CA2041436A1

CA2041436A1 - Coking decanted oil and other heavy oils to produce a superior quality of needle-grade coke

Info

Publication number: CA2041436A1
Application number: CA002041436A
Authority: CA
Inventors: Stephen C. Paspek; Jeffrey B. Hauser; Harry A. Adams; Sim Romero
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1990-05-04
Filing date: 1991-04-29
Publication date: 1991-11-05
Also published as: EP0455504B1; EP0455504A1; DE69108440T2; DE69108440D1; ATE120479T1

Abstract

ABSTRACT

The present invention is directed to a process for the production of needle-coke having a low CTE value. The process involves coking a petroleum feedstock at temperature below the point where CTE values for the coke rapidly increase for a time sufficient to obtain a relatively low CTE coke.

Description

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SA~-P-44~7 THE SP~CIFICATION

COKING DECANTED OIL AND OTHER HEAVY OILS
TO P~ODUCE A SUPERIOR QUALITY OF NEEDLE-GRADE COKE

BACKGROUND OF T~E INVENTION

The invention relates to a process for production of needle-coke used in the manufacture of graphite electrodes for the steel industry.
More par~icularly, this invention relates to a process for making needle-coke having the purity and physical properties necessary to meet the 5 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 10 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, 15 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.

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SAM-P-4427 Page ~

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 5 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 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 15 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 produc~ion and high quality/low 20 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; the disclosures of vhich are incorporated herein by reference. Such processes involve heating certain 25 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.
30 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.
~ ue to the complex nature of reactions occurring in a coke drum, it is impossible to specify the reactions at work on a molecular level.

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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 5 is explained in "0ptimum 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 10 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 conditiolls will lead 15 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 20 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 ~5 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 5 unique process which oper~tes 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 10 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 15 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 20 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 25 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 30 magnitude of increase would include a 100% increase in CTE value over a 20C 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 coket the process results in more uniform coke, i.e., more consistent CTE values throughout the drum because 5 of a more narrow residence time distribution. Also, the process minimiæes 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 15 accompanying Tables. While the inventive process will be described in connection with a preferred procedure, it will be understood that it is not intended to limit the invention to that embodiment or procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the 20 invention defined by the appended claims.
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 25 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 30 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 ?J~

SA~-~-4427 Page 6 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 5 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 dru~ 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 15 cracker decanted oil, a heavy cycle oil, or a filtered decanted oi1. Most preferably, the feedstock is a fluid cat cracker decanted oil.
Furthermore, blends of the above feedstoclcs 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 20 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 25 threshold temperature can in fact cover a range of temperatures of about 10-20C 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 400C to about 600C. More preferably, the coking temperature is between 420C and 510C. Most preferably, the temperature is in the range of about 460C to about 500C. However, the temperature is deyendent upon the feedstock and must be determined for 2 ~ 3 ~

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 a~ or below the threshold 5 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, 10 the coking occurs too rapidly to allow coalescence and s~all 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 15 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 20 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 with the coking system. In a still further embodiment, the vapor is used 25 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 30 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 2 ~

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 5 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 dic~ate, the reaction time can be 10 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 15 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 tlme.
The following examples demonstrate the invention.

Example I

Six feedstocks were studied to determine the effect of 25 tempera~ure on various types of feedstock. The experi~entation 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 lOOcc stainless steel pressure vessel. The top of the vessel is sealed by deforming a copper gasket when 30 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 ~'J ~ 3~ S

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 5 decanted oil fractions and blends thereof, were coked at a temperature range of 460C-525C for 16 hours.

TABLE I
Feedstock Properties Feedstock Beta/Aromatic Wt. ~ pentane ~ Aromatic ~ydrogen/
Number Hydrogen Insolubles Carbon %Carbon Carbon 1 0.59 5.0 71.8 90.9 1.08 2 1.01 3.4 66.2 90.5 0.97 3 0.~0 <.10 ---- ----- ----0.80 5.0 ---- ---- ----0.50 <.10 ---- ---- ----6 0.50 5.0 ---- --_ ____ Table II displays the results for the various feedstocks. All of the feedstocks display about the same low CTE value at 4~0C or below and about the same high CTE value at 510C 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 15C
temperature increase. This suggests that for feedstock 3, 490C is already past the threshold maximum coking temperature, while 475C is 30 below the threshold temperature. The results from the other feedstocks indicate a threshold temperature less than 510C. 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 35 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.

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Additional experiments were conducted at 460C 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 480C at 16 hours coking time, suggesting that there is at least a 20C temperature 5 "window of operability" that can produce a low CTE needle coke for these feedstocks.

