SAR - Review

1 Introduction

The Powder Flow Working Group was charged by IFPRI to produce a review of research challenges on the flow of powders. It includes academics (Robert Behringer, Michel Louge, James McElwaine, Robert Pfeffer, Sankaran Sundaresan, and Jeorg Schwedes), industrial advisors (Karl Jacob, Thomas Halsey, James Michaels, and Paul Mort), and IFPRI officials (Nikolaas de Jaeger, Roger Place). The group held a meeting on October 21 and 22, 2004 in Newark, NJ to kick-off the activity. Triantafillos (Lakis) Mountziaris represented the NSF.

The group produced two principal deliverables. First, it wrote a series of short articles that are collected in this report. Second, for the NSF’s benefit, it outlined broad research directions that the agency should consider to advance knowledge on powder flow that is relevant to industrial problems.

This report is organized from the small scale to the flow scale. Its articles summarize a state of understanding and recommend further work. We begin with considerations of interparticle forces; we then discuss the role of compressibility and cohesion in setting bulk properties; we focus on stresses; we explore similarity and scaling; we sketch numerical techniques; and we delineate regimes of granular flows. We close by discussing chute flows and by considering effects of the interstitial fluid.

The internal structure of a particle

The internal structure of a particle is an important attribute that determines macroscopic properties such as bulk density, effective mechanical properties, or dissolution rate. In this review article, the experimental and computational methods for the analysis of particle structure are summarized. After a critical review of the advantages and limitations of instrumental methods for two- and three-dimensional structure visualization, such as SEM and x-ray microtomography, several computational algorithms for the evaluation of quantitative descriptors from digital structure images are introduced. The idea of structure “fingerprint”, i.e. a set of numerical values uniquely defining the structure, is proposed. Finally, the complete powder structure characterization methodology is demonstrated on two application examples. The key conclusion of this review is that a wide portfolio of experimental and computational methods for structure characterization exists and is ready for practical use. However, at present our ability to quantitatively characterize particle structure exceeds our understanding of the relationships between processing, structure and end-use application properties of particles, which should be the focus of further research.

Keywords

• microstructure
• image analysis
• integral characteristics
• porous media
• product design

Currently, the equilibrium forces between ideal molecularly smooth surfaces in inert vapors are reasonably well-understood. But for non-spherical, polydisperse, and particles whose surfaces are not smooth even on the nano-scale, fundamental understanding is still rudimentary. In addition, flow involves changes in both the normal (e.g., adhesion) and lateral (e.g., shear or friction) forces between particles, which move in a complex 3D trajectory relative to each other. The situation becomes further complicated for interactions in condensable vapors, e.g., in humid air or air containing condensable organic molecules, where capillary condensation (liquid bridges) and capillary forces are now also involved.

More complex still, for particles that are not perfectly rigid or elastic, but viscoelastic or ‘anelastic’, slow deformations and rearrangements can occur over time due to the high local stresses (both compressive and tensile) on these particles, leading to ‘dynamic aging effects’ such as slow deformations and creeping flow, and various types of instabilities (in addition to the normal type of sudden slip instability). The aim of this review or report is to discuss these outstanding issues in terms of the equilibrium and dynamic (non-equilibrium or history-dependent) forces between particles, and what further experiments and theoretical modeling need to be done to enable us to control these forces and predict particle (powder, granular) flow.

Thus, where appropriate, each section ends with a paragraph entitled Outstanding issues and challenges describing the writer’s assessment of the important unresolved issues discussed in that section. At the very least, it is hoped that the various experiments and modeling strategies proposed will allow for scaling up of laboratory experiments to predict behavior under field conditions. This calls for ‘scaling theories’ or equations giving the critical conditions, e.g., to flow, in terms of the various length scales involved (particle size, rms roughness, shape anisotropy, Kelvin radius determined by the relative humidity, height of sample, etc.) and other relevant parameters (particle composition, elastic properties, extraneous vibrations, previous history of the sample, temperature, etc.). The focus will therefore be on interparticle interactions in typical atmospheric conditions. Colloidal interactions (in solution) will not be covered except in cases where particles are separated by thin layers of liquids capillary-condensed from undersaturated vapors.

