Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 47-52
47
Measurement of Collection Efficiency of Amosite Fibers James W. Gentry" Department of Chemical Engineering, Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742
Kvetoslav R. Spurny, Helmut Oplela, and Gerhard Weiss Institut fur Aerobiologie, Grafschaff, West Germany
This paper presents a discussion of two methods for measuring the collection efficiency for fibers with nuclepore filters--gravimetric, in which the mass of particles is estimated by the measured activity of 55Feand 59Fe,and optically, in which the concentration before and after the filter is measured. Electron microscopy is used to qualitively evaluate mechanisms. Because of the greatly enhanced penetration of fibers in comparison to isometric particles, correlations based on spherical particles are not applicable. Potential industrial applications of this study lie in sampling
for potential carcinogens (asbestos), in size selective preparation of fibers for biological experiments, and in the physical separation of organic crystals.
where either monodisperse polystyrene latex particles, condensation on nucleii (La Mer generators), or evaporation of uniform droplets (Berglund-Liu) (Berglund, 1973; Liu, 1974) produce an aerosol with a very narrow size distribution. For nonspherical particles-either fibers or platelets-it is necessary to use aerosols which are produced by mechanical shear of mineral samples. When asbestos is used, the particles cover a wide range of fiber lengths and diameters as well as containing a large fraction (15-9570) of agglomerates. For the experiments used in this study, a Spurny (1975, 1976a,b) vibrating bed generator was used. With this instrument, the fiber size distribution and concentration can be influenced by three control variables: the flow rate through the vibrating bed, the frequency, and the amplitude of the vibrations imparted to the bed. Experiments at different settings of the control parameters indicate that only a rough control on the fiber distribution is possible-by increasing the amplitude of the vibration, a larger concentration of fibers as well as more very long (Lp > 20 pm) fibers can be produced. It was found that a relatively narrow range of frequencies (35-100 Hz)was suitable for amosite. Below or above these frequencies, very few fibers were generated. At the higher frequencies, we obtained no counts with a Model 202 Royco. For all the experiments reported here, t h e control variables were maintained constant. T h e flow rate through the fluidized bed was 1 L f m i n , t h e frequency was 55 H z , and t h e a m plitude was 241 pm. One of the principal advantages of the vibrating bed generator in contrast to generators used in animal inhalation experiments is the lower concentration of agglomerates. Experiments with a cascade impactor indicated that 15% to 20% of the aerosols (as measured by the radioactivity of a radioactively labeled amosite) was in the form of agglomerates. A similar result was obtained from examining the fibers with a scanning electron microscope (SEM). (Spurny, 1978). In the studies reported here, we placed an 8-pm nuclepore filter (NPF) between the test section and the generator. This is indicated in the schematic diagram of the experiment (Figure 1) and represents a modification of the experimental design in previously reported experiments. We believe that a prefilter is essential if the aerosol is to be free of agglomerates (Gentry,
Introduction Measurements of the collection efficiency using fibers rather than monodisperse, isometric particles pose a number of difficulties: (1)The fibers have a wide range of length and diameters. (2) Since the fibers are generated by mechanical shear of a mineral sample, there are considerable agglomerates. (3) Measurement techniques may be sensitive to fiber orientation. (4) Collection efficiency depends on fiber orientation. Presented in this paper are comparisons among three methods for measuring the penetration through nuclepore filters. The fibers were generated from powdered amosite. Amosite, a type of asbestos, is characterized for the most part by long, straight fibers. Unlike chrysotile (another type of asbestos), the fibers are solid and they are more nearly cylindrical than crocidolite. Nuclepore filters, because of the uniformity of pore size, present a more suitable test filter (Spurny, 1976d). The system of amosite fibers with nuclepore filters presents the simplest practical system for the filtration of nonspherical particles. There are two direct applications in which nuclepore filters would be used to filter fibers-for sampling dusts in ambient atmospheres and industrial work spaces where one would be especially interested in determining the concentration of asbestos fibers, and secondly, in the preparation of fibers of a given size for animal inhalation studies. For both applications, it is important that the filtration characteristics can be accurately predicted. Because fibers have a tendency to align in order to penetrate filters, it is necessary to perform experiments with fibers rather than depend on extrapolations from correlations based upon isometric or spherical particles. Fiber Generation The design criteria for an aerosol generator requires that the generator produce: (1) an aerosol with physical properties (size distribution, elemental composition, shape ...) that remain constant during an experiment (4-6 h), (2) an aerosol with a narrow size distribution that is free of agglomerates, and (3) an aerosol in which the concentration and/or size distribution can be controlled by control variables (flow rates, generator parameters, gas composition, etc.). For spherical or isometric particles, these criteria are usually met-especially for aerosols in the 0.1-10 pm range 0196-4321/80/1219-0047$01 .OO/O
0
1980 American Chemical Society
48
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 1, 1980
..
