Solid electrolyte ionic conductor oxygen production with power generation

A process for producing an oxygen-depleted gas stream and a high-pressure gas stream containing oxygen and steam by compressing a feed gas stream containing elemental oxygen, heating the feed gas stream, and separating the heated feed gas stream using one or more ion transport modules into the oxygen-depleted gas stream on a retentate side and an oxygen gas stream on a permeate side of an ion transport membrane. The permeate side is purged using a high-pressure purge gas stream containing steam to produce the high-pressure gas stream containing oxygen and steam, which is directed to a turbine to recover power and produce an expanded, lower-pressure gas stream containing oxygen and steam.

FIELD OF THE INVENTION 
The invention relates to the use of solid electrolyte ionic conductor 
systems in gas separating systems and, in particular, to employing steam 
from an integrated Rankine cycle to purge the permeate side of solid 
electrolyte ionic conducting membranes to enhance the efficiency of the 
process and produce an oxygen and steam gas stream which can be readily 
separated to obtain a pure oxygen product while simultaneously generating 
power. 
BACKGROUND OF THE INVENTION 
The well-known Rankine vapor power cycle or its modifications (for example, 
reheat and regenerative cycles, dual pressure cycle, and cogeneration 
cycles) are currently used to produce electrical power. In these systems, 
steam is generally the working fluid of choice because of its easy 
availability, chemical stability, and relatively low cost. During the 
cycle, heat is added to the system to generate steam at high pressure 
which in turn is expanded through a turbine to generate power. 
Gas turbine power cycles are analogous to vapor power cycles in that the 
individual processes are steady flow processes carried out in separate 
components. The working fluid in a gas turbine power cycle, however, is 
generally air or the products of combustion of fuel and air. Air is a 
mixture of gases which may contain varying amounts of water vapor and, at 
sea level, has the following approximate composition by volume: oxygen 
(20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of 
other trace gases. If a fuel is used in such a system, heat is generated 
within the system by the fuel being combusted in a compressed air stream, 
and the resultant combustion products gas stream is expanded through a gas 
turbine to produce power. 
The metallurgical temperature limit on the gas turbine blades necessitates 
a gas turbine operation with a very high air-to-fuel ratio. In a 
conventional gas turbine system, the nitrogen in the feed air and the 
excess oxygen present in the combustion products gas stream act as heat 
sinks and thereby lower the temperature of the combustion products gas 
stream. As a result, the exhaust gas stream from the gas turbine power 
cycle contains excess oxygen in which additional fuel could be burnt. 
These hot exhaust gases could also be used to preheat the compressed feed 
air or may be used to generate steam that can be employed in a vapor power 
cycle. The latter combined power plant is generally referred to as COGAS 
plant. 
It is also possible to recover some or most of the oxygen not used to 
support combustion from a gas turbine cycle using ion transport membrane 
technology. Most oxygen generating systems utilize cryogenic gas 
separation methods (high purity, large scale) or membrane and adsorptive 
separation techniques (90-95% purity, small-medium scale). Conventional 
non-cryogenic bulk oxygen separation systems, for example, organic polymer 
membrane systems, are typically very power intensive, and are usually 
suitable only for the production of small quantities of oxygen-enriched 
air (for example, 50% oxygen). Although some of these conventional 
processes recover a part of the power utilized in producing the product, 
they do not produce any net power. In addition, conventional oxygen 
separation processes operate at low temperatures (less than 100.degree. 
C.), and do not benefit significantly from integration with a power 
generation process. 
An entirely different type of membrane, however, can be made from certain 
inorganic oxides. These solid electrolyte membranes are made from 
inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and 
analogous oxides having a fluorite or perovskite structure. Although the 
potential for these oxide ceramic materials as gas separation membranes is 
great, there are certain problems in their use. One of the larger problems 
is that all of the known oxide ceramic materials exhibit appreciable 
oxygen ion conductivity only at elevated temperatures. They usually must 
be operated well above 700.degree. F. (370.degree. C.), generally in the 
800.degree. F. to 1850.degree. F. (425-1000.degree. C.) range. This 
limitation remains despite much research to find materials that work well 
at lower temperatures. Solid electrolyte ionic conductor technology is 
described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, 
entitled Staged Electrolyte Membrane, and U.S. Pat. No. 5,733,435, 
entitled Pressure Driven Solid Electrolyte Membrane Gas Separation Method, 
which are both hereby incorporated by reference to more fully describe the 
state of the art. The elevated temperatures of operation, however, make 
ion transport processes well suited for integration with high temperature 
processes such as vapor-based, gas-based, or combined power cycles. 
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product 
Oxygen from Turbine Power Generation, relates to a process for generating 
net power using a combustion turbine, accompanied by the recovery of 
by-product oxygen-enriched gas. Air is compressed and heated, at least a 
portion of the air is combusted and a portion of the oxygen is removed 
from the air or combustion effluent using an air separator. The oxygen 
lean combustion effluent is expanded through a turbine to produce power. 
In an alternative embodiment, the effluent from the turbine is used to 
produce steam to generate additional power. In this process, the type of 
fuel is generally limited to "clean" fuels such as natural gas, oils, or 
synthesis gas. 
Chen, U.S. Pat. No. 5,035,727, entitled Oxygen Extraction from Externally 
Fired Gas Turbines, relates to a process for recovering high purity oxygen 
from an externally fired power generating gas turbine cycle. While this 
process is similar to Hegarty (described above), Chen differs in the use 
of an externally fired gas turbine so that other types of fuels such as 
coal or biomass may be used. 
Chen et al., U.S. Pat. No. 5,174,866, entitled 
Oxygen Recovery from Turbine Exhaust Using Solid Electrolyte Membrane, and 
Chen et al., U.S. Pat. No. 5,118,395, entitled Oxygen Recovery from 
Turbine Exhaust Using Solid Electrolyte Membrane, both relate to processes 
for extracting high purity oxygen from gas turbine exhaust streams by 
passing the gas turbine exhaust over an oxygen ion conducting membrane. In 
these processes, the oxygen separator employing an oxygen ion conducting 
membrane is placed downstream of some or all stages of the gas turbine, 
instead of upstream as in earlier patents. An electrically-driven ion 
transport unit is proposed when the turbine exhaust pressure is low. The 
exhaust stream from the oxygen separator is optionally expanded through an 
additional gas turbine stage. 
