|Title of Invention||
A PROCESS FOR MAKING PETROLEUM DERIVED PITCH SUITABLE FOR CONVERSION INTO CARBON FIBRES
|Abstract||An improved process for making petroleum derived pitch suitable for conversion into carbon fibres which comprises prefractionating the pyrolysis tar by known method to obtain high boiling residue, thermal soaking of said residue with hydrogen donor solvent such as herein described at a temperature in the range of 300-370' C under inert atmosphere for a period in the range of 3-6 hrs to obtain petroleum pitch.|
|Full Text||The present invention relates to an improved process for making petroleum derived pitch suitable for conversion into carbon fibres.
Pitch is a fusible carbonaceous mass and is a complex mixture of thousands of organic compounds mainly of polynuclear aromatic hydrocarbons with sulphur and nitrogen incorporated into the aromatic structures as heteroatoms. Short aliphatic chains are also present on the typical pitch molecules. These pitches have a wide range of industrial applications particularly in the manufacturing of high commercial value carbon materials such as graphite electrodes and high modulus carbon fibres. Carbon fibre is a high priced material and used in new generation light weight aircrafts, sports equipments and fashion articles. In carbon fibre making process, pitch is allowed to melt to a carefully controlled viscosity and then passed through a number of fine capillaries under pressure to produce the filament like fibres as the pitch re-solidifies. The green carbon fibres thus obtained is stabilized by oxidation in air followed by carbonization in an inert atmosphere.
As is well known to those skilled in the art, pitches suitable for carbon fibre production are commonly produced by thermal soaking of aromatics rich feedstocks obtained from coal tar and petroleum sources [Dolmatov et.al., Chem. and Technol. of Fuels and Oils, 24, 5 (1988); Komine et.al., Jpn. Patent JP 03, 02, 298 (1991), CA 115:34285e (1991); Archakova et.al. USSR patent Su 1, 333, 685, (1987); Dickakian, ACS Symp. Ser. 303 126 (1986). CA 104 : 209650x (1986)]. Thermal soaking is
generally carried out at temperature range of 350-500"C, at atmospheric pressure with continuous purging of inert gas.
In another commonly employed process thermal soaking of feedstock may be carried out under reduced pressure [(Turner et.al. Fuel. 66 1481 (1987)]. However, this process is disadvantageous because it gives lower yield of pitch due to more volatilization of high boiling hydrocarbons.
Attempts have been made to carry out thermal soaking of feedstock under high pressure [Dolmatov, Solid Fuel Chem., 23(4) 109 (1989)]. However, this process has some operational problems due to pressure assembly.
Attempts have also been made to carry out thermal soaking of feedstocks aided by free radical initiator. Although this process reduces thermal soaking period for getting quality pitch but often it is very difficult to control polymerization and condenstation reactions and yields pitch of high Quinoline Insolubles (QI) values and softening point which make it less suitable for spinning purposes in carbon fibre manufacture.
Present process differs from hitherto known processes in method of preparation. In previous methods petroleum pitch is first prepared and then hydrogenated to lower the softening point, viscosity and unmeltable quinoline insolubles to make them suitable for carbon fibre but in present process a low softening point and quinoline insoluble petroleum pitch is produced in a in-situ manner (in a single step) by adding a hydrogen donor solvent during thermal soaking step.
The main object of the present invention is to provide an improved process for making petroleum derived pitch suitable for conversion into carbon fibres which obviates the drawbacks as detailed above.
Another object of the present invention is to provide an improved process for making petroleum derived pitch of low softening point suitable for conversion into carbon Fibres.
Still another object of the present invention is to provide an improved process for making petroleum derived pitch of low softening point and high coking value suitable for conversion into carbon fibre.
Yet another object of the present invention is to provide an improved process for making petroleum derived pitch suitable for conversion into carbon fibre, at relatively low temperature 300-370°C.
The above objects are accomplished by prefractionation of feed stock (pyrolysis tar) followed by thermal soaking aided by hydrogen donor solvent such as cyclohexane, tetralin, decalin, tetrahydroquinoline (THQ), 9,10 dihydroanthracene, tetra-hydrofluoranthene, hydrogenated aromatics and extracts. Thermal soaking temperature, residence period and inert gas purge rate are the vital parameters of this process.
Accordingly the present invention provides an improved process for making petroleum derived pitch suitable for conversion into carbon fibres which comprises prefractionating the pyrolysis tar by known method to obtain high boiling residue, thermal soaking of said residue with hydrogen donor solvent such as herein described at a temperature in the
range of 300-370°C under inert atmosphere for a period in the range of 3-6 hrs to obtain petroleum pitch..
