This dissertation is concerned with aspects of the newly-proposed approach to nonlinear acoustics in which the Lagrangian description of gas motion is followed. It contains a systematic survey of the approach which leads to the so-called dynamic piston problem. Then new situations regarding the piston problem are studied. These situations cover cases of varying applied pressure and results concerning the formation of shock discontinuities are presented. / Dissertation (MSc)--University of Pretoria, 2013. / Mathematics and Applied Mathematics / unrestricted

The formation and oxidation of transition metal nitrides and carbides is reviewed and the crystal structures and types of bonding are discussed. Types of nitrides and carbides are categorized in terms of physical and chemical properties and type of bonding. The principles of sintering are summarised. Tlie theory and applications of thermal analytical techniques are reviewed. Surface area determination and estimation of average crystallite size by the BET method utilizing the adsorption of nitrogen gas at -196 X are explained along with the application of x-ray dififractometry ajid scanning electron microscopy to work in this area. In this present research selected transition metal nitrides and carbides have been oxidised in air and carbon dioxide. Activation energies have been determined for these reactions from isothennal oxidations utilizing the Arrhenius equation and from oxidations at different heating rates utilizing the Kissinger equation. Kinetic schemes for the isothermal oxidations have been proposed based on two models of the reactions, that is half order kinetics in which the rate of reaction is determined by the diffijsion of oxidising gas througli the oxide product and two-thirds order kinetics in which the reaction takes place at the surface of a spherical particle of diminishing size as the reaction pioceeds. Surface area measurements and electron microscopy have been utilized to study the ability of the product formed during the oxidations to sinter. X-ray diffractometry has been used to identify the crystal phases present in the initial nitride or carbide and in the oxidation products. The activation energies of the carbides were found to be lower than that found for the respective nitrides. At low temperatures the carbides oxidised more extensively than the respective nitrides, but at high temperatures the situation was reversed. This is explained in terms of the difference in the preexponential term of the Arrhenius equation. Tlie kinetics were found to be dependent on whether the oxide produced was structurally compatible with the remaining reactant and whether the oxide produced was able to sinter at the temperature used in the experiment.

The limited availability of fossil resourses mandates the development of new energy vectors, which is one of the Grand Challenges of the 21st Century [1]. Biocatalytic energy conversion is a promising solution to meet the increased energy demand of industrialized societies. Applications of biocatalysis in the gas-phase are so far limited to production of fine chemicals and pharmaceuticals. However, this technology has the potential for large scale biocatalytic applications [2], e.g. for the formation of novel energy carriers. The so-called solid/gas biocatalysis is defined as the application of a biocatalyst immobilized on solid-phase support acting on gaseous substrates [3]. This process combines the advantages of bio-catalysis (green chemistry, mild reaction conditions, high specicity & selectivity) and heterogeneous dynamic gas-phase processes (low diffusion limitation, high conversion, simple scale-up). This work presents the modifications of a PID Microactivity Reference reactor in order to make it suitable for solid/gas biocatalysis. The reactor design requirements are based on previously published laboratory scale solid/gas systems with a feed of saturated vapors [4]. These vapors are produced in saturation flasks, which were designed and optimized during this project. Other modifications included relocation of the gas mixing chamber, redesigning the location and heating mechanism for the reactor tube, and heating of the outlet gas line. The modified reactor system was verified based on the Candida antarctica lipase B catalyzed transesterication of ethyl acetate with 1-hexanol to hexyl acetate and ethanol and results were compared to liquid-phase model reactions. Products were analyzed on line by a gas chromatograph with a flame ionization detector. C. antarc- tica physisorbed on silica particles produced a 50% conversion of hexanol at 40 C in the gas-phase. A commercial immobilized lipase from Iris Biotech produced 99% and 97% conversions of hexanol in similar experiments. This project achieved its goal to design, establish and successfully verify a solid/- gas biocatalysis reactor. Future work will target optimization of the reactor's operating conditions and the development of whole cell catalysts for energy production reactions. Potential experiments include the study of hydrogenolytic carbon dioxide reduction to methanol by free enzymes or methanogenic organisms [5], and the investigation of hydrogen production by water splitting of algae or cyanobacteria.

