U.S. patent application number 10/644249 was filed with the patent office on 2005-02-24 for metal loaded carbon filaments.
This patent application is currently assigned to ConocoPhillips Company. Invention is credited to Allison, Joe D., Ramani, Sriram, Rangarajan, Priya.
Application Number | 20050042163 10/644249 |
Document ID | / |
Family ID | 34194042 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050042163 |
Kind Code |
A1 |
Allison, Joe D. ; et
al. |
February 24, 2005 |
Metal loaded carbon filaments
Abstract
Metal loaded carbon filaments and a process for making the same
are provided. This process includes forming metal on carbon
filaments produced from at least one carbon-containing compound,
e.g., an alkane or an alkene. The metal may be formed on surfaces
of previously formed carbon filaments by, for example,
electroplating, impregnation, or chemical vapor deposition.
Alternatively, the carbon filaments and the metal may be formed
concurrently, resulting in the metal being incorporated in the
carbon filaments. An article of manufacture is also provided that
includes a carbon filament having metal disposed thereon. The
article of manufacture may be, for example, a high surface area
catalyst, an electronic element, and a composite material having
enhanced electrical properties.
Inventors: |
Allison, Joe D.; (Ponca
City, OK) ; Ramani, Sriram; (Ponca City, OK) ;
Rangarajan, Priya; (Ponca City, OK) |
Correspondence
Address: |
CONOCOPHILIPS COMPANY
P.O. BOX 2443
BARTLESVILLE
OK
74004
US
|
Assignee: |
ConocoPhillips Company
Houston
TX
|
Family ID: |
34194042 |
Appl. No.: |
10/644249 |
Filed: |
August 20, 2003 |
Current U.S.
Class: |
423/447.3 |
Current CPC
Class: |
D01F 11/14 20130101;
D01F 9/1273 20130101; D01F 11/121 20130101; B82Y 30/00 20130101;
D01F 9/1271 20130101; H01B 1/04 20130101; D01F 11/123 20130101;
D01F 11/127 20130101; D06M 11/83 20130101 |
Class at
Publication: |
423/447.3 |
International
Class: |
D01F 009/12 |
Claims
What is claimed is:
1. A process for producing metal loaded carbon filaments,
comprising depositing metal on carbon filaments produced from at
least one carbon-containing feed.
2. The process of claim 1 wherein the carbon-containing feed
comprises an alkane found in natural gas.
3. The process of claim 1 wherein the carbon-containing feed
comprises an alkene.
4. The process of claim 1 wherein the carbon filaments are produced
before the metal is deposited on the carbon filaments.
5. The process of claim 1 wherein the carbon filaments and the
metal are formed concurrently such that the metal is incorporated
into the carbon filaments.
6. The process of claim 1 wherein the metal is formed by
electroplating.
7. The process of claim 1 wherein the metal is formed by
impregnation.
8. The process of claim 7 wherein a metal precursor of the metal
comprises a hydroxide, an oxide, a nitrate, a chloride, an organic
moiety, or combinations thereof.
9. The process of claim 1 wherein the metal is formed by chemical
vapor deposition.
10. The process of claim 1 wherein the carbon filaments and the
metal are formed in a single reactor.
11. The process of claim 10 wherein a hydrocarbon and a volatile
organometallic compound are concurrently fed to the single
reactor.
12. The process of claim 10 wherein the volatile organometallic
compound comprises silicon, and wherein the volatile organometallic
compound is fed to the single reactor to form silicon coated carbon
filaments.
13. The process of claim 1 wherein the carbon filaments and the
metal are formed in different reactors.
14. A carbon-based structure comprising a carbon filament and metal
disposed on the carbon filament.
15. The carbon-based structure of claim 14 wherein the metal is
incorporated in the carbon filament.
16. The carbon-based structure of claim 14 wherein the metal is
positioned on an outside surface of the carbon filament.
17. The carbon-based structure of claim 14, being capable of
storing a carbon-containing gaseous compound.
18. The carbon-based structure of claim 14, being capable of
storing a hydrogen compound.
19. An article of manufacture comprising a carbon filament having
metal disposed thereon.
20. The article of manufacture of claim 19 wherein the metal is
incorporated in the carbon filament.
21. The article of manufacture of claim 19 wherein the metal is
positioned on an outside surface of the carbon filament.
22. The article of manufacture of claim 19 wherein the article of
manufacture is a catalyst.
23. The article of manufacture of claim 19 wherein the article of
manufacture is a Fischer-Tropsch catalyst.
24. The article of manufacture of claim 19 wherein the article of
manufacture is an oxidative dehydrogenation catalyst.
25. The article of manufacture of claim 19 wherein the article of
manufacture is an alcohol production catalyst.
26. The article of manufacture of claim 19 wherein the article of
manufacture is a composite material.
