Mineral oils: processes and products – Chemical conversion of hydrocarbons – With prevention or removal of deleterious carbon...
Reexamination Certificate
1995-10-25
2002-12-24
Yildirim, Bekir L. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With prevention or removal of deleterious carbon...
C585S648000, C585S649000, C585S650000
Reexamination Certificate
active
06497809
ABSTRACT:
The present invention generally relates to processes for the thermal cracking of hydrocarbons and, specifically, to a method for prolonging the effectiveness of a treated pyrolytic cracking tube in inhibiting the formation of coke during pyrolytic cracking of hydrocarbons.
In a process for producing olefin compounds, a fluid stream containing a saturated hydrocarbon such as ethane, propane, butane, pentane, naphtha, or mixtures of two or more thereof is fed into a thermal (or pyrolytic) cracking furnace. A diluent fluid such as steam is usually combined with the hydrocarbon feed material being introduced into the cracking furnace.
Within the cracking furnace, the saturated hydrocarbons are converted into olefinic compounds. For example, an ethane stream is introduced into the cracking furnace wherein it is converted into ethylene and appreciable amounts of other hydrocarbons. A propane stream is introduced into the cracking furnace wherein it is converted to ethylene and propylene, and appreciable amounts of other hydrocarbons. Similarly, a mixture of saturated hydrocarbons containing ethane, propane, butane, pentane and naphtha is converted to a mixture of olefinic compounds containing ethylene, propylene, butenes, pentenes, and naphthalene. Olefinic compounds are an important class of industrial chemicals. For example, ethylene is a monomer or comonomer for making polyethylene. Other uses of olefinic compounds are well known to those skilled in the art.
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 as an effluent 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 generally forms 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 also 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 of the equipment in a cracking process system 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.
There are certain methods known by those skilled in the art for inhibiting or reducing the formation of coke on Metals. For instance, in U.S. Pat. No. 4,692,234 a method for reducing the formation of coke on the metal surfaces of a cracking process system is described whereby such metal surfaces are treated with an Antifoulant containing tin and silicon.
One phenomenon associated with the utilization of a tin and silicon antifoulant has been the loss of effectiveness of a treatment of the metal surfaces of cracking tubes during their use. While the treatment of cracking tubes with an antifoulant is observed to beneficially reduce the coking rate when the tubes are used to thermally crack hydrocarbons, it has also been observed that the treatment loses its effectiveness during such use. It was not until the discovery of the cause of this rapid loss in treatment effectiveness that a solution was obtainable.
It is, thus, an object of this invention to provide a method for prolonging the effectiveness of treated cracking tubes in resisting the formation of coke during the cracking of hydrocarbons.
The present invention is a method for cracking hydrocarbons using a cracking tube treated for the resistance of coke formation. A cracking tube, which has been treated with a tin and silicon antifoulant material to thereby deposit upon the surfaces thereof tin and silicon, is operated under cracking conditions while passing a desulfurized hydrocarbon feed through such treated tube. The use of a desulfurized or low sulfur feed in the treated tube reduces the rate in the loss of the effectiveness of the antifoulant treatment.
Another embodiment of this invention includes a method for prolonging the effectiveness in resisting coke formation of a pyrolytic cracking tube, treated for the resistance of coke formation, when the treated pyrolytic cracking tube is utilized in cracking hydrocarbons. The method includes desulfurizing a hydrocarbon feed containing a concentration of sulfur to remove at least a portion of the concentration of sulfur to provide a desulfurized hydrocarbon feed. The desulfurized hydrocarbon feed is then passed through the treated pyrolytic cracking tube, having deposited on the surface thereof tin and silicon, operated under suitable cracking conditions.
Other objects and advantages of the invention will be apparent from the detailed description of the invention and the claims.
It is been discovered that the presence of sulfur compounds in a pyrolytic cracking unit feed has a negative impact upon the effectiveness of a treated pyrolytic cracking tube to resist coke formation during its use. Specifically, it has been found that the sulfur in the pyrolytic cracking unit feed interacts with the tin, deposited by a treatment method on the surface of the pyrolytic cracking tubes, so as to strip the tin from the surface of the treated pyrolytic cracking tubes. The stripping of the tin from the treated pyrolytic cracking tube surface results in reducing the effectiveness of the treated pyrolytic cracking tube in resisting the formation of coke during cracking operation. The discovery of this previously unknown mechanism allows the inventors hereof to develop a solution to the problem of sulfur stripping of the tin deposited on the pyrolytic cracking tube surface by an antifoulant treatment method.
The treated pyrolytic cracking tube of the inventive method is a standard pyrolytic cracking furnace tube treated with an antifoulant material, or antifoulant, selected from a group consisting of tin, silicon and mixtures of tin and silicon. Any form of silicon and tin can be utilized as antifoulant material. Elemental silicon, inorganic silicon compounds and organic silicon compounds as well as mixtures of two or more thereof arc suitable sources of silicon. The term “silicon” as used herein refers to any one of these silicon sources, but the preferred silicon source is organic silicon (organosilicon) compounds. Elemental tin, inorganic tin compounds and organic tin compounds as well as mixtures of two or more thereof are suitable sources of tin. The term “tin” as used herein refers to any one of these tin sources, but the preferred tin source is organic tin (organotin) compounds.
Examples of organic silicon (organosilicon) compounds that may be used include compounds of the formula
wherein R
1
, R
2
, R
3
, and R
4
are selected independently from the group consisting of hydrogen, halogen, hydrocarbyl, and oxyhydrocarbyl and wherein the compound's bonding may be either ionic or covalent. The hydrocarbyl and oxyhydrocarbyl radicals can have from 1-20 carbon atoms which may be substituted with halogen, nitrogen, phosphorus, or sulfur. Exemplary hydrocarbyl radicals are alkyl, alkenyl, cycloalkyl, aryl, and combinations thereof, such as alkylaryl or alkylcycloalkyl. Exemplary oxyhydrocarbyl radicals are alkoxide, phenoxide, carboxylate, ketocarboxylate and diketone (dione).
Suitable organic silicon compounds include trimethylsilane, tetram
Brown Ronald E.
Harper Timothy P.
Reed Larry E.
Phillips Petroleum Company
Stewart Charles W.
Yildirim Bekir L.
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