ENTRAINMENT AND ELUTRIATION MODELING IN BUBBLING FLUIDIZED-BEDS

In bubbling fluidized bed combustion and catalytic cracking, elutriati on is a major cause of inefficiency, while it is highly desirable, for instance, in sludge incineration. Whether the intention is to quench or promote elutriation, the involved phenomena must be properly known if the process is to be efficiently controlled. In this work, entrainm ent and elutriation are reviewed and a modelling proposal is developed . The main concern relates to bubbling beds of wide spread particle si ze distributions under vigorous fluidization. The particles are subdiv ided into three classes: critical fines, smalls and larges. Critical f ines are particles whose terminal velocities are much lower than the g as superficial velocity, and are assumed to be instantaneously entrain ed at the bed surface. Smalls and larges are particles whose terminal velocities are respectively fewer and higher than the gas superficial velocity, and are assumed to be entrained through the bubble wake ejec tion mechanism. The bed surface entrainment model presents parameters which must be evaluated from empirical data. Comparison to experiment shows that, for U/U(nf)greater than or equal to 10, f(w) xi(k) should increase with U/U-mb and a correlation is proposed to account for this variation. The elutriation model is based on phenomenological observa tions from experiment, coming out with parameters which must be set fo r specific situations.