1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E. [email protected].

1

CE 583 – Control of Primary Particulates

Jeff Kuo, Ph.D., P.E. [email protected]

2

Content

Wall Collection Devices Gravity Settlers Centrifugal Separators Electrostatic Precipitators (ESP)

Dividing Collection Devices Surface Filters Depth Filters Filter Media Scrubber for Particulate Control

Choosing a Collector

3

Introduction

Many primary particles (asbestos and heavy metals) are more toxic.Many primary particles are respirable – health concern.Wall collection devices: driving the particles to a solid wall where they form agglomerates – gravity settler, cyclones, and ESP.Dividing collection devices: divide the flow into small parts where they can collect the particles – surface and depth filters, and scrubbers.

4

Wall Collection Devices –Gravity settlers

A long chamber through which the contaminated gas passes slowly, allowing time for particles to settle by gravity.Unsophisticated, easy to construct, little maintenance, treating very dirty gases (smelters and metallurgical processes), easy math.

5

Wall Collection Devices –Gravity settlers

Cross-sectional area (WH) > duct much lower velocity. Baffles spread the inflow evenly.Two ideal (limiting cases) Plug (block) flow model: unmixed. Mixed model

6

Gravity Settlers – Plug model

horizontalparticleeightidth

gasgas V

HW

QV ,

gas

engthresidence V

Lt

gast V

LVtV terminal = distance settling vertical

Particle removal efficiency related to residence time in chamber terminal settling velocity (Stokes’

law) distance to travel before hitting

wall

gaseight

engthplug VH

VL terminal

7

Gravity Settlers – Mixed model

Totally mixed in z-direction lead to decrease in (as gas move away from the inlet, C in a cross-section is homogeneous, so some particles still stay on the top, while the plug model particles will be more concentrated toward the lower sections).

]exp[1

exp1 terminal

flowplug

gaseight

engthmixed VH

VL

8

Gravity Settlers – Ex. 9.1

Find -D relationship for a gravity settler (H = 2 m, L = 10 m, Vavg = 1 m/s).

For 1- particle (how about 50-?)

44 1003.3]1003.3exp[1 mixed

45

262

terminal

1003.3)1)(2)(108.1(18

)2000()10)(81.9)(10(

18

HV

LgD

VH

VL

gaseight

engthplug

9

Gravity Settlers

Gravity settling is effective for large particles (>100), in reasonably sized chambers.To increase : making L larger (expensive), H smaller (hard to clean), Vavg smaller (expensive), increasing g.Increasing g: centrifugal.Horizontal elutriators: small gravity settlers used for particle sampling.

]exp[1

exp1 terminal

flowplug

gaseight

engthmixed VH

VL

10

Centrifugal Separators

Ex. 9.2: A particle travels with a gas stream with velocity of 60 fps (18.3 m/s) and r = 1 ft.

rmr

VmF

adius

asslcentrifuga

2circular2

1122.32

1/60/ 22

mg

rmV

F

Fc

gravity

lcentrifuga

smr

DVV c

t /0068.0)3048.0)(108.1(18

)2000()10()3.18(

18 5

26222

Ex 9.3: Find the terminal velocity of 1- particle

Cyclone (cyclone separator): most widely used particle collection device in the world.

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Rectangular gas inlet (2x as high as wide) tangentially to the vertical cylindrical body.The gas spirals around the outer part of the cylindrical body with downward component, then turns and spirals upward.The particles are driven to the wall by the centrifugal force.Dimensions are based on Do.

12


Inlet stream has a “height” Wi in the radial direction – the max. distance the particle needs to the wall.Length of flow path = NDo. (N = number of turns that gas makes traversing the outer helix = 5 typical).

ductinlet ofcircular )( eightidth

gas

HW

QV

)2/(W

distance stopping Stokes'

9 i

2terminal

NW

DNV

VW

VDN

i

c

circularidth

cycloneplug

plugmixed exp1

r

DVV c

t

18

22

13


Ex. 9.4: Compute -diameter relation for a cyclone separator with Wi = 0.5’, Vc = 60 fps and N =5.For 1- (how about 10-?)

0232.0)/000672.0018.0)(5.0(9

)8.124()1028.3)(60)(5(

9

262

cpsftlbmcpW

DNV

i

cplug

0230.00232.0exp1 mixed

5106.4

)/2000)(/60)(5(2

)/108.1)(5.0(9

2

9 63

5

mmkgsft

smkgft

NV

WD

particlec

inletcut

Cut diameter: diameter of a particle for which efficiency curve has the value of 50%.

