Venturi Scrubbers1

1

Venturi Scrubbers

• Basics

• Design Parameters

• Pressure Drop

• Particle Collection Efficiency

• Example Design Calculations

Advantages/Disadvantages

• Relatively small • Simple to operate • Low capital costs

• Comparable operating and maintenance costs to esp, baghouse

• Handles sticky, flammable, corrosive PM

• Independent of particle resistivity

• Higher energy costs

• Lower flow rates than esp, baghouse (105 _acfm)

• Potential for downstream corrosion and visible plumes • Produces sludge

3

In the fixed-throat venturi, the gas stream enters a converging section where it is

accelerated toward the throat section. Liquid droplets are also introduced into the

converging section. Owing to inertia, they have a different velocity relative to the smaller particles. The particles in the gas stream are collected when they impact upon the drops.

Venturi Basics

Exhaust gas

Liquid injection

5

7

• All wet scrubbers produce

entrained droplets.

• These droplets contain the

contaminants and must be

removed downstream

• This is referred to as mist

elimination or entrainment

separation.

• A cyclone is typically used for

the small droplets generated in

a venturi

To determine if a wet scrubber system is working properly, field personnel should observe if possible:

Outlet Gas Stream Opacity, but take into consideration the presence

of water droplets,

the Temperature Difference between the Gas Inlet and Outlet, the Liquid Flow Rate into the scrubber, and

Pressure Drop changes in the wet scrubbers and mist eliminators.

As with any inspection of an air pollution control device, attention must be given to the system’s:

Records & Physical Condition, and Compliance with Applicable Rules.

Wet scrubber systems used for air pollution control have many safety considerations including: Inhalation Hazards and Corrosive Liquids.

9

• The pressure drop across the mist eliminator provides an excellent indicator of its physical condition

• A decrease in the pressure drop across the mist eliminator may indicate structural failure

• The performance of the mist eliminator can also be evaluated by observing the stack and areas adjacent to the stack

• Rain-out of droplets around the stack, mud-lips and discolored streaks at the stack discharge, or heavy drainage from open ports all indicate a poorly performing mist eliminator.

Typical Venturi Scrubber

Design Parameters

• Liquid to Gas Ratio

(10–30 gallons/1000 acf )

• Gas Velocity at “Throat”

(60 – 150 m/s)

• Gas Pressure Drop

(< 80 inches H

₂

O)

• Inlet Particle Size

(>0.2 micrometers)

Liquid-to-Gas Ratio

• Higher L/G, higher η

• L/G optimal at 7–10 gal/1000ft

3

• L/G > 10 increases ∆P and operating costs

Due to their inertia, the larger water droplets respond slowly to changes in the surrounding gas velocity, whereas the smaller pollution particles respond

rapidly. The difference in velocity, the “relative velocity” causes impaction of the particles onto the droplets.

Relative Velocity

•Increased relative velocity,

increased η

•Highest gas velocity at center of

throat (150 – 500 ft/s)

•High relative velocities in throat

and downstream of throat

Pressure Drop

• ∆P = 10 to 80 in. w.c.

• ∆P > 45 in w.c. does not typically increase η in standard designs

13

Gonsalves, JAS et al Journal of Hazardous Materials B81 (2001) 123-140

Pressure Drop

(

2

)

water throat gas

Q

P

v

Q





∆ ∝ 

_



_





Calvert’s Model:

P

5.4 10

x

4

L

_sat

(

v

_throat2

)

G

ρ

−

 

∆ =

_{ }

 

∆P = pressure drop, inches w.c.

L/G = liquid to gas ratio, gal per 1000 ft3

ρ_sat = saturated gas stream density, lb/ft3 v_throat = gas velocity at throat, ft/sec

In general:

Assumptions:

• All liquid forms droplets

• Droplet acceleration only contribution to ∆P • All droplets have no initial axial velocity • Drops reach gas velocity in throat

15 sat throat throat sat

Q

P

v

K

A

ρ

∆

=

=

V_throat = gas velocity in venturi throat (ft/sec)

Qsat = gas volumetric flow rate at saturated conditions (ft3_/sec)

A_throat = cross-sectional area of venturi throat (ft2₎

∆P = pressure drop across venturi (inches H₂O) ρ_sat = density of gas at saturated conditions (lb/ft3) K = empirical constant to account for energy losses from Calvert’s equation:

1850 K L G =      

where L/G = liquid to gas ratio (gallons per 1000 ft3₎

throat throat inlet inlet

v

A

A

v

=

From mass continuity of the gas, the throat area is given as:

For optimal pressure recovery, the length of the throat area is

taken as 3 times the throat diameter and the length of the

diverging section is > 4 times the throat diameter.

