U.S. patent application number 17/229910 was filed with the patent office on 2021-12-02 for enhancements to a dual column nitrogen producing cryogenic air separation unit.
The applicant listed for this patent is Neil M. Prosser, Zhengrong Xu. Invention is credited to Neil M. Prosser, Zhengrong Xu.
Application Number | 20210372696 17/229910 |
Document ID | / |
Family ID | 1000005571355 |
Filed Date | 2021-12-02 |
United States Patent
Application |
20210372696 |
Kind Code |
A1 |
Xu; Zhengrong ; et
al. |
December 2, 2021 |
ENHANCEMENTS TO A DUAL COLUMN NITROGEN PRODUCING CRYOGENIC AIR
SEPARATION UNIT
Abstract
Enhancements to a dual column, nitrogen producing cryogenic air
separation unit are provided. Such enhancements include an improved
air separation cycle that uses three condenser-reboilers and
recycles a portion of the vapor from one or more of the
condenser-reboilers to the incoming feed stream and or the
compressed purified air streams to yield improvements in the
performance of such dual column, nitrogen producing cryogenic air
separation units in terms of overall nitrogen recovery as well as
power consumption.
Inventors: |
Xu; Zhengrong; (East
Amherst, NY) ; Prosser; Neil M.; (Lockport,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Xu; Zhengrong
Prosser; Neil M. |
East Amherst
Lockport |
NY
NY |
US
US |
|
|
Family ID: |
1000005571355 |
Appl. No.: |
17/229910 |
Filed: |
April 14, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63029909 |
May 26, 2020 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 3/0257 20130101;
F25J 2200/34 20130101; F25J 2280/10 20130101; F25J 3/04084
20130101; F25J 2210/40 20130101; F25J 5/005 20130101; F25J 1/0047
20130101; F25J 1/0012 20130101; F25J 2250/00 20130101; F25J 3/04763
20130101; F25J 2200/04 20130101 |
International
Class: |
F25J 3/02 20060101
F25J003/02; F25J 1/00 20060101 F25J001/00; F25J 3/04 20060101
F25J003/04; F25J 5/00 20060101 F25J005/00 |
Claims
1. An air separation unit comprising: a main air compression system
configured for receiving a stream of incoming feed air and
producing a compressed air stream; an adsorption based pre-purifier
unit configured for removing impurities from the compressed air
stream and producing a compressed, purified air stream; a main heat
exchange system configured to cool the compressed and purified air
stream to temperatures suitable for fractional distillation; a
distillation column system comprises a higher pressure column and a
lower pressure column linked in a heat transfer relationship via a
first condenser-reboiler; wherein the higher pressure column is
configured to receive the cooled, compressed, purified air stream
and produce a nitrogen enriched overhead and an oxygen-enriched
kettle stream; wherein the lower pressure column is configured and
produce a lower pressure nitrogen product stream, an overhead
stream and an oxygen-enriched bottoms; wherein the first
condenser-reboiler is configured to condense a first portion of the
nitrogen enriched overhead from the higher pressure column against
the oxygen-enriched bottoms from the lower pressure column to
produce a nitrogen reflux stream for the higher pressure column and
an ascending vapor stream in the lower pressure column from the
boil-off of the oxygen-enriched bottoms; wherein the distillation
column system further comprises a second condenser-reboiler
operatively associated with the higher pressure column and
configured to condense a second portion of the nitrogen enriched
overhead from the higher pressure column against a first split
portion of the oxygen-enriched kettle stream from the higher
pressure column to produce a liquid nitrogen stream and a recycle
stream from the boil-off of the oxygen-enriched kettle stream;
wherein a second split portion of the oxygen-enriched kettle stream
is introduced into the lower pressure column at an intermediate
location; wherein a third portion of the nitrogen enriched overhead
from the higher pressure column is taken as a medium/high nitrogen
product stream; wherein the distillation column system further
comprises a third condenser-reboiler operatively associated with
the lower pressure column and configured to condense the nitrogen
overhead from the lower pressure column against the oxygen bottoms
from the lower pressure column to produce a nitrogen reflux stream
for the lower pressure column and a waste stream; and wherein the
recycle stream is recycled to: (i) the main air compression system
and combined with the incoming feed air stream; (ii) a location
upstream of the main heat exchange system and combined with the
compressed, pre-purified air stream; or (iii) to the main heat
exchanger system.
