Measurement of Collection Efficiency of Amosite Fibers

Atmospheric Pollution 1978, Proceedings of the 13th International Colloquium, Paris, France, April 25-28,1978, M.M. Benarie (Ed.), Studies in Environmentof Science, Volume 1 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

107

MEASUREMENT OF COLLECTION EFFICIENCY OF AMOSITE FIBERS J . GENTRY*, K .

SPURNY, G. W E I S S , H . OPIELA

I n s t i t u t e f u r Aerobiologie, Grafschaft

,

Germany

*Department o f Chemical Engineering, University o f Maryland, College Park, U .S . A . ABSTRACT

The c o l l e c t i o n e f f i c i e n c y of amosite f i b e r s was measured f o r nuclepore f i l t e r s as a function of NPF pore s i z e and flow r a t e by four d i f f e r e n t measurement techniques

-

? -

a g r a v i m e t r i c determination based on t h e a c t i v i t y of "Fe,

a com-

p a r a t i v e f i b e r count using SEM, t h e use of a modified Royco with a v a r i a b l e flow r a t e , and a C l i m e t .

Comparisons of t h e four methods are made, and t h e i r appro-

p r i a t e n e s s i s discussed.

Measurements i n d i c a t e t h a t t h e e f f i c i e n c y i s a weak

function of flow r a t e . INTRODUCTION

Amosite i s a type of a s b e s t o s c h a r a c t e r i z e d by t h e following p r o p e r t i e s :

1.

The f i b e r s a r e s o l i d , rod-like with a comparatively uniform diameter.

[ref. 1 1 2.

The mineral amosite contains considerable Fe and can be e a s i l y l a b e l e d .

3.

C o r r e l a t i o n s of t h e aerodynamic diameter ( a s measured by a s p i r a l cen-

t r i f u g e ) show a p r o p o r t i o n a l i t y t o approximately t h e 1/6 power with respect t o the aspect r a t i o .

[ r e f . 21

Experimental measurements using amosite ( o r any non-spherical p a r t i c l e ) pose s e v e r a l unique d i f f i c u l t i e s .

More than one v a r i a b l e ( i . e . length and diameter)

a r e r e q u i r e d t o c h a r a c t e r i z e t h e f i b e r s , t h e degree of s c a t t e r e d l i g h t depends on t h e o r i e n t a t i o n as well a s t h e geometric p r o p e r t i e s of f i b e r s , t h e s i z e d i s t r i b u t i o n tends t o be more polydisperse than i s o m e t r i c p a r t i c l e s , and t h e f i b e r s a r e e a s i l y o r i e n t e d by v a r i a t i o n s i n t h e v e l o c i t y f i e l d .

[ r e f . 31

EXPERIMENTAL D E S I G N

The experimental apparatus can be subdivided i n t o t h r e e s e c t i o n s

- f i b e r gen-

e r a t i o n , agglomerate removal, and t h e measurement of t h e p e n e t r a t i o n .

The p r i n -

c i p l e o b j e c t i v e s of t h e design a r e t o generate a reproducible aerosol with a narrow s i z e d i s t r i b u t i o n of i n d i v i d u a l f i b e r s and a method of measuring and i n t e r p r e t i n g t h e p e n e t r a t i o n unambiguously. The f i b e r s w e r e generated from a ground, dry powder of amosite using a Spurny v i b r a t i n g bed g e n e r a t o r [ r e f . 4 1 .

Dry a i r p e r c o l a t e s through t h e bed of

*

powdered amosite which i s being shaken a t a constant frequency and amplitude. The c h a r a c t e r i s t i c s of t h e a e r o s o l l e a v i n g t h e generator can be c o n t r o l l e d by t h e flow r a t e through t h e bed and t h e frequency and amplitude o f t h e v i b r a t i o n s . Examination of t h e f i b e r s with an e l e c t r o n microscope show a narrow d i s t r i b u t i o n of diameter, a s l i g h t l y broader d i s t r i b u t i o n o f l e n g t h s , and a s t i l l broader d i s t r i b u t i o n of t h e c a l c u l a t e d aerodynamic diameter. The o b j e c t i v e of t h e a e r o s o l conditioning s e c t i o n between t h e f i l t e r and generator i s t o remove agglomerates (approximately 30% by weight) and t o minimize t h e e f f e c t of e l e c t r i c a l charge.

