U.S. patent application number 14/629842 was filed with the patent office on 2016-08-25 for system and method for integrated air separation and liquefaction.
The applicant listed for this patent is Henry E. Howard. Invention is credited to Henry E. Howard.
Application Number | 20160245585 14/629842 |
Document ID | / |
Family ID | 54478247 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160245585 |
Kind Code |
A1 |
Howard; Henry E. |
August 25, 2016 |
SYSTEM AND METHOD FOR INTEGRATED AIR SEPARATION AND
LIQUEFACTION
Abstract
A high efficiency and high volume air liquefaction system and
method is disclosed wherein the cryogenic air separation plant
includes a warm gas turbo-expansion cycle to supply the
supplemental refrigeration required to produce liquid products in
excess of about 15% of the incoming feed air.
Inventors: |
Howard; Henry E.; (Grand
Island, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Howard; Henry E. |
Grand Island |
NY |
US |
|
|
Family ID: |
54478247 |
Appl. No.: |
14/629842 |
Filed: |
February 24, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2270/14 20130101;
F25J 2245/40 20130101; F25J 2270/42 20130101; F25J 3/04175
20130101; F25J 3/04412 20130101; F25J 2270/80 20130101; F25J
2270/40 20130101; F25J 3/04278 20130101; F25J 2270/902 20130101;
F25J 3/04296 20130101; F25J 2270/90 20130101; F25J 3/04345
20130101; F25J 3/04393 20130101; F25J 2245/42 20130101; F25J 3/0409
20130101; F25J 3/04381 20130101 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1. A method of separating a feed mixture comprised of air
components in a cryogenic separation plant to produce liquid
products, the cryogenic separation plant having a main heat
exchanger and a distillation column system, the method comprising
the steps of: producing a pressurized and purified feed stream;
directing a first portion of the compressed and purified feed
stream to the main heat exchanger to cool the first portion of the
compressed and purified feed stream; liquefying the cooled first
portion of the compressed and purified feed stream to produce a
liquid feed stream suitable for rectification in the distillation
column system; diverting a second portion of the compressed and
purified feed stream to a turbo- expander section of the cryogenic
separation plant; partially cooling at least a portion of the
second compressed and purified feed stream in the main heat
exchanger; expanding the partially cooled second portion of the
compressed and purified feed stream in a turbo-expander to produce
work and a gaseous exhaust stream at a pressure suitable for the
rectification in the distillation column system; conducting a
cryogenic distillation process to separate the first and second
portions in the distillation column system thereby producing at
least one liquid product stream; compressing a working fluid in a
supplemental refrigeration circuit using the work produced by the
turbo-expansion of the partially cooled second portion of the
compressed and purified feed stream; cooling and expanding the
compressed working fluid in a turbo-expander disposed within the
supplemental refrigeration circuit; warming the expanded working
fluid by way of heat exchange with at least a portion of the feed
stream to impart a portion of the refrigeration required by the
cryogenic separation plant; and recirculating at least a portion of
the warmed working fluid to a compressor section within the
supplemental refrigeration circuit after having been warmed by way
of said heat exchange.
2. The method of claim 1 wherein the working fluid is selected from
the group consisting of air, nitrogen, or a mixture of air
constituents having an oxygen content not greater than air.
3. The method of claim 1 wherein the step of cooling the compressed
working fluid further comprises cooling the compressed working
fluid in one or more stages of cooling prior to expanding the
compressed working fluid in the turboexpander.
4. The method of claim 1 wherein the step of cooling the compressed
working fluid further comprises partially cooling the compressed
working fluid in the main heat exchanger.
5. The method of claim 1 wherein the step of compressing the
working fluid in a compressor section within the supplemental
refrigeration circuit further comprises: compressing the working
fluid in a first booster compressor coupled to the turboexpander
section of the cryogenic separation plant; and further compressing
the working fluid in a second booster compressor coupled to the
turboexpander disposed within the supplemental refrigeration
circuit.
6. The method of claim 1 wherein the compressed and purified feed
stream is at a pressure of greater than the critical pressure of
the feed stream.
7. The method of claim 1 wherein the first portion of the
compressed and purified feed stream directed to the main heat
exchanger is less than about 35 percent by volume of the compressed
and purified feed stream.
