Reactive purge for solid electrolyte membrane gas separation

A system and process for producing a high-purity product from a feed stream containing elemental oxygen by applying the feed stream to at least one separator including a feed zone and a permeate zone separated by a solid electrolyte membrane, and driving a portion of oxygen contained in the feed stream from the feed zone to the permeate zone via the membrane by applying to the permeate zone a reactive purge stream containing a reactive gas which combines with oxygen to establish a lower partial pressure of oxygen in that zone. Oxygen-depleted retentate is withdrawn as a high-purity product stream.

FIELD OF THE INVENTION 
This invention relates to apparatus and procedures for separating oxygen 
from a mixed gas feed stream and, more particularly, to employing a 
reactive purge stream with a solid electrolyte membrane for removing 
oxygen to purify the feed stream. 
BACKGROUND OF THE INVENTION 
Solid electrolyte membranes are made from inorganic oxides, typified by 
calcium or yttrium-stabilized zirconium and analogous oxides having a 
fluorite or perovskite structure. At elevated temperatures, these 
materials contain mobile oxygen-ion vacancies. When an electric field is 
applied across such an oxide membrane, the membrane will transport oxygen 
ions and only oxygen ions and thus act as a membrane with an infinite 
selectivity for oxygen. Such membranes are attractive for use in air 
separation processes. More recently, materials have been reported that 
exhibit both ionic and electronic conductivity. A membrane exhibiting such 
a mixed conduction characteristic can transport oxygen when subjected to a 
differential partial pressure of oxygen, without the need for an applied 
electric field or external electrodes. 
In an oxygen ion conducting inorganic oxide, oxygen transport occurs due to 
a presence of oxygen vacancies in the oxide. For materials that exhibit 
only ionic conductivity, electrodes must be applied to opposed surfaces of 
the oxide membrane and the electronic current is carried by an external 
circuit Electrons must be supplied (and removed at the other side of an 
oxide membrane) to make the reaction proceed. 
For mixed conductor materials that exhibit both ionic and electronic 
conductivity, the countercurrent to the flow of oxygen vacancies is an 
internal flow of electrons, rather than by an electrical current through 
an external circuit. The entire transport is driven by oxygen partial 
pressures in the streams adjacent opposite sides of a mixed conduction 
inorganic oxide membrane. In the absence of a purge stream, the "permeate" 
stream that carries the oxygen away from the membrane is "pure" oxygen, 
and both the feed and the retentate streams must be at a high pressure (or 
the "permeate" stream at a very low pressure) to create a driving force 
for the oxygen transport While such an unpurged membrane is attractive for 
the removal of larger quantities of oxygen from inert gas streams, the 
oxygen recovery is limited by pressures that can be applied. Even then, 
the degree of purification that can be obtained is limited. 
In the patent art, there are a number of teachings regarding the use of 
solid electrolyte inorganic oxide membranes. Chen et al. in U.S. Pat. No. 
5,035,726 describe the use of solid electrolyte membrane systems for 
removing oxygen from crude argon feed streams. Chen et al. employ an 
electrically-driven ionic conductor to achieve gas separation. Chen et al. 
also mention the possibility of using mixed conductor membranes operated 
by maintaining an oxygen pressure on the feed side. Chen et al. further 
teach that oxygen exiting from the permeate side of an electrically-driven 
ionic membrane may either be removed as a pure oxygen stream or mixed with 
a suitable "sweep" gas such as nitrogen. 
Mazanec et al. in U.S. Pat. No. 5,160,713 describe oxygen separation 
processes employing a bismuth-containing mixed metal oxide membrane. 
Mazanec et al. state generally that the separated oxygen can be collected 
for recovery or reacted with an oxygen-consuming substance. The 
oxygen-depleted retentate apparently is discarded. 
In U.S. Pat. No. 5,306,411, Mazanec et al. disclose a number of uses of a 
solid electrolyte membrane in an electrochemical reactor. It is mentioned 
that nitrous oxides and sulfur oxides in flue or exhaust gases can be 
converted into nitrogen gas and elemental sulfur, respectively. It is also 
mentioned that a reactant gas such as light hydrocarbon gas can be mixed 
with an inert diluent gas which does not interfere with the desired 
reaction, although the reason for providing such a mixture is not stated. 
The Mazanec patents do not disclose processes to produce a high-purity 
product from an oxygen-containing stream. 
