La réaction de transestérification des huiles végétales avec de l'éthanol permet la production d’esters éthyliques dont les applications industrielles sont, à ce jour, essentiellement cosmétiques ou alimentaires. Pour ouvrir le champ des applications aux biocarburants (pour substituer les carburants actuels issus de ressources fossiles), il est apparu nécessaire de développer un procédé de transestérification plus performant pour être économiquement rentable. Selon le schéma réactionnel et les propriétés thermocinétiques du système, les limites des procédés batchs existants pourraient être franchies en utilisant des procédés continus. Le système étudié est complexe en raison des changements d’équilibres de phase notamment et de la présence simultanée de différents phénomènes (mélange, transferts de chaleur et de matière, réactions principales et compétitives) qui doivent être précisément contrôlés. Pour concevoir correctement un procédé continu et acquérir de nombreuses données, les microréacteurs apparaissent comme un outil approprié à cette transposition. Dans ce travail, nous avons transposé la réaction batch dans un dispositif microstructuré continu (tube PFA de diamètre interne 508 μm) induisant un meilleur contrôle des transferts de chaleur et de matière. L’étude de l’influence des conditions de fonctionnement (débits des réactifs, rapport molaire initial huile/éthanol, température…) a permis de trouver des paramètres réactionnels favorables qui permettent d’atteindre des conversions et rendements élevés. Dans ces conditions, nous avons montré qu’il est possible d’acquérir des données cinétiques dès les premières secondes de réaction, ce qui n’était pas réalisable en réacteur batch conventionnel. Pour acquérir ces données en batch et en microréacteurs, nous avons développé une méthode d’analyse en ligne par spectroscopie proche infrarouge en s’appuyant sur la chromatographie en phase gazeuse comme méthode de référence. Des modèles PLS ont alors été établis pour quantifier en ligne les teneurs en composés majoritaires lors de la réaction de transestérification de l’huile de tournesol hautement oléique avec l’éthanol. A partir de ces données, les phénomènes mis en jeu ont été modélisés et les constantes cinétiques ainsi que les coefficients de transfert de cette réaction ont été déterminés. Le modèle a ensuite été utilisé pour simuler des réactions avec d’autres conditions opératoires et il nous a permis de travailler sur la séparation des produits de la réaction. / Transesterification reaction of vegetable oil with ethanol leads to ethyl esters, used to date for applications principally in food and cosmetic industry. To open the application field to biofuels (to substitute current fuels resulting from fossil resources), process efficiency has to be developed to be economically profitable. According to the reaction scheme and thermokinetic properties, limits of current batch processes can be overcome by carrying out continuous processes. The studied system is complex due in particular to phase equilibrium changes as well as simultaneous presence of various phenomena (mixing, heat and mass transfers, principal and competitive reactions) which have to be precisely controlled. Therefore, microreactors appear as the appropriate tool for transposition to a continuous process and acquisition of numerous data. In this work, batch reaction was transposed to a micro-scaled continuous device (PFA tube of 508 μm internal diameter), inducing better heat and mass transfer. Study of the influence of the operational conditions (reactants flow, initial ethanol to oil molar ratio, temperature...) revealed the favourable reaction parameters necessary to reach high conversions and yields. In these conditions, we showed the possibility of acquiring kinetic data at the first seconds of the reaction, which was not feasible in a conventional batch process. To acquire data in batch and microreactors, an on-line analysis method by Near InfraRed (NIR) spectroscopy was developed by using gas chromatography as a reference method. PLS models were then set up to quantify the major compounds contents on-line during the transesterification reaction of high oleic sunflower oil with ethanol. These data were used to model occurring phenomena and to determine kinetic constants and transfer coefficients. The model was subsequently used to simulate reactions with other operational conditions and also to work on the separation of reaction products.