8. The method of claim 1 wherein the second portion of the
compressed and purified feed stream diverted to the turboexpander
section of the cryogenic separation plant is greater than about 65
percent by volume of the compressed and purified feed stream.
9. The method of claim 1 wherein the step of producing a compressed
and purified feed stream further comprises the steps of:
compressing an air feed stream in a multistage main air compression
section of the cryogenic separation plant to produced a compressed
feed air stream; and purifying the compressed feed air stream in a
pre-purification unit of the cryogenic separation plant to produce
the compressed and purified feed stream.
10. The method of claim 9 wherein the working fluid comprises a
portion of the compressed feed air stream diverted from within the
multistage main air compression section of the cryogenic separation
plant.
11. The method of claim 1 wherein the distillation column system
comprises at least two columns wherein the feed mixture is
fractionally distilling into their component parts to produce a
plurality of product streams and waste streams, including the at
least one liquid product stream.
12. A supplemental refrigeration system for producing liquefied
products in a cryogenic separation plant comprising: an intake
conduit configured to receive a working fluid; a first compressor
coupled to the intake conduit and configured to compress the
working fluid, the first compressor mechanically coupled to a first
turboexpander of the cryogenic separation plant and using the work
produced by the first turboexpander of the cryogenic separation
plant; a second compressor coupled to the first compressor and
configured to further compress the compressed working fluid; a
second turboexpander operatively coupled to the second compressor
configured to expand the further compressed working fluid to
generate an expanded working fluid; a heat exchanger configured to
receive the expanded working fluid from the second turboexpander
and warm the expanded working fluid to impart a portion of the
refrigeration required by the cryogenic separation plant; and a
recirculating conduit configured to return the warmed expanded
working fluid from the heat exchanger to the first compressor
section.
13. The supplemental refrigeration system of claim 12 wherein the
heat exchanger is a main heat exchanger of the cryogenic separation
plant and the expanded working fluid imparts the portion of the
refrigeration required by the cryogenic separation plant via the
main heat exchanger.
14. The supplemental refrigeration system of claim 12 wherein the
heat exchanger is an auxiliary heat exchanger and the expanded
working fluid is warmed via indirect heat exchange with a boosted
compressed stream from the cryogenic separation plant.
15. The supplemental refrigeration system of claim 12 further
comprising one or more aftercoolers configured to cool the
compressed working fluid and/or the further compressed working
fluid.
16. The supplemental refrigeration system of claim 12 wherein the
first compressor further comprises a booster compressor coupled to
a lower column turboexpander of the cryogenic separation plant.
17. The supplemental refrigeration system of claim 12 further
comprising a supplemental cooling circuit and wherein the further
compressed working fluid is cooled via indirect heat exchange with
expanded working fluid.
18. The supplemental refrigeration system of claim 17 wherein the
supplemental cooling circuit further comprises passages within the
main heat exchanger of the cryogenic separation plant.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a system and method for
producing one or more liquid product streams by cryogenic
rectification of air or other feed stream, and more particularly, a
system and method for converting upwards of 20% of the incoming air
or other feed stream into one or more liquid product streams.
BACKGROUND OF THE INVENTION
[0002] Oxygen and nitrogen are produced commercially in large
quantities by the cryogenic distillation of air in air separation
units (ASU) where incoming feed air is compressed; purified of
contaminants; and then cooled in a main heat exchanger to a
temperature suitable for its rectification. The compressed,
purified and cooled air stream is then cryogenically distilled
typically in a thermally integrated dual pressure column system in
which air is separated into at least oxygen and nitrogen-rich
product streams. As well known in the art, additional columns may
be employed to produce argon or additionally rare gases.
[0003] Oxygen that is separated from the incoming air feed can be
taken as a liquid product that can be produced in the low pressure
column as an oxygen-rich liquid column bottoms. Liquid product can
additionally be taken from part of the nitrogen-rich liquid used in
refluxing the columns. As known in the art, the oxygen liquid
product can be pumped and then in part taken as a pressurized
liquid product, and also heated in the main heat exchanger to
produce an oxygen product as a vapor or as a supercritical fluid
depending on the degree to which the oxygen is pressurized by the
pumping. The liquid nitrogen can similarly be pumped and taken as
either as pressurized liquid product, a high pressure vapor or a
supercritical fluid.
