This study investigated the transport coefficients involved in mass and heat transfer during the drying of a porous solid partially saturated with multicomponent mixtures.  It included the coefficients governing liquid transport through the solid, the matrix of multicomponent diffusion coefficients in the liquid phase, and the effective thermal conductivity.  As it is not possible to determine these coefficients by theoretical considerations alone and considerable experimental work is required to determine them in a broad range of process conditions, the principle of this study has been the use of mathematical models complemented with some empirical parameters.  These empirical parameters were determined by comparison between measurements in specially designed experiments and the results of mathematical models that describe the process.  In addition, the application of the multicomponent diffusion coefficients is described in two cases where liquid diffusion is important: convective evaporation of a multicomponent stationary liquid film and a falling film. To study liquid transport through the solid, isothermal drying experiments were performed to determine the transient composition profiles and total liquid content of sand samples wetted with ternary liquid mixtures with different initial compositions and temperatures.  A mathematical model including mass transfer by capillary movement of the liquid and interactive diffusion in both the gas and liquid phases was developed.  To simulate the capillary movement of liquid mixtures, parameters experimentally determined for single liquids were weighed according to liquid composition. A fairly good agreement between theoretical and experimental liquid composition profiles was obtained considering that axial dispersion was included in the model. To study the matrix of multicomponent diffusion coefficients in the liquid phase, the redistribution of liquid composition in a partially filled tube exposed to a longitudinal temperature gradient was analysed.  Experimental work was carried out using two main ternary mixtures with different initial compositions and temperature gradients.  Experimental data were compared with the results of a theoretical model that describes the steady-state liquid composition distribution in a partially filled non-isothermal tube to find the empirical exponent that modifies the matrix of thermodynamic factors.  Correlations for the exponents as a function of temperature were determined for each particular multicomponent mixture. The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures was studied by measuring the liquid composition, liquid content and temperature distributions in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side.  Simulations performed at a quasi steady state were compared with experiments to estimate the adjusting geometric parameter of Krischer’s model for effective thermal conductivity, which includes the contribution of the evaporation-diffusion-condensation mechanism. The results revealed that a resistance corresponding to a parallel arrangement between the phases seems to dominate in this case. In the study of the convective drying of a multicomponent stationary liquid film, the equations describing interactive mass transfer were decoupled by a similarity transformation and solved simultaneously with a conduction equation by the method of variable separation.  Variations of physical properties along the process trajectory were taken into account by a stepwise application of the solution in time intervals with averaged coefficients from previous time steps.  Despite simplifications, the analytical solution gives a good insight into the selectivity of the drying process and is computationally fast.  On the other hand, numerical simulations of the convective evaporation of the multicomponent falling liquid film into an inert gas with a co-current flow arrangement of the phases almost always revealed a transition from liquid-phase-controlled conditions to a process in which neither the gas nor the liquid completely controls the evaporation. The results obtained in this work would be useful in implementing models to improve the design, process exploration and optimisation of dryers by incorporating the solid-side effects to describe the drying of liquid mixtures along the whole process. / QC 20110124

<p>Energy is one of the basic needs of human beings and is extremely crucial for continued development of human life. Our work, leisure and our economic, social and physical welfare all depend on the sufficient, uninterrupted supply of energy. Therefore, it is essential to provide adequate and affordable energy for improving human welfare and raising living standards. Global concern over environmental climate change linked to fossil fuel consumption has increased pressure to generate power from renewable sources [1]. Although substantial advances in renewable energy technologies have been made, significant challenges remain in developing integrated renewable energy systems due primarily to mismatch between load demand and source capabilities [2]. The output from renewable energy sources such as photo-voltaic, wind, tidal, and micro-hydro fluctuate on an hourly, daily, and seasonal basis. As a result, these devices are not well suited for directly powering loads that require a uniform and uninterrupted supply of input energy.</p>

