Modeling of maize breakage in hammer mills of different scales through a population balance approach

Abstract

The grinding of maize grains is an important process in the industry of poultry feed, being the most commonly used cereal in diet worldwide. Hammer mills are usually used for this process because of their simplicity, easy operation and low maintenance cost. However, the grinding process in hammer mills itself is complex and not completely understood. Hammer mills are impact-type crushers that comprise a rotating shaft fitted with fixed or pivoted hammers and mounted in a cylindrical chamber. The particles are fed into the chamber by gravity and exit the cylinder when they are small enough to pass through a screen located at the bottom. Comminution in hammer mills is influenced by design variables as the rotor shaft configuration (i.e., vertical or horizontal shaft), the hammer design and placement (i.e., distance between hammers and screen), the screen opening size and type (i.e., round, square, mesh type), and by operating variables as material feed rate and rotor speed. The resulting particle size distribution depends not only on material properties but also on design and operation variables of the mill.In this work, a simulator for a grinding process is presented. The simulator is based on a population balance model (PBM) that allows quantifying changes in particle size distributions (PSDs) and the mass geometric mean diameter (Dgw) as a function of mill operation variables (screen opening, mill rotor speed and feed rate).This model was developed completely in the gPROMS® Model Builder, using experimental data collected from a pilot-scale mill to fit the parameters of the breakage and classification functions, considering the effects of rotor speed and screen opening size. The gPROMS® in-built parameter estimation tool was used for parameter fitting and statistical test analysis. The model was validated with both pilot- and industrial- scales steady-state data, which also included variations in feed rates. The developed model was used to predict the dynamic behavior of the product mass geometric mean diameter and mill hold-up as a function of disturbances in the feed rate and the rotor speed. These results are highly valuable since they indicate the feasibility of using the fitted model to predict with confidence new operating points, study industrial milling performance and optimize hammer mill operation reducing the need to carry out expensive and time-consuming experimental tests.