Effect of pulsed electric field on electrodialysis of a NaCl solution in sub-limiting current regime

Abstract

In spite of growing applications of pulsed electric fields (PEF) in membrane separation processes, there are only few theoretical studies treating mass transfer. In this paper, we propose a 1D mathematical model, which adequately describes PEF mode electrodialysis (ED) with ion-exchange membranes at sub-limiting currents. The results of simulation are compared with experimental data obtained for a laboratory-scale electrodialysis stack. We show experimentally and theoretically that the average current density and mass transfer under PEF of a sufficiently high frequency (>1Hz in the used conditions) are higher than those in conventional steady state DС mode, if a same average voltage is applied. The advantage increases with frequency and reaches a maximum at about 100Hz. When applying a pulse after a pause, we benefit by low ohmic resistance and low diffusion potential drop caused by partial concentration restoration in close vicinity of the membrane. This allows passage of an instantaneous current of a high density, which can essentially exceed the limiting current density (jlim) in steady state DC conditions. However, at low frequencies this gain rapidly vanishes by increasing concentration polarization during the pulse, thereby the mass transfer in PEF mode is lower than that in DС mode. The gain is close to zero at low currents due to linearity of I–V curve and increases with increasing current. However, within the model, the average current density cannot be higher than ilim. The gain in mass transfer rate at high frequencies increases with decreasing duty cycle, but with this the energy consumption for electrodialysis desalination in PEF mode increases, it is always higher than the energy consumption in continuous DС mode.