|Publication number||US3759822 A|
|Publication date||Sep 18, 1973|
|Filing date||Oct 27, 1971|
|Priority date||Oct 27, 1971|
|Publication number||US 3759822 A, US 3759822A, US-A-3759822, US3759822 A, US3759822A|
|Original Assignee||Union Oil Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (19), Classifications (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent COKING A FEEDSTOCK COMPRISING A PYROLY- SIS TAR AND A HEAVY CRACKED OIL Hillis 0. Folkins, Claremont, Califi, as s ignor to Union Oil Company of California, Los Angeles, Calif. No Drawing. Filed Oct. 27, 1971, Ser. No. 193,149 Int. Cl. Cg 9/14 US. Cl. 208-131 7 Claims ABSTRACT OF THE DISCLOSURE The process for the production of a high quality petro leum coke which can be graphitized to obtain a graphite having a desirably low coefficient of thermal expansion, commonly referred to as needle graphite, is described. The method comprises the coking of a blend of a thermally or catalytically cracked heavy oil having a high aromatic content as a result of the thermal or catalytic cracking with a quantity of a pyrolysis tar which is obtained as a byproduct from the high temperature, diluent cracking of petroleum distillates to produce olefins. It has been observed that a synergism exists between the pyrolysis tar and cracked heavy oil so that the pyrolysis tars can be used in much greater quantities than would be predicted from the characteristics of the cokes produced by the coking of the individual components. In a typical embodiment, a blend comprising from to 60 percent of pyrolysis tar and 40 to 80 percent of a decant oil from catalytic cracking is thermally coked by heating to a temperature from 850 to about 1000 F. and passing the heated stock to a drum and permitting the stock to crack to form vapors and coke therein at temperatures of from 830 to about 875 F.
DESCRIPTION OF THE INVENTION This invention relates to a method for coking of hydrocarbons and, in particular, relates to a method for coking of hydrocarbons to produce a coke which is a needle graphite precursor.
Needle graphite, i.e., graphite having a low coefiicient of thermal expansion, is a premium product in demand for use in the metallurgical industries for graphite electrodes, etc. It is generally known that such graphite can be produced by the graphitization of petroleum coke which is obtained from the delayed coking of highly aromatic feedstocks. Thus, US. patents such as 2,922,755 and 2,775,549 disclose delayed coking processes wherein the feedstock is a highly thermally or catalytically cracked heavy oil. During the recycling and repeated cracking, this stock becomes more aromatic and refractory and is, theretore, a very desirable stock for use in a delayed coking process for the production of the premium coke. Decant oil from catalytic cracking is the best and most available of such heavy oils. Recent innovations in catalytic cracking such as the new molecular sieve catalysts and modern hydrogenation processes have resulted in a substantially greater conversion of the cracking stocks to lighter distillates and a corresponding decrease in the quantity of decant oil which is available for delayed coking operation. Consequently, the available feedstock for the conventional production of premium coke which yields graphites of desirably low coeflicient of thermal expansion values has diminished and there is no reasonable expectation of a reversal of this trend.
There has also occurred an increase in the production of pyrolysis tars which are high-boiling residual byproducts formed in the high temperature diluent cracking of petroleum gases and distillates such as naphthas and gas oils to prepare olefins. The high demand for low molecular weight olefins for use in the petrochemical indusice tries has increased the production of the pyrolysis tar byproduct. Although these pyrolysis tars have a high aromatics content as a result of their high temperature cracking, they are not desirable feedstocks for delayed coking operations because they also have a high olefin content. This olefinic content renders the pyrolysis tars prone to deposit coke in the heater tubes and equipment used in a delayed coking operation so that considerable difiiculty is experienced when these stocks are heated to the temperatures necessary to effect delayed coking. In addition, the pyrolysis tars are not as good a precursor for quality coke as are the aforementioned aromatic heavy oils since they yield cokes which, when graphitized, have considerably greater coefiicients of thermal expansion. Although several patents; 3,451,921; 3,460,907; 3,326,796; and 3,547,804; have issued on the use of the pyrolysis tars in a delayed coking operation for the production of premium coke, the difficulties in handling and coking of these charge stocks have essentially prohibited their commercial use in a delayed coking operation.