Kang et al., U.S. Pat. No. 5,562,754, entitled Integrated High Temperature 
Method for Oxygen Production describes oxygen production by ion transport 
membrane where the ion transport separator is located between two 
independently controlled direct, i.e. involving combustion, or indirect 
heating units. The permeate side of the ion transport membrane may be 
swept with steam. A stream of oxygen-containing gas preferably is heated 
in a direct-fired combustor, passed through the retentate zone of the ion 
transport membrane, and then directed to a gas turbine to generate power. 
This non-permeate stream is then discarded as exhaust. 
Kang et al., U.S. Pat. No. 5,565,017, entitled High Temperature Oxygen 
Production with Steam and Power Generation, relates to a system 
integrating an ion transport membrane with a gas turbine to recover energy 
from the retentate gas stream after it is heated and steam is added. Water 
is added to the retentate gas stream from the ion transport module prior 
to the gas turbine to increase the mass flow in the turbine. This permits 
the ion transport module and the gas turbine to each operate at its 
optimum temperature. 
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature 
Method for Oxygen Production, describes compression and heating of feed 
air in a first heating step (using heat exchanger and combustor) before 
passing the heated, compressed air through an oxygen separator employing a 
mixed conducting oxide. The retentate gas stream from the ion transport 
module is heated in a second heating step before expanding it through a 
gas turbine to recover power. The hot exhaust gases from the gas turbine 
are used to produce steam that is expanded through a steam turbine to 
generate additional power. In these processes, the operating temperatures 
of the ion transport module and the gas turbine are independently 
maintained by controlling the rate of heat addition in the first and 
second heating steps. 
None of the referenced patents have addressed the integration of ion 
transport membranes into Rankine power cycles and/or contemplate purging 
the permeate side of the ion transport membrane with elevated pressure 
steam and recovering oxygen at elevated pressure as taught by co-filed 
application U.S. Ser. No. 08/972,410, entitled Solid Electrolyte Ionic 
Conductor Oxygen Production with Steam Purge, Attorney Docket No. 20214, 
by Prasad et al. The prior art has taught that ion transport membranes can 
be used to recover part of the oxygen not required for combustion from the 
compressed air stream in gas turbine cycles, however, this is accomplished 
at the expense of compressing additional feed air to replace the oxygen 
removed together with the capital costs associated with the oxygen removal 
system. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to enable the efficient recovery 
of oxygen from an air feed stream that is part of a power cycle process. 
It is a further object of the invention to enable use of a steam-purged ion 
transport module to produce a crude nitrogen stream as the retentate from 
which the residual oxygen is subsequently removed in a reactively purged 
ion transport separator to produce a high purity nitrogen co-product 
stream as desired. 
Yet another object of the invention is to enable recovery of oxygen at an 
elevated intermediate pressure using the instant process without the need 
for an oxygen compressor by providing the steam purge stream at a high 
pressure and condensing out the water while at an intermediate elevated 
pressure after expansion in a steam turbine. 
It is another object of the invention to maximize power production by a 
Rankine cycle and improve energy utilization by inserting one or more high 
pressure steam expansion stages upstream of the ion transport separator 
purge inlet. 
It is a further object of the invention to enable simplification of the 
system, while producing co-product nitrogen at elevated pressure, by 
expanding in a turbine a portion of the retentate stream proportioned to 
generate sufficient power to drive the feed air compressor. 
SUMMARY OF THE INVENTION 
The invention comprises a process for producing an oxygen-depleted gas 
stream and a high-pressure gas stream containing oxygen and steam from a 
feed gas stream containing elemental oxygen. The feed gas stream is 
compressed and heated, and the heated feed gas stream is separated using 
at least a first ion transport module including an ion transport membrane 
into the oxygen-depleted gas stream on a retentate side and an 
oxygen-containing gas stream on a permeate side of the ion transport 
membrane. The permeate side is purged using a high-pressure purge gas 
stream containing steam to produce the high-pressure gas stream containing 
oxygen and the purge gas. The exiting high-pressure permeate gas stream 
containing oxygen and steam is expanded in a turbine to recover power and 
produce a lower pressure gas stream containing oxygen and steam. 
Preferably, the expanded lower-pressure gas stream containing oxygen and 
steam is separated into an oxygen gas stream by condensing out the water 
in a water- or air-cooled condenser. 
In a preferred embodiment of the invention, the heat contained in the 
retentate stream is recovered in a recuperative heat exchanger to preheat 
the incoming air. The high-pressure purge gas stream is directed through a 
second turbine and is superheated sufficiently to avoid condensation 
during expansion in the second turbine. The high temperature retentate 
stream preferably is expanded in an additional turbine with or without 
additional heat addition. 
In another preferred embodiment the retentate stream from the steam purged 
ion transport membrane is further processed in a deoxo stage, consisting 
of a reactively purged ion transport membrane, to produce a high purity 
nitrogen co-product stream.

DETAILED DESCRIPTION OF THE INVENTION 
The essence of the invention is to install an ion transport oxygen 
separator in a steam-based or a combined-cycle power generation 
configuration, such that the permeate side of the ion transport membrane 
is purged with high pressure steam. Such a steam purge enhances oxygen 
transport across the ion transport membrane and oxygen recovery from the 
feed gas stream, which typically is air. After the permeate gas stream 
containing steam and oxygen is expanded in a steam turbine and finally 
cooled, water condenses out of the gas stream and an oxygen gas stream, 
saturated with water vapor but otherwise pure, is obtained. In another 
part of the configuration, the retentate gas stream which has been 
partially depleted of oxygen in the ion transport module, may be combusted 
or externally heated using a fuel, and expanded in a gas turbine to 
produce more power. The exhaust from the gas turbine is generally hot 
enough to be utilized to assist in the generation of steam that will be 
utilized in the steam-based power generation part of the process or can be 
used in preheating the air feed to the separator. 
This novel method produces oxygen at a very low incremental power cost, 
which is attractive compared to other methods of oxygen production. The 
gases from which oxygen needs to be separated can be made available at 
relatively high temperature (greater than 400.degree. C.), whereas current 
commercial oxygen production processes typically operate at temperatures 
below 100.degree. C. Because of this limitation, conventional oxygen 
separation methods do not gain significant efficiencies by integration 
with a power generation process. Thus, it appears that novel gas 
separation processes employing oxygen ion conductors have the promise of 
highly synergistic integration with power generation processes which can 
dramatically lower the cost of oxygen. 