The above-identified patent and technical literature do not disclose means 
for reducing pressure, membrane area, electrical power, or compressor 
power to levels required for practical application of solid electrolyte 
membranes to the separation and purification of product gases by 
controlled permeation of oxygen. 
OBJECTS OF THE INVENTION 
It is therefore an object of this invention to provide an improved system 
for producing a high-purity retentate stream employing at least one 
oxygen-ion-conducting solid electrolyte membrane and a reactive purge to 
decrease the concentration of oxygen on the permeate side of the membrane 
and thereby increase the driving potential for oxygen ion transport across 
the membrane. 
It is another object of this invention to provide such a system wherein 
pressure or power requirements are reduced from those exhibited by the 
prior art. 
A still further object of this invention to provide such a system which 
enables reduced membrane area or reduced purge flow rates. 
SUMMARY OF THE INVENTION 
This invention comprises a process for producing a high-purity product from 
a feed stream containing elemental oxygen by applying the feed stream to 
at least one separator including a first feed zone and a first permeate 
zone separated by a solid electrolyte membrane capable of transporting 
oxygen ions, driving a first portion of oxygen contained in the feed 
stream from the feed zone to the permeate zone through the membrane by 
applying a reactive purge stream to the permeate zone to remove oxygen 
therefrom and establish a lower partial pressure of oxygen in the permeate 
zone, and withdrawing oxygen-depleted retentate as a product stream after 
oxygen has been removed from the feed zone. 
In a preferred embodiment, the separator described above is positioned as a 
second stage and the feed stream is initially directed to a second feed 
zone of a second separator, the second separator being positioned as a 
first stage and having a second permeate zone separated from the second 
feed zone by a second solid electrolyte membrane. Preferably, one or both 
stages are also purged with at least one type of a diluent stream. More 
preferably, at least a portion of output of the first permeate zone is 
directed to mix with the reactive purge stream. 
As used herein the term "elemental oxygen" means any oxygen that is 
uncombined with any other element in the Periodic Table. While typically 
in diatomic form, elemental oxygen includes single oxygen atoms, triatomic 
ozone, and other forms uncombined with other elements. 
The term "high-purity" refers to a product stream which contains less than 
five percent by volume of elemental oxygen. Preferably the product is at 
least 99.0% pure, more preferably 99.9% pure, and most preferably at least 
99.99% pure, where "pure" indicates an absence of elemental oxygen.

DETAILED DESCRIPTION OF THE INVENTION 
Purification system 10 according to this invention, FIG. 1, includes a 
four-port separator 12 having a first feed zone 14 and a first permeate 
zone 16 separated by a solid electrolyte oxygen-ion conducting membrane 
18. An oxygen-containing feed stream 20 is applied to the first feed zone 
14. The feed stream 20 optionally is compressed by compressor 22, warmed 
by heat exchanger 24, and/or preheated by trim heater 26, shown in 
phantom. 
Oxygen ions are transported across membrane 18 when the oxygen partial 
pressure P.sub.1 in feed zone 14 is greater than the oxygen partial 
pressure P.sub.2 in permeate zone 16. Oxygen-depleted product stream 30 is 
obtained from feed zone 14 and a permeate stream 32 is obtained from 
permeate zone 16. 
Heat from streams 30,32 optionally is transferred to feed stream 20 through 
heat exchanger 24. It is desirable to recover the heat using a heat 
exchanger to warm the feed stream prior to contacting the first 
electrolyte membrane. 
Vacuum pump 36, shown in phantom, optionally assists withdrawal of permeate 
stream 32 from permeate zone 16. Typically, the permeate must be cooled to 
below 100.degree. C., preferably below 50.degree. C., before it reaches a 
vacuum pump. 
Alternatively, the hot gas permeate stream 32 is expanded through an 
expander 37, shown in phantom, to produce power, and then passed through 
heat exchanger 24 for heat recovery. In this case, total pressure in the 
permeate zone 16 is greater than atmospheric pressure. 
A reactive gas purge stream 34 is applied to permeate zone 16 in 
counter-current flow to feed stream 20 in this construction. 
Counter-current flow of the purge stream is more desireable than 
co-current flow when not all of the oxygen is removed by reaction in the 
permeate zone 16. However, co-current or cross-flow arrangements may also 
be used. 
