More Complex Distillation Systems

In the example distillation system considered in Chapters 3 and 4 we studied the binary propane/isobutane separation in a single distillation column. This is a fairly ideal system from the standpoint of vapor-liquid equilibrium, and it has only two components, a single feed and two product streams. In this chapter we will show that the steady-state simulation methods can be extended to multicomponent nonideal systems and to more complex column configurations.

Other methods of analysis can also be applied to these more complex systems. In particular, we will find that the use of ternary diagrams provides very useful insight into the design of these complex systems. In addition, the effects of various design parameters in these nonideal systems are sometimes counterintuitive and significantly different from those in an ideal system.

5.1 METHYL ACETATE/METHANOL/WATER SYSTEM

We start with a ternary system that is fairly nonideal. Mixtures of methyl acetate, methanol, and water are generated in the production of polyvinyl alcohol. Both the methyl acetate and the methanol must be recovered for recycling or for further processing.

5.1.1 Vapor-Liquid Equilibrium

Methyl acetate and methanol form a homogeneous minimum-boiling azeotrope at 1.1 atm with a composition of 66.4 mol% methyl acetate and a temperature of 329 K. This means that the overhead product from a distillation column separating this binary mixture cannot have a methyl acetate composition greater than this azeotropic composition. In addition,

Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.

Distillation Composition ControlAspen Plus Azeotrope
Figure 5.1 (a) Opening binary analysis; (b) selecting type of analysis.

methyl acetate and water form a homogeneous minimum-boiling azeotrope at 1.1 atm with a composition of 88.84 mol% methyl acetate and a temperature of 333.8 K.

As discussed in Chapter 1, Aspen Plus has several nice analysis tools. To look at binary VLE, go to the top toolbar and click Tools, Analysis, Property, and Binary. The window shown in Figure 5.1a opens, and clicking OK opens the window shown in Figure 5.1b, on which we can specify the type of analysis. The dropdown menu at the top left gives three choices. Since distillation columns run at fixed pressures, the Txy diagram is the most appropriate.

We set the pressure at 1.1 atm (see Fig. 5.2) because, as we will find out shortly, this pressure will give a reflux drum temperature (325 K) that is high enough to permit the use of inexpensive cooling water in the condenser. Next we select the two components of interest, MEAC and MEOH, by using the dropdown menu arrows. Note that the Wilson physical property package has been specified. Clicking the Go button at the bottom of the window produces the Txy diagram shown in Figure 5.3. The minimum-boiling homogeneous azeotrope at 66.4 mol% methyl acetate and a temperature of 329 K is shown. Repeating for the other two binary pairs produces the Txy diagrams shown in

Txy Diagram
Figure 5.2 Specifying pressure.
Methyl Acetate And Water Diagram

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Liquid/Vapor Molefrac MEAC

Figure 5.3 Txy diagram for methyl acetate/methanol.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Liquid/Vapor Molefrac MEAC

Figure 5.3 Txy diagram for methyl acetate/methanol.

Figure 5.4 Txy diagram for methyl acetate/water.

Figures 5.4 and 5.5. The pure component boiling points at 1.1 atm can be found by referring to the table in Binary Analysis Results. Methyl acetate is 332.13 K, methanol is 340.13 K and water is 375.86 K.

Vapor-liquid equilibrium information for the ternary system can be seen by going up to the top toolbar and clicking Tools, Analysis, Properties, and Residue. The window shown in Figure 5.6a opens, and pressure and the three components of interest are specified. Clicking the Go button at the bottom of the window produces an equilateral ternary

Txy Diagram
Figure 5.5 Txy diagram for methanol/water.
Methanol Water Equilibrium

lb) Residue curve for ME AC/ME OH/WATER

Ternary Azeotropes

Figure 5.6 (a) Ternary residue curve analysis; (b) residue curves.

Azeotrope

Figure 5.6 (a) Ternary residue curve analysis; (b) residue curves.

diagram that shows the residue curves (see Fig. 5.6b). The residue curve lines lead from the minimum-boiling azeotrope to the highest-boiling component, which is water. Note the distillation boundary that runs between the two azeotropes.

As discussed in Chapter 1, Aspen Split provides additional insight. Click on Tools, Analysis, and Aspen Split. Two choices can be made: Azeotropic Search and Ternary Diagrams. Selecting the first opens the window shown in Figure 5.7a, where the pressure and components are specified. Clicking on Azeotropes in the list on the far left of the window under Output produces the results shown in Figure 5.7b. Clicking Report gives information about the two azeotropes (Fig. 5.7c).

The second alternative in Aspen Split is to select Ternary Diagrams. This opens the window shown in Figure 5.8a, on which the pressure and the three components are specified. Then clicking Ternary Plot at the bottom of the list on the left side of the window opens a right triangle, which displays boiling points of the pure components, the compositions and temperatures of the azeotropes, and the distillation boundaries (see Fig. 5.8b).

To add the residue curve lines, right-click on the graph, select Add and Curve. The cursor becomes a cross. Move it to a spot on the diagram and left-click. A residue curve through that point is drawn. Figure 5.8c gives the ternary diagram with four residue curves added. Note that a grid has also been added. This is achieved by clicking the bottom icon on the far right that is Toggle Grid.

Figure 5.7 (a) Azeotropic search; (b) azeotropic results; (c) report.

104 MORE COMPLEX DISTILLATION SYSTEMS (C) Physical Property Model: WILSON Valid Phase: VAP-LIQ

Mixture Investigated For Azeotropes At A Pressure Of 1.1 ATM

Comp ID

Component Name

Classification

Temperature

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  • prima gardner
    How to draw txy diagram for methanol water?
    8 years ago

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