Chapter eleven - Nanofiber filter reuse—repeated loading–cleaning cycles

Abstract

A nanofiber filter can be effectively cleaned using backpulse–backblow after being loaded with aerosols during gas filtration. In principle, the filter can be cleaned and reloaded indefinitely. Unfortunately, after each cleaning there are additional trapped residual aerosols that cannot be purged resulting in higher residual pressure drop as compared to that of a clean filter. This residual pressure drop, which indicates the residual entrapped aerosols in the filter mainly in the skin layer, increases over loading-and-cleaning cycles. When the residual pressure drop becomes excessive, the filter needs to be replaced. To study the behavior of the skin layer in the nanofiber with repeated loading and cleaning, tests have been conducted on a polyamide 6 nanofiber filter with a mean fiber diameter of 280nm. The pressure drop during repeated loading (using polydispersed NaCl aerosols less than 500nm) and cleaning (using backpulse–backblow) is closely monitored. The aerosol loading process has been repeated six times in between cleaning for the test nanofiber filter. The skin layer for the same nanofiber filter is studied using the analytical capillary, bridging, and cake models previously developed. The initial loading on the test nanofiber filter follows the low-skin model with pressure drop increasing modestly with aerosol deposit in a concave upward behavior, while the second and subsequent loading cycles follow the high-skin model with pressure drop increasing with aerosol deposit in a concave downward behavior. By matching the models with the measured pressure drop excursion for five subsequent loading cycles, the porosity and diameter of flowable pores, thus the permeability of the skin layer for the test filter can all be determined. Starting with a clean filter, the porosity in the skin layer is found to be 0.95 dropping off to 0.6–0.7 after the second loading-and-cleaning cycle. The initial capillary diameter of the flowable pores in the skin layer is found to be 1.2µm dropping to half the value at the end of first loading-and-cleaning cycle. Based on these values, the permeability of the skin layer has dropped to 21% and 14%, respectively, of its original value when the filter has been cleaned after the first and second aerosol loading cycle. Thereafter, the permeability of the skin layer in the nanofiber filter stays reasonably constant from the third to sixth loading cycles approaching a state of quasi-equilibrium.