Process for the production of mesophase

An improved process for producing an anisotropic pitch product suitable for carbon fiber manufacture. A carbonaceous feedstock substantially free of mesophase pitch is heated at elevated temperature while passing an oxidatively reactive sparging gas such as air through the feedstock. The oxidatively treated feedstock, which contains isotropic pitch, is solvent fractionated to recover a solid pitch which on fusion becomes an anisotropic pitch product having from 50 to 100 percent by volume mesophase. In one aspect of the invention the carbonaceous feedstock is oxidatively treated in a melt phase at a lower temperature and the resulting isotropic pitch is then heated at a higher temperature in a melt phase in the presence or absence of a non-oxidative sparging gas prior to solvent fractionation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to an improved process for producing a 
carbonaceous pitch product having a mesophase content ranging from about 
50 to 100 percent, which is suitable for carbon fiber manufacture. More 
particularly, the invention relates to a process for making mesophase 
containing pitch capable of producing high strength carbon fibers, by 
contacting a feedstock with an oxidative gas at an elevated temperature to 
prepare an isotropic pitch and thereafter solvent fractionating the 
isotropic pitch to recover a mesophase pitch product suitable for carbon 
fiber manufacture. 
2. The Prior Art 
In recent years extensive patent literature has evolved concerning the 
conversion of carbonaceous pitch feed material into a mesophase-containing 
pitch which is suitable for the manufacture of carbon fibers having 
desirable modulus of elasticity, tensile strength, and elongation 
characteristics. 
U.S. Pat. No. 4,209,500 (issued to Chwastiak) is directed to the production 
of a high mesophase pitch that can be employed in the manufacture of 
carbon fibers. This patent is one of a series of patents pertaining to a 
process for producing mesophase pitches suitable for carbon fiber 
production. Each of these patents broadly involves heat treating or heat 
soaking the carbonaceous feed while agitating and/or passing an inert gas 
therethrough so as to produce a more suitable pitch product for the 
manufacture of carbon fibers. 
As set forth in the Chwastiak patent, earlier U.S. Pat. Nos. 3,976,729 and 
4,017,327 issued to Lewis et al involve agitating the carbonaceous 
starting material during the heat treatment. The use of an inert sparge 
gas during heat treatment is found in U.S. Pat. Nos. 3,974,264 and 
4,026,788 issued to McHenry. Stirring or agitating the starting material 
while sparging with an inert gas is also disclosed in the McHenry patents. 
U.S. Pat. No. 4,277,324 (Greenwood) discloses converting an isotropic pitch 
to an anisotropic (mesophase) pitch by solvent fractionation. Isotropic 
pitch is first mixed with an organic fluxing solvent. Suspended insoluble 
solids in the flux mixture are then removed by physical means, such as, 
filtration. The solids-free flux liquid is then treated with an 
antisolvent to precipitate a mesophase pitch. The patent further discloses 
heat soaking the isotropic pitch at 350.degree. C. to 450.degree. C. prior 
to solvent fractionation. 
U.S. Pat. No. 4,283,269 (Greenwood) discloses a process similar to that of 
4,277,324 except that the heat soaking step is carried out on the fluxed 
pitch. 
Japanese Patent 65090/85 discloses heating a carbonaceous feed to 
350.degree.-500.degree. C. in the presence of an oxidizing gas to prepare 
a mesophase pitch. 
U.S. Pat. No. 4,464,248 (Dickakian) discloses a catalytic heat soak 
preparation of an isotropic pitch which is then solvent fractionated to 
produce a mesophase pitch. 
U.S. Pat. No. 3,595,946 (Joo et al) and U.S. Pat. No. 4,066,737 (Romavacek) 
call for the use of an oxidative reactive material, such as air to produce 
a heavy isotropic pitch which is used to make carbon fibers. 
U.S. Pat. No. 4,474,617 (Nemura et al) describes treating low mesophase 
content pitch with oxidizing gas at a temperature of 200.degree. to 
350.degree. C. to produce an improved carbon fiber. 
