Challenges in the Simulation of Drying in Fluid Bed Granulation

Fluid bed granulation is faced with a high level of complexity due to the simultaneous occurrence of agglomeration, breakage, and drying. These complexities should be thoroughly investigated through particle–particle, particle–droplet, and particle–fluid interactions to understand the process better. The present contribution focuses on the importance of drying and the associated challenges when modeling a granulation process. To do so, initially, we will present a summary of the numerical approaches, from micro-scale to macro-scale, used for the simulation of drying and agglomeration in fluid bed granulators. Depending on the modeled scale, each approach features several advantages and challenges. We classified the imposed challenges based on their contributions to the drying rate. Then, we critically scrutinized how these challenges have been addressed in the literature. Our review identifies some of the main challenges related to (i) the interaction of droplets with particles; (ii) the drying kinetics of granules and its dependence on agglomeration/breakage processes; as well as (iii) the determination of drying rates. Concerning the latter, specifically the surface area available for drying needs to be differentiated based on the state of the liquid in the granule: we propose to do this in the form of surface liquid, pore liquid, and the liquid bridging the primary particles.