In this study, the feasibility of a NaA zeolite membrane for the gas phase separation of steam and hydrogen mixtures was determined. The Fischer-Tropsch (FT) process, which produces high value fuels and chemicals from coal and natural gas, can be greatly improved upon by the selective removal of water from the FT reactor product stream. According to the FT reaction kinetics, the rate of reaction increases with the partial pressure of hydrogen but is adversely affected the presence of water in the reactor product stream. Chemisorbed water on the surface of the metal catalysts also enhances deactivation due to sintering and fouling. The use of a zeolite membrane reactor is well equipped to serve the purpose of in-situ water removal as it can facilitate the separation of chemical components from one another in the presence of catalytic reactions. The LTA type zeolite membrane NaA or zeolite 4A, in particular, is well suited for the separation of polar (H2O) from non-polar (H2) molecules because of its high hydrophilicity. NaA has also been identified as an excellent candidate for selective water removal applications due its high adsorption affinity and capacity for water. The NaA membrane used in this study was manufactured by means of the in-situ crystallisation method where the growth of crystals on the inside surface of a centrifugally casted a-alumina support was favoured. Scanning electron microscopy (SEM) analyses performed on the membrane after a double hydrothermal synthesis indicated that the surface topology was rough and that the zeolite crystals formed were not uniform in size. Overall, the membrane thickness varied between 6.5 and 8.0 flm. An evaluation of the membrane quality was made possible through permeation experiments involving SF6 and Hz. The calculated Hz/SF6 permselectivity in this study was found to be 9.78, which despite being higher than the Knudsen diffusion selectivity of 8.54, confirmed the presence of intercrystalline defects or non-zeolitic pores in the membrane. Experiments concerning pure component and binary mixture permeation of steam and hydrogen through the supported NaA membrane were conducted over a temperature range of 115°C to 160 °c for binary hydrogen/steam mixtures, 25°C to 160°C for pure hydrogen and 130°C to 170°C for pure steam. For the permeation of pure component hydrogen, a local maximum in its permeance having a value of 224 x 10'°8 mol.m,z.s'!.Pa'! was reached at a system pressure and temperature of 6.875 bar and 75°C respectively. For the permeation of pure component steam through NaA, the effects of capillary condensation in the pores and defects of the zeolite membrane resulted in a decrease in steam permeance as a function of absolute pressure for temperatures lower than 160 °c. Once the effects of capillary condensation had receded, maxima in the steam permeances as a function of temperature corresponding to values of 70 x 10,08, 65 X 10,08 and 75 x 10,08 mol.m•2.s'I.Pa'l were found for the 182.5, 197.5 and 222.5 kPa isobars respectively. These observations collaborated well with the description of surface diffusion with permeation taking place in the Langmuir (strong adsorption) regime. Permeation experiments through NaA as function of temperature were conducted for a 90 mol% steam -10 mol% hydrogen (90-10) binary mixture as well as for a 60-40 mixture of these two. At low temperatures the permeation of hydrogen was completely suppressed by the condensed steam resulting in an almost perfect separation. The Kelvin equation was used to estimate the pore size of the defects which was found to range between 1.86 and 2.45 nm. The temperature range over which these defects in the membrane were assumed to become unblocked (i.e. assuming when the first breakthrough of hydrogen occurred), were determined to be between 140 to 148 °c and between 128 to 130 °c for the 90-10 and 60-40 mixtures respectively. The mixture selectivities (towards water) between 115 °c and 130 °c were found to be immensely high (much greater than 1000) for both the 90-10 and 60-40 mixtures, while the ideal selectivities were calculated to be less than lover the same temperature range. At 140 °c, the selectivity towards water for the 9010 mixture was still greater than 1000; however for the 60-40 mixture at this temperature, an inversion of selectivity towards H2 had already taken place. The breakthrough in H2 permeance occurs at a much lower temperature when the feed mixture contains a lower concentration of water. Since the partial pressure of steam will be reduced, larger pores will become unblocked at lower temperatures according to the Kelvin equation. / Thesis (M. Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2008.

