U.S. patent application number 11/717389 was filed with the patent office on 2008-09-18 for air separation method.
Invention is credited to Richard John Jibb, Neil Mark Prosser.
Application Number | 20080223077 11/717389 |
Document ID | / |
Family ID | 39760361 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080223077 |
Kind Code |
A1 |
Prosser; Neil Mark ; et
al. |
September 18, 2008 |
Air separation method
Abstract
An air separation method in which a liquid air stream, produced
by vaporizing a pumped liquid oxygen stream, is introduced into a
lower pressure column and optionally, a higher pressure column of
an air separation unit. The liquid air stream is subcooled by
extracting a main air feed to the higher pressure column from a
main heat exchanger at a temperature warmer than the liquid air
stream to increase argon recovery in an argon column connected to
the lower pressure column. This temperature is selected such that
the liquid air stream approaches an average temperature of the
return streams being fed into the main heat exchanger from the
higher and lower pressure columns at a range between about 0.2K and
about 3K.
Inventors: |
Prosser; Neil Mark;
(Lockport, NY) ; Jibb; Richard John; (Amherst,
NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
39760361 |
Appl. No.: |
11/717389 |
Filed: |
March 13, 2007 |
Current U.S.
Class: |
62/646 |
Current CPC
Class: |
F25J 3/04296 20130101;
F25J 3/04303 20130101; F25J 3/04678 20130101; F25J 3/04309
20130101; F25J 2240/10 20130101; F25J 3/04187 20130101; F25J
3/04412 20130101; F25J 3/04236 20130101; F25J 3/04393 20130101;
F25J 3/04387 20130101; F25J 3/0409 20130101; F25J 2290/12
20130101 |
Class at
Publication: |
62/646 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A method of separating air comprising: producing a first
compressed and purified air stream and a second compressed and
purified air stream having a higher pressure than the first
compressed and purified air stream; cooling the first compressed
and purified air stream and the second compressed and purified air
stream in a main heat exchanger, through indirect heat exchange
with return streams produced in an air separation unit that include
at least part of a pumped liquid oxygen stream, thereby to produce
a main feed air stream and a liquid air stream; introducing the
main feed air stream into a higher pressure column of the air
separation unit, expanding the liquid air stream and introducing at
least part of the liquid air stream into a lower pressure column of
the air separation unit; introducing an argon-rich stream from the
lower pressure column into an argon separation zone formed by-at
least one column to produce an argon containing column overhead and
an argon containing product stream composed of the argon containing
column overhead; subcooling a crude liquid oxygen stream composed
of liquid column bottoms of the higher pressure column and a
nitrogen-rich liquid stream composed of liquefied nitrogen column
overhead of the higher pressure column and introducing at least
part of the crude liquid oxygen stream and at least part of the
nitrogen-rich liquid stream into the lower pressure column; and the
main feed air stream being extracted from the main heat exchanger
at a temperature warmer than the liquid air stream and introduced
into the higher pressure column at least at about said temperature,
thereby subcooling the liquid air stream and increasing the liquid
content thereof after having been expanded to improve the liquid to
vapor ratio in the lower pressure column and thereby to increase
the argon recovery, the temperature being selected such that the
liquid air stream has an approach temperature approaching that of
an average temperature of the return streams of no less than a
range of between about 0.2K and about 3K, the average temperature
being a calculated temperature at which a product of flow and
enthalpy of the return streams at a cold end of the main heat
exchanger is equal to the product of the flow and the enthalpy of
the return streams at their actual temperatures.
2. The method of claim 1, wherein the range is between about 0.4K
and about 2K.
3. The method of claim 1, wherein the temperature of the main feed
air stream is in a range of between about 6K and about 25K warmer
than the liquid air stream.
4. The method of claim 1, wherein the temperature of the main feed
air stream is in a range of between about 8K and about 15K warmer
than the liquid air stream.
5. The method of claim 4, wherein the range is between about 0.4K
and about 2K.
6. The method of claim 1, wherein: the liquid air stream is
expanded to a pressure suitable for its introduction into an
intermediate location of the higher pressure column; the liquid air
stream is divided into a first subsidiary liquid stream and a
second subsidiary liquid stream; the first subsidiary liquid stream
is introduced into the higher pressure column; and the second
subsidiary liquid stream is expanded and introduced into the lower
pressure column above a point of discharge of the argon-rich stream
to the argon column.
7. The method of claim 1, wherein: a third compressed and purified
air stream is produced; the third compressed and purified air
stream is partially cooled within the main heat exchanger and
introduced into a turboexpander to produce an exhaust stream for
generation of refrigeration; and the exhaust stream is introduced
into the lower pressure column.
