Title: Process and apparatus for the recovery of krypton and/or xenon
Abstract: Krypton and/or xenon is separated crudely from a mixture comprising oxygen and at least one rare gas selected from the group consisting of krypton and xenon in a process comprising feeding said mixture or a mixture derived therefrom to a rare gas recovery system and separating said mixture feed in said rare gas recovery system into rare gas-lean gaseous oxygen ("GOX") and rare gas-enriched product. The process is characterized in that at least about 50 mol % of said mixture is fed to the rare gas recovery system in the gaseous phase provided that, when said mixture feed is separated by selective adsorption, the concentration of xenon in the mixture feed is no greater than 50 times the concentration of xenon in air. One advantage of a preferred embodiment of the present invention is that it can easily be retrofitted to existing pumped LOX cycle ASUs.
Patent Number: 6,848,269 Issued on 02/01/2005 to Higginbotham, et al.
| Inventors:
|
Higginbotham; Paul (Guildford, GB);
Hayes; Kelvin Graham (East Horsley, GB);
O'Connor; Declan Patrick (Chessington, GB)
|
| Assignee:
|
Air Products and Chemicals, Inc. (Allentown, PA)
|
| Appl. No.:
|
723226 |
| Filed:
|
November 26, 2003 |
| Current U.S. Class: |
62/648; 62/925; 95/127 |
| Intern'l Class: |
F25J 003/00 |
| Field of Search: |
62/643,648,925
95/127
|
References Cited [Referenced By]
U.S. Patent Documents
| 2698523 | Jan., 1955 | Hnilicka | 62/648.
|
| 3191393 | Jun., 1965 | Dennis | 62/648.
|
| 3751934 | Aug., 1973 | Frischbier | 62/653.
|
| 3768270 | Oct., 1973 | Schuftan.
| |
| 3779028 | Dec., 1973 | Schuftan et al. | 62/648.
|
| 3971640 | Jul., 1976 | Golovko.
| |
| 4277363 | Jul., 1981 | Duhayon et al. | 588/1.
|
| 4568528 | Feb., 1986 | Cheung | 423/262.
|
| 4574006 | Mar., 1986 | Cheung | 62/648.
|
| 4805412 | Feb., 1989 | Colley et al. | 62/22.
|
| 5039500 | Aug., 1991 | Shino et al. | 423/262.
|
| 5122173 | Jun., 1992 | Agrawal et al.
| |
| 5833737 | Nov., 1998 | Tamura et al. | 95/98.
|
| 5913893 | Jun., 1999 | Gary et al. | 62/636.
|
| 6301929 | Oct., 2001 | Lochner | 62/643.
|
| 6378333 | Apr., 2002 | Dray | 62/648.
|
| 6658894 | Dec., 2003 | Golden et al. | 62/652.
|
| Foreign Patent Documents |
| 566151 | Dec., 1932 | DE.
| |
| 19855485 | Jun., 1999 | DE | .
|
| 1308680 | May., 2003 | EP.
| |
| 812397 | Apr., 1959 | GB.
| |
| 09002808 | Apr., 1997 | JP | .
|
Other References
Research Disclosure Sep. 1999, Publication No. 42517, p. 1158, "Crude
Krypton/Xenon Recovery from a Pumped LOX AUS Cycle", Disclosed
Anonymously.
Anonymous: "Crude Krypton/Xenon Recovery from a Pumped-LOX ASU Cycle" (Sep.
1999) Research Disclosure, Kenneth Mason Publications, Hampshire, GB, vol.
425, NR. 17 XP007124763 ISSN: 0374-4353 -whole document.
|
Primary Examiner: Doerrler; William C.
Attorney, Agent or Firm: Jones, II; Willard
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. Ser. No. 10/317,441 filed on 12
Dec. 2002 now U.S. Pat. No. 6,694,775.
Claims
What is claimed is:
1. A process for the recovery of at least one rare gas selected from the
group consisting of krypton and xenon from a mixture comprising oxygen and
at least one rare gas selected from the group consisting of krypton and
xenon, said process comprising:
separating feed air in a cryogenic air separation unit ("ASU") into
nitrogen-rich overhead vapor and liquid oxygen ("LOX");
pressurising at least a portion of said LOX to provide pressurized LOX;
at least partially vaporizing at least a portion of said pressurized LOX to
provide said mixture such that at least about 50 mol % of said mixture is
in the gaseous phase;
feeding said mixture or a mixture derived therefrom at a pressure greater
than the pressure of the part of the ASU producing said LOX to a rare gas
recovery system; and
separating said mixture feed in said rare gas recovery system into rare
gas-lean gaseous oxygen ("GOX") and rare gas-enriched product, provided
that, when said mixture feed is separated by selective adsorption, the
concentration of xenon in the mixture feed is no greater than 50 times the
concentration of xenon in air,
wherein the rare gas recovery system is an adsorber system, said process
comprising contacting said mixture feed with rare gas selective adsorbent
material in the adsorber system to effect the separation.
2. The process according to claim 1 wherein the process is selected from
the group consisting of a pressure swing adsorption ("PSA") process or a
temperature swing adsorption ("TSA") process.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of air separation and
has particular reference to the crude recovery of at least one rare gas
selected from the group consisting of krypton and xenon from an oxygen
product of an air separation.
Krypton and xenon are present in air at very low concentrations, typically
about 1.14 parts per million ("ppm") and about 0.087 ppm respectively.
They are both valuable gases and, thus, there is an economic incentive to
maximise their recovery in an air separation process.
In typical cryogenic air distillation processes, krypton and xenon
concentrate in the liquid oxygen ("LOX") product taken from the bottom of
the low pressure ("LP") distillation column as they are far less volatile
than oxygen. The smaller the LOX flow, therefore, the more concentrated
the krypton and xenon in this product.