[0004] In order to operate the distillation column system,
refrigeration must be supplied to offset ambient heat leakage, warm
end heat exchange losses and to allow the production of liquid
products. Supplemental refrigeration is typically supplied by
expanding part of the air, a waste or product stream from the low
pressure column within a turbo-expander to generate a cold exhaust
stream. The cold exhaust stream is then introduced into the
distillation column or the main heat exchanger. External
refrigeration can also be imparted by refrigerant streams
introduced into the main heat exchanger. Refrigeration can also be
generated through closed loop, external refrigeration cycles.
[0005] Many times it is desirable to produce significant fractions
of the incoming feed air, preferably between about 10% and 15% of
the feed air, as a liquid product. Liquid oxygen, liquid nitrogen
and/or liquid argon may be produced directly from the distillation
column system in what is known as an air liquefaction process. Such
liquid products are typically distributed and sold as merchant
products and may significantly enhance the profitability of an air
separation unit. However, the air liquefaction process requires
additional refrigeration to be supplied to the air separation unit
which drives up the capital costs and power consumption of the air
separation unit. In order to produce significant liquid fractions
of more than about 10% of the feed air, it is often necessary to
use two or more gas turbo-expanders to effectively deliver the
additional supplemental refrigeration to the air separation unit
over a range of temperatures. The optimal distribution of
refrigeration with respect to temperature reduces overall cycle
power consumption.
[0006] There are a number of critical process design aspects to
integrated air distillation and liquefaction processes. For
example, since a cost effective means is required to generate the
necessary compression power for the turbo-expansion, the shaft work
of turbo-expansion must be used in a manner that extracts the
maximum value to the air separation unit. Compression power and
capital cost problems associated with the distillation column based
air liquefaction processes are particularly challenging in small to
medium sized air separation plants.
[0007] The use of warm recycle compression-expansion circuits in
such small to medium sized plants to improve the air liquefaction
process has been tried but is somewhat expensive and introduces
unwanted process complexities to the overall air separation
process. See, for example, U.S. Pat. Nos. 4,883,518; 5,287,704;
5,400,600; 5,454,226; 5,758,515; 5,806,341; 6,257,020; and
8,397,535.
[0008] Accordingly, there is a need for the development of small
and medium air separation plants and processes with improved liquid
production capabilities. Specifically, there is a need to lower the
operating and capital costs associated with warm recycle
compression systems used in small to medium sized plants while
maintaining high liquefaction efficiency. In addition, there is
also the need to further push the liquid production capacity of
small and medium sized air separation plants over 15% of the
incoming feed air.
SUMMARY OF THE INVENTION
[0009] The present invention may be characterized as a method of
separating a feed mixture comprised of air components in a
cryogenic separation plant having a main heat exchanger and a
distillation column system, the method comprising the steps of: (i)
producing a pressurized and purified feed stream; (ii) directing a
first portion of the compressed and purified feed stream to the
main heat exchanger to cool the first portion of the compressed and
purified feed stream; (iii) liquefying the cooled first portion of
the compressed and purified feed stream to produce a liquid feed
stream suitable for rectification in the distillation column
system; (iv) diverting a second portion of the compressed and
purified feed stream to a turboexpander section of the cryogenic
separation plant; (v) partially cooling at least a portion of the
second compressed and purified feed stream in the main heat
exchanger; (vi) expanding the partially cooled second portion of
the compressed and purified feed stream in the turboexpander to
produce work and a gaseous exhaust stream at a temperature suitable
for the rectification in the distillation column system; (vii)
conducting a cryogenic distillation process to separate the first
and second portions in the distillation column system thereby
producing at least one liquid product stream; (viii) compressing a
working fluid in a supplemental refrigeration circuit using the
work produced by the turbo-expansion of the partially cooled second
portion of the compressed and purified feed stream; (ix) cooling
and expanding the compressed working fluid in a turboexpander
disposed within the supplemental refrigeration circuit; (x)
directing the expanded working fluid to the main heat exchanger and
warming the expanded working fluid in the main heat exchanger to
impart a portion of the refrigeration required by the cryogenic
separation plant; and (xi) recirculating at least a portion of the
warmed working fluid to a compressor section within the
supplemental refrigeration circuit after having passed through the
main heat exchanger or otherwise exchanged heat with at least one
cooling stream directed to the air separation plant/unit.
