U.S. patent application number 13/674393 was filed with the patent office on 2013-06-06 for air separation method and apparatus.
The applicant listed for this patent is Thomas J. Bergman, JR., Henry Edward Howard, Matthew R. Watt. Invention is credited to Thomas J. Bergman, JR., Henry Edward Howard, Matthew R. Watt.
Application Number | 20130139547 13/674393 |
Document ID | / |
Family ID | 47263591 |
Filed Date | 2013-06-06 |
United States Patent
Application |
20130139547 |
Kind Code |
A1 |
Howard; Henry Edward ; et
al. |
June 6, 2013 |
AIR SEPARATION METHOD AND APPARATUS
Abstract
A method and apparatus to produce oxygen and nitrogen
co-products in which a compressed a compressed and purified air
stream is cooled, fully or partially condensed and then rectified
in a main distillation column to form a nitrogen-rich vapor column
overhead and crude liquid oxygen. A crude liquid oxygen stream is
depressurized and then stripped in an auxiliary distillation column
with a stripping gas to produce an oxygen-rich liquid. The
nitrogen-rich vapor column overhead from the main distillation
column is used to form a nitrogen product and the crude liquid
oxygen is partially vaporized to produce the stripping gas, a
residual oxygen-rich liquid and liquid nitrogen reflux to the main
distillation column. The oxygen product is formed from the residual
oxygen-rich liquid by either providing the heat exchange duty in
condensing the compressed and purified air stream or by condensing
nitrogen-rich vapor used in refluxing the main distillation
column.
Inventors: |
Howard; Henry Edward; (Grand
Island, NY) ; Watt; Matthew R.; (Grand Island,
NY) ; Bergman, JR.; Thomas J.; (Clarence Center,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Howard; Henry Edward
Watt; Matthew R.
Bergman, JR.; Thomas J. |
Grand Island
Grand Island
Clarence Center |
NY
NY
NY |
US
US
US |
|
|
Family ID: |
47263591 |
Appl. No.: |
13/674393 |
Filed: |
November 12, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13311038 |
Dec 5, 2011 |
|
|
|
13674393 |
|
|
|
|
Current U.S.
Class: |
62/643 |
Current CPC
Class: |
F25J 3/04254 20130101;
F25J 2200/20 20130101; F25J 3/04284 20130101; F25J 3/04884
20130101; F25J 2210/50 20130101; F25J 2240/44 20130101; F25J 3/0409
20130101; F25J 2210/42 20130101; F25J 2250/02 20130101; F25J
2205/02 20130101; F25J 2270/02 20130101; F25J 3/04206 20130101;
F25J 3/0423 20130101; F25J 3/04103 20130101; F25J 3/04212 20130101;
F25J 3/0443 20130101 |
Class at
Publication: |
62/643 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1. A method of separating air to produce oxygen and nitrogen
co-products, said method comprising: cooling a compressed and
purified stream comprising the air; rectifying the compressed and
purified stream within a main distillation column to produce a
nitrogen-rich vapor column overhead and crude liquid oxygen;
producing an oxygen-rich liquid and an auxiliary column overhead
containing not less than 5.0 percent oxygen on a volume basis
within an auxiliary distillation column by, at least in part,
depressurizing a crude liquid oxygen stream composed of the crude
liquid oxygen, stripping the crude liquid oxygen stream within the
auxiliary distillation column with an ascending stripping gas and
partially vaporizing the oxygen-rich liquid through indirect heat
exchange with a nitrogen-rich vapor stream composed of the
nitrogen-rich vapor column overhead, thereby producing a liquid
nitrogen stream, the stripping gas and a residual oxygen-rich
liquid; refluxing the main distillation column with at least part
of the liquid nitrogen stream; forming an oxygen-rich vapor
fraction from the residual oxygen-rich liquid by indirectly
exchanging heat between a stream of the residual oxygen-rich liquid
with a gaseous stream having a nitrogen concentration no less than
that of air so that the stream of the residual oxygen-rich liquid
partially vaporizes; forming: an oxygen product stream from the
vapor fraction; a nitrogen product stream from the nitrogen-rich
vapor column overhead; and a waste stream from the auxiliary column
overhead; and passing the oxygen product stream, the nitrogen
product stream and the waste stream in indirect heat exchange with
the compressed and purified stream.
2. The method of claim 1, wherein the auxiliary column overhead is
solely produced by the stripping of the crude liquid oxygen stream
within an auxiliary distillation column.
3. The method of claim 1, wherein: the main distillation column is
refluxed with part of the liquid nitrogen stream; the stripping of
the crude liquid oxygen stream takes place within a stripping
section of auxiliary distillation column; stripping the crude
liquid oxygen stream within the auxiliary distillation column
produces a nitrogen and oxygen containing vapor stream; and the
nitrogen and oxygen containing vapor stream is rectified within the
auxiliary distillation column within a rectification section of the
auxiliary distillation column located above the stripping section
by introducing the nitrogen and oxygen containing vapor stream into
the rectification section and refluxing the auxiliary distillation
column and therefore, the rectification section with a further part
of the liquid nitrogen stream, thereby increasing recovery of
oxygen within the residual oxygen-rich liquid.
4. The method of claim 1, wherein: the oxygen-rich liquid is
collected within the auxiliary distillation column; and the
oxygen-rich liquid is partially vaporized by passing an oxygen-rich
liquid stream composed of the oxygen-rich liquid and the
nitrogen-rich vapor stream through a once-through heat exchanger to
form the stripping gas and the residual oxygen-rich liquid that
collects as a column bottoms of the auxiliary distillation
column.
5. The method of claim 4, wherein: the gaseous stream is the
compressed and purified stream; the compressed and purified stream
is partially condensed in a condenser; the oxygen-rich vapor
fraction is formed by: collecting the stream of the residual
oxygen-rich liquid in a separation vessel; introducing a liquid
phase stream, formed of a liquid phase produced within the
separation vessel, into the condenser and partially vaporizing the
liquid phase stream in the condenser through indirect heat exchange
with the compressed and purified stream, thereby producing a
two-phase stream from the liquid phase stream; introducing the two
phase stream into the separation vessel and disengaging liquid and
vapor phases of the two phase stream within the separation vessel
to form an oxygen-rich vapor fraction and the liquid phase together
with the stream of the residual oxygen-rich liquid collected in the
separation vessel; and the oxygen product stream is formed by
discharging a stream of the oxygen-rich vapor fraction from the
separation vessel.
6. The method of claim 5, wherein the condenser is located in a
main distillation column bottom region such that condensed air
mixes with downcoming liquid produced by the rectification to
thereby produce the crude liquid oxygen as a column bottoms in the
main distillation column.
