U.S. patent application number 17/580830 was filed with the patent office on 2022-05-12 for method for flexible recovery of argon from a cryogenic air separation unit.
The applicant listed for this patent is Richard D. Lenz, Yang Luo, Neil M. Prosser, Kevin J. Saboda. Invention is credited to Richard D. Lenz, Yang Luo, Neil M. Prosser, Kevin J. Saboda.
Application Number | 20220146195 17/580830 |
Document ID | / |
Family ID | 1000006093639 |
Filed Date | 2022-05-12 |
United States Patent
Application |
20220146195 |
Kind Code |
A1 |
Prosser; Neil M. ; et
al. |
May 12, 2022 |
METHOD FOR FLEXIBLE RECOVERY OF ARGON FROM A CRYOGENIC AIR
SEPARATION UNIT
Abstract
A method for flexible production of argon from a cryogenic air
separation unit is provided. The disclosed cryogenic air separation
unit is capable of operating in a `no-argon` or `low-argon` mode
when argon demand is low or non-existent and then switching to
operating in a `high-argon` mode when argon is needed. The recovery
of the argon products from the air separation unit is adjusted by
varying the percentages of dirty shelf nitrogen and clean shelf
nitrogen in the reflux stream directed to the lower pressure
column. The cryogenic air separation unit and associated method
also provides an efficient argon production/rejection process that
minimizes the power consumption when the cryogenic air separation
unit is operating in a `no-argon` or `low-argon` mode yet maintains
the capability to produce higher volumes of argon products at full
design capacity to meet argon product demands.
Inventors: |
Prosser; Neil M.; (Lockport,
NY) ; Luo; Yang; (Amherst, NY) ; Lenz; Richard
D.; (Tonawanda, NY) ; Saboda; Kevin J.; (Long
Beach, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Prosser; Neil M.
Luo; Yang
Lenz; Richard D.
Saboda; Kevin J. |
Lockport
Amherst
Tonawanda
Long Beach |
NY
NY
NY
CA |
US
US
US
US |
|
|
Family ID: |
1000006093639 |
Appl. No.: |
17/580830 |
Filed: |
January 21, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15859979 |
Jan 2, 2018 |
11262125 |
|
|
17580830 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 3/04969 20130101;
F25J 3/04412 20130101; B01D 2256/18 20130101; F25J 3/04884
20130101; F25J 3/04218 20130101; F25J 2250/58 20130101; F25J
3/04963 20130101; F25J 3/0409 20130101; F25J 3/04733 20130101; F25J
2200/94 20130101; F25J 3/048 20130101; F25J 3/04175 20130101; F25J
2205/82 20130101; F25J 3/04678 20130101; F25J 2250/02 20130101;
F25J 3/04703 20130101; F25J 3/0423 20130101; F25J 3/04721 20130101;
F25J 2205/60 20130101; B01D 2257/102 20130101; B01D 2257/104
20130101; F25J 3/04296 20130101; F25J 2250/52 20130101; F25J
2245/58 20130101; F25J 3/04236 20130101; F25J 2245/42 20130101;
F25J 3/04303 20130101 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1. A method of producing an argon product in an air separation
unit, the method comprising the steps of: producing a stream of
compressed and purified air; cooling the compressed and purified
air stream in a main heat exchanger system; rectifying the cooled,
compressed and purified air stream in a distillation column system
configured to produce a plurality of products including one or more
oxygen products and the argon product, wherein the distillation
column system comprises a higher pressure column and a lower
pressure column linked in a heat transfer relationship via a
condenser-reboiler and an argon column, extracting a clean shelf
nitrogen stream and a dirty shelf nitrogen stream from the
distillation column system subcooling the clean shelf nitrogen
stream and the dirty shelf nitrogen stream in one or more nitrogen
subcoolers; directing the subcooled clean shelf nitrogen stream to
an uppermost location of the lower pressure column and the
subcooled dirty shelf nitrogen stream to another location of the
lower pressure column at or below the uppermost location; wherein
the air separation unit operates in a first operating mode having a
ratio of subcooled clean shelf nitrogen directed to the lower
pressure column to subcooled dirty shelf nitrogen directed to the
lower pressure column of less than 1.5 and the argon recovery
within the air separation unit is less than a predetermined
recovery level; and wherein the air separation unit operates in a
second operating mode having a ratio of subcooled clean shelf
nitrogen directed to the lower pressure column to subcooled dirty
shelf nitrogen directed to the lower pressure column of greater
than or equal to 1.5 and the argon recovery within the air
separation unit is higher than the predetermined recovery level;
opening and/or closing one or more valves configured to regulate
flow of the clean shelf nitrogen stream and the dirty shelf
nitrogen stream through a plurality of discrete passages in the one
or more nitrogen subcoolers to switch the flow of the clean shelf
nitrogen stream and the dirty shelf nitrogen stream through the
plurality of discrete passages, wherein one or more of the
plurality of discrete passages switches between sub cooling the
dirty shelf nitrogen stream and sub cooling the clean shelf
nitrogen stream; wherein the air separation unit switches between
the first operating mode and the second operating mode by such
opening and/or closing of the one or more valves resulting in an
adjustment in the recovery of the argon product from the
distillation column system; and wherein a power consumption of the
air separation unit is lower in the first operating mode than in
the second operating mode.
2. The method of claim 1, further comprising the steps of: pumping
an oxygen-rich liquid from the lower pressure column to produce a
pumped liquid oxygen stream; warming at least part of the pumped
liquid oxygen stream in the main heat exchange system to produce an
oxygen-rich gaseous product stream; and taking a portion of the
pumped liquid oxygen stream to produce a liquid oxygen product
stream; wherein the one or more oxygen products further comprise
the oxygen-rich gaseous product stream and the liquid oxygen
product stream.
3. The method of claim 1, wherein the distillation column system is
further configured to produce a liquid nitrogen product stream
comprised of a portion of the clean shelf nitrogen stream.
4. The method of claim 3, further comprising the steps of:
diverting a portion of a nitrogen overhead from the higher pressure
column to the main heat exchanger system; and warming the diverted
portion of the nitrogen overhead in the main heat exchange system
to form a gaseous nitrogen product stream; wherein the recovery of
the one or more oxygen products, the one or more nitrogen products,
and the argon product from the distillation column system is
adjusted when the at least one of the plurality of discrete
passages in the plurality of heat exchange cores switches between
sub cooling the dirty shelf nitrogen stream and subcooling the
clean shelf nitrogen stream and a flow of the diverted portion of
the nitrogen overhead to the main heat exchanger system is varied;
and wherein the one or more nitrogen products further comprise the
gaseous nitrogen product stream and the liquid nitrogen product
stream.
5. The method of claim 1, further comprising the steps of:
splitting the stream of compressed and purified air into at least a
first part of the compressed and purified air stream and a second
part of the compressed and purified air stream; further compressing
the first part of the compressed and purified air stream in a
booster compressor arrangement to produce a boosted pressure air
stream; and cooling the boosted pressure air stream in the main
heat exchange system; and directing the boosted pressure air stream
to the distillation column system.
