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Circulating fluidized bed hydrodynamics in a riser of square cross-section

University of British Columbia

Hydrodynamic experiments were carried out in a cold-model circulating fluidized bed riser of 146 mm x 146 mm square cross-section and 9.14 m height. Ottawa sand particles of mean diameter 213 urn, particle density 2640 kg/rn 3and loosely packed bed voidage 0.43 were used as the bed material. Fibre optic probes were employed to measure both local time-mean voidage and particle velocity. A core-annulus flow structure was found to exist in the square riser as for risers of circular cross-section investigated by earlier workers. The voidage generally decreases laterally from the axis towards the wall. However, M-shaped lateral voidage profiles, which can also be distinguished in earlier reported data from circular risers, were found in a number of cases, with voidage first increasing from the wall, reaching a maximum, then decreasing somewhat towards the axis. In the corners, the voidage was lower while the descending particle velocity was higher than for other near-wall locations at the same cross-section. Bimodal and tri-modal probability distributions of particle concentration were found in some cases. The velocity of those particles which are ascending was found to be low at the wall and to increase towards the axis; there was a maximum in the lateral profile of descending particle velocity at a small distance from the wall. The velocity of particle downflow was found to be in the range 0.8 to 1.5 m/s. A theoretical model has been established to predict the velocity of descending clusters near the wall. The model suggests that the aspect ratio of ellipsoidal clusters has only a limited influence on the cluster velocity. The thickness of the outer annular zone first decreased with height until a minimum was reached 3 to 4 m above the distributor, then increased towards the top because net solids mass flux was outward towards the wall near the bottom and inward towards the axis near the top of the column. Significant differences were found between core-annulus boundaries defined as the location of zero vertical solids flux and zero vertical mean particle velocity, with the annulus thickness based on the latter definition being smaller. Wall roughness, introduced by affixing sand paper to the entire wall surface, was found to lead to an increase in voidage near the wall, while having little influence on the voidage near the axis. More uniform lateral voidage profiles were obtained for the rough-walled riser. Neither bimodal nor trimodal probability distributions of particle concentration were found for the roughwalled column, nor were there any M-shaped voidage profiles. Wall roughness increased the ascending particle velocity somewhat, while having little influence on the descending particle velocity near the wall. In a riser with simulated membrane walls, the valleys formed by the fin and two adjacent membrane tubes protect particles from the gas. Particle streamers near the wall then tend to move downwards in the fin region, leading to higher voidages and increase descending particle velocities than in the crest region or for flat smooth walls. It was found that wall features such as roughness and membrane tubes have significant influence on the gas-solids flow structure near the wall of circulating fluidized bed risers. Little influence of wall roughness and membrane tube could be detected near the axis of the column. A sampling probe and piezoelectric probe were used to measure lateral solids mass flux and momentum flux. Except at the very bottom of the riser, cross-flow fluxes were always substantially lower than (axial) net circulation fluxes, but high enough to assure considerable interchange between the wall and core regions. Lateral fluxes were highest at the bottom of the riser, relatively constant at intermediate levels, then increased slightly near the top. The solids momentum flux was found to increase with height in the lower part of the column, and then decrease in the upper part. The lateral particle velocity was as high as 2 to 3 m/s on the axis of the riser.

Thesis/Dissertation

application/pdf

2009-04-22

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http://hdl.handle.net/2429/7472

Chemical and Biological Engineering

University of British Columbia

UBCV

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