TABLE II
CTE Value of Coking Feedstocks at 16 Hours Product Coke Feedstock No.Temperature CCTE x 10-6/C
1 460 -.04 1 480 -.01 1 510 .86 1 525 1.05 2 460 .10 2 480 .08 2 510 .87 3 475 .01 3 490 l.OS
3 510 1.22 4 480 -.10 480 -.10 510 .73 6 480 -.04 6 510 l.OQ

Example II

Reaction time effects on CTE were determined for feedstocks 1 and 2 using the above described micro-coker system. The feedstocks were 35 subjected to varying coking times at at 460C and 480C. Three sets of time behavior micro-coking experiments were run, including 8, 16, 64 hour coking times at 460C coking temperature for feedstock 1. Also, 12 and 16 hour coking time experiments were run for feedstock 2 at a 460C coking temperature. Finally, 8, 10, and 16 hour coking times were tested for 2 ~

feedstock 2 at 480 coking temperature. The results are displayed in Table III.
The experiments with feedstock 1 were conducte~ at short (8-hour) and long (64-hour) coking times at 460C. The 8 hour experiment was 5 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, 460C run with feedstock 1 did ~ot develop sufficient coke structure and had significant amounts of partially-condensed solid pitch present which ~ormed into low 10 quality coke in the calciner. ~his is unacceptable for producing quality needle coke, therefore, 8 hours is below the necessary minimum coking time at 460C for feedstock 1. However, when feedstock 1 was coked for 16 and 64 hours at 460C, the CTE was no longer a strong function of time-at-temperature, because both experiments resulted in low CTE values. This 15 established a key finding: low CTE coke will occur given enough time and will stay a low CTE value with additional time at 460C for feedstock 1 coke, and presumably for other ~eedstocks as well.
The results from the feedstock 2 reaction time studies at 460C
and 480C show that a much longer time is necessary at 460C to achieve 20 low CTE coke, approximately 12-16 hours, while at 480C about 8 hours or less is required. This is indicated by Table III, where CTE decreases from 12 to 16 hours at 460C, but remains constant during this time period at 480C within experimental error.
The threshold reaction time can be determined for any particular 25 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 ti~e, at which point, CT~ values will change only slightly with increasing time-at-30 temperature.

TABLE III
Effect of time at 460C, and 480C For Two Feeds 5 Feedstock No. Temperature C Time (Hours) _TE x 10-6/C
1 460 8 >1 1 460 16 -.04 l 460 64 -.34 10 2 460 12 .34 2 460 16 .10 2 480 8 .04 2 4~0 10 .21 2 480 16 .08 Thus it is apparent that there has been provided, in accordance with the invention, a process that fully satisfies the object, aims, and advantages set forth above. Uhile the invention has been described in conjunction with specific embodiments thereof, it is evident that many 20 alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations a followed in the scope, spirit and broad scope of the appended claims.

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Claims

Claim 1. A process for forming coke comprising;

a) introducing a heated petroleum feedstock into a coking drum;
b) maintaining the coke drum contents at a temperature below the threshold temperature in said coking drum during said introduction;
c) introducing a heated vapor stream to said coke drum to maintain said coke at a temperature sufficient for coking but below the threshold temperature;
d) maintaining the introduction of said heated vapor for a time necessary to reach a threshold reaction time for said drum contents; and e) removing said coke from said coking drum.
Claim 2. The process of claim 1, wherein said petroleum feedstock is selected from the group consisting of decanted oils, heavy cycle oils, or mixtures thereof.
Claim 3. The process of claim 2, wherein said petroleum feedstock is a decanted oil.
Claim 4. The process of claim 1, wherein said threshold temperature is between about 400°C and about 600°C.
Claim 5. The process of claim 4, wherein said threshold temperature is between 460°C and 500°C.
Claim 6. The process of claim 1, 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.
Claim 7. The process of claim 6, wherein said hydrocarbon with a low coking tendency comprises fluid cat cracker light cycle oil.
Claim 8. The process of claim 6, wherein said hydrocarbon with a low coking tendency comprises coker heavy gas oil.
Claim 9. The process of claim 1, wherein said vapor is recycled through a bubble tower in combination with the coking system.
Claim 10. The process of claim 1, wherein said vapor is obtained from a fractionation tower not connected to the coking system.
Claim 11. The process of claim 6, wherein said threshold reaction time is between about 8 hours and about 16 hours.
Claim 12. The process of claim 1, wherein said threshold reaction time is not exceeded by more than about one hour.
Claim 13. The process of claim 1, 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.