6. The method of claim 5, further comprising the steps of expanding
a portion of the second part of the compressed and purified air
stream or a portion of the boosted pressure air stream in a
turboexpander arrangement to form an exhaust stream and directing
the exhaust stream to the distillation column system;
7. The method of claim 1, wherein the argon column arrangement
further comprises an argon distillation column, an argon condenser,
and an argon refining system and wherein the method further
comprises the steps of: directing an oxygen-argon containing stream
from the lower pressure column to the argon distillation column;
rectifying the oxygen-argon containing stream in the argon
distillation column to produce an argon-rich vapor stream and an
oxygen-rich bottoms stream; returning the oxygen-rich bottoms
stream from the argon distillation column to an intermediate
location of the lower pressure column; condensing the argon-rich
vapor stream in the argon condenser against a subcooled liquid
oxygen to produce a crude argon stream; and refining the crude
argon stream in the argon refining system to produce the argon
product.
8. The method of claim 7, wherein the argon refining system further
comprises a liquid phase argon adsorption system.
9. The method of claim 7, wherein the argon refining system further
comprises a gaseous phase argon pressure swing adsorption
system.
10. The method of claim 7, wherein the argon distillation column
further comprises a superstaged argon distillation column.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application that
claims the benefit of and priority to U.S. patent application Ser.
No. 15/859,979 filed on Jan. 2, 2018, the disclosure of which is
incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention relates to the recovery of products
from a cryogenic air separation unit, and more particularly, to a
system and method for the flexible recovery of argon from a
cryogenic air separation unit. Still more particularly, the present
invention relates to the flexible recovery of argon and other
products from a cryogenic air separation unit through the use of
clean shelf nitrogen and/or dirty shelf nitrogen as the reflux
stream to the lower pressure column and varying the percentage of
dirty shelf nitrogen in the total nitrogen reflux stream to achieve
the desired product recovery and realize power saving efficiencies
when operating in no argon recovery mode or low argon recovery
mode.
BACKGROUND
[0003] Argon is a highly inert element used in the some
high-temperature industrial processes, such as steel-making where
ordinarily non-reactive substances become reactive. Argon is also
used in various types of metal fabrication processes such as arc
welding as well as in the electronics industry, for example in
silicon crystals growing processes. Still other uses of argon
include medical, scientific, preservation and lighting
applications. While argon constitutes only a minor portion of
ambient air (i.e. about 0.93%), it possesses a relatively high
value compared to the oxygen and nitrogen products that are also
recovered from cryogenic air separation units.
[0004] Most modern air separation units are designed to produce
argon from the Linde-type double column arrangement by extracting
an argon rich vapor draw from the lower pressure column and
directing the stream to an argon column to recover the argon,
typically in a "superstaged" distillation process that produces
argon at merchant liquid purities (e.g. about 1 ppm oxygen) in
roughly 180 stages of separation. Alternatively, argon may be
produced at lower or intermediary purities (e.g. about 1% to 2%
oxygen) in a "crude" argon column arrangement in roughly 50 stages
of separation. Most cryogenic air separation units are designed
with a predefined and narrow argon production range, often with an
argon recovery of up to 90% or more of the available argon in the
incoming feed air.
[0005] While most cryogenic air separation units have a design life
of several decades or more, the argon production requirements over
the life of the cryogenic air separation unit often will vary, and
such variance can be significant. For example, when an air
separation unit is initially built the need for merchant argon
product to be sold to regional customers may not exist or, if such
need does exist, is usually lower than the merchant argon product
requirements after 5 years or even 10 years of plant operation.
This is known as the argon product ramp where lower argon
production or no argon production is needed in early years of air
separation unit plant operation but higher levels of argon
production up to full argon production levels are needed several
years later.
[0006] In addition, changes in local or regional argon product
demand may also occur over the operational life of the cryogenic
air separation unit. For example, a local or regional market may be
saturated with excess argon capacity or argon over-supply for
extended periods of time in which full production of argon product
is not warranted. Conversely, such local or regional markets may
also experience argon shortages from time to time, in which full
production of argon product from the cryogenic air separation unit
is needed.
[0007] Current cryogenic air separation unit plant designs do not
have the flexibility to make significantly different argon
production range to address the argon ramp phenomena or to meet
argon product demand changes. Even where cryogenic air separation
units can vary argon production, such plants fail to capture the
potential power savings when operating in the low argon mode
compared to operating in the high-argon or full argon production
mode.
[0008] What is needed, therefore, is an improved argon recovery
process or arrangement which can enhance the flexibility,
performance and cost-effectiveness of argon recovery in cryogenic
air separation units throughout the operational life of the plant.
In particular, what is needed is a cryogenic air separation unit
that is flexible in that it can operate in a `low-argon` or
`no-argon` mode when the argon product requirements or argon
product demand are low and realize operational power savings during
such modes yet also operate in a `high argon` mode when argon
product requirements are high.
SUMMARY
[0009] The present invention may be broadly characterized as a
method of producing one or more oxygen products, one or more
nitrogen products, and an argon product in an air separation unit,
the method comprising the steps of: (a) producing a stream of
compressed and purified air; (b) cooling the compressed and
purified air stream in a main heat exchanger system; (c) rectifying
the cooled, compressed and purified air stream in a distillation
column system configured to produce a plurality of products
including one or more oxygen products and the argon product,
wherein the distillation column system comprises a higher pressure
column and a lower pressure column linked in a heat transfer
relationship via a condenser-reboiler and an argon column; (d)
extracting a clean shelf nitrogen stream and a dirty shelf nitrogen
stream from the distillation column system; (e) subcooling the
clean shelf nitrogen stream and the dirty shelf nitrogen stream in
one or more nitrogen subcoolers; (f) directing the subcooled clean
shelf nitrogen stream to an uppermost location of the lower
pressure column and the subcooled dirty shelf nitrogen stream to
another location of the lower pressure column at or below the
uppermost location; and (g) opening and/or closing one or more
valves configured to regulate flow of the clean shelf nitrogen
stream and the dirty shelf nitrogen stream through a plurality of
discrete passages in the one or more nitrogen subcoolers to switch
the flow of the clean shelf nitrogen stream and the dirty shelf
nitrogen stream through the plurality of discrete passages, wherein
one or more of the plurality of discrete passages switches between
subcooling the dirty shelf nitrogen stream and subcooling the clean
shelf nitrogen stream.
[0010] In the above-described method, the air separation unit
operates in a first operating mode having a ratio of subcooled
clean shelf nitrogen directed to the lower pressure column to
subcooled dirty shelf nitrogen directed to the lower pressure
column of less than 1.5. The air separation unit also operates in a
second operating mode having a ratio of subcooled clean shelf
nitrogen directed to the lower pressure column to subcooled dirty
shelf nitrogen directed to the lower pressure column of greater
than or equal to 1.5 and the argon recovery within the air
separation unit is higher than the predetermined recovery level.
The air separation unit switches between the first operating mode
and the second operating mode by the opening and/or closing of the
one or more valves as set forth above resulting in an adjustment in
the recovery of the argon product from the distillation column
system.
[0011] In some embodiments, the first operating mode is a
`no-argon` or `low-argon` mode when argon demand is low or
non-existent and then switches to the second operating mode which
is a `high-argon` mode when requirements for the production of the
argon products are increased. Note the overall power consumption of
the air separation unit is generally lower in the first operating
mode than in the second operating mode.
