P. CHILUKURI, Shell Global Solutions, Amsterdam, The Netherlands; and
A. BHATTACHARYA, Shell Global Solutions, Bangalore, India
Gas development projects face growing challenges from increasingly sour resources with relatively high levels of carbon dioxide (CO2), hydrogen sulfide (H2S), mercaptans and carbonyl sulfide (COS), as well as tighter sales specifications and stricter environmental emissions standards. The removal of trace components, such as COS and thiol (RSH), can have a detrimental effect on project value, as considerable capital expenditures (CAPEX) and operational expenditures (OPEX) investments must be made.
Predominantly, mercaptans are either removed in an acid gas removal unit (AGRU) with the use of hybrid solvents, or slipped to a downstream dehydration/mercaptans removal adsorption unit using molecular sieves (molsieves). While the former lineup involves a single process unit to remove all sulphur compounds (H2S, RSH and COS) and CO2, the latter process lineup involves multiple process steps, including an AGRU absorber with aqueous amines to remove H2S and CO2.
It also involves a molecular sieve downstream unit to dehydrate and remove mercaptans, in combination with a dedicated physical or hybrid solvent for the removal of mercaptans from molsieve regeneration gas.
A perception exists that the use of hybrid solvents reduces revenue through hydrocarbon absorption losses. Simple hydrocarbon-loss percentages around the AGRU absorber provide some insight, but this data must be used cautiously, as it alone will not tell the full story.
Previous work has shown that, in most cases, higher hydrocarbon losses through coabsorption with hybrid solvents compared with using aqueous amines, such as methyl diethanolamine (MDEA), are offset by savings in reboiler energy consumption. This article goes a step further by modeling the full lifecycle costs of the two conventionally used process lineups so that informed solvent selection and capital investment decisions can be made. Capital and operating cost savings associated with hybrid solvents are also discussed. These savings further tip the economic balance in favor of hybrid solvent systems.