An apparatus is shown for contacting a bed of particulate material with a cross flowing fluid, which maintains the bed of particulate material within a retention volume. The apparatus includes partitions for retaining particles, with apertures disposed within the partitions. The apertures are covered by louvers that extend above the edges of the apertures to prevent solid particles from spilling through inlet apertures. And the apparatus includes baffles to inhibit erosion of the particles by redirecting the inlet gas flow.