Simulation of Gas-Solid Flow in a Modified Internally Circulating Fluidized Bed By Using a Multi-Scale CFD Approach

Particle flow and mixing affect considerably the performance of a gas-solid fluidized bed. In particular, the regeneration of a fluid catalytic cracking (FCC) unit heavily depends on the fast and symmetrical mixing of catalyst to keep homogeneous distribution of bed temperature. In the present work, the hydrodynamics of a modified internally circulating fluidized bed (MICFB), which acts as a particle mixer, was numerically investigated by using a multi-scale computational fluid dynamics (CFD) with the structure－dependent EMMS drag.

The MICFB consisted of a bubbling fluidized bed with ID of 300 mm and height of 3 m. A draft tube with ID of 219 mm with four symmetrical rectangular slots was coaxially mounted in the bed. A FCC catalyst having a mean particle diameter of 76 μm and a density of 1498 kg/m³ was used in the simulation. The bed was fluidized with air of density of 1.225 kg/m³ and viscosity of 1.7 kg/(m·s). The superficial gas velocity in the draft tube was 0.3 and 0.4 m/s, whereas in the annulus was 0.05 m/s. The initial stacking height of the bed was 1.2 m and the particle volume fraction was 0.6. The hexahedron grids were generated by using the Gambit® 2.3 and then the simulation was performed by using Fluent® 6.3.26. The mesh number was varied, totaling 200,000 (Mesh 1), 500000 (Mesh 2) and 800000 (Mesh 3), respectively, for grid independency test. The no-slip boundary condition was used to the gas phase while the partial slip boundary was used to the solid phase with a specularity coefficient of 0.5. The discretization methods used was the second-order upwind scheme. The pressure and velocity were coupled with the famous SIMPLE scheme. The time step was equal to 0.0005 s. All the simulations lasted for 40 s in physical time and the time-averaged variables were obtained over the last 10 s.

Three sets of meshes were tested and the Mesh 2 was eventually selected. The radial density distributions at different axial heights were obtained in these simulations and compared with the experimental data. Based on the predicted flow field and the density distribution we can see that the inner structures of the MICFB, that is, the central feeding tube and the rectangular slots, may promote the mixing of gas and solid. The comparison with available data shows fair agreement.