ARR - Annual Report
Work carried out this year is essentially focussed on 3-D image analysis. New 3-D shape descriptors completing the range of those introduced last year are defined. A dissolution index is first proposed. It gives an information of how morphology influence the dissolution process. We also implement the decomposition of a particle within its maximum inscribed spheres. From this, we extent in 3-D the computation of particle roundness and roughness. In parallel, we define a new method to quantify and to illustrate the uncertainty related to the assessment of particle size distribution using image analysis.
The two year research have allowed the definition of 3-D size and shape descriptors with a range equivallent to the 2-D one. The 3-D descriptors are optimised in terms of sensitivity and robustness. It will constitute practical tools for the future 3-D size and shape characterization. The methodology developped to quantify particle size distribution uncertainty will be useful for practionners using image analysis (either 2-D or 3-D) to determine the number of particles that must be analysed.
On the other hand, we completely finish the evaluation of 3-D imaging techniques regarding particle characterization specification. Future work will be dedicated to the creation of a new 3-D imaging technique adapted to routine 3-D particle characterization. Finally, X-ray micro-tomography and 3-D image analysis are applied on a real case, size and shape characterization of Zn particles used in battery. Thanks to a satisfying 3-D particle dispersion, the size and the shape of 4500 particles are characterized. The comparaison of the results with 2-D image analysis results and prior knowledges gives credence to the 3-D size and shape characterization.
During the third year of the research, we plan to perform 3-D size and shape characterization of severals real samples. This will fully validate the methods implemented. Another field for future research is the quanfication of cristallinity both in 2-D and 3-D. This issue have not been fully resolved even if it is important in pharmaceutic application.
This report addresses the properties of flow in a funnel or hopper. One aspect of this work 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 benefitted from collaborative interactions with Paul Mort of P&G.
The Flowdex tester consists of a cylinder with a small hole at the bottom. The material to be tested resides in this cylinder. The typical dimensions are a half dozen centimeters for the cylinder diameter, and about 10 cm for its height. A release mechanism provides a rapid opening mechanism for the plug at the bottom of the cylinder.
1. Introduction
A proposal of the Swiss Federal Institute of Technology Zurich (ETH), Laboratory of Food Process Engineering (years 4-6) on the topic "Quantitative Analysis of Structural Transformation in Extrusion Processing" has been accepted for support by IFPRI in June 2005 and started in April 2006. The present report covers the time frame from December 06 to May 2008.
Background to Surface Forces
In the first part of this report we provide some background to surface forces in order to explain the main considerations that apply when high concentration of electrolytes are present. That is the electrostatic component of the surface force is greatly reduced and the surface forces and therefore the stability of a system is dependent upon the balance between the repulsive hydration force and the attractive van der Waals forces.
Investigation of Interaction Forces
An investigation of the interaction forces in NaCl solutions at both low, intermediate and high salt concentrations illustrates this point further. Additionally the frictional forces in these systems have been measured in order to test the proposed approach of using the frictional and adhesive forces to characterize the hydration force in systems where the hydration force cannot be easily measured directly. This approach is validated.
New Technique for Solvent Structure Evaluation
The development of a new technique for the sensitive evaluation of solvent structure is described, termed photon pressure AFM. This technique is able to resolve the solvent structure adjacent to a solid material and observe alterations to that structure arising from the presence of cations in solution. It promises to provide exquisite detail on the interaction of ions with mineral surfaces.
Challenges and Future Directions
A major challenge has been to produce mineral surfaces that are sufficiently idealized in terms of geometry and surface finish to employ in these studies. To this end the technique of ALD is very promising. Funds from IFPRI have been employed to apply for a research grant to acquire this instrumentation.
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.