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.