L_throat

L_diverging

inlet

17

Calvert’s model over-predicts pressure

drop, worse for short throats

18

(

)

(

)

(

1

)

( )

2

2

gas gas gas gas drop

gas eq

m

f v

dP

v

dv

mv

dv

dx

D

ρ

+

−

=

+

+

Gas kinetics _{Drop kinetics} Film kinetics

L

m

G

=

(dimensionless)

Relatively Simple Numerical Model

R.H. Boll, Ind. Eng. Chem. Fundam. 12 (1973) 40 : See supplemental reading on course website

f = friction factor

D

_eq

= hydraulic diameter

Solve numerically for small dx, moving in the downstream direction while conserving mass and momentum:

19

Boll’s model works better

20

Collection Efficiency

•

Main removal mechanism is impaction

•

Particles < 0.1 µm mainly diffusion

•

Efficiency decreases exponentially with

decreasing particle size

Predicting Collection Efficiency

• Manufacturer Performance Curves

• Contact Power Theory

23

Performance Curve

(specific to a given

venturi geometry)

EPA Handbook Control Technologies for Hazardous Air Pollutants EPA625/6-91/014 June 1991

•

Reported pressure drop across venturi

•

Performance curve applicable to venturi

•

Reported collection efficiency

Contact Power Theory

All scrubbers give the same level of particle collection

at the same level of power consumption (Lapple &

Kamack)

25

T G L mech

P

=

P

+

P

+

P

P_T = total contact power,

P_G = power due to pressure drop of gas passing through the scrubber, P_L = power due to the scrubber liquid atomization, and

P_mech = power due to mechanical devices to increase contact, i.e., a rotor.

26

Contact Power Theory

0.157

G

P

=

∆

P

0.583

L L

L

P

p

G

 

=

_{ }

 

ΔP = pressure drop across venturi, in w.c.

p

_L

= liquid inlet pressure in pounds per square inch

*

L/G = liquid to gas ratio in gallons per cubic feet (gal/ft3)

27

Contact Power Theory

(

)

1 exp

P

_T

β

η

= −

−

α

Davis, W.T. Ed., Air Pollution Engineering Manual (2nd Edition), John Wiley & Sons, Inc., New York, 2000; Theodore, Louis and Anthony Buonicore, Ed., Air Pollution Control Equipment: Selection, Design, Operation, and Maintenance, Prentice-Hall, Inc. Englewood Cliffs, New Jersey, 1982.

Calvert Cut Diameter Model

( )

{

}

exp 1.3 0.518 ln

cut

d

=

−

∆

P

2

1

exp

ln(2)

j j j cut

d

Pt

d

η













= −

=

−

_

_



_

_







(

)

j _j

Pt

=

∫

Pt

mass

29

This program estimates the outlet size distribution from a Venturi Scrubber operating at a specified pressure drop. The particle penetration estimates are based upon the empirical model of Calvert et al ("Scrubber Handbook" NTIS Publication No. PB-213-016, NTIS, Springfield, VA, 1972)

Input parameters

pressure drop (cm H2O)= 50 inlet mass conc. (g/m3) = 1.00

sigma g = 1.50

MMAD (um)= 10.00

maximum throat velocity of gas (m/sec) 67.00

0 20 40 60 80 100 0.1 1.0 10.0 100.0 diameter, um % R em o v al

Calvert’s Cut Diameter Model

*

Example Design Calculations

30

31

Municipal Waste Incinerator

• Stack Gas flow rate: 110,000 acfm

• Stack Gas temperature: 400 F

• Moisture content: 5% by volume

• Dry gas MW = 29

• Particle mean size: 1 micrometer

• Required efficiency: 99.9%

32

Line of Maximum Possible Humidity Value (saturated; both vapor &

liquid present)

Vapor Only Not possible

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Humidification of Inlet Gas

lb-mole water 18lb water lb-mole gas lb water

0.05 =0.031

lb-mole gas lb-mole water 29 lb dry gas lb dry gas

in H = _ __ _    sat sat

400 F

T

127

lb water

H = 0.10

lb dry gas

in

T

F

=

=

From psychrometric chart:

Humidified air in the venturi

Air entering the venturi 5% by volume

Entering

scrubber

Leaving

scrubber

Adiabatic

Cooling &

Humidification

As water is

added

upstream of

the scrubber,

the water

evaporates,

cooling the

gas until it is

saturated

35

Venturi Scrubbers are sized based upon either the dry gas inlet volumetric flow rate or the saturated gas flow rate. Here we are using the saturated gas flow rate.