27. The article of manufacture of claim 19 wherein the article of
manufacture is an electrical element.
28. The article of manufacture of claim 19, being capable of
storing a gaseous compound.
29. The article of manufacture of claim 19, being capable of
storing hydrogen.
30. A carbon-based structure formed by the process of claim 1.
31. The carbon-based structure of claim 30, comprising at least 1
wt. % of the metal per total weight of the carbon filaments.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
FIELD OF THE INVENTION
[0004] This invention generally relates to the production of carbon
filaments. More specifically, the invention relates to metal loaded
carbon filaments and a process for making the same.
BACKGROUND OF THE INVENTION
[0005] Natural gas reserves have been found in remote areas where
it is uneconomical to develop the reserves due to the lack of local
markets for the gas and the high cost of transporting the gas to
distant markets. This high cost is often related to the extremely
low temperatures needed to liquefy the highly volatile gas during
transport. An alternative is to locally convert the natural gas to
products that can be transported more cost effectively.
[0006] Natural gas comprises several components, including alkanes,
i.e., saturated hydrocarbons (compounds containing hydrogen [H] and
carbon [C]) whose molecules contain carbon atoms linked together by
single bonds. The simplest alkanes are methane (CH.sub.4), ethane
(CH.sub.3CH.sub.3), and propane (CH.sub.3CH.sub.2CH.sub.3).
Exemplary products that natural gas can be used to produce are
carbon filaments, which are typically less than about 50 nanometers
(nm) in size. One process for forming carbon filaments involves
converting alkanes in natural gas to products such as alkenes (also
known as olefins) or carbon monoxide (CO), followed by converting
the alkenes and/or the CO to carbon filaments. Alkenes are
unsaturated hydrocarbons whose molecules contain one or more pairs
of carbon atoms linked together by a double bond. Generally,
alkenes are commonly represented by the chemical formula
CH.sub.2.dbd.CHR, where C is a carbon atom, H is a hydrogen atom,
and R is an atom or pendant molecular group of varying composition.
In the ODH process, alkanes are dehydrogenated in the presence of
oxygen (O.sub.2) and an ODH catalyst to form alkenes, CO, and
H.sub.2. The alkenes and/or the CO are then thermally decomposed in
the presence of a metal catalyst to form carbon filaments.
Producing carbon filaments in this manner depends upon an upstream
alkenes-generating process to supply the feed components for carbon
filament growth.
[0007] In contrast, another process for producing carbon filaments
involves converting alkanes in natural gas directly to carbon
filaments and thus avoids the costs associated with the
intermediate step of converting alkanes to alkenes and CO. The
direct conversion of alkanes to carbon filaments is also performed
using a metal catalyst.
[0008] Carbon filaments are known for their outstanding mechanical
properties such as having relatively high surface areas, aspect
ratios, and mechanical strength. Thus, researchers have found
useful applications for carbon filaments. For example, they are
commonly combined within a polymer matrix to form an engineered
composite material. However, the current number of applications of
conventional carbon filaments is limited. Therefore, a need exists
to develop carbon filaments with properties that allow them to be
used for a wide variety of new applications, such as conductive
materials or gas storage which are better served by
metal-containing carbon filaments.
SUMMARY OF THE INVENTION
[0009] According to an embodiment, a process for producing metal
loaded carbon filaments includes forming metal on carbon filaments
produced from at least one carbon-containing feed. The
carbon-containing feed may comprise an alkane, an alkene, carbon
monoxide (CO), or carbon dioxide (CO.sub.2). The metal may be
formed on surfaces of previously formed carbon filaments by, for
example, electroplating, impregnation, or chemical vapor
deposition. Alternatively, the carbon filaments and the metal may
be formed concurrently, resulting in the metal being incorporated
in the carbon filaments.
[0010] In another embodiment, a carbon-based structure comprises a
carbon filament and metal positioned on the carbon filament. In
particular, the metal may be incorporated in the carbon filament,
or alternatively, it may be positioned on an outside surface of the
carbon filament. The carbon-based structure is capable of storing
hydrogen and/or natural gas. In yet another embodiment, an article
of manufacture includes a carbon filament having metal disposed
thereon. The article of manufacture may be, for example, a catalyst
or an electrical element.
DESCRIPTION OF DRAWINGS
[0011] The invention, together with further advantages thereof, may
best be understood by reference to the following description taken
in conjunction with the accompanying drawings in which:
[0012] FIG. 1 is a process flow diagram of an embodiment, wherein
hydrocarbons found in natural gas are converted to carbon
filaments, followed by forming metal on the carbon filaments.
[0013] FIG. 2 is a process flow diagram of an alternative
embodiment to the embodiment shown in FIG. 1.
[0014] FIG. 3 is a process flow diagram of an embodiment, wherein
alkenes produced by oxidative dehydrogenation of hydrocarbons are
converted to carbon filaments, followed by forming metal on the
carbon filaments.
