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.

Characteristics in catalytic conversion of asphaltenes In petroleum heavy residues were studied in the hydrotreating process. A Boscan crude, an Athabasca bitumen, and a Khafji vacuum residue were tested as typical feedstocks. Various analyses were made to obtain the properties of asphaltenes before and after the reaction, e.g., changes of heteroatoms such as sulfur and metals, and decreases of molecular weight. The characteristic changes of asphaltene molecules were also investigated by electron spin resonance (ESR) and X-ray analyses. The association and coordination of vanadyl in asphaltenes were studied by the temperature dependence on the ESR spectra, and the sizes of the stacked crystallites and the aggregated asphaltene micelles were measured with X-ray diffraction and small-angle scattering. In the asphaltene cracking mechanism, it was clarified that the main reactions were the destruction of asphaltene micelles caused by vanadium removal and the depolymerization of asphaltene molecules by removal of heteroatoms such as sulfur.

A new catalytic hydrotreating process, the Asphaltenic Bottom Cracking (ABC) process, for heavy residual oils has been Investigated In the relation between catalysis and chemical structure. A proprietary catalyst has been developed which Is capable of hydrocracking asphaltenes Into heptane-soluble materials and decreasing the vanadium content of heavy crudes and residues at a lower hydrogen consumption than a commercial hydrodesulfurization (HDS) catalyst and without change In activity In a six-month test. Various heavy feedstocks were tested in a catalytic reactor (ABC section) and a solvent deasphalting unit (SDA). Precipitated asphaltenes were recycled. Reactivities of various residues and a proposed mechanism are discussed. This ABC process will be most useful as a step preceding an existing hydrocracking process in the upgrading of residues with high asphaltenes and metals contents. In addition, the application of this technology Is described.

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