Investigation of the figure of merit for filters with a single nanofiber layer on a substrate

We investigate filters composed of a layer of nanofibers on a substrate made of micrometer fibers and compare the performance of such nanofiber media to conventional micrometer fibrous filters. The performance of the nanofiber filters is evaluated using the figure of merit, which represents the ratio between the filtration efficiency and the pressure drop. Filtration tests were performed on four samples with different nanofiber solidities. As the nanofiber solidity increases, the filtration efficiency and the pressure drop both increase. We develop a numerical model to simulate the nanofiber filters. When the nanofiber solidity is appropriately adjusted, the pressure drop computed from the model is in good agreement with experimental results. Filtration efficiency for the nanofibers due to interception, inertial impaction and diffusion can be computed from the model. The simulation results are in good agreement with experiments for 20–780 nm particles but discrepancies exist for particles smaller than 20 nm. Our results show that nanofiber filters have better figure of merit for particles larger than about 100 nm compared to conventional fiberglass filters. For particles smaller than 100 nm, nanofiber filters do not perform better than conventional fiberglass filters.     

Characterization of filter media prepared from aligned nanofibers for fine dust screen

Nanofibers for fine dust filters of four structures (random, aligned, orthogonal, and nanofiber net) were prepared by electrospinning method using polymers such as PAN and PA6. While conventional electret filters experienced deterioration problems in fine dust(PM_1.0) capture as its surface charge decayed, the electrospun nanofibers prepared contributed to the removal capacity. The filters from aligned fibers showed high quality factors ( q _F : filter performance indicator) and filtration efficiency from 22 to 50% depending on particle size than simple electret media at a face velocity of 15.92 cm/s. The fiber structure of nanofiber net (NFN) presented almost absolute collection efficiency, particularly on dust particles smaller than 300 nm. Furthermore, the composite filters which are composed both of a commercial electret mask filters and nanofiber nets effectively enhanced the overall filtration efficiency by 59.46%, resulting in more than 99% for PM_1.0. Consequently, electrospun polymer nanofibers offer a promising plausible mask filter material with air permeability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48166.     

Experimental Study of Submicrometer and Ultrafine Particle Penetration and Pressure Drop for High Efficiency Filters

The fractional penetration of submicrometer particles through five high-efficiency glass fiber filters, one composite fiber filter, and two membrane filters was measured for particles of 0.004–0.42-μm diameter at filter face velocities ranging from 0.5 to 20 cm/s. The glass fiber filters all had approximately the same thickness and weight per unit area, and were rated 93% to 99.999% efficient using the conventional 0.3-μm dioctyl phthalate (DOP) test. The challenge aerosols were electrically classified monodisperse DOP in the diameter range of 0.032–0.42 μm, and polydisperse silver condensation aerosols having diameters of ～ 0.004–0.01 μm. Aerosol penetration through these media was found to be generally consistent with current theory for collection by diffusion and interception over the particle size and velocity range studied. Using a filter figure of merit calculated for penetration by 0.1- and 0.3-μm particles to facilitate comparison, all of the filters except one tetrafluoroethylene membrane filter (which was slightly better) had about the same performance.     

Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters

Conventional, mechanical fibrous filters made of microfibers exhibit a local minimum of fractional collection efficiency in the aerosol particle size-range between 100 and 500 nm, which is called the most penetrating particle size (MPPS). Simple theoretical calculations predict that this efficiency may be significantly increased using nanofibrous media. The main objective of this paper is an experimental verification of these expectations and simultaneously checking whether this anticipated gain in the filtration efficiency is not overpaid with an excessive pressure drop. For this purpose we developed a modified melt-blown technology, which allowed us to produce filters composed of micrometer as well as nanometer sized fibers. One conventional microfibrous filter and five nanofibrous filters were examined. The complete structural characteristics, pressure drop and efficiency of removal of aerosol particles with diameters 10-500 nm were determined for all media. The results of the experiments confirmed that using nanofibrous filters a significant growth of filtration efficiency for the MPPS range can be achieved and the pressure drop rises moderately. Simultaneously, we noticed a shift of the MPPS towards smaller particles. Consequently, the quality factor for bilayer systems composed of a microfibrous support and a nanofibrous facial layer was considerably higher than this one for a conventional microfibrous filter alone. Additionally, it was found that utilization of many-layer nanofibrous filters combined with a single microfibrous backing layer is even more profitable from the quality factor standpoint. Comparing experimental results with theoretical calculations based on the single-fiber theory we concluded that for microfibrous filters a fairly good agreement can be obtained if the resistance-equivalent fiber diameter is used in calculations instead of the mean count diameter determined from the SEM images analysis; in the latter case, filtration efficiency computed theoretically is slightly overestimated. This is even more evident for nanofibrous media, suggesting that in such case a structural filter inhomogeneity has a strong influence on the filter efficiency and its resistance and one should strive for minimization of this effect manufacturing nanofibrous filters as homogeneous as possible. We can finally conclude that fibrous filters containing nanofibers, which are produced using the melt-blown technique, are very promising and economic tools to enhance filtration of the most penetrating aerosol particles.     

