Investigation of Filter Bypass Leakage and a Test for Aerosol Sampling Cassettes

Plastic filter cassettes (37 and 25 mm), which are press fitted together to seal and hold a filter in place, are commonly used for sampling aerosols. Aerosol bypass leakage around the filter has been reported by several researchers and attempts have been made to test for leakage and to reduce the likelihood of leakage by improving cassette design. Under typical sampling conditions, there is often no indication to the user that leakage may have occurred. In the present study, a particle count leak test was developed that used a particle counter that measured the particle number concentration of ambient aerosol (primarily submicrometer particles) upstream and downstream of the filter cassette. The relationship between leak test results and particle loss from the filter depended on particle size and type in a complex fashion. The mechanisms of particle loss were investigated and the losses increased for particles above 2 w m and were much greater for solid and fume aerosols than for oil droplets. Although the test could not be used to predict particle mass loss during sampling, the test was a sensitive indicator of cassette bypass leakage and was used to establish compression pressures needed for proper assembly of these cassettes.     

Classification of particles in particle-laden stream through a stainless steel fibrous filter

This investigation experimentally explores the penetration curve of particles shot onto a stainless steel fibrous filter or a flat surface. The effect of the pore size of the stainless steel fibrous filter, with or without an oil coating, on the particle penetration was examined at various flow rates, nozzle diameters and dimensionless particle diameters, Sqrt(Stk). The penetration of the flat surface by particles was also determined for comparison. Experimental results demonstrate that oleic acid particles larger than Sqrt(Stk)₅₀ are collected on the stainless steel fibrous filter with a low penetration, while smaller particles stay in the particle-laden stream with high penetration. The penetration of potassium chloride particles exceeds that of oleic acid particles, because potassium chloride particles bounce off the stainless steel fibrous filter and the flat surface. Particles bounce off the metal filter less easily than the flat surface. Coating the stainless steel fibrous filter with oil effectively reduces problems of particle bounce. The potassium chloride particles sucked the coated oil forming a small mountain on the surface. When the loaded particle mass on the coated stainless steel fibrous filter ranges between 0.4 and 2.3 mg, Sqrt(Stk)₅₀ is a constant 0.35.     

Selective Particle Deposition in Crossflow Filtration

Abstract

To study the mechanism of particle deposition in crossflow filtration, hydrodynamic forces exerted on a spherical particle touching the surface of filter medium are analyzed to derive the critical selective cut-diameter of the deposited particles under various crossflow velocities and filtration rates in a crossflow filtration system. Experimental data of turbulent crossflow filtration of dilute light calcium carbonate suspension agree with the prediction of this theory within 30% error under the crossflow velocity of from 0.57 to 1.14 m/s. Equations to estimate the characteristics of crossflow filtration, such as steady-state filtration rate and average specific resistance of cake, are also presented.     

Experimental and Modeling Studies of the Stream-Wise Filter Vibration Effect on the Filtration Efficiency

The stream-wise vibration effect of a fibrous filter is studied experimentally and numerically for the purpose of evaluating filtration efficiency. The particle sizes range from 0.02 to 10 μ m and the face velocity ranges from 3 to 10 cm/s. The vibrational peak velocity also varied from 0 to 50 cm/s. The filtration efficiency for this wide size range is obtained by combining the individual test results for fine particles (0.02 to 0.5 μ m) and large particles (0.5 to 10.0 μ m). For the fine particle experiment, Arizona Road Dust (ARD) test particles are generated by an atomizer after an ultrasonic process and measured by a Scanning Mobility Particle Sizer (SMPS). For the large particle experiment, the test particles are generated by a fluidized bed and measured by an Aerodynamic Particle Sizer (APS). When the particles are generated by the atomizer after ultrasonicating, the majority of the particles are in nano scale without the agglomerates on the large particle surface, while particles generated by the fluidized bed are mostly in micro-scale because many nanoparticles are agglomerated on large particle surface. The filtration efficiency increases with the vibrational peak velocity in the impaction-dominant region (D _p > 0.1 μ m) and diffusion-dominant region (D _p < 0.1 μ m), due to the increased relative velocity between the particle and the filter fiber and the increased diffusion intensity from turbulence around the fiber, respectively. A model for the filter vibration effect is established with a modified Stokes number for the impaction-dominant region and an empirical analysis for the diffusion-dominant region.     

