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Polarization in the operation of electrodialyzer

release time:2019-11-02 17:18:02

The phenomenon of polarization occurring in the electrodialyzer refers to the phenomenon that some of the solution is different from the whole solution in the part close to the exchange membrane during the energization process. The phenomenon of polarization is that when ions move toward the electrode, their migration speed in the membrane and in the solution is not equal. At this time, the membrane conductance in the fresh water chamber is often much larger than the conductance of the solution, and therefore, the velocity of the ions passing through the exchange membrane. It is much faster than it migrates in solution, and as a result, the surface of the exchange membrane along one side of the fresh water chamber contains a lower concentration of ions from a thin layer of water than the average ion concentration in the entire fresh water chamber; similarly, In the concentrated water chamber, since ions cannot pass through the exchange membrane, a large amount of ions are concentrated on the surface, so the concentration of ions in a thin layer of water on the surface of the exchange membrane is higher than the average ion concentration in the entire concentrated water chamber, which is the ion exchange membrane. Opportunity phenomenon.
 
Polarization is unfavorable to the operation of electrodialyzers, and its hazard can be summarized into the following three aspects:
1. Increase power consumption due to increased resistance. Although the concentration of ions in the concentrated water chamber close to the surface of the exchange membrane can reduce the electrical resistance, it can not offset the increase in the ion concentration in the fresh water chamber immediately below the surface of the exchange membrane, so the total resistance is still increased. Therefore, the consumption of electric energy is increased. This phenomenon is more prominent when the current density is relatively large and the water flow velocity is relatively small. This is because the flow rate is too small to generate sufficient agitation in the water flow tank, so the phenomenon that the ion concentration is uneven is more remarkable.
2. The ionization of water in the concentrated water chamber occurs. When the concentration of ions in the fresh water chamber is relatively small and the current density is relatively large, the concentration of ions in the surface of the exchange membrane of the fresh water chamber is smaller due to the polarization phenomenon, which cannot meet the requirements of power transmission. More water is ionized into hydrogen ions and hydroxide ions, participating in the movement through the yang and the yin membrane. Thus, part of the current is used in the ionization of water, and a portion of the electrical energy is consumed in vain.
3. Cause scaling on the membrane. As indicated above, the result of polarization causes the water in the fresh water chamber to ionize into hydrogen ions and hydroxide ions. The produced hydroxide ions will enter the concentrated water chamber through the negative membrane, so that the water layer on the surface of the thick film is alkaline, so it is easy to produce precipitates such as Mg(OH)2 and CaCO3, and form scales. . Similarly, in the vicinity of the positive film of the fresh water chamber, since hydrogen ions permeate through the membrane and are transferred to the concentrated water chamber, the hydroxide ions remaining here also raise the pH, so that precipitates such as iron hydroxide are generated. .
As a result of fouling, the permeation area is reduced, the water flow resistance and resistance are increased, and the power consumption is increased. In order to reduce scale, it is often necessary to periodically replace the electrode, pickle the membrane with acid, and even remove the electrodialyzer to remove scale.