Method for providing a tube having coke formation and carbon monoxide inhibiting properties when used for the thermal cracking of hydrocarbons

Abstract: The rate of formation of carbon on the surfaces of thermal cracking tubes and the production of carbon monoxide during thermal cracking of hydrocarbons are inhibited by the use of cracking tubes treated with an antifoulant, including tin compound, silicon compound and sulfur compounds in the presence of a reducing gas such as hydrogen. Additionally, the concentration of carbon monoxide in a pyrolytic cracking process product stream is reduced by the treatment of the thermal cracking tubes of such process with a reducing gas having a concentration of a sulfur compound.

Description: The present invention generally relates to processes for the thermal cracking of hydrocarbons and, specifically, to a method for providing a tube of a thermal cracking furnace having coke formation inhibiting properties when used for the thermal cracking of hydrocarbons. In a process for producing an olefin compound, 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 furnace, the saturated hydrocarbon is converted into an olefinic compound. For example, an ethane stream introduced into the cracking furnace is converted into ethylene and appreciable amounts of other hydrocarbons. A propane stream introduced into the furnace 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. WO-A-9 522 588, which reference has to be considered under Article 54(3) EPC, discloses a heat exchange surface in reactors and/or heat exchangers for processing installations for hydrocarbons and other organic compounds at high temperatures in the gaseous phase. The metallic surfaces that come into contact with the organic substances are treated with a mixture of a silicon- and sulfur-containing product under specific conditions. The reference also discloses a process for producing a catalytically inactivated metallic surface in chemical reactors and/or heat exchangers. EP-A-0 540 084 relates to a method for treating a tube for the pyrolysis of hydrocarbons, in order to reduce the rate of coke formation, by decomposing a non-oxygen containing silicon organometallic precursor in the vapor phase, in an inert or reducing gas atmosphere to form a layer of ceramic material on a surface of the tube. WO-A-9 215 653 discloses a method for reforming hydrocarbons comprising contacting the hydrocarbons with a reforming catalyst in a reactor system of improved resistance to carburization and metal dusting under conditions of low sulfur. As a result of the thermal cracking of a hydrocarbon, the cracked product stream can also contain appreciable quantities of pyrolytic products other than the olefinic compounds including, for example, carbon monoxide. It is undesirable to have an excessively high concentration of carbon monoxide in a cracked product stream; because, it can cause the olefinic product to be "off-spec" due to such concentration. Thus, it is desirable and important to maintain the concentration of carbon monoxide in a cracked product stream as low as possible. Another problem encountered in thermal cracking operations is in the formation and laydown of carbon or coke upon the tube and equipment surfaces of a thermal cracking furnace. This buildup of coke on the surfaces of the cracking furnace tubes can result in an excessive pressure drop across such tubes thereby necessitating costly furnace shutdown in order to decoke or to remove the coke buildup. Therefore, any reduction in the rate of coke formation and coke buildup is desirable in that it increases the run length of a cracking furnace between decokings. It is thus an object of this invention to provide an improved process for cracking saturated hydrocarbons to produce olefinic end-products. Another object of this invention is to provide a process for reducing the formation of coke in a process for cracking saturated hydrocarbons. A still further object of this invention is to improve the economic efficiency of operating a cracking process for cracking saturated hydrocarbons by providing a method for treating the tubes of a cracking furnace so as to provide treated tubes having coke formation inhibiting properties. In accordance with the invention, a method for treating a tube of a thermal cracking furnace as defined in claim 1, is provided. In this method, a tube of a thermal cracking furnace is treated with an antifoulant composition so as to provide a treated tube having properties which inhibit the formation of coke when utilized in a thermal cracking operation. The method for treating the thermal cracking tube includes contacting under an atmosphere of a reducing gas, the tube with the antifoulant composition which comprises a compound selected from combinations of a tin compound, and a silicon compound. In the accompanying drawing: FIG. 1 provides a schematic representation of the cracking furnace section of