Claims
- 1. A composition which comprises antimony 2-ethylhexanoate and chromium (III) 2-ethylhexanoate, wherein the concentration of chromium in said composition is in the range of about 5 mole percent to about 90 mole percent.
Parent Case Info
This application is a division of application Ser. No. 523,540, filed Aug. 16, 1983 now U.S. Pat. No. 4,507,196.
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 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 heterogenous 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 peiodically 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 organic chromium compounds, a combination of tin and elemental chromium or an organic chromium compound (elemental chromium or an organic compound of chromium are referred to hereinafter as "chromium"), a combination of chromium and antimony, and a combination of tin, antimony and chromium 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 (39)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 134555 |
Mar 1985 |
EPX |
Non-Patent Literature Citations (5)
| Entry |
| Merritt, W. F. "Trace Element Compositions . . .," Nucl. Tech. Groundwater Pollut. Res., Proc. Advis. Group Meet., 1976 (Publd. 1980) IAEA: Vienna, Austria, pp. 153-158. |
| Hansen, Max. Constitution of Binary Alloys, McGraw-Hill, 2nd Ed. (1958) pp. 558-559. |
| Chemical Abstract 96:10499 "Bearing Alloy . . ." vol. 96 (1982) p. 254. |
| Chemical Abstract 79:35451, "Effect of Preparation Conditions" vol. 79 (1973), p. 246. |
| Gmelins Handbook Der Anorganischen Chemie, Tiel A and Lieferung 2, pp. 710-726, 727 (Translation). |
Divisions (1)
|
Number |
Date |
Country |
| Parent |
523540 |
Aug 1983 |
|
Patent Grant
4863892