7. The method of claim 6, wherein: the main distillation column is
refluxed with part of the liquid nitrogen stream; and the waste
stream indirectly exchanges heat with the crude liquid oxygen
stream such that the crude liquid oxygen stream is subcooled prior
to being depressurized.
8. The method of claim 4, wherein: the gaseous stream is composed
of the nitrogen-rich vapor column overhead; the heat is indirectly
exchanged between the stream of the residual oxygen-rich liquid and
the gaseous stream by depressurizing a liquid oxygen enriched
stream and passing the stream of the residual oxygen-rich liquid in
indirect heat exchange with the gaseous stream within a
thermo-siphon reboiler thereby producing the vapor fraction from
the partial vaporization of the stream of the residual oxygen-rich
liquid and a condensate stream through condensation of the gas
stream; and the condensate stream is introduced into the main
distillation column as reflux along with the liquid nitrogen
stream.
9. The method of claim 8, wherein the waste stream passes in
indirect heat exchange with the crude liquid oxygen stream prior to
the depressurization of the crude liquid oxygen stream so that the
crude liquid oxygen stream is subcooled.
10. The method of claim 1, wherein the stream of the residual
oxygen-rich liquid is pressurized such that the oxygen product
stream is also pressurized.
11. The method of claim 1, wherein a liquid nitrogen refrigeration
stream is introduced into the main distillation column to impart
refrigeration.
12. An apparatus for separating air to produce oxygen and nitrogen
co-products, said apparatus comprising: a main heat exchanger
configured to cool a compressed and purified stream comprising the
air; a main distillation column configured to rectify the
compressed and purified stream to produce a nitrogen-rich vapor
column overhead and crude liquid oxygen; an auxiliary distillation
column connected to the main distillation column and configured
such that a crude liquid oxygen stream, composed of the crude
liquid oxygen, is stripped with an ascending stripping gas within
the auxiliary distillation column and an oxygen-rich liquid and an
auxiliary column overhead containing not less than 5.0 percent
oxygen by volume are produced, at least in part, as a result of the
stripping of the crude liquid oxygen stream; an expansion valve
positioned between the main distillation column and an auxiliary
distillation column such that the crude liquid oxygen stream is
depressurized prior to introduction into the auxiliary distillation
column; means for partially vaporizing the oxygen-rich liquid
through indirect heat exchange with a nitrogen-rich vapor stream
composed of the nitrogen-rich vapor column overhead, thereby
producing a liquid nitrogen stream, the stripping gas and a
residual oxygen-rich liquid; the oxygen-rich liquid partial
vaporization means connected to the main distillation column such
that the main distillation column is refluxed with at least part of
the liquid nitrogen stream; the main heat exchanger connected to
the main distillation column and the auxiliary distillation column
so that a nitrogen product stream composed of the nitrogen-rich
vapor column overhead and a waste stream formed from the auxiliary
column overhead of the auxiliary distillation column indirectly
exchange heat with the compressed and purified air stream; means
for indirectly exchanging heat between a stream of the residual
oxygen-rich liquid with a gaseous stream having a nitrogen
concentration no less than that of air so that the stream of the
residual oxygen-rich liquid partially vaporizes and means for
forming an oxygen-rich vapor fraction from the stream of the
residual oxygen-rich liquid after having been partially vaporized;
and the main heat exchanger connected to the oxygen-rich vapor
fraction forming means, the main distillation column and the
auxiliary distillation column such that an oxygen product stream,
composed of the oxygen-rich vapor fraction, a nitrogen product
stream, composed of the nitrogen-rich vapor column overhead, and a
waste stream, composed of the auxiliary column overhead of the
auxiliary distillation column, pass within the main heat exchanger,
in indirect heat exchange with the compressed and purified
stream.
13. The apparatus of claim 12, wherein the auxiliary column is
solely provided with a stripping section where the stripping of the
crude liquid oxygen stream takes place.
14. The apparatus of claim 12, wherein: the auxiliary column has a
stripping section and a rectification section, located above the
stripping section; the stripping of the crude liquid oxygen stream
takes place within a stripping section of auxiliary distillation
column and a nitrogen and oxygen containing vapor stream is
produced in the stripping section that enters the rectification
section for rectification of the nitrogen and oxygen containing
vapor stream, thereby increasing recovery of oxygen within the
residual oxygen-rich liquid; the oxygen-rich liquid partial
vaporization means is connected to the main distillation column
such that the main distillation column is refluxed with part of the
liquid nitrogen stream and is also connected to the auxiliary
distillation column such that the auxiliary distillation column and
therefore, the rectification section is refluxed with a further
part of the liquid nitrogen stream; and another expansion valve is
positioned between the oxygen-rich liquid partial vaporization
means and the auxiliary distillation column so that pressure of the
further part of the liquid nitrogen stream is reduced to that of
the auxiliary distillation column.
15. The apparatus of claim 12, wherein: the auxiliary distillation
column has means for collecting the oxygen-rich liquid; the
oxygen-rich liquid partial vaporization means is a once-through
heat exchanger connected to an auxiliary distillation column and
the oxygen-rich liquid collecting means such that the oxygen-rich
liquid is partially vaporized within the once-through heat
exchanger through passage of an oxygen-rich liquid stream, composed
of the oxygen-rich liquid and the residual oxygen-rich liquid
collects as a column bottoms of the auxiliary distillation column;
and the main distillation column connected to the once-through heat
exchanger such that the nitrogen-rich vapor stream is condensed
within the once-through heat exchanger.
16. The apparatus of claim 15, wherein: the gaseous stream is the
compressed and purified air stream; the residual oxygen-rich liquid
heat exchange means and the oxygen-rich vapor fraction forming
means is a condenser connected to the main heat exchanger such that
the compressed and purified stream is partially condensed and a
separation vessel; the separation vessel is connected to the
auxiliary distillation column such that the stream of the residual
oxygen-rich liquid collects in the separation vessel; the
separation vessel is connected to the condenser so that a liquid
phase stream, composed of a liquid phase produced within the
separation vessel is partially vaporized in the condenser to
produce a two-phase stream that is introduced into the separation
vessel, liquid and vapor phases of the two phase stream are
disengaged within the separation vessel to form the oxygen-rich
vapor fraction and the liquid phase; and the main heat exchanger is
connected to the separation vessel so that the oxygen product
stream is formed from the oxygen-rich vapor fraction.
17. The apparatus of claim 16, wherein the condenser is located in
a bottom region of the main distillation column such that condensed
air mixes with downcoming liquid produced by the rectifying of the
compressed and purified stream to produce the crude liquid oxygen
as a column bottoms in the main distillation column;
18. The apparatus of claim 17, wherein: the once-through heat
exchanger is connected to the main distillation column such that
the main distillation column is refluxed with part of the liquid
nitrogen stream; and a subcooling heat exchanger is connected to
the once-through heat exchanger, the auxiliary distillation column
and the expansion valve such that the waste stream indirectly
exchanges heat with the crude liquid oxygen stream within the
subcooling heat exchanger and the crude liquid oxygen stream is
subcooled prior to passage through the expansion valve.
