Properties of Gases and Vapors AND VOC Incineratorsweb.nmsu.edu/~dwdubois/16_lecture_CEE452.pdf ·...
Transcript of Properties of Gases and Vapors AND VOC Incineratorsweb.nmsu.edu/~dwdubois/16_lecture_CEE452.pdf ·...
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Today’s Lecture
• Definition of physical properties and chemical reactions
• VOCs control devices– Incinerators
• Physical/Chemical Processes• Designs
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Gases and vapors
VAPORS (e.g. VOCs)
GASES (e.g. O2, NOx, SO2)
Evaporation
Sublimation
Deposition
Condensation
If mixture, thenVapor pressure
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Vapor pressure• Vapor pressure is a force (pressure) exerted by
the gaseous phase of a two phase—gas/liquid or gas/solid system.
• All liquids and solids have vapor pressure at all temperatures except at absolute zero, -459°F (-273°C).
• Equilibrium (saturation) vapor pressure is the pressure of a vapor in an enclosed place where the two phase system are in equilibrium state.
• For a given substance, vapor pressure depends on the temperature, pressure, and on the nature of the substance (Clausius-Clayperon). As temperature increases so does the vapor pressure.
• At a constant temperature and pressure existing inter-molecular forces of the substance are the determining factors of the vapor pressure (e.g. hydrogen bonds for OH-containing molecules).
BTR
HP +⋅
Δ−=ln
Vaporization enthalpy
Compound-specific constant
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Partial Vapor PressureRault Law: Vapor pressure of a solution is dependent on the molar fraction and the vapor pressure of each component.
nnsolution xPxPxPP ⋅++⋅+⋅= ....2211
Partial (individual) vapor pressure (for ideal solutions)
111 γ⋅⋅= xPPpartialFor non-ideal solution:
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DiffusionDiffusion: Flow from the high concentration “solution” to the low concentration “solution” to make the composition uniform.
Flow α Concentration
xDJ
∂∂⋅−=φ
D for gases: 10-5 to 10-4 m2/s, D for liquids: 10-10 to 10-9 m2/s.
Diffusion coefficients are inversely proportional to total pressure (but diffusion is the same).
Diffusion coefficients increase with increasing temperature
For a steady-state:
2
2
xD
t ∂
∂⋅=
∂∂ φφ
For non-steady-state:
dtdCD
AM
⋅−=
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Gas/solid and gas/liquid interactions
• Absorption (diffusion of a gas/vapor in a liquid)• Adsorption (diffusion of a gas/vapor in a solid)
Gas
Liquid/Solid
Liquids: Henry’s Law: Pgas=HCliquid
Solids: Depends on the gas and solid propertiesChemisorption: Chemical transformation of gases (e.g. catalysts)
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Chemical reactions• All reactions are possible !!!!!
– Reactants. Different reactants react at different speeds. – Catalyst that contributes a needed substance to the reaction. – Entropy. It is the measure of energy not available for work in the
reaction that becomes energy moved to disorder. – Reaction conditions. The temperature, humidity, and barometric
pressure will affect the reaction.
How (Thermodynamics) and When (Kinetics)
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Chemical reactions
Kinetics: Reaction rate [ ] [ ]ba BAkr ⋅⋅= 11
[ ] [ ]dc DCkr ⋅⋅=− 21
dDcCbBaA +↔+
RTEeAk /−⋅=
Activation energy
Thermodynamics: [ ] [ ][ ] [ ]ba
dcc
BADC
kkK
⋅
⋅==
2
1
For gases bB
aA
dD
cC
pPPPP
kkK
⋅
⋅==
2
1RS
RTHK p
Δ−⎟⎠⎞
⎜⎝⎛ Δ−=ln
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Incinerator• Combustion (oxidation) of VOCs
– Thermal oxidation (isothermal or non-isothermal) – Catalytic oxidation
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Reactions, kinetic and thermodynamics
OHyxCOOyxHC yx 222 24 +→⎟⎠⎞⎜
⎝⎛ ++
( ) ( )22222 2224 NfeSOfHXOHfbaCOOdfbeaXSONHC fedcba +++⎟
⎠⎞⎜
⎝⎛ −+→⎟
⎠⎞⎜
⎝⎛ −−+++
In case of mixture of
Oxidation (combustion) is not an one-step reaction
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Methane oxidation
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224
2224
21
2
22
COOCO
OHCOOCH
OHCOOCH
→+
+→+
+→+
