Simulação numérica da transferência de calor e massa em sistema bifásico multicomponente: uma abordagem baseada nas equações de Maxwell-Stefan

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Química, Florianópolis, 2013.Esta dissertação de mestrado originou-se da necessidade de avaliação de estratégias numéricas que permitam uma adequada predição in silico dos fenômenos de transferência de calor e massa em sistemas multicomponentes e multifásicos. Em particular, o foco do trabalho consistiu na implementação de modelos robustos em ferramenta de CFD. Para tanto, as equações de Maxwell-Stefan em conjunto com a lei de Fick generalizada foram codificadas na linguagem de programação C e o código gerado foi dinamicamente acoplado ao código comercial ANSYS® CFD (FLUENT®), versão 14.0. Determinou-se, então, a composição e a temperatura de equilíbrio em um sistema vapor-líquido, sendo cada fase composta por uma mistura de quatro hidrocarbonetos puros (metano, n-pentano, n-hexano e n-octano). Levou-se em consideração todas as características inerentes a misturas multicomponentes (a exemplo da correção para altas taxas e da contribuição de todos os gradientes de composição para a taxa de transferência da espécie i), o que introduz uma complexidade considerável ao sistema quando comparada à solução de escoamentos envolvendo misturas binárias. Os resultados obtidos nos estudos com a ferramenta de CFD foram verificados com predições obtidas do simulador de processos comercial em estado estacionário denominado PRO/II®, versão 8.2. Um estudo preliminar com a mistura binária água/ar foi conduzido com o objetivo de validar os resultados obtidos com o código CFD por meio de comparação direta com dados fornecidos pela carta psicrométrica padrão. Abstract : This dissertation was originated from the need of the evaluation of numerical strategies that may allow an adequate prediction in silico of heat and mass transfer phenomena in multicomponent and multiphase systems. In particular, the aim of the work has been to implement robust models in a CFD tool. Therefore, the Maxwell-Stefan's equations in conjunction with the generalized Fick's law have been codified in the C programming language and the code generated has been dynamically coupled to the commercial code ANSYS® CFD (FLUENT®), version 14.0. It has been determined, then, the equilibrium composition and temperature in a vapor-liquid system, in which each phase has been composed by a mixture of four pure hydrocarbons (methane, n-pentane, n-hexane and octane). All the inherent characteristics of multicomponent mixtures (e.g., the high flux correction and the contribution of all composition gradients to the transport rate of species i) have been taken into account, which introduces considerable complexity to the system when compared to the solution of flow involving binary mixtures. The results obtained in the studies conducted with the CFD code have been verified with predictions obtained from the steady state process simulator named PRO/II®, version 8.2. A preliminary study with the binary mixture water/air has been carried out with the aim to validate the results obtained with the CFD code by means of direct comparison with data provided by standard psychrometric chart