In each case t h e a e r o s o l stream leaving

the generator passed through an impaction s t a g e . ported,

In most of t h e experiments r e -

[ r e f . 51 e i t h e r an Andersen impactor o r a p r e f i l t e r

(8.0um) NPF was used.

The b e s t t e s t a e r o s o l was t h a t l e a v i n g t h e NPF p r e f i l t e r s t a g e which contained almost no agglomerates. well.

Unfortunately most of t h e long f i b e r s were removed a s

To minimize e l e c t r o s t a t i c e f f e c t s , t h e a e r o s o l was passed through a 8 5 K r

i o n i z a t i o n source.

For g r a v i m e t r i c measurements, t h i s s t a g e w a s by passed.

In

t h e s e measurements amosite l a b e l e d with 55Fe and 59Fe w a s used. The measurement procedure depended on whether g r a v i m e t r i c o r o p t i c a l methods a r e used. erator.

I n t h e g r a v i m e t r i c measurements, l a b e l e d amosite i s used i n t h e genThe t e s t f i l t e r s t a g e c o n s i s t s of two s t a g e s

-

t h e f i r s t with t h e t e s t

NPF and t h e second an a b s o l u t e f i l t e r c o n s i s t i n g of a 0.2um membrane f i l t e r with

a paper f i l t e r support.

The p e n e t r a t i o n was t h e a c t i v i t y of t h e backing f i l t e r s

divided by t h e a c t i v i t y measured from both s t a g e s .

The f i l t e r holders were de-

signed so t h a t t h e r e w a s no support s c r e e n . The o p t i c a l measurements were with e i t h e r a Royco (modified t o allow an i n creased flow r a t e ) o r a C l i m e t .

The b a s i c i d e a was t o pass a sample through t h e

o p t i c a l instrument which was drawn e i t h e r b e f o r e o r a f t e r t h e f i l t e r .

In these

experiments t h e a e r o s o l s were passed through t h e n e u t r a l i z a t i o n stage and no backing f i l t e r was used.

Provisions were made f o r a d d i t i o n a l d i l l u t i o n a i r when

higher flow r a t e s through t h e f i l t e r were necessary.

A s p e c i a l l y designed mani-

f o l d was incorporated i n t o t h e design so t h a t t h e sampling r a t e and t h e flow r a t e through t h e f i l t e r were independent. ELECTRON MICROSCOPY

For some samples, f i b e r s c o l l e c t e d on NPF were photographed using e l e c t r o n microscope (SEM)

.

The f i b e r s were then s i z e d and counted.

a scanning Approxi-

mately 100 f i b e r s w e r e used i n each sample. From e l e c t r o n microscopy it had been previously found t h a t t h e r e w a s l i t t l e

s i z e dependence of t h e d e p o s i t i o n p a t t e r n i n Andersen impactors and mostly agglomerates were deposited on t h e two s t a g e s .

Also, it was demonstrated t h a t

f i b e r s acquired a p r e f e r e n t i a l alignment f o r p e n e t r a t i n g NPF.

[ r e f . 51

A comparison of t h e f i b e r s passing through a NPF showed t h a t t h e average

length t o diameter r a t i o changed l i t t l e ( a decrease by approximately 20%) while t h e length and width decreased by over 5 0 % . The aerodynamic diameter decreases from approximately 2.0um t o 1.0Um.

The d i s t r i b u t i o n of diameters narrows a f t e r

passing through t h e f i l t e r . GRAVIMETRIC MEASUREMENTS

I n t h e gravimetric measurements, i r r a d i a t e d amosite was used i n t h e Spurny generator a s source m a t e r i a l f o r t h e a e r o s o l .

Previously reported measurements

[ r e f . 51 with a s e r i e s o f 6 NPF arranged i n a sequence of descending pore s i z e i n d i c a t e d t h a t f i b e r s were a b l e t o p e n e t r a t e pores with diameters l e s s than 15% of t h e f i b e r l e n g t h .

For t h i s experiment a t a low flow r a t e , t h e e f f i c i e n c y

ranged from 0 . 4 t o 0.75

(8um N P F ) .

No p r e f i l t e r was used.

More extensive measurements f o r

s i z e s of NPF over a wide range of flow r a t e s a r e r e p o r t e d i n f i g u r e 1.

For

each of t h e s e experiments a 8 u m NPF was used a s a p r e f i l t e r .

