Coking decanted oil and other heavy oils to produce a superior quality of needle-grade coke

Abstract: A process for the production of needle-grade coke having a low coefficient of thermal expansion (CTE) value involves coking a petroleum feedstock in a coking drum at temperature below the threshold temperature where CTE values for the coke rapidly increase for a time sufficient to obtain a relatively low CTE coke. The petroleum feedstock can be decanted oil or heavy cycle oil. The threshold temperature can be from about 400°C to 600°C. A heated vapor stream is introduced into the drum to maintain the coke at a temperature sufficient for coking but below the threshold temperature.

Description: The invention relates to a process for production of needle-coke used in the manufacture of graphite electrodes for the steel industry. More particularly, this invention relates to a process for making needle-coke having the purity and physical properties necessary to meet the stringent quality criteria of graphite electrodes. Specifically, a low coefficient of thermal expansion (CTE) is one of the most critical parameters of quality coke. The needle-coke obtained in practice of the process of the present invention is particularly well-suited for use as graphite electrodes in the steel industry. The low coefficient of thermal expansion found in the coke obtained in practice of the present invention allows for the construction of superior graphite electrodes. Throughout the Specification, references will be made to the use of the needle-coke as used in the production of graphite electrodes for the steel industry, and certain prior art coke cases will be discussed. However, it should be realized that the invention could be used in the production of other coke materials, as well as high quality needle-coke. DESCRIPTION OF THE ART In the production of needle-coke used in the manufacture of graphite electrodes for the steel industry there are stringent quality criteria regarding its purity and physical properties. In particular, a low coefficient of thermal expansion is one of the most critical parameters of coke quality. The low CTE is necessary to give electrodes sufficient resistance to thermal shock. Current performance requirements necessitate that the CTE of the coke have a value of between 0.0 to 0.3 x 10⁻⁶ per degree Centigrade. Coke having CTE values greater than about 0.4 to 0.5 x 10⁻⁶ per degrees Centigrade has poor quality needles and is therefore unsuitable for steel electrodes. In the production of needle coke, there are competing interests. High temperature leads to increased reaction rates, shorter reaction times, and maximum productivity. However, the coke is of a low quality. Low temperatures, in contrast, result in slower reaction rates, longer reaction times, and reduced productivity, but tend to yield higher quality coke. Therefore it is necessary in the art to reach an acceptable point between low quality/high quantity coke production and high quality/low quantity coke production which allows production of the greatest amount of coke meeting necessary industry standards. Processes for producing coke are well-known. See for example, U.S. patent Numbers 3,745,110 and 3,836,434. Such processes involve heating certain petroleum hydrocarbon streams to elevated temperatures and rapidly running the hot hydrocarbons into the bottom of a relatively quiescent chamber known as a coking drum. As the hydrocarbons are charged into the coking drum they undergo coking, i.e., they undergo a chemical reaction and a physical change from a liquid to a solid. In addition, U.S. Patent No. 4,547,284 teaches that premium coke is made by filling a drum at a low temperature and then raising temperature during a heat soak cycle using a heated vapor. US Patent No.4,822,479 describes a process that involves calculating a minimum coking severity level based upon the percentage of aromatic-form carbon atoms in the feedstock. An expression is then used to identify minimum coking temperatures, soaking temperatures and reaction times that satisfy the required severity level, and produces coke having CTE and VBD values in the premium coke range. However, the generally accepted route to needle-coke from an oil is a series of carbonization reactions that first transforms the oil into a pitch, which then forms a liquid crystal called mesophase, which subsequently orients and solidifies into a needle structure. This process is explained in "Optimum Carbonization Conditions Needed to Form Needle-Coke", Mochida, I., Oil and Gas Journal , May 2, 1988. Mochida indicates that to produce low CTE needle-coke, the proper feedstock and proper operating conditions for that feedstock are important. He proposes that to form low CTE content needle-coke it is important to first form small spheres of mesophase pitch, to maintain a sufficiently low viscosity to allow the mesophase spheres to coalesce into large domains, and to produce sufficient gas evolution at the right time in the reaction cycle to orient the mesophase domains into the desired needle-like structure. Failure to meet all of these conditions will lead to a more amorphous structure which has a significantly higher CTE. A new process for making coke has been found by Applicants, wherein unique temperature and reaction times i.e., time at temperature, have been developed to form low CTE needle-coke. In particular, Applicants have learned that there is a specific threshold temperature range, above which, coking will result in unexpectedly high CTE values. In addition, Applicants have discovered a minimum threshold reaction time, above which further reaction time does not significantly effect the CTE value of the coke. Accordingly, Applicants have established a new and improved coking process, wherein, a high quality low CTE value coke is produced by utilizing temperatures below the threshold temperature range for reaction time sufficient to achieve a low CTE value. SUMMARY OF THE INVENTION Accordingly, it is a primary object of this invention to provide a new and improved process for the production of needle-coke. It is a further object of this invention to produce needle-coke having a reduced CTE. One use would then be its conversion into high quality graphite electrodes for the steel industry. A still further object of the present invention is to provide a unique process which operates at the temperature and the time conditions newly discovered which produce high yields of suitable quality needle-coke. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious to one skilled in the art from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing objects in accordance with the purpose of the invention, which is embodied and broadly described herein, the process of this invention comprises introducing a heated petroleum feedstock into a coking drum, maintaining the temperature of the drum contents in a range near but below the CTE threshold temperature during the balance of the filling cycle, and maintaining the temperature of the drum contents at about the same temperature during the post-fill portion of the cycle by passing a heated vapor through the coke drum for sufficient time to allow the drum contents to properly react, orient, and solidify into a solid product with the desired properties. The time at temperature during fill in combination with the vapor introduction time should be at least the threshold reaction time. The threshold temperature, as described herein, basically encompasses the highest temperature at which coke can be produced while maintaining an acceptable CTE. The temperature at which a rapid CTE increase occurs will vary with the feedstock. Generally, however, the magnitude of increase would include a 100% increase in CTE value over a 20°C temperature rise. In general, as the reaction time increases, the drum contents continue to react and orient, forming a product with improved physical properties such as a lower CTE. When the time-at-temperature exceeds the threshold reaction time, further improvements in CTE with increasing time are minimal. In addition to high quality coke, the process results in more uniform coke, i.e., more consistent CTE values throughout the drum because of a more narrow residence time distribution. Also, the process minimizes reaction time by operating at the highest temperature possible while meeting coke product quality specifications. Accordingly, economic advantages are realized. DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the preferred embodiment of the invention and the examples which are illustrated in the accompanying Tables. Due to the complex nature of reactions occurring in a coke drum, it is impossible to specify the reaction network on a molecular level. Although not wishing to be bound by theory, the generally accepted route to form needle-coke from a hydrocarbon feedstock is a series of carbonization reactions that first transform the oil to a pitch, which then forms a liquid crystal called mesophase, which subsequently orients and solidifies into a needle structure. The present invention is a new and improved process for coke production. The process comprises heating a petroleum feedstock to a temperature necessary to maintain the temperature of the drum contents at a level sufficient for coking but below the threshold temperature and introducing the heated feedstock to a coking drum. Often it is desirable to heat the feedstock above its threshold temperature due to the inevitable cooling experienced by the feedstock in transit from furnace outlet to coke drum inlet and due to exothermic cracking reactions. This process is enhanced by filling the coking drum as rapidly as the physical constraints of the system allow. After the coke drum is filled to the desired level with the heated feedstock, a heated vapor is introduced to the coke drum. The vapor is introduced at a temperature sufficient to maintain the contents of the coke drum at a temperature near to but below the threshold temperature. The introduction of the vapor is conducted for at least the threshold reaction time. Following introduction of the vapor the formed coke can be stripped using steam, light hydrocarbons, or other solvents and removed from the drum as is known in the art. In accordance with the present invention, the feed can be any type of petroleum feedstock. Preferably, the feedstock is a fluid cat cracker decanted oil, a heavy cycle oil, or a filtered decanted oil. Most preferably, the feedstock is a fluid cat cracker decanted oil. Furthermore, blends of the above feedstocks can be utilized. The temperature to which the feed is heated is determined for each particular feed depending on the desired temperature range of the drum contents to obtain sufficiently low CTE in the product coke to meet product specifications. It is desirable to maintain the coke drum contents at the threshold temperature. The threshold temperature is the point at which increased temperature leads to rapidly increasing CTE values. The threshold temperature can in fact cover a range of temperatures of about 10°-20°C over which the CTE value of the coke begins its rapid increase, and above which CTE rapidly increases. The threshold temperature for a given feedstock is also a function of the drum pressure, recycle ratios and other parameters known to one skilled in the art. When the feedstock is a decanted oil, the coking temperature is preferably in the range of about 400°C to about 600°C. More preferably, the coking temperature is between 420°C and 510°C. Most preferably, the temperature is in the range of about 460°C to about 500°C. However, the temperature is dependent upon the feedstock and must be determined for each individual feedstock. This