8. The method of claim 5, wherein: a fourth compressed and purified
air stream is produced by extracting the fourth compressed and
purified air stream from an intermediate stage of a compressor used
in forming the second compressed and purified stream; and the
fourth compressed and purified stream is expanded within another
turboexpander and combined with the first compressed and purified
air stream within the main heat exchanger to increase liquid
production.
9. The method of claim 1, wherein a nitrogen column overhead stream
composed of the nitrogen column overhead is partially warmed within
the main heat exchanger, expanded within a turboexpander to produce
an exhaust stream for generation of refrigeration and the exhaust
stream is introduced into the main heat exchanger and fully warmed
therein.
10. The method of claim 1 or claim 5 or claim 6 or claim 7 or claim
8 or claim 9, wherein the liquid air stream is introduced into a
liquid turbine to expand said liquid air stream to the pressure
suitable for its introduction into an intermediate location of the
higher pressure column.
11. The method of claim 1, wherein the crude liquid oxygen stream
and the nitrogen-rich liquid stream are subcooled through indirect
heat exchange with the return streams that are formed from a
nitrogen-rich vapor stream composed of column overhead of the lower
pressure column and a waste vapor stream enriched in nitrogen to a
lesser extent than the nitrogen-rich vapor stream, the
nitrogen-rich vapor stream and the waste vapor stream being
introduced into the main heat exchanger after having subcooled the
crude liquid oxygen stream and the nitrogen-rich liquid stream.
12. The method of claim 1, wherein: a first part of the crude
liquid oxygen stream is expanded and introduced into the lower
pressure column and a second part of the crude liquid oxygen stream
indirectly exchanges heat with an argon column overhead stream
composed of the argon column overhead, thereby condensing the argon
column overhead stream and partially vaporizing the second part of
the crude liquid oxygen stream; liquid and vapor fraction streams
resulting from partial vaporization of the crude liquid oxygen
stream are introduced into the lower pressure column; and part of
the argon column overhead stream after having been condensed forms
the argon product stream and a remaining part thereof after
condensation is returned to the argon separation zone as reflux.
Description
FIELD OF THE INVENTION
[0001] A method for separating air in which a pressurized oxygen
product is produced by vaporizing a pumped liquid oxygen stream
against liquefying an air stream in a main heat exchanger and an
argon product is produced in an argon separation zone connected to
a lower pressure column that is operatively associated in a heat
transfer relationship with a higher pressure column. More
particularly, the present invention relates to such a method in
which a main feed air stream to the higher pressure column is
withdrawn from the main heat exchanger at a temperature that is
warmer than the liquid air stream to subcool the liquid air stream,
thereby to increase argon recovery.
BACKGROUND OF THE INVENTION
[0002] The separation of air into nitrogen, oxygen and argon
fractions have been conducted in air separation units in which air
is compressed, purified and cooled in a main heat exchanger to a
temperature suitable for its rectification. The air is introduced
into a higher pressure column of a double column arrangement also
having a lower pressure column in a heat transfer relationship with
the higher pressure column. Nitrogen and oxygen products may be
extracted from the higher and lower pressure columns.
[0003] An argon-rich stream can be removed from the lower pressure
column and introduced into an argon column to produce argon-rich
column overhead. The argon-rich column overhead is condensed,
typically with the use of all or part of a crude liquid oxygen
stream, produced as a column bottoms of the higher pressure column,
to generate liquid reflux for the argon column. A portion of the
argon-rich column overhead is taken as an argon product.
[0004] It is also known to produce a high pressure oxygen product
in such an arrangement by pressurizing an oxygen-rich stream
composed of a liquid oxygen column bottoms produced in the lower
pressure column by pumping the stream and vaporizing it in the main
heat exchanger against liquefying an air stream that constitutes
part of the air that has been compressed to a high pressure. The
resultant liquid air stream is expanded and introduced into the
lower pressure column or both the higher and lower pressure
columns.
[0005] An example of such a plant is disclosed in U.S. Pat. No.
6,293,126. In this patent, the main feed air stream is withdrawn
from the main heat exchanger at a temperature warmer than that of
the air stream that is further compressed and liquefied to produce
the liquid air stream. In an attempt to simplify the construction
of such a plant, the crude liquid oxygen stream is not subcooled
either prior to its use in condensing argon reflux or its
introduction into the lower pressure column. As a result, there
exists a greater vapor fraction of the crude liquid oxygen stream
entering the lower pressure column after expansion than would have
otherwise occurred had the crude liquid oxygen stream been
subcooled. Thus, the liquid to vapor ratio at a point in the lower
pressure column above the point at which the argon-rich stream is
extracted for further refinement in the argon column is less than
would otherwise have been possible. Moreover, extracting a main air
stream at a warmer temperature than the liquid air stream decreases
the temperature of the liquid air stream to an extent that it can
approach the temperature of the return streams used to cool the
incoming air. As a result, the compression requirements for the air
stream that is further compressed and liquefied are usually greater
than the flow and/or pressure that would otherwise have been
required had the main feed air stream not been withdrawn at the
warmer temperature. The further subcooling of the liquid air stream
tends to compensate for the reduced liquid to vapor ratio in the
low pressure column. This results in more power being consumed in
such a plant without any increase in argon recovery.