Reactive gas utilized according to the present invention preferably 
comprises any gas that is capable of reacting in stoichiometric or 
superstoichiometric (fuel-rich) conditions with elemental oxygen or oxygen 
ions to yield an equilibrium oxygen partial pressure, at the operating 
conditions of the separator, of less than 10.sup.-4 atmosphere. Reactive 
purge stream 34 includes a reactive gas such as natural gas, H.sub.2, CO, 
CH.sub.4, CH.sub.3 OH, or other gas that reacts or otherwise combines with 
oxygen to decrease the quantity of elemental oxygen in permeate zone 16 to 
lower oxygen partial pressure P.sub.2. The term "gas" refers to substances 
which are in gaseous or vapor form at the operating temperature of the 
oxygen separation system. 
Oxygen separation procedures employing SELIC membranes generally require 
that the feed stream (and the temperature of the membrane) be at an 
elevated level, e.g. 400.degree. C. to 1200.degree. C., preferably 
500.degree. C. to 1000.degree. C., for efficient transport of oxygen ions 
across the membranes. The term "SELIC" refers to solid electrolyte ionic, 
mixed, or dual-phase conductors that can transport oxide ions. Separation 
procedures according to the present invention typically utilize a reactive 
gas which combines with oxygen in an exothermic reaction. 
More heat may be generated in a combustion reaction than would be 
desireable for proper operation of the SELIC membrane The reaction is 
controlled in one construction by blending an oxygen-depleted diluent 
component stream 38 shown in phantom. Suitable diluent components include 
argon, nitrogen, steam, and carbon dioxide. 
The diluent is selected to control temperature rise by increasing the heat 
capacity of the combined stream 42, to slow the rate of reaction within 
permeate zone 16 by reducing the temperature or concentration of 
reactants, and/or to make conditions within permeate zone 16 less reducing 
Permeate zone 16 is a reaction zone according to the present invention, 
and rendering the gases less reducing increases the chemical stability of 
the membrane 18. 
In this construction, separator operation is further enhanced by diverting 
a portion 41 of product stream 30 through valve 40 to purge permeate zone 
16. The diluent effects described above can be achieved by the product 
purge if the product stream is sufficiently oxygen-depleted. In one 
construction stream 38 and/or stream 41 comprise ten to ninety-five 
percent of blended stream 42. The actual percentage is selected based on 
the relative costs of diluent and reactive gas, the oxygen reactivity of 
the reactive gas, the maximum temperature desired in the reactor, the 
desired heat release of the reaction, and the types and thicknesses of the 
SELIC membrane. 
In another construction, a portion of output from permeate zone 16 is 
directed through valve 46 as exhaust recirculation stream 48, both shown 
in phantom, to mix with reactive purge stream 34 prior to applying stream 
34 into permeate zone 16. Several important benefits may be achieved by 
recirculating the exhaust gas as shown in phantom. Water vapor or carbon 
dioxide in the exhaust stream 48 can diminish or suppress coke (carbon) 
formation and deposition which otherwise might foul the surface of SELIC 
membrane 18 and diminish its performance. In the absence of species such 
as water and carbon dioxide, coking is likely when high-temperature, 
hydrocarbon fuel-rich conditions occur. These conditions are especially 
likely near purge inlet 42 because reactive purge stream 34 initially is 
fuel-rich at inlet 42 and becomes fuel-depleted only as it approaches 
outlet 44. 
Another benefit of recirculating the exhaust gas stream when the fuel is 
incompletely combusted is that hydrogen, carbon monoxide, hydrocarbons, or 
other combustibles are recycled for more complete combustion to improve 
fuel efficiency and to reduce undesired emissions. Recirculating hydrogen, 
which is particularly reactive, will produce improved performance 
especially near purge inlet 42. Exhaust recirculation stream 48 also 
reduces the need for an external diluent 38 or for product purge 41. 
Additionally, recirculation stream 48 can be used to regulate temperatures 
within separator 12 by either adding heat to the stream 48 or rejecting 
heat from the stream 48, such as by using heat exchanger device 49, prior 
to mixing with reactive purge stream 34. Otherwise, a heat exchanger or 
other external heating mechanism may be needed for reactive purge stream 
34. Exhaust recirculation therefore can improve stability, control, and 
overall operation of a purification system according to the present 
invention. 
