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Particulate Control-2Fabric Filters
Particulate Scrubbers
Lecture notes adapted from Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu
Fabric Filters
• Well known and accepted method for separating dry particles from a gas stream
• Many different types of fabrics, different ways of configuring bags in a baghouse and different ways of flowing the air through the bags.
• There are 3 common types of baghouse based on cleaning method– Reverse-air– Shaker– Pulse-jet
Fabric Filters
Fabric FiltersA shaker baghouse
Filter compartements
Fabric Filters
Fabric Filters
Filtration Theory
Filtration Theory
Filtration Theory
Figure 6.2 pp 186
Filtration Theory
Filtration Theory
Design Considerations
Cleaning Cycles
• tf: time interval between two cleanings of the same compartment
• tr: time interval between cleanings of any two compartment
Variation of pressure drop with time
Time
P
Pm
tr
tc
Cleaning Cycles
Maximum Filtering Velocities in Shaker or Reverse Air Baghouses
• Table 6.1
Dusts Max. Filtering V (ft/min)
Activated charcoal, carbon black, detergents, metal fumes
1.5
Aluminum oxide, carbon, fertilizer, graphite, iron ore, lime, paint pigments, fly ash, dyes
2
Aluminum, clay, coke, charcoal, cocoa, lead oide, mica soap, sugar, talc
2.25
Bauxite,ceramics,chorme ore, feldsapr, blour, flint, glass, gypsum, plastics, cement
2.5
Asbestos, limestone, quartz, silica 2.75
Cork, feeds and grain, marble, oyster shell, salt 3-3.25
Leather, paper, tobacco, wood 3.5
Fabric SelectionFabric Max Temp, C Acid resistance Base resistance
Dynel 71 Good Good
Cotton 82 Poor Good
Wool 93 Good Poor
Nylon 93 Poor Good
Polypropylene 93 Excellent Excellent
Orlon 127 Good Fair
Dacron 135 Good Fair
Teflon 204 Excellent Excellent
Glass 288 Good Good
Table 6.2
Pulse Jet Filters
• Introduced 45 years ago captured one-half of the industrial air filtration market
• Air is filtered through the bags from outside to the inside, a cage inside each bag prevents the bag from collapsing
• The bags are cleaned by short blast of high pressure air (90-100 psi)
• Each bag is pulsed every few minutes• On stream use
Pulse Jet Filters
• There are no compartments and thus no extra bags which reduces size and cost (for a large coal-fired power plant, the baghouse is so large that it is designed with separate compartments)
• Since bags are placed from the top, no need to provide walkways between rows of bags (reducing the size)
• Felted fabrics can be used at much higher air to cloth ratio (higher filtering velocities)
Pulse Jet Filters
• Table 6.5. Maximum Filtering Velocities for Various Dust or Fumes Dusts or Fumes Maximum Filtering Velocity
(ft/min)Carbon, Graphite, Metallurgical Fumes, Soap, Detergents;Zinc oxide
5-6
Cement (Raw), Clay (Green), Plastics, paitn Pigments, Starch, Sugar, Wood, Gypsum, Zinc
7-8
Aluminum oxide, cement (finished), Clay (vitrifies), Lime, Limestone, Mica,Quartz, soybean, Talc
9-11
Cocoa, Cholocate,Flour,Grains, Leather Dust, Sawdust,tobacco
12-14
Advantages
Disadvantages
Example
Example
Example
Other Considerations
• Temperature and Humidity : Fabrics have different maximum allowable teperatures. Low T can cause condensation of acid and/or blinding of the fabric with wet dust
• Chemical nature of gas: Different fabrics hav different resistance to acisd or alkalies
• Fire/explosion: Some fabric are flammable; Some dust are explosive
• Dust Handling: dust removal rate, conveyor system, and hopper slope should all be considered
Wet Scrubbers
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Particulate Scrubbers
• Types of scrubbers: spray chamber and venturi scrubber
• Theory and design consideration
• Pressure drop
• Contacting power
Reading: Chap. 7
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Spray Chamber
Re
circ
ula
ted
wa
ter
Water to settling basin and recycle pumpVertical spray chamber (countercurrent flow)
Collecting medium: Liquid drops Wetted surface
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Cyclone Spray Chamber &
Impingement Scrubber
Flagan & Seinfeld, Fundamental of AirPollution Engineering, 1988
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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
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Theory: Spray Chamber
Droplet concentration in the chamber
dcd
L
dc
dd VAd
Q
VA
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
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Volume of air that flows through the cross-section area of a single droplet during the time dt
dzV
VddtVd
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
Qdz
V
Vd
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
d
Qdz
V
VddN ,3
2 6
4
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QL
QG
zN
2/dzzN
2/dzzN
Total number of particles removed per second over dx
2/,2/, dzzpdzzpcGp nnAVdN
G
dtdd
GtddG
dtdLd
Q
VA
VVdQ
zVQ
exp
)(2
3expP
Particle penetration in a countercurrent vertical spray chamber
Cross-sectional area of all the droplets
Gtdd
Ld
dcd
Lcd VVd
zQd
VAd
QzAA
2
3
4
6 2
3
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)(
1012.6expP
4
GtddG
dtdLd VVdQ
zVQ
If QL in gal/min and QG in cfm, z in ft and dd in m
G
dtdd
d
d
G
Ld Q
VAz
dQ
Q exp
2
3expP
Particle penetration in a cross-flow spray chamber
Q: How do we have higher collection efficiency?Q: What are the collection mechanisms (we need it for d)?
