Wet Air Oxidationkexhu.people.ust.hk/ceng371/371-00-7.pdf · Wet Air Oxidation Wet air oxidation...
Transcript of Wet Air Oxidationkexhu.people.ust.hk/ceng371/371-00-7.pdf · Wet Air Oxidation Wet air oxidation...
CENG 4710 Environmental Control Professor Xijun Hu
128
Wet Air Oxidation
Wet air oxidation (WAO) is a well-established
technique for wastewater treatment particularly toxic
and high concentration organic wastewater. WAO
involves the liquid phase oxidation of organics or
oxidizable inorganic components at elevated
temperatures (125-320 oC) and pressures (0.5 – 20
MPa) using a gaseous source of oxygen (usually air).
Enhanced solubility of oxygen in aqueous solutions
at elevated temperature and pressure provides a
strong driving force for oxidation. The elevated
pressures are required to keep water in the liquid
state. Water also acts as a moderant by providing a
medium for heat transfer and removing excess heat
by evaporation.
In WAO
• Carbon → CO2
• H → H2O
• N → NH3, NO3 or N2
• Halogen and sulfur → inorganic halides and
sulfates
The degree of oxidation depends on
• Temperature
• Oxygen partial pressure
• Residence time
• Oxidizability of the pollutants
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for sampling
Schematic diagram of a batch wet air oxidation reactor
The operating costs are almost entirely for power to
compress air and high pressure liquid pumping.
WAO becomes self-sustaining with no auxiliary fuel
requirement when the COD (chemical oxygen
demand) is above 20,000 mg/L. Incineration
(combustion) becomes self-sustaining when the
COD is in the range of 300,000 – 400,000 mg/L.
Adding a catalyst can achieve the same or better
oxidation efficiency at lower reaction temperatures
and pressures so reducing the operation cost. When a
catalyst is used, the process is called catalytic wet air
oxidation (CWAO).
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Schematic diagram of a continuous WAO reactor.
In most applications, WAO is not used as a complete
treatment method, but only as a pretreatment step
where the wastewater is rendered nontoxic and the
COD is reduced sufficiently, so that biological
treatment becomes applicable for the final treatment.
For industrial wastewater treatment, COD or TOC
(total organic carbon) is often used to characterize
the wastewater and to test the efficiency of the WAO
process.
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WAO of cotton desizing wastewater at 290 oC.
0 20 40 60 80 100 120 140 160
CO
D R
educt
ion (
%)
0
10
20
30
40
50
60
70
80
Reaction Time (min)
0 20 40 60 80 100 120 140 160
TO
C R
emoval
(%
)
0
10
20
30
40
50
60
70
80
without O2
with O2
without O2
with O2
The organics in wastewater are stable to heating
but oxidizable by oxygen
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Effect of reaction temperature on the WAO of cotton
desizing wastewater at 1.5 MPa partial oxygen pressure.
0.5 MPa1 MPa1.5 MPa
2 MPa3 MPa
PO
2
0.5 MPa1 MPa1.5 MPa
2 MPa3 MPa
PO
2
0 50 100 150
CO
D R
educt
ion (
%)
0
10
20
30
40
50
60
70
80
Reaction time (min)
0 50 100 150
TO
C R
emoval
(%
)
0
10
20
30
40
50
60
70
80
150 oC
200 oC
240 oC
270 oC
290 oC
150 oC
200 oC
240 oC
270 oC
290 oC
• WAO is better at a higher temperature • Near 80% COD and TOC removals at 290°C
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Effect of reaction pressure on the WAO of cotton desizing
wastewater at 240oC.
0 50 100 150
CO
D R
educt
ion (
%)
0
10
20
30
40
50
60
70
Reaction time (min)
0 50 100 150
TO
C R
emoval
(%
)
0
10
20
30
40
50
60
70
0.375 MPa0.75 MPa1.125 MPa
1.5 MPa2.25 MPa
PO2
0.375 MPa0.75 MPa1.125 MPa
1.5 MPa2.25 MPa
PO2
• WAO is better at a higher oxygen partial
pressure
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WAO of chemical fibre desizing wastewater at 2 MPa
partial oxygen pressure at various temperature.
