A method of converting a reactor having inlet and outlet ports and a manhole from axial flow to axial/radial or radial flow is described comprising introducing through said manhole perforate catalyst restraining means and attaching said catalyst restraining means to one of said ports so as to define an enclosure extending from said port for part of the length of the reactor, charging to said reactor a particulate catalyst material so as to build up a catalyst bed around said catalyst restraining means, with, for at least a major part of the height of said bed, a region between the exterior surface of said bed and the interior walls of the reactor, and charging to said region a particulate material that presents less resistance to flow than said catalyst.