Note: Descriptions are shown in the official language in which they were submitted.
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TECHNICAL FIELD OF THE INVENTION
Applicants have discovered solid state compositions with
enhanced product flux, and processes which exploit this
property. Membranes, formed from perovskitic or multi-phase
structures, with a chemically active coating have demonstrated
exceptionally high rates of fluid flux. One application is the
separation of oxygen from oxygen-containing feeds at elevated
temperatures. The membranes are conductors of oxygen ions and
electrons, and are substantially stable in air over the
temperature range of 25oC to the operating temperature of the
membrane.
BACKGROUND OF THE INVENTION
Solid state membranes formed from ion conducting
materials are beginning to show promise for use in commercial
processes for separating, purifying and converting industrial
fluids, notably for oxygen separation and purification.
Envisioned applications range from small scale oxygen pumps
for medical use to large gas generation and purification
plants. Conversion processes are numerous, and include
catalytic oxidation, catalytic reduction, thermal treating,
distillation, extraction, and the like. Fluid separation
technology encompasses two distinctly different membrane
materials, solid electrolytes and mixed conductors. Membranes
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formed from mixed conductors are preferred over solid
electrolytes in processes for fluid separations because mixed
conductors conduct ions of the desired product fluid as well
as electrons, and can be operated without external circuitry
such as electrodes, interconnects and power-supplies. In
contrast, solid electrolytes conduct only product fluid ions,
and external circuitry is needed to maintain the flow of
electrons to maintain the membrane ionization/deionization
process. Such circuitry can add to unit cost, as well as
complicate cell geometry.
Membranes formed from solid electrolytes and mixed
conducting oxides can be designed to be selective towards
specified product fluids, such as oxygen, nitrogen, argon, and
the like, and can transport product fluid ions through
dynamically formed anion vacancies in the solid lattice when
operated at elevated temperatures, typically above about
500°C. Examples of solid electrolytes include yttria-
stabilized zirconia (YSZ) and bismuth oxide for oxygen
separation. Examples of mixed conductors include titania-
doped YSZ, praseodymia-modified YSZ, and, more importantly,
various mixed metal oxides some of which possess the
perovskite structure. Japanese Patent Application No. 61-
21717 discloses membranes formed from multicomponent metallic
oxides having the perovskite structure represented by the
formula Lal_xSrxCol_yFey03_d wherein x ranges from 0.1 to 1.0,
y ranges from 0.05 to 1.0 and d ranges from 0.5 to 0. Some
other pertinent perovskite structures have been described in
copending application, Ser. No. 08/311,295, owned by the
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Assignee of record herein.
Membranes formed from mixed conducting oxides which are
operated at elevated temperatures can be used to selectively
separate product fluids from a feedstock when a difference in
product fluid partial pressures exists on opposite sides of
the membrane. For example, oxygen transport occurs as
molecular oxygen is dissociated into oxygen ions, which ions
migrate to the low oxygen partial pressure side of the
membrane where the ions recombine to form oxygen molecules, or
react with a reactive fluid, and electrons migrate through the
membrane in a direction opposite the oxygen ions to conserve
charge.
The rate at which product fluid ions permeate through a
membrane is mainly controlled by three factors. They are (a)
the kinetic rate of the feed side interfacial product fluid
ion exchange, i.e., the rate at which product fluid molecules
in the feed are converted to mobile ions at the surface of the
feed side of the membrane; (b) the diffusion rates of product
fluid ions and electrons within the membrane; and (c) the
kinetic rate of the permeate side interfacial product fluid
exchange, i.e., the~rate at which product fluid ions in the
membrane are converted back to product fluid molecules and
released on the permeate side of the membrane, or react with a
reactive fluid, such as hydrogen, methane, carbon monoxide,
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C1-C5 saturated and unsaturated hydrocarbons, ammonia, and the
like.
U.S. Patent No. 5,240,480 to Thorogood, et al,~
~ addressed the kinetic rate
of the feed side interfacial gas exchange by controlling the
pore size of the porous structure supporting a non-porous
dense layer. Numerous references, such as U.S. Patent
4,330,633 to Yoshisato et al, Japanese Kokai No. 56[1981)-
92,103 to Yamaji, et al, and the article by Teraoka and
coworkers, Chem. Letters, The Chem. Soc. of Japan, pp. 503-506
(1988) describe materials with enhanced ionic and electronic
conductive properties.
