Note: Descriptions are shown in the official language in which they were submitted.
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POROUS SUPPORT LAYER
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.
Field of the Invention
(0002) The present invention relates to a porous support layer for a composite
oxygen transport membrane and to a composite oxygen transport membrane in
which catalyst particles, selected to promote oxidation of a combustible
substance,
are located within an intermediate porous layer that is in turn located
between a
dense layer and a porous support layer and within the porous support.
Background
(0003) Oxygen transport membranes function by transporting oxygen ions
through a material that is capable of conducting oxygen ions and electrons at
elevated temperatures. Such materials can be mixed conducting in that they
conduct both oxygen ions and electrons or a mixture of materials that include
an
ionic conductor capable of primarily conducting oxygen ions and an electronic
conductor with the primary function of transporting 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, a metal or metal alloy or a mixture of the
perovskite type
material, the metal or metal alloy.
(0004) When a partial pressure difference of oxygen is applied on opposite
sides
of such a membrane, oxygen ions will ionize on one surface of the membrane and
emerge on the opposite side of the membrane and recombine into elemental
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oxygen. The free electrons resulting from the combination will be transported
back through the membrane to ionize the oxygen. The partial pressure
difference
can be produced by providing the oxygen containing feed to the membrane at a
positive pressure or by supplying a combustible substance to the side of the
membrane opposing the oxygen containing feed or a combination of the two
methods.
(0005) Typically, oxygen transport membranes are composite structures that
include a dense layer composed of the mixed conductor or the two phases of
materials and one or more porous supporting layers. Since the resistance to
oxygen ion 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 that, although can be active, that is oxygen ion or electron
conducting, the 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 react with the oxygen. 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.
(0006) U.S. Patent No. 7,229,537 attempts to solve such problems by providing
a
support with cylindrical or conical pores that are not connected and an
intermediate porous layer located between the dense layer and the support that
distributes the oxygen to the pores within the support. Porous supports can
also
be made by freeze casting techniques, as described in 10, No. 3, Advanced
Engineering Materials, "Freeze-Casting of Porous Ceramics: A Review of
Current Achievements and Issues" (2008) by Deville, pp. 155-169. In freeze
casting, a liquid suspension is frozen. The frozen liquid phase is then
sublimated
from a solid to a vapor under reduced pressure. The resulting structure is
sintered
to consolidate and densify the structure. This leads to a porous structure
having
pores extending in one direction and that have a low tortuosity. Such supports
have been used to form electrode layers in solid oxide fuel cells. In addition
to
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the porous support layers, a porous surface exchange layer can be located on
the
opposite side of the dense layer to enhance reduction of the oxygen into
oxygen
ions. Such a composite membrane is illustrated in US Patent 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.
(0007) As mentioned above, the oxygen partial pressure difference can be
created
by combusting a fuel or other combustible substance with the separated oxygen.
The resulting heat will heat the oxygen transport membrane up to operational
temperature and excess heat can be used for other purposes, for example,
heating
a fluid, for example, raising steam in a boiler or in the combustible
substance
itself. While perovskite structured materials will exhibit a high oxygen flux,
such
materials tend to be very fragile under operational conditions such as in the
heating of a fluid. This is because the perovskite type materials will have a
variable stoichiometry with respect to oxygen. In air it will have one value
and in
the presence of a fuel that is undergoing combustion it will have another
value.
The end result is that at the fuel side, the material will tend to expand
relative to
the air side and a dense layer will therefore, tend to fracture. In order to
overcome
this problem, a mixture of materials can be used in which an ionic conductor
is
provided to conduct the oxygen ions and an electronic conductor is used to
conduct the electrons. Where the ionic conductor is a fluorite structured
material,
this chemical expansion is restrained, and therefore the membrane will be less
susceptible to structural failure. However, the problem with the use of a
fluorite
structure material, such as a stabilized zirconia, is that such a material has
lower
oxygen ion conductivity. As a result, far more oxygen transport membrane
elements are required for such a dual phase type of membrane as compared with
one that is formed from a single phase perovskite type material.
(0008) As will be discussed, the present invention provides a porous support
layer
that makes it possible to obtain a robust oxygen transport membrane.
