Modeling of Gravitational Wet Scrubbers with Electrostatic Enhancement

Conventional gravitational wet scrubbers, which generally perform removal of fine particles with low efficiency, cannot meet new standards for pollution emissions. One way of improving the collection efficiency of fine particles is to impose additional electrostatic forces upon particles by means of particle‐charging, or droplet‐charging, or even opposite‐charging of particles and droplets. A Monte Carlo method for population balance modeling is presented to describe the particle removal processes of gravitational wet scrubbers with electrostatic enhancement, in such a way that the grade collection efficiency and particle size distribution are calculated quantitatively. Numerical results show that, the grade collection efficiency of submicron particles is only ca. 5 % in conventional wet scrubbers. However, it reaches ca. 25 % in particle‐charging wet scrubbers, ca. 70 % in droplet‐charging wet scrubbers, and even above 99 % in opposite‐charging wet scrubbers. Furthermore, population balance modeling is used to optimize the operational parameters of the droplet‐charging wet scrubbers by means of the quantitative comparison of the grade collection efficiency. It is found that the operational parameters that are beneficial to the high‐efficiency removal of fine particles are faster gas velocity, slower droplet velocity, larger liquid‐to‐gas flow ratio, larger charge‐to‐mass ratio of droplets, smaller geometric mean diameter and smaller geometric standard deviation of droplets.     

Experimental Study of Particle Collection by Small Cyclones

A new set of experimental data on the particle collection characteristics of small cyclones is reported. The collection efficiency for particles ranging from 2 to 10 μm in diameter was measured systematically for nine cyclones at flow rates ranging from 8.8 to 18.4 L/min. Special emphasis was given to the effects of the exit tube size and of the cyclone body size on the particle collection efficiency. The size ratio of the exit tube to the cyclone body was varied from 0.24 to 0.80. The experimental results show that the stiffness of the particle collection cutoff with size does not change noticeably with a change in the cyclone body size while operation of a cyclone at a low flow rate can cause the particle collection characteristics to become less stiff. It was also found that the exit tube diameter influences the particle collection efficiency substantially, with results showing that as the exit tube size is decreased, the collection efficiency increases. A large cyclone body size increases the efficiency. However, when the cyclone body is increased excessively, the collection efficiency appears to decrease somewhat. The experimental data were compared with existing cyclone theories and Barth's (1956) theory was found to be in good agreement. Finally, the exit tube was found to affect substantially the pressure drop of cyclones. As the exit tube size increased, the pressure drop decreased. However, when the exit tube size was further increased until it approached the body size, the pressure drop increased again.     

A Cylindrical Thermal Precipitator with a Particle Size-Selective Inlet

We designed a thermal precipitator in a cylindrical configuration with a size-selective inlet, and investigated its performance in experiments using differential mobility analyzer (DMA)-classified particles of sodium chloride (NaCl) and polystyrene latex (PSL). Our investigation was performed in two parts: (1) using the size-selective inlet to determine the best inlet-to-wall distance for optimal impaction of 1 μm particles; (2) using a simple inlet tube to measure particle collection via thermophoresis over a size range from 40 nm to 1000 nm. The results showed that the inlet had a particle cut-off curve, with a 50% particle cut-off Stokes number of 0.238, resulting in removing particles with sizes larger than 1 μm at an aerosol flow rate of 1.5 lpm. The thermophoretic particle collection efficiency in the prototype was measured without the size-selective inlet installed. The size dependence of the collection efficiency was negligible for particles with diameters ≤300 nm and became noticeable for those with diameters >300 nm. An analytical model was further developed to estimate the particle collection efficiency due to thermophoresis of the prototype under various aerosol flow rates and temperature gradients. For particles with diameters less than 400 nm, reasonable agreement was obtained between the measured data and the collection efficiency calculated from the developed analytical model. It was further concluded that the derived formula for the calculation of thermophoretic particle collection efficiency could serve as the backbone for future design of thermal precipitators in any configuration, when combined with the proper formula for the dimensionless thermophoretic particle velocity.

