Joseph C. Elgin, o f Princeton University, graduated from the University o f Virginia where he received his Ch.E. in 1944 and his M.S. in 7926, In 1929 he obtained his Ph.D. from Princeton, where he held Du Pont and Proctor Fellowships, and became an instructor in chemical engineering. A t this time he was Research Fellow of the American Petroleum Institute. H e has been chairman of the Department o f Chemical Engineering since 7936 and became a full professor in 1939. In addition, Elgin is a licensed engineer o f the State of N e w Jersey. His wartime affiliations included investigating and consulting work for the National Defense Research Committee; chief, Polymer Development, O f f i c e o f t h e Rubber Director; and work on the M a n h a t t a n Project. He is a member o f the American Chemical Society, American Institute of Chemical Engineers, Society for Promotion o f Engineering Zducation, and American Institute o f M e t a l l u r g i c a l and M i n i n g Engineers.
SOLVEI~TEXT Joseph ERHAPS the most important advances in the solvent extraction field in recent years have been a wider recognition of its importance both as an industrial operation and as a subject for chemical engineering investigation, and a better understanding of its scientific and engineering principles. As a unit operation, solvent extraction has assumed a chemical engiiicering status comparable with its companion operations, distillation and absorption. Rapidly developing in engincering application in the late thirties, by 1942 when the writer first summarized its status ( l j ) , the essential scientific and engineering bases and the design and operating methods for extracting with solvents had already becn developed, and a number of important industrial applications made. More recent progress hnr been in the direction of developing the quantitative engineering [lata neccssary for design, in cxpanding and improving industrial cistraction operations already introduced, and in developing n tiumber of new solvent extraction applications. Combined with distillation in the operation known as extractive distillation, solvent extraction provides a separation method for close-boiling compounds and azeotropic components which has rccently assumed major industrial importance. I n this operation t he distillation of a difficultly separable liquid mixture is carried out in the presence of a relatively high-boiling solvent, passed countercurrently into the top of the fractionating column and usually possessing a selective solvent action toward a component o f the mixturc being separated. The increasing industrial :ipplication of this operation, together with the progress made toward developing its quantitative chemical engineering design 1)riiiciples (4, 6, 10, IC), have bcen a major development of the past few years. I t has been widely employed in the petroleum industry for the separation of aromatics from nonaromatic hydrocarbons--e.g., thc Shell Oil process (14) for recovering nitrationgrade toluenc, using phenol as the extracting liquid, and for separating paraffins, olefins, and diolefins (for example, separation of butadiene from other C, homologs through the use of furfural). DESIGNDATA. Although rcccnt yeais h a w witnessed a rapid expansion of solvent extraction operations, the theory of this operation is still not so me11 understood as for thc companion vapor-liquid processes of distillation and absorption. A major lack of engineering design data still exists, especially for mass transfer rate coefficients and the performance of large-scale liquid-liquid contacting equipment. The rational development of chemical engineering design for solvent extraction operations requires extensive knowledge of the complex phase equilibria involved in liquid-liquid and liquid-solid systems. These are con-
P
26
C. Elgin siderably more complicated and difficult to treat, both cxperimentally and theoretically, than vapor-liquid operations, and fewer generalizations have yet been possible. There is a need for the accumulation of extensive phase equilibrium d a t a for extraction systems, particularly on the effect of temperature and pressure conditions and with a wide variety of solvent,s. A number of studies in the past few years have, however, contributed notably t o progress in these directions. PHASE EQUILIBRIUhl DATA. I n 1942 Smith (40) summarized the liquid systems for which phase equilibrium d a t a then existed, the majority involving water as one component. Recent studies (6, 7 , 12, 18, 19, $5, 89, 56,86,S 9 , C l ) have contributed data on a variety of new systems. I n particular, the work of Hixson and Bockelmann ( 1 9 ) and Drew and Hixson (13) on phase relations of systems of fatty acids, triglyceride esters of fatty acids, and some other compounds with liquefied gases, such as propane, have contributed materially t o solvent extraction progress. The formcr investigators measured the mutual solubility and the critical solution temperatures for various triglycerides--e.g., tristearin, tripalmitin, a,nd tricaprylin-and for oleic acid and refined cottonseed oil with liquid propane. Thcir results correlate the critical solution temperature with the number of carbon atoms in the compound and give an understanding of the phase behavior in the vicinity of the critical