2. The air separation unit of claim 1 further comprising an upper
column turbine circuit having a booster compressor and a turbine;
and wherein a portion of the compressed, purified air stream is
diverted to the upper column turbine circuit as a turbine air
stream and the booster compressor is configured for further
compressing the turbine air stream; turbine air stream main heat
exchange system is further configured for partially cooling the
further compressed turbine air stream; and wherein the turbine is
configured to expand the partially cooled turbine air stream to
produce an exhaust stream that is introduced into the lower
pressure column
3. The air separation unit of claim 1 further comprising: a recycle
compressor configured to compress the recycle stream and direct the
compressed recycle stream to an inter-stage location of the main
air compression system; and the main heat exchange system is
configured to cool the compressed, purified air in part via
indirect heat exchange with the recycle stream.
4. The air separation unit of claim 1 further comprising: a recycle
compressor configured to compress the recycle stream and direct the
compressed recycle stream the location upstream of the main heat
exchange system where the compressed recycle stream is combined
with the compressed, pre-purified air stream; and the main heat
exchange system is configured to cool the compressed, purified air
in part via indirect heat exchange with the recycle stream.
5. The air separation unit of claim 1 further comprising: a recycle
compressor configured to compress the recycle stream and direct the
compressed recycle stream to the main heat exchange system where
the compressed recycle stream is further cooled; and wherein the
further cooled recycle stream is introduced into the higher
pressure column of the distillation column system.
6. The air separation unit of claim 5 wherein the recycle
compressor is a cold compressor driven by a booster loaded turbine
and wherein the booster loaded turbine is configured to expand a
diverted portion of the medium/high nitrogen product stream to
produce an exhaust stream from the booster loaded turbine that is
combined with the lower pressure nitrogen product stream.
7. The air separation unit of claim 1 wherein the liquid nitrogen
stream is reintroduced to the higher pressure column with the
nitrogen reflux stream for the higher pressure column.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
provisional patent application Ser. No. 63/029,909 filed May 26,
2020 the disclosure of which is incorporated by reference.
TECHNICAL FIELD
[0002] The present inventions relates to enhancements to a dual
column, nitrogen producing cryogenic air separation unit, and more
particularly to improvements in the performance of such dual
column, nitrogen producing air separation units in terms of overall
nitrogen recovery as well as power consumption. The performance
improvements are generally attributable to an enhanced air
separation cycle that uses three condenser-reboilers and recycles a
portion of the vapor from one or more of the condenser-reboilers to
the incoming feed stream and or the compressed, purified air
streams.
BACKGROUND
[0003] Industrial gas customers often seek nitrogen product slates
at volumes and pressures that typically require very large
cryogenic air separation units. Such large scale or high volume
nitrogen producing air separation units often use a dual
distillation column arrangement, including a higher pressure column
and a lower pressure column in which gaseous nitrogen products are
withdrawn from the distillation columns at relatively high
pressures or at two different pressures. In the conventional dual
column nitrogen producing air separation unit, the higher pressure
column and lower pressure column are thermally linked in a heat
transfer relationship by a main condenser, which liquefies a
portion of the nitrogen-enriched vapor from the overhead of the
higher pressure column to be used as reflux to the higher pressure
column. Supplemental refrigeration for such conventional nitrogen
producing air separation cycles is typically provided via an upper
column turbine arrangement. An example of a large volume nitrogen
producing air separation unit is disclosed in U.S. Pat. No.
4,453,957.
[0004] Over the course of the past several decades numerous
improvements to such large volume nitrogen producing cryogenic air
separation units have been developed to address shortcomings in the
performance of such large-scale nitrogen producing air separation
cycles.
[0005] For example, U.S. Pat. No. 5,098,457 discloses a double
distillation column arrangement for large volume nitrogen
production where the main condenser is not driven by reboiling a
portion of the lower pressure bottoms liquid, but rather the main
condenser is driven by a portion of the kettle liquid from the
higher pressure column. More specifically, U.S. Pat. No. 5,098,457
discloses a split kettle arrangement wherein a portion of the
kettle liquid from the higher pressure column is re-boiled in the
main condenser and another portion of the kettle liquid from the
higher pressure column is directed to an intermediate location on
the lower pressure column.
