Modeling of submicron particle filtration in an electret monolith filter with rectangular cross-section microchannels

Electret monolith filters have the advantage of low pressure drop and high filtration efficiency. In such filters, the filtration of submicron aerosol particles occurs as air passes through millions of microchannels. This article investigates the flow and filtration mechanisms in a representative rectangular microchannel of an electret monolith filter. An improved incompressible lattice Boltzmann method with Bhatnagar–Gross–Krook (traditionally shortened as LBGK) and lattice velocity D3Q15 model is employed to simulate no-slip and slip flows in the rectangular microchannels of a monolith filter. We considered mono-disperse submicron particles and one-way coupling (particle motion was affected by the flow, but the presence of particles did not affect the flow). Based on flow computations, the effects of key dimensionless parameters (Reynolds number, Knudsen number, Stokes number and the dimensionless length of the channel) on the total capture efficiency of mono-disperse submicron particles were investigated. Our results indicate that the optimal monolith filter should be characterized by a Knudsen number between 0.022 and 0.044, and that the dimensionless length of the channel should be between 4 and 8.

Copyright © 2016 American Association for Aerosol Research     

Effect of xylene exposure on the performance of electret filter media

Performance degradation of electret filter media when exposed to xylene was investigated using a custom-made exposure apparatus. Three types of electret filter media were exposed to xylene in liquid and gas phases. Experimental data indicated that the penetration percentage of di-octyl-phtalate (DOP) aerosols through electret filter media was not influenced by exposure to xylene vapors for periods up to 8 h. The filtration performance for all three types of electret filter media decreased over 30% when the samples were exposed to liquid xylene. Pressure drop changes of the filter media prior and after exposure to liquid xylene were statistically negligible. Neither morphological changes nor degradation of the polymer fibers were observed after exposure of the filter media samples to liquid xylene. Thermodynamic simulations were performed to determine the molar flux of liquid and gas phases through the filter media specimens. Experimental results obtained via capillary gas chromatography were within 4% of the predicted values. The decrease in performance was attributed to changes in the density and spatial distribution of the electret charges on the surface of the polymer fibers.     

Effects of relative humidity and particle type on the performance and service life of automobile cabin air filters

Cabin air filters are the main barrier for protecting automobile passengers from on road particulate matter. There are many studies about the evaluation of their performance in terms of filtration efficiency. However, the knowledge about the loading capacity of them is still lacking. Meanwhile, there has been no quantitative method to estimate the proper filter service life time. This study focuses on testing the loading capacity of different types of cabin air filters under the conditions of different relative humidity values and particle types. The results indicate that when the relative humidity increases, the activated carbon coated filters can adsorb significant amounts of water with no significant increase of the pressure drop. The normal fibrous filters show in contrast negligible water adsorbance. Compared with the filters loaded by Arizona road dust only, loading the filters by Arizona road dust and soot particles simultaneously will result in the steeper loading curves as well as the shift of most penetrating particle size to the smaller diameter. Finally, a new method to estimate the proper service life time of the cabin air filters is suggested based on the loading curves.

© 2016 American Association for Aerosol Research     

Estimation of collection efficiency enhancement factor for an electret fiber with dust load

In a previous study to investigate how the morphology of particles accumulating on an electret fiber evolves and affects the collection efficiency of the filter at dust-loaded condition, a three-dimensional stochastic model is utilized to simulate the deposition and agglomeration of particles on a cylindrical electret fiber via two different electrical effects, namely, induced force (for uncharged particles) and Coulombic force (for charged particles). In the present study, the additional effect of Brownian diffusion is incorporated in the model and the morphology of particle agglomerates obtained in the simulated results is found to agree well with experimental observations obtained by Hiragi and Kanaoka et al. for both uncharged and charged particles. In addition, the ratio of dust-loaded collection efficiency, η, to the clean-fiber collection efficiency, η₀, can still be approximated as linear function in the case of weak electrical effects. However, when the electrical parameters are large, the normalized collection efficiency has to be represented by two linear relations, i.e., at low dust load and high dust load. Estimates of the initial collection efficiency and efficiency enhancement factor are given graphically and tabulated as function of Peclet number, the interception parameter and the two electrical parameters.     

