Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Jesse Zhu

Abstract

The reactor performances and hydrodynamics were systematically studied in a multifunctional fluidized-bed system which included a bubbling fluidized bed (BFB), a turbulent fluidized bed (TFB) as well as a newly identified circulating turbulent fluidized bed (CTFB) using the same batch of activated FCC particles. Catalytic ozone decomposition was employed as the model reaction to experimentally investigate the reactor performances of BFB, TFB and CTFB. The complete mappings of ozone concentration were obtained in these fluidized beds, showing close relationship with the solids holdup distributions. In the BFBs and TFBs, the study of scale-up effect revealed that the static bed height had almost no influence on the ozone concentration distributions, whereas the bed diameter affected the concentration distributions, especially in the bubbling regime. This work, for the first time, examined the reactor performance of a CTFB: the ozone concentration decreased along the axial direction with a large portion of reaction happening in the entrance section, while the “centre-high” and “wall-low” radial profile of ozone concentration was presented.

Comprehensive evaluations on reactor performance were then conducted across the full spectrum of the commonly used fluidized-bed reactors, including BFB, TFB, CTFB, riser and downer, in order to illustrate the superior and inferior features for each. The clear correlations between ozone concentrations and solids holdups confirmed that the reactor performances were essentially controlled by the flow structures including gas/solids behaviour and distributions. Furthermore, the CTFB and downer showed a comparable reactor performance that was very close to that of a plug-flow reactor, resulting from the uniform flow structures with little backmixing. While the TFB demonstrated favourable reactor performance, the CTFB is still superior in reactor efficiency. The further deviation of the BFB and riser from a plug-flow reactor was due to the significant gas bypassing and backmixing. The performances of the various fluidized beds were then quantitatively characterized by gas-solids contact efficiencies.

The hydrodynamics of the BFB, TFB and CTFB were also studied in order to help understand their reactor performances. An optical fibre probe was used to obtain the spatial distribution (i.e., axial and radial profiles of time-average data) and the temporal variation (i.e., time-serial data) of solid holdup. By analysing the instantaneous signals of solids holdup, the BFB was found to be dominated by a dense (solid) phase with a discrete dilute (bubble) phase, while the TFB exhibited a dynamic flow structure with the comparable dense (cluster) and dilute (void) phases. The CTFB experienced even more transient behaviour over the TFB, causing more interfacial activities. In addition, the CTFB successfully achieved a gas/solids upflow with solids circulation rates as high as 300 kg/m2s while maintaining a dense bed with solids holdup ranging from 0.25 to 0.35. The CTFB possesses many hydrodynamic advantages, such as uniform axial flow structure, homogeneously inter-diffused dilute/dense phases and no net solids downflow, leading to very favourable mass transfer and highly efficient gas-solids contact.


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