The present invention enables the integration of steam purged ion transport 
membranes for the separation of oxygen from air with Rankine steam, 
Brayton gas, and combined Brayton and Rankine steam power cycles. The key 
advantages of the processes proposed here are as follows: 
The present invention uses steam at elevated pressure as a purge gas, 
thereby reducing the effective partial pressure of oxygen on the 
purge-side. This enhances the driving force across the ion transport 
membrane, and effects a higher oxygen flux and a lower membrane area 
requirement. In practice it also makes possible higher recovery of oxygen 
contained in the air and permits, if so desired, recovery of oxygen at an 
elevated pressure, or expansion of the purge stream and recovery of power. 
Alternately, the stream can be expanded to an intermediate pressure and 
oxygen recovered at that intermediate pressure. Purging at an elevated 
pressure also reduces or eliminates the pressure differential across the 
ion transport membrane and eases structural design and sealing. 
By changing the amount of steam used for purging, the amount of oxygen 
recovered can be varied. In fact, as mentioned earlier most of the oxygen 
in the feed gas can be recovered. Also the membrane area may be reduced. 
Adaptation to gas turbine cycles is easy since the operating temperatures 
of ion transport separator and gas turbine can be uncoupled and a typical 
gas turbine processes a significant amount of excess air. Therefore the 
fraction of oxygen removed from the turbine air is a small portion of the 
total flow. 
Oxygen produced in the configuration is diluted with steam, making it 
easier and safer to handle. By withdrawing the steam and oxygen gas stream 
from the steam turbine exhaust at an intermediate pressure and condensing 
the steam, it is possible to obtain oxygen gas at a higher intermediate 
pressure albeit at the expense of reduced power generation. By-product 
nitrogen can be produced by deployment of a deoxo reactor in combination 
with or without employment of externally fired heaters. The co-production 
of power can be raised and optimized by suitably integrating a high 
pressure stage steam turbine upstream of the ion transport module. 
Several embodiments incorporating an ion transport membrane into a gas 
turbine system, including retrofit of a turbine power generation system, 
are disclosed in Prasad et al., U.S. Pat. No. 5,852,925, which is a 
divisional of U.S. Ser. No. 08/490,362, now abandoned, both of which are 
incorporated herein by reference. 
The ion transport membrane employed in the oxygen separator discussed 
herein is a solid electrolyte ionic conductor. Ion transport materials 
that transport oxygen ions are deemed useful for the separation of oxygen 
from gas mixtures. Certain ion transport materials are mixed conductors, 
conducting both oxygen ions and electrons. At elevated temperatures, these 
materials contain mobile oxygen ion vacancies that provide conduction 
sites for selective transport of oxygen ions through the material. The 
transport is driven by the partial pressure ratio of oxygen across the 
membrane: oxygen ions flow from the side with high oxygen partial pressure 
to that with low oxygen partial pressure. Ionization of oxygen to oxygen 
ions takes place on the cathode or retentate side of the membrane, and the 
ions are then transported across the ion transport membrane. The oxygen 
ions deionize on the permeate side of the membrane, releasing oxygen 
molecules. For materials that exhibit only ionic conductivity, external 
electrodes are placed on the surfaces of the electrolyte and the 
electronic current is carried in an external circuit. In mixed conducting 
materials electrons are transported to the cathode internally, thus 
completing the circuit and obviating the need for external electrodes. 
Dual phase conductors, in which an oxygen-ion conductor is mixed with an 
electronic conductor, may also be used for the same applications. 
Table I is a partial list of ion transport materials of interest for oxygen 
separation. 
______________________________________ 
Material composition 
______________________________________ 
1. (La.sub.1-x Sr.sub.x)(Co.sub.1-y Fe.sub.y) O.sub.3-.delta. (0 
.ltoreq. x .ltoreq. 1, .delta. from stoichimetry) 
2. SrMnO.sub.3-.delta. 
SrMn.sub.1-x Co.sub.x O.sub.3-.delta. (0 .ltoreq. x .ltoreq. 1, 
.delta. from stoichimetry) 
Sr.sub.1-x Na.sub.x MnO.sub.3-.delta. 
3. BaFe.sub.0.5 Co.sub.0.5 YO.sub.3 
SrCeO.sub.3 
YBa.sub.2 Cu.sub.3 O.sub.7-.beta. (0 .ltoreq. .beta. .ltoreq. 1, 
.beta. from stoichimetry) 
4. La.sub.0.2 Ba.sub.0.8 Co.sub.0.8 Fe.sub.0.2 O.sub.2.6, Pr.sub.0.2 
Ba.sub.0.8 Co.sub.0.8 Fe.sub.0.2 O.sub.2.6 
5. A.sub.x A'.sub.x,A".sub.x,B.sub.y B'.sub.y B".sub.y,O.sub.3-z 
(x,x',x",y,y',y" all in 0-1 range, z 
from stoichiometry--, 
6. (a) Co-La-Bi type: 
Cobalt oxide 15-75 mole % 
Lanthanum oxide 
13-45 mole % 
Bismuth oxide 
17-50 mole % 
(b) Co-Sr-Ce type: 
Cobalt oxide 15-40 mole % 
Strontium oxide 
40-55 mole % 
Cerium oxide 15-40 mole % 
(c) Co-Sr-Bi type: 
Cobalt oxide 10-40 mole % 
Strontium oxide 
5-50 mole % 
Bismuth oxide 
35-70 mole % 
(d) Co-La-Ce type: 
Cobalt oxide 10-40 mole % 
Lanthanum oxide 
10-40 mole % 
Cerium oxide 30-70 mole % 
(e) Co-La-Sr-Bi type: 
Cobalt oxide 15-70 mole % 
Lanthanum oxide 
1-40 mole % 
Strontium oxide 
1-40 mole % 
Bismuth oxide 
25-50 mole % 
(f) Co-La-Sr-Ce type: 
Cobalt oxide 10-40 mole % 
Lanthanum oxide 
1-35 mole % 
Strontium oxide 
1-35 mole % 
Cerium oxide 0-70 mole % 
7. Bi.sub.2-x-y M'.sub.x M.sub.y O.sub.3-.delta. (0 .ltoreq. x .ltoreq. 
1, 0 .ltoreq. y .ltoreq. 1, .delta. from stoichimetry) 
where: M'=Er, Y, Tm, Yb, Tb, Lu, Nd, Sm, Dy, Sr, Hf, Th, Ta, Nb, 
Pb, Sn, In, Ca, Sr, La and mixtures thereof 
M = Mn Fe, Co, Ni, Cu and mixtures thereof 
8. BaCe.sub.1-x Gd.sub.x O.sub.3-x/2 where, 
x equals from zero to about 1. 
