Continuous Countercurrent Extraction with Reflux

Whereas in ordinary countercurrent operation the richest possible extract product leaving the plant is at best only in equilibrium with the feed solution, the use of reflux at the extract end of the cascade can provide a product even richer, as in the rectifying section of a distillation column. Reflux is not needed at the raffinate end of the cascade since, unlike distillation, where heat must be carried in from the reboiler by a vapor reflux, in extraction the solvent (the analog of heat) can enter without a carrier stream. 

Let Ae represent the net rate of flow outward from this section. Then, for its (A + C) content,  

Combining equations (7-25), (7-27), and (7-28) we get an expression for the internal reflux ratio at any stage, 

The external reflux ratio can be calculated from equation (7-29) 

Material balances may also be written for the whole cascade. A balance for (A + C) is of the form 

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For purposes of computation, it is easiest to recall the similarity between the adsorption operation and continuous countercurrent extraction with reflux. Solid adsorbent as the added insoluble phase is analogous to extraction solvent, the adsorbate is analogous to the solvent-free extract, and the fluid stream is similar to the raffinate. Computations can then be made using the methods and equations of Chap. 10 [Eqs. (10.31) to (10.34) and (10.39) to (10.50) with Fig. 10.28]. Some simplification is possible, however, thanks to the complete insolubility of adsorbent in the mixture to be separated. 

A flow diagram for countercurrent extraction with reflux is shown in Fig. 20.14. To emphasize the analogy between this method and fractionation, it is assumed that the cascade is a plate column. Any other kind of cascade, however, may be used. The method requires that sufficient solvent be removed from the extract leaving the cascade to form a raffinate, part of which is returned to the cascade as reflux, the remainder being withdrawn from the plant as a product. Raffinate is withdrawn from the cascade as bottoms product, and fresh solvent is admitted directly to the bottom of the cascade. None of the bottom raffinate needs to be returned as reflux, for the number of stages required is the same whether or not any of the raffinate is recycled to the bottom of the cascade. The situation is not the same as in continuous distillation, in which part of the bottoms must be vaporized to supply heat to the column. 

Countercurrent multiple contact with reflux. This is a continuous operation an d()g()ll to fractional distillation. A cascade of stages is employed, with feed solution to be separated customarily entering somewhere in the middle of the cascade and extracting solvent at one end. Extract and raffinate phases flow countercurrently, with reflux provided at both ends of the cascade. Alternatively, reflux may be used only at one end of the cascade, corresponding to the enriching or stripping practices of distillation. 

These operations are all similar in that the mixture to be separated is brou t into contact with another insoluble phase, the adsorbent solid, and the unequal distribution of the ori al constituents between the adsorbed phase on the solid surface and the bulk of the fluid then permits a s aration to be made. All the techniques previously found valuable in the contact of insoluble fluids are useful in adsorption. Thus we have batchwise single-stage and continuous multistage separations and separations analogous to countercurrent absorption and stripping in the field of gas-liquid contact and to rectification and extraction with the use of reflux. In addition, the rigidity and immobility of a bed of solid... 

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