Air Pollution Control Technologies

monkeyresultMécanique

22 févr. 2014 (il y a 3 années et 10 mois)

96 vue(s)

Air Pollution Control
Technologies

VOC Incinerators



VOCs


Includes pure hydrocarbons and partially
oxidized compounds (organic acids,
aldehydes, ketones)


There are hunderds of individual
compounds with each with its own
properties and chracteristics


Combustion processes, various industrial
operations and solvent evaporation are the
main sources

VOC Incineration


Vapor incinerators are also called thermal
oxidizers and afterburners


They can be also used successfully for air
polluted with small particles of solids or
liquids.


Incineration can be used for odor control,
to destroy a toxic compound, or to reduce
the quantity of photochemically reactive
VOCs released to the atmosphere


VOC Incineration


VOC vapors might be in a concentrated
stream (such as emergency relief gases in
a petroleum refinery) or might be a dilute
mixture in air ( such as from a paint
-
drying
oven)


In the case of a dilute fume in air two
methods are used


Direct thermal incineration


Catalytic oxidation


VOC Incineration


In some sources (such as printing presses
or parts
-
painting stations), emissions need
to be captured by hoods and a common
duct system and routed to a thermal
oxidizer (TO)


Total VOC reduction in this case depends
on both the destruction efficiency of TO
and capture efficiency


Total efficiency will be equal to destruction
efficiency times capture efficiency


VOC Incineration


The main disadvantage of the VOC
incineration is the high fuel cost


Some of the products of combustion of
some VOCs are themselves pollutants


When a chlorinated HC is burned HCl and Cl
2

will be emitted and then additional control for
these pollutants might become neccessary


Alternatives to VOC Incineration


Biofilter


Wet scrubber


Recovery of the vapors (recompression,
condensation, carbon adsroption or liquid
absorption)

If very stringent control objective exists, only
incineration can achieve that



e.g. Reduction in concentration from 10,000
ppm to 5 ppm for some malodorous organic
sulfur compounds.


Oxidation Chemistry


Consider only the case of a premixed
dilute stream of a pure HC in air


C
x
H
y

+
(b)O
2
+3.76(b)N
2

xCO
2
+(y/2)H
2
O+3.76(b)
N
2



C
x
H
y

the general formula for any HC



b= x+(y/4)


3.76=the number of moles of N2 present in
air for every mole of O2

Oxidation Chemistry


(b)O
2
+3.76(b)N
2

xCO
2
+(y/2)H
2
O+3.76(b)
N
2



The actual detailed mechanisms of
combustion are complex and do not occur
in a single step as might be inferred from
above eqn.



The mechanism involves a branching
chain reaction and further complicated by
the larger number of possible
intermediates for higher order HCs.

Oxidation Chemistry


(b)O
2
+3.76(b)N
2

xCO
2
+(y/2)H
2
O+3.76(b)
N
2



The actual detailed mechanisms of
combustion are complex and do not occur
in a single step as might be inferred from
above eqn.



The mechanism involves a branching
chain reaction and further complicated by
the larger number of possible
intermediates for higher order HCs.

Major Reactions in CH4 Oxidation

CH
4
+O
2

CH
3
∙+HO
2


CH
3
∙+O
2

CH
2
O∙+OH

CH
4
+OH

CH
3

∙+ H
2
O

CH
2
O+OH

HCO ∙+ H
2
O

CH
2
+O
2

HO
2

∙+ HCO ∙

HCO+O
2

CO+HO
2



HO
2
+CH
4

H
2
O
2
+CH
3



HO
2
+CH
2
O

H
2
O
2
+HCO ∙

OH ∙

wall

CH
2
O ∙

wall


Chain propagation

Chain branching

Chain propagation

Chain initation

Chain termination

Developing Global Models


Instead, global models ignores many of
the detailed steps and ties the kinetics to
the main stable reactants and products


Since CO is a very stable intermediate, the
simplest two step model for HC oxidation
is:

