Particle systems may be characterised by a knowledge of the dimensions of the individual particles. By using the Feret's diameter distribution and the random filament distribution of an iron powder the characterisation of a system of the powder particles, constrained in a cylindrical die, has been achieved. The Monte Carlo method has been used to simulate the transmission of force through the model created by this characterisation. The results of 500 Monte Carlo simulations have been used to produce a description of the force due to one surface particle at various points in the system. Using this diagram together with a knowledge of the spatial distribution of the surface particles and the dimensions of the die it was possible to produce: (1) curves of side wall pressure vs. depth in the die for various loads; (2) curves of applied to transmitted pressure for various height:diameter ratios; (3) curves of friction loss at the walls vs. depth for different height:diameter ratios. The differences between the experimentally determined values and the values predicted on the basis of the model have been explained in terms of the assumptions made, and suggestions made as to how these may be eliminated.

It is not yet possible to predict how stress will be distributed in a given particulate system. Continuum mechanics provide elegant equations describing stress distribution in solid continua which due to lack of a suitable theory are often applied to particle systems. Soil mechanics on the other hand recognises the particle nature of such systems and produce solutions based on empirical failure criteria. However the way in which stresses are propagated in a bed of particles is not understood. This thesis investigates how stress is distributed in dry cohesionless particle beds with the aid of the photoelastic technique.

Particulates are present in the atmosphere as a result of both natural and man-made processes. Typical particulates include windblown soil, sea salt, sulphur, nitr0gen and hydrocarbon complexes, ammonium sulphate and nitrate, carbonaceous matter, biological debris, metal oxides, trace metals, and extra-terrestrial magnetic and radioactive compounds. Natural processes such as cloud formation, rainfall, and sedimentation cleanse the atmosphere of these particulates and, in so doing, form particle groups or agglomerates which can contain many unit particles. For the purpose of atmospheric research and, in particular, the physical tracing of pollution sources, particulates are a useful emission indicator. Consequently, if one wishes to use them effectively, it is necessary to separate the agglomerate into its independent particle units prior to analysis and identification. This is no simple matter, as the particles are held together by strong physical forces. The purpose of this thesis is to investigate a method of separating particulates for physical tracer studies, but the atmospheric aerosol was considered too complex a model for the initial studies.

A method has been developed by which the process engineer will be able to design continuous mixing systems and to explore mixing strategies, thus enabling him to take advantage of the benefits of the continuous mixing of particulate solids. The method is based on the prediction of the degree of smoothing out of periodic fluctuations in composition from a knowledge of the residence time distribution of the mixer and the nature of the fluctuations. Experimental verification of the method has been carried out in a ribbon blade continuous mixer using both free flowing sand and cohesive chalk. The residence time distributions were measured by adding an impulse of coloured tracer and measuring the outgoing composition, continuously, by a reflectivity device. Models involving a proportion of piston flow and of simple (or perfect) mixers in series were found to fit the experimental distributions. Periodic fluctuations were approximated by a Fourier Series and the amplitude reduction of rectangular periodic impulses were computed and compared with the measured amplitude reduction. With few exceptions the agreement was satisfactory for engineering purposes. It is claimed that the method can be used for designing continuous mixing systems and it is shown how a minor ingredient can be added in periodic impulses rather than a continuous stream to produce a satisfactory mixture.

Wavelet transforms are used to explore the dynamic evolution of granular material by post-processing data from DEM simulations. Data sets generated from two different physical scenarios were analysed: particles in vibration cells and samples under compression. It is shown how the wavelet multiresolution analysis (MRA) helps to reveal the particle motions within a vibration box subject to applied vibrations. Two combinations of amplitude and frequency vibration were applied in this work: low frequency with high amplitude and high frequency with low amplitude. Both cases give the same acceleration amplitude of approximately 7.8g where g is the acceleration due to gravity and both mono-sized particles and binary mixture were considered. The root mean square of the fluctuating velocity and the packing fraction of the two data sets are also consistent with the analysis results of the wavelets technique. Furthermore, it is shown how the MRA techniques were used to capture the position of inhomogeneities in granular material response by analysis of data sets from simulations of biaxial and triaxial compression tests. In the case of biaxial compression, results from the wavelet analysis are compared with plots of the cumulative rotation of the particles. In the triaxial compression test where a bonded material is considered the wavelet analysis is compared with plots of the position of broken bonds in the sample. Moreover, it is shown that the MRA techniques can aid in finding the time/strain scales on which significant events occur. This information could be used to determine how frequently to output data on stress, strain-rate (velocity) and packing voidage distributions during compression, shear and flow of the granular bulk. Furthermore, the wavelet technique should be considered for applications involving quantitative analysis of experimental data generated by the other advanced experimental techniques such as photo-elasticity and X-ray computed tomography techniques.