[0015] FIG. 4 is a process flow diagram of an alternative
embodiment to the embodiment shown in FIG. 3.
[0016] FIG. 5 depicts a SEM picture of pretreated carbon filaments
before metal modification.
[0017] FIG. 6 depicts a SEM picture of carbon filaments coated with
lithium oxide by impregnation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] In the embodiments shown in FIGS. 1-4, carbon filaments
produced from hydrocarbons are loaded with metal for use in various
applications. FIG. 1 depicts an embodiment in which the carbon
filaments are produced from alkanes recovered from a gas plant,
followed by loading metal on the carbon filaments in a separate
reactor. In an alternative embodiment to those shown in FIGS. 1 and
2, natural gas is fed directly to CF reactor 16 or CVD/CF reactor
30, respectively, without first being processed in a gas plant 12.
Referring to FIG. 1, the carbon filaments are first produced by
feeding a natural gas stream 10 comprising alkanes to a gas plant
12. Gas plant 12 includes a separator, e.g., a hydrocarbon splitter
for processing feed stream 10 into at least a methane fraction and
one or more additional fractions comprising ethane, propane, and
butanes and heavier hydrocarbons. Typically, feed stream 14
comprising a mixture of one or more of ethane, propane, and butanes
and heavier hydrocarbons is passed from gas plant 12 to a carbon
filament (CF) reactor 16, and the methane fraction is fed to
another process, for example a synthesis gas production process not
shown.
[0019] Within CF reactor 16, feed stream 14 contacts a CF catalyst,
i.e., any suitable catalyst for producing carbon filaments from
alkanes. As a result, the alkanes present in feed stream 14
decompose, thereby forming carbon filaments that may be, for
example, less than about 50 nm in diameter. Reaction products
produced in CF reactor 16 comprise carbon filaments, H.sub.2, and
unconverted hydrocarbons. The H.sub.2 produced in CF reactor 16 may
be recovered using any known separation technique such as membrane
separation. Carbon filaments are removed from CF reactor 16 via
product stream 18, and H.sub.2 is removed from CF reactor 16 via
by-product stream 20. Although not shown, by-product stream 20 can
be passed to processes that require H.sub.2, e.g., a
Fischer-Tropsch process, a hydrotreater, and a hydrocracker. The
unconverted hydrocarbons recovered from CF reactor 16 may be
further processed via a recycle stream (not shown) to the CF
reactor.
[0020] A nitrogen (N.sub.2) stream 13 and/or a H.sub.2 stream 15
may optionally be fed to CF reactor 16 to improve the heat
distribution and contact between the hydrocarbon gases and the CF
catalyst, and also to improve certain properties of the carbon
filament product. In this embodiment, the molar ratio of carbon to
H.sub.2 (C:H.sub.2) being fed to CF reactor 16 preferably ranges
from about 1:5 to about 1:0.1, more preferably from about 1:3 to
about 1:0.3 and most preferably from about 1:1 to about 1:0.5. The
molar ratio of carbon to N.sub.2 (C:N.sub.2) being fed to CF
reactor 16 preferably ranges from about 1:2 to about 1:0.1, more
preferably from about 1:1 to about 1:0.2, and most preferably from
about 1:0.5 to about 1:0.3.
[0021] The CF catalyst contained within CF reactor 16 may be a
metal catalyst, which is defined herein as comprising elemental
iron, nickel, cobalt, copper, or chromium; alloys comprising the
foregoing metals; oxides of the forgoing metals and alloys; and
combinations of the foregoing metals, alloys, and oxides. The CF
catalyst may be optimized to convert alkanes such as ethane and
propane into carbon filaments. Examples of catalysts that may be
employed in CF reactor 16 are metals such as nickel and cobalt and
commercially available alloys such as MONEL alloy 400 (Ni--Cu) and
NICHROME alloy (Ni--Cr). The CF catalyst may take the form of any
appropriate structure such as a wire, disk, gauze, mesh, sheet,
sphere, rod, or inert support coated with metal. Further, the CF
catalyst may be arranged in a fixed bed, or it may form a fluidized
bed within CF reactor 16.
[0022] The CF reactor 16 is configured to support the particular CF
catalyst being used and thus may be a fixed bed reactor or a
fluidized bed reactor. It is also configured to accommodate
harvesting of the carbon filaments upon completion of their growth
cycle and to provide for the removal of the carbon filaments from
the reactor vessel. The CF reactor 16 may be a continuous reactor,
allowing the CF process to operate continuously, or alternatively
it may be a batch reactor. A suitable continuous reactor is shown
in FIG. 6 of Tibbetts, Vapor Grown Carbon Fibers, NATO ASI Series
E: Applied Sciences, Vol. 177, pp. 78 (1989), which is incorporated
by reference herein in its entirety.
