Granular Filtration of Airborne NaCl Nanoparticles and Carbon Nanotubes

Filtration is one of the primary technologies for nanoparticle control. However, conventional filtration techniques are evaluated mostly for solid mass removal and do not show promising effectiveness for filtration of ultrafine particles. Granular beds can offer an alternative approach to remove ultrafine particles from the gas. They can offer high filtration efficiency of particles in the submicrometer. The effects of electrostatic forces on filtration efficiency are assumed to be insignificant and neglectable since the nanoparticles are very small and the charges on them are limited. All of that is based on conventional filtration theory developed from single fiber models. However, for granular bed filters, the effects of electrostatic forces are not proved yet. The particles can take charges when they are generated and/or during their transportation. Moreover, the electrostatic forces between particles and particles, and between particles and filter media may have effects on particle transportation in the air and deposition on the surfaces of the filter media. This work focuses on granular filtration of airborne NaCl nanoparticles and carbon nanotubes. It aims at understanding the effects of electrostatic forces on nanoparticles and to better understand the granular filtration mechanisms of single-walled carbon nanotubes. The effect of charging was investigated using nanosized NaCl particles. The range of particles tested was about 6-250 nm in diameter under the room conditions. Three sizes of homogeneous glass beads (2, 4 and 6 mm in diameter) were used in the tests each at three media thicknesses (2.5, 7.6 and 12.7 cm), and at three flow rates (23, 45 and 65 liters per minute). The results showed that the effects of electrostatic forces had opposite effects on filtration efficiency of particles smaller and greater than 20 nm. Electrostatic forces improved the filtration efficiency of large salt particles and reduced that of smaller ones. The granular filtration efficiency of carbon nanotubes was investigated with a straight column, and the lengths of the nanotubes were about 1- 100 µm. The granular filters were made of the same homogenous glass beads (2, 4 and 6 mm in diameter), three air face velocities of 12.0, 16.8 and 25.0 cm/s and three bed thicknesses of 10, 20 and 40 cm. Results showed the overall granular bed filtration efficiency decreases with increasing particle size, which indicated that the dominant mechanism is Brownian diffusion. The flow rates have an adverse effect on the filtration efficiency. As the bed thickness increased, the filtration efficiency increased. And the fine granule media diameter is a favorable factor for increasing filtration efficiency. Four theoretical models were compared with the experimental results of filtration of carbon nanotubes. The UBE model had a good agreement with the experiment results by 2 mm glass beads. And Tardos’s model fitted the experiment results better by 4 mm and 6 mm glass beads