U.S. patent application number 12/067672 was filed with the patent office on 2008-09-18 for process and apparatus for the separation of air by cryogenic distillation.
This patent application is currently assigned to L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exloitation Des Procedes Georges Claude. Invention is credited to Jean-Pierre Tranier.
Application Number | 20080223075 12/067672 |
Document ID | / |
Family ID | 35809642 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080223075 |
Kind Code |
A1 |
Tranier; Jean-Pierre |
September 18, 2008 |
Process and Apparatus for the Separation of Air by Cryogenic
Distillation
Abstract
A process for separating air by cryogenic distillation in a
column system comprising a high pressure column and a low pressure
column comprises compressing all the feed air in a first compressor
to a first outlet pressure, sending a first part of the air at the
first outlet pressure to a second compressor and compressing the
air to a second outlet pressure, cooling at least part of the air
at the second outlet pressure in a heat exchanger liquefying at
least part of the air at the second outlet pressure and sending the
liquefied air to at least one column of the column system wherein
at least 50% of the liquefied air sent to the column system has
been compressed in the second compressor, cooling a second part of
the air at the first outlet pressure in the heat exchanger and
expanding at least part of the second part of the air in an
expander from the first outlet pressure to the pressure of a column
of column system and sending the expanded air to that column, at
least partially vaporizing an auxiliary fluid, eventually further
warming said auxiliary fluid in the heat exchanger, sending at
least part of this auxiliary fluid to a third compressor to a third
outlet pressure, introducing at least part of said auxiliary fluid
at said third outlet pressure in the heat exchanger, cooling said
auxiliary fluid and at least partially liquefying said auxiliary
fluid, removing said auxiliary stream from the heat exchanger and
expanding it to a fourth pressure level before reintroducing it in
the heat exchanger where it will be partially vaporized as
above-mentioned, removing liquid from a column of the column system
and vaporizing the liquid by heat exchange in the heat
exchanger.
Inventors: |
Tranier; Jean-Pierre;
(L'Hay-Les-Roses, FR) |
Correspondence
Address: |
AIR LIQUIDE;Intellectual Property
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Assignee: |
L'Air Liquide Societe Anonyme Pour
L'Etude Et L'Exloitation Des Procedes Georges Claude
Paris, Cedex 7
FR
|
Family ID: |
35809642 |
Appl. No.: |
12/067672 |
Filed: |
September 21, 2006 |
PCT Filed: |
September 21, 2006 |
PCT NO: |
PCT/EP2006/066601 |
371 Date: |
March 21, 2008 |
Current U.S.
Class: |
62/644 |
Current CPC
Class: |
F25J 3/04303 20130101;
F25J 2270/66 20130101; F25J 3/04381 20130101; F25J 3/04412
20130101; F25J 2230/08 20130101; F25J 3/04393 20130101; F25J
2270/902 20130101; F25J 3/04296 20130101; F25J 3/04109 20130101;
F25J 2290/12 20130101; F25J 3/04309 20130101; F25J 3/04145
20130101; F25J 3/0409 20130101; F25J 3/04278 20130101; F25J 2230/20
20130101; F25J 3/04024 20130101; F25J 2270/12 20130101; F25J
3/04139 20130101 |
Class at
Publication: |
62/644 |
International
Class: |
F25J 3/04 20060101
F25J003/04; F25J 3/00 20060101 F25J003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 2005 |
EP |
05108826.8 |
Claims
1-8. (canceled)
8. A process for separating air by cryogenic distillation in a
column system comprising a high pressure column and a low pressure
column comprising the steps of: i) compressing all the feed air in
a first compressor to a first outlet pressure; ii) sending a first
part of the air at the first outlet pressure to a second compressor
and compressing the air to a second outlet pressure; iii) cooling
at least part of the air at the second outlet pressure in a heat
exchanger; iv) cooling a second part of the air at the first outlet
pressure in the heat exchanger and expanding at least part of the
second part of the air in an expander from the first outlet
pressure to the pressure of a column of column system and sending
the expanded air to that column; v) removing liquid from a column
of the column system, pressurizing the liquid and vaporizing the
liquid by heat exchange in the heat exchanger; and vi) at least
partially vaporizing an auxiliary fluid in the heat exchanger,
eventually further warming said auxiliary fluid in the heat
exchanger, sending at least part of this auxiliary fluid to a third
compressor to be compressed to a third outlet pressure, introducing
at least part of said auxiliary fluid at said third outlet pressure
in the heat exchanger, cooling said auxiliary fluid and at least
partially liquefying said auxiliary fluid, removing said auxiliary
stream from the heat exchanger and expanding it to a fourth
pressure level before reintroducing it in the heat exchanger for
the afore mentioned at least partial vaporization step.
