Molecular sieve abstract
Molecular sieve carbon fibers capable of separating and purifying
nitrogen from air in large quantities are provided. The carbon fibers
have diameters of 5 to 50 .mu.m and micropores opening directly
at the surface of the carbon fibers with pore sizes of 0.5 nm or
less. They are capable of separating nitrogen at a purity of 98%
or higher, even 99.9% or higher from air, etc., with a relatively
low adsorbing pressure and deadsorbing vacuum. The molecular sieve
carbon fibers do not deteriorate during operation since they are
resistant to division or powdering and the adsorbing pressure and
deadsorbing vacuum are relatively low.
Molecular sieve claims
We claim:
1. A process for producing molecular sieve carbon fibers, comprising
the steps of:
spinning a material selected from the group consisting of oil residue
and coal residue to form green fibers having diameters in the range
of 5 to 50 .mu.m;
treating said green fibers in air at a temperature in a range of
250.degree. to 450.degree. C. to introduce oxygen into said green
fibers so that an amount of oxygen introduced in said fibers which
is in a range of 8 to 28% by weight of the total weight of said
fibers after the oxygen is introduced;
cartionizing said fibers in an inactive atmosphere at a temperature
of 500.degree. to 750.degree. C. whereby a component of said material,
including oxygen atoms introduced thereto in said treating step
is released from said fibers to form a number of micropores opening
directly at the surface of said fibers, which micropores have diameters
in the range of 0.5 nm and below and substantially no pores larger
than 0.5 nm in size.
2. The process of claim 1 wherein said material is an oil residue
pitch.
3. The process of claim 1 wherein said material is coal tar.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitrogenoxygen separating molecular
sieve and separator, more specifically, a molecular sieve comprised
of carbon fibers capable of separating and purifying a high purity
of nitrogen gas from air and a separator using such a sieve.
2. Description of the Related Art
A large quantity of nitrogen gas is used in industry for creating
inert atmospheres in heating furnaces, etc. It is known to prepare
nitrogen gas by liquefying air and fractionally distilling nitrogen
gas from the liquid air using the difference of the liquefying temperatures.
Nitrogen gas prepared by liquefaction and fractional distillation
of air is, however, relatively expensive.
A nitrogen gas generating apparatus using an inorganic material
of the zeolite series is also known, but a drying device is necessary
in the apparatus at a stage before an adsorbing column due to the
high water adsorption of zeolite, making the cost of the apparatus
high.
Nitrogen gas generators using molecular sieves are also commercially
available. Nitrogen gas generation with molecular sieves enables
preparation of nitrogen gas in large quantities and a very low cost
in relation to nitrogen gas generation by liquefaction and fractional
distillation of air.
A typical molecular sieve material used in commercially available
nitrogen gas generators is "molecular sieve carbon", a
particulate carbon material. Various processes have been disclosed
for producing such molecular sieve particulate carbon. In one process,
raw materials of a phenol or furan series resin are adsorbed in
the surfaces of a porous carbon adsorbent and then polymerized and/or
condensed. Carbonization is then carried out to form a fine porous
structure in the adsorbent (Japanese Examined Patent Publication
(Kokoku) No. 49-37036). In another process, a hydrocarbon which
will produce carbon by thermal decomposition is added to coke, which
is then heat-treated to deposit carbon into the fine pores of the
coke (Japanese Examined Patent Publication (Kokoku) No. 52-18675).
In still another process, an organic material which is tacky at
room temperatures is mixed with fine particulate coal char, which
is then granulated and carbonized (Japanese Unexamined Patent Publication
(Kokai) No. 57-175715).
The pore sizes of fine pores of a nitrogenoxygen separating molecular
sieve, however, must be controlled to within the range of 0.5 nm
or less, preferably from approximately 0.35 nm to approximately
0.5 nm. All methods for producing a nitrogen-oxygen separating molecular
sieve from particulate carbon involve very complicated and highly
controlled procedures.
