env415 air pollution control engineering

8/14/2019 ENV415 Air Pollution Control Engineering

http://slidepdf.com/reader/full/env415-air-pollution-control-engineering 1/98



Ai r Regulations and Publ ic Pol icy(TÜRK YE)

Air Regu lat ions and Public Policy(TÜRK YE)





http://www.cevreonline.com/index.htm http://www.testmer.com.tr/alt_kategori.asp?id=134


Air Regu lat ions and Public Policy (EU)

Ai r Regulations and Publ ic Pol icy (EU)

http://www.epa.gov/air/criteria.html

Air Regu lat ions and Public Policy (USA-EPA)



h t t p : / / e c .

e u r o p a . e u / e n v i r o n m e n t / a i r / q u a l i t y / s t a n d a r d s . h

t m

Ai r Regulations and Publ ic Pol icy (EU) Air Regu lat ions and Public Policy (UK)

Ai r Regulations and Publ ic Pol icy (UK) Air Regu lat ions and Public Policy (UK)



21

What are the Pollutants?

National Ambient Ai r Quality Standards (NAAQS)

Primary pollutant: emitted directlySecondary pollutant: formed by chemical reactions among primary pollutants

*VOC: Volatile Organic Compound; 1health related; 2welfare related

Reading: Chap 1.1-1.4, 1.8-1.9

Pollutant Average Time Primary Standard 1 Secondary Standard 2

PM10 24-hour 150 µg/m3 Same as Primary

PM2.5 Annual 15 µg/m3 Same as Primary24-hour 35 µg/m3 Same as Primary

SO2 Annual 0.03 ppmv

24-hour 0.14 ppmv 3-hour 0.5 ppm (1300 µg/m3)

NO2 Annual 0.053 ppmv (100 µg/m3) Same as Primary

CO 8-hour 9 ppmv (10 mg/m3) None1-hour 35 ppmv (40 mg/m3) None

O3 8-hour 0.075 ppmv (2008 std) Same as Primary

8-hour 0.08 ppmv (1997 std) Same as Primary1-hour 0.12 ppmv Same as Primary

Pb Rolling 3-month average 0.15 µg/m3 (2008 std) Same as PrimaryQuarterly average 1.5 µg/m3 Same as Primary

22

Air Pollut ion Index (API) /Pollutants Standard Index (PSI)

• Segmented linear function connected by straight lines• Overall API is the maximum of all the sub-index values

API Value Air Quali ty Descriptor

0-50 Good

51-100 Moderate

101-199 Unhealthful

200-299 Very unhealthful

300 Hazardous

http://airnow.gov/index.cfm?action=airnow.national

23

Air Pollu tion Ind ex

0

100

200

300

400

500

0 1000 2000 3000 4000Pollutant Conc (ug/m3)

P S I S u b I n

d e x

TSP

SO2

TSP X SO2

CO

O3

NO2

Value 24 hr TSP

g/m3

24 hr SO2

g/m3

TSPXSO2

g/m3)2

8 hr CO

mg/m3

8 hr O3

g/m3

1 hr NO2

g/m3

0 0 0 N/A 0 0 N/A

50 75 80 N/A 5 118 N/A

100 260 365 N/A 10 235 N/A

200 375 800 65,000 17 400 1130

300 625 1600 261,000 34 800 2260

400 875 2100 393,000 46 1000 3000

500 1000 2620 490,000 57.5 1200 3750

Calculate the API of the following: 7 mg/m3 CO, 300 g/m3 TSP, 300 g/m3 SO2

24

Sources

Q: What are the most important sources for each criteria pollutant?What are the major sources in Alachua County?

Source Category CO NOx VOCs SO2 PM10 PM2.5

1998 200 8 1998 2008 199 8 2008 1998 2008 1998 2008 199 8 2008

Transportation Sources

Highway Vehicles 73.2 38.9 8.6 5.2 3.4 5.9 <0.1 0.3 0.3 0.2 0.1 0.2Off-Highway 23.7 18.0 4.2 4.3 2.6 2.7 0.5 0.4 0.3 0.3 0.3 0.3

Stationary Sources

Fuel CombustionElectric Utilities 0.5 0.7 6.2 3.0 <0.1 <0.1 7.6 13.4 0.2 0.5 0.5 0.1Industrial Furnaces 1.2 1.2 3.0 1.8 0.1 0.2 1.7 2.7 0.2 0.3 0.2 0.1Residential and Other 2.7 3.4 1.1 7.3 1.3 0.9 0.6 0.6 0.4 0.5 0.4 0.4

Industrial ProcessesChemical and Allied Product MFG 1.1 0.3 0.1 <0.1 0.2 0.4 0.3 0.3 <0.1 <0.1 <0.1 <0.1Petroleum & Related Industries 0.4 0.4 <0.1 <0.1 0.6 0.5 0.2 0.3 0.2 0.1 <0.1 <0.1Metal Processing 1.7 0.9 <0.1 <0.1 <0.1 <0.1 0.2 0.4 <0.1 <0.1 <0.1 0.1Other Industrial Processes 0.6 0.5 0.5 0.4 0.4 0.4 0.3 0.4 0.3 1.0 0.4 0.2Solvent Utilization <0.1 <0.1 <0.1 <0.1 4.2 5.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1Storage & Transport <0.1 0.1 <0.1 <0.1 1.3 1.3 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

Waste Disposal & Recycling 3.1 1.6 0.2 0.1 0.4 0.5 <0.1 <0.1 0.5 0.3 0.3 0.5

Miscellaneous 0.7 11.7 0.2 0.2 1.3 0.7 <0.1 <0.1 20.3 11.5 2.7 4.3

Total 115.4 77.7 24.3 16.3 15.9 18.8 11.4 18.9 22.9 14.8 4.9 6.3

Miscellaneous PM is mainly from fugitive, agricultural sources and fire

National US Emissions, 1998 and 2008 (millions of tons per year)



25

Source Performance Standards• New Source Performance Standards (NSPS’s):

mass emission rates applied to new sources forspecific pollutants from specific sources

• National Emission Standards for Hazardous Air Pollutants (NESHAP’s): emission rates forhazardous air pollutants that do not have NAAQSbut may result in “an increase in seriousirreversible, or in capacitating, reversible illness”.

• Standards can be based on mass, energy orprocess weight Why?

• Tier II Standards: emission rates per mile formobile sources

26

Prevention of Significant Deterioration• Prevent polluting clean air up to the ambient standards

by simply locating industry in clean air region

• Class I: Pristine; Class II: Almost all other; Class III:

Industrialized

Federal PSD Allowable Increments

Any difference between classes?

75371024-hr max

37195Annual geometric meanPM-10

700512253-hour max

18291524-hr max

40202Annual average

SO2

Class IIIClass IIClass IPollutant

Allowable increment g/m3

Q: Can I move my factory to Yellowstone NP where the air

quality is better than NAAQS?

27

Criteria for Selecting Appropriate AirPollution Control Device

• Degree of reduction of emissions

required to meet emission standards

• Process and effluent characteristics• Equipment capacities, efficiency and

limitations

• Capital investment and operation costs

28

Structure of the class

Fundamentals

Particulate Control

Devices

Gas PropertiesParticle Characteristics

Energy

CycloneElectrostatic Precipitator (ESP)

Fabric Filter Particulate Scrubber

Mobile Source

AdsorptionAbsorptionIncineration NOx, SOx

Gas Control

Devices



29

Termal Power Plant

30

31 32



33 34

35 36

Method Absorbent/

Adsorbent

Byproducts

Wet Type

NaOH or Na2SO3

solution Na2SO3, NaNO3, SO2,gypsum

NH3-Water (NH4)2SO4, SO2, gypsum,S

Slaked lime orlimestone slurry

Gypsum

Mg(OH)2-slurry SO2, gypsum (blendedwith slaked lime slurry)

Basic Al2(SO4)3-solution gypsum

Gypsum

Dilute-H2SO4 Gypsum

Dry Type Activated carbon (NH4)2SO4, gypsum, S,H2SO4

Flue Gas Desulphurization Methods (FGD)



37

Flue Gas Desulphurization Methods (FGD)

38

39

Air Pollu tion Control

Termal power plant

40

Yanma Sonrası CO2 Giderme



27.11.2012 1

Properties of Gases

• Ideal gas law

• Unit of concentration

• Vapor pressure & partial pressure

• Solubility

• Energy

• Humidity & psychrometric chart

• Cooling

Reading:

Chap 1.5-1.7, 10, 7.3, 8.5

27.11.2012 2

Ideal Gas Law

Other references:

1. CRC Handbook of Chemistry & Physics

2. Perry’s Chemical Engineers’ Handbook

T RnQP

T R MW P

T R MW

M

T RnV P

82.057

8.3148.314

8.314

27.11.2012 3

Unit of Concentration

Is 1 g/cm3 SO2 equal to 1 ppb SO2?

The annual standard of NO2 is 100 g/m3. What is the

concentration in ppb?

Is “ppm” molar basis, volume basis or mass basis?

What’s the difference between “ ACFM” and “SCFM” ?

27.11.2012 4

Vapor Pressure

The pressure required to maintain a vapor in equilibrium

with the condensed vapor (liquid or solid) with a flat

surface at a specified temperature

(Saturation)

Vapor PressureTime to reach equilibrium

How does vapor p ressure change if the temperature increases?

T C

B AT Pv

)(log Pv in mmHg and T in oC (if Table 9.2 is used)



27.11.2012 5 27.11.2012 6

What is the vapor pr essure of water at 20 oC? If the

measurement is conducted on Mars (the atmosphericpressure is about 0.006 atm), what will be the value?

27.11.2012 7 27.11.2012 8

Partial Pressure: the pressure that a gas (or vapor) in a

mixture of gases would exert if it were to occupy the entire

volume occupied by the mixture

T A A P yP

)100( )(

S RH T P

PS

v

A

Supersaturation: S > 1 ( RH > 100%)

Saturation Ratio(or relative humidity for water)

y A: mole fraction of component A

in the mixture in the gas phase

PT : total pressure of the system

Q: After a shower at dusk, the temperature starts to drop. How do PV and P A change correspondingly? Can you predict the weather at dawn?

1 mole of

O2 @ 1 atm

4 moles of N2

Q: How much is PO2?



27.11.2012 9

SolubilityHenry’s Law: the concentration of a gas dissolved in solution

is proportional to its partial pressure at a constant temperature

A A HxP

A A x H y 1or

H : pressure/

mole fraction in liquid

H 1: mole fraction in vapor/

mole fraction in liquid

H

Q: Is H a constant at a given temperature?

27.11.2012 10

Solubility Data for NH3 in Water

27.11.2012 11

Solubility Data for SO2 in Water

27.11.2012 12

Energy

• Heat Capacity: the amount of energy required toraise the temperature of the substance by 1 oC.

Q: Which one requires mor e energy to raise 1 oC?Water or copper?

• Specific Heat Capacity (Cp or Cv): heat capacity on

a unit mass basis (also a molar basis for gas)• Enthalpy: the difference of energy compared to that

at reference temperature (25 oC or 60 oF) – see TableB.7

• For liquid like water, the latent heat (HL, energy ofvaporization) also needs to be included.

)( 0T T C H p

Q: How much energy can we extract from 2 kg of air cooling from 100 to 50oC? (Table A.1 next page)

Q: Is energy important in APCD? Why?



27.11.2012 13

Handout

27.11.2012 14

Humidity in Air/Water Mixture

• The state of an air/water mixture is

determined by pressure, temperature

& humidity

• Psychometric Chart (Figures B.3 &

B.4 or Handout)

– Dry bulb temperature

– Wet bulb temperature: the

temperature at which a thermometer

with a wet wick wrapped around thebulb stabilizes

Q: Why is T DB always higher than T WB?

Q: Properties of TDB of 40 oC and TWB of 30 oC?

Q: If a stream of moist air is cooled and humidified adiabaticallyfrom TDB of 40 oC and TWB of 30 oC to TDB of 32 oC, how much is the change of

water per pound of dry air?

http://www.usatoday.com/weather/wsling.htm

27.11.2012 15http://howard.engr.siu.edu/staff1/tech/MET/ET401/LAB/psychro_carrier_si.jpg

27.11.2012 16

Cooling

• Dilution with ambient air (pros & cons?)

– Mass balance and energy balance

• Injection with water: remember to add latent

heat for vaporization (pros & cons?)

mixair gas

mixmixair air gasgas

mmm

H m H m H m

gas

air

air mix

mixgas

gasair T

T

T T

T T QQ

Q: Why is cooling of exhaust necessary?

Q: What options do we have?



27.11.2012 17

• Heat Exchanger: large U-tubes to transfer heat to ambient

air (water) by convection and radiation (pros & cons?)

LM x

ch p

ch p

T UA H

T T C M H

T T C M H

)(

)(

22222

11111 H 1: rate of heat given off by hot fluid

H 2: rate of heat absorbed by cold fluid

H x: rate of heat exchanged

U : overall heat transfer coefficient A: heat transfer area

T LM : log mean temperature difference27.11.2012 18

cc

hh

cchh LM

im

w

T T

T T

T T T T T

hk

x

h

U

21

21

2121

0

ln

11

1

ho: heat transfer coef. (outside tube)

hi: heat transfer coef. (inside tube)

xw: tube wall thickness

k m: tube thermal conductivity

Values can be found in Table 8.8 and reference books.

