Dissertations / Theses on the topic 'Bubble column'

Includes abstract.
Includes bibliographical references (leaves 96-99).
The bubble column reactor is commonly used in industry, although the fluid dynamics inside are not well understood. The challenges associated with solving multi phase flow problems arise from the complexity of the governing equations which have to be solved, which are typically mass, momentum and energy balances. These time-dependent problems need to include effects of turbulence and are computationally expensive when simulating the hydrodynamics of large bubble columns. In an attempt to reduce the computational expense in solving bubble column reactor models, a "cell" model is proposed which predicts the velocity flow field in the vicinity of a single spherical bubble. It is intended that this model would form the fundamental building block in a macroscale model framework that does predict the flow of multiple bubbles in the whole column. The non-linear Navier-Stokes (NVS) equations are used to model fluid flow around the bubble. This study focusses on the Reynolds number range where the linear Stokes equations can be used to accurately predict the flow around the bubble. The Stokes equations are mathematically easier to solve than the NVS equations and are thus less computationally expensive. The validity of the NVS model was tested against experimental data for the flow of water around a solid sphere and was found to be in close agreement for the Reynolds number range 25 to 80. The simulation results from the Stokes flow model were compared with those from the NVS flow model and were similar at Reynolds numbers below 1. The application is then in the partitioning of the bubble column into regions governed by either Stokes or NVS equations.

Considerable progress in understand and predicting turbulent bubbly flow in bubble column reactors has been advanced over the last two decades or so using a combination of model development, computational techniques and well-designed experiments. However, there remain many modelling uncertainties mainly associated with inadequate physical prescriptions rather than numerical schemes. The present project addresses some of these questions, in particular in relation to the interactions between the deformable rising bubbles and the turbulent eddies, with the later which from liquid shear flow or in the wakes of bubbles. Recent literature on existing models and experimental studies of bubble column reactors is reviewed in Chapter 1. It appears that the correlations and phenomenal models developed from early-stage experimental studies have been implemented into CFD modelling, and in return, accelerates the developments of theoretical understandings of the flow characteristics in the bubble columns. The research efforts made from both CFD modelling and experimental studies to understand the complicated mechanisms of gas-liquid interactions have been summarised in this chapter. In chapter 2, the inlet conditions, as one of the important issues in the CFD simulations of bubble columns, have been addressed. A kinetic inlet model is proposed, which considers the effects of number and size of holes on the gas spargers, the volume flow rate, and the gas-phase velocity profile. The proposed model achieves similar accuracy as modelling the real sparger holes while the computational costs have been significantly reduced. Chapter 3 applies a CFD-PBM method to investigate the influence of various shapes of bubbles on the bubble breakage rate and bubble size distribution. Bubbles are classified into spherical, ellipsoidal and spherical-capped shapes, and explicitly calculated in the breakage kernel. The correlation of aspect ratio of ellipsoidal bubbles is developed base on dimensionless numbers, summarising the effect of buoyancy, surface tension, and viscosity. The surface energy and pressure head have been adopted as two competing breakage mechanisms with the energy density constraint has been used as the breakage criterion. The simulation results demonstrate improvements in the estimations of gas holdup, liquid velocity, and bubble size distribution, as well as strong enhancements in mass transfer prediction. The effects of the turbulent kinetic energy spectrum for the turbulent bubbly flow on the bubble breakage are considered in Chapter 4. The κ-3 power law scaling behaviour of bubble induced turbulence is considered together with the Kolmogorov -5/3 law to characterise the turbulent eddies that interact with the subsequent bubbles. A characteristic length scale Λ is used to approximately separate the shear turbulence and bubble induced turbulence. The implementation of the modified breakage model into CFD modelling shows a great improvement in the prediction of bubble breakage rate, which believes to be competitive to the results obtained from Chen et al. (2004) that has artificially increase of breakage rate by 10 times. In Chapter 5, the approaching velocities of collision bubbles that are under the influence of shear turbulence and bubble induced turbulence are clearly distinguished. The turbulence dissipation rate that strongly affects the estimation of collision time has been calculated by taking into account the turbulence generation and dissipation in the wakes of bubbles, especially considering the anisotropic feature of bubble induced turbulence in the Reynolds stress turbulence model by using extra source terms. The modified coalescence model properly addresses the coalescence rate for different sizes of binary bubble coalescence. Chapter 6 presents the experimental study of the spatial velocity fluctuations and the turbulence energy spectrum in the wakes of bubbles by using PIV and highspeed imaging techniques. The experimental results clearly demonstrate the existence of the κ-3 power law scaling region due to bubble induced turbulence. The theoretical analysis successfully shows that the scaling exponent of -3 to be robust from three different aspect. In sum, some important issues of the gas-liquid interactions in turbulent bubbly flows have been addressed in this project. The implication is that the liquid phase turbulence is strongly affected by the size and shape of rising bubbles. Meanwhile, it can be found from the turbulence energy spectrum that the behaviours of turbulent eddies in the wakes of bubbles are very different from those in shear flow, thereby strongly influencing the kernels of bubble coalescence and breakage and hence the model predicted bubble size distributions.

A new fast response conductivity meter was developed and tested. The "five time constant" of the meter is 0.08 s which meets the requirement for measurements under the dynamic conditions relevant to this work.
In a laboratory column, a bubble interface was created by introducing a step change of gas flow, and the rising velocity of this interface, $u sb{in},$ was measured using a conductivity method with the new conductivity meter. A measurement of the three-dimensional bubble swarm velocity in the column was obtained by interpolation from the $u sb{in}$ measured as a function of $J sb{g2} vert J sb{g} sb1 ,$ where $J sb{g} sb1$ and $J sb{g} sb2$ are the superficial gas velocities before and after a step change of gas flowrate, respectively. This velocity was referred to as the hindered velocity, $u sb{h}.$ The buoyancy velocity, $u sb0 ,$ was readily determined by switching off the gas, i.e. $u sb0 = u sb{in}$ at $J sb{g} sb2 = 0.$
The average gas velocity, $u sb{g},$ was corrected to the local average gas velocity, $u sb{g,loc},$ to obtain the average gas velocity under the local pressure conditions at a given vertical position in the column. The experimental results showed that $u sb{h}$ was significantly less than $u sb{g,loc}$ (and $u sb{g}).$ This is because the $u sb{h}$ is the three-dimensional bubble swarm velocity and $u sb{g,loc}$ is the one-dimensional bubble swarm velocity. Unlike $u sb{g,loc},$ the $u sb{h}$ was constant along the column, which was supported by theoretical momentum analysis. The $u sb{h}$ is proposed as the key characteristic swarm velocity of the system.
For the air-water only system in the two-dimensional domain, using parabolic models for gas holdup and liquid circulation velocity profiles over the cross section of the column, the $u sb{h}$ could be fitted to the experimental data. For the air-water-frother system, the $u sb{h}$ could not be fitted to the experimental data which is attributed to the air bubbles adopting a circulatory flow pattern.
In the air-water only system under batch operation, Nicklin's derivation (1962), i.e. $u sb{g} = u sb0 + J sb{g},$ was supported only under restrictive conditions, namely $u sb{g}$ and $J sb{g}$ must be measured at atmospheric pressure. Considering the local values, the experiments showed that $u sb{g,loc}$ was not equal to $u sb0 + J sb{g,loc}.$ In the presence of frothers under batch or countercurrent operation, the experiments showed that Nicklin's derivation was not applicable even if atmospheric values of $u sb{g}$ and $J sb{g}$ were used.

