Venturi Scrubbers1

Venturi Scrubbers • • • • •

Basics Design Parameters Pressure Drop Particle Collection Efficiency Example Design Calculations

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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

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Venturi Basics 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.

Exhaust gas

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Liquid injection

Atomizing Spray

Liquid Entrainment 4

Photos at: http://members.aol.com/apcutk/index.htm

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“Pease Anthony” design

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Mist Eliminator • 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

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System Parameter Checklist 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.

Mist Eliminator Performance • 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.

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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 H2O) Inlet Particle Size (>0.2 micrometers) Energy Consumption (4-12 Watts/cfm)

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Liquid-to-Gas Ratio • Higher L/G, higher η • L/G optimal at 7–10 gal/1000ft3 • L/G > 10 increases ∆P and operating costs

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Relative Velocity 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.

•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

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Pressure Drop • ∆P = 10 to 80 in. w.c. • ∆P > 45 in w.c. does not typically increase η in standard designs

Throat exit

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

Pressure Drop In general:

 Qwater ∆P ∝   Qgas 

 2  vthroat 

(

)

L 2 Calvert’s Model: ∆P = 5.4 x10   ρ sat vthroat G ∆P = pressure drop, inches w.c. −4

(

)

L/G = liquid to gas ratio, gal per 1000 ft3 ρsat = saturated gas stream density, lb/ft3 vthroat = gas velocity at throat, ft/sec

Assumptions:

• • • • •

All liquid forms droplets Droplet acceleration only contribution to ∆P All droplets have no initial axial velocity Drops reach gas velocity in throat Flow is 1-D, incompressible, adiabatic 14

Calvert’s Model: Throat Velocity vs. ∆P Qsat ∆P vthroat = K = Athroat ρ sat Vthroat = gas velocity in venturi throat (ft/sec) Qsat = gas volumetric flow rate at saturated conditions (ft3/sec) Athroat = cross-sectional area of venturi throat (ft2) ∆P = pressure drop across venturi (inches H2O) ρsat = 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)

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

Athroat = Ainlet

vthroat vinlet

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. Ldiverging inlet

Lthroat 16

Calvert’s model over-predicts pressure drop, worse for short throats

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

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

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

dP − = vgas ( dvgas ) + mvgas ( dvdrop ) +

ρ gas

Gas kinetics

Drop kinetics

L m= (dimensionless) G

( ) dx

2 m + 1 f v ( ) gas

2 Deq

Film kinetics

f = friction factor Deq = hydraulic diameter

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Boll’s model works better

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

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

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Collection Efficiency • Main removal mechanism is impaction • Particles < 0.1 µm mainly diffusion • Efficiency decreases exponentially with decreasing particle size

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Predicting Collection Efficiency • Manufacturer Performance Curves • Contact Power Theory • Calvert Cut Diameter Method

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Performance Curve (specific to a given venturi geometry)

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

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Permit Application Information • Reported pressure drop across venturi • Performance curve applicable to venturi • Reported collection efficiency

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Contact Power Theory All scrubbers give the same level of particle collection at the same level of power consumption (Lapple & Kamack)

PT = PG + PL + Pmech PT = total contact power, PG = power due to pressure drop of gas passing through the scrubber, PL = power due to the scrubber liquid atomization, and Pmech = power due to mechanical devices to increase contact, i.e., a rotor.

25 Lapple, C.E. and H.J. Kamack, “Performance of Wet Dust Scrubbers”, Chemical Engineering Progress, vol. 51, March 1955.

Contact Power Theory = PG 0.157 ∆P L PL = 0.583 pL   G ΔP = pressure drop across venturi, in w.c. pL = liquid inlet pressure in pounds per square inch* L/G = liquid to gas ratio in gallons per cubic feet (gal/ft3)

*0.5 – 2 psig

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Contact Power Theory

η= 1 − exp ( −α PT

β

)

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.

