ARR - Annual Report

Executive Summary

The research is focused on the Mechanics of Powders and the ultimate goal of the work is to develop a quantitative description of active flows of fine powders. The study is centered on the “intermediate” regime of flow where both frictional and inertial effects are important. The main application is in the area of small-size, rough and/or cohesive powders that are industrially relevant.

The novelty of the project is to study a relatively large range of materials and flow geometries to gain meaningful insight. We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and elastic), used in an axial-flow Couette and in two Jenike-type shearing devices to measure stresses and their fluctuations as a function of geometry and shear rate.

From stress transmission tests in the Jenike-type devices, we found that a layer of granules (powder) transmits normal stresses only if sheared and large fluctuations are introduced for the case of large particles. When the particles are small, of the order of hundreds of microns or less, or are cohesive and form a cake upon compression, the fluctuations are diminished and stresses are transmitted without significant alteration.

The further goal to develop constitutive equations for the intermediate regime of flow was also undertaken. The axial-flow Couette device was used as a “rheometer” to characterize the flowability of powders, develop a constitutive equation and use it in a continuum-type theoretical model to predict flow patterns, velocity distributions and forces on boundaries such as stationary walls and moving pedals. The PI is collaborating with several groups of simulators (DEM and MDS) and mathematicians to implement these models and compare theoretical results to computations. The best results so far, were obtained for a relatively fast moving Jenike-type cell where large, spherical particles where sheared against a smooth wall. Further results in the Couette device using a continuous-type theory (on the lines proposed by the work of Tardos, 1997) are also very promising.

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 article/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.

This third report will focus on advanced 3D shape analysis. It will first present the shape analysis philosophy that is backing the whole project before entering into software developments using mathematical morphology. Finally an application of 3D cristallinity computations on nanoparticles acquired by STEM is briefly presented.

This report addresses the properties of flow and jamming in a funnel or hopper. The first part of this project, to date, involves understanding flow properties using a quasi-2D hopper geometry, photoelastic techniques, and high speed video.

The second part of the project involves using theoretical approaches that, to some extent, borrow from recent work in the physics community on the so-called jamming transition.

The third aspect of this work, a direct application to an IFPRI industrial partner, is to better understand and interpret results using the Flowdex tester. This device is used to characterize flowability, and in particular to characterize that property for various powders of interest to P&G.

This work has benefited considerably from collaborative interactions with Dr. Paul Mort of P&G. IFPRI funds were used primarily for the support of Ph.D. student Junyao Tang, who is working fulltime on this project. In addition, an undergraduate Sepher Sagdiphour has carried out a significant number of measurements.

In many suspensions, slurries and complex fluid formulations of industrial interest, the colloidal scale interparticle forces are attractive. These attractions set up a complex microstructure with slow dynamics that determine properties such as yield stress, stability and shear-rate dependent viscosity. In particular industrial situations, these properties may be more or less desirable; however, in every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as function of time, is of paramount interest in process and product development. Because industrial materials are comprised of colloids with heterogeneous interparticle forces, polydisperse size distribution and non-uniform shape, the effect of these parameters on the microstructural origin of yield stress and stability should be assessed. Current capabilities for prediction of rheology from underlying microstructure is largely limited to linear properties in systems with well-characterized interparticle forces and monodisperse particle sizes.

In this project we are developing tools for characterization of the microstructural evolution in gelled colloidal systems that can be applied to non-linear rheological and stability phenomena, such as stress-induced yielding and ravitationally-induced collapse. These microstructural characterization tools can be applied to develop a quantitative link between microstructure and bulk suspension properties, such as the gel modulus, yield stress and delayed sedimentation.

This project will expand our current knowledge of attractive systems by explicitly considering the effects of size polydispersity on the gel microstructure and bulk rheology, thus enabling us to bridge the results of model systems to relevant industrial materials. Microstructure will be quantified by direct visualization with confocal and optical microscopy during flow and sedimentation. Concomitant measurements of non-linear rheological properties such as the yield stress on identical systems will also be conducted. Interactions and forces will be probed by laser tweezers microrheology. Systematic exploration of polydispersity effects will be accomplished by mixing fractions of monodisperse colloids in known amounts to achieve a suspension with well-characterized polydispersity.