100
75
y c
b
U 501 LL
25
--
~
1977, 1978). Electron micrographs of the aerosol stream leaving the prefilter indicated that the agglomerates as well as many of the longer fibers were removed. Approximately 75% by mass of the aerosol leaving the generator was removed by the prefilter. Every effort was made to produce a reproducible aerosol. The operating variables-flow rate, vibration frequency, and vibration amplitude-were maintained constant. Several experiments were performed to check the reproducibility of the aerosol. Optical measurements with a Model 202 Royco showed no differences in the size distribution. Examination of particles collected on a nuclepore filter and examined with a scanning electron microscope indicated the distribution was constant within experimental scatter. There is one important difference in the experiments reported here. The procedures must be modified when a radioactively labeled fiber is used. T o avoid contamination, a different generator, prefilter assembly, and test section must be used. For these experiments, the test section consisted of the test NPF and a backing “absolute” filter consisting of a 0.2-pm membrane filter. The pressure drops across the N P F and the membrane filter were monitored during all experiments. It was found that the pressure drop across the NPF was 5 1 0 % of the pressure drop across the membrane filter. For all the experiments in which the penetration was measured optically, an unlabeled amosite was used. The penetration was obtained by measuring the ratio of concentrations in the streams passing through the filter and bypassing the filter. The particles were passed through an 85Kr source so as to minimize electrostatic effects. Previous experiments with a Model 202 Royco indicated no differences greater than experimental scatter between experiments where particles were passed through the neutralizer and those where the neutralizer was bypassed. Since the labeled asbestos was already emitting ions, we decided that it was unnecessary to pass these fibers through an ion source (Gentry, 1978b). I t should be emphasized that the mineral samplesalthough from a common source-were different for the optical measurements and those labeled with 55Fe(t,p = 2.7 years) and 59Fe ( t l j z = 45 d) (Timbrell, 1972). SEM-the one method of comparison between the fibers-produced by the two generators showed that the fiber shapes, diameters, and size distributions appeared to be the same within experimental scatter. Gravimetric Measurements
Gravimetric measurements are defined as measurements based on the mass of aerosol collected. Although the most widely used gravimetric method is based on the total weight of sample collected on a filter, other gravimetric methods would include trace element analysis by neutron
‘P
~
>
Figure 1. Schematic diagram of experiment.
.
-
-
5pm 3pm~
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 1, 1980 49
OPTICAL MEASUREMENTS
CLIMET-30p NPF AMOSITE -77
10
20
30
40
50
FLOW RATE (LITERSfMIN)
Figure 4. Collection efficiency as a function of flow rate and optical diameter (3.0-wm NPF diameter). OPTICAL MEASUREMENTS CLIMET- 50pm NPF
MOSlTE - 77
Figure 3. Clogging of nuclepore filters with amosite fibers.
