are extended t o high pressures t o make possible more intelligent control over certain industrial processes. Y - X - P - T curves are evaluated up t o a temperature of 275" C. a t saturation pressures. Critical temperatures and pressures of the system are also obtained.
ETHANOL-WATER SYSTEM
JOHN GRISWOLD, J. D. HANEYl, AND v. A. KLEIN2
Vapor-Liquid Properties at High Pressures
The Jniversity of Texas, Austin, Texas
ERTAIK operations used in the manufacture of an-
system determined with it are reported in another article (1A). The pressure gage had a total range of 1500 pounds and was graduated in 10-pound divisions. Experimental pressures were read to 1 pound, and the gage was checked against a dead-weight tester. Temperatures were determined by calibrated iron-constantan thermocouples and a low-range potentiometer. The accuracy of the temperature observations was approximately 0.5" A combination check on thermocouples and gage was obtained by observing the vapor pressure-temperature curve for water in the same apparatus, up to 1500 pounds pressure. The ethanol used throughout the work was U. S. P. grade material. Distilled water was taken from the laboratory supply. Analysis of samples was obtained from densities at 20" C., determined by the balanceplummet-thermostat method of Osborne, McKelvy, and Bearce (8).
C
hydrous alcohol, ethylene from alcohol, silica aerogel, and other products depend on the high-pressure vaporliquid behavior of ethanol-water. Data of satisfactory accuracy and pressure range have not heretofore been available. VAPOR-LIQUID EQUILIBRIA
e.
Vapor-liquid equilibria up to 15 atmospheres were reported by Grumbt (3). However, a study of his data shows serious scattering of the points and self-inconsistencies which he attributed to refluxing in the vapor line of his apparatus. The vapor-liquid equilibria of the system were determined a t several constant temperatures with an all-steel recirculation type apparatus. The development of this apparatus and the high-pressure vapor-liquid equilibrium of the benzene-toluene The photograph shows a control panel for continuous rectification of 190 proof alcohol from wheat mash, a t the plant of Joseph E.Seagram & Sons, Inc.: section of column may be seen in the background.
1 2
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Present address, Joseph E. Seagiam & Sons, Inc., Lawrenceburg, Ind. Present address, Dow Chemical Company, Freeport, Texas.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I
I
-A . VAPOR- L iaum
t
EQUIL IERIUM
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Vol. 35, No. 6
A
I
C R I T I C A L TFhPER.4 TURES
*-f-f3''
I
E THANO1 - WA TER CONSTANT TEMPERAT U K S
4.VAPOR-LIQUID
PER CCNT ETHANOL
ro
20
30
40
so
60
70
80
30
10
20
30
40
50
60
70
80
90
EQUILIBRIUM
OF
FTHA NOL- WA TER CONSTANT PRESSURES
-7
--b CAREY 8 LEWIS
A
v
20
30
4,O
50
Figure 1
CALCULATED F EXPERIMENTAL
THANOL IN
IO
(1
NOYES 8 WARFEL (
iiauio 60
70
80
90
The experimental vapor-liquid equilibrium data are summarized in Table I and plotted as isothermal Y-X curves on Figure 1A. To develop the constant pressure Y-X plot of Figure lB, pressure isotherms were plotted and then curves of vapor composition against pressure for several constant liquid compositions were constructed. Interpolation of the latter curves gave the isopiestic Y-X graph (Figure 1B). CRITICAL TEMPERATURES
Critical temperatures were determined b y observing the behavior of mixtures of known composition when sealed into glass tubes and heated. The sealing technique and the heater were described in an earlier article ( 2 ) . The sample tubes were of Pyrex, 4 mm. 0.d. with a 1-mm. wall thickness and
an over-all length of about 60 mm. The observed temperatures (after emergent stem corrections) were accurate to approximately 1" C. The constant volume behavior of this system as the critical state is approached was found to differ somewhat from that characteristic of pure compounds and hydrocarbon mixtures. The tubes were charged with alcohol solution to approximately one third their volume a t room temperature and were then sealed. I n the determination idiich followed, the liquid volume or meniscus level rose with temperature. TT'ithin the last 1" C. below the critical, the meniscus rose from about two thirds of the tube height to completely fill the tube. The vapor phase apparently became zero, and disappearance of the meniscus could not be observed. On slow cooling from 1' C. above this temperature, a white cloud 1%-ouldsuddenly appear, quickly condense, and reveal a liquid meniscus. These rising and falling temperatures differed by less than 1' C. for all cases in which the tubes were charged to between 20 and 40 per cent of their volume a t room conditions. This temperature is therefore taken as the true critical. Further support of this hypothesis is obtained from a study of relations for relative vapor and liquid volumes a t constant total
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1943
volume. Experimental critical temperatures are summarized in Table I1 and plotted on Figure 1C. The figure also shows the recent data of White (IO), which lie 2" to 5" C. lower than the new values. The critical temperature of this system approaches linearity with composition (on thc weight basis) much more closely than doe? that of binary hydrocarbon mixtures ( 7 ) .
