FR2518565A1 - TUBE FOR THERMAL CRACKING OR REFORMING OF HYDROCARBONS - Google Patents

Abstract

TUBE FOR THERMAL CRACKING OR REFORMING OF HYDROCARBONS. IN THE REACTOR TUBE FOR CRACKING AND THERMAL REFORMING OF HYDROCARBONS, THE REACTION LAYER (1) IN CONTACT WITH THE HYDROCARBONS IS FORMED BY A STEEL RESISTANT TO HEAT AND CONSISTING OF, IN PERCENTAGES BY WEIGHT, 0.01 TO 1, 50DE C, UP TO 3 OF SI, UP TO 15 MN, FROM 13 TO 30 OF CR, UP TO 0.15 OF N, THE COMPLEMENT BEING ESSENTIALLY FE AND THE COATING LAYER (2) COVERING THE SAID LAYER REACTION IS SHAPED BY A STEEL RESISTANT TO HEAT TO FE-CR-NI AND FUSED WITH THE REACTION LAYER AT ITS INTERFACE WITH THIS LATTER.

Description

Tube for thermal cracking or reforming of hydrocarbons.

  The present invention relates to a reactor tube for thermal cracking or reforming of hydrocarbons, in particular the reactor tube which prevents deposition and accumulation

  solid carbon accompanied by a chemical reaction of the hy-

  The hydrocarbon forms on its wall and avoids carburization.

  The reactor for thermal cracking and reforming of hydro-

  carbides used here has a tubular shape and is traversed by hydrocarbons in a liquid or gaseous form at pressures

  and high temperatures for cracking or thermal reforming

  In the presence or absence of a catalyst layer The material hitherto used for such reactors is heat-resistant Fe-Cr-Ni austenitic steel containing a large amount of Ni and Cr and is generally used for equipment used at elevated temperatures It is common practice to increase the Ni content to increase the heat resistance property of the tube material in front of the tube.

  be used at higher temperatures.

  Since the thermal cracking or reforming of the hydrocarbons is accompanied by solid carbon deposition, when the reaction is prolonged using such a Fe-Cr-Ni steel reactor tube as mentioned above, solid carbon deposits and accumulates inevitably on the surface of the wall (inner surface of the wall, outer surface of the wall, or on the inner surface as well as on the outer surface of the wall depending on how it is used the reactor tube) to be in contact with the hydrocarbons When no action is taken for such a solid carbon deposit, not only does it obstruct the passage through the tube of the hydrocarbon-containing fluid, but significantly reduces the overall coefficient heat transfer with respect to the heat of reaction supplied to the tubes from outside or removed from this tube to the outside, and it is therefore difficult to continue the operation It follows that a periodic shutdown is necessary to remove carbon deposits using various so-called decoking processes, although the reactor must operate continuously as a rule In addition, the conventional reactor tube mentioned above raises problems -that a deterioration of the material constituting the tb e as a result of a

  carburetion through the surface of the reaction wall, particularly

  especially a considerable reduction in the

  danger of cracks arising from fragility

  sation of the tube material under high pressures.

  In order to solve the above problems, the Applicant has engaged in intensive research and has found that the cause of the large carbon deposits in the heat-resistant Fe-Cr-Ni steel reactor tube lies in the fact that Ni contained in the steel acts catalytically by accelerating the deposition of solid carbon on the tube surface via the hydrocarbons and that there is a correlation between the amount of solid carbon deposition and the Neither of the material of the tube and reducing this Ni content

  neutralize and prevent the deposition of solid carbon on the.

  tube surface With regard to carburation, when steel

  of the tube contains an appropriate quantity of Mn and Nb, the fuel

  from the surface of the tube wall is effectively reduced and deterioration of the

material constituting the tube.

