Simple modeling of Donnan separation processes

The objective of this work is to analyze the kinetics of Donnan dialysis processes by simplified models enabling convenient characterization of the major features of separations of practical interest. The steps involved in a Donnan separation process are convective mass transfer of ions from the feed solution compartment to the solution-membrane interface, diffusion of the ions through the ion exchange membrane to the membrane receiver side and convective mass transfer of the ions from the membrane-solution interface to the receiver solution compartment. Two simple models are derived to characterize limiting transport mechanisms - a model describing transport under mass transfer control and a model describing transport under membrane diffusion control. Experimental data covering a range of Reynolds numbers and a range of solution concentrations can then serve to identify mass transfer parameters, solution concentrations and membrane diffusivities prevailing in the system under various operating conditions. Application of these concepts is fully illustrated by derivation of the two limiting models for sulfate ions transport from feed to receiver by chloride stripping ions and by providing experimental evidence in full support of the proposed approach.