La détermination des cinétiques réactionnelles est incontournable en génie de la réaction. Sans quoi on ne peut dimensionner convenablement un réacteur. Cependant, la complexité de certains systèmes réactionnels nécessite de fournir des efforts expérimentaux souvent rédhibitoires, en termes de moyens comme de temps. Des méthodologies de planification expérimentales pour la modélisation cinétique existent. Les objectifs de ces méthodes peuvent être la détermination précise des paramètres d’un modèle ou la discrimination de plusieurs modèles potentiels. Notre objectif est d’étudier des méthodologies pour discriminer entre des modèles et d’éprouver ces méthodologies sur des applications numériques et/ou expérimentales. Ces méthodologies s’appuient sur un processus itératif, qui, étape par étape, mène à la sélection d’un modèle, à la détermination de ses paramètres et à sa validation. Une fois la méthodologie précisée, un premier cas d’étude « numérique » concerne un système réactionnel, dont plusieurs modèles sont proposés, basés sur différents schémas réactionnels. L’objectif étant de trouver le bon schéma par application de la méthodologie. Ensuite un cas d’étude expérimentale est traité : l’acylation catalytique du ferrocène. Sa cinétique n’est pas connue. La méthodologie appliquée mènera à la proposition de plusieurs modèles cinétiques et à la sélection du plus adapté. Un dernier cas d’étude s’intéresse à la modélisation des réactions exothermique, en particulier en micro/milli réacteur continu. Un focus est d’abord fait sur une modélisation adaptée à ces systèmes réactionnels, à travers l’utilisation de nombres adimensionnels réduisant les degrés de liberté, et permettant une analyse synthétique du comportement du réacteur. Par la suite, plusieurs applications « numériques » de la méthodologie de discrimination de modèles sont proposés, dont l’objectif est la discrimination du comportement thermique des réacteurs. La méthodologie expérimentale utilise efficacement les données déjà accumulées sur les modèles, pour choisir au mieux chaque nouvelle expérience en fonction des objectifs ciblés. Elle permet de cibler efficacement les meilleures expériences pour atteindre l’objectif fixé. Les efforts expérimentaux sont ainsi optimisés, ainsi que la recherche de modèles cinétiques et thermiques. / Knowledge on kinetics is essential for chemical reactor modelling. Yet when chemical systems are very complex, development of good kinetic models leads to expensive and time consuming experiments, often prohibitive. Our goal is to develop efficient numerical methods to design the optimal experiments to select the best model among many possible candidates while precisely estimating its kinetics parameters. The gain is double: reduction of experiments and acquisition of more accurate information. Several study cases will enable the assessment of these methodologies. Optimal experimental design methodologies are based on iterative procedures,leading to the selection of the more accurate model, and to the identification of its parameters. In the present work, a first numerical study case is chosen as an illustration of the method, consisting on the discrimination of four synthesis pathway that are potential candidates to describe a reactional system. It is showed how the developed method can smartly choose experiments to lead to the choice of the accurate pathway. The second study case is the experimental study of the catalytic acetylation of ferrocene, for which any accurate kinetic models have not been found yet. Thanks to the iterative design of experiments, it is possible to characterize, very quickly, the order of reaction and how the catalyst effect has to be considered. The last part of this work deals with exothermic reactions and the coupling between thermal transfer and chemical reactions in milli/micro-reactors. The use of dimensionless numbers is proposed to reduce the number of parameters implied in such systems and to analyse the thermal behaviour of microreactors. Then, the aim is to illustrate how to discriminate thermal behaviours using the discriminatory methodology, through several study cases. Those examples demonstrate that iterative design of experiments is an efficient method to find the best experiments to solve the issues for selecting a model among others and for determining the associated parameters. This offers the advantages to reduce the experimental efforts in time and in matter, and thus to unlock modelling of many complex chemical systems.

