Application of centrifugal filter to aerosol size distribution measurement

In the present work, the centrifugal filter proposed by the authors was applied to classify aerosol particles followed by the detection of total mass or number concentrations so as to measure the size distribution of aerosol particles. The structure and operating condition of the centrifugal filter were optimized in order to attain sharp separation curves with various cut-off sizes between 0.3 and 10 μm. The aerosol penetrating the centrifugal filter at various rotation speeds was measured with a photometer to determine the total mass concentration. The virtue of this system is that the cut-off size is varied just by scanning the rotation speed of filter and that it can be applied to the measurement of high concentration aerosols without dilution by choosing an appropriate filter medium. As a result, the centrifugal filter was successfully applied to measure the size distribution of solid particles in size ranging from 0.3 to 10 μm.

Copyright © 2017 American Association for Aerosol Research     

Effects of size and shape on filtration of TiO2 nanoparticles

Titanium dioxide (TiO₂) is one of the most widely used nanoscale materials to date and could result in human exposures. The main objective of this study was to perform detailed characterization of TiO₂ agglomerate particles and how these properties influence particle penetration in a screen filter. Transmission electron microscope (TEM) photos showed compact agglomerates of nanoscale primary particles. The projected area diameter was close to the mobility diameter, where the length was about 25% larger than the mobility diameter. The mean aspect ratio of TiO₂ agglomerate was constant between 1.39 and 1.55. Using the tandem differential mobility analyzer-aerosol particles mass analyzer (DMA-APM) technique, we were able to measure aerodynamic diameter, mass, and fractal dimension. The value of fractal dimension based on mass and mobility diameter was 2.8. Penetration of classified TiO₂ particles through a screen filter was measured. Penetration increased with increasing mobility diameter and flow rate indicating that diffusion and interception were the main filtration mechanism. The measured physical dimensions, mobility diameter, and aerodynamic diameter were used in a single-fiber filtration theory for the fan model filter to predict the penetration of TiO₂ particles. The interception parameter was the key to estimate the penetration. Experimental penetration data were in best agreement with the model in which the maximum length was used to calculate the interception model. This result was consistent with the assumption that the rotation time of a non-spherical particle of small aspect ratio was much less than the transport time for the particle to pass through the filter fiber.

© 2017 American Association for Aerosol Research     

Improvements to dust filtration through acoustic agglomeration and atomization

Based on the theories of acoustic agglomeration and dust wet removal, an experimental apparatus was constructed to study the combined effects of acoustic agglomeration and atomization humidification in the pretreatment process to analyze the filtration performance of filter material. According to the concentration of coal-fired fly ash chosen in the experiments, the proper amount of atomization humidification and the proper sound pressure level (SPL) were determined. Under the relative humidity (RH) of 69% and with SPL in the range of 100 dB to 135 dB, the removal efficiency of fly-ash, the compressibility of the fly-ash particle layer on the filter media, and the performance of pulse filter cleaning were studied. The results indicate that the combined effects of sound fields and atomization humidification can effectively remove PM₁₀ and PM_2.5, and change the interaction and movement of particles, which can improve the pore structure of the fly-ash particle layer and increase the porosity of the dust layer. The results also indicate that with the proper amount of atomization humidification and appropriate SPL, the joint acoustic-atomization pretreatment can delay the filter material blocking, which reduces the pulse filter cleaning frequency and extends the filter cleaning cycle. It can also reduce the residual resistance after filter cleaning and prolong the operating lifetime of the filter media.