U.S. Patents 4,791,079 and 4,827,071 to Hazbun,
__ addressed the kinetic rate
of the permeate side interfacial gas exchange by utilizing a
two-layer membrane in which one layer was an impervious mixed
ion and electron conducting ceramic associated with a porous
layer containing a selective hydrocarbon oxidation catalyst.
a
Typical of metal oxide membrane references is Japanese
Patent Application 61-21717, described above. When an.oxygen-
containing gaseous mixture at a high oxygen partial pressure
is applied to one side of a membrane having a dense layer
formed from the enumerated oxide, oxygen will adsorb and
dissociate on the membrane surface, become ionized and diffuse
through the solid and deionize, associate and desorb as an
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oxygen gas stream at a lower oxygen partial pressure on the
other side of the membrane.
The necessary circuit of electrons to supply this
ionization/deionization process is maintained internally in
the oxide via its electronic conductivity. This type of
separation process is described as particularly suitable for
separating oxygen from a gas stream containing a relatively
high partial pressure of oxygen, i.e., greater than or equal
to 0.2 atm. Multicomponent metallic oxides which demonstrate
both oxygen ionic conductivity and electronic conductivity
typically demonstrate an oxygen ionic conductivity ranging
from 0.01 ohm-lcm-1 to 100 ohm-lcm 1 and an electronic
conductivity ranging from about 1 ohm-lcm-1 to 100 ohm-lcm-1
under operating conditions.
Some multicomponent metallic oxides are primarily or
solely oxygen ionic conductors at elevated temperatures. An
example is (Y203)0.1(Zr2~3)0.9 which has an oxygen ionic
conductivity of about 0.06 ohm-1 cm-1 at 1000oC and an ionic
transport number (the ratio of the ionic conductivity to the
total conductivity) close to 1. European Patent Application
EP 0399833A1 describes a membrane formed from a composite of
this oxide with a separate electronically conducting phase,
such as platinum or another noble metal. The electronic
conducting phase will provide the return supply of electrons
through the structure allowing oxygen to be ionically
conducted through the composite membrane under a partial
pressure gradient driving force.
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Another category of multicomponent metallic oxides
exhibit primarily or solely electronic conductivity at
elevated temperatures and their ionic transport numbers are
close to zero. An example is PrxInyOz which is described in
European Patent Application EP 0,399,833 Al. Such materials
may be used in a composite membrane with a separate oxygen
ionic conducting phase such as a stabilized Zr02. A membrane
constructed from a composite of this type may also be used to
separate oxygen from an oxygen-containing stream, such as air,
by applying an oxygen partial pressure gradient as the driving
force. Typically, the multicomponent oxide electronic
conductor is placed in intimate contact with an oxygen ionic
conductor.
Organic polymeric membranes may also be used for fluid
separation. However, membranes formed from mixed conducting
oxides offer substantially superior selectivity for such key
products as oxygen when compared to polymeric membranes. The
value of such improved selectivity must be weighed against the
higher costs associated with building and operating plants
employing membranes formed from mixed conducting oxides which
plants require heat exchangers, high temperature seals and
other costly equipment. Typical prior art membranes formed
from mixed conducting oxides do not exhibit sufficient
permeance (defined as a ratio of permeability to thickness) to
justify their use in commercial fluid separation applications.
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Oxygen permeance through solid state membranes is -known
to increase proportionally with decreasing membrane thickness,
and mechanically stable, relatively thin membrane structures
have been widely studied.
A second article by Teraoka et al, Jour. Ceram. Soc.
Japan. International Ed, Vol 97, pp. 458-462, (1989) and J.
Ceram. Soc. Japan, International Ed, Vol 97, pp. 523-529,
(1989), for example, describes solid state gas separation
membranes formed by depositing a dense, nonporous mixed
conducting oxide layer, referred to as "the dense layer", onto
a porous mixed conducting support. The relatively thick
porous mixed conducting support provides mechanical stability
for the thin, relatively fragile dense, nonporous mixed
conducting layer. Structural failures due to thermo-
mechanical stresses experienced by the membranes during
fabrication and use were substantially minimized due to the
chemical compatibility of the respective membrane layers.
Based upon considerations limited to dense layer thickness,
Teraoka and coworkers expected the oxygen flux to increase by
a factor of 10 for a membrane having a mixed conducting porous
layer and a thin mixed conducting dense layer compared to a
standard single layered dense, sintered mixed conducting disk.
However, they obtained an increase of less than a factor of
two.
Perovskitic structures include true perovskites that have
a three dimensional cubic array of small diameter metal ion
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octahedra, as well as structures that incorporate a
perovskite-like layers or layer, i.e., a two dimensional array
of small diameter metal ion octahedra arranged in a two
dimensional square array. These perovskite-like arrays are
charge stabilized by larger diameter metal ions, or other
charged layers. Examples of perovskitic structures include
cubic perovskites, brownmillerites, Aurivillius phases, and
the like. A description of the relation between perovskites
and some of the various perovskitic phases is presented in L.
