Influence of Interparticle Forces on Powder Behaviour
The role played by interparticle forces in influencing powder behaviour in a wide range of situations is reviewed. By examination of a variety of experimental evidence, theoretical predictions and the results of simulation studies, conclusions are drawn regarding the influence of interparticle forces on powder behaviour. The experimental studies reviewed include magnetic particles in a variable magnetic field, the behaviour of particulate systems with added liquid, particles within an imposed electric field, thermally induced interparticle forces, natural interparticle forces under varying humidity and the surprising influence of small forces on large particles. The range of powder behaviour and properties studied includes flowability, dispersion, shear properties, packing fraction, fluidization properties, angles of static and dynamic repose, properties of particle streams falling in gas and the behaviour of segregating particulate systems.
Brief Bio Details
Martin Rhodes holds a Bachelor’s degree in chemical engineering and a PhD in particle technology from Bradford University in the UK, industrial experience in chemical and combustion engineering and many years experience as an academic at Bradford and Monash Universities. He has research interests in various aspects of gas fluidization and particle technology, areas in which he has many refereed publications in journals and international conference proceedings. Martin has served on the editorial boards of Powder Technology and KONA and on the advisory board of Advanced Powder Technology. Martin has a keen interest in particle technology education. He has published four books and a CDROM on Laboratory Demonstrations and has directed continuing education courses for industry in the UK and Australia. He was co-founder of the Australasian Particle Technology Society. Martin is Emeritus Professor in the Department of Chemical Engineering at Monash University, Honorary Professor at the University of Queensland and Visiting Professor at the University of Leeds.
Cohesive powder flow
Professor Raffaella Ocone
Particulate flows play a central role in a number of natural phenomena (e.g., landslides, pyroclastic flows) as well as in pharmaceutical, chemical and energy conversion industries. Many particulate flows consist of cohesive particles, particles which by some mechanism are attracted to one another. Mechanisms that may cause cohesion in a particulate flow are: chemical bonding, electric charging, liquid bridging and van der Waals forces. When particles are wet or are in a moisture-rich environment, capillary forces may be important: these forces are generated by condensed moisture on the particle surface. The behaviour of wet particles differs significantly from that of dry particles. Capillary forces, brought about by what are often referred to as “liquid bridges”, are typically stronger than other type of cohesive forces.
In this talk we will present our current work on particles made cohesive through the existence of liquid bridges. We will present some rheological studies and some numerical simulations all aimed to elucidate the role that liquid bridges play on the “flowability” of particles. We also discuss some modelling efforts and how they could help bridge the gap between the particle and equipment scale. Whilst we recognise that quasi-static and rapid flows lye on well established theories, the intermediate regime between those two limits is still not well understood. The models that we present are aimed to understand the physics which is responsible of the bulk properties of powders flowing at the intermediate regime; the bulk behaviour and the bulk properties (such as agglomeration) strongly affect the performance of powders at the equipment scale (their macroscopic behaviour). As an example, in slightly wet particles, the liquid bridges between particles result in the reduction of inter-particle friction (lubrication) by switching the frictional contacts to fluid shear resistance. We use a shear cell apparatus to characterise powder stresses. These results are compared with discrete element modelling simulation results. In discrete element modelling, we solve Newton’s equation of motion for each particle and particle-particle collision is modeled by soft-sphere model. Finally, we use these results to propose coarse-grained models for large-scale applications
Research: Previous work over 25 years has covered modelling complex systems, spanning from solid/gas suspensions, to complex reaction networks. A particular area of current research is Hydrodynamics of granular materials and particle laden flow: Early studies aimed at formulating a predictive model to be used as a design and control tool and able to handle unwanted phenomena as particle segregation, backmixing and poor chemical conversion. The model developed in our group is extensively used to understand how solid particles behave when flowing in industrial equipment and applied to the hydrodynamics of pneumatic conveying, fluidised and bubbling beds. The model is able to handle large scale systems and therefore can be used in industrial plants. One of the completed projects dealt with the experimental characterisation (by means of Electrical Capacitance Tomography) and the mathematical description of the physics of the intermediate regime which develops between dense and dilute flow, which are very common in pneumatic conveying and in many industrial equipment dealing with transport of solid particles. Our work was included in a review undertaken by the DOE.
