3. Electronic Theses and Dissertations (ETDs) - All submissions

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Fosfomycin susceptibility of uropathogens at Charlotte Maxeke Johannesburg Academic Hospital (CMJAH)
(2018) Mothibi, Lesego Moetsana
Urinary tract infection (UTIs) have become the most common bacterial infections, equally found in the community and in hospital. Antibiotics such as ciproflocacin, trimethoprim/sulfamethoxazole, and nitrofurantion were used successfully as therapy for UTIs.
Lipase-catalysed hydrolysis of morita-baylis-hillman adducts
(2019) Mguni, Lindelo Mthabisi
Biocatalysis is the use of biological systems, such as enzymes, to perform chemical transformations on organic compounds. These enzymes catalyse reactions as whole cell systems or in isolated forms and have been found to exhibit high regio- and stereoselectivity towards chiral compounds. Lipases have been extensively used to catalyse kinetic resolutions of chiral compounds such as the Morita-Baylis-Hillman (MBH) adducts. The MBH adducts and their esters are important intermediates in organic synthesis and have been found to be valuable in the production of biologically active compounds. In this study, we expressed and partially purified the Pseudomonas fluorescens P26504 lipase in an active and soluble form to catalyse the kinetic resolution on MBH acetates to obtain enantiopure MBH adducts. The Pseudomonas fluorescens P26504 lipase was overexpressed in BL21 (DE3) pLysS cells at 25 °C for 16 hours, with 1mM IPTG concentration. Enzymatic assays were conducted after partial purification using p-nitrophenyl esters. The recombinant enzyme was highly active towards short chain esters and showed moderate activity towards medium chained esters. The Morita-Baylis-Hillman reaction was conducted, giving rise to racemic MBH adducts derived from benzaldehyde and hydro-cinnamaldehyde. The second step of the reaction was acetylation, producing chiral MBH acetates. A lipase-catalysed kinetic resolution was set-up, using the partially purified recombinant P. fluorescens P26504 lipase and the MBH acetates. TLC plate analysis showed that the recombinant lipase was able to hydrolyse both MBH acetates. However, further studies can be done to determine the enantioselectivity of the recombinant P. fluorescens P26504 lipase using chiral HPLC, which is more definitive.
An investigation of the enzymatic kinetic resolution of Morita-Baylis-Hillman adducts and their further functionalisation
(2019) Juma, Wanyama Peter
The Morita-Baylis-Hillman reaction (MBHR) is a carbon-carbon bond forming reaction that affords multifunctional Morita-Baylis-Hillman adducts (MBHA) with various synthetic applications. Unfortunately, many of the synthetic applications of these adducts cannot be realised because these adducts are formed in racemic form. This thesis has investigated functionalisation of enantiopure MBHA obtained using biocatalytic methods. The first part of the thesis describes the use of enantiopure aldehydes N-Boc-L-phenylalaninal and N-Boc-D-phenylalaninal to synthesize several Morita-Baylis-Hillman adducts in order to obtain diastereomers that would be separable by chromatographic methods. Unfortunately, this approach proved unsuccessful due to racemization of the aldehydes or MBHA under the reaction conditions applied. The second approach described is the resolution of racemic MBH acetates and esters using different enzymes. This exercise led to the identification of several lipases that were able to resolve racemic MBH acetates with excellent enantiomeric excess (ee) values and enantiomeric ratios (E). Racemic MBH adducts derived from the reaction of acrylonitrile with benzaldehyde, cinnamaldehyde and hydrocinnamaldehyde were successfully resolved. In each case the (+)-alcohol products were isolated in 94 - 97% ee after lipase-mediated enzymatic kinetic resolution of the corresponding acetates. Mosher’s ester derivatisation protocol was used to determine the absolute configuration of the resolved adducts, which was found to be (S). A lipase from Pseudomonas fluorescens, and Candida antarctica type B were found to be the best-performing enzymes. The last part of the thesis investigated the use of nitrogen nucleophiles for Michael addition to MBH adducts. The process confirmed that the use of nitrogen nucleophiles on TBS protected MBH adducts afforded nucleophilic addition products of high diastereoselectivity. The use of one of the enantiopure isolated MBH adducts in a diastereoselective Michael addition reaction with benzylamine led to a significantly enantio-enriched final product.
