Table 6.3). The tempering results in a smaller moisture gradient and faster drying rates in subsequent drying stages (Tables 4.6, 4.10, and 4.14). The effect of tempering level on drying capacity and fuel efficiency of CCF dryers is investigated in Chapter Seven. Most importantly, the tempering treatment aids in maintaining rice with a high head yield (Steffe and Singh, 1980). The tempering process relieves the stresses imposed upon rice kernels during the drying process. Fig. 6.3 shows the simulated air and rice temperature and moisture content versus dryer length and time (Test 13, Table 6.3) in the Blount/CCF three-stage concurrentflow rice dryer; Table 6.23 presents 'a comparison between experimental and simulated drying of medium-grain rough rice for the same test. The maximum rice temperatures in the top, middle, and bottom stages are 62, 60, and 52 C, respectively (Table 6.23). The outlet rice temperatures in the top, middle, and bottom stages are approximately 40 C. The rice kernels are exposed to temperatures above 45 C for approximately nine, nine, and four minutes in the three 188 'q°M % ‘quaiuoo alnistow .hoxnv moan zoamucouasocoo owmumloohap 0:» :H nv.o oHnmb .mm .02 umohv mafia paw camcoH Hoxuv momuo> pcoucoo ouspmflos bum moofip cam hflmv onouwnomsou woumHsafim v.0 mH ofi 5n wH mm on Hm mm mm om.m no .N mn.~ he .msme nv.H Ln I-II m.n a .cpmcog H N ‘xh ...—..— ..-fi-‘ . - I i 1 - I I I I ..J. I-I l l l I I ‘M~---o.h-m .1 -_ t M .. ;: a2me % t - l . l . l I A I l .. . . , A . . ' u I . g. . .. , .-.—r-‘._—.-4r. _._._. -m ...f--.‘ Cd - _ .. .. . . . ,. , i. l . . . l . .' ‘ . . - ~. . I ' i I b—fim‘- _ I I I , - .I ~ I - .... “.4 -“-*-—_ r..- n 1 l - z.::w. ,- I- .-u..q. u. : _. t .M _ . INT... 2.... Im.. .IM. . .w— u . .. V. .17 .5; ;_ A. A mwz_ I .M;_._m .;.. . “bl: - wagllI-- .: Ila. Ilfllfi -a . . w.wgfil hzuhzou. ..g. .; IllITIltll... III . «I! .imll. . Um ., . 1.. 45%.... IT. IT. In --...IMI I”: --..T.» a..- I -.....fi: ,-..,_..,_.....l I... 17.- s. If... .....- .. ...“...w... ._ _ . . I. _. t. . a .. ..- :58 ..«wu;; me. .E «#:9.. -.v-.u; .- 31%;;Islr mmacmaoz . ..N_“ h __ “IL . m ,M > 1m. l_r.. -I..m-.--_ .II. IIIIMII we. ..... . w _ . ._; .~ ..m M a --lluw _ mwmo Zephom Idl . 4 . , Ig:;--¢szmazmc . _; ozoumm 4.03:5)! I! - . I-I.uzummazme a cmmau . a _ z~>mo mop . . . . .IIuv v c . . .- ..:, cm ov ow ow com ‘axnuexadmal ONH C v H 30 00H owa com CNN 189 Table 6.2h Comparison between experimental and simulated drying of medium-grain rough rice in a three-stage concurrentflow dryer in Williams, CA. l98]. Input/output variables, Test number: 23, Table 6.h and parameters Experimental Simulated Grainflow rate (tons/hr/mZ) Grainflow speed (m/hr) Inlet moisture content (2 w.b.) Ambient temperature (C) Ambient relative humidity (2) Inlet rice temperature (C) NkNN NU'I-dth OOOOWJP FIRST STAGE Bed depth (m) Tempering length (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature (C) - 63.0 Outlet rice temperature (C) 52.0 h2.0 Outlet air temperature (C) h6.0 h2.0 Outlet relative humidity (3) - 9l.O -Static pressure (kPa) 2.5 2.5 Outlet moisture content (2 w.b.) - 2l.2 Stage fuel efficiency (kJ/EgHZO) - 6,l8h.0 N O 0 WC ##m-i dON-d $££QND.§IA§£ Bed depth (m) Tempering length (m) Inlet air temperature (C) l Airflow rate (m3/min/m2) Maximum rice temperature (C) - Outlet rice temperature (C) 52.0 Outlet air temperature (C) h6.0 Outlet relative humidity (2) - Static pressure (kPa) 3.9 Outlet moisture content (2 w.b.) - Sta e fuel efficiency (kJ/kg H20) - rm loam- #ONN Ntt’N o e e e WQWHO‘O‘U’ OOWOOOO . 3’ m—. THIRD STAGE Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature (C) - Outlet rice temperature (C) h0.0 Outlet air temperature (C) 39.0 Outlet relative humidity (X) - Static pressure (kPa) 3.5 Outlet moisture content (2 w.b.) - Sta e fuel efficiency (kJ/kg H20) - d MN 0 U0? W O‘md ant-rm e o o o e WU'IUDQUIU'IN ONU‘IOOOO 0 U1 m-n _COOLER Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Outlet moisture content (2 w.b.) l5 2 Percentage points removed (2 w.b.) 7.8 Dryer fuel efficiency (kJ/kg H20) h,2h7.0 -IN mdd row we 190 drying stages. A tempering level of 94 percent is achieved in each of the two tempering zones. It can be seen (Test 13, Table 6.5) that even at such high kernel temperatures (for a very short period of time) the rice maintained its head yield (only 0.9 percentage points decrease). Fig. 6.4 and Table 6.24 show data, similar to Fig. 6.3 and Table 6.23, for test number 23 (Table 6.4). The maximum rice temperatures in the top, middle, and bottom stages are 63, 73, and 67 C, respectively (Table 6.24). The rice kernels are exposed for temperatures above 45 C for approximately nine, seventeen, and twenty minutes. Tempering levels of 94 and 96 percent are achieVed in the two tempering zones. The use of such high temperatures caused a decrease in the head yield by 22.4 percentage points. Excessive kernel temperature and consequently high moisture removal rates (1.8, 3.2, 2.3, and 1.1 % in the top, middle, bottom, and cooling stages, respectively) detereorated the rice quality. Fig. 6.5 shows the temperature (air and long-grain rice) and moisture content versus dryer length and time in a two-stage CCF dryer (Test 1, 1981, Table 6.8). Most of the moisture is removed in the first three minutes of each drying stage; the rice temperatures are above 45 C for 4.5 minutes in the first stage and for 1.8 minutes in the second stage. Good agreement is obtained between experimental and simulated moisture contents, temperatures, 191 'q‘m % ‘1ua1uoa axnistow .hoxap moan moo ammumuosu ago a“ mw.o waamb .a .02 umohv mafia paw camcofi poxhv msmno> unmucou ouaumwos ppm Aoofin paw Hfimu annumnmmEmu woumfisaflm m.o .mfim mw.H cm.fl H; .mefie n~.o o man m.o a .numcos Has a . _ o ON cc 1 a .w co m Du 1 n I .ma cm 0 ezmhzou - 3 mmzhmuoz ooH HZMHZOU “mapmuoz TH .. m ONH H ._ mwmT “ Hmmua I ;,..M;;;. ace fl a A, . _. , I . ..H .-IIIT.§.IIJ-I-JI-.Tin _ f V . _ : 0.3” 192 Table 6.25 Comparison between experimental and simulated drying of long-grain rough rice in a two-stage concurrentflow dryer in Edna, TX, l98l. Input/output variables and parameters Test number: l, Table 6.8 Grainflow rate (tons/hr/m2) Grainflow speed (m/hr) Ambient temperature (C) Inlet rice temperature (C) Inlet moisture content (2 w.b.) Ambient relative humidity (2) FIRST STAGE Bed depth (m) Tempering length (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature Outlet relative humidity Static pressure (kPa) (C) Outlet rice temperature (C) Outlet air temperature (C) (3) Outlet moisture content (z w.b.) Stage fuel efficiency (kJ/kg H20) SECOND STAGE Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Maximum rice temperature Outlet relative humidity Static pressure (kPa) (C) Outlet rice temperature (C) Outlet air temperature (C) (3) Outlet moisture content (3 w.b.) 1 Stage fuel efficiency (kJ/EgHZOM COOLER Bed depth (m) Inlet air temperature (C) Airflow rate (m3/min/m2) Outlet moisture content (2 w.b.) Percentage points removed (8 w.b.) Dryer fuel efficiency (kJ/kg H20) Experimental Simulated 2.3 h.l 16.6 27.0 60.0 27.0 l.l 5.2 l35.0 35-0 - 55-0 55.0 h0.0 A3.0 A0.0 ' 73-0 1.8 l.8 - lh.9 - h,126.0 l.2 77-0 37.0 - 50.0 h0.0 h0.0 38.0 A0.0 - 59-0 2.l 2.l - l3.7 - 3.305-0 1.7 27.0 18.3 l3.0 l3.2 3.6 3.h 3.689-0 3 575-0 193 and dryer efficiencies (Table 6.25). There is a disagreement between the experimental and simulated outlet rice temperatures (55 versus 43 C) in Table 6.25. The value for the experimental outlet rice temperature (55 C) is not realistic. Rice at such temperatures should have presented' a very low moisture content; this was not observed. The maximum kernel temperatures reached are 55 and 50 C in the two stages. The first drying stage exhibits a fuel efficiency of 4,796 kJ/kg of water removed compared to the second stage (3,305 kJ/kg). Between stages the rice is tempered at 40 C; this represents a tempering level of 96 percent. The head yield of the milled sample was increased from 59.7 to 62.4 % in this particular test. 6.2.2 Crossflow Drying Table 6.26 shows the simulation results of a crossflow dryer with a grain mixing device located at 6.2 m from the top of the dryer. Fig. 6.6 shows the simulated average rice temperature and moisture content versus time and dryer length in the crossflow dryer. Table 6.27 compares the experimental and simulated drying of long-grain rice in a' crossflow dryer (Lot 73, first pass, Table 6.15). It takes 17.4 minutes for the rice to go through the dryer (15.2 m long). The average rice temperature increases from 26.7 to 194 Table 6.26 Simulation results of a crossflow dryer with grain mixing device (turn-flow) drying long-grain rice. DEPTH TIME AVE RICE EXHAUST EXHAUST EXHAUST AVE MOISTURE TEMP AIR TEMP ABS HUM REL HUM CONTENT (m) (hr) (C) (C) * (kg/kg) (2) (2 w.b .) (Lot 73. first pass, Table 6.15) 0.0 0.00 26.7 26.7 0.0171 66.0 16.5 1.3 0.02 32.3 26.7 0.0222 100.0 16.h 2.8 0.05 3A.7 26.7 0.0222 100.0 16.3 h.3 0.08 36.8 27.5 0.0233 100.0 16.2 6.1 0.12 39.0 28.9 0.0259 100.0 16.0 Grain mixing device (turn-flow) 6.7 0.13 “1.8 29.2 0.0259 100.0 16.0 8.3 0.16 93.9 30.9 0.0283 100.0 15.8 9.8 0.19 “5.3 32.5 0.0316 100.0 15.6 11.“ 0.22 h6.6 33.2 0.0329 100.0 15.5 12.8 0.25 h7.6 33.h 0.0333 100.0 15.2 15.2 0.29 h9.2 33.8 0.03h0 100.0 1h.9 (Lot 73. second pass, Table 6.15) 0.0 0.00 31.1 30.6 0.0166 60.0 15.7 1.3 0.02 39.1 30.h 0.0273 98.1 1h.5 2.8 0:05 3h.7 29.2 0.0256 98.8 1h.h “.3 0.08 35.6 28.3 0.0250 97.0 1h.2 6.1 0.12 36.9 28.2 0.02A5 96.0 1h.1 Grain mixing device (turn-flow) 6.7 0.13 38.2 28.3 0.027hh 100.0 16.0 8.3 0.16 39.5 29.5 0.0250 96.0 13.9 9.8 0.19 . 50.3 30.7 0.0252 90.5 13.8 11.“ 0.22 - 60.8 31.0 0.0255 88.7 13.6 12.8 0.25 Al.“ 31.1 0.0255 87.3 13.5 15.2 0.29 h2.2 31.2 0.025h 86.1 13. 195 30 ‘alnieladmal ON on em mm me be om l—sxfl E . m ma no cog HHII ON ma - ulllll. ucoucoo onsumwoe Spams“: ucoucoo ousumfios ommnm>< ucmucou ousumfloa E—JEHKNZ / onsumnomsou mafia ommnm>< 0H .II .262 .ossp oH - .Hoxnp onMmmono m :fi numcofi Hoxun wcm mafia msmuo> ucoucoo manumwos paw ouspmhomEou moan woumflasflm o.o .wflm —--_b_-—h__ -‘ —— ouw>ov _ onmI:H:H _ m 1 am o J o v o mH «a mH 0H 5H wH 'q‘m % ‘1uaiuoo axnzstow 196 49.2 C; the exhaust air temperature increases from 26.7 to 33.8 C. The simulation model assumes airflow reversal below the turnflow device. The exhaust air temperature of the crossflow dryer is lower (lower absolute humidities) than the concurrentflow dryer (higher absolute humidities). No condensation takes place in a properly operated concurrentflow dryer; condensation easily takes place in crossflow and fixed-bed dryers. The relative humidity of the exhaust air is 100 percent; this is due to the decrease in temperature of the drying air as it passes through the drying bed (decrease in the drying rate). There is very little heating of the grain on the exhaust side of the drying column. The average moisture content decreases by 1.5 percentage points. The small decrease in moisture content is due to the high grain velocity through the dryer (52.7 m/hr). Despite the presence of a turn-flow device a moisture gradient (difference between the maximum and minimum moisture content, Table 6.27) of 4.5 percentage points is developed in the crossflow dryer. The moisture gradient is well illustrated in Fig. 6.6. It was shown in Figs. 6.2 through 6.5 that there is no moisture gradient in a concurrentflow dryer since all kernels undergo the same drying treatment. 197 Table 6.27 Comparison between experimental and simulated drying of long-grain rice in a crossflow dryer in Edna, TX, 1982. Input/output variables and parameters Lot 73 f irst pass Experimental Simulated Grainflow rate (tons/hr/mZ) Grainflow speed (m/hr) Inlet moisture content (2 w.b.) Ambient temperature (C) Ambient relative humidity (2) Inlet rice temperature (C) 3 5 1 2 6 2 6 0.6 2.7 -5 9.h 6.0 6.7 m Column thickness (m) Column length (m) Airflow rate (m3/min/m2) Inlet air temperature (C) Outlet rice temperature (C) minimum average maximum Outlet moisture content (2 w.b.) minimum average maximum Fuel efficiency(kJ/kg H20) 29.0 m-: NNO‘O NNO-‘Ufl O‘WN NO‘O": NNOCD MONO \‘d—n—a 0003' 5.“ BOTTOM $759; Column thickness (m) Column length (m) Airflow rate (m3/min/m2) Inlet air temperature (C) Outlet rice temperature (C) minimum average maximum Outlet moisture content (2 w.b.) minimum average maximum Fuel efficiency (kJ/kg H20) Percentage points removed (2 w.b.) Dryer fuel efficiency (kJ/kg H20) 1.8 h,231.0 (”PM come: 0 O wnm 12.0 1h.9 16.3 3.531-0 1.5 “.073. 198 The crossflow dryer fuel efficiency before the turn-flow is 5,472 kJ/kg and 3,531 kJ/kg after the turn-flow device. The simulated outlet rice temperature is higher (67.2 C) than the measured value (63.0 C) because the latter was measured 0.04 m inside the grain stream (Fig. 5.1). It is probable that the first rice kernels (next to the dryer screen) on the inlet air side of the drying column reach the inlet air temperature. The drying of such kernels takes place in the first couple of minutes (high air temperature and low relative humidity); the kernels approach the inlet air temperature as they travel throughout the rest of the dryer. 6.2.3 Fixed-Bed Drying The effects of inlet air temperature and airflow rate on the drying time, energy efficiency (fuel plus fan), and moisture content gradient in a fixed-bed (in-bin) drying system is investigated. For the simulations the standard input conditions are: (1) an inlet ambient and rice temperature of 20 C was employed, (2) the relative humidity of the ambient air was 70 percent, (3) natural gas was the fuel used to increase the drying air temperature to 25, 30, and 35 C, (4) the average inlet and outlet moisture content in the dryer was 24 and 13 percent, respectively, and (5) a 199 Table 6.28 Simulation results of a fixed-bed dryer with medium-grain rough rice at 25 C and 0.08 m3/min/m3. RICE AIR ABS REL DEPTH MOISTURE TEMP TEMP HUM HUM CONTENT (m) (C) (C) (kg/ kg) (2) (2 wb) After 50 hours of drying 0.09 25.0 25.0 0.0102 51.7 12.1 0.h6 2h.9 2h.9 0.010h 52.7 12.7 0.85 2h.0 2h.0 0.0109 58.3 1b.5 1.22 21.9 21.9 0.0119 72.5 17.7 1.58 19.3 19.3 0.0130 92.6 21.5 1.98 19.1 19.1 0.0136 98.2 23.6 2.35 19.2 19.2 0.01h0 100.0 23.8 2.71 19.3 19.3 0.01h0 100.0 2h.0 3.00 19.3 19.3 0.01h0 100.0 2h.0 After 100 hours of drying 0.09 25.0 25.0 0.0102 51.8 12.0 0.h6 25.0 25.0 0.0102 51.8 12.0 0.85 25.0 25.0 0.0102 51.8 12.0 1.22 25.0 25.0 0.010h 52.8 12.3 1.58 2h.9 2h.9 0.0106 5h.0 12.9 1.98 2h.0 2h.0 0.0110 59.h 1h.h 2.35 22.1 22.1 0.0120 72.3 17.9 2.71 18.9 18.9 0.0135 98.5 23.6. 