Catalyst activator, in particular, a catalyst activator for heat conditioning a catalyst. The activator includes a vessel for containing a catalyst charge having an internal diameter of at least 1.2 meters and/or an internal volume of at least 5 m3; a fluidisation grid plate disposed in the vessel, the fluidisation grid plate having an upper major surface and a lower major surface; an array of generally conical depressions in the upper major surface that overlap by less than 17%; and an array of holes perforating the fluidisation grid plate, the holes extending from at least some of the generally conical depressions through the lower surface.