(70cf) Numerical and Experimental Investigation of a Countercurrent Staged Fluidized Bed

Fluidized beds are widely adopted as reactors, granulators, or heat/mass transfer units in many industries, including the production of foodstuffs, pharmaceuticals, petrochemicals and other products. However, most of the operations are batch operations and may not be adopted in continuous operations. Although circulating fluidized beds (CFB) are continuous operating fluidized beds and are commonly used in petrochemical cracking reactions, the residence time of solids is usually short (usually few seconds). Here, we study continuous fluidized beds with moderate solids residence time (say few minutes) because of their wide applications in drying , heat recovery and other processes. In this work, A countercurrent multi-stage fluidized bed with moderate particle residence time is studied both numerically and experimentally. Horizontal perforated plates were added to section the column and a traditional fluidized bed was modified as the countercurrent continuous multi-stage fluidized bed. Since the diameter of the holes on the plate is about 3 ~ 4 times larger than the particle diameter, the particles can discharge through the holes and a countercurrent operation is achieved. The flow regimes and the hydrodynamics of an isothermal fluidized bed on a dual flow distributor are simulated using Computation Fluid Dynamics (CFD). The gas and solid phases are modeled as interpenetrating fluids, obeying the mass and momentum balance equations and are coupled via the drag force, the virtual mass force and the energy dissipation due to particle fluctuation. In CFD simulations, the bed on the dual flow distributor changed from a non-growing state, a dilute state, a bubbling state to a flooding state with the increase of the gas velocities or the solids feeding flux. The solids discharging mechanisms, including raining, switching and dumping, are also well modeled. Experimental results show that a stable bed (ID=3cm, opening ratio=0.2) exists at: 0.35 m/s≦ gas velocity (us) ≦1.70 m/s; 0.05 kg/s?m2≦ feeding rate (F) ≦1.50 kg/s?m2 and is presented as an operation map. The operation map is different between a single-stage bed and a two-stage bed and the difference is due to the discharging probability. An on-line image-analysis technique was developed to determine the particle residence time. The particle residence time increases with the increase of the gas velocity and the number of the stages. The addition of a plate increases 40% of the mean residence time. When comparing the stable operation range of such a system, the 2D simulated results agree qualitatively with the 3D experimental data.