Seek help regarding hydrochloric acid falling film
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Question: Why is recirculation not used for the absorption of hydrochloric acid in falling film processes, whereas it is necessary for the absorption of chlorinated exhaust gases in such processes? Can a non-circulating flow meet the minimum wetting flow rate for falling film tubes?
Whether to cycle or not actually depends on the concentration you want to achieve.
Hydrochloric acid is a volatile acid; if 31% concentration hydrochloric acid is subjected to cyclic absorption, unnecessary volatilization losses will occur. If properly designed, a falling film absorber should be able to meet the minimum wetting flow rate of the falling film tubes even without recycling 31% hydrochloric acid. The absorption of chlorinated exhaust gases using alkaline solutions to produce sodium hypochlorite is a form of chemical absorption, and the volatility of 10% sodium hypochlorite solution is much lower than that of 31% hydrochloric acid; therefore, cyclic absorption can be employed.
……It is likely related to the amount of hydrogen chloride in the chlorinated exhaust gases and their stability; when the amount is sufficient and the volume of the exhaust gases remains stable, it is possible to adjust the flow rate of the absorbent liquid used. After passing through the graphite absorber, the liquid goes directly into the hydrochloric acid tank, and the concentration of the resulting acid falls within the required range. :)
In aqueous-phase chlorination processes such as CPE, CPP, and CPVC, the hydrogen chloride generated during the reaction essentially remains in the aqueous phase within the chlorination reactor. After filtration, dilute hydrochloric acid with a concentration of approximately 10–15% is obtained； The chlorinated off-gases leaving the chlorination reactor mainly consist of unreacted chlorine gas and inert gas components such as nitrogen; these are generally absorbed using an alkaline solution to produce sodium hypochlorite solution.