This invention relates to a process for isolating mesophase pitch.
Background and Summary of the Invention
It is well known that carbon fibers having excellent properties suitable
for commercial use can be produced from mesophase pitch. Mesophase pitch derived
carbon fibers are light weight, strong, stiff, thermally and electrically conductive,
and both chemically and thermally inert. The mesophase-derived carbon fibers perform
well as reinforcements in composites, and have found use in aerospace applications
and quality sporting equipment.
Low cost carbon fibers produced from isotropic pitch exhibit little
molecular orientation and relatively poor mechanical properties. In contrast, carbon
fibers produced from mesophase pitch exhibit highly preferred molecular orientation
and relatively excellent mechanical properties.
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
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 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.
The term "mesogens" means mesophase-forming materials or mesophase
Mesophase pitch is not ordinarily available in existing hydrocarbon
fractions, such as refining fractions, or in coal fractions, such as coal tars.
Mesophase pitch, however, may be derived from isotropic pitch containing mesogens.
Isotropic pitch containing mesogens is usually prepared by the treatment of aromatic
feedstocks. Such treatment, which is well known in the art, may involve one or
more heat soaking steps, with or without agitation, and with or without gas sparging
or purging. Gas sparging may be carried out with an inert gas or with an oxidative
gas, or with both types of operations. Numerous patents describe various aspects
of the treatment of aromatic containing feedstocks to obtain isotropic pitch.
Included are: U.S. Patent Nos. 4,209,500, heat soaking; 3,976,729 and 4,017,327,
agitation during heat treatment; 3,974,264 and 4,026,788, inert sparge gas during
heat treatment; 4,283,269, heat soaking of fluxed pitch; Japanese Patent 65090/85,
heating in the presence of an oxidizing gas; 4,464,248, catalytic heat soaking;
3,595,946 and 4,066,737, use of oxidative reactive material; and 4,474,617, use
of oxidizing gas; and many others.
Mesophase pitch may be obtained from isotropic pitch containing mesogens
by solvent fractionation, which 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
- (3) Diluting the flux filtrate with an anti-solvent (comix solvent) to precipitate
a mesophase-forming (mesogen containing) pitch.
- (4) Washing and drying the precipitated pitch.
- (5) Fusing the precipitated pitch to form mesophase.
The solvent fractionation procedure described is well known in the
art and is set forth in some detail in numerous patents, including U.S. Patent
No. 4,277,324, which is incorporated herein by reference. This patent sets forth
numerous solvents and anti-solvents which can be employed in solvent fractionation
and the operating conditions and procedures which may be used.
Separation of mesogens from isotropic pitch may also be effected
by the solvent extraction process described in U.S. Patent 4,208,267. In this patent
fractionation is accomplished without fluxing or flux filtration. The mesogen-containing
isotropic pitch is extracted with a comix type solvent and the mesogens are collected
as an insoluble residue. Solvents used in this process are similar to those employed
in the process of U.S. Patent 4,277,324.
It is desirable to provide an alternative process for obtaining mesophase
pitch from isotropic pitch which does not involve the use of a comix solvent, and
thus eliminates the need for storage and pumping facilities for two solvents and
separation facilities for separating the solvents.
According to the present invention, isotropic pitch containing mesogens
is combined with a solvent and subjected to dense phase or supercritical conditions
to effect phase separation of the mesogens from the pitch. In one aspect of the
invention, isotropic pitch containing mesogens is fluxed with a solvent to solubilize
the mesogens, the flux mixture is then filtered to remove insolubles, and the
solubilized mesogens are phase separated from the flux mixture under dense phase
or supercritical conditions of temperature and pressure. The dense phase or supercritical
conditions employed are such that the mesogens are recovered as mesophase.
U.S. Patent No. 4,581,124 discloses treatment of a pitch (containing
a substantial amount of mesophase, i.e. 5 to 25 weight percent) with solvent extraction
under supercritical conditions to recover a mesophase rich pitch containing at
least 30 percent mesophase and preferably at least 50 percent mesophase by weight.
Japanese Patent No. 60-170694 discloses the preparation of precursor
pitch for carbon fibers by extracting coal tar pitch with an aromatic solvent in
a critical state. The extracted pitch is then subjected to heat treatment with
sparging of inert gas to give the desired product.