Q: What is the dilution air flow rate needed (in cfm at 90oF) to cool 50,000 acfm of air from 1200 oF to 500 oF?

27.11.2012 19

Quick Reflection

27.11.2012 20

Dilution with Ambient Air

)(

(3) )(

)(

0,

0,

0,

T T C H

T T C H

T T C H

mixmix pmix

air air pair

gasgas pgas

(2)

(1)

mixair gas


mmm

H m H m H m

Heat capacity definition

Energy balance and mass balance

Density definition

mixmixmix

air air air

gasgasgas

Qm

Qm

Qm

(4)



27.11.2012 21

For a common air pollution scenario where pollutant

concentration is dilute, gas heat capacity can be assumed to be

the same as air heat capacity

(5) ,,, mix pair pgas p C C C

(6) gas

air air gas


T

T

T T T

The density of gas depends on the temperature

T R MW P

Plug (3) & (4) into (1) and (2)

(7) 0,

0,0,

T T C QQ

T T C QT T C Q

mixmix pair air gasgas

air air pair air gasgas pgasgas

27.11.2012 22

gas

air

air mix

mixgas

gasair T

T

T T

T T QQ

Use (5) in (7), it becomes

(8) 0

00

T T QQ

T T QT T Q

mixair air gasgas

air air air gasgasgas

Use (6) in (8), it then becomes

(9) 0

00

T T QT

T Q

T T QT T T

T Q

mixair air

gas

air air gas

air air air gas

gas

air air gas

Cancel air and regroup terms

(10) 0000 T T T T QT T T T T

T Q

air mixair mixgas

gas

air gas



2012/11/27 1

Particle Characteristics

• Aerosol size

• Aerosol size distribution

• Representative size

• Weighted distribution

• Log-normal distribution

Reading: Chap 3.1-3.2

2012/11/27 2

Q: How do we characterize a particle?

Size, Shape, Density, Composition (toxicity, corrosivity,

reactivity), Phase (liquid, solid)

Coal fly ash particles Iron oxide particles from arc welding

Q: Why should we care the particle size?

Q: How do we determine particle size?

Characterizing an Aerosol Particle

2012/11/27 3

d p > 10 m 1 < d p <10 m d p < 1 m

Health Concern: deposition in our respiratory system

Control Device’s Collection Efficiency: depends on particle size

Lifetime of Aerosols in the Atmosphere: particle size

Environmental Quality (e.g. visibility): particle size

2012/11/27 4

(see also Figure 3.1)

Size Range of Aerosol Particles

Hinds, Aerosol Technology, 1999



2012/11/27 5

Q: How do we characterize particle”S”?

• Concentration:

– Number concentration by counting

– Mass concentration by weight measurement

• Size

• Spread

Particle size distribution

Ex: Particles in room air:

N = 104 #/cm3 M = 5.23610-6 g/cc d p = 10-3 cm = 10 m

Aerosol Size Dist ribution

Q: Does this mean all the 104 particles in the 1 cc air are 10 m? Whatis the effect if we use this size to represent the system (e.g. in inhalation

system)? How can we better describe this aerosol system?

2012/11/27 6

Particle Size Distribution• Monodisperse - All the particles are of the same size

• Polydisperse - Particles are of more than one sizes (more realistic)

Typical data from measurement:

Can also be mass?

Lower Limit

m

Upper Limit

m

Count orFrequency,

ni(#)

NormalizedCount, h

i'

(#/ dp)

Fraction,

f i(#/total)

Fraction/size

(1/m)

Percent

(%)

Cumul.Percent

(%)

0 4 104 26 0.104 0.026 10.4 10.4

4 6 160 80 0.16 0.08 16 26.4

6 8 161 80.5 0.161 0.0805 16.1 42.5

8 9 75 75 0.075 0.075 7.5 50

9 10 67 67 0.067 0.067 6.7 56.7

10 14 186 46.5 0.186 0.0465 18.6 75.3

14 16 61 30.5 0.061 0.0305 6.1 81.4

16 20 79 19.75 0.079 0.01975 7.9 89.3

20 35 103 6.867 0.103 0.0069 10.3 99.6

35 50 4 0.267 0.004 0.0003 0.4 100

50 100 0 0 0 0 0 100

ni= N = 1000 f

i= 1 100

2012/11/27 7

dpi

(m)

0 10 20 30 40 50

F r e q u e

n c y / C o u n t

0

50

100

150

200

Q: Which size has the highest concentration?

Frequency (or count ) versus particle size

ii Count n

F r e q u e n c y

o r C o u n t

Lower

Limit m

Upper

Limit m

Count or

Frequency,

n i (#)

0 4 104

4 6 160

6 8 161

8 9 75

9 10 67

10 14 186

14 16 61

16 20 79

20 35 103

35 50 4

50 100 0

n i = N = 1000

2012/11/27 8

Normalized Count, Frequency/ dp versus particle size

dpi

( m )

0 10 20 30 40 50

n i ( d p i )

S i z e D

i s t r i b u t i o n F u n c t i o n

( f r e q u

e n c y /

d

p

0

20

40

60

80

i

pii d h N )'( p

i

p

i

id

n

d

Count h

'

Q: What is the value of the total area?

Lower

Limit m

Upper

Limit m

Normalized

Count, hi'

(#/ dp)

0 4 26

4 6 80

6 8 80.5

8 9 75

9 10 67

10 14 46.5

14 16 30.5

16 20 19.75

20 35 6.867

35 50 0.267

50 100 0



2012/11/27 9

Fraction/ dp versus particle size

dpi (m)

0 10 20 30 40 50

f i ( d

p i )

P r o b a b i l i t y D e n s i t y F u n c t i o n

( f r a c t i o n /

d

p i )

0.00

0.02

0.04

0.06

0.08

Q: What is the value of

the total area?

p

i

p

ii

i

i

d

f

N d

n

N h

N

n f

11'

Lower

Limit m

Upper

Limit m

Fraction,

f i (#/total)

Fraction/size

(1/m)

0 4 0.104 0.026

4 6 0.16 0.08

6 8 0.161 0.0805

8 9 0.075 0.075

9 10 0.067 0.067

10 14 0.186 0.0465

14 16 0.061 0.0305

16 20 0.079 0.01975

20 35 0.103 0.0069

35 50 0.004 0.0003

50 100 0 0

f i = 1

2012/11/27 10

• MEAN (arithmetic average):

the sum of all the particles sizes divided by the number of

particles

• MEDIAN:

The diameter for which 50% of the total are smaller and

50% are larger; the diameter corresponds to a

cumulative fraction of 50%

• MODE:

Most frequent size; setting the derivative of thefrequency function to 0 and solving for d p.

0

)( p p p

i

pii p

p dd d f d

n

d n

N

d d

Representative Size

2012/11/27 11

• GEOMETRIC MEAN:

the Nth root of the product of N values

Expressed in terms of ln(d p)

• Very commonly used because an aerosol system typically

covers a wide size range from 0.001 to 1000 m

d d d d d pg p

n

p

n

p

n N

pi

n d N

pi 1 2 3

1 11 2 3 ...

/( )

/

N

d nd

N

d n

d

pii

pg

pii

pg

lnexp

ln

ln

2012/11/27 12

Weighted Distributions• Why do we need distributions other than number

distribution?

• What are the other distributions?

• Definition: frequency of the property (e.g. number, mass)

contributed by particles of the size interval

• What is the effect?

Ex. A system containing spherical particles Number Concentration: Mass Concentration:

100 #/cc 1m & =1.91g/cm3 10-10 g/cc 1m

1 #/cc 10m 10-9 g/cc 10m

Q: Do we have “more” 1 m or 10 m particles

(i.e. are the majority 1 or 10 m)?

Q: How will it impact the PSD we see?



2012/11/27 13

dp (m)

0 10 20 30 40 50

M a s s f r a c t i o n / m

0.00

0.01

0.02

0.03

0.04

dp (m)

0 10 20 30 40 50

q ( d

p ) P r o b a b i l i t y D e n s i t y F u n c t i o n

0.00

0.02

0.04

0.06

0.08

0.10

Number Distribution Mass Distribution

Q: What is the mode size of the above distribution?

• Count Mean Diameter:

• Mass Mean Diameter:

0

)( p p p

i

pii

pn dd d nd

n

d nd

0

)( p p p

i

pii

pm dd d md

md md

Q: Which one should we use? Mass, number or other?

2012/11/27 14

Lognormal PSD• Normal Distribution: widely used elsewhere (e.g. student

grade), but typically not for aerosol science, because

– most aerosols exhibit a skewed distribution function

– if a wide size range is covered, a certain fraction of the

particles may have negative values due to symmetry.

2/1

2

2

2

1

2exp

2

1

N

d d n

d d

dd

df

p pi

p p

p

standard deviation

Q: If you just developed a particle control device that can collect 100% for

dp> 5 um and 0% for dp < 5 um, how will you report your device’s collection

efficiency? Tests were done using the particles shown on page 13.

2012/11/27 15

• The application of a lognromal distribution has no

theoretical basis, but has been found to be applicable

to most single source aerosols

• Usefuel for particle of a wide range of values

(largest/smaller size > 10)

• How to use it? Simply replace d p by ln(d p).

lnln

d n d

N pg

i pi geometric mean diameter

ln(ln ln )

g

i pi pgn d d

N

2

1geometric standard deviation

2

2

)(ln2

lnlnexp

ln2

1

ln g

pg p

g p

d d

d d

df

Q: How much is g

for monodisperse

aerosol?

Q: What’s the unit of g?

2012/11/27 16

• Features of Lognormal PSD

)/ln(

lnlnln

%50%84

%50%84

d d

d d g

For a given distribution, the

geometric standard deviation

remains constant

(nondimensional) for all

weighted distributions.

Q: If g = 1.5,

how much is d 84% / d 16%?

)/ln(ln2 %50%7.97 d d g

Size range (m) 0-2 2-5 5-9 9-15 15-25 >25

Mass (mg) 4.5 179.5 368 276 73.5 18.5

Size range (m) Mass fraction (m j) Cumulative percent

0-2 0.0049 0.5

2-5 0.195 20.0

5-9 0.4 60.0

9-15 0.3 90.0

15-25 0.08 98.0

>25 0.02 100

Measurement from a cascade impactor

Is this a log-normal distribution?

What’s d 50%? g?

Download log-probability graph



2012/11/27 17

Log-Probability Graph

dp (m)

0 .1 0 .2 0 .3 0 .4 0 .5 0. 60 .70 .80 .91 2 3 4 5 6 7 8 9 10 2 0 3 0 40 5 0 6 07 08 09010 0

C u m u l a t i v e % l e

s s t h a n d

p

0.01

0.05

0.10.2

0.5

1

2

5

10

20

30

40

50

60

70

80

90

95

98

99

99.899.9

99.99

2012/11/27 18

Quick Reflection



2012/11/2 1

Motion of Aerosol

• Newton’s Resistance Law and Stokes’ Law

• Cunningham Slip Correction Factor

• Settling Velocity, Mechanical Mobility

• Particle Acceleration

• Aerodynamic Diameter

• Settling Chamber

• Brownian Motion & Diffusion

Reading: Chap. 3.3-3.4

http://aerosol.ees.ufl.edu/aerosol_trans/section01.html

2012/11/2 2

History

History: Galileo’s (1564-1642)

experiment in Pizza tower

Newton’s Resistance Law: The force is proportional

to the gas pushed away and the relative velocity between

the sphere and the gas (negligible viscous force)

)2

(8

222 pg

p D p pg D D V AC V d C F

V p

C D = 0.44 for Re p > 1000

Tim Tebow can be a good air pollution engineer

because he knows how to control the movement

of particles in the air …..

2012/11/2 3

p p D d V F 3

p p pg

Dd V

C Re

2424

Stokes Law: negligible inertial force

compared to viscous force (Re p < 1)

Reynolds Number:inertial force/frictional force

p pg

p

d V Re

Two major parameters: V & d p

Q: Plot C D as a function of Re p for Newton’s Law

Q: Plot C D as a function of Re p for Stokes’ Law

Transition Regime

6

Re1

Re

243/2

p

p

DC

2012/11/2 4

Q: What is the max velocity of a particle to be in the Stokes regime?

