Thèses sur le sujet « Supercriticall fluids »

2014 - 2015
In the last decade, the application of microparticles, nanoparticles and composite microparticles involved several industrial fields. Conventional micronization techniques, such as jet milling, spray drying, liquid antisolvent precipitation and solvent evaporation are sometimes not suitable, since the produced particles are irregular, with broad size distribution, could be degraded due to mechanical or thermal stresses and polluted with organic solvents or other toxic substances. In this context, supercritical fluids (SCFs) based techniques have been proposed as an alternative to traditional processes thanks to the specific characteristics of SCFs, mainly solvent power and liquid-like densities with gas-like transport properties, that can be tuned varying pressure and temperature. Among supercritical assisted micronization techniques, Supercritical Antisolvent (SAS) precipitation has been successfully used to obtain microparticles and nanoparticles of several kinds of compounds, such as pharmaceuticals, coloring matters, polymers and biopolymers. In this process carbon dioxide (CO2) is used as an antisolvent at supercritical conditions: a solution containing the product to be micronized is injected into the precipitation chamber, saturated with supercritical carbon dioxide under the chosen conditions of temperature and pressure. CO2, in contact with the solution, forms a mixture in which the product is insoluble, causing the precipitation... [edited by author]
XIV n.s.

Diese Arbeit zeigt die Verwendung ueberkritischer Fluide als analytisches Werkzeug fuer den Transport einer Gruppe nichtfluechtiger Molekuele, naemlich Nucleobasen, in die Gasphase. Das am haeufigsten verwendete ueberkritische Fluid ist Kohlendioxid, welches sich jedoch als zu ineffizient bei der Aufloesung von Nucleobasen herausstellte. Deshalb wurde ein Gemisch aus Ethylen mit Ethanol als Cosolvens als ueberkritisches Loesungsmittel verwendet. Für die Erkennung des kritischen Punktes reiner Fluide oder verduennter Fluidmischungen wurde eine neue Methode entwickelt. Die Verschiebung des kritischen Punktes von Ethylen durch Zugabe von Ethanol wurde experimentell ermittelt und mit der Zustandsgleichung von Soave Redlich Kwong in Beziehung gesetzt. Fuer einen Molenbruch des Cosolvens Ethanol von 0.054 erhoeht sich die kritische Temperatur nur um 5,5 C, wohingegen die Theorie eine Erhoehung um 10 C vorhersagt. Fuenf biologisch relevante Nucleobasen wurden mit Hilfe von 3% Ethanol als Cosolvens in ueberkritischem Ethylen geloest. Die Zusammensetzung des Ueberschall-Molekularstrahles der expandierten Loesung wurde mit einem Quadrupol-Massenspektrometer quantitativ analysiert. Das Signalverhaeltnis der Nucleobasen zu Ethylen lag in der Groessenordnung von 10^-4 bis 10^-5. Diese Nucleobasen wurden auch auf Oberflaechen abgeschieden, sowohl durch Hochdruckexpansion der ueberkritischen Loesungen, als auch durch Verdampfung von alkoholischen Loesungen (nach der ’Drop Casting’ Methode). Die dabei entstehenden Morphologien wurden ex-situ mittels Rasterkraftmikroskopie untersucht. Die Ursachen dieser Unterschiede werden anhand der relevanten Nukleationsmechanismen diskutiert.
This work highlights the use of supercritical fluids (SCF) as an analytical tool for the transfer of a group of non-volatile molecules, namely nucleobases, into the gas phase. The most commonly used SCF carbon dioxide was found inefficient in dissolving the nucleobases. Therefore, a mixture of ethylene (p_c = 50.6 bar and T_c = 9.35 C) with a cosolvent was used as the SC solvent. A new bracketing method was developed for detecting the critical point (CP) of pure fluids and diluted mixtures of fluids. The shift in CP of ethylene on addition of ethanol was determined and related to theoretical calculations by using the Soave Redlich Kwong equation of state. Comparing the experimental results to theoretical methods for calculating the CP showed large deviations. The critical temperature shifted by only 5.5 C when the mole fraction of the cosolvent i.e. ethanol was 0.054. Five biologically relevant were dissolved in SC ethylene using 3% of ethanol as cosolvent. The supersonic molecular beam composition of the expanded solution was analyzed quantitatively using a quadrupole mass spectrometer and the ratio of the nucleobases to ethylene in the beam was found to be of the order of 10^-4 to 10^-5. Surface deposition of the nucleobases through SCF solutions was carried out and the morphology was recorded using Atomic Force Microscopy. Remarkable differences were observed while comparing the morphology obtained after deposition using rapid expansion of supercritical solutions (RESS) and drop casting method. These differences are discussed in terms of diffusion, rate of evaporation of the solvent, degree of supersaturation, and the nucleation process.

