Continuous particle mass measurement by recording the pressure drop in nuclepore filters

J. AemsolSci. Vol. 29, Suppl. I. pp. S1183-SI 184. 1998 Q 1998 Published by Elsevier Science Ltd. All tights reserved Printed in Great Britain 0@21-8502/98$19.00+0.00

Pergamon

CONTINUOUS

PARTICLE MASS MEASUREMENT BY RECORDING THE PRESSURE DROP IN NUCLEPORE FILTERS

CONSTANTINOS

SIOUTASt, PETROS KOUTRAKIS”, PEN-YAU WANG”, PETER BABICH”, and JACK M. WOLFON”

* University of Southern California, 3620 South Vermont Avenue, Los Angeles, CA 90089; “Harvard University, School of Public Health, 665 Huntington Avenue, Boston, MA 02115;USA KEYWORDS Pressure Drop; Continuous Mass Measurement; Nuclepore filters In the past three years we have attempted to develop a continuous particle mass monitor. This method is based on the measurement of the increase in the pressure drop with particle loading across porous membrane filters (NucleporeTM) over time (Koutrakis et al., 1995). As part of this development, we investigated both experimentally and theoretically the effect of parameters such as particle size and chemical composition, as well as filter face velocity and pore size, on the increase in the pressure drop with particle loading. In this paper, we describe the results of these investigations. The relationship between pressure drop across the particle-sampling Nuclepore filter per unit time as a function of particle concentration and size was investigated for various pore diameters and sampling flow rates. For each experiment, the total increase in the pressure drop across the filter in one hour was divided by the mass concentration measured with the reference filter gravimetrically. This new variable, AP/c,/t was plotted as a function of particle diameter. Results from the experimental measurements of the pressure drop per unit time and mass concentration are shown in Figure 1. The results show similar trends on the dependence of the quantity (AP/c,/t) on particle size. In general, (AP/c,/t) decreases slightly with particle size for particles from about 0.2 to 1.O pm for any pore size and face velocity. There is a sharp increase in (AP/c,/t) as particle size becomes smaller than 0.2 pm, and with the slope becoming about 3050% higher than the 0.2-l .O pm average at about 0.1 pm, and nearly doubling at 0.05 pm. Furthermore, depending on the filter face velocity, the value of (AP/c,,,/t) decreases sharply for the larger particles. Figure

1. Tests with a Sum pore Filter

Fig. 2. Measured vs. Predicted Pressure Drop Per Unit Time and 16

-

C

- 52 unls

- - T% - -26 cm/s

u 12 f! 3 2 =

8 4

0 0.01

0.1

Particle

1

Diameter

10

0

4 Predicted=4.78

(urn) S1183

8

12 U CcA0.51(D”3)

16

S1184

Abstracts

of the 5th International

Aerosol

Conference

1998

The sharp decrease in (AP/c,/t) for particles larger than 1.O pm is due to impaction of these particles on the inter-pore surfaces of the NucleporeTM filter. In order to better illustrate the dependence of the increase in pressure drop with loading, we performed multiple regression of (AP/c,/t) on the filter face velocity (U), the Cunningham slip correction factor (C,) and the filter pore diameter (D). The results of the regression (Figure 2) show that the increase in the pressure drop across the filter with loading can be expressed as: APIC /t=4.78 u cc”.5/D3 A ve,“, high correlation coefficient was obtained (R2=0.97). The effect of particle density on pressure drop was also investigated in a set of experiments involving monodisperse and polydisperse particles of different densities. Three different densities of particles, including polystyrene (pp=l .05 g/cm3), sodium chloride (pp=2.2 g/cm3) and silica (pp=2.6 g/cm3) were tested. The results of the density effect tests are shown in Table 1. Results from these experiments indicated that the pressure drop depends on the inverse of the square root of particle density. The pressure drop (per unit time and mass concentration) was subsequently normalized for the different particle densities, by multiplying by the square root of the particle density. This normalized pressure drop per unit time and mass concentration (i.e., AP(p,)“~sIC,,,/t) is also shown in the last column in Table 1. TABLE 1. Density Effect on the Rate of Pressure Drop Increase as a Function of Particle Mass Concentration. Filter Pore Diameter: 5 pm

Face

Particle Type

Particle Size Rage (W

Density (p/cd

AFxyt (inches of H20/pg/m31hr)

API&,/t (Normalized) ( inches of H20/pg/m31hr)

PSL

0.2-1.5

1.05

0.0031 (io.ooo2)

0.0032

Silica

OS-O.9

2.6

0.0021 (*0.0001)

0.0033

Sodium Chloride

0.2-1.8

2.2

0.0022 (*0.0002)

0.0032

PSL

0.2-1.5

1.05

0.0014 (*0.0001)

0.0014

Silica

OS-l.0

2.6

0.0009 (*0.0001)

0.0015

Velocity (cm

52

24

ACKNOWLEDGMENTS This work was supported by the Electric Power Research Institute (award number W09 152-O 1) and the Center for Indoor Air Research (award number 96-OSA). REFERENCES Koutrakis, P., Wang, P.Y., Wolfson, J.M., and Sioutas, C. (1995). U.S. Patent No: $571,945