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

Seawater is generally comprised of minerals, salts and other components. As such, this water is never suitable for various uses like human consumption. Therefore, this water undergoes purification to remove the pollutants. Among the numerous available techniques, electro-dialysis is widely used in desalination processes. It constitutes an ion exchange membrane that uses the influence of the electrical potential to transport salt ions from one solution to another. However, for electro-dialysis systems, the optimal operating conditions are determined by the current density.

Unfortunately, current density above the limit results in the depletion of the ions on the membrane which may further degrade the ion transportation function of the membrane. To this end, researchers have been looking for better alternatives for protecting the membrane and enhancing the operation of electro-dialysis systems and have identified increasing the current density limit as a promising solution.

Recently, researchers at Shizuoka University: Professor Yoshihiko Sano, Dr. Xiaohui Bai, Mr. Shuzen Amagai and Professor Akira Nakayama investigated the performance of porous spacers for increasing the limiting current density for electrodialysis systems. They were interested to at the importance of the porous spacers in increasing the limiting current density of the electro-dialysis systems. Their research work is currently published in the research journal, Desalination.

Briefly, the research team began their studies by using the filter press electro-dialysis system by filling the ceramic foams in both concentrated and dilute channels. Based on the Faraday’s law, the measuring system was validated taking into consideration the ionic mass balance. Furthermore, a comparison of the cases with and without the spacers was conducted to determine the effects of the porous spacers on the current density and stack voltage.

The authors observed that electro-dialysis systems with the porous spacers achieved a higher limiting current density as compared to those without. For instance, it was approximate to be about 1.8 to 3.3 times higher. This was attributed to the fluid mixing in the porous materials that further resulted in an enhanced mechanical dispersion. On the other hand, increasing the velocity can lead to a corresponding increase in the limiting current density. However, the increase ration of a system with a porous spacer to that without is much higher as compared to that of a conventional mesh spacer.

Furthermore, the addition of the porous spacer did not affect the electrical resistance due to the suppression of the concentration polarization. It was, however, possible to reduce the electrical power in the high-velocity region because the porous spacers resulted in lower stack voltages. Additionally, the presently introduced porous spacer achieved a higher permeability as compared to the initially used spacer.

By introduction of the porous spacer, the study by Shizuoka University researchers is set to advance desalination processes in several industrial and domestic applications of electro-dialysis systems. This due to the fact that they will significantly enhance the limiting current density and electrical resistance. Consequently, the increased permeability will lead to a decrease in the pumping power thus reducing the energy cost of the systems.