Fundamental studies of vibrated fluidized beds

Mención Internacional en el título de doctorLa fluidización es un proceso ampliamente utilizado en las industrias química, energética y de tratamiento de materiales debido a su buen rendimiento en el mezclado de sólidos y a su buena eficiencia de contacto, tanto sólido-sólido como gas-sólido. Entre las operaciones que hacen uso de lechos fluidizados se encuentran, por ejemplo, el craqueo catalítico (FCC), gasificación, combustión de combustibles sólidos, síntesis de Fischer-Tropsch, secado, granulación y recubrimiento de partículas. Sin embargo, la facilidad con la que las partículas fluidizan puede verse afectada por diversos factores. Por ejemplo, las partículas finas tienden a aglomerarse, lo que puede llegar a defluidizar el lecho o a formar caminos preferentes de gas en el mismo. Se han empleado múltiples estrategias para eliminar la aglomeración y mejorar la homogeneidad de fluidización. Entre estas estrategias se encuentra la vibración de un lecho fluidizado convencional, la cual es una tecnología prometedora consistente en la introducción de energía cinética en el sistema mediante la vibración mecánica de la vasija del lecho. Esta vibración proporciona la energía necesaria para romper los enlaces entre partículas, prevenir su aglomeración y evitar la canalización del gas en el lecho. A pesar de sus ventajas, la vibración introduce complejidades en la dinámica del lecho que están aún lejos de ser plenamente comprendidas. El conocimiento de estos fenómenos físicos complejos derivados de la vibración de un lecho fluidizado podría ser utilizado para la mejora del diseño y control de los lechos fluidizados vibrantes existentes, así como para aumentar su rango de operación en nuevas aplicaciones. Por todo lo anterior, un estudio de carácter fundamental del efecto de la vibración en el movimiento de la fase densa del lecho y de las burbujas presentes en el sistema es de suma importancia para entender la dinámica de este tipo de dispositivos. Este es el objetivo de la presente tesis doctoral, cuya estructura y resultados principales se indican en los párrafos siguientes.../ ... En resumen, la presente tesis doctoral revela, tanto experimentalmente como con la ayuda de modelos numéricos, que (i) la compresibilidad del gas afecta a las oscilaciones de la fase densa de un lecho fluidizado vibrado, (ii) la presencia ondas de compresión y de expansión de sólidos y de gas causadas por la la vibración de la vasija domina el comportamiento de burbujas aisladas en el lecho, (iii) estas ondas se generan en la base del lecho y viajan en dirección ascendente modificando el comportamiento medio y oscilatorio de las propiedades de las burbujas en función de la distancia al distribuidor, (iv) esto es aplicable a lechos operados en régimen burbujeante en los que existe una continua interacción entre burbujas y (v) la vibración introduce un grado extra de libertad que permite controlar la segregación y modificar los patrones de movimiento de las partículas en el lecho.Fluidization is a process extensively used in the energy, chemical and materials processing industries owing to the good performance in solid mixing and the high solid-solid and gassolid contact efficiencies it provides. Among the operations making use of gas fluidized beds are fluid catalytic cracking (FCC), gasification, combustion of solid fuels, Fischer- Tropsch synthesis, drying, granulation and coating. However, the ease with which particles fluidize may be affected by diverse factors. For example, fine particles tend to agglomerate, which can end up defluidizing the bed. Several strategies have been employed to eliminate agglomeration and improve the fluidization homogeneity. Among these strategies, vibration of a conventional fluidized bed is a promising technology consisting in introducing kinetic energy to the system by mechanical vibration of the bed vessel. Vibration provides the necessary energy to break interparticle bonds and prevent agglomeration and gas channeling. Despite its advantages, vibration introduces complexities in the dynamics of the bed that are still far from being fully understood. Knowledge of these complex physical phenomena arising from vibration of a fluidized bed could be used to improve design and control of the existing vibrated beds and to increase their range of operation to new applications. Therefore, a fundamental study of the effect of vibration on the bulk motion and the bubbles rising in a fluidized bed is paramount to understand the dynamics in this kind of gas-solids systems. This is the aim of the present dissertation, whose structure and main results are described in the following paragraphs ... / ... In summary, the present PhD thesis principally reveals, both experimentally and with the aid of numerical models, that (i) gas compressibility affects the oscillations of the bed bulk in vibrated fluidized beds, (ii) the presence of compression and expansion waves of solids and gas caused by the vibration of the bed vessel commands the behavior of isolated bubbles in the bed, (iii) these waves are generated at the base of the bed and travel upwards modifying the mean and the oscillatory behavior of bubble characteristics as a function of the distance to the distributor, (iv) this is also applicable to beds in bubbling regime with multiple interacting bubbles and (v) vibration introduces an extra degree of freedom to control segregation and modify the patterns of particle motion in the bed.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: Miguel Alejandro Menéndez Sastre.- Secretario: David Jordi Pallarés I Tella.- Vocal: J. Ruud Van Omme