[0012] In some preferred embodiments, the low-argon mode may
alternatively be characterized as when molar flow rate of the dirty
shelf nitrogen in the reflux streams divided by the sum of the
molar flow rates of dirty shelf nitrogen and clean shelf nitrogen
used in the reflux streams is greater than about 0.40 and an argon
recovery within the air separation unit is generally less than
about 80%. On the other hand, the high-argon mode may alternatively
be characterized as when the molar flow rate of the dirty shelf
nitrogen in the reflux stream divided by the sum of the molar flow
rates of dirty shelf nitrogen and clean shelf nitrogen used in the
total nitrogen reflux stream is less than about 0.40 and an argon
recovery within the air separation unit is generally greater than
about 80%.
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 is believed that the invention will be better
understood when taken in connection with the accompanying drawings
in which:
[0014] FIG. 1 is a schematic process flow diagram of an air
separation unit in accordance with an embodiment of the present
invention;
[0015] FIG. 2 is a schematic process flow diagram of an air
separation unit in accordance with another embodiment of the
present invention;
[0016] FIG. 3 is a schematic process flow diagram of an air
separation unit in accordance with yet another embodiment of the
present invention;
[0017] FIG. 4 is a schematic process flow diagram of an air
separation unit in accordance with still another embodiment of the
present invention;
[0018] FIG. 5 is a partial schematic process flow diagram of an
embodiment of a subcooler arrangement that is suitable for use with
the present invention when the air separation unit is operating in
a `no argon` operating mode or a `low argon` operating mode;
[0019] FIG. 6 is a partial schematic process flow diagram of the
subcooler arrangement of FIG. 5 when the air separation unit is
operating in a `high argon` operating mode;
[0020] FIG. 7 is a partial schematic process flow diagram of
another embodiment of a subcooler arrangement that is suitable for
use with the present invention when the air separation unit is
operating in a `no argon` operating mode or a `low argon` operating
mode;
[0021] FIG. 8 is a partial schematic process flow diagram of the
subcooler arrangement of FIG. 7 when the air separation unit is
operating in a `high argon` operating mode;
[0022] FIG. 9 is a partial schematic process flow diagram of yet
another embodiment of a subcooler arrangement that is suitable for
use with the present invention when the air separation unit is
operating in a `no argon` operating mode or a `low argon` operating
mode;
[0023] FIG. 10 is a partial schematic process flow diagram of the
subcooler arrangement of FIG. 9 when the air separation unit is
operating in a `high argon` operating mode;
[0024] FIG. 11 is a partial schematic process flow diagram of still
another embodiment of a subcooler arrangement that is suitable for
use with the present invention when the air separation unit is
operating in a `no argon` operating mode or a `low argon` operating
mode; and
[0025] FIG. 12 is a partial schematic process flow diagram of the
subcooler arrangement of FIG. 11 when the air separation unit is
operating in a `high argon` operating mode.
DETAILED DESCRIPTION
[0026] The present system and method provides a flexible cryogenic
air separation unit design that provides: (i) an efficient argon
rejection process that minimizes the power consumption when the
cryogenic air separation unit is not producing argon; (ii) an
efficient and low cost means to increase argon production when some
argon is needed; and (iii) the capability to produce argon at full
design capacity using added argon refining systems and methods.
Alternatively, the present system and method provides a flexible
cryogenic air separation unit design that provides: (i) an
efficient argon producing process that minimizes the power
consumption when argon production at full design capacity is not
needed; and (ii) the capability to produce argon at full design
capacity.
[0027] ASU process design options for flexible argon production and
high energy efficiency are proposed. The preferred embodiments of
the present system and methods employ the same lower pressure
distillation column arrangement and the same higher pressure
distillation column arrangement for either argon making or
non-argon making operating modes.
[0028] If argon production is desired, various argon refining
options, including but not limited to an argon distillation column
arrangement, catalytic deoxo, liquid or gas phase argon adsorption
purification, or any combination thereof, may be included in or
added to the integrated with the air separation unit, and more
particularly integrated with other portions of the distillation
column system If no argon production is needed, the argon refining
systems can be eliminated.
[0029] Where the argon refining options have been designed in or
added to the cryogenic air separation unit, the amount of argon
production may be adjusted by varying the percentage of dirty shelf
nitrogen used in the nitrogen reflux stream to lower pressure
column. In preferred embodiments, the total nitrogen reflux stream
to lower pressure column is comprised of a subcooled stream of
clean shelf nitrogen from the higher pressure column, a subcooled
stream of dirty shelf nitrogen, or a combination of the clean shelf
nitrogen and dirty shelf nitrogen. The amount of argon production
can also be adjusted by varying the amount of clean shelf vapor
drawn as a gaseous nitrogen product from the overhead of higher
pressure column or clean shelf liquid withdrawn from the main
condenser or from the top of higher pressure column.
[0030] Turning to FIG. 1, there is shown a simplified schematic
illustration of a cryogenic air separation plant also commonly
referred to as an air separation unit 10. In a broad sense, the
depicted air separation units include a main feed air compression
train or system 20, a turbine air circuit 30, a booster air circuit
40, a main or primary heat exchanger system 50, and a distillation
column system 70. As used herein, the main feed air compression
train, the turbine air circuit, and the booster air circuit,
collectively comprise the `warm-end` air compression circuit.
Similarly, the main or primary heat exchanger, portions of the
turbine based refrigeration circuit and portions of the
distillation column system are referred to as the `cold-end`
systems or equipment that are typically housed in one or more
insulated cold boxes.
[0031] In the main feed compression train shown in FIGS. 1 through
4, the incoming feed air 22 is typically drawn through an air
suction filter house (ASFH) and is compressed in a multi-stage,
intercooled main air compressor arrangement 24 to a pressure that
can be between about 5 bar(a) and about 15 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 26 exiting the main air compressor
arrangement 24 is fed to an aftercooler not shown) with integral
demister to remove the free moisture in the incoming feed air
stream. The heat of compression from the final stages of
compression for the main air compressor arrangement 24 is removed
in aftercoolers by cooling the compressed feed air with cooling
tower water. The condensate from this aftercooler as well as some
of the intercoolers in the main air compression arrangement is
preferably piped to a condensate tank and used to supply water to
other portions of the air separation plant. Further air cooling in
the aftercoolers may be provided by chilled water generated by
mechanical chillers, absorption chillers, or other techniques.
[0032] The cool, dry compressed air feed 26 is then purified in a
pre-purification unit 28 to remove high boiling contaminants from
the cool, dry compressed air feed. A 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
and/or pressure swing adsorption cycle in which moisture and other
impurities, such as carbon dioxide, water vapor and hydrocarbons,
are adsorbed. While one of the beds is used for pre-purification of
the cool, dry compressed air feed while the other bed is
regenerated, preferably with a portion of the waste nitrogen from
the air separation unit. The two beds switch service periodically.
Particulates are removed from the compressed, pre-purified feed air
in a dust filter disposed downstream of the pre-purification unit
28 to produce the compressed, purified air stream 29.
