Determination of Aerosol Size Distributions by Jet Impactor-Light Scattering Technique JOSEPH K. THOMPSON Naval Research laboratory, Washington
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25, D. C. Over narrotv size ranges. nhere the effect of this variation in the light scattering function is minimized, the intensity of scattered light is proportional to the particle concentration. The studies reported hcre were concerned primarily n ith particle sizes larger than 1 micron, where the total scattered light intensity is a function of the square of the particle diameter. I n the conventional method of using jet impactors, material is collected by impaction and then the quantity is determined by visual counting, weighing, or a suitable chemical process. I n the light scattering method described here, the collected material is disregarded and the aerosol particles that escape impaction are measured.
Particle size distributions of polydisperse oil aerosols are determined by using single jet impactors together with a light scattering meter. The aerosol i s drawn from a sampling probe through the jet impactor and then into the light scattering meter. The jet impactor removes particles larger than a certain size range, identified b y the characteristic diameter of the impactor. The light scattering meter measures directly the fraction of aerosol particles remaining airborne. By sampling in turn through each of a series of jet impactors having a range of characteristic diameters, size distribution data in terms of scattered light intensity are obtained. By determining particle size distribution before and after any obstruction in an air duct, such as a filter, the removal efficiency of that obstruction can be calculated for various particle size intervals.
APPARATUS AND METHOD
The apparatus was arranged as shown in Figure 1. The jet impactors were made according to the design of Ranz and Kong ( 3 ) . These impactors had rectangular jets with a jet-to-plate spacing of three times the jet width. The light scattering measurements were made d h the Naval Research Laboratory light scattering meter, u-hich has been described by Knudson and K h i t e ( I ) . This is a forward-scattering instrument which has a limiting sensitivity of about lo4 particles per liter for dioctyl phthalate (DOP) aerosols of 0.3-micron diameter. The aerosol being sampled was drawn a t a measured rate either through the jet impactor, A , or through the direct sampling tube, B , and then through the light scattering cell, C. The intensity of scattered light was indicated by the meter unit, D. The light scattering meter n-as first adjusted to read 100 on the aerosol sample d r a n n from the direct sampling tube, B. Then a sample was drawn through the jet impactor; the re-
A
is a device for the inertial removal of aerosol particks larger than a certain size range, identified by the characteristic diameter of the impactor. For a given aerosol material the characteristic diameter is fixed by the diniensions of the impactor and the velocity of the air through the jet. The light scattering niet'er is a n instrument for measuring the intensity of scattertd light,. The tot'al intensity of scattered light from spherical aerosol part,icles of a given isotropic material is a function of ~i pon-er of t'he particle diameter. This pon-er of the diameter in the light scattering function varies from the sixth pon-cr for particles less than about 0.3 micron to the second pon.er for particles 1 micron and larger. JET IhlPACT(1R
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Figure 1. Arrangement of apparatus for par-
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ticle size studies
sulting light scattering meter reading was a measure of the aerosol particles that escaped impaction. The difference between the tn.0 meter readings was a measure of the aerosol particles that were retained. STUDY
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VARIABLES
The performance of six jet impactors was studied by measuring their impaction efficiencies a t various flow rates for monodisperse dioctyl phthalate aerosols ranging from 0.8 to 1.2 microns in diameter. The variables-velocity, particle diameter, and jet widthwere combined into the dimensionless inertial parameter, I), described by Ranz and Kong ( 3 ) :
\\here C is a correction factor for the fluid resistance of air to particle motion, p is the density of the particle, v is the velocity, d, is the diameter of the particle, 7 is the coefficient of viscosity of air, and d, is the width of the jet. The results of the performance studies are shown in Figure 2 , where impaction efficiency is plotted against the square root of the dimensionless inertial parameter, +. The plotted points represent measurements made in this study. The dotted curve is taken from the report of Ranz and \Tong ( 3 ) and represents their data, which were obtained by analysis of the impacted material. The individual points are not shown for the Ranz and Wong curve, but the spread was about the same as for the light scattering data reported here. The light scattering technique for measuring particle concentrations of monodisperse aerosols is \\-ell established. Therefore, the agreement betn-een the light scattering data presented here and the Ranz and Kong curve confirms by an independent method that the inertial parameter expression accurately describes the performance of these jet impactors. Figure 3 shows the theoretical variation of impaction efficiency with particle size for the series of jet impactors used in this study. To construct these curves, the efficiency for various particle sizes was calculated for each impactor by substituting the appropriate values of d,, d,, p , and 2) in the inertial parameter VOL. 29, NO. 12, DECEMBER 1 9 5 7
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expression. The characteristic diameter which marks the theoretical sharp division between particle sizes that are impacted and those that escape impaction is designated for each impactor by a dotted line as the diameter for which the impaction efficiency is 50%. If the total area of the graph in Figure 3 is taken as representing the scattered light intensity measured for a hypothetical polydisperse aerosol with a special size distribution function, the area between the vertical coordinate and a given efficiency curve corresponds to the scattered light intensity measured for the fraction of the aerosol particles passed by that impactor. The area between two adjacent efficiency curves corresponds to the scattered light intensity for particles in the size interval betxeen the two impactors. As the impactors are run singly and not in cascade, it is desirable for adjacent efficiency curves to overlap. The narrower the size interval becomes between two successive stages, the less will be the effect of a varying power of the particle diameter in the light scattering function.
