Hydrodynamics of a two dimensional gas-solid fluidized bed reactor was investigated by applying computational fluid dynamics (CFD). Unsteady behavior of this multiphase flow was simulated by using the Eulerian-Eulerian model coupled with kinetic theory of granular flow. Momentum exchange coefficients are calculated by using the Gidaspow drag functions. Simulation results were compared with the experimental data in order to validate the CFD model. Good agreement between the numerical results and experimental data was observed under different operating conditions. The effect of mixture composition, inlet gas velocity, restitution coefficient of particles and bed deviation on gas-solid flow behavior were investigated. A mixture of particles with diameter of I and 2 mm and density of 2400 and 2500 kg/m(3) were fluidized under inlet gas velocity of 0.5-2.5 m/s. An increase in mass fraction of small particles from 45% to 59%, led to an increase in the bed height and a decrease in average diameter of particles in the bed. Restitution coefficient differences can be the cause of segregation in a bed including particles with the same size and density. Time averaged radial profiles were smoother in higher gas velocities. Bed deviation did not affect the mixing rate, but it could be beneficial to simplify the particle derivation process on the bed. Appropriate ranges of the Courant number were used to attain reasonable results and decrease the computational cost (C) 2013 Elsevier B.V. All rights reserved.