• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 69
  • 28
  • 14
  • 8
  • 6
  • 4
  • 2
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 210
  • 30
  • 25
  • 24
  • 19
  • 18
  • 16
  • 15
  • 14
  • 13
  • 12
  • 12
  • 11
  • 11
  • 11
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
71

Evaluation of the enhanced thermal fluid conductivity for gas flow through structured packed pebble beds / T.L. Kgame

Kgame, Tumelo Lazarus January 2010 (has links)
The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.
72

A ger?ncia de leitos em um hospital polo da regi?o ampliada de sa?de Jequitinhonha

Souza, Danielle Mandacaru January 2016 (has links)
Data de aprova??o ausente. / Disponibiliza??o do trabalho em conte?do parcial, conforme Termo de Autoriza??o. / Submitted by Jos? Henrique Henrique (jose.neves@ufvjm.edu.br) on 2017-03-10T19:29:54Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) danielle_mandacaru_souza_parcial.pdf: 114895 bytes, checksum: c25bdadc0c4a16e8b838134a138a653f (MD5) / Approved for entry into archive by Rodrigo Martins Cruz (rodrigo.cruz@ufvjm.edu.br) on 2017-03-30T16:43:01Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) danielle_mandacaru_souza_parcial.pdf: 114895 bytes, checksum: c25bdadc0c4a16e8b838134a138a653f (MD5) / Made available in DSpace on 2017-03-30T16:43:01Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) danielle_mandacaru_souza_parcial.pdf: 114895 bytes, checksum: c25bdadc0c4a16e8b838134a138a653f (MD5) Previous issue date: 2016 / O presente estudo buscou identificar a funcionalidade da Ger?ncia de leitos em um hospital polo da Regi?o Ampliada de Sa?de Jequitinhonha e da Regi?o de Sa?de Diamantina. Com abordagem qualitativa, o estudo foi realizado por meio de entrevistas gravadas e informa??es coletadas dos Sistemas de Processamento de Dados SPDATA. Entrevistou-se 89 colaboradores, sendo quatorze enfermeiros, quinze m?dicos, nove administrativos, doze funcion?rios da higieniza??o, sete profissionais da recep??o e trinta e tr?s t?cnicos de enfermagem. Realizou-se a an?lise de conte?do das entrevistas com base na metodologia de Minayo. Organizaram-se as informa??es coletadas em categorias tem?ticas: A funcionalidade da gest?o de leitos na organiza??o dos processos de entrada e sa?da dos pacientes; A responsabilidade da comunica??o como ferramenta para a instala??o do processo de Gest?o de Leitos; O direcionamento dos pacientes para a cl?nica certa ap?s a implanta??o da GL; A gest?o de leitos e a hotelaria hospitalar; Organiza??o do servi?o hospitalar ap?s a Gest?o de Leitos; Influ?ncia da Gest?o de Leitos na organiza??o das cirurgias eletivas. Esse estudo mostrou que o gerenciamento e melhoria do fluxo de pacientes ao longo das interna??es hospitalares s?o importantes, que o servi?o de Gerenciamento de Leitos buscou utilizar cada leito hospitalar em sua m?xima, diminuiu a espera para as interna??es, tanto eletivas quanto de urg?ncias. Com esse servi?o, a equipe multidisciplinar tem mais seguran?a na gera??o das informa??es sobre o leito que ser? ofertado para o paciente, al?m de tornar o processo de interna??o r?pido e eficiente. / Disserta??o (Mestrado) ? Programa de P?s-gradua??o em Ensino em Sa?de, Universidade Federal dos Vales do Jequitinhonha e Mucuri, [2016]. / This study sought to identify the functionality of beds Management on a pole hospital Jequitinhonha Extended Health Region and the Diamantina Health Region. With a qualitative approach, the study was conducted through recorded interviews and information collected from SPDATA Data Processing Systems. She interviewed 89 employees, with fourteen nurses, fifteen doctors, nine administrative, twelve employees of hygiene, seven professional reception and thirty-three nursing technicians. We conducted content analysis of the interviews based on Minayo methodology. Organized the information collected in thematic categories: The functionality of bed management in the organization of the procedures and exit of patients; The responsibility of communication as a tool for installing the bed management process; The direction of patients to the right clinic after the implementation of the bed management; The management of beds and hospital catering; the hospital service organization after he bed management; bed management influence in the organization of elective surgeries. This study showed that the management and improve the flow of patients through the hospital admissions are important, the beds Management service sought to use every hospital bed at its maximum, decreased waiting for hospitalizations, both elective as emergency. With this service, the multidisciplinary team has more security in the generation of information about the bed which will be offered to the patient, in addition to making the quick and efficient admission process.
73

Utilização de leitos de drenagem no desaguamento de lodos anaeróbios. / Anaerobic sludge dewatering by modified drying beds.

