Mass transfer rates in the electrodialyzer with U-shaped channels

Abstract

As a link in the chain of the study on hydrodynamic problems in the electrodialysis process, mass transfer rates in a U-turn were investigated to understand the secondary flow effect on the limiting current density. Mass transport in the electrolyte solution was described by a general mass transfer equation and an equation for potential gradient, and the boundary conditions were given by initial concentration and a relation between current density and concentration gradient. Using the concentration profile fitted to a third order polynomial and the velocity field derived from the creeping flow assumption, governing equations were solved by an integral method to yield asymptotic solutions representing very low and high polarization cases. From the theoretical results, the desalting rate of the U-turn was observed to be slight higher than that of the straight channel, and the limiting current density evaluated by the present theory was most conservative since the membrane surface concentration was the lowest comparing with other studies. Experiments were carried out to examine the theory. At low flow velocities, the agreement between two was good, and the limiting current density at higher flow velocities deviated from the theory due to the increased secondary flow velocity. It was thought that the secondary flow induced boundary layer skimming in the radial direction with a result of reducing polarization at the membrane surface.