Due to the inherent features of raw materials of porous particulate
carbons, molecular sieve particulate carbons have the construction
of fine pores as shown in FIG. 1 which includes macropores (with
a large pore size) 1 transitional pores (with intermediate pore
sizes) 2 and micropores (with finest pore sizes) 3 continuously
from the surface into the interior of a particulate carbon. Thus,
effective micropores 3 having a pore size of from 0.35 nm to 0.5
nm are formed only in the interior of the particles. Therefore,
a relatively high adsorbing pressure and deadsorbing vacuum are
necessary for pressure swing adsorption in a nitrogen generator.
The high adsorbing pressure and deadsorbing vacuum necessitate expensive
vacuum pumps and other devices. Further, a high adsorbing pressure
and/or deadsorbing vacuum can split or powder molecular sieve particulate
carbons due to the mechanically weak pore structure of the molecular
sieve particulate carbons
Further, molecular sieve particulate carbons have a relatively
small effective geometrical surface area, increasing the size of
a nitrogen generator.
A process for producing a molecular sieve in the form of fiber,
i.e., a molecular sieve carbon fiber, has also been disclosed (Japanese
Unexamined Patent Publication (Kokai) No. 57 -101024). This process
includes melt spinning a material depolymerized from coal, produced
by a special method, followed by infusibilization and slight carbonization
and activation. This process, however, requires a special raw material
for spinning and, more importantly, cannot produce carbon fibers
having a narrow distribution of a pore size in the range of 0.5
nm or less with a large effective fine pore volume. It activates
the carbon fibers from the outside with steam, etc. Steam, etc.
at a low temperature of 650.degree. C. to 700.degree. C. is not
effective for activation. Therefore, even if effective fine pores
having a pore size of 0.35 to 0.5 nm may be formed, they are few
in number. Activation at a higher temperature to increase the volume
of pores makes those pore sizes too large, i.e., larger than 0.5
nm. Thus, control of activation is extremely difficult or impossible.
In any case, activation of fibers from the outside is not sufficient
for providing molecular sieve carbon fibers effective for separating
nitrogen and oxygen. The examples or other portions of the above
patent publication, therefore, describe or mention only separation
between benzene and cyclohexane, not between nitrogen and oxygen.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nitrogen-oxygen
separating molecular sieve and separator capable of separating and
purifying high purity nitrogen gas.
Another object of present invention is to provide a nitrogen-oxygen
separating molecular sieve and separator operable for separating
and purifying high purity nitrogen gas under a relatively low adsorbing
pressure and deadsorbing vacuum.
A further object of the present invention is to provide a nitrogen-oxygen
separating molecular sieve and separator not susceptable to division
or powdering of the molecular sieve.
The present invention resides in a nitrogen-oxygen separating molecular
sieve, and, a separator using the same, including carbon fibers
of a diameter in the range from 5 to 50 .mu.m and many micropores
opening directly at the surface of the carbon fibers and having
pore sizes of 0.5 nm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial sectional view of a molecular sieve particulate
carbon, illustrating the pore structure thereof;
FIGS. 2a and 2b are side and partial sectional views of a molecular
sieve carbon fiber, the latter illustrating the pore structure of
the molecular sieve carbon fiber according to the present invention;
FIG. 3 shows adsorption characteristics of molecular sieve carbon
fibers; and
FIGS. 4 and 5 show adsorption pressure and deadsorption vacuum
characteristics of molecular sieves according to the present invention
and prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the exact purity of nitrogen gas used in large quantities
in industry, such as for heat treatment, depends on the individual
application, separating and purifying high purity nitrogen gas from
a gas mixture containing nitrogen and oxygen such as air requires
that the molecular sieve carbon material have fine pores of pore
sizes in the range of from approximately 0.35 nm to approximately
0.5 nm, in particular, near 0.4 nm, since the molecules of nitrogen
and oxygen are very close in size. (Generally, nitrogen molecules
are at the longest 0.41 nm and at the shortest 0.30 nm and oxygen
molecules are at the longest 0.38 nm and at the shortest 0.28 nm.)
