Molecular sieve abstract
A process for manufacturing a carbon molecular sieve having both
increased gas adsorption capacity and selectivity, which entails:
(a) adding coal tar, coal tar pitch, or a combination thereof to
powdered coconut shell charcoal as a binder; (b) pelletizing the
mixture and carbonizing the same at about 600.degree.-900.degree.;
(c) immersing the pellets in mineral acid solution, thereby sustantially
removing soluble ingredients containing alkaline metal compounds
therefrom; (d) drying the immersed pellets; (e) adding to the dried
pellets a fraction of cresote which is distilled at a temperature
of 140.degree. to 260.degree. C., in an amount sufficient to increase
both said gas adsorption capacity and selectivity; (f) heating the
pellets to about 600.degree.-900.degree. C., for about 10-60 minutes;
and (g) cooling the pellets in an inert gas.
Molecular sieve claims
What is claimed as new and is intended to be secured by Letters
Patent is:
1. A process for manufacturing a carbon molecular sieve having
both increased gas adsorption capacity and selectivity; comprising:
(a) adding coal tar, coal tar pitch or a combination thereof to
powdered coconut shell charcoal as a binder;
(b) pelletizing said mixture and carbonizing the same at about
600.degree.-900.degree. C.;
(c) immersing the pellets in mineral acid solution, thereby substantially
removing soluble ingredients comprising alkaline metal compounds
therefrom;
(d) drying the immersed pellets;
(e) adding to the dried pellets a fraction of cresote which is
distilled at a temperature of 140.degree.-260.degree. C., in an
amount sufficient to increase both said gas adsorption capacity
and selectivity;
(f) heating the pellets to about 600.degree.-900.degree. C., for
about 10-60 minutes; and
(g) cooling the pellets in an inert gas.
2. The process for manufacturing a carbon molecular sieve according
to claim 1 wherein the amount of the fraction of cresote is 2-8%
of the dried pellets.
3. The process for manufacturing a carbon molecular sieve according
to claim 1 wherein about 20-30% of coal tar or coal tar pitch or
a combination thereof is added to the powdered coconut shell charcoal.
4. The process for manufacturing a carbon molecular sieve according
to claim 1 wherein said mineral acid solution is about 0.4-0.6N
HCl.
5. The process for manufacturing a carbon molecular sieve according
to claim 2 wherein the amount of the creosote fraction is 3-8%
of the dried pellets.
6. The process for manufacturing a carbon molecular sieve according
to claim 1 which further comprises, after adding creosote to the
dried pellets stirring said creosote and said pellets at a temperature
of 200.degree.-400.degree. C. for about 20-25 minutes.
7. The process for manufacturing a carbon molecular sieve according
to claim 1 wherein the heating of said pellets after adding creosote
thereto is effected such that the temperature is increased at a
rate of about 10.degree.-15.degree. C./minute.
8. A process for manufacturing a carbon molecular sieve having
both increased gas adsorption capacity and selectivity, comprising:
(a) adding coal tar, coal tar pitch, or a combination thereof to
powdered coconut shell charcoal as a binder;
(b) pelletizing said mixture and carbonizing the same at about
600.degree.-900.degree. C.;
(c) immersing in the pellets in mineral acid solution, thereby
substantially removing soluble ingredients comprising alkaline metal
compounds therefrom;
(d) drying the immersed pellets;
(e) adding to the dried pellets 23-dimethyl naphthalene or 24-xylenol
or quinoline, in an amount sufficient to increase both said gas
adsorption capacity and selectivity;
(f) heating the pellets to about 600.degree.-900.degree. C., for
about 10-60 minutes; and
(g) cooling the pellets in an inert gas.
9. The process for manufacturing a carbon molecular sieve according
to claim 8 wherein the amount of 23-dimethyl naphthalene or 24-xylenol
or quinoline is 3-7% of the dried pellets.
Molecular sieve description
BACKGROUND OF THE INVENTION
The singular character of the surface of carbon derived from its
microporous structure, are well known, among them, character of
molecular sieve is worth noticing which the selective adsorptive
capacity changes depending on the size of the micropore of the surface.
The pressure swing adsorption process is one of the most important
uses of a carbon molecular sieve which is adapted to separation
of gaseous mixture by making use of the difference of the character
of selective adsorption.