3.00 18.8 18.8 0.0136 100.0 2h.1 After 128 hours of drying 0.09 25.0 25.0 0.0102 51.8 12.0 0.h6 25.0 25.0 0.0102 51.8 1210 0.85 25.0 25.0 0.0102 51.8 12.0 1.22 25.0 25.0 0.0102 51.8 12.0 1.58 25.0 25.0 0.0102 51.8 12.0 1.98 25.0 25.0 0.0103 52.2 12.3 2.35 2h.7 2h.7 0.0105 5h.3 13.1 2.71 23.5 23.5 0.0111 61.8 15.h 3.00 21.h 21.6 0.0123 76.9 19.6 200 bed depth of 3.0 m was used. The drying model results compare well with data presented by Calderwood (1979). 6 Nine simulation runs were conducted in order to predict the performance of the fixed-bed rice dryer. Table 6.28 shows the results of computer output of a fixed-bed dryer with medium-grain rough rice dried at 25 C for 50, 100, and 128 hours. After 50 hours of drying the drying front is between 1.0 and 2.0 meters in the drying bed. The relative humidity of the drying air before the drying front is the equilibrium relative humidity (51.7 percent). The relative humidity after the drying front is 100 percent. The wet bulb temperature approaches the dry bulb temperature as the air moves to the top of the dryer. After 100 hours of drying the drying front is between 2.0 and 3.0 meters in the drying bed. Condensation occurs at the top layer (moisture content increases from 24.0 to 24.1 percent). For lower initial moisture contents (20.0 percent or below) more water will be condensed at the top layers of the drying bed. After 128 hours of drying the rice at the bottom layers reached the equilibrium moisture content (12.0 percent) at the inlet air temperature of 25 C and 51.7 percent relative humidity. The air and the rice temperature are the same throughout the drying bed. It can be seen that the drying front has not completely reached 201 the top of the dryer; this happened because an average final moisture content of 13 percent was specified to stop the simulation. Fig. 6.7 shows that the drying time is much more affected at lower airflow rates. Higher inlet air temperatures increase the dryer capacity at a decreasing rate; higher dryer capacities are obtained with higher inlet air temperatures. Fig. 6.8 shows that the energy efficiency of a fixed-bed dryer is improved at lower temperatures. For the 3.0 m bed depth under investigation lower airflow rates result in a better energy efficiency but a lower drying capacity (Fig. 6.7). The lower energy efficiency (high energy cosumption per kg of water removed) at increased airflows is partially due to the increased power requirement for the inlet air fan. It can be seen (Fig. 6.8) that at higher temperatures the power requirement is less of a limiting factor; this is true for high temperature dryer such as the concurrentflow dryer. At higher temperatures and low airflows there is more overdrying of rice; this results in lower energy efficiencies (Fig. 6.8). Fig. 6.9 shows that lower drying temperatures result in lower moisture content gradients (difference between the moisture content at the top and bottom of the dryer). Higher airflow rates decrease the moisture gradient Drying time, hr 202 Medium—grain rough rice Drying conditions: Ambient temperature, 202C Inlet rice temperature, 20 C Ambient relative humidity, 70 % Initial moisture content, 24 % w.b. Final moisture content, 13 % w.b. Bed depth, 3 m 240 220 200 180 160 140 120 100 80 60 O 0.04 0.08 0.12 Airflow rate, m3/s/m3 Fig. 6.7 Drying time at different inlet air temperatures and airflow rates for a 3 m fixed—bed rough rice dryer. 203 Medium-grain rough rice Drying conditions: Ambient temperature, 20°C Inlet rice temperature, 20°C Ambient relative humidity, 70 % Initial moisture content, 24 % w.b. Final moisture content, 13 % w.b. Bed depth, 3 m 5,000 4,000 3,000 2,000 Energy efficiency, kJ/kg H20 1,000 Airflow rate, m3/s/m3 Fig. 6.8 Energy efficiency at different inlet air temperatures and airflow rates for a 3 m fixed-bed rough rice dryer. 204 Medium-grain rough rice drying conditions: Ambient temperature, 202C Inlet rice temperature, 20 C Ambient relative humidity, 70 % Initial moisture content, 24 % w.b. Final moisture content, 13 % w.b. Bed depth, 3 m 20 18 5...: O‘ ...: A H N H 0 Moisture content gradient (top minus bottom of dryer), % w.b. 0.04 0.08 0.12 Airflow rate, ms/s/m3 Fig. 6.9 Moisture content gradient at different inlet air temperatures and airflow rates for a 3 m fixed-bed rough rice dryer. 205 especially at higher temperatures. The high moisture gradients at higher temperatures occurred because of the overdrying of the bottom layer and underdrying of the top layer (simulation stopped when the average moisture content of 13 percent was reached). 6.3 Comparison of Rice Drying Methods The fixed-bed (in-bin) method of rice drying is widely used on farms. The rice is dried and stored without the need for moving the grain, making the operation much simpler. The drying process is slow (low capacity system). If satisfactory results are to be obtained, a high level of management and understanding of the principles of drying is. required. The drying rate and energy consumption are determined by the quality and quantity of the drying air. During the drying process a smaller moisture gradient is developed within individual kernels than during crossflow and concurrentflow drying since the moisture is removed slowly from the center to the surface of the kernel. When the ambient temperature is too low and/or the relative humidity is too high, supplemental heat is required to accelerate the drying process and avoid mold development. The added heat may result in overdrying of the bottom layers. The rate of movement of the drying front should be 206 fast enough to dry the top of the bin before mold development can start. In-bin drying systems provide low cost drying (and storing) especially under California conditions. The average energy efficiency of an on-farm rice drying facility (from 22.0 to 13.0 percent moisture content) in Southwest Louisiana in 1980/1981 was 3,000 kJ/kg of water removed (Verma, 1982). The use of simulation models will provide a better understanding of the in-bin drying process; the use of controllers can make the system even more efficient and eaSier to manage. In the multipass crossflow drying system the dryer is only used when the kernel surfaces are relatively moist. Higher inlet air temperatures (up to 85 C) result in higher drying capacity and higher system reliability (less dependence on weather conditions). Like in-bin systems crossflow dryers are of simple design. Grain on the inlet side, dries first, and, by the time it leaves the dryer has reached the inlet air temperature and a exessively low moisture content. Grain on the exhaust side remains cooler and wetter. Mixing of the rice after.drying is essential; the mixing of low/high moisture kernels results in kernel surface fissures due to moisture adsorption/desorption during tempering. Calderwood (1979a) showed that there was no reduction of head rice yields when blending 12.0 and , t .1! kgnt‘gr I 207 22.0 percent moisture content rice; however, severe reduction in head yield (12.0 to 20.0 percentage points) resulted from mixing overdried rice at a moisture content of 8.0 percent or lower with rice at a moisture content of 18.0 percent or higher. Four to six drying passes are required to dry rice from 24.0 to 13.0 percent moisture content in a crossflow dryer. Between passes the rice is tempered for approximately 24 hours in tempering bins (see Fig. 6.10). From a logistical point of view, it is desirable to complete the drying and tempering process in the minimum number of passes. This reduces the number of bins needed for wet storage (tempering), decreases the handling of rice, decreases the labor costs, and improves the energy efficiency of the entire operation. Multistage concurrentflow dryers (Fig. 3.20) can achieve this objective. Crossflow dryers are limited to relatively low inlet air temperatures (up to 85 C) compared to concurrentflow dryers (100 to 180 C) due to grain overdrying and quality deterioration. One or two percentage points decrease in head yield per drying pass are typical of crossflow drying systems. The overdrying and underdrying of grain results in lower energy efficiencies (5,000 to 6,000 kJ/kg of water removed) compared to concurrentflow dryers (3,500 to 4,500 kJ/kg of water removed). 208 Elevator Elevator A B To bulk .—1 storage bins Conveyor + TEMPERING BINS Drier O 1 .2 .3 .4 Wet paddy \ \N‘l L— Conveyor Fig. 6.10 Direction of flow and equipment required for a multipass crossflow rice drying system (De Padua, 1976). 209 Data presented by Calderwood and Webb (1971) indicate the shorter retention time along with increased air temperatures (82 C) increases dryer capacity and maintained head yields; the increases airflow rates (182 m3/min/metric ton of rice) resulted in no apparent change in milling yield. The performance of a crossflow dryer at increased temperatures and grainflow rates in presented in Tables 6.16 and 6.17. They show that such improvements are probable. Concurrentflow dryers .can be operated at higher temperatures than fixed-bed and crossflow dryers without excessive grain damage resulting in a higher energy efficiency. In a concurrentflow dryer there is a rapid conversion of air sensible heat into latent heat of vaporization of water evaporated from the grain. This cools the air and ensures that the maximum temperature reached by the rice kernels is well below the air inlet temperature (Fig. 6.1). Compared to crossflow drying, this type of drying is efficient because: (1) high temperatures minimize the quantity of drying air needed, (2) the proportion of heat wasted in the exhaust air is minimized (low total airflow), (3) every kernel receives the same treatment (no energy is wasted in overdrying grain), (4) ability to precisely control the maximum and the outlet kernel temperatures. 210 Rice in the 18.0 to 21.0 percent moisture content range can be dried in CCF dryers in one drying pass without overdrying the grain. The three drying stages plus the cooling stage in a three-stage concurrentflow dryer are equivalent to three to four passes in a crossflow dryer. Rice above 21.0 to 22.0 percent moisture content requires two drying passes in a concurrentflow dryer. This is equivalent to about six or seven drying passes in a conventional crossflow dryer. Increased capacity and simplification of the drying operation (built-in tempering) are advantages of concurrentflow dryers. CHAPTER 7 DESIGN ANALYSIS OF CONCURRENTFLOW RICE DRYING 7.1 Standard Conditions for Dryer Simulations Table 7.1 presents the standard conditions to be used in the concurrentflow dryer simulations. Two inlet moisture contents are considered; for the two-pass and the one-pass systems inlet moisture contents of 23.5 and 18.0 percent are employed. Every kernel is assumed to have the same uniformly distributed moisture content. A final moisture content of 13.0 percent is designed for since it is desirable for safe storage. Ambient air and rice temperatures of 20 C are employed. A relative humidity of 60 percent is assumed to be the average during the drying season 0 211 212 Table 7.1 Standard conditions for concurrentflow dryer simulation. Inlet moisture content, 2 w.b. two pass . 23.5 one pass 18.0 Outlet moisture content, 2 w.b. 13.0 Ambient temperature, C 20.0 Ambient relative humidity, X 60.0 Inlet rice temperature, C 20.0 Table 7.2 Standard inputs for the simulation runs to investigate the effect of bed depth on concurrentflow dryer performance. Inlet moisture content. 3 w.b. 23 Ambient air temperature, C 20 Ambient relative humidity, 2 60. Inlet rice temperature, C 20 Grainflow rate, tons/hr/mZ 2 Inlet air temperature, C top stage 160.0 middle stage 120.0 bottom stage 90.0 Tempering length, m first tempering zone 5.2 second tempering zone 5.2 213 7.2 Effect of Bed Depth The power required for the fans of a concurrentflow rice dryer is relatively high mainly due to the relatively large bed depths in the three drying stages. Decreasing these depths will not only affect the power requirement of the three fans but also moisture removal rates, grain temperatures, and energy efficiencies. Besides, it may slightly affect the rice quality. The reduction of the drying beds should be considered ‘when: (l) the power available for the motors is the limiting factor, and (2) an increased dryer capacity is desired. Simulations with the concurrentflow drying model were conducted to investigate the effects of bed depth(s) on: i (1) power requirement, (2) moisture removal rate, (3) maximum rice temperature, (4) outlet rice temperature, (5) dryer fuel efficiency, (6) dryer energy efficiency (energy to heat air and to power fans), (7) dryer capacity, and (8) rice type. Table 7.2 presents the standard inputs for the 14 simulation runs with medium- and long-grain rice) to investigate the effect of bed depth on CCF dryer performance. 