  The present invention has been designed based on the above analysis. The present invention makes it possible to obtain a reactor tube whose reaction layer (inner layer of the wall) to be in contact with the hydrocarbon is formed by heat-resistant and Ni-free Fe-Cr steel; heat-resistant Fe-Cr-Ni steel containing up to 10% Ni, so that it does not substantially cause said catalytic action accelerating the deposition of solid carbon, and whose reaction layer is covered by the outer layer formed of the conventional material used in equipment operating in high temperatures, such as heat-resistant Fe-Cr-Ni steel. this double-layer structure for the reactor tube, the applicant has managed to neutralize as much as possible the solid carbon deposit resulting from the reaction and to ensure stable operation without proceeding to the decoking for a prolonged time while maintaining the required characteristics of the reactor tube used in the presence of high temperatures and pressures. More specifically, the present invention makes it possible to provide a reactor tube for the thermal cracking or reforming of hydrocarbon, in which the deposition of solid carbon accompanied by the reaction can be avoided by forming the reaction layer in the zone. reaction vessel to be in contact with hydrocarbons with a heat-resistant steel and comprising, in percent by weight, 0.01-1.5% C, up to 3% Si, up to 15% Mn , 13 to 30% of Cr, up to 0, 15% of N, the balance being essentially Fe, and forming a coating layer which overlies and melds with said reaction layer at the separation surface with a heat resistant steel and comprising, in weight percent, 0.1 to 0.6% C, up to 2.5% Si, up to 2% Mn, 20 to 30% Cr. , 18 to 40% of Ni, up to 0.15% of N, the complement being

essentially Fe.

  Another object of the present invention is to provide a reactor tube for the thermal cracking or reforming of hydrocarbon, in which a solid carbon deposit is avoided on the surface of the tube and the carburization is neutralized through the surface of the tube. forming the reaction layer of the reactor tube in the reaction zone to be in contact with hydrocarbons with a heat resistant Fe-Cr-Mn-Nb steel and comprising, in weight percent, 0.3 to 1 , 5% C, up to 3% Si, 6 to 15% Mn, 20 to 30% Cr, up to 3% Nb, up to 0.15% N, the complement being essentially Fe, or with a heat-resistant Fe-Cr-Mn-Nb-Ni steel obtained by replacing a certain amount of Fe with Ni in a proportion of up to 10% and forming the coating layer

  which covers said reaction layer and which merges with

  the separation surface with a resistant Fe-Cr-Ni steel

  both by heat and by weight percent, 0.1 to 0.6% C, up to 2.5% Si, up to 2% Mn, 20 to 30% Cr, 18%. 40% of Ni, up to 0.15% of N, the balance being essentially Fe, or with a heat-resistant Fe-Cr-Ni steel and obtained by replacing a small amount of Fe with one or more of elements selected from Mo, W and Nb, in one

  combined amount up to 5% by weight.

  The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a partially cutaway front elevational view showing a reactor tube according to the present invention; Figure 2 is a sectional view through II-II of Figure 1, Figures 3 and 4 are sectional views of a reaction tube according to other examples of the invention; Fig. 5 is a graph showing the correlation between the Ni content of the material constituting the reactor tube and the amount of solid carbon deposition on the surface of the reaction layer; Fig. 6 is a graph showing the increase in the amount of carbon as a function of the penetration depth of the carburization in the reaction layer; Figure 7 is a graph showing the amount of

  depositing solid carbon on the surface of the reaction layer.

  When the reaction zone of the tube, which is brought into

  contact with hydrocarbons, is on the inner surface

  of the tube, the reaction layer 1 located inside as illustrated

  FIGS. 1 and 2 are formed of Fe-Cr ferritic or Ni-free grit-type heat-resistant steel, or Ferritic Fe-Cr type Ni heat-resisting steel. , ferritic-austenitic or martensitic

  containing up to about 10% Ni.

  The above-mentioned heat-resistant Fe-Cr steel may be, for example, the steel formed from 13 to 30% Cr (% by weight, as well as from below), 0.01 to 1.5% C, up to 2.5% Si, up to 2.0% Mn, up to 0.15% N, the complement being essentially Fe, or the steel in which a certain amount of Fe is replaced by one or more of Mo, W and Nb in a combined amount of up to 5.0% so that characteristics are obtained.

even better of the material.

  The coating layer 2, which covers the outer side of said reaction layer 1 and which is formed of steel

  austenitic heat-resistant Fe-Cr-Ni austenitic

  For tubes of this type, fusion with the aforesaid reaction layer 1 at the location of the separation surface so that a double-layer structure is obtained. On the other hand, when the reaction zone of the tube contact with the hydrocarbons is on the outer surface of the tube, the reaction layer 1 having the above-mentioned chemical compositions is formed on the outer side and the coating layer 2 having the above-mentioned chemical compositions is formed on the outer surface of the tube;

  inner side as shown in Figure 3.

