JP3320506B2 - Titanium tube for petroleum refining plant with excellent resistance to hydrogen absorption in aqueous hydrogen sulfide solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a state, and are not related to oil refineries for titanium tube having excellent hydrogen absorption resistance in a hydrogen sulfide solution, the heat exchanger tubing for desulfurization facilities of petroleum refining plant, Useful as piping.

[0002]

2. Description of the Related Art Titanium is widely used as an industrial material in chemical plants, power plants, seawater desalination plants, etc. due to its excellent corrosion resistance. Furthermore, since it has a high specific strength, it is also used as a material for aircraft and the like.

On the other hand, a drawback of titanium is embrittlement due to hydrogen absorption. Titanium readily absorbs hydrogen when hydrogen molecules or atoms are present on its surface, so that brittle titanium hydride is formed inside.
And, as the amount of this hydride increases, titanium becomes brittle and eventually breaks with very little force. The environment in which titanium absorbs hydrogen is, for example, in high-temperature steam such as a turbine blade of a power plant or in high-temperature hydrogen gas such as in a chemical plant.

In addition, hydrogen absorption of titanium occurs even in an aqueous solution of an acid or the like. This is the time of cathodic protection, corrosion of galvanic corrosion and titanium itself, 2H + ⁺ 2e titanium surface as a cathode reaction ^- → the reaction of H ₂ takes place, a part of generated hydrogen is to be absorbed into the titanium. There are very many literatures on hydrogen absorption of titanium in such an aqueous solution, and as a method for preventing this, for example, as disclosed in JP-A-63-210286 and JP-A-3-243759, the A method of forming a thick titanium oxide film or a titanium nitride layer is known.

[0005] On the other hand, even in the same aqueous solution, the hydrogen absorption behavior of titanium in a hydrogen sulfide aqueous solution is hardly known.
To the inventor's knowledge, "Anti-corrosion technology" 29 (198
0) pp. 113-121, “Corrosion” vo
l. 35, no. 8 (1979) pp. 378-382 and "Kobe Steel Engineering" vol. 35, no. 4, 63-66
There is only a literature on the page, and these only deal mainly with hydrogen absorption when titanium and a different metal come into contact in an aqueous hydrogen sulfide solution, and no measures are taken to prevent hydrogen absorption. Absent.

[0006]

In a petroleum refining plant, since it is necessary to remove sulfur from crude oil, hydrogen is added to a fraction obtained by fractionating crude oil into naphtha, light / light oil, heavy oil, etc., and sulfuration is performed. Separate in the form of hydrogen. The separated hydrogen sulfide flows inside the piping and the heat exchanger together with a fluid such as hydrogen gas or a hydrogenated fraction. A solution containing hydrogen sulfide is generally highly corrosive and not durable with ordinary metallic materials such as copper alloy and stainless steel. Titanium is completely resistant to hydrogen sulfide, but has the problem of hydrogen absorption.

Therefore, at present, there is no problem in terms of general corrosion resistance to this hydrogen sulfide environment, but it is necessary to use a titanium material which causes hydrogen absorption while being aware of it and paying attention to embrittlement, or The only alternative is to use inexpensive carbon steel and replace it frequently in a short period of time. However, frequent replacement of carbon steel pipes and the like is troublesome, and there are many problems in terms of stopping work. A typical example of a plant handling an aqueous solution of hydrogen sulfide is a petroleum refining plant, in which shell and tube type heat exchangers are about to be replaced by titanium. For this heat exchanger, a welded pipe of titanium is used. There is a demand for using a general titanium tube having excellent corrosion resistance. However, titanium pipes used in plants such as petroleum refining are mainly welded pipes obtained by welding cold-rolled titanium materials manufactured by repeating a cold rolling-annealing process. It has been found that even when such a normal titanium tube is immersed alone in a hydrogen sulfide solution without coming into contact with a dissimilar metal, the hydrogen absorption occurs violently.

An object of the present invention is to develop a titanium pipe for a petroleum refining plant which exhibits excellent resistance to hydrogen absorption in an aqueous hydrogen sulfide solution.

[0009]

DISCLOSURE OF THE INVENTION The present inventors have conducted intensive studies on how to reduce the hydrogen absorption of a titanium welded tube in an aqueous hydrogen sulfide solution and extend its life. For the first time, we find here that there is a significant relationship between state and tissue and the rate of hydrogen absorption,
The present invention has been completed.

