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
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
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.