9. One of the materials of A.sub..delta. A'.sub.t B.sub.u B'.sub.v 
B".sub.w O.sub.x family whose 
composition is 
disclosed in U.S. Pat. No. 5,306,411 (Mazanec et al.) as follows: 
A represents a lanthanide or Y, or a mixture thereof; 
A' represents an alkaline earth metal or a mixture thereof; 
B represents Fe; 
B' represents Cr or Ti, or a mixture thereof; 
B" represents Mn, Co, V, Ni or Cu, or a mixture thereof; 
and s, t, u, v, w, and x are numbers such that: 
s/t equals from about 0.01 to about 100; 
u equals from about 0.01 to about 1; 
v equals from zero to about 1; 
w equals from zero to about 1; 
x equals a number that satisfies the valences of the A, A', B, B', 
B" 
in the formula; and 0.9 &lt; (s + t)/(u + v + w) &lt; 1.1 
10. One of the materials of La.sub.1-x Sr.sub.x Cu.sub.1-y M.sub.y 
O.sub.3-.delta. family, where: 
M represents Fe or Co; 
x equals from zero to about 1; 
y equals from zero to about 1; 
.delta. equals a number that satisfies the valences of La, Sr, Cu, 
and M 
in the formula. 
11. One of the materials of Ce.sub.1-x A.sub.x O.sub.2-.delta. family, 
where: 
A represents a lanthanide, Ru, or Y; or a mixture thereof; 
x equals from zero to about 1; 
.delta. equals a number that satisfies the valences of Ce and A in 
the 
formula. 
12. One of the materials of Sr.sub.1-x Bi.sub.x FeO.sub.3-.delta. family, 
where: 
A represents a lanthanide or Y, or a mixture thereof; 
x equals from zero to about 1; 
.delta. equals a number that satisfies the valences of Sr and B; in 
the 
formula. 
13. One of the materials of Sr.sub.x Fe.sub.y Co.sub.z O.sub.w family, 
where: 
x equals from zero to about 1; 
y equals from zero to about 1; 
z equals from zero to about 1; 
w equals a number that satisfies the valences of Sr, Fe and Co in 
the formula. 
14. Dual phase mixed conductors (electronic/ionic): 
(Pd).sub.0.5 /(YSZ).sub.0.5 
(Pt).sub.0.5 /(YSZ).sub.0.5 
(B - MgLaCrO.sub.x).sub.0.5 (YSZ).sub.0.5 
(IN.sub.90% Pt.sub.10%).sub.0.6 /(YSZ).sub.0.5 
(IN.sub.90% Pt.sub.10%).sub.0.5 /(YSZ).sub.0.5 
(IN.sub.95% Pr.sub.2.5% Zr.sub.2.5%).sub.0.5 /(YSZ).sub.0.5 
Any of the materials described in 1-13, to which a high temperature 
metallic phase (e.g., Pd, Pt, Ag, Au, Ti, Ta, W) is 
______________________________________ 
added. 
It is relatively easy to use the basic ion transport separation process to 
remove nearly all of the oxygen from the feed gas stream to produce a 
nitrogen product gas stream, particularly if the permeate side of the ion 
transport membrane can be purged with an oxygen-free stream. It is, 
however, more difficult to efficiently recover oxygen as the product using 
this basic process. For example, if pure oxygen is withdrawn as the 
permeate gas stream at atmospheric pressure, the amount of oxygen that can 
be recovered is limited by the partial oxygen pressure of the exiting 
retentate. Therefore, the feed air stream must be at a pressure well in 
excess of 5 atm. At this level oxygen partial pressures are equal at the 
anode and cathode and oxygen recovery zero. If a steam purge is used a 
positive driving force can be maintained for oxygen transfer at 5 atm or 
even lower air pressure. A usually impractical alternate process would 
involve vacuum pumping of the permeate side of the ion transport membrane 
in order to maintain the driving force for the permeation process without 
contaminating the oxygen product gas stream. 
Processes according to the present invention use a pressurized superheated 
steam from a Rankine cycle system to purge the permeate side of the ion 
transport membrane. With an adequate flow of steam, the partial pressure 
of oxygen in the permeate gas stream can be reduced to a low value, thus 
permitting oxygen permeation to occur when the feed gas stream is at a 
lower pressure. By cooling the permeate gas stream, the water therein can 
be condensed and recycled, leaving the residual oxygen to be recovered. 
This cooled oxygen gas stream will contain some residual water vapor but 
is otherwise pure. It can be used directly as the product, or it can be 
dried further, for example, in a polymeric membrane or pressure or 
temperature swing adsorption (PSA) postpurifier. The oxygen product gas 
stream will be at the saturation pressure of the condensing steam, which 
can be adjusted to be at a low or intermediate level, depending on the 
optimization of product and power needs. Nitrogen can also be obtained as 
the product or coproduct at a moderate or high pressure level. 
Since generating steam is energy intensive especially with respect to the 
heat of vaporization it is advantageous to use the steam to generate power 
for driving plant associated compressors and or export. Such a power cycle 
also has the ability to use available low level heat to reduce the energy 
required for raising steam. The invention permits a high degree of freedom 
in selecting pressure levels for the feed gas depending on whether the gas 
is part of a gas turbine cycle, or whether a co-product is required at a 
certain pressure level. 
A preferred embodiment of the invention produces a high purity nitrogen 
co-product. In this case the steam purged ion transport membrane recovers 
the bulk of the oxygen contained in the feed leaving a residual amount of 
oxygen to be removed by a downstream deoxo in the form of a reactively 
purged ion transport membrane. In this case it is advantageous to control 
the amount of oxygen to be reacted in the deoxo at a level sufficient to 
provide the necessary heat to balance requirements of the gas cycle. Since 
the oxygen reaction takes place on permeate side, the retentate stream is 
not contaminated with products of combustion and a high purity nitrogen 
co-product can be obtained. The nitrogen will be at pressure and all or 
part of the stream can be expanded in a turbine to generate power 
depending on how much of the nitrogen product is required at pressure. 
There could be an advantage, with respect to system simplicity, in 
expanding just sufficient nitrogen to drive the air compressor leaving the 
remainder as an elevated pressure co-product. In case the product nitrogen 
can contain small residual amounts of oxygen, the deoxo can be eliminated 
and the steam purged ion transport membrane used to refine the feed to the 
permissible residual oxygen content. An externally fired heater upstream 
or downstream from the membrane can be used to balance the heat losses 
from the system. One of the advantages of the steam purged systems is 
that, since it provides such high recovery, it limits the need for 
compressing and processing a significant amount of excess air. 