Carbon capture and storage technologies are necessary to start lowering greenhouse gas emissions while continuing to utilize existing thermal power generation infrastructure. Calcium looping is a promising technology based on cyclic calcination/carbonation reactions which utilizes limestone as a sorbent. Steam is present in combustion flue gas and in the calciner used for sorbent regeneration. The effect of steam during calcination on sorbent performance has not been extensively studied in the literature. Here, experiments were conducted using a thermogravimetric analyzer (TGA) and subsequently a dual-fluidized bed pilot plant to determine the effect of steam injection during calcination on sorbent reactivity during carbonation. In a TGA, various levels of steam (0-40% vol.) were injected during sorbent regeneration throughout 15 calcination/carbonation cycles. All concentrations of steam were found to increase sorbent reactivity during carbonation. A level of 15% steam during calcination had the largest impact. Steam changes the morphology of the sorbent during calcination, likely by shifting the pore volume to larger pores, resulting in a structure which has an increased carrying capacity. This effect was then examined at the pilot scale to determine if the phase contacting patterns and solids heat-up rates in a fluidized bed were factors. Three levels of steam (0%, 15%, 65%) were injected during sorbent regeneration throughout 5 hours of steady state operation. Again, all levels of steam were found to increase sorbent reactivity and reduce the required sorbent make-up rate with the best performance seen at 65% steam.

Carbon capture and storage technologies are necessary to start lowering greenhouse gas emissions while continuing to utilize existing thermal power generation infrastructure. Calcium looping is a promising technology based on cyclic calcination/carbonation reactions which utilizes limestone as a sorbent. Steam is present in combustion flue gas and in the calciner used for sorbent regeneration. The effect of steam during calcination on sorbent performance has not been extensively studied in the literature. Here, experiments were conducted using a thermogravimetric analyzer (TGA) and subsequently a dual-fluidized bed pilot plant to determine the effect of steam injection during calcination on sorbent reactivity during carbonation. In a TGA, various levels of steam (0-40% vol.) were injected during sorbent regeneration throughout 15 calcination/carbonation cycles. All concentrations of steam were found to increase sorbent reactivity during carbonation. A level of 15% steam during calcination had the largest impact. Steam changes the morphology of the sorbent during calcination, likely by shifting the pore volume to larger pores, resulting in a structure which has an increased carrying capacity. This effect was then examined at the pilot scale to determine if the phase contacting patterns and solids heat-up rates in a fluidized bed were factors. Three levels of steam (0%, 15%, 65%) were injected during sorbent regeneration throughout 5 hours of steady state operation. Again, all levels of steam were found to increase sorbent reactivity and reduce the required sorbent make-up rate with the best performance seen at 65% steam.

Sulfur dioxide (SO₂), a well-known pollutant emitted from fossil fuel combustion, has major adverse health and environmental impacts. It is harmful at low concentration with a permissible exposure limit of two ppm for the eight-hour time-weighted average (TWA) value. Fortunately, its atmospheric concentration, like other air pollutants, has gradually reduced in Canada in the past years. However, despite the well-established flue gas desulfurization technologies, they have the disadvantages of being energy-intensive, not very efficient to achieve very low concentrations (at ppm level) and they operate at high temperatures. Moreover, emission standards are becoming more stringent. Novel methods are therefore investigated to capture SO₂, such as adsorption processes using zeolites and metal oxides (e.g., Iron (Fe) and Vanadium (V) based) which tend to sustain wide ranges of temperatures and pressures. Graphene oxide (GO) was also shown to physisorb SO₂ at low temperatures. In this work, we propose to metal functionalize GO as a step forward on the path for efficient SO₂ capture, by promoting the SO₂ oxidation reaction into sulfur trioxide (SO₃) for increased capacity due to a possible higher affinity with the surface. The GO has a high surface area, high porosity, and controllable surface chemistry. The aim is to achieve outlet concentration of SO₂ as low as 1 ppm through combined physisorption and reaction promoted that the presence of GO and metal, at low operating temperature. Iron oxide functionalized GO was synthesized using two different techniques: a polyol process (GO-FeₓOᵧ-P) and using a hydrolysis method (GO-FeₓOᵧ-H). The characterization analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), performed on the materials before and after SO₂ reaction show changes on the surface due to metal adding and to the sulfur capture. The breakthrough curves and the capacity calculations of the performed experiments have shown that with the addition of FeₓOᵧ on the surface of GO, the capturing capacity increases by a factor of three to four, indicating a possible change in the capturing mechanism. The evaluation of the temperature effect (from room temperature to 100℃) showed an increasing trend in the capture capacity for SO₂ with an increase in temperature, for both functionalized and non-functionalized GO, indicating it is not driven only by surface adsorption. The presence of sulfur species captured from the gas stream has been confirmed by energy-dispersive X-ray (EDXS) analysis. The future work would be focused on the investigation of the mechanisms and capturing phenomenon and the regeneration step for the materials in order to further improve the capturing capacity and process applicability.

