Problem of removing free chlorine during shutdown of ion membrane electrolyzers
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We are using Woody ion-exchange membrane electrolyzers. After shutting down the system, the concentration of brine is reduced from 305 g/l to 275 g/l, and the temperature is kept above 75°C; polarization is applied to remove free chlorine. Questions: 1. Why is it necessary to dilute the brine concentration? 2. Why must the temperature be maintained, and why cannot free chlorine be removed while the temperature is being lowered? Why wait until the free chlorine is completely gone before cooling down? One more question: Does using a polarized power supply while driving generate free chlorine? After polarization, if a problem is detected with the cell, the system is stopped (no power is supplied); is it necessary to remove free chlorine again?
1. Why dilute the saltwater concentration? To ensure a concentration difference between the anode and cathode solutions and prevent membrane bubbling caused by reverse ion migration. Furthermore, after the electrolyzer is shut down, the temperature drops; if the saturated brine is not diluted, crystals may precipitate in the brine, thereby scratching the ion exchange membrane. 2. Why is it necessary to maintain the temperature? Why can’t free chlorine be removed while the temperature is being lowered? Why wait until the free chlorine is completely gone before cooling down? I think maintaining a certain temperature might be more beneficial for the removal of free chlorine. One more question: Does using a polarized power supply while driving generate free chlorine? After polarization, if a problem is detected with the cell, the system is stopped (no power is supplied); is it necessary to remove free chlorine again? When a polarization power supply is applied, free chlorine generally does not form because the polarization voltage of around 1.8 is relatively low. So it’s not necessary.
1. Why dilute the saltwater concentration? To ensure a concentration difference between the anode and cathode solutions and prevent membrane bubbling caused by reverse ion migration. Furthermore, after the electrolyzer is shut down, the temperature drops; if the saturated brine is not diluted, crystals may precipitate in the brine, thereby scratching the ion exchange membrane. 2. Why is it necessary to maintain the temperature? Why can’t free chlorine be removed while the temperature is being lowered? Why wait until the free chlorine is completely gone before cooling down? I think maintaining a certain temperature might be more beneficial for the removal of free chlorine. One more question: Does using a polarized power supply while driving generate free chlorine? After polarization, if a problem is detected with the cell, the system is stopped (no power is supplied); is it necessary to remove free chlorine again? When a polarization power supply is applied, free chlorine generally does not form because the polarization voltage of around 1.8 is relatively low. So it’s not necessary.
1. Why dilute the saltwater concentration? To ensure a concentration difference between the anode and cathode solutions and prevent membrane bubbling caused by reverse ion migration. Furthermore, after the electrolyzer is shut down, the temperature drops; if the saturated brine is not diluted, crystals may precipitate in the brine, thereby scratching the ion exchange membrane. 2. Why is it necessary to maintain the temperature? Why can’t free chlorine be removed while the temperature is being lowered? Why wait until the free chlorine is completely gone before cooling down? I think maintaining a certain temperature might be more beneficial for the removal of free chlorine. One more question: Does using a polarized power supply while driving generate free chlorine? After polarization, if a problem is detected with the cell, the system is stopped (no power is supplied); is it necessary to remove free chlorine again? When a polarization power supply is applied, free chlorine generally does not form because the polarization voltage of around 1.8 is relatively low. So it’s not necessary.
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I think it is possible to remove free chlorine while cooling down.
The process of using brine circulation to remove free chlorine is itself a cooling process; typically, the steam used for heat exchange in the cathode liquid plate is not heated, and the temperatures on both the cathode and anode sides gradually decrease. In my opinion, this cooling process should not be too rapid, and the valve controlling the cooling water flow to the cathode should not be opened too wide, in order to avoid large temperature differences across the ion membrane, which can cause wrinkles. This phenomenon tends to occur in older electrolyzers.
During system shutdown*, free chlorine is removed while the system cools down; this process usually takes one and a half hours.
The operating procedures for cells in Wood’s plants differ from those in Clorox’s plants. Although we don’t use Wood’s procedures, we do have cells at Ineos, and their operating methods are quite similar to Wood’s. It requires the use of brine at a temperature above 70°C to remove free chlorine when parking, and since this type of tank does not have a circulation system for fresh brine, free chlorine can be completely removed within one hour. After the free chlorine is removed, depolarization current can be reduced, the temperature can be lowered (with brackish water), and the tank can be sealed. The reason for requiring a temperature above 70°C is that at such temperatures, free chlorine causes significant corrosion of the electrode mesh; therefore, by removing free chlorine, the polarization current can be reduced, which is different from the operation in chlorine engineering cells.