In cryogenic air distillation processes in which most of the oxygen product
is removed from the LP column in the gas phase, it is possible to make
sure that very little krypton and xenon is lost in the gaseous oxygen
("GOX") by removing the GOX several distillation stages above the bottom
of the LP column. These bottom guard stages are used mainly to prevent
excessive loss of krypton which is substantially more volatile than xenon.
Almost all of the krypton and xenon entering the air separation plant can
then be recovered in the LOX product, which is a very small proportion of
the total oxygen flow. This LOX product can then be processed to produce a
purified rare gas product. In the event that it is primarily a xenon
product that is required, one could dispense with the bottom guard stages
and still recover much of the krypton and almost all of the xenon entering
the plant in the LOX product.
If the LOX flow from the distillation process is much greater, for example
when all the oxygen is withdrawn from the distillation column as LOX,
pumped to the required pressure and evaporated in the main heat exchanger,
the loss of krypton and xenon is much greater, even when the LOX is taken
several stages up the LP column, separately from the liquid stream in
which is concentrated the rare gas components. Essentially all of the
krypton and xenon entering the air separation plant flows down the LP
column to the sump of the LP column in the descending liquid, so any
liquid withdrawal will remove a portion of the krypton and xenon
proportional to the total liquid withdrawn as product. This will typically
lead to losses of about 30% of these valuable products.
It is desirable, therefore, to increase the recovery of krypton and xenon
from an air separation plant in which at least part of the oxygen product
is withdrawn as LOX.
When a plant withdraws the main oxygen product as a vapor from the LP
column, the krypton and xenon can be recovered by processing the LOX
product as described above. However, for existing pumped LOX cycle plants,
there is usually no small stream having concentrated rare gas components
as all oxygen products are generally withdrawn from the bottom of the LP
column. Therefore, as krypton and xenon are so valuable, it is also
desirable to be able to retrofit rare gas recovery systems to existing
plants.
In addition, it is desirable to provide a xenon and/or krypton recovery
plant, which can process rare gas-enriched feed streams from an external
source.
U.S. Pat. No. 4,805,412 (Colley; published on 21st Feb. 1989) discloses a
process and apparatus for the cryogenic distillation of air with reduced
loss of krypton and xenon. Oxygen is withdrawn from the LP column of the
distillation system and is fed to a primary krypton column for extraction
of its krypton and xenon content. The main feed to the primary krypton
column is a stream of LOX but a small stream of GOX is also taken from the
LP column and is fed without pressure adjustment to the krypton column.
The LP column and the primary krypton column operate at substantially the
same pressure. A portion of the krypton-lean overhead vapor is condensed
and fed to the primary krypton column as descending wash liquid.
U.S. Pat. No. 6,301,929 (Lochner, published on 16th Oct. 2001) discloses an
air separation process in which a rare gas-lean LOX stream and a rare
gas-enriched LOX stream are formed. The two liquid streams are pumped to a
rare gas separation column operating at GOX product pressure. The rare
gas-lean LOX stream is passed as reflux to the top of the column and the
rare gas-enriched stream is passed to a lower section of the column. Rare
gas-lean GOX product is withdrawn as overhead from the column and a
further rare gas-enriched bottoms liquid stream is withdrawn. The
reboiler/condenser in the sump of the column is sized to vaporize almost
all the oxygen feed streams. As the oxygen feed streams are liquid, the
reboiler/condenser must be large to vaporize all of the feed.
Research Disclosure No. 42517 (disclosed anonymously in September 1999)
discloses an air separation process in which the oxygen product is removed
from the column system as LOX. The LOX stream is pumped to the oxygen
product pressure and divided into two steams. The first stream is passed
as reflux to the top of a rare gas column and the second stream is passed
to a lower zone of the column. The relative proportions of the two streams
are determined such that the column can reject methane. Rare gas-lean GOX
product is withdrawn as overhead from the rare gas column and a rare
gas-enriched bottoms liquid stream is withdrawn. The reboiler/condenser in
the sump of the rare gas column must be sized to vaporize almost all the
oxygen feed streams. As the oxygen feed streams are liquid, the
reboiler/condenser must be large to vaporize all of the feed.
DE-A-19855485 (Lochner; published on 10th Jun. 1999) discloses an air
separation process in which rare gas-lean LOX and rare gas-enriched LOX
are formed in the LP column. The two liquid streams are pumped to a rare
gas column, the lean stream being passed as reflux to the top of the
column and the enriched stream being passed to a lower section of the
column. In addition, some gaseous nitrogen ("GAN") is added to the bottom
of the rare gas column to strip liquid descending the column. Rare
gas-lean GOX overhead is returned to the LP column and a further rare
gas-enriched LOX stream is withdrawn.
U.S. Pat. No. 6,378,333 (Dray; published on 30th Apr. 2002) discloses an
air separation process in which a first LOX stream having a xenon
component is passed from the LP column to the upper portion of a xenon
concentrator column as reflux. In the xenon concentrator column, the LOX
feed is separated into xenon-rich bottoms liquid and xenon-lean GOX
overhead. A second LOX stream having a xenon component is withdrawn from
LP column, pressurized and partially vaporized against a portion of feed
air. Typically, the liquid fraction from this partial vaporization will be
also passed as feed to the xenon concentrator column.
U.S. Pat. No. 5,913,893 (Gary et al, published on 22nd Jun. 1999) discloses
a method of purification of a cryogenic fluid, especially liquid helium,
by filtration and/or adsorption. The impurities are filtered/adsorbed from
the fluid but are not available as a valuable product.