14


For a typical cyclone, Dcut ~ 5.If gas contains few particles <5 cyclone is the first choice (low cost and easy maintenance). Not good for sticky particles such as tar droplets.Efficiency increases (Dcut decreases) with increasing Vcircular. But, P~ V2

circular.

Reduce inlet duct Width (and diameter in proportion)Split flow into multiple cyclones to keep Vcircular

constantIf Wi = 0.125’ Dcut = 2.3.

15


Eq. 9.18 is not a good predictor for (9.19 is a little better one).An empirical data-fitting equation Is more satisfactory. )189.(

9

2

EqW

DNV

i

cplug

]19.9.[exp1 Eqplugmixed cutD

Dr

2

2

1 r

r

16

Cyclone Collection Efficiency with Particle Size Distribution

Collection efficiency varies with particle terminal velocity, which in turn varies with particle diameter D and densityEx 9.6: Performance computation for a cyclone separator of Dcut = 5 m with log normally distributed particle size: Dmass mean = 20 m, = 1.25.

Divide the distribution into 10 fractions.Find z (= number of standard deviation).p = penetration

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Overall ~ 81% Mass mean diameter that passes thru the cyclone?The diameter corresponds to half of 0.1859 ~0.2014 of the mean diameter ~ 4.

2014.0)25.1282.1exp()exp( zDD

mean

1396.0)5/201007.0(1

)5/201007.0(

)]/)(/[(1

)]/)(/[(

)/(1

)/(

2

2

2

2

2

2

cutmeanmean

cutmeanmean

cut

cut

DDDD

DDDD

DD

DD

19

Cyclone – Pressure drop

Vi = velocity at the inlet to the cyclone (~1.5x the V in the duct approaching the cyclone).K ~8 for most cyclones.Ex. 9-8: A cyclone has a reported pressure loss of 8 velocity head and Vi = 60 fps.

)2

(2igVKP

OHm

N

mkg

sN

s

m

m

kg

psiin

ft

ftlbm

slbf

s

ft

ft

lbmP

22

22

3

22

23

"4.61606)()29.18)(20.1)(2

8(

23.0)12

)(2.32

()60)(075.0)(2

8(

Blower before cyclone: particles get into bearings and collect on blades; after cyclone: air may be sucked in and re-entrain particles due to vacuum.

20

CheapNo moving parts (low maintenance)Removes solid or liquid particles (non-corrosive particles)Harsh conditions (high temperatures)Time-proven technology (1940s)

Low efficiency for small particles (Dp<10 um)High pressure drops High operating costsCan’t do sticky particles

General Cyclone Thoughts

IMPACTION!Mechanism=

streamlines

BrownianMotion(diffusion)

impaction

interception

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Electrostatic Precipitators (ESP)

ESPs are effective on much smaller particles. Viscous resistance (Stokes’ law) ~ D.For gravity settlers or cyclones: driving force ~D3.For ESPs: electrostatic force ~D2.It’ hard for ESPs to collect smaller particles (~ 1/D), but still easier than cyclones and settlers.Give the particles an electrostatic charge and put them in an electrostatic field.Rows of wires held at –40,000 V and plates are electrically grounded.On the plates, particles lose their charge and form a cake – removed by rappers, or a film of water.

22

How Do ESPs Work?

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How Do ESPs Work?

Two stage esp (www.airwater.com)

www.state.ia.us

One stage esp (www.zet.com)

www.bha.com

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ESPs(Cottrell precipitators)

In a typical ESP, the distance between wire and plate is 4 – 6”. The field strength near the wire would be much higher because much small surface area.

)105(,

4001.0

40..;

distance,

m

MVgeometryofbecausewiretheatHigher

m

kV

m

kVge

voltageapplied

x

VEstrengthField

25


H : the height through which particles must travel, at right angles to gas flow, before hitting wallL : distance traveled by gas in the collection device. The H will be small in ESP, the velocity of particles much higher because of the electrostatic force.

26


Corona discharge at the wire: electrons collide with gas molecules, knock out electrons (ionizing the gas) knock more electrons loose to form a steady corona discharge. Field charging away from the wire: as electrons fly towards wall, they collide with particles and captured by particles, negatively charged particles attracted to wall and discharged there. Diffusion charging: for particles smaller than ~0.15 , the interaction with electrons is mainly due to their random motion as a result of electron-gas molecule collisions (not due to electric field).

27

ESPs –Maximum charge on a particle

Ex. 9-9: How many electronic charges on 1- ( = 6 and Eo = 300 kV/m)? How about 1/3-particles?

mVstrengthfieldlocalEmdiameterparticleD

spacefreeformV

Ctypermittivi

coulombsinEDq

particlesair

/,:;,:.

