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Many processes within the chemical industry produce hydrogen chloride or sulphur dioxide as waste gases. As these processes are often carried out batchwise the gas streams arising from them are very variable in both quantity and composition.

In many countries there are strict maximum emission levels for these waste gases, which mean that they must be removed from any exhaust air streams. In the UK, typical limits as specified by HMIP are 30 mg/m3 for HCI, and 300 mg/m3 for SO2

The illustration is of an HCI/SO2 absorber system.

IIn the first column HCI is absorbed in water to make hydrochloric acid. In the second column, SO2 is scrubbed with caustic soda to form sodium sulphite. The scrubbed exhaust gas stream leaves the plant from the top of the second column.Other variants of this process are also possible. In one of these HCI and SO2 are absorbed in a caustic soda scrubber to produce a solution of sodium chloride and sodium sulphite.

Absorption of HCI

Hydrogen chloride can be readily absorbed in water to produce hydrochloric acid. This may then be sold or reused on the same site. If it is to be sold the acid must have a composition of about 30 wt% and be free from impurities. Because sulphur dioxide dissolves relatively easily in hydrochloric acid (1 to 4 wt% depending on operating conditions) a separate process stage for the separation of sulphur dioxide and any other impurities present could be necessary under certain circum-stances.

The absorption of hydrogen chloride is carried out by adiabatic absorption. The liquid is at its boiling point throughtout the absorber. Heat is released during the absorption and water vapour is condensed in the condenser at the top of the column.

Because the liquid is at its boiling point throughout the column the volatile impurity content of the HCI product is minimised. An additional steam stripper may be used to produce a pure acid with an impurity level of only a few ppm.

The product acid from the adiabatic absorption process will typically have an HCI content of about 30 wt%. If 38 wt% HCI is required then a falling film absorber should be used instead. In this process the heat of absorption is removed during the absorption, which is carried out in a vertically installed shell and tube heat exchanger.

Plate column are generally used for the adiabatic absorption process because of the low liquid loading. Where there is significant fluctuation in the gas loading, a packed column with several scrubbing stages is preferble. Borosilicate glass has proved itself to be the best material in many HCI absorption installations.

Absorption of SO2

Sulphur dioxide is much less soluble in water (or hydrochloric acid) than hydrogen chloride gas and a separate processing stage is required to process it.

The sulphur dioxide gas is scrubbed with caustic soda to from sodium sulphite which is discharged to the site waste water treatment plant. Because sodium sulphite has a limited solubility in water, dilute caustic soda is used as the scrubbing liquid.

The sodium sulphite will oxidise slowly in the waste water treatment plant to form sodium sulphate. Alternatively it may be desirable to perform this oxdiation in the SO2-absorption column using an appropriate oxidising agent.