U.S. patent application number 15/433409 was filed with the patent office on 2017-08-10 for synthesis of ordered microporous activated carbons by chemical vapor deposition.
The applicant listed for this patent is Korea Advanced Institute of Science and Technology, Saudi Arabian Oil Company. Invention is credited to Minkee CHOI, Seokin CHOI, Cemal ERCAN, Rashid M. OTHMAN, Yuguo WANG.
Application Number | 20170225146 15/433409 |
Document ID | / |
Family ID | 54601991 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170225146 |
Kind Code |
A1 |
WANG; Yuguo ; et
al. |
August 10, 2017 |
SYNTHESIS OF ORDERED MICROPOROUS ACTIVATED CARBONS BY CHEMICAL
VAPOR DEPOSITION
Abstract
Embodiments provide a methane microporous carbon adsorbent
including a thermally-treated CVD carbon having a shape in the form
of a negative replica of a crystalline zeolite has a BET specific
surface area, a micropore volume, a micropore to mesopore volume
ratio, a stored methane value and a methane delivered value and a
sequential carbon synthesis method for forming the methane
microporous carbon adsorbent. Introducing an organic precursor gas
for a chemical vapor deposition (CVD) period to a crystalline
zeolite that is maintained at a CVD temperature forms the
carbon-zeolite composite. Introducing a non-reactive gas for a
thermal treatment period to the carbon-zeolite composite maintained
at a thermal treatment temperature forms the thermally-treated
carbon-zeolite composite. Introducing an aqueous strong mineral
acid mixture to the thermally-treated carbon-zeolite composite
forms the methane microporous carbon adsorbent.
Inventors: |
WANG; Yuguo; (Dhahran,
SA) ; ERCAN; Cemal; (Dhahran, SA) ; OTHMAN;
Rashid M.; (Khobar, SA) ; CHOI; Minkee;
(Daejeon, KR) ; CHOI; Seokin; (Daejeon,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Saudi Arabian Oil Company
Korea Advanced Institute of Science and Technology |
Dhahran
Daejeon |
|
SA
KR |
|
|
Family ID: |
54601991 |
Appl. No.: |
15/433409 |
Filed: |
February 15, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14513707 |
Oct 14, 2014 |
9604194 |
|
|
15433409 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02P 20/133 20151101;
B01J 20/3214 20130101; B01J 2219/00105 20130101; B01J 20/28066
20130101; B01J 20/28073 20130101; C01P 2006/14 20130101; B01J
20/3078 20130101; C01B 32/05 20170801; B01D 2259/4525 20130101;
C01P 2006/12 20130101; B01D 2257/7025 20130101; B01J 2219/00094
20130101; Y02C 20/20 20130101; B01J 20/3204 20130101; C10L 3/06
20130101; B01J 2219/0011 20130101; B01J 2219/00157 20130101; B01J
19/2415 20130101; B01J 2219/00135 20130101; B01J 20/20 20130101;
B01J 3/008 20130101; B01J 2219/00159 20130101; B01J 20/3057
20130101; B01D 2253/102 20130101; B01J 20/28045 20130101; B01J
20/324 20130101; C01B 32/306 20170801; B01J 20/28076 20130101; Y02P
20/134 20151101 |
International
Class: |
B01J 20/20 20060101
B01J020/20; C10L 3/06 20060101 C10L003/06; B01J 20/32 20060101
B01J020/32; B01J 20/28 20060101 B01J020/28; B01J 20/30 20060101
B01J020/30 |
Claims
1. A methane microporous carbon adsorbent, comprising: a
thermally-treated carbon template of a crystalline zeolite having a
shape in the form of a negative replica of the crystalline zeolite,
a BET specific surface area, a micropore volume, a micropore to
mesopore volume ratio, a stored methane value and a methane
delivered value.
2. The methane microporous carbon adsorbent of claim 1, wherein the
shape is orthogonal with a mid-edge length in a range of 8 .mu.m to
20 .mu.m.
3. The methane microporous carbon adsorbent of claim 1, wherein the
shape is in the form of the negative replica of the crystalline
zeolite that is selected from the group consisting of FAU, EMT, BEA
zeolite structures, and combinations of zeolite structures
thereof.
4. The methane microporous carbon adsorbent of claim 1, wherein the
BET specific surface area is in a range of from 2500 m.sup.2/g to
3100 m.sup.2/g.
5. The methane microporous carbon adsorbent of claim 1, wherein the
micropore volume is in a range of from 0.95 cm.sup.3/g to 1.19
cm.sup.3/g as determined by the Dubinin-Radushkevich equation.
6. The methane microporous carbon adsorbent of claim 1, wherein the
micropore to mesopore volume ratio is in a range of from 4 to
6.
7. The methane microporous carbon adsorbent of claim 1, wherein the
stored methane value is in a range of from 172 mg/g to 192
mg/g.
8. The methane microporous carbon adsorbent of claim 1, wherein the
methane delivered value is in a range of from 152 mg/g to 171 mg/g
in a pressure range of from 1 bar to 40 bar.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of U.S. patent
application Ser. No. 14/513,707, filed on Oct. 14, 2014, entitled
"Synthesis of Ordered Microporous Activated Carbons by Chemical
Vapor Deposition," which is hereby incorporated by reference in its
entirety into this application.
BACKGROUND
[0002] Field
[0003] Embodiments relate to microporous carbons. More
specifically, embodiments relate to the formation of microporous
carbons and use in natural gas storage and transportation
systems.
[0004] Description of the Related Art
[0005] Natural gas is the portable and preferred fuel of choice
around the world. Natural gas burns more completely than other
traditional fuels, including petroleum and coal; therefore, the
combustion of natural gas is comparatively less harmful to the
environment. Natural gas and similar products, including LNG,
propane and other compressed-gas fuels, are much more efficient in
engine and turbine combustion systems. Pipelines are the
traditional and most cost-effective means of transporting natural
gas from the producer to the consumer.
[0006] When producing electricity or natural gas for non-commercial
users, a significant problem arises for natural gas transportation
networks: diurnal demand. People, unlike manufacturing plants or
facilities, tend not to be steady energy users throughout the day.
People consume greater amounts of electricity during the day and
into the early evening and much less at night and into the early
morning. The higher rates of consumption form a "peak period of
demand" and the lower rate of consumption creates a "non-peak
period of demand". This daily trend occurs throughout the year.
During different seasons, however, the length of each period
(longer or shorter periods of natural light requiring reduced or
greater amounts of artificial light, respectively) and the
amplitude of the period (for example, greater amounts demanded at
higher and lower temperatures versus more moderate temperatures)
can change the amount of demand during the diurnal period. The
location of the demand also has an impact upon the diurnal demand.
In cooler environments, overall daily electrical and natural gas
demand is lower in the summer months and higher in winter months as
consumers use heating equipment. In warmer environments, the daily
demand trends are opposite as consumer use air conditioning units
when it is hot.
[0007] Swinging electrical and natural gas consumption--not only in
daily use but also in seasonal differences--results in variability
across the natural gas transportation and production system.
Natural gas production, however, is nearly constant. The
supply-demand gap between natural gas production and total
consumption results in a "gas demand lag". The lag, without
intervention, manifests itself as system pressure increases and
decreases ("swings") across the natural gas transportation
grid.
[0008] Electrical generation facilities prefer constant,
high-pressure natural gas as a feedstock. Pressure swings in
natural gas feed can damage the electrical generation equipment,
especially rotational equipment, including gas turbines, due to
sudden inappropriate feed-to-fuel ratios that cause equipment
slowdowns while under load.
[0009] A solution to mitigating the pressure swings in gas
transportation networks is provided for in U.S. Pat. App. Pub. No.
2013/0283854 (published Oct. 31, 2013) (Wang, et al.), titled
"Adsorbed Natural Gas Storage Facility", which uses a microporous
adsorbent to adsorb and desorb natural gas.
[0010] Microporous adsorbents for adsorbed natural gas (ANG)
storage include activated carbons, metal-organic frameworks (MOFs),
zeolites and other organic or inorganic porous solids. MOFs have
been reported to have surface areas up to 4000 meters squared per
gram (m.sup.2/g) and absolute methane adsorption capacities as high
as 230 volume to volume (v/v) absolute methane adsorption at 290 K
and 35 bar (sometimes referred to as the "storage amount" ratio or
the "amount stored" ratio). There is some question, however, as to
whether this high number of absolute methane adsorption is
accurate. Several operational issues limit the practical use of
MOFs in natural gas adsorption-desorption systems. Methane, once
adsorbed into the framework, is strongly bound, so for desorption
temperatures as high as 100.degree. C. may be required to free the
adsorbed methane. MOFs are known to have a reduced hydrothermal
stability, so heating them to release methane repeatedly will
eventually degrade the framework. MOFs also are intolerant to
natural gas impurities such as hydrogen sulfide,
black/carbon-silicone powder and mercaptans, which are common in
natural gas.
[0011] Metal oxide adsorbents such as zeolites tend to adsorb less
methane than activated carbon materials at similar conditions. MOs
possess a smaller surface area--reportedly less than about 800
m.sup.2/g. Zeolites also have hydrophilic surfaces relative to
activate carbon material that makes them adsorb water over other
constituents in a natural gas stream.
[0012] Activated carbon materials have surface areas in a range up
to about 3000 m.sup.2/g and are relatively thermally and chemically
stable materials. Activated carbons are known in the industry to
have an absolute methane adsorption capacity in a range of from
about 130 to about 180 v/v methane adsorption at 290 K and 35
bar.
[0013] There are several limitations to using activated carbon
materials in an ANG application. Activated carbon materials have
generally a lower packing density than other materials due to the
presence of meso- (2<d<50 nanometers (nm)) and macro-sized
pores (>50 nm). Larger micropores are generated upon formation
of the activated carbon material due to excessive carbon burn-off
during the carbon activation process. The irregular morphology of
carbon particles with high surface areas tends to cause the dense
packing of particles, leaving voids between particles. The optimum
pore diameter for ANG is from about 1.1 to about 1.2 nm. The meso-
and macro-sized pores do not contribute to natural gas adsorption
but do count as part of the material volume, resulting in lower
packing density. Useful activated carbon materials have a bulk
density is in a range of from about 0.20 to about 0.75 grams per
cubic centimeter (g/cm.sup.3).
[0014] Another issue is slow mass transport through microporous
materials. Activated carbon materials having microporous can
exhibit slow kinetic adsorption-desorption behavior due to slow
mass transport. Slow mass transport can be attributed to large
micropore volumes with smaller-than-useful pore diameters for
adsorbing methane and a lack of connectivity between surface pore
aperture openings (also known as "dead end pores"). Pressure and
temperature changes can help accelerate the mass transfer to and
from the microporous material.
[0015] Another limitation is the number of potential materials
useful to design the activated carbon materials. Activated carbon
materials are produced by chemical combustion of non-porous carbon
precursors in a controlled manner. Although this method provides an
economic way of producing material in the macro sense of a
controlled reaction, rational and systematic design of specific and
regular carbon pore structures is not possible due to the highly
variable combustion process on the micro level. Structure
parameters including surface area, pore diameter and micropore
volume are strongly related to one another and are difficult to
control separately. As an example, a high degree of burn-off
achieves a large carbon surface area, which is positive for
increasing gas storage capacity. The high degree of burn-off,
however, also results in the unavoidable enlargement of pore
diameters, which decreases the adsorption strength and packing
density of the adsorbents per unit volume.
[0016] It is desirable to develop a method for forming an activated
carbon material, the activated carbon material, and a method of its
use that maintains or improves upon the packing density, the mass
transport and the adsorptive strength of activated carbon materials
while maintaining or improving upon the surface area and absolute
methane adsorption capacities of activated carbon materials. Ease
of use and handling of the activated carbon material and simplicity
of manufacturing are also desirable characteristics.
SUMMARY
[0017] A methane microporous carbon adsorbent comprising a
thermally-treated carbon template of a crystalline zeolite having a
shape in the form of a negative replica of the crystalline zeolite
and has a BET specific surface area, a micropore volume, a
micropore to mesopore volume ratio, a stored methane value and a
methane delivered value.
[0018] A sequential carbon synthesis method for forming a methane
microporous carbon adsorbent includes introducing an organic
precursor gas made of an organic precursor for a chemical vapor
deposition (CVD) period to a crystalline zeolite that is maintained
at a CVD temperature such that the carbon-zeolite composite forms.
The introduced organic precursor adsorbs via CVD into the
crystalline zeolite. The organic precursor converts into carbon
within the crystalline zeolite. The carbon within the crystalline
zeolite forms a carbon template of the zeolite. The method includes
introducing a non-reactive gas for a thermal treatment period to
the carbon-zeolite composite maintained at a thermal treatment
temperature such that a thermally-treated carbon-zeolite composite
forms. The carbon template of the zeolite within the crystalline
zeolite converts into a thermally-treated carbon template of the
zeolite. The method includes introducing an aqueous strong mineral
acid mixture to the thermally-treated carbon-zeolite composite such
that the methane microporous carbon adsorbent forms. The methane
microporous carbon adsorbent is a negative replica of the
crystalline zeolite, has a BET specific surface area, a micropore
volume, a micropore to mesopore volume ratio, a stored methane
value and a methane delivered value.
[0019] An embodiment of the method includes introducing the organic
precursor gas for a second CVD period to the thermally-treated
carbon-zeolite composite. The thermally-treated carbon-zeolite
composite is maintained at a second CVD temperature. A second
carbon-zeolite composite forms. The organic precursor adsorbs via
CVD into the thermally-treated carbon-zeolite composite. The
organic precursor converts into carbon within the thermally-treated
carbon-zeolite composite and forms with the thermally-treated
carbon template of the zeolite a second carbon template of the
zeolite. The embodiment of the method includes introducing the
non-reactive gas for a second thermal treatment period to the
second carbon-zeolite composite. The second carbon-zeolite
composite is maintained at a second thermal treatment temperature.
The second thermally-treated carbon-zeolite composite forms. The
second carbon template of the zeolite within the second
carbon-zeolite composite converts into a second thermally-treated
carbon template of the zeolite. In this embodiment of the method,
the aqueous strong mineral acid mixture is introduced to the second
thermally-treated carbon-zeolite composite instead of the
thermally-treated carbon-zeolite composite.
