ARR - Annual Report
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.
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 record the dependence of the mean drag on granular agitation.
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.
After a literature review, this second annual report describes the new axisymmetric experimental apparatus that we have designed for this project, and it outlines theories that we will employ for the interpretation of its data.
Introduction
Crystallography concerns both the internal and external form of a crystal [1]. Crystalline solids are an essential part of our modern technological environment, being important components of pharmaceuticals, foods, cosmetics, metals, ceramics and plastics. The process of crystallisation is used both for purification and as a separation process for the production of particular materials.
The customary way of forming crystals through suspension processes always relies on the usage of solvents in which solution phase is used as a media for homogenisation of the starting composition as well as enabling the molecular assembly processes. These solvents have been found to influence the crystallisation processes to the point of altering both the nucleation rate and the crystal morphologies and state of aggregation of the end product. Despite this, our knowledge and the understanding of the nature of the molecular assembly processes in supersaturated solutions, as well as the interaction of solvents with the crystal faces is severely lacking.
In the fine chemicals and pharmaceutical industries, where products are of high value, organic solvents are routinely used [2]. The ability of solvents to manipulate the structure and morphology of the crystals formed becomes invaluable. This becomes even more important when the drug or the dyestuffs are polymorphic in which case changing the solvent can result in a different polymorph crystallising more than one crystal structure. Modifying the polymorph can alter its physical behaviour. For example, in the case of a drug the rate of uptake in the body can increase making one polymorph more desirable over the other. Thus understanding the interactions between a solvent and solute as well as the fundamental theories lying behind the whole solution crystallisation can increase the performance of the final product as well as extending our ability to select solvents for crystallisation control.
Executive Summary
This report is an integral part of an effort to develop a computational platform to virtually synthesize and test particle compacts based only on the bulk and surface properties of the particles prior to the consolidation process. This virtual manufacturing and testing facility (VMTF) includes die filling, compaction –particle rearrangement and particle deformation (elastic and inelastic)–, compact ejection and subsequent mechanical testing. The current simulation platform is based on a multiscale approach, which bridges systematically the micro and meso-scale. The VMTF will provide the ability to reproduce the behavior of current products but more importantly, it will enable the simulation of systems never yet manufactured, virtually screening the best manufacturing conditions and particle/granule properties for a desired compact behavior or application. During this year we will continue the development of the subsequent modules of die-ejection and mechanical testing.
The specific content of this report includes a numerical study of the mechanical behavior of systems composed by particles with different sizes and materials subjected to consolidation. The simulation methodology is based on a mixed discrete/continuum approach which allows to systematically bridge the microscale response (particle and inter-particle scale) with the mesoscale and macroscopic behavior (container/sample scale). The methodology is particularly suitable for describing the post-rearrangement regime where consolidation proceeds mostly by elastic and inelastic deformation. This formulation is able to provide quantitative estimates of the evolution of macroscopic variables, such as pressure and density, while following microlevel processes, such as local coordination number and loading paths. This methodology is applied to polydispersed systems composed by particles with different nonlinear properties. The predictions are in general agreement with the experimental data during both loading and unloading cycle.
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 accurate 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 was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (ie due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was made also with the FDP section of work. However, with the 3-year extension to the research grant, a substantial amount of work now can be completed in the FDP section, as well as completing particular outstanding issues in the LVSF section.
Several difficulties were encountered during the course of the first 3 years of the project (eg unexpected results and phenomena) and delayed progress in certain areas. In some cases, it was not possible to complete particular scheduled tasks (eg testing aluminium and mild steel pipes with a wide range of granular solids). In other cases, it was necessary to pursue new research issues (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 has been successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. For example, the new transport boundary and pressure drop models have been found quite accurate for the poly pellet type materials tested to date and also have been able to explain some of the interesting and unexpected phenomena encountered during the experimental stages of the project.
Unfortunately, due to the various problems and delays to date, as well as the new discoveries and developments, the numerous pipe wall materials and bulk solids planned originally for the LVSF section were not able not been tested, preventing further confirmation of model accuracy and validity. Additional work is planned over the next 12 months for this purpose (eg testing at least one other granular material with properties different to the poly pellets).
A significant amount of additional time will be needed for the expected relatively more complex FDP section of work. For example, only one product and a few different pipelines were able to be tested by the end of the initial 3-year period. The 3-year extension will allow this to be pursued in greater detail (eg with other powders and pipeline configurations), as well as the commencement of investigations into modelling techniques.
This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.