EQUILIBRIUM STILL FOR MISCIBLE LI Data on Ethylene Dichloride-Toluene and Ethanol-Water C. A. JONES1, E. M. SCHOENBORN, AND A. P. COLEURN University of Delaware, Newark, Del.
A new apparatus for determining vaporliquid equilibria of miscible mixtures is composed of a residue chamber, a condensate chamber, and a flash boiler to vaporize the stream returning from the latter to the former. This arrangement ensures that a uniform concentration exists in the residue chamber at equilibrium. Vapor-liquid equilibrium data at 760 mm. pressure are presented for ethylene dichloride-toluene and ethanol-water. Isothermal data at 50" and 60' C. are given for the latter system. Consistency of results with this apparatus is shown by the close agreement
T
HE design of distillation and other contacting equipment requires reliable vapor-liquid equilibrium data. Although relatively few ideal solutions are known whose equilibrium relations can be calculated from vapor pressure-temperature data of the pure components, by far the larger number of systems of industrial importance are nonideal; and attempts to predict the equilibrium compositions of such mixtures from theoretical considerations alone have not proved successful. It has been the practice to determine such data experimentally under various conditions, and numerous kinds of apparatus have been devised. While existing equipment has been used with some degree of success, certain sources of error appear to be inherent in each type, and there is real need for an apparatus which is not only free from many of the usual sources of error, but is also simple in design, construction, and operation. The purpose of this paper is t o describe a new type of vapor-liquid equilibrium still which offers improvements in these directions and to present results obtained with it on two representative systems, ethylene dichloride-toluene and ethanol-water, under various conditions. The new still is a modification of the convenient recirculation type. A common type of equilibrium still, described by Sameshima (9),Othmer (7), Carey and Lewis ( S ) , and others, involves a still, condenser, condensate trap, and overflow line from the latter back to the still. Various means are employed to prevent refluxing of vapors. Two possible sources of error in this apparatus are: (a) The vapors leaving the liquid surface may not be in equilibrium with the main body of liquid in the still, inasmuch as they may be formed from a small portion of the liquid whose composition has been changed by the process of simple distillation. (b) The liquid being returned from the condensate trap is different in composition from that in the still and, unless mixing is instantaneous, may 1
Now in the Air Corps, United States Army.
666
of the ethylene dichloride-toluene data with Raoult's law. The data on ethanolwater at one atmosphere pressure are in good agreement with those of Carey and Lewis (3). Evaluation of the isothermal data on ethanol-water in terms of the van Laar equations indicates that these equations do not fit this system exactly, although they are a reasonable approximation. The activity coefficients and the v a n Laar constants, furthermore, show a negligible change with temperature over the range 50' t o 60' C. i n agreement with predictions from available heat of solution data. flash owing t o its lower boiling point. I n any case this liquid return causes a more or less heterogeneous mixture in the still and samples withdrawn may not be exactly representative. T o eliminate these two sources of error, a development was made some years ago by one of the authors as described by Chilton ( 5 ) . The idea was t o arrange so that the liquid in the residue chamber would have a uniform composition and the vapor leaving this liquid would not be generated from the residue liquid. These results were achieved by bubbling the vapors off the still through a trap which is almost surrounded by the same vapors to minimize heat loss. At steady etate the vapors would undergo no change in composition as they bubbled through the trap; final equilibrium and liquid uniformity in the trap were thus ensured. This type of still was further developed by Scatchard, Raymond, and Gilmann (10) who obtained extremely precise data on a number of systems. I n spite of its improvement in principle, this still offered certain difficulties in operation; unless the slight heat loss from the trap exactly balanced the heat gain from the surrounding vapor which is a t a slightly greater enthalpy because of the pressure drop required for it to bubble through the trap, the quantity of liquid in the trap would increase or decrease slowly. Any such quantity change would prevent the exact attainment of equilibrium. While Scatchard et al. (10) were able to eliminate this objectionable feature by allowing the liquid level in the inner trap to adjust itself until the heat loss and gain exactly balanced, it seemed desirable to make revision which would automatically prevent a change in liquid content of the trap. EQUILIBRIUM STILL
The new vapor-liquid equilibrium still is illustrated in Figures 1 and 2. It differs essentially from those just men-
June, 1943
INDUSTRIAL AND ENGINEERING
The vapor line leading from ihe residue chamber to the condenser is wound with a separate coil, kept a t a higher temperature than that of the vapor to prevent refluxing.
