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.

A crescente demanda por biocombustíveis e a concorrência dos combustíveis fósseis torna necessária a otimização dos meios já existentes e o desenvolvimento de novas tecnologias para produção de biocombustíveis, principalmente em projetos envolvendo biorreatores e fotobiorreatores (FBR) industriais. A dinâmica dos fluidos computacional (CFD) vem sendo utilizada em vários trabalhos para o estudo de parâmetros fluidodinâmicos que podem influenciar no rendimento dos processos químicos envolvidos, como tensão de cisalhamento, perfis de velocidade, tempo de residência e a influência da geometria sobre esses parâmetros. Contudo, não existe ainda um número abrangente de trabalhos que utilize técnicas de otimização acopladas com a resolução numérica do problema fluidodinâmico. Em alguns estudos, algoritmos de otimização são utilizados para determinar os melhores coeficientes das reações químicas. No entanto, não há estudos, até o momento, que reportem a otimização multiobjetivo simultânea dos parâmetros geométricos e do escoamento aplicados a equipamentos da indústria sucro-energética. Neste contexto, o presente trabalho de pesquisa tem como objetivo contribuir para o avanço científico e tecnológico através da implementação de um software aberto (PyCFD-O) que permita o acoplamento CFD-otimização e o desenvolvimento das bases de uma metodologia de projeto otimizado bem como de operação de biorreatores e FBRs de escala industrial. O PyCFD-O foi testado em dois estudos de caso que podem ser estendidos a um fermentador contínuo e um FBR. Os parâmetros geométricos de ambos os reatores foram otimizados de forma a minimizar simultaneamente a tensão de cisalhamento e a variância da distribuição do tempo de residência. O software PyCFD-O mostrou-se robusto, revelando que o processo global de otimização realiza de fato a busca pela fronteira de Pareto. Além da obtenção das geometrias otimizadas, também foram discutidos a influência dos parâmetros geométricos na hidrodinâmica do escoamento em ambos os casos. / The growing demand for biofuels and its competition with fossil fuels create the need to optimize the existing resources and development of new technologies for production of biofuels, particularly in projects involving industrial bioreactors and photobioreactors (PBR). Computational fluid dynamics (CFD) has been used in several studies for the study of fluid dynamics parameters that can influence the performance of the chemical process involved, such as shear stress, velocity profiles, residence time and the influence of these parameters on the reactor geometry. However, there are lacks of studies that utilize optimization techniques coupled with the numerical resolution of the fluid dynamic problem. The use of optimization algorithms has been reported in some cases, but there have not been reports on studies combining the optimization of flow parameters and multiobjective algorithms to choose ideal geometric parameters applied to the equipment of the sugar-energy industry. In this context, this research project aims to contribute to the advancement of scientific and technological knowledge trhough the implementation of open source software (PyCFD-O) for the CFD-optimization coupling and the development of the bases of a methodology for optimal design and operation of industrial scale bioreactors and PBR. The PyCFD-O software was tested in two case studies with characteristics that can be extended to a continuos fermenter and PBR. The geometric parameters of both reactors were simultaneously optimized in order to minimize the shear stress and the variance of residence time distribuition. The PyCFD-O software showed robustness, revealing that overall optimization process actually performs the search of Pareto frontier. In addition to the geometry optimization, the influence of the geometrical parameters of the hydrodynamic of the flow was discussed in both case studies.