U.S. Patent No. 4,277,324 discloses converting an isotropic pitch
to 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-forming pitch which
is fused to form mesophase. The patent further discloses heat soaking the pitch
prior to solvent fractionation.
U.S. Patent No. 4,208,267 discloses extracting isotropic pitches
with a comix (antisolvent) solvent to provide a solvent insoluble fraction. This
fraction when heated to 230°C to 400°C is converted to greater than 75% mesophase.
Brief Description of the Drawing
Figure 1 is a schematic diagram of a process unit suitable for producing
mesophase pitch which illustrates the invention.
Detailed Description of the Invention
Suitable isotropic pitches for use in carrying out the process of
the invention may be obtained by various treatments of heavy aromatic fractions,
including heat soaking. While heavy fractions generally may be used, the preferred
materials are petroleum pitches as previously defined. On a weight basis, particularly
useful pitches 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 are normally present in such pitches,
it is important that these other elements do not exceed about 5 percent by weight
of the pitch. Also, these particularly useful pitches typically will have an average
molecular weight on the order of about 200 to about 1000.
Useful starting materials in addition to the preferred petroleum
pitches include ethylene cracker tars, coal derivatives, petroleum thermal tars,
and aromatic distillates having a boiling range of from 650 to 950°F (343 to 510°C).
When heat soaking is employed to obtain suitable isotropic pitch,
this procedure is usually accomplished at a temperature in the range of about 370
to about 500°C for about 0.10 to about 240 hours. Lower soak temperatures require
longer soak times and vice versa. The preferred soaking conditions are from about
2 to about 24 hours at a temperature range of about 390 to about 430°C. As mentioned
previously, the heat soaking step may be carried out with or without agitation
and with or without the presence of a sparge or purge gas.
In the preferred aspect of the invention, isotropic pitch containing
mesogens is mixed with a fluxing solvent and is fluxed to solubilize the mesogens.The
use of this preferred aspect is now discussed.
A variety of solvents are suitable for use as the fluxing material.
They include such compounds as aromatics such as benzene and naphthalene, naptheno-aromatics
such as tetralin and 9,10-dihydroanthracene, alkyl aromatics such as toluene, xylenes
and methyl naphthalenes, hetero-aromatics such as pyridine, quinoline and tetrahydrofuran;
and combinations thereof. Also suitable are simple halo carbons, including chloro
and fluoro derivatives of paraffin hydrocarbons containing 1 to 4 carbon atoms
such as chloroform and trichloroethane and halogenated aromatics such as trichlorobenzene.
In general, any organic solvent having a critical temperature below 500°C, which
is non-reactive with the pitch and which, when mixed with the pitch in sufficient
amounts, is capable of solubilizing the mesogens may be used in carrying out the
process of the invention. At temperatures above 500°C undesirable reactions can
take place with or between aromatic compounds in the pitch.
The amount of fluxing solvent used will vary depending upon the temperature
at which mixing is conducted and the composition of the pitch. In general, the
amount of solvent used will be in the range of between 0.05 parts by weight of
solvent per part by weight of pitch to 2.5 parts by weight of solvent per part
by weight of pitch. Preferably, the weight ratio of flux solvent to pitch will
be in the range of from 0.7 to 1 to 1.5 to 1. The fluxing operation is usually
carried out at an elevated temperature and at sufficient pressure to maintain the
system in the liquid state. Mixing or agitation may be provided during the fluxing
operation to aid in the solubilization of the mesogens. Usually the fluxing operation
is performed at a temperature in the range of between 30 and 150°C and for a time
period of between 0.1 and 2.0 hours. However, fluxing may be carried out up to
the boiling point of the solvent at system pressure. If desired, the flux mixture
may be stored in tankage indefinitely.
Upon completion of the fluxing step, the solubilized mesogens may
be separated from the insoluble portion of the pitch by the usual techniques of
sedimentation, centrifugation or filtration. If filtration is the selected separation
technique used, a filter aid may be employed, if desired, to facilitate the separation
of the fluid material from the solids.
