Acidification of Feed Brine

Acidification of brine before sending it to the electrolyzers is the standard but not universal practice. Purified brine is alkaline and contains carbonate added in excess during chemical treatment. Both OH and COi reduce the effective anode current efficiency by consuming chlorine in the cells. The addition of HCl to the brine neutralizes the contained OH and decomposes the CO to CO2 and water. More chlorine is produced with no increase in electrical load. Addition of excess acid can even neutralize some of the OH that enters the anolyte by leakage back through diaphragms or membranes. Section 4.4.3 gives a more fundamental explanation of the effects of acid addition. 

This improvement in yield may be illusory in a plant that produces its own HCl for this purpose. While more chlorine is produced in the cells, it is then consumed in HCl synthesis to allow the return of the acid to the process. In a plant without an HCl unit. 

This section discusses the technical aspects of brine acidification and considers the requirements for acid supply. 

operation of the cells can be safe down to a feed brine pH less than one 

there is more scatter of the data in Fig. 7.78(b), probably reflecting the fact that the anolyte pH is the more fundamental variable 

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The most important impurity in chlorine is oxygen, particularly so in its major application, the synthesis of ethylene dichloride. As discussed in Section 9.6.1 and in parts of Chapter 11, acidification of feed brine is a common practice that helps to reduce the oxygen content of the cell gas. The acid of choice is HCl, and this is frequently produced on site from the cell product gases. 

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