MXPA98003146A - Diffusion coated furnace tubes for the production of ethylene - Google Patents

Abstract

The inner surface of ethylene furnace tubes is diffusion coated with a sufficient amount of chromium or chromium and silicon to form a first coating having a thickness of at least two mils. This coating is then cleaned, neutralized, and grit blasted. Then a second coating of a sufficient amount of aluminum or aluminum and silicon is diffused onto the first coating to form a total coating thickness of at least five mils. The surface of the second coating is cleaned and polished to remove the nickel and iron-rich overlay which is present as a result of the coating process and to provide a smooth uniform surface. When ethylene is produced using furnace tubes which are coated in this manner less coking occurs.

Description

COATED PIPE OVENS FOR DISTRIBUTION FOR THE PRODUCTION OF ETHYLENE FIELD OF THE INVENTION The invention relates to a coating containing chrome-aluminum-silicon diffused on the surface of steel and super-alloys to provide improved corrosion resistance.

BACKGROUND OF THE INVENTION Ethylene is produced by passing a feed containing naphtha and other distillates through an oven comprised of a series of tubes. To achieve the resistance to the creep and the desired oxidation, these tubes are made of superior alloys such as forged alloy 800 series and cast alloys in a centrifugal form such as HP, HK and HH alloys. The feed enters the furnace at a temperature of approximately 100 ° F (37.78 ° C), where it is heated to approximately 1650 ° F (898.89 ° C). During the process, pyrolytic coke is produced. Part of the coke builds up on the walls of the furnace tubes. The nickel in the tubes reacts with the coke to form long filament-like structures, which extend from the walls of the tubes, called catalytic coke. These strands tend to P1195 / 98MX trap the pyrolytic coke that passes through the tubes to form a complex amorphous coke coating on the inner wall of the furnace tubes. This coating acts as an insulator that reduces the temperature of the internal walls of the furnace tubes. Consequently, the furnace must be cleaned periodically to remove this coating. This cleaning is often called decoquizado. In many places the tubes should be cleaned every six weeks. The technique has attempted to control catalytic coking by selecting alloys with a high content of chromium and silicon or, by applying a coating of chromium or aluminum or a ceramic coating to the inner walls of the furnace tube. However, the higher chromium content introduces instability in the alloy structures. Aluminum coatings have found limited success over forged alloys with process temperatures not exceeding 1600 ° F. At higher temperatures, interdiffusion and exfoliation occur. Ceramic coatings suffer from cracking and exfoliation. Coatings of two or more materials have also been proposed for metals used in high temperature process applications. Japanese Patent 80029151 presents a method for applying a P119S / 98MX chromium-aluminum-silicon coating. This coating is produced by a cementing process with chromium packing followed by a cementing process with aluminum-silicon packing. It is said that the coated metal is useful for jet engines, gas turbines and internal combustion engines. In US Pat. No. 3,365,327, a method for steam diffusion coating of metal articles with aluminum-chromium-silicon to provide corrosion resistance at high temperature for gas turbine and refinery applications is presented. of oil. U.S. Patent Nos. 4,500,364 and 4,310,574 disclose methods for applying an aluminum-silicon coating for high temperature process applications. This technique includes a coating with pulp followed by burning or calcination at high temperature. None of these references teaches that these coatings are useful for ethylene furnace tubes. Packing cementing is a well-known technique for applying coatings by diffusion to metal surfaces. This process includes placing a packed mixture in close contact with the surface to be coated and subsequently heating the entire unit at an elevated temperature for a period of time.

