Suspension bed and fluidized bed

In the suspended-bed process, fine powdered catalyst is premixed with the feedstock, and then both are fed into the reactor along with hydrogen; they flow from bottom to top, where hydrocracking takes place. The catalyst remains suspended in the liquid phase and exits the reactor from the top along with the reaction products. Suspension bed reactors are generally of cylindrical design without a catalyst bed ； In a fluidized bed reactor, the catalyst is located within the reactor; a catalyst supply pipe is provided at the upper part of the reactor, while a catalyst discharge outlet is situated at the lower part (to remove old catalyst from the bottom and add new catalyst from the top). Hydrogen and feed oil enter the reactor from its lower part; as they pass through the catalyst-packed bed via the grid distributor, the void space between the catalyst particles increases gradually with rising flow rate, causing the volume of the catalyst bed to expand. The height of the catalyst bed is controlled by the flow rate of the circulating liquid (a circulation line is provided within the reactor; the reaction products are drawn out from the top of the reactor, pumped to the bottom of the reactor where they mix with the feedstock before entering the reactor again, thereby enabling control of the feedstock flow rate).

Can it be understood that a suspended-bed reactor requires finer catalysts compared to a bubbling-bed reactor, in order to keep the catalyst in a suspended state within the feed oil, whereas a bubbling-bed reactor only needs the flow rate of the liquid to create a fluidized bed.

The catalysts used in suspended-bed reactors are often at the nanoscale, and their addition amount is small, generally not exceeding 1% of the feed material; they are added along with the feed, undergo reaction and distillation in a single pass, and then are discarded after use. In a fluidized bed, the catalyst is typically located within the reactor, and the expansion of the bed layer is controlled to prevent the catalyst from escaping out of the reactor.

In hydrogenation reactions, the catalyst in a suspended-bed reactor moves along with the reactants, whereas in a bubbling-bed reactor, the catalyst remains inside the reactor.

Suspension beds also have catalysts at the micron scale

Some have a circulation pump while others do not; the catalyst can be added and removed on a daily basis, and there is also a type that does not require the catalyst to be removed