Basic principles of wet oxidation

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1 Basic principles

Wet Oxidation (WAO) generally operates under high temperature (120～374℃) and high pressure (0.
5～20MPa) operating conditions.
In the liquid phase, oxygen or air is used as the oxidant to oxidize the organic matter in dissolved or suspended state in the water.
A treatment method of reduced inorganic substances, the final product is carbon dioxide and water
.

The wet oxidation process is more complicated, and it is generally believed that there are 2 main steps:

① The mass transfer process of oxygen in the air from gas phase to liquid phase;

②Chemical reaction between dissolved oxygen and substrate
.

If the mass transfer process affects the overall reaction rate, it can be eliminated by strengthening stirring

Under high temperature and high pressure, the physical properties of water and oxygen as an oxidant have changed, as shown in Table 3-1
.

From room temperature to 100°C, the solubility of oxygen decreases with increasing temperature, but at high temperatures, this property of oxygen changes

Table 3-1 Physical properties of water and oxygen under high temperature and high pressure

The chemical reaction mechanism of WAO is very complicated.

It is generally believed that the WAO reaction is a free radical reaction.

The entire reaction process is as follows
.

(1) The initiation of the chain generates free radicals from the reactant molecules.
In this process, oxygen generates H ₂ O ₂ through thermal reaction .
The reaction is as follows:

RH+O ₂ →R·+HOO· (3-1)

2RH+O ₂ →2R·+H ₂ O ₂ (3-2)

H ₂ O ₂ +M→2HO·(M is catalyst) (3-3)

(2) Chain development is an alternating process of interaction between free radicals and molecules
.

R·+O ₂ →ROO· (3-4)

ROO·+RH→ROOH+R· (3-5)

RH+OH·→R·+H ₂ O (3-6)

(3) Chain termination If free radicals collide to generate stable molecules, the chain terminates
.

R·+R·→RR (3-7)

ROO·+R·→ROOR (3-8)

ROO·+ROO·+H ₂ O→ROH+ROOH+O ₂                  (3-9)

The role of the free radicals produced by the chain reaction in the above stages in the reaction process mainly depends on the composition of the organic matter in the wastewater, the oxidant used and other test conditions
.

The formation of H ₂ O ₂ in the reaction formula (3-2) shows that the wet oxidation reaction belongs to the free radical reaction mechanism
.

In the wet oxidation test of phenol-containing wastewater at 160℃, DO=640mg/L and phenol of 9400mg/L, Shibaeva et al.

RH+HOO·→R·+H ₂ O ₂                (3-10)

HOO·radical has high activity, but its concentration is very low under liquid phase oxidation conditions
.

However, it can be clearly seen from the above reaction process that it plays an extremely important role in the oxidation process of hydrocarbons and phenols

It should be pointed out that the generation of free radicals is not only through reaction formula (3-1) to formula (3-3), but also has many different explanations
.

Li and Tufano et al.

O·→O·+O· (3-11)

O·+H ₂ O→HO·+HO· (3-12)

RH+HO·→R·+H ₂ O (3-13)

R·+O ₂ →ROO· (3-14)

ROO·+RH→R·+ROOH (3-15)

From reaction formula (3-11) to formula (3-15), it can be seen that HO·radical is formed first, then HO·radical reacts with organic matter RH to form lower carboxylic acid ROOH, and ROOH is further oxidized to form CO ₂ and H ₂ O
.

Although the role of HO·radical in reaction formula (3-2) to formula (3-10) is not obvious, Shibaeva and Emanuel who advocated this reaction mechanism all confirmed the existence of reaction formula (3-13).
It is believed that the formation of HO· promotes the generation of R· radicals
.

Many scholars have also proved the importance of HO·radicals in combustion, ozonation, photocatalysis, and Fenton catalytic processes.
In addition, Shibaeva et al.
proved in their proposed mechanism that HO·s can also be formed through thermal homocracking reactions.
Free radicals:

RH+H ₂ O ₂ →R·H ₂ O+HO· (3-16)

Experiments such as Emanual proved that R· is directly proportional to the oxygen partial pressure of wet oxidation, but as the oxygen partial pressure increases, the concentration of R· will remain constant after reaching a certain value
.

When the oxygen partial pressure is low, the dissolved oxygen DO in the water is also low, and the reaction equations (3-3) to (3-4) slow down, leading to [R·]»[ROO·], which promotes the reaction equation (3-7) to occur

It can be seen from the above analysis that the rate of oxidation reaction is restricted by the concentration of free radicals
.

The rate and concentration of initial free radical formation determine the rate at which the oxidation reaction "automatically" proceeds
.
The enlightenment that can be obtained is that if hydrogen peroxide or some compounds with weak CH bonds (such as azo compounds) are added as a starter at the beginning of the reaction, the oxidation reaction can be accelerated
.
For example, under wet oxidation conditions, a small amount of H ₂ O _{2 is} added to form HO·.
This increased HO· shortens the induction period of the reaction and accelerates the oxidation rate
.
When the reaction progresses, during the proliferation and termination period, free radicals are consumed and reach a certain equilibrium concentration, and the reaction rate will also return to the initial rate
.

To increase the rate of initiation and reproduction of free radicals
.
Another effective method is to add transition metal compounds: metal ions with variable valence can gain or lose electrons from the saturated valence, leading to the generation of free radicals and accelerating the chain reaction
.

RH+M ⁿ⁺ →R·+M ^{(n-1) +} +H+ (3-17)

M + ROOH ^n-+ → M ^{(n-+.
1) +} + OH ^- + the RO · (3-18)

ROOH+Mn ⁺ →M ^(n-1)+ +H ⁺ +ROO· (3-19)

However, when the concentration of the catalyst M is too high, the oxidation reaction rate will be inhibited due to the formation of the following reaction, which is the reverse catalysis
.

ROO·+M ^(n-1)+ →ROOM ⁿ⁺                (3-20)

In the wet oxidation reaction, although the oxidation reaction is the main one, under the high temperature and high pressure system, hydrolysis, pyrolysis, dehydration, polymerization and other reactions also occur at the same time
.
Therefore, in the wet oxidation system, not only the radical reaction of breaking the CH bond at the α-C position of the polymer compound into a low-molecular compound, but also the breaking of the CC bond at the β or γ-C position occurs
.
Many intermediate products formed in the free radical reaction themselves also participate in the chain reaction in various ways
.