Relationship between ion membrane alkali production concentration and brine concentration entering the cell
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The current in the chlor-alkali cell remained unchanged, the amount of water added to the cathode stayed the same, as did the amount of salt water fed into the cell and the amount of salt water that was recycled. In other words, all flow rates and currents remained constant; I only changed the concentration of the salt water fed into the cell. Theoretically, with a constant current, KItnℓ should not change, so my alkali production should remain unchanged. Increasing the concentration of the salt water fed into the cell should result in a weaker salt water output (that is, an increase in the concentration of the weak salt water). But in reality, my alkali concentration continued to rise – could this be because fewer water molecules pass through along with the sodium ions, thereby causing the concentration to increase? Is there still an excess of sodium ions produced by electrolysis? Does the electrical efficiency still increase as the saltwater concentration rises?
Because each sodium ion carries 4 to 5 water molecules to the cathode as it passes through the ion membrane, I believe that as the concentration of brackish water at the outlet increases, the moisture content of the membrane decreases, and the amount of water that penetrates reduces. So it might be the increase in saltwater concentration where you are that has caused the rise in alkalinity.
The concentration of the brine fed into the tank should not be too high, as this can cause the ion exchange membrane to form bubbles. Moreover, by how much has the concentration of the brine fed into the tank increased, and by how much has the alkali concentration increased?
By increased concentration, I mean an increase that remains within the controlled range.
It shouldn’t be too noticeable that way; otherwise, it would be sufficient to simply increase the concentration of the saltwater entering the tank. With a constant current, the number of electrons passing through the electrolyzer should remain constant; as a result, the amount of hydrogen produced also remains constant. Even if more sodium ions pass through the ion membrane, this does not affect the material balance. You can monitor the power consumption of the electrolyzer over a few days; if the alkali concentration is high, the power consumption will definitely be high.
In actual production, indeed, 1) the concentration of the brine fed into the tank increases (with the flow rate of the brine remaining constant); for example, it goes from 300 g/l to 302 g/l, which leads to an increase in the concentration of caustic soda. 2. The flow rate into the tank increases (while the concentration of brine entering the tank remains unchanged), and the concentration of caustic soda increases. In these two cases, I believe that in either situation, the sodium ions in the brine entering the cell increase; accordingly, the amount of sodium ions passing through the ion membrane increases, which ultimately leads to an increase in the concentration of caustic soda.
With the current remaining constant, the total migration of Na and OH ions does not change! If current efficiency is not considered, that is, if the OH ions do not change, then the concentration of Na ions also remains unchanged! The only thing that changed was the amount of electromigration of water! Of course, an increase in saltwater concentration and membrane contraction also cause a slight increase in current efficiency!
Just looking at the concentration might lead to such results, but what about the actual production volume, electrical efficiency, and electricity consumption per ton of alkali produced? I think the power consumption per ton of alkali is the best indicator of performance; the others are not comprehensive enough
Personally, I think it is because the concentration gradient diffusion of water other than that carried by sodium ions decreases. An increase in the concentration of the anode solution has little effect on the ratio of the carboxylic acid layer to the sulfonic acid layer; since it is the carboxylic acid layer that determines the current efficiency, there might be less water carried by sodium ions, but it is not expected to be a significant decrease, otherwise the cell voltage would rise and the current efficiency would drop. When the vehicle is parked, water in the anode chamber can also migrate to the cathode chamber; I believe this phenomenon occurs as well when electricity is applied. As the concentration of the anode solution increases, it can be understood that the concentration of water decreases, and as a result, the amount of migration also decreases. It is said that there are also specialized water channels in the ion membrane. Regarding whether more sodium ions pass through the ion membrane, I think comparing the difference in concentration at the anode inlet and outlet before and after concentration adjustment could provide some guidance.