A first principles model of a gas-fluidized bed has been applied to calculate the hydrodynamics of a two-dimensional (2-D) bed with an orifice in the middle of a porous plate distributor. The advanced hydrodynamic model is based on a two fluid model approach in which both phases are considered to be continuous and fully interpenetrating. Conservation equations for mass, momentum and thermal energy have been solved numerically by a finite difference technique on a mini-computer. Our computer model calculates the porosity, the pressure, the fluidum phase temperature, the solid phase temperature and the velocity fields of both phases in 2-D Cartesian or axisymmetrical cylindrical coordinates. The new feature of the present model is the incorporation of Newtonian behaviour in the gas and solid phases. Our preliminary calculations indicate that the sensitivity of the computed bubble size with respect to the bed rheology (i.e. the solid phase viscosity) is quite small. However the bubble shape appears to be much more sensitive to the bed rheology. Results of the calculations have been compared with data obtained from an experimental cold-flow model (height: 1000 mm, width: 570 mm, depth: 15 mm).