Philosophiae Doctor - PhD / The production of biodiesel (methyl esters) from vegetable oils represents analternative means of producing liquid fuels from biomass, and one which is growing rapidly in commercial importance and relevance due to increase in petroleum prices and the environmental advantages the process offers. Commercially, biodiesel is produced from vegetable oils, as well as from waste cooking oils and animal fats. These oils are typically composed of C14-C20 fatty acid triglycerides. In order to produce a fuel that is suitable for use in diesel engines, these triglycerides are usually converted into the respective mono alkyl esters by base-catalyzed transesterification with short chain alcohol, usually methanol. In the first part of this study, the transesterification reactions of three different vegetable oils; sunflower (SFO), soybean (SBO) and waste cooking oil (WCO) with methanol was studied using potassium hydroxide as catalyst in a conventional batch process. The production of biodiesel from waste cooking oil was also studied via continuous operation systems (employing the use of low frequency ultrasonic technology and the jet loop reactor). The characterisation of the feedstock used and the methyl ester products were determined by different analytical techniques such as gas chromatography (GC), high performance liquid chromatography (HPLC) and thin layer chromatography (TLC). The effects of different reaction parameters (catalyst amount, methanol to oil ratio, reaction temperature, reaction time) on methyl ester/FAME yield were studied and the optimum reaction conditions of the different process systems were determined. The optimum reaction conditions for production of methyl esters via the batch process with the fresh oil samples (SFO and SBO) were established as follows: a reaction time of 60 min at 60 ºC with a methanol: oil ratio of 6:1 and 1.0 KOH % wt/wt of oil; while the optimum reaction conditions for the used oil (WCO) was observed at a reaction time of 90 min at 60 ºC, methanol: oil ratio of 6:1 and 1.5% KOH wt/wt of oil. The optimum reaction conditions for the transesterification of the WCO via ultrasound technology applied in a continuous system in this study were: a reaction time of 30 min, 30 ºC, 6:1 methanol/oil ratio and a 0.75 wt% (KOH) catalyst concentration. The ultrasound assisted transesterification reactions performed at optimum conditions on the different oil samples led to higher yields of methyl esters (96.8, 98.32 and 97.65 % for WCO, SFO and SBO respectively) compared to methyl esters yields (90, 95 and 96 % for WCO, SFO and SBO respectively) obtained when using conventional batch procedures. A considerable increase in yields of the methyl esters in the ultrasound assisted reaction process were obtained at room temperature, in a remarkably short time span (completed in 30 min) and with a lower amount of catalyst (0.75 wt % KOH) while the results from the continuous jet loop process system showed even better results, at an optimum reaction condition of 25 min of reaction, a methanol: oil ratio of 4:1 and a catalyst amount of 0.5 wt%. This new jet loop process allowed an added advantage of intense agitation for an efficient separation and adequate purification of the methyl esters phase at a reduced time of 30 min. The use of homogeneous catalysts in conventional processes poses many disadvantages; heterogeneous catalysts on the other hand are attractive on the basis that their use could enable the biodiesel production to be more readily performed as a continuous process resulting in low production costs. Consequently, a solid base catalyst (KNO3/FA) prepared from fly ash (obtained from Arnot coal power station, South Africa) and a new zeolite, FA/Na-X synthesized from the same fly ash were used as solid base catalysts in the transesterification reactions in the conversion of a variety of oil feedstock with methanol to methyl esters. Since fly ash is a waste product generated from the combustion of coal for power generation, its utilization in this manner would allow for its beneficiation (as a catalytic support material and raw material for zeolite synthesis) in an environmentally friendly way aimed at making the transesterification process reasonably viable. Arnot fly ash (AFA) was loaded with potassium (using potassium nitrate as precursor) via a wet impregnation method while the synthesized zeolite FA/Na-X was ion exchanged with potassium (using potassium acetate as precursor) to obtain the KNO3/FA and FA/K-X catalysts respectively. Several analytical techniques were applied for characterization purposes. The results of the XRD and XRF showed that the AFA predominantly contained some mineral phases such as quartz, mullite, calcite and lime. The high concentration of CaO in AFA was apparent to be beneficial for the use of fresh fly ash as a support material in the heterogeneous catalysed transesterification reactions. XRD characterisation of KNO3/FA results indicated that the structure of KNO3/FA gradually changed with the increase in KNO3 loading. The catalyst function was retained until the loading of KNO3 was over 10 %. IR spectra showed that the KNO3 was decomposed to K2O on the fly ash support during preparation at a calcination temperature of 500 ºC. The CO2-TPD of the KNO3/FA catalysts showed that two basic catalytic sites were generated which were responsible for high catalytic abilities observed in the transesterification reactions of sunflower oil to methyl esters. On the other hand, XRD results for the as- received zeolite synthesized from AFA showed typical diffraction peaks of zeolite NaX. SEM images of the FA /NaX showed nano platelets unique morphology different from well known pyramidal octahedral shaped crystal formation of faujasite zeolites and the morphology of the FA /KX zeolite did not show any significant difference after ion exchange. The fly ash derived zeolite NaX (FA /NaX) exhibited a high surface area of 320 m2/g. The application of the KNO3/FA catalysts in the conversion reactions to produce methyl esters (biodiesel) via transesterification reactions revealed methyl ester yield of 87.5 % with 10 wt% KNO3 at optimum reaction conditions of methanol: oil ratio of 15:1, 5 h reaction time, catalyst amount of 15 g and reaction temperature 160 °C, while with the use of the zeolite FA/K-X catalyst, a FAME yield of 83.53 % was obtained for 8 h using the ion exchanged Arnot fly ash zeolite NaX catalyst (FA/KX) at reaction conditions of methanol: oil ratio of 6:1, catalyst amount of 3 % wt/wt of oil and reaction temperature of 65 ºC. Several studies have been carried out on the production of biodiesel using different heterogeneous catalysts but this study has been able to uniquely demonstrate the utilization of South African Class F AFA both as a catalyst support and as a raw material for zeolite synthesis; these catalyst materials subsequently applied sucessfully as solid base catalysts in the production of biodiesel. / South Africa