214 Table 7.3 presents values for the static pressure and theoretical power requirement for four bed depths of medium-grain rough rice at six airflow rates. Table 7.4 presents similar data for long-grain rough rice. The effect of airflows on power requirement is plotted in Figs. 7.1 and 7.2. The power requirement decreases substantially by decreasing the bed depths. Also, the airflow rates can be increased to increase the dryer capacity. Three bed depths were used in the simulations of medium- and long-grain rice. Table 7.5 shows the data for medium-grain rice as Table 7.6 for long-grain rice. Run No. 1 (Table 7.5) represents the present design for the three-stage CCF dryer and has bed depths of 0.86, 1.16, and 1.31 m, for . the top, middle, and bottom stages, respectively. Typical airflow rates (40.0, 35.0, and 35.0 m3/min/m2) and grainflow rate (2.5 tons/hr/m2) were used. The input and output .of each stage for grain and air temperatures, moisture contents, static pressures, power requirement, and dryer fuel and energy efficiency are tabulated in Table 7.5 for medium-grain rice. For Run No. l the total moisture removed is 4.8 percentage points. The dryer fuel and energy efficiency are 4,289 and 4,470 kJ/kg of water removed. The total theoretical power requirement is 10.1 kW/m2. The maximum rice temperatures are predicted to be 55.0, 57.2, and 215 Table 7.3 Theoretical power requirements(a) as a function of airflow rates and static pressure(b) for four bed depths of medium-grain rough rice. Bed depth Airflow rate Static pressure Power requirement (m) (m3/min/m2) (kPa) (kW/m2) 25 1.9 1.6 30 2.5 2.5 1.31 35 3.1 ' 3.6 40 3.8 5.1 45 4.5 6.8 50 5.3 8.8 25 1.7 1.4 30 2.2 2.2 1 16 35 2.8 3.3 40 3.4 h.5 AS 4.0 6.0 50 h.7 7.8 25 1.1 0.9 30 1.4 1.h 0.76 35 1.8 2.1 to 2.2 2.9 AS 2.6 3.9 50 3.1 5.2 25 0.9 0.8 30 1.2 1.2 0.61 35 1.5 1.8 40 1.8 2.4 45 2.1 3.2 50 2.5 4.2 (a) Theoretical power requirement (50 percent efficiency) is calculated from the following relationship: (kW/m2).- 0.0333357 * (airflow rate, m3/min/m2) * (static pressure, kPa) (b) Static pressure from Henderson and Parsons (1974) with increased resistance as suggested by Calderwood (1973) (kPa) I 5280.0 * (Q, m3/min/m2) ** 1.5715 216 Table 7.4 Theoretical power requirements(a) as a function of airflow rates and static pressure(b) for four bed depths of long-grain rough rice. Bed depth Airflow rate Static pressure Power requirement (m) (m3/min/m2) (kPa) (kW/m2) 25 1.9 1.6 30 2.3 2.3 1.31 35 2.8 3.3 40 3.2 4.3 “5 3-7 5-6 50 4.2 7.0 25 1.7 1.4 30 2.1 2.1 1.16 35 2.5 1 2.9 #0 2.9 3.9 “5 3.3 5.0 50 3.7 6.2 25 1.1 0.9 30 1.4 1.4 0.76 35 1.6 1.9 #0 1.9 2.5 “5 2.2 3.3 50 2.4 h.0 25 0.9 ‘ 0.8 30 1.1 1.1 0.61 35 1.3 1.5 40 1.5 2.0 45 1.7 2.6 50 1.9 3.2 (a) Theoretical power requirement (50 percent efficiency) is calculated from the following relationship: (kH/m2) - 0.0333357 * (airflow rate. m3/min/m2) * (static pressure. kPa) (b) Static pressure from Calderwood (1973). Table b.10. 10 Power requirement, kW/m2 N Fig. 7.1 25 30 35 40 Airflow rate, ms/min/m2 Power requirement versus airflow rate rough rice (Table 7.3). 45 50 for medium-grain 10 Power requirement, kW/m2 Fig. 218 25 30 35 40 45 50 Airflow rate, m3/min/m2 7.2 Power requirement versus airflow rate for long-grain rough rice (Table 7.4). 219 Table 7.5 Simulations for the three-stage concurrentflow dryer with medium-grain rough rice investigating the effect of bed depths on dryer performance. Input/output variables Run ho. and parameters 1 2 3 4 5 6 Grainflow rate (tons/hr/m2) 2.5 2. 5 2.5 2.5 2. 5 2.7 Inlet moisture content (wa) 23.5 23. 5 23.5 23.5 23. 5 23.5 ‘FIRSI STAEE Bed depth (m) 0.86 O. 86 0. 61 0.86 0. 61 0.86 Tempering length (m) 5.2 5. 2 5. 2 5.2 5. 2 5.2 Inlet air temperature (C) 160.0 160. O 160. 0 160.0 160. 0 160.0 Airflow rate (m3/min/m2) 40.0 40. O 40.0 40.0 46.0 40.0 Maximum rice temperature (C) 55.0 55.0 55.0 55.0 58.8 53.0 Outlet rice temperature (C) 39.1 39.1 40.7 39.1 43.3 38.6 Outlet air temperature (C) 39.1 39.1 40.8 39.1 43.4 38.6 Outlet relative humidity (3) 78.8 78.8 67.0 78.8 57.6 78.9 Static pressure (kPa) 2.5 2.5 1.7 2.5 2.2 2.5 Power requirement (kW/m2) 3.3 3.3 2.4 3.3 3.3 3.3 Outlet moisture content (wa) 21.8 21.8 21.9 21.8 21. 5 22.0 1 Fuel efficiency (kJ/kg H20) 5.670 5,670 5,900 5,670 5, 523 5,785 SECOND SIAGE Bed depth (m) 1.16 0.76 O 61 0.76 0. 61 0.76 Tempering length (m) 5.2 5.2 5.2 5.2 5. 2 5. 2 Inlet air temperature (C) 120.0 120.0 120.0 120.0 120. 0 120. O Airflow rate (m3/min/m2) 35.0 35.0 35.0 41.0 45.0 41.0 Maximum rice temperature (C) 57.2 57.2 58.4 59.7 62.3 58.2 Outlet rice temperature (C) 40.4 42.2 43.4 42.6 47.1 42.9 Outlet air temperature (C) 40.4 42.2 43.4 42.6 47.1 42.9 Outlet relative humidity (2) 81.0 68.2 64.2 66.8 50.6 63.2 Static pressure (kPa) 2.8 1.8 1.4 2.3 2.1 2.3 Power requirement (kW/m2) 3.2 2.1 1.7 3.1 3.1 3.1 Outlet moisture content (wa) 20.0 20.1 20. 2 19.9 19.2 20. 2 , Fuel efficiency (kJ/kg H20) 3.457 3,606 3,798 3.790 3.599 3,764 THIRD STAGE Bed depth (m) 1.31 0.76 0.61 0.76 0.61 0.76 . Inlet air temperature (C) 90.0 90.0 90.0 90.0 90.0 90.0 . Airflow rate (m3/min/m2) 35.0 35 o 35 o 44.0 47.0 44.0 47.0 Maximum rice temperature (C) 51.7 53 0 53 9 55.6 59.4 55.0 56.1 Outlet rice temperature (C) 37.9 39.8 41.3 41.9 45.7 41.8 44.0 Outlet air temperature (C) 37.9 39.8 41.3 41.9 45.8 41.9 44.0 Outlet relative humidity (X) 78.0 68.6 61.9 55.5 43.8 56.6 49.4 Static pressure (kPa) 3.1 1.8 1.4 2.5 2.2 2.5 2.2 Power requirement (kW/m2) 3.6 2.1 1.7 3.7 3.5 3.7 3.5 Outlet moisture content (wa) 18.7 18.8 18.9 18.3 17.4 18.6 18.8 Fuel efficiency (kJ/kg H20) 3.535 3.608 3,518 3,592 3,517 3,435 3,422 Percentage points removed (3wb) 4.8 4.7 4.6 5.2 6.1 4.9 4.7 Dryer fuel efficiecy (kJ/kg) 4,299 4.380 4,499 4,366 4,246 4,326 4,405 Dryer energy efficiency (kJ/kg) 4,470 5,514 4,605 4,530 4,387 4,491 4,553 220 51.7 C in the top, middle, and bottom stages, respectively; the outlet rice temperatures are 39.1, 40.4, and 37.9 C. Fig. 7.3 shows the temperature distribution (air and medium-grain rice) and moisture content throughout the dryer in the standard three-stage CCF dryer. Fig. 7.4 illustrates similar process for long-grain rough rice. Run No. 2 is a simulation with the same airflows as in Run No. 1, but with bed depths in the middle and bottom stages equal to 0.76 m. The total moisture removed is 4.7 moisture percentage points for Run No. 2 and 4.6 points for Run No. 3. Dryer fuel efficiencies of 4,380 and 4,499 kJ/kg of water removed are obtained for Run No. 2 and Run No. 3, respectively; the dryer efficiencies for Run No. 2 and Run No. 3 are 4,514 and 4,605 kd/kg of water removed (comparable to 4,470 kJ/kg for Run No. 1). Similar maximum rice temperatures as in Run No. l are predicted. The’ outlet rice temperatures are increased by one to two degrees C for Run No. 3 when compared to Run No. l. The total theoretical power requirement is 7.5 kW/m2 for Run No. 2 and 5.8 kW/m2 for Run No. 3. This represents 26 and 43 percent decrease in power requirement when compared to Run No. l. 2 Runs No. 4 and 5 represent the CCF dryer with decreased bed depths (similar to Runs No. 2 and 3) operating at approximately the same power requirement as in Run No. 1. For Run No. 4 the airflows in the middle and .mm.n oHan .a .oz snag :mwmov noxnp moo owmumioonzu enmvcmum on» mo mean can cameo“ Hoxuv mamno> acoucoo ouaumflos use mooww camawissfipos pew away ousumnomaoh m.n .mam mo.m ow.m No.4 4; .mafie om.4 m4.o o nlfi. M- me.m~ . a .eumcos eo.o O N -L1-4-+‘ l l l l I >............_.... 4.---.. 4 C q. 1319A 221 . I‘ . . .-...- ..- -.A..¢r-‘... . - a , I O 0 cu ~o ‘exnieladmal o . . , . ¢ '77-'07?" , . . , . 4 I~ 1,..1 I 1 i 1 .14 -ii- ezmhzoo cod 0 .;:. 1 1 I 'Q'M % ‘1uaiuoo sinistow viii «:%;J;wzH>mo H nwszmmZuH. .-.‘..msos42_ -.-, emmsl ;. 4 wii-mze>moii:+ii-+i.oszMQZNH wze>ma.iw . ems .,.y zoeeom .._-, ozoumm .- mop..- 4 I j v o 1 0 o I i «4. , , .4 44.-.-.. 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H. _ : 4 ,., ._ A . ,4. , 4 -4 3_.- w W 4 4 4.4 .- ... , . 4 ., .‘4 .a_4 -..4 . 4.. 0.4 w -. mu acoucoo onsumHos can amUHH :Hmhm wcoH van “Hwy muspmuomsop v. n .mHm mo.m ow.N No.H 9: .oefih om.N mN.o .1 .11 I 1‘ Md mn.mH Nv.NH NN.h E .cpwcmg wo.w ow.o .--—.4 4...- o A - - ,_ . 4 :9--. mama i :I?,Er- _ r ‘ ...—1. -4....1 l 1 1 1 G. ‘ Lu a:p- ; .._.._.-T_44._ 1’*-‘ ‘ 1 1 .... -4..-- 1 .. . -. . , . ., + v .. . u -. - _b o .-. , ‘ . ‘ I o 2 J1 ~ | 7:3 1 . 1 l l ‘f’ 1 1 i 1 O \O 41 l - I 0 ‘ I 1 “......“ g D. 1 w t I .4.-__J___._¢.._ 1 O 1 ......L 1 ‘- — 4 O m ‘;~ 1 1 EZ+ .z.. 1 I- I 1 1 .... 2 LL] 1 F‘ Z Z : o '. U A 77717—“7 my»-.. quHzou . w . umaHmHOz.. -._ V91 I 1 l.l..l a: .3 .— 'U') u—c O z ooH A _ . . .4_4L1. a4 411.. 4 4 ' , 11 f. ._——-q—éqp*-p- 1.. A 1 0 o -—-—-L 1 o C 1 1 1 r L 1 1 1 I 1 1 I l + 1 1 1 fl 1 O Hhmyv.ww-4.._w4z.rx- “.m.:. ”=1. Hzmpzou .. .;:.x.. ...wmshmmoz ? CNN 1 1 1 . 1 I I A 1 'YiFéLé 1 1 1 1 1 1 I 1 ~9—~~ JV—‘—-‘- 1 1 f . ._ 1 ' " 1 1 4--...— ovH .. . . «0.0. v1. 191.0 .111. Volt - o v.tVJAA pl.“ , . O 1 ' V 444.4444“ .. ; . . 1 O 1 1 a . . . . . -..... “.----.rm-‘ - -- .4 . o - u . . ! . . I 9 ...—.4 -- V 1 ,1.)_._.-+._.. . . ¢ . . . 0 Goa ---u.- mw<4mv.-M.4 ,..:mz°~ .4 . _ mumo - “ quxm¢2mp - - --oz_>ao .4.,wszmazmpr . . «H: . zo++pm w -- .. ozoumm .-. M4QQHF ..._ Hm14mrg h owH . _ __4 _ m 4 4 4 _ _ 4 w 4 M 4 . m w 4 4,. = .-...jv..... H H _ 4 _ 4. H . . H _ 4 4 H 4 H H . i H .H CON 3 o ‘axnuexadmal 223 bottom stages are increased from 35.0 and 35.0 m3/min/m2 to 41.0 and 44.0 m3/min/m2. For Run No. 5 the airflows in the top, middle, and bottom stages are increase from 40.0, 35.0, and 35.0 m3/min/m2 to 46.0, 45.0, and 47.0 m3/min/m2. The total moisture removed is 5.2 percentage points for Run No. 4 and 6.1 points for Run No. 5. Run No. 4 presents comparable dryer energy efficiency (4,530 kJ/kg) as Run No. 1 (4,470 kJ/kg). Run No. 5 presents a slightly better dryer energy efficiency (4,387 kJ/kg) than Run No. l. The maximum and outlet rice temperatures are increased by approximately two to seven degrees C. Runs No. 6 and 7 are similar to Runs No. 4 and 5 but with increased grainflows in order to result in the same amount of moisture removal as in Run No. l. The grainflows are 2.7 tons/hr/mz for Run No. 6 and 3.05 tons/hr/m2 for Run No. 7. This represents increases of seven and 18 percent as compared to Run No. 1. The dryer energy efficiency for Run No. 6 is 4,491 kJ/kg as compared to 4,470 kJ/kg for Run No. l; the energy efficiency for Run No. 7 is 4,553 kJ/kg. The maximum rice temperatures for Run No. 6 are 53.0, 58.2, and 55.0 C as compared to 55.0, 57.2, and 51.7 C for Run No. 1; for Run No. 7 the maximum rice temperatures are 53.7, 59.6, and 56.1 C. The outlet rice temperatures for Run No. 6 are 38.6, 42.9, and 41.8 C as compared to 39.1, 40.4, and 37.9 C for Run No. 1; for Run No. 7 the outlet rice temperatures are 41.1, 45.2, and 224 44.0 C. Fig. 7.5 shows the temperature distribution (air and medium-grain rough rice) and moisture content throughout the dryer in the three-stage CCF dryer with decreased bed depths (0.61 m). Fig. 7.6 shows similar processes for long-grain rough rice. Table 7.6 gives the same data for long-grain rice as Table 7.5 for medium-grain rice. A decrease in the bed depths from 1.16 m to 0.76 m in the middle stage and from 1.31 to 0.76 m in the bottom stage for medium-grain rice results in a 26 percent savings in total theoretical power requirement (10.1 versus 7.5 kW/mz) the moisture removed is reduced by only 0.1 percentage points. For long-grain rice the same moisture removal was observed with a 27 percent saving in the total theoretical power requirement (9.1 versus 6.6 kW/mZ). A decrease in bed depth to 0.76 m in the middle and bottom stages at present power requirement design leads to an additional 0.4 percentage points of moisture removed from medium-grain rice and 0.9 percentage points for long-grain rice, at lower static pressures. A decrease in the bed depths for medium-grain rice to a standard value of 0.61 m results in a 43 perrcent savings in total theoretical power requirement (10.1 versus 5.8 kW/mZ) while removing only 0.2 fewer moisture percentage points (4.8 versus 4.6 percent). For long-grain rice the 225 'q'M % ‘1ua1uoo axnustow .fim. a ofiame .5 .oz gnaw mnumop won a He. o sud: noxhp moo mumum manna may mo mafia paw nausea uoxnp msmuo> acmucoo unaumfioa was moufin :fiwum-63wpoa paw Macy ousuauomaoh m.n .mflm mN.N NH.N 9: .oawh BH.H oo.H qt - xii 1- mN.NH No.HH E .numcmq Nv.o Hw.m mZON m ,. ... m. _ m mZON #1 azammazmc_ fl _ W omH wzm>mo “boom: 143+:+‘4 nzoumm . IT.W.,~.W T +az‘- hmmnu s h mrr H _ p . M g m . 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I M.A--- ..H 3.3..MM33. M3 . 3 31:. 333M4333mw—33wq3134.3.4.1:33141 - . OOH Aw 3333+ h2mkzoo-a 3 .3. W M .M MMMx . 333zf mmaemHoz-zq3ws.334.3#?-AW. -.M. .3- - 33.733 zfiJ. M ...;.. ... .3 M.. ...A ...M. .. . ON 131.333.33M-33*..e33.. MM .. g A. . M. AM M.... M A. _ .13.. . J ONH 1M3+H33M33333__M.33M .-.-1.3M 3.x _33M3 .3M; r33/Mzi A. ...-..- 3.3.. 3.31 ..-.r-..-;_ ..M MM .M . .fl .- M . . ._M :M b - T -. A 3 3 Ha MMMM»_3M ..M. M M.m~_ .M.. M. MwM M M. AAW .4 - ovfi 3-33.3MJUH1 —MJ3.3 ..M . M... M. M. w .. a M . —_ .. 3 W NN ..M M. MAM M_. M; .M:: - A e ..A,-:; M col 3M e-M 2;Mz- szN ,M -.. MZON 3: M muma .:333 szmma2me-33 W4 . wszma2mc3. ems -Mr M zop+om .M «M azouumM . MM “sooMz . M hmmaa M ach3 «N M. r-_ M.M M 3. _ . M M M 3 W-:-.M-r33M T -33M M M .3.M M M.-. r- M M. *.+u.1 com 3 o ‘axnieladmal Table 7.6 Simulations for the three- stage concurrentflow dryer with long-grain rough rice investigating the effect 227 of bed depths on dryer performance. Input/output variables Run No. and parameters l 3 h 7 Grainflow rate (tons/hr/m2) 2. 5 2.5 2. 5 3.1 Inlet moisture content (wa) 23. 5 23.5 23. 5 2L 5 FIRST STAGE Bed depth (m) 0.86 0.61 0.86 . 0.8l Tempering length (m) 5.2 5.2 5.2 .2 5.2 Inlet air temperature (C) l60.0 l60.0 l60.0 .0 l60.0 Airflow rate (m3/min/m2) no.0 no.0 no.0 . .o h6.03 Maximum rice temperature (C) 5h.7 5h.7 5h.7 .h 52.8 Outlet rice temperature (C) 37.9 39.1 37.9 .8 38.8 Outlet air temperature (C) 37.9 39.1 37.9 .8 38.8 Outlet relative humidity (2) 88.0 79.2 88.0 .1 77.h Static pressure (kPa) 2.l 1.5 l.5 .8 l.8 Power requirement (kW/m2) 2.8 2.0 2.8 .7 2.7 Outlet moisture content (Zwb) 2l.2 21.2 2l. 2 .7 2l.6 Fuel efficiency (kJ/EgHZO) h,l6h b.275 h, l6h h 015 4.59l §§§gnn STAGE Bed depth (m) 1.16 0.6) 0.76 0.6] O. 61 Tempering length (m) 5.2 5.2 5.2 .2 5. 2 Inlet air temperature (C) 120.0 120.0 120.0 .0 120. 0 Airflow rate (m3/min/m2) 35.0 35.0 kl.0 .0 h5. 0 Maximum rice temperature (C) 56.3 57.0 58.7 .h 57. 5 Outlet rice temperature (C) 38.9 60.8 h0.8 .l hi. 7 Outlet air temperature (C) 38.9 h0.8 h0.8 .2 bl. 8 Outlet relative humidity (2) 89.h 77.7 75.8 .2 70. 7. Static pressure (kPa) 2.5 1.3 l.9 .7 1.7 Power requirement (kW/m2) 2.9 1.5 2.6 .6 2. 