Copyright 2012 American Association for Aerosol Research     

Particle Collection Efficiency for Nylon Mesh Screens

Nylon mesh screens, unlike metal screens, are attractive as a collection substrate for nanoparticles because they can be digested or ashed prior to chemical analysis. A theoretical single-fiber efficiency expression developed for wire-mesh screens was evaluated for estimating the collection efficiency of 11–300 nm particles for nylon mesh screens. Pressure drop across the screens, the effect of particle morphology (spherical and highly fractal-like) on collection efficiency, and single-fiber efficiency were evaluated experimentally for three pore sizes (60, 100, and 180 μm) at three flow rates (2.5, 4, and 6 Lpm). The pressure drop across the screens was found to increase linearly with superficial velocity. The collection efficiency of the screens was found to vary by less than 4% regardless of particle morphology. Single-fiber efficiency calculated from experimental data was in good agreement with that estimated from theory for particles between 40 and 150 nm but deviated from theory for particles outside this size range. New coefficients for the single-fiber efficiency model were identified that minimized the sum of square error (SSE) between the values estimated with the model and those determined experimentally. Compared to the original theory, the SSE calculated using the modified theory was at least one order of magnitude lower for all screens and flow rates with the exception of the 60-μm pore screens at 2.5 Lpm, where the decrease was threefold.

Copyright 2012 American Association for Aerosol Research     

Understanding the Interaction of an Intense Laser Pulse with Nanoparticles: Application to the Quantification of Single Particle Mass Spectrometry

Understanding the characteristic behavior of ions produced from the interaction of a high energy laser pulse with nanoparticles is essential for quantitative determination of composition and size of nanoparticles from single particle mass spectrometry (SPMS). We employed a one-dimensional hydrodynamic model, where the laser field is coupled to the non-equilibrium time-dependent plasma hydrodynamics of the heated particles. We focus on regimes of laser width from 0.01 ns to 10 ns (532 nm wavelength, 100 mJ/pulse) and particle size (20–400 nm in diameter) most relevant to commonly used SPMS, and determine the properties of ions generated during the interaction with a strong laser pulse. We compare the simulation results with experiments conducted on aluminum nanoparticles.

The laser-particle interaction is separated into a “soft heating” regime followed by a hydrodynamic expansion. Simulation results showed that the ablation/ionization is effectively complete well before the laser ever reaches its peak intensity. As the pulse width decreased for a given pulse energy, the kinetic energy of ions increased, suggesting that too short a pulse laser (i.e., high laser intensity) would be undesirable because higher energetic ions lead to lower detection efficiency in the SPMS. Results also show that for particle sizes in the range of 100 nm ～ 400 nm, as particle size increased, the kinetic energy of ions produced from the particle increased with a power law relationship, consistent with experiment. Lastly our simulations indicated that ions from the surface of the particle are of higher energy, and therefore have lower detection efficiency.     

Thermophoretic Sampler and its Application in Ultrafine Particle Collection

A thermophoretic sampler is designed for the collection of particles smaller than 10 nm. The sampler is composed of heated and cooled surfaces separated by a gap of 0.1 mm; a bypass flow is introduced in the design to minimize the diffusional loss of nanoparticles in the upstream flow channel. Particles may be directly deposited on a 3 mm diameter TEM grid for chemical analysis or on other substrates for other purposes. Calculations show that at an inlet flow rate of 1.5 lpm and thermal gradient of 5 × 10⁵ K/m, a maximum collection efficiency of 41% can be achieved for a particle diameter of 1 nm. Ag particles with median size of 6 nm are used to characterize the thermophoretic sampler collection efficiency. The TEM images show that a sizeable number of particles less than 10 nm in diameter are collected, although they are not uniformly distributed on the grid, and the collection efficiency deduced from these deposited particles is much less than the theoretical estimation. Despite this, the efficiency is orders of magnitude higher than previous designs and it is easier to build.     

