Fixed-bed reactors have long been used in process industries. They contain catalyst, typically in pellet form, packed in a static bed. The syngas is then passed through the bed, where the reactions are induced as the gases contact the catalyst. Originally, fixed-bed reactors were the only commercially viable reactor type due to technological limitations. However, they also presented drawbacks mainly in the constraints existing in access to the catalyst material. Since the gas has to pass over the material the reaction is limited by the available surface area. This problem can be reduced by allowing more than one "bed" in the reactor for the gas to pass over, under, and/or through. The catalysts in fixed-bed reactors do not need to be as resilient, as they do not move in the bed. For the common situation encountered when a reaction process is exothermic, fixed-bed reactors demand cooling of the bed. If the excess heat is not dissipated from the reactor bed, it could eventually lead to deterioration and deactivation of the catalyst material. Fixed-bed reactors can be equipped with internal tubes where a heat transfer fluid, such as boiler feed water, can circulate inside the tubes to control the temperature rise in the reactor.
As opposed to fixed-bed reactors, fluidized-bed reactors contain their catalyst material in a fluid state. This fluid state can take on many different forms from having the catalyst suspended in an inert liquid, such as mineral oil, to having particles of the catalyst suspended by gas flowing through the reactor. There are several types of these reactors such as entrained bed, fluidized-bed, ebbulated bed or slurry bed depending on the velocity of the fluid in the reactor.
Fluidized-bed reactors solve some of the problems that are present in fixed-bed reactors. They allow for a better access to catalyst material since the catalyst particles are thoroughly mixed with the gas or fluid, allowing for the greatest possible surface area for reactions to take place. Heating issues are also helped due to the improved heat transfer resulting from the inherent characteristics of the fluidized state of the catalyst bed. This also includes reducing the potential for heat gradient build ups that can occur when the catalyst is immobile as in a fixed-bed. Fluidized-bed reactors can operate almost isothermally. They can also have internal tubes through which heat transfer fluids can be circulated to remove excess heat.
In fluidized-bed reactors, catalysts can be removed on line for regeneration or replacement without shutting down the reactor. A fixed-bed reactor must be shutdown to regenerate the catalyst or add new catalyst.