Segregation of particles in a tapered fluidized bed

Tapered fluidized beds are widely used in industrial operations to fluidize a wide range of particle sizes, and are thought to induce relatively strong particle mixing. Like their straight-sided counterparts, tapered fluidized beds are often considered as a homogeneous emulsion phase through which bubbles propagate. However, it has been shown that gas flow through the dense-phase of tapered beds is heterogeneous, generating a central fluidized core and unfluidized peripheral regions. When polydisperse particle mixtures are fluidized in a tapered bed, the structure becomes much more complex once the particles reach the minimum fluidization velocity (Umf), and a variety of segregation structures are generated. The aim of this study is to investigate how the two different types of structure (i.e. flow structure owing to the tapered shape of the bed and the segregation structures) interact and affect each other. Experiments were performed in a tapered (θ = 15°) planar bed using bidisperse mixtures of ballotini. The growth and extent of flow and segregation structures were measured, as well as the fabrics observed under different conditions. Under most conditions, the structure of the tapered bed is unaffected by the bed composition and segregation structures that form. An exception is at flow-rates just in excess of Umf when vertical columns of particles form, completely displacing larger-scale flow structures.The time scale for particle turnover in the central fluidized region is much shorter than that of particles captured within the peripheral regions. However, at sufficiently high gas flow-rates, uniform mixing can take place across the entire width of the bed.

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► Tapered fluidized beds (TFBs) are structurally heterogeneous
► They consist of a central fluidized core and unfluidized peripheral regions
► Fluidization of binary powders in a TFB generates a range of segregation structures
► Segregation behaviour does not differ greatly between the TFB and straight-sided bed
► Several types of segregation can occur simultaneously within a TFB