The tail gases from the Claus Units contain appreciable quantities of sulphur compounds which must be reduced before ultimate disposal of the gases to the atmosphere. The sulphur compounds in the tail gases are principally unreacted hydrogen sulphide (H2S) and sulphur dioxide with traces of carbonyl sulphide and carbon disulphide.
The Tail Gas Clean-Up Unit reduces these sulphur compounds to a level consistent with emission regulations. The Claus Unit tail gases from each train are combined, heated, and mixed with hydrogen. This mixture is then passed through a bed of cobalt-molybdenum catalyst. All of the sulphur compounds are reduced to hydrogen sulphide in this reaction.
The reactor effluent is then cooled in order to remove excess water vapour, which concentrates the hydrogen sulphide (H2S) in the gas phase. The liquid water produced is either recycled within the Upgrader or is fed to the Sour Water Stripping Unit. The cooled gas, now mainly nitrogen (from the Claus combustion air and the burned ammonia). H2S and excess hydrogen is sweetened with very lean alkanolamine solution in a low pressure amine absorber. The resulting sweet gas, containing only a few hundred parts per million of H2S, is then burned in the sulphur plant incinerator and released to the atmosphere via a tall stack. The rich alkanolamine solution is regenerated with stripping steam in a regenerator similar to the amine system regenerator. The H2S released is mixed with H2S from the main amine regenerators and is thus recycled to the Claus Units as feed gas.
The combination of the Claus Units and the Tail Gas Clean-Up Unit provides a design sulphur recovery of 99.9%
Used with permission (© SAIT Polytechnic, MacPhail School of Energy, 2009)