Orientador: Marco Aurélio Cremasco / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química / Made available in DSpace on 2018-08-21T17:25:48Z (GMT). No. of bitstreams: 1 VergelZapata_CarlosFernando_M.pdf: 3787074 bytes, checksum: c52cfd4ba974066436ff88b0a07c8b76 (MD5) Previous issue date: 2012 / Resumo: Os modelos matemáticos são utilizados como instrumentos de suporte operacional para melhorar e facilitar a otimização de um processo. No presente trabalho foi proposto um modelo simplificado para determinar a concentração de ozônio na água destilada por meio de um processo de ozonização em uma coluna de borbulhamento, em condições isotérmicas (21°C) e isobáricas (pressão atmosférica). O modelo foi baseado em uma cinética de reação de segunda ordem para a decomposição de ozônio na fase líquida. Para o desenvolvimento da modelagem a resistência do gás à transferência de massa e os efeitos da dispersão para as duas fases (gasosa-líquida) foram considerados desprezíveis. Além disso, assumiu-se composição constante de ozônio na fase gasosa. O modelo foi resolvido segundo as condições de contorno do processo, pelo método numérico Runge-Kutta de quarta ordem com a ajuda do software Matlab 7.0. Durante o processo de ozonização estudado o pH da solução não foi mantido constante, portanto no presente trabalho foi proposto uma cinética de reação de primeira ordem para a variação do pH, a qual representou satisfatoriamente. Diante dos resultados, observou-se que um aumento na concentração de ozônio na fase gasosa como condição inicial do processo e uma diminuição do pH da solução ao longo do tempo aumenta a concentração de ozônio na fase líquida. As distribuições de concentração de ozônio na água destilada obtidos da modelagem apresentaram concordância com os dados experimentais encontrados na literatura / Abstract: Mathematical models are used as operational support tools which improve and facilitate processes optimization. In this work, it was suggested a simplified model to determine the ozone concentration in distillated water, through an ozonation process in a bubble column under isothermal (21°C) and isobaric (atmospheric pressure) conditions. The model was based in second-order reaction kinetics of ozone decomposition in the liquid phase. For the modeling development, gas resistance to mass transfer and the dispersion effects for both phases (liquid-gas), were considered negligible. In addition, it was assumed a constant composition of ozone in the gaseous phase. The model was solved according the boundary conditions of the process, through the fourth-order Runge-Kutta numerical method supported on the software Matlab 7.0. During the ozonation process studied, the solutions pH wasn't maintained constant, therefore, it was proposed a first-order kinetics of reaction for the pH variation, which successfully represents. The results showed that an increase in the concentration of ozone in the gaseous phase as initial condition and a decrease of pH of solution through time increases the liquid phase ozone concentration. The ozone concentration distributions obtained of the modeling in distilled water, present agreement with the experimental data found in literature / Mestrado / Engenharia de Processos / Mestre em Engenharia Química

[pt] Neste trabalho é feita uma análise teórica e numérica da evaporação não isotérmica de um líquido contido em um recipiente cilíndrico parcialmente cheio, com paredes adiabáticas. Postula-se que a evaporação acontece em presença de convecção natural impulsionada por diferenças de massa específica, associadas com gradientes de temperatura e composição da mistura. Esta consiste de um gás e o vapor do líquido. Embora a formulação seja geral, o presente trabalho focaliza a evaporação de água para o ar. Estudou-se três situações. Um caso isotérmico, variante do clássico problema de difusão de Stefan, um Caso em que a temperatura do líquido é maior do que a temperatura ambiente e um terceiro caso no qual a temperatura do líquido é menor do que a do ambiente. Duas diferentes condições de contorno foram usadas na abertura do recipiente de modo a explorar a sensibilidade do escoamento às condições no topo. A distância entre a superfície do líquido e o topo variou de duas a dez vezes o raio do recipiente. Duas diferenças de temperatura entre o líquido e o ambiente foram investigadas, 3 graus Celsius e - 2 graus Celsius. O ar ambiente foi considerado como sendo muito seco ou muito úmido. Encontrou-se que, quando a temperatura do líquido é maior do que a temperatura ambiente, a taxa de evaporação alcança valores até quatro vezes maiores do que para o caso isotérmico. Para o caso em que a temperatura do líquido é menor do que a temperatura ambiente, a taxa de evaporação decresce para valores até duas vezes menores do que para o caso isotérmico. / [en] This work reports a theoretical and numerical analysis of the non-isothermal evaporation of a liquid contained in a partially filled cylinder vessel, with adiabatic walls. It is assumed that the evaporation occurs in the presence of natural convection driven by differences in specific mass associated with gradient of temperature and mixture composition. The mixture consist of a gas and the vapor of the evaporating liquid. Although the formulation is general, the specific focus of the present work is on the evaporation of water into air. Three situations were studied. An isothermal case, which is a variant of the classical Stefan diffusion problem, a case where the liquid temperature is higher than the ambient temperature, and a third case in which the liquid temperature is lower than the ambient. Two different boundary conditions were used at the openning of the vessel in a way to explore the sensitivity of the flow to the conditions on the top. The distance between the liquid surface and the top of the vessel varied from two to ten times the vessel radius. Two temperature differences between the liquid and the ambient were investigated, 3 degrees Celsius and - 2 degrees Celsius. The environmental air was considered to be either very dry or very wet. It was found that, when the liquid temperature is higher than the ambient temperature, the rate of evaporation can reach values up to four times larges than that for the isothermal case. For the case where the liquid temperature. is lower than the ambient temperature, the rate of evaporation decreases to values down to half of theisothermal case.

