trogen gas mixture instead of an SO?-air mixture is used to span the fluorescence instrument. As stated earlier and from Figure 5, use of an SOz-nitrogen span gas results in errors on the order of 30%. Carbon dioxide quenches the fluorescence to a lesser degree than oxygen, but by about the same order of magnitude. This is particularly important when the method is applied to combustion sources. Since the reduction of 0 2 levels in combustion sources is accompanied by a corresponding increase in COz levels, the net result is a minimizing of the quenching effect due to decreasing 0 2 levels. In fact, the spanning errors encountered with the analyzer used in this study could contribute a greater error than that resulting from any quenching effects. The fluorescence method for detecting SO2 emissions from combustion sources was thus adequate to monitor those emissions. Ideally, as has previously been noted ( 3 ) , the greatest accuracy can be obtained if the span gas consists of SO2 in a gas mixture characteristic of the source to be monitored. It has not been shown, however, that such SO2 mixtures remain stable over extended periods of time; therefore, care should be taken in using this procedure as a solution to the quenching problem. The introduction of NO2 a t levels found within a typical combustion source did not interfere significantly with the SOP measurements. The problem of arriving at a stable span value was the most serious drawback encountered in the present work. This, however, does not bear upon the fluorescence method and should be easily solved with a few instrumental modifications.
Acknowledgment We thank Nancy Reierson of EPA for performing the Method 6 analyses and Mike Moran and his staff of Northrop Services, Inc., for their operation of the SSSF for these experiments. Literature Cited (1) Okabe, H., Splitstone, P. L., Ball, J . J., J . Air Pollut. Control
Assoc., 23,514 (1973). (2) Zolner, W., Cieplinski, E., Helm, D., Thermo Electron Corp., TECO 40 Manual-Appendix A. (3) Wolfe, C. L., Giever, P. M., "Design, Fabrication and Evaluation of a Fluorescence Sulfur Dioxide Monitor", Research Appliance Co. and Stanford Research Institute, presented at APCA Meeting 68, June 15,1975. (4) International Biophysics Corp., Celesco Industries, private communication. ( 5 ) Homolya, J. B., J . Air Pollut. Control Assoc., 25,809 (1975). (6) Byerly, R., IEEE Trans. Nucl. Sci., NS-22,856 (1975). (7) Moran, M. J., "Simulated Stationary Source Facility Users Handbook", TN-262-1535, Northrop Services, Inc., Huntsville, Ala. (8) Strickler, S. J., Howell, D. B., J . Chem. Phys., 49, 1947 (1968). (9) Schwarz, F. P., Okabe, H., Whittaker, J. K., Anal. Chem., 46,1024 (1974). (10) Okabe, H., J. Am. Chem. SOC.,93,7095 (1971). (11) Fed. Regist., 36 (1591, 15704 (1971). (12) Hall, J.C., Jr., Blacet, F. E., J . Chem. Phys., 20, 1745 (1952). (13) Hui, M.-H., Rice,'S. A., Chem. Phys. Lett., 17,474 (1972). Receiued for review February 2, 1976. Accepted June 14, 1976. Mention o f trade names or commercial products does not constitute endorsement or recommendation for use by EPA.
Jet-Cone Impactors as Aerosol Particle Separators Jeffrey H. Schott and William E. R a m * Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minn. 55455
A jet-cone impactor, designed to eliminate particle bounce and reentrainment of collected aerosol material and to collect larger samples for analysis by removing impacted material to a secondary collection region with a portion of the gas flow, was calibrated experimentally for impaction surface angles of 45" and 60". Collection efficiencies were also calculated for a model impactor flow to determine effects of changing the impaction surface angle and jet Reynolds number. Sharpness of separation increased with increasing impaction surface angle, and impactor cut size decreased with increasing impaction surface angle. Variation of jet Reynolds number over a tenfold range had little effect on collection efficiency. Since agreement between experiment and theory was quite good, calculated collection efficiencies are recommended as a standard for this type of impactor.