[0006] U.S. Pat. No. 6,330,812 discloses another double
distillation column arrangement for large volume nitrogen
production that employs three condenser-reboilers including a
double main condenser configuration where both main condensers are
driven by reboiling kettle liquid from the higher pressure column
while the third condenser-reboiler associated with the lower
pressure column is driven by the oxygen-enriched liquid taken from
the bottom of the lower pressure column.
[0007] Finally, U.S. Pat. No. 6,257,019 discloses a triple
distillation column arrangement for large volume nitrogen
production. In addition to the conventional lower pressure
distillation column and higher pressure distillation column each
with a separate condenser-reboiler, the triple distillation column
arrangement also utilizes an intermediate pressure distillation
column and a third condenser operatively associated with the
intermediate pressure distillation column. The triple distillation
column arrangement is believed to demonstrate very high nitrogen
recoveries at comparatively lower power consumption levels.
However, a key disadvantage to the triple distillation column
arrangement is the higher capital costs associated with the
additional column, the third condenser/reboiler, and additional
compressors needed for the intermediate pressure column feed.
[0008] Other improvements to such large volume nitrogen producing
air separation units have been employed in applications requiring a
portion of the nitrogen to be provided as liquid nitrogen. In
applications where is no need or desire to make a liquid nitrogen
product from the air separation unit, the upper column turbine
arrangement disclosed in U.S. Pat. No. 4,453,957 is adequate.
However, in end-user applications where a liquid nitrogen product
is required or desired, employing the conventional upper column
turbine arrangement is economically impractical, as the arrangement
leads to high liquefaction unit power costs and unworkable
rangeability requirements. Such previous improvements included
arrangements that employ a lower column turbine arrangement, a
waste gas expansion arrangement, a nitrogen product expansion and
recycle arrangement, and a warm recycle turbine refrigeration
arrangement.
[0009] What is needed are further enhancements to such large-scale
nitrogen producing cryogenic air separation units to improve
nitrogen recovery and/or reduce the associated operating costs
(i.e. power costs) over the above-identified prior art systems and
previously disclosed improvements thereto.
SUMMARY OF THE INVENTION
[0010] The present invention may be characterized as an air
separation unit comprising: (a) a main air compression system
configured for receiving a stream of incoming feed air and
producing a compressed air stream; (b) an adsorption based
pre-purifier unit configured for removing impurities from the
compressed air stream and producing a compressed, purified air
stream; (c) a main heat exchange system configured to cool the
compressed and purified air stream to temperatures suitable for
fractional distillation; and (d) a distillation column system
comprises a higher pressure column and a lower pressure column
linked in a heat transfer relationship via at least three
condenser-reboilers. The distillation column system produces a
lower pressure nitrogen product stream, a medium/high pressure
nitrogen product stream, a waste stream and a recycle that is a
portion of the vapor from one or more of the condenser-reboilers
that is recycled to the incoming feed air stream and or the
compressed, purified air stream.
[0011] The higher pressure column is configured to receive the
cooled, compressed, purified air stream and produce a nitrogen
enriched overhead and an oxygen-enriched kettle stream while the
lower pressure column is configured and produce a lower pressure
nitrogen product stream, an overhead stream and an oxygen-enriched
bottoms. The first main condenser-reboiler of the three
condenser-reboilers is configured to condense a first portion of
the nitrogen enriched overhead from the higher pressure column
against the oxygen-enriched bottoms from the lower pressure column
to produce a nitrogen reflux stream for the higher pressure column
and an ascending vapor stream in the lower pressure column from the
boil-off of the oxygen-enriched bottoms. The second
condenser-reboiler is operatively associated with the higher
pressure column and configured to condense a second portion of the
nitrogen enriched overhead from the higher pressure column against
a first split portion of the oxygen-enriched kettle stream from the
higher pressure column to produce a liquid nitrogen stream and a
recycle stream from the boil-off of the oxygen-enriched kettle
stream. The third condenser-reboiler is operatively associated with
the lower pressure column and configured to condense the nitrogen
overhead from the lower pressure column against the oxygen bottoms
from the lower pressure column to produce a nitrogen reflux stream
for the lower pressure column and a waste stream. In addition, a
second split portion of the oxygen-enriched kettle stream is
introduced into the lower pressure column at an intermediate
location while a third portion of the nitrogen enriched overhead
from the higher pressure column is taken as a medium/high nitrogen
product stream.