Numerical simulations of rectangular shape bubble column reactors (BCR) are validated starting from preliminary simulations aimed at identifying proper simulation parameters for a given system and resulting up to the numerical simulation with mass transfer and chemical reactions. The transient, three dimensional simulations are carried out using FLUENT software and the results obtained for a system with low gas flow rate (48 L/h) indicated that we need enough fine mesh grid and appropriate closure of interfacial forces to predict reliably plume oscillation period, liquid axial velocity and gas holdup profiles. In case of high flow rate (260 L/h), we compared the results for the effect of different interfacial closure forces and change in inlet boundary condition for gas volume fraction. There is no change in hydrodynamic results when there is change in gas volume fraction at inlet boundary condition. The effect of virtual mass interfacial force on the simulation results is also negligible. However, the major effects of applying lift force on results of plume oscillation period, liquid axial velocity and gas holdup is predicted. For comparable simulation results to experimental data, it is suggested that requirement of enough fine grids and appropriate correlations for interfacial forces, especially the combination of drag and lift forces is necessary. To study the bubble size distribution in BCR the numerical simulations are carried out with QMOM population balance technique for air-water fluid system. After finalization of the generic moment boundary conditions with simulations with PBM using QMOM without breakage and coalescence phenomena, then we simulated the system with breakage and coalescence and eventually, the simulation results are compared with experimental and simulation data taken from the scientific literature. For better hydrodynamics results of BCR as compared to experimental results, the interfacial lift force with combination of drag force is predicted for QMOM. The discretization scheme for gas volume fraction and moments of first order upwind provided the expected results of bubble size distribution. The simulation result of QMOM with breakage and coalescence models were also in good agreement with hydrodynamics experimental results and simulation results of class methods and DQMOM for bubble size distribution results. The modelling of chemical absorption of pure CO2 gas in caustic solution is carried out in a rectangular BCR with identical simulation parameters settings of previous work. For applicability of available kinetic and physical data we developed concentration differential equations to estimate the species molar concentration with respect to time in MATLAB code. The obtained profiles of evaluation of concentration and pH were in similar fashion as compared to available CFD simulated concentration and pH profiles at a point in the bubble column with respect to time. CFD simulation taking into account the mass transfer and chemical reaction, the E-E approach is used with assumption of uniform bubble size for modelling of chemisorption of the CO2 gas bubbles into NaOH aqueous solution. The adopted models successfully predicted the hydrodynamics results and are in good agreement with experimental and simulation results, however, reaction processes results are not as per expectation and further improvement in adopted simulation methods is required for better results.

Dissolved air flotation (DAF) is a proven solid-liquid separation technology that is being used in water and wastewater treatment as an alternative process to conventional sedimentation operation. Due to its smaller footprint and ability to cater for higher liquid loading rates, it is ideal in many urban water treatment plants where space is limited and is usually designed for larger capacities. DAF systems generate microscale air bubbles to lift suspended particles in influent solution to the top of a rectangular tank and remove them by scrapers. Due to the recent focus on different applications of micro and nanoscale air bubbles across many industrial operations, researchers and engineers are trying to explore the application of DAF in other industries such as mineral ore processing, chemical industries, and sludge thickening operation in wastewater treatment plants (WWTPs). However, these applications have yielded low treatment efficiencies and less predictable performances, which demands more studies on the use of DAF in the high solid concentration of solid-liquid separation applications. While higher solid concentrations alter several physicochemical parameters in influent, an increase in slurry solution viscosity is a major concern. As a result, this thesis sought to identify the effect of viscosity on the microbubble (MB) size distribution and rise velocities which are identified as among the main factors affecting DAF performance. A laboratory-scale micro bubble generation system attached with a bubble column was designed and built to investigate the effect of viscosity on system dynamics. Shadow imaging technology and particle image velocimetry were utilised to measure bubble size distribution and bubble rise velocities respectively for different viscosities. Solutions were prepared by mixing commercially available Xanthan Gum powder in different concentrations. The results of these experiments identified interesting variations of bubble sizes and rise velocities concerning viscosity (ranging from 1 mPas to 67.6 mPas). An increase in viscosity reduced bubble sizes and narrowed size distribution (from 60 - 200 μm to 30 – 70 μm) while reducing mean bubble rise velocities from 57 mm/s to 9 mm/s. The results of these experimental studies were critically analysed, and it was identified that reduction of bubble coalescence in high viscous solutions resulted in smaller bubble sizes, while increased drag forces slow down the rise velocity of bubbles. Moreover, this study provides essential baseline information for future studies when trying to improve DAF efficiency in high solid content applications during solidliquid separation operations.

Abstract : The industrial partner of this project uses a slurry bubble reactor for the production of biogenic methanol. In the latter syngas is dispersed into the slurry continuous phase containing both liquid and solid phases. The rising bubbles containing a wide spectrum of the bubbles sizes, interact with the continuous phase due to the interface momentum transfer. The latter includes the drag, lift, wall lubrication and turbulent dispersion terms that require average bubble size, which needs to be calculated. One way to predict this average bubble size is by using population balance model (PBM), which can be coupled with the Eulerian framework. PBM also needs closure kernels for the bubble coalescence and bubble breakup. In this study, the influence of bubble coalescence and bubble breakup kernels have been studied in two- and three-phase system using eulerian approach, which solves momentum equation for each phase. The influence of the mesh sizes, number of bubble classes, numerical schemes, wall lubrication force and turbulent dispersion force are also included. In the two-phase system, results show that the Luo coalescence model needs to be tuned when used in combination with the Luo breakup kernel. The combination of the Luo coalescence and the Lehr breakup kernels (Luo-Lehr) show promising time-averaged radial profiles of gas holdup and axial liquid velocity as compared to empirical values. In the three-phase system, the combination of the Luo coalescence and the Lehr breakup kernels (Luo-Lehr) and the Luo coalescence and the Luo breakup kernels (Luo-Luo) predict convincing time-averaged radial profile of axial solid velocity as compared to experiments. However, at an elevated superficial gas velocity, a non-realistic behavior was predicted when compared to empirical observations. The sensitivity analysis results show that the 3 mm mesh size depicts a trend similar to the empirical values of the radial profiles of the gas holdup, axial liquid velocity, and solid axial velocity. The number of bubble classes influence the predicted bubble size distribution in the three-phase system while the numerical discretizing schemes have no influence on the results. The bench simulation results show that the inclusion of the turbulent dispersion term using a single porous tubular sparger influences the hydrodynamic behavior of the bubble column.
Le partenaire industriel de ce projet utilise un réacteur à suspension à trois phases pour la production de méthanol biogénique. Dans celui-ci, le gaz de synthèse est diffusé par barbotement dans la phase à suspension qui contient à la fois les phases liquide et solide. Les bulles en ascension présentent un large spectre de tailles et interagissent avec la phase à suspension en échangeant de la quantité de mouvement via leurs surfaces. Cet échange comprend les forces de trainé, de portance, de lubrification en proche parois et de dispersion par turbulence; lesquelles requièrent notamment le calcul de la taille moyenne des bulles. Une façon de prédire numériquement cette taille moyenne est de recourir à un modèle de bilan de population (PBM, de l’anglais Population Balance Model), qui peut être couplé avec un model multiphasique eulérien. Un tel PBM a requière des modèles de fermetures pour la coalescence et la rupture des bulles. Dans la présente étude, l'influence des modèles noyaux de coalescence et de rupture des bulles a été étudiée pour des systèmes à deux et à trois phases en utilisant l’approche eulérienne. L'influence de la taille du maillage, du nombre de classes de bulles, du schéma numérique, de la force de lubrification en proche parois et de la force de dispersion par turbulence sont également incluses. Dans un système bi-phasique, les résultats montrent que le modèle de coalescence Luo doit être ajusté lorsqu'il est utilisé en combinaison avec le noyau de rupture Luo. La combinaison des noyaux de coalescence Luo et de rupture Lehr (Luo-Lehr) montrent des profils radiaux moyennés dans le temps qui sont valides pour la concentration de gaz et la vitesse axiale du liquide par rapport aux mesures expérimentales. Dans le système triphasé, la combinaison des modèles noyaux de coalescence de Luo et de rupture de Lehr (Luo-Lehr) et de la coalescence de Luo et de rupture de Luo (Luo-Luo) prédisent des profils radiaux moyennés dans le temps qui sont valides pour la vitesse axiale moyenné dans le temps par rapport aux expériences. Cependant, à une vitesse de gaz superficielle élevée, ces profils prédisent un comportement non réaliste par rapport aux observations empiriques. Les résultats de l'analyse de sensibilité du maillage montrent qu’avec des cellules de 3 mm, le model prédit une tendance similaire aux valeurs empiriques pour les profils radiaux de concentration du gaz, de vitesse axiale du liquide et de vitesse axiale solide. Le nombre de classes de bulles influe sur les distributions prédites de taille de bulle dans le système triphasé alors que les schémas de discrétisation numériques n'ont aucune influence sur les résultats. Les résultats des simulations d’un banc d’essai avec diffuseur à bulles poreux montrent que tenir compte du terme de dispersion influence le comportement hydrodynamique de la colonne à bulles.