静电纺丝纤维滤材表征及其气液过滤性能

以聚丙烯腈与二甲基甲酰胺为原料配制纺丝溶液,采用静电纺丝技术制备玻璃纤维/聚丙烯腈纤维/玻璃纤维三层复合滤材,研究了纺丝溶液浓度与纺丝电压等参数对纤维形貌及尺寸的影响,分析了复合滤材的过滤性能. 结果表明,控制不同纺丝溶液浓度可得形貌不同的纤维,且溶液浓度越大纤维尺寸越大;纺丝电压对纤维形貌的影响较小,但增加纺丝电压使静电纺纤维层的孔径减小. 相比玻璃纤维滤材,复合滤材过滤效率明显提升,稳态效率最大可提升21%,最易穿透粒径效率最大可提升39%,但复合滤材孔径较小时,过程压降增加了一段跳跃阶段,纳米纤维层表面形成液膜,使复合滤材稳态压降升高.     

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.     

Improvement in nanofiber filtration by multiple thin layers of nanofiber mats

Nanofiber filtration is drawing great interest nowadays because of its large surface collection area as well as low air resistance. In this study, electrospun nanofiber mats of different thicknesses were evaluated for their filter quality factors. Shorter-term electrospun fiber mats exhibited a better quality factor than those longer-term electrospun ones. Multiple thin layers of nanofiber mats to improve the filter quality of the nanofiber filters were then evaluated. Filtration test results showed that the filter made up of multiple thin layers of nanofiber mats had a filter quality factor much higher than the single thick layer nanofiber mat. Better thickness uniformity in the multi-layer structure due to stacking compensation and smaller fiber diameters in nanofibers of short-term deposition time are two possible reasons for the improvement of the filter quality.     

Filter life comparison of different levels of nanofiber coated cleanable-surface filter for gas turbine

High-efficiency air filtration is a basic requirement for the most cost-effective operation of high-efficiency gas turbines. The filtration system protects the gas turbine from damaging debris. In gas turbine/dust collector applications, higher efficiency filtration could be achieved with nanofibers, which provide higher equipment protection than traditional media. With a nanofiber performance filter layer, the dust accumulates on the surface of the filtration media rather than within the media and could be cleaned off easily with a back pulse resulting in long filter life and a low-operating pressure drop. In this study five type of gas tribune nanofiber coated corrugated cellulose/synthetic filter media were developed. Nanofiber coating was adjusted for five filtration efficiency level, 50 ≤ E < 60, 60 ≤ E < 70, 70 ≤ E < 85, 85 ≤ E < 95 and 95 ≤ E, pore size and filter-life of the developed media were evaluated. One of the developed nanofiber coated media was also compared with two other commercial nanofiber coated gas tribune filter media, a glass fiber type filter media and a commercial fine fiber gas tribune filter media. It was seen that, with decreasing penetration levels due to nanofiber coating level, initial 30 cycle durations of filter life evaluation could reach about 229.9 to 250.7 min. Highest final cycle duration of 188.7 min belonged to cellulose/synthetic blend corrugated filter media with penetration of 13.66%. Nanofiber based surface filter media was cleaned up better than fine fiber media and final 30 cycle sequences were significantly higher. Surface of the nanofiber coated media was smoother when compared to fine fiber media and during the initial and final cycle test dust could not penetrate inside and could not hang to this smooth surface. So, with back pulse cleaning cake releasing have performed easily. It was also seen that, for higher filter life nanofiber coating should be uniform and robust to back pulse cleaning.     