Prior art processes for producing molecular sieve particulate carbons
cannot easily attain such a narrow distribution of fine pore sizes
since, as described before, molecular sieve particulate carbons
have a wide distribution of pore sizes from macropores to micropores
and effective micropores having pore sizes of 0.35 to 0.5 nm are
formed only in the interior of the particulate carbon.
The inventors found that carbon fibers with micropores having a
narrow distribution of pore sizes can be made by introducing some
kinds of material into green fibers after spinning, followed by
carbonizing the fibers at a relatively low temperature. FIG. 2 (a)
shows a carbon fiber 4 manufactured by the process mentioned above,
and FIG. 2(b) is an enlarged sectional view of a part of the carbon
fiber 4 encircled in FIG. 2(a). As shown in FIG. 2(b), a carbon
fiber manufactured by the process has micropores 5 opening directly
at the surface of the carbon fiber, that is, micropores are formed
at the surface of the carbon fiber without macropores or transitional
pores. The present invention was accomplished by applying such carbon
fibers as molecular sieves for separating nitrogen and oxygen.
Nitrogen-oxygen separating molecular sieve carbon fibers according
to the present invention have many advantages.
These carbon fibers have a narrow distribution of pore sizes in
the range of 0.5 nm or less, particularly from 0.35 to 0.5 nm, even
near 0.4 nm, due to the characteristics of the carbon fibers, the
crystallinity, and the orientation of the molecules. Carbon fibers
have a large effective geometrical surface area for forming desired
micropores. Micropores having desired pore sizes are formed directly
on the surfaces of the carbon fibers without the formation of macropores
or transitional pores. These features enable separation and purification
of nitrogen from air, etc. at a purity of 98 mole percent or higher,
further 99 mole percent or higher, even 99.9 mole percent or higher.
These high purities of nitrogen gas cannot be obtained by any carbon
fibers activated according to any prior art processes.
As stated before, the carbon fibers have a large effective geometrical
surface area for formation of desired micropores, and such micropores
are formed directly at the surface of the carbon fibers without
forming macropores or transitional pores. As a result, a relatively
low adsorbing pressure and deadsorbing vacuum, compared with molecular
sieve particulate carbons, can be used. This is advantageous since
relatively simple and less expensive vacuum pumps and other devices
can be used.
The absence of macropores and transitional pores and the relatively
low adsorbing pressure and deadsorbing vacuum, plus the high mechanical
strength and flexibility of carbon fibers, further prevent division
or powdering of molecular sieve carbons which occurs in the case
of molecular sieve particulate carbons.
The molecular sieve carbon fibers also can be manufactured by a
simple process compared with processes for manufacturing molecular
sieve particulate carbons.
The molecular sieve carbon fibers allow production of high purity
nitrogen gas in large quantities at a considerably lower cost compared
with the method of liquefaction and fractional distillation of air.
A kilogram of molecular sieve carbon fibers according to the present
invention, for example, can produce 1 liter of high purity nitrogen
gas (e.g., 99.9 mole %) per minute from air.
Nitrogen-oxygen molecular sieve carbon fibers according to the
present invention may be produced as below:
First, green fibers are spun from an organic material such as oil
or coal residues, for example, pitch or coal tar from crude oil
distillation or dry distillation of coal. Any conventional spinning
process, typically, melt spinning, may be used. The diameter of
the spun green fibers is preferably in the range from 5 to 50 .mu.m.
If the diameter of the fibers is too large, the fibers lose their
flexibility and become brittle. If too small, they become inconvenient
in handling. The length of green fibers is not particularly limited.
A material which can be driven out of the fibers during carbonization,
for example, oxygen or ozone, is introduced into the green fibers.
