Process for producing petroleum needle coke

Disclosed herein are a process for producing needle coke, which comprises reducing the ash content in a heavy oil obtained from fluid catalytic cracking of petroleum or in a hydrocarbon material mainly composed of said heavy oil to not more than 0.01 wt % by means of (1) filtration, (2) centrifugation, (3) electrostatic aggregation or (4) a combination thereof, and coking the thus treated heavy oil or hydrocarbon material with an ash content of not more than 0.01 wt %; and a needle coke produced by coking a heavy oil obtained from fluid catalytic cracking of petroleum or a hydrocarbon material mainly composed of said heavy oil.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for producing petroleum needle 
coke which is low in thermal expansion coefficient. 
Recently, a demand has been rising for providing needle coke with a low 
thermal expansion coefficient to suit with the certain use conditions of 
electrodes. 
Researches for meeting this demand had been carried out and as a result, 
needle coke having a lower thermal expansion coefficient than that of the 
conventional petroleum needle coke could be produced by removing quinoline 
insolubles from coal tar or coal tar pitch. 
However, increasing harshness of the use conditions of electrodes has 
enhanced the necessity for needle coke having an even lower thermal 
expansion coefficient. Various attempts for lowering the thermal expansion 
coefficient, for example, attempts for producing needle coke having a 
lower thermal expansion coefficient than that of coal needle coke by using 
petroleum materials have been conducted. Nevertheless, none of the 
proposed methods and techniques have been successful in terms of practical 
use, and there has yet been offered no commercial petroleum needle coke 
having a lower thermal expansion coefficient than that of coal needle 
coke. 
In the fluid catalytic cracking decant oil (hereinafter abbreviated as FCC 
decant oil) used as starting material in preparation of petroleum needle 
coke, there is contained usually about 0.02 to 0.03 wt % (200 to 300 ppm) 
or more of "ash", that is, fluid catalytic cracking catalyst (hereinafter 
abbreviated as FCC catalyst) such as silica-alumina catalyst, etc. Namely, 
it is known that the needle coke obtained from an FCC decant oil retaining 
an FCC catalyst in a high content is poor in properties such as thermal 
expansion coefficient. Therefore, it has been tried to remove the FCC 
catalyst from the FCC decant oil by suitable means such as static 
separation, etc. to reduce the FCC catalyst content to the above-mentioned 
range of about 200 to 300 ppm, and to thus treated FCC decant oil has been 
used as starting material for preparation of petroleum needle coke. 
However, the obtained needle coke was not sufficiently low in thermal 
expansion coefficient. 
Thus; the establishment of a process for easy commercial production of 
petroleum needle coke with a low thermal expansion coefficient and high 
quality has been demanded. 
As the result of the present inventors earnest researches on the subject 
matter, it has been found that the FCC catalyst detrimental to thermal 
expansion coefficient in the FCC decant oil can be easily removed by means 
of (1) filtration, (2) centrifugation and/or (3) electrostatic 
aggregation, and by using this treated PCC decant oil, needle coke having 
a thermal expansion coefficient equal to or smaller than that of coal 
needle coke can be obtained. The present invention was achieved on the 
basis of this finding. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an industrial process for 
producing easily and simply petroleum needle coke with a low thermal 
expansion coefficient and high quality. 
An another object of the present invention is to provide petroleum needle 
coke with a low thermal expansion coefficient and high quality. 
In a first aspect of the present invention, there is provided a process for 
producing needle coke, which comprises reducing the ash content in a heavy 
oil obtained from fluid catalytic cracking of petroleum or in a 
hydrocarbon material mainly composed of the said heavy oil to 0.01 wt % or 
less by means of (1) filtration, (2) centrifugation, (3) electrostatic 
aggregation or (4) combination thereof, and coking the thus treated heavy 
oil or hydrocarbon material with an ash content of not more than 0.01 wt. 
%. 
