A transient two-phase simulation reactor model for describing the hydrodynamics in a bubbling-fluidized bed was performed. A summary of the numerical method refined for over a decade is presented. The hydrodynamic model, based on the two-fluid theory, was validated with regards to the solids flow patterns in a fluidized bed operated under isothermal conditions. The predicted trends compared well with the experimental solids velocity field. The simulation appears to mimic experimental results accurately: both the simulation and the experiment show that the solids motion follows a 'gulf-stream' pattern with solids rising upward in the middle of the bed and falling downward near the side walls. The weak vortex patterns observed in the lower portion of the bed have also been simulated. Comparison of predicted and experimental axial solids velocities were in generally good agreement. This good agreement was found to be a function of the solids viscosity and not of the nonuniform inlet gas velocity.