Analytic solutions for filtration of polydisperse aerosols in fibrous filter

In this study, analytical solutions for penetration efficiency of a polydisperse aerosol in fibrous filter were derived employing Brownian diffusion and inertial impaction as removal mechanisms. Size distribution of aerosol particles was assumed to be represented by a log-normal function during the filtration. Derived solutions were compared with the exact solution, which is not based on the log-normal assumption, showing good agreement. Error resulting from the log-normal assumption was shown to be greater in the impaction-dominant regime than in the diffusion-dominant regime due to higher size dependency of collision kernel which destructed log-normal shape of size distribution. The penetration efficiency of the analytic solution initially decreases faster and then decreases slower than that of the exact solution in the diffusion-and intermediate dominant size regimes due to its polydispersity of particle distribution, while it overpredicted the particle removal in the impaction size range because of neglect of polidispersity effect. A new solution for the most penetrating particle diameter was also provided showing the dependence on filtration velocity, fiber volume fraction, and fiber size.     

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 Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was monitored by using pressure transducers, which were calibrated against an inclined manometer. Airflows of 5, 10 , 20, and 30 cm/s were used to study the flow dependency. The aerosol penetration results showed that the particles larger than 3 θ m did not penetrate the clean fabric filters tested in the present study. The loading curves (plots of pressure drop against sampling time) displayed three regions: an initial region of fast increase, a transition region, and a final linear region after the dust formation point. After the formation point of the dust cake, both fabric and fibrous filters shared the same slope (of the loading curves). The slope of different regions of the loading curves was determined by many factors, such as size of challenge aerosol, face found to be critical to the performance of the fabric filters. In order lower porosity, which caused an extra rise in pressure drop across velocity, surface treatment, and the compressibility of the dust cake forming on the filter. The method of final surface treatment was to avoid the unnecessary rise in air resistance, the melting clumps formed during final surface treatment should be as thin and narrow as possible, just enough to support the filter bag cleaning. From the standpoint of filter quality and energy consumption, the low filtration velocity has to be adopted whenever possible, because high filtration velocity not only led to lower filter quality (in particular for submicrometer-sized particles) but also created dust cake of lower porosity, which caused an extra rise in pressure drop across thet dust cake.     

Performance of fibrous filters during nanoparticle cake formation

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

Calibration and Testing of Samplers with Dry,Polydisperse Latex

A new method for measuring the collection efficiency of an aerosol sampler as a function of particle size has been developed, featuring the use of dry, polydisperse latex particles. Test aerosol is generated by placing a polydisperse latex powder sample into a fluidized bed of glass beads. An Aerodynamic Particle Sizer (APS) measures the particle size distribution entering and leaving the sampler's size-selector, yielding the penetration efficiency. The use of dry latex minimizes the ''phantom'' particle problem inherent with the APS by avoiding the generation of high concentrations of small particles such as those produced by nebulizers. In addition to having useful properties for determining particle size cutoff characteristics, including spherical shape, near-unit density, and white color, latex particles afford a test for the presence of particle bounce and reen trainment. A complete efficiency measurement can be made in a little over three minutes, facilitating experimentation with parameters such as sampler flow rate, which require repeated measurements. The method has been used extensively for the development and calibration of respirable and PM-2.5 samplers.     

Influence of Particle Characteristics on Filter Ripening

Abstract

To investigate the dependence of filter ripening on particle size and surface charge, multiple experiments were conducted under different particle destabilization conditions including pH control, alum, and polymer destabilization. Laboratory‐scale filtration experiments were performed at a filtration velocity of 5 m/h using spherical glass beads with mean diameter of 0.55 mm as collectors. Particle suspensions with a broad size distribution and a 1.7 µm mean particle size were filtered through a 10 cm depth filter column. Better initial solids removal was confirmed under favorable particle and collector conditions (i.e., under smaller surface charge), but better initial particle removal does not necessarily mean better overall particle removal efficiency. It was shown that changes of the particle size distribution (PSD) in the effluent can significantly influence overall particle removal efficiency. Chemical parameters such as zeta potential can be important during the initial stage of filtration, but their importance can decrease over time depending on the specific chemical conditions. The influent PSD and the removal of certain size particles during the initial stage of filtration can significantly influence ripening, which in turn, can influence the overall particle removal efficiency.     

