## Binary Vle Phase Diagrams

Two types of vapor-liquid equilibrium diagrams are widely used to represent data for two-component (binary) systems. The first is a "temperature versus x and y" diagram (Txy). The x term represents the liquid composition, usually expressed in terms of mole fraction. The y term represents the vapor composition. The second diagram is a plot of x versus y.

These types of diagrams are generated at a constant pressure. Since the pressure in a distillation column is relatively constant in most columns (the exception is vacuum distillation, in which the pressures at the top and bottom are significantly different in terms of absolute pressure level), a Txy diagram, and an xy diagram are convenient for the analysis of binary distillation systems.

Figure 1.2 gives the Txy diagram for the benzene/toluene system at a pressure of 1 atm. The abscissa shows the mole fraction of benzene; the ordinate, temperature. The lower curve is the "saturated liquid" line, which gives the mole fraction of benzene in the liquid phase x. The upper curve is the "saturated vapor" line, which gives the mole fraction of benzene in the vapor phase y. Drawing a horizontal line at some temperature and reading off the intersection of this line with the two curves give the compositions of the two phases. For example, at 370 K the value of x is 0.375 mole fraction benzene and the value of y is 0.586 mole fraction benzene. As expected, the vapor is richer in the lighter component.

At the leftmost point we have pure toluene (0 mole fraction benzene), so the boiling point of toluene at 1 atm can be read from the diagram (384.7 K). At the rightmost point we have pure benzene (1 mole fraction benzene), so the boiling point of benzene at 1 atm can be read from the diagram (353.0 K). In the region between the curves, there are two phases; in the region above the saturated vapor curve, there is only a single "superheated" vapor phase; in the region below the saturated liquid curve, there is only a single "subcooled" liquid phase. Figure 1.2 Txy diagram for benzene and toluene at 1 atm. Figure 1.3 Specifying Txy diagram parameters.

The diagram is easily generated in Aspen Plus by going to Tools on the upper toolbar and selecting Analysis, Property, and Binary. The window shown in Figure 1.3 opens and specifies the type of diagram and the pressure. Then we click the Go button.

The pressure in the Txy diagram given in Figure 1.2 is 1 atm. Results at several pressures can also be generated as illustrated in Figure 1.4. The higher the pressure, the higher the temperatures. Figure 1.4 Txy diagrams at two pressures. Figure 1.5 Using Plot Wizard to generate xy diagram. Figure 1.6 Using Plot Wizard to generate xy diagram.

Y-x for BENZENE/TOLUENE

Y-x for BENZENE/TOLUENE 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Liquid Molefrac BENZENE

Figure 1.7 xy diagram for benzene/toluene.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Liquid Molefrac BENZENE

Figure 1.7 xy diagram for benzene/toluene.

The other type of diagram, an xy diagram, is generated in Aspen Plus by clicking the Plot Wizard button at the bottom of the Binary Analysis Results window that also opens when the Go button is clicked to generate the Txy diagram. As shown in Figure 1.5, this window also gives a table of detailed information. The window shown in Figure 1.6 opens, and YX picture is selected. Clicking the Next and Finish buttons generates the xy diagram shown in Figure 1.7.

Y-x for PROPYLEN/PROPANE

Y-x for PROPYLEN/PROPANE Figure 1.8 xy diagram for propylene/propane.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Liquid Molefrac PROPYLEN

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Liquid Molefrac PROPYLEN

Figure 1.8 xy diagram for propylene/propane.

Figure 1.8 gives an xy diagram for the propylene/propane system. These components have boiling points that are quite close, which leads to a very difficult separation.

These diagrams provide valuable insight into the VLE of binary systems. They can be used for quantitative analysis of distillation columns, as we will demonstrate in Chapter 2. Three-component ternary systems can also be represented graphically, as discussed in Section 1.6.

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### Responses

• celendine
How to read a txy diagram?
8 years ago
• MEERI
How to read boiling point diagram?
8 years ago
• Aida
How to plot txy diagram?
8 years ago
• kathrin
How to read vapor liquid phase diagram?
8 years ago
• janet
How to read vaporliquid phase diagrams?
7 years ago
• Rosa
How to construct VLE diagram with ASPEN PLUS?
7 years ago
• Alice
How to generate VLE data using aspen plus?
7 years ago