Dehydration of Ethyl Alcohol by Fractional Liquid Extraction J
J
EDWARD G. SCHEIBEL Hoffmann-La Roche, Inc., Nutley,
T h e recovery of anhydrous ethyl alcohol from a dilute aqueous solution has been studied. Fractional liquid extraction is carried out by the use of a high boiling ketone and glycol and a high boiling aromatic hydrocarbon and glycol. Methods for estimating the distribution coefficients from other data and their application to equilibrium stage calculations are described. The complete process designs are presented for each pair of solvents. If an anhydrous alcohol denatured with 1 or 29” of an aromatic hydrocarbon is desired, glycol and o-xylene or any higher boiling aromatic are preferable. If pure absolute alcohol is required, glycol and methyl n-amyl ketone, or a ketone of the same or larger molecular weight, are preferable, and the initial cost and operating cost are
slightly greater. All separations involved in the process are simple and can be carried out in relatively few stages or trays. A considerable saving in operating expenses is possible over previous methods for producing anhydrous ethyl alcohol. Performance data are presented on this process in a multistage liquid extraction column; t h e optimum packing height in the laboratory column has been determined for these operations. As the separation obtained in the laboratory column compared favorably with the separation desired in a commercial unit, extrapolation of the performance of the column t o additional stages was minimized. The performance of the laboratory column is described and the results are interpreted on the basis of previously published data.
T
it is expected that with proper design the same performance can be realized on liquid extraction columns of large diameter with thirty t o fifty equilibrium stages. In addition t o improving the liquid extraction process, the new equipment opens up a relatively unexplored field of fractional liquid extraction (10). In this process a mixture can be separated by the countercurrent flow of two solvents, each having a preferential effect for one of the components of the mixture. By providing a sufficient number of equilibrium contacts between the solvents‘ it is possible to achieve a sharp separation of the components, even though the selectivity of the two solvents is small. This is analogous t o distillation, in which the vapor is somewhat selective for the more volatile component and the liquid is selective for the less volatile component, and with the proper number of plates a sharp separation can be obtained. Ethyl alcohol and water are appreciably different in their solubility characteristics and can probably be separated by any immiscible pair of solvents which do not react with the alcohol or water. The choice of suitable solvents then depends upon practical considerations. For simplified recovery of the pure components, so t h a t the solvents can be readily recycled to the extractor, the solvents should not form azeotropes with the ethyl alcohol and water. In order t o reduce the heat requirements, it is preferable to choose solvents t h a t are higher boiling than ethyl alcohol and water, so that the separation can be effected without distilling off all the solvent from the solution. Glycol and glycerol meet all these requirements and are selective for water. Higher boiling ketones and hydrocarbons are selective for ethyl alcohol and are not completely miscible with glycol or glycerol. Glycol was considered preferable t o glycerol because of its lower boiling point and lower viscosity at ordinary temperatures. Methyl n-amyl ketone was chosen because the lower boiling ketones, such as methyl isobutyl ketone, show a greater mutual solubility with glycol; the higher ketones boil at temperatures requiring higher steam pressures than those available in many chemical plants. Of the hydrocarbon solvents, o-xylene was chosen as the lowest boiling aromatic compound which does not form an azeotrope with ethyl alcohol and which can, therefore, be readily separated by distillation. Higher boiling aromatics would require solllc-
HE concentration of dilute ethyl alcohol produced by the fermentation of sugars was the subject of the earliest work on distillation. Ordinary distillation of this mixture cannot yield a concentration greater than the azeotropic mixture, which is 90 mole yo (4) or 96.7% by volume; in fact, it is so difficult to approach this concentration that 95% ethyl alcohol by volume is usually produced. Numerous physical and chemical methods have been developed for the dehydration of this alcohol, which is one of the oldest problems. With the use of fractional distillation at reduced pressure, it has been found that at a n absolute pressure of 100 mm. of mercury, the azeotrope contains only 0.4 mole % of water (4). The first application of azeotropic distillation was also the dehydration of ethyl alcohol by the addition of benzene (18). Numerous other entrainers which also form ternary azeotropes with ethyl alcohol and water, including n-hexane, cyclohexane, trichloroethylene ( d ) ,carbon tetrachloride, butyl chloride, and ethyl acetate, have been studied; these methods were summarized by Keyes (6). Ethyl ether has also been demonstrated as an entrainer for this separation (7, 16). This component forms a binary azeotrope with water but not with ethyl alcohol and i t does not form a ternary azeotrope with these components. Thus the overhead product consists of ethyl ether and water substantially free of ethyl alcohol. I n order t o obtain sufficient water in the azeotrope t o give appreciable phase separation of the overhead, it is necessary t o operate above atmospheric pressure. By operating at 130 pounds per square inch gage it is possible to re-use the heat from the condenser t o supply steam for concentrating the original weak alcohol feed. Thus this method has the advantage of utilizing the heat twice a t different temperature levels. PRELIMINARY DISCUSSION