The hydrodynamics of the downwards concurrent flotation column (CDFC) of the Jameson design has been studied. The effect of operating variables on the gas holdup in two- and three-phase mixtures was measured. To measure gas holdup, the isolating technique, conductivity and pressure techniques were employed. Gas fractions between 10 and 65% were achieved. These high holdups are a consequence of bubbles being forced downwards against their buoyancy. The high gas fraction may account for the fast flotation claimed for this cell.
The conductivity technique using Maxwell's equation gave a maximum error of 6%, in both two- and three-phase systems (considering the water-solids mixture as one phase).
The drift flux model was applied to try to correlate the data. Both two- and three-phase systems showed consistent trends. The model was used to estimate bubble size. In the Richardson and Zaki equation the m factor was in the range 2.9 to 3.1. A dimensionless drift flux was defined assuming $m=3$ which fitted the data. For three-phase systems, however, the results predicted a trend in bubble size that seemed opposite to observation. (Abstract shortened by UMI.)

The axial velocity profiles (local velocity versus time or position) of single bubbles in the absence and presence of flotation reagents such as frother were measuredin a water-jacketed transparent Plexiglas square (10 x 10 cm) column over a distance of 400 cm. The test liquid temperature was maintained uniform and constant at 30 °Cby water circulation in the jacket. Single bubbles, covering a size range of interest in flotation, were studied. A bubble generation frequency was selected such that velocitywas independent of frequency. To follow the bubble during its rise, a video camera supported on a track and capable of moving vertically at adjustable speeds was employed.
Dans une colonne de flottation carree (10 x 10 cm) a chemise d'eau faite de plexiglas transparent, on a mesurd, sur une distance de 400 cm, les profils de velocite axiale (velocite locale versus temps ou position) de bulles simples avec ou sans la presence de reactifs de flottation tel le moussant. En circulant de l'eau dans la chemise, la temperature du liquide a l'interieur de la colonne fut maintenue a une temperature constante de 30°C. On a etudie des bulles simples ayant des diametres d'intdret pour la flottation. Une frequence de Ondration des bulles fut choisie afm que la velocite soit independante de la frequence. Pour suivre la bulle lors de son ascension, on a utilisë une camera video montee sur rail et pouvant se placer verticalement a des vitesses variables. fr

Bubble columns are widely used in the bio-processing industry to perform large scale, aerobic fermentations. For this reason, there is a clear interest in optimising both the design and operation of such reactors. One cost-effective approach is the development of a Computational Fluid Dynamics (CFD) model of the process; the major advantage of this methodology being that it provides detailed information about the flow patterns within the column, knowledge which is difficult to experimentally obtain at an industrial scale. Such data are of particular value as it has been conjectured that poor distribution of nutrients leads to a reduction in the process yield. Hence, the aim of this work was the development of a CFD model capable of accurately describing flow in bubble columns operated in the industrially relevant heterogeneous flow regime (i.e. at superficial velocities greater than 0.1 m/s). In order to develop and validate such a model, it was necessary to obtain experimental data for both an air/water system as well as an air/fermentation media system, the latter being a topic rarely examined in the literature. Hence, a comprehensive experimental program was undertaken at both the bench-top (using a column 0.19 m in diameter and 1 m in height) and pilot-scales (using a column 0.39 m in diameter and 2 m in height). A comprehensive experimental dataset consisting of measurements of the mixing time, overall hold-up, bubble size distribution, as well as profiles of the local hold-up, liquid velocity and gas velocity was generated. Both experimental configurations were modelled using CFD; with the model predictions being in satisfactory agreement with the experimental data at both scales. The development of a predictive model capable of accurately describing the complex mixing patterns in bubble columns (both with and without the presence of surfactants) operating in the heterogeneous flow regime is seen as a key step in the design and optimisation of such systems.

The current work has developed a CFD model to characterise a pseudo-annular photocatalytic bubble column reactor. The model development was divided into three stages. Firstly, hydrodynamic assessment of the multiphase fluid flow in the vessel, which incorporated residence time distribution analysis both numerically and experimentally for validation purposes. Secondly, the radiation distribution of the UV source was completed. The final stage incorporated the kinetics for the degradation the model pollutant, sodium oxalate. The hydrodynamics were modelled using an Eulerian-Eulerian approach to the multiphase system with the standard k- turbulence model. This research established that there was significant deviation in the fluid behaviour in the pseudo-annular reactor when compared with traditional cylindrical columns due to the nature of the internal structure. The residence time distribution study showed almost completely mixed flow in the liquid phase, whereas the gas phase more closely represented plug flow behaviour. Whilst there was significant dependence on the superficial gas flow rate the mixing behaviour demonstrated negligible dependence on the liquid superficial velocity or the liquid flow direction, either co- or counter- current with respect to the gas phase. The light distribution was modelled using a conservative variant of the Discrete Ordinate method. The model examined the contribution to the incident radiation within the reactor of both the gas bubbles and titanium dioxide particles. This work has established the importance of the gas phase in evaluating the light distribution and showed that it should be included when examining the light distribution in a gas-liquid-solid three-phase system. An optimal catalyst loading for the vessel was established to be 1g/L. Integration of the kinetics of sodium oxalate degradation was the final step is developing the complete CFD model. Species transport equations were employed to describe the distribution of pollutant concentration within the vessel. Using a response surface methodology it was shown that the reaction rate exhibited a greater dependency on the lamp power that the lamp length, however, the converse was true with the quantum efficiency. This work highlights the complexity of completely modelling a photocatalytic system and has demonstrated the applicability of CFD for this purpose.

Column flotation as a concept was introduced approximately 90 years ago at Inspiration Copper Co., Arizona, with the first successful installation occurring at Les Mines Gaspe, Quebec, in 1981. Column flotation has since been applied to many other industries including deinking of recycled paper.
The research is a comparison of industrial bubble generating devices in a pilot and laboratory column using water/Dowfroth and pulp sampled on-line from a local deinking plant. The pilot column tested combinations of 6, 4 and 2 stainless steel (ss) porous spargers, and filter cloth and jetting sparger; the lab column used a single ss porous sparger. Long term tests on the pilot column were also done to evaluate maintenance issues.
Trends from the water/Dowfroth tests were used to predict results using pulp. Six ss spargers outperformed the other spargers in all cases. The performance of the lab column sparger matched 4 spargers, with the filter cloth performing marginally better than the jetting sparger or 2 spargers.
Gas holdup (Eg) and bubble surface area flux (Sb) gave good correlation with ink removal with all spargers failing within a narrow range. Surface area flux is suggested over Eg unless bubble diameter or superficial gas velocity are indeterminable. Sb > 100 s-1 gave ink removals equal to the plant Voith cells. An Sb below 40 s-1 gave zero ink removal. The lab and pilot column followed slightly different trends which was attributed to column diameter (i.e., wall effects).
The ss and filter cloth spargers present long term maintenance issues due to plugging. The performance of the 6 ss spargers decreased more quickly than any other during the long term tests, attributed to lower air velocities per pore.

The performance of ebullated bed hydroprocessors depends on the fluids distribution system and liquid recycle pan. Given that bubbles do not readily coalesce in the bed, the original bubble size distribution generated at the bubble cap distributor likely impacts buoyancy-based phase separation at the recycle pan. Gas entrained in the liquid recycle increases bed gas holdup at the expense of liquid holdup and product yield. The aim of this work was to investigate the impact of gas-liquid distribution system on resulting bubble properties and dynamics and incorporate a distributor sub-model into an existing fluid dynamics model of the industrial hydroprocessor. The size of initial bubbles formed in the plenum chamber was found to have negligible impact on phase holdups above the distributor. However, resulting bubble properties were found to depend on distributor geometry, distributor power dissipation and gas-liquid velocity ratio. In addition, a new set of scaling laws for gas-liquid distributors, based on dimensional analysis and similitude, was proposed. Geometric scaling was based on matching distributor fractional open area and ratios of critical dimensions. Dynamic similarity was based on matching three dimensionless groups and bubble coalescence behaviour. A bubble size distribution model was then developed. Both pressure and distributor were found to have an impact on individual bubble drag coefficients, as they both altered bubble size distribution. A novel drag model was thus also developed at industrially relevant conditions. Finally, a new gas-liquid distributor sub-model, including bubble size distribution and drag models previously developed, was incorporated into an overall fluid dynamics model of the hydroprocessor. The bubble size distribution model was also coupled with existing gas-liquid separation sub-model to better predict recycled gas and liquid fractions. A sensitivity analysis performed with the overall model revealed distributor configurations with potential of improving the processing capacity of the hydroprocessor.

