An enclosure in which there circulates a moving bed of solid particles (for example a moving-bed catalytic reactor such as a reforming reactor), is delimited in the enclosure by at least one grill. In the enclosure a fluid passes through the bed in a direction different from that of the flow of the assembly of the particles and issues from the bed by way of at least one grill. Disposed in the thickness of said bed in an intermediate wall (grill) such as to distribute the thrust forces produced by the fluid on the particles in the zones of the bed which are delimited in that way. Jamming of the particles against the grill or grills is greatly reduced and the admissible fluid flow rate is then decreased.