Vle Nonideality

Liquid-phase ideality (activity coefficients g = 1) occurs only when the components are quite similar. The benzene/toluene system is a common example. As shown in the sixth and seventh columns in Figure 1.5, the activity coefficients of both benzene and toluene are very close to unity.

However, if components are dissimilar, nonideal behavior occurs. Consider a mixture of methanol and water. Water is very polar. Methanol is polar on the OH end of the molecule, but the CH3 end is nonpolar. This results in some nonideality. Figure 1.13a gives the xy curve at 1 atm. Figure 1.13b gives a table showing how the activity coefficients of the two components vary over composition space. The Unifac physical property method is used. The g values range up to 2.3 for methanol at the x = 0 limit and 1.66 for water at x = 1. A plot of the activity coefficients can be generated by selecting the Gamma picture when using the Plot Wizard. The resulting plot is given in Figure 1.13c.

Now consider a mixture of ethanol and water. The CH3-CH2 end of the ethanol molecule is more nonpolar than the CH3 end of methanol. We would expect the nonideality to be more pronounced, which is exactly what the Txy diagram, the activity coefficient results, and the xy diagram given in Figure 1.14 show.

Note that the activity coefficient of ethanol at the x = 0 end (pure water) is very large (gEtOH = 6.75) and also that the xy curve shown in Figure 1.14c crosses the 45° line (x = y) at ^90 mol% ethanol. This indicates the presence of an azeotrope. Note also that the temperature at the azeotrope (351.0 K) is lower than the boiling point of ethanol (351.5 K).

An "azeotrope" is defined as a composition at which the liquid and vapor compositions are equal. Obviously, when this occurs, there can be no change in the liquid and vapor compositions from tray to tray in a distillation column. Therefore an azeotrope represents a "distillation boundary."

Azeotropes occur in binary, ternary, and multicomponent systems. They can be "homogeneous" (single liquid phase) or "heterogeneous" (two liquid phases). They can be "minimum boiling" or "maximum boiling." The ethanol/water azeotrope is a minimum-boiling homogeneous azeotrope.

The software Aspen Split provides a convenient method for calculating azeotropes. Go to Tools on the top toolbar, then select Aspen Split and Azeotropic Search. The window shown at the top of Figure 1.15 opens, on which the components and pressure level are specified. Clicking on Azeotropes opens the window shown at the bottom of Figure 1.15, which gives the calculated results: a homogeneous azeotrope at 78°C (351 K) with composition 89.3 mol% ethanol.

Up to this point we have been using Split as an analysis tool. Aspen Technology plans to phase out Split in new releases of their Engineering Suite and will offer another tool called "DISTIL," which has more capability. To illustrate some of the features of DISTIL, let us use it to study a system in which there is more dissimilarity of the molecules

Txy Plot For Binary Mixture
Figure 1.13 (a) Txy diagram for methanol/water; (b) activity coefficients for methanol/water; (c) activity coefficient plot for methanol/water.
Methanol And Water Boiling Point
Figure 1.13 Continued.

by looking at the n-butanol/water system. The normal boiling point of n-butanol is 398 K, while that of water is 373 K, so water is the low boiler in this system.

The DISTIL program is opened in the usual way. Clicking the Managers button on the top toolbar of the DISTIL window and clicking Fluid Package Manager opens the window shown at the top of Figure 1.16. Click the Add button. The window shown at the bottom of Figure 1.16 opens. On the Property Package page tab, the UNIFAC VLE package is selected with Ideal Gas. Then click the Select button on the right side of the window.

Diagram Ethanol
Figure 1.14 Txy diagram for ethanol/water (a), activity coefficient plot (b), and xy plot (c) for ethanol/water.
Ethanol Water TxyEthanol Diagram

Next click the Components page tab. Enter the name of a component in the Matches window and click Select to add each component to list on the left, as shown at the top of Figure 1.17.

Now click the Manager button again on the top toolbar and select Separation Manager—Separation and Azeotrope Analysis, which opens the window shown at the bottom Figure 1.17. With the Fluid Package highlighted under the Setup column on the left, the physical property package to be used is selected from the dropdown menu, and the components of interest are selected by clicking under the Selected column to

Aspen Fluid Package Tab
Figure 1.15 Aspen Split ethanol/water.
Aspen Fluid Package Tab
Figure 1.16 Setting up fluid package in DISTIL.
Aspen Fluid Package Tab
Figure 1.17 Selecting components in DISTIL.

place a green checkmark, not a red "x." Note that there is a message in the yellow box at the bottom of the window stating that the pressure needs to be specified. Clicking Options under the Setup column on the left opens the window shown at the top of Figure 1.18, where the pressure or a range of pressures is specified. Then click the Calculate button at the bottom of the window. Selecting the Compositions page tab or the Boiling Points page tab gives the results shown in Figure 1.19. Note that the azeotropic temperature (93°C) is lower than the boiling points of pure water (100oC) or butanol (118°c).

Various types of diagrams can be generated by going to the top toolbar and selecting Managers, Thermodynamic Workbench Manager, and Phase Equilibrium Properties. The window given at the top of Figure 1.20a opens, on which the fluids package and component are selected. Clicking the Plots page tab opens the window shown at the bottom of Figure 1.20a, on which pressure is set and the type of plot is specified. A Txy diagram is selected. Figure 1.20b gives activity coefficient and xy plots.

These results show a huge activity coefficient for butanol (yBuOH = 40) at the x = 1 point. The horizontal lines in the Txy diagram and the xy diagram indicate the presence of an heterogeneous azeotrope. The molecules are so dissimilar that two liquid phases are formed. At the azeotrope, the vapor composition is ~76 mol% water and the compositions of the two liquid phases are ~ 50 and ~ 97 mol% water.

Ethanol Condenser
Figure 1.18 Specifying pressure.

Up to this point we have considered only binary systems. In the following section ternary nonideal systems are explored using the capabilities of Aspen Split.

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