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The chemical miracle of superheated water: How did it become an ideal solvent for industrial reactions?
Superheated water is liquid water between 100°C and 374°C (705°F) that remains stable under pressure and cannot boil, often referred to as "subcritical water" or "supercritical water". Pressurized hot water. Due to its special physical and chemical properties, superheated water has gradually become an ideal solvent for industrial and analytical applications, and can replace traditional organic solvents, which will bring great benefits to environmental protection.
Superheated water exhibits many unique properties in chemical reactions, including the ability to act as a solvent, reagent, and catalyst.
Changes in properties and temperature
The properties of water change as its temperature changes, but superheated water changes more dramatically than would normally be expected. As the temperature of water increases, viscosity and surface tension decrease, while diffusivity increases with temperature. Furthermore, the autoionization of water increases with increasing temperature, resulting in a pKw close to 11 at 250 °C, indicating that both the hydrogen ion concentration and the hydroxide concentration are significantly increased while the pH remains neutral.
Explanation of unusual behavior
Water is a polar molecule with separation of positive and negative charge centers, which enables the water molecule to respond to electric fields. However, the strong hydrogen-bonding network in water restricts the arrangement of such molecules. Under superheated conditions, the continuous destruction of hydrogen bonds causes the relative dielectric constant of water to drop significantly, thereby reducing its ability to dissolve salts, but greatly increases its ability to dissolve organic compounds within a certain temperature range.
Solubility Improvement
Organic compounds
The solubility of organic molecules in superheated water increases dramatically with increasing temperature, partly because of a change in polarity that makes otherwise insoluble substances, such as polycyclic aromatic hydrocarbons (PAHs), more soluble at 225°C. The solubility is increased by five orders of magnitude, making superheated water more advantageous than other solvents when processing organic compounds.
Salts
Although the relative dielectric constant of superheated water decreases, many salts remain soluble until they approach the critical point. For example, the solubility of sodium chloride at 300°C reaches 37 wt%. However, when approaching the critical point, the solubility of these salts drops sharply.
Gas
Generally speaking, the solubility of a gas in water decreases as temperature increases, but this is not true before some critical temperature. In fact, gases such as nitrogen and oxygen can regain their solubility in superheated water above 90°C, making them extremely valuable for wet oxidation processes.
CorrosiveSuperheated water can be more corrosive than room temperature water, especially above 300°C, which requires the use of special corrosion-resistant alloy materials. Nevertheless, some reports indicate that carbon steel pipes have been used continuously for 20 years at 282°C with only minor corrosion.
The impact of stressBelow 300°C, water is relatively incompressible and pressure has limited effect on its physical properties. Since the pressure of superheated water directly affects the extraction rate and can even accelerate the extraction process of plant materials, superheated water has great potential for industrial applications.
Energy requirements
The energy requirements for heating water are significantly lower than those required to convert it into steam, making it more economical in the distillation process. For 1000 kg of water, the energy required to heat it from 25°C to 250°C is much less than that required to increase evaporation.
Extraction and reaction
Superheated water performs well in a variety of industrial reactions and can effectively carry out oxidation processes of organic compounds. In the presence of low oxygen, organic compounds remain stable in superheated water, making them ideal for green chemistry reactions.
Chromatographic analysis
In reversed-phase liquid chromatography, a mixture of water and methanol is often used as the mobile phase. Switching to superheated water allows separation over a wide temperature range, achieving good analytical results.
Superheated water has unlimited potential, and today's applications are undoubtedly just the tip of the iceberg. How can its environmental and industrial value be further expanded in the future?