0 20 40 60 80 100 120
CO
D R
educ
tion
(%)
0
10
20
30
40
50
60
70
80
90
Reaction time (min)
0 20 40 60 80 100 120
TO
C R
emov
al (
%)
0
10
20
30
40
50
60
70
80
150 oC
200 oC
240 oC
270 oC
150 oC
200 oC
240 oC
270 oC
Reaction time (min)
0 20 40 60 80 100 120
Bio
degr
adab
ility
(%
)
0
10
20
30
40
50
60
70
80
90
150 oC
200 oC
240 oC
270 oC
• WAO is better at a higher temperature
• 90% COD & 80% TOC removals at 270°
Biodegradability = BOD/COD
BOD = biochemical oxygen demand
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Possible Reaction Kinetics
• COD as reactant (C)
• Reaction mechanisms:
Wastewater CO & H Ok
Intermediate organicproducts (COD)
(COD)k
fast2 2
slow
kfast
• Rate data modeled by first order kinetics
− =dC
dtkC
where t is reaction time, and k is the specific reaction
rate constant which has the following temperature
dependency:
( )RTEkk /exp0 −= where k0 is a pre-exponential factor, E is the
activation energy, R is the universal gas constant and
T is the temperature in Kelvin. Integration gives
ktC
C=
0ln
where C0 is the initial COD value. By plotting
ln(C0/C) versus time, the slope is the specific
reaction rate constant k. A typical plot of the WAO
treatment of cotton desizing wastewater at a fixed
partial oxygen pressure of 1.5 MPa and four different
reaction temperatures is shown in the following
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figure. The data fit well into two straight lines for a
given temperature, indicating that oxidation proceeds
in two distinct steps: a fast initial reaction of large
molecules decomposed into intermediate products,
followed by a slow reaction of further oxidizing the
intermediate products into end products of low
molecular weight organic acids, carbon dioxide, and
water.
Reaction time (min)
0 20 40 60 80 100 120 140 160
LnC
0/C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
200 oC
240 oC
270 oC
290 oC
WAO of cotton desizing wastewater at 1.5 MPa partial
oxygen pressure (theoretical oxygen requirement).
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The specific rate constant k is a function of
temperature:
k kE
RT= −
0 exp or − = − +ln lnk k
E
RT0
1/T (K-1
)
00.0018 00.0020 00.0022
- ln
(K)
3
4
5
6
7
Kfast
Kslow
Effect of temperature on rate constants of cotton desizing
wastewater at 1.5 MPa partial oxygen pressure.
• The activation energies are:
Efast = 30 kJ/mol; Eslow = 9 kJ/mol
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If oxygen is not in excess, then k k PO
n= '
2
Reaction time (min)
0 20 40 60 80 100 120 140 160
InC
0/C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.375 MPa
0.75 MPa
1.125 MPa
1.5 MPa
2.25 MPa
PO2
WAO of cotton desizing wastewater at 240 oC.
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Oxygen partial pressure (MPa)
0 1 2 3
K
0.000
0.005
0.010
0.015
Kfast
Kslow
Effect of oxygen concentration on rate constants of cotton
desizing wastewater at 240 oC.
The slow reaction is independent of oxygen partial
pressure. The fast reaction strongly depends on the
oxygen supply when it is less than the theoretical
oxygen requirement (1.5 MPa), with excess oxygen,
even the fast reaction becomes independent of
oxygen partial pressure.
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For most WAO operations, the reaction is assumed
to consist of two steps: the decomposition of large
molecules into intermediate products and the further
oxidation of the intermediates into the end products
of carbon dioxide and water. If starch is assumed the
major content of the wastewater, which can be
hydrolyzed into glucose at first, and glucose is
oxidized into carbon dioxide and water thereafter.
Furthermore, it is assumed that a portion of the
organic compound is very difficult to be oxidized.
Therefore, the following reaction routes were
assumed as:
S K1 G k2 CO2 + H2O
k3
N
Where S is the substrate organic (starch) of
wastewater, G is glucose, and N is a non-oxidizable
organic. Reactions 1 and 3 do not change the COD
or TOC value of the solution.
