Browsing by Author "Lewis, Alison Emslie"
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- ItemOpen AccessAn analysis of the anti-avoidance provisions contained in the South African Income Tax Act(1995) Erasmus, Sean Fouracres; Lewis, Alison EmslieThe South African Income Tax Act No 58 of 1962 contains a number of specific as well as general anti-avoidance provisions. This dissertation places its main emphasis of focus on-the general anti-avoidance provisors contained in section 103 of the Act
- ItemOpen AccessAntisolvent gibbsite crystallisation from synthetic Bayer liquor(2009) Chiang, Yu-Lun; Lewis, Alison Emslie; Nathoo, JeetenThe current Bayer precipitation of gibbsite is notoriously slow and is considered to be the rate limiting step of the Bayer process. The present industrial reactor configuration involves precipitating gibbsite inside a series of large stainless steel agitated vessels due to the slow precipitation kinetics of gibbsite from pregnant Bayer liquor. The scope of this study is to investigate the effectiveness of using water as an antisolvent to increase the overall gibbsite yield and the associated particle behaviours under isothermal conditions. The experiments were performed in a series of batch experiments which were divided into two subsets. Firstly, the effect of different caustic concentrations present in the anti solvent on gibbsite yield was tested. Secondly, the effect of different seed loading (50 and 100 grams seeds per liter of synthetic liquor) on aggregation and fines formation were investigated. All batch experiments were conducted isothermally at 80°C under a closed system using a baffled stainless steel 316 agitated vessel. The experimental results indicated water to be a feasible medium for the isothermal antisolvent crystallisation of gibbsite from synthetic pregnant Bayer liquor. It was found that the extent of overall gibbsite yield exhibits a power law correlation to the change in the antisolvent caustic concentration with R2 ~1 (R2 = 0.99974). This finding is important as the regressed curve will be used to correlate gibbsite yield as a function of anti solvent purity. From the seed loading experiments, an Increase In seed loading under constant temperature and initial liquor supersaturation resulted in an overall decrease in product particle size due to a rise in inter-particulate attrition. However, crystal growth was not dominant during any period of the experiments with aggregation being more prominent for the system with the lower seed load. Gibbsite morphology determination from SEM revealed hexagonal solids which, in the presence of a higher seed loading indicated signs of fracture due to attrition. Quantitative evidence of this mechanism is shown USIng the discretised population balance method by Bramley et al. (1996) with the aid of the computational technique developed by Hounslow (2005) in Mathematica. Results generated using this method show decreasing aggregation with liquor de supersaturation and increased seed loading. The overall particle behaviours from the current investigation are validated by results obtained from previous gibbsite studies under very similar conditions. Although the gibbsite yield can be increased drastically in the presence of an anti solvent, the average particle size generated from this study is still too small to be commercially feasible due to the increase in initial supersaturation. Thus it is recommended that further particle size optimisation may be conducted inside a fluidised bed reactor where particles generally experience lower shear than inside a conventional agitated vessel.
- ItemOpen AccessControl of the desupersaturation reactor used in the SPARRO process(2003) Seewoo, Shilpa; Lewis, Alison Emslie; Van Hille, RobThe work described in this thesis focuses on the Slurry Precipitation And Recycle Reverse Osmosis (SPARRO) process. The process was designed to treat calcium sulphate (gypsum) scaling mine waters by encouraging preferentlal crystal growth onto introduced seeds, rather than on the reverse osmosis membranes. The major problem faced by the SPARRO process was the short membrane life span, possibly due to damage caused by contact with gypsum crystals. Gypsum exists in two extreme morphologies, plate and needle-like crystals, which are formed from high (0.2 M) and low (0.04 M) concentration of CaCl2 and Na2SO4 solutions respectively. This research reinvestigated the SPARRO process from a crystallisation perspective with the aims of: • Increasing the level of understanding of fundamental aspects of the SPARRO process. • Developing a technique to quantify the gypsum crystal morphology. • Developing design specifications for a 5 L lab scale desupersaturation reactor. • Developing and defining the critical parameters in controlling production of gypsum of a specified crystal size and morphology in the desupersaturation reactor.
