INDUSTRIAL AND ENGINEERING CHEMISTRY
,454
As the cost of conducting a commercial extraction is influenced by the occuirence of a solutrope, a change in temperature may be used to advantage to shift or eliminate solutropes. Whether it is best to shift the solutrope up or down depends on the coniposition of t h e feed and solvent streams and on the desired compositions for the exit streams. Sufficient data are now available for the present ternary system to permit the necessary calculations. ACKNOW LEDGAIENT
Vol. 47, No. 3
LITERATURE CITED (1) (2)
Bachman, I., IND. ENG.CHEnr.. ANAL.ED.,12, 38-9 (1940). Chattaway, F. D., and Backeberg, 0. G., J . Chem. SOC.,123,29993003 (1923).
"International Critical Tables," Vol. 3, p. 399, McGraw-Hill Book Co., Kew York, 1928. (4) Smith, J. C., Stibolt, V. D., and Day, R. W., ISD. ENG.C ~ M , , 43,190-4 (1951). ( 5 ) Teeter, H. M., and Bell, E. W.,Otg. Syntheses, 32, 20-2 (1952). (6) Treybal, 12. E., Weber, L. D., and Daley, J. F., IND.ENG.CHmr., (3)
38,817-21 (1946).
Valuable assistance and advice were given by Ervin Colton. Most of the experimental work n-as conducted by L. J. Lang., A. E. Lieb, M. J. McDaniel, and H. C. Walther.
(7) Vriens, G. N., and Medcalf, E. C., I b i d . , 45, 1098-104 (1953). (8) Westwater, J . W., and Audrieth, L. F., I b i d . , 46, 1281-4 (1954). RECEIYED for revieix. J u l y 28, 1 9 3
A C C S P T E D OC'TOBER
16, 1954
Vapor-Liquid Equilibria for System Methylcyclohexane-Toluene at Subatmospheric Pressures JAMES H. WEBER Department of Chemistry and Chemical Engineering, University of Nebraska, Lincoln 8, Neb.
APOR-LIQUID equilibrium data are of great importance to those engaged in the chemical industries, for many of their operations require fractionation by distillation. The system methylcyclohexane-toluene is of interest because it is often used t o calibrate distillation columns, and because the compounds are different types of hydrocarbons, one a saturated cycloparaffin and the other an aromatic, it is of interest t o determine the the deviation from ideality of a system containing these two compounds and the variation of the deviation from ideality of a system with changes of pressure. The vapor-liquid equilibrium data for the binary system methylcyclohexanc-toluene have been determined by Quiggle and Fenske (6) at a pressure of 1 atmosphere. It was the purpose of this study t o obtain similar information a t reduced pressures, and show, from these measurements, how this system deviates from ideal behavior.
The vacuum on the still ~ s a smaintained by a vacuum pump, measured by a differential manometer, and controlled by a manostat. The temperature was measured by a copper-constantan thermocouple used in conjunction with a Leeds anti Northrup Type K potentiometer. A thermocouple well, mad? of thin-walled glass tubing, extended from the top of the still to a point below the liquid level, and the thermocouple was located in the well in such a manner that its tip was just above the level of the quiescent liquid. When the liquid was boiling, the glass walls adjacent to the tip were alternately wet and dry. The temperatures are probably accurate within i 0.1" C., the pressures within i 2 mm. of mercury. Samples of the liquid and condensed vapor were removed through the stopcocks provided for the purpose by applying a vacuum slightly greater than that imposed on the still. The samples Tvere drawn into containers, which, in turn, were immersed in ice baths. Previous to the withdrawal of the liquid sample, a small amount of material was drained off, because it mas felt that good mixing did not' occur immediately above the stopcock.