© 2017 American Association for Aerosol Research     

Optimization of centrifugally spun thermoplastic polyurethane nanofibers for air filtration applications

While our knowledge of fiber formation by using conventional nanofiber spinning techniques has increased to a considerable extent, there are still few studies on centrifugal spinning either in academia or in the industry. Centrifugal spinning is a comparatively new method of producing fibers having nano- or microscale diameters. In this study, three main parameters (nozzle orifice diameter, rotational speed, polymer concentration) of centrifugal spinning were optimized to produce air filter media from thermoplastic polyurethane nanofibers. The effect of concentration of polymer solution was found to be a major contributor in TPU fibers optimization estimating 77.5%. After the optimization studies, the average fiber diameter of nanofiber sample produced at optimum conditions (22G needle as an orifice, 4000 rpm, and 10 wt% concentration of polymer solution) was 205 ± 84 nm. Aerosol filtration performance of the produced webs was analyzed. Filtration efficiency of the optimized sample was found to be 99.4% for 0.3 µm particle size at an expense of 98 Pa pressure drop.

Copyright © 2018 American Association for Aerosol Research     

Detection of RDX traces at the surface with sonic aerosol flow desorption

The use of a high-speed aerosol flow is proposed for sampling RDX from the surface followed by chromatographic analysis. The aerosol is generated from different solvents by means of a coaxial nebulizer. The effect of the aerosol flow parameters (solvent flowrate, an angle of the nebulizer inclination with respect to the surface) and various solvents (water, acetone, and hexane) on the efficiency of the RDX desorption was investigated. The optimal angle of the nebulizer was found to be 30°, under these conditions, the desorption of RDX from the surfaces of different structure (metal, glass, leather, cotton fabric, and paper) has also been studied. It is shown that under the action of an aerosol created using water and acetone, desorption from a smooth surface occurs most efficiently (1.5 times higher than with hexane). In this case, the sample removes almost completely (about 80%) by the aerosol flow in a few seconds. A relationship between the desorption efficiency and the amount of the solvent sprayed (that is the amount of aerosol particles in desorbing flow) has a characteristic maximum which location depends on the properties of the solvent spray. This effect is associated with a rate of solvent evaporation. Under optimal conditions for desorption of RDX from a smooth surface using an aqueous aerosol, an LOD of ～10?ng can be achieved. For porous and rough surfaces, the efficiency of the analyte desorption decreases (three times for leather and cotton fabric). The results of the experiments conducted allow one to conclude that the RDX solubility in the solvent used does not affect considerably the efficiency of the RDX desorption. It is assumed that small aerosol drops are very active and can capture the particles of the target analyte. This promotes the desorption of RDX molecules from the surface.

© 2018 American Association for Aerosol Research     

Characterization of nanosized silica size standards

Nanosized silica size standards produced with a sol–gel synthesis process were evaluated for particle size, effective density, and refractive index in this study. Particle size and effective density measurements were conducted following protocol from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Particle sizes were measured via electrical mobility analysis using a differential mobility analyzer (DMA) at sheath flow rates (Q_sh) of 3.0 and 6.0 L/min and a constant aerosol flow rate (Q_a) of 0.3 L/min. The measured mean and mode diameters agreed well with the labeled sizes in the size range 40–200 nm, with differences ranging from 0.03% to 0.8%, well within the labeled expanded uncertainties (95% confidence intervals) of 1.8%–2.2%. The coefficient of variation (CV) of the size distribution was 0.012–0.027 for 40–200 nm. Particle sizes measured for 20 nm and 30 nm standards showed size differences with respect to the certified sizes of 1.7% and 8.3% at Q_sh = 6.0 L/min, but the size distributions were narrow, with CV = 0.047–0.064. The average effective density for the range 40–200 nm measured with an aerosol particle mass analyzer (APM) was 1.9 g/cm³. The real component of the refractive index measured with an optical particle counter (OPC) was 1.41 at a wavelength of 633 nm. All properties (size, effective density, and refractive index) were stable and could be measured with good repeatability. From these evaluations, it was found that the nanosized silica size standards have good characteristics for use as size standards and constitute a feasible alternative to PSL particles.

