3D simulation of the electrodialysis process. Unobvious spacer filaments placement to increase the limiting current

Abstract

In the electrodialysis process, the concentration polarization phenomenon determines the value of the limiting current density and, consequently, significantly affects energy consumption. Commercial spacers are commonly used to mix the solution and enhance ion delivery to ion-exchange membranes, and its shape has a crucial impact on the electrodialysis optimization. The target features of spacers development are better mixing and lower shadow effect.

In a binary NaCl electrolyte solution with different diffusion coefficients of ions (D_Na≈1.5 D_Cl) the limiting current is determined by the cation-exchange membrane, according to Peers' equation, because its counterion has the minimal value of D in the solution. Recently using 2D simulation and corresponding experiment it was found that the limiting current of an electrodialysis channel with NaCl solution may be increased by displacing the spacer filaments towards the anion-exchange membrane. This arrangement increases the flow velocity of the solution near the surface of the cation-exchange membrane, thus, thereby reducing the boundary layer thickness near it.

In this work, using 3D simulation, we investigated several types of spacers. For the first time, an unobvious result was found: displacing the filaments toward the cation-exchange membrane in NaCl solution leads to an increase in the total limiting current and the effect is more pronounced then in the case of displacement toward the anion-exchange membrane. It is theoretically shown that in 3D systems, better solution mixing and ion delivery may be achieved in nonintuitive ways and the comprehensive analysis should be done to predict the system behavior.