Philosophiae Doctor - PhD / In South Africa, almost 90 % of the country’s electricity is generated from coal combustion. This reliance on coal for energy production is projected to continue in the near and medium term due to the increasing demand for industrial and domestic energy. During coal combustion, a large quantity of fly ash is produced as the main waste product and in South Africa approximately 36 - 37 million tons of fly ash is produced on a yearly basis. The management of huge quantities of fly ash has been and still is a continuing challenge that requires urgent intervention. In this regard, there exists an urgent need to maximize fly ash beneficiation, thus forming the motivation for this research. The overall objectives of this thesis was to synthesize high pure phase zeolites A and X from South African fly ash, study their formation mechanism, and explore the potential of mine waters during the synthesis process as well as developing new and efficient zeolite synthetic protocols by the use of ultrasound. In order to address these objectives, the research was designed in a sequential manner so that the preceding results could act as a platform for the attainment of the next objective. In this case, the identification and optimization of synthesis conditions for producing zeolite A and X acted as a basis for understanding the influence of use of mine waters as a substitute for pure water. This further laid the foundation for the in-situ ultrasonic monitoring of the formation process of zeolite A and X from fly ash. The final stages of the study involved use of ultrasonic energy as an ageing tool to improve the conditions obtained during the hydrothermal synthesis of zeolite A as well as investigate the potential to synthesize zeolites directly by use of ultrasound without the need for the fusion, aging or conventional hydrothermal treatment step. The result of the optimized synthesis conditions for producing zeolite A starting either from clear extract of fused fly ash or unseparated, fused South African class F fly ash slurry were molar regimes of 1 Al2O3 : 30.84 Na2O : 4 SiO2 : 414.42 H2O or 1 Al2O3 : 5.39 Na2O : 2.75 SiO2 : 111.82 H2O respectively and at a hydrothermal synthesis temperature of 100 °C for 2 hours. The optimized procedure was simple, efficient and resulted in a considerable improvement of the quality and phase purity of the zeolite A product when the clear extract of fused fly ash was used instead of starting from unseparated, fused fly ash slurry. On the other hand, the optimized synthesis conditions for preparing the typical octahedral shaped zeolite X from South African fly ash was found to be a molar regime of 1 Al2O3 : 4.90 Na2O : 3.63 SiO2 : 115.92 H2O at a hydrothermal synthesis temperature of 80 ºC for hours.

Please read the abstract in the section 00front of this document / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