To increase design possibilities and capabilities of impactors, this work considers the effects of Reynolds number, impaction surface angle, and split flow on collection efficiency of jet impactors with conical impaction surfaces. Effects of these quantities on collection efficiency were measured experimentally. Theoretical estimates of these effects were also obtained by observing the behavior of a mathematical model of the same impactors. 1250
Environmental Science & Technology
Experimental values of collection efficiencies were measured for conical impact surfaces with cone angles of 60" and 45". Split flow values were 0 and 10%. Jet Reynolds number was varied over a range from 1000-10 000. Effects on collection efficiency of two modifications were also studied. These modifications were holes in a plate separating a collecting and noncollecting region and a skimmer ring on the impact cone which changed the flow path of collected aerosol liquid. Collection efficiency was measured, with holes in the separating plate, for cone angles of 60" and 45' and split flows of 0 and 10%.The skimmer ring was used only for measurements with a cone angle of 45" and split flow of 10%. Theoretical collection efficiency curves were obtained from numerical solutions of the equations of motion of a model flow field. Computer programs developed by Marple ( I ) were used to construct solutions. Collection efficiency curves for impaction surface angles of go", 60°, and 45", jet Reynolds numbers of 1000, 3000,lO 000, and split flows of 0 and 10% were calculated. The effect of spacing ratio, N,, on collection efficiency has been studied both theoretically and experimentally. Investigations by Mercer and Stafford (2) and Marple ( I ) considered values of N , in a range from 0.125 to 10.0. For values of N , less than 1,decreasing N , increases the sharpness of separation and shifts the collection efficiency curve to lower values of N,+. However, position of the curve is very sensitive to small
changes of N , in this range. Between 1 and 5, N , has little effect on sharpness of separation or position of the efficiency curve. When N , is increased beyond 5, sharpness of separation decreases due to degradation of the jet before it impacts. N , was set at a value of 1.0 for the experimental and theoretical parts of this study.
Description of Jet-Cone Impactor General features and pertinent dimensions of a jet-cone impactor are shown in Figure 1. Dimensions are given in terms of jet diameter D, to show the configuration used in this study. The spindle diameter and the diameter of the hole in the separating plate are 2 Dc. The distance from the jet to the apex of the impact cone was set at D, ( N , = 1). The impaction surface angle, 8, is half the angle of the apex of the impact cone. For the configuration used in this study, the separating plate was positioned so that a particle bouncing specularly from the apex of the cone would be intercepted by the edge of the hole in the plate and be reflected into the collection region. The distance from the apex of the impact cone to the upper surface of the separating plate was specified, therefore, by
to produce a monodispersed aerosol from liquid diethylhexyl phthalate (DEP). Compressed air was passed through an absolute filter into a collision atomizer which had one air jet of 0.0135 ( = 0.343 mm) in. diam. Air pressure of 35 psig produced an air flow rate of 3.5 l./min through the generator. Polydispersed aerosol produced by the atomizer was passed through a 75 X 2 cm i.d. glass tube which had heating tape wrapped around the upper 20 cm. Voltage to the heating tape was regulated to give a power input of 137 W. This heating rate was sufficient to evaporate the polydispersed aerosol so that it could recon-
DC (1) tan(a - 2 8 ) When 8 equals 60" or 45O, D, has a value of 0.58 D, or zero. A section drawing the actual impactor is shown in Figure 2. Studies by Robles ( 3 ) confirmed that jet flow penetrates into the lower region of the impactor. Figure 3 shows modifications made to the impactor. The purpose of holes in the separating plate is to allow the gas flow to leave the lower region without turning back on itself in an annular jet of relatively high velocity. This reduces the probability that material which has collected on the lower surface of the separating plate will be reentrained. The skimmer ring changes the path of a 10% split flow so that liquid which flows off the edge of the impact cone can be entrained only in the split flow. D, =
Experimental Measurement of Collection Efficiency A condensation aerosol generator similar to that developed by Liu et al. ( 4 ) and built and operated by Perez ( 5 )was used
