Mechanical Wall-Treatment Method That Reduces Coke Formation, and Hydrocarbon Treatment Method

Abstract

The invention relates to a process for the treatment of a wall made of FeNiCr metal alloy of an industrial reactor which reduces the formation of coke on the said wall when it is subjected to operational conditions favourable to coking, the metal alloy comprising, within its structure, carbides, some of which show on the surface. The process comprises a mechanical stage of impact surface treatment, during which a surface of the wall is hammered by projection of particles under conditions suitable for obtaining covering of the carbides initially present at the surface by permanent plastic deformation of the surface, in particular of the chromium carbides.

Claims

1.-14. (canceled)

15. A process for the treatment of a wall made of FeNiCr metal alloy of an industrial reactor which reduces the formation of coke on the said wall when it is subjected to operational conditions favourable to coking, the metal alloy comprising, within its structure, carbides, some of which can show on the surface, the process comprising: a mechanical stage of impact surface treatment, during which a surface of the wall is hammered by projection of particles under conditions suitable for obtaining covering of the carbides initially present at the surface by permanent plastic deformation of the surface, the metal alloy containing at least 5% by weight of iron, at least 18% by weight of chromium, at least 25% by weight of nickel and at least 0.05% by weight of carbon.

16. The process according to claim 15, in which the particles used during the mechanical treatment stage are chosen from aluminium oxide particles, metal particles, beads made of material which is inert under the said operational conditions, or nesosilicate particles.

17. The process according to claim 15, in which the particles used during the mechanical treatment stage have a mean diameter of 100 to 500 m.

18. The process according to claim 15, in which, during the mechanical treatment stage, the particles are projected by a gaseous fluid under a pressure of 200 to 400 kPa.

19. The process according to claim 15, characterized in that it additionally comprises, before the mechanical surface treatment stage: a stage of chemical treatment of the surface of the wall to be treated, during which at least a part of the carbides initially present in the alloy, in particular at the surface, is removed by electrolytic dissolution.

20. The process according to claim 19, in which the chemical treatment stage is carried out under conditions suitable for dissolving at least a part of the carbides over a depth of at least 10 m.

21. The process according to claim 19, characterized in that the chemical surface treatment stage is carried out under conditions suitable for dissolving carbides chosen from chromium carbides, niobium carbide, when the alloy contains niobium, and carbonitrides, when the alloy contains nitrogen.

22. The process according to claim 19, in which the chemical treatment stage is carried out in an electrolysis cell comprising a solution chosen from an aqueous solution of an alkali metal hydroxide and an aqueous sulfuric acid solution.

23. The process according to claim 19, comprising at least one other chemical treatment stage, during which at least a part of the carbides initially present in the alloy, in particular at the surface, and not dissolved during a preceding chemical treatment stage is removed by electrolytic dissolution.

24. The process according to claim 19, in which: one chemical treatment stage is a stage of electrolytic dissolution of chromium carbides, another chemical treatment stage is a stage of electrolytic dissolution of niobium carbides, the metal alloy containing niobium.

25. The process according to claim 24, in which the electrochemical dissolution of the chromium carbides is carried out and then the electrochemical dissolution of the niobium carbides is carried out.

26. The process according to claim 15, characterized in that it comprises, after the mechanical surface treatment stage: an oxidation stage carried out under conditions suitable for forming a layer of oxide(s) on the surface which has been subjected to the mechanical treatment, in particular a layer containing one or more chromium oxides.

27. A process for the treatment of hydrocarbons under conditions capable of bringing about the formation of coke, characterized in that the hydrocarbons are brought into contact with a surface of a wall made of FeNiCr metal alloy, the metal alloy containing at least 5% by weight of iron, at least 18% by weight of chromium, at least 25% by weight of nickel and at least 0.05% by weight of carbon, the said surface of the metal wall being pretreated by a treatment process according to claim 14 so as to reduce the formation of a coke deposit.

28. A process for the treatment of hydrocarbons according to claim 27, in which the hydrocarbons are brought into contact with the surface of the metal wall at a temperature of 800 to 900 C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0085] The invention is now described by means of examples and with reference to the appended non-limiting drawings, in which:

[0086] FIG. 1 diagrammatically represents an electrolysis cell which can be used for the chemical surface treatment stage;

[0087] FIGS. 2 and 3 represent SEM photographs of sections of two samples which have been subjected to a selective dissolution electrochemical treatment;

[0088] FIGS. 4 to 7 are diagrammatic representations of the observations in section of samples which have respectively been subjected to: only a polishing (FIG. 4), an electrochemical dissolution treatment (FIG. 5), a mechanical surface treatment (FIG. 6), an electrochemical dissolution treatment followed by a mechanical surface treatment (FIG. 7);

[0089] FIGS. 8 to 11 are SEM photographs with a secondary electron detector (applied acceleration voltage of 20 kVFIG. 7-8, 10-11, or 25 kVFIG. 9) of sections of samples, according to two magnifications: [0090] a: magnification 35, scale of 500 m [0091] b: magnification 150, scale of 100 m.

