Aerosol Filtration Performance of Novel 3-Dimensional Nonwoven Composites

Abstract

The studies described in this dissertation focus on the design and aerosol filtration performance of novel filter media with homogeneous 3-dimensional distribution of all its components. Air purification by removal of aerosol and molecular contaminants is very important for improving breathing air quality and protecting expensive equipment and processes. Aerosol contaminants such as dust, pollen, airborne bacteria and fumes can significantly impair the quality of air. Other contaminants such as volatile organic compounds, sulfur dioxide, nitrogen oxides and carbon monoxide which are molecular in nature have to be removed using adsorbents or catalysts. A variety of filters, with different filtration efficiencies for aerosol and molecular contaminants, are used for air purification as per the requirements of their intended application. There are two main aspects of filters: (a) filter media and (b) filter media pleating and packaging. The filter media is the most basic component of filters which does the filtration. Depending on how the filter media is packaged, the filters can be made to have different performances even when using the same filter media. Carbon nanofibers/nanotubes are an important class of nanomaterials which have been intensively researched in the last two decades for a variety of applications such as polymer composites, microelectronics and batteries. Carbon nanofibers have not been studied for applications in aerosol filtration due to the lack of understanding in making carbon nanofiber composite filter media. The studies in this dissertation explain two novel methodologies of incorporating carbon nanofibers within the filter media, and demonstrate the enhancement in aerosol filtration performance due to nanofibers. Also, a predictive semi-empirical model was developed for aerosol filtration performance of a molecular filter media known as microfibrous materials. Filter media are made from fibers with diameters between 0.5 µm and 80 µm. Two different methods were used to obtain uniform 3-dimensional distribution of carbon nanofibers within the traditional fibrous filter media. One of these methods is described in Chapter 2, where carbon nanofibers were synthesized on the fibers of filter media. Sintered metal fiber filter media were made with 4-12 µm (dia.) nickel fibers. Nickel metal acted as the catalyst for synthesis of carbon nanofibers by the decomposition of acetylene at high temperatures. These filter media with nanofibers coated on the microfibers showed enhancements in filtration performance. However, it was found that the pores of the filter media closed with the nanofiber synthesis which led to limitations in enhancement of filtration performance. Another limitation was that these filter media could not be scaled up in a cost effective manner. The knowledge of carbon nanofiber synthesis required to make these filter media was gained by synthesis studies on nickel foil as described in Chapter 3. Process scalability of is an important factor to be considered for any applications involving nanomaterials. Chapter 4 describes a novel scalable process to incorporate pre-manufactured carbon nanofibers within filter media during their wet-lay formation. This methodology leads to uniform 3-distribution of nanofibers within the filter media. The resulting significant increase in filtration efficiency, even with small volume fractions of nanofibers, indicated that nanofibers acted as individual fibers within the filter media. The scalability of this process was shown by making more than 700 ft2 of the novel filter media using a continuous Fourdrinier pilot papermaking machine. A class of molecular filter media known as microfibrous materials is made of sorbent/catalyst particles immobilized within sinter-locked 3-dimensional matrices of microfibers. The sorbent/catalyst particles remove the molecular contaminants and the microfibers remove majority of the aerosol contaminants. Chapter 5 describes the formulation of semi-empirical models for estimating the aerosol filtration performance of these materials. Empirical factors were added to the existing models in literature to take into account the heterogeneity created by the relatively large particles within the matrices of microfibers. These semi-empirical models are powerful tools that can be used to design full scale filtration systems from microscopic scale after inclusions for pleating and packaging of filter media.