This introduction may be easily done by heating the green fibers
in an atmosphere containing the material to be introduced. Heating
in air is the most convenient. For example, heating in air at a
temperature in the range of 250.degree. C. to 450.degree. C. for
a period of time in the range of several minutes to several hours
or heating in an ozone atmosphere at a temperature in the range
of 50.degree. C. to 160.degree. C. for a period of time in the range
of several minutes to several hours may be used. The fibers preferably
contain 8% to 28% oxygen by weight of the total weight of the fibers
after this treatment.
This treatment introduces a higher amount of oxygen than the infusibilization
in the usual production of carbon fibers. Infusibilization was only
necessary for preventing fusion of green fibers during carbonization.
Also, the final carbon fibers were not supposed to have had pores.
Therefore, a relatively slight infusibilizing treatment was effected.
In contrast, in production of molecular sieve carbon fibers, an
introduction treatment stronger than infusibilization is used in
order to introduce oxygen, etc. into the fibers for later expulsion
to form the large number of micropores. As a result, a higher temperature
is preferable in the introduction treatment than the temperature
used in the infusibilization.
Finally, the green fibers are carbonized. This carbonization forms
micropores at the surface of the fibers by driving out the previously
introduced material and imparts a certain strength to the fibers.
The oxygen introduced in the fibers is emitted in the form of CO,
CO.sub.2 etc. This treatment, carbonization, is thus designed more
to form the desired micropores than to carbonize the fibers. A lower
temperature gives finer pores and a higher temperature coarser pores.
A temperature in the range of 500.degree. C. to 750.degree. C. is
preferable. The rate of temperature rise should be slow, for example,
10.degree. C. per minute or less.
This carbonization is carried out in an atmosphere inert to carbon
fiber material at the treatment temperature. Generally, inert gas
such as argon or nitrogen is used in carbonization in usual carbon
fiber production. However, combustion gas, i.e., a gaseous mixture
of steam, carbon dioxide, and nitrogen, may be used if the treatment
temperature is below approximately 700.degree. C. This is advantageous
since combustion gas is cheaper than an inert gas.
Thus, it is possible to form many (sufficient effective volume)
micropores having a pore size in the range of 0.5 nm or less at
the surface of carbon fibers by controlling the amount of oxygen,
etc., introduced in the green fibers and the temperature of carbonization.
The micropores are formed directly at the surface of the fibers
due to the crystallinity and the orientation of molecules of the
fibers, which are characteristics of the fibers.
EXAMPLE 1
Green fibers were melt spun in a conventional process from pitch
produced by residues from the heat-cracking of naphtha. Green fibers
having diameters of 10 .mu.m to 12 .mu.m were heated in air with
a rate of temperature rise of 2.degree. C./min and kept at 300`0
C. for 2 hours. The resultant fibers contained 12.1% oxygen by weight
of the total weight of the fibers. The fibers were heated in an
inert atmosphere (N.sub.2 ) with a rate of temperature rise of 2.degree.
C./min to 690.degree. C., kept at 690.degree. C. for 10 minutes,
and then cooled. Thus, molecular sieve carbon fibers were obtained.
EXAMPLE 2
The same green fibers as in Example 1 were heated in air with a
rate of temperature rise of 2.degree. C./min to 270.degree. C. and
kept for 2 hours at 270.degree. C. The temperature was then further
raised at a rate of 3.degree. C./min to 420.degree. C. and kept
for 5 minutes at 420.degree. C. At this time, the content of oxygen
was 20.7 wt. %. The fibers were heated in an inert atmosphere at
a rate of 4.degree. C./min and kept at 600.degree. C. for 1 hour.
EXAMPLE 3
Green fibers were centrifugally spun from pitch obtained by heat-treating
coal tar. The fibers were heated in air at a rate of 2.degree. C./min
and kept at 300.degree. C. for 1.5 hours. At that time, the content
of oxygen was 13.2 wt. %. The fibers were heated in an inert atmosphere
at a rate of 4.degree. C./min and kept at 680.degree. C. for 1 hour.