Purification system 50, FIG. 2, includes a first stage 52 having a second 
separator 53 and a second stage 54 having a first separator 55. Second 
stage 54 utilizes a reactive purge stream 56 which is a selected blend of 
reactive gas stream 57 and product purge stream 58. Alternatively, an 
external diluent can be substituted for product purge stream 58. The 
operation of first separator 55 therefore would be similar to that of 
first separator 12, FIG. 1, if separator 12 were positioned as a second 
stage. 
The ratio in oxygen partial pressures P.sub.1 and P.sub.2 of first feed 
zone 60, FIG. 2, and first permeate zone 61, respectively, is enhanced by 
the reaction of oxygen in permeate zone 61. Second separator 53, however, 
relies on a relatively high oxygen feed mole fraction X.sub.f in initial 
feed stream 51 and on a sufficient ratio in oxygen partial pressures 
P.sub.1 ' and P.sub.2 ' of second feed zone 62 and second permeate zone 63 
to achieve oxygen transport through membrane 76. The difference in oxygen 
partial pressures is established by compressor 64 which generates a high 
feed pressure, by oxygen-deficient purge stream 65, and/or by vacuum pump 
66. 
The intermediate retentate stream 67 from second separator 53 is directed 
to the first feed zone 60. Second stage 54 includes a microprocessor 68 in 
this construction which is electrically connected to sensors 69, 70 and to 
valve 71. Microprocessor 68 optimizes operation of first separator 55 
based on the flow rate and/or the mid-stage mole fraction X.sub.m of 
elemental oxygen of stream 67, as detected by inlet sensor 69, and the 
temperature of first permeate zone 61, as detected by exit sensor 70. In 
another construction, sensor 70 is positioned in permeate reaction zone 61 
instead of in exit stream 72. Changes in the sensed variables cause 
microprocessor 68 to adjust valve 71 to alter the amount of diluent 
product stream 58 which mixes with reactive gas stream 57, thereby 
changing the mixing ratio of reactive stream 56. 
In yet another construction, microprocessor 68 adjusts the flow rate of 
reactive gas stream 57 using a low-temperature valve (not shown). The 
low-temperature adjustable valve is much less expensive than the 
high-temperature valve 71, which in this construction can be replaced with 
an inexpensive fixed orifice to serve as a fixed valve 71. 
Some or all of exhaust stream 72 may be provided as stream 49, shown in 
phantom in FIG. 2, to combine with or to serve entirely as purge stream 
65. Some reactions may occur in second permeate zone 63, especially if 
exhaust stream 72 contains unburned fuel. 
Fuel is initially ignited in permeate zone 61 in one construction by 
ignitor 80. Electrical energy is delivered along line 82 to generate a 
spark in permeate zone 61. Initial combustion may be started by reaction 
of fuel 57 with purge stream 58; use of product purge to start combustion 
is especially appropriate for stream 56 when intermediate feed stream 67 
is air. 
Alternatively, initial external heat, such as from trim heater 26, FIG. 1, 
preheats the compressed feed streams 51 and/or 67, FIG. 2, and the 
membrane 74 to cause autoignition of the fuel 57. Autoignition of a 
hydrocarbon fuel such as methane depends on factors including its 
concentration and the concentration of elemental oxygen. Further, many 
SELIC membrane materials are catalytic, which may initiate and promote the 
combustion process and lower the autoignition temperature. Alternatively, 
an oxidation catalyst is introduced as granules or as a surface coating to 
promote oxidation reactions. Both heterogeneous surface reactions and 
homogeneous gas reactions may occur to consume oxygen 
The SELIC membrane may be prepared from a variety of materials including 
those listed in a related application disclosing two or more stages of 
solid electrolyte ionic and/or mixed conducting membranes, entitled 
"Pressure Driven Solid Electrolyte Membrane Gas Separation Method", U.S. 
Ser. No. 08/444,354, filed on May 18, 1995 now abandoned, which is 
incorporated herein by reference. Also incorporated herein by reference 
for their teachings are U.S. Pat. Nos. 5,160,713 and 5,306,411 of Mazanec 
et al. The SELIC membrane may include a non-SELIC structural support 
element, such as a porous metal or ceramic tube. 
