Multiple extraction stages

Multi-stage extraction is uncommon because solvents can be found that have highly favorable distribution coefficients that will get the job done in one or two stages. Two stages of extraction are quite common. One reason is that there may be emulsions that prevent good separation of the phases. If the emulsion carries some of the desired product to the second stage, it will get back into the rich phase and little will be discarded. Let's consider sending the depleted phase to the second stage while sending the enriched phase from the second stage to be the extractant in the first stage. The equations given for Component A are analogous to those for Component B :

The distribution coefficient definition is repeated. The equations for the stages are the distribution equation and a mass balance equation. For each stage, the distribution equation is simplified by letting the volume of the bottom phase = 1.0. The mass balance equation for Stage 1 says that the total mass of A is the amount added plus the amount coming as the rich top phase from Stage 2. The mass balance for Stage 2 recognizes that the only source of A is the depleted bottom phase from Stage 1.

We have four equations and four unknowns, but the terms for the bottom phase masses are very easily replaced by using the distribution equations. The algebra for two equations with two unknowns is left for you to solve and to compare with the computer program. Help with algebra. You should observe that yields improve slightly with two stages when the distribution coefficient is large and improve a lot when the distribution coefficient is small. Furthermore, there are interesting results from changing the solvent ratios that should be noted so that you can explain them on the exam.

while on sabbatical leave, ESB, Porto, Portugal July 1996