Modelling and experimental validation of a fluidized bed reactor freeboard region: application to natural gas combustion

A theoretical and experimental study of natural gas-air mixture combustion in a fluidized bed of sand particles is presented. The operating temperatures are lower than a critical temperature of 800 °C above which the combustion occurs in the vicinity of the fluidized bed. Our study focusses on the freeboard zone where most of the methane combustion takes place at such temperatures. Experimental results show the essential role of the projection zone in determining the global thermal efficiency of the reactor. The dense bed temperature, the fluidizing velocity and the mean particle diameter significantly affect the thermal behaviours. A model for natural gas-air mixture combustion in fluidized beds is proposed, counting for interactions between dense and dilute regions of the reactor [Pré et al. (1998)] supplemented with the freeboard region modelling of Kunii-Levenspiel (1990). Thermal exchanges due to the convection between gas and particles, and due to the conduction and radiation phenomena between the gas-particle suspension and the reactor walls are counted. The kinetic scheme for the methane conversion is that proposed by Dryer and Glassman (1973). Model predictions are in good agreement with the measurements

Couscous manufacture in fluidized bed by wet agglomeration of wheat semolina

Agglomeration is a key unitary operation in a number of industries (pharmaceutical, chemical, food, civil engineering). The agglomeration process controls the structural characteristics and final properties of agglomerates. One of the activities involving food agglomeration is the production of  couscous by agglomeration of durum wheat semolina. Couscous is considered the most important traditional dish among the Maghreb people. The industrial process of couscous production includes various stages, the most important of which is wet granulation of semolina, which contributes to the quality of the final product. The agglomeration of cereal powders from different origins (durum wheat, maize and barley) has been performed in a variety of equipment such as high shear mixers, drum mills and fluidized beds. However, the agglomeration of semolina in fluidized beds has had very limited study. The purpose of this research is, therefore, to study couscous production using durum wheat semolina in a fluidized bed equipped with a spray nozzle. The fluidized bed has the advantage of generating strong particle movement and intense mixing to increase the size of the granules evenly throughout the mass used.The efficiency of this process is determined by the couscous yield defined as the mass ratio of couscous to raw material. The results showed that couscous can be produced from semolina by wet fluidized bed agglomeration with a specific effect of fluidification air flow, liquid flow, bed temperature and spray liquid properties on the couscous quality (size, brittleness and morphology) as well as on yield. The latter rose by 60% when the water containing flour was sprayed. Furthermore, the results of this study showed that granules size changes directly with the liquid flow rate, while temperature and air flow have an opposite effect. It was also found that changing binder components have an effect on the quality of the agglomeration of the product. Key words: Couscous, agglomeration, fluidization, semolina, friability, granulation, atomization, size enlargement, dryin

Modelling of Heat Transfer in a Fluidized Bed Reactor Irradiated Indirectly by Concentrated Solar Energy

AbstractA two phases model of air heating in bubbling fluidized bed of sand particles with concentrated solar radiation as source of energy is developed. This model is based on the Kato and Wen's model (1969) for the hydrodynamic aspect which was modified to take account the thermic aspect. Nine algebraic equations were established in permanent regime for different heat and mass balances as well as heat losses against surrounding media. The Newton Raphson's method was used to solve this system of equations. Results have shown that the model developed is able to predict the temperature profiles of gas and particles in the bubble and emulsion phases, the wall temperature along the reactor, the heat flux transferred to the bed and heat losses by forced convection and radiation to surrounding air. The effects of the fluidizing air velocity, total mass of particles and the wind velocity on the thermal behaviours were examined. Model predictions seem reasonable looking for its comparison agreement with bibliographical data

Remediation of aged hydrocarbon contaminated soil by washing in fluidized bed column

Soil contamination with hydrocarbons represents a worldwide problem, especially for oil-rich countries. soil contamination becomes inevitable due to different accidents, aboveground spills, and leakage, threatening the fauna and flora. The purpose of this study is to remediate One-year aged contaminated soil with crude oil (23490 mg/kg) using the fluidization technique in a laboratory-scale column. Free water and surfactant solutions were used for washing at different operating conditions. The efficiency of the method was evaluated by the calculation of the total petroleum hydrocarbons (TPH) removal ratio. Without the addition of surfactant, the cleaning operation was not sufficiently efficient, especially at room temperature where the removal ratio was only about 18%. Raising the liquid temperature leads to some improvement where the TPH removal ratio reached 49% at 50°C. With the use of solutions containing Sodium Laureth Sulfate (SLES) as a surfactant, an important enhancement of removal ratio was noted, along with an important reduction in operating time, washing solution volume, and energy consumption. The use of alternatively working/stopping operation mode contributes to the improvement of efficiency. TPH removal ratios up to 99% were obtained under some favorable conditions. This research shows encouraging results for expanding towards the industrial level with clean and sustainable resources

