Aromatic pitch from asphaltene-free steam cracker tar fractions

A process and a product of the process for preparing a pitch suitable for carbon artifact manufacture features a pitch having a weight content of between 80 and 100 percent toluene insolubles. The pitch is derived from a deasphaltenated middle fraction of a steam cracker tar feedstock. The middle fraction is rich in 2, 3, 4, and 5 polycondensed aromatic rings. The pitch is characterized as being relatively free of impurities and ash.

FIELD OF THE INVENTION 
The present invention relates to the process for preparing a pitch used in 
carbon artifact manufacture such as carbon fiber production. More 
particularly, the present invention relates to a process for preparing a 
pitch with high liquid crystal fraction from a steam cracker tar 
distillate or a deasphaltenated steam cracker tar. 
BACKGROUND OF THE INVENTION 
As is well-known, carbon artifacts have been made by pyrolyzing a wide 
variety of organic materials. Indeed, one carbon artifact of particularly 
important commercial interest today is carbon fiber. Hence, specific 
reference is made herein to carbon fiber technology. Nevertheless, it 
should be appreciated that this invention has applicability to carbon 
artifact manufacturing generally, and most particularly, to the production 
of shaped carbon articles in the form of filaments, yarns, films, ribbons, 
sheets and the like. 
The use of carbon fibers for reinforcing plastic and metal matrices has 
gained considerable commercial acceptance. The exceptional properties of 
these reinforcing composite materials, such as their high strength to 
weight ratio, clearly offset their high preparation costs. It is generally 
accepted that large scale use of carbon fibers as a reinforcing material 
would gain even greater acceptance in the marketplace, if the costs of the 
fibers could be substantially reduced. Thus, formation of carbon fibers 
for relatively inexpensive carbonaceous pitches has received considerable 
attention in recent years. 
Many materials containing polycondensed aromatics can be converted at early 
stages of carbonization to a structurally ordered optically anisotropic 
spherical liquid crystal called mesophase. The presence of this ordered 
structure prior to carbonization is considered to be fundamental in 
obtaining a high quality carbon artifact. Thus, one of the first 
requirements of a feedstock material suitable for carbon artifact 
manufacture, and particularly for carbon fiber production, is its ability 
to be converted to a highly optically anisotropic material. 
In addition, suitable feedstocks for carbon artifacts manufacture, and in 
particular carbon fiber manufacture, should have relatively low softening 
points and sufficient viscosity suitable for shaping and spinning into 
desirable articles and fibers. 
Unfortunately, many carbonaceous pitches have relatively high softening 
points. Indeed, incipient coking frequently occurs in such materials at 
temperatures where they have sufficient viscosity for spinning. The 
presence of coke, infusible materials, and/or high softening point 
components are detrimental to the fiber making process. 
As is well-known, pitches have been prepared from the total tars obtained 
from steam cracking of gas oil or naphtha. In this regard, see, for 
example, U.S. Pat. Nos. 3,721,658 and 4,086,156. 
Steam cracker tar, like other heavy aromatics, is composed of a complex 
mixture of alkyl-substituted polycondensed aromatics. The chemical 
structure, molecular weight and aromatic ring distribution can be 
determined quantitatively using advanced analytical methods such as carbon 
and proton nuclear resonance spectroscopy or mass spectrometry. 
Steam cracker tar, like other heavy aromatics such as coal tars and tars 
from catalytic or fluid cracking, is composed of two major parts: (1) a 
low molecular oil; and (2) a high molecular weight fraction called 
asphaltene, which is insoluble in a paraffinic solvent. The asphaltene in 
steam cracker tar varies from 10-30 wt % depending on the type of 
feedstock being introduced into the cracker, the design of the cracker and 
the severity of the cracking. 
Asphaltenes can be determined quantitatively in steam cracker tar using 
n-heptane. 
The two aforementioned parts of steam cracker tar, i.e., the oil and the 
asphaltene, vary significantly in their chemical composition, molecular 
weight, melting characteristics and most importantly their coking 
characteristics. 
The asphaltene presence in the steam cracker tar tends to be detrimental to 
carbon artifact manufacture, because it produces coke in the pitch and 
more importantly it does not provide a pitch with a high liquid crystal 
content; i.e., it severely limits the composition of the pitch. 
