Computational study of gas-solid flow in riser section of circulating fluidized beds

Circulating fluidized bed riser reactors have found applications in many important industries, and contributions to the understanding the hydrodynamics of the gas-solid flows in riser reactor systems can significantly enhance the efficiency and productivity of such processes. The Computational Fluid Dynamics (CFD) package Fluent was used to simulate the riser of CFB using an Eulerian-Eulerian model. A User Defined Function/C++ code for the drag law closure of Arastoopour was developed and the predictions were compared to those obtained from the Wen-Yu drag closure available in the Fluent. It was shown that a two-dimensional riser model incorporating kinetic theory was capable of predicting the gas-solid flows and establishment of core-annular behavior. Gas-solid flow profiles for various mesh sizes were also obtained to assess solution accuracy. It was demonstrated that the computation time increased significantly when the mesh size was increased from 10 x 210 to 21 x 2604. The Arastoopour drag model generated better predictions than the Wen-Yu drag model for the conditions used in this study since it provided a more accurate description of solids downflow at the wall at smaller mesh sizes.