Extraction in Spray and
Packed Columns T. IC. SHERWOOD, J. E. EVANS, AND J. V. A. LONGCOR Massachusetts Institute of Technology, Cambridge, Mass.
Data are presented on liquid-liquid extraction from single drops which indicate that the interior of the drop is not stagnant but is considerably agitated. Data on extraction in spray and packed columns show an initial increase in extraction coefficient with increase in rate of flow of either continuous or discontinuous phase, presumably due to increase in interfacial surface as the holdup increases. A subsequent decrease in the coefficient at the highest flow rates is explained as the result of drop coalescence, this being especially noticeable at high rates of flow of the continuous phase. The coefficient is greater if the discontinuous phase does not wet the packing. The coefficients are largest in the packed column but the flooding rates are greatest in the spray (unpacked) column.
ECAUSE of its effectiveness as a complement to distilla-
B
tion in the separation of materials, liquid-liquid extraction has in recent years assumed considerable importance as one of the unit operations of chemical engineering. ‘Within a relatively few years the solvent refining of petroleum products has become common practice, and much has been done in developing the equipment necessary for large-scale operation. The development of the more theoretical aspects of extraction as a unit operation has been relatively slow as compared with the widespread adoption of the process on an industrial scale. Hunter and Nash (11-14) have described both the diffusional basis of extraction and the graphical methods of computation which are of value in making the necessary stoichiometric calculations. The latter have also been described with unusual clarity by Evans (8). The analogy between distillation and extraction and the meaning of “reflux” in extraction are discussed by Saal and Van Dyck (17) and by Thiele (go), and have been presented particularly well by Varteressian and Fenske (22, 23). Relatively little information is available in the literature with regard to the performance of extraction equipment. Fallah, Hunter, and Nash (9) and Strang, Hunter, and Nash (19) report data on extraction in a wetted-wall column, and on the flow conditions in such a column. Elgin and Browning (7) and Appel and Elgin (2) report investigations of countercurrent extraction in a spray column; acetic acid, isopropyl ether, and water were used in the first case, and benzoic acid, toluene, and water in the second. The latter investigation included a study of the operation of a packed column, but since 0.5-inch Berl saddles were used in a 2.03-inch i. d. column, channeling along the wall was doubtless large. Sherwood (18) reports data of Demo and Ewing (6) on extraction of acetic acid from water by benzene in a 3.55-inch i. d. tower packed with 0.5-inch carbon rings. Varteressian and Fenske (21) report data on extraction in the system benzene-ethyl alcoholwater in a 0.55-inch column packed with small metal chain and nickel wire rings. Rushton (16) describes the results of experiments in which oils were treated by countercurrent extraction with nitrobenzene in a 216/16-in~htower packed with various rings and saddles, to 1 inch in size.