1 m, 10 m, 100 m ( g = 1.2 kg/m3; = 1.81×10-5 Pa•s)



2012/11/2 5 2012/11/2 6

Categorize aerosol movement based on

interaction between the particle and gas molecules

Mean free path of gas ( ): average distance traveled by a gas

molecule between successive collisions; 0.066 m for air at STP

Ruler of aerosol movement:

)1.0( 52.21

3

md d

C

C

Vd F

p

p

c

c

p

D

(derivation for d p < 1 m @ STP)

1 mm 0.1 m

gas velocity at the surface of small particles is not zero --> slip

Cunningham Slip Correction Factor:

http://aerosol.ees.ufl.edu/aerosol_trans/section06.html

2012/11/2 7

Settling Velocity• When the drag force is equal and opposite to the

gravitational force

• Particle Mechanical Mobility (is a measure of the ease for an aerosol to have a

constant motion)

6

)(3

3gd

C

d V mgF F

pg p

C

p p

G D

1Refor18

2

p

c p p

TS

gC d V

p

C

D d

C

F

V B

3

)regimelaminarLaw,sStoke'(

Q: What is the physical meaning of B?

Q: Does a smaller or a larger particle have larger mobility?

the smaller the aerosol, the larger the mechanical mobility (i.e. easier to move)

Q: What is the impact of considering Cc on a particle’s

settling velocity? Why?

~ 0

2012/11/2 8

Relaxation Time: indication of the time required for a

particle to adjust/relax its velocity to a new condition of force

Terminal/Settling Velocity

18)

3(

6

2

3 C p p

p

C p

p C d

d

C d mB

)considered isonacceleratiotherif (or ag BF V GTS (Figure 3.8 in the book)

(remember B is an intrinsic property of a given aerosol)

Q: What other acceleration?

Transition Regime Turbulent Regime

29.043.0

71.014.171.0153.0

air

p p

TS

d gV

74.1

air

p p

TS

gd V

Small particles "relax" to new

environments (i.e. following the

flow well) in a very short time

larger particles are tend to stick to

their original path.



2012/11/2 9

If flow regime is unknown because V TS is unknown --> K factor 33.0

2

g p

p

gd K

Laminar K< 3.3

Turbulent K > 43.6

Q: Can we clean the dust in this classroom by waiting them to

settle down? How long will it take? Assume the particle

size is: 1 m, 10 m, 100 m. The room is 3 m high.

Fig 3-8 in the book

Q: For a 100 µm unit-density particle, which flow regime is

applicable?

2012/11/2 10

2012/11/2 11

Particle Acceleration

)1()(

)()(

)()(3

/

t

TS

p DG

eV t V dt

t dV t V g

dt

t dV md t V mgF F

FG=mg

t=0

V(t)=0

FG=mg

t=V(t)=?

FG=mg

t>3V(t)=VTS

F D=3 V(t)d p

F D=3 V TS d p

• Newton’s law

http://aerosol.ees.ufl.edu/aeros

ol_trans/section04.html

2012/11/2 12

Time for unit density particles to reach their terminal velocity

d p (m) 3 (ms) S * (cm)

0.01 0.00002 6.810-6

0.1 0.00026 8.810-5

1 0.011 3.610-3

10 0.94 0.23

100 92 12.7

* V0=1000 cm/s

Non-zero Initial velocity

V t V V V e f f

t ( ) ( )

/ 0

Stopping Distance

S V BmV 0 0

)1()()(/

0

t

f f eV V t V t x For Re0 < 1

Small aerosols adapt to the new environment (i.e. following

the flow well) in a very short time, almost instantly!!!



2012/11/2 13

Aerodynamic Diameter

• The Stokes diameter, d s, is the diameter of the sphere that has the

same density and settling velocity as the particle.

• The aerodynamic diameter, d a, is the diameter of the unit density

( 0=1 g/cm3) sphere that has the same settling velocity as the

particle.

1818

2

0

2

cacs p

TS

gC d gC d V

0

psa d d

Q: PM10 and PM2.5 are aerodynamic diameters. Why?

Is optical diameter or aerodynamic diameter more relevant?

2012/11/2 14

L HV

V

x

TS

Horizontal Elutriator/Settling Chamber(Plug flow model: no radial or axial mixing)

Q: If monodisperse aerosols are uniformly distributed at theentrance, what is the collection efficiency as a function of

VTS (dp)?

2012/11/2 15

Brownian Motion & Diffusion

• The primary transport mechanism for small particles (<

0.1 m); Important when transport distance is small: e.g.

filter, airway in human lung

• Definition:

– Brownian motion: irregular wiggling motion of a

particle caused by random bombardment of gas

molecules against the particle

– Diffusion: the net transport of the particles from a

region of higher concentration to a region of lower

concentration

http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/brownian/brownian.html

http://www.geocities.com/piratord/browni/Difus.html

2012/11/2 16

Fick’s First Law of Diffusion

The net flux of aerosols (the net number of particles

traveling through a unit area per unit time) is

proportional to the concentration gradient

J : flux (#/area/time)

D: diffusion coefficient (area/time)

n: particle number concentration (#/cm3)

dx

dn D J x

Fick’s Second Law of Diffusion

x

n D J

t

n2

2

The rate of loss of particles is proportional to the gradient of the flux.

driving force



2012/11/2 17

• Solve 1-D equation

Spread of particles over time and space

n

t D

n

x

2

2

11

2

1

16

1

Numbers on curves are values of Dt

2012/11/2 18

Solution Dt

x

Dt

N t xn o

4exp

2),(

2

Mean Square Displacement of particles

Result:

Dt

x

2

2

x N

x n x t dxo

2 21

( , )

Stokes-Einstein Equation for Diffusivi ty

p

c

d

kTC D 3

Q: How to have a larger diffusivity? Why?

Air, water, universe?



Cyclone

• Impactor, Stokes Number

• Cyclone Operation and Applications

• Standard Dimensions

• Lapple Theory

• Pressure Drop

• Arrangement, Discharge

Reading: Chap. 4 of Air Pollution Control-A Design Approach

http://aerosol.ees.ufl.edu/cyclone/section01.html

Stokes Number: the ratio of

stopping distance of a particle to a

characteristic dimension of the

obstacle. It’s a particle’s persistence

to the size of the obstacle.

j

c p p

jc D

UC d

D

U

d

S Stk

92/

2

Q: Does a smaller or a largerparticle possess higher persistence?

Impactor

f(Stk) efficiencyImpaction

2

j D

http://aerosol.ees.ufl.edu/cyclone/section03.html

U

Stk DC d

p

j

c

50

50

9

Q: How can we collect more smaller particles using the same nozzle?

Cyclone Operation

http://aerosol.ees.ufl.edu/cyclone/section05.htmlhttp://aerosol.ees.ufl.edu/cyclone/section05.html



Axial Inlet

Bottom Inlet

* Air Pollution

Control Equipment Calculation,

Theodore, CRC Press, 2008.

Cyclone Performance for Various Applicatio ns

* Air Pollution

Control Equipment ,Theodore &

Buonicore,

CRC Press, 1988.

Standard Cyclone

Dimensions

* Air Pollution

Control Equipment Calculation,

Theodore, CRC Press, 2008.

Standard Cyclone Dimensions

a

b

h

H

B http://aerosol.ees.ufl.edu/cyclone/section06.html



General guidelines:

• a < S

• b < (D-De)/2

• H > 3D

• Cone angle = 7o ~ 8o

•De/D = 0.4~0.5, H/De = 8, S/De = 1

Cyclones used for removing wood dust

Q: Why? Number of effective turns

2

1 h H h

a N e

Gas residence time

ie V DN t / Terminal velocity

D

V d t bV

ig p p

t

9

/

22

Smallest collected diameter

g pie p V N

b

d

9

a

b

h

H

B

Lapple Theory (plug f low)

Q: How can we reduce the smallest size further?

Vi

~0

~0

50% cut size

pie

pV N

bd

2

9%50

The collection efficiency

of any size d pj

2

%50 /1

1

pj p

jd d

Overall efficiency

j j f

Penetration

1P

Particle size ratio d p /d p50%

(%)

Q: Is the dp50% the median size of the

size distribution of the inflow particles?

Q: Is the laminar flow assumption valid?

Q: Is f j mass based?

Pressure Drop

Number of gas inlet velocity head

2

e

c H D

abK N

Static pressure drop

H ig N V P2

2

1

Power requirement

PQw f

K c = 16 for normal tangential inlet

= 7.5 for one with an inlet vane

Higher flow velocity results in a higher efficiency at the cost of a

higher pressure drop (and hence power)

Common ranges for pressure drops:

Low efficiency cyclone 2-4 inch of water

Medium efficiency cyclone 4-8 inch of water

High efficiency cyclone 8-10 inch of water



Example 4-5

• Design a conventional Lapple cyclone to function as a pre-

cleaner (η > 70%) on a gas stream that flows at 120

m3/min. ∆P < 3000 Pa. p = 1500 kg/m3, g = 1 kg/m3, = 0.07 kg/m-hr (1.944×10-5 kg/m-s). Specify your final

choice of body diameter, overall cyclone efficiency, inlet

gas velocity and pressure drop (assuming Kc = 14).

Size range (m) Mass % in size range

0-2 2.0

2-4 18.0

4-10 30.0

10-20 30.0

20-40 15.040-100 4.0

>100 1.0

Multi-cyclone Collector

APTI 442, US-EPA,

Chapter 2, 2003.

Arrangement

Series

Parallel/

BatteryAir Pollution Control Equipment, Theodore & Buonicore, CRC Press, 2008

Q: If each cyclone’s efficiency is 80%,

how much is the total efficiency for

this serial cyclone? Q: Advantages?

Disadvantages?

Discharge Valves

APTI 442, US-EPA,

Chapter 2, 2003.

Q: What may happen to

if equipped with a bad

discharge device?



Electrostatic Precipitator (ESP) Electrostatic Precipitator (ESP)





Electrostatic Precipitator (ESP)

• Electrical migration

• Electrical mobility

• Corona discharge

• ESP theory

• Charging mechanisms

• Ash resistivity

• Flue gas conditioning

• Power consumption

Reading: Chap. 5

Positive Negative

Republican Democrat

Love Hate

Ying Yang

Man Woman

Hell Heaven

Cation Anion

War Peace

Attraction Repel

Electrical Migration

– Statcoulomb (stC): the charge that causes a repulsive force of 1 dyne

when 2 equal charges are separated by 1 cm (3.3310‐10C)

– Unit charge: 4.8 10‐10stC (1.610‐19C)

2

21

r

qqK F E E E

F

q

E (q=ne)

Electric FieldCoulomb’s law



Millikan Experiment

(Robert Millikan,

US, 1868-1953;

Nobel Prize

Laureate, 1923)

Hinds, Aerosol Technology, 1999

http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan‐bio.html

Electrical Mobility

Electrical Mobility

• Terminal velocity in an electrical field

(electrical migration velocity/drift velocity)

c

TE p

C

V d qE

3

qEBd

qEC wV p

cTE

3

qBd

qC

E

V Z

p

cTE 3

(force balance) D E F F

(for Re < 1)

Q: What is the physical meaning of electrical mobility?

Q: When does a particle have a higher mobility?

Q:

Differenc

e between

cyclone

and ESPin terms

of forces

acting on

the

system?

What’s

the effect?

p

C

d

C B 3

Particle Mechanical Mobility

Positive Corona Negative Corona+ -

+ -

+

+ -

+

+

+

-

-

+

- +

- +

‐

-

‐

+

+

Corona Discharge

Step 1

Step 2

Step 3

Step 4

Collection Plate Collection Plate

Electron

Molecule

Particle

Electrode Electrode

Q: How can we generate charges?



Ozone generation ‐ http://www.mtcnet.net/~jdhogg/ozone/ozonation.html

1 2 3

1 2 3

(20) (12) (8)

Turbulent Flow with Lateral Mixing Model

Electrostatic Precipitator

• Deutsch‐Anderson

Equation

R

dt V

R

dt RV

N

dN TE TE 222

)2

exp()(

0 R

t V

N

t N TE

Q

AV P cTE exp11 Ac /Q: Specific Collection Area (SCA)

• Turbulent flow: uniformly mixing

• Perfect Collection

•The fraction of the particles removed in unit

time = the ratio of the area traveled by drift

velocity in unit time to the total cross‐section

Q: How to increase the efficiency?

Electrostatic Precipitator



Q: An ESP that treats 10,000 m3/min of air is

expected to be 98% efficient. The effective

drift velocity of the particles is 6.0 m/min. (a)

What is the total collection area? (b) Assuming

the plates are 6 m high and 3 m long, what is

the number of plates required?

6 m

3 mInternal Configuration: self-review

Charging Mechanism: Diffusion Charging

Random collisions between

ions and particles

kT

t N ecd

e

kT d n

ii p p

2

1ln

2

2

2

Q: Does q depend on time?

Does q depend on d p?

The total number of charges on a particle

(ci ~ 2.4104 cm/s)

neq

The total charges on a particle

Use esu, not SI units.

Charging Mechanism: Field Charging

• Bombardment of ions in the presence of a strong

field

eZ1

eZ

4

2

3

i

i

2

t N

t N

e

Ed n

i

i p

Total number of charges by field charging

Q : Is the charging rate dependent on particle size? On field strength? On time? On material?

Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999.

e

Ed n

p

s4

2

32

Saturation charge (Zi ~ 450 cm2/stV•s)

Comparison of Diffusion & Field Charging

Q: Does collection efficiency

increase as particle size increase

(because of a higher number of

charges)?

dp (um) ndiff nfield ntotal Zdiff ZField Z (stC•s/g)

0.01 0.10 0.02 0.12 0.66 0.10 0.76

0.02 0.30 0.06 0.36 0.49 0.11 0.60

0.05 1.1 0.40 1.50 0.31 0.12 0.43

0.1 2.8 1.6 4.38 0.23 0.13 0.36

0.2 7 6.5 13.2 0.18 0.17 0.35

0.5 21 40 61.2 0.15 0.30 0.45

1 48 161 209 0.16 0.52 0.68

2 108 646 754 0.16 0.98 1.14

5 311 4035 4346 0.18 2.34 2.52

10 683 16140 16824 0.20 4.61 4.80

20 1490 64562 66052 0.21 9.16 9.37

50 4134 403510 407644 0.23 22.78 23.0

Number of Charges vs dp

dp (um)

0.01 0.1 1 10

n

10-2

10-1

100

101

102

103

104

105

106

Diffusion charging

Field Charging

Nit = 107 s/cm3

= 5.1

E = 5 KV/cm

T = 298 K



ELectrical Mobility vs dp

dp (um)

0.01 0.1 1 10

Z

( s t C . s

/ g )

0.1

1

10 Diffusion charging

Field Charging

Combined Charging

Typical fly ash

size distribution

Q: If the ESP is used to collect the

fly ash, how will the particle sizedistribution at ESP outlet look like?

Resistivity/Conductivity

• Impact of particles’ resistivity on ESP’s performance:

• Factors: temperature, composition

• Flue gas conditioning

109 - 1010 ohm-cm is desired

Q: How does resistivity affect an ESP’s performance?

Dust Resistivity Effects of sulfur content and temperature on resistivity

Q: Is S in coal good or bad?



Water spray for cement kiln dust

Flue Gas Conditioning Effective drift velocity as a function of resistivity by measurement

Use the same Deutsch-Anderson Equation with new we.

Q: Estimate the total collection area required for a 95% efficient fly-ash ESP

that treats 8000 m3 /min. The ash resistivity is 1.6×1010 ohm-cm.

Good for moderate

collection efficiency

(90% ~ 95%)

High Eff iciency ESP (>95%)

Matts‐Ohnfeldt Equation

k

eC

wQ

Aexp1

Use k = 1 for fly ash

k = 0.5 or 0.6 for

industrial category

Rule of Thumb

• Below 95%, use Deutsch-Anderson Equation

• Above 99%, use Matts-Ohnfeldt Equation

• Between them, use an average

Q: In designing a high

efficiency ESP, a smaller

drift velocity is to be used.

Why?



Power Consumption

avgC C V I P

C

C e

A

kPw

Power density ~ 1-2 W/ft2

Q

kPC exp1

• Corona power

• Drift velocity

• Efficiency vs. Corona Power

k = 0.55 for Pc/Q in W/cfs up to 98.5%



Fabric Filters

• Filtration

• Fabric Selection

• Fabric Cleaning

• Air/Cloth Ratio, Filtration Velocity

• Filtration Mechanisms

• Pressure Drop and Design Consideration

• Nanofiber Filter

Reading: Chap. 6

Filtration

Packing density/solidity

Fiber filter

porosity-1 volumetotal

mefiber volu

For fiber filter, < 0.1

For woven filter, ~ 0.3

Q: Do filters function simply as sieves (to

collect particles larger than the sieve spacing)?

Filtration



Shaker Baghouse

Theodore & Buonicore,

Air Pollution Control Equipment,

CRC Press, 1988.

Frequency Several cycles/s

Motion type Simple harmonic

or sinusoidal

Peak

acceleration

1-10 gravity

Amplitude Fraction to a few

inches

Mode Off-stream

Duration 10-100 cycles, 30

s to a few minutes

Common bag diameter

5, 8, 12 in

Shaker Cleaning Parameters

Q: What are the common

problems encountered?

Reverse

Air

Baghouse

Reverse-Air

Q: Pros and Cons?Cleaning dust on baghouse

walls by traditional sledge-

hammering

Frequency Clean a compartment at a time,

sequencing 1 compartmentafter another; continuous or

initiated by a max.-pressure-drop switch

Motion Gent le collapse of bag(concave inward) upon

deflation; slowly repressurize acompartment after completion

of a backflush

Mode Off stream

Duration 1-2 min, incl. valve opening,closing & dust settling periods;reverse-air flow itself normally

10-30 s

Bag diameter 8, 12 inch; length 22, 30 ft

Bag tension 50-75 lb

Reverse-Air Cleaning Parameters

Reverse-Jet



Pulse-Jet Filters and Cleaning Pulse-Jet Frequency A row of bags at a time;

sequenced 1 row after

another; can sequence suchthat no adjacent rows clean

one after another; initiation of

cleaning can be triggered by

max-pressure-drop switch or

may be continuous

Motion Shock wave passes down bag;

bag distends from cage

momentarily

Mode On-stream; in difficult-to-

clean applications such as

coal-fired boilers, off-stream

compartment cleaning being

studied

Duration Compressed air (100 psi)

pulse duration 0.1 s; bag row

effectively off-line

Bag

diameter

5-6 in

Q: How can the blown-away particles by the on-line cleaning process be collected?

Q: Felted fabric or woven fabric?

Compartmentalized Pulse-Jet Baghouse and Venturis for Cleaning



Ai r/Cloth Ratio

A

QV

Filtration velocity

(average velocity)

Q: If thicker fabric is needed

to sustain the high force, is

its operating cost higher?

Filtration Mechanisms• Diffusion (Lee & Liu, 1982)

factor ichydrodynamKuwabara 44

3ln

2

1

number Peclet

158.2

2

0

3/2

Ku

D

U d Pe

PeKu

f

D

Lee, K.W. & Liu, B.Y.H., Aerosol Sci. Technol. ,

1:47-61, 1982

Q: How does efficiency change wrt d p?

Q: How to increase efficiency by diffusion?

• Impaction (Yeh & Liu, 1974)

4.0for5.27)286.29(

18

2

)(

8.2262.0

0

2

0

2

R R R J

d

U C d

d

U Stk

Ku

J Stk

f

c p p

f

I

f

p

d

d R

Yeh. H.C. & Liu, B.Y.H., J. Aerosol Sci.,

5:191-217, 1974

Q: How to increase impaction efficiency?

Q: How does efficiency change wrt d p?

( J = 2 for

R > 0.4)

• Interception (Krish & Stechkina, 1978)

2

2

)1(2

)2

1(1

11)1ln(2

2

1 R

R R

Ku

R R

Krish, A. A. & Stechkina, I. B., “The theory of Aerosol Filtration with Fibrous Filters”,

in Fundamentals of Aerosol Science, Ed. Shaw, D. T., Wiley, 1978.

Q: How to increase interception efficiency?

Fat Man’s Misery,

Mammoth Cave NP



• Gravitational Settling

• Total Single Fiber Efficiency

• Total Filter Efficiency

0

2

0

TS0

18

directionsamein theVand for U )1(

U

gC d

U

V G

RG

c p pTS

G

G R I D

G R I D

)1)(1)(1)(1(1

f

f

S d H P exp1)1(

4exp11

S f : Solidarity factor

H = 1mm

= 0.05

d f = 2m

U 0=10 cm/s

Q: Should we increase or decrease flow velocity in order to increase

collection efficiency for (a) tobacco smoke, (b) cement dust?

Filter efficiency for individual mechanismand combined mechanisms

dp (m)

0.01 0.1 1 10

E f f i

c i e n c y

0.0

0.2

0.4

0.6

0.8

1.0

Interception

Impaction

DiffusionGravitation

Total

Parallel

Flow

Operation

Q: How do you determine when to clean?



Pressure Drop (Filter Drag Model)

s p f PPPP

V LVt K V K 21

Areal Dust Density LVt W

Filter drag

V

PS W K K S 21

K 1 & K 2 to be determined empirically

(resistance factor)

P f : fabric pressure drop

P f : particle layer pressure dropPs: structure pressure drop

Time (min) P, Pa

0 150

5 380

10 505

20 610

30 690

60 990

Q: What is the pressure drop after 100 minutes of operation?

L = 5 g/m3 and V = 0.9 m/min.

K 2

K 1

tr

tc

tf

t j

Time

P r e s s u r e

pm

Time to filter

ccr f t t t N t )( N

QQ

N

Flow rate

11

N

QQ

N

Filtering velocity

C C

N N

NA

Q

A

QV

C C

N N

A N

Q

A

QV

)1(

11

Q: What are the parameters that affect our decision on the

number of compartments to be used?

Areal dust density

))(1( 1 c N r N j Lt V Lt V N W

Filter drag

j j W K K S 21

Actual filtering velocity

1 N N j V f V

Pressure drop

j jm j V S PP

N 1/ N f N V V f

3 0.874 0.8

5 0.767 0.71

10 0.67

12 0.6515 0.64

20 0.6220

Ex. Calculate the max pressure drop that must be supplied for

the following baghouse for a filtration time of 60 minutes: K1

= 1 inch H2O-min/ft, K2 = 0.003 inch H2O-min-ft/grain, tc = 4

min, 5 compartments, L = 10 grain/ft3, Q = 40000 ft3/min, Ac

= 4000 ft2/compartment.

Nanofiber Filter

Nano-alumina on microglass

Argonide

E-spun PAN nanofiber

Dr. Wolfgang Sigmund, MSE. UF



1

Particulate Scrubbers

• Types of scrubbers: spray chamber and

venturi scrubber

• Theory and design consideration

• Pressure drop

• Contacting power

Reading: Chap. 7

27.11.2012

WET SCRUBBERS: PARTICLE COLLECTION MECHANISM

Droplets collect particles by using one or more of several collection mechanisms.

These mechanisms are impaction, direct interception, diffusion, electrostatic

attraction, condensation, centrifugal force, and gravity. However, impaction and

diffusion are the two primary ones.

The particle's mass breaks away

from the streamlines and impact or

hit the droplet. Most scrubbers do

operate with gas stream velocities

well above 0.3 m/s. Therefore, at

these velocities, particles having

diameters greater than1.0 m are

collected by this mechanism.

Very small particles (less than 0.1

m in diameter) are so tiny that

they are bumped by gas

molecules as they move in the

exhaust stream. This bumping, or

bombardment, causes them to

fi rst move one way and thenanother in a random manner, or to

diffuse, through the gas. This

irregular motion can cause the

particles to collide with a droplet

and be collected.27.11.2012 2

WET SCRUBBERS: PARTICLE COLLECTION MECHANISM

27.11.2012 3

Packed towers are the simplest and most commonly used approaches to gas scrubbing. The

principle of this type of scrubber is to remove contaminants from the gas stream by passing

the stream through a packed structure which provides a large wetted surface area to induce

intimate contact between the gas and the scrubbing liquor. The contaminant is absorbed

into or reacted with the scrubbing liquor.

The packing of the tower is normally a proprietary loose fill random packing designed to

encourage dispersion of the liquid flow without tracking, to provide maximum contact area

for the ‘mass transfer’ interaction and to offer minimal back pressure to the gas flow. The

reactivity between the contaminant and the scrubbing liquor influences the system

designer’s determination of gas and liquor flow and the height and diameter of the packed bed.

A demister is fitted at the top of the tower to prevent entrainment of droplets of the

scrubbing liquor into the extraction system or stack.

Packed towers can be designed for very high efficiencies with relatively low capital and

running costs. The low pressure drop associated with packed bed scrubbers permits the use

of smaller more economical fans. Although efficiency may be affected, a packed tower will

usually function when gas or liquor flows vary from its original design parameters.

Packed towers can become clogged by insoluble particulates or the insoluble products of

chemical reactions and this scrubbing technique should not be used where this is cause for

concern.

PACKED TOWER SCRUBBING

27.11.2012 4



The principle of air stripping

has been used for a number

of years for the removal of

dissolved gases such as

carbon dioxide, hydrogen

sulphide and ammonia from

aqueous liquors. It has also

been used as a means of

increasing dissolved oxygen

content for the oxidation of

dissolved metals such as

iron. It was in the late

1970’s that this technology

was applied to the removalof volatile organic

compounds (VOCs) from

water.

PACKED TOWER STRIPPING

AND DEGASSING

27.11.2012 5

In a fluidized bed scrubber the typical single bed of a packed tower structure is replaced by

two or more shallow beds, and the high surface area angular packing are replaced by

hollow ellipsoids which are ‘fluidized’ by the gas stream. This is relatively fast moving

when compared to the velocity of the gas flow through a packed tower.

The residence time of the gas in the tower is thus rather less than in a packed tower but this

tends to be compensated by the higher mass transfer induced by the gas turbulence in the

‘fluidized’packing.

Fluidized bed scrubbers are not normally used for odour control because of the short

residence time of the gas flow within the tower.

A prime advantage of the fluidized bed is that the mobility of the packing minimizes the

aggregation of particulates and insoluble depositions. Very small particulates, 8μm and

below, will however tend to pass through a fluidized bed.