Thus, the art shows that it is known to heat soak a feed to form an 
isotropic pitch which yields mesophase pitch on solvent fractionation. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has now been found that when a 
carbonaceous feedstock substantially free of mesophase pitch is contacted 
with an oxidative gas under suitable conditions (including an elevated 
temperature), a product containing isotropic pitch is formed but is not 
further converted to mesophase pitch. Thereafter the isotropic pitch 
product is solvent fractionated, and a pitch product containing 50 to 100 
percent by volume mesophase, as determined by optical anisotropy, is 
obtained. The oxidative gas accelerates the formation of solvent 
fractionatable mesophase formers during the heating step. The pitch 
product from solvent fractionation provides fibers having high modulus and 
high tensile strength. In a two-step embodiment of the invention, the 
carbonaceous feedstock is contacted with the oxidative gas at a lower 
temperature level and the resulting isotropic pitch product is subjected 
to a heat soak at a higher temperature prior to solvent fractionation, 
said heat soak being carried out in a melt phase either in the presence or 
absence of a non-oxidative sparging gas. The use of melt phase allows 
thorough contacting of substantially all the pitch with the sparge gas, 
the melt pitch providing a substantially continuous melt phase. Thus, the 
present invention utilizes an oxidative acceleration of mesophase 
formation to yield equal amounts of mesophase pitch in less time. 
DETAILED DESCRIPTION OF THE INVENTION 
The carbonaceous feedstocks used in the process of the invention are heavy 
aromatic petroleum fractions and coal-derived heavy hydrocarbon fractions, 
including preferably materials designated as pitches. All of the 
feedstocks employed are substantially free of mesophase pitch. 
The term "pitch" as used herein means petroleum pitches, natural asphalt 
and heavy oil obtained as a by-product in the naphtha cracking industry, 
pitches of high carbon content obtained from petroleum asphalt and other 
substances having properties of pitches produced as by-products in various 
industrial production processes. 
The term "petroleum pitch" refers to the residuum carbonaceous material 
obtained from the thermal and catalytic cracking of petroleum distillates 
or residues. 
The term "anisotropic pitch or mesophase pitch" means pitch comprising 
molecules having an aromatic structure which through interaction have 
associated together to form optically ordered liquid crystals. 
The term "isotropic pitch" means pitch comprising molecules which are not 
aligned in optically ordered liquid crystals. Fibers produced from such 
pitches are inferior in quality to fibers made from mesophase pitches. 
The term "resin" is used to indicate the presence of mesophase-forming 
materials or mesophase precursors. The presence of resins is generally 
directly related to the insolubles content of the pitch, i.e. pentane or 
toluene insoluble content is directly related to the resin content of the 
pitch. 
Generally, feedstocks having a high degree of aromaticity are suitable for 
carrying out the present invention. Carbonaceous pitches having an 
aromatic carbon content of from about 40 percent to about 90 percent as 
determined by nuclear magnetic resonance spectroscopy are particularly 
useful in the process. So, too, are high boiling, highly aromatic streams 
containing such pitches or that are capable of being converted into such 
pitches. 
On a weight basis, useful feedstocks will contain from about 88 percent to 
about 93 percent carbon and from about 9 percent to about 4 percent 
hydrogen. While elements other than carbon and hydrogen, such as sulfur 
and nitrogen, to mention a few, are normally present in such pitches, it 
is important that these other elements do not exceed about 5 percent by 
weight of the feedstock. Also, these useful feedstocks typically will have 
an average molecular weight of the order of about 200 to about 1,000. 
In general, any petroleum or coal-derived heavy hydrocarbon fraction may be 
used as the carbonaceous feedstock in the process of this invention. 
Suitable feedstocks in addition to petroleum pitch include heavy aromatic 
petroleum streams, ethylene cracker tars, coal derivatives, petroleum 
thermal tars, fluid catalytic cracker residues, and aromatic distillates 
having a boiling range of from 650.degree.-950.degree. F. The use of 
petroleum pitch-type feed is preferred. 
As stated previously the process for the preparation of isotropic pitch to 
be subjected to solvent fractionation may be carried out in one step, i.e. 
by oxidative treatment at an elevated temperature above about 320.degree. 
F. Alternatively, the invention can be carried out in two steps, viz. by 
oxidative treatment at a lower temperature (below about 320.degree. F.), 
followed by heat soaking at a higher temperature (above about 320.degree. 
F.) sufficient to melt the pitch, with or without the use of a sparging 
non-oxidative gas, then subjected to solvent fractionation. Whichever 
process is employed, the preferred gas for the oxidative treatment of the 
carbonaceous feedstock is air or other mixtures of oxygen and nitrogen. 
Gases other than oxygen such as ozone, hydrogen peroxide, nitrogen 
dioxide, formic acid vapor and hydrogen chloride vapor, may be also used 
as the oxidative component in the process. These oxidative gases may be 
used alone or in admixture with inert (non-oxidative) components such as 
nitrogen, argon, xenon, helium, methane, hydrocarbon-based flue gas, 
steam, and mixtures thereof. In general, there can be employed any gas 
stream or a mixture of various gas streams with an appropriate oxidative 
component so that reaction with the feedstock molecules occurs to provide 
a carbonaceous material with increased resin content (mesophase 
precursors), but which is not converted to mesophase pitch. 