[0033] The compressed and purified air stream 29 is separated into
oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality
of distillation columns including a higher pressure column 72, a
lower pressure column 74, and an argon rejection column 76. Prior
to such distillation however, the compressed and pre-purified air
stream 29 is typically split into a plurality of feed air streams,
which may include a boiler air stream 42 and a turbine air stream
32. The boiler air stream 42 may be further compressed in a booster
compressor arrangement 44 and subsequently cooled in aftercooler 45
to form a boosted pressure air stream 46 which is then further
cooled to form a liquid or a dense phase fluid in the main heat
exchanger 52. Cooling or partially cooling of the air streams in
the main heat exchanger 52 is preferably accomplished by way of
indirect heat exchange with the warming streams which include the
oxygen stream 190, and nitrogen streams 191, 193, 195 from the
distillation column system 70 to produce cooled feed air streams 38
and 47.
[0034] The partially cooled feed air stream 38 is expanded in the
turbine 35 to produce exhaust stream 64 that is directed to the
higher pressure column 72. Refrigeration for the air separation
unit 10 is also typically generated by the turbine 35 and other
associated cold and/or warm turbine arrangements, such as closed
loop warm refrigeration circuits that are generally known in the
art. The fully cooled boosted pressure air liquid or dense phase
fluid stream 47 is divided into separate portions which are
expanded in expansion valve(s) 48, 49 prior to introduction into
the higher pressure column 72 and the lower pressure column 74,
respectively.
[0035] The main or primary heat exchanger 52 is preferably a brazed
aluminum plate-fin type heat exchanger. 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 small
air separation unit units, a heat exchanger comprising a single
core may be sufficient. For larger air separation unit units
handling higher flows, the heat exchanger may be constructed from
several cores which must be connected in parallel or series.
[0036] The turbine based refrigeration circuits 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 two-column or three column cryogenic air
distillation column systems. In the LCT arrangement shown in FIGS.
1 through 4, the compressed, cooled turbine air stream 32 is
preferably at a pressure in the range from between about 9 bar(a)
to about 60 bar(a). The compressed, cooled turbine air stream 32 is
directed or introduced into main or primary heat exchanger 52 in
which it is partially cooled to a temperature in a range of between
about 110 and about 200 Kelvin to form a partially cooled,
compressed turbine air stream 38 that is subsequently introduced
into a turbo-expander or turbine 35 to produce a cold exhaust
stream 64 that is introduced into the higher pressure column 72 of
distillation column system 70. The supplemental refrigeration
created by the expansion of the stream 38 is thus imparted directly
to the higher pressure column 72 thereby alleviating some of the
cooling duty of the main heat exchanger 52. Although not shown, in
some embodiments, the turbine 35 may be coupled with booster
compressor that is used to further compress the turbine air stream
32, either directly or by appropriate gearing.
[0037] While the turbine based refrigeration circuit illustrated in
FIGS. 1 through 4 are shown as a lower column turbine (LCT) circuit
where the expanded exhaust stream 64 is fed to the higher pressure
column 72 of the distillation column system 70, it is contemplated
that the turbine based refrigeration circuit alternatively may be
an upper column turbine (UCT) circuit where the turbine exhaust
stream is directed to the lower pressure column. Still further, the
turbine based refrigeration circuit may be some variant or
combination of an LCT circuit and UCT circuit.
[0038] Similarly, in an alternate embodiment that employs a UCT
arrangement (not shown), a portion of the purified and compressed
feed air may be partially cooled in the primary heat exchanger, and
then all or a portion of this partially cooled stream is diverted
to a turbo-expander. The expanded gas stream or exhaust stream from
the turbo-expander is then directed to the lower pressure column in
the two-column or multi-column cryogenic air distillation column
system. The cooling or supplemental refrigeration created by the
expansion of the exhaust stream is thus imparted directly to the
lower pressure column thereby alleviating some of the cooling duty
of the main heat exchanger.
[0039] The aforementioned components of the feed air streams,
namely oxygen, nitrogen, and argon are separated within the
distillation column system 70 that includes a higher pressure
column 72, a lower pressure column 74, an argon column 76, a
condenser-reboiler 75 and an argon condenser 78. The higher
pressure column 72 typically operates in the range from between
about 4.5 bar(a) to about 6.2 bar(a) whereas the lower pressure
column 74 operates at pressures between about 1.1 bar(a) to about
1.6 bar(a). The higher pressure column 72 and the lower pressure
column 74 are preferably inked in a heat transfer relationship such
that all or a portion of the nitrogen-rich vapor column overhead,
extracted from proximate the top of higher pressure column 72 as
stream 73, is condensed within a condenser-reboiler 75 located in
the base of lower pressure column 74 against boiling an oxygen-rich
liquid column bottoms 77. 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 containing stream 81 that is divided into a clean shelf
reflux stream 83 that may be used to reflux the lower pressure
column 74 to initiate the formation of descending liquid phase in
such lower pressure column 74 and a nitrogen-rich stream 85 that
refluxes the higher pressure column 72.
[0040] Exhaust stream 64 from the turbine 35 is introduced into the
higher pressure column 72 along with a portion of stream 47 for
rectification by contacting an ascending vapor phase of such
mixture within a plurality of mass transfer contacting elements,
illustrated as trays 71, with a descending liquid phase that is
initiated by reflux stream 85. This produces crude liquid oxygen
column bottoms 86, also known as kettle liquid, and the
nitrogen-rich column overhead 87, taken as clean shelf stream
89.
[0041] Lower pressure column 74 is also provided with a plurality
of mass transfer contacting elements, that can be trays or
structured packing or random packing or other known elements in the
art of cryogenic air separation. The contacting elements in the
lower pressure column 74 are illustrated as structured packing 79.
As stated previously, the separation occurring within lower
pressure column 74 produces oxygen-rich liquid column bottoms 77
extracted as an oxygen-rich liquid stream 90 and a nitrogen-rich
vapor column overhead 91 that is extracted as a nitrogen product
stream 95. As shown in the drawings, the oxygen-rich liquid stream
90 may be pumped via pump 180 and taken as a pumped liquid oxygen
product 185 or directed to the main heat exchanger 52 where it is
warmed to produce a gaseous oxygen product stream 190.
Additionally, a waste nitrogen stream 93 is also extracted from the
lower pressure column 74 to control the purity of nitrogen product
stream 95.
[0042] Both nitrogen product stream 95 and waste stream 93 are
passed through one or more subcooling units 99A, 99B. First
subcooler unit 99A is designed to subcool the kettle stream 88 and
the resulting subcooled kettle stream is expanded in valve 107 and
introduced as condensing medium to the argon condenser. The second
subcooler unit 99B is designed to subcool the clean shelf nitrogen
stream 83 and a dirty shelf nitrogen stream 84 drawn from a
location on the higher pressure column 72 below the nitrogen
overhead stream 87 draw. A portion of the subcooled clean shelf
nitrogen stream 104 may optionally be taken as a nitrogen liquid
product stream 98 and the remaining portion may be introduced into
lower pressure column 74 as a clean shelf reflux stream after
passing through expansion valve 96. The subcooled dirty shelf
nitrogen stream 94 may also be introduced into lower pressure
column 74 as a dirty shelf reflux stream after passing through
expansion valve 97. Subcooler unit 99A and subcooler unit 99B are
generally combined together in a single heat exchanger. However,
depending on the plant size and other factors, there may be
multiple subcooler heat exchangers arranged in parallel.