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Summary of the Invention
The present invention may be characterized as a porous support layer comprised
of an fluorite structured ionic conducting material having a porosity of
greater
than 20 percent and a microstructure exhibiting substantially uniform pore
size
distribution throughout the porous support layer, wherein the porous support
layer
is formed from a mixture comprising fluorite structured ionic conducting
material
or from a mixture comprising polymethyl methacrylate based pore forming
material and a fluorite structured ionic conducting material, said polymethyl
methacrylate based pore forming material and/or said fluorite structured ionic
conducting material having bi-modal or multi-modal particle sizes. The present
invention may further be characterized as a composite oxygen transport
membrane comprising (i) the above described porous support layer; (ii) an
intermediate porous layer often referred to as a fuel oxidation layer disposed
adjacent to the porous support layer and capable of conducting oxygen ions and
electrons to separate oxygen from an oxygen containing feed and comprising a
mixture of a fluorite structured ionic conductive material and electrically
conductive materials to conduct the oxygen ions and electrons, respectively;
(iii) a
dense separation layer capable of conducting oxygen ions and electrons to
separate oxygen from an oxygen containing feed, the dense layer adjacent to
the
intermediate porous layer and also comprising a mixture of a fluorite
structured
ionic conductive material and electrically conductive materials to conduct the
oxygen ions and electrons, respectively; and (iv) catalyst particles or a
solution
containing precursors of the catalyst particles located in pores of the porous
support layer and intermediate porous layer, the catalyst particles containing
a
catalyst selected to promote oxidation of a combustible substance in the
presence
of the separated oxygen transported through the dense layer and the
intermediate
porous layer to the porous support layer. The catalyst is preferably
gadolinium
doped ceria but may also be other catalysts that promote fuel oxidation. The
composite oxygen transport membrane may also include a porous surface
exchange layer or an air activation layer disposed or applied to the dense
separation layer on the side opposite to the intermediate porous layer or the
fuel
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oxidation layer. If used, the porous surface exchange layer or an air
activation
layer preferably has a thickness of between 10 and 40 microns and a porosity
of
between about 30 and 60 percent.
(0009) The intermediate porous layer or fuel oxidation layer preferably has a
thickness of between about 10 and 40 microns and a porosity of between about
20
and 50 percent whereas the dense layer has a thickness of between 10 and 50
microns. In one embodiment, the porous support layer is formed from a mixture
comprising, and preferably consisting essentially of, a fluorite structured
ionic
conducting material having a bi-modal or multimodal particle size distribution
and
optionally a pore forming material. Fluorite structured ionic conducting
material is
preferably stabilized zirconia, preferably yttria stabilized zirconia,
preferably
zirconia comprising at least 2mol% of yttrium oxide, preferably, from 2 to
5mol% of
yttrium oxide, or from 3 to 4mol% of yttrium oxide. It is understood that
"stabilized
zirconia" also includes "partially stabilized zirconia". The porous support
layer may
be formed from a mixture comprising 3mol% yttria stabilized zirconia, or 3YSZ
having a bi-modal or multimodal particle size distribution, and optionally a
pore
forming material. The pore forming material may be polymethyl methacrylate
based
pore forming material. The pore forming material may have a bi-modal or
multimodal particle size distribution. Preferably, the fluorite structured
ionic
conducting material having a bi-modal or multi-modal particle sizes comprises
at
least 30 weight percent of particles having a particle size greater than 2
microns
and/or at least 90 weight percent of particles having a particle size below 10
microns,
preferably below 8 microns or even below 7 microns. Such a material may be
obtained my mixing at least two powders having different median particle size.
In
one embodiment, the porous support layer is formed by mixing a first powder
having
a median particle size diameter (D50) of between 0.3 and 1.5 microns and at
least a
second powder having a median particle size diameter (D50) of between 2.0 and
6.0
microns. In one other embodiment, the porous support layer is formed from a
fluorite
structured ionic conducting material and polymethyl methacrylate based pore
forming material having a bi-modal or multimodal particle size distribution.
In all
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embodiments, the porous support layer has a preferred thickness of between
about
0.5 and 4 mm and porosity between about 20 and 40 percent.
(00010) Broadly
characterizing the preferred embodiments of the
composite oxygen transport membrane, the intermediate porous layer comprises a
mixture of about 60 percent by weight of (LauSrvCei-u-OwCrxMyVz03_8 with the
remainder Zrx,Scy,Az,02_8. Similarly, the dense separation layer comprises a
mixture of about 40 percent by weight of (LauSrvCei-u-OwCrxMyVz03_8 with the
remainder Zrx,Scy,Az,02_8. In the above formulations, u is from 0.7 to 0.9, v
is
from 0.1 to 0.3 and (1-u-v) is greater than or equal to zero, w is from 0.94
to 1, x
is from 0.5 to 0.77, M is Mn or Fe, y is from 0.2 to 0.5, z is from 0 to 0.03,
and
x+y+z =1, where y' is from 0.08 to 0.3, z' is from 0.01 to 0.03, x'+y'+z'=1
and A is
Y or Ce or mixtures of Y and Ce. The porous surface exchange layer or air
activation layer, if employed, can be is formed by a mixture of about 50
percent
by weight of (Lax-Sri_x-)y,,,M03_6, where x" is from 0.2 to 0.9, y'" is from
0.95 to
1, M is Mn or Fe, with the remainder Zrxiv Scy ivAz 1vO2_6, where yiv is from
0.08 to
0.3, ziv is from 0.01 to 0.03, xlv+ylv+ziv=1 and A is Y, Ce or mixtures
thereof.
(00011) More
specifically, one of the preferred embodiments of the
composite oxygen transport membrane includes an intermediate porous layer or
fuel oxidation layer that comprises about 60 percent by weight of
(La0.825Sr0.175)0.96Cr0.76Fe0.225V0.01503,5 Or (Lao.8Sro.A.95Cro.7Fe0.303_8
with the
remainder 10Sc1YSZ or 10SclCeSZ. Similarly, the dense separation layer
comprises about 40 percent by weight of
(Lao.825Sro.175)0.94Cro.72Mn0.26Vo.o203_8 Or
(La0.8Sr0.2)0.95Cr0.5Fe0.503_6, with the remainder 10Sc1YSZ or 10Sc1CeYSZ. The
porous surface exchange layer or air activation layer is formed by a mixture
of
about 50 percent by weight of (La0.8Sro.2)o.98Mn03_8 or La0.8Sr0.2Fe03_8,
remainder
10Sc1YSZ or 10SclCeSZ.
(00012) The
present invention may also be characterized as a product by
process wherein the product is a porous support layer. The process comprises:
(i)
fabricating a porous support layer comprised of an fluorite structured ionic
conducting material, the fabricating step including pore forming enhancement
step
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such that the porous support layer has a porosity of greater than about 20
percent
and a microstructure exhibiting substantially uniform pore size distribution
throughout the porous support layer.
(00013) The
present invention may also be characterized as a product by
process wherein the product is a composite oxygen transport membrane. The
process comprises: (i) ; (ii) applying an intermediate porous layer or fuel
oxidation layer on the porous support layer, (iii) applying a dense separation
layer
on the intermediate porous layer; and (iv) introducing catalyst particles or a
solution containing precursors of the catalyst particles to the porous support
layer
and intermediate porous layer, the catalyst particles containing a catalyst
selected
to promote oxidation of a combustible substance in the presence of the
separated
oxygen transported through the dense layer and the intermediate porous layer
to
the porous support layer.
(00014) Both
the intermediate porous layer and dense separation layer are
capable of conducting oxygen ions and electrons to separate oxygen from an
oxygen containing feed. Both layers comprise a mixture of a fluorite
structured
ionic conductive material and electrically conductive materials to conduct the
oxygen ions and electrons, respectively.
(00015) The
pore forming enhancement process involves the use of bi-
modal or multi-modal particle sizes of the polymethyl methacrylate based pore
forming material and/or the fluorite structured ionic conducting material of
the
porous support layer.
(00016) The
step of introducing catalyst particles or a solution containing
precursors of the catalyst particles to the porous support layer and
intermediate
porous layer may further comprise either: (a) adding catalyst particles
directly to
the mixture of materials used in the intermediate porous layer; or (b)
applying a
solution containing catalyst precursors to the porous support layer on a side
thereof opposite to the intermediate porous layer so that the solution
infiltrates or
impregnates the pores within the porous support layer and the intermediate
porous
layer with the solution containing catalyst precursors and heating the
composite
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oxygen transport membrane after the solution containing catalyst precursors
infiltrates the pores and to form the catalyst from the catalyst precursors.
(00017) Each of
the dense layer and the intermediate porous layer are
capable of conducting oxygen ions and electrons at an elevated operational
temperature to separate oxygen from an oxygen containing feed. The dense layer
and the intermediate porous layer comprising mixtures of a fluorite structured
ionic conductive material and electrically conductive materials to conduct
oxygen
ions and electrons, respectively.
(00018) The
porous support layer comprises, and preferably consists
essentially of, a fluorite structured ionic conducting material having a
porosity of
greater than about 20 percent and a microstructure exhibiting substantially
uniform pore size distribution throughout the porous support layer. Pores are
formed within the porous support layer using bi-modal or multi-modal particle
sizes of the polymethyl methacrylate based pore forming material and/or using
bi-
modal or multi-modal particle sizes of the fluorite structured ionic
conducting
material of the porous support layer.
