COKE FORMATION ON CATALYSTS DURING THE HYDROPROCESSING OF HEAVY OILS-ALPHA

Metal sulfide based hydrotreating catalysts used with heavy oils are known to deactivate during use. Initial deactivation is rapid and is associated with the deposition of carbonaceous material on the surface. Subsequent deactivation is slower and results mainly from the deposition of metals originally present in the feed onto the catalysts. The origins and nature of catalyst deactivation as a result of the deposition of carbonaceous materials is examined. The influence of various factors such as feed, catalyst and process parameters on the formation of coke during hydroprocessing is reviewed. The nature of the coke is discussed in some detail and is found to depend on the relative importance of different reactions that may occur during hydroprocessing. Initial coke deposits are suggested to result from the adsorption of asphaltenes on acidic sites on the catalyst. Subsequent reactions of these asphaltenes and further deposition of metals and coke occurs throughout the operational life of the catalyst. One major source of this coke is suggested to involve asphaltenes precipitated from solution. The relative importance of thermal and catalytic hydroprocesses in controlling asphaltene solubility is discussed and possible areas for further research designed to minimise coke are suggested.