In another preferred embodiment of the invention a high pressure steam 
expansion stage is inserted upstream of the inlet to the permeate side of 
the ion transport membrane. For system simplicity the process conditions 
for this stage are selected in such a way that all or most of the 
superheat, required for this stage to prevent a wet turbine exhaust, can 
be provided by the available sensible heat in the turbine exhaust from the 
second low pressure steam turbine stage which is located downstream from 
the ion transport membrane. The advantage of this arrangement is that it 
can generate a significant amount of extra power at marginally increased 
fuel expenditure. 
It should also be noted that the invention does not require the piping or 
handling of pure oxygen at high temperatures, which often presents serious 
safety hazards. Therefore, the need for special materials and procedures 
for handling high temperature pure oxygen is avoided and the claimed 
process should achieve greater safety. 
As noted above, the ion transport membrane will transport oxygen when there 
is a difference in oxygen partial pressure across it. In comparison to 
polymeric membranes, ion transport membranes have a higher flux and an 
infinite separation factor for oxygen with respect to nitrogen. The type 
of ion transport module used in the invention is usually a 4-port device, 
similar that even though the invention optimally operates with 
counter-current flow of permeate and retentate in the ion transport 
separator it is also applicable with other flow configurations, for 
example, co-current and crossflow configurations. 
The basic embodiment of the invention is illustrated by the schematic 
diagram in FIG. 1. During operation, oxygen-containing feed gas stream 10 
(generally air), usually at an elevated pressure, after having been heated 
to ion transport operating temperature, is introduced into ion transport 
module 14 containing ion transport membrane 16 having a retentate side 16A 
and a permeate side 16B. High-pressure steam purge stream 12 purges the 
permeate side 16B of ion transport membrane 16 to produce retentate gas 
stream 18 and permeate gas stream 21. The use of purge gas stream 12 
reduces the oxygen partial pressure on the permeate side 16B of ion 
transport membrane 16, and therefore enables efficient oxygen transport 
even when the retentate side 16A feed gas pressure is low. Because steam 
purge stream 12 mixes with and dilutes the oxygen that has permeated 
through ion transport membrane 16, permeate stream 21 that emerges from 
ion transport module 14 contains both steam and oxygen at high pressure. 
Permeate gas stream 21 is directed to energy extractor section 22 where 
steam turbine 24 generates power 28 and discharges gas stream 26 at a 
lower pressure. The remaining heat energy in gas stream 26 preferably is 
extracted and used, for example, to superheat steam and heat boiler feed 
water. Discharge gas stream 26 is directed to separator 29 to separate the 
gas stream into product oxygen stream 30 and discharge water stream 31. In 
addition, retentate gas stream 18 preferably is directed to heat extractor 
section 20 where energy is removed from retentate gas stream 18 by, for 
example, expansion through a gas turbine or transfer of heat to the feed 
stream. 
An embodiment of the invention is illustrated by a ore detailed schematic 
diagram in FIG. 2 which illustrates an integration of oxygen separation by 
a steam purged ion transport separator and an associated steam turbine 
with a gas turbine cycle. Feed gas 201, usually air, is compressed by 
compressor 202 to a pressure between 100 and 250 psig to produce 
compressed air stream 203. Stream 203 is then preheated in heat exchanger 
204 by waste heat contained in turbine exhaust 214. Resulting compressed 
and preheated air stream 205 preferably is heated further in combustor 206 
to the ion transport operating temperature of 800 to 1900.degree. F. 
(425-1035.degree. C.) by combusting a portion of the contained oxygen with 
fuel stream 208. Heated stream 207 enters the cathode or retentate side 
209A of ion transport separator 209. Here oxygen is transferred by ion 
transport across mixed conductor ion transport membrane 247 to the 
permeate side 209B of separator 209 with the driving force for the 
separation provided by the ratio of partial oxygen pressures across 
membrane 247. A significant fraction of the contained oxygen in the feed 
stream can be separated since typical gas turbine cycles must use excess 
air to limit the temperature rise in combustors to levels compatible with 
available materials of construction used in the turbine. 
Partially oxygen depleted retentate gas 210 is optionally further heated to 
the permissible turbine inlet temperature in combustor 211 using fuel 
stream 212. Gas stream 213, now at turbine inlet temperature, is then 
expanded in turbine 217 producing power which drives compressor 202 and 
usually a generator to generate power for export. Exhaust stream 214 
contains useful waste heat which preferably is used to preheat air stream 
203 in heat exchanger 204. Cooled stream 216 is discharged from the cycle. 
Optionally the waste heat is also utilized advantageously in the associated 
steam cycle. The example discussed later utilizes this option. In this 
embodiment at least a portion of the stream discharged from the gas 
turbine becomes stream 248 which provides the necessary heat for 
evaporating stream 225 in heat exchanger 249 which takes the place of the 
evaporator oil of a boiler which would be used in another option. Cooled 
stream 251 is discharged from the cycle. 
Processes according to the present invention purge the permeate side with 
high pressure superheated steam which is then expanded in a turbine to 
recover power. As shown in FIG. 2, high pressure superheated steam 232 is 
introduced to the permeate side 209B of separator 209, preferentially 
countercurrently to the feed stream 207. The presence of steam lowers the 
partial oxygen pressure on the permeate side 209B, which increases the 
driving force for oxygen transport, and thereby reduces the required area 
for ion transport membrane 247. The exiting permeate stream 234 is a 
mixture of steam and oxygen which is expanded in turbine 235 to recover 
power. The exhaust stream 236 from turbine 235 contains sensible heat 
which preferably is utilized to partially superheat high pressure steam 
228 in heat exchanger 229. Resulting stream 237 still contains sufficient 
heat to warm feed water stream 223 in heat exchanger 224. In externally 
cooled condenser 239 the major fraction of water in stream 238 is 
condensed to produce a mixture of water and saturated oxygen 240 which are 
separated in separator 241 to produce water stream 247 and oxygen stream 
242. 
Optionally, oxygen stream 242 is further cooled in cooler 243 resulting in 
cool oxygen stream 244 which is optionally compressed in compressor 245 to 
produce product oxygen stream 246 at a desired delivery pressure. If 
required, the product oxygen can be dried using a polymeric membrane or 
adsorption dryer. 
Water stream 247 exiting from separator 241 is joined with make-up water 
221 and then pumped to the desired pressure in water pump 222 to produce 
pressurized feed water stream 223 which is warmed in feed water heater 224 
and then, as warm water stream 225, is introduced to the evaporator heat 
exchanger 249 to produce saturated or slightly superheated steam 228. 
