1250
capillary trap (not shown), connected to one arm of a tee in the vapor inlet tube, and is discarded. I n the present apparatus i t was found expedient to insulate the boilers and vapor lines with rock wool so that heat losses would be small and constant. Rheostat-controlled electric heaters and ordinary 1000-cc. distillation flasks provided a simple and flexible arrangement for vapor generation.
N VIEW of the growing importance of industrial distillation
I
systems involving the miscible region of partially miscible liquids, an adequate method of measuring the vapor-liquid equilibrium of such systems is greatly to be desired. I n both the processes of azeotropic distillation and of liquid-liquid extraction, solvent is generally removed from one or more streams by s t r i p ping columns from which the condensed overhead separates into itwo layers. T o make an accurate and economic design, vaporliquid equilibrium of the miscible regions are required; although approximate methods of predicting these data are available (Z), insufficient data are available to test these methods adequately. This is probably because of the experimental difficulties encountered. There are two major difficulties encountered in studies of partially miecible systems in addition to those usually inherent in equilibrium stills (4). The first is t h a t the vapor from any but the most dilute samples will, on condensing, form an immiscible mixture. Thus the recirculation types of apparatus cannot be used in the normal manner, since the condensate, on separating into two layers, cannot be returned to the still with the two liquid phases in proper proportion. While a stirrer might be utilized to maintain the condensate well mixed, there would be some question as to the proper assimilation of the two-phase return liquid 'by the contents of the still. Stockhardt and Hull (7) eliminated recirculation merely by distilling off small quantities from a mixture of known composition after first refluxing in a tilting condenser, but this method involves slight errors caused by differential condenser holdup and, in addition, encounters the second major difficulty discussed below. The further difficulty offered by these systems lies in the great difference in composition between the vapor and liquid, and the small concentrations of the dilute component in the liquid. For example, in the miscible regions of isobutanol in water, which extends to about 2 mole per cent isobutanol, the vapor is from -fifteen to thirty times as rich in isobutanol as the liquid. Therefore, if an equilibrium study is undertaken where a liquid sample is distilled, its composition will change extremely rapidly as vapor is formed, and the arrival at the desired steadys t a t e conditions in the still becomes difficult or impossible. These particular difficulties have led the authors to an entirely different philosophy of approach to this problem. Instead of attempting to determine the vapor in equilibrium with given liquid samples, the present thought is to make u p certain vapor streams and to find the corresponding equilibrium liquids. Vapor streams of any composition can easily be formed b y mixing the vapors from separate stills containing the pure components, and a liquid can readily be brought into equilibrium by bubbling the mixed vapor through the liquid. This paper .describes such a still and supplies data taken to test the apparatus on the commercially important system, isobutanolwater.
EQUILIBRIUM CHAMBER
T o secure a liquid which is in true equilibrium with the vapor, the latter is bubbled through a quantity of the liquid in a suitable equilibrium chamber. The vapor enters the inner vessel (Figure 1) through tube C, the lower end of which, drawn into the shape of a small jet, dips slightly below the liquid surface. Passage of the vapor from the jet is directed upward and, with the aid of the small glass sleeve, D, surrounding it, causes smooth circulation of the liquid in the chamber and brings it rapidly t o equilibrium, The location of the jet with respect to the liquid surface is important since any increase in pressure due t o hydrostatic head, together with the frictional drop through the tube and jet, cause a small enthalpy gradient between the vapors in the two chambers. The dead-air cell, K , seems to prevent excessive heat transfer into the liquid, while the heat which is transferred into the vapor through the walls of the inlet and outlet vapor lines tends to superheat the vapor leaving the chamber and thus prevents partial condensation. Although the enthalpy gradient is small and of doubtful importance, it is present of necessity, and the above arrangement tends to reduce its effectiveness. Vapor leaving the equilibrium chamber through tube G is condensed and cooled in a suitable condenser. Saturation conditions and thermal equilibrium are further effected by inclusion of the equilibrium chamber in the outer vapor jacket, B. Heat losses from the unit are compensated for by condensation of a portion of the entering vapor, the remainder being automatically left saturated. Excess condensate is siphoned through capillary tube J and discarded. This vapor chamber serves the further purpose of providing a store of vapor of large volume compared to the throughput which tends to absorb any slight fluctuations in the composition of the vapors entering the equilibrium vessel, The entire unit is enclosed by the outer deadair jacket, L, and finally by a suitable insulating material such as rock wool. All parts described are constructed of glass to permit observation during operation, and they can be readily disassembled for ease in cleaning or minor adjustment. I n operation, liquid of the approximate equilibrium composition is first introduced into the equilibrium vessel through sampling tube H , and the liquid level is adjusted so as to facilitate a maximum of mixing by the vapor jet. When a run is begun, the initial liquid charge (about 15 cc.) is brought to the vapor temperature by condensation of s portion of the entering vapor. As the system approaches physical equilibrium, any heat requirements due t o change in liquid composition are balanced either by condensation or vaporization in the equilibrium chamber. When equilibrium has been established, the liquid and vapor are at the same temperature, and any gain or loss of heat from the system is balanced by heat flow to or from the outer vapor space. At this point the liquid level remains constant. A sensitive indication of the degree of equilibrium attained in the apparatus is also provided by thermocouple I whose junction is located in the well just above the vapor jet in the equilibrium chamber. When equilibrium is reached, the potentiometer shows no temperature variation greater than approximately 0.1" c.
