HYDRODESULFURIZATION REACTIVITIES OF NARROW-CUT FRACTIONS IN A GAS OIL

A gas oil (bp 227-377 degrees C) was fractionated by the distillation at intervals of 20 degrees C into five fractions. Each fraction was desulfurized over the NiMo and CoMo catalysts, respectively, in an autoclave reactor at 360 degrees C under a total pressure of 2.9 MPa to examine its hydrodesulfurization (HDS) reactivities. The fractions exhibited the different reactivities which were expressed by the pseudo-second-order kinetics, giving the decreasing rate constants of 0.636, 0.270, 0.118, 0.039, and 0.023 [min . wt %](-1) with an increase of their boiling point ranges. Composition analyses of the fractions indicated that alkyldibenzothiophenes, polyaromatic hydrocarbons, and nitrogen compounds in the heavier fractions restricted their reactivities. Steric hindrance of methyl groups as well as the competitive retardation of latter compounds are suggested. The NiMo catalyst showed higher activity for HDS of the heavier fractions than the CoMo through its higher hydrogenation activity, while the latter was slightly superior to the former in HDS of the lighter fractions. The polyaromatic species existing dominantly in the heaviest fraction (>340 degrees C) are responsible for the fluorescent color in the oils. On the basis of such understanding, a deep HDS process which combined a single-stage HDS for lighter fractions and a multistage HDS for the heavier fractions was proposed and examined. Such a process achieved the sulfur level below 0.05 wt % with the color removal and its superiority over the conventional HDS process was discussed.