Particle Control Techniques

David Leith

Dept. of Environmental Sciences and EngineeringUniversity of North Carolina at Chapel Hill

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

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

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

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

Design Parameters

Characterize collector performance

1. Collection Efficiency

2. Pressure Drop

3. Size and Initial Cost - not discussed here

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

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

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

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

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

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

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

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

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

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

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

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

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

Collection by Inertial Impaction

Stokes Number, Stk

0

1

D18

CVdStk cp

2

Impaction is important for big particles that move fast

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

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

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

+

-

+

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

Other Mechanisms: Less Important

Interception Gravity Radiometric forces

– Thermophoresis – Diffusiophoresis– Stephan flow– Photophoresis

BREAKTake five minutes,

then reconvene

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

Objective of Equipment Design 1. Determine efficiency, (d)

2. Determine pressure drop, P

Eff

ic’y

,

d

Particle Diameter

0

1

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

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

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

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?

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

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

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

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?

Filter Operation

particle

fiber

Particle collection on fibers

impaction, diffusion, electrostatics…

Need enough fibers to provide good coverage

gas

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

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

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

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

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

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

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)

Summary

Mechanisms cause particle collection– Impaction, diffusion, electrostatics

Collector Performance depends on:– Mechanisms,– Configuration of the device

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

Discussion