Furthermore, the subcooler units 99A and 99B are often combined
with primary heat exchanger 50. Subcooler units 99A and 99B in this
case are located at the cold end of the same heat exchanger as
primary heat exchanger 52. Then the number of subcooler units (99A
and 99B) in parallel corresponds to the number of primary heat
exchangers (52) in parallel.
[0043] Subcooler unit 99B comprises a nitrogen subcooler system 100
or arrangement having a plurality of valves, a header conduit or
manifold, and a one or more heat exchanging cores. As discussed in
more detail below with reference to FIGS. 5-12, the nitrogen
subcooler system 100 is preferably configured to subcool the clean
shelf nitrogen stream and the dirty shelf nitrogen stream via
indirect heat exchange with waste nitrogen stream 93 and nitrogen
product stream 95 taken from the lower pressure column 74. The
nitrogen subcooler system 100 is further configured to produce a
first subcooled nitrogen reflux stream 104 comprised of clean shelf
nitrogen, and/or a second subcooled nitrogen reflux stream 94
comprised of dirty shelf nitrogen. A portion of the first subcooled
nitrogen reflux stream 104 comprised of clean shelf nitrogen may be
introduced to the lower pressure column 74 at an uppermost location
while the second subcooled nitrogen reflux stream 94 comprised of
dirty shelf nitrogen is introduced to the lower pressure column 74
at either a first location proximate the uppermost location or at a
second location disposed below the uppermost location.
[0044] After passage through subcooling units 99A, 99B, nitrogen
stream 95 and waste stream 93 are fully warmed within main or
primary heat exchanger 52 to produce a warmed gaseous nitrogen
product stream 295 and a warmed nitrogen waste stream 293. Although
not shown, the warmed nitrogen waste stream 293 may be used to
regenerate the adsorbents within the pre-purification unit 28.
[0045] The argon column arrangement employed in the above-described
embodiments may be configured as an argon rejection column
integrated with the lower pressure column or may be a separate
`super-staged` argon rectification column. It is important to note
that when making an argon product in many conventional cryogenic
air separation units, an intermediate section of the lower pressure
column is typically under-utilized or unloaded because some of the
vapor is "bypassed" to the external crude argon or `superstaged`
column so that the flow area of this underutilized or unloaded
section of the lower pressure column required for distillation can
be reduced and somewhat less than the flow area for the remainder
of the lower pressure column sections. As a result, an argon
rejection column can actually be co-located in this under-utilized
or unloaded section of the lower pressure column structure by
designing a divided wall column having a main distillation section
and a partitioned argon rejection section at this location of the
lower pressure column structure. In such arrangement, a portion of
the vapor from the adjacent section of the lower pressure column
immediately below the divided wall column flows to the partitioned
argon rejection section. The remaining portion of the vapor from
the adjacent section of the lower pressure column immediately below
the divided wall column arrangement flows upward through to the
main distillation section. Design details about the preferred
divided wall column arrangements may be found in U.S. Provisional
Patent Application Ser. Nos. 62/550,262 and 62/550,269 both filed
Aug. 25, 2017 and U.S. patent application Ser. No. 15/057,148 filed
on Mar. 1, 2016, the disclosures of which are incorporated by
reference herein.
[0046] In embodiments with the argon rejection column integrated
within the lower pressure column structure, an intermediate section
of the column structure preferably contains a divided wall column
arrangement having a main distillation section and a partitioned
argon rejection section. The partitioned argon rejection section is
illustrated in FIGS. 1-4 as an argon column 76. The divided wall
argon column disposed within partitioned argon rejection section of
the lower pressure column structure operates at a pressure
comparable to the pressure within the lower pressure column. The
partitioned argon rejection section receives an upward flowing
argon and oxygen containing vapor feed 121 from the lower pressure
column 74, typically having a concentration of about 8% to 15% by
volume argon, and a down-flowing argon rich reflux 122 received
from an argon condenser 78. The partitioned argon column serves to
rectify the argon and oxygen containing vapor feed by separating
argon from the oxygen into an argon enriched overhead vapor 123 and
an oxygen-rich liquid stream 124 that that is released or returned
into lower pressure column 74 at a point below the divided wall
column arrangement. The mass transfer contacting elements 125
within the divided wall argon column arrangement can be structured
packing or trays.
[0047] All or a portion of resulting argon-rich vapor overhead 123
is then preferably directed as vapor stream 126 to the argon
condenser 78 also preferably disposed within the structure of the
lower pressure column 74 where it is condensed into a argon-rich
liquid stream. A portion of the resulting argon-rich liquid stream
is used as an argon-rich reflux stream 122 for the partitioned
argon rejection section and another portion may be optionally taken
an impure or crude liquid argon stream (not shown). In the depicted
embodiments, the argon-rich reflux stream 122 is directed back to
the uppermost portion of the argon column 76 and initiates the
descending argon liquid phase that contacts the ascending argon and
oxygen containing vapor feed 121.
[0048] The height of the partitioned argon rejection section is
preferably limited to accommodate between about 15 and 60 stages of
separation, and more preferably between 20 and 40 stages of
separation. While such limited number of separation stages is
sufficient for argon rectification needed to improve the oxygen
recovery of the cryogenic air separation unit, the resulting purity
of the argon vapor stream exiting the partitioned argon rejection
section is relatively low at about 4% to 25% oxygen, and more
preferably between 10% and 15% oxygen, with up to 1% nitrogen
impurities.
[0049] The argon condenser 78 is preferably configured as a
once-through condenser and is preferably disposed internal to the
lower pressure column 74, just above the divided wall arrangement
of the lower pressure column structure that forms the argon column.
This location of the argon condenser 78 is the natural feed point
for the kettle liquid and vapor, and the natural point to condense
the argon overhead vapor. As a result, this location is an ideal
location to house the argon condenser 78 to minimizing piping and
avoiding the need for a separator vessel for the two phase
partially boiled kettle stream. Alternatively, the argon condenser
78 may be disposed separately above the uppermost portion of lower
pressure column 74, or other location, although additional piping
may be required.
Argon Rejection and/or Recovery
[0050] The argon-rich stream withdrawn from the argon column can be
rejected or can be recovered by diverting all or a portion of the
impure argon-rich stream to an argon purification or refining
system or column. In the embodiment contemplating argon rejection
shown in FIG. 1, a portion of the impure argon-rich vapor stream
128 is withdrawn from the argon column 76 and optionally added to
the waste nitrogen stream 193 which is directed to the main heat
exchanger 52 to provide further refrigeration for the air
separation plant 10, thereby allowing increased oxygen recovery.
This particular arrangement is suitable for use in air separation
plants operating in a no argon operating mode or air separation
plants having no initial argon product requirements.
[0051] In an embodiment contemplating argon recovery shown in FIG.