9. The process of claim 8 wherein at least part of the first part
of the air is cooled upstream of the second compressor.
10. The process of claim 9 wherein at least part of the first part
of the air is cooled upstream of the second compressor in the heat
exchanger.
11. The process of claim 9 wherein at least part of the first part
of the air is cooled upstream of the second compressor in the heat
exchanger using a refrigeration unit.
12. The process of claim 8 wherein additional air is liquefied in
the heat exchanger at least one of the first and second
pressures.
13. The process of claim 8 wherein the third compressor compresses
an auxiliary fluid chosen from the group comprising containing at
least one of the following gases: He, H.sub.2, Ne, N.sub.2, CO, Ar,
O.sub.2, CH.sub.4, Kr, NO, Xe, CF.sub.4, HCF.sub.3, C.sub.2H.sub.4,
C.sub.2H.sub.6, C.sub.2F.sub.6, C.sub.3F.sub.8, N.sub.2O,
CO.sub.2.
14. The process of claim 13 wherein a principal component of the
auxiliary fluid is at least one of: Ar, O.sub.2, CH.sub.4 and
Kr.
15. An apparatus for the separation of air by cryogenic
distillation comprising: a) a column system; b) first, second and
third compressors; c) a first expander; d) a conduit for sending
air to the first compressor to form compressed air at a first
outlet pressure; e) a conduit for sending a first part of the air
at the first outlet pressure to the second compressor to form air
at a second outlet pressure; f) a heat exchanger, a conduit for
sending at least part of the air at the second outlet pressure to
the heat exchanger to form cooled compressed air at the second
outlet pressure; g) a conduit for removing liquefied air at the
second outlet pressure from the heat exchanger and for sending the
liquefied air to at least one column of the column system; h) a
conduit for removing a second part of the air at the first outlet
pressure from the heat exchanger and for sending at least part of
the second part of the air to the expander conduit for sending air
expanded in the expander to at least one column of column system;
i) a conduit for removing liquid from a column of the column
system, means for pressurizing at least part of the liquid to form
pressurized liquid and a conduit for sending at least part of the
pressurized liquid to the heat exchanger; and j) a refrigeration
cycle comprising the third compressor and a second expander, a
conduit for sending an auxiliary fluid from the third compressor to
the heat exchanger, a conduit for sending the auxiliary fluid from
the heat exchanger to the second expander, a conduit for sending
the auxiliary fluid from the second expander to the heat exchanger
and a conduit for sending the auxiliary fluid from the heat
exchanger to the third compressor.
Description
[0001] The present invention relates to a process and apparatus for
the separation of air by cryogenic distillation. It relates in
particular to processes and apparatus for producing oxygen and/or
nitrogen at elevated pressure.
[0002] Gaseous oxygen produced by air separation plants are usually
at elevated pressure about 20 to 50 bar. The basic distillation
scheme is usually a double column process producing oxygen at the
bottom of the low-pressure column operated at 1.4 to 4 bar. The
oxygen must be compressed to higher pressure either by oxygen
compressor or by the liquid pumping process. Because of the safety
issues associated with the oxygen compressors, most recent oxygen
plants are based on the liquid pumping process. In order to
vaporize liquid oxygen at elevated pressure there is a need for an
additional motor-driven booster compressor to raise a portion of
the feed air or nitrogen to higher pressure in the range of 40-80
bars. In essence, the booster replaces the oxygen compressor.
[0003] In the effort to reduce the complexity of an oxygen plant,
it is desirable to reduce the number of motor-driven compressors.
