Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-06-07
2002-04-09
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S011000
Reexamination Certificate
active
06368383
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of separating oxygen from an oxygen containing gas with the use of composite ceramic membranes. More particularly, the present invention relates to such a method in which the composite ceramic membrane has multiple layers comprising a dense layer, one or more active porous layers, and one or more porous support layers. Even more particularly, the present invention relates to such a method in which the pore radii, the distribution of pore radii, and thickness of the active porous layer are selected for optimum transport through the membrane.
BACKGROUND OF THE INVENTION
The development of ceramic materials that have high oxygen ion conductivity at elevated temperatures of ranging from between about 600° C., and about 1100° C. has created interest in employing membranes fabricated from these materials in separating oxygen from an oxygen containing gas stream. If the material or membrane also has electronic conductivity, electrons are returned within the membrane and the driving force for ion transfer is provided by a positive ratio of oxygen partial pressures at the feed or cathode side and the permeate or anode side respectively. Dense film membranes require reasonably thick (1 to 2 mm) membranes for structural integrity and, since the oxygen flux is inversely proportional to the membrane thickness, such membranes achieve only moderate oxygen flux levels. Providing a thin active membrane layer supported by a porous matrix, as in a composite membrane, minimizes the resistance of the dense layer. However, other types of resistance such as oxygen surface exchange and chemical potential drop in the support matrix become predominant limitations in such composite ceramic membranes. Composite ceramic membranes, in which one or several porous layers support a thin dense membrane film and in which the limitations mentioned above are addressed, provide significant opportunities for increasing the oxygen flux across the membrane over the levels that are able to be achieved by unsupported dense membranes.
The prior art suggests that there exists a dependency between oxygen flux and the pore size of the pores within the porous layers. For instance, T. Kenjo et al., “LaMnO
3
Air Cathodes Containing ZrO
2
Electrolyte for High Temperature Oxide Fuel Cells”, Solid State Ionics. 57, 1992, pgs. 259-302 discloses that enhancement of oxygen flux across a thin membrane requires an active electron and ion conducting layer adjacent to the dense electrolyte and suggests that flux may be enhanced by decreasing pore diameter. With respect to porous layer thickness it is further shown that there is a limiting thickness beyond which no further enhancement is realized. Similarly, Deng et al., “Transport in solid oxide porous electrodes: Effect of gas diffusion”, Solid State Ionics 80, 1995, pgs. 213-222, recognizes an optimum thickness of a porous layer for a maximum level of enhancement of oxygen flux and also, that the location of the optimum varies with the surface area of the pores per unit volume of the layer. U.S. Pat. No. 5,240,480 describes a composite membrane in which a plurality of porous layers support a dense layer. The pore radii of all of the porous layers are less than 10 microns and the pore radii of successive porous layers increase in a direction taken from the dense layer. The porous layer contiguous to the dense layer possesses both ionic and electronic conductivity and the pore radius of the active layer is sufficiently small so as to achieve a significant enhancement of gas phase to electrolyte surface exchange.
None of the prior art discusses the fact that real porous surfaces have not a single pore radius but a distribution of pore radii and that this distribution affects the level (relative to an optimum flux) that can be attained for a given thickness of the porous layer. As will be discussed, the inventors have found that transport through a composite membrane may be optimized beyond that thought possible in the prior art through a relationship of the thickness of the active porous layers to the pore radius and the definition of a permissible range in the standard deviation for the pore radius distribution from the optimum radius.
SUMMARY OF THE INVENTION
The present invention provides a method of separating oxygen from an oxygen containing gas with a composite membrane capable of conducting oxygen ions and electrons. In accordance with the method, the composite membrane is subjected to an operational temperature and the oxygen containing gas with a higher oxygen partial pressure at a cathode side thereof and a lower partial oxygen pressure established at an anode side thereof. It is to be noted that the operational temperature of such a membrane is well known and as indicated above, is typically within a range of between about 600° C. and about 1100° C. Lower and higher operational temperatures have been considered within the prior art. The oxygen containing gas can be air or a gas having oxygen in a bound state, for instance water. Further, the oxygen so separated from the oxygen containing gas can be recovered or further reacted to produce a product at the other side of the membrane, known as the anode side.
The composite membrane has a dense layer, at least one active porous layer contiguous to the dense layer, and at least one porous support layer. The term “active” as used in connection with a porous layer means a layer that is comprised of materials that have both oxygen ion and electron conducting conductivity and include both mixed conducting metallic oxides and multi phase mixtures of oxygen ion conducting metallic oxides and electron conducting oxides and/or electron conducting metals.
The active porous layer has a thickness and a distribution of pore radii. To obtain an oxygen flux at more than 80 percent of the maximum oxygen flux for that layer thickness, the standard deviation of the distribution of the log of the pore radii is equal to a product of 1.45 and a shape factor, the shape factor being greater than 0 and no greater than about 0.5. The thickness is about equal to the product of a constant and the area weighted average of the pore radii. The constant is a function of a material used to fabricate the active porous layer, the operational temperature, an oxygen partial pressure within the active porous layer, and a porosity and a tortuosity produced by the arrangement of pores in the porous layer.
Specifically, the constant for determining the optimum thickness in meters is determined from the relationship:
T
94
,
695
(
ⅇ
E
σ
-
E
k
16.63
(
1
T
-
1
1273
)
)
σ
ion
(
1
-
ϵ
)
k
P
O
2
0.5
ϵ
τ
where:
&sgr;
ion
is an ionic conductivity of the active porous layer [Ohm
−1
/m]
T is the operational temperature [K]
k is a surface exchange factor in the porous layer at 1273 K [mol O
2
/m
2
s bar
n
]
PO
2
is an oxygen partial pressure at said active porous layer [bar]
&egr; is a porosity of the active porous layer
E
&sgr;
is the activation energy for ion conductivity [J/mol]
E
k
is the activation energy for surface exchange [J/mol]
&tgr; is the tortuosity of the active porous layer.
Where thickness and radius are related by this constant, a maximum oxygen transport or flux can be effected through such a porous layer for a given set of operating conditions. As stated previously, when a porous membrane is fabricated, the pore size varies. The inventors have found, quite unexpectedly, that it is important to construct the membrane with a very specific pore size distribution to obtain an optimum membrane in terms of oxygen transport through the membrane.
The permissible standard deviation of the logarithm to the base 10 of the pore radii that yields more than 80 percent, and preferably more than 90 percent, of the maximum flux is the product of 1.45 times a shape factor that for 80 percent is greater than 0 and no greater than 0.5, and prefera
Gottzmann Christian Friedrich
Prasad Ravi
Van Hassel Bart Antonie
Virkar Anil V.
Praxair Technology Inc.
Rosenblum David M.
Spitzer Robert H.
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