Prediction of Dimensionless Cutsize for Size‐Fractionated Measurements of Particles that Impact on a Sintered Stainless‐Steel Filter

Abstract

This investigation experimentally studied the penetration curve of particles that impact on a sintered stainless‐steel filter with various pore sizes, sampling flow rates and jet diameters. The penetration curves were compared to those with an aluminum foil substrate. Test data reveal that when the sintered stainless‐steel filter has larger pore sizes (100 µm or 40 µm), the particle penetration, P(%), is lower and the curve is less steep than that obtained from the aluminum foil substrate. The penetration curve of the sintered stainless‐steel filter with smaller pore size (5 µm) is close to that of the aluminum foil substrate. The dimensionless cutsize‐shift (the ratio of the dimensionless cutsize of sintered stainless‐steel filter to that of aluminum foil) falls as the pore sizes and the Reynolds number increase. Experimental data were then compared with theoretical results, and theory over‐predicted the dimensionless cutsize‐shift. Hence, a regression equation for the dimensionless cutsize‐shift is proposed by fitting the experimental data. The discrepancy between the experimental data and the regression prediction is within 4%. The regression equation can be used to predict the dimensionless cutsize for the size‐fractionated measurements of particles that impact on a sintered stainless‐steel filter with various sized pores and Reynolds numbers.     

A Numerical Study of the Performance of an Aerosol Sampler with a Curved,Blunt, Multi-Orificed Inlet

The purpose of this study was to numerically simulate the performance of an aerosol sampler with a curved, blunt, multi-orificed inlet in order to understand the sampling characteristics of the first prototype of the button personal inhalable aerosol sampler ("button sampler"). Because the button sampler inlet design is too complicated to apply a three-dimensional model, an axisymmetric two-dimensional model was created to be similar in geometry and to simulate the major features of the airflow through the sampler when facing the wind. Particle trajectories were calculated in a variety of wind velocities and were categorized into 5 groups based on their interactions with the curved surface of the sampling plane. Empirical sampling efficiencies of the button sampler for 3 particle sizes were used to adjust the calculated sampling efficiencies in an attempt to improve the accuracy of the two-dimensional axisymmetric model in accounting for interactions between particles and the surface of the inlet of the button sampler. Sampling efficiencies for other particle sizes were then predicted. The results showed that sampling efficiency decreased with increasing particle size up to approximately 40 w m and then remained virtually unchanged at about 35% up to 100 w m. Although the efficiencies were lower than the American Conference of Governmental Industrial Hygienists' (ACGIH) inhalability curve for larger particles, the pattern of the predicted sampling efficiency was quite similar to the ACGIH inhalability curve. Sampling efficiencies for liquid aerosol particles larger than 15 w m were predicted to be noticeably lower than those for solid particles. The results also showed that the multi-orificed curved surface played an important role in establishing a pressure drop with desired flow alignment inside the sampler, thus greatly reducing the wind effect and significantly improving the uniformity of particle deposition on the filter. The less uniform deposition found at high wind velocity can be improved by increasing the sampling flow rate.     

