Process for preparing an anisotropic aromatic pitch

A process is described for preparing a highly anisotropic aromatic pitch characterized by a high content of toluene insolubles, a low melting point, a low viscosity, and low content of quinoline insolubles by heat soaking a petroleum pitch derived from catalytic cracking residue at high temperatures for a short time followed by stripping the distillable oils. The pitch made in accordance with this process will yield a highly anisotropic carbon on carbonization at elevated temperatures.

BACKGROUND OF THE INVENTION 
Aromatic pitches such as coal tar pitch or petroleum pitch are composed of 
a complex mixture of alkyl substituted polycondensed aromatics of high 
molecular weight and a high degree of aromatic ring condensation. These 
heavy aromatic products may further be characterized as having a softening 
point of 100.degree. to 130.degree. C., and high viscosity of 1000-5000 
cst at 160.degree. C. 
The important feature of pitch is that it can be transformed into a high 
strength carbon product on melting or carbonization. The microstructure of 
the carbon product produced is very much dependent on the type of pitch 
used, and may vary from a highly anisotropic structure having an ordered 
or crystalline structure, to an unordered or random isotropic structure. 
The anisotropic structure pitch is preferred for the production of carbon 
products such as carbon fiber or needle coke. 
Many types of pitches can be produced by varying the aromatic feedstock 
materials and the processes used in pitch manufacture. One simple method 
to characterize these pitches is by the use of solvent analysis, for 
example, the degree of insolubility in benzene, toluene, pyridine, 
quinoline, or anthracene. For the purpose of the present invention, 
aromatic pitches are characterized by their insolubilities in toluene and 
quinoline. 
Solvent analysis is a method which is universally used to define the type 
and composition of various pitches; the quantitative determination of 
insolubles in toluene and quinoline are two analytical protocols which 
have become standard in the industry. These insolubles represent the two 
major fractions of a pitch varying in aromaticity, degree of aromatic ring 
condensation, and coking characteristics. The different insoluble 
fractions of a pitch also differ in their physical characteristics such as 
melting, softening, and viscosity which is a critical requirement for 
carbon product manufacturing. 
The solubility analysis for determining quinoline insolubles is conducted 
according to ASTM D2318-66 protocol; the solubility analysis for 
determining toluene insolubilities is conducted by mixing 40 grams of a 
sample in 320 ml of toluene over an 18-hour time period, filtering, 
washing the insolubles in additional toluene, drying, and calculating the 
yield of insolubles as a percentage of initial sample. 
The most common feedstocks used for this production of pitches are the 
heavy aromatic residues obtained from coal carbonization, steam cracking, 
or the catalytic cracking processes of low molecular weight paraffinic 
hydrocarbons. 
The production of a highly aromatic pitch which will yield a non-ordered, 
isotropic carbon has previously been described in U.S. Pat. No. 3,721,658. 
More particularly, this patent describes a process for preparing an 
aromatic pitch by the catalytic air oxidation of an aromatic feedstock 
such as steam-cracked tar, at a temperature of 240.degree.-260.degree. C. 
This pitch has low toluene insolubles (about 15%) and very low quinoline 
insolubles. Because of the chemical structure of the pitch, and not 
because of the toluene or quinoline insolubles content, this pitch on 
melting or carbonizing will yield a highly isotropic carbon. 
Another example of aromatic pitch production may be found in U.S. Pat. No. 
4,086,156. This patent describes a method for preparing an aromatic pitch 
by the thermal treatment of steam cracker tar in the absence of oxygen and 
at a temperature of 380.degree.-390.degree. C. The pitch obtained by this 
method contains a low concentration of toluene and quinoline insolubles 
and will produce an isotropic carbon on melting or carbonizing. 
Unfortunately, the pitch produced from steam cracked tar is not very 
suitable for producing anistropic products. Only the pitches produced from 
catalytic cracking residue feedstocks have been found to be suitable. 
Examples of these suitable aromatic pitches are described in U.S. Pat. No. 
4,219,404. 
The residue obtained from the catalytic cracking processes of low molecular 
weight paraffinic hydrocarbons have been found to have the preferred 
physical and chemical characteristics for producing the pitch needed in 
the manufacture of anisotropic carbon products. More particularly, the 
typical physical and chemical characteristics of a suitable catalytic 
cracking residue feedstock material is presented in Table I. 
TABLE 1 
______________________________________ 
Characteristics of Catalytic Cracking Residue 
______________________________________ 
Physical Characteristics 
Viscosity cst at 210.degree. C. 
1.0-10.0 
Ash Content, wt. % 0.010-2.0 
Coking Value (wt. % at 550.degree. C.) 
