Investigation of Mixing/Segregation Patterns of Two Dissimilar Granular Solids in Fluidized BEDS By Capacitance Probes

Fluidized bed reactors are widely employed in industrial processes in which two or more type of solids are adopted. As example, in chemical looping combustion of solid fuels, the fuel reactor hosts the inert material, the solid fuel and the oxygen carriers. In most cases, solids present different features and operating conditions must be optimized to drive the fluidized bed reactor along the desired solids mixing status. A deep knowledge of the fluidization of dissimilar solids in size and density is required. As regards the experimental investigation of mixing/segregation of dissimilar solids in fluidized beds, the diagnostic tool for the measurement of solid concentration plays an important role.

In the present study, capacitance probes are adopted to assess the mixing between two dissimilar solids belonging to group B of Geldart classification, fluidized in the bubbling regime at 500°C. The technique enables the measurement of local mixture composition thanks to different dielectric properties of gas and solids and of the two chosen solids. However, the procedure to gather mixture composition in the emulsion phase of the dense bed at relevant operating conditions is not trivial. Indeed, the expansion state of the mixture dense phase is not known a priori, and its value is required to properly postprocess data from capacitance probes.

Different types of mixing and fluidization tests are carried out. Selected binary mixtures are characterized by analyzing capacitance probes signal during quenching tests, after fluidizing the bed for sufficient time. The void fraction adopted to postprocess the data is derived from loose beds bulk density measurements of mixtures of known composition. In a second type of experiment, capacitance probes signal is analyzed during the fluidization of the two solids. The voidage derives from blank characterization tests where single materials are fluidized. Finally, results from the two independent experiments are compared.