Continuous extraction

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The continuous extraction is a method of process engineering for the physical separation of substances by means of an extraction agent . The extractant is brought together in different variants with the mixture of carrier substance and substance to be separated and enriches itself with the substance to be separated. There are two variants of introducing the extractant, countercurrent and crosscurrent extraction.

Countercurrent extraction

Sketch of a mixer-settler apparatus with material flows

With countercurrent extraction, the carrier and the extractant are moved in opposite directions through the mixer tap in a mixer-settler . In the first stage, the heavily loaded carrier material flow is brought into contact with already enriched extractant, whereby a first refining takes place. The load on the carrier flow decreases with each stage. The loading of the extractant flow brought into contact therewith decreases in the same direction, so that finally in the last stage the already heavily depleted raffinate is dispersed with fresh, unloaded extractant . With this countercurrent process , a strong depletion of the raffinate is achieved with small amounts of extractant, which makes this variant very economical.

Liquid-liquid extraction

Liquid-liquid extraction consists of transferring the solvate (or several solvates) that is in the primary liquid solution into another, immiscible liquid ( solvent ). The solvent enriched with the solvate is called the extract , the diluted starting solution is called the raffinate .

The liquid-liquid extraction

The primary liquid solution and the solvent are contacted to effect transfer of the solvate. The two liquid phases (extract and raffinate) are separated by static decanting (mixer-settler) or centrifugal force.

Cross-flow extraction

In the case of cross-flow extraction, the carrier flow is dispersed in each individual stage with fresh, unloaded extractant. As a result, a higher depletion is achieved with a lower number of stages; however, significantly more extraction agent is also required.

Suitability of the methods

Both variants can be economically interesting. If extraction is to be carried out because another work-up method such as distillation cannot be used or this would not be economical, the countercurrent variant is usually more suitable. For separation problems where the extract is very valuable or where high purity is required, cross-flow extraction can be an economical alternative. A direct current extraction would theoretically be conceivable, but does not make sense, since only the phases that have just been separated from one another would always be mixed with one another in each stage, and it would not be possible to achieve a theoretical number of stages greater than 1.

literature

  • Burkhard Lohrengel: Introduction to the thermal separation process. Separation of gas, vapor and liquid mixtures . Oldenbourg Verlag, Munich 2007, ISBN 978-3-486-57799-0 .