Flow structures and mixing patterns in the freeboard of gas-fluidized bed reactors

Flow structures and gas mixing patterns establishing in the freeboard of a bubbling gas-fluidized bed have been investigated by means of a planar laser light scattering (PLLS) technique. Nondiffusive tracing of the flow structures has been accomplished by injection of an aerosol of micrometer-sized scattering particles at a preset axial position in the splash zone of the reactor. A digital image acquisition and analysis technique has been developed and used to characterize the macroscopic and microscopic features of the flow structures. Experiments have been carried out in the freely bubbling regime at gas superficial velocities ranging from incipient fluidization to U/Umf = 2.6. Bed solids consist of glass beads of two different size ranges belonging to group B of the Geldart classification of powders. Results are analyzed with the aim of determining effective gas dispersion coefficients and characteristic time-scales for gas micromixing in the splash zone as a function of the excess gas superficial velocity.