Investigating Granular Milling In a Hammer Mill: Experiments and Simulation

Investigating granular milling in a hammer mill: experiments and simulation

 

Shivangi. Naik, Bodhisattwa. Chaudhuri,

Department of Pharmaceutical Sciences,

University of Connecticut, USA

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. A comprehensive understanding of the fragmentation mechanism that occurs during comminution paved the development of a series of mechanistic and population balance based models. However, these models completely ignore the inter-particle and particle boundary interaction which dictates granular flow behavior. We numerically model a pilot-plant scale hammer mill using a Discrete Element Method (DEM) to study the breakage and kinematics of the particle motion within the hammer mill.

Simulations were carried out to study the effect of mill speed on kinetic energy of particles. In addition, parametric study was performed to understand the effect of hammer speed (rotational), feed rate and hammer-wall tolerance, on the final product size distribution. Below a critical hammer tip speed, a blending action rather than comminuting is observed. Increase in hammer tip speed causes higher frequency of impact of particles per unit time and higher specific energy of impact resulting in generation of much finer end product. With respect to feed rate, a narrow size distribution was obtained at lower feed rates. 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. Particle shape analysis revealed fragmentation to be the dominant mechanisms of size reduction at higher speeds. We observe that both the specific kinetic and strain energy of the particles (colliding with hammer) increase as the impact point becomes closer to the hammer-tip. Further investigation will be carried out to estimate the induced impact stress and specific energy of fragmentation (impact).