EXTRACT iON IN PACKED COLUMNS PULSED AND UNPULSED Thesis for the Degree of Ph. D. MICHIGAN STATE UN|VERSITY Vito Joseph Sarli 1960 This is to certify that the thesis entitled EXTRACTION IN PACKED COLUMNS PULSED AND UNPUISED presented by VITO J. SARLI has been accepted towards fulfillment of the requirements for PH. D. d CHEMICAL ENGINEERING ________ egree in_.. (”1 / ,1 ;”- I//’/ I Q ’.~ . ; (M /‘ /{. /-/{j 1:“ [,1"; {‘flli'a". 1 Major professor / 15, 1960 Ma h Date rc 0-169 LIBRARY Michigan State University EXTRACTION IN PACKED COLUMNS PULSED AND UNPULSED By VITO JOSEPH SARLI AN ABSTRACT Submitted to the School for Advanced Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemical Engineering 1960 ii VITO JOSEPH SARLI EXTRACTION IN PACKED COLUMNS, PULSED AND UNPULSED ABSTRACT This thesis presents a new method for calculating HTU (height of a transfer unit) in packed columns from the physical properties of the fluids and packing characteristics. To the author's knowledge no method has been presented previously for calculating HTU values without the use of experimental data. The calculations are based on hydrodynamic and mass transfer considerations of droplet, or disperse phase, flow in an immiscible continuous phase. Because of the fundamental nature of the analysis it should be possible to extend the method to other types of extractors such as spray, sieve plate, and baffle columns. In the experimental work of this thesis good agreement was ‘obtained'between the calculated and experimental BTU values for the transfer of acetone between water and carbon tetrachloride phases. The experimental values of HTU in this thesis and values of HTU reported by other investigators, deviated from the calculated values by an average of 30 percent in spite of a 20 fold variation in the values of HTU. From pulsed column operation the agreement between experimental and calculated values of HTU was of the same order of magnitude. How- ever, the basis for calculating the effect of pulse is not as sound :from a theoretical viewpoint as the rest of the calculations. iii VITO JOSEPH SARLI In the experimental work on pulsed columns in this thesis the height equivalent to a theoretical stage (HETS) is improved (i.e. lowered) when the pulse rate is increased, when total flow is reduced, and when packing size is reduced. Even at high pulse rates a 50 percent reduction in total flow causes a 20-25 percent reduction in HETS. HETS values are not affected much by changing the flow ratio or by changing the diameter of the column. At low flow rates better mass transfer was obtained when the phase with the larger volumetric rate was dispersed. At flow rates near the flood point or at high pulse rates, the choice of which phase is dispersed is immaterial. The method developed in this thesis to calculate HTU can also be used to calculate the flooding velocities in extraction columns. The agreement between experimental and calculated flooding rates is as good as is obtained from currently accepted flood point correlations. However, none of these correlations are extremely reliable. Extraneous effects such as localized packing orientation, bridging (especially in small size packing), or the tendency of droplets to coalesce or to resist coalescence can cause variations of two fold or more in flooding rates. It was found in this thesis that mass transfer strongly affects the tendency of droplets to coalesce. Transfer from the dispersed to the continuous phase promotes coalescence, while transfer in the reverse direction inhibits coalescence. Approved W @01_ EXTRACTION IN PACKED COLUMNS PULSED AND UNPULSED By VITO JOSEPH SARLI A THESIS Submitted to the School for Advanced Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemical Engineering 1960 V5fl??/$/ \v-v—vi ACKNOWLEDGEMENT The author wishes to express his sincere appreciation to Dr. Carl M. Cooper for his valuable guidance throughout the course of this work. Thanks are also extended to William B. Clippinger who assisted in the design and fabrication of the mechanical equipment necessary for this research. Many thanks are due to the staff of the Research Laboratories of United Aircraft Corporation for their skilled help in the production of this thesis. TABLE OF CONTENTS ABSTRACT . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGEMENT . . . . . . . . . . . . . . . . . LIST OF TABLES . . . . . . . . . . . . . . . . . LIST OF FIGURES . . . . . . . . . . . . . . . . . INTRODUCTION . . . . . . . . . . . . . . . . . . LIQUID-LIQUID CONTACTORS WITHOUT POWER AGITATORS Centrifugal contactors . . . . . . . . . . Liquid-liquid contactors . . . . . . . . . Spray columns . . . . . . . . . . . . . . . Packed columns . . . °.- . . . . . . . . . Flow rates . . . . . . . . . . . . . . . . Choice of dispersed phase . . . . . . . . . Packing size . . . . . . . . . . . . . . . Sieve plate columns . . . . . . . . . . . . LIQUID-LIQUID CONTACTORS WITH POWER AGITATORS . . Mixer-settlers . . . . . . . . . . . . . . Scheibel columns . . . . . . . . . . . . . Pulsed columns . . . . . . . . . . . . . . Sieve plate columns . . . . . . . . . . . . Pulsed spray columns . . . . . . . . . . . Pulsed packed columns . . . . . . .'. . . . FLOODING IN PACKED COLUMNS . . . . . . . . . . . SCOPE OF INVESTIGATION . . . . . ... . . . . . . THEORY AND CALCULATIONS . . . . . . . . . . . . . Mass transfer coefficient . . . . . . . . . Height of a transfer unit . . . . . . . . . Height equivalent to a theoretical stage, HETS Calculation of Tower Performance From Theoretical Considerations. HWdrodynamic Considerations . . . . . . . . . Terminal diameter, do . . . . . . . . . . . . . . . . . . . Terminal VEIOCity, Vt O O o O o o o o o e o o o o o o o o 0 vii Page ii vi xi xiii LA) O\O(I)O\U14=’UUUU |._.u 12 12 13 1h 18 18 21 23 22+ 25 25 26 27 27 29 TABLE OF CONTENTS (Contd.) Characteristic velocity, vo . . . . . . . . . . Holdup, x . . . . . . . . . . . . . . . . . . . Mean drop diameter, dvs . . . . . . . . . . . . . Interfacial area of contact, a . . . . . . . . . Mass Transfer Considerations . . . . . . . . . . . . Mass transfer in packed beds . . . . . . . . . . Modification of Mass Transfer Factors and Reynolds Numbers Application of the Ergun Correlation to Liquid-Liquid Mass Transfer . . . . . . . . . . . . . . . . . . . DISCRIPTION OF APPARATUS . . . . . . . . . . . . . . . Main COlUInn O O O O O O C O O O O O O O O O O O 0 Pulse generator . . . . . . . . . . . . . . . . . Packing material . . . . . . . . . . . . . . . . LiQUid-liqtlid 8yStem o o o o o o o o o e o o o o OPMTING PROCEURE O O O O O O O O O O O O O O O O O 0 Column operation . . . . . . . . . . . . . . . . Analytical Procedure . . . . . . . . . . . . . . EXPERIPTENTAL RESULTS 0 O O I O O O O O O O I O O O O 0 SECTION I - FLOODING CHARACTERISTICS IN PACKED COLUMNS Maximum Flow Rates Without Mass Transfer . . . . . . Numerical data: Tables I-III . . . . . . . . . . Visual Observations . . . . . . . . . . . . . . . Maximum Flow Rates With Mass Transfer . . . . . . . Numerical data: Tables IV and V. .l. . . . . . . Visual observations . . . . . . . . . . . . . . . Direction of Solute Transfer Considerations . . . . Effect of mass transfer on interfacial tension Effect of mass transfer on coalsence: Table VI . Effect of direction of transfer on column Operation: Table VII 0 O O O O O O O O C O O O O O O O O O 0 viii Page 30 32 3h 36 36 36 37 38 1L1 hl 1+7 1&9 1+9 52 5h 55 55 55 58 66 66 66 TO 70 71 71 TABLE or CONTENTS (Contd.) SECTION II - MASS TRANSFER IN PACKED COLUMNS . . . . . Mass Transfer in Unpulsed Packed Columns . . . . . . . Numerical data: Tables VIII and IX . . . . . . . Visml Observations O I O O O O O O O O O O O O I I Mass Transfer in Pulsed Packed Columns: Tables X - XIII . . . . The Effect of Inlet Tubes on Mass Transfer: Tables XIV and XV . SECTION III — CALCULATIONS BASED ON HYDRODYNAMIC AND MASS CONSIDERATIONS: TABLES XVI-XXIX . . . . . DISCUSSION 0 I O O O O O O O I O O O O O O O O O O O O 0 SECTION I - FLOODING CHARACTERISTICS IN PACKED COLUMNS . Maximum Flow Rates Without Mass Transfer . . . . . . . Effect of dirty packing in flooding . . . . . . . . Visual observations . . . . . . . . . . . . . . . . End section design . . . . . . . . . . . . . . . . Maximum Flow Rates With Mass Transfer . . . . . . . . Effect of mass transfer on flooding . . . . . . . . TRANSFER Comparison of flooding rates in pulsed columns: With and without mass transfer . . . . . . . . . . . . . . . Comparison of flooding rates in unpulsed columns: and without mass transfer . . . . . . . . . . . . . With Comparison of flooding rates with mass transfer: Pulsed and lmpflsed O O O 0 O O O O O O O O O O O O O O 0 Maximum Flows Calculated from Empirical Correlations . Effect of total flows on HETS . . . . . . . . . . . Effect of interface position on HETS . . . . . . . Unpulsed columns . . . . . . . . . . . . . . . . . Pulsed columns . . . . . . . . . . . . . . . . . . Choice of dispersed phase . . . . . . . . . . . . . Effect of flow ratio on HETS . . . . . . . . . . . Effect of column diameter on HETS . . . . . . . . . Effect of packing characteristics on HETS . . . . . ix Page 76 76 76 80 87 91 110 110 111 111 112 111+ 117 117 119 119 120 122 128 128 129 129 130 130 131 132 132 TABLE OF CONTENTS (Contd.) Unpulsed columns . . . . . . . . . . . . . . . . . . . P111866. COlmS O I I I I I I I I I I I I I I I I I I I HETS for inlet tubes . . . . . . . . . . . . . . . . . SECTION III - CALCULATION OF MASS TRANSFER IN PACKED COLUMNS Calculation of KGa . . . . . . . . . . . . . . . . . . Evaluation of droplet velocities, vO . . . . . . . . . Evaluation of distance of free fall . . . . . . . . . . Evaluation of drop diameters, dv0 . . . . . . . . . . . Comparison of characteristic drOp diameter and drop diameter in the packing . . . . . . . . . . . . . . . . . . . . Film.type flow vs droplets . . . . . . . . . . . . . . Holdup of dispersed phase; a comparison of equations at flooding . . . . . . . . . . . . . . . . . . . . . . . Effect of interface position on calculated KGa . . . . Calculations for pulsed columns . . . . . . . . . . . . CONCLUSI ONS 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 NOMENC MTLJRE I I I I I I I I I I I I I I I I I I I I I I I I BIBLIOGRAPIH I I I I I F I I I I I I I I I I I I I I I I I I APPENDIX A: Expressions for mass transfer . . . . . . . . . APPENDIX B: Characteristic velocity in terms of terminal velocity. APPENDIX C: Flooding velocity and holdup in columns . . . . APPENDIX D: Physical properties of CClu and water . ... . . Packing characteristics . . . . . . . . . . . . Distribution of acetone in water and CClu Page 132 132 131'. 1116 116 1&7 1A7 1A7 1A9 119 150 156 156 159 163 166 169 175 178 180 I80 181 xi LIST OF TABLES P I - UNFULSED RUNS AT FLOODING . . . . . . . . . . . . . . . ET II - PULSED AND UNPULSED RUNS AT LOODING . . . . . . . . . . 63 III — PULSED AND UNPULSED RUNS AT FLOODING . . . . . . . . . . 65 IV - PULSED AND UNPULSED RUNS AT FLOODING WITH MASS TRANSFER. 68 V - PULSED AND UNRULSED RUNS AT FLOODING WITH MASS TRANSFER. 69 VI - COALESCENCE OBSERVATIONS . . . . . . . . . . . . . . . . 72 VII - UNPULSED RUNS WHICH INDICATE THE EFFECT OF DIRECTION OF MASS TRANSFER ON FLOODING . . . . . . . . . . . . . . . 7h VIII - HETS FOR VARIABLE RATIOS OF CCLu/HQO AND MAXIMUM FLOW rI:ES 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 78 IX - HETS FOR NEAR CONSTANT RATIO CCLh AND REDUCED FLOW RATES 79 X - HETS FOR VARIABLE RATIOS OF CCLh/Heo AND MAXIMUM FLOW RATES IN PUT—SE CE OLIINW I I I I I I I I I I I I I I I I I I I I 82 XI - HETS FOR NEAR CONSTANT RATIO CCLh/HQO IN TWO DIFFERENT COLWANS I I I I I I I I I I I I I I I I I I I I I I I I 83 XII — HETS FOR NEAR CONSTANT RATIO CCLh/Heo IN PULSED COLUMN . 8A XIII — HETS FOR PULSED COLUMN AND NEAR CONSTANT RATIO OF COLA/H20 BELOW FLOODING . . . . . . . . . . . . . . . . 85 XIV - NIP IN 2 INCH DIAMETER COLUMN END SECTIONS . . . . . . . 89 XV - NIP IN THE INLET TUBES FOR 2 INCH DIAMETER COLUMN . . . 90 XVI - PHYSICAL PROPERTIES, CHARACTERISTIC DROP DIAMETERS, AND CHARACTERISTICS VELOCITIES . . . . . . . . . . . . . 96 XVII - CALCULATIONS FOR 3-PENTANOL-WATER . . . . . . . . . . . 97 XVIII - CAICULATIONS FOR BEMZENE-WATER . . . . . . . . . . . . . 98 XIX - CALCULATIONS FOR METHYL ISOBUTYL KEI‘ONE-WATER . . . . . 99 XX - CALCULATIONS FOR TOLUENE—WATER . . . . . . . . . . . . . lOO XXI XXII XXIII XXIV XXVI XXVII XXVIII XXIX XXXI XXXII XXXIII XXXIV XXXVI XXXVII LIST OF TABLES (Contd.) CALCULATIONS FOR CARBON TETRACHLORIDE-WATER AT FLOODING CALCULATIONS FOR CARBON TETRACHLORIDE-WATER AT FLOODING CALCULATIONS FOR CARBON TETRACHLORIDE-WATER BELOW FLOOD ING I I I I I I I I I .I I I I I I I I I I I I I I I CALCULATIONS FOR CARBON TETRACHLORIDE-WATER BELOW FI’OODING I I I I I I I I I I I I I I I I I I I I I I I I CALCULATIONS FOR CARBON TETRACHLORIDE—WATER BELOW F LOOD ING I I I I I I I I I I I I I I I I I I I I I I I I CAI-CIIIATI ONS OF KGa FROM HETS I I I I I I I I I I I I I CALCULATIONS FOR CARBON TETRACHLORIDE-WATER IN PULSED COLUW 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 CALCULATIONS FOR CARBON TETRACHLORIDE-WATER IN PULSED COLLWS I I I I I I I I I I I I I I I I I I I I I I I I CALCULATION OF KGa FROM HETS . . . . . . . . . . . . . . FLOOD POINT CALCULATION? BASED ON THE EQUATION PROPOSED BY HOFFING AND LOCKHART 2 . . CALCULATED AND EXPERIMENTAL FLOODING RATES IN UNPULSED COLIMS I I I I I I I I I I I I I I I I I I I I I I I I FLOOD POINT CALCULATIONS BASED ON THE EQUATION PROPOSED BY DELL AND PRATT<13) . . . . . . . . . . . . . . . . . FLOOD POINT CALCULATIONS BASED ON THE EQUATIONS OF CASE I AND CASE II FOR THE SYSTEM MIX-WATER . . . . . . SUMMARY OF CALCULATED KGa AND EXPERIMENTAL KGa VALUES FORCCLhandH20oeooooooeooooooooooo PROPERTIES OF CCLh AND WATER . . . . . . . . . . . . . . PACKING CHARACTERISTICS . . . . . . . . . . . . . . . . DISTRIBUTION OF ACETONE IN WATER AND CARBON TETRACHLORIDE xii Page 101 102 . 103 10h 105 106 107 108 109 123 12h 151 153 157 180 180 181 10 11 l2 13 1h 15 16 17 18 19 2C) 21. LIST OF FIGURES DETAILS OF MAIN COLUMN . . . . . . . . . . . . . . . . . . . DETAILS OF PACKING SUPPORT GRIDS . . . . . . . . . . . . . . ARRANGEMENT OF EQUIPMENT . . . . f . . . . . . . . . . . . . EFFECT OF TIME ON FLOODING . . . . . . . . . . . . . . . . . FLOODING IN UNPULSED COLUMNS - NO MASS TRANSFER . . . . . . FLOODING IN PULSED COLUMNS - WITH AND WITHOUT MASS TRANSFER FLOODING IN UNPULSED COLUMNS - WITH AND WITHOUT MASS TRANSFER FLOODING IN PULSED AND UNPULSED COLUMNS - WITH MASS TRANSFER HETS VS FLOW RATIO AT FLOODING IN PULSED AND UNPULSED COLUMNS HETS VS SUPERFICIAL VELOCITY IN TWO COLUMNS OF DIFFERENT DIAMETERS 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 HETS VS SUPERFICIAL VELOCITY FOR DIFFERENT INTERFACE POSITIONS IN UNPULSED COLUMN . . . . . . . . . . . . . . . . HETS VS SUPERFICIAL VELOCITY WITH DIFFERENT INTERFACE POSITIONS AND CONSTANT PULSATION RATE . . . . . . . . . . . xiii 115 116 125 126 127 135 136 137 HETS VS SUPERFICIAL VELOCITY WITH DIFFERENT INTERFACE POSITIONS AND CONSTANT PULSATION RATE . . . . . . . . . . . . . . . . HETS VS SUPERFICIAL VELOCITY AT DIFFERENT PULSATION RATES HETS VS SUPERFICIAL VELOCITY AT DIFFERENT PULSATION RATES . HETS VS SUPERFICIAL VELOCITY AT DIFFERENT PULSATION RATES . HETS VS SUPERFICIAL VELOCITY AT DIFFERENT PULSATION RATES . HETS VS SUPERFICIAL VELOCITY IN PULSED COLUMNS FOR TWO PACKING CHARACTERISTICS . . . . . . . . . . . . . . . . . HETS VS SUPERFICIAL VELOCITY IN UNPULSED COLUMNS FOR TWO PACKING CHARACTMISI'ICS I I I I I I I I I I I I I I I I I FLOODING CURVES FOR MIK-HQO; HOLDUP AT FLOODING FOR MIK-HQO DISTRIBUTION OF ACETONE IN WATER AND CARBON TETRACHLORIDE 139 11+O 1h1 1A2 1H3 1% 115 155 182 INTRODUCTION The importance of liquid-liquid extraction in packed columns for the separation and purification of chemicals has created an interest in and a need to understand mass transfer and fluid flow in packed beds. Liquid-liquid extraction in packed columns involves the counter- current flow of two immiscible liquids through the packing. One phase is usually denoted as the continuous phase and the other as the dispersed phase. The continuous phase fills the interconnected voids in the packed column and flows through the column as if it were passing through an irregularly shaped conduit. The dispersed phase is made up of droplets which pass through the continuous phase in tortuous paths determined by the distribution of the packing. The success of packed columns is dependent on the ability of the packing to modify favorably the flow pattern of the two phases. It increases turbulence in the continuous phase. It improves the distribution and increases the holdup of the disperse phase. It also distorts and sometimes breaks up the dispersed drops in their hindered movement, thus presenting fresh surfaces for mass transfer. If the disperse phase is the one which preferentially wets the packing, the transfer surface may be considerably extended. The larger the area that the two phases have in common, the higher is the