19. The apparatus of claim 15, wherein: the gaseous stream is
composed of the nitrogen-rich vapor; the stream of the residual
oxygen-rich liquid heat exchange means and the oxygen-rich vapor
fraction forming means are a thermo-siphon reboiler having a shell;
the shell connected to the auxiliary column to receive the stream
of the residual oxygen-rich liquid; another expansion valve is
positioned between the shell and the auxiliary column so that the
stream of the residual oxygen-rich liquid is depressurized; the
thermo-siphon reboiler connected to the main distillation column to
receive the gaseous stream and thereby condense the gaseous stream
through indirect heat exchange with the stream of the residual
oxygen-rich liquid and thereby form the oxygen-rich vapor fraction
within the shell and discharge a condensate stream to the main
distillation column as reflux along with the liquid nitrogen
stream; and the main heat exchanger is connected to the shell, the
main distillation column and the auxiliary distillation column such
that an oxygen product stream, formed from the vapor fraction, a
nitrogen product stream, formed from the nitrogen-rich vapor column
overhead, a waste stream, formed from the auxiliary column overhead
produced in the auxiliary distillation column pass within the main
heat exchanger, in indirect heat exchange with the compressed and
purified stream.
20. The apparatus of claim 19, wherein a subcooling heat exchanger
is positioned between the auxiliary distillation column, the main
distillation column and the main heat exchanger such that the waste
stream passes in indirect heat exchange with the crude liquid
oxygen stream prior to the depressurization of the crude liquid
oxygen stream and prior to the warming the waste stream in the main
heat exchanger.
21. The apparatus of claim 12 wherein the main distillation column
has a top inlet for introduction of a liquid nitrogen refrigeration
stream to impart refrigeration.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. patent
application Ser. No. 13/311,038, filed on Dec. 5, 2011, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
separating air in which oxygen and nitrogen products are produced
as co-products by rectifying compressed and purified air in a main
distillation column to produce the nitrogen product and stripping
crude liquid oxygen formed in the main distillation column within
an auxiliary distillation column to produce the oxygen product.
BACKGROUND OF THE INVENTION
[0003] Nitrogen is typically obtained at high purity by separating
nitrogen from air within a cryogenic air separation plant often
employing a single distillation column. In such a plant, air is
compressed and then purified of higher boiling contaminants to
produce a compressed and purified air stream. The compressed and
purified air stream is then cooled to a temperature suitable for
its cryogenic rectification within a main heat exchanger and then
introduced into a single distillation column operating at about 3
bara or higher. The air, within the distillation column, is
rectified to produce a nitrogen-rich vapor column overhead and an
oxygen-rich liquid column bottoms known as crude liquid oxygen or
kettle liquid. The oxygen-rich liquid is depressurized in an
expansion valve and then introduced into a heat exchanger to
condense a stream of the nitrogen-rich vapor column overhead and
thereby produce liquid nitrogen to reflux the distillation column.
The oxygen-rich liquid that is partially vaporized can be used to
generate refrigeration for the plant or alternatively,
refrigeration can be provided to the plant through liquid nitrogen
addition to the main column or a stream entering the main heat
exchanger.
[0004] In general, oxygen is not recovered from such single column
nitrogen plants. However, there is an interest in also recovering
oxygen from such plants. For example, float glass production
typically requires nitrogen and low purity oxygen in a flow ratio
of approximately 2:1. The oxygen is employed in the glass furnace
to boost recovery and is typically required at a low purity of
between 90 and 95 percent and at a pressure of between 10 and 20
psig. While such oxygen can be provided by a separate vacuum
pressure swing adsorption plant or through the vaporization of
delivered on-site liquid, the added expense is not justifiable. It
is to be noted that both oxygen and nitrogen can be produced by a
typical double column air separation plant having high and low
pressure columns operatively associated in a heat transfer
relationship. However, such a plant is not economical where, for
example, float glass is to be produced because of the requirement
of product compression and higher initial capital cost. The
incremental modification of a small single column nitrogen plant to
deliver both nitrogen and oxygen products can economically meet the
requirements of processes such as float glass production where the
nitrogen and oxygen products are required at pressure and at modest
flow rate, for example less than 100 kcfh of nitrogen.
[0005] In the prior art, there are examples of single column
nitrogen plants that have been modified to co-produce both nitrogen
and oxygen products. For example, in U.S. Pat. No. 4,783,210, a
stream of the crude liquid oxygen column bottoms is partially
vaporized in a heat exchanger used in condensing a stream of the
nitrogen-rich vapor column overhead produced in a distillation
column. The resulting nitrogen rich liquid is used to reflux the
column. A stream of the resulting liquid phase produced by the
partial vaporization of the crude liquid oxygen is then stripped in
a secondary or auxiliary distillation column to produce a column
bottoms, rich in oxygen, that can be taken as an oxygen product.
The auxiliary distillation column is reboiled with another stream
of the nitrogen-rich vapor which condenses in the reboiling and can
be taken as a liquid nitrogen product and also, used to reflux the
distillation column.
[0006] U.S. Pat. No. 5,074,898 discloses a single column nitrogen
generator with an auxiliary distillation column to produce an
oxygen product. In this patent, a crude liquid oxygen stream
generated in a main distillation column is stripped in an auxiliary
distillation column. The auxiliary distillation column is reboiled
with a stream of the nitrogen-rich vapor column overhead produced
in the main distillation column. This condenses the nitrogen-rich
vapor to produce reflux for the main distillation column. Residual
liquid generated in the auxiliary distillation column can be taken
as a liquid product along with part of the condensed nitrogen
vapor.
[0007] In both of these patents, the column bottoms is being used
to condense nitrogen, it necessarily must be at a lower pressure
than the nitrogen to accomplish nitrogen condensation. As a result,
the oxygen product is also at low pressure. Furthermore, since part
of the column bottoms is being taken as a product, there will be
less bottoms fluid to condense nitrogen. Consequently, nitrogen
production is limited. As will be discussed, the present invention
provides a method and apparatus for producing nitrogen and oxygen
co-products, that among other advantages, may allow the oxygen
product to be produced at elevated pressure. In another aspect of
the present invention, the auxiliary column bottoms is subjected to
a staged vaporization to enable a greater fraction of oxygen to be
obtained at lower power relative those processes contemplated in
the prior art.