Katz and R. Ward, Inorg. Chem. 3, 205-211, (1964)
These layered structures can accommodate vacancies of
oxygen ions, and the ordering of these vacancies can lead to
structural variations, such as the brownmillerite phase.
Brownmillerites are perovskites that have one sixth of the
oxygen ions missing with the resulting oxygen ion vacancies
ordered into continuous lines within the crystal. An example
is SrFe03_X, as described by S. Shin, M. Yonemura, and H. Ikawa
in Mater Res Bull 13, 1017-1021 (1978). Under conditions
where x = 0, the structure is a regular, cubic perovskite
structure. As conditions of temperature and pressure are
varied so that x increases, the oxygen vacancies that are
introduced are at first randomly scattered throughout the
crystal, or "disordered". However, as x approaches 0.5 the
vacancies can become "ordered", i.e., the vacancies form a
regular pattern throughout the crystal. When exactly one
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sixth of the oxygen ions are absent (X = 0.5) and the
resulting vacancies are "ordered", the phase is called a
brownmillerite.
Aurivillius phases, sometimes called lamellar
perovskites, consist of layers of perovskite wherein the
larger diameter metal canons have, in part or in toto, been
replace by layers of another oxide, commonly (gi2p2)2+, as
described in Catalysis Letters 16, p 203-210 (1992) by J.
Barrault, C. Grosset, M. Dion, M. Ganne and M. Tournoux.
Their general formula is [Bi202][An-lBn~3n+1]. where "A"
designates the larger diameter metal ions, and "B" designates
the smaller diameter metal ions. Wide latitude of
substitution is possible for the A and B metals in the
perovskite layer, and for Bi in the interleaving layers of
Bi202, as described by A. Castro, P. Millan, M. J. Martinez-
Lope and J. B. Torrance in Solid State Ionics 63-65, p 897-901
(1993), The so-called
superconductors, such as YBa2Cu30~-X, are also perovskitic
structures, with another type of ordered vacancies, as
described in W. Carrillo-Cabrera, H-D Wiemhofer and W. Gopel,
Solid State Ionics, 32/33, p 1172-1178 (1989).
Researchers are continuing their search for solid state
conductive membranes which exhibit superior flux without
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sacrificing mechanical and physical compatibility of the
composite membrane.
BRIEF SUNI~2ARY OF THE INVENTION
The present invention relates to novel mixed conductor
membranes which are capable of separating industrial fluid
streams. The membranes have a chemically active coating and a
structure and composition that forms a substantially
perovskitic structure, substantially stable in air over the
temperature range of 25oC to the operating temperature of the
membrane, such that enhanced flux is observed compared to
prior art solid state membranes. The upper range of membrane
operating temperature would be about 400°C for partial
oxidation of C2-C4 hydrocarbons and oxygen production
processes; about 700°C for selected partial oxidation and
oxygen production processes, as well as processes for the
removal of oxygen from industrial gases and industrial fluids
and about 850°C for previously mentioned processes under
certain circumstances, as well as for partial oxidation of
methane and natural gases.
While membranes are known which comprise a mixed
conducting oxide layer, the fluid impermeable membranes of the
present invention have a composition that forms a
substantially perovskitic structure. Such structures exhibit
enhanced flux, particularly of oxygen. A porous coating of
metal or metal oxide increases the kinetic rate of the feed
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side interfacial fluid exchange, the kinetic rate of the
permeate side interfacial fluid exchange, or both. Membranes
fabricated from such material and in such manner display
increased flux.
The fluid impermeable membranes according to the
invention are formed from a mixture of at least two different
metal oxides wherein the multicomponent metallic oxide form a
perovskitic structure which demonstrates electron conductivity
IO as well as product fluid ion conductivity at temperatures
greater than about 400°C. These materials are commonly
referred to as mixed conducting oxides.
Suitable mixed conducting oxides are represented by the
structure
~A1_xA~xl B 03-S
or
EBi2-yAy02-$')C~A1-xA~x)n-1Bn03n+1-$"~
wherein A is chosen from the group consisting of Ca, Sr, Ba,
Bi, Pb, Ca, K, Sb, Te, Na and mixtures thereof; A' is chosen
from the group consisting of La, Y, Ce, Pr, Nd, Pm, Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, and mixtures thereof; B
is chosen from the group consisting of Fe, Mg, Cr, V, Ti, Ni,
Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greater than
0.9, preferably not greater than about 0.6, more preferably
not greater than 0.4, most preferably not greater than 0.25; y
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is an integer from 0 to 2; n is an integer from 1 to 7; and 8,
8' and b" are determined by the valence of the metals.