efficiency of transfer of the solute which must pass :from one phase into the other. The countercurrent liquid-liquid exflzection column could be made more efficient by subdividing the droplets further so that the surface will be greater than that realized when this type of apparatus is used in its simplest form. The only energy available in a simple packed column for maintenance of disper- sion and agitation is provided by the velocity of the entering streams and the density difference of the two phases. This energy is small, usually insufficient to produce optimum subdivision of the droplets, maintain a high degree of turbulence in the continuous phase, and counteract the tendency of droplets to coalesce. Interaction of the two phases with insufficient energy results in a low interfacial area. A packed column is therefore less effective as a liquid-liquid extrac- tion column than it is as an absorber or distillation column because the gravity difference in gas-liquid systems is much greater. Several types of contacting devices have been developed and studied to circumvent the disadvantages inherent in continuous liquid- liquid contactors as a result of insufficient energy in the two streams. These devices supplement the gravitational energy by mechanical means. LIQUID-LIQUID CONTACTORS WITHOUT POWER AGITATORS A significant amount of research in liquid-liquid extraction and the various types of liquid-liquid contactors (of which the packed column is one) has been reported. Liquid-liquid contactors have been classified by Morello and Poffenberger<38), according to whether the separation of the two immiscible phases is caused by gravity, i.e. density difference of the immiscible liquids, or by centrifugal force. Industrially the former type is of greater importance and has received practically all of the attention of researchers in mass transfer studies. Liquid-liquid contactors dependent on gravity for separation of the two phases can be further classified as to whether the energy is inherent in the streams or is supplemented by mechanical devices. Centrifugal contactors. The most extensively used extractor which owes its success to centrifugal forces to enhance mass transfer and separation of phases is the Podbielniak, Inc., centrifugal con- tactor. The two liquids are brought into forced contact in a spiral confinement. A high speed rotor develops the centrifugal force and causes the liquids to flow countercurrently. In a paper by Bartels and Kleinman(3) it is reported that one centrifugal contactor is equivalent to four theoretical stages. Podbielniak, Inc., claims that 215 many as fifteen theoretical stages can be realized under certain ixieal conditions. Its main advantage is a short holdup time. Liquid-liquid contactors which are most extensively used for Twasearch studies are the spray column and the packed column. The LA \ \ .4 .\ latter, already introduced, possesses many similarities with the spray column. Spray columns. The spray column is the simplest of the continuous contacting devices. It consists of a vertical empty shell with nozzles for dispersing one of the liquids into fine droplets. The advantages of the spray column are its high capacity, ease of cleaning, and low cost. Its chief disadvantage is the inability to provide adequate transfer in a reasonable height of tower. The packed column generally gives a better efficiency than the spray column without packing<3u)(hu)(52). In the spray column a large fraction of the mass transfer takes place at the nozzle where the drops are formed. The rate of mass transfer diminishes after the droplet leaves the nozzle, due to stagnation of the com). surfaces formed in the dispersing nozzle The absence of pack- ing results in little opportunity for distortion of the drops, for formation of fresh droplet surfaces, for prevention of back mixing, and for creating turbulence in the continuous phase. Although the spray column is the simplest of continuous liquid- liquid contacting devices, the number of variables which appear to affect the rate of mass transfer is large. Physical properties of the fluids that exert an influence on extraction rates are viscosity, density, interfacial tension, and diffusivity. Also of prime importance is the direction of mass transfer and the phase which occurs as the dispersed phase. For spray columns it has been reported that the phase Inaving the larger flow rate should be made the dispersed phase; when the ratio of the two phases is nearly unity the phase receiving the solute should be dispersed(19)(22)(30)(58). As a result of increased holdup, a larger transfer area is obtained by dispersing the phase of greater flow rate. When mass transfer is from the continuous to the dispersed phase, a smaller drop size is experienced and coalescence does not take place as readily as in the reverse situation. Minard and Johnson<38), in studying the flow capacities of a four- inch diameter extraction column, present an empirical correlation for maximum throughput in terms of flow ratio and physical properties of the fluids. The reliability of the correlation is questioned by (57) Treybal in view of the small size of the column and the criterion of flooding used. Packed Columns. The packed column, which has been introduced already, has been the subject of many papers. The performance of packed columns is noticeably affected by the same variables that determine the efficiency of spray columns. In addition to the physical properties of the system, the following vari-' ables have great influence on mass transfer rate in packed liquid- liquid extraction columns: 1. Choice of dispersed phase. 2. Wetting characteristics of dispersed and continuous phase. 3. Size and type of packing. h. Height of tower and end section design. 5. Direction of mass transfer. 6. Holdup of dispersed phase. 7. Velocity of liquid flow. For solute transfer between two immiscible liquids in random packed towers the mass transfer area per cubic foot, a, and the mass transfer coefficient, k, cannot be measured separately. These two factors, which determine the capacity coefficient, are often combined into a mass transfer coefficient, ka, based on one cubic foot of packed volume rather than on one square foot of mass transfer area. The mass transfer coefficient, kGa or kLa, is related to two other frequently used quantities that give a measure of column performance, namely, the height of a transfer unit, HTU, and the height equivalent to a theoretical stage, HETS or HTS (see Appendix A). Certain basic trends can be noted from a study of the experimental data published; namely, that the quantities which measure the perform— ance of liquid-liquid contactors appear to depend more on the effect that the variables have on the mass transfer area, a, than on the mass transfer coefficient, k. Flow rates. The flow rate of either phase affects the over-all volumetric mass transfer coefficients, kGa or kLa. The principal effect of increasing the flow rate of either phase is to increase the holdup of the dispersed phase and consequently the area for mass transfer. The flow rate of the dispersed phase appears to influence the value of the interfacial area much more than the flow rate of the continuous ph8L_,_.,e(l)(29)( 52). Sherwood, Evans, and Longcor(52) studied the extraction of acetic acid from water by benzene, with benzene as the dispersed phase. The coefficient increased with increased flow rate of the continuous phase owing to increased holdup of the dispersed phase, until at the higher flow rates coalescence caused an actual reduction in the mass transfer area and volumetric mass transfer coefficient. Laddha and Smith<33> and Colburn and Welsh measured individual film coefficients and heights of transfer units. Two pure liquids of limited solubility were contacted and the approach to saturation of each phase was calculated in terms of "heights of individual transfer units". Flow rate of the continuous phase showed little influence on mass trans- fer coefficients. In every case the value of HTUd, height of transfer unit of the dispersed phase, was noted to be constant; whereas HTUc was reported to be strongly influenced by the flow rates. HTUC decreased as the ratio of dispersed to continuous phase flow rate increased. This' supports the findings of other investigators who have proposed that the phase of higher flow rate should be made the dispersed phase. In a later study reported by Leibsom and Beckman<35) , in which diethylamine was extracted from a water phase by toluene, the volumetric mass transfer coefficient, kLa, increased linearly with an increase in flow rate of the dispersed phase (toluene). The data indicated only a slight increase in volumetric mass transfer coefficient for an increase in flow rate of the continuous phase (water) in contrast to the effect of'increasing the dispersed phase flow rate. (26)(35)(36) that 3/8-inch packing is a transi- It is also suggested tion size for droplet behavior. For low dispersed phase rate the perform- ance of the column corresponded to that reported for l/2-inch packing and at higher dispersed phase rates the values for the mass transfer coefficient were smaller than those reported for the l/h-inch packing. Choice of dispersed phase. In some systems with a strong preferen- tial wetting of the packing by the dispersed phase there are no droplets. Under such conditions the dispersed phase flows through the continuous phase and along the packing in the form of a film or rivulets. The performance of liquid-liquid extraction equipment can be very different when the dispersed phase preferentially wets the packing, owing to the small increase in surface area with increased dispersed phase flow rate. For low dispersed phase flow rates the dropwise flow produces a higher interfacial area than the filmwise flow. This is due to channeling and nonuniform distribution of the disperse phase, expecially when the disperse phase flow rate is low. This phenomenon has been reported by several investigators<29><52> whose data indicate a higher volumetric mass transfer coefficient when the phase which preferentially wets the packing is made the continuous phase. It is possible, however, to observe a higher mass transfer coefficient with the preferential wet- ting phase dispersed when its flow rate is considerably higher than (1)(13)(30). the continuous phase For some systems a reversal of the dispersed and continuous phases has been observed when the phase that 3preferentially wets the packing is initially the dispersed phase. The reversal occurs with increased flow rate of the dispersed phase owing to. displacement of the "nonwetting" continuous phase by the "wetting" dis- persed phase(2). This phenomenon is more likely as flow rates approach flooding conditions. Packing size. The droplet size and the holdup are dependent on packing size in the column. The holdup and droplet size directly determine the interfacial area and consequently the volumetric mass transfer coefficient. In general it has been noted that the smaller the packing size, the (35)(52) (2h) have shown that better the mass transfer Gayler and Pratt the holdup decreased as the size of the packing was increased. A crit- ical size of packing was reported by several investigators(35)(36); namely, the droplet size was less influenced by packing l/2-inch or greater and the largest size drop was observed for l/h-inch packing. For l/A-inch Raschig rings the droplets rose through the packing as irregularly shaped drops that remained in the interstices of the pack- ing until impacted by one from behind. For l/2-inch Raschig rings and larger the drops appeared uniform in size and smaller than the drops Observed for the l/h-inch packing. The flow observed for 3/8-inch PaCking was Observed as a transition between the droplet behavior through l/h-inch rings and that through l/2-inch rings. At low flow rates of the phases the drops behaved as observed for l/2-inch and larger packing; however, as the dispersed or continuous phase flow rat . . . . e was increased the droplet Size increased and the behav10r was r-Q ~s ... - 10 similar to that observed for l/h-inch packing. In spite of the increased drop size for l/A-and 3/8-inch packings, the increased holdup of the dispersed phase and the increased agitation and turbulence of the con- .tinuous phase more than offset the adverse effect of the larger drops. In the study by Leibson and Beckman<35> it was reported that the column-to-packing ratio should be at least eight in order to eliminate a "wall effect" that results from a larger than normal void fraction of the packing. For a specific packing size, all other factors remain- ing constant, the mass transfer performance is improved as the column diameter is increased. If the wall effect is large the holdup of the dispersed phase is usually reduced owing to the abnormal localized high flow along the wall of the column. A lower retention or contact time is experienced, together with a reduction in interfacial area and volumetric mass transfer coefficient. The performance of a packed column for various kinds of packing material has been reported by several investigators<39)(hh)(52). A COlumn packed with unglazed ceramic Raschig rings indicated a higher efficiency than one packed with unglazed ceramic Berl saddles. The reverse effect was noted between ceramic saddles and carbon rings(52). In the latter situation, wetting characteristics of the packing by the dispersed phase may have been the controlling factor. .§ieve plate columns. In order to take advantage of the fact that a large fraction of the mass transfer takes place during and immediately after droplet formation, sieve plate columns have been d evelOped(57). Perforated plates spaced along the length of the 11 column cause the formation of fresh droplets and coalescence of the dispersed phase prior to passing through the succeeding plate. The con- tinuous phase is caused to flow across the plate and through a spout to the next plate. In order to obtain reasonable success in experimental columns of this type it is essential that proper flow rates ( a very narrow range) be maintained for the specific plate design under consid- eration. 12 LIQUID-LIQUID CONTACTORS WITH POWER AGITATORS Mixer-Settlers. The conditions for more efficient operation and performance of phase contactors can be improved by supplying additional energy for agitation and phase dispersion. When the energy is properly supplied both mass transfer area and mass transfer coefficient are substantially increased; and mass transfer capacity will in many cases be several times greater than can be realized without the application of external energy. The mixer-settler, using alternate chambers in series for agita- tion and decantation of the immiscible phases, was the first attempt at increasing mass transfer capacity. With a counterflow arrangement, equilibrium in each mixer can be easily reached and the performance of such equipment can be readily calculated. With a large number of contact stages, the mixer and settler arrangement is too bulky and expensive for practical application. Scheibel Columns. A modified.mixer-settler device was developed and studied by Scheibel et al.(h5)(h6)(h7). The unit consisted of a Vertical column with a central rotating shaft on which were mounted Stirrers. The tower was equipped with packed sections between the Stirrers. The packed sections served as calming sections where separa- tion of the phases took place. Each combination of mixing and.ca1min8 sections was equivalent to one stage. Scheibel columns often gave more than One theoretical stage for a combination of stirrer and calming s ection owing to the mass transfer that takes place in the packed section. 