Approximate-

l y 75% of t h e mass of t h e f i b e r i n c l u d i n g most of t h e agglomerates and very long f i l t e r s were removed i n t h i s stage.

The t e s t a e r o s o l was almost

agglomerate-free and more uniform i n s i z e than the a e r o s o l l e a v i n g t h e gen-.

251

1

erator.

0

..

. I

2

3

12p

1 5

4

FLOW RATE (I/MIN)

Over a wide range of v e l o c i t i e s

(1-4 l i t e r s / m i n ) t h e r e was no apprec i a b l e change i n p e n e t r a t i o n with flow r a t e , and a s t e a d y i n c r e a s e i n penet r a t i o n with pore s i z e .

FIG. 1 Collection Efficiency A s a Function of Flow Rate and Pore Size

Penetration

ranged from 98% f o r l u m NPF t o 2 3 % f o r 1 2 u m .

These r e s u l t s suggest t h a t most

f i b e r s a r e c o l l e c t e d by i n t e r c e p t i o n . OPTICAL MEASUREMENTS

The primary d i f f i c u l t y i n o p t i c a l measurements i s t h a t t h e r e i s considerable ambiguity i n i n t e r p r e t i n g t h e p e n e t r a t i o n due t o t h e p o l y d i s p e r s i t y of t h e fibers.

I t was found t h a t meaningful measurements

[ r e f . 51 of t h e p e n e t r a t i o n

could be made, i f t h e flow r a t e through t h e instrument were increased.

This

was a t t r i b u t e d t o t h e f i b e r s a c q u i r i n g a p r e f e r e n t i a l o r i e n t a t i o n a t t h e higher flow r a t e s .

Several of t h e newer instruments (Royco and Climet) operate a t

s u f f i c i e n t flow r a t e s t h a t according t o preliminary d a t a , m e a n i n g f ~ ~ measurel ments can be obtained without a l t e r i n q t h e instrument flow r a t e .

(It is

necessary t o design a s p e c i a l manifold so t h a t t h e flow r a t e through t h e NPF can be v a r i e d independently of t h e instrument flow r a t e ) . Measurements i n d i c a t e t h a t t h e p e n e t r a t i o n i n c r e a s e s with decreasing o p t i c a l diameter.

For t h e most p a r t , t h e e f f i c i e n c y i s a weak function of flow r a t e ,

although t h e r e are some measurements a t high flow rates which suggest t h e f i b e r s acquire a p r e f e r e n t i a l o r i e n t a t i o n a s they approach t h e f i l t e r .

Measurements

with t h e Climet and f o u r f i l t e r s i z e s (3pm, 5pm, 8 ~ m ,12um) i n d i c a t e t h a t the p e n e t r a t i o n measured by g r a v i m e t r i c methods correspond t o an o p t i c a l diameter ranging from 0.9 t o 1 . 4 u m with a mean value o f 1 . 2 u m .

S c a t t e r was l e s s for t h e

3pm and 5.um NPF.

SUMMARY O p t i c a l measurements with Climet and a modified Royco a s w e l l as gravimetric measurements of the p e n e t r a t i o n o f amosite showed l i t t l e change with flow r a t e except a t very high flow r a t e s .

The g r a v i m e t r i c measurements corresponded t o

t h e o p t i c a l measurements with an o p t i c a l diameter of approximately 1 . 2 um. For reproducible experiments it was found necessary t o make o p t i c a l measurements a t f a i r l y high instrument flow r a t e s and t o p r e f i l t e r t h e t e s t a e r o s o l with a 8 Um NPF.

The dominant c o l l e c t i a n mechanism appeared t o be i n t e r c e p t i o n .

REFERENCES

1.

K. Spurny, e t . a l . , Atmospheric P o l l u t i o n , ed. M. Benarie (1976) 459-469

2.

W.

3.

J . Gentry, K . Spurny, Aerosole i n Naturwissenschaft, Medizin, and Technik, Jahreskongress, GAF (1976) 38-42

4.

K . Spurny, C . Boose, D. Hochrainer, Staub, 35 (1975) 4 4

5.

J . Gentry, K . Spurny, J . Colloid I n t e r f a c e S c i . ,

StBber, H. F l a c h s b a r t , D. Hochramer, S t a u b , 30(1970) 270

(1978), i n p r e s s