The temperature employed in the one step oxidative process is above 
320.degree. C. and may be as high as about 500.degree. C., wherein the 
pitch is in a molten state, providing a substantially continuous melt 
phase and allowing substantially all the pitch to be contacted by the 
sparge gas. Preferably the oxidative process temperature range is between 
about 350.degree. C. and about 400.degree. C. The oxidative gas rate is at 
least 0.1 SCFH per pound of feed, preferably from about 1.0 to 20 SCFH. 
Sparging with the oxidative gas is generally carried out at atmospheric or 
slightly elevated pressures, e.g. about 1 to 3 atmospheres, but higher or 
lower pressures may be used if desired. The sparging time period may vary 
widely depending on the feedstock, gas feed rates, and the sparging 
temperature. Time periods from about 0.5 to about 32 hours or more may be 
used. Preferably the sparging time varies from about 2 to about 20 hours. 
It is important that the sparging time not be excessive since an extended 
time of oxidation at the temperatures used will produce a mesophase pitch 
or coke product rather than the desired isotropic product. 
The temperatures used in the oxidative step of the two step process are 
lower than those used in the one step process, but the pitch is still 
treated in a melt phase. Usually temperatures between about 200.degree. C. 
and about 350.degree. C. are employed, and preferably between about 
250.degree. C. and about 320.degree. C. The oxidative gas rate again is at 
least 0.1 SCFH per pound of feed and preferably varies from about 1.0 to 
about 20 SCFH. Since the pitch is treated as a melt, there is 
substantially total control between the pitch and the gas and "channeling" 
is largely avoided. Pressures employed are similar to those used in the 
one step process. The time of sparging with the oxidative gas may be from 
about 2 to about 100 hours depending on the other process variables 
employed. More usually the sparging time is between about 4 and about 32 
hours. 
At the relatively low temperatures employed in the oxidative phase of the 
two step process the materials formed give an isotropic pitch product 
rather than a mesophase pitch on solvent fractionation. Thus it is 
necessary to further treat the pitch resulting from the low temperature 
oxidation of the carbonaceous feed by subjecting it to a heat soak at a 
temperature higher than the temperature employed in the oxidative step. 
The temperatures and pressures used for the heat soak are generally the 
same as those employed in the one step oxidative process. The soaking time 
will be relatively short, usually from about 0.1 to about 8 hours, 
depending on the other process variables employed. Here again the time of 
treatment is controlled to provide an isotropic pitch rather than the 
mesophase pitch which would result from a more extended treatment. The 
two-step process may be preferred to the one-step process described to 
enhance the total yield of mesophase pitch. The two-step method of the 
present invention produces a higher conversion to mesophase pitch, based 
on the starting feedstock. 
Optionally, but not critically, the heat soak step can be carried out in 
melt phase in the presence of a non-oxidative sparging gas. Such a gas, 
when used, may be selected from the inert gases previously mentioned in 
the discussion of the one step oxidative process. In some instances it may 
be inconvenient to provide both an oxidative and a non-oxidative gas in 
the two-step process. In such event, the oxidative gas used in the first 
step may also be used as a sparging gas in the heat soak step, without 
detriment to the process. Of course, a different oxidative gas may also be 
used in each step of the two-step process, if desired. 
With completion of the oxidative treatment in the one step process (or the 
heat soak of the two step process), the isotropic carbonaceous feed is 
subjected to solvent fractionation, to produce, after fusion, a pitch 
suitable for spinning into carbon fibers. Solvent fractionation is carried 
out by the following steps: 
(1) Fluxing the isotropic pitch in a hot solvent. 
(2) Separating flux insolubles by filtration, centrifugation or other 
suitable means. 
(3) Diluting the flux filtrate with an anti-solvent to precipitate a 
mesophase forming pitch and washing and drying the precipitated pitch. 
After fusion, the pitch is identified as mesophase pitch. 
The solvent fractionation procedure described is well known in the art and 
is set forth in some detail in numerous patents including U.S. Pat. No. 
4,277,324, which is incorporated herein by reference. This patent sets 
forth the numerous solvents and anti-solvents which can be employed in 
solvent fractionation and the operating conditions and procedures which 
may be used. 
In some instances the temperatures and time periods employed in the single 
step oxidative treatment (or in the heat soak step of the two step 
process) may produce a residual product which contains some mesophase 
pitch. If this should occur, such mesophase pitch can be removed by the 
treatment of the isotropic pitch with the organic fluxing solvent, along 
with suspended insoluble solids and materials with high melting points. 