SOURCE O F VAPOR
T o supply a mixture of vapors of constant composition to the equilibrium liquid, the pure components are boiled separately at constant k e d rates in suitable distilling flasks. Any convenient source of heat can be used as long as the rate of heat input and, therefore, rate of vaporization can be controlled. The vapors thus generated are mixed and passed into the equilibrium chamber through tube A (Figure 1). Although the mixed vapor is apt to be slightly superheated, a slight loss of heat from the vapor line soon reduces it t o saturation conditions; a small amount of condensate formed is withdrawn through a suitable *
.
*
*
e
The photograph ou the facing page shows large distillation columns built by E. E. Badger & Sons Company for the separation of chemical products. The present addrese of t h e junior author, David Shilling, is t h e University of Wisconsin. Madison, Wis.
1251
INDUSTRIAL AND ENGINEERING CHEMISTRY
1252
Figure 1 Details of Equilibrium Chamber
equilibrium chamber was, in the present case, always a single-phase liquid; however, during operation with a vapor very near the azeotropic composition, the liquid, though single phase a t the boiling point, separated into two phases when withdrawn and cooled for analysis. Samples which were single phase at room temperat,ure (including most of the liquid samples and some of the vapor samples) were analyzed byrefractiveindex at 25" C. ( k 0 . 2 ' ) with a Zeiss dipping refractometer. The isobutanol used was a narrow-boiling fraction (107.8-108.0° C.) obtained by distillation a t high reflux ratio of a large quantity of the commercial material. The compositionrefractive index data of Colburn and Welsh (3) for isobutanol-water solutions were used since the same liquids were employedin bothstudies. Samples which formed two phases were analyzed by measuring the volumes of the two layers. The determinations were made directly in a graduated collecting cylinder, held (tightly stoppered) at 25" C. in a constanttemperature water bath for 15 to 30 minutes. The total volume of the samples taken ranged from 4 to 6 ml., and the collecting cylinder, previously calibrated with distilled water, could be read with a precision of about *0.05 ml. The solubility data employed in the analytical calculations were obtained from International Crit,ical Tables (3) : Isobutsnol in water layer at 25' C. Water in isobutanol layer at 25' C.
SAMPLING AND ANALYSIS
Since only a vapor of the exact heteroazeotropic composition can be in equilibrium with the two-phase liquid, the liquid in the
X
-MOLE % ETHYLEIdE
-
wt.% 8.25 16.57
Mole yo 2.14 45.0
Density data used in the calculations were determined for each of the two phases by means of the graduated collecting cylinder as follows: Saturated solutions were prepared by prolonged shak-
X
D ChLORISE
Data on Ethylene Dichloride-Toluene Experimental: 0 Jones et al.; Raoult's law.
Figure 2.
Vol. 35, No. 12
Figure 3.
-
MOLE
% !SOBUTANOL
Data on Isobutanol-Water 0 Experimental; X Stockhardt and Hull.
December, 1943
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ing of the two liquids in a separatory funnel a t approximately 25" C . Each layer was carefully introduced into the previously weighed cylinder and held in the constant-temperature bath until temperature equilibrium was again established; the volume was then read and the cylinder again reweighed. I n calculating cornpositions, the following average density values were used : f.?raram/ml. 0,983 0.836
Water saturated with ieobutanol, 25' C. Isobutanol saturated with water, 25O C.
shows a few of the points a t low concentrations of isobutanol and all the points at higher concentrations. The values all fall on a smooth curve, indicating consistency of the results. For comparison the data of Stockhardt and Hull (7) are shown, and agreement is good. Figure 4 is an enlarged diagram of the low isobutanol region. The data are remarkably close to a smooth curve. The points of Stockhardt and Hull are again in general agreement, although there is slightly more scattering of these values.