La cellulose est le polymère naturel le plus abondant. Cependant son utilisation est limitée par sa faible dissolution due à des liaisons hydrogènes intra et inter moléculaires. Jusqu’à aujourd’hui des solvants toxiques sont utilisés dans les procédés de dissolutions de la cellulose. Par conséquent de nouveaux solvants pour la dissolution de la cellulose ont été intensivement étudiés comme solutions de rechange de ces procédés polluants. Une des solutions est d’utiliser la technologie des fluides supercritiques utilisant le dioxyde de carbone. Malheureusement, la cellulose reste insoluble dans le CO2 dans les conditions supercritiques et il est donc important d'étudier un mélange binaire d’un co-solvant (organique ou liquide ionique) et le CO2 pour le développement d’un nouveau procédé. Cependant la connaissance du point fondamental des paramètres contrôlant le processus de dissolution dans ces fluides ralentit le développement de l’utilisation de cet outil propre et peu couteux en énergie. Nous avons donc utilisé la simulation de dynamique moléculaire pour caractériser le processus de dissolution de la cellulose dans ces fluides. Pour cela, nous nous sommes intéressés aux fluides supercritiques purs, puis aux mélanges des fluides supercritiques avec un co-solvant et enfin nous avons étudié le processus de dissolution de modèle de celluloses et de caractériser l’effet de la pression, la température, la composition du mélange ainsi que les propriétés structurales de ces modèles de cellulose sur le processus de dissolution
Cellulose is insoluble in neat supercritical CO2 and the main objective of this work was to investigate mixtures of scCO2 with polar cosolvents for the development of new processing technologies for the cellulose dissolution. The objective is achieved by studying the dissolution process of monomer of cellulose and its various polymorphs. The effect of the t/d parameters on the dissolution process was analyzed by molecular dynamics simulation. We begin with analyzing structure of pure supercritical fluids and mixtures of supercritical fluids/cosolvents using unconvential tools: Voronoi tesselations and nearest neighbours approach.Thermodynamics of the mixtures of scCO2/cosolvents is analysed in order to check the validity of the potential models used in our simulations for what the method of thermodynamic integration to calculate the energy, entropy and free energy of mixing was applied. To analyze the dissolution of cellulose we started from studying the solvation free energy of cellobiose(cellulose monomer) which was calculated from molecular dynamics simulations using free energy perturbation method. The influence of conformational degrees of freedom on solvation free energy of cellobiose was also considered.Finally, the direct dissolution of cellulose crystals models in well-known good cellulose solvent(1-ethyl-3-methylimidazolium chloride) and then considered supercritical solvents. It was found that various mixtures of CO2 with cosolvents do not dissolve cellulose but they can considerably affect its crystalline structure whereas ammonia fluid can dissolve cellulose and this process is significantly influenced by temperature, pressure and density

This Thesis describes how continuous flow reactors and heterogeneous catalysts have been used in conjunction with supercritical fluid solvents to develop a new process for carrying out catalytic organic reactions. The process exploits both the advantages of supercritical fluids (e.g. solvent tunability allowing facile product separation from the solvent) and heterogeneous catalysts and can be considered a 'Green' method of chemistry. Various reaction types have been explored, namely noble-metal catalysed hydrogenation reactions, supported-acid catalysed Friedel-Crafts alkylation reactions and supported-acid catalysed Friedel-Crafts acylation reactions. Chapter 1: Introduction This Chapter begins with an introduction to the concept of 'Green Chemistry' and its significance at the present time. The problems with the use of conventional solvents are highlighted, and examples of two 'greener' solvent systems - ionic liquids and supercritical fluids - are described. For each of these solvent systems, a brief review .of their use in more recent reaction chemistry is included. Chapter 2: Experimental This Chapter begins by describing the supercritical flow equipment developed at Nottingham in some detail, and includes equipment modifications which were made both to improve the effectiveness and safe operation of the equipment. The final sections of the Chapter describe the catalysts and analytical techniques used during the course of the research. Chapter 3: Continuous Hydrogenation in Supereritical Fluids This Chapter opens with a summary of some initial hydrogenation work carried out at Nottingham prior to this research. The results section reports a range of non-selective hydrogenation reactions, commencing with a detailed study of the hydrogenation of cyclohexene which was used to explore the capabilities of the equipment. Chapter 4: Continuous Selective Hydrogenation in Supercritical Fluids This Chapter is a continuation of Chapter 3 and begins with an introduction to different types of selective hydrogenation, then highlights some literature examples of selective hydrogenations conducted in supercritical fluids. The results section reports several selective hydrogenation reactions conducted at Nottingham. Chapter 5: Continuous Friedel-Crafts Alkylation and Acylation Reactions This Chapter begins with a discussion of the problems associated with conventional Friedel-Crafts chemistry, most of which are associated with the homogeneous catalysts normally used. A review of the use of various solid acid materials investigated for Friedel-Crafts type activity follows and this section concludes with literature examples of some supercritical Friedel-Crafts processes. The results section begins by reporting Friedel-Crafts alkylation reactions and closes with a brief amount ofFriedel-Crafts acylation.

Initially, the feasibility of using supercritical fluid chromatography as a means of analysing veterinary drugs was investigated. The classes of drugs that were studied included xanthins, corticosteroids and diuretics. Packed column supercritical fluid chromatography interfaced to mass spectrometry using the moving belt and thermospray interfaces was also investigated as a method of analysis. Standard mixture of the drugs were successfully separated and detected using supercritical fluid chromatography-mass spectrometry. Supercritical fluid extraction was investigated for its potential as an alternative extraction technique for horse feed. A variety of horse feeds were investigated using supercritical fluid extraction with supercritical fluid chromatography-mass spectrometry and gas chromatography-mass spectrometry as the analysis methods. Water was demonstrated to enhance extraction of caffeine and theobromine. Hordenine, caffeine and theobromine were successfully extracted from the different horse feeds. Finally, coupled supercritical fluid extraction - supercritical fluid chromatography interfaced to a mass spectrometer was investigated. It was used as a means of extracting and detecting corticosteroids from horse urine. Results obtained demonstrate the potential of coupled supercritical fluid systems in the field of drug analysis.