Figure 2 . Cross section of operating impactor
I
I INCH
Figure 1. Configuration of jet cone impactor
,
Figure 3. Detail of plate holes and skimmer ring to reduce reentrainment of collected film flow Volume 10, Number 13, December 1976
1251
dense on remaining nuclei before reaching the end of the tube. Only the center portion of the aerosol flow was used for calibration of the impactor. Excess aerosol was disposed of in an electrostatic precipitator which consisted of a fine wire surrounded by a grounded copper pipe. A 5-kV potential difference between the wire and the pipe produced a corona which charged the aerosol particles, and they collected on the inner surface of the pipe. Aerosol particle diameter was measured with a Minnesota “Owl” calibrated by the Mechanical Engineering Department of the University of Minnesota (6). The “Owl” measures the angular position of the first red band in the Tyndall spectra of a monodispersed aerosol. Particle size was calculated from Kitani’s (7) equation. The mass flow rate of aerosol coming from the generator was kept constant by adjusting the amount of diluting air to keep a constant pressure, measured on a manometer, at the generator outlet. Diluting air entered through an absolute filter and was regulated by a needle valve. Pressure within the impactor was measured by manometers. Aerosol particulate material leaving the impactor was collected in filters at the impactor exits. Flow from the bottom of the impactor was regulated by a needle valve and measured with a rotameter. Total gas flow through the impactor was regulated by a needle valve and measured by a wet test meter downstream of the vacuum pump. Experimental data were obtained in the following manner. First, the impactor was assembled taking care to ensure proper spacing between jet and spindle by clamping the impactor pieces sufficiently tight that the metal surfaces seated against each other. Glass fiber filters, Gelman No. 61694 Type A, whose weights have been recorded, were placed in Millipore filter holders connected to the impactor. The impactor was then connected to the vacuum pump. The impactor was first operated with air flow but without aerosol. Pressure drop across the jet was set, and the flow rate through the impactor was measured. The rotameter for split flow control was adjusted to give either 10% or no flow as desired. After these adjustments had been made, aerosol was fed to the impactor. Aerosol flow rate was adjusted immediately after the aerosol generator was connected to the impactor, that is, jet pressure drop, total flow rate, and split flow rate were checked, and necessary changes were made to return the pressure and flow rates to the desired values. At the conclusion of a run, aerosol flow and suction were stopped simultaneously. The glass filters were removed and weighed. Next, the impactor was disassembled, and collected liquid was washed out with absolute ethanol. Liquid which collected in the upper part of the impactor and on top of the dividing plate was combined in one petri dish. Liquid which collected in the lower part of the impactor and on the bottom of the dividing plate was combined in another dish. The petri dishes were then placed in an oven a t 50 “C to evaporate the ethanol solvent. After 6 h the dishes were removed, allowed to cool, and weighed. To ensure that only ethanol evaporated and not DEP, known weights of DEP were heated at 50 “C for 18 h. No difference in weight was detected, indicating there was no significant loss of aerosol material while removing the ethanol. Before the next run, the impactor was thoroughly washed in ethanol and dried with compressed air to prevent carryover of collected aerosol from one run to the next. Prior to each set of runs, petri dishes were cleaned, dried, and weighed. The dishes were cleaned by rinsing several times with ethanol, by soaking in hot chromic acid for several minutes, and by rinsing three times with distilled water. Finally, the dishes were dried a t 110 “C for 1 h. The quantities measured for each run were pressure drop across the accelerating jet, volumetric flow rate measured a t 1252
Environmental Science 8 Technology
