Hydrodesulfurization

Hydrodesulfurization (HDS) is a catalytic process that removes sulfur from fossil fuels, including crude oil fractions and petrochemical feeds, converting organosulfur compounds to hydrogen sulfide and saturating hydrocarbons to yield lower-sulfur fuels.

The process uses hydrogen and a supported metal sulfide catalyst, typically cobalt or nickel promoted molybdenum or tungsten on alumina or silica-alumina. It operates in fixed- or moving-bed reactors at elevated temperature and pressure, commonly around 340–420 °C and 30–100 bar hydrogen, with high hydrogen partial pressure to drive desulfurization and limit aromatics saturation.

Mechanistically, HDS proceeds via direct desulfurization and hydrogenation pathways, removing sulfur from thiols, sulfides, and thiophenes. The resulting H2S is separated and sent to sulfur recovery systems due to toxicity and odor.

Feedstocks include gas oils, diesel, jet fuel, and some naphtha and aromatics streams. The goal is to meet stringent fuel-sulfur specifications and protect downstream catalysts. Products are hydrotreated fuels with reduced sulfur; the offgas contains H2S processed in Claus or other sulfur-recovery units to recover elemental sulfur.

HDS is a core unit of hydrotreating, often combined with hydrocracking or hydrofinishing for deeper sulfur removal, especially for ultra-low-sulfur diesel. Catalyst aging, poisons from nitrogen compounds, and metal contaminants can reduce activity, necessitating regeneration or replacement.