[0023] Within CF reactor 16, the alkanes are contacted with the CF
catalyst in a reaction zone that is maintained at
conversion-promoting conditions effective to produce carbon
filaments. Preferably, conversion-promoting conditions are the
optimum flowrate, gas preheat and/or catalyst temperature.
Depending on the catalyst arrangement, preheating feed stream 14
may be preferred over preheating the catalyst. The temperature of
the gases contacting the catalyst preferably ranges from about
350.degree. C. to about 1000.degree. C., more preferably ranges
from about 450.degree. C. to about 800.degree. C., and most
preferably ranges from about 550.degree. C. to about 700.degree. C.
The CF reactor 16 may be operated at atmospheric or slightly
elevated pressures. The Gas Hourly Space Velocity (GHSV) preferably
ranges from about 1,000 hr.sup.-1 to about 100,000 hr.sup.-1, more
preferably from about 5,000 hr.sup.-1 to about 50,000 hr.sup.-1 and
most preferably from about 10,000 hr.sup.-1 to about 30,000
hr.sup.-1. The Gas Hourly Space Velocity is defined as the volume
of reactants per reaction zone volume per hour. The volume of
reactant gases is determined at standard conditions of pressure
(101 kPa) and temperature (0.degree. C.). In the case where CF
rector 16 is a fluidized bed reactor, the reaction zone volume is
defined as the total reaction zone volume, i.e., the expanded bed
volume in a fluidized system which comprises less than 100%
catalyst. On the other hand, if CF reactor 16 is a fixed bed
reactor, the reaction zone volume is volume of the catalyst
bed.
[0024] In addition, product stream 18, which contains the carbon
filaments produced by CF reactor 16, is fed to a metal loading unit
22 to form metal on the carbon filaments. Metal loading unit 22 may
include any known process for loading metal on the carbon
filaments. The temperature of the metal loading process may be less
than about 800.degree. C., and is preferably less than about
400.degree. C., to ensure that the carbon filaments do not become
damaged by exposure to high temperatures. When the temperature of
metal loading unit 22 is greater than 400.degree. C., it is
desirable to keep the molecular oxygen concentration in metal
loading unit 22 preferably below 15 mole (mol) %, more preferably
below 5 mol %, and most preferably below 1 mol %, to minimize
carbon oxidation. When the temperature of metal loading unit 22 is
lower than 400.degree. C., then there is no need to maintain the
molecular oxygen concentration below a certain value in metal
loading unit 22. The type and the amount of metal loaded on the
carbon filaments may vary depending on the end use application of
the carbon filaments and would be obvious to one skilled in the
art. Examples of suitable metals that may be formed on the carbon
filaments include: precious metals such as platinum (Pt), palladium
(Pd), ruthenium (Ru), and rhodium (Rh); other transition metals
such iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo), and
copper (Cu); alkali metals such as lithium (Li); other metals such
as silicon (Si); and combinations thereof. Preferably, the carbon
filaments are loaded with at least about 1 weight (wt.) % metal per
total weight of the carbon filaments.
[0025] An example of a suitable metal loading process is
electroplating. To perform electroplating, carbon filaments may be
suspended in an aqueous or organic solution containing a metal salt
such as Cu(NO.sub.3).sub.2, followed by placing the solution in a
cell containing a suitable cathode such as mercury (Hg). As the
carbon filaments migrate to the cathode, they become charged such
that they become part of the cathode. A voltage lower than the
reduction potential of the metal is applied to the cathode to
maintain a driving force such that the metal salt deposits on the
carbon filaments and becomes reduced.
[0026] Alternatively, metal may be loaded on the surfaces of the
carbon filaments by chemical vapor deposition (CVD). In CVD, carbon
filaments are passed into a reaction chamber containing one or more
volatile metallic compounds, e.g., organometallic compounds such as
nickel carbonyl, tetra ethyl ortho silicate (TEOS), molybdenum
oxide, methyl lithium, or butyl lithium. When the metal precursor
is TEOS, silicon carbide nanofibers are formed on the carbon
filaments. Carbon filaments are preferably less than about 1,000
nanometers (nm) in size, more preferably from about 5 to 500 nm,
and most preferably from about 5 to 200 nm. The carbon filaments
have an aspect ratio of length over diameter that is preferably
greater than 5, more preferably in the range of from about 10 to
about 2,000 nm. The filament aggregates (also called bundles) are
preferably less than 1 millimeter in size. Caron filaments are also
called carbon fibrils or carbon nanotubes; they may be hollow or
filled nanotubes and may contain a discontinuous or continuous
carbon overcoat. As used herein, nanofibers refer to carbon
filaments ranging from about 5 nm to about 500 nm in diameter,
preferably from about 50 nm to about 200 nm in diameter.
