Transition velocities in the riser of a circulating fluidized bed

We attempted to study the hydrodynamics and flow regime transitions in a circulating fluidized bed (CFB). In conventional gas-solids fluidization, the flow regimes include the fixed bed, bubbling fluidization and turbulent fluidization, in which the regimes mainly depend on the gas velocity. However, in a CFB, the flow regime depends on both the solids circulation rate and gas velocity. The flow regimes in a CFB include the high-velocity fluidized bed and pneumatic conveying. Experiments were conducted in a column of 0.052 m internal diameter and 1.2 m long covering a wide range of operating conditions. The transport velocity was estimated by emptying time and extrapolation techniques. It was noticed from the present experimental observations that Geldart B-type particles move more or less as individual particles, while Geldart A-type particles have a tendency to form clusters. Empirical correlations have been proposed for the transport velocity U_tr and the chocking velocity U_ch. A qualitative map of different flow regimes of a gas-solid fluidized bed has been proposed based transition velocities.