In this study, the feasibility of a NaA zeolite membrane for the gas phase separation of steam and hydrogen mixtures was determined. The Fischer-Tropsch (FT) process, which produces high value fuels and chemicals from coal and natural gas, can be greatly improved upon by the selective removal of water from the FT reactor product stream. According to the FT reaction kinetics, the rate of reaction increases with the partial pressure of hydrogen but is adversely affected the presence of water in the reactor product stream. Chemisorbed water on the surface of the metal catalysts also enhances deactivation due to sintering and fouling. The use of a zeolite membrane reactor is well equipped to serve the purpose of in-situ water removal as it can facilitate the separation of chemical components from one another in the presence of catalytic reactions. The LTA type zeolite membrane NaA or zeolite 4A, in particular, is well suited for the separation of polar (H2O) from non-polar (H2) molecules because of its high hydrophilicity. NaA has also been identified as an excellent candidate for selective water removal applications due its high adsorption affinity and capacity for water. The NaA membrane used in this study was manufactured by means of the in-situ crystallisation method where the growth of crystals on the inside surface of a centrifugally casted a-alumina support was favoured. Scanning electron microscopy (SEM) analyses performed on the membrane after a double hydrothermal synthesis indicated that the surface topology was rough and that the zeolite crystals formed were not uniform in size. Overall, the membrane thickness varied between 6.5 and 8.0 flm. An evaluation of the membrane quality was made possible through permeation experiments involving SF6 and Hz. The calculated Hz/SF6 permselectivity in this study was found to be 9.78, which despite being higher than the Knudsen diffusion selectivity of 8.54, confirmed the presence of intercrystalline defects or non-zeolitic pores in the membrane. Experiments concerning pure component and binary mixture permeation of steam and hydrogen through the supported NaA membrane were conducted over a temperature range of 115°C to 160 °c for binary hydrogen/steam mixtures, 25°C to 160°C for pure hydrogen and 130°C to 170°C for pure steam. For the permeation of pure component hydrogen, a local maximum in its permeance having a value of 224 x 10'°8 mol.m,z.s'!.Pa'! was reached at a system pressure and temperature of 6.875 bar and 75°C respectively. For the permeation of pure component steam through NaA, the effects of capillary condensation in the pores and defects of the zeolite membrane resulted in a decrease in steam permeance as a function of absolute pressure for temperatures lower than 160 °c. Once the effects of capillary condensation had receded, maxima in the steam permeances as a function of temperature corresponding to values of 70 x 10,08, 65 X 10,08 and 75 x 10,08 mol.m•2.s'I.Pa'l were found for the 182.5, 197.5 and 222.5 kPa isobars respectively. These observations collaborated well with the description of surface diffusion with permeation taking place in the Langmuir (strong adsorption) regime. Permeation experiments through NaA as function of temperature were conducted for a 90 mol% steam -10 mol% hydrogen (90-10) binary mixture as well as for a 60-40 mixture of these two. At low temperatures the permeation of hydrogen was completely suppressed by the condensed steam resulting in an almost perfect separation. The Kelvin equation was used to estimate the pore size of the defects which was found to range between 1.86 and 2.45 nm. The temperature range over which these defects in the membrane were assumed to become unblocked (i.e. assuming when the first breakthrough of hydrogen occurred), were determined to be between 140 to 148 °c and between 128 to 130 °c for the 90-10 and 60-40 mixtures respectively. The mixture selectivities (towards water) between 115 °c and 130 °c were found to be immensely high (much greater than 1000) for both the 90-10 and 60-40 mixtures, while the ideal selectivities were calculated to be less than lover the same temperature range. At 140 °c, the selectivity towards water for the 9010 mixture was still greater than 1000; however for the 60-40 mixture at this temperature, an inversion of selectivity towards H2 had already taken place. The breakthrough in H2 permeance occurs at a much lower temperature when the feed mixture contains a lower concentration of water. Since the partial pressure of steam will be reduced, larger pores will become unblocked at lower temperatures according to the Kelvin equation. / Thesis (M. Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2008.

An experimental study of phase transitions of long-chain n-alkanes induced by the effect of interfaces is described. ¶ The phase behaviour of long-chain n-alkanes (carbon number 14, 16, 17, 18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied in the vicinity of and down to several degrees below the bulk melting points, Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapour (0.97 [equal to or greater-than] p/p[subscript o] [equal to or greater-than] 0.997), studied capillary condensation transition, subsequent growth of capillary condensates between two surfaces, and phase transitions in both the adsorbed films and the condensates. By measuring the growth rate of the capillary condensates we have identified a transition in the lateral mobility of molecules in the adsorbed films on isolated mica surfaces. This transition to greater mobility occurs slightly above Tm for n-hexadecane, n-heptadecane and n-octadecane but several degrees below Tm for n-tetradecane, and is accompanied by a change in wetting behaviour and a measurable decrease in adsorbed film thickness for n-heptadecane and n-octadecane. Capillary condensates that form below Tm remain liquid, but may freeze if the degree of confinement is reduced by separation of the mica surfaces. An increase in the area of the liquid-vapour interface relative to that of the liquid-mica interface facilitates freezing in the case of the long-chain alkanes, which show surface freezing at the liquid-vapour interface. ¶ Although thermodynamic properties of the surface freezing transition have been rather well documented, the kinetics involved in formation of such ordered monolayers has so far received very little attention. We studied the surface tension of n-octadecane as a function of temperature in the vicinity of Tm, using the static Wilhelmy plate and the dynamic maximum bubble pressure methods. The two methods give different results on cooling paths, where nucleation of the surface ordered phase is involved, but agree on heating paths, where both methods measure properties of the equilibrium surface phase. On cooling paths, the surface of bubbles may supercool below the equilibrium surface freezing temperature. The onset of surface freezing is marked by a sharp drop in the surface tension. The transition is accompanied by an increased stability of the films resulting in longer bubble lifetimes at the liquid surface, which suggests that the mechanical properties of the surfaces change from liquid-like to solid-like. Our results suggest occurrence of supercooling of the monolayer itself.