Inkjet Aerosol Generator as Monodisperse Particle Number Standard

The AIST-inkjet aerosol generator (IAG) can generate highly monodisperse solid or liquid aerosol particles in the particle diameter range from 0.3 to 20 μm at precisely known particle generation rates. The device has been developed for evaluating the counting efficiencies of optical and condensation particle counters. Particle generation efficiency of the IAG is defined as the number of aerosol particles generated by one voltage pulse sent to an inkjet head. The 95% confidence interval of the efficiency were 0.998 ± 0.006 within the 0.4 to 10 μm particle diameter range. The efficiencies remained close to unity when the droplet generation rates were within 20–500 s^?1 and 100–900 s^?1 using ultrapure-water and isopropyl alcohol (IPA) as the solvent of the inkjet solution, respectively. The operating aerosol flowrate range of the IAG is currently 0.5 and 1.0 L/min. The coefficients of variations (C.V.) of the size distributions were 2 to 3% indicating the generated particles were highly monodisperse. The generated particle sizes were defined as the volume equivalent diameter, D_ve. The uncertainty analysis on the factors affecting D_ve indicated that 95% confidence interval of the D_ve is expected to be ±5%. The uncertainty of D_ve was entirely caused by the uncertainty of the average mass of a droplet. The reproducibility of particle sizes within 0.5 to 10 μm was evaluated using an aerodynamic particle sizer. The C.V. of the measured particle sizes were less than 6% and 4% when NaCl particles and ionic liquid droplets were generated, respectively.

Copyright 2014 American Association for Aerosol Research     

Particle fluctuations and dispersion in three-phase fluidized beds with viscous and low surface tension media

Particle fluctuations and dispersion were investigated in a three-phase (gas–liquid–solid) fluidized bed with an inside diameter of 0.102 m and height of 2.5 m. Effects of gas and liquid velocities, particle size (0.5–3.0 mm), viscosity (1.0–38×10⁻³ Pa s) and surface tension (52–72×10⁻³ N/m) of continuous liquid media on the fluctuating frequency and dispersion coefficient of fluidized particles were examined, by adopting the relaxation method base on the stochastic model. The fluctuations and dispersion of fluidized solid particles were successfully analyzed by means of the pressure drop variation with time, which was chosen as a state variable, based on the stochastic model. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas velocity, due to the increase of bubbling phenomena and bed porosity in which particles could move, fluctuate and travel. The frequency and dispersion coefficient of particles showed local maximum values with a variation of liquid velocity. The two values of fluctuating frequency and dispersion coefficient of particles increased with increase in particle size, but decreased with increase in liquid viscosity due to the restricted movement and motion of particles in the viscous liquid medium. Both fluctuating frequency and dispersion coefficient of particles increased with decrease in surface tension of liquid phase, due to the increase of bubbling phenomena with decrease in σ_L. The values of obtained particle dispersion coefficient were well correlated in terms of dimensionless groups as well as operating variables.     

An improved model for particle deposition in porous foams

Porous foam provides an inexpensive, light-weight and effective medium to capture physiologically-relevant aerosol fractions. It can be manufactured to have a wide range of properties relevant to aerosol deposition. A series of laboratory experiments were conducted to measure particle penetration though porous foam media of varying pore size and foam length. Both solid and liquid aerosols (0.01–10 μm diameter) were tested using a Sequenzial Mobility Particle Sizer or Aerodynamic Particle Sizer to count and size particles penetrating the foam. With this data, an existing semi-empirical model was improved upon to predict particle penetration through a foam of a given fiber diameter, and thickness. The model is based on three dimensionless parameters (St, Ng, Pe) that account for inertial, gravitational, and diffusive modes of deposition, respectively.     

