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Title: Effects of ultrafines on the hydrodynamics of gas fluidized beds
Author: Zahraei, Firoozeh
ISNI:       0000 0001 3576 5167
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1995
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It has been reported in the literature that the addition of small quantities of so called fines (usually defined as particles less than 10μm in diameter) to Group A/C and B materials can significantly change the fluidization characteristics of the host materials; e.g. suppress bubbling, cause very large bed expansions and reduce elutriation from the bed. This change in fluidization behaviour can be linked to the physical properties of the material. The aim of this project was to study, by a combination of experiments, the changes in fluidization behaviour of typical Group A and C materials with increasing quantities of ultrafines. The work was divided into two sections: (i) fluidization and (ii) systematic study of properties relevant to it. The fluidization experiments mainly consisted of measurements of minimum fluidization and minimum bubbling velocities as well as the bed expansion for a range of bed conditions. Bed collapse experiments were also carried out to determine the dense phase expansion together with the expansion due to bubbles. The latter basically included measurements of physical properties such as particle size and its distribution, pore size distribution and various densities of powders for several combinations of ultrafines and host materials. Cracking catalyst (FCC), silica and kieselguhr were the host materials while a range of fumed silica were used as the ultrafine in the experiments. Additives appeared to have a considerable effect on FCC and silica by increasing the expansion up to 50%, increasing the deaeration velocity and reducing the elutriation loss while the fluidization was much smoother in general. The data obtained with kieselguhr, a typical Group C material, indicated that under some conditions, the presence of the additives resulted in significant increases in bed expansion and improved the fluidization behaviour. The extent of these effects appeared to depend on the concentration of the ultrafines. Additives appeared to change the fluidization behaviour of the material by forming a particular network which leads to a looser bed structure. Different predictive criteria for transition from particulate to aggregate fluidization were used and compared to the experimental data. It was concluded that as well as the hydrodynamic effects, interparticle forces were of importance in the stability of the bed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical engineering