Wet Air Oxidationkexhu.people.ust.hk/ceng371/371-00-7.pdf · Wet Air Oxidation Wet air oxidation (WAO) is a well-established technique for wastewater treatment particularly toxic

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


129

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).


130

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.


131

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


132

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


133

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


134

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


135

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


136

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).


137

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


138

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.


139

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.


140

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.


141

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)


142

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)


143

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)


144

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)


145

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


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


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


148

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.


149

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 =

−


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

=


151

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


152

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


153

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


154

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.


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.


156

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


157

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


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


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.

Documents

Wet Air Oxidationkexhu.people.ust.hk/ceng371/371-00-7.pdf · Wet Air Oxidation Wet air oxidation (WAO) is a well-established technique for wastewater treatment particularly toxic