The commercial applications and types of photochromic are many and varied. In this work spiroxazines, spiropyrans and fulgides, of the type used in optical lenses, were examined using a variety of analytic techniques. Initially the feasibility of using supercritical fluid extraction (SFE) for the extraction of photochromics, and some UV stabilisers, from lens matrices was investigated for the first time. Packed column supercritical fluid chromatography combined with mass spectrometry (pSFC/MS), using both a moving belt and a thermospray interface, was assessed as a method for the separation and analysis of standard photochromics, prior to analysis of unknown photochromic lens extracts. Standard mixtures of the photochromics were successfully separated by pSFC and the mass spectrometric fragmentation patterns were recorded. The unknown photochromics, extracted by SFE, were separated by pSFC and their structures were then suggested from the mass spectra. A preliminary investigation was also carried out into the use of electrospray/mass spectrometry for the analysis of standard photochromics and also a nickel quencher (UV stabiliser). The lens extracts were also analysed by GC/MS and GC/Fourier-transform infrared, which were found to be complementary analytical techniques to pSFC/MS. Part of the work in this thesis concerned the development of a liquid chromatography/mass spectrometry particle beam interface for use with pSFC/MS. After physical adjustment of the interface, the operating parameters and sensitivity of the technique were investigated. pSFC/MS with the particle beam interface was then successfully used for the first time for the separation and analysis of standard mixtures of tocopherols, estrogens and benzodiazepines. Timbo powder (Derris root) was then extracted by SFE and the resulting extract was analysed using the developed technique.

The modification of a gas chromatograph for capillary supercritical fluid was demonstrated. The resulting instrument was used to characterize the performance of capillary columns manufactured specifically for SFC applications. The columns were tested for inertness, efficiency, and immobilization before and after extraction with the supercritical CO₂ solvent. Stable deactivation layers were obtained with polyethylene glycol pyrolysis of Carbowax 20M and Superox 20M. Crosslinking was evaluated with dicumyl peroxide (DCP) and azo-t-butane (ATB) as free radical initiators with OV-1701 as a stationary phase. ATB was found to yield more efficient crosslinking and had less effect on column polarity. Quantitative reproducibility of SFC was evaluated for alkanes, alcohols and acids on columns which were deactivated by polyethylene glycol pyrolysis or by mixed cyclic siloxanes. Both gave better reproducibility than an untreated column but the overall reproducibility of the polyglycol deactivation was better than the cyclic deactivation. Liquid CO₂ extraction was performed on a variety of spices and food products. This sample preparation proved useful in isolating CO₂ soluble materials from complex matrices which were insoluble in CO₂.
Ph. D.

This thesis contains details of experiments involving the use of supercritical fluids (SCF) and high pressure liquids acting as antisolvents and transport media. The thesis is divided into six Chapters, each detailing different aspects of SCF technology. Chapter 1: Provides the reader with general information concerning the properties of supercritical fluids. Information is given concerning the use of SCFs as solvents and antisolvents. Reviews of the PCA and GAS processes are also provided. Chapter 2: Describes the initial study of the preparation of C60(gas) intercalation compounds. PCA generates compounds of C₂H₄, C₂H₆, C₃H₆ and C₃H₈, which were characterised by JR. A novel GAS technique is used to allow collection of PXD, SEM and MAS ¹³C NMR data of these compounds. Chapter 3: Details a novel supercritical antisolvent technique which allows the generation of macroscopic crystals of intercalate species C60(C₂H₄) and C60(C₂H₆). Crystallographic data on these compounds is presented. Datasets for the intercalates C60(CO₂), C70(C₂H₆) and C70(CO₂) were also collected. Chapter 4 Contains preliminary information on the formation of ‘nanopeapods', that is, carbon nanotubes filled with C60. A new route to the formation of these materials at room temperature is shown. Chapter 5: Describes the impregnation of mesoporous silica with aluminium containing species using SCF as a solvent. Materials display unprecedented stability to steaming with high retention of structural ordering and surface area. Chapter 6: Describes the extraction of template molecules from mesoporous silica. The possibility of complete removal of surfactant using a SCF is shown. The effect of temperature and pressure effects are also measured. Appendices are available at the back of the thesis to describe the equipment, analytical techniques and some safety principles for use of SCF.

The aim of this work was to investigate the applicability of using supercritical (sc) hydrofluorocarbons (HFCs) as alternative solvents for hydrogenation and polymer modification processes. Solubility studies in binary and ternary systems have been carried out using both dielectrometry and gravimetric techniques and results show that a range of unsaturated carboxylic acids (crotonic acid, 6-methoxy-1-tetralone, methylsuccinic acid, alpha-acetamido-cinnamic acid and itaconic acid) have a high degree of solubility in 1,1,1,2-tetrafluoroethane (HFC 134a). The solubility results were modelled successfully using the Peng-Robinson equation of state (PR EOS) and this model was used to devise a separation methodology for itaconic acid and methylsuccinic acid. It is suggested that HFC 134a can be used as both the reaction medium and the extracting solvent, which enables in-line separation of compounds during sc synthesis.;The homogeneous asymmetric hydrogenation of a range of unsaturated substrates (itaconic acid, dimethyl itaconate, alpha-acetamido-cinnamic acid and trans-2-methyl-2-penetenoic acid) has been studied using a rhodium/MonoPhos catalytic system. High yields and enantiomeric excesses (ee's) have been observed and this, coupled with the separation technique, provides an effective method of asymmetric reduction, which greatly enhances the commercial applicability of this technology.;The infusion of difluoromethane (HFC 32) into polystyrene (PS) and polyethylene (PE) has been characterised and the results have been compared to those obtained for carbon dioxide. Significant plasticization was observed in the polymeric materials and it was shown that manipulation of the experimental temperature, pressure and depressurisation rate could cause significant changes in the morphology of the samples.;It is concluded that sc HFCs are promising alternatives to conventional organic solvents and are useful for a variety of processes. These media have accessible critical constants, relatively high dielectric constant values and are able to facilitate the dissolution of polar solutes and rhodium based catalysts without the need for co-solvents or fluorinated ponytails. Furthermore, the investigation suggests that reactions carried out in the sc regime can allow facile reagent/product separation and it is logical to assume that a similar methodology can be applied to catalyst recovery.