Significant cost reduction can be achieved if the booster can be
eliminated without much affecting the plant performance in terms of
power consumption. Furthermore, the air purification unit conceived
for a traditional oxygen plant would operate at about 5-7 bar which
is essentially the pressure of the high-pressure column, and it is
also desirable to raise this pressure to a higher level in order to
render the equipment more compact and less costly.
[0004] A cold compression process as described in U.S. Pat. No.
5,475,980 provides a technique to drive the oxygen plant with a
single air compressor. In this process, air to be distilled is
chilled in the main exchanger then further compressed by a booster
compressor driven by an expander exhausting into the high-pressure
column of a double column process. By doing so, the discharge
pressure of the air compressor is in the range of 15 bar which is
also quite advantageous for the purification unit. One
inconvenience of this approach is the increase of the size of the
main exchanger due to additional flow recycling which is typical
for the cold compression plant. One can reduce the size of the
exchanger by opening up the temperature approaches of the
exchanger. However, this would lead to inefficient power usage and
higher discharge pressure of the compressor, therefore increasing
its cost. An illustration of this prior art is presented in FIG. 1,
in which an oil brake is added to the system to dissipate the power
required for the refrigeration. In larger plants, a compressor
and/or a generator can replace the oil brake.
[0005] In FIG. 1 all the feed air is compressed in compressor 1,
purified in purification unit 2 and sent as stream 11 to the warm
end of the heat exchanger 5. All the feed air is cooled to an
intermediate temperature, removed from the heat exchanger as stream
7 and compressed in cold compressor 8. The compressed stream 9 is
sent back to the heat exchanger at a higher intermediate
temperature, cooled to a temperature lower than the inlet
temperature of the cold compressor 8 and divided in two. Stream 15
is sent to the Claude expander 13 which is braked by the compressor
8 and an oil brake. The rest of the air 10 is liquefied in the heat
exchanger and divided into two parts, one part being sent to the
high-pressure column 30 and the rest 34 being sent to the
low-pressure column 31.
[0006] An oxygen enriched liquid stream 28 is expanded and sent
from the high-pressure column to the low-pressure column. A
nitrogen enriched liquid stream 29 is expanded and sent from the
high-pressure column to the low-pressure column. High-pressure
gaseous nitrogen 14 is removed from the top of the high-pressure
column and warmed in the heat exchanger to form a product stream
24. Liquid oxygen 20 is removed from the bottom of the low pressure
column 31, pressurized by a pump 21 and sent as stream 22 to the
heat exchanger 5 where it vaporizes by heat exchange with the
pressurized air 10 to form gaseous pressurized oxygen 23. A top
nitrogen enriched gaseous stream 25 is removed from the
low-pressure column 31, warmed in the heat exchanger 5 and then
forms stream 26.
[0007] Some different versions of the cold compression process were
also described in prior art as in U.S. Pat. No. 5,379,598, U.S.
Pat. No. 5,596,885, U.S. Pat. No. 5,901,576 and U.S. Pat. No.
6,626,008.
[0008] In U.S. Pat. No. 5,379,598 a fraction of feed air is further
compressed by a booster compressor followed by a cold compressor to
yield a pressurized stream needed for the vaporization of oxygen.
This approach still has at least two compressors and the
purification unit still operates at low pressure.
[0009] In U.S. Pat. No. 5,596,885, a fraction of the feed air is
further compressed in a warm booster whilst at least part of the
air is further compressed in a cold booster. Air from both boosters
is liquefied and part of the cold compressed air is expanded in a
Claude expander.
[0010] U.S. Pat. No. 5,901,576 describes several arrangements of
cold compression schemes utilizing the expansion of vaporized rich
liquid of the bottom of the high-pressure column, or the expansion
of high-pressure nitrogen to drive the cold compressor. In some
cases, motor driven cold compressors were also used. These
processes also operate with feed air at about the high-pressure
column's pressure and in most cases a booster compressor is also
needed.
[0011] U.S. Pat. No. 6,626,008 describes a heat pump cycle
utilizing a cold compressor to improve the distillation process for
the production of low purity oxygen for a double vaporizer oxygen
process. Low air pressure and a booster compressor are also typical
for this kind of process.