point. D a t a for the ternary system, liquid propane-oleic acid-refined cottonseed oil, were also determined; it was possible t o show that, at a temperature close t o the critical for liquid propane, the separation of free fatty acids from a mixture with a vegetable oil such as refined cottonseed could be successfully made by extraction with this solvent. Drew and Hixson supplied similar dat,a for the niut,ual solubility and critical solution temperatures for binary systems of palmitic acid, stearic acid, and cetyl stearate with liquid propaiic and for the ternary system, propanc-palmitic acid-stearic acid. They correlated the available data for binary systems of these classes of aliphatic compounds with liquid propane, showing that the critical solution temperature is dependent upon molrcular weight. Based on their work, prediction can be made as t o the possibility of using liquid propane as a selective solvent for such high-molecular-weight compounds. The complexity of the equilibria with which solvent cxtraction is concerned and the labor involved in their direct, determination, which in some cases because of analytical difficulties is virtually impossible expcrimentally, emphasizes the development of gencralizations and of methods of data correlation and prediction for design purposes. The location and selection of a suitable or optimum solvent for a proposed extraction operation are usually
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 38, No. 1
complicated and laborious. Although the goal is still far in the future, important strides have been made in this direction in the past few years. Several methods of correlating tie line been developed (56,43) and applied to a number of 1 tems with considerable success. These, together with more recent works, are of material assistance in reducing the experimental data necessary for extraction design and in the prediction of suitable solvents. In 1942 Othmer and Tobias (36) presented a method for predicting equilibrium distribution in ternary systems from solubility and partial pressure data for the binary mixtures involved or, less accurately, from the latter data alone. Since then notable progress has been made in methods of treating, correlating, and predicting phase equilibria in both nonideal liquid-vapor and liquid-liquid systems, through thermodynamic treatments in terms of activity coefficients, by Carlson and Colburn (9), Treybal (@), Colburn and Schoenborn ( f O ) , Benedict and Rubin (4), White (47), and others. These investigators have supplied useful methods for estimating equilibrium distribution over the entire concentration range from a relatively few experimental determinations, for calculating tie lines from more readily available or measurable data (such as mutual solubilities and vapor pressures), as well as means for effecting some measure of classification and prediction of solvent behavior. Other recent studies provide assistance toward solving the problem of the preliminary qualitative selection of solvents for a proposed extraction separation. Ewe11 and co-workers ( I @ , employing the concept of the hydrogen bond, have classified liquids as t o types and given some insight into the behavior to be expected from their mixtures. The studies of critical solution temperatures in binary systems of paraffinic, olefinic, and cyclic hydrocarbons with a large number of organic solvents by Francis (17) and others (48) have provided a comprehensive qualitative knowledge of the selectivity t o be expected toward the components of hydrocarbon mixtures as well as a valuable tool for predicting suitable solvents for particular hydrocarbon separations. Specifically, Francis developed a number of useful generalities between the critical solution temperatures and mutual solubilities with hydrocarbons and the chemical structures of solvents. He also showed how the critical solution temperature may be used to predict the selectivity of solvents for the various types of hydrocarbons in their mixtures, such as in petroleum oils. The correlations of Drew and Hixson (19) and Hixson and Bockelmann (19) are also useful generalizations in dealing with proposed extraction separations involving liquid propane and similar liquefied gases. MASSTRANSFER RATEAND EQUIPMENT PERFORMANCE. With expanding industrial use, the need for design data on mass transfer rate in extraction and the quantitative performance of specific liquid-liquid contact equipment for industrially important extraction systems has become even more acute. Additional progress in this direction h a been made in the last few years, but comparatively few contributions have been published. Quantitative design correlations of mass transfer coefficients or H.T.U. with physical properties of the liquid phases involved, operating conditions, and mechanical design of extraction equipment are atill badly needed. Qualitatively, the mechanism of extraction and the dependence of extraction rate upon physical properties and dynamics involved are reasonably well understood through analogy with the similar, more extensively developed vapor-liquid operations. The major portion of the basic data reported on liquid-liquid contact equipment has been concerned with the performance of small scale units with particular emphasis on the effect of the flow rates of both phases on the transfer coefficients. It has been in terms of over-all mass transfer coefficients, H.T.U., or over-all diffusional resistances, rather than the individual film values more fundamental and useful for design. This situation has been largely due to the more difficult mechanics of handling liquid and solid phases, treating rate data, and separating over-all transfer rela-
