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 observe the development of localized inhomogeneities that are likely to be associated with the onset of clusters.

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.

This first annual report describes the experimental apparatus, outlines a theory and computer simulations to predict the flow, and specifies microgravity requirements for its implementation in Space.

In the IFPRI Annual Report of (1998), we described a novel method for selective deposition of nanometer-size metallic Au particles (ca. 1 nm) onto monodispersed well-defined metal oxide particles simply by heating a HAuCl, solution around pH 6 at 100 “C, without addition of any specific reducing agent. This was found to be a kind of catalytic reaction of the metal oxides. Similarly, we tried to extend this technique to the selective deposition of noble metal nanoparticles of the platinum group (Ru, Rh, Pd, Ir, and Pt) onto well-defined metal oxide particles, such as a-Fe,O, a-FeOOH, B-FeOOH, TiO, and ZrO. However, in contrast to the gold system, the noble metals in the platinum group were found to deposit onto the supports in the form of oxide or hydroxide by the aging at 100 “C, without being reduced to metallic particles. Metallic particles (l-4 nm) were finally obtained by reduction of the hydrous(oxide) precursor particles on the support powders with H, gas at 250 “C. The support particles in this case were found to play a decisive role in the acceleration of the selective deposition of the precursor particles and in the final formation of dense and well-dispersed metallic nanoparticles of the noble metals. In general, the specific surface area and surface roughness of the supports and their affinity to metal particles were found to be the decisive factors for obtaining well-dispersed metal particles by inhibiting the aggregation and/or sintering during the reduction process, Above all, the combination of Pt and monodispersed ellipsoidal TiO, particles (anatase) with a large specific surface area and a rough surface yielded well-dispersed nanoparticles of Pt as small as 1.3 5 0.5 nm. When it was used as a catalyst for hydrogenation of 1-octene to octane, an outstanding catalytic activity was shown over the other combinations. The Pt/TiO, catalyst prepared by the precursor deposition method in this study was superior to other Pt/TiO, catalysts prepared even by the best conventional methods, such as the ion-exchange method or the impregnation method.

We are now planning to study the correlation between the structural factors of heterogeneous catalysts and their catalytic performance, mainly as optical catalysts using new Pt/TiO, composite particles with nanosized titania supports precisely controlled in size, crystal habit, and crystal structure.

PVC ([CHZCHCI] n) powder with fine CaO and/or Ca(OIQ powder and PVDF ([CH$ZF&,) powder with NaOH powder were ground in air by a planetary ball mill to investigate their mechanochemical reactions. The ground mixture was washed with distilled water to remove soluble compounds in the ground product by filtration. Reaction yield was determined by measuring the concentration of halogen in the filtrate. In addition, for the system of PVDF-NaOH, the filtrate containing soluble organic compounds was regulated with HCl solution to decrease its pH to 2, then ethyl acetate was put in the filtrate to extract the organic compounds.

All the same, the grinding causes dehalogenating reaction, forming CaOHCl and polyetylene ([CH=CI&) from the former system, and NaF and organic phases such as [CH2-C=O], and [GC] from the latter one.

For the PVC and CaO/Ca(OH)Z system, the grinding enables to accelerate the dehydrochlorinating reactions between. However, the reactivity of CaO is superior to that of Ca(OHJ2, due to the formation of Hz0 in the PVC-Ca(OH)z system. PVC is transformed into partly dehydrochlorinated polymer, while CaO/Ca(OH)2 is changed into chloride form. The mechanochemically formed CaOHCl in the ground mixture can be removed out by washing with water. The dechlorination is improved with an increase in the molar ratio of (CaO/PVC) as well as grinding time. The impact energy of balls simulated by the PEM would be one of the important parameters governing the mechanochemical reaction between PVC and CaO/Ca(OH)2.

Mechanochemical reaction between PVDF and NaOH proceeds rapidly to transfer nearly half of total fluorine into NaF through the displacement of fluorine in PVDF by OH base. When both fluorine bases bound to the same carbon atom are replaced by OH-base, the dehydration takes place to generate water, which plays a significant role to cause the strong agglomeration of fine particles during grinding. About 90% of fluorine in PVDF can be transferred into NaF by grinding. Dehydration of polar CH2C(OH)2 base proceeds in two ways to form carbonyl (C=O) base and carbon double binding. This gives us water soluble and insoluble organic compounds formed in the ground product.

This report summarizes the activities conducted under IFPRI support from November 98 to November 99.

The main target of our work is to understand, characterize, and quantify the behavior of powders in the early stages of compaction attendant to the intrinsic properties of the material and the processing conditions. The project involves theoretical, numerical, and experimental components. It is framed within a collaborative effort with concentration on pharmaceutical manufacturing, but the results are relevant to powder compaction in many other industries.

The topics addressed in this report include:

• the micromechanics of particle rearrangement (Section 2);
• the energetics of particle rearrangement (Section 3);
• the effects of die roughness and particle deformability (Section 4);
• a brief precis of ongoing experimental work (Section 5).

