Particle Formation

Executive Summary

Agglomeration or granulation, as the name implies, is a process by which larger (millimeter or fractions of millimeter in diameter) granules are produced from fine (micron sized) powders in a mechanical agitator such as a drum, pen or high shear mixer or in a fluidized bed. Particle growth in these devices is facilitated by the use of a binder, i.e., a sticky fluid, a solution or a melt which upon dispersion in the powder mass and subsequent solidification at the interstices between particles generates stable granules. While powder size increase (agglomeration or granulation) is a widely used unit operation, few underlying physical principles describing the phenomena have been drawn. Successful granulation operation is therefore a largely haphazard undertaking. The present research attempts to lay a rational foundation to describe the mechanics of granulation by examining the process at the level of particle-binder-particle interaction, at the so-called microlevel.

The ultimate goal of the present work was to build a granulation model to predict granule size and growth rates from first principles using the properties of the powder, the binder and the characteristics of the mixer. It was discovered early in the project that liquid (binder) bridges formed between moving solid particles are the key to understanding of the many different processes taking place during granulation. It was also found that the study of these bridges, although attempted as far as their behavior with regard to surface tension effects is concerned, is not sufficiently developed and hence, a basic study of viscous effects in moving liquid bridges was undertaken. Furthermore, the phenomenon of particle coalescence and growth was studied using the theory of viscous liquid bridges developed earlier and regimes of granulation were defined in which both the growth rate and the limiting particle (granule) size were calculated. Three such regimes were identified, each characterized by a different dependence of the growth rate on such parameters as particle size, binder viscosity and surface tension and other parameters of lesser importance. All these quantities were incorporated in a dimensionless so-called Stokes (or Reynolds) number, characteristic values of which in turn delimit the different regimes.

Finally, the existence of the different growth and granule consolidation regimes was tested by experiments specially designed to isolate the important phenomena in question for each regime. As it clearly appears from the present work, the theory of coalescence regimes as presented above is only a framework which provides us with some basic insight into the phenomena of granule growth and consolidation but is not in fact a comprehensive model of granulation (although an attempt was made, to incorporate the above findings into a theory of defluidization of fluidized beds). One of the major practical achievements of the present work was the development of an instrument to characterize binders used in granulation and to measure binder strengthening times. These measured characteristics were then used during pilot scale fluid bed and drum granulation experiments to predict limiting granule diameters.

The present work did not provide a final solution for granulation theory but rather opened the field, presented a preliminary general framework and established the important lines of inquiry to be followed in the future. First and most important, is the measurement and/or prediction of shear forces in a mixer. This is essential since the knowledge of these forces is key to developing a comprehensive granule growth model by equating the disruptive and cohesive forces in the device. Introduction of particle and granule breakage into the overall theory through fracture mechanics is also paramount especially in such devices in which some drying of the granules occurs such as a fluid bed granulator or where the disruptive forces are very high such as in high shear mixers.

Executive Summary

1991-1992 began the first year of our three year study into the fundamental mechanisms responsible for effervescent atomization. Three issues were addressed during this period.

Early in the year we focused on spray behavior in the transition region where the two-phase flow that exits the nozzle as discrete gas bubbles in a continuous liquid is transformed into a continuous gas stream containing discrete liquid drops. Single-pulse holography was employed to observe the liquid breakup phenomena characteristic of effervescent atomization. A qualitative explanation of the mechanisms responsible for effervescent atomization is presented, based on this data. A quantitative model allowing calculation of spray mean drop size from nozzle geometry, operating conditions, and fluid rheology will be developed this year.

At mid-year we broadened our efforts to include an investigation of the interactions between the spray and its surroundings. In particular, we were interested in methods for improving the already superior energy efficiency of effervescent atomizers by minimizing the impact of the major energy loss mechanisms. A computational study was therefore performed to identify these major loss mechanisms and suggest methods for reducing their impact. We discovered that turbulent dissipation was the largest loss, followed by transformation of bulk kinetic energy into turbulent kinetic energy, and then entrainment of surrounding air. We concluded it was unlikely that turbulent dissipation and transformation of bulk kinetic energy into turbulent kinetic energy could be reduced. However, entrainment might be minimized by forming a more uniform distribution of smaller bubbles in the two-phase jet as it exits the nozzle. We will investigate spray-surroundings interactions more fully in 1992-1993.

Our most recent efforts have focused on the relationship between fluid rheological properties and spray mean drop size. This work was motivated by our previous study into the influence of polymer addition on nozzle performance. One result of that study was the conclusion that spray mean drop size was independent of changes in either consistency index or flow behavior index for a power law fluid. This year’s results show that it is fluid viscoelasticity that degrades nozzle performance. We will next develop a quantitative model that describes the performance of an effervescent atomizer when operating with viscoelastic fluids.

