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 (dp=70 μm, ρs=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 (ΔPm) decreases towards the wall in the range Ug=5–8 m/s, Gs=10–340 kg/m2 s. The thickness of the annular downflow layer based on ΔPm=0 reaches a maximum with increasing height. The annular downflow layer disappears locally with increasing solids mass flux (Gs) 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 Gs, and covers a wide Gs range right up to near the onset of the DSU regime.