[0010] The present invention may also be characterized as a
supplemental refrigeration system for producing liquefied products
in a cryogenic separation plant comprising: (a) an intake conduit
configured to receive a working fluid; (b) a first compressor
coupled to the intake conduit and configured to compress the
working fluid, the first compressor mechanically coupled to a first
turboexpander of the cryogenic separation plant and using the work
produced by the first turboexpander of the cryogenic separation
plant; (c) a second compressor coupled to the first compressor and
configured to further compress the compressed working fluid; (d) a
second turboexpander operatively coupled to the second compressor
configured to expand the further compressed working fluid to
generate an expanded working fluid; (e) a heat exchanger configured
to receive the expanded working fluid from the second turboexpander
and warm the expanded working fluid to impart a portion of the
refrigeration required by the cryogenic separation plant; and (f) a
recirculating conduit configured to return the warmed expanded
working fluid from the heat exchanger to the first compressor
section.
[0011] The compressed and purified feed stream is preferably
produced by first compressing an air feed stream in a multistage
main air compression system of the cryogenic separation plant to
produce a compressed feed air stream and then purifying the
compressed feed air stream in a pre-purification unit of the
cryogenic separation plant. In some embodiments of the present
system and method, the compressed and purified feed stream may be
at a pressure of greater than the critical pressure of the feed
stream.
[0012] The first portion of the compressed and purified feed air
stream directed to the main heat exchanger is preferably less than
about 35 percent by volume of the total compressed and purified
feed air stream whereas the second portion of the compressed and
purified feed stream diverted to the turbo-expander section of the
cryogenic separation plant is greater than about 65 percent by
volume of the compressed and purified feed stream.
[0013] In some embodiments of the present system and method, the
working fluid is compressed in a first booster compressor coupled
to the lower column turbo- expander or other turbo-expander of the
cryogenic separation plant and then further compressed in a second
booster compressor coupled to a turbo-expander disposed within the
supplemental refrigeration circuit.
[0014] Cooling of the compressed working fluid is preferably
accomplished in one or more stages of cooling prior to expanding
the compressed working fluid in the turbo-expander wherein one such
stage may comprise partially cooling the compressed working fluid
in the main heat exchanger. The compressed working fluid may also
be cooled using a supplemental cooling circuit wherein such cooling
occurs via indirect heat exchange with the expanded working fluid
or other refrigerating fluid or a vapor compression refrigerant,
such as R134a.
[0015] In some embodiments, the cooled, expanded working fluid
imparts a portion of the refrigeration required by the cryogenic
separation plant via the main heat exchanger. Alternatively, the
expanded working fluid may be warmed in an auxiliary heat exchanger
via indirect heat exchange with a boosted compressed stream that is
directed to the cryogenic separation plant thereby imparting a
portion of the refrigeration required by the cryogenic separation
plant.
[0016] One aspect that differentiates the present systems and
methods from the prior art systems and methods is that the shaft
work resulting from the turbo-expansion of the second air stream
provides the necessary power for compression of the warmed expanded
stream. Similarly, in some embodiments, the warmed expanded stream
is further compressed in a secondary compressor preferably imparted
by way of the shaft work generated through the turbo-expansion of a
third stream, namely the recycled and recompressed working fluid
stream. In the present system and method, the working fluid is
comprised of air, nitrogen or a mixture of air constituents having
an oxygen content not greater than air. The working fluid
preferably comprises a portion of the compressed feed air
stream.
[0017] Advantageously, the recycle compression necessary to power
warm level refrigeration is generated without an externally powered
compressor. The elimination of a separate drive means, power supply
and lubrication system offers a significant capital cost savings.
In addition, the main driver for product liquefaction is primarily
shifted toward the main air compressor. In particular, the
discharge pressure of the main feed air compressor system is
substantially increased over many conventional main feed air
compressor systems. By increasing the overall compression ratio on
the main feed air compressor system one can achieve enhance
liquefaction capability with a minimum in incremental capital
expense.
