Browsing by Subject "Particle"

Now showing 1 - 4 of 4
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    Aerosol particle electroscavenging by droplets.
    (2012-06) Zhang, Meng
    When water droplets precipitate under the action of gravity and frictional forces, they will collide with smaller aerosol particles and fall to the ground. Usually, either droplets or aerosol particles carry some electric charges, and some may be highly charged. Therefore, the electrostatic effect is a very important factor in particle scavenging. Wet scrubbers as air pollution control devices use the same theory as scavenging to remove both particulate and gas contaminants from the industrial exhaust streams. Electrostatic wet scrubbers were developed in an attempt to improve collection efficiency by raising the attraction force between droplets and particles. Very few numerical models have been developed to describe the phenomenon of particle collection by highly charged droplets when electrostatic force is dominant. In an attempt to understand the physics of scavenging, a new three-dimensional model has been developed to simulate neutral or charged particles collected by a group of neutral or charged droplets. The model can simulate the particle traveling through a matrix of droplets. Both the inertial effect and the electrostatic effect on particle scavenging have been considered. The collection efficiency can be estimated by utilizing this developed model. The effect on the collection efficiency by the size of the particle and of the droplet, the charges of the particle the droplet, and droplet distance have been investigated. A validation approach has also been developed and the study results have achieved good agreement with published data.
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    Biofuel combustion: a single particle approach including new tandem measurements.
    (2011-10) Dutcher, Dabrina D
    The physicochemical properties of aerosol particles are complex. They are often irregular in shape, and can contain complex mixtures of liquids and solids. By measuring multiple properties of a particle, it is possible to describe it more completely than is possible if only one property is evaluated. This is the principle behind the theme of this chapter: tandem aerosol measurements. The Aerosol Time-of-Flight Mass Spectrometer carries out tandem measurements of a particle's vacuum aerodynamic diameter and its composition. I describe here the use of the ATOFMS in series with instruments that measure other properties so as to provide still more information. These additional properties include particle mobility, mass, and "brightness" (i.e., the amount of light that it scatters when illuminated by a laser). In addition, we show that when the ATOFMS is used downstream of tandem differential mobility analyzer systems (TDMA), new information can be gained about species that affect a particle's hygroscopicity (HTDMA) or volatility (VTDMA). These novel instrument combinations yield information regarding the dependence of particle effective density, volatility, and hygroscopicity on particle composition. Additional information is presented about the relationship between particle mobility size and vacuum aerodynamic size for assorted particle types and about the unanticipated difficulties that I encountered when using the ATOFMS for tandem measurements. I discovered that the rotating seals in the aerosol particle mass analyzer (APM) contain compounds that volatilize and react with acidic particles. The ATOFMS is exceedingly sensitive to these reaction products, so much so that it is not possible to obtain meaningful information about the composition of the particles under investigation. This sensitivity may provide a sensitive means, however, to assess the particle acidity.
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    Evaluation and development of methods for measurement of penetration of filtering facepiece respirators
    (2015-07) Satish, Swathi
    Elevated concentrations of diesel exhaust have been linked to adverse health effects. Filtering facepiece respirators (FFRs) are widely used as a form of respiratory protection against diesel particulate matter (DPM) in occupational settings. The objective of this study was to evaluate NIOSH-certified R95 and P95 electret respirators challenged with Diesel exhaust and get a better understanding of the factors that influence penetration. Two techniques were employed for the measurement of penetration: (a) particle counting technique using a Scanning Mobility Particle Sizer (SMPS, TSI Inc.) which measures particle size distribution, and (b) Gravimetric analysis using polyfluortetraethylene (PTFE) and polypropylene (PP) filters. Gravimetric measurements using PP filters were variable compared to SMPS measurements and biased high as a result of the adsorption of gas-phase semi-volatile material. Relatively inert PTFE filters adsorbed less semi-volatile material resulting in more accurate measurements. To attempt to correct for these artifacts associated with adsorption of semi-volatile material, primary and secondary filters were used in series upstream and downstream of the FFR. Correcting for adsorption by subtracting the secondary mass from the primary mass improved the result for both PTFE and PP filters but this correction is subject to “equilibrium” conditions that depend on sampling time and the concentration of particles and semi-volatile material. Overall, the results demonstrate that great care must be taken when using filters to determine filtration efficiency of FFRs challenged with diesel exhaust. Pure PTFE or other filters that minimize adsorption of semi-volatile artifacts and two filters should be used in tandem to allow correction for adsorbed artifacts. Analysis of SMPS measurements indicated that the respirators behave differently for Diesel exhaust generated at light and heavy load on engine. At light load, the penetration of the R-95 and P-95 respirators showed a steep increase with time, exceeding the maximum allowed penetration of 5% after about 40 minutes. Whereas at heavy load, the respirators were found to have a relatively unchanging penetration (less than 5%) throughout the 90-minute test duration. This difference was attributed to the presence of a high concentration of organic carbon (OC) in Diesel exhaust which has a tendency to degrade the electric charges on the respirators, thus reducing the filtration enhancement from electrostatic attraction forces. To account for the complex nature of DPM and its varying properties with changes in operating and sampling condition, an oxidation-dilution tunnel was designed to produce Diesel exhaust with a controlled set of properties: elemental carbon (EC) concentration, OC concentration, EC/OC ratio and volume flow rate. This device was used to evaluate R-95 and P-95 respirators for solid Diesel exhaust aerosol. The methodology proved to be effective in controlling the EC concentration and total volume flow rate. Results showed that the R-95 and P-95 respirators were more than 95% efficient for solid Diesel exhaust aerosol. This thesis is divided into two parts. The first focuses on the measurement of penetration of FFRs for Diesel exhaust, the second on the development of a standard DPM generator for testing filtration systems.