Dans ce travail réalisé au Centre de Morphologie Mathématique and IFPEN, on s'intéresse à la microstructure et aux propriétés physiques d'alumines mésoporeuses. Il s'agit d'un supporte de catalyseur utilisés notamment dans les processus industriels de raffinage du pétrole. Fortement poreux, ce matériau est formé de ''plaquettes'' distribuées de manière désordonnée à l'échelle de la dizaine de nanomètres. Les propriétés de transport de masse du support de catalyseur sont fortement influencées par la morphologie de la microstructure poreuse. Ce travail porte sur la modélisation de la microstructure et des propriétés de transport des alumines mésoporeuses, à l'aide d'outils numériques et théoriques dérivés de l'analyse d'image et de la théorie des ensembles aléatoires. D'une part, on met en place des méthodes de caractérisation et de modélisation des microstructures, qui s'appuient sur, entre autre, des images obtenues par microscopie électronique en transmission (MET) et des courbes de porosimétrie azote. D'autre part, on utilise des méthodes d'homogénéisation numérique à champs complets par transformées de Fourier rapide (FFT).Dans un premier temps, le matériau est caractérisé expérimentalement par porosimétrie azote et résonance magnétique nucléaire à gradient de champ pulsé (RMN-GCP). Les images MET sont obtenus sur des échantillons d'épaisseur variable, filtrées et caractérisés par des fonctions de corrélation, notamment. Le bruit à haute fréquence issu de la membrane de carbone est identifié et pris en compte dans la modélisation de l'imagerie MET. À partir des images MET 2D, un modèle aléatoire à deux échelles est proposé pour représenter la microstructure 3D. Il prend en compte la forme des plaquettes d'alumines, leurs tailles, les effets d'alignement locaux et d'agrégation, qui sont identifiés numériquement. La procédure est validée à l'aide de comparaisons entre modèle et images expérimentales, en terme notamment de fonctions de corrélation et de surface spécifique, mesurées par porosimétrie azote.Dans un deuxième temps, une méthode de simulation des courbes d'isothermes de porosimétrie dans des milieux poreux périodiques ou aléatoires est développée. Basée sur des opérations morphologiques simples, elle étend un travail antérieur sur la porosimétrie au mercure. L'adsorption multicouche à basse pression est simulée à l'aide d'une dilatation tandis que les ménisques de l'interface vapeur-liquide intervenant pendant l'adsorption sont simulés à l'aide de fermetures de la phase solide par des éléments structurants sphériques. Pour simuler la désorption, une combinaison de fermetures et de bouchages de trou est utilisée. Le seuil de désorption est obtenu par une analyse de la percolation de la phase gazeuse. La méthode, d'abord validée sur des géométries simples, est comparée à des résultats antérieurs. Elle prédit une hystérésis et les distributions de pores associées à la porosimétrie. Nous l'appliquons aux modèles de microstructures 3D d'alumines mésoporeuses et proposons un modèle à trois échelles afin de rendre compte du seuil de pression pendant la désorption. En plus de la courbe de désorption, ce modèle reproduit les fonctions de corrélation mesurées sur les images MET.Dans un troisième temps, la diffusion de Fick, la perméabilité de Darcy, et les propriétés élastiques sont prédits à l'aide de calculs de champs complets par FFT sur des réalisations des modèles d'alumines mésoporeuses à deux et trois échelles. Les coefficients de diffusion effectifs et les facteurs de tortuosité sont prédits à partir de l'estimation du flux. Sont étudiés les effets de forme, d'alignement et d'agrégation des plaquettes sur les propriétés de diffusion à grande échelle. Les prédictions numériques sont validées au moyen des résultats expérimentaux obtenus par méthode RMN-GCP. / In a work made at Centre de Morphologie Mathématique and IFPEN, we study the microstructure and physical properties of mesoporous alumina. This is a catalyst carrier used in the petroleum refining industry. Highly porous, it contains disordered ''platelets'' at the nanoscale. The mass transport properties of the catalyst carrier are strongly influenced by the morphology of the porous microstructure. We focus on the modeling of the microstructure and of transport properties of mesoporous alumina, using numerical and theoretical tools derived from image analysis and random sets models. On the one hand, methods are developed to characterize and model the microstructure, by extracting and combining information from transmission electron microscope (TEM) images and nitrogen porosimetry curves, among others. On the other hand, the numerical homogenization relies on full-field Fourier transform computations (FFT).The material is first characterized experimentally by nitrogen