Design and Evaluation of Four-Stage Low-Pressure Cascade Impactor Using Electrical Measurement System

The low-pressure cascade impactor has been used to collect ultrafine particles that cannot be measured by conventional cascade impactors. Low-pressure cascade impactors resemble ordinary impactors, but are operated at reduced pressures of 0.05 ∼ 0.4 atm. Many kinds of low-pressure impactors have been developed by different researchers. However, it is still difficult to accurately design and evaluate the low-pressure cascade impactor.

In this study, a four-stage low-pressure cascade impactor for measuring the size distribution of submicron aerosol particles was designed and evaluated. To evaluate particle collection efficiency of each stage, an electrical measurement system was used. The cut-point diameters of Stages 1 through 4 were 0.238, 0.173, 0.111, and 0.063 μm in aerodynamic diameter. Stage 2 showed poor steepness of the collection efficiency curve and larger cut-point Stokes number than theory, which may be attributed to high nozzle velocity. The fluorometric method for particle collection efficiency measurement was shown to be unreliable for ultrafine particles.

The solid particle collection efficiency of the designed impactor was examined with different substrate conditioning methods. Porous metal substrate and silicon-coated substrate were tested with NaCl particles. It was shown that silicon coating did not effectively reduce the particle bounce because of high nozzle velocity, whereas the porous metal substrate considerably enhanced the particle collection efficiency.     

Influence of clean air and inlet configuration on the performance of slit nozzle virtual impactor

A two-partitioned horizontal inlet was developed for improving the collection efficiency and minimizing the wall loss problem in slit virtual impactor. The two-partitions were provided to simultaneously supply both aerosol and clean air to the virtual impactor. Both numerical and experimental investigations were carried out on the developed inlet configuration by considering different flowrate ratios of aerosol to clean air. The horizontal inlet was helpful in reducing the cutoff diameter, whereas the clean air prevented the particle deposition on the virtual impactor walls. The performance of two-partitioned horizontal inlet was compared with the conventional vertical inlet configuration for PM_2.5, PM₅ and PM₁₀ virtual impactors. All the operating conditions and geometric parameters, such as the inlet flowrate; the width of collection nozzle; the width, length and span of acceleration nozzle; and the distance between collection and acceleration nozzles, were kept the same and only the inlet configuration was changed. The major-to-total flowrate ratio was kept at 0.9 and minor-to-total flowrate ratio at 0.1. It was observed that by using the two-partitioned horizontal inlet configuration, the cutoff diameters for PM_2.5, PM₅ and PM₁₀ virtual impactors, were reduced by 16%, 10% and 11%, respectively, while the wall loss of particles near the cutoff size in all three cases were reduced from 16% to about 1%.     

Observation Evaluation of Nozzle Clogging in a Micro-orifice Impactor Used for Atmospheric Aerosol Sampling

A micro-orifice impactor uses micro-orifice nozzles to extend the cut sizes of the lower stages to as small as 0.18 μm in diameter without resorting to low pressures or creating excessive pressure drops across the impactor stages. In this work, the phenomenon of nozzle clogging caused by particle deposition was investigated experimentally for a commercial micro-orifice uniform deposit impactor (MOUDI, MSP model 100). Using an optical microscope, we observed that the micro-orifice nozzles of the lower three stages were partially clogged due to particle deposition during the atmospheric aerosol sampling. To examine the effect of nozzle clogging on the performance of the impactor, the pressure drop and the particle collection efficiency were evaluated for the lower three stages. The pressure drops across the clogged nozzles were higher than the nominal values given by the manufacturer. The particle collection efficiency of each stage was evaluated by using an electrical method for fine particles with diameters in the range of 0.1-0.6 μm. Monodisperse liquid dioctyl sebacate (DOS) particles were used as test aerosols. A Faraday cage was employed to measure the low-level current of the charged particles upstream and downstream of each stage. The collection efficiency curves shifted to correspondence to smaller orifice sizes, and the 50% cutoff sizes were much smaller than those given by the manufacturer for the three stages with nozzles less than 400 μm in diameter.     