13 The packed section, as well as serving as a calming section, pre- vents backmixing and internal circulation. The height of the packed section must be adequate to disengage the two phases for the rotor speeds used. As the rotor speed is increased the dispersion becomes finer with a corresponding increase in mass transfer area. With very high rotor speed emulsification or excessive backmixing is experienced, limiting the successful performance of the column. The flow capacity of such columns is strongly dependent on the rotor speed and is somewhat lower than the maximum reported in packed columns. The same factors that are considered for conventional packed columns in choosing the dispersed phase are important for mechanically agitated columns and have been considered by the investigators cited. Other variables that have been studied are: rotor speed, height of mixing section, height of calming section, and total throughput. Pulsed Columns. The pulsed column is another arrangement for combining countercurrent action and mechanical agitation. The mechan- ical agitation is superimposed as an up-and-down motion over the entire VOlume of the liquids at the same time that the liquids flow counter- currently as in a conventional liquid-liquid extraction column. Mechanical agitation may be supplied by means of a piston and cylinder located outside the column. Construction within the column may take any Of the conventional forms for performing the dispersing and coales- Cing Operations. For a series of sieve plates the liquids are forced iflnxmigh the small perforations causing a fine dispersion. In a packed lit column the droplets are dispersed by the forced contact with the packing, and increased turbulence is experienced by the continuous phase. Since the mechanical energy is caused by the pulse generator the intensity of agitation can be controlled by regulating the frequency and amplitude of reciprocating motion. Sieve Plate Columns. In 1935, van Dijck(‘7) was granted a patent for a pulsed column with a number of perforated plates so constructed that they could be moved up and down. The plates were not provided with downcomers or risers, so the continuous phase had to pass through the perforated plates. No appreciable flow was experienced when the pulsing mechanism was not in operation. Another arrangement described by van Dijck(17) required that the plates be fixed and the liquids pulsed. The latter design arrangement was investigated and reported in numerous articles. An excellent report was presented by Sege and Woodfield<50) who investigated a large number of variables involved in the operation of sieve plate columns. The system investigated was a 3-inch diameter Sieve plate column in which uranyl nitrate was extracted from water With tributyl phosphate. The effects of operating conditions and sieve plate design on extraction and flow capacity were discussed. For the pulsed sieve plate column three distinct types of phase- disPersion behavior were observed as a function of throughput rate and pulsing conditions. Mixer-settler type operation occurred at low t hrm-thuts and low pulsed frequencies. This type of operation was 15 characterized by a complete disengagement of the two immiscible phases between adjacent sieve plates. The less dense phase was dispersed dur- ing the upward movement of the fluids and the dense phase was dispersed during the downward movement. The emulsion type operation was experienced at higher throughputs and frequencies. It was characterized by uniform dispersion of the dispersed phase with no separation of the phases into distinct layers between the plates. The mass transfer capacity of the column was in- creased several times in this region because of increased interfacial area of contact and high degree of turbulence. At still higher throughputs and frequencies unstable operation was encountered. In this region the column flooded and the mass transfer capacity was reduced owing to coalescence of the dispersed phase in various regions of the column. In the same paper Sege and Woodfield reported that at low frequency and.amplitude the flow capacity of the column was equal to the pulsed Volume. At higher frequencies and.amplitudes the flow capacities were fOund to be less than the pulsed volume, and after a maximum flow was attained the throughput of the column was reduced with further in- creases in pulsed volumes. For a given flow rate two regions of flOOded conditions existed, one owing to insufficient pulsing and the other to excessive pulsing. Performance of the pulsed sieve plate column was found to be Ielatively insensitive to variations in flow rate provided that 16 "emulsion type" dispersion was maintained. An exception occurred at high and low flow rates where an increased value of HTU was experienced. The same considerations that applied to the choice of disperse and continuous phase for packed columns appear to hold for the pulsed sieve plate column operation. In another investigation reported by Sege and Woodfield on a 23.5- (51) inch column a louver plate redistributor was discussed to prevent channeling of the denser fluid at the top of a column. (12) In a paper by Cohen and Beyer ', the performance of a pulsed perforated plate column was discussed and data presented. The column used was 1 inch in diameter equipped with ten perforated plates at 2-inch intervals. The system studied was iso-amyl alcohol-boric acid feed solution and distilled water. The data presented showed trends similar to those reported by Sege and.Woodfield(50), i.e. the values of HETS (height equivalent to a theoretical stage) was relatively insensitive to flow variations at the higher pulse frequencies (characterized by emulsion type operation). A critical frequency-pulse combination was discussed above which little improvement in extraction performance was evident. Performance of the COlumn with pulse indicated that a reduction in height to one half or one third that required without pulse was possible. Higher rates of eXtI‘action were observed with water as the continuous phase. In a later paper presented by Edwards and Beyer(18) , flooding Characteristics of a perforated pulsed column were discussed. An anal- YS . . . :18 of column operation led to derivation of an equation for 17 predicting conditions of inadequate pulsing in the perforated plate column. Chantry, Von Berg,and.Wiegandt(ll) reported the data for some exploratory runs with a 1.57-inch diameter pulsed sieve plate column. The few data reported indicated trends similar to those reported by the investigators already mentioned. An HETS as low as 0.360 feet was obtained for the extraction of acetic acid from methyl isobutyl ketone with water under pulsed conditions. Griffith, Jasny, and Tupper<26> used a two-inch diameter sieve plate column, lhO inches long, to carry out the separation of cobalt from nickel by extraction with methylisobutyl ketone. Owneall HTU values from 1.1 inches at low flow rates to 3A inches at high flow rates were reported, compared with 2M and 57 inches respectively for the system operated as a spray column. The data reported were somewhat erratic, probably as a result of the complex multicomponent system investigated. Belaga and Bigelow used the system water-acetic acid—methyl isobutyl ketone. The evaluation of pulsed columns was carried out in a sieve plate column #5 inches long by l l/2 inches in diameter. The Plates spaced at l-inch intervals were drilled with l/32-inch holes, to give 23% free area. HTU values were found to range from 2.63 to 6-25 inches. The product of frequency and amplitude was considered to be a measure of the rate of pulsing. The amplitude and frequency were variedfrom 1/8 to 2 inches and 20 to 80 cycles per minute respectively. 18 The study was made with the aqueous phase dispersed and at a fixed flow ratio. Although the data were indicative of the same trends previously reported in this review, considerable variation existed within a family of curves. Thornton<5h> investigated the performance of a pulse sieve plate and packed column using the system toluene—acetone and water. Two types of pulsing units were discussed. Advantages were pointed out for a unit that relies on a pocket of air or inert gas to isolate the pulsing mechanism from the process liquors, over a unit that transmitted the pulse directly to the process fluids. Typical plots utilizing experi- mental data were presented for mass transfer performance and maximum throughputs. Pulsed Spray Columns. The performance of a spray column under (6) pulsed operation was reported by Billerbeck et al. The system investigated was methyl isobutyl ketone-acetic acid and water in a 1.5-inch diameter column. The performance indicated as much as eight— fold reduction in HTU as a result of pulsing. Pulsed Packed Columns. The application of pulsation to liquid- liquid extraction in packed and sieve plate columns has been carried Out in a series of investigations at Cornell University. In a publica- (11) tion by Chantry, Von Berg, and Wiegandt reporting the results of these studies the optimum operating conditions of pulse frequency and amplitude for one 3-component system at constant feed rate, the effect 0f Varied feed rate, and the effect of pulsation on flooding capacity 'were cited. 19 The packed column contained a 27-inch section with dumped l/h-inch porcelain Raschig rings. The system studied was methyl isobutyl ketone- acetic acid and water. In nearly all the runs, 20% by weight acetic acid solution in water was extracted by neutral solvent as the dispersed phase. It was reported that efficiency of the column increased with in- creased dispersed phase flow rate, and was independent of continuous phase flow rate when the system was pulsed. For constant flow rates, performance of the column measured in HETS, height equivalent to a theo- retical stage, passed through a minimum as either the pulse frequency or amplitude was increased. An HETS of 3.2 inches was reported. Two runs were made at flooding conditions with pulse, and compared with an unpulsed flooding run. The data showed a reduction in maximum throughput when the column was pulsed. In an earlier study reported by Feick and Anderson(21), benzoic acid was extracted from toluene with water, the latter being the con- tinuous phase fluid. The column employed was 36 inches long and 1 7/16 inches in diameter. Two types of packing material were used, l/2winch stainless steel McMahon saddles and 3/8-inch ceramic Raschig rings. The pulse amplitudes ranged from 1/16 to 1/h inch and the frequencies from 200 to 1000 cycles per minute. The increased values of ovenall mass transfer coefficients as a I‘esult of pulsing were considered to be caused either by the increase in interfacial area (owing to finer drops and increased holdup) or to the additional turbulence and its influence on the mass transfer 20 coefficient. The investigators assumed that the pulsing action had less influence on turbulence in the droplets than in the continuous phase. By making further runs with a solute, namely acetic acid, where the major diffusional resistance was in the dispersed toluene phase, the same order of increase in the mass transfer coefficient was observed as in the case of benzoic acid. On this basis it was concluded that the main effect of pulsation was to increase the interfacial area of contact. Mention was also made of the fact that maximum flow capacity of the pulsed column was reduced as a result of pulsation. Schuler<8><1u)<15><28>, analysis Flooding conditions, according to the des- cription presented by these investigators, occurred when both inlet streams were simultaneously tending to overflow into the outlet tubes of the opposite phases. Also it became difficult to distinguish a contin- uous from a discontinuous phase in the column at flooding. (2) Ballard and Piret , on the other hand, presented "transition point" data and reported that three types of behavior can occur, namely flooding, phase reversal, and slugging. The so-called "transition point" data were approximately 50 percent below the flooding rates obtained by the other workers. According to Pratt("3), the transition point data measured by Ballard and Piret correspond to the upper limit of the region of rapidly increasing holdup. Gayler, Pratt, and Roberts<23)(25) described three regimes of flow in packed liquid-liquid extraction columns for packing sizes greater than 1/2 inch. In the region of linear holdup, the holdup increases in Proportion to the dispersed phase flow rate. In the region of rapidly increasing holdup, the holdup increases sharply with slight increases in the disperse phase flow rate; this is evident above holdups of about 10 percent, and is interpreted by Pratt and co-workers as due to hindered settling of the droplets. The third region is the region of constant holdup, where coalescence of droplets occurs and droplet 22 shapes are changed in such a manner that the throughput can increase without further increase in holdup. This region is terminated by the flood point. Very little data exist on the affect of nonequilibrium distribution of solute on the flood point. Good evidence is presented by Callihan(lo) and other investigators<2h)(3o)(Su)(55) that markedly different flood- points are encountered for solute transfer. The direction of transfer also affects the flood point. Pratt and co-workers from A.E.R.E. Harwell, England, in recent years have contributed several papers in which a fundamental approach is presented for liquid-liquid extraction columns. Certain of their results have been mentioned above. However, the real importance of their studies is the presentation of hydrodynamic considerations basic to column oper- ations. Further considerations of their works are discussed in a later section of this thesis. SCOPE OF THE INVESTIGATION The investigations carried out in this research are divided into three areas of interest: 1. Investigation of certain previously overlooked phenomena that affect the maximum capacities of extraction columns. These include the effect of deposits that adhere to the packing elements, the influence of mass transfer, and the direction of mass transfer. Investigation of the effect of certain variables on HETS. These variables include flow rates, flow ratios, pulsation rates, position of the interface, and packing characteristics. Presentation of a theory, supporting calculations, and experimental data which demonstrate a general approach for the estimation of mass transfer in extraction. THEORY AND CALCULATIONS The measure of performance of liquid-liquid extraction equipment re- quires the determination, by measurement or calculation, of two types of information, namely flooding velocities and mass transfer characteristics. ‘looding velocities are correlated in terms of hydrodynamic principles and the physical properties of the liquids under consideration. Because of the complex mechanisms of countercurrent flow in columns randomly packed with a dispersing medium, the correlations normally presented are empirical or, at best, guided by dimensional analysis. The correlations for flow in packed extraction columns, expressed as maximum allowable throughput, i.e. flooding velocities, are directly applicable in design- ing for the diameter of a column. The required height of a column is dependent on the difficulty of transfer of the solute from one of the immiscible phases to the other. In extraction columns the difficulty of transfer is measured by the number of transfer stages or equilibrium stages. Because of the complex nature of mass transfer, research on towers has led to empirical correlaw tions. The correlations are useful for limited situations in design problems. In general the method for representing extraction performance in a Packed column makes use of at least one of the related terms commonly referred to as the mass transfer coefficients, KL or KG; the height of a transfer unit, HTU; the height equivalent to a theoretical stage, HETS; or'the height of a theoretical plate, HIP. The height of a column can 25 be fixed as the product of the number of theoretical plates, NTP, and the height of a theoretical plate, HTP. The latter value is determined ex— perimentally, as it is not easy to calculate it from ”a priori" arguments. The relationships between over-all mass transfer coefficient, height of transfer unit, and height of a theoretical plate are derived in Appendix A. Some of the more useful equations are presented below: (KL/KG) = D ('/Ks°) = (D/kLO) + ('/kG°) (l/KLO) = (l/kLO) + (I/D kGo) HTUOL = L/KLO A; HTUL = L/kLo A HTUOG = G/KGO A; HTUG = G/kGo A HTUOL = HTUL + (L/GD) (HTUG) HTUOG = HTUG + (HTUL) (DG/L) HTP = In (L/GD)/KGO A [(I/G)- (D/ L)] NTP = log [{[xz- (x./D )]/[x.