Shearing Interferometer Study of Shock Wave Impact on Membranes
(2019) Schulz, Jennifer
A study was conducted to investigate the flow eld characteristics that form for a shock wave propagating through a membrane. Analysis was also done to determine the strength of the shock wave reflected back into the system as well as the strength of the shock wave that was transmitted through the membrane. Multiple tests were done to analyse the e ects of di erent material types as well as di erent back pressures. Six di erent materials were used, each with three di erent back pressures. The flow field visualisation was done using both schlieren fow visualisation and shearing interferometry flow visualisation using a Sanderson prism. A comparison between the two forms of visualisation was done to determine which would provide the best qualitative understanding of the flow field. From this it was found that the shearing interferometry flow visualisation gave better insight into the density gradients as well as provided a clearer view of the flow characteristics. When analysing the flow eld characteristics for the early development of the transmitted shock wave, there were three principal characteristics that were present in all of the tests; a di racted shock wave, an expansion wave and a recompression shock wave. It was found that the prominence of these features varied depending on the material type as well as the type of back pressure. For the later ow development and membrane rupture, there were ve principal characteristics; small vortices, secondary shock waves, a vortex ring, expansion waves and a Mach disk. When analysing the strength of the re ected and transmitted shock waves, it was found that their strengths did not vary signi cantly for the di erent back pressures. The only exception was found when conducting tests using a balloon as a membrane. This was because the balloon de ected further into and out of the shock tube. The balloon tests were therefore done for a wider range of vacuum back pressures. It was found that the more the balloon de ected the stronger the transmitted shock wave produced, while the re ected shock wave remained approximately the same. A relationship between the strength of the re ected and transmitted shock wave was formed. It was found that as the strength of the re ected shock wave increased the strength of the transmitted shock wave decreased.
Three-dimensional shock wave reflection transition in steady flow
(2018) Surujhlal, Divek
This work details ﬁndings of an analytical, numerical and experimental investigation into the physical nature of three-dimensional shock reﬂection transition. Steady ﬂow shock reﬂections comprise two types: regular reﬂection and Mach reﬂection. Reﬂection studies have previously been conducted using double-wedge symmetrical test piece conﬁgurations. It had been found by previous researchers that the expansion waves resulting from the side edges of the wedges would inﬂuence the reﬂection plane. The three-dimensional nature of real experimental ﬂows gives rise to there generally being a coexistence of regular reﬂection (at the central portions) and Mach reﬂection (towards the outer peripheral portions) in between which transition occurs. It is the object of this work to understand three-dimensional transition for ﬂow ﬁelds in which edge eﬀects do not inﬂuence the reﬂection plane. Specially modiﬁed test piece geometry was developed for this purpose. Experimental tests were required for validation of the numerical models of the ﬂow ﬁeld. This was achieved by obtaining oblique shadowgraphs with optical orientation in both yaw and roll to assist in visualising the three-dimensional features of the ﬂow ﬁeld. These were compared with numerically reconstructed images at the same oblique orientations using a novel reconstruction technique. The main objective of this work was to identify the degree of correspondence of the threedimensional transition conditions to those of two-dimensional ﬂows. This was facilitated by means of reducing the three-dimensional analysis to an eﬀective two-dimensional one. It was found that the three-dimensional transition points occur at a higher eﬀective angle than predicted by twodimensional criteria, and tend towards two-dimensional criteria at reduced free-stream Mach numbers and increased model geometrical spreads. Another important aspect of this work was the nature of the intersection line in the vicinity of the transition point, i.e., the point of impingement of the incident wave and its Mach surface on the horizontal symmetry plane in between the test pieces. Here it was found that a cusp exists in the sweep proﬁle of the intersection line at the transition point. This was proved from a theoretical standpoint based on a model developed for the analysis of the ﬂow in the vicinity of transition. Evidence of this from the numerical and experimental results is given as well. Higher geometrical spreads and lower free-stream Mach numbers were found to create ﬂatter intersection line proﬁles at the horizontal symmetry plane on which the transition points were located further forward towards the apex of this line and which gave rise to greater transverse deﬂections for the streamlines passing through the regular reﬂection portions. Further discussion revolves around the nature of the shear and Mach surfaces. The Mach surface heights (representative of the triple line trajectories) are shown to increase monotonically. The shear layer edge trajectory, which originates at the sweep cusp, was found to show considerable transverse divergence but in keeping with the nature and extent of the transition cusp sweep differential, which in some cases was found to be large enough to cause a strong shock solution for the Mach reﬂection portion. In this case the shear surface edge trajectory diverted from trends seen for other models. The nature of the shear surface as a whole revealed interesting insights into the negative triple conﬁguration of the Mach reﬂection portions which comprised of convex Mach surfaces. This is in contrast to the geometry obtained for the Mach surface for full Mach reﬂection numerically studied with a highly-spread geometry. Here, the ﬂat Mach surface was found to increase monotonically towards the periphery in contrast to what was found for ﬂows with edge inﬂuences on the Mach surface. It is suggested that this is what precludes complex reﬂection (central Mach reﬂection, transitioning to regular reﬂection further out, with a further transition to Mach reﬂection at the periphery) from being obtained in such ﬂow ﬁelds
