Dr. V.V MAHAJANI

1
WET AIR OXIDATION
Dr. V.V MAHAJANI Professor of Chemical
Engineering, Institute of Chemical Technology,
Matunga, Mumbai 400 019
E.mail vvm_at_udct.org vvmahajani_at_gmail.com Phone
[phone] (Extn 2015)
V V M 0
2

• WELCOME
• TO ALL

v.v.mahajani, uict
3
CHEMICAL PROCESS INDUSTRY ( CPI) BIRDS EYEVIEW
UTILITIES GASEOUS WASTE RAW
MATERIALS PRODUCTS, By PRODUCTS, INTELLECTU
AL SOLID WASTE INPUTS LIQUID WASTE (
90 of water in)

CPI
V V M 1
4

• Chemical Engineers View
• BIO PROCESSES PHYSICO CHEMICAL PROCESSES
• Aerobic 1. SEPARATION 3. BULK
  MINERALIZATION
• Anaerobic Liquid / Liquid Extraction
  Incineration
• Precipitation Wet Air Oxidation
• Adsorption 4. POLISHING PROCESS
• Membrane Photo Chemical
• 2. REACTIVE DESTRUCTION Fenton
• Hydrotreatment Sonication
• Ozonation
• HYBRID PROCESSES INNOVATIVE COMBINATION OF
  ALL

V V M 2
5
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
V V M 3
6
WATER COSERVATION RESULTS IN CONCENTRATED WASTE
X NOT SUITABLE FOR BIO PROCESS

• OPTIONS AVAILABLE

• INCINERATION

• WET AIR OXIDATION

V V M 4
7

• 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.

V V M 5
8
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.

V V M 6
9
OXIDATION REACTIONFREE 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 C CO2 H2O N
N2, NH3, NO3,

H H2O P
PO4 X HX
(halogen) S SO42- O2
O2 Inorganic substances O2 Na2S
Na2SO4 Na2SO3
Na2SO4
V V M 7
10
OXIDATION POWER OF COMMON OXIDIZING
AGENTSRELATIVE 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
V V M 8
11

• 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.

V V M 9
12
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
V V M 10
13
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.
V V M 11
14

• The kinetics is then given as
• ? d(COD) k (COD)m (O2)n
• dt
• m ? 1 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.

V V M
11
15

• 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.

V V M 12
16
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

V V M 13
17
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

V V M 14
18

• Limitations
• Capital intensive due to exotic MOC.
• However, depreciation benefit makes it
  attractive!

V V M 15
19
OFFGAS
V V M 16
20
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

V V
M 17

21
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.

V V M 18
22

• 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.
• 
  

V V M 19
23
SUSTAINABLE DEVELOPMENT OF MANKIND
IS POSSIBLE ONLY WHEN
WE LEARN TO RESPECT THE DIGNITY OF
ENVIRONMENTAL PROTECTION