The phenomenon of pinning in an annular moving bed reactor with crossflow of gas

Abstract

Certain catalytic reactors with reversibly decaying catalysts use continuous replacement and regeneration of the catalyst. A bed of the catalyst pellets moves through the reactor between two co-axial cylinders and the reactant mixture passes radially, either inward or outward, through this bed. There is then an upper bound on the flow of reactants imposed by the mechanical phenomenon of “pinning”, in which the drag force exerted on the particles by the reaction mixture induces frictional forces between the particles and the downstream wall which are sufficient to prevent motion of the bed. A model has been developed to account for pinning behaviour in an annular moving bed reactor of finite length. It is assumed that the catalyst bed behaves as a Coulomb material, with forces of dry friction where it slides over solid surfaces. The results give an excellent qualitative account of the phenomenon, as observed in a small laboratory test system. The predicted gas pressure drops required for pinning are larger than those observed, but never by as much as a factor of two.