U.S. patent application number 12/116672 was filed with the patent office on 2008-11-27 for method and system for treating an oxygen-rich liquid bath collected at the foot of a cryogenic distillation column.
This patent application is currently assigned to L'Air Liquide Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'Etude Et L'Exploita. Invention is credited to Jean-Yves LEHMAN, Bernard SAULNIER.
Application Number | 20080289361 12/116672 |
Document ID | / |
Family ID | 33042046 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080289361 |
Kind Code |
A1 |
LEHMAN; Jean-Yves ; et
al. |
November 27, 2008 |
Method and System for Treating an Oxygen-Rich Liquid Bath Collected
at the Foot of a Cryogenic Distillation Column
Abstract
Methods and apparatus for treating a liquid bath of oxygen. A
liquid bath of at least 70 mol % of oxygen is located at the base
of a cryogenic distillation column. The bath is continuously boiled
by an aluminum reboiler. A portion of the liquid bath is purged to
prevent a build up of inflammable impurities in the bath. A portion
of the purge is sent to a second reboiler, which is less
inflammable than the first reboiler. Oxygen boiled by the second
reboiler is sent back to the column, and a portion of the oxygen
rich liquid bath treated by the second reboiler is also purged. The
invention also relates to an apparatus of carrying out this
method.
Inventors: |
LEHMAN; Jean-Yves;
(Maisons-Alfort, FR) ; SAULNIER; Bernard; (La
Garenne Colombes, FR) |
Correspondence
Address: |
AIR LIQUIDE;Intellectual Property
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Assignee: |
L'Air Liquide Societe Anonyme A
Directoire Et Conseil De Surveillance Pour L'Etude Et
L'Exploita
Paris
FR
|
Family ID: |
33042046 |
Appl. No.: |
12/116672 |
Filed: |
May 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10552124 |
Oct 5, 2005 |
7380414 |
|
|
PCT/FR04/50132 |
Mar 29, 2004 |
|
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|
12116672 |
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Current U.S.
Class: |
62/643 ; 62/648;
62/652 |
Current CPC
Class: |
F25J 3/0486 20130101;
F25J 3/04939 20130101; F25J 2250/20 20130101; F25J 3/04412
20130101; F25J 2290/44 20130101; F25J 3/04418 20130101; F25J
2250/02 20130101; Y10S 62/903 20130101; F25J 3/04884 20130101 |
Class at
Publication: |
62/643 ; 62/648;
62/652 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2003 |
FR |
03/50097 |
Claims
1-10. (canceled)
11. A method which may be used for treating a liquid bath of
oxygen, said method comprising: a) providing a liquid bath in the
bottom of a cryogenic distillation column or a column element,
wherein: 1) said liquid bath comprises at least about 70 mol %
oxygen; and 2) said column or column element forms part of a system
of columns that are used for the separation of air; b) boiling
continuously a portion of said liquid bath with at least one first
reboiler, wherein said first reboiler is made of aluminum; c)
purging a portion of said liquid bath, wherein said purging
prevents an excessive build up of inflammable impurities in said
bath; d) sending said purged portion to at least one second
reboiler, wherein said second reboiler is less inflammable than
said first reboiler; e) sending oxygen boiled by said second
reboiler back to said column; and f) purging a portion of the
oxygen rich liquid bath treated by said second reboiler.
12. The method of claim 11, wherein said purged portion sent to
said second reboiler is at least about 0.5 mol % of the total air
stream feeding said system of distillation columns.
13-14. (canceled)
15. The method of claim 11, wherein said purged portion of said
oxygen rich liquid bath treated by said second reboiler is less
than about 1% of the total air stream feeding said system of
distillation columns.
16. The method of claim 15, wherein said purged portion of said
oxygen rich liquid bath treated by said second reboiler is less
than about 0.2% of said total air stream feeding said distillation
columns.