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

The Council for Scientific and Industrial Research (CSIR) is developing a novel process to produce titanium metal at a lower cost than the current Kroll process used commercially. The technology initiated by the CSIR will benefit South Africa in achieving the long-term goal of establishing a competitive titanium metal industry. A bubble column reactor is one of the suitable reactors that were considered for the production of titanium metal. This reactor will be operated with a molten salt medium. Bubble columns are widely used in various fields of process engineering, such as oxidation, hydrogenation, fermentation, Fischer–Tropsch synthesis and waste water treatment. The advantages of these reactors over other multiphase reactors are simple construction, good mass and heat transfer, absence of moving parts and low operating costs. High heat transfer is important in reactors when high thermal duties are required. An appropriate measurement of the heat transfer coefficient is of primary importance for designing reactors that are highly exothermic or endothermic. An experimental test facility to measure wall heat transfer coefficients was constructed and operated. The experimental setup was operated with tap water, heat transfer oil 32 and lithium chloride–potassium chloride (LiCl–KCl) eutectic by bubbling argon gas through the liquids. The column was operated at a temperature of 40 oC for the water experiments, at 75, 103 and 170 oC for the heat transfer oil experiments, and at 450 oC for the molten salt experiments. All the experiments were run at superficial gas velocities in the range of 0.006 to 0.05 m/s. Three heating tapes, each connected to a corresponding variable AC voltage controller, were used to heat the column media. Heat transfer coefficients were determined by inducing a known heat flux through the column wall and measuring the temperature difference between the wall and the reactor contents. In order to balance the system, heat was removed by cooling water flowing through a copper tube on the inside of the column. Temperature differences between the column wall and the liquid were measured at five axial locations. A mechanistic model for estimating the kinematic turbulent viscosity and dispersion coefficient was developed from a mechanism of momentum exchange between large circulation cells. By analogy between heat and momentum transfer, these circulation cells also transfer heat from the wall to the liquid. There were some challenges when operating the bubble column with molten salt due to leakages on the welds and aggressive corrosion of the column. The experimental results were obtained when operating the column with water and heat transfer oil. It was found that the heat transfer coefficient increases with superficial gas velocity. The values of the heat transfer coefficient for the argon–water system were higher than those for the argon–heat transfer oil system. The heat transfer coefficients were also found to increase with an increase in temperature. Gas holdup increased with the superficial gas velocity. It was found that the estimated axial dispersion coefficients are within the range of those reported in the literature and the ratios of dispersion coefficients are in agreement with those in the literature. The estimated kinematic turbulent viscosities were comparable with those in the literature.
Dissertation (MEng)--University of Pretoria, 2014.
tm2015
Chemical Engineering
MEng
Unrestricted

Includes abstract.
Includes bibliographical references.
The intimate contact achieved between the gas and liquid phases in bubble columns, coupled with the inherent efficient mixing these reactors offer, yield excellent heat and mass transfer characteristics. These attributes have been exploited commercially for decades, however, due to the complexity of the underlying hydrodynamics, the prediction of bubble columns based on empirical models can be unreliable outside of the operating ranges used to fit these models. Computational Fluid Dynamics (CFD) has emerged as an attractive tool for simulating these reactors and is based on numerically approximating the fundamentally based Navier-Stokes equations on a discretized domain. The application of CFD has become more practical as the cost of computational resources has declined and has lead to the establishment of three distinct modelling approaches which have been evaluated for the purpose of bubble column simulation in a number of research papers over the past two decades. Here the Euler-Euler approach has been recommended for the simulation of large scale columns, however, this approach is based on the most assumptions and yields the least amount of flow field information. The Euler-Lagrange approach treats bubbles as discrete particles which allows for the incorporation of a deterministic bubble size distribution and the direct consideration of heat and mass transfer effects. The most fundamental approach, Direct Numerical Simulation (DNS), predicts flow properties at the bubble scale, however, is extremely computationally expensive and is therefore only practically applicable to the investigation of a very small number of bubbles. The objective of this study is to contribute to the simulation of gasliquid flow interaction occurring in bubble columns by proposing a novel technique for simulating bubble scale flow information at a significantly reduced computational expense. For this purpose, it is proposed to predict the micro-flow fields around individual bubbles, within an Euler-Lagrange framework, with an algebraic model termed the Bubble Cell Model (BCM). The high gradient regions around individual bubbles are thereby accounted for with an algebraic flow model that can be rapidly evaluated as opposed to the two-phase partial differential Navier-Stokes equations, thereby reducing the numerical complexity of the problem. Since no such flow models currently exist and accuracy and fast evaluation are imperative, a statistical approach to the construction of the BCM is justified.

Aquest treball presenta l'estudi numèric de fluxos turbulents bifàssics en un reactor de columna de bombolles en 3D utilitzant diferents models a diferents escales. En primer lloc se centra en la hidrodinàmica, las transicions del règim de flux i la transferència de massa utilitzant el model de barreja Euler-Euler k-ε per amplis rangs de velocitats superficials de gas. S'ha posat èmfasi en avaluar el rendiment d'aquest model i l'anàlisi de les transicions del règim de flux i el comportament del flux transitori dins del reactor de columna de bombolles. Es presenta la quantificació de les forces interfacials en diferents parts del reactor. Es comparen diferents models de l'estimació del coeficient global de transferència de massa, como són el model de penetració per lliscament i el model de cel·les de remolí, amb les dades experimentals per analitzar la transferència de massa. Els resultats revelen alguns dels trets característics dels règims de flux homogenis i heterogenis en la circulació de líquids, la retenció de gas, les fluctuacions turbulentes i la transferència de massa gas-líquid. Per als règims de flux transitori i turbulent s'han utilitzat les simulacions de grans remolins d'Euler-Euler per obtenir una resolució fiable de l'escala.
Este trabajo presenta el estudio numérico de flujos turbulentos bifásicos en un reactor de columna de burbujas en 3D utilizando diferentes modelos a diferentes escalas. En primer lugar se centra en la hidrodinámica, las transiciones del régimen de flujo y la transferencia de masa utilizando el modelo de mezcla Euler-Euler k-ε para amplios rangos de velocidades superficiales de gas. Se ha puesto énfasis se ha puesto en evaluar el rendimiento de este modelo y el análisis de las transiciones del régimen de flujo y el comportamiento del flujo transitorio dentro del reactor de columna de burbujas. Se presenta la cuantificación de las fuerzas interfaciales en diferentes partes del reactor. Se comparan diferentes modelos de la estimación del coeficiente global de transferencia de masa, como son el modelo de penetración por deslizamiento y el modelo de celdas de remolino, con los datos experimentales para analizar la transferencia de masa. Los resultados revelan algunos de los rasgos característicos de los regímenes de flujo homogéneos y heterogéneos en la circulación de líquidos, la retención de gas, las fluctuaciones turbulentas y la transferencia de masa gas-líquido.
This work presents numerical study turbulent two-phase flows in a 3D bubble column reactor using different models at different scales. The focus is first set on the hydrodynamics, flow regime transitions and mass transfer using the Euler-Euler mixture k-ε model at wide ranges of superficial gas velocities. The emphasis is to assess the performance of this model and the analysis of the flow regime transitions and the transient flow behavior inside the bubble column reactor. The quantification of the interfacial forces at different parts of the reactor are presented. Different models of the overall mass transfer coefficient estimation, namely the slip penetration model and the eddy cell model, are compared against the experimental data to analyze the mass transfer. The results reveal some of the characteristic features of homogeneous and heterogeneous flow regimes on the liquid circulation, gas holdup, turbulent fluctuations and gas-liquid mass transfer. For transient and turbulent flow regimes, Euler-Euler large eddy simulations were used for a reliable scale resolution. The flow is more dynamic, and more details of the instantaneous local flow structure have been obtained including large-scale structures and vortices developed in the bubble plume edge.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 109-114).
Heat and mass transfer processes governing the performance of bubble dehumidifier trays are studied in order to develop a predictive model and design rules for efficient and economical design of bubble column dehumidifiers for humidification-dehumidification (HDH) systems. As a result of their high heat transfer coefficients and large interfacial areas, bubble columns are an inexpensive and compact solution for dehumidification in HDH, which has promising applications in small-scale desalination and industrial water remediation. Performance parameters for dehumidifier design for HDH, including a device-specific parallel-flow effectiveness, are explained. A new model for the performance of single bubble trays is developed based on the rapid mixing in the column and the approximation of negligible gas-side resistance. An experiment is performed to measure the heat transfer coefficients outside cooling coils in shallow bubble columns, in which geometric parameters including liquid height and cylinder diameter, height, and horizontal position relative to the sparger orifices are varied. The highest heat transfer coefficients are recorded on cylinders placed in the coalescing region and directly above the sparger orifices. Heat flux and parallel-flow effectiveness of a bubble column dehumidifier are investigated experimentally to validate the model, which predicts the heat transfer rate well with an average absolute error of <3%. The independence of heat flux and effectiveness from liquid depth supports the assumption of negligible gas-side resistance to heat and mass transfer. Despite the mass exchange, the bubble column dehumidifier performs like a typical heat exchanger: the heat flux decreases and effectiveness increases with increasing coil area. The results of this study enable modeling and design of bubble column dehumidifiers for high heat recovery and low capital cost.
by Emily Winona Tow.
S.M.