Beam impact tests of a prototype target for the beam dump facility at CERN: Experimental setup and preliminary analysis of the online results

The beam dump facility (BDF) is a project for a new facility at CERN dedicated to high intensity beam dump and fixed target experiments. Currently in its design phase, the first aim of the facility is to search for light dark matter and hidden sector models with the Search for Hidden Particles (SHiP) experiment. At the core of the facility sits a dense target/dump, whose function is to absorb safely the 400 GeV/c Super Proton Synchrotron (SPS) beam and to maximize the production of charm and beauty mesons. An average power of 300 kW will be deposited on the target, which will be subjected to unprecedented conditions in terms of temperature, structural loads and irradiation. In order to provide a representative validation of the target design, a prototype target has been designed, manufactured, and tested under the SPS fixed-target proton beam during 2018, up to an average beam power of 50 kW, corresponding to 350 kJ per pulse. The present contribution details the target prototype design and experimental setup, as well as a first evaluation of the measurements performed during beam irradiation. The analysis of the collected data suggests that a representative reproduction of the operational conditions of the beam dump facility target was achieved during the prototype tests, which will be complemented by a postirradiation examination campaign during 2020. © 2019 authors. Published by the American Physical Society

Desarrollo de un prototipo biomédico para el estudio de la reestenosis y de stents activos pro-cicatrizantes (2)

La médecine cardiovasculaire s’attache au développement d’endoprothèses dénommées « stents ». Elles sont destinées à combattre les rétrécissements artériels et veineux mais il semble aujourd’hui nécessaire de les enduire de médicaments pour lutter contre le phénomène de recicatrisation excessive dit de resténose. Pour faire ça les chercheurs en médecine auront recours à un banc de test in vitro en cours de développement au LEMFI. Ce projet a comme objectif le développement et intégration dans un incubateur d’un banc biomédical pour étudier de stents. Après validation de la faisabilité du banc d’essai, ce projet offre le pas à suivre ainsi comme les différents éléments qui feront part d’un prototype intégré dans un incubateur.Ingeniería Industria

Bulk oscillation and velocity wave propagation in a vibrated fluidized bed at minimum fluidization conditions

The present work experimentally characterizes the behavior of the bed bulk and the solids velocity in a vertically vibrated pseudo-2D fluidized bed operated at minimum fluidization conditions. Measurements are undertaken combining Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV). Vibration at different amplitudes and frequencies is applied to the bed by the use of two vibro-motors symmetrically disposed at both sides of the bed vessel. The results show that both the center of mass of the bed and the bed surface oscillate with a frequency equal to that of the bed vessel. The bed surface oscillates in opposition of phase with the bed vessel, which reflects a cyclic compression and expansion of the bed bulk. The average solids velocity at each oscillation phase clearly shows that there exists a compression wave, produced by the impact of the bed bulk with the gas distributor, and an expansion wave, produced by the expansion of the bed bulk. Both waves travel upwards the bed bulk perturbing the velocity of particles along the bed height The waves span all the bed width and separate the bed bulk into two clearly distinguishable regions with different relative velocities. When the particles belonging to the region under the wave move upwards, the particles in the region above the wave move downwards and vice versa. The results also reveal that the compression wave generated at the bottom of the bed propagates at a velocity similar to the reported velocity of sound inside a fluidized bed. Far from the distributor, this wave velocity resulted to be nearly independent of the vibration amplitude and frequency for the range of conditions tested. These results can be useful for the understanding of the behavior of particles and bubbles in vibrated fluidized beds

Compressible-gas two-fluid modeling of isolated bubbles in a vertically vibrated fluidized bed and comparison with experiments

