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 entrainer.

• 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 separated.

• 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.

Applications

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

Differences

• 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 substance.

• 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.