diameter depends on fiber orientation as well as diameter and length. A series of experiments (Gentry, 1977) carried out with a Royco Model 202 modified so that the flow rate through the optical chamher could be increased suggest that a preferential alignment of the fibers was obtained at higher instrument flow rates. These experiments indicated the following. (1)The apparent mean diameter and standard deviation of the “optical diameter” for the fibers decreased as the flow rate increased. (The parameters were calculated under the assumption that the optical distribution was distributed as a log-normal.) (2) The standard deviation approaches the standard deviation of the “aerodynamic diameter” only at the higher flow rates (Q, > 2 L/min). (3) A similar experiment with isometric particles (monodisperse polystyrene latex aerosols) showed that the optical diameter decreased (this would be expected because of the shorter residence time) but the standard deviation increased with increased flow rate. (4) The fractional penetration through a Breslin diffusion battery (Breslin, 1971) was a unique function of the sedimentation parameter (aspredicted by Pich (1972) and confirmed by experiments with isometric particles) (Gentry, 1979) only a t the higher instrument velocities. Based on these experimental observations, the tentative conclusion was reached that the fibers acquire a preferential alignment a t higher instrument velocities. Certainly the evidence indicates that measurements at low instrument velocities are unreliable. The most important of the improvements in this series of experiments was the addition of the prefilter. Previous measurements had shown that 15-20% of the mass was in the form of agglomerates. By adding a prefilter, a test aerosol with few agglomerates was produced. In Figure 3, scanning electron micrographs of the fibers leaving the prefilter and collected on a 1-pm NPF are presented.
ob
10
20
30
40
I
50
FLOW RATE ILITERS/MIN )
Figure 5. Collection efficiency as a function of flow rate and optical diameter (5.0-prn NPF diameter).
As mentioned above, three independent flow rates are required to characterize the experiment: the flow rate through the vibrating bed (1L/min), the flow rate through the optical instrument (-4.5 L/min), and the flow rate through the test section. When optical measurements were used, the flow rate through the test filter ranged from 0.5 to 4 L/min. Since the filter cross-sectional area was 5.07 cm2, the superficial velocity ranged from 1.5 to 13 cm/s. For the measurements, a Model 208 Climet was used. Significant particle counts were obtained in three channels: 0.3, 0.5, and 1.0 pm. Approximately 68% of the counts were in the 0.3-pm channel, 25% in the 0.5-pm channel, 7% in the 1-pm channel, and less than 0.1% in the channels corresponding to larger particles. Too few counts were obtained in the channels above 1 pm in order to draw meaningful conclusions. The relative concentration of larger fibers-those with an “optical diameter” of 1pm or largel--are consistent with the frequencies counted with the scanning electron microscope (SEM). In these studies, four different pore sizes of NPF were used. These sizes were 3.0, 5.0, 8.0, and 12.0 Gm. The particles were passed through a 85Krsource in order to minimize electrostatic effects. The experimental results are presented in Figures 4-7 where the symbols represent the experimental measurements and the solid, horizontal lines represent the best fit of the experiments assuming that the collection efficiency is independent of flow rate. Several observations can he made regarding the experimental measurements. These are: (1)the collection efficiency appeared to be independent of flow rate for all three “optical” diameters; (2) the scatter in the data decreased as the pore size decreased. In regard to the second point, it should he noted that there is a time lapse of from 1to 4 min in the measure-
50
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19,
No. 1, 1980
I
u’
40 I ’ I
OPTICAL MEASUREMENTS CLIMET - 8 Opm NPF AMOSITE - 77
I
E
IOpm
30
E
IOpm
2
0
’
7
IO
IO -
OPTICAL MEASUREMENTS CLIMET - 120pm NPF AMOSITE - 77
O
05pm
i
0 3prn /
-
-
-
_ J 10
20
30
40
50
FLOW RATE (LITERS/MIN ) 10
20
30
FLOW RATE (LITERS/MIN
40
50
I
Figure 6. Collection efficiency as a function of flow rate and optical diameter (8.0-pm N P F diameter). AMOSITE FIBERS CLIMET
80 -
OI
/ ,
3 5 810 OPTICAL DIAMETER (pm)
Figure 7. Collection efficiency as a function of flow rate and optical diameter (12.0-pm N P F diameter).
