U.S. patent application number 14/444438 was filed with the patent office on 2016-01-28 for air separation method and apparatus.
The applicant listed for this patent is Yang Luo, Neil M. Prosser, Zhengrong Xu. Invention is credited to Yang Luo, Neil M. Prosser, Zhengrong Xu.
Application Number | 20160025408 14/444438 |
Document ID | / |
Family ID | 53490280 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160025408 |
Kind Code |
A1 |
Xu; Zhengrong ; et
al. |
January 28, 2016 |
AIR SEPARATION METHOD AND APPARATUS
Abstract
A method and apparatus for separating air by cryogenic
rectification in which cooled, compressed and purified air is
separated in a distillation column system having higher and lower
pressure columns operatively associated with one another in a heat
transfer relationship to produce an oxygen-rich liquid stream from
the lower pressure column. The oxygen-rich liquid stream is pumped
and heated through indirect heat exchange with a compressed heat
exchange stream to form a pressurized oxygen product stream. Part
of the air is sequentially and successively compressed in booster
compressors driven by turboexpanders to form the compressed heat
exchange stream while other parts of the air are expanded in
turboexpanders driving the booster compressors to form exhaust
streams that are introduced into both the higher and lower pressure
columns to generate refrigeration.
Inventors: |
Xu; Zhengrong; (East
Amherst, NY) ; Prosser; Neil M.; (Lockport, NY)
; Luo; Yang; (Amherst, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Xu; Zhengrong
Prosser; Neil M.
Luo; Yang |
East Amherst
Lockport
Amherst |
NY
NY
NY |
US
US
US |
|
|
Family ID: |
53490280 |
Appl. No.: |
14/444438 |
Filed: |
July 28, 2014 |
Current U.S.
Class: |
62/643 |
Current CPC
Class: |
F25J 3/04303 20130101;
F25J 2240/46 20130101; F25J 3/04678 20130101; F25J 3/04296
20130101; F25J 2215/54 20130101; F25J 2240/44 20130101; F25J
3/04393 20130101; F25J 3/04084 20130101; F25J 3/04412 20130101;
F25J 3/04175 20130101; F25J 3/0409 20130101 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1. A method of separating air within a cryogenic rectification
process, said method comprising: separating the air in the
cryogenic rectification process by cooling the air, after having
been compressed and purified and rectifying the air in a
distillation column system having a higher pressure column and a
lower pressure column operatively associated within one another in
a heat transfer relationship to produce return streams enriched in
components of the air that are warmed through indirect heat
exchange with the air to help cool the air and to produce product
streams; one of the product streams formed by withdrawing an
oxygen-rich liquid stream from a bottom region of the lower
pressure column, pumping at least part of the oxygen-rich liquid
stream to produce a pumped liquid oxygen stream and heating at
least part of the pumped liquid oxygen stream to form a pressurized
oxygen product stream, the at least part of the pumped liquid
oxygen stream constituting one of the return streams and the at
least part of the pumped liquid oxygen stream heated through
indirect heat exchange with a compressed heat exchange stream
composed of part of the air to be cooled and rectified in the
distillation column system; forming the compressed heat exchange
stream, a first exhaust stream and a second exhaust stream with the
use of a first booster loaded expander and a second booster loaded
expander having booster compressors driven by turboexpanders by
sequentially compressing the part of the air within the booster
compressors of the first booster loaded expander and the second
booster loaded expander to form the compressed heat exchange stream
and partially cooling and then expanding other parts of the air
within the turboexpanders to produce a first exhaust stream and a
second exhaust stream from expansion of the other parts of the air
within in the first booster loaded expander and the second booster
loaded expander, respectively; and introducing the first exhaust
stream into lower pressure column and the second exhaust stream
into the higher pressure column, thereby to impart refrigeration
into the cryogenic rectification process.
2. The method of claim 1, wherein: a first compressed air stream, a
second compressed air stream and a third compressed air stream are
formed, at least in part, by compressing and purifying the air to
produce a compressed and purified air stream and dividing the
compressed and purified air stream into the first compressed air
stream, the second compressed air stream and the third compressed
air stream, thereby to form the part of the air from the first
compressed air stream and the other parts of the air from the
second compressed air stream and the third compressed air stream;
the first compressed air stream is sequentially compressed within a
first and second booster compressor of the first booster loaded
expander and the second booster loaded expander to form the
compressed heat exchange stream; the second compressed air stream
is partially cooled and introduced into a first turboexpander of
the first booster loaded expander, thereby to produce the first
exhaust stream; the third compressed air stream is partially cooled
and introduced into a second turboexpander of the second booster
loaded expander, thereby to produce the second exhaust stream; and
the first compressed air stream and the second compressed air
stream is partially cooled in a main heat exchanger and the
compressed heat exchange stream condensed in the main heat
exchanger through indirect heat exchange with the at least part of
the pumped liquid oxygen stream to form a liquid air stream; the
liquid air stream is divided into first and second subsidiary
liquid air streams that are introduced into the higher pressure
column and the lower pressure column after having been reduced in
pressure compatible with the higher pressure column and the lower
pressure column.
3. The method of claim 2, wherein: the first compressed stream is
further compressed in a third booster compressor located upstream
of the first and second booster compressor; and the third
compressed air stream is further compressed in a forth booster
compressor located upstream of the second turboexpander.
4. The method of claim 1 or claim 2, wherein: the oxygen-rich
liquid stream is divided into a first oxygen-rich liquid subsidiary
stream and a second oxygen-rich liquid subsidiary stream; the first
oxygen-rich liquid subsidiary stream is pumped by a pump to produce
the pumped liquid oxygen stream; and the second oxygen-rich liquid
subsidiary stream is taken as a liquid product.
