Experiments on Particle Deposition in Sampling Lines

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

101

EXPERIMENTS O N P A R T I C L E DEPOSITION IN SAMPLING LINES iu'. GEIPEL

L e h r s t u h l f u r T h e r m i s c h e K r a f t a n l a g e n d e r TU Xiinchen

I I\] T RO DU C T I O N

The d u s t c o n t e n t o f f l u e g a s e s i s measured i n p a r t i a l f l o w s . The a e r o s o l i s drawn t o a f i l t e r t h r o u g h a s a m p l i n g t r a i n . The c o n c e n t r a t i o n i s c a l c u l a t e d by t h e c o l l e c t e d d u s t mass and t h e s u c k e d g a s volume. I n t h e s a m p l i n g l i n e a p a r t o f t h e d u s t may be d e p o s i t e d and s o a d v e r s l e y i n f l u e n c e s t h e m e a s u r e m e n t . T h i s e f f e c t depends on t h e geometry o f t h e t u b e and t h e p a r a m e t e r s o f t h e p a r t i c l e s and t h e two-phase f l o w . T h e r e a r e s e v e r a l i n v e s t i g a t i o n s on t h e d e p o s i t i o n o f p a r t i c l e s i n p i p e f l o w . Most o f t h e a u t h o r s t r i e d , however, t o p r e v e n t t h e r e - e n t r a i n m e n t o f p a r t i c l e s d e p o s i t e d . Because p a r t i c l e s r e - e n t r a i n i n r e a l t e s t s , i t was n o t s u p r e s s e d i n t h e s e e x p e r i m e n t s . Sampling t r a i n s were i n v e s t i g a t e d , which a r e u s e d i n f l u e g a s e s , t h a t means t u r b u l e n t f l o w w i t h p a r t i c l e s g r e a t e r t h a n 1 pm. The d e p o s i t e d mass i n t h e p i p e was m e a s u r e d b u t n o t t h e a l t e r a t i o n o f t h e p a r t i c l e size distribution. TIIEORETICAL DESCRIPTION O F THE PARTICLE DEPOSITION S t u d y i n g t h e volume e l e m e n t o f a p i p e t h e d e c r e a s e o f t h e p a r t i c l e concentration c of the gas-solid suspension i s described:

(L

pipe length; D

pipe diameter; w

gas v e l o c i t y )

The deposition v e l o c i t y k i s d e f i n e d by t h e q u o t i e n t o f t h e f l u x - o f deposited p a r t i c l e s t o the concentration: k = -

N F t c

(F

area; t

time; N

number o f p a r t i c l e s )

102

The amount o f t h e d e p o s i t i o n v e l o c i t y i s d e t e r m i n e d by t h e p a r t i c l e s , t h e p i p e f l o w , and t h e w a l l which a r e c o r r e l a t e d t o e a c h o t h e r . T h r e e i n f l u e n c e s may be d e s c r i b e d . F i r s t t h e p a r t i c l e s a r e t r a n s p o r t e d from t h e g a s f l o w t o t h e w a l l . Here t h e y a r e h e l d by a d h e s i v e f o r c e s . A l s o it i s p o s s i b l e t o r e - e n t r a i n s t i c k i n g p a r t i c l e s .

The r e s p o n d i n g

e q u i l i b r i u m causes t h e d e p o s i t i o n of i n t e r e s t . The t r a n s p o r t o f t h e p a r t i c l e s t o t h e w a l l i s done by g r a v i t a t i o n a l s e t t l i n g , eddy d i f f u s i o n , and e l e c t r o s t a t i c f o r c e s . The s e d i m e n t a t i o n i n t u r b u l e n t f l o w i s d e s c r i b e d by Fuchs 1 1 1

.

'The d e p o s i t i o n

v e l o c i t y k becomes:

(ws

s e t t l i n g velocity)

Many i n v e s t i g a t i o n s on eddy d i f f u s i o n d e p o s i t i o n r e l y on a work o f F r i e d l a n d e r 1 2 1 . H i s e x p e r i m e n t a l r e s u l t s a r e b e s t i n t e r p r e t e d by a n a n a l y s i s s i m i l i a r t o t h a t u s e d by von Karman f o r t h e r a t e o f t r a n s p o r t o f momentum and mass i n a t u r b u l e n t g a s s t r e a m . The c o e f f i c i e n t o f t h e mass t r a n s f e r k / w i s g i v e n f o r d i f f e r e n t v a l u e s o f a dimensionless stopping distance S x : c

The s t o p p i n g d i s t a n c e i s d e f i n e d :

(f

friction factor; p

mass ; dp

g a s d e n s i t y ; rl

p a r t i c l e diameter)

gas v i s c o s i t y ; m

P

particle

103

To e s t i m a t e t h e image f o r c e s b e t w e e n t h e p a r t i c l e s and t h e w a l l t h e l a w o f Coulomb may be u s e d . A c c o r d i n g t o Rumpf 131 t h e s e f o r c e s < 100 u m . D e p o s i t e d P p a r t i c l e s a r e h e l d t o t h e w a l l by van d e r Waals f o r c e s and l i q u i d

p l a y no r o l e on t h e a d h e s i o n o f p a r t i c l e s d

f i l m s . I f t h e r e a r e l i q u i d f i l m s between t h e p a r t i c l e s and t h e w a l l t h e c a p i l l a r y f o r c e s a r e h i g h e r t h a n van d e r Waals f o r c e s . P a r t i c l e s may b e r e - e n t r a i n e d by v i b r a t i o n s of t h e s a m p l i n g l i n e , r e b o u n d , a n d s h e a r s t r e s s e s o f t h e f l o w . The f o r c e s b e t w e e n t h e f l o w a n d t h e d e p o s i t e d p a r t i c l e s may b e e s t i m a t e d by d r a g f o r c e s a n d buayancy a c c o r d i n g t o measurements o f Rubin 1 5 1 . TEST PROCEDURE A p a r t i a l f l o w was i s o k i n e t i c l y drawn from t h e d u s t - a i r f l o w of t h e p i l o t p l a n t o f t h e I n s t i t u t e . The d u s t mass was c o l l e c t e d i n a glass-wad f i l t e r ( f i g . 1 ) . The g a s v e l o c i t y , t h e l e n g t h , and t h e diameter of t h e s t a i n l e s s s t e e l s a m p l i n g l i n e were v a r i e d . Each e x p e r i m e n t was s t a r -