As described above, a typical example of a plant that handles an aqueous solution of hydrogen sulfide is a petroleum refining plant, and in particular, a shell and tube type heat exchanger is about to be replaced with titanium. For this heat exchanger, a welded pipe of titanium is used. Since a titanium strip as a raw material of a welded pipe is usually manufactured by repeatedly performing a cold rolling-annealing step, for example, “Material and Process” No. 5, vol. 2 (198
9), page 1320 and JP-A-1-159364, titanium carbide, titanium nitride or titanium carbonitride is inevitably formed on the titanium surface. These titanium compounds improve the corrosion resistance of titanium,
JP-A-63-210286 and JP-A-3-243759.
As shown in the above publication, it was considered that it was also effective in absorbing hydrogen, and therefore, it was generally used without being removed. However, it has been found that these titanium compounds actually promote hydrogen absorption of titanium significantly in an aqueous solution of hydrogen sulfide, which is completely different from the conventional observation. Most of the titanium used in heat exchangers is welded titanium tubing, which has internal stress,
It was also found that setting this to a certain level or less makes hydrogen absorption more difficult to occur. It has been found that the crystal grain size also plays an important factor.

[0011] Based on this unexpected finding, the present invention provides
(1) There is no titanium carbide, titanium nitride and / or titanium carbonitride film on the surface and the internal stress is 8 kgf / m
oil refineries for titanium tube having a hydrogen absorption resistance in a hydrogen sulfide aqueous solution, characterized in that it has less than m ^2,
(2) a hydrogen absorption resistance in an aqueous hydrogen sulfide solution, characterized in that no titanium carbide, titanium nitride and / or titanium carbonitride film is present on the surface and the crystal grain size is not less than 15 μm and not more than 100 μm. A titanium pipe for a petroleum refining plant , and (3) no titanium carbide, titanium nitride and / or titanium carbonitride film on the surface, an internal stress of less than 8 kgf / mm ² , and a crystal grain 15 μm diameter
A petroleum refining plan having hydrogen absorption resistance in an aqueous hydrogen sulfide solution, characterized in that the thickness is not less than 100 μm.
The present invention provides a titanium pipe.

[0012]

Although the details of the hydrogen absorption mechanism of titanium in an aqueous hydrogen sulfide solution are unknown, the electrochemical investigation conducted by the present inventors has shown that the polarization curve (conditions) of titanium in an aqueous hydrogen sulfide solution shown in FIG. : H ₂ S = 6.16 wt%, temperature = 9
0 ° C), the titanium material (pickling material) from which the titanium carbide, titanium nitride and titanium carbonitride were removed by pickling the titanium cold-rolled material was used as the titanium material (non The amount of current at the time of cathode polarization is smaller than that of the processing material. The cathode reaction in this case is 2H ₂ S + 2e
→ H ₂ + 2HS ^−, and a small amount of current at the time of cathode polarization is considered to mean that hydrogen gas is hardly generated on titanium, and as a result, hydrogen is hardly absorbed by titanium. In other words, FIG. 1 shows that when titanium carbide, titanium nitride, and titanium carbonitride exist on the titanium surface, the surface portion becomes an active point and 2H ₂ S + 2e →
This indicates that the H ₂ + 2HS ⁻ reaction becomes active. Note that a similar phenomenon occurs even in a wet hydrogen sulfide gas environment.

Under such a background, more importantly, most of the titanium tubes used in the heat exchanger are welded titanium tubes, which require internal stress and reduce the internal stress to 8 kgf. / Mm ² , preferably 5 kgf
/ Mm ² or less, the hydrogen absorption resistance is further improved, and an industrially useful titanium tube can be obtained. When the internal stress is large, the crystal lattice of titanium is distorted, and hydrogen absorption easily occurs. Further, the crystal grain size also plays an important factor, and as the crystal grain size increases to 15 μm or more, the hydrogen absorption resistance also improves. Combination of both is effective.

As a method for removing the titanium nitride, titanium carbide, and titanium carbonitride films on the surface of a general cold-rolled titanium tube, pickling, surface polishing and the like are used. 750 ° C. or more for a long time, the titanium nitride, titanium carbide and titanium carbonitride on the surface can be eliminated by diffusing into titanium. Also, by appropriate combination of cold rolling and annealing conditions, titanium nitride,
It is also possible not to form titanium carbide and titanium carbonitride.