A aplicação da técnica de leito de jorro na secagem de pastas têm sido objeto de inúmeras investigações nos últimos anos, devido a possibilidade de obtenção de produtos de alta qualidade em uma operação de reduzido custo. No presente trabalho, avaliou-se a influência dessa técnica sobre as características e o desempenho de liberação de microcápsulas contendo diferentes fármacos modelo. Microcápsulas de caseína-pectina contendo indometacina e paracetamol foram preparadas por coacervação e posteriormente secas por leito de jorro e por spray drying. Após a secagem por leito de jorro, verificou-se uma redução na quantidade de indometacina recuperada a partir das microcápsulas. Não se observou segregação do fármaco durante a operação de secagem por leito de jorro, indicando que a reduzida recuperação do mesmo foi resultado do estabelecimento uma interação fármaco-polímero, a qual ocorre especificamente entre a caseína e a indometacina. A avaliação físico-química desses materiais indicou que a interação proposta possui natureza covalente e os estudos in vivo de atividade antiinflamatória das microcápsulas contendo indometacina mostraram a irreversibilidade dessa ligação em ambiente biológico. A metodologia de estímulo resposta foi avaliada e otimizada para a determinação do tempo de residência das microcápsulas em leito de jorro. Apesar de confiáveis, os valores de tempo de residência obtidos não puderam ser correlacionados à formação da interação fármaco-polímero, por se apresentarem estáveis quando da variação das condições operacionais do secador. Por outro lado, a vazão e a temperatura do ar de secagem mostraram influência significante sobre a formação da referida interação. Posteriormente à investigação da interação indometacina-caseína, foi avaliada a funcionalidade das microcápsulas obtidas por leito de jorro e por spray drying. Para tanto, foram realizados estudos de liberação in vitro, os quais mostraram a similaridade entre os perfis de dissolução da indometacina do paracetamol independentemente do método de secagem empregado. Uma exceção está na formulação não reticulada contendo indometacina, que apresentou um elevado nível de ruptura após a secagem por leito de jorro. Os dados obtidos na avaliação da funcionalidade dos sistemas juntamente com a constatação experimental do maior aproveitamento energético na secagem por leito de jorro, permitem considerar que a utilização dessa técnica na secagem de microcápsulas é promissora, visto que a interação fármaco-polímero discutida nesse trabalho é específica e dependente do tipo de fármaco encapsulado / The use of spouted bed technique in the drying of paste has been subject to many researches in the past years due to the possibility of obtaining high quality products at low cost. In present work the influence of this technique on microcapsule drug delivery systems performance was evaluated. Casein-pectin microcapsules with entrapped indomethacin and acetaminophen were prepared using the complex coacervation method. Thereafter the coacervates were dried using the spouted bed and spray drying techniques. After spouted bed drying an interaction between indomethacin and casein was observed, with reduction of drug recovery from microcapsules. The mass balance studies showed the absence of indomethacin segregation during spouted bed drying, which indicates drug-polymer interaction. This interaction was not observed for acetaminophen. The physico-chemical evaluation suggests that the indomethacin interaction had covalent nature and in vivo studies showed its stability under enzymatic biological conditions. The microcapsule residence time during spouted bed drying was determined using a validated and optimized stimulus response method. Despite the reliable results obtained it was not possible to determine the influence of residence times on drug-polymer interaction due to the stability of those values with operational conditions. On the other hand air temperature and air flow rate during spouted bed drying affected significantly the degree of drug polymer interaction. In order to evaluate the spouted bed and spray dried microcapsule?s performance a dissolution study was performed. The results showed very similar drug release, with exception for the non-crosslinked formulations dried in spouted bed. These microcapsules were ruptured by spouted bed mechanical stresses. The results obtained on the microcapsules performance evaluation and the characteristics of thermal efficiency and high evaporative capacity of spouted bed demonstrate the potential application of casein-pectin spouted bed drying

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

In urban areas, stormwater management ponds (SWPs) are built to mitigate polluted runoff. Although these ponds are increasing in numbers, their ecology is not well understood. Physical and chemical characteristics of 17 SWPs in the City of Ottawa were measured to determine the drivers of phytoplankton biomass (Chl. a) and primary production (PP). While total phosphorus was the best predictor of algal biomass in the ponds (as in lakes), the imperviousness of the catchment could also predict Chl. a. Planktonic PP in two ponds measured seasonally was more closely related to water residence time than to nutrient concentrations with rates approaching at times the theoretical maximum for aquatic systems. In one pond, whole ecosystem metabolism, estimated using diel changes in dissolved oxygen and δ18O-O2, suggested that these hypereutrophic systems were net sinks for carbon in the summer but likely sources to the atmosphere at other times of the year.

