A Study On Numerical Simulation Of The Electrodeionization Process

Electrodeionization (EDI) is a novel hybrid separation process combining electrodialysis (ED) and ion exchange (IX). EDI is becoming the major technology for producing high purity water and deep removal of heavy metal ions from waste water. However, there were few investigations about the influence of the concentration polarization on the water dissociation and the influence of the water dissociation on the removal and current efficiency of EDI process. The investigation on the elementary theory of the EDI process was not adequate.A numerical steady state model was established to simulate the process of EDI with the consideration of water dissociation. The full picture of the ionic transportation, ion exchange and the water dissociation process of the EDI were obtained. The influence of the difference of water dissociation of cation and anion membrane (CM and AM) on the EDI process, the influence of water dissociation at membrane surface on the removal and current efficiency and the water dissociation exaltation effect on the migration of salt ions were discussed. The water dissociation takes a very important role in EDI process. The investigator's opinion about the role of water dissociation in EDI process was that the regenerated resin (generated by water dissociation) was necessary for the deep removal of salt ions. However, there were few discussions about the necessity. We find that although the water dissociation makes the current efficiency decreased, it can enhance the migration of salt ions and the result is that the salt ion removal efficiency is improved. The product of water dissociation at the surface of AM could improve the migration of salt cations and the one of CM can improve the migration of salt anions.The EDI with dilute compartment fixed with cation resin bed only (CREDI) is usually adapted to remove heavy metal ions from waste water. We find that water dissociation current density at the surface of AM is much larger than the one of CM, and the result is that the exaltation effect of the water dissociation on cations is larger than the one on anions; therefore the removal efficiency of cations is much larger than the one of anions. The existing problem for the CREDI is the low current efficiency and the precipitation of metal hydroxides. According to the mechanism of exaltation effect of the water dissociation, we designed a layered bed stack configuration (cation resin, anion resin and mixed resin bed in turn (from the inlet to the outlet)), which was named as LayeredEDI. It is found that compared with the CREDI, LayeredEDI has larger current efficiency and less precipitation at the same cation removal efficiency.The EDI with the dilute compartment fixed with mixed resin bed only is adopted to produce ultrapure water. We investigated the influence of the water dissociation on the removal efficiency difference of salt cation and salt anion. The water dissociation current density at the surface of AM is also larger than the one at the surface of CM; therefore the removal efficiency of salt cation is larger than that of salt anions. In order to adjust the difference of the water dissociation current density between CM and AM, we assume that there is some catalytic group at the surface of CM. When the concentration of the catalytic group of the cation membrane is half of the one of the anion membrane, the difference between the removal efficiency of cation and anion is decreased obviously. The higher removal efficiency of both cation and anion is obtained at a much higher current efficiency.