— —(Y./D)]}[I—(L/cso)] +(L/GD) Mod (DO/L) Mass Transfer Coefficients. It is difficult to measure the indi- vidual mass transfer coefficients, kL and kG, and the mass transfer area per unit volume of tower, a. Instead, the factors kLa and kGa have been Evaluated experimentally as single combined quantities. It is even difficult to measure individual combined quantities kLa and kGa; there- fOre most experimental data has been reported in terms of over-all Volumetric mass transfer coefficients, KLa and KGa. Height of a Transfer Unit. Although the HTU is closely related to the’ Rees transfer coefficient, it is simpler to visualize. Its 26 dimension is simply length and its magmtude does not vary over wide limits. In packed towers the individual film coefficients, kGa and kLa, increase with G and L; however, the ratios G/kGa and L/kLa, and therefore the transfer units, are nearly independent of flow rates. The HTU can be stated as being that height of column which results in a change in concentration equal to the driving force. Height Equivalent to Theoretical Stage, HETS. The theoretical stage concept considers the tower to be subdivided into a number of equilibrium contacts or theoretical plates, NTP or NTS. When the column height is divided by NTP or NTS, HTP or HETS is obtained. The NTP in terms of end concentrations was originally derived for gas absorption by Kremser<32> and modified by Souders and Brown(53). The primary advantage of HETS over the HTU concept is that for fully developed turbulent flow the value of HETS is nearly independent of the flow ratio(lo). High throughput flow rates and pulsed column operation represent conditions at which fully turbulent flow is developed. Calculations of Tower Performance From Theoretical Considerations Instead of using an empirical approach to tower research, this thesis proposes to use theoretical calculations based on an understand- ing of the important variables on mass transfer. It is desirable whenever lpossible to calculate performance from theoretical considerations. Ex- J?erimental performance data is then taken to verify or test the theoretiw Qal cons iderat ions . The problem of calculating the amount of extraction in a packed 27 column leads to the necessity of accounting for the hydrodynamics of packed columns and the mass transfer across interfaces. Any theory that is proposed must take into account the resistance associated with the continuous phase, the resistance associated with the disperse phase, and the effective area for mass transfer. The approach formulated here is an attempt to calculate the indi- vidual resistance of films expressed in terms of kG and kL. Also, an effective transfer area calculation is proposed based on average drop diameters determined from free fall considerations and the quantity of liquid holdup in the tower. The effects of velocity on each of the resistances and transfer area are included. In general the packed column is considered to be analagous to a series of short spray columns, with lengths equal to the distance between packing contacts. The hydrodynamic principles which normally apply to spray columns are modified for the presence of packing. In the present development of the theory for calculating the performance of packed columns, it is assumed that the continuous phase is always that phase which preferentially wets the packing. The disperse phase therefore moves through the packing in the form of droplets. This leads to a limitation which will be discussed later. Hydrodynamic Considerations Terminal Diameter, do’ In packed liquidnliquid extractors free fall or rise of droplets through a continuous phase is always encountered to some extent. In a spray column no packing is present and the drops rise or fall unimpeded. In packed columns or sieve plate columns the 28 drops are in intermittent free fall. In the case of unimpeded free fall the drop rapidly attains its terminal velocity. In the case of impeded free fall the droplets accelerate and decelerate, and the average velocity is some fraction of the terminal velocity. For spray and sieve plate columns the drop assumes a size such that the surface tension forces counterbalance the drag or friction forces. This implies that large drops ensuing from inlet openings break up as the terminal velocity is approached. Small drops will tend to coalesce if contact occurs between them. On the average it can be expected that each drOp will assume some equilibrium size that satisfies the drag force re— sulting from its terminal velocity. For packed columns the average velocity is some fraction of the terminal velocity. Consequently the final drop size is somewhat larger than that ex- perienced in spray or sieve plate columns. This is a direct result of the reduced drag experienced at reduced velocity. In a paper by Pratt and co-workers(25), the following relationship is recommended for the characteristic diameter. (10 = .92 ./ r/gAp Pratt and co-workers refer to the characteristic diameter as the actual diameter of a drop emerging from a packed section when the continuous phase flow rate is zero, and the disperse phase flow rate is so small that only a few drOps are falling through the packing. The form of the rela- tionship can be justified by assuming that the surface tension effects counterbalance the drop weight as it falls through the packed space. 29 This relationship was determined from extensive measurements of drop diameters of several immiscible systems with different physical prcperties. An expression of the form proposed by Pratt and co-workers with an undetermined constant can be derived as follows: a. Large drops tend to break up at the terminal velocity because forces tending to distort them are large relative to the forces holding them together. b. The force which holds them together is proportional to the inter- facial tension and the drop circumference, ”dc 7'. c. The force which distorts the drop is proportional to the drag on the drop. The value at the terminal velocity is (W765)dO3A/’. d. The critical distortion at which the drop breaks up occurs when the distortion forces are a certain proportion of the forces holding the drop together. rdOTY=C(-r/6)dO3AP Solving for the diameter do, the resulting expression is d°=C./r/Apg Terminal Velocity, Vt' The terminal velocity is calculated from drag coefficients for the free settling of spheres with the characteristic diameters<25)\42). Stokes' Law relates the terminal velocity to the Characteristic diameter of spheres. The drag force, Fd equals '0! h- 43 E. ‘n. 'V 30 doAng Terminal settling velocity: Vt“"fi§;f' Fd, drag force, lbs g, gravity constant, ft/sec2 Vt, terminal velocity under action of gravity, ft/sec fL, viscosity of continuous phase, lbs/ft-sec Asp, difference in density between spheres and continuous phase, lb/ft3 v, velocity of drop Characteristic Velocity, v0. The velocity of a drop falling unim- peded attains the terminal velocity. In packed columns the average velocity of a drop is less than the terminal velocity. At zero continuous phase flow rate and small dispersed phase flow rate, the average velocity of fall or rise of a drop through the packing is referred to as the characteristic velocity, v0. The distance of free fall in packed columns is less than the nominal packing size. Pratt and co-workers(25) use drag coefficient data for calculating the average rate of fall or rise through a distance equal to 0.38 times the nominal packing size minus the Characteristic drop diameter, do. This checks well with measured velo- Cities except when the drop diameter is close to or greater than 0.38 times 'the packing size. Pratt's approach has merit since the characteristic velocity cannot EEIXceed the terminal velocity. The use of (0.38 dp-do) to express the EVVerage distance traversed.by drops between collisions seems quite a15bitrary. Its use is justified because it fits the experimental data prOpose the relationship ‘1 ‘1 l/v. There is some merit for this because it is shown by Gayler and Pmtt(2h) that the characteristic diameter, do, and characteristic 35 velocity, v have an inverse relationship. On this basis the mean drop 0, diameter at finite flow rates, dvs! may be expressed in terms of dO and V0, dvs = do vC/V Pratt and co-workers(2u) define V as the mean velocity of the droplets relative to the stationary packing. Expressed in terms of Vd, developed a correlation for mass transfer in packed beds in which he relates the mass transfer factor, [645(Sc)k]/V, to a Reynolds number, (6 V;>)/(aP,L). In order to make use of this correlation for extraction it is necessary to view the liquid-liquid contactor in terms of a packed bed with one-phase fluid flow. For a packed bed with one- phase liquid flow, the film through which the solute is transferred is stationary. The velocity of the fluid relative to the packing and the velocity of the fluid relative to the film are therefore the same. This velocity is given by V/e. In a packed column for liquid-liquid extraction, there are two moving fluid phases and the stationary packing. Each phase has a velocity relative to the packing and a velocity relative to the interface between the disperse phase and continuous phase. If the wflocity relative 39 to the packing and the velocity relative to the interface are the same, we have a situation analogous to a packed bed in which one fluid moves through the bed at this relative velocity. In order to apply the Ergun correlation to the flow of one of the phases in a packed liquid-liquid extractor, it is necessary to assume that the velocity of the fluid rela- tive to the packing and the velocity relative to interface are equal. The question arises whether the relative velocity of a phase should be equal to the actual phase velocity relative to packing or to the actual phase velocity relative to the liquid-liquid interface. we can reasonably expect that the velocity relative to the interface would have the most , effect in determining the resistance of the liquid film through which mass transfer occurs. There is no mass transfer to or from the packing. Thus in a relatively stationary continuous phase the velocity relative to the packing is near zero but the mass transfer coefficient in this case could be greatly increased by the rapid motion of dispersed phase droplets through it. The velocity of the phases relative to the interface is taken as the important velocity for mass transfer in a packed column. The sum of the velocities of the two phases relative to the interface must equal the velocity of either phase relative to the other. If the viscosities of the two fluids are equal, then the velocity of both phases relative to the interface are approximately equal. In the case where one fluid is very viscous, the velocity of the interface relative to the viscous phase will approach zero. to The mass transfer factor of either the disperse phase or the con- tinuous phase can be expressed as 6(Sc)k/ ur. If the velocities of the two phases relative to the interface are different, the value of ur will depend upon which phase is being considered. The velocity of each phase relative to the interface, when their viscosities are equal, is ur = urd = urc = l/2[Vn/éx + Vc/E(l-x)]. When the continuous phase preferentially wets the packing, its flow through the extractor is impeded and made more turbulent because it has to flow around particles of packing and also droplets of the disperse phase. In extending the Ergun correlation to this type of flow, the surface area of droplets must be taken into account as well as the surface of the packing. This may be done by adding the two surface areas together, so the Reynolds number expression for the continuous phase becomes: Rec = [s urcpfe (I—x)]/[(oq + od),uo] For disperse phase flow with continuous phase wetting the packing, the effective area is the area of the droplets only. The disperse phase does not come into direct contact with the packing. The Reynolds tauMber for the dispersed phase is then eXpressed as Red =[6 urdpfex]/[odp.cfl DESCRIPTION OF APPARATUS Main Column. The main column was constructed as shown in Figure l. It consisted of a Pyrex glass pipe two inches in internal diameter by six feet in length, fitted with expanded end sections four inches in internal diameter and thirteen inches in over-all length. The eXpanded end sections and the main column were standard stock items supplied by the Corning Glass Company. The end covers were constructed of l/h" stain- less steel, and had stainless steel fittings to accommodate inlet lines and pipes, exit lines, the pulse line, manometer taps, and the vent line. The end sections and covers were fitted with Teflon gaskets and connected with Corning aluminum flanges and inserts. The packing was supported on a three-inch diameter grid constructed from lengths of eighteen gauge stainless steel one-half inch wide,arranged on edge, and spaced one- fourth inch apart. The grids were provided with four legs (8-1/2 inches x 3/h-inch lB-gauge stainless steel) spot-welded to the edge of the circular grid support. The grid strips were also provided with saw-tooth edges to act as drip points to reduce the tendency for liquid to hang on the grid. The bottom packing support rested on the end cover and the ‘tOp packing support was held in place between the packing and the top end.cover. The upper support held the packing stationary when the column ‘WBS pulsed. The end sections provided sufficient reduction of exit stream 'velocities to permit complete separation of the two phases and to prevent the entering stream from being carried out with the exit fluid. The PaCkEd section of the column included part of the expanded chamber. The A2 FIG. I DETAILS OF MAIN COLUMN A. END FLANGES WITH COUPLINGS MAGNINEO ® @ INTO I/4" STEEL PLATE 9 © 8. WATER INLET I/4" ® ' I7 G) FITTINGS c. CGL4 OUTLET AND G DRAIN 3/4" FITTINGS 7 W (Ibfl D. NIANOMETER TAPS A I/4 FITTINGS _______® @— E. PULSE GENERATOR CONNECTOR 3/4" FITTINGS C P I] F. TEFLON GASKET (FLANGE NOT SHOWN) o (E) L G. PACKING SUPPORTS H. ADJUSTABLE OVERFLOW Q) LE6 ll 6) ATMOSPHERIC VENTS MANOMETER TUBE ' :D C) c. K. CCL4 INLET I/4 FITTINGS wATER OUTLET 3/4" FITTINGS I M. OVERFLow LEG PRESSURE BALANCE ASSEMBLY I"x3" GLASS REDUCER ANO 3/I6" PLATE WITH 3/4" COUPLINGS NOTE: ALL FITTINGS ARE 3I6 STAINLESS STEEL A3 inlet streams were introduced directly into the packing by passing the inlet tubes between two strips of the packing support. The inlet tubes were constructed from stainless steel tubing 3/16" in internal diameter, machined and flattened at one end to permit the tube to pass between the strips and to protrude one inch. Details of the packing support and inlet tubes are shown in Figure 2. This arrangement provided one-way flow past the packing support at reduced superficial velocities as recommended by Blending and Elgin(7). A schematic diagram Of the equipment is shown in Figure 3. Two glass- lined storage tanks of 250-gallon capacity each were used to contain the organic liquid; one contained the feed stream and the other collected the exit stream. Liquid in the collecting tank was pumped back to the feed tank and recirculated to the column before the start of each run. The water was contained in a stainless steel tank, and no attempt was made to collect it after passage through the column. The liquids were pumped by two centrifugal pumps, Eastern Industries, Model D-ll. The liquids passed through filters constructed Of three-inch diameter glass pipe packed with cheesecloth as filtering media. From the filters the liquids passed through l/8-inch needle valves to rotameters which measured the flows entering the column. The rotameters were fitted with three specially designed floats to permit measurement Of small variations in flow rates. Lines from the storage tanks to the rotameters were 5/8-inch polyethylene tubing. Inlet lines to the column from the rota- meters were l/h-inch polyethylene tubing. Exit lines from the column FIG.2 I411. DETAILS OF PACKING SUPPORT GRIDS ( ”"" “'” ,______ Ix." ..L..._.-- -..“V/ T CONSTRUCTION MATERIAL: ; I8 GAGE STAINLESS STEEL ,2 SERRATED Ih STRIPS :.*—_~ : N 1r—“[ 8 V2" I" IT If f N I“ to Io-3/4"—~I 3/4" v.0. STAINLESS TUBE 9 I/2" LONG MEASURED FROM INSIDE SURFACE OF END FLANGE, TOP SECTION FLATTENED #5 FIG. 3 $2... .2qu was? gun... used zeaeum foo mu>..<> 2.23 2:38 $3; .5528 we; 30.: we: $53524: mono Eoodam 2:54.. memo Eocene oziosu 2.4.3 «was ob 26> 92 5.33 cm»; ezm> 0.51.5015 3.. 39:53 39me $55.... cofimuzuo “when use. 33... cuequIcueuzfioc mix 33... .13 I I 55:32. 2:38 8on x23 zocomjou foo mx2<9> defined a "transition point" as the maximum throughput before sudden changes in the flow mechanism occurred. Above the transition point the column may flood or else the rates may change the flow characteristics to accommodate higher rates by a phase reversal. Ballard and Piret claimed that the "transition point" could be reproduced but above this point erratic column operation resulted. Series IX and X of Table III. The phenomenon of progressively in- creasing flow rates was not observed with lE-mm packing for the system carbon tetrachloride-water. The column operated more uniformly and smoothly with l2-mm rings than when 8-mm rings were used. Series VIII of Table II and Series XI of Table III. The continuous and dispersed phase were uniformly distributed in the packing for pulsed operation at the flood point. The drops were smaller for pulsed operation than for unpulsed operation. series I (10) Data from Callihan Series II 0 \O (D-xl 0\\n F’UU m H I 1 1 (a) 2.17 inch column packed with 8 mm Raschig rings TABLE I Unpulsed Runs at Flooding (b) packing height 30.75 inches (0) packing density 51.7 1bs/ft3 (d) no acetone, tap water Flow Rates ml/min V, ft/hr Water CClu Water CClh 650 3u3 55.8u 29.u6 670 173 57.56 1u.82 685 193 58.85 16.58 610 u63 52.u0 39.78 660 150 56.70 12.89 278 780 23.88 67.01 6u0 1A3 5u.98 12.29 6&0 193 5u.98 16.58 720.3 #07.0 61.88 3u.96 735.0 370.0 63.1u 31.78 #25.0 535-3 36.51 #5 99 366.9 361.7 31.52 31.10 358.0 395.3 30.76 3u.00 35h.5 #01.8 30.50 3u.50 227.7 1218.8 19.60 10h.7 5u2.9 205.5 h6.60 17.70 558.8 205.5 h8.00 17.70 625.5 1u0.u 53.70 12.10 683.0 1h5.7 58.70 12.50 61 Sum of sq roots Flow Ratio of superficial cc1u/H20 velocities 0.528 12.90 0.258 11.h3 0.282 11.7u 0.760 13.55 0.227 11.12 2.80 13.06 0.22u 10.92 0.302 11.19 0.565 13.78 0.503 13.58 1.259 12.83 0.986 11.19 1.10u 11.37 1.133 11.10 5.352 14.63 0.379 11.0u 0.368 11.1u 0.22u 10.82 0.213 11.20 62 TABLE I (Contd) Flow Rates Sum of sq roots Run ml/min V, ftghr Flow Ratio of superficial -__-. Water CClu Water CClu CClh/HZO velocities Series III 12 380.6 26u.5 32.70 22.72 0.695 10.h8 13 562.0 1h9.6 u8.28 12.85 0.266 10.53 1h 638.2 121.0 5u.83 10.u0 0.190 10.62 15 618.7 121.0 53.15 10.u0 0.196 10.51 16 278.0 A50.0 23.88 38.66 1.619 11.10 17 476.0 328.2 h0.89 28.20 0.689 11.70 18 181.h 827.5 15.58 71.09 u.562 12.37 Series IV 19 397.8 1269. 