[ ] [ ][ ] [ ]
[ ] [ ][ ] [ ] 2
122
21
22
241
2241
2
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OCOkr
OCOkr
OCHkr
OCHkr
CO
CO
CO
CH
⋅⋅=
⋅⋅−=
⋅⋅=
⋅⋅−=
[ ]COkrco 22=
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Important parameters (3τ)Temperature
High temps result efficient destruction of VOCs (T~ 1800 F)
kc1
=τ
TurbulenceAchieve good mixing between VOC and O2
TimeFor reactions to be completed
uL
QV
r ==τ
em D
L2=τ Diffusion coefficient
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Estimation of 3τLee et al., 1979, 1982 Temperature for 99% destruction efficiency (T99)
11109876
543219.993.758.628.661.873.4202.20592.02.806.710.1172.12594
WWWWWWWWWWWT
−+−+−−++++−=
W1 = carbon atomsW2 = aromatic compound (0/1)W3 = Double-bondW4 = nitrogen atomsW5 = autoignition temp.W6 = oxygen atomsW7 = sulfur atomsW8 = hydrogen/carbon ratioW9 = allyl compound W10 = C=Cl interactionsW11 = logτr
NH2
S
Cl OH
Temperature at which VOC will ignite without an external source
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Estimation of 3τ
'
'2
R
PSyZA O=
RTEeAk /−⋅=1.4600966.0 +−= MWE
Cooper et al., 1982
Z = Collision rate factorS = steric factor (=16/MW)y(O2) = molar fraction of oxygen
[ ][ ]
rk
in
out eHCHC τη −−=−= 11
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Mass balance for a typical burner
( ) ( ) ( ) ( ) 011
0
=−Δ+−Δ+−++
=−++
∑ fHMfHMhMhMhMhM
MMMM
VOCVOCGGEEBABAGGPAPA
EBAGPA
Fuel
Polluted Air
Air (Oxygen)
Exhaust
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Typical design values• Linear velocity = 10 -20 ft/s• Residence time < 1 sec (higher if medical)
uQDπ
=uDMWP
RTMQE
E 2)(
π==
ruL τ=
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Catalytic oxidizer-1
Fuel
Polluted Air
Air (Oxygen)
Exhaust
- Lower combustion temperature- Expensive materials- Lower pressure drop (as compared to scrubbers)
Gas
Solid
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Catalytic oxidizer-2
[ ][ ]
mLL
in
out eHCHC /11 −=−=η
( ) 322 Scfa
L =
Length of one mass transfer unit-catalyst-VOC interaction-temperature-pressure
If laminar flow: DudL
6.17
2=
If turbulent flow: Sc = Schmidt numberf = Fanning friction number
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Afterburner efficiency and fugitive emissions
Afterburner efficiency ~ Destruction of captured VOCs
Process A
Process B
Process C Process D Afterburner
Fugitive emissions
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Afterburner’s efficiency
• Fugitive emissions are nearly impossible to estimate
• EPA defined the total enclosure– Openings less than 5% of enclosure’s surface area and closed
during operations– Air flow should be from outside to inside– VOC sources far away from openings– Exhaust streams should go to the afterburner
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Heat Recovery
Energy/resource savings Generate primary/secondary profit
Decrease of Texhaust ~ 1000 FSavings of ~ 260 btu/lbsair
Sale steam/hot waterPre-heat VOC streamSecondary use of stream/hot water
Heat exchanger- Available surface- Thermal capacity
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FlaresFlares are emergency relief systems installed on the top of stacks to burn off unusable waste gas or flammable gas and liquids released by pressure relief valves during unplanned over-pressuring of plant equipment
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Flares operationSteam is injected into the
flame to:(a) reduce the formation of
black smoke (b) Create turbulent mixing(c) provide cooling of the
flare tip
Flaring/venting is a major source of greenhouse gases
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Costs• Afterburner cost
– Cost of thermal incinerator
– Cost of catalyst + AOC/replacement (if required)
– Cost of heat recovery exchanger
bQaP ⋅=
( ) CRFPPC lcc ⋅−=
QP ⋅+= 6.11000,220$
( )( )244.0 ln0672.0exp752.53 AAC ⋅⋅= −
For regenerative technology
For standalone heat exchanger
P = F.O.B cost (~80% of final DEC cost)Q = flow ratea,b = curve fit constantsPc, Pl = initial and replacement costs for catalystsCRF = Capital recovery factorA = heat exchange area