Validation of Discrete Element Modelling (DEM) on a Freeman FT4 rheometer using Electrical Capacitance Tomography to assess powder blending
Dr Giuseppe Forte
In this work, a FT4 Freeman powder rheometer was used in combination with Electrical Capacitance Tomography (ECT) to assess the mixing and segregation (de-mixing) process for populations of particles with different flow properties and size ratios. DEM simulations were utilised to simulate the blending process within theFreeman FT4 powder rheometer at different conditions. It was shown, in Figure, that the change in flow energy associated with segregation and mixing, and their dynamics, is dependent on the particle DEM frictional contact properties and particle size ratios (SR).It was shown that, for a binary mixture of particles, the flow energy presents a decreasing trend as the material is being mixed by the action of the blade.
Figure:DEM simulations of binary powder mixing in FT4 as function of size ratio (SR).
Experimental work was carried out to validate the modelling by assessing the influence of mixing or segregation on flowability results from a Freeman FT4 powder rheometer as well as the use of Electrical Capacitance Tomography (ECT) to measure powder mixing and mixedness. The Freeman FT4 chamber was equipped with two planes of eight electrodes each and measurements were taken at each mixing cycle. The ECT technique was used to reconstruct the internal material distribution of the FT4 based on the different relative electrical permittivity of the used powders. From the acquired tomograms, it was possible to distinguish, in agreement with measured flow energy, when either mixing or segregation was occurring. The particle blending for continuously sheared flows was investigated and the capability of ECT and powder flowabilty (as defined by the FT4 powder rheometer) to describe the distinct segregation and mixing dynamics of two particle species in a dynamically heterogeneous process was demonstrated.
Biography: Giuseppe Forte obtained his engineering doctorate from the University of Birmingham, where his research project was jointly funded by the Engineering and Physical Sciences Research Council (EPSRC) and Johnson Matthey. His current research, based at Johnson Matthey Technology Centre, Chilton, concerns the development of on-process measurement techniques for monitoring formulated products. His interests and expertise focus on industrial electrical tomography, acoustic spectroscopy and image processing.
Powder flow into a confined space: modelling and characterisation
Professor Charley Wu
University of Surrey
Many industrial processes involve deposition of powders into confided spaces, such as filling bins, silos and bags in bulk solids handling, filling dies in particulate product manufacturing. In these processes, powders are normally discharged from hopper-like equipment into specifically designed receiving-devices, which generally operate in open air, so that the air presence can have a significant impact on the deposition behavior. It is important to control the mass flow rate of powder during the discharge and content uniformity of the discharged powder mass. In this study, analytical models are developed to predict the overall mass flow rate during the flow of powder into confined spaces, based upon a modified Berveloo equation and air sensitivity classifications. Furthermore, a critical discharge rate can also be obtained from the model. Comparison with experimental data for die filling of fine powders reveals that the models can accurately predict not only the overall mass flow rate, but also the critical filling speed at which the confined space can be completely filled. Therefore, the models are useful tools for process development and formulation design in a number of industries.
Prof. Chuan-Yu (Charley) Wu is a Professor of Chemical Engineering at the Department of Chemical and Process Engineering, the University of Surrey. UK. He is currently an executive editor for Powder Technology, a leading peer reviewed journal on particle systems. He co-authored a monograph on “Particle Technology and Engineering” published by Elsevier in 2016 and edited two books entitled “Discrete Element Modelling of Particulate Media” and “Particulate Materials: Synthesis, Characterisation, Processing and Modelling” published by RSC publishing. He also edited five journal special issues and published over 100 scientific papers. He has given more than 60 invited presentations and seminars at international conferences, industrial companies and universities worldwide. Prof. Wu is a member of the advisory and editorial board for “Particuology”, “Acta Pharmaceutica Sinica B (APSB) ” and “Journal of Engineering”.
Prof. Wu has expertise in powder flow，particle technology, discrete element methods, finite element analysis, modelling and simulations, pharmaceutical engineering and granular dynamics. His research has been supported by global pharmaceutical companies including Pfizer, AstraZeneca, Sanofi and MSD, in addition to EPSRC, IFPRI and EU. He has recently coordinated a EU FP7-ITN consortium (IPROCOM) on predictive modelling of roll compaction that consists of 14 partners and 15 researchers from 8 European countries, with a total budget of €3.8m. He is also the scientist-in-charge (PI) of an FP7 MSCA IEF (THERMOPC) and a H2020 MSCA IF (DECRON).
Virtual formulation laboratory
Dr Csaba Sina
University of Leicester
Solid dose forms are the backbone of many manufacturing industries. In pharmaceutical therapeutics, tablets, capsules, dry powder inhalers and powders for re-suspension cover the vast majority of the £5.6Bn sales by this industry in the UK. Food (sales £67Bn) is the single largest industry of the UK manufacturing sector which totalled £365Bn sales in 2014 (Office of National Statistics). In all these manufacturing processes and in final use, the physical behaviour of the powder is at least as important as the chemistry. Stability, weight and content uniformity, manufacturing difficulties and variable performance are determined by decisions made during the formulation process Manufacturing problems are ubiquitous; the Rand report (by E.W. Merrow, 1981) examined powder processes and found on average 2 year over-runs to get to full productivity, and development costs 210% of estimates, due to incompatibility between powder behaviour and process design. In the intervening years, plant engineering techniques have developed, but the rationalisation of formulation decisions has never received more than cursory, empirical study.