ments of the concentrations after and before the test filter. This means that there is considerable uncertainty in the efficiency measurements as the penetration approaches 1. It is for this reason that there was considerable scatter in the measurements for the 12-pm NPF. This point is more clearly demonstrated in Figure 8 where the average collection efficiency is plotted as a function of “optical” diameter with the pore size of the N P F as a parameter. T h e data points are plotted with t h e assumption that the efficiency is independent of flow rate. (In light of the experimental data in Figure 4-7, this seems reasonable.) The vertical bars indicate the scatter in the measurements. These results show that scatter is less than 5% for the 3-pm and 5-pm NPF, it increases with pore size (probably reflecting the uncertainty in measurements), and decreases with increasing optical diameters. The latter effect may be due to the fact that the particle counts in the channels for 0.3 pm and 0.5 pm probably include isometric particles as well as fibers. The heavy triangles on the efficiency curves are the superimposed values obtained from the gravimetric measurements. Two points were apparent from the comparison of the gravimetric measurements presented in Figure 2 with the optical measurements in Figures 4-7. (a) The gravimetric measurements were significantly higher than the “optical” measurements as reported in any of the channels. However, the bulk of the mass in the distribution is contained in particles with optical diameters greater than 1 pm. Specifically, if the distribution of “optical diameters” were fit with a log-normal distribution, the mean volume would correspond to 1.17 pm. (This
Figure 8. Comparison of optical and gravimetric measurements (efficiency as a function of optical diameter and N P F diameter).
calculation is based on the assumption that the volume is proportional to the cube of the “optical diameter”.) (b) The “optical diameters” corresponding to the experiment gravimetric mean diameter (the solid triangles of Figure 8) are also 1.2 pm. Furthermore, there is no trend with changing pore size. (c) The measurements suggest the “optical diameter” can be correlated with the physical dimensions of the fibers and indicate that the contribution of very small fibers to the gravimetric measurements is slight. Electron Microscopy Previously reported results have demonstrated that electron microscopy can be useful in determining the morphology of fibers, in determining the elemental composition when used in combination with X-ray fluorescence, in demonstrating the enhanced penetration of fibers through NPF, and illustrating that fibers collected on the first stages of impactors are mostly agglomerates whereas those fibers collected on the last stages are single particles (Spumy, 1976~). The use of the SEM in the study of the collection efficiency of NPF is demonstrated with the fibers collected on a 5.0-pm NPF. The fibers passing through the filter were collected on a 1-pm NPF. The test aerosol consisted of amosite which was prefiltered through an 8.0-pm N P F to remove the agglomerates. The flow rate through the filter was 3.1 L/min. The collection efficiency measured by using a radioactively labeled amosite was 63.5%. Photographs were taken through the SEM of both the test filter and the backing filter. Approximately 100 fibers were counted and sized for each surface. In Figures 9 and 10, the cumulative fraction of fibers below a characteristic size are plotted as a function of diameter and length, respectively. The two curves represent the fibers collected on the test filter and those collected on the backing filter (the fibers passing through the filter). In Figure 9, the cumulative fraction is plotted as a function of fiber diameter. Below 0.6 pm, there is no difference between the fraction collected on the test or on the backing filter. For diameters greater than 0.6 pm, there is considerable discrepancy with significantly more thick fibers (dp> 1.0 pm) collected on the test filter than on the backing filter. In Figure 10, the cumulative fractions of fibers collected on the two surfaces are plotted as a function of fiber length. There is a slightly greater fraction of longer fibers collected on the test filter. This is probably due to the 10% of the fibers with lengths less than 1 pm which penetrated the test filter. The important conclusions to be drawn from these figures are that: (1) the length of fibers appears to have little influence on the collection efficiency, and ( 2 ) the diameter of the fibers plays a central role with very few fibers with diameters
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 1, 1980 I
NUMBER LXSTRIBUTION (MASS) IO0
51
STUDY
ELECTRON MICROSCOPE
I IO
-
IO
r
It
01
I
I
IO
90
CUMULATIVE
L.
IO
DIAMETER
NUMBER
99
I
99.99
FRACTION
Figure 11. Cumulative number distribution as a function of mass, before and after test filter (SEM).
STUDY
02
5pm NPF BACKING FILTER
14
(pm:
5pm NFF FILTER BACKING FILTER
loo/
Figure 9. Cumulative fraction of fibers with diameters less than D , before and after test filter (SEMI. 100-
~~
5 p r n “IF EI4CKING FILTER
MASS DISTRIBUTION STUDY @I
I
I
IO
CUMULATIVE
90
99
9999
MASS FRACTION
Figure 12. Cumulative mass distribution as a function of mass, before and after test filter (SEM).