5. The method of claim 4, wherein: a nitrogen-rich liquid stream is
pumped to produce a pumped liquid nitrogen stream and is warmed
through indirect heat exchange with the compressed heat exchange
stream to produce another of the product streams; and the pumped
liquid oxygen stream is divided into a first pumped oxygen stream
and a second pumped oxygen stream which are warmed through indirect
heat exchange with the compressed heat exchange stream and the
second pumped oxygen stream is passed through a valve prior to
being warmed so that pressurized oxygen products at two different
pressures are produced.
6. The method of claim 5, wherein: the higher pressure column and
the lower pressure column are thermally linked by a condenser
reboiler condensing nitrogen-rich vapor column overhead in the
higher pressure column through indirect heat exchange with the
oxygen-rich liquid column bottoms of the lower pressure column,
thereby producing nitrogen-rich reflux streams introduced, at least
in part, into the higher pressure column and the lower pressure
column as reflux; the distillation column system also has an argon
column connected to the lower pressure column to separate argon
from oxygen containing in a crude argon feed stream withdrawn from
the lower pressure column and fed to the argon column for
rectification; a kettle liquid stream composed of a crude liquid
oxygen column bottoms of the higher pressure column is partially
vaporized in an argon condenser connected to the argon column to
produce reflux for the argon column and a liquid argon-rich liquid
stream; liquid and vapor phase streams produced as a result of
partially vaporizing the kettle liquid stream are introduced into
the lower pressure column for further refinement; one of the
nitrogen-rich reflux streams and the kettle liquid streams are
subcooled in a subcooling heat exchanger; and a lower pressure
column, nitrogen-rich vapor column overhead stream and a waste
nitrogen stream are partially warmed in the subcooling heat
exchanger and further warmed within the main heat exchanger to help
cool the incoming air.
7. An air separation apparatus comprising: an air separation plant
having a main heat exchanger for cooling the air, after having been
compressed and purified and a distillation column system connected
to the main heat exchanger and having a higher pressure column and
a lower pressure column operatively associated within one another
in a heat transfer relationship and producing return streams
enriched in components of the air that are warmed within the main
heat exchanger through indirect heat exchange with the air to help
cool the air and to produce product streams; the air separation
plant having a pump connected to a bottom region of the lower
pressure column to pump at least part of an oxygen-rich liquid
stream to produce a pumped liquid oxygen stream and the pump also
connected to the main heat exchanger so that at least part of the
pumped liquid oxygen stream is heated within the main heat
exchanger as one of the return streams to form a pressurized oxygen
product stream constituting one of the product streams; the main
heat exchanger configured so that the at least part of the pumped
liquid oxygen stream is heated through indirect heat exchange with
a compressed heat exchange stream composed of part of the air to be
cooled and rectified in the distillation column system; and the air
separation plant also having a first booster loaded expander and a
second booster loaded expander comprising first and second booster
compressors connected to one another and to the main heat exchanger
so that part of the air is sequentially compressed within the first
and second booster compressors to form the compressed heat exchange
stream and first and second turboexpanders drive the first and
second booster compressors, respectively; the first and second
turboexpanders connected to the main heat exchanger so that other
parts of the air are expanded after having been partially cooled in
the main heat exchanger, thereby producing a first exhaust stream
and a second exhaust stream, respectively; and the first and second
turboexpanders connected to the distillation column system so that
the first exhaust stream is introduced into lower pressure column
and the second exhaust stream is introduced into the higher
pressure column, thereby to impart refrigeration into the air
separation plant.
8. The apparatus of claim 7, wherein: the air separation plant has
a main air compressor connected to a pre-purification unit to
produce a compressed and purified air stream; the first of the
booster compressors in flow communication with the pre-purification
unit so that the first compressed air stream is formed from part of
the compressed and purified air stream and is sequentially
compressed within a first and second booster compressors to form
the compressed heat exchange stream; the main heat exchanger is in
flow communication with the pre-purification unit so that the
second compressed air stream and the third compressed air stream
are formed from other parts of the compressed and purified air
stream and are partially cooled in the main heat exchanger; the
higher pressure column and the lower pressure column connected to
the main heat exchanger so that a liquid air stream, formed from
the compressed heat exchange stream indirectly exchanging heat with
the at least part of the pumped liquid oxygen stream, divides into
first and second subsidiary liquid air streams that are introduced
into the higher pressure column and the lower pressure column; and
expansion valves are positioned so that the first and second
subsidiary liquid air streams are reduced in pressure compatible
with that the higher pressure column and the lower pressure
column.
9. The apparatus of claim 7, wherein: a third booster compressor is
located between the pre-purification unit and the first of the
booster compressors so that the first compressed air stream is
further compressed in the third booster compressor; and a forth
booster compressor is located between the main heat exchanger and
pre-purification unit so that the third compressed air stream is
further compressed in the forth booster compressor prior to being
partially cooled in the main heat exchanger.
10. The apparatus of claim 7 or claim 8, wherein: a piping juncture
is located between the pump and the bottom region of the lower
pressure column so that the oxygen-rich liquid stream is divided
into a first oxygen-rich liquid subsidiary stream and a second
oxygen-rich liquid subsidiary stream; and the pump connected to the
piping juncture so that first oxygen-rich liquid subsidiary stream
is pumped by a pump to produce the pumped liquid oxygen stream and
the second oxygen-rich liquid subsidiary stream is able to be taken
as a liquid product.
11. The apparatus of claim 10, wherein: the main heat exchanger
also has passages to warm a pumped liquid nitrogen stream and a
first pumped oxygen stream and a second pumped oxygen stream
through indirect heat exchange with the compressed heat exchange
stream to produce other of the product streams and the pump is
connected to the passages so that pumped liquid oxygen stream is
divided into the first pumped oxygen stream and the second pumped
oxygen stream; an expansion valve is located between the pump and
one of the passages so that the second pumped oxygen stream is
passed through a valve prior to being warmed and pressurized oxygen
products at two different pressures are produced; and another pump
is located between the higher pressure column and the main heat
exchanger to pump a liquid nitrogen stream and thereby form the
pumped liquid nitrogen stream.
