Filtration of NaCl and WOx Nanoparticles using Wire Screens and Nanofibrous Filters

Abstract

Airborne nanoparticle filtration is essential for the protection of public health and the environment. The principles and fundamentals of air filtration have been validated with respect to micron particles; however, the mechanisms for airborne nanoparticle filtration are still uncertain. Conventional filtration theory states that diffusion dominates the behavior of submicron particles and that filtration efficiency increases inversely with the size of fine particles. This theory implies that nanoparticles can be effectively captured by properly designed air filters. However, some researchers have pointed out that single-digit nanosized airborne particles may behave like gas molecules upon impacting the filter media, if the kinetic energy is greater than the adhesion energy. As a result, such small nanoparticles may rebound from the filter media upon collision, in what is called thermal rebound. However, this phenomenon has not yet been observed in experimental studies, so uncertainties are still associated with the concept of thermal rebound, which itself has yet to be either proven or disproven. Despite the large amount of work done on nanoparticle filtration, there is still a gap between theory and experiments. This research aims to understand the interaction between nanoparticles and various filter media. The following tasks were done to achieve this goal: 1) Determining the performance of a scanning mobility particle sizer coupled with a Faraday cup electrometer (SMPS+E) for sizing airborne nanoparticles, 2) Implementing the nanoparticle filtration tests using wire screens for various particle number concentration distributions, 3) Developing a new thermal rebound model to determine the particle size at which thermal rebound occurs, 4) Characterizing PVA nanofibrous filters for nanoparticle removal, 5) Evaluating the effects of particle concentration on the filtration of PVA nanofibrous filters. Before conducting any filtration efficiency measurement, the performance of the scanning mobility particle sizer coupled with a Faraday cup electrometer (SMPS+E, GRIMM 7.860) was analyzed for NaCl and WOx particles at various particle number concentration distributions. The effects of instrument parameters, including the sheath air flow rate and sample air flow rate on particle number concentration distribution and on the lower and upper particle size detection limits were investigated theoretically and experimentally. For both types of nanoaerosol particles, the measurement of particle number concentration distribution depended on the selection of sheath air and sample air flow rate ratio, which depended on the initial particle concentrations and aerosol flow rate. Due to the high resolution of GRIMM SMPS+E for particle classification, no mobility shift was observed. The filtration efficiencies of nanoparticles with a broad size range and concentrations were then determined for uncharged micron-sized stainless-steel wire screens. Results showed that the filtration efficiency of WOx nanoparticles depends on particle number concentration distribution. For particles smaller than the mode size, the filtration efficiency followed the conventional theoretical model; however, the filtration efficiency deviated from that conventional model for larger particles. This result is likely due to the different morphology of WOx particles, which affects both particle charging and measurement performance. The next step was to develop a new filtration model by considering the possibility of thermal rebound effect. Theoretical analysis showed that, when nanoparticles collide on a solid filter media, it is more likely for plastic deformation to occur than elastic deformation does. Therefore, the nanoparticle filtration model was developed based on the assumption of plastic deformation of nanoparticles upon impaction to the surface of the filter media. Furthermore, results showed that the probability of thermal rebound increases inversely with relative humidity, which attenuates capillary force. The interactions between nanoparticles and various filter media are characterized by surface loading. Electrospun PVA nanofibrous filters were thus fabricated, then characterized in terms of fiber size distribution by SEM analysis using an automated tool, and their filtration performances were evaluated using NaCl nanoparticles. The higher the applied voltage and tip-to-collector distance and the shorter the deposition time, the higher the quality factors of the nanofibrous filters. Furthermore, the filter quality factor can be greatly improved by stacking up single layer filters made in a short deposition time. The electrospun PVA nanofibrous filters were then tested for sub-4 nm WOx particles with triple modal particle number concentration distributions at a low relative humidity (RH=2.9%). The upstream particle concentration affected the performance of nanofibrous filters, as it was higher at the lower particle concentration. The filtration efficiency for sub-2 nm particles showed that the particle critical diameter, below which thermal rebound may happen, is in the range of 1.07-1.17 nm. An analytical model was developed to predict the effects of particle concentration. Comparison between the developed model and experiments showed qualitative agreement; but more research is needed to further improve the model.