The subsequent treatment with the antisolvent provides a precipitated 
pitch in which mesophase forming molecules capable of combining to form 
the optically ordered liquid crystals which characterize mesophase pitch. 
The solvent fractionation treatment produces a solid pitch which on fusion 
becomes mesophase pitch which can be spun into continuous anisotropic 
carbon fibers by conventional procedures such as melt spinning, followed 
by the separate steps of thermosetting and carbonization. As indicated, 
these are known techniques and consequently they do not constitute 
critical features of the present invention.

The present invention will be more fully understood by reference to the 
following illustrative embodiments. 
EXAMPLE 1 
This example illustrates the one-step process of the present invention. A 
petroleum decant oil (900.degree. F+ residue) was used as a feedstock for 
this and the other Examples. The feedstock contained 3.8 percent toluene 
insolubles and less than 0.1 percent THF insolubles. In this example the 
feed was heated for 8 hours at 385.degree. C. A 2 percent oxygen in 
nitrogen gas stream was bubbled through the molten residue at 0.44 SCF per 
hour per pound of feed during the heating process. Oxidatively treated 
residual product containing isotropic pitch was obtained in 90 percent 
yield. The pitch also contained 31 percent toluene insolubles (TI) and 9 
percent THF insolubles (THFI). 
The treated pitch was solvent fractionated to produce a pitch suitable for 
spinning into carbon fibers. This was done by the following steps: 
(1) Fluxing the heat soaked pitch in an equal weight of hot toluene. 
(2) Filtering to remove flux insolubles. 
(3) Diluting the flux filtrate with 8 cubic centimeters (cc) per 1 gram (g) 
of pitch feed with a solvent composed of 20 volume percent heptane in 
toluene. 
(4) Cooling the solution to ambient and recovering the precipitated pitch 
by filtration. 
(5) Washing and drying of the pitch product. 
The resultant pitch obtained in 21 percent yield melted at 319.degree. C. 
The melted sample was cooled and identified as 100 percent mesophase. This 
pitch was spun into carbon fibers which were stabilized and then 
carbonized to 1850.degree. C. The fibers exhibited a tensile strength of 
409 Kpsi and a tensile modulus of 31 Mpsi. 
EXAMPLE 2 
The example further illustrates the one-step process of the present 
invention. Other samples of feedstock were oxidatively treated for 2, 4 
and 6 hours in three separate preparations. The process was carried out at 
385.degree. C. and 5 percent oxygen in nitrogen was bubbled through the 
molten reaction mixture at 0.44 SCF per hour per pound of feed. The yield 
and insolubles content of the oxidatively treated residues are shown in 
Table 1. Also shown are the yields from solvent fractionation of the 
oxidatively treated pitches to make mesophase pitches. The solvent 
fractionation conditions followed those described in Example 1. The 
mesophase pitches were each 100 percent mesophase. They were spun into 
carbon fibers which were stabilized and then carbonized. High strength 
high modulus fibers were produced as shown in the table. 
TABLE 1 
______________________________________ 
Example Number 
2A 2B 2C 
______________________________________ 
Heat Soak, hr @ 385.degree.C. 
2 4 6 
Residue (containing isotropic pitch) 
Yield, % 94 85 81 
Residue TI, % 18 32 65 
Residue THFI, % 5 11 18 
Solvent Fract. Yield, % 
21 24 25 
Meso. Pitch Melt Temp., .degree..C 
309 317 294 
Fiber Carb. Temp., .degree..C 
1850 1650 1850 
Carb. Fiber Tesnsile Str., Kpsi 
367 365 475 
Carbon Fiber Tensile Mod., Mpsi 
24 28 38 
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EXAMPLE 3 
This Example shows the effect of heat soaking in the absence of a reactive 
oxygen-containing gas. Petroleum decant oil residue feedstock was heat 
soaked in the molten state at 385.degree. C. for 8 hours while being blown 
with molten nitrogen at 0.44 SCF per hour per pound of feed. Heat soaked 
residual product containing isotropic pitch was obtained in 88 percent 
yield. This pitch contained 29 percent toluene insolubles and 11 percent 
THF insolubles. 
The heat soaked pitch was solvent fractionated by the procedure outlined in 
Example 1. Pitch suitable for spinning into carbon filters was isolated in 
24 percent yield. This pitch melted at 292.degree. C. and was 
characterized as 100 percent mesophase by optical microscopy. The 
stabilized and carbonized (1650.degree. C.) fibers from this pitch had a 
tensile strength of 439 Kpsi and a tensile modulus of 34 Mpsi. 