It is an object of this invention to provide a process for the production of petroleum coke which can be graphitized to a product having a low coeflicient of thermal expansion.
It is also an object of this invention to provide such a process which utilizes a large proportion of the pyrolysis tar byproduct of the high temperature diluent cracking of petroleum stocks.
It is a further object of this invention to provide such a process which is free of the coking difiiculties normally experienced when subjecting such pyrolysis tars to high temperatures.
It is a further object of this invention to provide a process for the production of coke which can be graphitized to products having low coeificients of thermal expansion which utilizes a substantially lesser quantity of thermally and catalytically cracked heavy oils than the conventional processes.
Other and related objects will be apparent from the following description of the invention.
It has now been discovered that pyrolysis tars can be heated to coking conditions of temperature and pressure without experiencing an objectionable quantity of coke deposition in the heating facilities it the pyrolysis tar is blended with a thermally or catalytically cracked heavy oil. It has been further discovered that significantly large quantities of pyrolysis tars can be blended with such heavy oils and the resultant blend can be thermally coked to produce vapors and dry coke which, when graphitized, yields a graphite having a surprisingly low coefiicient of thermal expansion. Specifically, it has been found that the coefiicient of thermal expansion of graphite produced by blends of a quantity of the pyrolysis tar with a thermally or catalytically cracked heavy oil is :not the expected additive value which would be extrapolated from the proportional addition of the coefiicients of thermal expansion values of cokes prepared by coking the individual stocks but, instead, is significantly lesser than such additive value.
Accordinngly, this invention comprises a method for the production of coke which is useful as a precursor for graphite having a desirably low coeflicient of thermal expansion value which comprises admixing a quantity of a pyrolysis tar obtained from the high temperature diluent cracking of a petroleum fraction to produce olefins and said pyrolysis tar with a highly aromatic, thermally or catalytically cracked heavy oil. The quantity of the pyrolysis tar in the blend is greater than the amount of the tar which would impart the same coefiicient of thermal expansion value to the graphite produced from the coke if the coefiicient of thermal expansion value of coke from the blended stocks were the additive result of the coeflicient of thermal expansion values of coke produced from the individual stocks.
The blended stocks are then heated to a temperature from about 920 to about 970 F. and at a pressure from about 25 to about 200 p.s.i.g., preferably from 50 to 150 p.s.i.g., and are then passed into a typical delayed coking drum where they are permitted to crack to form vapors and coke therein. The vapors are removed and recovered as valuable hydrocarbon distillates which are further processed in accordance with the particular refinery requirements. The coke is removed from the coking drum periodically whenever the coke has deposited to a sufficient level therein, by directing the coke charge to an alternate drum, disconnecting the drum from the process, cooling and removing of the coke therefrom.
The petroleum coke which is removed from the delayed coking drum typically has a volatile matter content of from 6.0 to about 10.0 weight percent and, in accordance with conventional processing, this coke is calcined by heating to a temperature from 1800 to about 2600 F. to remove the volatiles and produce a dry calcined coke which can be graphitized in the conventional manner by heating to a temperature up to about 4600 to about 5000 F., when necessary, in the presence of a suitable binder such as a coal tar pitch or other high-boiling residual products. Upon completing of the graphitizing process it will be observed that the product has a coefficient of thermal expansion of desirably low value, e.g., from about 3 to about 6x 10 F.