The challenges and specialised equipment associated with supercritical electrochemistry means standard reference electrodes (such as the saturated calomel electrode) cannot be used. The aim of the thesis is the investigation of a range of metallocenes as model redox systems against a Pt pseudo reference electrode in supercritical fluids. This work was then extended to the development of a new high pressure reactor. Initially, a range of metallocenes were investigated in liquid analogues of the supercritical fluids (acetonitrile and dichloromethane) to investigate their suitability. Cyclic voltammetry performed at both micro and macro electrodes were used to examine the behaviour of each individual redox couple. Electrochemistry of the metallocenes were then evaluated in supercritical fluids (supercritical carbon dioxide with acetonitrile and supercritical difluoromethane) for investigation as model redox systems. The diffusion coefficients have been determined at both micro and macro electrodes for both supercritical fluids. The implementation of baffled micro and macro electrodes has shown that the intrinsic convection (at the electrode surface) within supercritical fluids can be dampened. The diffusion of metallocenes in nanoporous aluminium oxide membranes (13 – 55 nm diameter cylindrical pores), in both supercritical fluids has also been investigated. This work was then extended to the development of a new high pressure plastic reactor leading to the first, successful, supercritical fluid electrodeposition of bismuth in the plastic reactor.

Doutoramento em Engenharia Química
Os coeficientes de difusão (D 12) são propriedades fundamentais na investigação e na indústria, mas a falta de dados experimentais e a inexistência de equações que os estimem com precisão e confiança em fases comprimidas ou condensadas constituem limitações importantes. Os objetivos principais deste trabalho compreendem: i) a compilação de uma grande base de dados para valores de D 12 de sistemas gasosos, líquidos e supercríticos; ii) o desenvolvimento e validação de novos modelos de coeficientes de difusão a diluição infinita, aplicáveis em amplas gamas de temperatura e densidade, para sistemas contendo componentes muito distintos em termos de polaridade, tamanho e simetria; iii) a montagem e teste de uma instalação experimental para medir coeficientes de difusão em líquidos e fluidos supercríticos. Relativamente à modelação, uma nova expressão para coeficientes de difusão a diluição infinita de esferas rígidas foi desenvolvida e validada usando dados de dinâmica molecular (desvio relativo absoluto médio, AARD = 4.44%) Foram também estudados os coeficientes de difusão binários de sistemas reais. Para tal, foi compilada uma extensa base de dados de difusividades de sistemas reais em gases e solventes densos (622 sistemas binários num total de 9407 pontos experimentais e 358 moléculas) e a mesma foi usada na validação dos novos modelos desenvolvidos nesta tese. Um conjunto de novos modelos foi proposto para o cálculo de coeficientes de difusão a diluição infinita usando diferentes abordagens: i) dois modelos de base molecular com um parâmetro específico para cada sistema, aplicáveis em sistemas gasosos, líquidos e supercríticos, em que natureza do solvente se encontra limitada a apolar ou fracamente polar (AARDs globais na gama 4.26-4.40%); ii) dois modelos de base molecular biparamétricos, aplicáveis em todos os estados físicos, para qualquer tipo de soluto diluído em qualquer solvente (apolar, fracamente polar e polar). Ambos os modelos dão origem a erros globais entre 2.74% e 3.65%; iii) uma correlação com um parâmetro, específica para coeficientes de difusão em dióxido de carbono supercrítico (SC-CO2) e água líquida (AARD = 3.56%); iv) nove correlações empíricas e semi-empíricas que envolvem dois parâmetros, dependentes apenas da temperatura e/ou densidade do solvente e/ou viscosidade do solvente. Estes últimos modelos são muito simples e exibem excelentes resultados (AARDs entre 2.78% e 4.44%) em sistemas líquidos e supercríticos; e v) duas equações preditivas para difusividades de solutos em SC-CO2, em que os erros globais de ambas são inferiores a 6.80%. No global, deve realçar-se o facto de os novos modelos abrangerem a grande variedade de sistemas e moléculas geralmente encontrados. Os resultados obtidos são consistentemente melhores do que os obtidos com os modelos e abordagens encontrados na literatura. No caso das correlações com um ou dois parâmetros, mostrou-se que estes mesmos parâmetros podem ser ajustados usando um conjunto muito pequeno de dados, e posteriormente serem utilizados na previsão de valores de D 12 longe do conjunto original de pontos. Uma nova instalação experimental para medir coeficientes de difusão binários por técnicas cromatográficas foi montada e testada. O equipamento, o procedimento experimental e os cálculos analíticos necessários à obtenção dos valores de D 12 pelo método de abertura do pico cromatográfico, foram avaliados através da medição de difusividades de tolueno e acetona em SC-CO2. Seguidamente, foram medidos coeficientes de difusão de eucaliptol em SC-CO2 nas gamas de 202 – 252 bar e 313.15 – 333.15 K. Os resultados experimentais foram analisados através de correlações e modelos preditivos para D12.
Diffusivities (D12) are fundamental properties both at research and industry levels, but the lack of experimental data and the non-existence of reliable and accurate equations to estimate them in compressed and condensed phases constitute important shortcomings. The main objectives of this work comprise: i) the compilation of a large database of D12 values in gas, liquid and supercritical systems; ii) the development and validation of new models for tracer diffusivities, applicable over wide ranges of temperature and density, for systems containing very distinct components in term of polarity, size and symmetry; iii) the installation and test of an experimental set-up to measure diffusion coefficients in liquids and supercritical fluids. Concerning modelling, a new accurate expression for tracer diffusion coefficients of hard sphere fluid was developed and validated using molecular dynamic data (average absolute relative deviation, AARD = 4.44%). The binary diffusion coefficients of real systems were also studied. An extensive database of diffusivities in gas and dense solvents was compiled (622 binary systems performing 9407 data points and comprehending 358 molecules) and used to validate the new models developed in this thesis. A set of new models were proposed for tracer diffusivities using different approaches: i) two molecularly-based models with one system-specific parameter that are applicable to gas, liquid, and supercritical systems, where the nature of solvent is limited to non-polar or weakly polar (global AARDs in the range 4.26-4.40%); ii) two molecularly-based models with two parameters, applicable in all physical states, for any solutes diluted in any type of solvent (non-polar, weakly-polar, and polar). Both models provide global errors between 2.74% and 3.65%; iii) one correlation with one parameter devoted to D 12 coefficients in supercritical carbon dioxide (SC-CO2) and liquid water (AARD = 3.56%); iv) nine empirical and semi-empirical correlations involving two parameters, dependent on temperature and/or solvent density and/or solvent viscosity. These models are very simple and provide accurate results (AARDs between 2.78% and 4.44%) in liquid and supercritical systems; and v) two predictive equations for diffusivities of solutes in SC-CO2 where the global deviations for both are inferior to 6.80%. In the whole, it may be emphasized that the new models cover the large variety of systems and molecules generally found. The results achieved are consistently better than those obtained by well known models and approaches taken from the literature. In the case of the 1- and 2-parameter correlations, it has been shown that such parameters can be fitted to a very small set of data, and subsequently used to predict D 12 values far from the original set of points. A new experimental set-up to measure binary diffusion coefficients by chromatographic techniques was designed and tested. The equipment, experimental procedure and analytical calculations to obtain the D 12 values by the chromatographic peak broadening technique were assessed by measuring diffusivities of toluene and acetone in SC-CO2. Then, the diffusivities of eucalyptol in SC-CO2 were determined in the ranges 202 – 252 bar and 313.15 – 333.15 K. The experimental data were analysed using D 12 predictive and correlation models.

