MFESI With Double Layers And Multiple Layers For High Purity Water Production

Membrane-free electrodeionization (MFEDI) is a novel deionization technique invited by our group. Ion exchange resins are packed between a pair of reticular electrodes in the MFEDI, which is operated in a batch countercurrent mode, alternating with a long upflow service step and a short downflow regeneration step. During service, MFEDI accomplished a deionization process just like traditional ion exchange. During regeneration, the purity water passed through the resin layer, which was supplied with a constant current simultaneously. MFEDI has a broad application prospect in high purity water production due to the easiness of its operation and the clean process.It is clearly that ion exchange resins are the heart of the MFEDI. The purification and regeneration performancees of the MFEDI depend highly on the properties and filling structure of the resins. At present, the application of the single layer MFEDI filled with mixed weak-acid and strong-base resins in high purity water production was restricted due to the poor purification performance and high regeneration voltage. Therefore, it is of great significance to study on resin selection of MFEDI and its filling structure in high purity water production.This thesis systematically studied the MFEDI with double layers and multiple layers for high purity water production. In the MFEDI with double layers, the bottom layer was filled with the mixed weak-acid and strong-base resins, while the upper layer was filled with the mixed strong-acid and strong-base resins. In the MFEDI with multiple layers, the whole resin layer was divided into five equal layers by inserting four pieces of thin strong-base resin layers. Major research works included three parts: resin selection, MFEDI with double layers for high purity water production, MFEDI with miltiple layers for high purity water production.Experimental results showed that the mixed weak-acid and strong-base resins and the mixed strong-acid and strong-base resins were selected to use in MFEDI for high purity water production due to their good deionization ability, perfect electrical regenerability, and high conductivity. The former can be placed in the bottom layer of MFEDI to increase the regeneration efficency due to its excellent electricall regenerability. The latter can be placed in the upper layer of MFEDI to gurrentee the water quality and to decrease the resistence of resins due to its good adsorption ability and conductivity. The performance of MFEDI for high purity water production was improved by using double layers. Effective regeneration was achieved, and the resins after repeated regeneration still remained their good efficacy for deionization.A total of20operational cycles were examined. The effluent conductivity was0.056?0.059?S/cm only, which met the quality requirement of the high purity water. The electrode polarity was reversed during regeneration to prevent the constant upward migration of cation or anion ions, and to get neutral concentrates. It was found that the contribution to resins regeneration was more than80%from the electrically enhanced water split and less than20%from the electrolysis reaction.It was demonstrated that the thin strong-base resin layers in MFEDI could effectively prevent the cation ions backward migration during regeneration. This could solve the problems, such as the decrease of the concentrate conductivity, the increase of the regeneration voltage, and the damage of the electrodes, caused by the electrode polarity reversal. MFEDI with mutiple layers was successfully used to produce high purity water. A total of30operational cycles were examined. The effluent conductivity was0.055?0.060?S/cm only; the water recovery was90%; and the energy consumption was0.71kWh/m3.