(00019) To aid
in the infiltration or impregnation process, a pressure may
be established on the second side of the porous support layer or the pores of
the
porous support layer and fuel oxidation layer may first be evacuated of air
using a
vacuum to further assist in wicking of the solution and prevent the
opportunity of
trapped air in the pores preventing wicking of the solution all the way
through the
support structure to the intermediate layer.
Brief Description of the Drawings
(00020) While
the specification concludes with claims distinctly pointing
out the subject matter that Applicants regard as their invention, it is
believed that
the invention will be better understood when taken in connection with the
accompanying drawings in which:
(00021) Fig. 1
is a cross-sectional schematic view of a composite oxygen
transport membrane element of the present invention that is fabricated in
accordance with a method of the present invention;
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(00022) Fig. 2 is an alternative embodiment of Fig. 1;
(00023) Fig. 3 is an alternative embodiment of Fig. 1;
(00024) Fig. 4 is an SEM micrograph image at 1000x magnification
showing a porous support layer comprised of 3YSZ with walnut shells as the
pore
forming material;
(00025) Fig. 5 is an SEM micrograph image at 2000x magnification
showing a porous support layer comprised of 3YSZ with multi-modal particle
sizes in accordance with the present invention.
Detailed Description
(00026) With reference to Fig. 1, a sectional view of a composite
oxygen
transport membrane element 1 in accordance with the present invention is
illustrated. As could be appreciated by those skilled in the art, such
composite
oxygen transport membrane element 1 could be in the form of a tube or a flat
plate. Such composite oxygen transport membrane element 1 would be one of a
series of such elements situated within a device to heat a fluid such as in a
boiler
or other reactor having such a heating requirement.
(00027) Composite oxygen transport membrane element 1 is provided with
a dense layer 10, a porous support layer 12 and an intermediate porous layer
14
located between the dense layer 10 and the porous support layer 12. A
preferable
option is, as illustrated, to also include a porous surface exchange layer 16
in
contact with the dense layer 10, opposite to the intermediate porous layer 14.
Catalyst particles 18 are located in the intermediate porous layer 14 that are
formed of a catalyst selected to promote oxidation of a combustible substance
in
the presence of oxygen separated by the composite membrane element 1. It is to
be noted that the term "combustible substance" as used herein means any
substance that is capable of being oxidized, including, but not limited, to a
fuel in
case of a boiler, a hydrocarbon containing substance for purposes of oxidizing
such substance for producing a hydrogen and carbon monoxide containing
synthesis gas or the synthesis gas itself for purposes of supplying heat to,
for
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example, a reformer. As such the term, "oxidizing" as used herein encompasses
both partial and full oxidation of the substance.
(00028)
Operationally, air or other oxygen containing fluid is contacted on
one side of the composite oxygen transport membrane element 1 and more
specifically, against the porous surface exchange layer 16 in the direction of
arrowhead "A". The porous surface exchange layer 16 is porous and is capable
of
mixed conduction of oxygen ions and electrons and functions to ionize some of
the oxygen. The oxygen that is not ionized at and within the porous surface
exchange layer 16, similarly, also ionizes at the adjacent surface of the
dense layer
which is also capable of such mixed conduction of oxygen ions and electrons.
The oxygen ions are transported through the dense layer 10 to intermediate
porous
layer 14 to be distributed to pores 20 of the porous support layer 12. It
should be
noted that in Figs. 1-3, the pores 20 within the porous support layer 12 are
shown
in an exaggerated manner. Some of the oxygen ions, upon passage through the
dense layer will recombine into elemental oxygen. The recombination of the
oxygen ions into elemental oxygen is accompanied by the loss of electrons that
flow back through the dense layer to ionize the oxygen at the opposite surface
thereof.
(00029) At the
same time, a combustible substance, for example a hydrogen
and carbon monoxide containing synthesis gas, is contacted on one side of the
porous support layer 12 located opposite to the intermediate porous layer 14
as
indicated by arrowhead "B". The combustible substance enters pores 20,
contacts
the oxygen and burns through combustion supported by oxygen. The combustion
is promoted by the catalyst that is present by way of catalyst particles 18.
(00030) The
presence of combustible fuel on the side of the composite
oxygen ion transport membrane element 1, specifically the side of the dense
layer
10 located adjacent to the intermediate porous layer 14 provides a lower
partial
pressure of oxygen. This lower partial pressure drives the oxygen ion
transport as
discussed above and also generates heat to heat the dense layer 10, the
intermediate porous layer 14 and the porous surface exchange layer 16 up to an
operational temperature at which the oxygen ions will be conducted. In
specific
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applications, the incoming oxygen containing stream can also be pressurized to
enhance the oxygen partial pressure difference between opposite sides of the
composite oxygen ion transport membrane element 1. Excess heat that is
generated by combustion of the combustible substance will be used in the
specific
application, for example, the heating of water into steam within a boiler or
to meet
the heating requirements for other endothermic reactions.
(00031) In the
embodiments described with reference to Figs. 1-3, the use
of a single phase mixed conducting material such as a perovskite structured
materials has the disadvantage of exhibiting chemical expansion, or in other
words, one side of a layer, at which the oxygen ions recombine into elemental
oxygen, will expand relative to the opposite side thereof. This resulting
stress can
cause failure of such a layer or separation of the layer from adjacent layers.
In
order to avoid this, the dense layer, the intermediate porous layer, and the
porous
surface exchange layer were all formed of a two phase system comprising a
fluorite structured material in one phase as the ionic conductor of the oxygen
ions
and an electronic conducting phase that in the illustrated embodiment is a
perovskite type material. In the described embodiments, the porous support
layer
12, 12', 12" have a thickness of between about 0.5 mm and about 4.0 mm, and
more preferably about 1.0 mm and are preferably formed only of a fluorite
structured material or of a fluorite structured material only with a PMMA
based
pore former material. As such, the porous support layers 12; 12' and 12"
preferably do not exhibit significant mixed conduction. The material used in
forming the porous support layer preferably have a thermal expansion
coefficient
in the range 9 x 10-6 cm/cm x Icl and 12 x 10-6 cm/cm x Icl in the temperature
range of 20 C to 1000 C; where "K" is the temperature in Kelvin.
(00032) As
discussed above, dense layers 10, 10', 10" or dense separation
layers function to separate oxygen from an oxygen containing feed exposed to
one
surface of the oxygen ion transport membrane 10 and contains an electronic and
ionic conducting phases. The dense separation layer also serves as a barrier
of
sorts to prevent mixing of the fuel on one side of the membrane with the air
or
oxygen containing feed stream on the other side of the membrane. As discussed
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above, the electronic phase in the dense layer is (LauSrvCei-u-OwCrxMyVz03-8
where u is from about 0.7 to about 0.9, v is from about 0.1 to about 0.3 and
(1-u-
v) is greater than or equal to zero, w is from about 0.94 to about 1, x is
from about
0.5 to about 0.77, M is Mn or Fe, y is from about 0.2 to about 0.5, z is from
about
0 to about 0.03, and x+y+z =1 ("LSCMV"). The ionic phase is Zrx,ScyAz,02-8
("YScZ"), where y' is from about 0.08 to about 0.3, z' is from about 0.01 to
about
0.03, x'+y'+z'=1 and A is Y or Ce or mixtures of Y and Ce. The variable "6" as
used in the formulas set forth below for the indicated substances, as would be
known in the art would have a value that would render such substances charge
neutral. It is to be noted, that since the quantity (1-u-v) can be equal to
zero,
cerium may not be present within an electronic phase of the present invention.
Preferably, the dense separation layer contains a mixture of 40 percent by
weight
(LaØ8 Sr0.2)0.95Cr0.5Fe0.5 03-8 remainder 10Sc1CeYSZ; or alternatively about
40
percent by weight of (Lao.825Sro.175)o.94Cro.72Mno.26Vo.o203_8, remainder
10Sc1YSZ.
As also mentioned above, in order to reduce the resistance to oxygen ion
transport, the dense layer should be made as thin as possible and in the
described
embodiment has a thickness of between about 10 microns and about 50 microns.
(00033) Porous
surface exchange layers 16, 16', 16" or air activation layers
are designed to enhance the surface exchange rate by enhancing the surface
area
of the dense layers 10, 10', 10" while providing a path for the resulting
oxygen
ions to diffuse through the mixed conducting oxide phase to the dense layer
and
for oxygen molecules to diffuse through the open pore spaces to the same. The
porous surface exchange layer 16, 16', 16" therefore, reduces the loss of
driving
force in the surface exchange process and thereby increases the achievable
oxygen
flux. As indicated above, it also can be a two-phase mixture containing an
electronic conductor composed of (Lax-Sri_x-)y,,,M03_6, where x" is from about
0.2
to about 0.9, y'" is from about 0.95 to 1, M is Mn or Fe; and an ionic
conductor
composed of Zrxiv Scy ivAz 1v02_6, where yiv is from about 0.08 to about 0.3,
ziv is
from about 0.01 to about 0.03, xiv+yiv+ziv=1 and A is Y, Ce or mixtures of Y
and
Ce. In the described embodiments, porous surface exchange layer is formed of a
mixture of about 50 percent by weight of (La0.8Sro.2)o.98Mn03_8, remainder
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10Sc1YSZ. The porous surface exchange layer is a porous layer and preferably
has a thickness of between about 10 microns and about 40 microns, a porosity
of
between about 30 percent and about 60 percent and an average pore diameter of
between about 1 microns and about 4 microns.
(00034) The
intermediate porous layer 14, 14', 14" is a fuel oxidation layer
and is a preferably formed of the same mixture as the dense layer 10, 10', 10"
and
preferably has an applied thickness of between about 10 microns and about 40
microns, a porosity of between about 25 percent and about 40 percent and an
average pore diameter of between about 0.5 microns and about 3 microns.
(00035) In
addition, incorporated within the intermediate porous layer 14,
14', 14" are catalyst particles 18, 18', 18". The catalyst particles 18, 18',
18" in
the described embodiments are preferably gadolinium doped ceria ("CGO") that
have a size of between about 0.1 and about 1 microns. Preferably, the
intermediate porous layers contain a mixture of about 60 percent by weight of
(La0.825Sro.175)o.96Cro.76Feo.225Vo.o1503_8, remainder 10Sc1YSZ. It is to be
noted
that intermediate porous layer as compared with the dense layer preferably may
contain iron in lieu of or in place of manganese, a lower A-site deficiency, a
lower
transition metal (iron) content on the B-site, and a slightly lower
concentration of
vanadium on the B-site. It has been found that the presence of iron in the
intermediate porous layer aids the combustion process and that the presence of
manganese at higher concentration and a higher A-site deficiency in the dense
layer improves electronic conductivity and sintering kinetics. If needed, a
higher
concentration of vanadium should be present in the dense layer because
vanadium
functions as a sintering aid, and is required to promote densification of the
dense
layer. Vanadium, if any, is required in lesser extent in the intermediate
porous
layer in order to match the shrinkage and thermal expansion characteristics
with
the dense layer.
(00036) The
porous support layer 12, 12', 12" can be formed from a past
mixture by known forming techniques including extrusion techniques and freeze
casting techniques. Although pores 20, 20', 20" in the porous support layer
are
indicated as being a regular network of non-interconnected pores, in fact
there
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exists some degree of connection between pores towards the intermediate porous
layer. In any event, the porous network and microstructure of the porous
support
layer should be controlled so as to promote or optimize the diffusion of the
combustible substance to the intermediate porous layer and the flow of
combustion products such as steam and carbon dioxide from the pores in a
direction opposite to that of arrowhead "B". The porosity of porous support
layers 14, 14', 14" should preferably be greater than about 20 percent for the
described embodiment as well as other possible embodiments of the present
invention.
(00037) The
porous support layers 12, 12', 12" are possibly fabricated
from 3YSZ material commercially available from various suppliers including
Tosoh Corporation and its affiliates, including Tosoh USA, with an address at
3600 Gantz Road, Grove City, Ohio. Advancements in the performance of the
porous support layers have been realized when using 3YSZ with multi-modal
particle sizes. Optionally, it could be combined with fugitive organic pore
former
materials, specifically polymethyl methacrylate (PMMA). When added, the pore
forming material is preferably a mixture comprising 30wt% carbon black with an
average particle size less than or equal to about 1 micron combined with 70wt%
PMMA pore formers having a narrow particle size distribution and an average
particle size of between about 0.8 microns and 5.0 microns. Although use of
the
PMMA pore formers with a narrow particle size distribution have shown
promising results, further pore optimization and microstructure optimization
may
be realized using bi-modal or multi-modal particle size distributions of the
PMMA based pore formers. For example, bi-modal or multimodal particle size
distribution of PMMA pore formers, including PMMA particles with average
particle diameters of 0.8 microns, 1.5 microns 3.0 microns and 5.0 microns are
contemplated.
(00038) As
described in more detail below, the preferred fabrication
process of the oxygen transport membrane is to form the porous support via an
extrusion process and subsequently bisque firing of the extruded porous
support.
The porous support is then coated with the active membrane layers, including
the
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intermediate porous layer and the dense layer, after which the coated porous
support assembly is dried and fired. The coated porous support assembly is
then
co-sintered at a final optimized sintering temperature and conditions.
(00039) An
important aspect or characteristic of the materials or
combination of materials selected for the porous support is its ability to
mitigate
creep while providing enough strength to be used in the oxygen transport
membrane applications, which can reach temperatures above 1000 C and very
high loads. It is also important to select porous support materials that when
sintered will demonstrate shrinkages that match or closely approximate the
shrinkage of the other layers of the oxygen transport membrane, including the
dense separation layer, and intermediate porous layer.
(00040) In a
preferred embodiment, the final optimized sintering
temperature and conditions are selected so as to match or closely approximate
the
shrinkage profiles of the porous support to the shrinkage profiles of the
dense
separation layer while minimizing any chemical interaction between the
materials
of the active membrane layers, the materials in the porous support layer, and
the
sintering atmosphere. Too high of a final optimized sintering temperature
tends to
promote unwanted chemical interactions between the membrane materials, the
porous support, and surrounding sintering atmosphere. Reducing atmospheres
during sintering using blends of hydrogen and nitrogen gas atmosphere can be
used to reduce unwanted chemical reactions but tend to be more costly
techniques
compared to sintering in air. Thus, an advantage to the oxygen transport
membrane of the disclosed embodiments is that some may be fully sintered in
air.
For example, a dense separation layer comprising
(La0.8Sro.2)o.95Cro.5Fe0.503_6 and
10Sc1CeSZ appears to sinters to full density in air at about 1400 C to1430 C.
(00041) As
shown in Fig. 5, the use fluorite structured ionic conducting
material having bi-modal or multi-modal particle sizes produced oxygen
transport
membranes with both high porosity and a substantially uniform pore size
distribution throughout the porous support layer. Figs. 4 and 5 are SEM
micrographs (respectively 1000x magnification and 2000x magnification) which
show greater pore uniformity and overall porous support layer microstructure
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uniformity when using a porous support comprised of fluorite structured ionic
conducting material having bi-modal or multi-modal particle sizes (Fig. 5)
than a
porous support comprised of 3YSZ and walnut shell pore formers (Fig. 4).
(00042) Porous support microstructure shown in Fig. 5 exhibited higher
strength, lower creep, and improved diffusion efficiency compared to a 3YSZ
porous
support having a single average particle size 3YSZ and larger size walnut
shell pore
formers (see Fig. 4).
(00043) It has also been observed that the disclosed porous support
layers
12, 12', 12" preferably have a permeability of between about 0.25 Darcy and
about 0.5 Darcy. Standard procedures for measuring the permeability of a
substrate in terms of Darcy number are outlined in ISO 4022. Porous support
layers 12, 12', 12" also preferably have a thickness of between about 0.5 mm
and
about 4 mm and an average pore size diameter of no greater than about 50
microns. Additionally, the porous support layers also have catalyst particles
18,
18', 18" located within pores 20, 20', 20" and preferably adjacent to the
intermediate porous layer for purposes of also promoting combustible substance
oxidation. The presence of the catalyst particles both within the intermediate
porous layer and within the porous support layer provides enhancement of
oxygen
flux and therefore generation of more heat via combustion that can be obtained
by
either providing catalyst particles within solely the intermediate porous
layer or
the porous support layer alone. It is to be noted that to a lesser extent,
catalyst
particles can also be located in region of the pores that are more remote from
the
intermediate porous layer, and therefore do not participate in promoting fuel
oxidation. However, the bulk of catalyst in a composite oxygen transport
element
of the present invention is, however, preferably located in the intermediate
porous
layer and within the pores adjacent or proximate to the intermediate porous
layer.
(00044) In forming a composite oxygen transport membrane element in
accordance with the present invention, the porous support 12, 12',12" is first
formed in a manner known in the art and as set forth in the references
discussed
above. For example, standard ceramic extrusion techniques can be employed to
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produce a porous support layer or structure in a tube configuration in a green
state
and then subjected to a bisque firing at 1050 C for about 4 hours to achieve
reasonable strength for further handling. After bisque firing, the resulting
tube
can be checked or tested for targeted porosity, strength, creep resistance
and, most
importantly, diffusivity characteristics. Alternatively, a freeze cast
supporting
structure could be formed as discussed in "Freeze-Casting of Porous Ceramics:
A
Review of Current Achievements and Issues" (2008) by Deville, pp. 155-169.
(00045) After
forming the green tube, intermediate porous layer 14, 14',
14" is then formed. A mixture of about 34 grams of powders having electronic
and ionic phases, LSCMV and 10Sc1YSZ, respectively, is prepared so that the
mixture contains generally equal proportions by volume of LSCMV and
10Sc1YSZ. Prior to forming the mixture, the catalyst particles, such as CGO,
are
so incorporated into the electronic phase LSCMV by forming deposits of such
particles on the electronic phase, for example, by precipitation. However, it
is
more preferable to form the catalyst particles within the intermediate porous
layer
by wicking a solution containing catalyst precursors through the porous
support
layer towards the intermediate porous layer after application of the membrane
active layers as described in more detail below. As such, there is no
requirement
to deposit particles of catalyst on the electronic phase. The electronic phase
particles are each about 0.3 microns prior to firing and the catalyst
particles are
about 0.1 microns or less and are present in a ratio by weight of about lOwt%.
