Investigation of the particle flowpattern and segregation in tapered fluidized bed granulators

The particle flowpattern and granule segregation in tapered fluidized beds have been studied using two techniques. The first technique is to fluidize beds of varying total mass and granule fractions, then defluidize them suddenly to “freeze” the composition, section the bed in layers, and determine the composition of each layer by sieving. The second technique is to track a radioactive particle mimicking a granule as it moves in the bed. The results show that the segregation behaviour of granules is complex, their behaviour changing from flotsam at low granule concentrations to slightly jetsam at higher concentrations. The flow in the tapered bed is very different from what is expected based on relations derived for cylindrical beds. In the tapered bed a central region of high bubble activity and upward flow was a dominant feature. This “gulf streaming” became more pronounced as the total bed mass, and therefore the bed height, was increased, resulting in a bed turn-over time almost independent of the total bed mass. Quantitative data are given for upward and downward particle velocities and flows, bed turnover times, and axial granule concentration profiles.