U.S. patent application number 16/768056 was filed with the patent office on 2020-11-12 for cryogenic distillation method and apparatus for producing pressurized air by means of expander booster in linkage with nitrogen expander for braking.
This patent application is currently assigned to L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des Procedes Georges Claude. The applicant listed for this patent is Alain BRIGLIA, Eric DAY, L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des Procedes Georges Claude, Fengjie XUE, Bowei ZHAO. Invention is credited to Alain BRIGLIA, Eric DAY, Fengjie XUE, Bowei ZHAO.
Application Number | 20200355429 16/768056 |
Document ID | / |
Family ID | 1000004989086 |
Filed Date | 2020-11-12 |
United States Patent
Application |
20200355429 |
Kind Code |
A1 |
ZHAO; Bowei ; et
al. |
November 12, 2020 |
CRYOGENIC DISTILLATION METHOD AND APPARATUS FOR PRODUCING
PRESSURIZED AIR BY MEANS OF EXPANDER BOOSTER IN LINKAGE WITH
NITROGEN EXPANDER FOR BRAKING
Abstract
Provided are a method and apparatus for producing nitrogen and
oxygen by means of cryogenic distillation of air. Nitrogen products
are extracted only from the top of a tower. If a customer needs
nitrogen with lower pressure, part of pure nitrogen that is
partially located at a first nitrogen product pressure is reheated
in a main heat exchanger, then decompressed to a second nitrogen
product pressure by means of a nitrogen expander, further reheated
by means of the main heat exchanger, and output as a low-pressure
nitrogen product. The nitrogen expander can be braked by an
expander booster for compressing air. By means of the method,
nitrogen with different pressures can be suitably produced, and the
energy consumption for producing the pressurized air can be reduced
by utilizing the expansion work of nitrogen.
Inventors: |
ZHAO; Bowei; (Zhejiang,
CN) ; BRIGLIA; Alain; (Zhejiang, CN) ; XUE;
Fengjie; (Zhejiang, CN) ; DAY; Eric;
(Zhejiang, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ZHAO; Bowei
BRIGLIA; Alain
XUE; Fengjie
DAY; Eric
L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des
Procedes Georges Claude |
Hangzhou, Zhejiang
Hangzhou, Zhejiang
Hangzhou, Zhejiang
Hangzhou, Zhejiang
Paris |
|
CN
CN
CN
CN
FR |
|
|
Assignee: |
L'Air Liquide, Societe Anonyme pour
l'Etude et l'Exploitation des Procedes Georges Claude
Paris
FR
|
Family ID: |
1000004989086 |
Appl. No.: |
16/768056 |
Filed: |
November 29, 2017 |
PCT Filed: |
November 29, 2017 |
PCT NO: |
PCT/CN2017/113525 |
371 Date: |
May 28, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 3/04412 20130101;
F25J 3/04218 20130101; F25J 2240/46 20130101; F25J 2240/10
20130101; F25J 3/04387 20130101; F25J 3/04309 20130101; F25J
3/04393 20130101; F25J 3/0409 20130101 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1-17. (canceled)
18. A method for producing nitrogen and oxygen by means of the
cryogenic distillation of air, comprising: (a) providing a first
tower working at a higher pressure and a second tower working at a
lower pressure, wherein the first tower and the second tower are
brought into communication by means of heat exchange via a main
condensing evaporator; (b) providing at least one air pre-cooling
system, one air purification system, one main air compressor, at
least one air booster, at least one main heat exchanger, and one
supercooler; (c) further pre-cooling and purifying an air feed gas,
which has been pressurized to a first pressure range by means of
the main air compressor, then sending part of the air feed gas to
the main heat exchanger for heat exchange with gas products
produced by means of rectification and then to the first tower,
subjecting the other part of the air feed gas to pressurization by
means of the air booster and several stages of expander boosters,
to heat exchange in the main heat exchanger with gas and liquid
products produced by means of rectification and then to expansion
or throttling for decompression to the first pressure range, and
then sending this other part to the first tower, or supercooling a
part by means of the supercooler, followed by throttling and
sending to the second tower; (d) rectifying the air feed gas in the
first tower, extracting oxygen-enriched liquid air at the bottom of
the first tower, pure liquid nitrogen at the top, and optionally
impure liquid nitrogen in the middle part, and sending same to the
supercooler for supercooling and then to the second tower as reflux
liquids; (e) extracting pure liquid oxygen in the main condensing
evaporator, and sending the pure liquid oxygen to a liquid oxygen
pump for pressurization and then to the main heat exchanger for
heat exchange with the air feed gas pressurized by means of the air
booster and the several stages of expander boosters, followed by
evaporation and vaporization for output as a product; (f)
extracting impure nitrogen from the second tower, and sending the
impure nitrogen to the supercooler for warming and then to the main
heat exchanger for further reheating; and (g) extracting pure