SUMMARY OF THE INVENTION
[0008] The present invention provides a method of separating air to
produce oxygen and nitrogen co-products. In accordance with such
method, a compressed and purified stream comprising the air is
cooled and rectified within a main distillation column to produce a
nitrogen-rich vapor column overhead and crude liquid oxygen. An
oxygen-rich liquid and an auxiliary column overhead containing not
less than 5.0 percent oxygen on a volume basis are produced within
an auxiliary distillation column by, at least in part,
depressurizing a crude liquid oxygen stream composed of the crude
liquid oxygen, stripping the crude liquid oxygen stream within the
auxiliary distillation column with an ascending stripping gas and
partially vaporizing the oxygen-rich liquid through indirect heat
exchange with a nitrogen-rich vapor stream composed of the
nitrogen-rich vapor column overhead. As a result, a liquid nitrogen
stream, the stripping gas and a residual oxygen-rich liquid are
thereby produced. The main distillation column is refluxed with at
least part of the liquid nitrogen stream. The phrase, "at least in
part" is used herein and in the claims to indicate that further
processing could take place within the auxiliary distillation
column to produce the oxygen-rich liquid and the auxiliary column
overhead. As set forth hereinafter, while the auxiliary column
could function solely to strip the crude liquid oxygen stream, it
could also function to rectify nitrogen and oxygen containing vapor
produced by such stripping to increase oxygen recovery.
[0009] An oxygen-rich vapor fraction is formed from the residual
oxygen-rich liquid by indirectly exchanging heat between a stream
of the residual oxygen-rich liquid with a gaseous stream having a
nitrogen concentration no less than that of air so that the stream
of the residual oxygen-rich liquid partially vaporizes. An oxygen
product stream is formed from the vapor fraction, a nitrogen
product stream is formed from the nitrogen-rich vapor and a waste
stream is formed from the auxiliary column overhead of the
auxiliary distillation column. The oxygen product stream, the
nitrogen product stream and the waste stream are passed in indirect
heat exchange with the compressed and purified stream.
[0010] As can be appreciated from the above discussion, since the
oxygen product stream is formed from a vapor fraction that is in
turn produced from the residual oxygen-rich liquid, the production
of the oxygen product is no longer directly coupled to the
production of reflux nitrogen to the main distillation column since
the residual oxygen-rich liquid is not used in condensing the
nitrogen-rich vapor from the main distillation column. As a
consequence, the removal of the residual oxygen-rich liquid does
not decrease nitrogen reflux to the main distillation column and
therefore, nitrogen production as in the prior art.
[0011] Additionally, since the residual oxygen-rich liquid is not
used in condensing nitrogen, such liquid can be produced at a
higher pressure.
[0012] As indicated above, the auxiliary column overhead can solely
be produced by the stripping of the crude liquid oxygen stream
within an auxiliary distillation column. Alternatively, the main
distillation column is refluxed with part of the liquid nitrogen
stream. In the latter case, the stripping of the crude liquid
oxygen stream takes place within a stripping section of the
auxiliary distillation column and the stripping produces a nitrogen
and oxygen containing vapor stream. The nitrogen and oxygen
containing vapor stream is rectified within the auxiliary
distillation column within a rectification section of the auxiliary
distillation column located above the stripping section by
introducing the nitrogen and oxygen containing vapor stream into
the rectification section and refluxing the auxiliary distillation
column and therefore, the rectification section with a further part
of the liquid nitrogen stream. The effect of this is to increase
recovery of oxygen within the residual oxygen-rich liquid by in
effect trapping the oxygen that would otherwise escape the
auxiliary column within the waste stream.
[0013] In either of the cases mentioned above, the oxygen-rich
liquid can be collected within the auxiliary distillation column.
The oxygen-rich liquid is partially vaporized by passing an
oxygen-rich liquid stream and the nitrogen-rich vapor stream
through a once-through heat exchanger to form the stripping gas and
the residual oxygen-rich liquid that collects as a column bottoms
of the auxiliary distillation column.
[0014] The gaseous stream can be the compressed and purified
stream. The compressed and purified stream is partially condensed
in a condenser and the stream of the residual oxygen-rich liquid is
collected in a separation vessel. A liquid phase stream, formed of
a liquid phase produced within the separation vessel, is introduced
into the condenser and partially vaporized in the condenser through
indirect heat exchange with the compressed and purified stream,
thereby producing a two-phase stream from the liquid phase stream.
The two phase stream is introduced into the separation vessel and
liquid and vapor phases of the two phase stream are disengaged
within the separation vessel to form an oxygen-rich vapor fraction
and the liquid phase together with the stream of the residual
oxygen-rich liquid collected in the separation vessel. The oxygen
product stream is then formed by discharging a stream of the
oxygen-rich vapor fraction from the separation vessel.
[0015] In the embodiment of the invention, described above, the
condenser can be located in a main distillation column bottom
region such that condensed air mixes with downcoming liquid
produced by the rectification to thereby produce the crude liquid
oxygen as a column bottoms in the main distillation column.
Further, the main distillation column can be refluxed with part of
the liquid nitrogen stream and the waste stream indirectly
exchanges heat with the crude liquid oxygen stream such that the
crude liquid oxygen stream is subcooled prior to being
depressurized.
[0016] As an alternative to forming the gaseous stream from the
compressed and purified stream, the gaseous stream can be composed
of the nitrogen-rich vapor column overhead. In such embodiment, the
heat is indirectly exchanged between the stream of the residual
oxygen-rich liquid and the gaseous stream by depressurizing a
liquid oxygen enriched stream and passing the stream of the
residual oxygen-rich liquid in indirect heat exchange with the
gaseous stream within a thermo-siphon reboiler. This produces the
vapor fraction from the partial vaporization of the stream of the
residual oxygen-rich liquid and a condensate stream through
condensation of the gas stream. The condensate stream is introduced
into the main distillation column as reflux along with the liquid
nitrogen stream.
[0017] In the above described embodiment, the waste stream can pass
in indirect heat exchange with the crude liquid oxygen stream prior
to the depressurization of the crude liquid oxygen stream so that
the crude liquid oxygen stream is subcooled.
[0018] In any embodiment of the present invention, the stream of
the residual oxygen-rich liquid can be pressurized such that the
oxygen product stream is also pressurized. Further, A liquid
nitrogen refrigeration stream can be introduced into the main
distillation column to impart refrigeration.
