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Thesis (doctoral)--Helsinki University of Technology, 2000. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.S.)--Ohio University, November, 1982. / Title from PDF t.p.

Thesis (M. Eng. Sc.)--University of Adelaide, Dept. of Chemical Engineering, 1989. / Includes bibliographical references (leaves 220-225).

Thesis (Ph. D.)--University of Adelaide, Faculty of Engineering, 1993. / Includes bibliographical references (leaves 315-326).

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2010. / Full text also avaiable online. Scroll down for electronic link.

Heat transfer in circulating fluidized beds was studied in both a 7.3 m high, 152 x 152 mm square, pilot-scale combustor and a 9.3 m high, 152 mm ID transparent cold model unit. Results were obtained for particles of mean size 171-299 µm at superficial gas velocities from 4 to 9.5 m/s and for solids circulation rates up to 70 kg/m².s. For the combustor, results obtained by using membrane walls and a vertical tube as heat transfer surfaces show a strong influence of the cross-sectional area-averaged suspension density on time-averaged, length-averaged suspension-to-surface heat transfer coefficient. The influence of superficial gas velocity is found to be small. Radiation becomes significant at suspension temperatures higher than 400 C and at low suspension densities. Heat transfer coefficients were also found to vary with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter, allowing seemingly discrepant published results to be reconciled. For the cold model unit, sudden and dramatic peaks in instantaneous heat transfer coefficients were measured using an instantaneous heat transfer probe. Simultaneous heat transfer and capacitance measurements suggest that these peaks are caused by the arrivals of particle strands at the heat transfer surface. Two-probe heat transfer measurements suggest the existence of a characteristic residence length for the strands at the wall in this column. A proposed heat transfer model, based on an overall core-annulus flow structure in the riser, and periodic formation, movement along the wall, and disintegration of strands in the annulus, gives reasonable agreement with a wide range of published data. It accounts successfully for the effects of heat transfer surface length and particle sizes. However, the effect of the heat transfer surface configuration on the flow pattern of particles must also be taken into account to give improved agreement with experimental data. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Heat transfer tubes suffer erosion when immersed in fluidized beds. This has caused problems, especially in fluidized bed combustors. The mechanism of erosion for horizontal tubes in fluidized beds is not well understood. The purpose of this study was to investigate the erosion mechanism in fluidized beds and to investigate the influence of operating parameters and the mechanical properties of the particles and tube materials. Horizontal tube erosion tests were carried out in a room temperature three-dimensional fluidized bed with a cross-section of 216 mm by 203 mm and height of 1.52 m. Sample rings of ten different materials were mounted on a solid bar and were weighed before and after each test to determine the erosion rate. The parameters tested were particle size (0.30 to 1.51 mm), particle sphericity (0.84 to 1.0), particle density, particle hardness, superficial air velocity (0.88 to 2.52 m/s), tube diameter (15 mm to 32 mm), tube configuration and material mechanical properties. Two additional types of experiments were also conducted to help understand the mechanism of erosion. In one particles were dropped freely in an empty column to impact on test specimens at different velocities determined by the dropping distance, in order to investigate erosion due to solid particle impact under known conditions. In the other the particle movement was filmed in the vicinity of a horizontal tube in a two-dimensional fluidized bed in order to investigate the particle flow pattern around a tube. A small number of tests were also conducted at high temperatures. The erosion of a horizontal tube in fluidized beds was found to be caused mainly by the impact of solid particles on the lower surface. Erosion was found to be strongly dependent on the particle impact velocity, which is closely related to the void (bubble or slug) rise velocity. The void rise velocity, in turn, is determined by the mean void size which depends on the superficial air velocity, column size and other fluidizing conditions. Particle diameter also has a strong influence on erosion. The target material Young's modulus appears to be the major mechanical property which is closely related to the erosion rate caused by solid impact erosion. Of the materials tested, all non-ferrous metals suffer much more erosion than ferrous metals. Localized high particle velocities due to jets and at bends or near feed points can be extremely harmful. The mechanism of erosion caused by low velocity (< 6m/s) solid particle impacts appears to be different than that caused by high velocity (> 30m/s) impacts reported in the literature, although there are some similarities in trends. The erosion at low impact velocities appears to be mainly due to a surface fatigue process, which, instead of plastically deforming a small amount of target material for every impact, deforms the target materials in the elastic range and causes them to crack on or underneath the surface leading to removal of materials. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