12. The apparatus of claim 11, wherein: the higher pressure column
and the lower pressure column are thermally linked by a condenser
reboiler condensing nitrogen-rich vapor column overhead in the
higher pressure column through indirect heat exchange with the
oxygen-rich liquid column bottoms of the lower pressure column,
thereby producing nitrogen-rich reflux streams; the higher pressure
column and the lower pressure column are connected to the condenser
reboiler so that the nitrogen-rich reflux streams are introduced,
at least in part, into the higher pressure column and the lower
pressure column as reflux; the distillation column system also has
an argon column connected to the lower pressure column so that a
crude argon feed stream from the lower pressure column is rectified
in the argon column to separate argon from oxygen contained in the
crude argon feed stream; an argon condenser is connected to the
argon column to produce reflux for the argon column and a liquid
argon-rich liquid stream; the argon condenser is connected to the
higher pressure column so that a kettle liquid stream composed of a
crude liquid oxygen column bottoms of the higher pressure column is
partially vaporized in the argon condenser; the argon condenser
connected to the lower pressure column so that liquid and vapor
phase streams produced as a result of partially vaporizing the
kettle liquid stream are introduced into the lower pressure column
for further refinement; a subcooling heat exchanger in flow
communication with the condenser reboiler and the higher pressure
column so that one of the nitrogen-rich reflux streams and the
kettle liquid streams are subcooled in a subcooling heat exchanger;
and the subcooling heat exchanger positioned between the lower
pressure column and the main heat exchanger so that a lower
pressure column, nitrogen-rich vapor column overhead stream and a
waste nitrogen stream are partially warmed in the subcooling heat
exchanger and further warmed within the main heat exchanger to help
cool the incoming air.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
separating air through a cryogenic rectification process in which a
pressurized oxygen product stream is formed by pumping an
oxygen-rich liquid stream to produce a pumped liquid oxygen stream
and warming the pumped liquid oxygen stream through indirect heat
exchange with a compressed heat exchange steam that is composed of
part of the air to be separated. More particularly, the present
invention relates to such a method and apparatus in which the
compressed heat exchange stream is formed by sequentially
compressing the air within booster compressors of a first booster
loaded expander and a second booster loaded expander that have
turboexpanders to expand partially cooled portions of the air to
produce exhaust streams that are introduced into thermally linked
higher and lower pressure columns to impart refrigeration into the
process.
BACKGROUND OF THE INVENTION
[0002] Air is separated into its component parts by means of a
cryogenic rectification process conducted in distillation columns
operated at cryogenic temperatures. The air in such a process is
first compressed in a main air compression system that may have a
series of compression stages linked to one another by intercoolers
to remove the heat of compression between stages. The compressed
air is then purified of higher boiling contaminants such as water
vapor, carbon dioxide and hydrocarbons within adsorbent beds
operated in an out-of-phase cycle where one adsorbent bed is
regenerated while another of the beds is adsorbing the impurities.
The cycle can be a temperature swing cycle, a pressure swing cycle
or a combination of both cycles. After purification the air is
cooled to a temperature suitable for its distillation and then
separated within distillation columns to produce oxygen-rich and
nitrogen-rich streams that are withdrawn from the columns and then
used in cooling the incoming air within a main heat exchanger. The
warmed streams constitute oxygen and nitrogen-rich products.
[0003] The distillation columns can include higher and lower
pressure columns. These distillation columns are so designated
given that the higher pressure column operates at a higher pressure
than the lower pressure column. The incoming air, after having been
purified and cooled, is introduced into the higher pressure column
and separated to produce a crude liquid oxygen column bottoms, also
known as kettle liquid and a nitrogen-rich vapor column overhead.
The crude liquid oxygen is further refined in the lower pressure
column to produce an oxygen-rich liquid column bottoms and another
nitrogen-rich vapor column overhead. The columns are thermally
linked by a condenser reboiler in which nitrogen-rich vapor
produced as the column overhead of the higher pressure column is
condensed through indirect heat exchange with the oxygen-rich
liquid column bottoms of the lower pressure column, thereby
producing liquid nitrogen reflux for the distillation columns and
boilup within the lower pressure column. An argon column can be
connected to the lower pressure column to separate and argon from
oxygen in a crude argon feed stream fed to the argon column from
the lower pressure column. If a pressurized oxygen product stream
is desired, a stream of the oxygen-rich liquid column bottoms of
the lower pressure column can be pumped to produce a pumped liquid
oxygen stream. The pumped liquid oxygen stream can be heated
through indirect heat exchange with a compressed heat exchange
stream composed of part of the air to be separated. The heat
exchange results in liquefaction of the air within the compressed
heat exchange stream and resulting liquid air can be introduced as
intermediate reflux into both the higher and lower pressure
columns.
[0004] The compressed heat exchange stream is commonly produced by
compressing a portion of the air in a booster compressor after the
air has been compressed and purified. Typically, this portion of
the air constitutes about 30 percent of the incoming air. The
remainder of the air, after having been cooled, is introduced into
the higher pressure column. Additionally, the air after having been
compressed and purified can be partially cooled and then expanded
in a turboexpander to produce an exhaust stream. The exhaust stream
is in turn introduced into the higher pressure column to impart
refrigeration and thereby balance losses at the warm end of a main
heat exchanger used in cooling the air and the export of
refrigeration accompanied by the production of liquid products that
are discharged from the plant as liquids.
