Excerpt from Fractional Distillation

In the distillation of petroleum, 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 points 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 vapour 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 behaviour of a mixture on 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 com position oi a mixture, very much greater accuracy is thereby attained.

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