The principal benefit of the use of an oxidative gas is more rapid 
formation of mesophase forming components during the oxidative treatment 
with no loss in fiber quality. In Example 3 (no oxygen) treatment for 8 
hours at 385.degree. F. produces heat soaked pitch yielding 24 percent 
mesophase. 
By comparison, in Example 2, treatment at the same temperature for only 4 
hours with an oxidative gas containing 5 percent oxygen produces heat 
soaked pitch yielding the same percent mesophase. 
Comparable fibers are obtained from the pitches in both examples. 
EXAMPLE 4 
This comparative example and Examples 5 and 6 illustrate the necessity for 
high temperature thermal treatment of the heat soaked pitch produced by 
low temperature (below 320.degree. F.) oxidative treatment when the 
objective is to produce high strength and high modulus carbon fibers. 
Petroleum decant oil residue was air blown at 2.0 SCF per hour per pound 
of feed for 16 hours at 250.degree. C. The product containing isotropic 
pitch obtained in 99.8 percent yield contained 13.9 percent toluene 
insolubles and 1.3 percent THF insolubles. 
The air blown pitch was solvent fractionated to produce a pitch suitable 
for spinning by the method described in Example 1. The pitch was recovered 
in 24.9 percent yield and melted at 297.degree. C. The product was an 
isotropic pitch (0 percent mesophase) after melting. This pitch was spun 
into carbon fibers which were stabilized and then carbonized at 
1800.degree. C. The fibers had a tensile strength of 115 Kpsi and a 
tensile modulus of 5.1 Mpsi. 
EXAMPLE 5 
In this example the isotropic pitch feedstock of Example 4 was air blown at 
300.degree. C. for 8 hours. The air rate was 2.0 SCF per hour per pound of 
feed. The product containing isotropic pitch recovered in 97.8 percent 
yield contained 30.1 percent toluene insolubles and 7.7 percent THF 
insolubles. 
The air blown pitch was solvent fractionated by the steps outlined in 
Example 1 to yield 35.4 percent of an isotropic pitch melting at 
307.degree. C. The pitch was spun into carbon fibers which were stabilized 
and then carbonized to 1800.degree. C. The fibers had a tensile strength 
of 150 Kpsi and a tensile modulus of 6.3 Mpsi. 
EXAMPLE 6 
This example shows the two-step process of the present invention. The 
feedstock of Example 4 was air blown at 250.degree. C. for 16 hours at an 
air rate of 1.0 SCF per hour per pound of feed. This was followed by 4 
hours of heat soak at 385.degree. C. while blowing the mixture with 
nitrogen at 2.0 SCF per hour per pound of feed. The residual product 
containing isotropic pitch recovered in 79.9 percent yield contained 33.4 
percent toluene insolubles and 11.5 percent THF insolubles. 
The heat treated pitch was solvent fractionated according to the steps 
outlined in Example 1. A mesophase pitch (100 percent anisotropic on 
fusion) was recovered in 28.4 percent yield. The mesophase melted at 
317.degree. C. The mesophase pitch was spun into carbon fibers which were 
stabilized and then carbonized to 1650.degree. C. The fibers had a tensile 
strength of 343 Kpsi and a tensile modulus of 20 Mpsi. 
A second test was carried out using the same procedure but without nitrogen 
blowing during the heat soak. The product containing isotropic pitch was 
recovered in 96.3 percent yield and contained 24 percent toluene 
insolubles and 11 percent THF insolubles. Upon solvent fractionation a 
mesophase pitch (100 percent anisotropic on fusion) was recovered in 26.1 
percent yield with a melting point of 323.degree. C. 
EXAMPLE 7 
A number of additional tests were carried out using the same procedures and 
gas rate of comparative Examples 4 and 5. The results of the oxidative 
treatment carried out in these tests are presented in Table 2. 
TABLE 2 
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Residue 
Reaction Reactive Gas O.sub.2 lnsolubles. % 
Sample 
Temp. .degree.C. 
Time Hr. Content. Vol % 
Toluene 
THF 
______________________________________ 
1 Feed None None 3.8 0.1 
2 250 8 2 5.8 0.2 
3 250 16 2 7.1 0.2 
4 200 8 20* 5.3 0.2 
5 200 16 20* 7.2 0.3 
6 250 8 20* 8.8 0.3 
7 300 16 20* 55.7 22.9 
______________________________________ 
*Air used as gas. 
The examples show that the oxygen treatment creates resin materials. 
Treatment of these increased insoluble feedstocks will allow production of 
mesophase materials according to the present invention. 
While certain embodiments and details have been shown for the purpose of 
illustrating the present invention, it will be apparent to those skilled 
in this art that various changes and modifications may be made herein 
without departing from the spirit or scope of the invention.