THE SYSTEM ANILINE-CHLOROBENZENE Equilibria between Liquid and Vapor at Pressures below Atmospheric K. E. COULTER, R. A. LINDSAY, AND E. M. BAKER University of Michigan, Ann Arbor, Mich.
The apparatus described is a modification of an equilibrium still ( I ) to permit the determination of liquid and vapor equilibria at pressures below atmospheric. The liquid-vapor equilibria for the system aniline-chlorobenzene at pressures below atmospheric are given to facilitate the design of vacuum distillation equipment. These equilibria present the unusual feature of a lower relative volatility with lower pressures.
T
HE organic industries continually find themselves faced with the problem of separating volatile reaction products by distillation at reduced pressure. Reduced-pressure distillation is dictated in these cases by the fact that one or more of the products are subject to decomposition at the normal boiling point. Such a condition exists for aniline in the aniline-chlorobenzene mixture used in manufacturing aniline. The design of distillation equipment for this and other like systems demands a knowledge of the reduced pressure equilibria between liquid and vapor. This paper presents the results of study of the equilibria between liquid and vapor at one eighth, one quarter, and one half atmosphere pressure in the system aniline-chlorobenzene. The aniline and chlorobenzene used were c. P. reagents with properties as specified by the AMERICANCHEMICAL SOCIETY.As a check, both the refractive index and the specific gravity of each were determined. The gravities (Table I) agreed with those in the literature. The molecular refractivities calculated by the Lorentz and Lorenz formula (30.5 for aniline and 31.16 for chlorobenzene a t 25' C.) agreed well with the theoretical values. Therefore i t was considered that the materials were sufficiently pure for the purpose of this investigation.
Distillation Apparatus
FIGURE 1. DIAGRAM OF TAH~ APPARATUS 1251
The apparatus used in this study may be regarded as a modification of the Baker still (1) to enable subatmospheric operation. It is shown in Figure 1. A is a 2-liter, three-neck distilling flask with all stoppers covered by tin foil to prevent attack by the aniline and chlorobenzene. This flask is set in an oil bath to the level indicated. B is the column c h a r a c t e r i s t i c of t h e Othmer apparatus and contains the specially constructed still head, C, to remove as distillate any vapors condensed in the upper portion. This still head prevents the return of any reflux to the surface of the still liquor, where it might otherwise
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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assured, and the vapor reaching the still head is certain to be TABLEI. SPECIFIC GRAVITIES FOR ANILINE-CHLOROBENZENE the same in composition as that leaving the still. MIXTURES Container Q is a surge vessel to maintain a constant presMole Fraction Specific Gravity, Specific Gravity, Mole Fraction sure in the apparatus. This pressure is maintained a t less Chlorobenzene 25/25' C. 25/25O C. Chlorobenzene than atmospheric by a vacuum pump connected through lead 1.0000 1.1054 0.2158 1,0402 2 and by a needle valve, V , which is used as a control to bleed 1.1036 0.0431 1,0256 0.9834 1.0981 0.0229 1.0233 0.9063 air into &. The difference between the still pressure and 0.0000 0.8090 1.0897 1.0211 0.4794 1.0622 that of the surrounding air is read on manometer W , connected t o the system by lead 8. Liquid samples are drawn through the water-cooled conIN THE SYSTEM ANILINE-CHLOROBBNTABLE11. EQUILIRRIA denser, K , into receiver 0, the condenser preventing flashing ZENE of the hot liquid. After a sufficient sample has been colMole Fraction of lected in 0, the three-way cock in line U is closed, and then Chlorobenzene Boiling Liauid Pressure, Point, Vapor the cock b e h e e n the receiver and the condenser is closed. 0 hlm. of Hg. c. X ?/ When the three-way cock in lead L7 is opened to the air, 0.010 0.090 9.79 the liquid sample may be withdrawn. When the three-way 0.018 10.62 0.162 0.104 9.30 0.519 cock G is closed, the condensed vapor is collected in F and is 0.202 8.31 0.680 recovered in a similar fashion by manipulating the three0.310 8.27 0.788 way cock in lead R. 0.438 7.32 0.851 84.2 0.450 6.98 0.851 The still was run for 20 minutes after the temperature and 82.8 0.934 0.745 4.84 75.3 pressure had become constant a t the desired value. The 0.876 3.49 0.961 .7 3. .. 1 3.37 0.880 0.961 pressure was determined by the barometric pressure and the manometer reading. At the end of the 20-minute period the 0.909 3.02 0.968 73.3 0,939 3.54 0.982 73.4 vapor sample was collected and the liquid sample mas with0.944 3.67 0.984 73.3 dran-n at the same time. 190
380
1ii:s
0.007 0.015
0.096
14.92 14.04
123.7 112.2 106.7 103.3 93.3
0.0944 0.167 0.324 0.394 0.744
0.505 0.650 0.796 0.832 0.966
9.79
91.9 91.4 91.7
0.860 0.876 0.900
0.962 0.962 0.934
4.12 3.58 1.57
