Publication: Fundamental studies of vibrated fluidized beds
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2016-12
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2016-12-12
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Abstract
La 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.
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.
Description
Mención Internacional en el título de doctor
Keywords
Fluidization, Numerical models, Vibrated fluidized bed, Bubble behaviors