(188c) New Developments in Membrane Solvent Extraction

Solvent extraction usually carried out by dispersing one phase as drops in the other phase needs phase coalescence after extraction; the devices used are mixer-settlers or tall columns in industrial operations. This dispersive operation requires a phase density difference and encounters problems of flooding, loading and low values of allowable phase flow rate ratios. Further, phase coalescence can be problematic especially if emulsion formation takes place. To bypass such problems, nondispersive solvent extraction via a porous hydrophobic membrane was developed sometime back. It is also called membrane solvent extraction (MSX). Here the organic phase flowing on one side of a porous hydrophobic membrane wetted the membrane pores whereas the aqueous phase flowing on the other side and not wetting the pores was maintained at the same or a higher pressure. There is a maximum allowable pressure difference after which aqueous phase breaks through and is dispersed in the organic phase; nondispersive operation is no longer possible. There are commercial units operating based on this concept. Countercurrent flow is used for high solute recovery in membrane solvent extraction as well. Inevitably there is significant pressure drop in the liquid flowing on each side of the membrane which may lead to or exceed maximum allowable phase pressure difference at both ends of the narrow flow channels encountered with hollow fiber membranes. To avoid such problems a new technique has been developed. We will describe this technique and illustrate membrane solvent extraction for different systems. A model developed will also be described.