laboratory liquid-liquid Extraction Column
Engirneyring Process development I
J. F. SHORT AND G.
H.TWIGG
THE DISTILLERS CO., LTD., RESEARCH AND DEVELOPMENT DEPARTMENT, GREAT BURGH, EPSOM, SURREY, ENGLAND
A more compact countercurrent liquid-liquid extraction column than the usual packed type was required for laboratory work and the evaluation of the column requirements needed to carry out a given separation. The column eventually used comprises a vertical tube with a concentric rotor forming an annular space for countercurrent flow of liquids. Because of the low height equivalent to a theoretical stage (0.6 to 4 inches, depending on the system and conditions of operation) it can be made compact and easily transportable, and yet works with high efficiency. By using different rotor diameters and appropriate rotor speeds, it is adaptable to a wide variety of systems. This type of column has proved useful in the quick evaluation of column requirements without the use of large quantities of materials, and as a handy tool for the organic chemist wishing to carry out multiple extractions or separations by extraction.
theoretical or perfect stage. The height equivalent to a theoietical or perfect stage (H.E.T.S.) corresponds to the height equivalent to theoretical plates (H.E.T.P.)of a distillation column. The present work on these columns has produced a small elperimental model having a height equivalent to a theoretical stage as low as 1 inch for a system xhich, in a laboratory size of packed column, gives height equivalent to a theoretical stage figures of over 1 foot. This efficiency is comparable with that shown in the vapor-liquid extraction field by a good laboratory distillation column, and is superior in this respect to any other type of liquidliquid extraction column so far reported. CONSTRUCTION
T
HE operation of liquid-liquid evtraction has been much neglected by comparieon with that of distillation because of the lack of efficient, easily operated apparatus. Organic preparations, particularly on a laboratory scale, frequently call for a simple extraction or stripping from a liquid mixture by a relatively immiscible solvent. Less frequently, the procedure is applied to the fractionation of compounds of similar chemical constitution. I n general, the operation is most efficiently carried out in a continuous countercurrent manner. A valuable contribution t o this operation of countercurrent fractionation has been made by Craig and coworkers (1, 6 , 1 2 ) . The application of the Craig machine, which incorporates a large number of individual stages, is limited however t o the treatment of small amounts of solutes. Ability to deal with larger quantities of material is shown by the apparatus of Johnson and Talbot (3, 4 ) which incorporates a series of separate extraction stages and can be operated countercurrently and continuously. It does not, however, have the simplicity of the present apparatus which can be used for simple strippings or can be readily modified for the fractional extraction process. The column is essentially a vertical tube with a rotating concentric cylindrical core forming an annular space for countercurrent flow of the phases. The one first described by Jantzen (2) showed considerable promise and was further developed by Ney and Lochte (7) and by Schutze and comorkers (8). Above a certain speed of rotation of the core, the motion of the liquid in the annular space becomes regulaiixed, like the series of vortices described by Taylor (10) and Lewis (6). A patent of the Shell Development Co. (9) describes experimental results on larger columns of this type, and it is clear from these and from the results on the present work that increase in size and throughput are attained a t the expense of a greater height equivalent to a
msTAlNLEss STEEL
Figure 1. Rotating Core Laboratory Liquid-Liquid Extraction Column
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TUBE. One form of the apparatus, which is illustrated in Figure I, is made of glass in three parts: a 4-inch length, A B , at the top with a side tube; a plain 2foot length, BC; and a 2-inch length, CD, a t the bottom. The best type of glass for a c c u r a t e work is Chance's Veridia precision-bore tubing, but, for a general apparatus any glass tubing is suitable if it is of reasonably uniform bore, sufficiently s t r a i g h t , and strong enough to stand a certain amount of vibration. The ends of the tubes are ground square and held together by sleeves of rubber or other flexible tubing. BEARINGS. These, a t B and C, were made from disks cut out of a 3/1e-inch s h e e t of Tufnol, which had a diameter equal t o the outside diameter of the glass tubqs. The greater part of each disk was cut away to allow passage of the liquids, a n d e a c h d i s k was drilled wit'h a central hole to accommodate the shaft. The upper bearing had a a/,,-inch central hole and the lower one a '/ginch, the lower tip of the shaft being turned down to l/g.inch to fit. Each
December 1951
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
bearing was securely held between the squared ends of the glass tubes by means of the rubber sleeves. ROTOR. With columns of this size, it was found that the maximum length of shaft between bearings is about 2 feet if whipping is not to occur a t some speeds of rotation. The shaft was formed from a suitable length of a/,~-inch stainless steel rod, the lower end being turned down for a length of a/8 inch to a diameter of l/g inch. The rotor may be of any suitable material, .glass or metal being the more convenient, and in this case it consisted of a length of 1foot 101/2inches of a standard size of stainless steel tubing. It was found advantageous to chamfer the ends of this tube to facilitate smooth entry of the liquids into the annular s ace. The tube and shaft were straightened as carefully as possibg for smooth running, and it was found satisfactory to mount the rotor on the shaft by short lengths of rubber tubing of suitable dimensions. With the help of a litt!e glycerol one length of rubber could be pushed to the middle of the rotor and another length inserted a t each end. It was found advisable to push the end rubbers about l / 8 inch clear of the tube ends to avoid mushrooming outward by the swelling action of organic solvents. The rotor was conveniently driven by a '/aO-hp. variable-speed electric motor.