ambient conditions, mass of aerosol collected in the upper portion of the impactor including the filter a t the exit (TOP), and mass of aerosol collected in the lower portion of the impactor also including the filter a t the exit (BOT). To convert these data to a form which showed collection efficiency as a function of the square root of impaction parameter, several assumptions and approximations were made. Collection efficiency was calculated from the mass of aerosol collected in the upper and lower portions of the impactor by the definition: BOT TOP+BOT Implicit in this definition is the assumption that all aerosol material which entered the impactor was collected either in the impactor or in the filters a t the impactor exits. T o confirm that all aerosol material was accounted for within the impactor or on the filters at the exits, the impactor was operated for 15 min with a constant mass rate of aerosol input on each run. The total amount of aerosol collected during each of the runs was the same, within experimental accuracy, indicating all aerosol material was collected somewhere in the impactor. Differences in volumetric flow rate with constant aerosol mass rate were accomplished by varying the amount of diluting air added. Output from the aerosol generator was checked over several time intervals at a constant outlet pressure of -1 in. of water gauge, and the output rate was constant. Values of the impaction parameter, NJ,,were calculated according to its definition ( N J , Cp, VoD, 2/18 pgDc).Particle diameter, particle density, and jet diameter had the values given below:
N E
’
cm D, = (1.1f 0.02) X p, = 1.00g/cm3 D, = 0.075 in. (0.19 cm) or 0.0375 in. (0.095 cm) Average jet velocity, V,, was obtained from the volumetric flow rate Q by assuming isentropic nozzle flow and using Equation 3, (3) Pressures Po and P are the absolute upstream and downstream nozzle pressures, respectively. For simplicity and without introducing any significant error, Po was taken as 29.9 in. of mercury for all runs. P was then 29.9 - hp where hp in inches of mercury was the measured pressure drop across the nozzle. According to Fuchs (81, the slip correction factor C is best given by an empirical formula when the particle diameter is of the same order as the mean free path of the gas. For D, of the order of 1p and air of normal temperatures and pressures, the formula for C reduces to an equation proposed by Cunningham, c = 1.0 0.189 X (4) DP For the 1.1-p particles used for calibration, a value of 1.17 was used for C . Values of C and pg, used in evaluating N J ,depend on the temperature and pressure of the gas. Since the flow region where impaction occurs is close to the impaction surface, temperature and pressure will be approximately the stagnation temperature and pressure, that is, to a good approximation, the inlet temperature of 300 K and inlet pressure of 29.9 in. Hg. Experimental results are shown in Figures 4-8. Figures 4, 5, and 8 show results for an impact surface angle of 45”. Figures 6 and 7 show results for an impact surface angle of 60”. Figure 8 was constructed from data taken with the skimmer
+
10
0
oQ@aWQ o
WOODO-
08 F
z
x
06
04
02
00 0.0
Dimensionless Size
Figure 4. Comparison of experimental and calculated collection efficiencies; cone angle 4 5 O , no split flow, auxiliary holes
001 00
I
02
I
04
I
I 06
Dimensionless
' Size
0.2
'
08
1
io
I
12
me
12
Figure 6. Comparison of experimental and calculated collection efficiencies; cone angle 60°,no split flow, auxiliary holes
00 00
02
06
04
fie
Figure 5. Comparison of experimental and calculated collection efficiencies: cone angle 45', 10% split flow, auxiliary holes
I .o
0.6 0.8 Dimensionless Size
04
dT,+
08
12
10
Dimensionless Size
Figure 7. Comparison of experimental and calculated collection efficiencies; cone angle 60°, 10% split flow, auxiliary holes
ring in place, a 45' impaction surface angle, and auxiliary holes in the separating plate. Theoretical Calculation of Collection Efficiency A theoretical estimate of the dependence of collection efficiency on particle size, cone angle, and Reynolds number was obtained from a mathematical model of the flow field and particle trajectories in the flow field. The Navier-Stokes equation for an incompressible Newtonian fluid was used to model the gas flow. Particle trajectories were described by Newton's second law of motion as generalized by Euler. Drag force on the particles was given by Stokes law with a slip correction factor, C, to account for deviations from Stokes law. Solutions for the flow field (9) were obtained for jet Reynolds numbers of 1000,3000,and 10 000 and impaction surface angles of 45', 60°, and 90'. Boundary conditions used for each solution were: flat velocity profile at the nozzle entrance with no radial velocity component, zero velocity at all solid surfaces, symmetry about the flow axis with no flow crossing the centerline, and lines of constant vorticity and stream function parallel to the impaction surface a t the exit. Numerical solu-
t 00 00
I
I 02
1
l 04
l
l
l
06
Dimensionless Size
I 08
I
1
1
10
1 12
fl,+,
