The world's matter can be manifested in a variety of different forms, the known material form is mainly six, namely solid, liquid, gaseous, plasma, Bose-Einstein condensed state and fermion condensed state, and the solid, liquid and gaseous three forms are the most familiar to us.
Most of the substances we see in our daily lives have three forms: solid, liquid and gaseous, and at different temperatures and pressures, these three forms can be transformed into each other, the most typical of which is water. At standard atmospheric pressure, the boiling point of water is 100 degrees Celsius, which means that at 100 degrees Celsius, water will become a gaseous form, that is, water vapor. 0 degrees Celsius is the freezing point of water, and at 0 degrees Celsius, water condenses into ice. If the air pressure rises, the freezing and boiling points of the water increase accordingly, and vice versa.
Is there only three forms of water? No, not only water, but many substances in special circumstances have a special form different from the three forms, which is called the "supercritical state".
What is a supercritical state? Taking water as an example, water has a critical temperature and critical pressure, and the so-called critical temperature refers to the maximum temperature at which water changes from gaseous to liquid. What does that mean? We know that under the standard atmospheric pressure, the boiling point of water is 100 degrees Celsius, but as the pressure increases, this temperature will also increase, but once the temperature exceeds the critical temperature, at this time, no matter how the pressure is increased, the gaseous water will no longer become liquid. Critical pressure is the minimum pressure required to vaporize water from gas to liquid at critical temperatures.
The critical temperature of water is 374.15 degrees Celsius, that is, once the temperature exceeds 374.15 degrees Celsius, no matter how the pressure is increased, the water will not change from gaseous to liquid.
The critical pressure of water is 22.05 MPa, which means that the minimum pressure required for water to convert from gaseous to liquid at 374.15 degrees Celsius is 22.05 MPa. So, what happens when the temperature and pressure of water are far beyond the critical value? Water will take on a special state, a supercritical state, and water in this state is called "supercritical water". Supercritical water is a special fluid that significantly increases the compressibility of supercritical water compared to ordinary liquid water, and also acquires the unique ability to dissolve other substances. Specifically, the difference between supercritical water and ordinary liquid water is mainly manifested in density, dielectric constant and viscosity.
At standard atmospheric pressure, the dynamic viscosity of ambient liquid water is 0.001 Pasecond, while the viscosity of supercritical water is 0.298x10∧-2 Pasecond, what does this mean? This means that supercritical water can be turned into a substance with high fluidity.
Having said all that, what is the use of supercritical water? The unique properties of supercritical water determine that it must have a wide range of uses, such as the treatment of organic waste. How is traditional organic waste handled? It is by incineration. The incineration method, as the name suggests, decomposes organic waste by high temperature incineration, but this treatment method will bring secondary pollution, but the use of "supercritical water combustion technology" can effectively solve this problem. Fuel or organic waste, after encountering an oxidizer, can undergo an oxidation reaction in supercritical water, and this violent oxidation reaction produces a "hydrothermal flame".
"Hydrothermal flames" are a very interesting phenomenon, in our opinion, it is that there is fire in the water, as if the water can really burn.
The use of supercritical water combustion technology to treat organic waste, not only will not produce secondary pollution due to high temperature, but also the removal rate can reach 99%, one time can be completed, no need for secondary rework. In addition to its combustible properties, supercritical water is also a high-quality "extractant", because supercritical water can have a good solubility ability, so it can extract flavors and oils from many natural substances. It is precisely because of the wide range of uses and great potential for development of supercritical water that scientists have been conducting research on supercritical water.
Early supercritical water was obtained in the lab, so scientists have always had the question, "Is there supercritical water in nature?"? Later, this question finally had an answer.
German scientists in the middle of the Atlantic Ocean in a seafloor hydrothermal vent accidentally found that there is natural supercritical water here, the reason is that the temperature here far exceeds the critical temperature of water, according to the measurement, the water temperature near the hydrothermal outlet reached 464 degrees Celsius, far beyond the critical temperature of water 374.15 degrees Celsius, which is a very exciting discovery for scientists, because it confirms that supercritical water is not artificially made, but objectively exists in nature. Of course, supercritical water is not the only substance that can be in a supercritical state, as long as the conditions are right, many substances can have a supercritical state.