Process for the production of high pressure nitrogen

A process is set forth for recovery of nitrogen from a feed gas stream, containing nitrogen and oxygen, using a cryogenic separation wherein a recycle stream having an oxygen content above that of the feed gas stream is recycled from the cryogenic separation to the feed gas stream without any intervening process step that would decrease the oxygen content of the recycle stream.

TECHNICAL FIELD 
The present invention is directed to the cryogenic separation of nitrogen 
from a feed gas stream containing nitrogen and oxygen. More specifically, 
the present invention is directed to recovering high purity nitrogen from 
air using a cryogenic separation with an unexpected efficiency increase 
achieved by appropriate recycle of a process stream. 
BACKGROUND OF THE PRIOR ART 
The use of nitrogen has become increasingly important in various industrial 
and commercial operations. For example, liquid nitrogen is used to freeze 
food, in the cryogenic recycling of tires and as a source of gaseous 
nitrogen for inerting. Gaseous nitrogen is used in applications such as 
secondary oil and gas recoveries and as a blanketing gas in metal 
refineries, metal working operations and chemical processes. In light of 
the increasing importance of nitrogen in such operations, it is desirable 
to provide a process which is both economical and efficient for producing 
nitrogen in the liquid an/or gas phase. 
High purity gaseous nitrogen is produced directly by well known cryogenic 
separation methods. U.S. Pat. No. 4,222,756 teaches a process and 
apparatus for producing gaseous nitrogen using multiple distillation 
columns and associated heat exchangers. Ruhemann and Limb, I. Chem. E. 
Symposium Series No. 79, page 320 (1983) advocate a preference for the use 
of the single distillation column instead of the typical double column for 
the production of gaseous nitrogen. 
Liquid nitrogen is typically produced by initially producing gaseous 
nitrogen in a cryogenic air separation unit and subsequently treating the 
gaseous nitrogen in a liquefier. Modified forms of cryogenic air 
separation units have been developed to directly produce liquid nitrogen. 
U.S. Pat. No. 4,152,130 discloses a method of producing liquid oxygen 
and/or liquid nitrogen. This method comprises providing a substantially 
dry and substantially carbon dioxide-free air stream, cryogenically 
treating the air stream to liquefy a portion of the air stream, and 
subsequently feeding the air stream into a fractionation column to 
separate the nitrogen and oxygen and withdrawing liquid oxygen and/or 
nitrogen from said column. 
Various process cycles using a single distillation column, with some 
boil-up at the bottom provided by the appropriate high pressure fluids, 
have also been suggested in the patent literature, for example, U.S. Pat. 
No. 4,400,188 and U.S. Pat. No. 4,464,188. 
In U.S. Pat. No. 4,595,405 a process for the cryogenic separation of 
nitrogen from air is taught, wherein the cryogenic separation is conducted 
in a single pressure distillation column. The oxygen enriched waste gas 
from the cryogenic separation is rewarmed, compressed to an elevated 
pressure and processed through a selective membrane separation to extract 
oxygen from the waste stream for recovery or removal, while returning a 
nitrogen enriched stream to the feed air to the cryogenic separation. This 
process entails the additional capital outlay for compression and membrane 
separation. It would be logical in that patented process, designed for the 
recovery of nitrogen, to recycle a nitrogen-enriched stream, after 
membrane treatment to remove it predominantly oxygen content, as is 
performed in that patent. 
In many of the cryogenic processes for recovery of nitrogen, the 
oxygen-enriched waste stream is removed from the cryogenic separation zone 
or distillation column and is reduced in pressure with the recovery of 
work in order to produce refrigeration for the feed stream being cooled 
for cryogenic separation. Often, there is more oxygen-enriched waste than 
is necessary to reduce in pressure with the recovery of work for the 
production of refrigeration. All of such waste cannot be processed 
accordingly without creating excess refrigeration. To avoid production of 
excess refrigeration, a portion of the waste stream is merely passed 
through an expansion valve, without the recovery of work, so as to 
minimize refrigeration production. This expansion without the recovery of 
work is a waste of the energy utilized to create the pressurized condition 
of that stream, as well as a waste of the nitrogen content of the stream. 