Lubrication theory plays a fundamental role in all mechanical design as well as applications to biomechanics. All machinery are composed of moving parts which must be protected against wear and damage. Without effective lubrication, maintenance cycles will be shortened to impractical levels resulting in increased costs and decreased reliability. The focus of the work presented here is on the lubrication of rotating machinery found in advanced power systems and designs involving micro-turbines. One of the earliest studies of lubrication is due to Osborne Reynolds in 1886 who recorded what is now regarded as the canonical equation governing all lubrication problems; this equation and its extensions have become known as the Reynolds equation. In the past century, Reynolds equation has been extended to include three-dimensional effects, unsteadiness, turbulence, variable material properties, non-newtonian fluids, multi-phase flows, wall slip, and thermal effects. The bulk of these studies have focused on highly viscous liquids, e.g., oils. In recent years there has been increasing interest in power systems using new working fluids, micro-turbines and non-fossil fuel heat sources. In many cases, the design of these systems employs the use of gases rather than liquids. The advantage of gases over liquids include the reduction of weight, the reduction of adverse effects due to fouling, and compatibility with power system working fluids. Most treatments of gas lubrication are based on the ideal, i.e., low pressure, gas theory and straightforward retro-fitting of the theory of liquid lubrication. However, the 21st Century has seen interest in gas lubrication at high pressures. At pressures and temperatures corresponding to the dense and supercritical gas regime, there is a strong dependence on gas properties and even singular behavior of fundamental transport properties. Simple extrapolations of the intuition and analyses of the ideal gas or liquid phase theory are no longer possible. The goal of this dissertation is to establish the correct form of the Reynolds equation valid for both low and high pressure gases and to explore the dynamics predicted by this new form of the Reynolds equation. The dissertation addresses five problems involving our new Reynolds equation. In the first, we establish the form appropriate for the simple benchmark problem of two-dimensional journal bearings. It is found that the material response is completely determined by a single thermodynamic parameter referred to as the "effective bulk modulus". The validity of our new Reynolds equation has been established using solutions to the full Navier-Stokes-Fourier equations. We have also provided analytical estimates for the range of validity of this Reynolds equation and provided a systematic derivation of the energy equation valid whenever the Reynolds equation holds. The next three problems considered here derive local and global results of interest in high speed lubrication studies. The results are based on a perturbation analysis of our Reynolds and energy equation resulting in simplified formulas and the explicit dependence of pressure, temperature, friction losses, load capacity, and heat transfer on the thermodynamic state and material properties. Our last problem examines high pressure gas lubrication in thrust bearings. We again derive the appropriate form of the Reynolds and energy equations for these intrinsically three-dimensional flows. A finite difference scheme is employed to solve the resultant (elliptic) Reynolds equation for both moderate and high-speed flows. This Reynolds equation is then solved using perturbation methods for high-speed flows. It is found that the flow structure is comprised of five boundary layer regions in addition to the main ``core'' region. The flow in two of these boundary layer regions is governed by a nonlinear heat equation and the flow in three of these boundary layers is governed by nonlinear relaxation equations. Finite difference schemes are employed to obtain detailed solutions in the boundary layers. A composite solution is developed which provides a single solution describing the flow in all six regions to the same accuracy as the individual solutions in their respective regions of validity. Overall, the key contributions are the establishment of the appropriate forms of the Reynolds equation for dense and supercritical flows, analytical solutions for quantities of practical interest, demonstrations of the roles played by various thermodynamic functions, the first detailed discussions of the physics of lubrication in dense and supercritical flows, and the discovery of boundary layer structures in flows associated with thrust bearings.
Doctor of Philosophy
Lubrication theory plays a fundamental role in all mechanical design as well as applications to biomechanics. All machinery are composed of moving parts which must be protected against wear and damage. Without eective lubrication, maintenance cycles will be shortened to impractical levels resulting in increased costs and decreased reliability. The focus of the work presented here is on the lubrication of rotating machinery found in advanced power systems and designs involving micro-turbines. One of the earliest studies of lubrication is due to Osborne Reynolds in 1886 who recorded what is now regarded as the canonical equation governing all lubrication problems; this equation and its extensions have become known as the Reynolds equation. In the past century, Reynolds equation has been extended to include three-dimensional eects, unsteadiness, turbulence, variable material properties, non-newtonian uids, multi-phase ows, wall slip, and thermal eects. The bulk of these studies have focused on highly viscous liquids, e.g., oils. In recent years there has been increasing interest in power systems using new working uids, micro-turbines and non-fossil fuel heat sources. In many cases, the design of these systems employs the use of gases rather than liquids. The advantage of gases over liquids include the reduction of weight, the reduction of adverse eects due to fouling, and compatibility with power system working uids. Most treatments of gas lubrication are based on the ideal, i.e., low pressure, gas theory and straightforward retro-tting of the theory of liquid lubrication. However, the 21st Century has seen interest in gas lubrication at high pressures. At pressures and temperatures corresponding to the dense and supercritical gas regime, there is a strong dependence on gas properties and even singular behavior of fundamental transport properties. Simple extrapolations of the intuition and analyses of the ideal gas or liquid phase theory are no longer possible. The goal of this dissertation is to establish the correct form of the Reynolds equation valid for both low and high pressure gases and to explore the dynamics predicted by this new form of the Reynolds equation. The dissertation addresses ve problems involving our new Reynolds equation. In the rst, we establish the form appropriate for the simple benchmark problem of two-dimensional journal bearings. It is found that the material response is completely determined by a single thermodynamic parameter referred to as the eective bulk modulus. The validity of our new Reynolds equation has been established using solutions to the full Navier-Stokes-Fourier equations. We have also provided analytical estimates for the range of validity of this Reynolds equation and provided a systematic derivation of the energy equation valid whenever the Reynolds equation holds. The next three problems considered here derive local and global results of interest in high speed lubrication studies. The results are based on a perturbation analysis of our Reynolds and energy equation resulting in simplied formulas and the explicit dependence of pressure, temperature, friction losses, load capacity, and heat transfer on the thermodynamic state and material properties. Our last problem examines high pressure gas lubrication in thrust bearings. We again derive the appropriate form of the Reynolds and energy equations for these intrinsically threedimensional ows. A nite dierence scheme is employed to solve the resultant (elliptic) Reynolds equation for both moderate and high-speed ows. This Reynolds equation is then solved using perturbation methods for high-speed ows. It is found that the ow structure is comprised of ve boundary layer regions in addition to the main core region. The ow in two of these boundary layer regions is governed by a nonlinear heat equation and the ow in three of these boundary layers is governed by nonlinear relaxation equations. Finite dierence schemes are employed to obtain detailed solutions in the boundary layers. A composite solution is developed which provides a single solution describing the ow in all six regions to the same accuracy as the individual solutions in their respective regions of validity. Overall, the key contributions are the establishment of the appropriate forms of the Reynolds equation for dense and supercritical ows, analytical solutions for quantities of practical interest, demonstrations of the roles played by various thermodynamic functions, the rst detailed discussions of the physics of lubrication in dense and supercritical ows, and the discovery of boundary layer structures in ows associated with thrust bearings.

