In the catalytic hydrotreating of heavy oil, it is necessary to crack the asphaltenes in the oil. However, since the asphaltenes are extremely large molecules, which consist of highly condensed heterocyclic and aromatic rings to which sulfur, nitrogen, oxygen, and metals (mainly vanadium and nickel) are bound, it is anticipated that their diffusion into catalyst pores will be difficult and that they will be main sources for coke and metal depositions on the catalyst surface. Therefore, to design the catalysts for hydrotreating of heavy oil, It Is important to understand the behavior of such a large molecule in the catalyst pores. In this paper, the diffusivities of standard polystyrenes and asphaltenes in several hydrotreating catalysts having the various pore sizes were measured by use of solid-liquid chromatography techniques, and the effect of the ratio of diffusing molecular diameter to the pore diameter was investigated on the effective diffusion coefficient. Furthermore, hydrotreating tests were performed for these catalysts with Boscan crude, which contains a large amount of metals and asphaltenes. As a result, a quantitative relation between the catalyst pore structure and the reactivities of asphaltene cracking and demetalation was obtained.
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