Evaluation of a Method to Measure Black Carbon Particles Suspended in Rainwater and Snow Samples

We conducted a detailed evaluation of a method for measuring the mass concentrations and size distributions of black carbon (BC) particles in rainwater and snow. The method uses an ultrasonic nebulizer (USN) and a single particle soot photometer (SP2). The USN disperses sample water into micron-size droplets at a constant rate and then dries them to release BC particles into the air. The masses of individual BC particles are measured by the SP2, using the laser-induced incandescence technique. The loss of BC particles during the extraction from liquid water to air depends on their sizes. We determined the size-dependent extraction efficiency using polystyrene latex (PSL) spheres with 12 different diameters between 107 and 1025 nm. The PSL concentrations in water were measured by the light extinction at 532 nm. The extraction efficiency of the USN showed a broad maximum of about 10% in the diameter range 200–500 nm and decreased substantially at larger sizes. The accuracy and reproducibility of the measured mass concentration of BC in sample water after long-term storage were about ±25% and ±35%, respectively. We tested the method by analyzing rainwater and surface snow samples collected in Okinawa and Sapporo, respectively. The measured number size distributions of BC in these samples showed negligible contributions of BC particles larger than 300 nm to the total number of BC particles. A dominant fraction of BC mass in these samples was observed in the diameter range 100–500 nm.

Copyright 2013 American Association for Aerosol Research     

A Granular Bed for Use in a Nanoparticle Respiratory Deposition Sampler

A granular bed was designed to collect nanoparticles as an alternative to nylon mesh screens for use in a nanoparticle respiratory deposition (NRD) sampler. The granular bed consisted of five layers in series: a coarse mesh, a large-bead layer, a small-bead layer, a second large-bead layer, and a second coarse mesh. The bed was designed to primarily collect particles in the small-bead layer, with the coarse mesh and large-bead layers designed to hold the collection layer in position. The collection efficiency of the granular bed was measured for varying depths of the small-bead layer and for test particles with different shape (cuboid, salt particles; and fractal, and stainless steel and welding particles). Experimental measurements of collection efficiency were compared to estimates of efficiency from theory and to the nanoparticulate matter (NPM) criterion, which was established to reflect the total deposition in the human respiratory system for particles smaller than 300 nm. The shape of the collection efficiency curve for the granular bed was similar to the NPM criterion in these experiments. The collection efficiency increased with increasing depth of the small-bead layer: the particle size associated with 50% collection efficiency, d₅₀, for salt particles was 25 nm for a depth of 2.2 mm, 35 nm for 3.2 mm, and 45 nm for 4.3 mm. The best-fit to the NPM criterion was found for the bed with a small-bead layer of 3.2 mm. Compared to cubic salt particles, the collection efficiency was higher for fractal-shaped particles larger than 50 nm, presumably due to increased interception.

Copyright 2015 American Association for Aerosol Research     

Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires

A unipolar charger with multiple discharging wires has been developed and investigated to enhance the extrinsic charging efficiency of nanoparticles by using sheath air near the wall of the charger. The applied voltage of the charger ranged from +4.0 to +10 kV, corresponding to corona current from 0.02 to 119.63 μA. Monodisperse NaCl particles of 10 ～ 50 nm and Ag particles of 2.5 ～ 10 nm in diameter were produced to test the performance of the charger with multiple discharging wires and to investigate the particle loss at different sheath flow rates, corona voltages and sheath air velocities. Results showed that the optimal efficiency in the charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate. The extrinsic charging efficiency increased from 2.86% to 86.3% in the charger as the particle diameter increasing from 2.5 to 50 nm. The TDMA (tandem-differential mobility analyzer) technique was used to investigate the charge distribution, and the charge distributions in the exit were obtained at the optimal operating condition.     