6 Outlet moisture content (Zwb) l9.l l9.l 18.6 .8 19. h Fuel efficiency (kJ/kgHZO) 23991 2,973 2,965 3, 029 THIRD STAGE Bed depth (m) l.3l 0.6l 0.76 0.6] Inlet air temperature (C) 90.0 90.0 90.0 90.0 Airflow rate (m3/min/m2) 35.0 35.0 hh.0 h7.0 Maximum rice temperature (C) 50.7 51.9 5h.h 53.6 Outlet rice temperature (C) 36.6 38.5 h0.0 ho. 6 Outlet air temperature (C) 36.6 38.5 h0.0 ho. 6 Outlet relative humidity (x) 83.5 73.9 62.5 6L 5 Static pressure (kPa) 2.9 1.3 2.l l.8 Power requirement (kW/m2) 3.h 1.5 3.] 2. 8 Outlet moisture content (wa) l7.5 l7.5 16.6 l7. 7 fuel efficiency (kJ/kg H20) 2,829 3.00h 2,90h 2, 893 Percentage points removed (zwb) 6.0 6.0 6.9 5. 8 Dryer fuel efficiency (kJ/kg) 3,hh6 3,h97 3,373 3, h35 Dryer energy efficiency (kJ/kg) 3,577 3,571 3,h81 3. 632 228 same moisture removal was observed with a 45 percent savings in the total theoretical power requirement (9.1 versus 5.0 kW/mz). A decrease in bed depths to 0.61 m at the present power requirement leads to an additional 1.3 percentage points of moisture removed for medium-grain rice and 1.9 percentage points for long-grain rice, at lower static pressures. ’ A decrease in the bed depths to 0.76 m in the middle and bottom stages at present power requirement design and constant moisture removal results in a seven percent increase in dryer capacity (2.5 versus 2.7 tons/hr/mZ) for medium-grain rice; for long-grain rice the dryer capacity is increased by 11 percent (2.5 versus 2.8 tons/hr/mz). A decrease in the bed depths to 0.61 m at present power requirement design and constant moisture removal results in a 18 percent increase in dryer capacity for medium-grain rice (2.5 versus 3.05 tons/hr/m2); for long-grain rice the dryer capacity is increased by 19 percent (2.5 versus 3.1 tons/hr/mz). From the simulation results with standard and decreased bed depths the following generalizations can be deducted: l) the power requirements are decreased substantially (45 percent) by decreasing the bed depths (Figs. 7.1 and 7.2). 229 2) with properly decreased bed depths almost the same amount of moisture is removed; also, more moisture is removed with the same power since the airflows are increased. 3) the dryer capacity is increased by 20 percent by decreasing the bed depths at constant power requirements and moisture removal. 4) the dryer fuel efficiency is decreased by an insignificant amount if the bed depths are decreased. 5) the maximum and outlet rice temperatures are approximately the same if the same airflows are used; the rice temperatures are increased slightly at increased airflows. 6) long-grain rice requires 21 percent less energy to be dried than medium-grain rice (3,560 kJ/kg, average of seven runs in Table 7.6 and 4,507 kJ/kg, average of seven runs in Table 7.5). 7) long-grain rice loses 22 percent more moisture than medium-grain rice under similar drying conditions. 8) the rice temperatures are slightly higher for medium-grain than for long-grain rice under similar operating conditions. 9) as indicated in Chapter Six (experimental results) long-grain rice maintained good quality during the drying process; this may be due to its lower initial moisture 230 content (16.0 to 18.0 percent); the drying of high moisture content (23.5 percent) long-grain rice needs more investigation; as indicated by Kunze and Calderwood (1980) long-grain rice fissures more rapidly than medium-grain rice and is more likely to decrease in milling yield during the drying process. 10) an engineering simulation study is not able to assess the effect of bed depth change in the CCF dryer on rice quality. However, it is believed, Runs No. l and 7 will produce rice with excellent quality characteristics since similar moisture removals, rice temperatures, and retention times are observed. 7.3 Effect of Tempering The tempering of single kernels of long-, medium-, and short-grain rough rice was investigated in Chapter Four. Tempering of single kernels of rough rice increases the drying rate in subsequent drying stages. Tempering times of about 30 minutes are sufficient at 45 C (approximate temperatures in the tempering zones of a CCF dryer). This section presents the results of tempering times and levels on moisture removal rate and dryer fuel efficiency for medium- and long-grain rough rice. Table 7.? presents three simulation runs for 5.2 m, 3.0 m, and “ J1. Jah- I u -231 Table 7.7 Simulations for the three-stage concurrentflow dryer with medium-grain rough rice investigating the effect of tempering on dryer performance. Input/output variables Run No. and parameters l 2 3 Grainflow rate (tons/hr/m2) 2.5 2.5 2.5 - Inlet moisture content (2wb) 23.5 23.5, 23.5 FIRST STAGE Bed depth (m) 0.86 0.86 0.86 Tempering length (m) 5.2 3.0 0.0 Inlet air temperature (C) l60.0 l60.0 l60.0 Airflow rate (m3/min/m2) h0.0 h0.0 h0.0 Maximum rice temperature (C) 55.0 55.0 55.0 Outlet rice temperature (C) 39.l 39.l 39.l Outlet air temperature (C) 39.1 39.l 39.l Outlet relative humidity (2) 78.8 78.8 78.8 Static pressure (kPa) 2.5 2.5 2.5 Power requirement (kW/m2) 3.3 3.3 3.3 Outlet moisture content (*wb) 2l.8 21.8 2l.8 Fuel efficiency (kJ/kg H20) 5.670 5.670 5,670 secou'o'TTAg * Bed depth (m) l.l6 l.l6 l.l6 Tempering length (m) 5.2 5.2 5.2 Inlet air temperature (C) l20.0 l20.0 l20.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 Maximum rice temperature (C) 57.2 57.2 57.2 Outlet rice temperature (C) h0.h A0.l 39.9 Outlet air temperature (C) h0.h #0.] A0.0 Outlet relative humidity (2) 8l.0 83.l 8h.6 Static pressure (kPa) 2.8 2.8 2.8 Power requirement (kW/m2) 3.2 3.2 3.2 Outlet moisture content (zwb) 20.0 20. 20.7 Fuel efficiency (kJ/kg H20) 3.h57 3.620 5,7hl ’ "rH I-RD 57' AG; Bed depth (m) 1.3] 1.3] 1.31 Inlet air temperature (C) 90.0 90.0 90.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 Maximum rice temperature (C) 5l.7 51.5 5l.h Outlet rice temperature (C) 37.9 37.9 38.0 Outlet air temperature (C) 37.9 37.9 38.0 Outlet relative humidity (2) 78.0 77.h 76.2 Static pressure (kPa) 3.l 3.l 3.l Power requirement (kW/m2) 3.6 3.6 3.6 Outlet moisture content (zwb) l8.7 18.8 19.6 Fuel efficiency (kJ/kg H20) 3,535 3.627 5,070 Percentage points removed (wa) h.8 A.7 3.9 Dryer fuel efficiency (kJ/kg) A.289 b.39l 5,252 232 0.0 m tempering lengths for medium-grain rough rice. Similar data for long-grain rough rice is presented in Table 7.8. Run No. 1 (Tables 7.7 and 7.8) represents the actual three-stage CCF dryer design (two tempering zones 5.2 m long). The grainflow rate of 2.5 tons/hr/mz gives a retention time in the tempering zones of 1.2 hours. A tempering level of 95 percent is achieved in the two temperihg zones when tempering medium-grain rough rice; for long-grain rough rice the tempering levels are 99 and 98 percent for the first and second tempering zones, respectively. For Run No. 2 (Tables 7.7 and 7.8) the tempering length is reduced from 5.2 m to 3.0 m in both tempering zones. The grainflow rate of 2.5 tons/hr/mz results in a retention time in the tempering zones of 0.7 hours. Tempering levels of 86 and 84 percent are achieved in the first and second tempering zones when tempering medium-grain rough rice; for long-grain rough rice the tempering levels are 96 and 95 percent. Reduced tempering lengths (3.0 m) have a minor effect on the moisture removal rate and dryer fuel efficiency. The fuel efficiency (3.0 m tempering lengths) for medium-grain rice is 4,391 kJ/kg as compared to 4,289 kJ/kg for the dryer with standard tempering zones (5.2 m). For long-grain rice the decreased tempering lengths result in an even smaller decrease in the 233 Table 7.8 Simulations for the three-stage concurrentflow dryer with long-grain rough rice investigating the effect of tempering on dryer performance. Input/output variables Run No. and parameters I 2 3 Grainflow rate (tons/hr/mZ) 2.5 2.5 2.5 Inlet moisture content (Zwb) 23.5 23.5 23. FIRST 5mg Bed depth (m) 0.86 0.86 0.86 Tempering length (m) 5.2 3.0 0.0 Inlet air temperature (C) l60.0 l60.0 l60.0 Airflow rate (m3/min/m2) h0.0 h0.0 h0.0 Maximum rice temperature (C) 5h.7 5h.7 5h.7 Outlet rice temperature (C) 37.9 37.9 37.9 Outlet air temperature (C) 37.9 37.9 37.9 Outlet relative humidity (2) 88.0 88.0 88.0 Static pressure (kPa) 2.l 2.l 2.l Power requirement (kW/m2) 2.8 2.8 2.8 Outlet moisture content (Zwb) 2l.2 21.2 21.2 Fuel efficiency (kJ/kg H20) h,l6h h,l6h h,l6h SECOND STAGE Bed depth (m) 1.16 l.l6 l.l6 Tempering length (m) 5.2 5.2 5.2 Inlet air temperature (C) l20.0 120.0 l20.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 Maximum rice temperature (C) 56.3 56.3 56.6 Outlet rice temperature (C) 38.9 38.8 39.0 Outlet air temperature (C) 38.9 38.8 39.] Outlet relative humidity (2) 89.h 90.7 88.A Static pressure (kPa) 2.5 2.5 2.5 Power requirement (kW/m2) 2.9 2.9 2.9 Outlet moisture content (wa) l9.l- l9.l l9.3 ' Fuel efficiency (kJ/kg;320) 2.991 3.036 3,A2l THIRD STAGE Bed depth (m) l.3l l.3l l.3l Inlet air temperature (C) 90.0 90.0 90.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 Maximum rice temperature (C) 50.7 50.6 50.9 Outlet rice temperature (C) 36.6 36.6 36.8 Outlet air temperature (C) 36.6 36.6 36.8 Outlet relative humidity (X) 83.5 83.1 82.A Static pressure (kPa) 2.9 2.9 2.9 Power requirement (kW/m2) 3.h 3.h 3.h Outlet moisture content (wa) 17.5 17.5 l7.9 Fuel efficiency (kJ/kg H20) 2,929 2.9Al 3,256 Percentage points removed (Zwb) 6.0 6.0 5.6 Dryer fuel efficiency (kJ/kg) 3,Ah6 3,h68 3,701 234 energy efficiency (3,468 versus 3,446 kJ/kg). For Run No. 3 (Table 7.7 and 7.8) no tempering zones are used. The effects of tempering are substantial especially for medium-grain rough rice. For medium-grain rough rice l.l fewer moisture percentage points are removed and the dryer fuel efficiency decreases from 4,289 kJ/kg to 5,252 kJ/kg; a less significant decrease in fuel efficiency is observed for long-grain rough rice (3,701 kJ/kg with no tempering vesus 3,446 kJ/kg with 5.2 m of tempering) at 0.4 percentage points decrease in moisture removed. The tempering of long-grain rough rice does not affect the drying rate and fuel efficiency in a concurrentflow dryer at tempering lengths over 3.0 m; this result is consistent with those found by Walker (1978). The reason for this is that long-grain rice dried in a deep-bed concurrentflow dryer has a fairly uniform final moisture content distribution (23.3 percent at the center and 16.2 percent at the surface of the kernel out of the first drying stage). Another reason is that long-grain rough rice has a smaller equivalent radius (0.001 m) than medium-grain rough rice (0.00156 m) and corn (0.00296 m). The tempering of medium-grain rough rice has a slight effect on moisture removal and fuel efficiency in a concurrentflow dryer at a tempering length of 3.0 m. This is expected since the moisture gradient of 9.4 percentage points (23.5 percent at the center and 14.1 percent at the 235 surface of the kernel) is observed in medium-grain rice after the first drying stage. Tempering lengths of 3.0 m represent a retention time of 25 and 60 minutes at grain velocities of 7.2 and 3.0 m/hr. According to Steffe et al. (1979) a tempering time of 36 minutes is sufficient to produce optimum quality in medium-grain rice. Gustafson et a1. (1982) found that for a thin-layer of dried corn approximately 67 percent of the breakage susceptability decrease due to tempering occurred in the first 15 minutes of tempering and 96 percent in the first 30 minutes. The' improvement on the moisture removal rate was 50 and 70 percent in the first 15 and 30 minutes of tempering, respectively. The short tempering associated with quality retention are explained by Litchfield et al. (1982). The maximum shear stresses were found to increase by a factor of three to five during the transition from drying to tempering of corn. Most of these stresses were decreased during the first 30 minutes of tempering. ' The tempering zones lengths of the CCF rice dryers can be reduced ‘from 5.2 to 3.0 m without effecting the rice temperatures, moisture removal rates, drying fuel efficiency, and most importantly, the quality aspects of medium- and long-grain rice. This will result in 4.4 and 4.2 m decrease in the total height of the three- and two-stage CCF dryers, decreasing the manufacturing costs. 236 The reduction in tempering lengths is justified because of the bed depth of the three drying stages. Beyond 0.6 m, the three drying beds act as three tempering zones (Figs. 7.3 and 7.4); the rate of moisture removed from the surface is zero. When the drying beds are decreased, as suggested in section 7.2, a smaller decrease in tempering length is recommended. The addition of a third tempering zone between the bottom drying stage and cooling stage should be considered. As indicated by Steffe et al. (1979), cooling the rice after two drying passes reduces the head yield by approximately two percentage points. Gustafson et al. (1982), indicated that tempering of corn before cooling also improves the efficiency of the drying process and maintains the product quality. The addition of a third tempering zone between the bottom drying stage and cooling stage in two- or three-stage CCF dryers should improve the drying efficiency and produce rice of higher quality (head yields). 7.4 Effect of Number of Stages Table 7.9 presents simulation runs for the two- and three-stage CCF dryers drying medium-grain rice from 23.5 to 13.0 and from 18.0 to 13.0 moisture content. The effect 237 Table 7.9 Simulations for the two- and three-stage concurrentflow dryers with medium-grain rough rice. Input/output variables Run No. and parameters l 2 3‘ 4 5 Grainflow rate (tons/hr/mZ) 2.3 2.7 1.9 2.2 2.6 Inlet moisture content (Zwb) 23.5 l8.0 23.5 l8.3 l8.0 l8 0 “"“r I Rs“"—T STAGE Bed depth (m) 0.86 0.86 0.86 0.86 0.86 0.86 Tempering length (m) 5.2 5.2 5.2 5.2 5.2 . .Inlet air temperature (C) 160.0 130.0 160.0 130.0 160.0 160.0 Airflow rate (m3/min/m2) 40.0 40.0 40.0 40.0 40.0 40.0 Maximum rice temperature (C) 57.2 63.4 62.4 66.8 57.3 63.3 Outlet rice temperature (C) 40.0 45.8 4l.6 46.9 42.6 45.2 Outlet air temperature (C) 40.0 45.8 4l.6 46.9 42.6 45. Outlet relative humidity (2) 76.l 57.3 72.8 53.8 58.l 52. Static pressure (kPa) 2.5 2.5 2.5 2.5 2.5 2. Power requirement (kW/m2) 3.3 3.3 3.3 3.3 3.3 3. Outlet moisture content (wa) 21.5 l6.2 20.8 I5.9 l6.5 l5. Fuel efficiency (kJ/kg H20) 5,235 4.422 4,696 4,068 7,023 6,02 SECOND STAGE Bed depth (m) l.l6 l.l6 - - l.l6 Tempering length (m) 5.2 5.2 - - 5.2 Inlet air temperature (C) l20.0 ll0.0 - - l20.0 Airflow rate (m3/min/m2) 35.0 35.0 - - 35.0 Maximum rice temperature (C) 59.2 60.6 - - 60.9 Outlet rice temperature (C) 4l.0 44.l - - 44.l Outlet air temperature (C) 4l.0 44.2 -. - 44.l Outlet relative humidity (x) 78.2 6l.5 - - 62.5 Static pressure (kPa) 2.8 2.8 - - 2.8 Power requirement (kW/m2) 3.2 3.2 - - 3.2 Outlet moisture content (2) l9.5 l4.8 - - I5.0 ‘ Fuel efficiency (kJ/kg H20) 3.394 4.272 - - 4,665 THIRD STAGE Bed depth (m) 1.3] 1.3] 1.31 1.3] 1.31 1.31 Inlet air temperature (C) 90.0 90.0 l20.0 ll0.0 90.0 ll0.