Coking and carburization resistant iron aluminides for...

Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...

Reexamination Certificate

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C208S132000, C585S648000, C585S649000, C585S650000, C585S920000, C422S200000, C422S201000, C420S077000, C420S079000, C420S081000

Reexamination Certificate

active

06830676

ABSTRACT:

BACKGROUND
1. Field of the Invention
The present invention is directed generally to apparatus for producing chemical products. More specifically, the present invention is directed to tubular products for cracking of hydrocarbon feedstock.
2. Background Information
Chemical reactions can be performed by flowing reagents along a tube maintained at high temperature and disposed inside a radiation zone of a furnace. More particularly, methods such as “steam pyrolysis” or “steam cracking” are known in which a diluent fluid such as steam is usually combined with a hydrocarbon feed and introduced into a cracking tube of a cracking furnace. Within the cracking furnace, the feedstock is converted to a gaseous mixture, which upon exiting the cracking furnace is cooled to remove most of the heavier gases and is compressed. The compressed mixture is routed through various distillation columns where the individual components such as ethylene are purified and separated. For example, in ethylene production, naphtha, ethane, butane or like starting material and steam are charged into a cracking tube as feedstock and radiantly heated from the outside to a high temperature in excess of 1000° C. to crack the material within the cracking tube.
One recognized problem in thermal cracking is the formation of coke that can foul the cracking tube. Coking is a surface phenomenon and is generally observed with iron and nickel-based alloys containing chromium in the range of 10 to 25 wt. %. Because coke is a poor thermal conductor, as coke is deposited in the cracking tube, higher furnace temperatures are required to maintain the gas temperature in the cracking zone at necessary levels. Higher temperatures can increase feed consumption and shorten cracking tube life. Additionally, coking can result in an excessive pressure drop across the cracking tube thereby adversely affecting cracking tube performance.
Under such conditions, it is necessary to stop the reaction on a regular basis in order to remove the deposits of by-products. In the particular case of a steam cracking reaction, such removal is performed by a decoking operation on the cracking tube, such as steam decoking and steam-air decoking. This operation consists in causing a mixture of air and steam to flow inside the cracking tube at a temperature that is sufficiently high to burn off and remove the coke. In practice, it is observed that a decoking operation takes a relatively long period of time, the total time required can approach a minimum of 48 hours, and that it is nevertheless desirable to decoke a tube at a high frequency, usually close to once every two or three months, and that such decoking operations result in a significant loss of production.
Another problem recognized in thermal cracking operations is carburization. Carburization results in the formation of carbides in the metal matrix or in grain boundaries of metallic components from exposure to a carbon containing atmosphere. Carburization is severe with alloys such as HP steels, INCO 803, and other materials that contain significant amounts of chromium and nickel with very little aluminum content in the alloy.
Carbides can embrittle steel walls in the cracking tube and the reaction system leading to metallurgical failure. In service, carburization can result in the loss of mechanical properties over time. Carburization can have an influence on the corrosion behavior as well, as carbon can react with chromium and locally deplete the metal matrix of chromium, making it more sensitive to corrosion. Since conventional cracking tubes have very little aluminum content, no diffusional resistance due to aluminum oxide (Al
2
O
3
) exists at the operating temperatures leading to carburization failure. However, because coking can lead to increased pressure in the tube and carburization can lead to the degradation of mechanical properties, the combined coking and carburization can lead to catastrophic failure, such as explosions, of tubes and is a safety hazard
A third problem associated with thermal cracking is materials based. The cracking tube undergoes expansion during the change in temperature of the material directly related to the coefficient of thermal expansion (&agr;) of the cracking tube material. However, such expansion can result in deleterious stresses forming in the metallurgical components of the reaction system due to the mismatch of the coefficient of thermal expansion between collocated and joined components of different material composition. In an extreme example, cracking tubes may expand by upwards of several percent, causing bowing, cracking, and even rupturing of the systems. This is particularly undesirable in cracking tubes using coatings or linings of materials with mismatched coefficients of thermal expansion, such as the chromium layer disposed on a cracking tube of HP-50 steel disclosed in U.S. Pat. No. 5,833,838.
A variety of solutions have been proposed for addressing the problem of coke formation and carburization in thermal cracking processes. Many of these are associated with using novel steel types, especially alloys. See for example, U.S. Pat. No. 4,762,681 to Tassen et al. and U.S. Pat. No. 4,976,932 to Maeda et al. Others utilize antifoulants, for example, U.S. Pat. No. 4,507,196 to Reed et al. which describes certain chromium antifoulants, and antifoulants which are combinations of chromium and tin; antimony and chromium; and tin, antimony, and chromium.
Methods to protect metal surfaces from carburization are also known. GB 1,604,604 to Perugini et al., discloses protecting metal surfaces against carburization by application by plasma spray deposition of a chromium layer. GB 1,149,163 to ICI, discloses methods of protecting against carburization including aluminizing and chrominizing steels containing iron, chromium, and nickel. U.S. Pat. No. 5,833,838 discloses the use of a Group VIB metal protective layer to improve the carburization and coking resistance of steels in cracking applications.
However, none of these approaches address the need to match the coefficient of thermal expansion of the various materials of the cracking tube nor doe they suggest the advantages of iron aluminide materials. Thus, it is advantageous to limit the deposition of by-products on the inside wall of the cracking tube and to inhibit the carburization of system metallurgical components. Additionally, it is advantageous that the cracking tube material exhibit excellent strength (especially in creep rupture strength) at high temperatures and oxidation resistance. Thirdly, it is desirable to have a cracking tube made from a material that exhibits a coefficient of thermal expansion that is compatible with other reaction system components.
SUMMARY OF THE INVENTION
The present invention relates to improvements in fouling resistant and corrosion resistant alloys which are useful as materials for thermal cracking or reforming reactor tubes for hydrocarbons, such as ethylene production cracking tubes and reformer tubes. More particularly, the invention relates to a lined pipe or tube having an inner lining of a fouling resistant and corrosion resistant alloy having high resistance to coking and carburization and a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of an outer body of at least a second material used in a cracking tube over the temperature range of ambient to about 1200° C.
Exemplary embodiments of the present invention are directed to providing a cracking tube with an iron aluminide first layer disposed as a lining on an inner surface of the cracking tube. The lining has high resistance to coking and carburization and a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of an outer tube.
In accordance with exemplary embodiments, an exemplary iron aluminide alloy for the lining includes 14-32 wt. % Al, 1-20 vol. %, preferably 10-14 vol. % transition metal oxides, optional ≦1 wt. % Cr, and the balance including Fe. Transition metal oxides include oxides of aluminum, ytt

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