Particle Formation

Introduction

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 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.

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 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.

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.

Future Goals.

For basic industrial control of shape, the enhanced image analysis feedback signal may be all that is required. One can envision using even simple proportional feedback control to adjust the impurity level to push the crystal shape back to the desired form once an excursion is detected. Even with the purposeful deletion of unclassifiable crystals, our sampling rate and image analysis is adequate to maintain control of the model system. These operational difficulties will only become less troublesome as the video cameras and imaging software continue to improve. We wish to push the analysis further, however. We understand the role of the impurity in the crystal structure. The essential understanding of the crystal structure comes from studies by Sherwood and coworkers [S, 71. With this microscopic understanding, we can model the dynamics of the transition of crystal shape in a distribution of crystals. We are constructing a population balance model with two internal coordinates that tracks crystal shape changes. This model is applicable to typical industrial crystallization processes. As far as we know, these results are the first demonstration of real time control of crystal shape. Although serious challenges remain, and the prototype process is somewhat idealistic, we hope these results inspire practitioners to think seriously about application of these principles in suitable industrial processes.

The goal of this research is to measure and regulate the shape and size of particles created by nucleation and growth processes in crystallizers.

In the final year of the grant, we implemented feedback control on a semi-batch crystallization in which an impurity free feed flows through the crystallizer and we regulate the flowrate of a habit modifier stream in order to maintain a desired shape.

At the 2000 IFPRI Annual meeting, we showed our first results in which without any prior knowledge of model parameters, a simple proportional-integral control algorithm is able to maintain a desired crystal shape and in doing so, determines the critical concentration of habit modifier required to maintain this shape. The prototypical system and process we selected is semi-batch crystallization of sodium chlorate (NaClOs). Sodium dithionate (NazS20s) is a habit modifier that influences the relative growth rates of 100 and i ii faces of the crystal. In the presence of at least 50 ppm sodium dithionate the growth of the iii faces is blocked by the impurity and the crystal shape changes from cubic to tetrahedral. Without impurity present, the 100 faces grow slower than the iii faces and the crystal shape changes from tetrahedral to cubic. The shape change is easy to detect with video images alone, though there are limitations with extracting useful numerical information from images for use as a signal for feedback control.

This prototypical process displays the following industrial characteristics.

1. Particle shape is affected by unmeasured disturbance variables.
2. Online sensing is available in the form of video images. The images are replete with bad data. Some particles are fused or broken; it is difficult to obtain representative samples; particle boundaries overlap each other; there are significant levels of process noise; and it is difficult to sample enough images to remove the effects of this noise through averaging. The standard image analysis software provides simple measures such as particle boxed area and aspect ratio; as we show later, these simple measures are inadequate signals for feedback control.
3. We can manipulate a process variable that also influences particle shape. Through this feedback policy, we maintain the desired shape in the face of the unmeasured disturbances. The video images are processed in real time to produce the feedback signal that is used for control.

Accomplishments

1. Developed and implemented a repeatable prototype process for illustrating online crystal shape control.
2. Added a higher level functionality to the standard image analysis software and tailored it to detect transitions between cubic and tetrahedral crystals in a slurry.
3. Implemented feedback control and maintained a desired shape in a semi-batch crystallization process.

As far as we know, these results are the first demonstration of real time control of crystal shape. Although serious challenges remain, and the prototype process is somewhat idealistic, we hope these results inspire practitioners to think seriously about application of these principles in suitable industrial processes.

Problems and suggestions associated with a progress of particle technology toward better-defined materials – engineered particulates – are presented.

In the first part of this review, coating technology was categorized from various angles, e.g., purpose, principle, methodology or instrumentation.

Dry, physical methods are divided into mechanical and sputtering, and spray coating inbetween. Typical machines and techniques are displayed. Emphasis is laid on the mechanochemical effects involved in the mechanical treatments. Deliberate introduction of mechanochemical reaction during coating is explained with some concrete examples.

Wet chemical methods are then described in detail. After explaining surface modification by surfactants or coupling agents, various colloid chemical routes are explained. Use of in situ reactions and related micro encapsulation are then referred.