  When the inner surface and the outer surface of the reactor tube both become the reaction zone, reaction layers 1.1 may be formed on both surfaces and a cover layer may be interposed between the two.

  reaction layers 1,1 as illustrated in FIG. 4.

  Figure 5 shows the correlation between the amount of solid carbon deposition (in mg / cm 2) and the content (% Ni) in the heat resistant Fe-Cr-Ni steel reactor tube (18%). Cr, 0.8% C, 1.5% Si, 1.1% Mn, 0.05% N, up to 35% Ni, 43.5 to 78.55% Fe) ( experimental conditions: quantity of ethane supplied: 400 cm 3 / min, S / C = 1.5, ethane gas transit time: 1 hour, inside diameter of the tube: 110 mm;

  temperature: 900 OC; S / C being H 20 mol / C atom).

  As can be seen from the drawing, the amount of solid carbon deposition increases as the Ni content in the tube material increases. For example, the Ni content of a Fe-Cr-Ni steel resistant to The heat used for the reactor tube material of this type is about 35% and coincides with the fact that it has not been possible to avoid

  significant deposition of solid carbon with the conventional reactor tube.

  This is because, as mentioned above, Ni on the wall surface of the tube acts catalytically to accelerate solid carbon deposition Based on this fact that the experiments confirm the maximum content of Ni in the present invention is limited to about 10.0% or preferably about 5% to neutralize and prevent solid carbon deposition as much as possible. The coating layer 2 which covers the reaction layer 1 may be made of heat-resistant Fe-Cr-Ni austenitic steel and which is usually used for the tube of this type. Examples of such steel chemical compositions may be be expressed in percent by weight, 20 to 30% Cr, 18 to 40% Ni, 0.01 to 0.6% C, up to 2.5% Si, up to

  2.0% Mn, up to 0.15% N, the complement being essential-

  Fe, or these chemical compositions may be those of steel in which a certain amount of Fe is replaced by one or more of the elements selected from Mo, W and Nb in one

combined content up to 5.0%.

  According to the present invention, the reaction layer 1 is made of Ni-free heat-resisting steel or containing Ni in proportions where this metal does not act substantially catalytically resulting in solid carbon deposition and the coating layer. 2 covers the reaction layer 1, the reaction layer and the coating layer being both

  united to one another by melting at the separation surface.

  The reactor tube thus has a double layer structure in which the coating layer 2 which covers

  said reaction layer 1 merges with this reaction layer

  1 to the separation surface and, thanks to these arrangements, the deposition of solid carbon on the surface of the wall of the reaction layer of the tube is effectively neutralized and the tube simultaneously has mechanical properties such as high toughness and a high resistance to

  creep rupture at high temperatures as well as the properties

  of Fe-Cr-Ni heat-resistant austenitic steel and, as a result, the reactor tube becomes a preferred tube

  to be used in the presence of pressures and tempera-

-18,565

7.

high eratures.

  Another embodiment of the present invention is

  the reactor tube having a reaction layer constituting

  primarily heat-resistant steel made from Ni-Fe-Cr-Mn-NI or heat-resistant Fe-Cr-Mn-Nb-Ni steel

  low nickel content containing up to 10% Ni.

  A preferred embodiment of said FeCr-Mn-Nb heat-resistant steel may be that composed, in percentage by weight, of 20 to 30% of Cr, 0.3 to 1.5% of C, up to 3% by weight. % Si, 6 to 15% Mn, up to 3% Nb, up to 0.15% N, the complement

being essentially Fe.

  A preferred example of low nickel nickel Fe-Cr-Mn-Nb-Ni heat resistant steel may be one in which Fe of said Fe-Cr-Mn-Nb heat resistant steel is partially replaced. by up to 10% Ni, i.e. heat-resistant steel compound, in weight percent, from 30% Cr, up to 10% Ni, 0.3-1 , 5% C, up to 3% Si, 6 to 15% Mn, up to 3% Nb, up to 0.15% N,

  the complement being essentially Fe.

  The heat resistant steel forming the coating layer

  It can be made of Fe-Cr-Ni heat-resistant austenitic steel generally used to form the tube material of the type described. For example, the steel comprising 20 to 30% by weight , 18 to 40% of Ni, 0.1 to 0.6% of C, up to 2.5% of Si, up to 2% of Mn, up to 0.15% of N, the complement being essentially Fe, or the steel having the above composition, but in which Fe has been replaced in part by one or more elements selected from Mo, W and Nb in

  a combined amount of up to 5%.