Saturated Gas Flow Rate

(

)

(

)

( )

(

)

460 460 sat sat in in w in dry in in gas sat in w w T Q Q Q T Q H H Q ρ ρ + = + + − = sat in in w sat in in gas w sat in Q Q Q T T H H ρ ρ = = = = = = = = =

Saturated emission stream flow rate, acfm Inlet emission stream flow rate, acfm Flow rate of water vapor added, acfm Temperature of saturated emission stream, F Temperature of inlet emission stream, F Density of emission stream, lb/ft3

Density of water vapor, lb/ft3

Absolute humidity of saturated emission stream, lb H₂O/lb dry air Absolute humidity of inlet emission stream, lb H₂O/lb dry air

(

)

P MW RT

ρ =

3

P = pressure of emission stream, atm MW = molecular weight of gas, lb/lb-mole

atm-ft R = gas constant, 0.7032 T = temperature of gas, lb mole R R − − 

(

1

)

dry in in ws Q =Q −B

36

Venturi Scrubbers are sized based upon either the dry gas inlet volumetric flow rate or the saturated gas flow rate. Here we are using the saturated gas flow rate.

Saturated Gas Flow Rate

(

)

(

460460

)

(

110, 000

) (

(

127400 460460

)

)

11, 500 86, 540 sat sat in in w in T Q Q Q acfm T + + = + = + = + + ( ) ( )( ) ( ) 3

1 atm 29 lb/lb-mole _{lb dry gas} 0.0676 0.7302 460 127 ft dry P MW RT ρ = = = +

(

1

)

110, 000 1 0.05

(

)

104, 500 dry in in ws Q = Q − B = − = acfm

( )

(

_{) (}

104, 500 0.0676 0.10 0.0316

₎₍

₎₍

₎

11, 504 0.042 dry in in gas sat in w w Q H H Q ρ acfm ρ − ₋ = = = ( ) ( )( ) ( ) 3

1 atm 18 lb/lb-mole _{lb water vapor} 0.042 0.7302 460 127 ft W P MW RT ρ = = = + Additional flow of evaporated water vapor

37

Venturi Scrubber Performance Curve

ΔP 47 inches w.c. at =99.9%

≅

η

Within suggested operating range

EPA Handbook Control Technologies for Hazardous Air Pollutants EPA625/6-91/014 June 1991

38

EPA Handbook Control Technologies for Hazardous Air Pollutants

Although a ∆P of 47” w.c.

is not typical for municipal

incinerators, it is still < 80”

w.c. and should work.

39 sat throat throat sat

Q

P

v

K

A

ρ

∆

=

=

V_throat = gas velocity in venturi throat (ft/sec)

Q_sat = gas volumetric flow rate at saturated conditions (actual ft3_/sec)

A_throat = cross-sectional area of venturi throat (ft2₎

∆P = pressure drop across venturi (inches H₂O) ρ_sat = density of gas at saturated conditions (lb/ft3) K = empirical constant to account for energy losses from Calvert’s empirical equation:

1850 K L G =      

where L/G = liquid to gas ratio (gallons per 1000 actual ft3₎

( )

1850 1850 9.62 20 K L G = = =      

• Gas Pressure Drop (< 80 inches H₂O) • Gas Velocity at “Throat” (60 – 150 m/s)

• Liquid to Gas Ratio (10–30 gallons/1000 acf ) • Inlet Particle Size (>0.2 micrometers)

47

9.62

263

/ sec

0.063

sat throat throat sat

Q

P

v

K

ft

A

ρ

∆

=

=

=

=

(

)

2

86, 540

5.4

263

/ sec 60 sec/ min

sat throat throat

Q

acfm

A

ft

v

ft

=

=

=

(

2

)

4

throat diameter=

5.4ft

= 2.6 ft

π

3 3

lb water 29 moles water ft water

0.1 0.16 0.16

lb dry gas 18 mole gas ft gas

ws

B =_ _ _= =  

 

( ) ft water vapor3 _{3 3} lb water ft dry gas3 ₃ lb dry gas₃ lb gas ₃ ( ) 1 0.16 (0.042 ) 0.84 0.0676 = 0.063

ft gas ft water vapor ft gas ft dry gas ft gas sat Bws w Bws gas

ρ = ρ + − ρ =_ _ +_ _ _

 

41

For optimal pressure recovery, the length of the throat area is

taken as 3 times the throat diameter and the length of the

diverging section is 4 times the throat diameter.

Throat and Diverging Sections

= 7.8 ft

= 10.4 ft L_diverging