WET AIR OXIDATION
Dr. V.V MAHAJANI
E.mail :vvm@udct.org
Professor of Chemical Engineering,
Institute of Chemical Technology,
Matunga, Mumbai 400 019
vvmahajani@gmail.com
Phone : (022) 2414 5616 (Extn 2015)
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WELCOME
TO ALL
v.v.mahajani, uict
CHEMICAL PROCESS INDUSTRY ( CPI)
BIRD’S EYEVIEW
UTILITIES
RAW MATERIALS
GASEOUS WASTE
CPI
PRODUCTS, By PRODUCTS,
INTELLECTUAL
SOLID WASTE
INPUTS
LIQUID WASTE (~ 90 % of water in)
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Chemical Engineer’s View
BIO PROCESSES
Aerobic
PHYSICO CHEMICAL PROCESSES
1. SEPARATION
Anaerobic
3. BULK MINERALIZATION
Liquid / Liquid Extraction
Incineration
Precipitation
Wet Air Oxidation
Adsorption
4. POLISHING PROCESS
Membrane
Photo Chemical
2. REACTIVE DESTRUCTION
Hydrotreatment
Fenton
Sonication
Ozonation

HYBRID PROCESSES :
INNOVATIVE COMBINATION OF ALL
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PROCESS PRE-VIEW
BIO-PROCESSES
MOST POPULAR PROCESSES OPERATING AT NEAR ATM PRESSURE AND
AMBIENT TEMPERATURE.
BIO GAS GENERATION FROM SPENT WASH OF A DISTILLERY UNIT
LIMITATIONS
SLOW RATES, LARGE VOLUME. HENCE, MORE FLOOR AREA REQD.
OFTEN NEED ENGINEERED MICRO-ORGANISMS
DO NOT PERMIT, INVARIABLY, SHOCK LOADS, TOXIC WASTES
NEEDS ELABORATE POLISHING TREATMENT FOR WATER RECYCLE
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WATER COSERVATION RESULTS IN
CONCENTRATED WASTE
X
NOT SUITABLE FOR BIO PROCESS
OPTIONS AVAILABLE:
INCINERATION
 WET AIR OXIDATION
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INCINERATION :
• HIGH OPERATING COST.
• LOWER CAPITAL INVESTMENT..
• WATER CAN NOT BE RECYCLED UNLESS TREATED.
• DEPRECIATION BENEFIT IS ONLY FOR CAPITAL
INVESTMENT AND NOT FOR OPERATING COST.
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WET AIR OXIDATION

MORE APPROPRIATELY : THERMAL PROCESS.

IT IS SUBCRITICAL OXIDATION PROCESS IN AN AQUEOUS MEDIUM
Water Tc = 374 0C & Pc = 217.6 atm

OXIDATION OF ORGANIC INORGANIC SUBSTRATE IN PRESENCE OF
MOLECULAR O2 T = 100 _ 250 0C; Pressure: O2 pressure 5 to 20 atm

O2 Solubility in water is minimum at near about 100oC.
Above 100 oC it is increasing with increase in
temperature.
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OXIDATION REACTION
FREE RADICAL MECHANISM
O2 + H2O
OH*
via OH* radical formation
NON SELECTIVE OXIDATION TO MINERALIZE OXIDIZABLE CONTAMINANTS
ORGANICS
O2
Ca Hb Nc
Pd Xe Sf
Og
H2O
C
N
H
P
X
S
O2
CO2
N2, NH3, NO3,
H2O
PO4
HX (halogen)
SO42O2
Inorganic substances
O2
Na2S
Na2SO3
Na2SO4
Na2SO4
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OXIDATION POWER OF COMMON OXIDIZING AGENTS
RELATIVE TO OXYGEN
O2
1.00
Cl2
1.06
ClO2
1.06
HOCl
1.24
H2O2
1.48
O3
1.68
OH* (hydroxyl radical) 2.33
F2
2.50
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• HIGHER OXIDATION POWER MEANS A RELATIVE LACK OF
SELECTIVITY.
This property IS USELESS for organic synthesis but the most
desirable in waste treatment.
SHE management does not allow use of “F”
WET Oxidation Technology is centered around OH*
radical as non-selective but powerful oxidizing
agent.
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INSIGHT INTO REACTION MECHANISM
Large molecular wt
O2
CO2 + H2O
organic substrate
low mol. wt organic acids
(Acetic, Propionic, Glyoxalic, Oxalic)
Complex Reactions
Intermediates are formed and can be slow to oxidize or mineralize to CO2
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KINETICS
The waste is characterized as: BOD (bio-chemical oxygen demand), COD ( chemical
oxygen demand ) & TOC ( total organic carbon )
Kinetics is presented in terms of COD / TOC reduction
Instead of having complex kinetics representing series and parallel reactions, a series
reaction approach is considered. We have found that a lumped parameter series
reaction in terms of COD is more design friendly
k1
k2
(COD)
(COD)
CO2 and H2O
Original
low mol. wt
Waste
intermediates
In majority of cases, the second reaction step (k2) is the rate limiting step.
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
The kinetics is then given as


