U.S. patent application number 11/354544 was filed with the patent office on 2006-06-29 for solar-thermal fluid-wall reaction processing.
Invention is credited to Carl Bingham, Karen J. Raska Buechler, Jaimee K. Dahl, Willy Grothe, Allan A. Lewandowski, Alan W. Weimer.
Application Number | 20060140848 11/354544 |
Document ID | / |
Family ID | 46282100 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060140848 |
Kind Code |
A1 |
Weimer; Alan W. ; et
al. |
June 29, 2006 |
Solar-thermal fluid-wall reaction processing
Abstract
The present invention provides a method for carrying out high
temperature thermal dissociation reactions requiring rapid-heating
and short residence times using solar energy. In particular, the
present invention provides a method for carrying out high
temperature thermal reactions such as dissociation of hydrocarbon
containing gases and hydrogen sulfide to produce hydrogen and dry
reforming of hydrocarbon containing gases with carbon dioxide. In
the methods of the invention where hydrocarbon containing gases are
dissociated, fine carbon black particles are also produced. The
present invention also provides solar-thermal reactors and
solar-thermal reactor systems.
Inventors: |
Weimer; Alan W.; (Niwot,
CO) ; Dahl; Jaimee K.; (Superior, CO) ;
Lewandowski; Allan A.; (Evergreen, CO) ; Bingham;
Carl; (Lakewood, CO) ; Buechler; Karen J. Raska;
(Westminster, CO) ; Grothe; Willy; (Boulder,
CO) |
Correspondence
Address: |
GREENLEE WINNER AND SULLIVAN P C
4875 PEARL EAST CIRCLE
SUITE 200
BOULDER
CO
80301
US
|
Family ID: |
46282100 |
Appl. No.: |
11/354544 |
Filed: |
February 14, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10383875 |
Mar 7, 2003 |
7033570 |
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11354544 |
Feb 14, 2006 |
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10239706 |
Feb 24, 2003 |
6872378 |
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PCT/US01/15160 |
May 8, 2001 |
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11354544 |
Feb 14, 2006 |
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60362563 |
Mar 7, 2002 |
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60203186 |
May 8, 2000 |
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Current U.S.
Class: |
423/457 ;
159/903 |
Current CPC
Class: |
B01J 2219/0272 20130101;
B01J 2219/0883 20130101; Y02P 20/133 20151101; B01J 8/087 20130101;
B01J 2208/00212 20130101; C01B 2203/0866 20130101; B01J 10/002
20130101; C01B 2203/049 20130101; C01B 3/24 20130101; C01B
2203/0272 20130101; C01B 3/344 20130101; Y02E 60/364 20130101; B01J
19/127 20130101; B01J 2208/00451 20130101; C01B 2203/0833 20130101;
F28C 3/12 20130101; C09C 1/48 20130101; B01J 2219/00006 20130101;
Y02E 10/41 20130101; C01B 2203/0216 20130101; C01B 2203/0805
20130101; B01J 2208/00513 20130101; B01J 8/0045 20130101; C01B 3/04
20130101; C01B 2203/085 20130101; C01B 17/0495 20130101; C01B
2203/04 20130101; B01J 2208/00522 20130101; B01J 2208/00752
20130101; Y02P 20/134 20151101; B01J 2208/0038 20130101; C01B
2203/0222 20130101; B01J 8/025 20130101; C01B 3/34 20130101; C01P
2006/80 20130101; Y02E 10/40 20130101; Y02E 60/36 20130101; F24S
20/20 20180501 |
Class at
Publication: |
423/457 ;
159/903 |
International
Class: |
C09C 1/48 20060101
C09C001/48 |
Goverment Interests
ACKNOWLEGEMENT OF GOVERNMENT SUPPORT
[0002] This invention was made, at least in part, with funding from
the United States Department of Energy under grant numbers
DE-FC36-99G010454 and DE-AC36-99G010337. The United States
Government has certain rights in this invention.
Claims
1. Carbon black produced by a method comprising the steps of: a)
providing a reactor comprising at least two reactor shells,
including an innermost and an outer shell, wherein the innermost
shell is substantially enclosed by each of the other reactor
shells, has an inlet and an outlet and is at least partially porous
and the outer shell is nonporous and at least partially
transparent; b) flowing a first gas stream comprising at least one
hydrocarbon reactant gas from the inlet to the outlet of the
innermost shell; c) flowing a second gas stream comprising a
non-dissociating gas inwardly through the pores of the innermost
shell; d) providing heat absorbing particles in the first gas
stream; e) heating the heat absorbing particles at least in part
with a source of concentrated sunlight through indirect solar
thermal heating; and f) transferring heat from the particles to the
first gas stream, thereby heating the reactant gas to a
sufficiently high temperature so that a desired amount of
conversion of the reactant gas occurs, thereby producing hydrogen
and carbon black, wherein the carbon black is substantially free of
ash and is substantially amorphous.
2. A high temperature solar-thermal reactor comprising: a) a first
inner shell which is at least partially porous, the first inner
shell having an inlet and an outlet; b) a second inner shell which
is nonporous and which substantially encloses the first inner
shell; c) a first gas plenum located substantially between the
first and second inner shell, the first plenum having an inlet and
an outlet, wherein the first plenum outlet is formed by the pores
of the first inner shell; d) an outer shell which is nonporous, at
least partially transparent, and which substantially encloses the
second inner shell; and e) a second gas plenum located
substantially between the second inner shell and the outer shell,
the second plenum having an inlet and an outlet, wherein the
reactor is heated at least in part by a source of concentrated
sunlight and the first inner shell is prevented from fluid
communication with the first and second gas plenums inside the
reactor, except for fluid communication between the first inner
shell and the first gas plenum through the pores of the first inner
shell.
3. The reactor of claim 2, wherein the first inner shell is made in
part of porous graphite.
4. The reactor of claim 2, wherein the second inner shell is made
of nonporous graphite.
5. The reactor of claim 2, wherein the at least partially
transparent outer shell is made of quartz.
6. The reactor of claim 5, wherein the inside wall of the outer
quartz shell is partially coated with silver.
7. The reactor of claim 2, wherein the at least partially
transparent outer shell is made of metal with a transparent section
made of quartz.
8. A high temperature solar thermal reactor system comprising the
reactor of claim 2 and a cooling zone downstream of the reactor,
wherein the downstream cooling zone has an inside dimension larger
than that of the first inner shell.
9. A high temperature solar thermal reactor system comprising the
reactor of claim 2 and a filtering device.
10. A high temperature solar thermal reactor system comprising the
reactor of claim 2 and a pressure swing adsorber or a membrane gas
separator.
11. A high temperature solar thermal reactor system comprising the
reactor of claim 2 and a device for feeding heat absorbing
particles into the inlet end of the first inner shell.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 10/383,875, filed Mar. 7, 2003, which takes
priority from U.S. provisional application Ser. No. 60/362,563,
filed Mar. 7, 2002, and which is a continuation-in-part of
application Ser. No. 10/239,706 filed Feb. 24, 2003, which is the
national stage of PCT application number PCT/US01/15160 filed May
8, 2001, which claims the benefit of U.S. provisional application
No. 60/203,186, filed May 8, 2000.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to solar-thermal reactors and
processes for carrying out high temperature chemical reactions.