The solid materials which are removed from the fluid pitch in this
preferred embodiment consist of materials such as coke and catalyst fines which
were present in the pitch e.g. prior to heat soaking or those insolubles generated
during heat soaking. If heat soaking conditions are not carefully controlled, mesophase
may be generated in the pitch during heat soaking. This mesophase is partially
lost in the process since it is predominantly insoluble in the flux mixture and
is removed with the other insolubles during the separation process. In the process
of the invention, isotropic pitch, which is substantially free of mesophase, is
preferred since this means that the prior treatment of the pitch has been accomplished
in a manner to provide for a maximum amount of mesogens in the pitch prior to solvent
In the preferred embodiment under discussion, after removal of the
solids from the system, the pitch solvent mixture containing dissolved mesogens
is subjected to supercritical temperature and pressure, i.e. temperature and pressure
at or above the critical temperature and critical pressure of the flux solvent
to effect phase separation of the mesogens from the pitch. In the case of toluene,
for example, the critical conditions are 319°C and 611 psia (4.21 MPa). The time
required to separate mesogens from the system will vary, depending on the particular
pitch and the solvent employed and the geometry of the separation vessel. If desired,
additional fluxing solvent may be added to the system. The amount of such added
solvent may be up to 12 parts of solvent by weight per part by weight of pitch
and preferably from 0.5 to 6 parts of solvent per part of pitch. If additional
fluxing solvent is added, agitation or mixing is desirable to promote intimate
In the prior art method of solvent fractionation of isotropic pitch,
which included the use of a comix or anti-solvent, a fusing operation served to
convert the mesogens to mesophase pitch. In the process of this invention, fusing
is not necessary to accomplish this conversion since the product obtained from
the supercritical phase separation step is mesophase rather than mesogens.
The supercritical conditions applied in carrying out the process
of the invention will vary depending on the solvent used, the composition of the
pitch and the temperature employed. The level of supercritical pressure may be
used to control the solubility of the pitch in the solvent and thus established
the yield and the melting point of the mesophase product. For example, at a given
temperature and solvent-to-pitch ratio, if the pressure on the system is increased,
the solubility of the pitch in the solvent also increases. This results in a lower
yield of higher melting point mesophase product. Lowering the pressure gives the
opposite result. Generally the supercritical temperature employed will be at or
somewhat above the critical temperature of the solvent, e.g. from 0 to about 100°C
above the solvent critical temperature. If desired, higher temperatures may be
used; however, they are not required. The pressure maintained on the system may
vary over a wide range since it is most conveniently used for controlling product
properties and yield. Thus the pressure applied on the system may be up to twice
as high as the critical pressure or higher if desired.
The temperature and pressure required for the process herein are
the same as or higher than the critical temperature and pressure of the solvent
used in the process. Suitable solvents are those solvents which have critical temperatures
in the range of from 100°C to 500°C. The upper temperature limit is controlled
by the thermal stability of the pitch and/or solvent mixture. The lower temperature
limit is set by the critical temperature of the particular solvent used. Preferred
solvents have critical temperatures above 200°C; however, other solvents such as
the halocarbons have lower critical temperatures. For example chlorotrifluoromethane
has a critical temperature of 29°C. The process temperature is typically up to
100°C above the critical temperature of the solvent or higher.
The process pressure is generally from 300 psig (2.0 MPa gauge) to
5,000 psig (34.5 MPa gauge), preferably from 500 psig (3.44 MPa gauge) to 3,000
psig (21 MPa gauge). It should be noted however, that some pitch/solvent process
systems may utilize higher or lower pressures. The system pressure varies over
a wide range since it is most conveniently used for controlling product properties
and yield. Thus, the pressure applied to the system may be up to twice as high
as the critical pressure of the solvent or higher.
The amount of solvent used in the process and the temperature employed
also affect the solubility of the pitch in the solvent which in turn affects the
melting point of the mesophase product. For example, increasing the amount of solvent
increases the amount of pitch solubilized and a similar effect is obtained with
increasing temperature. Both of these variations result in a reduced yield of
mesophase product of increased melting point.
Upon completion of phase separation of the mesogens (now mesophase)
from the pitch, flux solvent dissolved in the mesophase may be removed by reducing
the system pressure while maintaining the temperature at a sufficient level to
maintain the mesophase in the liquid state. Solvent removal is usually carried
out at a temperature of between 300 and 400°C for between 0.01 and 2 hours, depending
on the type of solvent removal procedure used. For example, with thin film evaporation
only very short residence times are required.
The mesophase pitch product obtained in the process of the invention
can be spun into continuous anisotropic carbon fibers by conventional procedures,
such as melt spinning, followed by the separate steps of stabilization and carbonization.
These are known techniques and consequently they do not constitute a critical feature
of the present invention.