6.0-10.0 
Asphaltene; (n-heptane insolubles), % 
0.1-12.0 
Toluene Insolubles, % 0.010-1.0 
Number Average Mol. wt. 220-290 
Elemental Analysis 
Carbon, % 88.0-90.32 
Hydrogen, % 7.74-7.40 
Oxygen, % 0.10-0.30 
Sulfur, % 1.0-4.5 
Carbon/Hydrogen Atomic Ratio 
0.90-1.0 
Chemical Analysis (by Carbon-13 Nuclear Magnetic 
Resonance Spectroscopy) 
Aromatic Carbon (atom %) 55-75 
Aromatic Ring Distribution (by Mass Spectroscopy) 
1 Ring (%) 1.2 
2 Rings (%) 23.6 
3 Rings (%) 37.5 
4 Rings (%) 31.8 
5 Rings (%) 3.8 
6 Rings (%) 0.9 
Molecular 
Weight Distribution (by Mass Spectroscopy) 
175-200 (%) 2.9 
220-225 (%) 13.4 
225-250 (%) 29.5 
250-275 (%) 23.1 
275-300 (%) 15.5 
300-325 (%) 6.8 
325-350 (%) 3.5 
Composition (by Clay-Silica Gel Chromatography) 
Aromatic, % 62.2 
Saturate, % 17.0 
Polar, % 18.3 
______________________________________ 
Petroleum pitches, though complex in their chemical structure, can be 
characterized using advanced modern analytical techniques. For example, we 
can determine quantitatively the various protons (aromatic, benzylic, 
aliphatic and naphthenic) present in a pitch by using a proton Nuclear 
Magnetic Resonance Spectroscopy (P-NMR). We can also determine 
quantitatively the various types of carbon atoms present in the pitch 
(aromatic carbon, benzylic carbon and paraffinic carbon) by using a 
carbon-NMR. We can also determine their molecular weight distribution by 
using a high temperature gel permeation chromatography. Another important 
chemical characteristic is the carbon/hydrogen atomic ratio which can be 
calculated from the carbon and the hydrogen elemental analysis. 
The process described in this patent deals with the production of a highly 
anisotropic aromatic pitch derived from catalytic cracking residue which 
has a high content of the desired fraction of toluene insolubles and a low 
level of quinoline insolubles, the toluene insoluble fraction having a low 
melting point and low viscosity. Our process also deals with the 
extraction of the toluene fraction from the pitch by solvent extraction. 
This preferred fraction in the pitch corresponding to approximately the 
toluene insolubles have been found to produce highly anisotropic carbon 
products (e.g. needle coke and carbon fiber) at elevated temperatures. For 
example, the pitch can be used to produce a carbon fiber by spinning at 
elevated temperature and pressure. This toluene insolubles fraction of the 
pitch has very high optical activity (as seen by polarized light 
microscopy), low melting point and low viscosity and it is considered to 
be part of liquid crystal (or mesophase) in the pitch. The quinoline 
insoluble fraction in the pitch was found to have a very high melting 
point (350.degree.-450.degree. C.) and high viscosity hampering subsequent 
manufacturing of carbon products and thus is regarded as undesirable when 
present in the pitch in substantial quantities (e.g. over 5% by weight). 
When a pitch is produced in a conventional manner from catalytic cracking 
residue which contains only the toluene insolubles fraction there is a 
major problem. When thermally treating the aromatic feed into a pitch, the 
toluene insolubes fraction start forming in the pitch initially, when the 
toluene insolubles level in the pitch reaches a certain level, the 
quinoline insolubles start forming in the pitch, presumably from the 
further condensation of the toluene insolubles. 
We have discovered a process where a petroleum pitch with a high toluene 
insolubles content can be produced without excessive quinoline insoluble 
formation. This process takes advantage of the variation in the rate of 
formation of the toluene and quinoline insolubles which can be varied by 
varying the thermal treatment temperature and time. 
SUMMARY OF THE INVENTION 
Basically the process comprises initially thermally treating a petroleum 
pitch derived from catalytic cracking residue at a very high temperature 
(410.degree. to 500.degree. C.) for a short time period (less than about 
120 minutes), effective to obtain a toluene insoluble level of at least 
30% by weight while minimizing the quinoline insoluble level to less than 
5% by weight, and then stripping the pitch to distill off all or part of 
the distillate oils in the pitch. The pitch is then preferably extracted 
with a suitable solvent to separate the toluene insoluble fraction.

DETAILED DESCRIPTION OF THE INVENTION 
For this process, the feed is petroleum pitch derived from a catalytic 
cracking residue, preferably with a low level of toluene insolubles (i.e. 