Determination of the retention efficiency of industrial air filters for particle sizes in the submicron range

Particle size distribution up to a maximum diameter of 0.1 μm has been produced in the presence of short-lived tracer Na-24 using an aerosol generator. Industrial air filters with a cross-section of 100 cm² have been exposed to very high particle fluxes such that particulate breakthroughs down to 10⁻⁸ can be assessed by measuring the activity ratio of the air before and after going through the filter.

This method, which involves installing an analytical filter (absolute filter) after the test filter, has a high accuracy and makes it possible to determine particle breakthrough under the same conditions of air flow velocity and pressure loss as in industrial processes. In comparison with on-the-spot-methods, e.g. the condensation nucleus counter, the above method includes passage across the entire filter area and excludes secondary aerosols.     

Mechanical and electret properties of polypropylene unwoven fabrics reinforced with POSS for electret filter materials

A novel polypropylene/Polyhedral oligomeric silsesquioxanes (PP/POSS) composite unwoven fabric with permanent electret was prepared through melt-blown process with corona charging. Field-emission scanning electron microscope (FESEM) and wide-angle X-ray diffraction (WAXRD) were employed to investigate the morphology of the composite fibers and the distribution of POSS nanoparticles on the surface of the fibers, respectively. POSS acted as nucleating agent and accelerated the crystallization process during nonisothermal cooling. The utmost stable charge density of PP/POSS melt-blown unwoven fabric was improved by 78.4% compared with the neat PP unwoven fabric. The maximum value of collection efficiency measured by monodisperse polystyrene aerosol (PSL) (particle size: 0.3 μm) collection could reach 97.36% for PP/POSS composite melt-blown unwoven fabric, which improved by 9% compared with neat PP melt-blown unwoven fabric. Moreover, both stress and elongation at break of the PP/POSS melt-blown unwoven fabrics were improved compared with PP unwoven fabric.     

Effect of microbubbles and particle size on the particle collection in the column flotation

Particle-bubble collection characteristics from microbubble behavior in column flotation have been studied theoretically and experimentally. A flotation model taking into account particle collection has been developed by particle-bubble collision followed by the particle sliding over the bubble during which attachment may occur. Bubble size and bubble swarm velocity were measured as a function of frother dosage and superficial gas velocity to estimate the collision and collection efficiency. Separation tests were carried out to compare with theoretical particle recovery. Fly ash particles in the size range of <38, 38-75, 75-125, >125 mm were used as separation test particles. Theoretical collision and collection efficiencies were estimated by experimental data on the bubble behavior such as bubble size, gas holdup and bubble swarm velocity. Collection efficiency improved with an increase of the bubble size and particle size but decreased in the particle size up to 52 mm. Also, flotation rate constants were estimated to predict the optimum separation condition. From the theoretical results on the flotation rate constant, optimum separation condition was estimated as bubble size of 0.3-0.4 mm and superficial gas velocity of 1.5-2.0 cm/s. A decrease of bubble size improved the collection efficiency but did not improve particle recovery.     

Penetration and pressure drop of a HEPA filter during loading with submicron liquid particles

Experiments and a model have been made of the effects of mass loading of a HEPA fiber filter during filtration of submicron liquid aerosol particles. The measurements reveal that penetration of the test medium increased during clogging by a liquid aerosol, irrespective of particle size within the chosen range (0.02–0.5 μm). The physicochemical properties of the test aerosol did not seem to affect this phenomenon. Application of a non-stationary filtration model by Payet [1991, Thèse, Université Paris, 150 p] (based on the correlation of Liu and Rubow [1990, 5th World Filt. Congress, Nice 3, 112] showed that the increase in penetration can be explained in part by an increase in interstitial velocity and in part by a decrease in the number of fibers available for capture of particles.     