[0020] The sequential carbon synthesis of the methane microporous
carbon adsorbent uses both a chemical vapor deposition (CVD) and a
post-thermal treatment procedure for introducing, carbonizing and
thermally treating small organic compounds acting as carbon
precursors in the pores of both small and large zeolite crystals.
The methane microporous carbon adsorbent has a microporous carbon
structure that is the negative replica of the zeolite structure in
which it forms. The methane microporous carbon adsorbent has a
well-defined micropore structure and a surface area similar to the
sacrificial zeolite.
[0021] "Graphitizing" does not mean that the carbon-zeolite
composite within the zeolite framework converts entirely into the
graphite form of carbon. Complete dehydrogenation of the
hydrocarbons and formation of the interlaced mono-carbon layer,
carbon-carbon bonded 3-dimensional structures occurs at
temperatures in excess of the temperatures used in this process.
Exposing the sacrificial zeolite framework to graphitization
temperatures would cause degradation of the zeolite. Temperatures
greater than 1373 K are known to cause certain zeolite structures
to physically collapse in a short period of exposure. Rather, the
deposited carbon forming the carbon template of the zeolite is more
strongly interconnected and rearranged into a matrix of stable
carbon-carbon bonds during thermal treatment as well as partially
dehydrogenated in the inert atmosphere. Therefore, if the term
"graphitization" or its related conjugates are used, it is in the
sense that the deposition of carbon and the thermal treatment of
the deposited carbon induce an elevated level of dehydrogenation
and the formation of an interlacing carbon-carbon bonding network
that is 3-dimensional but not to the extent that a pure graphene
network forms. This process of dehydrogenation and interlacing
occurs during both the deposition and the thermal treatment
periods.
[0022] An embodiment of the sequential carbon synthesis method
includes CVD of a zeolite while introducing an organic precursor at
a CVD temperature in a range of from about 800 K to about 900 K for
a CVD period in a range of from about 2 hours to about 9 hours. An
embodiment of the method includes where the CVD is followed by a
post-CVD thermal treatment at a thermal treatment temperature in a
range of from about 1100 K to about 1200 K for a thermal treatment
period of about 2 to 4 hours. In some embodiments of the method,
the CVD/post-CVD thermal treatment cycles are repeated. In some
embodiments of such methods, the CVD and periods between the first
and the later cycles are different. The resultant thermally-treated
carbon-zeolite composite is etched with an aqueous strong mineral
acid mixture to remove the sacrificial zeolite template. In an
embodiment of the method, the strong mineral acid is selected from
the group consisting of hydrochloric acid (HCl), hydrofluoric acid
(HF), nitric acid (HNO.sub.3), sulfuric acid (H.sub.2SO.sub.4) and
combinations thereof. In an embodiment of the method, the strong
mineral acid is a combination of HF and an acid selected from the
group consisting of HCl, HNO.sub.3, and H.sub.2SO.sub.4. In an
embodiment of the method, the resultant thermally-treated
carbon-zeolite composite is etched with a strong caustic to
sacrificially remove the zeolite template. In an embodiment of such
a method, the strong caustic is aqueous sodium hydroxide
(NaOH).sub.aq. The product methane microporous carbon adsorbent is
the negative carbon replica of the crystalline zeolite, which is an
inverse carbon matrix of the zeolite network. In embodiments that
perform at an additional CVD/post-CVD thermal treatment cycle, the
methane microporous carbon adsorbent has a more greatly ordered
carbon structure that has an increased BET specific surface area,
greater micropore volume and a reduced volume of mesopores than
methane microporous carbon adsorbents that only go through one
CVD/post-CVD thermal treatment cycle. This represents a greater
amount of order in the thermally-treated carbon template of the
zeolite. The densification of the deposited carbon during the
post-CVD thermal treatment forms strong carbon-carbon molecularly
bonded structures. The methane microporous carbon adsorbent that is
the negative carbon replica of the crystalline zeolite has a
methane adsorption and a delivery capacity, and is suitable for use
in ANG storage operations.
[0023] In principle, the larger the BET specific surface area or
the greater the micropore volume, the greater the methane
adsorption capacity is for a given adsorbent. However, the methane
adsorption capacity is also affected by pore size distribution,
micro-pore volume and packing density of the materials. As long as
there is no diffusion limitation, a high fraction of micropore
volume is better for methane storage.
[0024] The methane microporous carbon adsorbent is a negative
carbon replica. The synthesis platform is a crystalline zeolite
with micropore structures. The zeolite acts as a sacrificial
template for forming the methane microporous carbon adsorbent. FIG.
1 shows a simplified scheme showing the relationship between the
crystalline zeolite and the negative carbon replica that becomes
the methane microporous carbon adsorbent. A microporous crystalline
zeolite (a) is introduced for use as a sacrificial template. Small,
high carbon:hydrogen ratio organic molecules, including acetylene
(1:1), propylene and ethylene (1:2), and ethanol (1:3) are
introduced into the crystalline zeolite. The organic molecules are
carbonized while inside the zeolite micropores, forming a
carbon-zeolite composite (b). After carbon deposition, the zeolite
framework is removed by acid dissolution. The acid dissolution does
not affect the carbon template of the zeolite. The resultant (c)
can be a large, ordered methane microporous carbon adsorbent that
is a negative replica of the microporous zeolite.
[0025] An ANG storage facility for reducing the effect of diurnal
demand on a natural gas source includes an adsorption bed system.
The adsorption bed system has a methane storage capacity, contains
a methane microporous carbon adsorbent and is operable to both
adsorb onto and desorb methane from the methane microporous carbon
adsorbent. The ANG storage facility couples to a natural gas source
such that natural gas is introduced into the ANG storage facility
and desorbed methane is introduced into the natural gas source.
Optionally, the natural gas storage facility includes a temperature
control system and a compressor system.
[0026] A method of using the ANG storage facility includes
introducing natural gas into the ANG storage facility from a
natural gas source during a non-peak period of demand such that the
pressure within the natural gas source declines. The method
includes the step of operating the ANG storage facility during the
non-peak period of demand such that methane microporous carbon
adsorbent selectively separates methane from the introduced natural
gas and adsorbs the methane. The method includes maintaining the
ANG storage facility such that the adsorbed methane remains
adsorbed on the methane microporous carbon adsorbent until a peak
period of demand. The method also includes the steps of operating
the ANG storage facility during the period of peak demand such that
the methane microporous carbon adsorbent desorbs the adsorbed
methane. The method includes the step of introducing the desorbed
methane into the natural gas source during the period of peak
demand such that the pressure within the natural gas source
increases.
[0027] The adsorption natural gas storage facility is operable to
receive natural gas, to selectively separate methane from the
introduced natural gas and to store the methane via adsorption on
the methane microporous carbon adsorbent for a period. The ANG
storage facility is also operable to desorb and release the
adsorbed methane.
[0028] Introducing the methane to the natural gas source when
natural gas is in greater demand and removing methane from the
natural gas source when natural gas is not in demand reduces the
amplitude of the pressure swings in the natural gas source,
including a natural gas transportation system, caused by the
difference between diurnal demand and steady natural gas
production.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] These and other features, aspects, and advantages of the
embodiments are better understood with regard to the following
Detailed Description of the Preferred Embodiments, appended Claims,
and accompanying Figures, where:
[0030] FIG. 1 shows a simplified scheme showing the relationship
between (a) the crystalline zeolite and (b) the negative carbon
replica that becomes (c) the methane microporous carbon
adsorbent.
[0031] FIG. 2 is a cross-sectional diagram of a rotary tubular
furnace in a rotary tubular furnace system that is useful for
batch-performing several steps of the sequential carbon synthesis
method.
[0032] FIG. 3 is a process flow diagram of an embodiment of a
system for continuously performing an embodiment of the sequential
carbon synthesis method.
[0033] FIG. 4 is a process flow diagram of an embodiment of the
adsorbed natural gas (ANG) storage facility using the methane
microporous carbon adsorbent.
[0034] FIG. 5 is a graph showing traces of the X-ray Diffraction
(XRD) analysis for each synthesized large crystal NaX zeolite.
[0035] FIG. 6 shows scanning electron micrographs (SEMs) (a)-(c) of
each synthesized large NaX zeolite using TEA.
[0036] FIG. 7 is a graph showing traces of NH.sub.3 temperature
programmed desorption (TPD) profiles of the ion-exchanged CaX
zeolite and the commercial-grade NaX zeolite.
[0037] FIG. 8 is a graph showing traces of the XRD analysis for
several carbon templates of the zeolite made using CaX
zeolites.
[0038] FIG. 9 is a graph showing traces of nitrogen
adsorption-desorption isotherms of the carbon templates of the
zeolite formed.
[0039] FIG. 10 is a graph showing traces of the determined pore
size distribution using the non-local density function theory
(NLDFT) algorithm on the nitrogen adsorption-desorption isotherm
data shown in FIG. 9.
[0040] FIG. 11 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the LCaX zeolite, a
carbon-zeolite composite, thermally-treated carbon-zeolite
composites, and the resultant methane microporous carbon
adsorbent.
[0041] FIG. 12 is a graph showing traces of nitrogen
adsorption-desorption isotherms for carbon templates of the zeolite
made from LCaX-1023-2 and LCaX-873-4 and two methane microporous
carbon adsorbents made from LCaX-873-4H and LCaX-873-4H4H.
[0042] FIG. 13 is a graph showing traces of the determined pore
size distribution using the non-local density function theory
(NLDFT) algorithm on the nitrogen adsorption-desorption isotherm
data shown in FIG. 12.
[0043] FIG. 14 is a graph showing traces of the XRD analysis for
carbon templates of the zeolite made from LCaX-1023-2 and
LCaX-873-4 and methane microporous carbon adsorbents made from
LCaX-873-4H and LCaX-873-4H4H.
[0044] FIG. 15 shows SEMs of (a) BEA crystalline zeolite and (b)
methane microporous carbon adsorbents made using the BEA
zeolite.
[0045] FIG. 16 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the methane microporous carbon
adsorbent made from CaX and BEA zeolites.
[0046] FIG. 17 is a graph showing traces of the XRD analysis for
the methane microporous carbon adsorbent made from the CaX and BEA
zeolites.
[0047] FIG. 18 is a graph showing traces of the determined pore
size distribution using the NLDFT algorithm for four methane
microporous carbon adsorbents formed using two acetylene
CVD/post-CVD thermal treatment cycles.
[0048] FIG. 19 is a graph showing several traces of the methane
adsorption-desorption isotherms on a gravimetric basis for a carbon
templates of the zeolite and several methane microporous carbon
adsorbents at 298 K.
[0049] FIG. 20 shows SEMs (a)-(b) of methane microporous carbon
adsorbents made using calcium-ion substituted X zeolites.
[0050] FIG. 21 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the methane microporous carbon
adsorbents formed using CaX and NaX zeolites.
[0051] FIG. 22 is a graph showing traces of the determined pore
size distribution using the non-local density function theory
(NLDFT) algorithm on the nitrogen adsorption-desorption isotherm
data shown in FIG. 21.
[0052] FIG. 23 shows SEMs (a)-(b) of methane microporous carbon
adsorbents made using sodium X zeolites.
[0053] FIG. 24 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the NaX and the mass produced
NaX methane microporous carbon adsorbents.
[0054] FIGS. 1-24 and their description facilitate a better
understanding of the system and method of sequential carbon
synthesis as well as the system and method for use of the adsorbed
natural gas (ANG) storage facility. In no way should FIGS. 1-24
limit or define the scope of the embodiments.
[0055] FIGS. 1-4 are simple diagrams for ease of description.
Several of the graphs show traces and curves that are off-set from
the true y-axis value at y=0. This is done so for the sake of
clarity and is indicated in the Detailed Description and on each
Figure.
DETAILED DESCRIPTION
[0056] The Specification, which includes the Summary, Brief
Description of the Drawings and the Detailed Description, and the
appended Claims refer to particular features (including process or
method steps). Those of skill in the art understand that the
embodiments include all possible combinations and uses of
particular features described in the Specification. Those of skill
in the art understand that the embodiments are not limited to or by
the description of embodiments given in the Specification. The
inventive subject matter is not restricted except only in the
spirit of the Specification and appended Claims.
[0057] Those of skill in the art also understand that the
terminology used for describing particular embodiments does not
limit the scope or breadth of the embodiments. In interpreting the
Specification and appended Claims, all terms should be interpreted
in the broadest possible manner consistent with the context of each
term. All technical and scientific terms used in the Specification
and appended Claims have the same meaning as commonly understood by
one of ordinary skill in the art to which the embodiments belong
unless defined otherwise.
[0058] As used in the Specification and appended Claims, the
singular forms "a", "an" and "the" include plural references unless
the context clearly indicates otherwise. The verb "comprises" and
its conjugated forms should be interpreted as referring to
elements, components or steps in a non-exclusive manner, and the
embodiments illustrative disclosed suitably may be practiced in the
absence of any element which is not specifically disclosed,
including as "consisting essentially of" and "consisting of". The
referenced elements, components or steps may be present, utilized
or combined with other elements, components or steps not expressly
referenced. "Operable" and its various forms means fit for its
proper functioning and able to be used for its intended use.
"Detect" and its conjugated forms should be interpreted to mean the
identification of the presence or existence of a characteristic or
property. "Determine" and its conjugated forms should be
interpreted to mean the ascertainment or establishment through
analysis or calculation of a characteristic or property.
[0059] Spatial terms describe the relative position of an object or
a group of objects relative to another object or group of objects.
The spatial relationships apply along vertical and horizontal axes.
Orientation and relational words, including "upstream" and
"downstream" and other like terms are for descriptive convenience
and are not limiting unless otherwise indicated.
[0060] Where the Specification or the appended Claims provide a
range of values, it is understood that the interval encompasses
each intervening value between the upper limit and the lower limit
as well as the upper limit and the lower limit. Embodiments
encompass and bounds smaller ranges of the interval subject to any
specific exclusion provided.
[0061] Where the Specification and appended Claims reference a
method comprising two or more defined steps, the defined steps can
be carried out in any order or simultaneously except where the
context excludes that possibility.