PURITY O F CORIPOUNDS
The minimum time for a run was 1 hour, although i t is believed that equilibrium was attained in a shorter time. Equilibrium was said to have been reached when the temperature was constant for 15 minutes. On the early runs, two liquid and vapor samples were taken at, intervals of 10 minutes. Upon analysis, the sample showed a constancy of composition. Composit,ions were determined by measuring the refractive indices with an A4bberefractometer; the relationship between refractive index and composition was determined with the chemicals used in the experimental work. This instrument is precise to 0,0001 unit. For the system methylcyclohexane-toluene, a change of 0.0001 in the refractive index is equivalent to about 0.2% alteration in composition. This is beyond the accuracy of the experimental results. The compositions are believed t o bc accurate within 1 0 . 4 mole %.
The chemicals used in t h e experimental work were pure grade materials obtained from the Phillips Petroleum Co. , and had ' purity. Table I reports t h e physical a minimum of 99 mole % constants obtained on t h e chemicals used in the experimental work and, for comparison, the data on the pure compounds.
Table I. Properties of Pure Compounds Methycyclohexane Exgtl. Lit. Densizy, 25O
Refractive iEdex, 25' T , C . , vapor vressure, mm. H g 760 400 200
Toluene Exptl.
Lit.
0,7659
0.70506 (3)
0,8632
0.86231 ( 3 )
1.4208
1 ,42055 ( 3 )
1.4931
1.49413 ( 3 )
100.6 79.7 59.7
100.6 (6) 79.65 59.61
110 8 89.5 69.35
110.62(6) 89.48 69.50
VAPOR-LIQUID EQL'ILIBRIUlM DATA EXPERIMENTAL 31ETHOD
The vapor-liquid equilibrium data were determined in an Othmer still (4). The unit was equipped with a Friedrichs condenser, and an ethylene glycol solution a t 32" F. was used as coolant. These precautions ensured t h a t no vapors escaped from t h e condenser, but all were condensed and returned to the still.
Because there are many chances of experimental errors in the determination of vapor-liquid equilibrium data, great care should be taken in the measurements, and the resulting information should be subjected to consistency checks. I t is not sufficient to plot y, the mole fraction of the more volatile component in the vapor phase, against 2. the mole fraction of the more volatile
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1955
At low pressure, as is the case in the present work, f/P+ 1, a n d because the effect of pressure on the standard state liquid fugacity is negligible, i t can be assumed:
100
u
90
80 CL
8
5i
455
Po = f;
(9)
f: = P
(10)
Also a t low pressure:
70
I W
2
60
X
w
$ $
The effect of temperature on the standard state vapor fugacity is negligible. Substituting Equations 8, 9, and 10 in Equation 7 , the liquid phase activity coefficient of a component may be calculated from the expression:
50
40
I
L5.
30
+ z y
20
w
d =
y = -
YP XPO
The activity coefficients presented in this paper were determined b y the use of Equation 11. The vapor pressure d a t a used throughout t h e calculations are those presented in API 44 (6).
IO
o 0
IO
20
30
40
50
60
70
80
90
100
MOLE PER CENT METHYLCYCLOHEXANE LIQUID PHASE
Figure 1. z
-
y diagram for methylcyclohexanetoluene system
component in the liquid phase, and draw the apparent best curve through the numerous points. One is not assured t h a t the final results will be thermodynamically consistent. The experimental data were fitted to the Gibbs-Duhem equation, using its familiar form:
e By this means the consistency desired in the results was obtained. While the Gibbs-Duhem equation applies rigorously only at constant temperature and pressure, Beattp and Calingaert (1) have shown t h a t minor variations in temperature are unimportant. The liquid phase activity coefficient, 7,of a component in a mixture is defined as:
where U L is the liquid phase activity of the component and equal to ~L/?L. Hence Y =
6 T;
The vapor phase activity coefficient, gaseous mixture is defined as:
(3) ‘p,
of a component in a
60
0
I130 IO
20
60
50
40
(4)
ld
60
70
80
90
100
MOLE PER CENT METHYLCYCLOHEXANE
Figure 2. Diagram of temperature vs. composition for methylcyclohexane-toluene system
The activity coefficients, from Equation 11, were plotted against composition on semilogarithmic paper. The data were smoothed by using the van Laar solution ( 7 , 8) of the GibhsDuhem equation. This particular solution has been rearranged by Carlson and Colburn (2)in the following form:
+z)2
log y1 = ~A (1
a, ( P = -
+L
where avis the vapor phase activity of the component and equal to
jdf: Hence
fw ‘p=
?&
(5)
a t a n equilibrium condition between the liquid and vapor phases:
therefore,
‘p
Yf:
= Y
xfi
(7)
A t low pressures and moderate temperatures, ( P = l
This is a n application of the Lewis and Randall rule.