In a second aspect of the present invention, there is provided a process 
for producing needle coked which comprises reducing the ash content in a 
heavy oil obtained from fluid catalytic cracking of petroleum or in a 
hydrocarbon material mainly composed of the said heat oil to 0.01 wt % or 
less by means of (1) filtration, (2) centrifugation, (3) electrostatic 
aggregation or (4) a combination thereof, mixing the thus obtained heavy 
oil or hydrocarbon material having an ash content of not more than 0.01 wt 
% with a coal-tar heavy oil substantially free of quinoline insolubles, 
and coking the resulting mixture. 
In a third aspect of the present invention, there is provided needle coke 
produced by the process set forth in the first aspect of the invention. 
In a fourth aspect of the present invention, there is provided needle coke 
produced by the process set forth in the second aspect of the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
The FCC decant oil used as a starting material in preparation of petroleum 
needle coke in the present invention is an oil which is obtained as a 
by-product in the process of production of gasoline, LPG or the like 
through catalytic cracking of petroleum fractions such as light oil by use 
of a granulated catalyst. 
The "hydrocarbon material mainly composed of FCC decant oil", which is also 
usable as starting material of petroleum needle coke in the present 
invention, is a product obtained by mixing a hydrolysis oil, 
normal-pressure residual oil, reduced-pressure residual oil, coal tar, 
coal tar pitch and/or other organic materials with an FCC decant oil in a 
ratio of 30 to 90 parts by weight, preferably 40 to 80 parts by weight 
based on 100 parts by weight of the said FCC decant oil. 
As the methods usable for removing ash (reducing the ash content) in the 
present inventions (1) filtration and (2) centrifugation and/or (3) 
electrostatic aggregation may be exemplified. 
(1) The filtration mentioned above is a method in which an FCC decant oil 
or a hydrocarbon material mainly composed of the FCC decant oil 
(hereinafter referred to as starting material), which has been heated to a 
temperature of 100.degree. to 300.degree. C., preferably 150.degree. to 
250.degree. C., is passed through a membrane filter with a mesh size of 
not more than 3 .mu.m, preferably 0.1 to 1 .mu.m under a pressure of 1 to 
5 kg/cm.sup.2 G, preferably 1 to 3 kg/cm.sup.2 G. 
It is preferable to reduce the viscosity of the starting material for 
increasing filtration efficiency. But, when the starting material is 
heated to a temperature of more than 300.degree. C., the internal 
disturbance may be caused in the starting material. Also when the heating 
temperature is less than 100.degree. C., the viscosity of the starting 
material may elevate, thereby lowering the filtration efficiency. Still 
more, when the pressure applied for filtration is less than 1 kg/cm.sup.2 
G, the ash-filtering efficiency becomes low. When the pressure exceeds 5 
kg/cm.sup.2 G, some ash may be allowed to pass through the filter, 
resulting in incomplete filtration. When the mesh size of the membrane is 
greater than 3 .mu.m, some ash may also be allowed to pass through the 
filter to cause unsatisfactory filtration. 
(2) In the centrifugation, the starting material heated to a temperature of 
100.degree. to 300.degree. C. preferably 150.degree. to 250.degree. C. is 
centrifuged under a centrifuging force of not less than 7,000 G, 
preferably 8,000 to 10,000 G, at a temperature of 100 to 300 to separate 
ash. When the centrifuging force applied is less thin 7,000 G, the ash 
removing efficiency is unacceptably low. The centrifuging temperature 
exceeding 300.degree. C. is undesirable in view of durability of the 
centrifuge and probability of causing internal disturbance in the 
material. 
(3) The electrostatic aggregation is a method in which a voltage is applied 
to the ash particle as such as catalyst particles contained in the 
starting material to electrically charge them, so that they aggregate with 
each other; and the resultant aggregates are removed from the starting 
material by a known means. 
An example of this method is described below. At least two electrode plates 
having a sufficient surface area are disposed with a spacing of 1 to 1,000 
mm, preferably 50 to 300 mm between the electrode plates, and a voltage of 
1 to 100 kV, preferably 5 to 30 kV is applied across the electrode plates. 