C
x
H
y
+(x/2+y/4)O
2

xCO+(y/2)H
2
O 11.2

xCO+(x/2)O
2

xCO
2



11.3


Global Model Kinetics


Using Eqs 11.2 and 11.3 a global kinetic
model that is of the first order in each
reactant results in the rate equations:


r
HC
=
-
k
1
[HC][ O
2
]


r
CO
=xk
1
[HC][ O
2
]


k
2
[CO][ O
2
] and in the
presence of excess O2:



r
HC
=
-
k
1
[HC]



r
CO
=xk
1
[HC]


k
2
[CO]

A third equation for CO2:



r
CO2
=
-
k
2
[CO]

Global Model Kinetics


Those 3 equations represent a special case
of a general set of consecutive first order
irreversible reactions:






A

R

S

Levenspiel (1962) presented solutions for the
concentration of all components as a
function of dimensionless reaction time for
various k
2
/k
1
ratios.

k
2

k
1

The Three T’s


Temperature


Time


Turbulence

For good destruction afterburners should be
designed for temperatures 650
-
1100 C, for
residence times of 0.2
-
2 sec, and for flow
velocities of 20
-
40 ft/sec

Afterburner is now a standart as part of a hazardous
waste incinerators requiring %99.99 destruction
and removal efficiency of the Principal Organic
Hazardous constituents


The Three T’s


In mathematical sense 3Ts are related to 3
characteristic times:

t
C
=1/k


Chemical time

t
r
=V/Q=L/u Residence time

t
m
=L
2
/D
e


Mixing time

V: Volume of the reaction zone, m
3

Q: Flow rate (at T in the afterburner) , m
3
/s

L: Length of the reaction zone, m

u: gas velocity, m/s

D
e
: effective (turbulent) diffusion coef. m
2
/s


The Three T’s

Peclet number:



Pe= Mixing time/residence time

Damkohler number:



Da=Residence time/chemical time

If Pe is large and Da is small

mixing is the
rate controlling process in the afterburner

If Pe is small and Da is large

chemical
kinetics is the rate controlling process


Predicting VOC Kinetics


Kinetics are surely important to the proper
desing of an afterburner but kinetic data
are scarce and difficult and costly to obtain
by pilot studies


So past methods of determining the
desing or operating temperature of an
incinerator were very rough at best

Predicting VOC Kinetics

1. Ross’s Approach


Ross (1997) summarized the older methods by suggesting T
design

be set
several hunders degrees (F) above the VOC autoignition temperature

Autoignition T: the temperature at which cobmustible mixtures of the VOC in air
will ignite without an external source


Substance

Autoignition T,
(F)

Substance

Autoignition T,
(F)

Acetone

1000

Acrolein

453

Ethanol

799

Hydrogen
Sulfide

500

Cyclohexane

514

Styrene

915

Phenol

1319

Toluene

1026


Excessively high design Ts will result in very high estimated costs for purchasing
and operating a VOC incinerator

2. Lee and coworkers’ Approach


A purely statistical model to predict the Ts required to give various levels
of destruction in an isothermal plug flow afterburner based on their
experimental studies


T
99.9
= 594
-
12.2W
1
+117W
2
+71.6W
3
+80.2W
4
+0.592W
5
-
20.2W
6
-
420.3W
7
+87.1W
8
-
66.8W
9
+62.8W
10
-
75.3W
11


T
99
= 577
-
10W
1
+110.2W
2
+67.1W
3
+72.6W
4
+0.586W
5
-
23.4W
6
-
430.9W
7
+85.2W
8
-
82.2W
9
+65.5W
10
-
76.1W
11


T
99.9
= T for 99.9% destruction efficiency, F


W
1
=number of carbon atoms


W
2
= aromatic carbon flag (0=no,1=yes)


W
3
= C C double bond flag (0=no,1=yes)


W
4
= number of N atoms


W
5
= autoignition Temperature, F


W
6
= number of O atoms


W
7
= number of S atoms


W
8
= hydroge/carbon ratio


W
9
= allyl (2
-
propenyl) compound flag (0=no,1=yes)


W
10
= carbon
-
double
-
bond
-
chlorine interaction (0=no,1=yes)


W
11
= natural log of residence time


Predicting VOC Kinetics

3. Cooper, Alley and Overcamp’s Approach

Combined collision theory with empirical data and proposed a
method to predict an effective first order rate constant k for HC
incineration over the range from 940 to 1140 K.