Executive Summary

• Previous annual reports concentrated on the presentation of experimental data relating to the behaviour of fluidized beds of catalyst (Group A) powders. These related to the influence of mean particle size, addition of fines, type of gas, and temperature, on bubbling, bed expansion/density, minimum fluidization and mimimum bubbling velocities/voidages, and bed collapse characteristics.
• In this report we focus attention on the theoretical background of non-bubbling fluidized/aerated beds of powder in order (a) to provide a sound foundation for the interpretation of our experimental data and (b) to allow generalisations to be made for other powders and operational conditions.
• Both hydrodynamic and interparticle forces play a role in the behaviour of fine particle fluidization, with the latter assuming increasing importance as the mean particle size of the catalyst powder is reduced below about 70um. The van der Waals forces are evaluated with respect to particle size, particle roughness, and, in particular, gas adsorption. It is shown, theoretically and experimentally, that the use of CO2 and other strongly adsorbing gases at room temperature causes a considerable increase in the interparticle forces, to the extent that the catalyst can not be fluidized. As expected, the effect of adsorption disappeared at temperatures above about lOOoC, and this finding has consequences for experimental work in cold models, especially at high pressures.
• The trends shown by the experimental data are, on the whole, in accord with the theoretical predictions; however, because of the lack of fundamental data on, for example, the size of asperities on the particles, the Hamaker constant for FCC, (and the change in the value, if any, with temperature), it is not possible to make accurate quantitative predictions.
• By using experimental data, theoretical equations, and dimensional analysis, semi-empirical correlations which include powder cohesion have been developed. Work to improve these is continuing but much remains to be done.

Executive Summary

The primary objective of the Caltech program supported by IFPRI is to understand the dynamics of particles which do not coalesce immediately upon coagulation and, through that understanding, to guide the development of processes for production of particles with particular properties. Theoretical and experimental investigations of the dynamics of aggregate aerosols have been undertaken with IFPRI support. Following on our studies of the properties of agglomerate particles, we have developed models to describe the kinetics of agglomeration. The kinetics of agglomeration are determined by the mobilities of the agglomerate particles and by their collision cross sections. The lower density of agglomerates has competing effects on the coagulation kinetics &s compared with dense particles of equal mass: (i) the aerodynamic drag on the particles is increased due to the larger size of the particle of the same mass; (ii) the low density tends to increase the collision cross section of the agglomerate over that of the dense sphere. We have used fractal scaling concepts to evaluate the relationship between these measures of particle size and the particle structure.

The dynamics of aerosol formation and growth is well documented for particles that remain as dense spheres throughout their growth. We have generalized those models to account for aggregate structure, and have developed the first model for the collision frequency function that spans the entire range of particle Knudsen numbers and fractal dimensions. The particle size distribution is shown to approach an asymptotic form when the primary growth mechanism is coagulation, This asymptotic form is the so-called self-preseting particle size distribution. A dynamic exponent is defined that describes the rate of growth of the mean particle size. The dynamic exponent is shown to pass through a minimum in the transition regime, behavior that has not previously been described. Essential features of this asymptotic solution are observed experimentally, although direct comparison between experiment and theory is complicated by the transition between coalescing coagulation and agglomer&ion that took place in our experiments at about the same particle size as the transition between the free molecular and continuum size regimes.

A more complete description of aerosol agglomeration was made in numerical solutions to the coagulation equation using a modified sectional code. Comparison of the collision frequency functions of spheres and agglomerate particles reveals that the predominant effect is the increased collision cross section of the agglomerate, leading to a dramatic increase in the collision frequency of agglomerates as compared to dense particles of the same mass. Simulations that assume that particles coalesce completely up to a limiting size and do not coalesce at all beyond that size exhibit a broadening of the size distribution at that transitional size. Experimental evidence of that broadening is provided. The comparisons at this point can be considered only provisional in as much as direct experimental validation of the collision frequency function is still lacking.

Experimental studies have been conducted to explore a number of aspects of the evolution of aggregate aerosols. The structure-drag relationship has been explored by image analysis of transmission electron micrographs of mobility classified aggregate particles. For the low fractal dimensions of the aggregates studied (Df N 1.8), the drag on the particle scales well with the projected area. This result holds through most of the transition regime even though it is expected to be strictly valid only for particles much smaller than the mean free path of the gas molecules.