CHEMISTRY
667
The apparatus is built entirely of Pyrex to permit easy observation of the liquid levels during operation. Heat input to the three heating coils is simply and accurately controlled by ordinary slide-wire resistances or convenient transformers INDEX-COMPOSITION DATAFOR TABLE I. REFRACTIVE connected in series with each. For the system studied here, ETHYLENE DICHLORIDE-TOLUENE approximately 20, 15, and 10 watts were required by the Mole Fraction Refractive Refractive Mole Fraction flash boiler, residue chamber, and vapor line, respectively. Ethylene Index a t Ethylene Index a t Dichloride 250 c. Dichloride 25' C. Liquid from the distillate U-tube receiver flows under its own 0.5600 1.4670 0 1.4939 hydrostatic head through the three-way stopcock and capil0.6699 1.4611 0.1221 1.4881 0.7900 1.4547 0.2144 1.4841 lary tube into the flash boiler. The capillary tube serves the 0.9250 1.4469 0.3230 1.4790 essential function of smoothing the flow of liquid into the 1 .0000 1.4423 0.4663 1.4719 boiler and preventing undue fluctuations in vaporization rate during minor adjustments of the heat input. A Nichrome TABLE11. EXPERIMENTAL VAPOR-LIQUID EQUILIBRIUM DATA wire wound in the form of a spiral against the inside walls of FOR ETHYLENE DICHLORIDE-TOLUENE AT 760 MM. PRESSURE the boiling tube serves to distribute the liquid as a thin film T ~ Mole ~ Fraction ~ , Ethylene Dichloride Deviation Factors throughout the length of the tube and tends to preyept an t , OC. Liquid n Vapor VI YI Yl excess of liquid from being carried over into the residue re87.1 0.812 0.909 1.102 0.996 87.7 0.784 0.893 1.004 1.008 ceiver. During normal operation only a drop of liquid can 90.2 0.700 0.844 0.996 0.999 92.2 0.585 0.765 1.012 0.994 be seen a t the lower end of the flash tube; the presence of 1.000 92.6 0.568 0.750 0.986 liquid at this point thereby indicates that the vapor entering 95.9 0.479 0.674 I. 006 0,988 1.005 96.0 0.446 0.645 0.994 the receiver is not superheated appreciably. 96.8 0.415 1.008 0.614 0.993 Vapor enters the residue receiver through a smaller tube 97.8 0.375 1.006 0.578 0.973 99.3 0.365 0.989 0.565 0.960 which dips almost to the bottom of the chamber. The vapor 0.252 100.8 0.424 1.008 1.002 102.0 1.014 0.235 0.401 1.018 is, furthermore, directed upward and against the thermo107.0 0..... . 990 1.009 0.100 0.197 couple well. This ensures good agitation of the residue liquid 108.4 0.045 1.029 0.095 1.000 and provides for the accurate measurement of the equilibrium
668
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 35, No. 6
couple was connected to a semiprecision type Leeds-Northrup potentiometer reading to 0.001 millivolt so that the temperature readinrs were accurate t o approximately0.1' C. Dur&g isothermal runs it was necessary to control the pressure accurately to ensure constancy of boiling point of the mixture for every concentration of the charge in the still. An automatic vacuum regulator of the type designed by D. M. Smith and manufactured by Eck and Krebs (Catalog No. 4400) was connected to the apparatus at K. With this regulator it was possible to control the pressure so that the variation in the equilibrium boiling point was not greater than 0.5' C. between the various samples employed in the isothermal series. The vapor-liquid equilibrium data for the two binary systems investigated were evaluated in the light of the thermodynamic treatment recently described by Carlson and Colburn (4). Since a plot of the logarithm of activity coefficient against composition of each component of a binary system is remarkably sensitive to errors in experimental measurement, activity coefficients were calculated for all determinations. ETHYLENE DICHLORIDE-TOLUENE
Figure 1. Vapor-Liquid Equilibrium Still
temperature by means of a thermocouple. The equilibrium vapor flowing to the condenser is slightly superheated by the resistance windings on the vapor outlet tube to prevent the formation of reflux, and after condensing, returns to the distillate receiver to be recycled. Stopcocks B and C permit the removal of the distillate and residue samples for analysis. The vapor space of the residue receiver has no direct connection t o the outside except through the condenser to tube K . This tube acts as a vent during runs under atmospheric pressure or is connected to a suitable barostat during isothermal runs, It is also connected to the flash boiler through the three-way stopcock A in order that the pressure may be equalized prior to the taking of samples, Without this stopcock samples could not be withdrawn from either receiver without tending to siphon back into the flash boiler the liquid in the other reservoir. While the dimensions of the still can vary within rather wide limits, it appears best that the distillate and residue receivers be not much larger than is necessary for the production of the requisite volumes of sample desired. I n the present investigation (6) two such stills were employed. The smaller had receivers of about 5 ml. capacity, and the liquid recirculated rapidly so that equilibrium was reached in about 15 minutes. The larger still had receivers of about 15 ml. capacity, and about 40 minutes of operation mere necessary to insure reaching equilibrium. The smaller still was used on the experimental PROCEDURE.
