A radial flow reactor vessel is disclosed for use in gas purification, separation or reaction processes and most suitably used in prepurification processes. The reactor has internal baskets for confining a bed of active material. The baskets are rigidly supported at both the top and bottom ends of the reactor and have walls that are axially flexible and radially rigid. The vessel has multiple movable support columns designed to facilitate pre-stressing of the baskets to offset axial compressive loads induced from thermal cycling.