The highly aromatic feedstocks which is useful for blending with the pyrolysis tar in accordance with this invention comprises a thermally or catalytically cracked heavy oil. These stocks are heavy oils, i.e., have a high molecular weight from about 200 to 350 and a high boiling point. Typically, their initial boiling point is greater than about 350 F. and their 50 percent overhead boiling point is greater than about 450 F. This heavy oil is highly aromatic and has a high carbon to hydrogen ratio. The aromatic content of suitable oils is from 45 to about 95 percent, preferably from about 60 to 92 percent. Their carbon content is from 85 to 98 percent, preferably from 90 to 95 percent, and their hydrogen content is from 2 to 15 percent, preferably from 5 to 9 percent. Such a feedstock can be obtained from thermal or pressure tar, i.e., residue, which has a large quantity of high molecular weight fused ring aromatic hydrocarbons. Preferably, however, the heavy oil is a decant oil which is obtained from the fluid catalytic cracking of a petroleum feedstock. The catalytic cracking is conventionally performed on gas oil fractions obtained from the distillation of crude oil or the distillation of thermally or catalytically cracked products therefrom. The gas oil is subjected to temperatures from 850 to about 975 F. in the presence of a catalyst having a particle size from 40 to about 350 mesh which is suspended in the vapors of the gas oil within the catalytic cracking reactor. The vaporous eflluent from the catalytic cracking operation is condensed and fractionally distilled to obtain gas, gasoline and a higher boiling cycle oil. The cycle oil can be recycled to the catalytic cracking operation or can be subjected to thermal cracking. The common procedure is to recycle this cycle oil to the catalytic cracking Zone. During this processing the quantity of aromatics, which are more refractory to the catalytic cracking, builds up in the cycle stock and, with most operations, it is necessary to withdraw a portion of the cycle stock from the catalytic cracking operation. Since the cycle stock contains some finely subdivided catalyst which is entrained in the vapor effluent from the reactor, it is decanted to obtain a decant oil which is removed from the process. This decant oil is a very desirable and suitable feedstock for the production of premium coke in accordance with this invention.
Alternatively, the cycle stock from the catalytic cracking operation can be thermally cracked at a pressure from atmospheric up to about 1000 p.s.i.g. to produce gas, gasoline and a heavy high boiling refractory material (residue) which is commonly referred to as pressure tar. This pressure tar is also a very desirable and suitable feedstock for use by the process of this invention.
Instead of a thermal tar derived from the thermal cracking of catalytic cracking gas oil cycle stocks or decant oils, residues or thermal tars derived from the thermal cracking of virgin gas oils are excellent feedstocks for use by the process of this invention. Thermal cracking is usually carried out at 875 to 1050 F. and at pressures from about 150 to 1000 p.s.i.g. In some cases, thermal tar derived from the thermal cracking of reduced crudes can also serve as excellent feedstocks.
The pyrolysis tar which is employed as a component of the blend of the stock which is coked in accordance with this invention can be any tar produced by thermal cracking in pyrolysis furnaces to produce low molecular weight olefins. In general, olefins comprising chiefly ethylene with lesser amounts of propylene, butene and isobutylene are produced by the severe cracking of petroleum distillates or residues at temperatures from 1200 to about 1800 F., preferably from 1300 to about 1600" F. at pressures from atmospheric to about 15 p.s.i.g. and in the presence of a diluent gas. Typical diluents employed are low-boiling hydrocarbons such as methane, ethane or propane, although steam is the preferred and most commonly employed diluent. The products of this cracking operation are predominantly olefinic gases such as ethylene, propylene and butenes. A heavy pyrolysis tar is obtained from this cracking operation and is removed with the effiuent and separated by condensation therefrom. This pyrolysis tar has a high olefinic content and is therefore unstable upon subsequent heating and has a marked tendency to deposit coke on the heating tubes of furnaces employed for its subsequent conversion. The material, however, also has an appreciable content of condensed polycyclic aromatic hydrocarbons.
The coking process of this invention is fairly conventional in conditions and operations, the significant change comprising the application of the process to the particular blend of the pyrolysis tar and highly aromatic, thermally or catalytically cracked heavy oil which is described herein. The coking operation generally employs a furnace with heating tubes through which the oil to be coked is passed and heated to temperatures from about 900 to about 970 F., preferably from 920 to about 970 F. at pressures from 25 to about 200 p.s.i.g., preferably from about 50 to about 150 p.s.i.g. coil outlet or drum pressure. The feedstocks employed have a suificiently high boiling point that, during the heating operation in the furnace tubes, the stock is predominantly present as a liquid medium, i.e., little or no vaporization of the charge stock occurs during the heating operation. Upon reaching the desired preheat temperature, the charge stock is discharged into a coking vessel generally by being introduced into the bottom of the vessel and permitted to flow upwardly therethrough.