Nanoparticle collection efficiency of capillary pore membrane filters

The surface and overall collection efficiencies of capillary pore membrane filters were measured for sub-micrometer particles. Collection efficiencies were derived from the surface loadings of particles on filters measured by scanning electron microscopy and from airborne particle concentrations measured with a scanning mobility particle sizer. Tests used filters with nominal pore diameters of 0.4 and 0.8 μm and face velocities of 3.7 and 18.4 cm/s. Surface collection efficiencies were below 100% for particles smaller than 316 nm and below 55% for particles smaller than 100 nm. Overall collection efficiencies reached as low as 45% for 70 nm particles. For nanoparticles, collection efficiencies overall were substantially higher than those to the filter surface, indicating that deposition occurs to a large extent inside the filter pores. These results underscore the need to account for surface collection efficiency when deriving airborne concentrations from microscopic analysis of nanoparticles on capillary pore membrane filters.     

Penetration of Sub-50 nm Nanoparticles Through Electret HVAC Filters Used in Residence

Pleated electret HVAC filters are often used in residence to mitigate the particles that originate both indoors and outdoors. These filters are usually tested with particles larger than 300 nm. However, residential particles can contain a significant amount of nanoparticles with size below 50 nm due to cooking, smoking, cleaning, wood burning, and outdoor infiltration. In order to characterize the nanoparticle removal by electret HVAC filters, penetrations of 3–50 nm silver nanoparticles through five different flat sheet electret media used in commercial residential HVAC filters were tested with face velocities of 0.05, 0.5, and 1.0 m s^–1. Experimental results showed that all media had significantly high penetrations with 0.35–0.8 at the most penetrating particle sizes (MPPSs) for all three velocities, which were in the sizes of 10–30 nm. A model based on single fiber theory for particle penetration predictions was used and compared with the experimental data. Results showed that the model predicted the nanoparticle penetrations very well for all media and all face velocities tested. According to the model, for enhancing the nanoparticle efficiency of the current commercial HVAC filters, the fiber diameter should be reduced or the number of pleats should be increased. However, by doing these, pressure drop and cost may be largely increased. On the other hand, this study found the existing commercial mechanical HVAC filters were much capable for sub–50 nm nanoparticle removal when their minimum efficiency reporting values (MERVs) were larger than 13 and it is concluded mechanical HVAC filters can do a better job than electret ones. However, the quality factor analysis showed electret filters could be regarded as the best filter media for removing particles smaller than 300 nm.

Copyright 2015 American Association for Aerosol Research     

Air sampling filtration media: Collection efficiency for respirable size-selective sampling

The collection efficiencies of commonly used membrane air sampling filters in the ultrafine particle size range were investigated. Mixed cellulose ester (MCE; 0.45, 0.8, 1.2, and 5 μm pore sizes), polycarbonate (0.4, 0.8, 2, and 5 μm pore sizes), polytetrafluoroethylene (PTFE; 0.45, 1, 2, and 5 μm pore sizes), polyvinyl chloride (PVC; 0.8 and 5 μm pore sizes), and silver membrane (0.45, 0.8, 1.2, and 5 μm pore sizes) filters were exposed to polydisperse sodium chloride (NaCl) particles in the size range of 10–400 nm. Test aerosols were nebulized and introduced into a calm air chamber through a diffusion dryer and aerosol neutralizer. The testing filters (37 mm diameter) were mounted in a conductive polypropylene filter-holder (cassette) within a metal testing tube. The experiments were conducted at flow rates between 1.7 and 11.2 l min^?1. The particle size distributions of NaCl challenge aerosol were measured upstream and downstream of the test filters by a scanning mobility particle sizer (SMPS). Three different filters of each type with at least three repetitions for each pore size were tested. In general, the collection efficiency varied with airflow, pore size, and sampling duration. In addition, both collection efficiency and pressure drop increased with decreased pore size and increased sampling flow rate, but they differed among filter types and manufacturer. The present study confirmed that the MCE, PTFE, and PVC filters have a relatively high collection efficiency for challenge particles much smaller than their nominal pore size and are considerably more efficient than polycarbonate and silver membrane filters, especially at larger nominal pore sizes.     

