Catalyst – solid sorbent – or support therefor: product or process – In form of a membrane
Reexamination Certificate
1999-10-05
2001-04-10
Barry, Chester T. (Department: 1724)
Catalyst, solid sorbent, or support therefor: product or process
In form of a membrane
C502S302000, C502S303000, C502S304000, C502S305000, C502S306000, C502S308000, C502S309000, C502S311000, C502S325000, C502S326000, C502S328000, C095S056000, C585S520000, C210S763000, C210S510100, C210S500250
Reexamination Certificate
active
06214757
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas-impermeable, solid state materials fabricated into membranes for use in catalytic membrane reactors and more particularly to solid state oxygen anion- and electron-mediating membranes for use in catalytic membrane reactors for promoting partial or full oxidation of different chemical species, for decomposition of oxygen-containing species and for separation of oxygen from other gases. Solid state materials for use in the membranes of this invention include mixed metal oxide compounds having the brownmillerite crystal structure. Catalytic membrane reactions include, among others, the partial oxidation of methane or natural gas to synthesis gas.
BACKGROUND OF THE INVENTION
Catalytic membrane reactors using solid state membranes for the oxidation or decomposition of various chemical species have been studied and used previously. One potentially valuable use of such reactors is in the production of synthesis gas. See, for example, Cable et al. EP patent application 90305684.4 (published Nov. 28, 1990) and Mazanec et al. U.S. Pat. No. 5,306,411.
Synthesis gas, a mixture of CO and H
2
, is widely used as a feedstock in the chemical industry for production of bulk chemicals such as methanol, liquid fuel oxygenates and gasoline. Synthesis gas is currently produced from natural gas, i.e. methane, or other light hydrocarbons by steam reforming. In this technique, natural gas is mixed with steam and heated to high temperatures, and the heated mixture is passed over a catalyst, such as Ni on Al
2
O
3
, to form synthesis gas which is then collected. Steam reforming has two major disadvantages. First, the chemical reaction to produce CO and H
2
from steam (H
2
O) and natural gas (CH
4
) is endothermic, i.e. the reaction requires energy. Roughly one third of the natural gas consumed in the steam reforming process goes to produce heat to drive the reaction, rather than to produce CO and H
2
. Second, the ratio of H
2
,:CO in the synthesis gas produced by steam reforming is typically relatively high, from 3:1 up to about 5:1. For most efficient use in the synthesis of methanol, the ratio of H
2
:CO in synthesis gas should be adjusted to 2:1. Adjusting this ratio adds to the cost and complexity of the processing.
In contrast, the use of a catalytic reactor membrane for production of synthesis gas by partial oxidation of natural gas to CO and H
2
overcomes the disadvantages of steam reforming. First, the reaction to produce synthesis gas mediated by the catalytic membrane reactor (CH
4
+½O
2
→CO+2H
2
) is exothermic, i.e., the reaction gives off heat. The heat produced can then be beneficially used in a cogeneration facility. Second, the synthesis gas produced using a catalytic membrane reactor should have an H
2
:CO ratio of about 2:1. Additional processing steps are eliminated and all the natural gas consumed can be used to produce synthesis gas.
In a catalytic membrane reactor that facilitates oxidation/reduction reactions, a catalytic membrane separates an oxygen-containing gas from a reactant gas which is to be oxidized. Oxygen (O
2
) or other oxygen-containing species (for example, NO
x
or SO
x
) are reduced at one face of the membrane to oxygen anions that are then transported across the membrane to its other face in contact with the reactant gas. The reactant gas, for example methane, is oxidized, for example CH
4
to CO, by the oxygen anions with generation of electrons at the oxidation surface of the membrane.
Materials for membranes in catalytic membrane reactors must be conductors of oxygen anions, and the materials must be chemically and mechanically stable at the high operating temperatures and under the harsh conditions required for reactor operation. In addition, provision must be made in the reactor for electronic conduction to maintain membrane charge neutrality. Membrane materials of most interest are electron conductors, i.e., they conduct electrons.