The mixed conducting oxides are formed into fluid
impermeable membranes. At least one surface of the fluid
impermeable membrane is coated with a porous layer of metal or
metal oxide. The coating acts as a chemically active site
which enhances the kinetic rate of the interfacial fluid
exchange at the fluid impermeable membrane surface.
The current invention is directed towards a solid state
membrane, comprising a structure selected from the group
consisting of substantially perovskitic material, an intimate,
gas-impervious, multi-phase mixture of an
electronically-conductive phase and an oxygen ionconductive
phase, and combinations thereof; and a porous coating selected
from the group consisting of metals, metal oxides and
combinations thereof.
The current invention is also directed towards the use of
one or more membranes formed from the coated mixed conductors
described. Suitable uses of such membranes include processes
for the partial oxidation of Cl-C4 hydrocarbons, and oxygen
separation, production and removal from oxygen-containing
fluids, particularly air, or air diluted with other fluids.
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DETAILED DESCRIPTION OF THE INVENTION
The present invention relates perovskitic membranes with
a chemically active coating, and processes employing such
membranes. One such process is separating oxygen from oxygen-
containing feeds at elevated temperatures. The membranes are
conductors of product fluid ions and electrons, and are of a
composition that forms a substantially perovskitic structure.
Specific compositions stabilize a perovskitic structure in the
mixed conducting fluid impermeable membrane. Membranes
fabricated from such material display increased flux. More
particularly, a mixed conductor membrane wherein the fluid
impermeable membrane has the composition
[A1-XA'Xl B ~3-g Equation 1
or
[Bi2-yAy02-Sr][(A1-xA'x)n-lBn~3n+1-$~~J Equation 2
wherein A is chosen from the group consisting of Ca, Sr, Ba,
Bi, Pb, Ca, K, Sb, Te, Na and mixtures thereof; A' is chosen
from the group consisting of La, Y, Ce, Pr, Nd, Pm, Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, and mixtures thereof; B
is chosen from the group consisting of Fe, Mg, Cr, V, Ti, Ni,
Ta, Mn, Co, V, Cu, and mixtures thereof; x is not greater than
0.9; y is an integer from 0 to 2; n is an integer from 1 to 7;
and b, 8', and b" are determined by the valence of the metals;
wherein the perovskitic phase is substantially stable in air
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over the temperature range of 25-950oC; and coated with a
porous layer of metal or metal oxide selected from the group
consisting of platinum, silver, palladium, lead, cobalt,
nickel, copper, bismuth, samarium, indium, tin, praseodymium,
their oxides, and combinations of the same, has been shown to
exhibit unexpectedly high fluid transport fluxes, particularly
transport fluxes of oxygen.
Applicants' discovery can be more fully understood by
developing an understanding of the mechanism by which product
fluids are ionically transported through the mixed conducting
oxide membrane. Typical product fluids include industrial
gases, such as oxygen, nitrogen, argon, hydrogen, helium,
neon, air, carbon monoxide, carbon dioxide, synthesis gases
(mixtures of CO and H2), and the like. The product fluid flux
observed by conventional mixed conductor membranes is
controlled by surface kinetic limitations and bulk diffusion
limitations. Surface kinetic limitations are constraints to
product fluid flux caused by one or more of the many steps
involved in converting a feed fluid molecule on the feed side
of the mixed conductor membrane into mobile ions and
converting the ions back to product fluid molecules, or
reacting the ions with a reactant fluid molecule, on the
permeate side of the mixed conductor membrane. Bulk diffusion
limitations are constraints on fluid flux relating to the
diffusivity of product fluid ions through the fluid
impermeable membrane material.
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Membranes composed substantially of perovskitic phase
materials exhibit high overall product fluid flux. However,
the perovskitic phase is not formed in all mixed conducting
oxide materials or, if formed, is not stable over the required
range of fabricating and operating conditions. For instance,
membranes formed from hexagonal phase materials exhibit
little, if any, oxygen flux. To produce an effective oxygen
membrane, therefore, the membrane composition must maintain a
substantially high fraction of stable perovskitic phase in the
membrane at operating conditions.
To complement the enhanced bulk diffusion rate through
the fluid impermeable membrane, the kinetic rates of
interfacial fluid exchange must be examined. If the rate at
which the feed fluid is converted to mobile ions on the feed
side of the fluid impermeable membrane, or the rate at which
the mobile ions in the fluid impermeable membrane are
converted back to product fluid molecules, is slower than the
fluid impermeable membrane bulk diffusion rate, the overall
product permeation rate will be limited to the slowest of the
three processes.
