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Investigations on fine particle separation using an electrostatic nozzle scrubber
J. Aero:lol Sei., Vol. 23, Suppl. I, pp. $773~$777, 1992 Printed in Great Britain.
0021-8502/92 $5.00 + 0.00 Pergamon Press Ltd
INVESTIGATIONS ON FINE PARTICLE SEPARATION USING AN ELECTROSTATIC NOZZLE SCRUBBER
Marc S c h m i d t a n d Friedrich L0ffler I n s t i t u t ft~r M e c h a n i s c h e V e r f a h r e n s t e c h n i k u n d Mechanik, Universitglt Karlsruhe (TH), D-7500 Karlsruhe, G e r m a n y
ABSTRACT Numerical s i m u l a t i o n s of p a r t i c l e / d r o p i n t e r a c t i o n s shows a n i n c r e a s e d s e p a r a t i o n e s p e c i a l l y in the fine particle range. U s i n g t h i s p r i n c i p l e c h a r g e d p a r t i c l e s are d e p o s i t e d on o p p o s i t e l y e l e c t r o s t a t i c c h a r g e d drops. An e l e c t r o s t a t i c nozzle s c r u b b e r for particle s e p a r a t i o n w a s developed. In t h i s p a p e r the c h a r g i n g s y s t e m for both c o m p o n e n t s a n d first r e s u l t s of grade efficiency m e a s u r e m e n t s are presented.
KEYWORDS Aerosol m e c h a n i c s ; deposition; electrostatics; e m i s s i o n control; wet deposition; scrubber; spray; drops; particles
INTRODUCTION Wet s c r u b b e r s are u s e d in m a n y i n d u s t r i a l fields. To obtain a good sep a r a t i o n performance, n o r m a l l y large a m o u n t s of s c r u b b i n g fluid a n d a large p r e s s u r e drop are required. In most scrubbers, the s c r u b b i n g fluid is dispersed in the d u s t laden gas flow as droplets. The s e p a r a t i o n p r o c e s s r e s u l t s from collisions b e t w e e n p a r t i c l e s a n d drops. In the flow p a s t a drop, inertia and drag have a n effect on the particles. In addition, electrostatic field forces can act, d e p e n d i n g on the charge of the particle a n d the charge of the drop. Numerical s i m u l a t i o n s of p a r t i c l e / d r o p collisions a n d observations of particle trajectories in the flow p a s t a sphere showed t h a t electrostatic field forces can lead to a n inc r e a s e d e n h a n c e m e n t in collection efficiency, especially for small particles a n d small collision velocities (Schmidt a n d LOftier, 1992). For the realization of this principle, a nozzle s c r u b b e r is applied.
$773
M. SCHMIDTand F. L~FFLER
$774
ELECTROSTATIC CHARGING OF PARTICLES AND DROPS It's to t a k e care t h a t pa r t i c l e s a n d d r o p s c a r r y a c h a r g e t h a t is h i g h e n o u g h a n d h a s an opposite sign. The particles are c h a r g e d u s i n g a conv e n t i o n a l c o r o n a discharge. A s e t u p a n a l o g o u s to a t u b u l a r electrostatic p r e c i p i t a t o r is u s e d . By t h a t it is to t a k e care t h a t the d e s i g n of t h e c o u n t e r - e l e c t r o d e p r e v e n t s particle deposition. The s p r a y is c h a r g e d u s i n g a modified twin-fluid nozzle with i n t e r n a l mixing. S u c h a nozzle c o n s i s t s of a fluid nozzle a n d a n air nozzle. It atomizes a fluid j e t by c o m p r e s s d - a i r within a droplet formation zone. A ring electrode, c o n n e c t e d to a high voltage source, is placed a r o u n d this zone. The air nozzle is m a d e of PVC to e n s u r e electrical isolation of th e electrode. The b a s e for the electrostatic nozzle is a c o m m e r c i a l SU22-B nozzle from Spraying Systems. Figure I s h o w s a s e c t i o n a l view of a modified nozzle.
cap n u t air nozzle high voltage
fluid nozzle
ring electrode fluid inlet
compressed-air inlet Fig. l: Sectional view of a modified twin-fluid nozzle.