P1195 / 98MX of specified time. During heating, the coating material diffuses from the packaging onto the metal surface. A common bale mix used to create a chrome coating contains chrome, an inert filler such as alumina, and a halide activator such as ammonium chloride. The process of cementation with packing is particularly useful for coating internal walls of tubular structures. However, prior to the present invention, the technique has not created a packing cementing process that significantly reduces the formation of catalytic coke deposits on the internal walls of the ethylene furnace tube. The technique has also proposed the co-diffusion of chromium and silicon, of chromium and aluminum or of aluminum and silicon in a cementing process with one-step packaging. These methods have several disadvantages which include the difficulty in obtaining control of the coating composition process by diffusion and the non-uniform thickness of the coating by large scale diffusion, due to the heat transfer limitations of the packaging. Due to the temperature gradients found in a large retort packed with powder, laboratory processes are normally difficult to scale up to commercial processes in a way that provides the thickness of the P119S / 98MX diffusion coating and uniformity of composition over large components. Each time a metal alloy containing nickel, chromium and iron is coated using a diffusion process, an overlay or layer rich in nickel and iron is formed on the coating. In the past no efforts were made to remove this cover or overcoat. However, we have discovered that the overcoat promotes coking when it is present in ethylene furnace tubes. Accordingly, there is a need for an effective method for treating high alloy ethylene furnace tubes in order to reduce catalytic coking.

SUMMARY OF THE INVENTION We provide a method for coating the inner surface of ethylene furnace tubes, in which we diffuse a sufficient amount of chromium or chromium and silicon on the inner surface of the tube to form a first coating having a thickness of at least two thousandths of an inch. This surface of the coating is then cleaned, neutralized and sandblasted. Then we spread a sufficient amount of aluminum or aluminum and silicon over the first coating to form a second coating P1195 / 98MX having a total coating thickness of two stages of at least five thousandths of an inch. Finally, we clean and polish the second coating eliminating the overcoat rich in nickel and iron and providing a smooth and uniform surface. The coatings are preferably applied using cementing with packing or diffusion by thermal spraying. Other embodiments for transporting and applying the coating elements to the surface of the tube include inserts and gels of ceramic compounds.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a partially sectioned perspective view of an oven tube containing a package for applying a first coating in accordance with a first preferred embodiment; Figure 2 is a perspective view similar to Figure 1 showing the application of the second coating in accordance with the first preferred embodiment. Figure 3 is a cross-sectional view of a portion of an oven tube to which our coating has been applied; and Figure 4 is a perspective view showing an alternative method to apply our P119S / 98MX coating to a furnace tube.

DESCRIPTION OF PREFERRED MODALITIES We provide improved ethylene furnace tubes that will reduce pyrolytic coking and reduce decoking times in ethylene furnaces. These tubes are provided with a diffusion coating on their internal wall. The diffusion coating is applied in two stages. Referring to Figure 1, we illustrate an oven tube 2 that can be of any desired length and can include both straight portions as bends or return bends. The tube is filled with a packing mixture composition 4 containing chromium or chromium and silicon together with a binder such as aluminum oxide and an activator such as ammonium chloride. The caps 6 are placed on each end of the tube. The capped tube is then treated in a retort oven at a temperature and time high enough to form a chromium or chromium-silicon coating on the inner surface of the tube 2. After the diffusion coating has sufficiently cooled, we clean perfectly, neutralize and sandblast the coating. This provides a first coating surface that is receptive to the second stage of P1195 / 98MX coating. The second coating step is either an aluminum diffusion coating alone or a silicon aluminum combination. As shown in Figure 2, tube 2 is provided, which has an internal surface 8 containing the chromium or chromium-silicon coating indicated by the dotted surface. A spraying diffusion head 10 is inserted into the tube. This head provides a thermal spray 12 of aluminum or a combination of silicon aluminum. The spray forms the second coating on the first coating. In Figure 3, we show a cross section of the coated tube. The tube 2 has a first coating layer 9 of chromium or chromium and silicon. This coating must be at least 2 mils thick. At the top of the first coating 9, there is a second coating only of aluminum or a combination of silicon aluminum 11. The layer 11 must also have a thickness of at least 2 mils. Furthermore, we prefer that the combined thickness of the first coating and the second coating be at least 5 mils. After the application of the final layer 11, the internal surface is polished to remove the nickel-rich overcoat and iron, thereby minimizing the nucleation sites for coke deposition. The welding of P1195 / 98MX pipes are achieved using a special chamfered preparation and the typical welding and purge wire techniques historically used for the manufacture of the ethylene furnace tube. We have found that ethylene furnace tubes coated in accordance with the present invention have significantly less catalytic coking. For purposes of illustration, in Figure 3 we show two distinct layers 9 and 11 of uniform thickness. Those skilled in the art will understand that some diffusion will occur between the layers to create a strong bond. For the first stage coating, we created a diffusion chrome coating approximately 5 mils thick on the modified HP-40 Nb (Niobio) cast alloy tubes cleaned and eroded or sandblasted using a composition of packaging mixture of 48% by weight of chromium, 4% by weight of ammonium chloride, and 48% by weight of aluminum oxide. This packing was placed in the tubes, which were sealed in a retort and heated to 2200 ° F for 10 hours under an inert argon atmosphere. The surfaces of the tubes were then neutralized with an alkaline solution of pH 12, cleaned and eroded or abraded by jetting.