The to-date literature search on encapsulation and micro-encapsulation produces over 37000 references, with the earliest results originating back in the early 1900-th and as early as XIX century. The interest in encapsulation, seemingly non-fading since its inception, has been recently prompted by new requirements from myriad of industrial applications. In the last decade, the interest in encapsulation and micro-encapsulation has sprung anew with novel materials and technologies available for it: for example, developments in the area of polymeric microcapsules. That, in turn, ignited research in the area of release of encapsulated materials. Encapsulation evokes a great variety of not only methods, materials, techniques but also a large number of various industries and research institutions became interested in using it. Perhaps most notably, the scope of industries ranges from pharmaceutical, food, perfume to agriculture and materials. Besides drug delivery, the emphasis in this milieu is focused on more effective storage and delivery of materials. Notwithstanding, the encapsulation methods are equally important in research centers where it can be used for studying intracellular processes, exploring bio-chemical reactions in confined volumes, etc.

The report examines 3DParticle characterisation of crystals. In the following areas:

• 3D Digital aquisition of particles
• 3D Digital analysis of particles
• 3D Digital modelling of particles

Powder caking is considered as the undesired aggregation of particles resulting in the transformation of a free-flowing powder into a coherent solid mass. These might simply be large lumps that break down readily into their constituent primary particles. Alternatively caking may result in the complete and irreversible fusion of the entire particulate contents of a silo or other container. Whether mild or extreme, caking is a significant problem for a wide range of process industries in terms of loss of product quality or process performance, and can therefore have substantial impact on the financial health of a business. The aim of this review is to identify the current state of knowledge regarding the phenomenon of powder caking. Of particular concern are the underlying physical and chemical mechanisms, the role of process variables such as temperature, moisture content and consolidation and their dynamics, the range of experimental methods to assess caking and methods to prevent it. The main challenge of the subject is to be able to predict reliably the caking propensity of a powder product at a protracted time scale in the future, and this requires a detailed understanding of the factors listed above.

Throughout the review, areas for further research have been identified that will take us towards meeting this challenge. The role of interparticle forces in caking has been examined. Further work is required to characterise irreversible, non-equilibrium adhesive contact due to molecular rearrangement. The full role of piezoelectric, and pyroelectric charging in caking also requires further investigation.

A review of the formation of solid bridges between particles has identified two main processes; sintering and solvent evaporation. Research into sintering stems from the technologies of powder metallurgy and ceramic manufacture which involve elevated temperature and pressure. The applicability of this research to powder caking has hardly been addressed and its suitability not been examined. It is therefore recommended that further work is directed to develop the established concepts of sintering into the area of powder caking.

A small body of work has provided evidence of metastability in solid bridges causing the morphology of the bridge to evolve over protracted timescales. It is suspected that this condition is endemic in powder caking and therefore more research is recommended in this area. The formation of solid bridges from mixtures of solutes by solvent evaporation is a common phenomenon in caking. There are apparent contradictions regarding the nature of the solid bridge produced from mixed components which would benefit from further work.

The caking of amorphous powders has received a large amount of attention in the literature and models to predict caking kinetics are showing promise. The remaining uncertainty in this area relates to the behaviour of multi-component mixtures of particles. It is recommended that this area is targeted for further work. The published work relating to the dynamics of caking has been reviewed. Attempts at transient heat and moisture transfer modelling have been directed at materials that cake through dissolution and recrystallisation. More theoretical and experimental work is required in the area to develop a universal modelling tool to describe caking by this mechanism. The role of transient heat and mass transfer in caking by viscous flow, creep or sintering has not been addressed. These processes have been shown to be dependent on temperature and moisture content, and therefore it would be worthwhile to focus further work in this area. A review has been conducted of the wide range of tests to measure the strength and extent of powder caking. Of the conventional mechanical test methods, shear cell testing appears to be the most suitable, particularly if a cell was developed that had full humidity and temperature control by air percolation, and was instrumented to give changes in sample volume during time consolidation. For materials that cake by creeping, it is possible that creep testing could be reliably extrapolated to predict future caking propensity as long as the various creep mechanisms are adequately understood and accounted for. Recent developments in the application of indentation to measure powder flow could be applied to diagnose the early stages of caking. The method is sensitive, and requires very small amounts of material. It is recommended that the suitability of this technique for caking is considered in future work. The application of NMR measurements to caking looks a strong candidate for further investigation. It is recommended that the technique is coupled with more rigorous cake strength measurements. So far only amorphous materials have been studied. It would be interesting to apply NMR to the caking of crystalline or multi-component systems.

