WET AIR OXIDATION Dr. V.V MAHAJANI Professor of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019 11 E.mail :vvm@udct.org vvmahajani@gmail.com Phone : (022) 2414 5616 (Extn 2015) V V M 0

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CHEMICAL PROCESS INDUSTRY ( CPI) BIRD’S EYEVIEW UTILITIES GASEOUS WASTE RAW MATERIALS PRODUCTS, By PRODUCTS, INTELLECTUAL SOLID WASTE INPUTS LIQUID WASTE (~ 90 % of water in) CPI V V M 1

Chemical Engineer’s 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

LIMITATIONS V V M 3 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

WATER COSERVATION RESULTS IN CONCENTRATED WASTE X NOT SUITABLE FOR BIO PROCESS OPTIONS AVAILABLE: INCINERATION WET AIR OXIDATION V V M 4

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

MORE APPROPRIATELY : THERMAL PROCESS. 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

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 C CO2 H2O N N2, NH3, NO3, H H2O P PO4 X HX (halogen) S SO42- O2 O2 Inorganic substances Na2S Na2SO4 Na2SO3 Na2SO4 V V M 7

OXIDATION POWER OF COMMON OXIDIZING AGENTS RELATIVE TO OXYGEN 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

SHE management does not allow use of “F” 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

V V M 10 INSIGHT INTO REACTION MECHANISM organic substrate 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

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

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

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

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

Advantages and Limitations 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

Capital intensive due to exotic MOC. Limitations Capital intensive due to exotic MOC. However, depreciation benefit makes it attractive! V V M 15

Typical Continuous Wet Oxidation System for Liquid Waste ENERGY RECOVERY SYSTEM TREATED WATER EFFLUENT AIR WET OXIDATION REACTOR SATURATOR AIR COMPRESSOR Typical Continuous Wet Oxidation System for Liquid Waste BFW STEAM OFFGAS V V M 16

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

We can use following guidelines for water recycle in a chemical plant 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

Decide on treatment strategy. 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

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