Solids motion and mixing in high-density circulating fluidized beds

This study was directed to the hydrodynamics of vertical risers, such as those used in catalytic cracking and other chemical reactors. The work was carried out in risers of 0.2 m and 0.076 m diameters with FCC particles of mean diameter 70 mum, primarily using optical fibre probes and solid tracer response measurements.; The flow characteristics of high-density gas-solids systems were investigated in detail by measuring the local transient particle velocity, solids concentration and solids fluxes at different radial locations and axial positions in the 0.2 m diameter riser operating over a wide range of net solids circulation fluxes (120 kg/m2s < Gs < 350 kg/m 2s) and superficial gas velocities (5 m/s < Ug < 8 m/s). These data were used to generate radial and axial profiles of solids hold-up, particle velocity and solids flux and to show how the profiles were influenced by the operating conditions. The flow behaviour in the riser was observed to be a function of height as well as the operating conditions. With a change in operating conditions, the local particle velocity and solid fluxes in the relatively dilute core region of the riser changed more significantly than those in the wall region. The opposite trend was observed with respect to the local solid hold-up distributions. The effect of riser diameter on the local solids flow structure in high-density circulating fluidized bed systems was investigated by comparing the findings of this study with those reported by previous researchers in 76 mm and 104 mm diameter CFB risers. The increase in riser diameter in this study was observed to result in lower cross-sectional average solids holdup at the same superficial gas velocity and net circulation flux under high-density conditions. (Abstract shortened by UMI.)...