MflwwW—g (A .— *r’ll } l 1 Mll'n‘lfll \ HIWI M I t r \ 1 « 11-40); 10300 H yap-w- A STUDY OF THE MECHAMSM 0:? QRWFM’G SAND ON A HEATED SURFACE Thesis for the Degree cf M. S. WCHlGAN STATE COLLEGE Thomas Noel Tambiing 19953 TV Yr? 11W 1“ RIES IlJrlHulWWWWW!:Illul('il 3 1293 01774 9635 LIBRARY P Michigan State University This is to certifg that the thesis entitled A Study of tue Mecuanism of Drying Sand on a deated Surface presented liig Thomas Noel Tambling has been accepted towards fulfillment of the requirements for HasLerLs__deqnm hL_Chemical Engineering {1‘ f4 “i ,- / Major professor [Date August lit; 1953 -CM—.A PLACE IN REI'URN Box to remove this chedtout from your record. To AVOID FINE return on or before date due. MAY BE RECAUE with earIier due date ifrequested. DATE DUE DATE DUE. m A STUDY OF THE l-‘IJSli'LJISI-I OF DEX IN G SAID ON 2i. HEATED SUIfi‘ACE BY TriOl-L‘LS 1? 3L 'l‘gh-LBLING A THJJS IS Submitted to the School of Graduate Studies of Michigan State College of.Agriculture and.Applied Science in partial fulfillment of the requirements for the degree of. lvh'tSTER 0]? SC BEIGE Department of Chemical Engineering 1953 ACKIIO‘.‘TLEDG1~BN‘I‘S The author wishes to express his sincere thanks to Dr. Randall W. Ludt, under whose guidance, help, and consideration this investigation was undertaken. He also wishes to thank Mr. William Clippinger for his helo with mechanical details, and thanks also to the author's wife for her moral su920rt and help. 1‘ ‘ 5‘ ’ 5/“ .3; thumb II. III. IV. VI. VII. TABLE OF CONTEXT IHTRODUCTIOE. ‘4“ 331-3132;; 91003 J33. COECLUSIOH, BIBLIOGRAPHY. 55 57 58 IHTRODUCTION The purpose of this study lies in an attemot to secure a better understanding of the mechanism of drying on a heated surface. This paper has been limited to the investigation of the factors and mechanism involved in the process of drying sand on a heated plate. Drying itself is perhaps one of the oldest unit processes used by man. He has used it in the drying of hides, foods. clay- ware, etc.. throughout the ages. Although drying has a long history, serious studies into the mechanism of drying have been left to recent years. as a process in our oresent day world, it occurs in many of our chemical and industrial onerations. Drying is a princioal operation in the manufacture of rayon, celloohane, pigments, dyes, insecticides, fine chemicals. textiles, leather, wood products, ceramics, dried foods. soap, and paper (5). As a principal Opera- tion or some part of a process, drying can be an appreciable fraction of the total investment and processing cost. Many studies have been advanced in an effort to determine the mechanism and controlling factors involved in drying in a hope of better understanding. Emjirical equations (8), diffusion equations (5, 14), vapor pressure equations (6), suction equations (2, 7, 8), and/or caoillary flow equations (2, 7) are used in an attempt to use known data in the development of driers or to determine drying conditions. A consideration of the many different properties of solids should lead one to realize that no one single mechanism could be used to predict the movement of moisture and vapor in a solid during the drying process. To date most of the work done towards determining the mechanism of drying has been done in the field of air drying (6). Some work has been done in the field of infra—red, dielectric, and heated surfaces Crying. These last three methods of dryin do not seem to have been covered to the extent as has air drying. 5036 work on the drying mechanism has been published or written in the field of drying paper pulp on heated surfaces (6, 12). Lewis, McAdams, and Adams (4) analyzed data obtained on the drying of paper oulo on drum driers. Sherwood (IO) Presented a means of calculation of the influence of heat inflow through nondrying faces on the rate of drying. Stacey (13) and Sherwood and Cummings (11) have resorted and discussed data on the drying of various materials in pans, a method which is similar to drying on.heated surfaces. But little work seems to have been published dealing with the mechanism of drying granular solids on heated surfaces. Kirschbaum (3) has presented material on the theoretical principles of evaporative drying with heated surfaces. He developed basic equations and related them to the Mollier diagram of entha 9y versus moisture content. Further analysis of his work has not been presented for the lack of a translation of the article. McCready (6) has pro;osed two general cases for the drying of a wet slab on a heated surface. Case I A continuously decreasing temperature gradient extends from the hot surface, through the slab to the drying air. Case II The tenqerature gradient between the hot surface and the drying air gasses through a minimum t m)- erature in the slab and increases to the temqerature