Wet-air oxidation

Wet air oxidation Based on principle that organic compounds can be oxidized slowly at temperatures that are low compared with normal combustion temperatures (e.g. 572 degrees Fahrenheit versus 3,632 degrees Fahrenheit). The oxidation is carried out at high pressure, e.g. 1,000 per square inch, in the presence of water. 

Chemical Weapons/Explosive Waste/Unexploded Ordnance 

History The first patent of a WAO system is over 90-years old now. In 1911, Strehlenert obtained a patent for the treatment of sulphite liquor by oxidation with compressed air at 180°C. Industrial applications of the techniques started with patents granted independently to a Swedish company. The first known WAO plant was put up in 1958 by Borregaard in Norway for the treatment of sulphite liquors but was later closed down due to uneconomical operations. 

WAO is a well-established technique of importance for wastewater treatment, particularly toxic and highly organic wastewater. WAO involves liquid phase oxidation of organic 

Limitation cfWAO In the process of converting non-biodegradable waste into biodegradable carboxylic acids, different forms of intermediates are produced. To eliminate these intermediates, high temperature and high pressure are used. Corrosion can be observed due to vigorous condihons. This can be used as a pre-treatment method rather than a complete treatment plan. 

Advantage cfWAO The use of high temperature and pressure can be reduced by the use of catalysts. The thickness of the wall of the reactor can be reduced by using less vigorous operating conditions in the presence of a catalyst. The cost of the reactor design, preparation and maintenance can be reduced. 

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Wentworth process Weston cell Wet air oxidation Wet ball mill Wet-end additives Wet etching Wetfastness Wet grinding... 

G. Eriedhofen, H. Kerres, J. Rosembaum, and R. Thiel, "Wet Air Oxidation of Waste Water," report to Bundesministerium fur Eorshung und Technologic (BMFT), Dec. 1980. 

Wet Air Oxidation. With wet air oxidation, increased temperature and pressure are used to oxidize dilute concentrations of organics and some inorganics, such as cyanide, in aqueous wastes that contain too much water to be incinerated, but are too toxic to be treated biologically. In general, wet air oxidation provides primary treatment for wastewaters that are subsequendy treated by conventional methods. This technology can be used with wastes that are pumpable (slurries andUquids). 

Waste streams that are treated by wet air oxidation generally are those having dissolved or suspended organic concentrations from 500 to 50,000 mg/L. Below 500 mg/L, oxidation rates are too slow and above 50,000 mg/L, incineration may be more feasible. 

Temperature. The temperature for combustion processes must be balanced between the minimum temperature required to combust the original contaminants and any intermediate by-products completely and the maximum temperature at which the ash becomes molten. Typical operating temperatures for thermal processes are incineration (750—1650°C), catalytic incineration (315—550°C), pyrolysis (475—815°C), and wet air oxidation (150—260°C at 10,350 kPa) (15). Pyrolysis is thermal decomposition in the absence of oxygen or with less than the stoichiometric amount of oxygen required. Because exhaust gases from pyrolytic operations are somewhat "dirty" with particulate matter and organics, pyrolysis is not often used for hazardous wastes. 

Residence Time. Eor cost efficiency, residence time in the reactor should be minimized, but long enough to achieve complete combustion. Typical residence times for various thermal processes are incineration (0.1 s to 1.5 h), catalytic incineration (1 s), pyrolysis (12—15 min), and wet air oxidation (10— 30 min) (15). 

Wet air oxidation is based on a Hquid-phase oxidation between the organic material in the wastewater and oxygen suppHed by compressed air. The reaction takes place flamelessly in an enclosed vessel which is pressurized and at a high temperature, typically 13.79 x 10 Pa and 575°C. The system temperature, initiated by a start-up boiler, is maintained through auto thermal combustion of organics once the reaction starts. 

PAG sludge can be regenerated by wet air oxidation (WAO) or by a multiple-hearth furnace. Capacity losses might be high in WAO, particulady with low molecular weight organics. Weight loss in a furnace may exceed 20%. 

In wet-air oxidation, the aqueous mixture is heated under pressure ia the presence of air, which oxidi2es the organic material. The efficiency of the oxidation process is a function of reaction time and temperature. The oxidation products are generally less complex and can be treated by conventional biological methods (31). The reactor usually operates between 177 and 321°C with pressures of 2.52—20.8 MPa (350—3000 psig). 