Recent developments have indicated high oxygen consumption rates of about 35 g-mole/m³-min during oxidative pressure leaching. At such high oxygen consumption rates the mass transfer of dissolved oxygen at the gas-liquid interface may become rate-limiting. The objective of this study was to obtain an understanding of the gas-liquid mass transfer processes that take place in mechanically agitated pressure leaching systems. The classical reaction between sodium sulphite and dissolved oxygen to form sulphate at atmospheric pressure was used to determine the oxygen mass transfer rates in a 200-liter asymmetrical plastic tank, modelled after the shape of the first compartment of the zinc pressure leach. The effect of this asymmetry was compared with the work of Swiniarski who used a cylindrical symmetrical tank of similar volume. A number of process variables such as the impeller type and size, the impeller speed, the impeller immersion depth and the effect of full baffles that affect mixing were investigated. Also, the volumetric power consumption associated with the mass transfer rates were measured. The results indicate that the asymmetrical tank is at least 3.6 times more efficient in mass transfer than the symmetrical tank. There is a critical speed below which the mass transfer parameter, K[formula omitted], is almost zero and above which K[formula omitted] increases almost linearly with impeller tip speed. A simple energy balance model for bubble creation can predict the critical tip speed. It is shown that K[formula omitted] is enhanced at shallow depths, with a corresponding high mass transfer to energy ratio. The relative effectiveness of impeller types and sizes with regard to the use of power for gas-liquid mass transfer was established. Full baffles degrade the mass transfer rate at increased depth of impeller immersion. The results also add substantial support to the findings provided by DeGraaf [5] that: (i) The dimensionless correlations used in liquid mixing systems do not accurately predict dispersion rates by agitators. (ii) The optimum conditions for gas dispersion and the consequent generation of gas-liquid interfacial area are different from fluid mixing. (iii) The classical mixing power equations for impellers markedly overestimate power requirements during impeller gas dispersion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The mass and thermal transport in porous media play an important role in many engineering and geological processes. The hydrodynamic and thermal effects are two interesting aspects arising in the research of porous media. This thesis is concerned with numerical investigations of double-diffusive natural convective heat and mass transfer in saturated porous cavities with Soret and Dufour effects. An in-house FORTRAN code, named ALFARHANY, was developed for this study. The Darcy-Brinkman-Forchheimer (generalized) model with the Boussinesq approximation is used to solve the governing equations. In general, for high porosity (more than 0.6), Darcy law is not valid and the effects of inertia and viscosity force should be taken into account. Therefore, the generalized model is extremely suitable in describing all kinds of fluid flow in a porous medium. The numerical model adopted is based on the finite volume approach and the pressure velocity coupling is treated using the SIMPLE/SIMPLER algorithm as well as the alternating direction implicit (ADI) method was employed to solve the energy and species equations. Firstly, the model validation is accomplished through a comparison of the numerical solution with the reliable experimental, analytical/computational studies available in the literature. Additionally, transient conjugate natural convective heat transfer in two-dimensional porous square domain with finite wall thickness is investigated numerically. After that the effect of variable thermal conductivity and porosity investigated numerically for steady conjugate double-diffusive natural convective heat and mass transfer in two-dimensional variable porosity layer sandwiched between two walls. Then the work is extended to include the geometric effects. The results presented for two different studies (square and rectangular cavities) with the effect of inclination angle. Finally, the work is extended to include the Soret and Dufour effects on double-diffusive natural convection heat and mass transfer in a square porous cavity. In general, the results are presented over wide range of non-dimensional parameters including: the modified Rayleigh number (100 ≤ Ra* ≤ 1000), the Darcy number (10-6 ≤ Da ≤ 10-2), the Lewis number (0.1 ≤ Le ≤ 20), the buoyancy ratio (-5 ≤ N ≤ 5), the thermal conductivity ratio (0.1 ≤ Kr ≤ 10), the ratio of wall thickness to its height (0.1 ≤ D ≤ 0.4), the Soret parameter (-5 ≤ Sr ≤ 5), and the Dufour parameter (-2 ≤ Df ≤ 2).