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point. The ethanol-water azeotrope, which occurs a t 95.6 weight per cent alcohol at atmospheric pressure, lies at 95.3 weight per cent alcohol a t a total pressure of 1450 mm. (9) This indicates only a slow change in the azeotropic compo-
CRITICAL PRESSURES
Critical pressures were determined by the same procedure and with an apparatus similar to that used in an earlier article (9). The equipment (Figure 2) consisted of a steel bomb connected to a pressure gage by a loop of l/*-inch 0.d. annealed steel tubing. The bomb was suspended in a fused salt (IETS) bath by a sliding linkage to a motordriven cam. This gave the bomb vertical reciprocating agitation. The pressure gage described under "Vapor-Liquid Equilibria" was utilized when the pressures were below 1500 pounds. At higher pressures a 3000-pound gage graduated in 20-pound divisions and checked against a deadweight tester was used. The bomb was charged with 100 cc. of alcohol solution and heated to the boiling point to eliminate air. The gage tubing (used to vent air) was then connected to Figure 2. Apparatus for Determination of Critical Pressures the gage, and temperature-pressure data were taken through the critical temperature of the mixture, as read from Figure 1C. The usual range was from 20" below to 20" C. above the sition with temperature and pressure. Azeotropic composicritical temperature, in which six to eight observations were tions and behavior a t higher temperatures and pressures have made. The bath temperature and observed pressure were apparently not been reported. The critical temperature of constant for at least 10 minutes prior to readings. anhydrous ethanol is 243" C. From Figure 1C the critical composition at 250" C. is approximately 80 mole per cent alcohol, and a t 275" it is approximately 45 mole per cent. A minimum-boiling azeotrope must exhibit (a) no difference in composition between liquid and equilibrium vapor, and EQUILIBRIUM OF ETHANOLWATER TABLE I. VAPOR-LIQUID ( b ) an isothermal maximum pressure a t some definite compoTemperature, Mole %,Ethanol Mole 7% Ethanol Pressure c. in Liquid in Vapor Lb./Sq. In. kbs. sition. To show the aeeotropic behavior more clearly at 7.3 33.4 107 150 250 O C. and above, the pressure-composition diagram of 13.8 41.4 119 26.5 Figure 3 was constructed. It is evident that at 275" C. no 48.7 130 51.4 61.6 145 azeotrope exists. At 250" an equilibrium determination 63.9 69.5 147 gave 68.8 mole per cent alcohol in the liquid and 69.6 in the 5.8 24.7 300 200 11.4 33.8 331 vapor at a pressure of 1010 pounds. Since the critical 23.7 43.3 370 composition is 80 mole per cent alcohol and the critical 49.7 58.5 415 63.3 68.1 424 pressure is slightly above 1000 pounds a t 250" C., the presence 250
275
12.6 26.0 40.0
24.5 34.4 42.5
1176 1341 1492
The data were plotted and the pressures at the critical temperatures read from the plots. The results are included in Table I1 and plotted on Figure 1D. The critical pressure is seen to be substantially linear with weight per cent ethanol at concentrations below 70 per cent. BEHAVIOR OF AZEOTROPE
Separation of a mixture b y fractional distillation may be limited b y the existence of either an azeotrope or a critical
TABLE11. SUMMARIZED DATAFOR CRITICALTEMPERATURES AND PRESSURES Weight Ethanoy
Mole 7 Ethan07
100 94.0 88.7 84.0 79.0 74.1 69.2 64.4 61.0 55.1 49.6 45.1 40.0 35.8 26.9 24.0 18.7
100 86.1 75.5 67.3 59.4 52.8 46.8 41.5 38.0 32.4 27.8 24.4 20.6 17.9 12.6 11.0 8.3
Critical Temp C."
