ARR - Annual Report

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.

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.

The importance of explicitly considering interparticle interactions in the rheology of dense suspensions and particle slurries is well established, although the exact relationships between particle-level quantities and macroscopic rheology and stability are at best qualitative. Most of the understanding has been developed for low shear rheological (linear viscoclastic) properties and/or for dilute dispersions. This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces (due to the presence of the solvent) on the moderate to high shear rheological properties and shear stability of dispersions that span the colloidal to particulate range (colloidal dispersions to slurries). Of particular interest is the shear thickening transition and dilatancy, and how that explicitly depends on the strategy used to stabilize the dispersion or slurry (i.e. steric, electrostatic, polymeric stabilization), as well as the hydrodynamic forces important at higher shear rates. The research to date has the following components:

• Systematically explore the influence of the basic methods of particle stabilization on the shear thickening and dilatancy of well-characterized colloidal dispersions and slurries of non-colloidal particles. In this report, in particular, the effect of weak electrostatic stabilization forces on determining the onset and severity of shear thickening is examined.
• Determine the effects of particle size and concentration on the onset and severity of shear thickening.
• Employ this experimental data to test simplified two-particle models for the onset of shear thickening that are derived from simulations and theory.

The non–Newtonian rheology

The non–Newtonian rheology is calculated numerically to second order in the volume fraction in steady simple shear flows for Brownian spheres in the presence of hydrodynamic and excluded volume interactions. Previous analytical and numerical results for the low–shear structure and rheology are confirmed, demonstrating that the viscosity shear thins proportional to Pe2, where Pe is the dimensionless shear rate or P ́eclet number, due to the decreasing contribution of Brownian forces to the viscosity.

Large Pe Limit

In the large Pe limit remnants of Brownian diffusion balance convection in a boundary–layer in the compressive region of the flow. In consequence, the viscosity shear thickens when this boundary–layer coincides with the near–contact lubrication regime of the hydrodynamic interaction.

Wakes Formation

Wakes are formed at large Pe in the extensional zone downstream from the reference particle, leading to broken symmetry in the pair correlation function. As a result of this asymmetry and that in the boundary–layer, finite normal stress differences are obtained as well as positive departures in the osmotic pressure from its equilibrium value.

Normal Stress Difference

The first normal stress difference changes from positive to negative values as Pe is increased when the hard–sphere limit is approached. This unusual effect is caused by the hydrodynamic lubrication forces that maintain particles in close proximity well into the extensional quadrant of the flow.

Conclusion

The study demonstrates that many of the non–Newtonian effects observed in concentrated suspensions by experiments and by Stokesian Dynamics simulations are present also in dilute suspensions.