Skin layer in cyclic loading-cleaning of a nanofiber filter in filtering nano-aerosols.

Nano-aerosols from viruses to virgin pollutant particulates from combustion, 100 nm or smaller, are harmful to our health as they penetrate readily into our body causing various diseases. Nanofiber filter can capture effectively these nano-aerosols. However, over time the pressure drop increases dramatically and cleaning of the filter by backpulse/backblow is essential for filter reuse. The cyclic loading-and-cleaning of a nanofiber filter has been investigated for the first time experimentally and theoretically. The "skin" layer, a thin region upstream of the nanofiber filter, plays a pivoting role in controlling the pressure drop excursion of the filter. We model the skin layer to be made up of numerous fine capillaries and examine how continuous aerosols deposited in the capillaries affect rapid rise in pressure drop followed by bridging of aerosols across the capillary openings leading to more bridging and ultimately formation of cake on top of the bridges and filter surface. We have been able to describe the deposition of aerosols in the capillary pores for depth filtration, the deposition of aerosols in the cake (surface filtration), and the intermediate bridging regime between these two. We can depict the complete pressure drop excursion including the S-shaped curve behavior from depth filtration transiting to surface filtration for a filter with severe skin effect. Our prediction matches extremely well with the 6 cycles of loading/cleaning on a 280-nm nanofiber filter subject to challenging nano-aerosols, 50-400 nm. During cyclic loading and cleaning, the porosity and permeability in the skin layer for our experiment drop to 68% and 11-21% of their original values, respectively, and the effective pore diameter also drops from 1.2 to 0.6 µm.

Experimental investigation of backpulse and backblow cleaning of nanofiber filter loaded with nano-aerosols.

Nanofibrous filter have been proven effective to remove nano-aerosols with size less than 100 nm. Cleaning is required after long-term use; however, very little has been published on the subject. An experimental investigation has been launched to determine backpulse, backblow and combined backpulse-backblow on cleaning of a loaded nanofiber filter. Nylon 6 nanofiber filters were loaded with polydispersed NaCl particles, 60% < 100 nm and 90% < 160 nm, generated from an aerosol generator. Air jets in form of backpulse, backblow and their combined mode were used to clean a loaded filter. During cleaning, the filter cake was removed first for which the pressure drop across the loaded filter decreased rapidly, followed by loosely attached aerosols in the filter being removed with finite pressure drop reduction at a reasonable rate, ending in the final stage for which much lesser aerosols were being removed. Ultimately, the filter reached a residual pressure drop which was higher than that of the initial clean filter indicating residual aerosols were trapped both in the cake heel and filter. Backpulse has been found to be more effective in removing the cake from the filter surface, whereas backblow provides an added advantage of removing by convection of the detached aerosols away from the filter preventing recapture. The synergistic combination of backpulse-backblow provides the best cleaning performance of a nanofibrous filter loaded with nano-aerosols.