Steam 228 is further superheated in heat exchanger 229, recovering 
available heat from stream 236, and then introduced via line 230 to 
superheating coil 231 of externally fired heater 218 to raise its 
temperature to the operating temperature of ion transport separator 209. 
The resulting superheated stream 232 is then introduced into separator 
209. Combustion in externally fired heater 218 is sustained by combining 
fuel stream 219 and air stream 220 which finally exit as stack gas stream 
231. 
Some of the major advantages of the above system over an alternate system 
without the Rankine steam cycle are: a significant reduction in the 
required ion transport membrane area, the co-production of significant 
additional power by the steam turbine; and potential reduction and 
possible elimination of the total pressure difference across the membrane, 
to ease sealing between the retentate and permeate streams of separator 
209 as well as structural design of the separator. 
EXAMPLE 1 
In the following the calculated performance is compared for three gas 
turbine cycles with oxygen co-production: The system with the invention 
represented by FIG. 2 (Case A); a cycle where the gas turbine waste heat 
is used to raise steam for purging the permeate side of the ion transport 
generator and oxygen is delivered at an elevated pressure (Case B), and a 
gas turbine cycle with oxygen co-production and no steam generation where 
the turbine exhaust is used to regeneratively pre-heat the air (Case C). 
Process conditions: 
Oxygen Co-production: 1000 MNCFH 
Air Compressor Discharge Pressure: 185 psia 
Single Stage Air Compression at Ad.Eff=85% 
Ion Transport Inlet Temperature: 1650.degree. F. (900.degree. C.) 
Turbine Inlet Pressure 180 psia 
Turbine Exhaust Pressure 16 psia 
Turbine Inlet Temperature 2000.degree. F. (1090.degree. C.) 
Turbine Efficiency=90% 
Oxygen Recovery: 5.6% of Compressed Air Stream 
Cases with Steam Purge: 
Steam Pressure: 84 psia 
Steam raised: 172 lbs/1000 NCFH O2 
For Steam Turbine: 
Exhaust Pressure 16 psia 
Steam Turbine Efficiency: 90% 
Oxygen Product Pressure: 14.7 psia Cases A and C 82 psia Case B 
TABLE 2 
______________________________________ 
Performance Comparison 
Case A Case B Case C 
______________________________________ 
Net Power KW 75,000 55,500* 53,750 
Heat Required MM BTU/Hr 
606 527 442 
Heat Rate BTU/KW Hr 
8,080 9490 8220 
Eff. O2 Driving Force 
.45 .45 .115 
log(Po1/Po2) 
______________________________________ 
*Credit for O.sub.2 Compression to 82 psia 
The comparison in Table 2 clearly demonstrates the advantage of both steam 
purged cases over the non-purged case in terms of the available driving 
force for transporting O.sub.2 across the ion transport membrane. Since 
these values are proportional to ion transport area substantially more 
area will be required for the non purged gas turbine cycle. Case A, 
representing a process according to the invention, shows also a small 
advantage in energy utilization over Case C and a very significant 
advantage over Case B. 
A preferred embodiment of the invention, which incorporates additional 
inventive features, is illustrated in FIG. 3. This embodiment features the 
coproduction of nitrogen and the generation of additional power at minimal 
incremental fuel expenditure by addition of a high pressure steam turbine 
stage. 
Air 301 is compressed to a suitable pressure, such as between 100 and 300 
psia by compressor 302 to produce compressed air stream 303. Compressed 
air stream 303 is heated to ion transport operating temperature (typically 
between 700 and 1800.degree. F.) in heat exchanger 304 by heat sources 
such as available heat from various waste and product streams as well as 
heat generated in ion transport reactor 310. Heated and compressed air 305 
is introduced into the retentate side 306A of ion transport separator 306. 
Here oxygen is transferred across mixed conductor membrane 307 by ion 
transport driven by the ratio of partial oxygen pressures across membrane 
307. 
The permeate side 306B of separator 306 is purged by high pressure 
superheated steam. The dilution of the permeate gas with steam effectively 
reduces the partial oxygen pressure on the permeate side and therefore 
increases the driving force for oxygen transport. In an application that 
stresses the production of nitrogen and oxygen, the present invention 
enables maximization of the recovery of oxygen at minimum air flow since 
recovery is not limited by the exiting partial oxygen pressure of the 
retentate as in the nonpurged case. 
When co-production of nitrogen is desired, separator 306 removes the major 
portion of the contained oxygen from the feed stream. Since in this 
embodiment the residual oxygen is the oxidant for the reaction which 
provides the necessary heat for sustaining system operation, the residual 
amount of oxygen left in retentate stream 308 is a function of the heat 
demands of the system which in turn depends on the portion of retentate 
gas that is expanded in a turbine. 
Stream 308 is cooled in heat exchanger 304 against the feed stream 303 to a 
level which permits it to absorb the heat of reaction generated in ion 
transport reactor without exceeding the permissible maximum temperature of 
900 to 1850.degree. F. Partially cooled stream 309 enters the retentate 
side 310A of ion transport reactor 310. In this unit the residual oxygen 
in stream 309 is transported across membrane 356 and reacted with fuel in 
permeate stream 323 on the permeate side 310B of reactor 310. The reaction 
on the permeate side provides a very low oxygen partial pressure for 
efficient oxygen transport down to very low oxygen content of the 
retentate gas thereby producing a very high purity nitrogen stream 324. 
The heat of reaction is absorbed as by the temperature rise of the 
retentate gas with internal reactor heat transfer element 312 assuring 
that local membrane temperatures do not exceed permissible levels. A 
suitable design of reactor internal elements is discussed in U.S. Pat. No. 
5,820,655, which is hereby incorporated as a reference. 
In one embodiment all of the high pressure hot nitrogen gas 314 is 
recovered as high pressure product; in another embodiment, some or all of 
it can be expanded in a hot gas expander to generate power and deliver the 
nitrogen product at low pressure. From the point of view of system 
simplicity it may be advantageous to split stream 314 into two streams one 
of which, stream 315, is expanded in hot gas expansion turbine 354 to 
generate sufficient power to drive air compressor 302, expanded gas stream 
355 is delivered as a low pressure product 357 after recovery of waste 
heat in heat exchanger 304. Under most circumstances this leaves 
sufficient nitrogen 316 to be delivered as a high pressure product 321 
after recovery of waste heat in heat exchanger 304. Heat exchanger 304 is 
shown as one unit for the sake of simplicity but could optionally be 
divided into several exchangers to perform all the required heat transfer 
functions. 
