Ethanol Dehydration

For our second nonideal system we study a process that has extremely nonideal VLE behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351 K of composition 90 mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems, as we will see. So this complex example is a challenging simulation case.

The origins of the example go back over a century when a process to produce high-purity ethanol was needed. Ethanol is widely produced in fermentation processes. A typical mixture from a fermentation process has very low ethanol concentrations (4-6 mol%). If this mixture is fed to a distillation column operating at atmospheric pressure, high-purity water can be produced out the bottom but the ethanol purity of the distillate cannot exceed 90 mol% because of the azeotrope.

Some ingenious engineers came up with the idea of running the fermentation liquid through a conventional "preconcentrator" distillation column that takes most of the water out the bottom and produces a distillate that is perhaps 84 mol% ethanol and 16 mol% water. This binary stream is then fed into a second distillation column. Also fed to the top of this column as reflux is a stream that contains a high concentration (80 mol%) of benzene. The benzene serves as a "light entrainer" that goes overhead and preferentially takes water with it because the large dissimilarity between water and benzene renders the water very volatile. The ethanol goes out the bottom of this column despite the fact that water is the "heavier" component (normal boiling point of ethanol is 351.5 K, while that of water is 373.2 K).

The overhead vapor is a ternary mixture of all three components. When it is condensed, the repulsion between the water molecules and the organic benzene molecules is so great that two liquid phases form. The reflux drum becomes a decanter. The lighter organic liquid phase is pumped back to the column as organic reflux. The heavier aqueous phase contains significant amounts of ethanol and benzene, so it is fed to a second distillation column in which the water is removed in the bottoms stream. The distillate stream is recycled back to the first column.

The first item to explore is the complex VLLE of this heterogeneous vapor-liquid-liquid system.

5.2.1 VLLE Behavior

The phase equilibrium is reported to be well described by the Uniquac physical property package. Two binary Txy diagrams are given in Figures 5.17a and 5.17b. The two homogeneous minimum-boiling azeotropes are clearly shown. The ternary diagram (Fig. 5.17c) is generated using Aspen Split as described earlier in this chapter. A large part of the composition space has two liquid phases. The liquid-liquid equilibrium tielines are shown. The aqueous phase is on the left, and the organic phase is on the right.

Figure 5.17d gives a report of all of the azeotropes in this very nonideal VLLE system. Note that the ternary azeotrope is heterogeneous and has the lowest boiling point (337.17 K) of any of the azeotropes or of the pure components. This means that the overhead vapor from the first column will have a composition that is similar to that of this azeo-trope. Note also that the diagram is split up into three regions by the distillation boundaries that connect the four azeotropes. As you may recall from Chapter 2, these boundaries limit the separation that is attainable in a single column. The bottoms and distillate points must lie in the same region.

Distillation Column Azeotrope

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 ETHAN-01

T-xy for BENZEOl/ETHAN-01 - Plot


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  • mewael dahlak
    Why ethanol goes to the bottom of column?
    8 years ago

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