Effect of air blowing on the morphology and nanofiber properties of blowing‐assisted electrospun polycarbonates

Polycarbonate (PC) nanofibers are prepared using the air blowing‐assisted electrospinning process. The effects of air blowing pressure and PC solution concentration on the physical properties of fibers and the filtration performance of the nanofiber web are investigated. The air blowing‐assisted electrospinning process produces fewer beads and smaller nanofiber diameters compared with those obtained without air blowing. Uniform PC nanofibers with an average fiber diameter of about 0.170 μm are obtained using an applied voltage of 40 kV, an air blowing pressure of 0.3 MPa, a PC solution concentration of 16%, and a tip‐to‐collection‐screen distance (TCD) of 25 cm. The filtration efficiency improvement of the air blowing‐assisted electrospun web can be attributed to the narrow distribution of fiber diameter and small mean flow pore size of the electrospun web. Performance results show that the air blowing‐assisted electrospinning process can be applied to produce PC nanofiber mats with high‐quality filtration. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

基于纤维取向的纳米纤维滤料设计及其性能

通过基于霍夫变换的图像分析法获取静电纺纳米纤维取向分布信息,分析纤维取向对纳米纤维滤料性能的影响,并据此设计制备了中间为杂乱纤维层、两侧为相互垂直的取向纤维层构成的复合纳米纤维膜滤料。采用扫描电镜对纳米纤维膜形貌进行观察并获取SEM图像,进行了透气性、拉伸性能、孔径尺寸和过滤性能测试。结果表明,纳米纤维膜纤维分布方向拉伸断裂强度高,纤维取向各向异性比例理论值和实验值相吻合,纤维取向是影响纳米纤维膜力学各向异性的主要参数;取向纳米纤维膜滤料孔径较大且有许多微粒可逃逸的通道,其过滤效率和过滤阻力均较低,与文献中报道的数值模拟结果相一致;所设计制备的复合纳米纤维膜滤料结合了取向纳米纤维膜滤料力学性能优良和杂乱纳米纤维膜滤料过滤效率高的优点,其纵向和横向断裂强度分别为8.85MPa和8.71MPa,气流流速为25L/min时过滤效率高达99.691%。     

Analytical expressions for predicting capture efficiency of bimodal fibrous filters

In this work, a series of numerical simulations are formulated for studying the performance (collection efficiency and pressure drop) of filter media with bimodal diameter distributions. While there are numerous analytical expressions available for predicting performance of filters made up of fibers with a unimodal fiber diameter distribution, there are practically no simple relations for bimodal filters. In this paper, we report on the influence of the fiber diameter dissimilarity and the number (mass) fraction of each component on the performance of a bimodal filter. Our simulation results are utilized to establish a unimodal equivalent diameter for the bimodal media, thereby taking advantage of the existing expressions of unimodal filters for capture efficiency prediction. Our results indicate that the cube root relation of Tafreshi, Rahman, Jaganathan, Wang, and Pourdeyhimi (2009) offers the closest predictions for the range of particle diameters, coarse fiber number (mass) fractions, fiber diameter ratios, and solid volume fractions (SVF) considered in this work. Our study revealed that the figure of merit (FOM) of bimodal filters increases with increasing fiber diameter ratios for Brownian particles (d_p<100 nm), and decreases when challenged with larger particles. It has also been shown that when increasing the ratio of coarse fibers to fine fibers, FOM increases for Brownian particles, and decreases for larger particles.     