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To simplify the analysis, it is further assumed that
the reactions are kinetics controlled and the
dissolved oxygen concentration is a constant since
enough oxygen gas is supplied. The conversion
between the substrate organic and glucose is a fast
reversible reaction and reaches equilibrium very
quickly, represented by the equilibrium constant K1.
Reactions 2 and 3 are assumed to follow first order
kinetics. Let the total organic in the solution during
reaction be X, its removal rate is then
G k = dt
dX 2− (1)
where [G] stands for the concentration of glucose.
There exists equilibrium between the substrate and
glucose concentrations:
[G]=K1 [S] (2)
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where [S] is the concentration of starch. By
substituting Equation (2) into Equation (1), one
obtains
S k K = dt
dX 21− (3)
The total organic in the wastewater comprises S, G
and N:
X = [G] + [S] + [N] (4)
where [N] represents the concentration of non-
oxidizable product. Elimination of [G] by
substituting Equation (2) into Equation (4), we can
get
( )[N]XK+1
1 = [S]
1
− (5)
Thus, Equation (3) becomes
( )[N] - X K+1
k K =
dt
dX
1
21− (6)
Meanwhile
S k = dt
d[N]3 (7)
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Combining Equation (5) with Equation (7) we obtain
( )[N] - X K+1
k =
dt
d[N]
1
3 (8)
Dividing Equation (8) by Equation (6) yields
k K
k =
dX
d[N]
21
3− (9)
with the initial condition
t=0 [N]=0 (10)
The solution for Equations (9) and (10) is
X) - (X k K
k = [N] 0
21
3 (11)
Now we substitute Equation (11) into Equation (6) to
give
− X) - (X
k K
k-X
K+1
k K =
dt
dX 0
21
3
1
21 (12)
with the initial condition
t=0 X = X0 (13)
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where X0 is the total organic concentration in the
wastewater at time zero.
The solution for Equations (12) and (13) is
e k + k K
kK +
k + k K
k =
X
X t K + 1
k + k K -
321
21
321
3
0
1
321
(14)
If we assume the TOC value in the solution is
proportional to the total organic concentration in the
wastewater, X, i.e.
00 X
X
TOC
TOC= (15)
then the removal of TOC, TOC, would become
e k + k K
kK +
k + k K
k
TOC
T - 1 =
TOC
TOC
TOC
TOC1
TOC
TOC1
t K + 1
k + k K -
321
21
321
3
i
0
0i
0
iTOC
1
321
−=−=
OC
(16)
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where TOCi is the initial TOC value of the fresh
wastewater, which is different from the TOC value at
the reaction time t=0, TOC0, by a factor due to the
thermal decomposition.
Equation (16) is applied to simulate the WAO
treatment of natural fiber desizing wastewater at
different temperatures.
0.5 MPa1 MPa1.5 MPa
2 MPa3 MPa
PO
2
Reaction time (min)
0 50 100 150
TO
C R
emo
val
(%
)
0
10
20
30
40
50
60
70
80
150 oC
200 oC
240 oC
270 oC
290 oC
The model (lines) is in good agreement with the
experimental data. The kinetic parameters are
CENG 4710 Environmental Control Professor Xijun Hu
146
optimized from the experimental data by a least
square method, and listed in the following table.
Kinetic parameters of WAO of cotton desizing
wastewater at different temperatures
T ( oC) 150 200 240 270 290
K1 0.0231 0.0667 0.0758 0.0807 0.161
k2
(min-1)
0.0428 0.131 0.171 0.329 0.574
k3
(min-1)
2.411x10-4 9.8510-3 8.2110-3 0.0154 0.0428
From the small value of hydrolization equilibrium
constant, K1, at 150oC, it can be seen that starch does
not hydrolyze easily at low temperature. Once the
temperature is above 200oC, however, the effect of
reaction temperature on the rate of hydrolysis
becomes less significant. The equilibrium constant is
within the range of 0.067 to 0.08 for temperatures of
CENG 4710 Environmental Control Professor Xijun Hu
147
200 to 270oC. The temperature dependence of k2
and k3 are assumed to follow the arrhenius form:
)RT
E-exp(k=k
a0 (17)
where k0 is the pre-exponential factor, Ea is the
activation energy, and R is the gas constant.
