1188
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited (1) Allpress, J . Chem. Soo., 130, 1722 (1926). (2) Angeletti, Ann. Chim. applicata, 26, 234 (1936). (3) Browne, Am. Chem. J., 28, 462 (1906). (4) Browne, La. Agr. Expt. Sta., Bull. 91,17, 96 (1907). (5) Browne and Zerban, “Phy31cal and Chemical Methods of Sugar Analysis”, 3rd ed., p. 902, New York, John Wiley & Sons, 1941. (6) Coltof, Biochem. Z., 243,192 (1931). (7) Fischer and Baer, Helv. Chim. A c t a , 19,519 (1936). ( 8 ) Gayon and Dubourg. Ann. Inst. Pasteur, 15,527 (1901). (9) Haworth and Leitch, J . Chem. Soc., 113,194 (1918). (10) Helferich, Ber., 56,769 (1923). (11) Irvine and Paterson, J . Chem. Soc., 105, 920 (1914). (12) Kilp, Z . Spiritusind., 55, 188 (1932). (13) Levene and Kuna, J . Bid. Chem., 127, 51 (1939). (14) Levene and LaForge Ibid , 2 0 , 4 2 9 (1915). (15) Levene and Tipson, Ibid., 105,419 (1934). (15A) Lobry de Bruyn, C. A., and Alberda van Ekenstein, W. A., Rec. trau. chim., 16,258 (1897). (16) Meunier, Ann. chzm. phyn., [el 22,412 (1891). 21,858 (1929). (17) Nelson and Greenleaf, IND. EXG.CHEM., (18) Ohle and Just. Ber.. 68B,601 (1935). (19) Oldham and Rutherford, J . Am. Chem. SOL,54,366 (1932).
(20) (21) (22) (23) (24) (25) (2G) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36)
Vol. 34, No. 10
Peterson and Fred, J . Biol. Chem., 41,431; 42,273 (1920). Pette, Ber., 64,1587 (1931). Prinsen Geerligs, Intern. Sugar J., 40, 345 (1938). Purdie and Paul, J . Chem. Soc., 91,293 (1907). Raistrick and co-workers, T r a n s . R o y . SOC.(London), B220, 171 (193 1). Spoehr and Strain, J . B i d . Chem., 85, 365 (1929). Spoehr and Wilbur, Ibid., 69,421 (1927). Steigor and Reichstein, Helv. Chim. Acta., 18,794 (1935). Ibid., 19, 187 (1936). Stiles, Peterson, and Fred, J . Biol. Chem., 64,843 (1925). Tollenfl-Elmer, “Kurzes Haudbuch der Kohlenhydrate”, p. 199 (1935). Ibid.. p. 340; Vogel and Georg, “Tabellen der Zucker und ihrer Derivate”, 1931: Micheel, “Chemie der Zucker und Polysaccharide”, p. 70 (1939). Valentin. Collection Czechoslov. Chem. C b m m u n . , 3,499 (1931). Waterinan and van der E n t , Arch. Suikerind., 34,11,942 (1926). West and Holden, J . Am. Chem. Soc., 56, 930 (1934). Zerban, J . Assoc. Oficial A g r . Chem., 23,563 (1940). Zerban and Sattler, IND. ENG.CHEM.,ANAL.ED.,10,669 (1938).
PREBEKTED before the Division of Sugar Chemistry and Technology at the Atlantic City, N . J . 102nd Meeting of the A M E R I C A N CHEnfICAL SOCIETY,
Vapor-Liquid Equilibrium of Methanol-Ethanol-Vater Mechanism of Ethanol Dehydration Ternary vapor-liquid equilibria at atmospheric pressure are presented for the system methanol-ethanol-water. No ternary azeotrope was found. Requirements of a dehydration agent for the manufacture of anhydrous ethanol by distillation are discussed. A systematic procedure is used to determine from the ternary data how methanol behaves in this respect, with the conclusion that it is not suitable as a dehydration agent.
URING the investigation of a new approach to the problem of dehydrating ethanol by distillation, a fairly complete set of equilibrium data for the ternary system methanol-ethanol-water was obtained. These data are useful in distillation calculations for the recovery of the alcohols from their water solutions such as certain antifreezes and pharmaceuticals.
D
Ternary Equilibrium Forty ternary vapor-liquid equilibrium determinations were made in the original type Othmer still (8) with the addi1
Present address, Humble Oil and Refining Company, Baytown, Texas.