[0018] Another aspect that differentiates the present invention
from the prior art systems is the use or introduction of a warm gas
turbo-expansion as supplemental refrigeration. In general, it has
been found that the introduction of a warm gas turbo-expansion will
enable the production of an additional 5% to 10% of the incoming
feed air as liquefied product. This incremental liquid production
can be typically gained at low incremental power consumption which
provides the main economic advantage of the present system and
method. By recycling the streams only from the warm turbine, the
irreversibility associated with prior art cold recycle systems is
eliminated. In particular, the incremental liquefaction from about
10%-15% of the incoming feed air to greater than 15% of the
incoming feed air is achieved using warm turbo-expansion at roughly
half the unit power typically associated with cold turbo-expansion
that is used or contemplated in various prior art systems.
[0019] Yet another advantage of the present system and method over
many prior art liquefaction processes is that no separate
liquefaction heat exchanger is required to produce the greater than
about 15% of the air as liquid and more preferably greater than
about 18% of the air as liquid. In addition, the main heat
exchanger in the present system operates at high efficiency. In
general, the incremental liquefaction unit power consumption will
be on the order of about 10 kw-hr/kcf or less. This represents a
unit liquefaction power reduction of greater that about 35% when
compared to prior art systems that employs the liquefaction process
in a segregated liquefier.
[0020] In addition to lower power consumption, by producing a
significant liquid fraction, i.e., over 15% of the feed air
produced as liquid, the present system and method enables the near
complete liquefaction of the oxygen contained in the incoming feed
air. In such situations, the pump and piping associated with prior
art pumped liquid oxygen product stream can be eliminated with a
further appreciable capital cost savings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] 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:
[0022] FIG. 1 is a schematic process flow diagram of a cryogenic
rectification plant in accordance with the present invention for
carrying out a method of the present invention; and
[0023] FIG. 2 is a schematic process flow diagram of an alternate
embodiment of a cryogenic rectification plant in accordance with
the present invention for carrying out a method of the present
invention.
DETAILED DESCRIPTION
[0024] With reference to FIG. 1, a double column, cryogenic air
separation plant 10 is illustrated that is integrated with a closed
loop supplemental refrigeration circuit or warm recycle expansion
circuit 20, discussed hereinafter, to increase production of liquid
products such as liquid oxygen or liquid nitrogen. The operation of
this thermally linked, double column distillation system is well
known to the art of air separation. It is also understood by those
skilled in the art, that if argon were a necessary or desired
product, an argon column (or columns) could be incorporated into
the distillation system.
[0025] Feed air stream 1 is first compressed in a multi-stage air
compression system 100 with intercooling and condensate removal
(not shown for simplicity) to a substantially elevated pressure in
the range of about 40 to about 60 bar. The compressed air stream 2
is then directed to a pre-purification unit 110. The
pre-purification process undertaken by the pre-purification unit
110 may comprise several unit operations, including but not limited
to, direct contact water cooling, refrigeration based chilling,
direct contact with chilled water, phase separation and/or
absorption. In adsorption-based processes, the pre-purification
unit typically contains beds of alumina and/or molecular sieve
operating in accordance with a temperature and/or pressure swing
adsorption cycle in which moisture and other higher boiling
impurities are adsorbed. Also, as known in the art, such higher
boiling impurities are typically, carbon dioxide, water vapor and
hydrocarbons. While one bed is operating, another bed is
regenerated. The compressed, pre-purified air stream 3 is a clean
dry air stream that is subsequently split into two or more
portions.
[0026] A first portion of the compressed, pre-purified air stream
3, preferably about 65 to 80 volume % is taken as clean dry air
stream 32 which is directed to a main heat exchanger 200 where it
is partially cooled to a temperature in the range of about
160.degree. K to 220.degree. K and subsequently expanded in
turbo-expander 122. The turbine exhaust stream 37 exiting the
turbo-expander 122 is then introduced to the base of the moderate
to high pressure column 300 as a primary gaseous air feed. The
shaft work of turbo-expander 122 is imparted to a recycle
compression stage 505 which will be described in greater detail
below.
[0027] A second portion of the compressed, pre-purified air stream
3, preferably the remaining 20 to 35 volume % is taken as stream
21. Stream 21 is further cooled in the main heat exchanger 200 and
exits the main heat exchanger 200 as a dense phase liquid and
sub-cooled stream 22.
