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1 The nature of supercritical water
When water is in a supercritical state, its properties have undergone tremendous changes, which are mainly reflected in the following aspects
.
(1) Some macroscopic properties of hydrogen-bonded water in supercritical water are closely related to its microstructure, especially the bonding of hydrogen bonds between water molecules.
Therefore, the study of hydrogen bond structure has become the focus of research on the static structure of supercritical water
.
The physical and chemical properties of super (sub)critical water are mainly related to the fluid microstructure such as the structure of hydrogen bond clusters
(2) Density Liquid water is an incompressible fluid, and its density basically does not change with pressure, but slightly decreases with increasing temperature
.
However, the density of supercritical water changes not only with changes in temperature, but also with changes in pressure
It can be seen from the figure that in the supercritical region, the density of supercritical water is very sensitive to changes in temperature, and a small change in temperature will cause a large change in the density of supercritical water.
This phenomenon is particularly obvious near the critical point
.
The influence of temperature on the oxidation process of supercritical water is twofold
Figure 2-2 The density of water changes with temperature and pressure (Note: 1bar=105Pa)
(3) Dielectric constant The change of the dielectric constant causes a change in the dissolution capacity of supercritical water
.
In the standard state, due to the hydrogen bond, the dielectric constant of water is high, 78.
5
(4) Under the standard conditions of ion product, the ion product of water is 10 -14
.
Density and temperature have an influence on it, but the influence of density is the main one
The important characteristic of supercritical fluid is that its phase state, solubility, dielectric constant, ion product and other physical properties can be controlled by adjusting temperature and pressure
.
These physical properties are mostly functions of density
Due to the above-mentioned changes in physical properties, supercritical water behaves like a medium-strength non-polar organic solvent
.
Therefore, supercritical water can be completely miscible with non-polar substances (such as hydrocarbons) and other organic substances
Table 2-2 Comparison of solubility between supercritical water and ordinary water
2.
Supercritical water chemical reaction
Supercritical water has many unique properties
.
For example, strong solubility, high compressibility, etc.
, and water is non-toxic, cheap, and easy to separate from many products
.
Because in the actual process, many materials to be processed are originally aqueous solutions, in many cases it is not necessary to separate water from the final product, which makes supercritical water a very potential reaction medium
.
The chemical reaction of supercritical water has received extensive attention and increasing research
.
Table 2-3 shows the main types and application objects of supercritical water chemical reactions that have been developed and researched recently
.
Among the various supercritical water chemical reaction processes, the most in-depth research and industrial application is the use of SCWO to eliminate hazardous waste, including various toxic wastewater, organic waste, sludge, and human metabolic waste
.
Table 2-3 The main types and application objects of supercritical water chemical reactions