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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / M.S.

Hollow fibre and spiral wound membrane (SWM) modules are the most common commercially available membrane modules. The latter dominate especially for RO, NF and UF and are the focus of this study. The main difficulty these types of modules face is concentration polarisation. In SWM modules, the spacer meshes that keep the membrane leaves apart also help reduce the effects of concentration polarisation. The spacer filaments act as flow obstructions, and thus encourage flow destabilisation and increase mass transfer enhancement. One of the detrimental aspects of the use of spacers is an increase of pressure losses in SWM modules. This study analyses the mechanisms that give rise to mass transfer enhancement in narrow spacer-filled channels, and investigates the relationship between flow destabilisation, energy losses and mass transfer. It shows that the regions of high mass transfer on the membrane surface correlate mainly with those regions where the fluid flow is towards the membrane. Based on the insights gained from this analysis, a series of multi-layer spacer designs are proposed and evaluated. In this thesis, a Computational Fluid Dynamics (CFD) model was used to simulate steady and unsteady flows with mass transfer in two- and three-dimensional narrow channels containing spacers. A solute with a Schmidt number of 600 dissolving from the wall and channel Reynolds numbers up to 1683 were considered. A fully-developed concentration profile boundary condition was utilised in order to reduce the computational costs of the simulations. Time averaging and Fourier analysis were performed to gain insight into the dynamics of the different flow regimes encountered, ranging from steady flow to vortex shedding behind the spacer filaments. The relationships between 3D flow effects, vortical flow, pressure drop and mass transfer enhancement were explored. Greater mass transfer enhancement was found for the 3D geometries modelled, when compared with 2D geometries, due to wall shear perpendicular to the bulk flow and streamwise vortices. Form drag was identified as the main component of energy loss for the flow conditions analysed. Implications for the design of improved spacer meshes, such as extra layers of spacer filaments to direct the bulk flow towards the membrane walls, and filament profiles to reduce form drag are discussed.

Mass transfer processes in food systems, such as solute infusion, are poorly understood because of their complex nature. Food systems contain porous matrices and a variety of continuous phases within cellular tissues. Mass transfer processes are generally not pure diffusion: often convection, binding and obstructing diffusion will occur. Monte Carlo (MC) simulation has been increasingly used in life science and engineering to elucidate molecular transport in biological systems. However, there are few articles available discussing MC simulation in food processing, especially mass transfer. The main goal of this study was to show the inherent simplicity of the MC approach and its potential when combined with traditional experimental and theoretical approaches to better describe and understand mass transfer processes. A basic framework for MC random walk - simulation applied to a diffusion problem - is developed in this project. Infusion of two sizes of dextran macromolecules in fish muscle cells is used to apply the MC framework in combination with Fluorescence Recovery After Photobleaching experiments. Effective diffusivity coefficients within cells, considering the degree of obstruction due to the myofibrilar matrix, are assessed. Then, the results are used as input in a mathematical model that was developed for theoretical simulation of mass transfer in the multi-cellular tissue. Diffusivity values obtained by the MC framework had an SD of ±0.02 [µm²/s] around the true value of 0.25 [µm²/s]. MC results for degree of obstruction were 0.29 and 0.34 for dextran FD1OS and FD2OS, respectively, and the Devalues were 23.7 and 11.2 [µm2/s]. The statistical error in the estimation of D was estimated to be [22.8-24.6] and [9.7-12.7] (95% CI), where average experimental values of 24.3 [µm²/s] for FD1OS and 11.4 [µm²/s] for FD2OS were captured by the respective interval. The theoretical model showed a significant influence of the cell membrane characteristics and tissue porosity in both the degree of solute penetration and the solute distribution between intra- and extra-cellular space. The combined approach was successfully applied to a diffusion problem. Overall, it is expected that the present work will contribute towards the application of MC simulation in the field of Food Science and Engineering. / Graduation date: 2006

Hollow fibre and spiral wound membrane (SWM) modules are the most common commercially available membrane modules. The latter dominate especially for RO, NF and UF and are the focus of this study. The main difficulty these types of modules face is concentration polarisation. In SWM modules, the spacer meshes that keep the membrane leaves apart also help reduce the effects of concentration polarisation. The spacer filaments act as flow obstructions, and thus encourage flow destabilisation and increase mass transfer enhancement. One of the detrimental aspects of the use of spacers is an increase of pressure losses in SWM modules. This study analyses the mechanisms that give rise to mass transfer enhancement in narrow spacer-filled channels, and investigates the relationship between flow destabilisation, energy losses and mass transfer. It shows that the regions of high mass transfer on the membrane surface correlate mainly with those regions where the fluid flow is towards the membrane. Based on the insights gained from this analysis, a series of multi-layer spacer designs are proposed and evaluated. In this thesis, a Computational Fluid Dynamics (CFD) model was used to simulate steady and unsteady flows with mass transfer in two- and three-dimensional narrow channels containing spacers. A solute with a Schmidt number of 600 dissolving from the wall and channel Reynolds numbers up to 1683 were considered. A fully-developed concentration profile boundary condition was utilised in order to reduce the computational costs of the simulations. Time averaging and Fourier analysis were performed to gain insight into the dynamics of the different flow regimes encountered, ranging from steady flow to vortex shedding behind the spacer filaments. The relationships between 3D flow effects, vortical flow, pressure drop and mass transfer enhancement were explored. Greater mass transfer enhancement was found for the 3D geometries modelled, when compared with 2D geometries, due to wall shear perpendicular to the bulk flow and streamwise vortices. Form drag was identified as the main component of energy loss for the flow conditions analysed. Implications for the design of improved spacer meshes, such as extra layers of spacer filaments to direct the bulk flow towards the membrane walls, and filament profiles to reduce form drag are discussed.

