Claims
- 1. A composition, that is suitable for use as an antifoulant, comprising tin, antimony and aluminum, wherein said tin, antimony and aluminum are present in said composition in a form selected from the group consisting of elemental metals, organic, and inorganic compounds and in a form which is further characterized by being convertible to an oxide when placed in air having a temperature of about 700.degree. C. and wherein the concentration of antimony and aluminum in said composition is in the range of about 20 mole percent to about 60 mole percent for both said antimony and said aluminum.
- 2. A composition in accordance with claim 1 wherein said composition is in a solution and wherein the concentration of said composition in said solution is at least about 0.05 molar.
- 3. A composition in accordance with claim 2 wherein the concentration of said composition in said solution is in the range of about 0.3 molar to about 0.6 molar.
- 4. A composition in accordance with claim 2 wherein the solvent used to form the solution of said composition is selected from the group consisting of water, oxygen-containing organic liquids and aliphatic and aromatic hydrocarbons.
- 5. A composition in accordance with claim 1 wherein the tin is an organic compound containing tin, wherein the antimony is an organic compound of antimony and wherein the aluminum is an organic compound of aluminum.
- 6. A composition in accordance with claim 5 wherein said organic compound containing tin is a tetrahydrocarbyltin, wherein said organic compound of antimony is an antimony carboxylate and wherein said organic compound of aluminum is an aluminum alkoxide.
- 7. A composition in accordance with claim 6 wherein said tetrahydrocarbyltin is tetrabutyltin, said antimony carboxylate is antimony 2-ethylhexanoate and said aluminum alkoxide is aluminum isopropoxide.
Parent Case Info
This application is a division of application Ser. No. 632,934, filed July 20, 1984, now U.S. Pat. No. 4,545,893.
This invention relates to processes for the thermal cracking of a gaseous stream containing hydrocarbons. In one aspect this invention relates to a method for reducing the formation of carbon on the cracking tubes in furnaces used for the thermal cracking of a gaseous stream containing hydrocarbons and in any heat exchangers used to cool the effluent flowing from the furnaces. In another aspect this invention relates to particular antifoulants which are useful for reducing the rate of formation of carbon on the walls of such cracking tubes and in such heat exchangers.
The cracking furnace forms the heart of many chemical manufacturing processes. Often, the performance of the cracking furnace will carry the burden of the major profit potential of the entire manufacturing process. Thus, it is extremely desirable to maximize the performance of the cracking furnace.
In a manufacturing process such as the manufacture of ethylene, feed gas such as ethane and/or propane and/or naphtha is fed into the cracking furnace. A diluent fluid such as steam is usually combined with the feed material being provided to the cracking furnace. Within the furnace, the feed stream which has been combined with the diluent fluid is converted to a gaseous mixture which primarily contains hydrogen, methane, ethylene, propylene, butadiene, and small amounts of heavier gases. At the furnace exit this mixture is cooled, which allows removal of most of the heavier gases, and compressed.
The compressed mixture is routed through various distillation columns where the individual components such as ethylene are purified and separated. The separated products, of which ethylene is the major product, then leave the ethylene plant to be used in numerous other processes for the manufacture of a wide variety of secondary products.
The primary function of the cracking furnace is to convert the feed stream to ethylene and/or propylene. A semi-pure carbon which is termed "coke" is formed in the cracking furnace as a result of the furnace cracking operation. Coke is also formed in the heat exchangers used to cool the gaseous mixture flowing from the cracking furnace. Coke formation generally results from a combination of a homogeneous thermal reaction in the gas phase (thermal coking) and a heterogeneous catalytic reaction between the hydrocarbon in the gas phase and the metals in the walls of the cracking tubes or heat exchangers (catalytic coking).
Coke is generally referred to as forming on the metal surfaces of the cracking tubes which are contacted with the feed stream and on the metal surfaces of the heat exchangers which are contacted with the gaseous effluent from the cracking furnace. However, it should be recognized that coke may form on connecting conduits and other metal surfaces which are exposed to hydrocarbons at high temperatures. Thus, the term "Metals" will be used hereinafter to refer to all metal surfaces in a cracking process which are exposed to hydrocarbons and which are subject to coke deposition.
A normal operating procedure for a cracking furnace is to periodically shut down the furnace in order to burn out the deposits of coke. This downtime results in a substantial loss of production. In addition, coke is an excellent thermal insulator. Thus, as coke is deposited, higher furnace temperatures are required to maintain the gas temperature in the cracking zone at a desired level. Such higher temperatures increase fuel consumption and will eventually result in shorter tube life.
Another problem associated with carbon formation is erosion of the Metals, which occurs in two fashions. First, it is well known that in the formation of catalytic coke the metal catalyst particle is removed or displaced from the surface and entrained within the coke. This phenomenon results in extremely rapid metal loss and, ultimately, Metals failure. A second type of erosion is caused by carbon particles that are dislodged from the tube walls and enter the gas stream. The abrasive action of these particles can be particularly severe on the return bends in the furnace tube.
Yet another and more subtle effect of coke formation occurs when coke enters the furnace tube alloy in the form of a solid solution. The carbon then reacts with the chromium in the alloy and chromium carbide precipitates. This phenomena, known as carburization, causes the alloy to lose its original oxidation resistance, thereby becoming susceptible to chemical attack. The mechanical properties of the tube are also adversely affected. Carburization may also occur with respect to iron and nickel in the alloys.
It is thus an object of this invention to provide a method for reducing the formation of coke on the Metals. It is another object of this invention to provide particular antifoulants which are useful for reducing the formation of carbon on the Metals.
In accordance with the present invention, an antifoulant selected from the group consisting of a combination of tin and aluminum, a combination of aluminum and antimony or a combination of tin, antimony and aluminum is contacted with the Metals either by pretreating the Metals with the antifoulant, adding the antifoulant to the hydrocarbon feedstock flowing to the cracking furnace or both. The use of the antifoulant substantially reduces the formation of coke on the Metals which substantially reduces the adverse consequences which attend such coke formation.
US Referenced Citations (20)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 1001344 |
Aug 1965 |
GBX |
| 2066696 |
Jul 1981 |
GBX |
Non-Patent Literature Citations (2)
| Entry |
| Chemical Abstract 96:10499, "Bearing Alloy from an Aluminum-Tin Base . . . ", (1982), p. 254. |
| Hansen, M., Constitution of Binary Alloys, second edition, (1958), McGraw-Hill, N.Y., pp. 130-131. |
Divisions (1)
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Number |
Date |
Country |
| Parent |
632934 |
Jul 1984 |
|
Patent Grant
4686201
