Michael J. Hounslow

An experimental investigation of particle fragmentation using single particle impact studies

This paper presents the results of a comprehensive programme of experiments in which particles were impacted under controlled conditions against solid targets. The overall aim of these experiments was to gain an understanding of the fragmentation process of particle products in a pneumatic conveying system. A continuous air gun was used to examine the effect of particle velocity, impact angle, particle diameter, target material, target thickness and number of impacts on the fragmentation of spherical aluminium oxide particles. The effect of the impact velocity on the fragment size distribution was also examined. The results showed that decreasing the impact velocity, impact angle, target thickness, target hardness and particle size decreased the fragmentation rate. Quantifying the effect of these variables on particle fragmentation provides further understanding of how particles behave during pneumatic conveying, especially in terms of conveying velocities, bend geometry and bend surface material.

DIRECT EVIDENCE OF HETEROGENEITY DURING HIGH-SHEAR GRANULATION

Abstract Despite the volume of research concerning granulation, the knowledge of its rate behaviour is still a matter of some debate. Here, granulation is explored by looking at the interaction between granule properties and rate behaviour; process conditions are perturbed to induce different rate behaviour. Granules of calcium carbonate powder (mass-mean size of approximately 40 μm) and Polyethylene Glycol (PEG) 1500 are made in a 10-l vertical axis, high-shear mixer with the PEG added in two different ways: by pouring on and melting in. The resulting granules are cooled and sieved on a fourth-root-of-two progression of sieves to obtain the granule size distribution. Granules in specific size ranges are analysed using a thermo-gravimetric technique, mercury porosimetry and the primary particles are liberated from the binder in order that their size distribution may be examined using a light-scattering particle size analyser. The results show marked differences between the two methods of binder addition. The granules made by pouring on the binder exhibit bimodal weight distributions that are unimodal by the end of the experiment, larger granules, faster growth, low air incorporation into granules and uniform binder and primary particle size distribution (PSD) by the end of an experiment. The granules made by melting the binder in situ exhibit bimodal weight distributions throughout an experiment, smaller granules, slower growth, higher incorporation of air into granules and nonuniform binder and primary PSD throughout. Both methods of binder addition show considerable heterogeneity in the properties of granules. We interpret the heterogeneity of binder distribution by means of the nucleation theories of Schaefer and Mathiesen [T. Schaefer, C. Mathiesen, Int. J. Pharm., 139 (1996) 139] concluding that pour-on experiments follow the Schaefer and Mathiesen Immersion Hypothesis and that melt-in experiments follow the Schaefer and Mathiesen Dispersion Hypothesis. We put forward two competing hypotheses for the heterogeneity found in the distribution of primary particle sizes: the Preferential Nucleation Hypothesis and the Preferential Growth Hypothesis. We believe that the former applies to pour-on experiments and the latter to melt-in experiments.

Coupling granule properties and granulation rates in high-shear granulation

It is possible to link granulation rates to granule properties. The linkage is by multiple dimension population balance equations that, by means of simplifying assumptions, can be reduced to multiple one-dimensional (1-D) population balance equations (PBEs). Using simple physically based models, this paper demonstrates how multiple one-dimensional population balance equations can describe the results of high-shear granulation experiments of two different materials, calcium carbonate and lactose. Good agreement between experimental and simulated results was achieved enabling the granulation rates to be defined by two model parameters: the critical binder volume fraction and the aggregation rate constant. The modelling framework presented in this paper also provides a basis for the kinetic analysis of granulation experiments so that with further work, it is possible to determine the effect of process conditions and material properties on the model parameters.

Twin screw granulation: steps in granule growth.

The present work focuses on the study of the progression of granules in different compartments along the length of screws in a twin screw granulator (TSG). The effects of varying powder feed rate; liquid to solid ratio and viscosity of granulation liquid on properties of granules was studied. The bigger granules produced at the start of the process were found to change in terms of size, shape and strength along the screw length at all the conditions investigated. The granules became more spherical and their strength increased along the screw length. Tracer granules were also introduced in order to understand the role of kneading and conveying elements in the TSG. The kneading elements promoted consolidation and breakage while the conveying elements led to coalescence, breakage and some consolidation. The results presented here help to provide a qualitative and quantitative understanding of the twin screw granulation process.

Particle fragmentation in dilute phase pneumatic conveying

Using a continuous-flow gas gun, single particle impact studies were conducted to examine the characteristics of particle fragmentation. The effect of particle velocity, impact angle, particle size and number of impacts on the percentage of unbroken particles was investigated. A numerical simulation, combining particle trajectory with particle fragmentation characteristics was also developed to calculate the percentage of unbroken particles in a horizontal pipe and bend for the case of dilute phase pneumatic conveying. Validation of this model showed good agreement with the simulated and experimental results. The flexibility of this model will also enable particle breakage for any dilute pneumatic conveying configuration to be predicted.

