Axial and radial development of solids holdup in a high flux/density gas–solids circulating fluidized bed

Abstract

Detailed distributions of solids holdup in an extremely high density circulating fluidized bed riser with FCC particles are mapped by an optical fiber probe. The solids circulation rate reaches as high as 1000kg/m2s which has never been achieved before in an academic setting. When solids flux approaches 800kg/m2s, the axial flow structure becomes uniform and the cross-sectional mean solids holdup reaches 0.22 throughout the entire riser; the same reaching 0.32 at 1000kg/m2s. Compared to a typical core-annulus structure, the radial distributions of the solids holdup becomes much less uniform with a shrinking core and transits to a monotonic increasing profile towards the wall. Speed of flow development differs at various radial positions with almost instant development in the center even at the highest solids flux of 1000kg/m2s and then becoming slower towards the wall. Fluctuations in high density circulating fluidized beds are significantly greater than those in low density ones, leading to more vigorous interactions between gas and solids phases. As a result, better gas–solids contacting and mixing, plus the uniform axial profiles of solids holdup, provide better reactor performance for the high solids flux/density risers than low flux/density ones.