The fast-fluidized bed is characterized by intensive backmixing and nonhomogeneous flow patterns represented by two flow regions, i.e. a dense wall (or annular) region and a dilute core region. This segregated flow structure is undesired for certain chemical reaction applications. To reduce the backmixing and nonhomogeneity, superimposition of coarse particles onto a fine-particle circulating fluidized bed is attempted in this study. The effects of the coarse particles on the reactor performance are investigated in the light of a catalytic ozone decomposition reaction in the bed. The apparatus used consists of a riser, 102 mm in diameter and 6.32 m in height. FCC particles with a mean diameter of 58-mu-m, impregnated with ferric oxide, are used as catalysts. Polyethylene particles of 4.4 mm diameter and glass beads of 2 mm diameter are employed in the experiments as coarse particles. Both the hydrodynamic parameters and the ozone concentrations are measured under various operating conditions. Comparisons are made of the axial and radial distributions of ozone concentrations in the presence and absence of coarse particles. For a given Damkoehler number, the presence of coarse particles is found to improve ozone conversions in a riser with polyethylene particles at a high gas velocity, and with glass beads at a low gas velocity. A reactor model based on the core annulus flow structure is also proposed to account for the ozone conversion in the reactor.
