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A spiral jet mill was simulated using Discrete Element Method modelling and Computational Fluid Dynamics. The particle behaviour and fluid motion were analysed as a function of hold-up and grinding pressure. Particle collision energy was predicted to be prevalent along the bed surface and in front of the grinding jets, as shown through the collision data recorded. The bed itself affects the fluid flow field, as momentum is transferred to the particles. Increasing the grinding pressure does not result in a proportional increase in the kinetic energy of the particle system, as the high pressure jets begin to penetrate the bed with greater ease. The particle bed moves as 'plug-flow', with the layers of the bed closest to chamber wall. [ABSTRACT FROM AUTHOR]

Milling is an important process for tailoring the particle size distribution for enhanced attributes, such as dissolution, content uniformity, tableting, etc., especially for active pharmaceutical ingredients and excipients in pharmaceutical industries. Milling performance of particulate solids depends on the equipment operating conditions (geometry, process conditions and input energy etc.) as well as material properties (particle size, shape, and mechanical properties, such as Young's modulus, hardness and fracture toughness). In this paper the particle dynamics in a pin mill is analysed using Discrete Element Method (DEM), combined with a novel approach for assessing particle breakability by single particle impact testing. A sensitivity analysis is carried out addressing the effect of the milling conditions (rotational speed and feed particle flow rate), accounting for feed mechanical properties on the breakage behaviour of the particles. Particle collision energy spectra are calculated and shown to have a distribution with the upper tail end being close to the maximum energy associated with the collision with the rings. Breakage is primarily due to collisions with the rings, except for large particles that are comparable in size with the gap between the rings, nipping is also a contributory breakage mechanism. [ABSTRACT FROM AUTHOR]

Soft granular materials are assemblies of highly deformable grains interacting via surface forces. The large grain deformations of these materials differ them from hard granular systems, in which, their behaviors are essentially governed by grain rearrangements. In this paper, we study the uniaxial compression of soft granular materials using a numerical approach based on the Material Point Method allowing for large grain deformations, coupled with the Contact Dynamics method for the treatment of unilateral frictional contacts between grains. Considering the neo-Hookean and elasto-plastic grains, the compaction of 2D soft granular packings is analyzed. We focus essentially on the evolution of the packing vertical stress as a function of the packing fraction and the predictive models are proposed. [ABSTRACT FROM AUTHOR]

Knowledge of agglomerate strength is highly desirable for compression and tableting, dissolution and dispersion and mitigation of dust formation. The behaviour of agglomerates is affected by parameters such as density, agglomerate size, primary particle size, and interparticle bond strength. The method of agglomeration influences the evolution of structure, and this in turn affects its strength. Furthermore, the methods of strength characterisation, i.e. quasi-static or impact produce different results. To understand the role of structure and the influence of test method, agglomerate failure behaviour has been analysed by the use of the Distinct Element Method (DEM). We report on our work on the simulation of the breakage of the agglomerates for different porosities and impact conditions, where the role of impact speed and angle and type of contact bonding model have been evaluated. The adhesive contact model of JKR is used to form an agglomerate. The effect of the bonding level on the strength and size distribution of the clusters released as a result of failure has been investigated. This work also evaluates the effect of structure (porosity) on the strength of the agglomerates. [ABSTRACT FROM AUTHOR]

Coating of particulate solids by a thin film layer is of interest in many industrial applications such as seed and tablet coating. In seed processing, seeds are commonly coated with a protective coating layer consisting of fertilisers and crop protection products. Rotary drum batch coaters are typically used for this purpose. The coater consists of a cylindrical vessel with a rotating spray disk in the centre, onto which the coating liquid is fed. The seeds are driven around the vessel by its rotating base, and are mixed by two baffles; one on either side of the vessel. In the present study, DEM simulations are used to analyse the seed coating process. Corn seed is used as a model material and its shape is captured using X-Ray micro-tomography (XRT). The shape is incorporated into the simulations by the clumping multiple spheres to form a particle assembly. The coating uniformity of the seeds is predicted by implementing a coating model, whereby the coating mechanism is represented in the DEM by considering that once a droplet contacts a corn seed, it is removed from the simulation and its mass is attributed to the coating mass of the corn seed. The distribution of mass of sprayed spheres on the corn seeds and the coefficient of variation are evaluated for a range of process conditions, such as spinning disk rotational speed, droplets size and baffle arrangement and designs. In addition to evaluation of coating uniformity of particles, the scale-up rules for rotary drum batch coaters are investigated and reported. The outcome provides guidelines on scale-up rules and improvement of coating uniformity for rotary batch seed coaters. [ABSTRACT FROM AUTHOR]