© 2017 American Association for Aerosol Research     

Investigation of particle deposition on a micropatterned surface as an energy-efficient air cleaning technique in ventilation ducting systems

Abstract

Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation system can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This article proposes an air cleaning technique, aimed at submicron particles, by means of installing patterned surfaces on the walls of ventilation ducts, which can be easily cleaned by water and reused. The effect of patterned surfaces on particle deposition was studied numerically. In the numerical simulation, the Reynolds stress turbulent model was correlated at the near-wall regions by turbulent velocity fluctuation at the normal direction. Particle trajectory was solved by using Lagrangian particle tracking. The numerical model was then validated with a particle deposition experiment. A wind tunnel experiment was carried out to quantify the particle deposition on the semicircular micropatterns for a wide range of heights. Based on our numerical results, the semicircular pattern height of 500?µm with a pitch-to-height ratio (p/e) of 10 has 8.58 times enhancement of the energy efficiency compared with a high-efficiency particulate air filter. Our results indicated that adding surface micropatterns to ventilation ducting for submicron particle deposition is a possible energy-efficient air cleaning technique for practical usage.

Copyright © 2020 American Association for Aerosol Research     

Performance of nanofiber/microfiber hybrid air filter prepared by wet paper processing

High-performance air filters composed of a hybrid structure of nanofiber/microfiber were fabricated using wet paper processing. Two types of nanofibers (NF) with average diameters of 180 and 234?nm were mixed with a suspension of microfibers (11.5 and 11.7?µm) in various mixing fractions. Then, the suspension was filtered to fabricate hybridized fiber sheets with a known nanofiber/microfiber composition. The effects of NF diameter and mixing fraction on the performance of the hybrid filters were experimentally investigated. With increasing NF fraction, both the particle collection efficiency and the pressure drop increased. The quality factor (Q_f) was used to evaluate the performance of the prepared filters. As predicted by the single fiber filtration theory, the experimentally obtained Q_f was almost independent of the mixing fraction of the NF. The collection efficiency and pressure drop of the hybrid filters could be controlled by the NF fraction at the same Q_f. Moreover, the inhomogeneity factor of fiber packing (δ) did not significantly affect Q_f over the δ range from 3 to 23 for our filters. This implies that the lower particle capturing efficiency due to heterogeneous packing could be compensated by a decrease in the pressure drop, resulting in the same Q_f value. Therefore, Q_f for particles smaller than 100?nm, which are in the diffusion-controlled regime, can be increased by reducing the NF diameter.

Copyright © 2019 American Association for Aerosol Research     

Performance characteristics of air intake pleated panel filters for internal combustion engines in a two-stage configuration

Previous numerical studies that have used computational fluid dynamics (CFD) and experimental software to address the effects of the geometric parameters of pleats on the pressure drop and air flow rate through a fibrous filter are analyzed. The analysis establishes that using a test dust with gradually smaller particle sizes (10, 5, and 1?μm) results in a more intense increase in the filter pressure drop, thus decreasing the service life of the filter. The benefits of using a multicyclone as the first stage of air filtration are discussed. Selecting the air filter by determining the active surface of the filter medium A_c based on the allowable filtration rate is not sufficient; to select the filter medium of a motor vehicle air filter, the dust mass retained per unit of filtration area (mass loading of dust k_m) must be known for a specific allowable pressure drop Δp_fdop. New methods and conditions for determining the mass loading of dust k_m for filter paper and non-woven fabric in single-stage and two-stage filtration systems are presented. The characteristics of the separation efficiency and filtration performance as well as the pressure drop of a filter set comprising a single cyclone and a filter element with a specific filter medium surface are determined. The effects of the particle size distribution of the dust in the air downstream of the cyclone on the mass loading of dust k_m of the filter paper and non-woven fabric in a two-stage filtration system are presented. The mileage of a truck fitted with a single-stage or two-stage filtration system in a “multicyclone–panel filter” configuration is estimated based on the calculated mass loading of dust k_m of the filter paper and non-woven fabric.