Aerosol Synthesis and Characterization of Ultrafine Fullerene Particles

Abstract

Aerosol routes were used to produce fullerene particles in the nanometer size range. Particle formation mechanisms at processing temperatures of 400-700°C were studied by measuring particle number and mass size distributions in the gas phase by a differential mobility analyzer and a low-pressure impactor respectively. Subsequently, the foils of the impactor onto which fullerene particles were collected were examined by HPLC.     

阻尼器结构组成对碰撞阻尼性能的影响

研究阻尼器组成结构对带颗粒减振剂的碰撞阻尼器性能的影响,通过改变冲击器的直径和颗粒材料的类型,在一根悬臂梁上放置阻尼器,采用实验的方法获得各种成分组合条件下阻尼器的响应。结果表明,在带颗粒减振剂的碰撞阻尼器中,改变颗粒材料时,平均振幅降低率在72.7%~75.4%,高于不带颗粒时的62.7%;改变冲击器直径时,平均振幅降低率在72.1%~75.9%,远高于阻尼器中只有颗粒时的平均值33.3%。带颗粒减振剂的碰撞阻尼具有良好的减振性能,其减振效果好于传统的只带冲击器的碰撞阻尼器和只带颗粒的颗粒阻尼器;冲击器的直径和颗粒的材料类型对带颗粒减振剂的碰撞阻尼的减振性能影响均不大。     

Rebound behavior of nanoparticle-agglomerates

In general, the rebound behavior of particles depends on the particle/substrate material combination and the particle size. In the present investigation the rebound behavior of nanoparticle agglomerates is investigated in a low pressure impactor and compared to single spherical particles. For agglomerates, their structure and mechanical strength will also affect the rebound behavior. The rebound of openly structured agglomerates (fractal dimension D_f < 2) is determined by the primary particle size and the particle-substrate combination. The impact velocity required for rebound (critical velocity) is independent of the agglomerate size and equal to the critical velocity of single spherical particles having the same size as the primary particles. In case of agglomerate fragmentation no rebound was observed for openly structured agglomerates. For denser agglomerates (D_f > 2), the critical impact velocity decreases with increasing agglomerate size, where the decrease is more accentuated for higher fractal dimensions, finally approaching the behavior of spheres.     

Separation of flocculated ultrafine coal by enhanced gravity separator

The separation of ultrafine coal is inefficient due to the low settling velocity in centrifugal field. The feasibility of increasing particle size by flocculation to increase the separation efficiency is verified. The effect of flocculant on the size distribution of ultrafine coal was tested. The effect of flocculant dose on deash and desulfurization efficiency was studied to determine the appropriate dosage. Further, influence of the main operating parameters of the concentrator, including centrifugal force and water counter pressure on the separation performance, was studied. In addition, a comparative test was designed to verify the stability of the flocs. Results showed that the size of ultrafine coal particles could be effectively increased by the addition of flocculant, and the yield of ?0.045 µm fraction was decreased from 50% to 17% when the flocculant dosage was 10 g/t. The combustible material recovery of clean coal increased from 58% to 66%; meanwhile, the ash content reduced from 16.7% to 14.6%. In addition, combustible matter recovery and desulphurization efficiency decreased with the increase of centrifugal force, while they increased with the water counter pressure. Results of comparison tests of ultrafine coal with flocculant pretreatment and artificial coal with the same size composition showed that the latter had greater combustible recovery and desulfurization efficiency.     

