The way catalysts are used vary widely, Some examples include polymer production and different types of reactors for solid homogeneous catalysts..
The reactor may have multiple configurations including one large bed or several beds in series.
This is normally used where the process has to be carried out at high temperature where hot gases are on the outside of the tubes. Tubular reactors are also used where the process is highly exothermic and coolant is on the outside of the tube and where it is difficult to obtain and maintain the temperatures if a single bed was utilized.
Fluidised bed reactors are typically multiple vessel units whereby under normal operation, the catalyst will migrate between vessels. This allows the catalyst to be moved through the vessel and into regeneration sections that will regenerate the catalyst and return it to the top of the reactor. A common fluid bed reactor is for Fluid Catalytic Cracking (FCC) used to crack heavy hydrocarbon feedstocks.