determination can be accomplished by the process described in the following examples. Without wishing to be bound by theory, Applicants believe that low CTE values are obtained in the product coke at or below the threshold temperatures because the mesophase is given sufficient time at the necessary viscosity to permit coalescence into large domains. Furthermore, gas generation occurs during the correct portion of the polymerization reaction cycle to align the large mesophase domains which ultimately solidify into aligned needle structures. At high temperatures, the coking occurs too rapidly to allow coalescence and small domain mosaic mesophase structures are generated which have a higher CTE. It is critical to delay the solidification until coalescence and orientation occur to avoid production of high CTE value coke. The temperature in the coke drum is maintained after drum fill by sending a vapor with a low coking tendency through the coke drum. Preferably, the vapor is a hydrocarbon, steam, nitrogen, refinery gas, carbon dioxide or any inert gases or mixtures thereof. More preferably, the vapor is a refinery derived light hydrocarbon stream for example fluid cat cracker light cycle oil, coker heavy gas oil, or mixtures thereof. In one embodiment the vapor is recycled within the process, wherein the vapor stream is obtained from a bubble tower which is in combination with the coking drum system. In another embodiment, the vapor is recycled outside the coker unit operation, i.e., a fractionation tower not in combination vith the coking system. In a still further embodiment, the vapor is used on a once through basis. It has also been determined that reaction time is a crucial factor to the production of low CTE value coke. It has been found that insufficient reaction times can lead to insufficient development and solidification of the coke structure leading to significant amounts of sparsely condensed solid pitch which form poor quality coke in a calciner. This material will not meet typical needle-coke specifications. Furthermore, it has been determined that CTE values are not strongly influenced by additional time-at-temperature exceeding a certain minimum reaction time, herein described as the threshold reaction time. More particularly, longer time-at-temperature results in little change in CTE once the threshold reaction time for a particular feedstock at a particular temperature is reached. This establishes that low CTE coke will form given enough time and will remain low CTE coke even when exposed to very long time-at-temperature. Therefore, operation at a temperature below the threshold range, as described above, in combination with a cycle time slightly above the threshold reaction time results in an efficient production of needle-coke having a low CTE value. If other requirements dictate, the reaction time can be adjusted. For example, since the drum is filled gradually, the upper portion of the coke experiences a shorter coking time. Accordingly, it is up to the individual operation to determine if the most beneficial procedure involves coking only a portion of the coke for the threshold reaction time. For example, the lower 90% of the drum, which is filled first, may be coked for the threshold reaction time and form higher quality coke, while the upper 10% of the drum is coked for less than the threshold time and is of lesser quality. The upper 10% may be sacrificed in quality to obtain the lower 90% in a shorter period of time. The following examples demonstrate the invention. Example I Six feedstocks were studied to determine the effect of temperature on various types of feedstock. The experimentation was performed in a micro-coker system. This system consists of a glass tube sealed at one end and filled with the desired coking feedstock. This filled tube is placed in a custom-built 100cc stainless steel pressure vessel. The top of the vessel is sealed by deforming a copper gasket when the screw cap is tightened. The vessel is then connected to a gas/liquid separator and a back pressure regulator. The system is pressurized to the desired operating pressure, and the vessel is placed in a fluidized sandbath set to the desired operating temperature. Gases and vaporized liquids exit through the top of the vessel and are separated in the gas/liquid separator. The 1/8 inch tube connecting the vessel and the separator serves as a heat exchanger to condense the liquids. Gases leave the system through the regulator as it maintains a constant pressure. The six feedstocks, as described in Table 1, consisting of decanted oil fractions and blends thereof, were coked at a temperature range of 460°C-525°C for 16 hours. Table II displays the results for the various feedstocks. All of the feedstocks display about the same low CTE value at 480°C or below and about the same high CTE value at 510°C or above. Table II shows a dramatic increase in CTE at temperatures above the threshold temperature. Coke from feedstock 3, for example, shows a dramatic CTE increase from 0.1 to 1.05 X 10⁻⁶/°C over only a 15°C temperature increase. This suggests that for feedstock 3, 490°C is already past the threshold maximum coking temperature, while 475°C is below the threshold temperature. The results from the other feedstocks indicate a threshold temperature less than 510°C. The particular threshold temperature for any given feedstock can be determined by this method using micro-coker experiments. More particularly, coking operations can be conducted on a feedstock at gradually increasing temperatures, and the resulting needle-coke can be analyzed to determine CTE values. The threshold temperature point or range will appear as that temperature or temperatures where CTE values rapidly increase with increasing temperature. Additional experiments were conducted at 460°C on feedstock 1 and feedstock 2 in an effort to explore