Fundamental studies of vibrated fluidized beds
- Autores: Eduardo Cano Pleite
- Directores de la Tesis: Antonio Acosta Iborra (dir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Miguel Menendez Sastre (presid.), David Pallarès Tella (secret.), J. Ruud Van Ommen (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Mecánica y de Organización Industrial por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
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 gas-solid 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. Nevertheless, 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 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 its 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 indicated in the following paragraphs.
The first chapter of the dissertation briefly introduces the fluidization phenomena and the experimental and numerical methods typically used to characterize conventional fluidized beds. Chapter 2 summarizes the experimental, numerical and data processing strategies followed for the fundamental characterization of vibrated beds carried out in this work.
The effect of vibration of the bed vessel on the behavior of the solid bulk of a bed of small thickness (pseudo-2D bed) aerated at minimum fluidization conditions is presented in Chapter 3. In addition to the experimental evidence, results arising from a one-dimensional model specifically developed in this chapter and from two-fluid model simulations are analyzed. These analyses serve primarily as a basis to understand the effect of vibration on the bed bulk when no bubbles are present in the system, and secondly as a starting point to later interpret the behavior of the more complex vibrated bubbling beds. The obtained results show that, in addition to gas pressure waves, there exist compression and expansion waves of the solids phase that travel upwards in the bed and change the velocity of the particles along the bed height. These waves are caused by the interaction of the bottom of the bed bulk with the distributor. Both the experiments and the numerical methods show that vibration of the bed vessel promotes a cyclic compression and expansion of the bed bulk and different relative velocity of particles between the upper and the lower sections of the bed.
In Chapter 4, the motion of a bubble rising alone in a pseudo-2D vibrated fluidized bed is analyzed. This is done in a twofold way: experimentally (by means of Digital Image Analysis and Particle Image Velocimetry) and numerically (by means of Two-Fluid Model simulations). The experiments reveal that the mean and oscillatory behaviors of bubbles are strongly affected by the cyclic oscillation of the bed bulk. In particular, for the particles studied, increasing the vibration amplitude decreases the bubble velocity for a given bubble size. Vibration also promotes the presence of a phase delay of the bubble characteristics as a function of the bubble position along the bed height. Besides, the numerical simulations reveal the importance of considering the gas compressibility on the correct prediction of the oscillations of the bed bulk and the phase delay of bubble characteristics. It is found that oscillations of bubble characteristics are transmitted at a velocity similar to the sound velocity in a fluidized bed, which again manifests that the gas compressibility plays an important role in the transmission of compression-expansion waves along the bed height.
The pressure distribution and the motion of solids around bubbles rising alone in the bed are experimentally characterized in Chapter 5. The pressure perturbation caused by the bubble rising in the vibrated bed was found similar to that expected for a bubble rising in a bed without vibration and in accordance with the Davidson \& Harrison's potential flow model. This suggests that the gas pressure fluctuations produced by vibration of the bed do not interfere coherently with the pressure perturbation produced by the bubble. The applicability of the Davidson \& Harrison's potential flow model is also qualitatively confirmed with regard to the motion of solids around a bubble that rises in a vibrated bed. The experiments of Chapter 5 are also used to give an explanation to the characteristic wavy shape of bubbles in the vibrated fluidized bed. The results also reveal that the volume of particles dragged by the wake increases when increasing the vibration amplitude and frequency and is strongly affected by the oscillatory motion of the bed vessel.
Chapter 6 studies a vibrated bed operated in bubbling regime, showing that the mean behavior of bubbles is strongly affected by vibration. 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 analysis of the oscillatory behavior of bubbles in the bed reveals that the phase delay of bubble characteristics observed for isolated bubbles is preserved even if the bed is operated in bubbling regime and bubbles continuously interact with each other.
Finally, in Chapter 7, two applications of vibrated fluidized beds are studied. The first application comprises an experimental study of the particle segregation in a vibrated fluidized bed filled with particles of approximately the same size and different densities. It is found that vibration and the gas superficial velocity play counteracting roles on segregation. Vibration alone led to mixing, whereas low vibration strengths, in combination with the injection of gas, promote percolation that causes the segregation of the denser particles to the lower section of the bed. The second application is devoted to the experimental characterization of the different granular patterns appearing when vibrating a bed of triangular shape, with implications to beds of conical geometry. The results show that there is a combination of the vibration strength and the gas superficial velocity in which the direction of the conventional gulf stream circulation of particles inside the bed can be reversed.
In summary, the present PhD thesis reveals, both experimentally and with the aid of numerical models, that (i) gas compressibility affects the oscillations of the bed bulk, (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 induces an extra degree of freedom to control segregation and modify the patterns of particle motion in the bed.