It is apparent from the limited literature on performance of countercurrent columns that no general correlation of the data for design purposes may be expected until more published information is available. The present article presents the results of a continuation of the work begun by Demo and Ewing (6) on extraction in a 3.55-inch column packed with 0.5- and 1-inch carbon rings and 0.5-inch Berl saddles, as well as with an unpacked spray column. The investigation is analogous to those of Elgin and Browning and of Appel and Elgin in that definite three-component systems were used, and similar to the work of Rushton in that several packings were investigated. Although the tower was larger than those used by these investigators, the ratio of tower diameter to packing size may have been too small in the case of 1-inch packing. It is sometimes assumed that this ratio should be 8 or larger if the results are to be considered typical of the performance of towers of large cross section. Simultaneously with the study of the packed tower, an experimental investigation on extraction from single drops was undertaken, This is also described, as it throws light on the mechanism of diffusion into the dispersed phase. I n both studies the solute was acetic acid which was extracted from water by benzene and by methyl isobutyl ketone. ,
Procedure for Extraction from Single Drops
Solvent containing acetic acid was introduced through a glass nozzle mounted vertically at the bottom end of a glass column, 1.74 inches i. d. and 60 inches tall. The solvent drops formin at the nozzle tip rose through water which filled the unpackes column. The solvent feed was controlled by dropping from an analytical buret into a side tube connected t o the glass nozzle. The level in this side tube was maintained constant by close observation and regulation of the buret cock. This provided a uniform feed rate and an accurate measure of the amount of solvent introduced t o the column. The to of the column was fitted with a cork stopper carrying a 0.55-incR bent glass tube from which the solvent was withdrawn t o a measuring buret. The under side of this stopper was hollowed to form a cone-shaped receiver for the solvent drops arriving at the top of the column and so preventing any holdup of drops under this stopper. A small amount of water was introduced at the bottom of the column t o force some water out with the solvent leaving the top and thus to retain in the combined top product all acid present in the solvent. Both phases removed from the top were titrated, and the acid found was assumed to 1144
SEPTEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
have been present in the solvent drops arriving at the top. I n using the ketone a correction was made for the small acid concentration of the water phase in the tower, but with benzene this correction was quite negligible. I n both cases the water in the tower was changed after each run. Total material balances checked within 2 per cent in all tests. The acid content of inlet and outlet solvent phases, outlet water phase, and water in the column a t the end of the run were obtained by titration with standard sodium hydroxide using the method described below for the samples from the packed column. The water in the column at the end of the run was sampled at both top and bottom of the column in the runs with the ketone. Rate of solvent and amounts of water fed and of solvent and aqueous products were obtained from the buret readings. The rate of drop formation was obtained by counting the drops formed in 5 minutes. "Time of contact was taken as the average time of drop formation and rise obtained by several measurements with a stop watch on single drops. Several nozzles were used to obtain different drop sizes; the smallest nozzle was a 0.0010-inch i. d. stainless steel hypodermic needle. The other nozzles were fire-polished glass tubes. The effective height of the column was varied from 2.0 to 57.7 inches by adjusting the position of the lower stopper carrying the nozzle. All runs were made a t 22-28' C. (71.6-82.4' F.).
1145
or lowered as desired. The elevation of this overflow determined the position of the interface between phases in the tower, which could be controlled easily in this way. This overflow device in the raffinate line is indicated in Figure 1. Both feed liquids were pumped continuously from storage vessels at floor level to head boxes situated on a platform about 14 feet above the floor. The feed to the column was by gravity from these constant-head supply vessels, the overflow in each case being returned to the storage vessels. Calibrated orifices were inserted in each feed line, as Figure 1indicates, and fitted with petcocks to remove air from the manometer leads. Dyed butyl phthalate was used as a manometer fluid. Glass carboys were used for acid storage, with a copper head box and 0.59-inch glass lines. Steel drums were used for the solvent, with a/,-inch iron pipe lines.