  In the above examples of the present invention, the chemical compositions of Ni-Fe-Cr or Fe-Cr-Ni heat resistant steel and Fe-Cr r-Mn-Nb or Ni-low Fe-Cr-Mn-Nb-Ni constituting the reaction layer 1, and

  austenitic heat resistant steel to Fe-Cr-Ni

  killing the coating layer 2 are only given as

  purely illustrative and may also be subject to

: '

  and appropriate changes, such as an increase in

  or a decrease in the proportions of the components beyond the ranges mentioned above or a small addition of components to those described above or a removal of components among those described above - Figure 6 shows the results of a carburetion test carried out to find the influence of Mn and Nb contained in the material of the tube during a carburation to which was subjected

the reactor tube.

  Carburation conditions: carburizing treatment through the surface of the inner wall of the tube using an agent

solid carburization.

  Processing temperature: 11000 C Processing time: 500 hours The following three sample tubes A, B and C reactor have

been used for the test.

  Reactor tube A (double layer tube structure)

Inner reaction layer: -

  Thickness of the layer: 2 mm Fe-Cr-Mn-NbNi heat-resistant steel with low nickel content (25% Cr, 5% Ni, 0.6% C, 2% Si, 8- , 1% Mn, 0.45% Nb and 0.05% N) Outer Coating Layer: - Layer Thickness :: 10 mm Fe-Cr-Ni Heat Resistant Steel (25%) Cr,% Ni, 0.48% C, 1.5% Si, 1% Mn, and 0.05% N) Reactor Tube B (Double Layer Tube Structure) Inner Reaction Layer: Thickness layer: 2 mm Fe-Cr-Ni heat-resistant steel with low nickel content (25% Cr, 5% Ni, 1- 0% C, 2.0% Si, 1.1 % Mn and 0.05% N) Outer Coating Layer: Layer Thickness: 10 mm Same heat-resistant Fe-Cr-Ni steel as used for reactor tube A described above. Reactor tube C (single-layer tube equivalent to conventional reactor tube generally used) Thickness of layer: 12 mm Same Fe-Cr-Ni heat-resistant steel as used for diaper outside coating

  reactor tube A described above.

  In FIG. 6, the curves A, B and C show respectively

  The results obtained with reactor tubes A, B and C can be seen in this figure, in the case of reactor tube C having a single-layer structure in a material equivalent to those used for conventional reactor tubes. (0.4 C 25 Cr 1 Mn 35 Ni), the carbon increases beyond 2% due to the carburization at the surface of the tube wall, which indicates a significant carburization in

  direction of the inside of the tube wall.

  While in the case of the reactor tube B whose inner layer is formed of a low nickel material (1 C 25 Cr 1 Mn 5 Ni), the carbon increase

  is significantly lower than that of the reactor tube-

  described above In the case of the tube A of the developer, wherein the inner layer is of a material with a low content of

  nickel, containing Nb and a large amount of Mn (0.6 C 25 Cr -

  8 Mn 0.5 Nb 5 Ni), the carbon increase due to the carburization is extremely low, this increase being less than about 0.3% The neutralization effect of the carburization is better when the Mn content is higher. As a result, to obtain an anti-carburetion effect using Mn and Nb, the reaction layer 1 facing the reaction zone is formed into a material containing Nb and in large amounts. of Mn The content of Mn is

  at least 6% However, if the content of Mn.

  is exaggerated, ductility decreases significantly and

  molded products may crack at the moment of solidification.

  during the molding process and, therefore, the

  Maximum limit of the Mn content is defined as 15%.

  When Nb is contained in large quantities, the SIGMA phase precipitates during use at high temperatures and the ductility decreases noticeably, the maximum limit of Nb

  therefore being defined as -3%.

  FIG. 7 is a graph showing a comparison of the amounts of solid carbon deposition on the tube wall surface during a thermal cracking and reforming hydrocarbon reaction test, in which the inside of the tube is used as reaction zone and the three kinds of tube

  D, E and F reactor are made of a uni-large material.

  quantity of Mn and Nb or a material with a low Mn content. Test conditions: Amount of treated ethane: 400 cm / min; S / C = 1.5; temperature: 9000 C; duration 1 hour; and

  internal diameter of the reactor tube 110 mm.

