Centripetal Particle Size Classification Klaus Willeke” and James D. Blanchard Aerosol Research Laboratory, Institute of Environmental Health, University of Cincinnati, Cincinnati, Ohio 45267
A new particle-size classification technique has been developed in which airborne particles are inertially size-separated in a centripetal force field. T h e air sample is drawn into the classifier through an annular ring and is then accelerated in a cone-shaped (channel toward the classifier’s center line. Upon leaving the ]primary orifice of diameter W1 a t the apex, the aerosol flow converges and passes down into the collection orifice of diameter Wz, which is coaxial with t h e primary orifice and located a t a distance S from it. High-inertia particles overshoot the collection orifice while low-inertia particles penetrate through it. The particle penetration efficiency curve for W2IW1 = 0.42 and SIW1 = 0.25 has a size dependence close t o the ACGIH curve for respirable sampling. This technique has promise as a simple preclassifier for ambient or industrial particulate air sampling. Size classification of airborne particulate matter is performed in order to determine the aerodynamic size distribution of a n aerosol in ambient environments and in confined spaces such as industrial plants. Size classification techniques are also used to sample a n aerosol above or below a desirable particle size, or to split the aerosol into two or more particle size fractions for gravimetric and chemical analysis. Particle size analysis or sampling within certain particle size ranges is generally performed aerodynamically whenever the environmental sample is t o be related to the aerodynamic deposition pattern of particles in the human respiratory tract. Among the available aerodynamic impaction methods, t h e solid-plate impactor removes particles above a desirable cut size and passes the smaller ones (1-3). T h e cyclone and the centrifuge size discriminate through removal of t h e highinertia particles in a centrifugal force field ( 4 , 5 ) .T h e virtual impactor, also called the centripeter or dichotomous sampler, fractionates the particle size distribution a t a desirable cut size a n d retains both fractions in t h e airborne state (6, 7 ) . T h e opposing-jet classifier, still under development, also retains the size fractions in their airborne state, but may have the cut size increased or decreased through adjustments of t h e flow rates (8-10). I n the elutriator, the larger size fraction is removed gravitationally (11 ) . T h e centripetal particle size classification technique described here may, before closer examination, have the appearance of being the inverse of the centripeter (6). In the new technique, the small particle fraction is withdrawn along the center line, while in the centripeter the large particle fraction is withdrawn along the center line. Removal of t h e large size fraction in a n annular centripetal force field may make this new technique pairticularly suitable as a n inlet design for respirable particle samplers or for “inhalable particulate” samplers. While sampling for inhalable particulate matter, defined as airborne particles 1 1 5 p m in aerodynamic equivalent diameter, has been proposed as a new size-specific standard (12), no suitable inlet design has been found and agreed upon a t this time. T h e development of the centripetal particle-size classifier was motivated by a n observation made during the calibration of a slotted cascade impactor (13, 14). I t was observed t h a t particle deposits were located slightly below the slot entrance along both inner walls of those slots where t h e aerosol flow entered from both sides a t 90’ t o the slot flow. In end slots, where the aerosol flow enters only from one side, deposits were observed only on the inner wall opposite to t h e flow entry. 0013-936X/80/09 14.-0461$0 1.OO/O
@ 1980 American Chemical Society
Willeke (13) attributed these losses to sideways impaction. Calculations of the aerodynamic stopping distance proved t h a t high inertia particles could overshoot the center line of t h e slot flow and impact onto the opposite wall. T h e goal of this research was, therefore, to design and test a centripetal device which would pass the particles below a certain particle size and remove the particles above this size, while having a circular inlet geometry suitable as an inlet for particulate samplers. Experimental