A two part study was undertaken to explain the performance of cyclones operated in circulating fluidized bed combustion (CFBC) systems. In the first part, collection efficiency tests were performed on a one-ninth scale polyacrylic cyclone model of the industrial scale cyclone at the 22 MWe CFBC facility at Chatham, New Brunswick. Emphasis was placed on scale-up considerations, loading effects, inlet geometry effects, and flow visualization trials. Experiments were performed at room temperature with inlet velocities between 3.7 and 5.5 m/s, solids loading between 0.05 and 7.5 mass solids/mass air with two different solids systems. There was disappointing agreement between the results from the Chatham unit, scaled according to Stokes Number scaling, and the findings obtained from the cold model unit. There was a minimum in the particle collection efficiency for particles of diameter 2.5 to 3.0 µm, apparently associated with agglomeration effects in the cyclone. Particle collection efficiency was found to increase with increased particle loading for the conditions studied. Changes in the inlet geometry gave inconclusive results. The experimental results were limited by problems associated with feeding and recycling the fines solids system used. In the second part radial gas concentration profiles of a secondary cyclone serving the UBC pilot scale Circulating Fluidized Bed Combustor were performed at temperatures of about 870 ℃. Concentrations of O₂ , CO₂ , NO[formula omitted] , CH₄ , CO and SO₂ were measured. An increase in [CO], and to a lesser extent [CO₂], was measured near the cyclone wall. There appeared to be little radial variation in the concentration of other species. Further work is required to allow the cold model to operate continuously, with particles which can be fed more freely, and to obtain radial gas concentration profiles within the primary cyclone of the UBC CFBC system. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

In gas-solid fluidized bed reactors, such as those employed for polyethylene production, the generation of electrostatic charge is almost unavoidable. Electrostatic charges are generated due to the continuous contacts between particles and particles and the reactor wall. In such processes, accumulation of electrostatic charge causes a layer of particles to adhere to the reactor wall, a problem known as “sheeting” in polyolefin industry. Sheeting results in frequent reactor shutdowns for clean-up and in turn significant economic loss. The overall focus of this research is to better understand the underlying mechanisms of charge generation in gas-solid fluidized beds to ultimately be able to find means to reduce or eliminate this problem. The specific objective of this thesis is to determine the effect of fluidizing particle size on the degree of bed electrification and reactor wall coating. The experimental program involved the fluidization of polyethylene resins received directly from commercial reactors (i.e., having a wide size distribution of 20-1500 micron), as well as mono-sized large particles (600-710 micron) and binary mixture of small particles (200-300 micron and 300-425 micron with fractions up to 20 wt%) and large particles (600-710 micron). Experiments were carried out under atmospheric conditions in 3D fluidization columns housing two Faraday Cups for electrostatic charge measurement. For all conditions, the charge, mass and size distribution of particles fouled on the reactor wall as well as the layer thickness were measured and compared. Fluidization of the resins as received resulted in a certain size of particles (400 µm and smaller) to adhere to the column wall. For binary mixtures, the particles layer formed on the reactor wall mainly consisted of the smaller particles. Although the extent of wall coating declined as the amount of the smaller particles increased, but the smaller particles had a much higher net specific charge and thus replaced the large particles within the wall coating. Such high charge of small particles accumulated on the column wall in turn prevented the wall coating growth due to repelling the oppositely charged particles to the bulk of the bed. Regardless of the charge polarity of the bulk and wall particles, the wall fouling formation mechanism was found to be similar. Between the two sizes of small particles tested, the 212-300 micron particles gained a higher net specific charge than 300-425 micron particles. Bipolar charging due to small and large particles contacts was detected within the bulk of the bed and the wall coating.