Influence of ion concentration on the kinetics of electrodialysis with bipolar membranes

This work aims at the development of a mathematical model that describes the behaviour of electrodialysis on bipolar membranes (EDBM) systems in a wide range of solute concentrations. The mathematical model based on the application of the Nernst-Planck equation, accounts for the influence of diffusion and migration phenomena, and allows quantification of their relative importance in the mass transport flux as a function of the operating Conditions. In order to check the validity of the proposed model with experimental results, the obtention of hydrochloric acid and sodium hydroxide from sodium chloride was selected as a model system. The variables of the experimental design were the current density (250-1000 A m(-2)) and the number of cell pairs (I and 4). The contribution of diffusion to the total ionic flux was as high as 25% depending on the concentration level of the transported species. The diffusion coefficient of sodium and chloride ions did not remain constant within the whole range of experimental variables, describing the variation by means of an exponential equation D-i = D-0 exp(-tau C-m), that was incorporated into the mathematical model. (C) 2007 Elsevier B.V. All rights reserved.