An increasing demand for electricity in modem society has resulted in the burning of large quantities of coal and ultimately the production of large quantities of fly ash. The petrochemical industry, if based on coal in a country such as South Africa, also produce large quantities of fly ash. In a semi-arid country like South Africa, there is a need to recover water. Processes currently in use for the recovery of wastewater produce large quantities of brines. These brines are stored in waste dams, which are not only expensive to maintain, but also pose a potential threat to the environment. The process of geosynthesis led to the development of a new type of material, namely geopolymers. Geopolymers can best be viewed as a polymeric silicon-oxygen¬aluminium framework with alternating silicon and aluminium tetrahedra joined together in three directions by sharing all the oxygen atoms. Cations such as Na+, K+, Ca2+ and H3O+ must be present in the framework cavities to balance the negative charge generated by the Al3+ in IV-fold co-ordination. It was attempted in this study to manufacture a geopolymeric binder, supplying most of the ingredients through waste materials. In the first set of experiments, matrices containing different amounts of fly ash, kaolinite, sodium hydroxide, sodium silicate and brine or water were synthesised by mixing and heating at 50°C for 24 hours. Compressive strength measurements showed a maximum strength of 4.05 MPa after 28 days. Leaching tests indicated that sodium was the best stabilised showing a stabilisation of between 30 and 40% (70 to 60% of the sodium initially added leached out again). The anions were stabilised to a lesser extent. Infrared spectra obtained confirmed an aluminosilicate structure. The second set of experiments was done to obtain the optimum curing conditions. Matrices containing the same amounts of fly ash, kaolinite, sodium hydroxide, sodium silicate and brine or water were cured at different temperatures and for different time periods. The matrices containing water showed a maximum compressive strength of 7.25 MPa after 28 days when cured at 60°C for 48 hours, while their brine-containing counterparts showed a maximum compressive strength of 7.76 MPa after 28 days when cured at 70°C for 72 hours. Infrared spectra obtained confirmed an aluminosilicate structure while X-ray diffraction patterns obtained indicated a largely amorphous product. In the third set of experiments matrices containing different amounts of fly ash, metakaolinite, sodium hydroxide, sodium silicate and brine or water were synthesised by mixing and heating at the optimum conditions determined previously. Compressive strength measurements indicated a maximum strength of 1.45 MPa after 28 days. Leaching tests indicated a higher stabilisation of the cations than in the first set of experiments. Potassium was the best stabilised, showing a stabilisation of above 80%. The anions were again stabilised to a lesser extent. Infrared spectra obtained confirmed an aluminosilicate structure while X-ray diffraction patterns obtained indicated a largely amorphous material. / Dissertation (MSc (Chemistry))--University of Pretoria, 2007. / Chemistry / unrestricted

Please read the abstract in the section 00front of this document / Dissertation (MSc Agric (Pasture Science))--University of Pretoria, 2005. / Plant Production and Soil Science / unrestricted

Please read the abstract in the section 00front of this document. / Thesis (PhD (Chemistry))--University of Pretoria, 2008. / Chemistry / unrestricted

Please read the abstract in the section 00front of this document / Thesis (PhD (Entomology))--University of Pretoria, 2005. / Zoology and Entomology / unrestricted

No description available.

