KINETICS OF LIQUID MEMBRANE EXTRACTION IN SYSTEMS WITH VARIABLE DISTRIBUTION COEFFICIENT

The effect of a variable distribution coefficient on the calculation o f mass transfer coefficients in liquid membranes is analyzed. The extr action and stripping of lactate by ion-exchange with a quaternary ammo nium salt (Aliquat 336) is used as a case study, but it may be extende d for any system where an ion-exchange reaction is present. In this ca se, the distribution coefficient can only be considered constant if th e equilibrium constant of the reaction, K-e, is equal to 1 or if the c hange of the aqueous solute concentration during the extraction (or th e stripping) process is in the linear region of the equilibrium curve. The deviation between the values of the mass transfer coefficients ca lculated by using either a variable distribution coefficient or a cons tant distribution coefficient depends on the region of the equilibrium curve considered. The higher deviations on the mass transfer coeffici ents take place when the equilibrium curve diverges from linearity. Th e simultaneous extraction and stripping processes are studied using a bulk liquid membrane, but an equivalent procedure can be applied to ho llow fibre contactors, which are more adequate for practical applicati on as they provide an high surface area to volume ratio (A/V).