Radiant tube in cracking furnaces

Abstract

Radiant tube in a cracking furnace is disclosed. Hydrocarbons are cracked into ethylene in the tube through which hydrocarbons flow, which tube is heated from the outside to a temperature where cracking occurs. The tube is made from a FeCrAl material containing, in addition to Fe, 10 to 25% by weight of Cr, 1 to 10% by weight of Al and 1.5 to 5% by weight of Mo. Optionally, the material comprises up to a total of 2.2 wt. %, preferably up to 2.0 wt. % and more preferably up to 1.0 wt. %, of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

Description

FIELD OF THE DISCLOSURE

The present disclosure relates to a radiant tube formed of a material that meets requirements such as creep strength, ductility and service life in cracking furnaces, preferably furnaces for ethylene cracking.

BACKGROUND

In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.

In cracking furnaces or ethylene furnaces, hydrocarbons are cracked into ethylene, H₂C═H₂C, which constitutes a raw material for the plastics industry.

The cracking takes place by hydrocarbons being led through a radiant tube, which has been heated by means of surrounding burner to sufficiently high temperature, for instance, about 1,100° C., for ethylene being formed in the tube.

Typically, such a tube should be able to resist said high temperatures during a long time. Therefore, the tube usually is made from a FeCrAl material. One reason for the same material being used is, in addition to the heat-and-cold resistance, that a dense aluminum oxide Al₂O₃ is formed on the outside and inside of the tube. This oxide prevents Fe contacting the gas flowing in the tube when Fe acts as a catalyst for coke formation in the tube. If coke is formed in the tube, the same is deposited on the inside of the tube, the flow of gas in the tube becoming smaller and the heat transfer to the interior of the tube becoming lower. Coke may also entirely clog the tube.

In Swedish Patent No. 467 414, a FeCrAl material is disclosed where it is stated that the creep strength is increased by the fact that the material is alloyed with cobalt, nickel, silicon, manganese, zirconium and titanium as well as a small quantity of yttrium and hafnium, and then the material is heat-treated at minimum 1050° C. until very elongate granules are formed having a length of at least 5 mm and a ratio of length to cross-section of at least 10 to 1. Radiant tubes are manufactured from this material.

In the Swedish Patent No. 513 989, a method for the manufacture of a FeCrAl material by gas atomization is disclosed. A problem in gas atomization is that in the manufacture of a FeCrAl material containing titanium, small particles of TiN and TiC are formed in the charge before the atomization, which particles are caught on the melt nozzle that is used for the atomization and that the same is clogged. Swedish Patent No. 513 989 proposes that the charge that is to be atomized being brought to contain 0.05 to 0.50% by weight of tantalum and simultaneously less than 0.10% by weight of titanium. In Swedish Patent No. 513 989, it is stated that the material may contain molybdenum (Mo).

Above, it was stated that the creep strength was increased by means of a material according to Swedish Patent No. 467 414. However, there are additionally increased requirements on creep strength, ductility, as well as increased requirements on the service life of said tube. It would also be desirable to be able to manufacture longer tubes, which resist the high temperature during longer time than known tubes.

SUMMARY

Thus, the present disclosure relates to a radiant tube in a cracking furnace, where hydrocarbons are cracked into ethylene in a tube through which hydrocarbons flow, which tube is heated from the outside to a temperature where cracking of hydrocarbons occurs, and is characterized in that the tube is made from a FeCrAl material containing, in addition to Fe, 10 to 25% by weight of Cr, 1 to 10% by weight of Al and 1.5 to 5% by weight of Mo. Optionally, the material comprises up to a total of 2.2 wt. %, preferably up to 2.0 wt. % and more preferably up to 1.0 wt. %, of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Present embodiments of a radiant tube is found in a cracking furnace, where hydrocarbons are cracked into ethylene in a tube through which hydrocarbons flow. The tube is heated from the outside to a temperature where cracking occurs. Typically, the temperature of the tube, at the inlet end of the gas, is 900° C. and at the outlet end 1,125° C.

According to an exemplary embodiment, the tube is made from a FeCrAl material containing, in addition to Fe, 10 to 25% by weight of Cr, 1 to 10% by weight of Al and 1.5 to 5% by weight of Mo.

It is preferred that the material contains smaller fractions of one or more of the alloying materials, tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen. For example, in exemplary embodiments, the material comprises up to a total of 2.2 wt. %, preferably up to 2.0 wt. % and more preferably up to 1.0 wt. %, of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

According to a highly preferred embodiment, the material contains 2 to 3.5% by weight of molybdenum. The reason that the content of molybdenum should not be too high, but yet give strength-wise advantages in comparison with the material according to said Swedish Patent No. 467 414, is that at too high a content of molybdenum, a volatile molybdenum oxide MoO₃ is formed, which is gasified and evaporated. Thereby, the content of molybdenum is decreased, with a decreased strength as a consequence.

According to a preferred embodiment, the tube is made from a material having a creep strength high enough that the time to break of the tube at a temperature of 1,100° C. and a load of 2.2 MPa exceeds 100,000 hours. This corresponds to substantially a twice as high a creep strength in comparison with tubes made from the material disclosed in the Swedish Patent No. 467 414.

The higher creep strength entails a corresponding considerable increase of the service life of the tube.

This higher strength means that the tube may be made in a longer length than conventional cracker tubes. A cracker tube may typically be 10 to 17 meters long. Such a tube consists of two or more tubes welded together in the longitudinal direction. According to a preferred embodiment, the tube is formed in one piece having a length exceeding 10 meters.

Additional exemplary embodiments of the tube may be varied in respect of the material composition, above all regarding alloying materials having a low percentage in the material.

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A radiant tube in a furnace for cracking hydrocarbons, the radiant tube formed of a material comprising:

10 to 25 wt. % Cr;

1 to 10 wt. % Al;

1.5 to 5 wt. % Mo; and

balance Fe.

2. The radiant tube of claim 1, wherein the material comprises up to a total of 2.2 wt. % of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

3. The radiant tube of claim 2, wherein the material comprises up to a total of 2.0 wt. % of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

4. The radiant tube of claim 3, wherein the material comprises up to a total of 1.0 wt. % of one or more alloying materials selected from the group consisting of tantalum, hafnium, zirconium, yttrium, nitrogen, carbon and oxygen.

5. The radiant tube of claim 1, wherein the material comprises 2 to 3.5 wt. % of molybdenum.

6. The radiant tube of claim 1, wherein the material has a creep strength sufficiently high to produce a time to break of the tube in excess of 100,000 hours at a temperature of 1100° C. and a load of 2.2 MPa.