More particularly, it relates to a rapid-heating, short residence
time solar-thermal process for carrying out highly endothermic
dissociation reactions to produce hydrogen or hydrogen containing
gases. Most particularly, it relates to those dissociation
reactions wherein a solid particulate material is produced by the
dissociation of a gaseous precursor.
[0004] There is a significant interest to develop benign processes
for producing hydrogen that can be used as a fuel to power fuel
cell vehicles. Such processes should reduce the amount of
greenhouse gases produced, thus, minimizing impact on the
environment. However, current methods for producing hydrogen incur
a large environmental liability, because fossil fuels are burned to
supply the energy to reform natural gas (primarily methane,
CH.sub.4) to produce hydrogen (H.sub.2).
[0005] High temperatures above approximately 1500 K are required
for producing hydrogen and carbon black at high rates by the direct
thermal dissociation of methane [CH.sub.4+heat.fwdarw.C+2H.sub.2]
(reaction 1), ethane [C.sub.2H.sub.6+heat.fwdarw.2C+3H.sub.2]
(reaction 2), propane [C.sub.3H.sub.8+heat.fwdarw.3C+4H.sub.2]
(reaction 3), or, in general, a mixture of gases such as natural
gas generically represented as CxHy
[C.sub.xH.sub.y+heat.fwdarw.xC+(y/2)H.sub.2] (reaction 4).
[0006] Hydrogen can also be produced by the dry reforming of
methane with carbon dioxide
[CH.sub.4+CO.sub.2.fwdarw.2CO+2H.sub.2]. It is also possible to
carry out dissociation of methane simultaneously with the dry
reforming of methane if excess methane is present relative to that
required to react carbon dioxide. Such processes are useful since
they can provide for a high hydrogen content synthesis gas by
utilizing natural gas from natural gas wells that contain a high
concentration of carbon dioxide (typically 10 to 20 volume %
CO.sub.2) or using landfill biogas (30 to 40 volume %
CO.sub.2).
[0007] Hydrogen can also be produced by the thermal dissociation of
hydrogen sulfide [H.sub.2S+heat.fwdarw.H.sub.2+S] (reaction 5).
[0008] For these types of dissociation reactions, a solid (either C
or S) is formed as a co-product (with H.sub.2) of the reaction.
Often, the solid that is formed is in the state of fine particles.
These particles have a tendency to deposit along the walls of
reaction vessels or cooling chambers where the dissociation is
occurring. If deposition occurs along the inside walls of the
heated reactor, the particles tend to aggregate and crystallize.
For the case of carbon deposition, the normally amorphous
ultra-fine particles will grow in size and graphitize. Large
graphitic carbon particles are less reactive compared to more
amorphous fine sized particles and, hence, are of lower value.
Furthermore, deposition on the reactor walls can cause plugging of
the reactor and eventual shutdown of the process, thus, preventing
continuous operation. In addition, carbon deposition on an outer
transparent wall of a solar reactor can lead to overheating of the
reactor wall.
[0009] U.S. Pat. No. 4,552,741, to Buck et al., reports carbon
dioxide reforming of methane in a system comprising two catalytic
reactors. One of the catalytic reactors is heatable with solar
energy. In the abstract, the reactors are stated to be "filled with
a catalyst".
[0010] U.S. Pat. No. 5,647,877 reports solar energy gasification of
solid carbonaceous material in a liquid dispersion. The solid
carbonaceous material is heated by solar energy and transfers heat
to a surrounding liquid. Hydrogen is produced in the process by the
decomposition/gasification of the hydrocarbon (coal) particles.
[0011] EP 0675075A reports the use of solar energy to generate
hydrogen from water. In the reported process, water is reduced to
hydrogen with a metal, followed by reduction of the metal oxide
with a reducing agent.
[0012] Hence, there is a need to develop high temperature
environmentally benign processes for the production of H.sub.2 by
thermal dissociation of hydrocarbon gases, such as natural gas, and
to prevent the deposition of the products of dissociation on
reactor walls.
SUMMARY OF THE INVENTION
[0013] The present invention provides a method for carrying out
high temperature thermal dissociation reactions requiring
rapid-heating and short residence times using solar energy. In
particular, the present invention provides a method for carrying
out high temperature thermal reactions such as dissociation of
hydrocarbon containing gases and hydrogen sulfide to produce
hydrogen and dry reforming of hydrocarbon containing gases with
carbon dioxide. In the methods of the invention where hydrocarbon
containing gases are dissociated, fine carbon black particles are
also produced. The methods of the invention reduce or prevent the
produced carbon black from depositing along the inside wall of the
reactor or cooling zone. The present invention also provides
solar-thermal aerosol transport reactors and solar-thermal reactor
systems. The present invention also provides systems and methods
for separating the produced carbon black from the product gases,
purifying the hydrogen produced by the dissociation reaction, and
using the carbon black and hydrogen to generate electricity.
[0014] There is an enormous environmental benefit for carrying out
high temperature dissociation reactions directly without the
combustion of carbonaceous fuels. Thus, the present invention
provides a continuous cost-effective, solar-based method of
deriving hydrogen and fine carbon black particles from hydrocarbon
gases. The process does not result in increased environmental
damage due to burning fossil fuels.
[0015] The process of the present invention uses concentrated
sunlight to transfer heat at extremely high rates by radiation heat
transfer to inert radiation absorbing particles flowing in dilute
phase in the process gas. The heating to the particles is generally
carried out indirectly from a heated wall or series of walls which
are themselves heated indirectly or heated directly by
solar-thermal radiative heating. The inside most wall ("reaction")
is at least partially fabricated of a porous refractory material
with a compatible "fluid-wall" gas flowing inward, thus, providing
a blanket of gas and preventing deposition of particles on the
inside wall. The particles subsequently become radiators themselves
and heat flowing gases by conduction, thereby providing the energy
to carry out highly endothermic gas phase dissociation reactions.
The radiative coupling to heat flowing radiation absorbing
particles is beneficial because the gases to be heated are
themselves transparent to radiative heating. Preferably, the gases
and the particles flow co-currently to maximize the temperature and
heating rate of the gases. It is possible for the absorber
particles to either be fed into the process with the reactant gas
or to be generated in-situ by the reaction itself.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross-sectional view of the central portion of a
solar-thermally heated fluid-wall reactor having three walls. The
innermost wall of the reactor is a porous "reactor" wall, the next
outermost wall of the reactor is a solid "heating" wall, and the
outermost wall of the reactor is a transparent "protection
wall".
[0017] FIG. 2 is an overall cross-section of another reactor of the
invention.
[0018] FIG. 3 is a cross-section of a reactor having a transparent
window in the outer shell.
[0019] FIG. 4 is a schematic of a solar-thermal natural gas
dissociation system employing a solar thermally heated fluid-wall
reactor of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The invention provides a method for carrying out high
temperature thermal dissociation reactions requiring rapid-heating
and short residence times using solar energy. In particular, the
method of the invention allows production of hydrogen and hydrogen
containing gases through thermal dissociation of a gas comprising
hydrocarbon gases or mixtures thereof (such as natural gas) and/or
hydrogen sulfide. The methods of the invention also allow
production of hydrogen through dry reforming of methane with carbon
dioxide. The invention also provides high temperature solar
reactors.
