What is the exact difference between a suspended bed and a fluidized bed?

Hydrogenation in a heavy oil slurry bed 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 heater, it enters the slurry bed hydroprocessing reactor; a large amount of recycled hydrogen and additional fresh hydrogen also enter the slurry bed reactor after being heated in a hydrogen heater ； 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-pressure 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 fed back into cyclic cracking or sent directly out of 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 uniformly 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 only, there is no issue of catalyst deactivation. As a result, the suspended-bed hydrogenation unit offers great operational flexibility and a long operational lifespan. High single-pass conversion rate, high yield of diesel fractions, and high cetane number are among the notable features of the slurry bed hydrogenation process.

In both fluidized bed and suspended bed systems, the catalyst is in a suspended state. Have you ever seen water boil? When water reaches a boiling point, it is in a state of agitation, and the same is true for fluidized beds. A suspended bed refers to a situation where the catalyst is suspended within the material, with certain gaps between the catalyst particles. The catalyst in a fluidized bed suffers more wear than that in a suspended bed.

Can fixed-bed treatment be used in the event of power or air supply outages?