Predicting the dissolution rates of immiscible contaminants in groundwater is crucial for developing environmental remediation strategies, but quantitative modeling efforts are inherently subject to multiple uncertainties. These include unknown residual amounts of non-aqueous phase liquids (NAPL) and source zone dimensions, inconsistent historical monitoring of contaminant mass discharge, and the mathematical simulation of field-scale mass transfer processes. Effective methods for simulating NAPL dissolution must therefore be able to assimilate a variety of data through physical and scalable mass transfer parameters to quantify and reduce site-specific uncertainties. This investigation coupled upscaled and numerical mass transfer modeling with uncertainty analyses to understand and develop data-assimilation and parameter-scaling methods for characterizing NAPL source zones and predicting depletion timeframes. Parameters of key interest regulating kinetic NAPL persistence and contaminant fluxes are residual mass and saturation, but neither can be measured directly at field sites. However, monitoring and characterization measurements can constrain source zone dimensions, where NAPL mass is distributed. This work evaluated the worth of source zone delineation and dissolution monitoring for estimating NAPL mass and mass transfer coefficients at multiple scales of spatial resolution. Mass transfer processes in controlled laboratory and field experiments were analyzed by simulating monitored dissolved-phase concentrations through the parameterization of explicit and lumped system properties in volume-averaged (VA) and numerical models of NAPL dissolution, respectively. Both methods were coupled with uncertainty analysis tools to investigate the relationship between data availability and model design for accurately constraining system parameters and predictions. The modeling approaches were also combined for reproducing experimental bulk effluent rates in discretized domains, explicitly parameterizing mass transfer coefficients at multiple grid scales. Research findings linked dissolved-phase monitoring signatures to model estimates of NAPL persistence, supported by source zone delineation data. The accurate characterization of source zone properties and kinetic dissolution rates, governing NAPL longevity, was achieved by adjusting model parameterization complexity to data availability. While multistage effluent rates accurately constrained explicit-process parameters in VA models, spatially-varying lumped-process parameters estimated from late dissolution stages also constrained unbiased predictions of NAPL depletion. Advantages of the numerical method included the simultaneous assimilation of bulk and high-resolution monitoring data for characterizing the distribution of residual NAPL mass and dissolution rates, whereas the VA method predicted source dissipation timeframes from delineation data alone. Additionally, comparative modeling analyses resulted in a methodology for scaling VA mass transfer coefficients to simulate NAPL dissolution and longevity at multiple grid resolutions. This research suggests feasibility in empirical constraining of lumped-process parameters by applying VA concepts to numerical mass transfer and transport models, enabling the assimilation of monitoring and source delineation data to reduce site-specific uncertainties. / Doctor of Philosophy / Predicting the dissolution rates of immiscible contaminants in groundwater is crucial for developing environmental restoration strategies, but quantitative modeling efforts are inherently subject to multiple uncertainties. These include unknown mass and dimensions of contaminant source zones, inconsistent groundwater monitoring, and the mathematical simulation of physical processes controlling dissolution rates at field scales. Effective simulation methods must therefore be able to leverage a variety of data through rate-limiting parameters suitable for quantifying and reducing uncertainties at contaminated sites. This investigation integrated mathematical modeling with uncertainty analyses to understand and develop data-driven approaches for characterizing contaminant source zones and predicting dissolution rates at multiple measurement scales. Parameters of key interest regulating the lifespan of source zones are the distribution and amount of residual contaminant mass, which cannot be measured directly at field sites. However, monitoring and site characterization measurements can constrain source zone dimensions, where contaminant mass is distributed. This work evaluated the worth of source zone delineation and groundwater monitoring for estimating contaminant mass and dissolution rates at multiple measurement scales. Rate-limiting processes in controlled laboratory and field experiments were analyzed by simulating monitored groundwater concentrations through the explicit and lumped representation of system properties in volume-averaged (VA) and numerical models of contaminant dissolution, respectively. Both methods were coupled with uncertainty analysis tools to investigate the relationship between data availability and model design for accurately constraining system parameters and predictions. The approaches were also combined for predicting average contaminant concentrations at multiple scales of spatial resolution. Research findings linked groundwater monitoring profiles to model estimates of contaminant persistence, supported by source zone delineation data. The accurate characterization of source zone properties and contaminant dissolution rates was achieved by adjusting model complexity to data availability. While monitoring profiles indicating multi-rate contaminant dissolution accurately constrained explicit-process parameters in VA models, spatially-varying lumped parameters estimated from late dissolution stages also constrained unbiased predictions of source mass depletion. Advantages of the numerical method included the simultaneous utilization of average and spatially-detailed monitoring data for characterizing the distribution of contaminant mass and dissolution rates, whereas the VA method predicted source longevity timeframes from delineation data alone. Additionally, comparative modeling analyses resulted in a methodology for scaling estimable VA parameters to predict contaminant dissolution rates at multiple scales of spatial resolution. This research suggests feasibility in empirical constraining of lumped parameters by applying VA concepts to numerical models, enabling a comprehensive data-driven methodology to quantify environmental risk and support groundwater cleanup designs.