[0005] As can be appreciated, the booster compressor, used in
compressing the air and thereby forming the compressed heat
exchange stream, consumes electrical power and thus, represents
part of the ongoing expense in producing a pressurized oxygen
product. In order to decrease such expense, it is known in the
prior art to compress part or all of the air at a sub-ambient
temperature to produce the pressures required in suitably boosting
pressure of the compressed air to form the compressed heat exchange
stream. In such "cold compression," all or a portion of the
compressed air, after having been partially cooled to a temperature
intermediate the warm and cold ends of the main heat exchanger, is
compressed at the intermediate temperature and then reintroduced
into the main heat exchanger at a temperature level at which the
air passes through a phase transition from a liquid to a vapor or a
supercritical fluid. Since the air at such point is cold and
therefore, has a greater density than the ambient air, less power
is expended in compressing the air than had the air been solely
compressed at the warm end of the main heat exchanger. For example,
U.S. Pat. No. 5,475,980 discloses a cryogenic air separation
process to produce a pressurized product stream in which part of
the air being cooled is withdrawn from an intermediate location of
the main heat exchanger used in cooling the air. The withdrawn air
is then compressed by a compressor and reintroduced back into the
main heat exchanger at a location thereof at which the oxygen
vaporizes. After having been partially cooled, the air that has
been reintroduced into the main heat exchanger is then expanded in
a turboexpander coupled to the compressor so that no external
energy will be required in compressing the air at the intermediate
location of the main heat exchanger. The resulting exhaust stream,
which is a two phase flow, is then introduced into a phase
separator. A liquid phase stream is introduced into the higher
pressure column and a vapor phase stream is partially warmed,
expanded and then introduced into the lower pressure column to
impart additional refrigeration into the process. As can be
appreciated, such a process introduces complexity and expense into
the main heat exchanger used in carrying out the process because of
the intermediate outlets and inlets that are necessarily required
to withdraw and reintroduced air back into the heat exchanger.
[0006] As will be discussed, the present invention provided a
method and apparatus for separating air through cryogenic
rectification and producing a pressurized oxygen product that among
other advantages is energy efficient and can utilize conventional
warm end heat exchange equipment that is less complex and
therefore, expensive than prior art equipment discussed above.
SUMMARY OF THE INVENTION
[0007] The present invention provides a method of separating air
within a cryogenic rectification process in which the air is
separated by cooling the air, after having been compressed and
purified and rectifying the air in a distillation column system
having a higher pressure column and a lower pressure column
operatively associated within one another in a heat transfer
relationship. Return streams, enriched in components of the air,
are produced that are warmed through indirect heat exchange with
the air to help cool the air and to produce product streams. One of
the product streams is formed by withdrawing an oxygen-rich liquid
stream from a bottom region of the lower pressure column, pumping
at least part of the oxygen-rich liquid stream to produce a pumped
liquid oxygen stream and heating at least part of the pumped liquid
oxygen stream to form a pressurized oxygen product stream. The at
least part of the pumped liquid oxygen stream constitutes one of
the return streams and the at least part of the pumped liquid
oxygen stream is heated through indirect heat exchange with a
compressed heat exchange stream composed of part of the air to be
cooled and rectified in the distillation column system. The
compressed heat exchange stream, a first exhaust stream and a
second exhaust stream are formed with the use of a first booster
loaded expander and a second booster loaded expander having booster
compressors driven by turboexpanders. In this regard, the term,
"booster loaded expander" as used herein and in the claims means a
turboexpander coupled directly to a booster compressor so that the
work of expansion is dissipated in powering the booster compressor.
Part of the air is sequentially compressed within the booster
compressors of the first booster loaded expander and the second
booster loaded expander to form the compressed heat exchange
stream. Other parts of the air are partially cooled and then
expanded within the turboexpanders to produce a first exhaust
stream and a second exhaust stream from expansion of the other
parts of the air within in the first booster loaded expander and
the second booster loaded expander, respectively. The first exhaust
stream is introduced into lower pressure column and the second
exhaust stream is introduced into the higher pressure column,
thereby to impart refrigeration into the cryogenic rectification
process.
[0008] As compared with the prior art, since the air need not be
extracted and then reintroduced into the main heat exchanger, the
design of the main heat exchanger design can be simpler and
therefore, less expensive, than prior art heat exchangers where
cold compression is utilized. Furthermore, even though cold
compression is not used in the present invention, since the energy
required to compress the air in forming the compressed heat
exchange stream is recovered in turboexpanders coupled to the
compressors, the overall energy efficiency of the process is better
than or at least equal to that of prior art cold compression
techniques to make the present invention attractive from the
standpoint of energy consumption.
[0009] Preferably, a first compressed air stream, a second
compressed air stream and a third compressed air stream can be
formed, at least in part, by compressing and purifying the air to
produce a compressed and purified air stream and dividing the
compressed and purified air stream into the first compressed air
stream, the second compressed air stream and the third compressed
air stream. The part of the air that is thereby compressed is thus
formed from the first compressed air stream and the other parts of
the air that are expanded are formed from the second compressed air
stream and the third compressed air stream. More specifically, the
first compressed air stream is sequentially compressed within a
first and second booster compressor of the first booster loaded
expander and the second booster loaded expander to form the
compressed heat exchange stream. The second compressed air stream
is partially cooled and introduced into a first turboexpander of
the first booster loaded expander, thereby to produce the first
exhaust stream and the third compressed air stream is partially
cooled and introduced into a second turboexpander of the second
booster loaded expander, thereby to produce the second exhaust
stream. The first compressed air stream and the second compressed
air stream are partially cooled in a main heat exchanger and the
compressed heat exchange stream condensed in the main heat
exchanger through indirect heat exchange with the at least part of
the pumped liquid oxygen stream to form a liquid air stream. The
liquid air stream is divided into first and second subsidiary air
streams that are introduced into the higher pressure column and the
lower pressure column after having been reduced in pressure
compatible with the higher pressure column and the lower pressure
column. Preferably, the first compressed stream is further
compressed in a third booster compressor located upstream of the
first and second booster compressor and the third compressed air
stream is further compressed in a forth booster compressor located
upstream of the second turboexpander.