The present work was initiated to investigate the surface- and interfacial phenomena for iron and slag/iron systems. The aim was to understand the mechanism of the effect of surface active elements on surface and interfacial properties. In the present work, the adsorption of oxygen and sulfur on iron surface as well as adatom surface movements were studied based on the ab initio method. BCC iron melting phenomena and sulfur diffusion in molten iron were investigated by Monte Carlo simulations. The impact of oxygen potential on interfacial mass transfer was carried out by X-ray sessile drop method. Firstly, the structural, electronic and magnetic properties as well as thermodynamic stability were studied by Density functional theory (DFT). The hollow site was found to be the most stable adsorption site both for oxygen and sulfur adsorbed on iron (100) surface, which is in agreement with the experiment. The relaxation geometries and difference charge density of the different adsorption systems were calculated to analyze the interaction and bonding properties between Fe and O/S. It can be found that the charge redistribution was related to the geometry relaxation. In addition, the sulfur coverage is considered from a quarter of one monolayer (1ML) to a full monolayer. It was found that the work function and its change Δφ increased with S coverage, in very good agreement with experiment. Due to a recent discussion regarding the influence of charge transfer on Δφ, it is shown in the present work that the increase in Δφ can be explained by the increasing surface dipole moment as a function of S coverage. S strongly interacts with the surface Fe layer and decreases the surface magnetic moment as the S coverage increases. Secondly, a two dimensional (2D) gas model based on density functional calculations combined with thermodynamics and statistical physics, was proposed to simulate the movement of the surface active elements, viz. oxygen and sulfur atoms on the Fe(100) surface. The average velocity of oxygen and sulfur atoms was found to be related to the vibration frequencies and energy barrier in the final expression developed. The calculated results were based on the density function and thermodynamics &amp; statistical physics theories. In addition, this 2D gas model can be used to simulate and give an atomic view of the complex interfacial phenomena in the steelmaking refining process. A distance dependent atomistic Monte Carlo model was developed for studying the iron melting phenomenon as well as effect of sulfur on molten iron surface. The effect of boundary conditions on the melting process of an ensemble of bcc iron atoms has been investigated using a Lennard-Jones distance dependent pair potential. The stability of melting process was energetically and spatially analyzed under fixed wall and free surface conditions and the effects of short and long-range interactions were discussed. The role of boundary conditions was significantly reduced when long-range interactions were used in the simulation. This model was further developed for investigating the effect of sulfur on molten iron surface. A combination of fixed wall and free surface boundary condition was found to well-represent the molten bath configuration while considering the second nearest neighbor interactions. Calculations concerning the diffusion of sulfur on molten surface were carried out as a function of temperature and sulfur concentration. Our results show that sulfur atoms tended to diffuse away from the surface into the liquid bulk and the diffusion rate increased by increasing temperature. Finally, impact of oxygen potential on sulfur mass transfer at slag/metal interface, was carried out by X-ray sessile drop method. The movement of sulfur at the slag/metal interface was monitored in dynamic mode at temperature 1873 K under non-equilibrium conditions. The experiments were carried out with pure iron and CaO-SiO2-Al2O3-FeO slag (alumina saturated at the experimental temperature) contained in alumina crucibles with well-controlled partial pressures of oxygen and sulfur. As the partial pressure of oxygen increased, it was found that interfacial velocity as well as the oscillation amplitude increased. The thermo-physical and thermo-chemical properties of slag were also found to influence interfacial velocity. / QC 20101123

En el caso del almacenamiento de los residuos radioactivos los flujos osmóticos pueden ser relevantes y requieren un análisis en detalle. El residuo nuclear bituminizado (BW) será almacenado mediante contenedores en cavidades excavadas en la Boom Clay, que es una arcilla marina que presenta propiedades favorables para limitar y retrasar la migración de los contaminantes radioactivos. La interacción entre los dos materiales es un proceso acoplado químico-hidro-mecánico y depende de la respuesta hidromecánica de la Boom Clay y del BW. En condiciones de almacenamiento, el contacto del BW, que contienen cantidades importantes de NaNO3, con el agua subterránea induce la hidratación por gradientes osmóticos y el consiguiente hinchamiento, además de la difusión de la sal disuelta hacia la Boom Clay. Se pueden distinguir dos tipos de afecciones: la perturbación geomecánica causada por el hinchamiento del BW y el aumento de presión en el BW y cambio de las distribución de tensiones en la roca, y la perturbación físico química por la migración de grandes cantidades de sales. El objetivo de esta tesis es: (i) Mejorar la comprensión de los procesos que controlan la absorción de agua y el consecuente hinchamiento del BW que contengan sales (NaNO3), y (ii) Investigar los posibles efectos de la concentración de fluidos de los poros sobre el hinchamiento, la compresibilidad y comportamiento de corte de la Boom Clay. En primer lugar, se ha desarrollado una formulación para el análisis de la deformación inducida por la disolución de sales en medio poroso con contacto con agua. Las ecuaciones planteadas incluyen los flujos acoplados de agua y soluto. Se presenta también un trabajo teórico que ayuda a la comprensión del comportamiento mecánico del BW. Se considera este material como una mezcla de bitumen y cristales de NaNO3. Se ha desarrollado un modelo elasto-viscoplástico que describe el comportamiento de fluencia del BW considerando el comportamiento de