Applicants have discovered a novel composition which
stabilizes the perovskitic phase in fluid impermeable
membranes with compositions previously unable to sustain a
stable perovskitic phase over the range from ambient
temperature and pressure in air to the conditions used for
product fluid separation. In particular, specific
compositions stabilize a substantially cubic perovskite layer
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in a substantially perovskitic structure in ABO materials.
Porous coatings on one or more surface of the cubic perovskite
material increases the rate of fluid adsorption, ionization,
recombination or desorption, as well as increasing the overall
fluid flux rate.
The present invention provides coated membranes, and
permits the fabrication of mixed conductor oxide structures
that are substantially perovskitic phase. Membranes made from
such material and in such manner exhibit relatively high
overall bulk diffusion rates.
The claimed membranes comprise the composition
described in Equation 1 and Equation 2, having no connected
through porosity, a substantially stable perovskitic structure
in air at 25-950oC, coated with a porous layer of metal, metal
oxides, or mixtures of metals and metal oxides, and the
capability of conducting electrons and product fluid ions at
operating temperatures.
Multicomponent metallic oxides suitable for practicing
the present invention are referred to as "mixed" conducting
oxides because such multicomponent metallic oxides conduct
electrons as well as product fluid ions at elevated
temperatures. Suitable mixed conducting oxides are
represented by the compositions of Equation 1 and 2, which
yield a substantially stable cubic perovskite structure in air
at 25oC to the operating point of material. Materials
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described in the prior art exhibit significantly lower product
fluid fluxes.
The thickness of the material can be varied to ensure
sufficient mechanical strength of the membrane. As discussed
previously, thinner membranes increase the overall bulk
diffusion rate for a given membrane material. To exploit this
phenomena, thinner membranes may be supported by one or more
porous supports. The minimum thickness of unsupported mixed
conductor membranes of Applicants' invention is about 0.01 mm,
preferably about 0.05 mm, most preferably about 0.1 mm. The
maximum thickness of unsupported mixed conductor membranes of
Applicants' invention is about 10 mm, preferably about 2 mm,
most preferably about 1 mm.
The minimum thickness of supported mixed conductor
membranes of Applicants' invention is about 0.0005 mm,
preferably about 0.001 mm, most preferably about 0.01 mm. The
maximum thickness of supported mixed conductor membranes of
Applicants' invention is about 2 mm, preferably about 1 mm,
most preferably about 0.1 mm.
In addition to the increased product fluid flux, the
membranes of the present invention exhibit stability over a
temperature range from 25oC to the operating temperature of
the membrane and an oxygen partial pressure range from 1 to
about 1x10-6 atmosphere (absolute) without undergoing phase
transitions. Substantially stable perovskitic structures
include all structures with no less than 90~ perovskitic phase
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material, preferably no less than 95% perovskitic phase
material, and most preferably no less than 98o perovskitic
phase material, which do not exhibit permanent phase
transitions over a temperature range from 25oC to 950oC and an
oxygen partial pressure range from 1 to about 1x10-6
atmosphere (absolute).
Applicants' invention also includes chemically-active
coated membranes of an intimate, fluid-impervious, multi-phase
mixture of any electronically-conducting material with any
product fluid ion-conducting material and/or a gas impervious
"single phase" mixed metal oxide having a perovskite structure
and having both electron-conductive and product fluid
ion-conductive properties. The phrase "fluid-impervious" is
defined herein to mean "substantially fluid-impervious or
gas-tight" in that the mixture does not permit a substantial
amount of fluid to pass through the membrane (i.e., the
membrane is non-porous, rather than porous, with respect to
the relevant fluids). In some cases, a minor degree of
perviousness to fluids might be acceptable or unavoidable,
such as when hydrogen gas is present.
The term "mixtures" in relation to the solid multi-
component membrane includes materials comprised of two or more
solid phases, and single-phase materials in which the atoms of
the various elements are intermingled in the same solid phase,
such as in yttria-stabilized zirconia. The phrase
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"mufti-phase mixture" refers to a composition which contains
two or more solid phases interspersed without forming a single
phase solution.
In other words, the mufti-phase mixture is "multiphase",
because the electronically-conductive material and the product
fluid ion-conductive material are present as at least two
solid phases in the fluid impervious solid membrane, such that
the atoms of the various components of the mufti-component
membrane are, for the most part, not intermingled in the same
solid phase.
The mufti-phase solid membrane of the present invention
differs substantially from "doped" materials known in the art.
