Local Mass Transfer during Electrodialysis with Ion-Exchange Membranes and Spacers

Mass transfer during electrodialysis with ion-conducting spacers in the intermembrane gap is modeled experimentally using a local-distribution analysis of solutions. Due to emergence of back-flow regions near the spacers, local quantities (diffusion layer thicknesses, surface concentrations, Sherwood numbers) are distributed nonuniformly over the channel length. In a smooth channel, due to concentration polarization, local mass transfer rates steadily decrease along the solution supply coordinate, but introducing ion-conducting spacers into the channel intensifies mass transfer because of periodic interruption of diffusion layers and formation of recirculation zones. Effect of geometrical size of spacers on the mass transfer is studied. It is found that the mass transfer rate is maximum for ion-conducting spacers with the ratio 3.5 < l/h< 4.0.