Residue hydroprocessing technology

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Currently, the main technologies for residue hydroprocessing are fixed-bed hydrogenation and slurry-bed hydrogenation. (1) Fixed-bed hydrogenation process: The fixed-bed hydrogenation process involves the use of different types of catalysts in various beds to remove metal impurities as well as sulfur and nitrogen elements from heavy oil, and to modify its heavier components. Taking isolated island vacuum residue as an example, the demetallization rate is 70%–75%, the desulfurization rate is 85%–90%, the denitration rate is 65%–70%, and the carbon residue removal rate is 55%–60%. The conversion rate of gums can reach 60%–65%, and both density and viscosity are reduced to a certain extent. Therefore, the unconverted oil resulting from this process (hereinafter referred to as hydrodesorption tail oil) has excellent properties, making it an excellent blending component for catalytic cracking. Since the first bed layer of the fixed-bed hydrogenation reactor tends to clog, resulting in pressure drops that affect the operating cycle of the plant, it is required that the total metal content in the feedstock be less than 150 μg/g, and the asphaltenes content should not exceed 5%. The fixed-bed hydrogenation process has a low one-pass conversion rate, requiring large-scale heavy oil catalytic cracking and diesel hydroprocessing units to complement it, resulting in a low proportion of diesel and gasoline in the products. (2) Suspended-bed hydrogenation process: Heavy oil suspended-bed hydrogenation is relatively simpler compared to the fixed-bed hydrogenation process. After the heavy oil feedstock is mixed with the catalyst, it is mixed again with a small amount of recycled hydrogen and additional fresh hydrogen. Through heat exchange with the reactor outlet stream and heating in a hydrogen burner, it enters the suspended-bed hydrogenation reactor; a large amount of recycled hydrogen and additional fresh hydrogen also enter the suspended-bed reactor after being heated in a hydrogen burner ； Hydrogen and the light fractions in the reaction products enter the high-pressure separator from the top of the reactor, while the heavy fractions in the reaction products enter the low-pressure separator from the bottom of the reactor. Thereafter, they are sent to respective distillation units depending on their density. The reduced-wax oil is returned to be mixed with fresh feedstock for cyclic cracking; naphtha and diesel undergo hydrorefining to yield products. The hydrogenation tail oil is processed before being sent back for cyclic cracking or discharged directly from the plant for further utilization and processing. The catalysts for the suspended-bed hydrogenation process are simpler and cheaper than those for fixed-bed hydrogenation. After being dispersed, the catalyst is evenly mixed into the feed heavy oil; the amount of catalyst added amounts to 200–1,000 μg/g of the feed heavy oil (based on the metal content). It is used once only and not recycled, remaining ultimately in the hydrogenation tail oil. This process is suitable for the upgrading of low-quality heavy oils with high metal content, high residue, and high asphaltenes. It is characterized by high temperatures (430–460°C), high space velocities (0.5–1.0 h-1), and moderate operating pressures (10–12 MPa). The reaction involves primarily thermal cracking, with hydrogenation playing a secondary role; catalysts are added to prevent the radicals generated during intense cracking reactions from undergoing condensation reactions, thereby minimizing coking. The suspended-bed hydrogenation reactor has a simple structure; its interior is a hollow tank without any catalyst bed, which eliminates the problems of catalyst bed clogging and pressure drop. There is also no risk of the reactor overheating. Additionally, since the catalyst passes through once, there is no issue of catalyst deactivation. As a result, the suspended-bed hydrogenation unit offers great operational flexibility and a long operational lifespan. One of the notable features of the suspended-bed hydrogenation process is its high one-pass conversion rate, high yield of diesel fractions, and high cetane number.