8 particulate collectors - wet scrubbers
TRANSCRIPT
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
1/38
AIR POLLUTION CONTROLENGINEERING
Particulate collection: wet scrubbers
Prof. Stefano CERNUSCHI
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
2/38
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
Particulate collection: wet scrubbers
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
3/38
DIIAR Sez. Ambientale
WET SCRUBBERSOperating principles
Total particulate collection efficiency of the system dependent on 1,2,3
3Particulate collection: wet scrubbers
1+2 3
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
4/38
DIIAR Sez. Ambientale
WET SCRUBBERSOperating principles
Particle collection mechanisms
4Particulate collection: wet scrubbers
Liquid condensation(solid particle acting ascondensation nuclei for
supersaturated gas)
Moving droplet
Moving dropletDirect impact
Inertial impactMoving droplet
Moving dropletDiffusion impact
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
5/38
DIIAR Sez. Ambientale
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
5Particulate collection: wet scrubbers
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
6/38
DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Spray towers
6Particulate collection: wet scrubbers
Counterflow
Crossflow
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
7/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Spray towers
7Particulate collection: wet scrubbers
Cocurrent flow with cyclone separator
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
8/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Plate towers
8Particulate collection: wet scrubbers
Cocurrent flow
Countercurrent flow
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
9/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Packed bed towers
9Particulate collection: wet scrubbers
Fixed bed Floating bed
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
10/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Jet systems
10Particulate collection: wet scrubbers
Preformed liquidspray in cocurrent flow
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
11/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Jet systems
11Particulate collection: wet scrubbers
Preformed liquid
cyclonic scrubber
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
12/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Venturi
12Particulate collection: wet scrubbers
Raw gas inlet
Clean gas outlet
Water feed
Venturi
Mist eliminatorGas flux
Liquid flux
Particulate removal(wet cyclone)
Liquid
drainage
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
13/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Venturi
13Particulate collection: wet scrubbers
Variable throatregulation
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
14/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Venturi
14Particulate collection: wet scrubbers
Raw gas inlet
Venturi
Wet cyclone withplate separation aid
SC S
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
15/38DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
Venturi
15Particulate collection: wet scrubbers
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
16/38DIIAR Sez. Ambientale
WET SCRUBBERSOperating principles
Particle collection mechanisms: rainstorm model
16Particulate collection: wet scrubbers
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
17/38DIIAR Sez. Ambientale
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
17Particulate collection: wet scrubbers
particles massremoved
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
18/38DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
19/38DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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-
Particulate collection: wet scrubbers
d p p
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
20/38DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
Particulate collection: wet scrubbers
y,p
y,p2
p py,g
18 tv
dvdv-
2p
DSp
D
d v= / =
1N D
8DXDD
Xs
d p p
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
21/38DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
Particulate collection: wet scrubbers
DD Xs
2
tSpherical obstacle (drops) : =
7
N
+ 0.N
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
22/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
23/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
23Particulate collection: wet scrubbers
vtD
vG
vD
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
24/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
24Particulate collection: wet scrubbers
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
25/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
25Particulate collection: wet scrubbers
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
26/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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)
26Particulate collection: wet scrubbers
high N, high t
vD,fixed
vG
vD,rel
WET SCRUBBERS
http://upload.wikimedia.org/wikipedia/commons/2/2a/Adjthroatplunger.jpg -
8/21/2019 8 Particulate Collectors - Wet Scrubbers
27/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
27Particulate collection: wet scrubbers
vD,fixed
vG
vD,relvD,rel = vG - v D,fixed
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
28/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
29/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
Venturi scrubbers
29Particulate collection: wet scrubbers
Water feed
vD,fixed
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
30/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
30Particulate collection: wet scrubbers
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
31/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
31Particulate collection: wet scrubbers
Optimal dropdiameter
D,relp p
D
2
G
d Cu
18 N=
vD
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
32/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
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
32Particulate collection: wet scrubbers
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
33/38
DIIAR Sez. Ambientale
WET SCRUBBERSCollection efficiency
Pressure dropWet scrubbers: higher P, higher removal efficiencies
33Particulate collection: wet scrubbers
Spray towers
Venturi
VENTURI WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
34/38
DIIAR Sez. Ambientale
VENTURI WET SCRUBBERSGeneral design procedure
Particulate collection: wet scrubbers
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
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
35/38
DIIAR Sez. Ambientale
WET SCRUBBERSSystem configurations
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)
35Particulate collection: wet scrubbers
Sieve type separators(mesh-pad of woven or
random metal or plastic fibers)
Impingementseparators
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
36/38
DIIAR Sez. Ambientale
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
36Particulate collection: wet scrubbers
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
37/38
DIIAR Sez. Ambientale
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
37Particulate collection: wet scrubbers
WET SCRUBBERS
-
8/21/2019 8 Particulate Collectors - Wet Scrubbers
38/38
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