current research Aerosol Filtration by Means of Nuclepore Filters Structural and Filtration Properties KvCtoslav R.
James P. Lodge, Jr., Evelyn R. Frank, and David C. Sheesley
Laboratory of Atmospheric Sciences, National Center for Atmospheric Research, Boulder, Colo. 80302.
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The studies of Fleischer, Price, and their collaborators led to the development of a new type of filter material, a porous analytical filter trade-named Nuclepore. This resembles membrane filters in many of its properties, such as the relationship between air flow rate and pressure drop. One previously developed theory of filtration by membrane filters approximated their structure as a group of parallel capillaries. This model appeared to be an exact description of the structure of Nuclepore filters. Accordingly, the filtration properties of Nuclepore were determined and compared with an extension of the previous filtration theory. Both the initial performance and the change in properties caused by previously collected material were adequately accounted for by the theory.
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n the past. those concerned with the collection and analysis of aerosols have had only two types of filters available to them, each with its advantages and limitations. Fiber filters. the true filter papers, have been known longest. are the least expensive, and are probably the most widely used. In the past 20 years, the so-called membrane filters have been available (Spurn$. 1967) and their properties and uses have been described and verified (Spurn$. 1965. 1966; Spurn9 and Pich. 1964, 1965). While studying radiation damage in solids. a group at the General Electric Laboratories in Pleasanton. Calif.. recently developed a technique to produce small holes in susceptible material (Fleischer, Price. et al., 1965; Price and Walker. 1962). An obvious application was the preparation of a new type of porous filter. A thin (lO-pm.) sheet of polycarbonate was placed in contact with uranium sheets and put into a nuclear reactor. The neutron flux caused fission of uranium-235, and the fission fragments bombarded the film? leaving tracks of damage. Subsequent treatment in an etching bath removed the damaged material and enlarged the pores to a degree determined by the type of bath reagents and the duration
Visitor. Atmospheric Chemistry Program, NCAR, 1966/ 1967. Permanent affiliation, Czechoslovak Academy of Sciences. Institute of Physical Chemistry, Prague.
and temperature of treatment. Suitable geometry \\as devised to limit the effective fragments to those traveling nearly perpendicular to the film surface. The material, christened Nuclepore. is now available M ith reasonably uniform graded pore diameters between 0.5 and 8.0 Ilm. in a matrix nith a density of 0.95 gram per cc. Many models used in filtration theory-e.g., Spurn? and Pich. 1965-have approximated the structure of membrane filters as bundles of parallel capillaries. Recognizing the close congruence between these models and the actual structure of Nuclepore filters, we have studied the filters both theoretically and experimentally. Whereas previous membrane filters had three separate structures (upper surface, interior. and lower surface. Figure 1 ) . the new material, while the pore location is random, has a simple, uniform structure throughout (Figure 2 ) . The upper surface is smooth, the pores are circular in cross section, and, of importance in working with the filters. the matrix is mechanically very strong. Theory In view of the resemblance between the Nuclepore structure and that assumed in the filtration theory of Spurn? and Pich ( 1964), we have applied and extended this theory to cover the time dependence of pressure drop and efficiency-Le., the effect of collected particles on further filter performance. As in previous studies (Hampl and Spurn$. 1966), it was necessary to consider three regimes of gas flow through the pores of the filter. If, as usual. the Knudsen number is represented by
Kn = l / R where 1 = mean free path in centimeters and N = pore radius in centimeters, then for the aiscous flow region, where Kn 0.01, the equation of Twomey (1962) was used: err = I - 0.81904 exp( -3.6j68ND)
;
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0.09752 exp( -22.3045~7,) - 0.03248 exp( -S6.95ND) - 0.0157 exp(-107.6ND) - , . , (13) Once again, variation of efficiency with time was computed by substitution of the appropriate R , values for R , in Equation l l . The partial efficiency of interception, E K , was similarly obtained by the expression of Natanson (1957) : ER
N R (~ NE)
(14) (15)
Nx= r / R o
where
When A;J, 1 , the efficiency is obviously 1, and the formula is invalid. Efficiencies of filters that have collected particles already- on them are again obtained by substituting R , from Equation 4 or 5 for R , in Equation 15. The separate partial efficiencies were combined by an extension of the argument of Spurn); and Pich (1965). However. this overestimated the total efficiency. E. Investigation showed that this overestimate was caused by the terms containing ER. Hence, a u-eighting factor, 8, which was then evaluated empirically, was included in those terms. Finally. negligibly small terms were eliminated. The final expression was E
t,
- ET,
t8
€ -~ E i
‘
ED
- 8Ei
‘ ER
(16)
The parameter 6 had a value of 0.15. implying that the actual contribution of interception to the collection process was only 15% of that given by theory.