P119S / 98MX sand. For the coating of the second stage, we thermally spray with wire and arc, the surfaces of the chromed tubes with 5 to 7 mils of an alloy composed of 88% by weight of aluminum and 12% by weight of silicon. The resulting coated HP-40 tubes were thermally treated by diffusion under an internal argon atmosphere at 2000 ° F (1,093.33 ° C) for 3 hours. Upon completion of the diffusion heat treatment, the tubes were cleaned and abraded or eroded with sandblasting. Metallographic evaluation of a modified HP-40 Nb cast alloy tube coated with chromium-aluminum-silicon diffusion revealed an average coating thickness of 15 thousandths of an inch as determined by light microscopy, with a composition that includes 75% by weight of chromium, 2% by weight of aluminum and 17% by weight of silicon on the surface, as determined by scanning electron microscopy / dispersive energy spectrometry. At five thousandths in the coating, the composition was shifted to 10% by weight of chromium, 26% by weight of aluminum and 2% by weight of silicon. The nominal composition of the base alloy was reached at a depth of 18 thousandths of an inch below the surface of the coating. Thermal cycling experiments were conducted on modified HP-40 Nb tubes with coating P1195 / 98MX chromium-aluminum-silicon diffusion. These experiments involved heating in a furnace with an atmosphere of air from room temperature to 1850 ° F (1010 ° C) at a rate or speed of 9 ° F (-1278 ° C) / min, holding it at 1850 ° F (1010 ° C). C) for two hours, and then cooling all night by turning off the oven. A total of 60 cycles were conducted. The samples were weighed initially and after every five cycles and also at the end of the test. Signs of flake formation, discoloration, exfoliation, etc. were also examined visually. With optical and scanning electron microscopes, small sections of coated test specimens were examined and after the thermal cycles. There was no exfoliation or internal oxidation of the coating by diffusion of chromium-aluminum-silicon onto the modified HP-40 Nb substrate, which frequently occurs when only aluminum or aluminum-silicon is diffused and subjected to severe thermal cycles. The integrity of the coating by diffusion was exceptional. There was some interdiffusion (continued diffusion) of the elements of the coating. After the 60 thermal cycles, the thickness of the diffusion zone increased by 5 to 10 percent or, one thousandth of an inch. As an alternative to a packaging mix, P119S / 98MX can be used a composite ceramic or composite metal insert. As shown in Figure 4, this insert 20 is placed inside the tube 2. The tube is then capped or wrapped with tape and heated at an elevated temperature for a period of time to form the coating by diffusion. The composite insert will contain selected proportions of chromium-silicon or aluminum-silicon with an activator, an inert filler and a binder. After the tube 22 containing the insert 20 is heated for a sufficient period of time to form the desired coating by diffusion, the tube is cooled and the insert 20 is removed. After which, the coating is cleaned, neutralized and cleaned or sandblasted. The second coating containing aluminum or a silicon aluminum combination is then applied. This second coating can be applied using spray deposition as shown in Figure 2 or with cementing by packing or using an insert or composite gel. The present method is useful for both cast and forged kiln tubes. Our test revealed that the ethylene oven tubes coated in accordance with the present method resist catalytic coking better than other coated tubes currently in use. We attribute P1195 / 98MX this performance to the fact that our coating and coating process minimizes the nickel and iron present on the surface of the tube. While we have described and illustrated certain currently preferred embodiments of our methods for diffusion coating of ethylene furnace tubes, it should be understood in a distinctive manner that our invention is not limited thereto but can be incorporated in a variety of ways within the scope of the invention. scope of the following claims.

