The inventions relate to controlling the particle size, and gas flows in moving beds of particles containing metallic iron and/or nickel, wherein the metallic iron or nickel are reactants in carbonylation reactions with carbon monoxide (a component of the flowing gas). The inventions' use is to increase the rates of production of iron carbonyl and nickel carbonyl. The inventions use cross-flow funnels containing moving beds of carbonylation particles (i.e., that contain iron and nickel) and regulation of the removal of those particles from the bottoms of the funnels. Cross-flow refers to the horizontal flow of the carbonylation gas (containing carbon monoxide) through the downward moving beds of carbonylation particles held in the cross-flow funnels.