Characterization of the solids crossflow in a bubbling fluidized bed

Journal article, 2024

The horizontal transport of solids in bubbling fluidized beds that have a solids crossflow is characterized by applying magnetic tracer measurements and modeling techniques to determine: the contributions of convective and dispersive forms of transport of solids; the residence time distributions of the solids; the feasibility of achieving a plug-flow or well-stirred tank behavior of the solids flow; and the overall fluidization quality. The latter is quantified by determining the extent of de-fluidized zones under varying operational conditions. The experiments are conducted in a bubbling fluidized bed with different rates of forced horizontal flow of solids, applying different bed heights and fluidization velocities. The setup is designed and operated in accordance with Glicksman's full set of scaling laws for fluidized beds, allowing scaling-up of the results to hot, large-scale conditions that resembling, for example, indirect gasification. The assessment of horizontal solids flow involves the sampling of a ferromagnetic tracer using impedance measurements at distinct locations within the bed to: i) fit the convection-dispersion transport equation and, thereby, determine the horizontal dispersion coefficient and velocity of the solids; and ii) feed a deconvolution routine for studying reduced-order (simplified) representations of the solids flow through compartment models. The results from the fitting to the convection-dispersion equation show a strong and close-to-linear correlation between the horizontal solids dispersion coefficient and the forced horizontal solids velocity. This strong interdependency may be attributable to increased shear-related mixing at higher bed-wall shear rates, and it implies a greater challenge linked to attaining a convection-controlled (plug flow) pattern for the solids crossflow. The residence time distributions obtained reveal the limitations of the convection-dispersion equation in providing a general description of the solids flow. Compartment model fitting, when applied to the observed residence time distributions, reveals that an increase in solids crossflow or, to a lesser extent, increased bed height leads to improved fluidization quality.