On the metal deposition process during the hydrodemetallation of vanadyl-tetraphenylporphyrin

Abstract

This paper focuses on the metal deposition process during hydrodemetallisation (HDM) of vanadyl-tetraphenylporphyrin (VO-TPP) under industrial conditions. In catalyst pellets of a wide pore, low loaded molybdenum on silica, the vanadium deposition process was determined with EPMA and HREM. The effect of quinoline and H2S on the vanadium deposition profile is studied and an attempt is made to simulate the deposition profiles based on intrinsic reaction kinetics and percolation concepts. A reference vanadium deposition experiment is carried out in order to assess the influence of quinoline and H2S. Quinoline showed to decrease the rate of metal removal, the amount of vanadium deposited is lower as compared to the reference experiment. The shape of the vanadium deposition profiles is similar in both cases. A deposition maximum is observed in the centre of the pellet, indicating that the vanadium deposition process is not diffusion limited and that a sequential reaction mechanism applies for VO-TPP HDM. Low H2S partial pressure resulted in different vanadium deposition profiles as a function of the axial position in the reactor. At the inlet of the reactor, similar shaped profiles as the reference experiment were found, however, at the outlet of the reactor a shift towards M-shaped profiles was found indicating a diffusion limited vanadium deposition process. This shift in vanadium deposition profiles is explained by the build-up of the last intermediate resulting in a higher metal deposition rate. Simulations of the vanadium deposition profiles showed reasonable results at reactor inlet conditions. With HREM, vanadium deposits were found in a layered structure both in the surrounding of the active phase, causing active site poisoning, and as isolated vanadium sulfide clusters, likely causing active site generation. In our vanadium deposition experiments (up to 9 wt.% vanadium) no significant catalyst deactivation occurred, indicating that both effects are compensated.