Wet oxidation is a form of hydrothermal treatment. It is the
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
of dissolved or suspended components in
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
using
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
as the
oxidizer
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
. It is referred to as "Wet Air Oxidation" (WAO) when
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
is used. The oxidation reactions occur in
superheated water
Superheated water is liquid water under pressure at temperatures between the usual boiling point, and the critical temperature, . It is also known as "subcritical water" or "pressurized hot water". Superheated water is stable because of overpres ...
at a temperature above the normal boiling point of water (100 °C), but below the critical point (374 °C).
The system must be maintained under pressure to avoid excessive evaporation of water. This is done to control energy consumption due to the
latent heat of vaporization
The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
. It is also done because liquid water is necessary for most of the oxidation reactions to occur. Compounds oxidize under wet oxidation conditions that would not oxidize under dry conditions at the same temperature and pressure.
Wet oxidation has been used commercially for around 60 years. It is used predominantly for treating wastewater. It is often referred to as Zimpro (from ZIMmerman PROcess), after Fred J. Zimmermann who commercialized it in the mid 20th century.
System Description
Commercial systems typically use a
bubble column reactor
A bubble column reactor is an apparatus used to generate and control gas-liquid chemical reactions. It consists of a vertically-arranged cylindrical column filled with liquid, at the bottom of which gas is inserted.
Principle
The introduction ...
, where air is bubbled through a vertical column that is liquid full of the hot and pressurized wastewater. Fresh wastewater enters the bottom of the column and oxidized wastewater exits the top. The heat released during the oxidation is used to maintain the operating temperature.
WAO is a liquid phase reaction using dissolved oxygen in water to oxidize wastewater contaminants. The dissolved oxygen is typically supplied using pressurized air, but pure oxygen can also be used. The oxidation reaction generally occurs at moderate temperatures of 150°-320 °C and at pressures from 10 to 220 bar. The process converts organic contaminants to carbon dioxide, water, and biodegradable short chain organic acids. Inorganic constituents such as sulfides and cyanides are converted to non-reactive inorganic compounds.
In the WAO reaction, complex organic molecules, including biological refractory compounds, are broken into simpler organic compounds or to a complete mineralized state (CO
2, NH
3, Cl
−, SO
4−2, PO
4−3)
. Simple organic compounds such as low molecular weight carboxylic acids and mineralized reaction products may be present in the WAO effluents. Because of this, the WAO effluent generally requires post treatment prior to discharge. WAO effluents are typically readily biodegradable and exhibit high values for
BOD:
COD
Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...
ratios. Standard treatment techniques such as activated sludge biotreatment are typically used with WAO for complete treatment.
Catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
can be used in the WAO system to enhance treatment and achieve a higher COD destruction. Heterogeneous and homogenous catalysts have been used. Heterogeneous catalysts are based on precious metals deposited on a stable substrate. Homogenous catalysts are dissolved transition metals. Several processes, such as Ciba-Geigy, LOPROX, and ATHOS utilize a homogenous catalyst.
[Luck, F. Wet air oxidation: past, present and future. ''Catalysis Today'' 1999, ''53'', 81-91.] Mixed metal catalysts, such a Ce/Mn, Co/Ce, Ag/Ce, have also been effective in improving the treatment achieved in a WAO system.
A special type of wet oxidation process was the so-called "VerTech process" system. A system of this type operated in
Apeldoorn
Apeldoorn (; Dutch Low Saxon: ) is a municipality and city in the province of Gelderland in the centre of the Netherlands. It is located about 60 km east of Utrecht, 60 km west of Enschede, 25 km north of Arnhem and 35 km south of Zwolle. The ...
,
Netherlands
)
, anthem = ( en, "William of Nassau")
, image_map =
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of the Netherlands
, established_title = Before independence
, established_date = Spanish Netherl ...
between 1994 and 2004. The system was installed in a below-ground pressure vessel (also called a gravity
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
or GPV). The pressure was supplied by feeding the material to a reactor with a depth of . The deep shaft reactor also served as a heat exchanger, so no pre-heating was required. The operating temperature was about 270 °C with a pressure of about . The installation was eventually shut down due to operational problems.