fluencia de sus constituyentes. El modelo constitutivo elasto-viscoplástico ha sido implementado en el programa CODE_BRIGHT. Los resultados se han comparado con observaciones experimentales. Se ha estudiado el comportamiento a largo plazo del BW en contacto con agua al simular ensayos de hinchamiento por absorción de agua bajo condiciones confinadas. El análisis numérico ha demostrado ser capaz de proporcionar una representación satisfactoria de los principales patrones observados en su comportamiento. En lo que respecta al segundo objetivo de la tesis, se ha propuesto una formulación para el análisis de las deformaciones inducidas por procesos osmóticos en un medio poroso de doble estructura. Esta formulación distingue dentro del material un nivel micro-estructural y otro macro-estructural con cambios químicos que tienen un efecto significativo en la micro-estructura. Se han obtenido las ecuaciones básicas que describen los flujos acoplados de agua y solutos y el transporte de sus componentes a través de los macroporos así como las ecuaciones de balance de masa para agua y soluto en los macroporos y microporos. La formulación propuesta ha sido aplicada particularmente para analizar cualitativamente el efecto de la succión osmótica sobre el hinchamiento de los suelos arcillosos. Se han analizado los efectos a corto y largo plazo. Se ha investigado también la influencia del aumento de la concentración del fluido en los poros sobre las propiedades geotécnicas y el comportamiento de la Boom Clay no saturada. Se ha llevado a cabo un programa sistemático de investigación experimental, con control de succión osmótica y matricial, con el fin de investigar el efecto del incremento de la concentración del fluido de poros sobre la resistencia de corte y el cambio volumétrico bajo condiciones edométricas. Se ha observado, que bajo condiciones parcialmente saturadas, un cambio en la salinidad provoca una disminución en la compresibilidad y en la resistencia de corte del material. / For deep storage of high-level nuclear waste osmotic flows can be significant and so require a careful analysis. In Belgium, The bituminized nuclear waste (BW) named Eurobitum contained in metallic drums will be placed inside a tunnel or a shaft excavated in the Boom Clay, which is 100 m thick marine clay presenting favourable properties to limit and delay the migration of the leached radionuclides over extended periods of time. In Geological disposal conditions, contact of the bituminized radioactive waste which contains high amounts of highly soluble salt (NaNO3) with groundwater will result in water uptake and swelling of the waste and in subsequent diffusion of the dissolved salt through the host clay formation. Basically, two types of disturbance can be distinguished: A geo-mechanical perturbation, caused by the swelling of the waste and the increase of the pressure in and around the waste and a physico-chemical perturbation by the release of large amounts of NaNO3 and other soluble salts. In this context the aim of this thesis is: (i) to improve the understanding of the processes controlling the water uptake and the subsequent swelling of bituminized waste containing soluble salts (NaNO3), and (ii) to investigate of the possible effects of the increase of pore fluid concentration on swelling, compressibility and shear behaviour of Boom Clay. A formulation has been proposed for the analysis of deformation induced by dissolution of salts in porous media in contact with water. The equations include the effect of coupled transport phenomena and the formulation has been included as an extension in the coupled THM program CODE_BRIGHT. A theoretical and experimental work aiming at understanding the mechanical behaviour of the Bituminized Waste has been presented.This material is considered for this purpose as a mixture of bitumen and crystals of NaNO3. An elasto-viscoplastic model has been developed that describes the creep behaviour of BW considering the constituents' creep behaviour. The elasto-viscoplastic constitutive model has been implemented into CODE_BRIGHT. The modelling results have been compared with the experimental data. The impact of osmotic forces on the swelling of the material has been investigated by simulating water uptake swelling tests under confined conditions and comparing the predictions with experimental results. The numerical analysis has proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour. In regard to the second objective of this thesis, a formulation has been proposed for the analysis of deformations induced by osmotic processes in double structure porous media. The formulation is based on the distinction within the material of a microstructural and a macrostructural levels with chemical changes having a significant effect on the microstructure. A macroscopic description of the system is provided. Then the basic equations describing coupled flows of water and solutes and the transport of its components through macropores and mass balance equations for water and solute in macro and micro pores have been obtained. The proposed formulation has been particularly applied to analyze qualitatively the effect of osmotic suction on swelling of clayey soils. Transient and long term effects have been analyzed. The influence of pore fluid concentration on the geotechnical properties and behavior of Boom Clay under partially saturated conditions has been investigated. A systematic experimental research program involving osmotic suction and matric suction controlled experiments has been carried to investigate the effect of the increase of pore fluid concentration on shear strength and on the volume change behaviour under odometer stress state conditions. It has been observed that under partially saturated conditions a change in salinity causes a decrease in compressibility and shear strength.