EXAMPLE 4
The same green fibers as in Example 1 were treated for 30 minutes
at 70.degree. C. in an atmosphere containing 1000 ppm of ozone and
then heated in air at a rate of 10.degree. C./min to 400.degree.
C. At that time, the content of oxygen was 19.8 wt%. The fibers
were heated in an inert atmosphere at a rate of 4.degree. C./min
and kept at 610.degree. C. for 1 hour.
To examine the sizes of fine pores of the molecular sieve carbon
fibers of Example 1 adsorption isotherms were determined with carbon
dioxide, n-butane, and isobutane at 25.degree. C. The least size
of the molecule of carbon dioxide is 0.33 nm, that of n-butane 0.43
nm, and that of isobutane 0.50 nm. The resultant adsorption isotherms
are shown in FIG. 3. As seen in FIG. 3 carbon dioxide (0.33 nm)
and n-butane (0.43 nm) were adsorbed in a large quantity, while
isobutane (0.50 nm) was not substantially adsorbed. The molecular
sieve carbon fibers of Examples 2 to 4 gave similar results as in
FIG. 3.
Thus, it was confirmed that the molecular sieve carbon fibers of
Examples 1 to 4 had many micropores of 0.5 nm or less in size and
had substantially no pores larger than 0.5 nm in size.
The nitrogen-oxygen separating capability of the molecular sieve
carbon fibers of Example 1 was tested using the pressure swing adsorption
method. The apparatus used included two parallel adsorbing columns
having an inner diameter of 40 mm.phi. and a length of 110 mm filled
with molecular sieve carbon fibers, means for supplying air to the
adsorbing columns, a vacuum pump for evacuating the adsorbing columns,
and solenoid valves at the inlet and outlet of the adsorbing columns.
Air was supplied to one of the two columns for adsorbing oxygen
from the air and producing purified nitrogen gas, while the other
column was evacuated for deadsorbing the oxygen which had been adsorbed
by the carbon fibers in a previous adsorbing stage. The valves were
automatically switched by a timer so that adsorbing and deadsorbing
operations were alternately performed in the two columns and every
fixed time period. The outlet of the columns was provided with a
regulating valve so as to control the flow rate of the gas at 100
ml/min and with an analysator for continuously determining the concentration
of oxygen.
In the test, the adsorbing pressure was varied with a fixed deadsorbing
vacuum (100 mmHg), which was used prior to each adsorption operation.
The deadsorbing vacuum was varied with a fixed adsorbing pressure
(2 kg/cm.sup.2), which was used after each deadsorption operation.
The results are shown in FIGS. 4 and 5. In FIGS. 4 and 5 results
from similar tests carried out using molecular sieve particulate
carbons (KURARY CHEMICAL CO. LTD.) are included for comparison.
From FIGS. 4 and 5 it is seen that the molecular sieve carbon
fibers according to the present invention can separate and purify
nitrogen to a desired purity with a lower adsorbing pressure and
deadsorbing vacuum than in the case of molecular sieve particulate
carbons. The molecular sieve carbon fibers filled simply in the
columns enabled excellent separating and purification of nitrogen
over a long time period without deterioration of performance or
division or powdering of the carbon fibers.
Since molecular sieve particulate carbons are divided or powdered
during molecular sieve operation, they must be filled in adsorbing
columns in a special way with special measurements. These special
measurements are not necessary in the case of molecular sieve carbon
fibers according to the present invention.
Prior art carbon fibers manufactured by activating carbon fibers
after carbonization thereof cannot provide molecular sieves for
separating and purifying nitrogen from air, etc. particularly at
the high purity attained by molecular sieve carbon fibers according
to the present invention.
The nitrogen-oxygen separating molecular sieve carbon fibers and
nitrogen-oxygen separator according to the present invention may
be also used for separating and purifying oxygen gas. |