Transient H2S content rise in the effluent of a natural gas treating unit: Role of COS hydrolysis and heat stable amine salts

Abstract

In response to expanding requests towards natural gas usage as a key energy source and vital feedstock, revamping current gas refineries to provide additional capacity and meet the sustainability limitations is considered a crucial essence. In this work, the behavior of a gas refinery after switching its amine solution from diethanolamine to methyldiethanolamine was evaluated. The plant's main goals by this change, i.e., increase sweetening capacity and lowering energy consumption intensity was fulfilled. However, after some overhaul services, the hydrogen sulfide content of final treated gas raised from permissible limit, i.e., 3 ppm at least for three initial months of operation, and then progressively reduced to around 2 ppm. Systematic investigation of plant data shows that carbonyl sulfide hydrolysis in the dehumidification section as downstream of the sweetening section would increase the hydrogen sulfide content of the treated stream. Meanwhile, the gradual formation of heat stable amine salts in amine solution promotes its absorption capacity, which can counterpoise the effect of further hydrogen sulfide generation due to carbonyl sulfide hydrolysis in the subsequent section. Monitoring heat stable amine salts concentration in amine solution suggests that there should be a minimum level of 0.25 wt.% of these compounds to achieve suitable hydrogen sulfide content in final treated gas.