A study is made of the simultaneous heat and mass transfer processes which occur within a finite wood cylinder of circular cross-section when it is convectively dried. Governing transport equations are developed allowing for the functional dependence of both thermal and moisture diffusivities on moisture content and temperature. Derivative type boundary conditions are included in the analysis. The equations are formulated using a cylindrical coordinate system because it is well suited to modelling wood's anisotropy. In the case considered, the axial coordinate direction is aligned with the wood-grain. Due to the coupling and non-linearity present in the transport and boundary equations, an implicit finite-difference solution scheme is formulated. The three-time-level scheme uses an equation splitting technique to simplify its solution on the computer. A mathematical wood-model, as available in the literature, is refined and used to determine moisture and thermal diffusivities, and mass transfer boundary conditions. Results from this wood-model apply to softwoods below the fiber-saturation moisture content. Combined diffusivities of liquid and vapour are calculated for the radial and axial directions and results for the radial direction are compared to those found from diffusion experiments. Desorptional isotherms are used in the mass transfer boundary condition equations to relate surface humidity and moisture content. The wood-model is used in the mass transfer equation to determine wood drying behaviour under isothermal conditions. Initial moisture contents are uniform and equal to the fiber-saturation value. Solutions are presented to give local and average moisture content as a function of time as well as moisture content profiles. The effect of drying temperature was investigated for one case of wood density and shrinkage. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

With the increasing pressure of providing reliable potable water in a sustainable way, it is important to understand water phase change phenomena (condensation and evaporation) as the water phase change is involved in many processes such as membrane distillation and solar still which can be a feasible choice of supplementing the present potable water access. In the present thesis, we first elucidate the role of wettability of water condensation substrate by combining the droplet growth dynamics and droplet population evolution. The results show that wettability has a negligible effect on water condensation rate in an atmospheric environment. After confirming the role of substrate wettability, we provide a quantitative analysis of the effect of substrate geometry on water condensation in the atmospheric environment. The analysis can help to predict the efficiency of water condensation rate with a given substrate of a certain geometry with the aid of computational simulation tools. The results show that water condensation can be increased by 40% by rationally designing the geometry of the condensation surface. However, the condensation rate in the atmospheric environment is relatively slow due to the presence of non-condensable gas. In order to increase the condensation rate, a relatively pure vapor environment is desired, in which condensed water will be the major heat transfer barrier. Coalescence induced jumping of condensed droplets on superhydrophobic surfaces is an interesting phenomenon to help faster removal of condensed droplets. However, it is still not clear how to optimize the overall heat transfer efficiency by condensation on such surfaces. We observed an interesting phenomenon on a superhydrophobic nano-cones array, on which water preferentially condenses within larger cavities among the nanocones. Droplets growing form larger cavities have larger growth rate. This finding can possibly provide a solution to optimizing heat transfer efficiency. Finally, a nylon-carbon black composite is prepared by electrospinning to enhance water evaporation under solar radiation. The composite shows an interesting light absorption property. In a wet state, the composite can absorb around 94% of the incident sunlight. The composite also shows strong mechanical and chemical stability. Thus, the composite is considered to be a practical candidate to be applied in the solar distillation process.

