Effects of Catalyst Properties on Hydrodesulfurization Activity for Sulfur Removal from Fluid Catalytic Cracking Decant Oils

Removing sulfur from larger ring systems in fluid catalytic cracking decant oils used as needle coke feedstock is the most effective way of reducing the needle coke sulfur content. The large sulfur compounds found in decant oil are incorporated into coke in larger proportions than smaller sulfur compounds upon carbonization. The desirable outcome of decant oil hydrodesulfurization is, therefore, removing sulfur selectively from large polyaromatic ring systems with minimum hydrogen consumption. This study investigates the effects of catalyst properties on hydrodesulfurization activity to remove sulfur from decant oils. Two decant oils (DO-HS and DO-LS) representing a high (2.5 wt %) and low (0.9 wt %) sulfur content decant oil and their vacuum distillation fractions were hydrotreated in a fixed-bed flow reactor. Four catalysts (with varying average pore sizes, promoter atoms, and supports) were prepared with sequential incipient wetness impregnation to evaluate their activities for hydrodesulfurization and hydrogenation of decant oils. An increase in the average pore diameter from 7 to 14 nm for CoMo catalysts supported on Al2O3 proved capable of meeting the desired requirements for hydrodesulfurization of decant oil used in needle coke production. Of the four catalysts evaluated, CoMo supported on TiO2 outperformed the other three catalysts supported on Al2O3; however, focus was placed on the Al2O3-supported catalysts as a result of the superior mechanical integrity and proven longevity of Al2O3 in hydrodesulfurization reactors. It was shown by proton nuclear magnetic resonance that promoting Mo supported on Al2O3 with Ni instead of Co results in equivalent hydrogenation activity and decreased desulfurization. Upon carbonization of treated oils, the sulfur content of the resulting coke increased from the feed treated with a CoMo catalyst supported on Al2O3 with an average pore diameter of 7 nm, whereas coke produced from feeds treated over the CoMo catalyst supported on Al2O3 with an average pore diameter of 14 nm had a lower sulfur content compared to the feed. Therefore, with a proper catalyst design, sulfur in decant oil that tends to be retained in the coke can selectively be removed. Thus, hydrodesulfurization can favor the direct desulfurization route over the hydrogenation route by employing high reaction temperatures and modest hydrogen pressures.