On shock wave diffraction from non-orthogonal apertures
(2018) Paton, Randall Tyrone
The diffraction of shock waves has been explored in many contexts in which either the shock wave is plane and the confi ning volume has complex geometry or where the shock wave has some non-plane geometry, typically something like spherical since this is the approximate shape of the waves generated by blasts. However, these studies have not considered more complex initial wave shapes or exit geometries. This study therefore addresses this de ciency in two ways. The dynamic conical shock wave was originally proposed as a mechanism for the initiation of fusion whereby the focusing of the shock wave near the axis of symmetry would produce the high temperatures and pressures required. This was explored numerically and theoretically, as no experimental method was considered viable, and it was found that the regular re ection of a shock wave de ned by conical wave geometry is not stable and will revert to an irregular re ection pattern at the axis. Three primary geometries were identi ed distinguished by the number of irregular re ections formed and in uenced by the induced vortical ow. The current study used a developed experimental apparatus to study conical shock waves and an additional, new re ection pattern, named the von Neumann type (vN-type) for the similarity to the von Neumann re ection of weak plane waves, was identi ed. In addition, instability of the conical shear layer present as a result of the irregular re ection at the axis of the shock wave was identi ed experimentally which resembles the Kelvin-Helmholtz instability previously only studied in two-dimensional con gurations of compressible ow. Dynamic bending of the central jet from the re ection axis was also tested as a function of upstream occlusion in the shock tube and this suggests possible future work in compressible jet actuation. The current study also considered the diffraction of plane shock waves at the inclined or curved exits of shock tubes, which are more general examples of duct interaction of the sort that might be found in engines or ventilation systems. This was done experimentally using novel, open test sections for a conventional shock tube and for a limited number of computational cases. These ows are characteristically different from the diffraction of shock waves from tubes of complex cross-section studied to date where the exit plane is still normal to the direction of travel of the wave. This is because the shock wave still undergoes simultaneous diffraction at all points around the edge of the tube in such a case while in this study the wave diffracts at different times around the tube periphery. This affects the strength of both the emerging incident wave and the diffraction and thus an atypical formation of the jet and vortex takes place. In the case of the inclined plane exit of the shock tube, two primary phenomena were noted: de ection of the jet and change of the jet cone angle; and variations in the vortex diffraction behaviour. For the former effect a large inclination of the exit from the normal increased the spread of the jet and the inclination away from the tube axis. Also, a system of secondary shock waves forms in the jet due to the expansion fans formed at the diffraction edge, typical of under expanded jets but becomes weaker as exit surface inclination increases. The second effect noted is of the increased curvature as a function of time after diffraction for higher inclinations, due to the much stronger induced velocities for the portion of the vortex shed on the obtuse upstream edge. The vortex loop also loses coherence with increasing inclination because of the weak vortex shedding at the downstream edge of the tube. The results for the curved exit are similar although the effects are not as extreme since the limiting diffraction angles are lower than for the extreme plane cases due to the characteristics of circular geometry. In the extreme case of part of the shock tube exit being tangent to the exit surface, the vortex again does not form a closed loop but rather terminates in the exit surface. This was particularly tested here with a plane section at the tangent point. However, the secondary re ection of the diffracted shock wave due to the curvature of the surface toward the diffracted wave, which tends to disrupt the vortex, means that an internal diffraction with a fully closed tube would result in a short-lived vortex loop. In both of the latter cases the vortex loop, or arch if it cannot close into a loop, is part of the physical mechanism whereby a jet ow exiting a pipe adjusts to being a di use ow along the exit surface. This is accomplished by the spreading of the sheet of vorticity, which is the boundary layer in the pipe and the jet boundary outside of it, by the origination of turbulence in the breakdown of the vortex arch or loop. The diffraction of shock waves from non-orthogonal apertures demonstrates features previously unidenti ed and suggests complex ow patterns which simpli ed two-dimensional analysis cannot describe.