[0021] In particular, the invention provides a high temperature
solar-thermal reactor comprising [0022] a. a first inner shell
which is at least partially porous, the first inner shell having an
inlet and an outlet; [0023] b. a second inner shell which is
nonporous and which substantially encloses the first inner shell;
[0024] c. a first gas plenum located substantially between the
first and second inner shell, the first plenum having an inlet and
an outlet, wherein the first plenum outlet is formed by the pores
of the first inner shell; [0025] d. an outer shell which is
nonporous, at least partially transparent, and which substantially
encloses the second inner shell; and [0026] e. a second gas plenum
located substantially between the second inner shell and the outer
shell, the second plenum having an inlet and an outlet, wherein the
reactor is heated at least in part by a source of concentrated
sunlight and the first inner shell is prevented from fluid
communication with the first and second gas plenums inside the
reactor, except for fluid communication between the first inner
shell and the first gas plenum through the pores of the first inner
shell.
[0027] FIG. 1 is a cross-section of the central portion of a
reactor present invention. In the figures, the same numbers are
used to identify like features. In the configuration shown in FIG.
1, the reactor (1) has a first, innermost, inner shell (3) which is
at least partially porous, a second inner shell (5) which is
non-porous, and an outer shell (7) which is at least partially
transparent to solar radiation and is also non-porous. As used
herein, "shells" encompass tubes, pipes or chambers which are
elongated along a longitudinal axis. As used herein, a "porous"
shell region permits gas flow through the walls of the region while
a "nonporous" shell region does not. In a reactor with three
shells, the first inner shell is substantially enclosed by the
second inner shell and the outer shell and the second inner shell
is substantially enclosed by the outer shell. As used herein,
"substantially encloses" means that one shell is enclosed by
another for most of the length of the shell. The ends of a shell
that is substantially enclosed by another may extend past the ends
of the other shell (e.g. the ends of the first inner shell may
extend past the ends of the second inner shell and/or the outer
shell). FIG. 1 illustrates an embodiment where the "shells" are
concentric tubes of circular cross-section.
[0028] FIG. 1 also illustrates the central portion of the first
(41) and second (43) gas plenums. During operation of the reactor,
gases are flowed through the first inner shell and the two gas
plenums by connecting each of the respective inlets to at least one
gas source. The porous region(s) of the first inner shell serve as
an outlet to the first gas plenum. FIG. 1 illustrates three gas
streams, a first gas stream (21) flowing through the first inner
shell, a second gas stream (23) flowing through the first plenum,
and a third gas stream (25) flowing through the second plenum.
Preferably, the first gas stream is prevented from mixing with the
third gas stream within the reactor and mixing between the first
and second streams is limited to mixing within the first inner
shell due to flow of gas from the second gas stream through the
porous region(s) of the first inner shell. In other words, the
first inner shell is preferably prevented from fluid communication
with the first and second plenum inside the reactor, except for
fluid communication between the first inner shell and the first
plenum through the porous region(s) of the first inner shell. In
addition, during operation of the reactor fluid communication
between the first inner shell and the first plenum is primarily in
the direction from the first plenum to the first inner shell. The
pressure within the first plenum is high enough to overcome the
resistance of the porous first inner shell and still have a
pressure (at the instant the gas from the second gas stream leaves
the pore) greater than the pressure inside the shell. Restricting
fluid communication between the first inner shell and the first and
second plenum can prevent deposition of particulate reaction
products on the other shells and reduce the amount of gas from the
second and third gas streams which enters the first inner shell.
The overall volumetric flow rate of gases through the first inner
shell can affect the residence time and the production throughput
of the reactor. If the second and third gas streams are different,
it is also preferred to prevent mixing of the second and third gas
streams within the reactor.
[0029] In one embodiment, mixing of the gas streams is restricted
by seals. If the inner and outer shells are tubes as shown in FIG.
1, the plenums are further defined by these seals, since they serve
to define the gas volume. Statement that a plenum is located
"substantially between" two shells encompasses an extension of the
plenum beyond the two shells into a sealing structure. In addition,
a plenum being "substantially located" between two shells
encompasses reactor configurations where other reactor elements,
for example thermal insulation, are also located between the two
shells. FIG. 2 illustrates one sealing configuration which can be
used to prevent mixing of the gas streams within the reactor. In
FIG. 2, the inlet and outlets for the inner and outer shells are
illustrated as part of the sealing structures (31) and (33). The
inner shell inlet (11) and outlet (12) are substantially sealed
from the first plenum (41) and second plenum (43). FIG. 2 also
shows the first plenum inlet (13), with the outlet of the first
plenum being the porous region of the inner shell, and second
plenum inlet (15) and outlet (16). The sealing structures shown in
FIG. 2 are cooled with water (34) to prevent heat damage to the
fitting and sealing materials. Other suitable seal configurations
are known to those skilled in the art. Furthermore, the seal
configuration may be different at the inlet and outlet ends of the
reactor.
[0030] The reactor shown in FIG. 1 is operated generally as
follows. Concentrated solar-thermal radiation (91) passes through
the outer "protection" shell (7) and directly heats the second
inner "heating" shell (5). The nonporous heating shell re-radiates
from its inner wall and heats the first inner "reaction" shell (3).
Hence, the inner "reaction" shell (3) is heated indirectly by
concentrated sunlight from the surrounding "heating" shell (5). The
inner "reaction" shell (3) re-radiates from the inner wall and
heats the radiation absorber particles (27) and first gas stream
(21) flowing through it. When heated, the first gas stream
undergoes the desired reaction(s). As the first gas stream is
heated and the desired reaction(s) occur, one or more product gases
are added to the gas stream. A second gas stream (23) of
non-oxidizing and non-dissociating "fluid-wall" gas flows in the
annular region between the central "heating" shell and the inner
"reaction" shell. The "fluid-wall" gas enters the first plenum
between the inner and outer shell through an inlet and exits the
plenum through an outlet. One outlet of the first plenum is the
porous section of the inner shell. An additional outlet for the
first plenum may be used, so long as sufficient gas flow is
provided through the porous section of the inner shell. The
"fluid-wall" gas flows through the pores of the porous section of
the "reaction" shell (3), exits radially along the inside of the
"reaction" shell and provides for an inner "fluid-wall" gas blanket
(29) that prevents deposition of dissociation product particles on
the inside wall of the "reaction" shell. After entering the first
inner shell, the "fluid-wall" gas exits through the outlet of the
first inner shell. A third gas stream (25) of non-oxidizing and
non-dissociating "purge" gas flows in the annular region between
the outer "protection" shell and the center "heating" shell, thus
preventing oxidation of the central "heating" shell and any
insulation that may be present between the "protection" and
"heating" shell.
[0031] In another embodiment, the reactor comprises [0032] a) an
inner shell which is at least partially porous, the inner shell
having an inlet and an outlet; [0033] b) an outer shell which is
nonporous, at least partially transparent, and which substantially
encloses the second inner shell; and [0034] c) a gas plenum located
substantially between the inner and outer shell, the plenum having
an inlet and an outlet, [0035] wherein the reactor is heated at
least in part by a source of concentrated sunlight and the only
fluid communication between the inner shell and the gas plenum
inside the reactor occurs through the pores of the inner shell.