tribution by count is shown by curve 3 as frequency weighted with diameter squared. The visually determined frequency plot (curve I) shows a geometric mean diameter of 1.8 microns and a geometric standard deviation of 3.5. The location and slope of the light scattering size distribution curve (curve
visual count. The samples for the visual count were collected by means of a four-stage cascade impactor of the type described by May (2). The visually determined cumulative frequency distribution of the dioctyl phthalate aerosol is shown by curve 1 of Figure 4. I n addition, the cumulative size dis-
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RESULTS
The particle size distribution of a polydisperse aerosol may be determined by a procedure similar to that described for the jet impactor performance studies. The light scattering meter is set to read 100 on the sample drawn from the direct sampling tube. Then light scattering measurements are made on samples from each of a series of impactors whose characteristic diameters cover a suitable range of particle sizes. The resulting series of light scattering measurements gives the per cent of light scattered by particles smaller than the stated size, where the stated size for each measurement is the characteristic diameter of the impactor. I n other words, the light scattering measurements give directly a cumulative size distribution which is weighted in terms of light scattering power of the particles. The size distribution of a polydisperse dioctyl phthalate aerosol was determined by the jet impactor-light scattering method. The characteristic diameters of the six jet impactors ranged from 0.8 to 7.2 microns. This divided the total particle size range of the aerosol into seven intervals. The weighted size distribution given by the light scattering measurements is shown as a log probability plot by curve 2 of Figure 4. This straight line shows a weighted geometric mean diameter of 3.8 microns and a geometric standard deviation of 2.7. For the purpose of comparison the size distribution of the same dioctyl phthalate aerosol was determined by 1848
ANALYTICAL CHEMISTRY
INERTIAL
PARAMETER
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Figure 2. Variation of impaction efficiency with inertial parameter
Figure 3.
Variation of impaction efficiency with particle diameter
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Figure 4.
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Cumulative particle size distribution of a DOP aerosol
1. Diameter us. per cent of total number of particles by count 2. Diameter us. per cent of total observed scattered light intensity 3. Diameter us. per cent of total number of particles by count,
weighted in diameter squared
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Figure 5.
Effect of filter on particle size distribution of DOP aerosol
Diameter us. per cent of total observed scattered light intensity
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Figure 6. Effect of filter on particle size distribution and concentration of DOP aerosol
2, Figure 4) relative to the visuallydetermined curves (curves 1 and 3) indicate that the light scattering data are weighted with a fractional power of the diameter, and that this power changes from about 0.7 to about 0.3 over the size range studied. Because the theoretical total scattered light intensity from a particle is proportional to the square of the particle diameter in this size range, the measured intensity obviously does not follow the theoretical relationship. This discrepancy can be attributed to the fact that the NRL light scattering meter is a forn-ard-scattering instrument and hence does not measure the total flux of scattered light. The intensity of light scattered by a given particle varies with scattering angle; hence, the intensity of light scattered a t a given angle does not necessarily follow the square law relation with particle diameter. The relation can be expected to be different for each scattering cell with a different viewing angle. The difference in slope (geometric standard deviation) shown in Figure 4 between the light scattering distribution curve and the visually determined curves indicates that the apparent size distribution given directly by light scattering is narrow and skewed toward small sizes when compared to the distribution by visual count. If it is desired, the light scattering distribution data can be converted to show approximately the unweighted distribution as follows. The relative light scattering intensity from particles in each size interval between successive impactors in the series is obtained as the difference between the successive light scattering measurements. The weighting factor which affects the light scattering intensity is calculated for each size interval; this factor consists of the particle diameter a t the center of the size interval raised to the power indicated by the relative location of the light scattering and the visual count curves shown in Figure 4. Dividing the relative light scattering intensity for each size interval by the corresponding Iveighting factor gives a series of quantities which are approximately proportional to the unweighted frequency distribution. This conversion process is empirical, and the factors used are applicable only to a given light scattering cell and a given aerosol material. The apparent distortion in particle size distributions measured directly by the light scattering method does not interfere with using the technique for making comparison measurements of related aerosol size distributions. In comparison studies the distortion affects all measurements in the same way; therefore, relative concentrations VOL. 29, NO. 12, DECEMBER 1957
