ARR - Annual Report
It appeared that usual image analysis size parameters such as the equivalent area disc (or sphere) diameter, often used in image analysis software, did not give meaningful results in regard of the true physical dimensions of the particles. The existence of two populations could not be highlighted with that size descriptor.
Five products containing two types of particles with different shapes were prepared. 2D dynamic image analysis and laser diffraction results were compared to 3D image analysis results, taken as the reference.
The influence of particle shape on particle size distribution results obtained by different techniques was investigated. Such studies were already done in the past, but the products prepared here were mixes of two metallic powders presenting extreme shape properties (sphere-like and fibre-like particles).
Most of the time, industrialists are willing to characterize their products with the best accuracy and reproducibility. But the results obtained by different techniques may vary, and the interpretation of those results may not always be correct, depending on the characteristics of the analyzed products.
The results presented in this report are part of a research aiming to determine the influence of particle shape on the physical properties of powders (such as flowability and compaction).
A complete experimental determination of the drying and degradation kinetics to get better knowledge of the mechanisms involved in transforming a droplet to the particle is the main aim of the whole project. Accomplishment of this task requires the application of a special equipment which would make it possible to take samples and to ensure residence time long enough. The main difference of our approach and the approach encountered in the literature in a systematic investigation of spray drying process for chemical and biological systems is involving in situ analysis of the properties of continuous and dispersed phases from atomization to collection of dry product.
A typical systematic analysis of a spray drying process contains the following steps (Nath and Satpathy, 1998):
- atomizer performance studies,
- parametric sensitivity of spray dryer studies,
- powder property studies,
- thermal inactivation studies,
- post drying studies.
We proposed to make an additional step for deeper analysis of the spray drying process; to study spray and heat carrier properties during drying process (inside the dryer). Only this kind of analysis enables understanding of transferring mechanisms from droplet to particle connecting operational parameters of the process (temperature, humidity, moisture content) and current structure of spray (particle size distribution, particle velocities, etc.).
Spray drying is one of the best theoretically developed drying methods, particularly in the area of calculation of hydrodynamics in the two-phase flow: solid particles-gas. The process of spray drying is, however, such a complex phenomenon that so far no model describing it correctly has been proposed. The gravest errors in the calculation of spray drying are caused by an incorrect determination of drying kinetics and improper model of flow turbulence (Bahu, 1992).
Due to broad variety of materials being dried in spray dryers, it seems difficult to develop general principles concerning the kinetics of water removal from these materials (Masters, 1991). It is necessary to determine individual drying kinetics for each material separately. In a typical schematic of spray drying, a particle stops shrinking and the formation of a rigid structure starts when a critical moisture content is reached. Since that moment on the particle may not change its size, it may break down, disintegrate and agglomerate. All these phenomena affect the coefficients of aerodynamic drag and heat and mass transfer which have a significant influence on the drying process. Some materials may behave in a quite different way, e.g. the colloidal-capillary-porous bodies reveal high resistance to vapour diffusion on the surface; this may cause swelling of the particle in the initial period of drying (particles of milk).
The key problem in spray drying which has not been solved yet is the determination of drying kinetics and degradation kinetics for heat sensitive products. The lack of appropriate experimental investigations is due to technical problems in carrying them out. The residence time of particles in the spray dryer does not usually exceed 30 seconds, and is often even shorter. Thus the whole process of dehydration, formation of solid structure, degradation, etc. takes a very short time. Therefore, a few attempts made so far to determine the kinetics of product drying and degradation have been restricted to the analysis of relevant parameters only at the dryer inlet and outlet, e.g. Alizondo and Labuza (1974), Johnson and Etzel ( 1994).
Step I of this project is a preliminary stage to enable extensive studies on kinetics and degradation of dewatering of selected products in a disperse system when particle residence time does not exceed a few seconds.
As follows from the research we have carried out so far, the range of measurements performed in the experimental rig must be extended by increasing the residence time of sprayed material in the measuring section and reducing the risk of material deposition on the walls. To achieve this the diameter of the measuring section should be changed from 30 to 50 cm which would enable drying in a broad range of initial process parameters (mainly in a wide range of feeding rates and atomization angles).
The project steps and related milestones for the first year period as defined in the project proposal are:
- Set up of model suspension systems and of basic laboratory analytical procedures for testing microstructure, rheology and mechanical extrudate product properties (6 months) described in chapters 1-3
- Set-up / adjustment of extrusion system and first experiments with the selected model systems (6 months) described in chapters 4 and 5
Executive Summary
The aim of the project is to establish a relationship between the product properties and feed material and the mill functions for milling of organic solids. The specific objectives are:
- a) To characterise the physical, mechanical, and thermal properties of organic feed materials (material function) at the single particle level, and to examine the effects of temperature and humidity on these properties,
- b) To investigate the breakage behaviour of single organic particles at quasi-static and dynamic conditions under the influences of temperature and humidity,
- c) To investigate the bulk milling behaviour of model organic solids and mill hydrodynamics (mill function),
- d) To characterise the properties of milled product, and to correlate the product properties to material and mill functions.