nitrogen at a first nitrogen product pressure from the top of the
first tower and sending the pure nitrogen to the main heat
exchanger for reheating, wherein a part of pure nitrogen at the
first nitrogen product pressure is partially reheated in the main
heat exchanger, decompressed to a second nitrogen product pressure
by means of a nitrogen expander, then further reheated by means of
the main heat exchanger, and output as a product; and the nitrogen
expander is braked by means of a first expander booster, the first
expander booster further pressurizes the part of air feed gas that
has been pressurized by means of the air booster and reaches a
second pressure range to a third pressure range, and the air feed
gas within the third pressure range enters, directly or optionally
after undergoing further pressurization, the main heat exchanger
for heat exchange with the gas and liquid products produced by
means of rectification, and is then decompressed to the first
pressure range by means of a liquid expander and then sent to the
first tower, or a part of decompressed liquid is supercooled by
means of the supercooler, then throttled and sent to the second
tower.
19. The method of claim 18, wherein the main heat exchangers
include a high pressure plate heat exchanger and a low pressure
plate heat exchanger, or an integral combined heat exchanger.
20. The method of claim 19, wherein the part of air feed gas that
has been pressurized by means of the air booster and reaches the
second pressure range is partially cooled in the main heat
exchanger, then decompressed to the first pressure range by means
of an air expander, and then sent to the first tower.
21. The method of claim 20, wherein the nitrogen expander is braked
by means of a second expander booster, the second expander booster
further pressurizes the air feed gas that has been pressurized by
means of the air booster and reaches the second pressure range to
the third pressure range, and the air feed gas within the third
pressure range enters the main heat exchanger for heat exchange
with the gas and liquid products produced by means of
rectification, and is then decompressed to the first pressure range
by means of the liquid expander and then sent to the first
tower.
22. The method of claim 21, wherein the air feed gas that has been
pressurized by means of the air booster and reaches the second
pressure range is divided into three parts, wherein a first part is
pressurized to the third pressure range by means of the first
expander booster; a second part is partially cooled in the main
heat exchanger, then decompressed to the first pressure range by
means of the air expander, and then sent to the first tower; and a
third part is pressurized to the third pressure range by means of
the second expander booster, the pressurized first part and third
part of air feed gas are combined, then sent to the main heat
exchanger for heat exchange with the gas and liquid products
produced by means of rectification, then decompressed to the first
pressure range by means of the liquid expander and then sent to the
first tower.
23. The method of claim 20, wherein the nitrogen expander is braked
by means of a second expander booster, the second expander booster
further pressurizes the air feed gas that has been pressurized by
means of the first expander booster, the air feed gas is sent to
the main heat exchanger for heat exchange with the gas and liquid
products produced by means of rectification, then decompressed to
the first pressure range by means of the liquid expander and then
sent to the first tower.
24. The method of claim 23, wherein the air feed gas that has been
pressurized by means of the first expander booster is all sent to
the second expander booster for further pressurization.
25. The method of claim 21, wherein the liquid expander is braked
by means of an electric generator.
26. The method of claim 18, wherein a part of the liquid air feed
gas within the first pressure range cooled by means of the main
heat exchanger is supercooled by means of the supercooler, then
throttled and sent to the second tower as a reflux liquid.
27. The method of claim 18, wherein the pure liquid oxygen
extracted in the main condensing evaporator is partially
supercooled by means of the supercooler and then sent to a liquid
oxygen storage tank.
28. The method of claim 18, wherein the pure liquid oxygen
extracted in the main condensing evaporator is pressurized by means
of the liquid oxygen pump, and a part is expanded or throttled for
decompression, and then sent to the main heat exchanger for heat
exchange with the air feed gas, followed by evaporation and
vaporization for output as a product.
29. The method of claim 18, wherein after the part of pure nitrogen
at the first nitrogen product pressure is completely reheated in
the main heat exchanger, part of this part of pure nitrogen is
output as a first nitrogen product, and the other part is
pressurized to a third nitrogen product pressure by means of the
nitrogen booster and output as a third nitrogen product.
