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# THEORY OF FRACTIONAL DISTILLATION

The process of separation of one liquid from another liquid (binary mixture) having different boiling points by distillation is termed fractional distillation. The information, whether a particular solutions of two liquids can be separated by distillation o not, is provided by the study of liquid-vapour equilibrium diagrams at constant pressure, say atmospheric pressure. The separation is possible only when the vapour phase has a composition different from that of the boiling liquid mixture.

Liquid pairs of type-I (Ideal solutions): The boiling temperature composition curves for liquid and vapour phases in the case of binary solutions of first type are represented in Fig. 5.2.

Suppose a solution of composition x is heated. When the temperature Tx is reached, the boiling will start. At this temperature the vapour coming off from x will have the composition x1. Since x1 is richer in B than, x, the composition of the residual liquid will become richer will become richer in A. Let the new composition be y. This liquid cannot boil at temperature Tx but will require higher temperature Ty. The vapour coming off at temperature Ty will also be richer in B as represented by y1. Hence the composition of the residue will again be enriched in A. Thus, if the process is allowed to continue, the boiling point of pure liquid A. At the same time the solution becomes more and more richer in A. If the process is continued for a sufficiently long time, pure liquid A can be obtained.

Now, if the initial vapours are condensed, the solution obtained will have the composition x1 and boils at temperature Tz. Evidently, the distillate is richer in B than before. If the process of condensation and redistilling is continued, the final distillate will be pure B component.

Thus, the two liquids forming a solution of Type I can be separated by fractional distillation.

Solution of type-II (Positive deviations from Raoult’s law): The boiling temperature-composition curves of the liquid and vapour phases have been shown in Fig. 5.3. The two curves meet at a minimum point C where the liquid and vapour phases have the same composition. The liquid mixture at point C will boil at constant temperature T without undergoing any change in composition. Such a mixture which boils at constant temperature and distils over completely at the same temperature without any change in composition, is called constant boiling mixture or azeotropic mixture.

Consider the distillation of a mixture of composition x. The vapour given off has the composition x1. The composition of residual liquid will shift towards A. In the mean time the composition of the distillate shifts towards C. ultimately by repeated fractional distillation, the mixture of composition C will be obtained as distillate and pure liquid A will be left as residue. It will never be possible to have pure B.

When a mixture of composition y is distilled, the vapour given off has the composition y1, i.e., the composition of residual liquid will shift towards B. Ultimately the mixture of composition C will be obtained as distillate and pure liquid B will be left as residue. It will never be possible to have pure A.
There are several quid pairs which form minimum boiling point azeotropes. Some examples are given in the following table.

Table: Some azeotropic mixtures
 Mixture % Composition of azeotrope Boiling point (pressure = 1 atm) 1. 2. 3. 4. Water-Ethanol Pyridine-Water Ethanol-Benzene Acetic acid-Toluene 95.97 Ethanol 57.00 Pyridine 32.40 Ethanol 28.00 Acetic-acid 78.13oC 92.6oC 67.8oC 105.4oC

Solutions of type-III (Negative deviations from Raoult’s law): The boiling temperature-composition curves for the

liquid and vapour phases have been shown in Fig. 5.4. The curves meet at point C. At this point both liquid and vapour phases have same composition. The constant-boiling mixture has maximum boiling point.

Consider distillation of a mixture of composition x. The vapour coming off is richer in A as indicated by composition x1. The composition of the residual liquid shifts towards C. As the distillation proceeds, the composition of the distillate moves towards A and that of residue towards. C.

Similarly, a mixture of composition lying between B and C, say y, on distillation will gie vapour richer in B as indicated by composition y1. The composition of residual liquid shifts towards C and distillate towards A on repeated distillation.

It is never possible to separate a mixture completely into the pure components A and B. It mainly gives a constant boiling mixture (azeotropes) which can never be separated by distillation.

There are several liquid pairs which form maximum boiling point azeotrope. Some examples are tabulated below:

 Mixture % composition of azeotrope Boiling point (pressure = 1 atm) 1. Nitric acid-Water 68% Nitric acid 125.5°C 2. Acetic acid-Pyridine 65% Pyridine 139.0° C 3. Chloroform-Aceton 80% Chloroform 65.0° C 4. Hydrogen chloride-Water 79.8% Water 108.6° C

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