C e n t r i p e t a l Particle-Size Classifier. Figure 1 shows a schematic representation of the centripetal particle size classification technique. The inlet consists of a plunger separated by distance H from a plate with a central primary orifice of diameter W1. T h e aerosol flow is centripetally accelerated in t h e annular channel a t angle H perpendicular to the center line. The cone-shaped jet between the surfaces of t h e plunger and the orifice plate converges and passes down into a collection orifice of diameter Wr, which is coaxial with t h e primary orifice and located a t a distance S from it. T h e high-velocity aerosol particles emanating from the primary orifice deviate from the airstream lines depending on the magnitude of their inertia. High-inertia particles overshoot the center line and the collection orifice and impact onto the top surface of the collection cup, while low-inertia particles enter t h e collection cup with the airstream. T h e airflow is established by drawing air through the collection cup. There is no airflow withdrawn in the region between the two orifices. In order to evaluate the magnitude of the overshoot effect, we have made a linear approximation and have calculated the aerodynamic stopping distance ( 1 5 ) for t h e flow conditions a t the exit of the primary orifice. For the flow conditions given in Figure 1 and used for collecting the data shown in subsequent figures, particles in the size range of about 3-5 p m have a good probability of being eliminated from the airstream if the collection orifice diameter, Wp,is smaller than the primary orifice diameter, W1, and if the two orifices are separated within a certain range of distances, S . Test P r o c e d u r e . T h e experimental setup ( 1 6 ) is schematically shown in Figure 2. Liquid oleic acid test aerosols were generated (1 7 ) by a vibrating orifice monodisperseaerosol generator (Thermo-Systems, Inc., St. Paul, Minn.) and were discharged to near Boltzmann equilibrium by a IO-mCi, 85Kr radioactive source. Flow rate Q1 was extracted from the aerosol generator and was ducted into the upper plenum of the classifier. The aerosol flow entered the classification stage through 45 holes of 3.2 m m diameter each arranged uniformly on a 5.7 cm diameter circle. Internal losses were eliminated by rounding all corners. T h e collection cup was 5.72 cm (2.25 in.) in diameter, 7.6 cm (3 in.) high, and had 0.08 mm thick shim stock as a n orifice plate. The collection cup was centered inside a 7.5 cm diameter housing by 3 adjustment screws and was vertically translated by a micrometer screw. T h e particles t h a t penetrated into the collection cup were counted by a n optical single particle counter coupled to a multichannel analyzer and a printer. Measurements were made a t the standard Q1 = Q p = 118 cm3/s (0.25 cfm) flow rate of the optical particle counter or a t Q1 = Q 2 Q 3 = 189 cm3/s (0.40 cmf) through addition of an external pump; see Figure 2.
+
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ANNULAR AEROSOL INFLOW .I
I Figure 1. Schematic of centripetal particle size classification. Stop distances for particles up to 4.5 p n aerodynamic diameter are caiculated using the primary orifice: V = 51 15 cm/s, H = 0.033 cm, W1 = 0.358 cm, 0 = 20°, 0 = 189 cm3/s, Re = 1120 (based on chamber height, H, and circumferential diameter, W,)
Kr-85 CHARGE NEUTRALIZER
( ~ L U T E R
NO ZZLE-TO- CUP / N O 2 Z L E DIP, METER, S / W,
Figure 3. Mean particle penetration efficiency for different diameters of oleic acid test particles: H = 0.033 cm; W, = 0.358 cm; 0 = 20'; 0 = 189 cm3/s PRIMARY ORIFICE COLLECTION CUP
Figure 2. Experimental setup: VOMAG = vibrating orifice monodisperse aerosol generator; OPC = optical particle counter; MCA = multichannel analyzer In order to get the unclassified number count, a base-line chamber without the classification stage was put in place of the classifier. The base-line chamber count was verified to be the same as the particle count obtained from the primary orifice with the collection cup removed.