a pyrolytic cracking process system in which the tubes of such system are treated by the novel method described herein. Other objects and advantages of the invention will be apparent from the following detailed description of the invention and the appended claims thereof. The process of this invention involves the pyrolytic cracking of hydrocarbons to produce desirable hydrocarbon end-products. A hydrocarbon stream is fed or charged to pyrolytic cracking furnace means wherein the hydrocarbon stream is subjected to a severe, high-temperature environment to produce cracked gases. The hydrocarbon stream can comprise any type of hydrocarbon that is suitable for pyrolytic cracking to olefin compounds. Preferably, however, the hydrocarbon stream can comprise paraffin hydrocarbons selected from the group consisting of ethane, propane, butane, pentane, naphtha, and mixtures of any two or more thereof. Naphtha can generally be described as a complex hydrocarbon mixture having a boiling range of from 82 to 204°C (180°F to 400°F) as determined by the standard testing methods of the American Society of Testing Materials (ASTM). The cracking furnace means of the method according to the invention can be any suitable thermal cracking furnace known in the art. The various cracking furnaces are well known to those skilled in the art of cracking technology and the choice of a suitable cracking furnace for use in a cracking process is generally a matter of preference. Such cracking furnaces, however, are equipped with at least one cracking tube to which the hydrocarbon feedstock is charged or fed. The cracking tube provides for and defines a cracking zone contained within the cracking furnace. The cracking furnace is utilized to release the heat energy required to provide for the necessary cracking temperature within the cracking zone in order to induce the cracking reactions therein. Each cracking tube can have any geometry which suitably defines a volume in which cracking reactions can take place and, thus, will have an inside surface. The term "cracking temperature" as used herein is defined as being the temperature within the cracking zone defined by a cracking tube. The outside wall temperature of the cracking tube can, thus, be higher than the cracking temperature and possibly substantially higher due to heat transfer considerations. Typical pressures within the cracking zone will generally be in the range of from 135.5 to 273.5 kPa (5 psig to 25 psig) and, preferably from 170 to 239 kPa (10 psig to 20 psig). As an optional feature of the invention, the hydrocarbon feed being charged to pyrolytic cracking furnace means can be intimately mixed with a diluent prior to entering pyrolytic cracking furnace means. This diluent can serve several positive functions, one of which includes providing desirable reaction conditions within pyrolytic cracking furnace means for producing the desired reactant end-products. The diluent does this by providing for a lower partial pressure of hydrocarbon feed fluid thereby enhancing the cracking reactions necessary for obtaining the desired olefin products while reducing the amount of undesirable reaction products such as hydrogen and methane. Also, the lower partial pressure resulting from the mixture of the diluent fluid helps in minimizing the amount of coke deposits that form on the furnace tubes. While any suitable diluent fluid that provides these benefits can be used, the preferred diluent fluid is steam. The cracking reactions induced by pyrolytic cracking furnace means can take place at any suitable temperature that will provide the necessary cracking to the desirable end-products or the desired feed conversion. The actual cracking temperature utilized will depend upon the composition of the hydrocarbon feed stream and the desired feed conversion. Generally, the cracking temperature can range upwardly to 1093°C (2000°F) or greater depending upon the amount of cracking or conversion desired and the molecular weight of the feedstock being cracked. Preferably, however, the cracking temperature will be in the range of from 649 to 1038°C (1200°F to 1900°F). Most preferably, the cracking temperature can be in the range from 816 to 982°C (1500°F to 1800°F). A cracked gas stream or cracked hydrocarbons or cracked hydrocarbon stream from pyrolytic cracking furnace means will generally be a mixture of hydrocarbons in the gaseous phase. This mixture of gaseous hydrocarbons can comprise not only the desirable olefin compounds, such as ethylene, propylene, butylene, and amylene; but, also, the cracked hydrocarbon stream can contain undesirable contaminating components, which include carbon monoxide. It is generally observed that at the beginning or start of the charging of a feedstock to either a virgin cracking tube or a cracking tube that has freshly been regenerated by decoking, the concentration of undesirable carbon monoxide in the cracked hydrocarbon stream