(8)
The smoothing operation required that values be assumed for A and B; the initial values of these van Laar constants were determined directly from the experimental data. The activity coefficients calculated were then used to recalculate the composition by the equation:
The compositions so determined were compared with those used in Equations 12 and 13, which, naturally, depended on the values of A and B originally assumed. When the compositions obtained by the two methods were in good agreement over the entire range, the proper values of A and B had been selected.
INDUSTRIAL AND ENGINEERING CHEMISTRY
456 Table 11. T o ,C.
Yl 71 Pressure, 200 Mm. Hg 0.002 0.007 I:& 0.017 0,039 0.074 0,143 1.47 0.1085 0.187 1.34 1.35 0.153 0 257 1.23 0,210 0.3123 0.282 0.3875 1 18 0,3295 0.4383 1.16 0,4805 0,5775 1.12 1 11 0.510 0,6045 0.595 0.673 1.06 0.8865 0,741 1.04 0.6925 0.7526 1.04 0.773 0.817 1.03 0.830 0.863 1 .OO 0.8975 0.9175 1.01 0.8845 0.9025 1.01 0.9875 0.995 1.03 Pressure, 400 M m . Hg 0.0005 0,0013 1.93 0.054 0.087 1.22 0.1145 0.182 1.24 0.193 0,2805 1.17 0.358 1.15 0.257 1.10 0.357 0,455 0.413 0.508 1.08 0.482 0.568 1.06 1.04 0,5485 0.622 0.637 0 , 691 1.02 0.7145 0.696 1.02 1.02 0.832 0.798 1.00 0.899 0.883 0.992 0,990 1.00
69.3 68.7 67.3 66.6 65.7 64.8 63.8 63.4 61.6 81.4 61.1 60.7 60.7 60.3 60.15 60.1 60.0 59.7 89.4 88.6 87.6 86.45 85.7 84.4 83.7 83.0 82.4 81.6 81.25 80.5 80.2 79.7
close agreement between the final smoothed data and the experimentalresults. I n considering Figure 3, one must realize -that very small experimental errors are greatly magnified in this type of presentation, especially when the activity coefficients are small. Carlson and Colburn ( 2 ) point out that, as a general rule, systems of organic liquids approach Raoult's law as a limit as the temperature is increased. This study shows the conclusion to be true for the system methylcyclohexane-toluene over the range of experimentation. Table IV reports the values of the van Laar constants, A and B, for the three pressures thus far investigated. Table 1- shows the increase in activity coefficients a t constant composition with a decrease in pressure from 760 to 200 mm of mercury. The increase in activity coefficient of methylcyclohevane is significant, while that of toluene is somewhat smaller. There is little difference in the activity coefficients a t 760 and 400 mm. of mercury, although there is a general trend to larger coefficients a t the lower pressure. This is most evident There the coefficients are of greatest magnitude.
Experimental Data
21
7 2
1.00 1.005 1.00 1.01 1.01 1.03 1.06 1.05 1.10 1.10 1.10 1.19 1.13 1.14 1.23 1.15 1.21
...
1.00 0.99 0.99 0.99 0.98 1 .00 1.01 1.03 1.06 1.09 1.11 1.12 1.18 1.10
NOMESC LATURE
A = constant of van Laar equation
B = constant of van Laar equation T = temperature, C. P = total pressure, mm. mercury Po = vapor pressure, mm. mercury O
i z 0
a
= activity
f"
=
fugacity of pure component in standard state, a t temperature and pressure of system f = fugacity of a component in a mixture 5 = mole fraction in liquid phase y = mole fraction in vapor phase y = liquid phase activity coefficient, defined by Equation 2 q = vapor phase activity coefficient, defined by Equation 4
P
w
Vol. 47, No. 3
1.0
LL LL
Table 111. Smoothed Vapor-Liquid Equilibrium and Activity Coefficient Data T o ,C. 51 Ul 71 Y?