Study of a Magnetic Filter System for the Characterization of Particle Magnetic Property

A magnetic filter system has been constructed, and its performance has been investigated, to measure the magnetic property of monodisperse γ -Fe ₂ O ₃ particles in the size range from 100 to 300 nm. In the system, SS 430 screens are placed in the magnetic filter element and exposed to a strong external magnetic field generated by an electric coil. The high magnetic field gradient resulted from magnetized fine wires enhances the collection of magnetic particles in addition to the particle collection via the diffusion mechanism. The particle concentrations at the upstream and downstream of the magnetic filter element were measured by an Ultrafine Condensation Particle Counter (UCPC, TSI model 3025A). Particle penetration obtained in the experiment is a function of particle size, particle magnetic property, and wire magnetization. To retrieve the magnetic property of characterized particles from the measured penetration data, a numerical model was further developed using the finite element package COMSOL Multiphysics 3.5. In this modeling, a single mesh screen is assumed to be represented by unit cells. The flow, the magnetic fields, and particle trajectory were solved in a unit cell. The relationship between particle penetration and magnetic property can then be obtained via this model for the given particle size, aerosol flowrate, and external magnetic field strength. The numerical model was first validated by comparing the experimental penetration with the simulation results for the case of 100, 150, and 250 nm γ -Fe ₂ O ₃ particles having the magnetic susceptibility characterized by Vibrating Sample Magnetometer (VSM). The magnetic susceptibilities of other sizes from 100 to 300 nm were then derived from this model according to the measured penetration data.     

Experimental Investigation of Particle Removal from Surfaces by Pulsed Air Jets

This work presents an experimental study of particle removal from surfaces by means of a pulsed air jet directed toward the particle-laden surface. During the experiments, solid particles were dispersed over the surface, forming a layer of particles that did not touch each other. Under these conditions, resuspension of an individual particle was independent of the number of particles and their location. We attempt to explain the observed phenomena by analogy to heat transfer enhancement by pulsed jets. It is expected that since pulsed jets are effective in surface cooling, their application to improved surface cleaning should be promising. For a pulsed jet, we investigated the effect of pulse frequency on particle removal. It was found that particle removal efficiency could be significantly affected by the frequency of the jet. In particular, for a fixed jet velocity, the efficiency increases with frequency, reaches a maximum, and then decreases.     

Characterization of pigment-leached antifouling coatings using BET surface area measurements and mercury porosimetry

In this work BET surface area measurements and mercury porosimetry are used to characterize leached layers formed when seawater-soluble pigments (Cu₂O and ZnO) dissolve during accelerated leaching of simple antifouling coatings. Measurements on single-pigment coatings show that an increasing fraction of Cu₂O or ZnO pigment particles becomes unavailable for dissolution when the concentration of the pigment decreases in the coating and the interparticle distance in the binder matrix becomes larger. Experimental data for a coating initially containing a mixture of Cu₂O and TiO₂ pigments suggest that a substantial fraction of the smaller and inert TiO₂ particles may be lost from the coating upon dissolution of the larger Cu₂O particles. This inert particle translocation effect is important to take into account when interpreting polishing and leaching data and when developing mathematical models of antifouling coating behaviour because the active binder surface area and porosity of the leached layer are substantially increased. A similar effect was not observed for a coating with a mixture of ZnO and TiO₂ pigments. The two experimental methods are expected to be useful for practical analysis of leaching of seawater-soluble components from commercial antifouling coatings.     

INFLUENCE OF THE PARTICLE SIZE AND SUPERFICIAL GAS VELOCITY ON THE SUBLIMATION OF PURE SUBSTANCES IN FLUIDIZED BEDS OF DIFFERENT SIZES

Abstract

In the present study the sublimation of large solid carbon dioxide particles inside fluidized beds of fine particles is investigated. A model which takes the surface area of the sublimable particles into account is used to describe the sublimation kinetics. Based on this model, the results of different experiments, namely single particle experiments using a precision scale, batch experiments in a laboratory-scale fluidized bed and continuous experiments in a larger circulating fluidized bed are compared. The main focus of the study is to evaluate the influences of the particle size, of the inert bed material, of the bed temperature and of the superficial gas velocity, respectively.     

Modified Hallimond Tube for Flotation Kinetics Measurements

Abstract

Flotation is an important method for separating particles of different chemical and physical natures from pulverized minerals. Flotation depends basically on the formation of a gas/liquid/solid interface where the solid particle becomes attached to a gas bubble as a result of its hydrophobic surface characteristics (1).     