This project proposes to develop a Virtual Formulation Laboratory (VFL), a software tool for prediction and optimisation of manufacturability and stability of advanced solids-based formulations. The team has established expertise in powder flow, mixing and compaction which will be brought together for the first time to link formulation variables with manufacturability predictions.
The OVERALL AIMS of the project are (a) to develop the science base for understanding of surfaces, particulate structures and bulk behaviour to address physical, chemical and mechanical stability during processing and storage and (b) to incorporate these into a software tool (VFL) which accounts for a wide range of material types, particle structures and blend systems to enable the formulator to test the effects of formulation changes in virtual space and check fortential problems covering the majority of manufacturing difficulties experienced in production plants.
The VISION for VFL is to be employed widely in the development process of every new formulated powder product in food, pharmaceuticals and fine chemicals within five years of the completion of this project.
VFL will consider four processes: powder flow, mixing, compaction and storage; and will predict four manufacturability problems: poor flow/flooding, segregation/heterogeneity, powder caking and strength/breakage of compacts These account for the majority of practical problems in the processing of solid particulate materials
The OVERALL OBJECTIVES of the project are: (a) to fill the gaps in formulation science to link molecule to manufacturability, which will be achieved through experimental characterisation and numerical modelling, and (b) establish methodologies to deal with new materials, so that the virtual lab could make predictions for formulations with new materials without extensive experimental characterisation or numerical modelling. This will be achieved through developing functional relationships based on the scientific outcomes of the above investigations, while identifying the limits and uncertainties of these relationships.
Powder flow issues in advanced digital design of pharmaceutical therapeutics
Professor Mojtaba Ghadiri
University of Leeds
We study cohesive powder flow behaviour under both quasistatic and dynamic flow regimes as part of the Advanced Digital Design of Pharmaceutical Therapeutics (ADDoPT) programme (https://www.addopt.org/), a collaborative project supported by the Advanced Manufacturing Supply Chains Initiative (AMSCI) and Medicines Manufacturing Industry Partnership (MMIP), and led by Process Systems Enterprise (PSE). Development of predictive models for flow initiation, arching and flow rules under dynamic conditions (intermediate flow regime) is underway using Discrete Element Method. The focus is on the use of realistic particle properties for the simulations, such as shape and adhesion, as predicted by molecular dynamics simulations in other parts of the ADDoPT programme, and validated by experimental work. We report on the ongoing work on modelling of the Schulze annular shear cell and Freeman Technology FT4 and their relation with powder flow in screw feeders, the latter being of great interest to the continuous processing of pharmaceutical powders.
Research: Engineering Science of Processes involving Particulate Matter
Attrition, comminution, agglomeration, dispersion, compaction, nanopowder processing, electrical effects, mechanics of particle motion and super-critical fluid processing
The focal point of my work is the development of relationships between microscopic and macroscopic properties and phenomena; i.e. the way in which the microstructure of particulate solids and the micromechanics of their behaviour in process equipment influence the performance of the process and the characteristics of the product. The ultimate objective is to provide a basis for systematic design of particulate products and of related processes.
Attrition and Comminution: Damage mechanics of particulate solids is being investigated in relation to attrition and comminution, and erosion and abrasion of surfaces. Current work includes attrition of granules and encapsulates, milling and processing of pharmaceutical powders. The emphasis is on the characterisation of single particle damage under impact and quasi-static loading. Application of the results to the analysis of macroscopic bulk behaviour is done by the use of Distinct Element Analysis.
Agglomeration: The influence of scaling-up of the high shear mixer granulators on the evolved structure of granules is under investigation in a large multi-tasks project addressing experimental and modelling aspects of the flow field within the granulator and the characteristics of the granules formed as influenced by the scale of operation.
Electrical Effects: Work on this line originates from the development of the Electromechanical Valve for Solids (EVS), which is a novel device for flow control of granular materials using the electrical clamping phenomenon. Current work addresses the coalescence of water droplets in oils, electro-spraying and prilling of highly viscous suspensions to reduce andcontrol the droplet size and tribo-charging of pharmaceutical powders.