The submitted diploma thesis is focused on finding optimal inside arrangement of multi-chamber septic tank using the conductive method. The diploma thesis brings detailed description and measuring results, including the testing of several different arrangements of the standard circular three chamber septic tank in the project TAČR with the working title "Anasep" and newly developed four-chamber septic tank with several innovative features. The results are then categorized according to own terminology for ease application in the practice. The described method may be a suitable alternative for other technically and economically less available methods. The method shows very clearly whether the inner space of the septic tank sufficient use its volume to the velocity field distribution which has a direct impact on the rate of sedimentation of suspended solids and therefore affect the life of the downstream filtration equipment. Measured data processing and graphical outputs were processed in the programme Minitab. Graphical display of all variants of septic tanks was carried out using SketchUp. Visualization of a septic tank in 2D and 3D space plane was solved in COMSOL Multiphysic with the application of CFD module. The conclusion is based on achievements discussed the applicability of the described method and it presented hydraulically efficient multi-chamber septic tank, which is currently copyrighted solution, developed by a team of solvers in the project ANASEP.

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.

Zincor, a refinery in South Africa, uses the conventional roast-leachelectrowinning process to produce zinc metal. The roasting process of ZnS concentrate makes use of four Lurgi fluidized bed roasters to produce calcine (contains ZnO and ZnFe2O4 as zinc products) and SO2 gas. The roasting plant consists of two 18 m2 and two 35 m2 cross sectional area roasters. Prior to 1996, Zincor utilized air as the only oxidant and fluidizing medium in its roasters. The maximum dry feed rates that the roasters could process were 6.5 t/d.m2. In an attempt to increase production, oxygen enrichment was first trialled and then introduced into the fluidizing air. The ability of oxygen enrichment to increase the rate of the ZnS oxidation reaction allowed higher feed rates to the roasters. This was successful and oxygen enrichment was permanently implemented. That enabled dry feed rates to be maintained at 7.0 t/d.m2 and 7.3 t/d.m2 for the small and big roasters respectively. Oxygen enrichment up to 26% in the fluidizing air is utilized. Due to the highly competitive nature of the zinc industry, innovative processing techniques are necessary to be competitive. The aim of this study is to determine whether oxygen enriched air can be reduced by introducing micro-pelletized concentrate into the roaster feed blend, whilst maintaining current roaster feed rates and calcine quality. This study was executed in four parts. Firstly, the role entrainment played in influencing average particle residence time. Secondly, a study of production methods for stable micro-pellets. Thirdly, a study of the influence of oxygen enrichment and particle size on the roasting of micropellets. The fourth part of the study was introduction of micro-pellets into the Zincor roasters to determine whether oxygen enrichment could be reduced. The particle size distribution of a blend of feed concentrate to the roasters is 50% passing approximately 48 μm. This leads to entrainment values between 87% and 91%. The micro-pelletization process reduces the –500 μm fraction from 87% to 10%, which degrades to 30% during roasting. This requires that approximately 48% of the concentrate needs to be micro-pelletized to restore the 70% designed entrainment target. It was determined that entrained particles spend on average 0.46 hr to 2.44 hr in the bed compared to particles in the overflow that have residence times between 3.93 and 4.00 hr. The calculated times for entrainable particles are somewhat higher and for the bed overflow lower compared to the values measured by Spira, 1970. The required reaction time for micropellets was found to be far below their residence time inside a Zincor roaster. With a load of 20% micro-pellets introduced into the feed concentrate, the oxygen enrichment could be reduced by 60%. The quality of the calcine produced was maintained above the target of 98.8% ZnS to ZnO conversion. The results of this study have shown that the use of micro-pelletization of concentrate at Zincor reduces entrainment of particles successfully. Manipulation of entrainment through micro-pelletization can be used successfully to reduce oxygen enrichment, whilst improving production and maintaining quality at Zincor. Copyright / Dissertation (MEng)--University of Pretoria, 2009. / Materials Science and Metallurgical Engineering / unrestricted