[0027] The CVD reaction chamber is operated at any suitable
conditions for decomposing and condensing the organometallic
compounds, thereby forming metal radicals and/or ions that adsorb
on the surfaces of the carbon filaments. The configuration and
operation of CVD reactors are known in the art. The particular gas
flowrate, reactor temperature, and reactor pressure (typically
sub-atmospheric) employed can vary depending on, for example, the
type of metal being deposited. Additional disclosure regarding the
CVD process can be found in Campbell, Stephen A. The Science and
Engineering of Microelectronic Fabrication. New York: Oxford
University Press, 2001, which is incorporated by reference herein
in its entirety.
[0028] Yet another process that may be used to load metal on the
surfaces of the carbon filaments is wet impregnation. A wet
impregnation method that can be used to add metal to carbon
filaments comprises the following steps: a) dissolving at least one
metal precursor in a solvent to produce a metallic solution; b)
depositing said metallic solution onto carbon filaments; c)
optionally filtering the carbon filaments; d) drying the carbon
filaments at a temperature in the range of from about 80.degree. C.
to about 150.degree. C.; and e) heat-treating the dried carbon
filaments at a temperature below about 800.degree. C. The metal
precursor preferably comprises a metal ion and a counter-ion, such
as a hydroxide, a nitrate, an oxide, a chloride, or an organic
moiety. The preferred metal precursor comprises Group VIII metals,
more preferably Li, Pt, Pd, Ni, Cu, and Sn. The solvent is
preferably water or an organic medium such as methanol, acetone,
ethanol, or toluene. Depositing is preferably done by impregnation,
such as incipient wetness impregnation. The heat treatment step e)
is preferably performed in an inert atmosphere, such as an
atmosphere comprising nitrogen and argon.
[0029] A product stream 24 recovered from metal loading unit 22
contains carbon filaments coated with metal. If desired, the carbon
filaments may be passed back to CF reactor 16 (indicated by a
dashed line 26) to repeat the carbon growth and metal loading
processes. Multiple stages of metal loading and carbon growth can
be repeated as many times as desired to form multiple layers of
carbon and metal and to form branched carbon filaments. Since
deposited metals are growth points leading to multidimensional
carbon products of varying morphology, the number of stages and/or
recycles may be selected based on the morphologies necessary for
desired applications that require, for example, layered
metal-carbon structures.
[0030] FIG. 2 depicts another embodiment like that shown in FIG. 1
except that a single CVD/CF reactor 30 is used instead of separate
CF and CVD reactors. Alkanes from stream 14 and vaporized metallic
compounds (combined with stream 14 or fed separately) are passed
concurrently to CVD/CF reactor 30. As described previously, stream
14 may optionally contain small amounts of N.sub.2 and H.sub.2 in
the aforementioned ratios. Within CVD/CF reactor 30, the metal is
deposited on the carbon filaments as they are being formed such
that the metal becomes incorporated within the carbon matrix of the
filaments. Alternatively, a single compound, e.g., TEOS, may be fed
to CVD/CF reactor 30 to form metal carbide nanofibers such as
silicon carbide nanofibers.
[0031] The CVD/CF reactor 30 is configured to support both the CVD
and carbon growth processes and to accommodate removal of metal
filled carbon filaments from the reaction chamber via product
stream 32. The reaction chamber may be operated at conditions
appropriate for both the CVD process and the carbon growth process.
In particular, it may be operated at a temperature in the range of
from about 350.degree. C. to about 800.degree. C., more preferably
in the range of from about 450.degree. C. to about 750.degree. C.,
and most preferably in the range of from about 550.degree. C. to
about 700.degree. C.; and it may be operated at a pressure in the
range of from about 0 atm to about 20 atm (about 0 to 2,000 kPa),
more preferably in the range of from about 0 atm to about 10 atm
(about 0 to 1,000 kPa), and most preferably in the range of from
about 0 atm to about 5 atm (about 0 to 500 kPa). The GHSV
preferably ranges from about 1,000 hr.sup.-1 to about 100,000
hr.sup.-1, more preferably from about 5,000 hr.sup.-1 to about
50,000 hr.sup.-1 and most preferably from about 10,000 hr.sup.-1 to
about 30,000 hr.sup.-1. In addition to product stream 32, a stream
34 consisting essentially of H.sub.2 may also exit CVD/CF reactor
30.