1-15%) and preferably derived from catalytic cracking of low molecular 
weight paraffinic hydrocarbons. 
The selection of the process conditions, specifically heat soaking at a 
high temperature and short time, leading to the production of high 
mesophase pitches with high toluene insolubles fractions, i.e. at least 
30%, preferably at least 40% by weight of the pitch, but not the undesired 
quinoline insolubles, i.e. less than 5%, preferably less than 3% by weight 
of the pitch, is critical. These conditions comprise a high temperature of 
410.degree. to 500.degree. C., preferably 420.degree. to 450.degree. C., 
for a period of time less than 120 minutes, preferably 1 to 30 minutes, 
effective to obtain the toluene insoluble level of at least 30% by weight 
while minimizing the quinoline insoluble level to less than 5% by weight. 
An oxygen-free atmosphere such as nitrogen, hydrogen, or hydrocarbon is 
preferred to prevent undesirable reaction during heat soaking. Generally 
speaking, the higher the temperature is, the shorter is the time required. 
When the pitch is heat soaked for excessive time periods the quinoline 
insoluble level becomes excessive resulting in a pitch with an increased 
melting point and viscosity hampering subsequent manufacturing of carbon 
products. 
After the high temperature, short time heat soak, the pitch is then 
stripped of the distillate (aromatic) oils in the pitch. These distillate 
oils provide the important function during the high temperature heat soak 
of acting as a diluent to prevent excessive coke and quinoline insoluble 
formation. A minor amount of the distillate oils could be removed prior to 
heat soaking, i.e. removal of distillate oils at a level of less than 5% 
by weight of the pitch, but preferably no distillate oils are removed 
prior to heat soaking. 
The stripping of the distillate oils from the heat soaked pitch is an 
important step of our process. The stripping can be carried out by 
distillation in the presence of an inert gas (e.g. nitrogen or steam), but 
is preferably carried out by vacuum stripping in the temperature range of 
250.degree. to 400.degree. C., for example by cooling the pitch to about 
300.degree. C. under nitrogen atmosphere and then heating gradually to 
360.degree.-370.degree. C. under a reduced pressure of 0.1 to 65 mm Hg and 
vigorous agitation to avoid cracking. The removal of the aromatic oils 
from the pitch lead to increasing the carbon precursor (toluene 
insolubles) yield while lowering the melting point and the viscosity of 
the extracted precursor. 
Many variations of the basic process can be made which will lead to the 
production of a pitch with a high toluene insolubles fraction and low 
quinoline insolubles. For example, the first stage of the process (the 
thermal treatment or heat soaking) can be carried out under reduced 
pressure (e.g. 100-700 mm Hg) although it is preferably carried out at 
atmospheric pressure. 
The toluene insoluble fraction present in the high mesophase pitch prepared 
according to the process described in this invention has many of the 
physical and chemical characteristics desired for a precursor feed for the 
production of anisotropic carbon products such as carbon fiber or needle 
coke. The resultant toluene insoluble fraction should comprise: 
Low melting point of less than 325.degree. C., preferably less than 
300.degree. C., and low viscosity of less than 1000 cst at 360.degree. C. 
The physical characteristics of low melting temperature and low viscosity 
are key to the manufacturing of carbon products at temperatures below 
390.degree.-400.degree. C. where decomposition could take place; 
Very high aromaticity and degree of aromatic ring condensation as indicated 
by the high aromaticity of the pitch (86% aromatic carbon atom) and the 
high carbon/hydrogen atomic ratio (1.70 to 1.80) are important to the 
desired anisotropic structure development; 
Very high optical activity of at least 80%, preferably 100%, which is a 
measure of the desired anisotropic structure development; and 
Very good thermal stability as these fractions have been subjected to a 
high temperature during the initial thermal treatment and then subjected 
to reduced pressure to remove lower molecular weight hydrocarbons. 
Thus not only does the process of this invention prepare a precursor 
(toluene insoluble fraction) at a high yield, but critically results in a 
precursor with characteristics which are important to the subsequent 
manufacturing of quality anisotropic carbon products. 
The following method was used to prepare pitches described by the process 
of this invention. For pitch production a pilot unit consisting of an 
electrically heated metal reactor capable of operation under reduced 
pressure (e.g. 1-65 mm Hg) was used equipped with an agitator, nitrogen 
inlet, and a distillate recovery system to condense and collect the 
distillate during the thermal treatment and the vacuum stripping stages. 