W
0
o
>. k 2
89.5 87.85 86.4 85.1 83.9 82.8 81.95 81.15 80.45 80.00 79.7
I-
o a '"0
20 40 60 80 100 MOLE % METHYLCYCLOHEXANE LIQYID PHASE
Figure 3. Diagram of log activity coefficient composition for methylcyclohexanetoluene system
vs.
69.35 66.7 64.95 63.5 62.3 61.5 61.0 60.6 60.25 59.9 59.7
This procedure requires good thermal measurements, for the vapor pressures used in Equation 14 are those of the pure materials a t the measured temperatures. The vapor phase compositions were calculated from the equation: Y1 =
XiYlPP ~
P
Pressure, 400 Rlm. 0.00 0.00 0.10 0.158 0.20 0.290 0.30 0,395 0.40 0.491 0.50 0.575 0.60 0.G63 0.70 0.744 0.80 0.827 0.00 0,910 1.00 1,000 Pressure, 200 Mm. 0.00 0.00 0.10 0.178 0.20 0,308 0.30 0.409 0.507 0.40 0.50 0,590 0.60 0.673 0.70 0.748 0.80 0.831 0.90 0.913 1.00 1.000
Hg
1.28 1.22 1.13 1.10 1,066 1.04 1,025 1.01 1.01 1.00 1.00 Hg 1.55 1.38 1,265 1.185 1.15 1.10 1.065 1.03 1.01 1.00 1 .00
1 00 1. O O 1.00 1.01 1.03 1.06 1.08 1.13 1.17 1.22 1.23
1.00 1.01 1.025 1,055 1.08 1.11 1 . 135 1.175 1.22 1.26 1.33
(15) Table IV.
By following the procedure outlined above, the results a t any one pressure are assured t o be internally consistent, for they obey the Gibbs-Duhem relationship and the Van Laar equation.
colvcLU SIONS I n Table I1 the exoerimental data are Dresented and in Table 111, the smoothed data. These data a k shown in graphical form in Figures 1 and 2, a n z--y diagram and a diagram of temperature vs. composition respectively. Figure 3, a semilog plot, reports data on calculated activitv coefficients vs. composition as well as the activity coefficients determined from the experimental data. These figures, particularly 1 and 2, show the
760 (6) 1 19 1 21
A B
Table V.
Van Laar Constants for Smoothed Data Pressure, M m I-Ig 400 1 28 1 23
200 1 55 1 33
Comparison of Activity Coefficients at Constant Comnosition Pressure, bIm Hg 760 (6) 200 z1
71
72
Y1
*f?
0 40 0 60 0 80
1 06 1 03 1 00
1 05 1 10 1 18
1 16 1 065 1 01
1 08 1 135 1 22
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1955
Subscripts L refers t o liquid phase o refers to vapor phase 1 refers t o more volatile component, methylcyclohexane 2 refers t o less volatile component, toluene LITERATURE CITED
(1) Beatty, H. A., and Calingaert, G., IND. ENG. CHEM.,26, 904 (1934). ~----, (2) Carlson, H. C., and Colburn, A. P., Ibid., 34, 581 (1942).
457
(3) Driesbach, R. R., "Physical Properties of Chemical Substances," Dow Chemical Co., Midland, Mich., 1952, ' (4) Othmer, D. F., IND.ENG.CHEM.,ANAL.ED.,20, 763 (1948). (5) Rossini, F. D., and coworkers, Natl. Bur. Standards, Ciro. C461 (1948) ; American Petroleum Institute Research Project 44. (6) Quiggle, D., and Fenske, M. R., J. Am. Chem. SOC.,59, 1829 (1937). (7) Van Lsar, J. J., 2. phys. Chem., 72, 723 (1910). (8) zbid., 185,35 (1929)., RECEIVED for review May 11, 1954.
ACCEPTED October 18. 1954.
