Azeotropic Distillation | With Two Examples and Applications

You might have gone through different types of distillation techniques. Of them azeotropic distillation is quite interesting one.

This method is used to separate azeotropic solutions. As per Wikipedia, an azeotrope is a mixture which cannot be separated by simple distillation.

In general, most liquid mixtures can be separated by fractional distillation (FD). During the fractional distillation, one of the components gets into the vapor phase. This vapor is separated, cooled and collected in a receiver. The other components of the mixture remain back.

But this does not happen with azeotropic solutions. When these solutions are subjected to FD, both mixtures are present in the vapor form. Hence, they cannot be separated. This is due to similar vapor pressures of liquids. To solve this problem, the azeotropic distillation technique is employed.

Here, to an azeotropic solution a third substance is added. This addition changes the relative volatility of the components. For example, to a mixture of water and alcohol (called rectified spirit) glycerin is added. The addition of glycerin forms a new azeotrope between water and glycerin. This is because both are more polar than ethyl alcohol.

The result is that the volatility of ethyl alcohol is enhanced. Hence, it can be distilled out separately leaving behind water and glycerin.

Instead, if you add benzene to the above mixture, it forms a new azeotrope with ethyl alcohol. So, water can be removed from the mixture. Later, even the azeotrope breaks and you can remove benzene to get absolute alcohol.

This distillation requires a simple distillation apparatus. Hence, the method is less expensive and also faster.

Procedure of distillation

1. Take 100ml of rectified spirit and determine the concentration of water in it. Also, note down it refractive index

2. Add 5ml of glycerine to the above solution and transfer the entire mixture to round bottom flask.

3. Assemble the flask to the simple distillation apparatus as shown below.

Azeotropic Distillation - Instrument or apparatus

4. Heat the mixture and note the temperature of alcohol distillation.

5. The alcohol vapors rise into the condenser and then cool down separately into receiver flask.

6. Note down the refractive index of absolute alcohol to determine the percentage.

Applications of azeotropic distillation

Azeotropic distillation is mainly used for

1. Preparation of Absolute alcohol. Alcohol is available in different concentration like 70%, 90% in combination with water. Absolute alcohol is 100% alcohol without any parts of water.

2. This distillation is also used in petroleum refineries.

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