Effect of a porous spacer on the limiting current density in an electro-dialysis desalination

Abstract

A series of experiments have been carried out to examine the performance of porous spacers proposed for increasing the limiting current density, since it is of fundamental importance to find an optimal operate condition in an electro-dialysis. The fluid mixing in the porous material can play an important role to suppress the concentration polarization and achieve a high limiting current density. The effect of porous spacers filled in both dilute and concentrate channels on the stack voltage and the limiting current density has been investigated by comparing the cases with and without porous spacers. It has been found that the limiting current density with porous spacers is 1.8 to 3.3 times higher than that without a porous spacer as a result of mechanical dispersion caused by fluid mixing in porous materials. Furthermore, the increase ratio of limiting current density with the present spacers to that without spacer is higher than that of conventional mesh spacers. Moreover, it was found that the electrical resistance does not increase even when inserting the present porous spacers by comparing that of conventional mesh spacers, since the present porous spacer can suppress the concentration polarization concerned with electrical resistance. On the other hand, the increase in the pumping power turns to be negligible small by comparing the electrical power for an electro-dialysis since permeability of the present spacers is sufficiently high. In this study, it has been proven that the insertion of porous spacers is quite useful in terms of limiting current density, electrical resistance and pumping power.