Carbon dioxide serves as co-substrate in the production of succinic acid by Actinobacillus succinogenes. The transient concentration of dissolved CO2 in the broth (CCO2) controls the uptake of CO2 in the cell. Based on CCO2 , three distinct regimes could be identified in which the behaviour of the organism differed with CCO2 availability. When CCO2 was higher than 8.4 mM (44.4% saturated at an atmospheric pressure of 86 kPa), there was no evidence of CO2 limiting succinic acid productivity and flux to succinic acid remained constant. When CCO2 decreased below 8.4 mM a decrease in the succinic acid production and glucose consumption rates was observed to 28.01% and 19.89% of their original value respectively, at the lowest CCO2 value investigated. Below a CCO2 of 4 mM (21.16% saturated at an atmospheric pressure of 86 kPa), the productivity continued to decrease along with a shift in the total carbon flux from the succinic acid-producing pathway (C4-pathway) to the by-product-producing pathway (C3-pathway). The fraction of total carbon flux directed to the C4-pathway decreased from 0.48 to 0.33 at the lowest CCO2 value investigated. Although the by-product acetic acid concentration decreased to 88% of the original value, formic acid remained relatively stable and the ethanol concentration increased from an average of 0.26 g.L-1 to 1 g.L-1. The organism starts producing ethanol in order to satisfy the redox balance when the C4-pathway becomes less active. It was calculated that the flux shift to the C3-pathway does not favour ATP production. The organism is, however, still viable at the very low ATP production rates found at very low values of CCO2. Since succinic acid production is not limited at relatively low values of CCO2 (44.4% saturation), adequate CO2 supply to the fermenter can be achieved without major CO2 sparging which is beneficial from an industrial processing perspective. / Dissertation (MEng)--University of Pretoria, 2016. / Chemical Engineering / MEng / Unrestricted

Casting is a substantial part of modern manufacturing and production, typically used in the production of aluminum alloys. The high pressure die casting process is extremely suitable for mass production. Due to the high volume, wasted time and resources during the production cycle become more significant. Aluminum die castings require the die to be at elevated temperatures to produce acceptable castings. When the inner surfaces of a die are cold, the outer shell of the casting will cool too rapidly, and solidification of the outer shell occurs before the aluminum has time to uniformly fill the cavities. Therefore, without the die being within the proper temperature range, the castings produced will have significant issues in porosity and casting incompleteness. Furthermore, stresses are introduced to the casting surfaces when warm-up shots are used to raise the temperature prior to production. In the present work, research is conducted on designing a heating method for a casting die used in the manufacturing of an automotive transmission intermediate plate. An electric, short wave infrared heating system is simple and effective for the purpose. By utilizing an electric infrared heater in combination with a flat mirror reflector, the aluminum high pressure die casting die was heated to 300 ◦C surface temperature within 30 minutes. Further research can be done to optimize heat flux distribution and minimize energy consumption.

Please read the abstract in the dissertation / Dissertation (MEng)--University of Pretoria, 2012. / gm2014 / Chemical Engineering / unrestricted

Mass transfer in a porous solid, partially saturated with asingle solvent and multicomponent liquid mixtures, has beenexperimentally and theoretically studied. A porous materialcontaining single liquids and mixtures of organic solvents wasisothermally dried. Experiments were performed using a jacketedwind tunnel, through which a humidity andtemperature-controlled air stream flowed. The wetted porousmaterial was placed in a cylindrical vessel, whose top isexposed to the air stream until the material became dried to acertain extent. Drying experiments with the single solventswater, methanol, ethanol and 2-propanol, were performed atdifferent temperatures and transient liquid content profileswere determined. In isothermal drying experiments with liquidmixtures,the transient concentration profiles of thecomponents along the cylindrical sample as well as the totalliquid content were determined. The liquid mixtures examinedwere water-methanolethanol and isopropanol-methanol-ethanol.Two different temperatures and initial compositions were usedin the experiments. Mathematical models that describe nonsteadystate isothermal drying of a solid containing single liquidsand multicomponent liquid mixtures were developed. In the solidwetted with a single liquid, capillary movement of the liquidwas the main mechanism responsible for mass transfer. In thesolid containing liquid mixtures, interactive diffusion inliquid phase was superimposed to the capillary movement of theliquid mixture. In addition, interactive diffusion of thevapours in empty pores was considered. The parameters todescribe the retention properties of the solid and thecapillary movement of the liquid were determined by comparingtheoretical and experimental liquid content profiles obtainedduring drying of the solid wetted with single liquids. Tosimulate the transport of the liquid mixtures these parameterswhere weighed according to liquid composition. A fairly goodagreement between theoretical and experimental liquidcomposition profiles was obtained if axial dispersion isincluded in the model when the moisture consists of amixture. Keywords:Internal mass transfer, capillary flow,multicomponent, diffusion, solvent mixtures