[0006] As will be discussed, the present invention provides a
method for separating air in which argon recovery is increased over
that possible in prior art air separation systems, such as
discussed above, while minimizing the amount of excess power that
is necessarily used in increasing the argon recovery.
SUMMARY OF THE INVENTION
[0007] The present invention provides a method of separating
air.
[0008] In accordance with the method, a first compressed and
purified air stream and a second compressed and purified air stream
are produced. The second compressed and purified air stream has a
higher pressure than the first compressed and purified air stream.
These streams are cooled within a main heat exchanger through
indirect heat exchange with return streams that are produced in an
air separation unit. The return streams include at least part of a
pumped liquid oxygen stream and as a result of the indirect heat
exchange, a main feed air stream and a liquid air are produced from
the compressed and purified air.
[0009] The main feed air stream is introduced into a higher
pressure column of the air separation unit and the liquid air
stream is expanded and at least part of the liquid air stream is
introduced into a lower pressure column of the air separation unit.
An argon-rich stream from the lower pressure column is introduced
into an argon separation zone formed by at least one column to
produce an argon containing column overhead and an argon containing
product stream composed of the argon containing column overhead. It
is to be noted, that the term "argon separation zone" as used
herein and in the claims includes a single argon column, often
referred to in the art as a crude argon column, as well as columns
in series that provide a sufficient number of separation stages
that the argon product has very low levels of oxygen, typically
less than about 10 ppm.
[0010] A crude liquid oxygen stream composed of liquid column
bottoms of the higher pressure column and a nitrogen-rich liquid
stream composed of liquefied nitrogen column overhead of the higher
pressure column are subcooled. At least part of the crude liquid
oxygen stream and at least part of the nitrogen-rich liquid stream
are introduced into the lower pressure column.
[0011] The main feed air stream is extracted from the main heat
exchanger at a temperature warmer than that of the liquid air
stream and introduced into the higher pressure column at least at
about such temperature. Preferably, the temperature of the main
feed air stream is in a range of between about 6K and about 25K
warmer than the liquid air stream and more preferably, in a range
of between about 8K and about 15K warmer than the liquid air
stream.
[0012] The effect of this is to subcool the liquid air stream,
thereby increasing the liquid content thereof after having been
expanded to improve the liquid to vapor ratio in the lower pressure
column and thereby to increase the argon recovery. It is to be
noted that unlike the prior art, there can be no simplification
such as by not subcooling the crude liquid oxygen stream. If such
stream were not subcooled, argon recovery would suffer in that the
liquid to vapor ratio above the point of introduction of the liquid
air stream or part thereof would be less due to evolution of vapor
during the expansion of the same. Moreover, unlike the prior art,
the temperature of the main feed air stream is selected such that
the liquid air stream has an approach temperature approaching that
of an average temperature of the return streams of no less than a
range of between about 0.2K and about 3K, and preferably between
0.4K and 2K. The average temperature is a calculated temperature at
which a product of flow and enthalpy of the return streams at a
cold end of the main heat exchanger is equal to the product of the
flow and the enthalpy of the return streams at their actual
temperatures. As will be discussed, it has been found by the
inventors herein that if this temperature is made any smaller,
given the fact that a main heat exchanger is only of finite size,
the compression requirement for the second compressed and purified
air stream will increase with no appreciable increase in the argon
recovery.
[0013] In order to overcome warm end and heat leakage, as well
known in the art, refrigeration must be generated. There are a
number of ways to do this that are compatible with the present
invention. For example, a third compressed and purified air stream
can be produced. The third compressed and purified air stream can
be partially cooled within the main heat exchanger and introduced
into a turboexpander to produce an exhaust stream for generation of
the refrigeration. The exhaust stream can then be introduced into
the lower pressure column. A fourth compressed and purified air
stream can be produced by extracting the fourth compressed and
purified air stream from an intermediate stage of a compressor used
in forming the second compressed and purified stream. The fourth
compressed and purified stream is expanded within another
turboexpander and combined with the first compressed and purified
air stream within the main heat exchanger to increase liquid
production.
