AZEOTROPIC DATA FOR BINARY MIXTURES

J. Gmehling, J. Menke, J. Krafczyk, K. Fischer, J.-C. Fontaine and H. V. Kehiaian

In this table, important parameters are given for 808 azeotropic binary mixtures.

Binary homogeneous (single-phase) liquid mixtures having an extremum (maximum or minimum) vapor pressure P at constant temperature T, as a function of composition, are called azeotropic mixtures, or simply azeotropes. The composition is usually expressed as mole fractions, where x₁ for component 1 in the liquid phase and y₁ for component 1 in the vapor phase are identical. Mixtures that do not show a maximum or minimum are called zeotropic. A maximum (minimum) of the P(x₁) or P(y₁) curves corresponds to a minimum (maximum) of the boiling temperature T at constant P, plotted as a function of x₁ or y₁ [see T(x₁) and T(y₁) curves, Types I and III, in Figure 1]. Azeotropes in which the pressure is a maximum (temperature is a minimum) are often called positive azeotropes, while pressure-minimum (temperature-maximum) azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature T_Az, pressure P_Az, and the vapor-phase composition y_1,Az, which is the same as the liquid-phase composition x_1,Az.

In the two-phase liquid-liquid region of partially miscible (heterogeneous) mixtures, the vapor pressure at constant T (or the boiling temperature at constant P) is independent of the global composition x₁ of the two coexisting liquid phases between the equilibrium compositions x₁′ and x₁″ (x₁′ < x₁″).

The constant vapor pressure (boiling temperature) above the two-phase region of certain partially miscible mixtures is usually larger (smaller) than the vapor pressure (boiling temperature) at any other liquid-phase composition in the homogeneous region. In this case, the vapor-phase composition is inside the miscibility gap. Mixtures of this type are called heteroazeotropic mixtures, or simply heteroazeotropes. (Figure 1, Type II), as opposed to the other types of azeotropes, called homoazeotropes.

Only in a few cases partially miscible mixtures present a positive or negative azeotropic point in the single-phase region, outside the miscibility gap, similar to the azeotropic points of homogeneous mixtures (Figure 1, Types IV and VI).

A few binary mixtures, for example the system perfluorobenzene + benzene, may present two azeotropic points at constant temperature (pressure), a positive and a negative one. They are called double azeotropic mixtures, or simply double azeotropes. (Figure 1, Type V).

The knowledge of the occurrence of azeotropic points in binary and higher systems is of special importance for the design of distillation processes. The number of theoretical stages of a distillation column required for the separation depends on the separation factor α₁₂, i.e., the ratio of the K_i -factors (K_i = y_i/x_i ) of the components i (i = 1, 2). The required separation factor can be calculated with the following simplified relation (Ref. 1)

α₁₂ = K₁/K₂ = (y₁/x₁)/(y₂/x₂) = (γ₁P₁^s)/(γ₂P₂^s)      (1)

where γ_i is the activity coefficient of component i in the liquid phase and P_i^s is the vapor pressure of the pure component i.

In distillation processes, only the difference between the separation factor and unity (α₁₂ – 1) can be exploited for the separation. If the separation factor is close to unity, a large number of theoretical stages is required for the separation. If the binary system to be separated shows an azeotropic point (α₁₂ = 1), the separation is impossible by ordinary distillation, even with an infinitely large number of stages.

Following eq. (1) azeotropic behavior will always occur in homogeneous binary systems when the vapor pressure ratio P₁^s /P₂^s is equal to the ratio of the activity coefficients γ₂/γ₁.

Various thermodynamic methods based on g^E—models (Wilson, NRTL, UNIQUAC) or group contribution methods (UNIFAC, modified UNIFAC, ASOG, PSRK) can be used for either calculating or predicting the required activity coefficients for the components under given conditions of temperature and composition (Ref. 2).

Because of the importance of azeotropic data for the design of distillation processes, compilations have been available in book form for quite some time (Refs. 3-7). The most recent printed data collection was published in 1994 (Ref. 8). A revised and extended version appeared in 2004 (Ref. 9).

A collection of approximately 47,400 zeotropic and azeotropic data sets, compiled from 6600 references, are stored in a comprehensive computerized data bank (Ref. 10). The references from the above-mentioned compilations and from the vapor-liquid equilibrium part of the Dortmund Data Bank (Ref. 11) were supplemented by references found from CAS online searches, private communications, data from industry, etc. Over 24,000 zeotropic data and over 20,000 azeotropic data are available for binary systems. Nearly 90% of the binary azeotropic data show a pressure maximum. In most cases (ca. 90%) these are homogeneous azeotropes, and in approximately 7–8% of the cases heterogeneous azeotropes are reported. Less than 10% of the data stored show a pressure minimum. Approximately 21,000 of the datasets stored were published after 1970.

The table below provides information about azeotropes for selected binary systems. Mixtures are listed alphabetically by the name of the first component, followed by the name of the second component. For convenience in searching, each row is duplicated with the components reversed. 

Column headings for the table are as follows.

The explanation of the type of azeotrope is given by the following codes.

O: Homogeneous azeotrope in a completely miscible system
L: Homogeneous azeotrope in a partially miscible system
E: Heterogeneous azeotrope
X: Pressure maximum
N: Pressure minimum
D: Double azeotrope
C: System contains a supercritical compound

References

1. Gmehling, J. and Brehm, A., Grundoperationen, Thieme-Verlag, Stuttgart, 1996.
2. Gmehling, J. and Kolbe, B., Thermodynamik, VCH-Verlag, Weinheim, 1992.
3. Lecat, M., Doctoral Dissertation, 1908.
4. Lecat, M., L’Azeotropisme, Monograph, L’Auteur, Brussel, 1918.
5. Lecat, M., Tables Azeotropiques, Monograph, Lamertin, Brussel 1949.
6. Ogorodnikov, S. K., Lesteva, T. M., and Kogan V. B., Azeotropic Mixtures, Khimia, Leningrad, 1971.
7. Horsley, L. H., Azeotropic Data III, American Chemical Society, Washington, 1973. [https://doi.org/10.1021/ba-1973-0116]
8. Gmehling, J., Menke, J., Krafczyk, J., and Fischer, K., Azeotropic Data, 2 Volumes, VCH Verlag, Weinheim, 1994.
9. Gmehling, J., Menke, J., Krafczyk, J., and Fischer, K., Azeotropic Data, Second Edition, 3 Volumes, VCH Verlag, Weinheim, 2004.
10. Gmehling, J., Menke, J., Krafczyk, J., and Fischer, K., A Data Bank for Azeotropic Data, Status and Applications, Fluid Phase Equilib. 103, 51, 1995. [https://doi.org/10.1016/0378-3812(94)02569-M]
11. Dortmund Data Bank, http://www.ddbst.de

Azeotropic Binary Mixtures: Temperature, Pressure, and Composition