8 particulate collectors - wet scrubbers

8/21/2019 8 Particulate Collectors - Wet Scrubbers

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AIR POLLUTION CONTROLENGINEERING

Particulate collection: wet scrubbers

Prof. Stefano CERNUSCHI


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DIIAR Sez. Ambientale

WET SCRUBBERSOperating principles

Particulate removal through utilization of water (or liquid solutions) for wetting solids and incorporate particles within liquid phaseOriginally developed for gaseous pollutants removalParticle collection strongly dependent on surface to volume ratio ofgas/liquid interface liquid droplets : sprayed (coarse) or atomized (very fine) liquid films

combined films/droplets in high turbulence regionsProcess stages1. liquid phase dispersion in drops, films, vortex for increasing gas/liquid interphase

surface2. solid capture by liquid through interception and/or liquid condensation3. liquid separation from clean flue gas

Total particulate collection efficiency of the system affected by 1, 2 and 3above

2



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Total particulate collection efficiency of the system dependent on 1,2,3

3Particulate collection: wet scrubbers

1+2 3


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Particle collection mechanisms


Liquid condensation(solid particle acting ascondensation nuclei for

supersaturated gas)

Moving droplet

Moving dropletDirect impact

Inertial impactMoving droplet

Moving dropletDiffusion impact


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WET SCRUBBERSSystem configurations

Spray towers cylindrical columns equipped with pressure nozzles, in co-current, counter-current

or cross-flow configuration with or without packing material (fixed, floating) simple system, low pressure drop low efficiencyPacked column filled with fixed or floating elements with high specific surface to promote theformation of a liquid filmJet systems

preformed liquid spray vortex systems

Venturi scrubbers



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Spray towers


Counterflow

Crossflow


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Spray towers


Cocurrent flow with cyclone separator




Plate towers


Cocurrent flow

Countercurrent flow




Packed bed towers


Fixed bed Floating bed




Jet systems


Preformed liquidspray in cocurrent flow




Jet systems


Preformed liquid

cyclonic scrubber




Venturi


Raw gas inlet

Clean gas outlet

Water feed

Venturi

Mist eliminatorGas flux

Liquid flux

Particulate removal(wet cyclone)

Liquid

drainage




Venturi


Variable throatregulation




Venturi


Raw gas inlet

Venturi

Wet cyclone withplate separation aid

SC S




Venturi


WET SCRUBBERS




Particle collection mechanisms: rainstorm model


Particle concentration c(uniform)

Volume swept bysingle spherical drop V D Mass of particles captured

by single drop

Number of drops/time N D (uniform)

WET SCRUBBERS



WET SCRUBBERSCollection efficiency

Mass balance on particles in total volumedcdt =

mass transferred per drop number drops/time

volume

Mass transferred per drop = Mass intercepted capture efficiency =

4 D2

Dzc t

dcdt

= 4D2Dzc t ND

xyz =

4D2Dc t

ND xy

= 4

D2Dc tND

xy

6 D3

D

6D3

D

=

= 1.5c tDD

ND 6 D3

D

xy

SinceND 6 D

3D

xy =

QL A

with Q L = volume flow rate of liquid and A = horizontal cross section of gas/liquidcontact region, then

dcdt

= 1.5c tDD

QL A


particles massremoved

WET SCRUBBERS




Mass balance on particles in total volumedcdt = 1.5

c tDD

QL A

dcc

= 1.5tDD

QL A

dt

c

c0=exp 1.5

t

DDQL

A

t

higher removal with increasing rainfall intensity [ QL A t], increasing removal efficiency

[t], decreasing drop size [ DD]

Extension to scrubbersFor any scrubber, time interval t = gas residence time = Az/Q G:

E dp = 1 exp 1.5tDD

QLQG

z

Efficiency dependence withscrubber configuration, design and operating parameters (liquid-gas ratio Q L /Q G,height z, droplet dimension D D)

particle capture efficiency t 18Particulate collection: wet scrubbers

WET SCRUBBERS




Particle capture efficiency dependent on inertial forces that tend particles to hit obstacle rather than move over

with flue gas stream

Momentum balance in particle trajectory towards target obstacle ( y direction )[FE - F A = m p dv/dt = 0 ]y FE = inertial; F A = drag

mpdvy,p /dt = 3 d pvy with vy is the relative gas/particle velocity: vy = v y,g - vy,p For spherical particles:

dvy,p /dt = 18 (vy,g - vy,p )/ pd2p

19

y,p

y,p2

p py,g

18 tv

dvdv-


d p p

WET SCRUBBERS




Particle capture efficiency

t = time required for y-forces to move particle over obstacle (first approx.),proportional to time required for gas flow to pass over obstacle t Db /v