17. An apparatus which may be used for the cryogenic distillation
of air, said apparatus comprising: a) a cryogenic distillation
column or column element, wherein said column comprises a sump; b)
at least one first aluminum reboiler for treating an oxygen rich
liquid bath, wherein said first aluminum reboiler is located in
said sump; c) a first purge, wherein: 1) said first purge sends at
least a portion of said bath into at least one second reboiler; and
2) said second reboiler is less inflammable than said first
reboiler d) a conduit which sends oxygen vaporized by said second
reboiler back into said column; and e) a second purge, wherein said
second purge purges a portion of said bath in said second
reboiler.
18. The apparatus of claim 17, wherein said second reboiler is
located at the bottom of a heat exchanger outside of said
column.
19. The apparatus of claim 17, further comprising a partition
located in said column, wherein: a) said partition divides said
sump into a first compartment and a second compartment; b) said
first reboiler is located in said first compartment; c) said second
reboiler is located in said second compartment; and d) said
partition has a height such that said second compartment is fed
oxygen rich liquid by overflow from said first compartment.
20. (canceled)
21. The apparatus of claim 17, wherein: a) said column comprising
said sump is a low pressure column of a double column; b) said
double column comprises both said low pressure column and a medium
pressure column; c) said low pressure and said medium pressure
columns are thermally coupled to each other by said first reboiler;
d) said first reboiler is warmed by a nitrogen enriched gas from
said medium pressure column.
22. The apparatus of claim 21, wherein said second reboiler is
warmed by a nitrogen enriched gas from said medium pressure
column.
23. An apparatus which may be used for cryogenic distillation of
air, said apparatus comprising: a) a low pressure column, wherein
said low pressure column comprises a sump; b) at least one first
aluminum reboiler for treating an oxygen rich liquid bath, wherein
said first aluminum reboiler is located in said sump; c) a medium
pressure column, wherein said medium pressure column is thermally
coupled to said low pressure column by said first aluminum
reboiler; d) at least one partition located in said sump; e) at
least one second reboiler, wherein: 1) said second reboiler is
located in said sump; 2) said partition separates said first
reboiler and said second reboiler; and 3) said second reboiler is
less inflammable than said first reboiler f) a first purge, wherein
said first purge sends at least a portion of said oxygen rich
liquid bath to said second reboiler; g) a conduit which sends
oxygen vaporized by said second reboiler back into said low
pressure column; h) a second purge, wherein said second purge
purges at least a portion of said oxygen rich liquid bath in said
second reboiler; and i) a nitrogen conduit connecting said medium
pressure column with said first reboiler.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S.
application Ser. No. 10/552,124, filed on Oct. 5, 2008, which is a
.sctn. 371 of International PCT Application PCT/FR04/50132, filed
Mar. 29, 2007, which claimed priority to French Application
03/50097, filed on Apr. 10, 2003. Each of the aforementioned
related patent applications is herein incorporated by
reference.
BACKGROUND
[0002] The invention relates to the field of cryogenic air
separation and more particularly to cryogenic methods in which an
oxygen-rich liquid bath has to be boiled.
[0003] The cryogenic distillation of air is carried out in
distillation columns, and in the sump of some of these columns an
oxygen-rich liquid is collected, in particular in the low-pressure
column of a system of columns, such as a double air separation
column. This oxygen-rich liquid is continuously boiled so as to
provide reboil for the column, by means of a reboiler that is
installed in the sump and fed with a heat-transfer fluid, such as
the gaseous nitrogen collected at the top of the column.
[0004] This boiling of the oxygen progressively results in a
progressive increase in the concentration in the liquid bath
treated by the reboiler of impurities heavier than oxygen. These
compounds include light hydrocarbons, CO.sub.2 and nitrogen oxides.
This concentration is dangerous which, in certain zones of the
reboiler where the liquid oxygen is completely boiled off, a
deposit of hydrocarbons in the pure state may be produced on the
reboiler, resulting in combustion of said hydrocarbons. This
combustion may propagate to the aluminum which, for cost and energy
efficiency reasons, is generally the base material from which the
reboiler is made. Moreover, the build-up of inert compounds may
also be dangerous when these compounds solidify in a quantity such
that they block the channels of the reboiler. It is then necessary
to shut down the installation in order to restore it to correct
operation.