In Japanese Patent Publication No. 38-25969 wherein, a Principle
and Process of pressure swing adsorption and one of its application
example--separation of nitrogen and oxygen gas from air with activated
carbon--are disclosed. The surface of carbonaceous material naturally
has more or less the property of a molecular sieve, derived from
its microporous structure. In 1948 P. H. Emmett first prepared
a carbon molecular sieve by dry distillation of polyvinylidenchloride
resin. (P. H. Emmett: Chem. Rev. vol. 43 P. 69).
Usually, the diameter of micropores on the surface of carbon prepared
by conventional processes is usually greater than 10-20 .ANG., so
micropores of this size are too large to separate molecules of small
size, for example separation of nitrogen and oxygen gas from air,
namely the property of molecular sieve is inadequate. Several processes
of manufacturing carbon molecular sieve which is applicable to separation
of oxygen and nitrogen gas from air, have been disclosed, wherein
micropores are partially packed with fine carbon particles and their
size become narrow, thus the property of a molecular sieve is increased.
For example, such a process is disclosed in Japanese Patent Publication
No. 49-37036 wherein small amounts of prepolymer of phenol resin
or furan resin are added to activated carbon, polymerized, carbonized
by heating at 400.degree.-1000.degree. C., thus micropore on activated
carbon is partially packed with carbonaceous material and given
the property of molecular sieving. In Japanese Patent Publication
No. 52-18675 coke of which volatile material content is less than
5%, is heated to 600.degree.-900.degree. C. in a furnace, then hydrocarbon
vapour, such as benzene, toluene and producer gas, is introduced
therein, soot generated by decomposition of hydrocarbon partially
packs the micropores of coke, thus a molecular sieve made of coke
is formed. In this process, decomposition of hydrocarbon vapour
in the furnace is considered to be an indispensable requirement.
In Japanese Patent Application Laid Open No. 49-106982 a method
to narrow the size of micropores of coke is described, wherein an
organic compound in vapour or in solution is adsorbed onto the surface
of coke, thus, the micropore is partially packed. In Japanese Patent
Application Laid Open No. 56-130226 a method to narrow the size
of the micropores of carbon of size of more than 0.5 nm, is described
wherein a concentration of hydrocarbon in vapour of less than 2%,
is adsorbed on the surface of carbon and heated, thus the soot generated
by the decomposition of hydrocarbon adhered to its surface and narrowed
the micropores. In this treatment, hydrocarbon is necessary to have
molecular size of more than 0.5 nm such as methane, ethane and styrene.
This method comprises to decompose gaseous hydrocarbon and precipitate
its soot in the micropores of activated carbon or to adsorb gaseous
hydrocarbon in these micropores then decompose in it, consequently
narrows their size with soot, thus improves the carbon molecular
sieving property. In this process, it is necessary to introduce
the gaseous hydrocarbon into the furnace and decompose, precipitate
soot on adsorbent, so with this process it is complicated and difficult
to obtain a high quality carbon molecular sieve stably. At the same
time, cost of the product becomes expensive.
In Nenryokyokaishi vol. 60 No. 654 page 859-864 (1982), a method
to improve the molecular sieving property of Yallourn char is described,
wherein, 4-5% of exhaust liquor of sulfite pulp and coal tar pitch
are added to Yallourn char then pelletized, heated to 600.degree.-700.degree.
C. at the speed of 10.degree. C. min.sup.-1 in nitrogen gas and
maintained for 1 hour. In this process, the hardness of the pellet
is inadequate because of the exhaust liquor of sulfite pulp used
as a binder, and it is difficult to maintain the molecular sieving
property of the product constantly.