[0092] FIGS. 8a, 8b show photographs of a reference sample, FIGS. 9a, 9b show photographs of a sample which has been subjected to an electrochemical dissolution treatment, FIGS. 10a, 10b show photographs of a sample which has been subjected to a mechanical alumina blasting treatment, FIGS. 11a, 11b show photographs of a sample which has been subjected to an electrochemical dissolution treatment followed by a mechanical alumina blasting treatment.

DETAILED DESCRIPTION OF THE FIGURES

[0093] FIG. 1 diagrammatically represents an electrolysis cell 1. A difference in electric potential is applied between two electrodes 2, 3 immersed in an electrolytic solution 4. The positive terminal is the anode 2, the site of an oxidation, and the negative terminal is the cathode 3, the site of a reduction. A direct current generator 5, connected to the anode 2 and to the cathode 3, provides the current.

[0094] The substance to be dissolved has to be located on the anode 2 (+ terminal). The space between the two electrodes 2, 3 is, for example, approximately 1 cm. For the cathode (the terminal), a simple metal plate can be used. The electrolyte 4 will, for example, be a sodium hydroxide solution.

EXAMPLES

[0095] Samples made of metal alloy of HP modified 25-35 type and of 35-45 type were tested. These alloys are composed of an FeNiCr austenitic matrix within which niobium carbides (NbC) and chromium carbides (Cr.sub.7C.sub.3) precipitate. The characteristics of the metal alloys of the samples used are given in Table 1 below.

TABLE-US-00001 TABLE 1 Typical chemical composition (% by weight) of the materials used Cr Ni Fe C Si Mn Nb HP 25-35 25 35 26 0.5 1.4 1.6 0.5 HP 35-45 35 45 15 0.5 2.5 1.6 0.4

[0096] The samples used are plaques with dimensions of 830 mm (samples C1 to C5) and 825 mm (samples C6 to C9) and with a thickness of 2 mm obtained by electrical discharge machining to the core of 5 cm portions of new steam cracking tubes, with an initial thickness of 8 mm. The initial surface state is a crude machining state.

[0097] The tubes from which the tested samples result were manufactured by centrifugal casting.

[0098] Each tested sample was polished by means of SiC-based abrasive papers in the following order of fineness: 600, 800, 1200 and 2400.

Characterization Techniques Used

[0099] Scanning electron microscopy (SEM) for observation of the surfaces and of sections. The SEMs used are the Philips XL 20 SEM and the Zeiss Supra 55 VP SEM. [0100] Ion beam cutting: cross sections are produced by ion beam cutting with a beam of defocused ions. This technique uses accelerated argon ions to tear off the material, thus making possible a very fine and contamination-free surface polishing. The samples are adhesively bonded to titanium masks using a silver lacquer formed of fine silver platelets in suspension in a solvent.

Example 1

Electrochemical Treatment of the Surface

[0101] In this example, the sample is subjected to an electrolytic dissolution chemical treatment.

[0102] The sample to be tested is placed at the anode of an electrolysis cell, such as described in FIG. 1, the cathode being a metal plate made of stainless steel or of graphite, with dimensions similar to or greater than those of the sample. The anode and cathode are separated by a distance of approximately 1 cm, the plates being substantially parallel inside the electrolysis cell.

[0103] An electrolytic solution is prepared by dissolving, with mechanical stirring, 135 g of NaOH (in the form of pellets) in 11 of distilled water and then the electrolysis cell is filled with the solution obtained. The chloride content of the solution is less than 10 ppm by weight.

[0104] A potential difference is applied between the anode (sample) and the cathode.

[0105] Two series of five and four tests were carried out on HP 25-35 alloys, which were all polished before being placed in the solution. The conditions used for each test are collated in Table 2.

TABLE-US-00002 TABLE 2 Parameters of the electrolytic decompositions Current Sample Duration Voltage intensity name (hours) (V) (A) Comments C1 20 6 15 Evaporation of the C2 2 8 16 solution before the 3 6 10 end of the duration C3 15 6.5 12 C4 15 6 10 C5 20 6 10 Current Sample Duration Voltage intensity Depth of dissolution of name (hours) (V) (A) the chromium carbides C6 2 3.72 5 approximately 30 m C7 5 3.3 4.14 approximately 50 m 5 4.75 8.34 approximately 50 m C8 15 4.4 8.3 approximately 72 m C9 24 3.45 5 approximately 100 m

[0106] It will be noted that the current intensity does not appear to influence the depth of dissolution of the chromium carbides, unlike the duration of the dissolution.

