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Influence of air bubble size on float–sink of spheres in a gas–solid fluidized bed

https://doi.org/10.1016/j.apt.2011.08.002Get rights and content

Abstract

The float–sink of density adjusted spheres of different diameter (10–40 mm) in a gas–solid fluidized bed was investigated at various bed heights (50–200 mm). The maximum density of floating spheres (ρfloat) and the minimum density of sinking spheres (ρsink) were determined by the float–sink experiments. The fluidized bed density (ρfb) was measured using the height and cross section of the fluidized bed and total weight of the fluidized media. The diameter of air bubbles at the bed surface was measured at each bed height, and was normalized by the sphere diameter. It was found that the value of ρfbρfloat approaches zero as the normalized bubble diameter decreases from 4 to 0.5 regardless of the sphere diameter. The value of ρsinkρfb for sphere diameter = 10 mm approaches zero as the normalized bubble diameter decreases from 4 to 1.5, whereas the value for sphere diameter = 20–40 mm rises from zero as the normalized bubble diameter decreases from 1.5 to 0.5. The float and sink of spheres basically tend to follow the fluidized bed density with decreasing the normalized bubble diameter. However, relatively larger spheres do not sink based on the density difference as the normalized bubble diameter decreases, which may be due to that the fluidized bed viscosity becomes larger as the normalized bed diameter decreases.

Highlights

► We investigated the float–sink of different sized spheres in a gas–solid fluidized bed at various bed heights. ► The float–sink of smaller sized spheres does not follow the fluidized bed density at higher bed height. ► The float–sink of smaller sized spheres is stabilized by lowering the bed height. ► The stability of the spheres’ float–sink depends on the relationship between the sphere size and the bubble size.

Introduction

Development of dry separation techniques is strongly demanded as the substitution for the general wet separations using large quantities of water, because drought due to global warming is a critical issue and frugal water usage is unavoidable. The gas–solid fluidized bed, which has liquid-like properties such as density and viscosity [1], is one of the possibilities to replace the wet separations [2], [3], [4], [5], [6], [7]. The float–sink of objects in the fluidized bed can be applied as the dry separation based on the density difference. We performed fundamental and practical studies on the dry separation using the fluidized bed [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], and continuous separators based on our results are commercialized. We reported that the float–sink of lump iron ore particles smaller than 17.6 mm is unstable, although those with size greater than 17.6 mm are sharply separated [18]. Recently, we also reported that when the bed height is lowered, the float–sink of the smaller sized ones is stabilized [19]. The fluctuation of the surface height of the fluidized bed was focused as the determinant factor for the stabilization; however, decisive conclusion was not obtained. In this study, we focused on the size of air bubbles at the fluidized bed surface. The float–sink experiments using density adjusted different sized spheres and the measurement of the fluidized bed density were performed by changing the bed height. We measured the size of the air bubbles, and investigated the correlation between the spheres’ float–sink and the size of air bubble. In the previous study [19], a mixture of zircon sand and iron powder was used as the fluidized medium with the aim of lump iron ore particles separation. Here we used not the mixture but only the zircon sand as the fluidized medium.

Section snippets

Experimental

Zircon sand (bulk density = 2900 kg/m3 and size = +90–355 μm) (RASA CORPORATION) was used as the fluidized medium. The apparatus used previously [18], [19] was used in this study; the apparatus consisted of a cylindrical column (inner diameter = 290 mm and height = 530 mm) and an air distributor with a textile felt held between two perforated metal plates. The zircon sand was put into the column as the bed height h = 50–200 mm, and was fluidized by compressed air at u0/umf = 1.7; u0 and umf were 5.6 and 3.3 

Results and discussion

Fig. 1 shows hsp as a function of ρsp; the dotted line shows the fluidized bed density. At each h and Dsp, hsp is equal to 1 for smaller ρsp and is equal to 0 for larger ρsp, indicating that the spheres of smaller density completely float and those of larger density completely sink. The spheres of Dsp = 40, 30 and 20 mm are sharply separated, and the float–sink occurs around the fluidized bed density. The spheres of Dsp = 10 mm does not show sharp separation at h = 200 mm, but the float–sink becomes

Conclusion

In summary, the maximum density of spheres floating in the fluidized bed approaches the fluidized bed density regardless of the sphere diameter as the bubble diameter normalized by the sphere diameter is decreased by lowering the bed height. The minimum density of smaller spheres sinking in the fluidized bed approaches the fluidized bed with the decrease of the normalized bubble diameter. On the other hand, the minimum density of larger spheres sinking in the fluidized bed becomes larger than

Acknowledgements

This study was supported by Industrial Technology Research Grant Program in 2008 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the Core-to-Core Program promoted by Japan Society for the Promotion of Science (Project No. 18004).

References (19)

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