For ease of construction and improved performance, it is preferred that 
both SELIC membranes 74 and 76 are mixed conducting membranes. When SELIC 
membrane 74 is a pure ionic conductor membrane, as shown in FIG. 2 for 
illustration purposes, an external electrical circuit 83 is provided 
including cathode 84, anode 86, and connecting wire 88 to complete the 
circuit and thereby provide an electrical connection across the SELIC 
membrane. Oxygen ions are driven across SELIC membrane 74 by the oxygen 
chemical potential gradient to produce an EMF (electromotive force) that 
drives current in circuit 83. Alternatively, an external EMF such as a 
power supply is additionally applied to enhance oxygen ion movement 
Pressure driven processes are attractive for situations where large 
quantities of oxygen are to be permeated through a mixed conduction oxide 
membrane. In principle, the pressure driven process can also be used for 
removal of trace oxygen from the feed stream. This requires the oxygen 
partial pressure on the permeate side to be reduced to a level below that 
in the product stream. In practice, this can be accomplished by 
compressing the feed stream to a very high pressure, applying a very low 
vacuum level to the permeate, using a purge gas stream with a sufficiently 
low oxygen concentration, and/or using a reactive purge according to the 
present invention. 
The use of very high feed pressures or very low permeate pressures are 
power and capital intensive. Hence, non-purged pressure-driven processes 
tend to be economically unattractive for the removal of oxygen to achieve 
a very low concentration in the product. By contrast, the large currents 
required by conventional electrically driven processes make them too 
energy intensive to be attractive for the removal of large oxygen 
quantities. 
A multiple stage system according to the present invention is preferred to 
enable use of different types of SELIC membranes, different grades of 
reactive gas and/or purge gas, or different combinations of negative 
pressure and purge. Each stage may contain one or more SELIC membranes in 
feed series or feed parallel arrangement; the stages are in feed series 
arrangement. 
In multiple stage systems according to this invention, pure ionic SELIC 
membranes can be placed in different arrangements with mixed conductor 
membranes, preferably having an ionic membrane downstream of a mixed 
conductor membrane. This arrangement optimizes the ability of the 
preceding mixed conductor membrane to remove large amounts of oxygen from 
an oxygen-rich feed stream by a pressure-driven process, and the ability 
of the successive ionic membrane with electrodes and external circuitry to 
extract oxygen from a low-oxygen feed stream by the reactively purged 
process. 
Without a purge gas that has very low oxygen partial pressure, mixed 
conductors are not as suitable for extracting oxygen down to very low 
oxygen partial pressures. Ionic conductors with electrodes and external 
circuitry in inert purge configurations are inefficient and require large 
amounts of membrane area, making them very capital intensive if used to 
remove large amounts of oxygen. Ionic conductors in reactive purge 
configurations require much less area but would consume significant 
amounts of fuel and generate high temperatures if used to remove high 
concentrations of oxygen. 
Different types of SELIC membranes utilized for multiple stage systems 
according to this invention include membranes formed advantageously of 
different ionic or mixed conductor materials. In one construction, for 
example, a first stage membrane includes a mixed conductor perovskite 
which exhibits high oxygen ion conductivity but is unstable at very low 
oxygen partial pressures. The second stage must be comprised of a material 
which is characterized by high stability at very low oxygen partial 
pressure, even though such a material typically has a lower oxygen ion 
conductivity than that of the first stage SELIC membrane. Examples of 
mixed conducting materials of this type are disclosed in U.S. Pat. No. 
5,306,411 (Mazanec et al.). Materials used in the second stage must 
typically be stable at oxygen partial pressures of below 10.sup.-10 atm, 
which would typically be present in some areas of the permeate zone during 
reaction. 
Alternatively, a material such as yttria-stabilized zirconia 
"YSZ"(ZrO.sub.2 with 8% by weight of Y.sub.2 O.sub.3), which exhibits a 
much lower oxygen ion conductivity but is stable at low oxygen partial 
pressures, is used in the second stage. In this case, the second stage 
would be reaction purged and would have an external electrical circuit. 
One or more SELIC materials can be combined together in a single membrane, 
such as one of the multiphase mixtures disclosed in U.S. Pat. No. 
5,306,411 (Mazanec et al.), to tailor that membrane for the requirements 
of a particular stage. Further, different mechanical configuration can be 
used, such as a cross-flow geometry in the first stage, or in an 
ionic-only second stage, in which permeate is withdrawn at right-angles to 
feed and retentate flows. 