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 35.0 35.0 35 0 Maximum rice temperature (C) 53.0 54.9 63.4 63.8 55.l 63 2 Outlet rice temperature (C) 38.2 4I.8 4I.l 45.2 41.4 44.3 Outlet air temperature (C) 38.2 4I.8 4l.l 45.3 41.4 44.3 Outlet relative humidity (3) 76.l 57.5 81.4 56.5 59.3 58. Static pressure (kPa) 3.l 3.l 3.l 3.l 3.l 3. Power requirement (kW/m2) 3.6 3.6 3.6 3.6 3.6 3. Outlet moisture content (2wb) l8.l l3.7 l8.3 l4.0 l3.9 l3. Fuel efficiency (kJ/kg H20) 3,602 4,320 3,45l 4,008 4,487 4.l Moisture out of COOLER (zwb) - 13.2 - 13.5 13.4 13. Percentage points removed (twb) 5.4 4.8 5.2 4.8 4.6 4. Dryer fuel efficiency (kJ/kg) 4,l5 3,92l 4,12 3,607 4,898 4.5 238 of double passing when drying from 23.5 to 13.0 percent moisture content is discussed in the next section. Runs No. l and 3 show that rice is dried from 23.5 to approximately 18.0 percent in three- and two-stage CCF dryers, respectively. The three-stage dryer employes inlet air temperatures of 160, 120, and 90 C in the top, middle, and bottom stages, respectively. The total moisture removed is 5.4 percentage points with a fuel efficiency of 4,153 kJ/kg of water removed. The maximum rice temperatures are 57.2, 59.2, and 53.0 C in the three stages; the outlet rice temperatures are 40.0, 41.0, and 38.2 C. The dryer capacity is 2.3 tons of dry matter per hour per square meter of bed area. The two-stage dryer employes inlet air temperatures of 160 and 120 C in the top and bottom stages. The total moisture removed is 5.2 percentage points with approximately the same fuel efficiency (4,124 kJ/kg). The maximum rice temperatures are increased (62.4 and 63.4 C) due to the decreased grainflow rate (1.9 tons/hr/m2) in order to remove the same percentage points of moisture. In order to avoid grain damage the inlet air temperatures have to be decreased in a two-stage CCF dryer. This results in a lower capacity and slightly better fuel efficiency. A similar trend is observed when drying lower moisture content rice (from 18.0 to 14.0 percent). 239 The counterflow cooling was designed for single pass drying of agricultural products such as corn (low grainflow rates). The counterflow cooler is underdesigned for drying rice, especially for the double pass system; besides, the resistance of rice to airflow is much higher than for corn allowing less air to flow through the drying bed. One alternative for increased cooling of rice in a double pass system is to increase the size of the fan motor of the cooling stage. Medium-grain rice can be dried in a three-stage CCF dryer (Run fio. 5, Table 7.9) from 18.0 to 13.4 percent at a fuel efficiency of 4,898 kJ/kg and a dryer capacity of 2.6 tons/hr/mz. The maximum kernel temperatures are 57.3, 60.9, and 55.1 C in the top, middle, and bottom stages. The two stage CCF dryer (Run No. 6, Table 7.9) has a better fuel efficiency (4,593 kJ/kg), lower capacity (2.5 tons/hr/mZ), and higher maximum kernel temperatures (63.3 and 63.2 C). Table 7.10 presents simulation runs for the two- and three-stage CCF dryers drying long-grain rice from 23.5 to 13.0 percent and from 18.0 to 13.0 percent moisture content. Since long-grain rice dries faster than medium-grain rice (section 7.2) there is less over-drying of the rice kernels. Runs No. 1, 3, 5, and 6 show that long-grain rice can be successfully dried in two- and 240 Table 7.10 Simulations for the two- and three-stage concurrentflow dryers with long-grain rough rice. J— Input/output variables Run No. and parameters l 2 3 4 5 Grainflow rate (tons/hr/m2) 2.7 3.3 2 2 2.6 3.1 Inlet moisture content (2wb) 23.5 18.0 i;} 5 18.; 18.0 18 0 FIRST STAGE Bed depth (m) 0.86 0.86 0.86 0.86 0.86 0.86 Tempering length (m) 5.2 5.2 5.2 5.2 5.2 . Inlet air temperature (C) 160.0 I30.0 160.0 130.0 160.0 I60. Airflow rate (m3/min/m2) 40.0 40.0 40.0 40.0 40.0 40. Maximum rice temperature (C) 52.8 56.4 57.9 62.2 52.6 58. Outlet rice temperature (C) 37.3 41.6 38.7 - 43.8 39.0 41. Outlet air temperature (C) 37.3 4I.6 38.7 43.8 39.0 41. Outlet relative humidity (2) 89.3 70.3 87.6 65.1 72.8 66. Static pressure (kPa) 2.1 2.1 2.1 2.1 2.1 . Power requirement (kW/m2) 2.8 2.8 2.8 2.8 2.8 . Outlet moisture content (wa) 21.5 16.3 20.7 15.7 16.6 15 FueI efficiency (kJ/kg H20) 4,661 3,854 3,967 3,300 6,143 4,96 SECOND STAGE Bed depth (m) 1.16 1.16 - - l.l6 Tempering length (m) 5.2 5.2 - - 5.2 Inlet air temperature (C) 120.0 ll0.0 - - 120.0 Airflow rate (m3/min/m2) 35.0 35.0 - - 35.0 Maximum rice temperature (C) 54.7 54.9 - - 55.8 Outlet rice temperature (C) 38.6 41.1 - - 41.2 Outlet air temperature (C) 38.6 41.1 - - 41.2 Outlet relative humidity (2) 89.9 72.1 - - 72.4 Static pressure (kPa) 2.5 2.5 - - 2.5 Power requirement (kW/m2) 2.9 2.9 - - 2.9 Outlet moisture content (Zwb) 19.6 14.9 - - 15.0 Fuel efficiency (kJ/kggflZO) 2,993 3,510 - - 3,766 THIRD STAGE Bed depth (m) 1.31 1.31 1.31 1 31 1.31 1.31 Inlet air temperature (C) 90.0 90.0 120.0 110.0 90.0 110.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 35.0 35.0 35.0 Maximum rice temperature (C) 49.7 50.8 58.8 59.4 51.3 58.1 Outlet rice temperature (C) 36.4 39.0 39.2 42.0 39.2 41.3 Outlet air temperature (C) 36.4 39.1 39.2 42.0 39.2 41.3 Outlet relative humidity (X) 84.6 68.9 88.7 69.5 7.6 69.0 Static pressure (kPa) 2.9 2.9 2.9 2.9 2.9 . Power requirement (kW/m2) 3.4 3.4 3.4 3.4 3.4 Outlet moisture content (zwb) 18.0 14.0 18.2 13.8 13.9 1 Fuel efficiency (kJ/kg H20) 2,805 4,173 2,928 3.327 3,686 3. Moisture out of COOLER (zwb) - 13.5 - 13.3 13.4 1 Percentage points removed (wa) 5.5 4.5 5.3 4.9 4.6 Dryer fuel efficiency (kJ/kg) 3,502 3,410 3,490 3,000 4,107 3, 5 241 three-stage dryers without grain damage, at a higher capacity, and considerable lower fuel efficiency than medium-grain rice varieties. 7.5 Effect of Multipassing The experimental and simulated data presented in Chapter Six show that high moisture content (over 24.0 percent) rice can not be dried in one pass. Excessive heating. of rice kernels and consequent moisture removal rates detereorate the rice quality. Lower moisture content rice (18.0 percent) can be dried in one pass in a two-stage dryer as shown in Run No. 6 (Tables 7.9 and 7.10). High moisture content rice over 24.0 percent is better dried in two drying passes. Runs No. 1 through 4 present double pass drying of medium-grain (Table 7.9) and long-grain rice (Table 7.10). Medium-grain rice is dried in two drying passes from 23.5 to 13.2 percent in a three-stage CCF dryer without affecting the rice quality provided that the grainflow rates are 2.3 and 2.7 tons/hr/mz for the first and second drying passes, respectively. Decreased inlet air temperatures (130 and 110 C) should be used in the second drying pass (Run No. 2) to avoid overheating of rice kernels. A fuel efficiency of 4,015 kJ/kg can be expected 242 (combined fuel efficiency of Runs No. l and 2, Table 7.9). Long-grain rice is dried in two drying passes from 23.5 to 13.5 percent in a three-stage CCF dryer without affecting the rice quality, provided that the grainflow rates are 2.7 and 3.3 tons/hr/mz for the first and second drying passes, respectively. A fuel efficiency of 3,452 kJ/kg can be expected (combined fuel efficiency of Runs No. l and 2, Table 7.10). The grainflow rates for the two-stage CCF dryer are 2.2 and 2.6 tons/hr/mz for the first and second drying pass; the fuel efficiency is 3,268 kJ/kg (combined fuel efficiency of Runs No. 3 and 4, Table 7.10). Medium-grain rice can be dried in one drying pass from 18.0 to 13.4 percent in a three-stage CCF dryer without damaging the rice quality at a grainflow rate of 2.6 tons/hr/mz. A fuel efficiency of 4,898 kJ/kg can be expected (Run No. 5, Table 7.9). For long-grain rice (Run No. 5, Table 7.10) the grainflow rate and fuel efficiency are 3.1 tons/hr/m2 and 4,107 kJ/kg, respectively; a grainflow rate of 2.5 tons/hr/m2 is used in a two-stage CCF dryer (Run No. 6, Table 6.10) with a slightly improved fuel efficiency (3,589 kJ/kg). Runs No. 3, 4, and 6 (Table 7.9) show that medium-grain rice reaches excessive temperatures in a two stage dryer. To maintain quality, reduced temperatures and grainflow rates should be used. Another alternative is to 243 dry 23.5 percent moisture content rice in three drying passes. Three-stage CCF dryers can be used successfully to dry medium-grain and long-grain rice in one or two drying passes at fuel efficiencies of about 4,000 kJ/kg and 3,500 kJ/kg, respectively. This is a remarkable gain as compared to crossflow dryers (four to six drying passes) and fuel efficiencies on the order of 7,000 kJ/kg of water removed. This is possible since each drying stage of a three-stage concurrentflow dryer can be viewed as one pass in a crossflow dryer. 7.6 Effect of Initial Moisture Content The inlet moisture content of the grain supplied by the farmer varies from load to load. This affects the performance of a CCF dryer (moisture removal rate and fuel efficiency). Runs No. l and 5 (Tables 7.9 and 7.10) show the effect of drying rice with an initial moisture content of 23.5 as compared to 18.0 percent. For medium-grain rice, under similar drying conditions, the fuel consumption is increased (higher energy consumption) from 4,153 kJ/kg of water removed (Run No. 1, Table 7.9) to 4,898 kJ/kg of water removed (Run No. 5, Table 7.9). For long-grain rice the fuel consumption is increased from 3,502 kJ/kg to 4,107 244 kJ/kg of water removed. Increasing the initial moisture content results in a reduction in the allowable maximum and outlet rice temperatures when drying seed rice in a CCF dryer. Also, it results in an increase in the allowable maximum and outlet rice temperatures? when high head yields are desirable. Under comparable drying conditions higher initial moisture content rice dried in a CCF dryer results in lower maximum and outlet rice temperatures than low initial moisture content rice. The initial moisture content will determine the number of drying passes (one or two passes, sections 7.4 and 7.7). 7.7 Effect of Inlet Air Temperature and Airflow Rate The effect of drying air temperature -on moisture removal, maximum kernel temperature, outlet kernel temperature, and energy efficiency in a one-stage (top stage, 0.86 m of bed depth) CCF dryer with medium-grain rice was investigated. The top stage is analyzed since the temperature stresses are higher. The standard conditions are listed in Table 7.1. An initial moisture content of 23.5 percent is used; the grainflow rate is 2.5 tons/hr/mz. The inlet air temperatures employed are 140, 160, and 245 180 C; the airflow rates are 30.0, 40.0, and 50.0 m3/min/m2. Fig. 7.7 shows that moisture removal increases as the temperature and airflow rate increase. The maximum rice temperature (Fig. 7.8) and the outlet rice temperature (Fig. 7.9) also increase as the inlet air temperature and airflow rate increase. The increases in moisture removal, maximum rice temperature, and oulet rice temperature are more pronounced at higher airflow rates (50.0 m3/min/m2). Fig. 7.10 shows the energy efficiency increasing (less energy consumption) with increasing temperatures and airflows. The energy efficiency at 160 C inlet air temperature and 40.0 m3/min/m2 is abonormally high. The explanation for this behavior is the shift in the equilibrium moisture content model (from 11.0 to 12.0 percent, dry basis) at 42.5 C and 60 percent relative humidity (the two models are discontinuous at 42.5 C). Figs. 7.7 through 7.10 show that there is a compromise between energy efficiency and rice temperatures (quality of the dried rice). Dalpasquale (1981) presented a sensitivity analysis for the MSU concurrentflow model when drying soybeans. The effects of drying variables on the concurrentflow dryer performance were also investigated by Walker (1978). Similar responses are expected from the concurrentflow rice drying model presented in this dissertation. Fig. Fig. 246 . Grainflow Rate = 2.5 tons/hr/m2 77'.I i . - I N 9.: Moisture removal, % w.b 0 3O 4O 3 2 SO Airflow rate, m /min/m 7.7 Moisture removal from medium-grain rough rice at different inlet air temperatures and airflow rates in the top stage of a three-stage CCF rice dryer. 2 A' i 7O Grainflow Rate = 2.5 tons/hr/m2 S p U " ’7 ‘ “ “T“ 180°C 0 . i 2‘ 6O _ 160°C 3 ..... ; +4 “ f a ......... .1 .o o 140 C o. ‘ ; E _ _ o ........ i +’ 50 ______ d) U ...... cr-I a 5 ..... ,5 4O 3 O O 30 4O 3 2 SO Airflow rate, m /min/m 7.8 Maximum medium-grain rough rice temperature at different inlet air temperatures and airflow rates in the top stage of a three-stage CCF rice dryer. 247 U o a; so I ‘ V I I t fl I 7*r.< '7—"‘l ('11 ...: I I E 4“ Grainflow Rate = 2.5 tons/hr/m2 i%n4: ~ :0 :3 45 IL: ’ ‘f ‘ ' J, 3 ° : 3.]; 180 c d.) * ‘“f.‘-",'°‘“‘" . 2‘40 ‘t . 0) t ‘“ acu;3‘ ;_ 3 35 ‘._.I._.;....3 3”- -: 3 i 3’ .3 i“ a) 30 1' i ‘— ' ' I-I ‘; , ; : ;_ H 31~- up :3 fl 1 <3 0 i V' O 30 4O 3 2 50 Airflow rate, m /min/m Fig. 7.9 Outlet medium-grain rough rice temperature at different inlet air temperatures and airflow rates in the top stage of a three-stage CCF rice dryer. Grainflow Rate = 2.5 tons/hr/m2 d in I 7,000 6,000 ........ 5,000 1;;;_. 2{.¥{1 ['1 4;: .. 'éfg§. I 160°C Dryer energy efficiency, kJ/kg H20 0 3O 40 SO Airflow rate, mS/min/m2 Fig. 7.10 Dryer energy efficiency with medium—grain rough rice at different inlet air temperatures and airflow rates in the top stage of a three- stage CCF rice dryer. 