  The reactor tubes D and E have a double-layer structure formed of a reaction layer (thickness 2 mm) inside and a coating layer (thickness 10 mm) outside the inner layer in the tube D The reactor vessel is formed of a low nickel nickel FeCr-Mn-Nb-Ni heat-resisting steel and containing Nb and a large amount of Mn, while the reaction layer of the E-tube

  The reactor is made of Fe-Cr-heat resistant steel.

  Neither low in nickel and containing very little Mn.

  Both coating layers are formed by Fe-Cr-Ni heat-resistant steel typically used as

  material for the reactor tube of the type described.

  The reactor tube F is the conventional single-ply reactor tube which consists of the same material used for the coating layer of reactor tubes A and B described above. Chemical compositions of the reactor material The respective reactor tubes are as follows: reactor reaction tube D: 24.2% Cr, 4.8% Ni, 0.56% C, 1.9% Si, 8.81% Mn, 0.51% Nb and 0.05% N. Reaction layer of the reactor tube E; 25.2% Cr, 4.3% Ni, 0.96% C, 1.76% Si, 1.34% Mn and 0.05% N. Tube F reactor: 25.1 % Cr, 35.5% Ni, 0.43% C, 1.3% Si, 1.2% Mn, and 0.05% N, D, E, and F in Figure 7 show the results obtained with the aforesaid tubes D, E and F of the reactor. It is seen in this figure that the amount of solid carbon deposition on the surface of the wall of the reactor D tube, the reaction layer of which is formed by a steel resistant to the Ni-containing Fe-Cr-Mn-Nb-Ni heat containing Nb and a large amount of Mn is considerably lower than that of the reactor tube F formed by a conventional Fe-Cr chabur resistant steel -Ni, exactly as in the case of the reactor tube E comprising a reaction layer formed by a low-nickel Fe-Cr-Ni heat-resistant steel containing

  less than Mn and free of Nb, which indicates an anti-

  According to the foregoing, it is understood that even when a quantity of Mn as important as that described above is contained together with Nb in a low-grade Fe-Cr-Ni heat-resistant steel, nickel in which the Ni content is limited to 10%, the effect preventing the deposition of solid carbon is not hindered It will be understood from these tests that the reactor tube whose reaction layer opposite the reaction zone is formed Spar a Fe-Cr-Mn-Nb or Fe-Cr-Mn-Nb-Ni heat resistant steel with Ni, Mn and Nb contents defined as above is not subject only at very low levels of solid carbon deposition on the tube wall surface and has excellent characteristics with respect to its resistance to carburation. The reasons for specifying, as mentioned above, the

  C in the heat resistant steel, are as follows.

  In heat resistant steel with Fe-Cr-Mn-Nb or Fe-

  Cr-Mn-Nb-Ni, if the C content is too low, the SIGMA phase

- ': 12

  In addition, the lower the C content, the higher the solidification temperature of the molten alloy is, and, as a result of the centrifugal casting of the structural reactor tube, precipitates during use at elevated temperatures and the ductility decreases significantly. in the manner described hereinafter, tube which is the object of the present invention, the molten alloy of the reaction layer solidifies rapidly after molding and, as a result, a poorer melting takes place at separation between the reaction layer and the coating layer and it is then

  difficult to obtain a molding of the reaction tube with a -

  Good quality double-layered structure Such difficulties can be overcome by increasing the C content. However, when the C content is high, the material constituting the coating layer deteriorates as a result of diffusion transfer of the content. The carbon content of the reaction layer to the coating layer during the use of the reactor at elevated temperatures therefore, the C content is defined as 0.3 to 1.5% of the metal elements contained in the layer. of reaction, the contents of Cr, Si and N are

determined as follows': -

  Cr, in coexistence with Ni, has the effect of making

  nitrate the cast steel structure and thereby increase

  toughness at high temperatures and increase the resistance

  The Cr content must be at least 20% to obtain the required toughness and oxidation resistance at higher temperatures, especially at 1000 ° C. The above effect is reinforced as the Cr content increases. increases, but when it becomes too high, the decrease in ductility after use becomes excessive and, therefore, the

upper limit is 30%.

