Unstable sinking of spheres at higher air velocity in a gas-solid fluidized bed

Abstract

Float-sink of large objects (on order of cm) in a gas-solid fluidized bed of powder (on order of 100 s of microns) based on density difference has been utilized for dry density separation in industry. The air velocity u0/umf is one of the important factors operating the fluidized bed, where u0 and umf are the superficial air velocity and the minimum fluidization air velocity, respectively. It is empirically known that the sinking of heavy objects is “occasionally” unstable in the fluidized bed combustor, for which the higher air velocity u0/umf > 4 is used. Unstable sinking means heavy objects that are expected to sink but sometimes do not. However, the precise conditions at which the unstable sinking occurs are not clear. In this study, we investigated the float-sink characteristics at a given air velocity u0/umf = 2–7 using glass beads of size Dgb = 425–600 μm and 600–850 μm as the fluidized powder bed media. The float-sink experiments were carried out at the bed height hgb = 150 mm and 75 mm using density adjusted spheres (diameter = 30 mm). We found that the spheres stably float or sink based on density difference at Dgb = 425–600 μm &hgb = 150 mm and at Dgb = 600–850 μm &hgb = 75 mm. However, the unstable sinking does occur at u0/umf > 4 at Dgb = 600–850 μm &hgb = 150 mm. These results indicate that the powder size and the bed height are key factors to induce the unstable sinking at the higher air velocity.