Für die Modellierung der Hochdruck-Partialoxidation von Heiz- und Schweröl werden zunächst Untersuchungen des Einsatzstoffes durchgeführt, um deren Verdampfungs- und Pyrolyseverhalten zu bestimmen. Dazu wird das Verfahren der Hochdruck-Thermogravimetrie verwendet. Mit Hilfe der numerischen Strömungsmechanik unter Anwendung von detaillierten Reaktionsmechanismen und eines umfangreichen Partikelmodells werden die Zustände im Reaktor modelliert. Die Validierung mit Verweilzeitmessungen wird demonstriert und damit verbundene Probleme aufgezeigt. Anhand von Untersuchungen am Vergasungsrückstand kann die Existenz von Cenosphären (Tropfenrückstände) und von in der Flamme gebildetem Ruß nachgewiesen werden.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / To study the dispersion of the coffee fruits during the drying, a vibrated tray drier with recycle was used. The dryer consists basically of four sections: drying vertical tunnel, vibration system, system of warm air supply to the drying tunnel and recycle system of coffee fruits. The drying tunnel contains four perforated trays through which the coffee fruits and air flow out, in cross flow. The coffee fruits drying was carried out using two experimental design, where the studied variables were, for the first design: coffee fruits temperature (40; 45 and 50ºC); mass of coffee fruits (11.5; 12.5 and 13.5 kg) and air mass rate (7; 8 and 9 kg air/min); the varieties of coffee fruits were: Acaiá, Catuaí and Mundo Novo; and for the second design: mass of coffee fruits (10; 12 and 14 kg) and air mass rate (7; 8 and 9 kg air/min), with coffee fruits temperature around 45ºC and the coffee variety Mundo Novo. For the first experimental design, the coffee fruits temperature only influenced significantly the drying time, where the largest temperature level reduces in 26.77 h the drying time. For the second experimental design, the studied variables were not significant on drying time. Coffee fruit sphericity, density, sticky decreased with the decrease of the moisture content. With reference to the quality of coffee grain, the best operational conditions were obtained with greater coffee fruits mass and air mass rate. The coffee fruits flow in the drying tunnel is promoted by vibration of the trays coupled to electromagnetic vibrators. The vibration amplitude was determined by an accelerometer connected to a signal conditioner and an analogical oscilloscope. The vibration amplitude decreased with the reduction of the coffee fruits moisture content due to the shrinkage and decrease of the mass, sticky, density and particle size coffee fruits. The coffee fruits mass rate and the residence time distribution were determined (RTD), during the drying. At the end of the drying, the flow occurs with easiness due to: absence of sticky of the coffee fruits; decrease of the particle mass and particle vibration damping decrease, due to particle rigidity acquired in the drying. The dispersion coefficient (Ez) was determined by Taylor Dispersion Model, Free Dispersion Model and Modified Free Dispersion Model. For the great majority of the experiments, the model that better fitted the experimental data (greater coefficient of correlation) was the model of the Modified Free Dispersion. The dispersion coefficient (Modified Free Dispersion) ranged from 1.31×10-4 to 68.67×10-4 m2/s. The Péclet number ranged from 1.15 to 31.00. / Para estudar a dispersão dos frutos de café durante a secagem, utilizou-se um secador de bandejas vibradas com reciclo. O secador consiste basicamente de quatro seções: túnel vertical de secagem, sistema de vibração, sistema de injeção de ar aquecido no túnel de secagem e sistema de reciclo dos frutos de café. O túnel de secagem contém quatro bandejas perfuradas por onde escoam os frutos de café e o ar, em fluxo cruzado. A secagem dos frutos de café foi realizada utilizando-se dois planejamentos experimentais, onde as variáveis estudadas foram, para o primeiro planejamento de secagem: temperatura dos frutos de café (40; 45 e 50ºC); massa de frutos de café alimentada (11,5; 12,5 e 13,5 kg) e vazão de ar de secagem (7; 8 e 9 kg ar/min), a variedade dos frutos de café foram: Acaiá, Catuaí e Mundo Novo; e para o segundo planejamento de secagem: massa de frutos de café (10; 12 e 14 kg) e vazão de ar (7; 8 e 9 kg ar/min), mantendo-se a temperatura dos frutos em 45ºC e a variedade Mundo Novo. Para o primeiro planejamento somente a temperatura dos frutos influenciou significativamente o tempo