Nanofiber Inclusion and Fiber Pore Volume Potential to Affect Fundamental Filtration of NaCl Aerosols

Abstract

Sea salt aerosols are generated when the high-speed vessels travelling in the ocean collide with white caps from wake or bow of the ships. These aerosols have salt particulates ranging from 0.1 to 10 microns. These high-speed vessels have many air breathing equipment such as SOFS, turbines etc. If unfiltered, these salt particulates will flow into these equipment and deposit on the turbines or cathodes of fuel cells, deleteriously affecting its performance and decreasing its lifetime. Therefore, it is critical to remove these particulates before it is consumed by the air breathing equipment.

Commercial filter media have been used to address this problem. However, the commercial filter media are surface filters without any depth. These filters can hold limited amount of salt particles, before reaching unacceptable levels of pressure drop. Newly designed vessels are anticipated to ingest even higher level of impurities, further increasing the rate of performance degradation. Hence the development of new and improved filtration materials and methodologies are vital.

As a solution to this problem, an attempt was made in our lab to synthesize and evaluate a novel three-dimensional, asymmetric, porous, microfibrous filter media consisting of nanofibers embedded within microfibrous network. Nano-fiber flocs made of Vapor Grown Carbon Fibers (VGCFs) fibers were embedded with activated carbon fibers and polyester fibers to synthesize these novel filter media. Activated carbon fibers are porous in nature and hydrophilic. They can wick the salt aerosol into its pore via capillary forces. Hence this porous nature of the activated carbon helps to trap even more particulates, compared to a non-porous fiber. As long as the nano-fibers are wet, the salt loading process continues until the saturation occurs in the pores. This is a novel phenomenon, because a significant amount of salt particulates can be trapped inside the pores without any pressure drop penalty. Hence in AU nano-floc filter media, the pores of activated carbon and the interstitial spaces between fibers are utilized first, before surface filtration takes place. This results in the particle loading capacity of AU nano-floc media to be significantly higher than that of the commercial GOTs media. The loading of particles into the pores and the interstitial spaces between the fibers is referred as volume filtration mechanism. The volume filtration mechanism contributes in holding significant amount of salt particulates without reaching unacceptable levels of pressure drops.

This work discusses the efforts and summarizes the results associated with the inclusion of nanofiber for enhanced salt removal, the potential of porous nature of activated carbon fibers in salt entrapment and the regeneration ability of nanofiber filters. Fundamental filtration performance metrics such as filtration removal efficiency, Quality Factor (QF) etc. have been used to quantify the results. Additionally, another critical filtration parameter i.e. particle loading capacity has also been included in the studies. Experiments were performed to study the filtration performance of nano-floc filters on both wet and dry sea salt aerosols over a range of relative humidity and face velocity encountered by high-speed vessels. In addition, inclusion of hydrophobic material Polytetrafluoroethylene (PTFE) was also investigated to examine its ability to prevent aqueous flooding, enhance filtration and resist clogging. Most importantly, these novel wet-laid nanofiber filters were tested under relevant Navy test conditions and performance metrics were obtained and compared against the commercial GOTS media.