THE MINIMUM BUBBLING VELOCITY OF FLUIDIZED BEDS OPERATING AT HIGH TEMPERATURE

The paper studies the influence of operating temperature on the transition of a fluidized bed to the bubblingregime. Fluidization experiments and collapse tests were performed in a 54mm i.d. transparent column onsamples of FCC, silica sand and corundum, at temperatures ranging from 30 to 500°C. All the solidsinvestigated, included in group A of Geldart classification, exhibited homogeneous expansion afterfluidization at all temperatures. Depending on the technique adopted to identify the velocity at which thebed enters the bubbling regime, different trends of umb versus T are observed. Such results are analysedtogether with the expansion behaviour of particle systems of various densities, illustrated by diagrams builtwith data of fluidization and collapse experiments. The role exerted by thermally induced interparticle forcesin delaying the onset of steady bubbling is highlighted, as well as the opportunity of adopting a definition ofumb of practical meaning.