This thesis describes the investigation of phase behaviour of binary and ternary mixtures at high pressure. The particular applications chosen to be explored in this phase behaviour investigation were supercritical fluid electrodeposition (SCFED) and carbon capture and storage (CCS). Chapter 1 introduces the phase behaviour of mixtures. Chapter 2 describes the equipment and analytical techniques used throughout this thesis including the high-pressure variable-volume view cell, electrical conductivity cell, high pressure FTIR cell, and high-pressure optical fiber phase analyser. Chapter 3 details the solubility and conductivity investigation of several supporting electrolytes in difluoromethane (CH2F2), which provided an electrochemical bath with sufficient conductivity for electrodeposition in supercritical fluids. The most effective supporting electrolyte amongst the eight ionic compounds tested was [N(nC4H9)4][Al(OC(CF3)3)4] which was found to give a moderate solubility and the highest conductivity (222 Scm2mol-1) in CH2F2. [N(nC4H9)4][Al(OC(CF3)3)4] was followed by [N(nC4H9)4][FAP] and [N(nCH3)4][FAP], , making all of them to be satisfactory potential supporting electrolytes for SCFED. Chapter 4 describes the investigation of water solubility in CO2/N2 mixtures relevant to the CCS process. The scope of the investigation covers a wide pressure range and two levels of N2 (xN2= 0.05 and xN2= 0.10). This experimental study was conducted by using the FTIR technique as described in further detail in Chapter 2. It was found that the presence of N2 in CO2 lowered the solubility of H2O in supercritical CO2 with N2 compared to pure CO2. The solubility of water also decreases significantly when the concentration of N2 is increased from 5% to 10%. Chapter 5 further explores the role of phase behaviour in the application of CCS with the investigation of the phase envelope of the ternary mixtures of CO2 and permanent gases (Ar, N2, and H2). Three ternary mixtures were measured (90% CO2 + 5% N2 + 5% Ar, 98% CO2 + 1% N2 + 1% Ar, and 95% CO2 + 3% H2 + 2% Ar) by using the fiber optic reflectometer, as described in further detail in Chapter 2. The experimental data presented in this part also have been used to validate the equation of state for the CCS applications. It was found that the phase envelope of CO2 shifted to a higher pressure and the two-phase region become broader with the presence of permanent gases. Overall, both GERG-2004 and gSAFT provide a good agreement between the predicted and experimental data for all the ternary mixtures investigated, with the exception of the bubble-point line for the 3%H2 + 2%Ar + 95% CO2 mixture. Finally, Chapter 6 summarises the research that was conducted in this thesis. It also evaluates the progress made towards achieving the aims initially set-up in Chapter 1.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2001.
Current methods for wax fractionation result in products with large polydispersity, and due to the high temperatures required, thermal degradation of the wax is often incurred. The need for an alternative process thus exists. The purpose of this project is to investigate the technical viability of supercritical fluid processing as an alternative wax fractionation technology. The main aims of this project are to select a suitable supercritical solvent, to conduct binary phase equilibrium experiments, to determine if the process is technically viable and to investigate the ability of various equations of state to correlate the phase equilibrium data. Based on limited data from the literature, propane and a propane rich LPG (Liquefied Petroleum Gas) were selected as suitable solvents. Literature data for propane and high molecular weight alkanes is scares and incomplete, thus necessitating experimental measurements. A phase equilibrium cell was designed, constructed and commissioned. The cell was designed for pressures up to 500 bar and temperatures to 200 oC, and with the aid of an endoscope, the phase transitions were detected visually. The measurements correspond well to literature values from reliable research groups. Phase equilibrium data sets for propane with nC32, nC36, nC38, nC40, nC44, nC46, nC54 and nC60 as well as LP Gas with nC36 were measured. At temperatures just above the melting point of the alkanes, the phase transition pressures can be considered to be moderate, which will positively impact the economics of the process. The phase transition pressure increases with increasing carbon number, the relationship being found to be linear when the pressure is plotted as a function of carbon number at constant mass fractions and temperature. The increase in phase transition pressure with increasing carbon number indicates that the solvent will be able to selectively fractionate the wax. At higher temperatures the gradient of the line is larger and may thus lead to improved selectivity. The higher temperatures will also lead to better mass transfer. The linear relationship indicates that limited extrapolation to higher carbon numbers may be possible. However, this needs to be verified experimentally. The inability to measure the critical point and vapour pressure curves of the higher molecular weight normal alkanes, as well as the inability of cubic equations of state to predict liquid volumes and to capture the chain specific effects such as internal rotations, results in cubic equations of state requiring large interaction parameters to fit the data. The alternative, statistical mechanical equations of state, have difficulty in predicting the critical point of the solvent correctly and thus overpredicts the mixture critical point, yet require smaller interaction parameters to fit the data. Further work is required to improve the predictability of these non-cubic equations of state. This project has proven that wax fractionation by supercritical extraction with propane is technically feasible.

