Void blowing by gas flow through a bed of particulate material is avoided by the use of a cover plate that eliminates void spaces at the top of the bed and a vertical duct that allows a head of particulate material to be maintained above the cover plate. The particle bed is arranged in the usual fashion using inner and outer screens to hold particulate material in a vertically extended bed into which particulate material is intermittently added and withdrawn. The top of the particulate bed is contacted by an imperforate cover plate that is sloped up to an inlet opening that has a smaller cross-sectional area than the area of the bed. The cover plate has a slope which is greater than the angle of the repose of the particles so that particles filling the top section of the bed will be free of void spaces. The cover plate also has a relatively small inlet opening so that a small volume of catalyst particles can provide a head of particles above the inlet opening to resist upward pressure that otherwise forms voids at the inlet screen surface. This cover plate arrangement also minimizes bed heights and overall reactor height necessary to provide the head of particles for resisting upward pressure.