Vapor pressure data for isobutanol and for water and barometric corrections were taken from the Chemical Engineers' Handbook (6). The temperature in the equilibrium chamber was measured with a copper-constantan thermocouple and semiprecision type potentiometer.
TABLEI. EXPERIMENTAL VAPOR-LIQUID EQUILIBRIUM DATA FOR ETHYLENE DICHLORIDE-TOLUENE Pressure
Mm. H i 772.7 772.7 763.3 763.8 763.3
Temp., t
O
C.
88 2 98.3 88.2 105.7 104.4
Mole % ' Ethylene Dichloride Liquid ZI Vapor VI 71 .O 85.3 31.4 50.3 67.8 83.8 11.7 21.8 12.3 24.0
RESULTS
T A B L11. ~ EXPERIMENTAL VAPOR-LIQUIDEQUILIBRIUM DATA FOR IBOBUTANOL~ATER No. 2a
Pressure Temp., Mm. Hg OC. 756.2
C
d e
3b
93.53 90.00 96.05 98.12
Mole % Isobutanol Activity Coefficient Liquid 51 Vapor YL yl YP
0.65 1.52 0.27 0.11
757.0
b
2.08 1.00 1.92 1.93
31.6 22.6 31 0 30.4
33.1 43.8 33.8 32.4
763.8
97.6 90.3 90.5
0.20 0.91 0.90
6.25 22.45 21.8
46.6 49.2 48.0
1.020 1.120 1.112
762.0
98.95 96.80 96.73
93.8 90.6 91.1
68.9 61.0 59.8
1.037 1.032 1.010
5.17 4.65 5.06
760.0
92.3 90.75 89.25 90.55 89.25
75.9 69.3 57.5 69.3 37.1
44.8 40.3 34.7 40.3 32.0
1.080 1.135 1.247 1.143 1.781
8.04 2.74 2.28 2.75 1.603
761.6
89.61
.....
......
.....
C
7b d 8
8e
d
i
I
9
5.26 1.22 6.75 88.0 90.9 89.0 89.4
5 ba b
1.031 1.040 1.060 1.029
761.4
e
4a
18.62 29.0 9.00 8.27
1.013 1.005 1.022 1.090 1.088 1,055 1.051
C
1%:;
32.5
c
30
8'
B g.0 W
i * I
10
To determine the dependability of the still, several preliminary runs were made on the system ethylene dichloride-toluene, since this mixture had been found to obey Raoult's law closely at atmospheric pressure ( 4 ) . The results are given in Table I and are plotted in Figure 2. Since the data show fairly good agreement with Raoult's law, it was concluded that the apparatus involved no persistent or unaccountable errors and that the precision of the data obtained was primarily dependent upon the quality of control exercised. The experimental data on the isobutanol-water system are given in Table I1 and plotted in Figures 3, 4, and 5. Figure 3
R~~
1253
0
10 .
x ~i~~~~4.
-
Z.o
3.0
MOLE % ISOBUTANOL
Data on Isobutanol-Water in ~~~i~~ of ~a~~~ Isobutanol 0 Experimental; X Stockhardt and Hull.