of the hot surface and drying air. Exoerimentally he has dried pager gull under the conditions of Case I and Case II. Under Case I drying, during the time of constant drying, local tenierature in the slab remained constant until a critical moisture content was aoiroached, and then the temperatures de- creased. In the falling rate period, a zone of vaporization which was originally formed beneath the closed face of the slab advanced toward the Open face. Between this zone of vaporization and the Open face, there was a free water zone. In the first part of the falling rate, the temperatures in the slab decreased with a de- creasing temperature gradient, and the temaerature difference between the hot surface and the slab increased. In the latter part of the falling rate period, there was an increasing temqera— ture difference across the zone of vagorization with a decreasing temperature difference between the hot plate and the interface. In the zone of free water the temperature difference decreased. Under Case II drying. the drying process was similar to that in case I except that there were two zones of vagorization: one starting at the closed face and the other starting at the open face of the slab. These two zones of vaporization moved towards each other throughout the falling rate period. McCready also made a study on influencing factors and con- cluded that the factors that have the greatest influence on the rate of drying of pager are the temperature of the hot surface and the thickness of the oaoer. The temgerature of the drying air has little effect on the rate of drying, as most of the heat required for the drying process is sup1lied from the hot surface. Changing the relative humidity of the air has little effect on the rate of drying oroviding the difference between the hot surface and the drying air temoeratures is large. Pearse, Oliver, and Hewitt (8) agree that during the con- stant rate period drying takes place at the exposed surface and is a function of the diffusion of water vapor through the air film. They apgly emgirical relationships which are found to hold for the evaporation of water from free surfaces. They hold that the forces which are likely to affect the movement of free water in a solid are those due to gravity, friction, convection, and capillarity. The relative magnitude of these forces degends upon the structure of the bed and of its comgonent parts, and ugon any factors which may cause a change in the ghysical groperties of the water such as temperature. Also, if the bed is exoosed to a temgerature gradient, movement of water by distillation may occur. Any unbalanced gressures would in effect cause a movement of water. In a continuation of Ceaglske and Hougen's work (2), Pearse, Oliver, and Newitt have summarized the following conclusions as to particle size and controlling forces as a result of their ex- perimental work in air drying: Particle Size (cm.) Forces Controlling 10‘l Gravitational and Capillary. 10-2 Gravitational and Capillary. 10:? Capillary. 10 + Capillary. 10-5 Caoillary and Frictional. 10"6 Capillary and Frictional. While the work of Pearse, Oliver, and Hewitt (8) is more applicable to relatively coarse-granular solids; Oliver and Hewitt (7) continued the work with beds composed of fine particles. They came to the conclusion from the work with fine and relatively coarse granular solids composed of non-porous particles that the moisture movement is governed by the structure of the bed. There is a distinction of beds into two categories, namely (1) beds in which the movement of moisture is controlled solely by capillary forces, and (2) beds in which the capillary forces are limited by va)oriza- tion within the pores and moisture movement is largeLy by vapor diffusion. A third classification might be made in which the structure of the bed is such that, under one set of conditions, capillary forces are controlling and, under another set, vaporization effects become pronounced. 0w. EKPLgLiIICflII‘JJ PAOCAD'LI4E The sand used for the solid camezfrom sandy lake beds n ar Caro, Michigan. The screen analysis with standard screens was as follows: Screen no. Cumulative percent retained 40 6.# 60 33.5 100 39 .6 200 96.9 The sand collected on screen E0. 