12. air pollution part 2

8/8/2019 12. Air Pollution Part 2

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AIR POLLUTION

Roz Ayu bt Ismail

Hasmida bt Mohd Nasir

Norul Fatihah bt Mohammed NoahMohamad Amirrun Nazrie bin Suhaimi


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Indoor Air Quality

y Referring to the air quality within and around buildings and structures,especially as it relates to the health and comfort of building occupants.

y Can be affected by microbial contaminants, gases, particulates, or anymass or energy stressor.

y Common Pollutant:

- Radon- Molds

- Carbon Monoxide

- Volatile Organic Compound

- Asbestos Fibers

- Carbon Dioxide

- Ozone

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


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y Developed by Indoor EnvironmentManagement Branch, USEnvironment al Protection Agency

(EPA).y Analyzing the impact of sources,

sinks, ventilation, and air cleanerson indoor air quality.

y Allowed calculation of indoor

concentrations as a function oftime.

Indoor Air Quality Modeling

http://www.epa.gov/appcdwww/iemb/model.htm


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Mass ala ce

Volume = V

Co ce tratio = C

Q, Ca Q, C

Si kSource

Emission

Rate =E

Decay

Rate = k

ACCUMULATION = IN + GENERATION OUT - CONSUMPTION


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Mass ala ce

Rate ofpolluta t

i crease ibox

=Rate of polluta t

e teri g boxfrom outside

+Rate of polluta te teri g box fromi door emissio

-Rate of polluta tleavi g the box by

leakage to outdoors-

Rate of polluta tleavi g box by

decay

Volume = VCo ce tratio = C

, Ca , C

Si kSource

Emission

Rate =E

Decay

Rate = k

ACCUMULATION = IN + GENERATION OUT - CONSUMPTION


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Mass ala ce

Volume = VCo ce tratio = C

, Ca , C

Si kSource

Emission

Rate =E

Decay

Rate = k

Rate ofpolluta t

i crease ibox

=Rate of polluta t

e teri g boxfrom outside

+Rate of polluta te teri g box fromi door emissio

-Rate of polluta tleavi g the box by

leakage to outdoors-

Rate of polluta tleavi g box by

decay


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Equation 7-27 (pg 598)


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General Solution for Equation 7-27:

Steady-state Solution for Equation 7-27:

k=0, ambient concentration negligible,initial indoor concentration=0:


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7.10AIR POLLUTION CONTROLOF STATIONARY SOURCES

Gaseous pollutant FGD

Control Technologies for Nitrogen

Oxides Particulate Pollutants

Control Technologies for Mercury


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Gaseous Pollutant

Absorption

Adsorption

Combustion


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Absorption

y Transfer pollutant from gas phase to liquidphase.(mass transfer process)

y The removal of the pollutant gas takes place inthree steps:I. Diffusion of the pollutant gas to the surface of the

liquid.

II. Transfer across the gas/liquid interface.

III. Diffusion of the dissolved gas away from the

interface into the liquid.y The example is spray chamber and

tower/column.(see figure 7.26 and 7.27)


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y Amount of the absorption for a nonreactive solution is govern bypartial pressure.

y Henry's law give relationship between partial pressure andconcentration.

Pg = KHCequi

y Where,

p = Partial pressure of the solute in the gas above the solution.

c = Concentration of the solute

kH = constant with the dimensions of pressure divided byconcentration.

y Eq. above implies that Pg is must increase as the liquid accumulatesmore pollutant or else it will come out of solution.


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y Since the liquid is removing pollutant from thegas phases, this means the partial pressure isdecreasing as gas is cleaned. Reverse what we

want to happen.y The easiest way to settle this problems by run

the gas and liquid in countercurrent flow.


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Mass balance equation (look figure 7-28)

(Gm1)(y1) (Gm2)(y2) = (Lm1)(x1)- (Lm2)(x2)

Where ,Gm1 , Gm2 = total gas f low into and out of the column respectivelyY1 ,y2 = mole fraction of pollutant in a gas

Lm1 , Lm2 = total liquid flow

X1 , x2 = mole fraction of pollutant in liquid

Variable to design packed tower is gas f low rate, liquid f lowrate and the height of the tower.

The height of the tower equation is :

Zt = (Hog)(Nog) (7-45) See example 7-7.


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Adsorption

y The gas is bonded to a solid (mass transfer)

y Pressure vessels having a fixed bed are used to hold theadsorbent (figure 7-29)

y Common adsorbents is active carbon (charcoal), molecularsieves, silica gel, and activated alumina.

y The common property of these adsorbents is a large activesurface area per unit volume after treatment.

y They are very effectives for hydrocarbon pollutant. Inaddition, they can capture H2S and SO2.

y One special form of molecular sieve can also capture NO2.

y Except active carbons, adsorbents have a drawback that theypreferentially select water before any of the pollutant. So,

water must remove from the gas before it is treated.


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y All adsorbents are subject to destruction at hightemperature (1500C for active carbon, 6000C formolecular sieves, 4000C for silica gel, and 5000C foractivated alumina ). At this temperatures they are veryinefficient and in fact, their activity is regenerated!

y Relation between the amount of pollutant adsorbedand the equilibrium pressure at constant temperatureis called an adsorption isotherm. Equation byLangmuir :

W = __aCR*__

1 + bCg*The time to breakthrough:

tB = Zt v

f


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Combustion

y Alternatives method of control when thecontaminant in the gas stream is oxidizable to aninert gas.

y Use Direct Flame incineration method if:1. Gas stream have net heating value (NHV)greater than 3.7 MJ/m3.2. None of the byproducts of combustion betoxic.(eg. Trichloroethylene producesphosgene,which was used as a poison gas in world

war 1.)y Flame incineration is applied to varnish cooking,

meat-smokehouse, and paint bake-ovenemissions.


