Further measurements of flow dynamics in a high-density circulating fluidized bed riser

Abstract

Local voidages were determined for conditions corresponding to dense suspension upflow and fast fluidization in a riser of diameter 76.2 mm and height 6.1 m using a reflective-type optical fiber probe at superficial air velocities between 4 and 8 m/s and solids circulation fluxes up to 425 kg/m 2 s. High-density flow of cross-sectional mean volumetric solids concentration of about 0.2±0.05 was achieved in the riser with fluid catalytic cracking (FCC) particles of mean diameter 70 μm and density 1600 kg/m 3. Local time-mean voidages were nearly as low as ε mf at the wall and as high as 0.9 at the axis. As in our other recent work, solids refluxing near the wall of dilute CFB risers no longer existed for high-density conditions. Statistical analysis of local voidage fluctuations was used to characterize the local flow dynamics. The core behaves as a relatively uniform dilute flow, interspersed with infrequent particle structures. The number and density of these structures increases with radius. Maximum heterogeneity for high-density conditions occurs at some distance from the wall, unlike dilute risers where the flow is least uniform at the wall. Intermittency index vs. local voidage plots are “bell-shaped”, with a maximum where the local time-mean voidage is approximately 0.75.