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Poor flowability hampers particle processing in many instances; causing a reduction in process efficiency, increased manufacturing costs and leading to excessive waste. The resistance of powders to flow can arise from a number of particle properties, such as cohesion, friction, shape and roughness. The objective of this work is to determine to what extent the flowability of powders is affected by their particle shape and roughness. The Distinct Element Method (DEM) is used to precisely control the shape and roughness of the particles. Needle-like particles are represented by the method of overlapping spheres, introduced by Favier et al. (1999). The fractional overlap of these spheres is varied, along with the number of spheres per particle, to produce particles with ranges of roughness and aspect ratio, respectively. The flowability of these particles is assessed by simulating two systems: a standard shear box, and indentation of a powder bed with a spherical indenter - introduced by Hassanpour and Ghadiri (2007). The particles are generated with a random orientation and allowed to settle under gravity before they are consolidated by a platen to a desired stress. The results show that particles with a large aspect ratio experience reduced shear stresses. This behaviour may be caused by particles aligning, thus allowing them to slide past one another more easily. An increase in particle roughness promotes interlocking, for a given aspect ratio rougher particles are shown to experience increased shear stresses. This work provides a robust analysis of the influence of aspect ratio and roughness on the flowability of powders, thus allowing the flowability of real materials to be better understood. [ABSTRACT FROM AUTHOR]

In many particle processes there exist both stagnant and flowing regions. Shear bands naturally form between these regions as a transition from zero to full velocity. Particles within shear bands are prone to breakage through surface erosion, chipping and fragmentation when exposed to substantial shear stresses. The precise nature and extent of breakage is difficult to describe mechanistically, due to the distribution of particle strengths and forces acting upon them. Consequently the breakage of particles within shear bands is typically defined empirically using an attrition shear cell. The use of such empirical relationships to predict breakage in larger processes is severely limited in the literature. This paper describes a method of predicting bulk breakage in an agitated vessel by establishing particle breakage caused by applied stress and strain in a shear cell. The Distinct Element Method (DEM) is utilised to estimate the distribution of stresses and strains within the agitated bed, this distribution is then applied to the breakage relationship to predict total attrition. The DEM analysis requires a number of measurement cells to be considered within the bed. It is imperative that the dimensions of these cells are comparable to that of a naturally occurring shear band. With this measure in place the method outlined here describes the experimental breakage well. This method can be applied to any system where particles break due to shear deformation. [ABSTRACT FROM AUTHOR]

Rigorous models of elasto-plastic contact deformation are time-consuming in numerical calculations for the Distinct Element Method and quite often unnecessary to represent actual contact deformation of common particulate systems. In this work a simple linear elastic-plastic-adhesive contact model for spherical particles is proposed, whereby the loading cycle is a linear plastic deformation and the unloading is elastic with a higher stiffness compared to the plastic deformation. The adhesive behaviour is considered once the unloading contact force reaches the pull-off force, at which point the contact deforms with negative elastic-adhesive stiffness. In order to account for increase in adhesion due to plastic deformation, the pull-off force is evaluated using negative linear plastic-adhesive stiffness. The model is applied to compression of spherical particles with elastic-plastic-adhesive contacts for which sensitivity analyses of the model parameters on work of compaction are carried out. As the ratio of elastic to plastic stiffness is increased, the plastic component of the total work increases for a given strain and the elastic component decreases. Large stiffness ratio values imply particles undergoing larger plastic work for a given strain. By increasing interface energy, the plastic work increases for a given solid fraction, however the elastic work does not change. In this case, the maximum tensile force is increased therefore the work of adhesion is increased. [ABSTRACT FROM AUTHOR]

The undesirable breakage of seeds during processing may result in quality degradation. Seeds experience a portfolio of shear and impact stresses as they flow through various machinery, and this may cause surface damage as well as integral damage. An in-depth study and understanding of the microscopic mechanisms of the various processes is needed to investigate and address the problem of breakage. The main aim of this work is to carry out a parametric study of the effect of sliding and rolling friction on the flow field of seeds in a seed coater device by modeling particles motion using Distinct Element Method (DEM). It was found that sliding friction plays an important role in changing the flow pattern and particles solid fraction in a specified measurement cell. However, study of particle rolling friction showed that flow pattern and solid fraction of particles will not be affected once the coefficient of rolling friction exceeds a value of 0.1. [ABSTRACT FROM AUTHOR]