Model materials used in the work include aspirin, α-lactose monohydrate (α-LM), sucrose, sorbitol, starch, and microcrystalline cellulose (MCC). These materials cover a fairly wide range of physical, mechanical and thermal properties, hence ensuring generality of the results to be achieved. This report summarises the work done over the past three years, including the single particle breakage studies using the impact tester under both ambient and sub-ambient conditions, measurements of Young’s modulus, hardness and fracture toughness of single particles of some model materials using the nano-indentation method, characterisation of some product particles using the Dynamic Vapour Sortion (DVS), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD), analysis of the bulk milling behaviour of the model materials in a single ball simulating mill under both ambient and sub-ambient conditions, theoretical analysis of the mill dynamics, distinct element modelling of particle and milling ball motion to establish the mill function, investigation into the use of a flow aid (Aerosil) on the bulk milling, and population balance modelling of the milling of aspirin in collaboration with Du Pont. The main findings are summarised in the following:
Executive Summary
Our goals for the past year have focused on developing and refining an experiment to directly and quantitatively measure thermal rupture forces in a depletion system. Keeping in mind that we want to relate these force measurements to the macroscopic rheology of the gels, we have conducted these experiments using a new model system that will enable us to measure the rupture forces, observe the microstructure using confocal microscopy, and also measure the rheology of the gels. This work is being performed in close collaboration with Professor Michael Solomon, University of Michigan.
Executive Summary
Controlling the elasticity of gelled systems has a large effect on their overall consistency and flow behavior. Previously, we demonstrated a general correlation between the rheology and structural rigidity of model colloidal gels in the nonlinear regime. This earlier work suggests that both central and tangential interactions (in the form of interparticle attraction and friction) play an important role in the flow properties of many colloidal and granular systems. Here, we adopt a two-pronged approach to exploring the effect of such interactions on the measured elasticity of gelled systems. In collaboration with Eric Furst and his group, we develop a model gel system that allows us to perform direct force measurements in parallel with bulk rheometry experiments. These interparticle interactions dictate the underlying gel microstructure and ultimately affect the overall elasticity. We also develop model PMMA colloids with controlled surface roughness to determine its effect on gel rheology. Our work suggests that roughness enhances the linear elasticity of gels, but the effect is significant only at high volume fractions.
Preface
This report deals with the part of the 'Slurry Rheology" project that was carried out at the R.U.Leuven (Belgium). This part of the investigation started in September 1981. The main body of the work on the rheology project was executed at the University of Loughborough. As soon as the corresponding report is available a global report on the rheology project will be prepared.
Introduction
Static electrification of particles takes place in various kinds of particle handling processes. For example, particles are charged so heavily during gas- solids pipe flow that electric sparks are observed periodically through a transpearent part of the conveyor line.
Movements of charged small particles are affected by the charge itself, and the electrification causes various undesirable effects such as adhesion between particles and the wall of process equipments. The equipments are also electrified by impaction between particles and the wall. It may be difficult to reduce the potential of the equipments to zero level because of the electrical resistance and the capacitance between the equipments and the ground. The potential can be lowered by suitable grounding, even the grounding of the equipments can not prevent the electrification of particles.
On the contrary, the grounding may accelerates the electrification of particles. Besides such undesirable effects as adhesion and dust explosion, electrification of particles is utilized positively in electrostatic precipitation, electrostatic separation, electrophotography, and so on.
In the next section, we will briefly discuss the general idea of contact electrification, then the electrification of a particle by impaction on a metal plate, and electrification process of a particle in gas-solids pipe flow.
Summary
Powders containing particles of sub-micron size are normally agglomerated, and the size and strength of the agglomerates depend on the chemical nature of the material and its surface chemistry, primary particle size and shape and their distributions, and the atmospheric environment. The dispersion of such agglomerates in liquid media involves their breakdown into primary particles and the replacement of solid/gas by solid/liquid interfaces. Maintaining a high degree of dispersion requires that flocculation of dispersed primary particles must be prevented. Sometimes flocculation is desirable, and various types of flocculated structures are possible which differ in strength.
This project aims at understanding the forces that determine powder agglomerate strength and their relation to the energy required for dispersion in liquid media in terms of both chemical (wetting) and mechanical (milling) effects. Once these are understood and the conditions for effective dispersion are established, flocculation will be induced and the strength of floes measured.