3u.17 109.0 3.190 15.15 20 1010. 569.0 86.77 #8.88 0.563 16.30 21 1361. 328.2 116.9 28.20 0.2u1 16.11 22 66u.5 836.1 57.09 71.83 1.258 16.03 23 2h1.5 1591. 20.7 136.7 6.588 16.25 2h 1617. 20u.0 138.9 17.52 0.126 15.97 25 1500. 183.0 128.9 15.72 0.122 15.31 Series V 26 999.7 507.9 85.88 A3.63 0.508 15.86 27 1728. 257.6 1h8.u 22.13 0.1u9 16.90 28 1379. 269.0 118.5 23.11 0.195 15.71 29 1601. 150.0 137.5 12.89 0.09u 1h.29 30 1011. h85.0 86.86 hi.67 0.h78 15.77 31 689.h 753.6 59.23 6u.7u 1.093 15.73 32 321.0 1302. 27.58 111.8 n.056 15.81 33 338.h 1319. 29.07 113.3 3.898 16.03 3h 790.2 293.0 67.88 25.17 0.371 13.26 TABLE II Pulsed and Unpulsed Runs at Flooding (a) 2 inch column with expanded end sections (b) inlets directly in packing (c) packing height 81.5 inches ‘ (d) 8 mm Raschig rings, packing density h9.12 lbs/ft5 (e) no acetone in feed streams Flow Rates Sum of sq roots jflgL ml/min Vz ftghr Flow Ratio of superficial Water CClu Water CClu CClu/Hzo velocities Series VI Tap water 35 lst Day #87.0 218.0 u2.28 21.16 o.uus 11.10 36 2nd 527.0 218.0 51.16 21.16 0.h1u 11.75 37 2nd 360.0 770.0 31.9u 7u.75 2.139 1h.56 38 3rd 582.0 216.0 56.50 20.97 0.371 12.10 39 hth 655.0 218.0 63.10 21.16 0.335 12.51 ho 5th 675.0 218.0 65.52 21.16 0.323 12.69 hi 5th 360.0 810.0 3u.95 78.63 2.250 . 15.77 u2 583.0 315.0 56.60 30.58 0.5u0 13.05 13 635.0 130.0 61.6u 12.62 0.205 11.u0 uh 6h8.0 130.0 62.91 12.62 0.201 11.h8 u5 285.0 935.0 27.67 90.77 3.281 1h.87 u6 510.0 h90.0 u9.51 A7.57 0.961 13.9u M7 575.0 218.0 55.82 21.16 0.379 12.07 Series VII Distilled water A8 367.7 397.h 35.6 38.6 1.081 12.18 u9 355.0 526.3 31.5 5h.7 1.u82 13.27 50 3uu.o 569.1 33.u 55.2 1.65h 13.21 51 395.0 363 2 33-5 35.3 1-053 11.73 52 326.5 362.0 31.7 35.2 1.108 11.56 53 325.5 u7u.5 31.6 u6.1 1.u58 12.u1 5h 179.u 839.0 17.h 81.5 n.677 13.20 55 185.0 825.0 18.0 80.1 u.u59 12.19 56 320.2 5h1.0 31.1 52.5 1.690 12.83 57 31u.0 5u8.0 30.5 53.2 1.7h5 12.81 58 387.0 299.0 37.6 29.0 0.773 11.52 59 5h8.0 200.0 53.2 19.u 0.365 11.69 60 525.0 113.0 51.0 11.0 0.215 10.h6 Series VIII 125 cycles/min, 5 mm amplitude, distilled water 61 62 63 6h 65 553-0 862.0 1111.0 308.8 93.9 253.6 159.0 103.0 3uu.5 u15.u 53-69 83.68 107.8 29.98 9.12 TABLE II (Contd.) Flow Rates lemin Vz ftghr Water CClu Water CClu 2u.62 15.11 10.30 33.hh u0.33 Flow Ratio Sum of sq roots of superficial velocities 0.u58 0.18u 0.093 1.116 u.uzu 6h TABLE III Pulsed and Unpulsed Runs at Flooding (a) 2 inch column with expanded end sections (b) inlets directly in packing (c) packing height 81.5 inches (d) 12 mm Raschig rings, packing density 25.9l lbs/ft3 (e) no acetone in feed streams Flow Rates Sum of sq roots mlzmin V, ft/hr Flow Ratio of superficial EBEL Water CClh Water CClLL CClh/HEO velocities Series IX Tap water 66 lst day 1530. 760.0 148.5 73.78 .497 20.77 67 2nd 1530. 755.0 148.5 73.30 .493 20.74 68 3rd 1530. 755.0 148.5 73.30 .493 20.74 69 3rd 2760. 230.0 267.9 22.33 .083 21.09 70 3rd 2200. 445.0 213.6 43.20 .202 21.17 71 4th 1550. 760.0 150.5 7 .78 .490 21.86 Series X Distilled water 72 2244. 144.0 217.8 13.98 0.064 18.49 73 2180. 242.0 211.6 23.49 0.111 19.41 74 1880. 520.0 182.5 50.48 0.276 20.62 75 1620. 750.0 157.3 72.81 0.463 21.07 76 1280. 970.0 124.3 94.17 0.758 20.85 77 1140. 1436.0 110.7 139.40 1.260 22.32 78 1040. 1405.0 101.0 136.40 1.351 21.72 79 750. 1775.0 72.81 172.30 2.367 21.65 Series XI 125 cycles/min, 5 mm amplitude, distilled water 80 880. 1417.0 85.43 137.60 1.610 81 1880. 730.0 182.50 70.86 0.388 82 1504. 937.5 146.00 91.01 0.623 83 2400. 523.0 233.00 50.77 0.218 66 B. Maximum Flow Rates With Mass Transfer Numerical Data: Table IV. In Iable'IV flood point data with mass transfer are listed. The solute, acetone, was present in the entering carbon tetrachloride phase in concentrations of approximately one percent. The column in use was 2 inches in diameter with expanded ends and with inlets located in the packing. The 81.5-inch high section was packed with 8-mm Raschig rings. Series XII of Table IV lists data collected at flooding with no pulse at various Cat/H20 flow ratios. Series XIII of Table IV presents data collected at flooding at a pulsation rate of 125 cycles per minute and men amplitude. Table V. In Table V flood point data with mass transfer are presented. The solute was acetone entering with the carbon tetrachloride in concen- tration of approximately one percent. The 81.5 inches of packed section in.the 2-inch diameter column consisted of l2-mm Raschig rings. Series XIV of Table V lists data collected at flooding with no pulse at various Cali/H2O flow ratios. §§ries XV of Table V presents data collected at flooding at a pulsa- tion rate of 125 cycles per minute and S-mm amplitude. Visual Observations: §§ries XII through XV. At high carbon tetrachloride to water flow ratios large drops were observed in.most of the packing and small drops in the very bottom portion of the packing. The small drops appeared to be . 0f the same Size as those observed in earlier runs with no mass transfer. 67 At low ratios of carbon tetrachloride to water the small drops were present in a proportionately larger zone of the packed section. TABLE IV Pulsed and Unpulsed Runs at Flooding with Mass Transfer (a) 2 inch column with eXpanded end sections (b) inlets directly in packing (c) packing height 81.5 inches (d) 8 mm Raschig rings, packing density £9.12 lbs/ft3 (e) 1 percent acetone in CClu feed Flow Rates Sum of sq roots Run ml/min V, ft/hr Flow Ratio of superficial Water CClh Water C014 CClu/Hgo velocities Series XII no pulse 84 410.8 847.6 39.88 82.28 2.063 15.39 85 606.2 538.6 58.85 52.29 0.888 14.90 86 987.5 271.0 95.87 26.31 0.274 14.92 87 927.5 249.2 90.04 24.19 0.269 14.41 88 889.2 317.1 86.32 30.78 0.357 14.83 89 904.4 258.3 87.80 25.08 0.289 14.38 90 980.0 110.0 95.14 10.68 0.112 13.02 91 888.0 158.5 86.21 15.39 0.179 13.20 92 877.0 176.0 85.14 17.09 0 201 13.35 93 868.0 213.0 84.26 20.68 0.245 13.72 94 965.0 110.0 93.68 10.68 0.114 12.95 95 900.0 225.0 87.37 21.84 0.250 14.01 96 890.0 256.4 86.40 24.89 0.289 14.28 97 870.0 304.7 84.46 29.58 0.351 14.60 Series XIII 125 cycles/min, 5 mm amplitude 95 302.6 1102.0 29.38 107.00 3. 4 15.76 99 480.0 833.3 46.60 80.90 1.74 15.81 135 520.6 756.0 50.54 73.39 1.45 15.68 131 566.2 635.4 54.97 61.68 1.1 15.27 1:2 558.7 523.5 54.24 50.82 0.937 14.49 133 510.7 417.8 49.58 40.56 0.818 13.40 139 397.2 363.7 38.56 35.31 0.916 12.16 13; 542.8 253.3 52.70 24.59 0.466 12.22 }13 1053.0 110.0 102.2 10.67 0.104 13.37 *3? 439.8 304.6 42.70 29.57 0.693 11.97 TABLE V Pulsed and Unpulsed Runs at Flooding with Mass Transfer (a) 2 inch column with expanded end sections (b) inlets directly in packing (c) packing height 81.5 inches (d) 12 mm Raschig rings, packing density 25.91 1bs/ft3 (e) 1 percent acetone in CClu feed Flow Rates Sum of sq roots Run ml/min V, ftghr Flow Ratio of superficial water CClu Water 0014 CClg/H 0 velocities Series VIV no pulse 108 1452. 1022. 141.0 99.2 0.704 21.83 109 2160. 440. 209.7 42.7 0.204 21.01 110 1630. 650. 158.2 63.1 0.399 20.51 111 1620. 760. 157.3 73.8 0.469 21.12 112 1226. 1230. 119.0 119.4 1.003 21.83 113 2040. 538. 198.0 52.2 0.264 21.29 114 1590. 717. 154.4 69.6 0.451 20.75 115 1000. 1390. 97.1 134.9 1.390 21.47 132 810. 1690. 78.63 164.1 2.080 21.62 Series XV 125 cycles/min, 5 mm amplitude 116 1340. 1450.0 130.1 140.8 1.082 117 1640. 1085.0 159.2 105.3 0.661 118 2145. 710.0 208.2 68.9 0.331 119 1990. 810.0 193.0 78.6 0.407 120 1800. 910.0 174.7 88.3 0.506 121 2810. 445.0 272.8 43.2 0.158 122 1570. 1290.0 152.3 125.2 0.822 70 C. Direction of Solute Transfer Considerations The observations and data reported in Subsection B, above, indicated that mass transfer under some conditions contributes to the attainment of high flow rates and large drop formation. It was not clear whether the phenomena encountered were peculiar to the HQO-CClh-acetone system. It was not clear that the maximum flow rates are the same with the direction of transfer from the water phase to the carbon tetrachloride phase and vice versa. Further the question was raised whether the flood point is the same if it were approached with either phase the disperse phase or the continuous phase. It is difficult to observe these variations in the column, because at flooding the disperse phase and continuous phase appear to be uniformly distributed throughout the packed section. In order to clarify some of these points bench scale experiments were performed to supplement the column operating data. The experiments performed in this section include: bench scale investigations of interfacial tension and coalescence, with additional data from column operation. Effect of mass transfer on interfacial tension. A set of experiments was carried out to determine if mass transfer markedly reduced the inter- facial tension between the carbon tetrachloride and water layer. It did not. Interfacial tension observations were made with a stalagmometer. The observations were carried out for carbon tetrachloride and water: in the absence of acetone, with acetone transfer from carbon tetrachloride to water, 71 with acetone transfer from water to carbon tetrachloride, with acetone distributed between the two phases in equilibrium concentrations. The largest interfacial tension occurred in the absence of acetone, the smallest when acetone was distributed between the phases in equilibrium concentrations. Intermediate interfacial tensions were observed when acetone transfer occurred from either the water phase or carbon tetra- chloride phase. The observed differences were practically negligible for the small acetone concentrations investigated. Effect of mass transfer on coalescence. Table'VI presents the ob- servations of qualitative bench scale experiments on the effect of mass transfer on coalescence rate. The table includes observations made in the absence of solute, with solute transfer from either phase, and with solute distributed between the phases in equilibrium concentrations. For these experiments the disperse phase was injected with a hypo- dermic syringe into the continuous phase. Effect of direction of transfer on column operation. Table VII, Series XVI presents flooding data for carbon tetrachloride and water, in the absence of acetone, with acetone distributed between the two phase in equilibrium concentrations, with acetone transfer from carbon tetra- chloride to water, and with acetone transfer from water to carbon tetra- chloride. The same column and packing characteristics reported in Table V were used for these runs. Disperse ase CClh (CClh + Acetone) (H20 + Acetone) (cc1h + Acetone) H20 (H20 + Acetone) CClu (COIL + HAc) (H20 + HAO) (CClu + HAC) s" l\‘ <3 (320 + HA0) f-h‘, 2.1.}. TABLE VI Coalescence Observations Continuous Phase H2O CClu (H2O + Acetone) (CClu + Acetone) H20 (001“ + Acetone) CClh (H20 4' Acetone) (320 + HAc) (cc1h + HAc) (320 + na~> Observations Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision 72 TABLE VI (Contd) Disperse Phase Toluene H20 (Toluene + Acetone) (H20 + Acetone) (Toluene + Acetone) H20 (320 + Acetone) Toluene Toluene H20 (Toluene + HAc) H20 (H20 + HAc) Toluene Continuous Phase H2O Toluene (H20 + Acetone) (Toluene + Acetone) H20 (Toluene + Acetone) Toluene (H20 + Acetone) H20 Toluene H20 (Toluene + HAc) Toluene (H20 + HAc) Observations Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision Large drop formation and rapid coalescence Disperse phase in fine state of subdivision - ~ \I to}... ~Ih> m.®MH cow mN:H :NH maoae Hesse .ceaaoa o: mm.om em.ss m.osa cow mesa mma H>x moaatm mesoeaoo mmfipwooam> mH00 hmpmz :Hoo hmpmz Haaeaeacaam to aamae .> cas\es cam mpOOH dm mo 85m mopwm 30am wqacooah no awhmcdhe and: Mo acepacasm mo pocmmm one cacaaeaH noes: case attendee HH> WHEN? 75 SECTION II MASS TRANSFER IN PACKED COLUMNS In this research considerable datalxme been collected to investigate the efficiency of mass transfer in packed columns. Datatfe reported in this section based on experiments performed on two sizes of Raschig rings in a 2-inch diameter column. The packing height in the column measured 81.5 inches, except where noted. The concentration of acetone in the feed carbon tetrachloride was maintained at approximately one percent. In this range the distribution coefficient can be considered constant. N0 acetone entered with the water phase. The data represent a wide variety of operating conditions. For example, in Subsection A, efficiency of mass transfer at different flow ratios for unpulsed conditions is reported for two sizes of packing. The efficiency of mass transfer for two sizes of packing, and at flow rates below flooding is reported. Mass transfer for different positions of the interface at reduced flow rates have been investigated. All of the above variations have been repeated for a variety of pulsation rates. These data are reported in Subsection B. Subsection C presents the contribution of the inlet tubes to mass transfer. 76 A. Mass Transfer in Unpulsed Packed Columns The column and packing characteristics for Tabme VIII and IX are the same as for Tables IV and V. Numerical Data: Table VIII, Series XVII and XVIII. HETS data at flooding and for various ratios of CClu/HQO are tabulated for 8-mm and 12-mm Raschig rings. Table IX. In Table IX HETS data are tabulated for 8-mm and 12-mm Raschig rings at nearly constant CClu/H2O flow ratios and reduced flow rates. Series XIX of Table IX. HETS data are tabulated for 8-mm.Raschig rings. The intermediate interface position was determined by the position of the overflow leg at flooding when the flow ratio of CClu/HEO was approximately 2. The position of the overflow leg was not changed for the reduced flow rate runs. This procedure was used throughout this research where data are presented for runs below flooding with an intermediate interface. Series XX of Table IX. HETS data are tabulated for l2-mm Raschig rings. The interface was located at an intermediate point. Series XXI of Table IX. HETS data are tabulated for l2-mm Raschig rings with carbon tetrachloride as the continuous phase. Visual Observations: Table IX, Series XIX, XX. At low flow rates with an intermediate interface, carbon tetrachloride is the continuous phase in the lower portion of the column. Water is the continuous phase in the higher portion of the column. Carbon tetrachloride falls through the continuous water 77 phase as discrete drops. Water may or may not rise through the carbon tetrachloride phase as drops. When the water rate is very low it rises as a film along the packing. At higher water rates a portion of the water flow rises as a film and a portion rises as drops. 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The Effect of Inlet Tubes on Mass Transfer The data reported here were obtained by performing runs on the two end sections attached together and packed with 8-mm Raschig rings. The inlet tubes were placed directly in the packing to give the same configura- tion that was used in the column. The packed section in the attached end section measured 9.5 inches between packing supports. The bulk density of the packing was determined to be h9.5h lb/ft3. Table XIV presents NTP in the end sections for pulsed and unpulsed conditions for a limited ratio of CClh/H2O and a large range of flow rates. The flow rates span the range from very low rates to flooding. Series XXXII of Table XIV lists NTP for unpulsed operation. Series XXXIII of Table XIV lists NTP for a pulsation rate of 125 cycles per minute with S-mm amplitude. Series XXXIV of Table XV lists NTP for a pulsation rate of 125 cycles per minute with lO-mm amplitude. Table XV presents the contribution of the inlet tubes for the 2-inch diameter column end sections. Calculations for the inlet tube effect were determined for pulsed and unpulsed column operation. Column l of Table XV presents NTP data obtained from pulsed and unpulsed column operation. The NTP vs superficial velocities from Series XXV and XIX were plotted and smoothed values of NTP at various superficial velocities were read from the plot. Column 7 of Table XV lists the superficial velocities. Column 2 of Table XV lists the NTP in the end sections for the same 88 operating conditions to which Column 1 applies. The NTP in the end sec- tions vs superficial velocity from Series XXXIII and XXXII were plotted. The NTP at superficial velocities listed in Column 7 were read. Column fiiof Table XV lists NTP in the length of column (72 inches) between the end sections. This is obtained by subtracting Column 2 from Column 1. Column h of Table XV lists the calculated HETS for the packing between the end sections. This is obtained by dividing NTP of Column 3 into 72 inches. Column 5 of Table XV lists the NTP in the packing of the end sections. This is obtained by dividing HETS of Column h into 9.5 inches. Column 6 of Table xv lists the NTP contributed by the inlet tubes. This is given by the difference of Column 2 and Column 5. 