Le phénomène de carbonatation des matériaux cimentaires est l'une des causes majeures de la corrosion des armatures de structures en béton armé. Ce phénomène est étudié depuis de nombreuses années sur les ciments Portland ordinaires CEM I, et les mécanismes sont relativement bien identifiés. Néanmoins, on remarque que si l'on substitue une partie du ciment par des ajouts tels que des cendres volantes, la réaction pouzzolanique ou les réactions d'hydratation qui s'en suivront amèneront à un contenu molaire plus faible en CHet aboutiront à la création d'une plus grande quantité d'hydrates de type C-S-H. Le pouvoir tampon qu'exerce la portlandite sur le pH de la solution interstitielle sera affaibli et le matériau cimentaire sera potentiellement plus sensible à la présence de CO2 au travers d'une carbonatation des C-S-H qui sera plus marquée. D'un point de vue physique, les évolutions microstructurales induites par un niveau élevé de carbonatation des C-S-H deviennent complexes et peuvent accélérer la diffusion du CO2. Cette thèse a ainsi pour but de caractériser le comportement vis-à-vis de la carbonatation des ciments contenant de forts dosages en cendres volantes et de développer une modélisation des systèmes cimentaires correspondants. Des pâtes de ciment et mortiers ont été formulés avec des rapports E/C variables et différents taux volumiques de substitution en cendre volante. Après une longue cure endogène, des essais de carbonatation accélérée ont été réalisés (10% de CO2, 25°C et 63% HR). À diverses échéances, des essais destructifs (analyse thermique, porosimétrie au mercure et projection de phénolphtaléine) et non-destructifs (gammadensimétrie) ont permis de quantifier le dioxyde de carbone fixé dans chaque type d'hydrate (CH et C-S-H), les changements de microstructure induits (porosité, distribution poreuse), ainsi que l'eau de structure libérée par carbonatation. On a ainsi pu relier les changements de microstructure et la libération d'eau avec les niveaux de carbonatation de la portlandite et des C-S-H.Dans un second temps, la plateforme de modélisation, Bil (sous licence GPL), développée à l'Ifsttar a été utilisée comme support pour le développement d'un modèle aux volumes finis. Il permet de décrire simultanément des réactions chimiques couplées à un transport de matière. Les lois de comportement chimiques - microstructurales (évolution du volume molaire des C-S-H en fonction de leur état de décalcification) et hydriques (eau relarguée par la carbonatation) mises en évidence par la campagne expérimentale ont pu être ainsi introduites dans le modèle. La cinétique de dissolution de la portlandite est paramétrée par une réduction d'accessibilité des amas de cristaux de CH qui, au cours du temps, se recouvrent d'une gangue de calcite de moins en moins perméable. La contribution des C-S-H est prise en compte. Une approche thermodynamique originale permet de décrire leur état de décalcification à l'équilibre au cours de la carbonatation. Au final, de nombreuses espèces chimiques, ainsi que leur spéciation, sont introduites dans le modèle, notamment les alcalins qui ont un effet marqué sur le pH / Reaction of gaseous atmospheric CO2 with calcium-bearing phases in concrete infrastructure components is known to cause a lowering of alkalinity, leading to depassivation and corrosion of rebars. Carbonation mechanism is quite well understood from a physico-chemical point of view, especially in the case of materials made of OPC. Nonetheless the impact of supplementary cementitious materials (SCM), such as fly-ash, on carbonation is still an active research field. The pozzolanic reaction between CH and fly ash implies a lower portlandite content and a higher C-S-H content. Whilst CH is buffering the pH, its lower content in these materials may lead to a lower resistance to carbonation and to a higher contribution of C-S-H in terms of microstructural changes. Thus, this PhD thesis aims at understanding the effect of cement substitution by high contents of fly ash and develop a numerical model describing the carbonation of these cementitious materials. Accelerated carbonation tests (10% CO2, 25°C and 63% RH) were performed on various cement pastes containing fly ash (0%, 30% and 60% of volumic substitution and water-to-cement ratio before substitution of 0.45 and 0.6). Carbonation profiles were assessed by destructive and non-destructive methods such as thermogravimetric analysis and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was studied at different ages of CO2 exposure. By correlating microstructure changes with the degree of carbonation of each hydration product related to the formation of calcium carbonate, we are able to propose analytical relationships linking the decrease in porosity and the amount of released water to the carbonation level of CH and C-S-H.The modeling platform Bil (GPL) developed at Ifsttar was used to develop a reactive transport modeling of atmospheric carbonation, using a finite volume method. We introduced in the model the constitutive equations we highlighted using the experimental data. Microstructure evolution was quantified, taking into account the effect of the progressive decalcification of C-S-H linked to their molar volume, as well as the quantity of water released by carbonation. Combined with a kinetic formulation of CH dissolution, C-S-H decalcification was described by an original thermodynamic approach. In the end, many chemical species were introduced in the model, such as alkalis which strongly affect pH

Les systèmes d’aide à la décision existants sont modélisés selon un modèle multidimensionneldédié à l’analyse. Leurs principales limitations sont leur structure statique, leur volume et lefait qu’ils ne prennent pas en compte ni l’évolution des sources de données ni celle desbesoins d’analyse. Dans cette thèse, nous proposons une architecture dynamique pourl’analyse en ligne à la demande qui est différente d’un entreposage physique de données dansune base cible avec un modèle figé. Les données dans notre architecture peuvent continuer àévoluer et dans leur contenu et dans leur structure selon l’activité ou l’environnement qu’ellesdécrivent. Le recueil de données et leur structuration sous forme de contextes d’analyse se faitdonc au moment où l’on souhaite effectuer les diverses analyses envisagées. L’accès auxsources de données ne se fera alors qu’à ce moment.Pour pouvoir mettre en oeuvre cette architecture, nous envisageons une solution selon deuxgrands axes :– La construction d’un système d’intégration de données fondé sur une approche parmédiation. Afin de rendre la recherche de données pertinente, il fallait décrire defaçon plus proche de la réalité les sources de données. Pour cela, nous avons optépour l’utilisation des ontologies comme modèle de description des sources ainsique de leurs relations.– La mise en place d’un dispositif de création de contextes d’analyse à la demandesous forme de cubes de données. Ce dispositif est basé également sur l’utilisationde l’ontologie du domaine décisionnel et d’un entrepôt de cubes de données. / Current decisional systems are modelled according to a multidimensional model which, isdedicated to on-line analysis. Their principal limitations lie in their structure, their volume andthat they do not take into account data sources and analysis needs evolution. In this thesis, wepropose a dynamic architecture for on-line analysis on-the-fly which is different fromwarehousing data in a target base with a fixed model.In our architecture, data can continue to evolve in their sources according to the activity thatthey describe. Collecting and structuring data in analysis contexts is when we want to makeanalysis.To implement this architecture, we consider a solution composed of two main parts:- The construction of a data integration system by mediation based on ontologies.- The implementation of a dispositive to building analysis contexts on-the-fly which isbased on ontologies to describe the decisional domain.