RESIDUUM HYDROCRACKING WITH SUPPORTED AND DISPERSED CATALYSTS - STABLE HYDROGEN AND CARBON-ISOTOPE STUDIES ON HYDROGENATION AND CATALYST DEACTIVATION

The generally accepted mode of action of Ni- or Co-Mo/Al2O3 catalysts during hydrocracking of residua is that the catalyst participates in t he hydrogenation of aromatics which results in heteroatom removal and CCR conversion. Earlier studies have shown that under hydrocracking co nditions with residua, the catalyst loses the ability to hydrogenate a romatics within hours, and thereafter catalysis proceeds by a differen t mechanism. The established techniques of stable isotope analyses hav e been used to provide additional support for the proposed mechanism. The isotope studies showed that in the absence of a catalyst, there wa s no incorporation of gaseous hydrogen into the residuum fraction and only a small amount into the distillates. With a fresh catalyst, hydro gen was incorporated into all fractions initially but hydrogenation of residuum lasted for only 4 h. When bitumen residuum was hydrogenated under mild conditions, the added hydrogen was not lost under hydrocrac king conditions, Addition of gaseous hydrogen to the residuum was depe ndent on the concentration of catalyst for dispersed catalysts, and ca talysts previously used under mild conditions in the pilot plant did n ot show any hydrogen transfer from the gas phase to the residuum fract ion. In all cases, carbon-to-carbon bond breaking was correlated with the incorporation of gaseous hydrogen.