A ampliação de escala na produção de fármacos é um dos principais gargalos na indústria químico-farmacêutica. A Tecnologia de Microrreatores (TMR) soluciona este problema através da perspectiva de desenvolver uma metodologia dentro do laboratório que facilmente pode ser implementada em escala de produção industrial através do aumento do número de microrreatores arranjados em paralelo ou numbering-up. Além disso o uso do microrreator apresenta diversas vantagens, tais como excelente controle de troca térmica, homogeneização mais eficiente, aumento da velocidade da reação, alta conversão e seletividade, segurança ao se trabalhar com reagentes e produtos tóxicos, além da redução da geração de resíduos. O objetivo deste trabalho foi transpor a reação de síntese de um derivado da tiazolidina-2,4-diona, um intermediário para produção de fármacos no combate à diabetes, do processo batelada para microrreator em fluxo contínuo. Através dos resultados foi determinado que não existe a necessidade de mais de 5,5h para se obter o rendimento máximo (96%) da reação em batelada com o n-propanol como melhor solvente a uma velocidade inicial de 1,25 mmol/L.min e utilizando piperidina como melhor base na concentração ideal de 0,053 M. A transposição para o microrreator mostrou que os processos apresentam resultados semelhantes quando utilizada a temperatura de ebulição normal do solvente. Porém, como o microrreator possibilita o aumento da temperatura, foi obtido um rendimento de 76% a 160°C em 20 min de tempo médio de residência utilizando n-propanol como solvente, mostrando um aumento nominal de rendimento de 47%, se comparado com o processo batelada. Esses resultados contribuíram para uma produção cerca de 3 vezes maior atingindo o valor de 3,47 mg/min. Pôde-se concluir que o uso de microrreator deve ser melhor difundido nas indústrias químico-farmacêuticas podendo suprir as produções dos reatores batelada, com maior segurança e eficiência, gerando menos resíduos e ocupando uma área física muito menor. / Scaling-up in drug production is one of the main bottlenecks in the chemical-pharmaceutical industry. Microreactor Technology (MRT) solves this problem from the perspective of developing a methodology within the laboratory that can easily be implemented on an industrial scale by increasing the number of microreactors arranged in parallel or numbering-up. In addition, the use of microreactor has several advantages, such as excellent thermal exchange control, more efficient homogenization, increased reaction rate, high conversion and selectivity, safety when working with reagents and toxic products, as well as reduction of waste generation. The main objective of this work was to transpose the reaction of a thiazolidine-2,4-dione derivative, an intermediate for the production of drugs against diabetes, from the batch process to the microreactor in a continuous flow. From the results it was determined that there is no need for more than 5.5h to obtain the maximum yield (96%) in the batch reaction with n-propanol as the best solvent and with an initial reaction rate of 1.25 mmol/L.min using piperidine as the best basis at the optimum concentration of 0.053 M. Transposition to the microreactor showed that the processes show similar results when used at the normal boiling temperature of the solvent. However, as the microreactor enables operation with increased temperature, yield of 76% at 16 °C in 20 min of mean residence time was obtained using n-propanol as the solvent, showing a nominal yield increase of 47% when compared to the batch process. These results contributed to about 3 times higher production reaching a value of 3.47 mg/min. It could be concluded that the use of microreactor should be better disseminated in the chemical-pharmaceutical industries, being able to supply batch reactor productions with greater safety and efficiency, generating less waste and occupying a much smaller physical area.