[0062] When a patent or a publication is referenced in this
disclosure, the reference is incorporated by reference and in its
entirety to the extent that it does not contradict statements made
in this disclosure.
[0063] FIG. 2 is a cross-sectional diagram of a rotary tubular
furnace in a rotary tubular furnace system. The rotary tubular
furnace system is useful for batch-performing several steps of the
sequential carbon synthesis method. Rotary tubular furnace system
100 includes rotary tubular furnace 110. Rotary tubular furnace 110
contains cylindrical stainless-steel container 112. Cylindrical
stainless-steel container 112 has several interior baffles 114
mounted along the interior surface for the length of cylindrical
stainless-steel container 112. The rotation of cylindrical
stainless-steel container 112 causes interior baffles 114 to
contact and collide with solid material 116 previously introduced
into cylindrical stainless-steel container 112. The repeated
contacting and collision with interior baffles 114 distributes
solid material 116 in a random manner within the interior of
cylindrical stainless-steel container 112 that facilitates
solid-gas mixing, distributes heat and prevents solids from
adhering to one another. Cylindrical stainless-steel container 112
couples to inlet gas tubular 118 at first circular end 120 and
outlet gas tubular 122 at second circular end 124. During
operation, cylindrical stainless-steel container 112 rotates around
a lengthwise axis (arrows 126) formed by the coupling of inlet gas
tubular 118, cylindrical stainless-steel container 112 and outlet
gas tubular 122.
[0064] Rotary tubular furnace 110 contains cylindrical
stainless-steel container 112 within cylindrical shell 128. Heating
units 130 are fixed along the exterior of the lengthwise portion of
cylindrical shell 128 such that they are operable to transmit
thermal energy into the interior of cylindrical shell 128.
Thermocouples 132 are located in heating units 130, on cylindrical
shell 128, and inside cylindrical shell 128. Temperature controller
134, which electrically couples to heating units 130 and signally
to thermocouples 132, is operable to monitor the temperature values
provided by thermocouples 132 and adjust the transmission of
thermal energy into cylindrical shell 128 such that rotary tubular
furnace system 100 is operable to maintain a set temperature for
cylindrical stainless-steel container 112 during operation.
[0065] Rotary tubular furnace system 100 includes organic precursor
source 136 and non-reactive gas source 138. Rotary tubular furnace
system 100 is operable to selectively feed organic precursor source
136, non-reactive gas source 138 or both simultaneously to rotary
tubular furnace 110. Mixer 140 is downstream of both organic
precursor source 136 and non-reactive gas source 138 and is
operable to blend the two source gases together into a homogeneous
mixture when both are introduced simultaneously.
[0066] Using rotary tubular furnace system 100 to perform the
sequential carbon synthesis includes introducing a crystalline
zeolite into cylindrical stainless-steel container 112. The
crystalline zeolite acts as a sacrificial template for forming the
methane microporous carbon adsorbent. Cylindrical shell 128 is
closed such that it is air tight. Cylindrical stainless-steel
container 112 is set in rotation (arrows 126) such that the
crystalline zeolite is mixed using the interior baffles 114.
Non-reactive gas is introduced into cylindrical stainless-steel
container 112 from non-reactive gas source 138 through inlet gas
tubular 118 to purge the atmosphere within cylindrical
stainless-steel container 112 through outlet gas tubular 122 and
fill cylindrical stainless-steel container 112 with non-reactive
gas. A first chemical vapor deposition (CVD) temperature is set
using temperature controller 134, which raises in a steady and
controlled manner the temperature of cylindrical stainless-steel
container 112 until the first CVD temperature is detected through
thermocouples 132.
[0067] Upon achieving the first CVD temperature, a mixture of the
organic precursor and the non-reactive gas is introduced into
cylindrical stainless-steel container 112 to fill and maintain
cylindrical stainless-steel container 112 with the first CVD gas
mixture. The organic precursor gas is introduced into cylindrical
stainless-steel container 112 through inlet gas tubular 118 from
organic precursor source 136 after mixing with non-reactive gas in
mixer 140. The crystalline zeolite is exposed to the first CVD gas
mixture for a first CVD period at the first CVD temperature such
that the introduced organic precursor is adsorbed via CVD into the
crystalline zeolite, the organic precursor converts into a
deposited carbon that negatively replicates the crystalline
zeolite, and a first carbon-zeolite composite forms.
[0068] After the elapse of the first CVD period, a first thermal
treatment temperature is set using temperature controller 134 to
raise the operating temperature of the cylindrical stainless-steel
container 112 to the first thermal treatment temperature. In
addition, the organic precursor source 136 is isolated such that
only the non-reactive gas is introduced into cylindrical
stainless-steel container 112. Upon achieving the first thermal
treatment temperature, the introduction of non-reactive gas and the
first thermal treatment temperature is maintained for a first
thermal treatment period. During the first thermal treatment
period, the deposited carbon within the carbon-zeolite composite
converts into a thermally-treated carbon that negatively replicates
the crystalline zeolite, and the first thermally-treated carbon
template of the zeolite forms.
[0069] The processes of CVD and post-CVD thermal treatment are
repeated such that a second thermally-treated carbon template of
the zeolite forms. The CVD occurs for a second CVD period at the
second CVD temperature. The post-CVD thermal treatment occurs at a
second thermal treatment temperature for a second thermal treatment
period. After expiration of the second thermal treatment period,
the introduction of non-reactive gas continues and cylindrical
stainless-steel container 112 is permitted to cool to room
temperature. Upon reaching room temperature, the rotation of
cylindrical stainless-steel container 112 is halted and solid
material 116, which includes second thermally-treated
carbon-zeolite composite, is recovered from rotary tubular furnace
system 100.
[0070] After recovery of the second thermally-treated
carbon-zeolite composite, an aqueous strong mineral acid mixture is
introduced to the second thermally-treated carbon-zeolite
composite. The aqueous strong mineral acid mixture etches the
crystalline zeolite away from the second thermally-treated carbon
template of the zeolite, forming the methane microporous carbon
adsorbent.
[0071] FIG. 3 is a process flow diagram of an embodiment of a
system for continuously performing an embodiment of the sequential
carbon synthesis method. Sequential carbon synthesis system 200
includes first CVD/thermal treatment system 202 (dotted box),
second CVD/thermal treatment system 204 (dotted box) and recovery
system 206 (dotted box). First CVD/thermal treatment system 202 and
second CVD/thermal treatment system 204 are coupled in series.
First CVD/thermal treatment system 202 is operable to form the
first thermally-treated carbon-zeolite composite from the
introduced crystalline zeolite, and second CVD/thermal treatment
system 204 is operable to form the second thermally-treated
carbon-zeolite composite using the first thermally-treated
carbon-zeolite composite. Recovery system 206 is operable to form
the methane microporous carbon adsorbent from the first
thermally-treated carbon-zeolite composite, the second
thermally-treated carbon-zeolite composite, and combinations
thereof.
[0072] Several streams are introduced into sequential carbon
synthesis system 200 to support the formation of the negative
replica that is the methane microporous carbon adsorbent.
Sequential carbon synthesis system 200 processes crystalline
zeolite introduced through first feed line 210 from a source
outside of the process into a thermally-treated carbon-zeolite
composite. CVD gas supply lines 212 and 214 introduce from a source
outside of the process a gas that includes the organic precursor.
Acetylene, ethylene, propylene and ethanol are useful organic
precursors. In an embodiment of the method, the organic precursor
is selected from the group consisting of acetylene, ethylene,
propylene, ethanol and combinations thereof. In an embodiment of
the method, the organic precursor gas further comprises a
non-reactive gas. Neutral gas supply lines 216 and 218 introduce
from a source outside of the process a gas that has no reactivity
with the carbon-zeolite composite during post-CVD thermal treatment
at the thermal treatment temperature. Non-reactive gases include
noble gases such as helium and argon. In an embodiment of the
method, the non-reactive gas is selected from the group consisting
of helium, argon and combinations thereof. Acid supply line 220
introduces from a source outside of the process an aqueous strong
mineral acid mixture for removing the sacrificial crystalline
zeolite from a thermally-treated carbon-zeolite composite. Aqueous
strong mineral acid mixture includes aqueous mixtures of HCl and
HF.
[0073] The produced methane microporous carbon adsorbent--a
negative carbon replica of the introduced crystalline
zeolite--passes from sequential carbon synthesis system 200 through
adsorbent product line 222. Several streams also pass from
sequential carbon synthesis system 200 as byproducts of the
process. Spent CVD gas recovery lines 224 and 226 direct recovered
CVD gas, which contains unused organic precursor and hydrogen from
the carbonization of the organic precursor, to systems outside of
the process for separation and recovery. The recovered CVD gas may
also contain a non-reactive gas worth recovering in processes
outside of sequential carbon synthesis system 200. Helium is a
highly-limited natural resource that many countries consider a
strategic material. Argon is a useful non-reactive gas and is easy
to separate from the organic species. Neutral gas recovery lines
228 and 230 direct a mixture of non-reactive gas introduced through
neutral gas supply lines 216 and 218 and hydrogen from post-CVD
thermal treatment to systems outside of the process for recovery
and purification. Spent acid recovery line 232 passes a spent
aqueous strong mineral acid mixture to systems outside of the
process for regeneration or neutralization. The spent aqueous
strong mineral acid mixture contains dissolved aluminum and silicon
from etching the sacrificial zeolite to form the methane
microporous carbon adsorbent.
[0074] Sequential carbon synthesis system 200 introduces
crystalline zeolite into first zeolite hopper 240 of first
CVD/thermal treatment system 202 through first feed line 210. First
zeolite hopper 240 couples to and meters the crystalline zeolite
into first CVD reactor 242 using solids feed line 244. Sequential
carbon synthesis system 200 introduces the organic precursor into
the first CVD reactor 242 as part of an organic precursor gas using
CVD gas supply line 212. In an embodiment of the method, the
organic precursor gas includes a non-reactive gas. First CVD
reactor 242 can be a number of known reactor types for mixing
solids and gases together where the solids require a certain
residence time within the reactor, including a moving bed type
reactor or a fluidized bed reactor. First CVD reactor 242 is shown
with perforated plate 246 (dashed line) such that the organic
precursor gas is introduced through CVD gas supply line 212 below
the stack of crystalline zeolites (not shown), which are in various
stages of adsorption and carbonization. The organic precursor gas
moves upward from the bottom of first CVD reactor 242 to the top,
interacting with the introduced zeolite. The formed first
carbon-zeolite composite passes from first CVD reactor 242 through
carbonized composite line 248. The spent CVD gas passes from the
top of first CVD reactor 242 through spent CVD gas line 250. Spent
CVD gas line 250 couples to and feeds into spent CVD gas recovery
line 224.
[0075] Sequential carbon synthesis system 200 operates first CVD
reactor 242 such that the introduced organic precursor is adsorbed
via chemical vapor deposition (CVD) into the crystalline zeolite,
the organic precursor converts into a deposited carbon that
negatively replicates the crystalline zeolite, and the first
carbon-zeolite composite forms. Sequential carbon synthesis system
200 maintains first CVD reactor 242 at a first CVD temperature. In
an embodiment of the method, the first CVD temperature is in a
range of from about 800 K to about 900 K. Sequential carbon
synthesis system 200 maintains the crystalline zeolite within first
CVD reactor 242 for a first CVD period. In an embodiment of the
method, the first CVD period is in range of from about 2 hours to
about 9 hours.
[0076] Carbonized composite line 248 couples first CVD reactor 242
to purge vessel 252 and conveys the first carbon-zeolite composite
into purge vessel 252. Sequential carbon synthesis system 200
operates purge vessel 252 to remove any remaining organic precursor
from the first carbon-zeolite composite for recovery and reuse. Any
recovered organic precursor is conveyed to spent CVD gas recovery
line 224 through recovered gas line 254. In an embodiment of the
method, the purge vessel is maintained at a sub-atmospheric
pressure. A gas that is non-reactive with the first carbon-zeolite
composite can be introduced to purge the first carbon-zeolite
composite, including helium and argon. In an embodiment of the
method, a purge gas that is non-reactive with the first
carbon-zeolite composite is introduced into the purge vessel.
Sequential carbon synthesis system 200 passes the purged first
carbon-zeolite composite from purge vessel 252 using first thermal
treatment feed line 256.
[0077] First thermal treatment feed line 256 couples purge vessel
252 to first post-CVD thermal treatment unit 258. Sequential carbon
synthesis system 200 introduces the purged first carbon-zeolite
composite into first post-CVD thermal treatment unit 258. Neutral
gas supply line 216 introduces the non-reactive gas into first
post-CVD thermal treatment unit 258. First post-CVD thermal
treatment unit 258 can be a number of known reactor types for
mixing solids and gases together where the solids require a certain
residence time within the reactor as previously described. The
formed first thermally-treated carbon-zeolite composite passes from
first post-CVD thermal treatment unit 258 through first treatment
product line 268. In doing so, the first thermally-treated
carbon-zeolite composite passes from first CVD/thermal treatment
system 202. The spent thermal treatment gas, which comprises
non-reactive gas as well as evolved hydrogen from the thermal
treatment of the carbon-zeolite composite, passes from the top of
first post-CVD thermal treatment unit 258 through neutral gas
recovery line 228.
[0078] Sequential carbon synthesis system 200 operates first
post-CVD thermal treatment unit 258 such that the carbon template
of the zeolite within the first carbon-zeolite composite converts
into a thermally-treated carbon template of the zeolite that
negatively replicates the zeolite. Sequential carbon synthesis
system 200 maintains first post-CVD thermal treatment unit 258 at a
first thermal treatment temperature. In an embodiment of the
method, the first thermal treatment temperature is in a range of
from about 1100 K to about 1200 K. Sequential carbon synthesis
system 200 maintains the purged first carbon-zeolite composite
within first post-CVD thermal treatment unit 258 for a first
thermal treatment period. In an embodiment of the method, the first
thermal treatment period is in range of from about 2 hours to about
4 hours.