[0028] Part of liquid sub-cooled stream 22 is depressurized by way
of expansion valve 400 and is directed as incoming stream 23 into
an intermediary location in the moderate to high pressure column
300. A second part of liquid sub-cooled stream 22 is directed
through expansion valve 420 and introduced as incoming stream 24
into an intermediary location of low pressure column 310. It should
be noted that dense phase expanders (e.g. liquid turbines) can be
employed to supplement plant refrigeration in lieu of simple
Joule-Thompson (J-T) expansion valves. Such dense phase expanders
(not shown) may be employed upon stream 23 and stream 24 prior to
or in lieu of expansion valves 400/420. Alternatively, the entire
stream 22 may be expanded in a liquid turbine.
[0029] Columns 300 and 310 represent distillation columns in which
vapor and liquid are counter-currently contacted in order to affect
a gas/liquid mass-transfer based separation of the respective feed
streams. As is well known to the art, columns 300 and 310 will
preferably employ packing (e.g. structured packing or dumped
packing) or trays or combinations thereof. The moderate to high
pressure column 300 and the low pressure column 310 are linked in a
heat transfer relationship wherein the overhead of 300 exchanges
latent heat with the bottoms liquid of column 310.
[0030] In the illustrated embodiment, air streams 37 and 23 are
directed to the moderate to high pressure column 300 which
separates the respective streams into a nitrogen rich vapor column
overhead and an oxygen rich bottoms stream 40. The nitrogen-rich
vapor column overhead, extracted from the top of the moderate to
high pressure column 300 as a stream 50, is condensed within a
condenser-reboiler 220 located in the base of low pressure column
310 against boiling an oxygen-rich liquid column bottoms of column
310 where the latent heat of condensation is thereby imparted to
the oxygen rich bottoms fluid of column 310. The boiling of
oxygen-rich liquid column bottoms initiates the formation of an
ascending vapor phase within low pressure column 310. The
condensation produces a liquid nitrogen stream 51 that is divided
into streams 56, 55 that reflux the higher pressure column 300 and
the lower pressure column 310, respectively to initiate the
formation of descending liquid phases in such columns. An oxygen
enriched liquid 40 is also withdrawn from column 300 and is then
directed through pressure reduction valve 430 prior to entry into
low pressure column 310 as stream 41.
[0031] Low pressure column 310 operates at a pressure in the range
of about 1.1 to 1.5 bar. Nitrogen rich liquid stream 52 is first
subcooled in exchanger 210 and exits as stream 53 which may be
split into a product liquid stream 54 and the reflux liquid stream
55. The reflux nitrogen stream 55 is expanded through valve 435. It
should be noted that oxygen rich bottoms stream 40 may
alternatively be subcooled prior to expansion valve 430. Subcooling
of the oxygen rich bottoms stream 40 may be accomplished by way of
an additional subcooler or an extension of subcooler 210. It should
be noted that subcooler 210 may also be integrated into main heat
exchanger 200.
[0032] Within the low pressure column 310, incoming streams 55, 24
and 41 are further separated into nitrogen-rich overhead streams 60
and 70 and an oxygen rich bottoms liquid 80. Nitrogen-rich streams
60, 70 are warmed to about ambient temperatures by indirect heat
exchange within subcooler 210 and/or main heat exchanger 200. The
resulting warmed nitrogen-rich streams subsequently emerge as
warmed, lower pressure nitrogen streams 62 and 72. It should be
noted that warmed, low pressure nitrogen stream 62 may be taken as
a co-product nitrogen stream and compressed as necessary. The
warmed, low pressure nitrogen stream 72 often finds use as a
purge/sweep fluid for purposes of regenerating adsorbent systems
which may form part of pre-purification unit 110.
[0033] An oxygen rich liquid stream 80 is extracted from the base
of lower pressure column 310. This oxygen rich liquid stream 80 is
then compressed by a combination of the gravitational head and by
mechanical pump 440 to form a pressurized liquid oxygen stream 81.
The pressurized liquid oxygen stream 81 may then be split into an
oxygen product liquid stream 84 and directed to storage (not shown)
as well as liquid oxygen stream 82. Liquid oxygen stream 82 is
shown as directed to passages within the main heat exchanger 200
where it is vaporized and warmed to near ambient temperatures and
exits as a gaseous oxygen product stream 86 and may be directed to
a pipeline or utilized directly.
