JP4344458B2 - Corrosion prevention method for supercritical equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for preventing corrosion of a supercritical apparatus that oxidatively decomposes waste containing a hardly decomposable compound such as PCB and dioxin and a harmful substance using water in a supercritical state.
[0002]
[Prior art]
Supercritical water oxidation (SCWO) has attracted attention as a new treatment method for waste containing the above-mentioned hardly decomposable compounds and harmful substances. This method is a method in which waste is introduced into water in a supercritical state exceeding the critical point and oxidatively decomposed. In the supercritical state, both the dielectric constant and the ionic dissociation constant rapidly decrease, and water behaves like a non-polar high-density gas. When waste is put into this supercritical water, organic compounds are decomposed into water and CO ₂ by complete oxidation, and organic compounds containing heteroatoms such as N and S are heterogeneous while suppressing the generation of NO _X and SO _X. Atoms are converted to acids, salts, oxides, etc. On the other hand, since an inorganic compound hardly dissolves in supercritical water, this supercritical water oxidation method is suitable as an oxidative decomposition method for organic waste.
[0003]
The supercritical equipment used for this purpose is equipped with a reaction vessel that can maintain a pressure of 20 MPa or more and a temperature of 300 to 600 ° C., and waste is sent into the reaction vessel by a high-pressure pump and oxidatively decomposed. The However, since the inner wall surface of the reaction vessel is always exposed to high temperature and pressure, it is easily corroded. Therefore, although measures such as making the reaction vessel made of stainless steel and applying a titanium lining have been taken, the initial cost is high, and it is inevitable that corrosion gradually proceeds. In addition, since the wear progresses rapidly near the outlet of the reaction vessel, the durability is still insufficient even when a titanium lining is applied. For this reason, there is a problem that the reaction vessel must be periodically disassembled and the lining must be replaced, and the running cost is increased.
[0004]
[Problems to be solved by the invention]
The present invention has been made to solve the above-mentioned conventional problems and to provide a method for preventing corrosion of a supercritical apparatus that can economically prevent corrosion of the inner wall surface of a reaction vessel without relying on a lining.
[0005]
[Means for Solving the Problems]
In order to solve the above problems, the corrosion prevention method of the supercritical apparatus of the present invention maintains a pressure of 20 MPa or more and a temperature of 300 to 600 ° C. in order to decompose the waste by the supercritical water oxidation method. In a reaction vessel in which water is in a supercritical state, by mass%, SiO ₂ : 40 to 60%, Al ₂ O ₃ : 10 to 30%, CaO: 10 to 30%, Fe ₂ O ₃ : 10 to Inorganic powder particles having a composition of 30% are fed and reacted at 200 to 500 ° C. to form an inorganic oxide coating layer on the wall of the reaction vessel.
[0006]
According to the present invention, an inorganic oxide coating layer having excellent corrosion resistance is formed on the wall surface of the reaction vessel by reacting the inorganic powder particles fed into the reaction vessel. It is possible to prevent corrosion and wear of the wall surface. By performing this reaction at an appropriate timing, the wall surface of the reaction vessel can always be repaired, and semi-permanent corrosion can be prevented.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram conceptually showing a main part of a supercritical apparatus, wherein 1 is a reaction vessel filled with water and oxygen as an oxidizing agent, and 2 is a slurry containing organic waste is fed into the reaction vessel 1 The high-pressure pump 3 is a separation device for the decomposition product discharged from the reaction vessel 1. The reaction vessel 1 is made of stainless steel such as SUS 304 or SUS 316, and the inside is maintained at a pressure of 20 MPa or more and a temperature of 300 to 600 ° C. by a heating means and a pressurizing means (not shown), and water is in a supercritical state. It is said. The organic waste is oxidatively decomposed by contact with supercritical water, and the decomposed product is separated from the oxidizing agent by the separation device 3 and taken out as a salt.
[0008]
The above configuration is the same as that of the conventional apparatus, but in the present invention, the tank 4 for storing the inorganic particles and the inorganic particles stored in the tank 4 are supplied into the reaction vessel 1. A pump 5 is installed separately. Granular material of inorganic are those containing _{SiO 2, Al 2 O 3,} CaO, Fe 2 O 3, the composition of SiO in a weight ratio of _{2: 40 ~60%, Al 2} O 3: 10~30%, CaO: 10 to 30% and Fe ₂ O ₃ : 10 to 30% are preferable. This inorganic granular material is fed into the reaction vessel 1 by the pump 5 while the reaction vessel 1 is empty, and reacts. The glassy coating layer containing Fe ₃ O ₄ on the inner wall surface of the reaction vessel 1 6 is formed. The coating layer 6 is made of an inorganic oxide having low solubility, and exhibits excellent corrosion resistance.