To
the mixture, 100 grams of toluene, 20 grams of the binder of the type
mentioned
above, 400 grams of 1.5 mm diameter YSZ grinding media are added. The
mixture is then milled for about 6 hours to form a slurry (d50 of about 0.34
m).
About 6 grams of carbon black having a particle size of about d50 = 0.8 m is
then
added to the slurry and milled for additional 2 hours. An additional 10 grams
of
toluene and about 10 grams of additional binder is added to the slurry and
mixed
for between about 1.5 and about 2 hours. The inner wall of the green tube
formed
above is then coated by pouring the slurry, holding once for about 5 seconds
and
pouring out the residual back to the bottle. The coated green tube is then
dried
and fired at 850 C for 1 hour in air.
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(00046) The
dense layer 10, 10', 10" is then applied. A mixture weighing
about 40 grams is prepared that contains the same powders as used in forming
the
intermediate porous layer, discussed above, except that the ratio between
LSCMV
and 10Sc1YSZ is about 40/60 by volume, 2.4 grams of cobalt nitrate
{Co(NO3)2.6H20}, 95 grams of toluene, 5 grams of ethanol, 20 grams of the
binder identified above, 400 grams of 1.5 mm diameter YSZ grinding media are
then added to the mixture and the same is milled for about 10 hours to form a
slurry (d50 ¨ 0.34 m). Again, about 10 grams of toluene and about 10 grams of
binder are added to the slurry and mixed for about 1.5 and about 2 hours. The
inner wall of the tube is then coated by pouring the slurry, holding once for
about
seconds and pouring out the residual back to the bottle. The coated green tube
is then stored dry prior to firing the layers in a controlled environment.
(00047) The
coated green tube is then placed on a C-setter in a horizontal
tube furnace and porous alumina tubes impregnated with chromium nitrate are
placed close to the coated tube to saturate the environment with chromium
vapor.
The tubes are heated in static air to about 800 C for binder burnout and, if
necessary, the sintering environment is switched to an atmosphere of a
saturated
nitrogen mixture (nitrogen and water vapor) that contains about 4 percent by
volume of hydrogen to allow the vanadium containing electronic conducting
perovskite structured materials to properly sinter. The tube is held at about
1350 C to 1430 C for about 8 hours and then cooled in nitrogen to complete the
sintering of the materials. The sintered tube is then checked for leaks
wherein the
helium leak rates should be lower than 10-7Pa.
(00048) Surface
exchange layer 16 is then applied. A mixture of powders
is prepared that contains about 35g of equal amounts of ionic and electronic
phases having chemical formulas of Zr0.80Sc0.18Y0.0202_8 and La0.8Sr0.2Fe03-8,
respectively. To this mixture, about 100 grams of toluene, 20 grams of the
binder
identified above, about 400 grams of 1.5 mm diameter YSZ grinding media are
added and the resultant mixture is milled for about 14 hours to form a slurry
(dso ¨
0.4 m). About six grams of carbon black are added to the slurry and milled for
additional 2 hours. A mixture of about 10 grams of toluene and about 10 grams
of
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the binder are then added to the slurry and mixed for between about 1.5 and
about
2 hours. The inner wall of the tube is then coated by pouring the slurry,
holding
twice for about 10 seconds and then pouring out the residual back to the
bottle.
The coated tube is then dried and fired at 1100 C for two hours in air.
(00049) The
structure formed in the manner described above is in a fully
sintered state and the catalyst is then further applied by wicking a solution
containing catalyst precursors in the direction of arrowhead B at the side of
the
porous support opposite to the intermediate porous layer. The solution can be
an
aqueous metal ion solution containing about 20 mol% Gd(NO3)3 and 80 mol%
Ce(NO3)3. A pressure can be established on the side of the porous support
layer
to assist in the infiltration of the solution. In addition, the pores can
first be
evacuated of air using a vacuum to further assist in wicking of the solution
and
prevent the opportunity of trapped air in the pores preventing wicking of the
solution all the way through the porous support layer to the intermediate
porous
layer. The resulting composite oxygen transport membrane 1 in such state can
be
directly placed into service or further fired prior to being placed into
service so
that the catalyst particles, in this case Ce0.8Gd0.202_8 are formed in the
porous
support layer adjacent to the intermediate porous layer and as described
above,
within the intermediate porous layer itself. The firing to form Ce0.8Gd0.202-8
would take place at a temperature of about 850 C and would take about 1 hour
to
form the catalyst particles.
(00050) Although
the present invention has been described with reference
to a preferred embodiment, 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.
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