Fractional Solvent Extraction between Immiscible Solvents GRAPHICAL CALCULATION EDGAR L. COMPERE
AND
development
ADA RYLAND
LOUISIANA STATE UNIVERSITY, BATON ROUGE, LA.
T
HE possibility of sepa-
.ri
rating solutes by fractional solvent extraction has been increasingly appreciated in recent times; a good summary of t h e presentstatus in the field has been pre(4). If two sented by craig have different distribution ratios between two immiscible solvents, it is possible
A
cyclic countercurrent batch extraction procedure similar to that of O’Keeffe is presented, which results in Separation Of Solutes Paralleling that of a continuous countercurrent process. By continuing the cycles, i t should be possible to separate any desired amount of material at constant purity of the products. A trial and error method of graphical calculation based on that of Varteressian and Fenske is given. This procedure permits the selection of operating conditions and the prediction of results for the separation of solutes which do not necessarily have constant distribution ratios. A comparison of predicted and observed experimental results in the fivestage separation of phenol and picric acid, using benzene and weak aqueous sodium &loride solution as the solvents, indicates the method to be valid.
t;zy
wlkt
t ~ ~ l illustration), the light solvent is withdrawn a s p r o d u c t . Then quantity of light with 901ff1 vent, aLZ, is and H , is vithdrawn. To the funnel is then added not only H2 but also Fz, the second portion of feed solution, as indicated in the diagram; the funnel is shaken and H2 is withdrawn. Ha is added, shaken, withdrawn, and set aside. Except for the first portion of feed solution, the volumes of all portions of feed solution and light and heavy solvent are constant.
to One Of the to be Preferentially extracted by the lighter and the other by the heavier solvent, by suitable adjustment of the volume ratio of the two solvents. This paper presents an application of the graphical method of Varteressian and Fenske ( 1 3 ) to the problem of predicting the degree of sewration of the solutes under various operating conditions. The method is also presented ’by Elgin (6). This discussion is restricted t o the case of solvents which are not miscible, or whose miscibility is not altered by the presence of the solutes. Also, it is assumed that the solutes do not interact and consequently their respective distribution ratios are not affected by the presence of the other solute. On the other hand, the method does not require the distribution ratio to be constant as solute concentration varies, and thus permits consideration of cases where the solute associates or dissociates in either solvent. Concentration levels will be held sufficiently low in general to permit use of molar units rather than mole fractions.
At the end of several such cycles the compositions of the various stages will become constant, and the contents of the stages indicated by A , B , C , D,and E in Figure 1 will be the same as those in the corresponding stages in the continuous countercurrent system with integral stages, shown in Figure 2. Examples of such a system are the mixers and centrifugal separators of Bartels and Kleiman (1)and the highly efficient stirred column of Scheibel (IO). It is to the upper and lower parts of the system shown in Figure 2 that the method of Varteressian and Fenske (13) applies. In addition, for a continuous countercurrent system with differential stages-e.g., a packed column-this method will represent a good approximate approach, using the height of equivalent theoretical plate concept in the usual manner. The end results of these operations are the same as those of O’Keeffe, Dolliver, and Stiller
HEAVY BOLV EN T
LIGHT
SOLVENT
HEAVY
\\J
SOLVENT
'\
SOLVENT
""I,
Volumes Pure light solvent into first stage Light solvent in feed solution Pure heavy solvent into last stage Heavy solvent in feed solution Concentration of given solute, moles per liter In light solvent, in feed solution In light solvent, in product In heavy solvent, in feed solution In heavy solvent, in product
where LY Scheme for Cyclic Countercurrent Fractional Extraction
I t has been shown by Varteressian and Fenske ( I S ) that on such a diagram operating lines may be established whose slope is fixed by the solvent ratio, and that the operating line will be straight if the solvents are not miscible. Progression from one extraction stage to the next is represented by a series of horizontal and vertical steps between the operating and the equilibrium line in a fashion analogous to the McCabe-Thiele method in distillation calculations. Thus with an assumed extract composition and given solvent ratio, i t is possible to estimate the composition of the feed stage. This may be done on either side of the feed stage, the operating lines falling, respectively, above and below the equilibrium line. The intercepts of the lines (upper line on Y axis, lower on X axis) must account for the amount of solute in the feed (material balance requirement). When by trial and error a pair of operating lines has been found which predicts the same composition in the feed stage as well as meets the material b a l a n c e requirement, the intercepts on the two axes predict the solute conPURE H E A V Y c e n t r a t i o n in the SOLVENT two effluent solvent streams. The m a t e r i a l balance requirement may be solved graphically, as shown in t h e M - 3 : MLXER A N D i l l u s t r a t i o n . The SEPARATOR use of reflux is not FEED SOLUTION considered in this paper. A parallel comp u t a t i o n of t h e c o m p o s i t i o n of effluent streams may be carried out 1 M-S / E separately for the second solute, using the same s o l v e n t Figure 2. Continuous Counterratios and thus current Fractional Extraction operating lines of System
YP XI X P
Slope of upper operating line Slope of lower operating line Separation factor, for two solutes cy"
Figure 1.