Azeotropes of 2-Butoxyethanol with Alkyl Benzenes
c
WILLIAM F. KIEFFER AND RICHARD A. HOLROYDI The College of Wooster, Wooster, Ohio
T
HE monoethers of ethylene glycol are known t o be com-
pletely miscible with hydrocarbons, yet t o form solutions whose positive deviations from Raoult's law are of sufficient magnitude t o form minimum boiling azeotropes. The azeotropes of 2-ethoxyethanol with homologous alkyl benzenes have been reported (3) and have been shown t o follow closely the correlation proposed by Skolnik ( 6 ) . This research reports the comparable series of azeotropes between the higher boiling 2-butoxyethanol (171.2' C.) and the correspondingly higher boiling hydrocarbons from cumene (152.4O C.) t o n-butylbenzene (183.4' C.). Lecat ( 4 ) tabulates data for azeotropes with mesitylene, p-cymene, and n-butylbenzene. Horsley ( 2 ) includes these and
IO0
I
I
I
I
I
I
I
I
l
90 80700050
W
-
40 -
a
0
30-
hl
4
z 20
-
W
3 v) 0
2
j 100
f
180 160 170 150 BOILING TEMPERATURE OC.
X Lecat data
MATERIALS AND APPARATUS
The 2-butoxyethanol was obtained from t h e Carbide and Carbon Chemicals Co., Union Carbide and Carbon Corp. The butylbenzenes were provided by the Phillips Petroleum Co. through the courtesy of Fred Frey. The Paragon Division, Matheson Co., Inc., was t h e source of t h e p-cymene and oxylene. Mesitylene was from the Eastman Kodak Co. -411 materials were purified by distillation in a Todd fractionation assembly (7') operated at a reflux ratio of 10 to 1 . Pressure on t h e column was maintained at 760 =t1 mm. b y a manostat device. Mesitylene and o-xylene were also purified by fractional crystallization. Samples boiling within less than a 0.1' C. range were utilized. T h e normal boiling temperatures agreed within 0.1' C. with those re orted by t h e American Petroleum Research Project 44 (1).. #h e refractive indices, measured with a precision of f0.0002 on an Abbe refractometer thermostated a t 20" f 0.1 ' C., duplicated t h e published values except for a variation of f0.0004 for n-butylbenzene, - 0.0007 for p-cymene, and - 0.0007 for mesitylene. T h e procedure followed was to survey each binary system to determine vapor-liquid equilibria compositions at prevailing atmospheric pressures, usually about 740 mm. A Rogers, Knight, and Choppin ( 5 ) apparatus was used with either a fractional degree thermometer or a Beckmann thermometer. The refractive index of each sample was referred to prepared calibration curves for each system t o determine compositions. After each azeotrope concentration was ascertained from t h e conventional plots (vapor composition vs. liquid composition), it was confirmed for pressures of 760 mm. A 20-ml. sample of t h e azeotrope was prepared and distilled in the same Todd column t h a t was used for t h e purification of t h e starting materials. The temperature for t h e constant boiling mixture was determined to f0.05' C. The limit of error in evaluating compositions was f0.3 mole yo. All temperatures were read on thermometers calibrated against a National Bureau of Standards certified thermometer graduated in 0.1" C.
I
190
Figure 1. Azeotopes of %-butoxyethano1 with alkyl benzenes correlated by the method of Slcolnik
0 This research
data from Lecat on mixtures with propylbenzene and pseudocumene (1,2,4trimethylbenzene). T h e systems with mesitylene, p-cymene, and n-butylbenzene have been reinvestigated and those with see-butylbenzene, terl-butylbenzene, and cumene have been studied. Attempts t o identify a n azeotrope with the low-boiling o-xylene (144.4' C.) were inconclusive and tend t o confirm the expectation that 2-butoxyethanol boils too high (171.2" C.) to be an azeotrope former with this hydrocarbon.
RESULTS
The data for six azeotropes are summarized in Table I and presented graphically in Figure 1. Azeotrope composition is 1 Present address, Department of Chemistry, University of Rochester, Rochester, N. Y.