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.     

Migration and Deposition of Submicron Particles in Crossflow Microfiltration

Abstract

The migration and deposition of submicron particles in laminar crossflow microfiltration is simulated by integrating the Langevin equation. The effects of operating conditions on the particle trajectories are discussed. It is found that the Brownian motion of particles plays an important role in particle migration under a smaller crossflow velocity of suspension or a smaller filtration rate. Based on the simulated trajectories of particles, the transported flux of particles arriving at the membrane surface can be estimated. The particle flux increases with an increase of filtration rate and with a decrease of particle diameter; however, the effect of crossflow velocity on the particle flux is not obvious. The forces exerted on particles are analyzed to estimate the probability of particle deposition on the membrane surface. The probability of particle deposition increases with an increase of filtration rate, with a decrease of crossflow velocity, with a decrease of particle diameter, or with an increase of zeta potential on the particle surfaces. The simulated results of packing structures of particles on the membrane surface at the initial stage of filtration show that a looser packing can be found under a larger crossflow velocity, a smaller filtration rate, or a smaller diameter of filtered particles. Crossflow micro-filtration experiments are carried out to demonstrate the reliability of the proposed theory. The deviation between the predicted and experimental data of filtration rate at the initial period of filtration is less than 10% when the Reynolds number of the suspension flow ranges from 100 to 500.     

Development of an Automatic Beta Gauge Particulate Sampler with Filter Cassette Mechanism

A beta gauge particulate sampler for measuring the aerosol mass concentration in the ambient air is described. The instrument is automatically calibrated with two self-calibration mass standards during each sampling period, while it samples particles continuously with minimum sampling dead-time loss. Key design features of the instrument based on the attenuation of beta radiation include filter cassette mechanism, auto-calibration system, low sampling dead-time, high sensitivity, and straightforward audit procedures. The instrument consists of three main components: PM 10 inlet, mechanical filter movement system, and control and data processing system. The mechanical filter movement system includes particle collection system with filter cassette magazine, g -ray measuring module and particle sampling module, auto-calibration system, and flow control system. The control and data processing system performs filter cassette movement control, sampling pump control, and data analysis. The instrument has been tested in the field to compare the measurement results with those by gravimetric mass measurement. The developed beta gauge instrument has been proved to be an efficient measuring guage for the ambient particulate mass determination.     

Effect of Pore Size and Particle Size Distribution on Granular Bed Filtration and Microfiltration

Abstract

The paper reviews the effect of particle size distribution and pore size distribution on granular bed filter and crossflow microfiltration performance. The experimental results of the granular bed filter with pollen particles in suspension showed that the presence of large particles improved the filter efficiency of smaller particles in suspension. Microfiltration results with bi and tri‐modal latex suspensions showed that the permeate flux and the quality were significantly affected by the particle size and its distribution, especially when the particle size was smaller than the pore size of the membrane. The mathematical model simulation results of granular bed filtration show that media pore size distribution is an important parameter of filtration for the particle removal and pressure drop across the filter.     

Capillary-bridge forces between solid particles: Insights from lattice Boltzmann simulations

Liquid capillary-bridge formation between solid particles has a critical influence on the rheological properties of granular materials and, in particular, on the efficiency of fluidized bed reactors. The available analytical and semi-analytical methods have inherent limitations, and often do not cover important aspects, like the presence of non-axisymmetric bridges. Here, we conduct numerical simulations of the capillary bridge formation between equally and unequally sized solid particles using the lattice Boltzmann method, and provide an assessment of the accuracy of different families of analytical models. We find that some of the models taken into account are shown to perform better than others. However, all of them fail to predict the capillary force for contact angles larger than π/2, where a repulsive capillary force attempts to push the solid particle outward to minimize the surface energy, especially at a small separation distance. We then apply the most suitable model to study the impact of capillary interactions on particle clustering using a coupled lattice Boltzmann and Discrete Element method.     