1085.8:)(

64:,0006.1:;constantdielectric:

23

0

120

02

0

electronsC

electronsC

q

300)10602.1

(1088.1

)000,300()10)(1085.8(8

63

1917

2612

28

ESPs – Drift velocity (terminal settling velocity under electrostatic force)

Force on particle: F = qEP (EP, local electric field strength)

Resulting terminal settling velocity (with Stokes law for drag force)

Ex. 9-10:

pEEDF 02

023

22

0EDwvt

smw /033.0)108.1(

)8/6()103)(1085.8)(10(5

25126

29

ESPs – Drift velocity

w ~ E2 (E ~ wire voltage/wire-to-plate distance). One can raise the voltage or reduce distance, but limitation is sparking (most set for ~50 – 100 sparks/minute).The drift velocity is only ~5x as fast of Vc of cyclone. Why ESPs are much more effective?The drift velocity ~D for ESPs and ~D2 for cyclones. To achieve high V in cyclones, one must have high gas V. Typical gas V ~ 3-5 fps for ESPs ( ~ 3 to 10 s), while V ~ 60 fps ( ~ 0.5 s) for cyclones.

22

0EDwvt

30

ESPs – Collection Efficiency

Block (plug) flow:

Mixed flow:

Q

wAAwithLh

hH

QwithV

HV

LvwrotewesettlinggravityFor

plugavg

avg

tblock

,.

Replace

:

5.0~,modifiedexp1

)exp(1

kADQ

wA

equationAndersenDeutschQ

wA

k

mixed

mixed

31


Ex. 9.11: Compute -diameter relation for an ESP that has particles with =6 and A/Q = 0.2 min/ft. For 1- particle:

Efficiency =99.8% for D = 5 Drift velocity is a function of DThe cut-diameter ~ 0.5 .Log(p) vs. A/Q is linear.

73.0)]602.0)(109.0(exp[1

)exp(1

Q

wAmixed

22

0EDwvt

32


Ex. 9.12: Estimate w for coal containing 1% S.From the figure at = 99.5% A/Q = 0.31 min/ft

smfps

ft

ftQA

pw

/086.028.0

min/1.17

min/31.0

005.0ln

/

ln

33

ESPs –Performance & Cake Resistivity

High resistivity ash (elemental S): large Vcake , small Vwire,

poor charging, low - electron flow within cake, back corona

Low resistivity ash (carbon black): small Vcake , weak attraction to collection plate, re-entrainment

Back corona is a conversion of electrostatic energy to thermal energy that will cause minor gas explosion blow the cake off the plate.The practical resistivity range: > 107 and < 2 x 1010 ohm-cm.

34

ESP –Performance and Cake Resistivity

Little can be done on low resistivity ash.Remedies for high resistivity ash:- Higher T, hot ESP (improves conduction of some materials in the ash under high T)- Gas conditioning, add hygroscopic components to gas to improve surface conductivity. Some S in coal is converted to SO3 (absorbs water). Coal ash is basic needs acidic conditioner. NH3 works for acidic Portland cement ash.

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ESPs –Performance

Ex 9.13: If of an ESP = 90%. How much must we increase the surface area to have = 99%?

From 90% to 99.9% triple the area. However, w is ~ diameter (harder to remove the fines).Ex. 9-14: Use the modified D-A equation with k =2, the area needs to be quadrupled (not 2x).

2

)exp(01.01

)exp(1.01

existing

new

newnewnew

existingexistingexisting

AA

Q

wAp

Q

wAp

36

ESPs –Performance

Ex 9.15: An ESP has two identical sections in parallel, each receive ½ of gas flow and = 95%. If the flow is mal-distributed into 1/3 and 2/3, = ?

074.0)(

0111.0)3/1

2/1(995.2exp[

1057.0)3/2

2/1(995.2exp[

995.2);exp(05.0

212211

2

1

ppQQpQpQ

p

p

Q

wA

Q

wAp

It shows the importance of flow distribution.

37

ESPs –Performance

The typical linear V inside an ESP ~ 3 to 5 fps and pressure drop is 0.1 – 0.5” water.The technology is established with up to 99.5%+.Wet-ESP can have higher w, more complex and the collected aren’t dry powder (but it seems worthwhile)

38

High for even small particlesLow P even with high flowDry or wet collectionWide range of temperatureLow operating costs

Take up lots of spaceHigh capital costNot flexible to changeMay need a pre-cleaner at high concentrations…cyclone?

