3D structure design and simulation for efficient particles capture: The influence of nanofiber diameter and distribution

Highlights

• 3D filter structures design with different nanofiber diameter and distribution.

• Simulation of filters with high filtration efficiency and low pressure drop.

• Experimental verification of simulated results on filtration effect.

Abstract

Software simulation is a convenient and efficient way to design and check different air filter structures with high efficiency and low pressure drop. In this work, nanofiber filters of different diameters ranging from 100 to 900 nm were designed to check their influence on filtration efficiency, pressure drop and quality factor (QF). Slip-flow effect of air molecules was considered on the surface of single fiber. Then, filters with different diameter distributions were constructed to study the filtration efficiency discrepancy when the filter thickness and porosity were kept equal. With a rotation of the filters composed of nanofibers of 500 nm in diameter in the computational domain, the filtration efficiency and QF increased steadily. The simulation results were partially verified by electrospun cellulose acetate nanofiber filter, and meanwhile provide with new insights into the filter structure design of high filtration efficiency with low pressure drop.

Introduction

Particulate matter 2.5 (PM 2.5) refers to tiny particles with an equivalent diameter of less than or equal to 2.5 μm in the atmosphere, which have large specific surface area, strong activity, and easy to attach toxic and harmful substances. Fig. 1 shows the distribution of the global median PM2.5 simulated concentration from World Health Organization [1]. Particles of 10 μm in diameter are usually deposited in the upper respiratory tract, and they can penetrate deep into the bronchioles and alveoli once the size are less than 2 μm, posing a great threat to human health [2]. PM 2.5 mainly come from human activities such as industrial pollution, secondary inorganic aerosol, coal combustion, biomass burning, traffic and waste incineration emission as well as the natural soil dust [3]. The composition changes with different seasons. Except for controlling air pollution from various original sources, the demand for air purified with filters is increasing with booming industrial development.

High filtration efficiency, low pressure drops and large dust holding capacity are the ideal properties for effective air filtration. Reducing the fiber diameter in filter is an efficient way to increase filtration efficiency [4]. Electrospun nanofiber filters offer a promising method for PM 2.5 capture due to their large specific surface area, small fiber diameter and tortuous pore architecture. They have fibers with diameters ranging from 50-2000 nm and mean pore sizes of around several hundred nanometers to several microns [5]. Whereas the increase of filtration efficiency with thinner fibers is desirable, the increase of pressure drop that goes hand in hand with it is unwanted. Accordingly, quality factor (QF) is applied to optimally balance the effect of filtration efficiency ($\eta$) and pressure drop ($\Delta P$) [1,4].

$$QF=-\frac{ln\left(1-\text{η}\right)}{\Delta P}$$

Plenty of researches have been done focusing on filtration efficiency and QF. Gao et al. [6] developed a ternary structure built from scaffold nanofibers, microspheres and thin nanofibers. The filtration efficiency could go up to 99.99 % with 126.7 Pa pressure drop. The inclusion of microspheres enlarged the inter-fiber voids while the thin nanofibers increased the filter surface area for efficient particle capture. For high efficiency particulate air (HEPA) filters with a minimum removal efficiency of 99.97 % for particles greater or equal to 0.3 μm, a reduction of fiber diameter is an efficient way to increase the filtration efficiency [7]. The decrease of fiber diameter to around 60-100 nm was found to be most effective for slip-effect of air molecules on the periphery of nanofibers for low air resistance [8]. As an alternative method, the introduction of carbon nanotubes (CNTs) [9,10] or cellulose nanocrystals (CNCs) [11] in electrospun nanofiber membranes not only enhanced their mechanical properties, but also greatly reduced the pressure drop. The diameter of CNTs or CNCs (from several nanometers to dozens of nanometers) is less than the mean free path of the air molecules (66 nm) which alters the Brownian diffusion of aerosol particles [12]. The incorporation of zein nanoparticles on cellulose nanofibers could also efficiently remove particles with hierarchical structure from nano to micron scale [13].

Except for structure optimization, surface modification such as electret electrospun nanofiber air filter through the implementation of corona charging, tribo-charging or low-energy-electron-beam bombardment are also regarded as promising materials for efficient particulate matter (PM) capture [[14], [15], [16]]. However, further improvement on the long-term effectiveness with high filtration efficiency is needed [17]. Besides, surface modification with sputtering copper or carbon on electrospun nanofiber with higher dipole moment would have better removal efficiencies to particulate matter [18]. A movable air filter system generating a high electric field would even reduce pressure drop to only several Pascal with removal efficiency over 99 % [19].

With regard to structure optimization, the hierarchical structure of nanofibers would increase the removal efficiency with tortuous pore channels to separate different sized PM [20]. A layer by layer structure with alternate different average fiber diameters was also designed to increase the filtration efficiency and QF [21]. High quality factor was also derived from the heterogeneous structure during the capture of various sizes of sodium hydroxide aerosols. In addition, due to a more regular pore size distribution, aligned nanofiber filter showed higher probability for particle capture than randomly oriented nanofiber membrane [22]. In this work, the influence of nanofiber diameter and its distribution on filtration properties were studied by simulation. Different 3D filter structures were designed and simulated subsequently in COMSOL Multiphysics 5.4 to check the corresponding filtration properties. The filter thickness, fiber diameter, diameter variation coefficient (CV) and membrane rotating angle θ were adjusted to study their relationships with filtration efficiency, pressure drop and QF. Finally, cellulose acetate (CA) nanofiber filters with different diameter and distribution were prepared by needleless electrospinning to verify the simulation results.