BRIEF SUMMARY OF INVENTION
It is an object of this invention to provide a process for manufacturing
a carbon molecular sieve, which has improved selectivity especially
applied to separation of gases of small molecular size, for example,
separation of nitrogen gas from air. This process comprises pelletizing
powder of coconut shell charcoal containing small amounts of coal
tar as a binder, carbonizing, washing in mineral acid solution,
then adding specified amounts of creosote distilled in a specified
range of temperature or specified amounts of 23 dimethylnaphthalene
or 24 xylenol or quinoline, heating at a specified temperature
and time, after then, cooling in an inert gas.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have investigated ways to improve the molecular sieving
property of activated carbon for a long time. In this research,
the following information was obtained: an aggregation of fine carbon
crystallite could be grown when a coconut charcoal containing easily
crystallizing carbonaceous substance, such as coal tar pitch, was
heat treated at a specified temperature and time, then these crystallites
grown in the micropore diminish the micropore size. According to
these findings, we developed the process for manufacturing a carbon
molecular sieve, (U.S. Pat. No. 4458022 Japanese Pat. No. 1342860)
wherein, pelletizing coconut shell charcoal powder with coal tar
pitch and/or coal tar as a binder, carbonizing at about 750.degree.-900.degree.
C., immersing the pellets in a dilute mineral acid solution adding
about 1-3% coal tar pitch or/and coal tar, heating at 950.degree.-1000.degree.
C. for 10-60 minutes, cooling in an inert gas.
One of the most important uses of molecular sieving carbon is an
adsorbent for pressure swing adsorption process, wherein the degree
of character of selective adsorption is the most important factor,
therefore, the carbon molecular sieve having a high selective adsorptive
capacity have been desired eagerly. In general as the selective
adsorption increases, the adsorption capacity of gas decreases,
namely, there is a necessary trade-off between these properties.
Then, the inventors have investigated ways to overcome the difficulty,
namely, the influence of the material which is added to the carbon
base and the temperature of the heat treatment on the character
of selective adsorption and the adsorption capacity of gas. From
this research, the following information was obtained that the selective
adsorptive capacity of the carbon molecular sieve is increased remarkably
with the fraction of creosote in the specified range of temperature
and the heat treatment of the pellet in the specified range of temperature
and time, though the adsorption capacity of gas is slightly decreased.
From these findings, we achieved the present invention, that is,
the process for manufacturing an improved carbon molecular sieve,
which comprises, adding coal tar pitch and/or coal tar to coconut
shell charcoal powder as a binder, pelletizing, carbonizing at 600.degree.-900.degree.
C., immersing in mineral acid solution, washing, drying then penetrating
the fraction of creosote distilled in the range of temperature of
140.degree.-260.degree. C., heating to 600.degree.-900.degree. C.,
then maintaining 10-60 minutes at that temperature, after that,
cooling it in an inert gas. Furthermore instead of the fraction
of creosote, 23 dimethyl naphthalene, 24 xylenol, quinoline are
more suitable for this purpose.
In this specification creasote means the middle oil and the heavier
fractions of coal tar in which naphthalene, anthracene, tar acids
and tar bases are removed by crystalization.
In detail, in the present invention, pelletized carbon made of
coconut shell charcoal is specified as a raw material, wherein,
coal tar pitch and/or coal tar is added to powder of coconut shell
charcoal as a binder, then pelletized and carbonized. Pellets of
carbon molecular sieve carbon for separation of gases are preferred
to have the uniform shape and must have the required hardness. Various
kinds of wooden char can be used as raw materials such as lignite,
smokeless coal, wood, coke other than coconut shell charcoal, however,
these materials can not give practical hardness. On the other hand,
pelletized carbon molecular sieve made of coal had adequate hardness,
but lesser adsorption capacity and selectivity, when it is used
as adsorbent for separation of small size gases. When the carbon
molecular sieve is made of wooden raw material, the mean diameter
of micropores depends on manufacturing conditions, however, it is
possible that their diameters diminish to 12-15 .ANG. under appropriate
conditions.
When the molecular sieving carbon is made of coal, it is too difficult
to make the mean diameter of micropore smaller than 20 .ANG. under
any conditions. However, the diameter of small molecular weight
gas is usually less than 5 .ANG., so it is necessary to diminish
the size of micropore to around this size.
In the pelletizing process, 20-30% of coal tar pitch and/or coal
tar is added to the coconut shell char coal as a binder then mixing
and pelletizing by conventional method. Coconut shell charcoal used
as raw material is preferred to be crushed more than ones for other
uses.
Then, it is necessary to dry well at 600.degree.-900.degree. C.
and immersed in mineral acid solution to remove soluble ingredients.