Figure 8. Comparison of experimental and calculated collection efficiencies: cone angle 4 5 O , 10% split flow, auxiliary holes, skimmer ring Volume 10, Number 13, December 1976
1253
tions were generated by expressing the equations in a finite difference form and using a relaxation method devised by Gosman et al. (IO). The computer programs used were developed by Marple ( I ) . Particle trajectories were obtained by integrating finite difference forms of trajectory equations in a step-by-step procedure. Although fluid velocity components were known, the trajectory equations had to be integrated simultaneously since both velocity components were dependent on particle position. Again computer programs used were developed by Marple ( 1 ) . Critical impaction parameters for various starting positions were found by trial and error. A particle was started a t the impactor entrance with a given value of N,J,and was followed through the impactor until it either impacted or reached the exit. If the particle impacted, another was started at the same position with a smaller value of N+.If the original particle did not impact, N,J,was increased for the next particle. This procedure was repeated until two values of N+were found so that for one value particles impacted and for the other value they did not. The difference between these values of N$was repeatedly halved until the critical value was known within f0.002. The particle entrance position was then changed and the process repeated. In this manner, a series of critical values of N+for various particle starting positions was found. To construct efficiency curves, a function, F(ro),of starting position was used. F:(ro)is defined as the fraction of total flow which passed through a circular area between the impactor centerline and the particle initial position. Since a particle with a value of N+smaller than N+cwill not impact, the efficiency a t any value of N+ is the fraction of flow with N J , ~ smaller than the chosen N$.If N $ is a monotonically increasing function of F ( r o ) ,the efficiency curve and curve for F(r,) are identical. Results of efficiency calculations (9) are shown in Figures 4-8 and in Tables I and 11.Each figure has curves for Reynolds numbers of 1000 (short-dashed line), 3000 (long-short-dashed line), and 10 000 (solid line). Efficiency curves were also obtained for impactor operation with 10%of the total flow withdrawn from the bottom of the impactor (Figures 5 , 7 , and 8). Two methods of calculating collection efficiency were used to model two limiting types of flows within the collection region. One case assumed there was no mixing of gas from which particles had been removed by impaction with the rest of the gas in the collection region. The other case assumed there was complete mixing in the collection region. Calculation of critical values of impaction parameter for the case of 10%split flow with no mixing was accomplished by modifying the program so that any particle which crossed the lo??streamline was considered to have been collected. For complete mixing, 10%of the particles with a given value of impaction parameter which did not impact were counted as being collected. Since the assumption of no mixing did not agree with experimental data, Figures 5 , 7 , and 8 show calculated efficiencies for 10%split flow with complete mixing assumed and jet Reynolds numbers of 1000, 3000, and 10 000.
Table 1. Comparison of Experimental and Calculated Values of ( #+o ) ' I 2 Cone angle
60' 60' 45' 45'
1254
split flow, O h
Exptl values
Calcd values, Jet Reynolds no. 10 000 3000 1000
10 0 10 0
0.385 0.367 0.415 0.388
0.356 0.350 0.393 0.375
Environmental Science & Technology
0.371 0.360 0.405 0.395
0.380 0.368 0.415 0.402
Discussion of Results Impaction surface cone angle, the use of 10% split flow or no split flow, and the presence or absence of auxiliary holes in the separating plate all appear to have an effect on the position of experimentally determined collection efficiency curves. Comparison of experimentally measured collection efficiencies for cone angles of 60" and 45' is difficult if viewed directly. To put the comparison on a uniform basis, data were as the abscissa replotted as smoothed curves with (N&V+o)1/2 rather than (N$)1/2. ( N , J , ~is) the ~ / ~value of a size parameter
Table II. Calculated Values of N7 N R =~ 1000
(NJ,)"'
0.27328 0.29790 0.31074 0.32547 0.36400 0.39091 0.41608 0.43981 0.46233 0.49968 0.59529 0.27328 0.29790 0.31074 0.32547 0.36400 0.39091 0.41608 0.43981 0.46233 0.49968 0.59529 0.26570 0.28721 0.29527 0.30567 0.33726 0.35750 0.38078 0.39882 0.42168 0.45207 0.54169 0.26750 0.28721 0.29527 0.30567 0.33726 0.35750 0.38078 0.39882 0.42168 0.45207 0.54169