Oxygen separation system 90, FIG. 3, includes a separator 92 having a feed 
zone 94, a permeate zone 96, and a SELIC membrane 98. A feed stream 100 is 
compressed by compressor 102, heated by heat exchanger 104, and heated as 
needed by trim heater 106 before delivery to feed zone 94. A portion of 
oxygen-depleted product stream 108 optionally is diverted through valve 
110 to be mixed with reactive purge stream 112. 
A diluent stream 114 consisting primarily of steam is mixed with reactive 
purge stream 112 at valve 116. The actual composition of the blended 
reactive purge stream 117 entering permeate zone 96 therefore can be 
adjusted by passing selected amounts of product purge through valve 110 
and diluent steam through valve 116. The amounts of steam and product 
diluents are adjustable to control temperature, improve membrane 
separation or process stability, and enhance performance. 
Preferably, as shown in phantom, some heat contained in exit stream 122 is 
transferred to feed stream 100 by directing some or all of the exhaust 
through valve 124 to obtain side stream 126 which, after passing through 
heat exchanger 104 to warm feed stream 100, is returned as stream 130 to 
rejoin stream 125 between boiler 132 and condenser 136. 
If oxygen is desired as a co-product, the purge gas composition is 
controlled such that the amount of oxygen near inlet port 118 will be low 
while the amount of oxygen near exit port 120 will be high. A portion of 
stream 126 can then be diverted to provide a low-purity oxygen product 
stream 128. 
Boiler 132 transfers heat from stream 125 to water 134, thereby generating 
steam 114. Alternatively, a steam stream 114 is supplied from an external 
source. In this construction, stream 125 is further cooled in condenser 
136 and water vapor is extracted in water separation chamber 138 to supply 
water stream 134; make-up water 140 is added as needed. Stream 125 thereby 
becomes water-depleted stream 142. Preferably, water obtained from 
separator 138 is conventionally treated to remove carbon dioxide or other 
undesirable species to reduce corrosion in the boiler system. A pump 141, 
shown in phantom, may be added to pressurize stream 134. 
If the amount of combustion in permeate zone 96 is small, that is, only a 
small portion of oxygen is removed by reaction, then stream 142 can serve 
as an oxygen product stream. If combustion is near stoichiometric or is 
super-stoichiometric (fuel-rich), then stream 142 can yield carbon 
dioxide, carbon monoxide, and/or hydrogen as products, for example. In 
another construction, stream 125 is directed elsewhere or discarded 
without extracting water vapor. 
System 150, FIG. 4, is suitable for bulk production of a 
low-oxygen-concentration retentate product 152, such as nitrogen product, 
from a feed stream 154 such as air. System 150 includes a first separator 
92' which serves as a second stage and a second separator 151 which serves 
as a first stage. Different purge configurations including reactive gas, 
diluent gas and/or product purge are utilizable for the second stage as 
described above regarding FIGS. 1-3. In this construction, the first stage 
optionally is purged with an oxygen-depleted stream 153. Some fuel may be 
added to stream 153 to enhance performance and to generate heat to offset 
heat losses. 
Feed stream 154 is compressed by compressor 156 and enters a heat exchanger 
158 where the temperature of feed stream 154 is elevated by heat exchange 
with product stream 152 and oxygen byproduct stream 160 from second 
separator 151. A trim heater 164 further elevates the feed stream 
temperature as desired. The heated feed stream is applied to second 
separator 151, and a second portion of entrained oxygen is driven from the 
feed zone 166 to the permeate zone 168 via a second SELIC membrane 170, 
preferably a mixed conducting membrane. The oxygen partial pressure 
P.sub.2 ' in the permeate zone optionally is lowered by reducing the back 
pressure of exit stream 160, purging with an oxygen-depleted gas, such as 
effluent from the second stage, or by using a vacuum pump (not shown). 
Pure oxygen or an oxygen-enriched stream is thereby obtained as byproduct 
stream 160. 
Feed stream output 172 is directed to a first feed zone 94' of first 
separator 92', and a first portion of oxygen, which is contained in the 
feed stream output 172 from the second feed zone 166, is driven into first 
permeate zone 96' through first SELIC membrane 98'. Oxygen-depleted 
nitrogen is obtained as product stream 152. 
