FRR - Final Report

The production of submicron and nanometer sized particles by wet grinding in stirred media mills was investigated. Small grinding media and, thus, small stress energies are required for an effective grinding, i.e. for a minimization of the specific energy. Moreover, the relations, particularly the so-called stress model, derived for grinding of coarser particles in stirred media mills are also valid for grinding of nanoparticles. However, for the production of alumina particles with sizes below approximately 200 nm an effective stabilization of the particles against agglomera-tion is necessary, especially, if yttrium-stabilised zirconium oxide grinding media are employed. An electrostatic stabilization with anorganic acids is effective if the surface charge is near the one of neutral chloride. This behaviour can be described by the so-called Hofmeister series. In case of organic acids the hydrocarbon chain should be as short as possible.

Beside a high grinding efficiency a low product contamination by grinding media wear is important. As known from grinding coarser particles grinding media wear at different operating conditions can be described by a so-called wear energy. Moreover, the grinding media wear can be minimized by optimizing the suspension viscosity and, thus, by adjusting the pH-value: If the viscosity is too low, the collisions of the grinding media are not damped and, thus, the wear is high. If the viscosity is too high, high concentrations and packing of the grinding media in front of the separation device occur causing high wear of the grinding media. Besides optimizing the viscosity the product contamination can be minimized by using coarse alumina particles as grinding media. First results show that nanoparticles can be produced by such an autogenous grinding process.

Besides the investigations on nanogrinding the mechanism of capturing particles between two grinding media were investigated using a specially designed model apparatus. Among others the measurements with this model apparatus showed that adhesion of product particles at the grinding media surface and decreasing product particle sizes increase the number of particles caught at one stress event.

Nanomilling is of great interest to many industries. Anosize particles are gradually being incorporated into a broad range of application fields, examples are fillers for paper and plastic coatings, pigments, ceramics for abrasive and structural applications, toners for photocopy and printing machines, pharmaceuticals. Advanced devices include electronic packages, ultra-thin-film optical devices, advanced fuel cell catalysts, molecular conductors, and biochips. Besides the direct synthesis of these materials by chemical methods, wet grinding in stirred media mills is a suitable method for the production of sub-micron particles. The main advantage of using wet grinding in stirred media mills over alternative grinding methods is the possibility to apply higher power densities necessary to produce very fine particles [1].

Crystalline solids are an essential part of our modern technological environment, since they are important components for many materials such as pharmaceuticals, foods, cosmetics, metals, ceramics and plastics. The customary way of forming crystals in the chemical industry is through suspension processes which rely on the use of solvents as media for homogenisation of the starting composition, and as an enabling environment for molecular assembly processes. Solvents have been found to influence crystallisation to the point of altering nucleation rates, crystal morphologies, crystal aggregation and the crystal structure of the end product. While significant progress has been made in understanding the origin of crystal morphology, our current knowledge of the molecular assembly processes leading to the nucleation of a particular crystal form in supersaturated solutions is poor. The work detailed in this final report deals with the role of solvent in nucleation processes and the mechanisms by which nucleation may be influenced by solvent choice.

The first part of the report deals with solution speciation in concentrated and supersaturated solutions and explores the link between the species present in solution and the crystal structure of the polymorph which nucleates. For this part of the study, carboxylic acids were selected as model materials and a combination of IR and Raman spectroscopy used identify the H-bonding motifs existing in their crystalline and solution phases. A significant portion of this work was devoted to the two monocarboxylic, tetrolic and benzoic acids and it was discovered that in some systems, for example tetrolic acid, there is a very clear link between the solution state of the molecule and its resulting crystal structures. In others, benzoic and mandelic acid it seem that irrespective of the state of the solute in solution it always nucleates the same structure.

