ARR - Annual Report
Summary
In the particle technology, it is fundamentally important to know the interaction and adhesive forces between particles and to find the correlation of those forces with the microscopic characteristics of particle surface, because these forces are the origin of many phenomena which particles exhibit in industrial processes. The aim of this project is to clarify in-sihc at the molecular level the microstructure of surfaces in solutions of industrial importance and the correlation with interaction and adhesive forces between surfaces, using not only an atomic force microscope (AFM) but also computer simulations.
It was planned to clarify the phenomena and mechanism on the subjects shown in the map of’ the following figure, which were considered to be fundamental in understanding the phenomena in industrial particle processes.
The objective of this work is systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance is to understanding the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we are seeking to develop new methods to control and manipulate the properties of their interfacial films.
Dynamic Mechanical Properties
Specifically, the dynamic mechanical properties of the resulting inter-facial film are being studied directly with special emphasis on how they depend on both vibration frequency and strain rate. A homebuilt apparatus is employed to this purpose with the following unique properties:
- Surface-surface spacing are variable from thousands of Angstroms to molecular contact. The surface force needed to produce this separation is measured while at the same time measuring shear nanorheology. The tip is imaged in situ directly during each experiment, therefore force can be normalized by area to produce stress.
- Oscillatory deformations with variable frequency in the range 0.01 to 103 rad-sec-1 can be applied, with deformations either in the shear direction or in the normal (pumping) direction.
- The amplitude of deformation can be varied from sub-Angstrom to thousands of Angstroms. The reason to use very small deformations is to produce a linear viscoelastic response (representative of the rest state, this can be studied by methods of equilibrium statistical thermodynamics). The reason to use very large deformations is to produce strongly nonlinear deformations characteristic of very high shear rates.
To the best of our knowledge, no other instrument with these properties exists in any other laboratory in the world. We would like to take this opportunity to encourage IPPRI members to suggest new systems that would be interesting to study with these unique methods. The main finding during Year I was to develop criteria with predictive power to understand whether opposed particles will move past one another with intermittent stick-slip motion or with smooth sliding. We found that stick-slip motion occurred only when thin films were deformed faster than their intrinsic relaxation time. The observation offered a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film.
The increasing particle-particle-interactions with increasing fineness are an essential problem during wet comminution in stirred media mills. These interactions have an influence on the stability of the product suspension towards agglomeration and on the rheology. Experimental results show that during comminution the measured particle size increases again after reaching a product fineness of approx. 500 nm, although a further increase of the specific surface can be determined (BET-method). The reasons for this phenomenon are the increasing particle-particle-interactions and spontaneous agglomeration.
Possibilities of the stabilization of the product suspension in the stirred media mill are therefore mainly discussed in this report. With respect to this, first results of sample preparation are mentioned. It is shown that during the comminution changes of the pH value, the conductivity and thus the ionic strength as well as the zeta potential occur in dependence of the materials of the grinding chamber lining and the stirrer discs as well as the grinding media material and the product material. In further investigations the product suspension shall be stabilized electrostatically during the grinding process.
The comminution progress as well as the electrochemical properties of the product suspension shall be characterized by online measurements. For this measurements a possible experimental set-up is introduced and discussed. Only after this steps the systematic investigations concerning the influence of the grinding media size are reasonable.
The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accur’ate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.
The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as mentioned in the original research grant application, it is unlikely that both the FDP and LVSF sections can be completed thoroughly in a single 3-year period (ie due to the amount of work involved). Hence, top priority has been given initially to the LVSF section of the project, although some progress also has been made with the FDP section of work.
Several difficulties were encountered during the course of the project (eg unexpected results and phenomena) and have delayed progress in various areas. In some cases, it was not possible to complete certain scheduled tasks (eg testing aluminium and mild steel pipe and wide range of granular solids). In other cases, it was necessary to pursue new work (eg rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project will be successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. Unfortunately, due to the various problems and delays to date, the full range of pipe wall materials and bulk solids will not be able to be tested - such work is necessary to confirm the accuracy and validity of the new models (eg majority of work to date has concentrated on poly pellets). Also, a significant amount of additional time will be needed for the relatively more complex FDP section of work (eg only one product and a few different pipelines will be able to be tested by the end of the initial 3-year period).
This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.
