Fluidized-bed melt granulation: population balance modelling to identify the aggregation kernel

MARTA VILLA; BERTIN, DIEGO; COTABARREN, IVANA; PIÑA, JULIANA; VERONICA BUCALA

Sheffield

Workshop; 7th International Granulation Workshop; 2015

Sheffield University

Within the operations that handle solids, granulation is considered as one of the most important processes. It allows producing granules with well-defined particle size distributions and shapes for many industries such as the pharmaceutical, agrochemical, detergent, food, etc. [1,2].Granulation processes are usually classified according to the binder nature as: wet, dry or melt.Nowadays, research in melt granulation has gained interest for many applications. Particularly for materials incompatible with water; the melt granulation avoids the use of solvents and the disadvantages associated with their recovery and final disposal, and minimizes the energy cost related to solvent evaporation [3,4]. Among the melt granulation technologies, the fluidized-bed melt granulation is widely used. The final granules size distributions strongly depend on the operating conditions, the seeds size and binder properties. Modellingwould allow improving the performance of the granulation process. However, the equation (population balance) that describes the particle size transformation as the granulation proceeds is quite complex involving kinetic parameters to describe the growth and breakage mechanisms and nucleation or elutriation phenomena.Cotabarren et al. [2] and Veliz et al. [5,6]explored the effect of the operating variables on the process performance and particle properties for urea melt granulation carried out in a batch fluidized bed, identifying the operating growth regions that led to pure coating or combined growth by agglomeration and coating. Based on experimental data, (which are representative of relatively big seeds particles with respect to droplets size, high binder/seeds mass ratios, bottom spray configuration, and binder and seeds of the same chemical nature), in this work the kinetic parameters related to pure coating and combined mechanisms of coating and aggregation are established. As suggested by several authors [7-9], the aggregation kernel can be represented by two factors: one depending exclusivelyon the particle size (φ) and the otheron the process operating conditions (β0). Two different formulations are proposed: 1) φ is considered independent of particle size (i.e., φ=1) and 2)φis represented by the EKE model [10]. For both cases, the kinetic parameterβ0 is adjusted for each experiment. Finally, a fitted mathematical expression forβ0as a function of the binder flowrate, fluidization air velocity, bed temperature, atomization air flowrate and initial particle sizes,which allows predicting the experimental particles size distribution, is presented.