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    Liquid-borne nanoparticle characterization and its application to nanometer-rated liquid filter evaluation
    (2013-08) Ling, Tsz Yan
    Nanoparticles are often found in liquid-borne dispersed phases, in addition to the airborne and surface-borne phases. Characterization techniques for nanoparticles are needed for the environmental, health and safety studies of nanomaterials. On the other hand, membrane filtration has been demonstrated to be effective for the removal of liquid-borne nanoparticles. Such technique has found many applications, including: 1) contamination control in liquids used in industries which require high level of cleanliness, 2) control of the release of engineered nanoparticles for environmental health and safety and 3) drinking water purification and disinfection. For evaluating filtration performance of nanometer-rated filters, reliable techniques for counting and sizing liquid-borne nanoparticles are desirable. The objectives of this thesis are to 1) explore methods for characterizing liquid-borne nanoparticles and 2) apply these methods to study nanoparticle filtration problems. In Chapter 2, calibration results of the Nanoparticle Tracking Analysis (NTA) technique in our lab are reported. The concentration measurements agree well with that estimated by suspension mass concentration within the range of 108-1010 particles/ml. To ensure the concentration measurements are made within the linear valid range, a single sample should be diluted and the NTA concentration measurements for the original and diluted samples should agree. Applying the NTA technique, the size distribution and concentration of nanoparticles in the water used in Abrasive Waterjet Machining (AWM) and Electrical Discharge Machining (EDM) processes were measured. The particles generally have a most probable size of 100-200 nm. The filtration systems of the AWM and EDM processes were found to remove of 70 and 90 % the nanoparticles present, respectively. However, the particle concentration of the filtered water from the AWM was still four times higher than that found in regular tap water. These nanoparticles are mostly agglomerated, according to the microscopy analysis. Since AWM and EDM are widely used, the handling and disposal of used filters collected with nanoparticles, release of nanoparticles to the sewer and potential use of higher performance filters for these processes will deserve further considerations. The development of an aerosolization technique to measure liquid-borne nanoparticles down to 30 nm and its application to filter evaluation is discussed in Chapter 3. This technique involves dispersing nanoparticle suspensions into airborne form with an atomizer or electrospray aerosol generator, and measuring the size and concentration by a differential mobility analyzer coupled to a condensation particle counter. With the electrospray aerosol generator, residue particles can be controlled to be less than 10 nm, allowing particles as small as 30 nm to be clearly distinguished from the size distribution measurements. Calibrations with 30, 50, 125 and 200 nm polystyrene latex particles showed that liquid-borne and airborne particle concentrations are proportionally related. This provides an effective way to quantify liquid-borne particles as small as 30 nm, which cannot be analyzed by state-of-the-art liquid particle counters. Comparing to NTA, the aerosolization technique gives more accurate representation for polydisperse size distribution. The aim of Chapter 4 is to study the filtration process of a model membrane filter, the nuclepore filter. Initial filtration efficiency experimentally measured using the aerosolization and NTA techniques are comparable with each other. The capillary tube model modified from that developed for aerosol filtration was found to be useful to represent the experimental results, when a sticking coefficient of 0.15 is incorporated. This suggests that for the polystyrene latex (PSL) particles-nuclepore filter-water system, only 15% of the particle collisions with the filter results in successful attachment. The small sticking coefficient found can be explained by the unfavorable surface interactions between the particles and the filter medium. The sieving mechanism, in which particles are removed when they are larger than the filter pore size, is primarily used to describe the filtration process. The capture of particles smaller than the pore size, by adsorption via diffusion and interception, becomes effective when the combined electrostatic and van der Waals interactions between particle and filter surface are favorable. In Chapter 5, retention efficiency of a 50 nm- rated Polytetrafluoroethylene (PTFE) filter against nanoparticles of different materials (gold, PSL and silica), sizes (80, 50 and 30 nm), concentrations (2×109 to 4×1011 particles/ml) and size distributions (monodisperse and polydisperse) was measured. The decreasing trend of retention efficiency as a function of particle loading can be readily explained by the sieving mechanism. Among different particle materials, silica shows much lower retention efficiency compared to PSL and gold particles of the same size. This observation can be explained with the DLVO theory, which suggests that higher ionic strength of PSL and gold suspensions causes a decrease in the magnitude of energy barrier and favors their adsorption to the filter surface. In addition, this study observed higher retention efficiency for mixed particles compared to monodisperse ones and there is less than 10% of re-entrainment of particles collected by adsorption. In Chapter 6, numerical simulations for change in membrane filter's retention efficiency in the presence of a particle previously captured within the filter are reported. Because of the complex porous structure of the membrane, the filter model is simplified into two cases: 1) capillary tube model and 2) single fiber model. Simulations for the capillary tube model show that a particle captured near the pore opening can increase the collection efficiency of 30 nm particles due to impaction and interception by about two times, depending on the relative size of the particles to the pore. From the single fiber model, a particle attached on the fiber can increase the combined impaction-interception single fiber efficiency by 10 times, depending on the location of the particle attached. The capillary tube simulation results can be incorporated into pore blockage model which considers filters as multiple layers of parallel pores. The single fiber simulations can explain the higher collection efficiency due to a previously captured particle. However, the single fiber theory alone cannot account for the observed decreasing trend with particle loading.