The present apparatus therefore comprises a number of 2-foot tubes of different bore, each complete with bearings and top and bottom sections, together with a selection of rotors of different outside diameters, each mounted on a shaft. Any rotor may thus be readily coupled with any of the tubes and the range of annulus widths available is from 0.7 to 5.2 mm. From this range it has been possible to select an' assembly which has operated satisfactorily with any system so far encountered. The free space in the 2-foot columns used in this work ranges from 55 to 170 ml., inclusive, of the volume of the adjustable overflow tube. Although all the combinations have not been assessed independently, the optimum efficiencies with two of them are given in Table I. The test system was the extraction of acetic acid from 5% aqueous solution by methyl isobutyl ketone and the number of stages computed by the graphical method of Varteressian and Fenske (11).
TABLEI. SPECIMEN TESTSON A %-FOOT EXTRACTION COLUMN
OPERATION
Annulus width mm. Optimum roto; speed, r.p.m. Free space, ml. Aqueous acetic acid, ml. per hour Ketone. ml. Der hour Phase dispersed Number of theoretical atages Acetic acid extracted, yo
For simple routine extractions, the liquids are most conveniently fed by gravity from small aspirator bottles or tap funnels. Lighter phase enters at the bottom of the column and passes upward to overflowat the side tube in the topmost section. Heavier phase is fed to the top of the column, at a point below the side tube, and passes out a t the bottom by way of an overflow tube whose level can be varied. Raising or lowering of the level of this overflow alters the position of the interphase in the column and so determines the phase which is to be dispersed. When starting up, enou h of the heavier phase is let into the column t o form a seal at &e bottom (this liquid may consist of solute-free heavy phase). Flows of both phases are then started and the rotor is set in motion. When both phases are overflowing, the interphase level may be finally adjusted. As the rotor speed increases, the droplets of disperse phase will be seen to form a welldefined banded pattern which actually comprises a series of horizontal vortex rings (6, IO),and the presence of this phenomenon indicates conditions of efficient extraction. Then, provided the flows do not exceed the flooding rates and the revolutions remain constant, the column will function smoothly for an indefinite period with little attention. Where it is important to strip the solute as thoroughly as possible from one of the phases, the column may be completely filled and started up with solute-free solvents; the normal feeds may then be started; and the apparatus may be finally flushed with solute-free solvents. Where economy of extracting solvent is of serious moment, the apparatus could easily be combined with means for evaporation of the solvent from the extract for recirculation through the column. Other refinements of operation will suggest themselves for special circumstances. It is not possible to lay down any simple rule to determine the phase ratio to be used in a particular extraction. If the solute can be quickly assayed in the stripped effluent, it is a simple matter to adjust the phase ratio during operation, and any inadequately stripped liquor can be recirculated without stopping. With little experience, the partition coefficient of the solute is the best guide in fixing the flow ratio. ADAPTABILITY
A point on which insufficient emphasis has been laid in previous publications is that the optimum conditions may vary in several respects with different liquid systems, a limitation which should always be remembered when dealing with liquid-liquid extraction columns. For this reason, a single apparatus with one size of rotor and tube is not likely to have the degree of adaptability and general usefulness for a variety of systems which ie required of a routine laboratory unit.
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I 1.8
1300 96 100
180 Ketone
34
96.8
I1 2.6
1300 121
450
800 Ketone 13 86.5
The efficiency of the extraction is illustrated by the figures for percentage of acid extracted in a eingle passage through the column. (The partition coefficient, solvent/water, is approximately 0.7 and is not very favorable for extraction into the ketone.) APPLICATIONS
'
The column was initially designed to help in the quantitative elucidation of liquid-liquid extraction problems, but its compactness and low free space suggested its possible aptitude for routine laboratory extractionb. It has the advantage of simpiicity of operation over a batch countercurrent operation of an equivalent number of stages. Experience has shown that trouble with emulsification is much less when operating countercurrently in a column, a point which will be appreciated by those with experience of extractions involving violent agitation. Another possible field of application is in the exhaustive extraction of materials from a considerable bulk of liquid such as f r e quently occurs in biochemical work. These are normally lengthy operations and in many cases time could undoubtedly be saved by countercurrent column technique. A smaller one-piece version of this column, with a free space of 15 to 20 ml., has proved particularly useful in dealing with the products of small laboratory experiments. It is easily transported and set up on a laboratory bench. LITERATURE CITED
(1) Craig, L. C., J . Bid. Chem., 155,519 (1944). (2) Jantzen, E., Dechema Mono., 5, No. 48, p. 114, 1932. (3) Johnson, J. D. A,, and Talbot, A., J . Chem. Soc., 1950, 1068. (4) Johnson, J. D. A., and Talbot, A,, Nature, 164, 1054 (1949). (.5) Lewis, Proc. Rog. Soc., A117,388 (1927-28). (6) Lieberman, S. V., J.Biol. Chem., 173,63 (1948). ENO.CHEM.,33,825 (1941). (7) Ney, W. O., and Lochte, H. L., IND. (8) Schutze, H. G., et al., IND. ENO. CREM.,ANAL. ED., 10, 675
,
(1938). (9) Shell Development Co., Brit. Patent 615,425 (1949). (10) Taylor, G. I., Phil Trans.,A223,289 (1923). (11) Varteressian, K. A., and Fenske, M. R., IND.ENG. CHEM.,28, 928 (1936). (12) Williamson, B., and Craig, L. C., J.Biot. Chem., 168, 687 (1947). RECEIVED March
21, 1961.
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