Discrete particle simulation of particulate systems: A review of major applications and findings

Model-based analysis of high shear wet granulation from batch to continuous processes in pharmaceutical production--a critical review

The manufacturing of pharmaceutical dosage forms, which has traditionally been a batch-wise process, is now also transformed into a series of continuous operations. Some operations such as tabletting and milling are already performed in continuous mode, while the adaptation towards a complete continuous production line is still hampered by complex steps such as granulation and drying which are considered to be too inflexible to handle potential product change-overs. Granulation is necessary in order to achieve good flowability properties and better control of drug content uniformity. This paper reviews modelling and supporting measurement tools for the high shear wet granulation (HSWG) process, which is an important granulation technique due to the inherent benefits and the suitability of this unit operation for the desired switch to continuous mode. For gaining improved insight into the complete system, particle-level mechanisms are required to be better understood, and linked with an appropriate meso- or macro-scale model. A brief review has been provided to understand the mechanisms of the granulation process at micro- or particle-level such as those involving wetting and nucleation, aggregation, breakage and consolidation. Further, population balance modelling (PBM) and the discrete element method (DEM), which are the current state-of-the-art methods for granulation modelling at micro- to meso-scale, are discussed. The DEM approach has a major role to play in future research as it bridges the gap between micro- and meso-scales. Furthermore, interesting developments in the measurement technologies are discussed with a focus towards inline measurements of the granulation process to obtain experimental data which are required for developing good models. Based on the current state of the developments, the review focuses on the twin-screw granulator as a device for continuous HSWG and attempts to critically evaluate the current process. As a result, a set of open research questions are identified. These questions need to be answered in the future in order to fill the knowledge gap that currently exists both at micro- and macro-scale, and which is currently limiting the further development of the process to its full potential in pharmaceutical applications.

Regime transitions of granular flow in a shear cell: a micromechanical study

The regime transitions of granular flow in a model shear cell are investigated numerically with a stress-controlled boundary condition. The correlations between the elastically and kinetically scaled stresses and the packing fraction are examined, and two packing fractions (0.58 and 0.50) are identified for the quasistatic to intermediate and intermediate to inertial regime transitions. The profiles and structures of contact networks and force chains among particles in different flow regimes are investigated. It is shown that the connectivity (coordination number) among particles and the homogeneity in the shear flow increase as the system goes through the inertial, intermediate, and then quasistatic regimes, and there is only little variation in the internal structure after the system has entered the quasistatic regime. Short-range force chains start to appear in the inertial regime, which also depend on the magnitude of the shear rate. The percolation of system-spanning force chains through the whole system is a characteristic of the onset of the quasistatic regime, which happens at a packing fraction that is close to the glass transition, i.e., about random loose packing (0.58) but far below the isotropic quasistatic (athermal) jamming packing fraction of random close packing (0.64). The tails of the probability density distribution P(f) of the scaled normal contact forces for the flows in different regimes are quantified by a stretched exponential P(f)=exp(-cf^{n}) with a remarkable finding that n ∼ 1.1 may be a potential demarcation point separating the quasistatic regime and the inertial or intermediate regimes.

Calculation of noncontact forces between silica nanospheres

Quantification of the interactions between nanoparticles is important in understanding their dynamic behaviors and many related phenomena. In this study, molecular dynamics simulation is used to calculate the interaction potentials (i.e., van der Waals attraction, Born repulsion, and electrostatic interaction) between two silica nanospheres of equal radius in the range of 0.975 to 5.137 nm. The results are compared with those obtained from the conventional Hamaker approach, leading to the development of modified formulas to calculate the van der Waals attraction and Born repulsion between nanospheres, respectively. Moreover, Coulomb's law is found to be valid for calculating the electrostatic potential between nanospheres. The developed formulas should be useful in the study of the dynamic behaviors of nanoparticle systems under different conditions.