Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-10-27
2003-03-25
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C178S038000, C427S250000, C427S252000, C427S376200, C427S376800
Reexamination Certificate
active
06537388
ABSTRACT:
FIELD OF INVENTION
The invention relates to an alloy system applied as a coating on metal tubes used in high temperature applications such as ethylene production to resist carburization, inhibit catalytic coke formation, and resist coke fouling.
BACKGROUND OF THE INVENTION
Ethylene is produced by passing a feedstock containing naphtha, ethane, and other distillates through a furnace comprised of a series of tubes. In the production of ethylene there are components that operate at elevated temperatures such as the cracking furnace, transfer piping, quench exchangers and transfer line exchangers (TLEs). These components are exposed to a high temperature environment that can operate in reducing or oxidizing or alternately between both conditions. To achieve desired creep strength, mechanical requirements, and oxidation resistance, the furnace tubes are made of higher alloys such as the wrought Alloy 800 series, and centrifugally or static cast alloys such as HK, HP and 45Ni—35Cr alloys. The feedstock enters the furnace at a temperature of about 1000° F. (540° C.) where it is heated to about 1650° F. (900° C.). During the process pyrolytic coke is produced. Some of the coke accumulates on the walls of the furnace tubes. Nickel and iron in the tubes react with the coke to form long whisker-like structures that extend from the walls of the tubes called catalytic coke. These strands tend to catch pyrolytic coke passing through the tubes to form a complex amorphous coke layer on the inner wall of the furnace tubes. This amorphous coke layer acts as an insulator increasing the temperature of the inner walls in order to deliver adequate heat to the process stream to crack the feedstock. Consequently, the furnace must be periodically cleaned to remove this layer of coke. This cleaning is often called de-coking. At many locations the tubes must be cleaned every 30 days.
1. Brief Description of the Prior Art
The art has attempted to control catalytic coking by the selection of high chromium and nickel alloys with significant silicon content or by applying a chromium or aluminum or ceramic coating to the inner walls of the furnace tube. However, higher chromium coatings introduce instability in the alloy structures. Aluminum coatings have found limited success on wrought alloys with process temperatures not exceeding 1650° F. (900° C.). At higher temperatures inter-diffusion and spalling can occur. Solid ceramic coatings suffer from cracking and spalling.
Coatings of two or more materials have also been proposed for metals used in high temperature process applications. Bessen in U.S. Pat. No. 4,087,589 discloses methods for applying a chromium coating, an aluminum coating or a chromium coating followed by an aluminum coating on a nickel base alloy. In Japanese Patent 80029151 there is disclosed a method for applying a chromium-aluminum-silicon coating. This coating is produced by a chromium pack cementation process followed by an aluminum-silicon pack cementation process. The coated metal is said to be useful for jet engines, gas turbines and internal combustion engines. In U.S. Pat. No. 3,365,327 there is disclosed a method for vapor diffusion coating metallic articles with aluminum-chromium-silicon to provide elevated temperature corrosion resistance for gas turbine and oil refinery applications. The technique involves a slurry coating followed by high temperature firing. There is no teaching in any of these references that such coatings would be useful for ethylene furnace tubes.
In our U.S. Pat. No. 5,873,951 we disclose a method for coating ethylene furnace tubes made from iron-nickel-chromium alloys in which we apply a chromium diffusion coating, clean and roughen that coating, apply a second coating containing aluminum that also has a nickel and iron-rich overlay and then we polish the second coating to remove that overlay. In our U.S. patent application Ser. No. 09/255,596 now U.S. Pat. No. 6,139,647 we disclose a method of coating products formed from an iron-nickel-chromium alloy in which we expose the surface of the alloy to hydrogen to remove diffusion limiting oxides and then diffuse chromium or other metals onto the prepared surface.
The prior art as a whole, thus, teaches various methods of applying coatings containing chromium or aluminum to nickel based alloy tubes and other products. The references also report the thickness of the coatings and even the principal elements in the coatings. Many of the prior art chromium and aluminum coatings have been effective to some extent in resisting corrosion and in reducing carburization and coking problems. Nevertheless, these coatings continually fail after some period of time, and others must be cleaned at regular intervals that could be as short as 30 days. Thus, there is a need for coatings which last longer and resist fouling or coke build-up for longer periods of time.
The failure of the art to develop longer lasting coating systems, particularly ethylene furnace tubes is the result of a lack of understanding of what happens to these coatings over time. Although several people have attempted to understand and explain why chromium or aluminum based coatings fail in high temperature applications, there has been only limited understanding of what makes a long-lasting coating for ethylene furnace tubes and other products exposed to a high temperature environment in reducing or oxidizing environments. We have learned that a greater understanding of these systems could only come from a long term study of chromium and aluminum based coatings on tubes used in an ethylene production furnace.
2. Carburization and Catalytic Coking
The production process of making ethylene from hydrocarbon feedstocks, such as ethane, propane, naphtha and mixed precursors, creates violent thermal cracking of the hydrocarbon feedstock causing the liberation of carbon species. The carbon is in the form of CO (carbon monoxide) and CO
2
(carbon dioxide) which has the propensity to diffuse into the base material. This action of carbon ingress causes the depletion of chromium due to the formation of chromium carbides and it exposes the other elements in the base material to the process environment.
The stages and mechanisms of carburization in iron-nickel-chromium alloys are taught in the paper “Carburization of high chromium alloys” by Ramanarayanan, Petkovic, Mumford and Ozekein, as well as in the paper “Carburization—introductory survey” by Rahmel, Grabke and Steinkusch. Ramanarayanan et al. discuss the oxygen chemical potential and the carbon chemical potential, the key point being that there must be enough of an oxide (Cr
2
O
4
-spinel or Cr
2
O
3
) to resist the ingress of carbon. Rahmel et al. discuss the need for sufficient chromium on the surface, and the instability of chromium oxides with materials that have less than 30 weight percent chromium in their base metallurgy. The solubility of carbon in chromium is determined by the amount of chromium in the substrate alloy. For example, the solubility of carbon in an alloy containing 32 weight percent chromium is less than 0.02 weight percent carbon. The solubility of carbon in chromium approaches zero when the chromium content is approximately 40 weight percent. Based on these data, the conclusion that can be drawn here is that if there is a stable Cr
2
O
3
layer then there will be no carbon ingress or permeation into the base material.
The exposure of nickel and iron to such high carbon activity process gases is known to cause what is called catalytic coking, growing from the metal species to form filamentous coke. This filamentous coke is most detrimental to the process efficiencies because there is extra CO generated and there is a rapid collection of naturally occurring thermal coke adhering and building up on the tube wall. This coke build-up creates short run times and limits the amount of ethylene that can be produced with a given amount of energy input. There is also the problem of long decoke times of a furnace due to the presence of catalytic coke and excess carbon in the base metal. All of th
Bayer George T.
Wynns Kim A.
Alon, Inc.
Buchanan Ingersoll P.C.
Oltmans Andrew L.
Sheehan John
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