Particulate Scrubbers
Reading: Chap. 7
• Types of scrubbers: spray chamber and
venturi scrubber
• Theory and design consideration
• Pressure drop
• Contacting power
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Collecting medium:
 Liquid drops
 Wetted surface
Recirculated water
Spray Chamber
Q: What parameters will affect
the collection efficiency?
Q: Any other arrangement of
air & water?
Water to settling basin and recycle pump
Vertical spray chamber (countercurrent flow)
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Q: Is the gas velocity of any concern? Is droplet size important?
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Cyclone Spray Chamber &
Impingement Scrubber
Q: Is used water
recirculated?
Flagan & Seinfeld, Fundamental of Air
Pollution Engineering, 1988
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Venturi Scrubber
High efficiency even for small particles
QL/QG: 0.001 - 0.003
VG: 60 - 120 m/s
Q: ESP for sticky, flammable or highly corrosive materials?
Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995.
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Theory: Spray Chamber
 3
Volume of each droplet  d  d d
6
Total number of droplets that pass the chamber per second
QL
QL
6QL
Nd 

 3
d  d 3 d d
d
6
VG
QL: volumetric liquid flow rate
Droplet concentration in the chamber
Nd
6QL
nd 
 3
AcVd d d AcVd
Vd
Vtd
Vd  Vtd  VG
Vd: droplet falling velocity relative to a fixed coordinate
Vtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow)
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At a given time dt, the distance a droplet falls is
dz  Vd dt
Volume of air that flows through the cross-section area of a single
droplet during the time dt
Vair, single
 2 
  2  Vtd
  d d Vtd dt   d d 
dz
4 
 4  Vd
Total effective volume of gas swept clean per second by all
droplets in dz
 d d2  Vtd 6QL

Vair,all   d 
dz 3
d d
 4  Vd
Total number of particles swept clean per second by all droplets in dz
 d d2  Vtd 6QL

dN p   d 
dz 3 n
d d
 4  Vd
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p, z
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Total number of particles removed per second over dx

dN p  VG Ac n p , z  dz / 2  n p , z  dz / 2

QL
Particle penetration in a countercurrent vertical
spray chamber

3QLVtd d z 

Pd  exp 
 2QG d d (Vtd  VG ) 
 AdVtd d 

 exp 
QG 

N
z dz / 2
N
N
z
z dz / 2
Cross-sectional area of all the droplets
 6QL
Ad   Ac z   3
 d d AcVd
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  d d2 
3QL z
  
 
  4  2d d Vtd  VG 
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QG
8
 6.12104 QLVtdd z 

Pd  exp 
QG d d (Vtd  VG ) 

If QL in gal/min and QG in cfm, z in ft and dd in mm
Particle penetration in a cross-flow spray chamber
 3  QL   d  
 AdVtd d 







Pd  exp  
z   exp 



QG 

 2  QG  d d  
Q: How do we have higher collection efficiency?
Q: What are the collection mechanisms (we need it for d)?
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Deposition of Particles on a Spherical Collector
Re 
mG
Sc 
G D
d dVtd  G
mG
Particle Reynolds #

dp
dd
St 
Particle Schmidt #
mL

mG
Diameter ratio
Cc  p d p2Vtd
18mG d d
Particle Stokes #
Viscosity ratio
Single droplet collection efficiency
d
(diffusion)
(interception)
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(impaction)
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Impaction only
 St 
d  I  

 St  0.35 
2
(Impaction parameter Kp is
used in textbook; Kp = 2 St)
Q: Why is there an
optimal size?
p = 2 g/cm3
Q: The operating condition of a vertical countercurrent spray chamber are: QL/QG = 1 L/m3,
VG = 20 cm/s, dd = 300 mm and z = 1 m. Calculate the collection efficiency of 8 mm
particles through this chamber. Assume atmospheric pressure, 25 oC and p of 1 g/cm3.
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Venturi Scrubbers: Calvert Design
Particle penetration through a venturi scrubber

Q V  d
Pd  exp L G L d

 55QG mG

 K po f  0.7 
0.49



0
.
7

K
f

1
.
4
ln


po

 0.7  K f
0
.
7



po
 1 



 K po 

Kpo=2St (aerodynamic diameter) using throat velocity
f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials
Sauter mean droplet diameter
0.5
0.45
1.5
 mL  
QL 


1000 
 597
0.5 
QG 
  L   
, L and m should be in cgs
k1 = 58600 if VG is in cm/s
QL and QG should be of the same unit
= 1920 if VG is in ft/s
k1
dd 
VG
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 
 
 L 
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Pressure Drop
Venturi Scrubber
QL
p  k V
QG
2
L G
k  2(1  X 2  X 4  X 2 )
3lt C Dd  G
X
1
16d d  L
lt: venturi throat length
X: dimensionless throat length
Ex: 10” water, 2 mm,  = ?
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Venturi scrubber collecting a metallurgical fume
Contacting Power
Approach
When compared at the same power
consumption, all scrubbers give the
same degree of collection of a given
dispersed dust, regardless of the
mechanisms involved and regardless
of whether the pressure drop is
obtained by high gas flow rate or high
water flow rate
Contacting power, hp/1000 cfm

  1  exp( Nt )
Nt  PT
Nt: Number of transfer unit
(unitless)
(PT in hp / 1000 acfm)
(1 inch of water = 0.1575 hp/1000 cfm)
Q: Tests of a venturi scrubber show the results Friction loss (in H2O)  (%)
listed on the right. Estimate the contacting
12.7
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power required to attain 97% efficiency.
38.1
89
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Quick Reflection
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