Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Mechanical and Materials Engineering

Supervisor

Dr. Jesse Zhu

2nd Supervisor

Dr. Dominic Pjonetk

Co-Supervisor

Abstract

Abstract

Inverse liquid-solid fluidized beds have recently received increased attention, particularly for use with wastewater treatment bioreactors (i.e., particle-supported biofilms). The flow behaviour of free-rising light particles is especially interesting because their drag coefficients deviate from the standard drag curve. For this reason, the work presented in this thesis was focussed on investigating the minimum fluidization velocity () and the steady-state bed voidage associated with four particles, with densities of 28, 122, 300, and 678 kg/m3, in a conventional inverse fluidization regime. All experimental measurements were completed using a large-scale system comprising a downer column with a diameter of 200 mm and a height of 4.5 m. Substantial deviations from the Wen and Yu correlation predictions were evident in the experimentally determined Umf values due to the limited range of particle properties. A modified Wen and Yu correlation is therefore proposed as a means of improving predictions related to free-rising light particles. The bed voidage associated with the particles studied was also explored experimentally. A proposed force balance model has been developed for predicting bed voidage based on an analysis of the liquid-solid interaction forces acting on a suspended particle. Within the range of solid particle properties examined, the proposed model has demonstrated adequate accuracy with respect to predicting bed voidage in inverse liquid-solid fluidized beds.

Summary for Lay Audience

Summary for a Lay Audience

An inverse liquid-solid fluidized bed (I-LSFB) refers to a two-phase system in which dispersed light particles whose density is less than the liquid density are suspended by a downward liquid flow in a bed. Due to the drag and gravitational forces overcoming the buoyancy force, all particles become fluidized. Because of their advantages that can result in enhanced liquid-solid contact efficiency, conventional I-LSFBs have recently become a target of increased attention, particularly with respect to their use in wastewater treatment bioreactors (i.e., particle-supported biofilms). The flow behaviour of free-rising light particles is especially interesting because of the deviation of their drag coefficients from the standard drag curve. This background provided the motivation for the focus of this work: an investigation of the hydrodynamics of minimum fluidization velocity and bed voidage in a conventional I-LSFB for four kinds of particles, with densities of 28, 122, 300, and 678 kg/m3. All the experimental measurements were acquired in a large-scale system comprising a downer column bioreactor with a diameter of 200 mm and a height of 4.5 m. The minimum fluidization velocities were investigated using two different measurement methods: identification of the frictional pressure gradient and ascertainment of the bed expansion height under a variety of superficial liquid velocities. Compared to predictions derived from the common correlation established by Wen and Yu (1966), substantial deviations were observed in the minimum experimental fluidization velocities found for particle densities of 28, 122, and 300 kg/m3. A modification to the Wen and Yu (1966) correlation is proposed as a means of improving the predictions associated with the experimental results produced for this research. The bed voidage, which is related to the bed expansion, was investigated experimentally and was found to increase with higher downward flow rates. A force balance model was developed for predicting bed voidage based on an analysis of the liquid-solid interaction forces acting on a suspended particle. In comparison with previous models, this model provides more reliable predictions of bed voidage and produces results that are more in agreement with the experimental results from this and previous studies.

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