[0012] Therefore it is a purpose of this invention to resolve the
inconveniences of the traditional process by providing a solution
to simplify the compression train and to reduce the size of the
purification unit. This can moreover be achieved with good power
consumption. The overall product cost of an oxygen plant can
therefore be reduced. The main improvement in power consumption is
due to the reduction in the cold compressor flow by using
essentially latent heat instead of specific heat.
[0013] All percentages listed are molar percentages.
[0014] According to the present invention, there is provided a
process for separating air by cryogenic distillation in a column
system comprising a high pressure column and a low pressure column
comprising the steps of:
[0015] i) compressing all the feed air in a first compressor to a
first outlet pressure
[0016] ii) sending a first part of the air at the first outlet
pressure to a second compressor and compressing the air to a second
outlet pressure
[0017] iii) cooling at least part of the air at the second outlet
pressure in a heat exchanger to form cooled compressed air at the
second outlet pressure, liquefying at least part of the air at the
second outlet pressure and sending the liquefied air to at least
one column of the column system
[0018] iv) cooling a second part of the air at the first outlet
pressure in the heat exchanger and expanding at least part of the
second part of the air in an expander from the first outlet
pressure to the pressure of a column of column system and sending
the expanded air to that column
[0019] v) removing liquid from a column of the column system,
pressurizing the liquid and vaporizing the liquid by heat exchange
in the heat exchanger
[0020] vi) at least partially vaporizing an auxiliary fluid,
eventually further warming said auxiliary fluid in the heat
exchanger, sending at least part of this auxiliary fluid to a third
compressor to be compressed to a third outlet pressure, introducing
at least part of said auxiliary fluid at said third outlet pressure
in the heat exchanger, cooling said auxiliary fluid and at least
partially liquefying said auxiliary fluid, removing said auxiliary
stream from the heat exchanger and expanding it to a fourth
pressure level before reintroducing it in the heat exchanger where
it will be partially vaporized as above-mentioned.
[0021] According to optional features of the invention: [0022]
additional air is liquefied in the heat exchanger at the first
pressure. [0023] the third compressor compresses an auxiliary fluid
containing at least one of the following gases: He, H.sub.2, Ne,
N.sub.2, CO, Ar, O.sub.2, CH.sub.4, Kr, NO, Xe, CF.sub.4,
HCF.sub.3, C.sub.2H.sub.4, C.sub.2H.sub.6, C.sub.2F.sub.6,
C.sub.3F.sub.8, N.sub.2O, CO.sub.2. [0024] the third compressor
compresses an auxiliary fluid whose principal component comprises
at least one of: Ar, O.sub.2, CH.sub.4 and Kr.
[0025] According to another aspect of the invention, there is
provided an apparatus for the separation of air by cryogenic
distillation comprising:
[0026] a) a column system
[0027] b) first, second and third compressors
[0028] c) an expander
[0029] d) a conduit for sending air to the first compressor to form
compressed air at a first outlet pressure
[0030] e) a conduit for sending a first part of the air at the
first outlet pressure to the second compressor to form air at a
second outlet pressure
[0031] f) a heat exchanger, a conduit for sending at least part of
the air at the second outlet pressure to the heat exchanger to form
cooled compressed air at the second outlet pressure,
[0032] g) a conduit for removing liquefied air at the second outlet
pressure from the heat exchanger and for sending the liquefied air
to at least one column of the column system
[0033] h) a conduit for removing a second part of the air at the
first outlet pressure from the heat exchanger and for sending at
least part of the second part of the air to the expander
[0034] i) a conduit for sending air expanded in the expander to at
least one column of column system
[0035] j) a conduit for removing liquid from a column of the column
system, means for pressurizing at least part of the liquid to form
pressurized liquid and a conduit for sending at least part of the
pressurized liquid to the heat exchanger and
[0036] k) a refrigeration cycle comprising the third compressor and
a second expander (16), a conduit for sending an auxiliary fluid
from the third compressor to the heat exchanger, a conduit for
sending the auxiliary fluid from the heat exchanger to the second
expander, a conduit for sending the auxiliary fluid from the second
expander to the heat exchanger and a conduit for sending the
auxiliary fluid from the heat exchanger to the third
compressor.