[0034] As discussed above, air separation plant 10 is capable of
producing liquid products, namely, a nitrogen-rich liquid stream
and a liquid oxygen product stream. In order to increase the
production of such products, additional refrigeration is supplied
by a refrigeration system that is illustrated as a closed loop
supplemental refrigeration circuit or warm recycle expansion
circuit 20 that may use air as the refrigerant. In this regard,
part of the compressed and purified air stream may be used to
charge the closed loop supplemental refrigeration circuit 20 by way
of conduit and valving (not shown). After having been charged.
[0035] FIG. 1 also depicts a warm recycle expansion circuit 20
configured to provide supplemental refrigeration. In the
supplemental refrigeration circuit or warm recycle expansion
circuit 20, a refrigerant stream 93 enters warm turbo-expander 520
at a temperature in the range of between about 298.degree. K to
220.degree. K. In the illustrated embodiments, stream 93 is cooled
within main heat exchanger 200 prior to expansion in turbo-expander
520. As illustrated, warm turbo-expander 520 serves to generate the
supplemental refrigeration through the expansion of refrigerant
stream 93 to a pressure in the range of between about 5 bar and
about 15 bar. The expanded and cooled turbo-expander exhaust stream
94 is directed to an intermediary location of main heat exchanger
200 where it is subsequently warmed to ambient temperature and
exits the main heat exchanger 200 as warmed refrigerant stream 95.
Warmed refrigerant stream 95 is then directed to recycle
compressors 505, 510 which may be cooled as necessary with
intercoolers 506 and 511, respectively. In the preferred
embodiment, compressor 505 is coupled to turbo-expander 122 whereas
compressor 510 is couple to turbo-expander 520. Coupled in this
regard indicates that the shaft work of expansion, indicated by a
dotted lines 600, 602 is imparted directly to the coupled
compressor wheels, preferably through a common shaft.
[0036] The pressurized recycle stream 92 exiting compressor 510
will typically exit at a pressure in the range of between about 40
bar and 60 bar. This pressurized recycle stream 92 may be further
cooled within intercooler 511 by cooling water, ambient air and/or
sub-ambient utility (e.g. chilled water) prior to cooling in main
heat exchanger 200 and prior to expansion in turbo-expander
520.
[0037] In the embodiment of FIG. 1, compressors 505 and 510 will
exhibit some small leakage and a means must be employed to
replenish the working fluid. Such replenishment may be accomplished
by supplying a make-up stream through a small line and associated
valves. For example, a stream of compressed, purified air may be
diverted from stream 3 and directed into the supplemental
refrigeration circuit or warm recycle expansion circuit 20 at a
suitable location. Depending upon the pressure of operation of
turbine 520, a stream of cold gas from the inlet or exhaust of
turbine 122 may be used as a make-up stream. Alternatively,
nitrogen can be used as a working fluid and the makeup stream may
be extracted from the lower column 300 or from a product nitrogen
compressor associated with stream 62.
[0038] FIG. 2 depicts an alternate embodiment of the present system
and method for producing liquefied products in a cryogenic air
separation plant. This alternate embodiment is arranged to further
increase the overall energy efficiency of the air separation
process. In many regards, the embodiments of FIG. 1 and FIG. 2 are
very similar and thus, only the differences between the embodiments
of FIG. 1 and FIG. 2 will discussed in detail in the paragraphs
that follow.
[0039] With reference to FIG. 2, a portion of the compressed,
purified air (e.g. about 7 percent of the incoming air feed) is
introduced as a supplemental stream 96 through flow control valve
512 and into the warm recycle expansion circuit 20. The
supplemental stream 96 is combined with the discharge of compressor
505 where it is further compressed, cooled and then expanded to an
elevated pressure. Alternatively, the supplemental stream 96 may be
introduced into the discharge of compressor 510 where it is then
cooled and then expanded. The operation of the supplemental
refrigeration circuit or warm recycle expansion circuit 20 in FIG.
2 is basically the same as discussed above with reference to FIG.
1. However, in the embodiment of FIG. 2, a stream 98 comparable in
flow to that of supplemental stream 96 (e.g. about 7% of the
incoming air feed) is diverted or extracted from the turbo-expander
exhaust stream 94. The pressure of diverted stream 98 will
typically be in the range of between about 6 bar and about 15 bar.