[0079] Sequential carbon synthesis system 200 includes solids feeds
splitter 270, which is operable to selectively direct the first
thermally-treated carbon-zeolite composite received from coupled
first treatment product line 268 towards zeolite reactor 272 via
reactor feed line 274 or towards second zeolite hopper 340 via
second feed line 310, or both in proportion at the same time.
Solids feeds splitter 270 is operable to direct none, at least a
portion of the first thermally-treated carbon-zeolite composite
towards zeolite reactor 272 and the remainder, if any, towards
second zeolite hopper 340.
[0080] The inclusion of a splitter allows the flexibility to
operate the sequential carbon synthesis system to form methane
microporous carbon adsorbents using only the first CVD/thermal
treatment system, the second CVD/thermal treatment systems, or both
simultaneously.
[0081] Recovery system 206 of sequential carbon synthesis system
200 includes zeolite reactor 272. Reactor feed line 274 couples
purge vessel 264 to zeolite reactor 272 and conveys the first
thermally-treated carbon-zeolite composite into zeolite reactor
272. Sequential carbon synthesis system 200 introduces the aqueous
strong mineral acid mixture into zeolite reactor 272 through acid
supply line 220. Zeolite reactor 272 is operable to form the
methane microporous carbon adsorbent. Zeolite reactor 272 etches
the crystalline zeolite of the first thermally-treated
carbon-zeolite composite from the thermally-treated carbon template
of the zeolite using the aqueous strong mineral acid mixture. In an
embodiment of the method, the first thermally-treated
carbon-zeolite composite is maintained within the zeolite rector
for a residence time in a range of from about an hour to about two
hours. In an embodiment of the method, the aqueous strong mineral
acid mixture comprises HCl and HF. The aqueous strong mineral acid
mixture upon reaction with the crystalline zeolite converts into
the spent aqueous strong mineral acid mixture. Sequential carbon
synthesis system 200 passes the suspension of methane microporous
carbon adsorbent and spent aqueous strong mineral acid mixture from
zeolite reactor 272 using zeolite reactor product line 276.
[0082] Recovery system 206 also includes adsorbent recovery unit
278. Zeolite reactor product line 276 couples zeolite reactor 272
to adsorbent recovery unit 278 and conveys the suspension of
methane microporous carbon adsorbent and spent aqueous strong
mineral acid mixture into adsorbent recovery unit 278. Adsorbent
recovery unit 278 is operable to separate the suspension into the
methane microporous carbon adsorbent and the spent aqueous strong
mineral acid mixture. The produced methane microporous carbon
adsorbent passes through adsorbent product line 222, and spent acid
recovery line 232 passes the spent aqueous strong mineral acid
mixture.
[0083] Sequential carbon synthesis system 200 introduces the first
thermally-treated carbon-zeolite composite into second CVD/thermal
treatment system 204 using second zeolite hopper 340 through second
feed line 310. Second zeolite hopper 340 couples to and meters the
first thermally-treated carbon-zeolite composite into second CVD
reactor 342 using solids feed line 344. Sequential carbon synthesis
system 200 introduces the organic precursor into the second CVD
reactor 342 as part of an organic precursor gas using CVD gas
supply line 214. In an embodiment of the method, the organic
precursor gas includes a non-reactive gas. Second CVD reactor 342
can be a number of known reactor types for mixing solids and gases
together where the solids require a certain residence time within
the reactor. Second CVD reactor 342 has perforated plate 246
(dashed line) such that the organic precursor gas is introduced
through CVD gas supply line 214 below the stack of
thermally-treated carbon-zeolite composite (not shown), which are
in various stages of adsorption and carbonization. The organic
precursor gas moves upward from the bottom of second CVD reactor
342 to the top, interacting with the introduced first
thermally-treated carbon-zeolite composite. The formed second
carbon-zeolite composite passes from second CVD reactor 342 through
carbonized composite line 348. The spent CVD gas passes from the
top of second CVD reactor 342 through spent CVD gas line 350. Spent
CVD gas line 350 couples to and feeds into spent CVD gas recovery
line 226.
[0084] Sequential carbon synthesis system 200 operates second CVD
reactor 342 such that the introduced organic precursor is adsorbed
via CVD into the first thermally-treated carbon-zeolite composite,
the organic precursor converts into carbon and the second
carbon-zeolite composite forms. The first thermally-treated
carbon-zeolite composite already contains the first
thermally-treated carbon template of the zeolite from the first
CVD/post-CVD thermal treatment. The newly deposited carbon further
enhances the accuracy of the negative carbon replica, although the
deposited carbon is not fully incorporated into the existing first
thermally-treated carbon template of the zeolite at the lower CVD
temperatures. Sequential carbon synthesis system 200 maintains
second CVD reactor 342 at a second CVD temperature. In an
embodiment of the method, the second CVD temperature is in a range
of from about 800 K to about 900 K. In an embodiment of the method,
the first CVD temperature and the second CVD temperature are the
same. Sequential carbon synthesis system 200 maintains the first
thermally-treated carbon-zeolite composite within second CVD
reactor 342 for a second CVD period. In an embodiment of the
method, the second CVD period is in range of from about 2 hours to
about 4 hours. In an embodiment of the method, the first CVD period
and the second CVD period are the same.
[0085] Carbonized composite line 348 couples second CVD reactor 342
to purge vessel 352 and conveys the second carbon-zeolite composite
into purge vessel 352. Sequential carbon synthesis system 200
operates purge vessel 352 in a similar manner as purge vessel 252.
Any recovered organic precursor is conveyed to spent CVD gas
recovery line 226 through recovered gas line 354. In an embodiment
of the method, the purge vessel is maintained at a sub-atmospheric
pressure. In an embodiment of the method, a purge gas that is
non-reactive with the second carbon-zeolite composite is introduced
into the purge vessel. Sequential carbon synthesis system 200
passes the purged second carbon-zeolite composite from purge vessel
352 using second thermal treatment feed line 356.
[0086] Second thermal treatment feed line 356 couples purge vessel
352 to second post-CVD thermal treatment unit 358 and introduces
the purged second carbon-zeolite composite into second post-CVD
thermal treatment unit 358. Sequential carbon synthesis system 200
introduces the non-reactive gas into second post-CVD thermal
treatment unit 358 using neutral gas supply line 218. Second
post-CVD thermal treatment unit 358 can be a number of known
reactor types for mixing solids and gases together where the solids
require a certain residence time within the reactor as previously
described. The formed second thermally-treated carbon-zeolite
composite passes from second post-CVD thermal treatment unit 358
through thermally-treated carbon-zeolite composite line 360. The
spent thermal treatment gas, which comprises non-reactive gas as
well as evolved hydrogen, passes from the top of second post-CVD
thermal treatment unit 358 through spent thermal treatment gas line
362. Spent thermal treatment gas line 362 couples to and feeds into
neutral gas recovery line 230.
[0087] Sequential carbon synthesis system 200 operates second
post-CVD thermal treatment unit 358 such that the deposited carbon
within the second carbon-zeolite composite converts into a
thermally-treated carbon template of the zeolite that is the
negative replica. The second thermally-treated carbon-zeolite
composite forms as a result. The deposited carbon from the second
CVD period is fully incorporated during the second thermal
treatment period into the first thermally-treated template of the
zeolite, thereby forming the second thermally-treated template of
the zeolite. In addition, additional calcination time provides
energy to the existing negative carbon replica to improve its
conformance to the crystalline zeolite structure, further improving
the accuracy of the negative replication. Sequential carbon
synthesis system 200 maintains second post-CVD thermal treatment
unit 358 at a second thermal treatment temperature. In an
embodiment of the method, the second thermal treatment temperature
is in a range of from about 1100 K to about 1200 K. In an
embodiment of the method, the second thermal treatment temperature
is the same as the first thermal treatment temperature. Sequential
carbon synthesis system 200 maintains the purged second
carbon-zeolite composite within second post-CVD thermal treatment
unit 358 for a second thermal treatment period. In an embodiment of
the method, the second thermal treatment period is in range of from
about 2 hours to about 4 hours. In an embodiment of the method, the
second thermal treatment period is the same as the first thermal
treatment period.
[0088] Thermally-treated carbon zeolite composite line 260 couples
second post-CVD thermal treatment unit 358 to purge vessel 364 and
conveys the second thermally-treated carbon-zeolite composite into
purge vessel 364. Sequential carbon synthesis system 200 operates
purge vessel 364 to remove any remaining non-reactive gas and
evolved hydrogen from second thermally-treated carbon-zeolite
composite. The degassing mitigates the need for gas recovery for
zeolite reactor 272, which operates using strong acids that can
partially volatilize at room conditions. Any recovered gas is
conveyed to neutral gas recovery line 230 through recovered gas
line 366. In an embodiment of the method, the purge vessel is
maintained at a sub-atmospheric pressure. Sequential carbon
synthesis system 200 passes the purged second thermally-treated
carbon-zeolite composite from purge vessel 364 using second
treatment product line 368. In doing so, the second
thermally-treated carbon-zeolite composite passes from second
CVD/thermal treatment system 204.
[0089] Second treatment product line 368 couples purge vessel 364
to zeolite reactor 272 and conveys the purged second
thermally-treated carbon-zeolite composite into zeolite reactor
272. Zeolite reactor 272 is operable to form the methane
microporous carbon adsorbent from the second thermally-treated
carbon-zeolite composite using the aqueous strong mineral acid
mixture. The methane microporous carbon adsorbent forms by etching
the crystalline zeolite of the second thermally-treated
carbon-zeolite composite from the thermally-treated carbon template
of the zeolite. In an embodiment of the method, the second
thermally-treated carbon-zeolite composite is maintained within the
zeolite rector for a residence time in a range of from about an
hour to about two hours. The aqueous strong mineral acid mixture
converts into the spent aqueous strong mineral acid mixture upon
reacting with the zeolite. Sequential carbon synthesis system 200
passes the suspension of methane microporous carbon adsorbent and
spent aqueous strong mineral acid mixture from zeolite reactor 272
using zeolite reactor product line 276.
[0090] FIG. 4 is a process flow diagram of an embodiment of the
adsorbed natural gas (ANG) storage facility using the methane
microporous carbon adsorbent. Natural gas storage facility 400
couples to compressed natural gas (CNG) transportation pipeline
402, which is a natural gas source, at upstream connection 404 and
downstream connection 406. Upstream isolation valve 408 and
downstream isolation valve 410 are operable to fluidly isolate
natural gas storage facility 400 from CNG transportation pipeline
402. Check valves 412 provide additional assurance that any fluid
flowing through natural gas storage facility 400 is one-way from
upstream connection 404 to downstream connection 406.
[0091] Solar power array 424 electrically couples using electrical
power conduit 426 to temperature control system 428. Solar power
array 424 provides electrical power such that temperature control
system 428 satisfies the temperature regulation requirements of
natural gas storage facility 400 during both the peak period of
demand and the non-peak period of demand.
[0092] Natural gas introduced into natural gas storage facility 400
passes through adsorption beds inlet isolation valve 440 into
adsorption bed system 436. Adsorption bed system 436 has several
separate adsorption bed 438 in parallel. Each adsorption bed 438
contains the methane microporous carbon adsorbent (not shown) for
retaining the light natural gas component during the non-peak
period of demand. Adsorption beds thermal jacket 442 surrounds the
exterior of and regulates the internal temperature of adsorption
bed 438. Desorbed methane passes from adsorption bed 438 through
adsorption beds outlet isolation valve 444.
[0093] Temperature control system 428 couples to adsorption beds
thermal jacket 442. Temperature control system 428 controls,
maintains and modifies the internal temperature of adsorption bed
438. Temperature control system 428 introduces a
temperature-modifying fluid into adsorption beds thermal jacket 442
via adsorption bed supply conduit 446. Heat transfers to and from
the temperature-modifying fluid in adsorption beds thermal jacket
442 to support the adsorption and desorption of methane from the
methane microporous carbon adsorbent (not shown) contained in
several adsorption bed 438. Spent temperature modifying fluid
returns from adsorption beds thermal jacket 442 via adsorption bed
return conduit 448.
[0094] Adsorption bed system 436 couples to storage facility
compressor 450 and compressor bypass valve 452 via adsorption beds
outlet isolation valve 444. Both storage facility compressor 450
and compressor bypass valve 452 provide access to CNG
transportation pipeline 402 from adsorption bed 438. Storage
facility compressor 450 is operable to pressurize and introduce the
desorbed methane into CNG transportation pipeline 402 through
discharge conduit 456. Compressor bypass valve 452 permits direct
connectivity between adsorption bed system 436 and CNG
transportation pipeline 402 through discharge conduit 456. Storage
facility compressor 450 is operable of reduce the pressure in
adsorption bed 438 to facilitate desorption and purging of adsorbed
methane from the methane microporous adsorbent.
[0095] During the non-peak period of demand, a detectable condition
triggers natural gas storage facility 400 to operate the isolation
valves, including upstream isolation valve 408, adsorption beds
inlet isolation valve 440 and downstream isolation valve 410, such
that a fluid pathway forms through natural gas storage facility
400. Pressure differences between CNG transportation pipeline 402
and adsorption bed system 436 motivates natural gas to flow from
CNG transportation pipeline 402 into several adsorption bed 438 of
adsorption bed system 436. At reduced temperatures and increasing
pressure (as more natural gas flows into natural gas storage
facility 400), the methane from the introduced natural gas is
selectively separated and adsorbed by the methane microporous
carbon adsorbent in adsorption bed system 436. The remainder flows
out of natural gas storage facility 400 back into CNG
transportation pipeline 402 via discharge conduit 456. Temperature
control system 428 supplies temperature-modifying fluid to
adsorption beds thermal jacket 442 to facilitate the selective
separation and adsorption of methane by the methane microporous
carbon adsorbent in several adsorption bed 438.
[0096] Either at the end of the non-peak period of demand or when
some other detectable condition is detected, upstream isolation
valve 408, adsorption beds inlet isolation valve 440, adsorption
beds outlet isolation valve 444 and downstream isolation valve 410
close to isolate natural gas storage facility 400 from CNG
transportation pipeline 402. Temperature control system 428
maintains a storage temperature in adsorption bed system 436 such
that the adsorbed methane remains adsorbed onto the methane
microporous carbon adsorbent in adsorption bed 438 until the peak
period of demand.