[0010] The oxygen-rich liquid stream can be divided into a first
oxygen-rich liquid subsidiary stream and a second oxygen-rich
liquid subsidiary stream. The first oxygen-rich liquid subsidiary
stream is pumped by a pump to produce the pumped liquid oxygen
stream and the second oxygen-rich liquid subsidiary stream is taken
as a liquid product. Additionally, a nitrogen-rich liquid stream
can be pumped to produce a pumped liquid nitrogen stream. This
stream is also warmed through indirect heat exchange with the
compressed heat exchange stream to produce another of the product
streams. The pumped liquid oxygen stream can be divided into a
first pumped oxygen stream and a second pumped oxygen stream which
are warmed through indirect heat exchange with the compressed heat
exchange stream. The second pumped oxygen stream can be passed
through a valve prior to being warmed so that pressurized oxygen
products at two different pressures are produced.
[0011] The higher pressure column and the lower pressure column can
be thermally linked by a condenser reboiler condensing
nitrogen-rich vapor column overhead in the higher pressure column
through indirect heat exchange with the oxygen-rich liquid column
bottoms of the lower pressure column, thereby producing
nitrogen-rich reflux streams introduced, at least in part, into the
higher pressure column and the lower pressure column as reflux. The
distillation column system can also have an argon column connected
to the lower pressure column to separate argon from oxygen
containing in a crude argon feed stream withdrawn from the lower
pressure column and fed to the argon column for rectification. A
kettle liquid stream composed of a crude liquid oxygen column
bottoms of the higher pressure column is partially vaporized in an
argon condenser connected to the argon column to produce reflux for
the argon column and a liquid argon-rich liquid stream. Liquid and
vapor phase streams produced as a result of partially vaporizing
the kettle liquid stream are introduced into the lower pressure
column for further refinement. One of the nitrogen-rich reflux
streams and the kettle liquid streams are subcooled in a subcooling
heat exchanger and a lower pressure column, nitrogen-rich vapor
column overhead stream and a waste nitrogen stream are partially
warmed in the subcooling heat exchanger and further warmed within
the main heat exchanger to help cool the incoming air.
[0012] In another aspect, the present invention provides and air
separation apparatus that comprises an air separation plant having
a main heat exchanger for cooling the air, after having been
compressed and purified and a distillation column system connected
to the main heat exchanger. The distillation column system has a
higher pressure column and a lower pressure column operatively
associated within one another in a heat transfer relationship and
producing return streams enriched in components of the air that are
warmed within the main heat exchanger through indirect heat
exchange with the air to help cool the air and to produce product
streams. The air separation plant has a pump connected to a bottom
region of the lower pressure column to pump at least part of an
oxygen-rich liquid stream and thereby to produce a pumped liquid
oxygen stream. The pump is also connected to the main heat
exchanger so that at least part of the pumped liquid oxygen stream
is heated within the main heat exchanger as one of the return
streams to form a pressurized oxygen product stream that
constitutes one of the product streams. The main heat exchanger is
configured so that the at least part of the pumped liquid oxygen
stream is heated through indirect heat exchange with a compressed
heat exchange stream composed of part of the air to be cooled and
rectified in the distillation column system. The air separation
plant also has a first booster loaded expander and a second booster
loaded expander comprising first and second booster compressors
connected to one another and to the main heat exchanger so that
part of the air is sequentially compressed within the first and
second booster compressors to form the compressed heat exchange
stream and first and second turboexpanders that drive the first and
second booster compressors, respectively. The first and second
turboexpanders are connected to the main heat exchanger so that
other parts of the air are expanded after having been partially
cooled in the main heat exchanger, thereby producing a first
exhaust stream and a second exhaust stream, respectively. The first
and second turboexpanders are connected to the distillation column
system so that the first exhaust stream is introduced into lower
pressure column and the second exhaust stream is introduced into
the higher pressure column, thereby to impart refrigeration into
the air separation plant.
[0013] The air separation plant can have a main air compressor
connected to a pre-purification unit to produce a compressed and
purified air stream. The first of the booster compressors is in
flow communication with the pre-purification unit so that the first
compressed air stream is formed from part of the compressed and
purified air stream and is sequentially compressed within a first
and second booster compressors to form the compressed heat exchange
stream. The main heat exchanger is in flow communication with the
pre-purification unit so that the second compressed air stream and
the third compressed air stream are formed from other parts of the
compressed and purified air stream and are partially cooled in the
main heat exchanger. The higher pressure column and the lower
pressure column are connected to the main heat exchanger so that a
liquid air stream, formed from the compressed heat exchange stream
indirectly exchanging heat with the at least part of the pumped
liquid oxygen stream, divides into first and second subsidiary
liquid air streams that are introduced into the higher pressure
column and the lower pressure column. Expansion valves are
positioned so that the first and second subsidiary liquid air
streams are reduced in pressure compatible with that the higher
pressure column and the lower pressure column. Further, a third
booster compressor can be located between the pre-purification unit
and the first of the booster compressors so that the first
compressed air stream is further compressed in the third booster
compressor. A forth booster compressor can be located between the
main heat exchanger and pre-purification unit so that the third
compressed air stream is further compressed in the forth booster
compressor prior to being partially cooled in the main heat
exchanger.
