Distillation in Chemical Engineering

Distillation is the process of separation of volatile liquid from non-volatile substance or more frequently, the separation of two or more liquids in different volatility. If only one component of a mixture is volatile, there is no difficulty in obtaining it in a pure state by distillation and in many cases the constituents of a mixture of two or more volatile liquids may be separated – though frequently at much cost of time and materials.

Quantitative Analysis by Distillation: The determination by ordinary analytical methods of the relative quantities of two or more organic compounds in a mixture is often a matter of great difficulty, but in many cases, the composition of the mixture may be ascertained approximately and not seldom, with considerable accuracy from the results of a single distillation, if a very efficient still head is employed.

Difficulties encountered: The subject of fractional distillation is full of interest owing to the fact that difficulties so frequently occur, not only in the experimental work, but also in interpreting the results obtained.

In the distillation of petroleum, with the object of separating pure substances, such difficulties are of common occurrence and are due to one or other of three causes:- A. to the presence of two substances, the boiling point of which are very close together, B. to the presence of one or more components in relatively very small quantity, C. to the formation of mixtures of constant boiling point.

The separation of two liquids which boil at temperatures even 20° or 30° apart, such as ethyl alcohol and water, or benzene and isobutyl alcohol, may be impossible owing to the formation of a mixture of minimum or less frequently, of maximum boiling point. It is indeed, only in the case of substances which are chemically closely related to each other that the statement can be definitely made that the difficulty of separating the components of a mixture diminishes as the difference between their boiling points increases.

In any other case, we must consider the relation between the boiling points, or the vapor pressures, of mixtures of the substances and their composition and unless something is known of the form of the curve representing one or other of these relations, it is impossible to predict whether the separation will be an easy one or indeed, whether it will be possible.

The form of these curves depends largely on the chemical relationship of the components and it is now possible in a moderate number of cases, to form an estimate, from the chemical constitution of the substances, of the extent to which the curves would deviate from the normal form, and therefore to predict the behavior of a mixture on distillation.

Fractional distillation: Fractional distillation is frequently a very tedious process and there is necessarily considerable loss of material by evaporation and by repeated transference from the receivers to the still, but a great amount of both time and material may be saved by the use of a very efficient still-head and when the object of the distillation is to ascertain the composition of a mixture, very much greater accuracy is thereby attained.

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