Molding is a simple manufacturing process whereby fluid fills a master tool and then solidifies in the shape of the tool cavity. The precise nature of material flow during molding has long allowed fabrication of plastic components with sizes 1 mm 1 m. Polymer molding with precise critical dimension control could enable scalable, inexpensive production of micro- and nanostructures for functional or lithographic use. This dissertation reports experiments and simulations on molding of polymer micro- and nanostructures at length scales 1 nm 1 mm. The research investigates two main areas: 1) mass transport during micromolding and 2) polymer mechanical properties during nanomolding at length scales 100 nm. Measurements and simulations of molding features of size 100 nm 1 mm show local mold geometry modulates location and rate of polymer shear and determines fill time. Dimensionless ratios of mold geometry, polymer thickness, and bulk material and process properties can predict flow by viscous or capillary forces, shape of polymer deformation, and mold fill time. Measurements and simulations of molding at length scales 100 nm show the importance of nanoscale physical processes distinct from bulk during mechanical processing. Continuum simulations of atomic force microscope nanoindentation accurately model sub-continuum polymer mechanical response but highlight the need for nanoscale material property measurements to accurately model deformation shape. The development of temperature-controlled nanoindentation enables characterization of nanoscale material properties. Nanoscale uniaxial compression and squeeze flow measurements of glassy and viscoelastic polymer show film thickness determines polymer entanglement with cooperative polymer motions distinct from those observed in bulk. This research allows predictive design of molding processes and highlights the importance of nanoscale mechanical properties that could aid understanding of polymer physics.

A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. The underlying idea involves utilization of thin nanoporous membranes for maintaining microscopically thin liquid films by capillary action, while providing a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor generated by evaporation is continuously removed by using a dry sweeping gas keeping the membrane outlet dry. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat. Performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and film thicknesses (~1-5µm) and by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functionalities of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations using this microfluidic device demonstrate heat fluxes in excess of 600W/cm2 at 90 C using water as the evaporating coolant. In order to further enhance the device performance, a comprehensive theoretical and computational analysis of heat and mass transfer at micro and nanoscales is carried out. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is performed. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to determine the factors limiting the performance of the MEMS device on a micro-scale, a device-level detailed computational analysis of heat and mass transfer is carried out, which is supported by experimental investigation. Identifying the contribution of various simultaneously occurring cooling mechanisms at different operating conditions, this analysis proposes utilization of hydrophilic membranes for maintaining very thin liquid films and further enhancement in vapor advection at the membrane outlet to achieve higher heat fluxes.

Separation of dissolved metals from aqueous solutions using liquid membrane technology is highly advantageous owing to the degree of separation achieved, efficiency and application potential. In the present investigation four types of liquid membranes - bulk liquid membrane (BLM), emulsion liquid membrane (ELM), electrostatic pseudo liquid membrane (ESPLIM) and unified liquid membrane (ULM) have been extensively studied, for their application in extraction and concentration of dissolved metals from effluents. Experiments were conducted with various metal systems to optimize both system and process conditions and to find out the effect of various parameters on the performance of the process. Different mass transport models were proposed for each type, taking diffusional and kinetic resistances into account. Models were extended for simultaneous extraction systems and were verified by different metal-carrier experiments. Good agreement was found between the concentration profiles obtained from the models and the experimental data, thereby establishing the validity of models for all the four types of liquid membranes. The stirred cell employed in BLM process eliminates emulsification and demulsification processes. It also provides simultaneous contact of the organic liquid membrane phase with aqueous feed and strip phases. Overall rate expressions for extraction and stripping in BLM are based on an assumed kinetic mechanism to explain the process qualitatively. It was found that the magnitude^ of diffusional and kinetic resistances determines the overall mass transfer coefficient. The relative magnitude of mass transfer coefficient, reaction rate constants and equilibrium constants enables to visualize the controlling regime of the process. The problem of low flux rate due to high diffusion resistances, inefficient operation and exorbitant costs encountered in bulk and supported liquid membranes (SLM) are overcome in an ELM. In the ELM process, an emulsion of organic membrane phase and aqueous inner phase, is dispersed in the continuous aqueous feed phase. This gives a highly selective and ultra thin liquid film generating a large mass transfer area for separation. Experimental