JOHN GRISWOLD
AND
J. A. DINWIDDIE]
T h e University of Texas, Austin, Texas
tion of an electrical resistance wire wound about the body of the apparatus to compensate heat loss to the surroundings and thereby avoid refluxing. The results are summarized in Table I. The analysis was by the refractive index, specific gravity, and boiling point method reported earlier (6). The data are plotted as Figures 1 and 2. Although a method of plotting the vapor-liquid equilibrium data on a single ternary diagram (1) could have been used, a scheme involving two plots mas developed which will be found somewhat easier to read. Figure 1 is a y-x or vapor-liquid equilibrium plot for methanol in the ternary solution. It was constructed by plotting the data of Table I on the coordinates indicated. The number opposite each point is the mole percentage of water in the equilibrium liquid. Lines of constant mole per cent water were then drawn in by interpolation. Fixing the concentrations of any two components of a ternary mixture fixes its composition. Figure 2 for ethanol in the ternary mixture is analogous t o Figure 1. The water concentration in the equilibrium ternary vapor is obtained by difference. (For example, a liquid containing 20 mole per cent methanol and 30 mole per cent ethanol contains 50 mole per cent water. From Figure 1, its equilibrium vapor contains 36.5 per cent methanol, and from Figure 2 the vapor contains 38.5 per cent ethanol. Water in the vapor = 100 36.5 - 38.5 = 25 per cent.) Inspection of Figures 1 and 2 shows that, with a few exceptions, the data fall within 2 mole per cent of their correct positions with respect to the lines of constant mole per cent water. Since the analytical accuracy was better than this,
October, 1942
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1189
t
Mo/ X
Mefhonol in Lipid
Fiouxs 1. METHANOL EQUILIBRIUM CURVEFOR TERNARY SYSTEM ME.~HAXOL-ETHANOL-WATER
Bsrom
No.
9
10
%i
--
Still Pot% MeOH %EtOH ~ t . 9 wt.e 70.0 63.0 54.0 10.0 19.0 7.0 2h.2 14.1 4 2 . 0 24.2 4.0 6.0 m . 0 64.1
TABLE I. TERNARY VAPOR-LIQUID EQUILIBRIUM DATA --Condensat% MeOH W
t
.
e
15.2 7.4 84.5 53.1 60.3 71.9 23.3 41.3 67.3 70.3
% EtOH W t . e
Barom- - - - - - S t i l i eter % MeOH NO.
htm:
Pot-
8.0 15.2 21.7 34.1 19.7 18.7 29.4 5.2 12.5 20.0
% EtOH W t . e 39.2 33.3 28.3 9.6 19.3 11.5 3.6 14.2 8.9 3.5
3.0 9.2 13.8 3.2 6.1 16.7 31.7 46.6 77.5 88.5
9.6 7.5 2.9 5.1 3.3 09.0 56.6 44.1 20.0 10.0
w t x e
---Condensate% MeOH
m~
% EtOH
TK-XZ
15.8 28.4 39.6 61.4 41.7 42.7 64.7 17.3 30.1 59.1
55.0 45.3 36.9 15.4 31.2 28.4
17.2 29.5 50.0 19.8 39.0 24.9 47.2 58.6 85.8 93.8
39.8 33.6 13.1
9.0
44.9 27.9
10.9
31.6
l5,l 63.6 46.6 30.3 13.2 5.7
1190
INDUSTRIAL AND ENGINEERING CHEMISTRY
the charts are probably accurate to 2 mole per cent for each alcohol except in the extremely concentrated zone. Barometric pressure was between.740 and 750 mm. for all of the experimental data. The effect of barometric pressure variation on equilibrium is less than the experimental error in this case. Boiling points for the system a t 760 mm. may be read from the analytical chart ( 5 ) .