248 7.8 Operation of Concurrentflow Rice Dryers The combination of inlet air temperatures, airflow rates, and grainflow rates in order to achieve a target moisture content, is presented in this section. The drying of medium-grain rough rice from 18.0 - 23.5 percent to approximately 13.5 percent using one or two passes is presented. Standard conditions (Table 7.1) were used in the simulations. The recommendations are for the standard CCF dryer designs (dryer dimensions are given in Chapter Five). The counterflow cooler is verified to remove 0.5 percentage points of moisture. Table 7.11 presents dryer settings for a three-stage concurrentflow rice dryer with medium-grain rough rice using a double pass system. Medium-grain rice (Setting No. 1) should be dried from 23.5 to 13.5 percent moisture content in two drying passes. The inlet air temperatures for the first drying pass are 150, 130, and 100 C for the top, middle, and bottom stages, respectively. The temperatures in the second drying pass are 130, 110, and 100 C. The airflow rates in the first drying pass are 40.0, 35.0, and 35.0 m3/min/m2 in the top, middle, and bottom stages, respectively. The airflows in the second drying pass are 35.0, 35.0, and 35.0 m3/min/m2. Table 7.11 Dryer settings for a three-stage concurrentflow rice 249 dryer with medium-grain rough rice using a double pass system. I Setting No. 1 2 Drying pass first second first secondlfirst second Initial moisture content (wa) 23.5 18.1 23.5 18.1 21.0 16.7 Grainflow rate (tons/hr/mZ) 2.3 2.7 2.6 3.2 3.0 3.5 FIRST STAGE Inlet air temperature (C) 150.0 130.0 170.0 140.0 170.0 140.0 Airflow rate (m3/min/m2) 40.0 35.0 40.0 40.0 40.0 35.0 Outlet moisture content (zwb) 21.7 16.6 21.7 16.4 19.6 15.5 Maximum rice temperature (C) 54.6 59.8 56.4 63.2 54.1 59.7 Outlet rice temperature (C) 38.7 44.2 40.0 46.4 40.2 45.9 SECOND STAGE Inlet air temperature (C) 130.0 110.0 140.0 130.0 140.0 120.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 35.0 35.0 35.0 Outlet moisture content (wa) 19.6 15.2 19.8 15.0 18.0 14.4 Maximum rice temperature (C) 60.5 59.3 61.8 63.2 60.5 60.0 Outlet rice temperature (C) 41.3 43.6 42.3 46.2 43.7 45.9 THIRD STAGE Inlet air temperature (C) 100.0 100.0 120.0 110.0 100.0 100.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 35.0 35.0 35.0 Outlet moisture content (zwb) 18.1 14.0 18.1 13.9 16.7 13.4 Maximum rice temperature (C) 55.7 56.8 59.6 59.2 55.2 56.3 Outlet rice temperature (C) 39.3 42.6 41.7 44.7 41.1 44.2 COOLING STAGE Inlet air temperature (C) 20.0 20.0 20.0 Airflow rate (m3/min/m2) - 20.0 - 20.0 - 20.0 Final moisture content (Zwb) 18.1 13.5 18.1 13.4 16.7 12.9 Dryer energy efficiency (kJ/kg) 4,453 4,158 4,502 4,155 4,96 4,204 250 The grainflow rate is 2.3 and 2.7 tons of dry matter per hour per square meter for the first and second pass, respectively. Setting No. 2 uses higher inlet air temperatures and grainflow rates; this represents a 14 percent increase in capacity compared to Setting No. 1. Slightly higher kernel temperatures are expected in Setting No. 2. The expected dryer efficiency is 4,300 kJ/kg of water removed for Settings No. l and 2. Setting No. 3 is for medium-grain rice drying from 21.0 to 13.0 percent moisture content. Table 7.12 presents dryer settings for the one pass system. Rice is dried from 21.0, 20.0, and 18.0 to approximately 13.5 percent moisture content in one drying pass. Slightly lower energy efficiencies are expected especially at lower initial moisture content -(Settings No. 3 and 4). In Setting No. 4 rice is dried from 18.0 to 13.6 percent moisture content in one pass in a two-stage concurrentflow dryer. The energy efficiency is slightly better for the two-stage dryer than for the three-stage dryer (Setting No. 3) due to the decreased grainflow rate (from 2.7 to 2.2 tons/hr/m2). The recommendations in Table 7.11 and 7.12 are for medium-grain rough rice. As indicated in section 7.2, long-grain rice dries 22 percent faster than medium-grain rice. The grainflow rates in Table 7.11 and 7.12 should be 251 Table 7.12 Dryer settings for two- and three- stage concurrentflow rice dryers with medium-grain rough rice using a single pass system. Setting No. l 2 3 4 Initial moisture content (*wb) 21.0 20.0 18.0 18.0 Grainflow rate (tons/hr/mZ) 2.2 2.7 2.2 FIRST STAGE Inlet air temperature (C) 160.0 160.0 160.0 160.0 Airflow rate (m3/min/m2) 40.0 40.0 40.0 40.0 Outlet moisture content (zwb) 18.5 17.9 16.6 16.0 Maximum rice temperature (C) 62.6 60.6 ' 56.3 61.9 Outlet rice temperature (C) 43.4 43.0 42.4 44.7 SECQND STAGE Inlet air temperature (C) 120.0 120.0 120.0 - Airflow rate (m3/min/m2) 35. 0 35.0 35.0 - Outlet moisture content (wa) 16. 2 15.9 15.1 - Maximum rice temperature (C) 64. 6 63.1 61.5 ‘ Outlet rice temperature (C) 43. 8 43.8 44.4 THIRD STAGE Inlet air temperature (C) 110.0 110.0 100.0 115.0 Airflow rate (m3/min/m2) 35.0 35.0 35.0 35.0 Outlet moisture content (2wb) 14.2 14.2 13.9 14.1 Maximum rice temperature (C) 62.9 61. 7 57.4 63. 6 Outlet rice temperature (C) 44.1 43. 8 42.9 44. 6 COOLING STAGE Inlet air temperature (C) 20.0 20.0 20.0 20.0 Airflow rate (m3/min/m2) 20.0 20.0 20.0 20.0 Final moisture content (*wb) 13.7 13.7 13. 4 13.6 Dryer energy efficiency (kJ/kg) 4,246 4,529 5.128 4,838 252 increased by approximately 20 percent in order to achieve the same moisture removal. Energy efficiencies of 3,500 kJ/kg of water removed can be expected when drying long-grain rough rice. CHAPTER 8 SUMMARY 8.1 Concurrentflow Rice Drying 8.1.1 Simulation Model A multistage steady state MSU concurrentflow drying model was modified. The best available equations for equilibrium moisture content, convective heat transfer coefficient, and thin-layer drying (based on the diffusion theory of a composite sphere) were used. The physical and thermal property constants of rough rice required are: kernel equivalent radius, dry bulk density, specific surface area of the drying bed, and specific heat. 253 254 Simulated and experimental results for long- and medium-grain rice are compared. The model predicts the drying behavior with sufficient accuracy to perform a design analysis of the concurrentflow rice drying process and to make recommendations for the operation of concurrentflow rice dryers. 8.1.2 Quality Aspects of Dried Rice Head yield, color, and seed viability were chosen as the criteria for determining the quality aspects of rough and parboiled rice dried in concurrentflow and crossflow dryers. Experimental results showed medium-grain rice can be dried in a three-stage concurrentflow dryer at six to eight moisture percentage points in one pass at inlet air temperatures as high as 177 C without affecting the head yield and 'color of milled rice. Higher temperatures (150 to 200 C) and lower grain velocities (3.0 to 4.0 m/hr) cause an increase in the moisture removal (eight to ten percentage points) and a decrease in the head yield. Long-grain rice can be dried in a two-stage concurrentflow dryer at inlet air temperatures of 150 C in the top stage and 80 C in the bottom stage without affecting the white rice head yield. Inlet air temperatures of 138 C in the top stage and 79 C in the 255 bottom stage with a grain velocity of 8.8 m/hr does not affect the seed viability. The seed viability decreased by 18 percentage points when the temperature in the top stage was increased to 149 C and the grain velocity decreased to 7.3 m/hr under similar operating conditions. Long-grain parboiled rice was successfully dried in a three-stage concurrentflow dryer in one pass. The quality of the concurrentflow dried rice is excellent. The head yield is only three percentage points lower than the total milling yield and the color is consistently lighter than of rotary dried parboiled rice. The excellent quality of the dried rice in the multistage concurrentflow dryers is due to the gentle and uniform treatment of the rice. Even at 177 C, the rice never reaches temperatures above 65 C due to the short period of time (15 to 30 seconds) that the kernels are subjected to the high air temperatures. Simulation results showed that inlet air temperatures of 177, 116, and 88 C in the top, middle, and bottom stages of a concurrentflow dryer result in maximum medium-grain rice temperatures of 62, 60, and 52 C. The outlet temperatures in the top, middle, and bottom stages are approximately 40 C. Even at‘ such high kernel temperatures (for a very short period of time) the rice maintained its head yield. Other simulation results showed that kernel temperatures of 63, 73, and 67 C in the top, middle, and bottom stages, respectively, caused 256 a drastic reduction in head yield (22 percentage points). Maximum long-grain rice kernel temperatures of 55 and 50 C in a two-stage concurrentflow dryer did not affect the head yield of milled rice. A second reason for the superior quality of concurrently dried rice is the tempering process built into the dryer. The moisture gradients are equalized after one to three percentage points removal of moisture from the kernels. The tempering process also relieves the stresses imposed upon rice kernels during the drying process. The expected fuel efficiency of a three-stage concurrentflow dryer with medium-grain rough rice is 4,200 kJ/kg of water removed; a two-stage concurrentflow dryer with long-grain rough rice has a fuel efficiency on the order of 3,500 kJ/kg of water removed. Long-grain rice dries faster than medium-grain rice mainly due to its smaller equivalent radius. 8.1.3 Recommended Design Changes Computer simulation was used to investigate the leffects of decreased bed depths and shorter tempering sections in concurrentflow rice dryers. 257 The dryer capacity is increased by 20 percent by decreasing the bed depths to 0.6 meters at constant power requirements (increased airflows) and moisture removal for long- and medium-grain rough rice. An engineering sensitivity study is not able to assess the affect of bed depth change in the concurrentflow dryer on rice quality. However, it is believed that decreased bed 'depths will result in excellent rice quality since similar moisture removals, rice temperatures, and less retention time are observed as in the standard bed depths. The tempering of long- and medium-grain rough rice has little effect on the drying rate and the fuel efficiency in a concurrentflow dryer at tempering lengths over 3.0 meters. Long-grain rice dried in a deep-bed concurrentflow dryer has a fairly uniform moisture content distribution (e.g. 23.3 percent at the center and 16.2 percent at the surface of the kernel at the end of the first drying stage). This is due to the smaller equivalent radius of long-grain rice (0.001 m) compared to medium-grain rice (0.00156 m) and corn (0.00296 m). The tempering zone lengths of the concurrentflow rice dryers can be reduced from 5.2 meters to 3.0 meters without affecting the rice temperatures, drying fuel efficiency, and most importantly, the quality aspects of long- and medium-grain rice. A reason to decrease the tempering zone 258 lengths is that beyond 0.6 meters the drying beds of the standard concurrentflow rice dryers act as tempering zones. The addition of a third tempering zone between the third drying stage and cooling stage has the potential of improving rice quality and dryer fuel efficiency. 8.1.4 Operation of Concurrentflow Rice Dryers Combinations of inlet air temperatures, airflow rates, and grainflow rates in order to achieve target final moisture contents have been presented. Medium-grain rough rice can be dried in a three-stage concurrentflow dryer from 23.5 to 13.4 percent moisture content in two drying passes. - The dryer capacity is 1.45 tons of dry matter per hour per square meter of dryer cross-sectional area. The maximum kernel temperatures and outlet kernel temperatures are expected to be about 60 and 43 C, respectively. If the inlet moisture content of the rice is 21.0 percent or below the drying can be done in one pass. The dryer capacity is increased by 20 percent when drying long-grain rough rice; this is accomplished by increasing the grainflow rate. Increased grainflow rates at the same temperatures and airflow rates will result in lower kernel temperatures and better rice quality for long-grain varieties compared to medium-grain rice. 259 8.2 Crossflow and Fixed-Bed Drying Models Current versions of the Michigan State University (MSU) crossflow and fixed-bed grain drying models were modified to simulate rice drying. The physical and thermal properties of rough rice are the same as those in the concurrentflow model. An empirical thin-layer equation was used. Experimental results obtained with a conventional (non-mixing) crossflow dryer were compared with the simulated results; good agreement was obtained. Experimental results showed that an average moisture removal per drying pass of 1.4 percentage points was associated with 1.2 percentage points average reduction in head yield. The crossflow dried rice kept its relatively good quality due to the short retention time (18 minutes on the average) and low moisture removal rate per drying pass. The tempering period of 24 hours currently used in Texas appears .excessively since it requires excessive space (storage bins). A fuel efficiency of 5,000 kJ/kg of water removed was obtained during the testing of _a commercial crossflow dryer operating without air recycling or cooling. 260 The effects of inlet air temperature and airflow rate on drying time, energy efficiency, and moisture content gradient (difference between moisture content in the top and bottom layers) in a fixed-bed drying system was investigated. The simulated results compare well with those presented in the literature. 