The FT4 powder rheometer of Freeman Technology is widely used nowadays in industry for characterisation of particle flow under dynamic conditions of shear strain rate. It measures the work (termed flow energy) required to penetrate a rotating impeller into a powder bed. However, little is known about its underlying powder mechanics, i.e. the relationship between the flow energy and the prevailing local shear stress. This has recently been studied, but only for very simple and ideal systems amenable to analysis by DEM. We analyse the effect of gas flow through the powder bed on the flow behaviour of cohesionless particles in FT4 by DEM-CFD simulation. The results show that the relative particle velocities induced by the mean shear speed, is of the same order as that produced by the root of granular temperature. The shear stress in both cases with and without gas flow could be quantified by the inertial number. The flow energy correlates well with the shear stress in front of the blade, and both increase with the inertial number and could be significantly reduced by the upward gas flow. [ABSTRACT FROM AUTHOR]

The Freeman FT4 Powder Rheometer has been reported to describe well the powder flow behaviour in instances where other techniques fail. We use DEM to simulate the FT4 operation for slightly cohesive large glass beads at a range of strain rates. The curved impeller is shown to be beneficial in comparison to a flat blade as the variation of shear stress across the blade is reduced. The shear stress in front of the blade correlates well with flow energy (which the device measures) for a range of tip speeds and is shown to increase approximately linearly with tip speed when operating beyond the quasi-static regime. [ABSTRACT FROM AUTHOR]

Spiral jet milling is a size reduction process driven by the fluid energy of high velocity gas jets. Inter-particle and particle-wall interactions are responsible for size reduction. The process is energy intensive, but inefficient. The underlying mechanisms for size reduction in the mill are also not very well understood. The optimum grinding conditions are still currently found by trial and error experimentation. In this work, the Discrete Element Method coupled with Computational Fluid Dynamics is used to investigate the effects of different parameters on the particle collisional behaviour in a spiral jet mill. These include the particle concentration in the grinding chamber, the particle size, and the fluid power input. We report on our work analysing the efficiency of energy transfer and how it can be improved by changing the milling conditions and particle properties. [ABSTRACT FROM AUTHOR]

Segregation of granules is an undesired phenomenon in which particles in a mixture separate from each other based on the differences in their physical and chemical properties. It is, therefore, crucial to control the homogeneity of the system by applying appropriate techniques. This requires a fundamental understanding of the underlying mechanisms. In this study, the effect of particle shape and cohesion has been analysed. As a model system prone to segregation, a ternary mixture of particles representing the common ingredients of home washing powders, namely, spray dried detergent powders, tetraacetylethylenediamine, and enzyme placebo (as the minor ingredient) during heap formation is modelled numerically by the Discrete Element Method (DEM) with an aim to investigate the effect of cohesion/adhesion of the minor components on segregation quality. Non-spherical particle shapes are created in DEM using the clumped-sphere method based on their X-ray tomograms. Experimentally, inter particle adhesion is generated by coating the minor ingredient (enzyme placebo) with Polyethylene Glycol 400 (PEG 400). The JKR theory is used to model the cohesion/adhesion of coated enzyme placebo particles in the simulation. Tests are carried out experimentally and simulated numerically by mixing the placebo particles (uncoated and coated) with the other ingredients and pouring them in a test box. The simulation and experimental results are compared qualitatively and quantitatively. It is found that coating the minor ingredient in the mixture reduces segregation significantly while the change in flowability of the system is negligible. [ABSTRACT FROM AUTHOR]

In many powder handling operations, inhomogeneity in powder mixtures caused by segregation could have significant adverse impact on the quality as well as economics of the production. Segregation of a minor component of a highly active substance could have serious deleterious effects, an example is the segregation of enzyme granules in detergent powders. In this study, the effects of particle properties and bulk cohesion on the segregation tendency of minor component are analysed. The minor component is made sticky while not adversely affecting the flowability of samples. The segregation extent is evaluated using image processing of the photographic records taken from the front face of the heap after the pouring process. The optimum average sieve cut size of components for which segregation could be reduced is reported. It is also shown that the extent of segregation is significantly reduced by applying a thin layer of liquid to the surfaces of minor component, promoting an ordered mixture. [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]

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]

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]

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]

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]

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]