Contributions to process intensification in microfluidic devices

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Química, Florianópolis, 2016.Abstract : In this work, answers to some gaps found in the literature on the field of process intensification in microfluidic devices were proposed. The behavior of a carbon-based composite photocatalyst, specifically a composite of TiO2 and graphene, immobilized on the inner walls of a microchannel reactor, was evaluated and compared with a system containing pristine TiO2. Additionally, a comprehensive computational simulation was performed, based on fundamental physics of semiconductors and considering the coupling of radiation distribution, fluid flow, mass transport and chemical reactions. Moreover, a numerical study was carried out aiming to determine optimal photocatalytic film thicknesses for different illumination mechanisms (backside illumination, BSI, and front-side illumination, FSI) as a function of relevant operational variables and parameters, namely the incident irradiation, the apparent first-order reaction constant, the effective diffusivity and the absorption coefficient. Finally, the possibility of numerically predict the effect of wall wettability on gas-liquid flow pattern developed in microfluidic devices was investigated.Dispositivos microfluídicos são baseados em microcanais nos quais o diâmetro efetivo é da ordem de centenas de micrômetros, resultando em elevada razão área/volume. Embora um considerável avanço tenha sido observado nessa área nas últimas décadas, resultando, inclusive, em aplicações industriais comercialmente disponíveis, ainda há importantes questões em aberto. Neste trabalho, respostas a algumas dessas questões foram propostas. Em particular, procurou-se determinar o comportamento de dispositivos microfluídicos aplicados à intensificação de processos fotocatalíticos considerando um fotocatalisador compósito (especificamente um compósito de dióxido de titânio e grafeno) imobilizado nas paredes internas. Tal sistema foi, então, comparado a um equivalente no qual dióxido de titânio puro foi imobilizado. As partículas de dióxido de titânio e do compósito de dióxido de titânio-grafeno foram depositadas por meio de um método térmico. Suspensões de TiO2 e TiO2- grafeno foram preparadas e injetadas ao longo de microcanais de chips microfluídicos comerciais construídos com vidro borossilicato. Os dispositivos foram, então, tratados termicamente para promover a evaporação do solvente (água) e a deposição do fotocatalisador nas paredes internas. O processo foi realizado ciclicamente para promover a formação de múltiplas camadas. A evolução da deposição foi avaliada pelo monitoramento dos perfis óticos dos sistemas. Azul de metileno foi usado como reagente modelo em ensaios de fotodegradação. Ensaios preliminares permitiram determinar o efeito dos fenômenos de adsorção e fotólise sobre o comportamento global. Nos experimentos de reação fotocatalisada observou-se que uma maior velocidade de reação inicial foi obtida no microrreator contendo fotocatalisador composto (TiO2-GR) imobilizado nas paredes internas, mas ambos os sistemas (TiO2 e TiO2- GR) exibiram velocidades de reação similares quando o estado estacionário foi alcançado. Verificou-se que a taxa de descolorização do azul de metileno no chip microfluídico foi, aproximadamente, uma ordem de magnitude maior que aquela reportada em sistemas macroscópicos equivalentes em condições experimentais similares. Além disso, investigou-se, neste trabalho, a possibilidade de avaliar teoricamente o comportamento de sistemas microfluídicos aplicados a processos fotocatalíticos com base na física fundamental de semicondutores, bem como a possibilidade de modelar computacionalmente os fenômenos acoplados (distribuição de intensidade luminosa, escoamento, transporte de massa e reação química) que ocorrem em reatores de microcanais (provendo uma estimativa para o desempenho do reator, dos pontos de vista global e local). O modelo computacional foi validado com os resultados experimentais. Na sequência, o modelo computacional foi aplicado para a predição da melhor espessura para o filme fotocatalítico imobilizado nas paredes internas de dispositivos microfluídicos em diferentes condições de iluminação (backside illumination, BSI, e front- side illumination, FSI) como função de variáveis operacionais e parâmetros relevantes, nomeadamente a irradiação incidente, a constante de velocidade de reação aparente de pseudo-primeira ordem, a difusividade efetiva e o coeficiente de absorção do fotocatalisador. Finalmente, a possibilidade de predizer numericamente o efeito da molhabilidade da parede sobre padrões de escoamento multifásicos desenvolvidos em microcanais foi avaliada. Tal modelo computacional pode ser utilizado como fonte de informação prévia sobre o impacto de diferentes propriedades do filme fotocatalítico na morfologia interfacial de escoamento gás-líquido em microrreatores fotoquímicos. Em particular, escoamentos gás-líquido isotérmicos (Taylor e estratificado) foram avaliados através do modelo volume of fluid (VOF). Microcanais com condições limites de hidrofilicidade e hidrofobicidade foram investigados tomando-se como base um referencial experimental disponível na literatura. Um estudo preliminar detalhado foi conduzido para a determinação da malha computacional ótima, capaz de permitir modelagem adequada do filme líquido formado entre as cavidades de gás e a parede sólida, no caso de Taylor flow. Os resultados numéricos foram comparados com dados experimentais (comprimento máximo de cavidade e área de cavidade, para o caso de Taylor flow, e espessura do filme gasoso no caso de escoamento estratificado) e algumas correlações disponíveis (comprimento máximo de cavidade e perda de carga por cavidade) e boa concordância foi observada. Nas mesmas condições de alimentação, o modelo foi capaz de captar os diferentes padrões de escoamento gás-líquido esperados quando o ângulo de contato da parede foi variado. Portanto, tal modelo computacional pode ser utilizado em estudos de scale out com o objetivo de projetar e otimizar reatores compactos modulares baseados na tecnologia de microcanais nos quais escoamento multifásico, particularmente gás e líquido, é estabelecido. Discussões acerca das limitações e de propostas futuras referentes ao desenvolvimento deste trabalho também são apresentadas

