Measuring the Particle Packing of l-Glutamic Acid Crystals through X-ray Computed Tomography for Understanding Powder Flow and Consolidation Behavior

The morphology of free-flowed and gravity consolidated crystal powder beds of the alpha and beta polymorphic forms of l-glutamic acid, together with a detailed analysis of particle density and microstructure within alpha form tablets using state -of-the-art X-ray computed tomography (XCT), is presented. The Carr’s index is measured to be 19.7 and 35.2 for the bulk powders of the prismatic alpha form and needle-like beta form, respectively, revealing the alpha forms increased powder flowability versus the beta form. XCT reveals the alpha form consolidates under gravity more efficiently than beta, where the final measured bed density of the alpha form is 0.724 g/cm3 compared to 0.248 g/cm3 for the beta form, which is found to be caused by the inability of the beta particles to pack efficiently along their needle axis. Tabletting studies reveal that the alpha form consolidates into compacts of intermediate tensile strength, whereas the beta form cannot be compacted under these conditions. XCT analysis of tablets formed from α-form crystals reveals two discrete density regimes, one low-density region of fine powder which accounts for 53.8% of the compact, and high-density regions of largely intact single crystals which account for 44.2% of the compact. Further analysis of the tablet microstructure reveals that the crystal particles are generally orientated with their basal {0 0 1} plane, normal to the compaction force and that small microcracks which appear within the particles generally occur perpendicular to the surface and are orientated through possible {1 1 0} and {1 0 1} fracture planes. XCT also reveals evidence for incipient transformation between the meta-stable alpha to stable beta phase at concentrations below that detected using laboratory X-ray diffraction. The results show that XCT can accurately measure the extent of tapping induced densification and reveals the powder bed mesostructure characteristics and tablet microstructure for the two polymorphic forms of alpha and beta l-glutamic acid

A physically consistent Discrete Element Method for arbitrary shapes using Volume-interacting Level Sets

The properties of granular materials depend strongly on the shapes of their individual constituent particles or granules. Nonetheless, incorporating the effects of non-spherical or complex particle shapes into existing modelling frameworks, such as the discrete element method (DEM), has remained challenging. In this work, we propose the volume-interaction level set DEM (VLS-DEM) approach for carrying out physically accurate simulations of particles with arbitrary geometries. VLS-DEM builds upon level set DEM (LS-DEM), both of which implicitly define the geometry of a particle through a discrete signed distance function. However, instead of using surface nodes to compute the interparticle forces, VLS-DEM uses the overlap volume as computed by an Octree integration algorithm. This addresses some of the shortcomings of LS-DEM in terms of the accuracy and precision of the shape description, and opens up the possibility for bottom-up parametrisation methods. A number of tests were performed to compare VLS-DEM with regular DEM, LS-DEM, and analytical models. VLS-DEM is shown to give physically accurate results, comparable to analytical theory, even for highly complex systems such as those with concave interlocked particles.This project has been financed by Novo Nordisk A/S (Bagsværd, Denmark)