Flow structure and thickness of annular downflow layer in a circulating fluidized bed riser

Flow structures were determined in a circulating fluidized bed (CFB) riser (0.203 m i.d.×5.9 m high) of FCC particles (d_p=70 μm, ρ_s=1700 kg/m³). A momentum probe was used to measure radial momentum flux profiles at several levels and to distinguish between upward and downward flow regions. Time-mean dynamic pressure (ΔP_m) decreases towards the wall in the range U_g=5-8 m/s, G_s=10-340 kg/m² s. The thickness of the annular downflow layer based on ΔP_m=0 reaches a maximum with increasing height. The annular downflow layer disappears locally with increasing solids mass flux (G_s) at a constant gas velocity, with achievement of the dense suspension upflow (DSU) regime. A new correlation is developed to predict the time-mean thickness of solids down-flowing layer based on solids mass flux and momentum flux. It successfully accounts for the variation of the annular layer thickness with height and G_s, and covers a wide G_s range right up to near the onset of the DSU regime.