In an embodiment of the present invention pyrolysis tar used as a feed may be obtained from olefin production unit in petrochemical plant and aromaticity of the pyrolysis tar may be in the range of 100-150 Bureau of Mines Correlation Index (BMCI).
In a feature of the present invention prefractionation of pyrolysis tar may be effect at a temperature in the range of 200-270°C.
In another embodiment of the present invention hydrogen donor solvent used may be such as cyclohexane, tetralin, decalin, tetrahydroquinoline (THQ), 9,10 dihydroanthracene, tetrahydrofluoranthene, hydrogenated aromatics and extracts.
In still another embodiment of the present invention thermal soaking of residue of prefractionation and hydrogen donor solvent may be performed at temperature 300-370°C.
In still another embodiment of the present invention inert gas may be used such as nitrogen, helium or argon at a flow rate of 60-100 ml per minute.
In carrying the process into effect, feed stock (pyrolysis tar) is first subjected to an atmospheric distillation to remove low molecular weight aromatics. This step was termed as prefractionation. The low molecular weight aromatics play a less significant role in
the formation of pitch. Further, rate of conversion of mono-aromatics into di-and poly-aromatics is lower than the conversion of di- and tri- into polyaromatics [Rudnick et al.. Energy & Fuels. 5 733 (1991)]. Moverover. removal of mono-, di-aromatics and their derivatives in prefractionation also improves the aromaticity (Measured by Bureau of Mines Correlation Index. BMC1) due to increased concentration of polyaromatics in feed Thus to make the carbon fibre precursor making process time saving, it is better to remove the mono-and some di-aromatics from the feed at this stage. Removal of low boiling aromatics provides a by-product which may be a good source of benzene, toluene, xylene (BTX) and more particularly for the recovery of naphthalene. The operating parameters of this step are distillation at 200-270°C temperature and atmospheric pressure and inert gas purge rate, 60-100 ml/min. The important step of this process is thermal soaking of a mixture of residual portion of prefractionation and a hydrogen donor solvent (1-3% wt) under properly optimized conditions to get a specific quality of pitch. Thermal soaking is carried out by taking reaction mixture in a reactor consisting of two subsequent overhead water condensers followed by an ice cooled trap to collect the maximum amount of condensable liquid from effluent gases generated during cracking ol hydrocarbons present in the feed. During thermal soaking, growth of aromatic cluster takes place under the influence of thermal energy via polymerization through condensation reactions [Zander et al. Fuel 72 1281 (1993)]. Thermal soaking is performed at temperature 300-370°C for the period of 3 to 6 hours under the continuous purging of inert gas at the rate of 60-100 ml/min, at atmospheric pressure. These operating conditions were optimized for 100 gm charged material. The entire thermal
soaking is carried out under very carefully controlled operating conditions to ensure the good quality product. The main role of inert gas purging during this step is to prevent oxidation along with fast removal of low boiling aromatics . In lab scale experiments inert gas purging also helps to make reaction mixture homogeneous but in commercial practice it is carried out by mechanical agitator.
For a complete understanding of the nature of invention and scientific approach of this process, it is necessary to look into the mechanistic part of the reactions which take place during the process.
Prefraclionation which is carried out at 200-270°C, is a purely physical separation, because of, at this temperature no bond breaking and making reactions take place [Americ et al. Proc. 13th World Petroleum Congress, John Wiley & Sons 3 199 (1992)|. Prefractionation of pyrolysis tar makes it concentrated with three or more membered ring aromatics which are more favoured structures for easy conversion into pitch.
Thermal soaking is the key step of this process and conducted at temperature 300-370°C. At this temperature cracking of alkyl side chains present on the aromatic ring is more prone rather than ring opening reactions. Such reactions produces very active carbon centres on the aromatic rings. These active centres (or free radicals) first undergoes intermolecular dehydrogenative polymerization followed by intramolecular dehydrocyclization and produce planar aromatic units having required range of average molecular weight, softening point and quinoline insolubles (QI), suitable for spinning. If such reactions continues, subsequent polymerization and condensation of these aromatics
(average molecular weight 450-550) leads to formation of larger aromatics having molecular weight 2000 or more. As the average molecular weight and softening point increases there is marked decrease in solubility of aromatics compounds in quinolinc resulting more value of undesirable QI contents. Thus to check the further growth of aromatic units beyond a certain extent (checked by average molecular weight. QI, softening point), addition of a hydrogen donor agent in the reaction mixture appears to be a solution. Hydrogen donor agent through hydrogen transfer reactions to active centers arrests further polymerization and condensation reactions and restricts the growth of aromatic units so that it could remain as pitch suitable for conversion into carbon fibre but not coke [Kubo et al. European patent EP 272038 (1988) CA:109:76470t (1988)].
