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ContentVapor pressure, Equilibrium Vapor Content, EvaporationVOCsControl by PreventionControl by Concentration and Recovery Condensation Adsorption Absorption (scrubbing)
Control by Oxidation Combustion Biological Oxidation (Biofiltration)
Choosing a Control Technology
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Introduction
2nd most widespread and diverse class of emissions after particulates.Main concern: participation in “smog” and ozone reaction and formation of secondary particles.Some are toxic, some are IR absorbers.More than 150 HAPs are VOCs.
Source Type (US, 1997)
Thousands tons/yr
% of total
Wood burning 1294 5.73
Industrial processing
1457 7.58
Waste disposal 449 2.34
VOC storage 1377 7.17
Solvent use 6483 33.74
Motor vehicles 7660 39.86
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Vapor Pressure, Equilibrium Vapor Content, Evaporation
VP = f(T). VP of water @ 20 oC = 0.023 atm = 17.5 torr.Normal Boiling Point: T at which VP = Patm.
VP of Hg ~10-6 atm @ 20 oC, still a concern (toxic).Metals such as Cd, Zn will vaporize in incinerators.Partial P in a mixture
VP of a pure compound
...pppP
pxPyp iiii
321
* Law sRaoult'
compound pure of VP
equation Antoine log
eq.Clapyron -Clausius log
*
*
*
p
CT
BAp
T
BAp
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VOCs
Boiling points up to 500 oF (260 oC)Organic compounds up to 12 carbon atomsEPA Definition (40 CFR51.100(s)): Any compound of carbon, excluding CO, CO2, carbonic acid, metal carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions (some notable exceptions: methane, ethane, 1,1,1-TCA, some CFCs, PCE).VOC and HC terms are often practically identical.Polar organics are ~100x more soluble than HC.For same family, solubility decreases with increasing MW.
)20(68@)0007.0(01.0* CFatmpsiap oo
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Control by Prevention
SubstitutionWater-based paint, CNG
Process ModificationFluidized-bed powder coating, electric-powered vehicles (but more emission during electricity generation), car-pool.
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Control by Prevention
Filling, Breathing and Empting LossesFilling (displacement): Gas being displaced is typical saturated with VOCs it has been in contact.Breathing (diurnal): T increase causes increase in VP, expansion of the liquid, vapor and the tank.Adjust Reid VP (RVP) of gasoline for different seasons.Leakage control/fugitive emissions.
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Control by Prevention
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Stage 2 control:Dual vapor return systemStage 1: w/o return from pump.
0.45 g/gal
4.54 g/gal
0.32 g/gal
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Control by Concentration and Recovery - Condensation
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Control by Concentration and Recovery - Adsorption
Absorption – the dissolution of gas molecules into bulk liquid.Adsorption – physical, reversible attachment of gas (or vapor) molecules to a solid surfaceChemisorption – the attractive forces between gas molecule and surface are closer to chemical bonding than physical attraction.The term “Sorption “ is used to avoid having to make the distinction,
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Control by Concentration and Recovery - Adsorption
Adsorbents used are high surface area ( ca. 1000 m2/g) micro-porous particles. Alumina (Al2O3), Silica (Si2O3), activated carbon.Adsorbent capacity, w* : mass of sorbate per mass of sorbent.
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Control by Concentration & Recovery - Adsorption
x-axis: (T ’L/18M)log(fs/f); y-axis = 100 w*/ ’L
’L: liquid density at normal boiling point, g/cm3
M: molecular weightfs/f: essentially the same as p*/pi
(vapor pressure/ partial pressure)
Isotherms for HCs on activated carbon and silica gel
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Control by Concentration & Recovery – Absorption (scrubbing)
VOC selectively absorbed by bringing gas and solvent in contact in a packed column.Solvent and VOC separated in another column operating at lower temperature or pressure.
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Control by Oxidation – Combustion (incineration)
Combustion responsible for most of the air pollutants of concern.Combustion can also be a solution if practiced properly (i.e. sufficient 3T’s)Incineration is the term used when the purpose of combustion is the destruction of pollutants rather then energy production.
2222
32
2222
1566
222
1
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SOOHOSH
OHCOOHC
COOCO
bO
aVOCCkCr
2
kCdt
dC
kCr VOC
;
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)exp(RT
EAk
Destruction efficiency
99.00%
99.90%
99.99% Penetration, C/C0
0.0100
0.0010
0.0001 T, F
k, 1/s
t, s
t, s
t, s
1000
1.10E-04
41865
62798
83730 1200
0.14
32.9
49.3
65.8
1400
38.59
0.119
0.179
0.239
Benzene
Relatively small changes in T result in large differences in t required for a particular . In most incinerators the are of the order of seconds. If the T drops, drops significantly.
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Incineration arrangements
A) simple thermal incinerator
B) regenerative heat recovery
C) catalytic incinerator
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Control by Oxidation – Incineration
The oxygen and combustible come in contact over a surface instead of being homogeneously mixed.Solid or liquid combustible usually first vaporizes: Evaporation = simple phase change Pyrolysis = breakdown of large combustible
molecules thus giving off smaller, more volatile molecules in an inert (or reducing atmosphere), i.e. in the absence of oxygen.
Both are endothermic processes. The required heat is supplied by the combustion of the vapor
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Control by Oxidation – Biofiltration
Biofilters: gas distributor pipes at the bottom, covered with several feet of soil/compost VOC dissolved in soil moisture.
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Control by Oxidation – Biofiltration
Typical: soil depth = 3 – 4 ft; void = 50%, upward gas velocity = 0.005 to 0.5 fps; residence time = 15 - 60 s.Better for polar compounds (soluble)Need moisture and nutrients.Protection of M/O (toxins, pH, T)Large footage.
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Would type of VOC matter?
VOC Control Choices
