Two-phase flow of interstitial air in a moving packed bed of granular solids is modelled using a distinct element (DE) technique which considers the Newtonian dynamics of particle motion passing through a radial flow field of air in a mass flow hopper. The air flow is assumed to be incompressible and the mass flux of air at any height within the hopper is assumed to be constant. These assumptions allow the simulation of air-retarded and air-assisted hows in mass flow hoppers without the need for an extensive development of the momentum balance calculations on an Eulerian fixed grid. The particle-particle and particle-hopper wall interactions are modelled using a Hertzian interaction law and a contact friction algorithm of the Mindlin analytic form (Langston er al., 1995, Chem. Engng Sci. 50, 967). Predictions of discharge rates in both air-retarded and air-assisted flows are compared with the continuum mechanics calculations based on the steady-state flow assumption. The DE simulation results indicate certain transient and oscillatory features of the flow fields which have not hitherto been demonstrated by the continuum theories. Furthermore, it is shown that air-assisted flow leads to increased wall stresses which reduce the bulk solids discharge rate for discharge through small orifices.