Oxygen anion conductivity in a material can result from the presence of oxygen anion defects. Defects are deviations from the ideal composition of a specific compound or deviations of atoms from their ideal positions. Of interest for this invention are defects due to loss of oxygen from a compound leading to empty oxygen sites, i.e. oxygen vacancies, in the crystal lattice. A mechanism of oxygen anion conduction is “jumping” of the oxygen anions from site to site. Oxygen vacancies in a material facilitate this “jumping” and thus, facilitate oxygen anion conduction. Oxygen anion defects can be inherent in the structure of a given material of a given stoichiometry and crystal structure or created in a membrane material through reactions between the membrane material and the gas to which it is exposed under the conditions of operation of the catalytic membrane reactor. In a given system with a given membrane material, both inherent and induced defects can occur.
Materials with inherent oxygen anion vacancies are generally preferred. Loss of oxygen from a membrane material by reaction to create vacancies typically has a large effect on the structure of the material. As oxygen is lost, the size of the crystal lattice increases on a microscopic level. These microscopic changes can lead to macroscopic size changes. Because membrane materials are hard, size increases lead to cracking making the membrane mechanically unstable and unusable.
Electronic conductivity in a reactor is necessary to maintain charge neutrality permitting anion conduction through the membrane. It can be achieved by adding an external circuit to a reactor which allows for current flow. U.S. Pat. Nos. 4,793,904, 4,802,958 and 4,933,054 (all of Mazanec et al.) relate to membrane reactors where electronic conductivity is provided by an external circuit. In these patents, the membrane materials, which arc compounds with general stoichiometry AO
2
, with fluorite structures, such as yttria-stabilized zirconia, exhibit oxygen-anion conductivity.
Electronic conductivity can also be achieved by doping oxygen-anion conducting materials with a metal ion, as illustrated by U.S. Pat. Nos. 4,791,079 and 4,827,071 (both of Hasbun), to generate dual (electrons and oxygen anions) conducting materials. The Hasbun membranes are composed of fluorites doped with transition metals, including titania- and ceria-doped yttria-stabilized zirconia. The disadvantage of this approach is that the dopant metal ions can act as traps for migrating oxygen anions, inhibiting the ionic conductivity of the membrane.
The preferred method for obtaining electronic conductivity is to use membrane materials which inherently possess this property. Dual conducting mixtures can be prepared by mixing an oxygen-conducting material with an electronically-conducting material to form a composite, multi-component, non-single phase material. Problems associated with this method include possible deterioration of conductivity due to reactivity between the different components of the mixture and possible mechanical instability, if the components have different thermal expansion properties.
Cable et al., in European patent application No. 90305684.4 and the corresponding U.S. Pat. No. 5,306,411 of Mazanec at al. report multi-component solid membranes for oxidation/reduction reactions including the production of synthesis gas. The specific multi-phase components are mixtures of an oxygen-conducting material and an electronically conductive material. The oxygen-anion conducting material of the mixture is described as a perovskite ABO
3,
including those materials where A and B represent a mixture of more than one metal ion, for example La
a
Sr
b
O
3
, La
a
Sr
b
Fe
b
O
3
, La
a
Ca
b
CoO
3
, SrCo
a
Fe
b
O
3
, and Gd
a
Sr
b
CoO
3
, where a and b are numbers and a +b=1. The electronically-conducting material of the mixture is one or more of a variety of metals, metal oxides, metal-doped metal oxides and including mixed metal oxides of a perovskite structure, for example, YBa
2
Cu
3
O
x
where x is a number from 6-7. Exemplified multi-co
Sammells Anthony F.
Schwartz Michael
White James H.
Barry Chester T.
Eltron Research, Inc.
Greenlee Winner and Sullivan P.C.
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