Reactivities of Sulfur Compounds in HDS

Recently, investigations have demonstrated that sulfur compounds remaining in diesel fuels at sulfur levels lower than 500 ppm are the dibenzothiophenes (DBTs) with alkyl substituents at the 4- and/or 6-position, and are lower in HDS reactivity . These species are termed refractory sulfur compounds. Both steric hindrance and electronic factors are responsible for the observed low reactivity of 4- and 6-subbstituted DBTs .  

To put these problems into perspective based on conventional approaches for HDS of diesel fuels, for reducing the sulfur level from current 500 ppmw to 15 ppmw (the regulation in 2006) by conventional HDS processing, the volume of catalyst bed will need to be increased by 3.2 times as that of the current HDS catalyst bed. This is consistent in general with the analysis on a t3 ical Co-Mo catalyst by Haldor Topsoe regarding the required increase in catalyst activity and bed temperature for further reduction of sulfur from 500 

The NPRA (National Petroleum Refiners Association) estimates that imposing a 15-ppmw sulfur maximum on diesel could result in a 10% to 20% shortfall in supply . The above trends and discussion indicate that the petroleum refining industry is facing a major challenge to meet the new stricter sulfur speeifieations for produeing ultra elean transportation fuels in the early 21 eentury when the quality of the emde oils continue to deeline in terms of inereased sulfur eontent and deereased API gravity. 

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