[0014] A piping juncture can be located between the pump and the
bottom region of the lower pressure column so that the oxygen-rich
liquid stream is divided into a first oxygen-rich liquid subsidiary
stream and a second oxygen-rich liquid subsidiary stream. The pump
is connected to the piping juncture so that first oxygen-rich
liquid subsidiary stream is pumped by a pump to produce the pumped
liquid oxygen stream and the second oxygen-rich liquid subsidiary
stream is able to be taken as a liquid product. The main heat
exchanger can also be provided with passages to warm a pumped
liquid nitrogen stream and a first pumped oxygen stream and a
second pumped oxygen stream through indirect heat exchange with the
compressed heat exchange stream to produce other of the product
streams and the pump is connected to the passages so that pumped
liquid oxygen stream is divided into the first pumped oxygen stream
and the second pumped oxygen stream. An expansion valve is located
between the pump and one of the passages so that the second pumped
oxygen stream is passed through a valve prior to being warmed and
pressurized oxygen products at two different pressures are
produced. Another pump is located between the higher pressure
column and the main heat exchanger to pump a liquid nitrogen stream
and thereby form the pumped liquid nitrogen stream.
[0015] The higher pressure column and the lower pressure column can
be thermally linked by a condenser reboiler condensing
nitrogen-rich vapor column overhead in the higher pressure column
through indirect heat exchange with the oxygen-rich liquid column
bottoms of the lower pressure column, thereby producing
nitrogen-rich reflux streams. The higher pressure column and the
lower pressure column are connected to the condenser reboiler so
that the nitrogen-rich reflux streams are introduced, at least in
part, into the higher pressure column and the lower pressure column
as reflux. The distillation column system also has an argon column
connected to the lower pressure column so that a crude argon feed
stream from the lower pressure column is rectified in the argon
column to separate argon from oxygen contained in the crude argon
feed stream and an argon condenser is connected to the argon column
to produce reflux for the argon column and a liquid argon-rich
liquid stream. The argon condenser is connected to the higher
pressure column so that a kettle liquid stream composed of a crude
liquid oxygen column bottoms of the higher pressure column is
partially vaporized in the argon condenser. The argon condenser is
also connected to the lower pressure column so that liquid and
vapor phase streams produced as a result of partially vaporizing
the kettle liquid stream are introduced into the lower pressure
column for further refinement. A subcooling heat exchanger is in
flow communication with the condenser reboiler and the higher
pressure column so that one of the nitrogen-rich reflux streams and
the kettle liquid streams are subcooled in a subcooling heat
exchanger. The subcooling heat exchanger is positioned between the
lower pressure column and the main heat exchanger so that a lower
pressure column, nitrogen-rich vapor column overhead stream and a
waste nitrogen stream are partially warmed in the subcooling heat
exchanger and further warmed within the main heat exchanger to help
cool the incoming air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] While the specification concludes with claims distinctly
pointing out the subject matter that Applicant regards, as his
invention, it is believed that the invention will be better
understood when takening in connection with the sole figure
illustrating a schematic diagraph of an apparatus designed to carry
out a method in accordance with the present invention.
DETAILED DESCRIPTION
[0017] With reference to the drawing, an air separation plant 1 is
illustrated that is designed to conduct a cryogenic rectification
process in accordance with the present invention. In apparatus 1, a
feed air stream 10 is compressed by a main air compressor 12 and
then purified in a pre-purification unit 14 ("PP") to produce a
compressed and purified air stream 16. Compressed and purified air
stream 16, in a manner that will be discussed in further detail, is
in part further compressed and expanded and cooled in a main heat
exchanger 17 and then rectified in a distillation column system 18
to produce product streams 125, 127, 145, 141 and 151.
[0018] More specifically, main air compressor 12 can be
multi-stage, intercooled integral gear compressors with condensate
removal between stages. Such a compressor has, in addition to
intercoolers, between stages, an after-cooler, not illustrated, for
removing the heat of compression. The pre-purification unit 14 is
designed to remove higher boiling impurities from the air such as
water vapor, carbon dioxide and hydrocarbons. As well known in the
art and as discussed above, such purification unit 14 can
incorporate adsorbent beds operating in an out of phase cycle that
is a temperature swing adsorption cycle or a pressure swing
adsorption cycle or combinations thereof.
[0019] At a piping junction 19, the compressed and purified air
stream 16 is divided into a first compressed air stream 20, a
second compressed air stream 22 and a third compressed air stream
24. First compressed air stream 20 is sequentially compressed by
compressors in a first booster loaded expander unit 26 and a second
booster loaded expander unit 28 to form a compressed heat exchange
stream 30 that is condensed through indirect heat exchange with
pumped oxygen, specifically first and second pumped oxygen streams
140 and 142 heated to form a high pressure oxygen product stream
141 and a medium pressure oxygen product stream. High pressure only
product stream 141 could be a supercritical fluid or a high
pressure vapor depending upon the degree to which it was
pressurized prior to being heated. The main heat exchanger 17 can
be of braised aluminum plate-fin construction. Although on only one
main heat exchanger 17 is illustrated, it is understood that the
main heat exchanger 17 could be several of such units in parallel.
Also, again depending on the pressures of the pressurized oxygen,
the main heat exchanger 17 could be divided into two units where
one would operate at high pressure and the other at lower pressure
in a so called banked arrangement of heat exchangers. The higher
pressure heat exchanger in a particularly high pressure application
could be a spirally wound unit.
[0020] The first booster loaded expander unit 26 has a
turboexpander 32 connected to a first booster compressor 34 by
means of a shaft 36 and the second booster loaded expander unit 28
has a second turboexpander 38 connected to a second booster
compressor 40 by means of a shaft 42. The connecting means, 36 and
42, can also be gear. In the illustrated embodiment, the first
compressed air stream 20 is also compressed by a third booster
compressor 44. After removal of the heat of compression by an
aftercooler 46, the first compressed stream 20 is further
compressed by first and second booster compressors 34 and 40 with
intermediate removal of the heat of compression by means of an
aftercooler 48. The resulting compressed heat exchange stream 30 is
also cooled by an aftercooler 50 to remove the heat of compression
prior to being introduced into the main heat exchanger 17 for
indirect heat exchange with the first pumped oxygen stream 140 and
the second pumped oxygen stream 142. The second compressed air
stream 22 is partially cooled in the main heat exchanger 17 prior
to being expanded in first turboexpander 32. The third compressed
air stream 24 can also be compressed in a forth booster compressor
52 and after removal of the heat of compression within an after
cooler 54 is partially cooled in the main heat exchanger 17 before
being expanded in second turboexpander 38.
