Definition and Detailed Explanation of Azeotropic Distillation

An azeotrope is a liquid mixture with an equilibrium vapour of the same composition as the liquid. The dew point and bubble point are identical in the azeotropic composition and the mixture. Close boiling components showing small deviations from ideality may form an azeotrope. An azeotrope exhibits a maximum Or a minimum boiling point relative to the boiling point of pure liquids and, thus classified as maximum boiling and minimum boiling azeotropes. 

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Azeotrope mixture be separated by distillation

Minimum boiling azeotrope will boil at a temperature higher than the boiling points of pure components. The temperature composition diagrams and equilibrium diagrams for liquid mixture forming azeotropes at constant pressure are shown below pictures. Here we will see that the equilibrium curve crosses the diagonal-which is an indication of the existence of an azeotrope, i. e. y = x, and the mixture will not be separated by ordinary distillation.

Minimum boiling azeotrope system carbon disulfide-acetone in one atmosphere

Maximum boiling azeotrope system chloroform-acetone at one-atmosphere pressure

Refer to Fig. For all the mixtures of composition less than P, the equilibrium vapour is richer in more volatile components than the liquid, while for all mixtures of composition greater than P the equilibrium vapour is less rich in more volatile components than the liquid. The mixture of composition P gives the vapour of composition identical to the liquid. In such cases, complete separation by ordinary fractionation may not be possible. The constituents of a binary azeotrope are separated completely by (1) adding a third component to the binary mixture and (2) changing the system pressure. 

Azeotrope Distillation Breaker

The third component (which is relatively volatile) added to a binary azeotrope usually forms a low-boiling azeotrope with one of the feed constituents and is withdrawn as the overhead product/distillate. The third component added to break the binary azeotrope to affect its separation into pure components is called the entertainer/azeotrope breaker. 

Azeotropic distillation ethanol-water benzene

In this process, the component added forms an azeotrope with one of the feed components and the azeotrope is withdrawn as either the overhead or bottom product.

Ethanol-water azeotrope 

A binary azeotrope system containing ethanol and water forms a minimum boiling azeotrope at 99% by weight ethanol and thus blocks the production of pure components by ordinary distillation. The separation into almost pure form is effected by adding benzene as an entertainer to this binary azeotrope. In the azeotrope distillation technique of this system, the alcohol-water mixture containing 96% alcohol is fed to a first column, benzene used as an entertainer forms a low boiling ternary azeotrope (containing benzene alcohol and water) which is removed as a top product and nearly pure alcohol is taken out as a bottom product. 

The overhead from the first column is condensed, and phase separation is achieved in a decanter. The benzene-rich phase is returned to the first distillation column and the water-rich phase is sent to a second distillation column. The overhead from the second column containing benzene in major proportion is fed back to the first column while the aqueous solution of alcohol-water azeotrope as an overhead product (to be mixed with the feed) and pure water is the bottom product. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane