U.S. patent application number 16/558917 was filed with the patent office on 2020-01-02 for dual function composite oxygen transport membrane.
The applicant listed for this patent is Uttam R. Doraswami, Sean M. Kelly, Jonathan A. Lane, Jiefeng Lin, Pawel Plonczak. Invention is credited to Uttam R. Doraswami, Sean M. Kelly, Jonathan A. Lane, Jiefeng Lin, Pawel Plonczak.
Application Number | 20200001248 16/558917 |
Document ID | / |
Family ID | 56684709 |
Filed Date | 2020-01-02 |
United States Patent
Application |
20200001248 |
Kind Code |
A1 |
Lin; Jiefeng ; et
al. |
January 2, 2020 |
DUAL FUNCTION COMPOSITE OXYGEN TRANSPORT MEMBRANE
Abstract
A dual function composite oxygen transport membrane having a
layered structure of mixed conducting oxygen transport materials on
a first side of a porous substrate and a reforming catalyst layer
on an opposing second side of the porous substrate. The layered
structure of the mixed conducting oxygen transport materials
contains an intermediate porous layer of mixed conducting oxygen
transport materials formed on the porous substrate with a dense
impervious layer of mixed conducting oxygen transport materials
over the intermediate porous layer, and an optional surface
exchange layer of mixed conducting oxygen transport materials over
the dense impervious layer. The layered structure and the reforming
catalyst layer are formed in separate steps.
Inventors: |
Lin; Jiefeng; (Rochester,
NY) ; Plonczak; Pawel; (Buffalo, NY) ; Kelly;
Sean M.; (Pittsford, NY) ; Doraswami; Uttam R.;
(Bangalore, IN) ; Lane; Jonathan A.; (Snyder,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lin; Jiefeng
Plonczak; Pawel
Kelly; Sean M.
Doraswami; Uttam R.
Lane; Jonathan A. |
Rochester
Buffalo
Pittsford
Bangalore
Snyder |
NY
NY
NY
IN
NY |
US
US
US
US
US |
|
|
Family ID: |
56684709 |
Appl. No.: |
16/558917 |
Filed: |
September 3, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14753815 |
Jun 29, 2015 |
10441922 |
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16558917 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2325/20 20130101;
B01D 67/0039 20130101; B01D 71/024 20130101; B01D 69/10 20130101;
B01D 69/04 20130101; B01D 67/0048 20130101; B01D 53/228 20130101;
B01J 23/892 20130101; B01D 69/12 20130101; B01D 2325/10 20130101;
B01J 23/462 20130101; B01D 71/022 20130101; B01D 2311/2696
20130101; B01J 35/065 20130101 |
International
Class: |
B01D 71/02 20060101
B01D071/02; B01D 67/00 20060101 B01D067/00; B01D 69/12 20060101
B01D069/12; B01J 23/46 20060101 B01J023/46; B01J 23/89 20060101
B01J023/89; B01J 35/06 20060101 B01J035/06 |
Goverment Interests
U.S. Government Rights
[0001] The invention disclosed and claimed herein was made with
United States Government support under Cooperative Agreement number
DE-FC26-07NT43088 awarded by the U.S. Department of Energy. The
United States Government has certain rights in this invention.
Claims
1-10. (canceled)
11. A method of forming a dual function composite oxygen transport
membrane, said method comprising: providing a porous substrate
having a first side and an opposing second side; forming a layered
structure of mixed conducting materials in a sintered state on the
first side of the porous substrate; coating a catalyst layer on the
opposing second side of the porous substrate for catalyzing
endothermic reactions.
12. The method of claim 11 wherein the layered structure of mixed
conducting materials comprises an intermediate porous layer, a
dense layer, and an optional surface exchange layer, and the
forming of the dense layer and the forming of the catalyst layer is
carried out in separate steps.
13. A method of forming a dual function composite oxygen transport
membrane, said method comprising: providing a porous substrate
having a first side and an opposing second side; forming an
intermediate porous layer on the first side of the porous
substrate; forming a dense layer over the intermediate porous
layer; forming a surface exchange layer over the dense layer; and
forming a catalyst layer on the opposing second side of the porous
substrate.
14. The method of claim 13 wherein the forming of the catalyst
layer is carried out after the forming of the surface exchange
layer.
15. The method of claim 13 wherein a catalyst layer coating step in
the forming of the catalyst layer is carried out prior to a high
temperature sintering step in the forming of the surface exchange
layer.
16. The method of claim 13 wherein a catalyst layer coating step in
the forming of the catalyst layer is carried out prior to a coating
step in the forming of the surface exchange layer.
17. The method of claim 13 wherein a catalyst layer coating step in
the forming of the catalyst layer is a wash-coating technique.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to a dual function composite
oxygen transport membrane and a method of manufacturing the article
itself. More specifically, the invention relates to a dual function
composite membrane having a ceramic substrate with a mixed
conducting dense layer on one side of the substrate for oxygen
transport and a catalyst layer on the opposing side of the
substrate for catalyzing endothermic reforming reactions. The
membrane is produced by depositing the mixed conducting dense layer
and the catalyst layer on the opposing sides of the substrate in
separate steps. The catalyst layer is formed using catalyst
material selected to promote endothermic reactions.
BACKGROUND
[0003] Composite oxygen transport membranes have been proposed for
a variety of uses that involve the production of essentially pure
oxygen by separation of oxygen from an oxygen containing feed
through oxygen transport through such membrane. For example, such
membranes can be used in combustion devices to support oxy-fuel
combustion or in reactors for partial oxidation reactions involving
the production of a synthesis gas or generation of heat to support
endothermic chemical reactions.
[0004] In such applications, the composite oxygen transport
membranes contain a dense layer of a mixed conducting material that
allows transport of both oxygen ions and electrons at elevated
temperatures. The dense layer is formed on a ceramic substrate that
functions as a porous support. The dense layer can be composed of a
mixed conductor or two phases of materials, an ionic phase to
conduct the oxygen ions and an electronic phase to conduct the
electrons. Typical mixed conductors are formed from doped
perovskite structured materials. In case of a mixture of materials,
the ionic conductor can be yttrium or scandium stabilized zirconia,
and the electronic conductor can be a perovskite structured
material that will transport electrons or can be a metal or metal
alloy or a mixture of the perovskite type material and metal or
metal alloy. Some known membranes also have additional layers such
as a porous surface exchange layer located on the feed side of the
dense layer to enhance reduction of the oxygen into oxygen ions,
and an intermediate porous layer on the opposing side of the dense
layer. Such a composite membrane is illustrated in U.S. Pat. No.
7,556,676 that utilizes two phase materials for the dense layer,
the porous surface exchange layer and the intermediate porous
layer. These layers are supported on a porous support that can be
formed of zirconia.
[0005] In order to minimize the resistance of the membrane to the
ionic transport, such membranes are made as thin as practical and
are supported on a porous support. Since the resistance to oxygen
transport is dependent on the thickness of the membrane, the dense
layer is made as thin as possible and therefore must be supported.
Another limiting factor to the performance of an oxygen transport
membrane concerns the supporting layers on either side of the dense
layer; these supporting layers may or may not be active for oxygen
ion or electron conducting. These layers themselves can consist of
a network of interconnected pores that can limit diffusion of the
oxygen, or fuel or other substance through the membrane to
facilitate oxygen transport and enhance oxygen flux across the
membrane. Therefore, such support layers are typically fabricated
with a graded porosity in which the pore size decreases in a
direction taken towards the dense layer or are made highly porous
throughout. The high porosity, however, tends to weaken such a
structure. The resulting composite oxygen transport membrane can be
fabricated as a planar element or as a tubular element in which the
dense layer is situated either on the inside surface or the outside
surface of the planar element or tube.
[0006] The composite oxygen transport membranes function by
transporting oxygen ions through a material that is capable of
conducting oxygen ions and electrons at elevated temperatures. An
oxygen containing stream flows on one side, retentate side of the
membrane, at least a portion of which contacts the membrane
surface. Oxygen in the contacting oxygen containing stream ionizes
on the membrane surface and the resultant oxygen ions are driven
through the mixed conducting material and emerge on the opposite
side thereof to recombine into elemental oxygen. In the
recombination, electrons are liberated and are transported back
through the membrane to the retentate side to begin the ionization
cycle. The permeated oxygen reacts with a fuel flowing on the
permeate side of the membrane. The combustion reactions produce
products such as synthesis gases by means of partial oxidation of
the fuel. It is to be noted that the combustion reactions by
combusting at least some of the permeated oxygen produce a
difference in oxygen partial pressure across the membrane that can
serve as a driving potential for oxygen transport across the
membrane. The combustion reactions also produce heat that is used
to raise the temperature of the membrane to an operational
temperature at which the oxygen transport can occur. Heat in excess
of that required to maintain the membrane at a desired operational
temperature can be utilized to supply heat to an industrial process
that requires heating. In syngas production applications the fuel
stream introduced on the permeate side typically contains
combustible species such as hydrogen, carbon monoxide, methane. In
some instances other hydrocarbons may also be present in the fuel
stream. Unreacted combustible gas leaves with the effluent on the
permeate side.
[0007] Use of oxidation catalysts have been proposed to enhance
syngas production. The oxidation catalysts can be incorporated
within mixed conducting layer through which oxygen transport occurs
or the oxidation catalysts can be disposed within the membrane as a
contiguous layer to the mixed conducting layer. For example, U.S.
Pat. No. 5,569,633 discloses surface catalyzed multi-layer ceramic
membranes having a dense mixed conducting multicomponent metallic
oxide layer with a first surface contiguous to a porous support
surface and a second surface coated with catalyst material to
enhance oxygen flux by catalyzing reactions with oxygen separated
from an oxygen containing feed gas. Unexpected benefit of higher
oxygen flux was observed upon coating the membrane surface in
contact with the oxygen containing feed gas with catalytic
material. However, such solutions utilizing oxidation catalysts
initially accelerate the oxygen flux but the performance
deteriorates due to the intense redox cycles experienced by the
oxidation catalyst material, resulting in membrane cracks and
functional layer delamination. U.S. Pat. No. 8,323,463 discussed
impregnating the intermediate porous layer including a layer of
porous support contiguous to the intermediate porous layer with
catalysts such as gadolinium doped ceria to promote oxidation of a
combustible substance, and thus increase oxygen flux. U.S. Pat. No.
4,791,079 advocated the integration of impervious mixed conducting
ceramic layer with a porous catalyst for hydrocarbon oxidation or
dehydrogenation. Lithium or sodium promoted manganese complexes
were suggested as preferred catalysts. U.S. Patent Publication No.
2006/0127656 applied a porous catalytic layer adjacent to the mixed
conducting dense layer for catalytic partial oxidation of
hydrocarbons.
[0008] Use of reforming catalysts has also been proposed to enhance
syngas production by converting the unreacted hydrocarbon present
on the permeate side. The reforming catalyst can be positioned
proximate to the membrane permeate side as distinct catalyst
elements separate from the membrane. Examples of such distinct
catalyst elements include structured catalyst inserts in the form
of pellets, foils, mesh structures, monoliths and the like.
However, such solutions add pressure drop and complexity. The need
continues to exist to advantageously deploy reforming catalyst to
get higher synthesis gas yield, convert more of the methane in feed
stream to synthesis gas by reforming reactions, and manage heat
released from combustion reactions within the membrane to support
endothermic reforming reactions. The reforming catalyst should not
adversely affect oxygen flux, neither introduce contaminants into
the mixed conducting oxygen transport layers nor cause structural
and/or functional degradation.
[0009] As will be discussed the present invention provides a dual
function composite oxygen transport membrane and a method of
manufacturing the article itself. More specifically, the invention
relates to a dual function composite membrane that separates oxygen
as well as catalyzes reforming reactions, wherein said dual
function composite membrane comprises a ceramic substrate with a
mixed conducting dense layer on one side of the substrate for
oxygen transport, and a catalyst layer on the opposing side of the
substrate for catalyzing endothermic reforming reactions. The
membrane is produced by depositing the mixed conducting dense layer
and the catalyst layer on the opposing sides of the substrate in
separate steps. The catalyst layer is formed using catalyst
material selected to promote endothermic reforming reactions
thereby to convert hydrocarbon in the permeate side reaction
mixture into syngas.
SUMMARY OF THE INVENTION
[0010] In one aspect, the present invention provides a dual
function composite oxygen transport membrane that at an elevated
temperature separates oxygen from an oxygen containing gas stream
contacting a first side of the membrane and converts a hydrocarbon
gas contacting a second side of the membrane into syngas by
catalyzing reforming reactions.
[0011] In accordance with this aspect of the present invention, the
dual function composite oxygen transport membrane comprises a
porous substrate having a first side and an opposing second side; a
layered structure comprising at least a dense layer to transport
oxygen ions and electrons; a layered structure comprising at least
a catalyst layer also referred to as a reforming catalyst layer or
a catalyst containing layer to catalyze reforming reactions. The
dense layer containing layered structure is provided on the first
side of the porous substrate forming the first side of the
membrane. The catalyst containing layered structure is provided on
the opposing second side of the porous substrate forming the second
side of the membrane. The porous substrate is an integral part of
the membrane, provides mechanical support for the layered
structures on the first side and the opposing second side of the
substrate, and separates the dense layer and the catalyst
containing layer.
[0012] In another aspect, the present invention provides a dual
function composite oxygen transport membrane comprising a porous
substrate having a first side and an opposing second side with a
defined thickness between the first side and the opposing second
side; a dense layer having electronic and ionic conducting phases
for oxygen transport across the dense layer; a catalyst layer to
catalyze conversion of a hydrocarbon gas upon contacting the
catalyst layer into syngas; wherein the dense layer is located on
the first side of the porous substrate and the catalyst layer is
located on the opposing second side of the porous substrate, spaced
apart from the dense layer, i.e, the dense layer and the catalyst
layer are separated at least by the porous substrate.
[0013] In yet another aspect the present invention provides a
method of forming a dual function composite oxygen transport
membrane. The method includes: forming a porous substrate having a
first side and an opposing second side with a defined thickness
between the first side and the opposing second side; forming a
plurality of mixed conducting layers (each having a defined
thickness and a defined composition) on the first side of the
porous substrate to provide oxygen by oxygen transport for oxy-fuel
combustion; forming a catalyst layer on the opposing second side of
the porous substrate wherein the catalyst layer catalyzes reforming
of a hydrocarbon gas to form syngas upon contacting the catalyst
layer.
[0014] In one embodiment of the present invention the dual function
composite oxygen transport membrane comprises a layered structure
of mixed conducting oxygen transport layers formed on a first side
of a porous support and a reforming catalyst layer also referred to
as a catalyst layer formed on an opposing second side of the porous
support. The layered structure of mixed conducting oxygen transport
layers contain at least a mixed conducting layer referred to as a
dense layer having an electronic phase and an ionic phase, wherein
the electronic phase comprising
(La.sub.1-xM.sub.x).sub.wCr.sub.1-y-zFe.sub.yM'.sub.zO.sub.3-.delta.,
where M: Ba, Sr, Ca; M': Co, Ni, Ru, x is from about 0.1 to about
0.5, w is from about 0.90 to about 1.0, y is from 0.00 to 1, z is
from about 0.00 to about 0.2, and .delta. renders the compound
charge neutral; and wherein the ionic phase comprises
Zr.sub.1-x'Sc.sub.x'A.sub.y'O.sub.2-.delta., where x' is from about
0.1 to about 0.22, y' is from about 0.01 to about 0.04, and A is Y
or Ce or mixtures of Y and Ce. The porous substrate can be formed
of Zr.sub.1-x''B.sub.x''O.sub.2-.delta., where x'' is from about
0.05 to about 0.13, B is Y or Sc or Al or Ce or mixtures of Y, Sc,
Al, and Ce. The catalyst layer can be formed of composites of
reforming catalyst active metals, catalyst promoters and catalyst
support materials. The catalyst metal can be one or more of nickel,
cobalt, rhenium, iridium, rhodium, ruthenium, palladium, platinum
or their combinations. The catalyst support materials are high
surface area ceramic composites such as Al.sub.2O.sub.3, ZnO.sub.2,
CeO.sub.2, TiO.sub.2, or mixture of these materials. The catalyst
promoters include CaO, La.sub.2O.sub.3, MgO, BaO, SrO,
Y.sub.2O.sub.3, K.sub.2O or mixtures of these materials. Catalyst
metal could also be doped in a high temperature stable structure
such as perovskite, pyrochlore, hexaaluminate, spinels, zeolite, or
mixture of these materials.
[0015] In another embodiment of the present invention the dual
function composite oxygen transport membrane further comprises an
intermediate porous layer between the dense layer and the first
side of the porous substrate wherein the intermediate porous layer
is comprised of an electronic phase and the ionic phase.
[0016] In yet another embodiment of the present invention the dual
function composite oxygen transport membrane further comprises a
surface exchange layer overlying the dense layer so that the dense
layer is located between the surface exchange layer and the
intermediate porous layer and wherein the surface exchange layer
comprises an electronic conductor and an ionic conductor; the
electronic conductor of the surface exchange layer further
comprises
(La.sub.1-xM.sub.x).sub.wCr.sub.1-y-zFe.sub.yM'.sub.zO.sub.3-.delta.,
where M: Ba, Sr, Ca; M': Co, Ni, Ru, x is from about 0.1 to about
0.5, w is from about 0.90 to about 1.0, y is from 0.00 to 1, z is
from about 0.00 to about 0.2, and .delta. renders the compound
charge neutral; and wherein the ionic phase comprises
Zr.sub.1-x'Sc.sub.x'A.sub.y'O.sub.2-.delta., where x' is from about
0.1 to about 0.22, y' is from about 0.01 to about 0.04, and A is Y
or Ce or mixtures of Y and Ce.
[0017] The dual function composite oxygen transport membrane in
some embodiments can be configured wherein: the electronic phase of
the dense layer comprises
(La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.3O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.7Fe.sub.0.3O.sub.3-.delta.
and the ionic phase of the dense layer comprises
Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta.; the electronic
phase of the intermediate porous layer comprises
(La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
or (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.3O.sub.3-6 or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.7Fe.sub.0.3O.sub.3-.delta.
and the ionic phase of the intermediate porous layer comprises
Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta.; the electronic
phase of the surface exchange layer comprises
(La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.3Fe.sub.0.7O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.3Fe.sub.0.7O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
or
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.;
and the ionic phase of the surface exchange layer comprises
Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta.; the porous
substrate further comprises
Zr.sub.0.923Y.sub.0.077O.sub.2-.delta..
[0018] The dual function composite oxygen transport membrane can be
configured wherein the ionic phase of the dense layer constitute
from about 35 percent to about 65 percent by volume of the dense
layer; the ionic phase of the intermediate porous layer constitute
from about 35 percent to about 65 percent by volume of the
intermediate porous layer; the ionic conductor of the surface
exchange layer constitute from about 35 percent to about 65 percent
by volume of the surface exchange layer.
[0019] The dual function composite oxygen transport membrane can be
configured, wherein: the porous substrate has a thickness from
about 0.7 mm to about 2.5 mm, an average pore size from about 0.5
microns to about 5 microns, and a porosity from about 20 percent to
about 50 percent; the intermediate porous layer has a thickness
from about 10 microns to about 100 microns, an average pore size
from about 0.1 microns to about 1 micron, and a porosity from about
25 percent to about 50 percent; and the surface exchange layer has
a thickness from about 10 microns to about 25 microns, an average
pore size from about 0.1 microns to about 1 micron, and a porosity
from about 25 percent to about 50 percent; and the catalyst layer
has a thickness from about 2 microns to 250 microns, an average
pore size from about 0.5 microns to about 10 micron, and a porosity
from about 50 percent to about 80 percent; preferably the catalyst
layer has a porosity greater than the porosity of the porous
substrate.
[0020] In certain embodiments of the invention the dual function
composite oxygen transport membrane can be formed following a
sequenced stepwise protocol that comprises forming a layered
structure of mixed conducting oxygen transport materials on a first
side of a porous substrate, and forming a catalyst layer on an
opposing second side of the porous substrate in separate steps.
Furthermore, the formation of a layered structure of mixed
conducting oxygen transport materials comprises contacting the
first side of the porous substrate with one or more slurries
containing some common and some different ingredients to build the
layered structure with layers differing in composition, properties
and/or surface characteristics. The layered structure formed is an
overlay structure wherein an intermediate porous layer is first
formed on the first side of the porous substrate, next a dense
layer is formed over the intermediate porous layer, and then a
surface exchange layer is optionally formed over the dense layer.
The catalyst layer is formed on the opposing second side of the
porous support using a wash-coating technique.
BRIEF DESCRIPTION OF THE DRAWING
[0021] While the specification concludes with claims distinctly
pointing out the subject matter that applicants regard as their
invention, it is believed that the invention would be better
understood when taken in connection with the accompanying figures
wherein like numbers denote same features throughout and
wherein:
[0022] FIG. 1 is a cross-sectional schematic view of a dual
function composite oxygen transport membrane;
[0023] FIG. 2 is a process flow diagram for the production of the
dual function composite oxygen transport membrane of the present
invention;
[0024] FIG. 3 is an alternate process flow diagram for the
production of the dual function composite oxygen transport membrane
of the present invention;
[0025] FIG. 4 is an alternate process flow diagram for the
production of the dual function composite oxygen transport membrane
of the present invention;
[0026] FIG. 5 thru FIG. 8 show SEM micrographs of cross-sections of
internal surfaces of catalyst layers formed according to present
invention.
DETAILED DESCRIPTION
[0027] Dual Function Composite Oxygen Transport Membrane
[0028] With reference to FIG. 1, a sectional, schematic view of a
dual function composite oxygen transport membrane 1 of the present
invention is illustrated. Dual function composite oxygen transport
membrane 1 has a porous substrate 10 that has a first side 18 and
an opposing second side 22. The porous substrate serves as a
building block of the dual function composite oxygen transport
membrane that supports layers of different functional materials
located on either side of the substrate. As could be appreciated by
those skilled in the art, such dual function composite oxygen
transport membrane 1 could be configured as a dual function
composite oxygen transport membrane element in the form of a tube
or a flat plate. Such composite oxygen transport membrane tube or
plate would be one of a series of such elements situated within a
device to carry out chemical conversions such as converting a
hydrocarbon gas into syngas by endothermic reforming reactions. In
an application such as desiring syngas as the product, the dual
function composite oxygen transport membrane can be configured as a
tube made up of a porous substrate (also referred to as porous
support) 10 with a plurality of oxygen transport mixed conducting
layers on the first side (outside surface also referred to as
exterior surface of the tube) 18, and a reforming catalyst layer on
the opposing second side (inside surface also referred to as
interior surface of the tube) 22.
[0029] Porous Support
[0030] The porous substrate 10 could be formed from partially
stabilized zirconia oxide e.g. 3, 4 or 5 mole % yttria stabilized
zirconia or fully stabilized zirconia. Alternatively the porous
substrate can be formed from a mixture of MgO and
MgAl.sub.2O.sub.4. Alternatively the porous substrate could be a
porous metal, although not part of the present invention. As would
be appreciated by those skilled in the art, porous substrate 10
also referred to as porous support or porous support layer should
provide as open an area as possible while still being able to be
structurally sound in its supporting function. Porous support
structures for application in composite oxygen transport membranes
are best characterized in terms of their porosity, strength and
effective oxygen diffusivity. The porous support forms the
mechanical support for the "active" membranes layers, so should
have sufficient strength at high temperatures. A typical support
structure in this application would have total porosity in the
range of about 20 to about 50%. An important property of the porous
substrate is the ability to allow gaseous species such as H.sub.2,
CO, CH.sub.4, H.sub.2O and CO.sub.2 to readily move through the
porous support structure to and from the membrane `active` layers.
The ability of the substrate to allow gaseous transport can be
characterized by effective oxygen diffusivity, D.sub.eff O2-N2. For
this application it has been determined that a D.sub.eff O2-N2 more
than 0.005 cm.sup.2/s measured at room temperature is preferred.
The porous substrate should also possess a thermal expansion
coefficient not more than 10% different from that of the membrane
`active` layers between room temperature and membrane operation
temperature.
[0031] Oxygen Transport Mixed Conducting Layers
[0032] The oxygen transport mixed conducting layers comprise a
first mixed conducting layer 12 also referred to as first layer or
intermediate porous layer or innermost mixed conducting layer, a
second mixed conducting layer 14 also referred to as second layer
or dense layer or impervious dense layer, and a third mixed
conducting layer 16 also referred to as third layer or surface
exchange layer or outermost mixed conducting layer. These layers
are formed on the first side 18 of the porous substrate 10. A
catalyst layer is formed on the opposing second side 22 of the
porous substrate. The dual function composite oxygen transport
membrane is specifically designed to function in an environment in
which air or oxygen containing stream is introduced and contacted
with the outermost mixed conducting layer on the first side 18, and
a fuel or other combustible substance is introduced and contacted
with the catalyst layer on the opposing second side 22 of the
porous substrate 10. The fuel is subjected to combustion supported
by permeated oxygen to provide the partial pressure difference
necessary to drive oxygen transport and also to heat the membrane
to an operational temperature at which oxygen transport will occur.
As such, the first layer 12, which, as will be discussed, serves as
a porous fuel oxidation layer at which fuel combusts with permeated
oxygen. This porous oxidation layer may optionally include a
combustion catalyst to promote combustion reactions. In this
regard, the term "fuel" when used in connection with this layer,
both herein and in the claims, is not intended to be limiting, but
rather, to indicate and include any substance that can be oxidized
through permeation of oxygen through the membrane. The second layer
14 is a gas tight active dense layer that is impervious to gas and
allows only ion transport, in this case principally oxygen ions,
and is commonly referred to as dense layer or dense separation
layer. The third layer 16 serves to initially reduce the oxygen in
oxygen containing gas such as air contacting the third layer into
oxygen ions and thus serves as a porous surface activation layer.
Each of the first layer 12, the second layer 14 and the third layer
16 after heating and sintering will preferably each have a
thickness of about 10 .mu.m to about 100 .mu.m.
[0033] Turning attention to the composition of the oxygen transport
mixed conducting layers, a stabilized zirconia, namely,
Zr.sub.1-x-yA.sub.xB.sub.yO.sub.2-.delta. is a common material in
all three "active" membrane layers, namely, the first layer 12, the
second layer 14 and the third layer 16. As mentioned above in all
of these layers oxygen transport occurs and as such, are "active".
In order to generate industrially relevant levels of oxygen ion
conductivity, A and B are typically Sc, Y, Ce, Al or Ca.
Preferably, such stabilized zirconia has a composition given by
formula: Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta., often
noted as 10Sc1YSZ in literature associated with this class of
membrane. However it should be noted that many different
combinations of Sc, Y, Ce, Al, Ca or other elements can be
substituted to achieve the same end. The first layer 12,
intermediate porous layer is configured to have a high surface area
where fuel can react with oxygen or oxygen ions that recombine and
become available. The second layer 14, the dense layer, functions
to separate oxygen from an oxygen containing feed in contact with
the third layer, porous surface exchange layer 16 and contains an
electronic and ionic conducting phases. As discussed above, the
electronic phase of
(La.sub.1-xSr.sub.x).sub.wCr.sub.1-y-zFe.sub.yM'.sub.zO.sub.3-.delta.,
where M' is a metal: Co, Ni, Ru, x is from about 0.1 to about 0.5,
w is from about 0.90 to about 1.0, y is from 0.00 to 1, z is from
about 0.00 to about 0.2, and .delta. renders the compound charge
neutral. The ionic phase is Zr.sub.1-x'-y'Sc.sub.x'A.sub.y',
O.sub.2-.delta., where x' is from about 0.1 to about 0.22, y' is
from about 0.01 to about 0.04 and A is Y or Ce or a mixture of Y
and Ce. The porous support layer 10 is formed of
Zr.sub.1-x'A.sub.x'O.sub.2-.delta., where x'' is from about 0.05 to
about 0.13, A is Y or Sc or Al or Ce or mixtures thereof. The third
layer 16, a surface exchange layer is formed from a mixture of
particles of
(Ln.sub.1-xA.sub.x).sub.wCr.sub.1-yB.sub.yO.sub.3-.delta. and
10Sc1YSZ and optionally pore formers. In this layer, Ln is La, Y,
Pr, Ce or Sm, A is Ca, Sr, Ba, B can be Mn, Fe, Co Al, Ti or
combinations thereof; w is 0.9 to 1.1, x is 0.1 to 0.4 and y is 0.1
to 0.6. The
(Ln.sub.1-xA.sub.x).sub.wCr.sub.1-yB.sub.yO.sub.3-.delta. and
10Sc1YSZ of this layer after sintering should be present within a
first volume ratio of between 2 to 3 and 4 to 1 on a volume
percentage basis.
[0034] Reforming Catalyst Layer
[0035] A reforming catalyst layer 30 is located on the second side
22 of the porous substrate 10, separated (spaced apart) from the
first layer 12 located on the first side 18 of the porous substrate
10. The formation of this catalyst layer on the second side 22 of
the substrate is carried out as a separate step after formation of
at least the dense layer 14 on the first side of the substrate.
Highly porous reforming catalyst layer accelerates the endothermic
hydrocarbon reforming to produce syngas. The separation between the
oxygen transport layer and the reforming catalyst layer protects
the metal catalysts from exposure to both oxidative and reducing
environments and avoids catalyst redox cycles and internal stress
buildup.
[0036] Common catalyst coating techniques such as wash-coating,
dip-coating, spray deposition, and tape-casting of suspension or
sol-gel catalyst slurry can be applied to form the catalyst layer
30. The ingredients of a coating slurry can include one or more of
the following: catalyst in the form of metal or metal oxide or
metal precursors such as metal nitrate, ceramic support oxides as
catalyst carriers, high temperature stabilizers and promoters,
organic binders such as polyvinyl butyral (PVB), and optionally one
or more pore formers (e.g., carbon black, walnut shell, and
Poly-methyl methacrylate with either aqueous or alcohol or toluene
solvents. Alternately mixtures of catalyst metal and ceramic
carrier powders or commercially available supported catalyst
powders can be milled down to desired particle size to prepare the
slurry for coating on the substrate layer. Yet another alternate is
to pre-coat the porous ceramic composites such as Al.sub.2O.sub.3,
YSZ, CeO.sub.2 on the substrate layer of the dual function
composite oxygen transport membrane and then impregnate the coated
porous ceramic composite with catalyst metal precursors.
[0037] The preferred reforming catalysts include nickel, cobalt,
rhenium, iridium, rhodium, ruthenium, palladium, platinum, or their
combinations. The catalyst carrier candidates could be high surface
area ceramic materials such as Al.sub.2O.sub.3, ZnO.sub.2,
CeO.sub.2, TiO.sub.2, pervoskite, pyrochlore, hexaaluminate
supports, or mixtures of these materials. The high temperature
promoters may include CaO, La.sub.2O.sub.3, MgO, BaO, SrO,
Y.sub.2O.sub.3, K.sub.2O, spinel structured materials, or mixtures
of these materials. Organic binders not only determine the coating
layer adhesion, but also affect the micro-tunnels in the catalyst
layer. So it is preferred to be pre-mixed with alcohol solvent
(e.g., 12 wt. % PVB in IPA) to enhance its homogenous mix before
adding into other ingredients.
[0038] If included, the pore former particle size and loading are
preferably in the ranges of 0.5 to 8 .mu.m and 15 wt % to 35 wt %,
respectively. These pore formers are determined to develop a highly
porous network of catalyst coating layer on the porous substrate
and prevent blockage of gas flow paths in both catalyst layer and
porous substrate. They facilitate desired porosity (preferably 55%
to 70% porosity). The particle size of ceramic oxides is preferred
to be close to or greater than the diameter of the support layer
microchannel to minimize particle impregnation into the support
layer and blockage of gas flow through the channel. Thickness of
porous catalyst coating can be controlled by slurry viscosity and
coating times and is preferred to be greater than about 5 microns,
more preferably in the range of about 40 microns to about 150
microns to provide a mechanically stable catalyst layer having
sufficient surface area to obtain desired methane conversion.
Catalyst layers that are thicker, for example greater than 200
microns, may be structurally less stable, developing cracks and/or
delaminate. It is preferred to have thermal shrinkage rate of the
catalyst layer to be the same or as close as possible to that of
the porous substrate to prevent layer delamination and/or cracking;
this can be achieved for example by proper choice of composition
and/or thickness of catalyst layer.
[0039] The catalyst coating process can be implemented at different
steps in the manufacturing of the dual function composite oxygen
transport membrane. As shown in FIG. 2, first all three oxygen
transport mixed conducting layers, namely intermediate porous
layer, dense layer, and surface exchange layer are formed and then
catalyst layer is coated. FIG. 3 show another approach in which
only intermediate porous layer and dense layer are first formed,
then catalyst layer coated on the inside of the tube followed by
surface exchange layer formation over the dense layer to complete
the oxygen transport membrane architecture on the outside of the
tube. Preferably the catalyst coating step should be introduced
after at least dense layer was formed to avoid adverse effects of
exposure for long periods of time to high temperatures required to
sinter the dense layer; formation of inactive spinel structure of
transitional metals such as NiAl.sub.2O.sub.4 in the catalyst layer
could be accelerated; the catalyst layer could lose porosity, pore
structures as well as surface area, and result in significant
catalyst activity reduction.
[0040] It is preferred to integrate catalyst coating right before
or after the surface exchange (cathode) layer coating, because
these two coating layers are on the opposite side of the membrane
and could be sintered by co-firing at the same time. The
thicknesses of intermediate mixed conducting porous (anode) layer,
dense layer, and surface exchange porous (cathode) layer of a dual
function composite oxygen transport membrane can be about 10 .mu.m
to 100 .mu.m each, while the catalyst layer with porosity of 70%
and pore size of 6 .mu.m can have a thickness of about 20 .mu.m to
200 .mu.m. Highly porous catalyst surface geometry offers reduced
diffusional resistance and provides significantly more catalytic
surface area.
[0041] Yet another approach, shown in FIG. 4 is to first form a
reactor element comprising at least a first porous support tube (or
some other geometry) with mixed conducting oxygen transport layers
on the outside surface and a second porous tube (or some other
geometry) also with mixed conducting oxygen transport layers on the
outside, that are coupled together to provide a continuous flow
path to a fluid introduced at one end of the first tube to exit at
the other end of the second tube. The catalyst layer is then
deposited on the inside surface of the porous support tubes that
already have undergone formation of the three oxygen transport
mixed conducting layers in a layered structure, namely intermediate
porous layer, dense layer, and surface exchange layer on the
outside surface of the substrate tube. Such reactor elements are
discussed in pending U.S. Patent Publication 2015/0098872, which is
incorporated herein by reference.
[0042] Catalyst Layer Benefits
[0043] The dual function composite oxygen transport membrane is
operated at relatively high temperature (above 950.degree. C.) and
can advantageously produce high quality of syngas while sustaining
high oxygen flux performance. Furthermore, the catalytic reforming
of hydrocarbon fuels by the dual function composite oxygen
transport membrane enhances syngas yield, considerably lowers
methane slip and could facilitate elimination of downstream methane
removal depending on syngas end use process.
[0044] The endothermic reforming of methane catalyzed by the dual
function composite oxygen transport membrane catalyst layer
produces hydrogen and carbon monoxide. Some of the hydrogen and/or
carbon monoxide produced can diffuse into the porous substrate that
is an integral part of the dual function composite oxygen transport
membrane, and react with oxygen permeating the dense layer within
the dual function composite oxygen transport membrane. The
exothermic oxidation reactions consume permeated oxygen,
facilitating a difference in partial pressure of oxygen across the
membrane.
[0045] The dual function composite oxygen transport membrane can
advantageously manage the heat released from oxy-combustion of fuel
species with permeated oxygen that occurs in and near the
intermediate porous layer. These exothermic reactions generate a
considerable amount of heat, some of which supports endothermic
reactions such as hydrocarbon reforming catalyzed by the catalyst
layer located on the porous substrate. The porous substrate
separating the intermediate porous layer and the catalyst layer may
have a thickness several orders in magnitude to that of any of
these layers. A temperature gradient exists with heat flowing from
the oxy-combustion reaction region to the endothermic reforming
region. This helps prevent dual function composite oxygen transport
membrane oxygen flux reduction due to over cooling from catalytic
reforming.
[0046] Fabrication Method
[0047] With reference to FIG. 2, the process flow for producing a
dual function composite oxygen transport membrane in accordance
with one aspect of the present invention is provided.
[0048] The porous substrate 10 is first formed in a manner known in
the art. For example, using an extrusion process the porous
substrate could be formed into a tube in a green state and then
subjected to a bisque firing at 1050.degree. C. for 4 hours to
achieve reasonable strength for further handling. After firing, the
resulting porous substrate tube can be checked for porosity and
permeability. Then oxygen transport mixed conducting layers, namely
intermediate porous layer 12, dense layer 14 and surface exchange
layer 16 can be formed on the porous substrate, for example as
discussed in U.S. Pat. No. 8,795,417.
[0049] Table 1 lists the ingredients used to form the oxygen
transport mixed conducting layers on a tubular porous substrate in
the examples described below. The ionic conductive and electronic
conductive materials used to form intermediate porous layer and
dense layer in the examples are same, however this need not be the
case. Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta.(d50<0.6
.mu.m; from Daiichi Kigenso Kagaku Kogyo Co., Ltd.) was used as
ionic conductive material and
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
perovskite powder (d50 in the range of about 0.30 .mu.m to about
0.35 .mu.m; Praxair Specialty Ceramics) was used as electronic
conductive material.
TABLE-US-00001 TABLE 1 Oxygen transport mixed conducting Ionic
conductive Electronic conductive Pore layer composite composite
Binder Solvent former Intermediate
Zr.sub.0.802Sc.sub.0.18Y.sub.0.018O.sub.2-.delta.
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
Ferro Toluene Carbon porous B73210 black layer Dense
Zr.sub.0.802Sc.sub.0.18Y.sub.0.018O.sub.2-.delta.
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
Ferro Toluene N/A layer B73210 Surface
Zr.sub.0.802Sc.sub.0.18Y.sub.0.018O.sub.2-.delta.
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.3Fe.sub.0.7O.sub.3-.delta.
Ferro Toluene Carbon exchange B73210 black layer
[0050] For the dense layer, a 120 g batch of slurry was prepared
using 51 g of
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.5Fe.sub.0.5O.sub.3-.delta.
mixed with 69 g of
Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta., 60 g Ferro
B73210 binder, 255 g Toluene and 1200 g of 1.5 mm diameter YSZ
milling media in a 32 oz NALGENE bottle. The mixture was milled for
about 2.25 hours or until the particle size of the mixture was in
the range 0.3-0.35 .mu.m. For the intermediate layer, slurry was
prepared by adding 18 g of carbon black (pore former) to the dense
layer recipe.
[0051] For the surface exchange layer 16, 51 g of electronic
conductive material
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.3Fe.sub.0.7O.sub.3-.delt-
a. perovskite powder (from Praxair Specialty Ceramics) was mixed
with 69 g of ionic conductive material
Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-.delta., 60 g Ferro
B73210 binder, 255 g Toluene, 18 g carbon black and 1200 g of 1.5
mm diameter YSZ milling media in a 32 oz NALGENE bottle. The
mixture was milled for about 2.25 hours or until the particle size
of the mixture was in the range 0.3-0.35 .mu.m.
[0052] The tubular porous substrate structure was first coated with
the intermediate porous layer by contacting the outside surface of
the tubular porous substrate structure with the intermediate layer
slurry, at least twice to ensure final thickness was in the range
of about 10 to about 30 The dried intermediate layer was then
coated by contacting with a dense layer slurry, at least two times
to ensure final thickness was in the range of about 10 .mu.m to
about 30 Resulting coated tubular structure was then dried at room
temperature for about 1 to 2 hours before sintering at an elevated
temperature above 1350.degree. C.-1400.degree. C. for 6 hours in a
nitrogen environment. The sintered dense layer was then subjected
to a surface exchange layer coating step by contacting the sintered
dense layer with a surface exchange layer slurry. This was followed
by a drying step (at room temperature for 1 to 2 hours), and a high
temperature sintering step (air fired at 1250.degree. C. for half
an hour) to complete the surface exchange layer formation.
[0053] Catalyst layer 30 can be formed preferably by a wash-coating
technique. As shown in FIG. 2, the catalyst layer formation step
can be introduced into the manufacturing process after surface
exchange layer formation. The catalyst formation step comprises a
catalyst layer coating step, followed by optional air drying and
organics burn-off. The catalyst layer coating step comprises
contacting the inside surface of the tubular porous substrate
structure with a catalyst layer slurry also referred to as catalyst
coating layer slurry. The air drying and organics burn-off can be
carried out as separate steps or combined into a single step. FIG.
3 shows an alternate process flow for producing a dual function
composite oxygen transport membrane wherein the catalyst layer
coating step is carried out prior to the surface exchange layer
high temperature sintering step, and preferably prior to the
surface exchange layer coating step. The catalyst layer organics
burn-off step and the surface exchange layer high temperature
sintering step can be merged into a single step or can be carried
out simultaneously while providing atmospheres and operating
conditions (temperatures, pressures, and flows) to the catalyst
layer that are appropriate for organics burn-off, and to the
surface exchange layer that are appropriate for high temperature
sintering. This way process efficiency gains, as well as capital
and operating cost savings can be achieved. FIG. 4 shows yet
another process flow wherein a plurality of oxygen transport
membrane elements having mixed conducting oxygen transport layers
on the outside surface are treated to form a catalyst layer on the
inside surface of each element, thereby transforming them into dual
function composite oxygen transport membrane reactor elements.
[0054] Table 2 lists the ingredients used to form catalyst layer in
the dual function composite oxygen transport membrane examples
described below.
TABLE-US-00002 TABLE 2 Active Metal Ceramic Pore Dispersant metal
precursor Promoter carrier Binder Solvent former agent Ni--Rh
Ni(NO.sub.3).sub.2.cndot.6H.sub.2O, TZ-4YS Alpha- 12 wt. % Ethanol
PMMA KD-2 Rh(NO.sub.3).sub.3 Al.sub.2O.sub.3 PVB in ethanol Ru
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.8Fe.sub.0.15Ru.sub.0.05O.sub.3--
.delta. N/A N/A Ferro Toluene Carbon KD-1 B73210 black
Example 1: Nickel-Rhodium Based Catalyst Layer after Surface
Exchange Layer Sintering (FIG. 2)
[0055] 25 g of Alpha-phase aluminum oxide (1 .mu.m average particle
size, 8 to 10 m.sup.2/g surface area, from Alfa Aesar) and 8.5 g of
TZ-4YS with 4 mole % yttria stabilized zirconia powder (0.5 .mu.m
average particle size, from Tosoh Corporation) were dispersed in
200 mL of ethanol and 7 mL of KD-2 dispersant agent (Hypermer.TM.).
Adding 500 g of 1.5 mm diameter YSZ milling media into the
container, the mixture was milled on the roller mill (170 to 175
rpm) for 2 hours. The final particle size of the slurry was in the
range of about 0.5 to about 0.8 .mu.m. Along with 10 g of pore
former poly(methyl methacrylate) PMMA with average particle size of
6 .mu.m, 30 g of nickel nitrate hexahydrate
Ni(NO.sub.3).sub.2.6H.sub.2O and 0.5 g of Rh(NO.sub.3).sub.3 (both
from Sigma-Aldrich) were added into the mixture and mixed for
additional one hour. 12% by weight of plastic binder polyvinyl
butyral powder was first dissolved in ethanol solvent to enhance
its homogenous mixing and then 150 mL of resulting binder solution
was slowly added into the slurry mixture. The resulting mixture was
further milled for 1.5 hours to form sol-gel slurry.
[0056] The above prepared sol-gel slurry can be used to form a
catalyst layer containing Ni and Rh as active metals. Alternately
the sol-gel slurry can be prepared without the addition of
Rh(NO.sub.3).sub.3 to form a catalyst layer containing Ni as the
active metal. The Ni and Rh containing, as well as, Ni only
catalyst layer can be formed on the inside of a tubular composite
oxygen transport membrane.
[0057] The sol-gel slurry prepared as described above and having a
viscosity preferably in the range of about 25 centipoise to about
50 centipoise was used to wash-coat a catalyst layer on the inside
surface of a yttria-stabilized zirconia (YSZ) porous substrate tube
already coated with oxygen transport mixed conducting layers on the
outside surface. The tube, 7 mm ID and 24 inches long had been made
from a YSZ paste by a conventional extrusion process followed by
bisque firing at elevated temperature. Tubes made this way can have
a wall thickness in the range of about 0.7 mm to about 2.5 mm,
sufficient to operate at elevated temperatures and pressures. The
particular tube used in this example had a wall thickness of 1 mm.
The porosity of tube is preferred to be within the range of 25 to
45% for this application. The particular tube used in this example
had a porosity of 34%. Oxygen transport mixed conducting layers,
namely: surface exchange layer, dense layer, and intermediate
porous layer formed on the outside surface of the porous support
(YSZ) tube contained mixed ionic and electronic conductive (MIEC)
dual-phase materials. After forming the intermediate layer and
dense layer on the YSZ support tube, the tube was dried at room
temperature and then sintered at an elevated temperature of about
1350.degree. C. to about 1400.degree. C. to have a thickness in the
range of about 10 microns to about 30 microns. Then after treating
the tube with surface exchange layer slurry, the tube was sintered
at an elevated temperature of about 1250.degree. C. to complete the
formation of surface exchange layer. The composite oxygen transport
membrane tubes prepared in this manner are preferred to have a
thickness in the range of about 10 microns to about 30 microns. The
particular tube used in this example had an intermediate layer
about 15 microns thick, a dense layer about 15 microns thick, and a
surface exchange layer about 10 microns thick. Prior to
wash-coating, the tube was inspected and appropriate measures taken
to remove any dust on the inside surface of the tube, for example
by blowing air through the tube. The tube vertically positioned and
with one end plugged was gradually filled with sol-gel slurry until
the inside of the tube was completely filled. The liquid level
slightly dropped due to potential migration of liquid into the
porous substrate by capillary action; as needed slurry was added to
keep the tube completely filled. After waiting for about a minute
the slurry was slowly drained out of the tube, and the tube dried
at room temperature by flowing air for about 30 minutes at a low
flow rate, in the range of about 10 scfh to about 40 scfh. An inert
dry gas can be used instead of air for drying. The organic binder
and pore former in the catalyst layer were burned off by vertically
fixing the catalyst coated tube in a furnace and heating at a ramp
rate of 2.degree. C./min to 600.degree. C. and holding at that
temperature for one hour. After the burn-off procedure the tube was
cooled to ambient temperature. Catalyst loading in the resulting
dual function composite oxygen transport membrane was 0.48 g, as
calculated by weighing the tube before wash-coating and after cool
down. The SEM microstructure of a cross-section of this catalyst
layer shown in FIG. 5 suggests catalyst layer thickness to be about
75 .mu.m.
Example 2: Thinner Nickel-Rhodium Based Catalyst Layer after
Surface Exchange Layer Sintering (FIG. 2)
[0058] Another porous tube with oxygen transport mixed conducting
layers formed on the outside was inspected, cleaned off any dust
and filled with catalyst layer sol-gel slurry prepared as described
above in Example 1. In this instance the sol-gel inside the tube
was held for about 5 seconds rather than for about one minute prior
to initiating the draining process. The tube was then subjected to
the same steps and conditions of: air drying, organic binder and
pore former burn off and cool down as described above. Catalyst
loading in the resulting dual function composite oxygen transport
membrane was 0.11 g, as calculated by weighing the tube before
wash-coating and after cool down. The SEM microstructure of a
cross-section of this catalyst layer shown in FIG. 6 suggests
catalyst layer thickness to be about 15 .mu.m. The sol-gel slurry
holding time in the tube prior to draining appears to be an
important factor in determining the catalyst layer thickness.
Example 3: Ru-Pervoskite Based Catalyst Layer after Surface
Exchange Layer Sintering (FIG. 2)
[0059] 25.5 g of
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.8Fe.sub.0.15Ru.sub.0.05O.sub.3-.d-
elta. (particles ranging from 0.2 microns to 0.4 microns, obtained
from Praxair Specialty Ceramics) was dispersed in 25 g of toluene
solvent (purity>99.5%) along with 5 g of plastic Ferrobinder.
Adding 200 g of 1.5 mm YSZ media into the slurry container, the
mixture was milled on the roller mill (170 to 175 rpm) for 2 hours.
The final particle size of the slurry was about 0.35 microns. Then
4.5 g of pore former such as carbon black (particle size ranging
from 0.5 microns to 1.0 micron) was added and milling of the
mixture continued for 1 hour. Finally 0.3 g of dispersant (KD-1)
dissolved in 15 g of solvent was added to the slurry mixture and
milling continued for additional 1 hour. The resulting sol-gel
slurry was then used to wash coat a 7 mm ID, 24 inches long YSZ
porous tube already coated with oxygen transport mixed conducting
layers following similar steps of inspecting, plugging one end,
filling, adding slurry to keep the tube completely filled, waiting
for about one minute, then draining liquid from the tube, air
drying, burning off of organic binder and pore former material, and
cool down. In this instance the catalyst loading was 0.6 g. The SEM
microstructure of a cross-section of this catalyst layer suggested
catalyst layer thickness to be about 62 .mu.m.
Example 4: Nickel-Rhodium Based Catalyst Layer and Surface Exchange
Layer Co-Firing (FIG. 3)
[0060] 25 g of Alpha-phase aluminum oxide (1 .mu.m average particle
size, 8 to 10 m.sup.2/g surface area, from Alfa Aesar) and 8.5 g of
TZ-4YS with 4 mole % yttria stabilized zirconia powder (0.5 .mu.m
average particle size, from Tosoh Corporation) were dispersed in
200 mL of ethanol and 7 mL of KD-2 dispersant agent (Hypermer.TM.).
Adding 500 g of 1.5 mm diameter YSZ milling media into the
container, the mixture was milled on the roller mill (170 to 175
rpm) for 2 hours. The final particle size of the slurry was in the
range of about 0.5 to about 0.8 Along with 10 g of pore former
poly(methyl methacrylate) PMMA with average particle size of 6
.mu.m, 30 g of nickel nitrate hexahydrate
Ni(NO.sub.3).sub.2.6H.sub.2O and 0.5 g of Rh(NO.sub.3).sub.3 (both
from Sigma-Aldrich) were added into the mixture and mixed for
additional one hour. 12% by weight of plastic binder polyvinyl
butyral powder was first dissolved in ethanol solvent to enhance
its homogenous mixing and then 150 mL of resulting binder solution
was slowly added into the slurry mixture. The resulting mixture was
further milled for 1.5 hours to form sol-gel slurry. The resulting
sol-gel slurry was then used to wash coat a 7 mm ID, 24 inches long
YSZ porous tube already coated with two of the three oxygen
transport mixed conducting layers, namely intermediate porous layer
and dense layer only. The wash coating steps were similar to that
described in Examples 1 and 2 above, namely: inspecting and
removing any dust, plugging one end, filling with sol-gel slurry,
adding slurry as needed to keep the tube completely filled, waiting
for about one minute, then draining liquid from the tube. The tube
was then air dried at room temperature for about 5 minutes with air
flowing at a low flow rate of 40 SCFH. Next the surface exchange
layer slurry prepared in a manner described above was used to coat
the outside of (over) the dense layer. To complete the formation of
the surface exchange layer as well as to burn off organic binders
and pore former materials in the catalyst layer and the surface
exchange layer the tube was first dried at room temperature for
about one hour to about two hours, then heated at a ramp rate of
2.degree. C./min to 1250.degree. C. in an air fired furnace and
held there for half an hour, and allowed to cool down. In this
instance the catalyst loading was 0.52 g. The SEM microstructure of
a cross-section of this catalyst layer suggested catalyst layer
thickness to be about 80 .mu.m.
Example 5: Ru-Pervoskite Based Catalyst Layer and Surface Exchange
Layer Co-Firing (FIG. 3)
[0061] 25.5 g of
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.8Fe.sub.0.15Ru.sub.0.05O.sub.3-.d-
elta. (particle size range from 0.2 microns to 0.4 microns) was
dispersed in 25 g of toluene solvent (purity>99.5%) along with 5
g of plastic Ferrobinder. Adding 200 g of 1.5 mm YSZ media into the
slurry container, the mixture was milled on the roller mill (170 to
175 rpm) for 2 hours. The final particle size of the slurry was
about 0.35 microns. Then 4.5 g of pore former such as carbon black
(particle size ranged from 0.5 microns to 1.0 micron) was added and
mixture further milled for 1 hour. Finally 0.3 g of dispersant
(KD-1) dissolved in 15 g of toluene was added to the slurry mixture
and milled for additional 1 hour. Similar to Example 4, the tube
used in this example (7 mm ID and 24 inches long YSZ porous tube)
had only intermediate porous layer and dense layer formed on it.
The catalyst layer formation steps of inspecting, plugging one end,
filling, adding slurry to keep the tube completely filled during
the entire duration of about one minute, and draining liquid were
similar. The tube was then air dried at room temperature for about
5 minutes with air flowing at a low flow rate of 40 SCFH. Next the
surface exchange layer slurry prepared in a manner described above
was used to coat over the dense layer. To complete the formation of
the surface exchange layer as well as to burn off organic binders
and pore former materials in the catalyst layer and the surface
exchange layer, the tube was first dried at room temperature for
about one hour to about two hours, then heated at a ramp rate of
2.degree. C./min to 1250.degree. C. in an air fired furnace and
held there for half an hour, and allowed to cool down. In this
instance the catalyst loading was 0.62 g. The SEM microstructure of
a cross-section of this catalyst layer shown in FIG. 7 suggests
catalyst layer thickness to be about 55 .mu.m.
Example 6: Thicker Ru-Pervoskite Based Catalyst Layer (FIG. 3)
[0062] 25.5 g of
(La.sub.0.8Sr.sub.0.2).sub.0.98Cr.sub.0.8Fe.sub.0.15Ru.sub.0.05O.sub.3-.d-
elta. (particle size range from 0.2 microns to 0.4 microns) was
dispersed in 25 g of toluene solvent (purity>99.5%) along with 5
g of plastic Ferrobinder. Adding 200 g of 1.5 mm YSZ media into the
slurry container, the mixture was milled on the roller mill (170 to
175 rpm) for 2 hours. The final particle size of the slurry was
about 0.35 microns. Then 4.5 g of pore former such as carbon black
(particle size ranged from 0.5 microns to 1.0 micron) was added and
mixture further milled for 1 hour. Finally 0.3 g of dispersant
(KD-1) dissolved in 15 g of toluene was added to the slurry mixture
and milled for additional 1 hour. Similar to Example 5, the tube
used in this example (7 mm ID and 24 inches long YSZ porous tube)
had only intermediate porous layer and dense layer formed on it.
The tube was subjected to catalyst layer formation steps of
inspecting, plugging one end, filling, and adding slurry to keep
the tube completely filled during the entire duration of about one
minute. The liquid was then drained and the tube was air dried for
five minutes and then filled again with sol-gel slurry. The tube
was kept completely filled by adding slurry as needed. After
waiting for about a minute, the liquid was drained. In a manner
similar to that described above for Example 5, the tube was then
air dried at room temperature for about 5 minutes with air flowing
at a low flow rate of 40 SCFH. Next the surface exchange layer
slurry prepared in a manner described above was used to coat the
outside of the dense layer. To complete the formation of the
surface exchange layer as well as to burn off organic binders and
pore former materials in the catalyst layer and the surface
exchange layer, the tube was first dried at room temperature for
about one hour to about two hours, then heated at a ramp rate of
2.degree. C./min to 1250.degree. C. in an air fired furnace and
held there for half an hour, and allowed to cool down. In this
instance the catalyst loading was 0.84 g. The SEM microstructure of
a cross-section of the catalyst layer shown in FIG. 8 indicates
cracking and delamination of catalyst layer, and suggests catalyst
layer thickness to be about 225 .mu.m where it remained intact.
Therefore, it is preferable to control the catalyst thickness
within the range of 40 to 150 .mu.m.
[0063] Dual Function Composite Oxygen Transport Membrane
Performance
[0064] The dual function composite oxygen transport membrane tubes
made in the examples described above with functional layered
structures on the outside surface and the inside surface were
tested separately using a standard bench-scale reactor setup. The
tube was vertically positioned inside a metal shell embedded in an
electrically heated chamber. The dual function composite oxygen
transport membrane tube was connected to a source of feed gas and
an effluent processing system for safely disposing off syngas
product. The tube was heated to an operational temperature of about
950.degree. C. The feed gas was prepared using CH.sub.4, CO,
H.sub.2, and CO.sub.2 from gas cylinders and steam from a steam
source. The results described below were obtained using a feed gas
containing 12 mole % CH.sub.4, 11 mole % CO, 52 mole % H.sub.2, 4
mole % CO.sub.2 and 21 mole % H.sub.2O. The feed gas was preheated
to about 350.degree. C. prior to feeding to the tube. The flow rate
of the feed gas was controlled at achieve a desired space velocity
of about 31,000 per hour. Heated air at about 200.degree. C. with a
flow rate of 30 SLPM was introduced into the metal shell to flow on
the outside of the dual function composite oxygen transport
membrane tube in a direction countercurrent to that of feed gas
flowing through the tube. The pressure inside the metal shell, that
is on the outside of the dual function membrane tube was maintained
around 5 psig, and the pressure inside the dual function membrane
tube was maintained at a desired value in the range of about 5 psig
to about 200 psig. The effluent containing reaction products and
unreacted feed species was cooled, water condensed out. The
resulting gas stream was sampled and analyzed using a gas
chromatograph (GC). The hot air stream leaving the chamber was also
cooled and then analyzed for oxygen content using a real-time
resolved oxygen analyzer. Table 3 summarizes the results after 100
hours of operation indicating the dual function membranes to have
considerably improved methane conversion relative to a membrane
that has only oxygen transport functionality. The oxygen transport
functionality as indicated by the oxygen flux after 100 hours of
stable operation of dual function composite oxygen transport
membrane tubes prepared in Examples 1, 3 thru 5 is similar to that
of a reference tube that had mixed conducting oxygen transport
layers on the outside surface without a catalyst layer on the
inside surface. The wash-coating procedure, standardized
wash-coating procedure used for forming catalyst layer in these
examples involved filling the tube with a slurry containing
catalyst layer ingredients, holding the slurry in the completely
filled tube for one minute, then draining the slurry followed by
air drying and organics burn-off in air. The tubular dual function
composite oxygen transport membrane made in Example 2 has similar
oxygen flux performance even though a slightly different procedure
was followed; the slurry in the completely filled tube was held for
considerably less time than one minute, resulting in a thin
catalyst layer. In Example 6, however the tube was again refilled
with the slurry, the catalyst layer formed was thicker, and the
oxygen flux is considerably lower than those of tubes prepared
following standardized wash-coating procedure. The thicker catalyst
layer could pose higher diffusional resistance to transport of fuel
species through the catalyst layer into the porous substrate
towards the intermediate porous layer for reaction with permeated
oxygen within the membrane, affecting the driving potential for
oxygen transport. The results in Table 3 also indicate that the
composite oxygen transport membranes with catalyst layer, that is
dual function composite oxygen transport membranes achieved
considerably higher methane conversion. The catalyst layer
thickness appears to be an important factor. The Example 2 membrane
that had a thinner catalyst layer, about 15 microns appears to
achieve relatively lower methane conversion compared to those
having catalyst layer thicknesses in the range of about 50 microns
to about 80 microns. The Example 6 membrane that had a thicker
catalyst layer of about 225 microns with cracks and delamination in
some cross sections, also had relatively lower methane
conversion.
TABLE-US-00003 TABLE 3 Catalyst layer thickness, Normalized O.sub.2
CH.sub.4 Example Catalyst type Fabrication Method microns Flux*
conversion, % Reference N/A FIG. 2 without N/A 1.00 4.3% catalyst
layer steps 1 Ni--Rh FIG. 2 75 0.98 98.6% 2 Ni--Rh FIG. 2 15 1.00
95.4% 3 Ru-Pervoskite FIG. 2 62 0.99 98.8% 4 Ni--Rh FIG. 3 80 0.99
98.2% 5 Ru-Pervoskite FIG. 3 55 1.01 98.9% 6 Ru-Pervoskite FIG. 3
225 0.83 93.8% *Normalized with respect to reference membrane
(without catalyst layer)
[0065] Although the present invention has been described with
reference to preferred embodiments, as will occur to those skilled
in the art, changes and additions to such embodiment can be made
without departing from the spirit and scope of the present
invention as set forth in the appended claims. The dual function
composite oxygen transport membrane, even though described in the
context of syngas production are not limited to such uses.
* * * * *