The present invention overcomes the drawbacks of the prior art in producing 
high purity nitrogen using a cryogenic separation technique, wherein 
efficiencies are derived by the use of recycle and pressure maintenance of 
certain process streams as set forth below. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is a process for the recovery of nitrogen from a feed 
gas stream containing nitrogen and oxygen whereby a pressurized condition 
is retained in an oxygen-enriched recycle process stream, comprising the 
steps of: compressing a feed gas stream containing nitrogen and oxygen to 
an elevated pressure, introducing the elevated pressure feed gas stream 
into a cryogenic separation zone to recover a high purity nitrogen product 
and an oxygen-enriched waste stream from said zone, removing an elevated 
pressure recycle stream having an oxygen content above that of the feed 
gas stream from said cryogenic separation zone, and without any 
intervening process steps to decrease the oxygen content of said recycle 
stream, recycling said stream to the feed gas stream for introduction into 
the cryogenic separation zone. 
Preferably, said feed gas stream is air. Additionally, said elevated 
pressure recycles stream can be at least a portion of said oxygen-enriched 
waste stream. 
The recycle stream can be introduced into said feed gas stream at an 
intermediate level of the compression of said feed gas stream. 
Preferably said feed gas stream, after mixing with the recycle stream and 
performing further compression on the combined feed stream, is pretreated 
to remove water and carbon dioxide. Alternatively, said recycle stream is 
recompressed to said pressure of said elevated pressure feed gas stream 
and said recycle stream is introduced into said feed gas stream downstream 
of said pretreatment. 
Preferably said high purity nitrogen product has a nitrogen content of at 
least 95%. Alternatively, said high purity nitrogen product has a nitrogen 
content of at least 99.5%. 
Preferably, a portion of said oxygen-enriched waste stream is let down in 
pressure across an expander with the recovery of work to produce 
refrigeration for said cryogenic separation zone. Optimally, a second 
portion of said waste stream is recycled as said recycle stream without 
any substantial pressure reduction. 
A preferred embodiment of the present invention is a process for the 
recovery of nitrogen from a feed air stream whereby a pressurized 
condition is retained in a portion of an oxygen-enriched waste stream 
which is recycled, comprising the steps of: compressing a feed air stream 
to an elevated pressure, pretreating said feed air stream to remove water 
and carbon dioxide therefrom, cooling the feed air stream by heat exchange 
against a rewarming process stream, introducing said cooled feed air 
stream into a cryogenic distillation zone, separating said feed air stream 
in said distillation zone into a high purity nitrogen product and an 
oxygen-enriched waste stream having an oxygen content above that of the 
feed air stream, reducing the pressure on a first portion of the said 
waste stream by passage through a turbine expander to produce 
refrigeration for cooling the feed air stream, and recycling a second 
portion of said waste stream to the feed air stream without substantial 
pressure reduction and without any intervening process step to decrease 
the oxygen content of said recycled second portion of said waste stream. 
Preferably, said cryogenic distillation zone has a single pressure stage 
distillation column. Alternatively, the cryogenic distillation zone can 
have multiple pressure stages in the distillation column. 
Preferably, an oxygen-enriched stream is removed from the base of said 
cryogenic distillation zone and is vaporized against a condensing 
nitrogen-rich stream removed from the top of said cryogenic distillation 
zone to produce reflux for said cryogenic distillation zone. 
Alternatively, liquid nitrogen product can be produced from the process of 
the present invention either with or without gaseous nitrogen product. 
Additionally, the high purity nitrogen product can be rewarmed against the 
feed air stream. If needed, a third portion of said waste stream is 
bypassed around said expander and reduced in pressure without the recovery 
of work.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is an efficient means to recover energy from the high 
pressure oxygen-enriched, nitrogen-depleted waste stream produced in a 
nitrogen production cryogenic separation plant. The process provides this 
efficiency by compressing at least a part of the oxygen-enriched waste 
stream and mixing it with the feed gas stream to the cryogenic separation 
plant. Alternatively, the waste stream can be mixed with the feed gas 
stream at an intermediate stage of the feed gas compression and the 
combined streams further compressed to the distillation zone pressure. 
For nitrogen producing cryogenic plants in the size range of 30 to 250 
tons/day (T/D), energy costs and capital-related investment cost play 
approximately equally important roles in the cost of producing nitrogen. 
The present invention increases the energy efficiency of such plants by 
8-9% with very minimal increases in capital investment. 
In nitrogen producing cryogenic air separation plants of the above size 
range, nitrogen is typically produced at elevated pressure from air by 
cryogenic distillation in a single distillation column operating at a 
single elevated pressure. When nitrogen is produced at high pressure. the 
oxygen-enriched waste stream from the column is also required to be 
produced at an elevated pressure greater than ambient pressure but less 
than the final feed gas pressure. This waste stream is expanded across an 
expander with recovery of work to provide refrigeration for the cryogenic 
facility. However, a large fraction of this gas is reduce din pressure 
across an expander bypass valve (J. T. valve) without the recovery of work 
to avoid producing excess refrigeration. This is an inefficient step from 
the perspective of energy utilization. In contrast, the present invention 
decreases the flow of the waste stream through the expander bypass valve. 
Instead, some of this elevated pressure, oxygen-enriched waste stream at a 
pressure intermediate of ambient and final feed gas pressure is brought 
out of the cryogenic separation facility or cold box and mixed with the 
feed air stream to form a combined oxygen-enriched feed to the cryogenic 
separation zone. This allows the recovery of some of the pressure energy 
and the nitrogen content in the oxygen-enriched, nitrogen-depleted waste 
stream. The present invention accomplishes this goal by compressing at 
least a part of this waste stream and mixing it with the feed gas stream 
to provide a combined feed gas stream of feed air and recycle gas to the 
cryogenic separation. The oxygen-enriched waste stream should be at a 
pressure greater than ambient prior to compression and mixing with the 
feed stream to the cryogenic separation. In a preferred mode of operation, 
and in order to minimize capital investment, the elevated pressure 
oxygen-enriched waste stream which is being recycled is fed to a suitable 
intermediate stage of the main feed gas compressor, and the combined feed 
gas stream resulting therefrom is then further compressed to form the feed 
gas stream to the cryogenic separation. Even though mixing of the 
oxygen-enriched waste stream with the feed gas stream leads to 
irreversible thermodynamic losses due to the oxygen enrichment of the 
total feed gas to the cryogenic separating zone, this improvement is 
beneficial to the present invention, resulting in: greater overall 
efficiency, improvements in overall nitrogen recovery based upon the fresh 
feed to the cryogenic separation zone, and minimization of capital 
requirements. 
The main aspects of the present invention can be briefly described with 
reference to FIG. 1. A feed gas stream containing nitrogen and oxygen, 
preferably air, is introduced in line 10 to a main feed gas compressor 12 
which typically has several stage of compression with intercooling. One 
such intermediate stage is identified as 30. The feed gas stream is mixed 
with a recycle stream to provide a combined feed gas stream. The feed gas 
stream at elevated pressure in line 14 is then pretreated in a 
pretreatment zone 16 to remove water, carbon dioxide and any hydrocarbons 
existing in the feed gas stream. These materials are removed in line 18. 
Typical pretreatment plants can include water chilling, refrigeration with 
a halofluorocarbon, such as a FREON refrigerant, as well as adsorption of 
residual materials on switching beds of molecular sieve material, all of 
which techniques are well documented in the prior art and require no 
specific disclosure herein. 
The feed gas stream, at elevated pressure after pretreatment, is introduced 
in line 20 to a cryogenic separation zone 22. The cryogenic separation 
zone typically includes main and auxiliary heat exchangers wherein the 
feed gas stream is cooled close to its dew point by indirect heat exchange 
with rewarming process streams, as well as a distillation column, which 
may be single or multiple pressure stage in configuration. A nitrogen 
product is removed in line 24 and can comprise gaseous nitrogen, and/or a 
separately recovered product of liquefied nitrogen. A waste stream 
comprising an oxygen-enriched gas is removed in line 26. Specifically, 
with regard to the present invention, an oxygen-enriched, 
nitrogen-depleted stream is removed from the cryogenic separation zone 22 
in line 28 under pressurized conditions below the final pressure of the 
feed gas stream but above ambient pressure, and without any intervening 
process steps to reduce its oxygen content, such stream is recycled to be 
mixed with the feed gas stream. The composition of this recycle stream 28 
may or may not be the same as that of the waste stream in line 26, and its 
oxygen content will be above that of air. 
FIG. 1 illustrates the recycle to an intermediate stage 30 of the main gas 
compressor 12. For this purpose, it may be necessary to reduce the 
pressure or boost the pressure of stream 28 to match the pressure of the 
intermediate stage 30 of the compressor 12. Power for this compressor can 
be derived from the expansion in an expander of the oxygenenriched waste. 