Over the period of the last year, we have investigated aspects of process model identification and model based control for granulation processes. Significant accomplishments from that period are reported in two journal publications which are highlighted here:

[Sanders, Hounslow, Doyle III, Powder Technology, in press, 2008]

The modeling work in this paper provides insight on improved control and design (including measurement selection) of a granulation processes. Two different control strategies (MPC and PID) are evaluated on an experimentally validated granulation model. This model is based on earlier work done at The University of Sheffield, UK and Organon, The Netherlands. The granulation kinetics were measured in a 10 liter batch granulator with an experimental design that included four process variables. The aggregation rates were extracted with a Discretized Population Balance (DPB) model. Knowledge of the kinetics was used to model a continuous (well mixed) granulator. The controller model for the Model Predictive Controller is a linearized state space model, derived from the nonlinear DPB model. It has the four process variables from the experimental design and a feed ratio as input variables. Since the DPB model describes the whole size distribution (GSD), different sets of output variables were chosen and compared. When measuring controller performance based on the full granule size distribution, a PID setup can actually produce results that fluctuate more than the open-loop response. An MPC controller improves stability on both process outputs and the full granule size distribution.

[Glaser, Sanders, Wang, Cameron, Litster, Poon, Ramachandran, Immanuel, Doyle III, Journal of Process Control, in press, 2008]

This paper details a methodology for the design of a Model Predictive Controller for a continuous granulation plant. The work is based on a nonlinear one-dimensional Population Balance Model (1D-PBM), which was parameterized using experimental step test data generated at a continuous granulation pilot plant installed at the University of Queensland, Australia. The main objective was to operate the granulator under optimal conditions while off-specification material was fed back into the granulator to increase the economy of the process. The final algorithm design combines elements of Model Predictive Control (MPC) with gain scheduling to cancel nonlinearities in the recycle flow. A model directly identified from the step test data was the basis for testing a model predictive controller. Simulations show that the efficiency and robustness of this granulation process can be improved by applying the proposed control strategy. Ongoing work focuses on the implementation of the proposed control strategy on a full scale industrial plant.

Our aims for the renewal period of this IFPRI project are reiterated here, and the main body of this document reports our progress against these aims.

The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.

1. The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called solvation or in aqueous solutions hydration forces.

2. The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.

Therefore much effort has concentrated on acquiring or producing suitable surfaces for investigation. With silica surfaces this has not posed any significant challenge but other surfaces have not met with success. To this end we have purchased an Atomic Layer Deposition system that will enable us to deposit materials that mimic the mineral surface of interest onto silica surfaces. This instrument has now arrived and been installed in our laboratory. However there have been considerable delays in the arrival of the precursor chemicals, therefore we have not been able to use the instrument for the preparation of mineral like surfaces to date. These chemicals have now just arrived and we can now proceed. We have continued to develop the photon pressure technique but we do not present any new data relevant to this study here as measurements using the photon pressure technique require ultra-smooth surfaces and we have already presented data on silica surfaces the only surfaces that have been sufficiently smooth for this use up to this time. These surfaces will now be available to us with the arrival of the precursor chemicals for the ALD system.

The overall aim of the project is to elucidate the effect of feed material properties, mill dynamics and prevailing environment on milling of organic materials. This requires the use of a multi-scale approach covering molecular scale, to single particle scale and the bulk scale.

The recently concluded IFPRI programme (IFPRI FRR 52-03) addressed the effect of material properties on single particle breakage and bulk milling, which enabled establishment of a relationship between the single particle properties and the bulk milling. The main aims of this follow-up programme are to understand single particle breakage behaviour from the molecular scale, and to bridge the gap between the properties and behaviour at the single particle scale and those at the molecular scale. The principle methodology used in the follow-up programme is to investigate the single particle breakage behaviour as a function of temperature, humidity and strain rate. The work over the past 12 months has led to the following results:

• Effect of temperature was investigated at a relative humidity of ~30%. The results show that the extent of impact breakage of aspirin particles increases with increasing temperature, whereas little effect of temperature is seen for sucrose.

• Effect of humidity was studied at the ambient temperature (20oC). The results show no clear influence of the relative humidity for sucrose and aspirin particles on their single particle breakage behaviour.

