CONRADSON CARBON RESIDUE CONVERSION DURING HYDROCRACKING OF ATHABASCABITUMEN - CATALYST MECHANISM AND DEACTIVATION

The activity of promoted molybdenum on gamma-alumina catalysts toward residuum, Conradson carbon residue (CCR), and heteroatom conversions a nd solids formation in the reactor during hydrotreating and hydrocrack ing of Athabasca bitumen has been investigated during the first day on stream by using batch reactors and for up to 30 days on stream using a continuous flow pilot plant. It is proposed that, under hydrotreatin g conditions, catalytic CCR conversion and catalytic metals removal ta ke place on different active sites from catalytic sulfur removal and t hat catalytic nitrogen removal takes place on different sites again. T he active sites on the catalyst which promote conversion of CCR and he teroatoms through conventional hydrogenation reactions were lost in a matter of hours under cracking conditions. After the hydrogenation sit es were deactivated, the catalyst remained active toward CCR and sulfu r removal, presumably through a different mechanism. CCR conversion wa s strongly correlated with residuum conversion and initially there was no selectivity between conversion of CCR residuum molecules and non-C CR residuum molecules. Selectivity was introduced as the catalyst deac tivated over 30-50 days with CCR conversion decreasing faster than res iduum conversion. It is proposed that the main role of catalyst during hydrocracking is to assist in the reaction of thermally generated phe nyl radicals with molecular hydrogen, resulting in the addition of a h ydrogen atom to condensed aromatic centers, and ultimately resulting i n the decomposition of the condensed aromatic unit to give gases and d istilate. Hydrogenation of aromatics does not play a significant role in the hydrocracking of Athabasca bitumen residuum. The catalyst was o nly indirectly involved in preventing solids formation in the reactor during hydroprocessing.