Agglomerates and granules of nanoparticles as filter media for submicron particles

Abstract

An experimental study on filtration of submicron solid and liquid aerosol particles by using a filter media composed of agglomerates or granules of nanoparticles is described. Fumed silica nanoagglomerates, carbon black granules, silica shells, activated carbon granules, glass beads and nanoporous hydrophobic aerogel were among the granular filter media tested and compared to a commercially available HEPA fiber-based filter. Other than the glass beads which were used for comparison purposes, the primary particle size of the agglomerates/granules is of nanometer scale, but they agglomerate to form porous structures of about several hundreds of microns which were customized as packed (deep bed) or fluidized bed filters and challenged against submicron solid and liquid aerosols. For packed bed filters, the size of the granules has been optimized to a range of 150–500 µm with a filter thickness of about 1–3 in. and superficial gas velocities of less than 4 cm/s. Fluidized beds required granules smaller than 150 µm and the height of the bed was in the range of 15–40 cm. The customized filters and a HEPA fiber-based filter were challenged simultaneously against the same aerosol at the same superficial gas velocities. When using carbon black or aerogel granules as filter media, collection efficiencies comparable or even higher than HEPA fiber-based filters are obtained, but with the advantage of extra filtration capacity due to the deep bed configuration and the absorption of liquids into the porosity of the media. A fluidized bed filter of aerogel granules not only provides higher collection efficiency and larger capacity than a HEPA fiber-based filter when challenged against both oil mist and solid aerosols but also has an extremely low pressure drop compared to a packed bed filter and can be operated continuously with respect to removing saturated granules and adding fresh ones.