BED EXPANSION AND SOLID MIXING IN HIGH VELOCITY FLUIDIZED BEDS.
Item
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Title
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BED EXPANSION AND SOLID MIXING IN HIGH VELOCITY FLUIDIZED BEDS.
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Identifier
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AAI8103912
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identifier
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8103912
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Creator
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AVIDAN, AMOS A.
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Date
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1980
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Language
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English
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Publisher
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City University of New York.
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Subject
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Engineering, Chemical | Energy
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Abstract
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High velocity gas fluidization is a unit operation which brings gas and solid into intimate contact. It includes the turbulent and fast fluidization regimes which lie beyond the low velocity bubbling or slugging regimes. Bed expansion and solid mixing were studied in 15.2 cm I.D. rigs, of high aspect ratios: a circulating system 8.5 m tall, and an expanded top bed, 3.5 m tall. Fine cracking catalysts of Geldart's group A classification were used and air at ambient conditions served as the fluidizing medium. The gas velocities investigated were in the range 0.075-6.5 m/s and the solid rate in the upper end of the fast fluidization regime topped 150 kg/m('2).s.;The Two-Phase Theory is known not to apply when the gas velocity is raised beyond the slugging regime: the bed structure becomes more homogenous. Particulate like fluidization describes these high velocity regimes. A modified Richardson-Zaki approach is used to correlate bed expansion. It is shown to confirm the more homogeneous nature of the turbulent and fast regimes, and the validity of the concept of clusters--or the tendency of fine particles to aggregate. This clustering phenomenon is quite complex.;Solid mixing was studied with a ferromagnetic tracer and modelled by a turbulent eddy diffusivity dispersion model. Results in the low velocity expanded top bed show a maximum for the axial dispersion coefficient in the slugging regime. The dispersion coefficients in the turbulent regime are uniform throughout the bed and depend on the mode of solid recirculation--they are higher than those in the slugging regime when the recirculation is from the top (the expanded top bed). The axial Peclet number goes through a minimum in the fast fluidization regime (a gas velocity of 3.5 m/s, a solid rate of 100 kg/m('2).s and a voidage of 0.82), qualitatively in agreement with results from liquid-solid systems. The dependence of the axial dispersion coefficient on bed diameter is shown to be linear by a steady state density fluctuation analysis. Some implications for heat transfer in a high velocity fluidized bed are discussed.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
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Program
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Engineering