Bubble columns are multiphase contactors, which are often used for the operation of multiphase reactions. These reactors are characterized by large interfacial areas, intense mixing in the column and dynamic flow structures. For the design of bubble columns numerous empirical correlations for hydrodynamic and mass transfer parameters exist, which however, only cover a specific design range, e.g. column diameter and reaction system. Furthermore, they are often based on integral measurements. High deviations occur especially at moderate and high gas holdup, since fluid dynamics and mass transfer are essentially determined by local effects, which has received little attention so far due to lack of appropriate measurement techniques. For this reason the design of bubble columns is carried out with considerable safety margins. Therefore, bubble columns have a high potential for savings in terms of efficiency and operating costs if the unterstanding of the hydrodynamics and mass transfer in bubble columns is improved. The fluid dynamics in bubble columns are essentially characterized by the bubble size distribution and with increasing gas holdup by enhanced bubble interaction, which leads to intense motion in the liquid phase. A cross-scale study of the fluid dynamics is necessary to better understand the factors influencing the global fluid dynamics and to optimize the design of bubble columns. The major limitations in the experimental investigation of dense bubbly flow are the available measurement techniques. As the fraction of the dispersed phase increases, the optical accessibility decreases and limits the selection of appropriate measurement techniques. This work deals with the experimental study of fluid dynamics and mass transfer with chemical reaction using tomographic measurement techniques. The experiments were performed in a cylindrical bubble column with 0.10 m inner diameter. The gas phase fluid dynamics were studied by means of ultrafast X-ray computed tomography (UFXCT). Additionally, the minimally intrusive wire-mesh sensor was qualified to assess chemical species concentrations during chemical absorption of CO2 and to derive local mass transfer related parameters. Among the global description of the fluid dynamics also local parameters of the dispersed phase were extracted. Advanced post-processing algorithms for ultrafast X-ray CT data allow a more accurate determination of the Sauter diameter and interfacial area due to the extraction of the bubbles surface area. The interaction of bubbles within dense bubbly flow was studied by determining and evaluating distance parameters of the bubbles. Based on that, the near order of bubbles was studied and conclusions on their preferential arrangement in dense bubbly flows were derived. Furthermore, the fluid dynamics and mass transfer were studied in the presence of a chemical reaction. For this purpose experiments on the chemical absorption of CO2 in sodium hydroxide solution were performed at various gas flow rates and initial pH values of the solution. The change of the fluid dynamics and the conversion of the chemical species is analyzed for different experimental conditions. A core topic of the present work is the determination of concentration of a chemical species during chemical absorption of CO2. Within this work, the wire-mesh sensor was qualified for this new field of application and was used to determine cross-sectional data of the species conversion. Through combined use of the wire-mesh sensor and ultrafast X-ray CT an extensive data base on the chemical absorption of CO$_2$ was obtained, which can be used for numerical validation of bubbly flows with gas holdups < 0.17 / Blasensäulen sind in der chemischen Industrie sehr häufig genutzte Kontaktapparate für die Durchführung von Mehrphasenreaktionen. Sie