Development of electric virtual impactor with variable sampling particle size range

An electric virtual impactor with a capability of sampling fine and ultrafine particles was developed and its performance was evaluated both numerically and experimentally. The electric virtual impactor was provided with metallic electrodes, to which electric voltage in the range of 75–9000 V was applied for creating an electric field within the virtual impactor. Particle electric mobility was utilized to sample ultrafine particles at the major outflow section, while particle inertia was employed to collect fine particles at the minor outflow section. Silver nanoparticles with known charge level and Arizona test dust were used to experimentally validate the performance of the electric virtual impactor. Numerical and experimental outcomes agreed well with each other. The upper cutoff size of the electric virtual impactor was fixed at about 2.6 μm, while the lower cutoff size varied from 7 nm to 110 nm depending on the applied electric voltage. As a result, the proposed electric virtual impactor was able to sample both fine and ultrafine particles of a desired particle size range.     

Numerical study on deposition of non-spherical shaped particles in cascade impactor

This study investigated the deposition of non-spherical particles in a cascade impactor using numerical simulations based on computational fluid dynamics and a discrete phase model (CFD-DPM). An optimum drag force model of non-spherical particles was used to calculate the dynamic behavior of the needle-shaped particles. The trajectory of these particles in an elbow pipe was computed and measured using a high-speed video camera. The computed trajectory agreed well with the experimental trajectory, and it was confirmed that the drag force model of non-spherical particles correctly expressed the drag force in the CFD-DPM numerical simulation. Next, the motion of the needle-shaped particles in a cascade impactor was numerically simulated and compared with that in the experimental results. The simulated classification efficiency agreed well with the experimental results. Additionally, the relationship between the aspect ratio of the needle-shaped particles and their behavior in the cascade impactor was numerically analyzed. The cut-off diameter decreased with the aspect ratio at a 50% classification efficiency in the cascade impactor. This was because the drag force of the particle was assumed to increase with the aspect ratio, and longer particles fell at a lower stage in the cascade impactor.     

Development of a Compact Electrostatic Nanoparticle Sampler for Offline Aerosol Characterization

A compact electrostatic nanoparticle sampler has been developed to support the offline analysis of nanoparticles via electron microscopy. The basic operational principle of the sampler is to electrically charge particles by mixing nanoparticles and unipolar ions produced by DC corona discharge, and electrostatically collecting charged particles. A parametric study was first performed to identify the optimal operating condition of the sampler: a total flow rate (i.e., the sum of the particle and ion carrier flow rates) of 1.0 lpm, an aerosol/ion carrier flow rate ratio of 1.0, and a collection voltage of 4.5 kV. Under the above condition, the sampler achieved a collection efficiency of more than 90 % for particles ranging from 50 to 500 nm. The effect of particle material on the sampler’s performance was also studied. The prototype had lower collection efficiencies for oleic acid particles than for sodium chloride particles in the size range from 50 to 150 nm, while achieving a comparable efficiency in the size range large than 150 nm. Effects of particle diameter, particle material, and total flow rate on the sampler’s collection efficiency are explained by the particle charging data, i.e., charging efficiencies and average charges per particle.     

Aerosol Sampling Efficiency in a Model Room

The sampling process of a model room with a suction nozzle in a calm or low movement environment was numerically simulated and experimentally analyzed. Computational fluid dynamics (CFD) software, Fluent (Fluent Inc.), was used for the numerical simulation. The flow was considered to be compressible and turbulent, and particles were considered to be spheres of constant density. A good agreement was found between the numerical and experimental results (maximum difference of 15% in the overlapping zone), and the numerical model was further extended and used for parametric analysis. The influence of sampling velocity and shape of the suction nozzle on sampling efficiency was investigated experimentally and numerically in a particle size range of 2–45 µm. It was found that sampling efficiency is smaller for V-shaped nozzles, mainly for lower velocities. Sampling efficiency was calculated for each particle diameter and for the whole particle size distribution as well. Sampling efficiency decreases as particle size increases. It is concluded that knowledge of sampling efficiency for each of the sampled particle sizes may indicate the concentration and size distribution in the sampled space.     

