Overall solids movement and solids residence time distribution in a CFB-riser

The residence time distribution (RTD) of solids is essential for the design of CFB reactors where conversion proceeds with time. The residence time di stribution for the solids is measured at different working conditions. The resulting average residence times are correlated as function of gas velocit y and solids circulation rate and are compared with literature data. In ord er to predict the RTD of the solids, the solids/gas flow is firstly describ ed by a plug flow with dispersion. A fitting procedure gave experimental Pe clet numbers, which were correlated as function of gas velocity and solids circulation rate. A more fundamental approach based on the core/annulus flo w structure is thereafter used to predict the residence time. The riser is divided in a dilute core with particles flowing upward and a denser film mo ving slowly downward along the wall. Particle interchange between the two r egions is described by a convective interchange flow. The model is used to predict experimental RTD-curves. The interchange flux between core and annu lus corresponds well with radial fluxes reported in literature. Although th e RTD is qualitatively well described, experimental curves show a higher di spersion than the calculated ones. To improve the core/annulus approach, fu rther research is necessary. (C) 2000 Elsevier Science Ltd. All rights rese rved.