Comparative Study of the Performance of Nine Filters Utilized in Filtration of Aerosols by Bubbling

A new process of air purification has been developed and studied experimentally (Agranovski et al. 1999). This process is based on passing air through a filter immersed in liquid. It leads to the formation of bubbles within the filter as the carrier gas passes through and thus provides alternate mechanisms for the removal of aerosols. It was identified (Agranovski et al. 1999) that for relatively large particles (0.3-2.7 w m) the efficiency of the filter utilized in wet filtration is higher than for dry filtration. In the current paper, experiments are described that investigate the performance of the technology for particles in the size range of 0.03 to 2.7 w m, to identify the importance of an alternative mechanism of purification (diffusion). Also, 9 different filters were employed to compare the efficiencies of the wet and dry filtration for different applications of the technology.     

Collection of Airborne Microorganisms into Liquid by Bubbling through Porous Medium

A new method for the removal of airborne particles by air bubbling through fibrous filters immersed into a liquid has recently been developed (Agranovski et al. 1999) and shown to be very efficient for cleaning air environments with ultra-fine aerosol particles. The principal objective of the present study was to evaluate the new bubbling technique for the collection of airborne bacteria into a liquid for subsequent physical and microbiological analysis. It was found that the technique is capable of achieving a physical collection efficiency of 98.5% or higher for particles larger than 0.3 w m in aerodynamic diameter. The physical collection efficiency of the prototype bubbler remained at that high level for 8 h of continuous operation with negligible variation of the pressure drop across the device. Evaporation of the collection fluid did not exceed 20% during 8 h, and the reaerosolization effect on the physical collection efficiency of the bubbler prototype was <8%. The recovery rate of gram-negative Pseudomonas fluorescens bacteria collected for 20 min was shown to be as high as 74% - 10%. Its decrease with time was not statistically significant: the recovery rate reached 63% - 15% and 58% - 16% after 4 and 8 h of continuous operation, respectively. Thus the bubbling technique was demonstrated to be suitable for collecting viable airborne bacteria even if they are sensitive to the stress.     

Development of a new personal air filter test system using a low-cost particulate matter (PM) sensor

Abstract

The extensive use of air filters has encouraged advances in both the fabrication and characterization of air filter technology. An affordable and accessible means of assessing the quality of air filters is greatly needed because of the high demand for these filters. We developed a personal air filter test (PAFT) system for measuring filter pressure drop, efficiency, and quality of filtration. The PAFT system utilizes a PM sensor (Sharp, GP2Y1010AU0F) to measure filtration efficiency. PM sensor performance was evaluated and optimized to guarantee its suitability for this application. The sensor performance evaluation studied the output responses to sampling flow, particle diameter, and PM sources. We also improved the sensor’s sensitivity. The experimental results show that the sensor had no significant influence on the sampling flow. The sensor output was highly dependent on the particle size and PM source, but its response curves remained linear, which was an advantage for filter efficiency measurement. We measured the efficiency of nanofiber filters of various efficiencies, comparing the results to a reference efficiency as measured by a CPC (TSI, 3772). The test resulted in a filtration coefficient (K_f), which was used to correct the PAFT efficiency measurement results. We also conducted filtration efficiency tests on commercial mask filters and the results showed good agreement to the reference, with a small average error of about 2.5%. The complete design of the PAFT and experimental methods is discussed in detail.

Copyright © 2020 American Association for Aerosol Research     

Combined Wettable/Non‐wettable Filter for Mist Purification

Previous studies by Agranovski and Braddock [1] show that wettable filters are very efficient at collecting small liquid aerosols. However, a major problem associated with wettable filters is the breakthrough process associated with the destruction of film covering the fibres, and the removal of relatively large droplets by the carrier gas passing through the rear surface of the filter. A non‐wettable filter is relatively efficient at collecting large droplets and discarding them down its front face [2]. These two types of filters can be combined in layers to make use of these distinctive properties. The best arrangement is where the carrier gas passes first through the wettable filter, and then through the non‐wettable filter. Where there is a contact between the two filters, interface effects assist the filtration and filter system drainage. The filtration mechanisms are enhanced when the filter box is tilted so that gravity partially opposes the drag forces on the liquid in the filter. In the current paper, the results of experimental analysis of combined filtration systems are presented, and further steps towards industrial design are discussed.     

