Wet oxidation
Encyclopedia
Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water
using oxygen
as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidation reactions occur in superheated water
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. 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.
. It is often referred to as the Zimpro process, after Fred T. Zimmermann who commercialized it in the mid 20th century.
Commercial systems typically use a bubble column reactor
, 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
.
The majority of commercial wet oxidation systems are used to treat industrial wastewaters, such as sulfide
laden spent caustic
streams. Almost as many systems are also used for treating biosolids, in order to pasteurize and to decrease volume of material to dispose of.
A special type of process was the so-called "VerTech process". Here was the required pressure supplied by installing the system in de below ground pressure vessel (Gravity Pressure Vessel GPV). The pressure was supplied by feeding the material to a reactor with a depth of 1200 meters. The deep shaft reactor also served as a heat exchanger, so no pre heating was required. The operating temperature was about 270 degrees Celsius, with a pressure of about 100 bar.
The installation was operational in Apeldoorn (the Netherlands) between 1994-2004, but was eventually shut down due to operational problems.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
using oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidation reactions occur in superheated water
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 and pressurized hot water...
at a temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
above the normal boiling point of water (100° C), but below the critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...
(374° C).
The system must be maintained under pressure to avoid excessive evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
of water. This is done to control energy consumption due to the latent heat of vaporization. It is also done because liquid water is necessary for most of the oxidation reactions to occur. Compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
oxidize under wet oxidation conditions that would not oxidize under dry conditions at the same temperature and pressure.
Commercial applications
Wet oxidation has been used commercially for around 60 years. It is used predominantly for treating wastewaterWastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
. It is often referred to as the Zimpro process, after Fred T. Zimmermann who commercialized it in the mid 20th century.
Commercial systems typically use a bubble column reactor
Bubble column reactor
A bubble column reactor is an apparatus used for gas-liquid reactions first applied by Helmut Gerstenberg. It consists of vertical arranged cylindrical columns. The introduction of gas takes place at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange. It is...
, 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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
.
The majority of commercial wet oxidation systems are used to treat industrial wastewaters, such as sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
laden spent caustic
Spent caustic
Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful . Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants...
streams. Almost as many systems are also used for treating biosolids, in order to pasteurize and to decrease volume of material to dispose of.
A special type of process was the so-called "VerTech process". Here was the required pressure supplied by installing the system in de below ground pressure vessel (Gravity Pressure Vessel GPV). The pressure was supplied by feeding the material to a reactor with a depth of 1200 meters. The deep shaft reactor also served as a heat exchanger, so no pre heating was required. The operating temperature was about 270 degrees Celsius, with a pressure of about 100 bar.
The installation was operational in Apeldoorn (the Netherlands) between 1994-2004, but was eventually shut down due to operational problems.