January, 1946
tions and coefficients into the individual film components, tha:t has been the case for vapor-liquid operations. Several recent stu&qs have, however, been progress in the desired direction. Data on the performance characteristics of specific designs of liquid-liquid contacting equipment with specific extraction systems have continued slowly to accumulate. Allerton, Strom, and Treybal(1) added further data on the performance of a perforated plate tower and a tower packed with I/2-inch carbon rings for extracting benzoic acid from toluene and kerosene by water. They reported over-all H.T.U. values, throughput, and holdup as a function of various operating variables. Knight (25) added data on the performance of a tower packed with 1/2-inch Berl saddles in extracting aqueous furfural solutions with tduene. Over-all H.T.U. values were correlated with flow rates, and a n equation relating H.T.U. to flow rate is given. Moulton and Walkey (89) supplied performance data for a 3.75-inch perforated plate column, extracting methyl ethyl ketone from gasoline with toluene. Their object was to investigate a case involving a nonideal system with large changes in the volumes of the phases taking place through the column. Results are reported as plate efficiencies as a function of flow rates and for two different plate spacings. The efficiencieswere quite low, and the ratio of the two feeds was found to be an important variable. Meissner and coworkers (SO) supplied data on transfer coefficients and H.T.U. for spray and packed towers (packed with saddles and Raschig rings) under various operating conditions in the continuous countercurrent extraction of water from methyl ethyl ketone with strong calcium chloride brines. Hoping to separate the individual film transfer coefficients, Treybal and Work (4.5) measured over-all extraction coefficients in wetted-wall columns. They found that both phases affect the individual film transfer Coefficients and that neither film resistance was controlling in their experiments. They were not successful in breaking down the over-all coefficients into the individual film components but were able to correlate the over-all coefficients on the basis of the flow of the core fluid relative to the tower wall. Comings and Briggs (18) studied several systems in both wetted-wall and packed columns, and were able t o develop equations for the dependence of the over-all transfer coefficients on the flow rates of each phase. By employing the rate of mutual solution in the binary system of isobutanol and water, Colburn and Welsh (11) were able to measure individual film coefficients in a packed tower and to show how these varied with the flow rates of the two phases in such a tower. In order to study the effect of flow rates over a wide range of phase velocities and the effect of the physical properties of the phases on the rate of mass transfer, Bergelin, Lockhart, and Brown (6) studied extraction in 8 horizontal tube or duct type extractor. Data on the effect of the respective phase velocities on the over-all transfer coefficient were obtained, and a number of interesting qualitative observations and conclusions were made regarding extraction mechanism and the effect of relative flow rates on the velocity distribution in each phase. In an important contribution to the extractor design problem Brinsmade and Bliss (8) reported the results of studying the extraction of acetic acid from methyl isobutyl ketone with water in a wetted-wall tower. They succeeded in separating the overall into individual film diffusional resistances. Their method should be useful in future work for breaking down the over-all resistance into film values. Results for the individual film coefficients were correlated well by the following: For core fluid:
kd
-k =
De
For wall fluid:
1.07 iRe,')0.67
I&=
0.00135 Rew'
(s)o'62
where d = tower diameter; D = diffusivity of solute
i I E2:ansfer
coefficient
INDUSTRIAL AND ENGINEERING CHEMISTRY
(Continued on page 37)
27
CENTRIFUGATION Any consistent set of units may be empl a, i s equal to (2?rNr)*/r,where N is th
centrifuge. Test data are needed to application of the Stokes law to settling i centrifuge. Informative studies could be made on the effect of particle size, feed rate, vis centration of slurry on the tive sizes and densities wou As a result of such experimental studies, sound methods of scale-up will become av@ilable. These dethods will not, in general, furnish sufficient information to make a choice between filters and centrifuges. Cost data on sucrh items as investment, labor, operation, and maintenance are also required. When all of these facts become available, it is then possible economic selection. It would appear that the engineering schools in are in a n excellent position to cooperate with the m of centrifugal equipment in developing a sound theoretical background for this unit operation.