[0009]
In order to perform this reaction, it is preferable to control the inner wall surface of the reaction vessel 1 to 200 to 500 ° C. If the temperature is lower than this range, the inorganic oxide is not formed. Conversely, if the temperature is higher than this range, the inorganic oxide is decomposed. For this reason, it is possible to form the inorganic oxide coating layer 6 intensively on a specific portion by controlling the wall temperature of the reaction vessel 1, and in particular near the outlet of the reaction vessel 1 where corrosion and wear are likely to proceed. Can be coated intensively.
[0010]
Moreover, the coating layer 6 of a preferable inorganic oxide can be formed by making the composition of an inorganic granular material into the above-mentioned numerical range. The reason is as follows.
The reason why SiO _{2 is set} to 40 to 60% is to form a glass that becomes a binder for ensuring adhesion, and if it is less than this range, it is difficult to become glassy, and if it exceeds this range, the melting point of the glass increases. Therefore, adhesion cannot be obtained at 200 to 500 ° C.
The reason why Al ₂ O _{3 is set} to 10 to 30% and CaO is set to 10 to 30% is to form a glass material having a low melting point and chemically durable. Below this range, the glass material is corroded. It becomes easy to be formed and the stable coating layer 6 cannot be formed, and if it exceeds this range, the adhesion is impaired.
The reason why the content of Fe ₂ O _{3 is set} to 10 to 30% is to effectively produce Fe ₃ O ₄ as an active ingredient. If it is less than this range, the amount of Fe ₃ O ₄ produced is insufficient and sufficient resistance is obtained. Corrosivity cannot be obtained, and if it exceeds this range, it becomes difficult to uniformly disperse it in the vitreous as a binder.
[0011]
The inorganic oxide coating layer 6 formed as described above hardly dissolves in supercritical water, and exhibits excellent durability against corrosion by supercritical water. In addition, since the coating layer 6 can be formed at an arbitrary timing without decomposing the reaction vessel 1, it is easy to send inorganic particles during the operation of decomposing waste by the supercritical water oxidation method. In addition, it is possible to prevent corrosion of the reaction vessel 1. The extra inorganic powder particles that have not reacted are recovered by the separation device 3, and therefore do not adversely affect the supercritical water oxidation of the subsequent waste.
Examples of the present invention are shown below.
[0012]
【Example】
Concentrated sewage sludge was continuously supplied over 40 hours to a supercritical apparatus with a reaction temperature of 500 ° C, and the amount of wear on the inner wall of the reaction vessel was measured during that time. At one time it was 3 μm.
Next, according to the present invention, a granular material having a composition of SiO ₂ 40%, Al ₂ O ₃ 30%, CaO 20%, Fe ₂ O ₃ 10% is supplied as a 10% slurry from another line into the reaction vessel, Simultaneously, 40% of H ₂ O ₂ was supplied as an oxygen source to form an inorganic oxide coating layer. As a result, even when the reaction vessel was made of SUS 304, the amount of wear on the inner wall surface was reduced to 2 μm under the same conditions as above. The same conditions as above apply when the inorganic oxide coating layer is formed in the same manner with the composition of the powder as SiO ₂ 50%, Al ₂ O ₃ 10%, CaO 10%, Fe ₂ O ₃ 30%. The amount of wear on the inner wall was confirmed to be 2 μm.
[0013]
【The invention's effect】
As described above, according to the present invention, it is possible to form an inorganic oxide coating layer on the wall surface by sending inorganic particles into the reaction vessel without decomposing the reaction vessel, thereby preventing corrosion of the reaction vessel. Can be achieved. The method of the present invention does not require an initial cost as compared with the conventional method of lining the titanium, and the inorganic powder to be used is inexpensive, so that the running cost is very low. Therefore, the present invention has an advantage that the corrosion prevention of the reaction vessel for decomposing the waste by the supercritical water oxidation method can be economically achieved.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reaction container, 2 High-pressure pump, 3 Separation device, 4 Tank which contains the inorganic granular material, 5 Pump, 6 Coating layer of inorganic oxide

Claims (1)

In order to decompose the waste by the supercritical water oxidation method , the pressure is maintained at a pressure of 20 MPa or more and a temperature of 300 to 600 ° C., and in a reaction vessel in which water is in a supercritical state , SiO ₂ : 40 ~ 60%, Al ₂ O ₃ : 10 ~ 30%, CaO: 10 ~ 30%, Fe ₂ O ₃ : Fe ~ 10 ~ 30% inorganic powders are fed and reacted at 200 ~ 500 ℃ A method for preventing corrosion of a supercritical apparatus, comprising forming a coating layer of an inorganic oxide on a wall surface of a reaction vessel.

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