The petroleum pitch derived from a catalytic cracking residue which was 
used in the examples was Ashland Pitch No. 240 which contained about 25 to 
28% by weight distillate oil, 6 to 8% toluene insolubles, 0.1 to 0.5% 
quinoline insolubles; with the toluene insoluble fraction having an 
optical activity of about 75% and a melting point of about 325.degree. C. 
Seventy pounds of Ashland Pitch No. 240 was introduced into the metal 
reactor, equipped with agitator, inlet for nitrogen and electrically 
heated to around 150.degree.-200.degree. C. under an atmosphere of 
nitrogen. When the pitch was softened, agitation was started and the pitch 
was then heated to the desired high temperature (420.degree.-450.degree. 
C.) under nitrogen atmosphere. The mixture was then thermally treated for 
the desired time and cooled to around 300.degree. C. The undesired 
distillate oils were then removed by heating the mixture gradually with 
agitation under reduced pressure (around 1-65 mm Hg). The pitch product 
was then cooled to around 200.degree. C. and pumped out of the reactor 
under nitrogen atmosphere. 
Examples 1 to 4 in Table II illustrate the usefulness of the vacuum 
stripping on the pitch composition to increase the level of toluene 
insolubles. 
TABLE II 
__________________________________________________________________________ 
Pitch Composition 
Pitch Composition 
Before Vacuum Stripping 
After Vacuum Stripping 
Heat Soak Conditions 
Toluene 
Quinoline 
Toluene 
Quinoline 
Temperature 
Time 
Insolubles 
Insolubles 
Insolubles 
Insolubles 
Example 
Feed (.degree.C.) 
(Hrs.) 
(%) (%) (%) (%) 
__________________________________________________________________________ 
1 Ashland 240 
410 1 21 1.5 32 1.0 
2 " 410 2 22 1.8 35 2.3 
3 " 420 1/2 20 1.4 43 1.0 
4 " 420 1 22 1.8 46 2.0 
__________________________________________________________________________ 
Pitches have been produced according to the process described in this 
invention using a 10-gallon reactor using a 70-lb. feed (Ashland Pitch No. 
240). Examples 5 to 10 in Table III illustrate the pitch production and 
provides the carbon precursor (toluene insolubles) and other key 
characteristics (melting point and viscosity). For comparison purposes, 
Examples 11 and 12 in Table III are given where no vacuum-stripping of 
pitch oil is made. 
TABLE III 
__________________________________________________________________________ 
Precursor (Toluene In- 
Vacuum 
Pitch Composition 
solubles Characteristics) 
Heat Soak Conditions 
Stripping 
Toluene 
Quinoline 
Melting 
Viscosity 
Optical 
Temperature 
Time 
(% Oil 
Insolubles 
Insolubles 
Point 
(Centistock 
Activity 
Example 
Feed (.degree.C.) 
(Min.) 
Removed) 
(%) (%) .degree.C. 
at 360.degree. C.) 
(%) 
__________________________________________________________________________ 
5 Ashland 240 
420 1 41.7 33 0.66 250-275 
560 -- 
6 " 420 15 34.2 35 0.69 250-275 
-- 100 
7 " 420 30 42.4 39 0.67 275-300 
348 100 
8 " 430 1 20.6 34 0.65 250-275 
-- 100 
9 " 430 7 27.3 41 0.95 250-275 
695 100 
10 " 430 15 21.8 43 1.51 250-275 
-- 100 
11 " 420 60 0 26 2.8 325-350 
5687 100 
12 " 400 120 0 22 1.4 325-350 
9659 100 
__________________________________________________________________________ 
As will be readily appreciated, the pitch produced in accordance with the 
present invention will contain materials insoluble in quinoline at 
75.degree. C. This quinoline insoluble material may consist of coke, ash, 
catalyst fines, and high softening point materials generated during heat 
soaking. Consequently, after removing the oil from the heat soaked vacuum 
or steam stripped pitch, undesirable high softening point components 
present in the resultant mixture are preferably removed. A particularly 
preferred technique for removing these components is disclosed in 
copending application Ser. No. 29,760, filed Apr. 13, 1979, which 
application is incorporated herein by reference. Basically, the heat 
soaked and de-oiled pitch is fluxed, that is, it is treated with an 
organic liquid in the range, for example, of from about 0.5 parts by 
weight of organic liquid per weight of pitch to about 3 parts by weight of 
fluxing liquid per weight of pitch, thereby providing a fluid pitch having 
substantially all the quinoline insoluble materials (including inorganic 
matter) suspended in the fluid in the form of readily separable solids. 
