Oxygen production process

Details Oxygen production process Oxygen production process

1995-06-07

2002-12-03

Gorgos, Kathryn (Department: 1741)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C423S579000, C204S295000, C096S006000, C096S011000

Reexamination Certificate

active

06488739

ABSTRACT:

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 production of oxygen. The membranes are conductors of oxygen ions and electrons, and are substantially stable in air over the temperature range of 25° C. 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 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 La
1−x
Sr
x
Co
1−y
Fe
y
O
3−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 Assignee of record herein. The subject matter of that application is incorporated herein by reference.
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, C
1
-C
5
saturated and unsaturated hydrocarbons, ammonia, and the like.
U.S. Pat. No. 5,240,480 to Thorogood, et al, incorporated herein by reference, 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. Pat. No. 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. Pat. Nos. 4,791,079 and 4,827,07 to Hazbun, incorporated herein by reference, 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.
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 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
−1
cm
−1
to 100 ohm
−1
cm
−1
and an electronic conductivity ranging from about 1 ohm
−1
cm
−1
to 100 ohm
−1
cm
−1
under operating conditions.
Some multicomponent metallic oxides are primarily or solely oxygen ionic conductors at elevated temperatures. An example is (Y
2
O
3
)
0.1
(Zr
2
O
3
)
0.9
which has an oxygen ionic conductivity of about 0.06 ohm
−1
cm
−1
at 1000° C. 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.
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 Pr
x
In
y
O
z
which is described in European Patent Application EP 0,399,833 A1. Such materials may be used in a composite membrane with a separate oxygen ionic conducting phase such as a stabilized ZrO
2
. 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 su

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