HAROLD G. CASSIDY Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut
A SUBSTANTIAL portion of the chemist's activities consists in the resolution of mixtures of one kind or another. The separation processes which are used extend in increasing subtlety from mechanical sorting of mixtures of macroscopic particles to those methods employed for separating isotopes. In ordinary laboratory practice separation methods of intermediate subtlety are commonly used. These methods are such as might be used for separating mixtures of homologues; or of isomers containing the same functional group, among organic substances; or of closely related inorganic substances. Of these methods, only those which utilize, for the separation, the distribution of the mixture into two phases will be dealt with here. This excludes such methods as centrifugation, electrophoresis, etc. It is the purpose here to point out in a systematic yet general way the similarities in principle and the differences in application of a number of these phasepair distribution processes. It is hoped that this will aid the understanding of existing separation processes of this type and assist in the devising of new processes. In separating the components of a mixture by means of distribution, two phases are employed. The components of the mixture distribute themselves into these phases in ratios different from the ratio in the original mixture. The distribution having occurred, the phases can he separated mechanically, and the components in A portion of this paper was presented as pert of the Cs.lco Lecture Series, at the Calco Chemical Division, American Cyanamid Company, Bound Brook, New Jersey.
C--Table
the new ratios recovered from each phase. The natures of the distributions will of necessity depend upon the kinds of phases used in the phase pair. The various possibilities are set forth in a cross-table (Table 1) together with some of the separation processes which rely on the particular distributions. (A few selected references are given for the various processes. The literature is too voluminous to be treated otherwise.) The actual separation process consists of certain definite operations which may be generalized to cover all of the distributions listed in Table 1 (15). Operation 1. The mixture to be separated, dissolved in or constituting one of the phases, is brought into contact with the other phase. Often, a rather specialized machine is used for this operation and the subsequent ones. Operation 2. Contact between the phases of the phase pair is maintained for a period to permit the components of the mixture to become distributed into the phases. This period may be that required to reach equilibrium, but its length is not necessarily governed by this end. Operation 3. The phases are separated by some mechanical means. Operation 4. The mixtures of substances, in new ratios of their components, are recovered from the phases separately by some suitable method. It is not necessary to list here the mechanical methods necessary for separating the phases in each case. In distillation they involve, for example, drawing off the
TABLE 1 of Possibilities for Separation Processes Involving Distribution, into Two Phases, with Some Examples of Proceases
Phases in wntacl
Gas or Vapor (V) Liquid (L) Solid (S)
Mobile (surface of a. liquid) (MI Immobile (surface of a solid) (I)
Gas or vapor (V)
v-v
Liquid (L)
Bulk Phases
-
~~
(Atmolysk (I), Gas diffusion (s)) v-L (Distillation (3), Absorption (4)) V-S
GL (Solvent-solvent extrrtotion (5)) GS
(CrystaUisation (7)) (Sublinmtion (6)) Surface Pbaaes L-M V-M (Emulsion separation (9)) (Foamseparation (9), Flotation (lo))
v-I
Solid (S)
(Gas a d s o ~ t i o n(11), Hypersorption (18))
GI
(Adsorption from solution IS) Chromatography
h 1
S-S (Enflewage (8))
SI
(Segregation in crystal masses)
JOURNAL OF CHEMICAL EDUCATION
liquid phase a t one point in the contacting machine (the still) and drawing off and condensing the vapor phase a t another point. Each of the distributions between phase pairs listed in Table 1can he made the basis of separation processes. Within one phase pair the separation processes can differ in the methods of obtaining contact (Operation 1) and in the types of machines used for bringing about the contact; in the period allowed for the distribution and its equilibrium or nonequilihrium result (Operation 2); in the methods of separating the phases (Operation 3); and in the recovery of the products of the separation (Operation 4). These methods of operation are utilized in connection with each phase pair, or if not actually used are conceivable. In connection with Operation 1 we can distinguish four ways of contacting the phase pairs: in a batchwise manner, a cascade manner, a differential counter-current manner, and a cocurrent manner (16). The batchwise method is in principle quite distinct from the differential countercurrent. The cascade application is a multiple stage, or multiplied batchwise, method which is analogous to a systematic batchwise fractionation such as may be carried out on the basis of a "diamond pattern" (17); the differential countercurrent is the limit which might in some cases he approached by a cascade operation in which the stages are very numerous. Examples of these processes for a number of solvent-pair distrihutions are given in Table 2. The cocurrent method, in which the members of the phase pair pass in the same direction, is industrially important but will not he considered here. I t seems to be essentially a batchwise process. These different ways of contacting phase pairs show, apart from their differencein appearance a very fundamental difference in principle (23). For a given quantity of the phases the batchwise is the least efficient and the differential countercurrent the most efficient, other things being equal. This can be seen from the following argument. Suppose that a solute with a distrihution ratio of 1 were extracted from one phase with an equal volume of the other; the amount extracted in a single hatch process carried to equilibrium would be 0.5 of the original amount of solute. If the second phase were used in two steps, using equal portions in each, the total solute extracted would he 0.555; in 4 steps 0.590,
