Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing nonmetal element
Reexamination Certificate
1999-02-02
2001-05-22
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing nonmetal element
C095S045000, C095S054000
Reexamination Certificate
active
06235187
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an oxygen separation method using ceramic compositions useful as solid electrolyte ion transport membranes. More particularly, the invention relates to a perovskite structure that contains the oxides of two lanthanides and two transition metals. The composition is oxygen deficient and maintains a cubic structure over a wide temperature range.
BACKGROUND OF THE INVENTION
The separation of oxygen from an oxygen containing gaseous stream is a process step in a number of commercially significant manufacturing operations. One method of oxygen separation utilizes a mixed conductor ceramic material. Oxygen ions and electrons are selectively transported through the non-porous ceramic membrane element that is impervious to other species. Suitable ceramics include mixed conductor perovskites and dual phase metal—metal oxide combinations. Exemplary ceramic compositions are disclosed in U.S. Pat. Nos. 5,702,959 (Mazanec, et al.), 5,712,220 (Carolan, et al.) and 5,733,435 (Prasad, et al.), all of which are incorporated by reference in their entireties herein.
The membrane elements have oxygen selectivity. “Oxygen selectivity” means that only oxygen ions are transported across the membrane with the exclusion of other elements, and ions thereof. The solid electrolyte membrane is made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium or analogous oxides having a fluorite or perovskite structure. Use of these membranes in gas purification applications is described in European Patent Application No. 778,069 entitled “Reactive Purge for Solid Electrolyte Membrane Gas Separation” by Prasad, et al.
The ceramic membrane element has the ability to transport oxygen ions and electrons at the prevailing oxygen partial pressure in a temperature range of from 450° C. to about 1200° C. when a chemical potential difference is maintained across the membrane element. This chemical potential difference is established by maintaining a positive ratio of oxygen partial pressures across the ion transport membrane. The oxygen partial pressure (P
O2
) is maintained at a higher value on the cathode side of the membrane, that is exposed to the oxygen-containing gas, than on the anode side, where transported oxygen is recovered. This positive P
O2
ratio may be obtained by reacting transported oxygen with an oxygen-consuming process or fuel gas. The oxygen ion conductivity of a mixed conductor perovskite ceramic membrane is typically in the range of between 0.01 and 100 S/cm where S (“Siemens”) is reciprocal of ohms (1/&OHgr;).
For effective application of a perovskite for oxygen separation, a number of requirements should be met. The perovskite should have a high oxygen flux, where flux is the rate of oxygen transport through the membrane structure. The perovskite must have a cubic crystalline structure over the entire range of operating temperatures. Perovskites with a hexagonal crystalline structure are not effective for oxygen transport. Some perovskites have a hexagonal crystalline structure at room temperature (nominally 20° C.) and undergo a phase transformation at an elevated temperature. In such a material, the phase transformation temperature represents the minimum temperature at which an oxygen separator containing that material as a membrane element may be operated. The perovskite structure must be chemically stable at the operating temperature and have a degree of mechanical stability.
A number of mixed oxide perovskites are disclosed as useful for oxygen separation. These perovskites are typically of the form ABO
3
where A is a lanthanide element, B is a transition metal and O is oxygen. A lanthanide, or rare earth element, is an element between atomic number 57 (lanthanum) and atomic number 71 (lutetium) in the Periodic Table of the Elements as specified by IUPAC. Typically, yttrium (atomic number 39) is included within the lanthanide group. The transition metals are those in Period 4, and between Groups II and III, of the Periodic Table of the Elements and include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. The A component and/or the B component may be doped with other materials to enhance stability and performance.
U.S. Pat. No. 5,648,304 by Mazanec, et al. discloses an oxygen selective perovskite represented by the formula
[A
1-x
A′
x
][Co
1-y-x
B
y
B′
z
]O
3-d
,
where
A is selected from the group consisting of calcium, strontium and barium;
A′ is selected from the lanthanide series defined as elements 57-71 on the Periodic Table of Elements as well as yttrium, thorium and uranium;
B is selected from the group consisting of iron, manganese, chromium, vanadium and titanium;
B′ is selected to be copper or nickel;
x is in the range of between about 0.0001 and 0.1;
y is in the range of from about 0.002 and 0.05;
z is in the range of from about 0.0005 and 0.3;
and
d is determined by the valence of the metals.
Mazenac et al. disclose that the addition of a relatively low concentration of specific transition metals stabilizes the perovskite as a cubic structure inhibiting the formation of hexagonal phase materials. The crystalline structure is disclosed as stable over a temperature range of 25° C. to 950° C.
U.S. Pat. No. 5,712,220 by Carolan, et al. discloses a perovskite effective for solid state oxygen separation devices represented by the structure
Ln
x
A′
x′
A″
x″
B
y
B′
y′
B″
y″
O
3-z
where
Ln is an element selected from the f block lanthanides;
A′ is selected from Group 2;
A″ is selected from Groups 1, 2, and 3 and the f block lanthanides;
B, B′ and B″ and independently selected from the d block transition metals, excluding titanium and chromium;
0<x<1;
0<x′<1;
0<x″<1
0<y<1.1;
0<y′<1.1;
0<y″<1.1;
x+x′+x″=1.0;
1.1>y+y′+y″>1.0; and
z is a number which renders the compound charge neutral where the elements are represented according to the Periodic Table of the Elements as adopted by IUPAC.
The structure disclosed by Carolen et al. has a B (transition metal) ratio (y+y′+y″/x+x′+x″) that is greater than 1. The structure is disclosed as having stability in an environment having high carbon dioxide and water vapor partial pressures.
U.S. Pat. No. 5,817,597 by Carolan et al. discloses a perovskite effective for solid state oxygen separation devices represented by the structure
Ln
x
A′
x′
Co
y
Fe
y′
Cu
y″
O
3-z
Where
Ln is an element selected from the f block lanthanides;
A′ is either strontium or calcium;
x, y and z are greater than 0;
x+x′=1
y+y′+y″=1;
0<y″<0.4; and
z is a number that renders the composition of matter charge neutral. The composition is disclosed as having a favorable balance of oxygen permeance and resistance to degradation under high oxygen partial pressure conditions. The B-site is stabilized by a combination on iron and copper.
Another perovskite structure suitable for use as an oxygen transport membrane is disclosed in Japanese Patent Office Kokai No. 61-21,717 that was published on Jan. 30, 1986. The Kokai discloses a metal oxide for oxygen transport membrane represented by the structure:
La
1-x
Sr
x
Co
1-y
Fe
y
O
3-&dgr;
where
x is between 0.1 and 1;
y is between 0.05 and 1; and
&dgr; is between 0.5 and 0.
A paper by Teraoka (
Chemistry Letters,
A publication of the Chemical Society of Japan, 1988) discloses a perovskite structure suitable for use as an oxygen transport membrane and discusses the effect of cation substitution on the oxygen permeability. One disclosed composition is La
0.6
Sr
0.4
Co
0.8
B′
0.2
O
3
where B′ is selected from the group consisting of manganese, iron, nickel, copper, cobalt and chromium.
In another field of endeavor, perovskites have been found to have superconductivity,
Anderson Harlan U.
Chen Chieh-Cheng
Kaus Ingeborg
Sprenkle Vincent
Bell Bruce F.
Praxair Technology Inc.
Rosenblum David M.
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