Law (1978) investigated s u c h a nozzle for the u se in a g r i c u l t u r a l s p r a y i n g of oily fluids. In his e x p e r i m e n t s the droplets have the opposite c h a r g e sign like t he ring electrode. He explains the droplet c h a r g i n g m e c h a n i s m in his e x p e r i m e n t s with electrostatic i n d u c t i o n . In o u r e x p e r i m e n t s the u s e d w a t e r droplets have the s a m e charge sign like the ring electrode. So c o n t a c t c h a r g i n g on t he ring electrode could be the m e c h a n i s m in o u r e x p e r i m e n t s . B u t t h e s e effects are not d i s c u s s e d in this paper. T h e y are object of f u r t h e r investigations. Detailed r e s u l t s c o n c e r n i n g th e droplet c h a r g e will be p u b l i s h e d later.
$775
Investigations on fine particle separation
EXPERIMENTAL SET-UP Figure 2 shows a commercial nozzle scrubber. It was modified by supplying it with electrostatic twin-fluid nozzles and a particle charger. optical partJcal counter
[~xll
Demister
raw gas
particle seeder
clean gas high voltage
krypton-85 source
compressed-air
fresh water particlecharger
t
y~ ,tL ..... !
n
i
,,l,
L
over flow/ outflow Fig. 2: Experimental set-up.
S776
M. SCHMIDT and F. I~FFLER
T h e p a r t i c l e s a r e a d d e d to t h e r a w g a s s t r e a m w i t h aid of a p a r t i c l e s e e d e r . To e n s u r e a d e f i n e d c h a r g i n g , a Kr85 s o u r c e n e u t r a l i z e s t h e m before t h e y e n t e r t h e c o r o n a c h a r g e r . After t h e nozzle s e c t i o n w i t h f o u r e l e c t r o s t a t i c t w i n - f l u i d n o z z l e s follows a d e m i s t e r . Two n o z z l e s a r e a r r a n g e d in c o - c u r r e n t a n d two in c o u n t e r - c u r r e n t flow d i r e c t i o n . T h e s a m p l i n g p r o b e is c o n n e c t e d w i t h a n o p t i c a l p a r t i c l e c o u n t e r to d e t e r m i n e g r a d e efficiencies d o w n to 0.3 ~m.
RESULTS F i r s t e x p e r i m e n t s a r e d o n e w i t h q u a r t z p a r t i c l e s in t h e size r a n g e 0.3 ~ m < x < 10 ~m. T h e r a w g a s flow t h r o u g h t h e s c r u b b e r is a b o u t 150 m a / h a n d t h e p r e s s u r e loss is 3 0 0 Pa. T h e nozzles o p e r a t e d w i t h a w a t e r p r e s s u r e of P w = 1.3 b a r a n d a n air p r e s s u r e of PL = 1.0 b a r . T h e w a s h i n g liquid flow is 2 7 5 1/h t h a t r e s u l t s in a very low specific w a s h i n g liquid c o n s u m p t i o n L of 1.8 liter w a t e r per c u b i c m e t e r r a w gas. T h e r i n g e l e c t r o d e of t h e n o z z l e s h a s a p o t e n t i a l of + 1 kV a n d t h e d i s c h a r g e e l e c t r o d e of t h e c o r o n a c h a r g e r h a s a p o t e n t i a l of - 18 kV.
1.0 ---0--
with electrostatics
0.9
= + , k V
i "
{ ~~
', ....
orona:-'kV, ........
0.8 0.7
-----t:>--without e l e c t r o s t a t i c s S
i i~
......
'"I
/
....
.....................~
....i - ~
0.6 T (x) 0 . 5 0.4 0.3 0.2 0.1 0.0 0.1
:
:
:
i
i
:, i i i l
:
:
:
1
~
i
!
}
i
i i i
}
}
10
x / ~m Fig. 3: Collection efficiencies for q u a r t z p a r t i c l e s w i t h L = 1.8 I / m 3.