60 was used for this study. This would place the diameter of the particles between the limits 0.25 - 0.42 mm. About one and seven tenths of a pound of 40-60 sand was weighed out on a beam balance. To this sand was added enough water to bring the total weight to two pounds. This placed the original moisture content at approximately 15 percent. In the first series of runs when the plate temperature was below the boiling point of water, the water added was at its boiling point. The purpose was an attemut to reduce the time to heat the sa ple to a constant drying rate. The sand and water were mixed thoroughly tOgether. The mixed sand sample was then placed in a mold consisting of an annular ring for the sides and a sheet of clear plastic for the bottom. The ring was one inch high and had an internal diameter of seven inches. The ring was made of galvanized iron sheet. The depth of the sand in this mold w.s 0.75 inches. The sand in this mold was then placed on a heated plate, and the elastic sheet was removed. A brass ring with an one inch internal diameter, 1.5 inches external diameter, and two legs 1.5 inches long, holding six thermocouoles evenLy spaced around the ring, was glaced in the middle of the sand slab. These thermocou.les, Leeds and Lorthruo, ho. 20, cogger— constantan, duglex, glass insulated, were spaced 1/16, 1/8, 5/16, 1/2, 11/16, and 15/16 inches from the heated plate. The thermo- couple 15/16 of a inch from the plate was 3/16 inches from the open or top surface of the sand slab. Its Purgose was to give one some idea of the temgerature of the air layer above the surface of the slab. An electric hot slate was used as the source of heat. It was connected through three variable rheostats so that the temgera— ture could be controlled throughout a run. Two circular steel plates one foot in diameter and each one 0.75 inches thick were placed on the hot jlate. The steel glates were used in hoges to get a better distribution of heat. The electric plate itself was subject to hot sgots, and the steel slates placed ugon it were an attemqt to remove this fault. Six thermocoueles were glaced half way between the two faces of the to; two circular slates. One was at the center, and the others at a distance of l, 2, 3, 4, and 5 inches respectively away from the center. A seventh thermocou le was placed about 1/16 of an inch from the too face and about one inch from the center. These thermocouqles were also Leed and Horthrup, Ho. 20, cogger-constuntun, duglex, and glass insulated wires. All thermocougles were connected throu¢b.throw switches to a Leeds and Northrup potentiometer using an ice solution as the cold junction. From a series of areliminazy runs, it was observed that the tenuersture drop in the steel slate and in the send at the same level from the center to the outside edges was within one degree F. at temperatures up to as high as 250 degrees F. As this temperature droo across the diameter of the steel :lete and sand was small, it was not considered necessary to take the temperature at various distsnces from the center. So the tenaeratures re- corded both in the steel jlate and sand bed were from the center. and these were considered to be the average ten erstures through- out the glate and sund slab. To protect the drying slab from air currents and to help set up natural connection currents, the drying appsrdtus was shielded. This shield extended 14 inches above the drying surface. This was done in hoges that it would lead to more uniform results. An one inch internal diameter COgger tube machined down to a wall thickness of 0.031 inches was used to take samules. SamJIes were taken at various time intervals bv dressing the copper tube down through the sand bed then renoving it with the senile within its shell. This ssmlle W¢S then divided into 1/8 of an inch layers, and out into nreviously weighed weighing bottles. This procedure was reduced to a minimum time, and the tine involved averaged aggroximately two minutes. These weighing bottles with the sand sat les were then weighed again and placed in an oven k gt at 240 degrees F. for an overnight geriod. The weighing bottles were removed, allowed to cool, and weighed the final time. To help determine the possible migration of water in the sand bed during the drying )rocess, a five gercent sodium chloride solution was used in Runs 23 and 24, and a two eercent solution was used in hun 25 as the moisture mixed with the sand. Samgles were taken in the grevious way. After the drying and final weigh- ing, the samgles were mixed with 50 ml. of distilled water to dissolve the NaCl. Mohr method for the determination of chloride was used to determine the chloride content in each sample. Temgeratures and moisture content were all recorded against time and against the distance from the heated surface. DATA AID G34" ‘fiq- LII IS 10 11 TABLE — I Run 12. Time Percent Moisture Content - Gms. Water Total min. Gms. 3. D. Sand aver. , Distance from Plate - Inches 0—1/8 1/34/4 HAL-318 318—1/2 yz-jja 5/8—314 90 16.7 15.1 15.8 15.6 15.9 15.4 15.9 No temoerature gradient was imgosed upon sand. It was allowed to stand for 90 minutes before samples were taken. TABLE - II Run 13. Pressure 744 m.m. Room temyerature 829E. Wet bulb temperature 6903. Time Temaerature - OF min. Plate Distance from Plate - Inches ins 1J8 ihé 142 inué 0 195 1.5 180 147 142 129 118 5. 178 157 154 144 134 10. 179 160 158 143 140 15. 