Membrane and use thereof

Details Membrane and use thereof Membrane and use thereof

2001-01-08

2003-01-07

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C096S004000

Reexamination Certificate

active

06503296

ABSTRACT:

This invention relates to a dense single phase membrane having both high ionic and electronic conductivity and capable of separating oxygen from an oxygen containing gaseous mixture and further use of the membrane.
Inorganic membranes are beginning to 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, solid electrolytes that are mixed conductors and ionic conductors. In both cases the transport is by anionic vacancies or interstitial defects in the electrolyte. In the case of pure ionic conductors, electrons have to be transported in an external circuit, while in the case of mixed conductors no external circuit is necessary as electrons are transported in the membrane material. The driving force for transport is in the mixed conductor case supplied by a difference in partial pressure of oxygen between the two sides of the membrane, while in the pure ionic case in addition an external electrical potential can be supplied.
Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen containing gaseous mixture when a difference in oxygen partial pressure exists across the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions which migrate to the low pressure side of the membrane and recombine 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 two factors, the diffusion rate within the membrane and the rate of interfacial oxygen exchange. Diffusion controlled oxygen permeability is known to increase proportionally with decreasing membrane thickness at high temperature (Fick's law). With decreasing thickness the surface exchange control becomes more important.
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-A-438902), production of synthesis gas described in U.S. Pat. No. 5,356,728 and enrichment of a sweep gas for fossile energy conversion with economical CO2 abatement described in the none published international patent Application Nos.: PCT/NO97/00170, PCT/NO97/00171 and PCT/NO97/00172 (Norsk Hydro ASA).
For the application of MCM (Mixed Conducting Membrane) technology, the membrane material must fulfill certain requirements in addition to being a good mixed conductor. These fall into the two categories of thermodynamic and mechanical stability. The membrane material must be thermodynamically stable over the appropriate temperature and oxygen partial pressure range. Furthermore, the membrane material must be stable towards the additional components in the gaseous phase, and towards any solid phase in contact with it (e.g. support material). This calls for different materials for different applications.
Previous reports on oxygen permeable membranes have dealt with perovskite related materials based on the general formula ABO
3−&dgr;
where A and B represent metal ions. &dgr; has a value between 0 and 1 indicating the concentration of oxygen vacancies. In the idealised form of the perovskite structure it is required that the sum of the valences of A ions and B ions equals 6. Materials known as “perovskites” are a class of materials which has an X-ray identifiable crystalline structure based upon the structure of the mineral perovskite, CaTiO
3
. Perovskite type oxides ABO
3−&dgr;
containing dopants on the A and B-site are promising materials for oxygen-permeable membranes. In such materials the oxygen ions are transported through the membrane via oxygen vacancies. Usually the large A-site cation is a trivalent rare earth, while the smaller B-cation is a transition metal (e.g. LaCoO
3−&dgr;
). The trivalent rare earth A-site cation is usually partially substituted by divalent alkaline earth (e.g. Sr), to increase the vacancy concentration, &dgr;/3, on the oxygen sub lattice. A similar increase in &dgr; can be accomplished by partial substitution of the B-site cation by a divalent cation (e.g. Zn, Mg), or more commonly by another mixed-valent transition metal (e.g. Fe, Ni, Cu). One of the first reported examples of such a material is La
0.8
Sr
0.2
Co
0.8
Fe
0.2
O
3−&dgr;
(Teraoka et al., Chem. Lett. (1985) 1743-1746). European patent application no. 95100306.0 (EP-A-663232) and U.S. Pat. No. 5,712,220 describe compositions of this type for oxygen separation.
When A is divalent and B is trivalent &dgr; will be close to 0.5. A number of these compounds adopt the brown millerite structure where the oxygen vacancies are ordered in layers. Compositions of this type are described in U.S. Pat. No. 5,714,091 and International patent application no.: PCT-US96/14841 for use as membranes in the partial oxidation reactors.
When separating oxygen from an oxygen containing gaseous mixture the membrane is a conductor of product fluid ions and electrons. When no direct oxidation process takes place on the product site of the membrane there is a relatively small difference in partial pressure of oxygen across the membrane, and accordingly the driving force is small. For such applications it is beneficial to use a membrane material where the defects are interstitial oxygen excess, with most of the stoichiometry change in the oxygen partial pressure range in question, rather than oxygen vacancies as in the perovskites. This will ensure a maximum of gradient in oxygen concentration in the material at small oxygen partial pressure gradients. The activation energy for transport of oxygen ions will most often be lower in the case of interstitials than in the case of vacancies.
The main object of the invention was to arrive at a membrane capable of separating oxygen from an oxygen containing gaseous mixture.
Another object of the invention was to arrive at a membrane comprising a material thermodynamically stable over the appropriate temperature and oxygen partial pressure range.
Furthermore, an object of the invention was to arrive at a membrane comprising a material possessing structures that can accommodate interstitial oxygen excess.
Furthermore, another object of the invention was to arrive at a membrane comprising a material showing very low chemical expansion.
Still another object of the invention was to arrive at a membrane stable towards the additional components in the gaseous phase.
Still another object of the invention was to arrive at a membrane stable towards any solid phase in contact with the membrane.
The inventors found that a dense single-phase membrane comprising of a mixed metal oxide material with interstitial oxygen excess represented by the formula:
A
y
A′
y′
A″
y″
B
x
B′
x′
B″
x″
B′″
x′″
O
4+&dgr;
where A, A′ and A″ are chosen from group 1, 2 and 3 and the lanthanides; and B, B′, B″ and B′″ are chosen from the transition metals according to the periodic table of the elements adopted by IUPAC wherein 0≦y≦2, 0≦y′≦2, 0≦y″≦2, 0≦x≦1, 0≦x′≦1, 0≦x″≦1, 0≦x′″≦1 and x and y each represents a number such that y+y′+y″=2, x+x′+x″+x′″=1 and &dgr; is a number where 0≦&dgr;<1 quantifying the oxygen excess has both high ionic and electronic conductivity and is capable of separating oxygen from an oxygen cont

Affiliated with

Also associated with

Rating

Membrane and use thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Membrane and use thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Membrane and use thereof will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3061534