Natural gas fields that contain higher amounts of carbon dioxide (CO2) than hydrogen sulfide (H2S) pose significant challenges for sulfur recovery units (SRUs) that utilize conventional Claus-based recovery processes. In such instances, the use of acid gas enrichment (AGE) technology may be required in order to increase the H2S content in the acid-gas stream to improve Claus plant performance and economically exploit the reserve.
The primary objective of AGE is to maximize CO2 slip in the reject gas steam while minimizing H2S leak. This results in an increase in overall enrichment and decreases the amount of sulfur emissions from the plant. AGE can also be used to minimize the volume of acid gas for reinjection or to debottleneck facilities in the event that new fields containing sour gas are to be developed.
Typically, this is done using an absorber, which treats the low-quality acid gas stream exiting the regenerator. Because these streams consist primarily of acid gases and water, it is not uncommon for the entire sour gas stream to be absorbed using an adequate amount of solvent. In some cases, enrichment can also be achieved by recycling a portion of the separated acid gas back into the contactor.
With regards to improving sulfur recovery through AGE, it’s important for operators to consider cost tradeoffs as a result of both higher solvent circulation and reboiler energy usage. In all cases, determining the feasibility of a specific AGE unit or technology will require a comprehensive evaluation of process parameters, such as lean solvent temperature, feed gas H2S-to-CO2 ratio, internal tower specifications, and type of solvent, as these inputs will ultimately dictate the overall efficiency and effectiveness of the enrichment process.