will be higher or reach a maximum concentration peak, which will herein be referred to as peak concentration. Once the carbon monoxide concentration in the cracked hydrocarbon stream reaches its peak or maximum concentration, over time it will gradually decrease in an almost asymptotic fashion to some reasonably uniform concentration. While the asymptotic concentration of carbon monoxide will often be sufficiently low to be within product specifications; often, the peak concentration will exceed specifications when there are no special efforts taken to prevent an excessive peak concentration of carbon monoxide. In untreated tubes, the peak concentration of carbon monoxide can exceed 9.0 weight percent of the cracked hydrocarbon stream. Conventionally treated tubes provide for a peak concentration in the range from 6 weight percent to 8.5 weight percent and an asymptotic concentration in the range of from 1 weight percent to 2 weight percent. A critical aspect of the method according to the invention includes the treatment or treating of the tubes of a cracking furnace by contacting the surfaces of such tubes with the antifoulant composition while under an atmosphere of a reducing gas and under suitable treatment conditions. It has been discovered that the coke formation inhibiting properties of a cracking tube are improved by treating such cracking tube with the antifoulant composition in a reducing gas atmosphere as opposed to treatment without the presence of a reducing gas. Thus, the use of the reducing gas is an important aspect of the method according to the invention. The reducing gas used in the method according to the invention can be any gas which can suitably be used in combination with the antifoulant composition during treatment so as to provide an enhancement in the ability of the treated tube to inhibit the formation of coke during cracking operation. The preferred reducing gas, however, is hydrogen. The antifoulant composition used to treat the tubes of the cracking furnace in the presence of a reducing gas such as hydrogen is a compound that provides for a treated tube having the desirable ability to inhibit the rate of coke formation as compared with an untreated tube or a tube treated in accordance with other known methods. Such antifoulant composition comprises a compound selected from mixtures of a tin compound, and a silicon compound. Any suitable form of silicon can be utilized as a silicon compound of the antifoulant composition. Elemental silicon, inorganic silicon compounds and organic silicon (organosilicon) compounds as well as mixtures of any two or more thereof are suitable sources of silicon. The term "silicon compound" generally refers to any one of these silicon sources. Examples of some inorganic silicon compounds that can be used include the halides, nitrides, hydrides, oxides and sulfides of silicon, silicic acids and alkali metal salts thereof. Of the inorganic silicon compounds, those which do not contain halogen are preferred. Examples of organic silicon 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 to 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, tetramethylsilane, tetraethylsilane, triethylchlorosilane, phenyltrimethylsilane, tetraphenylsilane, ethyltrimethoxysilane, propyltriethoxysilane, dodecyltrihexoxysilane, vinyltriethyoxysilane, tetramethoxyorthosilicate, tetraethoxyorthosilicate, polydimethylsiloxane, polydiethylsiloxane, polydihexylsiloxane, polycyclohexylsiloxane, polydiphenylsiloxane, polyphenylmethylsiloxane, 3-chloropropyltrimethoxysilane, and 3-aminopropyltriethoxysilane. At present hexamethyldisiloxane is preferred. Organic silicon compounds are particularly preferred because such compounds are soluble in the feed material and in the diluents which are preferred for preparing pretreatment solutions as will be more fully described hereinafter. Also, organic silicon compounds appear to have less of a tendency towards adverse effects on the cracking process than do inorganic silicon compounds. Any suitable form of tin can be utilized as the tin compound of the antifoulant composition. Elemental tin, inorganic tin compounds and organic tin (organotin) compounds as well as mixtures of any two or more thereof are suitable sources of tin. The term "tin compound" generally refers to any one of these tin sources. Examples of some inorganic tin compounds which can be used include tin oxides such as stannous oxide and stannic oxide; tin sulfides such as stannous sulfide and stannic sulfide; tin sulfates such as stannous sulfate and stannic sulfate; stannic acids such as metastannic acid and thiostannic acid; tin halides such as stannous fluoride, stannous chloride, stannous bromide, stannous iodide, stannic fluoride, stannic chloride, stannic bromide and stannic iodide; tin phosphates such as stannic phosphate; tin oxyhalides such as stannous oxychloride and stannic oxychloride; and the like. Of the inorganic tin