Packed Tower Solvent and aqueous streams were contacted in a 3.55-inch i. d. Pyrex glass tower, 66 inches long and mounted vertically. This was fitted with headers and distributing nozzles at both ends and operated empty as a spra tower or acked with one of three FIGURE 2. BRASSHEADER packing materials. I n a&ition to t%e tower, the necessary auxiliaries included storage vessels, feed and product lines, orifice meters, pumps, and an overflow device to control the interface The solvent extract was strip ed of acid for re-use by contactin between the two phases in the tower (Figure 1). with water or with a dilute sol%ion of sodium hydroxide. Acia In all runs the direction of diffusion was from aqueous layer raffinate was made up to 6.0 per cent for re-use by adding glacial to solvent layer-i. e., an aqueous solution, initially 6.0 per cent acetic acid. A layer of solvent was maintained a t all times above acetic acid, was extracted by either benzene or methyl isobutyl the solution in the acid storage carboys, and a layer of water was ketone. The aqueous layer, either feed or raffinate, will be rekept in the solvent drum. Since these vessels were agitated by ferred to as acid. Since the flow of one phase past the other is the continuous circulation to the head boxes, each feed liquid was necessarily by gravity, the heavier acid phase entered the tower maintained saturated with the other phase. a t the top and was withdrawn a t the bottom, while the solvent Before each run the feed liquids were recirculated through the passed in the reverse direction. head boxes for ap roximately one hour in order to saturate each At each end of the glass tower was a brass header consisting of phase with the otRer layer. The continuous phase was admitted a cylindrical chamber 3 inches high and 3.5 inches i. d. The inuntil the column was about three quarters full, and the second comin liquid was fed into the side of the chamber whence it enfeed was then started. The position of the interface was adtered &e tower through six 0.120-inch i. d. brass tubes extending justed by means of the overflow control, and the flows were set 2 inches into the tower and 2 inches into the header chamber. and held a t the desired rates. The These were spaced symmetrically position of the interface was ap roxi at a radius of 1 inch from the mately level with the ends of tge si; center of the tower. The outgoing small feed tubes either at top or liquid was withdrawn through a l/4bottom header, depending on which inch brass pipe leading from a hole phase was dispersed. After about in the center of the header plate. four com lete changes of the conOne of these headers is shown in tinuous p\ase in the column, judged Figure 2. The six small feed tubes sufficient to obtain steady state, a extend into the header chamber, set of four samples was obtained. with the supply tube feeding the A second set was then obtained from header at the right and the brass 5 to 15 minutes later, depending on pipe through which liquid was withthe flow rates, and the test was ended. drawn a t the left. The cover plate Since it was possible to adjust the (shown removed) was fitted with a temperature of the room, the t:sts petcock and a glass thermometer. were all made a t 25" * 2 " C. (77 * I n most of the runs with packing 3.6 ' F.). the packed height was 54 inches, with The acid samples were titrated 6-inches free space above and below with 1N sodium hydroxide, b means the packing. The packing rested on a S/~-inch-meshnickel wire grid. I n of thymol blue indicator. T%e bena few runs only 20 inches of packing zene extract was analyzed by shaking were used, with the free space above 50-cc. portions with an equal amount the packing increased to 40 inches. of water and titrating the mixture with 0.1 N caustic; thymol blue indiI n order to avoid the occurrence of appreciable extraction in the large cator was used and the mixture was free space above the packing in these shaken violently until the end point tests, the six upper feed tubes were was reached. c extended by 0.24-inch glass tubing to A faint blue in the water layer was E taken as an end point. As B check introduce the acid a short distance above the packing. A wood spacer it was found that the same end point was obtained when sufficient maintained the glass extensions in the same relative position as the ethyl alcohol was added to make short brass tubes previously dethe two phases completely miscible. scribed. The ketone extract was titrated in a PUMP PUMP The aqueous raffinate leaving the similar manner with 1 N caustic. column passed from the bottom The benzene feed was titrated with header through a swivel pi e to an 0.01 N caustic; the ketone feed with overflow vessel which could {e raised FIGURE 1. DIAGRAM OF APPARATUS 0.1 N caustic.
1
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1
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VOL. 31, NO. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
1146
Equilibrium data for the system benzene-acetic acid-water were available in the literature (6). Corresponding data for the system methyl isobutyl ketone-acetic acid-water were obtained experimentally. Whereas the acetic acid concentration in water is roughly thirty times as great as in the benzene phase in equilibrium, it was found that the distribution of acetic acid between methyl isobutyl ketone and water corresponds to about equal concentrations in both phases. The ketone is obviously a much better solvent than benzene for the removal of acetic acid from water. The mutual solubility and equilibrium data for the ketone system are given in Table I. TABLEI. EQUILIBRIUM AND MUTUAL SOLUBILITY DATAFOR SYSTEM ACETICACID-WATER-METHYL ISOBUTYL KETONE AT 25" c.
