• Energy Research

Abstract 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.

Abstract In the production process of pharmaceutical pellets with a narrow size distribution and a high sphericity, a combined extrusion-spheronization technique is frequently used. The rounding of the wet cylindrical extrudates in the spheronizer after the extrusion step is influenced by various interfering mechanisms, in particular plastic deformation, breakage, attrition and coalescence. Due to the complexity of these mechanisms which depend on the particle dynamics, there is no sufficient description of the particle rounding process in the spheronizer. In this study, the Discrete Element Method (DEM) which runs on the micro scale is coupled with a Particle Shape Evolution (PSE) model on the macro scale to describe how the particle shape changes due to collisions. For the DEM simulation a new contact model was used which was developed to capture the cyclic, dominant visco plastic deformation behaviour. Based on the DEM collision data, the changing particle shape was described in the PSE model by applying the proposed submodels for the different formation mechanisms. The resulting particle shapes obtained with this simulation framework are in a good agreement with experimental data.

AbstractFibrous depth filters are frequently used for the purification of gas streams with low dust loadings, as well as processes where a high initial filtration efficiency is required (e.g., clean rooms for aseptic production). One tool suitable for supporting the development of optimized filter media is the use of numerical simulations. The drawback of this technique is the high computational resources required. In this work, a new and fast approach based on a one‐dimensional model was applied. Structural characteristics (e.g., porosity distribution and fiber diameter) of two different filter media were successfully determined using a novel X‐ray microscope. These characteristics were incorporated in the filtration model, and their influence on the calculations was evaluated. It was found that the porosity distribution does have an impact on local (microscopic) deposition rates, but only a minor influence on the macroscopic filtration efficiency (around 3%). Benefits of the model are the application of measured structural data and the low computational expense. Compared to experimental data (VDI 3926 / ISO 11057), the prediction of the filtration efficiency can be improved by incorporating the structural data in the model.