Nanofibrous chitosan non-wovens for filtration applications

Chitosan containing nanofibrous filter media has the advantage of filtering material based on both its size and functionality. They can be potentially applicable in a wide variety of filtration applications ranging from water purification media to air filter media. We have fabricated nanofibrous filter media by electrospinning of chitosan/PEO blend solutions onto a spunbonded non-woven polypropylene substrate. Filter media with varying fiber diameter and filter basis weight were obtained. Heavy metal binding, anti-microbial and physical filtrations efficiencies of these chitosan based filter media were studied and correlated with the surface chemistry and physical characteristics of these nanofibrous filter media. Filtration efficiency of the nanofiber mats was strongly related to the size of the fibers and its surface chitosan content. Hexavalent chromium binding capacities up to 35 mg chromium/g chitosan were exhibited by chitosan based nanofibrous filter media along with a 2-3 log reduction in Escherichia coli bacteria cfu.     

聚乳酸复合空气过滤材料的制备及性能研究

以分子质量为7万的聚乳酸（PIA）为原料,分别用不同的溶剂制得两种纺丝液,并采用静电纺丝法将其分别纺在水刺无纺布和熔喷无纺布上,采用TSI8130仪器对阻力和效率进行测试,比较两者的过滤性能。结果表明：当纳米纤维膜厚度为2mm左右时,水刺非织造布与纳米纤维的复合材料过滤效率从近乎0提高到45．88％,当纳米材料厚度为1mm左右时,提高到26．12％;熔喷非织造布分别与1mm和2mm厚度的纳米纤维膜复合后过滤效率分别提高了23．7％和24．6％,但缺点是过滤阻力提高。     

Single-Fiber Interception Efficiency for Elliptical Fibers

Synthetic fibers with non-circular cross-sections are used in air filters. These fibers may offer performance advantages over traditional fibers with circular cross sections because they have more surface per unit volume of fiber upon which particles can collect. Starting with a solution for the velocity field around fibers with elliptical cross-sections, expressions for predicting the single-fiber efficiency for particle collection by the interception mechanism were developed for elliptical fibers. The interception efficiency predictions depend on filter solidity, fiber properties such as size, aspect ratio, and orientation of the cross section relative to the incoming flow, and particle diameter. The expressions demonstrate that single fiber interception efficiency for elliptical fibers generally increases with increasing particle diameter, increasing solidity, increasing aspect ratio, and as the major axis of the ellipse becomes more perpendicular to the incoming air flow.     

Performance of fibrous filters during nanoparticle cake formation

Fibrous filters are highly efficient in removing micrometer particles, but their performance in the nanometer particle range is still little known. The aim of this study was to evaluate pressure drop and collection efficiency during nanoparticles cake formation using commercial fibrous filters. The filter media used were High Efficiency Particulate Air (HEPA) and polyester filters. The aerosols were generated by a commercial inhaler using a 5 g/L solution of NaCl and the particles produced were in the size range from 6 to 800 nm, with a peak at around 40 nm. A superficial velocity (v_s) of 0.06 m/s was employed. During the filtration, the maximum pressure drop established was ?P ₌ ?P_f +980Pa, where ?P_f is the initial pressure drop of the filter. The collection efficiency was determined for a clean filter and for intermediate pressure drops. The filtration curves obtained showed that the HEPA filter provided greater surface filtration, compared to the polyester filter. Comparison of the collection efficiencies for clean filters revealed that the HEPA filter was highly efficient, even in the absence of cake, while the polyester filter showed initial collection efficiencies of between 20 and 40% for particles in the size range from 100 nm to 1000 nm. However, after formation of the filter cake, the collection efficiencies of both filters were almost 100% during the final stage of filtration. This shows that the fibrous filter can be applied in several industrial processes with highly efficient nanoparticle separation, after the formation of a thin layer cake filtration.     