Continuous Measurements of Ambient Particle Deposition in Human Subjects

The total deposition fraction (TDF) of fine and ultrafine aerosols was measured in a group of six healthy adults exposed to polydisperse ambient aerosols in Boston. Fifteen repeated inhalation-exhalation cycles were conducted during a given exposure session. Deposition efficiency for particles with aerodynamic diameter ranging from 63.5 to 2045 nm was determined using the average concentration of inhaled and exhaled particles measured during these cycles. Deposition efficiencies ranged from 7.3±18.7%(240-275 nm) to 98.6±28.1%(1545-2045 nm). Subjects exhibited similar deposition patterns with minimum efficiencies between 200-400 nm. Results from ANOVA and mixed-model regression analyses showed significant differences (p < 0.05) in particle deposition efficiency by particle size as well as among the subjects. Deposition efficiencies varied most among the subjects for particles between 100 and 1000 nm in size. A comparison with the ICRP model showed good agreement, with best agreement for male subjects and particle sizes <400 nm.     

Design and Performance Test of a Lab-Made Single-Stage Low-Pressure Impactor for Morphology Analysis of Diesel Exhaust Particles

A serial method is described for estimating the particle effective density and dynamic shape factor of particles, i.e., diesel exhaust particles (DEPs). For this purpose, we designed a single stage low-pressure impactor with a cutoff diameter of 130 nm. The collection efficiency curve of the impactor was obtained using mobility-classified sodium chloride (NaCl) particles as a function of the mobility diameter. Then by converting the mobility diameter of the NaCl particle into the aerodynamic equivalent diameter, the efficiency curve can be expressed as a function of the aerodynamic diameter. We also obtained the efficiency curve numerically by using a commercial computational fluid dynamics software package. After confirming the design and performance of the impactor (experimentally 135 nm and numerically 137 nm of cutoff diameter), we measured the currents carried by mobility-classified DEPs downstream and upstream of the impactor so that the collection efficiency value for DEP could be obtained at each mobility diameter of DEPs. By making this value equal to that of the efficiency curve, the relationship between the mobility diameter of DEPs and the aerodynamic diameter was obtained; this enabled us to determine the effective density and dynamic shape factor of DEPs. The effective density decreased from 1.06 to 0.51 g/cm³ and the dynamic shape factor increased from 1.28 to 1.64 as the particle size increased from 60 to 105 nm, regardless of the engine type or operating conditions.

Copyright 2015 American Association for Aerosol Research     

Size Distribution Measurement of Carbonaceous Particulate Matter Using a Low Pressure Impactor with Quartz Fiber Substrates

The collection characteristics of a small deposit area low pressure impactor (SDI) were studied in order to employ the impactor for size distribution measurements of carbonaceous matter. In this work, the SDI was calibrated for soft and porous quartz substrate material in a series of laboratory experiments. The collection efficiency curves were measured by using monodisperse dioctyl sebacate particles and by applying two different detection methods. One method was based on the detection of current carried by charged test particles, and the other measured number concentrations of particles in bipolar charge equilibrium by two condensation particle counters. Concerning the particle size corresponding to a 50% collection efficiency (D ₅₀ ), significant shifts toward smaller particle sizes were found for the quartz fiber substrates compared with the flat plates. Also the shapes of the collection efficiency curves differed considerably: quartz substrate gave less steep curves than plain impaction plates. The new calibration was applied to field data from urban and rural sites. Compared with the original calibration of the SDI, the new calibration changed the measured size distributions of organic and elemental carbon. In addition, a reasonable size-segregated mass closure was achieved by combining data from thermal-optical analysis and ion-chromatography.     