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Calvert Cut Diameter Model 2   dj    Pt j =− 1 η j =exp  − ln(2)    d cut     

d cut =exp {1.3 − 0.518ln ( ∆P )} Integrating over all inlet particle sizes

Pt = ∫ Pt j ( mass ) j 28

Calvert’s Cut Diameter Model* 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 50 1.00 1.50 10.00 67.00

% Removal

pressure drop (cm H2O)= inlet mass conc. (g/m3) = sigma g = MMAD (um)= maximum throat velocity of gas (m/sec)

100 80 60 40 20 0 0.1

1.0

10.0

100.0

diameter, um *spreadsheet located on course website

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Example Design Calculations

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

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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%

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

Not possible

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

Vapor Only

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Humidification of Inlet Gas 5% by volume

H in = 0.05

lb-mole water  18lb water   lb-mole gas  lb water  =0.031   lb-mole gas  lb-mole water   29 lb dry gas  lb dry gas

From psychrometric chart: Humidified air in the venturi

Air entering the venturi

Tin = 400 F Tsat = 127 F H sat = 0.10

lb water lb dry gas

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As water is added upstream of the scrubber, the water evaporates, cooling the gas until it is saturated Leaving scrubber

Adiabatic Cooling & Humidification

Entering scrubber 34

Saturated Gas Flow Rate 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. Tsat + 460 ) ( sat Qin Qin = + Qw dry (Tin + 460 ) Q = Qin (1 − Bws ) in

Qw =

(

in Qindry ρ gas

)(H

sat

− H in )

ρw

Qinsat =

Saturated emission stream flow rate, acfm

Qin =

Inlet emission stream flow rate, acfm

Qw =

Flow rate of water vapor added, acfm

Tsat =

Temperature of saturated emission stream, F

ρ= P

P ( MW ) RT

= pressure of emission stream, atm

MW = molecular weight of gas, lb/lb-mole

Tin =

Temperature of inlet emission stream, F

R

in = ρ gas

Density of emission stream, lb/ft3

T

ρw =

Density of water vapor, lb/ft3

H sat =

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

H in =

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

atm-ft 3 lb − mole −  R = temperature of gas,  R = gas constant, 0.7032

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Saturated Gas Flow Rate 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.

Qindry = Qin (1 − Bws ) = 110, 000 (1 − 0.05 ) = 104,500acfm = ρ dry = ρW

Qw

(

P ( MW ) = RT

(1 atm )( 29 lb/lb-mole ) lb dry gas = 0.0676 0.7302 ( 460 + 127 ) ft 3 P ( MW ) (1 atm )(18 lb/lb-mole ) lb water vapor = = 0.042 RT 0.7302 ( 460 + 127 ) ft 3

)

in Qindry ρ gas ( H sat − H in ) =

Qinsat Qin =

ρw

(104,500 )( 0.0676 )( 0.10 − 0.0316 ) = 0.042

(Tsat + 460 ) + Q 110, 000 (127 + 460 = ) + 11,500 = ( ) (Tin + 460 ) w ( 400 + 460 )

Additional flow of evaporated water vapor

11,504acfm

86,540acfm

Total flow of gas inside scrubber at saturated conditions.

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Venturi Scrubber Performance Curve

ΔP ≅ 47 inches w.c. at η =99.9% Within suggested operating range

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

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Although a ∆P of 47” w.c. is not typical for municipal incinerators, it is still < 80” w.c. and should work.

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

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Throat Velocity (used to size the scrubber)

Qsat ∆P vthroat = K = Athroat ρ sat Vthroat = gas velocity in venturi throat (ft/sec) Qsat = gas volumetric flow rate at saturated conditions (actual ft3/sec) Athroat = cross-sectional area of venturi throat (ft2) ∆P = pressure drop across venturi (inches H2O) ρsat = density of gas at saturated conditions (lb/ft3) K = empirical constant to account for energy losses from Calvert’s empirical equation: K=

1850 L   G

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

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= K

1850 = L   G

1850 = 9.62 ( 20 )

• • • •

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)

 lb water   29  moles water ft 3 water = = Bws  0.1 = 0.16 0.16   mole gas ft 3 gas  lb dry gas   18    ft 3 water vapor  lb water ft 3 dry gas   lb dry gas  lb gas = + (0.042 ) 0.84 ρ sat Bws ( ρ w ) + (1 − Bws )= ρ gas  0.16     0.0676 3  = 0.063 3 3 3 3 ft gas ft water vapor  ft gas   ft dry gas  ft gas  

= vthroat

A= throat

Qsat ∆P 47 = K = 9.62 = 263 ft / sec Athroat ρ sat 0.063

Qsat 86,540acfm = = 5.4 ft 2 vthroat 263 ft / sec ( 60sec/ min )

(

4 throat diameter= 5.4ft 2 π

)

= 2.6 ft 40

Throat and Diverging Sections 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.

Ldiverging = 10.4 ft

= 7.8 ft

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Condensation/Venturi Scrubber

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