Supercritical water oxidation technology
Supercritical water oxidation (SCWO) technology is a technology that can realize advanced oxidation treatment of a variety of organic wastes.
Supercritical water oxidation is to completely oxidize organic matter into clean H2O, CO2, and N2 substances through oxidation. S, P, etc. are converted into the highest valence salts for stabilization, and heavy metal oxidation stable solid phase exists in ash.
The principle of supercritical water oxidation technology is to use supercritical water as the reaction medium, after a homogeneous oxidation reaction, the organic matter is quickly converted into CO2, H2O, N2 and other harmless small molecules.
Supercritical water oxidation technology has achieved great success in the treatment of various wastewater and surplus sludge. Its disadvantages are harsh reaction conditions and strong corrosiveness to metals, as well as oxidation of certain chemically stable compounds. It also takes a long time.
In order to accelerate the reaction rate, reduce the reaction time, and lower the reaction temperature, so that the advantages of supercritical water oxidation technology are more obvious, many researchers are trying to introduce catalysts into the supercritical water oxidation process.
Detailed explanation of supercritical water principle
The so-called supercritical fluid refers to a special state of fluid matter.
When the fluid in vapor-liquid equilibrium is raised and pressured, thermal expansion causes the density of the liquid to decrease, and the increase in pressure makes the vapor-liquid two-phase interface disappear and become a homogeneous system, which is the critical point.
When the temperature and pressure of the fluid are higher than the critical temperature and critical pressure respectively, it is said to be in a supercritical state. Supercritical fluid has good fluidity similar to gas, but its density is much greater than that of gas, so it has many unique physical and chemical properties.
The critical point of water is 374.3℃ and pressure 22.064MPa. If the temperature and pressure of water are raised above the critical point, it is supercritical water. Its basic properties such as density, viscosity, conductivity, and dielectric constant are all comparable to ordinary Water is very different and exhibits properties similar to non-polar organic compounds.
Therefore, supercritical water can be completely miscible with non-polar substances (such as hydrocarbons) and other organic substances, while inorganic substances, especially salts, have very low ionization constants and solubility in supercritical water.
At the same time, supercritical water can be completely miscible with gases such as air, oxygen, nitrogen, and carbon dioxide.
Supercritical water oxidation
Since supercritical water is an excellent solvent for both organics and oxygen, the oxidation of organics can be carried out in an oxygen-rich homogeneous phase, and the reaction does not have limitations due to the need for phase shift. At the same time, the high reaction temperature of 400-600°C also accelerates the reaction speed, and can reach a destruction rate of more than 99% within a few seconds of reaction time.
The oxidation reaction of organic matter in supercritical water can be simply expressed as:
The supercritical water oxidation reaction is completely complete: organic carbon is converted to CO2, hydrogen is converted to H2O, halogen atoms are converted to halide ions, sulfur and phosphorus are converted to sulfate and phosphate, respectively, and nitrogen is converted to nitrate and nitrite ions or nitrogen. . Moreover, the supercritical water oxidation reaction is similar to a simple combustion process to some extent, and a large amount of heat is released during the oxidation process.
In order to further accelerate the reaction speed, reduce the reaction time and lower the reaction temperature, so that the supercritical water oxidation technology can give full play to its own advantages, research on the catalytic supercritical water oxidation technology for wastewater treatment is increasingly emerging.
Supercritical water oxidation characteristics
Advantages of supercritical water
- High efficiency and thorough treatment. Organic matter can be completely oxidized into non-toxic small molecule compounds such as carbon dioxide, water, nitrogen and salts under appropriate temperature, pressure and certain retention time. The removal rate of toxic substances is over 99.99% , In line with the requirements of fully enclosed treatment:
- Because SCWO is a homogeneous reaction carried out under high temperature and high pressure, the reaction rate is fast and the residence time is short (can be less than 1 min), so the reactor has a simple structure and small volume;
- It has a wide range of applications and can be applied to the treatment of various toxic substances and waste water;
- No 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;
- When the organic content exceeds 20%, it can rely on the self-oxidation exothermic during the reaction to maintain the required temperature for the reaction without additional heat supply. If the concentration is higher, more oxidation heat will be released, and this part of the heat energy can be recovered .
Disadvantages of supercritical water oxidation
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 conducted 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 necessary for the development, design and control of supercritical water oxidation processes. Compared with data, knowledge is far from meeting the requirements.
In the actual engineering design, in addition to considering the reaction kinetics of the system, some engineering factors must be paid attention to, such as corrosion, salt precipitation, catalyst use, heat transfer, etc.
In the supercritical water oxidation environment, it is more likely to cause metal corrosion than under normal conditions. High concentrations of dissolved oxygen, high temperature and pressure conditions, extreme pH values and certain types of inorganic ions can accelerate corrosion.
Corrosion will cause two problems. One is that the effluent after the reaction contains certain metal ions (such as chromium, etc.), which will affect the quality of the treatment; the other is that excessive corrosion will affect the normal operation of the pressure system. Under the conditions of 300～500℃, pH 2-9, and chloride concentration of 400mg/L, the corrosion of 13 alloys was studied experimentally.
The results show that pH has little effect on corrosion within a given temperature range. In the subcritical state of 300°C, due to the high dielectric constant of water and the solubility of inorganic salts, electrochemical corrosion is the main cause. When the temperature rises above 400°C, the dielectric constant of water and the solubility of salt drop rapidly, and chemical corrosion is dominated at this time.
In supercritical water oxidation, the acid and salt produced during the neutralization process of alkali are often added to the feed. Because the solubility of inorganic substances is very small under supercritical conditions, there will be salt precipitation during the process. Some salts have a high viscosity, which may cause blockage of the reactor or pipeline. Partial improvements can be achieved through optimization of the reactor form and proper operation. Pretreatment may be required for certain high-salt systems.
Catalysts are used in the research of supercritical water oxidation of some substances, mainly to increase the conversion rate of complex organics, shorten the reaction time or reduce the required reaction temperature. Most of the applicable catalysts are used in previous studies on wet air oxidation and subcritical water oxidation processes. Compared with homogeneous catalysis and heterogeneous catalysis, the overall effect of heterogeneous catalysis is better.
Because the properties of water vary greatly near the critical point, the heat transfer near the critical point must also be considered in the supercritical water oxidation process. Below the critical point temperature but close to the critical point, the kinematic viscosity of water is very low, natural convection increases when the temperature rises, and the thermal conductivity increases rapidly. 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.
Although there are still some problems to be solved in supercritical water oxidation technology, due to its outstanding advantages, more and more attention has been paid to the treatment of hazardous waste. It is a new treatment technology with broad development and application prospects. .