Powder Mechanics: The current activity aims to relate the macroscopic bulk behaviour to single particle properties for the analysis of flow, segregation, compaction, dispersion and mixing.
Nanopowder Processing Work on this line addresses the formulation of granules formed by nano-assemblies having optimum re-dispersion characteristics.
Supercritical Fluid Processing Spray coating of surfaces by droplets/fine particles and formation of fine powders by several methods using super-critical fluids is under development.
Powder Spreading in Additive Manufacturing
James Weir Fluid Laboratory, Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose St., Glasgow, UK G1 1XQ
Additive Manufacturing (AM) is an enabling technology that allows the production of bespoke parts that are impossible to manufacture with any other technique. The Powder Based AM is perhaps the most promising AM technique where a thin layer of powder is deposited (recoating process) on a fabrication surface to form a thin bed (~100 μm) with a roller or a blade (Fig.1). A laser (or electron) beam is then focused onto the bed which scans a raster pattern of a single layer of the part by melting and fusing the powder grains in its path. This process is then repeated to fabricate the product layer-upon-layer.
PBF offers substantial benefit for rapid production of prototypes and lately for weight-sensitive/multi-functional parts at small volumes , with almost arbitrary complexity. However, highly complex and multi-physics nature of the process is hindering the development of the technology and introduction of new materials . The powder bed preparation stage is perhaps the most challenging stages of the process. The objective is the preparation of a dense and smooth bed which is known to be crucial for a successful build.
Fig 1: A PBF device in operation (Courtesy of Waterloo University)
The effects of grain shape, cohesion, liquid bridges and van der Waals forces are not well understood in this process and currently, the industry relies on advanced production techniques to develop powders with near perfect sphericity and roundedness and advises to follow strict powder handling/storage procedures to ensure a successful build. The application of DEM to the spreading process in AM will be discussed in this talk (Fig. 2) to address some of the scientific/practical challenges that the industry is facing.
Fig 2: Simulation of Powder Spreading in AM
 I. Gibson, D. W. Rosen and B. Stucker, New York: Springer, 2010.
 S. Haeri, Y. Wang, O. Ghita and J. Sun, Powder Technology, pp. 45-54, 2017.
Cohesive powder flow modelling
Professor Colin Hare
University of Surrey
Powder flowability is most commonly assessed by use of a shear cell, however this technique loses its reliability at low stresses. In recent years several techniques have emerged to assess powder flow under low stress conditions, though none have yet been shown to have the reliability found for shear cell tests at higher stresses. Here we explore the method of ball indentation, by which a consolidated bed is penetrated by a spherical indenter and the flow resistance measured. This approach requires a smooth surface, which can typically be achieved at stresses as low as 0.1 kPa. The method directly measures the plastic flow resistance (hardness), which is equal to the unconfined yield stress multiplied by a constraint factor. For many years the approach has routinely been applied to continuum materials such as metals and ceramics, where constraint factor has been established to have a value of approximately three. However, for discrete powder beds the constraint factor has been shown to be material dependent1,2.
In this work the flowability measured by ball indentation is assessed for a wide range of powders, and compared to shear cell measurements to determine the constraint factor. The variation of constraint factor with stress and with particle properties is examined. Furthermore, the reliability of the approach is evaluated and compared to that of the shear cell.
1Wang, C., Hassanpour, A., Ghadiri, M. (2008), Particuology, 6, 282-5.
2Zafar, U. (2013), PhD thesis, University of Leeds.
Colin Hare is a lecturer in Chemical Engineering at the University of Surrey, specialising in powder flow, particle breakage and the Discrete Element Method (DEM). He completed his PhD “Particle Breakage in Agitated Vessels” in 2010, under the supervision of Prof. Mojtaba Ghadiri at the University of Leeds, sponsored by the EPSRC and GlaxoSmithKline. In 2012, Colin received the Young Researcher Award at the UK Particle Technology Forum. In 2014-2015 he undertook a one year Knowledge Transfer Secondment (KTS) to Procter and Gamble Newcastle Innovation Centre, developing DEM modelling and measurement capabilities to assess and improve battery manufacturing processes.He has been awarded three grants from the Interntaional Fine Particle Research Institute (IFPRI); one to collaborate with Prof. Jin Ooi (Edinburgh) on “Development of Grindability Tests”, and an initial and renewal grantas principal investigator to investigate “Flowability of Weakly Consolidated Powders”. Colin was awarded the IChemE Don Nicklin medal 2015, which recognises talented young chemical engineering researchers. Colin is a member of the EPSRC College and of the EPSRC Early Career Forum in Manufacturing Research. His research is focused on relating material properties and processing conditions to bulk powder behaviour, utilising experimental and computational techniques.
