Selective gas treating refers to the preferential removal of hydrogen sulphide from a sour gas stream while rejecting most of the accompanying CO2 or delaying the recovery of CO2 until a subsequent processing step. Selective treatment using chemical solvents is usually based on the more rapid pickup of H2S compared to CO2. Thus, the contact time between the solvent (MDEA) and sour gas is limited to permit removal of the H2S only to the degree required and then to stop the contact so that only minimum co-absorbtion of CO2 occurs. Increased selectivity for H2S over CO2 expands the regeneration capacity of an amine unit, reduces the energy required for treating and improves the H2S quality of the acid gas.
The selectivity of MDEA and related solvents can be influenced by:
- Contact temperature – colder processing (less than 90°F) or hotter processing (greater than 120°F) results in improved selectivity.
- Contact pressure – lower pressures improve selectivity.
- Feed gas CO2/ H2S ratio – higher ratios of CO2 to H2S favor selectivity.
- Total acid gas loading.
- Location of lean amine feed point on contactor tower.