[0097] During the peak period of demand, a condition detected by
natural gas storage facility 400 triggers it to operate adsorption
beds outlet isolation valve 444 and downstream isolation valve 410
such that a fluid pathway forms between CNG transportation pipeline
402 and adsorption bed system 436. Temperature control system 428
operates such that the temperature-modifying fluid is provided to
adsorption beds thermal jacket 442. The temperature-modifying fluid
facilitates desorption of adsorbed methane from the methane
microporous carbon adsorbent in the several adsorption bed 438,
forming desorbed methane.
[0098] Introduction of the desorbed methane into CNG transportation
pipeline 402 from adsorption bed 438 at times during the peak
period of demand optionally occurs without the need of compressive
assistance by opening compressor bypass valve 452. If a suitable
differential pressure between adsorption bed 438 and CNG
transportation pipeline 402 exists the desorbed methane optionally
flows from adsorption bed system 436 into CNG transportation
pipeline 402 without compression. When compression is used, closing
compressor bypass valve 452 and operating storage facility
compressor 450 provides motivation to the desorbed methane for
introduction into CNG transportation pipeline 402 during the peak
period of demand.
[0099] The operation of storage facility compressor 450 is operable
to form a sub-atmospheric pressure or "partial vacuum" in the
several adsorption bed 438 to facilitate desorption of methane in
preparation for the next adsorption cycle. Closing adsorption beds
outlet isolation valve 444 during the partial vacuum condition
causes adsorption bed 438 to retain the sub-atmospheric pressure
condition, which facilitates additional desorption of methane.
[0100] Natural gas storage facility 400 operations also includes
operating temperature control system 428 such that it provides
heating or cooling, shutting down storage facility compressor 450,
opening isolation valves for adsorption bed 438 to equalize
pressure and closing all other remaining isolation valves to
natural gas storage facility 400.
[0101] Experiments
[0102] Several experiments show the formation of methane
microporous carbon adsorbents from commercial-grade and large
crystalline zeolites. Useful methane microporous carbon adsorbents
are manufactured using a variety of crystalline zeolites, organic
precursors, CVD temperatures and periods, and post-CVD treatment
temperatures and periods. The variety of useful materials shows the
versatility of the sequential carbon synthesis method in forming a
relatively high surface area and micropore volume methane
microporous carbon adsorbents.
[0103] Synthesis of Large Crystal NaX Zeolites
(Si:Al=1.35-1.45)
[0104] This experiment shows the formation of "large" (versus
commercial-grade sizes of 1 to 2 .mu.m) crystal NaX zeolites. The
large NaX zeolites are useful to act as the sacrificial framework
for forming the methane microporous carbon adsorbent. In an
embodiment of the method, the method further comprises the step of
forming the crystalline zeolite.
[0105] In an embodiment of the method, the crystalline zeolite
comprises tri-ethanolamine (TEA). Large crystal NaX zeolites are
synthesized by adding TEA into a zeolite synthesis gel.
Na.sub.2SiO.sub.3.5H.sub.2O and sodium aluminate (55%
Al.sub.2O.sub.3 and 45% Na.sub.2O) are used as the silica and
alumina precursors, respectively. TEA is a complexing agent for
aluminum cations. The presence of TEA can retard the nucleation of
zeolite crystals compared with the growth process of the crystal,
resulting in larger crystals when included. The resultant gel
composition is in a molar ratio of about 4.76 Na.sub.2O:1.0
Al.sub.2O.sub.3:3.5 SiO.sub.2:454 H.sub.2O:n TEA, where n is varied
at three values for forming three different test gels: About 3, 5
and 7. Each resultant test gel is transferred to a polypropylene
bottle and hydrothermally crystallized at 373 K (Kelvin) for 72
hours. Each large NaX zeolite product is collected by filtration
and dried at 373 K.
[0106] FIG. 5 is a graph showing traces of the X-ray Diffraction
(XRD) analysis for each of the synthesized large NaX zeolites. Note
that for the sake of clarity in FIG. 5 that the individual traces
of NaX-7TEA and NaX-STEA are offset by a fixed value of Intensity
in counts per second (CPS). In reality, all three traces have the
same value at 2.theta.=0. The trace for NaX-STEA is offset by 6700
CPS; the trace for NaX-7TEA is offset by 12000 CPS. XRD analysis of
each large NaX zeolite shows that the NaX zeolite made with TEA
(n=3) is the only large NaX zeolite with XRD trace "peaks"
corresponding to those of a traditional NaX zeolite. No other
impurity phase peaks are observed with this zeolite. In contrast,
both large NaX zeolites made with TEA (n=5, 7) show XRD trace peaks
corresponding to a "P" zeolite (GIS phase) (see arrows FIG. 5) that
supplement the NaX zeolite phase peaks. The results indicate that
using less TEA as part of the gel composition is more useful for
forming large crystal NaX zeolites than greater amounts of the
compound.
[0107] FIG. 6 shows SEMs (a)-(c) of each synthesized large NaX
zeolite using TEA. The SEMs reveal the size of the zeolite
framework. Each of the SEMs (a)-(c) of FIG. 6 shows the large NaX
zeolites made with TEA having the typical orthogonal crystal
morphology of a NaX zeolite (approximately octahedral in
geometry--FAU) but with a crystal size distribution (left portion
of SEMs (a)-(c) of FIG. 6) such that the mid-edge length of the
octahedron is in the range of about 8 .mu.m (micrometers or
microns) to about 20 .mu.m. In an embodiment of the method, the
crystalline zeolite has a shape that is orthogonal with a mid-edge
length in a range of 8 .mu.m to 20 .mu.m. The crystal centered in
the close-up image on the right portion of SEM (a) of FIG. 6 shows
a crystalline zeolite with a mid-edge length of the octahedron of
about 14.9 .mu.m. The crystal centered in the close-up image on the
right portion of SEM (b) of FIG. 6, where TEA (n=5), shows a
crystalline zeolite with a mid-edge length of the octahedron of
about 11.8 .mu.m. The crystal centered in the close-up image on the
right portion of SEM (c) of FIG. 6, where TEA (n=7), shows a
crystalline zeolite with a mid-edge length of the octahedron of
about 9.68 .mu.m. Note that the left potion of SEMs (b)-(c) of FIG.
6 (TEA (n=5, 7 respectively)) both show combinations of P and X
crystalline zeolites merged (see arrows for SEMs (b)-(c) of FIG.
6). Large NaX zeolite using TEA (n=3) do not appear to show such
clustering of combined P and X zeolites.
[0108] Calcium Ion-Exchange of NaX Zeolite
[0109] In an embodiment of the method, the step of forming the
crystalline zeolite includes ion-exchanging a first crystalline
zeolite with calcium ions to form a second, ion-exchanged
crystalline zeolite. Calcium X zeolite (CaX) is prepared by
ion-exchange with a commercial-grade NaX zeolite (not the large
crystal NaX previously formed) by exchanging the sodium ions for
calcium ions. The commercial-grade NaX zeolites are small
crystallites having a mid-edge octahedral length in a range of from
about 1 to about 2 The resulting CaX zeolites are about the same
size.
[0110] About 10 g (grams) of commercial-grade NaX is constantly
stirred in 200 mL (milliliters) of 0.32 M (Molar)
Ca(NO.sub.3).sub.2 (calcium nitrate) solution for about 4 hours to
perform the ion-exchange.
[0111] FIG. 7 is a graph showing traces of NH.sub.3 (ammonia)
temperature programmed desorption (TPD) profiles of the
ion-exchanged CaX zeolite and the commercial-grade NaX zeolite.
FIG. 7 indicates that ion-exchange of NaX zeolite with
Ca.sup.2+ions generate acid sites that catalyze the selective
carbon deposition inside the zeolite micropores. These micropores
are directed towards small hydrocarbon molecules. The ion-exchanged
CaX zeolite shows two desorption peaks at 473 K and 653 K, which
indicates the presence of two types of acid sites. In contrast, the
commercial-grade NaX zeolite does not show any desorption profile,
which indicates that there are no acid sites.
[0112] The CaX zeolite also appears to have increased thermal
stability. Table 1 compares the thermal stability of
commercial-grade NaX zeolite and calcium-ion exchanged X
zeolite.
TABLE-US-00001 TABLE 1 Thermal stability of NaX and ion-exchanged
CaX crystal zeolites. Sample A.sub.z.sup.[1] T.sub.init.sup.[2]
(.degree.K) T.sub.0.5.sup.[3] (.degree.K) NaX 1 933 1043 Ca.sup.exX
0.93 983 1153 .sup.[1]Equivalent fraction of exchange cation in
zeolite. .sup.[2]Temperature at which structural degradation is
first observed from the X-ray powder pattern (K).
.sup.[3]Temperature at which the structure is 50% decomposed
(K).
[0113] Table 1 shows enhancement of the thermal stability of the
calcium-ion exchanged X zeolite (CaX) as seen in relative increases
in both T.sub.init and T.sub.0.5. This is a benefit for performing
CVD using a CaX ion-exchanged zeolite over a NaX zeolite: The
crystallinity of the CaX zeolite does not change even at 973 K,
which is useful given that CVD temperatures are in a range of from
about 873 K to about 973 K.
[0114] Carbon Deposition within CaX Zeolite
[0115] Carbon vapor deposition (CVD) is performed using a plug-flow
reactor. About one gram of zeolite (commercial-grade NaX zeolite,
previously-produced ion-exchanged CaX zeolite) is placed in the
plug-flow reactor. The temperature is increased within the reactor
in a controlled, gradual manner to a CVD temperature under
continuous helium flow. After stabilization at the CVD temperature
for about 30 minutes, the helium gas is changed over to the organic
precursor gas that is a combination of helium and organic
precursor.
[0116] Three different organic precursors are used for testing
three organic precursor gases for CVD: Propylene, ethanol and
acetylene. The kinetic diameters of both propylene and ethanol are
0.45 nm and acetylene is 0.33 nm. For introducing propylene as the
organic precursor, the organic precursor gas has a composition of 2
vol. % propylene in a He mixture. For introducing ethanol, the
organic precursor gas has a composition of ethanol-saturated helium
(room temperature; 6 kPa pressure). A bubbler is used to introduce
the helium through the liquid ethanol to form the saturated gas
mixture. For introducing acetylene, the organic precursor gas has a
composition of 2% vol. % acetylene in a He mixture. Each organic
precursor gas is introduced to each zeolite sample at a mass flow
rate of about 200 mL/minute per gram of zeolite. The organic
precursor gas is introduced and maintained at the mass flow rate
for a CVD period during which the organic precursor adsorbs into
and carbonizes within the zeolite at the CVD temperature, forming a
carbon-zeolite composite. After the CVD period has elapsed, the
introduced organic precursor gas is changed to the non-reactive gas
(pure helium) and the plug-flow reactor is permitted to cool to
room temperature.
[0117] Each resultant carbon-zeolite composite is treated with an
aqueous strong mineral acid mixture comprising 3.4 wt. % HCl and
3.3 wt. % HF acids. The resultant carbon-zeolite composite is
exposed to the aqueous strong mineral acid mixture twice at room
temperature for a 1 hour period. The resultant carbon template of
the zeolite--a negative replica of the zeolite--is filtered, washed
with deionized water and dried overnight at 373 K.
[0118] Effect of Organic Precursor on Forming a Carbon Template of
the Zeolite using the CaX Zeolite
[0119] Two different organic precursors--propylene and ethanol--are
applied at different CVD temperatures to form several carbon
templates of the zeolite. For this experiment, the following
designation code indicates the process used for manufacturing each
carbon template of the zeolite: "zeolite template-CVD
temperature/organic precursor/CVD time heat treatment", where
"zeolite template" is the ion and type of zeolite template used
(NaX, CaX). "CVD temperature" is in K for the four-hour period of
organic precursor introduction. "Organic precursor" is selected
from propylene ("P"), ethanol ("E") and acetylene ("A"). "CVD time
heat treatment" indicates the length of post-CVD heat treatment at
1123 K in hours. For example, "CaX-973P5" means a CaX zeolite
template at a CVD temperature of 973 K while introducing an organic
precursor gas containing propylene for a CVD period of 5 hours.
[0120] FIG. 8 is a graph showing traces of the XRD analysis for
several carbon templates of the zeolite made from a CaX zeolite.
Three carbon templates of the zeolite are formed: CaX-1073E6,
CaX-973E6 and CaX-973P5. Note that for the sake of clarity in FIG.
8 that the individual traces of CaX-1073E6 and CaX-973E6 are offset
by a fixed value of Intensity in CPS. In reality, all three traces
have a similar value at 2.theta.=0. The trace for CaX-973E6 is
offset by 10000 CPS; the trace for CaX-1073E6 is offset by 15000
CPS. The XRD patterns for all three carbon templates of the zeolite
as given in FIG. 8 show a broad peak around 20 in a range of from
about 5.degree. to about 6.degree.. The broad peak in at this 20
value indicates that all three carbon templates of the zeolite have
a structural micropore arrangement that is ordered and regular. A
sharp peak in 2.theta.=5-6.degree. range indicates that the carbon
templates of the zeolite have regularity in microform corresponding
with the structural ordering of (111) plane stacking X zeolite
(also known as the "FAU" zeolite structure). This suggests that
each carbon templates of the zeolite negatively replicates the
micropore structure of the sacrificial zeolite. Of the three carbon
templates of the zeolites, the negative replica formed from
CaX-973P5 shows the strongest and most highly-resolved peak at
2.theta.=5-6.degree.. The strong peak relative to the other two
carbon templates of the zeolite indicates that the negative replica
formed by CaX-973P5 is the most accurate representation of its
zeolite.