Ce travail vise non seulement à la conception et la fabrication de nouvelles puces microfluidiques pour l'oxydation de l'alcool benzylique, mais aussi au développement d'une méthode utilisant le plasma. Cette dernière est consacrée à la fonctionnalisation de surface avec un liant afin d'ancrer des particules de catalyseur. Les résultats montrent que le procédé PECVD est une méthode universelle permettant de déposer un nombre élevé de fonctions amines à partir de l’APTES sur différentes surfaces. Suite à l’étude des différents paramètres, des conditions optimales ont été trouvées. En effet, par rapport à la fonctionnalisation conventionnelle par voie humide, une meilleure hydrophilicité, une épaisseur de dépôt ainsi qu’une densité de groupements amines plus élevées ont été obtenus sur les échantillons traités. De plus, les résultats avec les AuNPs immobilisés sur la zéolite indiquent que l’APTES est un meilleur précurseur que le MPTES car il offre une plus grande teneur en or. Pour la zéolite et AuNPs@zéolite, les particules sont fonctionnalisées à l'aide du CES en tant qu’agent de liaison pour les amines protonées présentes sur la surface du COC ; ce dernier étant prétraité en utilisant le procédé PECVD. Le microréacteur à base d'or présente une sélectivité élevée stable au benzaldéhyde (~94%). Cependant, il montre également une conversion faible d'alcool benzylique (~20%). Le microréacteur type AuNPs@zéolites réalise la meilleure activité catalytique dans notre étude, car une sélectivité élevée par rapport au benzaldéhyde (>99%) est obtenue avec la conversion la plus élevée de l'alcool benzylique (~40%). / This work aims not only at designing and fabricating new microfluidic chips for benzyl alcohol oxidation, but also at developing a methodology of plasma devoted to the surface functionalization with linkage reagent in order to anchor catalyst particles in the next step. Results show that the PECVD method is a universal method that can deposit high amine content of APTES polymerized film on various substrate surfaces. Optimized plasma conditions for APTES deposition were found and lead to a better hydrophilicity of the substrates, a higher coating thickness, as well as a higher amine group density than the conventional wet chemistry method. In addition, the APTES depositions lead to a further higher coverage and amount of AuNPs when the pH value is 6.2. Moreover, the results of immobilizing AuNPs on zeolite indicate that APTES is a better linker than MPTES as it provides a higher amount of gold loading. For zeolite and AuNPs@zeolite deposition, the particles were functionalized with carboxyl group using CES as a linker for bounding the protonated amines on COC surface that is pre-modified using PECVD method. The latter coating is stable in hydrodynamic flows and could be further used in microfluidics. Finally, the gold \Y zeolite \AuNPs@zeolite microreactors are respectively connected into pre-designed microfluidic system. The gold type microreactor exhibits stable high selectivity to benzaldehyde (~94%). However, it also shows relative low benzyl alcohol conversion (~20%). The AuNPs@zeolites type microreactor performs the best catalytic activity in our study as a high benzaldehyde selectivity (>99%) is obtained with the highest benzyl alcohol conversion (~40%).

Over recent years the delivery of nanosized drug particles has shown potential in improving bioavailability. Drug nanosizing is achieved by ¿top-down¿ and by ¿bottom-up¿ approaches. Owing to limitations associated with the top-down techniques, such as high energy input, electrostatic effects, broad particle size distributions and contamination issues, great interest has been directed to alternative bottom up technologies. In this study, the hypothesis that microreactors can be used as a simple and cost-effective technique to generate organic nanosized products is tested using three steroids (hydrocortisone, prednisolone and budesonide). Arrested antisolvent nanoprecipitation using ethanol (solvent) and water (antisolvent) was conducted within the microreactors. To enable experimental design for the microreactor studies, solubility profiles in different ethanol-water combinations at 25 °C were explored. All three drugs¿ solubility increased with increasing ethanol concentration showing maxima at 80-90 % v/v ethanol-water mixtures. Because of the complex multivariate microfluidic process, artificial neural network modelling was then employed to identify the dominant relationships between the variables affecting nanoprecipitation (as inputs) and the drug particle size (as output). The antisolvent flow rate was found to have the major role in directing drug particle size. Based on these successful findings, the potential of preparing pharmaceutical nanosuspensions using microfluidic reactors was researched. A hydrocortisone (HC) nanosuspension (NS) was prepared by introducing the generated drug particles into an aqueous solution of stabilizers stirred at high speed with a propeller mixer. A tangential flow filtration system was then used to concentrate the prepared NS. Results showed that a stable narrow sized HC NS of amorphous spherical particles 500 ± 