Photocatalytic degradation of methyl violet in water using TiO2/Cellulose-N-MWCNTs
(2018) Mathebula, Xiluva
ABSTRACT TiO2-carbon based composites are of great significance in a wide range of applications including photocatalytic degradation. This is attributed to the high photodecomposition efficiency of the composites as compared to independent TiO2. Carbon materials such as cellulose polymer and multiwalled carbon nanotubes (MWCNTs) are considered as good supports for TiO2 owing to their unique properties such as lightweight, large surface area and high aspect ratio. Lately, the study of cellulose-MWCNTs composite has been an area of academic interest due to its large mass fraction, and prowess to facilitate toughening mechanisms in fiber bridging. However, a cost-effective method that can improve the dispersion and interfacial adhesion of the MWCNTs in the polymer is still required. Thus different modification methods of MWCNTs have been explored to increase the binding sites of the material. In this study, it was hypothesized that the cellulose’s potential as a TiO2 support can be improved by hybridizing it with MWCNTs resulting in high TiO2-C photocatalytic activity through synergistic effect. A catalytic decomposition of Fe-Co/CaCO3 was used over C2H2 to fabricate the MWCNTs. Thereafter, the MWCNTs were functionalized by (1) acid-treatment (referred to as fMWCNTs), (2) nitrogen doping by in situ and ex situ methods (referred to as in situ N-MWCNTs and ex situ N-MWCNTs, respectively) and (3) both acid treatment and nitrogen doping (referred to as in situ fN-MWCNTs and ex situ fN-MWCNTs). Moreover, cellulose-N-MWCNTs (C@fN-MWCNTs) hybrid was prepared by electrospinning a solution of cellulose acetate/in situ fN-MWCNTs (11/0.115) in DMAc at 25 kv and 1 mL/h. The prepared MWCNTs and cellulosic materials were further used as support materials of TiO2 in the photodegradation of methyl violet (MV 6B). The supported TiO2 catalysts were prepared by a sol-gel method and then analyzed using various techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy. iii TGA results revealed that in situ N-MWCNTs contained high impurities inclusive of as Fe, Co, Ca, and amorphous carbon which were identified by XRD analysis. Nevertheless, TGA, BET and TEM, showed that acid treatment of MWCNTs improves their purity, surface area and anchoring sites for the TiO2, respectively. Furthermore, SEM results showed that C@fNMWCNTs hybrid interacts with TiO2 better than cellulose fibers. This was in accord with the PL results which showed a reduction in the electron/hole recombination. However, the surface area of C@fN-MWCNTs was very low compared to cellulose fibers which resulted in low dye adsorption capacity by C@fN-MWCNTs. The photocatalytic degradation activity commercial TiO2 was enhanced by 3.7% and 5.6% after being supported on cellulose and C@fN-MWCNTs, respectively. Thus, incorporating in situ fNMWCNTs with cellulose did improve the cellulose’s potential as a TiO2 support. However, the overall photocatalytic degradation performance of TiO2/C@fN-MWCNTs was less than that of in situ TiO2/fN-MWCNTs. This may be due to the reduction in the surface area, which resulted in reduced adsorption and thus lowers degradation efficiency.