Analysis of Steady-State Filtration and Backpulse Process in a Hot-Gas Filter Vessel

The need to develop a technology for clean and efficient electric power generation has led to the development of advanced pressurized fluidized bed combustors (PFBC) and integrated gasification combined cycles (IGCC). The effective filtration of hot gases for removal of ash and sulfur sorbent, however, is the key to the success of these advanced coal energy systems. Recently, attention has been given to the use of ceramic candle filters for hot-gas cleaning. The ash cake formation on these filters needs to be removed by the backpusle for their successful operation. In this paper, steady-state filtration as well as the transient gas flow during the backpulse process in the integrated gasification and cleanup facility (IGCF) (located at the National Energy Technology Laboratory, NETL) is studied. The steady-state filtration condition is first evaluated, using a compressible heat-conducting flow analysis. Particle transport patterns are studied, and the deposition patterns of 1-30 w m particles on the ceramic filters and the vessel surfaces are analyzed. To simulate the backpulse process, the pressure at the filter exit is increased sharply in a period of about 0.01 s pressure. The stress transport model of the FLUENT code is used to evaluate the time evolution of the transient gas flow velocity, pressure and thermal fields, as well as turbulence intensities and stresses inside the candle filter and in the IGCF filter vessel. Contour plots of the hot-gas flow conditions from the start of the pressure buildup to its saturation level are presented. The results show the rapidly changing flow conditions during the initial stages of the backpulse. The pressure wave propagates along the length of the filter until a monotonic increase of pressure with time is achieved; that is, the pressure field inside the filter at the initial stages of the backpulse is strongly nonuniform. Therefore the potential for incomplete filter cake removal exists. Motions of particles that enter the vessel and/or are ejected from the candle filter during the backpulse process are also studied, and illustrative particle trajectories are presented.     

Removal of Xylene from Waste Gases using Biotrickling Filters

Two identical laboratory‐scale biotrickling filters, filled with different ceramic materials, were operated in order to investigate the removal of xylene from a waste gas stream. The biotrickling filter columns were seeded with pure bacteria identified as Bacillus firmus, which can utilize xylene as the sole carbon and energy source. The purification performance of the biotrickling filters was examined for xylene inlet concentrations C_g ≤ 3000 mg/m³ at different gas flow rates of 0.2 m³/h, 0.6 m³/h, and 1 m³/h, which correspond to gas empty bed residence times (EBRTs) of 84.8 s, 28.3 s, and 17.0 s, respectively. Both biofilters displayed a removal efficiency of no less than 95 % with the inlet xylene less than 3000 mg/m³ at the EBRTs of 84.8 and 28.3 s. When EBRT decreased to 17.0 s, the biofilter filled with ceramic particle type 2 had a better performance. The flow rate of trickling liquid has little effect on the removal efficiencies of the two filters. In the case of uneven distribution of trickling liquid in the packing materials, the performance of the biofilter can be improved by increasing the nitrogen nutrient supplement. Biomass quantity decreases as the depth of packing material increases in both biofilters, but the biofilter filled with ceramic particle type 1 had more alive bacteria per unit mass of packing material than the other.     

Removal of aerosols by bubbling through porous media submerged in organic liquid

Aerosols can be filtered by passing the carrier gas through a fibrous filter immersed in water (Agranovski, I. E., Myojo, T., & Braddock, R. D. (1999a). Bubble filtering through porous media. Aerosol Science and Technology, 31, 249-257. Using water as the irrigating fluid significantly increases the efficiency of filtration of particles and adds the possibility for simultaneous removal of alien gases from the carrier. Organic compounds (gaseous and particulate) form a significant proportion of pollutants in the atmosphere, and effective purification is needed for ambient air as well as for cleaning exhaust streams. Water does not have a high level of solubility for gaseous organic compounds, and alternative irrigating liquids need to be considered. Experiments were conducted using sunflower oil as the irrigating fluid. The filtration efficiencies of the oil are better than for water, for liquid di-ethyl-hexyl-sebacate particles. As the solubility of organic vapours is much higher in oil compared with the one in water, oil provides an excellent opportunity for utilizing as the irrigating liquid for high-efficient simultaneous removal of organic particles and vapours from air carrier.     