U.S. Pat. No. 5,039,500 (Shino et al, published on 13th Aug. 1991)
discloses the gasification of a small LOX purge stream taken from an air
separation unit ("ASU") and passing the gaseous stream through an adsorber
which selectively adsorbs xenon. Xenon is recovered during the
regeneration phase of the adsorber. The xenon concentration in the purge
stream is about 31 ppm, i.e. about 360 times the concentration of xenon in
air.
JP-A-09002808 (Takano et al; published on 7th Jan. 1997) discloses the
gasification of a small LOX purge stream taken from an ASU and passing the
gaseous stream through a first adsorber (which selectively adsorbs xenon)
and then through a second adsorber (which selectively adsorbs krypton).
Xenon and krypton are recovered during the regeneration phase of the
adsorbers.
It is well known in the art that krypton and xenon will concentrate in
oxygen liquid because of the extremely low volatility of these gases.
Thus, it is a requirement in the prior art that a LOX stream be processed
in order to recovery krypton and xenon. Most of the prior art additionally
provides a small oxygen purge stream concentrated in krypton and xenon so
that the crude recovery system will be smaller. There is no disclosure in
the aforementioned prior art of the recovery of krypton and xenon from a
warm product gaseous oxygen stream.
In prior art processes, if krypton and xenon recovery is required, it is
generally necessary to design the LP column of an ASU so that a small rare
gas-rich LOX purge can be withdrawn. Such modifications add significantly
to the necessary capital investment and to the height to the LP column.
It is desirable to overcome disadvantages of (and thereby improve on) the
exemplified prior art and to provide an air separation process that is
able to produce a rare gas-enriched product (for further processing into
purified krypton and/or xenon products) and a pure LOX product without
involving the capital expense and running costs of a large
reboiler/condenser or additional equipment such as an argon stripping
column.
BRIEF SUMMARY OF THE INVENTION
The inventors have realized that crude xenon recovery can be achieved by
contacting xenon- (and usually krypton-) containing vapor feed with a
reflux liquid, even if the vapor feed has a low concentration of krypton
and xenon and if the vapor feed is at a high pressure. Krypton can also be
recovered. The recovered product may then be further processed to provide
at least one purified krypton and/or xenon product. The expression "low
concentration" used in the context of krypton and xenon in the vapor feed
is intended to mean that the krypton and xenon concentration in the vapor
feed is lower than in prior art purge streams but higher than in air.
According to a first aspect of the present invention, there is provided a
process for the recovery of at least one rare gas selected from the group
consisting of krypton and xenon from a mixture comprising oxygen and at
least one rare gas selected from the group consisting of krypton and
xenon. The process comprises feeding said mixture or a mixture derived
therefrom to a rare gas recovery system and separating said mixture feed
in said rare gas recovery system into rare gas-lean GOX and rare
gas-enriched product The process is characterised in that at least about
50 mol % of said mixture is fed to the rare gas-recovery system in the
gaseous phase. When the mixture feed is separated by selective adsorption,
the concentration of xenon in the mixture feed is no greater than 50 times
the concentration of xenon in air.
The invention involves passing feed mixture, the bulk of which is oxygen
and at least about half of which is gaseous, to a crude rare gas recovery
system. The crude rare gas recovery system could be a column, a column
system, a heat exchanger or an adsorber but, whatever the nature of the
recovery system, krypton and/or xenon components are concentrated and the
concentrated product (and purified oxygen product) recovered. Rather than
processing a small purge stream rich in krypton and xenon, preferred
embodiments of the invention are intended to process a larger GOX stream
having a lower krypton and xenon concentration.
The feed is preferably at a pressure higher than that of the source from
which it is taken, e.g. the cryogenic air distillation column from which
it was originally withdrawn. The feed may be vaporized oxygen resulting
from a pumped LOX cycle in an ASU, GOX from the warm-end of the main
exchanger (possibly following compression) or may be from an oxygen
pipeline. LOX may also be fed to the crude recovery system to provide
refrigeration and/or reflux.
One advantage of the present invention is that it can be applied to
existing plants producing oxygen. Such plants may be easily retrofitted
with a crude recovery system according to the present invention such that
the krypton and xenon in the existing oxygen product streams may be
recovered without additional processing.
Preferably, at least 90 mol % of the mixture feed is gaseous. More
preferably, all of the mixture feed is gaseous.
The process is applicable to the production of xenon-enriched product,
krypton-enriched product and xenon- and krypton-enriched product.
The process may further comprise feeding a xenon-enriched stream to the
rare gas-recovery system. The enriched stream may be at least partially
gaseous or may be liquid and may be taken from a cryogenic air
distillation system, which would usually be a different system to that
generating the mixture.
The mixture may be taken from a GOX pipeline in which case it would usually
be under pressure and may not further require pressurization.
Alternatively, the mixture may be pressurized before being fed to the rare
gas recovery system, for example, if it is removed from a LP column of an
ASU.
The process may further comprise separating feed air in a cryogenic air
separation unit ("ASU") into nitrogen-rich overhead vapor and liquid
oxygen ("LOX"), pressurising at least a portion of said LOX to provide
pressurized LOX and at least partially vaporizing at least a portion of
said pressurized LOX to provide said mixture feed. In such processes, all
of said at least a portion of said pressurized LOX is preferably vaporized
to produce said mixture. The pressure of the mixture feed is preferably
greater than the operating pressure of the part of the ASU producing said
LOX. The LOX stream may be divided into at least two portions, each
portion being vaporized at different pressures before being fed to the
rare gas recovery system.
In one embodiment of the invention, the rare gas recovery system is a
gas-liquid contact separation system and the process comprises contacting
said mixture feed with LOX in the separation system to effect the
separation.