Experimental Study on the Effect of Fiber Orientation on Filter Quality

Most filtration models assume that the air stream runs perpendicularly to the orientation of the filter fibers. However, cigarette filters remove aerosol particles apparently by a different filter configuration in that the fiber orientation almost parallels the air streamlines. To focus on the effect of fiber orientation, cellulose acetate filters were used in this work to facilitate the filter performance comparison. A piece of original round cigarette filter was molded to form a cube. The same piece of filter was used for both perpendicular and parallel orientations, to avoid the variability caused by the non-uniform filter media distribution. DOP aerosol particles used in the tests were generated by either a constant output aerosol nebulizer or an ultrasonic atomizing nozzle. A Po-210 radiation source was used to neutralize the challenge aerosols to the Boltzmann charge equilibrium. A scanning mobility particle sizer (for < 0.8 μm) and an aerodynamic particle sizer (for > 0.8 μm) were used to measure aerosol number concentrations and size distributions upstream and downstream of the cigarette filters. The results showed that parallel and orthogonal filters behave similarly. However, the pressure drop across parallel filter was lower than for the perpendicular filter, indicating that the airflow is more laminar passing through the parallel filters. Possibly for the same reason, aerosol penetration through parallel filter was higher than the orthogonal filter, although the difference may not be statistically significant. When a comparison of the fiber orientation is based on filter quality, orthogonal filter performs better than parallel filter, if face velocity is lower than 60 cm/s. Parallel filter performs better only when the particles are smaller than the most penetrating size and under high face velocity.     

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     

Nanoparticle filtration by electrospun polymer fibers

Polyacrylonitrile (PAN) fibers with mean diameters in 270-400 nm range were prepared by electrospinning for use as a filter media. Compared to commercial filters made of polyolefin and glass, the fibers of electrospun filters were more uniform in diameter. The performance of electrospun filters was evaluated by measuring the penetration of monodisperse NaCl nanoparticles (below 80 nm in size) through the filters. It was found that electrospun filters could be made which had nanoparticle penetration values comparable to commercial filters but with substantially less filter mass. The penetration of nanoparticles through the electrospun filter media could be reduced by increasing the filter thickness, which is controlled by the collection time during the electrospinning process. Nanoparticle collection by electrostatic forces was found to be negligible for electrospun filters. Filter quality factors and single fiber collection efficiencies were found to be independent of filter thickness for electrospun filters, and the penetration of nanoparticles through electrospun filters was in better agreement with theoretical predictions than was the measured penetration through a commercial filter. This study shows that electrospinning is a promising technology for the production of high performance nanoparticle filters.     

A study on the effects of laminating temperature on the polymeric nanofiber web

The use of fine fiber has become an important design tool for filter media. Nanofibers-based filter media have some advantages such as lower energy consumption, longer filter life, high filtration capacity, easier maintenance, low weight rather than other filter media. The nanofibers-based filter media made up of fibers of diameter ranging from 100 to 1,000 nm can be conveniently produce by electrospinning technique. Common filter media have been prepared with a layer of fine fiber on typically forming the upstream or intake side of the media structure. The fine fiber increases the efficiency of filtration by trapping small particles, which increases the overall particulate filtration efficiency of the structure. Improved fine fiber structures have been developed in this study in which a controlled amount of fine fiber is placed on both sides of the media to result in an improvement in filter efficiency and a substantial improvement in lifetime. In the first part of this study, the production of electrospun nanofibers is investigated. In the second part, a different case studyis presented to show how they can be laminated for application as filter media. Response surface methodology (RSM) was used to obtain a quantitative relationship between selected electrospinning parameters and average fiber diameter and its distribution.     

On the pressure drop prediction of filter media composed of fibers with bimodal diameter distributions

In addition to collection efficiency, pressure drop is the most important characteristic of a filter medium. While there are numerous analytical expressions available for predicting the pressure drop of the filters made up of fibers with a unimodal fiber diameter distribution, there are not enough studies dedicated to filters composed of fibers with a bimodal (or multimodal) fiber diameter distribution. In this work, the pressure drop per unit thickness of filters made of bimodal fiber diameters is calculated by solving the Navier-Stokes equations in a series of 2-D geometries. These results are used to find the unimodal equivalent diameters of each bimodal filter that could be used in the existing expressions for calculating pressure drop. In agreement with the work of Brown and Thorpe [Brown, R.C., Thorpe, A., Glass-fiber filters with bimodal fiber size distributions. Powder Technology 118 (2001) 3-9.], it was found that the area-weighted averaging of the fiber diameters in a bimodal filter provides a relatively good estimation of its equivalent unimodal fiber diameter. We, however, noticed that in such an averaging the error percentage in the pressure drop prediction is sensitive to the fiber diameter ratios as well as the fraction of each fiber diameter in the bimodal filter. We, therefore, obtained a correction factor for the estimation of the unimodal equivalent diameters as a function of fiber diameter ratio and their number fractions.