Results and Discussion Particle Penetration Efficiencies with Plunger. Figure 3 shows the mean particle penetration efficiency for different liquid particle sizes. It was obtained by placing the collection cup initially against the primary orifice and then receding the cup incrementally. The penetration efficiency is defined as the number of particles counted downstream of the impaction cup divided by the base-line count with the impaction cup removed, as measured by the base-line chamber. As shown in Figure 3, the particle penetration efficiency is decreased as the particle size is increased, which one would predict when one considers the overshoot effect of high-inertia 462
Environmental Science & Technology
particles. The data of Figure 3 and other data we have obtained (16) show that the penetration efficiency rises to a maximum value at an S/W1 distance less than 0.5 and then decreases and approaches a plateau with increasing S / W1 distance. Our data show that the maximum penetration efficiency is only slightly dependent on SIW1 for different volumetric air flow rates. The shape of the curves in Figure 3 may be explained as follows: For S/W1 distances under 0.10-0.15, a portion of the primary orifice is blocked by the collection cup orifice, forcing the airflow to turn sharply into the collection cup and causing most of the particles to be lost by impaction onto the top surface of the cup. As the collection cup is withdrawn, an increased number of particles penetrate the cup, raising the penetration efficiency. As the collection cup is further receded, the top surface of the cup no longer lies in the direct line of sight for particles jetting out of the primary orifice. Smoke studies showed (16) that secondary air circulation develops in the plenum between the two orifices as the S/W1 distance is increased. Particles are thus removed through losses to the surfaces between the two orifice assemblies. As the SIW1 distance is increased to about 1.5, the number of particles lost for different particle sizes no longer increases significantly. In Figure 3 the collection orifice diameter is less than half the primary orifice diameter (W2/W1 = 0.42). The collection orifice cross section is therefore less than one-fourth of the primary orifice cross section, and the greatest pressure drop occurs across the collection orifice. For a collection orifice diameter almost as large as the primary orifice diameter ( W2/W1 = 0.92), the penetration efficiency was found (16) to be higher for all particle sizes with the peaks of the curves occurring near SIW1 = 0. The data of Figure 3 and those obtained for other orifice diameter ratios are more suitably converted into plots of particle penetration efficiency as a function of particle size
inn ,"",
100 I
la
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-
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0.50
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1.50
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a a
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1 2.0
I 3.0 AERODYNAMIC DIAMETER (pm) 100
I
,
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0
2.0 30 AERODYNAMIC DIAMETER ( p m )
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4.0
1
1
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1
1
-
3 ...
50
'
1
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0 25
> u
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c
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3 .o 2.0 AERODYNAMIC DIAMETER (pm)
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Figure 4. Perform,ance curves for the centripetal classifier. Dimensions and flow rate of Figure 3: (a) small orifice in collection cup; (b) medium-sized orifice; (c) orifice comparable to primary orifice size
with the nozzle-to-cup distance S as a parameter. Figure 4 reveals that the particle penetration efficiency is sharpest for small S/W1 distances where the overshoot effect is most prominent. If one defines the geometric standard deviation ug by the 84 and 16% efficiencies, then the curve for SIW1 = 0.25 and Wz/Wl = 0.42 in Figure 4a has an extrapolated ug
value of 1.7-1.9. One should note that the sampling efficiency curve for respirable dust as defined by the American Conference of Governmental Industrial Hygienists (18)has a ug of the same magnitude. A small adjustment in geometry and flow rate would shift the centripetal classification curve to coincide with the ACGIH curve. The technique described here Volume 14, Number 4, April 1980 463
100
~
~~
~
Table 1. Particle Diameter for 50% Penetration Efficiency through Centripetal Particle Size Classifier a flow rate, W ~ I W I cm3/s
80
-rf
0.42 0.42
> 0 z
0.70 0.70
w
0.92 0.92
6C IW A
z
118 189 118 189 118 189
partlcle diameter at 5 0 % penetratlon, wm no plunger, with plunger, S/wl = S/WI = 0.25 0.50 1.00 1.50 1.00
3.60
2.75 >4.50 3.85 >4.50 3.75
2.80 2.20 2.35 2.50 2.50 2.65
a W, = 0.358 cm. H = 0.033 cm. for SIW, = 0.25-1.50.
0 ta a
1.65 4.50
Values are approximately the same
+
W
w Z
a 40
Conclusion
_1 W
2 + a a
NO PLUNGER S / W , = I 00 20
e
b
-
I O
20
30
40
50
The centripetal classification technique was tested in the 1 . 5 4 5 - p m particle size range. When the collection cup orifice diameter is about half the primary orifice diameter, its particle penetration efficiency for a nozzle-to-cup distance of S/W1 = 0.25 is similar to that obtained with a cyclone used for respirable dust sampling. An inlet designed on the basis of the data shown may thus serve as an alternate to the cyclone for respirable dust sampling. Its axial symmetry and its simplicity may be considered as advantages over the cyclone when used as a preclassifier. A possible disadvantage may be that a classifier designed as a respirable sampler inlet may have a ’ higher pressure drop than a cyclone for a comparable cut size. In theory, this method should also be applicable for “inhalable particulate” sampling with a particle size cut at 15 pm. However, this requires further research. The centripetal classification technique has the advantage of being independent of the wind direction because of the annular Inlet design. Its dependence on wind speed needs to be determined. Acknowledgments We thank Mr. Alex Fodor for his expert machining of the classifier and Mr. Jozef Svetlik for his technical assistance. Literature Cited (1) May, K. R., J . Sci. Instrum., 22, 187-95 (1945)
