(264b) Computational Study of Needle-Shaped Particle Breakage Under the Agitation By Rotating Blades
The prediction of particle breakage is critical in some solids handling processes, such as filtration, drying, blending, and transport, in order to achieve the desired Particle Size Distribution (PSD) for granular materials. Recently, the Freeman FT4 Powder Rheometer has been applied to test the friability of Active Pharmaceutical Ingredients (APIs). In the tests using the FT4 powder rheometer, the powder bed under the compression of an applied weight on the top is agitated by rotating blades. After a certain time of agitation, the reduction in the mean particle size is used to classify the API powders as easy, medium, or hard to break. In this work, the Discrete Element Method (DEM) is employed to numerically model needle-shaped particle breakage in FT4 powder rheometers. The needle-shaped particle in DEM is formed by connecting a number of spheres in a straight line using elastic bonds, and the particle breaks at the bonds where the tensile or shear stresses exceed the corresponding strengths. In the numerical modeling, the evolutions of average particle size, PSD, and number of particles with each aspect ratio can be monitored. Therefore, the breakage kernel and daughter distribution function can be determined for the population balance model in order to describe the effect of particle breakage on the PSD.
The DEM simulation results show that the particle breakage rate depends on the friction between the particles and the base and top flat walls, the applied weight on the top of powder bed, and the blade rotational speed. With small particle-wall friction, the particle bed rotates like a solid body along with the blades and particle breakage rarely occurs. While with large particle-wall friction, retarded flow zones are obtained close to the base and top walls, and the particles are broken due to strong interactions with the blades. As a larger weight is applied to compress the powder bed, the interactions between the particles and walls/blades increase, leading to an increase in breakage rate. The breakage rate increases as the rotational speed of blades increases, due to the increased frequency of particle-blade interaction. However, the breakage per blade revolution is independent of rotational speed. It is also found that the increased number of particles and the average particle size follow power law relationships with the input work done by the blades, which is linearly proportional to the blade rotational speed, elapsed time, and the applied weight.
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