Optimization of a laval nozzle for energy-efficient cold spraying of microparticles

An application of cold spraying is the microstructuring of surfaces by particles in the size range of 1–10?µm, for example, to improve tribological properties. Since the particle acceleration is crucial for the cold spray process, this work is focused on the optimization of a Laval nozzle. Therefore, a nozzle was designed using analytic models for the fluid and particles behavior. The new nozzle design was improved by CFD simulations considering the effects of friction between the gas and the inner nozzle wall. Moreover, the effects of the distance between the nozzle outlet and the sample, as well as the particle position by entering the nozzle on the particle trajectories and impact velocity were studied numerically. The optimization of nozzle design has been validated experimentally by cold spraying small TiO₂ particles with mean size of 1?µm and larger Ti particles with the mean size of 10?µm. The optimized nozzle showed an improved spray performance. This work has been presented at the conference SMT30 (ID 267)     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Formulation and Evaluation of Insulin Dry Powder for Inhalation

Abstract

Purpose. Dry powder formulation of insulin for pulmonary administration was prepared to obtain increased drug deposition in the alveolar absorptive region. The deposition was studied by investigating the dispersion and deaggregation of insulin from the carrier lactose using an Andersen cascade impactor and twin stage impinger. The subsequent absorption following the deposition was studied by in vivo method. Methods. Insulin in solution with absorption promoters was lyophilized. The powder was incorporated with lactose of different grades and their combinations as carriers to deliver using an inhaler device. Solid-state characteristics of the carrier as well as the drug powder were assessed by particle size and distribution measurement. The flow properties such as moisture content, powder density, angle of repose, and carr's compressibility index of the powder mixture were determined. The aerosol behavior of the powder was studied by dispersion using rotahaler^© connected to a twin-stage impinger (TSI) and an eight-stage Andersen cascade impactor (ACI) operating at different flow rates of 30–90 l/min. The in vivo performance was studied by deliverance to the respiratory tract of guinea pigs. The intratracheal bioavailability with respective to intravenous route was calculated by measuring the blood glucose reduction. Results. The coarser particles of lactose in fractions of carrier containing a wide particle size distribution impacted in the preseperator of cascade impactor, and only the particle less than 10 µm size entered stage 0–stage 7. Formulation containing 1:1 mixture of Respitose ML006 (62%<50 µm) and Respitose ML003 (37.8%<50 µm) as carrier imparts well deaggregation of insulin, and higher deposition leads to 52.3% of fine particle fraction at 60 Lit/min and in vivo bioavailability of 82%. Conclusions. Insulin formulations containing 1:1 mixture of Respitose ML006 and Respitose ML003 as carrier can impart deeper deposition of drug particles and cause higher bioavailability. This suggests that carrier used in the formulation influenced the amount of insulin deposition in the alveolar region of the lung. Hence, it was concluded that the availability of insulin for systemic absorption depends on the particle size of the drug as well as the carrier lactose.     

Optimal design and performance evaluation of a metal fiber filter for capturing radioactive aerosols

ABSTRACT

Conventional high-efficiency particulate air (HEPA) filters made of glass fiber media are prone to recycling problem and restrictions in extreme environmental condition such as high flow rate, high temperature, and fire. Therefore, metal fiber filters with minimal maintenance can replace conventional HEPA filters. The objective of the study is to evaluate the theoretical and experimental characteristics of a SUS316L metal fiber filter made from the fiber diameter of 8 µm. Theoretical modeling for predicting the collection efficiency of the radioactive aerosol is performed on the metal fiber as a function of particle size, filter thickness, and flow rate. Comparison between the experimental and theoretical results demonstrates that they are in good agreement. Consequently, the model is later utilized for performance optimization of the metal fiber filter. Also the metal filter for collecting the radioactive aerosol is optimized at the particle collection efficiency of 99.97% in most penetrating particle size (MPPS) of region 0.3 µm which complies with the standards established for conventional glass fiber HEPA filters.     

Maximum Likelihood Estimation for the Andersen Sampler for Log-Normally Distributed Particles

Abstract

Given a sample of particles from cascade impactors the geometric mean and variance are commonly estimated by plotting cumulative particle size as a function of particle size on log probability paper, assuming a log normal distribution and drawing a line. Here, theoretical estimates are described base upon the likelihood of the sample, a simple numerical method is described to obtain these estimates.     