In this step, drying becomes inadequate under 600.degree. C. and
capacity of oxygen and nitrogen gas adsorption decreases when dried
at more than 900.degree. C.
For example, when the molecular sieving carbon is prepared except
immersing and washing in 0.6N HCl, the capacity of adsorption and
the character of selective adsorption are either decreased, that
is, 6.0 to 5.2 ml/g, 26 to 18 respectively.
Most of ingredients removed by washing with acid solution are alkaline
metal compounds contained in charcoal made of coconut shell and
these same compounds prevent the growth of aggregation of fine carbon
crystallites which partially pack the micropores in heat treatment,
so this said step is one of the most important one in the present
invention. The kind and concentration of acid is not specified,
however, 0.4-0.6N HCl is preferable based on experience. The difference
of adsorption speed between oxygen and nitrogen is hardly recognized
immediately after washing with acid and drying of the pelletized
charcoal.
Then a fraction of creosote which is distilled at the temperature
of 140.degree.-260.degree. C. is mixed well with the above-mentioned
charcoal to permit adequate penetration heated at 600.degree.-900.degree.
C. for 10-60 minutes. The content of creosote is not specified,
however, when it is decreased, gas adsorption capacity is increased,
but the character of selective adsorption is decreased, and when
it is increased the reverse takes place. A content of a creosote
fraction of about 2-8% is generally used. Therefore, the content
of the fraction of creosote is preferred to be 3-8%. It is preferable
to stir sufficiently the pelletized charcoal after adding said fraction
at 200.degree.-400.degree. C. for 20-25 minutes for the well-penetration
of creosote.
Heat treatment is preferably carried out in an inert gas stream.
The speed of the rising temperature is not specified, however, 10.degree.-15.degree.
C./minute is preferable.
Before adding said fraction, the pelletized charcoal has a high
gas adsorption capacity, though it is decreased steeply after addition
it, but increased again by heat treatment because of decomposition
and evaporaption of said fraction. When the temperature is less
than 600.degree. C., it does not obtain adequate molecular sieving
character. On the other hand, the temperature is higher than 900.degree.
C., the character of selective adsorption is increased remarkably
but the capacity of gas adsorption is decreased steeply, therefore
it can not be applied to pressure swing adsorption process. As shown
in example 4 5 and 6 when molecular sieving carbons are prepared
by adding same amounts of said fractions, heated at various temperatures
in the range of 600.degree.-900.degree. C., the capacity of oxygen
adsorption is decreased as the temperature rises, against the character
of selective adsorption is increased. Though the molecular sieving
carbon prepared through the heat treatment at 800.degree. C. showed
the best result in the test apparatus of pressure swing adsorption
process, however, this best temperature is considered to vary with
the content of said fraction and other factors. The preferable heat
treatment time varys with its temperature; it is insufficient under
10 minutes while the gas adsorption capacity is decreased steeply
when it exceeds 60 minutes.
In the present specification, the method of measurement of adsorption
capacity and selectivity which is introduced for evaluation of the
adsorbent, are as follows:
Sample of the adsorbent (about 10 g) is left in reduced pressure
for desorption more than 30 minutes then oxygen or nitrogen gas
of 1 atmosphere at 25.degree. C. is introduced and the adsorption
capacity is showed as volume of gas (ml/g) adsorbed in 60 seconds.
Selectivity is measured by the following procedure: at first, volume
of adsorbed oxygen gas in 5 seconds, at 25.degree. C., 1 atmosphere,
is measured, then necessary time of adsorption of the same volume
of nitrogen gas (expressed as TN.sub.2) are measured, and the value
(S=TN.sub.2 /5) is defined as selectivity. This value has a close
relationship with capacity of adsorbent when it is used for pressure
swing adsorption process.
From Table 1 the oxygen gas adsorption capacity and selectivity
(character of selective adsorption) of the carbon molecular sieve
prepared by the process of the present application are remarkably
improved in comparsion with one prepared by our prior application,
namely, these two properties are improved at the same time. This
is worth noticing because there is a remarkable antinomy between
these two properties of the sieve prepared by conventional procedure.
Such a carbon sieve is more effective to use for pressure swing
adsorption process.