Because powdered activated carbon is generally used in relatively small quantities, the spent carbon has often been disposed of in landfills. However, landfill disposal is becoming more restrictive environmentally and more costiy. Thus large consumers of powdered carbon find that regeneration is an attractive alternative. Examples of regeneration systems for powdered activated carbon include the Zimpro/Passavant wet air oxidation process (46), the multihearth furnace as used in the DuPont PACT process (47,48), and the Shirco infrared furnace (49,50). 

Also, wet air oxidation offers an alternative to conventional incineration for the destmction and detoxification of dilute ha2ardous and toxic waste waters. A 98% removal efficiency of dyehouse effluent has been claimed by wet air oxidation (203). 

Two other methods worth discussing are wet air oxidation and regeneration by steam. Wet oxidation may be defined as a process in which a substance in aqueous solution or suspension is oxidized by oxygen transferred from a gas phase in intimate contact with the liquid phase. The substance may be organic or inorganic in nature. In this broad definition, both the well known oxidation of ferrous salts to ferric salts by exposure of a solution to air at room temperature and the adsorption of oxygen by alkaline pyrogallol in the classical Orsat gas analysis would be considered wet oxidations. 

Most applications of commercial significance require some elevation of temperatures and pressures. A range of about 125 C (257 F) and 5 atm. to 320 C (608 °F) and 200 atm covers most cases. Frequently, air is the oxygen-containing gas, in which case the process may be termed wet-air oxidation (WAO). In the general case, including the use of pure oxygen, the broader term of wet oxidation (WO) is used. 

There are two basic processes for thermal tfeatment of sludge. One, wet air oxidation, is the flameless oxidation of sludge at temperatures of 450 to 550° F and pressures of about 1,200 psig. The other type, heat treatment, is similar but canied out at temperatures of 350 to 400° F and pressures of 150 to 300 psig. Wet air oxidation (WAO) reduces the sludge to an ash and heat treatment improves the dev/aterability of the sludge. The lower temperature and pressure heat treatment is more widely used than the oxidation process. 

Solubilization of a fraction of the influent-suspended solids can occur as a result of thermal conditioning. In low-pressure, wet-air oxidation, some of the organics present are oxidized as well. Solubilization of the volatile suspended solids produces a supernatant or filtrate of relatively high organic strength. 

Wet-air oxidation (also called liquid-phase thermal oxidation) is not a new technology it has been around for over forty years and has already demonstrated its great potential in wastewater treatment facilities. Despite this, there are some very important issues that remain to be addressed before a wet oxidation process can be scaled-up the kinetics of oxidation of many important hazardous compounds... 

The Catalytic Wet Air Oxidation (CWAO) process is capable of converting all organic contaminants ultimately to carbon dioxide and water, and can also remove oxidizable inorganic components such as cyanides and ammonia. The process uses air as the oxidant, which is mixed with the effluent and passed over a catalyst at elevated temperatures and pressures. If complete COD removal is not required, the air rate, temperature and pressure can be reduced, therefore reducing the operating cost. CWAO is particularly cost-effective for effluents that are highly concentrated... 

The CWAO process is a development of the wet air oxidation (WAO) process. Organic and some inorganic contaminants are oxidized in the liquid phase by contacting the liquid with high pressure air at temperatures which are typically between 120° C and 310° C. 

Wet Oxidation is the oxidation of soluble or suspended oxidizable components in an aqueous environment using oxygen (air) as the oxidizing agent. When air is used as the source of oxygen the process is referred to as wet air oxidation (WAO). The oxidation reactions occur at elevated temperatures and pressures. 

SCWO Supercritical Water Oxidation TKN Total Kjeldahl Nitrogen TOC Total Organic Carbon TSS Total Suspended Solids WAO Wet Air Oxidation... 

Hydrogenation reactions, particularly for the manufacture of fine chemicals, prevail in the research of three-phase processes. Examples are hydrogenation of citral (selectivity > 80% [86-88]) and 2-butyne-l,4-diol (conversion > 80% and selectivity > 97% [89]). Eor Pt/ACE the yield to n-sorbitol in hydrogenation of D-glucose exceeded 99.5% [90]. Water denitrification via hydrogenation of nitrites and nitrates was extensively studied using fiber-based catalysts [91-95]. An attempt to use fiber-structured catalysts for wet air oxidation of organics (4-nitrophenol as a model compound) in water was successful. TOC removal up to 90% was achieved [96]. 

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