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y Use catalytic incineratorif :

y Catalytic material enable oxidation to be carried outin gases that have an NHV less than 3.7MJ/m3

y Catalytic combustion is has successfully been applied

to printing-press, varnish cooking, and asphalt-oxidation emissions.

y Problem in design catalytic reactor is to determine thevolume and dimensions of the catalyst bed for a given

conversion and flow rate.y See example 7-9 that show how to estimating the

dimension and volume of the catalyst.


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Flue gas desulfurization (FGD)

Flue gas desulfurization systems fall into 2 broadcategories:

Nonregenerative reagent used to remove sulfuroxides is discarded

Regenerative reagent used is recovered and reused


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Nitrogen Oxide (NOx)

Result from combustion processes

Produced from :

oxidation of N2 bound in the fuel

Reaction of O2 and N2 in the combustion air (T>1600K)

Reaction of N2 in the combustion air with hydrocarbonradicals


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Control technologies for NOx

a) Prevent the formation of NOx during thecombustion process

By reduce the flame temperature

Alternatives:Minimizing operatingemperatureFuel switchingLow excess air

Flue gas recirculation

Lean combustionStaged combustionLow NOx burnersSecondary combustion

Water/steam injection


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b) Convert NOx formed during combustion into N2and O2

Selective catalytic reduction (SCR)

Combustion

process

NH3 injectedupstream of

catalyst bed

NH3

NOx N2 + O2


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Selective noncatalytic reduction (SNCR)

Urea injectedinto flue gas

(870-1090oC)

Ureaconvertedinto NH3

NH3

NOx N2 + O2


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Nonselective catalytic reduction (NSCR)

Use 3-way catalyst

Require reducing agent

Need larger boiler

Reduction capabilities

Prevention: 30-60 %SCR : 70-90 %

SNCR : 30-50 %


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Particulate Pollutants

Cyclones

y For particle sizes greater than 10m in diameter

How its works? Particle accelerated through a spiral motion

Imparts centrifugal force to the particles

Hurled out of the spinning gas

Impact on cylinder wall

Slide to the bottom of the cone

Removed through valving system


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Reverse flow cyclone


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Standard reverse flow cyclone proportions


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y The efficiency of collection of various particle sizes(L) can be determined from an empiricalexpression and efficiency graph (FIGURE 7-36)


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Where

d0.5 = cut diameter, the particle size forwhich the collection efficiency is 50%

= dynamic viscosity of gas, Pa.s

B = width of entrance, m

H = height of entrance, m

p = particle density, kg/m3s

Qg = gas flow rate, m3

/sU = effectiveness number of turns made in

traversing the cyclone


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y The value ofU may be determined approximately by thefollowing:

y Where L1 and L2 are the length of the cylinder and conerespectively


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y For further understanding, lets go through ex 10-7pg 617


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FILTERSy Use as control method when high efficiency

control of particles smaller than 5m isdesired

y 2 types are in use

i. The deep bed filter

ii. The baghouse


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The deep bed filter

y Resembles a furnace filter

y Used to intercept particles in the gas

streamy Preferable for relatively clean gases and

low volumes, such as air conditioningsystem.


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Baghouse

yPreferable for dirty

industrial gas with highvolumes


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Mechanically cleaned (shaker) baghouse (a) and pulse-jet-clean baghouse (b)


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y Fundamental mechanisms of collection includescreening and sieving

y Once a dust cake begins to form on the fabric,

sieving is probably the dominant mechanism.y The buildup of the dust cake also increases

the resistance to gas flow.

y At some point the pressure drop across the

filter bags reduces the gas flow anunacceptable level and the filters bags mustbe cleaned


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y Three methods use to clean the bags are

i. Mechanical shaking

ii. Reverse air flow

iii. Pulse-jet cleaning

y Mechanical shaking operate by directing the dirtygas into the inside of the bag.

y Reverse air flow cleaning a compartment is isolatedand a large volume of gas flow is forced

countercurrent to normal operationy Pulse-jet baghouses, the particulate matter is

collected outside of the bag.


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y The dust cake is removed by directing a pulsed jetof compress air into the bag.

y Example 7-11 (page 620)

y

Solution:1. Nothing that the air-to-cloth ratio units of m/s

are equivalent to m/s.m, calculate the net clotharea required with one compartment off line for

cleaning:A = Q/V

= 20 m/s/0.01m/s. m

= 2000 m


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2. The net number of bags is the total area dividedby the area of one bag:

2000 m/()(0.15 m)(12 m) = 353.67 or

354 bags3. With one-eighth of the bags off line, an additionalone-eighth of the net number required:

354bags/8 = 44.25 or 44 bags

4. The total number of bags is 354 + 44 = 398.


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Liquid scrubbing

y Used when the particulate matter to be collected iswet, corrosive, or very hot, the fabric filter may notwork.

y Typical scrubbing applications includei. Control of emission of talc dust

ii. Phosphoric acid mist

iii. Foundry cupola dustiv. Open heart steel furnace fumes.


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yPrinciple of operation of the liquidscrubber is that a differentialvelocity between the droplets ofcollecting liquid and the particulatepollutant allows the particle toimpinge onto the droplet.


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Venturi scrubber


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yHigh efficiency, dry collection ofparticles from hot gas streams can beobtained by electrostatic

y The EPS is usually constructed ofalternating plates and wires

y A large direct current potential

(30 75 kV) is established between theplates and wires

Electrostatic Precipitation (EPS)


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12. air pollution part 2

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