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Static and dynamic stressing of elastic, plastic and viscous granules

Applicant Dr.-Ing. Peter Müller, since 2/2016
Subject Area Mechanical Process Engineering
Term from 2014 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 265879638
 
Final Report Year 2018

Final Report Abstract

Mechanical behavior of zeolite 4A and alumina oxide granules and microcrystalline cellulose MCC pellets under quasi-static uniaxial compression loading, which occurs during common industrial processes, e.g. crushing, mixing, fluidized beds, pneumatic conveying, coating, apparatus filling and hopper discharge, have been studied. Influence of environmental humidity (considered by moisture content), varying granule velocities and several impacts (considered by different loading rates and repeated loadings, respectively), and stiffening and strengthening effects of coating, on the mechanical behavior of granules and pellets have been investigated. It has been clearly seen that moisture has a weakening effect of the mechanical properties of granules and pellets. Moistening of pellets causes a certain swelling in size and decreases contact stiffness and the characteristic strengths. At higher loading rates, the influence of moisture content present in the granule becomes much more dominant. Contact history, i.e. repeated loading at a unique contact area, causes strengthening in the direction of strain, however, internal micro-damages may develop, leading to macro-breakage at loads less than the load required to cause breakage in fresh granules. After repeated loading, macro-breakage and plastic micro-yield strength of dry and wet granules of different sizes have been reduced. Film coating of pellets of diameter d=1 mm with a coat thickness of 20 μm alters the stiffnesses and strengths of pellets, which is a combination of altered surface contact properties and a slight increase in size due to coating. Hydroxypropyl methylcellulose (HPMC) coatings are recommended for purposes where contact compliance is desired, since they exhibit larger ranges of elasticity, elastoplasticity and plasticity in comparison to Eudragit-coated and no-coated pellets, while Eudragit coatings are recommended for purposes where contact compliance is not important. The coefficient of restitution (CoR) and the contact time of the impact of steel, bronze and brass spheres with glass plates have been studied. Influence of material properties, geometry and impact velocity on the CoR and the contact time have been investigated using free-fall tests. The CoR of the impact of dominant elastic steel spheres with glass plates follows the theoretical predictions by the model of Zener, which describes the elastic impact of particles and plates of infinitely large length and width, considering the energy dissipation due to the propagation of a flexural wave in the plate. With increasing impact velocity, elastic-plastic bronze and brass particles begin to deform plastically and the CoR drops sharply. The velocity at which plastic deformation begins, i.e. yield velocity, has been determined for the impact of bronze spheres of 2 mm and 10.3 mm diameter and brass spheres of 3.5 mm diameter with glass plates. Contact time increases with increasing sphere diameter, and decreases with increasing impact velocity and plate thickness. This study presents comprehensive results of the compression and the impact of millimeter-sized spherical particles and plates. The statistically reliable results of this project can be used to improve and optimize industrial processes in which spherical particles are under compression loading, or impact between particles and walls of the apparatuses occur. With this advanced knowledge, apparatus designs can be improved to avoid loading rates, moisture contents, or impact velocities at which plastic yielding and/or breakage begins, e.g. in storage silos, pneumatic conveyors, catalyst reactors, discharge hoppers and mixers. Moreover, the results can be directly used for further discrete element method (DEM) simulations of realistic granule systems. Most materials do not exhibit perfectly elastic, but, at least in part, elastic-plastic and/or visco-elastic behavior. The existing models describing the elastic-plastic and the visco-elastic impact fail to calculate the CoR of the impact for various materials and geometries accurately. Furthermore, the contact time of elastic-plastic impact has not yet been reported in the literature. The inaccuracies of the existing models to predict the CoR and the contact time of the impact of different materials indicate the following area as recommendation for future work: A comprehensive study of the CoR and the contact time of the elastic-plastic and/or visco-elastic impact of a sphere with an infinitely large plate of finite thickness, i.e. combined plastic (or viscous) and bending effects.

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