Gas sweetening is the process for the removal of mainly acid gases (H2S and CO2) and, in addition, the simultaneous removal of sulphur organic species (RSH, COS, CS2) from process gas. It is an essential step of sour gas processing for natural gas treatment, NGL recovery, LNGs, refineries and petrochemicals in order to meet transport and market specifications, to comply with environmental regulations for emissions and to control corrosion. As a consequence, concentrations must be limited. For example, for a commercial natural gas the residual acid concentration is usually 4 ppmv of H2S and 2% CO2 with a mercaptans content lower than 15 mg/Sm3 whereas in natural gas liquefaction (LNG) the CO2 content must be 50 ppmv and the H2S lower than 4 ppmv.
The removal of acid gases (H2S and CO2) can be achieved with a wide range of different technologies. The chemical absorption of acid gases on amines is the most common but molecular sieves and non-regenerative adsorbents can be also used for specific applications. Chemical absorption of acid gases can be improved by use of activators or by combined use of amines and physical solvents allowing the removal of additional sulphur compounds (RSH, COS, CS2).
A list of available Gas Sweetening processes is the following:
Conventional Amines: MEA, DEA, MDEA, DIPA. The most widely used is the MDEA as it is more selective to H2S with lower regeneration heat while DEA is used in case total CO2 and H2S removal is required. A higher selectivity of H2S vs CO2 can be achieved with use of conventional MDEA by means of a special design with very low residence times in Amine Absorber reducing CO2 absorption and allowing a higher H2S/CO2 ratio in acid gas. The same principle is used for Acid Gas Enrichment upstream the Sulphur Recovery Units
Activated Amines: amine (MDEA or DIPA) is mixed with an activator to modify absorption and regeneration properties such as improving the efficiency of CO2 removal, improving the selectivity for H2S, reducing regeneration heat and allowing absorption of COS or RSH, etc.
Mixed Solvents: amines are mixed with physical solvents in order to enhance physical absorption of sulphur compounds
Molecular Sieves: normally used for the removal of mercaptans and H2S from gas phase at a concentration of a maximum of some hundreds of ppmv while CO2 can be efficiently removed at a max of 0,1 %. Mol. sieves are 5A type or larger and the typical process scheme is based on open loop regeneration (spent regeneration gas is not recycled). Mol. sieves can be also used for the sweetening of NGL or LPGs
Non-Regenerative Adsorbents: used for the removal of H2S at lower concentrations (typically lower than a hundred ppmv) and based on high activity metal oxides which by reaction with H2S are converted to stable sulphides.