[0121] Those of ordinary skill in gas adsorption research
understand and appreciate that there are several different testing
procedures for determining the surface characteristics for
carbon-zeolite composites, carbon templates of the zeolite,
thermally-treated carbon templates of the zeolite and methane
microporous carbon adsorbents. The article by Wang, et al.,
"Experimental and Theoretical Study of Methane Adsorption on
Granular Activate Carbons", AIChE Journal 782-788 (Vol. 58, Issue
3) ("Wang"), describes a process and an apparatus for
characterizing adsorbent materials using nitrogen porosimetry at 77
K to determine the nitrogen adsorption-desorption isotherms. BET
(Brunauer-Emmett-Teller) analysis provides specific surface area of
the carbon templates of the zeolite as a function of the changes to
relative nitrogen pressure (P/P.sub.0) during the isothermic
testing. The D-R (Dubinin-Radushkevich) equation uses the relative
nitrogen pressure data for determining the volume of each type of
pore (micro- and mesopores) present on the carbon templates of the
zeolite based upon molecular stacking mechanics if the diameter of
the pore is close to the working diameter of the molecule being
adsorbed and surface adsorption within the pore if the diameters
are dissimilar.
[0122] FIG. 9 is a graph showing traces of nitrogen
adsorption-desorption isotherms of the carbon templates of the
zeolite. Note that for the sake of clarity of all three traces that
the isotherm trace for CaX-973E6 has been off-set by an additional
adsorbed amount of 200 cm.sup.3/g and the isotherm trace for
CaX-1073E6 has been off-set by an additional adsorbed amount of 250
cm.sup.3/g at P/P.sub.0=0. The carbon templates of the zeolite
formed form CaX-973P5 shows the least deviation on the return leg
of the adsorption-desorption isotherm, whereas CaX-1073E6 shows the
greatest. This deviation may indicate a greater amount of mesopore
volume in the CaX-1073E6 carbon templates of the zeolite versus the
CaX-973P5 carbon templates of the zeolite.
[0123] FIG. 10 is a graph showing traces of the determined pore
size distribution using the non-local density function theory
(NLDFT) algorithm on the nitrogen adsorption-desorption isotherm
data shown in FIG. 9. Note that for the sake of clarity in FIG. 10
that the individual traces of CaX-1073E6 and CaX-973E6 carbon
templates of the zeolite are offset by a fixed value of dV.sub.p in
centimeters per gram (cm.sup.3/g). In reality, all three traces
have a similar value at W=0 nm. The trace for CaX-973E6 is offset
by 0.16 cm.sup.3/g; the trace for CaX-1073E6 is offset by 0.26
cm.sup.3/g. FIG. 10 shows that all three carbon templates of the
zeolite formed from CaX zeolites have dual porosity with both
micropores (about 1.5 to 2 nm in diameter) and mesopores (about 2
to 5 nm in diameter). Hydrogen has a kinetic diameter of 2.89 A and
methane has a kinetic diameter of 3.8 A. The trace of carbon
templates of the zeolite made from CaX-973P5 indicates that it has
the largest total micropore volume. The trace of carbon templates
of the zeolite made from CaX-1073E6 indicates the largest total
mesopore volume.
[0124] Table 2 provides surface area as well as micro- and mesopore
volume data on all three carbon templates of the zeolite made from
CaX in addition to a carbon template of the zeolite formed from
acetylene: CaX-1023A2. As shown in FIGS. 9 and 10 and given in
Table 2, the four carbon templates of the zeolite have dual
porosity (both meso- and micro-pores). The CaX zeolite only has a
microporous structure; therefore, the presence of mesopores,
especially at levels greater than about 0.40 cm.sup.3/g, indicates
a less-than-desirable negative replication of the zeolite. The
presence of mesopores indicates incomplete filing of the zeolite
micropores with the organic precursor, which leads to a more
poorly-defined replication of the pore structure.
TABLE-US-00002 TABLE 2 Pore structure and surface area properties
of several carbon templates of the zeolite using commercial-grade
sized ion-exhanged CaX zeolite. S.sub.BET.sup.[1]
V.sub.micro.sup.[2] V.sub.meso V.sub.total Sample (m.sup.2/g)
(cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) CaX-973P5 1915 0.75 0.34
1.09 CaX-973E6 1596 0.58 0.48 1.06 CaX-1073E6 1826 0.65 0.66 1.31
CaX-1023A2 2567 0.95 0.42 1.37 .sup.[1]Brunauer-Emmett-Teller (BET)
specific surface area. .sup.[2]Micropore volume determined using
the D-R equation.
[0125] In Table 2, the carbon template of the zeolite made from
CaX-1023A2 exhibits a greater surface area (2567 m.sup.2/g) than
the carbon template of the zeolite prepared using propylene (1900
m.sup.2/g) and ethanol (average 1792 m.sup.2/g). The carbon
template of the zeolite made from CaX-1023A2 shows the highest
total pore volume (1.37 cm.sup.3/g), the highest micropore volume
(0.95 cm.sup.3/g) and the highest micropore:mesopore volume ratio
of the four samples (2.26). The carbon template of the zeolite made
from CaX-973P5 has a similar micropore:mesopore volume ratio
(2.20).
[0126] Although not intending to be limited by theory, the data and
determinations shown in FIGS. 9 and 10 as well as Table 2 suggests
that the organic precursor--regardless of size--cannot diffuse into
the zeolite micropores greater than a certain amount the first time
it is introduced. There is a finite volume for each pore in the
zeolite that can take a limited amount of organic precursor
molecules. To maximize the amount of carbon present in a micropore
and to provide for a better characterization of the surface of the
zeolite (both in overall surface area and micropore volume), the
organic precursor should have both a small kinetic diameter to
maximize the number of molecules in the micropores as well as a
high ratio of carbon to other atoms (hydrogen, oxygen) such that
the amount of carbon atoms in each micropore of the zeolite is
maximized during CVD.
[0127] The results obtained appear to indicate that acetylene is
the best of the three organic precursors followed closely by
propylene. The carbon templatesof the zeolite synthesized using
acetylene at 1023 K for 2 hours shows a relatively high BET surface
area (2567 m.sup.2/g) and large micropore volume (about 1
cm.sup.3/g). Acetylene does have a smaller kinetic diameter (0.33
nm) to methane (0.38 nm). Cetylene has an optimum carbon: hydrogen
ratio (1:1) versus propylene (1:2) and ethanol (1:3 w/1 oxygen) and
its molecular shape is linear versus having non-linear bond angles
as propylene and ethanol, which makes their kinetic diameter
greater.
[0128] Introducing Acetylene Organic Precursor to Large CaX
Zeolites
[0129] Acetylene should be able to occupy any micropore that
methane can adsorb into; however, mesopores still formed in the
carbon templates of the zeolite formed from CaX-1023A2. In
addition, the use of large X zeolite templates may require longer
diffusion times through the zeolite. Large amounts of crystalline
zeolites (>1 g), whether small or large, may require techniques
to maximize the opportunity of diffusion into each zeolite with
zeolite particles contacting one another and inhibiting points of
vapor access into each structure. The use of greater CVD
temperatures (1023 K) may cause premature deposition of carbon by
acetylene before full diffusion into the sacrificial zeolite. The
triple bond between the two carbon atoms of acetylene already
contains a significant amount of bond energy that is fairly easy to
release and promote reaction relative to double-bonded compounds.
In combination with a large crystal zeolite or a bed of smaller
zeolites packed together, there may not be an adequate diffusion
period at the greater CVD temperatures to support the formation of
the carbon negative replica of the crystalline zeolite.
[0130] A method for introducing and carbonizing acetylene at a
lower CVD temperature and then thermally-treating the deposited
carbon at a temperature higher than the CVD temperature but lower
than a temperature where graphitizing occurs (+2000.degree. C.)
increases the density of the deposited carbon by converting loose
carbon into an interconnected carbon matrix) within the micropores
and on the surface of the sacrificial zeolite before the zeolite is
removed. Reduced temperature acetylene CVD (.ltoreq.873 K) deposits
the carbon within the zeolite, forming the carbon templates of the
zeolite. At a lower CVD temperature--less than 1000 K, and less
than 900 K--the carbon deposition should occur more uniformly than
at greater CVD temperatures by preventing carbonization before
penetration throughout the sacrificial zeolite. Heat treating the
carbon templates of the zeolite at a greater temperature (about
1123 K) in a non-reactive gas atmosphere dehydrogenates the
deposited carbon within the carbon-zeolite composite and increases
the amount of carbon-carbon bonding, forming a stronger and denser
composite structure of the thermally-treated carbon template of the
zeolite.
[0131] For this experiment, the following designation code
indicates the process used for manufacturing each carbon template
of the zeolite and methane microporious carbon adsorbent: "zeolite
template-CVD temperature-CVD time heat treatment", where "zeolite
template" is the ion used as part of the template zeolite (Na, Ca).
"CVD temperature" is in K for the four-hour period of organic
precursor introduction. "CVD time heat treatment" indicates the
length of post-CVD heat treatment at 1123 K in hours. If a second
"H" is present, this indicates that the organic precursor addition
and post-CVD heat treatment are repeated. If an "L" is present
before "zeolite template", that indicates that the zeolite template
is a large-crystal X zeolite synthesized with TEA (n=3) as
previously described instead of using the commercial-grade sized
(1-2 .mu.m) NaX or the similar sized ion-exchanged CaX zeolite. For
example, "LCaX-873-4H" indicates the methane microporous carbon
adsorbent is a synthesized using a large CaX zeolite with acetylene
at a CVD temperature of 873 K, a CVD period of 4 hours and is then
post-CVD thermal treatment at 1123 K for four hours.
"LCaX-873-4H4H" sample is a similarly synthesized methane
microporous carbon adsorbent, but the acetylene CVD application
temperature and period as well as the post-CVD heat treatment are
repeated a second time at similar conditions.
[0132] Table 3 shows structural properties of several carbon
templates of the zeolite and methane microporous carbon adsorbents
manufactured using large CaX zeolites under several different CVD
and post-CVD thermal treatments. Acetylene is the organic precursor
for all tests.
TABLE-US-00003 TABLE 3 Pore structure and surface area properties
of several carbon templates of the zeolite and methane microporous
carbon adsorbents formed using large-crystal ion- exchanged CaX
(LCaX) zeolite. S.sub.BET.sup.[1] V.sub.micro.sup.[2] V.sub.meso
V.sub.total Entry Sample (m.sup.2/g) (cm.sup.3/g) (cm.sup.3/g)
(cm.sup.3/g) 1 LCaX-1023-2.sup.[3] 2462 0.92 0.30 1.22 2
LCaX-1023-2.sup.[4] 2156 0.83 0.43 1.26 3 LCaX-973-3.sup.[3] 2381
0.93 0.31 1.24 4 LCaX-873-4.sup.[3] 841 0.33 0.12 0.45 5
LCaX-873-4H.sup.[3] 3049 1.12 0.45 1.57 6 LCaX-873-4H4H.sup.[3]
2830 1.10 0.23 1.33 7 LCaX-873-4H4H.sup.[4] 2840 1.12 0.21 1.33 8
LCaX-823-9H4H.sup.[4] 2950 1.17 0.18 1.35
.sup.[1]Brunauer-Emmett-Teller (BET) specific surface area.
.sup.[2]Micropore volume (V.sub.micro) calculated using D-R
equation. .sup.[3]1 gram zeolite used for acetylene CVD. .sup.[4]5
gram zeolite used for acetylene CVD.
[0133] Samples numbered 1, 3 and 4 in Table 3 show several
interesting effects on the produced methane microporous carbon
adsorbents that may have an impact upon commercial production of
methane microporous carbon adsorbents using large crystal zeolites.
The three aforementioned carbon templates of the zeolite indicate
that a relatively greater CVD temperature is useful in obtaining
both a greater overall BET specific surface area and a micropore
volume than lower CVD temperatures. Sample number 4 (LCaX-873-4)
carbon templates of the zeolite has a reduced BET surface area and
microporosity compared to samples 1 and 3 even with an additional
amount of CVD period (4 hours versus 2 or 3). Although not wanting
to be limited by theory, it is believed that the carbon template of
the zeolite formed using LCaX-873-4 did not sufficiently
interconnect at the CVD temperature of 873 K during the 4 hour CVD
period. This indicates that the zeolite micropores are fully filled
with deposited carbons that have some bonding but not with a
significantly interlaced 3-dimensional (3-D) structure. Upon
removal of the sacrificial zeolite using the aqueous strong mineral
acid mixture, the resultant carbon template of the zeolite
structure collapsed and was otherwise unusable as a structured
adsorbent.
[0134] Performing the same operation and adding a post-CVD thermal
treatment for four hours under a helium atmosphere at 1123 K before
removal of the zeolite framework (LCaX-873-4H, sample number 5)
improves not only the surface area of the methane microporous
carbon adsorbent over the carbon template of the zeolite by a
factor of 3.6 but also increases the micropore volume by a factor
of 3.4 versus sample number 4. These findings were unexpected and
further explored as disclosed.
[0135] Table 3 shows that additional post-CVD thermal treatment of
the carbon-zeolite composite, either through a post-CVD heat
treatment (sample number 5) or a secondary CVD treatment with
another post-CVD heat treatment (sample numbers 6-9) when using a
reduced CVD temperature (<900 K), provides a highly microporous
structure in the methane microporous carbon adsorbent that has
adequate structural integrity for removal of the sacrificial large
crystal zeolite without collapsing.
[0136] Comparing the results of methane microporous carbon
adsorbent sample numbers 6-8 with methane microporous carbon
adsorbent sample number 5 from Table 3, there is a reduction in the
mesopore volume for sample numbers 6-8 while comparatively
maintaining the BET specific surface area and micropore volume.
Sample numbers 6-8 have a micropore:mesopore volume ratio in a
range of from about 4.7 to about 6.5, which is an improvement over
the volume ratio of about 2.5 for sample number 5. Sample number 6
(LCaX-873-4H4H) has a reduced mesopore volume (0.23 cm.sup.3/g)
compared to sample number 5 (LCaX-873-4H; 0.45 cm.sup.3/g) just
with the performance of a second acetylene CVD/post-CVD thermal
treatment cycle before removing the zeolite template.
[0137] Methane microporous carbon adsorbent sample number 5,
LCaX-873-4H, has a greater surface area (3049 m.sup.2/g), micropore
volume (1.12 cm.sup.2/g) and micropore:mesopore volume ratio (2.49)
than that of carbon template of the zeolite sample number 1
(LCaX-1023-2). Comparatively, this indicates that the reduction of
the CVD temperature, lengthening the CVD period and applying a
post-CVD thermal treatment results in an improved negative replica
of the large zeolite. The methane microporous carbon adsorbent of
LCaX-873-4H shows the greatest total pore volume (1.57 cm.sup.3/g)
of all the samples.
