(333a) Challenges and Opportunities of Nano-Aerosol Filtration Using Nanofiber Filter

Chair Professor, Mechanical Engineering

The Hong Kong Polytechnic University, Hung Hom, Hong Kong

Fellow (AICHE, ASME, AFS, HKIE, HKAES)

Challenges and opportunities of nano-aerosol filtration using nanofiber filter

Abstract

Nano-aerosols/ultrafine particles (<100nm) in high concentration (50-500+ million/m³) are found in polluted air both outdoors and indoors. They are from combustion of fossil fuel from vehicles, industries and power plants. Another source of these nano-aerosols is the photochemical reaction under sunlight between the emitted NOx and VOC from vehicles, resulting in smog and haze, which becomes problematic for most congested cities. Air-borne seasonal flu and epidemic viruses are also in the same size range. By their small sizes, these invisible killers can infiltrate readily into our bodies from breathing leading to health problems and diseases, from cardiovascular diseases to mental/behavioural disorders.

Filters made of nanofibers, with diameter 100-300nm, are very effective to capture these invisible killers. Despite their large specific surfaces are advantageous providing effective diffusion and interception capture, they can also induce high pressure drop. This presentation discusses various innovative nanofiber technologies being researched, developed, and some of which are being commercialized on filtration of nano-aerosols to achieve high capture efficiency, low pressure drop, and high storage capacity.

For light aerosol loading as used in respirator and facemasks, to achieve high capture efficiency for a given aerosol size, a certain specific amount of nanofibers per square meter of filter is required. When the same specific amount of nanofibers is divided into multiple layers, each separated by a permeable scrim material, the pressure drop of the entire filter is much reduced as compared to the case with all nanofibers in a single layer. This is because the effective flow pore is much reduced when many layers of randomly oriented nanofibers are lay over each other. When permeable material is introduced between thin layers of nanofibers, this introduces macro-pores, which provides a relief to flow in an otherwise strict micro-pore structure, thus relieving the pressure drop. Another approach is to charge electrostatically the nanofibers, which is also quite challenging.

For extended aerosol loading as used in cabin filter, vehicles and HVAC, other than high efficiency and low pressure drop, the aerosol storage capacity becomes important. In fact, a cake forms on the nanofiber filter surface relatively quickly. The transition from depth to surface/cake filtration has been studied experimentally, theoretically and with CFD model. It has been found that more aerosols are captured and deposited at the upstream end of the filter forming a skin layer, followed by plugging of pores in the skin layer by aerosol deposit, building of dendritic bridges above the plugged pores of the capillaries in the skin layer, and finally cake forming above the filter. The cake formed above a nanofiber filter is quite dense, resulting in higher pressure drop across the entire filter. It can be made less dense and more permeable when a microfiber filter with specific fiber diameter and packing is installed upstream of the nanofiber filter. The cake formed from such composite filter is rather complicated. This interesting technology has been proven both experimentally and by CFD modelling.

Depth filtration on submicron aerosols and nano-aerosols using nanofiber filter as function of face velocity, filter thickness, and fiber packing density has been well established. This includes low Peclet number (Pe<10) condition for which diffusion dominates over convection/interception capture. Most experiments were almost exclusively ran based on sodium chloride aerosols or using organic liquid droplets. An interesting challenge is to demonstrate on the diesel aerosols from emission with even lower Peclet number, Pe<1, corresponding to highly diffusion dominant.

Another interesting challenge is to clean the filter after it is being loaded with nano-aerosols, we have developed backpulse-and-backblow to clean the filter without breaking the nanofibers. The filter can be reuse for loading again until it needs to be cleaned. We have also attempted to understand the complicated loading behaviour of the reuse filter after it has being cleaned.

All-in-all, there are many interesting challenges in filtering nano-aerosols using nanofiber filter, but they also present as possible opportunities for development of break-through technologies.

Biography of Wallace Woon-Fong LEUNG

Dr. Wallace W-F Leung received his MS and ScD both from MIT. For the past 40 years, he has worked on filtration and separation from ultrafiltration of protein solution, centrifugation of process stream, vacuum filtration of minerals, to air filtration. For 18 years, he was with Bird/Baker Hughes as Senior Research Scientist and Director of Process Technologies directing centrifugal separation/filtration. Earlier, he has worked for Gulf Oil and Schlumberger on flow in petroleum reservoirs. For the past 13 years, he is Chair Professor of Innovative Products and Technologies at The Hong Kong Polytechnic University. Besides air filtration, he is also interested in applications of nanofiber technologies in renewable energy (photovoltaics, hydrogen production using water spliting), clean air and water (photocatalysis), and health technologies (wound dressing).

Dr. Leung is a fellow of American Institute of Chemical Engineers, American Society of Mechanical Engineers, American Filtration and Separations Society, Hong Kong Institute of Engineers, and Hong Kong Academy of Engineering Sciences. He has 49 US patents and two books to his credit. He is the Chairman for the World Filtration Congress in 2004 (New Orleans) and 2020 (San Diego). He is also Chairman for the International Delegation on Filtration 2016-2020 with 12 member countries, overseeing the world Filtration Congress and promoting filtration and separation activities worldwide.