SUMMARY OF THE INVENTION 
This invention features an optically anisotropic pitch which is prepared 
from an asphaltene-free steam cracker tar middle distillate fraction by 
heat soaking the middle distillate fraction at 420.degree.-440.degree. C. 
between 2-6 hours at atmospheric pressure and then vacuum stripping the 
heat soaked mixture at temperatures from 370.degree.-420.degree. C. The 
pitch comprises approximately 80 to 100% toluene insolubles by weight and 
is further characterized as being relatively free of impurities and ash. 
It is an object of this invention to provide an improved pitch for 
manufacturing a carbon artifact. 
It is another object of the invention to provide a pitch for manufacturing 
carbon fibers which is more uniform, and which is free of ash and 
impurities. 
It is a further object of this invention to provide a pitch having high 
toluene insolubles, and which does not necessarily require Ti solvent 
extraction prior to spinning into fibers. 
These and other objects of this invention will be better understood and 
will become more apparent with reference to the following detailed 
description considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Generally speaking, the steam cracker tar which is used as a starting 
material in the process of the present invention is defined as the bottoms 
product obtained by cracking gas oils, particularly virgin gas oils, such 
as naphtha, at temperatures of from about 700.degree. C. to about 
1000.degree. C. A typical process steam cracks gas oil and naphtha, at 
temperatures of 800.degree. C. to 900.degree. C., with 50% to 70% 
conversion to C.sub.3 olefin and lighter hydrocarbons, by stripping at 
temperatures of about 200.degree. C. to 250.degree. C. for several 
seconds. The tar is obtained as a bottoms product. A gas oil is, of 
course, a liquid petroleum distillate with a viscosity and boiling range 
between kerosene and lubricating oil, and having a boiling range between 
about 200.degree. C. and 400.degree. C. Naphtha is a generic term for a 
refined, partly refined or unrefined liquid petroleum product of natural 
gas wherein not less than 10% distills below 175.degree. C. and not less 
than 95% distills below 240.degree. C., as determined by ASTM Method 
D-86. Steam cracker tars typically consist of alkyol substituted 
polycondensed aromatic compounds. 
Obviously, the characteristics of a steam cracker tar vary according to the 
feed in the steam cracking plant. 
Characteristics of typical steam cracker tars obtained from the steam 
cracking of naphtha, gas oil and desulfurized gas oil are respectively 
given in Table 1, below: 
TABLE 1 
__________________________________________________________________________ 
Physical and Chemical Characteristics of Steam Cracker 
Tars from Naphtha, Gas Oil and Desulfurized Gas Oil Cracking 
SCT from Gas 
SCT from Oil Cracking 
SCT from Desulfurized 
Naphtha Cracking 
Ex (1) 
Ex (2) 
Gas Oil Cracking 
__________________________________________________________________________ 
Physical Characteristics 
Viscosity cst @ 210.degree. F. 
13.9 19.3 
12.4 
25.0 
Coking Value at 550.degree. F. (%) 
12 16 24 25 
Toluene Insolubles (%) 
0.200 0.200 
0.250 
0.100 
n-Heptane Insolubles (%) 
3.5 16 20 15 
Pour Point (.degree.C.) 
+5 +5 -6 +6 
Ash (%) 0.003 0.003 
0.003 
0.003 
Chemical Structure (by 
carbon and proton NMR) 
Aromatic Carbon (atom %) 
65 72 71 74 
Aromatic Protons (%) 
34 42 42 38 
Benzylic Protons (%) 
40 44 46 47 
Paraffinic Protons (%) 
25 14 12 15 
Carbon/Hydrogen Atomic Ratio 
0.942 1.011 
1.079 
1.144 
Elemental Analysis 
Carbon (wt %) 91.60 90.31 
88.10 
90.61 
Hydrogen (wt %) 8.10 7.57 
6.80 
6.60 
Nitrogen (wt %) 0.15 0.10 
0.15 
0.18 
Oxygen (wt %) 0.20 0.22 
0.18 
0.19 
Sulfur (wt %) 0.06 1.5 4.0 1.5 
Iron (ppm) 0.003 0.003 
-- -- 
Vanadium (ppm) 0.000 0.001 
-- -- 
Silicon (ppm) 0.001 0.00 
-- -- 
Number Average Molecular Wt 
295 300 305 315 
Distillation Characteristics 
5% Vol 203 283 245 -- 
10% Vol 233 296 260 -- 
20% Vol 245 330 296 -- 
30% Vol 266 373 358 -- 
40% Vol 308 421 371 -- 
50% Vol 356 470 401 -- 
60% Vol -- 540 -- -- 
70% Vol -- 601 -- -- 
77% Vol -- 610 -- -- 
__________________________________________________________________________ 
In the process of the present invention, the steam cracker tars are 
fractionally distilled by heating to elevated temperatures at reduced 
pressures. For example, the stream cracker tar is heated to temperatures 
in the range of 130.degree. C. to 320.degree. C. at an approximate 
pressure of 10 mm of mercury. Basically, the steam cracker tar is 
separated into a middle distillate fraction having a boiling point at 760 
mm mercury in the range of from about 270.degree. C. to about 490.degree. 