89 as.mm com. em.mm mam. m.maa was. 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The results of the calculations based on hydrodynamic and mass transfer considerations of a dispersed phase flowing through a continuous phase in packed columns are presented in tables. The calculations require no new experimental data. The experimental data collected by this writer and previous investigators serve only as a check of the theory. The calculated performance charac- teristics are tabulated as KGa and the experimentally observed values are labeled KGa observed. Table XVI presents the calculations of characteristics drop diameters, terminal velocities of characteristic drop diameters, and characteristic velocities. The liquid systems, the pertinent physical properties, and column characteristics are also tabulated. These data are used in the calculations reported in subsequent tables. In several cases the physical properties and packing characteristics had to be estimated. Where certain valuascf packing characteristics were not available from the investigator cited, the writer supplied these data from other sources for similar packing sizes. In every case the appropriate references are given in the tables. Table XVII presents the results of calculations for the system 3-pentanol-water. The observed values for ka were reported by Lhadda and 92 Smith (33). It is to be noted that individual volumetric film coefficients are reported in this case. The holdup, x, was calculated from the equation referred to as Case II in the theory, i.e. [VdJ/X + Vc/(" fl] = €vo (I —x) . The velocity of the dispersed phase relative to the continuous phase is tabulated as [Vd/gx + VC/g (I -- x )J . The drop diameter and the surface area of the drops are listed as dvs and ad respectively. The Reynolds number for the continuous and dispersed phase and corresponding mass transfer factor are tabulated as Rec,d and [6 Sc ‘S/urlcfl respectively. Each of these terms corresponds to the properties of the phase beingconsidered. Table XVIII presents the results of calculations for the system benzene-water with acetic acid transfer from.the water phase. The experimental over-all volumetric mass transfer coefficients based on the benzene film measured by Sherwood and co-workers (52) are also tabulated. In addition to the variables presented in Table XVIII, Table XIX presents the diffusity, Df, for each of the phases and the distribution coefficient, D. The former is required to calculate the Schmidt number for the mass transfer factOr; the latter for the over-all mass transfer coefficient from the expression V/KC::(EV/kL) 4- (1//k0) . See Appendix A for the derivation. ,Table XIX lists the calculations for the system methyl isobutyl ketone-water with acetic acid transfer from the water phase. The observed 93 values of over-all volumetric mass transfer coefficients were measured by Sherwood and co-workers (52). Table XX lists the results of calculations for the system toluene- water with diethylamine transfer from the continuous water phase. Leibsan and Beckman (35) measured over-all mass transfer coefficients for this system. Table XXI lists the results of calculations for the system carbon tetrachloride-water with acetone transfer from the carbon tetrachloride phase at flooding. The conditions for which the calculations are tabulated are consistent with the experimental runs reported as Series XVIII. The measured values of over-all mass transfer coefficients are also listed. The calculations for holdup, x, in Table XXI were made by applying the equation, [Vd/X +VC/(i-X)] =6VO at flooded conditions. Appendix C presents the derivation for holdup at flooding. Table XXII. The results presented in Table XXII are for the same system.and conditions as reported in Table XXI. The difference lies in the holdup, x. In this table the holdup x was chosen as 0.5 in every case. This will be taken up in the discussion. Table XXIII lists the results of calculations for carbon tetrachloride- water with acetone transfer from.carbon tetrachloride below flooding. The conditions for the calculations are consistent with those reported for Series XXI. The experimental values of over-all mass transfer coeffi- cients are also listed. Table XXIV presents the results of calculations for the same system 9k and conditions as reported in Table XXIV. The differences in tabulated values are accounted for by the mechanism of flow assumed for the dispersed water phase. In Table XXIII, discrete drops were assumed for the dispersed phase. In Table XXIV, film type flow of the dispersed phase was assumed to allow for the preferential wetting characteristics of the water phase. Table XXV presents the calculations for the system carbon tetra- chloride-water with acetone transfer from the dispersed carbon tetra- chloride phase. The flow rates are below flooding. The values for the observed values of over-all volumetric mass transfer were not measured directly. They were calculated from HETS values for carbon tetrachloride as the continuous phase (Series XXI), and HETS values for the intermediate interphase (Series XX). The interphase position was hh.5 inches from the base of the packing. The expression which relates the HETS values and interface position is: (NTP)Intermediate=[37/(HETS)HQO] + [uh'S/(HETS)CClh]:[81'5/(HETS)IntermediateJ Table XXVI presents the calculations of over-all volumetric mass transfer coefficients from.HETS values. These values are reported as the measured values in TablesIXXPXXV- The equation which expresses the rela- tionship between them is, KGa = (G/A) [ln(L/GD)] /[(HETS) (l-GD/L)] . This expression is derived in Appendix A. Table XXVII lists the results of calculations in pulsed columns at reduced flow rates for the system.carbon tetrachloride and water with acetone transfer. The conditions for which the calculations were performed 95 are consistent with those reported for Series XXVII and XXIX. With pulsation it was observed and stated previously that the holdup is higher and drop diameter smaller than without pulsation. The theory which was developed for non-pulsed operation can be applied in pulsed columns after modifications. In order to be consistent with the observed phenomena of increased holdup and reduced drop diameter, the characteristic velocity was arbitrarily reduced by one half of the pulse velocity in the packing. Table XXVIII lists the results of calculations for the system carbon tetrachloride-water in a pulsed column at flooding. The calculated and measured values are for the operating conditions reported in Series XXIII. Table XXIX presents the calculations of over-all volumetric mass transfer coefficients from HETS values. 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was not: . . . . mm. m.m o.mmm amm mam -oaxsw.a ms.m saoo oma s a m n o m a a memo was oam. as mom amm ma moans; New s39 eosnomno box ox a e.ox oqo us , oaoaomv sswus 0 qumV ns\mps us-pe\uoa ousoauao mmm wox onm w on .ou: Awmv an a* msoscapcoo m.mom ammoo. osm om. m.mo mam nsoo. saoo waa m.wom sous: . oao. om. m.mm mam nsoo. aaoo oma 0 0.2m NJO PJPH hogan: HHHxHx mehwm mpw\ pm «um um .m>v AAquVW\o>v+Awa©>v QSUHOS “x unmpu .> an pm pm .06 mmammmao mom m vm\ >uo> mdoschcoo p£\pm 02m .Q> .xanOHo> medm on “new as m .CaE\moHoxo an ADV umcHUOOHm Adv mQESHoo comHsm CH umpm3uvwap0Hnowapme conuwo pom meoHpmHSUHwo HHH>xN mqm+o> ss\pm as mmm u oo\a spaooao> <\o meme asaoassomsm T38; magi 75E}; 0&3 u use memm soak aux mo coapdeono xHNx EBB. 110 DISCUSSION SECTION I FLOODING CHARACTERISTICS IN PACKED COLUMNS. Various studies in the literature report the maximum flows attainable in packed columns. In all cases much data are presented and attempts are made at correlating the results in terms of packing characteristics such as void fraction and effective area of packing, and physical properties of the liquids. At best the correlations are to be used with caution since the maximum flows in packed columns are frequently influenced greatly by various additional characteristics which are normally overlooked. Again, in small size packing much higher flow rates can be attained in dirty packing than in clean packing. For example in bothamall and large packing, mass transfer under certain conditions increases the maximum flows in the column and under other conditions reduces them. The flow capacity in the packing is very much influenced by the non- uniformity inherent in random packing of the column. The subsequent discussion explains these phenomena in terms of observations and.mea- surements which have been presented in Section I of the Experimental Results. 111 Maximum Flow Rates Without Mass Transfer Effect of_Dirty Packing op Flooding. The maximum flow rates in packed columns can sometimes be much greater for a dirty or conditioned packing than for a clean packing. Two distinct flooding curves are shown in Figure A. The lower curve represents flooding in the clean packing and the upper curve represents flooding in the dirty, or conditioned, packing. The dashed line in Figure 6 represents the flooding curve based on flood point data reported by Callihan for the system tap water-carbon tetra- chloride. The unusual curve resulting from the data reported by Callihan is apparently due to the different degrees of conditioning of the packing for each of the flood points. Continuous operation results in higher maximum flow rates. This effect is observed in Figure 5 by the points marked 0, l, 2, 3, h, and 5 days. It is apparent that a higher flooding curve could be measured for each of the 5 days. A sharp reduction in column capacity occurred when the dirty column.was cleaned by acid treating. Reference is made to Runs 1 to 3 in Table I. These points indicate higher flooding rates than the sub- sequent values. The runs which follow Run 3 are for the column when cleaned by acid treating. Run 3h also indicates a sharp reduction when the column is cleaned by acid treating. The increased capacity with continuous operation is apparently due to a bridge formation at the points of contact between the packing elements. The bridge is formed from solids, dissolved in the water, 112 that gradually precipitate and build up on the packing. Bridge formation reduces the randomness of flow by forming channels that aid rivulet-type flow. The solids that form the bridge are dissolved by acid treating and are broken down by pulsing the liquids. Evidence of the latter is given by Run 1+7 of Table II in the Experimental Results. It is difficult to bridge large packing sizes, such as l2-mm Raschig rings, since the individual packing elements are not nearly close enough in randomly packed situations. Therefore, it is expected that for clean water, such as distilled water, continuous operation will not result in increased column capacities. Also the capacities in large packing sizes are expected to be independent of time. These phenomena are observed in Figure 5. The lower curve represents flooding for the clean 8-mm.Raschig rings with distilled water. The intermediate curve represents flooding for dirty 8-mm Raschig rings. The upper curve represents flooding for both clean and dirty conditions for l2-mm.Raschig rings. Visual Observations at_Floodigg. Flooding in packed columns is characterized by increased holdup of the dispersed phase. There is evidence that this holdup is neither uniform nor constant. The continually changing holdup in some sections of the column results in nonuniform flow of the dispersed and continuous phases in the section. This could lead to velocities at the section which surpass the maximum allowable flow rates. Flooding could start at the section and grow until the entire column is flooded. However, it is also likely that the holdup will change at the section before the entire column is flooded and a redistribution 113 of the phases will occur. This phenomenon was observed and reported as erratic column behavior in the Experimental Results. This also explains the up and down movements of the interfaces at flooding when no external changes were made in the volume flow rates to the column. Regardless of which phase was maintained as the continuous phase below flooding, at flooding the carbon tetrachloride and the water phases were redistributed naturally until the continuous phase was predominantly water and the dispersed phase was carbon tetrachloride. Apparently this phenomenon is related to the continually changing holdup discussed above. Two explanations are offered for the apparent phase reversal phenomenon. At flooding the dispersed and continuous phases are present in the column in approximately equal proportions. It would seem that either phase could be dispersed and for the proper conditions an interchange between the dispersed and continuous phase could occur. It is apparent that in order to establish balanced conditions at flooding, the phase which preferentially wets the packing forces the nonwetting phase from the packing. The situation for maximum stability, then, is one in.which the dispersed phase is the nonwetting phase. Another explanation of these phenomena is possible. One can logically assume that in randomly packed columns it is improbable that uniform packing densities occur throughout. Apparently, there is a cross- section for which the bulk density of the packing material exceeds the bulk density in all other sections of the column. Maximum flow rates or flooding velocities are attained in this section before any other section. 11h The liquids, then, pile up on either side of the maximum packing density (minimum voids) cross-section. Above the minimum voids cross-section the heavy phase accumulates, and below the light phase builds up. The probability of having a very small voids cross-section increases with the column length. In the case of Raschig rings an orientation is possible which would completely block the flows. Yet one can calculate only an average voids fraction for the column. This situation casts some doubt on the general reliability of empirical correlations based on average packing characteristics for the calculation of flooding rates. Equations of this type have been reported by many (7)’ (8)’ (l4)’ (l5)’ (28). End Section Design. Dell and Pratt (15) reported that inlet tube placements were important for attaining high flooding rates. Flooding data have been reported in this research with inlet tubes placed directly in the packing and outside the packing. No apparent Changes in flooding rates were measured. This is observed in the upper curve of Figure l. The data for Series V and IV represents runs for the water inlet tube placed directly in the packing and OUUiie of the packing respectively. Blanding and Elgin (7), and Dell and Pratt (15), reported that expanded end sections were important for attaining high flow capacities. Before and including Series IV expanded and sections were not used. They were employed in all experimental runs after Series V. It was not apparent that the expanded end sections made any difference. -Other effects such as the effect of dirty or conditioned packing, non-uniform packing densities, non-uniform holdups, phase reversal, and mass transfer were of a much greater significance. 115 FIG.4 Om. 0v. 0m. 0m. 0: $5... .>s_oo.m> $23 00. 00 00 0s 00 On 0? 0m ON 0. 0 oziufi. 24w 4 U 0' x4 (<7 sexes. ouzoiazoo lo a > were..." a 02—20(n. owz _ .0200 A o» 2 mafia o 2.33:: 93. A... 8&3 e »m 8243... 2:38 : museum 0 23.3.20 .6 _ 3.8.». o M 0 0. ON on 0V 0m 00 0h 00 Om 00. 0: ON. 0m. 02.0001.“— 20 m2; m0 Pomuum 0v. 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Amve o aoo oapam soas .mpovwmapmm>ca m50anw> an cocawppo spud pcochawm HHw Mo thEESm m manomopmmh one Canon aon ov coumHma ma noag3 nmwhm mo opwchho «Amvm o.as u endows; amacm.om n saoo-omm_x EUwacx@ AK «concmB HwaowupopsH 00.H msm.o 00.H 00.H 0mm :Hoo .mo w«*axpawooma> moaphmmonm Hmoamxnm a \ .4- ON... . New; N.“ ’“ooofx o.aa. saw ow: v.8 x . ’Illlv_00> Fm. m. ”CWT? mawvmnflmvwnqu NNQ m.0_ mm m mm. m. 0N; F L .o . o\.aw a Q axpamcom ESUHA 0m: Awmv wasnxooH was meammom x9 pomomoam coHpmdwm map so domwm mcoprHdono chom UOOHm xxx mqm HoHonhmgsm so mpoom mamddm map 00 85m mCESHoo pmmHSQCD CH mmvmm mcHUOOHh prcmEHammxm was ompwHDOHwo Hxxx mqmda 125 FIG. 6 5:: Stooqu> mwhdi OoN OmN O¢N ONN OON on. 00. 0?. ON. 00. on 00 O¢ ON 0 o cub/Phat: 5w: oz N ON 110.? o/ o/ . 5..»z<¢s »»«z 1 lb? //\.5u»z<¢s »» 5.5» o — _ H H L H H _ 00- mmmmzst. mm<2 .50....53 024 It? mzszjoo ommusn. 2_ 02.0004... 55 :8 z. uzosuo< oz uH/la Mum-13A ’70:) 126 FIGJ ¢I\._I._ Ph.-comm) 15.43 OQN owN O¢N CNN OON 00. 00. 9. ON. OO. 00 OO 0 d 0 ON / O of 011/ 0.? 5.34.5 »» 5.5» o 7/ _ . . _ . _ _ . s... «mum-,3”: mmdz PDOIEB 024 It? szDJOO owmqan—ZD 2. 025004“. 127 FIG. 8 OON OON OVN OON 00. 5F. ..Eood.’ 52; 18... of on: OO. O0 00 O? ON LIII. II 0m043a ON 0] // 002.2 0.10m4m 002.2 0&0042 22 N. / m \ / /. to 3 8 8 ‘AJJOO'IBA '30:) .8 fill-Ills 05.. J3 z. uzosuo... 8. . . 52;. 2.62;. a: u. >x 5.5» o 5335.... a: » 5.25.26 5. as: 2255:... 52... 