We numerically modeled a pilot-plant scale hammer milling machine using Discrete Element Method (DEM). DEM is employed to study primarily the breakge and kinematics of the particle motion within the hammer mill. Moreover, we calculate the velocity, the stress field within the granular bed, breakage and the energy distribution of collisions. Algorithm of damage mechanics is incorporated to model weakening and breakage of material processed in the hammer mill. Incremental impact breakge of the particles is modeled as breakage due to single impact is certainly rare. Material properties of Cellulose and Lactose are used in the simulation.
Parametric study is performed to understand the effect of hammer speed (rotational), hammer-wall tolerance, material properties and specific exit classification conditions 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 generation of much finer end product. 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. Material with lower Young Modulus(cellulose) shows higher breakage rate than Lactose at constant vessel geometry and operating condition. Our model predicts the optimal operating condition relating the size distribution of the resultant progeny with the material properties, mill geometry and mill operating conditions. The net mill power is also derived from the hammer geometry, hammer tip velocities and impact forces.