156.7 155.6
0.0044 0.014
0.095 0.200
23.75 17.64
143.3 131.7 130.5 119.4 119.4
0,0768 0,161 0.220 0.392 0.452
0.470 0.646 0.716 0.840 0.862
10.65 9.52 8.95 8.15 7.57
111.7 109.4
0.736
0.933 0.961 0.960 0.967
5.00 4.01 3.37 3.05
1G3:9
0.860
0.877 0.906
0.176
9.26 8.12 7.62 4 62
vaporiae and give too rich a vapor phase. Therniometer D records the temperature of the vapors; condenser E condenses the vapors and delivers the condensate to the small receiver, F; any condensate from C also goes to receiver F. Except when vapor samples are being collected in F for subsequent analysis, the condensate passes through the three-way cock, G, and the air-cooled condenser, H , and is returned to the still a t I. Condenser H prevents the liquid from flashing back into bulb F when it comes into contact with the hot liquid of the bath. Thermometer M records the temperature of the liquid in the still, which is heated internally by an electric coil, L, of Chromel C vire connected in series through a rheostat to the 110-volt alternating-current line. The rheostat provides the control for the amount of vaporization in the still. The temperature of the oil bath is kept from 2-3" C. above the still temperature. The bath is agitated by ail electric stirrer to provide a uniform temperature distribution. The three necks of flask A are wound with aluminum foil and theu asbestos rope, Column B is wound with aluniinum foil, Chromel A electric heating coil, and then asbestos rope. The coil is kept a t such a temperature that thermometer 11 reads 1-2" C. higher than the still temperature; thus the vapor is superheated, the absence of refluxing in colimin H is
Analyses The samples of liquid and vapor were then analyzed by determining their specific gravities, and from these specific gravities the composition of each was read from Figure 2, drawn from the data of Table I. Table I was prepared by determining the specific gravities of solutions of aniline arid chlorobenzene of known composition. All specific gravity determinations were made a t 25' C. by the use of a 10-cc. pycnometer; this size permitted accurate analyses and yet did not require withdrawing a sample so large as to affect materially the composition in the still. This technique was checked previously by Raker et al. in earlier work with this apparatus.
3
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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I
I
BOILING
-u
22
20
I
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I
I
I
FIGURE 5 POINTS OF THE ANILINE- CIUOROBENZEM SYSTEM a - l a ATMOSPHERE PRESSURE 0 - V A ATMOSPHERE PRESSURE w I / 2 ATMOSPHERE PRESSURE
FIGURE A
VARIATION OF M WITH THE COMPOSITION OF THE LICUQ o-I/B ATMOSPHERE PRESSURE o - l a ATMOSPHERE PRESSURE a.l/2 ATMOSPHERE PRESSURE
-"",I
90
Bo
90 loo TEMPERATURE
, Analyses by refractive index appeared the more rapid and easy method, but previous work by these investigators indicated that minute unavoidable traces of organic impurities have large unpredictable effects on the refractive index. At the same time it was found that these same traces of impurities had no determinable effect on the specific gravity. The observed equilibrium data are presented in Table I1 and Figures 3 and 4. Figure 3 is the conventional equilibrium plot. Figure 4 shows the variation of the relative volatility, CY, with composition and pressure. Alpha is defined by the equation,
y a=- 1
- 11
X -
1-x
OC
am
1
I
where x and y represent the mole fraction of the more volatile component, chlorobenzene, in the liquidand vapor, respectively. Figure 5 shows the boiling point variations a t the observed pressures, and Figure 6 gives the variation of the vapor pressure of the pure components with temperature. Figure 6 was obtained by supplementing data from the International Critical Tables ( 2 ) for the normal boiling points with data obtained by extrapolating the curves of Figure 5 for the boiling points of the
pure components a t reduced pressures. In Figures 3 and 4 the experimental points are plotted for all pressures. To prevent confusion, in Figure 3 a curve is drawn only through the points for one quarter atmosphere. The data for one half and one eighth atmosphere define similar curves which lie, respectively, above and below the one-quarter-atmosphere curve. The results in Figures 3 and 4 present the fact that the relative volatility of the system aniline-chlorobenzene decreases slightly with reduced pressure. This is in marked contrast to the common premise that reduced pressure renders separation by distillation less difficult.
Literature Cited (1) Baker, E. A I . , Hubbard, R. 0. H., Huguet, J. H., and Michalowski, S.S.,IND.ENQ.CHEM.,31, 1260 (1939). (2) International Critical Tables, Vol. 111, pp. 220-1, New York, McGraw-Hill Book Co., 1928. BASEDupon a researoh report submitted by K. E. Coulter in partial fulfillment of the requirements for the degree of bachelor of science in ohemical engineering, University of Michigan, 1941.
