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CFD modeling of the hydrodynamics of circulating fluidized bed riser Almuttahar, Adnan M.

Abstract

A two dimensional computational fluid dynamics (CFD) model has been developed to simulate the hydrodynamics of gas-solid flow in a high density circulating fluidized bed (HDCFB) riser using the commercial CFD software, Fluent. The Eulerian-Granular multiphase model was applied, which treats both phases as a continuum while the governing equations of mass and momentum conservation were solved for gas and solid phases. The kinetic theory of granular flow was used to provide the closure relations for the governing equations for the solid phase. CFD modeling of the isothermal multiphase flow of air and fluid catalytic cracking (FCC) particles in a circulating fluidized bed (CFB) riser has been performed and compared to the experimental findings of particle volume fraction, particle axial velocity, and local particle solid flux profiles reported by J. Liu (Liu, PHD thesis, 2001 Department of Chemical and Biological Engineering, The University of British Colombia). The simulated profiles were overall in good qualitative agreement with the experiments, while similarly, the simulated particle axial velocities were in good quantitative and qualitative agreement with the experiments. However, due to the difficulties in modeling the solid segregation toward the wall accurately, the solid volume fraction was under predicted near the walls. The effect of different drag models including Gidaspow, Arastoopour, and Syamlal and O’Brien drag models on modeling results was investigated. All the drag models predicted quite similar flow hydrodynamics; however, the Syamlal and O’Brien drag model, which was modified based on the minimum fluidization velocity of the applied FCC particles, indicated better predictions of the solid volume fraction profiles at the core area. Different wall restitution coefficient values and solid slip conditions have been applied to study their effects on solid volume fraction distribution across the riser. While the wall restitution coefficient did not exhibit a significant effect on the riser hydrodynamics, the appropriate slip condition aided in predicting the solid segregation toward the wall. Using the free solid slip condition resulted in a better agreement with the experimental data of the solid volume fraction distribution near the walls. Finally, the model was evaluated comprehensively by comparing its predictions with experimental results reported for a circulating fluidized bed riser operating at a solid mass flux in the range of 94 to 550 kg/m²s and a superficial gas velocity in the range of 4 to 8 m/s. The model was capable of predicting the main gas-solid flow features in the HDCFB riser operating at a solid mass flux in the range of 254 to 455 kg/m²s. However, the model was incapable of accurately predicting the gas-solid flow behavior in a low density circulating fluidized bed riser with a solid mass flux of 94 kg/ m²s and risers operating in dense suspensipon up-flow regime with a solid mass flux of 550 kg/ m²s.

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