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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.
11
Centrifugal Separators
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
Centrifugal Separators
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
Centrifugal Separators
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
Centrifugal Separators
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
Centrifugal Separators
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
17
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
21
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.
23
How Do ESPs Work?
Two stage esp (www.airwater.com)
www.state.ia.us
One stage esp (www.zet.com)
www.bha.com
24
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
ESPs(Cottrell precipitators)
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
ESPs(Cottrell precipitators)
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
ESPs – Collection Efficiency
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
ESPs – Collection Efficiency
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.
35
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
39
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
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
51
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
53
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
Removal of Particles in a Co-flow Scrubber
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
58
Removal of Particles in a Co-flow Scrubber
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($$$
328245)1000
)(9806)](100)(5.0[(2
Ex. 9-26: For a scrubber using water as the scrubbing fluid, estimate the pressure drop: VG = QG/xy = 100 m/s and QL/QG = 0.001
OcmHatmm
N
mkg
sN
m
kg
Q
QV
A
yx
VQP
G
LLG
LGLGLL
224
2
32
2
2
1021.0~10
))(001.0)(1000()100(
61
Ex. 9-27: Dcut = 0.5 , QL/QG = 0.001, & C = 1.24, find gas velocity at the throat and P.
Daerodynamic cut diameter =
(0.5)(2*1.24)0.5 = 0.79V = 90 m/s (Fig. 9.27)P =~ 80 cm of water (Fig. 9.27)
62
Flammable and explosive dusts are OKGas adsorption and particle collectionCan do mistsCools hot gases (can feed to fabric filter if dried)Flexible Chemicals may become less nasty through reaction
Corrosion issues - water may increase corrosivityCreates wet waste stream- water pollution + $$$Need to remove collected particles from water before recirculatingHigh power input to generate well-dispersed droplets
General Scrubber Thoughts
64
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
65
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.