ARR - Annual Report
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.
As described in the contract proposal the first phase of work was to:
- I prepare and characterise a model dispersion
- develop or evaluate a suitable low stress viscometer
and progress in these in reported in section 2 and 3 of this report. The model dispersion was chosen to consist of well characterised particles in which the degree of flocculation/deflocculation could be controlled by suspension in a fluid whose viscosity could be modified thus altering the shear stresses present for a given strain rate. For reasons previosuly discussed it was decided to use a mono-disperse polystyrene latex as the model dispersion and this report describes how sufficient quantities for rheological experiments were synthesised, worked up and characterised. Work is still proceeding on the characterisation of these latices, particularly to measure their zeta potentials as a function of salt concentration and to determine the effect on this of high sucrose and gylcerol concentrations, these substances being chosen to alter the viscosity of the suspending fluid.
A requirement of the programme of work to be done under this contract is the ability to measure shear stress/strain properties at low strain rates and in oscillation at small amplitudes. To this end the University has purchased a Deer Rheometer and this report gives a preliminary assessment of this instrument. Section 4 of this report is an account of preliminary experiments to study 2 the effect on the viscosity of a calcium carbonate/water/Dispex system of increasing the viscosity of the suspending liquid. The results of this work are discussed in relations to a published suspension flow equation and to a theoretical model of the balance of interparticle and hydrodynamic forces on suspension stability.
Abstract
Pressure drop-velocity measurements for flow of water slurries comprised of various concentrations of narrow size fractions of fine glass beads in l.0", 1.5", 2.0" and 2.5" diameter straight pipes, and in an array of 1.0" and 2.0" pipe bends, fittings and valves were taken. Using these data friction loss coefficients, and equivalent lengths (in numbers of pipe diameters) were calculated for the various fittings. The experimental facility used in getting these data, the actual data and the preliminary analysis of the results are presented in this report.
Experience in powder handling industry have shown that some powders exhibit flow problems when transported through pipes and hoppers, and also the tendency of particles to adhere to conveyor belts and machinery. These problems have been thought to be the cause of electrostatic charge build up on the powder particles. In some cases it has been observed that two almost identical powders will behave differently, one may cause flow and other problems whilst the second may be quite acceptable.
The aim of this short study was to measure a few electrical properties of some powders which caused problems and to compare them with those similar powders which did not give rise to any problems. The properties studied were:
- (i) Tribocharging characteristics of the powders.
- (ii) Resistivity as a function of pressure.
- (iii) Dielectric constant and spectroscopic analysis.
The tribocharging behaviour of the powders was studied by passing small quantities through different tubes, for example, copper, aluminium, perspex etc, and measuring the charge generated as a function of powder velocity and tube length. The magnitude and polarity of charge acquired by the powder determines the nature and severity of the problem.
The resistivity and dielectric constant both play a role in determining the build up of charge and rate of discharge. The higher the resistivity of a powder the longer it takes for the charge to leak away from it. The greater the dielectric constant the higher the charge it would acquire. Both these parameters were measured and compared for different powders. Spectroscopic analysis such as infra-red spectra of the powders were also compared.