Removal of aerosols by bubbling through porous media submerged in organic liquid
Introduction
Modern industrial processes generate atmospheric wastes ranging from gases to coarse particles, with a variety of chemical and biological activities. The venturi scrubber is a wet technique which is used for the removal of gaseous pollutants, and can achieve efficiencies of 97% for particles down to in diameter (Cooper & Alley, 1994; Behie & Beckmans, 1974).
A removal of aerosols on the walls of bubble freely rising in a liquid has been studied by Remy (1936), Fuchs (1964), and more recently by Valdberg, Isianov, and Ialamov (1993). Fuchs (1964) modelled the processes of interception, inertia and diffusion, which are the major processes involved in filtration by bubbling. However, the bursting of the bubble may also create further aerosol particles such as film droplets and jet droplets (Yu-Mei Kuo & Chiu-Sen Wang, 1999). This process may result in the bubbler generating aerosols from its top surface.
A more recent technique involves immersing a fibrous filter on a sieve plate and passing the aerosol and carrier gas through the device (Agranovski, 1995; Agranovski, Myojo, & Braddock, 1997). High efficiencies above 99% have been reported for the device where water is used as the irrigating fluid (Agranovski, Myojo, & Braddock, 1999a). The performance of this device does not depend on the level of liquid above the filter, and thus the filtration processes are active inside the wet porous media. Agranovski et al. (1999a) showed that the carrier gas flows along narrow tortuous pathways through the wet filter and that inertia effects result in capture on impact on the liquid walls of the pathway. Interestingly, Yung-Sung Cheng, Yue Zhou, and Bean T. Chen (1999) found that inertial impaction was the main cause of particle deposition in the mouth, throat and lungs of humans.
Many pollutants in modern society involve organic substances which need to be filtered from exhaust streams or from ambient air. Water does not have a high solubility for organic compounds (Atkins, 1990). However, the wet-filter technology can use other liquids with better solubility for organic substances.
This paper investigates the use of a food oil as an irrigating liquid for the wet-filter technology. Experiments are made using a variety of filter media, and the results compared with the performance of the dry filters, and the case when the irrigating fluid is water.
Section snippets
Background
To investigate the flow paths of a carrier gas (air) through a fibrous filter submerged into a water layer, Agranovski et al. (1999b) used nuclear magnetic imaging. They found that air pathways were tortuous air tubes with diameters of the order of 0.5– for the investigated thick filter made out of polyester fibres with diameter with the packing density of 29%. Other observations have been made of the diameter of the air bubble as it detaches from the filter under slow flow
Experimental apparatus
The experimental set-up used in this investigation is shown in Fig. 1 and consisted of
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vacuum pump, air flow meter and air flow control valve to provide air at required flowrate,
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SLG-250 (Topaz, Germany) aerosol generator,
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filter chamber with plate, filter, restraining net and circulating oil,
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particle-concentration monitoring system, Aerodynamic Particle Sizer (APS3310, TSI, USA) with a diluter, flow and pressure meters,
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HEPA filters on the inlet and exhaust streams.
The aerosol generator provided
Observations
Photographs of the bubbling process were used to measure the bubble size for various flow rates and with water and then with oil (see Fig. 3). It was observed that the bubbles in oil are generally smaller than those for water, and tended to remain spherical. The upward flow of oil bubbles also tended to converge laterally and rise in a tapered formation. The bubbles in water tended to rise vertically with less convergence.
Measurements of the diameters of bubbles which had been just released and
Discussion
The results for the efficiencies for bubbling through oil and water indicate that inertial processes are becoming more dominant as the particle size increases. For particle sizes above , the efficiencies all tend to 100% and inertial forces dominate. At higher flow rates, inertial forces are also more important as the particles are less able to adjust to the air flow as it negotiates the flow tubes through the filter. For particle sizes below , diffusion processes will become more
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