© 2018 American Association for Aerosol Research     

Detachment of adhered particles from a cloth surface subjected to a rod strike

A mechanical impulse can cause adhered particles to detach from a surface. For various purposes, particle detachment may need to be enhanced or restricted. Unlike rigid solids, cloth material can be deformed or bent by a mechanical impulse. However, neither the cloth deformation nor the induced turbulent airflow has been well studied. This investigation experimentally measured the detachment of Arizona test dust (ATD) from cloth segments. The vertical margins of each cloth segment were fastened to a frame, and the cloth surface with the ATD adhered to the reverse side was struck with a rod. The cloth motion, induced airflow, and particle detachment were recorded by a high-speed camera. In addition, the displacement and acceleration of the cloth were monitored with a laser distance sensor. The mass percentages of detached particles from the cloth and the particle residual were weighed. Several factors that affected particle detachment were compared. The results revealed that the particle detachment was caused by a combination of the vibrating motion of the cloth surface, the hydrodynamic action of the induced turbulent airflows, and the particle agglomeration when the cloth was bent. A strike could even leave fewer residual particles when a much higher surface dust load had initially adhered to the cloth.

Copyright © 2019 American Association for Aerosol Research     

Collection efficiency of airborne fibers on nylon mesh screens with different pore sizes and configurations

Aerodynamic behavior of airborne fibers including high-aspect ratio particles plays an important role in aerosol filtration and lung deposition. Fiber length is considered to be an important parameter in causing toxicological responses of elongate mineral particles, including asbestos, as well as one of the factors affecting lung deposition. In order to estimate the toxicity of fibers as a function of fiber length, it is required to separate fibers by length and understand mechanisms related to fiber separation for use in toxicology studies. In this study, we used nylon mesh screens with different pore sizes as a separation method to remove long fibers and measured screen collection efficiency of glass fibers (a surrogate for asbestos) as a function of aerodynamic diameter with the aim to prepare toxicology samples free of long fibers and/or harvest long fibers from the screen. Two screen configurations ([i] without a laminar flow entrance length, and [ii] with the entrance length) were tested to investigate the effect of screen pore size (10, 20, and 60 µm) and screen configuration on collection efficiency of fibers. Screen collection efficiency (η) was obtained based on measurements of downstream concentrations of a test chamber either without or with a screen. The results showed that screen collection efficiency increases as screen pore size decreases from 60 to 10 µm for both cases with and without entrance lengths. For the screen configuration without entrance length, higher collection efficiency was obtained than the case with entrance length probably due to increased impaction caused by the close proximity of inlet to screen. In addition, the difference between the collection efficiencies for the different configurations was small in the aerodynamic size range below 3 µm while it increased in the size range from 3 to about 7 µm, indicating that as large aerodynamic diameter is associated with longer fibers, some differential selection of fibers is possible. Modified model collection efficiency for 10 and 20 µm screens based on the interception predicts well the measured data for the case with entrance length, indicating that the fiber deposition on these screens occurs dominantly through the interception mechanism in the micrometer size range under a given flow condition.     

Effects of an external electric field on the collection efficiency of air filters: Filtration mechanisms with an external e-field

An external electric field was applied on the filter to improve its collection efficiency, and the collection efficiencies of the different filters under various conditions were evaluated. Dominant electrical filtration mechanisms for each condition were investigated using experimental and theoretical approaches. Four types of air filters were used as test filters: a charged fiber filter, a low-grade filter with 50% collection efficiency in the most penetration particle size (MPPS) zone, and two high-grade filters with more than 95% collection efficiency in the MPPS zone. Three different particle charge states—neutralized, single-charged and uncharged—were considered. For neutralized particles, the external electric field led to a 14.5%p. and 2.5%p. increase in the collection efficiencies of the low-grade filter and charged fiber filter, respectively. With the electric field, the collection efficiency of the low-grade filter increased by 30%p. for single-charged particles. The electric field also affected the collection efficiencies of the charged filter and high-grade filters, but the effect was not significant. For uncharged particles, the electric field did not lead to a remarkable increase in the collection efficiencies of any of the filters. Through experimental and theoretical analysis, it was found that the polarization force imposed on the charged fiber was the dominant factor for the charged fiber filter regardless of application of the external electric field. The Coulombic force imposed on the electric field was the dominant factor for the low-grade filter, while both the Coulombic and the polarization forces affected the collection efficiency of the high-grade filter.

