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

112/04/22 Aerosol & Particulate Research Lab 30

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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112/04/22 Aerosol & Particulate Research Lab 32

Cyclone Spray Chamber &

Impingement Scrubber

Flagan & Seinfeld, Fundamental of AirPollution Engineering, 1988

112/04/22 Aerosol & Particulate Research Lab 33

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

112/04/22 Aerosol & Particulate Research Lab 34

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

112/04/22 Aerosol & Particulate Research Lab 35

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

112/04/22 Aerosol & Particulate Research Lab 36

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

112/04/22 Aerosol & Particulate Research Lab 37

)(

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

112/04/22 42

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

112/04/22 Aerosol & Particulate Research Lab 43

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

112/04/22 Aerosol & Particulate Research Lab 49

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