EP0889146B1 - Heat resistant chromized steel, process for the production thereof and its use in anti-coking applications - Google Patents

Description

The invention relates to steel objects having anti-coking properties, as well as the process used to obtain such objects by depositing a coating anti-coking on a matrix made of a steel, in general a refractory steel. The process is used for the production of parts that must resist coking in various refining or petrochemical processes.

The coated parts of the invention can be used for various processes of the refining and petrochemicals involving temperatures above 350 ° C: steam reforming, dehydrogenation, visbreaking, among others. More specifically, the invention applies to the manufacture of tubular steam cracking ovens intended for a long-term service at temperatures of the order of 800 to 1100 ° C.

The carbonaceous deposit that develops in the ovens during the conversion of hydrocarbon is generally called coke. This deposit of coke is harmful in industrial units. Indeed, the formation of coke on the walls of the tubes and reactors leads in particular to a reduction in heat exchanges, blockages significant and therefore increases in pressure drop. To keep a constant reaction temperature, it may be necessary to increase the temperature of the walls, which risks causing damage to the alloy constituting these walls. There is also a decrease in the selectivity of the installations and consequently of the performance.

It is therefore necessary to periodically stop the installations in order to proceed to a decoking. It is therefore economically interesting to develop materials or coatings capable of reducing the formation of coke.

Numerous documents describe the coke formation reaction in various reactions bringing hydrocarbons into contact with walls at an elevated temperature. This coking phenomenon is in particular widely described and studied in the context of thermal cracking of hydrocarbons. As background article we can cite in particular that written by Professor FROMENT and published in 1990 in the periodical Review of Chemical Engineering , volume 6, number 4, pages 292 to 328, whose title is "Coke formation in the thermal cracking of Hydrocarbons ". We can also cite a more recent article by BILLAUD, BROUTIN, BUSSON, GUERET and WEILL, published in the Revue of the Institut Français du Pétrole in 1992, volume 47, number 4, pages 537 to 549, under the title "Coke Formation during Hydrocarbons Pyrolysis ", for the first part, and in the same review in 1993, volume 48, number 2, pages 115 to 125, under the same title, for the second part.

To summarize the observations described in the prior art, it can be said that the coke formation during thermal cracking of hydrocarbons is a phenomenon complex involving different mechanisms, at least one of which involves reactions catalyzed by the presence of oxides of metallic elements such as nickel, iron or cobalt on the wall of the devices used for the implementation of said processes. These metallic elements are generally contained in large quantities in the refractory superalloys used due in particular to the thermal levels encountered at the wall of these devices. This catalytic mechanism is very preponderant: observations have shown that if this mechanism was inhibited, it was possible in the case steam cracking to increase by a factor of at least about 3 the cycle time between two stops for decoking the ovens necessary for the implementation of this process.

A number of documents report methods of inhibiting the formation of catalytic coke.

We know in particular JP-03-104843 application which describes a refractory steel anti-coking for ethylene cracking oven tube.

Mention may also be made of US Pat. No. 5,208,069, which describes a method for passivation of the metal surface of reactor tubes coming into contact with hydrocarbons by decomposition in situ (that is to say in the device definitively assembled) of a non-oxygenated organometallic derivative of silicon under conditions of formation of a thin layer of ceramic material on the surface of these tubes. This method, in which the deposition is carried out at atmospheric pressure or under slight depression, generally does not lead to obtaining a relatively uniform deposit over the entire length of the tubes because the rate of growth of the deposit is not uniform along the tube and therefore the thickness, or even the quality of the deposit, varies along the tube. These variations entail a risk of obtaining zones with large thicknesses and therefore of low adhesion and / or of zones in which the deposition of silicon carbide is of poor quality and therefore also of low adhesion. The pressure level at which - according to the examples of this patent - the vapor phase deposition of an organometallic derivative of silicon is much too high which does not allow to have a homogeneous deposition because the gas diffusion distances are much less important than under vacuum, that is to say for example at a pressure of less than 10 ^{+ 4} Pa. Furthermore, the deposited silicon carbide is a compound with a low coefficient of expansion, whereas the substrate employed usually has a coefficient much higher expansion, which leads over time and heating and cooling cycles a significant risk of loss of sealing, at least at certain points, of the coating of silicon carbide and consequently contact between the hydrocarbons and the superalloy which leads to an increase in the coking speed of the apparatus.

Mention may also be made of international application WO-A-95 / 18,849, which describes including objects used as components of cracking reactors coated with a protective layer against coking containing chromium. This layer is applied by different plating methods or by painting. Chrome coatings obtained do not have sufficient coking resistance, in particular in alternating coking and decoking cycles.

We have now discovered new anti-coking steels, which can be used in particular in the applications mentioned above and which do not have the drawbacks supra.

• un substrat en acier réfractaire renfermant au moins 0,2 % en masse de carbone ;
• une barrière de diffusion riche en carbone ;
• et une couche externe ayant une teneur de 90 à 99 % en masse de chrome ; et caractérisé en ce qu'il a été revêtu par une méthode de cémentation.