179 161 158 148 140 30. 178 160 158 150 142 40. 180 162 160 151 144 45. 181 163 161 152 144 60. 182 164 162 154 146 112 12 Air 15/16 92 102 114 114 117 113 119 122 13 TABLE — III Run 13. Pressure 744 mm. Room temperature 82°F. Wet bulb temperautre 69°F. Time Percent Moisture Content -_;msnhiéifir_. Total min. Gms. B. D. Band Aver. Distance from Plate - Inches 0—1/3 118-114 114—338 3/8-1/2 1/2-5/8 513—3431 15 12.7 12.71 12.3 11.6 11.43 11.44 12.03 30 9.02 9.60 9.34 9.62 9.53 9.70 9.47 45 6.72 7.05 7.03 7.36 6.92 7.80 7.13 60 3.78 4.26 4.18 4.33 4.32 3.77 4.11 14 Run 14. Pressure 742mm. Room temperature 90°F. Wet bulb temperature 78°F. \ Time Temperature - °F. min. Plate Distance from Plate - Inches 1/16 1/8 5/16 112 11/16 15/16 0 195 1.5 178 152 147 135 126 119 100 5 178 160 156 148 139 131 111 10 178 161 160 152 144 135 118 20 177 164 160 152 144 137 124 30 178 164 160 152 145 136 121 50 179 164 161 153 146 137 121 60 178 164 160 152 145 136 121 70 176 162 160 152 144 136 120 80 176 164 161 154 147 138 123 90 179 157 154 149 143 136 122 TnBLE V Run 14. Pressure 742 mm. doom temperature 900 F. wet bulb temperature 78° F. Time Percent Moisture Content - Gms. Water min. G'ms. B. D. Sand Distance from Plate - Inches 0-1/8 l/8-l/4 Mala 3/8411 112-518 5/8-314 O 30 10.20 11.02 10.25 11.30 11.40 10.40 60 4.20 4.43 5.47 5.40 5.10 4.96 90 0.135 0.40 1.32 2.10 0.85 0.85 15 Total Aver. 19.9 10.76 4.93 .834 TABLE - VI Run 15. Pressure 736 mm. Room temperature 39°F. Wet Bulb temperature 77°F. Time Temperature - 03. min. Plate Distance from plate - Inches Air 1/16 118 8/16 112 1/16 15/16 0 192 1.5 175 153 148 136 126 119 100 5 176 161 157 148 138 128 109 10 176 162 159 150 142 134 117 20 174 162 159 151 142 134 120 30 175 162 159 151 142 132 118 40 177 164 160 152 142 134 118 50 176 163 160 152 143 134 122 60 176 162 160 152 142 132 118 70 174 162 159 152 143 134 120 80 176 158 157 150 141 132 120 17 Run 15. Pressure 736 mm. Room temperature 89°F. Wet bulb temperature 77°F. Time Percent Hoisture Content - 935. Water Total min. Gus. 3. D. Sand fiver. Distance from Plate - Inches 0-1/8 1/8—1/4 15/4—35/8 3/8—15/2 1/2-5/‘3 54/3114 0 20.3 20 12.90 10.60 11.00 12.50 13.10 40 10.40 6.77 8.25 8.36 8.15 8.32 8.38 60 4.02 4.40 4.4 1.70 4.47 1.04 3.35 80 0.76 1.70 1.74 1.54 0.46 0.27 1.08 TnBLE — VIII Run 16. Pressure 729 mm. Room tem9erature 83°F. Wet bulb temperature 77°F. Time Temperature - OF. min. Plate Distance from Plate - Inches 1/16 1/8 5/16 1/2 11116 O 183 1 174 144 138 125 116 109 5 172 156 152 142 133 125 10 172 157 154 146 137 120 15 172 158 155 146 138 130 25 172 159 155 147 139 130 30 173 159 156 147 140 132 18 Air 15/16 95 105 109 113 115 115 19 I3 ABLE — IX Run 16. Pressure 729 mm. Room temperature 83°F. 0 Wet bulb temperature 77 F. Time Percent Moisture Content - Gas. Water Total min. Gms. 3. D. Sand Aver. Distance from Plate - Inches 0-1/8 15’8-1/4 114—338 313-112 112-5518413844 0 14.2 15 11.60 10.40 10.50 11.00 11.50 12.30 11.22 17 11.50 10.80 10.60 10.50 10.80 11.50 11.12 30 8.76 9.14 9.45 9.43 9.40 10.00 9.36 33 7.71 7.80 8.50 8.50 8.90 9.02 8.41 Run 18. Pressure Room temperature Wet bulb temperature Time min. 0 0.5 2.5 5.0 10.0 20.0 25.0 736 mm. 78°F. 70°F. Temperature - 0F. Distance from Plate - Inches 1/16 1[3 5/16 1/2 11j16 161 154 132 112 102 177 173 158 138 123 186 181 167 149 130 189 185 172 153 135 188 185 172 154 136 188 185 173 154 136 20 Air 15/16 88 103 111 118 120 TnBLE — XI Run 18. Pressure 736 mm. Room temperature 78°F. Wet bulb temperature 709?. Time Percent Moisture Content - Gms. Water min. Gms. 3. D. Sand Distance from Plate - Inches _ 0-1/8 1153—114» 1/4-35/8 318—112 1]2-5/8 5/8-314 11 10.30 10.30 10.20 10.60 10.70 10.20 13 9.80 9.60 9-97 9.80 9.77 9.75 27 4.83 5.30 5.50 5.58 5.60 5.20 29 3.75 -4.40 4.63 4.50 4.74 4.65 21 Total Aver. 10.72 9.78 5.34 4.45 23 $83 7 — XII dun 19. Pressure 733 mm. Room temperature 798E. Wet bulb temperature 67°F. Time Temperature - 0E. min. Plate Distance from Plate - Inches Air Q16 1/8 5116 112 11116 15/16 0 237 1.5 208 175 169 154 137 125 101 5 209 185 180 166 154 141 112 10 209 188 183 170 158 146 120 15 208 188 184 171 160 148 124 20 209 190 186 174 162 150 126 25 210 192 188 176 164 150 127 30 212 196 192 180 166 151 122 35 221 184 180 170 160 149 125 40 225 175 172 164 155 146 125 24 TABLE — XIII Run 19. Pressure 733 mm. Room temperature 79 F. Wet bulb temperature 6703. Time Percent Moisture Content - Gms. Hater Total min. Gms. d. D. Sand Aver. Distance from Plate - Inches Q.-_l_l.Ei._.l/ 8.-1/.4_114-31.8_3/.8_-_112_112-_i./8._1LL-114 0 17.1 10 11.4 11.6 11.3 11.2 11.20 12.8 11.58 20 7.35 8.25 7.67 8.3 9.02 9.5 8.35 30 1.93 4.01 4.34 4.72 4.22 4.95 4.03 40 0. 1.51 2.65 3.06 2.98 0.83 1.84 At about 34 min. a sam11e was taken from the surface. moisture equaled .8 . 