The method depends on MW and the type of the HC

Once k is found, T
design

can be obtained


'
'
2
/
R
P
Sy
Z
A
Ae
k
O
RT
E



A: pre
-
exponential factor

Z’: collision rate factor (from Figure 11.5 for alkanes, alkenes, and aromatics)

S=steric factor:
to account for the fact that some collisions are not effective in
producing reactions becasue of molecular geometry S=16/MW


yO2: mole fraction of O2 in the afterburner

P: absolute pressure, atm

R’: gas constant, 0.08206 L
-
atm/mol
-
K

Predicting VOC Kinetics

3. Cooper, Alley and Overcamp’s Approach

The activation energy E=
-
0.00966(MW)+46.1 (From figure 11.6)

Now k can be calculated for any desired temperature.

In an isothermal pluf flow reactor (PFR) the HC destruction efficiency,
the rate constant and the residence time are related as:

'
'
2
/
R
P
Sy
Z
A
Ae
k
O
RT
E



r
k
in
out
e
HC
HC
t






1
]
[
]
[
1
Predicting Overall Kinetics


The destruction of VOC occur quickly relative to CO
destruction

The kinetics of CO destruction have been studied by many
researchers

Howard published the following expression for CO oxidation
(valid for the range of 840
-
2360 K)


Destruction rate of CO =

1.3(10)
14
e
-
30,000/RT
{O
2
}
1/2

{H
2
O}
1/2
{CO}

Where { }indicates concentration in mol/cm
3

This equation can be combined with the VOC kinetic.

Example 11.1


Estimate the temperature required in an
isothermal plug flow incinerator with a
residence time of 0.5 sec to give 99.5%
destruction of toluene by using 3 methods
given.

Method 1.

autoignition T + 300 F=1026+300 = 1326 F

Example 11.1


Method 2. Lee et al.


T
99.9
= 594
-
12.2(7)+117+0+0+0.592(1026)
-
0
-
0+87.1(11.4)
-
0+0
-
75.3ln(0.5)


T
99
= 577
-
107+110.2+0+0+0.586(1026)
-
0
-
0+85.2(1.14)
-
0+0
-
76.1ln(0.5)


T99.5 can be calculated by taking a linear
average:

T99.5=(T99.9+T99)/2=1378 F

Example 11.1


Method 3. Cooper et al.

First calculate required value k



E=
-
0.00966(MW)+46.1

S=16/MW=16/92=0.174

Z’=2.85(10)11

Then calculate A



Now find T

1
5
.
0
6
.
10
)
005
.
0
ln(
5
.
0
1
995
.
0
1









s
k
e
e
k
k
r
t

1
10
11
)
10
(
07
.
9
08205
.
0
)
1
)(
15
.
0
)(
174
.
0
(
)
10
(
85
.
2
'
'
2




s
R
P
Sy
Z
A
O
F
K
A
k
R
E
T
Ae
k
RT
E
1331
995
)
)
10
(
07
.
9
/
6
.
10
ln(
1
987
.
1
45200
)
/
ln(
1
10
/








Catalytic Oxidation


Catalyst: is an element or a compound that
speeds up a reaction without undergoing
permananet change itself


Gaseous molecules diffuse to and adsorb
onto the surface of the catalyst


After reaction product gases desorb and
diffuse back into the bulk gas stream


The detailed mechanisms of the reaction
are not known but the reaction proceeds
much faster and/or at much lower T


Design Considerations for


Thermal Oxidizers


The process of a VOC thermal oxidizer
involves selection of


Operation T


Residence time


Sizing the device to achieve those with the
proper flow velocity

Factors:

O2 content

Type of operation (continous or intermittent)

Concentration of VOCs

Design Considerations for


Thermal Oxidizers


To minimize the cost, it is desirable to keep
the stream to be treated as low as possible


i.e. Not dilute VOC streams with too much
air


However most insurance regulations limit
the max VOC concentraiton to 25% of the
lower explosive limit (LEL) of the VOC

LEL and UEL Limits

LEL and UEL Limits

Design Considerations for


Thermal Oxidizers

Organic

LEL (%
by

volume

in
air
)

Acetone

2.15

Benzene

1.4

Ethane

3.2

Ethanol

3.3

Methane

5

Propane

2.4

Toluene

1.3

Design Considerations for


Thermal Oxidizers


Material

and

energy

balances

are

performed

to

the

device
to

calculate

the

flow

rate of
fuel

gas

required

to

raise

the

air

T at a
given

flow

rate
to

the

required

T.


