Fixed bed reactors have a catalyst bed that is held in place, as opposed to an ebulated bed or fluidized bed.

Consequently, the catalyst in fixed bed reactors can not be drawn off and regenerated during operation, so it gradually and progressively deactivates as coke is produced as a reaction by-product and enters and blocks the catalyst pores. This eventually forces the unit to be shut down so that the catalyst can be regenerated by burning off the coke or replaced by new catalyst.