Economic Extractive distillation (Definition, categories, Good Solvent selection criteria, Solvent screening)
Extractive distillation is defined as distillation in the present of a miscible, high boiling, relatively non-volatile 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 volatility.
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.
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Fig. 2 Extractive distillation with a heavy solvent which introduce no new azeotrope for a minimum boiling azeotrope. In some case, B can come off the top of 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,
- Minimum boiling azeotropes,
- Maximum boiling azeotropes
- Nonazeotrope mixtures.