A typical doping procedure involves adding small amounts of an
element, or its oxide (i.e., dopant), to a large amount of a
composition (i.e., host material), such that the atoms of the
dopant become permanently intermingled with the atoms of the
host material during the doping process, whereby the material
forms a single phase. The mufti-phase solid membrane of the
present invention, on the other hand, comprises a product
fluid ion conductive material and an electronically conductive
material that are not present in the dopant/host material
relationship described above, but are present in substantially
discrete phases. Hence, the solid membrane of the present
invention, rather than being a doped material, may be referred
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to as a two-phase, dual-conductor, multi-phase, or
mufti-component membrane.
The solid state membranes of the present invention may
have a first surface, a second surface, an electron-conductive
path between the first and second surfaces, and an oxygen ion-
conductive path between the first and second surfaces.
The mufti-phase membrane of the present invention can be
distinguished from the doped materials by such routine
procedures as electron microscopy, X=ray diffraction analysis,
X-ray adsorption mapping, electron diffraction analysis,
infrared analysis, etc., which can detect differences in
composition over a mufti-phase region of the membrane.
Typically, the product fluid ion-conducting materials or
phases are solid solutions (i.e., solid "electrolytes") formed
between oxides containing divalent and trivalent cations such
as calcium oxide, scandium oxide, yttrium oxide, lanthanum
oxide, etc., with oxides containing tetravalent cations such
as zirconia, thoria and ceria or the product fluid
ion-conducting materials or phases comprise a product fluid
ion-conductive mixed metal oxide of a perovskite structure.
Their higher ionic conductivity is believed to be due to the
existence of product fluid ion site vacancies. One product
fluid ion vacancy occurs for each divalent or each two
trivalent cations that are substituted for a tetravalent ion
in the lattice. Any of a large number of oxides such as
yttria stabilized zirconia, doped ceria, thoria-based
materials, or doped bismuth oxides may be used. Some of the
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known solid oxide transfer materials include Y203-stabilized
ZrOz, Ca0-stabilized Zr02, Sc203-stabilized Zr02, Y203-stabilized
Bi203, Y203-stabilized Ce02, Ca0-stabilized Ce02, ThOz, Y203-sta-
bilized ThOz, or ThOz, Zr02, BiZ03, CeOz, or HfOz stabilized by
addition of any one of the lanthanide oxides or CaO. Many
other oxides are known which have demonstrated product fluid
ion-conducting ability which could be used in the multi-phase
mixtures, and they are included in the present concept.
Preferred among these solid electrolytes are the
Y203- (yttria) and Ca0- (calcia) stabilized ZrOz ( zirconia)
materials. These two solid electrolytes are characterized by
their high ionic conductivity, their product fluid ion conduc-
tion over wide ranges of temperature and product fluid
pressure, and their relatively low cost.
Applicants have also discovered that since perovskitic
structures exhibit excellent electron-conductive and product
fluid ion-conductive properties, in multi-phase materials,
perovskitic materials may be used as the
electronically-conductive material, the product fluid
ion-conductive material, or both. The resulting multi-phase
mixture can be coated with a porous, chemically active
material to produce a solid state membrane with enhanced flux.
The present invention can consist of a solid state
membrane comprising an intimate, gas-impervious, multi-phase
mixture comprising from about 1 to about 75 parts by volume of
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the electronically-conductive phase and from about 25 to about
99 parts by volume of the product ion-conductive phase.
The porous coating comprises metal or metal oxide
selected from the group consisting of platinum, silver,
palladium, lead, cobalt, nickel, copper, bismuth, samarium,
indium, tin, praseodymium, their oxides, and combinations of
the same, where the coating exhibits oxygen ionic conductivity
less than about 1.0 ohm-lcm-1, preferably less than about 0.1
ohm-lcln 1, most preferably less than about 0.01 ohm-lcm 1
under operating conditions. The porous coating may be applied
using standard applications techniques including, but not
limited to spraying, dipping, laminating, pressing,
implanting, sputter deposition, chemical deposition, and the
like.
The membranes of the present invention can be used to
recover product fluid, such as oxygen, from a product fluid-
containing feed fluid by delivering the product fluid-
containing feed fluid into a first compartment which is
separated from a second compartment by the subject membrane,
establishing a positive product fluid partial pressure
difference between the first and second compartments by
producing an excess product fluid partial pressure in the
first compartment and/or by producing a reduced product fluid
partial pressure in the second compartment; contacting the
product fluid-containing feed fluid with the membrane at a
temperature greater than about 400°C to separate the product
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fluid-containing feed into a product fluid-enriched permeate
stream and a product fluid-depleted effluent stream.
A difference in product fluid partial pressure between
the first and second compartments provides the driving force
for effecting the separation when the process temperature is
elevated to a sufficient temperature to cause product fluid in
the product fluid-containing feed fluid residing in the first
compartment to adsorb onto the first surface of the membrane,
become ionized via the membrane and to be transported through
the fluid impermeable membrane in the ionic form. A product
fluid-enriched permeate is collected or reacts in the second
compartment wherein ionic product fluid is converted into the
neutral form by the release of electrons at the second surface
of the membrane, in the second compartment.
