Supercritical Water Oxidation Technology
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Supercritical Water Oxidation (SCWO) technology is a method capable of achieving deep oxidation treatment for a variety of organic wastes. SCWO involves the complete oxidation of organic matter into clean H2O through oxidation reactions.2O、CO2With N2Equalizing substances, such as S and P, are converted into the most valuable salt forms for stabilization. Heavy metals are stabilized in the solid phase and exist in the ash. The principle of Supercritical Water Oxidation (SCWO) technology is to use supercritical water as the reaction medium, where homogeneous oxidation reactions rapidly convert organic matter into CO.2、H2O、N2And other harmless small molecules.
Supercritical water oxidation has achieved significant success in treating various wastewater and residual sludge. However, its drawbacks include stringent reaction conditions, strong corrosiveness to metals, and a longer required time for oxidizing certain chemically stable compounds. To accelerate reaction speed, reduce reaction time, and lower reaction temperature, thereby making the advantages of supercritical water oxidation more pronounced, many researchers are attempting to introduce catalysts into the supercritical water oxidation process.
Principle
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Supercritical refers to a unique state of a fluid substance.[1]When a fluid at the liquid-vapor equilibrium is heated and pressurized, thermal expansion decreases the liquid density, while the increased pressure causes the disappearance of the phase interface between the liquid and vapor phases, forming a homogeneous system, which is the critical point. When the temperature and pressure of the fluid are both higher than the critical temperature and pressure, it is referred to as being in a supercritical state. Supercritical fluids have excellent fluidity like gases but have a much higher density, thus exhibiting many unique physical and chemical properties.
The critical point of water is at a temperature of 374.3°C and a pressure of 22.064 MPa. Once the temperature and pressure of water exceed this critical point, it becomes supercritical water. This form of water exhibits significantly different basic properties, such as density, viscosity, electrical conductivity, and dielectric constant, from ordinary water, and behaves similarly to non-polar organic compounds. Consequently, supercritical water can completely dissolve non-polar substances (like hydrocarbons) and other organic materials, while inorganic substances, particularly salts, have low ionic constants and solubility in supercritical water. Additionally, supercritical water can fully mix with gases such as air, oxygen, nitrogen, and carbon dioxide.
Due to the excellent solubility of supercritical water in both organic matter and oxygen, the oxidation of organic matter can be carried out in a homogeneous phase rich in oxygen, eliminating limitations caused by phase transfer requirements. Additionally, the high reaction temperature of 400-600°C accelerates the reaction rate, allowing for a destruction rate of over 99% within just a few seconds. The oxidation reaction of organic matter in supercritical water can be simply expressed as:
Acid + NaOH = Inorganic Substance
Supercritical Water Oxidation Reaction Achieves Complete and Thorough Conversion of Organic Carbon to CO2Hydrogen conversion to H2Oxygen atoms convert to halide ions, sulfur to sulfates, phosphorus to phosphates, and nitrogen to nitrate or nitrite ions, or nitrogen gas. Moreover, the supercritical water oxidation reaction is somewhat similar to a simple combustion process, releasing a large amount of heat during the oxidation process.
Research into catalytic supercritical water oxidation for wastewater treatment is gaining momentum as a means to further enhance reaction speed, reduce reaction time, and lower reaction temperature, enabling the technology to fully leverage its advantages.
Pros and Cons
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Advantages:
Highly efficient and thorough, organic matter can be fully oxidized into non-toxic compounds such as carbon dioxide, water, nitrogen, and salts under appropriate temperatures, pressures, and retention times. The removal rate of toxic substances exceeds 99.99%, meeting the requirements for closed-loop treatment.
(2) As SCWO involves a homogeneous reaction under high temperature and pressure, the reaction rate is fast, with a short residence time (less than 1 minute). Therefore, the reactor design is simple and compact.
(3) Broad applicability, suitable for the treatment of various toxic substances and waste water/waste.
(4) The process does not generate secondary pollution, resulting in clean by-products that require no further treatment. Inorganic salts can be separated from the water, and the treated wastewater can be fully recycled and reused.
(5) When the organic content exceeds 2%, the reaction can maintain the required temperature through self-oxidation heat released during the reaction process, eliminating the need for additional heat supply. If the concentration is higher, more oxidation heat is released, which can be recovered.
Shortcomings:
Despite the many advantages of supercritical water oxidation, its operation under high temperature and pressure undoubtedly imposes strict requirements on equipment materials. On the other hand, although some research has been conducted on the properties of supercritical water, the solubility of substances within it, and the kinetics and mechanisms of supercritical water chemical reactions, this is far from meeting the requirements for the necessary knowledge and data for the development, design, and control of the supercritical water oxidation process.
During actual engineering design, it is crucial to consider not only the reaction kinetics of the system but also various engineering factors such as corrosion, salt precipitation, catalyst usage, and heat transfer.
(1) Corrosion is more prone to occur in metal under supercritical water oxidation conditions compared to standard conditions. High concentrations of dissolved oxygen, high temperature and pressure, extreme pH levels, and certain types of inorganic ions can accelerate corrosion. Corrosion presents two main issues: first, the effluent after the reaction contains certain metal ions (such as chromium), which can affect the quality of treatment; second, excessive corrosion can impact the normal operation of the pressure system. Experimental research on the corrosion of 13 alloys was conducted under conditions of 300-500°C, pH 2-9, and chloride concentration of 400 mg/L. The results indicate that pH has little effect on corrosion within the given temperature range. At subcritical conditions below 300°C, primarily electrochemical corrosion occurs due to the high dielectric constant of water and solubility of inorganic salts. As the temperature rises above 400°C, the dielectric constant of water and solubility of salts rapidly decrease, leading to predominantly chemical corrosion.
(2) Salt Precipitation: In supercritical water oxidation, it is common to add alkali to neutralize the acid generated and the salt formed during the feedstock addition process. Due to the low solubility of inorganic substances under supercritical conditions, salt precipitation may occur during the process. Some salts have a high viscosity, which could potentially cause blockages in reactors or pipelines. This can be partially mitigated through optimization of the reactor design and appropriate operating methods. Pre-treatment may be necessary for certain high-salt content systems.
(3) Catalysts: Catalysts have been utilized in research on supercritical water oxidation of certain substances, primarily to enhance the conversion rate of complex organic compounds, shorten reaction times, or lower the required reaction temperature. The vast majority of applicable catalysts are those previously used in wet air oxidation and subcritical water oxidation process studies. Compared to homogeneous catalysis and heterogeneous catalysis, heterogeneous catalysis generally exhibits superior overall performance.
(4) Heat Transfer: Due to the significant changes in water's properties near the critical point, heat transfer issues in the supercritical water oxidation process must also be considered. Below the critical point temperature but close to it, water has a low kinematic viscosity, and natural convection increases with temperature, leading to a rapid increase in thermal conductivity. However, when the temperature is slightly above the critical point, the heat transfer coefficient sharply decreases, which may be due to the decrease in fluid density and the differences in physical properties between the bulk fluid and the fluid near the tube wall.
Although there are still some unresolved issues with supercritical water oxidation technology, its prominent advantages are gaining increasing attention in the treatment of hazardous waste, making it a promising new disposal technology with vast development and application prospects.