Entry port selection for detecting particle size differences in metered dose inhaler formulations using cascade impaction

Different sized glass entry ports were evaluated for their drug collection efficiency during aerodynamic particle sizing of metered dose inhalers (MDIs) using cascade impaction. A comparison was made between collection efficiency in the entry port, impactor plates, and filter using the 1 L, 2 L, and 20 L glass entry ports and the USP and twin impinger entry ports. Entry port losses were dependent on the size of entry port selected, with 1-2 L ports showing optimal recovery on impactor plates, compared to the USP entry port. The 1 L entry port was further compared with the USP entry port in its ability to discriminate between subtle changes in particle size distribution (PSD) in an investigational hydrofluoroalkane (HFA)-based MDI formulation. Deliberately induced differences during product manufacture were easily detected using the 1 L entry port with the Andersen cascade impactor. The USP port was unable to distinguish among products with small particle size differences. An alternative entry port such as the 1 L glass entry port used in this study may provide better means of characterizing the PSD during formulation development and stability testing of MDIs.     

Optical Probe for the In-Line Determination of Particle Shape,Size, and Velocity

A laboratory optical probe was developed to simultaneously determine the following particle characteristics: circularity, particle projection area, equivalent diameter of a circle, length of the particle outline or perimeter, maximum chord length, aspect ratio, and particle velocity. Using the projection area and the perimeter, the particle shape factor circularity can be determined. The aspect ratio was approximated by the ratio of the equivalent diameter to the maximum chord length. The basic measuring principle is multi-point scanning of the particle shadow image by a line of optical fibers. In addition, the particle velocity can be measured by a differential spatial filter of optical fibers. These fibers are step index fibers with a core diameter of 64 µm and cladding of 70 µm. The shadow image of a single particle was generated by a parallel laser beam. The uncertainty of the measured circularity and aspect ratio was investigated by using metal wires with diameters of 0.12 to 0.5 mm as test particles with known circularity and aspect ratio. The standard deviations were 1.9% for the circularity and 15.5% for the approximated aspect ratio. In addition, the optical probe system was investigated by measurements of solid particles with different shapes. As an example, the results of sand, marjoram seed, and metallic oxide particles are shown. Using 1000 sand particles, the correlation between equivalent diameter and particle velocity could be demonstrated. The presented configuration of the optical probe is applicable in the size range of 0.1 to 0.9 mm and up to a particle velocity of 5 m/s.     

Aerosol Synthesis and Characterization of Ultrafine Fullerene Particles

Aerosol routes were used to produce fullerene particles in the nanometer size range. Particle formation mechanisms at processing temperatures of 400-700°C were studied by measuring particle number and mass size distributions in the gas phase by a differential mobility analyzer and a low-pressure impactor respectively. Subsequently, the foils of the impactor onto which fullerene particles were collected were examined by HPLC.     

EXPERIMENTAL INVESTIGATION OF A WET DUST SCRUBBER: DUST COLLECTION BASED ON TURBULENT DIFFUSION

ABSTRACT

The “nozzle scrubber” is a wet scrubber in which the scrubbing water is dispersed in the dust laden gas stream by means of one or more pneumatic nozzles. This scrubber is distinguished by an excellent collection efficiency for submicron dust at an unusually low energy and water consumption. No well-defined theory exists for this process. The collection efficiency in the “nozzle scrubber” depends primarily on turbulent diffusion respectively on the interaction of particles and droplets induced by turbulence, and not on inertial separation as in the case of the venturi scrubber. A light scattering device was used to measure the particle distributions. The experimental set-up was built up in a technical scale. The influence of operation parameters, especially water consumption, residence time, and pressurized air, on the grade efficiency has been demonstrated by their systematic variation. The contribution of turbulent diffusion to the collection efficiency has been confirmed.