Hydrodesulfurization of Fluid Catalytic Cracking Decant Oils in a Laboratory Flow Reactor and Effect of Hydrodesulfurization on Subsequent Coking

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.