Characterization and evaluate the efficiency of different filter media in removing nanoparticles

Highlights

• HEPA_2 filter has the smaller fiber diameter than HEPA_1 and HEPA_3 filter.

• The lowest permeability constant was obtained by HEPA_2 filter.

• HEPA filters reached above 99% efficiency.

• Efficiency was evaluated for particles in the 7–289 nm range.

• HEPA_2 filter was highlighted when it was analyzed the cost benefit.

Abstract

Nowadays, removing nanoparticles in a filtration using fibrous filter media become a technique very useful. There are a lot of applications of these filters which can be found in air conditioner and clean rooms. Filter media used in this work have very similar characteristics and among them can be cited heterogeneous distribution and small diameter fibers. There are little studies about the performance of fibrous filter media in removing nanoparticles. So, the aim of this work was evaluate the fiber diameter, permeability and efficiency of HEPA (High Efficiency Particulate Air Filter) filters with glass and quartz fibers in 5 g/L sodium chloride solution by the technique of electric mobility. It was possible to simulate a contamination of nanoparticles through the experimental system in a stream of ultrapure air using surface speed 0.05 m/s and flow rate 1.59 L/min, which was done the particle count before and after passing through the filter media. HEPA_2 filter showed the lower permeability and the HEPA_1 filter the higher. In general, both filter media were very efficient in removing nanoparticles during the filtration because of the fibers distribution is heterogeneous. But HEPA_2 filter showed lower penetration of particles in the range of 7–289 nm comparing to the other filters.

Introduction

One side there is the concern with the control particulates with small dimensions and in other cases there is the advancement of technological applications involving nanoparticles. According to Aitken et al. [1], “nanoparticles” are particles with 0.1 μm (100 nm) dimension or less. For instance, combustion particles are usually in the 10–50 nm size range, but when they combine with other particles and agglomerate form larger particulates. The agglomerate particles may be broken down into smaller particles and release into air [2].

New researches aim to avoid pollutants affect the environment conciliating modernization and the industry grow without seriously harming the ecosystem. One way to avoid this is to find a way to hinder the arrival of nanoparticles to the ecosystem.

Several studies in epidemiology and toxicology say that as smaller as the size of the particles exposed in air more will be the occurrence of cardio-respiratory diseases [3]. According to EPA [4], this is explained by the fact that particles smaller than 10 μm are generally not removed by the upper respiratory system and, therefore, eventually affect the pulmonary alveoli and blood flow.

For this purpose, studies of processes capable of removing impurities contained in the air stream are made. Filtration is one of the most commonly processes used to capture particles from a gas stream. This process occurs by passing the aerosol through the filter medium in which the particles are being deposited on the filter surface. This is a process with high efficiency, easy operation, flexibility and economy [5].

The fibers of the filter media can be classified as microfibers and nanofibers. However, some authors define “nanofibers” as fibers with diameter less than 1 μm [6], [7]. But in this work we will consider the fibers used in the filter media as microfibers because they are measured approximately 0.5 μm.

HEPA filters with glass and quartz fibers are between the fibrous filter media that have been much used due to high efficiency of removing particles and can be used in air conditioner. These filters have similar characteristics but it isn’t known the most worth among them. Thus, this study aims to evaluate the fiber diameter, permeability and efficiency of different fibrous filter media in removing nanoparticles using sodium chloride solution.

Ascertain fiber diameter in order to analyse the filter medium is very important due to the possibility of particles' retention on the filter mainly when the fiber diameter is small.

Samples of the filter media were prepared in the Structural Characterization Laboratory DEMa at Federal University of São Carlos and analyzed by scanning electron microscope (SEM). It was possible to analyze them on a personal computer using the Image Pro Plus version 7.0 program after the images are generated by SEM-FEG and thus obtain the mean diameter of fibers of the filter media.

Images were divided into 12 equal vertical portions in order to obtain a large amount of fiber diameters to calculate the average diameter of each filter medium, as seen in Fig. 1.

Method of analysis consists in observing the width of the fiber at each intersection with the vertical lines in blue,¹ as can be seen in Fig. 2. Thus, the program operator traced straight perpendicular to the fibers that cross the vertical lines being represented by yellow lines and the program will automatically calculate this values, which are the fibers diameters represented by written in green. Analyzes were performed on all blue lines from left to right, thus formed a table of the entire calculated fibers diameter.

Six images were analyzed at different positions from each filter medium. It was measured approximately 50 fibers diameter in each image. It was used Eq. (1) to determine the average fibers diameter of HEPA filters. As we know these filters do not have homogeneous fibers.

$$\overline{D_f}=\frac{\sum n_i{d}_{\mathit{fi}}}{N}$$

$n_i$ is the number of fibers of determinate diameter, $d_{\mathit{fi}}$ is the fiber diameter in determinated band and N is the total number of fibers.

Van Osdell et al. [8] and Podgórski et al. [9], who used this method to determine the HEPA filter diameters, highlighted this as a simple method but hard-working and impossible to represent the real fiber diameter.

According to Hung and Leung [10] there are two ways to improve the filter’s quality. One is to make it more efficient in filtering out aerosol to increase the filtration efficiency and another one is to make it more permeable to reduce the pressure drop.

Forchheimer’s equation is used to evaluate the fluid flow in porous media. The first term of this equation refers to purely viscous effects and the second term to the inertial effects, according to equation [11]:

$$\frac{\mathrm{\Delta }P}{L}=\frac{\mu }{k_1}.v_s+\frac{{\rho }_g}{k_2}{v}_s^2$$

which L represents the thickness of the filter media, μ is the viscosity of the fluid, $k_1$ e $k_2$ are constants of permeability of filter media, ${\rho }_g$ represents the density of the gas and $v_s$ is the superficial velocity. ΔP is the pressure drop that is the difference between the inlet and outlet pressure of the filter during the passage of the air stream.

In this work it was used low filtration velocity, so the second term of Eq. (2) can be neglected and the Eq. (3) may be used [11]:

$$\frac{\mathrm{\Delta }P}{L}=\frac{\mu }{k_1}.v_s$$

Thus, the permeability of the porous filter media can be obtained by Darcy’s equation. This equation evaluates the flow of fluids in porous filter media relating pressure drop values with superficial velocity.

The collection efficiency is experimentally obtained through the technique of electric mobility in which the amount of particles is calculated before and after passing through the filter medium determined by the equation:

$$E=\frac{C_o-C_e}{C_0}$$

which Co and Ce represent the concentration of particles before and after passing through the filter media, respectively.