ONE of the most important problems with which present-day technique is faced is. that of sharply separating the groups of crude oil components. This separation can be made according to two principles, either according to molecular size or to chemical characteristics. The' former separation is usually effected by distillation, the latter by extraction.
Whereas great progress has been made both in the theory and practice of distillation, such progress is by no means perceptible to the same extent in the process of extraction. It is shown in this paper that there is a close relation between distillation and extraction and that the three types of distillation, viz.
have their analogies in
Extraction by simple shaking out;
Washing in counter-current;
A new form of extraction
which, contrary to the first two forms, just as complete rectification, makes it possible to produce the two components of a binary mixture in any degree of purity.1 This new process consists in introducing the mixture to be separated into the middle of the. apparatus. The extracting agent is admitted and the raffinate drawn off on one side, while on the other side the extract solution is, drawn off and part of the extract resulting from the removal of the former returned as reflux. If the extracting agent used is moderately selective and is only partially miscible with either component of the mixture tinder test at the working temperatures, complete rectification can be effected.
The theoretical treatment of the extraction is first developed on general lines. From a phase-theoretical point of view it is shown that it is possible to deduce an analogy between extraction and distillation from the phenomenon that the external pressure hardly effects extraction, so that the extra degree of freedom introduced by the extraction agent can be eliminated by fixing the external pressure. It is further shown that while heat content and volume determine the liquid-vapour system, heat content and amount of solvent determine the phases in extraction and that osmotic pressure in the latter case is equivalent to the external pressure in the former. The analogy with distillation is demonstrated by ternary graphs, by the existence of conditions such as minimum reflux ratio, the theoretical analogy to "azeotropes" and to the distillation above the critical temperature of one of the two components.
From the fact that the theoretical treatment of extractions can be pursued in exactly the same way as that of distillations, it follows that the conditions necessary to optimum methods for distillation must, properly transformed, imply those for extractions, and vice versa. Thus it is shown that, contrary to the usual practice, extraction should be carried out with a temperature gradient through the apparatus and that