The present invention provides a petroleum refining plastic having titanium carbide, titanium nitride or titanium carbonitride on its surface as well as a welded tube made of a general cold rolled titanium material as described above.
For pure titanium tubes and titanium alloy tubes in general.
In short, the above-mentioned means is used so that the area covered by these films is at least 1.0% of the total surface area so that no titanium carbide, titanium nitride or titanium carbonitride exists on the surface. You can remove them.

The internal stress of the titanium pipe can be reduced by heat treatment after pipe formation. 8k internal stress
It is necessary to be less than gf / mm ² , preferably 5 kgf / mm ² or less.

The crystal grain size can be adjusted to a desired grain size by adjusting the heat treatment conditions during the production of the titanium material. The lower limit is set to 15 μm at which the effect of hydrogen absorption is exhibited, and the upper limit of the crystal grain size is set to 100 μm in order to saturate the effect of hydrogen absorption and to eliminate adverse effects on workability and mechanical properties due to the excessively large particle size. .

If a scratch is present on the titanium surface for some reason, hydrogen is easily absorbed therefrom. Examination of the cross-sectional microstructure of the scratched titanium material immersed in the aqueous hydrogen sulfide solution showed that the hydride extended radially around the scratch. Such flaws are particularly likely to occur in the tube forming process and the heat exchanger assembling process. Also, not all flaws necessarily cause hydrogen absorption. According to the study of the present inventor, a flaw caused by abrasion of a hard object such as a flaw or a chip caused by a sharp blade or a projection causes hydrogen absorption. It has also been found that in general, when the depth of the flaw is 10 μm or more, severe hydrogen absorption occurs therefrom. Even if the surface is shallower than that, if the damage is caused by an extremely sharp object, hydrogen absorption may occur. In the vicinity of the scratched surface, the crystal state is disordered, and there are many lattice defects such as dislocations and vacancies, which become active sites, and H ₂ S →
This is because the reaction of H ⁺ + HS ⁻ or H ₂ S → 2H ⁺ + S ⁻ is likely to occur in that part, and the oxide film newly formed after the generation of the scratch is incomplete and the hydrogen atoms are easily transmitted. It is thought.

When the depth of the flaw exceeds 10 μm, the hydride is clearly observed, and as the depth of the flaw increases, more hydride is observed. Therefore, it is preferable to eliminate scratches on the titanium that have a depth of more than 10 μm.

When there is a surface flaw, it is preferable to remove the flaw surface by pickling or polishing, heat-treat at a temperature of 300 ° C. or higher, or inactivate the flaw by using both of them. . That is, by heat-treating the flaw portion, the disordered crystal state returns to a healthy state, and the sharp state of the flaw can be rounded to suppress hydrogen absorption. In addition, by removing these imperfect parts by removing the scratched surface by pickling or polishing, the sharpened point of the wound is made blunt to inactivate the wound and make a new titanium surface appear. As a result, hydrogen absorption can be suppressed to the same extent as where there is no flaw. By performing both the removal of the scratched surface and the heat treatment, the hydrogen absorption resistance is further improved.

The simplest and most effective method for removing the wound surface is acid pickling using an acid such as hydrofluoric acid. In addition to the pickling, a mechanical removal method such as polishing can be used. It is preferable to remove the wound surface by 0.1 μm or more.
In the case of polishing, it is necessary to pay attention to the polishing method. In other words, polishing itself creates new scratches, and if the polishing surface is rough, there is a possibility that new hydrogen absorption will occur therefrom. As a finish after the polishing, the polishing paper number should be at least 400, preferably buff-polished. It is preferable to delete the entire surface of the titanium material in order to inactivate hidden scratches as well.

If the surface of titanium is rough, hydrogen absorption tends to occur. Therefore, it is preferable that the surface roughness Rmax is 3.0 μm or less.

The titanium tube is useful as a heat exchanger, piping, etc. for desulfurization-related equipment in a petroleum refining plant. It is also useful in an environment of wet hydrogen sulfide gas.

[0024]

EXAMPLES The usefulness of the present invention will be described based on the following examples and comparative examples.