10
10 LENGTH (urn)
Figure 10. Cumulative fraction of fibers with lengths less than L , before and after test filter (SEM).
greater than 1 pm penetrating the NPF. In Figures 11 and 12, the cumulative number distribution and cumulative mass distribution are plotted as a function of particle mass. (In the graphs, a value of 1 on the ordinate corresponds to a prolate spheroid with a mass g.) For a log-normal distribution, the curves of 2.0 X would yield a straight line with a 50% intercept corresponding to the mean (diameterand the slope proportional to the standard deviation. Several points should be noted regarding these two graphs. First, the mas3 mean diameter of fibers collected on the test filter is large, having a value of 1.2 pm. (An aspect ratio of 10 corresponding to the mean value of the fibers counted by SEI? was assumed for this calculation.) This indicates why one would expect the gravimetric measurements to be disportionately influenced by the larger fibers which make up less than 5% of the fibers counted. Secondly, the narrowing of the decrease of the slope of the distributions measured on the backing filter in comparison to the test filters indicate that the fiber distribution passing through the filter is narrower than the initial distribution. Discussion of Results The principal conclusions drawn from this study were the following: (1)the collection efficiency of amosite fibers
by nuclepore filters is controlled by interception with diffusion and interception playing a minor role; ( 2 ) the fibers show a strong tendency to align with their major axis perpendicular to the face of the filter. The key variable in determining the probability of penetration is the fiber diameter; (3) there appears to be a basis for comparison of optical and gravimetric measurements; and (4)refinements in the experimental procedure-including the use of a prefilter before the test filter, improved flow controls, and a higher instrument flow rate-are requisites for meaningful experimental measurements with nonspherical particles. The main reason that we believe direct interception to be the principal mechanism of collection is that the collection efficiency is independent of flow rate. If inertial impaction were the primary collection mechanism, the collection efficiency should increase with flow rate. If diffusion were the predominant mechanism, the collection efficiency should decrease with flow rate. (In the case of diffusion, the length of the pores in the NPF and the fiber size suggest that diffusion would have a negligible effect. The independence of flow rate was observed for all sizes of fibers with the proviso that the “optical diameter” is proportional to the fiber size.) A second reason that we believe interception to be the dominant mechanism is that SEM showed that the thicker fibers (those with diameters greater than 1.0 pm) were preferentially removed by the 5.0-pm test filter. Because of the difference in the distribution of fiber diameters collected on the test filter and the backing fiiter, it was felt that the diameter was the key parameter in collection by interception. A limited number of SEM which indicated the fibers aligned perpendicular to the face of the filter was consistent with this model as was the fact that the only way fibers 10 to 20 pm in length could penetrate the openings was if they were aligned. I t is im-
52
Ind. Eng. Chem. Prod.
Res. Dev., Vol. 19, No. 1, 1980
portant to note that there was little difference in the size distribution according to length for the fibers collected on the filter or passing through the filter. The supporting evidence for the relationship between the collection efficiency measured gravimetrically and optically is derived primarily from Figure 8. The important point is that the "optical diameter" yielding the same collection efficiency as the gravimetric measurements is independent of the NPF pore diameter having a constant value of 1.2 pm. One limitation with our present measurements is that the channels for the "optical diameters" were all lower than 1.2 pm. However, the optical measurements indicated a mass mean diameter of 1.18 pm while the SEM indicated a mass mean diameter of approximately 1.2 pm. Consequently, one would expect the gravimetric efficiency to be influenced by the larger fibers. Certainly more measurements-especially with long, narrow fibers-are needed before a quantitative relation between optical and gravimetric measurements can be developed. John (1978) has proposed an expression to account for interception having the form
