Meeting Ultra-Low Sulphur Dioxide Emissions for Projects Based on Sour and Contaminated Gas Reserves

摘要

Designing gas treating line-ups for liquefied natural gas (LNG) and natural gas liquids (NGL) extraction facilities with low hydrogen sulphide (H2S) to carbon dioxide (CO2) ratios in the feed is challenging. The complexity increases by an order of magnitude as the emission requirements become very stringent. Sulphur dioxide (SO2) emission requirements vary from a more typical 150 to 50 ppmv. The acid gas from the acid gas removal units (AGRU) in these facilities i￡ typically very lean, whereas Claus sulphur recovery units (SRU) prefer rich acid gas feeds. The presence of benzene, toluene, ethylbenzene, and xylenes (BTEX) and mercaptans in the original feedstock add an additional layer of challenges. It is necessary to maintain a sufficiently high temperature in the Claus furnace for the efficient destruction of these components. This may not be easily attainable with lean acid gases. In this scenario, it may be necessary to add an acid gas enrichment unit (AGEU) to the processing line-up. However, including an AGEU in the line-up would introduce AGEU off-gas containing H2S and mercaptans, which would be routed directly to the incinerator. This increases the SO2 that is emitted to atmosphere, therefore making it very challenging to meet stringent SO2 emission (<50 ppmv) limits. The conventional gas processing line ups are inadequate when dealing with difficult natural gas reserves in low SO2 emission regimes. This is especially true in the presence of organic sulphur (mercaptans and carbonyl sulphide (COS)) and BTEX. While bulk COS removal can be achieved with secondary amines, removal of mercaptans will require hybrid solvents like Sulfinol. The integration of gas treating process units and new breakthrough technologies are necessary to achieve optimal designs. Shell's novel gas processing line-ups address these challenges head-on. This paper focuses on one such innovative integrated line-up and compares it with conventional line-ups for compliance with SO2 emission requirements. The chosen line-up involves the Shell CANSOLV* TGT+ (tail gas treating plus) system, which is an integrated sour gas treating solution that can ensure ultra-low SO2 emissions (less than 10 ppmv), even for natural gas reserves containing unfavourable H2S/CO2 feed ratios and significant BTEX and mercap-tan contamination. It also minimises the complexity of the process line-up. In addition, the Shell CANSOLV TGT+ line-up can improve operational flexibility and the gas processing operating window at comparable capital cost.