de secagem, onde o maior nível de temperatura reduz em 26,77 h o tempo de secagem. Para o segundo planejamento as variáveis estudadas não foram significativas para o tempo total de secagem dos frutos de café. Durante a secagem ocorre encolhimento dos frutos de café, diminuição da esfericidade, da pegajosidade, da densidade aparente e aumento da área superficial específica com a diminuição do conteúdo de umidade dos frutos de café. Em relação à qualidade do grão de café, as melhores condições operacionais foram obtidas com maior massa e maior vazão de ar de secagem. O escoamento do café no túnel de secagem é promovido pela vibração das bandejas acopladas a vibradores eletromagnéticos. A amplitude de vibração foi determinada por um acelerômetro acoplado a um condicionador de sinal e a um osciloscópio analógico. A amplitude vibracional diminuiu com a diminuição da umidade dos frutos de café devido ao encolhimento e à diminuição da massa, da pegajosidade, da densidade e do diâmetro dos frutos de café. Durante a secagem foram determinadas a vazão mássica dos frutos de café e a distribuição do tempo de residência (DTR). Ao final da secagem, o escoamento dos frutos de café ocorre com maior facilidade devido a: ausência de pegajosidade dos frutos de café; diminuição da massa das partículas e redução do amortecimento da vibração dos frutos de café devido à rigidez adquirida na secagem. O coeficiente de dispersão (Ez) foi determinado pelos modelos da Dispersão de Taylor, da Dispersão Livre e da Dispersão Livre Modificado. Para a grande maioria dos experimentos, o modelo que melhor se ajustou aos dados experimentais (maior coeficiente de correlação) foi o Modelo da Dispersão Livre Modificado. O coeficiente de dispersão dos frutos de café variou de 1,31×10-4 a 68,67×10-4 m2/s. O número de Péclet variou de 1,15 a 31,00. / Doutor em Engenharia Química

Este estudo apresenta a modelagem fluidodinâmica de um reator solar utilizado para Processos Oxidativos Avançados (POA). Desenvolveu-se um modelo que considera a fluidodinâmica, o campo de radiação e cinética da reação de actinometria química (ferrioxalato), em regime transiente. Essa modelagem foi feita utilizando-se o código de fluidodinâmica computacional PHOENICS. Para análise dos resultados de simulações com o modelo, consideraram-se os dados experimentais de actinometria química para um trecho do reator, constituído de dois tubos (hairpin), e de distribuição de tempos de residência (DTR), para o reator completo, constituído de dez tubos. Os dados experimentais foram obtidos por RIBEIRO (2009). O resultado da análise da distribuição do tempo de residência do reator completo mostrou que o modelo baseado em escoamento laminar apresentou uma maior aderência aos dados experimentais de DTR. Como os experimentos de actinometria foram realizados em trecho com dois tubos do reator, construiu-se a geometria do hairpin que apresentou uma DTR mais aderente aos modelos teóricos. Outra simplificação foi necessária para a modelagem do campo de radiação de forma mais precisa, adaptando-se o hairpin para um tubo reto simples. A partir dos resultados de actinometria química foi possível estimar, pelo modelo, a taxa de fótons incidentes na parede do reator. / This study consists of the fluid dynamic modeling of a solar reactor used in Advanced Oxidation Processes (AOP). The model was developed by considering fluid dynamics, radiation field and the kinetics of the chemical actinometry reaction (ferrioxalate) in transient regime. This modeling was developed using computational fluid dynamics (CFD) in PHOENICS. Simulation results based on the model were analyzed by comparing them with a set of chemical actinometry experimental data obtained by RIBEIRO (2009). This considered a reactor section constituted by two pipes (hairpin), and the residence time distribution (RTD) of the complete reactor, composed of ten pipes. Residence time distribution results showed that the laminar flow model presented a better fitting to experimental data. Since the actinometry experiments were carried out in a reactor section with two pipes, a new geometry was designed, which resulted in a better fitting of RTD results with theoretical models. In order to obtain a more precise radiation field model, another simplification was necessary, which consisted of assuming a straight cylindrical pipe geometry. The use of the chemical actinometry and the radiation field model enabled the estimation of the incident photons rate at reactor wall.