2
Methods


1.
The

traditional

approach
:
assume

an
isothermal

plug

flow

reactor

2.
The

use

of
computer

program
to

handle

the

calculations

and

allows

for

non
-
isothermal

operation

Method 1. Traditional Approach



Material and energy balance:

E
BA
PA
G
M
M
M
M
.
.
.
.
0




Burner

Flame
mixing
chamber

Reaction
chamber

Fuel
Gas (G)

Burner
air (BA)

Polluted
air (PA)

Exhaust
gas (E)

Material
Balance

L
i
VOC
c
VOC
G
c
G
E
E
G
G
q
X
H
M
H
M
h
M
h
M
h
M
h
M
i
i
BA
BA
PA
PA










)
(
)
(
0
.
.
.
.
.
.
Energy
Balance

Net heat of
combustion
Lower heating
value kj/kg

Xi:Fractional conversion of VOC
i

h: Specific enthalpy kj/kg

Mass flow
rates (kg/min)

Method 1. Traditional Approach



If we assume that enthalpy functions of all
streams similar to those for pure air and
consider all heat losses (q
L
)as a simple
fraction of the heat input (f
L
):

E
BA
PA
G
M
M
M
M
.
.
.
.
0




)
1
(
)
(
)
1
(
)
(
0
.
.
.
.
.
.
L
i
VOC
c
VOC
L
G
c
G
T
E
T
TG
G
T
f
X
H
M
f
H
M
h
M
h
M
h
M
h
M
i
i
E
BA
BA
PA
PA











)
(
)
1
(
)
(
)
1
(
)
(
)
(
)
(
.
.
.
.
G
E
i
i
BA
E
BA
PA
E
PA
T
T
L
G
c
L
i
VOC
c
VOC
T
T
T
T
G
h
h
f
H
f
X
H
M
h
h
M
h
h
M
M












Method 1. Traditional Approach



Fuel gas is mixed with the outside ambient air
in a preset ratio R
b
, then M
BA
=R
B
M
G
, also T of
burner air equals to T of fuel gas then the
equation takes its final form:

)
)(
1
(
)
1
(
)
(
)
1
(
)
(
)
(
.
.
.
BA
E
i
i
PA
E
PA
T
T
B
L
G
c
L
i
VOC
c
VOC
T
T
G
h
h
R
f
H
f
X
H
M
h
h
M
M











Example 11.2


Calculate the mass flow rate of CH4 required for an afterburner
to treat 2645 acfm of polluted air. It is estimated that the burner
will bring in 200 scfm of ouside air. The fuel gas enters at 80 F
and the burner air enters at 80 F. The lower heating value
(LHV) of CH4 is 21560 Btu/lb. Assume 10% overall heat loss
and ignore any heat gained by the oxidation of the pollutants.


SOLUTION

From Appendix Table B.2, densities of the inlet PA and BA are
0.060 and 0.074 lb/ft3.

M
PA

= 2465 acfm (0.060 lb/acf)=148 lb/min

M
BA

= 200 scfm (0.074 lb/acf)=14.8 lb/min

Appendix Table B.7 enthalpies:

h
TE
= 328 h
TBA
= 4.8 h
TBA
= 33.6 h
TG
= 4.8


Example 11.2

SOLUTION

M
PA

= 2465 acfm (0.060 lb/acf)=148 lb/min

M
BA

= 200 scfm (0.074 lb/acf)=14.8 lb/min

h
TE
= 328 h
TBA
= 4.8 h
TBA
= 33.6 h
TG
= 4.8 Btu/lb

f
L
:0.1




)
(
)
1
(
)
(
)
1
(
)
(
)
(
)
(
.
.
.
.
G
E
i
i
BA
E
BA
PA
E
PA
T
T
L
G
c
L
i
VOC
c
VOC
T
T
T
T
G
h
h
f
H
f
X
H
M
h
h
M
h
h
M
M












M
G

= 2.53 lb/min

Sizing the Device


To ensure adequate mixing and to
approach the condition of plug flow a
linear velocity of 20
-
40 ft/sec
recommended