The first permeate zone 96' is purged with reactive gas stream 112' which 
includes a desired mixture of diluent steam 114' and product nitrogen 
diverted through valve 110' as described above for FIG. 3. If available, a 
suitable external diluent can be used instead of passing product nitrogen 
through valve 110'. In general, the ratio of purge flow to product flow 
ranges from 0.05 to 5. 
A two-stage SELIC membrane system 210 for producing a high-purity product 
such as nitrogen from a feed stream such as air is shown schematically in 
FIG. 5. Preferably, both stages utilize mixed conductor SELIC membranes. 
Air stream 215 is compressed to five to ten bar by an externally powered 
compressor 216 and/or a compressor 218 which is connected to a shaft 220 
driven by an expansion turbine 222. Coolers 224 and 226 lower the 
temperature of air stream 215 to compensate for heat of compression. 
Contaminants such as water and carbon dioxide are removed from compressed 
air stream 215 in prepurifier 228, such as a thermal or pressure swing 
adsorption device or a polymeric membrane device. Decontaminated air 
stream 229 is heated regeneratively in heat exchanger 230 and then 
introduced as a heated feed stream 232 to feed zone 234 of first SELIC 
stage 212. In one construction, approximately 30% to 80%, preferably about 
40% to about 70%, of elemental oxygen present in feed stream 232 is 
transferred by an oxygen partial pressure ratio driving force to permeate 
zone 236 which is at a low total pressure. 
Intermediate retentate stream 238 is directed to feed zone 240 of second 
SELIC stage 214 where substantially all of the remaining elemental oxygen 
is transferred into reaction zone 242. High-purity nitrogen is withdrawn 
as product stream 244, which is passed through heat exchanger 230 to 
become cooled product stream 250. A portion 246, preferably 6% to 9%, of 
product stream 244 is diverted at an intermediate temperature through 
throttle 248 to serve as a low pressure regeneration gas in prepurifier 
228. 
Reactive purge stream 252 enters reaction zone 242 and consumes oxygen to 
lower the oxygen partial pressure and thereby maintain a high partial 
pressure ratio even at the high purity product end of second stage 214. 
Commercial production of nitrogen is achievable even with small-area SELIC 
membranes. 
Approximately 10% to 20% of high-purity product stream 244 is diverted as 
stream 253 through valve or orifice 254 to dilute pressurized reactive gas 
stream 256, such as methane. Preferably, reactive purge stream 252 
contains enough methane to react with all oxygen within reaction zone 242. 
In some cases it may be desirable to have a small amount of excess fuel in 
the second stage 214 to provide some reactive gas in the first stage 212 
to supplement its heating needs and to enhance removal of oxygen. 
Further, reactive purge stream 252 preferably is maintained at a total 
pressure close to, more preferably slightly below, the pressure of stream 
244. Similar total pressures on the feed and permeate sides within second 
SELIC stage 214 decrease mechanical stresses in the SELIC membrane and 
seals, and reduce potential sealing problems encountered during use of 
different high-temperature materials. 
High pressure, low oxygen permeate stream 258 is expanded through turbine 
222 to recover power, such as for driving compressor 218. In one 
construction, turbine 222 is an inexpensive turbocharger that has been 
modified as disclosed in U.S. Pat. No. 5,460,003 (Nenov), incorporated 
herein by reference. In another construction, turbine 222 is replaced by a 
throttling valve, located in stream 260 downstream of heat exchanger 230, 
to lower the pressure of permeate stream 258 at reduced capital costs. 
After expansion, cooled stream 258 becomes low pressure stream 260 which is 
reheated in heat exchanger 230 and is directed to purge the permeate zone 
236 of first SELIC stage 212. Exiting permeate stream 262 is also cooled 
against decontaminated feed air stream 229 and is then discharged to the 
atmosphere. 
Limiting reactive purging to the second stage 214 reduces fuel consumption 
and eases thermal management of system 210. System 210 can be designed to 
have excess heat available from the heat of reaction of the fuel injected 
into the second stage even with turbine expansion so that no other energy 
source is required for heating the air and maintaining the system at a 
desired temperature. 
Thermal management is enhanced in this construction by diverting a portion 
of exhaust stream 258 through valve 270 as a recirculation stream 272. 
Heat is removed at regions 274, 276 within heat exchanger 230 to warm 
expanded stream 260 and to externally remove heat from the second stage 
214; in another construction region 276 is a separate heat sink. To offset 
circuit pressure drop, cooler recirculating stream 278 is brought back to 
second stage purge inlet pressure by a small compressor 280. Cooling of 
the recirculating stream 272 by regions 274, 276 also enables use of a 
less expensive compressor 280. 
