w11 Control of Vocs

8/11/2019 w11 Control of Vocs

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INTRODUCTION•

VOC is the 2nd

most widespread and diverseclass of air pollutants after PM.• VOCs can be toxic, remain in the atm for a

long time.• Contributes to formation of photochemical

compounds, greenhouse effect and depletionof ozone layer.



MORE SOURCES• Household products including: paints, paint

strippers, and other solvents;• moth repellents and air fresheners;• stored fuels and automotive products;• cleansers and disinfectants;• wood preservatives;• aerosol sprays;• dry-cleaned clothing;



Figure show the contribution of VOC emissions from many sectors, excluding open sourcessuch as forest fires and dust from unpaved roads.



Do VOCs cause smog?• VOCs play a significant role in the formation of

ozone & PM in the atm.• The accumulation of ozone and particulates

results in smog, reduces visibility.



Subtopics• Control by prevention• Control by concentration and recovery• Control by oxidation



Control by Prevention

Substitution

Process Modification

Leakage Control



Switch from oil to water-basedpaints reduces VOC emissionsfrom painting, coating andprinting. But water-basedpaints are not durable.



Substitution

• Replace gasoline as a motor fuel withcompressed natural gas or propane.



Process Modification

• Replace gasoline powered vehicles withelectric powered vehicles.



Process Modification•

Replaceconventionalsolvent dependentcoating processwith newtechniques

• e.g. fluidized bedpowder coating,UV lithography,electrostatic spray.



Leakage Control

• Filling, breathing and emptying losses : workinglosses

• Tanks containing liq VOCs can emit VOC vaporswhile filling & emptying activities, and change inT & P.

• Figure 10.2: Vented tank. As liq enters the tank,

liq level rises, vapor space above in tank mustdecrease in volume. The air + vapor mixture mustbe vented out to prevent overpressure.



• When liquid is withdrawn from the tank, airwill flow in through the vent to fill the spaceto prevent tank from collapse due to vacuum.



• Petrol contains volatile organic compounds (VOCs) thatevaporate in storage tanks.

• During unloading of petrol to an underground storagetank or refueling of a vehicle, petrol vapor in the tank willbe displaced by the incoming petrol. Unless controlled,the petrol vapor will dissipate into the atmosphere. Major

harmful effects of VOCs from petrol filling stations wouldinclude:(a) Enhancing the formation of ozone & fine particulates

in the atmosphere, thus causing smog,(b) Potential health risk to the public as it

contains benzene, a carcinogen,(c) A nuisance to people in the vicinity.



Gasoline Leakage Control

• Filling, breathing and emptying losses forgasoline

• Example 10.7 illustrates that 470 000 gal/dayof gasoline is lost as vapor when gasoline istransferred from service station storage tanksto customers’ vehicles!

• Good nozzle design can reduce this lost.



Can the shape of the nozzle tell that it recoverspetrol vapor?

Yes. The shape of the vapour recovery nozzle is different from those without. It has avapour guard and holes at the nozzle spout to recover petrol vapour.



Tank design to reduce VOC emission

• Large amount of liq with high vapor pressurelike gasoline are stored in floating roof tanks(internal or external).



Seal leaks

• Many small emissions occur at the seal asleaks.

• Fig 10.7 shows 3 kinds of seals: static,compression/packed, as

• The seal used in pumps & valves at VOCprocessing facilities are more complexversions of b and c.



Control by Concentration andRecovery

• Most VOCs are valuable solvents or fuel.• It can be recovered in almost pure form by:

– Condensation – Adsorption – Absorption (Scrubbing)



Condensation

• VOC can be removed from a gas stream bycooling the stream to its dew point T, the VOCare condensed as a liquid.

• The dew point of the vapor-gas mixturedepends upon the condensation of the vapor.Therefore, keeps on decreasing as theconcentration of vap dec. The T must beprogressively reduced t recover morematerial.



Condensation• All vapors cannot be recovered by condensation alone.

Cost of cooling the gases is very high. Only recovermaterials at T that is economically viable.

• Condensation is only applicable for vapor-rich orsaturated gas mixture.

• For low conc of organic vap, the T required are too low,more often blow freezing points. Material freezes oncooling coil and needs frequent defrosting.

• Condensation serves as preliminary method prior toother treatments like adsorption or combustion.



Types of condensers• Two types: Direct and indirect• Indirect : In a shell & tube condenser , the

material recovered is in its pure form. Coolingmedium (water, refrigerant) passes thru tubes,vap kept on shell side.

• Indirect: Spray, packed, venturi or cyclonic units.Direct contact between water or liq and the vap-gas mixture. A mist of fine roplets form and canbe difficult to separate from the gas. May requiredemisters . The organic material can be recoveredby distillation/extraction/decantation.



Adsorption• The best method of separation when the gas present is

in small quantity (i.e. <600 ppm).• Serves as further polishing step to adsorb residual

solvent vapors after condensation/scrubbing.• Compounds get attached on a solid porous adsorbent.• After adsorption, adsorbents are regenerated by

steam/hot air/nitrogen.• Often several adsorption beds are used in large scale

applications e.g. collecting solvent vap from largepaint-drying oven or large printing press. See Fig 10.10.• When relative humidity is high, the adsorption capacity

of adsorbents gets reduced.



Absorption (Scrubbing)

• Vapors are scrubbed using water or liquids inwhich it is soluble. Commonly used are water,methanol, or higher alcohols, ketones, esters,

ethers.• Can be conducted in spray column, plate

columns, packed columns or venturi scrubber.•

May require further purification by exractionor distillation to recover the material andscrubbing liq.



Absorption

• The rate of transfer of an organic solvent fromgases depends on the relative conc of theorganic pollutants in the gas and liq. The

greater the difference, the higher the transferrate.• The conc diff is higher if liq and gas flow in

opposite directions (counter current).• T are kept low because solubility of gases/vap

is higher at low T.



Control by Oxidation

• The final fate of VOCs is mostly to be oxidizedto CO2 or H2O by:

– combustion (incineration at high T); or – Biological oxidation (biofiltration, low T); – Or in atm, forming ozone ad fine particulates.



Combustion/Incineration

• Three methods of incineration: – Direct flame – Thermal – CatalyticSee previous lecture slides on details of each

methods.



END OF SLIDE

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w11 Control of Vocs

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