NT
N R= ~ 3000 (N$)'/*
NT
h e = 10 000 (N$)"'
Cone angle = 45"; split flow = 0 % 0.03438 0.25554 0.03395 0.22978 0.06814 0.28173 0.06728 0.26456 0.10165 0.30051 0.10036 0.28448 0.13512 0.31572 0.13340 0.29527 0.26731 0.35531 0.26401 0.33726 0.39432 0.38283 0.38977 0.36614 0.51567 0.40466 0.51029 0.39290 0.63086 0.42902 0.62522 0.41415 0.73745 0.45207 0.73218 0.44158 0.83176 0.48702 0.82750 0.47401 0.90925 0.57391 0.90645 0.55733 Cone angle = 45'; split flow = 10% 0.13094 0.25554 0.13055 0.22978 0.16133 0.28173 0.16055 0.26456 0.19148 0.30051 0.19032 0.28448 0.22161 0.31572 0.22006 0.29527 0.34058 0.35531 0.33761 0.33726 0.45489 0.38283 0.45079 0.36614 0.56410 0.40466 0.55926 0.39290 0.66777 0.42902 0.66270 0.41415 0.76370 0.45207 0.75896 0.44158 0.84858 0.48702 0.84475 0.47401 0.91832 0.57391 0.91580 0.55733 Cone angle = 60'; split flow = 0% 0.03438 0.25554 0.03395 0.24300 0.06814 0.27612 0.06728 0.25858 0.10165 0.28448 0.10036 0.27328 0.13512 0.29527 0.13340 0.27893 0.26731 0.32786 0.26401 0.31324 0.39432 0.35310 0.38977 0.33957 0.51567 0.37248 0.51029 0.35531 0.63086 0.39091 0.62522 0.37873 0.73745 0.41231 0.73218 0.40077 0.83176 0.44335 0.82750 0.43265 0.90925 0.52559 0.90645 0.51051 Cone angle = 60'; split flow = 10% 0.13094 0.25554 0.13055 0.24300 0.16133 0.27612 0.16055 0.25858 0.19148 0.28448 0.19032 0.27328 0.22161 0.29527 0.22006 0.27893 0.34058 0.32786 0.33761 0.31324 0.45489 0.35310 0.45079 0.33957 0.56410 0.37248 0.55926 0.35531 0.66777 0.39091 0.66270 0.37873 0.76370 0.41231 0.75896 0.40077 0.84858 0.44335 0.84475 0.43265 0.91832 0.52559 0.91580 0.51051
0.03379 0.06695 0.09987 0.13275 0.26275 0.38799 0.50846 0.6 2295 0.73003 0.82572 0.90526 0.13041 0.16025 0.18988 0.21947 0.33647 0.449 19 0.55761 0.66065 0.75703 0.84315 0.91473 0.03379 0.06695 0.09987 0.13275 0.2 6275 0.3 8799 0.50846 0.62295 0.73003 0.82572 0.90526 0.13041 0.16025 0.18988 0.21947 0.33647 0.449 19 0.55761 0.66065 0.75703 0.84315 0.91473
a t 50% collection efficiency. For impactors of the same dimensions operated a t the same conditions, this ratio reduces to D,lD,, where D,, is the size of particle collected with 50% efficiency a t the given operating conditions. This change of abscissa scale allows comparison of collection efficiency as a sharpness of separation near the impactor cut size. These comparisons are shown in Figure 9. A curve for an impaction angle of 90' is also shown. Figure 9 indicates that sharpness of separation increases with increasing cone angle and decreases with increasing split flow. Both of these results are expected intuitively. A larger cone angle means the flow must change directions more rapidly and fewer particles of a given size can follow the flow and escape impaction. A greater percentage of split flow reduces the sharpness of separation by removing a larger number of unimpacted particles with the split flow. Difference in sharpness of separation is not large for a change in split flow from 0 to 10%.This permits an impactor to be used with 10%split flow, to carry impacted material to a secondary collection region, without significant loss in the sharpness of separation. Collection efficiencies calculated a t the same jet Reynolds number show the same trends as measured efficiencies when plotted on a function of ( N + / N $ p . Addition of auxiliary holes to the separating plate had a major effect on collection efficiency. Without auxiliary holes, impactor collection efficiencies did not approach loo%, reaching a maximum and decreasing at N+ > 1.After addition of auxiliary holes, collection efficiencies did, within experimental error, reach 100%.Lowered collection efficiencies a t higher impaction parameter (higher jet velocities), when there were no holes in the separating plate, were due to reentrainment of previously impacted material. Liquid aerosol particles form a film when they impact. When this film flows from the edge of the impact cone, the reverse high-velocity annular gas jet can carry reentrained material and film collected on the under side of the plate out of the collection region. With additional holes in the separating plate, the path of the gas flow is changed in the separating plate, and its escape velocity from underneath the plate is greatly reduced, eliminating reentrainment. A skimmer ring with 10%split flow further reduced reentrainment of collected aerosol material flowing from the edge of the impaction cone by directing it immediately into the split flow. Increased collection efficiency obtained by using a skimmer ring was primarily observed on the upper portion of the efficiency curve. Changing the jet Reynolds