results on membrane instability and emulsion swelling indicate that volumetric leakage rate depends linearly on the stirring speed and that the nature of surfactant does not have any appreciable effect on emulsion swelling. A general permeation model was developed taking into account the external mass transfer around the emulsion drop, diffusion in the drop, reaction at the aqueous-organic interface, leakage of the internal phase to the external phase due to membrane breakup and emulsion swelling due to osmotic pressure difference. Model equations with appropriate boundary conditions were numerically solved by orthogonal collocation technique for a set of model parameters obtained either from known correlations or from independent experiments. Comparison of the model predictions with experimental data from the batch permeation of chromium and other metals using carrier Alamine 336 or LIX 64N, shows that the model predictions are in very good agreement with the experimental findings. Further this model can be used to simulate the effects of various experimental conditions such as metal and hydrogen ion concentrations, carrier concentration, drop diameters, etc., for similar systems. Studies on ESPLIM were conducted with the aim of demonstrating the effectiveness of this new separation process and to develop a simple transport model for metal permeation. In the ESPLIM process, a high electrical field (3-5 kV A.C.) is used for phase dispersion. This system consists of a rectangular reactor filled with membrane solution divided into extraction and stripping cells by a centrally placed integrated type baffle which also acts as an electrode. Two more electrodes were placed in the extraction and stripping cells, where feed and strip phases are introduced from the top of the reactor. When high electrical field is applied across the electrodes, fine droplets of feed and strip are formed and are dispersed in extraction and stripping cells where simultaneous extraction and stripping occurs. The process can be viewed as simultaneous counter current extraction and stripping. The aqueous drops coalesce in the settlers at the bottom of the reactor and are removed continuously. Steady state mass transport model proposed for ESPLIM system accounts for the vertical counter-current extraction and stripping processes taking place in the extraction and stripping cells, together with the lateral transport process of the metal-complex and carrier across the two cells through the integrated baffle zone. The model equations were solved analytically to obtain concentration profiles as a function of the height of the reactor. The required parameters such as mass transfer coefficients, diffusion coefficients etc. were estimated using different correlations. Model predictions agreed remarkably well with the experimental data under various process conditions. From this investigation, it was found that ESPLIM is a simple, efficient and economical process and can be applied in a variety of situations. Based on a suitable combination of solvent extraction, dispersion and liquid membrane technique, a new type of separation system called " Unified Liquid Membrane " was developed. The ULM unit was designed and fabricated, and experiments were conducted to evaluate its performance. The ULM is basically derived from ESPLIM by changing the reactor, baffle design and dispersion technique. Aqueous feed and strip phases were atomized using compressed air through a fine nozzle and are dispersed on either side of an integrated baffle plate that divides the reactor into extraction and stripping cells. Tapering bottom of the reactor reduces the dead volume of the liquid in the settlers and the baffle plate remarkably reduces the leakage problem as well as the resistance through the baffle. Experiments were conducted using LIX 64N and Alamine 336 as carriers for copper and chromium and / or zinc. Mass transport model proposed considers both chemical and phase equilibria in extraction and stripping cells, vertical and lateral transport of carrier and complex across the extraction and stripping cells through the baffle zone. The model equations were solved using initial conditions at the top of the reactor, and equilibrium data for extraction and stripping cells. Effect of various experimental conditions and process parameters was simulated using this model and the model predictions are found to be in excellent agreement with the experimental data. The ULM system developed in this investigation overcomes the major limitations encountered with the other types of liquid membranes while retaining all the advantages of this technology. The problem of high mass transfer resistance from bulk phase to metal permeation as in the case of BLM was eliminated by good phase dispersion. Additional resistance to mass transport from solid membrane as in the case of SLM was removed by using an integrated baffle which also avoids problems of membrane instability, pore clogging and selectivity. The complex problems of emulsification and demulsification were completely eliminated making the system much simpler and efficient. Very good phase dispersion was obtained by atomization without the need for either stirring the whole system or application of high electrical field in the reactor. The membrane liquid within the integrated baffle elements allows easy transport of different species between extraction and stripping cells while completely preventing the mixing of the two aqueous phases. The problems of leakage, swelling and occlusion were avoided due to very short residence time of the aqueous drops in the reactor. It was found that the new ULM configuration is simple, elegant, highly efficient and superior to the other types of liquid membrane systems.

