Fluidized bed hydrodynamics by means of electrical capacitance tomography
Choudhary, Mohammad Omer
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Fluidized bed reactors are utilized in a variety of applications from hydrocarbon cracking to drying of mineral ore to coating of pharmaceutical pills. A proper understanding of bed hydrodynamics is essential to properly design, operate and control the process. Intrusive and non-intrusive measurement techniques have been utilized to study various aspects of fluidized bed behaviour. Electrical Capacitance Tomography (ECT) has been utilized to determine the distribution of gas-solid mixture in the bed. The Digital Image Analysis Technique (DIAT) is used to determine the bubble behavior in two-dimensional beds. The bubble behavior has not been studied by ECT. The premise of this research is to test a new technique by combining ECT solid fraction maps with image processing techniques to determine the bubble characteristics in the bed. Electrical Capacitance Tomography (ECT) was used to map the relative fractions of sand-air mixture in a 14 cm ID acrylic vessel at two different static bed heights. The voids were defined as the areas of gas-solid mixture with value of less than 0.25. Afterwards simple image analysis techniques were applied to isolate bubbles from the rest of the bed. The resulting data were converted into binary images to extract hydrodynamic information. The two main parameters of interest were the bubble diameter and its rise velocity. The experimental average velocities and average diameters matched the results obtained from respective correlations in the literature. However, large spreads existed for both these parameters due to the simultaneous presence of bubbles and slugs. The experimental fluidized bed did not transition fully into the turbulent regime but has the character of a hybrid bubbling-slugging regime. This is illustrated by experimental diameters which indicate presence of bubbles and slugs simultaneously. The same information was illustrated by experimental velocity-diameter envelopes (minimum and maximum values) which overlapped for higher flow conditions. The radial solid fraction distribution illustrated the same point with an inverted “M” profile which has been associated previously with regime transition. The path of bubble rise was identified, mainly rising through the midway point between the bed’s center and its inner edge. Five different types of bubbles were identified using the two dimensional binary images of voids. However, this technique can only be used for regimes where voids/bubbles are the main source of gas transfer to the surface of the bed.
DegreeMaster of Science (M.Sc.)
DepartmentChemical and Biological Engineering
CommitteeBaik, Oon-Doo; Phoenix, Aaron; Stanley, Kevin
Copyright DateJanuary 2015