Hydrodynamics and mass transfer around active particles in dense gas-fluidized beds

•Voidage by capacitance probes is measured through a statistical approach.•A lower (LV) and a higher (HV) voidage phase co-exist in the emulsion phase.•LV and HV-phase may refer to the bubble wake and the bulk of the emulsion phase.•Results confirm the need for hydrodynamic studies at industrial conditions.•Mass transfer and expansion patterns of the emulsion phase are closely related. The hydrodynamics of bubbling fluidized beds of Geldart B granular solids operated at ambient temperature and at 500 °C are investigated by uncooled capacitance probes at different gas superficial velocities. Statistical analysis of the time-series of the local bed voidage generates Probability Density Functions (PDF) that display multimodal patterns. A remarkable bimodal character is apparent in the part of the PDF that refers to the emulsion phase when the gas superficial velocity is increased. It is inferred that two phases with different porosity co-exist in the emulsion phase: a lower voidage LV-phase and a higher voidage HV-phase. The link between the LV- and HV-phases as assessed from PDF of capacitance signals and the actual flow patterns induced in the emulsion phase by the bubbles is open to scrutiny. Along a different path, the influence of the bed hydrodynamics and expansion patterns of the emulsion phase on the gas mass transfer around a freely moving coarse active particle is investigated. To this end, mass transfer-limited reaction experiments are performed, consisting in catalytic oxidation of carbon monoxide at 450 °C over Pt-loaded γ-alumina spheres immersed in the fluidized bed. The mass transfer coefficient around the freely moving active particles is measured and worked out to calculate the particle Sherwood (Sh) number. Results obtained from reaction experiments confirm the close relationship between mass transfer and expansion patterns of the emulsion phase, and the need for proper hydrodynamic characterization of the bed. A Frӧssling-type equation, Sh=2·∊e+K·(Ree/∊e)1/2·Sc1/3, well correlates the results. Sh increases with fluidization gas velocity and a best fitting is obtained using the bed voidage of the HV-region of the emulsion phase in the correlation.