[0014] As an alternative method of generating refrigeration, a
nitrogen column overhead stream composed of the nitrogen column
overhead can be partially warmed within the main heat exchanger and
then expanded within a turboexpander to produce an exhaust stream
for the generation of the refrigeration. The exhaust stream can
then be introduced into the main heat exchanger and then fully
warmed therein.
[0015] In any embodiment of the present invention, the liquid air
stream can be introduced into a liquid turbine to expand the liquid
air stream to a pressure suitable for its introduction into the
intermediate location of the higher pressure column.
[0016] The crude liquid oxygen stream and the nitrogen-rich liquid
stream can be subcooled through indirect heat exchanger with return
streams that are formed from a nitrogen-rich vapor stream composed
of column overhead of the lower pressure column and a waste vapor
stream enriched in nitrogen to a lesser extent than the
nitrogen-rich vapor stream. The nitrogen-rich vapor stream and the
waste vapor stream can be introduced into the main heat exchanger
after having subcooled the crude liquid oxygen stream and the
nitrogen-rich liquid stream.
[0017] A first part of the crude liquid oxygen stream can be
expanded and introduced into the lower pressure column and a second
part of the crude liquid oxygen stream can indirectly exchange heat
with an argon column overhead stream composed of the argon column
overhead. As a result, the argon column overhead stream can be
condensed and the second part of the crude liquid oxygen stream can
be partially vaporized. Liquid and vapor fraction streams resulting
from the partial vaporization of the crude liquid oxygen stream can
then be introduced into the lower pressure column. Part of the
argon column overhead stream after having been condensed can form
the argon product stream and a remaining part thereof after
condensation can be returned to the argon separation zone as
reflux.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] While the specification 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:
[0019] FIG. 1 is a schematic process flow diagram of an apparatus
for carrying out a method in accordance with the present
invention;
[0020] FIG. 2 is a graphical representation of the prior art
heating and cooling curves in a main heat exchanger;
[0021] FIG. 3 is a graphical representation of the heating and
cooling curves within a main heat exchanger that operates in
connection with an air separation method in accordance with the
present invention;
[0022] FIG. 4 is a fragmentary, schematic view of an alternative
embodiment of FIG. 1 showing an alternative embodiment for a
subcooling unit integrated with the main heat exchanger;
[0023] FIG. 5 is a fragmentary, schematic view of an alternative
embodiment of FIG. 1 employing expansion of a nitrogen-rich stream
to generate refrigeration; and
[0024] FIG. 6 is a fragmentary, schematic view of an alternative
embodiment of FIG. 1 employing further expansion to increase the
production of liquid.
[0025] In order to avoid repetition of the explanation of the
accompanying figures, the same reference numerals are used and
repeated in the figures where the description of particular
elements designated by the reference numerals are identical.
DETAILED DESCRIPTION
[0026] With reference to FIG. 1, an air separation plant 1 is
illustrated that is configured for carrying out a method in
accordance with the present invention is illustrated.
[0027] An air stream 10 is compressed by means of a main air
compressor 12. The air pressure of the resultant compressed stream
is set by the pressure of a higher pressure column 48 to be
discussed hereinafter and pressure drop. After cooling in an after
cooler 14 to remove the heat of compression, the air stream 10 is
purified within a purification unit 16 to remove higher boiling
impurities such as carbon dioxide and moisture that could freeze as
well as hydrocarbons that could collect to present a safety hazard.
Purification unit 16, as well known in the art, can be beds of
molecular sieve adsorbent operating out of phase in a known
temperature swing adsorption cycle to purify air stream 10.
[0028] The compression and purification of the air stream 10
produces compressed and purified air stream 18 that is divided to
produce a first compressed and purified air stream 20 that
constitutes the largest portion resulting from such division. A
part 22 of compressed and purified air stream 18 is further
compressed within a booster compressor 24 to produce a second
compressed and purified air stream 28. Part 22 of compressed and
purified air stream 18 typically has a flow rate in a range of
between about 24% and about 40% of compressed and purified air
stream 18. The discharge pressure of the booster compressor 24 is
set by the pressure of a pumped liquid oxygen stream 122 also to be
discussed hereinafter. When the pressure of second compressed and
purified air stream 28 is below its critical pressure, the pressure
is typically less than about 2.5 times the pressure of pumped
liquid oxygen stream 122. The heat of compression of second
compressed and purified air stream 28 is preferably removed by
after cooler 26.
[0029] As will be discussed, optionally, a further part 30 of
compressed air stream 18 is compressed within a booster compressor
32 to produce a third compressed and purified air stream 36 for
refrigeration purposes. The flow rate of the further part 30 of
compressed and purified air stream is typically in a range of
between about 5% and about 20% of that of compressed and purified
air stream 18. The heat of compression is preferably removed from
third compressed and purified air stream 36 by an after cooler 34.