Orientador: Julian Martínez / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia de Alimentos / Made available in DSpace on 2018-08-21T22:31:55Z (GMT). No. of bitstreams: 1 Silva_LuizPauloSales_M.pdf: 5215588 bytes, checksum: cb283a60a47e8463b3ab4b9e87526e92 (MD5) Previous issue date: 2013 / Resumo: Este projeto utilizou a tecnologia supercrítica no processo de extração, usando o dióxido de carbono como solvente. Esta tecnologia apresenta-se como uma alternativa para processos que usam solventes orgânicos tóxicos, além de respeitar os princípios da química verde, pautada nos conceitos de desenvolvimento sustentável. Com o objetivo de compreender melhor todos os mecanismos fenomenológicos envolvidos neste processo, bem como poder controlá-los visando à sua otimização, a modelagem matemática é uma opção bastante atrativa. As substâncias capsaicinoides, presente em grandes quantidades em várias espécies de pimenta, responsáveis pela sensação pungente e que, no entanto, possuem comprovadas ações benéficas ao organismo, foram definidas como substâncias alvo. Desta forma, para os estudos dos fenômenos de transferência de massa, três espécies de pimentas, Capsicum frutescens, Capsicum chinense, Capsicum boccatum, foram analisadas quanto aos seus teores de capsaicinoides. A espécie Capsicum frutescens apresentou o maior concentração destas substâncias e foi escolhida como a matéria-prima para etapas posteriores. Um planejamento experimental de extração supercrítica desta espécie de pimenta foi realizado variando a pressão e a temperatura. A partir destas extrações foi observado que a condição de extração de 15MPa e 313 K apresentou a melhor combinação entre rendimento e concentração de capsaicina. Cinéticas de extração realizadas nesta condição, porém variando a vazão de solvente, o diâmetro de partícula e o volume de extração, foram estudadas. Maiores taxas de extração foram obtidas nas maiores vazões e nos menores diâmetros de partícula e volume de leito de extração devido à maior importância do fenômeno convectivo. O modelo de partículas intactas e rompidas de Sovová (1994) foi usado para ajustar os dados experimentais das curvas e obter os parâmetros do modelo. Três tipos de modelagem foram realizadas: ajuste de cada curva individualmente; ajuste simultâneo gerando um conjunto de parâmetros para os pares das duplicatas; ajuste múltiplo que ajustou um valor único do parâmetro XK para cada conjunto de curvas com o mesmo diâmetro de partícula. Com os ajustes foi possível calcular o coeficiente convectivo de transferência de massa de cada condição e o respectivo valor do número de Sherwood experimental. Com os dados experimentais de cada condição foram calculados os números adimensionais de Reynolds e Schimdt. Com estes novos conjuntos de dados de números adimensionais foram realizadas novas modelagens matemáticas e, através destas, propostas novas correlações. Estas novas equações foram baseadas tanto na existência única da convecção forçada quanto na existência, mesmo que pouco significativa, da convecção natural. A eficácia destes novos modelos foi avaliada com a comparação dos coeficientes de transferência de massa convectivos calculados com aqueles ajustados das curvas experimentais, apresentando, em geral, boa similaridade. Por fim, uma extração em escala piloto realizada deu indícios, através dos resultados calculados das novas correlações, que a convecção natural nestas escalas não pode ser desprezada / Abstract: This workt used the supercritical technology in the process of extraction, using carbon dioxide as solvent. This technology is based on concepts of sustainable development and respects the principals of green chemistry. It appears as an alternative to processes that use toxic organic solvents. Mathematical modeling is an interesting tool to understand better all phenomenological mechanisms involved in this process and to be able to control and optimize them. Capsaicinoids, which are responsible for the pungent sensation caused by peppers, have well-known beneficial properties for human organism. These substances are present in large quantities in several pepper species. Capsaicinoids were chosen as target substances for the study of mass transfer phenomena. Capsaicinoid contents were analyzed for three pepper species: Capsicum frutescens, Capsicum chinense, Capsicum boccatum. The species Capsicum frutescens showed higher concentration of these substances and was chosen as raw material for further steps. An experimental design of supercritical extraction from this pepper species was carried out varying pressure and temperature. These extractions showed that the extraction condition of 15 MPa and 313 K gave the best combination of yield and capsaicin concentration. Therefore, extraction kinetics was studied under this condition, varying solvent flow rate, particle diameter and extraction bed volume. The highest extraction rates were obtained for high solvent flow rates, low particle diameters and low extraction bed volume. This can be explained by the greater importance of the convective phenomenon under these conditions. The Sovová¿s model (1994) for intact and broken particles was used to fit experimental data to curves and obtain model parameters. Three types of mathematical modeling were established: (1) fitting of each individual curve, (2) simultaneous fitting creating a set of parameters for pairs of duplicates, (3) multiple fitting that adjsuts a single value for the parameter XK for each set of curves with the same particle diameter. These fits allowed calculating the convective mass transfer coefficient for each condition and the respective values of the experimental Sherwood number. Experimental data was used to calculate dimensionless numbers of Reynolds and Schmidt for of each condition. Other mathematical modelings were performed using these new data sets of dimensionless numbers, which allowed proposing new correlations. These new equations were based on the existence of forced and free convection, even though the importance of the second phenomenon was considered small. The efficiency of these new models was assessed with a comparison of calculated convective mass transfer coefficients to those fitted from experimental curves. A good coherence was found between both. Finally, a pilot scale extraction was performed and the results obtained using the proposed correlations suggested that free convection cannot be neglected at such scales / Mestrado / Engenharia de Alimentos / Mestra em Engenharia de Alimentos