[0032] According to yet another embodiment shown in FIG. 3, alkenes
formed via oxidative dehydrogenation (ODH), dehydrogenation,
thermal cracking, or combinations thereof may be converted to
carbon filaments, followed by loading the carbon filaments with
metal. A description of a suitable integrated ODH/CF process can be
found in copending U.S. patent application Ser. No. 10/288,710,
filed Nov. 05, 2002 and entitled "INTEGRATED OXIDATIVE
DEHYDROGENATION/CARBON FILAMENT PRODUCTION PROCESS AND REACTOR
THEREFOR," which is incorporated by reference herein in its
entirety. In addition, dehydrogenation and thermal cracking of
hydrocarbons are well known in the art. A suitable thermal cracking
process is disclosed in U.S. Pat. No. 5,925,799, which is
incorporated by reference herein in its entirety. A suitable
dehydrogenation process is the Oleflex.TM. process of UOP LLC of
Des Plaines, Ill., as described in Oleflex.TM. Process for
Propylene Production. 1998.
http://www.uop.com/techsheets/oleflex.pdf, which is incorporated by
reference herein in its entirety.
[0033] Referring to FIG. 3, a feed stream 40 comprising
hydrocarbons and a feed stream 42 comprising molecular oxygen
(O.sub.2) are fed to an alkene synthesis reactor (ASR) 44 to
produce alkenes from the hydrocarbons. Alternatively, hydrocarbons
and oxygen may be combined into a single feed stream. Feed stream
40 may contain any gaseous hydrocarbons such as natural gas,
associated gas, light hydrocarbons having from 1 to 10 carbon
atoms, or naphtha. Preferably, feed stream 40 comprises at least
50% by volume light alkanes (e.g., methane, ethane, and propane),
which may be recovered from a gas plant for processing natural gas
into different fractions. Feed stream 42 may contain, for example,
pure oxygen, air, oxygen-enriched air, or oxygen mixed with a
diluent.
[0034] In preferred embodiments, ASR reactor 44 comprises a
catalyst, and at least a portion of the hydrocarbons undergo
catalytic dehydrogenation in the presence of the catalyst to
produce an effluent stream comprising CO, CO.sub.2, H.sub.2,
H.sub.2O, alkenes, and unconverted hydrocarbons. Preferably, the
catalyst is active in the ODH reaction of light hydrocarbons. A
separator (not shown) may be employed to separate the product gas
from the ASR reactor 44 into an alkene stream 46 comprising
substantially or alternatively consisting essentially of alkenes
for feeding to a CF reactor 50 and a synthesis gas stream 48
comprising substantially or alternatively consisting essentially of
H.sub.2 and CO for feeding to another downstream process.
Hydrocarbon conversion within the ASR reactor 44 typically is less
than 100 percent in a single pass, and thus the unconverted
hydrocarbons may optionally be separated and recycled back to feed
stream 40 (not shown). Alternatively, the unconverted hydrocarbons
may be fed to CF reactor 50.
[0035] Any suitable reactor configuration may be employed to
convert the reactants in the ASR reactor 44. When ASR reactor 44
comprises a dehydrogenation catalyst, one suitable configuration is
a fixed catalyst bed in which the catalyst is retained within a
reaction zone in a fixed arrangement. Dehydrogenation catalysts may
be employed in the fixed bed regime using fixed bed reaction
techniques known in the art.
[0036] In preferred embodiments, ASR reactor 44 contains an ODH
catalyst and is a short-contact time reactor, e.g., a millisecond
contact time reactor of the type used in synthesis gas production.
A general description of major considerations involved in operating
a reactor using millisecond contact times is given in U.S. Pat. No.
5,654,491, which is incorporated herein by reference. Additional
disclosure regarding suitable ASR reactors comprising an ODH
catalyst and the ODH process is provided in commonly owned
published U.S. Patent Application No. 2003/0040655 A1 (Ser. No.
10/106,709), entitled "Oxidative dehydrogenation of alkanes to
olefins using an oxide surface;" Schmidt et al, New Ways to Make
Old Chemicals, Vol. 46, No. 8 AIChE Journal p. 1492-95 (August
2000); Bodke et al, Oxidative Dehydrogenation of Ethane at
Millisecond Contact Times: Effect of H.sub.2 Addition, 191 Journal
of Catalysis p. 62-74 (2000); Iordanoglou et al, Oxygenates and
Olefins from Alkanes in a Single-Gauze Reactor at Short Contact
Times, 187 Journal of Catalysis p. 400-409 (1999); and Huff et al,
Production of Olefins by Oxidative Dehydrogenation of Propane and
Butane over Monoliths at Short Contact Times, 149 Journal of
Catalysis p. 127-141 (1994), each of which is incorporated by
reference herein in its entirety.