[0005] A partial solution to this problem could be to replace the
aluminum reboiler with a copper reboiler, which runs no risk of
catching fire in contact with hydrocarbons. However, this solution
would be expensive, in particular because the exchanger would have
to have substantially greater dimensions, for the same performance,
than an aluminum exchanger.
[0006] Another solution, conventionally adopted, consists in
purging a portion of the oxygen-rich liquid. Such a purge takes
place naturally if the installation is used to produce liquid
oxygen or gaseous oxygen at high pressure, by what is called the
"internal compression" method, or low-pressure gaseous oxygen.
However, if the oxygen is withdrawn from the column above the
reboiler (something which is the case in installations producing
krypton or xenon), or if the liquid oxygen withdrawn is only
partially vaporized and if its unvaporized portion is sent back
into the column, the problem rises in the same manner. Under these
conditions, it is necessary either to purge a large stream of
liquid oxygen, to send it through absorbers, in order to strip it
of its impurities, and to send it back into the reboiler, or to
withdraw only a small stream of liquid oxygen, but to discharge it
to the outside of the system without utilizing it. Since this
latter solution is costly in terms of wasted material and energy,
it is beneficial to minimize as far as possible the fraction of
liquid oxygen purged.
[0007] If the air treated by the cryogenic distillation
installation is very clean, the purge stream may be as low as 0.01%
of the total treated air stream. However, in common practice the
purge stream is from 0.1 to 0.2% of the total treated air stream.
The lower the purge stream, for the same initial air purity, the
higher the risk of a dangerous build-up of hydrocarbons and other
impurities in the oxygen-rich liquid. It is estimated in general
that, with a purge stream of 10% of the total treated air stream or
higher, there is no longer any danger in using an aluminum
reboiler.
[0008] One solution proposed by the document WO-A-99/39143 consists
in purging a fraction of oxygen-rich liquid that is sufficiently
large to ensure safe operation of the reboiler and in sending the
purged liquid into a second reboiler external to the installation,
in which high impurity contents of the concentrated liquid found
therein can be tolerated and in managing the corresponding risk.
This external reboiler may be periodically warmed to a relatively
high temperature so as to remove the impurities that are present
therein.
SUMMARY
[0009] The invention includes both methods and apparatus to achieve
the desired results, as described, but is not limited to the
various embodiments disclosed. The object of the invention is to
propose an alternative solution to that which has just been
described, in which any risk of explosion of any reboiler would be
eliminated and would be easier to manage, while still making it
possible to finally discharge out of the installation only a
minimal amount of treated air.
[0010] For this purpose, the subject of the invention is a method
of treating a liquid bath containing at least 70 mol % oxygen
collected in the bottom of a cryogenic distillation column or
column element forming part of a system of columns that is used for
the separation of air, in which said liquid bath is continuously
boiled by means of at least a first reboiler made of aluminum, a
portion of said oxygen-rich liquid bath is purged so as to prevent
an excessive build-up of inflammable impurities in said bath, said
purged portion is sent into at least a second reboiler, the oxygen
boiled by said second reboiler is sent back into said cryogenic
distillation column and a portion of the oxygen-rich liquid bath
treated by said second reboiler is purged, characterized in that
the second reboiler is, by its construction and/or its material,
less inflammable than the first reboiler.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a further understanding of the nature and objects for
the present invention, reference should be made to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like elements are given the same or analogous
reference numbers and wherein:
[0012] FIG. 1 illustrates a schematic representation, according to
one embodiment of the current invention, of a cryogenic air
distillation column;
[0013] FIG. 2a illustrates a schematic representation, according to
another embodiment of the current invention, of a cryogenic air
distillation column; and
[0014] FIG. 2b illustrates a sectional view of the embodiment shown
in FIG. 2a.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0015] The invention is a method of treating a liquid bath
containing at least 70 mol % oxygen collected in the bottom of a
cryogenic distillation column or column element forming part of a
system of columns that is used for the separation of air, in which
said liquid bath is continuously boiled by means of at least a
first reboiler made of aluminum, a portion of said oxygen-rich
liquid bath is purged so as to prevent an excessive build-up of
inflammable impurities in said bath, said purged portion is sent
into at least a second reboiler, the oxygen boiled by said second
reboiler is sent back into said cryogenic distillation column and a
portion of the oxygen-rich liquid bath treated by said second
reboiler is purged, characterized in that the second reboiler is,
by its construction and/or its material, less inflammable than the
first reboiler.