General ESP Thoughts

www.bha.com Power plants Cement plants Paper mills Steel foundries Indoor air quality

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Capital Costs depend on total plate area ‘A’ Purchase price = aAb

a=962, b=0.628 for 10,000 ft2 < A < 50,000 ft2

Installation cost : ~2.2*DEC

Operating Costs - depend on power consumptionFan pulling the air through the plates

Total delivered equipment cost (DEC)=1.18*(purchase price)

ESP - Costs

40

Dividing Collection Devices

Divide the flow into small parts and bring it in contact with large surface area Surface filters Depth filters Scrubbers

Surface filters: fine particles are caught on the sides of holes of a filter (a membrane – sheet steel, cloth, wire mesh or paper) and a cake is formed (the actual filter)

41

Dividing Collection Devices –Surface filters

Surface velocity (face velocity, approach velocity, superficial velocity, air to cloth ratio).Vs = Q/APressure drop for flow through porous media

Ptotal = Pfilter + Pcake

medium porous of thickness:x

medium porous ofty permeabili:k

viscosityfluid:

..

k

VxP s

42

Filters - What Happens to the Collected Particles?Shaker Pulse-

jetSonic horn

Different types of cleaning Main way to identify bag housesDifferent bag materials (woven vs. ‘felted’)Different cleaning frequency

Reverse air

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interior exterior

www.bha.com

45

Surface FiltersAs the cake builds up, the outlet C declines and stabilizing at a value about 0.001x the inlet C. The falls with increasing Vs (Figure 9.15).

At low Vs, they will also have high on fine particles (ESPs have difficulties to collect particles of 0.1 to 0.5).

46

Woven:Stronger tensile strengthLonger time between cleaning (1/2 hr- several hours)Hold more filter cake

Shaker and reverse air use woven materials

Pulse jet use felted materials

Felted:Less tensile strengthShort time between cleaning (every few minutes)Abrasive particles, smaller particles always

47

Depth Filters Depth filters collect particles

throughout the entire filter body.Mechanisms that contribute to particle capture: impaction, interception, and diffusion (Table 9-3).High-efficiency, particle-arresting (HEPA) filters – thrown-away type (no cleaning).

streamlines

BrownianMotion(diffusion)

impaction

interception

48

Depth Filters

Impaction parameter (separation number):

diametertarget:

18

2

b

bb

Stokess

D

D

VD

D

xN

target) theostraight t went particles all if

collected be d that woulparticles of(number

collected) particles of(number

49

Depth FiltersEx. 9-18 to 9-20: A cylindrical fiber 10 is placed perpendicular to a gas stream (V = 1 m/s) with C = 1 mg/m3 and d = 1. Find . Find for a row of parallel fibers with center-to-center spacing of 5 fibers. How about 100 rows?

9998.011

%4.8)42.0)(5

1(

1042.0)10(42.0collection of Rate

10)10)(10)(1(ratepossibleMaximum

617.0)10)(/108.1(18

)1()10)(2000(

18

100100100

88

835

55

262

rowrowsrows

row

b

bs

pp

sm

gsm

gCVD

smkgD

VDN

50

High efficiency for even small particlesWide variety of solid particle typesModular flexible design, flexible conditionsLow pressure drops

Take up lots of spaceBad for high T and corrosivityBad for moist conditionsPotential for fire/explosionNeed frequent cleaningNeed bag replacement

General Fabric Filter Thoughts

solid waste.dpwt.com

www.usairfiltration.com

Mining plant

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When Would I Use a Fabric Filter?

Size classification is not desired High efficiency is requiredValuable dry material needs to be recoveredRelatively low volumesRelatively low temperatures

Power plants Fertilizer Food processing Paper mills Ore processing

www.usairfiltration.com

Fibreboard plant

52

Scrubbers

Bring the flow of gas in contact with a large number of liquid droplets representing a large surface areaNatural occurrence: rainfall

tdropp CzDm

2

4

efficiency

collection

ionconcentrat

particle

rain of dropby

swept volume

rain of drop one

by collected

mass particle

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Scrubbers - Removal of particles from a volume of air during a rainstorm

Ex 9-22: Q/A = 0.1”/hr with Ddrop = 1 mm. Air contains dparticle = 3 m, C0 = 100 g/m3. C1-hr =?

Find Vt = 14 ft/s (4.2 m/s) for 1 mm raindrop

Calculate Ns (=0.23)

Find t ~ 0.23 (Fig. 9-18)

C/C0 = 0.43 C = 43 g/m3

mm/hr e.g. area,unit per rate rainfall :

5.1exp0

A

Q

AD

tQCC

L

drop

Lt

54

Removal of Particles in a Cross-flow Scrubber

Make Ddrop small, and/or z large

Both measures would result in some liquid droplets being carried out of the scrubber.

