Diagnosis of hydrodynamic regimes from large to micro-fluidized beds

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Micro-fluidized bed (MFB) technology is a newly emerging technique that is receiving increasing interest for various industrial applications. MFB is generally characterized by the use of inner diameters (Dt) of a few millimeters, and a large specific contact surface area between the walls and the fluidization system. These specific features have the advantage of enabling ultra-fast heat dissipation for exothermic reactions, as well as isothermal conditions. However, the wall effect also clearly prevails, leading to complicated hydrodynamic phenomena. In addition, the study of MFB is challenging, as a consequence of the difficulties encountered in its characterization, due to strong probe interference effects. In order to understand the wall effect and to make use of suitable fluidization conditions for practical applications of MFB, the present study first establishes a diagnostic methodology, and then applies this to study the influence of a reduction in Dt from large to micro-fluidized bed scales. Experiments were carried out in six glass columns with Dt ranging between 4 and 100 mm, using spherical Geldart group B particles (glass beads). The methodology described here is based on the analysis of pressure fluctuation measurements, which are used to monitor fluidization simultaneously in the time and frequency domains. Fixed bed, pseudo-homogeneous fluidization, bubbling, slugging, and turbulent fluidization regimes are identified in MFBs. When Dt is reduced from large-to micro-fluidized bed scales, minimum fluidization, bubbling and slugging are delayed, whereas the onset of turbulent fluidization occurs more rapidly. The late stage of turbulent fluidization covers a relatively wide range of gas velocities, and is found to have homogeneous fluidization structures, which are of interest for practical MFB applications such as Fischer–Tropsch synthesis.