ARR - Annual Report

This project’s objective is to bring unique experimental insight to the detailed interactions between a gas and dispersed particles. By informing recent theories for those interactions, notably those of Profs. Brady and Koch, this work will benefit a wide array of industrial processes involving gas-solid suspensions.

The research is made possible by our development of a unique experiment producing shearing flows of gas and solids in the absence of gravitational accelerations. The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed. Within that range, we shall characterize the viscous dissipation of the energy of the particle fluctuations and record the dependence of the mean drag on granular agitation.

We are developing a microgravity flow cell in which to study the interaction of a flowing gas with relatively massive particles that collide with each other and with the moving boundaries of the cell. Unlike Earth-bound flows where the gas velocity must be set to a value large enough to defeat the weight of particles, the duration and quality of microgravity on the Space Station will permit us to achieve suspensions where the agitation of the particles and the gas flow can be controlled independently by adjusting the pressure gradient along the flow and the relative motion of the boundaries.

After a literature review, this second annual report describes the new axisymmetric experimental apparatus that we have designed for this project, and it outlines theories that we will employ for the interpretation of its data.

Introduction

Crystallography concerns both the internal and external form of a crystal [1]. Crystalline solids are an essential part of our modern technological environment, being important components of pharmaceuticals, foods, cosmetics, metals, ceramics and plastics. The process of crystallisation is used both for purification and as a separation process for the production of particular materials.

The customary way of forming crystals through suspension processes always relies on the usage of solvents in which solution phase is used as a media for homogenisation of the starting composition as well as enabling the molecular assembly processes. These solvents have been found to influence the crystallisation processes to the point of altering both the nucleation rate and the crystal morphologies and state of aggregation of the end product. Despite this, our knowledge and the understanding of the nature of the molecular assembly processes in supersaturated solutions, as well as the interaction of solvents with the crystal faces is severely lacking.

In the fine chemicals and pharmaceutical industries, where products are of high value, organic solvents are routinely used [2]. The ability of solvents to manipulate the structure and morphology of the crystals formed becomes invaluable. This becomes even more important when the drug or the dyestuffs are polymorphic in which case changing the solvent can result in a different polymorph crystallising more than one crystal structure. Modifying the polymorph can alter its physical behaviour. For example, in the case of a drug the rate of uptake in the body can increase making one polymorph more desirable over the other. Thus understanding the interactions between a solvent and solute as well as the fundamental theories lying behind the whole solution crystallisation can increase the performance of the final product as well as extending our ability to select solvents for crystallisation control.

Executive Summary

This report is an integral part of an effort to develop a computational platform to virtually synthesize and test particle compacts based only on the bulk and surface properties of the particles prior to the consolidation process. This virtual manufacturing and testing facility (VMTF) includes die filling, compaction –particle rearrangement and particle deformation (elastic and inelastic)–, compact ejection and subsequent mechanical testing. The current simulation platform is based on a multiscale approach, which bridges systematically the micro and meso-scale. The VMTF will provide the ability to reproduce the behavior of current products but more importantly, it will enable the simulation of systems never yet manufactured, virtually screening the best manufacturing conditions and particle/granule properties for a desired compact behavior or application. During this year we will continue the development of the subsequent modules of die-ejection and mechanical testing.

The specific content of this report includes a numerical study of the mechanical behavior of systems composed by particles with different sizes and materials subjected to consolidation. The simulation methodology is based on a mixed discrete/continuum approach which allows to systematically bridge the microscale response (particle and inter-particle scale) with the mesoscale and macroscopic behavior (container/sample scale). The methodology is particularly suitable for describing the post-rearrangement regime where consolidation proceeds mostly by elastic and inelastic deformation. This formulation is able to provide quantitative estimates of the evolution of macroscopic variables, such as pressure and density, while following microlevel processes, such as local coordination number and loading paths. This methodology is applied to polydispersed systems composed by particles with different nonlinear properties. The predictions are in general agreement with the experimental data during both loading and unloading cycle.

The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accurate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.

The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (ie due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was made also with the FDP section of work. However, with the 3-year extension to the research grant, a substantial amount of work now can be completed in the FDP section, as well as completing particular outstanding issues in the LVSF section.

Several difficulties were encountered during the course of the first 3 years of the project (eg unexpected results and phenomena) and delayed progress in certain areas. In some cases, it was not possible to complete particular scheduled tasks (eg testing aluminium and mild steel pipes with a wide range of granular solids). In other cases, it was necessary to pursue new research issues (eg rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project has been successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. For example, the new transport boundary and pressure drop models have been found quite accurate for the poly pellet type materials tested to date and also have been able to explain some of the interesting and unexpected phenomena encountered during the experimental stages of the project.