Structural rearrangements during sintering of aggregates have been explored by heat treating aggregate particles while they are still entrained in the carrier gas. Particles are observed to retain the appearance of fractal clusters of smaller primary particles throughout coalescence that leads to many primary particles from the original particle being incorporated into a single primary particle in the sintered agglomerate. This observation raises serious questions about the inference of mechanisms of particle growth from structure measurements alone.

EXECUTIVE SUMMARY

The formation mechanisms of uniform, submicron, inorganic particles were explored in this three year study. This project was initiated after a detailed literature review demonstrated that insufficient data was available to provide the basis on which to test various growth models. A research program was developed to explore growth mechanisms of a variety of systems such that common themes resulting in narrow size distributions could be extracted. This program sought to determine if uniformity is the result of details of bond breaking and making during the chemical reaction or if there is an underlying physical chemistry which must be satisfied for uniform precipitates to be formed. Following a general summary and summaries from each material studied, a detailed discussion is presented.

1. In conjunction with US Department of Energy supported research, three experimental systems were investigated; i) silica from hydrolysis and condensation of tetraethylortho-silicate, ii) titania from hydrolysis and condensation of tetraethylortho-titanite and iii) gold from the reduction of auric acid with sodium citrate.

2. In each system, particle growth rates, rates of loss of solution phase metal containing species and colloidal properties of the growing particles were characterized. Particular attention was paid to developing methods of distinguishing between growth by molecular addition and growth by aggregation of smaller particles. Kinetic models were developed which linked particle growth with rates of reaction in solution and the effects of variables such as precursor concentration; ionic strength and pH.

3. This research program demonstrated that the LaMer model for the formation of uniform particles is rarely, if ever, followed. Indeed, developing reactor schemes based on the LaMer model can result in focusing on the wrong control variables.

4. The central result of this research program is that uniformity is a consequence of particle interaction potentials which act to generate a constant number density of colloidally stable particles early in the precipitation reaction. This result suggests that in the development of reactor conditions resulting in uniform precipitates, the correct control variable set must include factors which influence colloidal stability.

5. This research program demonstrates the importance of understanding size dependency of heterocoagulation and the interaction potentials of sub-10 nm particles. In particular, the ability of small adsorbed molecules to provide a steak barrier which prevents aggregation into the primary van der Waals minimum has been demonstrated: a result of tremendous technological significance.

In this IFPRI funded project we investigate the role of short range forces in controlling the morphology of sub-micron precipitates. Our progress can be summarized as:

1. Silicotungstic acid (STA) has been chosen as a model colloidal particle to mimic the properties of primary particles (nuclei) in precipitation reactions involving metal oxides.
2. STA forms crystal hydrates where the number of waters of hydration per STA molecule varies with the relative humidity of the vapor with which it is in equilibrium.
3. The waters of hydration of STA crystals are lost in discrete steps in solid/solid phase transitions involving changes in the crystal lattice.
4. The locations of the phase transitions can be related to the osmotic pressure which would have to be applied to hold the crystal at a particular solid’s volume fraction if it were equilibrated with pure water. These osmotic pressures are large (the highest hydrate is not formed until an osmotic pressure of approximately 300 atmospheres is applied at 25°C).
5. These studies demonstrate that large driving forces must be applied to squeeze solvent from between small metal oxide particles with a strong affinity for the continuous phase. Resulting from the small particle size and the particle/continuous phase affinity is a strong, short range repulsion which will completely dominate over electrostatic and Van der Waals forces.
6. Future studies will involve developing measures of the spatial dependence of the solvation interaction energy and the effects of varying continuous phase/particle affinity.
7. Our findings suggest that for precipitation reactions producing particles with a strong affinity for the continuous phase, classical nucleation events may never occur. The implications of this concept require better understanding of the role or solvent/particle interactions in controlling particle interaction potentials and molecular attachment rates to growing particles.

ABSTRACT

The effect of solvent surface tension, pH of water and surface chemistry of particles on agglomerate strength was studied using silica and titania agglomerates washed with different solvents. Aprotic solvents of various surface tension and water at various pH (with and without electrolyte) were employed to separate the effects of capillary pressure and particle-particle condensation reactions. The wetting behavior of the solvent was studied using both the imbibition method and the modified wilhelmy plate technique. The two techniques were compared to determine their limitations when applied to spherical particles. Agglomerate strength was determined using a calibrated ultrasonic field, mechanical shear and isostatic compaction. Organic groups present on the particle surface were identified using infra-red adsorption and 13C NMR and the effect of the groups on agglomerate strength was discussed. The strength and strength distribution of the agglomerates formed was found to depend on the solvent surface tension, wetting characteristic, pH and surface chemistry of the particle. The presence of electrolyte in the solvent was found to effect the extent and strength of agglomeration, specially at higher pH. Agglomerate strength was also found to increase with increasing capillary stresses, and pH of water.

