Hydrodynamics of circulating fluidized beds with risers of different shape and size

Abstract The hydrodynamics of risers of CFB units have been studied by means of a transparent column having a 0.120×0.012 m rectangular cross-section and two cylindrical columns having 0.120 m i.d. and 0.400 m i.d. Several solids, ranging in size from 48 μm to 300 μm and ranging in density from 1 330 kg m −3 to 2 600 kg m −3 , were tested. Instantaneous pressures were measured at several points along the risers by means of pressure transducers. Their output signals were processed to provide instantaneous and time-averaged voidage profiles. Unit flow structures made of ‘plugs’ and ‘slugs’ were detected in dense and transition regions of the risers, regardless of the size and the shape of the column and the type of solids. The dense and transition regions as a whole appear to form the inlet section of the risers. A descriptive model has been proposed to estimate the length of the dense and transition regions in terms of the riser's design and operating variables. The model is based on Taylor instability at the interface between ‘plugs’ and their upstream ‘slugs’.