In one arrangement of this embodiment, the gas-liquid contact separation
system is a gas-liquid contact column with no distillation stages and the
process comprises passing (e.g. bubbling) the mixture feed through LOX in
said column to effect the separation.
In another arrangement, the gas-liquid contact separation system is a
distillation system and the process comprises feeding said mixture to the
distillation system for separation into said rare gas-lean GOX as overhead
vapor and said rare gas-enriched product and feeding LOX to said
distillation system as reflux. Preferably, the mixture is superheated
prior to being fed to the distillation system.
Whether at least the majority of the mixture is fed to the distillation
system in either gaseous or liquid state, the rare gas-lean overhead may
be condensed, pressurised to a high pressure (e.g. using a pump) and then
revaporised.
Where the gas-liquid contact separation system is a distillation system,
the process may further comprise separating feed air in a cryogenic ASU
into nitrogen-rich overhead vapor and LOX, removing a stream of LOX from
the ASU, pressurizing at least a portion of said LOX stream to produce a
stream of pressurized LOX, dividing said pressurized LOX steam into a
major portion and a minor portion, at least partially vaporising said
major portion to provide said mixture and feeding said minor LOX portion
to the distillation system as reflux. All of said major portion is
preferably vaporized to produce said mixture. Where the distillation
system comprises a single distillation column, the process comprises
feeding said mixture to the column for separation into said rare
gas-enriched product and said rare gas-lean GOX and feeding said minor LOX
portion to said column as reflux.
Where the gas-liquid contact separation system is a distillation system,
the process may comprise separating feed air in a cryogenic ASU into
nitrogen-rich overhead vapor and LOX, removing a stream of LOX from the
ASU, pressurizing at least a portion of said LOX stream to produce a
stream of pressurized LOX, at least partially vaporising at least a
portion of said pressurized LOX steam to provide said mixture, condensing
at least a portion of said rare gas-lean GOX overhead vapor by indirect
heat exchange against a refrigerant to produce condensed overhead and
feeding at least a portion of the condensed overhead to the distillation
system as reflux. Preferably, all of said at least a portion of said
pressurized LOX stream is vaporized to produce said mixture. If the
distillation system comprises a single distillation column, then the
process may comprise feeding said mixture to the column for separation
into said rare gas-enriched product and said rare gas-lean GOX and feeding
at least a portion of said condensed overhead to said column as reflux.
The distillation system may comprise one or more distillation columns.
Usually, the system has only one column as this reduces the initial
capital investment but systems having either two or three columns may be
preferred in certain circumstances.
In one arrangement of this embodiment, the distillation system comprises at
least a higher pressure ("HP") distillation column and a lower pressure
("LP") distillation column. The HP and LP columns are thermally integrated
via a reboiler/condenser. The process comprises feeding said mixture to
said HP column where it is separated into rare gas-depleted overhead vapor
and rare gas-enriched bottoms liquid. Rare gas-enriched bottoms liquid is
fed to said LP column after pressure adjustment for separation into said
rare gas-lean GOX and said rare gas-enriched product. Rare gas-depleted
overhead vapor is at least partially condensed by indirect heat exchange
against rare gas-enriched product to produce at least partially condensed
rare gas-depleted overhead, at least a portion of which is fed to the HP
column as reflux. Liquid from or derived from the HP column is fed to the
LP column as reflux. LOX may be fed to the HP column as reflux. LOX for
reflux is typically produced by the separation of feed air in a cryogenic
ASU. The HP column may be reboiled by at least partially vaporizing rare
gas-enriched bottoms liquid by indirect heat exchange against a heating
fluid.
In another arrangement of this embodiment, the distillation system
comprises at least a higher pressure ("HP") distillation column, a medium
pressure ("MP") distillation column and a lower pressure ("LP")
distillation column. The HP and MP columns are thermally integrated via a
first reboiler/condenser and the MP and LP columns are thermally
integrated via a second reboiler/condenser. The process comprises feeding
said mixture to said HP column where it is separated into first rare
gas-depleted overhead vapor and first rare gas-enriched bottoms liquid.
First rare gas-enriched bottoms liquid is fed to said MP column after
pressure adjustment for separation into second rare gas-depleted overhead
vapor and second rare gas-enriched bottoms liquid. First rare gas-depleted
overhead vapor is at least partially condensed by indirect heat exchange
against second rare gas-enriched bottoms liquid to produce at least
partially condensed first rare gas-depleted overhead, at least a portion
of which is fed to the HP column as reflux. Liquid from or derived from
the HP column is fed to the MP column, the LP column or both as reflux.
Second rare gas-enriched bottoms liquid is fed to said LP column for
separation into said rare gas-lean GOX and said rare gas-enriched product
Second rare gas-depleted overhead vapor is at least partially condensed by
indirect heat exchange against said rare gas-enriched product to produce
at least partially condensed second rare gas-depleted overhead, at least a
portion of which is fed to the MP column as reflux. Liquid from or derived
from the MP column is fed to the LP column as reflux. The process may
further comprise feeding LOX to the HP column as reflux. The LOX for this
reflux is preferably produced by cryogenic separation of feed air.
In a further arrangement of this embodiment, the distillation system
comprises at least a first distillation column and a second distillation
column with first and second columns operating at the same pressure. The
process comprises feeding said mixture to said first column for separation
into rare gas-lean GOX and rare gas-enriched product, feeding said mixture
to said second column for separation into rare gas-lean GOX and rare
gas-enriched product, feeding LOX to said first column as reflux and
feeding at least one liquid selected from the group consisting of rare
gas-enriched product from the first column and LOX to said second column
as reflux. The process may further comprise dividing a stream of said
pressurized mixture into two equal portions and feeding one portion to
each column.