[0036] This reactor is operated as follows. Concentrated
solar-thermal radiation passes through the outer "protection" shell
and directly heats the inner "reaction" shell. The inner "reaction"
shell re-radiates from the inner wall and heats the radiation
absorber particles and first gas stream flowing through it. When
heated, the first gas stream undergoes the desired reaction(s). A
second gas stream of non-oxidizing and non-dissociating
"fluid-wall" gas flows in the annular region between the outer
"protection" shell and the inner "reaction" shell. The "fluid-wall"
gas enters the plenum between the inner and outer shell through an
inlet and exits the plenum through an outlet. The porous section of
the inner shell forms one outlet of the plenum. An additional
outlet for the plenum may be used, so long as sufficient gas flow
is provided through the porous section of the inner shell. The
"fluid wall" gas flows through the pores of the porous section of
the "reaction" shell, exits radially along the inside of the
"reaction" shell and provides for an inner "fluid-wall" gas blanket
that prevents deposition of dissociation product particles on the
inside wall of the "reaction" shell.
[0037] In general, the shells comprising the reactors of the
invention may be positioned vertically or horizontally, or in any
other spatial orientation. For the case of a vertical reaction
shell process, the apparatus may be arranged to provide upward or
downward flow of the gas stream and the cloud of particles. Upward
flow guarantees that aggregated particles will not be carried
through the reaction shell. Downward flow reduces the potential for
plugging in the solids feed line, if present. Preferably, the
reactor shell is positioned vertically and flow is downward.
[0038] The invention provides a method for carrying out a high
temperature chemical reaction process to produce hydrogen or
synthesis gas comprising the steps of: [0039] a) providing a
reactor comprising at least two reactor shells, including an
innermost and an outer shell, wherein the innermost shell is
substantially enclosed by each of the other reactor shells, has an
inlet and an outlet and is at least partially porous and the outer
shell is nonporous and at least partially transparent; [0040] b)
flowing a first gas stream comprising at least one reactant gas
from the inlet to the outlet of the innermost shell; [0041] c)
flowing a second gas stream comprising a non-dissociating gas
inwardly through the pores of the first inner shell; [0042] d)
providing heat absorbing particles in the first gas stream; [0043]
e) heating the heat absorbing particles at least in part with a
source of concentrated sunlight through indirect solar thermal
heating; and [0044] f) transferring heat from the particles to the
first gas stream, thereby heating the reactant gas to a
sufficiently high temperature so that a desired amount of
conversion of the reactant gas occurs, thereby producing hydrogen
or synthesis gas.
[0045] For a reactor having a first inner shell, a second inner
shell, and an outer shell, the invention provides a method
comprising the steps of: [0046] a) providing a reactor comprising a
first inner shell which is at least partially porous and has a
first inner shell inlet and outlet, a second inner shell which is
nonporous and substantially encloses the first inner shell, an
outer shell which is nonporous, at least partially transparent and
substantially encloses the second inner shell, a first plenum
substantially located between the first inner shell and the second
inner shell and having a first plenum inlet and outlet, and a
second plenum substantially located between the second inner shell
and the outer shell and having a second plenum inlet and outlet
wherein the first plenum outlet is formed by the pores of the first
inner shell and the first inner shell is prevented from fluid
communication with the first and second gas plenums inside the
reactor, except for fluid communication between the first inner
shell and the first gas plenum through the pores of the first inner
shell; [0047] b) flowing a first gas stream comprising at least one
reactant gas from the inlet to the outlet of the first inner shell;
[0048] c) flowing a second gas stream comprising a non-dissociating
gas through the inlet of the first plenum, thereby causing part of
the second gas stream to flow inwardly through the pores of the
first inner shell; [0049] d) flowing a third gas stream comprising
a non-dissociating, non-oxidizing gas from the inlet to the outlet
of the second plenum; [0050] e) providing heat absorbing particles
in the first gas stream; [0051] f) heating the heat absorbing
particles at least in part with a source of concentrated sunlight
through indirect solar thermal heating; and [0052] g) transferring
heat from the particles to the first gas stream, thereby heating
the reactant gas to a sufficiently high temperature so that a
desired amount of conversion of the reactant gas occurs, thereby
producing hydrogen or synthesis gas.
[0053] The innermost inner shell (the first inner shell in a
three-shell reactor) has an inlet and an outlet for the first gas
stream. The interior of the innermost shell defines a reaction
chamber within which the high temperature reaction takes place. The
innermost shell is capable of emitting sufficient radiant energy to
raise the temperature of the reactant gas(es) within the reaction
chamber to a level required to initiate and sustain the desired
chemical reaction. The innermost shell is made of a high
temperature refractory material. The refractory material
subsequently heats flowing radiation absorber particles flowing
through the first inner shell and is substantially chemically
unreactive with the particles or the reactant or product gases. A
preferred material for the innermost shell is graphite.
[0054] The innermost shell is at least partially porous. The
innermost shell may be wholly of porous material or may comprise
one or more regions of porous material. For example, the innermost
shell may take the form of a graphite tube having a central porous
region with nonporous ends. The porous region(s) of the innermost
shell are selected so that sufficient uniform flow of
non-dissociating gas occurs radially inward through the pores to
provide a fluid-wall protective blanket for the radially inward
surface of the innermost shell. The fluid-wall can prevent particle
deposition on the radially inward surface of the innermost shell.
The porosity of the porous region(s) can be varied and is selected
on the basis of the required gas flow and allowable pressure drop
to provide for a fluid-wall of gas to prevent deposition along the
inside wall of the reactor. The length of the porous section(s) of
the "reaction" shell can be varied and is determined by the zone
where particle deposition is most likely to occur. Likewise, the
placement of the porous section along the length of the "reaction"
shell is determined by the most likely location of particle
deposition. Preferably, the length of the porous section of the
"reaction" shell is limited to where it is needed to prevent wall
deposition of dissociation product particles. Too large of a porous
section will provide for too much fluid-wall gas entering the
interior of the innermost "reaction" shell. The entry of fluid-wall
gas into the "reaction" shell increases the overall volumetric flow
rate of gases through the "reaction" shell, thus reducing residence
time and limiting the production throughput of the reactor.
[0055] A second inner shell substantially enclosing the first inner
shell may be present, but is not required. If no second inner shell
is present the "reaction" shell is heated directly by concentrated
sunlight passing through the "protection" shell and "fluid wall"
gas is flowed in the plenum substantially located between the
"reaction" shell and the "protection" shell.
[0056] The use of a second inner shell offers several advantages.
The use of a nonporous second inner shell distances the "fluid
wall" gas from the outer "protection" shell, which can increase the
safety of the process when the "fluid wall" gas is a flammable gas
such as hydrogen. Furthermore, if the second inner shell is a tube
made of a material such as graphite, an electrical current can be
run from one end of the tube to the other and generate additional
heat for the process through resistance heating of the tube. This
additional heat can supplement the process at times when the source
of concentrated sunlight does not provide the desired amount of
energy (e.g. a cloudy day).
[0057] Typically, the second inner shell is composed of nonporous
high temperature refractory material. The second inner shell is
most preferably made of solid graphite. As previously discussed,
the second inner shell can function as a "heating" shell, since it
radiates heat to the innermost shell. In addition, the combination
of the first and the second inner shell at least partially defines
a first plenum or volume for the non-dissociating fluid wall
gas.