Modeling of SO2 removal in fabric filter

Fabric filters are involved in most semi-dry flue gas desulfurization process and represent ability of SO₂ removal. SO₂ removal efficiency in fabric filter after a semi-dry scrubber is investigated. Experimental results showed that SO₂ inlet concentration has little effect on SO₂ removal efficiency, SO₂ removal efficiency increases as flue gas inlet temperature increases and relative humidity affects SO₂ removal efficiency significantly. The kinetic model based on shrinking core theory has been presented. It is found that, in the beginning, when calcium hydroxide conversion ratio is less than 0.3, SO₂ removal process is mainly controlled by chemical reaction (Model-2); and when calcium hydroxide conversion ratio is greater than 0.3, SO₂ diffusion through product layer is rate limiting (Model-3). The experimental results in fabric filter are successfully correlated by Model-3.     

Air flows and pressure drop modelling for different pleated industrial filters

A dimensionless model was developed to determine the pressure drop across clean pleated filters, according to filter medium type, geometric characteristics of the pleating (distance between two pleats, pleat height, etc) and air flow parameters (filtration velocity, air density, etc). The model was derived from both experimental and numerical results obtained from nuclear and automotive filters — high efficiency particulate air (HEPA) and low efficiency particulate (LE), respectively. The major findings were that a more homogeneous air flow distribution occured over the surface of the pleated HEPA filter, while geometric characteristics had a greater influence on the initial pressure drop across the LE filter. The numerical model highlighted the fundamental importance of the filter medium's air flow resistance on air flow distribution.     

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.     

Comparative study of the elimination of toluene vapours in twin biotrickling filters using two microorganisms Bacillus cereus S1 and S2

BACKGROUND: To investigate the microbial degradation performance of organic pollutants in the atmosphere using a biotrickling filter, two microorganism strains, Bacillus cereus S1 and Bacillus cereus S2, were selected, identified and inoculated into a twin biotrickling filter for comparison. RESULTS: Both strains showed good performance towards the degradation of model organic pollutants when gas flow rates ranged from 100 to 600 L h⁻¹. For S1, the total maximum removal efficiency (RE) of toluene was maintained nearly 100% not only at gas flow rates of 100 L h⁻¹ corresponding to empty bed residence time (EBRT) 199.44 s, but also at gas flow rates of 200 L h⁻¹ (EBRT = 99.72 s) and 300 L h⁻¹ (EBRT = 66.48 s). However, S2 had a much lower degradation capability; near 100% removal efficiency was obtained only at the gas flow rate of 100 L h⁻¹ although both bacteria belong to the same Bacillus cereus. With further increase in gas flow rate, the total REs for both S1 and S2 decreased slightly at first and then dropped sharply to 46% and 35%, respectively, at an EBRT of 33.24 s, corresponding to a gas flow rate of 600 L h⁻¹. Starvation for between 2 and 10 days resulted in the re‐acclimation times of both strains ranging between 1.0 and 15.5 h. CONCLUSION: Strain S1 would be a better choice for inoculation into a biotrickling filter than strain S2, because of its much higher toluene removal capacity and rapid recovery to full performance. Copyright © 2008 Society of Chemical Industry     

Meso-scale structures of bidisperse mixtures of particles fluidized by a gas

Flow characteristics of bidisperse mixtures of particles fluidized by a gas predicted by the mixture based kinetic theory of [Garzó et al., 2007a] and [Garzó et al., 2007b] and the species based kinetic theory model of Iddir and Arastoopour (2005) are compared. Simulations were carried out in two- and three-dimensional periodic domains. Direct comparison of the meso-scale gas-particle flow structures, and the domain-averaged slip velocities and meso-scale stresses reveals that both mixture and species based kinetic theory models manifest similar predictions for all the size ratios examined in this study. A detailed analysis is presented in which we demonstrate when the species based theory of Iddir and Arastoopour (2005) will reduce to a mathematical form similar to the mixture framework of [Garzó et al., 2007a] and [Garzó et al., 2007b] . We also find that the flow characteristics obtained for bidisperse mixtures are very similar to those obtained for monodisperse systems having the same Sauter mean diameter for the cases examined; however, the domain-averaged properties of monodisperse and bidisperse gas-particle flows do demonstrate quantitative differences. The use of filtered two-fluid models that average over meso-scale flow structures has already been described in the literature; it is clear from the present study that such filtered models are needed for coarse-grid simulations of polydisperse systems as well.     

