Enhancement in NaCl aerosol filtration in a filter media through the use of porous fibers with multiscale, multidomain porosities: Unveiling novel filtration mechanisms governed by capillary and thermodynamic forces.

Abstract

Studying NaCl aerosol filtration holds significant importance for several reasons. It is commonly used as a standard challenge aerosol for testing air filter media because it resembles properties of many airborne particles. Additionally, filtration of NaCl aerosols plays a crucial role in various marine applications. In marine environment, which is abundant with NaCl aerosols, filtration of NaCl aerosols becomes imperative to protect air breathing equipment aboard high-speed marine vessels like LCACs and ships. Equipment such as gas turbines and Solid Oxide Fuel Cells rely on clean air for optimal performance. Failure to filter NaCl aerosols can lead to their deposition on these equipment, resulting in corrosion and damage, thus reducing their operational lifespan. Traditional fibrous filters employ solid fibers such as glass fibers which has no intra-fiber porosity. They capture particulates onto the fiber surface using mechanism/s like inertial impaction, interception and diffusion. NaCl is hygroscopic in nature and exists in either solid, hydrated or liquid states depending on Relative Humidity (RH). Porous hydrophilic fibers can be utilized to wick these hygroscopic particles into their wet (i.e. condensed) pores, potentially enhancing both the filtration performance and the loading capacity when used as filter fibers. Therefore, this study investigates novel NaCl filtration mechanisms which are uniquely available to filter media composed of nanoporous hydrophilic fibers. The first mechanism involves the dissolution of dry, solid NaCl aerosols deposited onto the surface of Activated Carbon Fibers (ACFs) into the wet intrafiber porosity of ACFs. ACFs, which are hydrophilic fibers, have intrafiber pores condensed with water, even at low RH conditions due to a combination of physisorption, chemisorption and capillary forces. The dry NaCl particles get dissolved and wicked into the water-filled pores, facilitated by the favorable thermodynamics of NaCl dissolution in water. The second mechanism involves direct wicking and capillary condensation of aqueous NaCl into the intrafiber porosity of ACFs. The third mechanism involves the utilization of nanofibers in the form of a floc, as opposed to a continuous layer spanning the entire flow path. Vapor Grown Carbon Fiber (VGCF) nanofiber floc has a high surface-to-volume ratio, and extensive intra-floc porosity. Traditional nanofiber filter media uses nanofibers as a surface layer which spans the entire flow path, increasing the flow resistance. If used in filtration of NaCl aerosols, these media will clog after brief use, and go into a high pressure drop state. The VGCF nanofiber flocs do not span the flow path, thus providing negligible flow resistance. The nanofiber flocs capture NaCl aerosols at low RH conditions via slip-flow, increasing filtration efficiency without anticipated increase in pressure drop. The flocs absorb wet NaCl aerosols in a sponge-like fashion. To elucidate these novel filtration mechanisms, a novel fibrous depth filter media comprising multi-scale, multi-domain porosity was designed by embedding VGCF nanofiber flocs into a matrix of ACFs, held together by a binder fiber. This media consists of three distinct porosity domains: 1) Intrafiber porosity of ACF (0.950 cm3/gm), 2) Intra-floc porosity of VGCF flocs (4.108 cm3/gm), and 3) Interfiber porosity among all fibers (11.7 cm3/gm). For comparative analysis, ACF filter media was synthesized without VGCF flocs to examine the impact of their absence on NaCl aerosol filtration. Similarly, filter media made of non-porous Graphite fibers were also included to understand the impact of the absence of intrafiber porosity on NaCl aerosol filtration. This thesis comprises three different studies to understand the filtration of NaCl aerosols. The first study involves NaCl filtration using ACF filter media and Graphite filter media. Filtration performance metrics including pressure drop, filtration efficiency, Quality Factor, and NaCl loading capacity were evaluated to understand the effect of intrafiber porosity on NaCl aerosol filtration. SEM and EDS analyses were performed to verify the movement of NaCl into the intrafiber pores of ACFs. The second study focuses on NaCl aerosol filtration using ACF media with and without VGCF flocs. Same filtration performance metrics were evaluated to assess the impact of nanofiber flocs on NaCl aerosol filtration. SEM and EDS analyses were also conducted to verify the transport of NaCl into the intrafloc porosity of VGCF flocs. Third study analyzes the deliquescence of NaCl aerosols and their interaction with ACFs, VGCF flocs and Graphite fibers. SEM and EDS (including Cl mapping) were performed to understand the interaction of aqueous NaCl with the three aforementioned filter fibers. The Quality Factor of ACF media with VGCF flocs was higher than Graphite media when filtering both low and high RH NaCl aerosols. This is due to slip-flow occurring at nanofiber surfaces of VGCF flocs enabling capture of NaCl aerosols without anticipated pressure drop. Due to wicking of NaCl in ACF nanopores combined with NaCl entrapment into highly porous VGCF flocs, the NaCl loading capacity of ACF media was 1200% (or 12X) higher than that of media made with non-porous Graphite fibers, on loading low RH NaCl aerosols. This capacity increased by another 315% (or 3.15X) when filtering high RH NaCl aerosols, due to ease of wicking liquid aerosol droplets into both ACF pores and VGCF flocs. The transport of NaCl inside the intrafiber nanopores of ACFs, as well as within the intra-floc porosity of VGCFs was confirmed by EDS analysis. The inclusion of VGCF nanofiber flocs into ACF media increased both the filtration efficiency and Quality Factor by a two-fold increase, compared to ACF media without VGCF flocs. This was observed at both low and high RH conditions. This is attributed to wall-slip at nanofiber surfaces of VGCF flocs, enabling efficient capture of NaCl aerosols without anticipated increase in pressure drop. Without VGCF flocs, the ACF media showed unacceptable performance (FE and QF). Furthermore, at high RH, ACF media without VGCF flocs never reached a breakthrough point. The NaCl loading in ACF media with VGCF flocs increased by 315% on going from low RH condition to high RH condition. This enhancement at high RH is primarily due to wicking of aqueous NaCl directly into the extensive pore volume of VGCF flocs in a sponge-like manner, and secondarily due to wicking of aqueous NaCl directly into the intrafiber nanopores of ACFs. EDS analysis showed higher Cl:C ratio in the flocs compared to ACFs, particularly under high RH conditions, confirming the importance of VGCF flocs in ACF media for NaCl aerosol filtration. More importantly, preferential loading of VGCF flocs was observed with chlorine mapping, further emphasizing the importance of VGCF flocs in NaCl aerosol filtration. This study also confirmed the deliquescence of NaCl particles on different filters and investigated the interaction of the deliquesced NaCl with various filter fibers including ACFs, VGCF flocs and Graphite fibers. Deliquescence of NaCl was observed in all these fibers on exposing humid (75% RH) air to the samples loaded with NaCl. Although optical microscopy was not able to capture the phenomenon of wicking of deliquesced NaCl into ACF pores, it was able to capture the wicking of aqueous NaCl into VGCF nanofiber flocs. The deliquesced NaCl remained on the surface of the Graphite fiber as droplets.