N = impact number: ratio between Stokes stopping distance and obstacle diameter

20

y,p

y,

b2 2

p p p py, pg

18tdvv

18D 1 d d vv - N


y,p

y,p2

p py,g

18 tv

dvdv-

2p

DSp

D

d v= / =

1N D

8DXDD

Xs

d p p

WET SCRUBBERS




Particle collection mechanisms

large integral large time and force for particles to go over obstacle, lesser capture: lower efficiencies for small N

small integral time and force less adequate for moving particles over obstacle,higher capture: higher efficiencies for large N

higher N: higher stopping distance with respect to obstacle diameter

higher capture efficiency t

21

y,g

y,p

y,p

dv

- v

1

v N


DD Xs

2

tSpherical obstacle (drops) : =

7

N

+ 0.N

WET SCRUBBERS


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Cross flow scrubbers

Rainstorm model

E dp = 1 exp 1.5tDD

QLQG

z

Higher E with small drops ( DD) tall device (z)but gas flows ( no still air like rainstorm) high entrainment by gas for small drops and/or tall devices low

efficiencies attainable in practice 22Particulate collection: wet scrubbers

WET SCRUBBERS


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Counterflow scrubbers

Distance travelled by drop with respect to fixed coordinates distance travelled by drop with respect to gas rainstorm model to be modified forconsidering absolute drop velocity v D with respect to fixed coordinates

vD = vtD - vG vtD = drop terminal settling velocity with respect to gas (relative velocity)vG = gas velocity


vtD

vG

vD

WET SCRUBBERS


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Counterflow scrubbers - mass balanceMass particles captured

volumetime [M] = mass particles transferred out gas

volumetime [T]

M = Volume swepttime Particle concentration Capture efficiency = VC t

T = Gas flow rate Particle concentration change = QG c

V = Total drops in system Volume "swept" by one drop

time= NVD

N =Dropstime

Average drops "absolute" residence time =QL

D3D6

z

vtDv G

VD= Drop cross section Drop velocity relative to gas = D2D

4 v tD

V = Q L 1.5DD z zvtDv G

Final mass balance (infinitesimal terms)

QL1.5DD

dz vtDvtDv Gc t = QG dc


WET SCRUBBERS


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Counterflow scrubbers - mass balancedc

c= 1.5

t

DD

QL

QG

vtDvtDv G

dz

E dp = 1 exp 1.5 tDD

QLQG

z vtD

vtDv G

Higher E with

small drops ( DD) tall device (z) downflow drops velocity v tD approximating upwards gas velocity v G (v D 0)

liquid drops retained in the scrubber: accumulation of liquid, flooding conditions

practical applications not suitable for high particulate removals

Comparison with cross flow scrubber: E dp = 1 exp 1.5tDD

QLQG

z

drops cover longer distance relative to gas with respect to fixedcoordinates addition of vtDv

tDv

G

term


WET SCRUBBERS


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Cocurrent scrubbers

vD,rel = relative drops velocity with respect to gas = vG - v D,fixed

geometrical arrangement for obtaining small drops high relative drop velocities v D no drops losses from gas entrainment (cross and

counterflow) neither flooding risks (counterflow)


high N, high t

vD,fixed

vG

vD,rel

WET SCRUBBERS
http://upload.wikimedia.org/wikipedia/commons/2/2a/Adjthroatplunger.jpg


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Cocurrent scrubbers

Mass balance (see counterflow)Mass particles captured

volumetime [M] = mass particles transferred out gas

volumetime [T]M = VC t T = QG c

V = Total drops in system Volume "swept" by one droptime = NV D

N =Drops

time Average drops "absolute" residence time =

QL

D3

D6

x

vGv D,rel

VD= Drop cross section Drop velocity relative to gas = D2D

4 vD,rel

QL1.5D

D

dx vD,relvG v

D,rel

c t = QG dc


vD,fixed

vG

vD,relvD,rel = vG - v D,fixed

WET SCRUBBERS


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Cocurrent flow scrubbersdc

c =

1.5

tDD

QLQG

vD,rel

vG vD,rel dx

Dependence of v D,rel, N, t, and drop size distribution D D with x (distance frominjection point)

empirical or semiempirical solutions 28Particulate collection: wet scrubbers

Integration over x

t N and N vd,rel t vD,rel

WET SCRUBBERS


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Venturi scrubbers


Water feed

vD,fixed


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Venturi scrubbers

Ed (d

p) = 1-exp-(k R N )

k = device configuration geometrical constantR = liquid/gas ratio (Q L /Q G)N= impact number = f(d p, DD, v G)

Impact number N

capture efficiency increase with N Venturi configured for achievinghigher stopping distances XS ( high v G with respect to v D,fixed) and lowerdrop diameters D D


Xd,a

XS X

S< X

d,a no capture

X

S>X

d,a captureXS

Xd,a

D,

2 2p p G p p GS

G

fixed

D D DG

d Cu v - d Cu vX1

v= =D D 18

ND8

WET SCRUBBERS


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Venturi scrubbers Average drop diameter D D Nukijama-Tanasawa empirical relationship for pneumatic liquid atomization