Figure 3. Measured pressure drop as a function of face velocity for Nuclepore filters
. . . .-Theoretical curves, derived from Hagen-Poiseuille law 1. R
2.
= 2.5 pm.
??= 4.0
ym. 3. R = 1.0pm.
4. 77 = 0.5 pm. 5. R = 0.4 pm. 6. = 0.25 pm.
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Volume 3, Number 5, May 1969 455
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4
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7
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.+.-.-e-*
____---
2
----: - *
I. ...............................................
O
I
2
4
3
9
5
1
I 1
6
t(10~sec)
Figure 4. Computed pressure drop change with filtration time 1. -Solid 2.
3. 4. 5.
6. 7, 8. 9.
........ Liquid
.........
i
" = OS4
q =5
I
Solid R o = 0.5 pm. Liquid __ R o = 0.25 pm.; r = 0.1 urn. -. , - R . = 0.25 em.; r = 0.001 pm. R o = 0.4 pm.; r = 0.1 pn, - - - R O = 0.5 pm.; r = 0.1 pm. ..........R o = 1.0 pm.; r = 0.1 pm.
..........
cm./sec.
r=O*lpm. '
Table I. Efficiencies of Three Different Nuclepore Filters Aerosol Filter 1 Filter 2 Filter 3 R = 0.25 pm, R = 0.40 pm. R = 0.50 pm. Particle L = 13 pm. L = 18 pm. Radii, L = 10 pm. P = 0.05 P = 0.03 P = 0.04 pm
0.001000 0.001 359 0.001 848 0.0025 12 0.003415 0.004642 0.0063 10 0.008577 0.01 1659 0.015849 0.021544 0.029286 0.03981 1 0.0541 17 0.073564 0.100000 0.135936 0.184785 0.25 1 189 0.341455 0.464 159 0.630957 0.857696 1.16591 4 1.584893 2.1 54435 2.928645 3.981072 5.41 1695 7.356423 10.000000 y = 7 . 5 cm./sec
T=291"K.
1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 0.994024 0.884257 0.801431 0.772089 0.798300 0.869800 0.962894 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo ii =
1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo
1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 0.962285 0.833821 0.668685 0.523636 0.419264 0.3561 89 0.331 043 0.341560 0.383730 0.461439 0.572563 0.709916 0.851971 0.9605 88 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo
1.oooooo 1.oooooo 0.898276 0.747581 0.609221 0.512955 0.463552 0.459291 0.498257 0.576369 0.689913 0.822648 0.942284 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo 1.oooooo
182.7 micropoises
c = 2 2 g cc. = 0 065 turn
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no = 105 em.-? Solid aerosol
pore length for various combinations of pore size and particle size. Figure 9 further explores the effects of flow rate. Figures 10 and 11 show the influence of air temperature and pressure. Finally, Figure 12 explores the time dependency phenomenon. assuming a particle concentration 10.7 cm.'3 Experimental Filter structure was studied by light and electron microscopy. For electron microscopic examination, small portions of filter surface were coated with silicon monoxide, which was then shadowed with chromium. The filter was subsequently dissolved with chloroform (Frank, Lodge, et al., 1968). Pore sizes and numbers were determined from micrographs of these replicas. Weights were measured aith a Cahn electrobalance. Experimental pressure drop and efficiency data were obtained with the apparatus shown in Figures 13 and 14. For measurement of pressure drop, air entered manifold 1 (Figure 13), passed through drying tubes 2 and 3, was filtered by F , , a membrane filter, and passed to manifold 4, manifold 10, and finally to filter holder, H. The pressure 456 Environmental Science & Technology
Figure 5. Computed dependency of efficiency on particle size and face velocity With increasing air velocity, efficiency minimum moves in direction of small particles and lesser efficiencies 1. q = 0.1 cm./sec. 2. q = 1.0 cm./sec. 3. q = 5.0 cm./sec. 4. q = 15.0 cm./sec.
