particle control techniques david leith dept. of environmental sciences and engineering university...
TRANSCRIPT
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Particle Control Techniques
David Leith
Dept. of Environmental Sciences and EngineeringUniversity of North Carolina at Chapel Hill
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Objectives of the Tutorial
1. Discuss particle control at an “intermediate” level
2. Provide some supplemental information and references
3. Provide some tools to help illustrate the concepts discussed
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Materials Provided
Outline of the Tutorial
CD with the following files:– This Powerpoint Presentation Control.ppt – Excel Spreadsheet: Control.xls – Supplemental Reading: Control.pdf– Tutorial Outline Outline.pdf
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Introductions
Who are you?
Is there a particular aspect of control technology that brings you here?
We will try to shape the tutorial somewhat to reflect your interests
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Outline of the Tutorial
1. Design Parameters Efficiency, pressure drop
2. Examples of Control Devices
3. Collection Mechanisms Impaction, diffusion, electrostatic attraction
BREAK
4. Control Equipment Cyclones, scrubbers, filters, ESPs
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Design Parameters
Characterize collector performance
1. Collection Efficiency
2. Pressure Drop
3. Size and Initial Cost - not discussed here
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Collection Efficiency, Efficiency: fraction of incoming particles collected Fractional Efficiency, (d): Efficiency vs. diameter
Overall Efficiency, :– Depends on fractional
efficiency, (d) and– Size distribution, F(d) E
ffic
’y,
dParticle Diameter
0
1
0
o )d(d)d(F)d(See Spreadsheet
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Difference in static pressure upstream vs downstream of collector
Fan operating cost = constant x P x Q
Pressure Drop, P
Collector To Fan
P
Air Flow, Q
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Outline of the Tutorial
Design Parameters Efficiency, pressure drop
Examples of Control Devices Collection Mechanisms
Impaction, diffusion, electrostatic attraction
Control Equipment Cyclones, scrubbers, filters, ESPs
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Inertial Collectors: Cyclones High inlet loadings
– Wet or dry particles
High , d > 10 m P 1 kPa (4” w.g.) Low initial cost Moderate operating cost Applications:
– Sawdust
– Rock dust
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Low Energy Scrubbers
High inlet loadings
– Wet or dry particles
Hot gases OK
High , d > 10 m
P 1 kPa (4” w.g.)
Moderate initial cost
Moderate operating cost
– water and slurry disposal
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High Energy Scrubbers
High inlet loadings
Hot gases OK
High , d > 0.5 m
P 10 kPa (40” w.g.)
Moderate initial cost
High operating cost
Applications:
– Metallurgical processes
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Cleanable Fabric Filters
Moderate inlet loadings
High , all particles
P 1.5 kPa (6” w.g.)
Moderate initial cost
Moderate operating cost
Applications:
– Dry dusts
– Power plants
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Disposable Media Filters
Very low inlet loadings
High , particles of all sizes
P 0.3 kPa (1” w.g.)
High replacement cost
Moderate operating cost
Applications:
– Cleanrooms
– Nuclear, drugs
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Electrostatic Precipitators: 2-Stage
Low inlet loadings
– Good for mist
High , d > 0.5 m
P 0.1 kPa (0.4” w.g.)
Moderate initial cost
Moderate operating cost
Applications:
– Indoor air quality
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Electrostatic Precipitators: 1-Stage
Moderate inlet loadings
High , d > 0.2 m
P 0.5 kPa (2” w.g.)
High initial cost
Moderate operating cost
Applications:
– Power plants
– Cement plants
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Outline of the Tutorial
Design Parameters Efficiency, pressure drop, size and cost
Examples of Control Devices Collection Mechanisms
Impaction, diffusion, electrostatic attraction
Control Equipment Cyclones, scrubbers, filters, ESPs
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Particle Collection Basics
Term in brackets is dimensionless group Particle distance depends on collection mechanism Collector distance depends on collector type
Collection = FDistance particle travels
Distance characteristic of collector
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Inertial Impaction
Particle deviates from gas streamline due to its inertia
Gas streamline
Object
Particle
D
dV
Impaction depends on Stokes Number, Stk
Stk = Particle stop distance
Dimension of target
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Collection by Inertial Impaction
Stokes Number, Stk
0
1
D18
CVdStk cp
2
Impaction is important for big particles that move fast
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Diffusion
Diffusion depends on inverse of Peclet Number, Pe
Particle deviates from gas streamline due to its Brownian Motion
Gas streamline
Object
Particle
D
d
Pe-1 = Particle diffusion distance
Dimension of target
V
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Collection by Diffusion
DV
DPe-1
d3
TkCD c
Diffusion is important at high temperatures for small particles that move slowly
Pe-1
0
1
TkCcV Dd3
Pe-1
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Electrostatic Attraction
Particle deviates from gas streamline due to Electrostatic Attraction
Electrostatic collection depends on:
Distance due to electrostatic force
Dimension of the collector
GasFlow
ElectricField
ChargedParticle
V
W
X
+
-
+
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Collection by Electrostatics
Electrostatics are important for charged particles in high electric fields
0
1
W t / X
W tX
n e E Cc t3 d X
=
n increases with E and d