In the Introduction these topics are motivated in the context of the overall goals of the project.

The main aim of this Lancaster University-Bradford University collaborative project is to understand the forces between a variety of dry materials at the single particle level, to relate these to the complimentary bulk powder flow measurements, and hence assess how far such single particle data are able to predict flow behaviour of real value to chemical engineers. In particular, we are interested in (a) the role of ambient conditions such as relative humidity, and properties such as particle size, roughness and surface condition, and (b) the role of both particle-particle interactions and particle-wall interactions, in flow and adhesion behaviour.

The selection of particulate material for recent study has been determined mainly by the following criteria:

1. Availability as particles of controlled shape, size, roughness and surface chemical condition, for use as model systems for adhesion and flow studies;
2. For single particle studies, the extent to which the bulk flow and compaction properties of the material had already been studied in testers at Bradford and in laboratories elsewhere;
3. Suggestions to us from other IFPRI members for materials of particular industrial relevance and interest to the powder flow community.

The details of the force-curve technology, using a commercial AFM, the details of the bulk powder flow experiments, sample preparation, humidity control, and other experimental details have been extensively described in previous IFPRI annual reviews, and will not be repeated here. To achieve the objectives stated above, our work during the first half of this third year (December 1998-May 1999) has fallen into three main areas:

1. At Lancaster, single-particle adhesion studies in well-defined model systems comprising relatively large glass spheres and flat surfaces, and related systems, using the force-distance software available with the Topometrix Explorer AFM.
2. Adhesion studies, using force-distance curves, of more cohesive and finer powders, mainly those whose bulk cohesion properties had already been extensively studied in several testers at Bradford and elsewhere, or powders of particular industrial relevance (zeolite, hydrated alumina). In particular, we hope to relate the effects of particle size, ambient conditions, and powder-wall adhesion noted in the bulk experiments to any difference we see in the single particle or small-scale adhesion experiments.
3. At Bradford and elsewhere, the continued improvement and standardisation of the bulk powder flow experiments in the Warren Spring-Bradford Cohesion Tester (WSBCT) using a small range of well-characterised powders, and in particular by comparative studies with several other testers in different laboratories.

Many details of the work in (1) - (3) above have already been described in our previous annual report (Year 2, ending 14 November 1998) or in the report for the 1999 AGM. Only the main conclusions, and some details not discussed in the Year 2 Report, will be given in this report. Ongoing experimental work on model systems, involving the adhesion between glass spheres and flat surfaces, and fitting the experimental results to theoretical models, is being pursued in collaboration with Dr J A S Cleaver and colleagues in the Department of Chemical and Process Engineering at the University of Surrey. The program of comparative studies of bulk powder flow and cohesion using various testers is now essentially complete and includes work at Albi in the laboratory of Professor J Dodds, using a ring-type shear cell developed by Schwedes. (Once certain software problems have been solved, we plan to include also data obtained at Delft data using the bi-axial cell being developed in Professor B Scarlett’s laboratory). The force curve studies at Lancaster on cohesive powders have been directed recently towards answering questions of more particular relevance to bulk cohesion testing, such as the role of particle-wall adhesion or particle size effects, but we have an ongoing interest in explaining some of the more unusual aspects of the force curves, e.g. humidity effects and long-range forces, in terms of the fundamental interactions involved.

The main innovation since the June 1999 AGM has been the introduction of single- particle frictional force measurements using the AFM to complement the normal adhesion measurements. Frictional forces are just as important to powder flow as these adhesion forces which we have studied exclusively in the first 2 years of the project, but we had not anticipated this major development to any extent in our “forward look” from previous reports. Cohesive materials studied so far include hydrated alumina and a limestone powder used as a standard material to calibrate cohesion testers. In particular, we are attempting to correlate single particle friction- load data with the corresponding shear stress-load plots from bulk cohesion testers. To this end, we have begun discussions with theoreticians at Surrey (Professor M Ghadiri, Dr S J Anthoni, Dr A Sharif) and elsewhere who have performed distinct element analysis to construct models for the statistics of transmission of normal and shear forces through a powder assembly. These models will be essential to provide the necessary links between single particle and bulk cohesion and flow. The current project has been granted a l-year extension, and it is expected that friction and modelling studies of this type will form the bulk of the last year of the project, to November 2000.

This year research on the rheological behavior of concentrated suspensions has focused on the theoretical description of shear thimling and thickening and the developmenb of the accelerated Stokesian Dynamics (ASD) method. The goal is to provide a quantitative microstructurally-based description of suspension rheology for a range of concentrations and particle- level interactions. The problem is addressed by both analytical and computational (simulation) approaches. Not all of the work described here has been supported by IFPRI; for example, the work on RSD has not. The report is intended to give an integrated view of our current efforts.

The process of disintegration of liquid/solid suspension sheets and jets is analysed in a fundamental manner and visualized by suitable measurement methods which allow qualitative and quantitative evaluation of the process. Supporting numerical analysis and theoretical derivations will contribute to a basic understanding and control of the suspension atomization process. Model suspensions based on water and water/glycerol mixtures with various suspended particles will be atomized by means of conventional and specifically designed atomizers.