64 nm diameter and zeta potential ¿18 ± 2.84 mV could be produced. The ocular bioavailability of a microfluidic precipitated HC NS (300 nm) was assessed and compared to a similar sized, milled HC NS and HC solution as a control. The precipitated and the milled NS achieved comparable AUC0-9h of 28.06 ± 4.08 and 30.95 ± 2.2, respectively, significantly (P < 0.01) higher than HC solution (15.86 ± 2.7). These results illustrate the opportunity to design sustained release ophthalmic formulations. Going nano via microfluidic precipitation was also exploited to tailor budesonide (BD) NS for pulmonary administration. The in vitro aerosolization by nebulization of a BD NS was studied in comparison with a commercial BD microsuspension. Overall, the fine particle fraction generated from BD NS (56.88 ± 3.37) was significantly (P < 0.05) higher than the marketed BD (38.04 ± 7.81). The mean mass aerodynamic diameter of BD NS aerosol (3.9 ± 0.48 ¿m) was significantly smaller (P < 0.05) than the microsuspension (6.2 ± 1.09 ¿m) indicating improved performance for BD NS. In conclusion, findings of this study support the hypothesis of using microfluidic nanoprecipitation as a promising and economical technique of drug nanosizing. / Egyptian Government (Ministry of High Education)

Rising energy demands and the continual push to find more energy efficient technologies have been the impetus for the investigation of waste heat recovery techniques. Diesel engine exhaust heat utilization has the potential to significantly reduce the consumption of fossil fuels and reduce the release of greenhouse gases, because diesel engines are ubiquitous in industry and transportation. The exhaust energy can used to provide refrigeration by implementing an organic Rankine cycle coupled with a vapor-compression cycle. A critical component in this system, and in any waste heat recovery system, is the heat exchanger that extracts the heat from the exhaust. In this study, a cross-flow microchannel heat exchanger was geometrically examined and thermally tested under laboratory conditions. The heat exchanger, referred to as the Heat Recovery Unit (HRU), was designed to transfer diesel exhaust energy to a heat transfer oil. Two methods were developed to measure the geometry of the microchannels. The first was based on image processing of microscope photographs, and the second involved an analysis of profilometer measurements. Both methods revealed that the exhaust channels (air channels) were, on average, smaller in cross-sectional area by 11% when compared to the design. The cross-sectional area of the oil channels were 8% smaller than their design. The hydraulic diameters for both channel geometries were close to their design. Hot air was used to simulate diesel engine exhaust. Thermal testing of the heat exchanger included measurements of heat transfer, effectiveness, air pressure drop, and oil pressure drop. The experimental results for the heat transfer and effectiveness agreed well with the model predictions. However, the measured air pressure drop and oil pressure drop were significantly higher than the model. The discrepancy was attributed to the model's ideal representation of the channel areas. Additionally, since the model did not account for the complex flow path of the oil stream, the measured oil pressure drop was much higher than the predicted pressure drop. The highest duty of the Heat Recovery Unit observed during the experimental tests was 12.3 kW and the highest effectiveness was 97.8%. To examine the flow distribution through the air channels, velocity measurements were collected at the outlet of the Heat Recovery Unit using a hot film anemometer. For unheated air flow, the profile measurements indicated that there was flow maldistribution. A temperature profile was measured and analyzed for a thermally loaded condition. / Graduation date: 2012

Ce projet de thèse s'inscrit dans la recherche et le développement de dispositifs de lutte contre les émissions de composés organiques volatils (Programme principal n°7). En effet, l'objet de ce projet est d'étudier et de développer une nouvelle génération de microréacteurs catalytiques structurés pour la dépollution d'effluents gazeux contenant des composés organiques volatils (COV) issus de sources dispersées et/ou confinées. La difficulté principale du traitement des effluents gazeux est souvent liée à la multiplicité des sources de pollution et donc à la nécessité de collecter tous les effluents pollués vers une unité de traitement de grande échelle. Tout procédé qui pourrait facilement s'adapter et répondre à des pollutions diverses et localisées quels que soient les polluants, le débit et l'application, constituerait une grande avancée technologique dans le traitement de la pollution de l'air. Ainsi, des microsystèmes catalytiques pourraient être mis en oeuvre pour le traitement de composés organiques volatils dans des environnements industriels divers comme des ateliers d'imprimerie, de séchage, de peinture, de vernissage et des ateliers de nettoyage à sec. Toutes ces applications présentent un intérêt majeur d'un point de vue environnemental et de santé publique et constituent des cas