The above-mentioned effect of said fraction was developed from
the comparison between the effect of decomposition of said fraction
in liquid or solid phase and that of hydrocarbons (benzene, toluene
etc.) in vapor phase. The inventors investigated whether the character
of selective adsorption is formed by precipitation of fine carbon
particles or by carbonization of said fraction. These particles
are caused through evaporation and decomposition of penetrated hydrocarbons
in vapor phase, while the carbonization of said fraction is carried
out in liquid phase or solid phase on the surface.
A molecular sieving carbon is prepared as in Example 1 except benzene
is used in place of said fraction and nitrogen gas is not introduced
during heat treatment. The properties of thus prepared carbon molecular
sieve are as follows: the capacity of gas adsorption is 7.2 ml/g
the character of selective adsorption is 28 namely, the character
of selective adsorption is formed to some extent. It is supposed
that benzene vapor stays around, decomposes to fine carbon particles
and precipitates on the surface. On the other hand, said heat treatment
is carried out in nitrogen gas stream so as to remove hydrocarbon
vapor immediately after evaporation, the character of selective
adsorption is as follows; the capacity of oxygen adsorption is 7.9
ml/g, the selectivity is S=1.3 (Comparative Example 4), and these
properties are about as same as the carbonizing charcoal (Comparative
Example 6), namely the selectivity are hardly formed. However, when
creosote or/and coal tar are added in stead of benzene, a considerable
selectivity is formed even heat treatment is carried out in nitrogen
gas stream, as shown in Example 1 Comparative Example 2 and 3.
Therefore it is obvious that the molecular sieving property is formed
through decomposition of penetrated carbon compound in liquid phase
or solid phase. In other words, in order to form high grade molecular
sieving property on the surface, it is necessary to penetrate carbon
compound which is at least partially carbonized in liquid phase
or solid phase during heat treatment. While carbon compound which
evaporates almost all during heat treatment, can not be used as
penetrating agent.
From this standpoint, the inventors investigated the influence
of fractions of coal tar on the molecular sieving property, and
found that creosote was preferable--especially the fraction distilled
in the temperature range of 140.degree.-260.degree. C. as the penetrating
agent. As coal tar is sticky, the upper limit of its content must
not exceed 1.5-2.5% and if it is added more than 2.5%, the property
of gas adsorption is scarcely formed through heat treatment. However,
when said fraction of creosote is used in stead of coal tar, even
though more than 6-8% is added, the property of gas adsorption is
formed to a considerable extent. It is supposed that a thin carbon
layer is formed uniformly on the surface; this is why to improve
the character of selective adsorption. If coal tar is used for penetrating
agent, a thick carbon layer is formed ununiformly on the surface
which is considered to choke micropore. The surface condition and
property of carbon layer are caused by macro molecular ingredient
contained in coal tar. It is necessary to make a thin and uniform
carbon layer coverd on the surface and the layer chokes the micropore
partially, but not completely.
Furthermore, the inventors investigated components contained in
said fraction distilled in the temperature range of 140.degree.-260.degree.
C., and found that 23 dimethyl naphthalene, 24 xylenol and quinoline
were particularly effective for this purpose in the amount of about
3 to 7% of the dried pellets. At the same time, carbon compound
which is decomposed and at least partially carbonized in liquid
phase or solid phase, are also used for this purpose.
From table 2 the carbonized charcoal has a high capacity of gas
adsorption, but hardly has a character of selective adsorption.
Said fraction adheres ununiformly on the surface immediately after
adding and heating for a while, gradually penetrated into it. This
condition of the surface corresponds to that of heat treatment at
80.degree. C. in table 2 and the character of selective adsorption
is formed a little, while the capacity of gas adsorption decreases
steeply. Then the capacity of gas adsorption and the character of
selective adsorption are improved at the same time in accordance
with rising of heat treatment temperature, namely, when the temperature
is 300.degree.-400.degree. C., a considerable character of selective
adsorption is formed and the temperature rises to 600.degree. C.
said character attained to more high value. In this procedure, when
said temperature is 300.degree.-400.degree. C., said fraction partially
evaporats and its vapor stays around the surface, but it is hardly
supposed that said vapor is decomposed to fine carbon particle and
precipitated on the surface at such a low temperature. On the other
hand, considering that the capacity of gas adsorption is increased
as the temperature rises, the condition of the surface is changed
by evaporation and decomposition and supposed to form the character
of the molecular sieving. Thus, it can be infered that the character
of molecular sieving is formed by the different mechanisms from
precipitation of fine carbon particle caused by decomposition of
hydrocarbons in vapor phase at high temperature.