[0016] According to other optional aspects: [0017] said purged
portion sent into said second reboiler represents at least 0.5 mol
% of the total air stream feeding the system of distillation
columns; [0018] said purged portion sent into said second reboiler
represents at least 10 mol %, preferably at least 20 mol %, of the
total air stream feeding the system of distillation columns; [0019]
oxygen-rich liquid treated by said second reboiler is purged as a
stream equal to at most 1% of the total air stream feeding the
system of distillation columns; and [0020] oxygen-rich liquid
treated by said second reboiler is purged as a stream equal to at
most 0.2% of the total air stream feeding said distillation
column.
[0021] The subject of the invention is also a cryogenic
distillation column or column element, in the sump of which at
least a first aluminum reboiler for treating an oxygen-rich liquid
bath is placed, comprising purge means for taking a portion of said
bath into at least a second reboiler, means for sending the oxygen
vaporized by said second reboiler back into said column, and means
for purging a portion of said bath sent into said second reboiler,
characterized in that the second reboiler is by its construction
and/or its material less inflammable than the first reboiler.
[0022] According to other aspects of the invention: [0023] said at
least second reboiler is placed in the bottom of a heat exchanger
placed outside said column; [0024] the cryogenic distillation
column or column element includes a partition that divides its sump
into a first compartment and a second compartment, in that said at
least first reboiler is placed in the first compartment, in that
said at least second reboiler is placed in the second compartment
and in that said partition has a height such that it allows the
second compartment to be fed with oxygen-rich liquid coming from
the first compartment by overflow; and [0025] the cryogenic
distillation column or column element as claimed in claim 8,
characterized in that it includes means for measuring the level of
oxygen-enriched liquid present in the compartments defined by the
partition.
[0026] The subject of the invention is also an air distillation
unit comprising a cryogenic distillation column as claimed in claim
6, 7, 8 or 9, characterized in that the column, in the sump of
which the first reboiler is placed, is the low-pressure column of a
double column comprising a medium-pressure column and the
low-pressure column, these columns being thermally coupled to each
other by means of the first reboiler, and comprising means for
sending a nitrogen-enriched gas from the medium-pressure column to
the first reboiler and optionally to the second reboiler.
[0027] As will have been understood, the basic idea of the
invention consists in purging the aluminum reboiler(s)
conventionally used by sending the purged liquid into at least one
other reboiler made of a metal such as copper, which may be placed
either on the inside or on the outside of the column. The copper
reboiler can tolerate, without posing a hazard, high concentrations
of impurities in the oxygen-rich liquid that it treats, and it is
possible to purge only a minimal amount of liquid from this copper
reboiler. The boiled oxygen is sent back into the column and an
excellent material balance is obtained in the operation of
cryogenically separating the initial mixture (generally air), while
still maintaining a very satisfactory level of operating safety of
the installation. Of course, copper is only one example of metal
that can be used to form the other reboiler--any other metal
exhibiting comparable noninflammability and thermal conductivity
characteristics could be used.
[0028] FIG. 1 shows a portion of a cryogenic air distillation
installation 1 comprising, as is known, two columns, one on top of
the other. The lower part of this installation is made up of a
medium-pressure column 2 and the upper part of the installation 1
is made up of a low-pressure column 3. These two columns are
separated by a partition 4. A liquid bath 5 very rich in oxygen (at
least 70%, with contents of 95% or higher commonly obtained)
collects in the bottom of the low-pressure column 3. This bottom of
the low-pressure column 3 also contains an aluminum reboiler 6. Its
function is to ensure that the liquid oxygen contained in the
liquid 5 is boiled, so as to provide reboil for the low-pressure
column 3. Inside this reboiler, heat exchange is provided by means
of nitrogen taken off from the top of the medium-pressure column 2
via a line 7 that introduces the nitrogen in the gaseous state into
the reboiler 6. As is known, the heat exchange inside the reboiler
causes this gaseous nitrogen to condense, which returns in liquid
form to the low-pressure column 2 via a line 8. As is also known, a
portion of the oxygen-rich liquid 5 is purged out of the
low-pressure column 3, by means of a line 9, so as to limit the
concentration of impurities in the oxygen-rich bath 5.