The characterisation of cohesive powder flowability is often required for reliable design and consistent operation of powder processes. This is commonly achieved by mechanical testing techniques on bulk powder, such as shear test, but these techniques require a relatively large amount of powder and are carried out at large pre-consolidation loads. Many industrial cases require small amounts of powders to be handled and processed, such as filling and dosing of capsules. In other cases, the availability of testing powders could be a limiting issue. It has been shown that under certain circumstances, indentation on a cohesive powder bed by a blunt indenter can give a measure of the resistance to powder flow (Hassanpour and Ghadiri 2007). In the present work, the ball indentation process is analysed by numerical simulations using DEM in order to investigate the operation window of the process in terms of indenter size and penetration depth. The flow resistance of the assembly, commonly termed hardness, is evaluated for a range of sample quantities and operation variables. A sensitivity analysis of bed height reveals that a minimum bed height of 20 particle diameters is required in order to achieve reliable measurements of hardness. It is also found that indenter sizes with diameters smaller than 16 particle diameters exhibit fluctuations in powder flow stress measurements. As the indenter size decreases, it moves closer to the size of bed particles. Therefore, rearrangements at the single particle level influence the force on the indenter, resulting in fluctuations, and possible compaction. [ABSTRACT FROM AUTHOR]

Results of a study of the influence of primary particle size on the strength, density and internal structure of granules produced in a high shear mixer granulator, Cyclomix (manufactured by Hosokawa Micron B.V., The Netherlands) are reported. Different grades of calcium carbonate powder (available commercially as Durcal 15, 40 and 65) were granulated in a 50 L granulator. Durcal 15 is the finest powder, d50 = 23 μm, and Durcal 65 is the coarsest one, d50 = 60 μm. An aqueous solution of polyethylene glycol was used as the binder. Granules produced from the three powder grades were dried and tested to ascertain their internal structure using X-ray Micro Tomography (XMT). The granules were also individually subjected to quasi-static compression to characterise their crushing strength. The envelop density of granules for each powder grade was also measured. The results show that the envelope density increases with the mean size of primary particles. It is found that a more uniform strength and density distributions are obtained for the coarsest powder grade and the granulation operating conditions for the finest grade, Durcal 15, produced the weakest granules. This is attributed to the presence of large pores and cavities in their cores, as observed by XMT. [ABSTRACT FROM AUTHOR]

A method for predicting particle breakage in agitated dryers is described. The method utilizes an estimation of stresses and strains occurring in a dryer bed sheared by an impeller using the Distinct Element Method (DEM). An assemblage of particles is then subjected to these stresses in a shear cell to assess the extent of attrition under a range of stresses and strains. Paracetamol particles in the size range 500–600 μm are used for the experimental work. The relationship of attrition with stress and strain is then incorporated into the distribution of stress and strain in the dryer estimated by DEM. The extent of attrition for a range of conditions including the impeller speed has been analysed. The prediction shows impeller speed to have limited effect on attrition within the range of speeds tested. [ABSTRACT FROM AUTHOR]

Tribo-electrification is an important phenomenon in powder handling, as it could cause hazards and nuisance, such as dust explosion, adhesion to walls and blockage of pipelines leading to powder losses and difficulties in flow control. At present, there are only a few methods available to predict the dynamic charging of bulk powders, and they are not suitable for testing a small quantity of powders or handling highly active powders, for which full isolation would be needed. In this paper, the operation of a simple test device is analysed. Tribo-electrification of a single spherical bead inside a sealed capsule is induced through a horizontal motion using a shaking device at various frequencies. The single particle contact charge obtained from experiments is incorporated in numerical simulations based on Distinct Element Method (DEM) and is used to predict the transient charge accumulation of an assemblage of beads. In the model, the tribo-charging of a single bead is related to the contact area of the bead on impact with the capsule wall. The effects of long range forces, space charge and image charge at boundary condition are investigated for bulk charging. It is shown that the inclusion of space charge effects and boundary condition into the DEM model leads to a better prediction of charge build-up when compared with the experimental results. [ABSTRACT FROM AUTHOR]

Powder flow could give rise to tribo-electrification, causing segregation and adhesion of particles to the containing walls. This leads to operational problems and reduction in process efficiency. In order to mitigate the undesirable effects of tribo-elecric charging, a fundamental understanding of charge transfer is essential. In order to model the charge transfer due to collision of particles against the walls, various simple experiments are carried out, based on which a model of single particle charging has been developed, which is then incorporated into Distinct Element method (DEM) to predict the bulk behavior. The experiments consist of shaking various numbers of plastic particles in a stainless steel cubic capsule. The parameter windows are number of particles and sizes of particles. During shaking, the particles get charge, when they contact the walls. The charge on individual particles accumulates until the saturation charge is reached. When the number of particles increases, the saturation level decreases because of space charge effect. In the simulation, LIGGGHTS is used for the DEM simulations, in which the model of charge transfer and space charging effect are incorporated. To calculate the electric force applied to particles due to electrostatic inductions from the walls and other charges on particles, the simulation charge method is used. The results of the simulations and experiments are compared, and simulation results capture the tendency of experimental results. [ABSTRACT FROM AUTHOR]