*
N. J.
The fractional liquid extraction process has been used very little because of the problems involved in obtaining a large number of equilibrium stages in liquid extraction equipment. I n a type of liquid extraction column (10,11) recently described, it is possible to obtain an equilibrium stage in about the same column height &s t h a t required by a theoretical plate in distillation. The comparison WM made on the basis of a laboratory columnJ and
1497
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
1498
Vol. 42, No. 8
water binary system were determined in previous work ( I O ) . The ternary constants in the Redlich and Kister equation were neglected. The activity coefficients for water in methyl n-amyl ketone and alcohol solutions did not vary greatly from the activity coefficients in methyl n-amyl ketone solution alone when based on the ketone concentrations. Over the range of concentrations studied up to 35 mole %, no appreciable difference was calculated for the activity coefficients of ethyl alcohol in methyl n-amyl ketone and in the methyl n-amyl ketonewater solution a t the same solvent concentration. Thus if the activity COefficients of ethyl alcohol and water in methyl n-amyl ketone are desired a t the particular solvent concentration, very little interpolation is required in Figure 1.
PO
IO 9
8
w
O S
TABLEI. BINARY CONSTANTS AND
FOR
ETHYL.kCOHOL,
\\'ATER,
GLYCOL SYSTEMS
(Redlich and Kister) Ethyl Alcohol-Water Biz .6
-
CONCENTRATION OF MErmnmrL KETONE SOLVENT MW
ClZ Dii
.4
= 0.560 = -0.150 E
-0.034
Water-Glycol
BZ3
czs
02s
0 = 0 = 0
Glycol-Ethyl Alcohol
Bai = c3l 0 3 1
0.315
= -0.060 = 0
FRACTION
Figure 1. Activity Coefficients of Ethyl Alcohol and Water from Methyl n-Amyl Ketone Solutions
what higher steam pressures for their recovery. Similarly, paraffinic and naphthenic compounds would have t o be higher boiling than the aromatics to avoid the formation of an azeotrope with ethyl alcohol. Accordingly, the dehydration of ethyl alcohol by fractional liquid extraction, using methyl n-amyl ketone and glycol and +xylene and glycol, has been studied and the process designs based on each system of solvents have been compared. METHYL n-AMYL K E T O N W L Y C O L SOLVENTS
Data on liquid distribution of ethyl alcohol and water between methyl n-amyl ketone and glycol are not available, but by simplifying assumptions that have been found applicable in several other systems (10) it is possible to predict the distribution data from vapor-liquid equilibria data. The method is based on the activity coefficient curves of the solutes from the two solvents. The necessary data for the ethyl alcohol-methyl n-amyl ketone system are not available, and it was assumed that the activity coefficient,s of ethyl alcohol from this solvent are the same as from acetone. The data of Gordon and Hines ( 3 )on the acetoneethyl alcohol system were correlated by the method of Redlich and Kister (8) and the curve shown in Figure 1 was calculated for the constants B = 0.300, C = -0.020, and D = 0 in their equation. The activity coefficients for water from methyl n-amyl ketone were calculated from the mutual solubility data ( 1 ) and three additional points of data were determined for the distribution of water between glycol and this ketone. The activity coefficients were calculated assuming that the water-glycol system was ideal, as indicated by the vapor-liquid equilibrium data a t 0.3 atmosphere (6). Any deviation of the glycol-water system from ideality would affect the activity coefficients from tmhe methyl n-amyl ketone calculated by this method, but if the curve is drawn through these points, the distribution data calculated from the curve by the same assumption would be correct. The use of the curve which was correlated t.o a thermodynamically sound equation allows the prediction of the effect of ot'her factors t i t h greater reliability, particularly in considering the thermodynamic relations of ternary systems. The constants for the binary activit,y coefficient curves, given in Table I,were used t o calculate the limiting coefficients of water from ethyl alcohol-methyl n-amyl ketone solutions and of ethyl alcohol from water-methyl n-amyl ketone solutions. The constants for the ethyl alcohol-
~
The activity coefficients of ethyl alcohol from glycol were not available in the literature and preliminary investigation showed that this system was nonideal. Three measurements of the distribution of ethyl alcohol between methyl n-amyl ketone and glycol were made and the activity coefficients calculated from these data are shown in Figure 2. The curve of the activity coefficients of ethyl alcohol from glycol was drawn through an additional point subsequently determined from the distribution between o-xylene and glycol. However, the three points determined cover the range of concentrations encountered in the present system. Similarly, by this method any errors in the assumed activity coefficient curve for ethyl alcohol from methyl n-amyl ketone are counterbalanced by a compensating error in the other curve and the distribution coefficients calculated from the curves will be in agreement with the observed coefficients. I n this case the use of these curves allows a thermodynamically sound calculation of the activity coefficients for the ternary system. By this method it is possible t o obtain a system of curves which are properly related to each other, but this does not imply that the calculated curves are necessarily correct, because the introduction of ternary constants may affect them Generally, the effect of the ternary constants is small and the uncertainty introduced by neglecting these additional constants is probably less than the experimental errors in the original binary data and the deviations i n generalized relationship which were the basis for these calculations. The binary constants used to calculate the additional curves on Figure 2 as described for Figure 1are given in Table 11.