- ItemOpen AccessCopper sulphide precipitation in a fluidised bed reactor(2002) Peterson, Karen Angeline; Lewis, Alison Emslie; Lacour, Stella; Van Hille, RobThe purpose of this thesis was an investigation into the feasibility of the removal of a single heavy metal (copper) by sulphide precipitation from a copper rich stream in a fluidised bed reactor, as well as developing an understanding of the behaviour of the copper sulphide system.
- ItemOpen AccessDevelopment of a brine treatment protocol using Eutectic Freeze Crystallization(2010) Randall, Dyllon G; Lewis, Alison EmslieHypersaline inorganic brines are generated from many global mining operations and the volume of these brines is increasing at an exponential rate. The environment and water resources in the vicinity of these mining operations are at a risk of being polluted as a result of this increase in brine volume. These are the key reasons why these brines need treatment...This thesis ultimately showed that EFC could be used to treat multi-component streams and that pure salts could be sequentially produced along with potable water.
- ItemRestrictedThe effect of a morphology modifier on the precipitation behaviour of nickel powder(Elsevier, 2006) Ntuli, Freeman; Lewis, Alison EmslieThe effect of a morphology modifier on the precipitation process of nickel powder was investigated in order to develop an understanding of its mechanism of action. Experiments were conducted on a pilot-plant scale using a 75-L autoclave with modifier dosages in the range of 0.25–5 vol%. Samples were collected from each successive batch reduction within a cycle and the powder was separated from the liquor before being washed and dried for subsequent analysis. The active particle rate processes were identified by transforming the particle size distribution (PSD) data into moments and from the change in surface area as measured by the BET method. Scanning Electron Microscopy (SEM) micrographs of the powder were used to observe the powder morphology and to validate the proposed particle rate processes and mechanism of action of the modifier. Evolution of the first moment (m0) and third moment (m3), equivalent to the total number of particles and volume, respectively, indicated that growth and aggregation were the major particle rate processes at a modifier dosage of 0.25 vol%. Breakage became apparent at dosage levels above 0.25 vol%. A decrease in BET surface area was noted in each cycle, indicating the presence of aggregation. The magnitude of decrease in the surface area indicated that the extent of aggregation decreased with increasing modifier dosage. SEM micrographs revealed that the powder was compact and aggregated at modifier dosages between 0.25 and 3 vol% and that loose porous powder was produced at 5 vol%. The modifier was found to inhibit growth, resulting in the formation of weaker agglomerate bridges leading to shear-induced breakage. This led to an increase in the surface area available for reduction. However, the effect of increased surface area in promoting reduction was outweighed by growth inhibition above a modifier dosage of 1 vol%. Thus, the number of attainable batch reductions increased when the modifier dosage was increased from 0.25 to 1 vol% and decreased with further increases in modifier dosage.
- ItemOpen AccessEffect of antiscalants during eutectic freeze crystallization of a reverse osmosis retentate(2015) Peters, Edward Michael; Rodriguez-Pascual, Marcos; Lewis, Alison EmslieEutectic Freeze Crystallization (EFC) is a separation technique which involves simultaneous crystallization of water and solute under eutectic conditions. It can be applied to treatment of various industrial aqueous streams containing dissolved organic and inorganic contaminants, such as reverse osmosis (RO) retentate brine streams. Since antiscalants are dosed in RO feed streams, these become concentrated in the retentate brine stream and could have an undesirable effect on crystallization kinetics of both ice and salt in EFC. In this study, the impact of a phosphonate antiscalant on the kinetic processes of nucleation and growth in EFC was investigated. Firstly, the effect of an antiscalant on the thermodynamic phase equilibria of a binary Na2SO4 aqueous solution was experimentally determined. The effect of the antiscalant on the nucleation and growth rates of both ice and salt in a continuous EFC process was then established for concentrations of 200, 350 and 500 mg/L of antiscalant. Product quality parameters such as the Crystal Size Distribution (CSD), morphology and purity of crystals were also measured since they are directly affected by the kinetic rate processes investigated.