work on ethylene dichloride-toluene where the samples were
analyzed b refractive index measurements, since in this case small sampfes were adequate for analysis. The larger still was used in the work on etlianol-water where analyses were made by density measurements and samples of about 10 ml. were required. Equilibrium temperatures were measured by a cop er con stantan thermocouple inserted into the thermocouple wei. -The cold junction was placed in 5 bath of meltmg ice. The thermo-
To determine the dependability of the still, a mixture was desired which would permit an absolute evaluation of results rather than a comparison with other experimental data which might be in error. To satisfy these requirements a mixture was selected which was believed to obey Raoult's law. Although benzene-toluene is considered the classical example of an ideal mixture, the refractive index and density ranges are small, and precise analysis is difficult. The system benzene-ethylene dichloride can be easily and precisely analyzed by refractive index or by density; but the boiling point range is small, and although the vapor-liquid equilibrium data in the literature indicate that Raoult's law is undoubtably followed, they are not too consistent. The system ethylene dichloride-toluene has the convenient boiling range of benzene-toluene and the precision in analysis of ethylene dichloride-benzene. It was believed that this system also would follow Raoult's law. The fact that no previous experimental data on vapor-liquid equilibrium were available against which to compare the present results, was not considered a serious disadvantage; any experimental faults would certainly cause Raoult-law deviations which would be different from the regular and consistent type of deviation characteristic of nonideal systems, as discussed by Carlson and Colburn (4). The liquids used were constant-boiling fractions taken from large volumes of these materials fractionally distilled in the laboratory. The toluene had a boiling point of 110.6' C. a t 760 mm. mercury and a refractive index of 1.4939 n':,". The ethylene dichloride had a boiling point of 83.65' C. and a refractive index of 1.4423 na,". Analyses of samples taken from the still were made with an Abbe refractometer kept a t a constant temperature of 25' C. Refractive index-composition data (Table I ) were obtained from experimental determinations made on samples of the same material used in the equilibrium studies.
V A P O R - L I QEQUILIBRIVM ~~D D.4TA TABLE111. EXPERIMENTAL FOR ETHANOL-WATER AT 760 MM,PRESSURE T t,O
~
c.
100 95.50 90.6 85.4 83.70 82.75 82.00
81.0 80.5 79.8 78.9 78.26 78.32
~ Mole ~ Fraction , Liquid XI 0 0.018 0,054 0.124 0.176 0.230 0.288 0.385 0.440 0.514 0.673 0.840 1.0
Ethanol Vapor ui 0 0.179 0.3375 0.470 0.514 0.542 0.570 0.612 0.633 0.657 0.735 0.850 1.0
Deviation Factors Yl
... 6.22 3.939 2.870 2.369 1.969 1.702 1.411 1.308 1.210
1.052. 1.017
...
