Transition from depth to surface filtration for a low-skin effect filter subject to continuous loading of nano-aerosols

Abstract

A thin, highly porous, nanofiber filter with low-pressure drop start out in “depth filtration” in a clean state with aerosols mostly captured by the fibers in the filter. For nanofibers with high capture efficiency, the fibers in the region upstream of the filter facing the challenging aerosol flow tends to capture more aerosols. The capture aerosols in turn can capture more incoming aerosols in this region leaving much less aerosols escaping downstream of the filter. Furthermore, these captured aerosols, forming dendritic structures, reduce the size of flowable pores in the region. This “skin region” upstream of the filter can be quite thin as compared to the entire filter thickness, yet it accounts for the large fraction of the aerosols captured in the filter as well as large fraction of the pressure drop across the filter. With more aerosols loading, the openings of the flowable pores in the skin region get blocked by aerosol dendrites that bridge across captured aerosols and fibers. These aerosol bridges subsequently build-up above the filter surface in the “surface filtration” regime. As more aerosol bridges stack-up on existing ones and interact with each other, they form ultimately a continuous cake layer on the filter surface. The pressure drop due to the skin effect, which is a combination of both pore filling in the skin region in the filter and the aerosol bridges, can be relatively low (i.e. low-skin effect) for low nanofiber packing density and a thin nanofiber filter. However, the establishment of the skin region during depth filtration and the aerosol bridging effect above the filter surface are still two essential processes prior to formation of the cake on the filter surface. Two experiments and related analysis on loading of thin nanofiber filters using nano-aerosols are used to investigate this scenario. It has been found that in the course of aerosol bridging, the pressure drop behaves predominantly concave upward with increasing specific aerosol deposit. This contrast with previous study with high-skin effect for a thick, less-porous nanofiber filter for which aerosol bridging predominantly yields a concave downward behavior with specific aerosol deposit during pressure drop excursion. Most importantly, the bridging model that predicts successfully the high-skin effect can also predict the low-skin effect where pressure drop has a concave upward behavior with increasing specific aerosol deposit. Finally, we have demonstrated the aerosol holding capacity for a thin, highly porous, nanofiber filter can still be significant, with a small fraction of the total aerosols captured by depth filtration in the filter, and majority of deposited aerosols above the filter in the surface filtration regime, partly in the aerosol bridges and mostly in the cake.