Unfortunately, due to the various problems and delays to date, as well as the new discoveries and developments, the numerous pipe wall materials and bulk solids planned originally for the LVSF section were not able not been tested, preventing further confirmation of model accuracy and validity. Additional work is planned over the next 12 months for this purpose (eg testing at least one other granular material with properties different to the poly pellets).

A significant amount of additional time will be needed for the expected relatively more complex FDP section of work. For example, only one product and a few different pipelines were able to be tested by the end of the initial 3-year period. The 3-year extension will allow this to be pursued in greater detail (eg with other powders and pipeline configurations), as well as the commencement of investigations into modelling techniques.

This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.

Fundamental theoretical and experimental works about grinding and crack formation in solids has been done by Schönert [1]. Based on this work he estimates the possible minimum particle size for grinding purposes to be in a range of 10 to 100 nm depending on the physical properties of the material. Investigations of the comminution of fused corundum (Al2O3) should answer the question whether and in which particle size range a lower limit of the grindability exists. During the investigations the increasing particle- particle-interactions with increasing fineness are an essential problem. Due to this interactions the viscosity of the product suspension increases and often spontaneous agglomeration of product particles occurs.

This report shows the experimental setup which allows the measurement of the most important electrochemical properties and the analysis of the particle size distribution of the product suspension as well as an adjustment of the pH-value for stabilization during the comminution process.

First results for comminution of fused corundum with different grinding media materials and grinding media sizes are shown. In addition, the influence of the electrostatic stabilization on the grinding progress is discussed.

This year research on the rheological behavior of concentrated suspensions has (1) addressed theoretically the effect of interparticle forces on suspension rheology, (2) developed the Accelerated Stokesian Dynamics (ASD) method, and (3) used ASD to study the effects of surface roughness and sliding friction on rheological behavior. The goal has been to provide a quantitative microstructurally-based description of suspension rheology for a range of concentrations and particle-level interactions. This year most of the effort has been on the numerical side. Please note that not all of the work described here has been supported by IFPRI; for example, the work on ASD has been primarily supported by NASA.

The present annual report composes of three parts; (1) mechanochemical (MC) syntheses of rare earth oxyhalides (ROX, R=La, Sm, Nd, Pr, X=F, Cl, Br) to form functional materials from inorganic systems, (2) MC syntheses of solid state solutions such as LaOCl1-xBrx from mixtures of rare earth oxides (R2O3) and rare earth halides (RX3, X=F, Cl and Br) from the inorganic systems, and (3) MC synthesis of LaOF from an inorganic-organic system (La2O3 and polyvinylidene fluorine (PVDF)). Regarding the first part, the grinding the constituent components (R2O3 and RF3) enables us to form ROF monophase in the product, and the reaction proceeds with an increase in grinding time, and the crystallite size of the product formed is about 15~20nm. As for the second part, the MC reaction proceeds as grinding progresses, forming LaOCl1-xBrx. Unit cell dimensions, a, c and lattice volume of the solutions, evolve linearly with an increase in x in the LaOCl1-xBrx series. Comparing unit cell dimensions of LaOX synthesized by MC reaction to those of LaOX synthesized by solid state reaction at high temperature, there is not any significant difference in the length of c, while a is shortened slightly. This may be attributed to the complex cation layer of (LaO)nn+ with a close relationship to a of the cell dimensions, being affected by the intensive grinding. As for the final part, the reaction through the substitution of F- by O2- results in defluorination of PVDF forming LaOF. This reaction proceeds with an increase in grinding time and is almost completed by about 240min. Mean size of the synthesized LaOF particles is sub-micron order, and the particles look like agglomerates. By prolonged grinding, agglomerated fine particles, consisting of LaOF and resultant organic materials, are transformed to the composites of rupture-like shape and these particles are covered with residual fine particles, progressively. Bonds such as C-O-H and C=C, are formed in the resultant organic phase in the ground mixture. The reaction yield reaches about 98 % at 240min. From these investigations, they have found a new route as well as something that leads perhaps to high possibility of application of this MC reaction to pharmaceutical field.

The future work will be focused on the grinding a mixture of a pharmaceutical raw material like talc with a binder such as cellulose to investigate MC reaction and molecular design between the two materials. In addition, the MC reaction and effect for food materials such as lactose will be investigated. In this study, co-grinding of lactose with cellulose with or without additive such as paracetamol will be made.