Os processos oxidativos avançados são promissores para a degradação de compostos orgânicos resistentes aos tratamentos convencionais, como o fenol. A fluidodinâmica computacional (CFD) tornou-se uma poderosa ferramenta para analisar processos fotoquímicos por resolver os balanços acoplados de quantidade de movimento, de massa e de radiação. O objetivo deste trabalho é investigar o processo UV/H2O2 num reator fotoquímico anular usando CFD e um modelo cinético mais realista. O modelo em CFD foi criado de forma progressiva. Inicialmente, foram determinados os campos de velocidade para três vazões (30, 60 e 100 L/h). Considerou-se dois diâmetros de lâmpada para reproduzir a configuração experimental do sistema. A discretização foi feita com malhas tetraédricas variando entre 390 000 e 1 200 000 elementos. Quatro modelos de turbulência RANS foram analisados: k-e, k-w, o shear stress transport (SST) e o modelo de tensões de Reynolds (RSM). O campo de velocidades foi validado comparando a DTR com seu levantamento experimental. A próxima etapa foi incluir o mecanismo de degradação de fenol proposto por Edalatmanesh, Dhib e Mehrvar (2008) no modelo em CFD. Trata-se de um modelo cinético baseado em equações dinâmicas para todas as espécies. O campo de radiação foi calculado pelo modelo radial e pela solução da equação de transporte de radiação através do método discrete transfer. As simulações reproduziram dados experimentais abrangendo uma larga gama de concentrações iniciais de fenol, razões molares H2O2/fenol e três potências de emissão das lâmpadas. O campo de velocidades obtido era dependente da vazão: o fluido pode manter movimento helicoidal sobre toda a extensão do reator ou se desenvolver como um escoamento pistonado. O modelo k-e não reproduziu bem o escoamento por não ser adequado para escoamentos rotativos. Os outros modelos geraram curvas de DTR com bom ajuste aos dados experimentais, especialmente o modelo k-w. O desvio médio entre as simulações de degradação de fenol e os dados experimentais é inferior a 8%. Verificou-se que, devido ao escoamento rotativo, os reagentes ficavam concentrados próximos à parede externa e migravam para a região da lâmpada ao longo do reator. A elevada intensidade de radiação na superfície da lâmpada criou uma camada ao seu redor na qual a fotólise do H2O2 ocorreu com grande taxa. Os radicais OH gerados nessa camada eram transportados para a região das paredes por convecção. Isso fez com que a maior parte do fenol fosse atacada na segunda metade do reator e gerou acúmulo do radical próximo à lâmpada na seção de saída do reator, já que o poluente já fora oxidado nessa área. O método discrete transfer previu intensidades de radiação maiores que o modelo radial, e, consequentemente, maior concentração de radicais OH. Os resultados satisfatórios indicam que CFD foi uma ferramenta adequada para analisar este escoamento reativo. / Advanced oxidation processes are a promising technology for degradation of organic compounds resistant to conventional treatments such as phenol. Computational fluid dynamics (CFD) has recently emerged as a powerful tool that allows a deeper understanding of photochemical processes in reactor engineering by solving the coupled momentum, mass and radiation balances. This work aimed to investigate the UV/H2O2 process in an annular photoreactor using CFD and a more realistic kinetic model. A progressive approach was used to develop the CFD reactor model. First, the velocity fields were determined for three volumetric flow rates (30, 60 and 100 L/h). Two lamp diameters were considered to reflect the experimental configuration of the system. Tetrahedral meshes varying form 390,000 to 1,200,000 elements were analyzed to achieve grid independence. For accounting turbulence effects, four RANS models were tested: k-e, k-w, the Shear Stress Transport (SST) and the Reynolds Stress models (RSM). The velocity field was validated through comparison to RTD experimental data. Next step was introducing the mechanism of phenol degradation proposed by Edalatmanesh, Dhib and Mehrvar (2008) into the CFD model. This kinetic model is based on dynamic equations for all species. The fluence rate field was calculated by the radial model and by solving the radiation transport equation with the discrete transfer method. Simulations reproduced experimental data spanning a wide range of initial phenol concentrations, H2O2/phenol molar ratios and three values for lamp power. It was found that the velocity field depends on the volumetric flow rate: either it maintains a swirling motion through the whole reactor or might develop like a plug flow. The k-e model did not represent the RTD data accurately, and the velocity field therefore, since it is not appropriate for swirling flows. The other turbulence models showed good match of RTD, especially the k-w model. Simulations of phenol degradation deviated less than 8% from experimental data. It was possible verified that, due to the swirling inlet effects, reactants got concentrated close to the outer wall and migrated on the lamp direction along the reactor path. High radiation intensities close to the lamp surface created a layer around it where photolysis of H2O2 took place with higher rates. OH radicals were generated in that layer and transported towards the outer wall by convection. This caused most of phenol to be consumed in the second half of the reactor and accumulation of the radical near the lamp and the reactor outlet, since the pollutant in this area was already oxidized. The discrete transfer method predicted higher incident radiation intensity than the radial model, and higher concentrations of OH radicals as a consequence. Satisfactory results indicated that CFD was an appropriate tool for analyzing this reactive flow.