Fernando Cintra Mortara 15 September 2011 (has links)
O método de desaguamento por leitos de drenagem foi desenvolvido por Cordeiro a partir do ano de 1993 (CORDEIRO, 2001) para o desaguamento de lodos de Estações de Tratamento de Água. A sua aplicação para lodos de lagoas de estabilização de esgotos foi feita por Fontana et al. (2007). Este trabalho teve por objetivo avaliar a utilização de leitos de drenagem no desaguamento de lodos produzidos em reatores UASB. Para tanto foram realizados ensaios em laboratório, utilizando testes do tempo de drenagem, para avaliar a influência de diferentes doses e tipos de polímeros no desaguamento do lodo. Alguns geotêxteis foram utilizados como meio filtrante nos funis de Buchner e também avaliados pelo teste do tempo de drenagem. Após escolhidas a manta e o polímero que apresentaram os melhores resultados em laboratório, foram desenvolvidos ensaios em escala piloto, com a utilização de três unidades cobertas de leitos de drenagem, cada um com dimensões de 1,58 m x 1,09 m por 0,5 m de altura. Para a avaliação do efeito das diferentes doses de polímero em escala piloto, foram utilizadas doses de polímero de 0 a 8 g/kg de sólidos totais (peso seco) e mantida a taxa de aplicação de sólidos em 15 kgST/m².ciclo (a concentração de sólidos no lodo foi de aproximadamente 27 g/L). Os resultados indicaram que com doses de polímero =2 g/kgST (peso seco) obtinha-se teor de sólidos no lodo de cerca de 13% após 1 dia e 20% após 10 a 15 dias. Para se atingir teor de sólidos de 25 a 30% o período de secagem necessário foi de cerca de 30 dias. Embora que mesmo sem a aplicação de polímeros o comportamento do lodo fosse semelhante ao de lodos condicionados com polímero, em relação à evolução do teor de sólidos em função dos períodos de secagem, observou-se certa dificuldade na remoção do lodo desaguado e principalmente na limpeza das mantas geotêxteis. Para avaliar diferentes taxas de aplicação superficial de sólidos, foram ensaiadas taxas entre 9 e 22 kgST/m².ciclo. A evolução do teor de sólidos, ao longo do período de secagem dos 3 ensaios realizados, indicou que taxas mais altas de aplicação de sólidos não diminuem as velocidades de secagem, sugerindo que, inclusive, taxas mais altas poderiam ser ensaiadas. / The method of dewatering sludge through Drainage Beds was originally developed in 1993 by Cordeiro (CORDEIRO, 2001) with sludge from Water Treatment Plants. Fontana et al. (2007) used sludge from wastewater stabilization ponds with positive results. The purpose of this research is the evaluation of the drainage bed method to dewater sludge from UASB reactors. Several laboratory trials were performed, measuring drainage times to assess the influence of different polymer types and doses on sludge dewatering. Few geotextiles were used as filter on the drainage time test to evaluate their performance. Once the polymer and the geotextile that best performed in the laboratory were chosen, several pilot experiments were performed with the use of three covered units of drainage beds, each measuring 1.58m x 1.09m (width) x 0.5m (height). For the evaluation of different polymer conditioning doses on a pilot scale, doses from 0 to 8 g/kg of total solids (dry weight) were used and the applied rate of solids kept at 15 kgTS/m².cycle (the concentration of solids in the sludge was of approximately 27 g/L). Results indicated that with the use of polymer doses =2 g/kgTS (dry weight), approximately 13% of total solids in the sludge was observed after one day, which increased to approximately 20% after 15 days. To reach 25% to 30% of total solids in the sludge, the drying time required was circa 30 days. Even without the application of polymers, the behavior of sludge in terms of total solids during the drying period was similar to the sludge conditioned with polymers. Although, with the unconditioned sludge, some difficulty to remove the dewatered sludge cake was observed, mainly in cleaning of the geotextile sleeves. In order to evaluate different application rates of solids, the pilot experiments included rates between 9 and 22 kgTS/m².cycle. The behavior of the total solids in the cake during the drying period of the three experiments suggested that the higher rates of application of solids does not reduce the drying speed, indicating that higher rates can be targeted in future research projects.
74