  If serves as a deoxidizer during the melting of the cast steel and also improves the anti-carburizing property However, the Si content should not exceed 3.0%, since a

  excess of Si leads to less good weldability.

  N serves, in the form of a solid solution, to stabilize

  and strengthen the austenitic phase, form a nitride and a carbon

  nitride with Nb and Cr, produces grains refined by nitride and carbonitride finely dispersed and precipitated and prevents

  grain growth, which helps to improve the resistance

  It is preferable that the upper limit of the Ni content is 0.15% since the presence of an excess of N allows excessive precipitation.

  nitride and carbonitride, the formation of

  nitrides and carbonitrides and a decrease in

resistance to solderability.

  Among Nb, Mo and W which are selectively contained lo in the coating layer, Nb contributes to the improvement of the moldability and also forms Nb-carbonitrides which disperse finely in the austenitic phase thereby reinforcing the austenitic matrix and greatly increasing the creep rupture strength, while making the molding structure thinner and improving the weldability However, when its content becomes too high, the breaking strength

  creep decreases on the contrary and ductility also decreases.

  Therefore, the Nb content does not exceed 5% -

  Nb usually contains Ta which is the element having the same effect as Nbet, therefore, the combined amount of

  Nb and Ta should not exceed 5% when Nb contains Ta.

  Mo and W also form carbonitrides and reinforce the austenitic structure in the same way as Nb, whereas their effect is enhanced by the co-presence of Nb. However, when the combined content of Nb + Mo + W exceeds 5%, it reduces the

  ductility as in the case of Nb alone and it is then decon-

  economically segregated Whether Mo and W are used independently or together, it is preferable that the combined Mo and / or W content does not exceed 5% Reactor tube

  having the double-layer structure according to this

  The reaction is preferably carried out by centrifugal molding. During molding, the molten metal formed from Fe-Cr-Ni heat-resistant steel containing a larger amount of Ni to form the outer coating layer is poured into the mold. for centrifugal molding so as to obtain the coating layer of desired thickness Immediately after, the solidification of this layer on the surface of the wall

  melted steel made of heat-resistant steel in

  Cr or Fe-Cr-Ni with low Ni content or Fe-Cr-Mn-Nb or Ni-low Fe-Cr-Mn-Nb-Ni for forming the inner reaction layer, is poured so that the reaction layer of desired thickness is molded. The rotation of the mold is then extended to complete the molding.

  process, the inner reaction layer and the coating layer

  The outer layer together form a thin layer having merged at the adjacent surfaces of these two layers, resulting in a tube having two layers united to one another metallurgically in the molding above, so that both If the layers undoubtedly fuse at their interface, it is preferable that the heat resistant steel of the reaction layer has a lower melting temperature than the heat resistant steel of the coating layer. the desirable mutual relation with respect to these melting temperatures by adjusting the chemical compositions of each heat-resistant steel, principally the carbon content of these steels, within the limits defined below, with respect to each other; There is no limitation

  specific with regard to other molding conditions.

  The molding temperature of the molten steel can be adjusted to a temperature of 150 ° C., for example above the melting temperature conventionally used in practice, and because of the need to protect the inner surface of the reaction layer. against the oxidation of the air, one can apply a flow

  appropriate according to the usual method.

  In conventional centrifugal casting of the double-layered tube, it is customary practice to mold the

  reaction before the inner surface of the coating layer

  It solidifies because, if the molten alloy for the reaction layer is poured after the coating layer has solidified to its inner surface, the merging of the two layers at their interface becomes incomplete and can not be obtained. However, in this process, solid bonding of these layers is possible, although a solid bond can be obtained between the two layers, the molten alloys constituting the two layers

18565

  are mixed in an exaggerated manner so that, not only does it become impossible to form each layer to the desired thickness, but, in addition, the composition of the alloy constituting the respective layers moves away from that of the Therefore, in the case of the double-layer tube of the present invention, the reaction layer is molded after the coating layer has solidified to the desired double-layered tube. at its inner surface so that an exaggerated mixing of the two layers can not occur and that the aforementioned defects accompanying this mixture can be avoided, despite this molding process (molding of the reaction layer after solidification of the inner surface of the coating layer), the two layers are firmly united to one another. The reason for this result is that the heat-resistant steel used for curing reaction tube of the present invention and whose composition is that defined above has a wide temperature range between the beginning and the end of the solidification and, therefore, even when the molten metal of the reaction layer comes into contact with the solidified inner surface of the coating layer, it does not solidify immediately so that an appropriate thickness of the fused layer is formed at the interface between the reaction layer and the coating layer. at the same time, the coating layer does not remelt excessively and the fused layer reaches the minimum thickness required by firmly bonding the two layers to one another and thus obtaining an ideal structure for

double layer.

