Catalyst deactivation in distillate hydrotreating (Part 1): Catalyst deactivation in the commercial vacuum gas oil hydrotreating unit

Deactivation of hydrotreating catalyst (HOP-471, Orient Catalyst) was studied in a commercial vacuum gas oil (VGO) hydrodesulfurization unit. Used catalysts were recovered from the catalyst bed at several depths in the reactor after one year of operation at 8 MPa 360-400°C and LHSV 2 l/h. The used catalysts were characterized by analysis of carbon and the metals (V and Ni) deposited on the catalyst, by adsorption of pyrene, and by two kinds of activity test in an autoclave and a down flow reactor to compare the results obtained with those of virgin, regenerated and grained used catalysts. According to the analytical result of the used catalyst, the amount and the density of the coke deposited increased with the depth of the catalyst bed (while the metal deposit decreased). Reduction in the surface area (S.A.) and pore volume (P.V.) of the catalyst with depth was not singly ascribed to the amount of coke deposited but also to its quality. The metals deposited also appeared to reduce the catalyst S.A. and P.V. The rate of adsorption of pyrene onto the catalyst was found to increase with its bed depth, indicating that deposition of bulky coke hinders diffusion. The catalyst at the top of the bed maintained 40% HDS activity while that at the bottom maintained 80% activity. And the regenerated catalyst at the top of the bed recovered only 20% of 60% recoverable HDS activity. However, the grained catalyst recovered 90% of HDS activity and it also recovered the normal rate of pyrene adsorption completely. Hence, it was concluded that deposition of coke and metal at the entrance of catalyst pore caused deactivation of both HDS activity and normal rate of pyrene adsorption. In contrast, HDN activity was found to increase with the depth of the catalyst bed, suggesting that deposition of Ni may provide new HDN active sites in molybdenum sulfate matrix on alumina support.