Particle size distribution sampler

Effects of carrier gas flow rate, dilution flow rate, and the combustion boat temperature were studied by sampling the aerosol stream with an electrical aerosol analyzer to obtain the particle size distribution. Filter samples were taken for chemical analysis to determine mass concentrations. Aerosol samples were also collected in an electrostatic sampler for electron microscopic examination. 

Previous work has shown that the 3.3-5.5 /xm coal fraction has a particle size distribution similar to that of respirable mine dust collected on personal samplers during mining operations (13). This fraction was used for the spark-source analyses. The coals used in this investigation are identified in Table I. The respirable dusts were obtained from personal sampler filters submitted to the Dust Group, Pittsburgh Technical Support Center, Federal Bureau of Mines. The samples were collected during actual mining operations. 

Due to their simplicity side-wall samplers are superficially attractive (Figure 1.37) but serious errors in concentration and particle size distribution can arise unless the particles are fine, the concentration is high and a very high sampling velocity is used. A projection extending from the pipe wall only marginally improves sampling efficiency [29]. 

A patent has been issued on an instrument operating in a similar way [34]. It consists of a particle suspension sampler, a settler and a weight or volume sensor. Particle size distribution is determined from sensor output and the time for the settling particles to pass the sensor. 

FIGURE 10.1 (a) Effect of the number of particles counted on the average mean primary particle diameter, r/j (circles), and Sauter mean primary particle diameter, (squares), as well as the geometric standard deviation (that is, a measure of the width of the size distribution, triangles), (b) The corresponding primary particle size distribution and a TEM picture of the investigated titania nanoparticles collected with a thermophoretic sampler directly from a premixed TiOj flame at a height of 0.5 cm above the burner. (Courtesy of H.K. Kammler and S.E. Pratsinis.)... 

Figure 9. Effect of sampling velocity ratio on the sample particle size distribution using a side-wall sampler. (Reproduced with permission from reference...

In the applications of gas-solid flows, measurements of particle mass fluxes, particle concentrations, gas and particle velocities, and particle aerodynamic size distributions are of utmost interest. The local particle mass flux is typically determined using the isokinetic sampling method as the first principle. With the particle velocity determined, the isokinetic sampling can also be used to directly measure the concentrations of airborne particles. For flows with extremely tiny particles such as aerosols, the particle velocity can be approximated as the same as the flow velocity. Otherwise, the particle velocity needs to be measured independently due to the slip effect between phases. In most applications of gas-solid flows, particles are polydispersed. Determination of particle size distribution hence becomes important. One typical instrument for the measurement of particle aerodynamic size distribution of particles is cascade impactor or cascade sampler. In this chapter, basic principles, applications, design and operation considerations of isokinetic sampling and cascade impaction are introduced. 

During limnological investigations of Lake Zurich, Switzerland, monthly measurements were made of the total particle count and the particle size distribution as a function of depth over a 12-month period. A Kemmerer sampler was used to collect a 1-L sample which was conserved with formaldehyde, stored at 4°C in the dark, and counted within 48 hr. Sample handling, preparation, and details of the microscope counting technique using a Zeiss Videomat image analyzer and particle counter are described in detail elsewhere (19). 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

With increased sensitivity, the size-segregating samplers have produced data that are most helpful in characterizing the size and composition of aerosols. The data from these samplers have been used to resolve a bimodal, and sometimes a trimodal, distribution of particles. 

A fourth example of data showing the particle distribution was a study that used the DRUM sampler at Grand Canyon National Park in 1984 (22). Recording the size distribution of sulfur was necessary in helping to understand the effects of sulfur on visibility degradation because there were two size modes one near 0.3 xm and one around 0.1 xm. These modes were not present simultaneously but appeared somewhat anticorrelated (see August 14 in Figure 7). 

A novel device was designed to estimate the dislodgeability of dust-associated pesticide residues by skin contact (Edwards and Lioy, 1999). Called the EL Sampler , the device consists of a spring-loaded assembly that permits the sampling medium to be pressed lightly (12g/cm or 1160 Pa) onto the surface to be monitored. A 10-cm x 15-cm Empore C-18 extraction membrane was used for the sampling medium. The material was chosen after controlled experiments on particle adhesion showed it to pick up the same distribution of test dust particle sizes as the human hand. In studies in which the EL sampler was pressed onto polyethylene surfaces coated with house dust and then sprayed with a solution of pesticides in 2-propanol, the device was found to collect 35%, 31 %, 32% and 18 %, respectively, of chlorpyrifos, diazinon (0,0-diethyl 0-[6-methyl-2-(l-methylethyl)-4-pyrimidinyl] phosphorothioate), malathion and atrazine (6-chloro-A -ethyl-A -isopropyl-l,3,5-triazine-2,4-diamine). Parallel studies with human hand presses (full hand at 6.8 kg = ca. 6900 Pa) yielded collection efficiencies of 42 %, 29 %, 43 % and 21 %, respectively. 

An alternative is the use of an optical method to measure particulate concentrations and size distributions. This technique has the obvious advantage of having a negligible effect on the particulates since the equipment would be external to the exhaust system. An optical method also has the potential to be much simpler to use since it would eliminate the need for elaborate and cumbersome systems containing probes, stack samplers, flow development tunnels, filters, and heat exchangers. In addition, final data from an optical system could be immediately obtained electronically as opposed to weighing the various filters in a particle impactor by hand, and as such, the optical analyzer is a real time instrument capable of following exhaust gas fluctuations and other nonsteady effects. 

Malvern ALPS 100 system liquid particle counter is a modular system that can be used with an autosampler for multiple samples or with an online sampler for direct measurement of flowing liquids. It can be used with sample volumes down to 0.5 ml and uses a built-in multi-channel analyzer to perform size distribution analyses of low concentration dispersions. Suitable for both aqueous liquids and solvents, it measures up to 50 size bands in the 2 to 100 pm or 3 to 150 pm size range with output from a built-in thermal printer or external dot matrix printer. 

Figure 2 Representative example of a mass distribution of ambient particulate matter as a function of particle diameter. Mass distribution per particle size interval is shown as Amass/A(logDa) (in Rgni ) plotted against particle size (ZJa) in micrometers. Tbe figure also shows the range of aerosol sizes included in various methods of aerosol measurement wide range aerosol classifiers (WRAC), total suspended particulate (TSP) samplers, PMjo and PM25 samplers (source Lippman and Schlesinger, 2000) (reproduced by permission of Annual Reviews from Annual Review of Public Health 2000, 21, 309-333).

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