Extractive Distillation

Extractive distillation is defined as distillation in the present of a miscible, high boiling, relatively nonvolatile component, the solvent, that forms no azeotropes with the other components in the mixture. It is widely used in the chemical and petrochemical industries for separating azeotropic, close-boiling, and others low relative volatility mixture.

Extractive distillation works because the solvent is specially chosen to interact differently with the components of the original mixture, thereby altering their relative volatilities. Because these interactions occur predominantly in the liquid phase , the solvent is continuously added near the top of the extractive distillation column so that an appreciable amount is present in the liquid phase on all of the trays below. The mixture to be separated is added through second feed point further down the column. In the extractive column, the component having the greater volatility, not necessarily the component having the lowest boiling point, is taken overhead as a relatively pure distillate. The other component leaves with the solvent via the column bottoms. The solvent is separated from the remaining components in a second distillation column and then recycled back to the first column.

There are several industrial application for homogeneous azeotropic distillation listed in the Encyclopedia of Separation Technology by Michael F. D., Jeffrey P. K.

Extractive distillations can be divided into three categories, each correspond to the different residue curve maps, the minimum boiling azeotropes, maximum boiling azeotropes and the nonazeotrope mixtures. Since our benzene-cyclohexane mixture to be separated is of the second type of mixture, i.e. the minimum boiling azeotrope, we will focus our attention on column sequencing this type of azeotropic separation method in the following section.

As in azeotropic distillation, design of extractive distillation system will also requires significant preliminary work including:

●      Choosing the solvent

●      Developing or finding necessary data, such as azeotropic condition or residue curve

●      Preliminary screening

●      Computer simulation

●      Small scale testing

For our example, we will consider the first four steps.