idéaux pour mettre en oeuvre ces nouveaux microsystèmes et démontrer leur efficacité. Les microréacteurs présentent un avantage certain en terme de conception de procédés. La possibilité d'associer des éléments microstructurés ou des petites unités permet d'adapter et d'intégrer le procédé catalytique de façon appropriée, indépendamment de l'échelle et l'effluent. De plus, le concept modulaire est évidemment plus sûr. Ainsi, les microréacteurs construits à partir de plaques microstructurées permettent une distribution du flux gazeux dans l'ensemble des microcanaux. En outre, en raison des dimensions caractéristiques des microcanaux, les dispositifs microstructurés permettent une grande diminution de la distance entre la zone catalytique et le fluide conduisant à de meilleures propriétés de transferts que les réacteurs à lit fixe. Enfin et surtout, les microréacteurs catalytiques permettent une manipulation plus sûre des produits dangereux, inflammables et même explosifs que dans les réacteurs conventionnels. En partenariat avec le Laboratoire d'Analyse et d'Architecture des Systèmes (LAAS-Toulouse), ce projet vise donc à concevoir et à développer des réacteurs catalytiques microstructurés suivant les objectifs définis précédemment. Ces microréacteurs sont constitués d'un empilement de plaques de silicium recouvertes d'une couche fine de platine utilisant des technologies de la microélectronique. Les technologies utilisées par le LAAS constituent une alternative aux dépôts catalytiques sur des microstructures existantes. De plus, un des avantages majeurs de l'utilisation des microtechnologies est la possibilité d'intégrer les sources chauffantes sur une plaque de silicium (résistances thermiques métalliques). Ainsi, l'intégration de sources chauffantes au coeur du microréacteur est une solution intéressante à la question des besoins énergétiques. Les travaux associés à ce projet de thèse visent à optimiser les performances et les conditions de fonctionnement de microréacteurs catalytiques et à mieux comprendre et appréhender les phénomènes impliqués. Les performances des microréacteurs conçus sont évaluées vis-à-vis de diverses molécules représentatives des émissions industrielles aux mélanges de COV avec pour objectif final une étude sur des effluents réels. / This thesis project is part of the research and development of devices to fight against the emission of volatile organics compounds.

O escopo deste trabalho foi o desenvolvimento de microrreatores em tecnologia LTCC para produção de biodiesel, com foco na otimização de uma geometria de micromisturador. Esta proposta é resultado das oportunidades identificadas em três áreas do conhecimento: Microtecnologia, Intensificação de processos e Biocombustíveis. A principal ferramenta de desenvolvimento desta proposta é a fluidodinâmica computacional. Os microcanais baseados em geometrias com sucessivos cotovelos foram os escolhidos, para a investigação computacional e experimental. A metodologia computacional desenvolvida para alcançar os objetivos propostos envolve as etapas de: definição de um padrão de comparação, projeto das distâncias entre cotovelos, escolha de uma geometria com base na comparação entre diversas geometrias baseadas em sucessivos cotovelos e a otimização da geometria em função dos parâmetros fluidodinâmicos. Paralelamente, ensaios para a produção de biodiesel foram realizados, bem como, a investigação da produção de emulsões para avaliar como uma etapa do processo de produção do biodiesel. A geometria escolhida e otimizada foi a serpentina 3D, o que permitiu a otimização do módulo de tempo de residência e o projeto do microrreator. Finalizando, um microrreator foi projetado com parâmetros ótimos, obtendo assim a intensificação de processo por meio de conceitos de microtecnologia, para aplicação na produção de biocombustíveis. / The scope of this work was the development of microreactors in LTCC technology for biodiesel production, with a focus on the optimization of a micromixer geometry. This proposal is resulted from the opportunities identified in three areas of knowledge: Microtechnology, processes intensification and Biofuels. The main tool for development of this proposal is the computational fluid dynamics (CFD). The microchannels geometry with successive elbows were chosen for computational and experimental research. The computational methodology developed to achieve the proposed goals involves the following steps: defining a standard of comparison, a project of the distances between elbows, a choice of geometry based on the comparison between different geometries based on successive elbows and geometry optimization for the parameters hydrodynamic. In addition, tests for the production of biodiesel were being made and the investigations of production of emulsions to evaluate a step in the producing of biodiesel process. The geometry was chosen and optimized serpentine 3D, allowing the optimization of residence time module and the design of the microreactor. Finally, a microreactor was designed with optimal parameters, thus obtaining the intensification process through microtechnology concepts for application in the biofuels production.

