CFD Modelling of an Electrodialysis Cell

Electrodialysis (ED) cells are based on the selective transport of ions through membranes which ideally exclude the co-ions, so that when a potential difference is applied to the cell all ions are transported in solution but only the counter ions can cross the membrane. Thus, concentration polarization occurs in solutions adjacent to membrane surfaces due to the different transport number between the membrane and the solution. The concentration profiles formed in the solution near the membrane depends on the mass transport fluxes and hydrodynamic pattern in the channel; operating conditions and geometrical configuration of the cell affect greatly the concentration polarization [1]. Therefore, the performance of ED cells depends not only on the membrane properties but also on the electrolyte hydrodynamics being important to provide an adequate flow patter avoiding channeling, stagnant zones, jet formation, among others flow deviations. Keeping a homogeneous flow patter minimizes variations of mass transfer fluxes and current density over the membrane surface preventing high polarization zones that could give rise to water dissociation, low efficiency and premature membrane failure. The examination of relevant mass transport mechanisms by modelling can help to evaluate the effect of design characteristics and operating conditions on the performance of an electrodialysis cell through the analysis of concentration, potential and current density distributions. In this task, CFD has proven to be a valuable tool for the analysis of flow and mass transfer in membrane separation systems [2]. Therefore, this work presents the modelling of a laboratory ED cell coupling hydrodynamics and mass transport, taking into consideration changes of Donnan potential. The flow pattern inside channels of ED cell were obtained by 3D CFD modeling and the results were used to calculate the residence time distribution (RTD) to compare with experimental RTD. The hydrodynamic behavior was used then to solve the mass transport model to obtain a description of the concentration distribution of each of the ionic species in the cell, as well as of the amounts derived as fluxes of each of the components, current density and Donnan potentials at every point of the membrane. [1] Gurreri L., Tamburini A., Cipollina A., Micale G., Ciofalo M., CFD prediction of concentration polarization phenomena in spacer-filled channels for reverse electrodialysis J. Membr. Sci. 468, 133–148 (2014). [2] Fimbres-Weihs G.A., Wiley D.E., Review of 3D CFD modeling of flow and mass transfer in narrow spacer-filled channels in membrane modules, Chem. Eng. Process. 49 759–781 (2010).