Influencing factors of current efficiency in Membrane Electrolyzer process
Membrane Electrolyzer is a key component of the ion-exchange membrane caustic soda process. The ion membrane should maintain high current efficiency and low cell voltage stably for a long time, reduce DC power consumption, and extend service life. The following briefly introduces the factors that affect current efficiency during the Membrane Electrolyzer process.
1. The influence of anolyte and catholyte concentration and temperature on current efficiency of Membrane Electrolyzer
The change of the anolyte concentration has a significant effect on the current efficiency. The concentration of NaCl in the anolyte decreases, and the current efficiency also decreases, because the decrease of NaCl concentration leads to an increase in the water content of the membrane, and the speed of OH- reverse osmosis increases and the current efficiency decreases. If the anolyte continues to run at a low concentration, it will cause the membrane to swell. In severe cases, it will cause membrane blistering and delamination, causing permanent damage to the membrane. The expansion of the membrane will cause Cl- to penetrate into the cathode compartment, resulting in excessively high salt content in the cathode alkali. At the same time, the concentration of NaCl decreases, and the number of water migration in the membrane will increase, which leads to an increase in the penetration of Cl- into the cathode chamber. When the anolyte concentration is too high, the water permeability of the membrane will decrease, the membrane voltage will increase, the power consumption will increase, and the membrane life will be reduced. Therefore, the mass concentration of the anolyte out of the tank is generally controlled at 200-220 g/L.
2. The influence of current density on the current efficiency of Membrane Electrolyzer
There is a temperature point at which the ideal current efficiency can be achieved under different current densities. The data shows that when the current of the ion-exchange membrane electrolyzer is 1.0～4.0 kA/m2, the current efficiency is rarely affected by the current density; but when the current density is lower than 1.0 kA/m2, the ratio of OH- to the anode side through the ion membrane Gradually increase, resulting in a decrease in current efficiency; at the same time, the running time under low current density should not be too long, otherwise it will cause the membrane performance to decline. Under the premise that the current density does not reach the limit current density, changes in a wide range have little effect on the current efficiency.
3. The influence of electrolyte flow on current efficiency of Membrane Electrolyzer
Stable and appropriate electrolyte flow is a necessary condition to ensure the smooth progress of Membrane Electrolyzer electrolysis process, and it is an effective way to improve current efficiency. The main purpose of ensuring the stability of the electrolysis flow is to prevent the ion membrane from blistering due to excessive local brine concentration or lack of liquid, and to reduce the reverse migration of OHˉ. At the same time, during the electrolysis process, the bubble effect has a significant influence on the voltage of the electrolytic cell. During the electrolysis process, a large amount of hydrogen is produced in the electrolytic cell, which increases the gas rate in the cell, and the amount of gas attached to the ion membrane and the electrolytic cell will inevitably increase, resulting in an increase in cell voltage.
4. The influence of pure water supply on current efficiency
The concentration of NaOH in the catholyte is controlled by the flow of pure water. When the pure water supply is abnormal, the concentration of NaOH will be unstable and the current efficiency will decrease. If the impurity content in the pure water is too high, it will cause the impurity content in the finished alkali to be too high, which will cause the quality of the product to decrease; at the same time, the impurity heavy metal ions are reduced by the current and adhere to the side of the cathode tank, causing the cell voltage to rise. High, resulting in lower current efficiency. Therefore, the supply of pure water is also a necessary condition to ensure the smooth operation of the ion-exchange membrane electrolyzer, as well as an effective measure to improve current efficiency.