Azeotropic and Extractive Distillation – Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Azeotropic and Extractive Distillation

• An azeotrope is a mixture of two or more liquids
(chemicals) in such a ratio that its composition cannot be changed by simple
distillation. This occurs because, when an azeotrope is boiled, the
resulting vapor has the same ratio of constituents as the original mixture.

• Because their composition is unchanged by
distillation, azeotropes are also called constant boiling mixtures.

• Positive azeotrope is 95.63 % ethanol and 4.37 % water
(by weight). Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope
boils at 78.2 °C, which is lower than either of its constituents.

• Negative azeotrope is hydrochloric acid at a
concentration of 20.2 % and 79.8 % water (by weight). Hydrogen chloride boils
at -84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher
than either of its constituents.

Now if in distillation azeotrop forms then we can’t separate
them so we add foreign material which is known as ENTRAINER. We choose
the entrainer in such a manner so that it can make a minimum boiling azeotrope
with one of the present component.

For Ex:- A+B make a azeotropes and we use C as a

• C makes a minimum boiling azeotropes with A

• Now if we start distillation then C+A behave as a one
component and B as other

• So we find C+A in distillate because it makes a minimum
boiling azeotropes and B as a bottom product

• So in this way you can separate azeotropes by distillation

For example:

• Benzene is added to the azeotropic mixture of water and
ethyl alcohol. Benzene breaks the mixture water-ethyl alcohol and form new
azeotrope between benzene and ethyl alcohol.

• The volatility of water is enhanced. On distillation,
water distills at 65.85 °C leaving EA and benzene behind.

• Boiling point of this binary mixture is 68.2 °C and
benzene gets distilled leaving pure alcohol behind. It can be distilled off at
78.3 °C.

• In Extractive distillation, the third substance
added to the azeotropic mixture is miscible, has high boiling point and
relatively non-volatile liquid compared to the components to be

• The solvent interacts with the components of the mixture
thereby causing their relative volatilities to change.

• Example:

• Separation of toluene from paraffin hydrocarbons of
approximately same molecular weights. The separation of toluene and isooctane
(example of hydrocarbon) is difficult.

• In the presence of phenol, the relative volatility of
isooctane increases, therefore, separation of toluene is relatively easy.


The liquor from fermentation process is a common source of
ethanol and contains approximately 8–10%. Petroleum refineries and distilleries
use these types of distillation


• Azeotropic distillation is a process of distillation
wherein you can add a certain component into the mixture to have a better
separation process. Usually, the certain component added into the mixture is
water or benzene, because these can help in increasing the volatility of a

• Extractive distillation is a distillation technique
wherein the capability of mixing or miscibility, the component of being
non-volatile, and even a high boiling point, could be the measurement of
separating a mixture without even forming an azeotrope. This kind of method is
usually used for mixtures with almost the same volatility.


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