Les évaporateurs à flot tombant (EFT) sont largement utilisés dans les industries chimiques, de la réfrigération, du raffinage du pétrole, et alimentaires. Dans l'industrie laitière, les EFT sont appliqués pour la concentration de solutions avant l'étape de séchage. Malgré l'importance économique du procédé d'évaporation sous vide dans la fabrication de produits laitiers déshydratés, la connaissance du procédé est essentiellement empirique. Des recherches visant à améliorer l'efficacité du procédé sont donc nécessaires. L'objectif de ce projet de doctorat est de caractériser expérimentalement un EFT lors de la concentration de produits laitiers, par des approches thermo et hydro-dynamiques, afin d'étudier les interactions entre les propriétés des produits et les paramètres opérationnels. Un évaporateur à flot tombant, simple effet, à l’échelle pilote, qui décrit le même processus que celui à l'échelle industrielle, d'un point de vue hydrodynamique, a été instrumenté et utilisé pour établir les bilans massiques et énergétiques. La capacité évaporatoire et le coefficient global de transfert de chaleur ont été calculés à partir des données expérimentales. Une méthodologie pour la détermination expérimentale des fonctions distribution des temps de séjour (DTS) a été développée. En effet, les fonctions de DTS fournissent des informations essentielles sur l'écoulement des produits lors de la concentration dans un EFT. L'augmentation de la concentration, du débit massique et de la distance parcourue par le produit entraîne une augmentation de la dispersion des particules dans le flux. Ces fonctions ont été modélisées par une combinaison de réacteurs en cascade, parfaitement agités. D’après l'interprétation de ce modèle, deux flux, un principal et un secondaire, correspondant à deux couches superposées de produit circulant à travers des tubes d'évaporateur, a été proposé. La méthodologie développée pour le calcul des fonctions de DTS a été appliqué pour la concentration de produits laitiers (lait écrémé, lactosérum doux et acide). Par la suite, l'étude a été étendue à la formation de l'encrassement pendant la concentration par évaporation sous vide. Il a été montré que le temps de séjour moyen était plus sensible pour identifier l'encrassement que le coefficient global de transfert de chaleur et la capacité évaporatoire. Ainsi, cette étude a souligné le rôle crucial de l’importance de la caractérisation des EFT sous vide afin d’en améliorer leurs performances et la qualité des produits qui en sont issu. / Falling film evaporators (FFE) are widely used in the chemical, refrigeration, petroleum refining, desalination and food industries. In the dairy industry FFE is applied for the concentration of solutions prior to the drying step. Despite the economic importance of the vacuum evaporation process in the manufacture of dairy dried products, the knowledge about the process is mostly empirical. Research aiming to improve the efficiency of the process is therefore necessary. The objective of this PhD project was to characterize experimentally a FFE during the concentration of dairy products by means of thermodynamic and hydrodynamic approaches, in order to study the interactions between the products properties and the operating parameters. A pilot-scale, single-stage falling film evaporator that describes the same process as that of an industrial scale from a hydrodynamic point of view was instrumented and used to establish the mass and energy balances. The evaporation rate and the overall heat transfer coefficient were calculated from the experimental data to follow up the process. A methodology for the determination of the experimental residence time distribution (RTD) functions was developed. RTD functions provide global information about the flow of the products during concentration in a FFE. Increasing of the concentration of skim milk, mass flow rate and the distance covered by the product resulted in an increase in the dispersion of the products particles. The experimental RTD functions were modelled by a combination of two perfectly mixed reactor tanks in series. From the interpretation of this model, two different flows, a main and a minor flow, were identified. The RTD methodology developed on skim milk was applied to sweet whey and lactic acid whey and the study was extended to the formation of fouling during a 5-hour concentration. The mean residence time was more sensitive to identify fouling than the overall heat transfer coefficient and the evaporation rate. This study emphasized the crucial role of process characterization to improve the performance of FFE and product quality.

