Competitive effects of hetero-atom containing compounds in the hydrodemetallisation of vanadyl-tetraphenyl-porphyrin

Abstract

Heavy oil residua contain large amounts of hetero-atoms, such as sulfur, nitrogen, oxygen and metals. The metals, mainly vanadium and nickel, are removed by catalytic hydrotreatment. Sulfur, nitrogen and oxygen containing compounds present in heavy oil residua can influence the rate at which the metals are removed, since they can adsorb competitively on the active site of the catalyst. Hence, the rate of the hydrogenation and ring cleavage steps in the hydrodemetallisation of the organo-metallic compounds may be slowed down. In the present work the competitive effect of hetero-atom containing compounds which are analogues of those occurring in heavy oil residua, i.e. quinoline, ammonia, benzofuran, water, dibenzothiophene and anthracene, has been investigated on the hydrodemetallisation of vanadyl-tetraphenyl-porphyrin. The spiked amount of hetero-atoms has been chosen near to those occurring in oil residua. The added hetero-atom containing compounds appeared to decrease the rate of metal removal. An increasing effect on the hydrogenation steps was found in the following sequence: dibenzothiophene < quinoline < anthracene ≈ benzofuran < ammonia ≈ water. For the lumped ring cleavage step the effect increased in the following sequence: dibenzothiophene < anthracene ≈ quinoline ≈ benzofuran < ammonia < water. The effect of quinoline, benzofuran, dibenzothiophene and anthracene is caused by competitive adsorption on the active sites of the catalyst. Water and ammonia, which have the most pronounced effect, are suggested to be co-ordinated to the vanadyl group in the porphyrin and, therefore, decrease the rate of metal removal. It is concluded that competitive adsorption of the reaction products from the ring cleavage of VO-TPP is negligible and that the presence of hetero-atom containing compounds in realistic feedstocks can decrease the rate of metal removal significantly.