DD = 4947/ vG + 29.7 R 1,5 operation at optimum drop size DD

too large D D less total area available for impactiontoo small D D rapid decrease in v D,rel (i.e. rapid acceleration to v G), lower time

for impaction


Optimal dropdiameter

D,relp p

D

2

G

d Cu

18 N=

vD

WET SCRUBBERS


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Pressure drop

Simplified momentum balance between gas inlet section 1 and outlet section 3 steady flow (Q G and Q L constant)

negligible friction losses with respect to energy loss by gas for accelerating liquid drops

SincevG3 v G1vL2 = 0vL3 = v G3, then:

Base expression for semi-empirical P equations:

k = throat geometry, drop diameter, units conversion factors


3 1 3 21 3 G G G G L L L LP -P A =Q v - v +Q v - v

2L L G L G L LL G2

G

Q v Q Q Q P = = = v A A Q

2 LL G

G

Q P = vQ

2 LL GG

Q P = k v

Q

WET SCRUBBERS


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Pressure dropWet scrubbers: higher P, higher removal efficiencies


Spray towers

Venturi

VENTURI WET SCRUBBERS


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VENTURI WET SCRUBBERSGeneral design procedure


INPUT DATAParticle characteristics density p size distribution (lognormal: M g, Sg)

Flue gas characteristics flow rate viscosity temperature particulate concentration moisture contentEmission limits

Collection efficiency E TI

INPUT DATA number of parallel devices

Throat cross section side L (squared)

L = (Q j /v) 1/2 diameter D (circular)

D = [4(Q j /v)/ ]1/2

drops diameter D D (Nukijama-Tanasawa)

RESULTSImpact number N

total collection efficiency E TD (numerical integration)

NO

OK

34

12

d p

TD d p p p0

E (d )= 1- exp(-K R N )E = E (d ) f(d ) d(d )

1.5D 4957D = 29.66 R v

change designparameters

INPUT DATA K constant for efficiency

RESULTSsaturated gas flow rate Q js

fan design power P V (kW)

YESGas flow rate/device

> 100000 m 3 /h

NO

ETD E TI

p 2p

D

vN= d

18 D

RESULTSPressure drop

-2 2

2p cm H O = 1.039 10 v R

INPUT DATA clean flue gas characteristics

(psychrometric chart)temperature T Sabsolute humidity U as

fan efficiency V

js V

V

Q pP =

100

WET SCRUBBERS


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Entrainment (mist) eliminatorsrequired by all wet scrubbing systems

different types depending on liquid droplet sizes inertial (impingement) separators for coarser drops (spray chambers, packed/plate towers) sieve type for finer drops (venturi)


Sieve type separators(mesh-pad of woven or

random metal or plastic fibers)

Impingementseparators

WET SCRUBBERS


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WET SCRUBBERSIndustrial applicability

Liquid properties particle and gas interactions with liquid: solubilization, flocculation, condensation,

froth formation could negatively influence process operation and affect separatedsludge and liquid by-products management and disposal

liquid viscosity affects process energy requirements outlet clean gas cooled and saturated with water vapor, with effects on plume

visibility and stack height

Gas characteristics gas/liquid interactions (even positive effects from simultaneous gaseous pollutants

absorption) higher and/or variable flow rates might be managed with parallel systems or

variable throat designs

high particle loadings and/or at high T might be treated with two stage systems:pretreatment (1 st stage) for removal of coarse fractions and gas cooling with lowerpressure drops, final treatment for fine fractions removal with higher pressuredrops on reduced gas flow rates


WET SCRUBBERS


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WET SCRUBBERSGeneral field of applications

Typical applications Wide gas flow rate variations

Humid dust Plants already equipped with a water treatment unit Fire or potentially explosive flue gas

Advantages High efficiency (> 99%) for fine particles (Venturi)

Simultaneous gaseous pollutants removal Suitable for hot and humid flue gas, adhesive particles Contained capital costs, low space requirement No fire or explosion risks Disadvantages

High operating cost (pressure drops) for Venturi scrubbers Liquid and solid (sludge) discharge Dust discharge as sludge Problems of corrosion, scaling, clogging Flue gas cooling and humidity saturation (visible plume) Noise impact


WET SCRUBBERS


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WET SCRUBBERSGeneral field of applications

General operating parameters

ParameterTypical range

Spray tower Vortex type VenturiCut diameter (m) 0.7 - 1.5 0.6 - 0.9 0.05 - 0.2Pressure loss (kPa) 0.6 - 2.5 1.5 - 3 3 - 20Gas velocity (m/s) 1 - 2 10 - 30 50 - 150

Liquid/gas ratio (l/m 3) 0.05 - 5 1 0.5 - 5Energy consumption (kWh/1000 m 3) 0.5 - 2 0.5 - 1 1.5 - 6

8 particulate collectors - wet scrubbers

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