Chemical and Biomolecular Engineering - Theses
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ItemInterfacial tension study and rheological characterisation of water-in-oil emulsion explosives( 2007)Highly concentrated emulsion explosives are complex colloidal systems that remain robust for very many years after production. In this work a poly-isobutyl succinic anhydride (PIBSA) derived emulsions have been studied to understand the extraordinary nature of the stability and flow of these emulsions. In this study a model sodium oleate emulsion system is developed to further confirmation of the nature emulsion explosives. Two PIBSA based emulsifiers were used to prepare the emulsions. The main emulsifier of interest was comprised of an amide, ester and imide head group attached to a PIBSA chain, the second was entirely comprised of an imide head group attached to a PIBSA chain. The pendant drop method was used to obtain dynamic interfacial tension measurements for an interface between of paraffin oil/PIBSA-imide and ammonium nitrate, and dodecane/sodium oleate and sodium nitrate. The IsoFit-WardTordai computer package that incorporated Langmuir, Frumkin, Reorientation and Aggregation adsorption isotherms was used to calculate diffusion coefficients for total time adsorption. Diffusion coefficients for short time adsorption with Langmuir and Frumkin isotherms were determined and long time adsorption with the Langmuir isotherm were calculated. All of the diffusion coefficients results suggested that direct diffusion controlled adsorption was not the mechanism these surface active agents employed, rather, the results suggested that an activation barrier controlled adsorption process dominated the kinetics of the interfacial adsorption. There is much evidence to suggest that rheological properties of highly concentrated emulsions have not been characterised correctly for some time. This is mainly due to wall depletion or wall slip effect phenomenon inherent to rheological investigations performed on smooth stainless steel surfaces. The effect of wall slip was eliminated through physical means by performing rheological investigations with a sand blasted cone and plate, and the use of the vane geometry as a rheomter. Another method to eliminate wall slip effects in steady shear for highly concentrated emulsions was formulated and established in this investigation. A two-stage Tikhonov regularisation procedure that converts steady shear data into rheological property functions has been developed. The two-stage method is able to obtain not only the shear-stress shear-rate function but also the slip velocity as a function of wall shear- stress. The method is such that it obtains the rheological functions over the maximum range of shear-rate covered by the data. The results obtained using the new method were compared to those obtained using the sand blasted cone and plate and vane geometry with good agreement being observed. Over a 17 month period, a PIBSA based emulsion was observed in order to determine if any dramatic rheological changes were taking place with time, the results obtained in both shear and dynamic analysis effectively concluded that the PIBSA emulsion explosive was extremely stable and the minor changes observed were within experimental uncertainty of the measurements. Various microscopy techniques were used to determine the droplet structure and size distribution within various emulsions. Optical microscopy, Qualitative Phase Imaging, Differential Interference Contrast imaging were employed, and it was found that the PIBSA and PIBSA-imide emulsions had a spherical shape rather than a hexagonal close packed structure that was previously assumed for these emulsion explosives. Images of the sodium oleate emulsion indicated that there were obvious destabilising mechanisms at work, affecting the longevity of the emulsion.
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ItemThe effect of shear on dewatering of flocculated suspensions( 2005)The ability to separate a suspension into its respective solid and liquid constituents is an important requirement in the chemical, wastewater and mineral industries. Typically, separation occurs in open, large diameter tanks known variously as thickeners, settlers or clarifiers. The design and operation of these devices have been based, until recently, on kinematic models and macroscopic mass balances. The problem with these approaches is that consolidation in the bed is not described accurately and consequently, the area required for thickening is often grossly overestimated. Recently, Buscall and White [24] proposed a 1−D phenomenological theory of dewatering that encompasses both sedimentation and consolidation, providing a more solid grounding for understanding, simulating and optimising dewatering in a range of devices, including thickeners. This theory identifies two important rheological parameters; a concentration dependent yield stress, Py (φ) and hindered settling function, R(φ). Despite representing a significant improvement over a kinematical approach, Buscall and White’s dewatering theory involves a number of simplifications so that in practise, simulations often underestimate dewatering in full sized thickeners [97, 153]. One aspect of thickening that is poorly understood is the effect of raking. At the base of the thickener, a rake transports the thickened sediment to the outlet. An additional effect from raking is to increase the average solid concentration in the underflow [33, 46]. Raking introduces normal and shear stresses, which cannot be described within a one-dimensional framework. Therefore, observed differences between predicted and measured thickener underflow concentrations are attributed to the action of the rake. The aim of this thesis is to develop a better understanding of how shear stresses effect compressional dewatering in both pilot and full scale thickening operations. Before attempting to quantify the effect of shear on dewatering, it was considered necessary to first establish that the 1-D model was capable of predicting dewatering in the absence of shear. Up until now, no known studies have been undertaken to validate the model under controlled conditions. To approximate one-dimensional flow with no shear, a tall narrow column with no moving parts was used. Two solid fluxes and several bed heights were studied, and the outputs from the column were compared with the 1-D model predictions. The results show that under ideal conditions, the model predicted the underflow solid concentration to within 10 %. The effect of shear on dewatering was investigated using a Couette shear device. Couette geometry was chosen to provide uniform shear. Since in Couette flow, no normal stresses act in the direction of rotation, the mechanism behind dewatering can investigated. These experiments showed that shear caused