[0012] In some embodiments, the recycle stream is compressed in a
recycle compressor and recycled back to the main air compression
system, preferably to an inter-stage location of the main air
compressor while in other embodiments the recycle stream is
compressed in a recycle compressor and recycled back to and/or
combined with the compressed, pre-purified air stream. Still other
embodiments contemplate the recycle compressor configured as a cold
compressor driven by a booster loaded turbine configured to expand
a diverted portion of the medium/high nitrogen product stream to
produce an exhaust stream from the booster loaded turbine that is
combined with the lower pressure nitrogen product stream. In all
embodiments, refrigeration is preferably supplied to the air
separation unit by use of an upper column turbine arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] While the present invention concludes with claims distinctly
pointing out the subject matter that Applicants regard as their
invention, it may be better understood when taken in connection
with the accompanying drawings in which:
[0014] FIG. 1 is a schematic process flow diagram of an embodiment
of a dual column, nitrogen producing cryogenic air separation unit
in accordance with an embodiment of the present invention;
[0015] FIG. 2 is a schematic process flow diagram of another
embodiment of the present dual column, nitrogen producing cryogenic
air separation unit; and
[0016] FIG. 3 is a schematic process flow diagram of yet another
embodiment of the present dual column, nitrogen producing cryogenic
air separation unit.
DETAILED DESCRIPTION
[0017] As discussed in more detail below, the disclosed cryogenic
air separation systems and methods provide certain performance
enhancements to large-scale, dual column, nitrogen producing
cryogenic air separation units targeted to increase nitrogen
recovery and reduce power consumption compared to prior art
large-scale, dual column, nitrogen producing cryogenic air
separation units.
[0018] Turning to FIG. 1, there is shown a schematic illustration
of the large volume, nitrogen producing cryogenic air separation
unit 10. In a broad sense, the depicted air separation unit
includes a main feed air compression train or system, a turbine air
circuit, a main heat exchange system, and a distillation column
system.
[0019] In the main feed compression train shown in FIG. 1, the
incoming feed air 22 is typically drawn through an air suction
filter house and is compressed in a multi-stage, intercooled main
air compressor arrangement 24 to a pressure that can be between
about 6.5 bar(a) and about 11 bar(a). This main air compressor
arrangement 24 may include integrally geared compressor stages or a
direct drive compressor stages, arranged in series or in parallel.
The compressed air stream 26 exiting the main air compressor
arrangement 24 is fed to a pre-purification unit 28 to remove
impurities including high boiling contaminants. The
pre-purification unit 28, as is well known in the art, typically
contains two beds of alumina and/or molecular sieve operating in
accordance with a temperature swing adsorption cycle in which
moisture and other impurities, such as carbon dioxide, water vapor
and hydrocarbons, are adsorbed. One or more additional layers of
catalysts and adsorbents may be included in the pre-purification
unit 28 to remove other impurities such as carbon monoxide, carbon
dioxide and hydrogen to produce the compressed, purified air stream
29. Particulates may be removed from the feed air in a dust filter
disposed upstream or downstream of the pre-purification unit
28.
[0020] As shown in FIG. 1, the compressed, purified air stream 29
may be split into a plurality of air streams, including a turbine
air stream 31 and a compressed, purified feed air stream 33.
Turbine air stream 31 may be further compressed in a turbine air
booster compressor 37 and subsequently cooled in an aftercooler 39
to form a boosted pressure turbine air stream which is then
partially directed to the main heat exchange system which includes
heat exchanger 52A where it is partially cooled. The partially
cooled, boosted pressure turbine air stream 38 exits heat exchanger
52A and is expanded in turbine 35 to produce exhaust stream 64 that
is directed to lower pressure column 74. In this manner, a portion
of the refrigeration for the air separation unit 10 is thus
provided by the expansion of the turbine air stream 38 in turbine
35. The remaining portion of the compressed, purified feed air
stream 33 is fully cooled in heat exchangers 52A and 52B and exits
the cold end of heat exchanger 52B as a fully cooled air stream 47.
The fully cooled air stream 47 is introduced into higher pressure
column 72 at a location proximate the bottom of the higher pressure
column 72.
