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A bi-directional coupling between the Discrete Element Method (DEM) and Population Balance Modelling (PBM) is suggested to model the failure of pharmaceutical tablets under abrasive conditions such as bulk flow of impact. DEM provides the description of kinetics and permits the extraction of collision energy distributions. Representation of the primary shape of the tablets is achieved by using the multi-sphere approach, a rigid assembly of systematically connected spheres to resemble non-spherical shape. Based on the collision energy distribution and the abrasion characteristics of the tablets by solving the PBM equations, the mass loss of tablets according to their mechanical resistance, as well as the fragment size distribution progeny can be computed. For the description of the abrasive failure characteristics, a multi-stage calibration procedure for the empirical breakage kernels of Vogel and Peukert as well as of Ghadiri and Zhang is also applied. The novelty of the presented approach is the bi-directional coupling: tablet and fragment distribution masses are dynamically updated for subsequent time steps in DEM by the particle replacement method according to the stress history of the system. The outcome of this methodology helps to get a deeper understanding of the process-property relationships, to evaluate the effect of any process or geometry modifications in processing operations, such as drum coating, and to reduce the requirements of experimental investigations.

Fine grinding in wet stirred media mills is a key operation in numerous industrial processes. During wet milling, grinding beads (typically millimetre scale) collide with each other under rapid agitation. These collisions transfer energy into the surrounding medium, a feed stream comprising a premixed slurry (a suspension of micron-sized particles), leading to a reduced particle size. Numerical simulation provides an opportunity to gain mechanistic insight by modelling the influences of grinding media, agitator speed and slurry rheology on the breakage. The multiphase nature of wet milling necessitates models that account for both the solid grinding beads and the slurry feed. Established fluid-particle coupled methods are in principle capable of doing this, but the need to resolve the large hydrodynamic lubrication forces between near-contacting beads demands extremely high fluid field resolution, leading to proliferating simulation size and cost. To address this challenge, a Discrete Element Method (DEM) simulation for a lab scale mill is presented in which the bead-bead interaction includes pairwise hydrodynamic lubrication in addition to frictional contact forces. The model accounts for multiphase effects in a simplified yet physically well-motivated way, circumventing the computational cost of a fully fluid-resolved model. A systematic model calibration against the laboratory mill is presented, and thereafter the model provides a good estimation for the empirical power draw across the full range of rotation speeds and considered feed viscosities. The differences in energy dissipation modes and the local distribution of collisions along the mill chamber are examined for the extreme viscosity condition, revealing that most energy dissipation is due to inter-particle forces acting tangentially (shearing and rolling) while highly energetic collisions (impact and torsion), relying on free flows and inter-bead mobility, are relatively unimportant.

Finishing and transport operations of pharmaceutical tablets such as coating and conveying processes can qualitatively be improved with a better understanding of the interplay of process parameters and product quality attributes. This study provides a first step towards the simulation of these processes with the calibration of the motion modelling of uniaxial-compressed cylindrical tablets using the multi-sphere approach (MS) within the Discrete Element Method (DEM). Only high accuracy in the representation of cylindrical tablet shape, especially regarding edges, yields a good agreement between simulated and experimental packing fraction. Sensitivity regarding model parameter calibration was evaluated based on tumbling drum tests which are of wide occurrence in the industry and the so-called pouring experiment extracting static and dynamic angles of repose, identified sliding and rolling friction as the most sensitive parameters. These were successfully calibrated by an iterative crossed analysis in tumbling drum and pouring tests. Additionally, a complete determination and subsequent calibration strategy is provided for Coefficient of Restitution and Young's modulus. Results of the final numerical modelling are qualitative and quantitative in agreement with experimental test case experiments. A final validation is performed through the simulation of an attrition tester with a single baffle, highlighting one of the main limitations of DEM: the problem-targeted nature of the operation of calibration.

The goal of the present study is to assess the extent of particle rearrangement and the degree of anisotropy for uni-axial compaction of microcrystalline cellulose (MCC) by Discrete Element method simulations. The elasto-plastic cohesive hysteretic Edinburgh contact model is hereby considered. First, a calibration strategy for all required collisional, frictional, or cohesion input parameters is presented. A dual approach combining direct determination tests (e.g., nano-indentation or inverse gas chromatography) and indirect fitting tests (e.g., tumbling drum test) provides the final set of property values. Second, the uni-axial compaction of MCC for a range of target compaction pressures and different primary particle dispersities (mono- and polydisperse) is conducted. The anisotropic behavior of compaction is analyzed by computing averaged quantities such as the deviatoric term and the maximum difference between characteristic roots of the fabric tensor. The results depict a remaining degree of anisotropy after compaction, depending on the maximum compaction pressure and particle dispersity, which does not fully recover after powder decompression.