Carbon dioxide (CO2) is a common working fluid for refrigeration systems. The triple point of CO2 (about −56 °C and 0.51 MPa) is often regarded as the lower operating limit for the con-ventional CO2 vapor compression systems, because below this temperature and pressure, solid CO2 could occur and block the system components. However, if the technical issue could be solved and a stable operation of a vapor compression cycle for heat absorption be-low the triple point pressure (or sublimation cycle) could be realized, there would be a great potential for CO2 to replace the common refrigerants with a very high environmental impact such as R-23 for refrigeration applications below −50 °C. The focus of this work is on the dis-cussion of the feasibly of the sublimation cycle regarding the energy efficiency and the block-ing issues. Seven different two-stage and three-stage CO2 sublimation systems are theoretically evalu-ated and compared to a two-stage R-23 system, which serves as a baseline. A calculation model for the systems is developed. The optimum intermediate pressures for each system as well as the high pressure for the systems in transcritical operations are calculated within the given temperature and pressure constraints. Multiple influence factors, such as the ambient temperature, compressor efficiency, are considered in determining the operating limit and evaluating the performance for each system. In order to find out the cause of the blockages in the sublimation system due to the solid CO2, the solid-gas flow is visualized through experiments. Different throttling devices are investi-gated under various inlet conditions. As the sublimator, a heated sight glass assembly is used. It is found that besides the inlet temperature and pressure condition, the tube wall in the down-stream section of the throttling devices has a great influence on the blockages. A larger heat flux also helps to reduce the blockage in the sublimator. Based on the knowledge gained from the theoretical investigation of the cycle variant and preliminary experiments, a cascade sublimation system is designed, constructed and tested. Despite the fact that the system still requires optimization in terms of energy efficiency and operation stability, it is capable of long continuous operation, and thus the basic feasibility of the sublimation cycle is verified. Finally, the further issues and improvement potentials for the heat transfer and sublimator are discussed.:Acknowledgment Abstract Contents Index of figures Index of tables List of abbreviations and symbols 1 Introduction 1.1 Background and Motivation 1.2 Objective and procedure 2 Fundamentals and state of the art 2.1 The R-744 sublimation cycle 2.2 Expansion into solid-gaseous region and critical flow 2.3 Sublimator and solid-gas two-phase flow 2.4 Summary 3 Thermodynamic analysis of sublimation systems 3.1 Definition of the cycle variants 3.1.1 The baseline system 3.1.2 R-744 cascade systems 3.1.3 R-744 booster systems 3.2 Boundary conditions 3.3 Description of the models 3.3.1 Compressor 3.3.2 Heat exchangers 3.3.3 Other components 3.3.4 Fluid properties 3.4 Process calculation and optimization 3.5 Results and discussion 3.5.1 General boundary conditions 3.5.2 Variable temperatures 3.5.3 Variable compressor efficiency 3.5.4 Variable pressure loss and superheating in the sublimator 3.6 Evaluation of the system variants 4 Experimental visualization of the solid-gas flow 4.1 Throttling below the triple point 4.1.1 Experimental setup - test rig I 4.1.2 Results and discussion 4.2 CO2-Sublimation in a horizontal channel 4.2.1 Experimental setup - test rig II 4.2.2 Results and discussion 4.3 Summary 5 Experimental investigation on the performance of a cascade sublimation system 5.1 Experimental setup – test rig III 5.1.1 The refrigerant cycles 5.1.2 The sublimating unit 5.2 Methodology 5.2.1 The measuring procedure 5.2.2 Data evaluation and uncertainty analysis 5.3 Results and discussion 5.3.1 Transient behavior 5.3.2 System performance 5.3.3 Compressor performance 5.3.4 Long period measurements 5.4 Summary 6 Existing issues and optimization potentials 6.1 Blockage-free operation at low wall temperatures 6.1.1 Supplementary experiment 6.1.2 Outlook 6.2 Heat transfer 6.2.1 Supplementary experiment 6.2.2 Outlook 7 Summary Literature Appendix A. Differential evolution A.1 Basics of differential evolution A.2 Convergence of the results for different system variants Appendix B. Mass flow rate from the capillary tubes B.1 Measurement of the mass flow rate B.2. Comparison of the results with the numerical model and correlations Appendix C. Supplement to the measurements of the test rig III C.1 Exemplary measurement of the R-23 operation C.2. Measurement of the air velocity for the sublimator Appendix D. Supplement to the measurements at low wall temperatures D.1. Calculation of the heat transfer coefficients for the airside D.2. Determination of the local sublimation heat transfer coefficients Publications during the PhD study