Combustion du gaz naturel en réacteur à lit fluidisé (étude expérimentale et modélisation de la zone dense et de la zone de désengagement)

La combustion in-situ de gaz naturel en réacteur à lit fluidisé concerne les procédés de régénération ou de traitements de déchets de natures diverses (sables de fonderies, boues à forts taux d'humidité ). Malgré ces nombreuses utilisations, l'aspect théorique des phénomènes mis en jeu lors de la combustion du gaz naturel en lit fluidisé reste encore mal compris. Ainsi, l'objectif de ce travail est l'élaboration d'une base théorique permettant de décrire les phénomènes mis en jeu lors de la combustion du gaz naturel en réacteur à lit fluidisé contenant des particules inertes. Ce travail est divisé en trois grandes parties. La première partie concerne une étude expérimentale portant sur la détermination des caractéristiques hydrodynamiques, notamment l'aspect du mélange gazeux lors d'une alimentation séparée du réacteur par deux courants de réactifs (air, gaz naturel). La deuxième partie est relative à l'acquisition de données concernant concernant l'influence de divers paramètres opératoires sur la combustion d'un pré-mélange air-gaz naturel. Nous avons examiné l'effet de la température du lit dense, de la vitesse superficielle du mélange gazeux, de la taille des particules, du facteur d'air et de la hauteur du lit au repos. A l'issue de cette étude expérimentale, nous avons conclu que parmi les grandeurs étudiées, la vitesse superficielle du mélange gazeux, la taille des particules et surtout la température du lit dense exercent un effet notable sur le processus de combustion. Pour des températures de lit infériers à 800C, les comportements hydrodynamique et thermique de la zone de désengagement conditionnent l'avancement de la réaction et les émissions de polluants gazeux. La dernière partie est consacrée à la modélisation du réacteur en tenant compte des phénomènes thermiques, hydrodynamiques et réactionnels qui se produisent aussi bien dans la zone dense que dans la zone de désengagement. Ce modèle reproduit d'une manière fidèle l'ensemble de nos observations expérimentales.TOULOUSE-ENSIACET (315552325) / SudocSudocFranceF

Lavage des sables contaminés par les hydrocarbures en colonne à lit fluidisé : Approche expérimentale

In this work, a method for washing sand contaminated by petroleum is described. It consists in extracting the pollutant from the soil grains using fresh water without any chemicals. Two sets of experiments were conducted: at small scale using test tubes and at large scale using one fluidizing column. Experiments with test tubes were performed in batch mode using dune sand of 312 µm mean diameter artificially contaminated with petroleum at an amount of 10% based on clean sand weight. The study of the effect of many operating parameters on decontamination efficiency has shown that water temperature and agitation intensity exerts remarkable effects. On the contrary, initial contaminant concentration and treatment duration exerts just little effects. Experiments using fluidising column were performed using the same contaminated sand in continuous mode. Sand grains were fluidised by ordinary water at a sufficiently low velocity to avoid any entrainment. Temperature was kept at 50 °C. Results showed that fluidisation is an efficient technique in view of high yield of petroleum recovery (up to 65% in mass). The absence of any chemical additive to water makes this operation both cheap and ecological.Au cours de ce travail, une méthode pratique de lavage des sables contaminés par les hydrocarbures (pétrole) en lit fluidisé a été étudiée. Cette méthode consiste à extraire le polluant du sol au moyen de l’eau ordinaire. Le principe repose sur la création d’un mouvement des particules de sable contaminées par circulation d’eau en mouvement ascendant dans une colonne munie d’un distributeur en plaque poreuse. L’eau entraine dans son mouvement le pétrole qui est par la suite récupéré en haut de colonne. Les mesures expérimentales ont révélé que cette méthode est tout à fait adaptée à ce genre de traitement. Les pourcentages de pétrole récupéré sont de l’ordre de 50 à 65 % cependant les teneurs résiduaires dans le sable restent relativement élevées par rapport aux normes environnementales. Parmi les paramètres opératoires examinés, la température de l’eau et le degré d’agitation du milieu fluidisé influencent de façon remarquable l’efficacité de la dépollution. Par contre, la durée de l’opération et le degré de pollution affectent très peu l’efficacité du traitement. L’absence de tout additif chimique à l’eau rend cette opération à la fois écologique et peu onéreuse

