Hydrodynamic studies of the flow of fine particles through a fluidized dense bed of coarse solids

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Talukdar, Jaideep
Engineering; General; Chemical
thesis / dissertation description
This study explains the hydrodynamics of a circulating fluidized bed (CFB) system, the Battelle Multi-Solids Fluidized Bed System (MSFB). It consists of a circulating fluidized bed of fine particles superimposed on a bubbling bed of coarse solids.One way to characterize such a system is to describe the mechanism of gas-solid flow through the bed. The gas flow in systems like these is through bubbles or slugs (regions of voids containing little or no solids). Bubbles are typically characterized by their size (length or diameter), their rise velocity, and their frequency.Another task of the initial phase of this study is to characterize an L-valve, a solids-recirculating device commonly used in an MSFB.Next, the mechanism of fine particle movement through a bubbling region of coarse fluidized solids is studied in considerable detail. Bubble characteristics are studied in a variety of systems of coarse particles with fines passing through at high velocity.Amongst numerous optical, electrical and other techniques available for the study of the passage of bubbles, the pressure fluctuation technique is the most robust. In this investigation, pressure probes are connected to pressure transducers which are in turn linked to an on-line data acquisition system supported on a microcomputer. A commercially available software package (Notebook) is used to sample pressure at specified points in the fluidized bed at extremely fast rates, of up to 200 Hz. This resulted in pressure-time traces which are analysed to give bubble length, bubble rise velocity, and bubble frequency.Another important objective of this study is to estimate the fine particle residence time in the dense bed section. A defluidization technique is utilized in experimentally measuring the solids holdup in the dense bed.A mathematical model is developed from first principles, based on a momentum balance on the fine particles. An empirical correlation is also developed, so that the residence time of fine particles may be predicted for any fine particle-coarse particle system, based on system dependent parameters. The model is in fair agreement with experimental data.The 'gross' or macroscopic aspects of an MSFB are also investigated. These include pressure drop in the dense bed of coarse particles, dense bed height and dense bed voidage variation, with the passage of fines. (Abstract shortened by UMI.).