-Mutual
Soly. Data-
Ketone
Water
Acid
%
%
%
1.55 3.7 10.5 17.4 26.0 37.6 51.6 66.4 81.6 97.9
98.45 76.8 57.5 48.4 39.6 29.1 19.2 12.0 6.5 2.12
19.5 32.0 34.2 34.4 33.3 29.2 21.6 11.9
0
-Equilibrium -Ketone LayerAcetic acid Density
%
G./cc.
1.87 8.9 17.3 24.6 30.8 33.6
0.798
0.804 0.807 0.809 0.811 0.812
Dat-Water Acetic aoid
% 2.85 11.7 20.5 26.2 32.8 34.6
LayerDensity G./cc.
0.995 0.996 0.998 0,999 1.000 1,001
0
Extraction from Single Drops The quantities varied were drop size, column height, inlet concentration of acid in solvent, and solvent feed rate. Both benzene and methyl isobutyl ketone were used. Six runs were made with an acid concentration of 0.0755 pound mole per cubic foot, and five runs a t a concentration of 0.0474 pound mole per cubic foot in the inlet ketone, all with a column height of 57.7 inches. Thirteen runs were made a t an inlet concentration of 0.0603 pound mole per cubic foot, with the column height varied from 2 to 57.7 inches. Several inlet nozzles were employed with each inlet acid concentration of the ketone. Drop diameters, calculated from the measured feed rate and the drop count on the assumption that the drops were spheres, varied from 0.0745 to 0.137 inch. This range of drop diameters is quite small compared with the range of nozzle diameters used. The smaller drops appeared to be spherical, but the larger drops were noticeably flat, with horizontal axes perhaps twice the vertical axes. Seven runs were made with benzene containing 0.00576 pound mole of acetic acid per cubic foot a t a column height of 57.7 inches. Twelve runs were made with 0.00374 pound mole of acid per cubic foot in benzene with column heights from 2 to 57.7 inches. The benzene drops were larger than those obtained with the ketone, the calculated diameters varying from 0,109 to 0.221 inch. The drop diameters were calculated on the basis of spherical drops, the drop volume being obtained by dividing the volumetric feed rate by the number of drops formed per unit time. In the runs with benzene the equilibrium concentrations in benzene corresponding to the observed water-phase concentrations were negligible, and the actual benzene concentrations could be taken as equal to the over-all concentration driving forces on the benzene basis. The calculated transfer coefficients are plotted against drop diameter on Figure 3; all the data shown are for the 57.7-inch column height. The values of K were obtained from the equation : where L
= ketone or benzene
rate, cu. ft./hr.
Cp, C1 = acid concentrations of drops at exit and inlet, respectively, lb. mole acetic acid/ cu. ft. A = calculated area term, in sq. ft., of total drops in column at any time, obtained from measured feed rate, drop count (no. of drops per min.), and measured time of formation and rise of drops from bottom to top.
The calculation of A is based on the assumption that the drops are spherical. The logarithmic mean driving force, AC, m. is based on C1 and Cz and the equilibrium concentrations in ketone or benzene corresponding to the observed concentrations in the aqueous phase a t the bottom and top of the column. From Figure 3 it is apparent that K increases with drop size for both systems, and that for the same drop size K Kis greater than K B . Variations in acid content of the inlet ketone have no effect on K K ,but two curves are obtained for K Bfor the two inlet acid concentrations in benzene. The correlation is essentially the same if K is plotted against Reynolds number for the rising drops. Since the liquid-phase diffusivities are probably almost equal in the two cases, it might be expected that K K and K B would not differ greatly. The interfacial tension for the benzene-water system (10) is between 33 and 35 dynes per cm., whereas that for the ketone (1) varies from 8.8 for ketone-water with no acid to 3.0 for the feed containing 0.075 pound mole of acid per cubic foot. Although the interfacial tension should influence the drop size, it is difficult to see how it might affect K for a given drop size. Except for minor differences in velocity of rise, the conditions outside the drop were essentially the same for both ketone and benzene, and it seems logical to look within the drop for an explanation of the observed differences in K for the two systems.