[0037] In embodiments in which the ASR reactor 44 comprises an ODH
catalyst, the hydrocarbon feedstock and the oxygen-containing gas
are contacted with the ODH catalyst in a reaction zone that is
maintained at conversion-promoting conditions effective to produce
alkenes. Feed streams 40 and 42 are preferably pre-heated before
contact with the ODH catalyst. The process is operated at
atmospheric or super atmospheric pressures, the latter being
preferred. The pressure may range from about 100 kPa to about
12,500 kPa, preferably from about 130 kPa to about 5,000 kPa. The
catalyst temperature may range from about 400.degree. C. to about
1200.degree. C., preferably from about 500.degree. C. to about
900.degree. C. The gas hourly space velocity (GHSV) for the
process, stated as normal liters of gas per kilogram of catalyst
per hour, ranges from about 20,000 hr.sup.-1 to at least about
100,000,000 hr.sup.-1, preferably from about 50,000 hr.sup.-1 to
about 50,000,000 hr.sup.-1. Residence time is inversely
proportional to space velocity, and high space velocity indicates
low residence time on the catalyst. In a preferred millisecond
contact time reactor, the residence time of the reactant gas
mixture with the ODH catalyst is no more than about 100
milliseconds.
[0038] ODH catalysts may be of any suitable composition and form,
including foam, monolith, gauze, noodles, spheres, pills or the
like, for operation at the desired gas velocities with minimal back
pressure. Typically, ODH catalysts contain a precious metal such as
platinum to promote the conversion of hydrocarbons to alkenes. For
example, U.S. Pat. No. 6,072,097 and WO Pub. No. 00/43336 describe
the use of platinum and chromium oxide-based monolith ODH catalysts
for ethylene production with SCTRs; U.S. Pat. No. 6,072,097
describes the use of Pt-coated monolith ODH catalysts for use in
SCTRs; and WO Patent No. 00/43336 describes the use of Cr, Cu, Mn
or this mixed oxide-loaded monolith as the primary ODH catalysts
promoted with less than 0.1% Pt, each of these references being
incorporated herein in their entirety. Alternative ODH catalysts
are available that do not contain any unoxidized metals and that
are activated by higher preheat temperatures. Examples of preferred
alternative ODH catalysts that do not contain any unoxidized metal
are disclosed in copending U.S. Pat. Applications 60/309,427, filed
Aug. 1, 2001 and entitled "Oxidative Dehydrogenation of Alkanes to
Olefins Using an Oxide Surface" and 60/324,346, filed Sep. 24, 2001
and entitled "Oxidative Dehydrogenation of Alkanes to Olefins Using
Non-Precious Metal Catalyst", which are incorporated by reference
herein in their entirety.
[0039] Referring again to FIG. 3, alkene stream 46 is passed to CF
reactor 50 to produce carbon filaments. As mentioned previously,
unconverted hydrocarbons from ASR reactor 44 may also be passed to
CF reactor 50. The growth and recovery of the carbon filaments is
performed in the same manner as described in reference to FIG. 1
with the exception that primarily alkenes rather than alkanes are
disassociated to form the carbon filaments. A H2 stream 54 and a
carbon filament stream 52 exit the CF process. The carbon filaments
from stream 52 are then subjected to a metal loading process 56 to
form metal thereon. Any of the previously described metal loading
processes, such as CVD, electroplating, or wet impregnation, may be
employed. As a result, carbon filaments coated with metal are
recovered from metal loading process 56 via stream 58. Optionally,
as indicated by dashed line 60, the metal coated carbon filaments
can be recycled back to CF reactor 50 and metal loading process 56
any many times as desired to form multiple layers of carbon
filaments and metal.
[0040] FIG. 4 depicts yet another embodiment like that shown in
FIG. 3 except that a single CVD/CF reactor 64 is used instead of
separate CF and CVD reactors. Alkenes from ASR reactor 44 and
vaporized metallic compounds are concurrently passed to CVD/CF
reactor 64. As a result, metal is deposited on the carbon filaments
as they are being formed such that the metal becomes incorporated
within the carbon matrix of the filaments. The CVD/CF reactor 64
may be operated in the same manner as CVD/CF reactor 30 of FIG. 2.
Carbon filaments filled with metal may be recovered from CVD/CF
reactor 64 via stream 66, and H.sub.2 may be recovered from CVD/CF
reactor 63 via stream 68.
[0041] The metal loaded carbon filaments formed in the processes
shown in FIGS. 1-4 exhibit excellent properties such as increased
electrical conductivity and enhanced H.sub.2 storage. For example,
they typically have resistivities in the range of from about
10.sup.-4 to about 10.sup.4 ohms/cm.sup.2, and they can typically
store from about 0.1--to about 10 wt. % H.sub.2 per total weight of
the carbon filaments. As such, they can be used to form improved
end use articles such as electronic elements (e.g., transistors,
sensors, and wires), composite materials having enhanced electrical
properties, and metal catalysts (e.g., Fischer-Tropsch catalysts,
ODH catalysts, and alcohol producing catalysts) having high surface
area carbon support structures. Those skilled in the art would know
how to make and use such end use articles. Further, the present
invention contemplates forming high strength materials such as
metal carbide nanofibers.
EXAMPLES
[0042] The invention having been generally described, the following
example is given as particular embodiments of the invention and to
demonstrate the practice and advantages thereof. It is understood
that the examples are given by way of illustration and are not
intended to limit the specification or the claims to follow in any
manner.