Residence time distributions have become an important analytical tool in the analysis of many types of flow systems. Residence time distributions have proven to be effective for analysing trickle bed reactors, as it allows determination of parameters under operating conditions allowing no interference of these conditions. By studying the residence time distribution a great amount of information can be obtained and therefore used to determine a number of hydrodynamic parameters. Due to recent findings that prewetting has a tremendous effect on a number of hydrodynamic parameters such as holdup, wetting efficiency and pressure drop, it is therefore the aim of this study to investigate the effect of trickle flow morphology or prewetting on a trickle bed reactor. The residence time distribution is obtained whereby hydrodynamic parameters are determined and therefore the effect the flow morphology has on various hydrodynamic parameters is highlighted. A number of methods were used to determine these parameters, namely that of the best-fit method, whereby the PDE model was used, and the method of moments. Operating conditions included varying gas and liquid flow rates for porous and non-porous catalyst particles at atmospheric pressure. The different prewetting procedures used during this work included the following: <ul><li>Non-wetted </li> <li>Levec-wetted </li> <li>Super-wetted</li></ul> From this investigation the following conclusions were made: <li>Prewetting has a great effect on the hydrodynamic parameters of trickle bed reactors</li> <li>The differences in prewetting can be attributed to differing flow morphologies for the different prewetted beds i.e. the dominant flow morphology for a non-wetted bed is that of rivulets and for prewetted beds that of film flow</li> <li>It was also found that at low liquid flow rates the flow morphology in prewetted beds changes from film flow to a combination of rivulet and film flow</li> <li>The different flow morphologies for prewetted and non prewetted beds was confirmed by the residence time distributions and various parameters obtained there from</li> <li>At low liquid flow rates the flow morphology becomes a more predominant factor in creating the tailing effect present in residence time distribution for prewetted beds</li> <li>The tailing effect in residence time distributions is a result of both internal diffusion and liquid flow morphology, where the liquid flow morphology is the more dominant factor</li> <li>The use of residence time distributions to determine a number of hydrodynamic parameters proved to be very useful and accurate by means of different methods, i.e. method of moments and best-fit method</li> <li>Differences in the liquid holdup determined from the method of moments and the weighing method confirmed that different flow morphologies exist for different prewetted beds</li> <li>An increase in the dispersion coefficient with prewetting was observed indicating that the amount of micro mixing is different for the different prewetted beds</li> <li>Differences in residence times and high values for the dynamic holdup, for the porous packing, confirmed that the PDE model does not model well the porous packing response curves due to the lack of internal diffusion and internal holdup in this model</li> <li>The dynamic-static mass transfer showed that film flow, as in prewetted beds, results in slower mass transfer as opposed to rivulet flow and therefore it is concluded that prewetting results in different flow morphologies.</li></ul> Following this study it is recommended that a residence time distribution model be used or developed that incorporates the effects of internal diffusion and internal holdup as present in porous catalyst particles. In addition, it was found that very few correlations could accurately predict hydrodynamic parameters due to the absence of the effect of prewetting and therefore it is recommended that correlations be developed that incorporate the effect of prewetting. / Dissertation (MEng)--University of Pretoria, 2008. / Chemical Engineering / unrestricted

Für die Modellierung der Hochdruck-Partialoxidation von Heiz- und Schweröl werden zunächst Untersuchungen des Einsatzstoffes durchgeführt, um deren Verdampfungs- und Pyrolyseverhalten zu bestimmen. Dazu wird das Verfahren der Hochdruck-Thermogravimetrie verwendet. Mit Hilfe der numerischen Strömungsmechanik unter Anwendung von detaillierten Reaktionsmechanismen und eines umfangreichen Partikelmodells werden die Zustände im Reaktor modelliert. Die Validierung mit Verweilzeitmessungen wird demonstriert und damit verbundene Probleme aufgezeigt. Anhand von Untersuchungen am Vergasungsrückstand kann die Existenz von Cenosphären (Tropfenrückstände) und von in der Flamme gebildetem Ruß nachgewiesen werden.

