(242e) The Onset of Bubbling in High Temperature Fluidized Beds

Recent research on fluidization of particle beds at superambient thermal levels has shown that operating temperature exerts its influence both on the properties of the fluidizing gas and of the solid phase. Particles subjected to high temperature undergo a visible change of behaviour apparently connected to the growth of thermally induced interparticle forces, an effect that can explain why the extension of measures, concepts and relationships valid at room level to thermal conditions typical of most industrial processes may become a serious source of errors. This difficulty is found both in the analysis of the transition to the fluidized state, which has been the subject of previous work of our research group in latest years, and in that of the onset of the bubbling regime, which constitutes the focus of present activity. Based on experiments on several types of solids covering a wide temperature range, the paper identifies the phenomena that accompany the development of the bubble phase past the incipient fluidization treshold. The approach followed is that of analysing the expansion behaviour of the bed, reconstructed by plotting on the same diagram in function of the superficial gas velocity values of the average voidage of the bed and of its dense phase voidage provided by fluidization and bed collapse experiments. The dependence on operating temperature of parameters related to the expansion ability of the particulate system as well as of its minimum bubbling velocity is discussed.