In another embodiment, the gas-liquid contact separation system is at least
one heat exchanger. In such an embodiment, the process comprises feeding
said mixture to the bottom of the or each heat exchanger, condensing a
portion of said mixture ascending through the passages of the or each heat
exchanger by indirect heat exchange against refrigerant to produce
condensed mixture and contacting ascending mixture with descending
condensed mixture in the passages to effect the separation by
dephlegmation. Preferably, the indirect heat exchange takes place in the
upper portion of the or each heat exchanger. The or each heat exchanger
may be reboiled by at least partially vaporizing rare gas-enriched product
by indirect heat exchange against a first heating fluid. The process may
further comprise warming the rare gas-lean GOX to ambient temperature by
indirect heat exchange within the or each heat exchanger against a second
heating fluid, said heat exchange taking place above the heat exchange to
produce the condensed mixture.
In a third embodiment, the rare gas recovery system is an adsorber system
and the process comprises contacting said mixture feed with rare gas
selective adsorbent material in the adsorber system to effect the
separation. The process may be either a pressure swing adsorption ("PSA")
process or a temperature swing adsorption ("TSA") process, both of which
are well-known in the art.
The concentration of xenon in the mixture feed is no more than 50 times,
preferably no more than 20 times and most preferably about 5 times, the
concentration of xenon in air.
The separation is usually a crude separation and the rare gas-enriched
product may be further processed to produce at least one product selected
from the group consisting of a purified xenon product, a purified krypton
product and a purified krypton and xenon product. Such further processing
steps are well known in the art and include combusting the rare
gas-enriched product to remove hydrocarbon compounds followed by further
purification of the resultant product by distillation.
At least one further adsorber may be used to remove hydrocarbon compounds,
carbon dioxide and/or nitrous oxide from LOX or GOX feed streams to the
rare gas recovery system or from intermediate or final LOX or GOX streams.
According to a second aspect of the present invention, there is provided
apparatus for the recovery of at least one rare gas selected from the
group consisting of krypton and xenon from a mixture comprising oxygen and
at least one rare gas selected from the group consisting of krypton and
xenon according to the first aspect, said apparatus comprising a cryogenic
ASU for separating feed air into nitrogen-rich overhead vapor and LOX,
pressurizing means for pressurizing at least a portion of said LOX to
provide pressurized LOX, vaporizing means for vaporizing at least about 50
mol % of said pressurized LOX to provide said mixture, and a rare gas
recovery system for separating said mixture into rare gas-lean GOX and
rare gas-enriched product.
The rare gas recovery system may be a gas-liquid contact column with no
distillation stages. The mixture is passed (e.g. bubbled) through LOX in
such a column.
The rare gas recovery system may also be a distillation system. Such
apparatus may further comprise means for superheating said mixture prior
to it being fed to the distillation system. The apparatus may further
comprise conduit means for feeding a portion of said pressurized LOX from
the pressurizing means (e.g. a pump) to the vaporizing means and further
conduit means for feeding the remaining portion of said pressurized LOX
from the pump to the distillation system as reflux. The apparatus may
further comprise heat exchange means for at least partially condensing a
portion of said rare gas-lean GOX overhead against a refrigerant to
provide at least partially condensed rare gas-lean GOX overhead and
conduit means for feeding at least a portion of said at least partially
condensed overhead to the distillation system as reflux.
In a preferred embodiment where the rare gas recovery system is a
distillation system, the apparatus may further comprise a first
distillation column for separating mixture into rare gas-lean GOX and rare
gas-enriched product, a second distillation column for separating mixture
into rare gas-lean GOX and rare gas-enriched product at the same pressure
as said first distillation column, conduit means for feeding LOX to the
first distillation column as reflux and conduit means for feeding at least
one liquid selected from the group consisting of rare gas-enriched product
from the first distillation column and LOX to the second distillation
column as reflux.
The rare gas recovery system may be at least one heat exchanger for
separating the mixture by dephlegmation. The apparatus may further
comprise first heat exchange means provided in the upper portion of the or
each heat exchanger for condensing ascending mixture by indirect heat
exchange against a refrigerant. The apparatus may further comprise second
heat exchange means, e.g. provided in the lower portion of the or each
heat exchanger, for vaporizing rare gas-enriched product by indirect heat
exchange against a first heating fluid. The apparatus may further comprise
third heat exchange means provided above the first heat exchange means in
the or each heat exchanger for warming rare gas-lean GOX to ambient
temperature by indirect heat exchange against a second heating fluid.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1A is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system is a single distillation
column;
FIG. 1B is a schematic representation of a different arrangement of the
embodiment of the present invention depicted in FIG. 1A;
FIG. 2A is a schematic representation of another arrangement of the
embodiment of the present invention depicted in FIG. 1;
FIG. 2B is a schematic representation of a different arrangement of the
embodiment of the present invention depicted in FIG. 2A;
FIG. 3 is a schematic representation of the distillation column depicted in
FIGS. 1A, 1B, 2A and 2B without the optional bottom section of
distillation stages;
FIG. 4 is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system has two distillation
columns operating at different pressures;
FIG. 5 is a schematic representation of a different arrangement of the
embodiment of the present invention depicted in FIG. 4;
FIG. 6 is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system has three distillation
columns, each column operating at a different pressure;
FIG. 7 is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system has two distillation
columns operating at the same pressure;
FIG. 8 is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system is a heat exchanger;
FIG. 9 is a schematic representation of a different arrangement of the
embodiment of the present invention depicted in FIG. 8; and
FIG. 10 is a schematic representation of an embodiment of the present
invention in which the rare gas recovery system comprises an absorber.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1A, LOX is withdrawn as stream 100 from the LP column 10
of a double column ASU. The LOX stream 100 is pumped by LOX pump 12 to
provide a pumped LOX stream 102 which contains essentially all the krypton
and xenon which has entered the double column ASU. The krypton and xenon
concentration in the LOX is about 5 times greater than that in atmospheric
air as the flow of stream 100 is typically 20% of the ASU feed air flow.