2, the impure argon-rich stream 129 containing between about and 4%
and 25% of oxygen impurities and up to about 1% nitrogen is
withdrawn from the argon rejection column 76 and diverted to
another argon distillation column 130 configured to refine the
impure argon-rich stream 129 and produce a higher purity argon
product stream 135. The impure argon-rich stream 129 taken from the
overhead of the argon rejection column 76 is introduced as the
ascending vapor in the argon distillation column 130. The argon
distillation column 130 is configured to rectify the impure
argon-rich stream and produce an oxygen-rich liquid bottoms 132, an
argon-rich overhead 133, and an argon product stream 135. The
oxygen-rich liquid bottoms 132 are pumped back to the argon
rejection column 76 as reflux stream 136 via pump 137. The
argon-rich overhead 133 is directed to argon condenser 178 as
stream 138 where it is condensed against a subcooled stream of
kettle oxygen 140 and returned as reflux stream 139 to the top
portion of the argon distillation column 130. The vaporized or
partially vaporized oxygen stream 142 exiting the argon condenser
178 is returned to the lower pressure column 74. When utilizing the
secondary argon distillation column arrangement, it should be noted
that there are numerous variations for integrating or coupling the
secondary argon distillation column with the remainder of the
distillation column system 70. In the argon production system of
FIG. 2, product argon stream 135 is produced directly from argon
distillation column 130. This method was first described in U.S.
Pat. No. 5,133,790. Alternatively, an additional distillation
column is required for final removal of the nitrogen impurity.
[0052] In an alternate embodiment contemplating argon recovery
shown in FIG. 3, the impure argon-rich stream 129 is withdrawn from
the argon column 76 and all or a portion is diverted as stream 144
to an adsorption based argon purification/refining system 150.
System 150 is most often implemented in order to change a no argon
production system as exemplified in FIG. 1 to an argon production
system. As such, all of the impure argon-rich stream 129 withdrawn
from the argon column 76 is diverted as stream 144 to the argon
purification/refining system 150. Device 160 depicts a
semi-permanent isolation device such as a blind flange. However,
device 160 alternatively is a valve so that the air separation
plant 10 can operate in an argon production mode and a no argon
production mode.
[0053] The adsorption based argon purification/refining process may
be a liquid phase adsorption process or a vapor phase adsorption
process. In the liquid phase adsorption based purification/refining
process, an impure argon-rich liquid stream can be withdrawn from
the argon condensing assembly and recovered by diverting a portion
of the argon-rich liquid stream to a liquid phase adsorption based
argon purification/refining system, as generally taught in U.S.
Pat. No. 9,222,727; the disclosure of which is incorporated by
reference herein.
[0054] The vapor phase adsorption based argon purification/refining
system 150 depicted in FIG. 3 includes one or more adsorbent beds
containing an adsorbent that is designed to remove oxygen
impurities and optionally nitrogen impurities from the impure
argon-rich stream 144. Pressure elevation of the impure argon-rich
stream 144, if necessary, is accomplished with a compressor or pump
(not shown). In the preferred embodiment, the adsorption of the
impurities produces a purified argon stream that may be delivered
as a purified argon vapor stream 145 in a series of process steps
comprising adsorption, equalization, blowdown, and pressurization.
As is well known in the art, the adsorption based argon refining or
purification systems generally employ an alternating adsorption
cycle having an on-line phase where the impure argon-rich stream
144 is purified within one or more adsorbent beds and an off-line
phase where the adsorbent contained in the adsorbent beds is
regenerated through desorption of the adsorbed impurities.
Optionally, a portion of the blowdown gas may be recycled as stream
146 back to argon rejection column 76.
[0055] In yet another alternate embodiment for argon recovery shown
in FIG. 4, the impure argon-rich stream 129 is withdrawn from the
argon column 76 and diverted as stream 146 to a catalytic deoxo
based argon purification/refining system 160 which produces a high
purity argon product 165.
Clean Shelf Nitrogen Reflux vs. Dirty Shelf Nitrogen Reflux
[0056] Increasing the flow of the subcooled nitrogen-rich reflux
stream to the lower pressure column of the air separation unit
benefits product recovery, especially argon recovery. Hence, a
lower shelf nitrogen vapor draw increases recovery because of the
higher reflux flow and a greater boil up rate in the main
condenser. The level of recovery improvement is also impacted by
the nitrogen purity in reflux stream. Higher nitrogen purity in
reflux stream generally produces a higher argon recovery. Clean
shelf nitrogen is much lower in argon content than dirty shelf
nitrogen and thus has higher nitrogen purity. With greater clean
shelf nitrogen use as the reflux stream to the lower pressure
column, the argon that is lost in the waste nitrogen stream becomes
significantly reduced.
[0057] Clean shelf nitrogen vapor is preferably drawn from the top
of the higher pressure column and condensed in the main condenser
reboiler to produce clean shelf nitrogen, a portion of which is
used as the reflux stream to the lower pressure column. Some clean
shelf nitrogen reflux will always be required if product quality
top hat nitrogen is withdrawn from the lower pressure column as a
product nitrogen stream. Dirty shelf nitrogen having a lower
nitrogen purity than clean shelf nitrogen is withdrawn from the
higher pressure column several stages below the top of the higher
pressure column. The position of the dirty shelf nitrogen draw may
be optimized to maximize the power savings during low argon or no
argon production modes. The high pressure column is capable of
producing greater flow rates of dirty shelf nitrogen than clean
shelf nitrogen. This leads to a power savings for the use of dirty
shelf nitrogen reflux to the low pressure column.
[0058] The ratio of clean shelf nitrogen to dirty shelf nitrogen
may be adjusted to match the required argon production from the air
separation unit and to optimize the associated process
efficiencies. In `high argon` recovery operation modes, the process
typically requires higher clean shelf nitrogen flows whereas in
`low argon` recovery operation modes or `no argon` recovery
operation modes, a higher percentage of dirty shelf nitrogen could
be used as part of the reflux stream directed to the lower pressure
column. Use of a higher content of dirty shelf nitrogen in the
reflux stream that is directed to the lower pressure column
significantly reduces the power consumption of the air separation
unit and improves the overall process efficiency. For example, the
lowest power consumption will generally occur when the dirty shelf
nitrogen draw from the higher pressure column for use as reflux in
the lower pressure column is maximized.
[0059] In other words, the flexibility in the argon product make is
achieved by varying the ratio of clean shelf nitrogen to dirty
shelf nitrogen that makes up the total reflux stream to the lower
pressure column. Controlling the ratio of clean shelf nitrogen to
dirty shelf nitrogen that makes up the total reflux stream to the
lower pressure column can be achieved with specific subcooler
arrangements as described in the paragraphs that follow.
[0060] Turning now to FIGS. 5-12, generic piping configuration and
banking arrangements for the air separation unit subcooler 100 is
shown using the same reference numerals to designate the same or
similar items in the respective Figs. The embodiments illustrated
in FIG. 5 and FIG. 6 use eight heat exchange cores for the
subcooler of the air separation unit, although fewer or more heat
exchange cores may be employed. The representations in FIGS. 5-12
are of subcooler unit 99B. As previously described, subcooler unit
99B is usually combined with subcooler unit 99A, and both are often
combined with primary heat exchanger 52. FIG. 5 depicts the
respective flows of clean shelf nitrogen and dirty shelf nitrogen
through the heat exchanger cores when the air separation unit is
operating in a `no argon` operating mode or a `low argon` operating
mode. FIG. 6 depicts the respective flows of clean shelf nitrogen
and dirty shelf nitrogen through the same heat exchanger cores when
the air separation unit is operating in a `high argon` operating
mode. Both the dirty shelf nitrogen streams and clean shelf
nitrogen streams in FIGS. 5 and 6 are subcooled in heat exchange
cores via indirect heat exchange with the waste nitrogen stream 193
and the gaseous nitrogen product stream 195.