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within the same size groups can be properly compared. As an example of the use of the jet impactor-light scattering technique for comparison of related size distributions, the performance of a coarse aerosol filter was studied. The filter was set into a suitable duct and exposed to a polydisperse diovtyl phthalate aerosol having a particle size range of approximately 1 to 20 microns in diameter. Aerosol sampling stations were located in the duct a short distance both upstream and donnstreani from the filter. Light scattering measurements ryere made on samples drawn from the direct sampling tube and from each inipactor in turn a t both sampling stations. For this Study the light scattering nieter n a s set to read 100 on the original aerosol upstream from the filter. The resulting series of readings shon-ed not only the change in particle size distribution of the aerosol, but also the change in gross concentration due to the filter. The results of the filter performance study are shown in Figures 5 and 6. Figure 5 is a log probability plot of tlie weighted size distributions given by the light scattering measurements. The solid line represents the particle size distribution of the aerosol ahead of the filter, and the dotted line represents that after the filter. There was a loss of 527, in gross concentration of aerosol
due to the filter. Because of this the light xattering meter readings for the downstream station were multiplied by the normalizing factor, 100 ’48, before plotting. Figure 5 s h o w weighted geometric mean diameters of 3.1 microns for the original aerosol and 1.6 for the aerosol after the filter. The geometric standard deviation of the aerosol before the filter rras 2.4; that after the filter was 1.6. The relative changes within size groups are shorn by the histogram in Figure 6. Here the differences between light scattering meter readings for successive jet impactors were divided by the m-idths of the respective particle size intervals to obtain the height of the blocks. For this purpose the meter readings \\-ere not normalized for the loss in gross concentration of the aerosol. The area of each block is proportional to the per cent of the aerosol particles, weighted in light scattering power, in that size range. The unshaded blocks represent the aerosol before the filter; the superimposed shaded blocks represent the aerosol after the filter. The removal efficiency of the filter for each particle size interval may be obtained from the difference between the respective block heights. It is apparent from Figure 6 that the filter efficiency was greatest for particle sizes larger than 3 microns. The principal limitation on the useful-
ness of this method of studying particle size distributions comes from the varying pon-er of the diameter in the relationship betn-een meter I esponse and particle diameter. Another limitation, which iyas not discussed here, is the fact that tlie absolute accuracy of jet impactors in separating particle sizes is not known. Khere these limitations do not interfere, as in comparison studies, the method is both useful and rapid. Only dioctyl phthalate aerosols u-ere used for the studies described here. The method can be applied to aerosols of other liquids and also to dusts, if precautions are taken to prevent re-entrainment of impacted material. The method n-ould not be useful for studying mixed dispersions of two or more materials having different densities, for the impaction process would not distinguish between large particles of low density and small particles of high density. LITERATURE CITED
Knudson, H. W., White, L., NRL Rept. P-2642 (1946). May, K. R., J . Sci. Instr. 22, 187-95 (1951). Ranz, W.E., Wong, J. B., Univ. Ill., Eng. Expt. Sta., AEC Contract No. AT(30-3)-28, Tech. Rept. 4 (July 31, 1951). RECEIVEDfor review October 23, 1956. Accepted ilugust 16, 1957. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, S. J., September 1956.
Carbon-Hydrogen Analysis of Coke on Catalysts S. G. HINDIN, J. K. LEE, and S. W. WELLER Houdry Process Corp., Marcus Hook, Pa.
b A method is proposed for the determination of carbon and hydrogen in coke on synthetic cracking catalysts or on platinum-containing reforming catalysts. The coke is burned with a known, limited volume of oxygen; hydrogen and carbon are determined b y measuring the vapor pressure in the reaction system after the water and carbon dioxide formed have been successively frozen. The method is comparable in accuracy with the conventional combustion train method for the determination of carbon, and it is considerably more accurate for the determination of hydrogen because it is independent of catalyst water content.
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METHOD used in petroleum company laboratories for the analysis of coke present on solid
N THE CONVESTIONAL
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ANALYTICAL CHEMISTRY
catalysts. such as cracking and reforniing catalysts, water and carbon dioxide formed in the combustion are determined by neighing. The carbon content is determined accurately by this method, but the hydrogen content can be in gross error because of the correction that must be made for catalyst “n-ater of constitution.” For example, if a cracking catalyst contains 1% coke and 1% “water,” only about one third of the total water collected during conibustion arises from hydrogen in the coke. The method presented here differs in principle from the conventional method. The coked catalyst is heated (in a closed system) n-ith a known, limited volume of oxygen, slightly in excess of that required for complete combustion. The amounts of \yater and carbon dioxide formed by combustion are determined from the vapor pressures in the system
after each combustion product has been separately frozen by a suitable lowtemperature bath. \Then it is desired to analyze coke on a platinum-containing reforming catalyst, the catalyst is treated with hydrogen, prior to combustion, to convert any platinum oxides to metal. A correction is subsequently applied for the amount of oxygen used to convert the metal to the oxide during the combustion. This procedure is Comparable in accuracy and precision TT ith the conventional method in the determination of carbon and considerably superior in the determination of hydrogen. Two factors contribute to this increased accuracy: The method is independent of catalyst 11-ater, and milligram amounts of hydrogen can be determined more accurately by measuring the volume of oxygen required for oxidation to water than by neighing the water formed.