(2) Merce’r,T. T., Ann. Occup Hyg., 6, 1-14 (1963). (3) Marple, V. A,, Willeke, K., Atmos. Enuiron., 10, 891-6 (1976). (4) Lippmann, M., Chan, T. L., Staub-Reinhalt. Luft, 39, 7-11 (1979). (5) Stober, W., Flachsbart, H., Enuiron. Sci. Technol., 3, 641-51 (1969). (6) Hounam. R. F.. Sherwood. R. J.. A m Ind. H w . Assoc. J.. 26. 122-31 (1965). ( 7 ) Dzubav. T. G.. Stevens, R. K., Enuiron. Sci. Technol., 9, 663-8 (1975). (8) Willeke, K., Pavlik, R. E., Enciron. Sci. Technol., 12, 563-6 (1978). (9) Pavlik, R. E., Willeke, K., A m . Ind. Hyg. Assoc. J., 39, 952-7 (1978). (10) Willeke, K., Pavlik, R. E., J . Aerosol Sci., 10, 1-10 (1979). (11) Dunmore. J. H.. Hamilton. R. J..Smith. D. S. G..J. Sci. Instrum.. 41,669-72 (1964): (12) . . Miller. F. J.. Gardner. D. E.. Graham, J. A,. Lee, R. E., Wilson, W. E., Bachmann, J. D., J . Air Pollut. Control Assoc., 29,610-$ (1979). (13) Willeke, K., Am. Ind. Hyg. Assoc. J., 36,683-91 (1975). (14) Willeke, K., McFeters, J. J., J. Colloid Interface Sci., 53,121-7 (1975). (15) Fuchs. N. A.. “The Mechanics of Aerosols”, Pereamon Press, New York, 1964. (16) Blanchard, J. D., M. S. Thesis, University of Cincinnati, Cincinnati, 1979. (17) Willeke, K., Ed., “Generation of Aerosols and Facilities for Exposure Experiments”, Ann Arbor Science Publications, Ann Arbor, 1980. “I
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(18) Lippman, M., in “Air Sampling Instruments for Evaluation of Atmospheric (?ontaminants”, 5th ed., American Conference of Government Industrial Hygienists, Cincinnati. 1978, Section G.
Received f o r recieu Octobcr 29, 19i.9).Acccjptcd J a n u a r y 21, 1980. This material i s based upon I ! ork irhich ic’as partiaily supported h x t h e A‘ational Science Foundatir~niiridcr Grant ,Yo. E.V(;77-0.1667.
Trace Element, Radionuclide, and Polynuclear Aromatic Hydrocarbon Concentrations in Unionidae Mussels from Northern Lake George Merrill Heit”, Catherine S. Klusek, and Kevin M. Miller Environmental Measurements Laboratory, U.S. Department of Energy, New York, N.Y. 10014 ~
Analyses of the soft tissues of three species of Unionidae mussels collected from northern Lake George, N.Y., showed t h a t these organisms concentrated Cd, Cu, Hg, Se, and Zn above t h e levels found in the sediment from which they were collected. Chromium, Ni, and P b occurred in the tissues at the concentration levels of the sediment, while As and Sn were found to be only a small fraction of the levels in sediment. The polynuclear aromatic hydrocarbons, phenanthrene, fluoranthene, pyrene, I-methylpyrene, perylene, and dibenzothiophene, were detected in some but not all of t h e mussel samples. Benz[a]ani,hracene, benzo[a]pyrene, and dibenzacridine were not found in any of the mussels. The mussels were found to accumulate radionuclides believed to have been deposited following the Chinese weapons tests of 1976 and 1977. Naturally occurring principal y-emitting radionuclides in the ‘W a n d ’j2Th series as well as 40K were not detected in the tissues. T h e use of pelecypods, such as the intertidal mussel M j ~ t i lus, to study t h e occurrence and bioavailability of metals, hydrocarbons, and transuranic elements in marine environments is well known (1-3). However, few reports on the levels of these substances in freshwater bivalves are available. Most of t h e studies which have been presented in the literature are concerned with the levels of a few trace elements in mussels collected from moderately polluted environments ( 4 - 7 ) . In this study we have looked a t three classes of trace substances released into the environment by energy-related activities: trace elements and polynuclear aromatic hydrocarbons (PAHs) from the incomplete combustion of fossil fuels (8,9) and radioactivity from various sources. I t was our aim to determine which of these substances are available to these filter-feeding members of the freshwater food web and t o observe t o what extent they are bioaccumulated under socalled “natural” conditions in which there is little stress from energy-related pollutants other than those deposited in the ecosystem by long-range atmospheric transport. Materials and Methods
Sampling Location. Three species of freshwater mussels were collected from Hearts Bay, northern Lake George, during October 1977. T h e lake is located in t h e eastern Adirondack Mountains of t h e New York State Adirondack Park. Lake George is 5 2 krri long and is divided essentially into two lakes with approximately equal volume basins by a natural constriction termed t h e “Narrows”. T h e southern basin is more eutrophic, receiving some local anthropogenic input from the resort area located at this end of the lake. T h e northern basin is essentially oligotrophic, receiving little input from local sources (10). I t has been stated t h a t the sediments presently being deposited in the northern basin of the lake are quite similar t o those deposited in the past by glacial action (11). We have previously presented evidence indicating t h a t the input of PAHs into the northern portion of t h e lake appears t o be t h e result of long-distance atmospheric transport of combustion-derived materials (8).
