ABSTRACT
Formation of coke like sediments or particulates is a serious problem in the hydroprocessing of heavy residues for high conversion. The sediments can cause both operability problems and rapid catalyst deactivation. The macromolecules of the heavy feedstocks such as asphaltenes are believed to contribute significantly to sediment formation and coke deposition. As part of an extensive research program on the factors which influence sludge or solids formation during residue hydroprocessing, we have examined the nature of changes that take place in the characteristics of the asphaltenic fraction of Kuwait vacuum residue under different operating conditions. The studies revealed that sediment formation is the result of reduction in solubilization efficiency of asphaltenes in the product medium compared with feedstock. Molecular size distribution of the product asphaltenes showed that operating at high temperatures enhanced depolymerization and fragmentation of asphaltenes to low molecular weight materials. A portion of the low molecular weight asphaltene fragments with relatively low H/C ratio resisted further cracking even at high temperatures and led to the formation of coke like sediments. Large pore catalysts were observed to reduce the problem of sediments formation. The role of catalyst pore size on asphaltenes conversion is discussed.160