25 TaBLE - XIV Run 21. Time Temperature - OF. min. Plate Distance from Plate - Inches Air 1116 118 55/16 1J2 15,36 15/16 0 267 .33 240 176 166 144 126 115 102 2.5 223 194 190 174 156 141 110 5.0 221 201 197 183 166 150 117 10. 221 203 199 137 171 154 123 15. 222 203 201 189 170 153 129 20 226 196 192 180 161 146 127 25. 229 180 178 170 158 146 127 30. 230 170 169 162 152 142 122 35. 230 165 164 158 148 136 122 40. 229 163 161 155 146 133 120 Pressure 740 mm. or Room temperature 87 3. Wet bulb temoerature 7289. TABLE - XV Run 21. Time Percent Moisture Content - Gms. Water min. Gus. B. D. Sand Distance from Plate - Inches 0-1/8 118—114 114—318 318-142 1/2-5/3 5/8—3/4 5 9.8 9.1 8.65 8.7 9.7 9.96 10 6.9 7.15 6.3 6.4 6.4 6.8 15 4.2 4.45 4.3 4.32 4.11 4.13 20 2.12 2.82 3.01 3.04 3.1 2.83 0‘ Pressure 740 mm. Room temperature 879F. Net bulb temgerature 72°F. 26 Total Aver. 2? TAJLE - XVI Run 22. Time Temperature - OF. min. Plate Distance from Plate — Inches Air 1116 133 511.5 112 115/16 15/16 0 259 .75 229 180 171 153 138 128 111 3.0 219 195 190 174 159 142 120 5. 218 201 196 181 166 148 120 10. 216 200 196 183 164 145 124 15. 215 197 192 180 161 145 128 20. 216 192 189 175 156 142 126 25. 217 177 174 165 152 140 124 30. 217 168 166 157 146 131 118 35. 216 163 160 153 144 127 119 40. 215 161 158 150 143 124 117 45. 215 158 155 148 141 126 117 50. 215 154 150 140 137 121 117 55. 215 158 154 143 130 124 119 60. 216 160 156 145 130 123 119 65. 214 162 157 146 133 124 120 70. 214 162 157 146 136 126 120 Pressure 744 mm. -. o doom temgerature 82 E. Wet bulb temperature 72°F. 28 TABLE — XVII Run 22. Time Percent Eoisture Content - Gms. Water Total min. _~ Gms. 3. D. Sand Aver. Distance from Plate - Inches 0-1/3 1734/4 144—313 343-112 1/2-5/3 513—314 0 16.28 5. 10.2 10.3 10.0 10.2 10.6 10.8 10.35 10 5.95 6.8 6.7 6.8 6.65 7.0 6.66 15 4.6 5.1 5.2 5.15 5.6 5.2 5.16 20 2.7 3.2 3.3 3.7 3.1 3.4 3.24 Pressure 744 mm. Room temperature 82°F. Wet bulb temperature 72°F. TABLE — XVIII fiun 23. Time Percent NaCl Dry Basis min. Distance from Plate - Inches 0-1/8 1/3-1/4 1/4-3/8 318-112 112-518 518—344 0. 5. 4.940 .480 .564 .612 .620 1.080 10. 4.480 .375 .419 .478 .535 1.275 15. 5.500 .362 3.72 .484 .425 1.380 20. 5.960 .216 1.96 .288 .372 1.885 Pressure 739 mm. 29 Aver. 1.337 Run 24. Time min. 10. 15. 20. Pressure Room temperature 0-1/8 3-97 30 TAJLE - XIX Percent NaCl Dry Basis Distance from Plate - Inches Aver. 113—1J4 1144318 3/8-112 1/21518 .118-3/4 .705 .385 .363 .302 .341 .938 .190 .189 .164 .255 1.120 .122 .105 .169 .143 1.08 .131 .0904 .0904 .154 1.395 737 mm. 85oF. Wet bulb temperature 78°F. 31 TABLE - XX Run 25. Time Percent HaCl Dry Basis min. Distance from Plate - Inches Aver. 0-1/8 1/3—1/4 114-3/8 3/8—1/2 1/2-5/8 518-314 0. .233 10. .397 .105 .084 .125 .149 .504 20. .767 .046 .050 .064 .079 .362 Pressure 738 mm. Room temperature 83°F. Wet bulb temperature 7793. TABLE - XXI dun 20. Time Temperature 0F. min. Plate Interface Term. Diff. .5 228 170 58 3. 213 193 20 5. 212 194 18 10. 211 198 13 15. 210 198 12 20. 211 200 11 25. 214 202 12 30. 222 190 32 40. 231 171 60 50. 233 160 73 65. 230 161 69 80. 227 169 53 TABLE — XXII Run 22. Time Temperature 01". min. Plate Interface Tern. 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I III LIIII ...-I- I 1.911 I I «IIIIIII4. . IIIHII I .. - “f 3... .« . 1.. I . .... r1..- ... .-....r .-. ....-. _ _.... .. ... . -_ . ._ . I. . I... ..4ITzI-I- II._-II.-I.I a.- * ..n.-.-.I-.....U.- .:-.-:--.-. ......-.-...._... _ .I - .... __ ... j..- ..... b . -1- I. .---.-_--.-...-_ .-..a- . 1.-.: r..- . -. 1...... ..-.I .- -.. II. ... _ . -. .-.... . .. LI ... .IIIF I IIEVIWII—IWLIFN. r + I w 4-14 I .III... N a Y I I“ + I¢I~ ... h< I .«. _ .. _. I.” .I .- . 6 —_.La_ I a. .. ._ L. _..1-1.’ ... . ...T. . II.«-: III I. In . _ I... «.6... 2.2:. II I . III III -888IrllflBI-XE ‘ a 3 I— had. DII)IIOI-I-DI II II nII'.OIbI-!I 'la. .t..r 3 In 8". ’l .oiancblvld‘ snap-n- ( D alt-a1. IO.) .latllio 'fi I..! )0 w. .1.I.3 I. .51. 0””.013 :ouno cal-I u-o’ouu E III. t0. to. (Ia. < I 2 I. COCI I Olo'ao. 0‘30. -l-u-o 0'5: .30]... .5 II". ’0 .1. I\ It. \'I ., : I. Ic¢l c u 3 I. an." .gucge 93.: ca: .52.: r. .3: O. =— x (.7... 2:53. .‘ I = In '91. .1 I B I. '6‘. '_"‘ — J +.~‘-- .. ... .-- ull IIIIIUII. l!!! o 0 .4 W. 3 I. Nun‘: IIIII O‘IIOOI ‘\ in, ‘0- ‘0 ‘1‘ - .. . .---_o a Irv . . 7 V y 7 _. .. , -I1:L.‘-+-- StplllIO’IIIl Iroot-Q .(.w.: I. an (I [Q fl ... I. so: .”Cl.i '1‘Ill II" C; ’\' in. lic‘.