Removal of PM2.5 entering through the ventilation duct in an automobile using a carbon fiber ionizer-assisted cabin air filter

This study evaluated the charging characteristics of a carbon fiber ionizer for PM2.5 and carried out particle capture laboratory tests after an ionizer was installed upstream of the media of an electret cabin air filter. When the ion concentration per particle (N_i) of the carbon fiber charger was 10⁶ ions/cm³, the average charge numbers for each particle were 1.54, 0.88, and 0.49 at 0.6, 1.2, and 1.8 m/s of face velocity, respectively (the particle charging times, τ, were 167, 83, and 56 ms, respectively). For these face velocities, the PM2.5 removal efficiencies of the filter media were 69.3%, 65.2% and 62.2%, respectively, but increased to 80.4%, 71.2% and 65.5%, respectively, when the ionizer was turned on. The carbon fiber ionizer was then installed in front of an electret cabin filter in the air conditioning system of an automobile, after which field tests were performed at a roadside area. For the same N_iτ used in the lab-scale tests, the effects of the carbon fiber ionizer on increasing PM2.5 %Reduction were mild as 9.4%, 4.0%, and 2.8% when the flow rates were at the second, fourth, and sixth levels, respectively (the face velocities were 0.6, 1.2, and 1.8 m/s, respectively). The PM2.5 %Reduction can be substantially increased by 20–21%, for a higher value of N_iτ (=1.0×10⁸ ions s/cm³), which is realized by increasing the power consumption of the carbon fiber ionizer.     

Effect of the oxygen reduction catalyst loading method on the performance of air breathable cathodes for microbial fuel cells

This paper presents three different methods of hydrothermal (HT), microwave (MW), and cyclic voltammetry (CV) used to load a catalyst on a cathode surface. In the HT and MW methods, a multiwall carbon nanotube (MWCNT) is used as a support material to fix the catalyst, while Nafion solution is used as a binder to load the catalyst on the cathode surface. For the third option, the CV method is used to directly load the catalysts on the cathode surface without any support material. The performances of the three cathodes are tested in an air breathable batch microbial fuel cell (MFC) and compared to that of a commercial carbon cloth cathode with platinum (Pt). The maximum power density of the MFC with the HT cathode is measured as 833 mW m^?2, which is higher than those of the CV and MW cathodes and slightly smaller than the MFC with the Pt cathode. The open circuit voltage of the MFC with the HT cathode is 610 mV, which is higher than those of MFCs with other cathodes, while the power density is higher than the MFCs of the MW and CV cathodes. In the case of the HT cathode, a conductive MWCNT network is well formed and entangled with the catalyst nanostructure of the cathode surface while the small ohmic and activation resistances of the HT cathode contribute to the good MFC performance.     

Effect of step-by-step depolarization on electret performance of OH defect-controlled ceramic hydroxyapatite

Ceramics electrets of hydroxyapatite (HAp) have been developed for both power generators and artificial bones. However, the formation mechanisms of the HAp electrets have not yet been completely understood. In particular, it is unclear how the OH⁻ ion-dipoles in HAp lattices contribute to electret performance. Herein, we show that HAp with controlled OH⁻ ion content exhibits excellent performance as an electret. We prepare OH⁻-defect-controlled HAp samples either by restricted dehydration during thermal process or by valence control through the substitution of Ca²⁺ with Nd³⁺. The experimental results prove that the electrets prepared by partially dehydrated HAp and Nd-substituted HAp with a small number of OH⁻ ions showed better performance. Our results suggest that the OH⁻ ion-dipole polarization contributes negatively to the electret performance.     

Characteristics of an electrostatic precipitator for submicron particles using non-metallic electrodes and collection plates

