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(11) | EP 0 244 227 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for producing pitch for the production of carbon fibers |
| (57) A process for producing pitch for the production of carbon fibers, which process
comprises subjecting a heavy oil obtained in fluid catalytic cracking of petroleum
and having a boiling point not lower than 200°C, to the action of a high gradient
magnetic separatorto capture and remove fine particles contained in the heavy oil,
and then heat-treating the heavy oil at a temperature in the range of 380° to 480°C
and a pressure in the range of 2 to 50 kg/cm2·G. |
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a process for producing pitch for the production of carbon fibers. Particularly, the present invention provides a process for producing pitch superior in performance as a starting material for the production of carbon fibers which process involves removing fine particles present in a heavy oil in a specific manner.
[0002] The production of carbon fibers from pitch is .known. Particularly, it is known that a carbon fiber having high strength and high elastic modulus is obtained by heat-treating petroleum pitch to obtain pitch containing an optically anisotropic liquid crystal called mesophase, melt-spinning the thus- obtained pitch into pitch fiber, and then subjecting the pitch fiber to an infusiblization treatment and then to a carbonization treatment, which may be further followed by a graphitization treatment (Japanese Patent Publication No.3567/1984).
[0003] However, carbon fibers obtained from pitch are inferior in tensile strength to those obtained from polyacrylonitrile, although they are superior in elastic modulus, and so their improvement has been desired.
[0004] Various fine particles are contained in petroleum heavy oils, and catalyst residue, etc. are contained in the so-called decant oil obtained by fluid catalytic cracking of petroleum. If these fine particles remain in spinning pitch, they will plug the spinneret during melt spinning, not only making it difficult to effect stable spinning but also causing deterioration in tensile strength of the resulting carbon fiber.
[0005] In other words, if such fine particles can be removed from the heavy oils, it is expected that there will be obtained carbon fiber having an improved tensile strength.
[0006] However, since those fine particles are extremely small in diameter, usually in the range of 0.1 to 30mp, they cannot be removed by the filter used in the petroleum refining industry. The use of a filter of fine mesh such as filter paper or membrane filter may be useful for removing such fine particles. However, the use of such a filter is disadvantageous in that the pressure loss is large and there occurs clogging, making it difficult to effect a long-time operation. Thus, in operation, it is not suitable for handling a large quantity of heavy oil even if there is made replacement of the filter.
[0007] Using a centrifugal separator has also been proposed as a method for removing the fine particles. This method, however, involves a problem in the heavy oil handling quantity, and the removal of the fine particles is not always satisfactory. For this reason, further improvement has been desired.
SUMMARY OF THE INVENTION
[0008] It is the object of the present invention to overcome the above-mentioned problems and particularly to provide a process for producing pitch useful as a = starting material for the production of carbon fibers of high quality by removing fine particles contained in a heavy oil in an extremely highly efficient manner.
[0009] The present invention resides in a process for producing pitch for the production of carbon fibers, characterized in that a heavy oil obtained in fluid catalytic cracking of petroleum and having a boiling point not lower than 200°C is treated by means of a high gradient magnetic separator to capture and remove fine particles contained therein and then heat-treated at a temperature of 380° to 480°C and a pressure of 2 to 50 kg/cm2·G.
[0010] By using the process of the present invention, not only can the fine particles content of the heavy oil be decreased to 5 ppm or less, but also the remaining fine particles can be reduced in diameter to not larger than 0.5mµ. Besides, it becomes. possible to handle a large quantity of heavy oil.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Heavy oils employable in the invention, obtained in fluid catalytic cracking of petroleums, are those by- produced in producing light oil such as-gasoline by fluid catalytic cracking of petroleums such as kerosene, gas oil, atmospheric bottom residue, vacuum distillate, or those resulting from hydrogenation thereof, in the presence of a catalyst such as a natural or synthetic silica-alumina or zeolite, usually at a temperature in the range of 450° to 550°C and at a pressure in the range of atmospheric pressure to 20 kg/cm2.G, the heavy oils boiling substantially in the range of 200° to 550°C, -preferably 300° to 550°C.