C. In a particularly preferred embodiment of the present invention, the 
distillate fraction of the steam cracker tar which is employed in forming 
a suitable carbonaceous pitch for carbon artifact manufacture, is that 
fraction boiling in the range of about 370.degree. to about 490.degree. C. 
at 760 mm of mercury. 
An ASTM D1160 distillation of a typical steam cracker tar is given in Table 
2, below: 
TABLE 2 
______________________________________ 
Vol % Vapor Temperature 
Vapor Temperature 
Distillate 
@ 10 mmHg .degree.G 
@ 760 mmHg .degree.G 
______________________________________ 
2 130 270 
5 140 277 
10 147 285 
20 165 307 
30 190 336 
40 216 368 
50 243 400 
60 282 444 
70 316 483 
71 320 490 
______________________________________ 
The middle fraction taken at distillate 370.degree.-490.degree. C. at 760 
mmHg has high aromaticity and narrow molecular weight. It contains no ash 
or solid particulate and does not contain high coking asphaltene. 
Chemically it is composed of polycondensed 2, 3, 4 and 5 aromatic rings. 
Table 3 below gives the physical and chemical characteristics of a typical 
middle distillate fraction of steam cracker tar: 
TABLE 3 
______________________________________ 
Characteristics of Steam Cracker Tar Distillate (370-490.degree. C.) 
______________________________________ 
1. Physical Characteristics 
Ash Content (%) = Nil 
Asphaltene (n-heptane insolubles) (%) 
= Nil 
Viscositty cps @ 99.degree. C. 
= 4.5 
Toluene Insolubles (%) = Nil 
Coking Value @ 550.degree. C. (%) 
= 2.0 
2. Chemical Structure (CMR and PMR) 
Aromatic Carbon (atom %) = 71 
Paraffinic Protons (%) = 22 
Benzylic Protons (%) = 41 
3. Elemental Analysis 
Carbon (wt %) = 90.7 
Hydrogen (wt %) = 7.3 
Oxygen (wt %) = 0.20 
Nitrogen (wt %) = 0.10 
Sulfur (wt %) = 1.6 
4. Number Average Mol. Wt (GPC) 
= 245 
5. Aromatic Ring Distribution (MS) 
1 Ring = 3.7 
2 Rings = 43.6 
3 Rings = 39.2 
4 Rings = 11.1 
5 Rings = 1.5 
6 Rings = 0.8 
7 Rings = 0.1 
Aromatics with Carbon and Hydrogen 
= 84.3 
Aromatics with Carbon, Hydrogen and Oxygen 
= 3.7 
Aromatics with Carbon, Hydrogen and Sulfur 
= 11.9 
6. Average Carbon Atom in Side Chain 
= 3.0 
______________________________________ 
The molecular structure of a typical steam cracker tar middle distillate 
fraction as determined by high resolution Mass Spectrometer, is given 
below in Table 4: 
TABLE 4 
______________________________________ 
Molecular Structure of a Typical 
Steam Cracker Tar Distillate 
Compound Type 
Typical Name Wt % 
______________________________________ 
CnH.sub.2n-8 
Indanes 0.6 
CnH.sub.2n-10 
Indenes 1.3 
CnH.sub.2n-12 
Naphthalenes 5.0 
CnH.sub.2n-14 
Naphthenonaphthalene 
9.1 
CnH.sub.2n-16 
Acenaphthalenes 17.2 
CnH.sub.2n-18 
Penanthrenes 29.0 
CnH.sub.2n-20 
Naphthenophenanthrenes 
8.8 
CnH.sub.2n-22 
Pyrenes 7.3 
CnH.sub.2n-24 
Chyrsenes 2.3 
CnH.sub.2n-26 
Cholanthrenes 0.9 
CnH.sub.2n-12 S 
Naphthenobenzothiophenes 
0.4 
CnH.sub.2n-14 S 
Indenothiophenes 0.6 
CnH.sub.2n-16 S 
Naphtnothiophenes 8.5 
CnH.sub.2n-18 S 
Naphthenonaphthothiophenes 
0.6 
CnH.sub.2n-20 S 0.5 
CnH.sub.2n-10 O 
Benzofurans 
CnH.sub.2n-16 O 
Naphthenofurans 2.8 
CnH.sub.2n-18 O 
Naphthenonaphthofurans 
