Abstract: In a mixed-phase hydrocarbon hydroprocessing catalytic reaction system, wherein a guard-bed of particulate matter is employed above one or more individual beds of catalyst particles, the adverse detrimental effects of excessive reaction zone pressure drop are overcome by sensing the differential pressure across the guard-bed. Vaporous stream components are withdrawn from the reaction chamber through a locus above the guard-bed and at a rate proportional to the differential pressure. These withdrawn vaporous components are reintroduced into a substantially particle-free void volume below the guard-bed and above the bed of catalyst particles.