Azeotropic ternary diagrams

The simplest form of ternary RCM, as exemplified for the ideal normal-paraffin system of pentane-hexane-heptane, is illustrated in Fig. 13-58 7, using a right-triangle diagram. Maps for all other non-azeotropic ternary mixtures are qiiahtatively similar. Each of the infinite number of possible residue curves originates at the pentane vertex, travels toward and then away from the hexane vertex, and terminates at the heptane vertex. 

If the azeotrope is not sensitive to changes in pressure, then an entrainer can be added to the distillation to alter in a favorable way the relative volatility of the key components. Before the separation of an azeotropic mixture using an entrainer is considered, the representation of azeotropic distillation in ternary diagrams needs to be introduced. 

Petlyuk FB, Kievskii VY and Serafimov LA (1975) Thermodynamic and Topological Analysis of the Phase Diagrams of Polyazeotropic Mixtures II. Algorithm for Construction of Structural Graphs for Azeotropic Ternary Mixtures, Russ J Phys Chem, 49 1836. 

We predicted their behavior earlier using infinite-dilution /f-values, with the results at 1 atm shown in Table VIII. Only the acetone and chloroform appear to display azeotropic behavior. With this information and that for pure species boiling points at the pressure of interest, we can sketch the ternary diagram for this mixture. We can also use a computer code to generate it, which was done for Fig. 25. We see that there is one maximum-boiling azeotrope between acetone and chloroform. 

The separation process depends on the nature of the vapor-liquid equilibrium relationships of the system, which can be represented on a ternary diagram. Figure 10.3a shows a ternary diagram at some fixed system pressure. Components A and B are close boilers, and A forms an azeotrope with the entrainer E. The curves in the triangle represent liquid isotherms. A corresponding vapor isotherm (not shown) could be drawn to represent the vapor at equilibrium with each liquid curve with tie lines joining vapor and liquid compositions at equilibrium. The temperature of the isotherms reaches a minimum at point Z that corresponds to the composition of the azeotrope formed between A and E. 

Figure 10.4b shows a ternary diagram of such a system. Entrainer E forms azeotropes... 

The composition of the ternary azeotrope and that of the feed determine the relative rates of entrainer and feed required to produce a pure product in the bottoms and the azeotropic composition in the overhead. Figure 10.5 shows a typical ternary diagram for ABE at expected process temperature and pressure. Point F represents the fresh feed, which is a binary mixture of A and B. Point Z represents the ternary azeotropic composition. A straight line drawn through E and F represents compositions obtained by mixing fresh feed with variable amounts of entrainer. The combined feed composition should be such that it would separate into pure component A and the ternary azeotrope. Therefore, the combined feed composition should fall on a straight line joining A and Z. The amount of entrainer added to the fresh feed should yield the composition represented by the intersection point F. ... 

In this process the entrainer is added as redux to the azeotropic column. The ternary azeotrope is taken as column overhead and condensed, whereupon it separates in the receiver into two liquid phases, one rich in the entrainer and the other rich in component B. Compositions below the curve in the ternary diagram (Figure 10.5) form two coexisting liquid phases, and those above the curve form one liquid phase. The tie lines connect compositions in the two liquid phases at equilibrium with each other. 

FIGURE 12.14 Ternary diagram of a system containing three binary azeotropes and one ternary azeotrope. Arrows denote direction the distillation will proceed in, based on starting mixtures. (J. Stichlmair, J. R. Fair, 1998. Distillation—Principles and Practices. New York Wiley-VCH.)... 

The ternary system of methyl acetate, methanol, and water illustrates the occurrence of a distillation boundary. Figures 1.27 and 1.28 give the binary azeotropes of the system at 20psia using NRTL physical properties. Figure 1.29 shows the ternary diagram with a distillation boundary. 

A more complex system is shown in Figures 1.30 and 1.31. Ethanol, water, and benzene display three binary azeotropes and one ternary azeotrope. The resulting ternary diagram (Fig. 1.31) has three distillation boundaries that separate the ternary space into three regions. 

Figure 5.3 gives the ternary diagram for the system. The acetone/methanol binary azeotrope is shown on the ordinate axis. The residue curves originate from this minimumboiling azeotrope and move to the heaviest component DMSO comer. Figure 5.4 shows the location of the distillate (D) and bottoms (B) products. The straight component-balance... 

Figure 9.13 gives a ternary diagram for the isopentane-methanol-TAME system at 4 bar. The phase equilibrium of this system is complex because of the existence of azeotropes. The UNIFAC physical property package in Aspen Plus is used to model the VLB in all units except the methanol/water column where the van Laar equations are used because of their ability to accurately match the experimental data. 

A pioneering paper by Ryan and Doherty explored several alternative heterogeneous azeotropic configurations using benzene as the entrainer. They examined two- and three-column flowsheets and concluded that the three-column flowsheet with a preconcentrator (beer still), an azeotropic column, and a recovery column was the economic optimum. They used approximate ternary diagram methods of analysis. 

The earhest book that concentrated exclusively on extractive and azeotropic distillation was published in 1964 by E. J. Hoffman. Ternary diagrams for many azeotropic systems were discussed, including the concepts of residue curves, and alternative flowsheet configurations were presented. 

A typically way to separate a binary mixture containing an azeotrope is to add a third component to the system. Aspen can easily generate the following useful ternary diagrams for the purpose of analysis and design. 

Figure 8.10 illustrates the ternary diagram for this system. The two binary azeotropes produce a distillation boundary. Residue curves are shown, which indicate that a column operating in the lower region will have a bottoms product that is mostly TAME and a distillate product that is somewhere near the iCs/methanol azeotrope. A column operating in the upper region will have a bottoms product that is mostly methanol and a distillate product that is somewhere near the iCs/methanol azeotrope. 

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