Analysis of an Extractive Distillation Column with Three Design Variables

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Consider an extractive distillation column operating at atmospheric pressure with 55 stages, a partial reboiler, and a total condenser. It is used to separate an acetone/methanol azeotrope using water as an entrainer.

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Pure entrainer is fed to the column at stage 24. (We count the stages from the top.) The lower feed is composed of an equimolar mixture of acetone and methanol and is fed to the column at stage 39. The lower feed flow rate is set to .

This Demonstration solves the MESH equations (material, equilibria, summations, and heat) for the distillation column, and displays the composition and temperature profiles for user-set values of the reflux ratio, the entrainer feed temperature, and the entrainer feed flow rate. The distillate flow rate is set equal to .

The Demonstration gives the composition and temperature profiles in the three sections of the extractive distillation column, which are the rectifying, extractive, and stripping sections (shown by the green, red, and blue panes, respectively). The acetone, methanol, and water compositions are indicated in orange, green, and brown, respectively. The calculation shows excellent agreement with results obtained with Aspen HYSYS,a commercial simulation package.

The performance of the column is displayed using contour plots of constant acetone purity in the distillate stream in the plane of two of the three process variables: the reflux ratio, the entrainer feed temperature, and the entrainer feed flow rate.

One important result (seen in the first contour plot tab) is that there is an optimum finite value of the reflux ratio at a given entrainer feed temperature. That is, the separation is not improved by simply increasing the reflux ratio. Indeed, separation may even become unfeasible at infinite reflux in some cases.

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Contributed by: Housam Binous, Naim Faqir, Adamu Sagir, and Brian G. Higgins (November 2012)
Open content licensed under CC BY-NC-SA


Snapshots


Details

Expressions for pure component vapor and liquid enthalpies were adapted from Aspen HYSYS.

The mixture is assumed to obey modified Raoult's law, and activity coefficients are predicted using the Wilson model [4].

References

[1] M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, Boston: McGraw-Hill, 2001.

[2] E. J. Henley and J. D. Seader, Equilibrium-Stage Separation Operations in Chemical Engineering, New York: Wiley, 1981.

[3] W. L. Luyben and I.-L. Chien, Design and Control of Distillation Systems for Separating Azeotropes, Hoboken, NJ: Wiley, 2010.

[4] G. M. Wilson, "Vapor-Liquid Equilibrium XI: A New Expression for the Excess Free Energy of Mixing," Journal of the American Chemical Society, 86(2), 1964 pp. 127–130.



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