In verfahrenstechnischen Anlagen der Flüssig-Flüssig-Stoffübertragung kommt es an der Phasengrenze zwischen den nicht mischbaren Flüssigphasen häufig zur Ausbildung hydrodynamischer Instabilitäten. Sie sind mit komplexen Geschwindigkeitsfeldern in den Flüssigphasen, insbesondere in den grenzschichtnahen Regionen verbunden und führen zu einem starken Anstieg der pro Zeiteinheit übertragenen Stoffmenge. Die Lösung der Diffusionsgleichung reicht in diesem Fall zur Vorausberechnung des für Auslegungszwecke bedeutsamen Stoffdurchgangskoeffizienten nicht mehr aus. Chemische Reaktionen stellen Quellen oder Senken von Wärme und Stoff dar, die das Auftreten von Instabilitäten begünstigen und die mathematische Beschreibung zusätzlich erschweren. Im Rahmen der vorliegenden Arbeit wurden experimentelle und numerische Untersuchungen zum Flüssig-Flüssig-Stoffübergang in einem vertikalen Kapillarspalt durchgeführt. Reaktionsfreie Stoffübergänge und solche mit einer exothermen chemischen Reaktion an der Phasengrenze zeigten eine große Vielfalt von Konvektionsstrukturen, wie Rollzellen, Thermiken und das doppeldiffusive Fingerregime. Die Visualisierung der Transportvorgänge erfolgte durch das Schattenschlierenverfahren. Die Beobachtungen wurden hinsichtlich geometrischer Eigenschaften von Konvektionsstrukturen sowie deren zeitlicher Änderung ausgewertet. Dazu zählten insbesondere das Längenwachstum von Thermiken und horizontale Wellenlängen von Fingerstrukturen. Zur mathematischen Beschreibung der Phänomene im Kapillarspalt wurde ein Modell entwickelt, welches auf den gekoppelten, zweidimensionalen Transportgleichungen von Impuls, Wärme und Stoff beruht. Es berücksichtigt dichte- und grenzflächenspannungsgetriebene Instabilitätsmechanismen sowie die besonderen Durchströmungseigenschaften des Kapillarspalts. Die Phasengrenze wurde als eben angenommen. Die Lösung der Modellgleichungen erfolgt auf numerischem Wege durch ein Computerprogramm. Das Modell ist in der Lage, die beobachteten Instabilitätsphänomene qualitativ richtig wiederzugeben. Mit Hilfe von Simulationsrechnungen konnte der Mechanismus aufgeklärt werden, der zum schnelleren Rückgang des Stoffdurchgangskoeffizienten im Rollzellenregime der rein grenzflächenspannungsgetrieben Instabilität im Vergleich zum Vorgang mit überlagerter Dichtekonvektion führt. Des Weiteren gelang der Nachweis des doppeldiffusiven Fingerregimes beim Stoffübergang mit exothermer Grenzflächenreaktion. Die berechnete Erhöhung des Stoffdurchgangskoeffizienten stimmt mit Angaben in experimentellen Arbeiten anderer Autoren überein.

<p>Energy is one of the basic needs of human beings and is extremely crucial for continued development of human life. Our work, leisure and our economic, social and physical welfare all depend on the sufficient, uninterrupted supply of energy. Therefore, it is essential to provide adequate and affordable energy for improving human welfare and raising living standards. Global concern over environmental climate change linked to fossil fuel consumption has increased pressure to generate power from renewable sources [1]. Although substantial advances in renewable energy technologies have been made, significant challenges remain in developing integrated renewable energy systems due primarily to mismatch between load demand and source capabilities [2]. The output from renewable energy sources such as photo-voltaic, wind, tidal, and micro-hydro fluctuate on an hourly, daily, and seasonal basis. As a result, these devices are not well suited for directly powering loads that require a uniform and uninterrupted supply of input energy.</p>

La gazéification est aujourd'hui l'une des stratégies les plus prometteuses pour valoriser les déchets en énergie. Cette technologie thermo-chimique permet une réduction de 95 % de la masse des intrants et génère des cendres inertes ainsi que du gaz de synthèse (syngaz). Le syngaz est un combustible gazeux composé principalement de monoxyde de carbone (CO), d'hydrogène (H2) et de dioxyde de carbone (CO2). Le syngaz peut être utilisé pour produire de la chaleur et de l'électricité. Il est également la pierre angulaire d'un grand nombre de produits à haute valeur ajoutée, allant de l'éthanol à l'ammoniac et l'hydrogène pur. Les applications en aval de la production de syngaz sont dictées par son pouvoir calorifique, lui-même dépendant de la teneur du gaz en H2. L’augmentation du contenu du syngaz en H2 est rendu possible par la conversion catalytique à la vapeur d’eau, largement répandu dans le cadre du reformage du méthane pour la production d'hydrogène. Au cours de cette réaction, le