[0107] Under the conditions tested, observation with an SEM of the surface of the samples C1 to C4 shows a dissolution of the chromium carbides Cr.sub.7C.sub.3 but no dissolution of the niobium carbides NbC. The austenitic matrix remains intact.

SEM Observation of the Samples in Cross Section

[0108] Sections of the different samples were observed with an SEM.

[0109] FIGS. 2 and 3 are photographs of the sample C4 dissolved for 15 h (FIG. 2) and of the sample C5 dissolved for 20 h (FIG. 3). The acceleration voltage applied for the measurement is 15 kV, the magnification is 619 (FIG. 2) and 629 (FIG. 3) and the scale is 10 m. In the sample C4, cavities are observed over a depth of approximately 40 m, which appears to indicate the existence of interconnected carbide networks. In the sample C5, the cavities extend over a depth of 80 m. Chromium carbides still exist between 50 and 80 m, which appears to indicate that the network of carbides is not completely interconnected. Table 2 indicates, for the samples C6 to C9, the maximum depth down to which dissolution of the chromium carbides was observed.

[0110] It is thus possible to influence the depth of the carbides attacked by modifying the electrolysis conditions.

[0111] FIGS. 4 and 5 diagrammatically represent typical observations of a section of an untreated sample (FIG. 4) and of a sample which has been subjected to a chemical treatment (FIG. 5). In these diagrams, the black parts correspond to the chromium carbides and the grey parts to the niobium carbides.

[0112] Thus, the presence of niobium carbides (NbC), of chromium carbides (Cr.sub.7C.sub.3) and of cavities is noted in FIG. 5. Niobium carbides are observed in the cavities. Without wishing to be committed to a theory, during the electrolytic dissolution, the solution might spread by dissolving the chromium carbides resulting from the interconnected networks but while retaining the niobium carbides (NbC). In addition, it is observed that the cavities are not completely empty. A chemical analysis by SEM/EDX (Energy Dispersive X-ray Spectrometry) shows that the chromium carbides have been partially dissolved. The presence of oxygen inside the cavities is also observed, which leads it to be believed that there is formation of oxide or of hydroxide, probably originating from the electrolytic solution.

Example 2

Mechanical Surface Treatment/Shot Peening

[0113] A polished HP 25-35 alloy sample is subjected to shot peening in a sleeve sandblasting chamber. The parameters used are as follows: [0114] Particles: glass beads 100-200 m [0115] Projection distance: approximately 15 cm [0116] Duration of the projection: 15 seconds for a sample of a few cm.sup.2 [0117] Carrier gas: compressed air under a controlled pressure of 2.5 to 3.5 bars, nozzle diameter 6 to 8 mm, 40 litres of particles in a closed circuit.

[0118] A sample M1 is obtained.

Example 3

Mechanical Surface Treatment/Sandblasting (Alumina Blasting)

[0119] A polished HP 25-35 alloy sample is subjected to alumina blasting in a sleeve sandblasting chamber. The parameters used are as follows: [0120] Particles: brown corundum (Al.sub.2O.sub.3) 250-400 m [0121] Projection distance: approximately 15 cm [0122] Duration of the projection: 15 seconds for a sample of a few cm.sup.2 [0123] Carrier gas: compressed air under a controlled pressure of 2.5 to 3.5 bars, nozzle diameter 6 to 8 mm, 40 litres of abrasive particles in a closed circuit.

[0124] A sample M3 is obtained.

[0125] FIG. 6 diagrammatically represents the typical observation of a section of a sample which has been subjected to a mechanical treatment. It is noted that the chromium carbides are no longer in direct contact with the surface.

Example 4

Chemical Treatment+Mechanical Treatment/Shot Peening

[0126] The sample C4 of Example 1 is subjected to the same shot peening treatment as that described in Example 2. A sample CM4 is obtained.

Example 5

Chemical Treatment+Mechanical Treatment/Alumina Blasting

[0127] The sample C4 of Example 1 is subjected to the same alumina blasting treatment as that described in Example 3. A sample CM5 is obtained.

[0128] FIG. 7 diagrammatically represents the typical observation of a section of an alloy sample which has been subjected to a chemical and mechanical treatment. It is noted that the chromium carbides are no longer in direct contact with the surface and that the cavities formed by the electrochemical dissolution have been at least partly closed for the majority of them.