The coking vessel has an overhead line from which vaporous products from the coking reaction can be Withdrawn to a fractionator. The remaining residue undergoes cracking and becomes reduced to dry coke and vapors which are removed overhead, condensed, fractionated and processed in accordance with the particular re finerys requirements.
The dry coke accumulates in the coking vessel until the vessel has become substantially filled with coke and, at that time, the charge stock is diverted into another vessel. The filled coke drum is then cooled, opened and the coke is removed therefrom by the use of water jets, drills, rams or other equipment for dislodging the coke from the vessel. In a typical coking operation with the charge stocks of this invention, the yield of coke produced comprises from about 15 to about weight percent of the charge stocks.
It has been observed that the coke prepared by the delayed coking in the manner aforedescribed using a charge stock comprising entirely the aforementioned aromatic heavy oils, e.g., the deoant oil or thermal tar, will produce coke which, when graphitized, yields a product having a coefficient of thermal expansion of very low value, e.g., from 3 to about 5 10' F. The delayed coking of a charge stock comprised entirely of the pyrolysis tar under the conditions described herein will yield coke which, when graphitized, produces a graphite having a significantly higher coefficient of thermal expansion, e.g., from 10 to about 25 l0" F. The latter product has a thermal expansion which is too great to permit its use as a premium graphite. It has also been observed that when the pyrolysis tar is subjected to the preheating operation described herein, it deposits large amounts of coke in the furnace tubes and requires frequent interruption of the heating operation to remove this coke from the heating tubes. Accordingly, this material is not a highly desirable feedstock for delayed coking operations.
It has now been observed that when a quantity of the aforementioned pyrolysis tar is blended with the aromatic, thermally or catalytically cracked heavy oil and the :blend is subjected to the aforementioned coking conditions, a premium coke can be produced which can be processed into graphite that will have a lower coefficient of thermal expansion than would be predicted from the knowledge of the coefficient of thermal expansions of graphite prepared from coke obtained by coking the unblended components separately. In other words, although the pyrolysis tar produces coke which, when graphitized, has a high coefiicient of thermal expansion, a large quantity of this material can be blended with the thermally or catalytically cracked heavy oil without appreciably increasing the coeflicient of thermal exapnsion of graphite prepared by coking of these stocks. It has also been observed that the blend of pyrolysis tar and thermally or catalytioally cracked heavy oil can be preheated in the conventional furnace heaters without encountering any objectionable amount of coke deposition in the heater tubes, contrary to the behavior of the pyrolysis tar when this material alone is preheated to the requisite coking temperatures.
Accordingly, this invention comprises blending pyrolysis tar with thermally or catalytically cracked heavy oil and coking the blend. Preferably, the pyrolysis tar is present in the blend in a quantity which is greater than the amount of the tar that would impart the coefficient of thermal expansion value to the graphite from coke produced by the blend if the coefficient of thermal expansion values of such coke were the additive result of the coefficient of thermal expansion values of graphite made from coke prepared from the individual stocks. Preferably, the amount of pyrolysis tar in the blend comprises from to about 75 weight percent; most preferably from to about 60 percent. The coke produced by such an operation, when graphitized, will be observed to have a disproportionally lower coefficient of thermal expansion than would be expected from the additive values of the coeflicient of thermal expansion of cokes prepared from the individual components.
The invention will now be described by reference to the following examples which will illustrate a mode of practice of the invention and demonstrate results obtainable thereby.
