A reactor includes: a shell with first and second ends, an fluid inlet at the first end for receiving a process fluid, and an outlet at the second end for discharging a reacted process fluid, and a plurality of elongate containment units within the shell for containing a particulate catalyst or sorbent, each containment unit including two perforate members defining a space in which the particulate catalyst or sorbent may be placed, the perforate members mounted between two fluid-impermeable end members, wherein one end member extends across the containment unit to provide a closed end and the other end member closes the space thereby providing an open end through which a process fluid may enter or exit the containment unit, and a header assembly connected to the open ends of the containment units and either the fluid inlet or fluid outlet. A process using the reactor is also described.