A portion of product stream 321 (preferably 5 to 15%) may be throttled down 
by valve 318 to a lower pressure stream 319 and mixed with fuel stream 320 
as a diluent to produce stream 317. Stream 317 is heated in heat exchanger 
322 against permeate waste stream 324 and introduced to the permeate side 
of ion transport reactor 310 as stream 323 where the fuel contained in the 
stream reacts with the oxygen permeating across membrane 356. Hot permeate 
waste stream 324 is discharged as stream 325 after recovery of waste heat 
in exchanger 322. 
Alternatively, at least a portion of the permeate discharge from the ion 
transport reactor can become stream 324B, shown in phantom, which joins 
stream 315 prior to expansion through turbine 354. In this case the 
reactive purge stream is at or near retentate pressure and throttling 
valve 318 is now a proportioning valve. Valve 368, also shown in phantom, 
may be inserted to adjust the pressure of stream 315 to that of stream 
324B. The alternate arrangement increases the amount of nitrogen recovered 
at pressure, at the expense of contaminating the low pressure nitrogen 
with products of combustion, since expansion of the permeate stream can be 
substituted for that of some of the high pressure nitrogen. 
As in FIG. 2, the permeate side 306B of ion transport separator 306 is 
purged with high pressure superheated steam 326 preferably at a pressure 
of at least 50 psia, more preferably at least 150 psia. The steam circuit 
is similar to that of FIG. 2 except that a high pressure steam turbine 
stage has been inserted upstream of separator 306. The details of the 
steam circuit are as follows: Feed water 341 is pumped to an elevated 
pressure (typically 300 to 1200 psig) by pump 342 then heated as stream 
343 in feed water heater 333 against available waste heat and introduced 
via stream 345 to the evaporator section 346 of boiler 347. High pressure 
steam 348, at or near saturation temperature, is superheated in heat 
exchanger 331 to a sufficiently high temperature to avoid condensation 
during expansion as stream 349 in steam turbine 350 to an intermediate 
pressure level. It is advantageous to select operating conditions for the 
steam turbines in such a fashion that all or most of the superheat for 
stream 349 can be provided by the available sensible heat from the exhaust 
330 of the low pressure stage 329. The discharge 351 from turbine 350, now 
near saturation temperature at an intermediate pressure, is partially 
superheated by transfer of residual available waste heat in exchanger 331, 
which is shown as a single exchanger but could consist of two separate 
units, and then as stream 352 is further superheated in superheating coil 
353 of boiler 347 to produce superheated steam 326 at ion transport 
operating temperature. 
Stream 326 is introduced to the permeate side 306B of separator 306 where 
it picks up oxygen transferred from the retentate side 306A of separator 
306. The steam-oxygen mixture 328 is expanded in the low pressure stage 
turbine 329 to produce power. Since the discharge 330 from this turbine is 
still at a substantially elevated temperature the sensible heat of stream 
330 is recovered in heat exchanger 331 and feed water heater 333, via 
stream 332, to produce stream 334 which is introduced to externally cooled 
condenser 335 where the major portion of the contained water will condense 
to produce a water oxygen mixture 336. Stream 336 is separated in 
separator 337 into oxygen, saturated with water, and oxygen 338 which 
optionally can be further cooled, compressed, dried and delivered as 
product 338A. 
Water stream 339 discharged from separator 337 joins make-up water 340 and 
is recycled to the system. Combustion in boiler 347 is sustained by fuel 
stream 358 and air or oxygen stream 359 and produces flue gas waste stream 
360. 
The advantages of the embodiment represented in FIG. 3 are significant as 
Example 2 will show. The employment of a high pressure steam purge in the 
ion transport separator permits high oxygen recovery which is not limited 
by the oxygen partial pressure of the exiting retentate and as a result 
minimizes the amount of excess air that has to be processed in the complex 
system and therefore minimizes investment. The use of an ion transport 
reactor as a combined deoxo and combustor avoids contamination of the 
nitrogen co-product with products of combustion. Characteristics of ion 
transport reactors where oxygen reacts on the surface of the anode of the 
ion transport membrane permits effective combustion at low oxygen-fuel 
ratios and limits generation of NOX (nitrous oxides) since less or no 
nitrogen is present and the reaction takes place on the anode surface of 
the membrane which with proper heat transfer design can be maintained at 
temperatures substantially below typical flame temperatures of 
conventional combustors. 
Splitting the retentate flow into a stream to be delivered at pressure and 
one to be expanded in such way that just sufficient power is generated to 
drive the air compressor provides for a simple, effective system since it 
avoids an additional machine to compress nitrogen to the desired product 
pressure. Use of a two-stage steam turbine expansion with reheat of the 
first stage exhaust and use of the intermediate pressure stream as the 
purge gas for the separator provides for an energy efficient Rankine cycle 
without need for designing the ion transport module for very high 
pressures. Normally a high pressure would be required for efficient energy 
production from a steam stream at the high temperatures at which ion 
transport systems have to operate. 
System design is simplified by proportioning the operating variables of a 
system according to the present invention in such a way that most or all 
of the superheat required in the feed to the high pressure turbine, to 
prevent condensation during expansion, is provided by available sensible 
heat in the second low pressure stage exhaust. The combined system can 
produce oxygen, high purity nitrogen and co-generated power at very 
attractive fuel consumption, as illustrated by the following example. 
EXAMPLE 2 
Oxygen Product: 1000 MNCFH 
Nitrogen Co-Product at 145 PSIA: 1670 MM NCFH 
Air Compressor Discharge pressure: 155 psia 
Stages of Air Compression: 4 (a compressor driven by a hot gas expander 
rather than a gas turbine) 
Air Compressor Efficiency: 85% 
Hot Gas Turbine Inlet Pressure: 150 psia 
Hot Gas turbine Inlet Temperature: 1750.degree. F. (955.degree. C.) 
Hot Gas Turbine Exhaust Pressure: 16 psia 
Hot Gas Turbine Efficiency: 90% 
Ion Transport Separator Inlet Temperature: 1650.degree. F. (900.degree. C.) 
Ion Transport Deoxo Outlet Temperature: 1750.degree. F. 
Deoxo Fuel Dilution: 79 MNCFH N2 
High Pressure Steam Turbine Inlet Pressure: 1000 psia 
High Temperature Turbine Inlet Temperature: 805.degree. F. (430.degree. C.) 
Low Pressure Turbine Inlet Pressure: 150 psia 
Low pressure Turbine Inlet Temperature: 1650.degree. F. 