[0061] In the `no argon` operating mode or the `low argon`
operating mode depicted in FIG. 5, roughly 62.5% of the heat
exchanger cores are dedicated to sub-cooling the dirty shelf
nitrogen 84 whereas roughly 37.5% of the heat exchanger cores 120
are dedicated to sub-cooling the clean shelf nitrogen 83. As seen
therein, valves 110, 210 are set in the `open` position while
valves 112, 212 are set in the `closed` position so that five (5)
of the eight (8) subcooler cores pass dirty shelf nitrogen from the
inlet manifold 115 through the heat exchange cores to the exit
manifold 215 and three (3) of the eight (8) subcooler 99A cores
pass clean shelf nitrogen from the inlet manifold 115 through the
heat exchange cores to the exit manifold 215.
[0062] Conversely, in the `high argon` operating mode depicted in
FIG. 6, roughly 12.5% of the heat exchanger cores are dedicated to
sub-cooling the dirty shelf nitrogen 84 whereas roughly 87.5% of
the heat exchanger cores are dedicated to sub-cooling the clean
shelf nitrogen 83. As seen in FIG. 6, valves 110, 210 are set in
the `closed` position while valves 112, 212 are set in the `open`
position so that one (1) of the eight (8) subcooler cores pass
dirty shelf nitrogen 84 from a section of the inlet manifold 115
through the heat exchange core 120 to a section of the exit
manifold 215 and seven (7) of the eight (8) subcooler cores pass
clean shelf nitrogen 83 from the inlet manifold 115 through heat
exchange cores 120 to a section of exit manifold 215.
[0063] The dirty shelf nitrogen 84 is received from a draw point in
the higher pressure column 72 to a section of the inlet manifold
115 where it is distributed among one or more of the subcooler
cores 120 and then withdrawn from a section of the exit manifold
215, recombined, and passed through valve 97 before it flows in a
pipe up to a feed point in the lower pressure column 74. Likewise,
the clean shelf nitrogen is received as stream 83 to a section of
the inlet manifold 115 where it is distributed among one or more of
the subcooler cores 120 and then withdrawn from a section of the
exit manifold 215. A portion of the subcooled clean shelf nitrogen
83 is passed through valve 96 before it flows in a pipe up to a
feed point in the lower pressure column 74 while the remainder of
the clean shelf nitrogen, if any, is passed through a control valve
101 and taken as a liquid nitrogen product stream 98.
[0064] The valves depicted in the illustrated embodiments are
preferably gate valves, ball valves, or other type of valves that
exhibit very low pressure drop when fully open. Using valves that
exhibit very low pressure drop when fully open is important because
any significant pressure drop could tend to compromise the
distribution of the shelf liquid amongst the heat exchange cores.
Likewise, the valves could be manual valves or could be automatic
on-off valves if switching between the various operating modes is
expected to be done on a more frequent basis. While single valves
are depicted, each valve may instead be a combination of valves
called a "block and bleed" arrangement that is well known in the
industry. Such a configuration can be used to minimize the
possibility of leakage. Alternatively, a blind flange or
"spectacle" blind flange could be used in place of the valves.
[0065] FIG. 7 and FIG. 8 show another example of an eight (8) heat
exchanger core banked subcooler arrangement. In the `no argon`
operating mode or the `low argon` operating mode depicted in FIG.
7, the dirty shelf nitrogen 84 and clean shelf nitrogen 83 flows
are split in in an identical fashion as discussed above with
reference to FIG. 5 with five (5) of the eight (8) heat exchange
cores 120 designated for dirty shelf nitrogen. Valves 111, 211 are
set in the `open` position while valves 112, 212 are set in the
`closed` position so that only three (3) of the eight (8) heat
exchange cores pass clean shelf nitrogen 83 from the inlet manifold
115 through the heat exchange cores 120 to the exit manifold
215.
[0066] However, in the `high argon` operating mode generally
depicted in FIG. 8, all the subcooler heat exchange cores 120 are
configured to receive clean shelf nitrogen 83 from the inlet
manifold 115 and to pass the sub-cooled clean shelf nitrogen 83 to
the exit manifold. Hence, the valves 111 and 211 are disposed in
the main dirty shelf nitrogen conduits rather than in the inlet
manifold 115 and exit manifold 215 and are in the `closed` position
such that no dirty shelf nitrogen 84 is used in this embodiment of
the `high argon` operating mode. Similar to the embodiment of FIG.
6, a portion of the subcooled clean shelf nitrogen 83 is passed
through valve 96 before it flows in a pipe up to a feed point in
the lower pressure column 74 while the remainder of the clean shelf
nitrogen, if any, is passed through a control valve 101 and may be
taken as a liquid nitrogen product stream 98.
[0067] The embodiments shown in FIG. 9 and FIG. 10 show another
example of an eight (8) heat exchanger core banked subcooler
arrangement with cross-tied shelf transfer lines. This embodiment
differs from the embodiment shown in FIGS. 5 and 6 in that there is
a shelf transfer line 201, and shelf transfer valves 203, 205 as
well as two (2) separate transfer conduits 216, 217 from the exit
manifold 215 to the lower pressure column 74 and separate feed
points 202, 204, 206, 208 to the lower pressure column 74.
[0068] In the `no argon` operating mode or the `low argon`
operating mode depicted in FIG. 9, the dirty shelf nitrogen 84 and
clean shelf nitrogen 83 flows are split with three (3) of the eight
(8) heat exchange cores 120 designated for clean shelf nitrogen 83
and five (5) of the eight (8) heat exchange cores 120 designated
for dirty shelf nitrogen 84. In these operating modes, transfer
valve 203 is in the `open` position while transfer valve 205 is in
the `closed` position. The clean shelf nitrogen 83 is directed to
the lower pressure column 74 via transfer valve 203 and shelf
transfer line 201 to a small diameter conduit 216 with valve 218
and introduced into lower pressure column 74 at an uppermost
location 202. The dirty shelf nitrogen 84 is directed to lower
pressure column 74 via a large diameter conduit 217 with valve 219
and introduced into lower pressure column 74 at a second location
208 that is below uppermost location 202.
[0069] In the `high argon` operating mode depicted in FIG. 10, the
dirty shelf nitrogen and clean shelf nitrogen flows are split with
seven (7) of the eight (8) heat exchange cores 120 designated for
clean shelf nitrogen 83 and one (1) of the eight (8) heat exchange
cores 120 designated for dirty shelf nitrogen 84. Transfer valve
203 is in the `closed` position while transfer valve 205 is in the
`open` position. The clean shelf nitrogen 83 is directed to the
lower pressure column 74 via the large diameter conduit 217 with
valve 219 and introduced into the lower pressure column 74 at the
uppermost location 204. The dirty shelf nitrogen 84 is directed to
transfer valve 205 to the small diameter conduit 216 with valve 218
and introduced into the lower pressure column 74 at the second
location 206 that is below the uppermost location 202. Preferably,
the clean shelf nitrogen should be valved to feed at a point above
the top hat of the lower pressure column whereas the dirty shelf
stream should be valved to feed at a point proximate the same
location as the waste nitrogen draw. Conduit diameters are
preferably selected to achieve stable flow throughout the design
flow ranges. For example, the embodiment of FIGS. 9 and 10, the
small pipe diameter will be designed with a flow range of about 3:1
and the large pipe diameter be designed with a flow range of about
5:7 to ensure a stable two phase flows through these conduits at
the targeted flow ranges.