0.44 
CnH.sub.2n-20 O 
Acenaphthyenofurans 
0.2 
______________________________________ 
Another method to prepare an asphaltene-free steam cracker tar fraction is 
by removing the asphaltene from steam cracker tar by a solvent extraction 
of the asphaltene with a paraffinic solvent such as n-heptane, iso-octane, 
n-pentene, or pet-ether. Table 5, below, gives the characteristics of a 
deasphaltenated oil obtained from a steam cracker tar using n-heptane as a 
solvent (Feed:solvent ratio=1:30): 
TABLE 5 
______________________________________ 
The Preparation of Deasphaltenated 
Steam Cracker Tar 
Deasphaltenated 
Steam Steam 
Cracker Tar 
Cracker Tar 
1 2 1 2 
______________________________________ 
Weight (%) 100 100 80 82 
Sp. Gr. @ 15.degree. C. 
1.112 1.117 1.084 1.073 
Coking Value @ 550.degree. C. 
18.1 18.8 7.8 7.3 
Viscosity (cps) @ 100.degree. F. 
779 925 33.0 22.2 
Ash Content (%) 0.003 0.004 Nil Nil 
Asphaltene (%) 20.0 18.0 1.0 1.2 
(n-heptane insolubles) 
Carbon (%) 87.2 86.6 86.7 87.22 
Hydrogen (%) 6.7 6.6 6.91 7.22 
Oxygen (%) 0.32 0.31 0.46 0.21 
Sulfur (%) 3.7 5.3 4.5 4.5 
Aromatic Carbon (atom %) 
73 72 70 71 
C/H Atomic Ratio 
1.07 1.10 1.04 1.00 
______________________________________ 
After separating the steam cracker tar middle fraction distillate, the 
middle fraction distillate is heat soaked at temperatures in the range of 
about 400.degree. C. to 500.degree. C. Optionally and preferably, the heat 
soaking is conducted at temperatures in the range of about 390.degree. C. 
to about 450.degree. C., and most preferably at temperatures in the range 
of about 410.degree. C. to about 440.degree. C. In general, heat soaking 
is conducted for times ranging from one minute to about twenty hours, and 
preferably from about two to six hours. In the practice of the present 
invention, it is particularly preferred that heat soaking be done in an 
atmosphere such as nitrogen, or alternatively in hydrogen atmosphere. Heat 
soaking also may be conducted at reduced pressures in the range of from 
about 50 to 100 mm of mercury. 
After heat soaking the distillate, the heat soaked distillate is then 
heated in a vacuum at temperatures generally about 400.degree. C. and 
typically in the range of about 370.degree. C. to 420.degree. C., at 
pressures below atmospheric pressure, generally in the range of about 1.0 
to 100 mm mercury. This additional heating removes at least part of the 
oil present in the heat soaked distillate. Typically, from about 90 to 
100% of the oil which is present in the heat soaked distillate is removed. 
As can be readily appreciated, the severity of the heat soaking conditions 
outlined above, will affect the nature of the pitch produced. The higher 
the temperature chosen for heat soaking, and the longer the duration of 
the heat soaking process, the greater the amount of toluene insoluble 
components that will be generated in the pitch. 
Aromatic pitch can be characterized by various instrumental techniques. The 
aromaticity of pitch prepared from steam cracker tar distillate is very 
high, around 87% (measured by carbon NMR). These pitches have high C/H 
atomic ratio and contain little or no oil. 
