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Properties of Gases and Vapors AND

VOC Incinerators

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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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VOCs controls

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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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Heat exchange systems

100100%13

12 ⋅−−

=⋅ΔΔ

=TTTT

TTE

av

recrec

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Heat exchange systems

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Regenerative heat exchanger

Efficiency: 80-95%

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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