Advances in electronics fabrication, coupled with the demand for increased computing power, have driven the demand for innovative cooling solutions to dissipate waste heat generated by these devices. To meet future demands, research and development has focused on robust and stable two-phase heat transfer devices. A confined impinging jet is explored as means of utilizing two-phase heat transfer while minimizing flow instabilities observed in microchannel devices. The test configuration consists of a 4 mm diameter jet of water that impinges on a 38 mm diameter heated aluminum surface. Experimental parameters include inlet mass flow rates from 150 to 600 g/min, nozzle-to-surface spacing from 1 to 8 mm, and input heat fluxes from 0 to 90 W/cm2. Results were used to assess the influence of the testing parameters on the heat transfer performance and stability characteristics of a two-phase confined impinging jet. Stability characteristics were explored using power spectral densities (PSDs) of the inlet pressure time series data. Confined impinging jets, over the range of conditions tested, were found to be stable and an efficient means of removing large amounts of waste heat. The radial geometry of the confined jet allows the fluid to expand as it flows radially away from the nozzle, which suppresses instabilities found in microchannel array geometries. Conditions of the heater surface were found to strongly influence two-phase performance. Analysis of PSDs, for stable operation, showed dominate frequencies in the range of 1-4 Hz, which were attributed to generated vapor expanding in the outlet plenum and the subsequent collapse as it condensed. A stability indicator was developed by inducing artificial instabilities into the system by varying the amount of cross sectional area available for outlet vapor removal and compared to the results for stable operation. / Graduation date: 2012

Distributed hydrogen generation from liquid hydrocarbon fuels to supply portable fuel cells presents an attractive, high energy density alternative to current battery technology. Traditional unit operation reactor design for hydrogen generation becomes inadequate with decrease in scale because of the unique challenges of size and weight minimization. To address the challenge of reactor scale-down, the concept of multifunctional reactors has emerged, in which synergistic combination of different unit operations is explored to achieve improved performance. The direct droplet impingement reactor (DDIR) studied here is based on this approach in which the liquid feed is atomized using a regularly spaced array of droplet generators with unparalleled control over droplet characteristics, followed by vaporization and reaction directly on the catalyst surface. Considering each droplet generator in the array as a unit cell, a comprehensive, first-principles model of the DDIR has been developed by considering the intimately coupled processes of 1) droplet transport, heating, evaporation, and impingement on the catalyst surface, 2) liquid reagent film formation, capillary penetration, and vaporization within the catalyst layer, and 3) gas phase heat and mass transfer and catalytic reactions. Simulations are performed to investigate the effect of reactor operating parameters on performance. Experimental validation of the model is carried out by visualizing droplet impingement and liquid film accumulation while simultaneously monitoring reaction product composition over a range of operating conditions. Results suggest an optimal unit cell shape for reaction selectivity based on a balance between reagent back diffusion and catalyst bed thermal resistance. Further, achieving a target throughput is best accomplished by adding together a larger number unit cells with optimized geometry and lower throughput (per unit cell) to more effectively spread heat and avoid hotspots at the catalyst interface. At the same time, conditions must be satisfied for ensuring droplet impingement on the catalyst surface, which become more stringent as unit cell throughput is decreased.