- ItemOpen AccessThe effect of energy input on precipitation in an oscillating grid reactor(2010) Mokgethi, Botlhe Tshimologo; Deglon, David A; Lewis, Alison EmslieThe move to cleaner production has led to the process industry aiming at producing chemicals in a pure form and at the lowest possible cost. In practice this has resulted in a shift from treating processes as "black boxes" towards developing more concise models which reveal what happens within these "black boxes". This shift can be discerned at plant level for better control mechanisms, at a reactor design level to incorporate the effect of hydrodynamics and even at a molecular level when designing a tailor-made catalyst.
- ItemOpen AccessThe effect of energy input on the aggregation rate in an oscillating multi-grid reactor(2014) Willis, Brian Mathew; Lewis, Alison Emslie; Rodriguez-Pascual , MarcosAggregation is a size enlargement mechanism which can be either desirable or inconvenient depending on the process involved. It involves the coming together or collision of two or more particles to form a larger single particle. It is a mechanism which is poorly understood and much research is still required before the intricacies of the process are fully grasped. Although it is known that aggregation is influenced by energy input (amongst other factors), the relationship between the two has previously been measured in anisotropic turbulent environments. Thus although aggregation is a function of local energy input, it has yet to be measured in environments where the local energy dissipation is well understood, or else it has been studied in low Reynolds number environments. This thesis aims to address this deficiency by studying the effect of local energy input on the aggregation rate in a well characterised environment of isotropic and homogenous turbulence.
- ItemMetadata onlyEffect of Operating Conditions on Ice Characteristics in Continuous Eutectic Freeze Crystallization(Wiley-VCH Verlag, 2014) Chivavava, Jemitias; Rodriguez Pascual, Marcos; Lewis, Alison EmslieThe efficacy of eutectic freeze crystallization (EFC) in crystallizers and in subsequent downstream washing or filtration steps is a direct function of size and morphology of the crystalline products. These product characteristics are influenced by supersaturation and residence time. To investigate the effect of residence time and supercooling on ice product characteristics in a continuous EFC process, crystallization experiments were performed in an EFC crystallizer with Na2SO4 aqueous solution. Crystal size distribution and morphology during the crystallization process were analyzed by digital image processing. Longer residence times enhanced the mean crystal size and disk-shaped ice crystals were formed. Increasing supercooling resulted in a larger mean ice equivalent diameter.
- ItemOpen AccessEffect of operating conditions on product quality in continuous eutectic freeze crystallization(2013) Chivavava, Jemitias; Lewis, Alison EmslieThe efficiency of gravitational separation in a staged continuous Eutectic Freeze Crystallization process is largely influenced by the quality of solid products exiting the crystallizer. The particle size distribution and shape of the crystalline products affect the efficiency of the washing and filtration steps hence the ultimate purity of the product water and or salts. These characteristics are the net effect of the crystallizer operating conditions such as the level of supersaturation, residence time and magma density. In order to control the mentioned product characteristics, the mechanistic interaction of crystallizer operating conditions and product morphology and size needs to be understood. This work was aimed at investigating the effect of residence time and supersaturation on the product quality in continuous Eutectic Freeze Crystallization and a 2L glass crystallizer equipped with scrapers was employed for the investigations.
- ItemOpen AccessExploration of the mechanism of rhodium(III) co-precipitation with copper sulfide (at low rhodium concentrations) incorporating the cationic substitution reaction path(2007) McGeorge, Barry; Lewis, Alison EmslieThis study is a preliminary investigation into the mechanism and kinetics of Rh3+ coprecipitation with CuS upon aqueous thiosulphate addition to acidic base metal sulphate solutions, where Rh3+ concentration is two orders of magnitude lower than Cu2+, over 50 - 150 °C. The heterogeneous cationic substitution has been identified as a new precipitation path in metal sulfide co-precipitation, namely, the reaction between more soluble, co-precipitated metal sulphide (CuS) and the less soluble cation in solution (Rh3+), with the large KSP difference providing the chemical driving force.
