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1 Advantages of supercritical water oxidation technology
Compared with other treatment technologies, supercritical water oxidation technology has obvious advantages
.
①High efficiency, thorough treatment, organic matter can be completely oxidized into non-toxic small molecule compounds such as carbon dioxide , water, nitrogen and salt under proper temperature, pressure and certain retention time.
The removal rate of toxic substances is 99.
99% Above, it meets the requirements of fully enclosed treatment
.
② Because SCWO is a homogeneous reaction under high temperature and high pressure, the reaction rate is fast and the residence time is short (may be less than 1 min), so the reactor has a simple structure and small volume
.
③It has a wide range of applications and can be applied to the treatment of various toxic substances and waste water
.
④No secondary pollution is formed, product cleaning does not require further treatment, and inorganic salts can be separated from the water, and the treated wastewater can be completely recycled
.
⑤ When the organic content exceeds 2%, it can rely on the self-oxidation exotherm during the reaction to maintain the required temperature for the reaction
.
There is no need to supply additional heat.
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2 Problems in the supercritical water oxidation method
However, although the supercritical water oxidation method has many advantages, its high temperature and high pressure operating conditions undoubtedly put forward strict requirements on equipment materials
.
On the other hand, although some studies have been carried out on the properties of supercritical water and the solubility of substances in it and the kinetics and mechanism of supercritical water chemical reactions, these are related to the knowledge necessary for the development, design and control of critical water oxidation processes.
(1) Corrosion The operating conditions of supercritical water oxidation are harsh.
High concentration of dissolved oxygen, high temperature, high pressure, and the presence of acids, alkalis, and inorganic ions in the wastewater can all accelerate the corrosion of the container
.
Many kinds of materials, including expensive special alloys and ceramic products, have been used to manufacture SCWO reactors
In order to solve the corrosion problem, the reactor needs to be improved
.
In terms of the material of the reactor, it is necessary to use special materials such as titanium-nickel alloy to manufacture the reaction equipment.
In order to solve the corrosion problem, the nature of the reaction material can also be changed, such as changing the chlorine content in the feed material by the dilution method, and changing the pH value of the material by the alkali neutralization method
.
Although the above measures can partially alleviate the corrosion of SCWO, the substantial increase in the treatment volume brought about by the dilution method leads to the decline of the technical and economic feasibility; the salt produced by the neutralization method will precipitate in the SCWO reactor and related pipelines, causing The system is blocked, which affects the normal operation of SCWO
In addition, the development of new catalysts is also a feasible method to solve the corrosion problem
.
The addition of the catalyst can increase the supercritical water oxidation reaction rate, reduce the reaction time, lower the reaction temperature, control the reaction route and reaction products, and reduce the corrosion of the reactor
(2) There are two reasons for the clogging of equipment and pipelines caused by the precipitation of inorganic salts and metal oxides, which can cause salt to precipitate in the SCWO reactor:
① Inorganic salts dissolved in wastewater at room temperature and pressure are precipitated due to a sharp drop in solubility in water under supercritical conditions;
②SCWO reactants and reaction products contain acidic substances, and the presence of acidic substances will seriously corrode the reactor.
In order to slow down the corrosion of the reactor, an appropriate amount of alkali is often added for neutralization, and the resulting salt will be in the reactor.
Precipitate out
.
The viscosity of these salts is very large, which will reduce the heat transfer rate and increase the pressure drop of the system.
(3) There is a lack of necessary basic data.
Some people study the most suitable reaction conditions for different organic substances SCWO, but the research scope is usually limited to model compounds.
There are few studies on specific industrial wastewater, and the necessary basic data are lacking, such as the phase equilibrium of supercritical water.
Data, SCWO kinetic parameters
.
Due to the harsh reaction conditions, it is difficult to conduct online analysis of SCWO intermediate products, and the possible intermediate reactions can only be judged by speculation
(4) The operating cost is high.
It is reported in the literature that the cost of treating 1 ton of pollutants containing about 10% (mass fraction) of organic matter is as high as 300 US dollars
.
The cost of oxidizer accounts for the main part
.
In addition, the complex and corrosion-resistant reactors required to process special materials are expensive to build
.
In the supercritical water oxidation reaction, the system carries a large amount of heat energy.
How to efficiently recover this part of heat energy and reduce its operating cost is a problem that must be solved in industrialization
.
(5) Heat transfer Because the properties of water vary greatly near the critical point, the problem of heat transfer near the critical point must also be considered during the oxidation of supercritical water
.
When the temperature is below the critical point but close to the critical point, the kinematic viscosity of water is very low.
When the temperature rises, the convection increases, and the thermal conductivity increases rapidly.
Therefore, convection heat transfer is the mainstay in the reactor.
If there is good heat transfer Conditions can promote the progress of the reaction and improve the efficiency of the reaction
.
But when the temperature does not exceed the critical point much, the heat transfer coefficient drops sharply, which may be caused by the drop in fluid density and the difference in physical properties between the main fluid and the fluid at the tube wall
.