The pumped LOX stream 102 is divided into two portions. The major portion
106 is vaporized to at least a quality of 0.9 (i.e. 90% of the stream is
gaseous) in the heat exchanger 14 against a condensing compressed air
stream 130. Ideally, the major portion 106 is completely vaporized and
slightly superheated. The resultant vaporized stream 108 is passed to a
lower zone of a single crude rare gas recovery column 16. The minor
portion 104 of the pumped LOX is passed as reflux to the top of the crude
column 16. The flow of reflux stream 104 is determined by the desired
recovery of krypton and xenon.
In the crude distillation column 16, separation is effected into overhead
rare gas-lean GOX, which is withdrawn as stream 110 and warmed in the heat
exchanger 14 to provide GOX stream 112, and rare gas-enriched bottoms
liquid, which is withdrawn as product stream 120 and which can be further
purified by known methods.
In heat exchanger 14, the LOX stream 106 is warmed and vaporized against a
compressed feed air stream 130 (although other pressurized streams such as
high pressure nitrogen could be used instead). Most of the compressed feed
air stream 130 is withdrawn from the heat exchanger 14 as a stream 136 of
liquefied air but a portion 132 is withdrawn from the heat exchanger 14
and liquefied in crude column reboiler 18 to provide a further stream 134
of liquified air. The two liquefied air streams are combined and the
combined stream 138 is fed to the ASU double column system.
It should be noted that it is not necessary to the invention that the
heating fluid for reboiler 18 should have the same composition as the
fluid condensed by the boiling LOX in heat exchanger 14. In addition, the
reboiler 18 may be provided outside the column 16 rather than in the sump
of the column as shown. Further, if reboiler 18 is provided outside column
16, then it may be fed directly with liquid from column 16 or it may be
located inside a pot fed by liquid from column 16. It should also be noted
that, provided oxygen feed stream 108 is sufficiently superheated, then
column 16 may not require a reboiler. The descending reflux liquid would
then be largely vaporized by desuperheating of the feed stream 108.
Higher reflux flows in column 16 more easily wash the krypton and xenon
components from the rising oxygen vapor. As stated, it is preferred that
stream 108 should be completely gaseous. One reason for this is that any
liquid in stream 108 will have to be vaporized in the reboiler 18 which
necessitates increasing the size of the reboiler.
Crude column 16 is shown having two distillation sections. In the upper
section, krypton and xenon are washed from the rising gas by the reflux
liquid. The lower section is optional and serves to increase the
concentration of krypton and xenon in the bottoms liquid. Although reflux
for the crude column is shown as provided by LOX feed stream 104, this
source of reflux could be supplemented by or replaced by use of a column
condenser, driven by an appropriate refrigerant.
In principle, column 16 does not need any distillation stages at all.
Oxygen feed stream 108 may be bubbled through liquid in the sump of column
16. Much of the xenon in the oxygen feed would transfer to the sump
liquid. If the oxygen feed 108 is two-phase, then most of the xenon in the
feed would be in the liquid phase. However, having at least one
distillation stage, in addition to the reboiler, is highly preferred.
An optional rare gas-enriched stream 140, which may originate from a
distillation column different from that from which the oxygen feed stream
108 originates, may be fed to the column 16. Typically, if present, stream
140 may be a rare gas-rich purge stream from a different air separation
plant and it may be liquid or at least partially gaseous. This optional
rare gas-enriched feed stream is applicable to any embodiment of the
invention.
The process depicted in FIG. 1B is similar to that depicted in FIG. 1A and
the corresponding features have been given the same numerical legends.
In the crude distillation column 16, separation is effected into overhead
rare gas-lean GOX, and rare gas-enriched bottoms liquid, which is
withdrawn as product stream 120 and which can be further purified by known
methods. The overhead rare gas-lean GOX is condensed in overhead condenser
20. A portion 109 of the resulting liquid is returned to the distillation
column 16 as reflux and the remaining portion 111 is pumped to a higher
pressure (preferably supercritical) in pump 13, vaporised (if
sub-critical) and warmed in the heat exchanger 14 to provide GOX stream
112. The warming (and evaporation if present) of this GOX stream is
primarily against compressed air stream 160, which is cooled (and, if
sub-critical condensed) to form stream 162 which is fed to the ASU double
column system.
In heat exchanger 14, the LOX stream 106 is warmed and vaporized against a
compressed feed or recycle air stream 130 (although other pressurized
streams such as high pressure nitrogen could be used instead). Most of the
compressed air stream 130 is withdrawn from the heat exchanger 14 as a
stream 136 of liquefied air but a portion 132 is withdrawn from the heat
exchanger 14 and liquefied in crude column reboiler 18 to provide a
further stream 134 of liquified air. The two liquefied air streams are
combined and the combined stream 137 is reduced in pressure and fed to the
overhead condenser 20 of the distillation column 16. Here it is partially
evaporated to produce a vapour stream 150 which is warmed in heat
exchanger 14 to form stream 152. The remaining liquid 138 is fed to the
ASU double column system. The vapour stream 152 may be recompressed to
form all or part of the compressed air feed stream 130
It should be noted that it is not necessary to the invention that the
heating fluid for reboiler 18 should have the same composition as the
fluid condensed by the boiling LOX in heat exchanger 14 or the cooling
fluid for condenser 20. It should also be noted that condenser 20 need not
be located on the column, and its duty could be performed in the main heat
exchanger 14.
