Acoustic Filtration of Fine Aerosol Particles
The Challenge
Removal of fine particles PM 2.5 (smaller than 2,5 microns) from open-air is at present, one of the most important challenges in air pollution abatement. These particles are very dangerous because of their ability to penetrate deeply into the lungs, their long resistance in suspension in the air and their character generally toxic due to both their origin (condensation of chemically active elements) and their ability to act as vehicle of noxious agents as virous, bacteria, dioxins, etc. Lung damaging dust can be as small as 0.5 microns, and airborne virous can be as small as 0.3 microns. On average, in open-air urban areas more than 99% of the total number of particles are characterised by a diameter smaller than 1 micron. Heavier and larger particles will settle out, and never even make it to air filters. Then the effective way to improve indoor and outdoor air is to remove micropollutants. The problem is that the conventional filters are inefficient to capture submicron particles. The best air filters called HEPA (High Efficiency Particulate Air), are effective at removing particulate matter like pollen and dust matter, but small pollutants less than 0.3 microns, like viruses and bacteria, can escape to them. On the other hand, microorganisms collected on HEPA filters can multiply and get released back into air.
How ultrasounds win the challenge:
The PUSONICS power airborne ultrasonic plate-transducers are efficient to agglomerate micron and sub-micronic particle matter, increasing their size.
An intense ultrasonic field applied to an aerosol induces particle interactions that lead to the formation of larger particle agglomerates. This growth of the aerosol particle size distribution favors the collection of the tiny particles for conventional methods.
The stronger effect is the relative motion of particles of different sizes or densities along the ultrasound field. The particle entrainment by the sound field makes the bigger and heavier particles move slower and the smaller and lighter particles move faster.
Examples of aersols tested:
carbon black smoke, smoke from burning rubber, black soot, glycol fog, mixture of black soot and glycol fog, coal combustion fumes, diesel exhausts, SiO2 and TiO2
Size range of the particles tested: 0.2 to 2.5 micron
Particle growth after processing: Up to 60 times size increase with 5 sec treatment time at 21 kHz and about 163 dB, depending on the parameters of the aerosol.
Case Studies
Preconditionning systems for coal combustion and diesel exhausts:
An agglomeration chamber with four plate-transducers installed prior to an electrostatic filter in a gasoil power plant.
Exposure time of 2 seconds inside the chamber improved the retention efficiency of the electrostatic filter about 40% for both micro and sub-micron sized particles.
Where to apply Pusonics technology for air cleaning?
- Preconditioning of conventional filters, with flow rates of the order of 1000-2000m3/h
- Outdoors and indoors applications, with particulate number concentrations of 104 – 105 particles/cm3
- Industries, public building, residences…
References
"Ultrasonic agglomeration and preconditioning of aerosol particles for environmental and other applications" E. Riera, I. González-Gómez, G. Rodriguez and J.A. Gallego-Juárez, Chapter 34, pp 1023-1058 in "Power Ultrasonics" J.A. Gallego-Juárez and K.F. Graff, (Eds), WP-Elsevier, Cambridge, UK 2015. https://doi.org/10.1016/B978-1-78242-028-6.00034-X
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