The issue of low chlorine purity and high free chlorine levels in brackish water
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This post was last edited by sunjl1981 on 2013-1-6 at 23:38. Since the plant was restarted after its recent major maintenance, the free chlorine content in the brine coming out of the electrolyzers is on the high side, ranging from 1200–1400 mg/l, whereas it used to be kept around 900 mg/l. At the same time, the purity of chlorine gas is low, at 97–97.5%, while it used to be above 98%. There are no abnormalities in the slot pressure; it is no different from before. The pH value of the brackish water coming out of the electrolyzer is the same as before; it’s a bit high, around 4.0. Our company uses electrolyzers from Beihua Machinery and membranes from Asahi Kasei; they have been in use for just one year. Currently, hydrochloric acid is not added to the anode solution in the tank; the manufacturer requires it to be added after two years of operation. With this situation occurring now, I’m thinking about whether hydrochloric acid should be added to the anode solution in the tank. If you all have any good suggestions, please share them. Let’s discuss the reasons behind this phenomenon and the solutions. # ， ， &
1. The most likely explanation is that during the shutdown for major maintenance, after the membrane was washed, its swelling properties changed, and some of the impurities on its surface were removed； 2. It will recover after running for some time ； 3. Under normal circumstances, the cell voltage decreases and the purity of chlorine decreases, which is manifested as a decline in anode efficiency ； 3. As for the extent of recovery, it depends on whether any damage occurred during parking and the severity of that damage. This post was last edited by yzhms on 2008-1-25 18:08.]
Regarding what the original poster mentioned, in my opinion: 1. If it is indeed as the poster said, problems might have occurred during major repairs, startup/shutdown processes, or maintenance (uncertain). 2. Regarding the addition of acid, it can be done at any time; the earlier it is done, the better. As long as the rate at which acid is added is controlled, that’s fine! 3. Check whether there is any change in cathode efficiency to assess the performance of the carboxylic acid layer. 4. The films of Asahi Kasei’s F4400 series were relatively thin in the past, and this phenomenon was quite common.
I think it’s better to add the acid earlier, so it will be ready sooner. But I don’t know why the manufacturer has such a requirement: no hydrochloric acid for two years.
Surface analysis: Not adding acid for two years can save nearly 2 million yuan (based on a scale of 60,000 tons and operation at the designed current density)
I think when adding acid, it is better to follow the advice of the membrane manufacturers.
You say: and the loss of some surface impurities； May I ask: if the impurities are removed, shouldn’t the membrane’s performance improve? How can the problem I mentioned still occur? Now, the issue I mentioned has returned to normal levels, as you said.
1. Our understanding is that this is the case, but no membrane supplier will agree to it； 2. If iron is present in the saline solution, it may form deposits on the anode surface; this can be improved by rinsing (personal understanding) ； 3. During the rinsing process, the complex salts formed by substances such as sodium sulfate that accumulate during production are reduced, which leads to a decrease in membrane efficiency ； 4. A greater impact may be the difference in operating conditions of the membrane during shutdown and during normal production ； 5. If there are no other major factors, such a decline can be reversed.
Let’s learn it together*; I’ve also thought about this issue