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www.elsevier.com/locate/j aerosci

S e s s i o n 4 D - F i l t r a t i o n

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U T I L I S A T I O N OF W E T F I B R O U S M E D I A F O R F I L T R A T I O N OF S T I C K Y A E R O S O L P A R T I C L E S

I.E. AGRANOVSKI and J. M. WHITCOMBE

Faculty of Environmental Sciences Griffith University, Nathan Campus, Queensland Australia 4111

Keywords: Aerosol, Sticky particle, Wettability, Air Quality

INTRODUCTION

Agranovski and Braddock (1998) examined the use of wettable filter fibres for the removal of liquid aerosols. The use of wettable filter material enables liquid aerosols to be trapped by the filter, and drained away under gravity. As more liquid aerosols are trapped an equilibrium layer of liquid is formed on the surface of each fibre. This layer increases the rate of filtration as the layer of liquid begins to trap particles before they reach the filter fibre (Agranovski and Braddock, 1998).

It is proposed that by artificially generating a thin liquid film around a filter fibre, filtration rates can be increased for a wide range of particles, not just liquid aerosols (Whitcombe, 1999). It is common for many chemical and metallurgical industries to generate large quantities of sub-micron 'sticky' particles of pollutants (Whitcombe, 1999). Often the use of conventional wet scrubber technology or dry filtration is ineffective in removing these pollutants to required levels (Cooper and Alley, 1994).

Conventional wet scrubbers are of limited use in such environments as they can not achieve relatively high removal rates (> 95%) for particles down to 1 micrometer in diameter (Cooper and Alley, 1994:220). Dry filtration is also limited in its application because sticky particles are not easily cleaned off and can block dry filter systems causing an increase in pressure drop and the eventual failure in the system (Whitcombe, 1999).

PROPOSED TE CHNOL OGY

By artificially generating a thin liquid layer on wettable fibres the sticky particles will be filtered out of the gas stream by the liquid, and not the fibre as is done in dry filtration. Once the particle is trapped in the liquid film, it is removed as the flim is continuously regenerated by the supply of fresh liquid from above. This continual supply of liquid along the exterior surface of the filter ensures all sticky particles are removed from the filter face, thus preventing blockages and allowing the continually cleaning of the filter's surface. In addition to particle removal, a high efficient removal of gaseous contaminates can also be achieved by "liquid film" technology.

The galvanizing industry, which currently utilise wet scrubber technology, was selected as a representative industry for this study of feasibility of wet filtration in a commercial setting. This facility generates particles of zinc and zinc components in a range between 0.1 ~tm and 10~tm.

DESIGN M E T H O D O L O G Y

The current water separator located down stream of the wet scrubber was identified as the optimal location for the new wet filtration system. The large, hollow separator provides a suitable space to house the filters, eliminating the need for new infrastructure inside the facility. Refer to Figure 1 for a

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Figure 1: Wet filtration system

stylised schematic of the system. The irrigation system is designed to enable each bag to remain continually wet while in use. To reduce water usage at the site, the fresh make up water used to top up the recycled water tank at the base of the separator is utilised for the irrigation system of the filters, allowing fresh water to be continually supplied to each filter. To optimize the use o f water, each filter has been equipped with its own irrigation system located at the top of the filter. The irrigation system consisted of a plastic pipe loop with spray nozzles strategically located around it. Nozzles were selected and installed to ensure the total external surface of the filter is continually supplied with water, allowing the entire filter to remain wet and drain continuously.

RESULTS

Before the installation of the wet filtration system the removal efficiency at the galvanising facility was approximately 79%, based on total particle loading rates and 80% for chloride components.

After the wet filtration system was installed at the commercially operating facility removal efficiencies increased to approximately 98.6% for total particle loading and 98% for chloride components, refer to Table 1 below. The increase in removal efficiency of 19.6% for particles and 18% for chloride was achieved with only a slight increase in pressure drop across the system. Table 1: Results of test conducted after wet filtration installation

Sampling Air Temp Relative Particles Zinc Comp Chloride Comp. Location (°C) Hum (%) (g/Nm 3) (g/Nm 3) (g/Nm 3)

efore Inlet stack 45.8 7.6 0.274432 0.069608 0.097549

earrangement Outlet stack 27.1 64.9 0.057902 0.036865 0.019231

tier Inlet stack 66.4 2.6 0.506452 0.163723 0.119345

earrangement Outlet stack 31.6 6 8 . 3 0.0066996 0.003125 0.001959

CONCLUSION

The use of a wet filtration process for the removal of sub micron sticky particles has been successfully trialed at a commercially operating strip galvanising facility and achieve a removal rate of 98.6% and 98% for total particles and chloride components respectively. These removal rates were achieved at a facility with a long history of producing semi-wet sticky particles, over a wide particle size distribution between 0.01~tm and 30.01am. The use of a wet filtration system can be seen as a very effective tool to allow existing commercial facilities to augment their existing wet scrubber technology without large capital expense.

REFERENCES

Agranovski I. E., and Braddock, R.D. (1998) Filtration of Liquid Aerosols on Wet-table Fibrous Filters AIEChE Journal Vol. 44, 12:2775 Cooper and Alley (1994) Air Pollution Control: A Design Approach, Waveland Press Inc, USA Mycock J., McKenna J. & Theodore L. (1995) Handbook of Air Pollution Control Engineering and Technology, Lewis publishers, USA. Whitcombe, J. (1999) Design of air pollution control technology for use in the removal offine metallic particles in the galvanising industry, Undergraduate Thesis, Griffith University, Nathan, Australia