compounds those which do not contain halogen are preferred as the source of tin. Examples of some organic tin compounds which can be used include tin carboxylates such as stannous formate, stannous acetate, stannous butyrate, stannous octoate, stannous decanoate, stannous oxalate, stannous benzoate, and stannous cyclohexanecarboxylate; tin thiocarboxylates such as stannous thioacetate and stannous dithioacetate; dihydrocarbyltin bis(hydrocarbyl mercaptoalkanoates) such as dibutyltin bis(isooctylmercaptoacetate) and dipropyltin bis(butyl mercaptoacetate); tin thiocarbonates such as stannous O-ethyl dithiocarbonate; tin carbonates such as stannous propyl carbonate; tetrahydrocarbyltin compounds such as tetramethyltin, tetrabutyltin, tetraoctyltin, tetradodecyltin, and tetraphenyltin; dihydrocarbyltin oxides such as dipropyltin oxide; dibutyltin oxide, dioctyltin oxide, and diphenyltin oxide; dihydrocarbyltin bis(hydrocarbyl mercaptide)s such as dibutyltin bis(dodecyl mercaptide); tin salts of phenolic compounds such as stannous thiophenoxide; tin sulfonates such as stannous benzenesulfonate and stannous-p-toluenesulfonate; tin carbamates such as stannous diethylcarbamate; tin thiocarbamates such as stannous propylthiocarbamate and stannous diethyldithiocarbamate; tin phosphites such as stannous diphenyl phosphite; tin phosphates such as stannous dipropyl phosphate; tin thiophosphates such as stannous, O,O-dipropyl thiophosphate, stannous O,O-dipropyl dithiophosphate and stannic O,O-dipropyl dithiophosphate, dihydrocarbyltin bis(O,O-dihydrocarbyl thiophosphate)s such as dibutyltin bis(O,O-dipropyldithiophosphate); and the like. At present tetrabutyltin is preferred. Again, as with silicon, organic tin compounds are preferred over inorganic compounds. The tubes treated with the antifoulant composition in the presence of a reducing gas will have properties providing for a significantly greater suppression of the rate of coke formation, when used under cracking conditions than tubes treated exclusively with the antifoulant composition but without the presence of a reducing gas. A preferred procedure for pretreating the tubes of the cracking furnace includes charging to the inlet of the cracking furnace tubes a reducing gas such as hydrogen containing therein a concentration of the antifoulant composition. The concentration of antifoulant composition in the reducing gas can be in the range of from 1 ppmw to 10,000 ppmw, preferably from 10 ppmw to 1000 ppmw and, most preferably, from 20 to 200 ppmw. The temperature conditions under which the reducing gas, having the concentration of the antifoulant composition, is contacted with the cracking tubes can include a contacting temperature in the range upwardly to 1093°C (2000°F). In any event, the contacting temperature must be such that the surfaces of the cracker tubes are properly passivated and include a contacting temperature in the range of from 149 to 1093°C (300°F to 2000°F), preferably, from 204 to 982°C (400°F to 1800°F) and, most preferably, from 260 to 871°C (500°F to 1600°F). In a further preferred embodiment, the contacting step is conducted at a temperature in the range of from 538 to 704°C (1000 to 1300°F). The contacting pressure is not believed to be a critical process condition, but it can be in the range of from atmospheric to 3.55 MPa (500 psig). Preferably, the contacting pressure can be in the range of from 170 kPa to 2.17 MPa (10 psig to 300 psig) and, most preferably, 0.239 to 1.136 MPa (20 psig to 150 psig). The reducing gas stream having a concentration of antifoulant composition is contacted with or charged to the cracker tubes for a period of time sufficient to provide treated tubes, which when placed in cracking service, will provide for the reduced rate of coke formation, relative to untreated tubes or tubes treated with the antifoulant without the presence of a reducing gas. Such time period for pretreating the cracker tubes is influenced by the specific geometry of the cracking furnace including its tubes; but, generally, the pretreating time period can range upwardly to 12 hours, and longer if required. But, preferably, the period of time for the pretreating can be in the range of from 0.1 hours to 12 hours and, most preferably, from 0.5 hours to 10 hours. Once the tubes of a cracking furnace are treated in accordance with the procedure described herein, a hydrocarbon feedstock is charged to the inlet of such treated tubes. The tubes are maintained under cracking conditions so as to provide for a cracked product stream exiting the outlet of the treated tubes. An important benefit that results from the treatment of cracker tubes by the method according to the invention utilizing the antifoulant composition is a reduction in the rate of coke formation in comparison with the coke formation rate with untreated tubes or tubes treated with an antifoulant composition but without the presence of a reducing gas