the effect of a much lower coking temperature on CTE. The data shows a low CTE between 460° and 480°C at 16 hours coking time, suggesting that there is at least a 20°C temperature "window of operability" that can produce a low CTE needle coke for these feedstocks. Example II Reaction time effects on CTE were determined for feedstocks 1 am 2 using the above described micro-coker system. The feedstocks were subjected to varying coking times at at 460°C and 480°C. Three sets of time behavior micro-coking experiments were run, including 8, 16, 64 hour coking times at 460°C coking temperature for feedstock 1. Also, 12 and 16 hour coking time experiments were run for feedstock 2 at a 460°C coking temperature. Finally, 8, 10, and 16 hour coking times were tested for feedstock 2 at 480° coking temperature. The results are displayed in Table III. The experiments with feedstock 1 were conducted at short (8-hour) and long (64-hour) coking times at 460°C. The 8 hour experiment was chosen to simulate the coke at the point when solidification was just about complete. The 64 hour experiment was chosen to see if any changes occur to the coke long after solidification. The 8-hour, 460°C run with feedstock 1 did not develop sufficient coke structure and had significant amounts of partially-condensed solid pitch present which formed into low quality coke in the calciner. This is unacceptable for producing quality needle coke, therefore, 8 hours is below the necessary minimum coking time at 460°C for feedstock 1. However, when feedstock 1 was coked for 16 and 64 hours at 460°C, the CTE was no longer a strong function of time-at-temperature, because both experiments resulted in low CTE values. This established a key finding: low CTE coke will occur given enough time and will stay a low CTE value with additional time at 460°C for feedstock 1 coke, and presumably for other feedstocks as well. The results from the feedstock 2 reaction time studies at 460°C and 480°C show that a much longer time is necessary at 460°C to achieve low CTE coke, approximately 12-16 hours, while at 480°C about 8 hours or less is required. This is indicated by Table III, where CTE decreases from 12 to 16 hours at 460°C, but remains constant during this time period at 480°C within experimental error. The threshold reaction time can be determined for any particular feedstock by coking the feedstock at a particular temperature, preferably just below the threshold temperature for various periods of time and analyzing the resultant needle-coke to determine CTE values. The CTE values should decrease over time to the threshold reaction time, at which point, CTE values will change only slightly with increasing time-at-temperature. Effect of time at 460°C, and 480°C For Two Feeds Feedstock No. Temperature °C Time (Hours) CTE x 10⁻⁶/°C 1 460 8 >1 1 460 16 -.04 1 460 64 -.34 2 460 12 .34 2 460 16 .10 2 480 8 .04 2 480 10 .21 2 480 16 .08

Claims: A process for forming coke comprising; (a) determining the threshold temperature for a petroleum feedstock by measuring the coefficient of thermal expansion of coke formed from said feedstock at various coking temperatures, wherein said threshold temperature is the coking temperature at which the coefficient of thermal expansion rapidly increases in conjunction with a narrow range of coking temperature increase; (b) heating said petroleum feedstock and introducing said petroleum feedstock into a coking drum; (c) maintaining the coke drum contents at a temperature below said threshold temperature; (d) introducing a heated vapor stream to said coke drum to maintain said coke at a temperature sufficient for coking but below said threshold temperature; (e) maintaining the introduction of said heated vapor for a time necessary to reach a threshold reaction time for said drum contents; and (f) removing said coke from said coking drum. A process as claimed in claim 1, wherein said petroleum feedstock is selected from the group consisting of decanted oils, heavy cycle oils, or mixtures thereof. A process as claimed in claim 2, wherein said petroleum feedstock is decanted oil. A process as claimed in any one of the preceding claims wherein said threshold temperature is between 400°C and 600°C. A process as claimed in claim 4, wherein said threshold temperature is between 460°C and 500°C. A process as claimed in any one of the preceding claims wherein said vapor is selected from the group consisting of a hydrocarbon with a low coking tendency, nitrogen, inert gases, carbon dioxide, refinery gas, steam, or mixtures thereof. A process as claimed in claim 6, wherein said hydrocarbon with a low coking tendency comprises fluid cat cracker light cycle oil. A process as claimed in claim 6, wherein said hydrocarbon with a low coking tendency comprises coker heavy gas oil. A process as claimed in any one of the preceding claims, wherein said vapor is recycled through a bubble tower in combination with the coking system. A process as claimed in any one of the preceding claims, wherein said vapor is obtained from a fractionation tower not connected to the coking system. A process as claimed in any one of the preceding claims, wherein said threshold reaction time is between 8 hours and 16 hours. A process as claimed in any one of the preceding claims, wherein said threshold reaction time is not exceeded by more than about one hour. A process as claimed in any one of the preceding claims, wherein a substantial portion of said coke drum contents are maintained for said threshold reaction time at a temperature sufficient for coking but below the threshold temperature.

Inventors: ADAMS HARRY A, HAUSER JEFFREY B, PASPEK STEPHEN C, ROMERO SIM, ADAMS, HARRY A., HAUSER, JEFFREY B., PASPEK, STEPHEN C., ROMERO, SIM
Applicants: STANDARD OIL CO OHIO, THE STANDARD OIL COMPANY
Application year: 1991
Main classification: C10B55
IPC codes: C10B55