[0019] The present invention also provides an apparatus for
separating air to produce the oxygen and nitrogen co-products. Such
apparatus includes a main heat exchanger, a main distillation
column and an auxiliary distillation column. The main heat
exchanger is configured to cool a compressed and purified stream
comprising the air and the main distillation column is configured
to rectify the compressed and purified stream to produce a
nitrogen-rich vapor column overhead and crude liquid oxygen. The
auxiliary distillation column is connected to the main distillation
column and configured such that a crude liquid oxygen stream,
composed of the crude liquid oxygen, is stripped with an ascending
stripping gas within the auxiliary distillation column and an
oxygen-rich liquid and an auxiliary column overhead containing not
less than 5.0 percent oxygen by volume are produced, at least in
part, as a result of the stripping of the crude liquid oxygen
stream. An expansion valve is positioned between the main
distillation column and an auxiliary distillation column such that
the crude liquid oxygen stream is depressurized prior to
introduction into the auxiliary distillation column.
[0020] A means is provided for partially vaporizing the oxygen-rich
liquid through indirect heat exchange with a nitrogen-rich vapor
stream composed of the nitrogen-rich vapor column overhead, thereby
producing a liquid nitrogen stream, the stripping gas and a
residual oxygen-rich liquid. The oxygen-rich liquid partial
vaporization means is connected to the main distillation column
such that the main distillation column is refluxed with at least
part of the liquid nitrogen stream. The main heat exchanger is
connected to the main distillation column and the auxiliary
distillation column so that a nitrogen product stream formed from
the nitrogen-rich vapor and a waste stream formed from the
auxiliary column overhead of the auxiliary distillation column
indirectly exchange heat with the compressed and purified air
stream. A means is also provided for indirectly exchanging heat
between a stream of the residual oxygen-rich liquid with a gaseous
stream having a nitrogen concentration no less than that of air so
that the stream of the residual oxygen-rich liquid partially
vaporizes. Further, a means is provided for forming an oxygen-rich
vapor fraction from the stream of the residual oxygen-rich liquid
after having been partially vaporized. The main heat exchanger is
connected to the oxygen-rich vapor fraction forming means, the main
distillation column and the auxiliary distillation column such that
an oxygen product stream, composed of the oxygen-rich vapor
fraction, a nitrogen product stream, composed of the nitrogen-rich
vapor column overhead, and a waste stream, composed of the
auxiliary column overhead of the auxiliary distillation column,
pass within the main heat exchanger, in indirect heat exchange with
the compressed and purified stream.
[0021] The auxiliary column can be solely provided with a stripping
section where the stripping of the crude liquid oxygen stream takes
place. As an alternative, the auxiliary column can be provided with
a stripping section and a rectification section, located above the
stripping section. In the latter case, the stripping of the crude
liquid oxygen stream takes place within a stripping section of
auxiliary distillation column and a nitrogen and oxygen containing
vapor stream is produced in the stripping section that enters the
rectification section for rectification of the nitrogen and oxygen
containing vapor stream. This rectification is provided to thereby
increase recovery of oxygen within the residual oxygen-rich liquid.
The oxygen-rich liquid partial vaporization means is connected to
the main distillation column such that the main distillation column
is refluxed with part of the liquid nitrogen stream and is also
connected to the auxiliary distillation column such that the
auxiliary distillation column and therefore, the rectification
section is refluxed with a further part of the liquid nitrogen
stream. Another expansion valve is positioned between the
oxygen-rich liquid partial vaporization means and the auxiliary
distillation column so that pressure of the further part of the
liquid nitrogen stream is reduced to that of the auxiliary
distillation column.
[0022] In any embodiment of the present invention, the auxiliary
distillation column can be provided with means for collecting the
oxygen-rich liquid. The oxygen-rich liquid partial vaporization
means is a once-through heat exchanger connected to an auxiliary
distillation column and the oxygen-rich liquid collecting means
such that the oxygen-rich liquid is partially vaporized within the
once-through heat exchanger through passage of an oxygen-rich
liquid stream, composed of the oxygen-rich liquid and the residual
oxygen-rich liquid collects as a column bottoms of the auxiliary
distillation column. The main distillation column is connected to
the once-through heat exchanger such that the nitrogen-rich vapor
stream is condensed within the once-through heat exchanger.
[0023] The gaseous stream can be the compressed and purified air
stream. In such case, the stream of the residual oxygen-rich liquid
heat exchange means and the oxygen-rich vapor fraction forming
means is a condenser and a separation vessel. The condenser is
connected to the main heat exchanger such that the compressed and
purified stream is partially condensed. The separation vessel is
connected to the auxiliary distillation column such that the stream
of the residual oxygen-rich liquid collects in the separation
vessel. The separation vessel is connected to the condenser so that
a liquid phase stream, composed of a liquid phase produced within
the separation vessel is partially vaporized in the condenser to
produce a two-phase stream that is introduced into the separation
vessel. Liquid and vapor phases of the two phase stream are
disengaged within the separation vessel to form the oxygen-rich
vapor fraction and the liquid phase and the main heat exchanger is
connected to the separation vessel so that the oxygen product
stream is formed from the oxygen-rich vapor fraction.
[0024] In the above embodiment of the present invention, the
condenser can be located in a bottom region of the main
distillation column such that condensed air mixes with downcoming
liquid produced by the rectifying of the compressed and purified
stream to produce the crude liquid oxygen as a column bottoms in
the main distillation column. Further, the once-through heat
exchanger can be connected to the main distillation column such
that the main distillation column is refluxed with part of the
liquid nitrogen stream. A subcooling heat exchanger is connected to
the once-through heat exchanger, the auxiliary distillation column
and the expansion valve such that the waste stream indirectly
exchanges heat with the crude liquid oxygen stream within the
subcooling heat exchanger and the crude liquid oxygen stream is
subcooled prior to passage through the expansion valve.
[0025] As an alternative, the gaseous stream can be composed of the
nitrogen-rich vapor. In such case, the stream of the residual
oxygen-rich liquid heat exchange means and the oxygen-rich vapor
fraction forming means are formed by a thermo-siphon reboiler
having a shell. The shell is connected to the auxiliary column to
receive the stream of the residual oxygen-rich liquid and another
expansion valve is positioned between the shell and the auxiliary
column so that the stream of the residual oxygen-rich liquid is
depressurized. The thermo-siphon reboiler is connected to the main
distillation column to receive the gaseous stream and thereby
condense the gaseous stream through indirect heat exchange with the
stream of the residual oxygen-rich liquid and thereby form the
oxygen-rich vapor fraction within the shell and discharge a
condensate stream to the main distillation column as reflux along
with the liquid nitrogen stream. The main heat exchanger is
connected to the shell, the main distillation column and the
auxiliary distillation column such that an oxygen product stream,
formed from the vapor fraction, a nitrogen product stream, formed
from the nitrogen-rich vapor column overhead, a waste stream,
formed from the auxiliary column overhead produced in the auxiliary
distillation column all pass within the main heat exchanger, in
indirect heat exchange with the compressed and purified stream.