Mussel Species. The species of mussels collected, in order of abundance, were L a m p s i l u s radiata, Elliptio c,omplanatus, a n d A n o d o n a t a g r a n d i s . All of the organisms were gathered in about 3 m of water by divers and frozen shortly after collection. T h e sediment in which they were collected was grey in color, somewhat gritty, and composed of a mixture of‘fine sand and silty clays. Analysis. T h e mussels and sediment were analyzed for trace elements and polynuclear aromatic hydrocarhons by contractor laboratories. Radionuclides in the mussels were measured in our laboratory. Trace E l e m e n t s . Fifteen-eighteen soit tissues from each species were separately analyzed for trace elements. The individual tissue weighed between 2 and 12 g (wet weight). Each sample was homogenized by hand using a Teflon spatula and then dried a t 60 “C. The percent moisture was determined. T h e samples were wet ashed with concentrated nitric acid in acid-precleansed Teflon beakers over a hot plate. After 6-8 h, t o complete digestion, the samples were taken to near dryness, redissolved in 10%nitric acid, and diluted to a known volume. T h e samples were analyzed for As, Cd, Cr, Cu, Ni, Se, S n , and Zn. Sediment samples to be analyzed for the same elements were digested in a similar manner except that concentrated hydrofluoric and perchloric acids were used. Sediment and tissue samples to be analyzed for Hg were wet-digested overnight in Pyrex beakers in a water bath at 60 “C, using sulfuric and nitric acids. J u s t prior to analysis the samples were further oxidized by the addition of potassium permanganate and potassium persulfate. All of the analyses were performed by atomic absorption spectrophotometry (AAS); however, depending upon the elements to be analyzed, different procedures were used. Cadmium, Cu, and Zn were analyzed by flame AAS, Hg by cold vapor flameless AAS, and Cr and Ni using a heated graphite furnace and deuterium background corrector. Arsenic and Se were also analyzed by graphite furnace AAS; however, the sample aliquots were first converted to hydrides using oxalic acid and sodium borohydride. The metal hydride was purged into the graphite furnace with helium gas. R a d i o n u c l i d e s . The soft tissues of 3 2 Elliptio mussels were sealed collectively in a 210-cm:j Teflon-lined aluminum tuna can and analyzed by y spectroscopy. Owing to the limited amount of mussel tissue and sediment available, only the E l l i p t i o species was analyzed for radionuclides. T h e sample was counted for 4000 min with a 130-cm:’ Ge(Li) y - r a y detector in conjunction with a 4000-channel analyzer and a programmable calculator (12). Polynuclear Aromatic Hydrocarbons. Twelve composite soft tissue samples consisting of 12-50 mussels (Elliptio and Larnpsilus), weighing between 50 and 200 g wet weight, were homogenized and saponified with 4 N sodium hydroxide at 90 “C for 2 h and then extracted three times with ether. T h e ether extracts were combined, dried on magnesium sulfate, concentrated on a modified Kuderna-Danish apparatus, and diluted with methylene chloride. The concentrated ether extract was then fractionated by BioReads 5x8 gel permeation chromatography and the aromatic fraction analyzed by
This article not subject to U.S. Copyright. Published 1980 American Chemical Society
Volume 14, Number 4, April 1980
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