[0070] FIG. 11 and FIG. 12 show another example of a parallel and
banked subcooler arrangement. In this embodiment, the two parallel
subcoolers and associated heat exchanger cores need not be
identically configured. Using this arrangement in the `no argon`
operating mode or the `low argon` operating mode as generally
depicted in FIG. 11, the dirty shelf nitrogen and clean shelf
nitrogen flows are split with three (3) of the four (4) flow paths
designated for dirty shelf nitrogen 84. Valves 110, 210 are set in
the `open` position while valves 112, 212 are set in the `closed`
position so that the flow through the remaining flow path 120 is
clean shelf nitrogen 83. On the other hand, using this arrangement
in the `high argon` operating mode as generally depicted in FIG.
12, the majority of the flow paths 120 are dedicated to sub-cooling
the clean shelf nitrogen 83 while the remaining flow path 120 is
sub-cooling dirty shelf nitrogen 84. Valves 110, 210 are set in a
`closed` position while valves 112, 212 are set in an `open`
position
[0071] In order to achieve the desired flow split between clean
shelf nitrogen 83 and dirty shelf nitrogen 84 in the embodiments of
FIGS. 11 and 12, split headers 221, 222 are used for each of the
parallel banked subcoolers. The parallel banked subcooler
arrangement in FIGS. 11 and 12 have a first split header 221 to
divide the shelf nitrogen flow in the first subcooler into 25.0%
and 75.0% fractions and a second split header 222 to divide the
shelf nitrogen flow in the second subcooler into 50.0% and 50.0%
fractions. Suitable manifolding of the parallel, banked subcooler
arrangements are contemplated that would allow very flexible splits
or even customized splits between dirty shelf nitrogen and clean
shelf nitrogen. Note that the apportionment of dirty shelf nitrogen
and clean shelf nitrogen within the subcooler units described in
FIGS. 11 and 12 can apply for a single core and for more than two
cores. It should also be noted that while customized split header
designs described are preferred for achieving the desired flow
splits of dirty shelf nitrogen and clean shelf nitrogen, customized
designs of the subcoolers could alternatively be used to apportion
the flows of dirty shelf nitrogen and clean shelf nitrogen for each
mode. However, by customizing the split header designs, the designs
of each subcooler need not be customized. Finally, implicit in the
description of apportioned dirty shelf liquid and clean shelf
liquid flows amongst the banked parallel subcoolers is a thermal
equivalence of the two streams. While dirty shelf liquid and clean
shelf liquid are not exactly thermally equivalent, they are very
similar and can be treated as thermally equivalent.
Examples
[0072] For various embodiments of the present system and method for
flexible recovery of argon, a number of process simulations were
run using various air separation unit operating models to
characterize: (i) the impact of shifting between `low argon`
operating mode and `high argon` operating mode on overall power
consumption/savings and argon recovery in a large oxygen producing
air separation unit; (ii) the impact of drawing shelf vapor on
power consumption/savings and argon recovery in a large oxygen
producing air separation unit; and (iii) the impact of using
various splits of dirty shelf nitrogen and clean shelf nitrogen as
the reflux stream to the lower pressure column on the power
consumption/savings and argon recovery of a large oxygen producing
air separation unit.
[0073] Table 1 shows the results of the computer based process
simulation for the present systems and associated methods described
above. For the process simulations, a 1300 ton per day (TPD) pumped
liquid oxygen plant having a lower column turbine was simulated.
The modeling parameters included different selected air separation
unit operating modes (i.e. `low argon` mode and `high argon` mode)
as well as selected liquid product makes. Such parameter selections
involved varying the dirty shelf nitrogen ratio (i.e. the molar
flow rate of the dirty shelf nitrogen used in the reflux stream
divided by the sum of the molar flow rates of dirty shelf nitrogen
and clean shelf nitrogen used in the reflux stream) and varying the
shelf vapor ratio (i.e. molar flow rate of shelf vapor withdrawn as
gaseous nitrogen product divided by the molar flow rate of the
liquid oxygen taken from the lower pressure column).
TABLE-US-00001 TABLE 1 Impact of Shelf Vapor, Dirty Shelf, and
Clean Shelf on Power Consumption and Argon Recovery Shelf Vapor
Dirty Shelf Argon Power Savings ASU Plant ASU Liquid Argon Ratio
Ratio Recovery (%) vs No DS & Description Make Mode [SV/O2]
[DS/(DS + CS)] (%) No Shelf Vapor 1300 TPD 2% Liquid Low .74 .47 68
6.9 Oxygen Plant 1300 TPD 2% Liquid High .33 .30 88 3.5 Oxygen
Plant 1300 TPD 6% Liquid Low .78 .49 67 8.1 Oxygen Plant 1300 TPD
6% Liquid High .19 .15 93 3.2 Oxygen Plant
[0074] As seen in Table 1, for a given product slate, i.e. 6%
liquid, and selection of a `low argon` operating mode with the
dirty shelf ratio of 0.49 and a shelf vapor ratio of 0.78; the
argon recovery is only 67% but the power savings of 8.1% may be
realized compared to the base case of no shelf vapor taken from the
higher pressure column and all clean shelf nitrogen used as reflux
to the lower pressure column. For the same product slate, i.e. 6%
liquid, and selection of a `high argon` operating mode with the
dirty shelf ratio of 0.15 and a shelf vapor ratio of 0.19; the
argon recovery is about 93% but with a lower power savings of only
3.2% compared to the base case of no shelf vapor taken from the
higher pressure column and all clean shelf nitrogen used as reflux
to the lower pressure column.
[0075] For a given product slate of only 2% liquid and the air
separation unit operating in a `low argon` operating mode with the
dirty shelf ratio of 0.47 and a shelf vapor ratio of 0.74; the
argon recovery is only 68% but the power savings of 6.9% may be
realized compared to the base case of no shelf vapor taken from the
higher pressure column and all clean shelf nitrogen used as reflux
to the lower pressure column. For the same product slate, i.e. 2%
liquid and selection of the `high argon` operating mode with the
dirty shelf ratio of 0.30 and a shelf vapor ratio of 0.33; the
argon recovery is about 88% but with a lower power savings of only
3.5% compared to the base case of no shelf vapor taken from the
higher pressure column and all clean shelf nitrogen used as reflux
to the lower pressure column.
[0076] It should be noted that the above example is for
illustration purposes only. The results could be very different
depending on a number of factors, for example the specifics of the
air separation process and design, the products required, the cost
of power, and the market value of argon.
[0077] Although the present system for the flexible recovery of
argon in an air separation unit has been disclosed with reference
to one or more preferred embodiments and methods associated
therewith, as would occur to those skilled in the art that numerous
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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