[0026] In the embodiment discussed above, the subcooling heat
exchanger is positioned between the auxiliary distillation column,
the main distillation column and the main heat exchanger such that
the waste stream passes in indirect heat exchange with the crude
liquid oxygen stream prior to the depressurization of the crude
liquid oxygen stream and prior to the warming the waste stream in
the main heat exchanger.
[0027] In any embodiment of the present invention, the main
distillation column can be provided with a top inlet for
introduction of a liquid nitrogen refrigeration stream to impart
refrigeration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] While the present invention concludes with claims distinctly
pointing out the subject matter that Applicants regard as their
invention, it is believed that the present invention will be better
understood when taken in connection with the accompanying drawings
in which:
[0029] FIG. 1 is a schematic illustration of an air separation
plant for carrying out a method in accordance with the present
invention;
[0030] FIG. 2 is a schematic illustration of an alternative
embodiment of an air separation plant for carrying out a method in
accordance with the present invention; and
[0031] FIG. 3 is a schematic illustration of a yet further
alternative embodiment of an air separation plant for carrying out
a method in accordance with the present invention.
DETAILED DESCRIPTION
[0032] With reference to FIG. 1, an air separation plant 1 is
illustrated that is capable of co-producing oxygen and nitrogen
products ("N.sub.2" and "O.sub.2").
[0033] A compressed and purified air stream 10 is cooled in a main
heat exchanger 12 to a temperature suitable for its rectification
at or near saturation. As can be appreciated, air separation plant
1 could be part of an installation requiring compressed air and as
such, compressed and purified air stream 10 could be formed from
part of the air produced in such installation. Alternatively, such
stream could be generated by compressing the air, typically from
between 70 psia to 90 psia, and then purifying the air of higher
boiling contaminants that would solidify or concentrate at
cryogenic temperatures; for instance, carbon dioxide, water vapor
and hydrocarbons. Main heat exchanger 12 can be of conventional
brazed aluminum plate fin construction. As would occur to those
skilled in the art, main heat exchanger 12 could be formed from a
plurality of units operating in parallel.
[0034] The compressed and purified air stream 10, after having been
cooled, is introduced into a condenser 14 located in a bottom
region 16 of a main distillation column 18. Distillation column 18
contains mass-transfer contacting elements such as trays,
structured packing, random packing or a combination of such
elements that are generally indicated by reference numeral 20.
Preferably, the incoming air is partially condensed within
condenser 14 to produce a vapor fraction 22 that ascends into the
mass-transfer contacting elements 20 to contact a descending liquid
phase that produces a nitrogen-rich vapor column overhead 23.
Nitrogen-rich vapor column overhead 23 will typically be
essentially pure nitrogen. The partial condensation of the air
produces a liquid fraction 24 that constitutes about 20 percent of
the incoming air and collects along with the liquid that has
descending within elements 20 within the bottom region 16 of main
distillation column 18 as crude liquid oxygen 26.
[0035] It is to be noted, alternative means for condensing the
incoming air would be to locate condenser 14 outside of the main
distillation column 18. In connection with such means, the liquid
fraction 24 and the liquid having descended within distillation
column 20 could be separately combined to form the crude liquid
oxygen 26 or they could be subcooled and fed separately to column
34. In another such means, the compressed and purified air stream
could be introduced directly into the main distillation column
shell. The air would be allowed to mix with vapor within the column
and the recirculated vapor 22 from the condenser 14. In such
configuration, the condenser would have no condensing-side
headering, namely, the gas would simply be induced to flow into the
exchanger through condensation. As a further alternative, a portion
of the cooled air stream 10 could be directed to the shell of
column 18 and a portion piped directly to condenser 14. Such a
configuration would allow a modulation of the product oxygen and
nitrogen flows. However, this configuration would require
additional valves and control means. A further means would be to
employ a separate stream of air that could be further compressed.
The separate air stream would be split at either the warm end, if
further compressed, or if not so compressed, at the cold end. The
stream would then be totally condensed within the condenser 14. The
condensed air would then be introduced into an interstage location
of the main distillation column 18 through suitable piping. The
condensed air would not be collected in the sump of the column and
instead, liquefied air would be fed several stages up from the
bottom of the main distillation column 14.
[0036] A crude liquid oxygen stream 28 composed of the crude liquid
oxygen 26 is then preferably subcooled within a subcooling heat
exchanger 30 and depressurized within an expansion valve 32,
positioned between the main distillation column 18 and the
auxiliary column 34, to a pressure of preferably 15 to 25 psia. The
crude liquid oxygen stream 28 is then stripped within an auxiliary
distillation column 34 by contacting such stream with an ascending
stripping gas stream 52 within mass transfer contacting elements 36
that could be trays, structured packing, random packing or a
combination of such elements. The stripping produces an auxiliary
column overhead 38 that typically contains between 80 percent to 90
percent by volume of nitrogen and a residual oxygen-rich liquid 56
that typically contains 85 percent to 98 percent by volume of
oxygen.
[0037] The oxygen-rich liquid 40 can be collected within a
collection tray 42 or by similar means in which downcoming liquid
is collected and rising vapor is allowed to pass through and into
the mass transfer contacting elements 36. A stream of the
oxygen-rich liquid 44, composed of the oxygen-rich liquid 40, is
partially vaporized through indirect heat exchange with a
nitrogen-rich vapor stream 46 composed of the nitrogen-rich vapor
23 within a once-through heat exchanger 48. As a result of the
indirect heat exchange, a liquid nitrogen stream 50 is produced.
The stream of the oxygen-rich liquid 44 is partially vaporized to
produce the stripping gas 52 and a residual gas liquid stream 54
that collects in the bottom of the auxiliary distillation column 34
as a residual oxygen-rich liquid 56. It is to be noted that in
alternative embodiment, a two phase stream from the boiling side of
the once-through heat exchanger could be extracted from the
auxiliary distillation column 34 and directed to a separate phase
separator to produce the residual oxygen rich liquid stream 68 and
a vapor phase would then be piped back to the auxiliary
distillation column 34 to form the stripping gas 52. Although not
preferred, this would be equivalent to that shown in the
Figures.
[0038] As an alternative means to the once-through heat exchanger
48 and the collection tray 42, a thermo-siphon reboiler situated in
the base of the auxiliary column 34 could be employed to partially
vaporize the oxygen-rich liquid 40 falling from the mass-transfer
contacting elements 36 into the base of the auxiliary column 34.
The core of such reboiler would sit within the residual oxygen-rich
liquid and produce the stripping gas through vaporization of the
oxygen-rich liquid 40. This would not, however, be preferred given
that such a heat exchanger would not operate with the temperature
differences of a once-through heat exchanger; and consequently, the
incoming air would have to be compressed to a higher pressure.