Vol. 34, No. 10
distillation proceeded. Likewise, the overhead temperature increased regularly for both mixtures. It is therefore concluded that the system contains no ternary azeotrope. The fractionation efficiency of the column (roughly 10 transfer units) was inadequate to draw final conclusions ahout the action of methanol as a dehydration agent. Mechanism of Alcohol Dehydration
Examination for Ternary Azeotrope Vapor-liquid equilibria of the constituent binary mixtures ethanol-TTater (4),methanol-water (4), and ethanol-methanol (6) are available in the literature. Only the ethanol-water mixture forms an azeotrope. Inspection of Figures 1 and 2 reveals no indication of a ternary azeotrope. To obtain positive information, two ternary mixtures (one lorn in water and one high in water) were charged to a still and fractionated, The attached colunin was l1/z X 41 inches (3.8X 104 em.) in packed section, containing 1/4-inch (6.35-mm.) porcelain Berl saddles, and n’as fitted with a total condenser, condensate trap, and sampling device. Composition of distillate samples changed uniformly and continuously as the
Anhydrous ethanol is manufactured by the fractionation of an aqueous feed (having approximately the azeotropic composition) in a column to which a dehydration liquid is introduced near the top or with the reflux. Benzene, ethyl ether, and trichloroethylene are commercial dehydration agents (9). Each of these has the property of forming an aqueous azeotrope which boils at a iower temperature than does 95 per cent alcohol. Thus the water is separated from the alcohol, and the latter is withdrawn from the bottom of the column a8 an anhydrous product. There is another possible mechanism of dehydrating alcohol by a nonazeotropic distillation, which has apparently not been reported heretofore. It is well established that certain ternary
October, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1191
dehydrate the binary azeotrope. General conclusions as to the suitability of methanol as a dehydration agent may be drawn from the procedure following: Let x
mole fraction of a component in liquid mixture y = mole fraction of same component in its equilibrium vapor a = relative volatility of component A with respect to component B = (Y/x)A/(Y/x)B =
Normally ethanol is more volatile than water, or the relative volatility is greater than unity (a > 1.0). At the azeotropic composition, a becomes unity, or the liquid and equilibrium vapor have the same composition. To dehydrate alcohol by removing water in the form of an azeotrope a t the top of a fractionating column, cy for alcohol-water must be less than unity at the molar water-alcohol ratio, R, of 1.18 (corresponding to the binary alcohol-water azeotrope), and also for compositions near this value. R is plotted against a on Figure 3. Ternary vapor-liquid equilibrium data for the benzeneB alcohol-water system have been published ( 2 ) . These are also plotted on Figure 3 with curves of constant mole per cent benzene drawn in by interpolation. Benzene fulfills the requirement for dehydration of ethanol. Ternary data for corresponding ether and trichloroethylene systems must show the same effect. The possibility of separating anhydrous alcohol as an overhead binary mixture with some third agent was not mentioned in two recent review articles (7, IO). To satisfy this condition, the relative volatility of alcohol to water must be greater than unity at the azeotropic water-alcohol ratio and a t compositions containing less water. With methanol as the third component, relative volatilities for ethanol-water were calculated from the data of Table I and plotted as Figure 4. In dilute ethanol solutions small amounts of methanol are seen to increase the relative volatility of ethanol to water, FIQURE 3 (Above). EFFECT OF BENZENEON RELATIYE VOLATILITY OF while larger amounts of methanol depress it. ETHANOLWATER SYSTEM The effect fades out as the water content deFIGURE4 (Below). EFFECT OF METHANOL ON RELATIVE VOLATILITY creases, and becomes negligible a t the waterOF ETHANOL-WATER SYSTEM ethanol azeotrork ratio. k The significait conclusion from Figure 4 is that the effect of methanol upon the ethanol-water azeotrope-is slight. Therefore methanol is not suitable as a systems whose components form either homogeneous or dehydration agent for ethanol. The possibility remains that heterogeneous binary azeotropes have no ternary azeotrope. some other agent exists which will more efficiently destroy The present ternary mixture and the butanol-butyl acetatethe ethanol-water azeotrope in the manner described, so that water system (8)fall into this category. it would be practicable as a dehydration agent. This method I n the case of a ternary system containing ethanol and may be applied to find separating agents for other homowater, if the third component forms a regular binary solution geneous azeotropes. with ethanol and with boiling points lower than that of the ethanol-water azeotrope (78.15’ C.),it should be possible to Literature Cited dehydrate the ethanol by fractionating the ternary mixture into a dry binary overhead product and a water bottom. (1) Baker et al., IND. ENQ.CHBM.,31, 1263 (1939). (2) Barbaudy, J. Chern. Phys., 24, 22 (1927). Methanol is an obvious agent to test in this way, since it (3) Brunjes and Furnas, IND.ENQ.CHEM.,28,573 (1936). does not form an azeotrope with water, and its binary solution (4) Cornell and Montonna, Ibid., 25, 1331 (1933). with ethanol is almost ideal. The fractional distillation ex(6) Griswold and Dinwiddie, IND. ENQ. CAEM., ANAL. ED., 14, periments mentioned earlier should have separated water as 299 (1942). (6) Hausbrand, “Die Wirkungsweise der rectifisier und distillier the still residue. Apparate”, Berlin, Julius Springer, 1921. Nearly anhydrous mixtures were obtained overhead a t the (7) Keyes, D.B., IND. ENQ.CHEM.,33, 1019 (1941). first of each run, and the still contained dilute alcohol before (8) Othmer, D.F.,Ibid., 20,743 (1928). it went dry in both cases. However, fractionation was in(9) Ibid., 32, 1688 (1940). adequate to estimate the amount of methanol necessary to (io) rw., 33, 1106(1941).
f
h