Transformations diagénétiques des dépôts silicoclastiques FA du bassin de Franceville au Gabon (2.2-2.0 Ga) par l'invasion de solutions oxydo-réductrices / Diagenetic transformations and redox fluids invasion in siliciclastic FA formation, Franceville basin (2.2-2.0 Ga), Gabon

Bankole, Olabode Modupe 16 December 2015 (has links)
Dans le bassin de Franceville (Gabon), les sédiments détritiques non-métamorphosés d’âge Paléoprotérozoïque (2,15 Ga) des formations FA et FB ont fait l’objet d’une étude faciologique, pétrographique et géochimique. L’objectif était de déterminer l’origine de ces matériaux clastiques et des minéralisations uranifères associées, mais aussi d’en retracer l’histoire diagénétique à travers l’évolution des paléoconditions rédox et des fluides qui les ont percolés.La pétrographie des faciès montre que la minéralogie et la texture initiales de ces sédiments ont été fortement modifiées au cours de la diagenèse précoce. Au toît de la formation FA, les quartz-arénites faiblement granoclassées ont été très tôt cimentées par du quartz, s’opposant ainsi à l’action ultérieure de la diagenèse d’enfouissement. Dans les arènes arkosiques, la séquence paragénétique liée aux ciments authigènes suggère que les interactions fluides-roches ont été polyphasées et que les éléments chimiques nécessaires à leur précipitation trouvent leur origine dans l’altération de minéraux détritiques. Pour un ensemble lithologique donné, les effets de la diagenèse varient peu d’un faciès à l’autre, ce qui indique un fort contrôle des paramètres initiaux tels que la minéralogie, la texture et la composition du fluide poral. Par conséquent, ces faciès sédimentaires nous informent directement sur les propriétés hydrologiques qui régnaient dans le bassin sédimentaire, et sur la nature de la diagenèse précoce qui les a affectés.Les données pétrographiques et géochimiques montrent que la précipitation de l’hématite en lits ferrugineux a débuté juste après le dépôt des sédiments, dont l’altération des minéraux primaires a libéré le fer, plus tard redistribué durant la diagenèse. Ainsi, la mise en évidence d’une corrélation positive entre les valeurs de "delta" 56Fe et les rapports Fe/Mg mesurés sur échantillon total suggère que le fer se répartit entre deux pôles que sont l’hématite authigène et les silicates porteur du fer. En revanche, l’absence de relation entre les rapports Fe3+/FeT et les compositions isotopiques du fer démontre que des oxydes riches en isotopes lourds du fer préexistaient dans les sédiments lorsque la diagenèse précoce a débuté. Le fer présent initialement à l’état réduit dans des faciès sédimentaires verts s’est progressivement oxydé au cours de la diagenèse, processus à l’origine de la formation des lits ferrugineux.Dans la formation FA du Bassin de Franceville, les gisements uranifères résultent de la libération de l’uranium contenu à l’origine dans les grès oxydés des dépôts fluviatiles inférieurs puis de sa migration et de son piégeage dans les grès réduits et les mudstones silteux des formations de deltas de marée supérieures, où il s’est concentré par un mécanisme de type roll-front.L’analyse des éléments chimiques majeurs et traces des formations FA et FB indique que leurs matériaux constitutifs trouvent leur source dans des roches ignées felsiques. Tandis que les diagrammes de terres rares normalisés aux chondrites, les anomalies négatives en Eu et les rapports GdN/YbN plaident plus précisément en faveur des roches archéennes et paléoprotérozoïques. Enfin, des diagrammes discrimants et des rapports en éléments majeurs et traces montrent que, si la plupart des grès de la formation FA se sont déposés sur une marge passive, les mudstones et les grès fins des formations FA et FB ont sédimenté sur une marge continentale active en relation avec la tectonique paléoprotérozoïque de la Ceinture orogénique du Centre-Ouest de l’Afrique. / The FA and FB Formations clastic sediments have been subjected to detailed facies, petrographic, and geochemical analyses in relation to diagenesis, fluid flow, paleo-redox conditions, provenance, and uranium mineralization during the evolution of the unmetamorphosed Paleoproterozoic (ca 2.15 Ga) Franceville Basin, Gabon. Lithofacies analyses in combination with petrographic studies indicate that the original mineralogical and textural properties of the sediments have been greatly modified during diagenesis. The moderately sorted quartz arenite at the top of FA underwent early quartz cementation; thus preventing it from subsequent burial diagenetic processes. The inferred paragenetic sequence of authigenic cements in the arkosic arenites suggest a multiphase of fluid-rock interactions with most of the ions needed for their precipitations likely sourced during alteration of detrital precursors. The observed slight variations in the diagenetic pathways in different lithofacies associations are closely related to primary mineralogy, texture, and nature of pore fluid. This relationship suggests that depositional facies can provide an insight into the diagenetic pathways and hydrologic properties of sediments in sedimentary basins. Petrographic features coupled with whole rock geochemical and iron isotope analyses suggest that hematite precipitation in the red beds started after sediment deposition with the iron internally derived by alteration of iron-bearing minerals and redistributed during late diagenesis. Positive correlation between Fe/Mg ratio and "delta" 56Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates. The lack of relationship between Fe3+/FeT ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis or burial. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a roll-front type in the FA Formation of the Franceville Basin. Major and trace element geochemical data for the FA and FB formation are consistent with sediments derived exclusively from felsic igneous source. Chondrite-normalized patterns with high LREE/HREE ratios, negative Eu anomalies, and GdN/YbN ratios favour mixture of Archean and Post-Archean felsic sources. Discriminant diagrams and elemental ratios of major and trace elements suggest deposition of most of the FA sandstone in a passive margin, while the mudstones and fine-grained sandstones of FA and FB were deposited in an active continental margin during Paleoproterozoic tectonic regimes of the West Central African Belt.
75

