The protection of metals, particularly alloys, from exposure to carbon at elevated temperatures is encountered in many industrial environments. Two problems commonly encountered are carburization of the metal and carbon buildup on the metal. Both problems are of particular concern in a furnace employed in thermal cracking a stream of hydrocarbons to produce olefins. Therefore, the invention is described with reference to this operation.
At the heart of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs a serpentine array of tubing. This array is composed of lengths of tubing and fittings that may total several hundred meters in length. The array of tubing is heated to a carefully monitored temperature by the fire box.
A stream of feedstock is forced through the heated tubing under pressure and at a high velocity, and the product is quenched as it exits. For olefin production, the feedstock is frequently diluted with steam. The mixture is passed through the tubing array which is commonly operated at a temperature of at least 750.degree. C. During this passage, a carboniferous residue is formed and deposits on the tube walls and fittings.
The carbon deposits initially in a fibrous form. It is thought this results from a catalytic action, primarily due to nickel and iron in the tube alloy. The fibrous carbon appears to form a mat on the tube wall. This traps pyrolitic coke particles that form in the gas stream. The result is buildup of a dense, coke deposit on the tube wall. This carbon buildup is commonly referred to as "coking."
A short range concern is the thermal insulating effect of the carbon buildup on the tube wall. This necessitates continually increasing the fire box temperature to maintain a steady temperature in the hydrocarbon stream passing through the furnace. Ultimately, the fire box and the tube wall reach temperatures where operation must be discontinued. The carbon must then be removed in a procedure referred to as decoking.
A longer range concern is the effect of the carbon on the metal tubes in the cracking furnace. A gradual embrittlement of the metal is observed with consequent loss of mechanical strength. Since the operation is conducted under considerable pressure and tensile load, the danger of tube rupture arises. Both pressure and tensile load tend to be relatively constant factors. However, as a metal tube becomes weak due to embrittlement, these factors become significant. It then becomes necessary to shut the operation down and completely rebuild the furnace with new tubing.
It has been proposed to apply a glass-ceramic coating to a metal surface to protect the metal from embrittlement due to carburization. It has also been proposed to employ a glass-ceramic coating to lessen the tendency for carbon deposition (coking) to occur during a thermal cracking process.
TABLE I sets forth, in weight percent on an oxide basis as calculated from the precursor glass batch, the compositions for several different glass-ceramics proposed as protective coatings. Examples 1-6 illustrate alkaline earth metal alumino borates or borosilicates. Examples 7-14 illustrate alkaline earth metal silicates which may contain minor amounts of alumina or zirconia.
TABLE I SiO.sub.2 B.sub.2 O.sub.3 Al.sub.2 O.sub.3 BaO MgO CaO ZnO ZrO.sub.2 MnO.sub.2 SrO NiO F 1 -- 19.1 27.9 42.0 11.0 -- -- -- -- -- -- -- 2 -- 25.4 18.6 56.0 -- -- -- -- -- -- -- 6 3 17.5 20.2 29.7 -- -- 32.6 -- -- -- -- -- -- 4 9.6 22.2 32.5 -- -- 35.8 -- -- -- -- -- -- 5 30.6 12.7 3.8 15.9 23.5 -- 13.5 -- -- -- -- -- 6 -- 27.0 19.8 29.7 7.8 -- 15.8 -- -- -- -- -- 7 32.0 -- -- 40.9 -- -- -- 8.2 18.9 -- -- -- 8 33.9 -- 2.9 43.3 -- -- -- -- 20.0 -- -- -- 9 33.2 4.8 -- 42.4 -- -- -- -- 19.6 -- -- -- 10 65.0 -- 6.9 -- -- -- -- -- -- 28.1 -- -- 11 47.2 -- -- -- -- -- -- 12.1 -- 40.7 -- -- 12 54.1 -- 5.7 -- -- -- -- -- -- 23.3 16.8 -- 13 38.3 -- -- -- -- -- -- 5.9 22.7 33.1 -- -- 14 62.7 -- 5.3 32.0 -- -- -- -- -- -- -- --
The efficacy of a glass-ceramic coating for these respective purposes was tested on a section of tubing in a thermal cracking furnace. A tendency for sections of the coating to separate from the metal was observed. This undesirable occurrence was thought to be occasioned by an expansion mismatch between the metal and the glass-ceramic coating. The metal was an austenitic metal having a CTE of about 180.times.10.sup.-7 /.degree. C. (RT-800.degree. C.), whereas the glass-ceramic had a CTE of about 130.times.10.sup.-7 /.degree. C.
It was recognized that this disparity in CTEs created high compressive stresses in the coating. In accordance with standard enameling experience, this would normally be considered desirable. However, the conditions that prevail during operation of a thermal cracking furnace are rather severe. In particular, the furnace undergoes rapid thermal change during a decoking cycle.
It is a purpose of the present invention to provide a solution to the delamination of a glass-ceramic coating from a metal on which it is deposited. Another purpose is to provide a means of promoting adherence of a glass-ceramic coating to a metal body having a high CTE. A further purpose is to provide a component for a thermal cracking furnace that has a glass-ceramic coating in combination with a coating having a higher CTE. Another purpose is to provide an improved method of protecting a metal body with a glass-ceramic coating.