Thus, the invention relates to a steel object having anti-coking properties, characterized in that it comprises:

• a refractory steel substrate containing at least 0.2% by mass of carbon;
• a carbon rich diffusion barrier;
• and an outer layer having a content of 90 to 99% by mass of chromium; and characterized in that it has been coated by a cementation method.

The substrate (or matrix) generally consists of a refractory steel, containing preferably from 0.20 to 0.80% (percent) by mass of carbon.

More particularly, this steel has grains of austenitic structure at ambient temperature. A specific family of steel considered in the invention is that Manaurite® (registered trademark of MANOIR INDUSTRIES).

Typical examples of steels which can be used in the invention have the main characteristics indicated in the following table (the compositions are in% by weight): VS mn Yes Or Cr Fe additions ASTM A297 HK Manaurite 20® 0.30 to 0.65 1-2 1-2.5 18-24 23-36 compl. Ti. Nb. W ASTM A297 HL 0.20 to 0.60 1-2 1-2 18-22 28-32 compl. ASTM A297 HN 0.20-0.50 1-2 1-2 23-27 19-23 compl. ASTM A297 HP 0,35-0.75 1-2 1-2,5 33-37 24-28 compl. Manaurite 36XS® 0.35 to 0.60 1-1.5 1-2 33-38 23-28 compl. Nb, W Manaurite XM® 0.35 to 0.60 1-1.5 1-2 33-38 23-28 compl. Nb, Ti, Zr Manaurite XT® 0.35-0.45 1-1.5 1-2 42-46 32-37 compl. Nb Manaurite XTM® 0.40 to 0.45 1-2 1-2 43-48 34-37 compl. Nb, Ti

As indicated above, the invention is more particularly considered refractory steel objects coated by thermal chromium plating methods, such as chromisation by cementation, in particular by pack cementation or by gas phase carburizing.

The pack-cementation process is well known to those skilled in the art. He was described in numerous documents. For example in the United States patent US-A-5,589,220, the authors point out that pack-cementation is a derived process CVD (abbreviation for "Chemical Vapor Deposition"), which consists of heating, in closed or open enclosure containing the metal part to be coated, a "pack" at one high temperature for a fixed period, during which a deposit diffusion occurs on said metal part. The cementation "pack" in the closed or open enclosure is protected from oxidation by an inert atmosphere or reductive. The cementation "pack" consists of a piece or a substrate of metal or of alloy to be coated, surrounded by the elements to be deposited (in the form of a metal or a master alloy), a halide activating salt and a pulverulent filler. We use a inert gas such as argon or hydrogen as the gaseous environment of "pack". Once the "pack" is heated to a fairly high temperature, the salt activator reacts with metal or master alloy powder to form vapors metal halide. The metal halide vapors diffuse towards the surface of the metal or substrate through the gas phase of the porous "pack". On the surface of substrate, a reaction step takes place which results in the deposition of the desired element and in the formation, by diffusion in the solid state of a protective coating on the surface of the metal. The surface reaction can be quite complex and involve adsorption, dissociation and / or diffusion of molecular species.

In addition, gas carburization consists of heating to a high temperature in an open enclosure containing the metal part during a fixed duration, during which a diffusion deposit occurs on said piece metallic by a chromium halide gas generated by the action of a halide and / or of its hydride on a bed of chromium or chromium alloy granules from 0.1 mm to 50 mm in diameter.

This metallizing gas is conveyed by a gas carrying the bed of granules to said piece by a specific distributor-diffuser.

The carrier gas is a gas such as argon or hydrogen as protective gaseous environment.

The process of metallizing gas supply to said metal part is governed by the rules of fluid dynamics and limits the formation of chromium deposits.

The balanced choice of physical parameters (temperature, duration of treatment, mass flows of gaseous species) and of the chemical activity of the halide of chromium, optimized by the ratio of solid masses present (halide / chromium in granules), with or without the presence of its hydride, stabilizes the chemical activity of the metallizing halide and the conditions for forming the coating.

The mass concentration of chromium in the granules is at least fifty for hundred.

According to the invention, one operates more particularly under the conditions of pack-cementation shown below.

The parts to be coated are placed in boxes containing a cement made up chromium powders (30 to 40% by mass) and alumina (60 to 70% by mass) and a activating halide (0.1 to 2% by mass relative to all of the two powders) under an atmosphere, for example hydrogen or argon. Heat treatment isotherm is then carried out at a temperature of 900 to 1200 ° C.

The objects coated according to the invention can be used in general as building materials for tubular bundles of pyrolysis reactors of hydrocarbons, with or without the presence of water vapor, more particularly for steam reforming or steam cracking reactors. They can also be used as building materials for tube bundles for process furnaces petroleum or petrochemical, such as visbreaking. They can still be used in the coating of ferrules and / or internal of fixed bed reactors for treatments petrochemicals, such as for example dehydrogenation or reforming.

In applications of this type, the coated objects according to the invention exhibit improved anti-coking properties. In addition, the deposits obtained do not deteriorate during severe thermal cycles between room temperature and for example 1000 ° C., with heating and cooling rates of 500 ° C / h.

The examples given below illustrate the invention.