The co-processing of fine-particle agglomerates and liquids is common in industrial practice. In many applications, the processing goal is the reduction of the agglomerate size (or possibly complete breakage of the agglomerate into its constituent particles) and distribution of the fragments throughout the liquid medium. To accomplish this, hydrodynamic shear can be applied to the suspension by various mechanical means. The underlying motivation for this research is to obtain a fundamental understanding of the various factors that influence the dispersion behavior of agglomerates. Attainment of such an understanding may facilitate the development of interfacial engineering strategies aimed at improving the outcome of dispersion processes or to the design of more efficient dispersion equipment.
Our basic 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 emphasis of the work supported under this IFPRI grant involves investigation of how certain time-dependent (dynamic) phenomena affect the outcome of dispersion processes. Such time-dependent effects are inherent in several aspects of the dispersion process. For instance, in practical processing equipment, the agglomerates are subject to complex shear histories. The contacting of particles and agglomerates with processing liquids leads to wetting and spreading phenomena that change over the course of time. Also, for soluble materials, dissolution is a time-dependent effect.
In 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, we performed additional modifications to the experimental device and refined our experimental procedures. This allowed us to complete additional experimental studies that highlight the dramatic influence of time-dependent effects on the outcome of dispersion. In addition, we performed a thorough modeling study of the flow and shear fields present within our experimental device. This modeling enables us to analyze the results of our experiments, and to understand the limitations of our dispersion chamber.
This report summarises progress in IFPRl project 37 in 1999/2000. Karen Hapgood has completed her PhD (November, 2000) and much of this report summarises the significant results of her research on wetting and nucleation. Some very interesting in progress results on the dynamic mechanical properties of wet granules are also included.
A new model for predicting drop penetration time from powder and liquid binder properties is presented. This model takes account of the presence of macrovoids in loosely packed powder beds and introduces the concept of an effective porosity and effective pore size seen by liquid drops penetrating into the bed by capillary action. The effective pore size is smaller than the Kozeny model capillary size by 2 to 4 times for lactose powders and an order of magnitude for very fine zinc oxide and titanium dioxide powders. Model predicted penetration times are compared with experimental data for a wide range of binder and powder properties. Predictions are within an order of magnitude for all powders with good agreement for lactose and glass ballotini. This is much better than the existing literature models.
A simple model to predict the fraction of agglomerates formed in the spray zone as a dictionof dimensionless spray flux is developed using spatial statistics:
The equation is in very good agreement with both Monte Carlo simulations of drop coverage on a powder surface and experimental nucleation experiments.
The conceptual nucleation regime presented in report 37-02 is extended and compared with results From granulation experiments in 1 litre and 25 litre laboratory mixer granulators. Experiments confirm that the narrowest granule size distributions are produced in the drop controlled regime where there is both low dimensionless spray flux and short drop penetration time. The implications for granulator design and scale up are discussed.
The first set of results from detailed measurement of the dynamic mechanical properties of wet mass pellets are presented. Experiments with glass ballotini powders and a wide range of liquid binders confirm that peak flow stress is a strong function strain rate. All results can be collapsed onto a single correlation between dimensionless stress and capillary number. Similar results are shown for crushed silica powders. This work is the first step to develop a generalised correlation that relates wet mass constitutive properties to the formulation properties.
The main research goals for years 4 to 6 of the project are briefly discussed.
Summary
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, 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 axisymmetric Couette cell producing shearing flows of gas and agitated solids in the absence of gravitational accelerations (Fig. 1). The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed.
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 gas pressure gradient along the flow and the relative motion of the boundaries.
According to the proposal there were the three following aims we wanted to achieve in the first year of the project extension:
- Extension of the co-current experimental rig to a co- and counter-current spray drying system
- Design, construction and testing of a small-scale device for determination of drying kinetics parameters
- Elaborating of CFD model for scaling-up of spray drying process
The process of disintegration of liquid/solid suspension jets and sheets by atomization is analysed in a fundamental manner and visualized by suitable measurement method which allow qualitative and quantitative evaluation of the process. Supporting numerical analysis and theoretical derivations will contribute to basic understanding and control of the suspension atomization process. Model suspensions will be atomized by means of conventional and specifically designed atomizers.
The third year activities which are reported here include:
- Extending the stability analysis to predict the primary droplet break-up
- Experimental investigations of suspension atomization in twin-fluid atomizer
- Performing experiments with a new rotary-atomizer
Model suspensions based on water, water/glycerol mixture and water/CMC- (carboxymetylcellulose) mixture with suspended glass particles have been atomized.