A positive product fluid partial pressure difference
between the first and second compartments can be created by
compressing the feed fluid, such as air or other oxygen-
containing fluid in an oxygen separation process, in the first
compartment to a pressure sufficient to recover the product
fluid-enriched permeate stream at a pressure of greater than
or equal to about one atmosphere. Typical pressures range
from about 15 psia to about 250 psia and the optimum pressure
will vary depending upon the amount of product fluid in the
product fluid-containing feed. Conventional compressors can
be utilized to achieve the necessary product fluid partial
pressure. Alternately, a positive product fluid partial
pressure difference between the first and second compartments
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can be achieved by evacuating the second compartment to_a
pressure sufficient to recover the product fluid-enriched
permeate. Evacuation of the second compartment may be
achieved mechanically, using compressors, pumps and the like;
chemically, by reacting the product fluid-enriched permeate;
thermally, by cooling the product fluid-enriched permeate; or
by other methods known in the art. Additionally, the present
invention may utilize an increase of product fluid partial
pressure in the first compartment while simultaneously
reducing the product fluid partial pressure in the second
compartment, by the means described above. The relative
pressures may also be varied during operation, as necessary to
optimize product fluid separation, or necessitated by process
which supply feeds to, or accept product streams from, the two
compartments.
Recovery of the product fluid-enriched permeate may be
effected by storing the substantially product fluid-enriched
permeate in a suitable container or transferring the same to
another process. For oxygen production processes, the product
fluid-enriched permeate typically comprises pure oxygen or
high purity oxygen defined as generally containing at least
about 90 volo 02, preferably more than about 95 volo 02 and
especially more than 99 vol% 02.
Although oxygen separation and purification is described
herein for illustrative purposes, the present invention may be
used in similar fashion for separation, purification and
reaction of other product fluids including, but not limited
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to, nitrogen, argon, hydrogen, helium, neon, air, carbon
monoxide, carbon dioxide, synthesis gases, and the like
The following example is provided to further illustrate
Applicants' invention. The example is illustrative and is not
intended to limit the scope of the appended claims.
EXAMPLE
Example 1
A mixed conductor fluid impermeable membrane of
nominal composition [Lap.2Sr0.g][Cop_lFep.~Crp.2Mgo.o1]03-$ was
prepared in a manner similar to the examples described in U.S.
Patent No. 5, 061, 682,
4232.60 grams of Sr(N03)2, 773.80 grams of La203 (Alpha, dried
at 850°C), 6927.80 grams of Fe(N03)~9H~0), 730.50 grams of
Co(N03)3~6H20, and 64.10 grams of Mg(N03)2 were added to
approximately 30 liters of deionized water containing
dissolved sucrose.
A portable spray-dryer was used to spray-dry the ceramic
precursor solution described above. A suitable portable
spray-dryer is available from Niro Atomizer of Columbia, Md.
The spray-dryer includes a centrifugal atomizer capable of
speeds up to 40,000 rpm. The atomizer sits near the top of a
drying chamber that, has an inner diameter of 2 feet, 7 inches,
with a 2-foot cylindrical height and a 60° conical bottom.
The centrifugal atomizer and drying chamber are made from
stainless steel. The drying chamber is coupled to an electric
air heater for providing drying air to the drying chamber.
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The drying air is drawn through the drying chamber by a blower
positioned downstream from the drying chamber. The spray-
dryer includes a cyclone separator that receives the drying
air and dry product from the bottom of the drying chamber.
The cyclone separator separates the dry product from the
exhausted drying air. The bottom of the cyclone separator
includes an outlet that allows the dried particles to
gravitate into a vertically oriented tube furnace capable of
maintaining an air temperature of about 300°-450° C. The dried
particles are pyrolyzed in the tube furnace. The tube furnace
has a height sufficient to provide a residence time for the
freely gravitating particles of about 0.5 to 2.0 seconds. The
bottom of the tube furnace communicates with a collection
chamber where the ceramic particles are collected.
The ceramic precursor solution described above was
introduced into the spray-dryer chamber at a flow rate of
about 1.8 liters per hour. The centrifugal atomizer spinning
at about 30,000 RPM broke up the precursor solution into small
droplets having a diameter on the order of about 20-50
microns. The air flow through the drying chamber and cyclone
ranged between about 35-40 standard cubic feet per minute.