‘ DISCUSSION There are three periods of drying: (1) a heating-up period where the solid is being heated to a constant rate; (2) the constant rate period during which the water is being vaporized at a constant rate, and (3) the falling rate period in which the dnying rate is continuously decreasing. Drying includes all three periods pro- viding that the critical moisture content has not been reached before the constant rate is involved. The Heating-up Period Koisture content was higher at the bottom of the slab before the slab was placed on the hot surface (see Table I). In this tyge of distribution. gr.vitdtional forces were controlling, and this accounted for the greater concentration of wg+er at the bottom of the slab. When the wet slab was ulnced on the hot surface, flash vagorization took place at the interface between the hot plate and the closed face of the sand slab. There was a great deal of vaporization. In fact, the system functioned as a sacked distillation column. The vapor passed up through the bed condensing as it traveled. The use of sodium showed the effects of this heating-up ueriod. In dun 23, the original sodium chloride content was 1.337 percent. During the five minutes before a sangle was taken, the sodium was depleted to an average of 0.675 percent for the top 5/8 inches of the slab. The total thickness of the slab was 3/4 inches. The migration of sodium chloride must be accompanied by a similar movement of liquid water. If the water is traveling down- ward as shown by the data, there must be a diffusion of water vaqor up through the bed from the heated surface. This vapor condensed, diluting the solution of salt, and in turn carried the salt back to the bottom surface. This explains the high concen— tration of salt at the hot plate. A rapid heating-up period followed placing the slab on the heated surface. The temgerature increased to a maximum, at which time constant drying started. The difiusion of water vapor through the slab accounted for the relatively rapid heating—up period. The plate temperature decreased to a minimum during tAe heating-up geriod. If the electric oower was not controlled at this minimum point, the slate temperature started to increase. This decrease was caused by the vagorization of the water when the wet slab was placed on the plate. Also, the slab was at room temperature and had to be heated up until it was in equilibrium with the hot plate. During this heating-us geriod, the moisture was distributed in an inverted bell shape manner. That is, the moisture was in greater concentration at the two faces. This tyne of curve was characteristic of samples taken during the heating—up period as shown by the data. Then the curvature of the moisture distribution 51 gradient reached a relatively flat position, the heating-up period ceased and the constant rate period started. The Constant Rate Period The constant rate period was characterized by constant con- ditions. The temperature reached a maximum in the slab and remained relatively constant. The temperature gradient was also constant during tnis period. The temgerature difference between the hot plate and the closed face remained constant. Also. the moisture during this period was distributed evenLy across the slab. There was very little moisture gradient. The temgerature in this period remained constant until the falling rate deriod was agproached. There were slight variations caused by external conditions such as the plate temperature and air currents. The temperature difference at the hot surface interface was constant. The temperature differences remained within the limits of ten and fifteen degrees for the constant rate period regardless of the plate temaeratures that were used. Graph 15 shows this correlation between temperature difference and plate temxerature when one was plotted against the other. These temperature differences and plate temperatures were taken from different runs duriig the constant rate Jeriod. Transitional Period There was no definite break between the constant rate geriod and the falling rate period. Rather there was a transitional period during which the closed face started to dry out faster than the rest 52 of the slab. In other words, there was a lower moisture concen- tration at the heated surface. In this period between the constant and falling rate periods, the zone of vagorizution started to move ugward toward the too surface of the slab. The graphs of moisture versus distance show that this movement of the zone of vaporization was characterized by a gernbola type curve. The curve extends from a low at the hot surf ce interfdce, to a high point, and then to a low concentration of moisture. This movement of the zone of vaporization towards the Open face is also sudjorted by HcCreedy (6). The Falling hate Period In the falling rate period, the temperature declined. It decreased until the minimum was reached snd then started to rise. The temgereture gre‘ient decreased in such a manner that the tem- perature near the hot plate decreased the most. The temuerature at the open face remained constant for a geriod of time and then decreased also. As in Run 22, the temperature at the nir interface remained constant for twenty minutes (see grayh 10). The temgernture difference