Importance of characterization from non-conventional points of view, e.g., homogeneity of the coating between a lot, within a lot, within a treating chamber or vessel in a machine or even within a single particle is emphasized with concrete examples of analyses. Evaluation of chemical interaction at the host-guest interface and within a guest or shell layer are refined as well.

Examples of particle coating application are given from various area including electronics, optonics, magnetic materials and pharmaceutics. Representative case studies carried out in the author’s own laboratory are introduced. They will cover most of the categories of coating referred above.

Throughout the entire review, the author pays special attention on the microscopic and chemical views to fabricate and to evaluate the well-defined coated products to fulfill the concept of the modern engineered particulates.

Executive Summary

Uniform anatase-type TiOl nanoparticles were prepared by Gel-Sol method, in which a condensed aqueous solution of Ti-triethanolamine (TEOA) complex is first aged at 100 “C for 24 h for the hydrolysis of the Ti-TEOA complex to Ti(OH)4 gel network, and then aged at 140 “C for 72 h for the nucleation and growth of the final product by gradual dissolution of the Ti(OH)4 gel. The Ti-TEOA complex was previously prepared by mixing titanium isopropoxide (TIPO) directly with TEOA in a dry box at a molar ratio 1:2. Typically, uniform TiOz particles of ca. 21 nm in mean diameter were obtained by aging an aqueous solution of 0.25 mol dm” TIP0 stabilized with 0.50 mol dmT3 TEOA (initial pH = 9.5) at 100 “C for 24 h, followed by aging at 140 “C for 72 h. Effect of pH on the particle size was remarkable, since the mean diameter of cuboidal particles increased from ca. 5 to 30 nm with increasing pH from 1 to 11.5. However, no TiO2 particles were obtained over pH 12 even after the 2nd aging for 72 h. If TIP0 is not stabilized by TEOA in advance, Ti(OH)4 flock is formed instantly on mixing TIP0 with water at room temperature, and it is completely converted to ill-defined polydispersed TiOl particle of ca. 9 nm in mean diameter after the 1 st aging at 100 “C for 24 h. From the reduction of the reaction rate with increasing pH, irrespective of the presence or absence of TEOA, the increasing particle size with pH is basically elucidated in terms of the reduction of the nucleation rate by the lowered concentration of precursor complexes to TiOZ particles with increasing pH. In addition, this pH effect on the final particle size of TiOz was pronounced by the presence of TEOA, since TEOA liberated after the 1st aging significantly lowered the solubility of the produced Ti(OH)4 gel by adsorption, and since this etTect of TEOA was 111 enhanced with increasing pH. The particle size was also varied systematically by adding different amounts of seeds. Dramatic increase in the formation of TiOz with the increasing amount of seeds provided us with information that the dissolution of the Ti(OH)4 gel is not the rate-determining step of the particle growth of TiOz, and that the particles are grown by deposition of monomeric solute and not by aggregative deposition of particulate matters such as hypothetical primary particles of TiO2. It was also found that the particle shape was changed from cuboidal form to rod-like one when the pH was increased to 11 S. This result was explained in terms of the adsorption of TEOA to the crystal planes parallel to the c-axis of the anatase crystals.

As an application of thus-prepared well defined TiOz nanoparticles, their catalytic performance as a photocatalyst for water photolysis was studied. For this purpose, a special reactor was designed for precise measurement of the quantum efficiency of the photo-excited electrons and holes. Using this reactor with Pt/TiOz catalysts prepared by the selective deposition of Pt onto TiOz particles precisely controlled in size and shape, we performed preliminary experiments for the effects of the particle size and shape of TiOz, Pt loading, pH, atmospheric pressure, and concentration of electrolyte on the quantum efficiency. As a consequence, we found significant effects of these factors. However, we also found that the most imminent issue to be resolved is the surface modification of the Ti02 particles to introduce separate electron and hole trapping centers for the prevention of the drastic recombination of photoelectrons and holes.