Patents and technical literature show that low softening point and quinoline insolubles pitches suitable for carbon fibres are prepared by hydrogenation of high softening pitches through catalysts, hydrogen gas or hydrogen donor solvent [Yamada et al. Jpn. Patent 58, 18421 (1983). Mochida et al. J. Jpn. Pet. Inst. 30(1) 31 (1981 )|. Takeuchi et al. .1. Jpn. Pet. Inst. 37(2) 136 (1994)]. The present invention provides a single step process for making low softening point pitches suitable for carbon fibres and is the example of insitu hydrogenation of pitch.
The reaction mechanism of this invention may be explained as follows :
The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention.
Feedstock for pitch was prepared by charging 580 gms of pyrolysis tar of BMCI 128 in a glass reactor and distilling it at 200-270°C under continuous purging of nitrogen gas at the flow rate of 60-100 ml/min. A residue obtained which is 78% by weight is used as feedstock for pitch.
In the following examples unless it is indicated that there was some other feed, the same residue of prefractionation was subjected to processing for making pitch.
Petroleum pitch was prepared by thermal soaking of a mixture of residue of prefractionation (98 gm) of BMCI 154 and tetralin (2.07 gm) at 335°C for a period of 4.5 hrs in a glass reactor under the continuous purging of inert gas, nitrogen at a flow rate of 95 ml/min to obtain the pitch of softening point 135°C and coking value 42.38% wt. The pitch yield was 39.70 gm.
The same procedure of experiment 2 was repeated except a little lower nitrogen flow rate 80 ml/min and higher thermal soaking period, 5 hrs. As a result of 41.93 gm pitch was obtained to have softening point 124°C and coking value 40.78 % wt.
The same procedure of example 2 was repeated except providing thermal soaking period 3 hrs. As a result of pitch of softening point 116°C was obtained in 41.36 % wt yield.
The same procedure of pitch preparation was used as described in example 2 except providing thermal soaking duration of 6 hrs and a little higher nitrogen flow rate 100 ml/min. A pitch of 1390C softening point with 37.50% wt yield was obtained.
The same procedure of example 2 was repeated except another hydrogen donor solvent decalin (2.01 gm). As a result of 43.04 gm pitch was obtained which was found to have softening point 120°C and coking value 39.76 % wt.
The main advantages of the present invention are :
1. This improve process provides single step process for making petroleum derived pitch suitable for carbon fibres and eliminates a costly and high pressure hydrogenation step used in hietherto known processes.
2. This invention provides a low cost precursor for producing carbon fibre. Another most widely used precursor. Polyacrylonitrile (PAN) is relatively expensive as compared to petroleum pitch precursor (Mochida et al. CHEMTECH, Feb 95, p. 29).
3. This process provides two liquid by-products which can effectively be utilised further. Light distillate which is produced during prefractionation is a rich source (35-40% wt) of naphthalene. Heavy distillate, which is produced during thermal soaking is a good feedstock for carbon black.
We claim :
1. An improved process for making petroleum derived pitch suitable for conversion into carbon fibres which comprises prefractionating the pyrolysis tar by known method to obtain high boiling residue, thermal soaking of said residue with hydrogen donor solvent such as herein described at a temperature in the range of 300-370°C under inert atmosphere for a period in the range of 3-6 hrs to obtain petroleum pitch.
2. A process as claimed in claim 1 wherein pyrolysis tar used is a by-product of naphtha cracking operation in olefin production plant and aromatics.
3. A process as claimed in claim 1 & 2 wherein pyrolysis tar used is of an aromaticity within the range of 100-150 BMCI.
4. A process as claimed in claim 1 to 3 wherein temperature used in prefractionation is within the range of 200-270°C.
5. A process as claimed in claims 1 to 4 wherein hydrogen donor solvent used is such as cyclohexane, tetralin, decalin, tetrahydroquinoline (THQ), 9,10 dihydroanthracene, tetra-hydrofluoranthene, hydrogenated aromatics .
6. A process as claimed in claims 1 to 5 wherein inert gas used is such as nitrogen, helium or argon.
7. An improved process for making petroleum derived pitch suitable for conversion into
carbon fibers substantially as described herein with reference examples.
|Indian Patent Application Number||2449/DEL/1995|
|PG Journal Number||31/2009|
|Date of Filing||29-Dec-1995|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI-110001, INDIA.|
|PCT International Classification Number||C10C 003/00|
|PCT International Application Number||N/A|
|PCT International Filing date|