Ce travail de thèse porte sur l’étude et le développement de matériaux adaptés pour la conversion et le stockage thermochimique de l’énergie solaire concentrée à haute température (400-1200°C), en utilisant des réactions solide-gaz réversibles. Ce type de stockage peut être associé aux centrales solaires thermodynamiques pour la génération d’électricité. Une étude bibliographique a permis d’identifier et de sélectionner les matériaux les plus adaptés possédant une densité d’énergie élevée pour le stockage thermochimique, suivant les critères de domaine de température et de non-toxicité requis. Les matériaux sélectionnés sont des oxydes métalliques (de Fe, Mn, Co, Cu), ainsi que des carbonates et des hydroxydes (de Ca, Sr, Ba). Les travaux ont porté ensuite sur les équilibres thermodynamiques des systèmes afin de prévoir les températures de transition et capacités de stockage théoriques. Une étude expérimentale a également été effectuée avec pour objectifs de déterminer précisément les niveaux de température, capacités de stockage en oxygène et enthalpies pour chaque réaction, et de démontrer leur réversibilité complète sur plusieurs cycles successifs. Des oxydes métalliques mixtes (systèmes binaires de Co-Cu, Co-Fe, Mn-Fe, Mn-Co, Mn-Cu) et des pérovskites substituées sur sites A et B ont été développés afin d’optimiser les propriétés redox des matériaux pour le stockage thermochimique. Concernant les carbonates et les hydroxydes de Ca, Sr, Ba, l’addition d’un agent stabilisant (MgO) a permis d’améliorer la stabilité des matériaux et la réversibilité des réactions au cours des cycles. Enfin, un nouveau réacteur thermochimique solaire, permettant la conversion en continu de particules réactives solides, a été validé expérimentalement et optimisé dans le cas de la décomposition de CaCO3 pour le stockage de l’énergie solaire. / This PhD thesis deals with the study and development of suitable materials for thermochemical conversion and storage of concentrated solar energy at high temperature (400-1200°C), by using reversible solid-gas reactions. This type of storage can be associated with solar thermal power plants. A bibliographic survey was performed to identify and select the most promising materials with a high energy storage density for thermochemical storage, based on different required criteria. The selected materials are metal oxides (of Fe, Mn, Co, Cu), carbonates and hydroxides (of Ca, Sr, Ba). The work then focused on the thermodynamic equilibria to determine the theoretical transition temperatures and energy storage capacities of the selected materials. An experimental study was carried out in order to determine the reaction temperatures, oxygen storage capacities and enthalpies for each reaction, and to demonstrate their complete reversibility upon cycling. Mixed metal oxides (binary systems of Mn-Fe, Co-Fe, Co-Cu, Mn-Cu, Mn-Co) and A- and B-site substituted perovskites were developed to optimize their thermochemical energy storage properties. Regarding carbonates and hydroxides of Ca, Sr, Ba, the addition of a stabilizing agent (MgO) allowed improving the materials cycling stability and reactions reversibility during successive cycles. Finally, a novel lab-scale solar reactor was experimentally tested in order to investigate the continuous decomposition of CaCO3 particles for thermochemical solar energy storage application.

Maitriser la logistique de la chaine du froid à des températures de -20°C/-30°C reste un enjeu majeur de sécurité sanitaire. Des solutions auto-réfrigérées basées sur des systèmes thermochimiques sont très adaptées à la mise en température rapide de caissons isothermes et de leur maintien pendant plusieurs heures. Différents concepts et configurations de procédés répondant à cette problématique sont proposés. Leur modélisation dynamique de type nodale, impliquant une gestion thermodynamique d'un ou plusieurs réacteurs, a permis d'analyser leur comportement et d'évaluer leur pertinence. Les configurations les plus performantes sont sélectionnées et analysées afin d'établir un pré-dimensionnement industriel du procédé. D'autre part, la fiabilité de ces systèmes thermochimiques repose sur la qualité des transferts de masse et de chaleur au sein des réacteurs. A cette fin, une approche de diagnostic de dysfonctionnements possibles de réacteurs est développée. La méthodologie proposée s'appuie sur la comparaison de la réponse expérimentale du réacteur testé à celle modélisée d'un réacteur opérant dans les mêmes conditions opératoires présentant ou non des défauts. Une base de données de comportements de réacteurs défaillants, établie par simulation de défauts de typologie et d'intensité connues, permet ainsi la détection et l'identification rapide de possibles dysfonctionnements de réacteurs issus de la chaine de fabrication. / A better cold chain logistics understanding and control is a major safety issue in deep freezing processes within -20°C/-30°C. Self-cooling solutions based on thermochemical systems appear very suitable for rapid cooling of isothermal containers and its temperature regulation for several hours. Different process concepts and configurations are presented to tackle this problem. Through their dynamic nodal modeling, involving thermodynamic management of one or various reactors, an analysis of their behaviors is performed to evaluate their relevance. A selection of the best performing configurations leads us to an industrial pre-design.To undertake quality measures, a diagnosis approach for possible reactors malfunctions is developed. The methodology is based on a comparison of experimental responses between tested reactor and reference one under the same operating conditions. A database of simulated faulty behaviors will allow the detection and identification of possible malfunctions of reactors coming from the production line.