Le séchage est une opération incontournable pour la valorisation énergétique des boues résiduaires. La flexibilité pour ajuster la teneur en matière sèche finale de la boue est un critère important pour le choix d'une technologie. Cet objectif est difficile à atteindre pour les sécheurs à palettes. La modélisation du processus est alors essentielle. Malheureusement, le comportement rhéologique des boues est complexe et la mécanique des fluides numérique est hors de portée. La notion de Distribution des Temps de Séjour est employée ici pour caractériser l'écoulement. Un protocole fiable et reproductible a été établi et mis en œuvre sur un pilote de laboratoire. Des injections Dirac d'oxyde de titane et de sels métalliques, avec la spectrométrie de fluorescence X comme méthode de détection, ont été employées pour caractériser les DTS du solide anhydre et de la boue humide. Pré-Mélanger la boue pâteuse, pour disperser le traceur par exemple, modifie la structure du matériau. Ceci a été mis en évidence par des mesures de distribution en taille des particules et par des caractérisations rhéologiques. Cependant, des expériences de séchage en batch ont montré que ce pré-Mélange n'a aucune influence sur la cinétique et sur la phase plastique. Nous avons montré que le solide anhydre et le solide humide s'écoulent de la même manière. Une seconde méthode, basée sur une détection par conductimétrie, a alors été développée. Plus facile à mettre en œuvre et moins onéreuse, cette méthode s'avère tout aussi fiable que la première. L'influence de la durée de stockage de la boue, avant séchage, a été évaluée. Le temps de séjour de la boue dans le sécheur double quand la durée de stockage passe de 24h à 48h. Finalement, un modèle d'écoulement, basé sur la théorie de chaînes de Markov, a été développé. L'écoulement du solide anhydre est décrit par une chaîne de n cellules parfaitement mélangées, n correspondant au nombre de palettes. Les probabilités de transition entre les cellules sont régies par deux paramètres : le ratio de recyclage interne, R, et la masse de solides retenus, MS. R est déterminé par la relation de Van der Laan et MS est identifié par ajustement du modèle aux données expérimentales. Le modèle décrit de manière satisfaisante les DTS. La masse de solides retenus identifiée est toujours plus faible que la quantité mesurée expérimentalement. Une partie de la boue, collée aux parois du sécheur et au rotor, agit comme un volume mort. / Drying is an unavoidable operation prior to sludge valorization in incineration, pyrolysis or gasification. The flexibility to adapt the solid content of the dried sludge to the demand is a major requirement of any drying system. This objective is difficult to reach for paddle dryers. Modeling the process is thus essential. Unfortunately, sludge rheological behavior is complex and computational fluid dynamics is out of reach for the time being. The concept of Residence Time Distribution (RTD) is used here to investigate sludge flow pattern in a paddle dryer. A reliable and reproducible protocol was established and implemented on a lab-Scale continuous dryer. Pulse injections of titanium oxide and of salt metals, with X-Ray fluorescence spectroscopy as detection method, were used to characterize the RTD of anhydrous solid and wet sludge, respectively. Premixing the pasty sludge, for tracer powder dispersion for instance, changes the structure of the material. This was highlighted through the measurements of particle size distributions and characterization of rheological properties. However, drying experiments performed in batch emphasized that premixing does not have any influence on the kinetic and the sticky phase. The RTD curves of the anhydrous solid are superimposed on those of the moist sludge. Consequently, a simpler protocol, based on pulse injection of chloride sodium and offline conductivity measurements, was established. Easier to implement in industry and cheaper, this method proves to be as reliable as the first one. The influence of storage duration prior to drying was assessed. The mean residence time doubles when the storage duration changes from 24h to 48h. Finally, a model based on the theory of Markov chains has been developed to represent the RTD. The flow of anhydrous solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles. The transition probabilities between the cells are governed by two parameters: the ratio of internal recirculation, R, and the solids hold-Up, MS. R is determined from the Van der Laan's relation and MS is identified by fitting the model to the experimental RTD. The model describes the flow pattern with a good accuracy. The computed hold-Up is lower than the experimental one. Part of the sludge is stuck to the walls of the dryer, acting as dead volumes in the process.