Executive Summary

The goals of this research program for the period 1991 through 1994 are:

1. to identify the fundamental mechanisms responsible for effervescent atomization,
2. to quantify the impact that variations in the two-phase flow pattern at the nozzle exit have on the mechanisms responsible for effervescent atomization,
3. to formulate a theoretical model for the transition from a dispersed gas in liquid system, i.e. bubbly or slug flow in the nozzle, to a dispersed liquid in gas system, i.e. spray,
4. to develop a model describing the evolution of droplets as they propagate through a series of shock and expansion fronts.

Work during 1992-1993 focused mainly on identification of the fundamental mechanisms responsible for effervescent atomization, and on the relationship between internal two-phase flow and the mechanisms responsible for effervescent atomization. Both Newtonian and non-Newtonian fluids were considered.

For the Newtonian fluids, we were able to show that spray mean drop sizes decreases rapidly & lower air-liquid ratios because the near nozzle spray structure evolves from a sequence of single bubbles undergoing rapid expansion to an annular tree as air-liquid ratio is increased. The latter results in large caps of liquid which do not break up into small drops. We were also able to show that increasing air-liquid ratio above about 10% (on a mass basis) will lead to only a slight increase in nozzle performance because no further evolution of the annular tree structure is accomplished.

For the non-Newtonian fluids, we have shown that it is the presence of fluid viscoelasticity that degrades atomizer performance and have provided a preliminary correlation between viscoelasticity and atomization. We have also developed a performance map relating polymer molecular weight, polymer concentration, and mean drop size to serve as a rough guideline when applying effervescent atomizer to non-Newtonian systems.

We also considered spray-surroundings interactions during 1992-1993. In particular, we have measured the rate at which effervescent sprays entrain surrounding air using a direct technique. It was found that the rate of entrainment increases linearly with axial distance and depends strongly on the initial gas-to-liquid mass flow ratio (GLR). Entrainment results were correlated using a dimensionless group based on spray momentum flux. The correlation indicates that spray structure may significantly affect entrainment behavior. The correlation may be used to predict entrainment in effervescent atomization of low viscosity fluids.

Executive Summary

The primary objective of the Caltech program supported by IFPRI is to understand the dynamics of particles which do not coalesce immediately upon coagulation and, through that understanding, to guide the development of processes for production of particles with particular properties. Theoretical and experimental investigations of the dynamics of aggre- gate aerosols have been undertaken with IFPRI support. Following on our studies of the properties of agglomerate particles, we have developed models to describe the kinetics of agglomeration. The kinetics of agglomeration are determined by the mobilities of the ag- glomerate particles and by their collision cross sections. The lower density of agglomerates has competing effects on the coagulation kinetics as compared with dense particles of equal mass:

• The aerodynamic drag on the particles is increased due to the larger size of the particle of the same mass;
• The low density tends to increase the collision cross section of the agglomerate over that of the dense sphere.

We have used fractal scaling concepts to evaluate the relationship between these measures of particle size and the particle structure. Our theoretical investigations suggest that the collision frequency of aggregate particles is enhanced over that of dense spheres of equal mass.

This report describes experiments that are being undertaken:

1. To probe the structural rearrangements that take place when aggregate particles sinter;
2. To measure the col- lision frequency for aggregate particles as a function of particle size and structure.

The ex- periments are performed using model particles: bispheres for idealized sintering experiments and aggregates produced by low temperature aerosol coagulation for studies of aggregate sintering and aggregation kinetics.

Executive Summary

The primary objective of the project is to model and experimentally verify the fundamental aspects of mechanically agitated granulation processes using binders. Specific aims will be to develop predictive models which take into account the chemical and physical properties of the material components (substrate and binder), and probe critical scale-up factors.

In the initial period of the programme of work, focus is on a literature review, with particular attention paid to solid; liquid interactions and in particular how these can be examined via surface free energy approaches. The review, which forms the major component of the report, demonstrates the potential for predicting solid-liquid interactions from surface free energy considerations and identifies the critical role played by such material interactions in the granulation process.

Initial experimental work has involved the commissioning and testing of a new mixer torque rheometer (MTR). The MTR has been shown to be a valuable tool in monitoring directly and in real-time the rheological properties of the wet massing substrate:binder system under test. The new MTR has provided improved facilities, and control via a dedicated microcomputer, and initial practical work has demonstrated the versatility of the instrument.