Ef = [ ~ N -RN R ~ ] ~ / ~ based on the flow model of Happel and Brenner (1973). NR is the ratio of particle to pore diameter. For the four larger pore sizes, one obtains an average particle size of 2.4 pm. This value is substantially larger than the mass mean fiber diameter. The mass mean aerodynamic diameter of the fibers used in our study would be approximately 3.0 ym although (Balzer, 1970; Stober, 1970) there is no reason for believing that the collection efficiency would correlate with the aerodynamic diameter. If the expression were used, the particle diameter would be 1.8pm. The first expression gives a smaller standard deviation. The improvements in the experimental procedure were demonstrated by the absence of agglomerates from the samples examined with the SEM. The necessity for higher flow rates through optical instruments had been described previously. Our results do indicate less scatter between replicate measurements. Conclusion This paper presents the results for a series of experiments in which the collection efficiency of amosite fibers is measured both gravimetrically (using a radioactively labeled amosite) and optically. Examinations of the fibers with a scanning electron microscope indicated that after prefiltration with an 8-pm NPF, the test aerosol was free of agglomerates. The significant findings for this study include the following: (1)The collection efficiency of NPF is independent
of flow rate. Both optical and gravimetric measurements indicated the same conclusion. (2) The collection efficiency measured gravimetrically correspond to an optical diameter of 12 pm. (3) Examination with the electron microscope of particles collected on the test filter and backing filter indicated little change with length. On the other hand, fibers with larger diameters were preferentially collected. (4) It was necessary to prefilter the aerosol before the test section in order to remove agglomerates. ( 5 ) It is necessary to control the flow rate through the vibrating bed, through the test filter, and through the optical sensor independently. In addition, our results suggest two other tentative conclusions: (1)The primary mechanism of collection is interception with the fiber diameter being the most significant parameter. (2) Measurements of the collection efficiency can be correlated with gravimetric measurements provided that the instrument flow rate is in excess of 3 L/min. Acknowledgment J. W. Gentry wishes to acknowledge financial support under National Science Foundation Grant No. ENG 78 00738. In addition, the computer time for this project was supported in full through the facilities of the Computer Science Center of the University of Maryland. Literature Cited Balzer, S. L., Trans. Ind. Health Found. Annual Meeting, 44, 87 (1970). Berglund. R. N., Liu, B., Envlron. Sci. Techno/., 7 , 147 (1973). Breslin, A. J., Guggenheim, S. F., George, A. C., Staub, 31, 313 (1971). Dzubay, T. G., "X-ray Fluorescence Analysis of Environmental Samples", Ann Arbor Sci. Publ., Mich., 1977. Gentry, J. W., Spurny, K. R., Aerosole in Naturwlssenschaft Medizln und Technlk, Proc. 4th Tagung, GAF, 36-42 (1977). Gentry, J. W., Spurny, K. R., J. ColloidInterface Sci., 65, 174 (1978a). Gentry, J., Spurny, K.. Weiss, G., Opieb, H., "Atmospherlc Pollution 1978", M. Benarie, Ed., pp 107-110, Elsevier, 1978b. Gentry, J. W., Shen, P.:,Spurny, K., J. Aerosol Sci., 10, 113-122 (1979). Happel. J.. Brenner, H., Low Reynolds Number Hydrodynamics", 2nd ed, pp 150-153, Noordhoof, Leyden, 1973. John, W., Reischl, G., Goren, S..Plotkin, D., Atmos. Environ., 12, 2015-2019 (1978). Liu. B. Y . H., Berglund. R. N., Agarwal, J. K., Atmos. Environ., 8 , 717-732 (1974). Pich, J.. J. Aerosol Sci., 3, 351 (1972). Spurny, K. R., Boose, C., Hochrainer, D., Staub-Reinhalt Luft, 35, 440 (1975). Spurny, K. R., Boose, C., Hochrainer, D., Mnig, F. J., Zentbl. B a M . Paras!%&, Abt. I. Orig. B . , 161, 362 (1976a). Spurny, K. R., Boose, C., Hochrainer, D., Monig, F. J., Ann. Occup. Kyg.. 19, 85 (1976b). Spurny. K. R., Stober, W., Opiela, H., Weiss, G., "Atmospheric Pollution", M. Benarie, Ed., pp 459-469, Elsevier, Amsterdam, 1976c. SDurnv, K. R., Stober, W., Ackermann, E. R., Lodge, J. P., Spurny, K. R., Jr., ' J . Air Pollut. Contr. Assn., 26, 496-498 (1976d). Spurny, K. R., Gentry, J. W., Stober, W., Fund. AerosolSci., 257-324 (1978). Stober, W., Flachsbart, H., Hochrainer. D., Staub, 30, 277 (1970). Timbrell, V., "UICC Standard Reference Asbestos Samples", 1972.
Received f o r reuiew June 13, 1979 Accepted August 28, 1979
A presentation based on the work presented in this paper was given at the Colloid Symposium in Rolla, Mo., June 11-13, 1979.
