The behaviour of freely bubbling fluidized beds
Geldart, Derek
Geldart, Derek
Publication Date
End of Embargo
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The University of Bradford theses are licenced under a Creative Commons Licence.
Peer-Reviewed
Open Access status
Accepted for publication
Institution
University of Bradford
Department
Postgraduate School of Studies in Powder Technology
Awarded
1971
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Additional title
Bubble sizes in cylindrical equipment are satisfactorily predicted from theory and from two-dimensional studies. The distribution of gas between dense and bubble phases is calculated from bed expansion data
Abstract
Gas fluidized beds have provided fascinating and infuriating problems for both the academic and industrial engineer. Although
fluidization phenomena have been the subject of many hundreds of papers we still cannot dispense with experimentation in order to predict bed behaviour even for relatively simple operations, let alone complex chemical reactions.
This thesis provides a broad but brief scrutiny of the fluidization behaviour of a wide variety of powders and then focuses on one specific but important question: how can we predict the sizes of bubbles under given conditions in a large fluidized bed?
Experimental work on a wide variety of sands and other materials has been carried out in cylindrical columns 5 cm and 50.8 cm diameter, and in addition films from two two-dimensional beds have been analysed.
Fluidized solids have been classified into four groups characterised by density differences Pg-Pf, mean particle size, and gas viscosity.
A critical examination of the results of previous workers together with the new experimental work has shown that within one of the largest groups, bubble size is independent both of mean particle size and - more significantly - of particle size distribution.
An equation has been developed which relates bubble size to the gas distributor design, distance above the distributor and the excess gas velocity U-U0 , and good agreement is obtained with published data relating to larger beds fitted with commercial distributors.
Measurements of bed expansion combined with the analysis of films from two- and three-dimensional equipment have proved useful in evaluating the distribution of gas between dense and bubble phases.
For a given value of U-U0, the visible bubble flow rate decreases as particle size increases. This is because the through-flow of gas from bubble to bubble is proportional to the minimum fluidization velocity U0. As a result both bubble concentration and bed expansion decline with increasing particle size but bubble size is not affected.
An equation has been developed which enables the visible bubble flow rate to be calculated from measurements of bed expansion and U-U0 alone.
bubble concentration, frequency and size have been compared at the surfaces of two- and three-dimensional beds under corresponding conditions of distributor design, bed height and gas velocity. Bubble sizes are significantly larger in the three-dimensional equipment and this is believed to be due to the predominance of out-of-line coalescence. A transformation procedure has been developed for the prediction of dimensional bubble sizes from two-dimensional data, and has been successfully applied to the only sand so far tested.
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Type
Thesis
Qualification name
PhD