number changes the position and, to a lesser extent, the slope of the collection efficiency curves, as can be seen by examining calculated collection efficiency curves for several Reynolds numbers. In each of Figures 4-8, calculated efficiencies are shown for Reynolds numbers of 1000,3000,and 10 OOO. Increasing the jet Reynolds number displaces the collection efficiency curves to lower values of (N$)II2,that is, to smaller particle sizes. The displacement is relatively small, however. For an order of magnitude change in jet Reynolds number, from 1000 to 10 000, the change in (N+)I12a t 50% collection efficiency is only 5%. As the jet Reynolds number increases, the slope of the collection efficiency curve also increases slightly. Overall, the effect of jet Reynolds number on collection efficiency is small, and variation in Reynolds number during impactor operation can usually be ignored. Experimental data are superimposed on calculated results in Figures 4-8 to test the appropriateness of the model used for calculation. In all cases, agreement is quite good a t low values of impaction parameter. At high values of impaction parameter there is good agreement between experimental data and extrapolated numerical results. Agreement is not as good
for intermediate values of impaction parameter. This is most likely due to a small reentrainment effect still present in the experimental data. Comparison of experimental data, using the skimmer ring and calculated results for the same cone angle and split flow condition, Figures 5 and 8, substantiates this conclusion. Recommendations Several recommendations concerning the design, use, and interpretation of results for jet-cone impactors are made here. First, the impactor of Figure 2 is not intended for general use. I t was constructed for ease of calibration. Impactors of this type should be carefully designed for each application using the results of this work as guidelines. The specific character of the particles being collected will determine which features are necessary to retain large samples of impacted material in a collection chamber. The system of Figure 2 was not challenged with a dry monodisperse powder, although the cone was located with limits of a first reflected bounce in mind. If particles have an irregular shape, there will be a dispersion about the reflected angle. In such cases and for solid particles in general, the usefulness of this type of impactor will be highly specific, and reliability must depend on actual tests. Second, it is undesirable to operate impactors of this type in series. Some aerosol particles which did not impact collected on the walls in the upper portion of the impactor. The amount which collected on the walls could not be correlated with impactor operating conditions. Operation in series would lead to uncertainties in collection efficiencies of all stages past the first. The degree of uncertainty would increase with the number of stages. Third, criteria should be given for determining the number and area of auxiliary holes to be used in the separating plate. Since the purpose of the holes is to reduce the velocity of the gas leaving the collection region, it is only necessary to specify the additional area that the holes must provide. Their arrangement, as long as it is symmetrical, is unimportant. It is recommended that the additional area provided by the holes be of the order of 50 times the jet area. Although the gravitational force on the aerosol particles is relatively small, it is desirable to operate the impactor with flow vertically downward onto the impaction surface to take advantage of gravity. Under these circumstances the upward velocity through the
IO
F
z
08
x C
:
c
06
L
W C
E 04
-
0 0
02
00 01
03
30
IO
Reduced
Impaction
IO 0
Parameter
( N ~ I N ~ ~ ) ' ' ~
Flgure 9. Sharpness of separation-experimental
-___
90' No split flow
60' No split flow
_ _ - _60
10% Split flow
-45' No split flow
4 5 O 10% Spht flow
Volume 10, Number 13, December 1976
1255