It is to be pointed out that main air compressor 12 and booster
compressor 24 are preferably multi-stage machines with inter-stage
cooling. Booster compressor 32 is a single stage machine powered by
turbine 62. Compressors 12 and 24 are usually powered by an
external source, usually an electric motor.
[0030] First compressed and purified air stream 20 and second
compressed and purified air stream 28 are cooled within the main
heat exchanger 40 to produce a main feed air stream 42 that is at
or near its dew point and a liquid air stream 44. As will be
discussed, first compressed and purified air stream 20 and second
compressed and purified air stream 28 are cooled by indirect heat
exchange with return streams, produced in the air separation unit
46, that are enriched in oxygen and nitrogen. It is to be pointed
out that the present invention contemplates that second compressed
and purified air stream 28 could be above the critical pressure. In
such case, the cooling of such stream would produce a dense phase
vapor in a process known as "pseudo liquefaction" in that no actual
liquid phase would be produced. Therefore, the term "liquefaction"
or the term "liquid" when used in connection with liquid air stream
44 herein and in the claims contemplates both a pseudo liquefaction
that produces a dense phase vapor and an actual liquefaction that
produces a liquid.
[0031] The main feed air stream 42 is introduced into a bottom
region of a higher pressure column 48 of air separation unit 46
that operates at a higher pressure than a lower pressure column 50
of air separation unit. Air separation unit 46 also includes an
argon column 52 that provides an argon separation zone for
refinement of argon to produce an argon containing column overhead
from which argon product is extracted. Argon column 52 in a proper
case could be replaced with a series of columns to present a
sufficient number of stages of separation to substantially separate
the oxygen as described above.
[0032] Although not illustrated, it is understood that higher
pressure column 48, lower pressure column 50 and argon column 52
contain mass transfer elements to contact liquid and vapor phases
of the mixtures to be separated within such columns. These mass
transfer elements can be known structured packing or sieve trays,
dumped packing or combinations thereof.
[0033] Liquid air stream 44 is introduced into a liquid expansion
device 54 and is expanded to a pressure suitable for its
introduction into an intermediate location of higher pressure
column 48 above main feed air stream 42. Liquid expansion device
54, as illustrated, is preferably a liquid turbine in which the
work of expansion can be recovered in an electric generator, used
to drive a compressor or dissipated as heat with an oil brake. It
is understood that liquid expansion device 54 could be an expansion
valve. After expansion, liquid air stream 44 is divided into first
subsidiary liquid stream 56 and a second subsidiary liquid stream
58. The second subsidiary liquid stream 58 is introduced into the
higher pressure column 48. As such, the discharge pressure of
liquid expansion device 54 is set at a pressure of the higher
pressure column 48 plus pressure drop. The first subsidiary liquid
stream 56 is reduced in pressure by an expansion valve 60 and then
introduced into lower pressure column 50. As would occur to those
skilled in the art, all of the liquid air stream 44 could be
introduced into the lower pressure column 50 and expanded to a
suitable pressure for such purposes.
[0034] In order to refrigerate the process and thus, overcome warm
end losses, third compressed and purified air stream 36 after
removal of the heat of compression is partially cooled within the
main heat exchanger 40. By partially cooled, what is meant is that
the stream is cooled to a temperature that is between the warm and
cold end temperatures of main heat exchanger 40. The resultant
third compressed air stream 36 after having been partially cooled
is then introduced into a turboexpander 62 to produce an exhaust
stream 64 that is introduced into the lower pressure column 50. As
is apparent from the illustration, the pressure of exhaust stream
64 is set at the pressure of the lower pressure column 50.
[0035] The separation of the air within the higher pressure column
48 produces a nitrogen column overhead that is rich in nitrogen.
Additionally, a crude liquid oxygen column bottoms is produced
within higher pressure column 48 that is enriched in oxygen. A
nitrogen-rich vapor stream 66, composed of the nitrogen-rich column
overhead, is introduced into a condenser reboiler 68 that is
located within a bottom region of lower pressure column to vaporize
oxygen-rich liquid collecting as liquid column bottoms within lower
pressure column 50 against condensing the nitrogen-rich vapor
stream 66 to produce the nitrogen-rich liquid stream 70. Part 72 of
nitrogen-rich liquid stream 70 is introduced back into the top of
higher pressure column 48 as reflux and a part 74 of the
nitrogen-rich liquid stream 70 is subcooled along with crude liquid
oxygen stream 76 composed of the crude liquid oxygen column bottoms
of higher pressure column 48 in a subcooling unit 78.
[0036] Part 74 of nitrogen-rich liquid stream 70 is divided into
first and second subsidiary nitrogen streams 80 and 82. Second
subsidiary liquid nitrogen stream 82 can be taken as a product.