zeichnen sich durch eine hohe Gas-Flüssig-Aus-tauschfläche, starke Vermischung und dynamische Strömungsstrukturen aus. Bisher basiert die Auslegung von Blasensäulen vorrangig auf empirischen Korrelationen. Hohe Abweichungen dieser Korrelationen treten vor allem bei moderaten und hohen Gasgehalten auf, da die Fluiddynamik und der Stofftransport wesentlich durch lokale Effekte bestimmt sind, die bisher aufgrund fehlender Messtechnik nur wenig Beachtung finden. Aus diesem Grund erfolgt die bisherige Auslegung von Blasensäulenreaktoren mit erheblichen Sicherheitszuschlägen. Daraus lässt sich ableiten, dass Blasensäulen in Bezug auf deren Effizienz und die Höhe der Betriebskosten ein enormes Einsparungspotential aufweisen, wenn das Verständnis von Hydrodynamik und Stofftransport in Blasensäulen verbessert wird. Die Fluiddynamik in Blasensäulen wird wesentlich durch die Blasengrößenverteilung und bei steigendem Gasgehalt durch die durch zunehmende Blaseninteraktion induzierte Flüssigkeitsbewegung charakterisiert. Eine skalenübergreifende Untersuchungen der Fluiddynamik ist erforderlich, um die Einflussfaktoren der globalen Fluiddynamik besser verstehen zu können und die Auslegungsgleichungen optimieren zu können. Die größte Limitierung der experimentellen Untersuchung von dichten Blasenströmung stellt die verfügbare Messtechnik dar. Mit zunehmenden Anteil der Dispersphase nimmt die optische Zugänglichkeit ab und schränkt die Möglichkeiten bei der Auswahl geeigneter Messtechniken ein. Diese Arbeit beschäftigt sich mit der experimentellen Untersuchung der Fluiddynamik und des Stofftransports bei chemischer Reaktion mit Hilfe tomographischer Messverfahren. Die Experimente wurden in einer zylindrischen Blasensäule mit 0,10 m Durchmesser durchgeführt. Die Fluiddynamik der Gasphase wurde mit Hilfe der ultraschnellen Röntgen-Computertomographie (UFXCT) untersucht. Zusätzlich wurde der minimal-intrusive Gittersensor qualifiziert, um die Konzentrationen einer relvanten chemischen Spezies während der chemischen Absorption von CO2 zu bestimmen und stofftransportbezogene Parameter auf lokaler Ebene berechnen. Neben der räumlich und zeitlich gemittelten Beschreibung der Fluiddynamik wurden auch lokale Parameter der dispersen Phasen bestimmt. Fortgeschrittene Auswertungsalgorithmen ermöglichen eine genauere Berechnung des Sauterdurchmessers und der Phasengrenzfläche durch direkte Bestimmung der Blasenoberfläche. Die Interaktion von Blasen in der dichten Blasenströmung wurde durch die Berechnung von Abstandsparametern untersucht. Dies ermöglicht die Analyse der Nahordnung von Blasen und lässt damit Rückschlüsse auf deren bevorzugte Anordnung in dichten Blasenströmungen zu. Darüber hinaus wurden die Fluiddynamik und zum Stofftransport in Gegenwart einer chemischen Reaktion untersucht. Dazu wurden Experimente zur chemischen Absorption von CO2 n Natronlauge in der Blasensäule bei verschiedenen Gasdurchsätzen und unterschiedlichen initialen pH-Werten der Lösung durchgeführt. Die Veränderung der Fluiddynamik der Gasphase und die Umwandlung der chemischen Spezies wurden unter verschiedenen Betriebsbedingungen analysiert. Ein Kernthema der Arbeit stellt die Erfassung der Konzentration einer chemischen Spezies während der chemischen Absorption dar. Dazu wurde im Rahmen dieser Arbeit der Gittersensor für dieses neue Anwendungsgebiet qualifiziert und eingesetzt, um querschnittsaufgelöste Daten über den Verbrauch der charakteristischen Spezies zu ermitteln. Durch den kombinierten Einsatz der Gittersensormesstechnik und der ultraschnellen Röntgentomographie wurden umfangreiche Daten zur Hydrodnamik und zum Stofftransport während der chemischen Absorption von CO2 erhalten. Diese stellen eine umfassende Datenbasis zur numerischen Validierung von Blasenströmungen mit Gasgehalten bis zu 17% dar.