[0138] The result indicates that incomplete filling of zeolite
micropores with the organic precursor before the carbon is
thermally deposited leads to the formation of mesopores in the
carbon templates of the zeolite. The sequential carbon synthesis
method allows a reliable means of producing and reproducing methane
microporous carbon adsorbents regardless of the zeolite amount
(that is, bed thickness) used. Compare sample numbers 6 and 7,
which use 1 gram and 5 grams of material, respectively.
[0139] Decreasing the acetylene CVD temperature to 823 K and
increasing the first CVD period, a methane microporous carbon
adsorbent with slightly enhanced BET surface area and micropore
volume is synthesized (sample number 8; LCaX-823-9H4H). Methane
microporous carbon adsorbent sample numbers 5-8 indicate that a CVD
temperature in in a range of from about 800 K to about 900 K
provides an appropriate combination of both dispersion of acetylene
and carbonization not only into small amounts of the large CaX
zeolites but also into layered beds of the sacrificial zeolites
(sample numbers 7 and 8). Lengthening the CVD period within the
limited lower temperature range appears to improve the BET specific
surface and the micropore:mesopore ratio. Although not intending to
be limited by theory, it is believed that the acetylene more
thoroughly penetrates into the pore structure and forming the first
carbon template of the zeolite before the first thermal treatment
cycle. At a CVD temperature less than 773 K using acetylene, the
carbon template of the zeolite forms within the carbon-zeolite
composite, but the process requires a CVD period that is not
practical for commercial methane microporous carbon adsorbent
production.
[0140] FIG. 11 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the LCaX zeolite, a
carbon-zeolite composite, thermally-treated carbon-zeolite
composites, and the resultant methane microporous carbon adsorbent.
Isotherms for the LCaX zeolite, the carbon-zeolite composite for
Sample 4, the thermally-treated carbon-zeolite for Sample 5, and
the methane microporous carbon adsorbent that is Sample 6 are
represented. The nitrogen isotherms are performed at different
points along the carbonization and thermal treatment process.
Except for a pristine large CaX zeolite, that is, prior to carbon
deposition using acetylene, the carbon template of the
zeolite--both pre- and post-thermal treatment--is maintained within
the zeolite; the zeolite is not removed during isothermal
testing.
[0141] FIG. 11 shows pristine LCaX zeolite having the greatest
adsorbed amount, but this is obvious as it has fully-open zeolite
pores. Sample number 4 (LCaX-873-4) carbon-zeolite composite shows
that after a CVD period of 4 hours at a CVD temperature of 873 K
using acetylene, the adsorption capacity of the carbon-zeolite
composite is reduced by about 80% (measuring at P/P.sub.0=1). The
adsorption amount drops because the pores of the zeolite are
clogged with deposition carbon in the carbon-zeolite composite.
Sample number 5 (LCaX-873-4H) is a thermally-treated carbon-zeolite
composite that is thermal treated at 1123 K for four hours under a
helium atmosphere. The process appears to have regenerated about
25% of the LCaX zeolite micropore volume. The thermal treatment
also appears to have increased the density of the carbon template
of the zeolite within the thermally-treated carbon-zeolite
composite. The thermally-treated carbon template of the zeolite
forms a more interlinked carbon-carbon structure, which causes the
carbon network within the zeolite to shrink. This permits more
nitrogen to penetrate into the thermally-treated carbon-zeolite
template (LCaX-873-4H).
[0142] Because many of the zeolite micropores are regenerated after
the post-CVD thermal treatment (the deposited carbon dehydrogenates
and the network of interlaced carbons physically shrinks as
carbon-carbon bonding becomes more prevalent), a second acetylene
CVD/post-CVD thermal treatment cycle penetrates the carbon-zeolite
composite and fills the newly exposed and remaining micropores.
After performing the second acetylene CVD/post-CVD thermal
treatment cycle, the micropores of the carbon-zeolite composite
sample number 6 (LCaX-873-4H4H) are almost filled with the
thermally-treated carbon template of the zeolite. Using the data
previously presented in Table 3, the micropore:mesopore volume
ratio is greater than 4 for LCaX-873-4H4H.
[0143] FIGS. 12-14 show analysis of two types of the methane
microporous carbon adsorbents and two carbon templates of the
zeolite using LCaX as the zeolite template and acetylene as the
organic precursor. Each is made using 5 grams of the LCaX zeolite.
FIG. 12 is a graph showing traces of nitrogen adsorption-desorption
isotherms for carbon templates of the zeolite made from LCaX-1023-2
and LCaX-873-4 and two methane microporous carbon adsorbents made
from LCaX-873-4H and LCaX-873-4H4H. FIG. 13 is a graph showing
traces of the determined pore size distribution using the non-local
density function theory (NLDFT) algorithm on the nitrogen
adsorption-desorption isotherm data shown in FIG. 12. Note that for
the sake of clarity in FIG. 13 that the individual traces of carbon
template of the zeolite using LCaX-873-4, and the methane
microporous carbon adsorbents LCaX-873-4H and LCaX-873-4H4H, are
offset by a fixed value of dV.sub.p in cm.sup.3/g. In reality, all
four traces have a similar value at W=0 nm. The trace for
LCaX-873-4 is offset by 0.30 cm.sup.3/g; the trace for LCaX-873-4H
is offset by 0.43 cm.sup.3/g; the trace for LCaX-873-4H4H is offset
by 0.85 cm.sup.3/g. FIG. 14 is a graph showing traces of the XRD
analysis for carbon templates of the zeolite made from LCaX-1023-2
and LCaX-873-4 and methane microporous carbon adsorbents made from
LCaX-873-4H and LCaX-873-4H4H. Note that for the sake of clarity in
FIG. 14 that the individual traces of LCaX-873-4, LCaX-873-4H and
LCaX-873-4H4H offset by a fixed value of Intensity in CPS. In
reality, all three traces have a similar value at 20=0. The trace
for LCaX-873-4 is offset by 30000 CPS; the trace for LCaX-873-4H is
offset by 50000 CPS; the trace for LCaX-873-4H4H is offset by 72000
CPS.
[0144] The most precise negative replica of the LCaX zeolite
structure (LCaX-873-4H4H) appears to show a classic Type I isotherm
and nearing saturation at a reduced nitrogen partial pressure
(P/P.sub.0>0.1) in FIG. 12. The LCaX zeolite with no deposited
carbon shows a similar Type I isotherm curve in FIG. 11. The
methane microporous carbon adsorbent formed by LCaX-873-4H, which
is synthesized in a single cycle of acetylene CVD/post-CVD thermal
treatment, shows a comparatively greater total pore volume than the
double-cycled LCaX-873-4H4H methane microporous carbon adsorbent in
FIG. 12. FIG. 12 shows that a large amount of adsorption for the
LCaX-873-4H methane microporous carbon adsorbent occurs in a
partial pressure range of P/P.sub.0>0.1. FIG. 13 confirms that
the increased adsorption amount by LCaX-873-4H methane microporous
carbon adsorbent is due to the presence of additional pore volume
in the mesopore range (the rounded hump that spreads along the
track at values>2 nm, which indicates pore sizes outside of the
micropore range)
[0145] The methane microporous carbon adsorbents (LCaX-873-4H and
LCaX-873-4H4H) show in FIG. 13 a large spike (narrow, intense) in
pore size distribution in the micropore regime (W<2 nm). FIG. 14
shows that the carbon templates of the zeolite and methane
microporous carbon adsorbents that performed well either have a
great CVD temperature and no post-CVD treatment (LCaX-1023-2) or
have at least one post-CVD thermal treatment cycle (LCaX-873-4H,
LCaX-873-4H4H) show a response in intensity at about
2.theta.=6.3.degree. in the XRD trace. The methane microporous
carbon adsorbent from LCaX-873-4H4H shows a very sharp peak at this
value. This indicates that the adsorbent has an ordered microporous
structure very similar to the template zeolite (see FIG. 5 for the
large NaX zeolite having a TEA (n=3); FIG. 8 for a similar bump for
the commercial-grade sized ion-exchanged CaX replicas). The
presence of the sharp XRD intensity peak at 20=6.3.degree. is
useful for indicating the precision of the negative replication of
the sacrificial zeolite structure (that is, efficiency of carbon
deposition and thermal treatment).
[0146] Forming Methane Microporous Carbon Adsorbents from
Commercial BEA and Commercial-Grade Sized CaX Zeolites
[0147] A commercial BEA zeolite is obtained from Zeolyst Int'l
(Conshohocken, PA) having a Si:Al molar ratio of about 19. The BEA
zeolite is a round-shaped particle with a size distribution in a
range of from about 500 nm to about 1 .mu.m. A CaX zeolite
(commercial-grade sized (1-2 .mu.m) Ca.sup.+2 ion-exchanged NaX
zeolite) is also used and is manufactured as previously described.
Each zeolite goes through a similar sequential carbon synthesis
method: a first CVD process using acetylene at a CVD temperature of
823 K for a first CVD period of 9 hours, a first post-CVD thermal
treatment in a helium atmosphere at 1123 K for four hours, a second
CVD with acetylene at a CVD temperature of 823 K for a second CVD
period of 4 hours, a second post-CVD thermal treatment in a helium
atmosphere at 1123 K for four hours. The sacrificial zeolite
frameworks are etched away in several aqueous strong mineral acid
mixture washes. The resultant methane microporous carbon adsorbents
are recovered. Testing on the methane microporous carbon adsorbents
are presented in Table 4.
TABLE-US-00004 TABLE 4 Pore structure and surface area properties
of the methane microporous carbon adsorbents formed from BEA and
commercial-grade sized CaX zeolites. S.sub.BET.sup.[1]
V.sub.micro.sup.[2] V.sub.meso V.sub.total Sample (m.sup.2/g)
(cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) CaX-823-9H4H 2933 1.18 0.28
1.46 BEA-823-9H4H 2940 1.19 0.31 1.50
.sup.[1]Brunauer-Emmett-Teller (BET) specific surface area.
.sup.[2]Micropore volume (V.sub.micro) calculated using D-R
equation.
[0148] FIG. 15 shows SEMs of (a) BEA crystalline zeolite and (b)
methane microporous carbon adsorbents made using the BEA zeolite.
(a) of FIG. 15 is a SEM of the commercially-obtained BEA zeolite.
(b) of FIG. 15 is a SEM of the methane microporous carbon
adsorbents made using the commercially-obtained BEA zeolite of SEM
(a).
[0149] Both methane microporous carbon adsorbents given Table 4
show high BET specific surface area (about 3000 m.sup.2/g) as well
as micropore volume (about 1.2 cm.sup.3/g). The methane microporous
carbon adsorbent formed from CaX has a micropore:mesopore volume
ratio of 4.21. The methane microporous carbon adsorbent formed from
BEA has a micropore:mesopore volume ratio of 3.84.
[0150] FIGS. 16 and 17 show analysis of both types of methane
microporous carbon adsorbent given in Table 4. FIG. 16 is a graph
showing traces of nitrogen adsorption-desorption isotherms for the
methane microporous carbon adsorbent made from CaX and BEA
zeolites. Both materials show Type I N.sub.2 adsorption-desorption
isotherms. FIG. 17 is a graph showing traces of the XRD analysis
for the methane microporous carbon adsorbent made from the CaX and
BEA zeolites. Note that for the sake of clarity in FIG. 17 that the
individual trace of CaX-823-9H4H is offset by a fixed value of
Intensity in CPS. In reality, the traces have a similar value at
2.theta.=0. The trace for CaX-823-9H4H is offset by 40000 CPS. FIG.
17 shows very sharp XRD peak in the low angle regime
(2.theta.<10.degree.). Both FIGS. 17 and 18 as well as the ratio
of micropore:mesopore volume each indicate that the methane
microporous carbon adsorbents are negative replicas that closely
resemble each of their sacrificial zeolite in ordered micropore
structure.
[0151] FIG. 18 is a graph showing traces of the determined pore
size distribution using the NLDFT algorithm for four methane
microporous carbon adsorbents formed using two acetylene
CVD/post-CVD thermal treatment cycles. Note that for the sake of
clarity in FIG. 18 that the individual traces for CaX-823-9H4H,
CaX-873-4H4H and LCaX-823-9H4H are offset by a fixed value of
dV.sub.p in cm.sup.3/g. In reality, all four traces have a similar
value at W=0 nm. The trace for CaX-823-9H4H is offset by 0.22
cm.sup.3/g; the trace for CaX-873-4H4H is offset by 0.50
cm.sup.3/g; the trace for LCaX-823-9H4H is offset by 0.90
cm.sup.3/g. FIG. 18 shows four different negative replicas using
three different sacrificial zeolites, two different CVD
temperatures, and two different first CVD periods. FIG. 18
demonstrates is that all four methods--even with different small
pore zeolites, CVD temperature and CVD period--show a very narrow
pore size distribution in the micropore range. FIG. 18 also shows
that the mesopore pore range for these methane microporous carbon
adsorbent is relatively insignificant.
[0152] The results of this experiment indicate that other
crystalline zeolite structures may be used as sacrificial templates
for forming the methane microporous carbon adsorbent. The
experiments have shown that NaX, CaX and BEA zeolites are useful in
forming the methane microporous carbon adsorbent. FAU, which
include the commercial-grade sized NaX, the large NaX (LNaX), the
large and commercial-grade sized ion-exchanged NaX (CaX and LCaX),
and NaY; EMT, which is similar to FAU; and BEA zeolite structures
are all 12-membered ring structures and have 3-dimentional pore
connectivity, which are suitable to act as the framework for
forming the 3-dimentional negative replica. In an embodiment of the
method, the crystalline zeolite is selected from the group
consisting of FAU, EMT and BEA zeolite structures. In an embodiment
of the methane microporous carbon adsorbent, the shape is in the
form of the negative replica of a crystalline zeolite that is
selected from the group consisting of FAU, EMT, BEA zeolite
structures, and combinations of the zeolite structures.