[0058] Additional inner shells can be used in the process. If used,
they are sized and positioned so that the innermost shell is
enclosed by each of the other reactor shells (i.e. the reactor
shells are substantially "nested" one inside the other). If
additional inner shells are used, "purge" gas can be used to
prevent oxidation of these shells as well.
[0059] The outer "protection" shell is at least in part transparent
or semi-transparent to the concentrated sunlight, thereby allowing
concentrated sunlight to flow through and heat the inner shell(s)
of the reactor. The "protection" shell is made of a high
temperature material that is oxidation resistant. A suitable
material for the transparent portion of the outer shell is quartz.
The transparent portion of the outer shell may be a transparent
section, window or opening to allow the concentrated sunlight into
the vessel. The shell wall transparent area, allowing for
concentrated sunlight entry and subsequent solar thermal heating,
should be selected to provide heating during the desired reaction
residence time requirements for the process.
[0060] The outer shell may be made entirely of quartz. In this
case, the sections of the internal wall of the shell where sunlight
is not being concentrated and entering the vessel, may be coated
with a reflective material, such as silver, to keep the
concentrated sunlight inside the reactor. If such a reflective
coating is used, there must be an uncoated transparent section,
window or opening to allow the concentrated sunlight into the
vessel.
[0061] Alternatively, the outer "protection" shell may be made of a
refractory non-transparent material with a section containing a
transparent window where concentrated sunlight can enter, as
schematically illustrated in FIG. 3. In the configuration shown in
FIG. 3, both a first (3) and a second inner shell (5) are
substantially enclosed by the outer shell. The "heating" shell (5)
is directly exposed to concentrated sunlight in the section of the
shell located in the path of the sun through the transparent
section (9) of the "protection" shell (7). As shown in FIG. 3, the
"heating" shell may be surrounded by refractory insulation (6) in
the region where it is not directly exposed to concentrated
sunlight via the transparent section. The insulation may be
concentrically placed and extends substantially from the "heating"
shell to the concentric "protection" shell, although it may not
completely fill the space between the heating shell and the
protection shell. The refractory insulation can be a combination of
graphite insulation near the "heating" shell and an alumina type
refractory insulation near the "protection" shell. This design
arrangement allows concentrated sunlight to enter through a
transparent section and heat the "heating" shell while the
surrounding insulation reduces losses of ultraviolet radiation from
the "heating" shell, thereby increasing the efficiency of the
process. It is also possible to provide cooling of the outer metal
refractory "protection" shell, particularly in the region
immediately surrounding the transparent window allowing
concentrated sunlight to directly heat the "heating" shell. The
non-transparent refractory material may be a metal with a
sufficiently high melting point, such as steel. In FIG. 3, the
inlet (11) to the first inner shell is shown as having a feed gas
inlet (18) and optional heat absorbing particle feed inlet (19).
The particle feed inlet is not required if the heat absorbing
particles are wholly generated by the dissociation process.
[0062] The combination of the outermost inner shell and the outer
shell at least partially defines a plenum or volume for gas. If no
second inner shell is used in the reactor, non-dissociating fluid
wall gas flows in the space between the outer shell and the inner
shell. Otherwise, a non-oxidizing and non-dissociating "purge" gas
typically flows between a second plenum substantially located
between the outer shell and the second inner shell to protect the
second inner "heating" shell from oxidation.
[0063] The first gas stream initially comprises at least one
reactant selected from CH.sub.4, C.sub.2H.sub.6, C.sub.3H.sub.8,
generally C.sub.xH.sub.y, H.sub.2S, natural gas, or a combination
thereof. The first gas stream may contain substantial amounts of
carbon dioxide, as may be present in "biogases" such as landfill
gas. Landfill gases may contain as much as 40% carbon dioxide. The
first gas stream may also initially comprise a non-reactive gaseous
component. For example, in lab-scale tests, methane is sometimes
diluted with argon for safety reasons. As the gas stream is heated
and the reaction or reactions occur, one or more product gases are
added to the gas stream. These product gases comprise H.sub.2 and,
depending on the composition of the reactant gases, may also
comprise incomplete dissociation products such as C.sub.2H.sub.2,
C.sub.2H.sub.4, or other gases. In the case of reactions (1 to 4),
additional carbon particles are also produced, and in the case of
reaction (5), elemental sulfur is produced. A preferred reactant
gas stream is natural gas or one containing natural gas. A most
preferable reactant gas stream is natural gas which is free of
mercaptans and hydrogen sulfide.
[0064] In the method of the invention, the first gas stream is
heated to a sufficiently high temperature within the reactor that
the desired amount of conversion of the reactant gas(es) is
obtained. Hydrogen formation may take place below this temperature.
Preferably the first gas stream is heated to at least about 1500 K
within the reactor. As used herein, the use of "about" with
reference to a temperature implies that the temperature is within
25 K of the stated temperature. In other embodiments, the reactant
gas is heated to about 2100 K or heated to a maximum temperature in
the range between about 1500 K and about 2700 K or between about
1800 K and about 2400 K. The temperature inside the innermost shell
of the reactor can be measured with a thermocouple. Alternatively,
temperatures inside the reactor can be measured with an optical
pyrometer. For a three-shell reactor, the hot zone temperature
measured with an optical pyrometer is typically the temperature of
the nonporous "heating" shell, since the "heating" shell encloses
the "reaction" shell in the hot zone. The temperature inside the
inner "reaction" shell may be less than that of the "heating" shell
due to thermal losses due to heating the porous shell and the gases
in the first plenum and the reaction shell.
[0065] The reactors and methods of the invention allow conversion
of at least about 30% of a hydrocarbon or hydrogen sulfide reactant
gas. As used herein, the amount of conversion is the ratio of the
moles of reactant gas reacted to the moles of reactant gas
supplied. In various embodiments, the reactors and methods of the
invention can produce at least 50% or at least 70% conversion of
reactant gas.
[0066] As used herein, the "residence time" is the time that the
reactant gas(es) spend in the hot zone of the innermost "reaction"
shell The hot zone length may be estimated as the length of the
reactor directly irradiated by the source of concentrated sunlight.
The residence time depends on the flow rate of the first gas stream
containing the reactant gas(es), the flow rate of the fluid wall
gas through the pores of the inner shell, the reactant gas
temperature and the degree of conversion of the reactant gas(es).
The residence time may be calculated through modeling or estimated
by averaging the residence times of the components of the gas
stream flowing through the innermost tube at reaction temperature
and assuming that half of the actual conversion occurs over the
entire length of the hot zone and contributes to the formation of
additional moles of gas (e.g., 2 moles of hydrogen are formed for
every mole of methane converted). In the methods of the invention,
the residence time is preferably between about 1 and about 50
milliseconds. More preferably, the residence time is between about
5 and about 30 ms. Most preferably, the residence time is between
about 10 and about 20 ms.