1/T (K-1
)
0.0018 0.0020 0.0022 0.0024
ln(k
)
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
ln(k2)
ln(k3)
k2 and k3 are with the following equations
CENG 4710 Environmental Control Professor Xijun Hu
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T
4106 - 1.87=)ln(k2 (18)
T
7728 - 2.35=)ln(k3 (19)
The activation energy for the oxidation of glucose,
34.1 kJ/mol, is much larger than 25 kJ/mol, a value
where mass transfer resistance can be ignored.
Therefore, the reactions here are indeed kinetics
controlled. On the other hand, the value of
activation energy obtained here is smaller than the
value reported in the literature for the oxidation of
glucose. This means that the fast-formed
intermediates of this kind of wastewater are easier to
oxidize than the pure glucose. The activation energy
for the conversion of the original organic to the non-
oxidizable product is 64.2 kJ/mol, much larger than
that for the oxidation of glucose. This implies that
oxidation is the major reaction.
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Other possible WAO reaction mechanisms
During WAO, the long molecules are oxidized to
various intermediates products. Most of the initial
intermediates formed (except the low molecular
weight carboxylic acids) are unstable and further
oxidized to end products (CO2, etc.) or to low
molecular carboxylic acids (mainly acetic acid). The
low molecular carboxylic acids are resistant to
further oxidation. Thus, the organics in the effluent
from a WAO system can be divided into three
groups:
A: all initial & relatively unstable intermediates
B: refractory intermediates like acetic acid
C: oxidation end products
A + O2 ----k1-------→ C (CO2 + H2O)
k2 k3
B + O2
Assume oxygen is in excess, we may have
AAA CkCk
dt
dC+=− 21
BAB CkCk
dt
dC−= 32
12
1 /0,11
nO
RTECekk =
−
22
2 /0,22
nO
RTECekk =
−
CENG 4710 Environmental Control Professor Xijun Hu
150
32
3 /0,33
nO
RTECekk =
−
tkkAA eCC
+−=
)(0,
21
][)(
0,321
2
0,
213
3
tkktkA
tkBB
eeCkkk
k
eCC
+−−
−
−−+
+
=
CB,0 can be assumed to be zero:
( ) e
k - k + k
k - k +
e k - k + k
k =
C
C+C
tk + k -
321
31
k-
321
2
0,A,0
BA
21
3t
BC+
The COD or TOC in wastewater should be CA + CB,
so
( ) e
k - k + k
k - k + e
k - k + k
k
TOC
TOC tk + k -
321
31 tk -
321
2
0
213
=
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Improved WAO
The efficiency of WAO can be improved by various
means, such as adding a catalyst or using a stronger
oxidant.
Catalytic wet air oxidation (CWAO)
The catalyst used may be metal salt solution, metal
oxide powders, or porous solid supported metals.
By using metal ion solutions and metal oxide
powders as catalysts in the treatment of wastewater,
The benefits:
• Higher COD and TOC removals
• Lower reaction temperature and total pressure
The disadvantages:
• Cause secondary pollutants
The solution:
• Immobilize metals onto granular porous solids
• Used catalysts can be recovered by filtration
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0 20 40 60 80 100 120
CO
D R
emo
val
(%
)
0
20
40
60
80
Reaction time (min)
0 20 40 60 80 100 120
TO
C R
emo
val
(%
)
0
10
20
30
40
50
60
Cu(NO3)2
FeSO4
Mn(NO3)2
CuSO4
No Catalyst
Cu(NO3)2
FeSO4
Mn(NO3)2
CuSO4
No Catalyst
Effect of catalysts on the CWAO of dyeing and printing
wastewater at 200oC, p O2=2.65 MPa.
