JPS6321718B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to reaction tubes used for thermal decomposition and reforming reactions of hydrocarbons, and in particular to the prevention of adhesion of precipitated solid carbon to the surface of reaction tubes during chemical reactions of hydrocarbons. This invention relates to a reaction tube that can prevent accumulation.

The reactor for thermal decomposition and reforming of hydrocarbons is tubular in shape, and liquid or gaseous hydrocarbons are passed through the tube under high temperature and high pressure to perform thermal decomposition or reformation with or without the presence of a catalyst layer. The material for the reaction tubes constituting the reactor is Fe-Cr-Ni austenitic heat-resistant steel containing large amounts of Ni and Cr, which is commonly used as a material for high-temperature equipment. The higher the temperature is, the higher the Ni content is usually used.

This thermal decomposition and reforming reaction of hydrocarbons involves precipitation of solid carbon, but if these reactions are continued using the Fe-Cr-Ni austenitic heat-resistant steel reaction tube, The above precipitated solid carbon inevitably adheres to the inner wall surface of the reaction tube (the surface facing the space where chemical reactions occur. However, depending on the usage conditions of the reaction tube, it may be the outer wall surface or both inner and outer walls of the reaction tube). causes accumulation. If this solid carbon precipitation is left untreated, it will not only obstruct the flow of fluid containing hydrocarbons inside the pipe, but also reduce the overall heat transfer coefficient when supplying or removing reaction heat from outside the pipe to carry out the reaction. This results in a significant drop in the concentration, making it difficult to continue operating the reactor. For this reason, even reactors that normally operate continuously for long periods of time are forced to temporarily suspend operations and periodically perform so-called decoking, which is the removal of deposited carbon using various methods.

In order to address the above-mentioned problem, the inventors continued intensive research on the precipitation phenomenon of solid carbon on the tube wall surface, and as a result, they found that a significant amount of solid carbon was deposited in the conventional Fe-Cr-Ni heat-resistant steel reaction tube. This is because the Ni contained in the tube material acts as a catalyst to promote carbon precipitation from hydrocarbons in contact with the tube surface, and the amount of solid carbon deposited and the amount of solid carbon in the tube material are It was found that there is a certain correlation between the Ni content and the Ni content, and that by reducing the Ni content, solid carbon precipitation on the tube surface can be suppressed and prevented.

This invention was completed based on the above knowledge, and when the inside of the tube is used as a reaction zone, the inner wall surface (inner layer) of the reaction tube that comes into contact with hydrocarbons contains Ni. Fe-Cr heat-resistant steel that does not contain Fe-Cr, or Fe whose Ni content is limited to a range that does not substantially exhibit the catalytic effect of solid carbon precipitation.
- Made of Cr-Ni heat-resistant steel, and this inner layer is made of high-temperature equipment material, such as a laminated outer layer made of Fe-Cr-Ni austenitic heat-resistant steel, which is the conventional pipe material. This product provides a reaction tube coated with a double layered structure, which maintains the necessary characteristics as a reaction tube used under high temperature and high pressure, while also providing a reaction tube that is coated with We succeeded in suppressing and preventing the precipitation of solid carbon as much as possible and guaranteeing stable operation over a long period of time without the need for decoking.

This invention will be explained in detail below.

In the reaction tube of the present invention, the inner layer in contact with hydrocarbons is made of Fe-Cr ferrite type or martensitic heat-resistant steel, or Fe-Cr-Ni ferrite or ferrite-austenite with a Ni content of about 10% or less. Made of mold or martensitic heat-resistant steel.