First subsidiary liquid nitrogen stream 80 is reduced in pressure
by an expansion valve 84 and then introduced into the top of lower
pressure column 50. As would occur to those skilled in the art, all
of part 74 of nitrogen-rich liquid stream 70 could be introduced
into lower pressure column 50.
[0037] An argon-rich stream 86 as a vapor is introduced into argon
column 52. Argon-rich stream 86 will typically contain between
about 5% and about 20% argon. An argon-rich column overhead is
extracted as an argon-rich vapor stream 88 and condensed within a
heat exchanger 90 located within a shell 92. The resultant
argon-rich liquid stream 94, as a stream 96, is introduced back
into argon column 52 as reflux and an argon product stream 98 can
be extracted as an argon product. The resultant argon lean liquid
stream 100 is returned to lower pressure column 50.
[0038] Depending upon the number of stages of argon column 52,
argon-rich column overhead and therefore the argon product stream
98 can be a crude stream that requires further processing for
purification purposes. As known in the art, such a crude stream can
be further processed to remove residual oxygen in a de-oxo unit and
then in a nitrogen column to remove any residual nitrogen.
[0039] Crude liquid oxygen stream 76 after having been subcooled is
then divided and a first part 102 of such stream can be expanded
within an expansion valve 104 and directly introduced into lower
pressure column 50. A second part 106 can be expanded within an
expansion valve 108 and then introduced into the heat exchanger 92
in indirect heat exchange with argon-rich vapor stream 88 to
condense the same. The resultant vapor stream 110 can be introduced
into the lower pressure column 50 along with a liquid stream
112.
[0040] Crude liquid oxygen stream 76 and second part 74 of
nitrogen-rich liquid stream 70 are subcooled within subcooling unit
78 through indirect heat exchange with nitrogen column overhead
stream 114 and a waste stream 116 having a lesser concentration of
nitrogen than nitrogen column overhead stream 114. At the same time
an oxygen-rich stream 118, extracted from the bottom of the lower
pressure column 50, can be pumped by a pump 120 to produce a pumped
liquid oxygen stream 122. The pumped oxygen can also be above its
critical pressure and therefore is a dense phase or "pseudo
liquid." The first part 124 thereof can be introduced into the main
heat exchanger 40 for the liquefaction of second compressed air
stream 28. Also introduced into main heat exchanger are other
return streams such as nitrogen column overhead stream 114 and
waste stream 116. These return streams also serve to cool the
incoming first compressed and purified air stream 20 to produce the
main feed air stream 42 and to partly cool the third compressed air
stream 36. It is to be pointed out that embodiments of the present
invention are possible in which waste stream 116 is not removed.
This results in the nitrogen column overhead stream 114 having a
lower concentration of nitrogen and thus forming a waste stream. In
the illustrated embodiment, however, column overhead stream 114,
waste stream 116 and first part 124 of pumped liquid oxygen stream
122 consist of the return streams of the process.
[0041] Nitrogen column overhead stream 114 and the vaporized first
part 124 of the pumped liquid oxygen stream form nitrogen and
pressurized oxygen products. The second part 126 of pumped liquid
oxygen stream 122 can optionally be taken as a liquid product.
[0042] As indicated above, the first compressed air stream 20 is
not fully cooled within main heat exchanger 40. Rather, it is
withdrawn to produce main feed air stream 42 having a warmer
temperature than the second compressed air stream 28 upon its
liquefaction and discharge as liquid air stream 44 from main heat
exchanger 40. As mentioned above, this causes the subcooling of
liquid air stream 44. The temperature of main feed air stream 42 is
preferably in a range of between about 6K and about 25K warmer than
liquid air stream 44. A more preferred range is between about 8K
and about 15K.
[0043] With reference to FIG. 2, the temperature profile within the
main heat exchanger 40 is shown in which the first compressed air
stream 20 is fully cooled and is thus withdrawn after having fully
traversed the main heat exchanger 40. In this particular prior art
operation, there exists a temperature difference in the cold end of
main heat exchanger of about 6.2K.
[0044] With reference to FIG. 3, the temperature profile within
main heat exchanger 40 is shown in accordance with the present
invention. Withdrawal of compressed and purified air stream 20 at
the warmer temperature and therefore, production of main feed air
stream 42 at the warmer temperature results in a steeper cooling
profile because all that remains within the main heat exchanger 40
to be cooled is second compressed and purified air stream 28 which
results in the production of liquid air stream 24 at a subcooled
temperature. As a result, less vaporization occurs due to the
expansion of liquid air stream 44 within expander 54 and the first
subsidiary liquid stream 56 after passage through valve 60 and
second subsidiary liquid stream 58 has a greater liquid content
upon its introduction into higher pressure column 48. Main feed air
stream 42 is warmer entering the high pressure column. This results
in greater liquid-vapor traffic and therefore an increase in the
production of nitrogen-rich vapor in the top of higher pressure
column 48. The greater liquid content of first subsidiary air
stream 56 produces an increased liquid to vapor ratio below the
point of introduction into the lower pressure column 50.