Alternatively to FIG. 1, the recycle stream 28 may be recycled with 
additional separate recompression to a pressure condition allowing the 
recycle to be introduced into line 20. This provides the benefit of 
down-sizing the pretreatment facility 16, while incurring the cost of 
separate recompression equipment in line 28. 
The advantage of performing the process as illustrated in FIG. 1 is that 
the oxygen-enriched stream of line 28 would traditionally be reduced in 
pressure either for refrigeration or through a bypass JT valve in the 
prior art during the process of removal of such a waste stream in a 
nitrogen generating process. The present invention retains the pressurized 
condition of the oxygen-enriched recycle stream in line 28 and blends the 
same with the feed gas stream to the separation zone 22 despite the fact 
that the recycle is oxygen enriched and the feed gas stream is introduced 
to the cryogenic separation zone 22 to produce nitrogen product. The water 
stream in line 26 may also constitute a desirable product stream if oxygen 
concentrations meet and use applications. 
The unexpected results of the present invention are that the recovery of 
the minor amount of nitrogen in the recycle stream 28 and the recoupment 
of the pressurized condition of such stream provides efficiencies which 
overcome the thermodynamic inefficiency of recycling an oxygen-enriched 
stream to the feed gas into a nitrogen producing process. In summary, the 
inefficiency of mixing the oxygen-enriched recycle with the feed gas 
stream is less than the inefficiency of reducing its pressure across a 
valve and venting to atmosphere. 
The present invention will now be described with reference to a preferred 
detailed embodiment illustrated in FIG. 2. A feed air stream 210 is 
introduced into a multistage main air compressor 212 and elevated in 
pressure to approximately 124.4 psia in line 214. A recycle stream 228 is 
introduced in one of the intermediate stages of the compressor to provide 
the combined feed gas stream. The combined feed gas stream is cooled by 
indirect heat exchange with cooling water in aftercooler 213. The feed gas 
stream is further cooled in a refrigerated heat exchanger 215 to condense 
water, which is removed in phase separation vessel 217. Residual water and 
carbon dioxide, as well as trace hydrocarbons, are removed from the feed 
gas stream in a mole sieve switching bed adsorption system 219, wherein 
the feed is passed through one parallel bed until regeneration is required 
and then the feed is switched to pass through the other bed while 
regeneration occurs. Such a switching adsorptive clean-up is well known in 
the art and does not require greater elaboration. The aftercooler 213, the 
refrigerated cooler 215, the phase separation vessel 217 and the switching 
adsorptive beds 219 collectively constitute a pretreatment stage 216. 
The elevated pressure, clean and dry feed gas stream in line 220 is then 
introduced into the main heat exchanger 233 to be cooled against rewarming 
gaseous nitrogen, a recycle stream and a waste stream. The cooled feed gas 
stream at -269.degree. F. is introduced in line 225 into a single pressure 
stage distillation column 227 which is constructed with the appropriate 
traditional trays for countercurrent rectification. Vapor which is slowly 
enriching in nitrogen ascends the column 227, while liquid slowly 
enriching in oxygen descends the column. An oxygen-enriched stream is 
removed from the base of the column 227 in line 237 and reduced in 
pressure through valve 239 before being introduced to the overhead of the 
column to provide cooling by indirect heat exchange in a 
boiling/condensing heat exchanger 231. Vaporous nitrogen enriched gas 
passes from the distillation column 227 overheads into the heat exchanger 
231 and is condensed against the rewarming oxygen-enriched gas and is 
returned as liquid for reflux in line 233 and as a liquid nitrogen product 
(LIN) in line 235. The remaining vaporous nitrogen having a high purity of 
at least 95%, and preferably at least 99.5%, is removed in line 229 and 
rewarmed in the main heat exchanger 223 against the feed gas stream in 
line 220. The high purity rewarmed nitrogen gas (GAN) is removed as a 
product at a pressure of 115 psia in line 224. 