Low Pressure Turbine Exhaust Pressure: 16 psia 
Steam Turbine Efficiencies: 90% 
Steam Condensing Pressure: 14.7 psia 
Steam Generated: 303 M lbs/Hr 
Results: 
Oxygen Recovery: 83% of O2 in Feed Air 
N2 Co-Product at 145 psia: 1,670 M NCFH 
Net Power Generated: 58,560 KW 
Heat Required: 490 MM BTU/Hr 
Heat Rate: 8370 BTU/KW Hr 
Heat Rate with Credit for N2 Compr.: 7850 BTU/ KW Hr H.R. with Credit for 
Sep. Pwr. (7KW/MNCFH): 7060 BTU/KW Hr Eff. Avg. O2 Transport Driving 
Force: 0.308 
Example 2 illustrates very attractive performance numbers for the 
embodiment of the invention as shown in FIG. 3. The use of a steam purge 
for the permeate side of the ion transport separator permits very high 
recovery of oxygen and therefore limits the amount of excess air that has 
to be processed by the system. Compared to Example 1, the amount of air is 
smaller by a factor of 3.1:1. The use of an ion transport reactor, which 
performs the deoxo function and at the same time generates the heat 
required for the air circuit, permits recovery of 90% of the nitrogen 
contained in the feed air at high product purity with a very simple cycle. 
41% of the product nitrogen can be recovered at high pressure. The 
employment of a two-stage steam turbine cycle permits achieving very 
attractive energy utilization rates. The rates are significantly better 
than for the best case of Example 1 even though the former employs higher 
peak temperatures. 
It should be noted that the cycles considered herein for illustration are 
Rankine and Brayton-Joule cycles, although the invention is not intended 
to be so limited. Thus, the embodiments of the invention discussed herein 
may be modified to incorporate other cycles known to those of skill in the 
art, for example, multiple reheat and regenerative cycles, dual pressure 
cycles, and cogeneration cycles, without departing from the spirit of the 
invention. Also it will be obvious to those skilled in the art that in 
practical applications that there be many opportunities to provide further 
integration of the Brayton and Rankine portions of the proposed 
embodiments especially with respect to optimum utilization of waste heat. 
Also it will be obvious to those skilled in the art that inlet conditions 
to various components can be varied to accommodate material considerations 
without departing from the intent of the invention, for instance cooling 
the purge stream exiting the ion transport membrane, prior to introduction 
to a steam turbine or cooling the separator or deoxo retentate prior to 
admission to a hot gas expander. 
The processes of the invention can be driven partially by low-level heat 
and thus can be integrated with other processes that produce heat, 
including processes that utilize the product oxygen in producing the heat. 
When operated at a high feed pressure, the invention can be integrated 
with gas turbines or other devices for the production of power from the 
high pressure product or waste streams as shown in the examples. Typical 
ranges for operating parameters of the ion transport module are as 
follows: 
Temperature: For the ion transport module, the temperature will typically 
be between 700.degree. F. to 2000.degree. F. (370 to 1100.degree. C.) 
range, and preferably between 1000.degree. F. and 1850.degree. F. (535 to 
1010.degree. C.) range. The steam turbine will typically operate between 
400 and 1700.degree. F. range. 
The gas turbine will typically operate between 1300 and 2600.degree. F. 
Pressure: The purge-side pressure will be typically be between 1 atm and 40 
atm, and preferably between 1 atm and 12 atm. The feed-side pressure will 
be between 1 atm and 40 atm if pressurized nitrogen is needed as a product 
or if the ion transport exhaust gas goes to a gas turbine for power 
generation, and 1 atm to 10 atm otherwise. The feed-side and permeate side 
pressures may be made substantially equal to reduce sealing requirements 
between the sides of the membrane. Gas turbine inlet pressure will 
typically be between 5 atm and 40 atm, and the exhaust pressure will 
typically be between 1 atm and 1.5 atm. The steam generator will typically 
generate steam at a pressure of 3 atm to 80 atm for the Rankine cycle. 
Oxygen Ion Conductivity of the Ion Transport Membrane: Typically in the 
0.01-100 S/cm range (1 S=1/Ohm). 
Thickness of the Ion Transport Membrane: Ion transport membrane can be 
employed in the form of a dense film, or a thin film supported on a porous 
substrate. The thickness (t) of the ion transport membrane/layer is 
typically less than 5000 microns, preferably it is less than 1000 microns, 
and most preferably it is less than 100 microns. 
Configuration: The ion transport membrane elements may typically be tubular 
or planar. 
If high purity (greater than 99.9%) nitrogen is to be produced, it may not 
be practical to carry out the separation using one ion transport module 
stage and postpurification may have to be used as in Example 2. For 
example, oxygen and low purity nitrogen can be produced in the ion 
transport stage and a postpurifier may be employed to remove most or all 
of the residual oxygen from the low purity nitrogen retentate from the ion 
transport module stage. The post purifier could be a traditional deoxo 
system (for example, one employing H.sub.2 -based deoxo), or 
preferentially another ion transport module as in Example 2. 
If a second ion transport module stage is employed for postpurification, it 
may use a reactive purge gas, for example, methane, to purge the permeate 
side of the ion transport membrane. Such a reactive purge gas greatly 
reduces the partial pressure of oxygen on the purge side of the ion 
transport membrane, thereby effecting an enhanced oxygen transport and 
requiring a much smaller ion transport membrane area than if a nonreactive 
purge gas were used. A reactive purge gas may also provide all or part of 
the heat input necessary to maintain a high operating temperature in the 
ion transport module stage. It is also possible, although less desirable, 
to use a product purge scheme in the second ion transport module stage, 
whereby a part of the high purity nitrogen retentate gas stream is 
recycled to sweep the anode side of the ion transport module. The ion 
transport membrane area requirement in the product purge system is much 
higher than that in a reactively purged system. 
The amount of oxygen separated in the ion transport module may easily be 
altered as desired by varying the feed gas stream pressure, the feed air 
flow rate, and/or the ion transport operating temperature. Although 
pressure-driven ion transport modules are preferred for the simplicity of 
their design, the ideas described herein are applicable to systems 
utilizing either an electrically-driven ion transport membrane or a 
pressure-driven ion transport membrane. 
Specific features of the invention are shown in one or more of the drawings 
for convenience only, as each feature may be combined with other features 
in accordance with the invention. In addition, various changes and 
modifications may be made to the examples given without departing from the 
spirit of the invention. Alternative embodiments will be recognized by 
those skilled in the art and they are intended to be included within the 
scope of the claims.