[0067] In the methods of the invention, heat absorbing particles
are provided in the first gas stream. The radiation absorbing
particles are heated indirectly by solar-thermal heating, and they
must be easily separated from the gas after processing. Typically,
these radiation-absorbing particles are carbon black. As used
herein, "indirect" heating means that the heating is by radiation
from a heated wall that is itself heated indirectly or directly by
solar-radiation. In one embodiment, the particles are fine carbon
black particles. Carbon black is chemically stable at extremely
high temperatures and can be easily separated from the flowing
process gas using a filter and/or cyclone separator. Because carbon
is produced according to hydrocarbon dissociation reactions, it is
compatible with the hydrocarbon dissociation type of reactions to
be carried out in the process for producing H.sub.2. Preferably,
the particles comprise recycled carbon black synthesized according
to the dissociation reactions of the present invention. More
preferably, the particles are carbon black particles generated
in-situ from the dissociation of a reactant gas. In this manner,
the carbon black particles can be produced in situ via dissociation
reactions of gaseous hydrocarbons, thereby eliminating the need to
feed the particles into the reactor. Sulfur particles produced from
dissociation of hydogen sulfide are also suitable for use as heat
absorbing particles.
[0068] The radiation absorbing particles must be dispersed in the
reactor apparatus, and the form of dispersion is important. The
particles should flow as a dust or particle cloud through the
apparatus, dispersed in a dispersing process gas. They should have
a fine primary particle size, preferably in the sub-micron size
range, and be non-agglomerated, thus providing the highest surface
area possible for solar radiation absorption. The heat absorbing
particles may be provided as a result of the dissociation reaction.
For example, carbon black particles can be produced in situ via
dissociation reactions of gaseous hydrocarbons. The particles
produced via reactions 1-5 using the methods of the invention
typically have a primary particle size less than about 50 nm and
are essentially amorphous. Carbon black particles produced using
the methods of the invention are essentially ash-free and may be
more amorphous than those produced using other commercially
available carbon black producing processes. The particles may also
be provided by feeding the particles into the reactor.
[0069] When the particles are provided by feeding preformed
particles into the reactor, several different methods can be used
to disperse the particles. The particles can be dispersed
mechanically, such as by shearing on the surface of a rotating drum
or brush. Alternatively, the particles can be dispersed using the
shear provided by high velocity gas exiting with the particles from
a feed injection tube. Experience has shown that the exiting "tip
speed" from the injection tube should be at least 10 m/s to provide
the shear necessary for complete dispersion of fine powders.
Particles generated in-situ are inherently well dispersed in the
process.
[0070] The process gas used for dispersing the particles must be
compatible with the reaction process or easily separated after
processing. It may be a mixture of recycle gases from the process.
Preferred dispersing process gases comprise natural gas,
C.sub.xH.sub.y, CH.sub.4, or H.sub.2, or a combination thereof.
[0071] In general, the radiation absorbing particles flow
co-currently with the flowing gas stream through a reaction shell
to maximize heat transfer from the particles to the gas. The shell
may be oriented horizontally or vertically. For the case of a
vertical reaction shell process, the flow direction may be upward
or downward. Upward flow guarantees that aggregated particles will
not be carried through the reaction shell, and downward flow
reduces the potential for plugging in the solids feed line. A
preferred flow direction is downward with particles generated
internally and separated downstream.
[0072] The second gas stream used to provide the "fluid-wall"
blanket gas flowing inward from the porous "reaction" shell wall is
preferably a non-dissociating gas so as not to plug the pores of
the porous wall. The fluid-wall gas is also selected to be
compatible with the reactants and the products, i.e., so that it
will not interfere with the reaction or be difficult to separate
from the gas stream exiting the reaction shell. The fluid wall gas
is preferentially a product of the reaction being carried out.
Hydrogen (H.sub.2) is a preferred fluid wall gas when carrying out
reactions (1) through (5). The H.sub.2 may be recycled from a
downstream purification process. Inert gases, such as N.sub.2 or
argon are also suitable for use as the second gas stream.
[0073] In reactors having a first and second inner shell and an
outer shell, a third gas stream comprising a non-oxidizing and
non-dissociating "purge" gas flows between the outermost
"transparent or semi-transparent "protection" shell and the solid
"heating" shell. This "purge" gas can be a hydrogen or an inert gas
such as N.sub.2 or argon.
[0074] In the methods of the invention, heat absorbing particles
are heated at least in part with a source of concentrated sunlight
(91). The reactors may be heated by solar energy alone or by a
combination of solar energy and resistance heating of one of the
shells of the reactor. The source of concentrated sunlight (91) may
be a solar concentrator (50), as shown in FIGS. 2 and 3. These two
figures also show unconcentrated sunlight (90) entering the solar
concentrator. Preferably, the solar concentrator of the apparatus
is designed to optimize the amount of solar thermal heating for the
process. Solar fluxes between about 1500 and about 2000 kW/m.sup.2
have been shown to be sufficient to heat the particles to
temperatures between 1675 and 1875 K. More preferably, solar fluxes
between about 2000 and 5000 kW/m.sup.2 are desired to achieve even
higher temperatures and reactor throughputs. Most preferably,
reaction temperatures are approximately 2100 K.
[0075] The sunlight can be provided in the form of a collimated
beam (spot) source, a concentric annular source distributed
circumferentially around the reactor, or in the form of a
linearized slot source providing heating axially along the length
of reactor. The light can be redirected and focused or defocused
with various optical components to provide the concentration on or
in the reactor as required. An example of a suitable solar
concentrator for use in the present invention is the High-Flux
Solar Furnace (HFSF) at the National Renewable Energy Laboratory
(NREL) in Golden, Colo. The HFSF uses a series of mirrors that
concentrate sunlight to an intensified focused beam at power levels
of 10 kW into an approximate diameter of 10 cm. The HFSF is
described in Lewandowski, Bingham, O'Gallagher, Winston and Sagie,
"Performance characterization of the SERI Hi-Flux Solar Furnace,"
Solar Energy Materials 24 (1991), 550-563. The furnace design is
described starting at page 551, wherein it is stated, [0076] The
performance objectives set for the HFSF resulted in a unique
design. To enable support of varied research objectives, designers
made the HFSF capable of achieving extremely high flux
concentrations in a two-stage configuration and of generating a
wide range of flux concentrations. A stationary focal point was
mandatory because of the nature of many anticipated experiments. It
was also desirable to move the focal point off axis. An off-axis
system would allow for considerable flexibility in size and bulk of
experiments and would eliminate blockage and consequent reduction
in power. [0077] In particular, achieving high flux concentration
in a two-stage configuration (an imaging primary in conjunction
with a nonimaging secondary concentrator) dictates a longer f/D
[ratio of focal length to diameter]for the primary [concentrator]
than for typical single-stage furnaces. Typical dish concentrators
used in almost all existing solar furnaces are about f/D=0.6. To
effectively achieve high flux concentration, a two-stage system
must have an f/D=2. Values higher than this will not achieve
significantly higher concentration due to increased losses in the
secondary concentrator. Values lower than this will result in a
reduction of maximum achievable two-stage flux. At low values of
f/D, the single stage peak flux can be quite high, but the flux
profiles are also very peaked and the average flux is relatively
low. With a longer f/D, two-stage system, the average flux can be
considerably higher than in any single-stage system. The final
design of the HFSF has an effective f/D of 1.85. At this f/D, it
was also possible to move the focal point considerably off axis
(.about.30.degree.) with very little degradation in system
performance. This was because of the longer f/D and partly because
of the multi-faceted design of the primary concentrator. This
off-axis angle allows the focal point and a large area around it to
be completely removed from the beam between the heliostat and the
primary concentrator.