[0021] Cooling the compressed, purified feed air stream 33 and
partially cooling the boosted pressure turbine air stream in the
heat exchangers 52A and 52B is preferably accomplished by way of
indirect heat exchange with the warming streams which include the
medium/high pressure nitrogen product stream 105, the lower
pressure nitrogen product stream 110 and a recycle stream 100 from
the distillation column system to produce cooled air streams
suitable for rectification in the distillation column system.
[0022] The heat exchangers 52A and 52B are preferably brazed
aluminum plate-fin type heat exchangers. Such heat exchangers are
advantageous due to their compact design, high heat transfer rates
and their ability to process multiple streams. They are
manufactured as fully brazed and welded pressure vessels. For
larger air separation units handling higher flows, the heat
exchanger may be constructed from several cores which must be
generally connected in series as illustrated in the drawings.
[0023] The turbine based refrigeration circuit used in cryogenic
air separation units are often referred to as either a lower column
turbine (LCT) arrangement or an upper column turbine (UCT)
arrangement which are used to provide refrigeration to a cryogenic
air distillation column systems. In the UCT arrangements shown in
FIGS. 1-3, the boosted turbine air stream is preferably at a
pressure in the range from between about 6 bar(a) to about 10.7
bar(a) and partially cooled to a temperature in a range of between
about 140 K and about 220 K. This cooled, compressed turbine air
stream that is introduced into the turbine to produce an expanded,
cold exhaust stream 64 that is then introduced into the lower
pressure column of the distillation column system. The supplemental
refrigeration created by the expansion of the turbine air stream is
thus imparted directly to the lower pressure column thereby
alleviating some of the cooling duty of the main heat exchanger. In
some embodiments, the turbine may be coupled with a compressor,
either directly or by appropriate gearing.
[0024] While the turbine based refrigeration circuit illustrated in
the FIGS. 1-3 is shown as an upper column turbine (UCT) circuit
where the turbine exhaust stream 64 is directed to the lower
pressure column, it is contemplated that the turbine based
refrigeration circuit alternatively may be a lower column turbine
(LCT) circuit or a partial lower column (PLCT) where the expanded
exhaust stream is fed to the higher pressure column of the
distillation column system.
[0025] The illustrated distillation column system includes a higher
pressure column 72, a lower pressure column 74, a first main
condenser-reboiler 75, a second condenser-reboiler 85 and a third
condenser-reboiler 95. The higher pressure column 72 typically
operates in the range from between about 7 bar(a) to about 12
bar(a) whereas lower pressure column 74 operates at pressures
between about 4.5 bar(a) to about 7 bar(a). Cooled feed air stream
47 is preferably a vapor air stream slightly above its dew point,
although it may be at or slightly below its dew point, that is fed
into the higher pressure column 72 for rectification resulting from
mass transfer between an ascending vapor phase and a descending
liquid phase that is initiated by a nitrogen based reflux stream.
This separation process within the higher pressure column 72
produces a nitrogen-rich column overhead 89 and crude
oxygen-enriched bottoms liquid also known as kettle liquid 80 which
is taken as kettle stream 88.
[0026] The higher pressure column 72 and the lower pressure column
74 are preferably linked in a heat transfer relationship via the
first main condenser-reboiler 75 wherein a first portion 73 of the
nitrogen-rich vapor column overhead extracted from the higher
pressure column 72 is condensed within the first main
condenser-reboiler 75 shown as a once-through heat exchanger being
located in the base of lower pressure column 74 against the
oxygen-rich liquid column bottoms 77 residing in the bottom of the
lower pressure column 74. The boiling of oxygen-rich liquid column
bottoms 77 initiates the formation of an ascending vapor phase
within lower pressure column 74. The condensation produces a liquid
nitrogen stream 81 that is used to reflux the lower pressure column
74 to initiate the formation of descending liquid phase therein. If
desired, a portion of the reflux stream may be withdrawn as liquid
product.
[0027] A second portion 83 of the nitrogen-rich vapor column
overhead extracted from the higher pressure column 72 is condensed
within the second condenser-reboiler 85 shown as a once-through
heat exchanger disposed in a separate condenser vessel 84. The
second condenser-reboiler 85 is operatively associated with the
higher pressure column 72 and configured to condense the second
portion 83 of the nitrogen enriched overhead from the higher
pressure column 72 against a subcooled first split portion 86 of
the oxygen-enriched kettle stream 88 from the higher pressure
column 72 to produce a liquid nitrogen stream 82 and a recycle
stream 100 from the boil-off of the oxygen-enriched kettle stream.