[pt] Foi estudado a subida de bolhas de ar formadas diretamente dentro de uma camada de água destilada disposta a baixo de um fluido Newtoniano superior imiscível. Modificou-se as propriedades do fluido superior, obtendo-se seis fluidos diferentes e foi verificado a diferentes e foi verificado a diferença de comportamento de subida da bolha de ai em cada um desses fluidos. Notaram-se três tipos de comportamento durante ascensões em função do tamanho de bolha: a) Bolhas pequenas: ficam retidas algum tempo na interface para subir em seguida sem carregar nenhuma película de água da fase mais densa. Limite: D menor que 3,4 mais ou menos 0,2 mm mais ou menos 0,2 mm. b) Bolhas médias atravessam a interface líquida e carregam uma película da fase mais densa envolvendo a bolha de ar até a superfície livre do líquido superior, que não deve estar a uma altura inferior a 50 D, de maneira que nos assegure estar a superfície livre na faixa de deslocamento permanente. Limite: 3,4 mais ou menos 0,2 mm menor que D menor que 5,5 mais ou menos 0,3 mm c) Bolhas grandes: atravessam a interface e carregam a película até uma certa altura, quando se desprende da bolha de ar. Limite: D maior que 5,5 mais ou menos 0,3 mm Os limites encontrados independiam da viscosidade do meio superior. Para todos os líquidos foram os mesmos. Fixou-se mais objetivamente o estudo sobre os comportamentos b e c, e foi verificada a grande influência que a película de água exerce sobre a bolha. A modificação do valor da viscosidade tem uma influência sobre (a) a forma pela qual as bolhas atravessam a interface; (b) a forma pela qual as bolhas carregam a a película de água e (c) o destacamento entre a bolha e a película de água. Para fluido de lata viscosidade, a passagem é lenta e forma uma coluna líquida que se quebra em gotículas e o destacamento da película de água e suave. Para líquidos de pequena viscosidade o comportamento é oposto. A passagem pela interface é abrupta e o destacamento é violento. As curvas criadas para descrever a altura em que ocorria o desprendimento em relação ao diâmetro das bolhas para cada um dos líquidos superiores estudados, mostram que para um mesmo diâmetro, a diminuição da viscosidade causa um desprendimento a uma altura maior. Para uma mesma viscosidade de (mesmo líquido) o aumento do diâmetro corresponde a uma menor altura de desprendimento entre a bolha de ar e a película de água. As experiências feitas para medir a transferência de massas de água pela bolha de ar permitiram que verificássemos que uma quantidade maior de água é transferida quando o líquido superior tem alta viscosidade. Da mesma maneira o aumento de diâmetro de bolha implica em um aumento de transferência de massa. / [en] The rising of air bubbles released in distilled water beneath na imiscible Newtoniam fluid was studied in an imiscible Newtonian fluid was fluid ws studied in this work. The properties of the upper fluid were modified and rising of the bubble in each of the six different fluids was noted. There were thereee types of behavior witch were a function of the bubbles diameter: a)Small size bubbles: remained temporary ar the interface and then rise upper phase without any perceptible water element envelopping them Limit: D less than 3,4 more or less 0,2 mm b)Medium size bubbles: passes directly through the liquid-liquid interface and now enveloped with a waer layer of variable thikness rise in the upper phase and reach the free surface with the coating intact. The medium size bubbles rise at a distance of over 50D. Limit: 3,4 more or less 0,2 mm less than D less than 5,5 more or less 0,3 mm c)Large size bubbles: passed directly trrough the liquid interface. At a certain height above the interface the water layer datached from the air bubbles. Limit: D more than 5,5 more or less 0,2 mm The critical diameters were found to depend on the viscosity of the upper phase. This study was restrict to cases b and c where the large influence of the envelopping water layer datached from the air bubble. This study was restrict to cases b and c where the large influence of the envelopping water layer over the air bubble was observed. The change of the visconsity value has an influence on : (a) the way in which the bubble and the water layer and; (b) the way in which the bubble lift the the water layer and (c) the detachment between the bubble and the water layer. For fluid of high viscosity, the passage is slow and makes a liquid colimn that breakes into passage is slow and makes a liquid column that breakes into droplets and the detachmenr of the water layer is smooth. For liquid os samall viscosity this transitional behavior is the oppoite. The detachment is violent. The curves which describe the height of detachment as function of the diameter, for each one fo the upper liquids show that: for a given diameter also have a inverse variation. The investigation made to measure the water mass transfer phenomena caused by the bubbles showed that the mass transfer will be larger for the higher viscosity fluides and the larger diameter bubbles.

