particulate scrubbers
DESCRIPTION
Particulate Scrubbers. Types of scrubbers: spray chamber and venturi scrubber Theory and design consideration Pressure drop Contacting power. Reading: Chap. 7. www.wpclipart.com/weather/happy_rain_cloud.png . Spray Chamber. Collecting medium: Liquid drops Wetted surface. - PowerPoint PPT PresentationTRANSCRIPT
04/22/2023 Aerosol & Particulate Research Lab 1
Particulate Scrubbers
• Types of scrubbers: spray chamber and venturi scrubber
• Theory and design consideration• Pressure drop• Contacting power
Reading: Chap. 7
www.wpclipart.com/weather/happy_rain_cloud.png
04/22/2023 Aerosol & Particulate Research Lab 2
Spray Chamber
Rec
ircul
ated
wat
er
Water to settling basin and recycle pumpVertical spray chamber (countercurrent flow)
Collecting medium:· Liquid drops· Wetted surface
What parameters will affect the collection efficiency?Any other arrangement of air & water?
04/22/2023 Aerosol & Particulate Research Lab 3
Is the gas velocity of any concern? Is droplet size important?
04/22/2023 Aerosol & Particulate Research Lab 4
Cyclone Spray Chamber & Impingement Scrubber
Flagan & Seinfeld, Fundamental of AirPollution Engineering, 1988
Is used water recirculated?
04/22/2023 Aerosol & Particulate Research Lab 5
Venturi Scrubber
Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995.
High efficiency even for small particlesQL/QG: 0.001 - 0.003 VG: 60 - 120 m/s
ESP for sticky, flammable or highly corrosive materials?
04/22/2023 Aerosol & Particulate Research Lab 6
Theory: Spray Chamber
Droplet concentration in the chamber
dcd
L
dc
dd VAd
QVA
Nn 3
6
Vd: droplet falling velocity relative to a fixed coordinateVtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow)
Volume of each droplet3
6 dd d
Total number of droplets that pass the chamber per second
33
6
6d
L
d
L
d
Ld d
Q
d
QQN
Vtd
VG
Vd
Gtdd VVV
QL: volumetric liquid flow rate
04/22/2023 Aerosol & Particulate Research Lab 7
Volume of air that flows through the cross-section area of a single droplet during the time dt
dzVVddtVd
d
tddtdd
22
single air, 44V
Total effective volume of gas swept clean per second by all droplets in dz
3
2
allair,6
4V
d
L
d
tddd d
QdzVVd
At a given time dt, the distance a droplet falls is
dtVdz d
Total number of particles swept clean per second by all droplets in dz
zpd
L
d
tdddp n
dQdz
VVddN ,3
2 64
04/22/2023 Aerosol & Particulate Research Lab 8
QL
QG
zn
2/dzzn
2/dzzn
Total number of particles removed per second over dx
2/,2/, dzzpdzzpcGp nnAVdN
G
dtdd
GtddG
dtdLd
QVA
VVdQzVQ
exp
)(23expP
Particle penetration in a countercurrent vertical spray chamber
Cross-sectional area of all the droplets
Gtdd
Ld
dcd
Lcd VVd
zQdVAd
QzAA
23
46 2
3
04/22/2023 Aerosol & Particulate Research Lab 9
)(1012.6expP
4
GtddG
dtdLd VVdQ
zVQ
If QL in gal/min and QG in cfm, z in ft and dd in mm
G
dtdd
d
d
G
Ld Q
VAzdQ
Q exp23expP
Particle penetration in a cross-flow spray chamber
How do we have higher collection efficiency?What are the collection mechanisms (we need it for d)?
04/22/2023 Aerosol & Particulate Research Lab 10
04/22/2023 Aerosol & Particulate Research Lab 11
Single droplet collection efficiencyd
p
dd
G
L
mm Diameter ratio Viscosity ratio
d
G
GtddVdm
Re D
ScG
G
m
dG
tdppc
dVdC
Stm
18
2
Particle Reynolds # Particle Schmidt # Particle Stokes #
Deposition of Particles on a Spherical Collector
(diffusion)
(interception) (impaction)
04/22/2023 Aerosol & Particulate Research Lab 12
Impaction only2
35.0
StSt
Id
p = 2 g/cm3
(Impaction parameter Kp is used in textbook; Kp = 2 St)
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.
Why is there an optimal size?
04/22/2023 Aerosol & Particulate Research Lab 13
Venturi Scrubbers: Calvert Design
popo
popo
GG
dLGLd KfK
fKfK
QdVQ 1
7.049.0
7.07.0
ln4.17.055
expPm
Particle penetration through a venturi scrubber
Kpo=2St (aerodynamic diameter) using throat velocity f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials
Sauter mean droplet diameter
5.145.0
5.0
5.0
1 1000597
G
L
L
L
LGd Q
QVkd
m
k1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s
, L and m should be in cgsQL and QG should be of the same unit
04/22/2023 Aerosol & Particulate Research Lab 14
Pressure Drop
Venturi Scrubber
Ex: 10” water, 2 mm, = ?
1163
)1(2 242
2
Ld
GDdt
G
LGL
dClX
XXXk
QQVkp
lt: venturi throat length X: dimensionless throat length
04/22/2023 Aerosol & Particulate Research Lab 15
Contacting Power Approach
)exp(1 tN Tt PN Contacting power, hp/1000 cfm
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
(PT in hp / 1000 acfm)Nt: Number of transfer unit (unitless) (1 inch of water = 0.1575 hp/1000 cfm)
Venturi scrubber collecting a metallurgical fume
Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency.
Friction loss (in H2O) (%)
12.7 5638.1 89
04/22/2023 Aerosol & Particulate Research Lab 16
04/22/2023 Aerosol & Particulate Research Lab 17
Quick Reflection