O esgotamento das reservas de petróleo, bem como o impacto ambiental que o seu processamento provoca, tem induzido a busca por fontes alternativas de energia para substituir os fósseis de petróleo como combustível automotivo. No Brasil, o crescimento do agronegócio e o consequente uso dos seus produtos e resíduos de fontes vegetais, favoreceram incríveis descobertas tais como matérias-primas para biocombustíveis, incluindo bioetanol derivado de cana de açúcar e biodiesel de óleos vegetais. Neste sentido, o biodiesel tem recebido bastante destaque nos últimos anos. O objetivo do presente estudo foi a para produção de biodiesel de óleo de soja empregando metanol ou etanol como álcoois pressurizados, além da análise da influencia da adição de água nas melhores condições alcançadas. Para tal, uma unidade experimental de bancada foi construída. Para investigação das variáveis que influenciam o rendimento das reações no sistema estudado, um planejamento fatorial 23 foi adotado, no qual foram investigados os efeitos da temperatura (180 e 300°C), do tempo de residência (10 minutos e 1 hora), da razão molar óleo:etanol (1:10 e 1:50), e da concentração de água (0 a 10% massa em relação ao óleo) sobre a conversão em ésteres da reação, que foi monitorada por cromatografia gasosa de alta resolução. Observou-se que a temperatura tem forte influência na conversão em ésteres da reação, com os melhores resultados para metanol ou etanol (95% de conversão) tendo sido obtidos na temperatura de 300°C. A adição de agua (2,5 e 5,0% em relação ao óleo) ocasionou um leve aumento na conversão (98%) para ambos metanol e etanol. Então, conclui-se então, que condições similares no rendimento do biodiesel de óleo de soja em reator batelada foi obtido utilizando metanol ou etanol pressurizados em condições supercriticas. Este trabalho também propõe uma nova metodologia para análise de acilgliceróis.
The depletion of petroleum reserves coupled with the environmental impact caused by the form of its industrial processing lead a frantic search for alternative energy sources to replace fossil petroleum as automotive fuel. In Brazil, the growth of agribusiness and the consequent use of its products and resides of vegetable sources, brought exciting discoveries such as feedstock biofuels, including bioethanol derived from sugar cane and biodiesel from vegetable oil. In this since, the biodiesel has been receiving singular attention at last years. The objective of this present study was producing biodiesel from soybean oil using methanol or ethanol as alcohol pressurized beyond the analysis of the influence of added water under optimum conditions achieved. For such, an experimental unity was built. To investigate the variables influencing in the yield of the reaction in the system studied, an experimental planning using 23 arranged was adopted, in which was investigated the effects of temperature (220 and 300°C) residence time (10 minutes and 1 hour) and molar ration (oil/alcohol) (1:10 and 1:50). After, the addition of water to the system (0 to 10% in relation to oil) was also evaluated on the conversion to esters, which was monitored by high resolution gas chromatography. It was observed that temperature has a strong influence on the conversion of esters with the best results for methanol or ethanol (95% conversion) were obtained at a temperature of 300°C. Addition of water (2,5 and 5,0% in relation to oil) caused a slight increase in conversion (98%) for both methanol and ethanol. Then was concluded that similar results in the yields of biodiesel from soybean oil in batch reactor was obtained from both methanol or ethanol pressurized as supercritical conditions. This work proposes a new methodology for acylglycerols analysis.