Example I The experiments were performed in a laboratory coking unit in which the oil is preheated in a coking coil 0.40 inch in internal diameter and 10 feet in length. The preheated oil is discharged into a cylindrical chamber having a 6 inch inside diameter and 36 inches in height. The oil is discharged into the bottom of the cylindrical chamber which is in upright position and the coke accumulates therein as the level of oil rises in this chamber. The top of the vessel has a vapor withdrawal conduit of 0.4 inch in diameter and vapors are removed from the drum. In the processing, the oil is preheated to a temperature of from 900 to 970 F. and the pressure on the heating coil is maintained with a pressure control valve at the discharge of the heating coil. The pressure in the coke drum is maintained at about 50 p.s.i.g. The temperature of the liquid within the coke drum varies somewhat depending on the preheat temperature, the coke drum pressure, and the nature of the feedstock.
In the first experiment the coking is performed on a decant oil obtained as the cycle stock of a fluidized catalytic cracking process; in the second experiment the coking is performed on a pyrolysis. tar obtained from the high temperature diluent cracking of naphtha to produce olefins, while in the third experiment the coking is performed on a mixture of these two stocks blended in an equal weight mixture. The pyrolysis tar has the following properties:
Gravity API-.. 1 Average molecular weight 225 Pour point F 15 Sulfur percent 0.3 Nitrogen percent 0.003 Carbon "percent" 93 Hydrogen "percent" 7 Viscosity SSU/ 210 F. 45 Carbon residue "percent" 11 Initial boiling point F-.. 383 50 percent boiling point F 590 95 percent boiling point F 915 Aromatic content percent 90.5 Aliphatic olefins "percent" 2.1 Saturates percent 0.3 Olefinic materials 1 percent 69.5
1 Present as alkenyl aromatics.
The coke obtained from the coking experiments is weighed to determine the yield of coke and samples of the coke are then calcined by heating to a temperature of about 2500 F., the calcined coke is comminuted and blended with a coal tar binder, the blend is extruded and the extrudate is baked by heating to a temperature of about 1800 F. The baked carbon is then graphitized by heating to a temperature of about 4900 F. in an electrical graphite resistance furnace to obtain a sample of graphite. The graphite sample is analyzed to determine its coefficient of thermal expansion over a range of temperatures from 50 to C. using an instrument having a differential transformer unit: manufactured by the Admerco Corporation.
The following table sets forth the results obtained by these experiments:
TABLE 1 Cok- Temp. Coke Graph- Expenment ing F. yield, rte CIE number Charge stock coil drum percent X10 1 Deeant oil 950 876 26.9 4.0 2 Pyrolysis tar A 930 853 17. 2 12. 3 3 Decant oil 50%, 900 861 25.4 4.5
pyrolysis tar A 50%.
also experienced in preheatingof the pyrolysis tar to the necessary preheat temperature and a number of attempts to coke this tar were unsuccessful because of the premature coking and excessive coke deposition which occurred in the laboratory preheater.
The undesirable characteristics of the pyrolysis tar upon coking were not observed when the pyrolysis tar was admixed with an equal weight amount of decant oil in Experiment No. 3. The mixture did not deposit any excess quantities of coke in the preheating coil and an attractive yield of coke, 25.4 weight percent, was obtained in the experiment. When this coke was processed into graphite the coefficient of thermal expansion of the graphitized product was 4.5 10' a value which is not significantly higher than the 4.0 10 observed upon the graphitizing of coke obtained from a charge stock comprising entirely decant oil. This result was quite surprising since the arithmetic blend of the coefficient of thermal expansions of the pyrolysis tar and decant oil is 8.3 and this would normally have been the expected coefficient of thermal expansion obtained from an equal weight mixture of the two charge stocks. This can be calculated as the sum of the products of the weight fraction times the coefiicient of thermal expansion for each coke. This is calculated as: 0.50 (4.0)+0.50 (12.3)=8.3.
Example 11 The preceding experiment is repeated except that the pyrolysis tar is replaced by another pyrolysis tar obtained from the high temperature diluent cracking of gas oil to produce ethylene. This tar has the following properties:
The graphite product is analyzed to determine its coefficient of thermal expansion over a range of temperatures from 25 to 125 C. using an instrument having a differential transformer sensing unit manufactured by the Labtronics Division of Theta Corporation. The following results are obtained:
TABLE 2 Cok- Temp. Coke Graph- Experiment ing F. yield, ite GTE number Charge stock coil drum percent X10- 4 Decant oil 950 877 26. 4. 0 5 Pyrolysis tar B 920 845 23.1 11.7 6 Decent oil 50%, 920 856 29. 9 5. 0
pyrolysis tar B The preceding data evidence that the graphite product obtained from the coke prepared from the blend of pyrolysis tar and decant oil had a disproportionately low coeflicient of thermal expansion. These data with a different pyrolysis tar confirm the data of Example 1.
