Impact of Processing Heavy Coker Gas Oils in Hydrocracking Units

摘要

The hydrocracking process unit is one of the most versatile refinery upgrading units thatrnproduces a wide range of products including ultra-low sulfur, low aromatics diesel fuel and highrnsmoke point kerosene. Chevron Lummus Global’s (CLG) hydrocracking experience began withrnChevron’s invention of modern hydrocracking, ISOCRACKING, more than 50 years ago. As anrnentity since 1993, CLG has repeatedly been the first to introduce technologies and catalysts thatrnenable refiners to economically, safety and reliably meet today’s tough new standards forrnproducing cleaner transportation fuels.rnWith the onset of new grass roots refineries configured with delayed coking as the primary lowrncost residuum upgrading unit and hydrocracking as the gas oil upgrading unit, hydrocracking ofrncoker-derived products becomes a key factor in refining operations. When faced with inquiriesrnfrom the refining industry, licensors of delayed coking technologies are challenged to maximizernliquid yields and, as such, typically employ low to ultra-low recycle rates and high recycle cutrnpoints in the design of the delayed coking unit. However, these designs result in a very highrnboiling incremental liquid yield of heavy coker gas oil (HCGO) which contains very highrnconcentrations of polycyclic aromatic compounds and other contaminants. These compoundsrntend to rapidly deactivate hydrocracking and/or hydrotreating catalysts and significantly reducernthe catalyst cycle time. Hydrocracker licensors have responded to this by emphasizing a lowerrnHCGO TBP end point, lower asphaltenes, lower Conradson carbon, and lower metals (Ni+V)rncontent. These properties are directly related to the end point of the HCGO. Despite extensivernplanning, quality of coker gas oils can vary significantly with drum swings. As a result, thernhydrocracker will routinely see some amount of coker gas oil with a quality significantly worsernthan planned. If catalyst lives are to be maintained at a given run length, hydroprocessingrndesign parameters must be adjusted, resulting in directionally higher costs.rnBecause of this effect, the coker operating parameters have to be optimized to balance cokerrnand hydrocracking costs against the incremental overall product yield and reduced cokernproduction. In essentially every case such an optimization will reduce the recycle cut point andrnincrease the recycle rate relative to a design based solely on coker considerations. The savingsrnin overall investment and operating cost pay for the slight loss of liquid yield and increase inrncoke production. Directionally, the optimization is aimed at balancing the overall liquid yieldrnagainst total investment and operating costs for the coker/hydrocracker combination.rnOver the years, CLG has developed innovative process configurations to effectively process thernhigh boiling vacuum gas oil (VGO)/HCGO gas oils while maximizing distillates yield withrnoptimum hydrogen utilization via the ISOCRACKING and ISOTREATING processes andrncatalysts.rnThis paper outlines the strategies employed by CLG to hydrocrack high boiling VGO/HCGOrnblends. Reference to commercial operation is included in this paper.