/
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2
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Figure 8. Computed dependency of efficiency on pore length Figure 6. Computed dependency of efficiency on particle size and pore size With increasing pore size, efficiency minimum decreases and curves widen 1. R. = 0.4 pm.
3. R o = 1.0 Fm. 4. 5.
R,,= 2.5 Fm. R, = 4.0 pm.
(
Efficiency increases with filter thickness, most pronounced for smallest particle sizes 1. R O = 0.4 pm.; r = 0.1 pm. 2. R O = 1.0 pm.; r = 0.2 pm. 3. R O = 2.5 pm.; r = 1.0 pm. 4. R O = 4.0 pm.; r = 1.0 pn.
s = 21 g.jcc.
2
3
\
q
P Figure 7. Computed dependency of efficiency on filter pososity Not a parameter under control of experimenter, but largelj go\erned by pore size 1. R o = 0.4 pm.; r = 0.1 pm. 2. R. = 1.0 pm.:I' = 0.2 pm. 3. R o = 2.5 em.; r = 1.0 pm. 4. R o = 4.0 em.;r = 1.0 um.
drop Mas measured on manometer M Z . flou rates uere shown by the various rotameters in the system. and system pressure uas given by manometer M I . The time dependency of pressure drop was determined by adding aerosols from generators 5 to 9, discussed in more detail belou. For the measurement of efficiency, radioactively labeled aerosols were used (Spurn9 and Hampl, 1965. 1967; Spurn); and Lodge, 1968). Generator 5 contained a plati-
[cm/secJ
Figure 9. Computed dependency of efficiency on flow rate As particle size increases, efficiency minimum moves in direction of smaller air velocities, while value at minimum increases 1. r = 0.001 pm. 2. I = 0.005 p.m. 3. r =0.01 pm. R,, =. 4.0 pm. 4. r = 0.05 pm. P = 0.05 5. r = 0 . 1 pm. 5 = 2.2 g./cc. 6. r = 0.5 pm.
1
7. r = 1.0 urn.
r
num uire filament labeled with a mixture of platinum isotopes by neutron activation. Generator 6 produced a pyrophosphoric acid aerosol containing phosphorus-32. Selenium and selenium oxide aerosols, labeled with selenium-75. were produced in generator 7. Generator 8 was used to produce oil aerosols on nuclei of sodium chlorideNa2*, produced by generator 9. The output of generator 9 could also be used directly. Details of all the above genVolume 3, Number 5, M a y 1969
457
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c
1.0 I
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0.8
0.6
0.4
2
31
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j I
I
400
I
I
800
T
l I
1200
I
1,
1 1
I
1600
[OK1
Figure 10. Computed effect of temperature on efficiency Curves again have a minimum, position of which is a function of pore and particle size 1. R . = 2.5 pm.; r = 1.0 pm. 2. Ro = 0.25 pm.; r = 0.1 pm. 3. R o = 1.0 pm.; r = 0.2 pm.