8.3 Comparison of Rice Drying Methods Fixed-bed drying systems present a low capacity and an excellent energy efficiency (1,500 to 3,000 kJ/kg of water removed). Temperature and relative humidity of the drying air must be controlled to avoid overdrying of the bottom layers (maintaining quality of the product). The use of microprocessors to control the temperature and relative humidity of the drying air will make this system more efficient, easier to manage, and thus, more competitive with other drying systems. Multipass crossflow dryers have a higher capacity and are more reliable but have decreased dryer efficiencies (about 5,000 kJ/kg of water removed) compared to fixed-bed drying systems. Crossflow dryers are limited to medium inlet air temperatures (up to 80 C). Best results for head yield and capacity are obtained in crossflow dryers when high inlet air temperatures coupled with high grainflow 261 rates are used (decreased retention time to approximately 18 minutes, thus increasing the number of drying passes). Even at higher grainflow rates there are temperature and moisture gradients across the drying columns in a crossflow dryer. Maximum kernel temperatures are (60 to 80 C) higher in crossflow as compared to concurrentflow dryers (50 to 60 C). Concurrentflow dryers can be operated at higher inlet air temperatures (150 to 180 C) than fixed-bed (ambient to 40 C) and crossflow dryers (up to 80 C) resulting in higher energy efficiencies (3,500 to 4,500 kJ/kg of water removed) without excessive grain damage (less than four percentage points reduction in head yield during the drying process). The drying operation in concurrentflow dryers can be performed in one or two passes compared to four to six passes for crossflow dryers. Concurrentflow dryers present the potential of producing the best quality grain due to the ability of icontrolling the moisture removal and rice temperature of each kernel in each drying stage. CHAPTER 9 CONCLUSIONS 1. A steady state concurrentflow rice drying model was developed and validated. The model was used to analyze the drying process and make design/operation recommendations for concurrentflow rice dryers for medium- and long-grain rough rice. 2. The quality aspects of the rice dried in concurrentflow dryers are excellent. Rough rice maintained head yield, color, and seed viability during the drying process. Parboiled rice presented high head yields (70 percent); with a color consistently lighter than of rice dried in rotary dryers. 3. The bed depths of the three drying stages of the standard design of concurrentflow dryers should be decreased to a uniform length of 0.6 meters; allowing the use of higher airflows and grainflow rates. A 20 percent 262 263 increase in dryer capacity will be gained without affecting the rice quality. The tempering zone lengths of concurrentflow rice dryers can be reduced from 5.2 meters to 3.0 meters without affecting the moisture removal, dryer efficiency, and quality aspects of long- and medium-grain rice. The capacity of concurrentflow dryers is 20 percent higher for long-grain rice than for medium-grain rice under similar operating conditions. 4. Several combinations of inlet air temperature, airflow, and grainflow rate in order to achieve target final moisture contents are recommended. Long- and medium-grain rough rice can be dried in three-stage concurrentflow dryers from 23.5 to 13.5 percent moisture content in two drying passes. The drying of 18.0 - 21.0 percent moisture content can be done in one drying pass. The drying of 15.0 - 18.0 percent moisture content rice can be done in a two-stage dryer. 5. Three multistage concurrentflow dryers were experimentally tested with long- and medium-grain rough and with long-grain parboiled rice. A conventional crossflow dryer was tested with long-grain rough rice. Concurrentflow dryers have a fuel efficiency of 3,500 to 4,200 kJ/kg of water removed and preserve the grain quality; higher system capacity can be expected when the drying and tempering operations are done in two drying 264 passes. Similar fuel efficiencies are obtained during rough and parboiled rice drying. The crossflow rice dryer presented an average fuel efficiency of 5,200 kJ/kg of water removed; on the average the head yield of the crossflow dried rice decreased from 54.8 to 52.1 percent when drying from 16.5 to 13.5 percent moisture content. 6. Good agreement is obtained when comparing the experimental and simulated results for crossflow and fixed-bed drying of rice. The modified MSU crossflow and fixed-bed drying models can be used to study the drying behavior in such dryers. Thus, the proper recommendations for the operation of crossflow and fixed-bed dryers can be obtained through computer simulation. 7. Fixed-bed drying systems have a low capacity, a good drying efficiency (1,500 to 3,000 kJ/kg of water removed) and result in variable product quality. Crossflow dryers have a higher drying capacity and a higher reliability and an increased energy consumption (5,000 kJ/kg of water removed) compared to fixed-bed drying systems. Concurrentflow dryers of rice have a higher capacity and a better energy efficiency than crossflow and fixed-bed dryers. Concurrentflow dryers have the potential of producing the best quality grain due to the potential of precisely controlling the moisture removal rate and rice temperatures of individual kernels in each drying stage. CHAPTER 1 O RECOMMENDATIONS FOR FURTHER RESEARCH 1. Rice physical and thermal properties: a) determination of pressure drop as a function of airflow rate (high airflow rates, 20 - 60 m3/min/m2), moisture content, percentage' of fines, rice type, and degree of packing. b) determination of convective heat transfer coefficient for a bed of long-, medium-, and short-grain rough rice (high airflow rates, 20 - 60 m3/min/m2). 2. Optimization of concurrentflow dryer design, with respect to grain quality, and energy efficiency: a) development of a quality model as a function of kernel temperature, retention time, and moisture removal rate. The number and pattern of kernel fissures for inlet and outlet samples of brown and milled rice should be considered. Modeling seed viability during' the drying 265 266 process is being developed in the United Kingdom (Nellist, 1981). b) development of an energy efficiency model as a function of the drying variables. 3. Development of an unsteady state concurrentflow drying model for optimum control studies. The problem is to obtain a constant outlet moisture content when the inlet moisture content of the grain supplied by the farmer varies from load to load. The objective is to use the minimum thermal energy per ton of dried grain and to obtain maximum product quality. Microprocessors can be used to control the moisture content (through temperature sensing) out of each drying state. 4. Development of a simulation model for concurrentflow drying of parboiled rice. The physical and thermal properties of parboiled rice (as discussed in section 3.2 for rough rice) should be determined. 5. The measurement of rice temperatures in the tempering zones of concurrentflow rice dryers should be conducted. This will determine if there is a decrease or an increase in the rice temperature as the rice passes through the tempering zone. L I S T O F R E F E R E N C E S Adair, C. R. 1972. Production and utilization of rice. In: Rice Chemistry and Technology. D. F. Houston (ed). 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Pfost. 1967. Adsorption and desorption of water vapor by cereal grains and their products. Transactions of the ASAE lO(4):545-551,555. Crank, J. 1974. The Mathematics of Diffusion. Oxford University Press, Ely House, London, W.l. Dalpasquale, V. A. 1981. Drying of soybeans in continuous-flow dryers and fixed-bed drying systems. Unpublished Ph.D. thesis. Agr. Eng. Dept., Michigan State University, East Lansing, MI. De Padua, D. B. 1976. Drying. In: Rice Postharvest Technology. E. V. Araullo, D. B. De Padua, and M. Graham (eds). International Development Research Centre, Ottowa, Canada. 270 Dealson, D. L. 1980. Recommendations for drying rough rice in Louisiana. Unpublished report. Cooperative Extension Service, Louisiana State University, Baton-Rouge, LA. Dorfman, E. and J. L. V. Rosa. 1980. Ponto de colheita e temperatura de secagem na qualidade do arroz. Lavoura arrozeira. Jan/Fev. In Portuguese. Evans, T. W. 1970. Simulation of counter-flow drying. Unpublished M.S. thesis. Agr. Eng. 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' Lavoura Arrozeira. 1982. Industrializacao do arroz. Jul/Agos. In Portuguese. Litchfield, J. B., M. R. Okos and M. Fortes. 1982. Design of corn drying systems to minimize kernel . stress. ASAE paper number 82-3552. ASAE, St. Joseph, MI. Lorenzen, R. T. 1958. The effect of moisture on weight-volume relationships of small grains. Paper, Slst Annual Convention ASAE, Santa Barbara, CA. Lu, J. J. and T. T. Chang. 1980. Rice in its temporal and spatial perspectives. In: Rice Production And Utilization. B. S. Luh (ed). AVI Publishing Co., Westport, CT. Misra, M. K. and D. B. Brooker. 1978. Thin-layer drying of shelled corn. ASAE paper number 78-3002. ASAE, St. Joseph, MI. Morita, T. and R. P. Singh. 1977. Physical and thermal properties of short-grain rough rice. ASAE paper number 77-3510. ASAE, St. Joseph, MI. illl‘l-IIIEAIIIIFIIIIIIIIIIIIIIIIJIIII llii'll'l‘l'ill}.."|'3r 272 Morse, M. D., J. H. Lindt, E. A. Oelke, M. D. Brandon, and R. G. Curley. 1967. The effect of grain moisture content at time of harvest on yield and milling quality of rice. Rice Journal. January, 16-20. Nellist, M. E. 1981. Predicting the viability of seeds dried with heated air. Seed Science and Technology 9(2):359-372. O'Callaghan, J. R., D. J. Menzies, and P. H. Bailey. 1971. Digital simulation of agricultural dryer performance. J. Agric. Engng. Res. 16:223. Overhultz, D. G., G. M. White, H. E. Hamilton, and I. J. Ross. 1973. Drying soybeans with heated air. Transactions of the ASAE 16(1):112-113. Ozeike, G. O. I. and L. Otten. 1981. Theoretical analysis of the tempering phase of a cyclic drying process. Transactions of the ASAE 24(6):1590-1594. Page, G. 1949. Factors influencing the maximum rates of air drying shelled corn in thin layers. M.S. thesis. Purdue University, Lafayette, IN. Paulsen, M. R. and T. L. Thompson. 1973. Drying analysis of grain sorghum. Transactions of the ASAE 16(3):537-540. Pfost, H. B., S. G. Mauer, D. S. Chung and G. A. 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In: Rice: Production And Utilization. B. S. Luh (ed). AVI Publishing Co., Westport, CT. Steffe, J. F. 1979. Moisture diffusion and tempering on the drying of rough rice. Unpublished Ph.D. thesis, Agr. Eng. Dept., University of California, Davis, CA. 274 Steffe, J. P., R. P. Singh and A. S. Bakshi. 1979. Influence of tempering time and cooling on rice milling yields and moisture removal. Transactions of the ASAE 21(2):361-366. Steffe, J. F. and R. P. Singh. 1980a. Liquid diffusivity of rough rice components. Transactions of the ASAE 23(3):?67-774,782. Steffe, J. F. and R. P. Singh. 1980b. Theoretical and practical aspects of rough rice tempering. Transactions of the ASAE 23(3):775-782. Steffe, J. F. and R. P. Singh. 1980c. Note on volumetric reduction of short grain rice during drying. Cereal Chemistry 57(2):148-150. Steffe, J. F. and R. P. Singh. 1982. Diffusion coefficients for predicting rice drying behavior. J. Agric. Engng. Res., 27:498-493. Thompson, T. L., G. H. Foster and R. M. Peart. 1969. Comparison of concurrent-flow, crossflow, and counterflow grain drying methods. USDA Mrktg. Res. Rep. 841. Thompson, T. F. 1972. Temporary storage of high-moisture shelled corn using continuous aeration. Transactions of the ASAE 15(2):333-337. Thompson, T. L., R. M. Peart and G. H. Foster. 1968. Mathematical simulation of corn dryin -- a new model. Transactions of the ASAE 11(4 :582. Thorne, B. and J. J. Kelly. 1980. A mathematical model for the rotary dryer. In: Drying'80. Developments In Drying. Vol. 1. A. S. Mujumdar (ed). McGraw Hill International Book Company, New York, NY. USDA. 1976. United States standards for rough and brown rice for processing and milling of rice. USDA, AMS, Washington, DC. USDA. 1982. Rice market news. 63(26):5. Veja. 1980. O campo conta com os lucros da supersafra. Veja No. 603. Editora Abril. 26 de marco. In Portuguese. 275 Vemuganti, G. R. and H. B. Pfost. 1980. Physical properties related to drying 20 food grains. ASAE paper number 80-3539. ASAE, St. Joseph, MI. Verma, L. R. 1982. Energy used in on-farm rice drying. ASAE paper number 82-3010. ASAE, St. Joseph, MI. Walker, L. P. 1978. Process analysis of a multistage concurrentflow rice dryer. Unpublished Ph.D. thesis. Agr. Eng. Dept., Michigan State University. East Lansing, MI. Wang, C. Y. 1978. Simulation of thin-layer and deep-bed drying of rough rice. Unpublished Ph.D. thesis. Agr. Eng. Dept., University of California, Davis, CA. . Wang, C. Y., T. R. Rumsey and R. P. Singh. 1979. Convective heat transfer coefficient in a pached bed of rice. ASAE paper number 79-3040. ASAE, St. Joseph, MI. Wang, C. Y. and R. P. Singh. 1978. A single layer drying equation for rough rice. ASAE paper number 78-3001. ASAE, St. Joseph, MI. Wang, C. Y. and R. P. Singh. 1978a. Computer aided simulation of rice drying. Paper presented at the 71th Annual Meeting of the American Institute of Chemical Engineers, Miami Beach, FL, Nov. 12-16. Wasserman, T. and D. L. Calderwood. 1972. Rough rice drying. In: Rice Chemistry And Technology. D. F. Houston (ed). American Association of Cereal Chemists, St. Paul, MN. Webb, B. D. and R. A. Stermer. 1972. Criteria of rice quality. In: Rice Chemistry And Technology. D. F. Houston (ed). American Association of Cereal Chemists, St. Paul, MN. Webb, B. D. 1975. Cooking, processing, and milling qualities of rice. In: Six Decades Of Rice Research In Texas. Texas Agr. Exp. Sta. Res. Monog. 4. Webb, B. D. 1980. Rice quality and grades. In: Rice: Production And Utilization. B. S. Luh (ed). AVI Publishing Co., Westport , CT. 276 White, G. M., T. C. Bridges, O. L. Lower and I. J. Ross. 1978. Seed coat damage in thin-layer drying of soybeans as affected by drying conditions. ASAE paper number 78-3052. ASAE, St. Joseph, MI. Willson, J. H. 1979. Rice in California. Butte County Rice Growers Association, Richvale, CA. Wratten, F. T., W. D. Poole, J. L. Chesness, S. Bal and V. Ramarao. 1969. Physical and thermal properties of rough rice. Transactions of the ASAE 12(6):801-803. Zuritz, C., R. P. Singh, S. M. Moini and S. M. Henderson. 1979. Desorption isothers of rough rice from 10 C to 40 C. Transactions of the ASAE 22(2):433-436. 