YZ
:
1 000
0.990 1.042 1.084 1,148 1.194 1,298 1.378 1.530 1.820 2.160
lune, 1943
INDUSTRIAL A N D E N G I N E E R T N G C H E M I S T R Y
Experimentally determined boiling points, vapor and liquid compositions, and calculated activity coefficients are presented in Table 11. The boiling points were corrected slightly to a total pressure of 760 mm. Vapor pressure data used in computing the deviation factors were taken from Perry (8) and International Critical Tables, and plotted on an enlarged scale over the temperature range in question. The deviation factors appear to vary slightly among themselves; but with one or two exceptions, possibly where equilibrium had not been fully attained, all values of y are close to 1.000. Furthermore, no general trend, either positive or negative, can be observed as in the usual case of nonideal solutions. Therefore, the system ethylene dichloride toluene appears to be ideal and to obey Raoult's law closely. Since the deviation factors are so close to unity, they were
Figure 2.
669
ETHANOLWATER
The performance of the apparatus with ethanol-water was also tested since the experimental equilibrium data for this system are well established. The ethanol available was sufficiently pure; it had a boiling point of 78.3" C. at 760 mm. and a density of 0.7893 at 20' C. The ethanol-water samples were analyzed by density measurements in a 10-ml. specific gravity bottle with ground-in thermometer and stoppered capillary tube. The bottle was carefully filled at exactly 20" C. and rapidly weighed. Distilled water from the Iaboratory supply was used in making up all samples. At low ethanol concentrations some difficulty was encountered in the operation of the still; the high surface tension of the mixture appeared to cause an uneven flow of distillate into the flash boiler. This difficultywas remedied, however,
Diagram of Apparatus
not plotted. Vapor-liquid compositions, however, were computed from vapor pressure data by Raoult's lam and are plotted, together with the experimentally determined values, in Figure 3. The good agreement on this basis is evident. Therefore, the system ethylene dichloride-toluene follows Raoult's law as was expected, and the new equilibrium still yields reliable results.
by inserting the capillary tube at the entrance to, and the Nichrome spiral in, the flash-boiling tube to smooth out the flow. Data obtained a t 760 mm., together with the calculated deviation factors, are presented in Table 111. Consistent positive deviations from Raoult's law are apparent. The y-curves for this system are plotted in Figure 4 and vapor-
670
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE
c.
t,
1 ' .
HEATSO F
Vol. 35, No. 6
DILUTIONFOR
S O L U T I O N AT INFINITE
ETHANOL-WATER
Lz
Ll 1/T
L1
Lz
2
i
2.3R
Calculated from Data of Bosjnakovio and Grumbt (#) 0.00366 0.00354 0.003415 0.00330 0.00320 0.003099 0,003001 0,00292 0.002835 0.002755 0.00268
0
10 20 30 40 50 60
70 80 90 100 0 17.33 42.06
3595 3200 3020 2420 2080 1700 1250 870 540 282 -49
45s 361 285 260 105 50 185 -230 -285 309 -273
-
-
787 700 661 530 455 372 273 190.7 118.2 61.7 -10.7
Calculated from Data of Bose (1) 0.00366 3780 401 816 0.00844 2890 189 629 0,00318 2080 10 453
100.1 79.0 62.4 56.9 23.0 11.0 40.5 -50.3 -62.4 -67.6 -59.6 87.1 41.1 2.2
MOLE FRACTION N V L E N E DICHLORIDE IN LIQUID
Figure 3.
x-y Diagram for Ethylene Dichloride-Toluene
at 760 Mm. Pressure
Solid line, calculated from Raoult's law; circles, experimental data.
liquid compositions in Figure 5 . Both sets were calculated in the same manner from identical vapor pressure data (8). The data of this investigation are thus in good agreement with those of Carey and Lewis, and both sets of results are undoubtedly reliable. Carey and Lewis (3) utilized the recirculation type of equilibrium still, with special arrangements to prevent refluxing and to obtain correct samples, as well as a careful technique. The agreement of the new data with those mentioned is an indication that, with the precautions taken, reliable results can be obtained with their type of spparatus.
MOLE FRACTION ETHANOL IN LIQUID
Figure 4. Raoult's Law Deviation Factors for Ethanol-Water a t 760 Mm. Pressure Solid line, data of Carey a n d Lewis (3); circles, experimental data.