252;. ...... » ...x 5.5» o 55.. 2.8»? a... u. 2.. 5.5» o 5...... oz 5:... 252;. ...... o ..x 5.5» o _ a L F L L _ L _ _ T o 3 OO. 295242... mm42 It; mZZDJOO oumufiaz: 024 owmqaa 2. 02.0004“. Om. 128 SECTION II MASS TRANSFER IN PACKED COLUMNS In this section HETS values for various flows, pulsation rates, packing characteristics and other conditions such as interface positions are discussed. The data have been presented in Tables VIII through XV of the Experimental Results. Effect of. Total. {laws on EELS. In pulsed and unpulsed columns HETS values are strongly influenced'by the total flows of the two phases. This is Observed in Figures 10 through 19. In pulsed columns HETS values are poorest (i.e., largest) at flooding and they improve (become smaller) with decreasing total flows. Even at high pulsation rates HETS values are not insensitive to total flows. The changes in HETS are at least as large at high pulsation rates as the changes at low pulsation rates for the same percentage changes in total flows. For example, from Figure 13, with.water as the continuous phase, HETS is 2.2 feet at flooding. At 50 percent of maximum total flow HETS is 1.67 feet. The change in HETS is 2h.l percent at the high pulsation rate of I25 cycles/min, 25 mm.ampe£es. From.Figure 12, with water as the continuous phase, HETS is 3.85 feet at fIOOding. At 50 percent of the maximnm'total flow, HETS is 2.92 feet. The change in HETS is 23.2 Percent at lower pulsation rate of 125 cycles/min, lO mm.amppaea~ Also the largest relative reduction in HETS occurs when water is the continuous phase, and the least when CClL is continuous. These results are in contrast to Observations reported for sieve plate columns in which 129 investigators and observed in the present research also. Figure 9 presents HETS as a function of flow ratio for pulsed and unpulsed operation. The curves in Figure 9 indicate a con- stant HETS for flow ratios above 0.5. The low values of HETS are appar- ently due to a reduction in maximum total flow. The reduction in flow rates is the result of small drOps that form when the mass transfer rate is re- duced in some section of the column. Small errors in the analysis of acetone concentrations in low con- centration exit streams introduce large errors in HETS. It is believed that the spread of values at the low ratios of CClu/HEO is due to the very low concentrations of acetone, which had to be analyzed. This is Observed in Tables VIII and X of the Experimental Results. Effect of Column gameter 93 HETS. In pulsed columns HETS is not :influenced by column diameter, i.e. the "wall effect" appears to have a ruegligible influence on HETS. Pulsing of the liquids distributes the czcxndnuous and dispersed phase uniformly in the column, thus reducing <3<>nsiderably the tendency towards channeling. This is Observed in Figure, .113. HETS is plotted as a function of total flow rates for two different <341£nmeter columns. The packing in each case is 8 mm Raschig rings. Effect _o_f._’ Backing Eharacteristics 2n REES. Unpulsed Columns. In unpulsed columns the efficiency of transfer b81¢:er flooding is not greatly different for the two types of packing Elnriloyed in this research. The total flow rates have a greater influence CH1 IHTTS tbsn.the packing characteristics. This is Observed in Figure 19. 133 At flooding, the high values of HETS for the l2-mm packing and the low HETS values in 8-mm packing are directly due to the total flows. The flow capacity of the l2-mm packed column is considerably higher than that of the 8-mm column. It has been observed throughout this discussion that high values of total flow are accompanied by high values of HETS and vice versa. The values of HETS at flooding for the 8—mm and 12-m packing are ob- served in Figure 9. The flow capacities with mass transfer are observed in Figure 8 of Section I. Pulsed Columns. At and below flooding, efficiency of mass transfer is increased sharply in 8—mm packings with pulsation (Figures 9, 18, 19). The increase in efficiency for lE-mm packing, however, is not as great for the same pulsation rates (Figures 9, 18, 19). For the 8-mm Raschig rings, this. is due to the sharp decrease in drop diameters and increased holdup of the dispersed phase. Also, some improvement can be expected as a result of increased turbulence during pulsation. For the l2-mm packing, changes in drop diameter and holdup are not as pronounced because of the large voids encountered in 12—mm packing relative to 8-mm packing. The large discrepancies are apparently due to the effect of packing c=haracteristics on the mechanism of flow, and the mechanism by which small d«Peps are formed in pulsed operations. In small size packing, the drop Size is influenced not only by,-the physical properties of the liquids but also by the. size of Openings or voids in the packed column. In contrast to this, the drOp size is not influenced greatly by large size packings. The voids are already larger than the average equilibrium drop size. In 13h small size packing the drOps apparently are slowed up considerably by the packing and must distort to pass through the voids. During pul- sation, the drOps in the small packing are broken down by collisions. In the large packing the drops simply pass through the large voids as they normally would when no pulsation is imposed on the system. Therefore, the increase in transfer area is considerably greater in small size pack- ing than in large size packing. HETS for Inlet Tubes. The simple inlet tubes employed in all the runs of this research did not contribute to HETS. This is apparently due to the single opening in the tubes and the placement of the tubes directly into the packing. This is observed in Table XV. The number of transfer stages, NTP, in the inlet tubes are very small for both pulsed and unpulsed operation. This is not in agreement with those investigators who claim that significant transfer occurs during drop formation at the inlets to the extraction zones of spray columnso m~w~ 1 71.54.1114}.-. i :4: - _ ._ H . l .I. ’1'. .-ul...‘|v||. ,‘Ilt.n.tl.l...l 1'1.“ Lw.l $2.: 95qu 22o ..xx mmium o 32;. Exams. :2 u. :33. 3.3.... o 14.1- . . i . _ _ v m N _ 0 . a . a 4. a fl . i l . _ --., Flui‘lhl'lclii—II'iLIIl 1115.111- --boil. “ 1 T1 L1111I‘ 31.1.,£.1+.111 11 o - .lr.. lllwmiulllll mozE oiomam 3:2 m =3. wmimm o . I NMJDQ OZ mg.¢ 0.10m<¢ 22 N. ...)x memm 0 . IIIIIFII" .91.I|||II11J. I1 '71 LT. 1L1 .lvl m. -.- ...... a» _ F H Fl. wZZDJOo oumqaaza oz< cumusa 2. @2503“. .2 0:3“. so: m> E: 1".1'1'111'1 9.11I .IOII 1.119 I] .9111! w 1 V H” mm 1 1| -olnltllll , w 332.22 o , w i w ,1. . . 1 . 1 .1 ..-- - 1 . c. . _. , ... m. i _ I _ _ m m . a . i l a _. _ l _ l _ .. l . .r ” Till}?! I w .11, 4 J» ...... 111.1 11,11;--!111..1..l1- ..1 i A. . _ i . 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Ch 00 ON. om On 0? 0m ON 9 T $3HON1 w. ‘SIBH FIG. l0 a a l l l m . i l a m. 1 +1-1.11r- 1+. 11, $ .111. 1-.--1.*.1 -1. . .H A l 1 H _ l a . i _ , _ a H i m l M 1 1 L_r111l.lu -55] 11||||+111. 1 1111 |1LY11|11 .1 1w- 1 11 1 f 1 :911 . 1. 1 _ M m _ m I M , ”kl H11LH11111 1 931-1 + k : k1 -1 1.1 l h . i l i P. i any _ n. :1--. 1+1 . 11.11 1 q l r m \ m I M M _ i W. m H i 1L. ---.--l- 1..- 1-. - I . . _ _ n H \ : l M i M _ 55:50 3.82. ~n.m o H m I _ 55.23 3.82. ~n.n O P111111”- 1 .3 25.3.3; 55.25 3:2... 3...“ o _ oLX 2: 32.3 V 1 33224 22 n w .z_z\mm..o>o 3. “use 29533 1% ~Ro~xxcwoo "SSE 33.. F, Sun. «48 z. 2358 o\o_ mmmhmzda hzmmwmma “.0 $2231.00 03». z. >h_oo.._m> 44.0....mma3m m> mew: FIG. II HETS VS SUPERFICIAL VELOCITY FOR DIFFERENT INTERFACE POSITIONS IN UNPULSED COLUMN now RATIO: ecu/1120 z 2 12 MM RASCHIG RINGS FERIES MI 0064 commuous muss SERIES xx INTERMEDIATE INTERFACE —— -— H20 commuous PHASE, CALCULATED rRoM xx1, XX ___ __ _ _ --- ”--..-. : _ ----.i-.._-_-._,_,_--_. “"“"”““'T’“ "T Y Y 1 Y Y 1 . 137 b I—~~— 4 . e? '— I I u. m“ .. _..__ I- I w : I ‘ I 4 "T T 3 all 1 1 ; F I ". - 1 + y 1f I ’ T. 2 .._.__..._-._-__i_.---_ -1 -, _ 1.. i. ; , I .; .1 I I I ‘ I ' i i I I l i l j _ i J' 2 C 40 60 80 100 120 140 160 ISO 200 240 TOTAL SUPERFICIAL VELOCITY, FT/HR arch {hoods usoimmanm 439 0% 00... 0mm 0mm 0% 0mm com 8. ow. oz 03 oo_ om om ow 0m 0 i 138 fl « l . _, _ I 1 N 13‘313H 3.5:. .3022:on 38 xix 3:5» 83: maoozczoo ON: 53:. «Baum uoqazupz. 34.3252. ..>: museum 325 0.5.0.: 2: N. N R 0m: \vuuu 6:5. 30.: 33.624 :2 0. £53386 3. ”20.233. m._.._._oo..w> J<_o_ammn5m m> whm: FIG.I2 139 FKLI3 xxx: ghee-g, .EoCmmasm ...Soh O¢m ONm 00m 0mm OGN OVN ONN OON Om. 0w. 0?. ON. 00. 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L -1 .1..- wozE 0.10mdm 22 N. xx mw.¢wm. mozz... oiowdm 22 m x.x mmem . mo._._001_m> J<.O_n_mm&3m m> mkwI FIG.|9 ID 13‘513H 1116 SECTION III CALCULATIONS OF MASS TRANSFER IN PACKED COLUMS In this section a brief discussion of the calculations of mass transfer coefficients from fundamental and semitheoretical considerations is presented. The over-all mass transfer coefficient, KGa, calculated by the method outlined in Theory and Calculations of this dissertation, is in V good agreement with the observed or experimental values. The agreement I is good not only for the data collected by this investigator but also for 'previously reported extraction data in packed columns. The calculations and experimental values of KCa have been tabulated in Section III of Experimental Results. It is difficult to pass Judgement on the general reliability of each of the intermediate results in the analysis since many are not subject to measurement without elaborate and extended research. For example, drop diameters, drop velocities, holdup, effective mass transfer areas, average distance of drop fall between collisions, and velocities of each phase relative to the interface are some of the intermediate results which were not measured directly. However, the analysis which makes use of these intermediate results does indicate the merits of the over-all approach. The agreement between observed and calculated results is considerably better than was originally expected. Calculation of KGa. It is seen, from Tables XVI to XXV, XXVII and XXVIII, that the analysis proposed in the Theory and Calculations 1117 section has considerable value in predicting KGa. The method for cal- culating values of KGa can be considered as two separate calculations. First, the mass transfer area, a, can be determined if the droplet diameters, d , and the disperse phase holdup, x, are known. Second, vs the mass transfer coefficient, KG can be determined from kL, kG and D, i.e. l/KG=D/kL + l/kG. The resume which follows is not to be construed as a recipe for arriving at final answers but rather a recapitulation of the general approach. This approach could well be extended to other types of extractors. Evaluation of Droplet Velocities, voi, The velocity of droplets relative to the continuous phase depends on droplet diameters, dif- ference in density of the two phases, viscosities, and distance of fall. Theoretical calculations can be made provided that the drop diameters are known. In packed.beds the droplet velocity is some fraction of the Stokes' Law terminal velocity. The exact relation used in the calculations is given in Appendix B. Evaluation of Distance of Free Fall. The drops fall an average distance which is a fraction of the height of the voids in the packed bed. In this work the distance recommended by Pratt and co-workers is used; namely s=O.38dp-O.92(7%Af’g)%. This imposes the limitation that the packing size must be at least equal to or greater than 2.h2 (7%Af’g)%. Evaluation Of Drop Diameters, d‘Vfl' lI'he drop diameters are deter- Inined'by the drag forces that break down the drops and surface forces tunat hold the drops together. .A drop diameter is calculated from the 1118 balance of the forces acting on the drop. The drop diameter resulting from the balance of drag and surface tension forces is corrected for the effect of the liquid velocities to give the diameter of the drop in the packed column, dVS=O.92(7%§P8)% (vO/v). Evaluation of Holdup, x. Below flooding, the holdup x is a function of the flow ratios and fluid velocities. It is determined from the expression, Vd/ex + VC/e(l-x)=vo(l-x). At flooding the holdup is taken as 0.5 when the flow ratios of the phases are within normal Operating limits, i.e. 0.5‘2 Vd/VC‘Z 1.5. Also at flooding the holdup can be estimated from the relationship x=(R—f§)/(R-1). Evaluation of kG and kL. These mass transfer coefficients are evaluated from mass transfer factors, J, explained in terms of the Schmidt number, Sc=Dffl#%% and Reynolds number, Re=DpurF%u. For the Ergun correlation used in this thesis the mass transfer factor, J, is [6(Sc)k/ur] c,d and the Reynolds number, Rec’d is (Bur-cI"c€(l-x)/(ap-1~ad),1.l.C or 6urdfhex/ad;td, depending on the phase considered. Evaluation of urc.rd. The velocities, urc and urd are the effective velocities of the continuous and dispersed phases in the packing relative to the interface. When the viscosity of the two phases are equal urc=urd=ur° When the viscosities of the phases are not equal the Velocities urc and urd assume different values. The more viscous phase attains a velocity relative to the interface which is less than the ‘Velocity attained by the less viscous phase. For this thesis an inverse Irelationship is assumed between the velocities and viscosities. 1119 Comparison of Characteristic Drop Diameters and Drop Diameters in the Packing. According to the theory the smaller the velocity in the packing between the dispersed and continuous phase the smaller the drag forces. The drop sizes that accompany small drag forces are large. This is observed in all of the tables by comparing drop sizes dO and dv‘. The former value, d , is the characteristic or terminal drop size 0 that corresponds to the maximum or characteristic velocity, v0, between the drop and continuous phase in the packing. The latter, dvp, corre- sponds to drop diameter at the fluid velocity, v, in the packing. Increased flows are accompanied by increased holdup of the dispersed phase and consequently smaller values of fluid velocities in the packing. This is observed in all of the tables in Section III of the Experimental Results. These results are fortunate because as the holdup is increased the drop size is increased. This compensating factor apparently cor- rects somewhat for the normal crowding of droplets at high holdup. However it may not account for the abnormal variations in coalescence that result from the direction of mass transfer. Film Type Flow vs Droplets. For the situation that the dispersed phase wets the packing, the mechanism of flow could be different than when the dispersed phase is the nonwetting phase. Film type flow and droplets can occur simultaneously when the dispersed phase preferentially wets the packing. Film type flow means that the wetting phase spreads over the surface of the packing and flow is along the surface of the :packing. 150 The fraction of the dispersed phase that flows as film and the fraction that flows as droplets are difficult to determine. However it is interesting to compare the limiting cases: i.e., 100 percent droplet flow and 100 percent film type flow. For film type flow the area for transfer is approximately the area of the packing regardless of the flow ratio and holdup. However the effective area for transfer is apparently less because of stagnation zones in the packing. This is observed by comparing the results in Tables XXIII and.XXIX. For drop flow the calculated values of KGa are smaller than the observed values. For film flow the calculated values are larger than the observed values. Holdup of Dispersed Phase; A Comparison of Equations at Flooding. The experimental flooding rates appear to support the fact that the sum of the square roots of the superficial velocities is nearly a constant. This is observed in Table XXXI which presents data from this research and Table XXXII which presents flooding data from Dell and Pratt for the system methyl isobutyl ketone and water. In Appendix D it is seen that the equation referred to as Case I, i.e. Vd/EX + Vc/e(l-x)=v0, leads to a constant (V? +.V§) at flooding. On the other hand Case II, i.e. Vd/ex + Vc/e(l-x)=vo(l-x) does not work. On this basis it is apparent that Case I is closer to the actual results in variation with flow ratios. In packed unpulsed columns the holdup is better approximated.by using the results of Case I, namely that, x=(R-,/R)/(R-l). 151 mm.ma o.mma o.aa ooH. oo.m mm.bH H.mma 4.0m mama. mm.: Hm.sa n.mmH mw.aa ammo. o.wa ww.oH 0.:m :.ma mmm. mm.a oo.ea n.moa mo.ma ooa. o.m nm.ma H.mo m.mb mmn. mam. mo.wa m.mma m.0m com. 0.: os.mH m.nm m.mm ems. em. oo.oa o.am o.ao oon. o.H mm.ma m.mH m.maa mam. ea. mm.nH m.mm n.mHH mmm. m. nas>+wo> a BEL a> Ao>\§+3\a fl Nesta; “a SD.“ 8> Aug/$33} o>\e> Amav HmpcmeHuoexm covdHSUHmo Hon m1oH x m.» omega o.on tz o». u o ”new... .. rims. ..-... ...me 3 .... é .. mmm M.