Investigations on fine particle separation
$777
Figure 3 s h o w s collection efficiencies for the o p e r a t i n g c o n d i t i o n s described above. In the case w i t h o u t electrostatics the collection efficiency over t h e particle d i a m e t e r h a s a S-like s h a p e t h a t is typical for inertial capture. The very low w a s h i n g liquid c o n s u m p t i o n leads to a poor collection efficiency with a cut size of 4 gm. The charging of the particles and t h e d r o p s yields to a c o n s i d e r a b l e i n c r e a s e of collection efficiency. The curve h a s a flatter s h a p e t h a t i n d i c a t e s electrostatic effects. With the s a m e w a s h i n g liquid c o n s u m p t i o n and the same p r e s s u r e loss the cut size r e d u c e s to 0.4 gm. O t h e r e x p e r i m e n t s to investigate the effect of image forces are done. In t h e s e investigations either the drops or the particles were charged. It can be stated t h a t the charging of the drops, has only a weak effect, w h e n the particles are u n c h a r g e d . If only the particles are c h a r g e d an i n c r e a s e d collection efficiency due to an i n c r e a s e d deposition also on t h e walls of t h e s c r u b b e r could be s t a t e d . F u r t h e r e x p e r i m e n t s are n e c e s s a r y to q u a n t i f y this effect as the influence of other operating c o n d i t i o n s of the nozzles a n d the scrubber. Using the principle of electrostatic e n h a n c e d scrubbing, it can be stated, t h a t it is possible to i n c r e a s e the s e p a r a t i o n p e r f o r m a n c e c o n s i d e r a b l y w i t h o u t the disadvantage of an increased pressure loss.
ACKNOWLEDGEMENTS The a u t h o r s t h a n k the Deutsche Forschungsgemeinschafl and KT Kunststofftechnik GmbH D-5210 Troisdorf for the friendly s u p p o r t of this project.
REFERENCES Law, S.E. (1978). E m b e d d e d - E l e c t r o d e E l e c t r o s t a t i c - l n d u c t i o n SprayC h a r g i n g Nozzle: Theoretical and E n g i n e e r i n g Design. TRANSACTIONS of the ASAE, pp. 1096-1104. S c h m i d t , M., LSffler, F. (1992). C a l c u l a t i o n of particle d e p o s i t i o n on c h a r g e d droplets. Preprints: 2. Europ. Symp. Separation of Particles from Gases (PARTEC 92) N~rnberg. pp. 21-41
0021-8502/92 $5.00 + 0.00 Pergamon Press Ltd
INVESTIGATIONS ON FINE PARTICLE SEPARATION USING AN ELECTROSTATIC NOZZLE SCRUBBER
Marc S c h m i d t a n d Friedrich L0ffler I n s t i t u t ft~r M e c h a n i s c h e V e r f a h r e n s t e c h n i k u n d Mechanik, Universitglt Karlsruhe (TH), D-7500 Karlsruhe, G e r m a n y
ABSTRACT Numerical s i m u l a t i o n s of p a r t i c l e / d r o p i n t e r a c t i o n s shows a n i n c r e a s e d s e p a r a t i o n e s p e c i a l l y in the fine particle range. U s i n g t h i s p r i n c i p l e c h a r g e d p a r t i c l e s are d e p o s i t e d on o p p o s i t e l y e l e c t r o s t a t i c c h a r g e d drops. An e l e c t r o s t a t i c nozzle s c r u b b e r for particle s e p a r a t i o n w a s developed. In t h i s p a p e r the c h a r g i n g s y s t e m for both c o m p o n e n t s a n d first r e s u l t s of grade efficiency m e a s u r e m e n t s are presented.
KEYWORDS Aerosol m e c h a n i c s ; deposition; electrostatics; e m i s s i o n control; wet deposition; scrubber; spray; drops; particles
INTRODUCTION Wet s c r u b b e r s are u s e d in m a n y i n d u s t r i a l fields. To obtain a good sep a r a t i o n performance, n o r m a l l y large a m o u n t s of s c r u b b i n g fluid a n d a large p r e s s u r e drop are required. In most scrubbers, the s c r u b b i n g fluid is dispersed in the d u s t laden gas flow as droplets. The s e p a r a t i o n p r o c e s s r e s u l t s from collisions b e t w e e n p a r t i c l e s a n d drops. In the flow p a s t a drop, inertia and drag have a n effect on the particles. In addition, electrostatic field forces can act, d e p e n d i n g on the charge of the particle a n d the charge of the drop. Numerical s i m u l a t i o n s of p a r t i c l e / d r o p collisions a n d observations of particle trajectories in the flow p a s t a sphere showed t h a t electrostatic field forces can lead to a n inc r e a s e d e n h a n c e m e n t in collection efficiency, especially for small particles a n d small collision velocities (Schmidt a n d LOftier, 1992). For the realization of this principle, a nozzle s c r u b b e r is applied.