A more definitive test of the equilibrium data, as previously pointed out ( I ) , is t o calculate from them the activity coefficients, y1 and 7 9 , of the two components, and to plot them as shown on Figure 5 where
Here again the data are well correlated by smooth curves, particularly in the concentrated isobutanol region. At low concentrations of isobutanol, the activity coefficients of the concentrated component, water, would be expected to be practically unity, whereas they fall slightly high. This discrepancy is believed to be due to slight inaccuracies in the temperature readings which are apparently somewhat low. Correction of these temperatures were made by Shilling (6) to values which result in activity coefficients of unity for the water in this region; this lowered slightly the activity coefficients of isobutanol in this region, but did not change the liquid and vapor compositions. The observed temperatures in the concentrated isobutanol region are undoubtedly correct, since the activity coefficient curve of the concentrated component approaches unity in the expected asymptotic manner.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
0‘
20
40
X -MOLE
60 80 % ISOBUTANOL
100
Figure 5. Activity Coefficients Calculated from Experimental Isobutanol-Water Data
An attempt was made to “fit” the activity coefficients with curves representing the van Laar equations:
Vol. 35, No. 12
problem of securing reliable equilibrium data on partially miscible systems. It is obvious, however, that application of the apparatus need not be limited necessarily to such binary partially miscible systems. TERNARY AND MULTICOMPONENT SYSTEMS.Because of its inherent simplicity, this type of apparatus seems particularly well adapted to the study of systems of more than two components, especially those known to possess a heteroazeotrope. Modification can be made simply by the use of an additional boiler for each component with but little increase in complexity of operation or control. The relative ease and rapidity with which a vapor which condenses to a two-phase liquid can frequently be analyzed by measurement of the volumes of the respective layers is an advantage not possessed by the usual recirculation type of still. MISCIBLEMIXTERES. As indicated by the equilibrium data presented for ethylene dichloride-toluene, completely miscible systems of two or more components can be reliably evaluated with the still described, provided sufficient quantities of the pure components are available. Khile the present method utilizes somewhat greater amounts of material than do previous ones, this is not a serious disadvantage in most plant laboratories where the mixtures obtained may find use in subsequent distillation operations. MODIFIEDEQUILIBRIUM CHAMBER. The seeming complexity of the equilibrium chamber can be reduced by eliminating the outer jackets which serve to maintain thermal equilibrium, and replacing them by a coil of suitable resistance wire surrounding the inner vessel as employed in the still for miscible Rystems ( 4 ) . The proper balancing of heat loss from the vessel would be achieved by controlling the height of the liquid, which should not change for some time as equilibrium is approached. The means of introducing vapor into the equilibrium liquid where a short glass tube permits smooth liquid circulation, results in a steady liquid level so that a slight increase or decrease in liquid height is readily apparent. Local superheating of the vapors evolved through tube G can similarly be affect*edby electric winding in order to ensure the elimination of reflux a t this point where insulation may not prove adequate. I t is hoped that the apparatus and method described will stimulate the acquisition of much needed vapor-liquid equilibrium data for those systems without which the rigorous treatment of distillation column design and performance is impossible or of doubtful vdue. NOMENCLATURE
log
YZ
B
=I
(1
+ gy
(3)
The nearest fit was obtained by trial and error with equations having the following constants: A = 1.7 and B = 0.7. These curves were in close agreement for the water layer, but deviated from the data by 0 to 15 per cent for the isobutanol layer. The method discussed by Carlson and Colburn ( 1 ) for predicting van Laar constants from solubility data gives (for the boiling-point solubility values of 2.13 and 40.25 mole per cent isobutanol, respectively, at 90” C.) the follow-ing: A = 1.66 and B = 0.4. These values obviously do not represent the experimental data and cast doubt on this method of prediction as well as o n the validity of the van Laar equations for systems where A and B differ so widely. Additional data are required to find whether this limitation on the use of the van Laar equations applies to other partially miscible systems where the mutual molal solubilities are so unsymmetrical in the two phases. DISCUSSION OF RESULTS
The consistency of the experimental data as subjected to rigorous interpretation supports this new method of approach to the
van Laar constants in Equations 1 and 2 A, B P total pressure, mm. Hg P1,Ps vapor pressures of components 1 and 2, respectively, mm. Hg t = temperature, O C. = mole % of components 1 and 2, respectively, in liquid 21, z 2 yl, yz = mole yo of components 1 and 2, respectively, in vapor yl, yB = activity coefficients of components 1 and 2, respectively = = =
LITERATURE CITED
(1) Carlson, H. C.,and Colburn, A. P., IND.EWQ.CHEW,34, 581-9
(1942). Colburn, A. P., and Welsh, D. G., Trans. Am.Inst. Chem. Engrs. 38, 179-202 (1942). (3) International Critical Tables, Vol. 111, p. 385, New York, MaGraw-Hill Book Co., 1927. (4) Jones, C. A., Schoenborn, E. M., and Colburn, A. P., IND.ENG. CHEW,35,666-72 (1943). (5) Perry, J. H., Chemical Engineers’ Handbook, 2nd ed., New York, McGraw-Hill Book Co., 1941. (6) Shilling, David, thesis, Univ. of Del., 1942. (7) Stockhardt, J. S., and Hull, C. M., IWD.ENG.CHEX.,23, 1438-40 (2)
(1931).
PRESENTED before the Division of Industrial and
Engineering Chemistry at the 106th Meeting of the AMERICANCHEMICAL SOCIETY, Pittsburgh, Penna.