Impact breakage of fertiliser granules

Abstract Systematic data are presented for the single impact failure of 3.2, 5.3, and 7.2 mm fertiliser granules over a wide range of impact speeds and angles. The probability of failure was found to change only slowly between 90° (normal) impact and 50°, but decreased rapidly below 50°. The probability of failure increased with increasing size of granules. The effect of impact velocity on the mean, median and the proportion of the largest-sized fragments were examined. Two distinct forms of normal impact damage were identified, corresponding to low and high impact velocities, and the mechanisms of failure are discussed.

Aggregation during precipitation from solution: an experimental investigation using Poiseuille flow

Aggregation is an important crystal size enlargement mechanism during precipitation and the rate of aggregation is affected by the various process conditions such as solution composition and the hydrodynamics of the system. In this paper, an investigation of the effect of hydrodynamics as well as solution composition on the aggregation of calcium oxalate monohydrate crystals is reported using a novel Poiseuille Flow Crystalliser (PFC). The well-characterised hydrodynamics of the PFC allows the prediction of collision rates using Smoluchowskis theory and therefore the quantification of the efficiency of aggregation. At low shear rates, i.e. in the high-efficiency regime, it is found that the aggregation rate constant varies linearly with shear rate providing good validation for Smoluchowskis theory. Further, at increasing shear rates a maximum in the value of the aggregation rate constant is observed confirming the disruptive role of fluid shear rate in impeding aggregation. In the investigation of the effect of solution composition on aggregation, it is found that the aggregation rate constant increases with the growth rate, but is independent of the species ion ratio. Finally, we find that dimensionless strength is a powerful correlating variable in determining aggregation efficiency.

Twin screw wet granulation: Binder delivery.

The effects of three ways of binder delivery into the twin screw granulator (TSG) on the residence time, torque, properties of granules (size, shape, strength) and binder distribution were studied. The binder distribution was visualised through the transparent barrel using high speed imaging as well as quantified using offline technique. Furthermore, the effect of binder delivery and the change of screw configuration (conveying elements only and conveying elements with kneading elements) on the surface velocity of granules across the screw channel were investigated using particle image velocimetry (PIV). The binder was delivered in three ways; all solid binder incorporated with powder mixture, 50% of solid binder mixed with powder mixture and 50% mixed with water, all the solid binder dissolved in water. Incorporation of all solid binder with powder mixture resulted in the relatively longer residence time and higher torque, narrower granule size distribution, more spherical granules, weaker big-sized granules, stronger small-sized granules and better binder distribution compared to that in other two ways. The surface velocity of granules showed variation from one screw to another as a result of uneven liquid distribution as well as shown a reduction while introducing the kneading elements into the screw configuration.

Alignment mechanisms between particles in crystalline aggregates

This paper examines the effect of mechanisms leading to alignment between the individual particles which make up aggregated crystal systems. It is postulated that when crystals collide in a suspension, their relative orientations have an effect on the probability that the crystals will stick together. Scanning and transmission electron microscopes have been used to study aggregates produced during the precipitation of calcite, and the relative orientations of crystallites within the aggregates have been measured. In a calcite sample produced at low solution ionic strength, it is found that more than half of the crystallite pairs measured show some special alignment between the crystallites. Aggregates taken from moderate ionic strength experiments do not show any noticeable degree of alignment. It is proposed that this alignment in the aggregates made at low ionic strength is caused by an ability of the crystallites to re-align themselves into a more favourable energy state before they are fixed into place.

A mechanistic model for amorphous protein aggregation of immunoglobulin-like domains.

Protein aggregation is associated with many debilitating diseases including Alzheimer’s, Parkinson’s, and light-chain amyloidosis (AL). Additionally, such aggregation is a major problem in an industrial setting where antibody therapeutics often require high local concentrations of protein domains to be stable for substantial periods of time. However, despite a plethora of research in this field, dating back over 50 years, there is still no consensus on the mechanistic basis for protein aggregation. Here we use experimental data to derive a mechanistic model that well describes the aggregation of Titin I27, an immunoglobulin-like domain. Importantly, we find that models that are suitable for nucleated fibril formation do not fit our aggregation data. Instead, we show that aggregation proceeds via the addition of activated dimers, and that the rate of aggregation is dependent on the surface area of the aggregate. Moreover, we suggest that the “lag time” seen in these studies is not the time needed for a nucleation event to occur, but rather it is the time taken for the concentration of activated dimers to cross a particular solubility limit. These findings are reminiscent of the Finke–Watzky aggregation mechanism, originally based on nanocluster formation and suggest that amorphous aggregation processes may require mechanistic schemes that are substantially different from those of linear fibril formation.