In this work the size and motion of isolated bubbles in a vertically vibrated fluidized bed are numerically investigated by means of two-fluid model simulations. The oscillations of the bed bulk and the bubble diameter and velocity are compared with experimental results of a pseudo-2D bed using an averaging of cycles method to account for the intrinsic unsteadiness caused by vibration. The effects of gas compressibility and the air plenum of the vibrated bed are also numerically investigated. The results show that the two-fluid model simulations resorting to a compressible gas model are able to reproduce both the cyclic compression and expansion of the bed bulk and the bubble oscillations observed in the experiments. In contrast, the simulations with the incompressible gas model fail to reproduce these effects. The presence of the air plenum in the numerical model diminishes the amplitude of the bed and bubble oscillations and improves their resemblance to the experiments. In the simulations with compressible gas, a phase delay is found between the bed displacement and the oscillation of bubble characteristics. In harmony with experiments, the phase delay is smaller in the lower half of the bed (i.e. close to the distributor) than in the upper half. This effect is not reproduced by the simulations with incompressible gas-phase. These results suggest that the phase delay in vibrated beds is caused by the compression of the gas phase, which leads to compression-expansion waves traveling through the bed. The simulations also confirm that the amplitude of vibration influences the magnitude of the bubble diameter and velocity oscillations, whereas the delay of the bubble characteristics is mainly affected by the bed vibration frequency.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad

Effect of vertical vibration and particle size on the solids hold-up and mean bubble behavior in a pseudo-2D fluidized bed

The solids hold-up and mean bubble behavior in a vertically-vibrated fluidized bed are experimentally studied in the present work by Means of Digital Image Analysis (DIA) for four different powders with Geldart classifications A, B and A/B. The bed has a small thickness (i.e. pseudo-2D bed) and operates in bubbling regime subject to a wide range of gas superficial velocities, vibration frequencies and vibration amplitudes. Mean parameters of the bed and the bubbles, such as solids hold-up, bubble fraction, bubble number density and bubble diameter and velocity, are characterized here by averaging the results over time and space. The results reveal that vibration of the bed promotes a confinement of the bubble path to the central section of the bed. This bubble confinement is more intense for the smallest particles tested and for high vibration strengths and creates two different bubble regimes in the bed. In particular, close to the distributor, the bubble velocity decreases when increasing the vibration amplitude of the bed vessel because bubbles are smaller and less confined, and they behave like isolated bubbles. The behavior of bubbles changes when they are far from the distributor, where the interaction between bubbles becomes greater due to their bigger size and the confinement of bubbles induced by vibration. This confinement promotes coalescence of bubbles. It is shown that consideration of these two different regimes of bubble dynamics allows to shed light on understanding the apparently contradictory results encountered in the literature regarding bubble behavior in bubbling vibrated fluidized beds.This work has been partially funded by the Universidad Carlos III de Madrid, Spain, Ayudas a la Movilidad 2014.Publicad

Effect of particle shape on biomass pyrolysis in a bubbling fluidized bed

The effect of biomass particle shape on the conversion of beech wood during pyrolysis in a bubbling fluidized bed (BFB) was experimentally quantified. A lab-scale BFB installed on a high-precision scale was used to characterize the mass loss of the biomass particles immersed in the bed. The scale could monitor the mass loss of the beech wood particles while moving freely inside the bed, which was operated at 2.5 times the minimum fluidization velocity of the bed material employed. The tests were performed at 500 and 600 °C using beech wood particles of the same mass, but different in shape. All particles used were cylindrical in shape, with the same mass, and differing in their aspect ratio, analyzing particles from typical biomass chips to standard biomass pellets. The experimental results indicate that the velocity of pyrolysis for the different particles is proportional to the characteristic heat transfer length of the particles, with pyrolysis times ranging from 27 to 53 s for a bed temperature of 600 °C and from 43 to 85 s for a bed temperature of 500 °C. The minimum pyrolysis time was obtained for particles with a diameter of 20 mm and a length of 2 mm pyrolyzing in a bed at 600 °C, whereas the maximum pyrolysis time corresponds to particles of 10 mm in diameter and 8 mm in length converting in a bed at 500 °C. Estimations of the conversion time obtained from a Shrinking Unreacted Particle Model (SUPM), assuming a constant density and reducing volume of biomass during conversion, and a Uniform Conversion Model (UCM), considering uniform volume and decreasing density of biomass along the conversion process, were compared to experimental measurements of the conversion time. Qualitative agreement was found between the experimental values and the predictions of the conversion time from the simplified models, obtaining in all cases conversion times proportional to the characteristic length of heat transfer of each particle shape