- ItemMetadata onlyHeat and mass transfer effects of ice growth mechanisms in pure water and aqueous solutions(American Chemical Society, 2014) Kapembwa, Michael; Rodriguez-Pascual, Marcos; Lewis, Alison EmslieInteractions between heat and mass diffusion determine growth mechanisms during ice crystallization. The effects of heat and mass transfer on ice growth in pure water and magnesium sulfate solution were investigated by studying the evolution of the gradient of the refractive index using color Schlieren deflectometry. For pure water, the gradient of the refractive index of water was used to calculate the temperature and therefore the local supersaturation. Its effect on the ice crystal growth rate and morphology was studied. It was found that, for local supersaturations greater than 2.8, the morphology was dendritic ice, with a growth rate 2 orders of magnitude higher than that for layered growth. During dendritic growth, 3−16% of the heat of crystallization diffused to the liquid side, which is counter to current understanding. At the transition (between the time of partial melting of the dendritic ice and the beginning of the layered ice growth), a higher supersaturation than that responsible for layered growth was observed. For ice growth from an aqueous salt solution, a mass and thermal diffusion boundary layer in front of the growing ice was created by diffusion of the solutes from the ice and by the release of heat of crystallization.
- ItemOpen AccessHeat and mass transfer effects of ice growth mechanisms in water and aqueous solutions(2013) Kapembwa, Michael; Randall, Dyllon G; Rodriguez-Pascual , Marcos; Lewis, Alison EmslieResearch into ice crystallization processes is an important area of study. The desire to improve product quality and efficiency of processes involving ice crystallization in industries such as desalination by freezing, freeze drying, freeze concentration and freeze crystallization for food processing, requires insight into the ice growth mechanisms. More so, a novel technology called Eutectic Freeze Crystallization, where water is recovered in the form of ice, requires that ice crystals are of high purity as this directly determines the quality of the water obtained. During ice crystallization, ice growth mechanisms play an important role in determining the structure, size and morphology of ice which have an effect on separation processes and product purity. Heat and mass transfer play a fundamental role in ice growth processes as they affect the thermodynamics and kinetics of the crystallization process. Ice growth experiments were carried out in pure water, in 8.4 wt% and 16.8 wt% magnesium sulphate and in 8.4 wt% sodium nitrate using a 10x5x31 mm test cell made of Plexi-glass®. The Colour Schlieren optical technique was used to conduct the experiments. This is because of its capability to map refractive index gradients related to either temperature or/and concentration gradients of the solution during crystal growth.
- ItemOpen AccessA hydrodynamic study of nickel suspension in stirred tanks(2005) Aoyi, Ochieng; Lewis, Alison EmslieMany studies on solid-liquid mixing have been dedicated to low density particles at low solids concentrations. In the present work, computational fluid dynamics (CFD) simulation and experimental methods were employed to study suspension of high density particles (nickel) at high solids concentration in water. The work first focused on establishing the velocity field in a liquid-only system and then progressed to a solid-liquid system. In the liquid-only system, the influence of tank geometry and simulation strategies, including turbulence models, on fluid flow pattern and mixing was investigated in a tank stirred by a Rushton turbine. The standard k-f. model gave better overall predictions of mean velocity fields than the k-ro and RNG k-f. models. The CFD simulation and experimental results obtained with the laser Doppler velocimetry (LDV) method showed that mixing time and homogenization energy decreased with a decrease in the impeller bottom clearance. It was further shown that there is a bottom clearance range in which a draft tube can aid mixing in a tank stirred by the Rushton turbine. In the solid-liquid system, a hydrofoil impeller was used to investigate the influence of simulation strategies, particle properties and hydrodynamic operating conditions on mixing features such as the off-bottom solids suspension, cloud height, solids concentration distribution and local particle size distribution. The simulation results were compared with experimental ones, in which the off-bottom solids suspension was determined visually and an optical attenuation technique was employed to determine the cloud height and solids concentration distribution. The local particle size distribution (PSD) in the tank was measured by a laser diffraction method. A better agreement between the simulation and experimental results was obtained with drag models that account for the solids loading or free stream turbulence than those that do not. It was shown that the Stokes law applies up to a diameter of 150 ~m for the nickel particles. A CFD simulation strategy for studying mixing of high density solids is proposed and it is shown that a CFD simulation method can be used to develop empirical models that predict mixing features. A CFD simulation approach that takes particle size into account gives predictions that are more representative of practical applications than the mono-size particle simulation approach. Reactor configurations and hydrodynamic parameters that improve mixing were identified. These can also aid optimal design of mixing systems.