Commercial Applications
Spent Caustic Treatment
The majority of commercial wet oxidation systems are used to treat industrial wastewater, such as
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
laden
spent caustic Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful (or spent). Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants. The contaminants have consumed the majo ...
streams from ethylene and LPG production as well as naphthenic and cresylic spent caustics from refinery applications.
Typical classification of WAO treatment systems.
Low temperature WAO systems oxidize sulfides to thiosulfate and sulfate but high concentrations of thiosulfate are present in the treated effluent. The mid temperature systems fully oxidize sulfides to sulfate and mercaptans are oxidized to sulfonic acids. For sulfidic spent caustics, this results in a high chemical oxygen demand (COD) destruction (>90%). High temperature systems are used to oxidize organic compounds that are present in naphthenic and cresylic spent caustics.
Sewage Sludge Treatment
Almost as many systems are also used for treating
biosolids
Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use ...
, in order to
pasteurize
Pasteurization or pasteurisation is a process of food preservation in which packaged and non-packaged foods (such as milk and fruit juices) are treated with mild heat, usually to less than , to eliminate pathogens and extend shelf life. ...
and to decrease volume of material for disposal. The thermal conditioning occurs at temperatures of 210 – 240 °C. A 4% dry solid slurry can be processed in a WAO system where it is disinfected and the treated effluent can be dewatered to 55% dry solids using a filter press.
Other Applications
Wet air oxidation has also been used to treat a variety of other industrial process waters and wastewaters which include:
· Hazardous Waste
·
Kinetic Hydrate Inhibitors (KHI) from produced water
[Kumfer, B.; Clark, M.; Cook, S.; Garza, T.; Jackson, S. “Treatment of Produced Water Containing KHI by Wet Air Oxidation”, International Conference on Gas Hydrates (ICGH), Beijing, China, 28 July- 1 August 2014.]
·
Polyol ether/styrene monomer (POSM) wastewater
[Patria, L.; Maugans, C.; Ellis, C.; Belkhodja, M.; Cretenot, D. Luck, F., Copa, B; Wet air oxidation processes. In ''Advanced Oxidation Processes for Water and Wastewater Treatment'', Parsons, S.; IWA Publishing: London, 2004, 247-274.]
· Ammonium sulfate crystallizer
mother liquor
The mother liquor (or spent liquor) is the solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approx ...
· Pharmaceutical wastewater
· Cyanide Wastewater
·
Powdered Activated Carbon regeneration
See also
*
Supercritical water oxidation
Supercritical water oxidation (SCWO) is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point (thermodynamics), critical point. Under these conditions water becomes a fluid with unique propert ...
*
Incineration
Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other high ...
*
List of waste-water treatment technologies
This page consists of a list of wastewater treatment technologies:
See also
*Agricultural wastewater treatment
*Industrial wastewater treatment
*List of solid waste treatment technologies
* Waste treatment technologies
*Water purification
*Sewa ...
*
Powdered activated carbon treatment
Powdered Activated Carbon Treatment (PACT) is a wastewater technology in which powdered activated carbon is added to an anaerobic or aerobic treatment system.Jafarinejad, S. “Activated sludge combined with powdered activated carbon (PACT process ...
References
*Zimmermann, F. ''Waste Disposal'', US Patent 2665249, 1950.
*Mishra, V.; Mahajani, V.; Joshi, J. "Wet Air Oxidation", ''Ind. Eng. Chem. Res.", 34, 2-48, 1995.
*Maugans C.; Ellis, C. "Wet Air Oxidation: A Review of Commercial Sub-Critical Hydrothermal Treatment", Twenty First Annual International Conference on Incineration and Thermal Treatment Technologies, New Orleans, May 13–17, 2002.
WAO History Paper
*Patria, L.; Maugans, C.; Ellis, C.; Belkhodja, M.; Cretenot, D.; Luck, F.; Copa, B. "Wet Air Oxidation Processes", ''Advanced Oxidation Processes for Water and Wastewater Treatment'', S. Parsons Editor, pp 247–274. 2004, IWA Publishing.
*Giudici, D.; Maugans, C. "Improvement of Industrial Synthesis of Methyl Methacrylate Application of a Wet Air Oxidation Process (WAO)"
MMA WAO Paper
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