As a specific example of the above Fe-Cr heat-resistant steel, Cr13
~30% (weight%, same below), C0.01% or more, 1.5
%, Si 2.5% or less, Mn 2.0% or less, N 0.15% or less, the balance being Fe, or Mo, W with a part of Fe being 5.0% or less for the purpose of further improving material properties. , or Nb, and one or more elements of Mo, W or Nb are substituted with the above component elements.
Examples include heat-resistant steel having a composition containing 5.0% or less of carbon dioxide. It should be noted that as the C content of the inner layer alloy that comes into contact with hydrocarbons increases, the amount of carbon deposited on its surface tends to increase. The outer layer alloy material deteriorates due to diffusion of carbon into the outer layer. In order to avoid these disadvantages, it is advantageous for the inner layer alloy to have a lower C content. Therefore, in this invention, the C content of the inner layer alloy is set to less than 1.5%.

Contains Ni instead of the Fe-Cr heat-resistant steel mentioned above
When forming the tube inner layer using Fe-Cr-Ni heat-resistant steel, specifically, it is sufficient to use a steel with a composition in which part of the Fe in the above Fe-Cr steel composition is replaced with Ni. However, its Ni content is about 10%
It must be within the following range. Figure 1 shows
Fe-Cr-Ni heat-resistant steel (Cr18%, C0.8%, Si1.5
%, Mn1.1%, N0.05%, Ni0~35%, Fe43.5~
78.55%) and the Ni content (%) in ^a reaction tube. (Experimental conditions: Ethane supply amount 400c.c./
min, S/C1.5, temperature 900℃).

As shown in the figure, the amount of precipitation increases as the Ni content of the pipe material increases. By the way, the Ni content of the Fe-Cr-Ni heat-resistant steel conventionally used as reaction tube material is approximately 35%, which coincides with the fact that a significant accumulation of solid carbon was unavoidable. This is because, as mentioned above, Ni on the surface of the tube wall exhibits a catalytic effect on the precipitation of solid carbon. In this invention, based on this experimentally confirmed fact, in order to suppress and prevent solid carbon precipitation as much as possible, the upper limit of the Ni content is set at about 10.0%, and more preferably at about 5.0% or less. .

It goes without saying that the chemical compositions of the Fe-Cr series and Fe-Cr-Ni series heat-resistant steels are specified in consideration of the characteristics required as materials for hydrocarbon chemical reactors; These are examples, and as long as the purpose of the present invention is not lost, appropriate modifications and changes may be made, such as slight increase or decrease in the amount of each component element exceeding the above composition range, addition or deletion of other component elements in small amounts, etc.
Modifications are useful as well.

The reaction tube of this invention is based on the above Fe-Cr system or
The inner layer is made of Fe-Cr-Ni heat-resistant steel with Ni10.0% or less.
-Has a double laminated structure coated with Ni-based heat-resistant steel. The heat-resistant steel that forms this outer layer is Fe--
Cr—Ni austenitic heat-resistant steel can be used. As a specific example, Cr20~30%,
Ni18~40%, C0.1~0.6%, Si2.5% or less, Mn2.0
% or less, N0.15% or less, the balance is Fe, or a part of this Fe is mixed with Mo, 5.0% or less,
Examples include steel compositions containing 5.0% or less of one or more of Mo, W, or Nb together with the above-mentioned component elements by substitution with one or more of W or Nb.
Of course, as long as the purpose of the present invention is not lost, various component compositions in which the amount of each element is slightly increased or decreased, new additions or deletions are made are similarly useful.

By forming such a double-laminated tube, precipitation of solid carbon on the inner wall surface of the reaction tube is effectively suppressed, as well as Fe-Cr-Ni in the outer layer.
The mechanical properties such as high-temperature strength and high-temperature creep rupture strength possessed by the austenitic heat-resistant steel are added, making it even more preferable as a reaction tube used at high temperatures and high pressures.

