Membrane and use thereof

Details Membrane and use thereof Membrane and use thereof

2002-06-27

2004-09-07

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C096S004000, C096S011000

Reexamination Certificate

active

06786952

ABSTRACT:

The present invention relates to a solid multicomponent membrane which is particularly suited as dense oxygen separation membrane in applications with high driving forces for oxygen transport.
Inorganic membranes show promise for use in commercial processes for separating oxygen from an oxygen containing gaseous mixture. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle plants. This technology encompasses two different kinds of membrane materials; oxygen ion conductors and mixed oxygen ion and electronic conductors. In both cases the oxygen ion transport is by oxygen ion vacancies or interstitial oxygen in the membrane material. In the case of mixed conductors electrons are also transported in the membrane material.
Membranes formed from mixed conducting oxides can be used to selectively separate oxygen from an oxygen containing gaseous mixture at elevated temperatures. Oxygen transport occurs when a difference in the chemical potential of oxygen (&Dgr;logp
O2
) exists across the membrane. On the high oxygen partial pressure side of the membrane, molecular oxygen dissociates into oxygen ions which migrate to the low oxygen partial pressure side of the membrane and recombine there to form oxygen molecules. Electrons migrate through the membrane in the opposite direction to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by three processes; (I) the rate of oxygen exchange at the high oxygen partial pressure surface of the membrane, (II) the oxygen diffusion rate within the membrane and (III) the rate of oxygen exchange on the low oxygen partial pressure surface of the membrane. If the rate of oxygen permeation is controlled by the oxygen diffusion rate, the oxygen permeability is known to be inversely proportional to the membrane thickness (Fick's law). If the membrane thickness is decreased below a certain critical membrane thickness which depends on temperature and other process parameters, surface oxygen exchange on one or both membrane surfaces will become oxygen permeation rate limiting. The rate of oxygen permeation is then independent of the membrane thickness.
During recent years the use of dense mixed conducting membranes in various processes has been described. Examples are oxygen production described in European Patent Application No. 95100243.5 (EP-A-663230), U.S. Pat. No. 5,240,480, U.S. Pat. No. 5,447,555, U.S. Pat. No. 5,516,359 and U.S. Pat. No. 5,108,465, partial oxidation of hydrocarbons described in U.S. Pat. No. 5,714,091 and European Patent Application No. 90134083.8 (EP-A438902), production of synthesis gas described in U.S. Pat. No. 5,356,728 and enrichment of a sweep gas for fossil energy conversion with economical CO
2
abatement as described in PCT/NO97/00170, PCT/NO97/00171 and PCT/NO97/00172.
For the application of MCM (Mixed Conducting Membrane) technology, the membrane material must fulfil certain requirements in addition to being a good mixed conductor. These fall into three categories; thermodynamic stability under static conditions, thermodynamic stability under dynamic conditions, and mechanical stability. The membrane material must be thermodynamically stable under any static condition within the appropriate temperature and oxygen partial pressure range. Furthermore, the membrane material must be stable against reaction with the additional components in the gaseous phase (e.g. CO
2
, H
2
O, NO
x
, SO
x
), and any solid phase in contact with it (e.g., seals and support material). This calls for different materials for different applications.
A membrane material that fulfils all the stability requirements under static conditions, may still be unstable when it is placed in a potential gradient. Any multi-component material kept in a potential gradient, e.g. oxygen partial pressure gradient or electrical potential gradient, will be subjected to driving forces acting to demix or decompose the material. These phenomena are called kinetic demixing and kinetic decomposition and are well described in the literature (e.g., Schmalzried, H. and Laqua, W., Oxidation of Metals 15 (1981) 339).
Kinetic demixing acts to gradually change the cationic composition of the membrane along the axis parallel to the applied potential. This phenomenon will always occur in materials where a mixture of cations are present on the same sublattice. Kinetic demixing may or may not reduce the performance and lifetime of the membrane.
Kinetic decomposition implies a total breakdown of the compound or compounds comprising the membrane, and results in the appearance of decomposition compounds on the membrane surface. This phenomenon occurs in all multicomponent materials when placed in a potential gradient exceeding a certain critical magnitude. A membrane kept in an oxygen partial pressure gradient large enough for kinetic decomposition to take place, will have its performance and lifetime reduced. Those skilled in the art recognize the phenomenon of kinetic decomposition as one of the major critical parameters in developing durable membranes, particularly for processes involving large potential gradients across the membrane.
Furthermore, when the membrane is placed in an oxygen chemical potential gradient and it responds by establishing a gradient in the concentration of oxygen vacancies or interstitials parallel to the direction of the applied potential, the membrane experiences mechanical stress with the strain plane perpendicular to the direction of the applied potential gradient. This mechanical stress is caused by a phenomenon referred to as chemical expansion, which can be defined as the dependency of the unit cell volume of the nonstoichiometric oxide on the oxygen stoichiometry. When the chemical expansion exceeds a critical limit, and gives rise to mechanical stress exceeding a critical limit governed by the membrane package design, a mechanical failure of the membrane package may result Those skilled in the art recognize the phenomenon of chemical expansion as one of the major critical parameters in developing durable membrane packages.
Two prior art processes can be put forward as particularly relevant to the present invention: the production of synthesis gas in which an oxygen containing gas is fed to the first side of a membrane, whereby pure oxygen is transported through the membrane, and the so produced oxygen partially oxidizes a hydrocarbon containing gas supplied to the second side of the membrane; and fossil energy conversion with economical CO
2
abatement (e.g. PCT/NO97/00172) where an oxygen containing gas is fed to the first side of a membrane, whereby pure oxygen is transported through the membrane, and the produced oxygen oxidizes a hydrocarbon containing gas supplied to the second side of the membrane.
The process conditions of the relevant process define the environs of the membrane and play a determining role in the selection of membrane material. Examples of typical process parameters for the two said processes are given in Tables 1 and 2, respectively. Both processes are characterized by a logp
O2
gradient across the membrane of well above 10 decades. Furthermore, both processes call for membrane materials that have a high stability against reaction with CO
2
under reducing conditions, as the CO
2
pressure is well above 1 bar.
TABLE 1
Example of process parameters for an MCM syngas
production process
Fuel side
Air side
Temperature
750-950° C.
750-950° C.
Total pressure
30 bar
1.5 bar
p
O2
10
−17
bar
0.03-0.23 bar
p
CO2
3-5 bar
0.04-0.05 bar
Other major components
H
2
, CO, H
2
O, CH
4
N
2
TABLE 2
Example of process parameters for an MCM power
production process
Fuel side
Air side
Temperature
1100-1200° C.
1100-1200° C.
Total pressure
12-32
bar
10-30
bar
p
O2
appr. 10
−12
bar
0.5-5
bar
p
CO2
0-12
bar
<2
bar
Other major components
H
2
O, CH
4
N
2
During recent years dense mixed conducting membranes have been described.
U.S. Pat. No. 5,306,411 discloses a solid, gas-impervious, electron-conductive, oxyge

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