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 Utr and the chocking velocity Uch. A qualitative map of different flow regimes of a gas-solid fluidized bed has been proposed based transition velocities.