porosimetry and pulse-field gradient nuclear magnetic resonance (PFG-NMR). TEM images, obtained on samples of various thicknesses are filtered and measured in terms of correlation function. The high-frequency noise caused by carbon membrane support is identified and integrated in the TEM image model. Based on the 2D TEM images, a two-scale random set model of 3D microstructure is developed. It takes into account the platelet shape, platelet size, local alignments and aggregations effects which are numerically identified. The procedure is validated by comparing the model and experimental images in terms of correlation function and specific surface area estimated by nitrogen porosimetry.Next, a procedure is proposed to simulate porosimetry isotherms in general porous media, including random microstructures. Based on simple morphological operations, it extends an earlier approach of mercury porosimetry. Multilayer adsorption at low pressure is simulated by a dilation operation whereas the menisci of the vapor-liquid interface occurring during adsorption are simulated by closing the solid phase with spherical structuring elements. To simulate desorption, a combination of closing and hole-filling operations is used. The desorption threshold is obtained from a percolation analysis of the gaseous phase. The method, validated first on simple geometries, is compared to previous results of the literature, allowing us to predict the hysteresis and pore size distribution associated to porosimetry. It is applied on 3D microstructures of mesoporous alumina. To account for the pressure threshold during desorption, we propose a refined three-scale model for mesoporous alumina, that reproduces the correlation function and the desorption branch of porosimetry isotherms.Finally, Fick diffusion, Darcy permeability, and elastic moduli are numerically predicted using the FFT method and the two-scale and three-scale models of mesoporous alumina. The hindering effects in diffusion are estimated by the Renkin's equation. The effective diffusion coefficients and the tortuosity factors are estimated from the flux field, taking into account hindering effects. The effects of platelet shape, alignment and aggregation on the diffusion property are studied. The numerical estimation is validated from experimental PFG-NMR results.

[pt] A produtividade de poços produtores de gás, que operam com pressões de fundo inferiores à pressão de orvalho, é afetada pelo aparecimento da saturação de líquido em seus entornos. Para entender esse fenômeno, conhecido como bloqueio por condensado, os simuladores em escala de poros são ferramentas úteis na investigação dos parâmetros que influenciam na quantidade e na distribuição da saturação de condensado, assim como seus efeitos na redução do fluxo de gás. Esse trabalho apresenta um modelo de rede de capilares composicional e isotérmico para o estudo do escoamento de gás retrógrado em meios porosos. Forças capilares e gravitacionais não foram consideradas. O escoamento monofásico é comutado para bifásico de padrão anular quando a pressão e a composição do fluido atingem um critério de estabilidade. O método de Newton é aplicado para resolver as equações de fluxo e consistência dos volumes e calcular o transporte de cada um dos componentes ao longo da rede. As propriedades do fluido e o comportamento do escoamento foram testadas contra os resultados de um simulador termodinâmico comercial e soluções analíticas, respectivamente. Após validação, o simulador foi utilizado para obter curvas de permeabilidade relativa gás-líquido através da despressurização de uma rede 2D e alguns resultados são discutidos. / [en] Gas well deliverability in retrograde gas reservoirs is affected by the appearance of liquid saturation around the wellbore when the bottom-hole pressure is below the dew point. Pore-scale simulators are used to model this phenomenon, known as condensate blockage, and to investigate parameters that ifluence the amount and the distribution of condensate saturation, as well as how it chokes the gas flow. Here, a fully-implicit isothermal compositional pore-scale network model is presented for retrograde gas flow in porous media. Capillary and gravitational forces are neglected. The model shifts from single-phase flow to annular flow regime when the pressure and the fluid composition reach a stability criteria. Newton s method is applied on flow and volume consistency equations to calculate the transport of each component through the network. Fluid properties and flow behavior were tested against a commercial thermodynamic simulator and analytical solutions respectively. After validation, the simulator was used to predict gas-liquid relative permeability from a depletion process in a 2D network and some results are discussed.