Sweetening is a critical procedure in gas processing facilities. Carbon dioxide (CO2) and hydrogen sulfide (H2S) are common impurities in natural gas. These impurities need to be removed to different levels by sweetening process to remove contaminants; primarily H2S because of its corrosiveness and CO2 for its lack of heating value.
Gas sweetening units are found in gas production sites, gas processing plants, LNG facilities, refineries, petrochemicals, etc. This robust process is widely used to treat a range of gas streams, including natural, associated, tail and flare gas.
Adsorption and chemical absorption are frequently used to remove acidic components from a gas stream. Molecular sieve system is the most renowned method for adsorption processes which has the capability to adsorb CO2, H2S and mercaptans. On the other hand, chemical absorption processes work based on chemical reactions of acidic components with a material in the solvent solution. Alkanolamines and caustic are the most generally accepted and widely used of the many solvents available because of their reactivity and availability at low cost.
In a typical system, feed gas enters the top of the molecular sieve bed and flows downwards through the molecular sieve material to remove contaminants. The sweetened gas then exits the bottom of the unit, ready for downstream processing. In a two-vessel system, one adsorber is in use while the other is being regenerated for reuse, with flow automatically switched between the two units to optimize system performance.
In a gas sweetening unit using alkanolamines, the Inlet gas enters the bottom of the absorber column flowing upwards with the aqueous amine solution where the amine chemically absorbs H2S and CO2 to produce a sweetened gas stream. The sweet gas exits at the top of the absorber column while Rich amine solution containing H2S and CO2 enters the regeneration section causing the acid gas to be stripped from the amine and the regenerated lean amine is recycled into the absorption section.
Caustic wash is an alternative way to remove H2S and CO2 as well as mercaptans. Contrary to reversible amine reaction with acidic contaminants, reactions of H2S and CO2 with NaOH are very exothermic and cannot be reversed easily, so the used caustic is routed to spent caustic treatment unit. However, in case of mercaptans, caustic can be regenerated through the mercaptan oxidation process.
The design of acid gas removal package will vary to meet the specification of sweet gas which is achievable by controlling of foaming and degradation during the process, working in optimized operational conditions, employing suitable solvent concentration in case of chemical absorption and suitable molecular sieve material in case of adsorption.