$773
M. SCHMIDTand F. L~FFLER
$774
ELECTROSTATIC CHARGING OF PARTICLES AND DROPS It's to t a k e care t h a t pa r t i c l e s a n d d r o p s c a r r y a c h a r g e t h a t is h i g h e n o u g h a n d h a s an opposite sign. The particles are c h a r g e d u s i n g a conv e n t i o n a l c o r o n a discharge. A s e t u p a n a l o g o u s to a t u b u l a r electrostatic p r e c i p i t a t o r is u s e d . By t h a t it is to t a k e care t h a t the d e s i g n of t h e c o u n t e r - e l e c t r o d e p r e v e n t s particle deposition. The s p r a y is c h a r g e d u s i n g a modified twin-fluid nozzle with i n t e r n a l mixing. S u c h a nozzle c o n s i s t s of a fluid nozzle a n d a n air nozzle. It atomizes a fluid j e t by c o m p r e s s d - a i r within a droplet formation zone. A ring electrode, c o n n e c t e d to a high voltage source, is placed a r o u n d this zone. The air nozzle is m a d e of PVC to e n s u r e electrical isolation of th e electrode. The b a s e for the electrostatic nozzle is a c o m m e r c i a l SU22-B nozzle from Spraying Systems. Figure I s h o w s a s e c t i o n a l view of a modified nozzle.
cap n u t air nozzle high voltage
fluid nozzle
ring electrode fluid inlet
compressed-air inlet Fig. l: Sectional view of a modified twin-fluid nozzle.
Law (1978) investigated s u c h a nozzle for the u se in a g r i c u l t u r a l s p r a y i n g of oily fluids. In his e x p e r i m e n t s the droplets have the opposite c h a r g e sign like t he ring electrode. He explains the droplet c h a r g i n g m e c h a n i s m in his e x p e r i m e n t s with electrostatic i n d u c t i o n . In o u r e x p e r i m e n t s the u s e d w a t e r droplets have the s a m e charge sign like the ring electrode. So c o n t a c t c h a r g i n g on t he ring electrode could be the m e c h a n i s m in o u r e x p e r i m e n t s . B u t t h e s e effects are not d i s c u s s e d in this paper. T h e y are object of f u r t h e r investigations. Detailed r e s u l t s c o n c e r n i n g th e droplet c h a r g e will be p u b l i s h e d later.
$775
Investigations on fine particle separation
EXPERIMENTAL SET-UP Figure 2 shows a commercial nozzle scrubber. It was modified by supplying it with electrostatic twin-fluid nozzles and a particle charger. optical partJcal counter
[~xll
Demister
raw gas
particle seeder
clean gas high voltage
krypton-85 source
compressed-air
fresh water particlecharger
t
y~ ,tL ..... !
n
i
,,l,
L
over flow/ outflow Fig. 2: Experimental set-up.
S776
M. SCHMIDT and F. I~FFLER
T h e p a r t i c l e s a r e a d d e d to t h e r a w g a s s t r e a m w i t h aid of a p a r t i c l e s e e d e r . To e n s u r e a d e f i n e d c h a r g i n g , a Kr85 s o u r c e n e u t r a l i z e s t h e m before t h e y e n t e r t h e c o r o n a c h a r g e r . After t h e nozzle s e c t i o n w i t h f o u r e l e c t r o s t a t i c t w i n - f l u i d n o z z l e s follows a d e m i s t e r . Two n o z z l e s a r e a r r a n g e d in c o - c u r r e n t a n d two in c o u n t e r - c u r r e n t flow d i r e c t i o n . T h e s a m p l i n g p r o b e is c o n n e c t e d w i t h a n o p t i c a l p a r t i c l e c o u n t e r to d e t e r m i n e g r a d e efficiencies d o w n to 0.3 ~m.