Reducing Particle Bounce and Loading Effect for a Multi-Hole Impactor

Aerosol sampling is used to evaluate the health hazards associated with particles deposited in the human breathing system. Impactors, which are extensively employed as aerosol samplers, have low collection efficiency because of particle bounce. The impaction plate is typically coated with oil or grease to prevent particle bounce. However, such coating materials cannot sustain long-term heavy particle loading.

In this study, the impaction plate was recessed, forming a cavity filled with Trypticase Soy Agar (TSA) to reduce particle bounce and re-entrainment. An ultrasonic atomizing nozzle was employed to generate challenge aerosols. An Aerodynamic Particle Sizer (APS) was utilized to measure the number concentrations and the size distributions upstream and downstream of the size-selective devices. A multi-hole impactor and Personal Environmental Monitor PM _2.5 (PEM–PM _2.5 ) were used to evaluate particle bounce and heavy particle loading. Liquid type-Dioctyl phthalate (DOP), soluble solid type-potassium sodium tartrate tetrahydrate (PST) and insoluble solid type-polymethyl methacrylate (PMMA) were investigated, as were different impaction surfaces/surface combinations. The multi-hole impactor coated with silicone oil was compared with a TSA-filled plate. Laboratory results demonstrate that the solid PST particles bounced off the TSA-filled plate less than off the silicone-coated aluminum plate. This study also used a 700-μm-thick layer of silicone oil to prevent TSA dehydration. The experimental results revealed that the silicone-TSA double layer minimized PST particle bounce during the two-hour heavy sampling (mass concentration was around 7.22 mg/m ³ ). Moreover, the PEM-PM _2.5 impactor yielded consistent results when the silicone-TSA double layer method was used. These results are useful for designing bounce-free impaction substrates during heavy load sampling.     

Soot Particle Studies—Instrument Inter-Comparison—Project Overview

An inter-comparison study of instruments designed to measure the microphysical and optical properties of soot particles was completed. The following mass-based instruments were tested: Couette Centrifugal Particle Mass Analyzer (CPMA), Time-of-Flight Aerosol Mass Spectrometer—Scanning Mobility Particle Sizer (AMS-SMPS), Single Particle Soot Photometer (SP2), Soot Particle-Aerosol Mass Spectrometer (SP-AMS) and Photoelectric Aerosol Sensor (PAS2000CE). Optical instruments measured absorption (photoacoustic, interferometric, and filter-based), scattering (in situ), and extinction (light attenuation within an optical cavity). The study covered an experimental matrix consisting of 318 runs that systematically tested the performance of instruments across a range of parameters including: fuel equivalence ratio (1.8 ≤ φ ≤ 5), particle shape (mass-mobility exponent ( D _fm ), 2.0 ≤ D _fm ≤ 3.0), particle mobility size (30 ≤ d _m ≤ 300 nm), black carbon mass (0.07 ≤ m _BC ≤ 4.2 fg) and particle chemical composition. In selected runs, particles were coated with sulfuric acid or dioctyl sebacate (DOS) (0.5 ≤ Δ r _ve ≤ 201 nm) where Δ r _ve is the change in the volume equivalent radius due to the coating material. The effect of non-absorbing coatings on instrument response was determined. Changes in the morphology of fractal soot particles were monitored during coating and denuding processes and the effect of particle shape on instrument response was determined. The combination of optical and mass based measurements was used to determine the mass specific absorption coefficient for denuded soot particles. The single scattering albedo of the particles was also measured. An overview of the experiments and sample results are presented.     

Effects of Particle Shape and Size Distribution on Sorption and Flow Performance in Electrically Stabilized Expanded Beds

Abstract

Use of the electrically stabilized expanded bed is an approach to the improvement of the performance of processes in beds of solid sorbents. Particle motion in a fluidized bed of nonconducting particles such as molecular sieves is halted when the bed is “frozen” by a strong electric field imposed across the bed. This allows sorption performance to approach that of fixed beds while maintaining the low pressure drops which—are characteristic of fluidized beds. Beds with axial and radial electrode geometries function equally well in the sorption of carbon dioxide (C0₂2) from nitrogen (N₂). The effects of particle shape and particle size distribution on bed stabilization are presented. Particle shape was varied from spherical to the irregular forms that are characteristic of crushed particles. The sorption performance was not appreciably different with particles having various shapes.     