Antimicrobial Air Filtration Using Airborne Sophora Flavescens Natural-Product Nanoparticles

We investigated nanoparticle generation from a natural plant extract using the aerosol technique of the nebulization-thermal drying process, and tested its usefulness for antimicrobial air filtration. Sophora flavescens Ait. ethanolic extract was prepared as an antimicrobial natural-product suspension. Suspension droplets were generated using a single-jet Collison nebulizer, passed through an active carbon absorber to remove ethanol, and mixed and dried with sheath air. For drying, natural-product particles were exposed to 200°C for ～1 s. Finally, particles were introduced into a scanning mobility particle sizer, and their size distribution and morphology were analyzed. For application of natural-product particles to antimicrobial air filtration, the nanosized particles generated were deposited continuously onto air filter medium at various times. Physical characteristics (filtration efficiency, pressure drop, and fiber morphology by scanning electron microscopy), and biological characteristics (antimicrobial tests against Staphylococcus epidermidis, Bacillus subtilis, and Escherichia coli bioaerosols) were then evaluated. We also analyzed the chemical composition of particles deposited on the filter surface. The results showed that the nanoparticles generated were spherical and demonstrated a polydisperse size distribution, ranging from several tens to several hundred nanometers. Although the filter pressure drop increased with the amount of nanoparticle on the filter, the bioaerosol filtration efficiency and antimicrobial activity were enhanced. In particular, the S. flavescens natural-product nanoparticle-deposited filters were more effective for removal of Gram-positive than Gram-negative bioaerosols. These results are promising for the implementation of this new technology for control of air quality against hazardous bioaerosols.     

Gas/Particle Partition Measurements of PAH at Hazelrigg,U.K.

During September and October, 1998, the new Integrated Organic Gas and Particle Sampler (IOGAPS), was operated at Hazelrigg, UK, the field measurement station of the University of Lancaster. Gas/particle partition ratios of twenty-two 2–5 ring PAH were determined using both the IOGAPS (in which the gas phase is collected before the particle phase) and a low flow sampler which utilized the traditional filter-sorbent geometry. For compounds of intermediate volatility, less partitioning to the gas phase was observed when the denuder was used. The denuder (8- channel, 60 cm, 16.7 L min^?1 air sampling rate) trapped small amounts of several non-volatile PAH. This result is consistent with particle diffusion losses of 5 to 10% for particles less than 0.05 μm under the flow conditions in the denuder. The 60-cm denuder was probably longer than necessary for the flow rate used. During the sampling, both glass fiber and Teflon-coated glass fiber filters were used. Without a denuder in front of a glass fiber filter, the fine particulate mass (PM 2.5) measurements showed a major positive bias that has been attributed to adsorption of gases by the filter. Teflon-coated glass fiber filters were not subject to this artifact problem, and equal masses were collected on filters from the denuded and non-denuded air flows.     

CO2 mass transfer and conversion to biomass in a horizontal gas–liquid photobioreactor

This study deals with CO₂ mass transfers and biomass conversion in an industrial horizontal tubular photobioreactor. An analytical approach is used to determine an expression modeling the influence of CO₂ mass transfers on the overall biomass conversion efficiency for a given culture broth, heat and light conditions. Fluid mechanics and mass transfer are predicted with a classical two-phase flow approach (Taitel and Dukler, 1976) combined with a dissolution correlation developed and tested in the laboratory (Valiorgue et al., 2011). The influence of the stripping gas, removing the excess of oxygen in the liquid, on the conversion to biomass efficiency is shown to be not negligible. The expression is used to evaluate how the photobioreactor's design and process parameters can be tuned in order to improve biomass conversion efficiency. The biomass conversion efficiency evolution with the photobioreactor's length was found to behave asymptotically and it was explained by the relative orders of magnitude of gas dissolution and gas stripping. It has been shown that the gas flow rate for stripping and therefore the oxygen removal will be limited when further increasing the industrial photobioreactor's length for a given objective of CO₂ conversion to biomass efficiency.     

Impact of Relative Humidity on HVAC Filters Loaded with Hygroscopic and Non-Hygroscopic Particles

The key characteristics of an air filter—flow resistance and filtration efficiency—are strongly affected by captured particles. The impact of exposing loaded heating, ventilating, and air conditioning air filters to a relative humidity (RH) other than that experienced during loading is investigated to develop an understanding of the role of RH throughout filter operation. Flat sheets of commercial filter media were loaded with hygroscopic, non-hygroscopic, or a mixture of particles in a laboratory apparatus. When filters loaded with hygroscopic particles in dry air were exposed to an elevated RH of 40%, the flow resistance reduced by up to 47%, depending on the filter being tested. Investigation of filter efficiency before and after changes in RH in the same samples shows reductions of up to 11 percentage points in the 130-nm size range. Further increasing RH causes additional drops in flow resistance and efficiency whereas reverting back to a lower humidity does not change the filter characteristics. The irreversibility of the particle-loaded filter characteristics implies that the RH increases are associated with an irreversible change in the particle structure. The response to humidity was reduced if an aerosol mixture of hygroscopic and non-hygroscopic particles is used. Exposure of filters loaded with only non-hygroscopic particles does not show the same dependence on RH. Small increases in growth factor for RH changes below deliquescence, causing morphological changes in captured particle aggregates, is a potential explanation for the changes observed.