Pressurized recirculation stream 282 is then mixed with reactive gas stream 
256 to tailor the temperature and content of reactive purge stream 252. 
For example, if reactive purge stream 252 contains methane slightly above 
the stochiometric requirements of permeate zone 242, some residual 
hydrogen will be present in recirculation stream 282 to ease lighting off 
the reaction near the purge inlet of permeate zone 242. 
EXAMPLE 
Production of high purity N.sub.2 from a nitrogen feed stream containing 2% 
oxygen using reactive purge is quantified below in Table I for a single 
stage SELIC system similar to separator 12, FIG. 1, without the equipment 
shown in phantom. The process pressure and feed flow were established by a 
pressurized source of feed nitrogen. The reactive purge stream was also at 
1.1 atm pressure and consisted of 40% hydrogen and 60% nitrogen simulating 
a blending of hydrogen with a portion of the product nitrogen as a diluent 
purge. The resulting product stream was 99.9999% oxygen-free nitrogen. 
TABLE I 
______________________________________ 
Material One of the materials of A.sub.s A'.sub.t B.sub.u B'.sub.v 
B".sub.w O.sub.x 
family whose composition is disclosed in 
U.S. Pat. 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 
SELIC Area 14 cm.sup.2 
Thickness 0.13 cm 
Process 1.1 atm 
pressure 
Process 1000.degree. C. 
temp. 
Feed flow 750 sccm of (2% O.sub.2 in N.sub.2) 
Purge flow 250 sccm of (40% H.sub.2 in N.sub.2) 
Feed O.sub.2 2% 
conc. 
Product O.sub.2 
&lt;1 ppm 
conc. 
______________________________________ 
It is shown above that efficient processes and apparatus can be designed to 
remove oxygen from a gas stream using as membranes solid oxide 
electrolytes which transport oxygen ions. By employing electrolytes that 
also have significant electronic conductivity (i.e. mixed conductors), the 
separation process can be pressure driven, without a need for electrodes 
and applied electrical voltages. The use of reactive purging, with or 
without vacuum pumping, on the permeate side greatly increases the 
capability and efficiency of the pressure-driven process 
Reactive purging can also permit an ionic conductor with electrodes and an 
external circuit to be used for high-purity retentate production. In such 
a scheme, power can be produced in that stage as a co-product. 
Significant improvements in operation may be achieved by conducting the 
purification process in two or more stages with the successive stages 
operating at lower partial pressures of oxygen on both the feed and 
permeate sides. Progressively lower oxygen partial pressures on the 
permeate side can be created by purging with gas streams containing 
progressively lower oxygen concentrations and/or progressively higher 
quantities or qualities of reactive gas as described above, and/or by 
vacuum pumping to progressively lower pressures. 
For producing high-purity nitrogen from air, for example, the first stage 
preferably removes about 30% to about 80% of oxygen contained in the feed 
stream, and more preferably removes about 40% to about 70% of the oxygen. 
The reactive purge stream preferably is at a lower pressure than that of 
the feed stream, and more preferably is at a slightly lower pressure to 
facilitate sealing and to reduce mechanical stresses. The SELIC membrane 
or membranes in the first stage are selected to achieve high oxygen 
conductivity at relatively high oxygen partial pressures and the SELIC 
membrane or membranes in the second stage are selected for stability at 
relatively low oxygen partial pressure. 
By combining an initial mixed conductor SELIC stage with a subsequent 
ionic-only conductor SELIC stage, the mixed conductor stage removes the 
bulk of the oxygen whereas the ionic conductor stage removes the last 
traces of oxygen to produce a high purity oxygen-free product such as 
nitrogen or argon. 
While diluent streams such as exhaust recirculation stream 282 and product 
purge stream 253, FIG. 5, have been described as preferably being mixed 
together with the reactive gas stream prior to applying the combined purge 
stream through a single inlet to the permeate zone, one or more diluent 
streams may be introduced through separate inlets in other constructions 
according to the present invention. The diluent streams can be mixed with 
the reactive gas during or after introduction of the reactive gas into the 
permeate zone. 
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. Alternative embodiments will be 
recognized by those skilled in the art and are intended to be included 
within the scope of the claims.