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MICROMIXING in CHEMICAL REACTORS Test Reactions

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MICROMIXING in CHEMICAL REACTORS Test Reactions MICROMIXING IN CHEMICAL REACTORS Test Reactions Ph.D. Dissertation in Biological and Chemical Engineering by Maria Isabel da Silva Nunes Departamento de Engenharia Química Faculdade de Engenharia Universidade do Porto October 2007 To Lara and Quim "Problems worthy of attack prove their worth by fighting back." Paul Erdos (1913-1996) Acknowledgements I appreciate the support of Professor José Carlos Brito Lopes and Professor Madalena Maria Gomes de Queiroz Dias by the supervision of this work. They were not only mentors but also good friends. I wish to thank Professor Alírio E. Rodrigues, Director of the LSRE, where this work was carried out. I also wish to thank Professor John R. Bourne and to Professor Lúcia Santos for numerous and productive discussions. To my friends Vera and Vi a special thanks. My gratitude to all colleagues in the LSRE that made this work possible with their help and support, specially: António Martins, Paulo Laranjeira and Ricardo Jorge Santos and also to the following colleagues from DAO - Universidade de Aveiro: Ana Paula Gomes, Arlindo Matos, Carlos Borrego, José Figueiredo and Luís Tarelho. To my daughter, brother, parents and husband who deserve special credit for their affection and support. Financial support for this work was in part provided by the research project FCT/POCTI71999/EQU/34151 for which the author is grateful. The author acknowledges her Ph.D. scholarship awarded by FCT, PRAXIS XXI/BD/9296/96. Abstract The azo coupling between 1-naphthol and diazotized sulfanilic acid and simultaneous azo coupling of 1- and 2-naphthol with diazotized sulfanilic acid have been widely used to study the influence of mixing on the product distribution of fast chemical reactions with fluid of low viscosities. The aim of this work is to study the influence of viscosity on the kinetics of those test reactions, for their subsequent application in the micromixing assessment studies in two reactors: NETmix® static mixer and mixing chamber of a RIM (Reaction Injection Molding) machine. The kinetics studies were performed in both aqueous non-viscous and viscous media. The results obtained in the first study allowed clarifying some controversy found in the literature around the rate constants and the absorption spectrum of one product. For the second study it was necessary to choose a water-soluble additive to raise the solution viscosity, ensuring at the same time that no other properties were changed (e.g., Newtonian behavior, inertness, pH). Several polymeric and non-polymeric thickeners were studied, and a polyurethane solution was found to be a good choice. The rate constants for aqueous viscous medium can not be directly correlated with those in aqueous non-viscous medium maybe due to chemical interferences of the additive or to mixing limitations in the mixing chamber of the stopped-flow equipment used. For this equipment a methodology was developed for the mathematical treatment of kinetic data and the stopped-flow dead time and stoppage time determination, which corrects for the concentration gradients within the optical cell. This method also allows the simultaneous determination of rate constant. The results obtained for the mixing characterization study in the NETmix® static mixer using the azo coupling of 1-naphthol in aqueous solution showed that the Reynolds number has a great influence on the mixing degree, and that this influence is more relevant for Re < 200 . For higher values of Re the mixer behaves as a perfectly mixed reactor. In the RIM machine the test reaction was the simultaneous coupling of 1- and 2-naphthol in the viscous medium. The increase of Reynolds number enhances the mixing intensity inside the mixing chamber, where a transition regime was identified (100 < Re ≤125 ), from segregated to mixing state. Resumo O acoplamento azo entre o 1-naftol e o ácido sulfanílico diazotizado e o simultâneo acoplamento azo do 1- e 2-naftol com o ácido sulfanílico diazotizado têm sido muito usados para estudar a influência da mistura na distribuição produtos de reacções químicas rápidas, com fluidos de baixa viscosidade. O objectivo deste trabalho é estudar a influência da viscosidade na cinética destas reacções teste, para a sua posterior aplicação em estudos de caracterização da micromistura em dois reactores: misturador estático NETmix® e câmara the mistura de uma máquina de RIM (Reaction Injection Molding). Os estudos cinéticos foram levados a cabo em meios aquoso não-viscoso e viscoso. Os resultados obtidos no primeiro estudo permitiram clarificar alguma controvérsia encontrada na literatura à cerca das constantes cinéticas e no espectro de absorvância de um produto. Para segundo estudo foi necessário escolher um aditivo, solúvel em água, para aumentar a viscosidade da solução, assegurando simultaneamente que outras propriedades permaneciam inalteradas (por exemplo, comportamento Newtoniano, inerte, pH). Foram estudados alguns espessantes poliméricos e não poliméricos e uma solução de poliuretano foi seleccionada como sendo uma boa escolha. As constantes cinéticas em meio aquoso viscoso não podem ser directamente correlacionadas com as em meio aquoso não-viscoso, talvez devido a interferências químicas do aditivo ou a limitações de mistura na câmara de mistura to equipamento stopped-flow usado. Para este equipamento, foi desenvolvida uma metodologia para o tratamento matemático dos dados cinéticos e para a determinação do tempo morto e do tempo de paragem, o qual corrige os gradientes de concentração dentro da célula óptica. Este método também permite a determinação simultânea da constante cinética. Os resultados obtidos no estudo de caracterização da mistura no misturador estático NETmix®, usando a reacção de acoplamento azo do 1-naftol em solução aquosa, mostraram que o número de Reynolds tem uma grande influência no grau de mistura e que esta é mais relevante para Re < 200 . O misturador comporta-se como um reactor de mistura perfeita para valores mais elevados de Re . O simultâneo acoplamento do 1- e 2-naftol em meio viscoso foi a reacção teste usada na máquina de RIM. O aumento do número de Reynolds melhora a intensidade da mistura no interior da câmara de mistura, onde foi indentificado um regime de transição (100 < Re ≤125), do estado de segregação para o de mistura. Table of Contents Page Table of Contents........................................................................................................................ i List of Figures ........................................................................................................................... v List of Tables............................................................................................................................ xi Notation.................................................................................................................................... xv 1. INTRODUCTION .............................................................................................................1 1.1 Motivation and Relevance.........................................................................................1 1.2 Thesis Objectives and Layout....................................................................................3 2. MICROMIXING: STATE-OF-THE-ART .....................................................................7 2.1 Introduction................................................................................................................7 2.2 Historical Perspective ................................................................................................9 2.3 The Importance of Mixing .......................................................................................12 2.4 Monitoring Mixing Quality......................................................................................13 2.4.1 Physical Methods.........................................................................................13 2.4.2 Chemical Methods.......................................................................................15 2.4.2.1 Single Fast Reactions.....................................................................18 2.4.2.2 Multi-Step Fast Reactions..............................................................19 2.5 Test Systems and Micromixing Modeling...............................................................23 2.6 Conclusion ...............................................................................................................25 3. THE STOPPED-FLOW TECHNIQUE ........................................................................27 3.1 Introduction..............................................................................................................27 3.2 Stopped-Flow Equipment ........................................................................................29 3.2.1 Setup and Operation.....................................................................................29 3.2.1.1 Spectrophotometer .........................................................................30 3.2.1.2 Sample Handling Unit....................................................................31 3.2.1.3 Workstation....................................................................................31 3.2.1.4 Operation Conditions .....................................................................32 3.2.2 Limitations of the Stopped-Flow Technique................................................32 3.2.3 Stopped-Flow Dynamics Modeling for Pseudo-First Order Kinetics..........34 3.2.3.1
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