CO est converti en H2 et CO2 selon : CO + H2O → CO2 + H2. Ce processus est possible grâce à des catalyseurs métalliques mis en contact avec le CO et de la vapeur. La conversion catalytique à la vapeur d’eau a jusqu'ici été réservé pour de grandes installations industrielles car elle nécessite un capital et des charges d’exploitations très importantes. Par conséquent, les installations de plus petite échelle et traitant des intrants de faible qualité (biomasse, déchets, boues ...), n'ont pas accès à cette technologie. Ainsi, la seule utilisation de leur syngaz à faible pouvoir calorifique, est limitée à la génération de chaleur ou, tout au plus, d'électricité. Afin de permettre à ces installations une gamme d’application plus vaste de leurs syngaz, une alternative économique à base de catalyseur biologique est proposée par l’utilisation de bactéries hyperthermophiles hydrogénogènes. L'objectif de cette thèse est d'utiliser Carboxydothermus hydrogenoformans, une bactérie thermophile carboxydotrophe hydrogénogène comme catalyseur biologique pour la conversion du monoxyde de carbone en hydrogène. Pour cela, l’impact d'un phénomène de biominéralisation sur la production d’H2 a été étudié. Ensuite, la faisabilité et les limites de l’utilisation de la souche dans un bioréacteur ont été évaluées. Tout d'abord, la caractérisation de la phase inorganique prédominante lorsque C. hydrogenoformans est inoculé dans le milieu DSMZ, a révélé une biominéralisation de phosphate de calcium (CaP) cristallin en deux phases. L’analyse par diffraction des rayons X et spectrométrie infrarouge à transformée de Fourier de ce matériau biphasique indique une signature caractéristique de la Mg-whitlockite, alors que les images obtenues par microscopie électronique à transmission ont montré l'existence de nanotiges cristallines s’apparentant à de l’hydroxyapatite. Dans les deux cas, le mode de biominéralisation semble être biologiquement induit plutôt que contrôlé. L'impact du précipité de CaP endogène sur le transfert de masse du CO et la production d’H2 a ensuite été étudié. Les résultats ont été comparés aux valeurs obtenues dans un milieu où aucune précipitation n'est observée. Dans le milieu DSMZ, le KLa apparent (0.22 ± 0.005 min-1) et le rendement de production d’H2 (89.11 ± 6.69 %) étaient plus élevés que ceux obtenus avec le milieu modifié (0.19 ± 0.015 min-1 et 82.60 ± 3.62% respectivement). La présence du précipité n'a eu aucune incidence sur l'activité microbienne. En somme, le précipité de CaP offre une nouvelle stratégie pour améliorer les performances de transfert de masse du CO en utilisant les propriétés hydrophobes de gaz. En second lieu, la conversion du CO en H2 par la souche Carboxydothermus hydrogenoformans fut étudiée et optimisée dans un réacteur gazosiphon de 35 L. Parmi toutes les conditions opérationnelles, le paramètre majeur fut le ratio du débit de recirculation du gaz sur le débit d'alimentation en CO (QR:Qin). Ce ratio impacte à la fois l'activité biologique et le taux de transfert de masse gaz-liquide. En effet, au dessus d’un ratio de 40, les performances de conversion du CO en H2 sont limitées par l’activité biologique alors qu’en dessous, elles sont limitées par le transfert de masse. Cela se concrétise par une efficacité de conversion maximale de 90.4 ± 0.3 % et une activité spécifique de 2.7 ± 0.4 molCO·g–1VSS·d–1. Malgré des résultats prometteurs, les performances du bioréacteur ont été limitées par une faible densité cellulaire, typique de la croissance planctonique de C. hydrogenoformans. Cette limite est le facteur le plus contraignant pour des taux de charge de CO plus élevés. Ces performances ont été comparées à celles obtenues dans un réacteur à fibres creuses (BRFC) inoculé par la souche. En dépit d’une densité cellulaire et d’une activité volumétrique plus élevées, les performances du BRFC à tout le moins cinétiquement limitées quand elles n’étaient pas impactées par le transfert de masse, l'encrassement et le vieillissement de la membrane. Afin de parer à la dégénérescence de C. hydrogenoformans en cas de pénurie de CO, la croissance de la bactérie sur pyruvate en tant que seule source de carbone a été également caractérisée. Fait intéressant, en présence simultanée de pyruvate et de CO, C. hydrogenoformans n’a amorcé la consommation de pyruvate qu’une fois le CO épuisé. Cela a été attribué à un mécanisme d'inhibition du métabolisme du pyruvate par le CO, faisant ainsi du pyruvate le candidat idéal pour un système in situ de secours. / Gasification is today one of the most promising strategies to recover energy from waste. This