In addition to the conventional solvent fluxing, the process of this
invention also includes enhanced fluxing. Enhanced fluxing employs elevated temperatures
and pressures up to the critical conditions for the flux mixture. Enhanced fluxing
offers higher solubility leading to improved yields. It also offers process advantages
such as greater compatibility with the supercritical conditions employed in the
process and easier flux filtering of less viscous mixtures. The solvent ratio employed
with enhanced fluxing will vary from between 0.5 and 2.5 parts by weight of solvent
per part of weight by pitch.
After removal of the solvent the liquid mesophase recovered under
the supercritical conditions of the invention may be spun directly, or alternatively
this material may be cooled to a solid phase material for transport in storage.
If desired, the mesophase product may be solvent washed and dried as in the conventional
two solvent process.
In the preferred aspect of the invention, as afore-described, solvent
fluxing of the heat soaked isotropic pitch and filtration of the flux mixture removes
inorganic contaminants and flux insoluble components from the desired product.
This results in a high quality mesophase having a very low quinoline insolubles
content. Dense phase or supercritical separation of the mesogens from the pitch
may also be effected without the fluxing or filtration steps to provide a desirable
mesophase product. While the mesophase obtained by this simplified process is not
of as high quality as that resulting from fluxing and filtration, it is suitable
for use in many applications and is of higher quality than mesophase obtained from
isotropic pitch by other processes such as gas sparging, gravity separation. In
this aspect of the invention the heat soaked isotropic pitch containing mesogens
is combined with the solvent in a suitable manner. For example, the pitch may be
melted and combined with heated solvent and the combination then subjected to supercritical
conditions. Alternatively the pitch may be subjected to supercritical conditions
of the particular solvent used and then combined with solvent, also provided under
supercritical conditions. After they are combined the pitch and solvent may be
subjected to mixing or agitation to provide an intimate admixture of the materials
prior to effecting phase separation. Thereafter the procedure followed is the same
as that previously described for the preferred embodiment of the invention subsequent
to the filtration step. The solvents employed in this aspect of the invention may
be the same as those previously listed for the preferred embodiment. The amount
of solvent used is up to about 12 parts per part by weight of pitch and preferably
from 0.5 to 8.0 parts of solvent per part of pitch.
The process of the invention may be further exemplified by reference
to the flow scheme shown in Figure 1. Referring to the Figure 1,filtered flux liquid,
which is a mixture of isotropic pitch, solvent, and solubilized mesogens, is introduced
through line 1 to mixer 5 and is joined by solvent provided via line 28. Both of
these streams are increased in pressure and temperature to supercritical conditions
prior to their introduction to the mixer. After thorough mixing the materials are
introduced to phase separator 4, wherein phase separation takes place to provide
a mixture of isotropic pitch and solvent in the upper portion of the separator
and mesophase containing dissolved solvent in the lower portion of the separator.
The bottom phase in the separator is removed through line 6 and introduced to
stripper 8 where separation and recovery of the solvent is effected. For this purpose,
stripping gas is introduced to the stripper through line 10. Mesophase pitch product
is withdrawn from the bottom of the stripper through line 12 and stripping gas
and solvent are removed overhead through line 14 and passed to flash drum 16.
The solvent and stripping gas in the flash drum are joined by isotropic pitch and
solvent removed overhead from phase separator 4 through line 18. In the flash drum
conditions of temperature and pressure are maintained to provide separation of
solvent and stripping gas from the isotropic pitch, which is withdrawn from the
bottom of the flash drum through line 20. The solvent and stripping gas are taken
overhead through line 22 and introduced to separator 24 where the solvent and stripping
gas are separated. The gas is withdrawn overhead through line 30 and solvent is
removed from the bottom of the separator and is recycled to the fluxing operation
through line 26. A part of the solvent is also transferred through line 28 for
combination with the filtered flux entering mixer 5 as previously described.
The drawing has been described by reference to the preferred embodiment
of the invention; however, the same process procedure may be followed when fluxing
and filtration are not employed. In this case the feed to mixer 5 through line
1 is isotropic pitch containing mesogens rather than filtered flux.
The following examples illustrate the results obtained in carrying
out the invention.