(Examples and Comparative Examples) J as shown in Table 1
Using a pure titanium welded pipe equivalent to IS1 class, a test material subjected to various treatments as shown in Table 2 was subjected to a hydrogen absorption test in an aqueous hydrogen sulfide solution. Table 3 shows the composition of the aqueous hydrogen sulfide solution used. This is a solution taken from a working oil refinery plant. This solution was stored in a sealed container together with the test materials, heated to 110 ° C., and immersed for 2400 hours. In addition, the titanium tube where titanium carbonitride does not exist on the surface,
It was produced by pickling a titanium tube.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

Titanium tubes with reduced internal stress were manufactured by vacuum annealing at various temperature conditions. As shown in FIG. 2, the internal stress of the titanium
°), its opposite side (180 °) and its middle (90 °
°) A strain gauge is attached to each part in advance, and a cut is made in the vicinity of the strain gauge.
And the internal stress in the longitudinal direction (X direction) was calculated.
Specifically, the internal stress was determined using the following formula.

[0030]

(Equation 1)

[0031]

(Equation 2)

In these equations, the symbols are defined as follows: ε _X2 : Longitudinal measured strain ε _X1 : Longitudinal initial strain (before cutting) ε _Y2 : Longitudinal measured strain ε _Y1 : Longitudinal initial strain (Before cutting) ν: Poisson's ratio E: modulus of longitudinal elasticity σ _X : internal stress in the longitudinal direction σ _Y : internal stress in the circumferential direction

Table 4 shows the results of hydrogen analysis after immersion of various test materials.
Shown in

[0034]

[Table 4]

The specimen (No. 1) in which titanium carbonitride exists on the surface and the residual stress exceeds 8 kgf / mm ² is 7
It causes a very large amount of hydrogen absorption at 63 ppm. On the other hand, the test material from which titanium carbonitride was removed (No.
In the case of 2), the amount of hydrogen absorption is remarkably reduced, and it can be seen that the effect of absorbing hydrogen is further enhanced by reducing the residual stress of the titanium tube to less than 8 kgf / mm ² . Preferably, by setting the residual stress of the titanium tube to 5 kgf / mm ² or less, almost no hydrogen absorption occurs. Next, even if the residual stress exceeds 8 kgf / mm ² , it can be seen that as the crystal grain size increases (Nos. 6 to 9), the hydrogen absorption resistance is also improved. The effect appears 2
The lower limit is set to 0 μm, and the upper limit of the crystal grain size is set to 100 μm in order to saturate the effect of resistance to hydrogen absorption and to eliminate adverse effects on workability and mechanical properties due to an excessively large grain size. Finally, it can be seen that the test materials (Nos. 4 and 5) having a low residual stress and a crystal grain size of 40 μm also exhibited excellent characteristics with almost no hydrogen absorption.

[0036]

According to the present invention, a titanium material having an extremely low hydrogen absorption amount in an aqueous hydrogen sulfide solution can be obtained. In a petroleum refining plant, we succeeded in further improving the practicality of using titanium tubes with good corrosion resistance.

[Brief description of the drawings]

FIG. 1 shows a polarization curve in a hydrogen sulfide aqueous solution of a titanium material (pickling material) from which titanium carbide, titanium nitride, and titanium carbonitride have been removed by pickling, and a titanium material in which they are present (untreated material). It is a graph.

FIG. 2 is an explanatory diagram showing a method for measuring internal stress.

Claims (3)

(57) [Claims] 1. The method according to claim 1, wherein the surface is free of titanium carbide, titanium nitride and / or titanium carbonitride film and has an internal stress of
A titanium pipe for a petroleum refining plant having hydrogen absorption resistance in an aqueous hydrogen sulfide solution, characterized by having a gf / mm ^{2 or} less. 2. A titanium carbide, titanium nitride and / or titanium carbonitride film is not present on the surface and the crystal grain size is 15
oil refining process with hydrogen absorption resistance in an aqueous solution of hydrogen sulfide, characterized in that the thickness is not less than 100 μm and not more than 100 μm.
Titanium tube for runt . 3. There is no titanium carbide, titanium nitride and / or titanium carbonitride film on the surface, and the internal stress is 8 kgf.
/ Mm and less than ^2, and oil refineries for titanium <br/> down tube having a hydrogen absorption resistance in a hydrogen sulfide aqueous solution, characterized in that the grain size and 100μm or less than 15 [mu] m.