Copyright © 2017 American Association for Aerosol Research     

Filtration efficiency analysis of fibrous filters: Experimental and theoretical study on the sampling of agglomerate particles emitted from a GDI engine

Fibrous filters are commonly used for aerosol purification and sampling. The filtration efficiency has been extensively studied using standard aerosol generators, yet the literature on experimental data and theoretical study concerning the filtration of agglomerates from real engines remains scarce. A filtration efficiency test system was developed to determine the filtration efficiency of two types of filters (uncoated and fluorocarbon coated) loaded by particulate matter (PM) emissions from a gasoline direct injection (GDI) engine. The experimental results showed that the filtration efficiency in terms of PM mass and number increased over time for both types of filters. The fractional efficiency (penetration efficiency) curves for the test fibrous filters rendered a U-shaped curve for particle sizes from 70 to 500 nm, and the most penetrating particulate size (MPPS) decreased over time. A small fraction of accumulation mode particles with the size between 70 nm to 500 nm penetrated the filters while almost all nucleation mode particles with the size below 50 nm were captured by the filters. The filtration efficiency derived from an empirical model based on classical single-fiber theory for laden filters generally agreed with the experimental data for the first 500 s, but suffered a significant deviation by approximately one order of magnitude at 948 s. A better estimate of the filtration efficiency trend with the maximum deviation of about 20% (except for large particles at the high end of the measurement spectra) was obtained by using a revised model which incorporates the effects of the increase in filter solidity, local velocity, dynamic shape factor and effective total length of fibers during the filtration process.

© 2017 American Association for Aerosol Research     

Optimized detection of particulates from liquid samples in the aerosol phase: Focus on black carbon

Operational parameters for a single particle soot photometer (SP2) and a CETAC Marin-5 nebulizer were optimized for detection of particulates aerosolized from liquid samples. The sensitivity of nebulization efficiency on nebulizer input gas pressure, liquid sample flow rate, and alcohol doping of the sample were explored. The nebulization efficiency of the Marin-5 was found to be roughly independent of applied gas pressure once above a minimum pressure. The nebulization efficiency changed by ～50% for an order of magnitude change in liquid sample flow. Doping the sample with isopropyl alcohol at a 1:1 ratio results in a ～50% relative increase in nebulization efficiency over a broad range of liquid flows. These results should apply to all particulate materials in the size range studied. SP2 operational parameters including sheath and sample flow were explored to optimize detection of refractory black carbon (rBC) specifically via coupling to the nebulizer. The SP2 tested samples up to 5 cc s?1 with 100% detection of rBC in its size range of detection, with increased sample jet spread and corresponding lack of detected rBC in the air at higher flows, leading to a total undetected rBC mass fraction of ～15% at 16 cc s?1. Varying sheath flow does not improve this result, which is significant because under reasonable Marin-5 operating conditions, the SP2 only samples a fraction of the total air flow out of the nebulizer. Recommended operational parameters for cases of sample with low rBC loadings are presented: first, when very little liquid sample is available; second, when considerable sample is available.

© 2017 American Association for Aerosol Research     

Influence of fiber orientation distribution on performance of aerosol filtration media

This work is conducted to better our understanding of the influence of fibers’ in-plane and through-plane orientations on pressure drop and collection efficiency of fibrous media. The Stokes flow equations are numerically solved in virtual, 3-D, fibrous geometries with varying in-plane and/or through-plane orientations. Pressure drop and aerosol collection efficiency characteristics of such media are calculated and compared with available studies from the literature. Our results indicate that pressure drop and submicron particle capture efficiency of common fibrous filters with a fiber diameter of about 10 μm are independent of the in-plane orientation of the fibers, but decrease with increasing the fibers’ through-plane orientation. More interestingly, it was found that filters with higher through-plane fiber orientations have a higher figure of merit if challenged with nanoparticles. The figure of merit of these media, however, decreases as the particle size increases, reversing the effect of fibers’ through-plane orientation. It was also shown that when the diameter of the particles is comparable to that of the fibers, collection efficiency increases with decreasing the fibers’ in-plane orientation, while the pressure drop remains almost unchanged. This indicates that decreasing the fibers’ in-plane orientation increased the figure of merit of media made of nanofibers.     