With the higher gas velocity the tower diameter can be narrower and a more compact unit

designed. Additional costs are, however, incurred with more expensive packing, the

complexity of the structure, and the capital outlay and running costs of larger fans.

The efficient function of the fluidized bed depends on the velocity of the gas stream being

maintained between specified minimum and maximum levels – and this would typically be

a narrower range than in a packed tower.

FLUIDISED BED SCRUBBING

27.11.2012 6

7

Spray Chamber

R e c i r c u l a t e d w a t e r

Water to settling basin and recycle pump

Vertical spray chamber (countercurrent flow)

Collecting medium:

Liquid drops

Wetted surface

Q: What parameters will affect

the collection efficiency?

Q: Any other arrangement of

air & water?

27.11.2012 8

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

27.11.2012



9

Cyclone Spray Chamber &

Impingement Scrubber

Flagan & Seinfeld, Fundamental of Air

Pollution Engineering, 1988

Q: Is used water

recirculated?

27.11.2012

Venturi scrubbing is a most effective

technique for the removal of

particulates from a gas stream, even

down to sub-micron size. Scrubbing

liquor and gas stream are brought

together in turbulent contact within

the venturi throat and the particulates

are forced into the atomized liquor.

Venturi scrubbing may be adequate to

handle some more reactive

contaminants but to reduce fumes to

acceptable levels it may be necessary

to effect further treatment using a

packed tower scrubber. Combination

units incorporating both types of

scrubber using a common sump are

most practical in some circumstances.

Venturi scrubbing is typically a cost

effective and efficient approach to

removing particulates.

VENTURI SCRUBBERS

27.11.2012 10

Type of Wet Scrubbers

27.11.2012 11 12

Venturi Scrubber

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

High efficiency even for small particles

Q: ESP for sticky, flammable or highly corrosive materials?

QL/QG: 0.001 - 0.003 VG: 60 - 120 m/s

27.11.2012



13

THEORY: Spray Chamber

Droplet concentration in the chamber

d cd

L

d c

d d

V Ad

Q

V A

N n

3

6

V d : droplet falling velocity relative to a fixed coordinate

V dt : droplet terminal settling velocity in still air (i.e. relative to the gas flow)

Volume (m3) of each droplet3

6 d d d

Total number of droplets (Nd ) that pass the chamber per second

33

6

6d

L

d

L

d

Ld

d

Q

d

QQ N

V dt

V G

V d

Gdt d V V V

Q L: volumetric liquid flow rate (m3/s)

27.11.2012 27.11.2012 14


27.11.2012 15


27.11.2012 16




2127.11.2012 22

Single droplet collection efficiency

d

p

d

d

G

L

Diameter ratio Viscosity ratio

d

G

Gdt d V d

Re

DSc

G

G

d G

dt p pc

d

V d C St

18

2

Particle Reynolds # Particle Schmidt # Particle Stokes #

Deposition of Particles on a Spherical Collector

(diffusion)

(interception) (impaction)

27.11.2012

23

Impaction only

2

35.0

St

St I d

p = 2 g/cm3

Q: Why is there an

optimal size?

Q: The operating condition of a vertical countercurrent spray chamber are: Q L /QG = 1 L/m3,

V G = 20 cm/s, d d = 300 m and z = 1 m. Calculate the collection efficiency of 8m

particles through this chamber. Assume atmospheric pressure, 25 oC and p of 1 g/cm3.

(Impaction parameterK p is

used in textbook; K p = 2 St )

27.11.2012 24

Venturi Scrubbers: Calvert Design

po po

po

po

GG

d LG Ld

K f K

f K f K

Q

d V Q 1

7.0

49.0

7.0

7.0ln4.17.0

55expP

Particle penetration through a venturi scrubber

K po=2St (aerodynamic diameter) using throat velocity

f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials

Sauter mean droplet diameter (Nukiyama and Tanasawa), d d

5.145.0

5.0

5.0

1 1000597

G

L

L

L

LG

d Q

Q

V

k d

k 1 = 58600 if V G is in cm/s

= 1920 if V G is in ft/s

, L and should be in cgs

Q L and QG should be of the same unit

27.11.2012



25

f

f r p

D

Ld

G Dd t

G

LG L

vd

C

d

C l X

X X X k

Q

QV k p

Re

(Re)

4

Re

24

116

3

)1(2

3/1

242

2

Pressure Drop

Venturi Scrubber

lt : venturi throat length X : dimensionless throat length

Ex: 10” water, 2 m, = ?27.11.2012 26

Contacting Power

Approach

)exp(1 t N

T t P N

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)

N t : Number of transfer unit (unitless)

(1 inch of water = 0.1575 hp/1000 cfm)

Venturi scrubber collecting a metallurgical fume

Q: 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 56

38.1 8927.11.2012

27

T t P N

27.11.2012



• Crystalline zeolite

• Uniform pores to selectively separate compounds by size & shape

Air Pollution Engineering Manual., 1992

Properties of Activated AluminaBulk Density

Granules 38-42 lb/ft3

Pellets 54-58 lb/ft3

Specific Heat 0.21-0.25 BTU/lboFPore Volume 0.29-0.37 cm3/gSurface Area 210-360 m2/gAverage Pore Diameter 18-48 ÅRegeneration Temperature (Steaming) 200-250 oCM axi mu m Al lowabl e T empera tur e 500 oC

Properties of Molecular Sieves

Anhydrous SodiumAluminosilicate

Anhydrous CalciumAluminosilicate

AnhydrousAluminosilicate

Type 4A 5A 13XDensity in bulk (lb/ft3) 44 44 38Specific Heat (BTU/lboF) 0.19 0.19 -Effective diameter of pores (Å) 4 5 13Regeneration Temperature (oC) 200-300 200-300 200-300Maximum Allowable Temperature (oC) 600 600 600

Adsorption Mechanism

Rate of adsorption

Rate of desorption

)1( f pk r aa

f k r d d

At equilibriumd a

a

k pk

pk f

Mono-layer coverage f k m a ' (m: mass of adsorbate adsorbed per unit mass of adsorbent)

Langmuir Isotherm

Adsorption Isotherm: the mass of adsorbate per unitmass of adsorbent at equilibrium & at a given

temperature

( f : fraction of surfacearea covered)

f

1- f

p

m

pk

k

k m

p

pk

pk m

1

2

12

1 1

1

Langmuir Isotherm

( p: partial pressure of the adsorbate, i.e., toluene)

Q: Low P? High P?



Freundlich Isotherm

n pk m

Q: Calculate the equilibrium adsorptivity of 1000 ppm toluene in

air on 4X10 mesh activated carbon at 298 K and 1 atm.

Effects of Humidi tyIsotherm for toluene & trichloroethylene

and water vapor (individual)

Amount of trichloroethylene adsorbedas a function of relative humidity

Q: How can we adjust the system to reduce the impact of humidity?

Fixed-Bed Adsorpt ion System

Q: How will the OUTLET concentration

as a function of TIME look like?



Regeneration

Theodore & Buonicore, 1988

Q: In addition to steam, what else can we use?Q: Typically only 30 ~ 40% of the equilibrium isotherm is used. Why is that?

• A well-designed system has steam consumption in the range of 1 to 4 lbof steam/lb of recovered solvent or 0.2 to 0.4 lb of steam/lb of carbon

• In a continuous operation, a minimum of 2 adsorption units is required.

Q: Three-units? Any advantage?

Q: How will you select the regeneration time?

Rotary Bed

System

Mycock et al., 1995



Fluidized-Bed

System

Q: Benefits?

Pressure Drop

75.1'

)1(150

')1( 2

3

Gd G D

d Pg

p

gg p

Typical operating range:

< 20 in H2O; 20 < V < 100 ft/min==> determine theMaximum Adsorbent Bed Depth

P: pressure drop (lb/ft2) D: bed depth (ft) : void fractionG’: gas mass flux (lb/ft2-hr) g: gas viscosity (lb/ft-hr)d p: carbon particle diameter (ft)

Q: Why?

Union Carbide Empirical Equation56.1

10037.0

V DP

P: bed pressure drop, in H2OV : gas velocity, ~60-140 ft/min D: bed depth, ~5-50 inchesd p: 4X6 mesh sized carbon

Minimum Adsorbent Bed Depth

Need to be at least longer than the MTZ

S

B

s C

C D X

MTZ 11

1

C B: breakthrough capacity %C S : saturation capacity % X S : degree of saturation in the MTZ (usually 50%) D: bed depth

Other Systems: Nongenerablehpsl

Thin-bed adsorber

Canister adsorber

Mycock et al., 1995

Q: What need to be known to start the design of an adsorption

bed system?



Exercise I

• An exhaust stream contains 1880 ppm of n- pentane at 95 oF. The flow rate to be treatedis 5500 acfm. Carbon capacity is 3.5 lb n- pentane/100 lb AC. Carbon density is 30lb/ft3. 2-bed system: 1 hr for adsorption andthe other hr for regeneration.

• Q: Mass flow rate of n-pentane? Volumeof carbon bed? Flow velocity? Steam

requirement? Pressure drop?

Exercise II

• Conditions: 10,000 acfm of air @ 77 oF at 1atm containing 2000 ppm toluene (MW =92) to be treated. 95% removal efficiency by4X10 mesh carbon expected (density ofcarbon = 30 lb/ft3)

• Q: how many lb/hr of toluene to beremoved?

• Q: If regeneration at 212 oF, what’s theworking capacity?

• Q: Design an adsorption system with maxP of 8 inH2O, 4 hr cycle, two beds.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

40 50 60 70 80 90 100 110

IV Allowable bed depth 8

inH2O / DP in Column II

V Required bed depth Carbon

volume (143 ft3) / area in

Column III

I II III IV V

Velocity P C ro ss -S ec tio na l Are a Allo wa ble be d d ep th R eq uire d b ed de pth

fpm in H2O / Q (10,000 a cfm) / 8 inH2O / C a rbon volume (143 ft3) /

ft o f b e d d e pt h V el i n C o lu mn I P in Co lum n II a re a in Co lum n III

40 2 250.00 4.00 0.62

50 2.7 200.00 2.96 0.775

60 3.7 166.67 2.16 0.93

70 4.6 142.86 1.74 1.09

80 5.4 125.00 1.48 1.24

90 6.5 111.11 1.23 1.4

100 7.6 100.00 1.05 1.55



Absorption

• Definition

• Equipment

• Packing materials

• Design considerations:

– Mass balance

– High gas flow

– Mass flow

• Concentrated systems

• HTU (Height of a Transfer Unit) and NTU

Reading: Chap. 13

img.alibaba.com/photo/50678451/Ceramic_Metal_...

Definition

Transfer of a gaseous component

(absorbate) from the gas phase to a

liquid (absorbent) phase through a

gas-liquid interface.

Q: What are the key parameters that affect the effectiveness?

Q: How can we improve absorption efficiency?

Mass transfer rate: gas phase controlled absorption

liquid phase controlled absorption

Q: Does it matter if it’s gas phase or liquid phase controlled?

Equipment

Mist

Eliminator

Liquid

SprayPacking

Liquid outlet

Dirty gas in

Spray

nozzle

Clean gas out

Countercurrent

packed tower

Spray tower

Mycocket al., 1995

Redistributor Q: Limitations of a

spray tower? Q: Why redistributor?

Clean gas out

Dirty gas in

Berl

Saddle

Intalox

SaddleRaschig

Ring

Lessing

RingPall

Ring

Tellerette

Three-bed cross

flow packed tower

Liquid sprayDry Cell

Packing

Mycock et al., 1995Q: Criteria for good packing materials?



Design considerations:

What are known? What are we looking for?

Mass Balance

In = Out

Slope of OperatingLine = Lm /Gm

1221 mmmm LG LG

2121 x x L y yG mm

Lm: molar liquid flow rate

Gm: molar gas flow rate x: mole fraction of solute in pure liquid

y: mole fraction of solute in inert gas

(for a dilute system)

Gas in

Liquid out

Gas out

Liquid in

Clean water Dirty water

Clean air

Dirty air

Generally, actual liquid

flow rates are specified

at 25 to 100% greater

than the required

minimum.

• G = 84.9 m3/min (= 3538 mole/min). Pure water is used

to remove SO2 gas. The inlet gas contains 3% SO2 by

volume. Henry’s law constant is 42.7 (mole fraction of SO2 in

air/mole fraction of SO2 in water). Determine the minimum water

flow rate (in kg/min) to achieve 90% removal efficiency.

Q: How much is X2 if fresh

water is used? What if afraction of water is recycled?