Gdrop

Lt

QD

zQCC

5.1exp0

55

Removal of Particles in a Counter-flow Scrubber

As Vt VG , C 0

But, this means droplets are nearly stationary with respect to the container flooding

)(

5.1exp0

Gt

t

Gdrop

Lt

VV

V

QD

zQCC

56

Removal of Particles in a Co-flow Scrubber

Need high relative velocity between gas and droplets without loosing the droplets or equipment flooding.IDEA: Introduce water droplets at right angles to gas but let them go out with the gas, then separate them in a cyclone.This is a modification of the way a cross-flow scrubber is operated.

57


Idea is to increase velocity difference between particles and droplets and thus improve impaction.Venturi design is widely used because it saves fan power.

www.environmental-center.com

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Integration difficult because VG, Vrel, t all change with xDdrop is non-uniform, and not constant with x

dxVV

V

Q

Q

DC

dC

relG

rel

G

Lt

drop )(

5.1

59

ScrubbersEx. 9-23: In a venturi scrubber the throat V = 122 m/s. Particles to be removed = 1 and drop D = 100. QL/QG = 0.001. At a point Vrel = 0.9 VG, what is the rate of decrease in C in the gas phase?

. travelledmmevery 12.4% decrease

gas in the ionsconcentrat Particle

124.0124

)9.0

9.0)(10)(92.0)(

10

5.1(

dC/C

18)-9 Fig. (from 92.0

78.6)10)(108.1)(18(

)1229.0()10)(2000(

18

34

45

26

2

mmm

VV

V

dx

D

VDN

GG

G

t

b

ps

60

Scrubbers – Pressure dropEx. 9-25: A venturi scrubber has a throat area of 0.5 m2, a throat velocity of 100 m/s, and P = 100 cm water (9806 N/m2). Assuming motor&blower = 100%, find the power required.

yrkWhyrhkw

hpkwmN

skw

m

NPQP G

/300,107$)/05.0)($/8760)(245($$$

328245)1000

)(9806)](100)(5.0[(2

Ex. 9-26: For a scrubber using water as the scrubbing fluid, estimate the pressure drop: VG = QG/xy = 100 m/s and QL/QG = 0.001

OcmHatmm

N

mkg

sN

m

kg

Q

QV

A

QQ

yx

VQP

G

LLG

LGLGLL

224

2

32

2

2

1021.0~10

))(001.0)(1000()100(

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Ex. 9-27: Dcut = 0.5 , QL/QG = 0.001, & C = 1.24, find gas velocity at the throat and P.

Daerodynamic cut diameter =

(0.5)(2*1.24)0.5 = 0.79V = 90 m/s (Fig. 9.27)P =~ 80 cm of water (Fig. 9.27)

62

Flammable and explosive dusts are OKGas adsorption and particle collectionCan do mistsCools hot gases (can feed to fabric filter if dried)Flexible Chemicals may become less nasty through reaction

Corrosion issues - water may increase corrosivityCreates wet waste stream- water pollution + $$$Need to remove collected particles from water before recirculatingHigh power input to generate well-dispersed droplets

General Scrubber Thoughts

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What happens to the collected particles?

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When Would I Use a Scrubber???Wet particles that are in hot gas streamCorrosive particlesVery fine particles requiring high efficiencyParticles are with gases that also need to be removedCombustible gasesCooling is desirable and added moisture is not bad

Power plants Paper mills Food industry Cosmetics Steel/metal industry

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Choosing a collector

Small or occasional flow throwaway device (also a good final cleanup device).Sticky particles throwaway or into liquid.Particles that adhere well to each other but not to solid surfaces are easy to collect.Electrical properties of particles are of paramount importance in ESPs.For non-sticky particles >5 cyclones.For particles <5 ESPs, filters, and scrubbers.For large flows, pumping cost makes scrubbers $$$.Corrosion resistance and acid dew point must always be considered.

66

Summary

Gravity settlers, cyclones, ESPs drive particles to wall, similar design equations.Filters and scrubbers divide the flow. Different design equations.Surface filters for heavy laden gas streams; depth filters for final clean-up of air, or clean gas, or for fine liquid drops that coalesce on them and then drop off.To collect small particles, a scrubber must have a very large relative velocity between gas and liquid. co-flow scrubbers venturi scrubbers.

1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E. [email protected].

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Transcript of 1 CE 583 – Control of Primary Particulates Jeff Kuo, Ph.D., P.E. [email protected].