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Single droplet collection efficiencyd
p
d
d
G
L
Diameter ratio Viscosity ratio
d
G
GtddVd
ReD
ScG
G
dG
tdppc
d
VdCSt
18
2
Particle Reynolds # Particle Schmidt # Particle Stokes #
Deposition of Particles on a Spherical Collector
(diffusion)
(interception) (impaction)
Impaction only
2
35.0
St
StId
p = 2 g/cm3
(Impaction parameter Kp is used in textbook; Kp = 2 St)
Venturi Scrubber
• Use intertial impaction of suspended particles on water droplet formed by gas atomization
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Venturi Scrubbers: Calvert Design
popo
popo
GG
dLGLd KfK
fKfK
Q
dVQ 1
7.0
49.0
7.0
7.0ln4.17.0
55expP
Particle penetration through a venturi scrubber
Kpo=2St (aerodynamic diameter) using throat velocity f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materialsAtomization produces a wide distribution of droplet size. However using the Sauter mean droplet diameter (dd) equation can be solved with satisfactory results.
5.145.0
5.0
5.0
1 1000597
G
L
L
L
LGd Q
Q
V
kd
k1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s
in dyne/cm, L in g/cm3 and should be in poiseQL and QG should be of the same unit
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116
3
)1(2 242
2
Ld
GDdt
G
LGL
d
ClX
XXXk
Q
QVkp
Pressure Drop
Venturi Scrubber
lt: venturi throat length X: dimensionless throat length
Ex: 10” water, 2 m, = ?
44
Contacting Power Approach
)exp(1 tN
Tt PN
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 :contacting power in hp / 1000 cfm) and : coefficient and exponent of PT
Nt: Number of transfer unit (unitless)
PT should be determined from the friciton loss across the wetted portion of the scrubber.
Contacting Power Approach
Venturi scrubber collecting a metallurgical fume
Contacting power, hp/cfm
Example
)exp(1 tN Tt PN (PT contacting power in hp / 1000 acfm)Nt: Number of transfer unit
(unitless) (1 inch of water = 0.1575 hp/1000 cfm)
Q: 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 56
38.1 89
Example
)exp(1 tN
Tt PN
Convert friction loss to contacting power (hp/1000 cfm): 1 in H20 =0.1575 hp/1000cfm
Friction loss (in H2O)
PT hp/1000cfm
12.7 2
38.1 6
(%) Nt
56 0.821
89 2.207
97 3.506
)207.2ln(6lnln6207.2
)821.0ln(2lnln2821.0
Example
Tt PN
Substractin Eq A from Eq B:
)207.2ln(6lnln6207.2
)821.0ln(2lnln2821.0
A
B
cfmhpP
P
T
T
1000/10
44.0506.3 90.0
44.06
207.2
90.0
)2/6ln()2ln6(ln989.0
90.0
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Problem 7.1
Solution
2
35.0
St
StId
dG
tdppc
d
VdCSt
18
2
Impaction parameter Kp is used in textbook
dG
tdaw
dG
tdppc
dG
tdppcP d
Vd
d
VdC
d
VdCStK
991822
222
w
ppa p
dd
22
Determine the density of water and the viscosity of the air at 80 °F from Appendix B
)(2
3expP
GtddG
dtdLd VVdQ
zVQ
Solution
)(2
3expP
GtddG
dtdLd VVdQ
zVQ
Solution
2
35.0
St
StId