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 15 ppm over CoMo/Al2O3–SiO2 (catalyst X) at 340 °C and 5 MPa (H2) for 2 h. At the same conditions, AM-GO and AH-GO could be desulfurized to 70 and 78 ppm, 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 (1 h) enhanced the AH-GO HDS during the second-stage (1 h) to 9 ppm. However, replacing the hydrogen in the second-stage with 5% H2S in hydrogen inhibited the HDS, resulting in product sulfur content of 15 ppm. 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.