Hydrodynamic similarity of solids motion and mixing in bubbling fluidized beds

Solids motion and solids mixing behavior are important in many fluidized-bed applications, and yet, in the evaluation of hydrodynamic similarity criteria, relatively few studies have considered these phenomena directly. This work reports on a series of physical experiments involving solids motion and solids mixing undertaken in a number of freely bubbling fluidized beds scaled using the “simplified” scaling criteria. In the first set of experiments, the motion of large neutrally buoyant particles in bubbling beds of a finer material was compared using a simple float-tracer experiment in beds that were 300 mm, 600 mm, and 1,560 mm in dia. The distribution of circulation times for the float-tracers was found to scale well across the majority of gas velocities considered when the scaling law was followed, but discrepancies were noted when either the bulk bed material or large tracer particle sizes were mismatched. These results demonstrate that the circulation of large neutrally buoyant particles within the correctly scaled bubbling beds is indeed a “scaleable” phenomenon across a wide range of bed sizes, and this has useful consequences for fluidized-bed applications involving floating bodies. In the second set of experiments, solids downflow velocities were compared in scaled beds that were 146 mm and 300 mm in dia. using a novel application of electrical–capacitance tomography (ECT). The measured solids downflow velocities showed qualitative agreement with the scaling law and correlations in the literature. The novel tracer technique was only moderately successful in this application, but is worthy of further investigation, as the approach may serve to enhance the capabilities of existing ECT equipment.