[0039] The liquid nitrogen stream 50, in its entirely, or a part
thereof, as illustrated, can be introduced into the main
distillation column 18 as a reflux stream 58. The remainder, as a
further liquid nitrogen stream 60 (depressurized through valve 63)
and a waste stream 62, composed of the auxiliary column overhead
38, can be first introduced into subcooling heat exchanger 30 in
indirect heat exchange with the crude liquid oxygen stream 28 for
purposes of the subcooling thereof and then passed in indirect heat
exchange with the incoming compressed and purified air stream 10 to
help cool the same within the main heat exchanger 12.
Alternatively, stream 60 and stream 62 may be combined and warmed.
After having been warmed, the further liquid nitrogen stream 60
would be discharged from main heat exchanger 12 as a gaseous
nitrogen co-product ("N.sub.2") and the wastestream 62 could be
used in the regeneration of adsorbents employed in a
pre-purification unit used to pre-purify the incoming air and used
in connection with air separation plant 1. Furthermore, it is also
possible to recycle all or part of such stream by recompressing it
and combining it with the incoming air. It should be noted that
utilization of stream 60 is optional, the vaporization of liquid
nitrogen stream 60 serves to provide additional subcooling too
stream 28 and hence enables greater oxygen production.
[0040] It should be noted that liquid nitrogen reflux stream 60
could be extracted from an interstage location of column 20 prior
to passage through valve 63.
[0041] It is also to be noted that the subcooling heat exchanger 30
or 30' to be discussed could be integrated with the main heat
exchanger 12. It is equally possible that an embodiment of the
present invention could be constructed without the use of such
subcooling heat exchangers 30 or 30'. In any case, in the
illustrated embodiment, the further liquid nitrogen stream 60 can
be depressurized in an expansion valve 63 before passing through
subcooling heat exchanger 30 and main heat exchanger 12.
[0042] The nitrogen co-product is obtained by warming a nitrogen
product stream 64, composed of the nitrogen-rich vapor column
overhead 23, within the main heat exchanger 12, through indirect
heat exchange with the incoming compressed and purified air stream
10 to also help cool the same. As illustrated, nitrogen product
stream 64 and nitrogen-rich vapor stream 46 are produced by
dividing a nitrogen-rich vapor stream 66 removed from the top of
main distillation column 18 into the two foregoing streams.
However, the nitrogen product stream 64 and the nitrogen-rich vapor
stream 46 could be separately removed from the main distillation
column 18. It is to be noted that since all of the liquid nitrogen
stream 50 could be introduced into the main distillation column 18
as reflux, the nitrogen co-product could be formed solely from the
warmed nitrogen product stream 64.
[0043] The oxygen co-product is produced by removing a stream of
the residual oxygen-enriched liquid 68, composed of the residual
oxygen-rich liquid 56, and introducing such stream into a
separation vessel 70. A liquid phase stream 72, composed of the
liquid phase 74, is partially vaporized in the condenser 14 to
produce a two-phase stream 76 through indirect heat exchange with
the incoming compressed and purified air stream 10, thereby
partially condensing the compressed and purified air stream 10.
Liquid and vapor fractions of the two phase stream 76 are
disengaged within the separation vessel to form a vapor fraction
and a liquid phase 74 produced by the disengagement with the stream
and addition of the residual oxygen-rich liquid 68. A control valve
(not shown) may be employed to modulate the operating pressure of
vessel 70. The oxygen product stream is formed from a vapor phase
stream 80 composed of the vapor fraction 78. A drain stream 82 that
is passed through a valve 84 can optionally be collected as a
liquid product (or directed as a contaminant drain vaporizer).
Alternatively a portion of stream 68 may be extracted as liquid
product and sent to suitable storage.
[0044] It is to be noted that the stream of the residual
oxygen-rich liquid 68 can be pressurized as a result of the
auxiliary distillation column 34 being situated at a height above
the main distillation column 18. This results in a pressurized
oxygen product. A yet further alterative, is to pressurize the
stream of the residual oxygen-rich liquid by means of a mechanical
pump or the use of the pump in connection with the liquid head
produced by situating the auxiliary distillation column 34 above
the main distillation column 18.
[0045] As illustrated, condenser 14 acts as a thermo-siphon. As an
alternative means for forming the oxygen product, it is possible
that separation vessel 70 could be constructed to house the
condenser 14. In such case the two-phase stream would exit the
condenser into the interior of the separation vessel 70.
[0046] However, it is believed that the illustrated embodiment is
more cost effective to such an arrangement. Additionally, if more
than roughly 20 percent of the oxygen product were to be taken as a
liquid product by way of drain stream 82, then it would be possible
to reconfigure the condenser 14 as a once-through heat exchanger
with a potential further savings in power consumption. In such an
arrangement, the stream of the residual oxygen-rich liquid 68 would
be piped directly to the condenser 14. The partially vaporized
product would then be separated in a phase separation vessel and
there would not be a return stream, such as oxygen-enriched liquid
stream being introduced into the condenser 14.
[0047] With reference to FIG. 2, an alternative embodiment of the
present invention is illustrated as an air separation plant 2. It
is to be noted that where the elements illustrated in FIG. 2 would
have the same description as in the discussion above with respect
to FIG. 1, such elements shown in FIG. 2 will employ the same
reference numbers as those in FIG. 1; and for the sake of brevity,
will not be further discussed. Air separation plant 2 is
specifically designed to allow a greater fraction of the nitrogen
contained in air to be recovered as product. This is accomplished
through an increase in liquid nitrogen reflux to the main
distillation column 18.
[0048] In air separation plant 2, the compressed and purified air
stream 10 after having been cooled within main heat exchanger 12 is
introduced into main distillation column 20 and rectified to
produce a nitrogen-rich vapor column overhead 23 and a crude liquid
oxygen column bottoms 26'. A crude liquid oxygen stream 28' can
optionally be subcooled in a subcooling unit 30' and stripped
within auxiliary distillation column 34 after having been
depressurized within expansion valve 32. A first nitrogen-rich
vapor stream 46 is condensed in the once-through heat exchanger 48
to produce a first liquid nitrogen stream 50 and residual
oxygen-rich liquid 56 in the manner described with respect to air
separation plant 1. The resulting stream of the residual
oxygen-rich liquid 68 produced in auxiliary distillation column 34
is introduced into a thermo-siphon reboiler 86 after having been
depressurized in an expansion valve 90. The stream of the residual
oxygen-rich liquid 68 is partially vaporized through indirect heat
exchange with a gaseous stream 92 that can be composed of the
nitrogen-rich vapor column overhead 23 of main distillation column
18. Gaseous stream 92 passes into a core 94 located within a shell
96 of the thermo-siphon reboiler 86 to accomplish the heat
exchange. The gaseous stream 92 is condensed to produce a
condensate stream 98 that is introduced into the main distillation
column 18 as part of the reflux for such column. All of the liquid
nitrogen stream 50, produced in the once-through heat exchanger 48,
is introduced into the main distillation column 18 as column
reflux. This staged condensation of the nitrogen-rich vapor column
overhead will increase the liquid nitrogen reflux and therefore,
the increased ability of the air separation plant 2 to produce the
nitrogen product.
