Worldwide trends in crude oil supply have been indicating the declining availability of conventional crude. This trend has been offset by the increasing production of heavy crude. For heavy crude, the yield of distillate fractions can be increased by upgrading distillation residues. A number of thermal processes (e.g., visbreaking, delayed-, fluid-and flexi-coking) and asphaltenes and metals separation processes (e.g., deasphalting), the so-called carbon-rejecting processes, have been used on a commercial scale for several decades (1, 2). Heavy feeds can also be upgraded by hydroprocessing, the so-called hydrogen addition option (3, 4). This requires the presence of hydrogen and an active catalyst. Compared with thermal processes, hydroprocessing operations are more flexible, giving higher yields of liquid fractions. However, the costs of high-pressure equipment, catalysts and H ₂ required for hydroprocessing have to be offset by the increased yields and quality of liquid products. The optimum hydroprocessing operation can be achieved by properly matching the type of reactor and catalyst with the properties of heavy feeds. Several types of catalytic reactors, i.e., fixed bed, moving bed and ebullated bed reactors, have been available and used commercially. The efforts to develop entirely new catalytic phases and reactors with the aim to increase the efficiency of hydroprocessing of heavy crudes have been made continuously.

In spite of some similarities, the hydroprocessing of heavy feeds differs markedly from that of light feeds. This results from the presence of high molecular weight asphaltenic molecules and resins, as well as that of the organometallic compounds in...