Summary

In the particle technology, it is fundamentally important to know the interaction and adhesive forces between particles and to find the correlation of those forces with the microscopic characteristics of particle surface, because these forces are the origin of many phenomena which particles exhibit in industrial processes. The aim of this project is to clarify in-situ at the molecular level the microstructure of surfaces in solutions of industrial importance and the correlation with interaction and adhesive forces between surfaces, using not only an atomic force microscope (AFM) but also computer simulations.

It was planned to clarify the phenomena and mechanism on the subjects shown in the map of the following figure, which were considered to be fundamental in understanding the phenomena in industrial particle processes.

Executive Summary

Work is reported of relevance to the pharmaceutical, chemical, mineral and other manufacturing processes against our objectives of developing and demonstrating the use of multi-sensors for mapping of microstructure and flow properties of concentrated suspensions and dispersions. The goal is to use this information for control of particulate product formulation in which on-line measurement enables new or more consistent products to be manufactured. The measurement tools used have included electrical conductivity tomography, X-ray tomography and photography and ultrasound measurements. Principal achievements of the research and team are reported under the following headings:

Software Tools

• Development of a new algorithm for enhances data analysis and image reconstruction, that is more robust to fluctuations in the background electrical properties of the process mixture.
• Computation of local changes in axial and radial solid concentration profiles.
• Measurement of velocity of features/flow structures in the mixture.
• An example case study for hydraulic conveying.

Instrument and Technique Development

• Simpler (single) electrode arrays for pipe-based measurements.
• Alternative impedance measurements yielding absolute images for paste extrusion in barrel and die.
• Demonstration of 3d imaging and reconstruction from a limited electrode data set in a narrow bore die.
• Use of X-ray microtomography with a digital simulation approach to enable prediction of properties of complex structure materials.

Model Validation Case Studies

Here the goal has been to use the multi-sensor measurement method to validate models. Two examples have been progressed-

• Slurry hydrotransport modelling and measurement using electrical tomography (above)
• Slurry transport within a small diameter hydrocyclone, comparing CFD and tomographic measurement (electrical and XC-ray photographs)

The forward plan for 2002/3 is outlined with our focus on measurement and quantification of product microstructure in particulate materials and products.

Introduction

This research focuses on the dispersion of clusters of fine particles through the application of hydrodynamic shear. In many industrial applications, the processing goal is the production of breakdown of the particle agglomerate into small fragments (or possibly complete breakage of the agglomerate into its constituent particles) followed by distribution of the fragments throughout the liquid medium. Our ultimate goal for this work is to obtain a fundamental understanding of the various factors that influence the dispersion behavior of fine-particle agglomerates. Once such an understanding is achieved, chemical or mechanical strategies aimed at improving the outcome of dispersion processes or the design of more efficient dispersion equipment can be attempted.

Our general approach has been to study the dispersion behavior of well-characterized single agglomerates in controlled flow fields. This allows us to establish the links between the fundamental properties of an agglomerate and dispersion characteristics such as critical shear stress for dispersion, mode and kinetics of fragmentation, and the evolution of the fragment size distribution.

A major emphasis of the research supported under this IFPRI grant involves investigation of the role of certain time-dependent (dynamic) phenomena in governing dispersion. Such time-dependent effects are inherent in many aspects of the industrial practice of dispersion. For example, when agglomerates are contacted with processing liquids, wetting and spreading of liquid throughout the agglomerate structure occurs simultaneously with dispersion. Also, particle agglomerates are subject to complex, non-steady shear histories in practical processing equipment. Also, in some cases, dissolution of the particles or binder liquids may occur on a time scale comparable to that for dispersion itself.

In the first year of this IFPRI grant, the bulk of the research effort was devoted to the development of a new experimental approach for the investigation of the influence of dynamic effects on dispersion behavior. This entailed the design (and redesign) of a dynamic dispersion chamber, and construction of it and the ancillary equipment. Preliminary experiments were done to validate the experimental techniques and to refine the analytical procedures. In the second year, we performed additional modifications to the experimental device and refined our experimental procedures. This allowed us to complete additional experimental studies that highlight the dramatic influence of time-dependent effects on the outcome of dispersion. In addition, we performed a thorough modeling study of the flow and shear fields present within our experimental device. This modeling enables us to analyze the results of our experiments, and to understand the limitations of our dispersion experiments with oscillatory flow. In the third year, we continued to perform experimental studies that shed light on the fundamentals of dispersion processes. These studies are leading us toward a mapping of dispersion regime on which different types of dispersion behavior can be related to agglomerate and flow characteristics. In addition, we attempted to study the dispersion behavior of agglomerate samples that were obtained from IFPRI member companies, and found that our experimental methods divulged a significant non-uniformity in the agglomerate structure. Such non-uniformity makes difficult the interpretation of the outcome of dispersion experiments with these materials.