Continuous milli-scale and micro-scale structures such as FlowPlate® microreactors have emerged as a promising element of process intensification due to their inherently effective rates of mass and heat transfer. These microfluidic devices have proven to be a preferred solution in place of energy-intensive batch processes for certain pathways of fine chemical and pharmaceutical synthesis, most notably fast reactions taking place on the scale of milliseconds to seconds. Computational fluid dynamics (CFD) has become an increasingly valuable tool in the field of microreactor design and optimization for its ability to locally map complex fluid flow patterns and resolve microscopic scales of reactive mixing that are challenging to characterize experimentally. The primary objective of this research was thus to develop and validate a mathematical model for the simulation of chaotic flow and homogeneous mixing in continuous microreactors. The model needed to be versatile enough to handle transition between flow regimes within a given reactor as well as the coexistence of both chaotic and laminar flow patterns in the micromixing elements that comprise said reactors. This was successfully achieved through the implementation of a k-ω SST (shear-stress transport) turbulence model that accounts for the impact of small-scale temporal and spatial fluctuations generated in the micromixer geometries studied herein; be it a liquid-liquid mixer (LLM), a serpentine (SZ) or a tangential (TG) mixer. In a first CFD study, the computational predictions were validated based on excellent agreement with experimental pressure loss (R^2 > 0.997) and residence time distribution (RTD) data (R^2 > 0.97) in several LL microreactors at Reynolds numbers ranging from 210 to 2140. Furthermore, the local velocity distribution and streamlines were mapped across the 3D domain of these reactors and it was discovered, based on the emergence of advective recirculation zones and turbulent dispersion, that a drastic change in flow behaviour occurred in these mixing elements at a Reynolds number of about 640. The interspacing of LLM elements with straight microchannels proved to be a suitable approach to modulating pressure loss while concurrently maintaining the chaotic secondary flow patterns generated from the mixers. In a second CFD study, the impact of micromixer geometry on the local velocity fields and advective transport performance was investigated both from a macromixing and micromixing perspective. Like the LLM, the SZ and TG mixers conferred chaotic secondary flow patterns at characteristic Reynolds numbers between 500 and 1000. As such, it was concluded that it would be ideal to operate these mixers at water flow rates of at least 30 ml/min. Contour plots of the velocity magnitude coupled with the computation of RTD showed that the SZ virtually mimics a plug-flow profile over a volume of 77 mm3 or greater at 50 g/min. The RTD of the LLM and TG resembles that of a mixed flow pattern given that approximately 65-80% of their fluid volume is occupied by recirculation zones. As such, it required 65 LLMs in series (3105 mm3) and 80 TGs (1142 mm3) to approach the same pattern as 10 SZs (77 mm3) from a macromixing perspective. Micromixing time distributions (MTD) were also characterized by locally computing the decay time of small-scale segregation (t_SSS) as a function of flow rate, wherein higher flow rates generated lower characteristic mixing times. The TG and LLM conferred the broadest range of mixing times, spanning nearly four orders of magnitude in the range of [0.02 ms, 10 ms], whereas the SZ generated a much narrower MTD ranging between [0.024 ms, 0.69 ms]. Finally, the impact of geometry and flow conditions on reaction yield was assessed by characterizing the extent of a finite-rate reaction relative to an infinitely fast reaction taking place in parallel. The calculated yield for the competitive-parallel reaction scheme showed that the second Damköhler number (Dall) computed based on the mean tSSS provides useful information about whether the process will be limited by the intrinsic rate of reaction or by the rate of mass transfer, even though the reaction process is controlled by a combination of the RTD as well as loss of LSS and SSS. It was concluded that the change in MTD as a function of power dissipation should coincide with the reaction yield response, and that any deviation in that relationship is because of macroscopic blending of reactants in the entrance region.

No description available.