[0043] Carbon filaments were prepared using a tube furnace with a
quartz reactor tube containing a Monel screen (1"W.times.6"L).
First, the tube furnace was heated to 650.degree. C. under N.sub.2
flow at 200 mL/min. Once the temperature reached 650.degree. C.,
ethylene flowing at 200 mL/min and N.sub.2 flowing at 60 mL/min
were fed to the furnace, and the exit gases were analyzed using a
gas chromatograph (GC) containing a thermal conductivity detector
(TCD) for hydrocarbon products. The reaction was continued at
650.degree. C. for a time period ranging from 1 hour to 4 hours,
with 2 hours being the standard run length. After this time, the
gas flows were stopped, and the reactor was allowed to cool down to
room temperature. Then the reactor was opened, and the carbon
sample was brushed off from the Monel screen inside a glove box to
prevent carbon fines from affecting the lab atmosphere. On an
average, about 5-10 grams of carbon were made per hour at these run
conditions.
[0044] The following impregnation method was employed to deposit Li
on carbon filaments: (a) the carbon filaments were sieved using a
14-mesh screen to separate the coarse filaments from the fines; (b)
the coarse sample was washed with distilled and de-ionized (DDI)
water and then with acetone and dried at 100.degree. C. overnight
to remove the volatiles; (c) the dried coarse sample was sonicated
in an ultrasonic bath for 20 minutes using TRITON X-100 surfactant
(commercially available from E.I. DuPont de Nemours and Company)
diluted with water at a volume ratio of 1:100, allowed to stand for
1 hour, and filtered by applying vacuum; and (d) the residue on the
filter paper was dried in a convection oven using air at about
100.degree. C. overnight. Steps (a)-(d) were performed to clean the
sample and collect higher purity carbon filament bundles. FIG. 5
depicts a SEM picture of the pretreated carbon filament sample
before metal modification.
[0045] Next, an ALDRICH 25,427-4 aqueous solution containing
lithium hydroxide in DDI water (available from Aldrich Chemical
Co.) was added to the pretreated carbon filament sample such that
the sample contained 5 wt. % Li based on the weight of the carbon.
The sample was then stirred while heating at 70 to 80.degree. C. on
the hotplate for 3 hours and dried in a convection oven in air at
about 100.degree. C. overnight, followed by calcination in air at
250.degree. C. for 3 hours. The resulting sample after calcination
contained a coating of lithium oxide on carbon and is shown in the
SEM picture in FIG. 6.
[0046] Based on the SEM image shown in FIG. 6, the impregnation
method did not result in uniform coating of Li on the carbon. Most
of the carbon filaments were covered with lithium oxide (a bright
colored coat). Some of the filaments also showed incorporation of
the metal oxide inside the filaments since the hollow portions
appeared to be covered.
[0047] The following table shows the BET surface areas of carbon
filament samples prepared as a function of reaction temperature and
feed composition using Monel catalyst screens, wherein BET surface
area measurements are well known in the art. These samples were not
coated with any metal compounds. This table is intended only to
show the variation that can be achieved in the filament properties
by controlling the process parameters and is not meant to limit the
scope of the invention. Any of these filaments could be coated with
metal/metal oxide/metal compounds as desired to achieve the
required surface areas.
1TABLE 1 BET surface areas of carbon filaments (before metal
deposition) Feed composition Reaction BET surface area of the (wt.
%), temperature resulting carbon filaments Catalyst (.degree. C.)
(m.sup.2/g) 100% ethylene, Monel screen 500 114 100% ethylene,
Monel screen 650 345 100% ethylene, Monel screen 800 221 100%
ethylene, Ni screen 750 137
[0048] While the preferred embodiments of the invention have been
shown and described, modifications thereof can be made by one
skilled in the art without departing from the spirit and teachings
of the invention. The embodiments described herein are exemplary
only, and are not intended to be limiting. Many variations and
modifications of the invention disclosed herein are possible and
are within the scope of the invention. Use of the term "optionally"
with respect to any element of a claim is intended to mean that the
subject element is required, or alternatively, is not required.
Both alternatives are intended to be within the scope of the
claims.
[0049] Accordingly, the scope of protection is not limited by the
description set out above, but is only limited by the claims which
follow, that scope including all equivalents of the subject matter
of the claims. Each and every claim is incorporated into the
specification as an embodiment of the present invention. Thus, the
claims are a further description and are an addition to the
preferred embodiments of the present invention. The discussion of a
reference in the Description of Related Art is not an admission
that it is prior art to the present invention, especially any
reference that may have a publication date after the priority date
of this application. The disclosures of all patents, patent
applications, and publications cited herein are hereby incorporated
by reference, to the extent that they provide exemplary,
procedural, or other details supplementary to those set forth
herein.
* * * * *
References