Mass or heat transfer inside a spherical gas bubble at low to moderate Reynolds number

Mass (or heat) transfer inside a spherical gas bubble rising through a stationary liquid is investigated by direct numerical simulation. Simulations were carried out for bubble Reynolds number ranging from 0.1 to 100 and for Péclet numbers ranging from 1 to 2000. The study focuses on the effect of the bubble Reynolds number on both the interfacial transfer and the saturation time of the concentration inside the bubble. We show that the maximum velocity Umax at the bubble interface is the pertinent velocity to describe both internal and external transfers. The corresponding Sherwood (or Nusselt) numbers and the saturation time can be described by a sigmoid function depending on the Péclet number Pemax = Umaxdb/D (db and D being the bubble diameter and the corresponding diffusion coefficient)

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Dynamics and mass transfer of rising bubbles in a homogenous swarm at large gas volume fraction

The present work focuses on the collective effect on both bubble dynamics and mass transfer in a dense homogeneous bubble swarm for gas volume fractions ↵ up to 30%. The experimental investigation is carried out with air bubbles rising in a square column filled with water. Bubble size and shape are determined by means of a high-speed camera equipped with a telecentric lens. Gas volume fraction and bubble velocity are measured by using a dual-tip optical probe. The combination of these two techniques allows us to determine the interfacial area between the gas and the liquid. The transfer of oxygen from the bubbles to the water is measured from the time evolution of the concentration of oxygen dissolved in water, which is obtained by means of the gassing-out method. Concerning the bubble dynamics, the average vertical velocity is observed to decrease with α in agreement with previous experimental and numerical investigations, while the bubble agitation turns out to be weakly dependent on α. Concerning mass transfer, the Sherwood number is found to be very close to that of a single bubble rising at the same Reynolds number, provided the latter is based on the average vertical bubble velocity, which accounts for the effect of the gas volume fraction on the bubble rise velocity. This conclusion is valid for situations where the diffusion coefficient of the gas in the liquid is very low (high Péclet number) and the dissolved gas is well mixed at the scale of the bubble. It is understood by considering that the transfer occurs at the front part of the bubbles through a diffusion layer which is very thin compared with all flow length scales and where the flow remains similar to that of a single rising bubbl

Elaboration et validation d'un modèle de simulation dynamique d'un chenal d'oxydation (couplage du modèle ASM1 et du modèle piston avec dispersion)

Le but de ce travail est le développement d un modèle 1D pour la simulation du fonctionnement d un chenal d oxydation. Le couplage du modèle hydrodynamique et des cinétiques biologiques permettront une meilleure description du procédé. L hydrodynamique dans le chenal est modélisée par un réacteur piston à dispersion axiale. Le transfert d oxygène et les réactions biologiques sont aussi intégrés dans le modèle. Les réactions biologiques sont représentées par le modèle biologique ASM1 développé par Henze 1987 tenant compte de la dégradation du carbone et celle de l azote. C est un modèle tout à fait adapté pour la modélisation des processus biologique mis en jeu dans un chenal d oxydation partiellement aéré. La validation du modèle développé a été effectuée par la comparaison avec les mesures effectuées sur une installation pilote de volume 1 m3 (Lesage 2002). Cette validation est faite en trois étapes : validation de l hydrodynamique par la simulation de la distribution du temps de séjour, validation du transfert d oxygène par la simulation de remontée d oxygène en eau claire et validation du modèle biologique par la simulation de la réponse réspiromètrique. Le modèle développé est ensuite utilisé pour la simulation du fonctionnement d une station à taille réelle celle de Mahrès (1300 m3). Des mesures des concentrations en oxygène dissous dans le chenal de Mahrès ont permis de distinguer une hétérogénéité spatiale et temporelle. La simulation à échelle réelle consiste à reproduire ces gradients de concentrations existant au sein du chenal après la détermination des paramètres du modèle par le biais de mesures expérimentales sur la station citée et par le calage du modèle cinétique de dégradation de l azote. Ce travail dénote l importance du modèle développé pour la simulation d un procédé à boue activée avec une hydrodynamique particulière et une aération par alternance de zone où la zone anoxie est nécessaire pour la réaction de dénitrificationThe aim of this study is to obtain a validated model for the description of an oxidation ditch system, which could be used as a decision tool. A spatio-temporal 1D model which integrates hydrodynamic, oxygen transfer and biologic kinetics was developed. This work provides a prediction of COD and nitrogen fractions and dissolved oxygen concentrations described by widely used ASM1 model (Henze et al., 1987). This model was validated on the experimental result obtained by a campaign measurement in lab scale activated sludge study having a volume of 1m3 (Lesage, 2002). This validation is made in three steps: hydrodynamic validation by the simulation of the residence time distribution, oxygen transfer validation by simulating oxygen rise in clear water and the biological model validation by the simulation of oxygen uptake rate. The model developed was used to simulate a full scale plant of 1300 m3. Such model can well predicts the plug flow behavior of dissolved oxygen exiting in large scale oxidation ditch and has the ability to simulate dynamic behavior of wastewater components. The full scale simulation is done after model parameters identification and calibration of nitrogen kinetics model parameters. This model can be used to perform real time prediction which can help to optimize nitrification and denitrification occurring in the ditch under dynamic loadTOULOUSE-INSA-Bib. electronique (315559905) / SudocSudocFranceF

