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Inertial Separation of Airborne Nanoparticles with Fibrous Filters

Yoshio Otani, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Kanazawa, Japan, J. C. Kim, Chemical Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Kanazawa, Japan, and Naoko Tajima, Kanomax, 2-1 Shimizu, Suita, NH, Japan.

INTRODUCTION Nanoparticles such as carbon nanotubes, fullerenes, titanium dioxides are prospective nanomaterials with various industrial applications. However, the health effects of these nanoparticles when inhaled are of great concern especially for those who are working in the manufacturing processes. The health effect may result from the combination of the size and the chemical compositions. When these nanoparticles are dispersed into the air at a high concentration, they readily agglomerate or attach to ambient particles so as to form larger particles. Therefore, it is essential to separate these agglomerated particles from the primary nanoparticles for assessing the health effect when conducting the exposure study. Moreover, the information on the composition of atmospheric nanoparticles with respect to particle size is the key in order to investigate the health effects of atmospheric nanoparticles. Several aerosol classifiers for nanoparticles, such as a differential mobility analyzer and a low pressure impactor, are available at present, but the mass of nanoparticles obtained as a result of classification is so small that the amount is not enough for the accurate composition analyses. The present work proposes to use fibrous filters for separating nanoparticles from the larger particles for the subsequent exposure study and chemical analyses. Filters are commonly used to collect particles in all size ranges, and never used for the classification of particles. However, by passing aerosol through a filter at a high filtration velocity, it is possible to collect submicron particles by inertial filtration while maximizing the penetration of nanoparticles by suppressing the Brownian deposition in the filter.

EXPERIMENTS Stainless steel fiber mat was used as a filter material since it resists at high filtration velocities of several m/s. Polydisperse zinc chloride particle were generated by an evaporation-condensation type aerosol generator and passed though the filter. The inlet and outlet concentrations were measured by using a scanning mobility particle sizer (TSI Inc., Model 3081) for the determination of classification performance of filters.

RESULTS AND DISCUSSION By using the stainless steel fiber mat with the diameter of 8 micrometers, it is possible to have 50% cutoff diameter of particles as small as 100 nm at the filtration velocity of 50 m/s. The pressure drop across the filter was about 5 kPa. The pressure drop is relatively small compared to the low pressure impactors so that the HEPA filters placed downstream of the inertial filters can collect particles smaller than 100 nm without any significant changes in particle composition due to the evaporation of volatile species under the reduced pressures. Furthermore, by increasing the filtration area of inertial filter, we can achieve high sampling flow rate to collect particle masses sufficient to the chemical analysis, which in turn make it possible to measure the composition changes in particles with a high time resolution.

CONCLUSIONS The present work proposes a new concept of the usage of a fibrous filter, i.e., the utilization of fibrous filters for the classification of nanoparticles. The present work confirmed that the filter employed in the present work can separate particles smaller than 100 nm at the filtration velocity of 50 m/s. By selecting an appropriate filter structure and fiber diameter, it is possible to develop a classifier for separating smaller particles.