[0037] According to further optional aspects of the invention, the
apparatus may include a further expander and means for sending
nitrogen from a column of the column system or air to the further
expander.
[0038] In this case, one of the second and third compressors may be
coupled to the expander and the other of the second and third
compressors may be coupled to the further expander.
[0039] At least one of the second and third compressors is coupled
to the air expander.
[0040] Preferably the conduit for sending a first part of the air
at the first outlet pressure to the second compressor is connected
to an intermediate point of the heat exchanger.
[0041] Preferably the second and third compressors are connected in
series.
[0042] The expander may be chosen from the group including an air
expander whose outlet is connected to the high pressure column, an
air expander whose outlet is connected to the low pressure column,
a high pressure nitrogen expander and a low pressure nitrogen
expander.
[0043] The apparatus may include a further expander chosen from the
group including an air expander whose outlet is connected to the
high pressure column, an air expander whose outlet is connected to
the low pressure column, a high pressure nitrogen expander and a
low pressure nitrogen expander.
[0044] Preferably the further expander is coupled to one of the
second and third expanders.
[0045] The invention will now be described in greater detail with
reference to FIGS. 2, 3, 5 and 6 which are process flow diagrams
representing cryogenic air separation processes according to the
invention, FIG. 4 which is a heat exchange diagram and to FIG. 7
which shows a coupling system for compressors and expanders in a
process according to the invention.
[0046] In the embodiment of FIG. 2, atmospheric air is compressed
by the air compressor 1 and purified in the purification unit 2 to
yield an air stream (stream 11) free of impurities such as moisture
and carbon dioxide that can freeze in the cryogenic equipment. A
first portion of this air is compressed in a booster brake
compressor 3 to raise its pressure further. This pressurized first
portion (stream 4) is then cooled in the main exchanger 5 to
condense to form a liquefied air stream (stream 27), which is fed
to at least one of the distillation columns, following expansion in
a valve. The air may liquefy within or downstream the main
exchanger depending on the pressure used. An auxiliary fluid
mixture 6 of krypton (90%) and oxygen (10%) is introduced in heat
exchanger 5 when it is vaporized and slightly warmed after
vaporization to yield a cold auxiliary gaseous stream at an
intermediate temperature T1. At least a portion of this cold
auxiliary stream (stream 7) is sent to a cold brake compressor 8 at
temperature T1 to be compressed to raise its pressure (stream 9).
Stream 9 is then sent back to the exchanger at temperature T2 which
is greater than T1 and cooled in exchanger 5 to condense to form a
liquefied auxiliary stream (stream 10), which is expanded in a
valve 16 to form stream 6. A phase separator could be added if
stream 6 is a two-phase fluid, the liquid phase being introduced in
heat exchanger 5 and the vapor phase mixed with stream 7. The
second portion of stream 11 (stream 12) is cooled in exchanger 5 to
yield stream 15, which is sent to the expander 13 at an inlet
temperature of T3, for expansion into the high pressure column. It
is preferable that the power generated by expander 13 be used to
drive the booster brake compressor 3. The rest of stream 12 is
liquefied as stream 33 and sent to the high pressure column 30.
Nitrogen rich gas 14 can be extracted from the high pressure column
30, warmed in exchanger 5 to form stream 17, which is then expanded
in expander 18 having an inlet temperature T4. The power of
expander 18 can be preferably used to drive the cold booster brake
compressor 8. The exhaust of expander 18 (stream 19) then returns
to the cold end of exchanger 5 to be re-heated to close to ambient
temperature forming stream 24. Pump 21 boosts the pressure of
liquid oxygen product 20 extracted at the bottom of the low
pressure column 31 to the desired pressure then sends pressurized
oxygen stream 22 to exchanger 5 for vaporization and heating to
yield the oxygen product 23. The double column system is a
traditional type of two-column process as described in numerous
patents or papers on air separation technology having a high
pressure column 30 and a low pressure column 31, thermally linked
by a reboiler-condenser at the bottom of the low pressure column.
An argon column (not shown) can be used with the double column
system to provide a concentrated argon stream.
