Investigation of mixing/segregation patterns of two dissimilar granular solids in fluidized beds by capacitance probes

• Capacitance probes are employed innovatively in FBs of bidisperse solids. • A new method for data postprocessing is proposed and validated. • The dielectric constant of a binary mixture of solids is accurately predicted. • Steady-state tracer concentration profiles are gathered by a statistical approach. • Evolution of tracer concentration with time can hardly be tracked. The present study addresses mixing/segregation phenomena relevant to several chemical processes in bubbling fluidized bed reactors. Mixing between two dissimilar solids belonging to group B and D of Geldart classification is investigated in a lab-scale fluidized bed at high temperature. Capacitance probes are selected and tested innovatively to assess the transient phase of the mixing process between the two solids and the measurement of steady local mixture composition thanks to dissimilar dielectric properties of gas and solids and of the two chosen solids. A new procedure to gather quantitative results has been used considering expansion of the emulsion phase voidage and validated based on experimental data measured at 500 °C in the bubbling fluidized bed regime. The tests in transient conditions comprise the determination of two axial dispersion coefficients, D s , m a c r o a , on average, 3.1 × 10 - 2 m 2 / s , and D s a , ranging between 1.8 and 3.1 × 10 - 3 m 2 / s , referred to the tracers batch crossing the bed vertically without mixing with the rest of the bed and to the tracer batch disaggregating and reaching local steady-state concentrations in the dense bed, respectively. New insights are mainly revealed with reference to the phenomenology at steady-state conditions. In particular, axial and radial segregation patterns could be determined. Critical issues and corresponding improvements of the technique are highlighted and are mainly related to the properties of the two dissimilar solids and the capacitance probes amplification system.