[0029] According to the invention, this liquid oxygen purged via
the line 9 is introduced into a heat exchanger 10. In the
embodiment shown in FIG. 1, this exchanger 10 is located outside
the cryogenic separation installation. It is made up of a tank 11
in the bottom of which oxygen-rich liquid 12 is deposited. The
bottom of the tank 11 also contains a copper reboiler 13, the role
of which is to boil off the oxygen contained in the bath 12. This
copper reboiler 13 is, like the aluminum reboiler 6 of the
cryogenic separation installation 1, supplied with gaseous nitrogen
taken off from the medium-pressure column by means of a line 14.
This gaseous nitrogen condenses in the copper reboiler 13, and a
line 15 withdraws the nitrogen from the reboiler 13 and returns it
to the medium-pressure column 2. A line 16 tapped off the top of
the exchanger 10 returns the gaseous oxygen into the low-pressure
column 3, while a line 17 purges a fraction of the liquid 12, this
fraction therefore constituting the only amount of oxygen-rich
liquid discharged from the entire installation.
[0030] The copper reboiler 13 may be replaced with a reboiler made
of copper or made of another metal, such as aluminum, but which by
its construction is less inflammable than the reboiler 6, for
example the second reboiler may be a tubular reboiler.
[0031] The second reboiler is located inside the cold box that
serves to insulate the column system 1.
[0032] The stream of oxygen-rich liquid 5 sent via the line 9 into
the exchanger 10 is an operating parameter of the installation that
can be controlled at will by the user. If it is desired to ensure
that, whatever the initial cleanliness of the air treated by the
distillation installation 1, there is strictly no hazard in this
liquid 5 having an excessively high concentration of impurities,
the stream of liquid 5 sent into the line 9 must be greater than or
equal to 10% of the total quantity of air treated by the column 1.
Of course, if air having a relatively high initial purity is
treated, a substantially smaller purge stream in permissible. A
purge stream of oxygen-rich liquid 5 into the exchanger 10 of at
least 0.5% is accepted as being a good overall balance between
economic considerations (which recommend a small stream in order
not to have to have an excessively large exchanger 10) and safety
considerations (which recommend a high purge stream in order to
ensure that too high an impurity concentration in the oxygen-rich
liquid 5 of the low-pressure column 3 is not exceeded).
[0033] The other important parameter of the installation according
to the invention that has to be controlled is the purge stream of
oxygen-rich liquid 12 present inside the exchanger 10 and
discharged via the line 17. It is this purge stream that represents
the only part of the materials treated by the installation that
will be discharged and finally lost, if it does not undergo a
subsequent treatment. Of course, it is advantageous to limit this
purge stream to the lowest possible value, compatible with the safe
operating requirements of the installation, and in particular of
the exchanger 10. Since the reboiler 13 of this exchanger 10 is
made of copper, it is capable of tolerating very substantially
higher inflammable impurity concentrations than an aluminum
reboiler could. Under these conditions, a purge stream passing via
the line 17 of generally less than 1% of the total air stream
treated by the installation is imposed. An economic calculation
shows that above this 1% value, it often becomes less expensive in
terms of energy to carry out irreversible boiling of the
oxygen-rich liquid 5 purged outside the installation. This said,
even with air treated by the installation that is initially highly
laden with inflammable impurities, it is possible in complete
safety to purge quantity of oxygen-rich liquid via the line 17 of
the exchanger 10 of less than 0.2% of the total quantity of air
treated by the installation.