Yf
=
=
(Y,/X,) solute I (Y,/X,) solute I1
separation factor per stage and n = number of stages
The slopes given above are algcbraic slopes, because it is usually desirable to use a different spacing of coordinates along the two axes of the equilibrium diagram in order to gain precision in calculation. The material balance condition is:
Y/Lj
+ HjXj
=
(Ho
+ H I ) Xp + ( L O+ L j )
Y p
This may be plotted as a straight line on the diagram, provided feed conditions and solvent ratios are known. The respective efffuent concentrations for any chosen operating lines will represent a point on this line. The line is easiest established by calculating its intercepts, assuming successively = 0, Y, = 0, and calculating the value of the other coordinate from the above equation.
x,
ILLUSTR 4TION
The procedure may be illustrated by the separation of phenol and picric acid, using benzene and aqueous 0.1 Rf sodium chloride solutions as solvent. It is recognized that these substances could actually be more readily separated by other means, such as treatment with buffered alkali. However, they have similar distribution ratios between these solvents, analysis of the various
El
1 .o
!=
\ -I
i
o.8
d
3 0.6 N 3 m
$
0.4
0
" 0.2 W
3 s n.n -.0.0 0.05 0.1 0 0.1 5 0.20 0.25 SOLUTE CONCN. IN AQUEOUS 0.1 M NaCI, MOLE/LITER
Figure 3. Equilibrium Distribution Lines for Phenol and Picric Acid between Benzene and Aqueous Sodium Chloride Solution at 25" C.
January 1951
241
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.25
-i Y
:
0.20
0
I W
W
0.15 W
m
f
x
0.1 0
8 0 u
4
2
0,05
a
-
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). U
0.00 ri
0.01
000 X = PICRIC
ACIO
0 02 0.03 CONC. IN AQUEOUS W I O
0.04
0.05
N ~ C I , M O L E S / L.
Figure 4. Graphical Computation of Fractional Extraction of Picric Acid with Benzene and Aqueous Sodium Chloride Solution in Five Countercurrent Stages Feed concentration (to third stage), picric acid in benzene, Y j = 0.0278 mole per liter Feed volume, benzene, L j = 30 ml. Pure benzene t o first stage, Lo = 12 ml. Pure aqueous NaCl solution t o last stage, Ha = 100 ml.
phases is not difficult, and picric acid shows a curved equilibrium distribution line while t h a t of phenol is less curved up t o high concentrations. Thus they serve well as a test of the proposed graphical calculation method. A benzene solution, 0.278 molar in picric acid and 1.03 molar in phenol, was taken, in order to test the degree of separation that might be achieved in five stages. Table I presents the basic computations and results, and the graphical computation for each component is given in Figures 4 and 5. Extractions were carried out as given in Figure 1,until products of the eighth cycle were available, it being assumed t h a t the system was stable by then. Product concentrations a t the end of the fifth and eighth cycles were substantially the same. Determination of phenol was carried out by the bromination method of Siggia ( I d ) . Picric acid was determined by titration with sodium hydroxide, using bromothymol blue indicator.
