(553d) Experiment and DEM Based Investigation of Breakage of Pharmaceutical Granules
Particle size reduction of dry granular material by mechanical means, also known as milling or comminution, is undoubtedly a very important unit operation in pharmaceutical, agricultural, food, mineral and paper industries. As comminution is a stochastic and a non-linear process, an attempt was made to understand this complicated process by conducting parametric studies experimentally and computationally using Discrete Element Method (DEM). Studies were performed lactose spheres to understand the effect of hammer speed (rotational), feed rate and hammer-wall tolerance. The size and shape of the resultant progeny of particles were analyzed by sieve analysis and microscope/image analysis techniques respectively. Greater size reduction was observed at higher speeds and low feed rates owing to the greater centrifugal force experienced by the particles and longer mean free path lengths respectively. Particle shape analysis revealed fragmentation to be the dominant mechanism of size reduction at higher speeds. Increase in impeller wall tolerance resulted in rolling mode regime of powder bed which was found to be significant at low impeller speeds. DEM simulations were carried out to study the effect of mill parameters on fragmentation. Below a critical hammer tip speed, a blending action rather than comminution was observed. The feed rate determines the hold up of material in sizing chamber and hence energy required for size reduction. At low hold-up, longer path lengths are achieved by particles resulting in higher impact velocity and hence a finer size distribution. At higher hold up the number of collisions is high, but the kinetic energy per particle is low leading to poor breakage probability. To develop an understanding of how grain breakage evolves in a granular system under impact the components of stress tensor were also determined from the DEM simulations.