Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Jesse Zhu

Abstract

The fluidization of particles in upwards and inverse Liquid-Solid Circulating Fluidized Bed is carried out to investigate the hydrodynamic characteristics when using “heavy” and “light” particles, whose densities are higher and lower than that of the surrounding liquid respectively. Generally, the solids are fluidized upwards in the former case, whereas, the downwards fluidization is preferred in the latter scenario.

In the Liquid-Solid Circulating Fluidized Bed (LSCFB) riser, where the upwards fluidization takes place, the effects of particle properties on solids holdup are investigated experimentally based on three parameters: superficial liquid velocity, normalized superficial liquid velocity and excess superficial liquid velocity. The results show that the excess superficial liquid velocity (Ul-Ut), among those three parameters, is a more appropriate parameter to evaluate the effects of the particle properties on the solids holdup, facilitating general comparisons for different types of particles. Then such particle property effects are studied analytically by incorporating operating parameters and particle properties into a mathematical model, showing excellent agreement with the experimental results. By this model, the transition velocity demarcating the circulating fluidization regime and the transport regime is determined to complete the flow regime map in liquid-solid fluidization systems.

In the Inverse Liquid-Solid Circulating Fluidized Bed (ILSCFB) downer, where the inverse fluidization takes place, under the circulating fluidization regime, the hydrodynamic characteristics are investigated experimentally by the fluidization of Styrofoam and Hollow Glassbeads. For both types of particles, axial solids holdup distribution is quite uniform, while radial solids holdup distribution is slightly non-uniform with slight increase adjacent to the wall under various operating conditions, but no obvious “core-annulus” structure is observed. Such solids holdup distribution pattern is closely related to the solids circulation rate, superficial liquid velocity, local liquid and particle velocity which are also measured for Styrofoam particles. It is shown that the radial profiles of both liquid and particle velocities are slightly non-uniform, higher at the center region while lower adjacent to the wall, influenced by solids circulation rate and superficial liquid velocity. The local slip velocity derived from local liquid and particle velocities is found to be very close to the single particle terminal velocity and one-dimensional slip velocity deduced from the superficial liquid, solids velocities and cross-sectional average solids holdup, suggesting that there is no obvious clustering phenomenon and solids segregation in ILSCFB downer under various operating conditions.

The hydrodynamics under the inverse conventional fluidization regime are also studied by examining the bed voidage and dimensionless bed expansion. A new mathematical model correlating Archimedes and Reynolds number is proposed for the prediction of the bed voidage and dimensionless bed expansion in both inverse and upwards liquid-solid fluidization system, exhibiting better accuracy than that of the well knownRichardsonand Zaki equation.

The comparisons of the hydrodynamics in ILSCFB and LSCFB are also made based on the force balance discussion, enabling the comparison of inverse and upwards circulating fluidization of particles. Then the generalized flow regime map is developed in terms of dimensionless superficial velocity and dimensionless particle size by determining the demarcations of different flow regimes quantitatively.


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