An isotropic feedstock was prepared by heat soaking an 850+°F (454
+ °C) cut of decant oil from an FCC unit for six hours at 741°F (394 °C). The heat
soaked pitch was then fluxed by conventional means by combining the pitch and flux
solvent (toluene) in about equal amounts at the reflux temperature of toluene.
Flux filtration of the mixture removed particles down to submicron size. The filtered
flux liquid was then vacuum distilled to remove the toluene. A clean, solid heat
soaked pitch with a hot stage melting point of 123°C resulted from this procedure.
285 gm of this pitch were mixed with an initial 950 gm of toluene in a 2-liter
high pressure stirred autoclave. The system was heated to a processing temperature
of 340°C under autogenous pressure. Upon reaching the operating temperature, 834
gm of additional toluene were added to raise the operating pressure to 1215 psia
(8.38 MPa). The resulting mixture of about 22.8 percent pitch in toluene was then
agitated at 500 rpm for a period of one hour. Processing conditions during agitation
were 340°C and 1215 psia (8.38 MPa) pressure. After one hour, the agitator was
turned off and the mixture was permitted to equilibrate and settle for 30 minutes.
Following the settling period, samples were obtained at operating pressure from
the top and bottom of the autoclave using heated sample containers. These samples
were the basis of all subsequent analyses.
The top equilibrated phase was 81.9 weight percent toluene, with
the remainder being extracted pitch oils. The bottom phase was 24.9 weight percent
toluene, with the remainder being non-volatile mesophase pitch. Product yield in
the bottom phase as a percentage of feed weight was 27 percent on a toluene-free
basis. The non-volatile material from the bottom phase was removed from the sample
container and heated to 360°C and held for 30 minutes under vacuum to remove the
The mesophase content of the product from the bottom phase by hot
stage examination was determined from a polished section, using optical image analysis.
The product was 100 percent mesophase. The hot stage melting point of the material
was 337°C. The material was. successfully press spun into a continuous fiber at
a spinning temperature of 360°C. The fiber was stabilized and carbonized by conventional
means. Properties from samples of the fiber were as follows:
Example 2 (Comparative Example)
These properties are indicative of a fiber of acceptable quality.
Tensile Strength (Kpsi)
320 (2.2 GPa)
33 (228 GPa)
A 1000 gm sample of the heat-soaked aromatic pitch prepared in Example
1 was fluxed 1:1 in toluene at 110°C. Flux filtering netted 4.6% insolubles. The
flux filtrate was diluted with comix solvent (toluene/heptane) at a ratio of 8
ml per gram of pitch feed. This rejection mixture was cooled to 30°C and the precipitate
was isolated by filtration, washed and dried. The yield, melting temperature and
mesophase content of the precipitate and the toluene:heptane comix ratio are shown
Melting temp., °C
Mesophase content, %
The properties of the mesophase pitch obtained in this example using
the prior art solvent fractionation process are comparable to the 27 wt% yield,
337°C melting temperature and 100 percent mesophase content obtained in Example
1 using the process of the invention.
The comix toluene:heptane ratio may be used to control the melting
point of the precipitate. Increasing the amount of heptane during rejection will
precipitate a softer (lower melting) product and result in a slightly higher yield.
Two tests were carried out with the feedstock of Example 1. Heat
soaking, flux filtration and recovery of mesophase were carried out in the same
manner and under the same conditions as in Example 1, except that the operating
pressure and solvent-to-pitch ratio were varied as shown in the following table.
*Percent heat soaked pitch in mixture of solvent and pitch subjected
to supercritical conditions of temperature and pressure.
Percent Heat Soaked Pitch*
Mesophase Hot Stage Melt Temp. °C
Control (Ex. 1)
1215 (8.38 MPa)
2710 (18.68 MPa)
1420 (9.79 MPa)
Test 1 illustrates the effect of pressure on solubility and thus
the pitch melting point. Increasing the pressure increases the solubility of the
pitch in the solvent which provides a separated mesophase product having a higher
Test 2 illustrates the effect of solvent-to-pitch ratio on solubility
and the mesophase melting point. Reducing the amount of solvent decreases the solubility
of the pitch in the solvent which results in a separated mesophase product of lower
While certain embodiments and details have been shown for the purpose
of illustrating the present invention, it will be apparent to those skilled in
the art that various changes and modifications may be made herein within the scope
of the appended claims.
In the process of the invention all of the above variables interact
and are controlled to provide the desired mesophase product and ultimately the
properties of the fiber made from such product.