CONSTAIVTS FOR ETHYL ALCOHOL,WATER, TARLE 11. BINARY AND METHYL ~ - A & I YKETONE L SYSTEMS Ethyl AlcoholTVa t er Bir = 0.560 C u = -0,150 Dir = -0.084
(Redlich and Kister) Water-Methyl n-Amyl Ketone Bna = 1 . 7 6 c 2 3 = 1.01 Ow = 0 . 3 8
Methyl n-.4myl Ketone-Ethyl Alcohol Bai = 0.300 Ca1 = -0,020 031 = 0
By means of Figures 1 and 2 it is possible to calculate the distribution coefficients for water and ethyl alcohol between glycol and methyl n-amyl ketone. A sample calculation of these distribution coefficients is given in Table I11 for a 60 mole % ' ethyl alcohol and 40 mole % water mixture present a t a total concentration of 0.05 mole per mole of methyl n-amy1 ketone. The basic curves were calculated by neglecting the mutual solubility
August 1950
1499
INDUSTRIAL AND ENGINEERING CHEMISTRY
two pure components by countercurrent contact between two solvents. The equilibrium stage calculations are carried out by alternate material balances and equilibrium calculations (10)and t h e basic equations were derived for the relationships above and below the feed. CONC~NTRATIONS ABOVE FEED STAGE.
where
Yn+l= concentration of a com onent in the lighter solvent
P
+
Figure 2. Activity Coefficients of Ethyl Alcohol and Water from Glycol Solutions
of the solvents and the effect of this factor on the activity COefficients. Thus the distribution coefficients are calculated on the same assumptions. The activity coefficients of ethyl alcohol and water from methyl n-amyl ketone are determined from Figure 1 a t a solvent concentration of 0.952, and by interpolating between the two curves for the activity coefficient of water. Thus the activity of the water and ethyl alcohol in the methyl namyl ketone solution can be calculated a t equilibrium conditions. These activities must be equal to the activities of the two components in the glycol solution. The concentrations in the glycol phase are determined from Figure 2 by trial and error. At an assumed solvent concentration and an assumed ratio of ethyl alcohol to water in the solution the activity coefficients of these two components are observed from Figure 2. Calculation of the concentrations of the respective components must agree with the previously assumed ratio and their total concentration must give the assumed solvent concentration by difference. The agreement must be such that a t the h a 1 calculated values of the concentrations the activity coefficients determined from Figure 2 would not be measurably different from the activity coefficients used in the calculations. Otherwise a second trial should be made with the new activity coefficients. With practice one or two trials will suffice to establish the equilibrium conditions. The mole ratio of the ethyl alcohol and water t o the glycol solvent is then calculated as given in Table 111. The distribution coefficient is the ratio of the concentration in the lighter methyl n-amyl ketone solvent to the concentration in the heavier glycol solvent. In order to simplify the subsequent stage calculations, concentrations should be expressed on a mole ratio basis (10). Figure 3 summarizes the distribution coefficients calculated for this system of solvents.
leaving n l t h stage be ow the top, moles per mole of solvent Y , = concentration of the component in the lighter solvent leaving nthstage below the top, moles per mole of solvent Yp = concentration of the component in the overhead product stream, moles per mole of solvent H , = rate of heavier solvent, moles per hour L, = rate of lighter solvent, moles per hour D, = distribution coefficient of component a t nth stage below the top expressed as ratio of concentration in the lighter solvent t o concentration in the heavier solvent. Concentrations are expressed on a mole ratio basis
If all concentrations are expressed as mole ratios instead of
TABLB111. SAMPLE CALCULATION OF DISTRIBUTION COBFFICIBNTS OF ETHYL ALCOHOLAND WATERBETWEEN METHYL +AMYLKETONEAND GLYCOL
Methyl n-Amyl Ketone Solution Y?
