A study on the hydrodynamics of a circulating fluidized bed using CFD as a simulation tool

Abstract

A two dimensional computational fluid dynamics (CFD) model has been proposed to simulate the hydrodynamics of gas- solid flow in a circulating fluidized bed (CFB) riser adopting Eulerian approach using the commercial CFD software, Fluent. Present work has proposed a new drag model to calculate the gas- solid momentum transfer coefficient. The Kinetictheory of granular flow was used to provide the closure relations for the governing equations to find out the stress tensor and pressure for the solid phase. Simulation results are compared with the experimental results available in the literature. 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 in the literature. The effect of different drag models including Gidaspow, Arastoopour, and Syamlal and O Brien drag models on modeling results were analysed. All the drag models predicted quite similar flow hydrodynamics; however the Syamlal and O Brien drag model, which was modified and used in the present study based on the minimum fluidization velocity of the solid particles has indicated better predictions of the solid volume fraction profiles at the core area. 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 of 8 m/s. 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 suspension up-flow regime with a solid mass flux of 550 kg/m2s. newline newline newline