30. An apparatus for producing nitrogen and oxygen by means of the
cryogenic distillation of air, comprising: (a) a first tower
working at a higher pressure and a second tower working at a lower
pressure, wherein the first tower and the second tower are brought
into communication by means of heat exchange via a main condensing
evaporator; (b) at least one main air compressor, one air
pre-cooling system, one air purification system, one air booster, a
first expander booster, at least one main heat exchanger, one
nitrogen expander, at least one liquid expander, one liquid oxygen
pump and one supercooler; (c) a pipeline for sending an air feed
gas to the first tower via the main air compressor, the air
pre-cooling system, the air purification system and the main heat
exchanger; (d) a pipeline for sending oxygen-enriched liquid air at
the bottom of the first tower to the supercooler for supercooling
and to the second tower; (e) a pipeline for sending pure liquid
nitrogen at the top of the first tower to the supercooler for
supercooling and to the upper part of the second tower; optionally,
a pipeline for sending impure liquid nitrogen in the middle part of
the first tower to the supercooler for supercooling and to the
second tower; (g) a pipeline for extracting impure nitrogen from
the second tower and sending the impure nitrogen to the supercooler
for warming and to the main heat exchanger for reheating; (h) a
pipeline for extracting pure liquid oxygen from the main condensing
evaporator and sending the pure liquid oxygen to the liquid oxygen
pump for pressurization and then through the main heat exchanger;
and (i) a pipeline for extracting pure nitrogen from the top of the
first tower and sending the pure nitrogen to the main heat
exchanger; and (j) a pipeline for sending a part of pure nitrogen,
which has been reheated, from the main heat exchanger to the
nitrogen expander, and returning the expanded pure nitrogen to the
main heat exchanger for reheating, wherein the nitrogen expander is
braked by means of the first expander booster, and the main air
compressor, the air booster and the first expander booster are
sequentially connected in series, and are connected to the main
heat exchanger, the liquid expander and then the first tower via a
pipeline.
31. The apparatus of claim 30, further comprising a pipeline for
bringing the air booster into communication with the main heat
exchanger, and a pipeline that penetrates out from the middle part
of the main heat exchanger and connects the air expander and the
first tower in sequence.
32. The apparatus of claim 31, wherein the nitrogen expander is
braked by means of a second expander booster, and the apparatus
further comprises a pipeline for bringing the air booster into
communication with the second expander booster, and then
sequentially connecting the main heat exchanger, the liquid
expander and the first tower.
33. The apparatus of claim 32, wherein the air booster is directly
connected to the second expander booster, or the air booster is
connected to the second expander booster via the first expander
booster.
34. The apparatus of claim 30, wherein the main heat exchangers
include a high pressure plate heat exchanger and a low pressure
plate heat exchanger, or an integral combined heat exchanger.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a .sctn. 371 of International PCT
Application PCT/CN2017/113525, filed Nov. 29, 2017, which is herein
incorporated by reference in its entirety.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a low-temperature
rectification air separation process and device.
BACKGROUND OF THE INVENTION
[0003] The use of cryogenic distillation to separate air into
nitrogen and oxygen products is a common and mature technology. At
least two air separation towers operating at different pressures--a
medium pressure tower and a low pressure tower--are brought into
communication by means of heat exchange via a main condensing
evaporator. A pressurized, purified and cooled air feed gas is
input into the medium pressure tower and/or the low pressure tower,
and by means of rectification, gaseous and/or liquid nitrogen and
oxygen are obtained. All or part of the nitrogen and oxygen undergo
heat exchange with the air feed gas in the main heat exchanger to
obtain gaseous nitrogen and oxygen products at room temperature.
The design of an air separation device and process is generally
based on the requirements of customers for the status, pressure and
output of nitrogen and oxygen products.
[0004] When it is necessary to produce oxygen and/or nitrogen
products with a higher pressure--for example, greater than 40 bara,
an external pressurization method by which room temperature oxygen
or nitrogen, which has been reheated by means of the main heat
exchanger, is pressurized by means of a corresponding booster, or
an internal pressurization method by which low-temperature liquid
oxygen or liquid nitrogen is boosted to a desired pressure by means
of a pump and then reheated by means of the main heat exchanger may
be selected. In particular, when producing high pressure oxygen,
due to production safety and equipment cost considerations, the
internal pressurization process is generally used.
