NOVELTY - Catalytic reactor comprises a bed of particulate catalyst around a central region connected to an inlet or outlet port, and offering less resistance to flow than the catalyst particles. The central region has a height equal to the height of the bed, the exterior surface of which has a cross section less than that of the reactor, thereby leaving a space between the bed and the reactor walls. This space is filled with particulate material presenting less resistance to flow than the catalyst particles.    USE - As a catalytic reactor for adiabatic reactions, such as low temperature steam reforming, water gas shift, methanol synthesis, methanation, ammonia synthesis and hydrogenation reactions.    ADVANTAGE - The use of a region of particulate material with lower fluid flow resistance than the catalyst around a central region prevents the pressure differential between the inlet and outlet of the reactor becoming too high. The fluid flow rate is also increased, without increasing the pressure differential. The structure of the reactor is simplified, and the catalyst can be more easily replenished than in a conventional radial flow reactor due to the manhole and flexible restrainer.    DETAILED DESCRIPTION - Catalytic reactor comprises inlet and outlet ports and a bed of particulate catalyst around a central region in communication with one of the ports and offering less resistance to flow than the catalyst particles. The central region has a height equal to at least a major part of the height of the bed, where the exterior surface of the bed has a cross section less than that of the reactor, thereby leaving a space between the exterior surface of the bed and the reactor walls. This space is filled with particulate material presenting less resistance to flow than the catalyst particles.    INDEPENDENT CLAIMS are included for;    (1) A method of charging a reactor, by using a perforated catalyst restrainer to define the central region. The gap formed when a second particle restrainer is placed between the first restrainer and the reactor wall is filled with lower flow resistance particulate material;    (2) A method of converting a catalyst reactor from axial flow to axial/radial flow, by attaching the perforated catalyst restrainer to the inlet or outlet to define an enclosure subsequently charged; and    (3) A process of passing a process fluid through an inlet port of a reactor, then through a series of regions to the outlet.