Heat and mass transfer study in fluidized bed granulation—Prediction of entry length

Fluidized bed granulation is a process by which granules or coated particles are produced in a single piece of equipment by spraying a binder as solution, suspension, or melt on the fluidized powder bed. Heat and mass transfer correlation useful for designing a granulator has been derived based on the equivalence of evaporation rate of the liquid to the heat transferred from hot gas to particles:(m/A)Dp2λLmf(1−εmf)(Tg−Tl)Kg=hDpKg.This equation is applied to data on granulation experiments by different workers to calculate Reynolds number and Nusselt number to obtain a relation between heat and mass transfer from gas to particles during granulation on a logarithmic scale from which the following empirical relation is obtained:Nu=0.0205Re1.3876Nu=0.0205Re1.3876which is comparable to Kothari's correlationNu=0.03Re1.3.Nu=0.03Re1.3.By using the heat and mass transfer correlation obtained, the entry length, that is the length of granulator up to which effective heat transfer from gas to bed particles takes place, is estimated, which is also validated with experimental study. The correct estimation of entry length is useful in optimal design of a granulator.