Modeling the effect of packing density on filtration performances in hollow fiber microfiltration module: a spatial study of cake growth

This study continues from a previous work on the impact of packing density on the fluid flow distribution in a hollow fiber module [1]. A numerical model was developed to simulate the growth of a particle cake along the surface of a hollow fiber membrane and the subsequent fluid flow during a microfiltration operation. The model accounts for the continuous change in porous domain (cake and porous wall) geometry and permeability as long as filtration occurs. The effect module packing density has upon cake growth is carefully analyzed both for inside/out (I/O) and outside/in (O/I) filtration modes. The results exhibit significant differences in the time variations of cake spatial distribution along the fiber as a function of packing density for both filtration modes. Then a confrontation between forward filtration and backward filtration velocities offers some conclusion on the effect of packing density on the backwash efficiency. This in turn underlines the importance of design parameters in the filtration performance of a hollow fiber module

Image processing for the experimental investigation of dense dispersed flows : Application to bubbly flows

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Image processing for the experimental investigation of dense dispersed flows: application to bubbly flows

In this work an image processing technique is proposed to improve the measurement of ellipsoidal ob- jects, such as bubbles in dispersed flows. This novel algorithm devoted to the measurement of bubble size, shape and trajectory is applied to binarised images from a gray-level gradient filter. To improve data statistics, an ellipse fitting method is employed to take into account truncated bubbles at the image edges. Then, an original approach is proposed to enable the segmentation of overlapping bubbles. The complete algorithm is evaluated on synthetic images and on real images for an air-bubble swarm within water. This new and robust methodology enables to increase substantially (more than 40%) the number of bubbles detected and thus to improve data convergence

On single bubble mass transfer in a volatile liquid

We consider single bubble mass transfer of an non-condensible gas into a volatile liquid phase in indus- trial conditions, as observed for example in hydrocarbons liquid phase oxidation processes. Instantaneous bubble size, shape and velocity are measured using image processing with a particle tracking method. The mass transfer rate nitrogen into hot and pressurized liquid cyclohexane is deduced from the bubble volume decrease rate and is compared to literature correlations valid under isothermal conditions. Experiments are performed in a pressurized reactor for P = 20 bar, 30° ≤ T ≤ 150°C and bubble Reynolds number Re = O(10—100). The analysis of bubble rise dynamics shows that the gas-liquid system studied can be considered as a clean system. The mass transfer results are found to follow isothermal correlations predictions excepted for ambient temperature for which liquid evaporation in bubbles is shown to be coupled with mass transfer. This phenomena seems to be a consequence of having a high Lewis number

Economic Multi-Objective Dynamic Optimization (EMODO) as a Decision-Making tool in Biomethanation Process

In biological methanation, the methane produced by anaerobic digestion (AD) is upgraded with the addition of syngas. The successful implementation of biomethanation requires the optimization of the production to be competitive in economic terms against chemical processes. Optimization is an arduous task, especially when it is desired to optimize multiple objectives that can be conflicting, such as yields, productivities, process times, and profit gains, among others. This work aims to implement an Economic Multi-Objective Dynamic Optimization (EMODO) approach as a decision-making tool for adequately operating the biomethanation process. The proposed EMODO strategy was based on a previously developed dynamic model for biomethanation. This strategy effectively optimized the ???????? and the ???????????? ???????????? by manipulating the inlet flow rates of gas ( ?? ?????? ???? ) and liquid ( ?? ?????? ???? ). The strategy also highlights the conflicting behavior of economic objectives and the dependence on substrates. The dynamic optimization improves the response time of the model smoothing the transitions between stages and achieving well adaptation to disturbances regarding the cost of the substrates and the selling prices of the products