Extractive distillation

The extractive rectification is a method in chemical process technology for the separation of azeotropic or close-boiling mixtures. These mixtures with low relative volatility of the components involved can be described by normal rectification either did not separate ( azeotropes ) or only with disproportionate effort.

Method

The extractive uses of an additive, the so-called entraining agent that selectively increases the volatility of a component or significantly alters the activity coefficients of the substances to be separated in different directions and thus the separation factor

γ: activity coefficient Ps: saturation vapor pressure of the pure substance

Changed and as far away from 1 (= azeotrope ). The entraining agent should also not introduce new azeotrope, so none of the form an azeotrope substances to be separated, and it should have a higher boiling point or vapor pressure having a significantly lower than the substances to be separated. Thus, the additive can be removed with any of the substances to be separated on the bottom of the column and a second column of this residue ( regenerated). The entraining agent is then fed back to the extraction column.

Example

A typical example is the separation of an extractive azeotropic mixture of benzene and cyclohexane. By the addition of significant quantities of an additive (in the example up to 40 mole percent of N-methyl- 2-pyrrolidone), the azeotrope removed ( liquid composition x = vapor composition y), and separation is made possible.

Benzene / cyclohexane 20 mole percent NMP

Benzene / cyclohexane 40 mole percent of NMP

Development of a technical system

Typically, the entrainer, so that usually is a high boiler is fed near the top of the column and the mixture to be separated into a lower bottom, in order to achieve the best possible mixing.

More basic requirement for the choice of a suitable entrainer for the extractive rectification is the complete miscibility with all components of the mixture to be separated. Also, the material should be inexpensive, available and non-toxic and have a low viscosity.

Entrainer selection

The selection of a suitable material, with the aid of factual databases for vapor -liquid equilibria and activity coefficients (in particular, activity coefficients at infinite dilution, γ ∞ ) be done. Ionic liquids can be used as an entraining agent used. For the selection of the entrainer - tailored to the separation problem - models for the estimation of activity coefficients, such as UNIFAC, used.