The solids flow in the CFB-riser quantified by single radioactive particle tracking

Single radioactive particle tracking was used to measure the overall solids residence time (and its distribution) in the riser of a CFB, operating at superficial air velocities (U) of 1 to 9 ms(-1) and solids circulation fluxes (G) between 20 to 600 kg m(-2)s(-1). The results demonstrate that the particle motion and mixing differ according to the operating mode of the riser, with a fairly constant velocity throughout the riser achieved in the dilute or dense riser flow, but with a significant amount of back-mixing for intermediate values of U and/or G. This back-mixing is due to the core-annulus mode of particle flow. Whereas experimental results and theoretical predictions are in fair agreement for the dilute and dense riser flow, the core-annulus regime needs to account for a U and G dependent slip factor (phi), in excess of the commonly proposed value phi=2, especially at U-U-TR < 2 ms(-1). Moreover, the previously published riser operation diagram is confirmed by the experiments, although a further analysis of the core-annulus regime is needed. (C) 2011 Elsevier B.V. All rights reserved.