Abstract: A multistage catalytic reactor of unitized internal construction whereby the vertical support columns for the division plates and the catalyst support grids comprise a unitized structure from the top to the bottom of the reactor and are fixedly attached at their respective ends to the top and bottom of the reactor. The reactor further includes horizontal beams for support of a catalyst support grid or mesh, at least some of said beams being mounted for movement relative to said vertical columns through windows or openings in at least a portion of said columns. The beams are of such length that they either (i) span as a chord across the reactor or (ii) at least one end of the beam abuts a vertical column with sufficient clearance to allow for expansion. At each junction between a beam and a window in a vertical column as well as where a beam end abuts a vertical column, the beam is supported on a rocker means affixed to the column so that the beam is free to move as it expands and contracts.