[0078] When the outer shell is wholly transparent or has a window
which extends completely around the shell, the concentrated
sunlight is preferably distributed circumferentially around the
reactor using at least one secondary concentrator. Depending upon
the length of the reaction shell, multiple secondary concentrators
may be stacked along the entire length of the reaction shell. For
the HFSF described above, a secondary concentrator that is capable
of delivering 7.4 kW of the 10 kW available (74% efficiency)
circumferentially around a 2.54 cm diameter.times.9.4 cm long
reaction tube has been designed, constructed, and interfaced to the
reactor.
[0079] The invention also provides reactor systems which combine
the reactor of the invention with one or more other system
elements. System elements useful for use in the present invention
include, but are not limited to, particle dispersion and feeding
devices, sources of concentrated solar energy, cooling zones,
filtering devices, various purification devices, hydrogen storage
devices, and thermophotovoltaic devices.
[0080] In one embodiment, a cooling zone is located downstream of
the aerosol transport reactor. The cooling zone is preferably
expanded and of a larger diameter than the inner "reaction" shell.
The cooling zone is preferably ajacketed steel tube with a coolant
flowing within the jacket of the tube. The function of the cooling
zone is to provide a larger volume where product gases and
dissociated product particles can be cooled. The purpose of the
expanded tube is twofold. First, it provides for a reduced velocity
of product gas and entrained particles flowing through it and,
hence, an increased residence time for cooling. Second, the
expanded design allows for dissociated product particles to be
cooled while flowing in the gaseous space, thus, reducing
thermophoretic deposition of fine particles on the cooler wall.
This design reduces the tendency for dissociated product particles
to deposit along the cooling zone wall.
[0081] Additionally, a system element for removing the solid
dissociation products from the gas stream can be provided. Carbon
black particles are preferably removed from the gas stream after
the gas exits the reaction chamber. The carbon black may be removed
from the gas stream by suitable methods as known in the art, such
as by filtration, cyclonic separation, or a combination thereof.
Some of the carbon black particles may be recycled in the process,
preferably providing absorber surfaces for heating the gas. The
carbon black may be sold as a product or may be used as a raw
material to supply a carbon conversion fuel cell for generating
electricity. The carbon black from the solar-thermal dissociation
process is fine sized and preferably substantially free of sulfur
and ash. Hence, it is a preferred feed stock for supplying a carbon
conversion fuel cell.
[0082] The system may also comprise a system element for separating
hydrogen from non-dissociated gaseous components or otherwise
purifying the hydrogen produced by the process.
[0083] The product gas that is separated from the heat absorbing
particles can be purified using a pressure swing adsorber (PSA),
membrane or some other type of gas separation device that will
separate hydrogen from non-dissociated gaseous reactants (i.e.
CH.sub.4, C.sub.xH.sub.y natural gas, etc.) or byproducts of
reaction (e.g. acetylene, etc). Some of the purified hydrogen can
be recycled to the process, preferably as the "fluid-wall" gas and
"purge gas". Some of the recycled hydrogen can be fed to an
upstream hydrogenator to hydrogenate mercaptans that may have been
added to the natural gas. The hydrogenated mercaptans are then
removed along with H.sub.2S in a molecular sieve such as an
adsorption bed of zinc oxide particles. The bulk of the hydrogen
will be used in downstream processes, preferably to supply fuel
cell batteries for stationary generation of electricity or for on
board transportation applications involving fuel cell vehicles. The
purified hydrogen exiting the separation device (PSA or membrane)
may supply a hydrogen pipeline at lower pressure or may be
compressed and stored in a storage tank, such as at a service
station for servicing fuel cell vehicles.
[0084] FIG. 4 illustrates one system comprising a hydrogenation and
ZnO bed (70), solar-thermal fluid wall reactor (1), a cooling zone
(60) immediately following the reactor, a baghouse filter (62), a
low pressure compressor (64), a pressure swing adsorber (66), and a
high pressure compressor (68). The high pressure compressor can
additionally be connected to a hydrogen storage device, not shown
in FIG. 4.
[0085] In another embodiment, a reactor with a transparent outer
shell such as a quartz tube may be coupled to one or more
thermophotovoltaic devices. Thermal radiation from the outermost
inner shell of the reactor (e.g. from a graphite heating tube in a
three shell reactor) which passes out through the transparent outer
shell can be used to power the thermophotovoltaic devices. The
thermophotovoltaic devices are placed in locations not shielded by
the solar concentrator.
[0086] Those of ordinary skill in the art will appreciate that
starting materials, reactor components, reactor system components,
and procedures other than those specifically exemplified can be
employed in the practice of this invention without resort to undue
experimentation. The skilled artisan will be further aware of
materials, methods and procedures which are functional equivalents
of the materials, methods and procedures specifically exemplified
herein. All such art-known functional equivalents are intended to
be encompassed by this invention.
[0087] All references cited herein are incorporated by reference
herein to the extent that they are not inconsistent with the
teachings herein.
EXAMPLES
Example 1
Operation of a Three-Shell Reactor
[0088] In accordance with the present invention, a concentric
three-tube aerosol transport reactor was constructed and vertically
interfaced to the HFSF at NREL. The aerosol transport reactor
consisted of an outer 5.1 cm outside diameter.times.4 mm
thick.times.24 cm long quartz "protection" tube, a central 2.4 cm
outside diameter.times.4 mm thick.times.35.6 cm long graphite
"heating" tube, and a 1.8 cm outside diameter.times.6 mm
thick.times.44 cm long graphite "reaction" tube. The "reaction"
tube consisted of a 30 cm long porous graphite section with 7 cm of
solid graphite tube on both ends of the "reaction" tube. The
porosity of the graphite tube was 49% with a permeability of air
(at standard temperature and pressure (STP)) of 1
ft.sup.3/ft.sup.2/min. It was a sunny day. A secondary concentrator
delivered 7.4 kW of solar-thermal power over a 9.4 cm length. The
concentrator was positioned concentrically around the outer quartz
"protection" tube. A 99% methane/1% argon gas was fed at a rate of
4 standard liters per minute (slpm) into the top of the graphite
"reaction" tube and flowed downward. Hydrogen "fluid-wall" gas was
fed at a rate of 1 slpm to the annular region between the inner
"reaction" tube and the central "heating" tube. The hydrogen flowed
within the annular region and through the porous section of the
"reaction" tube and exited radially inward providing a fluid-wall
of hydrogen along the inside "reaction" tube wall. Argon "purge"
gas flowed at a rate of 2 slpm in the annular region between the
outer quartz "protection" tube and the central solid graphite
"heating" tube. The argon prevented oxidation of the graphite
"heating" tube. No carbon black absorber particles were fed to the
inner "reaction" tube. The temperature of the reactor as measured
by a Type B thermocouple inserted in the hot zone was 1873 K. Feed
gas was flowed for approximately 1 hour. A downstream gas
chromatograph analyzed the steady state composition of the exiting
stream, after the 1 slpm "fluid-wall" hydrogen was subtracted out.
A downstream flowmeter measured the gas flow rate as 3.2 slpm
(after subtracting out the 1 slpm fluid-wall H.sub.2). The
unreacted methane content was 30 mole % with the remaining gas
essentially hydrogen. This corresponded to a conversion of 76% of
the feed methane for a residence time of approximately 0.03
seconds. The system was taken off sun and allowed to cool. No
dissociated carbon was found to be deposited anywhere along the
inside wall of the "reaction" tube. The product carbon black
collected downstream was analyzed by x-ray diffraction and found to
be essentially amorphous carbon black with a primary particle size
between approximately 20 and 40 nanometers. This example
illustrates that the fluid-wall reactor tube prevented deposition
of reaction products within the reactor and allowed continuous
operation.
