Reasons for the increase in cell voltage in zero-gap electrolyzers

This is the first time I’ve heard of such a situation. Could you explain in detail what happened at that time?

Is it when stopping and starting the vehicle, or does it increase suddenly while in operation? It seems there’s a problem either when stopping or starting the vehicle

The description of the phenomenon is not comprehensive enough; there are a few points to note. If the rise is sudden and uniform, it is generally due to saltwater. If the rise is gradual and the individual units differ, it is usually a case of first decreasing and then increasing, which is generally a problem related to zero-offset adjustment. When the membrane is damaged, the voltage drops first.

Why does the cell voltage drop first when the membrane is damaged?

I’m sorry, guys; I didn’t explain it clearly enough. I’ll explain in detail. Our company uses Asahi Kasei R-230 electrolyzers; starting from 2012, we have gradually replaced them with zero-gap electrolyzers. A major overhaul was carried out on June 4 this year, and the units were put back into operation on June 12. Before stopping, the DC power consumption per ton of alkali was around 2150–2160, with an average efficiency of the electrolyzer at around 95.6%. On June 2, it was found that the levels of calcium and magnesium were again above the limit, at 161 PPB; measures were taken to reduce these levels. The plant was shut down for major repairs on June 4, and after the repairs, the membranes and electrolyzers were replaced and inspected. On June 12, driving was carried out; due to the fact that the maintenance tasks were not completed satisfactorily, the operation was not smooth, with repeated stops of individual tanks. During the start-up and shutdown process of tank D, the polarization power supply was activated, but the polarization procedure was not followed as specified on three occasions – namely, the flow rates of brine and caustic soda did not change. After the stoppage, purified brine was used for circulation, which led to a galvanic cell reaction. After driving normally, it was found that the voltage in slot D had increased by 18 volts; the voltages in the other slots increased slightly but not significantly – slot B saw an increase of 7 volts, slot F an increase of 6 volts, and slot H an increase of 3 volts. The voltages in the remaining slots did not change. The current was 13 KA. The specific consumption of caustic soda rose to 2200. Could everyone analyze whether this is related to the fact that the polarization procedure wasn’t run? Is it still caused by early-stage calcium and magnesium pollution? Which reason is more significant?

When the membrane is contaminated with calcium and magnesium, the tank temperature rises simultaneously; the failure to activate the polarization rectifier in a timely manner has an impact on the electrode coating, which also leads to an increase in tank temperature. What is the operating current?

Based on your description, it seems that calcium and magnesium contamination is the main cause. The polarization process was not carried out as specified on 3 occasions, but saltwater and caustic soda are still in circulation. The fact that the flow rate hasn’t changed likely means it hasn’t reached the set value; you can address this by increasing the time allowed for the process to proceed. If the saltwater is still in circulation, it means that the anode solution is constantly being replaced. This situation has an impact on the electrolyzer, but it does not cause such a significant increase in cell voltage; therefore, calcium and magnesium contamination is the most likely explanation.

With such high levels of calcium and magnesium, why don’t you stop production immediately and reduce output? ! This causes irreversible damage to the ion membrane, which is the main reason for the increased power consumption and reduced membrane efficiency there. When the machine is stopped, the polarization current does not function as it should, causing destructive damage to the cathode coating of the electrolyzer; this is also the reason for the increase in voltage in cell D. Therefore, the operations on the ion membrane must be carried out with utmost precision; making a mistake can result in permanent damage.

In all versions, calcium and magnesium contamination should be widespread; why does it only affect certain tanks?

So, regarding the D-slot, I personally think it is related to the operation of the polarization current. If it has no impact on the slot voltage, then why is a polarization rectifier needed?