Usually crude oil extracted from geological sources is accompanied by hydrogen sulphide gas (H2S). This H2S is separated from rest of the oil at the atmospheric distillation unit along with lighter hydrocarbons and collectively termed as 'sour gas'. H2S is separated from hydrocarbons at the amine treatment unit and H2S rich gas flow is sent to sulphur recovery unit (SRU) for extracting elemental sulphur from H2S gas.
The basic process used in SRU is 'Claus Process' which involves combustion of a part of H2S gas to form SO2. Later this SO2 is reacted (conversion) with H2S in presence of catalyst (Cobalt / molybdenum / aluminum oxide ) to form elemental sulphur and steam.
The main chemical reactions involved are:
2H2S + 3O2 → 2SO2 + 2H2O (Combustion reaction)
4H2S + 2SO2 → 6S + 4H2O (Conversion reaction)
Note that around one third of the H2S is first combusted and then reacted with rest of the H2S to give elemental sulphur and steam as end products.
Generally upto 97% of H2S to elemental sulphur is achievable with the use of multiple conversion reactors. If the remaining H2S has to be removed from the tail gas due to environmental concerns, the remaining tail gas can be sent to a Tail Gas Treatment (TGT) absorber.