• Channeling: the gas or liquid flow is much greater at some

points than at others

• Loading: the liquid flow is reduced due to the increased gas

flow; liquid is held in the void space between packing

• Flooding: the liquid stops flowing altogether and collects in

the top of the column due to very high gas flow

Problems with high gas flow

• Gas flow rate is 3538 mole/min and the minimum liquid flow rate

is 2448 kg/min to remove SO2 gas. The operating liquid rate is

50% more than the minimum. The packing material selected is 2”

ceramic Intalox Saddles. Find the tower diameter and pressure

drop based on 75% of flooding velocity for the gas velocity.Properties of air:: molecular weight: 29 g/mole; density: 1.17×10-3

g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp. L

G

G

L

g

F G

LG

L

1.02)'(

L: mass flow rate

of liquid

G: mass flow rate

of gas

G’: mass flux of gas

per cross sectional

area of column

F : Packing factor

: specific gravity

of the scrubbing

liquid

L: liquid viscosity

(in cP; 0.8 for water)

(dimensionless)

Mass Transfer

differenceionconcentrat

arealinterfacia

/d transferre

massof ratek Flux

Two-Film Theory (microscopic view)

I GG p pk J

L I L C C k J

I I HC p

LG

LG

HC pk H k

J

//1

1

Cussler, “Diffusion”, Cambridge U. Press, 1991.

C C k A M J i /

pG

C I

p I

C L

)(timearea

mass

J: flux

k : mass transfer coefficient

(gas phase flux)

(liquid phase flux)

(overall flux)

*

*

p pK

C C K J

GOG

LOL

LG

OG

G L

OL

k H k K

H k k K

//1

1/1/1

1

L

G

HC p

H

pC

*

*

Macroscopic analysis of a packed tower

outflowminusinsoluteof flow

soluteof onaccumulati

dz

dx L

dz

dyG mm ''0

Mole balance on the solute over the

differential volume of tower

)('

'11 y y

L

G x x

m

m

(equivalent concentration

to the bulk gas pressure)

(equivalent pressure to the

bulk concentration in liquid)

(overall liquid phase MT coefficient)

(overall gas phase MT coefficient)

1

2

L’m: molar flux

of liquid

G’m: molar flux

of gas



Mole balance on the solute in the gas only

absorption bylostsolute

outflowminusinflowsolute

onaccumulatisolute

*)('0 y yaPK dz

dyG OGm

1

*

'

0

y

yOG

m Z

Z y y

dy

aPK

Gdz Z Hx y *

Z Z mmOG

m

Z Z mmOG

Hx y

Hx y

L HGaPK

G

Hx y

Hx y

L H GaPK Z

11

11

ln'/'1

1'

ln'/'/1

11

NTU?HTU?

a: packing area per volume

(tower height)

11'

' x x

G

L y y

m

m

Mass balance

Equilibrium

Hx y *

1

*

' y

yOG

m

Z y y

dy

aPK

G Z

x1 , y1*

x1 , y1

x Z , y Z *

x Z , y Z

Alternative solution:

*

*11

**

111

ln

;'

z z

z z LM

LM

z

OG

m

y y y y

y y y y y

y

y y

aPK

G Z

Assumptions for dilute/soluble systems?

Pure amine

Lm = 0.46 gmole/s

0.04% CO2

1.27% CO2

Gm = 2.31

gmole/s

C* = 7.3%

CO2 in amine

Q: A Packed tower using organic amine at 14 oC

to absorb CO2. The entering gas contains 1.27%

CO2 and is in equilibrium with a solution of

amine containing 7.3% mole CO2. The gas

leaves containing 0.04% CO2. The amine,

flowing counter-currently, enters pure. Gasflow rate is 2.31 gmole/s and liquid flow rate is

0.46 gmole/s. The tower’s cross-sectional area

is 0.84 m2. K OGa = 9.34×10-6 s-1atm-1cm-3. The

pressure is 1 atm. Determine the tower height

that can achieve this goal.

Absorption of concentrated vapor

Mole balance on the controlled volume

)'()'(0 x Ldz

d yG

dz

d mm

Gas flux

yGG mm

11'' 0

Liquid flux

x L L

mm1

1'' 0

1

1

0

0

1

1

1

1

0

0

1

1

11'

'

11

11'

'

1

x

x

x

x

G

L

y

y

x

x

x

x

G

L

y

y

y

m

m

m

m

x1 , y1

x1 , y1*

x Z , y Z *

x Z , y Z



Mole balance on the gas in a differential tower volume

*)(

1

'0

2

0 y yaPK dz

dy

y

GOG

m

NTU HTU

y y y

dy

aPK

Gdz Z

y

yOG

m Z

Z

1

*)1(

'2

0

0

aPK

G HTU

OG

m0'

1

*)()1(

2

y

y Z y y y

dy NTU

HTU

(ft)

HTU

For a given packing material and

pollutant, HTU does not change much.



Combustion Kinetics (Global models)

n

A A kC

dt

dC C A: concentration of pollutant A

k : kinetic rate constant

n: reaction order

If pollutant concentration is much less than O2 concentration,

n can be assumed to be 1.

kt C

C

A

A exp0

Arrhenius Equation for rate constant

RT E Ak act /exp E act : activation energy (cal/mol) R: gas constant (1.987 cal/gmole K)

Determination of A and E act

'

'2

R

P yS Z A

O

Z’ collision rate factor

S : Steric factor (ineffective collision)

yO2: mole fraction of O2

in the afterburner

P: pressure (atm)

R’: gas constant(0.08205 l-atm/mol oK)

MW S

16

1.4600966.0 MW E act

Q: How long does it take to incinerate 99% toluene (MW = 96)

at 1000 K (O2 mole fraction = 0.15)?

(Kcal/mol)

Thermal Incinerator Design –Isothermal Plug Flow Reactor

Knowns? Unknowns?

mwaste gas

mburner air

m fuel

T 1

mexhaust T 2



Mass Balance:

air exhaust air burner fuelgaswaste mmmm

Out In

Energy Balance

00 T T C mT T C Q

H Q H mq

p pg

gh

qh: heat rate required

Q: gas volume flow rate

g: gas density

H : enthalpy (Table B.7)C p: specific heat

T 0: reference temperature (25 oC or 60 oF)

.

(Stoichiometry)

Steady State Energy Balance

losscombustionVOC VOC VOC combustion fuel

air burner air burner fuel fuelgaswastegaswasteexhaust exhaust

q H m X H m

H m H m H m H m

X : fractional conversion of VOC

Assume (1) the enthalpy functions of all streams are similar to that

for air

(2) the heat loss is a fraction of the heat input

0T T C H p

losscombustionVOC VOC VOC combustion fuelloss f H m X H mq

Combining mass balance and heat loss relationship with energy balance

Heat of Combustion

Higher Heating Value (HHV) =

Lower Heating Value (LHV) + Latent Heat of Water (Table B.6)

1 BTU/lb = 2.326 KJ/kg

)()1(/

)1(

)()(

fuelexhaust losscombustion

combustionVOC VOC lossVOC

air burner exhaust air burner gaswasteexhaust gaswaste

fuel

H H f H

H m f X

H H m H H mm

lossVOC VOC VOC combustion fuel

VOC VOC VOC combustion fuel

air burner b fuel fuelgaswastegaswaste

exhaust air burner fuelgaswaste

f H m X H m

H m X H m

H m H m H m

H mmm

combustion

combustion

air urner

Then group items by each material flow

Sizing the Incinerator

T

exhaust I

Q D

v

4

Incinerator diameter Qexhaust : exhaust flow rate

vT : gas velocity in the incinerator

(20 ~ 40 ft/s)

Length of Incinerator

r T L v r : residence time (0.2 ~ 2.0 s)

Length/diameter = 2 ~ 3 Q: Stoichiometric air? Excess air? Effects?

Q: What will happen if velocity

is too low?

A waste gas stream of 2465 acfm enters the afterburner at 200 F.

The desired exhaust temperature is 1350 F. It is estimated that the

burner will bring in 200 acfm of outside air. Both the burner air and

the fuel gas enter at 80 F. Assuming 10% overall heat loss and

ignoring any benefits of the oxidation of the pollutants, calculate the

required mass flow rate of methane.



Catalytic Oxidizers

• Advantages of Catalytic Oxidation – Lower fuel requirements

– Lower NOx, CO and CO2 emissions

• Disadvantages – Subject to deactivation – thermal aging (sintering of support and

deposited catalyst)

– Presence of poisons and suppressants problematic – "poison" isirreversible, fast-acting: P, Bi, As, Hg, Pb;

slow-acting: Fe, Sn, Si

– Suppressants – halogens, S

– Masking – covering or blocking access to sites - PM

– Erosion – e.g. particles in gas stream

Q: How can it do that?

Q: How does it look like?

Examples

of Catalyst

Supports

Catalytic OxidizerReduces activation energy

• Heat required to raise temperature of gases is lower



Catalytic Converter

(automobiles)

Washcoats onautomotive catalyst

Automotive

catalyst Structural

design including

honeycomb

support and

mounting can

Catalytic Oxidizers continued

• Types of Catalysts

– Precious Metal: Pt, Pd, Rh – more prevalent with lower operating T and

shorter residence time

– Base Metal: Cu, Cr, Mn, Co – in some cases designed for more

resistant to certain contaminants, Cl

• Operating temperature

– Too low, get products of incomplete combustion (PICs)

– Too high, increases thermal aging (operate < 1200 oF to avoid

"sintering")

– Normally when getting high destruction effi ciency,

• k chemical rxn >> k mass transfer

Q: What design parameters are we looking for?

Q: Pollutant concentration as a function of distance from the catalyst surface: (1)

reaction on catalyst surface is slow; (2) reaction on catalyst surface is very fast?

Mass transfer limited design

• For mass transfer limited case gas phase resistance controls rateof destruction – use mass transfer coeff. k m



Mass transfer model – Retallick ref.

• Material balance

Integrate from 0<x< L

Define mass transfer unit

length, Lm, N = number of

transfer unit

N = L/Lm hence C(x) = C o e -N

Q*dC=-a*A c*dx*k m*C

C ak

dx

dC m

/exp)( 0 xak C xC m

ak L

m

m

Q: Physical meaning of Lm?

Laminar Flow Solution

• For laminar flow conditions, molecular diffusivity governsand Nusselt number (Nu

m) is available in the literature =

4.4 circular holes, 4.1 parallel plates.

Hence substituting for km

For a given destructionefficiency (penetration)

N can be determined toyield bed length L.

d D Nuk mm /

D

d

D Nu

d

ak L

mm

m6.174

22

Q: A honeycomb catalytic converter has an effective hole

diameter of 0.059 inch and 72% open area. The flow

contains toluene (diffusivity: 0.084 cm2 /s) and its velocity

is 20 ft/s. Determine the length of the incinerator in

order to have 99% destruction efficiency.

Total amount of catalyst surface area is critical in

assuring effective destruction:• Catalyst surface area/gas volume flow rate: 0.2 – 0.5

ft2/scfm

• Exhaust flow rate/volume of catalyst (space velocity):

50,000 – 100,000 hr -1

Q: What is the inverse of space velocity?

Hazardous Waste Incineration

• 40 CFR, Part 261, Subpart O

– For each principal organic hazardous constituent(POHC) in the waste stream, there must be at least99.99% (four 9’s) destruction and removalefficiency (DRE).

– At least 99% of the hydrogen chloride must beremoved if the emissions are more than 1.8 kg/hr.

– Particulate emissions must not exceed 180 mg/m3

corrected to 7% oxygen in the stack gas

– Waste containing chlorinated dioxins, chlorinateddibenzofurans and chlorinated phenols (RCRDcodes F020-F028) require a 99.9999% (six 9’s)DRE.



NOx

• Thermal NOx vs Fuel NOx

• Strategies for Combustion Modifications

– Off stoichiometric combustion, flue gas

recirculation, water injection, gas reburning, low

NOx burner

• Flue Gas Treatment

– SCR, SNR, Absorption, Adsorption

Reading: Chap 16

27.11.2012 1 27.11.2012 2

NOx

Thermal NOx vs Fuel NOx

• Thermal NOx: formed by reaction between N2 and O2

in the air; sensitive to temperature

– Fast formation rate at high temperature

– Fast cooling rate freezes formed NOx

• Fuel NOx: formed from combustion of fuels containing

organic nitrogen in the fuel; dependent on localcombustion conditions and nitrogen content in the fuel

• NOx Control:

– combustion modification --> prevent formation

– flue gas treatment --> treat formed NOx

Q: Can absorption, adsorption or incineration be used to

control NOx?

27.11.2012 3

Strategies for Combustion Modification

• Reduce peak

temperatures of the

flame zone

• Reduce gas

residence time in

the flame zone

Q: Temperature as a

function of equivalent ratio?

tricStoichiomeair

fuel/

air

fuel:)(ratioEquivalent

27.11.2012 4



• Off-Stoichiometric

Combustion/Staged combustion:

combusting the fuel in two or more

steps. Fuel rich then fuel lean.

• Flue gas recirculation: reroute

some of the flue gas back to the

furnace; lower O2 and allow NOx

to proceed the “frozen” reactions

• Water injection: reduce flame

temperature; energy penalty

Modification of Operating Conditions

Why?

http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket)

27.11.2012 5

• Gas reburning: injection of natural gas into the boilerabove the main burner to create a fuel-rich reburn zone;hydrocarbon radicals react with NOx to reduce NOx to N2.

http://www.lanl.gov/projects/cctc/factsheets/eerco/gasreburndemo.html

27.11.2012 6

• Low-NOx burner: inhibit NOx formation by

controlling the mixing of fuel and air; lean excess

air and off-stoichiometric combustion

27.11.2012 7

Flue Gas Treatment

• Selective Catalytic Reduction (SCR)

O H N O NH NO

O H N O NH NO

22

catalystsupported OVorTiO

232

22

catalystsupported OVorTiO

23

6342

6444

522

522

Q: Should a SCR reactor be

installed before or after

particle control devices?

