Supercritical water refers to the water when the air pressure and temperature reach a certain value, and the density of water expanded due to high temperature is exactly the same as that of water vapor compressed due to high pressure.At this time, the liquid and gas of water are indistinguishable and completely blended together, becoming a new kind of liquid with high pressure and high temperature.
1) Any pure substance has its unique critical state
2) When the pressure is higher than the critical pressure, there is no vaporization section in the isobaric heating process, and the liquid directly changes from unsaturated state to overheated state
3) No matter how high the pressure is, it is impossible to liquefy the gas when the temperature is higher than the critical temperature
4) Under critical conditions, superfluid characteristics may exist
5) Near the critical state, water and steam haveLarge specific constant pressure heat capacitycharacteristic
Saturated vapor pressure and critical state
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There is attraction between actual gas molecules. Any gas will liquefy at a certain temperature and pressure, but ideal gas cannot liquefy[2]。
Gas liquefaction generally requires cooling and pressurization: cooling can reduceMolecular thermal motionDiscrete tendency;Pressurization can reduce the distance between molecules and increase the intermolecular attraction.
There is a temperature limit for gas liquefaction:
Critical temperature Tc: the maximum temperature allowed for pressurized gas liquefaction
Critical pressure pc: the minimum pressure required for gas liquefaction at critical temperature
Critical volume Vc: molar volume of substance at critical temperature and pressure
Tc, pc, Vc are collectively referred to as the critical parameters of gases, and are a kind of characteristic parameters of substances
The following table shows the critical parameters of several gas critical states
Tc / ℃
pc / MPa
Vc / dm3·mol-1
He
- 268
zero point two two seven
zero point zero five seven three
N2
- 147
three point three nine
zero point zero eight nine five
H2O
three hundred and seventy-four
twenty-two point zero five
zero point zero five six three
CO2
thirty-one
seven point three seven five
zero point zero nine four zero
The liquefaction process and critical state of the actual gas can be shown on the isothermal p - Vm diagram drawn in the experiment, as shown in the Critical State Curve.
Critical state curve
(1) Isotherms of T>Tc
When T>Tc, the gas cannot be liquefied. The isotherm indicates the pVT change of the gas state.
The isotherm is smooth, and there is no abrupt point of slope.
Compared with the hyperbola pVm=RT of the ideal gas at the same temperature, it can reflect the degree of deviation of the actual gas from the ideal behavior.
(2) Isotherms for T<Tc
There is a horizontal section on the isotherm, where the pressure remains unchanged while the system volume changesThe pressure of the horizontal section increases with the increase of temperature, and the length of the horizontal section decreasesWhen the temperature is critical, the horizontal line shrinks to a point CThe coordinates of point C are Tc, pc, Vc, and are called critical points
In the Critical State Curve:
The horizontal line corresponds to the gas-liquid two-phase equilibrium state;
Low pressure red line corresponds to gas state;
High pressure blue line corresponds to liquid
(3) T=isotherm and critical point of Tc
The coordinates at critical point C are Tc, pc, Vc
Vc (l)=Vc (g), there is no difference between gas and liquid
Point C is a horizontal inflection point, and its mathematical feature is that the first and second derivatives of this point are both zero.
Saturation curve: connect the state points of saturated vapor and liquid at various temperatures into two curves
All the state points in the saturation curve correspond to the gas-liquid equilibrium state
example
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waterCritical temperature T=647.30K, critical pressure Pc=22.1287 MPa, critical specific volume vc=0.00317 m3/kg.In the range of gas-liquid two-phase equilibrium coexistence, including the critical pointSpecific heat capacity at constant pressure、Coefficient of volumetric thermal expansion、Isothermal compression coefficientandAdiabatic indexAll tend to infinity.
According to the principle of nuclear fission, in nuclear fission, scientists will use neutrons as "shells" to bombard atomic nuclei of materials such as U235,NucleusWill split into two new nuclei.Because there is mass loss in this fission process, according toEinsteinOfMass energy equationAnd energy will be released.At the same time,Nuclear fissionNew neutrons will be released during the process. If properly controlled, these neutrons can continue to attack the atomic nucleus to ensurenuclear fission reaction No pause.
The so-called critical state isNuclear fissionThe number of new neutrons produced is just enough for the reactor to continue fission.IfNeutron numberIf there are too many, the operation of the reactor will be unstable, and even there is a danger of explosion in serious cases;Conversely, if the number of neutrons is too small,Fission reactionWill stop.
In order to avoid danger, nuclear reactors have taken various measures to absorb surplus neutrons. Adding boron in cooling water is one of them.
Supercritical water
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German scientists recently investigated a high-temperature hydrothermal vent at the bottom of the Atlantic Ocean and found that the highest water temperature near the vent reached 464 ° C. This is not only the highest temperature liquid found in nature so far, but also the first time to observe water in a supercritical state under natural conditions.This is the first time that humans have observed the existence of supercritical water in the natural state. Previously, people could only reach the supercritical state of water through technology in the laboratory.
The so-called supercritical water refers to the water when the air pressure and temperature reach a certain value, and the density of the water expanded due to high temperature is exactly the same as that of the water vapor compressed due to high pressure.At this time, the liquid and gas of water are indistinguishable and completely blended together, becoming a new kind of liquid with high pressure and high temperature.Andrea pointed out that supercritical water has two remarkable characteristics.First, it has extremely strong oxidation capacity. Put the substances to be treated into supercritical water, fill with oxygen andhydrogen peroxideThis material will be oxidized and hydrolyzed.Some can also spontaneously ignite, emitting flames in water.Another characteristic is that it can be mixed with oil and other substances, and has a broader fusion ability.These characteristics enable supercritical water to produce strange functions.
It is very meaningful to study supercritical water.Many countries in the world are carrying out research, development and utilization of supercritical water, especially Germany and Japan.Germany has developed a technology that can use supercritical water to treat pollutants.[1]