Distribution, Abundance and Movement of Fish among Seagrass and Mangrove Habitats in Biscayne Bay

Goebel, Patrick C 17 March 2016 (has links)
Inshore tropical and subtropical estuaries harbor a relatively high abundance and diversity of organisms. Specifically within estuaries, mangrove and seagrass habitats provide shelter and food for a plethora of organisms, through some or all their life histories. Given the biological connection between offshore coral reefs and coastal estuaries, there is a critical need to understand the underlying processes that determine distribution and abundance patterns within mangrove-seagrass habitats. The predatory fish assemblage within the mangrove and seagrass beds of Biscayne Bay, Florida (USA), was examined over 24-hr. time periods along a distance and habitat gradient from the mangrove edge and nearshore environment (0–300 m) to farshore (301–700 m) seagrass beds. This thesis also investigated the occurrence, distribution and timing of reef fish movement between offshore coral reef habitat and inshore seagrass beds over 24-hr periods. Results indicate that fish predators differed over both the sampling period and with distance from mangrove edge. The results also demonstrated reef fishes move into Biscayne Bay at dusk and exit at dawn by utilizing Broad Creek Channel as a passageway. This work supports the idea of diel migration of selected reef fishes to inshore seagrass beds and highlights the importance of connective channels between habitats. The results suggest that the degradation or loss of seagrass habitat could differentially impact the life-history stages of reef fish species.
76

An assessment of chiropractic adjustment beds as reservoirs for normal flora and infectious bacterial pathogens at a chiropractic teaching clinic