[0049] As can be appreciated, other means in place of thermo-siphon
reboiler 86 could be employed including a once-through heat
exchanger if the drain stream 106 were of sufficient flow.
Furthermore, although not illustrated, in place of the
nitrogen-rich vapor, a stream having a nitrogen concentration no
less than air could be introduced into the core 94 of the
thermo-siphon reboiler 86. For example, it is possible to compress
an air stream and liquefy such stream in the main heat exchanger
12. Thereafter, it could be introduced into an intermediate
location of the main distillation column 18. A gaseous stream
having about the same composition as such liquid stream could then
be passed into the core 94 of the thermo-siphon reboiler 86 and
condensed. The resulting condensate stream would be introduced into
a location of the main distillation column at which the composition
of the downcoming column liquid was the same or nearly the same as
such condensate or introduced into the auxiliary distillation
column 34 for stripping and production of the oxygen
co-product.
[0050] The partial vaporization of the stream of the residual
oxygen-rich liquid 68 creates vapor and liquid fractions that
collect in the shell as a vapor fraction 100 and a liquid phase
102. A vapor phase stream 104, composed of the vapor fraction 100,
is passed in indirect heat exchange with the incoming compressed
and purified air stream 10, within main heat exchanger 12, to warm
the vapor phase stream 100, to produce the oxygen product
("O.sub.2") and to help cool the compressed and purified air stream
10. A drain stream 106 composed of the liquid phase 102 can be
depressurized in a valve 108 and collected as a liquid oxygen
product. In such embodiment, optionally, the waste stream 62 can be
passed in indirect heat exchange with the crude liquid oxygen
stream 28' within the subcooling heat exchanger 30'. The waste
stream 62 is also, subsequently passed in indirect heat exchange
with the compressed and purified air stream 10, within main heat
exchanger 12, to warm the waste stream 62 and help cool the
compressed and purified air stream 10. The waste stream 62 could be
recycled back into the incoming air. This may be accomplished after
warming in exchanger 12 by direct mixing with feed air or after
further compression. As in air separation plant 1, the nitrogen
product stream 64, composed of the nitrogen-rich vapor column
overhead 23, is warmed within the main heat exchanger 12 through
indirect heat exchange with the incoming compressed and purified
air stream 10 to also help cool the same.
[0051] With reference to both FIG. 1 and FIG. 2, refrigeration is
imparted to either air separation plant 1 or air separation plant 2
by means of a liquid nitrogen stream 110 that is passed through a
flow control valve 112 and then introduced into a top inlet 114
located in the top of main distillation column 18. Such stream can
be obtained from an on-site storage tank and may be of high or low
pressure. The addition of such stream compensates for ambient heat
leak as well as warm end losses incurred with the operation of main
heat exchanger 12. It should be noted that liquid stream 110 could
be introduced directly into one of the vapor streams entering heat
exchangers 30' or 12. As could be appreciated, a portion of the
incoming air could be expanded prior to introduction into the main
distillation column 18. In this regard, a portion of the compressed
and purified air stream after partial cooling within main heat
exchanger 12 could be expanded and then mixed with waste gas stream
62. Another possibility would be to back-pressure the distillation
column system of the main distillation column 18 and the auxiliary
distillation column 34 and waste stream 62 could be partially
warmed within main heat exchanger 12, expanded in a turboexpander
and the further warmed within main heat exchanger 12. The use of a
turboexpander may afford the possibility of taking a liquid product
from the column system. A portion of the liquid nitrogen obtained
from condensers 48 or 96 may be sent to suitable storage. A yet
further alternative is that a liquid oxygen stream could be
introduced or combined with the stream of the residual oxygen-rich
liquid 68 and then introduced into the shell 96 of the
thermo-siphon reboiler 86.
[0052] With reference to FIG. 3, an air separation plant 1' is
illustrated that constitutes an alternative embodiment to air
separation plant 1 shown in FIG. 1. In air separation plant 1, an
auxiliary column 34' is shown that is provided with a stripping
section 36a that can be provided with mass transfer contacting
elements 36 as discussed above with respect to FIG. 1. The crude
liquid oxygen stream 28 is stripped within such stripping section
36a as described above and rather than directly producing an
auxiliary column overhead, produces a nitrogen and oxygen
containing vapor. Auxiliary column 34a is also provided with a
rectification section 36b situated above the stripping section 36a
to rectify such nitrogen and oxygen containing vapor and thereby
recover some of the oxygen that would otherwise have been vented
from the plant in the waste stream 62. This rectification section
is refluxed by liquid nitrogen stream 60' that is introduced into
the top of auxiliary column 34' after having been depressurized by
a valve 63'. As can be appreciated, the same modification could be
made to the plant shown in FIG. 2. In such case, the auxiliary
column would be modified in a like manner to that shown in FIG. 3.
In case of the FIG. 2 embodiment, part of the condensate stream 98
would be used to reflux the auxiliary column 34'.
[0053] As can be appreciated, all that is contemplated is
recovering some of the oxygen that would otherwise be lost in the
waste to increase oxygen recovery. However, a balance must be
struck between increased oxygen recovery and the fact that liquid
nitrogen is being taken from the liquid nitrogen that would
otherwise have been used in refluxing the main distillation column
and therefore, the production of nitrogen. Any greater
rectification would be accompanied by an increase in the flow of
liquid nitrogen stream 60' and therefore, a decrease in the
available product nitrogen 64 since the reflux rate of the main
column must be maintained in order to maintain product nitrogen
purity. A more concrete manner in stating such a limitation is that
the performance of the rectification section 36b, when taken in
connection with that of the stripping section 36a should be that
the auxiliary column overhead 38 within the auxiliary column 36
should contain no less than 5.0 percent oxygen on a volume basis.
In order to accomplish this, typically, a ratio of the flow rate of
the liquid nitrogen stream 60' to the total available nitrogen flow
from column 18 (the sum of streams 60 and 64) should be between 0.1
and 0.4 and a liquid to vapor ratio in the rectification section
36b of between 0.23 or less.
[0054] While the present invention has been described in reference
to a preferred embodiments, as will occur to those skilled in the
art, numerous changes, additions and omissions can be made without
departing from the spirit and scope of the present invention as set
forth in the appended claims.
* * * * *