or 18 per cent more than the single-batch extraction. It is evident that the differential countercurrent method would be most efficient,for in the passage of one phase past the other the extracting phase would leave the other in the most complete state of exhaustion possible. The separation processes involving differences at the level of Operation 2 are not quite so numerous a t present as might be considered possible. For each different method of contacting there could he imagined processes which differ by using either equilibrium or nonequilibrium distrihutions. Some examples of these methods are collected in Table 3. I t will be evident, also, that the procedure under each method of contacting may be varied in another way without changing the principle of the method. For example the cascade method, as well as its limit, the differentialcountercurrent method, may he carried out either in a nonsteady-state operation or in a steady-state operation. In the former a given quantity of material is distributed, possibly to equilibrium, and then fractions are removed, more or less slowly, until the entire quantity of material has been exhausted or until a determined state of the separation has been reached. In the latter type of operation the mixture is fed into the system a t the same rate at which fractions are drawn off, a steady state being achieved throughout the machine being used for the separation. This type of system is sometimes called an "open" system (36); it is characterized by a stable gradient of some kind between the point of ingress of the mixture to the machine and the points of egress of the fractions. Because of this gradient the steady state is not an equilibrium state over-all since the latter condition would require the intensities of all forms of energy to he uniform. A closer approach to equilibrium in cascade and differential countercurrent processes can be achieved through the device of reflux, applied at both ends of the system. (This is not the sole purpose of reflux.) Differences in the mode of carrying out Operations 3 and 4 are of no particular interest here, nor will those separation processes be discussed in which several different types of distribution are utilized simultaneously. These latter are covered here in principle, though not explicitly. Two questions may be raised a t this point: Why do the molecules of the mixture distribute themselves be-
TABLE 2 Example. of Methods of Operation by Which the Various Indicated Phase Pairs Am Utilized in Separation Precesses Phase uairs utilized Method of operatin Batchwise
Cascade (multi-stage) Differential countercurrent
v-L
L-L
Distillation
Sduentsolvent eztrmtion
L-I Adsorption
L-S CrystaUizatin
Simple crystaliisation Distillation of a volatile Simple liquid-liquid ex- Simple decolorization traction (5) (18) substance from an essentidly nonvolatile solvent Bubble-cap tower operrw Syjtematic repeated ex- Systematic repeated ad- Systematicrecrystallizsr tion ( 8 , 4) traction ( 1 8 ) sorption ($0) tion ($1) Packed tower operation Packed tower extraction Chromatography ( I d ) , Counterourrent crystal@,4) (19) Hypersorption ( 1 8 ) lieation ($$)
MAY, 1950
243 -
A further complication in separations is provided by the existence of constant-distribution mixtures. In crystallization these may be eutectics (St), or weakly hound complexes which are not separated under the P~oeessrelying on ubtual NaequilibrizLm conditions of most crystallieations (e. g., low temperaPhase pair equilibrium process V-L Fractional distillation Nonequilihriumevapn- ture). The analogue in distillations is the existence of ration (SL), molecu- azeotropes (33). These may be broken in a variety of under reflux (8. . , 1..) lar distillation (#5) ways (54). In adsorption the analogue is found in the GI Simple decoloriaation Probably mast chro(1s) matographic proc- mixtures which are adsorbed in the same ratio as they esses (1L) exist in the bulk phase (35). It may he imagined that these can be separated by nonequilibrium chromatogtween two phases in contact, and what factors control raphy (as aeeotropes may be separated by nonequilibthe relative amount of a given substance in each phase? rium distillation (24). It is evident from this analysis that all these distribuThe latter question can be discussed only in connection tion processes are analogous in principle. In practice with the individual processes because of the multiplicity of factors involved and because these differwith they differ with the nature of the phase pair, which the phase pairs in use. The former question has been governs the kind of machine used in carrying out the attacked in many ways. The molecules of the mix- separation, and with the manner of contacting the ture distribute themselves between the phases in a phase pairs. They differ also in many details of operaspontaneous process which is of itself irreversible. tion which can only he discussed under the individual The tendency of the molecules to distribute themselves processes. can be measured, as it occurs, as a rate. It can also be ACKNOWLEDGMENT measured as a potential which will be greater the fnrI am indebted to Professor Charles A. Walker for ther removed the initial state of the system is from the helpful discussion of some of the points in this paper. eauilibrium state. The esca~inatendency of molecules from one phase into the othkr can be judged from the REFERENCES equilibrium distribution. It is sometimes measured in (1) URBAIN, E., AND R. URBAIN,Compt. rend., 176, 166 (19231. terms of fugacity (26). The drive toward distribution NF.,Chem. , &Met. Eng., 52,Dec., 98(1945). See (2) H O O E R ~J. alsoBARRER, R. M., Trans. Faraday 8oc., 35,644 (1939). of the components between the phases is sometimes ex"The Chemical Tech(3) GRUSE,W. A,, AND D. R. STEVENS, plained as being due to the tendency to increase the nology of Petroleum," McGraw-Hill, New York, 1942, entropy of the system. I t is sometimes measured in p. 259 et seq. KIRSCHBADX, E., "Distillation and Rectiterms of the probability of finding the molecule in one fication," Chemical Publishing Co., New York, 1948. ROBINSON, C. S., AND E. R. GILLILLNO, "Elements of or the other phase. Fractional Distillation," 3rd ed., McGraw-Hill, New The forces driving the distribution can be dealt with York, 1939. bv the methods of rate theorv " (27). . . The results which (4) SEERWOOD, T. K., "Absorption and Extraction," McGravare obtained in this way are generally applicable to all Hill, New York, 1937. See also FENSKE,M. R., C. S. distributions. The chemical basis for explaining the C m r s o ~ AND . D. QUIGGLE. Znd. Eno. Chem... 39.. 1322 (1947). distributions are less general. As stated above they T.G., AND A. W. NASH,J. Chem. Sac. 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TABLE 3
Some Processes Which Differin Closeness of Approach to Equilibrium
JOURNAL OF CHEMICAL EDUCATION
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