d(COD)
dt
m1;
=
k (COD)m (O2)n
n varies with 0.5 to 1.0
CATALYSTS
Wet air oxidation reactions can be catalyzed by

homogeneous catalysts

heterogeneous catalysts
to reduce SEVERITY of operating conditions.
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 CATALYST CHARACTERIZATION

Homogeneous catalysts

The catalyst should be such that complete oxidation of
substrate is possible to CO2 and H2O.

It should be compatible with MOC of the reactor.

It should be easily recoverable.
CATALYST RECOVERY
Homogeneous catalysts could be recovered by
Precipitation
Ion exchange technique
Liquid emulsion membrane process
The leached catalyst and support can be recovered also by
the above techniques.
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Heterogeneous catalysts
– Cu, Co, Mn, Fe, Ru could be supported on suitable support such as
Al2O3, SiO2 and TiO2
– Temperatures are around 200 oC and there exists acetic acid as an
intermediate. This could result in extraction/leaching of the catalyst
element into treated aqueous stream.
– Leaching of support also may take place.
We have observed:

Cu salts are very good for complete mineralization

Co and Fe are not able to oxidize acetic acid as effectively as copper
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Advantages and Limitations
Advantages

It can handle concentrated waste COD 10,000-500,000 mg/l

It can handle toxic chemicals cyanides, sulphides and
priority pollutants

Waste with high TDS can be handled

Energy integration possible

Very less space, even it can be underground.

Lower operating cost
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Limitations

Capital intensive due to exotic MOC.
However, depreciation benefit makes it attractive!
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STEAM
ENERGY RECOVERY
SYSTEM
OFFGAS
AIR COMPRESSOR
AIR
BFW
AIR
SATURATOR
WET OXIDATION
REACTOR
BFW
EFFLUENT
ENERGY RECOVERY
SYSTEM
TREATED
WATER
Typical Continuous Wet Oxidation System for Liquid Waste
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Integration with other waste treatment processes:
It is possible to have hybrid systems to realize economic advantage
of the waste treatment process.
1 Membrane – WAO
2 WAO - Membrane
3 Sonication – WAO
4 Fenton – WAO
5 Biological treatment – WAO
6 WAO - Biological treatment
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A SYSTEMATIC APPROACH FOR WATER TREATMENT FOR RECYCLE
We can use following guidelines for water recycle in a chemical plant
Identify contribution of water bill in the cost of production.

Identify the scenario around your project with special reference to
availability of water in future, considering your future
requirements due to expansion.

Take water balance in your plant.

Identify all water outlets such as plant effluent, utility blow
downs, water used in administrative block, canteen etc. Please
note that one can do little to evaporation loss in cooling tower.

Have detailed analysis of each effluent stream and decide which
can be used for recycle and which can be used for purging. It
may be possible to use purge water for gardening and
horticulture.
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 Have specifications for water use at all process blocks in
the project.
For instance, specifications for water used
for washing filters would be totally different from that used
as boiler feed water generating steam for captive power
generation also.
 Decide on treatment strategy.
 Since each effluent stream is unique, carry out bench scale
studies.
 Carry out detailed technoeconomic feasibility study to
ensure that set goals or targets could be achieved /
realized.
 Implement the project without any delays.
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SUSTAINABLE DEVELOPMENT OF MANKIND
IS POSSIBLE ONLY WHEN
WE LEARN TO RESPECT THE DIGNITY
OF ENVIRONMENTAL PROTECTION