• Effect of repeated impacts on single particle breakage (fatigue tests) was investigated using sucrose at the ambient temperature and ~30% relative humidity. The results show that the extent of breakage increases first with increasing number of impacts, reaches a maximum after a certain number of impact, and then followed by a slow decrease with a further increase in the number of impacts. The impact velocity has a great effect on the maximum value of the cumulative extent of breakage; a higher impact velocity gives a higher peak breakage extent; however, the number of impacts needed to reach the peak breakage extent also increases with increasing impact velocity.

We have employed Atomic Layer Deposition (ALD) to produce mineral like surfaces that are extremely smooth for use in surface force studies. Our investigations to date have focused on alumina and titania surfaces.

Alumina Surfaces

Alumina surfaces were found to be unstable in aqueous solution – they slowly dissolved. This prevented surface force investigations in simple aqueous solutions. However the surface could be passivated against dissolution through the adsorption of short chain carboxylic acids. These acids are of industrial interest as they have significant effects on the rheology of alumina dispersions. Examination of the surface forces between alumina surfaces in solutions of muconic acids revealed DLVO type forces under some conditions Non‐DLVO forces where a strong attraction was evident between the surfaces and is significantly stronger than van der Waals attraction. This attraction was attributed to the formation of a capillary consisting of an oil‐like muconic acid phase forming between the surfaces. This phase change is induced by the close proximity of the surfaces and is possible because the muconic acid is present at concentrations that approach the solubility limit in these solutions. The presence of a capillary between the surfaces results in a strong attraction. Attempts were made to form stable alumina surfaces that would enable surface force measurements to be conducted in water and electrolyte solutions. This included looking at much thicker layers and using different binding layers (such as titania). To date none have been successful. We are still pursuing this though it may be possible that all alumina surfaces – not just ALD surfaces – have this property. A slow rate of dissolution would not be revealed in many studies and therefore may have previously gone unnoticed. Evidence from Optical Reflectometer (OR) shows that the surface dissolves at a rate of ~8 nm per hour. So indeed the rate of dissolution is slow, but sufficient to prevent surface force or optical reflectometry measurements.

Titania Surfaces

In contrast, titania surfaces are stable and this has allowed us to perform a range of surface force studies at both low and high salt concentrations. At low salt concentrations a long range, pH dependent electrostatic force was observed. This data could be fit using the DLVO theory, which enabled the surface potential to be determined. This showed that the isoelectric point was between pH 5 and pH 6. At short range a repulsive interaction dominated the attractive van der Waals force. This is attributed to hydration forces. At high salt concentrations adhesion was seen that was dependent on both the pH and specific salt present. We find that this trend does not follow the Hofmeister series.

In many suspensions, slurries and complex fluid formulations of industrial interest, the colloidal scale interparticle forces are attractive. These attractions set up a complex microstructure with slow dynamics that determine properties such as yield stress, stability and shear-rate dependent viscosity. In particular industrial situations, these properties may be more or less desirable; however, in every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as function of time, is of paramount interest in process and product development. Because industrial materials are comprised of colloids with heterogeneous interparticle forces, polydisperse size distribution and non-uniform shape, the effect of these parameters on the microstructural origin of yield stress and stability should be assessed. Current capabilities for prediction of rheology from underlying microstructure is largely limited to linear properties in systems with well-characterized interparticle forces and monodisperse particle sizes.

In this project we are developing tools for characterization of the microstructural evolution in gelled colloidal systems that can be applied to non-linear rheological and stability phenomena, such as stress-induced yielding and gravitationally-induced collapse. These microstructural characterization tools can be applied to develop a quantitative link between microstructure and bulk suspension properties, such as the gel modulus, yield stress and delayed sedimentation. This project will expand our current knowledge of attractive systems by explicitly considering the effects of size polydispersity on the gel microstructure and bulk rheology, thus enabling us to bridge the results of model systems to relevant industrial materials. Microstructure will be quantified by direct visualization with confocal and optical microscopy during flow and sedimentation. Concomitant measurements of non-linear rheological properties such as the yield stress on identical systems will also be conducted. Interactions and forces will be probed by laser tweezers microrheology. Systematic exploration of polydispersity effects will be accomplished by mixing fractions of monodisperse colloids in known amounts to achieve a suspension with well-characterized polydispersity.