“ ofipmfiuwpowamno omwwwmwwwsmmmw amaze .x..s0fimcm9 mpM\mnH mm m v we H ammcfiu manomwm Hmaommpoch panama was ad swab Sosa m .nesaoo AMHV mofisvfiq was mofipmahmpompmso nesaoo .2 .3 H.L, so. . :.T. ......) ..LflLfl2: Amav pumpm was Hamo hp oomOQOLm cofipmswm on» so vomwm mcofipwHSUHmo pcflom vooam HHxxx mqm<9 152 On the other hand at flooding for normal flow ratios, i.e. not far from unity, a value of 0.5 for holdup may serve Just as well. This is observed in Figure 20 which indicates that the holdup is 0.5 at flow ratios of unity for either phase dispersed. KGa calculations based on x=0.5 and x=(R-/R)/(R-l) are presented in Tables XXI and XXII. Figure 20 also includes flooding curves which indicate the varia- tions in the sum of the square roots of the superficial velocities and variations in holdup for either phase dispersed. The system employed for the calculations is MIK and water. In addition to the equations of CasesI and II, the empirical equation of Dell and Pratt(13) was employed to calculate a flooding curve for comparison with his experimental data for the system MIX-H20. As expected, good agreement is observed since the data was used to establish the equation. In this research it was discussed ear1ier that regardless of which phase is dispersed below flooding, at flooding the water phase, which preferentially wets the packing, is continuous. In certain situations this involves a phase reversal. If however the nonwetting phase flow rate exceeds the wetting phase flow rate by a large amount, as by a factor of 10 or greater, it appears logical that the former would be the c=ntinuous phase. According to Figure 20 for MIK/Héo equal to 10, if MIK is dispersed the holdup would be near 0.8, an improbable situation. with phase reversal the holdup would be closer to 0.2 a more likely Situation. Therefore, it is possible for phase reversal to. occur, when the flooding rates are approached, so that the nonwetting phase becomes 153 mo.om m.mmm m.mm ms. ooa.o o.m mo.om m.wsa s.:a so. oom.o 0.: so.oa n.mm mmm.o con. mmoo.o Aoooa mo.om m.mma b.mw mm. mmm.o o.m mo.ma o.mm om.m mos. amo.o 0.0a mo.om n.noa m.nm com. osa.o oa.a om.ma :.mm mn.om mma. om.o 0.: mo.om a.mm o.moa com. smm.o om.o 0:.ma m.mn o.aa mmm. on.o o.a mo.om m.mw o.>ma nae. www.o om.o mm.wa m.aa o.am mam. swo.o n.o mo.om w.on o.ana mam. mms.o mm.o oo.wa na.ma o.mma sea. am.o a.o mo.om m.mm o.aom am. mow.o ba.o no.om o o.moa o oo.a o mo.om o o.moa o oo.a 0 mafia? Sines page}, x aommestaV} o>\e> We>+mo> HEMP, BEE; x Aomm>\oaaz>+a~\a o>\e> HH mmwo H mmmo mom.o n.w .uexpa aom u o> «ma: "swarm emmamaman AX1HVO>V Axnav\o> + x\w> "HH ammo o>v C?.C\o> + X\o> "H mwmo popm31tz Empmhm map hog HH ommo new H mmmo mo mCOHpQSvm map so oomwm msoapmHSQHmo vcaom cooah HHHE Emma. 15h mm.ma o.mwm o oo.a oo.a o. mm.ma o.mam mm.mm ms. om.o o.m mm.ma o.osa m.ma em. om.o 0.: aw.m o.aa wa.m com. mm.o o.ooa mm.ma m.mma m.mo mm. smm.o o.m mm.ma o.aa as.m mos. am.o 0.0a mm.ma m.ooa a.am on. amm.o oa.a mm.ma m.ms mn.ma mma. ow.o 0.: mm.ma m.om w.ooa on. asa.o oo.a oo.aa m.mn m.mn mmm. om.o o.a mm.ma a.nm m.oma aaa. mmm.o 00.0 mm.na m.ma m.w® New. mmm.o n.o mm ma m.mn m.maa mam. mmm.o em.o mm.sa mm.sa m.wsa sea. amo.o a.o mm.ma m.0m o.ama am. mma.o ma.o mm.ma o o.mwm o o o mm.ma o o.mwm o o o www.mt> ae\pm.s> pe\pa.o> x Aomm>\eaz>+av\a o>\s> me>+mt> pa\pc.s> aa\pm.o> m: Aomm>\aazs+av\a o>\s> HH mmmo H mmmo mow.o n w.~pn\pm mm: H o> momm "mmmnm domhmmmfim a see ES aaa Ao~=>\.:: 155 > + :2 o._ no owe We no no am no ~.o ..o flw No.4 o>\ ..> . m ...o. v18\3\ E $1 . x £33 11 a. c-5>.m\.-$. x .a 83 .. no. owxii mom \ a ozaood .2 .530: no. \\ / :3... 3.4 .33 5 20.533 :02... 8.2.533“ o1 oumxwama x34 \ mhzam 4 1.39. .H mm u 1. 1 _vv\¢z+?v\o>v uH wmdo o~:-x.z mo“. 83.8 3.83.. H oumcuama xi FIG.20 156 continuous. Apparently in this research the flow ratios were not far enough away from unity to observe the nonwetting phase as the continuous phase at flooding. Effect of Interface Position on Calculated KGa. It is interesting to observe that the semitheoretical calculations predict the trends of KGa values which are consistent with the choice of the dispersed phase. Table )OCXIV summarizes some of the calculated and observed KGa values for various situations of interface position. It is seen that calculated KGa values are highest (HETS smallest), when CCl)4 is dispersed and smallest (HETS highest) when water flows as drops. This is also observed experimentally. For the intermediate interphase situation it will be recalled from Section II of this discussion that HETS values are intermediate between the values observed for a continuous water phase and continuous CClLL phase (see Figure 11). This is also observed in Table XXXIV, when it is allowed that the calculated KGa results for the CClh dispersed phase lie somewhere between the values for the droplet flow and film type flow Situation. Calculations for Pulsed Columns. With pulsing, the agreement between calculated and observed values are fair to good. It is difficult to correct for the pulsing action on the hydrodynamics of flow and the resulting drop sizes. The results are tabulated in Tables XXVII and )QWIII of the Experimental Results. The modification of the calculations (see Section III of Experimental Results) for the pulsing action was made consistent with the observed 157 n.mma-oo.mma :.ama-oo.ama ao.mm -om.mn ma.mm 10a.am .33: hpaooao> Hmaoamummsm O.NH m0.0H mn.m n~.m 3a manna om>hmmno muonsapcoo .omm aux ea.m 00.0 mm.m mm.m mH.ma O#.OH mfi.m Om.:H mm.» mm.» am.m sm.ma w®.m m®.: m:~.a mm.>a NO.H mw.m mhm.o N.®H >KN manna xx moapmm e5 manna >39 93.8. dopmgoawo 3% 3me 93.6 30H.“ mdoscapCOU .omm vo>hompo machomav Edam max mommnoVCH Umpddfioamo mpmaumEhoch wsoscapcoo :Hoo .3. «or 0mm Op :Hoo scum coupoMmcmnu oCOpoo< on soapmmadg on .mCHUOOHM Sodom ADV m n omEaaoo 63E scam A3 Omm UCG JHUU MOM mmSHd> wox HQ¥C®EHH§ Ufld 60$ UdegUHdU ..HO bgm >Hxxx mqmpompo mooscapcoa :Hoo aux 158 phenomena, that the holdup is higher and the drop size is smaller. This can be accomplished in the equations by reducing the characteristic velocity of the drop(by an amount equivalent to % the pulse velocity). There is Justification for this because it is logical that the maximum fluid velocity of the fluids relative to each other are reduced with increased pulsing. In fact, for extremely high pulsing rates the drops will remain suspended in the continuous phase and the velocity between the phases becomes zero. Another approach is offered for future consideration. The drop sizes in unpulsed columns are dependent on the acceleration of gravity. In pulsed situations the effective acceleration is increased. If this is considered in the theory smaller drops would be calculated with increased acceleration resulting from increased pulsing. This would lead to smaller maximum fluid velocities such as the characteristic velocity, v0, and higher holdups. The re search: 1. 159 CONCLUSIONS following conclusions have been made as a result of this A theory is presented which leads to calculations of over-all mass transfer coefficients, KGa, and HTU that are in good agreement with experimental results. The agreement is good for the results of this research as well as for results reported by other investigators with various extraction systems. Mass transfer from the dispersed to the continuous phase aids coalescence. High throughput rates are obtainable in packed columns when coalescence of the small drops occurs as a result of mass transfer from the dispersed to the continuous phase. In the absence of mass transfer, or when mass transfer occurs from the continuous to the dispersed phase coales- cence of small drops is inhibited. A reduction in maximum flow rates in packed columns results when the small drops do not tend toward coalescence, as a result of mass transfer from the continuous to the dispersed phase. Unusual variations in flooding rates occur when the flow ratio of CClh/HEO is low and mass transfer rates are reduced in some section of the column. The flooding curves are 10. 11. 12. 160 S-shaped. These variations in flooding rates are explained in terms of mass transfer and coalescence phenomena. In pulsed columns with mass transfer from the dispersed to the continuous phase the S-shaped flooding curves are more pro- nounced than in unpulsed columns. This is due to the increased efficiency of mass transfer with pulsing. In small size packing continuous operation with liquids that contain impurities which precipitate on the packing may lead to increased flooding rates with time. This is explained by bridging of the packing, reducing the randomness of flow. In large size packing the individual packing elements are separated enough so that bridging of the packing elements does not occur. Pulsing of the liquids inhibits bridge formation. Phase reversal sometimes occurs in packed columns. Two explanations have been offered: One is based on preferential wetting of the packing by one of the liquids, the other on a minimum voids cross-section that results from random packing. Phase reversal does not occur with pulsing. The equation of Hoffing and Lockhart, representative of empirical equations that are frequently used to predict flooding rates in packed columns, must be used with caution. These correlations do not consider the effects of mass trans- fer, dirty packing, or localized minimum voids that occur in random packing. 13. 111. 15. 16. 17. 18. 19. 161 The efficiency of mass transfer (i.e. low HETS) in both pulsed and unpulsed situations is increased with reduced total flows. Even at high pulsing rates, the increase in efficiency as a result of reduced throughputs can be as high as at low pulsing rates. A 50 percent reduction in total flow is accompanied by as much as a 25 percent reduction in HETS for low and high pulsing rates. HETS values are not dependent on flow ratios. In this research the efficiency of transfer is highest when the water phase is the dispersed phase and lowest when the CClu phase is dispersed. Calculations'from the theory ~' agree with this also. HETS at high pulsing rates is the same regardless of which phase is dispersed. HETS in pulsed columns is independent of the column diameter, i.e. the wall effect is negligible with pulsing. In unpulsed columns the efficiency of transfer below flooding is not strongly dependent on the packing size. The total flow rates have a greater influence on HETS than the packing characteristics. With pulsing the efficiency of mass transfer increases sharply in small size packing. The increase is not nearly as great in large size packing. This is due to the drops breaking down by collision in the small size packing. In the large packing 20. 162 the voids are considerably larger and the drops simply pass through as they normally would in unpulsed situations. Expanded end sections and single inlet tubes had little influence on flooding rates and the mass transfer efficiencies in this research. ad NTUOL NTUOG 163 NOMENCLATURE column cross section ft2 surface area of drops, disperse phase, per unit column volume ft"'l surface area of packing per unit volume of column ft"'1 distribution coefficient D a Y/X* = Y/X = Y/X coefficient of diffusion ft2/hr nominal packing size ft characteristic diameter of drOp ft mean diameter of drOp ft drag force lb light phase which flows up the column ft3/hr gravity ft/hr2 column height above the base ft over-all height of transfer unit--liquid or heavy phase film ft over-all height of transfer unit-~gas or light phase film ft or HTP, height of theoretical plate ft mass transfer coefficient for liquid or heavy phase film, gas or light phase film defined by equation (1), (2) in Appendix.A. ft/hr over-all mass transfer coefficients based on liquid or heavy phase film, gas or light phase film defined by equation (3), (h) in Appendix A. ft/hr heavy phase which flows down the column ft3/hr number of transfer units over-all n. er of transfer units based on liquid or heavy phase film 2‘ x2 (X-X*)'l dx 1 over-all number of transfer units based on gas or light phase film Y1Y2 (Y*-Y)'l (II I NTP Red Sc =#/DfP urd urc ur=urc=urd Vt VdAEx: VC/€(l-x) X Y Xiin 16h number of theoretical plates ratio of superficial velocities of disperse and continuous phase Vd/Vc abscissa for Ergun Correlation Rec, Red (Dpr)Au.Reynolds number [611“; pc 6 (1“ ”111% + ad) lie] 1 [Burdpd 6‘1de I‘d] average distance that a drop falls in a column s e .38Dp-.92 ( 7 )"2 for packed column ft 2356' time hr Schmidt number velocity of continuous phase relative to interface ft/hr velocity of disperse phase relative to interface ft/hr when viscosity of both phases are equal phase velocity relative to empty column ft/hr continuous phase velocity relative to empty column ft/hr disperse phase velocity relative to empty column ft/hr characteristic velocity of drops ft/hr mean droplet velocity relative to packing ft/hr mean droplet velocity relative to continuous phase ft/hr terminal settling velocity ft/hr actual velocity of dispersed, continuous phase relative to packing ft/hr concentration in heavy phase lb-mol/ft3 concentration in light phase lb-mol/ft3 concentration at the interface lb-mol/ft3 165 X*,Y* concentration in equilibrium with Y,X lb-mol/ft3 x volume fraction of voids occupied by disperse phase l-x volume fraction of voids occupied by continuous phase G volume fraction of column occupied by voids l-c volume fraction of column occupied by packing ex volume fraction of column occupied by disperse phase €(l-x) volume fraction of column occupied by continuous phase 7 interfacial tension between phases lb/hr2 1i viscosity lb/ft-hr density lb/ft3 [SP density difference of phases lb/ft3 Subscripts refers to continuous phase d refers to disperse phase 1 refers to interphase p refers to packing ( )1 subscript 1 refers to bottom of column' ( )2 subscript 2 refers to top of column 10. 11. 12. 13. 111. 15. 16 . 17. 18. 19. 20. 21. 166 BIBLIOGRAPHY Appel, F.J. and J.C. 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Geankoplis, Ind. Eng. Chem., 5Q, 1763 (1951). Wicks, C.E. and R.B. Beckman, A.I.Ch.E. Journal, 1, A26 (1955). Wilke, G.R., Chem. Eng. Progr., 35, 218 (1919). 169 APPENDIX A EXPRESSIONS FOR MASS TRANSFER In this section some of the relationships commonly used to express tower performance are developed. Consider a tower in which two immiscible fluids are continuously contacted. The composition at any cross-section of the tower will be Y and X for the light phase, G, and heavy phase, L, respectively. Assumptions: a. The immiscible liquids are separated by an interface bounded by the heavy phase liquid film.and light phase liquid film. b. The flow rate of each stream, L and G, remains constant throughout the tower. c. The distribution coefficient, D, is constant. d. The driving force is the concentration X and Y in lb-mols/ft3- Within a section of tower dH the rate of transfer of solute, lb-mols/hr. can.be expressed by the relationships: 6.112 Ma de = GdY = kLaAdH1X-Xi) (l) = ksaAde-Y) (2) A (H = KLaAdH1x-x“) (3) 6.1!. 1.,xI = KGaAdH(Y*-Y) (1+) The mass transfer coefficients kL, kG are defined by equations (1) and (2); the over-all mass transfer coefficients KL, KC are defined 170 by equations (3) and (h). The combined expressions kLa, kGa, KLa, KGa are often referred to as mass transfer or over-all mass transfer co- efficients based on unit volume of tower. Integrating equations(3) and (h): 2 NTUOL =f| [ax/1x-wo1] = (KLCAHVL (5) 2 NTUOG =1; [dY/(DX‘Y)] = (KeaAH1/G (6) Applying GdY = de from (1): «ox-v1 = (DG/L)dY-dY = [1os-1.)/1.]av d(X-Y/D) = dX-(L/DG)dX = [(DG-L)/DG]dX Substituting for dX and dY in (5) and (6); mums [DG/(DG-U] f1 d1X-Y/D)]/(X-Y/D) (7) = [DG/(DG-L)] 1n[1xz-x§)/1x.-x’f)] 2 NTU . 1. - - - o. [/1013 1.1]f' [d1ox v1]/1ox v1 (8) = [L/(DG-L)] In [1Yf-Y21/(Yf-m] Using D = Y2/X2* = YE/Xe: 1xz-x§1/1x.—x‘.‘1 = (flag/($11.1 (8a) From (5), (6), (7), (8), and definition of HTUzH/NTU: NTUOL/NTUOG 3 06/1. 3 HTUgg/HTUOL 3 (KL/L1/(KG/G) (9) 1=11 - fR—1/(R‘I) a 1 x vc/K 1 Vd/K 1 {Vc/K 13 + Chi/“)2 7L- _- 1 -.-_~-.--.- .4. _. _._._____,- t :_.:_._,_._--_ -.--1 - ,_ < -- - ~24 0 0 3 1.0 : 0 1 0.5 0.414 1 0.344 0.172 1 1 , 050 1 0.25 1 0.25 1 2 1 0.586 1 0.171 ' 0.342 1 100 1 0.91 2 0.00827 . 0.827 1 1 L111. _ ___'_ .0. .... 1 O I "00 ' ... CASE III R/[(l--n)x]-I~I/(l-u)2 = K/Vc = (R-Rx+x)/[x(l-x)2] Vc = Kx (I-x)2/(R-Rx +x) dV/dx = K [(R-Rx+x) (l-4x+3x2)- 111111-1112 (I-R)]/(R-Rx+x)2 0.0I 0.|0 l.0 4.0 0 2 2 2 3 (R-Rx+x-4Rx+4Rx -4x +3Rx ~3Rx + 3313- x+2x2 -— 113+ Rx-Zsz + R13) 0.064 0.! 67 0.333 0.423 0.50 2 (2-2R)x + 3Rx - R R-4Rx- 2x2 + {>sz + 2113- 2R):3 R -4Rx+ (SR-2113+ (2-2R1x3 Vc/K 1 LC 0.762 0.464 0.l48 0.05l2 - (-3141 Jsaz— 8R2 + SR )/(4-4R) (~3R2/m)/(4-4R) [-3 + W]/[14m1-4] Vd/K ? (we/K) 0 0.0076 0.046 0. I 48 0.2048 ' ..-.-.___._.L --. I79 180 m.maa ofimm. Hm.mm mnm.m 0mm:. mmwm. -.::H xmnzm ea ma H.mmm ammo. ma.m: :mm.m owma. mmmz. >w.>ma mmwau mafiq ea m mpm\pa mpg\mpm cowpomgm zpfimcmm use as aw mp0\mpa 000< ufio> gasm mafim\mma< wsfim\0EdHo> wcfim\pz mpfimcmm Hmflgmpwz mwcfim wfinommm moHBmHmMHoNNN mum<9 m:.m :®.mm mufihoagomypma cophwo mod 2 mmo.a m:.m 0:.mw hmpwz mun\mpH 0:-00\mpa pm\mna sofimcma HmwommpmpsH «hpamoomw> pfimcmm AOOmmnooomv mma<3 mz< iHoo mo mmHemmmomm >xxx mqm