The rewarmed oxygen-enriched gas from the overhead boiling/condensing heat 
exchanger 231 is removed in line 243 at a pressure of 53 psia and 
-283.4.degree. F. This stream is utilized to produce the refrigeration for 
the cryogenic separation. To achieve this refrigeration, a first portion 
of the waste stream in line 243 is removed in line 245 for pressure 
reduction. The remaining waste stream in line 247 is partially rewarmed in 
the main heat exchanger 223 before some of the remaining waste is 
separated in line 249 for combination with the first portion in line 245, 
which is combined in line 251. Most of the waste stream in line 251 is 
reduced in pressure with the recovery of work by expanding in an expander 
turbine 257 resulting in significant cooling of the resulting low pressure 
gas. A third portion of the waste gas stream in line 253 is bypassed 
around the expander turbine 257 and is reduced in pressure without 
recovery of work in a bypass valve operating with the Joule-Thompson 
effect identified as 255. This bypassed third portion of the waste stream 
is reduced in pressure without recovery of work in order to avoid excess 
refrigeration and is combined with the turbine-expanded waste stream in 
line 259. This waste stream in line 259 comprises the main refrigeration 
source in the main heat exchanger 223, wherein the gas is rewarmed against 
the cooling feed gas stream in line 220. The low pressure oxygen-enriched 
waste gas stream is removed in line 226 and vented. A portion of this 
stream 226 can be used to regenerate molecular sieve pretreatment beds if 
they are included in the facility. Stream 226 could also be a useful 
product if its oxygen content is appropriate for end use applications. 
A second portion of the oxygen-enriched waste gas stream is diverted around 
the pressure reduction valve 255 and expander turbine 257 and without any 
further process steps, such as membrane separation which would affect or 
specifically decrease the oxygen content of the gas, is recycled in line 
228 back to the feed air stream. The recycle stream can be introduced into 
an intermediate stage 230 of the multistage main air compressor 212, so as 
to recoup its pressure value and its nitrogen value. 
Although it would appear inconsistent in a nitrogen recovery cryogenic 
separation to introduce an oxygen-enriched stream into the feed air stream 
to the separation, it has been unexpectedly found by the present inventors 
that the recited recycle reduces the relative power requirements of the 
process over a cycle with no recycle and actually increases the recovery 
of nitrogen based upon fresh air feed to the overall process. This is 
based upon the fact that the extent of recycle only diminishes the 
nitrogen content of the feed 225 going to the distillation column 227 to 
result in a nitrogen entry concentration of 72%. If only air were fed to 
the distillation column, the nitrogen concentration would be 79%. This 
inefficiency of performing the recycle is found to be less than the 
inefficiency of reducing the pressure of the recycle stream across the JT 
valve 255 and venting that stream as a waste stream. This advantage is 
manifested in the relationship between the distillation column 227, the 
refrigeration source 255 and 257, and the main heat exchanger 223, all of 
which make up the cryogenic separation zone or cold box 222. 
In order to demonstrate the value of performing a recycle of even an 
oxygen-enriched waste gas stream to the feed air stream, the following 
comparison of the prior art without recycle is made with three embodiments 
of the present invention utilizing such a recycle. 
EXAMPLE 1 
Calculations were done by computer simulation of a process as shown in FIG. 
2 wherein no recycle in line 228 was performed and some of the waste gas 
is expanded across expander 257 and the remaining waste gas is passed 
through the bypass valve 255. The inefficiency herein is due to the gas 
required to be passed through the bypass valve 255 without recoupment of 
energy and which is thereafter merely vented from the process. The 
calculation produced 87 T/D of gaseous nitrogen at 115 psia and 1.7 T/D of 
liquid nitrogen. The ambient conditions used where 14.7 psia, 70.degree. 
F., and 50% relative humidity. Some of the pertinent results are 
illustrated in Table 1 below. It is seen that a large flow (about 32% of 
the feed air) bypasses the expansion turbine and the amount of nitrogen 
recovered relative to the total nitrogen feed air is 52.8%. 
EXAMPLE 2 
In this example, computer simulation calculations were done according to 
the present invention as embodied in the process shown in FIG. 2. These 
examples included the recycle of a portion of the waste stream in line 228 
without any attempt to decrease the oxygen enriched character of the 
stream. The product mix and conditions were the same as those given for 
Example 1 above. In this process, the amount of the recycle stream 228 can 
be controlled. When a smaller amount is recycled, a larger amount of flow 
is expanded across the expander bypass valve and vice versa. The 
concentration of oxygen in the main air compressor discharge is also 
dependent on the recycle flow. The concentration of oxygen increases with 
an increase in the recycle flow and decreases with a decrease in the 
recycle flow. The following three cases were performed for different 
recycle flows by computer simulation under Example 2 as set forth below. 
