Effect of addition of sulphur and phosphorus on heavy oil hydrotreatment with dispersed molybdenum-based catalysts

The influence of the addition of sulphur and phosphorus on the activity of finely divided molybdenum-based catalysts formed in situ was investigated with regard to the hydrotreatment of Morichal crude oil (Venezuela). Reactions were carried out using a batch reactor at 470°C. The catalyst precursors used were molybdenum naphthenate, cobalt octylate and di-2-ethylhexyl phosphate. An increase in elemental sulphur addition led to increased catalytic activity, especially in the presence of phosphorus. It was found that the hydrogen sulphide that was generated from elemental sulphur can accelerate the decomposition of the precursors at temperatures below 240°C during the period of heating of the reactor. It was suggested that hydrogen sulphide is essential for the catalyst species to be sufficiently sulphided and activated without suffering possible coke poisoning. Phosphorus increased the hydrodesulphurization activity in amounts up to ca. 15·10 −4 mol P per 70 g of feed, but further addition decreased the activity. The optimum level was unchanged irrespective of the molybdenum concentrations employed. In addition, the vanadium removal improved remarkably with increase in the amount of phosphorus added. It is concluded that phosphorus interacted strongly with the vanadium compounds in the feed and prevented the catalyst species from being deactivated by the deposition of vanadium.