3D Numerical simulation of natural gas combustion in a fluidized bed reactor

International audienceThis study presents detailed 3D unsteady CFD simulations, performed using NEPTUNE_CFD V1.08@Tlse code, of air-methane combustion in a dense fluidized bed containing inert particles. Predictions are compared with the experimental data reported in Dounit et al. (2001a, 2001b, 2008). In their study, the authors investigated the behavior of natural gas combustion process in dense fluidized bed and in the freeboard at temperatures lower than a critical value (< 850°C). The main outputs of the experiments are vertical profiles of gaseous-species concentrations and gas temperature. In CFD simulations, an Euler-Euler approach is used to compute separately gas and solid phases flows (primary variables for each phase are volume fraction, velocity, enthalpy, mass fraction), with detailed closure models to account for fluid-particle and particle-particle mass, momentum and energy transfers. Gaseous combustion is modeled by a two-step mechanism following Arrhenius-type equations (Dryer & Glassman, 1973, Westbrook and Dryer, 1981). The Eulerian modeling takes into account the energy exchange by radiation between the gas, the particles and the reactor walls as well. The 3D unsteady simulations are analyzed to characterize dynamic flow behavior, thermal spatial distribution and interphase equilibrium. Time-averaged quantities are computed to compare predictions with the available experimental measurements

3D Numerical simulation of natural gas combustion in a fluidized bed reactor

International audienceThis study presents detailed 3D unsteady CFD simulations, performed using NEPTUNE_CFD V1.08@Tlse code, of air-methane combustion in a dense fluidized bed containing inert particles. Predictions are compared with the experimental data reported in Dounit et al. (2001a, 2001b, 2008). In their study, the authors investigated the behavior of natural gas combustion process in dense fluidized bed and in the freeboard at temperatures lower than a critical value (< 850°C). The main outputs of the experiments are vertical profiles of gaseous-species concentrations and gas temperature. In CFD simulations, an Euler-Euler approach is used to compute separately gas and solid phases flows (primary variables for each phase are volume fraction, velocity, enthalpy, mass fraction), with detailed closure models to account for fluid-particle and particle-particle mass, momentum and energy transfers. Gaseous combustion is modeled by a two-step mechanism following Arrhenius-type equations (Dryer & Glassman, 1973, Westbrook and Dryer, 1981). The Eulerian modeling takes into account the energy exchange by radiation between the gas, the particles and the reactor walls as well. The 3D unsteady simulations are analyzed to characterize dynamic flow behavior, thermal spatial distribution and interphase equilibrium. Time-averaged quantities are computed to compare predictions with the available experimental measurements

Numerical Simulation of Multiphase Reactive Flows

This study deals with mathematical modeling and numerical simulations of reactive multiphase flows in dense fluidized beds. These flows involve complex physical mechanisms related to the coupling between the bed hydrodynamic and the reactions,which are still poorly understood. In this context, numerical simulations can provide explanatory access to the underlying physics taking place in the reactor, thus supplementing the experimental results. The present contribution focuses on the natural gas combustion in a dense fluidized bed reactor, for which experimental results are available from the literature (Dounit, 2001; Dounit et al., 2001, 2008). In their experiments, the authors pointed out the essential role played by the particle projection zone, above the bed surface, in the global thermal efficiency of the reactor operating at relatively low temperatures (600°C–800°C). In the present study, this point is further investigated by analyzing the results obtained by the numerical simulations. The unsteady 3D numerical simulations were performed using NEPTUNE_CFD code which is based on an Euler–Euler approach; the latter computes both the gas and the particulate phases in an Eulerian framework, accounting for specific closures modeling the interphase momentum and energy transfers. Time-averaged quantities were then computed and compared with the available experimental measurements. Numerical results (especially the gas temperature) were found to be very sensitive to the mesh refinement for the selected operating point. A further analysis at mesoscopic and macroscopic scales was carried out. This analysis pointed out the crucial role of the toroidal loop, which extends from the bed, close to the ejection zone, to the freeboard, on the fuel conversion. In numerical simulations, this loop must be accurately reproduced in order to provide reliable combustion predictions