Thesis (MEng)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: The expansion of the global fuels industry has caused an increase in the quantity of hydrocarbons produced as a by-product of refinery gas-to-liquid processes. Conversion of hydrocarbons to higher value products is possible using bioprocesses, which are sustainable and environmentally benign. Due to the deficiency of oxygen in the alkane molecule, the supply of sufficient oxygen through aeration is a major obstacle for the optimization of hydrocarbon bioprocesses. While the oxygen solubility is increased in the presence of hydrocarbons, under certain process conditions, the enhanced solubility is outweighed by an increase in viscosity, causing a depression in overall volumetric oxygen transfer coefficient (KLa). The rate at which oxygen is transferred is defined in terms of a concentration driving force (oxygen solubility) and the overall volumetric oxygen transfer coefficient (KLa). The KLa term comprises an oxygen transfer coefficient (KL) and the gas-liquid interfacial area (a), which are dependent on the uid properties and system hydrodynamics. This behaviour is not well understood for hydrocarbon bioprocesses and in a bubble column reactor (BCR). To provide an understanding of oxygen transfer behaviour, a model hydrocarbon bioprocess was developed using a BCR with a porous sparger. To evaluate the interfacial area, the Sauter mean bubble diameter (D32) was measured using an image analysis algorithm and gas holdup (ϵG) was measured by the change in liquid height in the column. Together the D32 and ϵG were used in the calculation of interfacial area in the column. The KLa was evaluated with incorporation of the probe response lag, allowing more accurate representation of the KLa behaviour. The probe response lag was measured at all experimental conditions to ensure accuracy and reliability of data. The model hydrocarbon bioprocess employed C14-20 alkane-aqueous dispersions (2.5 - 20 vol% hydrocarbon) with suspended solids (0.5 - 6 g/l) at discrete super ficial gas velocity (uG) (1 - 3 cm/s). For systems with inert solids (corn our, dp = 13.36 m), the interfacial area and KLa were measured and the behaviour of KLa was described by separation of the in uences of interfacial area and oxygen transfer coefficient (KL). To further the understanding of oxygen transfer behaviour, non-viable yeast cells (dp = 5.059 m) were used as the dispersed solid phase and interfacial area behaviour was determined. This interfacial area behaviour was compared with the behaviour of systems with inert solids to understand the differences with change in solids type. In systems using inert solids, a linear relationship was found between G and uG. An empirical correlation fo rthe prediction of this behaviour showed an accuracy of 83.34% across the experimental range. The interfacial area showed a similar relationship with uG and the empirical correlation provided an accuracy of 78.8% for prediction across the experimental range. In inert solids dispersions, the KLa increased with uG as the result of an increase in interfacial area as well as increases in KL. An increase in solids loading indicated an initial increase in KLa, due to the in uence of liquid-film penetration on KL, followed by a decrease in KL at solids loading greater than 2.5 g/l, due to diffusion blocking effects. In systems with yeast dispersions, the presence of surfactant molecules in the media inhibited coalescence up to a yeast loading of about 3.5 g/l, and resulted in a decrease in D32. Above this yeast loading, the fine yeast particles increased the apparent viscosity of the dispersion sufficiently to overcome the in uence of surfactant and increase the D32. The behaviour of G in yeast dispersions was similar to that found with inert solids and demonstrated a linear increase with uG. However, in yeast dispersions, the interaction between alkane concentration and yeast loading caused a slight increase in dispersion viscosity and therefore G. An empirical correlation to predict G behaviour with increased uG was developed with an accuracy of 72.55% for the experimental range considered. Comparison of yeast and inert solids dispersions indicated a 37.5% lower G in yeast dispersions compared to inert solids as a result of the apparent viscosity introduced by finer solid particles. This G and D32 data resulted in a linear increase in interfacial area with uG with no significant in uence of alkane concentration and yeast loading. This interfacial area was on average 6.7% lower than interfacial area found in inert solid dispersions as a likely consequence of the apparent viscosity with finer particles. This study provides a fundamental understanding of the parameters which underpin oxygen transfer in a model hydrocarbon bioprocess BCR under discrete hydrodynamic conditions. This fundamental understanding provides a basis for further investigation of hydrocarbon bioprocesses and the prediction of KLa behaviour in these systems.
AFRIKAANSE OPSOMMING: Die uitbreiding van die internasionale brandstofbedryf het 'n toename veroorsaak in die hoeveelheid koolwaterstowwe geproduseer as 'n deur-produk van raffinadery gas-tot-vloeistof prosesse. Omskakeling van koolwaterstowwe na hoër waarde produkte is moontlik met behulp van bioprosesse, wat volhoubaar en omgewingsvriendelik is. As gevolg van die tekort aan suurstof in die alkaan molekule, is die verskaffing van voldoende suurstof deur deurlugting 'n groot uitdaging vir die optimalisering van koolwaterstof bioprosesse. Terwyl die suurstof oplosbaarheid verhoog in die teenwoordigheid van koolwaterstowwe, onder sekere proses voorwaardes is die verhoogde oplosbaarheid oortref deur 'n toename in viskositeit, wat 'n depressive veroorsaak in die algehele volumetriese suurstofoordragkoëffisiënt (KLa). Die suurstof oordrag tempo word gedefinieer in terme van 'n konsentrasie dryfkrag (suurstof oplosbaarheid) en KLa. Die KLa term behels 'n suurstofoordragkoëffisiënt (KL) en die gas-vloeistof oppervlakarea (a), wat afhanklik is van die vloeistof eienskappe en stelsel hidrodinamika. Hierdie gedrag is nie goed verstaan vir koolwaterstof bioprosesse nie, asook in kolom reaktors (BCR). Om 'n begrip van suurstof oordrag gedrag te voorsien, is 'n model koolwaterstof bioproses ontwikkel met 'n BCR met 'n poreuse besproeier. Om die oppervlakarea te evalueer, is die gemiddelde Sauter deursnit (D32) gemeet deur 'n foto-analise algoritme en gas vasvanging ( G) is gemeet deur die verandering in vloeibare hoogte in die kolom. Saam is die D32 en G gebruik in die berekening van die oppervlakarea in die kolom. Die KLa is geëvalueer met insluiting van die meter se reaksie sloering, om n meer akkurate voorstelling van die KLa gedrag te bereken. Die meter reaksie sloering was gemeet op alle eksperimentele toestande om die akkuraatheid en betroubaarheid van data te verseker. Die model koolwaterstof bioproses gebruik n-C14-20 alkaan-water dispersies (2.5 - 20 vol% koolwaterstof) solide partikels (0.5 - 6 g/l) op diskrete oppervlakkige gas snelhede (1 - 3 cm/s). Vir stelsels met inerte solides (koring meel, dp = 13.36 m), is die oppervlakarea en KLa gemeet en die gedrag van KLa beskryf deur skeiding van die invloede van oppervlakarea en KL. Om die begrip van suurstof oordrag se gedrag te bevorder, is nie-lewensvatbare gisselle (dp = 5.059 m) gebruik as die verspreide solide fase en oppervlakarea is bepaal. Hierdie oppervlakarea gedrag is vergelyk met die van stelsels met inerte solides om die verskille met verandering in solide tipes te verstaan. In stelsels met inerte solides, is 'n line^ere verwantskap gevind tussen G en uG. 'n Empiriese korrelasie vir die voorspelling van hierdie gedrag is opgestel met 'n akkuraatheid van 83.34% in die eksperimentele reeks. Die oppervlakarea het 'n soortgelyke verhouding met uG en die empiriese korrelasie verskaf 'n akkuraatheid van 78,8% vir die voorspelling van oppervlakarea oor die eksperimentele reeks. In inerte solide dispersies, het die KLa toegeneem met uG as die gevolg van 'n toename in grens oppervlak asook stygings in KL. 'n Toename in solides belading het n aanvanklike styging in KLa aangedui, as gevolg van die invloed van die vloeistof-film penetrasie op KL, gevolg deur 'n afname in KL op vastestowwe ladings groter as 2.5 g/l, te danke aan diffusie blokkeer effekte. In stelsels met gis dispersies, het die teenwoordigheid van benattings molekules in die media samesmelting geïnhibeer tot 'n gis lading van ongeveer 3.5 g/l, en het gelei tot 'n afname in D32. Bo hierdie gis lading, het die fyn gis partikels die skynbare viskositeit van die verspreiding verhoog genoegsaam om die invloed van benattings molekules te oorkom en die D32 te verhoog. Die gedrag van G in gis dispersies was soortgelyk aan die van inerte solides en dui op 'n lineêre toename met uG. Maar in gis dispersies, het die interaksie tussen alkaan konsentrasie en gis lading 'n effense toename veroorsaak in die verstrooiing viskositeit en dus in G. 'n Empiriese korrelasie is ontwikkel om G gedrag te voorspel en het 'n akkuraatheid van 72,55% vir die eksperimentele verskeidenheid beskou. Vergelyking van gis en inerte patrikel dispersies wys 'n 37.5% laer G in gis dispersies in vergelyking met inerte vaste stowwe as 'n gevolg van die skynbare viskositeit bekendgestel deur fyner vastestowwe partikels. Hierdie G en D32 data het gelei tot 'n linere toename in grens oppervlak met uG met geen beduidende invloed van alkaan konsentrasie en gis lading nie. Die oppervlakarea was gemiddeld 6.7% laer as oppervlakarea gevind in inerte partikel dispersies as 'n waarskynlike gevolg van die skynbare viskositeit met fyner partikels. Hierdie studie bied 'n fundamentele begrip van die veranderlikes wat die suurstof oordrag definieer in 'n model koolwaterstof bioproses BCR onder diskrete hidrodinamiese voorwaardes. Hierdie fundamentele begrip bied n basis vir verdere ondersoek van koolwaterstof bioprosesse en en die voorspelling van KLa gedrag in hierdie stelsels.