recovery of acetpse and etbyl Jcohol from dilute aqueous solutions with solvents m c h methyl isobutyl kstone, and isoamyl alcohol of butylene gly001-from dilute fermentation liquors with methylvi acetate. The use of multistage extraction systems involving mixers requires a knowledge of the degree of mixing and the power requirements ch equipment for proper design. An important contribut this end was made recently by Miller and Mann (31). They studied the power requirements and degree of mixing for seven agitatolt designs for two-phase immiscible liquid mixtuies in unbamed tanks. I n addition t o extensive design and power data, their work provides a correlation of the power required to agitate systems of two immiscible liquids with the operating oharaoteristics and design of the system in terms of a plot of a power function V.S. Reynolds number. They also developed the application of the results t o the design of larger scale equipment. APPLICATIONS.Steady advances in the applications of solvent, extraction t o industrially important separations have been made” in the expansion and improvement of commercial installations using the older extraction processes, such as in lubricating oii and vegetable oil refining; the introduction of several industrially
LITERATURE CITED
(1) Anonymous, Chsm. & Met. Eng,, 50, 1 (2) Beams, J. W..“Science in Progress”, Yale Univ. Press, 1940. (3) Bellin, M. I., and Libina, B. I,, Khim. M NO. 7,3-9. (4) Dietz, T. J., J . Franklin Inst., 236,451(6) Dietz, T. J., and Kishbaugh, T. V., Ibid., (6) Faucher, G . H., Oliphant, S. C.+and Bo ENO.CEXOM., ANAL.ED., 1 (7) Hausen, H.,2. Ver. deut, Ing (8)Hosking, J. S.,J. Council Sci. Ind. Research. 17,234 (1944 (9) Howe, A. F.,U. S. Patent 2,37Q,353(Feb. 27, 1946). (10) Moore, D.H., Rm.Sci. Instrumenla, 14,296-7 (1943). ( 1 1 ) Mulliken, R. S., J . Am. Chem. SOC,,44,1033-8 (1922). (12) Neuman, J. J., U, 9.Patent 2,341,230(Teb. 8.1944). (13) Oliphant, 5. C., Houssiere, Am. Inst. Mining Met. Engrs., Tech. Pub. 1539 (1942). (14) Riegel, E.K.,“Chemical Machinery”, New York, Reinhold Pub. Gorp., 1944. (16) Sanchez,F.G., Proc. 14th Assoc. Sugar Tech. Cuba, 19 (16) Sanchez, F.G., U. S. Patent 2,335,794(Nov. 30, 194 “Atomia Energy for Military Purposes”, Prince(17) Smyth, H. D,, ton, N. J., Princeton Univ. Press, 1945. U. 8.Patent 2,368,876 (Feb. (18) Terrados, E.P. Y,, (19) Terrill, H. M.,J. Franklin I @ . , 237,73-6 (1944) (20) Veksler, G.M., K h h . Mmhinostroenie, 1940,No. -8. (21) Vilter, E.F., U. S. Patent 2,360,465,(Oct, 17, 1944). (22) Wilsmann, W., Chem. A p p . , 29, 186-90, (23) YarmolinskiX, M. B., Sa&har,1940, No.
and separation of come use of liquid propane and employed. Other substitution of liquiddistillation, evaooratiofi t o dryness, and extraction of the dry residue, in the production of vitamins (21) and similar pharmaceuticals. I n this field salting-out procedures have been superseded by more efficient and eeonomical extraction methods, Solvent extraction is also an important operation as an alternate t o adsmptbn in the concentration and purification of penicillin (83s). cids and glyceride esters solvent action is a p treatment of oil
ean with furfural and other solvents ging) and unsaturated (drying) commercial operation. to the treatment of other ch ~LIcottonseed. linseed. and vegetable oils (2.3.13.18. ,,. coconut, and t o the separation ‘and refining of free fatty acids derived from them. Several commercial processes making use of some type of extraction with organic solvents, such as methanol, pane, furfural, and acetone, have also been developed. o separate high-molecular-weight fatty acids such &s ic. Little information, is available, but the work ffixson (13) and Hirson and Bockelmann (19) gives insight in some cases into their technical bases. Smith and Funk (39) showed that liquid-liquid solvent extraction with mixed organic solvents, such aa various glycol mixtures with methanol, can be commercially feasible for recovering aromatic components from hydrocarbon mixtures such as gasoline, although the recovery and purity of the aromatic extract is limited. Solvent extraction has been applied t o the recovery of itaconic acid from fermentation liquors with organia solvents r
SOLVENT EXTRACTION CONTINUED FROM PAQS 27
Re
-
modified Reynolds number density nscosity Subscripts = core and wall fluids, c and w, respectively =
p ~.r=