The extraction of oil from fixed beds of Canola seed ( Brassica napus and Brassica campestris ) was studied using carbon dioxide at temperatures and pressures ranging from 25 to 90°C and 10-36 MPa respectively. The highest oil solubility in the CO₂ (11 mg/g CO₂) was observed at 36 MPa and 55°C. The equilibrium oil concentration in the CO₂ phase, was found to be independent of the oil concentration in the seed phase. The extracts were found to be essentially free from phosphorus (<7ppm) and their fatty acid content did not change significantly as the extraction progressed. The total amount of oil recovered from the seeds by CO₂ extraction depended upon the seed pre-treatment. For commercially flaked seed, this amount was comparable to that recoverable by conventional hexane extraction. The CO₂ extraction of simple triglycerides at 36 MPa and 55°C was investigated. The solubilities of tripalmitolein, triolein, and trieicosenoin were 20 mg/g CO₂, 10 mg/g CO₂, and 4 mg/g CO₂ respectively. The composition of CO₂ extracts of an equimolar mixture of the above triglycerides was also studied. It was found that the concentration of each triglyceride in the extract was equal to the product of its mole-fraction in the mixture and its solubility in the CO₂. Equations governing the mass transfer from the Canola seed to the CO₂ solvent were developed. A transient one-dimensional mathematical model based on these equations was used to obtain concentration profiles of oil in both the solvent and seed phases, and to determine the overall volumetric mass transfer coefficient. The calculated concentrations and extraction rates were in good agreement with experimental results. The overall volumetric mass transfer coefficient for the initial constant rate period was found to be proportional to the 0.54 power of interstitial velocity.
Graduate and Postdoctoral Studies
Graduate

There has been considerable interest in the field of Supercritical Fluid Chromatography (SFC) and it is fast becoming recognised as a very useful analytical tool, offering the speed of separation found with gas chromatography (GC) with the solvating power of high performance liquid chromatography (HPLC). The mechanism of retention in SFC is unclear and a number of competing interactions can be considered to be occurring. studies into the retention of test solutes exhibiting a variety of functional groups in different systems have been carried out using a simple home-made SFC system constructed using a pye-Unicam GC oven and a HPLC pump.

The oral route for drug administration offers an efficient and convenient method for drug delivery. However, there is an assortment of drugs which exhibit narrow absorption windows in the upper small intestine and as a result demonstrate limited bioavailabilities. One approach in the improvement of bioavailability in these cases is to retain the delivery system proximal to the absorption window for a prolonged period of time. Although controlled release products are widely available on the market, marketed gastroretentive systems remain elusive. This work explores the manufacture and characterisation of a multi-unit gastroretentive system utilising the biocompatible polymer poly (lactic-coglycolic acid). The novel PGSS technique enables the production of PLGA particles whilst omitting the use of volatile organic solvents. Morphological and microCT analyses of the particles revealed a highly porous matrix with porosity values in the order of 30-40%. The relationship between porosity, density and in vitro floating ability for particles with sizes between 100-2000 J.1m revealed that particle size plays an important role; larger microparticles possess decreased density, higher porosity and increased buoyancy. Encapsulation of two model drugs, riboflavin and furosemide, was carried out during the processing step with high encapsulation efficiencies (80-100%) being revealed. Release of the drugs in PBS (pH7.4) was found to be sustained over a period of 24 hours with a decrease in cumulative release in simulated gastric fluid (pHl.2). The introduction of the hydrophilic polymer poly(ethylene glycol) was found to modulate release rate; PEG with a molecular weight equal of more than 3 KDa increased the rate of release in PBS media up to 20% over hrs, however this was not observed for release in SGF. A comparison of morphology prior to and following exposure to the release media confirms that the emergence of intricate porous channels on exposure to the release medium is related to an increase in release rate. In order to augment the gastroretentive potential of the system the mucoadhesive polymer, chitosan was incorporated both as a post processing surface modification and as part of the initial formulation. ToF-SIMS surface analysis confirmed the presence of chitosan at the surface of the particles in both cases. Initially the potential for the particles to interact with mucus was evaluated utilising in vitro tests. The presence of chitosan significantly improved adsorption of mucin to particles, as well as enhancing adhesion of particles to a mucus producing epithelial cell layer. The thiolated chitosan derivative chitosan-N-acetyl-cysteine demonstrated an increase in adhesion of mucin solution; however the modified chitosan resulted in a decrease in adhesion to mucus producing cell line which was considered to be a result of the mucolytic actions it may exert on the mucus layer. Oral administration of buoyant particles to a rat model improved the pharmacokinetics of the anti-hypoglycaemia drug metformin, with addition of mucoadhesive properties providing further improvement. This study demonstrates that introduction of buoyancy and mucoadhesion functionalities to particles prepared by the PGSS method could improve delivery of drugs demonstrating narrow absorption windows in the upper small intestine.