Additionally, the greater production of nitrogen-rich vapor at the
top of higher pressure column 48 results in more liquid being
produced into lower pressure column 50 as reflux by virtue of
increased production of second part 74 of liquid nitrogen-rich
stream 70. In the present invention, since the crude liquid oxygen
stream 76 is also subcooled, a greater liquid fraction of this
stream after expansion is also able to be introduced into lower
pressure column 50. The resultant overall greater liquid to vapor
ratio within lower pressure column 50 results in more argon being
present within argon-rich stream 86 and therefore, a greater rate
argon recovery. It is to be noted, that the same also will increase
the oxygen recovery, albeit to a lesser extent. However, since,
typically, the oxygen is being supplied to customers under supply
contracts, the plant can be operated to meet commercial needs by
decreasing the degree of main air compression to also lower the
overall power requirements of a method conducted in accordance with
the present invention while still taking advantage of the increased
argon recovery possible in the inventive method disclosed
herein.
[0045] However, as main feed air stream 20 becomes progressively
warmer, the temperature of the liquid air stream 42 becomes
progressively lower. In order to prevent the heating and cooling
curves within the main heat exchanger 40 from crossing, more air
will have to be compressed within booster compressor 24 thereby
increasing the power requirements of the plant. Increasing flow of
30 is another way of compensating for the smaller temperature
difference at the cold end of heat exchanger 40. This tends to
increase total power and decrease argon recovery. It has been found
by the inventors herein that the withdrawal of the main feed air
stream 20 from the main heat exchanger 40 at a specific, predefined
temperature, allow the temperature of liquid air stream to be
controlled so as to approach the temperatures of the return
streams, namely, nitrogen column overhead stream 114, waste stream
116 and pumped liquid oxygen stream 124. Such control thereby
allows for an increase in argon recovery without unnecessary
increases in the power requirements for the compression of the air.
In a typical plate-fin heat exchanger, the main feed air stream 42
should be withdrawn from the main heat exchanger 40 at a
temperature such that liquid air stream 44 has a temperature that
approaches that of the average temperature of the return streams by
no less than a range of between about 0.2K and 3K, and preferably
between 0.4K and 2K. Below this range in temperature, power
requirements rapidly increase without any appreciable increase in
argon recovery. As mentioned above, this "average temperature" is
calculated to be a temperature at which the flow times the enthalpy
is equal to the flow times the enthalpy of such return streams at
their actual temperature at the cold end of the main heat exchanger
40. In the illustrated embodiment, the return streams at the cold
end of main heat exchanger 40 are first part 124 of pumped liquid
oxygen stream 122, and nitrogen column overhead stream 114 and
waste stream 116 at the warm end of subcooling unit 78. It is to be
noted that if any additional streams are withdrawn from the column
system and then fed to main heat exchanger 40, then such streams
would be counted in such calculation of the average temperature. As
would be known, the control of such temperature of main feed air
stream 44 is effectuated by design of the main heat exchanger 40
and more specifically, the location of an outlet thereof to
discharge main feed air stream 42 therefrom.
[0046] With reference to FIG. 4, in an alternative embodiment of
the air separation plant shown in FIG. 2, main heat exchanger 40
and subcooling unit 28 can be combined into a single unit 40'. The
air separation plant illustrated in FIG. 4 otherwise functions in a
manner set forth for the apparatus of FIG. 1.
[0047] With reference to FIG. 5, an alternative embodiment of the
air separation plant shown in FIG. 1 is illustrated. A nitrogen
enriched vapor stream 130 can be extracted from nitrogen-rich vapor
stream 66 and a remaining portion 67 of nitrogen-rich vapor stream
66 can be introduced into condenser reboiler 68. Nitrogen enriched
vapor stream 130 is introduced into main heat exchanger 40'' in
which it is partially warmed and then introduced into a
turboexpander 132 coupled to a generator 134. The resultant cooled
exhaust stream 136 is introduced into the main heat exchanger 40''
that is provided with a passage to fully warm such stream and
thereby refrigerate the process. Other than the alternative method
of generating refrigeration, the plant illustrated in FIG. 5 is
otherwise identical to that shown in FIG. 1.