Example III To illustrate the effect of blending of other charge stocks with decant oil, two other experiments were performed. In these experiments a commonly employed coking feedstock was used which is a heavy coker gas oil fraction having a boiling range of from about 600 to about 956 F. with a midpoint boiling point of 776 F. This material was obtained as a heavy gas oil distillate from a fractionator having a feed of the condensate from a delayed coking operation. In the first experiment, the heavy coker gas oil was coked separately and in the second experiment a mixture of 30 percent of the coker gas oil was blended with 70 percent of the decant oil and the resultant blend was subjected to the coking conditions. The following table summarizes the conditions of coking and the results obtained, including the results on the coefficient of thermal expansion of graphites prepared from tar was blended with the decant oil, the mixture of the heavy coker gas oil with decant oil produced a coke which, when graphitized, yielded a product having a higher coefiicient of thermal expansion than would have been expected from the thermal expansion coefficients of graphite from the pure stocks. Thus, the arithmetic blend of coefficient of thermal expansions in proportion to the percentages of the components in the change stock indicated that a coefficient of thermal expansion of about 6.3 should have been expected. In contrast to this result, the coefiicient of thermal expansion of the product was 7.8, indicating that this property of graphite cannot be predicted from aknowledge of the graphites produced by coking of the individual components.
The preceding examples are intended to fully illustrate a mode of practice of the invention and to demonstrate results obtainable thereby. It is not intended that the invention be unduly limited by these examples, but instead, it is intended that the invention be defined by the materials and method steps and their obvious equivalents set forth in the following claims.
1. A method for production of coke useful as a precursor for graphite having a desirably low coefficient of thermal expansion value which comprises admixing a quantity of a pyrolysis tar obtained from the high temperature diluent cracking of a petroleum fraction at temperatures from 1200 to 1800 F. to produce olefins with a heavy oil derived from the thermal or catalytical cracking of a petroleum feedstock at temperatures less than 1050 F. and having an aromatic content greater than 45 percent and an initial boiling point above about 350 F.; the quantity of said pyrolysis tar in said blend being from 20 to 60 percent of the blend and greater than the amount of said tar which would impart said coefficient of thermal expansion value to graphite from said coke if said value were the additive result of the coefficient of thermal expansion values of graphites prepared from cokes obtained from the individual tar and heavy oil; heating the blended stocks which consist essentially of the aforesaid tar and heavy oil to a temperature from 850 to about 1000 F. and passing the heated stocks to a drum and permitting the stocks to crackand form vapors and said coke therein.
2. The method of claim 1 wherein said quantity of pyrolysis tar is insufficient to increase the coeflicient of thermal expansion by an amount greater than 50 percent of the coefficient of thermal expansion observed for graphite prepared from coke obtained by coking of said heavy oil.
3. The method of claim 1 wherein said quantity of pyrolysis tar is less than the amount which would effect a 25 percent increase in the coefficient of thermal eX- pansion value of graphite prepared from coke obtained by coking of said heavy oil.
4. The method of claim 1 wherein said pyrolysis tar is obtained from the high temperature cracking of a petroleum naphtha to low molecular weight olefins.
5. The method of claim 1 wherein said pyrolysis tar is obtained from the high temperature cracking of a petro- 5 leum gas oil to low molecular weight olefins.
6. The method of claim 1 wherein said heavy oil is a decant oil from catalytic cracking.
7. The method of claim 1 wherein said heavy oil is a residue from thermal cracking.
10 References Cited UNITED STATES PATENTS 3,547,804 12/ 1970 Noguchi et al. 208-431 3,617,514 11/1971 Marla! 208-431 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 208-46
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