Predicting product properties of fluidized bed spray granulation using CFD-DEM simulations

CFD-DEM is the most promising way of describing the micro- and meso-scale behavior of fluidized beds. In particular, the interaction of single particles with both flow and other particles, as well as liquid droplets can be resolved, while preserving their individual fate. Fluidized bed spray granulation is the preferred way of generating functional, high value granular products. By injecting a solution or suspension onto a particle bed, the particles grow layer by layer. Here, the drying conditions have a major influence on the structure of the layers, due to the pre-concentration of droplets that cause changes in the viscosity and velocity of impinging droplets. This affects the spreading behavior of droplets on the surface, together with the crystallization occurring within the droplets themselves in the case of solutions, resulting in either porous or dense layers. As these phenomena are well accessible in CFD-DEM simulations, their combination can be a powerful tool in the design of granulators that produce tailor-made particles. In this work, a compressible CFD-DEM solver, capable of modelling heat transfer, spray injection and evaporation, is presented and validated against experimental data. A simple concept for tracking the progress of the structure-determining processes taking place within the droplets and on the surface of the wetted particles is presented. Calibration granulation experiments from literature were recreated in simulations and used to correlate between resulting shell porosity and micro-scale event timescales.

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Predicting product properties of fluidized bed spray granulation using CFD-DEM simulations