EDI principle and influencing factors

Electrodeionization (EDI) technology is a new type of membrane separation technology that combines electrodialysis and ion exchange.

Its main features are:

  1. resin is regenerated without the use of acids and alkalis, and clean production is achieved;
  2. equipment It is self-regenerating at the same time of operation, so it is equivalent to continuously obtained regenerated mixed bed ion exchange column, which can achieve continuous deep desalination;
  3. good water quality, low water production cost, and convenient daily operation and management.

Schematic diagram of the principle of electrodeionization The fresh water chamber of the electrodialyser is filled with mixed anion and cation resins, and the electrodialysis and ion exchange are placed in a container to organically combine the two. The ions in the water are first adsorbed on the resin particles due to the exchange effect, and then formed by the resin particles under the action of the electric field; the ion transmission channel; migrates to the membrane surface and enters the concentration chamber through the ion exchange membrane. The polarization in the contacted diffusion layer causes the hydrolysis to become H + and OH-. Most of them play a role in regeneration of the resin except for part of the load current, so that the three processes of ion exchange, ion migration, and electrical regeneration occur together. Promote each other to realize the process of continuous ion removal.

The influence of raw water conductivity on the desalination effect is that when the influent flow rate is 120L / h, changing the raw water conductivity can obtain the relationship between the effluent conductivity and the raw water conductivity. The effect of raw water conductivity on the quality of produced water can be seen, under the same operating current, as the raw water conductivity increases, the conductivity of EDI effluent also increases. Because the conductivity of the raw water is low, the content of ions is also low, and the low ion concentration makes the potential gradient formed on the surface of the resin and the membrane in the pale chamber also large, which leads to enhanced dissociation of water and increased limit current. The large amount of H and OH makes the regeneration effect of the anion and cation exchange resin filled in the light room good.

When the conductivity of the raw water is 21.5μS / cm, the conductivity of the EDI effluent has been very small (0.1 ~ 0.05μS / cm) with the increase of the operating current. This is because the smaller the conductivity of the raw water, the more intense the hydrolysis , The more H and OH produced, the better the effect of resin electrical regeneration (to maintain good exchange performance). When the operating current continues to increase, H and OH are used for load current in addition to regenerating the resin, so the degree of hydrolysis in the light chamber continues to increase, so that the ion exchange and the regeneration of the resin gradually reach equilibrium, and the water conductivity becoming steady. Therefore, the conductivity of raw water is one of the most important factors affecting the quality of produced water.

When the influent conductivity is high, the quality of the produced water decreases with the increase of the operating current. Taking the curve of the raw water conductivity as 100 μS / cm as an example, when the operating current gradually increases from 0 to 5A, the EDI effluent The conductivity rises from 0.17μS / cm to about 0.5μS / cm (water quality is reduced), the reason is that the concentration polarization is low at high salinity, the hydrolysis is weak, and the resin is hardly regenerated. Exchange plays a major role, and the resin is saturated with salt ions in a short time, and then the resin mainly plays a role in enhancing ion migration.Regardless of whether the influent salt content is high or low, the EDI equipment of the second and fifth stages has a good desalination effect (desalination rate 99%), and the conductivity of the effluent can reach the high purity water standard

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