A method of adapting an axial flow reaction vessel having opposed ports to an opposed axial flow reaction vessel includes installing a process fluid collection system within the body of the vessel in fluid communication with one or more of the ports; providing the vessel with a bed of particulate catalyst or sorbent containing a layer of inert particulate material around the process fluid collection system; and adapting the feed to the vessel through one or more of the ports such that a process fluid fed to the vessel is passed axially and in the opposite direction through the fixed bed of catalyst or sorbent and is collected by the process fluid collection system disposed centrally within the bed and in fluid communication with one or more of the ports.