For industrial uses such as pressure swing adsorption process,
a more effective molecular sieving carbon is required, thus high
temperature heat treatment (600.degree.-900.degree. C.) is adopted
for this purpose. When said pelletized charcoal containing carbon
compound is heat treated at such a high temperature, a considerable
amounts of vapor stays around, thus fine carbon particles may precipitate
on the surface, however, even if heat treatment is carried out in
nitrogen gas stream at high temperature, the character of the molecular
sieve is considered to be formed to high extent. This consideration
is already mentioned concerning the result of forementioned example.
Both the capacity of gas adsorption and the character of selective
adsorption are improved at the same time by heat treatment at low
temperature (Table 2). However, for industrial use, a carbon molecular
sieve needs a high selectivity and hardness, so it is necessary
to prepared through high temperature heat treatment. At such high
temperature these two properties still show antimony only a little.
However, it is clear that values of both characters are either very
high grade as shown in the results of Example 4-6. Thus prepared
molecular sieve satisfies well for practical use, therefore, the
problem is substancially solved.
The other process for manufacturing molecular sieving carbon is
known wherein hydrocarbon vapor is introduced into a carbonaceous
base at high temperature, for example methane, ethane and benzene,
and it easily decomposes to free fine carbon particles, then precipitates
on the surface, thus the character of molecular sieving is formed.
Furthermore, infering this procedure, when a molecular sieving carbon
is prepared by heat treatment of the carbonaceous base containing
coal tar or pitch etc., the property of molecular sieving is possible
to regard to be formed by precipitation of fine carbon particles
caused by decomposition in vapor phase. However, it is clear that
said property is mainly formed derived from the change of condition
of the surface as beforementioned.
When the molecular sieving carbon is prepared practically, the
heat treatment is carried out during introduction of a small amounts
of inert gas, for example, nitrogen gas, thus sometimes evaporated
vapor is not removed completely and a small amounts of decomposed
fine carbon particles may precipitates on the surface. However,
the formation of the molecular sieving property is considered to
be a little depend on said precipitation.
It is necessary to cool the carbon molecular sieve in inert gas
after heat treatment. In this process, cooling temperature is preferred
to be less than 300.degree. C., when it is removed from the furnace
and contacted with air at a temperature higher than 300.degree.
C., adsorption capacity of oxygen remarkably decreases.
In the present invention, the carbonized pellet prepared by adding
coal tar or pitch to charcoal powder as a binder and carbonized,
scarcely has the character of selective adsorption. However, the
character of excellent selective adsorption is formed through immersing
in dilute acid solution, adding specified fraction and heat treatment.
In this procedure, the development of said specified fraction bears
a very important role. Thus, both the capacity of gas adsorption
and the character of selective adsorption are improved either to
very high grade.
Thus prepared carbon molecular sieve by the present invention is
applied preferable for the separation of nitrogen and oxygen gas
by pressure swing adsorption process, at the same time, it is available
for separation of gases of which size of molecule is less than 5
.ANG. by the same process, based on difference of the size.
In the present specification, the method of measurement of adsorption
capacity and the selectivity which is introduced for evaluation
of the adsorbent, are described before. Furthermore, the evaluation
with a small size pressure swing adsorption apparatus is carried
out for the practical use.
adsorption column; 2 set, each set 1.05 l (packed molecular sieving
carbon
adsorption pressure; 6.5 Kg/cm.sup.2 (G), 25.degree. C.
desorption pressure; 760 mmHg
adsorption time, desorption time; each 2 minutes
SV (Space velocity); 2.0 1.5 minute.sup.-1.
SV means the value that volume of the separated nitrogen gas at
standard condition is divided by volume of the molecular sieving
carbon packed in one adsorption column. However, experimental results
obtained with said apparatus fluctuates to some extent, so as a
rule, the property of molecular sieving is evaluated by the capacity
of gas adsorption and the selectivity and the result of said apparatus
is considered as reference. |