CLAUS UNIT NITROGEN AND ACID GAS CO-FEED OPERATION TO MITIGATE RELIABDLITY AND ENVIRONMENTAL SENSITIVITIES AT TURNDOWN CONDITIONS

摘要

Switching Claus sulfur recovery unit(SRU)operation between acid gas and natural gas firing is performed as part of SRU shutdown,startup,or hot-standby when acid gas production is below the unit's turndown capacity.Firing a SRU on natural gas frequently,or for prolonged periods,can result in serious equipment and process reliability issues.These issues include burner damage,refractory spalling from thermal cycling,catalyst deactivation,and/or soot accumulation leading to capacity reduction.If acid gas rates are low for extended periods,shutting down a SRU after natural gas firing can also lead to accelerated equipment corrosion and plugging of sulfur rundown lines. At one of ExxonMobil's refineries,a SRU was frequently placed on hot-standby,firing natural gas,during planned and unplanned refinery unit outages.On many occasions,site acid gas production dropped below the combined turndown limit of the SRU trains,and one specific train was designated as the"swing"unit.Placing the swing unit on natural gas hot-standby became more frequent as the site shifted from processing high to low sulfur crudes,reducing acid gas production to historical lows.Prior to the most recent turnaround,the hydraulic capacity of the swing unit was reduced by-40% due to severe sooting caused by frequent natural gas firing operations. A strategy of co-feeding nitrogen(N2)with acid gas(AG)was developed and successfully tested at multiple ExxonMobil and Imperial Oil sites to mitigate the reliability concerns associated with natural gas hot-standby operation and to minimize acid gas flaring when operating below the turndown limit.At one ExxonMobil refinery,applying N2-acid gas co-feed as part of a wider strategy increased the overall turndown capability by approximately 35%,allowing the unit to continue mnning instead of going into natural gas hot-standby.Supplementing the N2 co-feed with minimal natural gas co-firing has also been performed to prevent low air flow trips and mitigate other process sensitivities during this mode of operation. This paper discusses the reliability and environmental drivers,"development,and successful implementation of the N2-AG co-feed strategy at a number of ExxonMobil and Imperial Oil sites.This strategy can be used during unit upsets,turnarounds,or re-start operations following site-wide shutdowns such as those caused by hurricanes or power outages.