Computational Modeling of Ionic Transport in Continuous and Batch Electrodialysis

Abstract

ABSTRACTWe describe the transport of ions and dissociation of a single salt and a solvent solution in an electrodialysis (ED) stack. An ED stack basic unit is made of a two-compartment cell: dilute and concentrate. We use the fundamental principles of electrochemistry, transport phenomena, and thermodynamics to describe mechanisms and to predict the performance of the ED process. We propose and analyze three model formulations for a single salt (KC1). The first and the second models are for a one- and two-dimensional continuous ED, and the third examines batch ED. The models include the effect of the superficial velocity in the boundary layer near the ion-exchange membranes. We examine the diffusion and electro- migration of ions in the polarization region and consider electromigration and convection in the bulk region. We show that the ionic surface concentration of both membranes in the dilute compartment is affected by two parameters: flow rate and current density. We also show that in the dilute compartment, concentration changes along the x-axis are greater than along the y-axis because the ionic flux along the x-axis is greater and is in the direction of the current. In simulations where the KC1 dilute concentration ranges from 200 to 500 mol m-3 with a constant concentrate concentration, the dilute voltage drop accounts for more than 36% of the total voltage drop. This value is reduced to 7% as the dilute concentration increases and contributions of both ion-exchange membrane drops account for more than 50% of the total cell voltage drop. All the three models were validated experimentally.