Influence of gas velocity on the particle collection and reentrainment in an air-cleaning electrostatic precipitator

Particle deposition and reentrainment experiments were performed in a two-stage electrostatic precipitator (ESP), consisting of positive corona precharger and collecting electrode sections. Attention was focused on studying the indoor air pollution deposition and reentrainment into six size ranges from 0.3 to >10?μm. Tests were performed in an office room (200?m³) for airflow velocities from 1.4 to 8?m/s. The effect of airflow velocity on the collection efficiency of the ESP was investigated both experimentally and analytically to study reentrainment phenomena in a turbulent flow. A stationary two-dimensional analytical model was carried out by modeling the particle transport. The boundary conditions for charged particles on collecting and repelling electrodes were determined by physical considerations, including chaotic and drift motions, the reflection of charged particles from a surface, and the reentrainment of charged particles. A decrease in the experimental collection efficiency for large particle diameters (≥0.5?μm), as compared to the theoretical prediction, was interpreted as the reentrainment of particles. The size-resolved dust reentrainment fluxes from the collecting electrode were evaluated in two limiting cases, considering that either the reentrained particles are not charged or that they are charged as the particles in the deposition flux. Dimensional analysis is applied to these results, introducing the wall friction velocity as a universal parameter that determines the flow character. In general, the particles with diameters <5?μm and >5?μm exhibit different reentrainment behavior.

Copyright © 2018 American Association for Aerosol Research     

Filtration of fly ash using fluidized bed at 300–500 °C

The filtration of the coal-burning fly ash using fluidized beds with silica sand of 770 μm under temperatures of 36, 300, 400, and 500 °C was studied. The variations of the outlet concentration and particle size distribution (PSD) with time were measured to evaluate the dynamic characteristics of the process. Experimental results showed that the overall collection efficiency decayed with the operation time, revealed the effect of the elutriation of fly ash on particle filtration. The collection efficiency rose when the temperature increased from 36 °C to 500 °C. The strong attrition at high temperature released more small particles than that at room temperature, increased the concentration of the particles less than 10 μm (PM₁₀) at high temperature. The removal efficiency of the particles in a size of 4–7 μm, not the submicron particles, is the lowest because they are most easily elutriated from fluidized beds.     

Using stereo XPTV to determine cylindrical particle distribution and velocity in a binary fluidized bed

Nonspherical particles are commonly found when processing biomass or municipal solid waste. In this study, cylindrical particles are used as generic nonspherical particles and are co-fluidized with small spherical particles. X-ray particle tracking velocimetry is used to track the three-dimensional particle position and velocity of a single tagged cylindrical particle over a long time period in the binary fluidized bed. The effects of superficial gas velocity (u _f), cylindrical particle mass fraction (α), particle sphericity (Φ), and bed material size on the cylindrical tracer particle location and velocity are investigated. Overall, the cylindrical particles are found in the near-wall region more often than in the bed center region. Increasing the superficial gas velocity u _f provide a slight improvement in the uniformity of the vertical and horizontal distributions. Increasing the cylindrical particle mass fraction α causes the bed mixing conditions to transition from complete mixing into partial mixing. © 2018 American Institute of Chemical Engineers AIChE J, 65: 520–535, 2019     

Modelling the Effect of Particle Size in Microfiltration

Abstract

Particle deposition at the filter surface in microfiltration is studied to better understand the effect of particle size on cake morphology and permeability reduction. Numerical simulations are carried out on a Hele Shaw cell which consists of a representative unit element of a two dimensional spatially periodic flat plate with pores. The particle concentration in the fluid is assumed to be low so that particles enter one by one into the computation domain. Particles follow the flow streamlines under creeping flow conditions from a random initial location until they are subjected to physico‐chemical interactions near the filter surface or a particle already deposited. The computational domain consists of two regions: a fluid region and a porous medium region, i.e. the particle cake. The flow over the two regions of the Hele Shaw cell is computed using the Darcy model, including the variations of the permeability field due to the cake formation. Results show that both the permeability and the filtration efficiency are affected significantly by particle size.     