Referring now to FIG. 2A, a stream 200 of LOX is withdrawn from a pumped
LOX ASU coldbox 20 and pumped by LOX pump 22 to provide a pumped LOX
stream which is divided into two portions. The major portion is vaporized
within the coldbox 20 to provide a stream 204 of warm pressurized GOX.
This stream contains most of the krypton and xenon which entered the
pumped LOX ASU. The krypton and xenon concentration in the GOX is about 5
times higher than in atmospheric air as the flow of stream 200 is
typically 20% of the ASU feed air flow. GOX stream 204 is cooled in heat
exchanger 24 to provide stream 208 having a quality of at least 0.9.
Ideally, stream 208 should be superheated. Stream 208 is fed to a lower
section of a single crude rare gas recovery column 26. The remaining
portion 202 of the pumped LOX stream from pump 22 is optionally warmed in
heat exchanger 29 and fed as stream 203 to the top of column 26 to provide
reflux and refrigeration.
In the crude distillation column 26, separation is effected into overhead
rare gas-lean GOX, which is removed as stream 210 and warmed in heat
exchanger 24 to provide GOX product stream 212, and rare gas-enriched
bottoms liquid, which is withdrawn as a product stream 220 and which can
be further purified by known methods.
A minor portion 230 of the pumped LOX ASU compressed feed air is cooled in
heat exchanger 24 and the cooled stream 232 is fed to reboiler 28 where it
is condensed. A stream 234 of condensed air is returned to the pumped LOX
ASU, optionally after exchanging heat with LOX stream.202 in subcooler 29.
Reboiler 28 may be located outside column 26, possibly inside its own pot
if desired. Stream 230 does not have to be a boosted air stream. Any
suitable pressurised stream (e.g. boosted nitrogen) could be used to
provide the heating duty for reboiler 28. Further, the boosted stream
providing the reboiler heating duty could be supplied directly to the
reboiler as a cold stream from the main ASU 30, rather than a warm stream
which is cooled in exchanger 24. Exchanger 24 might have other streams
associated with it, e.g. stream 220 might be vaporised and warmed in said
exchanger prior to its further purification.
The flow of reflux stream 203 is determined by the desired recovery of
krypton and xenon. As in FIG. 1, it is preferred that stream 208 be
completely gaseous as any liquid in stream 208 would have to be vaporized
by the reboiler 28 which would necessitate increasing the size of the
reboiler.
Column 26 is depicted as having two distillation sections. In the upper
section, krypton and xenon are washed from the rising gas by the reflux
liquid. The lower section is optional and serves to increase the
concentration of krypton and xenon in the bottoms liquid. Although reflux
for the crude column is shown as LOX feed stream 203, this source of
reflux could be supplemented by or replaced by the use of a column
condenser, driven by an appropriate refrigerant.
It can be readily appreciated from this figure that the invention is suited
to the simple retrofit of a krypton and xenon recovery system to an
existing pumped LOX ASU. GOX stream 204 and the compressed feed air stream
230 are both warm streams and, thus, are readily accessible for such a
retrofit. The LOX stream 202 for reflux and refrigeration is readily
supplied by connection outside the coldbox to the discharge line of the
LOX pump 22. The compressed air, liquefied in the crude recovery
equipment, is sent as line 234 to the ASU coldbox. It is simple to design
the coldbox to easily route this stream to join the main ASU liquefied
compressed feed air stream. Thus, retrofit of a rare gas recovery system
is readily available for minimal initial capital investment.
The process depicted in FIG. 2B is similar to that depicted in FIG. 2A and
corresponding features have been given the same numerical legends. A
stream 200 of LOX is withdrawn from a pumped LOX ASU coldbox 20 and pumped
by LOX pump 22 to provide a pumped LOX stream which is divided into two
portions. The major portion is vaporized within the coldbox 20 to provide
a stream 204 of warm pressurized GOX, which may be at a supercritical
pressure. This stream contains most of the krypton and xenon which entered
the pumped LOX ASU. The krypton and xenon concentration in the GOX is
about 5 times higher than in atmospheric air as the flow of stream 200 is
typically 20% of the ASU feed air flow. GOX stream 204 is cooled (and
condensed if subcritical) in heat exchanger 24 to provide stream 205 which
is a liquid or supercritical dense-phase stream. A portion 250 of stream
205 is mixed with LOX stream 202 and the remaining portion 207 is reduced
in pressure and evaporated in the heat exchanger 24 primarily by
condensing compressed air stream 230 to form stream 208 having a quality
of at least 0.9. Ideally, stream 208 should be superheated. Stream 208 is
fed to a lower section of a single crude rare gas recovery column 26. The
remaining portion 202 of the pumped LOX stream from pump 22 is mixed with
LOX stream 250, optionally heated in heat exchanger 24, reduced in
pressure and fed as stream 203 to the top of column 26 to provide reflux
and refrigeration.
In the crude distillation column 26, separation is effected into overhead
rare gas-lean GOX, which is removed as stream 210 and rare gas-enriched
bottoms liquid, which is withdrawn as a product stream 220 and which can
be further purified by known methods. GOX stream 210 is condensed in heat
exchanger 24, pumped to pressure in pump 23 and the pumped stream 211
re-warmed (and evaporated if sub-critical) in heat exchanger 24 primarily
against the incoming GOX stream 204 to provide GOX product stream 212. The
pressure of stream 212 is similar to stream 204, and may, if the streams
are supercritical, be higher.