P119S / 98MX

Claims (20)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A method for coating a surface of a metal product formed from an alloy. containing nickel, chromium and iron, comprising: a. Spreading a sufficient amount of chromium on the surface to form a first coating having a thickness of at least two thousandths of an inch; b. clean the first coating; c. roughing the first coating; d. spreading a sufficient amount of aluminum on the first coating to form a second coating of aluminum having a thickness of at least two thousandths of an inch and a superimposed layer rich in nickel and iron; and e. polish the second coating to eliminate the overlay rich in nickel and iron. The method according to claim 1, wherein the first coating and the second coating have a combined thickness of at least 5 mils. 3. The method according to claim 1, which P1195 / 98MX also comprises the step of co-diffusing silicon with chromium to form the first coating. The method according to claim 1, which also comprises the step of co-diffusing silicon with aluminum to form the second coating. The method according to claim 1, wherein at least one of the first coating and the second coating are applied by a surface chemical diffusion process. 6. The method according to claim 5, wherein the surface chemical diffusion process is the packing cementation. The method according to claim 1, wherein at least the first coating and the second coating are applied by thermal spraying. The method according to claim 1, wherein the surface is an inner wall of a tube and at least one of the first coating or the second coating is applied by: a. place a packaged mixture containing at least one of chromium or aluminum in the tube; b. close both ends of the tube; and c. heating the tube to a high temperature for a sufficient time to create the coating by diffusion on the inner wall. P1195 / 98MX 9. The method according to claim 1, wherein the surface is an inner wall of a tube and at least one of the first coating or the second coating is applied by: a. placing in the tube an insert formed from a compound of: one of a metal or a ceramic, at least one coating material selected from the group consisting of aluminum, chromium and silicon, an activator, a filler, and a binder; b. close both ends of the tube; and c. heating the tube to an elevated temperature for a sufficient time to create a diffusion coating on the inner wall. The method according to claim 1, wherein the roughing of the first coating is achieved by cleaning or eroding with sandblasting. 11. An improved tube for ethylene furnaces comprising a tubular member formed of an alloy containing nickel, chromium and iron and a coating on an inner surface of the tubular member, the coating is formed by the steps of: P1195 / 98MX a. spreading a sufficient amount of chromium on the inner surface to form a first coating having a thickness of at least two thousandths of an inch; b. clean the first coating; c. roughing the first coating; d. spreading a sufficient amount of aluminum on the first coating to form a first coating of aluminum having a thickness of at least two thousandths of an inch and an overlay rich in nickel and iron; and e. polish the second coating to eliminate the overlay rich in nickel and iron. 12. The improved tube according to claim 11, wherein the first coating and the second coating have a combined thickness of at least 5 mils. The improved tube according to claim 11, wherein the silicon is co-diffused with chromium to form the first coating. 14. The improved tube according to claim 11, wherein the silicon is co-diffused with aluminum to form the second coating. 15. The improved tube according to claim 11, wherein the roughing of the first coating is achieved P119S / 98MX by eroding or sand blasting. 16. An improved ethylene furnace of the type comprising a plurality of tubes formed of an alloy containing nickel, chromium and iron and in which the feed is passed and heated, wherein the improvement comprises a coating on an inner surface of at least one of the tubes, the coating is formed by the steps of: a. spreading a sufficient amount of chromium on the inner surface to form a first coating having a thickness of at least two thousandths of an inch; b. clean the first coating; c. roughing the first coating; d. spreading a sufficient amount of aluminum on the first coating to form a second coating of aluminum having a thickness of at least two thousandths of an inch and a superimposed layer rich in nickel and iron; and e. polish the second coating to eliminate the overlay rich in nickel and iron. The improved ethylene furnace according to claim 16, wherein the first coating and the second coating have a combined thickness of at least 5 mils. P1195 / 98MX 18. The improved ethylene furnace according to claim 16, wherein the silicon is co-diffused with chromium to form the first coating. 19. The improved ethylene furnace according to claim 16, wherein the silicon is co-diffused with aluminum to form the second coating. The improved ethylene furnace according to claim 16, wherein the roughing of the first coating is achieved by sandblasting or erosion. P1195 / 98MX