Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

Jun Oshitani, Shunsuke Kato, Takuya Tsuji, Kimiaki Washino, Shusaku Harada, Hirokazu Kajiwara, Kei Matsuoka and George V. Franks

Depending on their density, large objects will either float or sink in a gas–solid fluidized bed due to the
liquid–like properties and density of the fluidized bed. The float–sink technology has been applied to dry
density separations in industry. It is important for optimized industrial application to understand how
the air velocity and the powder bed height affect the float–sink as the key operating factors. In this study,
we investigated the float–sink of spheres of various density by varying the air velocity and the powder
bed height. Also, we obtained the local fluidized bed density and the buoyancy force working on the
sphere at various heights. We used the weight of a stainless-steel sphere in the fluidized bed to estimate
the local fluidized bed density and the buoyancy force based on Archimedes principle. We found that the
spheres float–sink behavior changes dramatically with the air velocity and the powder bed height and
that the spheres float–sink behavior is correlated to DF = Fb – Fg, where Fb is the buoyancy force and
Fg is the gravity force acting on the sphere. We also found that the fluidized bed density is not constant
as a function of height when the air velocity is relatively large; the local fluidized bed density is interestingly
either minimal at approximately mid-height or surprisingly, gradually increases with height within
the fluidized bed at higher air velocities. The possible reasons are discussed by considering the local variation
of the motion of air bubbles and the fluidized medium which affect the fluid force acting on the
sphere in the fluidized bed.

Advanced Powder Technology

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