<p>Kyle Weideman PhD Thesis</p>

This work was concerned with the determination of mass transfer and kinetic parameters of ozone reactions with four organic compounds from different families, namely reactive dye RO16, triclocarban, naphthalene and methanol. In order to understand the mechanisms of ozone reactions with the organic pollutants, a radical scavenger (t-butanol) was used and the pH was varied from 2 to 9. Ozone solubility (CAL*) is an important parameter that affects both mass transfer rates and chemical reaction kinetics. In order to determine accurate values of the CAL* in the current work, a set of experiments were devised and a correlation between CAL* and the gas phase ozone concentration of the form CAL*(mol/L) = 0.0456 CO3 (g/m3 NTP) was obtained at 20°C. This work has also revealed that t-butanol did not only inhibit hydroxyl radical reactions but also increased mass transfer due to it increasing the specific surface area (aL). Values of the aL were determined to be 2.7 and 3.5 m2/m3 in the absence and presence of t-butanol respectively. It was noticed that the volumetric mass transfer coefficient (kLa) has increased following the addition of t-butanol. Ozone decomposition was studied at pH values of 2 to 9 in a 500 mL reactor initially saturated with ozone. Ozone decomposition was found to follow a second order reaction at pH values less than 7 whilst it was first order at pH 9. When the t-butanol was added, the decomposition of ozone progressed at a lower reaction order of 1.5 for pH values less than 7 and at the same order without t-butanol at pH 9. Ozone decomposition was found significant at high pHs due to high hydroxide ion concentration, which promotes ozone decomposition at high pHs. The reaction rate constant (k) of RO16 ozonation in the absence of t-butanol was determined. The result suggests that RO16 degradation occurs solely by molecular ozone and indirect reactions by radicals are insignificant. The chemical reaction of triclocarban with ozone was found to follow second order reaction kinetics. The degradation of naphthalene using the liquid/gas-ozone (LGO) system was studied. This result showed that hydroxyl radicals seemed to have limited effect on naphthalene degradation which was also observed when a radical scavenger (t-butanol) was used. Reaction rate constants were calculated and were found around 100 times higher than values reported in the literature due to differences in experimental conditions. From the results of the experimental investigation on the degradation of methanol by ozone it was found that the rate constant (k) of the degradation reaction increased at pH 9. The reaction stoichiometry was found to have a value of 1 mol/mol. The two steps of the liquid/solid-ozone (LSO) system were studied on beds of silica gel and a zeolitic material (D915) and the ozone adsorption process was modeled and found that particle rate controls ozone adsorption step but liquid rate controls the water treatment step. Ozone desorption with pure deionised water was studied. The water flow rate was found to accelerate the desorption rates but pH was found to decrease the desorption rates. In contrast, the effect of pH was insignificant in the presence of t-butanol. Determination of the adsorption isotherms for RO16, naphthalene and methanol revealed that RO16 did not exhibit adsorption on silica gel, but both naphthalene and methanol showed adsorption on D915 described by Langmuir model. / Education Service Department of the Libyan Government

<p> An investigation of the technique of obtaining the wall shear stress in a two-phase flow, by measuring the mass transfer coefficient at the wall with the electrochemical method, has been completed.</p> <p> The experiments involved the measurement of flow rates, pressure drops, void fractions and mass transfer coefficients, for a cocurrent upwards gas-liquid flow in a vertical tube, 13 mm in diameter. The liquid phase was an electrolyte consisting of 1.0 to 3.0 molar sodium hydroxide, and 0.005 to 0.010 equimolar potassium ferricyanide and potassium ferrocyanide. The gas phase was nitrogen. The flow regimes studied were slug, churn and annular.</p> <p> Emphasis is placed on the measurements obtained with the electrochemical method. Its application, advantages and disadvantages are detailed. A series of single-phase experiments were performed to explore the characteristics of the method and to serve as benchmarks for the two-phase experiments.</p> <p> The space-time-averaged values of the mass transfer coefficient were found to give the wall shear stresses to an accuracy of ±20%. Frequency analysis of the local fluctuating values indicate that measurements of the local mass transfer coefficient can be used for flow regime identification.</p> <p> The theoretical flow regime map of Dukler and Taitel successfully predicted the flow regimes. The correlations of Griffith and Wallis, and Lockhart and Martinelli as modified by Davis, predicted the pressure drops and void fractions to an accuracy ±15% when applied to the appropriate flow regimes. As a further exercise, the force interactions between the phases, referred to as the interfacial shear terms, were calculated from both the measured and predicted void fractions and pressure drops.</p> / Thesis / Master of Engineering (MEngr)

<p> Using a single bubble suspended in a liquid flow regime, the effects of velocity, bubble size, and surfactant levels on the gas transfer process across the bubble interface were investigated. Mass transfer data reported in the literature for non-circulating carbon dioxide bubbles was verified. A mathematical model predicting the mass transfer process for the single bubble system used in this study was formulated and this model provided a reasonable fit for experimental data obtained for the dissolution of a carbon dioxide bubble into an aqueous solution of a second sparingly soluble gas. The fate of a hypothetical air bubble in an aerator was briefly considered.</p> / Thesis / Master of Engineering (MEngr)

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