On-stream catalyst replacement hydroprocessing method wherein an upstream mixture of hydrogen and hydrocarbon liquid counter flows through a downwardly moving bed of hydroprocessing catalyst in a reactor vessel. The mixed feed stream of hydrogen and liquid hydrocarbon components enters a surge zone between the lower end of the reactor and a plenum zone to form a common pool under a conical support for the lower end of the downflowing catalyst bed. The mixed feed enters the plenum chamber through a plurality of passageways extending downwardly from the plenum zone to the same depth adjacent the lower end of the surge zone so that the liquid component normally prevents hydrogen from establishing independent paths before entering the plenum zone. Separation of the hydrogen and hydrocarbon liquid components from the mixed feed is thus assured to occur in the plenum zone directly below the pervious conical support to form a plurality of stepped concentric local reservoir rings under the conical support. Such rings are spaced apart sufficiently to create adjacent and alternate separate feed rings of hydrogen and hydrocarbon liquid. The depth of the lower end of each ring is intermediate the depth of the two radially adjacent concentric rings to maintain uniform feed of hydrogen and hydrocarbon liquid across the full cross-sectional area of the downflowing catalyst particles.