Academic literature on the topic 'Bubble column reactor'

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.<br>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.<br>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 (MEng)--Stellenbosch University, 2015.<br>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.<br>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.

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.<br>Master of Science

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

Research into hydrate production technologies has increased in the past years. While many technologies have been presented, there is no consensus on which reactor design is best for each potential application. A direct experimental comparison of hydrate production technologies has been carried out in between a variety of reactor configurations at similar driving force conditions. Three main reactor types were used: a stirred tank, a fixed bed and a bubble column and compared different phase contacting patterns for the stirred tank and bubble column. In the initial phase of hydrate formation in a stirred tank, formation was mass and heat transfer limited at the lower stirring speed, and heat transfer limited at the higher stirring speed. After more than 10% of the water had been converted to hydrate, formation was mass transfer limited regardless of the other conditions. Neither the use of a gas inducing impeller, nor a 10 wt% particle slurry significantly affected hydrate formation rates; however, the particle slurry did lower the induction time. Due to the poor scale-up of impeller power consumption in a stirred tank, a semi-batch fixed bed was studied since it does not require any power input for mixing. The significantly slower rates of formation observed in the semi-batch fixed bed, as well as the lost reactor capacity to particles, mean that this type of system would require a much larger reactor. Faster volume and power normalized rates of hydrate formation were observed in the bubble column than in a stirred tank at similar mass transfer driving force conditions. Higher conversions of water to hydrate were observed in the bubble column because mixing was accomplished by bubbling gas from the bottom rather than by an impeller. The highest conversions of water and gas were achieved during a later stage of accelerated hydrate formation, indicating an optimal hydrate fraction for continuously operated bubble column reactors. The second stage of hydrate formation occurred more frequently at higher gas flowratess. Therefore, the increased water conversion and single-pass gas conversion justify the increased energy input required by the higher gas flowrate. Balancing the rates of mass transfer and heat removal was also critical for optimal bubble column as insufficient mass transfer would result in a lower rate of formation and insufficient heat transfer would cause previously formed hydrates to dissociate. The addition of 10wt% glass beads to the reactor promoted hydrate formation; however, it did not do so sufficiently to make up for the loss in reactor capacity or the increased energy requirement.

A cocurrent downflow contactor (CDC) has been constructed in and potential as a reactions. Prior to studying the CDC as a reactor its performance order reactor to investigate its for use with behaviour gas-liquid-solid was investigated with respect to its hydrodynamic behaviour with liquids of varying viscosity and surface tension. Liquid systems employed were water based, containing various concentrations of sodium alginate, glycerol, and I-propanol. Parameters such as minimum inlet liquid velocity, column liquid velocity, pressure drop, bubble size, gas holdup, and gas liquid interfacial area were determined. Empirical relationships were derived enabling minimum inlet velocity, maximum column velocity, pressure drop and gas holdup to be calculated. It was observed that gas holdup values in the range of 0.5-0.6 could be obtained and these are much greater than those attainable with conventional stirred or bubble column systems. The gaS-liquid mass transfer characteristics of the CDC were studied by the absorption of oxygen into water and a dimensional analysis of data allowed the overall mass transfer coefficient to be determined and correlated with experimental data. The hydrogenation of itaconic acid (CSHS04) using a 5% and 10%w/w Pd/Charcoal catalyst was studied at 15-50oC and 1 bar absolute in a stirred reactor and at 200C and 1-3 bars absolute in the CDC reactor using the following solvents: water, methanol, 2-propanol and aqueous glycerol. It was found that gas-liquid mass transfer rate control could be eliminated, due to the efficient agitation and high gasliquid interfacial areas generated. The rate controlling steps were found to be those of liquid-solid mass transfer and surface reaction, both steps being almost equally important in the CDC reactor. In the case of the stirred reactor, the liquid-solid mass transfer contribution depended upon the solvent and ranged from 5% (methanol) to 40% (water). Finally it was observed that on a laboratory scale the stirred and CDC reactor can give similar performances. However, in industrial or pilot plant scale, the CDC could retain its efficiency, giving high values of overall mass transfer coefficients and overcome the inefficient mixing of large scale stirred reactors.