Tan refining 'of petroleum fractions by the use of solvents is becoming an operation of steadily, increasing significance, and it is desirable that the, process should develop on scientific lines.

It is. essential that -in, an industrial operation it should be possible to reach reasonably correct conclusions as to the size and nature of the equipment required, the results of the process, and the capital .and running costs well in advance of the actual installation. Such conclusions can only be reached by means of mathematical. calculation based on established facts.

This paper is an attempt to show how such mathematical computation can be founded on physical laws. - The problem is exceedingly complex and therefore only the ;simpler aspects of it are examined.. The complex cases, however, have not been entirely neglected and suggestions for solving some of these are-put. forward. An attempt has been made in the first portion of this paper to classify and compare extraction methods and to show how the process results obtainable by such methods can' be forecasted. In the second part of the paper process equipment has been dealt with. in a similar manner, CLASSIFICATION OF EXTRACTION METHODS.

The methods employed for contacting two liquid phases in a solvent extraction process may be classified as follows :

  1. 'Single contact processes.

  2. Co-current multiple contact processes.

  3. Counter-current contact processes.

In the single contact process, contact or mixing of the treated and the solvent liquid is carried out in a - FIGURE I - single stage or operation. After contact the two phases are separated. The process may be carried out either by batch or continuous working. In the latter case the two liquids are mixed continuously in a suitable mixing device and separated by centrifuges or by continuous settling.

In the co-current multiple contact process, mixing and separation of the two liquids, exactly as in the previous method, is followed by contact of the treated liquid, after separation, with fresh solvent. This operation is repeated as often as required. The process may be carried out in batches, or semi-continuous working as shown in Fig. 1 may be employed.

In the counter-current process the treated liquid is contacted with solvent which has been employed in a previous contacting stage, except in the final stage of * Department of Oil Engineering and Refining, University of Birmingham. the process where the treated liquid is extracted with fresh solvent. This effect is most readily produced by continuous working, where the two liquids are made to flow counter-currently through some suitable form of apparatus. This may consist of a vertical tower, the incoming liquid to be treated entering at the bottom and leaving at the top, wh

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