Cracking Furnace

Abstract: Furnace for cracking of hydrocarbons containing one or more tubes, which are made from an alloy having 15-30 weight % Cr, 3-10 weight % aluminium, not more than weight 1% of one or more of zirconium, titanium, hafnium, cerium and calcium, the balance being iron. The insides of the tubes may be covered by aluminium oxide layers obtained by preoxidation of the tubes.

Description: This invention is for a furnace for cracking of hydrocarbons. Such a furnace has one or mostly several tubes, through which the hydrocarbons flow during intensive heating and cracking. Furnaces according to the invention have tubes which make possible longer operational times between exchange of tubes and higher working temperature in the furnace than is possible by prior art furnace designs. Prior art furnaces for cracking of hydrocarbons have tubes made from nickel base alloys with relatively high chromium contents. This brings with it several disadvantages as the tube material is expensive and does not have a fully satisfying resistance to carburization and formation of carbides, primarily chromium carbide. Further the shape durability of these tubes, which are designated as high temperature material, is not fully sufficient in some applications. A cracker is used for cracking of hydrocarbons. The starting material can be e.g. nafta or propane mixed with a smaller amount of steam. When the gases pass through the tubes in the cracking furnace its temperature is increased up to about 850°C. Among important products which are obtained are ethylene and propene. Further hydrogen, methane, buthene and other hydrocarbons are obtained. In order to avoid unwanted reactions it is essential that the heating is very rapid and that the products which are obtained are thereafter rapidly cooled. The residence time in the furnace is only a few tenth of a second. The temperature in the furnace is 1100 - 1200 °C and the temperature of the goods in the tubes in the furnace can be more than 1100 °C. Heating of the furnace can be performed by burning gases from the cracking process, e.g. hydrogen and methane and a furnace may be equipped with a great number of burners, which can be positioned in the bottom and sides of the furnace. The tubes which are used in the furnace shall have the ability to withstand the high temperatures with a good shape durability. They must also be resistant against oxidation and corrosion in order to tolerate the atmosphere in the furnace. The carbon potential inside the tubes in the furnace is very high and the tube material should therefore be resistant against carburization and formation of carbides. Small amounts of sulphur are often added to the starting materials and the tubes must then also be resistant to sulphur and sulphur compounds. On the inside of the tubes there are also deposits of carbon and coke which may cause local temperature variations. These deposits may be removed suitably by oxidation with steam. The present invention is directed to a furnace having tubes of a material which has considerably improved resistance against the conditions in the furnace. A furnace according to the invention has the characteristics mentioned in claim 1. Other embodiments of the invention have the characteristics which are mentioned in the dependent claims. A furnace according to the invention has tubes made from an alloy having 15-30 weight % chromium, 3-10 weight % aluminium, balance mainly iron. The alloy also comprises the usual impurities and possible smaller amounts of other alloying components. When these tubes are exposed to oxidizing conditions at high temperature, aluminium oxide is formed on the surface and suitably at least the inside of the tubes have a layer of aluminium oxide before the furnace is used in production. In spite of the very high carbon potential inside the tubes during the process it has shown that such tubes have a very good resistance to carburization and formation of carbides such as chromium carbide. The tubes also have excellent resistance against sulphur and sulphur compounds which are added to the hydrocarbons in small amounts in order to prevent carburization of the tube material. A furnace according to the invention also has such properties that the addition of sulphur can be unnecessary. Suitably the tubes are in many cases made from an alloy which also includes up to 1 weight % of one or more of yttrium, zirconium, titanium, hafnium, cerium and calcium. Such additives have been found to improve the properties of the aluminium oxide layer. It has also turned out that among others the shape durability is very good when seemless tubes, produced by extrusion, are used. For this purpose it is advantageous to use billets made by powder metallurgical methods. Such tubes have high heat resistance by extremely high temperatures. The temperature of the goods in the tubes may with acceptable shape durability be up to about 1300 °C, which is considerably higher than what has hereto been possible in this kind of furnaces. It should be mentioned that heating tubes for furnaces, comprising extruded, seamless tubes of a FeCrAl alloy, are previously known as such, e.g. from EP-A-0321427. However, these tubes are heated at the inside by means of an electric resistance element or by a gas burner disposed at the inside of the tube, which is designed to radiate heat from the outside thereof to the furnace chamber. Accordingly, such a furnace is totally different from the one according to the present invention, where hydrocarbons flow through the interior of the tubes so as to be heated and cracked. Furthermore, the documents GB-A-382355, EP-A-0091526 and EP-A-0035369 also disclose substantially nickel-free alloys for use in furnaces at elevated temperatures. However, none of these documents indicate the use of seamless steel tubes formed by extrusion of powder metallurgical billets. Nor do these documents indicate an aluminium oxide layer obtained by preoxidation at the inside surface of each tube before the furnace is taken into operation. The materials which are used for the tubes of a furnace according to the invention have, compared to prior art materials, a high electrical resistance. It is therefore possible to perform the heating wholly or partly by passing current directly through the tubes. The heat transfer from the walls of the tubes to the gas inside the tubes is mainly by radiation. As mentioned above it is essential that the heating is very rapid and it may therefore be suitable to enlarge the radiating internal surface of the tubes by making the insides with projections in the shape of longitudinal bars or ribs. When extruding these can be directly obtained by the shape of the extrusion dies.

Claims: A furnace for cracking hydrocarbons, comprising one or more heat radiaton tubes, through which the hydrocarbons flow during intensive heating and cracking, said tubes being made from a FeCrAl alloy, characterized in that said alloy comprises 15-30 weight% Cr, 3-10 weight% Al, the balance being mainly iron and minor amounts of other alloying components, the inside surfaces of the tubes are covered by aluminium oxide layers obtained by preoxidation of the tubes before the furnace is taken into operation, and the tubes are seamless and are produced by extrusion of powder metallurgical billets so as to permit operating temperatures up to 1300°C, the heat being transferred to the flowing hydrocarbons mainly by inward radiation from the inside wall surfaces of the tubes. Furnace according to claim 1, characterized in that the alloy comprises 15-30% weight% Cr, 3 - 10 weight% Al and a total of not more than 1 weight% of one or more of yttrium, zirconium, titanium, hafnium, cerium and calcium. Furnace according to any one of the preceding claims, characterized in that the heating is obtained by direct current flow in the walls of the tubes. Furnace according to any one of the preceding claims, characterized in that the inside walls of the tubes have protrusions in order to enlarge the heating surface.

Inventors: OLSSON JAN-OLOV, OLSSON, JAN-OLOV
Applicants: KANTHAL AB
Application year: 1991
Main classification: C10G9
IPC codes: F28F1, C22C38, C10G9