between the plate and the inter- face incrensed during the falling rate period. It increhsed to a maximum and then decreased. This increased temgerdture difference was caused by increased thermal resistance. The incredsed thermal resistance wls due to the drying out of the sand at the interface. Graph 11 shows how this temperature difference changed during the drying process. The moisture distribution during the falling rate geriod was a bell shajed curve. The moisture concentration was the greatest in the center portion of the sand slab. The slab was drying out at both surfaces. As drying proceeded, the two surfaces became com- pletely dry. The temperature decrease of the falling rate period occurred between the limits of 3.4 to 6 percent moisture, dry basis. A correlation of the moisture content and the temperature showed that the temperature in the slab decreased at a critica moisture content. Graghs 12 and 13 show that this critiCal point occurred within the same range for four different olate temjeratures. Drying dates The drying rate was the gre test for the hi her plate tem- '- (I) perature. When average moisture co tent was plotted against time, the sloge of the curve gave the drying rate. Graph 9 shows that the sloge of the curve for the higher glate teugeratures is greater than the sloges for lower temperatures. The steeper the sloge of the curve, the greater'the drying rate. Zones of Vaporization Vaporization of moisture was taking place at both surfaces of the sand bed. This was indicated by thecirying out of the two surfaces. Also, the data on the movement of sodium chloride showed that water was moving towards both surfaces. The greatest movement of water was in the direction of the heated surface. This movement of water to these surfaces indicated that vaqorization was taking place there. Sources of Possible Error A certain amount of error was probably introduced while takingssamoles of sand. Vaaorization of moisture most likely con- tinued even after the sample was renovedzfrom the slab. To reduce this source of inaccuracy. the time element was reduced to a minimum. It was felt that the tempera ures recorded were within the limit of glue or minus one degree. It is possible that there may have been several occasions when this did not hold true. Probably some error may have resulted in the construction and extension of curves. COHCLUSIOK Heating-up Period 1. hoisture content was higher at the bottom of the slab before the slab was glaced on the hot surface. 2. On placing the sand slab on the hot glnte, flash vagori- 2 tion took place. 3. a ragid heating-up geriod followed glacing the slab on the heated plate, and the temperature increased to a maximum at which time constant drying started. 4. The glate tem erature decreased to a minimum. 5. The moisture was distributed in an inverted bell shaoe manner. The Constant Rate Period 1. The temgerature reached a maximum in the slab and remained relatively constant. 2. The temperature gradient was constant. 3. The temperature difference between the hot plate and the closed face remained constant. Q. The moisture was distributed evenly across the slab. f“. Transitional Period 1. There was no definite break between the constant rate period and the falling rate geriod. 2. The zone of vaporization started to move toward the toy surface of the slab. The Falling date Period 1. The temoerature reached a minimum and then increased. 2. At first the temaerature decreased faster at the heated surface than the ooen face. 3. The temperature difference between the glate and the interface increased to a iaximum and then decreased. 4. The moisture distribution was a bell shaped curve. 5. The temperature decrease of the falling rate oeriod occurred between the limits of 3.h to 6 percent moisture, dry basis. Drying Rates 1. The drying rate was the greatest for the higher plate temoerature. Zones of Vagorization 1. There were two zones of vaporization. One starting at the hot plate interface, and the other starting at the ogen face. 2. The greatest movement of liquid whter was toward the heated surface. 10. ll. 12. 13. 1"“; o 57 BIBLIOGjnPEY Badger, fl. L. and McCabe, W. 3., Elements of Chemical Engineering, 2nd ed., McGraw-Hill (1935). Ceaglske, E. H. and Hongen, 0. A., "The Drying of Granular Solids", Trans. nn. Inst. Chem. Eng., 11, 283-314 (1937). Kirschbauu, Emil, Z. Ver. dent. Ing., Verfahrenstech, Ho. 3, 84-7 (1943)- Lewis, Hondams, and Adams, Pulo and Peder Nag. (Canada), 21. 122 (1927). Marshall, U. R., Jr., "Drying" in Encycloaedia of Chemical Technolopg; The Interscience anyclogedia, Inc., V01. j, 232-255 (1950). 