Q: Why is it called “selective”?

Temperature ~ 300 - 400 oC

http://www.lanl.gov/projects/cctc/

factsheets/scr/selcatreddemo.html

Also good for Hg emission control!!!

27.11.2012 8



O H NOO NH

O H N O NO NH

223

2223

6454

6444

Temperature ~ 800 - 1000 oC

Q: Disadvantages of

SCR and SNR?Above 1000 oC

• Selective Noncatalytic Reduction (SNR)

222

32

2222

32

22 2

OO H OH HO

HNO NO HO

O H HOO H OH

HNO NOOH

HONO NOOH

OH O H

• Wet Absorption: generally good for only NO2

because NO is insoluble. Need to oxidize NO first.

27.11.2012 9

• Dry Sorption

– Activated carbon (220 ~ 230 oC): reduce NOx to N2;

oxidize SO2 to H2SO4 if NH3 is injected, and carbon is

thermally regenerated to remove concentrated H2SO4

– Shell Flue Gas Treating System (~ 400 oC)

– Alkali Metal and Alkali Earth Metal based sorbents:

form metal nitrates (e.g. NaNO3, Mg(NO3)2)

CuOOCu

O H SOCu H CuSO

O H N O NH NO

CuSOSOOCuO

2

2224

22

catalystsasCuSOorCuO

23

422

5.0

22

6444

5.0

4

27.11.2012 10

27.11.2012 11

Chambers Medical -

Dry Sorbent InjectionProcess Flow

For removal of SO2and HCl

27.11.2012 12



27.11.2012 13 27.11.2012 14

Active carbon

and Sodium-

Bi-Carbonate

are feeded to

the flue gas in

the dry

scrubbing

system

27.11.2012 15

http://www.ergapc.co.uk/

Odour Scrubbing System

27.11.2012 16



27.11.2012 17

Odour Scrubbing System

27.11.2012 18

Ash Treatment



Control of SO2

• Strategies for SO2 removal

• Formation Prevention

– Low sulfur fuel, Fuel desulfurization,

• Flue Gas Treatment

– Wet scrubbing, Dry scrubbing, Spray drying, Dual

Alkali, Wellman-Lord Process

Reading: Chap 15

27.11.2012 1

Strategies for SO2 Removal

Main Option Suboption Examples of Processes

Do not create SO2 Low-sulfur fuel

Desulfurize the fuel Oil desulfurization, Coal

cleaning

SO2 scrubbing:

Throwaway Wet scrubbing Lime, Limestone, Dual alkali,

Misubishi, Bischoff, Forced

oxidation (w/ gypsum disposal)

Dry scrubbing Lime spray drying, Lime

injection, Trona, Nahcolite

Regenerative Wet processes Absorption with water(smelters), Wellman-Lord,

MgO, Citrate, Aqueous

carbonate, SULF-x,

CONOSOx, Forced oxidation

(w/ gympsum sales)Dry processes Activated carbon adsorption,

Copper oxide adsorption

27.11.2012 2

Fuel Desulfurization

Oil & natural gas

RS H H S R 2

reactioncatalytic

2

Claus process

S O H SOS H

SOO H OS H

322

23

2

reaction catalytic

22

2222

Coal

Mineral sulfate

--> wash away

Organic sulfur ???

--> SO2 removal

IGCC – Integrated

GasificationCombined

CycleSO2 Removal

High concentration (e.g. smelting)

Absorbing SO2 to make H2SO4

Low concentration (< 2000 ppm)

Flue gas desulfurization

(for sale; a commodity,not a pollutant)

27.11.2012 3

Limestone (Wet) Scrubbing

O H gCOCaSOCa HSOsCaCO

gCO HSOCaSOO H sCaCO

223

2

33

23

2

223

)(22)(

)(22)(

Q: Advantages and disadvantages compared to water?

LSO I SO L

C C k J ,,22

Mass transfer in liquid film

Overall mass transfer coefficient

LG

OGk H k

K //1

1

Main reactions converting SO2 to bisulfite in the liquid film

32

2

32

322

2 HSOO H SOSO

HSO H O H SO

: enhancement factor

precipitate

Q: Does mass transfer

increase as increases?

(packed tower)

(effluent

hold tank)

27.11.2012 4



27.11.2012 5

Q: Is it better to have a high pH?

27.11.2012 6

Lime (Wet) Scrubbing

O H CaSOOO H CaSOO H CaSOOH CaSO H

SO H O H SOOH CaO H CaO

24223

23232

3222

22

25.022)(

Dual Alkali

NaOH yCaSO xCaSO

O H Na ySO xSOCaCOCaO

O H ySO xSO NaSO NaOH SO Na

34

2

2

3

2

43

2

2

3

2

4232

5.0/

5.0/

(recycled)

Q: How does it compare to limestone scrubbing?

Q: Advantages and disadvantages?

27.11.2012 7

Lime-Spray Drying

Same as lime scrubbing except the water evaporates

before the droplets reach the bottom of the tower

Dry scrubbing

Direct injection of pulverized lime or limestone, also

trona (natural Na2CO3) or nahcolite (natural NaHCO3)

Q: Advantage of dry methods over wet methods?

Ca2+

SO2

Ca2+

CaSO4

CaO

CaSO4

CaO

27.11.2012 8



27.11.2012 9

Dry scrubbing

http://wpca.info/library.php

27.11.2012 10

Dry scrubbing-Process Layout

Atomizer

Wellman-Lord (W-L) process

(1) Flue gas pretreatment: (venturi prescrubber) to remove

particulates, SO3 and HCl

(2) SO2 absorption by Na2SO3 solution

3422332

42232

32232

225.0

2

NaHSOSO NaO H SOSO Na

SO NaOSO Na

NaHSOO H SOSO Na

(3) Purge treatment: centrifuge the slurry to remove solids

(4) Na2SO3 regeneration

232232

223232

COSO NaSOCO Na

O H SOSO Na NaHSOheat

(averagely, 1 mole Na2SO3 for 42 moles of SO2)

No further SO2 absorption

(concentrated, 85%)

(make-up sodium)

Claus process

27.11.2012 11 27.11.2012 12

Westvaco process

In the Westvaco process, the activated-carbon catalyst with the adsorbed/absorbed H2SO4

is regenerated by bringing it into contact with H2S in a fluidized bed at 150 oC when the

H2SO4 is converted to elemental sulphur. A fraction of elemental sulphur is recovered from

the carbon by direct vaporization in a stripper. The remaining sulphur is reacted with

hydrogen to provide the H2S require in the earlier step.

Adsorber/

Absorber Reducer Stripper

H2S

production

Unit

O H S S H SO H

SO H OO H SO

2

carbonactivated

242

42

carbonactivated

222

443

5.0

Clean Gas

Flue gas

containing SO2 H2S from

Natural gas

Activated Carbon

H2S



27.11.2012 13

Diagram of gas suspension absorption system.



27.11.2012 1

Control in Mobile Sources

• Types of mobile sources

• Impact of mobile sources

• Emission standards

• Types of engines: gasoline vs diesel, 4-strokes vs

2-strokes

• Emission Control: technology and policy

• Add-on Control Device – catalytic converter

• Hybrid Vehicles and Fuel Cells

Reading: Chap 15

Gasoline engines emit carbon monoxide (CO), nitrogen oxides (NOx),

hydrocarbons (HC), particulate matter (PM), and lead (where leaded gasoline isused), as well as other toxics such as benzene, 1,3 butadiene, and formaldehyde.

27.11.2012 2

Mobile Sources

AIRCRAFT

http://www.flightradar24.com/

27.11.2012 3

Mobile Sources

http://www.flightradar24.com/

27.11.2012 4

Mobile Sources

Ship Track



27.11.2012 5

Emission Standards Note

: Standards are applicable over

the “useful life” of the vehicle,

which is defined as 50,000

miles or five years for

automobiles. The durability of

the emissions control device

must be demonstrated over this

distance within allowed

deterioration factors. Figures in

parenthesis apply to a useful

life of 100,000 mile, or ten

years beyond the first 50,000

miles.

a. Non-methane hydrocarbons. b. The U.S. Environmental Protection Agency (EPA) could delay implementation of tier 2

standards until 2006.

Source: CONCAWE (Conservation of Clean Air and Water in Europe). 1994. Motor

Vehicle Emission Regulations and Fuel Specifications–1994 Update. Report 4/94.

Q:What is the current ES?

27.11.2012 6

Light-Duty Vehicle and Light-Duty Truck Emission Standards

27.11.2012 7

Q: Gasoline vehicles for ULEV and ZEV?

Q: NMOG?

27.11.2012 8

• Heavy Duty Vehicles

– For the same power output, only 70% mass compared to

gasoline engines

– Less volatile, Lower CO2 emission

– Lower operating cost, 2/3 of an equivalent gasoline truck

Lloyd and Cackette, JAWMA, 51, 2001, p809-847.

Q: Gasoline penalty in Europe?VMT: Vehicle Miles Traveled



27.11.2012 9

• Heavy Duty Vehicles (cont.)

– Contains S (500 ppmw): new regulation just kicked in 2006

– Lower operating speed, slower thrust/acceleration

– Lean exhaust: catalytic converters don’t work

– Also standards for PM, smoke opacity

– Account for 5% of vehicles, account for 35% visibility

reduction; Off-road vehicles emit 2 times PM

(regulated in CA from 1996)

– Soot from diesel engines is the 2nd largest contributor to

global warming

– Relative health risk of diesel exhaust exposure ~ 1.4

Q: Emission regulations for diesel engines?

Heavy-Duty Truck and Bus Engine

Emission Standards

27.11.2012 10

HC CO NOx PM

Heavy‐Duty Diesel Truck Engines 1.3 15.5 4.0 0.10

Urban Bus Engines 1.3 15.5 4.0 0.05

EPA 1998 Standards

NMHC THC CO NOx PM

Heavy‐Duty Diesel Truck Engines (1994) 1.2 1,3 15.5 5.0 0.10

Urban Bus Engines (1996) 1.2 1.3 15.5 4.0 0.05

CA Standards

EPA Standards for MY 2004

NMHC+NOx NMHC

Option 1 2.4 n/a

Option 2 2.5 0.5

27.11.2012 11

Motorcycles

• Small number in US (0.6% of HC, 0.1% NOx

and <0.1% of PM of all mobile sources); large

in developing countries

• Higher emissions per mile than a car or even a

SUV

Class Engine Size

(cc)

Implementation

Date

HC

(g/km)

HC+NOx

(g/km)

CO

(g/km)

I < 170 2006 1.0 - 12.0

II 170-279 2006 1.0 - 12.0

III > 280 2006 - 1.4 12.0

2010 - 0.8 12.0

Highway Motorcycle Exhaust Emission Standards

http://www.epa.gov/otaq/roadbike.htm

Thailand Tutu

27.11.2012 12

4-Stroke Internal Combustion

Premixed gasoline vapor

Combustion occurs

every two revolutions

Animation: http://www.allstar.fiu.edu/aerojava/pic6-3.htm



27.11.2012 13

Octane Rating• A measure of gasoline’s resistance to engine knock (rattling or

pinging sound in cylinders due to premature burning)

– Heptane: 0

– Iso-octane: 100

– Anti-knock agents: Tetraethyl lead (Pb(C2H5)4), highly branched

alkanes, aromatics

• (R+M)/2

– Research Octane Number (RON): test engine running at 600 rpm – Motor Octane Number (MON): test engine at 900 rpm

C C C C C C C

C C C C C

C C

C

Q: What if a lower grade than required is used?

Q: Does my car perform better if I use a higher grade than required?

http://en.wikipedia.org/wiki/

Image:09-03-06-Octane.jpg

27.11.2012 14

Compression stroke Power stroke

Q: Why two strokes are not good?