- L-

t e d w i t h a "zero-measure-

Test pipe

Filter

ttt w m

ment" t o d e t e r m i n e t h e pump

d u s t mass mo.

In this

c a s e t h e f i l t e r was i n s t a l l e d i n t h e sampling l i n e adjacent t o the

Pilot Plant

Fig. 1 . Schematic arrangement of t h e equipment

t e s t f a c i l i t y . To measure the deposition, the pipe was f i t t e d . The d u s t mass s t i l l passing m

f was c o l l e c t e d i n t h e f i l t e r . From b o t h measurements t h e d e p o s t i o n r a t e A was c a l c u l a t e d : A =

"0

mO

mf

-

C

- l - T

(7) 0

The g a s v e l o c i t y i n t h e p i p e was m e a s u r e d w i t h an o r i f i c e . Q u a r t z , l i m e s t o n e , a n d f l y a s h were u s e d as t e s t d u s t s . A f t e r e a c h e x p e r i ment t h e s a m p l i n g l i n e was c l e a n e d c a r e f u l l y .

104

RESULTS In diagramm 2 the deposition in a horizontal pipe was plotted versus the gas velocity (Re = wD/v). The deposition of coarse particles differs appreciably from that of fine dusts. Increasing

1.04

I

Limestone

0

o

3pm 12pm

3 11 18

L= D=

0

10000

Re

20 000

-

Fig. 2. Deposition rate of the test dusts the gas velocity the deposition of big particles decreases. Fine particles show a distinct maximum deposition (Re w 8000). At low Reynolds numbers the forces of deposition by eddy diffusion prevail. The re-entrainment by the shear stresses of the flow sets in. Big parti1,o cles settle out. At high velocity settling is dis- horizontal turbed. In addition the particles sticking at the wall are re-entrained. A A fraction of coarse fly ash and one of fine limestone were sucked through a horizontal and Re a vertical sampling line (fig. 3). In the vertical Fig. 3 . Horizontal and vertical pipe there is no gravisampling line

105

1.0 '

L\:

d,=3,,m

D=6mm

,' ,.Go-'\, //

0.5A

/

-

-

L = 2,7m

y 1'5

'A

/

/'

\. \.

\.

b\ 7

it was not possible to investigate how the deposition of coarse particles varies with the diameter of the tube. Diagramm 4, however, shows experiments with a fine fraction. Following eq. ( 1 ) the deposition decreases if the

DISCUSSION OF RESULTS Finally these results are compared to the theories given first. Diagramm 5 shows measurements and calculations of the rate of deposition by sedimentation and eddy diffusion. The settled mass of big particles decreases with increasing gas velocity. Re-entrainment and rebound 1 7 1 make the measured values smaller than the ones calculated. At low gas velocities the deposition of fine particles is equal to the calculated value by eddy diffusion. Re-entrainment p r e vails if the gas velocity is increased to about Re = 9000 (D = 10 mm), and the deposition gets smaller. It is not possible to calculate this point. To estimate the adhesive and drag forces, which are responsible for re-entrainment, in practical sampling lines the para-

106

meters of t h e p a r t i c l e s a r e unknown. S i m i l a r v e l o c i t i e s a r e r e p o r t e d by Eddy diffusion Settling - Experiment

D = l O mm

\2

0'

10000

Re

20000

I

Chang 16 [ a n d Se hm e l 181 f o r t h e b e g i n n i n g o f reentrainment. Sampling t r a i n s should n o t b e t o s m a l l (Dw10 m m ) . To s a m p l e c o a r s e p a r t i c l e s avoid horizontal lines. The d e p o s i t e d mass i s

small i f t h e g a s v e l o c i t y w

F i g . 5 . Measured and c a l c u l a t e d Je P O s i t i o n

> 30

m/s.

Heating of the

l i n e prevents liquid films between t h e p a r t i c l e s and the w a l l of the pipe.

REFERENCES 1 N . A . F u c h s , The M e c h a n i c s o f A e r o s o l s , Pe rga m on P r e s s , O x f o r d 1Yo4, p.264 2 S.K. F r i e d l a n d e r , H . J . J o h n s t o n e , Ind.Engng.Chem.49(1957)1151-56 3 H . Rumpf, Chemie I n g . T e c i i n . , 4 6 ( 1 9 7 4 ) 1 - 1 0 4 El. S c h u b e r t , Chemie I n g . T e c h n . , 4 6 ( 1 9 7 4 ) 3 3 3 - 3 3 4 5 G . R u b i n , F . L o f f l e r , Chernie 1 n g . T e c h n . MS 3 7 3 - 7 6 ( 1 9 7 6 ) 6 [I. C h a n g , T . S m i t h , A m . I n d . H y g . A s s . J . , 33(1972)722-728 7 L . F . Forney, L.A. Spielman, J . C o l l o i d . I n t e r f . S c i . , 52(1975)468-478 8 G.A. Sehmel, Am.Ing.Ilyg.Ass.J., 31(1970)758-771