Then the starting material heated to a temperature of 100.degree. to 
300.degree. C., preferably 150.degree. to 250.degree. C. is passed between 
the electrode plates to electrically charge and aggregate the ash 
particles such as fine catalyst particles contained therein with each 
other. The thus treated material is subjected to the above-described 
filtration and/or centrifugation to remove ash. 
When the spacing between the electrode plates exceeds 1,000 mm or when the 
voltage applied is less than 1 kV, electric charging and aggregation of 
the ash particles are insufficient, resulting in unsatisfactory ash 
removal. Also, when the distance between the electrode plates is less than 
1 mm, the starting material is unable to pass between the plates. 
Application of a voltage exceeding 100 kV causes internal disturbance 
(phenomenon of bubble generation by volatilization of low boiling-point 
materials) in the material. 
For effecting efficient aggregation and removal of the ash particles such 
as fine catalyst particles in the starting material, it is preferable to 
lower the viscosity of the material by heating. However, heating more than 
300.degree. C. is undesirable as it may cause internal disturbance in the 
material. Addition of a light oil such as naphthalene oil and creosote oil 
is also a recommendable method for lowering the viscosity of the starting 
material. 
As a result of the above ash removing treatment, the ash content in the 
starting material is reduced to not more than 0.01 wt %, preferably not 
more than 0.005 wt %, more preferably not more than 0.002 wt %. 
The thus treated material is charged into a delayed coker and coked therein 
at a temperature of 450.degree. to 500.degree. C. to obtain green coke. 
This green coke is calcined at a temperature of 1,200.degree. to 
1,500.degree. C. by using a rotary kiln, rotary hearth electric furnace, 
shaft kiln or the like to obtain needle coke. 
A coal-tar heavy oil substantially free of quinoline insolubles may be 
mixed with the above treated starting material having an ash content of 
not more than 0.01 wt % in an amount of 30 to 95 parts by weight, 
preferably 40 to 80 parts by weight based on 100 parts by weight of the 
starting material to prepare a coking raw material. 
Typical examples of the coal-tar heavy oil are ordinary coal tar which is 
generated as a by-product in the process of coke production and coal tar 
pitch with a softening point of not more than 100.degree. C. 
The "substantially free of quinoline insolubles" means that the content of 
the quinoline insolubles is not more than 0.1 wt %. The known methods 
(such as disclosed in DE 2638992) can be applied for removing the 
quinoline insolubles from the coal tar heavy oil. 
The thermal expansion coefficient of the electrode produced from the needle 
coke obtained in the manner described above is not mote than 
5.5.times.10.sup.-7 /.degree.C., preferably not more than 
5.3.times.10.sup.-7 /.degree.C., more preferably not more than 
4.9.times.10.sup.-7 /.degree.C. 
The needle coke obtained according to the process of the present invention 
is useful as an electrode material Because of its small thermal expansion 
coefficient.

EXAMPLES 
The present invention is further described below with reference to the 
embodiments thereof. 
The thermal expansion coefficient was determined in the following way. The 
calcined coke was adjusted in particle size and added with 2% of iron 
oxide as inhibitor, and after one-hour mixing by a kneeder, the resultant 
mixture was molded into a labo-electrode, which was then calcined at a 
temperature of 1,000.degree. C. and further subjected to a graphitizing 
treatment at a temperature of 2,800.degree. C. The thermal expansion 
coefficient of the resulting product was measured. 
Example 1 
An FCC decant oil (ash content:0.024 wt %) was heated to a temperature of 
150.degree. C. and passed through a membrane filter having 0.5 .mu.m of a 
membrane size under a pressure of 4 kg/cm.sup.2 G remove ash. The 
resulting FCC decant oil was coked in an autoclave at a temperature of 
500.degree. C. for 24 hours under a pressure of 3 kg/cm.sup.2 G and 
calcined at a temperature of 1,400.degree. C. The results are shown in 
Table 1. 
Example 2 
An FCC decant oil (ash content: 0.024 wt %) was heated to a temperature of 
100.degree. C. and centrifuged by a self-discharging-type disc centrifuge 
with a centrifugal force of 10,000 G (G means g.multidot.cm/sec.sup.2) to 
remove ash. The resulting FCC decant oil was coked in the same way as in 
Example 1. The results are shown in Table 1. 
