Supercritical water oxidation principle and reaction mechanism

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1 Principles of supercritical water oxidation

The main principles of the supercritical water oxidation is the use of supercritical water as a medium to oxidative decomposition of organic matter, SCWO major oxidant used air, O ₂ , H ₂ O ₂ , KMn04 _.
4 and KMn04 _.
4 + O ₂ and the like
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In the supercritical water oxidation process, since supercritical water is an excellent solvent for organics and oxygen, the oxidation of organics can be carried out in an oxygen-rich homogeneous phase, and the reaction will not be restricted due to phase transfer

Organic compound+O ₂ →CO ₂ +H ₂ O (2-1)

Heteroatoms in organic compounds → acids, salts, oxides (2-2)

Acid+NaOH→Inorganic salt (2-3)

The supercritical water oxidation reaction is completely complete
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Organic carbon is converted to CO ₂ , hydrogen is converted to water, halogen atoms are converted to halide ions, sulfur and phosphorus are converted to sulfate and phosphate , respectively , and nitrogen is converted to nitrate and nitrite ions or nitrogen

At present, many compounds, including nitrobenzene , urea, cyanide, phenols, acetic acid and ammonia, have been tested in supercritical water oxidation, and they have all been proved to be effective
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In addition, studies on rocket propellant, nerve gas and mustard gas have also been conducted, which proved that the above-mentioned substances can be processed into non-toxic and simplest small molecules after oxidation with supercritical water

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2 Supercritical water oxidation reaction mechanism

Regarding the mechanism of supercritical water oxidation reaction, it is generally not involved in the early research, and it was gradually paid attention to until later
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The more typical reaction mechanism of supercritical water oxidation is free radical reaction theory

RH+O ₂ →R·+HO ₂ · (2-4)

RH+HO ₂ ·→R·+H ₂ O ₂ (2-5)

Hydrogen peroxide is further broken down into hydroxyl groups

H ₂ O ₂ +M→2HO· (2-6)

M is the medium in the reaction system, mainly water.
Under the reaction conditions, H ₂ O ₂ can also be pyrolyzed into hydroxyl groups
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Due to the strong electrophilicity of the hydroxyl group, it can interact with almost all hydrogen-containing compounds

RH+HO·→R·+H ₂ O (2-7)

The free radicals (R·) produced in formulas (2-4), formulas (2-5), and formulas (2-7) can react with oxygen to generate peroxide radicals, which can further obtain hydrogen atoms to form Peroxide
.

R·+O ₂ →ROO· (2-8)

ROO·+RHROOH+R· (2-9)

Typically a peroxide compound generating the decomposition smaller molecules, which break up rapidly to generate final formic or acetic far, formic and acetic acids are ultimately oxidized to CO.
'S ₂ and H ₂ O
.

It is worth mentioning that different oxidants such as oxygen or hydrogen peroxide have different free radical initiation processes

The commonly used reaction devices for studying supercritical state equilibrium, thermodynamics and kinetic analysis usually do not have a viewport for observing the reaction.
The product needs to be analyzed and measured after the reaction is completed, so in-situ reaction information cannot be obtained.
, It is not easy to study the reaction state and reaction mechanism under supercritical conditions
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At present, the most commonly used in-situ reaction technology is one of the important methods for in-depth study of the reaction process in ultra (near) critical water.