EXAMPLE 1

A piece of Manaurite XM® refractory steel having the following composition is treated by pack-hardening. VS mn Yes Or Cr Fe additions Manaurite XM® 0.35 to 0.60 1-1.5 1-2 33-38 23-28 compl. Nb, Ti, Zr

The quantitative analysis of the various elements was carried out on the chromized part, for the matrix (or substrate) itself, for the edge thereof, and for the deposit of chromium at its internal edge and at its external edge. The contents of the various elements are indicated in the following table:

EXAMPLE 2 (comparative)

Chromium was deposited electrolytically. To do this, we have immersed a sample of Manaurite XM® refractory steel serving as cathode, in a chromate bath and the chromate ions were reduced to metallic chromium on the cathode. A chromed steel was obtained, designated hereafter by Manaurite XM® chromed by electrolysis.

EXAMPLE 3

The tests were carried out under steam cracking conditions at 800 ° C. with a hydrocarbon and water charge. Coking speed was monitored by analysis TGA. The samples coated on their entire face are placed in a perfectly homogeneous steam cracking reactor suspended from the bane of the thermobalance. After a few hours of experience, the speed stabilizes at a value called asymptotic, characteristic of the reactivity of the deposit vis-à-vis coking. A air decoking is then carried out in order to follow the behavior during cycles coking-decoking-coking, etc.

A chromium-plated Manaurite XM® refractory steel sample is used as described in Example 1 and, for comparison. a refractory steel sample Manaurite XM® itself, as well as a sample of steel of the same type coated by CVD a layer of titanium carbide and a layer of silicon carbide, a sample steel of the same type coated by CVD with a layer of titanium carbide and a sample of Manaurite XM® steel chromed by electrolysis.

The test is carried out at 800 ° C., with a charge of hexane and water for a conversion of approximately 30%. The results indicated in Table 4 below were obtained. Asymptotic coking speed (g / m ² h) ^1st coking ^2nd coking Manaurite XM® 1.00 1.05 Manaurite XM® chromed by electrolysis 0.60 0.65 Manaurite XM® / TiC 0.46 0.47 Manaurite XM® / TiC / SiC 0.19 0.50 Manaurite XM® chrome 0.20 0.20

From these results, it appears in particular that the use of Manaurite XM® chromized glass of Example 1 according to the invention leads, at the end of the first coking, to a coking speed much lower than that observed with the Manaurite XM® before chromization (5 times weaker) or that observed with Manaurite XM® electrolytic or that the Manaurite XM® coated with a layer of titanium carbide. The ManauriteXM® chromized in Example 1 also gives good results after second coking. For its part, a steel coated with titanium carbide and carbide silicon gives poor results from the second coking.

EXAMPLE 4

Thermal cycling resistance tests were carried out in order to simulate the thermal shocks that industrial steam cracking tubes can undergo. These tests have were carried out in a muffle furnace equipped with an air injection. The samples have undergone 15 thermal heating and cooling cycles, from room temperature to 1000 ° C (and vice versa), with a heating (or cooling) speed of 500 ° C per hour. During the first 15 cycles, the parts are examined after each cycle.

The chromium-coated Manaurite XM® refractory steel as described in Example 1 was evaluated according to the previous protocol. After 145 cycles (which corresponds to cumulative thermal cycling on industrial plant for an estimated service life of 10 years), this steel has not degraded. During the first 15 cycles, the parts are examined after each cycle. Metallographic examinations confirm the integrity of the coating.

For comparison, tests of resistance to thermal cycling were carried out. under the same conditions with a coating of titanium carbide alone: after 10 cycles, the total disappearance of the layer was observed. By the way, with a coating of titanium carbide and silicon carbide, the layer of silicon carbide after 5 cycles.

Claims (10)

1. A coated steel article exhibiting anti-coking properties, characterized in that it comprises:

   a refractory steel substrate comprising at least 0.2% by weight of carbon;

   a carbon-rich diffusion barrier; and

   an outer layer containing 90% to 99% by weight of carbon,

   characterized in that it is coated using a cementation method.
2. An article according to claim 1, characterized in that it has been coated by pack cementation or by gas phase cementation.
3. An article according to claim 1 or claim 2, characterized in that the amount of carbon in the substrate is 0.2% to 0.8% by weight.
4. An article according to any one of claims 1 to 3, characterized in that the substrate has the following composition: C Mn Si Ni Cr Fe additions 0,35-0,60 1-1,5 1-2 33-38 23-28 balance Nb, Ti, Zr
5. Use of an article according to any one of claims 1 to 4 as a constituent material in tube bundles of hydrocarbon pyrolysis reactors, in the presence or absence of steam.
6. Use according to claim 5, characterized in that said reactors are steam reforming reactors.
7. Use according to claim 5, characterized in that said reactors are steam cracking reactors.
8. Use of an article according to any one of claims 1 to 4 as a constituent material of tube bundles for stills for oil or petrochemical treatments.
9. Use according to claim 8, characterized in that the treatment is a visbreaking treatment.
10. Use of an article according to any one of claims 1 to 4 as a constituent material of rings and/or baffles in fixed bed reactors for petrochemical treatments.