Multistage crossflow extraction

Extraction processes can be improved by multiple treating the raffinate product with fresh solvent. In Fig. 6.2-3 a four stage crossflow process is depicted. The raffinate from each stage is mixed with fresh solvent. 

As can be seen from the graphical constmction the concentration of the transfer component B in the raffinate is reduced step by step to very low values. However, the coneentration in the extract also falls down making the recoveiy of a pure flac-tion of 5 by further separation processes more difBcult. The analogous process of multiple crossflow leaching is shown in Fig. 6.2-4. The thermodynamics of both types of extraction are completely equivalent. 

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The methods used to analyze mass transfer in leaching systems depend on the methods of contacting used and how they are combined in multistage equipment. Appropriate methods of analysis may be the same for two superficially different types of contacting. Thus, crossflow extraction can be analyzed in terms of fixed-bed extraction and, if the two-phase flow in cocurrent slurry extraction is sufficiently pluglike, it... 

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