The second portion of the work moves on from the nature of the solution to explore the impact of solvent on the nucleation process in the polymorphic system p-aminobenzoic acid (PABA). PABA has two crystalline polymorphic forms, and which are related enantiotropically having a transition temperature of 240C. It was thus envisaged that in this system the nucleation of the two forms could be studied at the transition temperature where thermodynamic effects would be identical for each and solvent influences could be systematically explored. Solvent selection was found to have a significant impact on the ability to nucleate the polymorph. Only in water was it possible to nucleate both forms; all other solvents favoured the form irrespective of the temperature. This outcome was rationalised in terms of the solution species and the impact of solvent on the growth of the phase.

Wet powders consist of solid particles which are bound by a binder liquid. In contrast to pastes they have a non-negligible volume fraction of air which signifcantly influences their flow behavior. Wet powder systems are highly cohesive. Capillary and surface tension forces are responsible for bonding forces exceeding Van-der-Waals adhesion forces by about one order of magnitude. At direct particle contact points normal force dependent frictional forces are present whereas particle surfaces which are lubricated by binder liquid facilitate a relative movement between particles. Due to high packing densities, sterical effects become essential when wet powders are subjected to high normal stresses. During extrusion the wet powder is forced into a reduced cross-sectional area (die). This accounts for a relative movement between the particles in the die inlet region. While the core of the powder in the (cylindrical) barrel flows in the form of an accelerated plug into the die, the particles in the outer regions are subjected to shear resulting in a structural change. The packing density is increased, air inculsions are compressed or pressed into the core resulting an increased saturation. The particles form a kind of shear layers. In ideal case the wet powder is transformed from the powdery state (3P) into a suspension state (2S). This is reered to as 3P2S transition. In real systems air is often still present but the system shows suspension like flow behavior with drastically reduced shear stresses (3S).

The transformed material in the die inlet region constitutes a shear zone along which material slides into the die. Before a steady state is reached this shear zone grows with time because of the lower flow velocity near the wall compared to the core. Within the scope of this IFPRI project hydrophilic wet powder systems were systematically investigated with respect to their flow behavior during extrusion and their microstructure as a function of process and material parameters. As solids the silica powders SEPASILr B 5/63, SEPASILr B 20/100 and SIKRONr B 800 and the glass ballotini SPHERIGLASSr 3000 were used. They vary in particle size distributions, particle shapes and in solid-liquid interfacial tensions. Watery Nutrioser solutions with varying concentrations were applied as binder liquids. They show Newtonian flow behavior. In the experimental part of this project critical stresses were quantifed which have to be surmounted during ram extrusion before a steady state is reached. These stresses depend on process and material parameters and are a measure of the flowability of wet powders. It was shown that shear in the barrel and the die can be neglected. The die length has no impact on flow profiles and on critical stresses. The ability of a wet powder to flow can be significantly improved by subjecting them to a mechanical energy input. Shear helps to cover the particles by uniform liquid layers resulting in reduced interparticle friction. Uniaxial compaction can improve the flowability if air cells are compressed to such an extent that the saturation exceeds about 80%. However, in case of kneaded wet powder systems no reduction of the resistance against flow by uniaxial compaction is observed. At low strain rates there is no impact of the binder liquid viscosity on shear stresses.

Throughout the course of this grant, we have carried out extensive experiments, and to a certain extent, theoretical studies, that pertain to the flow of grains (typically tri-disperse disks) in a quasi-2D hopper. This material was summarized extensively for the IFPRI-NSF Collaboratory, and the present report is based largely on that summary. I note that since the submission of the report for the Collaboratory, we have carried out additional studies of the velocity and density fields for our hopper flows. These studies use both particle tracking (PIV) and convolution techniques to obtain the velocity field on relatively short time scales, i.e. time scales short enough to capture detailed fluctuations. These fluctuations are of interest to the extent that they play a role in determining the overall flow properties. The two different velocimetry techniques yield data on finer and coarser spatial scales, respectively for the PIV and convolution methods. This work is nearing completion and will be part of a comprehensive manuscript that we are now preparing. Initial phases of this work were described in Jamming and Flow in 2D Hoppers, J. Tang, S. Saghdipour, and R.P. Behringer, Powders and Grains p. 515-518 (2009), ed. M. Nakagawa and S. Luding. It is also anticipated that the outcome of the IFPRI-NSF Collaboratory, which will be based, in part, on these studies will lead to an additional publication. This work has also been presented at roughly a half-dozen technical meetings (outside IFPRI). Finally, we have recently obtained a DEM code from Prof. Marcos Salazar (U of Bourgogne) which we are adapting to our hopper system. Hence, we will be able to also contribute to the modeling effort associated with the Collaboratory, and I anticipate using this code for the studies in the new IFPRI Grant.

