Vapor-liquid equilibrium heterogeneous azeotrope

FIG. 13-8 Vapor-liquid equilibrium data for an n-butanol-water system at 101.3 kPa (1 atm) phase splitting and heterogeneous-azeotrope formation. 

A collection of approximately 47,400 zeotropic and azeotropic data sets, compiled from 6600 references, are stored in a comprehensive computerized data bank (Reference 10). The references from the above-mentioned compilations and from the vapor-liquid equilibrium part of the Dortmund Data Bank (Reference 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 data sets stored were published after 1970. 

An extensive tabulation of azeotropes has been compiled by Horsley. An older compilation is that of Lecat. Certain nonideal vapor-liquid equilibrium models are useful for predicting azeotropic behavior of binary systems in particular, the model of Renon and Prausnitz is useful in this regard because it can handle the two liquid phases associated with heterogeneous azeotropes. The Horsley book also contains guidelines for the prediction of azeotrr res. 

Three types of binary equilibrium cui ves are shown in Fig. 13-27. The y-x diagram is almost always plotted for the component that is the more volatile (denoted by the subscript 1) in the region where distillation is to take place. Cui ve A shows the most usual case, in which component 1 remains more volatile over the entire composition range. Cui ve B is typical of many systems (ethanol-water, for example) in which the component that is more volatile at lowvalues of X becomes less volatile than the other component at high values of X. The vapor and liquid compositions are identical for the homogeneous azeotrope where cui ve B crosses the 45° diagonal. A heterogeneous azeotrope is formed with two liquid phases by cui ve C,... 

Figure 8-7. System with heterogeneous azeotrope-two liquid phases in the equilibrium with one vapor phase. Used by permission. Smith, B.D., Design of Equilibrium Stage Processes, McGraw-Hiii, New York (1963), all rights reserved.

FIG. 13-18 Typical binary equilibrium curves. Curve A, system with normal volatility. Curve B, system with homogeneous azeotrope (one liquid phase). Curve C, system with heterogeneous azeotrope (two liquid phases in equilibrium with one vapor phase). 

The formation of azeotropes due to deviations from Raoult s law was discussed in Section 1.3. An azeotrope is a mixture that, at a given pressure (the azeotropic pressure), boils at a constant temperature (the azeotropic temperature) and has the same composition (the azeotropic composition) in the equilibrium vapor and liquid phases. Homogeneous azeotropes are those that form one liquid phase at equilibrium with the vapor heterogeneous azeotropes are those that form two liquid phases at equilibrium with each other and the vapor. 

Homogeneous azeotropes occur in a great many binary mixtures, and tables of azeotropic temperatures, pressures, and compositions can be found in the compilations by Horsley [6] and by Gmehling et al. [7]. Such azeotropes occur when one vapor phase is in equilibrium with one liquid phase. In addition, extrema in isothermal Pxy and isobaric Txy diagrams occur in some three-phase VLLE situations for binary mixtures. These are called heterogeneous azeotropes. But at heterogeneous azeotropes the composition of the vapor is rarely the same as that of either liquid these situations are discussed in 9.3.7. 

This means that at any given pressure, such as P2 in Figure 9.15, three-phase VLLE occurs at only one temperature. That temperature identifies a heterogeneous azeotrope at the pressure P2 the azeotropic temperature is the lowest at which vapor can exist in equilibrium with liquid. At this T, boiling of the two-phase liquid will produce a vapor of fixed composition, regardless of the overall composition of the system. 

The liquid-phase nonideality is so large that a heterogeneous azeotrope is formed. The molecules are so dissimilar that two liquid phases are formed. The composition of the vapor is 75.17mol% water at 1 atm. The compositions of the two liquid phases that are in equilibrium with this vapor are 43.86 and 98.05 mol% water, respectively. 

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