Finally the published work relating to anti-caking agents has been reviewed. The mechanisms by which these reportedly operate are various including:

• competing with the host powder for available moisture,
• acting as a surface barrier between the host particles, (preventing the formation of liquid bridges, decreasing inter-particle friction, dissipating electrostatic forces, or inhibiting crystal growth of solid bridges),
• increasing the Tg of an amorphous phase, or
• forming a moisture-protective barrier on the surface of hygroscopic powders using e.g. lipids.

All of these mechanisms could potentially be deployed to reduce caking in multi-component formulations, and therefore further research in this area is strongly recommended.

The shape of a crystalline solid has a major impact on its downstream processing and on its end-use properties and product functionality, issues that are becoming increasingly important in the pharmaceutical and life science, as well as the specialty and fine chemical industries. Though it is widely known that improved crystal shapes can be achieved by varying the conditions of crystallization (e.g., solvent type, additive and impurity levels, etc), there is far less understanding of how to effect such a change. Until recently, most methods for predicting crystal shapes were based exclusively on the internal crystal structure, and hence could not account for solvent or impurity effects. New approaches, however, over the possibility of accurately predicting the effects of solvents. We review models for predicting crystal shape, and evaluate their utility for process and product design.

1. INTRODUCTION

Agglomeration, which is a size enlargement unit process, has applications in a wide range of industries, including mineral processing, pharmaceutical, detergent, agricultural, food processing and speciality chemical processing. The products of the agglomeration process are generally called agglomerates or granules if the size enlargement process is granulation or crystal if the size enlargement process if crystallization. Typically powders are agglomerated together to form larger particles with desired properties for instance improved flow properties, low dust formation during handling, better solubility. Industrial consumers of solid particulate products desire better flowability, metering properties, better product stability and lower environmental hazards [1]. Whatever the end use of the agglomerates it is important that they retain the engineered properties up to the point of final use. They should conform to the specifications they are produced according to even after undergoing several handling and transportation processes. This is not usually the case as some of the agglomerates fail during the transportation and other handling process caused by breakage of the particles. It becomes imperative that the agglomerates be made with sufficient strength to survive that handling and transportation process. The strength of the agglomerate depends on its structure and composition which in turn depends the process and formulation variables.

The mixing of powders and granular materials is of central importance for the quality and performance of a wide range of products. However, process design and operation are very difficult, being largely based on judgment rather than science. There are not even tabulated data to tell how the quality of mixtures depends on mixer selection. Design depends on experience, not science.

There are no sound scale-up laws for a given equipment type, largely because particle size needs to be included in any dimensional analysis. Design is not possible by applying physical principles. There is no reliable equation to describe the flow of single component powders nor an equation for predicting the structure of multi-component mixtures. In most cases, measurement has been difficult because the materials are optically opaque. Much work in the research literature has been questionable because sampling results are affected by sample size.

Modern experimental techniques and modeling work have provided a good deal of information on the behaviour of many of the pieces of equipment, though these have been small in size. The focus has also been restricted to single and two components. However, the studies have enhanced knowledge of physical behaviour. For example, for a wide range of equipment when operating at lower velocities, mixing is determined by the number of revolutions of the mixer, not the time. Observations of flow structure have led to a few specific models that should scale with equipment size. Measurement techniques are becoming more effective in giving internal flow patterns and in measuring powder composition.

For cohesionless materials, DEM (Discrete Element Method) codes are now being used to describe flow patterns on the scale of 10,000 to 250,000 particles. A strategy that embraces the effects of particle size, equipment size and internal geometry, is advocated for the future. The aim would be to elucidate engineering principles of general utility. As part of the overall approach, the findings must be backed by experiment. For cohesive materials, there is scope to develop methods coming from population balance modeling. There is also scope to develop an understanding by subjecting well defined cohesive materials to clear patterns of strain.

It may now be possible to use the methods of digital photography to obtain data which can be fed into a method of mixture characterisation that is free of the problems of sample size. Together with an understanding of the relationship between observation at a surface and the average of a flow as a whole, such a method would, if successful, be of immense utility. At the least, performance charts for industrial equipment would finally become available.

The next stage of development is to build on the emerging knowledge and methods so that the basics for design are laid down. Design then becomes predictable and operation subject to effective control of performance.