Stream 230 of the pumped LOX ASU compressed feed air is split into two
portions, a minor portion 231 and a major portion 260. Stream 231 is
cooled in heat exchanger 24 and the cooled stream 232 is fed to reboiler
28 where it is condensed to form stream 234. Stream 260 is cooled and
condensed in heat exchanger 24, primarily by evaporating oxygen stream
207, to form stream 262, which is mixed with stream 234 and reduced in
pressure to form stream 264. This stream is evaporated and warmed in heat
exchanger 24 to form stream 266, primarily by condensing GOX stream 210
and cooling air streams 260 and 261. Reboiler 28 may be located outside
column 26, possibly inside its own pot if desired. The return air stream
266 is at a lower pressure than feed stream 230 and may be recompressed in
a separate compressor (not shown) to form all or part of stream 230, or,
as shown, may be returned to the suction of the compression stage 270
following the stage 268 from which stream 230 is withdrawn. Stream 230
will be a major portion of the flow through the compressor 268, so that
little inefficiency is introduced by reducing the pressure of the
remaining portion which goes directly to the following compression stage
270.
It can also be readily appreciated from this figure that the invention is
suited to the simple retrofit of a krypton and xenon recovery system to an
existing pumped LOX ASU. GOX stream 204 and the compressed feed air stream
230 and return air stream 266 are all warm streams and, thus, are readily
accessible for such a retrofit. The LOX stream 202 for reflux and
refrigeration is readily supplied by connection outside the coldbox to the
discharge line of the LOX pump 22. Thus, retrofit of a rare gas recovery
system is readily available for minimal initial capital investment.
FIG. 3 depicts a crude krypton and xenon recovery column 36 that is similar
to that used in the processes depicted in FIGS. 1 and 2. Column 36 differs
from the columns of FIGS. 1 and 2 in that the optional bottom section of
stages are omitted.
The main rare gas-containing oxygen feed to column 36 is stream 308 which
is at least 90% gaseous. The oxygen feed is separated in column 36 into
rare gas-lean GOX overhead, which is removed as stream 310, and rare
gas-enriched LOX bottoms, which is removed as stream 320. Column 36 is
refluxed by a stream 304 of LOX fed to the top of the column. The flow of
LOX reflux stream 304 determines the losses of xenon and particularly of
the more volatile krypton in the overhead vapor. Selection of the LOX
reflux/feed GOX flow determines the liquid to vapor ("L/V") flow ratio.
It should be noted that, in FIG. 3, stream 304 is typically a fraction of
the pumped LOX and thereby contains the same krypton and xenon
concentrations as the feed GOX stream 308. Thus, the overhead GOX stream
310 will be in equilibrium with LOX stream 304 and so will contain some
krypton and xenon. It is within the scope of the invention to replace or
supplement stream 304 by use of a column condenser in which part of the
overhead gas is condensed against a suitable refrigerant and returned to
the column as reflux, optionally with a small fraction withdrawn as rare
gas-lean LOX product.
FIG. 3 shows only a single column section but, optionally, there may be a
bottom section located between feed 308 and reboiler 38. Such an
additional column section would help to concentrate the krypton and xenon
in the bottom product.
A stream 332 of heating fluid, e.g. compressed feed air, is condensed in
reboiler 38 against boiling rare gas-enriched liquid in the sump of the
column 36 providing a condensed heating fluid stream 334. It should be
noted that the reboiler 38 could be deleted provided that the feed stream
308 is sufficiently superheated. The refrigeration necessary to
desuperheat the feed stream would then be provided by vaporizing
downflowing LOX in the column such that only stream 320 remained.
FIG. 4 depicts a crude rare gas recovery system comprising a HP column 40
thermally integrated with a LP pressure column 42 via a reboiler 44.
Oxygen feed stream 400, having a vapor quality of at least 0.9 and
containing more krypton and xenon than atmospheric air, is fed to a lower
portion of HP column 40. Ideally, feed stream 400 is in the gaseous state.
In HP column 40, separation is effected into overhead rare gas-depleted
vapor and rare gas-enriched bottoms liquid. The enriched bottoms liquid is
withdrawn as stream 404 and is fed to a lower zone of LP column 42 where
it is separated into rare gas-lean product, which is removed as stream 408
and further processed if required to provide purified krypton and/or xenon
products, and rare gas-enriched overhead vapor, which is removed as stream
406. The overhead vapor from HP column 40 is at least partially condensed
in reboiler 44 against boiling rare gas-lean product to provide at least
partially condensed overhead. A portion of the condensed overhead is used
as reflux for HP column 40 and the remainder is fed as stream 402 to LP
column 42 as reflux. A portion of stream 402 can optionally be withdrawn
as rare gas-depleted LOX product in stream 412.
Typically, the flowrate of stream 402 will be approximately half of the
flow leaving reboiler 44, so that the L/V ratio in both columns is
similar. The L/V ratio will be about 0.5 if oxygen feed stream 400 is all
vapor as preferred. The higher the L/V ratio, the more difficult it is for
xenon and particularly for krypton to escape from the column system.
The pressure difference between the two columns will tend to be small
because the composition of fluids on both side of reboiler 44 will be
similar. Thus, there will be issues to consider in liquid transfer from HP
column 40 to LP column 42. One or more pumps might be necessary to
accomplish this transfer. Alternatively, GOX vapor could be injected into
the transfer lines thereby using "vapor lift" to accomplish the liquid
transfer.
The columns are depicted as stacked with LP column 42 located above HP
column 40. However, the invention also applies to arrangements where the
two columns are side by side or even if HP column 40 were stacked above LP
column 42.
The process represented in FIG. 4 uses only a single column section in
columns 40 and 42 but, optionally, there could be more than one section.
For example, In LP column 42 there could be a bottom section between feed
404 and the reboiler 44, which would help to concentrate the krypton and
xenon in the bottom product.
The approximate column L
