A validation and mesh refinement study has been performed for the simulation of a freely bubbling bed of fluid catalytic cracking (FCC) catalyst operating at superficial gas velocities in the range of 0.05 to 0.20 m/s, using the two-fluid computational fluid dynamics (CFD) code Multiphase Flow with Interphase eXchanges (MFIX). The simulation results have been compared to experimental data collected using an electrical capacitance tomography (ECT) imaging system as well as data from the literature. A comparison has been made based on the bed expansion, and bubble diameters and rise velocities. A novel method has been used to extract bubble properties from ECT data and simulation results. It was found that a modified gas-solids drag law, corresponding to an effective particle agglomerate diameter in the range of 135 to 170 mum for FCC particles of actual 75 mum mean diameter, was required to adequately predict the fluidization behavior observed experimentally. These findings support the argument that cohesive interparticle forces lead to agglomeration of FCC catalyst powder and significantly affect the fluidization quality. The model is the first presented in the open literature to successfully apply CFD to obtain realistic predictions of FCC bubbling bed behavior. (C) 2003 Elsevier Science B.V. All rights reserved.