A catalyst activator vessel for heat conditioning a catalyst is disclosed. The activator vessel includes inner and outer vessels, a perforated, normally generally horizontal grid plate within the inner vessel, and a fluid path extending through the grid plate within the vessel. The inner vessel can have an inside diameter of at least 50 inches (1.27 m). The space between the inner and outer vessels defines a flue. The perforated grid plate within the inner vessel can have an upper surface perforated with a pattern of overlapping, generally conical depressions and a lower surface, optionally overlapping by at least 17%. The fluid path extends upwardly through the perforated grid plate and is a conduit along which a fluid flows through the grid plate. The fluid will fluidize a particulate material, such as a catalyst, disposed above the grid plate in the inner vessel.