The diverted stream 98 is further cooled and liquefied by way of
main heat exchanger 200. The resulting liquefied air stream 97 is
then depressurized through valve 513 and is introduced into an
intermediary location of the low pressure column 310. It should be
noted that the resulting liquid stream 97 may also be introduced
into the higher pressure column 300 or combined with stream 23
and/or stream 24 prior to column entry. In general, between about
5% to about 10% of the air can be fed to the column system by way
of diverted stream 98. In the embodiment illustrated in FIG. 2, the
primary liquefaction air flow is reduced to about 18 percent of the
total feed air which, in turn, reduces the total power consumption
of the illustrated air separation process by about 1.5 percent
compared to the embodiment of FIG. 1.
[0040] Although the above-described supplemental refrigeration
circuit or warm recycle expansion circuit 20 preferably employs air
as a working fluid or refrigerant, other working fluids such as
nitrogen, argon or another refrigerant may be employed. In
applications where nitrogen or air are the working fluids in the
warm recycle expansion circuit, the recycle expansion circuit need
not operate in a closed circuit but may also be operated in an open
or partially open circuit. In its broadest terms, the working fluid
can be any fluid mixture with oxygen content no more than air and
preferably a dew point below the exhaust temperature of the warm
turbine. Alternative fluid components may include fluorocarbons and
inorganics such as carbon dioxide.
[0041] In order to enhance the refrigerating effect of the warm
recycle expansion circuit 20, stream 92 may be cooled within
intercooler 511 by way of chilled water or by way of a separate
refrigeration system (not shown) using a commercially available low
temperature refrigerant like ammonia or R134a. A particularly
advantageous arrangement would involve the use of nitrogen as a
working fluid for the warm expansion within the recycle expansion
circuit 20 and chilled water or a low temperature refrigerant in
intercoolers 506 and 511.
[0042] Although not shown, it may also be advantageous to introduce
additional compression stages into the supplemental refrigeration
circuit or warm recycle expansion circuit 20. In particular, a
booster compression stage may be configured upstream or downstream
of the inlet and/or the outlet of compressors 505 and 510. Also, a
variable high speed motor may be operatively coupled to compressors
505 and 510 and configured to increase the compression ratio/power
of compressor stages within the warm recycle expansion circuit 20.
Alternatively, a high speed generator may be configured so that
shaft work of expander 122 may partly be recouped as electrical
energy. When using the high speed generator arrangement, it may be
advantageous to periodically shut down or turn down the warm
turbo-expander 520 and warm recycle expansion circuit 20 from
operation diverting the flow within the supplemental refrigeration
circuit or warm recycle expansion circuit 20 using a recirculation
or bypass circuit (not shown).
[0043] It is to be noted that although the air separation plant 10
is illustrated as having a high pressure column and a low pressure
column connected in a heat transfer relationship by provision of
condenser-reboiler 220, other types of air separation plants are
possible. For example, the present system and method can be used in
low purity oxygen plants where the high pressure column and low
pressure column are not connected in a latent heat transfer
relationship as shown in FIGS. 1 and 2. Rather, the lowermost
reboil of the low pressure column in a low purity oxygen plant is
typically provided by the condensation or partial condensation of a
compressed air stream that is afterwards fed into the high pressure
column.
[0044] It should be noted that any number of alternative air
distillation processes and configurations may be employed with the
present system and method. In particular, argon may be recovered
from the base double column system. Additional columns may be
employed for purposes of reducing power consumption, for the
recovery of rare gases, the further purification of argon or the
production of ultra-high purity products.
[0045] Also, the production of a high pressure, pumped oxygen
stream as shown in FIGS. 1 and 2 is optional as it is entirely
possible to extract a cold gas from the low pressure column 310 and
warm it directly in the main heat exchanger 200 while venting the
oxygen or compressing a portion of the produced oxygen for
subsequent use. Alternatively, gaseous oxygen can be extracted from
column 310 and directed into the waste nitrogen stream 71 that is
subsequently warmed to ambient temperatures.
[0046] From the foregoing, it should be appreciated that the
present invention thus provides a system and method for high
efficiency separation of an incoming feed air stream in a cryogenic
separation plant having a warm gas turbo-expansion cycle to supply
the supplemental refrigeration required to produce liquid products
in excess of about 15% of the incoming feed air. While the
invention herein disclosed has been described by means of specific
embodiments and processes associated therewith, numerous
modifications and variations can be made thereto by those skilled
in the art without departing from the scope of the invention as set
forth in the appended claims or sacrificing all of its features and
advantages.
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