[0048] With reference to FIG. 6, a yet further alternative
embodiment of the air separation plant illustrated in FIG. 1 is
shown. In such embodiment, a fourth compressed air stream 150 is
taken from an intermediate stage of the booster compressor 24,
preferably, the first or second stage thereof. The resulting fourth
compressed air stream 150 is then compressed within a compressor
152 to produce compressed air stream 154 that, after removal of
heat of compression within an after cooler 156, is introduced into
a turbine 158 to produce an exhaust stream 160 that is combined
with first compressed air stream 20 at an intermediate location and
temperature level of a main heat exchanger 40''' having an inlet
provided for such purpose. This results in a capability to produce
more liquid than the plant shown in FIG. 1. Other than the
modification outlined in this paragraph, the remainder of the plant
would otherwise be identical to the air separation plant 1 shown in
FIG. 1.
[0049] The following are calculated examples of the operation of
air separation plant 1, as illustrated in FIG. 1, that is conducted
in accordance with a method of the present invention (Table 1) and
a prior art method in which the main feed air stream 42 is
withdrawn from the main heat exchanger 40 at the cold end
temperature of the main heat exchanger 40 (Table 2). In both
examples, the plants are designed to produce a unitized gaseous
oxygen flow of 1000 (first part 124 of pumped liquid oxygen stream
122 after vaporization in main heat exchanger 40) and a unitized
liquid oxygen flow of 34 (second part 126 of pumped liquid oxygen
stream 122).
TABLE-US-00001 TABLE 1 Pressure, Percent Stream Ref. No. Flow
Temperature, K psia Composition vapor 18 4948 282.0 88.0 air 100 20
2815 282.0 88.0 air 100 28 (after cooling 1453 305.4 1100 air 100
in after cooler 26) 42 2815 108.9 84.0 air 100 44 1453 97.9 1099
air 0 58 436 96.2 83.7 air 0 56 (after valve 60) 1017 82.0 20.1 air
14.8 36 (after discharge 679 144.9 136.8 air 100 from main heat
exchanger 40) 64 679 89.2 20.2 air 100 82 34.0 81.9 83.0 99.9998% 0
N.sub.2 + Ar 98 36.1 89.1 17.8 99.9998% 0 Ar 126 34.0 96.3 450
99.6% O.sub.2 0 124 (after 1000 291.0 446 99.6% O.sub.2 100
vaporization within main heat exchanger 40) 116 (after being 815
291.0 17.2 98.6% N.sub.2 100 fully warmed within main heat
exchanger 40) 114 (after being 3029 291.0 16.9 99.9999% 100 fully
warmed N.sub.2 + Ar within main heat exchanger 40)
TABLE-US-00002 TABLE 2 Pressure, Percent Stream No. Flow
Temperature, K psia Composition vapor 18 4968 282.0 88.0 air 100 20
2863 282.0 88.0 air 100 28 (after cooling 1426 305.4 1100 air 100
in after cooler 26) 42 2863 103.4 84.0 air 100 44 1426 103.4 1099
air 0 58 428 98.1 83.7 air 3.9 56 (after valve 60) 998 82.1 20.1
air 20.2 36 (after discharge 679 144.9 136.8 air 100 from main heat
exchanger 40) 64 679 89.2 20.2 air 100 82 34.0 82.0 83.0 99.9998% 0
N.sub.2 + Ar 98 34.4 89.1 17.8 99.9998% 0 Ar 126 34.0 96.3 450
99.6% O.sub.2 0 124 (after 1000 290.7 446 99.6% O.sub.2 100
vaporization within main heat exchanger 40) 116 (after being 941
290.7 17.2 98.1% N.sub.2 100 fully warmed within main heat
exchanger 40) 114 (after being 2925 290.7 16.9 99.9999% 100 fully
warmed N.sub.2 + Ar within main heat exchanger 40)
[0050] By way of comparison, the argon recovery of the present
invention, as shown in Table 1, is 78.1%. The argon recovery for a
prior art method, represented in Table 2, is 74.1%. Likewise, the
oxygen recovery from Table 1 is 99.3%, the oxygen recovery in Table
2 is 98.9%. The lower degree of flash off of streams 56 and 58 as
they enter the higher and lower pressure distillation columns 48
and 60, for the present invention as shown in Table 1 (percent
vapor), and the warmer temperature of main feed air stream 42, lead
to the improved product stream recoveries. The reduced flash off is
a result of the lower temperature of liquid air stream 44 in the
present invention. In Table 1, the flow of second compressed and
purified air stream 28 is required to be 1.9% higher than in Table
3. As a result, the power consumption for the present invention is
slightly higher than in the prior art.
[0051] While the invention has been described with reference to a
preferred embodiment, as will occur to those skilled in the art,
numerous changes, additions and omissions can be made without
departing from the spirit and the scope of the present invention as
recited in the appended claims.
* * * * *