Why is recirculated absorption not used for the absorption of hydrochloric acid in a falling film process?
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Question: Why is recirculation not used for the absorption of hydrochloric acid in falling film process, whereas it is necessary for the absorption of chlorinated exhaust gases in the falling film process? Can a non-circulating flow meet the minimum wetting flow rate for falling film tubes?
The source of hydrogen chloride gas for synthesizing hydrochloric acid is the hydrogen chloride synthesis furnace, and the flow rate of hydrogen chloride in this furnace can be controlled stably. When absorbing hydrochloric acid, it is sufficient to regulate the ratio between the flow rate of the absorbent water and that of the hydrogen chloride gas in order to obtain hydrochloric acid with a stable concentration; therefore, no continuous circulation is required to achieve the desired concentration of hydrochloric acid. The process of producing hydrochloric acid from hydrogen chloride generated as a by-product of the chlorination reaction is different; the flow rate of hydrogen chloride changes at various points during the chlorination process. The peak flow rate of hydrogen chloride can be several times, or even dozens of times, higher than the low flow rates. Therefore, it is difficult to achieve the desired concentration of hydrochloric acid in just one step without using a recirculating absorption system. As a result, processes for producing hydrochloric acid from hydrogen chloride generated as a by-product of chlorination reactions are usually designed to utilize recirculating absorption.
If required, unstable chlorination reactions, as well as the by-product hydrochloric acid, can also be designed to produce hydrochloric acid of a specific concentration directly, without recycling it.