Usually the residence time 0.4
-
0.9 sec is
enough


For hazardous waste incinerator residence
time must be 2 sec or longer


For biomedical waste incinerator
residence time must be 1 sec or longer


Sizing the Device


Length of the reaction chamber (L) is given by


L=u
t
r


The volumetric flow rate (Q)from the ideal gas
law:



The diameter of the chamber (D)


E
E
E
E
MW
P
RT
M
Q
)
(
.

u
Q
D
E

.
4

Example 11.3


Specify the length and the diameter of the
afterburner of Example 11.2 given that the
design velocity in the main chamber is 15
ft/sec and the desired residence time is 1
sec

Solution


Length of the reaction chamber

L=u
t
r
=15(1.0) = 15 ft

M
E
=148+14.8+2.5=165 lb/min


The volumetric flow rate (Q)from the ideal gas
law:




The diameter of the chamber (D)


min
/
7790
)
28
(
0
.
1
)
1810
)(
73
.
0
)(
165
(
)
(
3
.
ft
MW
P
RT
M
Q
E
E
E
E



in
u
Q
D
E
40
)
60
)(
15
(
)
7790
/
4
4
.
.





General Approach


Start with equation 11.21 and 22 for a small slice of the
afterburner


Treat each small slice as a CSTR


The fraction combusted depends on T, k and residence
time in the CSTR


Assume an inlet T and kinetic model of 11.6 and 11.7 for
both VOC and CO


Because of the heat losses and gains Texit and Tinlet
are different


Use iterative appraoch to find outlet T and CO and VOC
concentrations


Repeat the procedure for all small CSTRs


If the outlet VOC and CO concentrations are not those
desired, start with a new assumed inlet T.

Design Considerations for

Catalytic Oxidizers


Can reduce the required temperatures by
hunders of degrees and can save
considerable amount of space for
equipment as compared with thermal
oxidizers


Catalysis is usually a noble metal such as
palladium or platinum deposited on an
alumina support in a configuration to give
miniumum pressure drop


The pressure drop consideration is often
cirtical for incinerator design

Design Considerations for

Catalytic Oxidizers


A honeycomb arrangement typically results
in a pressure drop of 0.05
-
0.5” H2O/inch of
bed depth


Packed bed of 1/8” diameter pellets 1.0
-
10”
H2O/inch of bed depth

Design Considerations for

Catalytic Oxidizers


Catalyst activity refers to the degree to
which a chemical reaction rate is increased
compared with the same reaction without
the catalyst. They can also be very
selective. Such activity and selectivity
translated into lower operating
temperatuerd required for the desired
percentage of destruction.

Design Considerations for

Catalytic Oxidizers


Table 11.4 Some T used for catalytic
incineration


Compound

Reported

Temperatures
, C

H2

20, 121

CO

150, 260, 315

n
-
pentane

310

n
-
decane

260

benzene

227,302

toluene

230,302

MEK

282,340

Basic Design Equation


[VOC]
L
/[VOC]
0
=e
-
L/Lm



[VOC]
L :
concentration of the VOC at length L

[VOC]
0
= inlet concentration of the VOC

L=length of catalyst bed

L
m
=length of one mass transfer unit

If Re<1000 Then



Lm=ud
2
/17.6D

u: linear velocity in the channel

d=effective diameter of the channel

D=diffusivity, m2/s

Basic Design Equation


[VOC]
L
/[VOC]
0
=e
-
L/Lm



[VOC]
L :
concentration of the VOC at length L

[VOC]
0
= inlet concentration of the VOC

L=length of catalyst bed

L
m
=length of one mass transfer unit

If Re<1000 Then



Lm=ud
2
/17.6D

u: linear velocity in the channel

d=effective diameter of the channel

D=diffusivity, m2/s

Basic Design Equation


If the flow is turbulent then



Lm=2/fa(Sc)
2/3

f: Fanning friction factor

a=surface area per unit voluem of bed, m
-
1

Sc=Schmidt number (m/pD)


The calculated length (usually 2
-
10” for 99% removal)should
be doubled for the safety of the design

Catalyst surface area 0.2
-
0.5 ft2 per waste gas