[0047] The above temperatures T1, T2, T3 and T4 are provided as the
preferred arrangement. Above, going from the hottest temperature to
the coldest the temperatures are T2, T5, T1 and T3. Depending upon
the pressure of the vaporized oxygen and the pressure of the column
system the order of these temperatures can be modified to optimize
the performance of the process.
[0048] It is useful to note the booster brake compressors 3 is a
single stage compressor and is usually provided as part of the
expander-booster package and therefore its construction is much
simpler and its cost structure much lower than the stand-alone or
motor-driven booster compressor. However if necessary, compressor 3
may be a stand-alone or motor-driven booster compressor. Compressor
8 could be either a stand-alone or motor driven booster compressor
with one to four stages depending upon the pressure of stream 4 and
stream 23. It could be driven directly by expander 18 (alternately
expander 13) at the same speed or through a gear to optimize the
performances of the booster and expander.
[0049] The range of the process variables of the embodiment of FIG.
2 is as follows:
[0050] Stream 11 pressure: about 9 to 17 bar a
[0051] Stream 4 pressure: about 16 to 50 bar a
[0052] Stream 9 pressure: about 5 to 20 bar a in case of a mixture
rich in krypton
[0053] T1: about -110.degree. C. to -165.degree. C.
[0054] The flow compressed by the booster brake compressor 8 can be
reduced by optionally extracting some of stream 12 as liquefied air
flow 33. As such, less power is required to drive the booster brake
compressor 8 and some power savings can be achieved. The amount of
air liquefied at the first pressure should not be more than 50% of
the liquefied air sent to the column system, preferably not more
than 40%, more preferably not more than 35%.
[0055] It is common practice in air separation technology to
substitute the nitrogen expander with an air expander. The
embodiment of FIG. 3 describes such an arrangement: after the first
compressor, the portion 12 of stream 11 is cooled in exchanger 5
and part of this stream is extracted to yield stream 50, which is
sent to expander 52 for expansion into the low pressure column 31.
The power of expander 52 is preferably used to drive the cold
compressor 8. It is useful to note that one can also opt to divide
stream 12 before exchanger 5 and send the corresponding air stream
to a separate passage in exchanger 5 then cool and expand it in
expander 52 into the column. FIG. 4 shows the exchange diagram
corresponding to the process of FIG. 3.
[0056] The above technique can be modified slightly as described in
FIG. 5: a portion 53 of the air at the exhaust stream 54 of
expander 13 can be warmed in the exchanger 5 then send to the
expander 52 for expansion into the low pressure column. In
situations where there is some condensation in stream 54, one can
extract the gas feeding the expander 52 by adding a vapor-liquid
separator or even better, use the sump of the high pressure column
as a separator, in this case, the gas feeding the expander is
extracted at the sump of the high pressure column.
[0057] In many situations where there is a need for a significant
amount of nitrogen rich gas product at elevated pressure, it is no
longer economical to utilize the nitrogen rich gas expander 18.
Instead as shown in FIG. 6 the nitrogen rich gas 14 can be
extracted and produced directly off the high pressure column 30 to
yield the nitrogen product 41. In those situations one can opt to
raise the pressure of compressor 1 to increase the power output of
the expander 13 to cover the lack of refrigeration caused by the
elimination of the nitrogen expander. To further simplify the
expander and booster brake compressors arrangement, the tandem
expander and booster brakes can be mechanically integrated into a
single train: the power of the expander 13 drives the two
compressor brakes 3 (single stage) and 8 (double stage). In
addition, a motor and/or generator 60 can extract or add mechanical
power to the system depending on the performance and production
expected from the plant at a certain time. Depending upon the flows
and pressures of the expander and booster brake compressors a speed
changer (gear) can be used to optimize the system performance. An
illustration of the arrangement with gear is presented in FIG. 7. A
further expander 18, 52 could also be added to such a system.
[0058] The process may be modified to vaporize pumped liquid
nitrogen as an additional stream or as a stream replacing the
pumped oxygen stream.
[0059] The illustrated processes show double column systems but it
will be readily understood that the invention applies to triple
column systems.
[0060] In the case where the double or triple column systems
operate at elevated pressures, some of the low pressure nitrogen
may be expanded in an expander 18.
* * * * *