CASE I 
In this embodiment, 109 pound moles/hr of the oxygen-enriched, 
nitrogen-depleted waste gas stream containing about 40% oxygen is recycled 
to the main air compressor. The flow through the expander bypass valve is 
now 107 pound moles/hr as compared to 203 pound moles/hr for Example 1 
(see table 1). Due to this recycle flow, the amount of feed air flow is 
decreased to 551 pound moles/hr. The concentration of oxygen in the stream 
at the discharge of the main air compressor is 24%. High concentrations of 
oxygen present safety hazards and require expensive compressors, whereas 
the concentration of 24% oxygen allows the use of a less expensive 
apparatus. In comparison, for fresh air feed, the oxygen concentration on 
a dry basis would be 21%. As seen from Table 1, the power consumed by this 
Case I of Example 2 is about 6.5% lower than the calculated power for 
Example 1 without a recycle. This is substantial savings in power using 
the recycle, especially considering the fact that the additional capital 
investment for Case I of this Example 2 is minimal with regard to the 
requirements of Example 1. 
CASE II 
The recycle flow of the oxygen-enriched, nitrogen-depleted waste stream in 
this case is increased to 156 pound moles/hr. It has about 42.6% oxygen 
and the concentration of oxygen in the main air compressor discharge is 
26%. The process details for this Case II are set forth in Table 1. The 
flow through the expander bypass valve is further reduced to 75 pound 
moles/hr. A remarkable power savings of about 8% is observed for this Case 
II over the Example 1 case using no recycle. 
CASE III 
When the recycle flow is further increased to 221 pound moles/hr, the 
concentration of oxygen in the main air compressor discharge increases to 
29%. The expander bypass flow decreases to 39 pound moles/hr. There is 
some more energy savings for this Case III; however, as compared to Case 
II, it is a small increase. The overall energy savings as compared to that 
calculated for Example 1 above is 8.5% for this Case III. 
The energy savings for all three cases of Example 2, demonstrating the 
recycle of the present invention as illustrated in FIG. 2, are quite 
encouraging and prove the applicability of the present invention. The 
results of the efficiency comparison with recycle versus non-recycle as 
set forth in Table 1 below: 
TABLE 1 
______________________________________ 
Pertinent Calculation Results for Examples 1 & 2 
Product: 87 T/D GAN at 115 psia 
1.7 T/D LIN 
Example 2 
Example 1 
Case I Case II Case III 
______________________________________ 
Oxygen in Feed to Cold 
21 24 26 29 
Box (%) 
(Stream 20 or 225) 
Oxygen in Waste (%) 
35.6 40.1 42.6 46.4 
(Stream 26 or 226) 
Recycle Stream Flow 
-- 109 156 221 
(lb moles/hr) 
(Stream 28 or 228) 
Expander Bypass Flow 
203 107 75 39 
(lb moles/hr) 
(Stream 253) 
Feed Air Flow 639 551 518 480 
(lb moles/hr) 
(Stream 10 or 210) 
N.sub.2 Recovery from Air 
52.8 61.3 65.2 70.3 
Feed (%) 
Relative Power 
1.0 0.935 0.919 0.913 
______________________________________ 
The prior art processes which fail to use a recycle stream are a tradeoff 
between capital and energy costs. In a plant size in the range of 30 to 
250 T/D of nitrogen contained in the product gas, any process is designed 
to minimize the number of equipment items of significant capital cost. As 
a result, in order to produce high pressure, gaseous nitrogen product, no 
gaseous nitrogen compressor is used. Also, in certain applications, due to 
the possibility of contamination of the gaseous nitrogen, it is not 
advisable to use a product compressor on ultra high purity nitrogen from 
the cryogenic separation zone. Either of these considerations leads to a 
process with significant energy losses, since a substantial amount of 
oxygen-enriched waste gas must be expanded across a bypass valve, to the 
exclusion of any recycle without substantial pressure reduction. In 
contrast, the present invention provides a scheme to limit the amount of 
gas expanded across this valve, without significant additional capital 
requirements, such as the membrane used in the prior art, which nitrogen 
enriches the waste which is recycles. Instead, the present invention is 
designed to take a significant fraction of the oxygen-enriched waste gas 
out of the cryogenic separation zone at a high pressure and mixes this gas 
with feed gas stream at a suitable stage either in the main feed gas 
compressor or downstream of the feed gas stream pretreatment zone. This 
allows the process of the present invention to take advantage of reduced 
power requirements, lower capital costs, and increased recovery in 
comparison to the prior art. 
The scope of the present invention should be ascertained from the claims 
which follow.