Ce travail de recherche vise à mesurer et à contrôler la distribution du diamètre des bulles (DDB) dans une colonne de flottation. L'objectif se décompose en trois parties. La première phase vise à estimer correctement la taille des bulles grâce à des algorithmes de traitement d'images prises par une caméra. La DDB obtenue est ensuite modélisée par une distribution log-normale qui est définie par deux paramètres, la moyenne et l'écart type. Deuxièmement, grâce au nouveau système de mesure et à la représentation log-normale, un modèle dynamique à gains non linéaires (structure de Wiener) dont les sorties sont la moyenne et l'écart-type de la DDB est estimé. Une bonne concordance entre la réponse du système expérimental et celle du modèle identié est observée. Finalement, une stratégie de commande prédictive contrainte basée sur le modèle de Wiener est conçue afin de réguler la BSD. Les résultats de laboratoire obtenus sont très satisfaisants.

Includes bibliographical references.
A bubble column reactor is a vertical cylindrical vessel used for gas-liquid reactions. Bubble Columns have several applications in industry due to certain obvious advantages such as high gas-liquid interfacial area, high heat and mass transfer rates, low maintenance requirements and operating costs. On the other hand, attempts at modelling and simulation are complicated by lack of understanding of hydrodynamics and mass transfer characteristics. This complicates design scale-up and industrial usage. Many studies and models have attempted to evolve understanding of the hydrodynamic complexity in Bubble Columns reactors. A closer look at these studies and models reveals a variety of solution methods for different systems (Frössling, 1938; Clift et al., 1978; Hughmark, 1967; Dutta, 2007; Ranz and Marshall, 1952; Benitez, 2009; Buwa et al., 2006; Suzzia et al., 2009; Wylock et al., 2011). Numerous correlations (Frössling, 1938; Clift et al., 1978; Hughmark, 1967; Dutta, 2007; Ranz and Marshall, 1952; Benitez, 2009; Buwa et al., 2006) exist but to date in literature, there is no general approach to determining accurate estimates of average mass transfer coefficient values. Good estimates of the average mass transfer coefficient will improve the predictive capacity of the associated models. Recent attempts at modelling micro-scale bubble-fluid interaction resulted in the Bubble Cell Model, BCM, (Coetzee et al., 2009) which simulates the velocity vector field around a single gas bubble in a flowing fluid stream using a Semi-Analytical model. The aim of the present study is to extend the BCM applications by integrating the mass balance into the framework to predict the average mass transfer coefficient in bubble columns. A nitrogen-water steady state system was simulated in an axisymmetric grid where mass transfer occurs between the gas and liquid.

Investigations into the modelling techniques that depict the transport of discrete phases (gas bubbles or solid particles) and model biochemical reactions in a bubble column reactor are discussed here. The mixture model was used to calculate gas-liquid, solid-liquid and gasliquid-solid interactions. Multiphase flow is a difficult phenomenon to capture, particularly in bubble columns where the major driving force is caused by the injection of gas bubbles. The gas bubbles cause a large density difference to occur that results in transient multi-dimensional fluid motion. Standard design procedures do not account for the transient motion, due to the simplifying assumptions of steady plug flow. Computational fluid dynamics (CFD) can assist in expanding the understanding of complex flows in bubble columns by characterising the flow phenomena for many geometrical configurations. Therefore, CFD has a role in the education of chemical and biochemical engineers, providing the examples of flow phenomena that many engineers may not experience, even through experimentation. The performance of the mixture model was investigated for three domains (plane, rectangular and cylindrical) and three flow models (laminar, k-e turbulence and the Reynolds stresses). mThis investigation raised many questions about how gas-liquid interactions are captured numerically. To answer some of these questions the analogy between thermal convection in a cavity and gas-liquid flow in bubble columns was invoked. This involved modelling the buoyant motion of air in a narrow cavity for a number of turbulence schemes. The difference in density was caused by a temperature gradient that acted across the width of the cavity. Multiple vortices were obtained when the Reynolds stresses were utilised with the addition of a basic flow profile after each time step. To implement the three-phase models an alternative mixture model was developed and compared against a commercially available mixture model for three turbulence schemes. The scheme where just the Reynolds stresses model was employed, predicted the transient motion of the fluids quite well for both mixture models. Solid-liquid and then alternative formulations of gas-liquid-solid model were compared against one another. The alternative form of the mixture model was found to perform particularly well for both gas and solid phase transport when calculating two and three-phase flow. The improvement in the solutions obtained was a result of the inclusion of the Reynolds stresses model and differences in the mixture models employed. The differences between the alternative mixture models were found in the volume fraction equation (flux and deviatoric stress tensor terms) and the viscosity formulation for the mixture phase.