277 APPENDIX A .30an own-Eco he ...-2!. ad .35 2o... .9. £838 :21 o 62 .m: up... 5 8.: ._ .82 .33: can: In... a... 3 >1!- .o.. 3. 3.5: 82F . 1.35.8. 818: .....2 33.... as... 38... 2:31. . 23:30 33 azocsfip ..o 35.338 E 3 .5 ”Coco .588 93aeo333o 3398588 ace 3.23 "953.. ..o .38 ..o :33:— 23 6.539: ..o 333.. 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S 8 ..... fl 62 .m... .5508 5 8...... ....3 3...: .... ......m a... .... .. ... ..... . ...z ...... 2.5..» 8... «:5... 8m. 58.—om 58.05 :_ .2552 :_ 25:52 a _ . :65: 58¢ a a: .58 5 5280...: 855255 3:959:55 M053 253 .5: £5on 30h. 526 55.2 5.60 £25 .....on voguv-.82 .5552 @0555 5:: $5358 5 3.53 not 3 ...oES. :55 n:- .vouc:5t.e8: .53 I... 5.5: :55qu 38.8... :2: new. 8:— _5=_2 15: 1:88 85—0 2.: .5. £::.::.:=ao¢ 25.5 .5: 5.5.5 Saga .83: 2.8... 3.5.5 3.. 8:... 2...... 2.8% v... 2...... 281 APPENDIX B Table 8.1 Conversion factors Quantity Units Multiply by To get Airflow rate m2/min/m2 3.2808 ft3/min/ft2 Area m2 10.7639 ft2 Convective heat transfer coefficient kJ/hr/mZ/C 0.0h89 BTU/hr/ftZ/F Density kg/m3 0.062h 1b/ft3 Diffusion coefficient m2/hr 10.7639 ft2/hr Energy efficiency kJ/kg 0.h299 BTU/1b Grainflow rate kg/hr/mZ 0.20h8 lb/hr/ftZ Latent heat of vaporization kJ/kg 0.h299 BTU/lb Length m 3.2808 ft Mass kg 2.20h6 lb metric ton 2,20h.6 1b Power kw l.3h10 HP Specific heat kJ/kg/C 0.2388 BTU/lb/F Specific surface area m2/m3 0.30h8 ft2/ft3 Static pressure kPa b.0186 in. H20 Temperature difference C 1.8 F Thermal conductivity W/m/C 0.5778 BTU/hr/ft/F Thermal diffusivity m2/hr 10.7639 ft2/hr Velocity m/hr 3.2808 ft/hr Volume m3 35.3lh7 ft3 282 APPENDIX C Sample Run for the Three-Stage Concurrentflow Rice Dryer 0.0 = OUTPUT, 1. = INPUT-OUTPUT, 2. = DETAILEDI. UNIT TYPE(1=SI,2=ENGLISH)2. 2 GRAIN TYPE (0=STOP,1=LONG,2=MEDIUM,3=SHORT)2. 2 SIMULATE A CONCURRENTFLOW DRYER USING THE STEFFE DIFFUSION EQUATION FOR MGRICE WITH INPUT IN ENGLISH UNITS INPUT CONDITIONS: AMBIENT TEMP, F58. 58.0000 INLET MOISTURE CONTENT, WET BASIS PERCENT25.54 25.5400 GRAIN TEMPERATURE, F76. 76.0000 STAGE 1 INPUT CONDITIONS: STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. I INLET AIR TEMP, F275. 275.0000 INLET ABS HUM RATI0,009 .0090 AIRFLOW RATE, CFM/FT2 (AT AMBIENT CONDITIONS)120. 120.0000 GRAIN FLOW RATE, BU/HR/FT29.95 9.9500 DRYER LENGTH, FT3.6 . 3.6000 OUTPUT INTERVAL, FT.5 .5000 TEMPERING LENGTH, FT17. 17.0000 283 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0045 AIRFLOW RATE LB/HR/FT2 529.3 CFM/FT2 HEAT TRANSFER COEF, BTU/HRFTZF 9.579 EQUIL MC, WB PERCENT .01 DRY BASIS, DECIMAL .0001 INLET MC, DRY BASIS DECIMAL .3430 GRAIN VELOCITY, FT/HR 12.38 LB/HR/FT2 427.93 AT TIN 170.3 DEPTH TIME AIR ABS REL GRAIN MC MC TEMP HUM HUM TEMP WB DB FT HR F LB/LB DECIMAL F PERCENT DECIMAL 0.0000 0.0000 275.0 .0090 .0045 76.0 25.54 .3430 .5036 .0407 113.7 .0234 .3668 113.4 24.26 .3204 1.0023 .0810 106.8 .0294 .5562 106.7 24.02 .3162 1.5037 .1215 103.3 .0323 .6743 103.3 23.90 .3140 2.0024 .1618 101.6 .0338 .7426 101.5 23.81 .3124 2.5221 .2038 99.9 .0353 .8131 99.8 23.74 .3114 3.0024 .2426 98.6 .0364 .8690 98.6 23.72 .3110 3.5038 .2831 97.8 .0371 .9097 97.7 23.71 .3107 3.6002 .2908 97.7 .0372 .9142 97.7 23.70 .3107 THE MAX. GRAIN TEMP. IS 127.62929 THIS HAPPENS ATlA s. STATIC PRESSURE, IN OF H20 11.19 HORSEPOWER, HP/FT2 .42 ENERGY INPUTS, BTU/LB FAN (.5 EFF) 2.4 HEAT AIR 66.0 MOVE GRAIN 0.0 TOTAL 68.4 WATER REMOVED, LB/LB .0323 BTU/LB H20 2117.49 284 STAGE 2 INPUT CONDITIONS: . STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. 1 INLET AIR TEMP, F225. 225.0000 INLET ABS HUM RATI0.009 .0090 AIRFLOW RATE, CFM/FT2 (AT AMBIENT CONDITIONS)124. 124.0000 GRAIN FLOW RATE, BU/HR/FT29.95 9.9500 DRYER LENGTH, FT4. 4.0000 OUTPUT INTERVAL, FT.5 .5000 TEMPERING LENGTH, FT17. 17.0000 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0108 AIRFLOW RATE LB/HR/FT2 546.9 HEAT TRANSFER COEF, BTU/HRFTZF 9.996 EQUIL MC, WB PERCENT .08 DRY BASIS, DECIMAL .0008 INLET MC, DRY BASIS DECIMAL .3107 GRAIN VELOCITY, FT/HR 12.38 LB/HR/FTZ 427.93 CFM/FTZ AT TIN 164.0 DEPTH TIME AIR ABS REL GRAIN MC MC TEMP HUM HUM TEMP WB DB FT HR F LB/LB DECIMAL F PERCENT DECIMAL 0.0000 0.0000 225.0 .0090 .0108 97.7 23.70 .3107 .5233 .0423 119.3 .0214 .2868 119.1 22.33 .2875 1.0010 .0809 112.1 .0274 .4462 112.0 22.08 .2834 1.5024 .1214 107.8 .0309 .5667 107.7 21.93 .2809 2.0063 .1621 105.3 .0329 .6488 105.2 21.83 .2792 2.5103 .2028 103.4 .0345 .7159 103.3 21.75 .2780 3.0064 .2429 101.5 .0360 .7889 101.5 21.72 .2774 3.5077 .2834 100.7 .0367 .8236 100.7 21.69 .2769 4.0117 .3241 99.7 .0375 .8648 99.7 21.66 .2765 THE MAX. GRAIN TEMP. IS 131.90061 THIS HAPPENS AT LBS 285 STATIC PRESSURE, IN OF H20 13.05 HORSEPOWER, HP/FTZ .51 ENERGY INPUTS, BTU/LB FAN (.5 EFF) 2.9 HEAT AIR 52.5 MOVE GRAIN 0.0 TOTAL 123.8 WATER REMOVED, LB/LB .0665 BTU/LB H20 1861.04 STAGE 3 INPUT CONDITIONS: STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. 1 INLET AIR TEMP, F180. 180.0000 INLET ABS HUM RATI0.009 .0090 AIRFLOW RATE, CFM/FT2 (AT AMBIENT CONDITIONS)95. 95.0000 GRAIN FLOW RATE, BU/HR/FT29.95 9.9500 DRYER LENGTH, FT4.6 4.6000 OUTPUT INTERVAL, FT.5 .5000 TEMPERING LENGTH, FTO. 0.0000 286 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0272 AIRFLOW RATE LB/HR/FTZ 419.0 CFM/FT2 HEAT TRANSFER COEF, BTU/HRFTZF 7.070 EQUIL MC, WB PERCENT 1.82 DRY BASIS, DECIMAL .0185 INLET MC, DRY BASIS DECIMAL .2765 GRAIN VELOCITY, FT/HR 12.38 LB/HR/FTZ 427.93 AT TIN 117.4 DEPTH TIME AIR ABS REL GRAIN MC MC TEMP HUM HUM TEMP WB DB FT HR F LB/LB DECIMAL F PERCENT DECIMAL 0.0000 0.0000 180.0 .0090 .0272 99.7 21.66 .2765 .5187 .0419 105.8 .0221 .4355 105.7 20.83 .2630 1.0017 .0809 101.5 .0264 .5872 101.3 20.70 .2611 1.5083 .1218 99.6 .0281 .6593 99.6 20.62 .2598 2.0044 .1619 97.7 .0299 .7406 97.7 20.58 .2592 2.5005 .2020 96.5 .0312 .8018 96.3 20.56 .2588 3.0018 .2425 95.9 .0316 .8270 95.9 20.55 .2586 3.5005 .2828 95.4 .0321 .8524 95.4 20.54 .2584w 4.0045 .3235 94.8 .0327 .8821 94.8 20.53 .2583 4.5084 .3642 94.7 .0328 .8899 94.7 20.52 .2582 4.6002 .3716 94.6 .0329 .8923 94.6 20.52 .2582 THE MAx. GRAIN TEMP. 18 117.18358 THIS HAPPENS AT 10 s STATIC PRESSURE, IN or H20 10.14 ’“ ’ -»« HORSEPOWER, HP/FT2 .30 ENERGY INPUTS, BTU/LB FAN (.5 EFF) 1.7 HEAT AIR 29.4 MOVE GRAIN 0.0 TOTAL 154.9 WATER REMOVED, LB/LB .0848 BTU/LB H20 1826.70 287 0.0 = OUTPUT, 1. = INPUT-OUTPUT, 2. = DETAILEDI. UNIT TYPE(1=SI,2=ENGLISH)1. l GRAIN TYPE (0=STOP,1=LONG,2=MEDIUM,3=SHORT)2. 2 SIMULATE A CONCURRENTFLOW DRYER . USING THE STEFFE DIFFUSION EQUATION FOR MGRICE WITH INPUT IN SI UNITS INPUT CONDITIONS: AMBIENT TEMP, C14. 14.0000 INLET MOISTURE CONTENT, WET BASIS PERCENT25.54 25.5400 GRAIN TEMPERATURE, C24. 24.0000 STAGE 1 INPUT CONDITIONS: STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. 1 INLET AIR TEMP, C135. 135.0000 INLET ABS HUM RATI0.009 .0090 AIRFLOW RATE, M3/M2/MIN(AT AMBIENT CONDITIONS)36.5 . 36.5000 GRAIN FLOW RATE, MTON/HR/M22.09 2.0900 DRYER LENGTH, M1.1 1.1000 OUTPUT INTERVAL, M.1 .1000 TEMPERING LENGTH, M5.2 ' 5.2000 288 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0045 AIRFLOW RATE KG/HR/MZ 2583.0 M3/M2/MIN AT TIN 51.9 HEAT TRANSFER COEF, KJ/HR/M2/C 195.750 EQUIL MC, WB PERCENT .01 DRY BASIS, DECIMAL .0001 INLET MC, DRY BASIS DECIMAL .3430 GRAIN VELOCITY,‘M/HR 3.77 KG/HR/MZ 2090.00 ABS DEPTH TIME AIR REL GRAIN MC MC TEMP HUM HUM TEMP WB DB M HR C KG/KG DECIMAL C PERCENT DECIMAL 0.0000 0.0000 135.0 .0090 .0045 24.0 25.54 .3430 .1007 .0267 47.4 .0198 .2819 47.1 24.43 .3233 .2071 .0549 43.7 .0255 .4344 43.6 24.17 .3188 .3015 .0799 41.4 .0291 .5559 41.3 24.05 .3166 .4079 .1081 40.1 .0311 .6370 40.0 23.95 .3149 .5007 .1327 39.2 .0326 .6972 39.1 23.89 .3139 .6015 .1594 38.5 .0335 .7409 38.5 23.83 .3129 .7023 .1861 37.9 .0345 .7862 37.9 23.79 .3121 .8007 .2122 37.3 .0355 .8385 37.2 23.77 .3118 .9015 .2389 36.9 .0360 .8663 36.9 23.75 .3114 1.0015 .2654 36.5 .0367 .9022 36.5 23.74 .3113 1.1000 .2915 36.3 .0369 .9151 36.3 23.73 .3111 THE MAX. GRAIN TEMP. IS 52.84864 THIS HAPPENS AT 145 STATIC PRESSURE, CM OF H20 28.41 HORSEPOWER, HP/M2 4.54 ENERGY INPUTS, KJ/KG FAN (.5 EFF) 5.5 HEAT AIR 154.0 MOVE GRAIN 0.0 TOTAL 159.6 WATER REMOVED, KG/KG .0319 KJ/KG H20 5003.08 289 STAGE 2 INPUT CONDITIONS: STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. 1 INLET AIR TEMP, C107. 107.0000 INLET ABS HUM RATI0.009 .0090 AIRFLOW RATE, M3/M2/MIN(AT AMBIENT CONDITIONS)37.8 37.8000 GRAIN FLOW RATE, MTON/HR/M22.09 2.0900 DRYER LENGTH, M1.2 1.2000 OUTPUT INTERVAL, M.1 .1000 TEMPERING LENGTH, M5.2 5.2000 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0109 AIRFLOW RATE KG/HR/MZ 2674.9 M3/M2/MIN AT TIN 50.0 HEAT TRANSFER COEF, KJ/HR/MZ/C 204.861 EQUIL MC, WB PERCENT .10 DRY BASIS, DECIMAL .0010 INLET MC, DRY BASIS DECIMAL .3111 GRAIN VELOCITY, M/HR 3.77 KG/HR/MZ 2090.00 DEPTH TIME AIR ABS REL GRAIN MC MC TEMP- HUM HUM TEMP WB DB M HR C KG/KG DECIMAL C PERCENT DECIMAL 0.0000 0.0000 107.0 .0090 .0109 36.3 23.73 .3111 .1003 .0266 50.9 .0178 .2134 50.6 22.55 .2912 .2003 .0531 46.5 .0242 .3585 46.3 22.28 .2866 .3003 .0796 44.4 .0272 .4449 44.4 22.12 .2841 .4011 .1063 42.8 .0296 .5251 42.8 22.01 .2823 .5067 .1343 41.5 .0315 .5950 41.5 21.92 .2808 .6003 .1591 40.7 .0328 .6470 40.6 21.86 .2798 .7043 .1866 40.0 .0337 .6874 40.0 21.81 .2789 .8003 .2121 39.4 .0347 .7314 39.3 21.77 .2783 .9019 .2390 38.6 .0358 .7855 38.6 21.75 .2780 1.0011 .2653 38.3 .0361 .8065 38.4 21.73 .2777 1.1027 .2922 38.0 .0367 .8323 38.0 21.71 .2773 1.2019 .3185 37.7 .0371 .8521 37.7 21.70 .2771 THE MAM. GRAIN TEMP. 13 55.34846 THIS HAPPENS AT13 S 290 STATIC PRESSURE, CM OF H20 32.63 HORSEPOWER, HP/M2 5.40 ENERGY INPUTS, KJ/KG FAN (.5 EFF) 6.6 HEAT AIR 122.6 MOVE GRAIN 0.0 TOTAL 288.7 WATER REMOVED, KG/KG .0659 KJ/KG H20 4379.85 STAGE 3 INPUT CONDITIONS: STAGE TYPE (0=NEW ANALYSIS,1=CONCURRENT)1. 1 INLET AIR TEMP, C82. 82.0000 INLET ABS HUM RATI0.009 .0090 AIRFLOW RATE, M3/M2/MIN(AT AMBIENT CONDITIONS)29. 29.0000 GRAIN FLOW RATE, MTON/HR/M22.09 2.0900 DRYER LENGTH, M1.4 1.4000 OUTPUT INTERVAL, M.1 .1000 TEMPERING LENGTH, M0. 0.0000 291 PRELIMINARY CALCULATED VALUES REL HUM, DECIMAL .0274 AIRFLOW RATE KG/HR/MZ HEAT TRANSFER COEF, KJ/HR/M2/C 145.157 EQUIL MC, WB PERCENT 1.85 DRY BASIS, DECIMAL .0188 INLET MC, DRY BASIS DECIMAL .2771 GRAIN VELOCITY, M/HR 2052.2 M3/M2/MIN AT TIN 35.9 3.77 KG/HR/MZ 2090.00 DEPTH TIME AIR ABS REL GRAIN MC MC TEMP HUM HUM TEMP WB DB M HR C KG/KG DECIMAL C PERCENT DECIMAL 0.0000 0.0000 82.0 .0090 .0274 37.7 21.70 .2771 .1005 .0266 42.7 .0193 .3496 42.5 20.96 .2652 .2013 .0533 40.1 .0237 .4881 40.1 20.81 .2628 .3013 .0798 38.7 .0261 .5792 38.7 20.73 .2616 .4013 .1063 37.9 .0275 .6360 37.9 20.68 .2607 .5029 .1333 37.3 .0287 .6833 37.2 20.64 .2601 .6013 .1593 36.5 .0299 .7401 36.5 20.62 .2597 .7005 .1856 36.2 .0304 .7667 36.2 20.60 .2594 .8029 .2128 35.7 .0313 .8087 35.7 20.59 .2593 .9005 .2386 35.5 .0316 .8257 35.5 20.58 .2591 1.0013 .2653 35.3 .0320 .8446 35.3 20.57 .2590 1.1005 .2916 35.2 .0323 .8582 35.2 20.57 .2589 1.2093 .3205 35.1 .0325 .8689 35.0 20.56 .2588 1.3021 .3451 34.9 .0327 .8846 34.9 20.56 .2588 1.4000 .3710 34.8 .0329 .8933 34.8 20.55 .2587 THE MAX. GRAIN TEMP. IS 47.37366 THIS HAPPENS.NTJD S. STATIC PRESSURE, CM OF H20 25.78 HORSEPOWER, HP/M2 3.27 ENERGY INPUTS, KJ/KG FAN (.5 EFF) 4.0 HEAT AIR 68.8 MOVE GRAIN 0.0 TOTAL . 361.5 WATER REMOVED, KG/KG .0843 KJ/KG H2O 4288.88 '-'- J- 155 Table 6.5 Bulk density and milling yield for the successful tests(a) conducted with the three-stage CCF rice dryer at Williams, CA, and with medium-grain rough rice. Rough Rough Brown Brown White White rice rice rice rice rice rice Test bulk moisture total head total head No. density content yield yield yield yield (kg/m3) (X wb) (X) (2) (2) (2) I IN 558 12.5 79.9 74.9 66.4 52.6 1 OUT 581 12.8 80.7 74.4 65.8 48.5 2 IN 573 13.0 80.4 75.6 65.1 48.3 2 OUT 576 12.2 81.2 73.6 65.h 49.7 3 IN 574 11.7 81.1 71.7 66.8 47.2 3 OUT 582 11.7 81.5 71.8 67.5 45.9 A IN 577 12.2 80.9 74.5 65.1 45.8 4 OUT 576 11.2 80.5 74.1 65.8 47.0 5 IN 561 11.3 80.4 73.4 65.4 40.8 5 OUT 57h 11.1 80.6 71.6 67.9 47.7 6 IN 568 11.9 80.5 69.8 66.8 48.6 6 OUT 573 11.7 81.1 69.4 68.1 45.9 7 IN 572 12.0 81.2 69.1 66.4 45.1 7 OUT 576 12.0 81.5 70.2 66.9 45.9 8 IN 554 12.0 80.6 75.7 65.6 49.0 8 OUT 565 12.1 81.4 73.9 67.7 50.9 9 IN 583 12.5 82.3 75.7 66.4 50.5 9 OUT 570 12.0 80.7 73.6 65.7 49.2 10 IN 560 13.0 80.4 75.2 66.1 43.2 10 OUT 563 12.0 80.2 74.2 66.7 47.6 11 IN 563 12.0 80.2 74.2 66.7 47.6 11 OUT 576 12.5 81.1 71.6 67.0 45.4 12 IN 579 12.8 81.4 78.0 65.7 52.1 12 OUT - 12.5 81.6 77.2 66.8 54.8 13 IN 574 12.5 80.7 75.1 65.8 50.5 13 OUT 572 12.5 80.4 75.5 65.9 49.6 (a) A successful test is defined as one in which the decrease in head yield is less or equal to four percent. The bulk density was not measured. 156 12.0 percent moisture content (a slight increase in the bulk density was observed during the drying process). The average brown rice total yield for the inlet and outlet samples was 80.8 and 81.0 percent, respectively; the average brown rice head yield was 74.1 and 73.2 percent. The average white (milled) rice total yield for the inlet and outlet samples was 66.0 and 66.7 percent. The average white rice head yield was 47.8 and 48.3 percent. On the average there was a slight increase in the head yield of the milled outlet samples (0.5 percentage points). Table 6.6 presents data, similar to Table 6.5, for the unsuccessful tests. The average bulk density for the inlet and outlet samples was 571 kg/m3 at 12.5 % moisture content and 582 kg/m3 at 12.6 % moisture content, respectively. The average brown rice total yield for the inlet and outlet samples remained unchanged at 80.7 percent; the average head yield decreased from 75.5 percent (inlet samples) to 69.8 percent (outlet samples). The white rice total yield decreased from 66.5 to 65.3 percent. The white rice head yield decreased by 11.9 percentage points (from 51.0 % to 39.1 %). The white rice head yield of the inlet samples (Tables 6.5 and 6.6) was considerably lower than those for an average year (Table 3.22). The low head yields of the inlet and outlet rice samples resulted from poor quality of the undried rice (high number of kernel fissures). The "IIIITIIIIIIIIIIIIIIIII