Le présent travail porte sur une étude comparative des caractéristiques optiques et électriques des décharges glissantes se propageant aux interfaces solide/gaz sur des isolateurs de verre, de Bakélite et de résine époxy en présence des gaz N2, CO2 et SF6 et des mélanges SF6/N2 et SF6/CO2, sous tension continue et alternative (50 Hz), en géométrie pointe - plan. L’objectif est de mieux comprendre les mécanismes impliqués dans l’initiation des décharges partielles et leur évolution et développement en décharges surfaciques (glissantes) lesquelles peuvent conduire au contournement des composants et systèmes haute tension. Les résultats obtenus montrent que la morphologie et la longueur finale (d’arrêt ou d’extension maximale) des décharges surfaciques dépendent de la forme et de l’amplitude de la tension, de l’épaisseur et de la nature du solide isolant, du type du gaz/mélange et de sa pression. Il est montré que la longueur finale des décharges Lf augmente quasi-linéairement avec la tension. Lf diminue lorsque la pression du gaz et/ou l’épaisseur du solide augmentent. Cette longueur est plus courte dans le SF6 que dans le CO2 ou le N2 ; et elle diminue significativement lorsque le taux du SF6 dans le mélange de gaz augmente. Par ailleurs, pour une tension donnée, Lf augmente avec la constante diélectrique de l’isolant solide. La longueur finale des décharges est nettement plus élevée sous tension alternative que sous tension continue. La morphologie des décharges glissantes générées sous tension continue et alternative est généralement non radiale; leur orientation est fortement influencée par la présence des charges d’espace présentes ou déposées sur la surface de l’isolateur. Une analyse fractale des décharges glissantes obtenues expérimentalement sous tension continue est également réalisée et une corrélation entre la dimension fractale, la pression du gaz, la constante diélectrique et l’épaisseur du matériau solide est mise en évidence. En particulier, la dimension fractale augmente lorsque la constante diélectrique augmente et/ou l’épaisseur de l’isolant solide diminue et/ou la pression du gaz diminue. / This work deals with a comparative study of optical and electrical characteristics of creeping discharges propagating at solid/gas interfaces on insulators made of glass, Bakelite and epoxy resin in the presence of N2, CO2 and SF6 gases and SF6/N2 SF6/CO2 mixtures, under DC and AC (50 Hz) voltage, using a point - plane electrode arrangement. The objective is to better understand the mechanisms involved in the initiation of partial discharges and their evolution and development into discharges surface (creeping discharges) that can lead to flashover of components and high voltage systems. The results show that the morphology and final length (maximum extension or stopping length) depend on the shape and amplitude of the voltage, the thickness and the nature of the solid insulator, type of gas / mixture and its pressure. It is shown that the final length Lf increases quasi-linearly with the voltage. Lf decreases as the gas pressure increases and/ or the thickness of the solid increases. Lf is shorter in SF6 than in CO2 or N2, and it decreases significantly when the rate of SF6 in the gas mixture increases. Moreover, for a given voltage, Lf increases with the dielectric constant of the solid insulation. The final length of the discharge is much higher under AC voltage than under DC voltage. The morphology of creeping discharges generated under DC and AC voltage is generally not radial; and their orientation is strongly influenced by the presence of space charges present or deposited on the surface of the insulator. A fractal analysis of creeping discharges experimentally obtained under DC voltage is also carried out and a correlation between the fractal dimension, the pressure of the gas, the dielectric constant and the thickness of the solid material is highlighted. In particular, the fractal dimension increases when the dielectric constant increases and / or thickness of the solid insulator decreases and / or pressure of the gas decreases.