Theoretical and experimental study of the removal of dust particles from flue gases using two-phase steam-water droplet
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INTRODUCTION
T H E efficient collection of fine particulates (less than 3/~m in size) from effluent gases is a major environmental and technological problem facing the steel industry. It is generally held that conventional dust abatement techniques, such as the use of venturi scrubbers and electrostatic precipitators, work very well for particle sizes down to about 5 microns, but for smaller particles the energy consumption will become quite high, as reported by Young et al. and Natkins et al. 1.2 In the last few years a number of techniques have been proposed 3'4'5 for the removal of fine dust particles from effluent gas streams. Recent work done at Aeronetics Inc. 4 and Lone Star Steel Company 5 has shown that a two-phase mixture of steam and water droplets, obtained by flashing pressurized hot water through a nozzle, is an excellent means for the efficient scrubbing of submicron sized particles from effluent gases. In the aeronetics system the two-phase mixture of steam and water droplets is produced by flashing subcooled water through a convergent-divergent nozzle. On the other hand, in the Lone Star steel system the two-phase mixture of steam and water droplets is produced by atomizing water sprays with a supersonic steam jet. These latter methods of particulate scrubbing are attractive because the principal energy requirements of these systems may be obtained directly from hot flue gases. While there has been some reported practical experience with the operation of these systems, the fundamental aspects of droplet formation on the flashing of subcooled water and steam-water mixtures through nozzles has received very little attention up to the present. Previous work concerning two-phase steam-water flows ~ 9 has been primarily in the area of critical flows and on the prediction of critical flow
L. BRITTO, formerly a Graduate Student at Massachusetts Institute of Technology, is with IBM Corporation, Lexington. KY. J. SZEKELY is Professor, Department of Materials Science and Engineering, Massachusetts Institute of Technology. Cambridge, MA 02139. Manuscript submitted April 9, 1984
METALLURGICAL TRANSACTIONS B
rates, with little emphasis on measurement and prediction of droplet sizes and velocities. Furthermore, there has been no previous effort made to model the two-phase scrubber dust collection system, in order to predict dust collection efficiencies. The work to be described in this paper has two major components. One involves the experimental and theoretical study of droplet formation in the expansion of subcooled water and two-phase steam-water mixtures upon passage through a convergent/divergent nozzle. The other concerns the development of a theoretical model to predict dust collection efficiencies of a two-phase scrubber as a function of the various process parameters. These two components of the work are, of course, very closely interrelated, because as will be shown subsequently, the dust removal efficiency in these systems depends critically on the nature of the two-phase system, that is, the
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