Molignano, Laura (2023) Hydrodynamics and mixing of dissimilar granular solids in dense gas-fluidized beds. [Tesi di dottorato]

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Abstract

Gas-fluidized beds (FB) are frequently used as favorable reaction environments in many industrially relevant heterogeneous processes where intimate contact between a solid and a gas phase is required. Bed hydrodynamics strongly influence the course of heterogeneous and homogeneous reactions in gas-FBs as they affect mixing/segregation patterns of the fluidizing gas and of fluidized solids, as well as heat and mass transfer around active particles immersed in a bed of inert particles. Assessment of the mentioned phenomena is, therefore, a key prerequisite for successful design and operation of FB converters. The two-phase theory is of fundamental importance as a conceptual framework of aggregative fluidization for freely bubbling FBs of materials belonging to groups B and D of Geldart classification. Despite the vast popularity of the two-phase theory, it has long been recognized as being inaccurate under certain conditions, often resulting in an overestimation of the actual visible bubble flow. There is ample experimental evidence that properties of the emulsion phase in a freely bubbling FB depart from bed properties at incipient fluidization conditions. Nonetheless, systematic and comprehensive modelling of the emulsion phase expansion at gas superficial velocities in excess of incipient fluidization is still largely lacking. The scenario becomes even more complex when two or more dissimilar solids are fluidized, and operating conditions must be optimized so as to promote a prescribed degree of solids mixing. Characterization of hydrodynamics and mixing of single and polydisperse granular solids in FBs requires the setup of appropriate diagnostic tools, combining lack of intrusiveness, good sensitivity and reproducibility, robustness with respect to a broad range of process conditions. In the present PhD thesis, hydrodynamics and solids mixing in freely bubbling FBs of Geldart B/D particles are investigated. Home-made uncooled capacitance probes are purposely developed. Most of the study is based on experiments in a laboratory-scale unit, to pave the groundwork for ongoing research on a pilot-scale FB. In a first experimental campaign, four different bed materials are investigated both at ambient temperature and 500 °C, by varying the gas superficial velocities. The major effort consists in the analysis and interpretation of the time series of capacitance probe signals at different locations in the bubbling FB. Results provide clear evidence of the expansion of the emulsion phase of the bed, and of the possible presence of two previously unnoticed low- and high-voidage regions in the emulsion phase. The consequent increase in the associated superficial gas velocity with respect to minimum fluidization conditions is assessed through available correlations/balance equations, and interpreted in relation to the role of the bubble throughflow coefficient. A second experiment, based on diffusion-limited oxidation of carbon monoxide on a solid catalyst, is directed to the assessment of the effect of emulsion phase expansion on mass transfer around active particles immersed in beds of inert solids. Very good prediction of the Sherwood number dependence on superficial gas velocity is attained. Better estimates of the active particle Sherwood number are obtained if the Frössling-type correlation is implemented using the actual voidage and superficial gas velocity of the expanded emulsion phase. Results also support the possible usage of the Richardson-Zaki correlation applied to the emulsion phase of gas bubbling FBs. In a third experiment, capacitance probes are further employed, with a novel approach, to characterize mixing of dissimilar solids in gas FBs. Time-series of signals from capacitance probes are analyzed with the aim of assessing the transient dispersion of one solid component into the other. Steady-state spatially resolved profiles of the solids concentration are determined at different operating conditions of the gas FB. Results reveal advantages and limitations of the technique applied to bidisperse solids and show, for the investigated systems, a peculiar phenomenology, which resembles the layer inversion. Altogether, results are intended to provide not only a step forward in the understanding of dense gas FBs phenomenology but also widen the capabilities in design, scale-up and optimization of FB reactors.

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