A converter having an exterior shell with a central axis, at least one gas inlet and at least one gas outlet. The exterior shell includes a base and an interior support structure. A number of vertically stacked catalytic chambers within the exterior shell are each in communication with one or more gas inlets and gas outlets. Each chamber is defined by an inner wall of the external shell, a permeable catalytic bed support platform, a lower division plate spaced below the platform defining a gas retention plenum and an upper division plate above the platform. At least one of: the platform; the upper division plate; and the lower division plate have a toroid surface symmetric about the central axis with an outer periphery supported by the inner wall of the exterior shell and an inner periphery supported by the interior support structure.