Example 2
Operation of a Three-Shell Reactor with No "Fluid-Wall" Gas
Flow
[0089] The process conditions of Example 1 were repeated except
that no "fluid-wall" hydrogen gas was flowed through the porous
"reaction" tube. Within 8 minutes, the process was shut down due to
difficulties maintaining feed gas flow using mass flow controllers.
After cooling, the reactor was dismantled and inspected. It was
found that carbon was deposited inside of the "reaction" tube. The
carbon was analyzed by x-ray diffraction and found to contain a
large graphitic content. This comparative example illustrates that,
without the fluid-wall, the reactor plugs and prevents continuous
operation.
Example 3
Reactor Operation with Increased Fluid-Wall and Purge Gas Flow
Rates
[0090] The apparatus described in Example 1 was used except that
the "fluid-wall" gas was changed to argon. It was fed at a rate of
4 slpm through the porous tube wall. In addition, the argon "purge"
gas flow was increased to 10 slpm.
Examples 4 to 12
Reactor Operation with Varying Methane Flow Rate and Solar Flux
[0091] The apparatus described in Example 1 was used with the gas
flow rates given in Example 3. The nonporous "heating" wall
temperature was measured through a hole in the trough section of
the secondary concentrator using a pyrometer. For this apparatus,
the temperature of the wall of the nonporous carbon tube was
typically about 100-200 K higher than the temperature inside the
reaction tube. The solar flux was varied in order to achieve
heating wall temperatures of 1716, 1773, 1923, 2073, and 2140 K.
Although the "fluid-wall" argon flow rate was maintained at 4 slpm,
the methane flow rate was varied from 0.8 to 2.2 slpm. All flow
rates corresponded to average residence times between approximately
10 and 20 milliseconds. The dissociation (conversion) of methane to
hydrogen and carbon black was calculated from the measured
concentration of H.sub.2 and is reported in Table 1. TABLE-US-00001
TABLE 1 (Examples 4 to 12) Methane Dissociation Heating Initial
Conversion Wall Temperature Methane Flow Rate of Methane (K) (slpm)
(%) 1716 0.8 0 .+-. 5 1716 2.2 0 .+-. 5 1773 1 15 .+-. 5 1773 2 18
.+-. 5 1923 1.5 29 .+-. 5 2073 1 69 .+-. 5 2073 2 55 .+-. 5 2140
0.8 81 .+-. 5 2140 2.2 83 .+-. 5
[0092] This set of examples indicates that increasing the solar
flux, which in turn increases the heating wall temperature and the
temperature inside the reactor tube, results in an increase in the
thermal dissociation (conversion) of methane to H.sub.2 and carbon
black. The product carbon black for all runs was analyzed by x-ray
diffraction and transmission electron microscope images to
determine that it was amorphous carbon black with a primary
particle size of 20 to 40 nanometers.
Examples 13 to 24
Dry Reforming With Varying Total Feed Rate and Solar Flux
[0093] The apparatus described in Example 1 was used. During these
experiments, the argon "purge gas" was fed at a rate of 10 slpm.
The "fluid-wall" argon was fed at a rate of 4 slpm. The reactant
gas was maintained at a two to one CH.sub.4 to CO.sub.2 feed ratio.
Total flow rates of 1 and 2 slpm were used. By changing the solar
flux, the heating wall temperature was varied from 1873 to 2123 K,
with increments of 50 K. The conversion of methane was calculated
from the measured concentration of H.sub.2, and the conversion of
CO.sub.2 was calculated from the measured concentration of CO. Both
values are reported in Table 2. TABLE-US-00002 TABLE 2 Examples 13
to 24 (Dry Reforming and Dissociation) Total Flow Heating Wall Rate
of CH.sub.4 CH.sub.4 Conversion CO.sub.2 Conversion Temperature
(.sup.K) and CO.sub.2 (slpm) (%) (%) 1873 1 35 .+-. 14 17 .+-. 11
1925 1 47 .+-. 1 22 .+-. 2 1977 1 58 .+-. 1 35 .+-. 5 2025 1 65
.+-. 7 51 .+-. 18 2074 1 71 .+-. 6 65 .+-. 7 2108 1 69 .+-. 5 56
.+-. 16 1924 2 29 .+-. 5 19 .+-. 9 1924 2 20 .+-. 4 6 .+-. 3 1974 2
27 .+-. 2 8 .+-. 3 2022 2 39 .+-. 3 15 .+-. 3 1801 2 51 .+-. 7 30
.+-. 13 1831 2 55 .+-. 9 35 .+-. 16
[0094] The average residence time of all runs was approximately 10
milliseconds. This set of examples indicates that concentrated
sunlight can be used to carry out dry CO.sub.2 reforming of
CH.sub.4 reactions in short residence times. It is also evident
that both increased temperature and decreased reactant gas flow
rate result in higher conversion of CO.sub.2 to CO and CH.sub.4 to
H.sub.2.
Examples 25 to 30
Reactor Operation During Dry Reforming With Varying Total Feed Rate
and Methane to Carbon Dioxide Feed Ratio
[0095] The apparatus described in Example 1 was used with the flow
rates presented in Example 3. For a given day, the highest
available solar flux level was utilized. This resulted in heating
wall temperatures ranging from 2063 to 2115 K, as seen in Table 3.
Total CH.sub.4 and CO.sub.2 feed rates of 1 and 2 slpm were used.
Three CH.sub.4 to CO.sub.2 feed ratios were utilized: 1 to 1, 1.5
to 1, and 2 to 1. The conversion of methane was calculated from the
measured H.sub.2 concentration, and the CO.sub.2 conversion was
calculated from the measured CO concentration. Both values for each
experiment appear in Table 3. TABLE-US-00003 TABLE 3 Examples 25 to
30 (Simultaneous Dry Reforming and Dissociation) Total Flow Rate of
CH.sub.4 to CO.sub.2 Heating Wall CH.sub.4 CO.sub.2 CH.sub.4 and
Feed Ratio Temperature Conversion Conversion CO.sub.2 (slpm) (molar
volume) (.sup.K) (%) (%) 1 1:1 2063 64 .+-. 7 33 .+-. 7 1 1.5:1
2114 76 .+-. 2 64 .+-. 8 1 2:1 2108 69 .+-. 5 56 .+-. 16 2 1:1 2083
50 .+-. 2 20 .+-. 2 2 1.5:1 2115 58 .+-. 3 34 .+-. 6 2 2:1 2104 55
.+-. 9 35 .+-. 16
[0096] This set of experiments shows that the fluid-wall aerosol
flow reactor can be used to carry out dry CO.sub.2 reforming of
CH.sub.4 reactions with various reactant feed ratios. It also
indicates that for a given total flow rate, changing the feed ratio
does not significantly change the conversion of CH.sub.4 to H.sub.2
or the conversion of CO.sub.2 to CO. However, both conversion
values are increased when the total flow rate of CH.sub.4 and
CO.sub.2 is decreased from 2 slpm to 1 slpm.
* * * * *