- ItemOpen AccessIce scaling in continuous eutectic freeze crystallization(2016) Jooste, Debora; Lewis, Alison Emslie; Chivavava, JemitiasEutectic Freeze Crystallization (EFC) is a novel and potentially cost effective technique to treat industrial brines by the simultaneous crystallization of ice and salt under sub-eutectic conditions. Previous research has demonstrated that the formation of an ice scale layer on the cooling surfaces of indirectly cooled crystallizers severely decreases heat transfer. This increases the mechanical energy requirements and overall operational cost of the process. The energy efficiency of EFC as a wastewater treatment and resource recovery technology is, therefore, dependent on effective control of ice scaling. This research focused on determining the induction time, defined as the time between initial nucleation and scale layer formation, where shorter induction times are associated with more severe scaling tendencies. The experimental work was conducted in a hybrid crystallizer-separator with a 2 litre crystallization zone fitted with a mechanical scraping device. The effect of the driving force for heat transfer, scraper speed and the solute type and concentration of inorganic electrolyte impurities in a binary eutectic Naâ‚‚SOâ‚„-Hâ‚‚O system was investigated. Induction time decreased with an increase in the driving force for heat transfer, due to a lower wall temperature and an increased driving force for crystallization as a result of the higher heat flux. An increase in scraper speed resulted in an increase in induction time, due to the more frequent removal of the thermal boundary layer and better distribution of supersaturation and magma throughout the crystallizer. The induction time was found to be specific to dissolved ionic species as a result of unique electrostatic interactions between the cooled wall and ice layer surface. Induction time showed an increase with an increase in concentration of electrolyte impurities, due to the increased mass transfer limitation of solute molecules away from the growing ice front.
- ItemOpen AccessImpurities in crystals formed by eutectic freeze crystallization(2011) Apsey, Grant; Lewis, Alison EmslieA large concern facing South African industry, particularly the mining sector, is a suitable and sustainable means to deal with large volumes of liquid waste, comprised mostly of effluent water streams containing a variety of dissolved salts. A novel technique is being developed for the treatment of industrial waste brines; Eutectic Freeze Crystallization (EFC), can potentially separate these into pure water and its major constituent salts, which can be reused or sold. The feasibility of this technology is heavily invested in its capacity to produce pure salts. Thus, it is important to investigate the obtainable purity of these salts, before larger scale operations can begin. A major component of many South African waste brines is sodium sulphate. This work has therefore focused on the purity of sodium sulphate obtained from two different South African brines.
- ItemMetadata onlyIn Situ Growth Measurements of Sodium Sulfate during Cooling Crystallization(Wiley-VCH Verlag, 2014) Egan, Timothy; Rodriguez Pascual, Marcos; Lewis, Alison EmslieCombined rainbow Schlieren deflectometry (RSD) and liquid crystal thermography (LCT) served as in situ non-intrusive techniques to determine local concentration and temperature fields based on refractive index gradients in solution during crystallization of sodium sulfate from aqueous solution. The combination of RSD with LCT allowed the decoupling of concentration and temperature effects. Convective mass transport happened during periods of fast, dendritic growth, but even higher levels occurred during the densification of the dendritic crystals, indicating significant mass deposition in this phase. The convection developed mainly as a result of concentration gradients as opposed to temperature gradients. A supersaturation field map was generated and growth kinetics of the sodium sulfate salt was determined. The kinetics followed a power law relationship, with constants in line with those in the literature.
- ItemOpen AccessIndustrial application of eutectic freeze crystallization(2011) Jivanji, Rineshkumar Bharat; Lewis, Alison Emslie; Nathoo, JeetenSouth African industries are currently generating increasing quantities of hypersaline brine streams. These wastewater streams need to be processed in an ecologically friendly and economically favourable method to produce both purified water and salts.
- ItemOpen AccessIndustrial brine characterisation and modelling(2010) Zibi, Lindizwe Mthanjiswa; Lewis, Alison EmslieIndustrial wastewater contains many soluble inorganic and organic components and solid particles. This study focused on inorganic industrial hypersaline brines. Chemical engineering presents a variety of mechanical, thermal, biological and chemical processes capable of treating hypersaline brines to the standard required by legislation. However, some of these technologies are inefficient, costly and outdated and are not applicable in solving modern brine accumulation problems.