Example 3 
An FCC decant oil (ash content: 0.024 wt %) was passed between the 
electrode plates, across which a voltage of 10 kV has been applied, at a 
flow rate of 6.1/min and then treated by a self-discharging type disc 
centrifuge with a centrifugal force of 7,000 G to remove ash. The 
resulting decant oil was coked after the manner of example 1. The 
Example 4 
An FCC decant oil from which ash has been removed by the same method as 
used in Example 1 was mixed with a coal tar pitch having a softening point 
of 40.degree. C. and a solvent (a mixture of kerosene and an aromatic oil) 
having a solubility index of 70 (mixing ratio=1:0.6). Then a coal tar 
pitch from which the quinoline insolubles have been removed by static 
separation (at a temperature of 250.degree. C.) and then the solvent has 
been distilled away was mixed in the ratios shown in Table 2, and each 
mixture was coked in the same way as in Example 1. The results are shown 
in Table 2. 
Comparative Example 1 
An FCC decant oil (ash content: 0.024 wt %) from which ash has not been 
removed was coked in the same way as in Example 1. 
Comparative Example 2 
A coal tar pitch from which the quinoline insolubles have been removed by 
the method of Example 4 was coked in the same way as in Example 1. 
TABLE 1 
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Comp. 
Example 1 
Example 1 
Example 2 
Example 3 
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Ash removing 
Ash un- Filtra- Centrifu- 
Electrostatic 
method removed tion gation aggregation 
Ash content (%) 
0.024 0.001 0.004 0.002 
Thermal 6.7 4.8 5.2 4.9 
expansion 
coefficient 
(*10.sup.-7 .degree. C..sup.-1) 
Puffing (%) 
0.82 0.86 0.85 0.85 
(1700-2600.degree. C.) 
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(Note) Puffing means a ratio of an irreversible expansion of a baked 
electrode containing needle coke in the production of graphite electrodes 
The Puffing is shown by elongation (%) of a baked electrode in a directio 
vertical to the machine direction at a temperature of 1,700 to 
2,600.degree. C. 
TABLE 2 
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Coal tar pitch:petroeum heavy oil 
Mixing ratio 
100:0 75:25 50:50 25:75 0:100 
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Ash content (%) 
0.003 0.003 0.002 0.001 0.001 
Thermal 5.5 5.3 51 4.9 4.8 
expansion 
coefficient 
(*10.sup.-7 .degree. C..sup.-1) 
Puffing (%) 
0.71 1.47 1.21 1.02 0.86 
(1700-2600.degree. C.) 
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Example 5 
An FCC decant oil was heated to a temperature of 100.degree. C. and then 
treated by a self-discharging type disc centrifuge at a speed of 8,000 G. 
(corresponding to a centrifuging force of G) to remove ash. The thus 
treated FCC decant oil was mixed with a coal tar pitch having a softening 
point of 40.degree. C. and a solvent (a mixture of kerosene and an 
aromatic oil) having a solubility index of 70 (mixing ratio=1:0.6). Then a 
coal tar pitch from which the quinoline insolubles have been removed by 
static .separation (at a temperature of 250.degree. C.) and then the 
solvent has been distilled away was mixed in the ratios shown in Table 3, 
and each mixture was coked by a delayed coker at a temperature of 
500.degree. C. under a pressure of 3.5 kg/cm.sup.2 G for 24 hours and then 
calcined by a rotary kiln at a temperature of 1,500.degree. C. The results 
are shown in Table 3. 
TABLE 3 
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Coal tar pitch:petoleum heavy oil 
Mixing ratio 75:25 50:50 25:75 
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Ash content (%) 
0.003 0.002 0.001 
Thermal expansion 
3.5 3.6 3.8 
coefficient 
(*10.sup.-7 .degree. C..sup.-1) 
Puffing (%) 1.41 1.24 1.00 
(1700-2600.degree. C.) 
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