Kinetics of liquid membrane extraction in systems with variable distribution coefficient

The effect of a variable distribution coefficient on the calculation of mass transfer coefficients in liquid membranes is analyzed. The extraction and stripping of lactate by ion-exchange with a quaternary ammonium salt (Aliquat 336) is used as a case study, but it may be extended for any system where an ion-exchange reaction is present. In this case, the distribution coefficient can only be considered constant if the equilibrium constant of the reaction, K-e, is equal to 1 or if the change of the aqueous solute concentration during the extraction (or the stripping) process is in the linear region of the equilibrium curve. The deviation between the values of the mass transfer coefficients calculated by using either a variable distribution coefficient or a constant distribution coefficient depends on the region of the equilibrium curve considered. The higher deviations on the mass transfer coefficients take place when the equilibrium curve diverges from linearity. The simultaneous extraction and stripping processes are studied using a bulk liquid membrane, but an equivalent procedure can be applied to hollow fibre contactors, which are more adequate for practical application as they provide an high surface area to volume ratio (A/V).