• Use of catalysts greatly improves the oxidation
• The effectiveness of catalysts is
Cu(NO3)2 > CuSO4 > Mn(NO)2 > FeSO4
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PtO2
PtO2
0 20 40 60 80 100 120
CO
D R
emoval
(%
)
0
20
40
60
80
CuO
Fe2O
3
MnO2
PtO2
TiO2
No catalyst
Reaction time (min)
0 20 40 60 80 100 120
TO
C R
emov
al (
%)
0
20
40
60
CuO
Fe2O
3
MnO2
PtO2
TiO2
No catalyst
Effect of metal oxide catalysts on the CWAO of dyeing
and printing wastewater at 200oC, p O2=2.65 MPa.
• Use of catalysts greatly improves the oxidation
• The efficiency of catalysts is
CuO > Fe2O3 > TiO2 > MnO2 > PtO2
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0 20 40 60 80 100 120
TO
C R
emo
val
(%
)
0
20
40
60
0 20 40 60 80 100 120
CO
D R
emo
val
(%
)
0
20
40
60
80
Reaction time (min)
0 20 40 60 80 100 120
Co
lor
Rem
ov
al (
%)
0
20
40
60
80
Cu-Al2O
3
Cu(NO3)
2
No catalyst
CuO
Cu-Al2O
3
Cu(NO3)
2
No catalyst
CuO
Cu-Al2O
3
Cu(NO3)
2
No catalyst
CuO
Effect of various copper catalysts on the CWAO of dyeing
and printing wastewater at 200oC, p O2=2.65 MPa.
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155
Addition of H2O2 as promoter
0 20 40 60 80 100 120
TO
C O
xid
atio
n R
emo
val
(%
)
0
15
30
45
Reaction Time (min)
0 20 40 60 80 100 120Co
lor
Ox
idat
ion
Rem
ov
al (
%)
0
20
40
60
80
WAOCWAOPCWAO
WAO of dyeing wastewater at 200oC, p O2=2.65 MPa.
CWAO & PCWAO: Cu/AC (copper supported on
activated carbon) catalyst was used.
PCWAO: 10% H2O2 of the theoretical oxidation
requirement was added in additional to oxygen.
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Wet Peroxide Oxidation (WPO) Completely replace oxygen by H2O2.
0 15 30 45 60 75 90 105 120 135 150
TO
C O
xid
atio
n R
emoval
(%
)
0
20
40
60
80
Reaction Time (min)
0 15 30 45 60 75 90 105 120 135 150
Colo
ur
Oxid
atio
n R
emoval
(%
)
0
20
40
60
80
100
70oC
110oC
130oC
150oC
• Reaction is very fast
• High TOC & color removal at 130oC
• H2O2 is expensive
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0 20 40 60 80 100 120
TO
C O
xid
atio
n R
emoval
(%
)
0
20
40
60
80
Reaction Time (min)
0 20 40 60 80 100 120
Colo
ur
Oxid
atio
n R
emoval
(%
)
0
20
40
60
80
100
50%Qth
100%Qth
200%Qth
Effect of hydrogen peroxide dosage on WPO of
dyeing wastewater concentrate.
Increasing H2O2 dosage accelerates the TOC
reduction when it is below its theoretical amount.
However, when the H2O2 dosage is above the
theoretical requirements it little affects the final TOC
CENG 4710 Environmental Control Professor Xijun Hu
158
and color reductions, although the initial reaction
rate increases as the H2O2 dosage increases. This
indicates the maximum final TOC removal
efficiency can not be improved by increasing the
H2O2 dosage. The reason for this might be that the
excess H2O2 reacts with the hydroxyl radical to form
water and HO2 radical which will further react with
H2O2 to form water and hydroxyl radical. Therefore,
H2O2 is self-consumed.
•++→+•
•+→•+
OHOOHOHHO
HOOHOHOH
22222
2222
CENG 4710 Environmental Control Professor Xijun Hu
159
Catalytic Wet Peroxide Oxidation (CWPO)
0 20 40 60 80 100 120
TO
C O
xid
atio
n R
emo
val
(%
)
0
10
20
30
40
50
60
70
No Catalyst
Fe++
200mg/l
Cu++
200mg/l
AC-Cu 2g/l
Reaction Time (min)
0 20 40 60 80 100 120Co
lor
Ox
idat
ion
Rem
ov
al (
%)
0
10
20
30
40
50
60
70
80
Effect of catalyst on WPO of dyeing wastewater at 110oC.