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0 8 1
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I
O
h
1
i
-
E 04
-
I
I
I
2
3
I 4
I 5
I
6
Figure 12. Change in efficiency of filter with sampling duration (time dependency)
6.
-
I
0
t(103s e d
1. 2. 3. 4. 5.
06-
tI
7. 8.
1 1 ::idid1 Solid 1 Liquid Solid Liquid Solid Liquid
q = 1 cm./sec.
r = 0 . 2 pm. q = 10 cm./sec. r =0.1 pm.
R = 0.5 pm.
q = 25
R = 1.0 /anLm. r = 0.1 pm. no = 1 6 5 em,
cm./sec.
q=l~crn./sec.
no = 10s c m - 3
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1 Figure 11. Computed effect of atmospheric pressure on efficiency For aerosol sampllng in upper atmosphere, Blhatlon efficiency is much higher 1. R . = 2.5 pm.; r = 1.0 pm. 2. R o = 0.25 pm.; r = 0.1 pm. 3. R . = 1.0 bm.; r = 0.2 &m.
erators have been given by Spurn9 and Lodge (1968). The interconnections shown permitted use of any of the generators, all of which connect to manifold 10, with similar flows and the same flow measuring and pressure control devices. The interconnections also provided dilution air to inhibit coagulation of the aerosol. Aerosols used experimentally were obtained from any one of the generators in Figure 13, the criterion being size and phase-Le., liquid or solid. Liquid aerosols varying from the pyrophosphoric acid of mean radius 0.02 pm. and up in size were obtained from generator 6 or 8. Solid aerosols were generated in 5 , 7, and 9 and had a mean 458 Environmental Science & Technology
radius of 0.043 lLm. for the platinum oxide aerosol, 0.17 pm. for the selenium aerosol, or a variety of sizes with the NaCl generator (9, Figure 13). The size and density of the aerosol are indicated in any figure where experimental data are shown and this information is applicable. After passage through the measuring filter holder, H,the gas stream passed on to manifold 11, and then to safety filter F,, a membrane filter. The pressure drop of this portion of the system was balanced by pump P, which served to keep the pressure upstream of H near atmospheric. Details of filter holder H are shown in Figure 14. The aerosol entered at A , and the filtered air stream left as indicated. M is the point of attachment of the low-pressure leg of the manometer, M,, in Figure 13. The aerosol stream passed through the filter being tested, F, and then through a membrane filter, F1. Filter F1, a Millipore grade HA, was assumed to collect all particles passing through F. The two filters were monitored by end-window Geiger-Muller tubes, C , and Cp, which registered on the dual channel impulse counter, IC. Since geometry was identical in the two channels, the efficiency of filter F was given directly by the two counts, as indicated. The recorder, W , also permitted calculation of changes in efficiency with time.
H
Figure 13. Schematic diagram of experimental apparatus for pressure drop and efficiency determination 1, 4, 10, 11, Manifolds 2. Silica gel drying columns 3. Drierite drying columo Fl,FI,absolute filters; M I , Ma, M 3 , M 4 , Manometers 5. Platinum aerosol generator 6. Pyrophosphoric acid aerosol generator I. Selenium aerosol generator 8. Oil aerosol generator 9. Salt aerosol generator; P, Pump; H, filter holder (shown fn detail in Figure 14). Stopcocks and rotameters are denoted by usual symbols.
Results and Discussion In contrast to membrane and fibrous filters. the Nuclepore filter has an extremely narrow range of pore sizes. Figure 15 shows measured size distribution curves for the filters used in this study, together with the curve for a comparable membrane filter (Synthesia VUFS) . Table I1 includes the various measurements of mechanical properties of Nuclepore filters, together with similar values, where available and/ or applicable, for membrane and fibrous filters. From these data it can be seen that several of the pore size grades are well within the usual criteria of uniformity-e.g., geometric standard deviation 1.lo. Table I1 shows that the porosity of Nuclepore is an order of magnitude lower than that of membrane filters; yet, as