The air entering the drying chamber was preheated to about
375° C. As the small droplets were forcefully convected
toward the bottom of the drying chamber, they became fully
dehydrated down to a critical state of dehydration such that
their diameter was reduced to about 10.0 microns or less. The
temperature of the drying gas at the bottom of the drying
chamber was approximately 125° C., which ensures substantially
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all the water was removed from the particles in the spray-
dryer. The dried powder and drying air were then separated
from each other in the cyclone separator. The separated
powder fell due to gravity through the tube furnace, which was
preheated to about 490° C. The particles' residence time in
the furnace ranged from about 0.5-2.0 seconds. The
temperature in the tube furnace initiated the exothermic
anionic oxidation-reduction reaction between the nitrate ions
and the oxides in the individual particles. The combustion
by-products (C02 and water vapor) were passed through the
system and out the exhaust, while the reacted particles
dropped into the collection jar. About 1000 grams of
particles were collected.
The resulting powders were analyzed, and had the
composition [La0.19Sr0.8J[Cop.lFep.69Crp.2Mg0.Ol~~x~ A 190.10 g
portion of the resulting powder, 3.88 g polyvinyl butyral
resin (Monsanto, St. Louis MO), and 160 ml toluene were
charged, with 820 g of Zr02 media to a jar mill, and milled
for approximately 3 hours. 20 ml absolute ethanol was added,
and the slurry allowed to stand, without milling, overnight.
The product was filtered, and the resulting powder was dried
and screened to pass though a 60 mesh Tyler screen. X-ray
diffraction (XRD) of the powder showed that the material was
1000 cubic perovskite phase.
A 4 g portion of the screened powder was pressed into a
1-3/8" diameter disk under 28,000 psi applied pressure. The
disk was fired in air at 105oC for 15 minutes, the temperature
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increased to 1300oC over the course of 13 hours and maintained
for 1 hour, then cooled to ambient temperature.
The disk was polished on both sides with 500 grit SiC
with isopropanol to a final thickness of 1 mm. 3.0 wto Bi203
(Fluka) was added to platinum ink (Englehardt), and the
resulting mixture was diluted with toluene to form a low
viscosity coating fluid. Both sides of the polished disk were
coated with the fluid over an area of approximately 2.0 cm2.
The coated disk was heated in air to 1065°C over a period of
10.5 hours, then cooled to ambient temperature. The coated
disk was bonded to a 1 inch outside diameter mullite tube with
a 1/8" thick Pyrex ring, and the exposed surface area measured
to be approximately 2 cm2.
The mullite tube, disk, and gas handling equipment were
placed in a thermistatically controlled electric heater. The
disk was heated in stagnant air to 850oC as indicated by a
thermocouple affixed to the mullite tube approximately 1 cm
from the tube/disk bond. Air flow at the rate of 1.0 1/min
was initiated on one side of the disk, and helium permeate
feed flow at 150 cm3/min started on the other side of the
disk. The effluent helium permeate was analyzed using on-line
gas chromatography. The permeate was also analyzed for
nitrogen, to permit correction for any air leakage into the
permeate stream.
Oxygen flux of the membrane was calculated using the
expression:
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qp2 = (qp * (xp2p - 0.256 * XN2P~ * PO/760 * 273/TOl / 100
where q02 - Oxygen flux (cm3/min);
qp - Permeate exhaust flow rate (cm3/min);
x02p - Oxygen concentration in permeate exhaust (%);
xN2p - Nitrogen concentration in permeate exhaust
0
(o);
PO - Atmospheric pressure (mm Hg, abs.); and
TO - Ambient temperature (degrees K).
Oxygen flux was normalized to correct for membrane disk
thickness variations using the expression:
q'02 = q02 * L
where q~02 - Oxygen flux normalized for thickness
(cm3/min-mm) ;
q02 - Oxygen flux (cm3/min); and
L - Thickness of membrane disk (mm).
Oxygen flux per unit area was calculated by dividing the
oxygen flux normalized for thickness (q'02) by the membrane
disk area, measured in cm2.
Operating characteristics of the disk were evaluated for
over 50 hours. Test data are supplied in Table l, below.
Ambient temperature (To) was maintained at 293°K, and Po was
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741 mm Hg for all data points. The air feed rate was
maintained at 1000 sccm.
The data of Table 1 show the excellent long-term
stability of the material in air at elevated temperatures, and
the high oxygen flux.
TABLE 1
Time Membrane PermeatePermeate q02 q~02
Temp analysis
(hours)(Deg. C) (sccm) (X02P ) (XN2P)(cc/min)(cc%m2/min)
1 850 154 0.163 0.017 0.256 0.128
6 850 152 0.159 0.017 0.248 0.124
53 850 154 0.087 0.020 0.128 0.064
