Anomalous filtration of solid particles by Nuclepore filters

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Afmosphe~icEn~ironmenrVol. 12. pp. 1555- 1557. Q Pergamon Press Ltd. 1978.Printed in Great Britain.

[email protected]@io

SHORT COMMUNICATION ANOMALOUS FILTRATION

OF SOLID PARTICLES FILTERS*

W. JOHN and G.

BY NUCLEPORE

REISCHL

Air and Industrial Hygiene Laboratory, California Department of Health, Berkeley, CA 94704, U.S.A.

S.

COREN

and D.

PLOTKIN

Department of Chemical Engineering, University of California, Berkeley, CA 94720, U.S.A. (First

received 21 November 1977 and infinal form

13 January

1978)

Abstract-Because ofrecent interest in the use ofstacked Nuclepore filters as a particle size-selective sampler, the filtration efficiency of 8 pm pore Nuclepore filters has been measured for laboratory aerosols. Lower efficiencies are obtained for solid particles than for liquid particles, probably due to particle bounce. Interception appears to be the dominant filtration mechanism.

The use of a sequence of Nuclepore filters as a particle sizeselective sampler was suggested by Spurny et al. (1969 b) and studied by Melo and Phillips (1974). Recently, a two filter sampler for the separation of respirable and non-respirable particles in ambient air has been investigated by Parker et al. (1977 b) and Cahill et al. (1977). This attractively simple device produces samples suitable for X-ray fluorescence analysis. In attempting to validate the sequential filtration sampler, we have measured the filtration efficiency for laboratory aerosols.

aerosols were produced by a ~rglund-Liu vibrating orifice generator. The aerosols included viscous liquid particles, 75’;/, v/v glycerol, 25% v/v uranine and solid, spherical particles of methylene blue. The loaded filters were sonicated in alcohol-water solution; uranine was quantitated on a fluorimeter and methylene blue on a spectrophotometer. The efficiency of the 8 pm filter was determined by dividing the deposit on that filter by the sum of the deposits on both filters and the supporting grid for the 8 pm filter. The deposit on the plastic supporting grid, obtained by washing it with solvent, was approx 10% of the total, depending on particle size. The fitter loadings were kept relatively light to avoid loading problems.

MEASUREMENT

We have investigated the sequentia1 fiItration unit used by Cahill et al. (1977). consisting of an 8 pm pore size Nuclepore filter followed by a 0.4pm pore size filter, both held in a double 47 mm filter holder*. Monodisperse, neutralized * Work partially supported by the California Air Resources Board. t Nuclepore Corp., Pleasanton, CA, U.S.A.

RESULIS

AND DISCUSSION

Figure 1 shows that the collection efficiencies of the 8 pm Nuclepore filter for methylene blue and glycerol-uranine particles agree for small particle size but the efficiency for methylene blue falls significantly below that for glyceroluranine with increasing particle size. Finally, both efficiencies approach 100% as the particle diameter approaches the pore diameter. Figure 2 shows that the methylene blue efficiencies

lOOr

I YX

oGLYCEROL-URANINE 6c.-

X MRTRYLENE BLUE

60 -

I

0

I

I

2

I

I 4

I

I

I

6

AERODTRAMIC PARTICLE

DImER,

I 6

I

I

10

pm

Fig. 1. Measured filtration efficiency of 8 pm pore size Nuclepore filters for methylene blue particles compared to that for glycerol-uranine particles. The Bow rate was 5 i min- ’ (face velocity 6 cm s-l, based on an exposed area 42 mm dia.). 1555

1556

Short Commumcation

agree better with the glycerol-uranine effictencies when the face velocity is reduced from 6 cm s ’ to 2.4cm s- ‘. The lower efficiency for methylene blue may be explained by particle bounce. The observed increu.~e of efficiency with ~lrcrrtrsiny face velocity is consistent with this hypothesis. Microscopic examination showed that the glycerol-uranine particles were deposited mostly on the periphery of the pores. Thus, after bouncing, the methylene blue particles would have a high probability of penetrating the filter. The possibility of particle bounce was discussed by Parker rr trl. (1977). Some evidence of particle bounce, although inconclusive, was obtained by Parker and Buzzard (1977). Earlier. Spurny and Madelaine (1971) suggested that particle bounce might explain their observation of lowered efficiency for 0.357 pm latex particles at high flow rates. The glycerol-uranine efficiencies are well-fitted by a calculation using interception alone. For uniform flow. we calculate the efficiency for interception to be: I: = 2R - R=

the filtratton efhciency. although It 15 surprisingly close considering the simplicity of the calculation. A better fit is obtained by using a more realistic non-uniform flow from Happel and Brenner (1973) who calculated creeping flow through an orifice using oblate spheroidal coordinates. From the stream function. we obtain for the interception efficiency:

,: ; (:R - R2)J:,

(1)

where R is a dimensionless parameter defined as the ratio of the particle diameter to the pore diameter. Equation (I), shown as the dashed line in Fig. 3. somewhat overestimates

(AZLPM METHYLENE BLUE / x 5LPM GLYCEROL-URANINE5LPM-

1

I

0

I

I

2

I

I

4

I

I

I

I

8

6

I

10

AERODYNAMIC PARTICLE DIAMETER, )m

Fig. 2. Filtration efficiency for methylene blue particles taken at two flow rates, 2 I min I (triangles) and 5 I min-’ (crosses). The line representing the glycerol-uranine data is repeated from Fig. 1.

80 -

0

2

4

(2)

Shown as the solid line m Ftg. 3. Equatton (2) is a very good fit to the glycerol-uranine points. The more realistic flow near the hole edge improves the fit for small particles which must pass near the hole edge to be intercepted. This may explain the overestimation of interception obtained by Spurny et al. (1969a) who used a uniform flow calculation. The apparent absence of impaction in the present experiment may be explained qualitatively from a consideration of the streamlines. As a particle approaches the filter from a distance, it follows streamlines which curve toward the pore. In the region near the pore, the streamlines curve in the opposite direction. This tends to cancel drift across streamlines caused by inertia. an effect discussed by Smith and Phillips (1975). We suppose that the cancellation is fortuitously nearly complete for the present experimental parameters. Additional

6

8

GEOMETRICAL PARTICLE DIAMETER, ,uum

Fig. 3. Comparison of measured efficiency for glycerol-uranine particles (data points) and theoretical calculations ofinterception. Geometrical particle diameter, the appropriate variable for interception, is used as the abscissa. The dashed line is calculated for uniform flow [Equation (l)], the solid line for non-uniform flow [Equation (2)] through an orifice.

Short Communication experiments are necessary to confirm the absence of impaction and to determine its possible importance at higher flow rates.

The present work demonstrates that particle bounce is an important phenomenon in filtration by large pore size Nuclepore filters and that interception is apparently the dominant filtration mechanism. For the use of sequential filters in atmospheric sampling, these results imply that the particles are sized geometrically rather than aerodynamically. Particle bounce could lead to serious errors. In a more general sense, the present results show that additional work is necessary in order to improve understanding of the filtration process with Nuclepore filters. We are currently extending our investigation.

Department of Chemical Engineering. University of Calijbrnia, Berkeley, CA 94720, U.S.A.

Feeney P. J., Flocchini R. G., Goodart C., Shadoan D. J. and Wolfe G. W. (1977) Analysis of respirable fractions in -atmospheric particulates via sequential filtration. J. Air Pollut. Control. Ass. 21, 675-678.

CONCLUSIONS

Air and Industrial Hygiene Laboratory, California Department of Health, Berkeley, CA 94704, U.S.A.

1557

W. JOHN G.

REISCHL

S. GOREN D. F%OTKIN

REFERENCES

Cahill T. A., Ashbaugh L. L., Barone J. B., Eldred R. A.,

Happel J. and Brenner H. (1973) Low Reynolds Number Hydrodynamics, 2nd revised Edn, pp. 150-153. Noordhoff International, Leyden, The Netherlands. Melo 0. T. and Phillips C. R. (1974) Aerosol-size spectra by means ofmembrane filters. Environ. Sci. Technol. 8,67-71. Parker R. D. and Buzzard G. H. (1977) A filtration model for large pore Nuclepore filters. (Unpublished). Parker R. D., Buzzard G. H., Dzubay T. G. and Bell J. P. (1977) A two stage respirable aerosol sampler using Nuclepore filters in series. Atmospheric Environment 11, 617-621. Smith T. N. and Phillips C. R. (1975) Inertial collection of aerosol particles at circular aperture. Environ. Sci. Technol. 9, 564-568.

Spurny K. R., Lodge J. P., Jr., Frank E. R. and Sheesley D. C. (1969a) Aerosol filtration by means of Nuclepore filters: structural and filtration properties. Enoiron. Sci. Technol. 3,453-464.

Spurny K. R., Lodge J. P., Jr., Frank E. R. and Sheesley D. C. (1969b) Aerosol filtration by means of Nuclepore filters : aerosol sampling and measurement. Environ. Sci. Technol. 3, 464-468. Spurny K. and Madelaine G. (1971) Efficiency measurement of nuclear pore filters by means of latex aerosols. Collection Czechosloc. Chem. Commun. 36, 2857-2866.