602
V O L U M E 19, NO. 8
points of the components of an alcohol-ketone system with change in pressure. As a result of the study of these systems, it has been found that the methanol-acetone azeotrope exhibits the unusual phenomenon of becoming nonazeotropic a t both low and high pressures-that is, below 200-mm. pressure the system is nonazeotropic with methanol as the more volatile product, while above 15,000 mm. the system is nonazrotropic with acetone the more volatile component. Some of the equilibrium data for this system and two other alc~ohol-k?tonrazeotropps ar? shown in Figures l and 2.
The similarity of the diagrams for the different systems at suitable pressures is of interest. For example, the diagram for methanol-acetone a t 10,000 mm. corresponds approximately to the diagram for methanol-methyl ethyl ketone a t 1000 mm. and for methanol-methyl propyl ketone a t 100 mm. LITERATURE CITED
(1) Britton, E. C., Nutting, H. S., and Horsley, L. H. (to Dow Chemical Co.), U. S.Patent 2,324,256 (July 13, 1943).
Graphical Method for Predicting Effect of Pressure on Azeotropic Systems H. S . NUTTING
AND
L. H. HORSLEY, The Dow Chemical Company, Midland, Mich.
4
manner and could obtain the curves only by detailed experimental work. I t has becn found that the vapor pressure curves of azeotropes are straight lines when plotted on a Cos chart which permits determination of the complete vapor pressure curve from data a t two pressures. Since an azeotrope by definition has either a higher or a lower vapor pressure than that of any of the components, the azeotropic vapor pressure curve will always lie above or below the
RAPID and easily applicable method has been found for
indicating the effect of pressure on the composition and Imiling point of an azeotropic system. The method is based on the w e of the Cox vapor pressure chart ( 1 ) on tvhich the log of vapor pressure is plotted as a function of l / ( t o C. 230) to give a straight line over a wide range of pressures. Lecat ( 2 ) has considered the use of the vapor pressure curves of azeotropes to indicate the pressure a t which a system would become nonazeotropic. However, he plotted in the conventional .i
+
10000
10000
e
e
6
6
4
4
7---
__
----- -
2
2
.--~
-
LOO0 B 6 4 W
3
w 4
s
---
2
___ _-
100 -20
-id
0
2
100 IO
20
30 4 0
60
80
DO
EO
160
200
+o
240
1
-IO
o
IO
20
30
40
w
BO
100
IX)
IM)
200 240
-10
0
10
20
M
40
M)
80
00
I20
le0
200 240
1.000 8
a w
6
a
4
L 2
Hs
IO0
e 8 4
z 0
le0
I
[COX SCALE
TEMPERATURE
l/(T+230)]
C.
Figure 1. lzeotropic Vapor Pressure Curves of Methanol-Methyl Ethyl Ketone, Methanol-Acetone, Water-n-Propanol, and Water-Ethanol
AUGUST 1947
603
CA. B
C/‘A
//./
e/
a U
0
-1
AZEOTROPIO A T A L L P R E S S U R E S
AZEOTROPE DISAPPEARS AT P AND P ’ . COX
Figure 2.
SCALE
I/(T°C.+230)
Schematic Diagram of Vapor Pre-snrc Curbes of Binary Azeotropes
curves of the coniponent s. This is indicated schematicall) ill Figure 2 where A and B are vapor pressure curves of the components and C is the vapor pressure of the azeotrope. If curve C crosses either 3 or B , the azeotropic vapor pressure is no longer greater or less than any of the components and the system will become nonazeotropic at the point of intersection. On the other hand, if the azeotropic curve is parallel to the other curvps the system \vi11 he azeotropic up t o the critical pressure. The method ha6 bren successfully applied to numerous systems, four of whirh are shown in Figure 1. The azeotrope methanol-niethyl ethyl ketone became nonazeotropic a t 3000 mm. of mercury after it was predicted that this would occur at 2000 to 4000 mm. The azeotrope methanol-acetone was studied in detail after it was predicted that the azeotropism would disappear a t both low and high pressures. This system is nonazeotropic below 200 mm. of mercury and above 15,000 mm. compared to predicted limits of 200 to 500 mm. and 10,000 to 20,000 mni. While this is the only azeotropic system known to become nonazeotropic a t
hulh lo\\. aiid high pressures, there are indications that the phciiomenon occurs in several othcr systems, contrary to the conclusions of Lecat that such systems probably do not exist ( 3 ) . Caution should he used in extrapolating curves to very low pressures because of the possibility of curvature in the vapor pressure lilies over a manyfold range of pressures. In cases where only the normal azeotropic boiling point is knoxvn, it is possible to predict the effect of pressure on the systt’m by dran-ing the azeotrope curve through the normal boiling point with a slope equal to the average slopes of the component vapor pressure curves. This procedure will permit a fairly accurate prediction of rvhether the azeotrope will cease to exist below the critical pressure. LITERATURE CITED
( 1 ) Cox, I d . Eng. Chem., 15, 592 (1923). ( 2 ) Lecat, Ann. SOC. sci. Bruzelles, 49B,261-333 (1929). (3) Lecat, “Trait6 de Chimie Organique,” Vol. 1, p. 139, Paris,
Grigtiard. Mason et Cie., 1935.
Graphical Method for Predicting Azeotropism and Effect of Pressure on Azeotropic Constants L. H. HORSLEI , The Dow Chemical Company, Midland, Mich.
L
E C h T (2) has devised an analytical method for determining azeotropic boiling points and compositions for certain related groups of binary systems. The method is based on the fact that the composition and boiling point of an azeotrope are related to the relative boiling points of the two components. 1,rcat thus obtained a series of equation. of the foriii
+ I A I b + A2c L‘ = d + Ae + Azf
difference in boiling point of azeotrope and the lower boiling component u, b, . . . . f = constants for a given series of related azeotropes such as methanol-hydrocarbons S o t e that A may be positive or negative; I A 1 is always positive. 6
.-
=
- - ___
Table I.
8 = n
Pressure
M m . Hg
(boiliiig point of component A ) - (boiling point of component B ) 1 A = difference in boiling point of d and B (absolute value of A) C = azeotropic coniposition in weight percent d
where A
=
~
200 400 760 6,000 11,000 -~
C. 35 50 65 130 153
~
C 43 61 80
162 193
_
_
_
_
Effect of Pressure
Boiling Point Methanol Benzene O
---___
-
A
C. -9 -12 -15 -35 -44 O
Azeotropic Boiling Point Calcd. Found O C. C. 23 26 39 42 55 57 125 124 150 149
C Calcd. Found W e i g h t 70 30 34 33 36 39 40 54 55 64 63
_ _ _ ~ _ ~ _ _ _ _
~
