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If the hydrogen chloride gas produced can be economically processed into hydrochloric acid and the gas loading does not fall below a certain minimum value, which is 50% of the designed maximum, then adiabatic absorption is the most effective means of production.

Adiabatic absorbers produce, under normal operationg conditions, hydrochloric acid of 30-31% strength. This compares with a theoretical maximum of 35% under similar operating conditions.These absorbers as their name implies, operate without any heat input into the reaction zone. In the process hydrogen chloride gas is absorbed by fresh water flowing down the column.

 

The heat generated by this reaction vaporises approximately 30% of the water which rises to the head of the column with any other non-soluble components of the gas stream.

Pure hydrogen chloride streams containing only air, water vapour and/or non-condensibles, are condensed within the column and are returned together with the make up water to the packed section.

A typical arrangement is shown in Figure 2.

If the hydrogen chloride is contaminated with condensibles (other than water), i.e. the by-product of a chlorination process, an arrangement as shown in Figure 3 must be used. In this case the steam generated together with any impurities rises to the head of the column and is condensed in an external heat exchanger. In some cases it is then possible to separate aqueous and organic phases and return the water to the column.

Whilst approximately 80% of the heat of solution is removed by the condensers at the head of the column, the remainder is removed at the base before the product is run off. The heat exchanger supplied with the unit guarantees an acid outlet temperature of approximately 30 °C to 40 °C

The maximum possible gas loading and therefore the quantity of acid produced is the same for both types of absorber, and depends basically upon the type of packing material in the column.

Performance data for Corning Process Systems hydrogen chloride absorbers can be obtained from
Table 1.