Particulate Scrubbers

04/22/2023 Aerosol & Particulate Research Lab 1

Particulate Scrubbers

• Types of scrubbers: spray chamber and venturi scrubber

• Theory and design consideration• Pressure drop• Contacting power

Reading: Chap. 7

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Spray Chamber

Rec

ircul

ated

wat

er

Water to settling basin and recycle pumpVertical spray chamber (countercurrent flow)

Collecting medium:· Liquid drops· Wetted surface

What parameters will affect the collection efficiency?Any other arrangement of air & water?


Is the gas velocity of any concern? Is droplet size important?


Cyclone Spray Chamber & Impingement Scrubber

Flagan & Seinfeld, Fundamental of AirPollution Engineering, 1988

Is used water recirculated?


Venturi Scrubber

Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995.

High efficiency even for small particlesQL/QG: 0.001 - 0.003 VG: 60 - 120 m/s

ESP for sticky, flammable or highly corrosive materials?


Theory: Spray Chamber

Droplet concentration in the chamber

dcd

L

dc

dd VAd

QVA

Nn 3

6

Vd: droplet falling velocity relative to a fixed coordinateVtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow)

Volume of each droplet3

6 dd d

Total number of droplets that pass the chamber per second

33

6

6d

L

d

L

d

Ld d

Q

d

QQN

Vtd

VG

Vd

Gtdd VVV

QL: volumetric liquid flow rate


Volume of air that flows through the cross-section area of a single droplet during the time dt

dzVVddtVd

d

tddtdd

22

single air, 44V

Total effective volume of gas swept clean per second by all droplets in dz

3

2

allair,6

4V

d

L

d

tddd d

QdzVVd

At a given time dt, the distance a droplet falls is

dtVdz d

Total number of particles swept clean per second by all droplets in dz

zpd

L

d

tdddp n

dQdz

VVddN ,3

2 64


QL

QG

zn

2/dzzn

2/dzzn

Total number of particles removed per second over dx

2/,2/, dzzpdzzpcGp nnAVdN

G

dtdd

GtddG

dtdLd

QVA

VVdQzVQ

exp

)(23expP

Particle penetration in a countercurrent vertical spray chamber

Cross-sectional area of all the droplets

Gtdd

Ld

dcd

Lcd VVd

zQdVAd

QzAA

23

46 2

3


)(1012.6expP

4

GtddG

dtdLd VVdQ

zVQ

If QL in gal/min and QG in cfm, z in ft and dd in mm

G

dtdd

d

d

G

Ld Q

VAzdQ

Q exp23expP

Particle penetration in a cross-flow spray chamber

How do we have higher collection efficiency?What are the collection mechanisms (we need it for d)?


Single droplet collection efficiencyd

p

dd

G

L

mm Diameter ratio Viscosity ratio

d

G

GtddVdm

Re D

ScG

G

m

dG

tdppc

dVdC

Stm

18

2

Particle Reynolds # Particle Schmidt # Particle Stokes #

Deposition of Particles on a Spherical Collector

(diffusion)

(interception) (impaction)


Impaction only2

35.0

StSt

Id

p = 2 g/cm3

(Impaction parameter Kp is used in textbook; Kp = 2 St)

The operating condition of a vertical countercurrent spray chamber are: QL/QG = 1 L/m3, VG = 20 cm/s, dd = 300 mm and z = 1 m. Calculate the collection efficiency of 8 mm particles through this chamber. Assume atmospheric pressure, 25 oC and p of 1 g/cm3.

Why is there an optimal size?


Venturi Scrubbers: Calvert Design

popo

popo

GG

dLGLd KfK

fKfK

QdVQ 1

7.049.0

7.07.0

ln4.17.055

expPm

Particle penetration through a venturi scrubber

Kpo=2St (aerodynamic diameter) using throat velocity f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials

Sauter mean droplet diameter

5.145.0

5.0

5.0

1 1000597

G

L

L

L

LGd Q

QVkd

m

k1 = 58600 if VG is in cm/s = 1920 if VG is in ft/s

, L and m should be in cgsQL and QG should be of the same unit


Pressure Drop

Venturi Scrubber

Ex: 10” water, 2 mm, = ?

1163

)1(2 242

2

Ld

GDdt

G

LGL

dClX

XXXk

QQVkp

lt: venturi throat length X: dimensionless throat length


Contacting Power Approach

)exp(1 tN Tt PN Contacting power, hp/1000 cfm

When compared at the same power consumption, all scrubbers give the same degree of collection of a given dispersed dust, regardless of the mechanisms involved and regardless of whether the pressure drop is obtained by high gas flow rate or high water flow rate

(PT in hp / 1000 acfm)Nt: Number of transfer unit (unitless) (1 inch of water = 0.1575 hp/1000 cfm)

Venturi scrubber collecting a metallurgical fume

Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency.

Friction loss (in H2O) (%)

12.7 5638.1 89


Quick Reflection

Particulate Scrubbers

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