A Water Cyclone to Preserve Insoluble Aerosols in Liquid Flow—An Interface to Flow Cytometry to Detect Airborne Nucleic Acid

A miniature cyclone was designed to gently capture fine aerosols into a continuous liquid flow. The geometry of the cyclone was designed so that the friction of the turning air swirls a 100 μl volume of water at the base of the cone, creating a standing liquid vortex which coats the inside deposition surface. The collection efficiency of the cyclone was characterized as a function of insoluble particle size, both in stand-alone operation and preceded by aerosol growth by water vapor condensation. The aerosol growth lowered the smallest collected particle size and created synonymous sample-into-substrate material conditions at the point of impact. The cyclone collection efficiencies were higher than 88% for the fluorescent polystyrene latex bead diameter sizes 50–3000 nm. The cyclone was further interfaced to a flow cytometer to detect airborne nucleic acid (as a virus test aerosol) in the cyclone sample flow. The flow cytometer, which is commonly used for single cell identification via fluorescence, was modified to accept a continuous sample flow (nominal 60 μl min^?1) from the cyclone for real-time detection. A rod-shaped plant virus (Tobamovirus) and a protein-enveloped insect virus (Baculovirus) were aerosolized, collected by the cyclone, and stained inline using the nucleic acid dyes SYBR Green I, SYTO-9, and SYTO-24 (Molecular Probes, Inc.). In addition, an Environmental Scanning Electron Microscope (ESEM) was used to confirm the collection of single virus particles and qualitatively evaluate the degree to which the aerosolization and collection process affected the integrity of the virus.     

Inhibition of influenza virus activity by multivalent glycoarchitectures with matched sizes

We describe the synthesis of a series of sialic acid‐conjugated, polyglycerol‐based nanoparticles with diameters in the range of 1–100 nm. Particle sizes were varied along with the degree of functionalization to match the corresponding virus size and receptor multiplicity in order to achieve maximum efficiency. To build up these architectures, we used biocompatible, hyperbranched polyglycerols as scaffolds and recently developed polyglycerol‐based nanogels, the sizes of which can be varied between 2–4 nm and 40–100 nm, respectively. We demonstrate here that such multivalent nanoparticles inhibit influenza A virus cell binding and fusion and consequently infectivity. The potential of multivalency is evident from larger particles showing very efficient inhibition of viral infection up to 80 %. Indeed, both the size of the nanoparticle and the amount of ligand density are important determinants of inhibition efficiency. The inhibitory activity of the tested polymeric nanoparticles drastically increased with size. Particles with similar dimensions to the virus (50–100 nm) are exceedingly effective. We also observed a saturation point in degree of surface functionalization (i.e. ligand density), above which inhibition was not significantly improved. Our study emphasizes the importance of matching particle sizes and ligand densities to mimic biological surfaces and improve interactions; this is a vital concept underlying multivalent interactions.     

Effect of acrylic core–shell rubber particles on the particulate flow and toughening of PVC

Different types of acrylic core–shell rubber particles with a poly(butyl acrylate) (PBA) core and a grafted poly(methyl methacrylate) (PMMA) shell were synthesized. The average size of acrylic core–shell latex particles ranged from 100 to 170 nm in diameter, having the core gel content in the range of 35–80%. The melt blending behavior of the poly(vinyl chloride) (PVC) and the acrylic core–shell rubber materials having different average particle sizes and gel contents was investigated in a batch mixing process. Although the torque curves showed that the particulate flow of the PVC in the blends was dominant, some differences were observed when the size and gel content of the particles varied. This behavior can be attributed to differences in the plasticizing effect and dispersion state of various types of core–shell rubber particles, which can vary the gelatin process of the PVC in the mixing tool. On the other hand, the highest toughening efficiency was obtained using core–shell rubber particles with the smallest particle size (i.e., 100 nm). The results showed that increasing the gel content of the core–shell impact modifiers with the same particle size improved the particle dispersion state in the PVC matrix. The toughening efficiency decreased for the blends containing 100 and 170 nm rubber particles as the gel content increased. Nevertheless, unexpected behavior was observed for the blends containing 140 nm rubber particles. It was found that a high level of toughness could be achieved if the acrylic core–shell rubber particles as small as 100 nm had a lower gel content. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009