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 bubbling regime. Fluidization experiments and collapse tests were performed in a 54mm i.d. transparent column on samples of FCC, silica sand and corundum, at temperatures ranging from 30 to 500°C. All the solids investigated, included in group A of Geldart classification, exhibited homogeneous expansion after fluidization at all temperatures. Depending on the technique adopted to identify the velocity at which the bed enters the bubbling regime, different trends of umb versus T are observed. Such results are analysed together with the expansion behaviour of particle systems of various densities, illustrated by diagrams built with data of fluidization and collapse experiments. The role exerted by thermally induced interparticle forces in delaying the onset of steady bubbling is highlighted, as well as the opportunity of adopting a definition of umb of practical meaning.