Chemical and Biomolecular Engineering - Theses
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ItemDevelopment of an improved methodology for taphole development programs( 2004)Many industrial smelters are interested in improving their tapholes. At present, the key drivers for taphole development are to increase the throughput, improve the safety, streamline the operation and reduce the maintenance burden of tapholes. In the past, smelters have used a 'trial and error' approach to develop tapholes and in some cases hazardous and inoperable designs have been installed. To avoid this situation, some smelters have used heat transfer modelling as part of their taphole development programs, however the models that have been produced lack realistic boundary conditions, material properties and thorough validation. An improved methodology for the development of realistic taphole heat transfer models has been proposed and demonstrated in this work through a case study that was carried out on an industrial taphole. Boundary conditions including the tapping channel heat transfer coefficient were measured in the industrial taphole using a network of thermocouples. The thermocouples were installed during a regular maintenance shutdown and monitored continuously throughout an entire taphole campaign during which 23 taps occurred. The thermal conductivities of new and used samples of the taphole refractories were measured over the relevant range of operating temperatures using the laser flash technique. A 3D heat transfer model of the taphole was developed based on the measurements and the model was used to simulate the steady state and transient temperatures in the taphole over a complete tapping cycle. The model was validated against temperature data that was measured inside the taphole. Overall, the model was in good agreement with the measured data. For most areas of the taphole, the model predicted temperatures that were within 35°C of what was measured throughout the entire simulation. The validation in this work was more rigorous than the validation of any of the models presented in the available literature. The model demonstrated that realistic heat transfer models of industrial tapholes could be prepared when the relevant phenomena are understood and included in the model. The model assumed that the heat transfer coefficient measured in the tapping channel was uniform along the taphole. As a result, the model may have failed to capture a localised intense region of heat transfer in the tapping channel. It was recommended that further measurements be carried out in the taphole to investigate variations in the heat transfer coefficient along the tapping channel, and that the variations be subsequently incorporated into the model. A sensitivity analysis indicated that the model was most sensitive to the uncertainty in the contact resistances and the boundary conditions on the bath side of the model, which were not measured in this work. Further measurements were recommended to confirm the values used in the model. Further modelling was recommended to include phenomena that were relevant to taps other than the third tap that was the focus of the model. The phenomena were a worn tapping channel, aged taphole refractory, variations in the flow and freezing over. Further experiments and measurements were recommended to investigate the effect of flow variations and aged refractories on the heat transfer in the taphole.
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ItemThe role of calcium in geopolymerisation( 2004)The search for a new environmentally friendly construction material that will match the durability of ancient concrete has stimulated interest in geopolymerisation in recent times. Despite extensive research that has been conducted on various aspects of geopolymerisation, especially in enhancing the properties of resultant binders and understanding the chemical mechanisms involved, a number of questions remain to be answered. The role of calcium in geopolymerisation is one of them. It is known that the presence of a moderate amount of calcium in a geopolymeric binder will improve the resultant compressive strength. However, it is still unclear whether calcium will play a charge-balancing role within the aluminosilicate structure (to maintain electro-neutrality in a similar way as alkali metal cations, Na+ or K+, or perhaps an alternative role). In addition, the role of calcium in geopolymerisation is of great interest because it is proposed for the first time that calcium links geopolymer chemistry to, and distinguishes it from, ordinary Portland cement (OPC). (For complete abstract open document)
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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.