Colin Hare

Particle Breakage in Agitated Dryers

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.

Instantaneous Monitoring of Drill Bit Wear and Specific Energy as a Criteria for the Appropriate Time for Pulling Out Worn Bits

Current techniques for deciding when to pull out worn bits are based on speculation rather than reliable engineered methods. Two concepts have been previously reported in the literature to incorporate the effects of drilling parameters on forecasting the life of drill bits. Bit tooth flatness and specific energy approaches were used for assessing the bit tooth wear and predicting the rock formation and its properties being drilled. However, drilling shale formations as well as encountering abnormal geologic formations especially unconformities, reduces the reliability of these methods. In this paper, a modified technique based on combined bit dullness and specific energy have been used as trending tools for determining the status of the drill bit even in cases where drill torque data is unavailable. As case studies, currently producing oil wells from southern Iraq are analysed for bit wear. Estimated results of bit wear for each bit run were correlated with the qualitative field bit tooth flatness revealing a successful key index for the suitable time to pull out worn bits. Furthermore, literature values of unconfined compressive strength (UCS) of the drilled formations were compared with the computed mechanical specific energy (MSE) to validate the obtained results. Good agreement was observed making the study encouraging. The analysis is promising for evaluating drill bit selection and predicting the type of formation being drilled.

Impact breakage of pharmaceutical tablets

Tablets are the most common solid dosage form of pharmaceutical active ingredients due to their ease of use. Their dissolution behaviour depends on the particle size distribution and physicochemical properties of the formulation, and the compression process, which need to be optimised for producing consistently robust tablets, as weaker tablets are often prone to breakage during production, transport and end use. Tablet strength is typically determined by diametric compression and friability tests. The former gives rise to propagation of a crack on a plane along the compression axis, whilst the latter, carried out in a rotating drum, incurs surface damage and produces chips and debris. These tests produce different measures of strength, neither of which have been correlated with mechanical properties that are accountable for breakage, i.e. hardness, elastic modulus and fracture toughness. We propose a new method based on single tablet impact testing, following the work of Ghadiri and Zhang, 2002, who analysed particle damage by propagation of sub-surface lateral cracks and identified the fundamental form accountable for impact surface damage to be a lumped parameter related to hardness and fracture toughness. Microindentation, carried out separately, to determine fracture toughness led to complete failure of the tablets, hence an unreliable measurement of fracture toughness and no correlation with the experimental trend. In addition, by assuming the fracture toughness to be proportional to the square root of Youngs modulus, the indentation measurements do not correlate well with the impact breakage. The discrepancy between the impact and indentation methods is expected to be due to mechanical property variation across the tablet surface, and with strain rate. The impact method is a more suitable test to describe tablet propensity for attrition as it directly represents the failure mode tablets may experience during processing under well-defined conditions. In contrast, the friability test subjects tablets to a similar breakage mechanism but under less well-defined conditions, whilst the compression test represents a different failure mode that is not representative of stresses incurred during processing.

The influence of aspect ratio and roughness on flowability

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 Ghadir...

Assessing Flowability of Small Quantities of Cohesive Powder using Distinct Element Modelling

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.

A new contact model for modelling of elastic-plastic-adhesive spheres in distinct element method

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.

Analysis of seed processing by the distinct element method

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.

Prediction of bulk particle breakage due to naturally formed shear bands

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.