Executive Summary

This is the final report on work performed on the IFPRI project during the period September 2005 through October, 2010. The research is focused on the study of Powder Mechanics and the ultimate goal is to develop a quantitative description of active flows for a wide variety of powders. The study is centered on the slow, frictional and the dense, “intermediate” regimes of flow where both frictional and inertial effects are important. The novelty of the project is the study of a large range of materials and several flow geometries to gain meaningful insight. We report on a series of materials from simple (round beds) to complex (fine, odd-shaped, elastic and/or compressible), used in a shear cell of the Jenike type, an axial-flow Couette, a centripetal geometry characteristic of a “spheronizer” and a hopper flow with a moving discharge (characteristic of a tabletting device) to measure stresses and porosity (void fraction), and their fluctuations as a function of geometry and shear rate.

The development of constitutive model 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 theoretical model to predict flow patterns, velocity and porosity distributions and forces on boundaries such as stationary walls. The PI is collaborating with several groups of simulators (DEM and MDS) and mathematicians to implement these models and compare theoretical results to computations as well as develop a continuous model “in house” based on commercially available software (FLUENT) and applied to the “Speronizer” geometry. Good results are reporter for the “fast” Jenike cell (collaboration with the group of Professor Sundar Sundarasen of Princeton University, see section 3.3.3.), the Couette device, using a continuous-type theory emplying FeatFlow, a Finite Element (FE) model developed by a group of mathematicians at the University of Dortmund in Germany (Professor Stefan Turek, see Appendix A).

Summary

During the period 9/1/07 – 8/31/10, $123,000 of IFPRI support was allocated evenly between the University of Michigan and the University of Delaware for support of experimental studies of the microstructure of gelling. In this report we discuss results that have identified: (i) how microstructure in colloidal gels of monodisperse particles is modified by application of stresses that lead to rupture and yielding; (ii) how microstructure in systems of polydisperse particles is affected by sedimentation and delayed collapse.

Introduction

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. In every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as a 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 nonuniform shape, the effects 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.

Scope and Significance

Particulate materials manufactures use daily particle size distribution estimation methods (such as sieving and laser diffraction). In the last decade, 2D static and dynamic image analysis tools have become increasingly widespread among the industry and scientific research. These techniques allow more than the analysis of particle size; they also give access to particle shape. The need for standard size and shape parameters for particle characterization is obvious, and it is important to draw the attention of the end user to a few important aspects of image analysis results: image resolution, comparison of results from different particle size ranges, pertinence of the used size and shape parameters, etc.

In this work, a discussion of existing size and shape parameters is carried out, and new parameters have been defined for 2D, as well as for 3D image analysis. Also, these parameters were used to investigate the relations between particle shape and particle size distribution results, and physical properties of powders.

Executive Summary

This report summarizes the work done over six years in IFPRI project 51, ¡§Multi-Scale Approach to Modeling and Control of Granulation Processes¡¨. This project covers the research efforts of 4 research students/post-docs at UCSB, as well as collaborative endeavors with both Imperial College, and the Procter and Gamble Company. The studies span the following technical sub-themes:

• Efficient computational methods for multi-dimensional population balance models
• Empirical methods for identification of granulation models for process control
• Model-based control design for a granulation process
• Pilot plant testing of model identification and control methods

The subspace modeling work in this study 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 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.