9459

Volatility Inversion in Extractive Distillation

Consider an extractive distillation column operating at atmospheric pressure with 43 stages, a partial reboiler, and a total condenser. It is used to separate acetone and methanol using chlorobenzene as an entrainer.
Pure entrainer at is fed to the column at a flow rate, , to be set by the user at stage 17 (counting from the top). The lower feed is composed of an equimolar mixture of acetone and methanol. The lower feed, at , is located at stage 33 (counting from the top) and has a flow rate equal to .
The Demonstration solves the MESH equations (mass,equilibrium, summation, and heat) and displays the composition and temperature profiles for user-set values of the reflux ratio, . The second degree of freedom is set by taking a distillate flow rate equal to .
For this particular entrainer selection, one observes a phenomenon called volatility inversion. Indeed, methanol (b.p. ) exits the extractive distillation column as a pure distillate product, while the more volatile component acetone (b.p. ) leaves accompanied by the entrainer (i.e., chlorobenzene) at the bottom of the column.
Finally, a comparison of the present calculation with Aspen HYSYS shows excellent agreement.

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DETAILS

The 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.
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.
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