Comparison of 2-D and 3-D CFD simulations of bubbling fluidized beds with x-ray fluoroscopy and imaging experiments

Computational Fluid Dynamics (CFD) is a promising tool for the development of comprehensive models of gas-phase polymerization in fluidized bed reactors. However, CFD models of gas-solids flow in fluidized beds are still under development and validation of these models on relatively simple test cases is essential if we hope to achieve reliable predictions of the complex phenomena associated with polyolefin production in commercial reactors. In the present study, numerical simulations of bubbling fluidized beds were performed in two (2-D) and three-dimensions (3-D) using the Eulerian granular multiphase model in Fluent, a commercial CFD package. The CFD simulation results are validated against experiments conducted on a small diameter, low-pressure fluidization column with ideal particles (glass beads with a narrow PSD). X-ray fluoroscopy and image processing techniques were applied to obtain detailed information about the bubbles in the bed, e.g. frequency distribution, bubble diameter and axial velocity. The 2-D simulations showed reasonably good agreement with the experimental bubble data on the coarse meshes but the predictions deviated as the mesh was refined. Next, a novel “numerical” x-ray technique was introduced to facilitate a direct comparison of the bubble properties from the 3-D CFD simulations with the x-ray based measurements. The 3-D bubble sizes and frequency data is well predicted using this approach but bubble velocities are significantly overpredicted by the 3-D simulations. Further work is needed to refine the numerical x-ray approach and the x-ray fluoroscopy based imaging technique.