The first year activities which are reported here include:

• Experimental investigations of atomization in rotary atomizers
• Stability Analysis and experimental investigation of twin fluid atomization

In order to provide a necessary data base for analysis and comparison, in the first year mainly the fluid behaviour has been studied.

Agglomerates of fine particles are pervasive in industry. In many particle processing applications, the agglomerates are carried within a suspending fluid, and hydrodynamic shear is applied to break the agglomerate into fragments and to distribute the fragments throughout the suspending media. The underlying purpose of this research it to obtain a fundamental understanding of the various factors that influence this dispersion process. Such information can lead to the development of interfacial engineering strategies aimed at improving the outcome of dispersion processes, or to better design of dispersion equipment.

Our general approach is 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.

The specific focus of the work supported under the IFPRI grant involves investigation of how certain time-dependent (dynamic) behaviors can influence the outcome of dispersion processes. Dynamic effects can arise in several facets of the dispersion process. For instance, in practical processing equipment, complex shear histories are inherent. Also, the wetting and spreading of fluids associated with contacting particles and agglomerates with processing fluids are dynamic effects. Finally, for some materials, dissolution of the solids plays a significant role.

For 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 of this project, we have continued research into mixing and segregation behaviors in tumblers and shakers. Among the key results are the following.

1. We have determined that different blenders of industrial design share common dynamical behaviors, including

(a) extremely sharp transitions between segregation states, that are observed across wide length scales.

(b) a cut-off size ratio beyond which segregation mechanisms appear to shut down.

The existence of dynamical similarity across length scales (and in different geometries) opens up the possibility to produce new scaling relations for the practical scale-up of blenders of common design. The presence of a cut-off indicates new prospects for designing products to mitigate segregational tendencies.

2. We have identified previously unreported segregation phenomena due to differential particle density in shakers and have begun study of density-induced segregation in tumblers.

3. We have established that particle shape can have a significant effect on blending behavior for coarse grains. In particular,

(a) particles with different shape but nearly identical size and density segregate in tumblers and shakers. Combined shape- and density- variations produce novel and poorly understood effects;

(b) we have identified a new regime of behavior in which shape-induced segregation is reversible -- i.e. particles of different shape can be either mixed or separated at will.

4. We have developed cellular automata models that successfully predict segregation in rotating drums, rocking drums, drums tumbled end-over-end, and V-blender shells.

This report summarises progress in IFPRI project 37 in 1998/99. Significant progress has been made in studying (1) wetting and nucleation, and (2) consolidation and growth.

Ex-granulator Spray Flux Studies

Ex-granulator spray flux studies are complete. This study focuses on the nucleation zone, which is the area where the liquid binder and powder surface come into contact and form the initial nuclei. An equipment independent parameter, dimensionless spray flux ‘I‘, is defined to characterise the most important process parameters in the nucleation process: solution flowrate, powder flux, and binder drop size. Experiments with red dye and image analysis demonstrate that changes in dimensionless spray flux correlate with a measurable difference in powder surface coverage. Nucleation experiments show that spray flux controls the size and shape of the nuclei size distribution. At low ‘I”, the system operates in the drop controlled regime, where one drop forms one nucleus and the nuclei size distribution is narrow. At higher ‘I”, the powder surface is cakes creating a broader size distribution. For controlled nucleation with the narrowest possible size distribution, it is recommended that the dimensionless spray flux be less than 0.1 to be in the drop-controlled regime.

Drop Penetration Studies

Drop penetration studies are ongoing. A relatively simple drop penetration model accounts well for the effect of liquid properties on penetration time. However, the effect of powder bed structure is more complex and harder to predict quantitatively. This is the subject of ongoing work.

Coalescence Criteria for Deformable Granules

A new coalescence criteria for deformable granules has been developed. Granules may coalesce without plastic deformation (type I coalescence) or with plastic deformation (type II coalescence). The model helps explain a number of different observed granulation behaviours, identifies theoretically the key controlling groups for granule growth and gives a theoretical underpinning to the granulation regime map. The theoretical locations of the granule growth regime map boundaries proposed by Iveson and Litster (1998a) were also analysed leading to a better-quantified and improved regime map. Drum and mixer granulation data for a range of materials was used to validate the regime map.

Drum Granulation Data

The drum granulation data fitted the regime map well. However, the data from the large-scale mixers had Stokes deformation numbers Stdef which were several orders of magnitude too high. Hence the map is a useful tool for comparing the granulation behaviour of different materials in the same device. However, until we have a better understanding of the flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment. More work is required to better characterise the flow patterns and impact velocities in mixer type granulators.

New Techniques

Two new techniques have been developed to characterise the mechanics of wet powders and granules. These techniques will be used to characterise a wide variety of formulations in year 3 of the project.

Research Areas for 1999/2000

The report outlines the research areas for 1999/2000 as well as discussing longer-term research goals.