Liquid nitrogen stream 82 could be added to the liquid nitrogen
reflux stream 81 that is used to reflux the lower pressure column
74.
[0028] The remaining portion of the oxygen-enriched kettle stream,
referred to as the second split portion 87, is subcooled and then
flashed via valve 187 and introduced into an intermediate location
of the lower pressure column 74, a number of stages above the first
main condenser-reboiler 75. In addition, a third portion of the
nitrogen-rich vapor column overhead extracted from the higher
pressure column 72 which is not liquefied in either of the first
main condenser-reboiler or the second condenser-reboiler but is
withdrawn as a medium pressure or high pressure nitrogen product
stream 105 and warmed in heat exchangers 52A and 52B to produce a
warmed medium/high pressure nitrogen product stream 115.
[0029] In the lower pressure column 74, the ascending vapor phase
includes the boil-off from the first main condenser-reboiler 75 as
well as the exhaust stream 64 from the upper column turbine 35
introduced at an intermediate location of the lower pressure column
74. The descending liquid is initiated by nitrogen reflux stream 96
from the third condenser reboiler 95 which is released into the
lower pressure column 74.
[0030] Lower pressure column 74 is also provided with a plurality
of mass transfer contacting elements, that can be trays or
structured packing or other known elements in the art of cryogenic
air separation. The separation occurring within lower pressure
column 74 produces a nitrogen overhead 92, a lower pressure
nitrogen product stream 110 taken from a location proximate an
upper section of the lower pressure column several stages below the
overhead 92, and an oxygen-rich liquid column bottoms 77. The lower
pressure nitrogen product stream 110 is further warmed in heat
exchangers 52B, 52A to produce a lower pressure, warmed nitrogen
product stream 120.
[0031] As indicated above, the third condenser-reboiler 95 is
associated with the lower pressure column 74 and disposed in a
vessel 94. The third condenser-reboiler 95 is configured to
condense the nitrogen overhead 92 from the lower pressure column 74
against the portion of the oxygen bottoms liquid 77 that is not
reboiled. That portion of the oxygen bottoms liquid 77 from the
lower pressure column 74 is subcooled and the subcooled stream 176
is flashed via valve 177 with the resulting stream 178 into the
boiling side of the third condenser-reboiler 95. The condensed
liquid produced by the third condenser-reboiler 95 is nitrogen
reflux stream 96 used to reflux the lower pressure column 74 while
the vapor generated is withdrawn as waste stream 93 which is warmed
in heat exchanger 52B and the warmed waste stream 193 may be
utilized to regenerate the pre-purifier unit 28.
[0032] The recycle stream 100 is taken from the vapor stream
exiting the second condenser-reboiler 85 and is preferably recycled
back to the main air compression system and combined with the
incoming feed air stream 22. As shown in FIG. 1, the heat
exchangers 52A and 52B are configured to extract refrigeration from
the recycle stream 100 and cool the compressed, purified air in
part via indirect heat exchange with the recycle stream 100. The
warmed recycle stream 102 is then introduced to the main air
compressor 24, preferably at an inter-stage location, or optionally
compressed in a recycle compressor (not shown) and cooled in an
aftercooler (not shown) prior to combining with the feed air
stream.
[0033] Turning now to FIG. 2, there is shown partial schematic
diagrams of an alternate embodiment of the present system and
method. Many of the features, components and streams associated
with the nitrogen producing air separation unit shown in FIG. 2 are
similar or identical to those described above with reference to the
embodiment of FIG. 1 and for sake of brevity will not be repeated
here. The key differences between the nitrogen producing air
separation units illustrated in FIG. 2 compared to those in the
corresponding arrangement shown in FIG. 1 is the recycle stream is
compressed in recycle compressor 104 and cooled in aftercooler 103
and returned to a location downstream of the pre-purifier but
upstream of the main heat exchange system and combined with the
compressed, pre-purified air stream 29.
[0034] In the embodiment of FIG. 2, the recycle compressor 104 may
need to be sized and operated to fully compress the recycle stream
to pressures needed to combine with compressed, pre-purified air
stream whereas the embodiment of FIG. 1, the recycle pressure
exiting an optional recycle compressor may be lower as the recycle
stream will be further compressed in the main air compressor.