[0021] The expansion of the second compressed air stream 22 within
first turboexpander 32 produces a first exhaust stream 56 and the
expansion of the third compressed air stream 24 within second
turboexpander 38 produces a second exhaust stream 58. The first
exhaust stream 56 is introduced into a lower pressure column 60 of
the distillation column system 18 and the second exhaust stream 58
is introduced into a higher pressure column 62 of the distillation
column system 18 in order to impart the refrigeration generated by
such expansion into the cryogenic rectification process. The
compressed heat exchange stream 30 after having been cooled in the
main heat exchanger 17 is condensed to form a liquid air stream 64
that is divided into a first subsidiary liquid air stream 66 that
is introduced into the higher pressure column 62 and a second
subsidiary liquid air stream 68 that is introduced into the lower
pressure column 60 after having been expanded in a valve 70 to a
pressure compatible with its introduction into the lower pressure
column 60. The higher pressure column 62 will operate at a higher
pressure than the lower pressure column 60, typically 5.0-6.0
bar(a). The lower pressure column 60 will typically operate at a
pressure of 1.1 to 1.5 bar(a).
[0022] Although not illustrated, both the lower pressure column 60
and the higher pressure column 62 contain mass transfer contacting
elements in the form of known sieve trays or structured packing or
a combination of such types of elements. The mass transfer
contacting elements function to bring ascending vapor and
descending liquid phases of the air to be distilled in the columns.
In case of the higher pressure column 62, the ascending vapor phase
is initiated by the introduction of second exhaust stream which
becomes successively richer in nitrogen as it ascends. The
descending liquid phase becomes ever more rich in oxygen to form a
crude liquid oxygen column bottoms 72 also known as kettle liquid.
The lower pressure column 60 and the higher pressure column 62 are
thermally linked by means of a condenser reboiler 74 that serves to
condense a nitrogen-rich vapor stream 76 into a liquid nitrogen
stream 78. Nitrogen-rich vapor stream 76 is composed of
nitrogen-rich vapor column overhead produced as a result of the
distillation occurring within the higher pressure column 62. The
liquid nitrogen stream 78 is in turn divided into a first and
second subsidiary liquid nitrogen streams 80 and 82. Subsidiary
liquid nitrogen stream 80 serves as reflux to the higher pressure
column 62 and thus, initiates formation of the descending liquid
phase within such column. The lower pressure column 60 serves to
further refine the crude liquid oxygen column bottoms 72. For such
purposes, a crude liquid oxygen stream 84 after having been
subcooled in a subcooling heat exchanger 86 can be partially
vaporized in a manner to be discussed and introduced into the lower
pressure column 60. This produces an oxygen-rich liquid column
bottoms 86 in the lower pressure column 60 and a nitrogen-rich
vapor column overhead. The oxygen-rich liquid column bottoms 86 is
in turn partially vaporized by condenser reboiler 74 to initiate
formation of the ascending vapor phase. The second subsidiary
liquid nitrogen stream 82 after having been subcooled in the
subcooling heat exchanger is used in initiating formation of the
descending liquid phase. As illustrated, part of the second
subsidiary liquid nitrogen stream 82 can be reduced in pressure by
a valve 88 and taken as a liquid nitrogen product stream 90.
Another part of the second subsidiary liquid nitrogen stream 82 can
be used in forming the liquid nitrogen reflux stream 92 for the
lower pressure column 60. Liquid nitrogen reflux stream 92 is
reduced in pressure by means of a valve 94.
[0023] Also as illustrated and optionally, the distillation column
system 18 can include an argon column 96. An argon and oxygen
containing stream 98 is removed from the lower pressure column and
then introduced into the argon column 96 for rectification. An
oxygen containing column bottoms 100 is produced that is returned
to the lower pressure column 60 by means of an oxygen stream 102.
Also produced is an argon-rich column overhead that is condensed by
removal of an argon-rich vapor stream 104 and condensing the same
in an argon condenser 106 having a core 108 surrounded by a shell
110. The argon-rich liquid stream 112 resulting from the
condensation of the argon-rich vapor can be divided into a reflux
stream 114 and a subsidiary argon-rich liquid stream 116 that can
be further processed in a manner known in the art to produce an
argon product. For example, such further processing could be
conducted in another column to further separate the argon from the
oxygen. The condensation of the argon-rich vapor stream 104 is
brought about through indirect heat exchange with the crude liquid
oxygen stream 84 after having been subcooled. In this regard, the
crude liquid oxygen stream 84, after having been expanded by
passage through a valve 118, is introduced into the shell 110 to
condense the argon-rich vapor. This results in the partial
vaporization of the crude liquid oxygen stream 84. Vapor phase and
liquid phase streams 120 and 122, respectively, composed of the
liquid and vapor phases produced by the partial vaporization of the
crude liquid oxygen stream 84, are introduced into the lower
pressure column 60 for further refinement of the crude liquid
oxygen. It is understood that if argon column 96 were not present,
the crude liquid oxygen stream 84 would be directly introduced into
the lower pressure column 60. It is to be further pointed out here
that subcooling of such a stream is optional.