Clinoptilolite, a natural zeolite, is capable of removing heavy metals from acid rock drainage (ARD). However, previous studies have predominantly been on artificial solutions, with no previous work on sorbent regeneration. This study investigated a novel process for capturing ARD and regenerating clinoptilolite based on a slurry bubble column (SBC) for enhanced mass transfer. Uptake tests were performed with natural ARD and various sorbent particle sizes from 300 to 1400 μm in average diameter, superficial gas velocity from 0.08 to 0.23 m/s, initial aqueous pH from 2 to 6, Zn concentrations from 15 to 215 ppm and sorbent/solution ratio from 25 to 400 g/kg to test zinc uptake. To obtain favorable regeneration, zinc in loaded clinoptilolite was displaced by NaCl as regenerant. Regenerant concentrations from 10 to 40 g/kg, regenerant/sorbent ratios from 100 to 400 g/kg, particle sizes from 300 to 1400 μm, and initial regenerant pH from 2 to 6 were tested to find suitable regeneration conditions. To test the long-term viability of clinoptilolite sorbent, repeated capture-regeneration cycles were tested. It was shown that NaCl regenerant could be reused to minimize waste volume. Three removal-only cycles after 10 full cycles continuously decreased the accumulated zinc in the clinoptilolite, allowing the uptake capacity to be almost fully recovered. When 10 full cycles were tested after three removal stages, results were similar to those for the first 10 cycles. Only one-quarter of the regenerant was required to achieve a similar total zinc uptake when reused NaCl solution was utilized. During the remediation, dealumination of clinoptilolite was observed, under certain circumstances. Increased Al in the aqueous phase led to co-precipitation of Zn-Al colloid, enhanced by abundant sulfate in solution. The unit zinc uptake of the Al colloid was much higher than that of the raw clinoptilolite. Adsorption isotherms were fitted to the Langmuir model, and the overall aqueous species and surface uptake were explored by the PHREEQC model. The cyclic capture/regeneration process is promising that further development work is warranted.

Bubble columns and airlift reactors were modeled numerically to better understand the hydrodynamics and analyze the mixing characteristics for each configuration. An Eulerian-Eulerian approach was used to model air as the dispersed phase within a continuous phase of water using the commercial software FLUENT. The Schiller-Naumann drag model was employed along with virtual mass and the standard k-e turbulence model. The equations were discretized using the QUICK scheme and solved with the SIMPLE coupling algorithm. The flow regimes of a bubble column were investigated by varying the column diameter and the inlet gas velocity using two-dimensional simulations. The typical characteristics of a homogeneous, slug, and heterogeneous flow were shown by examining gas holdup. The flow field predicted using two-dimensional simulations of the airlift reactor showed a regular oscillation of the gas flow due to recirculation from the downcomer and connectors, whereas the bubble column oscillations were random and resulted in gas flow through the center of the column. The profiles of gas holdup, gas velocity, and liquid velocity showed that the airlift reactor flow was asymmetric and the bubble column flow was symmetric about the vertical axis of the column. The average gas holdup in a 10.2 cm diameter bubble column was calculated and the results for the two-dimensional simulation of varying inlet gas velocities were similar to published experimental results. The average gas holdup in the airlift reactor for the three-dimensional simulations compared well with the experiments, and the two-dimensional simulations underpredicted the average gas holdup.
Master of Science

Anaerobic digestion is one of the most promising management options for dairy manure treatment. Manure wastewater from dairy farms has been used for methane production for decades. However, performance failure due to inadequate mixing is routine. In general, the mixng of anaerobic digester is achived throguh mechnical stirring, liquid circulation, and gas circulation, among which the gas circulation proves to be the most effcient way. In this work, we studied the liquid flow pattern of two differetn type of gas-circualtion based anaerobic digesters, with the aim to understand the effects of hydrodynamic behavior of the digesting liquid on the anaerobic digestion performance, so a better mixing strategy can be provided. We used two 20-L gas circulation based anaerobic digesters with confined (gas-lift) and unconfined (bubble column) design. The anaerobic digestion performance and mixing behaviors were studied at different gas recirculation rate. It was found that the biogas production from the bubble column was constantly higher than that from gas-lift digester. However, the overall flow of the two digesters, which is indicated by residence time distribution (RTD), showed a similar pattern. Furhter investigation of local liquid flow behavior using Computational Fluid Dynamic (CFD) indicate that the bubble column accumulated higher portion of sludge in the bottom of the digester, which has a higher TS and VS, COD, and biomethane production potential than those from the gas-lift digester. This is the reason that the biogas production from the bubble column is higher than the gas-lift digester. Through this study, a thorough characterization of the flow behavior of the anaerobic digester were developed, and provided a deep insight of its influence on the anaerobic digestion performance.
Master of Science

This paper describes the modelling of bubbly flow in a bubble column considering non-drag forces, polydispersity and bubble induced turbulence using the Eulerian two-fluid approach. The set of used closure models describing the momentum exchange between the phases was chosen on basis of broad experiences in modelling bubbly flows at the Helmholtz-Zentrum Dresden-Rossendorf. Polydispersity is modeled using the inhomogeneous multiple size group (iMUSIG) model, which was developed by ANSYS/CFX and Helmholtz-Zentrum Dresden-Rossendorf. Through the importance of a comprehensive turbulence modeling for coalescence and break-up models, bubble induced turbulence models are investigated. A baseline has been used which was chosen on the basis of our previous work without any adjustments. Several variants taken from the literature are shown for comparison. Transient CFD simulations are compared with the experimental measurements and Large Eddy Simulations of Akbar et al. (2012).

Thesis (Ph. D.)--Ohio State University, 2005.
Title from first page of PDF file. Document formatted into pages; contains xxiii, 187 p.; also includes graphics Includes bibliographical references (p. 179-187). Available online via OhioLINK's ETD Center

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student submitted PDF version of thesis.
Includes bibliographical references (p. 67).
The Center for Clean Water and Clean Energy at MIT and KFUPM have been developing many novel desalination systems. One of the new technologies originating from the Lienhard Research Laboratory is the Humidification Dehumidification desalination system, or HDH. In many ways HDH resembles the natural rain cycle for producing fresh water, in that sea water is evaporated from oceans into humid air that travels up into the atmosphere, before condensing and producing precipitation. That precipitation is then collected as fresh drinking water. One of the main hindrances with carrier gas based desalination systems over traditional thermal desalination systems like multi-flash distillation (MSF) systems is that there is a large thermal resistance in the dehumidifier between the carrier gas and the condensing coils resulting in poor heat transfer rates. The proposed solution is to create a bubble column for improved condensation. The condensing coil will be submerged in a body of water while humid air is sent through a sieve plate to create bubbles in this body of water where it will condense directly. Firstly, a single stage bubble column was designed, built, modeled, and tested. The model theoretically predicts the effects of bubble diameter, superficial velocity, liquid height in column, inlet mole fraction of vapor, impact on coils, and particle integration. Through experimentation it was shown that it was possible to achieve heat transfer rates of of 4 kW1m2 up to 20 kW1m2 ; rates that are 10 to 30 times that of existing state-of-the-art dehumidifiers. Secondly, a multi stage bubble column was designed, built, modeled, and tested in a full HDH system. Multi-staging is done to improve the effectiveness of the system. A three stage column was able to achieve an effectiveness of 89.4%.
by Steven Lam.
S.B.