Advantageously, the pre-purifier unit 28 in FIG. 2 is likely
smaller and less costly than the pre-purifier unit in FIG. 1 due to
the increase in volume of incoming feed air due to the addition of
the recycle stream upstream of pre-purifier unit 28 in FIG. 1 but
downstream od the pre-purifier unit in the embodiment of FIG. 2.
Also, by avoiding the pre-purifier unit, the recycle stream of FIG.
2 does not bear the associated pressure drop.
[0035] The embodiment depicted in FIG. 3 shows a further
arrangement of the large-scale, dual column, nitrogen producing air
separation unit where the recycle compressor 202 is a cold
compressor configured to compress the cold recycle stream 200. The
cold compressed recycle stream 208 is cooled in heat exchanger 52B
and sent to the higher pressure column 72. In the illustrated
embodiment, the cold compressor 202 is driven by a booster loaded
turbine 205. The booster loaded turbine 205 is configured to expand
a diverted portion 207 of the medium/high nitrogen product stream
105 with the exhaust stream 206 from the booster loaded turbine 205
being returned to the lower pressure nitrogen product stream 110.
Again, as many of the features, components and streams associated
with the nitrogen producing air separation unit shown in FIG. 3 are
similar or identical to those described above with reference to the
embodiment of FIG. 1 the associated descriptions will not be
repeated here.
Examples
[0036] When compared against the various existing prior art air
separation cycles for large scale dual column, nitrogen producing
air separation units, the use of split kettle arrangement with
three condenser-reboilers and a portion of the vapor from the
second condenser-reboiler being recycled as contemplated by the
present systems and methods generally reduces power consumption of
the air separation unit by about 5.0% or more while concurrently
increasing nitrogen recovery. Computer model simulations of a prior
art dual column, nitrogen producing air separation unit against the
embodiments shown in FIGS. 1-3 and described herein are shown in
Table 1).
[0037] Admittedly, each of the proposed embodiments of the present
dual column, nitrogen producing air separation unit require an
increase in capital costs compared to the prior art air separation
units as a result of using three condenser-reboilers and the
recycle compressor (See FIGS. 1-3), as well as the optional boosted
loaded turbine (See FIG. 3), and increase size of the pre-purifier
unit (See FIG. 1). In addition, the proposed embodiments operate at
slightly higher pressures which must be factored into the designs
and costs associated therewith. Like many cryogenic air separation
unit designs, the design trade-offs to be considered is whether the
additional nitrogen recovery and reduce power costs of the
disclosed embodiments outweigh the additional capital costs and
higher pressure requirements for the components within air
separation unit.
TABLE-US-00001 TABLE 1 Prior Art Dual Column Dual Column Dual
Column Dual Column Nitrogen ASU Nitrogen ASU Nitrogen ASU Nitrogen
ASU of FIG. 1 of FIG. 2 of FIG. 3 Feed Air Flow (Nm.sup.3/h)
40385.3 36082.9 37089.2 36089.2 Feed Air Flow Pressure (psia) 139.4
151.9 178 169 Recycle Flow (% of Feed Air) 0.0% 34.6% 25% 35%
Recycle Pressure (psia) NA 91.3 115.6 101.1 High P Product Flow
(Nm.sup.3/h) 11025 15986.7 15126.3 8817.7 High P N2 Pressure (psia)
130 142.5 169 160 Lower P N2 Product Flow (Nm.sup.3/h) 14981 9812.9
1006.4 17635.3 Lower P N2 Pressure (psia) 60 61 61.5 61.5 Relative
Power Consumption Baseline -5.00% -5.43% -1.45% Nitrogen Recovery
(% of Feed Air) 64.0% 71.5% 69.8% 73.2%
[0038] In the computer model simulations run, it was also found
that the higher recycle flows may increase the oxygen content in
the recycle steam which, in turn, increases the oxygen content of
the feed air streams sent to the distillation column system. Thus,
by controlling the recycle flow rate, the air separation unit
operator has the ability to control the oxygen content in the
recycle flow so as to optimize performance of the nitrogen
producing, dual column air separation unit.
[0039] While the present enhancements to a large-scale, dual column
nitrogen producing air separation unit has been described with
reference to several preferred embodiments, it is understood that
numerous additions, changes and omissions can be made without
departing from the spirit and scope of the present inventions as
set forth in the appended claims.
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