DEM simulation of the mixing behavior in a spheronization process

Spherical pellets for pharmaceutical applications are widely produced by an extrusion-spheronization process. To achieve an equal, spherical pellet shape with a spheronization process, it is crucial that all pellets are exposed to similar stress conditions. However, in a spheronizer the pellets close to the friction plate are subjected to much higher stresses than pellets at the top of the torus, resulting in a strongly inhomogeneous stress distribution within the particle bed. Therefore, the product quality depends in particular on the mixing process in the spheronizer. In this study, the mixing behavior in a spheronization process is analyzed using DEM simulations. The real geometry and realistic process parameters of a lab scale spheronizer were investigated. To determine the mechanical properties of the wet pellets for the contact model, various single particle experiments were conducted with MCC-based pellets produced by extrusion-spheronization. The spatial mixing was characterized in different ways. Besides the determination of the degree of mixing based on statistical analysis, the Fokker-Planck equation was utilized. In this way the spatial distribution of the degree of mixing over the time was obtained. By using the poloidal distribution of the transport and dispersion coefficients of the Fokker-Planck equation the course of the degree of mixing in the different zones of the spheronizer was clarified.

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