thermo-chemical technology allows a 95% weight reduction of the input and generates inorganic inert ashes as well as a synthesis gas (syngas). Syngas is a gaseous fuel mainly composed of carbon monoxide (CO), hydrogen (H2) and carbon dioxide (CO2). Syngas can be burned to produce heat and electricity. It is also the building block of many high added- value products ranging from ethanol to ammonia and pure hydrogen. Downstream applications of syngas production will depend on its heating value, which is determined by its content in H2. Upgrading the H2 content in syngas is performed by the water-gas shift (WGS) reaction, widely utilized during methane reforming for hydrogen production. During the WGS reaction CO is converted to H2 and CO2 according to: CO + H2O → CO2 + H2. This process is achieved using a metallic catalyst in a heterogeneous gas-phase reaction with CO and steam. The WGS reaction has so far been reserved for large-scale gasification plants and requires high capital and operational expenditures. Hence, smaller scale plants that process low-grade materials (biomass, waste, sludge...), would not have access to such technology. The only possible outcome with the synthesis gas (syngas) produced and which generally has a poor heating value, is to generate heat or at best, electricity. In order to offer small plants access to the WGS reaction and to a higher range of products from their syngas, an alternative to the expensive and energy-intensive established catalyst-based WGS is here considered, such as extreme-thermophilic microbial processes carried out by hydrogenogens. The goal of this thesis was to use Carboxydothermus hydrogenoformans, a thermophilic carboxydotrophic hydrogenogenic bacterium as a biological catalyst for the WGS reaction. This was done by characterizing the impact of a growth-associated biomineralization phenomenon on H2 production and assessing the feasibility and limitations of using the strain in a bioreactor. First, characterization of the predominant inorganic phase when Carboxydothermus hydrogenoformans was inoculated in the DSMZ medium revealed the biomineralization of two crystalline CaP phases. The X-ray diffractometry peaks and Fourier transform infrared spectroscopy spectrum of this biphasic material consistently showed features characteristic of Mg-whitlockite, whereas transmission electron microscopy analysis showed the existence of hydroxyapatite-like nanorods crystals. In both cases, the mode of biomineralization appears to be biologically induced rather than biologically controlled. The impact of the endogenous CaP precipitate on CO mass transfer and H2 production was thus assessed and compared to a medium where no precipitation was observed. In the DSMZ medium, the apparent KLa (0.22 ±0.005 min-1) and H2 production yield (89.11 ±6.69%) were higher than the ones obtained in the modified medium (0.19 ±0.015 min-1 and 82.60 ±3.62% respectively). The presence of the precipitate had no impact on C. hydrogenoformans CO uptake. Overall, the CaP precipitate offers a novel strategy for gas-liquid mass transfer enhancement using CO hydrophobic properties. Second, the conversion of CO into H2 by C. hydrogenoformans was investigated and optimized in a 35 L gas-lift reactor. Upon all operational conditions, the ratio of gas recirculation over CO feed flow rates (QR:Qin) was the major parameter that impacted both biological activity and volumetric gas-liquid mass transfer. The CO conversion performance of the gas lift reactor was kinetically limited over a QR:Qin ratio of 40, and mass transfer limited below that ratio, resulting in a maximum conversion efficiency of 90.4±0.3% and a biological activity of 2.7±0.4 molCO· g–1VSS· day–1. Despite very promising results, CO conversion performance was limited by a low cell density, typical of C. hydrogenoformans planktonic growth. This limitation was found to be the most restrictive factor for higher CO loading rates. Results were compared to the performance of the strain inoculated in a hollow fiber membrane bioreactor where performance, despite the higher cell density and volumetric activity, was biokinetically limited, when not limited by gas–liquid mass transfer, membrane fouling and aging. To avoid any C. hydrogenoformans decay during potential CO shortages, growth of the bacterium on pyruvate as a sole carbon source was characterized. Interestingly, when grown simultaneously on pyruvate and CO, pyruvate consumption was initiated upon CO depletion. This was attributed to the inhibition of pyruvate oxidation by CO, making pyruvate the ideal candidate for an in-situ back-up system.