This thesis presents a detailed study into the role of surface chemistry in the high shear granulation of organic pharmaceutical solids. Crystalline materials with markedly different surface chemistries were selected as model compounds which were granulated with a polymeric binder. The surface wettability of primary powders was evaluated by employing the sessile drop contact angle measurements to evaluate the facet-specific surface energetics of macroscopic crystals, obtained via a solvent-mediated or a temperature-controlled crystallisation technique. A novel methodology using inverse gas chromatography (IGC) at finite concentrations (FC) was developed to evaluate surface energy distributions of the powder samples. The surface energetics of organic macroscopic crystals was found to be facet-specific, consistent with the local surface chemistry of the facet under examination. The rank order of hydrophilicity of specific facet was in good agreement with concentration of hydroxyl groups determined from X-ray photoelectron spectroscopy. IGC-FC was applied to measure [gamma]d/SV profile, which were in good agreement with the [gamma]d/SV values measured from contact angle on the corresponding single crystals. This technique was shown to be able to distinguish surface energy heterogeneity, homogeneity as well as subtle changes in the [gamma]d/SV distributions, due to variations in the crystal habits. The wettability of the model compounds was ranked based on the contact angle polarity and IGC surface energy heterogeneity distributions, and the spreadability of binder solution was assessed based on the thermodynamic spreading coefficients. The increase in surface wettability led to the increase in granule mean size and granule strength. Formulations with similar wetting behaviour resulted in similar granule size, whilst those formulations with dissimilar surface wettability were found to exhibit substantially different granulation behaviour. In conclusion, the materials surface chemistry plays a crucial role in granulation processes. Its thorough characterisation and understanding may lead to more controllable product attributes and improved final performances.

Granulation is a process for agglomeration where powder material is combined with liquid binder solution to facilitate the formation of larger, free-flowing granules. Granulation has become a mainstream process amongst the industries with applicability in numerous areas, which include the pharmaceuticals, mineral processing, fertilisers and in the production of a range of commodity products. A major driVing force for the production of granules from their ungranulated counterparts arises from the economic savings Le., increased bulk density permits savings to be made in transportation. and storage. Furthermore, granules may be tailored to possess certain desirable attributes that will suit the final application of the granules. Granulation is an example of a process that exhibits complex interactions between the underlying granulation phenomena such as nucleation, consolidation, aggregation and breakage. In addition, the granUle properties are distributed heterogeneously across the entire particle population posing as a particular challenge in generating a mathematical model that is able to accurately describe the granulation behaviour. The modelling approach used in this study is different from common practices, which tend to rely on heuristics and empiricism for the operation of the granulation process. This empirical approach signifies a disconnect from our understanding of the underlying physics of the process, which poses as a impediment towards the efficient operation of granulation processes. The work presented in this thesis attempts to address this disconnect by applying a threedimensional population balance with mechanistic representations for the underlying granulation rate processes. The population balance framework is ideally suited for this particular process, as it enables the evolution of the granules to be tracked with respect to differentiating particle traits, e.g. the granule size distribution. The selection of the desired properties is influenced by the importance of these particle properties on the end granule product, and also by their influence on key process mechanisms. A novel mechanistic nucleation kernel is developed incorporating fundamental material properties pertaining to the powder substrate and the liquid binder solution. The model form of the nucleation kernel is formulated by drawing a parallel with the collision/transition state theory. There are few literature reports on the inclusion of nucleation phenomena in the population balance models of granulation processes, let alone a mechanistic nucleation model. This study is one of the first in this regard. The recent recognition of the importance of the wetting kinetics and the nucleation thermodynamics on the nucleation phenomenon has been factored into the nucleation kernel by explicitly accounting for the effects of the liquid flow rate and the physicochemical properties of the material properties (surface tension, contact angle, and spreading coefficient). Batch granulation experiments were conducted obtaining granule measurements with respect to the size distribution, porosity and fractional binder content. Preliminary results for the validation of the population balance model with the experiment-measurements showed a good agreement, providing partial albeit valuable validation of the population balance model. This is also one of the first studies to model and validate a three-dimensional population balance model for granulation. Model based analyses were also carried out under a variety of processing conditions, which included the effects of changing formulations, droplet size effects, feed size distribution and the effects of powder and binder properties. The proposed model demonstrated the interactions for a range of feed formulations in tandem with granulating operating conditions, establishing qualitative agreement with similar findings derived from past experimental studies.

The process of pharmaceutical tabletting involves the application of stress onto the powdered drug ingredients contained within a rigid die. This compaction process will cause the powder to densify and form a tablet, which is then ejected from the die cavity. In reality, the tablet will incur some elastic strains during the compaction process and the tablet dimensional changes resulted due to the elastic relaxation of the tablet in the subsequent processing stages undermine the final tablet mechanical integrity. The ejection stage, where the tablet is extruded from the die, has not been studied extensively and is an important stage where any tabletting defects are immediately visible. It is then the purpose of this Thesis to experimentally elucidate the behaviour of the tablet elastic relaxation and its influence on the tablet mechanical integrity during the ejection, the final stage of the compaction process. The Thesis starts with highlighting the existence of the tablet elastic relaxation during the ejection stage that has been successfully studied on-line by the novel use of accurate laser sensors to record the tablet dimensional changes, with particular emphasis on the interrelationship of the tablet elastic relaxation with the observed ejection force and the tabletting defects of the Paracetamol tablet. The use of a lubricated die has also been shown to reduce the tablet elastic relaxation and the tabletting defects of the Paracetamol tablets. In terms of the time-dependent nature of the elastic relaxation, subtle time-dependent elastic relaxation behaviour has been observed and eludicated for the chosen time-dependent Starch material. A brief, but detailed and comprehensive material comparison consisting Aspirin, microcrystalline cellulose (Avicel) and Paracetamol is presented in terms of the elastic relaxation during the ejection stage. The observed tablet elastic relaxation is then interrelated with the tablet strength, which is determined by an indirect tensile test (Brazilian) and a direct tensile test. Finally, the presence of a localized elastic relaxation occurring at the interface of an Avicel bilayered tablet has been shown to undermine the strength of the bilayered tablet, where the extent of the interface weaknesses is clearly dependent upon the behaviour of the localized elastic relaxation. In conclusion, the Thesis has successfully demonstrated and elucidated the tablet elastic relaxation behaviour during the ejection stage and the consequences on the final tablet mechanical integrity.

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