Preparation of Granular Ceramic Filter and Prediction of Its Collection Efficiency

The silicon carbide (SiC) ceramic filter is the most favorable component to remove particulate matter from hot flue gas due to its high filtration efficiency and high thermal durability. The effect of SiC powder size on the physical properties and filtration performance to prepare high-performance granular ceramic filter media was investigated in this study. Disk-type filter media were prepared by mixing ceramic components followed by physical compression and sintering. The porosity and average pore diameter in the filter media increased with increasing powder size. However, the mechanical strength decreased with increasing powder size, while it increased with increasing physical compressive force. The filter performance factor, q_FM was introduced to evaluate the ceramic filter properties, and the SiC50 filter was the best of the ceramic filters prepared in this study. We also found that diffusion was a dominant collection mechanism for particles smaller than 0.7 μm, and direct interception and inertia were dominant collection mechanisms for particles larger than 0.7 μm in the SiC50 filter based on a single collector efficiency model. In addition, the predicted collection efficiencies showed reasonably good agreement with the experimental ones.

Copyright 2014 American Association for Aerosol Research     

Novel Instrument to Separate Large Inhalable Particles

Large inhalable particles are present in the workplace, yet few instruments exist to count and size such particles in situ. Inhalable-aerosol exposure can be evaluated using mass-based samplers such as the IOM or Button sampler, but these devices do not provide information on particle size distributions. Size-resolved samplers such as cascade impactors or the Aerodynamic Particle Sizer are limited to particle sizes <20 μm due to difficulties with particle aspiration and transmission losses. This work describes the development of two samplers capable of measuring the concentration and size distribution of airborne particles from 20 to 100 μm in aerodynamic diameter. One device is based on the principles of an upflow elutriator, whereas the other eliminates the potentially adverse effects of an upward-facing jet to separate particles from a quiescent airstream. Analytical models and computational fluid dynamics simulations were used to predict the performance of the two samplers. Sampling efficiencies of these devices were tested in a calm-air chamber with polydisperse, fluorescent microspheres (10–100 μm). Epifluorescent microscopy of settled dust was used to determine reference particle counts and sizes. Both devices are capable of size-selective sampling; however, the second sampler produced higher sampling efficiencies and sharper cut points compared to the simpler elutriator design. Experimental sampling efficiencies for both samplers showed good agreement with computational and analytical solutions. This work suggests that these devices can size-segregate inhalable aerosols in quiescent environments.

Copyright © 2015 American Association for Aerosol Research     

Investigation of particle deposition on a micropatterned surface as an energy-efficient air cleaning technique in ventilation ducting systems

Abstract

Building ventilation ducting systems play a core role in controlling indoor air quality by recirculating the indoor air and mixing with ambient air. The ventilation system can serve as an air cleaning system itself either through the filtration system or integrating other means, while at the same time, attention to energy consumption is needed. The high-efficiency fibrous filters in a conventional filtration system not only cause high-pressure drops that consume fan energy but also add to the high operation cost. This article proposes an air cleaning technique, aimed at submicron particles, by means of installing patterned surfaces on the walls of ventilation ducts, which can be easily cleaned by water and reused. The effect of patterned surfaces on particle deposition was studied numerically. In the numerical simulation, the Reynolds stress turbulent model was correlated at the near-wall regions by turbulent velocity fluctuation at the normal direction. Particle trajectory was solved by using Lagrangian particle tracking. The numerical model was then validated with a particle deposition experiment. A wind tunnel experiment was carried out to quantify the particle deposition on the semicircular micropatterns for a wide range of heights. Based on our numerical results, the semicircular pattern height of 500?µm with a pitch-to-height ratio (p/e) of 10 has 8.58 times enhancement of the energy efficiency compared with a high-efficiency particulate air filter. Our results indicated that adding surface micropatterns to ventilation ducting for submicron particle deposition is a possible energy-efficient air cleaning technique for practical usage.

Copyright © 2020 American Association for Aerosol Research     

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.