Oscillatory behavior of the bed bulk and the bubbles in a vertically vibrated pseudo-2D bed in bubbling regime

The effect of the bed vessel vibration on the oscillatory behavior of the bed bulk and the bubbles is experimentally studied in the present work by means of Digital Image Analysis (DIA) in a pseudo-2D bed. The bed material was three different powders of Geldart A, B and AFB classifications and was operated in bubbling regime for different superficial gas velocities and vibration amplitudes and frequencies. A tracking methodology was developed in order to follow the oscillatory motion of the bed bulk and each individual bubble in the system. This allowed the analysis of the interaction of the dense phase of the bed with the oscillations of the Nibble diameter, position and velocity. The results indicate that both the center of mass of the bed and the bubble characteristics follow the oscillation of the bed vessel with a similar frequency but with a phase delay. The amplitude and phase delay of the oscillation of the center of mass of the bed are more sensitive to variations of the vibration frequency than to variations of the vibration amplitude of the bed vessel. Both the amplitude and the frequency of the bed vessel vibration have a stronger impact on the bubble behavior of beds filled with small particles. The existence of a phase delay between the oscillations of bubble characteristics in the lower and upper sections of the bed indicates the existence of compression-expansion waves in the dense phase that modify the bubble behavior along the bed despite bubbles are interacting with each other. The presence of compression-expansion waves may shed light onto the different behaviors encountered for the mean bubble behavior in vibrated fluidized beds.This work has been partially funded by the Universidad Carlos III de Madrid, Ayudas a la Movilidad 2014

Partitioning of a wide bubbling fluidized bed with vertical internals to improve local mixing and bed material circulation

Industrial scale fluidized bed reactors are characterized by limited mixing rates, either local or global, especially when using low-pressure drop gas distributors to reduce operational costs. In this work, partitioning of wide beds using vertical internals is proposed as an effective technique to improve local mixing in large reactors, i.e., mixing in specific zones of the bed. The effect of the vertical internals height on local solids mixing within partitions was experimentally evaluated in a pseudo-2D bed by analyzing the velocity and flow structure of the solids and the circulation time within individual partitions. In the presence of internals, global mixing, i.e., mixing between neighboring partitions and across the entire reactor, may be reduced as vertical internals compartmentalize the bed. Thus, the effect of the internals height on global mixing was also quantified while using bed materials with the same properties, but differing in color, in the different partitions, and analyzing the time evolution of the concentration of solids. Furthermore, the effect of internals on bubbles was also evaluated for different internal heights. It was found that internals with a height between the gulf-stream height and the fixed bed height promote the appearance of vortex pair structures in each partition of the wide bed. These structures substantially improve local mixing within each partition, while global mixing between partitions is practically unaffected by the presence of these short internals.The research that led to this publication was conducted with the support of a US-Spain Fulbright grant co-sponsored by the Spanish Ministry of Universities ("Ministerio de Educación, Cultura y Deporte en el marco del Programa Estatal de Promoción del Talento y su Empleabilidad en I+D+i, Subprograma Estatal de Movilidad, del Plan Estatal de I+D+I"). The authors acknowledge the financial support by the Foundation Seed Fund MIT - Spain "la Caixa". Eduardo Cano-Pleite also acknowledges support from the CONEX-Plus program funded by Universidad Carlos III de Madrid and the European Union's Horizon 2020 program under the Marie Sklodowska-Curie grant agreement No. 801538

Modeling the motion of fuel particles in a fluidized bed

A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called 'recirculation cells', which are crucial for the lateral mixing. Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (...)This work was financed in part by the Swedish Gasification Centre (SFC) within the framework of the Centre for Indirect Gasification of Biomass (CIGB), and by the Swedish Energy Agency within the framework of project P-38347-2. This work was funded in part (receiver E. Cano-Pleite) by the CONEX-Plus program by Universidad Carlos III de Madrid, the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 801538 and from Programa de Atracción de Talento (Modalidad 2) de la Comunidad de Madrid (Spain), with number 2019-T2/AMB-15938