The reaction tube of the present invention having the double-layered structure is preferably manufactured by centrifugal casting. In order to cast a reaction tube by centrifugal force casting using heat-resistant steels having the chemical compositions described above as alloys for the inner and outer layers, first, the alloy for the outer layer (high Ni
Immediately after injecting the molten metal of the inner layer alloy (Fe-Cr-Ni heat-resistant steel) to form the desired thickness and solidifying up to the inner wall surface, It is sufficient to form an inner layer of the desired thickness by casting a molten metal of heat-resistant steel containing less than .0% Fe-Cr-Ni, thereby metallurgically bonding the inner and outer layers together at the boundary. A double-laminated tube can be obtained (in this case, the composition of each layer alloy (mainly the amount of C) is adjusted within the specified range so that the inner layer alloy has a lower melting point than the outer layer alloy. ). There are no special restrictions on other casting conditions; for example, the molten metal casting temperature for each layer is, as usual, approximately 150°C below its respective melting point.
It is only necessary to adjust the temperature to a high temperature, and if necessary,
A suitable slagging agent may be administered in a conventional manner to protect the inner surface of the outer layer from atmospheric oxidation.

In order for the double-laminated tube obtained by centrifugal casting to fully exhibit its characteristics, it goes without saying that the alloys in each layer should not mix with each other and a predetermined layer thickness should be ensured during casting. It is desirable that each layer is metallurgically completely adhered and strongly bonded at the boundary, and that the fused layer at the boundary has the minimum thickness necessary to obtain a strong bond. . From this point of view, in conventional centrifugal casting of double-laminated pipes, if the inner layer alloy is injected after solidifying up to the inner surface of the outer layer, fusion at the boundary between the two layers will be insufficient. Because of this concern, it is common practice to cast the inner layer while the inner surface is still in an unsolidified state. In that case, the adhesion between both layers is sufficient, but on the other hand, there is a drawback that the molten alloys of both layers mix excessively, making it difficult to form a good double-layered structure. However, in the centrifugally cast double-laminate tube of the present invention, although the inner layer is cast after the inner surface of the outer layer is solidified, the adhesion between both layers is excellent. The reason for this is that if the melting point of the inner layer alloy is lower than that of the outer layer alloy, the inner layer molten metal in contact with the solidified inner surface of the outer layer will not solidify immediately, and the interface will have an appropriate thickness. This is due to the fact that it can form a fusion layer. Moreover, at this time, the outer layer is not unnecessarily remelted, so there is no mixing of the alloy between the two layers, and the thickness of the fused layer is kept to the minimum necessary to obtain a strong bond. Stay,
Forms an ideal double laminated structure.

In addition, the double laminated structure can also be formed by, for example, using a combination of casting and thermal spraying to produce a cast pipe made of a predetermined alloy, and then thermally spraying the surface of the pipe with the predetermined alloy. Yes, it is possible, but if centrifugal casting is used, it not only ensures the integral bonding between both layers as described above, but also allows the thickness of each layer to be controlled as desired, and also allows for control over the chemistry of the alloy used. There is also an advantage that the component composition can be freely selected depending on the desired material properties.

To give an example of the above-mentioned reaction tube having a double-layered structure formed by centrifugal casting, in a high-frequency induction melting furnace, Fe-Cr-Ni heat-resistant steel molten metal (C0.45%, Si1. 2%, Mn1.1%, Cr26.5%,
Ni35.5%, N0.06%, balance Fe), and Fe-Cr heat-resistant steel molten metal (C1.2%, Si1.2%,
The molten alloy for the outer layer 35
Kg to form an outer layer with an outer diameter of 134 mm, a wall thickness of 25 mm, and a length of 500 mm. Immediately after the inner surface has solidified, 10 Kg of molten alloy for the inner layer is cast to form an inner layer with a wall thickness of 10 mm. As a result, a reaction tube consisting of two concentric layers in which the inner and outer layer alloys were not mixed and both layers were metallurgically bonded together was obtained.