Copyright 2015 American Association for Aerosol Research     

Comparison of Test Methods for Determining the Particle Removal Efficiency of Filters in Residential and Light-Commercial Central HVAC Systems

Central heating, ventilating, and air-conditioning (HVAC) filters are often the dominant mechanism for particle removal in buildings. However, little is known about filter performance in real environments, particularly in residential and light-commercial buildings where particle concentrations and compositions can be very different from laboratory test conditions. This article explores differences in HVAC filter test protocols and refines a whole-house method for in situ testing of filters for size-resolved particle removal efficiencies. Results from the in situ method are compared with those from a simple upstream–downstream method for three types of commercially available filters in an unoccupied test house. Results from both field methods are compared with standardized laboratory test results as measured by an independent laboratory and as reported by the manufacturer. In general, comparisons between filter efficiency as measured by the refined whole-house method and as measured by the upstream–downstream method resulted in similar values of particle removal efficiency for many particle sizes and compared well with standardized lab tests, although experimental uncertainties were generally greatest for the whole-house method. However, the refined whole-house method has the added benefit of allowing an investigation of more particle interactions in an indoor environment, including deposition to ductwork and other HVAC system components, exfiltration through duct leakage, and bypass airflow around filters. Both field methods can be used to investigate the effects of HVAC system characteristics and dust loading on filter efficiency in real environments.

Copyright 2012 American Association for Aerosol Research     

Study air–water system in horizontal loop geometry

To enhance the understanding of hydrodynamic of air–water multi-phase flow inside a toroidal geometry, experiments were carried out in horizontal torus reactor. Compared with vertical flow, the flow in horizontal milli torus reactor was characterized by one additional flow pattern. In vertical position two flow regimes are considered: not-dispersed and dispersed flow while in horizontal position three flow regimes have been distinguished: stratified flow, dispersed flow and mixed flow regimes. The mixing time is measured by a conductimetric method as described by (Benkhelifa et al., 2000). The effect of both superficial gas velocities and impeller rotation speeds has been studied. The mixing time has been decreased by increasing both the superficial gas velocity and the impeller rotation speed and has been shorter than the one given for the horizontal configuration. The axial dispersion inside the reactor was modelled by the Zhang's model. The obtained results are in a good agreement with Zhang's model.     

Purification and removal of Ascaris and Fasciola hepatica eggs from drinking water using roughing filters

The performance of a horizontal roughing filter (HRF) and a downflow roughing filter (DRF) in the removal of Ascaris and Fasciola hepatica eggs was investigated. The experiments were performed at three filtration rates of 0.5, 1.0 and 1.5 m/h and different influent concentrations of Ascaris and Fasciola hepatica eggs. Alteration of the filtration rate in the range of 0.5–1.5 m/h did not have significant influence on the effectiveness of the roughing filters. The HRF had higher efficiency in the removal of both Fasciola hepatica and Ascaris eggs in comparison with the DRF, so that the average efficiencies of the HRF for the removal of Fasciola hepatica and Ascaris eggs at filtration rate of 1.0 m/h were determined to be 89.0 and 57.3%, respectively, whereas the same values of the DRF were 77.2 and 52.5%, respectively. The straining was confirmed to be the main mechanism of helminth eggs removal by the roughing filters, because Fasciola hepatica eggs with larger size were removed more effectively than Ascaris eggs. The results of this study indicate that the roughing filters, especially the HRF, had promising performance in the removal of helminth eggs and could be used for water and secondary effluent treatment.     

塑料在汽车滤清器中的应用

针对汽车滤清器用金属材料特别是黑色金属材料存在密度大、易腐蚀、不耐磨、加工工艺要求高等缺点,采用改性聚丙烯、增强尼龙66、(丙烯腈／丁二烯／苯乙烯)共聚物(ABS)、聚甲醛等改性塑料和工程塑料制备了滤清器中的叶片环、扩散锥、汽油滤清器外壳、空气滤清器外壳、旋流管、中心管等部件,既提高了生产效率,又降低了成本,经济效益显著。