Humic acids adsorption and decomposition on Mn2O3 and α-Al2O3 nanoparticles in aqueous suspensions in the presence of ozone

© 2018 Elsevier Ltd. The removal and decomposition of humic acids (HAs) in the presence of ozone and aqueous suspensions of Mn2O3 and a-alumina (Al2O3) nanoparticles was investigated. Mn2O3 presented lower BET specific surface area (15.6m2 g-1 vs 45.8m2 g-1) but a higher point of zero charge (PZC) (5.9 vs 4.2) than α-Al2O3. Solution pH played a key role in the adsorption of HAs and catalytic oxidation on the surface of α-Al2O3 and Mn2O3 nanopart icles. The adsorption capacity of α-Al2O3 at the natural pH of HAs in water (pH 5.5) was up to2.903 gHAs g-1, but no adsorption occurred onto the Mn2O3 nanoparticles, due to the unfavorable surface charge at pH 5.5. In consequence, although Mn2O3 was a more efficient catalyst (khet=0.7 L-1 min-1 g-1) than α-Al2O3 (khet=0.2 L-1 min-1 g-1) for the decomposition of O3, Mn2O3 did not exhibited catalytic action duringthe ozonation of HAs at pH 5.5. Instead, the Mn2O3 catalytic action was significant at pH equal to PZC (catalytic ate constant ratio k1-HAcat/k1-HA=1.562). Overall, α-Al2O3 exhibited the highest catalytic removal rate of HAs during ozonation (k1-HAcat/ k1-HA=2.298) due to favorable surface charge and larger specific surface area. The main mechanism for HAs removal in the presence of α-Al2O3 involves simultaneous adsorption of both HAs and O3, the reaction of ozone from the bulk solution and the catalytic decomposition of HAs on the solid surface by ROS, through complex series-parallel reactions. The α-Al2O3 dosage up to 0.5 g L-1 required to remove HAs by catalytic ozonation was significantly lower than in other studies employing granular activated carbon, iron coated zeolite or γ-alumina catalysts

Sonoelectrochemistry: ultrasound-assisted organic electrosynthesis

The application of ultrasound with electrochemistry in organic chemistry (known as organic sonoelectrochemistry) accelerates the activation process of chemical reactions. This hybrid technology enhances electrical efficiency and modifies and increases the product yield. Moreover, it facilitates the mass transfer phenomena and the processes of cleaning, degassing, and activation of the electrode surfaces; maintains higher current densities for efficient chemical transformations; and also works efficiently for mixing of reactants in multiphase systems. The ultrasound technology has a prominent effect in heterogeneous reaction systems especially during the solid (electrode)–liquid (electrolytic mixture) interfacial cavitation process. The ultrasound technology gains attention due to its fundamental and positive effect in organic chemistry to make possible the challenging electrosynthetic processes. Herein, we report the sonoelectrosynthetic methods that will help researchers to understand and apply this methodology for scale-up of processes in organic synthesis and also in more modern innovative continuous-flow organic electrochemistry. Therefore, this study will provide valuable insight into the effects caused by ultrasound-assisted electrosynthesis and how this technology revolutionizes organic synthesis. It is believed that the hybrid sonoelectrochemical synthesis serves as a solution to the limitations of the commercialization of synthetic processes and offers a new, modern aspect in organic synthesis in a clean, hassle-free, and sustainable approach