RESULTS F i r s t e x p e r i m e n t s a r e d o n e w i t h q u a r t z p a r t i c l e s in t h e size r a n g e 0.3 ~ m < x < 10 ~m. T h e r a w g a s flow t h r o u g h t h e s c r u b b e r is a b o u t 150 m a / h a n d t h e p r e s s u r e loss is 3 0 0 Pa. T h e nozzles o p e r a t e d w i t h a w a t e r p r e s s u r e of P w = 1.3 b a r a n d a n air p r e s s u r e of PL = 1.0 b a r . T h e w a s h i n g liquid flow is 2 7 5 1/h t h a t r e s u l t s in a very low specific w a s h i n g liquid c o n s u m p t i o n L of 1.8 liter w a t e r per c u b i c m e t e r r a w gas. T h e r i n g e l e c t r o d e of t h e n o z z l e s h a s a p o t e n t i a l of + 1 kV a n d t h e d i s c h a r g e e l e c t r o d e of t h e c o r o n a c h a r g e r h a s a p o t e n t i a l of - 18 kV.
1.0 ---0--
with electrostatics
0.9
= + , k V
i "
{ ~~
', ....
orona:-'kV, ........
0.8 0.7
-----t:>--without e l e c t r o s t a t i c s S
i i~
......
'"I
/
....
.....................~
....i - ~
0.6 T (x) 0 . 5 0.4 0.3 0.2 0.1 0.0 0.1
:
:
:
i
i
:, i i i l
:
:
:
1
~
i
!
}
i
i i i
}
}
10
x / ~m Fig. 3: Collection efficiencies for q u a r t z p a r t i c l e s w i t h L = 1.8 I / m 3.
Investigations on fine particle separation
$777
Figure 3 s h o w s collection efficiencies for the o p e r a t i n g c o n d i t i o n s described above. In the case w i t h o u t electrostatics the collection efficiency over t h e particle d i a m e t e r h a s a S-like s h a p e t h a t is typical for inertial capture. The very low w a s h i n g liquid c o n s u m p t i o n leads to a poor collection efficiency with a cut size of 4 gm. The charging of the particles and t h e d r o p s yields to a c o n s i d e r a b l e i n c r e a s e of collection efficiency. The curve h a s a flatter s h a p e t h a t i n d i c a t e s electrostatic effects. With the s a m e w a s h i n g liquid c o n s u m p t i o n and the same p r e s s u r e loss the cut size r e d u c e s to 0.4 gm. O t h e r e x p e r i m e n t s to investigate the effect of image forces are done. In t h e s e investigations either the drops or the particles were charged. It can be stated t h a t the charging of the drops, has only a weak effect, w h e n the particles are u n c h a r g e d . If only the particles are c h a r g e d an i n c r e a s e d collection efficiency due to an i n c r e a s e d deposition also on t h e walls of t h e s c r u b b e r could be s t a t e d . F u r t h e r e x p e r i m e n t s are n e c e s s a r y to q u a n t i f y this effect as the influence of other operating c o n d i t i o n s of the nozzles a n d the scrubber. Using the principle of electrostatic e n h a n c e d scrubbing, it can be stated, t h a t it is possible to i n c r e a s e the s e p a r a t i o n p e r f o r m a n c e c o n s i d e r a b l y w i t h o u t the disadvantage of an increased pressure loss.
ACKNOWLEDGEMENTS The a u t h o r s t h a n k the Deutsche Forschungsgemeinschafl and KT Kunststofftechnik GmbH D-5210 Troisdorf for the friendly s u p p o r t of this project.
REFERENCES Law, S.E. (1978). E m b e d d e d - E l e c t r o d e E l e c t r o s t a t i c - l n d u c t i o n SprayC h a r g i n g Nozzle: Theoretical and E n g i n e e r i n g Design. TRANSACTIONS of the ASAE, pp. 1096-1104. S c h m i d t , M., LSffler, F. (1992). C a l c u l a t i o n of particle d e p o s i t i o n on c h a r g e d droplets. Preprints: 2. Europ. Symp. Separation of Particles from Gases (PARTEC 92) N~rnberg. pp. 21-41