Analysis and deep hydrodesulfurization reactivity of Saudi Arabian gas oils

Gas oils obtained from Arabian Light (AL-GO), Arabian Medium (AM-GO) and Arabian Heavy (AH-GO) crude oils were subjected to detailed analysis in terms of reactive and refractory sulfur, nitrogen, as well as aromatic species. Deep hydrodesulfurization (HDS) of these gas oils over SiO₂-Al₂O₃-supported CoMo and NiMo catalysts was studied using autoclave reactor either in one- or two-stage operations. AL-GO was easily and deeply desulfurized to 15ppm over CoMo/Al₂O₃-SiO₂ (catalyst X) at 340°C and 5MPa (H₂) for 2h. At the same conditions, AM-GO and AH-GO could be desulfurized to 70 and 78ppm, respectively. Two-staged HDS, by combining CoMo and NiMo catalysts, in successive steps resulted in effective deep HDS. The replacement of hydrogen atmosphere after the first-stage (1h) enhanced the AH-GO HDS during the second-stage (1h) to 9ppm. However, replacing the hydrogen in the second-stage with 5% H₂S in hydrogen inhibited the HDS, resulting in product sulfur content of 15ppm. Analysis of sulfur species indicate that significant fraction of reactive and refractory sulfur species were removed during the first-stage whereas the remaining refractory sulfur species were removed during the second-stage. Kinetic analysis indicates overwhelming influence of refractive sulfur species on the overall HDS. The results from this study show that two-stage scheme with optimum catalysts in series can be applied to overcome the difficulty to achieve deep HDS of AH-GO.