In a sulphur burning or metallurgical or spent H2SO4 regeneration plant where no Oleum is produced upstream, SO3 gas exits the fourth catalyst pass, is cooled down and enters the second (= final) absorption tower, where H2SO4 is distributed over ceramic packing to absorb the SO3 . When the gas exits the F.A.T. it exits to atmosphere, unless some Tail Gas Scrubber is running downstream. Due to the quite high inlet gas temperature and the relatively low inlet acid temperature, mist is formed thermodynamically and acid droplets are mechanically entrained from the acid distribution system or packing.
In a spent H2SO4 regeneration plant the mist and droplets combine typically as a load of 1,000 – 3,000 mg/m3. In sulphur burning or metallurgical plants the mist and droplets combine typically as a load of 750 – 2,000 mg/m3. The particle size spectrum will be sub-micron to medium sized ( 0.5 to 5 micron ), average 2 microns.
Prevention of air pollution, generally to meet local emission or stack opacity limits. Generally authorities require that exit mist loads should not exceed 20 mg/m3. Below 20 mg/m3 of exiting H2SO4, no stack plume is visible.