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Figure 8.3 Binary azeotropes.

Figure 8.4 Binary Txy diagrams.
Figure 8.6 The Txy diagram for iC5/methanol.
Figure 8.8 Activity coefficients for iC5/methanol.
Upper Region

Figure 8.10 Ternary diagram at 4 bar.

Figure 8.10 Ternary diagram at 4 bar.

Ternary Diagrams. Ternary diagrams are very useful in analyzing three-component systems and in helping to design columns, particularly for systems with complex vapor— liquid equilibrium. We illustrate some of their features in this section using the ternary iC5/ methanol/TAME system at 4 bar.

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 iC5/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 iC5/methanol azeotrope.

We can locate various streams on this ternary diagram. For example, plotting a point with coordinates zMeOH = 0.3 and ziC5 = 0.4 will indicate a feedstream to the column with these compositions. In the same way, the distillate and bottoms points can be located on the diagram. Because of the "ternary mixing rule," the feed coordinates must lie on a straight line connecting the distillate and bottoms coordinate points.

8.1.3 Conceptual Design

To generate ternary plots and to use them for design, Aspen Split is used. This software is imbedded in Aspen Plus and can be accessed by going to the toolbar and clicking Library and References. The window shown in Figure 8.11 opens in which the Aspen Split box should be checked. A new page tab will appear at the bottom of the process flow diagram next to those of the standard unit operation models, which is shown in

Figure 8.11 Accessing the Aspen Split library.

Figure 8.12. Selecting this page tab reveals the icon ConSep. Dragging and dropping this icon onto the process flowsheet window places a block there, which we labeled "Ternary." Feed and product streams are inserted in the normal way using Material STREAMS. The conditions of the feed are specified, as shown in Figure 8.13. The feed is 40 mol% iC5, 30 mol% methanol, and 30 mol% TAME in this illustrative example.

Next we open the Ternary block and select Inputs (Fig. 8.14). The components, pressure, and physical property package are specified. The Design mode is selected on the right side of the window. Then, the blue "N " (next) button is clicked to run the program. The window shown in Figure 8.15 opens up showing the Setup view. Selecting Analysis opens the window in Figure 8.16 in which the three boxes are checked to generate residue curves and distillation boundaries and to show the locations of azeotropes.

Selecting Specifications opens the window in Figure 8.17. There are 4 degrees of freedom. One of these is the reflux ratio, which is specified to be 5. That leaves three compositions that can be specified. Let us assume that we are trying to produce fairly pure TAME out the bottom of the column, so we specify that the bottoms composition is 0.1 mol% iC5 and 1 mol% methanol. Likewise, we want to lose very little TAME in the distillate, so we specify that the distillate is 0.1 mol% TAME. These are preliminary guesses that may or may not be attainable.

Finally, the Calculate button at the top of the window is clicked. If the overall component balances are not feasible with the compositions selected and with the given feed, an error message is displayed. If the balances are feasible, the window shown in Figure 8.18 opens. The program performs tray to tray calculations, starting from each

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Figure 8.12 Conceptual design page tab.

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