Lanthanides Effects on TiO2 Photocatalysts

Semiconductors have been evaluated to heterogeneous photocatalysis degradation of recalcitrant contaminants in aqueous media due to the capacity of mineralizing these compounds under UV or visible light irradiation. However, this process has the inherent feature of photogenerated charges recombination and the high bandgap energy of the electronic structure of some semiconductors that can reduce the formation of reactive oxygen species, which are responsible for the compound degradation. In this context, structural modifications in semiconductors have been proposed to enhance the photocatalytic activity, such as doping processes with elements that are capable of generating superficial defects that capture the formed electrons, avoiding the recombination, or increasing the density of –OH groups or water molecules on the surface of the catalyst, which can enhance the formation of hydroxyl radicals. Therefore, this brief review is proposed to show the role of lanthanides in the TiO2 doping and the synthesis method applied, as well as the results discussed in the literature

Gas bubbles have controversial effects on Taylor flow electrochemistry

Electrochemistry is currently resurging in popularity amongst synthetic chemists due to the unique opportunities it provides to activate organic molecules. Simultaneously, continuous-flow technology has been used to enable scalability and to increase the efficiency of the developed electrochemical processes. Many of these processes involve a gaseous reagent or byproduct generated during the electrochemical process. The presence of a gas phase in flow reactors may lead to the generation of a so-called Taylor flow regime, where gas bubbles and liquid segments alternate. While Taylor flow has almost exclusive positive effects in flow chemistry due to increased mass and heat transfer, we show herein that the ramifications of gas bubbles on flow electrochemistry are essentially negative. Computational fluid dynamics (CFD) was used to gain a detailed understanding of the effects induced by the gas phase on the electrochemical process, taking the reduction of furfural to furfuryl alcohol carried out in an in-house developed electrochemical reactor as benchmark. We show that the gas bubble presents a local situation with infinite electrical resistance leading to a temporary passivation of the electroactive surface, while its presence also intensifies the mixing in the liquid slug reducing mass transfer limitations. Essentially, the larger the bubble, the higher the energy losses become and the less efficient the reactor is used. This results in a higher overall energy consumption for the electrochemical process. Moreover, we investigated the residence time distribution in the liquid slug, and the effect of different operational conditions (bubble size, gas holdup, interelectrode distance, electrolyte velocity and species concentration) on the overpotential and current density, providing guidelines for reactor design and operation. Based on the results described herein, we also discuss potential solutions to increase the efficiency of the electrochemical flow reactor

On the performance of liquid-liquid Taylor flow electrochemistry in a microreactor – A CFD study

A comprehensive understanding of the underlying phenomena (coupled fluid flow, charge transfer, mass transfer and chemical reaction) is fundamental for a proper design, analysis and scale-out of chemical reactors when carrying out multiphase electro-organic transformations. In this study, we have explored the novel combination of organic electrochemical synthesis and computational fluid dynamics (CFD) to perform a systematic theoretical investigation concerning the effect of different operational parameters on the performance of organic-aqueous Taylor flow in electrochemical microreactors. The results indicate that operating at high concentrations of the rate-limiting species (>5 mol⋅m−3 for Di≥10-9 m2⋅s−1; 500 mol⋅m−3 for Di~10-10 m2⋅s−1) is beneficial for the reactor performance. However, excessively high concentrations (>500 mol⋅m−3) do not result in a further improvement in mass transfer and current/voltage relation. Higher diffusivities are also beneficial, but even in this scenario limiting current densities can be found when working at low concentrations. Overall, keeping an internal:external phase electrical conductivity ratio > 1 improves the reactor performance. Working at lower velocities can be beneficial in some scenarios, since higher limiting current densities can be obtained. However, the velocity impact on the reactor performance is not significant in some operating conditions (e.g., at higher concentrations and diffusivities). Finally, working with higher cell potentials is beneficial, but limiting current densities can be encountered at lower concentrations and diffusivities. Variables such as internal phase volume fraction, droplet length and interelectrode distance also have relevant impact on the reactor performance, but are subjected to the same conditioning factors previously mentioned. A comprehensive potential balance was also conducted, showing the relative importance of the activation, Ohmic and concentration overpotentials under different operating conditions. We believe the insights gained herein will be of interest to researchers in both academia and industry to develop more efficient electrochemical flow reactors for liquid–liquid transformations