Abstract
The application of pulsed electric field (PEF) in electrodialysis has been proven to be efficient for a number of effects: increasing mass transfer rate, mitigation of scaling and fouling, reducing water splitting. Recently, the improvement of the membrane permselectivity for specific counterions was discovered experimentally by the group of Laurent Bazinet (N. Lemay et al. J. Memb. Sci. 604, 117878 (2020)). To better understanding the effect of PEF in electrodialysis, simulations were performed using a non-stationary mathematical model based on the Nernst–Planck and Poisson equations. For the first time, it was not only the condition used when the current density is specified but also the condition when the voltage is set. A membrane and two adjacent diffusion layers are considered. It is shown that when applying the regime used by Lemay et al. (the same current density in conventional continuous current (CC) mode and during the pulses in PEF mode), there is a significant gain in specific permselectivity. It is explained by a reduction in the membrane concentration polarization in PEF mode. In the CC mode of electrodialysis, increasing current density leads to a loss in specific permselectivity: concentration profiles in the diffusion layers and membrane are formed in such a way that ion diffusion reduces the migration flux of the preferentially transferred ion and increases that of the poorly transferred ion. In PEF mode, the concentration profiles are partially restored during the pauses when the current is zero. However, if a different condition is used than the condition applied by Lemay et al., that is, when the same average current density is applied in both the PEF and CC modes, there is no gain in specific permeability. It is shown that within the framework of the applied mathematical model, the specific selectivity depends only on the average current density and does not depend on the mode of its application (CC or PEF mode).
Highlights
Electrodialysis (ED) is one of the rapidly progressing membrane methods for desalination, concentration and separation of aqueous solutions today
The use of current or voltage pulses alternating with pauses in ED, the so-called pulsed electric field (PEF) mode, leads to the mitigation of concentration polarization and, as a result, improves the performance of ED compared with conventional CC mode [5,7,8,13,14,15]
A mathematical model based on the Nernst–Planck and Poisson equations is reported to describe competitive counterion transport through an ion-exchange membrane
Summary
Electrodialysis (ED) is one of the rapidly progressing membrane methods for desalination, concentration and separation of aqueous solutions today. The use of current or voltage pulses alternating with pauses in ED, the so-called PEF mode, leads to the mitigation of concentration polarization and, as a result, improves the performance of ED compared with conventional CC mode [5,7,8,13,14,15]. Uzdenova et al [23] theoretically studied the effect of a PEF on mass transfer at overlimiting current regimes by applying the Nernst–Planck–Poisson–Navier–Stokes equations They showed that the intensive electroconvective vortices decay almost immediately after switching off the current. These results showed that the application of the PEF modes in ED is a promising way for the process intensification in the overlimiting current range It is hypothesized [26,27] that the main cause of improvement of ED performance when applying PEF in intensive current regimes is electroconvection. Two PEF modes are compared when current or voltage pulses are applied
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