Logtenberg, Jana January 2009 (has links)
Submitted in partial compliance with the requirements for a Master Degree in Technology: Chiropractic at the Durban University of Technology, 2009. / Background: Research has indicated the majority of bacteria on chiropractic adjustment beds (beds), can persist on dry inanimate surfaces for months. Thus, insufficient disinfection procedures create continuous sources of pathogens endangering patients and healthcare workers alike. This research study aimed to assess the beds as reservoirs for micro-organisms, at a chiropractic teaching clinic (clinic) in South Africa. Method: A selection of samples obtained from the headrests and armrests of the beds were serially diluted, plated in duplicate (using the spread plate technique) and incubated for 24-48 hours at 37°C. After inspection for the presence of micro-organisms, those present were enumerated to determine their quantities, the microbial build-up throughout the day, as well as the degree of the transmission from the patients to the beds during treatment. The incidence of the micro-organisms was established, along with their identities, using microscopic and macroscopic characteristics. These micro-organisms were also used to assess the efficacy of the disinfectant currently in use by the clinic. Results: Microbial growth was present on 89.4% of the beds sampled. The quantities of the micro-organisms increased significantly (p=0,027) from 7:30 am to 16:30 pm, with the median increasing from 25 colony forming units (cfu) / cm2 to 714 792 cfu/ cm2. The microbial build-up was highly significant (p<0.001), with a median of 346 cfu/ cm2 at 7:30 am and 10:30 am; increasing to 162 291 cfu/ cm2 by 13:30 pm and 250 million cfu/ cm2 by 16:30 pm. There was also a significant increase (p<0.001) in the quantity of micro-organisms during treatment with a median of 0 cfu/ cm2 before treatment that rose to 23 479 cfu/cm2 after treatment, indicating that the micro-organisms present on the beds were being deposited by the patient`s skin during the treatment. The most prevalent micro-organisms identified were Staphylococci and Serratia, with an average of 59% and 40% of colonies; while Micrococci and Bacilli were relatively uncommon. No growth was evident after 5 minutes of exposure to the disinfectant during the growth inhibition test. For the Kirby Bauer test, the average size of the zone of inhibition increased as the dilution decreased. The disinfectant is effective but more so against the Gram-positive than the Gram-negative bacteria. The disinfectant was 5,0, 5,5 and 5,6 times more effective than phenol in eradicating Staphylococci, Serratia and Bacilli, respectively. Conclusions and Recommendations: This study showed that micro-organisms were present on the beds. Staphylococci and Serratia have been implicated in many healthcare associated infections. The present disinfectant is effective, but should be used in between every patient. A different or additional disinfectant that is more effective against the Gram-negative bacteria should be considered for future use.
77

Anonymous needlework : uncovering British patchwork, 1680-1820

Long, Bridget January 2014 (has links)
During the eighteen century there was a significant growth in patchwork materially and linguistically. It was the century when patchwork was stitched at all levels of society and has been identified as the time when patchwork moved out from the small domestic world of decorative sewing into the wider public sphere, leaving behind other needlework as it became embedded in the language and writing of the period. This research examines the social and cultural contexts relating to the making of patchwork in the long eighteenth century and in doing so contributes to the story of women and their material lives in the period. Noted for its longevity, surviving as a widespread practice across the century, patchwork was a democratic needlework that was practiced by any woman capable of stitching a variety of fabric pieces together to make a larger whole. A widespread understanding of the term and familiarity with the practice enabled it to be employed successfully in the literal and figurative language of the period. Patchwork was heralded as a fashionable activity in the early eighteenth century, but was later used to represent the ideal of the moral and capable housewife, devoted to her sewing skills and thrifty in her practice. The figurative style of the period allowed the simultaneous use of the word in differing ways so that patchwork was used both positively and negatively in literature, drama, critical review, political debate and theoretical discourse.
78

Autoportante : Human architectures for public engagement

Cid, Celia January 2019 (has links)
Autoportante is a collection of stories of human architectures designed for public engagement, a study of the sense of responsibility people feel in appreciating, using and mantaining public and private structures that provide us with support. Hence, the word autoportante is defined by me as “one’s own automatic ability to care for support structures in society”. My research question being: does it come automatic for people to appreciate and care for support structures?  I have studied human behaviours towards supportive elements in society; objects, places and services (people) that help us in our everyday life. I asked myself whether there is a collective need to relate differently to these, by carrying notions of support structures, politics of care, housekeeping and hospitality throughout different design iterations.
79

Computational Fluid Dynamics Simulations of Radial Dispersion in Low N Fixed Bed Reactors