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Other Mechanisms: Less Important
Interception Gravity Radiometric forces
– Thermophoresis – Diffusiophoresis– Stephan flow– Photophoresis
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BREAKTake five minutes,
then reconvene
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Outline of the Tutorial
Design Parameters Efficiency, pressure drop, size and cost
Examples of Control Devices Collection Mechanisms
Impaction, diffusion, electrostatic attraction
Control Equipment Cyclones, scrubbers, filters, ESPs
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Objective of Equipment Design 1. Determine efficiency, (d)
2. Determine pressure drop, P
Eff
ic’y
,
d
Particle Diameter
0
1
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Cyclone Operation
Gas forms vortex, also flows toward axis
Centrifugal force , Fc, pushes out;Drag force, FD, pushes in
Big particles go out toward wall Small particles go in toward axis
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Cyclone Collection Mechanisms
(d) = f [ d50, cyclone dimensions ]
core
inp
2
5050
d18
VdStk
=
= f [cyclone dimensions]
Efficiency, d50, (d)
Pressure Drop, P [ ]dimensionscyclonef2V
2g
=P x
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Cyclone Equations
2max,tcp
cvZ
Q9d
53.125.0
2cD
De
D
ba
2
D52.0r
Br2if1
B
r2
1BD
hHSHZ c
cc
33.074.061.0
2inletmax,tD
H
D
De
D
bav1.6v
d
d1
1
c
2
22cD
baln05.1
D
baln21.5dln87.062.0ln
g
1v
2
1HP
L
2g
3/1
2D
BD
hD
HD
S
D
ba20H
Collection Pressure Drop
See Handout and Spreadsheet
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Other Issues with Cyclones
Wall erosionSticky particles
Good operation under extreme conditionsSampling cyclones are different
Performance optimization is possible– For given gas flow and pressure drop, what
size and shape gives maximum efficiency?
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Venturi Scrubber Operation
Small particles impact on large droplets
Relative velocitybetween drops andparticles causes impaction
Water in
Large droplets with cargo of particles collect in entrainment separator
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Venturi Collection Mechanism
Need enough liquid droplets to provide good coverage
Relative velocity causes particle impaction onto water droplets
Particles embedded in air have high velocity
Accelerating droplets have low velocity
D18
VdStk
Ccp2
p - d
d
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Venturi Equations
2
d35.0Stk
Stk
2/3
G
L
diw/gd
Q
Q918.0
VV
0050.0d
K35.0
)35.0/K(tanK
35.01K55.3
K41.2
)*V1(K35.0
)35.0/)*V1(K(tan)K/35.0
*V1(K55.3
)*V1(1.2)*V1(K4
CQ
Q
expPt
5.015.0
de
5.0de
1
de5.0
5.0de
5.1de
Digg
LL
)1XXX1(2*V 22de
Ld
gDit
d16
CL31X
2
w/g
d/g
w/g
d/gde
2w/g
g
LL
V
V
V
V1*VV
Q
QP
Collection Pressure Drop
See Handout and Spreadsheet
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Other Issues with Scrubbers
Water supply and treatment expensive Vapor plume can be a problem Effective entrainment separation necessary
Performance optimization is possible– For given gas flow and pressure drop, what
throat diameter and water use rate gives highest efficiency?
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Filter Operation
particle
fiber
Particle collection on fibers
impaction, diffusion, electrostatics…
Need enough fibers to provide good coverage
gas
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Filter Collection Mechanisms
Single fiber collection by:– Impaction, Stk– Diffusion, Pe-1
– Electrostatics, Wt/X
Fibers in filter combine through– Solidity (volume fraction of fibers)– Thickness– Fiber diameter
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Filter EquationsCollection Pressure Drop
44
3
2
lnKu
2
2IKu2
StkJ
f
Cp2
d18
CVdStk
D
VdPe
gf
...DRDIRT
32
D PeKu
158.2
Tfd
L
1
4exp1
2f
g
dKu
LV16P
See Handout and Spreadsheet
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Other Issues with Media Filters
No effective theory for dirty filters– Pressure drop increases with use– Efficiency increases (solids)
or decreases (liquids) with use Ineffective gaskets and holes in media occur
Pleated media provide optimum performance– Maximize filter surface; minimize filter thickness
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Electrostatic Precipitators
Particles acquire charge:
field charging due to ions thatfollow electric field lines
electric field
diffusion charging due tomolecular motion of ions
random motionof ions
Charged particles in electric field move toward collection plate
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High voltage to electrodes causes electric field
Electrodes spaced between grounded plates
Operation - One Stage Precipitator
Particles charge and collectat the same time
Field charges particles and moves them toward plates
Practical precipitators have manyflow channels that operate in parallel
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ESP Equations
Particle Charging Particle Collection
Tk
tNecdKln
eK
Tkd)t(n iiE
E 21
2
2
2
tNZeK
tNZeK
eK
dE)t(n
iiE
iiE
E 142
3 2
eK
dE)t(n
E42
3 2
d
CEenW c
3
Q
AWexp1
See Spreadsheet
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Other Issues with ESPs
Electrodes can deteriorate with time Electrical problems occur
– Back corona– Dust resistivity problems; gas conditioning
Gas flow problems occur– Ineffective gas distribution– Gas flow through hopper (sneakage)
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Summary
Mechanisms cause particle collection– Impaction, diffusion, electrostatics
Collector Performance depends on:– Mechanisms,– Configuration of the device
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Summary, Cont’d
Collector performance described with modelsBased on physics of mechanisms and collectors
Models are inexact but can often provide insight into collector performance
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Discussion
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