Example 6

Coking

[0129] Coking tests were carried out on the samples C4, M2, M3, CM4 and CM5 prepared in Examples 1 to 5, and also on a reference sample simply polished. The samples were brought to high temperature in the presence of a mixture of light hydrocarbons and of steam (similar to industrial conditions). They were thus subjected to conditions favouring the formation of coke.

[0130] Each sample was subjected to the following conditions: [0131] 1. Rise in temperature under dry argon (O.sub.2 impurities in the argon of approximately 3 ppm by volume) up to 900 C. (5 C./min), [0132] 2. Pre-oxidation of the samples 900 C., 1 h under synthetic air, [0133] 3. Flushing of the furnace with dry argon 30 min, [0134] 4. Coking of the samples: 45 minutes 860 C.: ethane+steam, [0135] 5. Flushing of the furnace under dry argon 30 min, [0136] 6. Halting of the heating system and slow cooling of the samples.

[0137] The samples were observed with an SEM. FIGS. 8a and 8b are photographs (magnifications 35 and 150 respectively) of the surface of the reference sample which has not been subjected to any specific treatment besides the initial polishing. The formation of coke at the surface is observed. FIGS. 9a and 9b are photographs of the sample M1-shot peened (magnifications 35 and 150 respectively), FIGS. 10a and 10b are photographs of the sample M2-alumina blasted (magnifications 35 and 150 respectively) and FIGS. 11a and 11b are photographs of the sample CM5 (magnifications 35 and 150 respectively).

[0138] The samples which have been subjected to a chemical treatment exhibit overall less coke than the reference sample. Coke is still observed over approximately 10% of the surface of the sample.

[0139] The sample which has been subjected to the most violent treatment (M2-alumina blasted) exhibits a dented surface with numerous protrusions due to the impacts of the projectiles.

[0140] The samples which have been subjected to a mechanical treatment exhibit less coke than the reference sample. The amounts of coke formed on the shot-peened sample (M1) and the alumina-blasted sample (M2) appear to be similar (see figures).

[0141] A notable reduction in the amount of coke is also observed for the samples which have been subjected to a chemical treatment prior to the mechanical treatment (samples CM4 and CM5), as may be made out in FIGS. 11a and 1 lb for the sample CM5.

Example 8

Electrochemical Treatment of the Surface

[0142] In this example, the sample is subjected to an electrolytic dissolution chemical treatment in order to remove the niobium carbides.

[0143] The sample to be tested is placed at the anode of an electrolysis cell of the same type as that represented in FIG. 1 and described in Example 1.

[0144] A 7.2 mol.Math.l.sup.1 electrolytic solution of sulfuric acid (H.sub.2SO.sub.4) is prepared, with which the electrolysis cell is filled.

[0145] A first test was carried out on an HP 25-35 alloy with dimensions of 825 mm and with a thickness of 2 mm, which was polished before being placed in the sulfuric acid solution.

[0146] A potential difference of the order of 0.8 V is applied between the anode (sample) and the cathode for 2 hours. The sample is subsequently rinsed with distilled water and then with ethanol, dried and stored in a case sheltered from scratches and from the air in a desiccator.

[0147] A second test was carried out under the same electrolysis conditions on a sample with the same dimensions and of the same alloy subjected beforehand to an electrolytic dissolution of the chromium carbides. The latter is carried out with a current density of 5 A.Math.in.sup.2 (0.775 A.Math.cm.sup.2) for 2 hours in a NaOH solution (135 g in the form of pellets in 1 l of water). The sample obtained is subsequently rinsed with distilled water and then with ethanol and dried before being introduced into the sulfuric acid solution for the dissolution of the niobium carbides.

[0148] An examination of the surface state by backscattered electron scanning electron microscopy (mode of imaging sensitive to chemical contrast) shows that there was no dissolution of the niobium carbides in the case of the first test.

[0149] On the contrary, a dissolution of all the carbides (chromium and niobium) is observed by examination of the surface state of the sample subjected beforehand to a dissolution of the chromium carbides. The absence of niobium carbides (NbC) at the surface was confirmed with an SEM by an EDX (Energy Dispersive X-ray) analysis. The chemical distribution of the niobium over the surface analysed shows a few places locally rich in Nb but not any niobium carbide.

[0150] Unlike a simple dissolution in sulfuric acid, the successive electrolytic decomposition of the chromium carbides and of the niobium carbides thus makes it possible to dissolve the NbCs at the surface. Without wishing to be committed to a theory, the electrolytic dissolution of the M.sub.23C.sub.6/M.sub.7C.sub.3 might come partially to lay bare the NbCs and to increase the free surface area in contact with the electrolyte of the second dissolution.