The suspended solids are then separated by filtration or the like, and the 
fluid pitch is then treated with an antisolvent, i.e., an organic liquid 
or mixture of organic liquids capable of precipitating and flocculating at 
least a substantial portion of the pitch free of quinoline insoluble 
solids. 
As will be appreciated, any antisolvent which will precipitate and 
flocculate the fluid pitch can be employed in the practice of the present 
invention. However, since it is particularly desirable in carbon fiber 
manufacture to use that fraction of the pitch which is readily convertible 
into an optically anisotropic phase and which has a low softening point 
and viscosity suitable for spinning, the antisolvent employed for 
precipitating the desired pitch fraction generally is selected from 
aromatic and alkyl substituted aromatic hydrocarbons and cyclic ethers and 
mixtures thereof. Examples of aromatic and alkyl substituted aromatic 
hydrocarbons include benzene, toluene, xylene, naphthalene, ethylbenzene, 
mesitylene, bi-phenyl and tetrahydronaphthalene. Representative examples 
of halogen substituted aromatic hydrocarbons include chlorobenzene, 
trichlorobenzene, bromobenzene, orthodichlorobenzene, and 
trichlorobiphenyl. Representative examples of cyclic ethers include furan 
and dioxane. Representative examples of mixtures of antisolvents include 
mixtures of compounds such as coal tar distillates, light aromatic gas 
oils and heavy aromatic gas oils. 
The amount of solvent employed will be sufficient to provide a solvent 
insoluble fraction capable of being thermally converted to an optically 
anisotropic material. Generally, from about 1 part of pitch to 4 parts of 
solvent to about 1 part by volume of pitch to about 16 parts of volume of 
solvent, depending upon the type of solvent, will be employed. After 
precipitating and flocculating the pitch, the solvent insoluble fraction 
is separated by typical techniques such as sedimentation, centrifugation, 
filtration and the like. 
Examples 13, 14, and 15 illustrate carbon precursor preparation by 
liquid-liquid extraction of the pitch with toluene. The pitch prepared in 
Example 9 was used as a feed for the extraction process. The pitch was 
crushed in small pieces with around 1/2" diameter and then mixed with 
toluene in 1:1 ratio in a first stage extraction. A small quantity of 
filter aid such as cilite was added. The mixture was then heated under 
nitrogen to reflux with mechanical agitation. The mixture was allowed to 
stand at reflux conditions for one hour and cooled to 
95.degree.-105.degree. C. It was filtered while hot through a cilite 
pre-coated sparkler filter. 
The filtrate was then diluted with toluene to bring the pitch:toluene ratio 
of 6:1 in a second stage extraction. The mixture was heated to reflux with 
agitation and allowed to cool slowly to room temperature (4-16 hours). The 
carbon insoluble was then filtered, washed with toluene and dried in a 
vacuum oven at 125.degree.-160.degree. C. for 24 hours. 
In Examples 13, 14, and 15 as illustrated in Table IV, the preparation of 
carbon precursors by extraction is shown. 
TABLE IV 
__________________________________________________________________________ 
First Extrac- 
Second Extrac- 
Toluene Insoluble Characteristics 
tion Stage 
tion Stage Viscosity 
Carbon/Hy- 
Optical 
(Pitch: Tol- 
(Pitch: Tol- 
Yield 
Melting Pt. 
(Centistock 
drogen Atomic 
Activity 
Example 
uene Ratio) 
uene Ratio) 
(%) (.degree.C.) 
at 360.degree. C.) 
Ratio (%) 
__________________________________________________________________________ 
13 1:1 1:6 19.0 
275-300 
695 1.70 100 
14 1:1 1:6 21.4 
275-300 
872 1.74 100 
15 1:1 1:6 22.1 
275-300 
744 -- 100 
__________________________________________________________________________ 
Examples 16, 17, and 18 illustrate the preparation of carbon fiber from 
insolubles precursor as prepared in Example 13. The carbon precursor 
powder was heated under nitrogen at around 350.degree. C. to agglomerate 
the powder into a solid mass. The solid carbon precursor is then 
transformed into fibers with 8 to 10 microns diameter by heating to around 
370.degree. C. and spinning through a spinarate. The green carbon fibers 
are then oxidized at elevated temperature in the presence of air and 
carbonized in nitrogen atmosphere. Details of carbon fiber characteristics 
are listed in Table V. 
TABLE V 
______________________________________ 
Fiber Tensile 
Diameter Strength Modulus Strain to 
Example (micron) (KSi) (MSi) Fail Ratio 
______________________________________ 
16 9.0 367 38.0 0.97 
17 8.2 391 39.9 0.98 
18 8.3 391 39.3 1.01 
______________________________________