  To produce the double-layered tube, it is also possible to use the method consisting, for example, of combining a centrifugal molding with a spraying and, in the first embodiment, by centrifugal molding, a molded tube comprising only a single layer. then it is covered with the desired alloy by spraying this alloy on the desired surface of the tube.

  the above-mentioned molding is used in the above-mentioned way.

  However, it is possible not only to obtain a solid bond between the two layers, but also to give a desired thickness to

16 -

  each layer and, in addition, choose the appropriate chemical composition for the alloy of each layer of material that this

  alloy satisfies the desired characteristics of the material.

  An example of manufacturing the reactor tube-according to the-

  The present invention by centrifugal casting may be one in which the molten metal formed of Ni-Fe-Cr-Ni heat-resistant steel (25.5%) is prepared in a high-frequency induction melting furnace. Cr, -35.0% Ni, 0.45% C, 1.0% Si, 0.8% Mn, 0.66% N, the complement was essentially Fe) used for the coating layer

  and the molten metal formed of Fe-Cr-heat resistant steel

  Mn-Nb (25.5% Cr, 0.6% C, 2.0% Si, 9.1% Mn, 0.45% Nb, 0.05% N, the complement being essentially Fe) used for the reaction layer, 20 kg of said alloy for the melted coating layer is poured into the mold in order to

  centrifugal molding so as to form a coating layer

  It is 134 mm in external diameter, 15 mm in thickness and 500 mm in length and, immediately after the solidification of the inner surface of this layer, 10 kg of molten alloy used for the reaction layer are poured. so as to form a reaction layer 10 mm thick thereby obtaining the reactor tube G having a concentric double-layer structure without the alloys of the outer and outer layers being mixed with each other and with the two

  metallurgically fused layers at their interface.

  Another method of manufacture according to the present invention is that in which, with the aid of the same process for the preparation of molten metal as that mentioned above, the same molten metal preparation process as mentioned above is used to prepare a molten alloy. high Ni-Fe-Cr-Ni heat-treated molten steel (26.0% Cr, 35.9% Ni, 0.44% C, 1.2% Si, 1.0 % Mn, 0.04% M, the complement

  being essentially Fe) for the alloy of the coating layer

  and a heat-resistant cast alloy of Fe-Cr-Mn-NbNi with low Ni content (25.3% Cr, 6.5% Ni, 0.55% C, % Si, 12.2% Mn, 0.65% Nb, 0.06% N, the balance being essentially Fe) for the alloy of the reaction layer and, under the same centrifugal molding conditions as those of the preceding examples, 20 kg of molten coating layer alloy and 10 kg of molten reaction layer alloy are molded to obtain the reactor tube H having a concentric double-layer structure without the inner layer alloys and Outer tubes are mixed with each other and with the two layers united to one another metallurgically. In the case of the aforesaid tubes G and H of the reactor, the inner reaction layers have been treated to obtain a thickness of 2 mm and an inside diameter of 101 mm and a solid carbon deposit test was carried out on each of these tubes (test anti-coking) and a carburization test In all these tests, the test conditions were the same as those used for the tests described previously. In the case of the reactor tube G, the amount of solid carbon deposition was 0, The carburized amount (the increment amount of C) measured at the depths of 0.5 mm, 1.5 mm, 2.5 mm and 5.5 mm from the surface of the wall of the tube was respectively 0.30%,

0.25%, 0.14% and 0.017%.

  In the case of the reactor tube H, the amount of solid carbon deposition was 0.12 mg / cm 2 and the carburized amount (the amount of C increanent) measured at depths of 0.5 mm, 1.5 mm , 2.5 mm and 5.5 mm were respectively 0.25%,

0.21%, 0.11% and 0.05%.

  Both the above-mentioned reactor tubes G and H both had anti-coking properties and anti-carburizing characteristics superior to those of the single-layer reactor tube formed solely by conventional heat-resistant steel and used for the coating. outside (see

  curve C in Figure 6 and column F in Figure 7).