Loading characteristics of filter pretreated with anionic surfactant for monodisperse solid particles

Anionic surfactant (sodium oleate, SO) was used to pretreat polypropylene fibrous filters to make them negatively charged. This work examines the effects on particle loading of an anionic surfactant-pretreated filter. Also, the effects of various factors, such as the particle size, the face velocity, concentration of the surfactant, and particle distribution (mono and poly) on the particle loading characteristics were evaluated.Experimental data reveal that the electric field of a filter treated with anionic surfactant (SO) could be directly measured using an electrofieldmeter, suggesting that the pretreatment with surfactant charged the filters. The results demonstrate that pretreating the filter with SO surfactant increases its particle-loading capacity. The clogging points of the untreated filter, and of the 0.01, 0.05 and 0.08 M SO-pretreated filter are 18, 23, 28 and 33 g/m². The loading behavior of the SO-pretreated filter depended on the size of the particles and the operating face velocity. Additionally, the loading behavior of the SO-pretreated filter with polydisperse particles is much the same as that with monodisperse particles. However, the clogging point of the SO-pretreated filter with polydisperse particles is higher than that with monodisperse equal size.     

The Effect of Particle Sedimentation on Gravity Filtration

ABSTRACT

Simulation of cake formation of mono-sized and dual-sized particles under gravitational sedimentation and filtration is presented. The dynamic analysis proposed by Lu and Hwang in 1993 is applied to examine the local cake properties formed under a falling head by considering the hindered settling effect of particles in the slurry and the variation of the pressure drop across the filter septum. Results of this study show that, at a given position in a cake, the solid compressive pressure reaches a maximum value and then decreases for a gravity filtration due to the decrease in the driving head. A cake constructed with dual-sized particles has a more compact structure than does one with mono-sized particles, and larger particles will form looser packing than will smaller ones for mono-sized particles. A dual-dispersed suspension with a lower fraction of large particles will result in the lowest cake porosity and the highest specific filtration resistance of cake. Comparison of the porosity distribution in filter cake formed by means of gravity filtration and constant head filtration shows that the porosity near the filter septum of gravity filtration has a convex behavior while that of constant head filtration has a tendency toward concavity. This discrepancy is mainly due to the change in the driving head during the filtration process. Both theoretical and experimental results show that the uniformity of particle size distributions in the filter cake will be much better when the relative settling velocity between large and fine particles is reduced.     

Experimental filtration efficiencies for large pore nuclepore filters

Experimental filtration data were collected in an effort to validate an impaction model previously developed and presented. Using a sampler with a 9.5 μm pore diameter Nuclepore filter, collection efficiencies were measured for both liquid and solid aerosols over a size range of 2–9 μm. Data for the liquid aerosol showed good agreement with the impaction model; however, data for the solid aerosol indicated an appreciably lower collection efficiency than predicted by the model. The liquid aerosol data validate the impaction model. The solid aerosol data indicate particle bounce or reintrainment subsequent to impact and underscore particle capture as a problem to be dealt with if the Nuclepore surface is to be used as a size selective filter.     

Solid particle erosion behaviour of high purity alumina ceramics

The solid particle erosion behaviour of a high purity, cold isostatically pressed ceramics, CIP-Al₂O₃, is studied in this paper. The influence of particle properties, such as hardness and shape, on erosion is examined, as well as the effect of varying the impingement angle of the erodent stream on the weight loss of alumina ceramics samples. Therefore, the erosive wear behaviour was studied at five different impact angles (30°, 45°, 60°, 75° and 90°), using SiC and SiO₂ particles as erodents.The material loss during solid particle erosion is measured by changes in surface roughness, surface morphology and mass loss.The surface roughness and topography of the eroded Al₂O₃ ceramics were recorded using a profilometer.Scanning electron microscopy (SEM) was used to examine the features of eroded surfaces and to ascertain erosion mechanisms of the tested alumina samples.The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles. It was found that maximum erosion by both types of particles occurs at an impact angle of 90°.