[0153] Comparative Methane Adsorption for Several Carbon
Adsorbents
[0154] Wang provides a description of the testing procedures and
the apparatus for determining the gravimetric basis for adsorption
isotherms. FIG. 19 is a graph showing several traces of the methane
adsorption isotherms on a gravimetric basis for a carbon template
of the zeolite and several methane microporous carbon adsorbents at
298 K. Each carbon template of the zeolite and the methane
microporous carbon adsorbents are manufactured using the code
provided in FIG. 19 described supra. The evaluation pressure range
is from about 0 to about 40 bar. The evaluation temperature is
maintained at 298 K. The value provided at the end of each isotherm
trace is the "CH.sub.4 stored" value. As shown in FIG. 19, the
methane microporous carbon adsorbent from CaX-823-9H4H, which does
not use large crystal zeolite, has the greatest methane stored
value on a weight basis. The greater CVD temperature with no
post-thermal treatment carbon template of the
zeolite--CaX-1023-2--has a relatively reduced methane stored value
at 40 bar pressure compared to the methane microporous carbon
adsorbents.
[0155] Table 5 shows the storage properties of the carbon template
of the zeolite, the five methane microporous carbon adsorbents, and
two known commercial activated carbon adsorbents. "Maxsorb.RTM.
3000" (Kansai Coke and Chemicals Co., Ltd; Japan) is a carbon
material (about 3000 m.sup.2/g) that is activated by exposure to a
solution of potassium hydroxide (KOH). "SRD-08016" is an activated
powdered carbon material supplied from Chemviron Carbon (Feluy,
Belgium).
TABLE-US-00005 TABLE 5 Pore structure and surface area properties
as well as determined methane adsorption properties of several
commercial activated carbon materials, a carbon template of a
zeolite, and several methane microporous carbon adsorbents.
CH.sub.4 CH.sub.4 CH.sub.4 CH.sub.4 CH.sub.4 S.sub.BET V.sub.micro
stored retained 1 deliv. deliv. deliv. Sample (m.sup.2/g)
(cm.sup.3/g) (mg/g) bar (wt. %) (mg/g) (v/v).sup.[1] (v/v).sup.[2]
Maxsorb .RTM. 3000 3180 1.31 180 10 162 102 89 SRD-08016 1840 0.74
124 16 104 86 76 CaX-1023-2 2567 0.95 161 12 142 103 71
CaX-823-9H4H 2933 1.15 192 11 171 104 73 CaX-873-4H4H 2631 1.06 172
11 152 109 67 LCaX-823-9H4H 2950 1.17 180 11 160 122 103
LCaX-873-4H 3049 1.12 174 12 153 105 73 LCaX-873-4H4H 2840 1.10 184
11 164 123 103 .sup.[1]Delivered CH.sub.4 amount calculated based
on packing density. .sup.[2]Delivered CH.sub.4 amount calculated
based on tap density.
[0156] FIG. 20 shows SEMs (a)-(b) of methane microporous carbon
adsorbents made using calcium-ion substituted X zeolites. (a) of
FIG. 20 is a SEM of methane microporous carbon adsorbents made
using a NaX calcium-ion substituted zeolite (CaX). (b) of FIG. 20
is a SEM of methane microporous carbon adsorbents made using an
LNaX calcium-ion substituted zeolite (LCaX). In an embodiment of
the adsorbent, the shape is orthogonal with a mid-edge length in a
range of 8 um to 20 um. As both SEMs (a)-(b) of FIG. 20 show, the
ion-exchange of Ca.sup.2+in the NaX and LNaX zeolites did not
affect the octahedral particle morphology of the resultant methane
microporous carbon adsorbents formed from CaX and LCaX. Instead, it
appears that on the "macro" level to be a negative replica of the
sacrificial crystalline zeolite.
[0157] Table 5 shows that Maxsorb the carbon template of the
zeolite, and the five methane microporous carbon adsorbents retain
an amount of methane in a range of from about 10 wt. % to about 12
wt. % residual amount of CH.sub.4 at 1 bar. The "delivered
CH.sub.4" amount represents the amount of methane that is adsorbed
and released between cycles of 1 bar and 40 bar, and is determined
by subtracting the adsorption amount detected at 1 bar from the
adsorption amount detected at 40 bar.
[0158] Both packing and tap densities are used for calculating the
volumetric CH.sub.4 adsorption amounts of the methane microporous
carbon adsorbents. The five methane microporous carbon adsorbents
and the carbon template of a zeolite given in Table 5 show similar
methane adsorption amounts on a gravimetric basis (FIG. 19), but
the determined volumetric values are different due to deviations in
packing and tap densities. Deviations appear more significant for
tap density. Although not intending to be limited by theory, the
methane microporous carbon adsorbents forming from the large
sacrificial zeolite particles (LCaX series) show greater overall
methane adsorption volume capacity than the materials created from
the smaller sacrificial zeolite particles (CaX series). On a gas
volume basis, therefore, the LCaX formed methane microporous carbon
adsorbents that provide greater methane adsorption even at a
reduced volume density than the CaX formed methane microporous
carbon adsorbents and the carbon template of the zeolite.
[0159] Methane microporous carbon adsorbents manufactured using
LCaX-823-9H4H and LCaX-873-4H4H according to Table 5 have a greater
methane adsorption volume capacity in a range of from about 10 vol.
% to about a 20 vol. % on either a packing density or a tap density
basis compared to Maxsorb.RTM. 3000.
[0160] Forming Negative Carbon Replicas of Commercial NaX
Zeolite
[0161] Table 1 shows that a commercial-grade NaX zeolite has a
lower thermal stability than an ion-exchanged CaX zeolite
(T.sub.init), and the discussion regarding Table 1 indicates that
NaX zeolites may be unsuitable for forming methane microporous
carbon adsorbents. Using a sequential carbon synthesis method
having a CVD temperature that is less than 900 K, however, provides
an opportunity to reexamine this assumption. Table 6 shows two
methane microporous carbon adsorbents: One made with CaX zeolite
and one made with NaX zeolite.
TABLE-US-00006 TABLE 6 Pore structure and surface area properties
of two methane microporous carbon adsorbents formed from
commercial-grade sized NaX and CaX zeolites. S.sub.BET.sup.[1]
V.sub.micro.sup.[2] V.sub.meso V.sub.total Sample (m.sup.2/g)
(cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) CaX-823-9H4H 2933 1.18 0.28
1.46 NaX-823-4H2H 2974 1.18 0.23 1.41
.sup.[1]Brunauer-Emmett-Teller (BET) specific surface area.
.sup.[2]Micropore volume (V.sub.micro) calculated using D-R
equation.
[0162] The post-CVD thermal treatment is performed twice for four
hours at 1123 K on the NaX zeolite. Although not wanting to be
limited by theory, it is believed that the deposited carbon
structure within the NaX zeolite after CVD has sufficient strength
to support the carbon-NaX zeolite composite even during the thermal
treatment post-CVD process such that the NaX zeolite framework
remains intact and does not degrade. The connected
thermally-treated carbons in and between the micropores of the NaX
zeolite internally stabilizes the zeolite structure while the
carbon becomes denser during the post-CVD treatment process. Table
6 shows that the methane microporous carbon adsorbent from the NaX
zeolite is very close to the 3000 m.sup.2/g BET specific surface
area value that one of ordinary skill in the art may describes as a
"super adsorbent" (.gtoreq.3000 m.sup.2/g).
[0163] Based upon the data presented in Tables 1-6 and FIGS. 2-22,
in an embodiment of the methane microporous carbon adsorbent the
BET specific surface area is in a range of from about 2500
m.sup.2/g to about 3100 m.sup.2/g. In an embodiment of the
adsorbent, the micropore volume is in a range of from 0.95
cm.sup.3/g to 1.19 cm.sup.3/g as determined by the
Dubinin-Radushkevich equation. In an embodiment of the adsorbent,
the micropore to mesopore volume ratio is in a range of from 4 to
6. In an embodiment of the adsorbent, the stored methane value is
in a range of from 172 mg/g to 192 mg/g. In an embodiment of the
adsorbent, the methane delivered value is a range of from 152 mg/g
to 171 mg/g in a pressure range from 1 bar to 40 bar.
[0164] FIGS. 21 and 22 show analysis of both types of methane
microporous carbon adsorbents given in Table 6. FIG. 21 is a graph
showing traces of nitrogen adsorption-desorption isotherms for the
methane microporous carbon adsorbents formed from CaX and NaX
zeolites. Both methane microporous carbon adsorbents show Type I
N.sub.2 adsorption-desorption isotherms. FIG. 22 is a graph showing
traces of the determined pore size distribution using the non-local
density function theory (NLDFT) algorithm on the nitrogen
adsorption-desorption isotherm data shown in FIG. 21. Note that for
the sake of clarity in FIG. 22 that the individual trace of
NaX-823-4H2H is offset by a fixed value of dV.sub.p in cm.sup.3/g.
In reality, the traces have the same value at W=0. FIG. The trace
for NaX-823-4H2H is offset by 0.25 cm.sup.3/g. In reality, all
traces have the same value at 20=0. FIG. 22 shows the methane
microporous carbon adsorbent made from the NaX zeolite having a
strong spike in the range of from about 1 nm to about 2 nm pore
width. FIG. The micropore:mesopore volume ratio for the methane
microporous carbon adsorbents using the NaX zeolite is about 5.13,
which is within the range of methane microporous carbon adsorbents
made from CaX and LCaX given in Table 3 and discussed supra.
[0165] FIG. 23 shows SEMs (a)-(b) of methane microporous carbon
adsorbents made using sodium X zeolites.(a) of FIG. 23 is a SEM of
methane microporous carbon adsorbents made using a commercial-grade
sized NaX zeolite. (b) of FIG. 23 is a SEM of methane microporous
carbon adsorbents made using a LNaX zeolite. The size difference of
the zeolite material does not affect the octahedral particle
morphology. In comparing (a)-(c) of FIG. 6 with (a)-(b) of FIG. 23,
one can observe that the change in size does not affect the
octahedral shape or the ability to adsorb the organic precursor for
forming the methane microporous carbon adsorbents.
[0166] Scaled Synthesis of Methane Microporous Carbon Adsorbents
using Commercial-Grade Size NaX zeolite
[0167] The rotary tubular furnace shown in FIG. 2 is used to
perform scaled synthesis of methane microporous carbon adsorbents
for amounts greater than 1-5 grams from commercial-grade NaX
zeolites. The size of the commercial-grade NaX zeolites is such
that the mid-edge length is about 2 About 50 grams of the
commercial-grade sized NaX zeolite and about 50 grams of cleansed
sea sand (washed in deionized water; particle sizes of about 15 to
about 20 mesh) are introduced into the cylindrical stainless-steel
container located in the center of the tubular furnace. The sea
sand is used to help solids mixing and to keep the NaX zeolites
from sticking together during carbon vapor deposition and thermal
treatment. The cylindrical container is purged with argon (a
non-reactive gas), the cylindrical container rotated and the
temperature within the cylindrical container ramped up to 823 K. As
the container rotates, the NaX zeolite and the sea sand particles
not only collide with one another but also the internal baffles of
the cylindrical container, causing the zeolite crystals to be
dropped through the atmosphere contained within the cylindrical
container every few seconds. Upon reaching the first CVD
temperature of 823 K, an organic precursor gas that is a mixture of
4 vol. % acetylene in argon (Ar) is introduced into the cylindrical
stainless-steel container at a flow rate of about 1000 mL/minute
and maintained for a first CVD period of about 7 hours. After the
first CVD period elapses, the introduced gas is switched from the
acetylene/Ar gas mixture to pure Ar, which is introduced at a rate
of 500 mL/minute. The temperature is ramped up to the post-CVD
thermal treatment temperature of 1123 K and maintained at the
thermal treatment conditions for about three hours. After three
hours of post-CVD thermal treatment, the cylindrical container is
permitted to cool partially down under Ar flow until reaching 823
K. Upon reaching the second CVD temperature of 823 K, the organic
gas mixture of 4 vol. % acetylene in Ar is reintroduced into the
cylindrical stainless-steel container at a flow rate of about 1000
mL/minute and maintained at that rate for a second CVD period of
about 4 hours. After the second CVD period elapses, the introduced
gas is switched from the organic precursor gas of acetylene/Ar
mixture to pure Ar, which is introduced at a rate of 500 mL/minute.
The temperature is ramped up to the second treatment temperature of
1123 K for the second post-CVD thermal treatment and maintained at
the treatment temperature for three hours. After three hours of the
second post-CVD thermal treatment, the cylindrical container is
permitted to cool down under Ar flow until reaching room
temperature. Upon reaching room temperature, the rotating cylinder
is halted and the thermally-treated carbon-zeolite composite is
collected. The thermally-treated carbon-zeolite composite is
separated from the sea sand using a sieve, and the NaX zeolite is
removed from the thermally-treated carbon-zeolite composite using
the aqueous strong mineral acid mixture containing HCl/HF
previously. The recovered methane microporous carbon adsorbents are
tested for surface and pore properties as well as for comparative
isothermal information.
TABLE-US-00007 TABLE 7 Pore structure and surface area properties
of two methane microporous carbon adsorbents formed from
commercial-grade sized NaX (1 gram) and scaled synthesis from
commercial- grade sized NaX (50 grams). S.sub.BET.sup.[1]
V.sub.micro.sup.[2] V.sub.meso V.sub.total Sample (m.sup.2/g)
(cm.sup.3/g) (cm.sup.3/g) (cm.sup.3/g) NaX-823-4H2H 2980 1.18 0.23
1.41 NaX-large scale synthesis 2810 1.04 0.39 1.43
Brunauer-Emmett-Teller (BET) specific surface area.
.sup.[2]Micropore volume (V.sub.micro) calculated using D-R
equation.
[0168] FIG. 24 is a graph showing traces of nitrogen
adsorption-desorption isotherms for the NaX and the mass produced
NaX methane microporous carbon adsorbents. As shown in Table 7, the
methane microporous carbon adsorbent synthesized in a scaled
synthesis process (50 grams of NaX zeolite) shows only a slight
reduction in surface area and micropore volume versus the methane
microporous carbon adsorbent synthesized using 1 g NaX zeolite in
the plug-flow reactor (NaX-823-4H2H).
* * * * *