holes and annular region should exceed the terminal settling velocity of all particles which should escape impaction. Fourth, a calibration standard for this type of impactor has to be specified. Two calibrations of jet-cone impactors were determined in this work. One was measured experimentally; the other was calculated by solving the appropriate equations for a model impactor. Since the two calibrations agree quite well, the calculated calibration given in Tables I and I1 is favored because it is specified in more detail. Calculated efficiency curves were obtained with jet Reynolds number as a parameter. Measured collection efficiencies were not taken with a constant jet Reynolds number. Calculated collection efficiencies were obtained directly as a function of impaction parameter. Since derived values of physical quantities were used to specify values of impaction parameter for experimental collection efficiencies, an additional uncertainty was introduced which is avoided by calculated results. Fifth, conditions in approach lines to the impactor should be considered when designing an impaction system and in analyzing the size distribution measured by the system. Collection of particles in approach lines can result in a different aerosol size distribution at the impactor inlet than the sampling point. Constraints must be placed on both the diameter and the length of the approach line to minimize collection of particles by gravity settling, impaction on elbows and bends, and Brownian or turbulent diffusion. Sixth, jet-cone impactors have potential uses other than measuring aerosol size distributions. For example, a single rough stage could be used to remove larger particles which would interfere with the analysis of submicron particles with sizes much smaller than those which can be measured by impactors. They could also be used in place of elutriation for the classification of nonfriable particles. Nomenclature
C = Cunningham slip correction factor, dimensionless (Equation 4)
D, = jet diameter (Figure 1) D, = particle diameter D, = distance from separating plate to spindle (Figure 1) N R= ~ V,D,pg/pg = jet Reynolds number Ns = distance from orifice to impact point divided by D,, spacing ratio N, = collection efficiency N+= Cp,VODp2/18pgDc = impaction parameter N+, = critical impaction parameter a t which N , = 0.5 V , = average gas velocity a t jet (Equation 3) 0 = impaction surface angle (half angle of apex of impact cone) pg = gas viscosity p s = gasdensity p p = particle density Literature Cited (1) Marple, V. A., “A Fundamental Study of Inertial Impactors”,
Particle Laboratory Publ. 144, Univ. of Minnesota, Minneapolis, Minn., 1970. (2) Mercer, T. T., Stafford, R. G., Ann. Occup. Hyg.,12,41 (1969). (3) Robles, J. R., MS thesis, University of Minnesota, Minneapolis, Minn., 1970. (4) Liu, B.Y.H., Whitby, K. T., Yu, H.H.S., J . Rech. Atrnos., 2,397 (1966). ( 5 ) Perez, M., MS thesis, University of Minnesota, Minneapolis, Minn., 1970. (6) Liu, B.Y.H., Marple, V. A., Yazdani, H., Enuiron. Sci. Technol., 3, 381 (1969). (7) Kitani, S., J . Colloid Sci., 15,287 (1960). (8) Fuchs, N. A,, “The Mechanics of Aerosols”, Pergamon, New York, N.Y., 1964. (9) Schott, J. H., MS thesis, University of Minnesota, Minneapolis, Minn., 1973. (10) Gosman, A. D., Pun, W. M., Runchal, A. K., Spalding, D. B., Woflshten, M., “Heat and Mass Transfer in Recirculating Flows”, Academic Press, New York, N.Y., 1969. Received for reuiew July 21,1975. Accepted June 21,1976. Financial support by the National Science Foundation under Grant GK 34414
Effect of pH on Exchange-Adsorption or Precipitation of Lead from Landfill Leachates by Clay Minerals Robert A. Griffln” and Neil F. Shimp Environmental Geology Laboratory, Illinois State Geological Survey, Natural Resources Building, Urbana, 111. 6180 1
Environmental legislation places industry in the United States in the position of having to handle and dispose of huge volumes of solid wastes, sludges, and liquid concentrates of pollutants. Landfilling is generally regarded as the most economical method for disposing of large volumes of wastes, and sanitary landfill operators are being asked to accept more industrial wastes. However, little information is presently available as to whether P b containing industrial wastes can be safely disposed of in landfills designed to contain municipal solid wastes. This paper reports the results of an investigation whose purpose was to determine the capacity of two major clay minerals for removing P b from solution and the effect municipal leachates have on this capacity at various pH values. Another purpose of the investigation was to gain insight into the mechanisms responsible for attenuation of P b as well as to evaluate the potential use of clay minerals as liners for waste disposal sites. 1256
Environmental Science & Technology
Experimental Clay Minerals. The clays used were kaolinite and montmorillonite. The kaolinite was collected in Pike County, Ill. The montmorillonite was southern bentonite from the American Colloid Co. These particular clays were chosen because their compositions more nearly represent the clay minerals found in soils than most reference clays, and they are obtainable in commercially available quantities. The clays were purified by sedimentation techniques until the