ISOTHERMAL STUDIES
The isothermal work required that the pressure on the still be adjusted until the equilibrium boiling point was essentially the same for each sample. Sufficiently low pressures were obtained in this work by using an aspirator pump connected to the water tap in the laboratory. The pressure regulator
TABLEIV. ISOTHERMAL VAPOR-LIQUID EQUILIBRICM DATA FOR ETBANOL-WATER Temp. t, C. 50.50 50.52 50.25 50.75 50.51 60.5 50.2 50.2 60.50
Pressure Mm. 133 157 164 177 200 196 207 220 225
60.60 60.65 60.70 60.65 60.70 60.65 60.50 60.65 60.65 60.80 60.65
219 249 298 325 342 344 343 363 364 366 362
Mole Fraction Ethanol Liquid ZI Vapor UI 0,046 0.290 0,093 0.424 0.1225 0.482 0.168 0,507 0.333 0.690 0,3425 0.586 0.513 0,649 0.824 0.846 0.910 0.90s 0.051 0.086 0.197 0.375 0.509 0.527 0.545 0.808 0.851 0.860 0.972
0.316 0,393 0.517 0.596 0.648 0.660 0.671 0.826 0.802 0.867 0.972
Deviation Factors yl
?a
3.64 3.15 2.85 2.46 1.542 1.459 1.162 1.001 0.980
1.041 1.050 1.030 1.070 1.295 1.300 1.690 2.06 2.320
3.845 3,135 2.155 1.421 1 198 1 181 1.171 1.022 1.015 1.010 0.998
1.028 1.079 1.161 1.366 1.591 1.608 1.621 2.135 2.19 2.26 2.35
I I
was isolated from the system by a cold chamber packed with a mixture of ice and salt. The trap effectively condensed any vapor which escaped from the still and gave an additional means of checking on the still losses during a run. It also prevented vapor from coming into contact with the pressure control device. At the beginning of a run the pressure was adjusted slowly to nearly the correct value, and the still was allowed to come to steady state as noted by constancy of the temperature readings. The pressure was then slowly changed until the boiling point of the mixture had reached the predetermined value, after which the still was again allowed to reach equilibrium. I n all runs, both isothermal and nonisothermal, the still was allowed to operate a n additional 20 to 30 minutes to ensure compIete attainment of equilibrium. Samples were carefully withdrawn from each receiver into ice-cooled stoppered bottlea, after the still had been vented to atmospheric pressure and stopcock A had been opened to equalize the pressure across the receiver. Isothermal data on ethanol-water were taken to permit a definite evaluation in terms of the van Laar equations; in the past it had been difficult to fit exactly the constant pressure results by these equations, and there was some doubt as to whether a variation of the constants with temperature was the reason. The isothermal results are given in Table IV,
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1943
0.4 0.5 0.6 0.7 0.8 MOLE FRACTION ETHANOL IN LIQUID
0.3
Figure 5.
x-y
0.9
1.0
Diagram for Ethanol-Water at 760 Mm.
0
Figure 6.
Solid fine, data of Carey and Lewis (3); circles, experimental data.
and are plotted as y-x curves in Figure 6 and as activity coefficients in Figure 7 . Very little difference was observed in the results over the temperature range studied-i. e., a t 50" and 60" C. Values of van Laar constants A and B were calculated from smoothed values of y at z values of 0.2, 0.4,0.6, and 0.8, and averaged; new curves were drawn, based on these averages. It is apparent that the agreement with the data is not satisfactory near the ends of the curves. We are forced to conclude that the van Laar equations do not fit this system exactly, although they are a reasonable approximation. It was hoped that the difference between the activity coefficient curves could be related to the theoretical effect of t e m p e r a ture. The temperature effect is a function of heats of solution. From values of molar heats of solution, L , a t infinite dilution we can predict the change in values of the van Law constants as follows (4): dA d(l/T)
or AI
=
0.1
0.2
67 1
0.3 0.4 0.5 0.6 0.7 MOLE FRACTION ETHANOL I N LIQUID
0.8
by Bose (1) a t O", 17.33", and 42.05" C. and by Bosjnakovic and Grumbt (2) at 0" t o 100" C. From these data heats of solution a t infinite dilution were calculated for each component (Table V) ; they are plotted as L/2.3 R against 1/T in Figure 8. The areas under the curves between the temperature limits of the experiments give the predicted values for change in the van Laar constants, with the results for this case of
- Am" = 0.034 Booo - Bm" = -0.004
-480'
where A applies to ethanol component and B to water, and the subscripts indicate temperature. I n this case the pre-
- A? =
50°
C.; A
Figure 7.