2-Stroke Engine

http://media-2.web.britannica.com/eb-media/10/310-004-58E43DBF.gif

27.11.2012 15

Diffusion flame

700 – 900 oC

from compression

Liquid diesel injection

Ignition

delay

27.11.2012 16

Emission Control

• Engine design, vehicleshape

• Fuel composition – Octane rating, oxygenated

fuel

– Fuel desulfurization

• Alternative fuel – natural gas, liquefied

petroleum gas, methanol,ethanol , bio-diesel,hydrogen

• Transportation control – Regulatory steps, public

transportation, economicincentive

• Regulation: Inspection

• Add-on control

• Alternative powergeneration – Solar, Electrical, Fuel Cell,

Hybrid



27.11.2012 21

Bio-diesel• Non-petroleum-based diesel fuel consisting of short chain alkyl (methyl

or ethyl) esters, made by transesterificationof vegetable oil or animalfat (tallow), which can be used (alone, or blended) in unmodified diesel-engine vehicles

• Transeterification: separate the fatty acids from the glycerol byreplacing the glycerol with short linear alcohols; typically requiresliquid catalyst

• Distinguished from the straight vegetable oil (SVO) used (alone, or blended) as fuels in some converted diesel vehicles

• Advantages: reduction in greenhouse gases; no sulfur; better lubricity

• Disadvantages: slightly lower energy density; may contain water;cloud point (gelling) higher the petroleum diesel; by-product glycerol;more expensive

• Emissions (B20):

• Algae as a feedstock http://en.wikipedia.org/wiki/Biodiesel

PM HC CO NOx

10% 21% 11% 2%

27.11.2012 22

Traffic Congestion

• In 2007, $78 billion drain on US economy, including

– 2.9 billion gallons of fuel

– 4.2 billion lost hours

Annual extra time on the roads

Rank Metro area Hours

1 Los Angeles 72

2 San Francisco 60

2 Washington, DC 60

2 Atlanta 60

8 Orlando 5411 Miami 50

Data provided by Texas Transportation Institute, http://mobility.tamu.edu/ums/

27.11.2012 23

Catalytic Converter (Gasoline)Pt, Pd

Two-way

Bloomfield, L., Scientific America, 2001

Exhaust Gas composition

27.11.2012 24

Pt, Rd

Three-way



27.11.2012 29

Hydrocarbon Vapor Emissions from Gasoline Distribution

AsifFaiz et al., 1996,

Air Pollution from Motor Vehicles27.11.2012 30

Older model

O2 for lean

environment

Newer Model

(1980’s in US)

Q: Cold-start emission more

serious? How to control it?

Lean NO catalyst:

Zeolite catalyst to reduce NO

Combined with lean-burn engine

O H N CO NO HC Zeolite

222

27.11.2012 31

Catalytic Oxidizer

27.11.2012 32

Thre-way Catalytic Converter



27.11.2012 33

Automotive catalyst structural design

27.11.2012 34

Washcoats on automotive catalyst

27.11.2012 35

Catalytic Oxidizer

27.11.2012 36

Catalyst Materials



27.11.2012 37

Diesel Particulate Filter

27.11.2012 38


27.11.2012 39


27.11.2012 40




27.11.2012 41

Clean School Diesel Bus Technology Options

Clean Fuel /

Clean Tech

Type of Engine % Reduction in

PM Emissions

% Reduction in

NOx Emissions

Approximate Cost of Technology

Ultra-Low

Sulfur Diesel

(ULSD)

New or Used About 5 to 9%

Enables the PM

filter to work

N/A 8 to 25 cents per gallon more than

regular diesel now

In June 2006, when ULSD will be

required nationwide, cost differential

will be much less

Particulate

Matter Filter

New or Used -

1995 or newer

models

60 to 90% N/A $5,000 to $10,000Must use ULSD fuel

Oxidation

Catalyst

New or Used 20 to 30% N/A $1,000 to $2,000 and can be used

with regular diesel

Biodiesel

Fuel

New or Used B 20 - 1 0%

B100 -40%

Biodiesel increases

NOx emissions of.

B20 blend +2%

B100 fuel +10%

B20 - 15 to 30 cents per gallon more than

regular diesel

B100 - 75 cents to $1.50 per gallon more than

regular diesel (B100 may not be an option for

cold climates)

EmulsifiedDiesel Fuel

New or Used About 50% About 10% 20 cents per ga llon more thanregular diesel fuel

http://www.epa.gov/cleanschoolbus/technology.htm

27.11.2012 42

Electric Car

• First electric car in 1835 by Professor

Sibrandus Stratingh of Groningen,

Netherlands

• First battery powered electric car in

1881 by Camille Faure in France

• Lost market to ICE circa 1910

• Government backed promotion (tax

credit) revived electric carThomas Edison and an electric car in1913 (National Museum of American

History)

Q: Is it really zero emission? Q: Why isn’t it commonly used?

Plug-In: uses a wall socket at night to charge and relies on an electric

motor to go many miles before sipping any gasoline – good for short-distance driving in cities

Q: Where do you find a wall socket when not at home?

http://www.pbs.org/now/shows/223/

Who killed the electric car (EV1)?Who killed the electric car (EV1)?

27.11.2012 43

Hybrid Electric Vehicle

• Use 2 sources of motive energy:

combustion of gasoline (Internal

Combustion Engine) & electrical

energy (Electric Motor energized by

a Battery)

– ICE for highway driving

– EM provides added power duringhill climbs, acceleration, and other

periods of high demand

• Regenerative Braking: converts

some of the kinetic energy into

electric energy; electric motor

becomes a generator for battery

• Also available for heavy-duty

hybrid vehicles (diesel-electric) http://www.nrel.gov/vehiclesandfuels/ahhps/

Automobile.Honda.com

27.11.2012 44

Advantages of Hybrid Electrics

• high performance

• long-range capacities

• high fuel efficiency

Fuel economy

Lower emissions

2007 Ford Focus 2007 Toyota Prius 2007 Honda Hybrid

MPG(city) 27 MPG(city) 51 MPG(city) 49

MPG(highway) 37 MPG(highway) 60 MPG(highway) 51

Cost: federal and state purchase incentives

Driving privileges

Q: Is hybrid popular?



27.11.2012 45

Fuel Cell Operation

http://en.wikipedia.org/wiki/Fuel_cell

http://www1.eere.energy.gov/hydrogenandfuelcells/

27.11.2012 46

• University of Florida Fuel Cell Research Lab

H2 source:

• H2 (gas, liquid, solidhydride) – Converted from other

energy sources (coal,solar, wind)

• Methanol (CH3OH)

• Gasoline

Limitation of H2 fuel

• Pipelines needed to convey hydrogen fuel not currently in place

• Retail fueling facilities must be placed throughout Q: Where?• Danger of H2 explosion (although gasoline explosion is also dangerous)

• Requires construction of hydrogen production facilities

27.11.2012 47

Hydrogen Fueling Station

President Bush touring a hydrogen

fueling station in DC in May 2005

Washington, DC (1st in US)

Los Angeles Int. airport, CA

Richmond, CA

Ann Arbor, MI (EPA)

Taylor, MI

50 stations in 15 states by end

of 2007

H2 fuel station in Iceland http://www.cnn.com/2007/TECH/science

/09/18/driving.iceland/index.html

27.11.2012 48

Methanol as an Alternative to Hydrogen

• Some production facilities already exist

• Distribution facilities can accommodate with slight modifications

• Dispensing facilities can accommodate with only slight

modifications

• Currently there is an abundance of methanol

Steam Reforming Equilibrium Reaction at 1atm and 300oC:

CH3OH + 1.5H2O 2.896H2 + 0.896CO2 + 0.104CO + 0.603H2O

Catalyzed by Phosphoric Acid Membrane (PAM)

Q: Are fuel cells used in stationary sources?



27.11.2012 49

Evaporative Loss – Refrigerant• CFC (Chlorofluorocarbons) – Ozone depleting chemical

phased out starting from 1987 Montreal Protocol

• Replacement: HCFCs (Hydrochlorofluorocarbons)

HFCs (Hydrofluorocarbons)

Q: Impact of stockpile and CFC in discarded units?

• Further ban on global warming gases

– EU: HFC-134a banned in new cars in 2011; any otherfluorinated gas with a GWP > 150 in all vehicles in 2017

– US: ???

• Replacement: nontoxic and nonflammable

– CO2 (requires new systems); DP-1 (GWP-40); Fluid H(GWP-10)

Compound CO2 CFC-12 HCFC HFC-134a

GWP 1 8500 1300-7000 1300

http://pubs.acs.org/isubscribe/journals/cen/83/i1

8/html/8318gov2.html?emFrom=emLoginhttp://www.epa.gov/ozone/ods.html

27.11.2012 50

Gasoline and Diesel Contnets



Indoor Air Pollution ControlA gram of pollution released indoors

produces about 1000 times more

exposure than one released outdoors

Rule of one thousand

Indoor Air Pollutants Indoor Air Pollutants



Indoor Air Pollutants

Smoke from solid fuels is a risk factor for

•Acute respiratory infections (ARI),

•Chronic obstructive pulmonary disease (COPD)

•Lung cancer (from coal smoke)

Risks from solid fuels

Environmental tobacco smoke (ETS) Environmental tobacco smoke (ETS)



American National Human Activity Pattern Survey (NHAPS)

and ARB surveys of children & adults

>11000 interviews over 2‐year period

• INDOORS 89%

• OUTDOORS 6%

• IN VEHICLES 5%

Where do we spend our time?

CO2,

CO,

SO2,

NO2,

Ozone (O3),

Hydrogen chloride (HCl),

Nitrous acid (NHO2),

Nitric acid vapour (HNO3)

Radon.

Inorganic gases contaminated indoor air

Organic gases contaminated indoor air Control of Indoor Air Quality

• Control of pollution sources

• Indoor air

purification

Indoor air quality (IAQ) can be achieved by two way

• Control of pollution sources is a most economical and effective approach

in improving IAQ to eliminate or reduce indoor pollution sources

• Indoor air purification is an important method of removing indoor pollutants and

improving IAQ under the circumstances that the ventilation and the control of

pollution sources are impossible.



Control of pollution sources

• Filtrating the outdoor air to prevent outdoor pollutants from entering the

room.

• Isolating the sites that may form pollution sources (e.g., copycat rooms, printer

rooms, kitchen and toilet) in order to avoid intercrossing infection, and using

the enforced ventilation when necessary.

• Making full use of pollution‐free or low‐pollution building materials and

decorating materials. Preventing building products with high pollution from

entering market by government legislating and setting up industry standard.

For the products in markets, government can label them with different grade.

The building materials and decorating

• materials with high pollution can be eliminated by market mechanism.

• Dust and liquid drops are the important medium for bacteria to spread. It is

necessary to

termly clean

the

components

that are

easy

to

be

infected

in

air

‐

conditioning systems (e.g., filter, heat exchanger and muffler) and to replace

them in time in order to avoid the aggradations of pollutants.

Indoor Air Purification

• Filtration

• Adsorption

• Photo‐catalytic Oxidation (PCO)

• Negative Air Ions (NAIs)

• Non

‐

thermal

plasma

(NTP)

The major methods of indoor air purification include

Filtration is a quite economical and efficient

method of improving IAQ.

Filters are important components in all AC

systems.

Filtration

Ozone removal: the chemical reactions between ozone and the particles

deposited on the filters ‐ lead to oxidation products such as formaldehyde,

carbonyls, formic acid, and ultra‐fine particles

DA: Air filtration systems could become a source of contamination from

micro‐organisms harmful to human health.

Solution: to prevent the accumulation and dispersion of microorganisms by

adding anti‐microbial agents on the surfaces of filter.

Adsorption

The adsorbents able to be used to purify indoor air mainly include activated carbon,

zeolite, activated alumina, silica gel, and molecular sieves.

Adsorption on activated carbon is an extensive method of purifying indoor air due to

its large specific area and high adsorption capacity.

At

low concentration

level,

the

advantages

of

activated

carbon with

abundant

micro‐

pores are more prominent.

Activated carbon fibers (ACF) exhibit a higher adsorption capacity and have faster

adsorption kinetics than granular activated carbon (GAC), 2 to 20 times faster than

on GAC.

ACF are easier to use than GAC since they can be formed in various forms such as

cloth and felt . Therefore, ACF are more suitable as an adsorbent for removing indoor

gaseous pollutants.



Adsorption

DISADVANTAGES

•ACF is quite expensive, and its price is 5–100 times higher than

that of GAC, which limits the wide application of ACF.

•Individual adsorbent does not have the ability to adsorb all

kinds of indoor pollutants, and only exhibits good adsorption

effects on some kinds of pollutants.

•The composition of indoor pollutants is quite complex, and

their concentrations are greatly different.

•Hence the adsorbents with wide adsorption range need to be

developed.

Adsorption

Photo‐catalytic Oxidation (PCO) Photo‐catalytic Oxidation (PCO)



Non‐thermal plasma (NTP) can be generated in different modes, such as direct current

corona discharge, pulse corona discharge, dielectric barrier discharge and glow

discharge.

The pulse corona discharge is usually used to generate NTP because it can reduce the

formation of O3

in the discharge process.

Atmospheric plasma discharges generate high energy electrons, while the background

gas remains close to room temperature .

The energetic electrons excite, dissociate and ionize gas molecules, which produce

chemically active species such as atomic oxygen, hydroxyl radicals and ozone.

These active species are capable to remove various indoor pollutants such as VOCs,

aerosol particles and microbe.

DA : NTP

leads

to

the

formation

of

byproducts

(e.g., CO,

O3,

NOX and

aerosol

particles)

in the process of indoor air purification.

Non‐thermal Plasma (NTP) Non‐thermal Plasma (NTP)

Non‐thermal Plasma (NTP)

The following is a summary of their (Non‐thermal plasma ) disadvantages:

• Exhibiting good purification effects on some kinds of

pollutants, but poor purification effects on other

pollutants

Non‐thermal Plasma (NTP)

env415 air pollution control engineering

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