[0024] A nitrogen-rich vapor stream 124, composed of nitrogen-rich
vapor column overhead of the lower pressure column 60 and a waste
nitrogen stream 126 can be removed from the lower pressure column
and then partially warmed in the subcooling heat exchanger 86 and
fully warmed in the main heat exchanger 17 to produce a nitrogen
product stream 125 and a waste nitrogen product stream 127 which
can be used in regenerating adsorbent beds of pre-purification unit
14. Additionally, an oxygen-rich liquid stream 128, composed of
residual oxygen-rich liquid 86, can be removed from the lower
pressure column 60. By means of a piping juncture 129, a part 130
of such stream can be expanded in a valve 132 and taken as an
oxygen-rich liquid product stream. Another part 134 of the
oxygen-rich liquid stream 128 can be pressurized by a pump 136 to
produce a pumped liquid oxygen stream 138. Pumped liquid oxygen
stream 138 can optionally be divided into a first pumped oxygen
stream 140 and a second pumped oxygen stream 142. Second pumped
oxygen stream 142 can be expanded in a valve 144 to a lower
pressure so that when first pumped oxygen stream 140 and second
pumped oxygen stream 142 are heated in main heat exchanger 17, high
pressure and medium pressure oxygen product streams 141 and 145 are
produced. Optionally, a nitrogen-rich liquid stream 146 composed of
the liquid nitrogen stream 78 can be removed from the higher
pressure column 62 and then pumped by a pump 148 to produce a
pumped liquid nitrogen stream 150. Pumped liquid nitrogen stream
150 can be heated to produce a pressurized nitrogen product stream
151.
[0025] The molar flow range of the first, second and third
compressed air streams 20, 22 and 24, respectively, as a percentage
of all of the incoming air, can be between 25.0 percent to 35.0
percent for the first compressed air stream 20, between 5.0 percent
and 8.0 percent for the second compressed air stream and between
60.0 percent and 67.0 percent for the third compressed air stream
24. Thus, most of the air enters the higher pressure column 62 as
the second exhaust stream 58 that is produced by expansion of the
third compressed air stream 24 after having been expanded. The flow
rate of the second compressed air stream 22 and its expansion in
the first turboexpander 32 to produce the first exhaust stream 56
that is introduced into the lower pressure column 60 has a far
lower flow rate. It is to be noted that the third compressed air
stream has to be compressed by the forth booster compressor 54 in
order to create a sufficiently large expansion ratio across the
second turboexpander 38 that will enable the second exhaust stream
to enter the higher pressure column 62 that operates at a higher
pressure than the lower pressure column 60. The second compressed
stream 22 which is used in forming the first exhaust stream has no
compression beyond the pressure imparted by the main air compressor
12 because the resulting exhaust stream 56 enters the lower
pressure column 60 which is operated at a lower pressure than the
higher pressure column 62. Consequently, the generation of the
first exhaust stream 56 is far more efficient than the generation
of the second exhaust stream 58 because there is no addition
compression required by, for instance, forth booster compressor 52.
The reason for the split in compressed air flow rates, set forth
above, is that the generation of refrigeration by an expander
exhausting into a higher pressure column will have the least effect
on the ability of the plant to produce liquids, for example, liquid
oxygen stream 130. Specifically, as mentioned above, the
distillation column systems 18 functions by separating the nitrogen
from the oxygen to produce a crude liquid oxygen column bottoms of
the higher pressure column 62 that is further refined in the lower
pressure column 60. As more air is diverted directly to the lower
pressure column 60, recovery will begin to suffer. Therefore,
although there is an energy penalty with the use of forth booster
compressor 52, it is a necessary energy penalty if liquid products
are to be produced in quantity. This being said, the production of
liquid products by the air separation plant 1 is entirely optional.
It is to be noted though that the use of the first turboexpander 32
exhausting into the lower pressure column 60 does to a limited
extent relieve the degree to which expansion need be generated by
the second turboexpander 38 exhausting into the higher pressure
column 62; and thus, in this respect an energy efficiency is
realized. However, as will be discussed, a process can be conducted
in air separation plant 1 in connection with the turboexpander 32
that will realize a more substantial benefit in connection with the
reduction of the size of the adsorbent beds used in
pre-purification unit 14.
[0026] The energy savings of the present invention is brought about
by the elimination of a booster compressor that is used in
compressing the compressed heat exchange stream 30 to the required
pressure from the pressure of the main air compressor 12. The air
in any case has to be expanded in turboexpanders to generate
refrigeration. The recapture of the work of expansion produced by
first and second turboexpanders 32 and 38 in the generation of
refrigeration by compressing the first compressed air stream to
form the compressed heat exchange stream thus saves energy that
would otherwise have been expended in such compression. It is to be
noted that the pressure of the pressurized oxygen product stream
141 sets the pressure required of the compressed heat exchange
stream 30. In the illustrated embodiment, the production of this
pressure requires that some external energy be expended namely in
third booster compressor 44. However, this is still less energy
that would have otherwise have been required had a separate booster
compressor been provided in creating the final pressure. In this
regard, depending upon the pressure required of the compressed heat
exchange stream, additional compression energy can be added by
means of operating main air compressor 12 at a slightly higher
pressure than would be normally used. Such higher pressure would
allow first booster compressor 44 to extract more energy from first
turboexpander 32 and thus create a higher pressure. As mentioned
above, this also would have the benefit of allowing for a reduction
in the size of the adsorbent beds of the prepurification unit
14.
[0027] It is to be noted that embodiments of the present invention
can be carried out that are more simple than that of air separation
plant 1. For instance, third and forth booster compressors 44 and
52 could be eliminated in such a simplified embodiment. In such an
embodiment, the different compressor operating pressure ranges of
first and second booster compressors 34 and 44 would be created
solely by means of the difference in air flows of the second and
third compressed air streams 22 and 24.
[0028] While the present invention has been discussed in relation
to a preferred embodiment, as would occur to those skilled in the
art, numerous additions, omission and changes thereto can be made
without departing from the spirit and scope of the invention as set
forth in the appended claims.
* * * * *