Glycol Solution
D
7
D.76,
Y,
activity 2, 21, activity YY, eoef- mole mole mole ooefratio fraction ficient activity ficient fraction Ethyl alcohol 0.03 0.0286 Water 0.02 0.0180
-0.05
0.0476
1 . 8 0 0.0515 12.5 0.237
X,
mole ratio
2 . 1 5 0.024 1 . 0 2 0.232
0.0323 0.312
0.256
0.3443
_-
distnbution coefficient 0.93
- 0.064
EQUILIBRIUM STAGE CALCULATIONS
Figure 4 is a schematic diagram of fractional liquid extraction column in which the feed is separated into L
TOTAL WNCENTRATION IN METHYL n -AMYi KETONE- MOLES ETHYL ALOOHOb t WATER /IO0 MOLES WM
Figure 3. Calculated Distribution Coefficients of Ethyl Alcohol and Water between Methyl n-Amyl Ketone and Glycol
1500
INDUSTRIAL AND ENGINEERING CHEMISTRY
mole fractions, the units of H and L are in moles of solvents rather than in total moles of the two liquid streams, and it is then apparent that H and L are constant in each section of the column. They may be different above and below the feed if a quantity of either of the solvents is introduced with the feed. Equation 1 may be multiplied through by L,, the flow rate of the lighter solvent; all the terms ~mjt will then be expressed as the f--:+-+ i) quantity of the particular comI I ponent. Thus L,,Y,,+I is the I I I quaritityin moles per hour of the I component leaving the II. lth I H,, stage in light solvent, L?,D,* L,,Y,, can be shown t o be the quantity in moles per hour of the component leaving the nth stage Figure 4. Schematic K epr e s e n t a t i o n of in the heavy solvent, and L A Y ,is Material Balances of the quantity of the component in Liquid Extraction the light solvent stream leaving CoILlTTln the t o D of the column. For purposes of these calcul~tioris the fresh solvents introduced into the column arc assurncd t o be free of any of the compone1it.r in the feed
+
I
~
-of B pure gas is a simple function of the viscosity and heat capacity) is riot rigorously proved as yet, and the best data indicate that it, m:ty be slightly in error. Because of this, Hirschfelder el ui. do not recommend the Chapman and Enskog ( 3 ) equation but instc:id recommend the use of the modified Eucken equation:
it, v a s felt that perhaps if the kinematic viseusit). u, (nionit~titui?i diffusivity) were replaced by the therrnal diffusivity t v L
the cquation
could be used toget,lier with Equation 1 to calculate niistuw inductivitv. This cquation was applied to 49 poitits :iriii 11ta.t value of the constant \vas fourid to bt. 1, I 14. Ttw i.c:sulti!iq equation lor binaries is 30.7 .
l -
0.06-
for gas mixtures as well as for the pure gases. HIT' is a function (6) of the gas and temperature, b u t under. all possible conditiolis it does not differ from unity by more than 0.5%. Although this
BtU
hr)(fl)('F)
-
0.04-
is a fair approximation for pure gases it fails completely for it misture of gases, as can be seen in Figure 1. Kennard (8) and several other authors recommend the equation k m = ki ($1)' K(ZIZ?) k p ( . ~ ) * (3)
+
-
--I(IRSCHFEL0ER e t . att61 E q 2
+
where K is a constant t o be determined for the pair of gasvs in question. Even using a K determined by least squares from the data, the agreement is poor as can be seen in Figure 2. Equation 1 appears to be the most satisfactory and since Buddenberg and Wilke ( 2 ) and Sutherland (10) have found this
4UTHORS E q s 11-16
0
O
U
02 C"2
D A T A O F 188s HIRST"'
0 4 X
L
0 6
M O L E F R A C T I O N H,
O B
10 HZ
Figure 1. Thermal Conductivity o f I I y d r o g ~ l Carbon Dioxide ILixtures d t 0 " c:.
