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    Home > Chemicals Industry > Chemical Technology > The nature and chemical reaction of supercritical water

    The nature and chemical reaction of supercritical water

    • Last Update: 2021-06-19
    • Source: Internet
    • Author: User
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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

    .
    As the temperature increases, the hydrogen bonds in the water are opened, and the intermolecular interaction force weakens

    .
    However, due to the lack of understanding of the structure and characteristics of supercritical water, the understanding of hydrogen bonds in supercritical water has been insufficient for a long time

    .
    Recent studies have shown that hydrogen bonds have special properties in the supercritical region

    .
    Using infrared spectroscopy to study the relationship between hydrogen bond and temperature in high-temperature water, and get the relationship between the relative strength of hydrogen bond (X) and temperature t, X=(-8.
    68×10-4)(t+273.
    15)+0.
    851, This formula describes the
    relationship between the hydrogen bonding degree of water and the temperature in the temperature range of 7 to 526°C and the density of 0.
    7 to 1.
    9 g/cm
    3
    .
    The degree of hydrogen bonding and temperature of water characterize the dependence of hydrogen bonding on temperature.
    In the range of 298 to 773 K, the degree of hydrogen bonding of water has a roughly linear relationship with temperature

    .
    At 298K, the hydrogen bonding degree X of water is about 0.
    55, which means that the hydrogen bonding in liquid water is about half of that in ice.
    At 673K, X is about 0.
    3, and even at 773K, the value of X is greater than 0.
    2

    .
    This shows that at higher temperatures, hydrogen bonds can still exist in water

    .

    (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

    .
    Therefore, the density of supercritical water can be controlled between gas and liquid by changing the temperature and pressure

    .
    The density of water changes with temperature and pressure as shown in Figure 2-2.
    The temperature and pressure required to reach a certain density can be determined

    .

    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

    .
    First, increasing the temperature will provide energy for the reaction molecules, increasing the number of activated molecules, the reaction rate constant will increase, and the reaction rate will increase; on the other hand, under supercritical conditions, the increase in temperature will cause the density of water to decrease, thereby reducing The concentration of reactants causes the reaction rate to slow down

    .
    In different temperature and pressure regions, these two effects have different degrees of influence on the reaction rate

    .
    In the region far from the critical point, the increase in the rate constant caused by the increase in temperature increases the reaction rate to a greater extent than the decrease in the reaction rate caused by the decrease in the reactant density, so the increase in temperature can accelerate the reaction rate

    .
    Similarly, the density of water changes with changes in pressure, which will cause changes in the concentration of reactants, thereby affecting the reaction rate

    .
    When the reaction order of the reactants in the reaction rate equation is positive, the increase in water density due to the increase in pressure increases the concentration of the reactants, thereby accelerating the reaction rate

    .

    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

    .
    The dielectric constant of water changes with changes in density and temperature

    .
    As the density increases, the dielectric constant increases; as the temperature increases, the dielectric constant decreases

    .
    The dielectric constant of supercritical water is similar to that of polar organics at normal temperature and pressure

    .
    Because the dielectric constant of water is very low at high temperatures, it is difficult for water to shield the electrostatic potential energy between ions, so the dissolved ions appear in the form of ion pairs

    .
    Under these conditions, water behaves more like a non-polar solvent, which also reveals that it can dissolve non-polar organics

    .

    (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 higher the density, the greater the ion product of water

    .
    Near the critical point, as the temperature increases, the density of water drops rapidly, resulting in a decrease in ion product

    .
    When far away from the critical point, the temperature has less influence on the density, the temperature rises, and the ion product increases

    .
    ACMitchell like that in 1000 ℃ and a density of 2g / cm & lt
    .
    3
    , the water is highly conductive electrolyte solution
    .

    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

    .
    In the supercritical state, because there is no gas-liquid phase transition, it can continuously change from a low-density state to a high-density state

    .
    In other words, using temperature and pressure as operating variables, it is easy to adjust the density, thereby controlling the fluid characteristics to achieve the purpose of use

    .

    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

    .
    However, the dissociation constant and solubility of inorganic substances (especially salts) in supercritical water are very low

    .
    For example, under the condition that the density of supercritical water does not exceed 0.
    325g/cm
    3 at 400~500℃, the ionization constant of NaCl is 10 -4 , and the solubility of NaCl at room temperature can reach 37% (mass fraction)
    .
    In addition, supercritical water can be completely miscible with air, nitrogen,
    oxygen , carbon dioxide and other gases, which is an important condition for supercritical water as an oxidation reaction medium
    .
    Table 2-2 shows the solubility comparison between supercritical water and ordinary water

    .

    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

     

     

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