Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. J. Zhu

2nd Supervisor

S. Dr. Barghi

Joint Supervisor

Abstract

Circulating Turbulent Fluidized Beds (CTFB) refer to fluidized beds integrated into high density circulating systems to simultaneously achieve highly efficient gas-solid interactions existing in turbulent fluidized beds and low solids backmixing featured by circulating fluidized beds. Hydrodynamics and micro flow structure were experimentally studied in a CTFB (3.6 m high and 0.104 m id) using 76 µm FCC particle with air velocities of 0.5 ~ 5.0 m/s and solids circulation rates of 0 ~ 420 kg/m2s The distributions of solids holdup were acquired using optical fibre probes and pressure transducers at sampling frequencies of 50 kHz × 131 s and 1 kHz × 400 s respectively. A Pseudo Bubble-Free Fluidized Bed was developed to dynamically calibrate the optical fibre probes. Based on statistical parameters, a Moment Consistency Data Processing Method (MCDPM) was proposed to calculate solids holdups of the dense and dilute phases from the experimental data. A Divided Phase Cross-Correlation Method (DPCCM) was adopted in cross-correlating the solids holdup signals of the dense and dilute phases to obtain the phase particle velocities.

MCDPM provided average solids holdups of the dense and dilute phases and the phase fractions over bubbling (BFB), turbulent (TFB), circulating turbulent (CTFB), high density circulating (HDCFB) and circulating (CFB) fluidized beds. The flow structure in terms of phase division and the micro flow characteristics were studied across all five regimes from low to high velocities, CTFB was found to have strong similarities with TFB.

Study on the detailed hydrodynamics and transition characteristics of the CTFB demonstrated that solids holdup distribution in CTFB was more homogeneous both axially and radially than that of other regimes, and the local solids flux and the local particle velocity were both proportional to the solids circulation rate. Microscopically, CTFB was characterized by dilute phase dominating flow in the centre and dense phase dominating flow in the annular region. Such flow structure was different from either dense phase dominating flow in BFB or dilute phase dominating flow in CFB. New criteria for the transition air velocities were proposed for CTFB. The results demonstrated that the onset transition velocity from BFB to CTFB remained nearly unchanged, and the ending transition air velocity from CTFB to CFB increased, with increasing solids circulation rate.

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