[0013] In such heavy oils are usually contained approximately 10 to 550 ppm of fine particles which contain iron and iron-, nickel- and vanadium compounds and which widely range in diameter from 0.1 to 100 mµ.
[0014] In the present invention, any of the heavy oils set forth above is subjected to the action of a high gradient magnetic separator to capture and remove fine particles present therein. Particularly, fine particles containing iron and iron-, nickel- and vanadium compounds can be removed effectively thereby
[0015] The said magnetic separator used in the present invention is designed so that when a ferromagnetic filler is placed in a space of uniform, high magnetic field and a high magnetic field gradient not smaller than 100 x 103 gauss/cm is created around the filler under magnetic field, ferromagnetic or paramagnetic fine particles are thereby magnetized on the surface of the filler, making it possible to separate them from weak paramagnetic or diamagnetic fine particles.
[0016] As the ferromagnetic filler mentioned above there usually is employed an aggregate of ferromagnetic fine lines such as steel wool-or steel net, having a diameter of 1 to 1,000 µm, preferably 10 to 500 pm, or expanded metal or steel beads, with steel wool being preferred.
[0017] As an example of the high gradient magnetic separator there may be mentioned "SALA-HGMS®" (trade name) manufactured and sold by Sala Magnetic Inc., U.S.A.
[0018] The fine particles present in the heavy oil are removed by the high gradient magnetic separator in such a manner that the heavy oil is introduced into the magnetic field space of the separator and the said fine particles are thereby magnetized to the ferromagnetic filler.
[0019] As variables in the operation of the high gradient magnetic separator there are magnetic field intensity, linear velocity and processing temperature. Optimum conditions are selected according to the kind and size of particles to be magnetized.
[0020] The magnetic field intensity represents the intensity of the magnetic field in the space where the filler is placed. Usually, it is not lower than 500 gauss, for example in the range of 500 to 20,000, preferably 1,000 to 20,000 gauss.
[0021] The processing temperature represents the temperature of the oil when introduced into the high gradient magnetic separator. It is usually in the range of room temperature to 400°C, preferably 50° to 250°C.
[0022] The linear velocity represents a linear velocity of the oil when passing through the magnetic field space. It is usually in the range of 0.01 to 100 cm/sec, preferably 0.1 to 20 cm/sec.
[0023] The heavy oil thus processed by the high gradient magnetic separator is then heat-treated (primary heat treatment) at a temperature in the range of 380°C to 480°C, preferably 400° to 450°C, and at a pressure in the range of 2 to 50 kg/cm2, preferably 5 to 30 kg/cm2. Thereafter, light components are distilled off to obtain pitch for the production of carbon fibers. The heat treating time is usually in the range of 5 minutes to 30 hours, preferably 0.5 to 10 hours.
[0024] The method of distilling off light components is not specially limited, but a thin film distillation method is adopted preferably, which is carried out usually at a temperature in the range of 250° to 500°C, preferably 300° to 400°C, under reduced pressure. The pressure is preferably not higher than 50 mmHg. The .thin film is not larger than 10 mm, preferably not larger than 5 mm, in thickness.
[0025] Distillate boiling not higher than 400°C is removed by the thin film distillation to obtain the carbon fiber producing pitch of the present invention.
[0026] For producing carbon fiber using the said pitch of the invention, the pitch is heat-treated (secondary heat treatment) at a temperature in the range of 340° to 450°C, preferably 370° to 420°C, at atmospheric pressure or under reduced pressure to obtain an optically anisotropic pitch as spinning pitch. The heat treating time is usually in the range of 1 to 50 hours, preferably 3 to 20 hours.
[0027] The secondary heat treatment may be carried out while introducing an inert gas such as nitrogen. In this case, the amount of such inert gas to be introduced is preferably in the range of 0.7 to 5.0 scfh/lb-pitch.
[0028] The proportion of the optically anisotropic phase in the spinning pitch is preferably in the range of 70% to 100%, more preferably 90% to 100%.
[0029] The spinning pitch thus obtained is melt-spun in a known manner and the resulting pitch fiber is subjected to infusiblization treatment and burning treatment to obtain carbon fiber.
[0030] The following examples are given to further illustrate the present invention, but the invention is not limited thereto.
