Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-11-28
2004-09-21
Fortuna, Ana (Department: 1723)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S046000, C210S500250, C210S510100, C210S502100, C429S103000, C585S818000, C204S400000
Reexamination Certificate
active
06793711
ABSTRACT:
BACKGROUND OF THE INVENTION
Molten salt electrolytes are employed for ion transport in fuel cells for the generation of electricity. See, for example, U.S. Pat. Nos. 4,480,017, 4,410,607 and 4,079,171. Molten carbonate electrolytes have been used extensively in fuel cell applications. See, for example, U.S. Pat. Nos. 5,354,627 and 5,989,740. Molten salt fuel cells can be supplied with reformed fuel gas from an external reformer system. Alternatively, molten salt fuel cells can incorporate an internal reforming catalyst, i.e., a steam reforming catalyst, to produce hydrogen-containing gas (e.g. synthesis gasp for use at the gas electrode side of the fuel cell for generation of electricity. See, for example, U.S. Pat. Nos. 5,075,277; 5,380,600; 5,622,790; and 6,090,312 for internal reforming fuel cells.
Molten salt electrolytes have also been employed in electrochemical cells for gas separation, e.g., for the separation of oxygen, via transport of oxygen-containing anions through the molten salt. See, U.S. Pat. No. 4,859,296. More specifically, molten nitrate salt electrolyte has been employed in an electrochemical cell for oxygen separation via transport of nitrate ion (U.S. Pat. No. 4,738,760).
Molten chloride salt electrolytes (lithium chloride and potassium chloride) have been employed in electrochemical cells for the recovery of chlorine from hydrogen chloride gas. See, Yoshizawa, S. Et al. (1971) J. Appl. Electrochem. 245-251. U.S. Pat. Nos. 5,618,405 and 5,928,489 report the removal and recovery of hydrogen halides from gas mixtures using molten halide salt electrolytes.
In electrochemical and fuel cells employing molten salt electrolytes, a porous electrolyte plate (or tile) is made from a porous non-conducting matrix impregnated with the molten salt and positioned between an anode and a cathode. The porous matrix is typically made of a refractory, non-electron-conducting, inorganic material, such as lithium aluminate or lithium titanate. The electrolyte plate conducts or mediates ions between the anode and the cathode via the molten salt. The molten salt is selected for transport or mediation of a desired ion, e.g., a carbonate salt is used for mediation of a carbonate anion or a chloride salt is used for transport of chloride ion. The molten salt electrolyte plate does not conduct electrons. The anode and cathode of the electrochemical or fuel cells are electrically connected through an external circuit for electron transport.
Catalytic membrane reactors using gas-impermeable solid state membranes for the oxidation or decomposition of various chemical species have been extensively studied. 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 and liquid fuel oxygenates.
Catalytic membrane reactors can also be employed for steam reforming of hydrocarbons. Steam reforming involves the following reactions illustrated with methane as the hydrocarbon:
CH
4
+H
2
O—→CO+2H
2
CH
4
+2H
2
O—→CO
2
+4H
2
CO+H
2
O—→CO
2
+H
2
(Water gas shift reaction).
U.S. Pat. No. 5,229,102 reports the production of CO
2
, and H
2
by steam reforming of a hydrocarbon in a catalytic ceramic membrane reformer. In the membrane reactor, H
2
and CO
2
are generated by passing hydrocarbon and steam into the reactor zone in contact with a steam reforming catalyst, e.g., Ni metal promoted with alkali metal. Hydrogen is removed by permeation (or diffusion) through the membrane increasing the efficiency of the reaction.
Catalytic membrane reactors employing gas-impermeable, ion conducting membranes can, for example, be used for oxidation/reduction reactions. For example, oxygen or an oxygen-containing species (such as NOx or SOx) can be reduced at the reduction surface of a catalytic membrane to oxygen-containing anions which are transported across the membrane to an oxidation surface where they react to oxidize a selected reduced species. Materials used in the membranes in such a reactor conduct oxygen-containing anions. Provision must be made in such reactors for electron conduction to maintain charge neutrality permitting anion conduction through the membrane. Electron conduction has been achieved by the use of external circuits for current flow (U.S. Pat. No. 4,793,004). Electron conductivity has also been achieved by doping oxygen-anion conducting ceramic materials with metal ions to generate a material that conducts electrons and oxygen anions. See, U.S. Pat. Nos. 4,791,079 and 4,827,071.
Alternatively, mixed-conducting composite materials can also be made by mixing an oxygen aniononducting material and an electronically-conducting material to form a multiple phase material that conducts both electrons and anions. A preferred method for obtaining mixed- (or dual-) conducting catalytic membranes is to use a membrane material that inherently conducts both electrons and ions. For example, a number of mixed metal oxide materials can be formed into gas impermeable mixed conducting membranes. See, for example, U.S. Pat. No. 6,033,632 and references cited therein.
The present invention relates to gas-impermeable mixed conducting membranes for use in a variety of catalytic membrane reactions and gas separation applications, which are formed by impregnating a porous electron-conducting matrix with a molten salt electrolyte. Ions mediated through these membranes facilitate gas separation and/or provide reactive species for the generation of desired value-added products in catalytic membrane reactors.
SUMMARY OF THE INVENTION
This invention relates to mixed-conducting membranes, i.e., membranes that conduct both ions and electrons, which behave as short-circuited electrochemical cells. The membrane comprises a porous electron-conducting matrix and a molten salt that conducts ions. The electron-conducting matrix is at least in part impregnated with molten salt to provide for ion transport through the membrane. The membrane comprises two external surfaces for contact, respectively, with reagent gas and reactant gas. Ions are transported from one external surface to the other external surface of the membrane. One or both of the external surfaces of the membrane can be catalytic. The external surfaces can be provided with adherent catalyst layers, or a three-dimensional catalyst can be provided in close proximity to one or both of the external membrane surfaces. Ions to be transported are formed at or near one external surface of the membrane in contact with reagent gas, transported through the membrane and released at the other external surface where they may react with reactant gas in contact with that surface.
In specific embodiments, one external surface of the membrane is an oxidizing surface and the other external surface is a reducing surface. The reducing surface is typically contacted with an oxidized or oxygen-containing gas. The oxidizing surface is typically contacted with a reduced gas.
Of particular interest are membranes in which the molten salt conducts certain oxide ions, particularly carbonate ion (CO
3
2−
). The membranes of this invention are useful generally for production of value-added products using reactive ions that are mediated through the membrane as reagents to convert lower-value starting materials (e.g., hydrocarbons). The membranes of this invention are also useful for the separation of gases and are of particular use for the separation of carbon dioxide from gas mixtures.
More specifically, the membranes of this invention are useful for the generation of carbonate ion from carbon dioxide- and oxygen-containing gas mixtures. The carbonate ion generated and mediated through the membrane is, in turn, useful as a reagent ion for reaction with reactant gases to produce desired products and particularly for partial oxidation of redu
Eltron Research, Inc.
Fortuna Ana
Greenlee Winner and Sullivan P.C.
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