Medeiros, Nicholas J 20 April 2015 (has links)
Fixed bed reactors are widely applicable in a range of chemical process industries. Their ease of use and simplified operation make them an attractive and preferred option in reactor selection, however the geometric complexities within the bed as a result of the unstructured packing has made the design of such beds historically based on pseudo-homogenous models together with correlation-based transport parameters. Low tube-to-particle diameter ratio (N) beds, in particular, are selected for highly exothermic or endothermic reactions, such as in methane steam reforming or alkane dehydrogenation. Due to the large fraction of tube to catalyst particle contact in these low N beds, wall effects induce a mass transfer boundary layer at the wall, and in the case of thermal beds, a simultaneous resistance to heat transfer. Computational Fluid Dynamics (CFD) has been shown to be an accurate tool for experimental validation and predictive analysis of packed beds, and may be used to derive more accurate design parameters for fixed bed reactors. More specifically, the elucidation of dispersion, or the transport of reactant and product within the bed due to molecular diffusion and convective flow is of fundamental interest to the design of fixed beds. Computational Fluid Dynamics was used in this research to study solute dispersion in eight beds of varying N at a range of particle Reynolds numbers in the laminar flow regime. In the first stage of research, flow development was simulated in three-dimensional packed beds of spheres. Then, the reactor wall was sectioned to include a boundary condition of pure methane, from which the solute could laterally disperse into the bed. In the second stage, a two-dimensional representation of the bed was created using the commercial Finite Element Analysis software COMSOL Multiphysics. In these models, axial velocity profiles and radial methane concentration profiles taken from the 3-D models were supplied, and a fitting procedure by use of the Levenberg-Marquardt Least-Squares optimization algorithm was completed to fit radial dispersion coefficients and near-wall mass transfer coefficients to the CFD data. These optimization runs were conducted for all N at a number of bed depths in each case. Two sub-studies were conducted in which a constant velocity profile and a local velocity profile were supplied to the 2-D model, and the optimization re-run. It was found that this two parameter model did not fully account for various mechanisms of dispersion in the bed, namely the increasing rate of dispersion from the tube wall boundary layer up to the bed center, but only accounted for a diffusive-dispersion at the wall and a constant-rate, convective-dispersion everywhere else in the bed. Length dependency of dispersion coefficients were also noted, particularly in the developing sections of the bed. Nevertheless, the combined CFD and optimization procedure proved to be an accurate and time-efficient procedure for the derivation of dispersion coefficients, which may then lend themselves to the standard design of packed bed reactors.
80

Computational Fluid Dynamics Simulations of Radial Dispersion in Low N Fixed Bed Reactors

Medeiros, Nicholas J 20 April 2015 (has links)
Fixed bed reactors are widely applicable in a range of chemical process industries. Their ease of use and simplified operation make them an attractive and preferred option in reactor selection, however the geometric complexities within the bed as a result of the unstructured packing has made the design of such beds historically based on pseudo-homogenous models together with correlation-based transport parameters. Low tube-to-particle diameter ratio (N) beds, in particular, are selected for highly exothermic or endothermic reactions, such as in methane steam reforming or alkane dehydrogenation. Due to the large fraction of tube to catalyst particle contact in these low N beds, wall effects induce a mass transfer boundary layer at the wall, and in the case of thermal beds, a simultaneous resistance to heat transfer. Computational Fluid Dynamics (CFD) has been shown to be an accurate tool for experimental validation and predictive analysis of packed beds, and may be used to derive more accurate design parameters for fixed bed reactors. More specifically, the elucidation of dispersion, or the transport of reactant and product within the bed due to molecular diffusion and convective flow is of fundamental interest to the design of fixed beds. Computational Fluid Dynamics was used in this research to study solute dispersion in eight beds of varying N at a range of particle Reynolds numbers in the laminar flow regime. In the first stage of research, flow development was simulated in three-dimensional packed beds of spheres. Then, the reactor wall was sectioned to include a boundary condition of pure methane, from which the solute could laterally disperse into the bed. In the second stage, a two-dimensional representation of the bed was created using the commercial Finite Element Analysis software COMSOL Multiphysics. In these models, axial velocity profiles and radial methane concentration profiles taken from the 3-D models were supplied, and a fitting procedure by use of the Levenberg-Marquardt Least-Squares optimization algorithm was completed to fit radial dispersion coefficients and near-wall mass transfer coefficients to the CFD data. These optimization runs were conducted for all N at a number of bed depths in each case. Two sub-studies were conducted in which a constant velocity profile and a local velocity profile were supplied to the 2-D model, and the optimization re-run. It was found that this two parameter model did not fully account for various mechanisms of dispersion in the bed, namely the increasing rate of dispersion from the tube wall boundary layer up to the bed center, but only accounted for a diffusive-dispersion at the wall and a constant-rate, convective-dispersion everywhere else in the bed. Length dependency of dispersion coefficients were also noted, particularly in the developing sections of the bed. Nevertheless, the combined CFD and optimization procedure proved to be an accurate and time-efficient procedure for the derivation of dispersion coefficients, which may then lend themselves to the standard design of packed bed reactors.

Page generated in 0.0287 seconds