  As previously mentioned, in the reactor tube according to the present invention, the reaction layer is heat-resistant Fe-Cr, Ni-Fe Fe-Cr-Ni steel, Fe-Cr-Mn-Nb or Ni-low Fe-Cr-MnNb-Ni and, therefore, solid carbon deposition on the wall of the tube

  as a result of the chemical reaction of hydrocarbons is neutral

  efficiently, especially when the reaction layer

  It is made of heat-resistant Fe-Cr-Mn-Nb or Fe-Cr-Mn-NbNi steel with low Ni content, solid carbon deposition and carburization are effectively neutralized. of the reaction is covered by the Ni-high Fe-Cr-Ni heat-resistant austenitic steel layer which fuses with said reaction layer, the reactor tube acquires the characteristics of high temperatures which enable it to withstand sufficient measure its use at temperatures exceeding 500 C and at

  pressures above atmospheric pressure.

  quent, this tube allows, when used for cracking

  high temperature and high pressure hydrocarbon

  or mixtures thereof with water vapor, an oxygen-containing gas, etc., to obtain low molecular weight hydrocarbons or for the manufacture of gaseous mixtures containing hydrogen or carbon, to maintain stable operation for a prolonged time without the various disadvantages caused by solid carbon deposits or by deterioration or damage to the

  material of the tube as a result of carburization.

  It is understood that the foregoing description has not

  been given purely as an illustration and not as a limitation

  and that variants or modifications may be

  within the scope of the present invention.

2518565 -

Claims (7)

  1 reactor tube for the thermal cracking or reforming of hydrocarbons, characterized in that: the reaction layer (1) in contact with the hydrocarbons is formed by a heat-resistant steel, comprising the following components in the following proportions expressed in percentages by weight: C: 0.01 to 1.5   If: up to 3 Mn: up to 15 Cr: from 13 to 30 N: up to 0,15   the complement being essentially Fe, and the coating layer   (2) which covers and fuses with said reaction layer at the interface between these two layers is formed by Fe-Cr-Ni heat-resistant steel, comprising the following components in the proportions shown below. after expressed in percentages by weight: C: 0.1 to 0.6   If: up to 2.5 Mn: up to 2 Cr: 20 to 30 Ni: 18 to 40 N: up to 0.15   the complement being essentially Fe.   2 reactor tube according to claim 1, characterized   characterized in that the reaction layer (1) is formed by Fe-Cr heat-resistant steel, the   If reaches up to 2.5% and the Mn content reaches up to 2.0%.   3 reactor tube according to claim 1,   characterized in that the reaction layer is formed by a Ni-low Fe-Cr-Ni heat-resistant steel, whose Si content is up to 2.5%, the Mn content reaches up to at 2.0% and in addition the Ni content reached up to 10%.   4 reactor tube according to claim 2 or 3,   characterized in that a certain amount of Fe is replaced in the reaction layer by one or more elements selected from Mo, W and Nb in a combined amount of up to 5%. Reactor tube according to Claim 1, characterized in that the reaction layer is formed of Fe-Cr-Mn-Nb heat resistant steel with a C content of 0.3 to 1.5. %, the Mn content of 6 to 15%, the Cr content of 20 to 30% and further the Nb content up to 3%. 6. The reactor tube according to claim 1, characterized in that the reaction layer is formed of a low nickel content Fe-Cr-Mn-Nb-Ni heat-resistant steel having a C content. is from 0.3 to 1.5%, the Mn content from 6 to 15%; the Cr content of 20 to 30% and of which the   Ni content reaches up to 10% and the N-content up to 3%.   7 Reactor tube according to any one of the   1 to 6, characterized in that a certain amount of Fe of the coating layer of the reactor tube is replaced by one or more of the elements selected from Mo, W and Nb in a combined amount of up to 5% in weight.   8 Reactor tube according to any one of the   1 to 7, characterized in that the layer of   reaction constitutes the inner layer and the coating layer.   This is the outer layer of the reactor tube.   9 Reactor tube according to any one of the   1 to 7, characterized in that the layer of   reaction constitutes the outer layer and the coating layer.   This is the inner layer of the reactor tube.   Reactor tube according to any one of the   1 to 7, characterized in that the layers of   reaction are formed both on the inside and on the outside.   of the reactor tube and the coating layer is   placed between the reaction layers.