during such treatment. This reduction in the rate of coke formation permits the treated cracker tubes to be used for longer run lengths before decoking is required. Now referring to FIG. 1, there is illustrated by schematic representation a cracking furnace section 10 of a pyrolytic cracking process system. Cracking furnace section 10 includes pyrolytic cracking means or cracking furnace 12 for providing heat energy required for inducing the cracking of hydrocarbons. Cracking furnace 12 defines both convection zone 14 and radiant zone 16. Respectively within such zones are convection coils as tubes 18 and radiant coils as tubes 20. A hydrocarbon feedstock is conducted to the inlet of convection tubes 18 by way of conduit 22, which is in fluid flow communication with convection tubes 18. Also, during the treatment of the tubes of cracking furnace 12, the mixture of hydrogen gas and antifoulant composition can also be conducted to the inlet of convection tubes 18 though conduit 22. The feed passes through the tubes of cracking furnace 12 wherein it is heated to a cracking temperature in order to induce cracking or, in the situation where the tubes are undergoing treatment, to the required treatment temperature. The effluent from cracking furnace 12 passes downstream through conduit 24 where it is further processed. To provide for the heat energy necessary to operate cracking furnace 12, fuel gas is conveyed through conduit 26 to burners 28 of cracking furnace 12 whereby the fuel gas is burned and heat energy is released. The following comparative example is provided to further illustrate the present invention. EXAMPLE This comparative example describes the experimental procedure used to obtain data pertaining to the addition of hydrogen (reducing atmosphere) with an antifoulant during pretreatment injection onto a cracking coil. The experimental apparatus included a 4.27 m (14') long, 8 pass coil made of 6.35 mm (1/4") O.D. Incoloy 800 tubing which was heated to the desired temperature in an electric tube furnace. In one run, 50 ppmm tetrabutyl tin (TBT) was injected with steam (37.5 mol/h) and nitrogen for a period of thirty minutes at an isothermal temperature of 704°C (1300°F) in the furnace. The injection was then discontinued and ethane was charged to the reactor at a rate of 745.5 g/h. Steam was charged with the ethane to the reactor at a rate of 223.5 g/h. Carbon monoxide in the cracked gas and pressure drop across the reactor coil were monitored continuously throughout the run of eighteen minutes. Coke production in the cracking coils was then measured by analyzing the carbon dioxide and carbon monoxide produced when burning out the coil with a steam/air mixture. In a subsequent run, 50 ppmm tetrabutyl tin was injected with 1.7 standard liters per minute hydrogen at identical conditions as the previous run. This injection was then stopped and ethane was charged to the reactor at identical conditions as the previous run. Again, carbon monoxide production in the cracked gas was monitored and coking rate in the furnace determined for this run which also lasted eighteen minutes. The coking rate as measured by the carbon dioxide produced on burning out of the reactor coil was 585 g/h, which was substantially less than the 1403 g/h measured for the run that injected TBT only. The carbon monoxide produced in the cracked gas during the runs was also significantly less for the run that injected the TBT/hydrogen mixture as compared to the TBT only run. The results are shown in Table I for both runs. These data show that adding the tetrabutyl tin compound in a reducing environment will significantly enhance the reduction of the coking rate and the production of carbon monoxide in the cracked gas. CO in Cracked Gas (Wt. %) Time (min.) TBT Only TBT/Hydrogen 6 0.024 0 9 0.09 0.076 12 1.232 0.514 15 2.35 2.4

Claims: A method for treating a tube of a thermal cracking furnace with an antifoulant composition under an atmosphere of a reducing gas so as to provide a treated tube having coke formation inhibiting properties, characterized by comprising: contacting said tube with an antifoulant composition comprising a compound selected from combinations of a tin compound, and a silicon compound. The method of claim 1, wherein said reducing gas comprises hydrogen. The method of claim 1 or 2, wherein the contacting step is conducted at a temperature in the range of from 538 to 704° C (1000 to 1300 °F). The method of any of the preceding claims, wherein the concentration of such antifoulant composition in said atmosphere of said reducing gas is in the range of from 1 to 10,000 ppmw.

Inventors: BROWN RONALD E, GREENWOOD GIL J, HARPER TIMOTHY P, REED LARRY E, SCHARRE MARK D, BROWN, RONALD E., GREENWOOD, GIL J., HARPER, TIMOTHY P., REED, LARRY E., SCHARRE, MARK D.
Applicants: PHILLIPS PETROLEUM CO, PHILLIPS PETROLEUM COMPANY
Application year: 1996
Main classification: C10G9
IPC codes: C10G9, C09K3