Stagewise discontinuous extraction

Modify the above program to model the case of a discontinuous five-stage extraction cascade, in which a continuous flow of aqueous phase L is passed through the cascade. The solvent is continuously recycled through the cascade and also through a solvent holding tank, of volume Vs, as shown below. This problem of a stagewise discontinuous extraction process has been solved analytically by Lelli (1966). 

With differential stagewise (discontinuous) extraction, the solvent Lg and mixture F are intensively mixed in a vessel (Fig. 6-11). The raffinate R and extract E are formed at the end of the extraction. The extract phase is continuously withdrawn, and then separated by distillation into the key component S and the recycled solvent L, for example. 

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