Computational fluid dynamics (CFD) modeling was used to predict the hydrodynamics of a column reactor. Bubble columns have applications across many engineering disciplines and improved modeling techniques help to increase the accuracy of numerical predictions. An Eulerian-Eulerian multi-fluid model was used to simulate fluidization and to capture the complex physics associated therewith. The commercial code ANSYS Fluent was used to study two-dimensional gas-liquid bubble columns. A comprehensive parameter study, including a detailed investigation of grid resolution was performed. Specific attention was paid to the bubble diameter, as it was shown to be related to cell size have significant effects on the characteristics of the flow. The parameters used to compare the two-dimensional (2D) cases to experimental results of Rampure, et. al. were then applied to a three-dimensional (3D) geometry. It was demonstrated that the increase in accuracy from 2D to 3D is negligible compared to the increase in CPU required to simulate the entire 3D domain. Additionally, the reaction chamber of a microbial fuel cell (MFC) was modeled and a preliminary parameter study investigating inlet velocity, temperature, and acetate concentration was conducted. MFCs are used in wastewater treatment and have the potential to treat water while simultaneously harvesting electricity. The spiral spacer and chemical reactions were modeled in a 3D geometry, and it was determined that inlet velocity was the most influential parameter that was simulated. There were also significant differences between the 2D and 3D geometries used to predict the MFC hydrodynamics.
Master of Science

Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD-simulations applying the multi-fluid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist. Therefore, reliable predictions of unknown bubbly flows are not yet possible with the multi-fluid approach. A strategy to overcome this problem is to define a baseline model in which the closure models including the model constants are fixed so that the limitations of the modeling can be evaluated by validating it on different experiments. Afterwards, the shortcomings are identified so that the baseline model can be stepwise improved without losing the validity for the already validated cases. This development of a baseline model is done in the present work by validating the baseline model developed at the Helmholtz-Zentrum Dresden-Rossendorf mainly basing on experimental data for bubbly pipe flows to bubble columns, bubble plumes and airlift reactors that are relevant in chemical and biological engineering applications. In the present work, a large variety of such setups is used for validation. The buoyancy driven bubbly flows showed thereby a transient behavior on the scale of the facility. Since such large scales are characterized by the geometry of the facility, turbulence models cannot describe them. Therefore, the transient simulation of bubbly flows with two equation models based on the unsteady Reynolds-averaged Navier–Stokes equations is investigated. In combination with the before mentioned baseline model these transient simulations can reproduce many experimental setups without fitting any model. Nevertheless, shortcomings are identified that need to be further investigated to improve the baseline model. For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, CFD-grade experiments in an airlift reactor were conducted in the present work. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed. The closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

A novel approach of obtaining bubble size and spatial distribution is developed by hybridising techniques of Electrical Resistance Tomography and the Gas Disengagement Technique using a Population Balance as a framework. As a result, detailed hydrodynamic predictions suitable for Bubble Column Reactor [BER] optimisation results with minimal computing effort. Electrical Resistance Tomography [ERT] is a technique for creating 3D images of objects occurring in space. The images are obtained through current stimulations through a body surface electrodes and measurements of resulting voltage signals due to interior spatial conductivity field distribution. The use of ERT imaging method for hydrodynamic parameter predictions in a BCR has a benefit of yielding high temporal resolution but low spatial resolution. The low spatial resolution in electrical imaging accounts for underestimated or overestimated hydrodynamic parameter predictions similar to results obtained from the use of alternative techniques. The population balance model [PBM] is a mathematical framework with which the spatial transport of properties of bubble population can be described. The PBM also allows for the description of the time-variant bubble population properties by a division of bubble population into size classes. Moreover, the PBM allows for the inclusion of models of bubble coalescence and breakage phenomena, which affect the distribution of bubble population properties during bubble swarming. The included source terms enable accurate modelling of the bubble evolution either in a steady or unsteady state fluid flow regime. The objective of the present study is to develop an ERT interpretation technique yielding a high accuracy reconstruction of bubble population distribution through coupling ERT measurements to a PBM. It is hypothesized that a higher accuracy interpretation of ERT measurements will result from coupling ERT measurements to a PBM. The ERT technique has the capacity to image the steady and time-dependent gas void fractions in column sections as bubbles swarm and during dynamic gas disengagement [DGD]. This ERT potential is explored in hybridizing ERT and a PBM in the present work.

In this study, the photocatalytic mineralization of the industrial dump-site leachate was evaluated using an internally-irradiated 18-Litre pilot-scale aerated annular bubble column photoreactor. The study includes evaluating the effect of catalyst loading, leachate initial concentration, initial solution pH, light intensity and oxygen partial pressure. The reaction runs were performed over a 48-hours period at room temperature and atmospheric pressure. Titanium catalyst loading was optimized to be 3 gL-1 where the reaction rate constant 20x10-6 mol L-1 min-1.Beyond this dosage, the effect of light scattering by the catalyst particles were noticed on dropping the degradation rate. Moreover, at high catalyst loading, particles aggregates reduce the interfacial area between the reaction solution and the photocatalyst resulting in significant reduction in the number of active sites on the catalyst surface. It is also noticed that when the initial leachate concentration is high, the number of the active sites are decreased because of their competitive adsorption on the TiO2 particles; while on the other hand, during the light intensity illumination period, the OH radicals formed on the catalyst surface are remaining constant as evidenced by constant hydroxyl production rate. Thus, the reactive O2 attacking the contaminants molecules decrease and simultaneously the overall photodegradation efficiency also decrease dramatically. The plot of the apparent reaction rate constant versus the initial leachate concentration exhibits almost a quadratic behaviour which has an optimum value at concentration of 50 mM. Finally, it was found that the degradation rate constant increased with O2 partial pressure until a maximum was obtained around 50% O2/N2 of gas feed composition. The drop in the rate beyond 50% can be explained by the fact that the dissolved oxygen molecular oxygen is strongly electrophilic and thus increasing the dissolved oxygen content probably reduced electron-hole recombination rate and hence the system was able to maintain favourable charge balance necessary for the photocatalytic-redox process. Moreover, in the presence of excess O2, the photocatalyst surface may become highly hydroxylated to the point of inhibiting the adsorption of organic species causing decrease in the degradation rate. Effect of upflow co-current and counter current continuous operation mode were performed in the 18-litre bubble column photoreactor for the photooxidation degradation tratment of the dump-site landfill leachate. The best situation is liquid flow rate at 800 mL min-1 and total gas flow rate at 5 Lmin-1 for the counter current operation, while for the up-flow co-current operation, the best situation is liquid flow rate at 600 mL min-1 and total gas flow rate at 5 Lmin-1

Les colonnes à bulles sont utilisées en minéralurgie et en traitement des eaux pour capturer différents types de particules. Leur capacité d’échange bénéficie aujourd’hui d’un regain d’intérêt pour assurer la production de micro-algues destinées à un usage médicinal, alimentaire, ou énergétique : les concentrations d’oxygène et de dioxyde de carbone peuvent être contrôlées grâce à la considérable aire interfaciale gaz-liquide dans la colonne à bulles. Une étude expérimentale en boucle fermée a été menée pour simuler le passage du gaz dans une succession de colonnes en série. Le modèle théorique associé confirme le rôle critique du diamètre des bulles lors du transfert de masse. Un générateur de micro-bulles (MBG) innovant a été conçu et testé. Le prototype est capable de produire des micro-bulles d’un diamètre moyen Dbubble = 0.252 mm. L’invention a été officiellement déclarée. Le dernier chapitre a pour objet l’amélioration des méthodes de traitement de Fluorescence Induite par Plan Laser (PLIF), qui permettent d’obtenir les coefficients de transfert de masse kl. La première correction présentée prend en compte les variations de l’extinction de la fluorescence due au pH pendant la calibration et a été évaluée sur une mesure de concentration de CO2 dans le sillage d’une bulle en ascension libre dans une colonne d’eau. La seconde correction proposée doit être appliquée quand la distance de la région de mesure où les variations de pH sont observées augmente. La nécessité de cette correction a été illustrée par un cas test dans le sillage d’un nuage de bulles en ascension libre dans une colonne d’eau
Bubble column are involved in many industrial fields ranging from chemical industry to mineral processing. It recently became an industrial stake for the production of micro-algae intended for medicinal use, food or energy: the oxygen and carbon dioxide concentrations can be controlled via the efficient mass transfer induced by the significant gas-liquid interfaciale area into the bubble column. Firstly, experimental closed-loop study has been carried out to simulate the passage of gas in a succession of columns in series. The associated theoretical model confirms the critical importance of the bubble diameter for mass transfer.Therefore, an innovative Micro-Bubble Generator (MBG) has been designed and tested. The prototype is able to produce micro-bubbles of average diameter Dbubble = 0.252 mm. The invention has been officially declared. The last chapter aims at improving data treatment methods for Planar Laser-Induced Fluorescence (PLIF), which enables to obtain experimentally mass transfer coefficient kl through concentration measurements. The first presented correction takes into account variations of the fluorescence extinction due to pH during the calibration step, and has been evaluated on CO2 concentration measurement in the wake of a free rising bubble. The second proposed correction should be applied when the length in the measurement region over which pH variations are observed increases: variations of the extinction coefficient will affect the local incident light intensity and therefore the measurements. The need for this correction has been illustrated on a test case in the wake of a cloud of free rising bubbles