dewaterability to improve up to a critical shear rate, beyond which dewaterability was adversely affected. The relationship between this critical shear rate and flocculation conditions was investigated by using different flocculant dosages. The shear modified Py (φ,γ) and R(φ,γ) can be input to the 1−D model, thereby incorporating shear indirectly. As a result, the model predicted an order of magnitude increase in solids flux. The above procedure was used to characterise the dewaterability of a real thickener feed as a function of shear rate. The optimum shear rate was determined by finding the minimum R(φ,γ). Then, Py(φ) and R(φ) were input into the thickener model. The predicted underflow concentration could then be compared against plant data. Even when shear is taken into account, the model still under predicts the performance of the thickener. To understand this result, the pilot column work was revisited since the control over experimental conditions was far greater. To introduce shear, concentric cylinders were installed in the column and rotated at a fixed speed. Thus, the effect of shear and bed height on underflow density were determined at different rates of shear. This showed that the underflow concentration increased with bed height; a result not expected based on the model prediction. The effects of shear on underflow density were secondary to bed height. The bed height dependence can only be explained if dewatering is not steady but changes over time. For a four metre bed height the residence time is eight times longer than a one metre bed. Improvements in dewatering could be related to time dependent restructuring of aggregates which would result in an associated change in R(φ). By fluidizing suspensions for times corresponding to the residence times in the tall column, R(φ) and Py(φ) could be determined, as functions of volume fraction and time. Aggregate properties including structure and density were measured before and after fluidization using focussed beam reflectance measurement (FBRM) and floc density analysis (FDA).
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ItemCharacterisation of shear upon dewaterability of colloidal suspensions( 2004)Solid-liquid separation is an important unit operation in many industrial processes. Research on the process optimisation and technical advancement of this operation is crucial to ensure a reliable and economical process. Work include developments in filtration theory and understanding of suspension behaviour are constantly investigated to ensure the process reaches the target. In this work, enhancement in dewatering was quantified with the use of shear. Shear, in this context, is a mechanism used to deform a suspension network such as the action of raking in thickeners. In order to understand the inter-play between shear and dewatering for colloidal networked suspension, the relationship between shear and compression rheology was investigated. The effect of shear was investigated in two situations, which are the presence of shear during and before dewatering. The former was achieved by applying an electric field to a suspension during (in-situ to) dewatering and the latter was achieved through varying shear condition for flocculated suspensions prior to dewatering. The analysis of dewatering properties was then performed by applying the consolidation theory of Landman and White (Landman et al. 1995; Landman and White 1997). The materials used in the study were two types of colloidal metal oxide particles namely AKP-30 alumina and Ajax kaolin. They were chosen due to their physical differences such as particle size and shape. The dispersion state of these suspensions from coagulated to dispersed, was fully controlled by manipulation of the particle surface chemistry. This changed the suspension micro-structure. Flocculated suspensions were prepared by the addition of a non-ionic, high molecular weight polyacrylamide to the coagulated AKP-30 alumina suspension. The study on shear rheology was investigated by employing steady shear and small amplitude oscillatory shear (SAOS) measurements. The use of these two methods allowed quantitative interpretation of a network deformation of a suspension which involved the transition between solid-like behaviour (before yielding) and liquid-like behaviour (after yielding). The network deformation can be characterised as either being of a brittle or ductile type. The use of Lissajous figures also aided the understanding of the deformation. The shear rheology of the metal oxide suspensions was found to depend critically on the extent of the inter-particle interactions. Both dispersed and coagulated suspensions show different rheological properties. In the case for coagulated suspensions, rheological parameters such as the shear and compressive yield stresses, and elastic modulus plateau value, all scaled, which indicate that these parameters arise from particle interaction. This is in contrast to the understanding of shear rheology on freshly flocculated suspension which is still limited, particularly for concentrations at close the gel point. The shear rheology of flocculated suspension was then compared to that of coagulated suspensions. Various degrees of flocculation conditions were investigated. Network deformation was found to show an identical pattern to that of coagulated suspensions but the network strength was found to increase with shear rate. The dewatering properties of dispersed and coagulated AKP-30 alumina and Ajax kaolin suspensions were compared. It was found that dispersed suspensions have lower compressibility (i.e. produce a higher final solids concentration at the same applied pressure) and permeability compared to coagulated suspensions. Comparison between the two model suspensions shows that AKP-30 alumina suspensions have better dewatering qualities compared to kaolin suspensions. The effect of flocculation conditions for AKP-30 alumina suspension was found to dramatically affect the settling rate but had only a small effect on dewatering at higher solids. A combination of dewatering methods was used and they gave excellent results for the prediction of the dewatering characteristics of suspensions for a wide range of solids concentration from close the gel point up to close to the maximum close packed concentration. Finally, electrically enhanced dewatering (EED) for Ajax kaolin suspension was compared to that of normal dewatering. Results showed significant dewaterability enhancement at pressures below 10 kPa. The application of EED was also investigated for an industrial sample of water treatment sludge. Similar results were noted as for Ajax kaolin, with an increase of equilibrium solids concentration and permeability with EED at a given pressure. In this context, EED delivered promising results in improving the dewatering properties of difficult-to-dewater suspensions.