The effect of low level sulfide addition and the performance of precipitated- iron Fischer-Tropsch catalysts
(2016-08-31) Bromfield, Tracy Carolyn
Precipitated-iron Fischer- Tropsch catalysts were sulfided in the range 500 - 20000 ppm S/Fe with an aqueous sulfide source (Na2S, (NB4)zS, (NB4)zS5) during the precipitation process. Sulfidation was performed at pH 10.75, 8.5 and 6.9. Sodium ions were removed by centrifugation, and atomic absorption analysis confirmed low sodium levels (0-51 ppm). Based on solution speciation models, ferrous sulfide (FeS) which formed from aqueous HS' species, was found to influence the iron-oxyhydroxide crystallite morphology. It is proposed that, when sulfide was added at pH 10.75, FeS molecules functioned as nuclei for crystallite growth, while a pH 6.9 they assisted 'with the aggregation of particles. The processes of nucleation and aggregation appeared to be in competition following sulfidation at pH 8.5, resulting in a composite morphology that produced an inactive catalyst. The bulk structure of the catalysts was elucidated using XRD, SEM and nitrogen porosimetry, All sulfided catalysts exhibited enhanced BET surface areas and total pore volumes with a maximum at 2000 ppm S (surface area = 166 m2/g,total pore volume = 0.254 cm3/g) compared to an unsulfided catalyst (surface area = 58 m2/g, total pore volume = 0.184 cm3/g), Furthermore, for any series of catalysts at the same level of sulfidation, the BET surface areas were observed to decrease as the pH of sulfide addition decreased. Increasing levels of sulfidation (to 20000 ppm) brought about an increase in crystallite size and therefore, improved crystallinity as determined by XRD measurements. Materials with larger crystallites possess smaller surface areas, and thus the crystallinity was found to increase as the pH of sulfidation decreased. Surface characterisation by XPS after calcination at 400°C and reduction (400°C), revealed sulfate species (169.4 eV) on catalysts sulfided with 500-2000 ppm, while sulfide species (162.O eV) emerged at higher sulfide content. No sulfates were observed on reduced catalysts following calcination at 200 C. [Abbreviated Abstract. Open document to view full version]
Chemical reactivity: teaching and learning problems and attempted solutions
(2016-06-10) Bapoo, Abdool Hamid
This thesis reports on school teaching and learning problems in the area of chemical re&ctivity and on an attempt to overcome some of these problems with an in-service course for teachers. rhase 1 of this research was a study of the conceptions of aspects of reactivity, rates of reaction and chemical equilibrium held by secondary school students. The nature and origin of the conceptual di:Eficltlties, i.n the above areas were investigated. The concepts of ret,ctivity and reactivity se.rd.e s are formally introduced at a standard 8 level. These concepts form a springboard for the understanding of more formal quantitative worl< done i.n rates of reaction and chemical equilibrium at a standard 10 level. Two multiple-choice questionnaires, a reactivity qUestionnaire and a i'a'Ceand (.,quilibrium qUestionnaire, were designed to probe for the nature and ori9i.n of views held )y st
Analysis and modelling of residence time distribution in a high speed gas reactor
(2015-07-02) Rabbitts, M C
Flow patterns and the associated residence time distribution are important factors affecting the performance of a chemical reactor. This dissertation investigates the flow patterns in a scale-model of a gas reactor where opposing inlet jets are used to create mixing. The effects of different vessel geometries and gas flowrates are investigated. The residence time distribution (RTD) was measured by means of radioactive tracer techniques. The extremely short mean re^’dence time in the vessel necessitated the use of sharp tracer pulses and a computerised digital recording system. Flow visualisation experiments were also performed to aio interpretation of the results. The results were analysed using the method of frequency analysis, in which the coefficients of the Fourier Series arc used to characterise the RTD curves. It was found that the shape of the RTD curve is unaffected by change in flowrate; i.e. the flow patterns are independent of flowrate over the range investigated. The geometry of the vessel however plays a major role. The effect of reducing the length/diameter ratio of the reactor is an increase in the mixedness in the vessel. Internal recirculation patterns are well-defined at a higher length/diameter ratio. The RTD curves were also used to set up mathematical flow models. Simple multi-parameter models were used, where the vessel was viewed as consisting of interconnected flow regions such as plug flow, stirred tank, and axial dispersion. Close fits of the mathematical models to the experimental RTD curves were obtained.

3. Electronic Theses and Dissertations (ETDs) - All submissions

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