TABLE I. SEPARATION OF PHENOL AND PICRIC ACID BY FIVE STAGESOF FRACTIONAL SOLVENTEXTRACTION AT 25’ C. (Nurnbe; of cycles, 8 ) Vol., MI. 30 12 100
Solvent Benzene Rolution t o feed stage L/ Pure benzene t o first sta e Aqueou’s 0.1 M NaCl tolfa’st stage, Ho
d
Separation of Phenol and Picrio Acid Concn., Moles/Liter Aotual Calod. Picric acid (I) In feed solution, Y/-I I n efauent benzene, YrI I n effluent aq. soln., X,-I Phenol (11) I n feed solution, Yj-I1 I n effluent benzene, Yn-I1 In effluent aq. soln., Xp-I1
0.278 0.134 0.0266
0.i32 0.0280
1.030 0.438 0.134
O.iO5 0.139
Separation Factor/Stage 1.09 1.10
I n effluent benzene I n effluent aq. soln.
Recovery, % of Input Picric acid Phenol 68 59 32 43
0
0.05
0.10
OS5
XnPHENOL CONC. I N AQUEOUS M/IO
020
0.2 5
NACL, MOLES/L.
Figure 5. Graphical Computation of Fractional Extraction of Phenol with Benzene and Aqueous Sodium Chloride Solution in Five Countercurrent Stages Feed concentration (to third stage), phenol i n benzene, Yf = 1.03
moles per liter Feed volume, benzene, Lf = 30 ml. Pure benzene to first stage, LO = 12 ml. Pure aqueous NaCl solution to last stage, HO = 100 ml.
The above selection of solvent ratios is not regarded as optimum. Actually, the curvature of the phenol equilibrium line was not recognized a t the time the experiment was planned, and solvent ratios were selected on the basis of a straight phenol equilibrium line. However, the separation factor per stage, (Y ( d ) , should remain fairly constant at this concentration level as solvent ratio is varied, and represents a good measure of the separation. It corresponds to the relative volatility in distillation calculations. I n the case of linear equilibrium distribution lines and operation under total reflux, it would be equal to the ratio of slopes of these lines, or t o the ratio of distribution coefficients. It is believed t h a t the method shows satisfactory agreement between predicted and observed values. It should be useful as a method of selecting ,operating conditions and predicting results for various solvent fractionation processes. LITERATURE CITED
(1) Bartels, C. R., and Kleiman, G., Chem. Eng. Progress, 45, 589 (1949). ( 2 ) Benedict, M., Ibid., 43, 41 (1947). (3) Bush, M. T., and Denaen, P. M., Anal. Chem., 20, 121 (1948). (4) Craig, L. C., Ibid., 21, 85 (1949). (5) Craig, L. C., J. Biol. Chcm., 155, 519 (1944). (6) Elgin, J. C. “Perry’s Handbook of Chemical Engineering,” 2nd ed., pp. 1327-44, New York, McGraw-Hill Book Co., 1941. (7) International Critical Tables, Vol. 111, pp. 427, 428, New York. McGraw-Hill Book Co., 1928. (8) Marvel, C. S., and Richards, J. C., Anal. Chem., 21, 1480 (1949). (9) O’Keeffe, A. E., Dolliver, M. A., and Stiller, E. T., J . Am. Chem. Boc., 71,2452 (1949). (10) Scheibel, E. G., Chem. Eng. Progress, 44, 681, 771 (1948). (11) Seidell, A,, “Solubilities of Organic Compounds,” 3rd ed., Vol. 11, pp. 330 ff., 386 ff., New York, D. Van Nostrand Co., 1941. (12) Siggia, S., “Quantitative Organic Analysis via Functional Groups.” w . 111. New York. John Wilev & Sons. 1949. (13) Varteressian, K. A., and Fenake, M. R., ”IND. ENG.CHEM.,28, 1363 (1936); 29, 270 (1937). RECEIVEDMarch 25, 1950. Presented before the Division of Analytical Chemistry a t the 117th Meeting of the AMERICANCHEMICALSOCIETY, Houston, Ter.
