Kinetic analysis and CFD simulations of the photocatalytic production of hydrogen in silicone microreactors from water-ethanol mixtures

Abstract

Silicone microreactors containing microchannels of 500 µm width in a single or triple stack configuration have been manufactured, coated with an Au/TiO2 photocatalyst and tested for the photocatalytic production of hydrogen from water-ethanol gaseous mixtures under UV irradiation. Computational fluid dynamics (CFD) simulations have revealed that the design of the distributing headers allowed for a homogeneous distribution of the gaseous stream within the channels of the microreactors. A rate equation for the photocatalytic reaction has been developed from the experimental results obtained with the single stack operated under different ethanol partial pressures, light irradiation intensities and contact times. The hydrogen photoproduction rate has been expressed in terms of a Langmuir-Hinshelwood-type equation that accurately describes the process considering that hydrogen is produced through the dehydrogenation of ethanol to acetaldehyde. This equation incorporates an apparent rate constant (kapp) that has been found to be proportional to the intrinsic kinetic rate constant (k), and that depends on the light intensity (I) as follows: kapp = k·I0.65. A three-dimensional isothermal CFD model has been developed in which the previously obtained kinetic equation has been implemented. The model adequately describes the production of hydrogen of both the single and triple stacks. Moreover, the specific hydrogen productions (i.e. per gram of catalyst) are very close for both stacks thus suggesting that the scaling-up of the process could be accomplished by simply numbering-up. However, small deviations between the experimental and predicted hydrogen production suggest that a fraction of the radiation is absorbed by the microreactor components which should be taken into account for scaling-up purposes.Postprint (author's final draft

Effect of temperature on the gas-phase photocatalytic H2 generation using microreactors under UVA and sunlight irradiation

“…The characterization of the Au/TiO2 photocatalyst and its deposition on the microchannels has been already studied by scanning electron microscopy, Xray diffraction, UV-vis reflectance spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy [20,21]. This methodology allowed to obtain a homogeneous Au/TiO2 layer (average thickness of ca.…”

“…The manufacture of silicone microreactors for photocatalytic reactions using 3D printed molds allows a simple, cheap and reproducible fabrication method of UV-light transparent microreactors with custom-made dimensions and flexible in design [20] We have recently reported a kinetic analysis for the gas-phase photogeneration of hydrogen at room temperature in a silicone microreactor coated with Au/TiO2 expressed in terms of the Langmuir-Hinshelwood (L-H) model. The apparent rate constant (kapp) was found to be proportional to the intrinsic kinetic rate constant (k), which depends on the light intensity (I) as 5 kapp=k•I 0.65 [21]. L-H can fit many photocatalytic processes, although it does not illustrate the microscopic process of photocatalysis.…”

Modelagem Da Produção Fotocatalítica De Hidrogênio Em Reator De Microcanais

“…Apesar dos progressos na fotocatálise, as tecnologias atuais de decomposição da água ainda apresentam baixa eficiência. Muitos estudos envolvem os reatores convencionais de macroescala que apresentam limitações no transporte de massa e de fótons, e os microrreatores aparecem como ferramentas adequadas para reações fotocatalíticas devido a suas características (AHMAD et al, 2015;CASTEDO et al, 2017;CHEN et al, 2015;JAYAMOHAN et al, 2015).…”

“…Reatores de microcanais, dispositivos muitas vezes chamados também de microrreatores ou reatores microestruturados, são dispositivos microfluídicos com diâmetro ou altura na faixa de 10-100 µm, sendo que a alta área interfacial específica leva a uma maior transferência de calor e massa. Além disso, apresentam maior homogeneidade na iluminação Revista Brasileira de Energias Renováveis, v.7, n.3, p. 319-327, 2018 espacial e melhor penetração da luz através de toda profundidade do reator em comparação aos reatores de larga escala (CASTEDO et al, 2017;CORBEL et al, 2014;ESKANDARLOO et al, 2015;JAYAMOHAN et al, 2015;MATSUSHITA et al, 2008).…”

“…A fluidodinâmica computacional tem se desenvolvido e se tornado, em muitos casos, uma alternativa de baixo custo à realização de experimentos com respeito a projetos de engenharia. É conhecido que o projeto de fabricação de fotorreatores é fator chave que afeta a velocidade de decomposição da água, e a fluidodinâmica computacional é uma ferramenta de simulação que permite predizer o comportamento de um reator químico, por exemplo, fornecendo o modelo que tenha sido validado antes de acordo com resultados experimentais e simulados (CASTEDO et al, 2017;DUTTA;CHAKRABORTY, 2012).…”