Implications of particle size to transient stage of deep bed filtration

This study was aimed at investigating the effect of particle size, mostly in the submicron range, on break-through stage of filtration. Latex beads, with diameters ranging from 0.46- to 2.967-μm were filtered through filter grains of diameters 0.1-, 0.175- and 0.45-mm. Experimental conditions were chosen so as to obtain breakthrough curves. The experimental results showed that the initial efficiency follows the pattern reported by previous experimental and theoretical studies, i.e., lower efficiency for 0.825-μm particles which fall in the range of critical size. However, the particle removal during the transient stage increased with an increase in particle size for the range of sizes studied. This pattern is qualitatively confirmed by the theoretical predictions of Vigneswaran and Chang (1986) model. This study also provides experimental verification of the effect of the ratio of particle size and grain size at different stages of filtration.     

金属杂质对马边磷矿酸解料浆过滤特性的影响

采用正交试验与单因素试验相结合的方法,就马边磷矿中金属杂质对磷矿的酸解及过滤过程的影响进行了研究,讨论金属杂质对磷矿转化率和料浆粘度、电导率、过滤强度的影响及变化规律,确定了铁镁铝杂质对过滤强度产生影响的强弱程度。     

Experimental study on mechanical filtration through tobacco columns: Influence of cut-filler shape and size distribution on filtration efficiencies

Tobacco columns are extreme examples of heterogenous packed beds, which have various cut-filler shapes and a wide range of size distribution. The behavior of mechanical filtration through tobacco columns has been investigated by using polystyrene latex (PSL) standard particles to compare the actual filtration efficiency with the predicted filtration efficiency calculated by theoretical equations for spherical packed beds. The influence of cut-filler shape and the range of cut-filler size distribution on filtration efficiency have been examined. The effect of diffusion in tobacco columns was lower and the effect of interception and inertia were higher than in spherical packed beds. These results show that a partially faster flow could have occurred in tobacco columns. It means that it is difficult to utilize the theoretical equations for spherical packed beds to heterogenous packed beds as proposed. Filtration efficiency through tobacco columns had a relationship with the factor that shows cut-filler shape and size distribution (r = 0.894, p < 0.05) and the factor that shows cut-filler size distribution (r = 0.683, p < 0.15). The factor showing cut-filler shape and size distribution was expected to be an effective factor of filtration efficiency for heterogenous packed beds. From these experiments, empirical equations that can be applied to tobacco columns have been proposed, and the prediction accuracy during burning was validated. It has been found that the prediction accuracy was precise, revealing the importance of taking the influence of cut-filler shape and size distribution into account in the filtration equations.

Copyright © 2016 American Association for Aerosol Research     

甲醇制芳烃催化剂及相关工艺研究进展

当前我国芳烃生产面临石油资源短缺等困难,而我国丰富的煤炭资源和当前过剩的甲醇产能为煤基甲醇制芳烃提供了价格低廉的原料,对于我国的能源安全具有重要意义。本文综述了近年来国内外甲醇制芳烃（MTA）技术的相关研究进展,分别介绍了MTA固定床、流化床技术;针对传统MTA过程产物复杂,分离能耗大,经济效益差等不足,归纳总结了更具经济性的甲醇一步法制对二甲苯（MTPX）以及甲醇一步法制对二甲苯联产低碳烯烃（MTO&PX）技术;针对传统MTA催化剂稳定性差等不足,对比介绍了甲醇直接制芳烃以及低碳烯烃制芳烃路线,结果表明甲醇经低碳烯烃制芳烃工艺路线具有催化剂寿命长、产品组成易调节等优势。最后,本文认为分子筛限域效应、分子筛表面修饰新技术、金属-酸双活性中心协同效应是未来MTA研究的重要方向。