(397a) 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).