[0034] The size of the exchanger 10 and of the copper reboiler 13
that contains it depend tightly on the stream of oxygen-rich liquid
5 that they have to treat. The greater this stream, the larger the
exchanger 10 and the reboiler 13 have to be. If the space available
outside the column 1 is relatively limited, the exchanger 10 can
only be small in size--under these conditions, the installation
will be able to treat only a rather limited stream of oxygen-rich
liquid 5. This type of installation, as shown in FIG. 1, is
therefore to be recommended more for cases in which the air treated
by the cryogenic separation column 1 already has at the start a
relatively high purity. Otherwise, it may be recommended to use an
installation according to the invention as shown in FIG. 2.
[0035] In this example, the sump of the low-pressure column 3 is
divided into two compartments by a partition 18 of height H. In the
example shown, the partition 18 forms a corner, the first
compartment 19 representing about three-quarters of the bottom of
the low-pressure column 3 and the second compartment 20
representing the remaining quarter. At least one aluminum reboiler
20, 21 or 23 is installed in the first compartment 19 (or several
of them, as in the example shown), and at least one copper reboiler
24 is installed in the compartment 20. The height H of the
partition 18 is calculated in such a way that the oxygen-rich
liquid 5 present in the first compartment 19, when the low-pressure
column 3 is operating in the steady state, spills over the top of
the partition 18 so as to pass into the second compartment 20. This
stream of liquid 5 flowing out of the first compartment 19 into the
second compartment 20 therefore represents the purge stream of the
oxygen-rich liquid. On entering the second compartment 20, the
purged liquid forms a bath 5', which is treated by the copper
reboiler 24. This treatment enriches the bath 5' with impurities.
Since the reboiler 24 is made of copper, this impurity enrichment
can be tolerated without prejudicing the safety operating
conditions of the installation. A line 25 purges the liquid 5' rich
in oxygen and in impurities present in the second compartment 20,
in a manner similar to the line 17 of the first embodiment of the
invention, shown in FIG. 1.
[0036] The copper reboiler 24 may be as large as permitted by the
internal space in the low-pressure column 3, relative to the size
of the aluminum reboiler or reboilers 21, 22, 23 needed for
treating the oxygen-rich bath 5. The installation is preferably
provided with means for detecting the levels reached by the
oxygen-rich liquid 5, 5' in the compartments 19, 20 defined by the
partition 18. In this way, the operation of the installation can be
controlled, especially by regulating the purge stream flowing in
the line 25, in particular so as to prevent the return of liquid
oxygen 5' concentrated in impurities into the first compartment 19
from the second compartment 20.
[0037] A gaseous oxygen stream (not illustrated) is withdrawn from
the bottom of the low-pressure column 3 and warmed in the exchange
line of the unit in order to form a gaseous product. The unit may
also produce liquid products. However, it is not possible to use
this kind of unit to produce gaseous oxygen by boiling a
pressurized liquid stream.
[0038] In order for the installation to be operated properly, it is
advantageous to give the inside of the column 3 a configuration
such that the impurity-depleted liquid oxygen flowing down the
column 3 preferentially runs into the first compartment 19
containing the aluminum reboiler or reboilers 21, 22, 23. Likewise,
it is recommended to promote mixing of this impurity-depleted
liquid oxygen with the liquid oxygen 5 already present in the first
compartment 19. As a variant, for all the examples that have been
described, it is possible to operate the various reboilers not
using gaseous nitrogen withdrawn from the top of the
medium-pressure column 2, but with air or any other heat-transfer
fluid whose feed would be independent of the rest of the cryogenic
separation column 1.
[0039] Of course, the invention is applicable to any type of
cryogenic distillation column in the sump of which an oxygen-rich
liquid requiring to be purged collects, the double-column
installation described being only a preferred example.
[0040] It will be understood that many additional changes in the
details, materials, steps and arrangement of parts, which have been
herein described in order to explain the nature of the invention,
may be made by those skilled in the art within the principle and
scope of the invention as expressed in the appended claims. Thus,
the present invention is not intended to be limited to the specific
embodiments in the examples given above.
* * * * *