We have developed a novel ESP that uses an anticorrosive carbon brush precharger and plastic collection plates into which metallic films are inserted. The collection efficiency of the ESP was measured using ultrafine KCl particles by varying the applied voltages, the number of channels in the charger, the gap between the collection plates, and the air flow rates. Tests of loading and cleaning on the collection plates were also conducted using JIS class # 8 dusts and KCl particles.The experimental results showed that the precharger (400×400×800 mm³) generated a lot of unipolar ions whilst producing negligible concentrations of ozone (<5 ppb), and that when the ESP was operated with 16 channels of ionizers and a 10 mm gap between the collection plates (400×400×185 mm³), it removed more than 95% of the ultrafine particles with a power consumption of only 5 W and a pressure drop of 5 Pa per 1200 m³/h at 2 m/s. It was also shown that by increasing the applied voltage and the number of channels in the charger, and by decreasing the gap between the collection plates, an improvement in the collection efficiency of the ESP could be achieved for a scale-up. It was also found that the collection efficiency for the ultrafine particles fell from approximately 95% to 50% after dust loading with 100 mg/m³ of the JIS dusts for 2 h, but then recovered perfectly to the efficiency of the initial state after the collection plates were sprayed with water at 25 L/min for 4 min.     

Effect of low gas velocity on the nanoparticle collection performance of an electrostatic precipitator

Gas velocity is a critical operational variable in the electrostatic precipitation process. However, studies concerning the collection of nanoparticles have ignored the effect of low gas velocities. Therefore, the aim of this work was to use a wire-plate electrostatic precipitator to investigate the effects of atypical gas velocities under different electric fields on the efficiency. Increasing of both variables enhanced the efficiency, under the conditions used. Unusual behaviors of the grade efficiency curves were associated with diffusional effects. The experimental overall efficiency data were compared with the Deutsch model, and effective migration velocity curves with maximum points were obtained.     

Performance of two shrouded probes for the collection of liquid aerosols in a wind tunnel optimized for high air speeds

Abstract

A combination of type A (high flow model) or B (low flow model) shrouded probe and appropriate isokinetic air-sampler (IAS) was tested in a wind tunnel that was optimized for high air speed testing using computational flow modeling. Liquid uranine aerosols (LUA) with AED (aerodynamic equivalent diameter) of 10?μm were generated at a constant flow rate using a vibrating orifice aerosol generator. The monodispersed aerosols were introduced into a wind tunnel at speeds of 5, 10, 15 and 20?m/s. The high flow (A) or low flow (B) model shrouded probe and the appropriate isokinetic air-sampler (IAS) was co-located to collect the LUA simultaneously during each treatment. After the test, the LUA deposited on the filters and inside the walls of the two air-samplers were collected and analyzed for fluorescence intensity units to determine the wall loss, transmission and aspiration ratios. While the type B shrouded probe had 20% (at 10?m/s) and 14.3% (at 15?m/s) higher wall loss ratios than model A, it had 16.1% (at 10?m/s) and 11.6% (at 15?m/s) higher transmission ratios compared to model A. Similarly, probe B had 17.6% (at 10?m/s) and 14.6% (at 15?m/s) higher aspiration ratios than probe A at similar air velocities. Overall, the wall loss, transmission and aspiration ratios of 10?µm AED ULA measured with two types of shrouded probes at 5, 10, 15 and 20?m/s air velocities in the optimized wind tunnel had good agreement with the range of standard data.

Copyright © 2020 American Association for Aerosol Research     

Impact of multilayering on the filtration performance of clean air filter media

Fibrous filter media are commonly used to remove airborne particles that are harmful to human health and the environment. Although filter media are often multilayered for various reasons, no systematic study of the impact of multilayering on filter media performance has been reported. In this paper, direct numerical simulations with the lattice Boltzmann method are used in order to shed light on the impact of multilayering on the performance of clean bimodal fibrous filter media in a Stokes flow regime. Virtual model clean filter media with up to eight layers and various fibre formulations are compared in terms of permeability or pressure drop, capture efficiency, and quality factor. A careful analysis of the results revealed that multilayering had no statistically significant impact on the performance of the clean filter media. At best, the impact of multilayering was similar to that of the inherent variability of such random structures. Fibre formulation was found to be a more efficient way of improving the performance of the filter media. Placing interlayered air gaps between fibrous layers also slightly improved the quality factor by facilitating the flow at the interfaces of the fibrous layers. These findings will guide future studies on the performance of multilayered filters with more complex flow conditions, such as those encountered with inertial or nanofibre-made filter media and with the fouling of filter media.