In theory these should bo general correlations, The results of a number of pilot plant studies of specific extraction processes have been recently reported (19,84, 36, 89) involving the refining of crude long-chain fatty acids by liquid propane extraction, extraction of aromatics from gaaolines with mixed solvents composed of methanol and ethylene glycol ; January, 1946
I
INDUSTRIAL A N D ENQINEERING CHEMISTRY
37
ouch as n-butanol (a7). A recent important large-scale application in the synthetic rubber polymerization plants is the liquidliquid extraction of inhibitor from butadiene (86)with caustic Eoda in a special type contactor prior t o charging it to the polymerizing reactors. Perhaps one of the major recent industrial applications of extraction has been in the recovery and purification of butadiene from other hydrocarbons in various petroleum processes for butadiene manufacture as a raw material for synthetic rubber production. Together with extractive distillation and absorption, liquid-liquid extraction with various solvents has been applied to separate butadiene from paraffins and other butadiene homologs at some stage in its manufacture. The details of these operations have not yet been revealed. Smith and Braun (58) supplied some information concerning the purification of butadiene by liquid-liquid solvent extraction with mixed solvents such as glycol-methanol solutions and solutions of complex cuprous halides [Na&u(CN),] in mixed glycol-methanol solvents. Aqueous ammonia solutions of cuprous acetate or chloride can also be used to purify butadiene by liquid-liquid extraction. One important manufacturing process for butadiene has employed liquid-liquid countercurrent stepwise extraction with aqueous copper ammonium acetate solution to recover butadiene from a refrigerated liquid C, cut. In the Russian S.K. process for manufacturing butadiene from alcohol, countercurrent liquidliquid extraction with water is stated to be employed to purify the crude butadiene from volatile aldehydes (42). Similarly, Qtlimer and co-workers (36) recently applied liquid-liquid extraction with solvents, such as butanol and methylvinylcarbinol acetate, to recover butylene glycol from aqueous fermentation liquors, which is then converted to butadiene through the diacetate. Turning to mother form of solvent extraction usually classified as leaching, the recovery of vegetable oils from oil-bearing seeds (sa, 18, 3‘3, 97) has rapidly assumed large commercial importance in recent years. A start on the development of design theory and data for such operations has been made by several investigators ($4, 55’). Continuous solvent extraction methods are being applied to the recovery of soybean oil on a large commercial scale, and are being currently developed for application t o tung, peanut, and cottonseed oils (28, 37). Considerable progress is also known to have been made in similar leaching processes for coca0 beans and the extraction of chlorophyll from dfalfa by the application of narrow-boiling pentane, hexane, and heptane cuts. Many other, less well known, applications of solvent extraction have undoubtedly been made in manufacturing processes for various chemicals. i t s immediate future, both as an industrial and as a chemical engineering unit operation, seems assured as the conditions affecting its utilization are more extensively revealed. Applications in the inorganic field have been made, and this direction has been as yet little explored.
LITERATURE CITED
Allerton, J., Strom, B. O., and Treybal, R. E., ‘L’rariu.Ant. Indt. Chem. Engrs., 39, 361 (1943).
Batchelder, A. H., U. S. Patent 2,235,795 (,June 9, 1942). Beach and Robinson, Ibid., 2,268,020 (Dec. 2, 1941). Benedict, M., and Rubin, L. C., TtarLa. Ana. Inut. C‘hetn. Engrs., 41, 353 (1945). ( 5 ) Benedict, M., et al., Ibid., 41, 371 (1946).
(6) Bergelin, O., Lockhart, F. J., and Brown, G. C., Ibitl., 39, 173 (1943). (7) Briggs, S. ‘X., and Comings, E. W., IND. ENO.CHJW.,35, 411 (1943). (8) Brinsmade, D. S., and Bliss, K., Trans. Ani. I m t . Chem. Engrv , 39, 679 (1943). (9) Callson, H. C., and Colburn, A. P., Tbid., 34, 681 (1942). (10) Colburn, A. P., and Schoenborn, 13. M., I b i d , 41, 421 (1945) (11) Colburn, A. P., and Welsh, D. G., Ibitl., 38, 179 (1942). (12) Comings, E. W., and Briggs, S. W., Ibid., 38, 143 (1942)(13) Drew, D. A., and Hixson, A. N., Ibid., 40, biri (1944). (14) D u m , C. L., et al., Ibid., 41, 631 (1946). (15) Elgin, J. C., Chem. d;r M e t . Eng.,49, 110 (1943). (16) Ewell, R. H., Harrison, J. M., and Beig, L., TND. ]
