Fluidized-bed melt granulation: Coating and agglomeration kinetics and growth regime prediction

Abstract

Mass and population balance equations are used to describe the spray-on melt granulation process in a batch fluidized bed where coating and agglomeration can occur simultaneously. Two different models are considered to describe the agglomeration kinetics: the Equipartition Kinetic Energy (EKE) and Size Independent (SI) kernels, while the coating kinetics is represented by a power law. Based on experimental data obtained under different operating conditions (melt and air atomization flowrates, air fluidization velocity, seed diameter and bed temperature), the coating and agglomeration kinetic parameters are fitted to correctly predict the final particle size distributions. It is concluded that the SI kernel and the zero-order coating kinetics best describe the agglomeration and coating mechanisms, respectively. A correlation to predict the SI kernel from macroscopic variables and properties of the system is proposed. This tool can be used to provide starting kernel factors for solving the PBEs governing spray-on melt granulation in fluidized-bed granulators. Finally, a criterion to predict the limit of agglomeration occurrence is formulated. The criterion, which is developed in terms of dimensionless numbers that depend on process conditions and take into account mass, heat and momentum transfer phenomena, is found to be appropriate to predict the growth regime for fluidized-bed spray-on melt granulation of urea (i.e., the specific studied system).