Solvent analysis is widely used to define or characterize the pitch 
composition and/or the liquid crystal fraction in the pitch. We define the 
pitch of this invention by the toluene insolubles content (by weight 
percent). The quinoline insolubles in the pitch is also a useful guide in 
defining the pitch characteristics. 
The inventive process can prepare pitches with a very high toluene 
insolubles content (80-100% by weight) and low quinoline insolubles 
content (0.1-15% by weight). This pitch content can only be produced 
because of the use of a middle distillate fraction which has a low 
molecular weight and contains 2, 3, 4 and 5 polycondensed aromatic rings. 
As is disclosed in U.S. Pat. No. 4,208,267, in carbon fiber manufacture, it 
is particularly beneficial to use a fraction of the pitch which is readily 
convertible into a deformable optically anisotropic phase. Consequently, 
in the process of the present invention, it is particularly preferred to 
isolate that fraction of the heat soaked and vacuum stripped steam cracker 
distillate which is readily convertible into a deformable optically 
anisotropic phase. The preferred technique for isolating that fraction of 
the pitch is set forth in U.S. Pat. No. 4,208,267, which patent is 
incorporated herein by reference. Basically, that process requires 
treatment of the pitch with the solvent system which consists of a solvent 
or mixture of solvents that has a solubility parameter of between 8.0 and 
9.5 and preferably between 8.7 and 9.2 at 25.degree. C. 
Also and more preferably when extracting a fraction of a completely 
de-oiled pitch prepared from steam cracker tar distillate, it is preferred 
to use a single solvent, such as toluene. The crushed or molten pitch is 
mixed with toluene at 1:2 to 1:16 pitch/toluene ratio, and the mixture is 
agitated for 3-20 hours at room temperature. The toluene insoluble 
fraction is then filtered, washed and dried. 
EXAMPLES 1-4 
The following experimental method was used: 
About 600 grams of a steam cracker tar middle distillate fraction was 
charged to an electrically heated reactor equipped with nitrogen injection 
and mechanical agitation. The feed is then heated to the desired 
temperature, 420.degree.-440.degree. C., under a blanket of nitrogen and 
allowed to react at that temperature for the desired time, 15 to 90 
minutes, with good agitation under nitrogen. 
The heat soaked mixture was then vacuum stripped at reduced pressure, 
0.2-1.0 mmHg, at a liquid temperature of 400.degree.-420.degree. C. to 
remove most, if not all, of the distillable oils. The vacuum stripped 
pitch is allowed to cool under reduced pressure and discharged. Results 
for these Examples 1-4, are listed in Table 6. 
The percent quinoline insolubles in the product pitch was deterined by the 
standard technique of quinoline extraction at 75.degree. C. (ASTM Test 
Method D2318/76). 
The toluene insoluble fraction of the pitch was determined by the following 
method: 
About 40 grams of the crushed pitch product were mixed for 18 hours at room 
temperature with 320 ml of toluene. The mixture was thereafter filtered 
using a 10-15 micron fritted glass filter. 
The filter cake was washed with 80 ml of toluene, reslurried and mixed for 
about four hours at room temperature with 120 ml of toluene, and then 
filtered using a 10-15 micron glass filter. 
The filter cake was washed with 80 ml of toluene followed by a wash with 80 
ml of heptane, and finally the solid was dried at 120.degree. C. in a 
vacuum for 24 hours. 
The above method for determining toluene insolubles is hereinafter referred 
to as the SEP method (an achronym for the standard extraction procedure). 
The toluene insolubles in the pitch can also be determined by a one stage 
extraction method, by simply agitating the pitch and toluene 
(pitch:toluene ratio=1:8) at room temperature for 4 hours and then 
filtering, washing and drying the extract. 
The optical anisotropicity of the pitch was determined by first heating the 
pitch to 375.degree. C. and then cooling the pitch. A sample of the pitch 
was then placed on a slide with Permount, a histological mounting medium 
sold by the Fisher Scientific Company, Fairlawn, New Jersey. A slip cover 
was placed over the slide by rotating the cover under had pressure. The 
mounted sample was crushed to a powder and evenly dispersed on the slide. 