Example 1
[0031] Hydrogenated oil of vacuum gas oil from Arabian crude oil.was. subjected to fluid catalytic cracking at 485°C using a silica alumina catalyst to obtain heavy oil (A), whose properties are as shown in Table 1.
[0032] Then, the heavy oil (A) was subjected to the action of a high gradient magnetic separator "SALA-HGMS ®" (trade name) under the following conditions to obtain processed oil (B):
[0033] Also processing was carried out under the same conditions as above except that the magnetic field intensity was changed to 20 kilogauss, to obtain processed oil (C).
[0034] The heavy oil (A) and the processed oils (B), (C) were found to have such ash contents as set out in Table 2.
Example 2
[0035] The processed oil (C) obtained in Example 1 was heat-treated at a temperature of 410°C and a pressure of 9 kg/cm for 3 hours. Then the heat-treated oil was subjected to thin film distillation at a temperature of 355°C, a pressure of 15 mmHg and at a film thickness of 3 mm to substantially distill off components boiling not higher than 490°C to obtain pitch for the production of carbon fiber.
[0036] Then, 30 g of the starting pitch thus obtained was stirred under introduction of nitrogen at a rate of 600 ml/min and heat-treated at 400°C, at atmospheric pressure, for 7 hours to obtain spinning pitch.
[0037] The spinning pitch thus obtained was melt-spun at 345°C into pitch fiber 12mµ in diameter. The pitch fiber was subjected to infusiblization, carbonization and graphitization treatments under the following conditions to obtain carbon fiber.
[0039] The infusiblization, carbonization and graphitization treatment were carried out under the following conditions.
o Infusiblization: Heating is made at a rate of 2°C/min from 50°C up to 300°C and cooling is made immediately after reaching 300°C, in an oxygen atmosphere.
o Carbonization: Heating is made at a rate of 25°C/min and the fiber for carbonization is held at 700°C for 1 minute, in a nitrogen atmosphere.
o Graphitization: Heating is made up to 2,500°C at a rate of 50°C/min in a current of nitrogen gas..
Comparative Example 1
[0040] Carbon fiber was produced in the same way as in Example 2 except that the heavy oil (A) was used in place of the processed oil (C). Results are as shown in Table 3.
Comparative Example 2
[0041] The heavy oil obtained in Example 1 was subjected to centrifugal separation at 150°C, 4,500 rpm to obtain processed oil (D). The ash content of the processed oil (D) was 15 ppm.
[0042] Then, carbon fiber was produced in the same way as in Example 2 except that the processed oil (D) was used in place of the processed oil (C). Results are as shown in Table 3.
EFFECT OF THE INVENTION
[0043] According to the process of the present invention, as set forth hereinabove, fine particles (ash) contained in heavy oils can be removed with extremely high efficiency. When melt spinning is done using the spinning pitch obtained by heat-treating the carbon fiber producing pitch of the invention, not only can the spinning be effected stably without causing yarn breakage, but also there is obtained carbon fiber of high strength and high elastic modulus.
1. A process for producing pitch for the production of carbon fibers, which process
comprises subjecting a heavy oil obtained in fluid catalytic cracking of petroleum
and having a boiling point not lower than 200°C, to the action of a high gradient
magnetic separator to capture and remove fine particles contained in the heavy oil,
and then heat-treating the heavy oil at a temperature in the range of 380°C to 480°C
and a pressure in the range of 2'to 50 kg/cm2.G.
2. A process according to Claim 1, wherein said magnetic separator has a space with
a ferromagnetic filler placed therein and has a magnetic field intensity in the range
of 500 to .20,000 in said space.
3. A proces according to Claim 1 or Claim 2, wherein the processing by said magnetic
separator is performed at a temperature in the range of room temperature of 400°C
and at a linear velocity in the range of 0.01 to 100 cm/sec.
4. A process according any of Claims 1 to 3, wherein light components are distilled
off after the heat treatment.
5. A process according to Claim 4, wherein the distilling-off of light components
is carried out by a thin film distillation method.
6. Carbon fiber produced from pitch obtained by the process of any of Claims 1 to
5.