8 m ficCready, D. W., ”Drying of Pulp and Paper', Pa3er Trade J., 191, No. 13, 65—71 (1935). Oliver, T. R. and Hewitt, D. L., "The Mechnnism of Dryi Solids, Part II", Trans. Inst. Chem. leg., 22, 9-18 ‘1 1-5; 1949). Pearse, J. 3., Oliver, T. 3., and Hewitt, D. M., "The hecha.ism olids, Part I", Trans. Inst. Chen. Lnn., ) Perry, J. 3., Chemical Engineers Handbook, 3rd ed., i"icGraw- Hill (1950). Sherwood, T. K., Ind. Eng. Chen., 22, 976 (1929). Sherwood and Cummings, "Mechanism of Drying of Clay", Ind. Ensiifihsno. 25; 311-315 (1933). Shroff, P. D., "Drying of Paper Puld on a Heated Surface," Un ublished 3.3. Thesis, Hich. State College (1949). ‘7‘ Stacey, A. 3., "Drying Rate Efficiency", rans. in. Inst. Chem. Enf., ié, 169—91 (1924). Walker, et. al., Princinles of Chemical Engineering, 3rd ed., mcGraw-Hill (1937). APPTLHJIX 58 Run 11. Pressure Room temperature Wet bulb temgerature 74°F. Time min. 10. 15. 20. 30. Plate 187 169 170 170 170 170 170 170 80°F. Temgerature - OF. Distance from Plate - Inches 1116 .118 5116 112 11/16 101 138 127 120 115 150 147 138 132 125 153 150 142 135 127 155 153 145 139 131 155 153 146 139 132 155 153 146 140 133 155 153 146 140 133 Air 15/15 104 108 108 112 114 115‘ 114 TABLE - XXIV Run 11. Time Percent Moisture Content - Gus. Water Total min. Gms. 3. D. Sand Aver. Distance from Plate - Inches 0—1/8 1/5—1/4 1L4tgja 3j3-1/2 1/2-5/3 5/3921# 0 15 .95 15 13-7 11.95 13.5 14. 19.1 14.1 13.05 30 10.7 9.85 10.5 10.93 11.9 12,93 11,10 Pressure 733 mm, Room temgerature 86°F. Wet bulb temperature 74°F. r3 51. J I .§ Run 20. Pressure 741 mm. Room temperature 78°F. Wet bulb temnerature 66°F. Time Temperature - OF. min. Plate Distance from Plate - Inches Air 1/15 118 5115 1/2 11115 15/16 0. ZQB .5 228 165 160 134 117 109 96 3. 213 188 182 165 146 132 107 5. 212 190 186 171 155 140 114 10. 211 193 189 176 161 145 122 15. 210 19% 191 178 162 143 124 20. 211 195 192 130 164 156 125 25. 214 199 189 186 166 150 126 30. 222 185 181 170 157 145 121 35. 229 171 163 150 150 137 113 40. 231 163 165 157 148 139 122 50. 233 158 156 150 141 128 113 65. 230 160 158 146 130 123 117 80. 227 166 163 151 137 122 118 95. 224 166 163 152 190 130 117 TABLE — XXVI Run 20. Pressure 741 mm. Room temgerature 78°F. Net bulb temperature 66°F. Time Percent Moisture Content - G23. Water min. Gms. 5. D. Sand Distance from Plate — Inches 0—1/8 1/3-1_/4_114-33/3 518—132 1312-5318 518—114 10. 10.7 10.2 9.93 10.45 11.38 12.3 20. 5.13 5.96 6.3 6.75 7.1 7.15 30. 2.34 3.54 4.1 4.36 4.52 4.39 40. 0.63 1.8 2.54 2.76 2.54 .96 Total Aver. 10.83 0\ be 3.87 1.87 Run 23. Pressure 739 mm. Time min. 5.0 10. 13. Plate 282 247 234 235 239 229 TABLE — XXVII Temaerature — Distance from Plate - Inches 1116 118 5116 112 11116 176 166 147 132 119 205 205 188 164 157 206 205 193 171 148 204 202 191 172 148 199 198 189 169 143 Air Tema. 15/16 107 121 128 131 131 TABLE — XXVII Run 23. Pressure 739 mm. Time Temaerature — 0?. Air min. Plate Distance from Plate - Inches Temo. 1116 118 5116 112 11116 15116 O 282 .5 247 176 166 147 132 119 107 5.0 234 205 205 188 164 157 121 10. 235 206 205 193 171 148 123 15. 239 204 202 191 172 148 131 18. 229 199 198 189 169 148 131 w‘..w1 r- Tnfihb XXVIII Run 23. Pressure 739 mm. Time Percent Moisture Content - Gns. Rater__ min. Gms.;B.IL Sand Distance from Plate - Inches 0-1/8 113—114 114—318 318—112 112-518 518-314 0. 5. 11.40 11.30 11.40 11.40 11.67 12.8 10. 5.90 10.00 10.00 10.20 10.23 11.17 15. 7.92 9.05 9.03 9.40 8.53 7.38 18. 4.92 6.38 6.85 7.33 6.44 5.84 Total Aver. 11.66 9.58 8.55 6 .30 Run 24. Pressure 737 mm. Room temperature 85°F. Wet bulb temgerature 78°F. Time Temgerature - 03. Air min. Plate Distance from Plate - Inches Temp. 1116 1/8 5116 112 11116 15/16 0. 296 1.5 247 203 188 166 164 134 106 3.5 240 5 . 240 208 208 194 180 152 120 10. 243 208 208 195 180 157 134 15 . 242 208 206 195 180 159 137 20. 243 202 198 186 172 154 138 30. 242 177 176 168 154 134 125 Run 24. Pressure 737 mm. doom temperature 85°F. Wet bulb temperature 78°F. Time Percent Moisture Content - Gus. Water Total min. Gms. 3. D. Sand Aver. Distance from Plate - Inches 0-118 113—114 114—313J18-112 112-518 518—314 O 5 7.72 8.28 8.55 8.85 8.18 8.74 8.39 10 4.90 5.96 7.61 6.01 6.25 6.11 6.13 15 2.52 3.34 3.86 3.91 4.33 4.55 3.75 20 0.45 1.67 2.55 2.69 2.63 2.33 2.07 TABLE - KXKI Run 25. Pressure 733 mm. 300m temperature 83°F. Wet bulb temperature 77°F. Time Percent Moisture Content - Gns. Water Total min. Gas. 3. D. Sand Aver. Distance from Plate - Inches 0—118 113—114 114-3/8 318—112 112-518 513—314 10 9.45 9.00 9.54 10.26 9.61 10.40 9.7 20 4.94 5.82 6.04 6.45 6.20 6.40 5.98 TABLE — XXXII Run 25. Pressure 738 mm. Room temgerature 83°F. Wet bulb temgerature 7 QT. Time Temperature - O 3. Air min. Plate Distance from Plate - Inches Temp. 1116 118 .5116 112 11116 15/16 0 276 l. 259 195 186 159 124 114 97 5. 236 206 203 190 168 147 115 10. 236 206 206 194 172 148 121 15. 235 206 204 193 169 143 126 20. 232 202 196 187 166 143 126 “H thCry-évtAi H 1.35.. MICHIGQN STQTE UNIV. LIBRQRIES 31293017749635