Furthermore, to give another example, as an alloy for the outer layer, a Fe-Cr-Ni heat-resistant steel molten metal (C0.43%, Si1.3
%, Mn1.1%, Cr26.0%, Ni35.8%, N0.04%,
The balance Fe) 35Kg, and the alloy for the inner layer is:
Fe-Cr-Ni heat-resistant steel molten metal specified as Ni10.0% or less (C0.8%, Si1.2%, Mn1.1%, Cr24.5%,
Using 7.5% Ni and 10 kg (balance Fe), centrifugal force casting was used to form an outer layer with the same dimensions as in the previous example, and immediately after the inner surface had solidified, the inner layer was cast. A reaction tube was obtained in which there was no mixing of alloys and both layers were metallurgically bonded together.

In addition, in the above explanation, a reaction tube used under conditions where the inner wall surface of the reaction tube comes into contact with hydrocarbons was taken as an example, but in the case of a reaction tube where the outer wall surface of the tube comes into contact with hydrocarbons, is in the outer layer, contrary to the example above.
It goes without saying that Fe-Cr type or Fe-Cr-Ni type heat-resistant steel with Ni10.0% or less should be applied.
In addition, when the pipe is used under conditions where both the inner and outer surfaces come into contact with hydrocarbons, the above steel materials are used for the outer and inner layers, and a conventional Fe- A triple laminated structure with Cr-Ni austenitic heat-resistant steel interposed therebetween may be used. In either case, it can be manufactured by centrifugal casting.

As described above, the reaction tube of the present invention has a surface in contact with hydrocarbons made of Fe-Cr or Ni.
Since it is made of Fe-Cr-Ni heat-resistant steel with a limited amount, it can effectively suppress and prevent the precipitation of solid carbon that accompanies the chemical reaction of hydrocarbons. Moreover, this Fe-Cr system or Fe-Cr-Ni system heat-resistant steel layer has a high Ni content that is integrally bonded to it.
Because it is reinforced with a layer of Cr-Ni heat-resistant steel, it has high-temperature properties that can withstand use at temperatures above 500°C and pressures above atmospheric pressure.

Therefore, the reaction tube of the present invention is suitable for the above-mentioned high temperature and
Under high pressure, hydrocarbons alone or together with water vapor,
It is used for thermal decomposition into low-molecular-weight carbides mixed with oxygen-containing gas, etc., or for the production of gaseous mixtures containing hydrogen, carbon oxide, etc., and suppresses the precipitation of solid carbon over a long period of time, ensuring stable operation. can be maintained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the Ni content of the reaction tube material and the amount of solid carbon deposited on the tube surface.

Claims (1)

[Claims] 1. The inner layer of the tube wall contains 0.01 to 1.5% C (weight%, same hereinafter), 2.5% or less Si, 2.0% or less Mn, 13 to 30% Cr.
%, N0.15% or less, with the balance being Fe,
Or Mo, W or with a part of Fe less than 5.0%
Fe substituted with one or more elements of Nb
Cr-based ferrite type or martensitic type heat-resistant steel, or C0.01~1.5%, Si2.5% or less, Mn2.0
% or less, Cr13~30%, Ni10.0% or less, N0.15% or less, and the balance is Fe, or one or more of Mo, W, or Nb with a part of Fe being 5.0% or less. It is made of Fe-Cr-Ni type ferritic type, ferrite-austenitic type or martensitic type heat-resistant steel substituted with elements, and the outer layer of the tube wall is C0.1~0.6%, Si2.5% or less, Mn2. 0% or less,
The composition is within the range of Cr20~30%, Ni18~40%, N0.15% or less, and the balance is Fe or a part of Fe is 5.0%.
1. A tube for thermal decomposition and reforming reactions of hydrocarbons, characterized in that it is made of Fe--Cr--Ni austenitic heat-resistant steel substituted with one or more elements of Mo, W, or Nb in an amount of up to %. 2. The tube for pyrolysis/reforming reaction of hydrocarbons according to claim 1, wherein the outer layer and the inner layer of the tube wall are formed by centrifugal casting.