This study investigates the hydrodesulfurization (HDS) of fluidized catalytically cracked decant oils used as feedstock for needle coke production. Three decant oils, representing a high (4.0 wt %), medium (2.5 wt %), and low (0.9 wt %) sulfur content, were hydrotreated in a fixed-bed flow reactor. Removing sulfur from larger ring systems in decant oils is the most effective way of reducing the needle coke sulfur content, because large aromatics are significant contributors to the coke product obtained from delayed coking. Two commercial catalysts with different pore size distributions were tested for their hydrodesulfurization activities, selectivities for specific sulfur-containing species, and hydrogenation of constituent polyaromatic hydrocarbons (PAHs) under different operating conditions. The decant oils and hydrotreated products were analyzed by GC/ MS to determine changes in molecular compositions of the feedstocks. Following HDS, the decant oils and their hydrotreated products were carbonized to produce a semicoke, and the coke was evaluated for mesophase formation and quality. The desirable outcome of decant oil HDS is sulfur removal, particularly from large polyaromatic ring systems, with minimum hydrogen consumption and hydrogenation. The results showed that the desired level of 0.5 wt % of sulfur in both low-and medium-sulfur decant oils could be achieved through HDS over a commercial CoMo catalyst. Furthermore, hydrogenation of the PAH during HDS appeared to slightly improve the mesophase development seen upon subsequent carbonization.

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.

Two decant oils with different sulfur contents and their vacuum distillation fractions were hydrotreated in a fixed-bed flow reactor to produce a feed with sufficiently low sulfur content for needle coke production. Products from hydrotreatment were subsequently carbonized in a tubing bomb reactor to characterize the carbonaceous mesophase development seen in the resulting semicoke. Although the purpose of the hydrotreatment is to reduce sulfur content, hydrogenation of aromatic compounds also takes place during the treatment, thus increasing the hydrogen consumption. Modest hydrogenation of decant oil from the hydrotreatment improved the mesophase development but resulted in a significant decrease of the semicoke yield upon carbonization of the treated product. As a remedy to conserve hydrogen during hydrotreatment and achieve higher coke yields in the subsequent carbonization, only the middle fraction from vacuum distillation of the decant oil was hydrotreated and blended back with the vacuum bottoms to simulate the coker feed. This scheme was successful to attain the desirable sulfur reduction in the feedstock without the penalty of a reduced coke yield upon carbonization or useless hydrogen consumption.

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