=
0.897, B
-
1.0
Isothermal x - y Diagram for Ethanol-Water A 5 0 ° C.; 0 60°C.
L 2TR
D a t a on h e a t s of s o l u t i o n of ethanol and water are given
0.8
MOLE FRACTION ETHANOL IN LIQUID
0.346
6OOC.; A
-
0.936, B = 0.380
Raoult-Law Isothermal Deviation Factors for Ethanol-Water
Solid line, experimental data; dashed line, calculated from average van Laar canstants.
672
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 35. No. 6
encourage further developments in this direction ACKZOWLEDGMENT
The authors gratefully acknowledge the valuable suggestions of T. H. Chilton, R. G. Nester, and D. M. Smith in the development of the final apparatus. NOMENCLATURE
A = arbitrary constant in van Laar equations equal to log y1 at 2, = 0 B = arbitrary constant in van Laar equations equal to log y2 at x2 = 0 L = molar heat of solution at infinite dilu-
tion, cal./gram mole R = gas constant, 1.987 calJ(mo1e) (" C.) T = absolute temperature, K. z = mole fraction in liquid y = mole fraction in vapor y = activity coefficient Subscripts 1. 2 = low and high boiling components, respectively LITERATURE CITED
E., International Critical Tables, 1-01. V, p. 159 (1929). Bosjnakovic, F., and G r u m b t , J. A,, (2) Forsch. Gebiete Ingenieurw., 2, 421-8 (1931). (3) Carey, J. S., and Lewis, 7.7.'. K., IXD.ENQ. CHEX., 24, 882-3 (1932). (4) Carlson, H. C., and Colburn, A. P., Ibid., 34,581-9 (1942). (5) Chilton, T. H., 4th Symposium on Chem. Eng. Education, Wilmington, Del., pp. 64-71 (1935). A., ihesis, Cniv. of Del., 1941. (0) Jones, C . ~ ENQ.CHEM.,20,743-6 (7) Othmer, D. F., IND. (1928). (8) Perry, J. H., Chemical Engineers Handbook, 2nd ed., New York, McGraw-Hill Book Co., 1941. Sameshima, J., J. Am. Chem. Soc., 40, 1483-90 (1918). Scatchard, G., Raymond, C. L., and Gilmann, H. H., Ibid., 60, 1275-8 (1938). (1) Bose,
Courtesu, The Lurnnus Company
Underside of a Bubble-Cap Plate for a Large Distillation Column, Showing Central Downpipe
dicted changes are too small to be significant. The data are in agreement with this prediction, in so far as the data also show a negligible change with temperature. From the heat of solution plots the activity coefficients of ethanol would be expected to increase slightly with increasing temperature at a decreasing rate up to about 100' C. The activity coefficients of water would be expected to increase extremely slightly with increasing temperature up to 40" C. and thereafter to decrease extremely slightly. The region where we could expect an appreciable effect of temperature for this system would be at low temperatures for ethanol; for example, AA = 0.007 per " C. a t 20" C., which means that the dilute-end value of the y-x curve for ethanol will increase 1.6 per cent for 1" C. increase in temperature in this range. CONCLUSION
The experimental data obtained with the new still show a consistency which indicates that reliable results can be obtained with it. On the other hand, the constant pressure results on ethanol-water are in exact agreement with data of Carey and Lewis; therefore the possible sources of error in their type of apparatus may not be important when careful precautions are made. In any case, the evaluation of equilibrium data in terms of theoretical relations requires a higher degree of accuracy in the data than is often obtained in casual types of apparatus, and it is hoped that this study may
0.0036
0.0034
0.0032
0,0030
0,0028
fi Figure 8. Heats of Solution at Infinite Dilution for Ethanol-Vater H From Bose (1); 0 from Bosjnalrovio and Grumbt
(2).