243.0 248.0 253.1 259.2 264.2 270.3 277.3 284.4 288.4 296.5 307.5 311.6 317.6 325.7 334.8 339.8 344.9
Weight % Ethanol 100 86.5 80.0 63.9 46.0 30.9 16.3
Critical Mole 7 Pressure Etheno? Lb./Sq. Ih. 100 71.5 61.0 40.9 25.0 14.9 7.1
925 1100 1220 1618 2060 2440 2830
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 35, No. 6
is an exceptional rather than a general situation (4). The system thus does not fall into either of two classes which exhibit retrograde condensation as enunciated by Katz and Kurata (6). A necessary condition for retrograde condensation is the existence of separate points of maximum pressure and of niaxiniuin temperature during the coexistence of liquid and rapor, by a mixture of some definite composition. This requires that the peak of a border curve be round rather than sharp. Although few of the present l--X data lie close to the critical locus, vapor and liquid isotherms extrapolate so near the same point on the locus curl-e that zones of retrograde condensation must be either extremely small or nonexistent. ACKNOWLEDGMEhT
S. S.Sutherland assisted in the construction of the diagrams.
LITEHATUHE CITED I
M O L E PER C h N T E T H A N O L
Figure 3
or absence of a n azeotrope between 70 and 80 mole per cent alcohol a t this temperature cannot be definitely ascertained. PRESSURE-TEMPERATURE RELATIONS
The most widely used method of obtalning high-pressure vapor-liquid equilibrium data on binary systems heretofore has been to observe dew and bubble points on samples of definite composition in a variable-volume calibrated glass pressure tube (6). The resulting P-V-T data are customarily plotted directly as envelope curves. The envelope for each composition is tangent to the critical locus curve. Vapor-liquid equilibria may be calculated from P-X-Y plots of dew and bubble point curres. On the other hand, the present method yields direct Y-X data. Hon-ever, a P-T diagram was constructed for ethanol-water (Figure 4) since i t is of interest for comparison with other systems similarly plotted. To develop this diagram, the data were interpolated from a P-X chart with pressure on a logaritlimic scale, The spacing of the isotherms was nearly linear with temperature, and P-T values for compositions of 25, 50, and 'it? niole per cent alcohol were read from the plot. The results along with vapor pressures of water, ethanol, and the critical locus are shown on Figure 4. The critical locus contains no point of higher pressure or higher temperature than the critical values for aater. This
( l j Carey and Lewis, IND. E B G . CHEM.,24, 882 (1932). (1A) Griswold, Andres, and Klein, Trans. A m . Inst. Chem. Engis., 39, 223 (1943) ; Petroleum Refinerg, 22, No. 6 (1943). (2) Griswold and Kasch, 1x11.EBG. CHEM.,34, 804 (1942). (3) Grumbt, J. A., Tech. Mech. Thermodgnam., 1, 309, 349 (1930). (4) Hougen and Watson, "Industrial Chemical Calculations", 2nd ed., pp. 406, 407, New York, John TTiley & Sons, 1936. (5) Katz and K u r a t a , IND. ENG. CHEM.,32, 817 (194CI) (6) K a y , W. B., Ibid., 30, 459 (1938). (7) Mayfield, F. D., I b i d . , 34, 844 (1942). (74.) Noyes and Warfel, J . A m . Chem. Soc , 23, 463 (1901). (8) Osborne, McKelvy, and Bearce, Bur. Standards, B d l . 9, 371 (1913). (9) Wade and Merriman, J . Chem. Sac., 99, 997 (1911). (10) White, J. F., Trans. A m . I n s t . Chem. Engrs., 38, 435 (1942).
Figure 4