Thereafter, the crushed sample was viewed under polarized light at a 
magnification of 200.times., and the percent optical anisotropicity was 
estimated. 
TABLE 6 
__________________________________________________________________________ 
Preparation of Steam Cracker Tar Distillate Pitch 
Pitch Composition Toluene Insoluble (SEP) 
Toluene 
Characteristics 
Vacuum Stripping Insolubles Vis- 
Ex- 
Heat Soaking Liquid Toluene 
Quinoline 
(One-Stage cosity 
(%) 
am- 
Temperature 
Time 
Pressure 
Temperature 
% Oil 
Insolubles 
Insolubles 
Method) cps 
Optical 
ple 
(.degree.C.) 
(hrs) 
(mmHg) 
(.degree.C.) 
Removed 
(SEP) (%) 
(%) (%) Tg.sup.(1) 
C/H 
360.degree. 
Activity 
__________________________________________________________________________ 
1 420 4 1.0 370 11.2 50.4 1.9 256 
1.86 
-- -- 
2 430 3 1.0 370 14.8 54.0 1.3 255 
1.80 
-- -- 
3 430 4 0.2 360 12.3 80.0 8.0 95 249 
1.83 
1,393 
75+ 
4 430 4 0.5 400 10.3 86.0 0.4 100 249 
1.80 
1,210 
-- 
__________________________________________________________________________ 
.sup.(1) Tg = Glass Transition Temperature 
Referring to the illustrative FIGURE, various feedstocks are shown 
including the deasphaltenated steam cracker tar bottom fraction of this 
invention. These feedstocks are shown divided into their corresponding 
percentages of useable (precursor) pitch materials, and non-useable 
(non-precursor) pitch materials. It is observed that when all the cat 
cracker bottom fractions are used to obtain precursor materials, only a 
small percentage of liquid crystal rich materials are obtained. For 
example, heat soaked Ashland Pitch is observed to contain only 
approximately 25 percent Ti precursor. 
Such a pitch material must be further treated to extract the useable Ti 
fraction. However, the problem with extracting the Ti content from such a 
pitch material is that it is very difficult to do this without also 
including the so-called "bad actors". In other words, the impurities and 
ash are also carried along. In addition, heat treating these low Ti 
materials will very often produce coke, which is detrimental to the 
spinning process. 
Therefore, the elimination of the "bad actors" and the coke producing 
substances in advance of further processing would not only be desirable in 
producing a trouble-free precursor material, but also should usually 
eliminate the need to perform an additional extraction step. 
Thus, it is observed that a feedstock material which uses only a middle 
fraction, i.e. distillate fractions (370.degree.-490.degree. C.), of a 
steam cracker tar bottom, will be virtually free of the "bad actors", and 
will contain between 80 and 100% Ti after heat soaking and vacuum 
stripping. Such precursor materials will be very uniform, relatively free 
of ash and impurities as further defined by a low quinoline insoluble 
content (less than 15% by weight), and will easily lend themselves to 
further controlled processing. 
As aforementioned, such precursors may not require an additional extraction 
step for the Ti. 
The FIGURE also represents similar results obtained from other feedstock 
materials such as Steam Cracker Tars (SCT) and Cat Cracker Bottoms (CCB). 
When the middle fractions of these feedstocks are separated, heat soaked, 
and vacuum stripped, it is observed that high content Ti substances are 
also produced. 
Thus, the invention is not necessarily limited to the starting materials, 
but rather to the realization of the need to prefractionate and separate 
the middle fractions from these materials, and to vacuum strip these 
fractions after heat soaking at temperatures generally in excess of 
400.degree. C. 
A pitch of this invention can be generally defined by the following solvent 
analysis: 
______________________________________ 
Solvent Analysis 
______________________________________ 
Toluene insolubles wt % 
80-100 
(SEP method) 
Quinoline insolubles wt % 
1.0-15 
(ASTM D2318-66) (preferably less than 5%) 
Aromaticity 80-90 
(% Aromatic carbon atom) 
Melting point (.degree.C.) 
150-250 
Glass Transition Temperature 
170-220 
(.degree.C.) (Tg) 
Ash wt % nil-0.1 
Optical Activity 70-100 
(% by polarized light 
microscopy) 
______________________________________ 
Having thus described this invention, what is desired to be protected by 
Letters Patent is presented in the following appended claims.