DISTILLATION
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istlllation and liquid-liquid extraction are two ' Both are widely used in labratory and plant applications. Of these two processes, extraction is by far the less widely understood. Somehow, the concept of solvent selectivity is more difficult than the concept of relative volatility. In addition, introduction of the solvent to the system adds a degree of freedom and complicates the data correlations. It is useful, therefore, that an analogy exists between distillation and extraction. Detailed development of that analogy results in better understanding of extraction, and identification of possible new areas for study. This article gives a side-by-side development of the analogy, comparing phase behavior, solvent and enthalpy balances, and flow schemes. A simple graphical procedure for determining solvent dosage requirement, often overlooked, is described. By comparing flow schemes, a new method for carrying out precision batch extraction-analogous to precision batch distillationis developed. This method should be useful for the analytical separation of a mixture according to polarity. The relationship can be summarized as:
D"extremely important separation processes.
THE
DISTl11ATOl N EKTRAITION ANALOGY
--Solvent in extraction is analogow to h a t energy in distillation ROBERT A.
WOOIDLE
-Extraction phase behavior, as shown on a solvent-composition diagram, is analogous to distillation phase behavior on an enthalpy-composition diagram solvent balance around an extraction system is analogous to an enthalpy balance around a distillation system
-A
-For most distillation flow schmes, there are analogous extraction flow s c h e r
In developing this analogy, it should not be forgotten that the processes are not identical. An extraction phase diagram may be more complicated than the vaporliquid counterpart. Several different solvents may be used in one process, while there i s only one kind of heat. Temperature profile of an extraction column is often controlled; pressure may be vaned also. It is important not to limit extraction schemes to those with a simple distillation counterpart. The Solvent-Heat Energy Analogy
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In extraction, the solvent should be treated as the equivalent of heat energy in distillation. This relationship was pointed out by Saal and Van Dyck (6) in 1933 and by Randall and Longtin (5) in 1938. The analogy can be seen qualitatively in Figure 1. For easier comparison, the solvent is assumed to be the lighter phase; thus it flows in the same direction as the heat energy. The analogy still holds for a heavy solvent. Inspection of the figure reveals:
SOLVfNT IN
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L PAFFIYATF
Distillation is analogous to Extraction Distillate Waste (or bottoms) Overhead condenser Bottoms cooler VOL 5 5
Extract Rafiate Extract stripper Raffinate stripper NO. 3 M A R C H 1 9 6 3
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DISTILLATION PHASE DIAGRAM
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EXTRACTION PHASE DIAGRAM
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"A
+B
WI. FRACTION B (SOlVfNl-FRfE]
at the arbitrarily chovn
POINT a':
The mixture i s heated to some ternperaturr below the bubble point by addition of heat, B B.1.u. per pound
POINT li':
POINT cible with S. R.fm;ns to Figurc 7c, if can bc scm thd th merinnrnporsibh extract concenlration carcsponds to pant X .. In Figclra 76, th merimurn theoretical extract concenhotimt is pure B, d h q h this conesponds to on iyimte number of stages. Oboiously, effectimss of extracf rgux wiN be limited in syshns of the type shown in Figures 7c and 7c
I t was shown that the solvent balance equations for extraction define two lines on the equilibrium diagram. These two lines have one point in common, and are further restricted so that one line passes through (x,, y = 0) and the other passes through (x,, y = value set by phase diagram). It can easily be shown that for a given phase diagram-i.e., for a given system at a given temperature and pressure-and for a given charge composition x,, three additional quantities will completely define the two lines and, therefore, the separation process. Values may be assigned to any three of the following variables: extract composition, ratlinate composition, solvent-feed ratio, solvent-ratlinate ratio, reflux ratio, and number of stages. A common design problem is: given target values of x I , x,, and Q/F, determine the reflux ratio and number of stages required. A common practical problem is: given an existing contactor (with a given number of stages) and an allowable Q/F to meet a target, xI, determine the refined oil yield (equivalent to determining x.) and the corresponding reflux ratio. I n each case the requisite number of conditions are specified and thus each is capable of solution, provided the phase diagram and x, are specified. LITERATURE CITED (1) Badger, W. L., Banehero,
J. T., “Introduction to Chemical Engineering,” McGraw-Hill, New York, 1955. (2) Maloney, J. O., Schuhert, A. E., Tram. A.1.Ch.E. 36, 741 (1940). (3) Perry, John H. (ed.),Chemical Enginem’ Handbcak (3rd ed.), McGraw-Hill, New York, 1950. (4) Patrol. &fiwr39, No. 9,234 (1960). 30, (5) Randall, Mede, and Longtin, Bruce, IND. ENO.CHEM. 1063,1188,1311 (1938).
(6) Sad, R. N. J., Van Dyck, W. J. D., World Petroleum CongKss, Vol. 11, pp. 352-8, 1933. (7) Skelland, A. H. P., IND.ENO. CHUM. 53, 799-800 (1961). 22
INDUSTRIAL A N D ENGINEERING CHEMISTRY