Research: Colin's expertise are in particle technology, where he relates particle properties to process performance, with particular focus on powder flow and particle breakage. He carried out a knowledge transfer secondment to Procter & Gamble during 2014-15, exploring “Advanced Manufacturing Based on the Discrete Element Method”. In 2014 he was awarded a research grant by the International Fine Particle Research Institute (IFPRI) to carry out a “Flowability Assessment of Weakly Consolidated Powders”, receiving further funding from IFPRI in 2017 to continue this work.
Summary of research interests: Powder flowability, particularly at low stresses and/or high strain rates; Predicting and optimising particle breakage/attrition mitigation; Mechanical property characterisation; The Discrete Element Method (DEM); Particle coating by powders or liquids.
CHARACTERISATION OF COHESIVE POWDERS BY THE RAINING BED METHOD
Department of Informatics, Modeling, Electronics and Systems Engineering, University of Calabria, 87036 Arcavacata di Rende (CS), Italy.
The capacity of a granular material or a powder to flow under the action of gravity or other forces in a given set of conditions is an essential requirement in nearly all stages of solid handling and processing. However, solid ‘flowability’ is not an inherent property of a material, as it depends on the factors that define the state of the material (degree of consolidation, interparticle forces, interaction with fluids, etc.) and its capacity to resist stresses typical of the intended application.
Determining, predicting or improving solid flowability is of crucial importance when dealing with cohesive bulks: various characterization techniques have thus been devised to assess their flow properties and there is an increasing interest to compare the relevant results and transfer them from one field of application to another.
Among these techniques, the Raining Bed Method, recently developed after an original idea by Buysman and Peersman, has proved capable to determine the tensile strength of a cohesive powder at low levels of consolidation stress and in the presence of interstitial gas flow. Such conditions, hardly reproducible in other ways, are of sure interest in gravity discharge, gas fluidisation, powder dispersion and a number of other industrial operations. The characteristics of the technique are illustrated together with some examples of the results achievable. Both the potentiality of the technique and the difficulties to overcome on the way of its further development are discussed.
Short profile of Prof. B. Formisani
Brunello Formisani graduated in Chemical Engineering in 1979 from the University “Federico II” of Naples, Italy, where he started his career of scientist with a grant. In 1982, he joined the Department of Chemical and Materials Engineering at the University of Calabria, where he holds the chair of Chemical Plant Design since 2004. His research interests are in the field of particle fluidization and solids processing; current activities are related to segregation phenomena in multicomponent fluidisation, confined fluidised systems, fluidisation at high temperatures and processing of solids subject to interparticle forces.
FROM PROCESS ANALYTICS TO TRICKLE DOWN TECHNOLOGY
Centre for Process Innovation (CPI) – The National Formulation Centre, The Coxon Building, John Walker Road NETPark, Sedgefield, UK.
Formulated products such as detergents, cosmetics, paints, adhesives, and lubricants are ubiquitous in both commercial and consumer facing markets. It is estimated that the combined global market for formulated products is around $1000bn and design and manufacture of formulated products is a significant value-adding step, with a value multiplier ranging anywhere from 3 – 100 compared to the basic ingredients of a product. By working with a wide range of market sectors, knowledge partners and the UK’s most innovative technology companies, CPI are tasked to meet key challenges for the formulating industries and drive cross-sector innovation and value chain synergies.
The ‘complex particles’ technical group within CPI are undertaking a transformational project showcasing twin-screw granulation for the pharmaceutical industry. Building on the quality-by-design philosophy adopted by the pharmaceutical industry, this project will serve to highlight a number of prominent themes within powder technology including batch-to-continuous manufacturing, process analytics, and Industry 4.0. Experimental results from this demonstration will feed in to modelling projects aiming to couple discrete element method approaches with population balance modelling for improved simulation of the continuous granulation process.
CPI has identified continuous powder processing, powder characterisation, and the packaging of both bulk and consumer goods as areas where it can provide support to industry, specifically SMEs. By building capability in these areas, coupled with such transformational projects as mentioned above, it is believed that the state-of the-art powder processing technology and techniques can be made increasingly accessible to more sectors of the UK economy.
Timothy Addison works as an engineer for the National Formulation Centre platform within The Centre for Process Innovation. He obtained his doctorate from the University of Leeds in 2009 and hence undertook postdoctoral studies with Procter and Gamble. During his time with BHR group he worked on a range of projects from private consultancy to large collaborative FP7 projects. More recently, he worked on formulation and process development projects for Johnson Matthey. His areas of expertise include colloidal characterisation, formulation, and encapsulation.