[0005] During the internal pressurization process, a high pressure
warm stream is required for evaporating and vaporizing the high
pressure liquid oxygen in the main heat exchanger, wherein this
warm stream is generally a high pressure air feed gas, or may also
be circulating high pressure nitrogen. If a high pressure air feed
gas is used, the air feed gas that has reached a pressure for the
medium pressure tower by means of a main air compressor needs to be
further pressurized to a higher pressure by means of a booster, and
this is an energy-consuming process.
SUMMARY OF THE INVENTION
[0006] A technical problem to be solved by certain embodiments of
the present invention is to improve the utilization of the
rectification capacity of an air separation tower, especially a low
pressure tower.
[0007] Another technical problem to be solved by certain
embodiments of the present invention is to reduce the energy
consumption required for the pressurization of an air feed gas.
[0008] Still another technical problem to be solved by certain
embodiments of the present invention is how to flexibly provide
nitrogen with different output and pressure to a customer.
[0009] In one aspect, the present invention provides a method for
producing nitrogen and oxygen by means of the cryogenic
distillation of air, comprising: providing a first tower working at
a higher pressure, i.e. a medium pressure tower, and a second tower
working at a lower pressure, i.e. a lower pressure tower, wherein
the first tower and the second tower are brought into communication
by means of heat exchange via a main condensing evaporator;
providing at least one air pre-cooling system, one air purification
system, one main air compressor, at least one air booster, at least
one main heat exchanger, and one supercooler; further treating an
air feed gas, which has been pressurized to a first pressure range
by means of the main air compressor, by means of the pre-cooling
system and the purification system, then sending part of the air
feed gas to the main heat exchanger for heat exchange with gas
products produced by means of rectification and then to the first
tower, subjecting the other part of the air feed gas to
pressurization by means of the air booster and several stages of
expander boosters, to heat exchange in the main heat exchanger with
gas and liquid products produced by means of rectification and then
to expansion or throttling for decompression to the first pressure
range, and then sending this other part to the first tower, or
supercooling a part of separated liquid by means of the
supercooler, followed by throttling and sending to the second
tower; rectifying the air feed gas in the first tower, extracting
oxygen-enriched liquid air at the bottom of the first tower, pure
liquid nitrogen at the top, and optionally impure liquid nitrogen
in the middle part, and sending same to the supercooler for
supercooling and then to the second tower as reflux liquids;
extracting pure liquid oxygen in the main condensing evaporator,
and sending the pure liquid oxygen to a liquid oxygen pump for
pressurization and then to the main heat exchanger for heat
exchange with the air feed gas pressurized by means of the air
booster and the several stages of expander boosters, followed by
evaporation and vaporization for output as a product; extracting
impure nitrogen from the second tower, and sending the impure
nitrogen to the supercooler for warming and then to the main heat
exchanger for further reheating; extracting pure nitrogen at a
first nitrogen product pressure from the top of the first tower and
sending the pure nitrogen to the main heat exchanger for reheating.
In the method, a part of pure nitrogen at the first nitrogen
product pressure is partially reheated in the main heat exchanger,
decompressed to a second nitrogen product pressure by means of a
nitrogen expander, then further reheated by means of the main heat
exchanger, and output as a product; and the nitrogen expander is
braked by means of a first expander booster, the first expander
booster further pressurizes the part of air feed gas that has been
pressurized by means of the air booster and reaches a second
pressure range to a third pressure range, and the air feed gas
within the third pressure range enters, directly or optionally
after undergoing further pressurization, the main heat exchanger
for heat exchange with the gas and liquid products produced by
means of rectification, and is then decompressed to the first
pressure range by means of a liquid expander and then sent to the
first tower, or a part of liquid is supercooled by means of the
supercooler, then throttled and sent to the second tower.
[0010] In another aspect, the present invention further provides an
apparatus for producing nitrogen and oxygen by means of the
cryogenic distillation of air, comprising: a first tower working at
a higher pressure and a second tower working at a lower pressure,
wherein the first tower and the second tower are brought into
communication by means of heat exchange via a main condensing
evaporator; at least one air pre-cooling system, one air
purification system, one main air compressor, one air booster, a
first expander booster, at least one main heat exchanger, one
nitrogen expander, at least one liquid expander, one liquid oxygen
pump and one supercooler; a pipeline for sending an air feed gas to
the first tower via the main air compressor, the air pre-cooling
system, the air purification system and the main heat exchanger; a
pipeline for sending oxygen-enriched liquid air at the bottom of
the first tower to the supercooler for supercooling and to the
second tower; a pipeline for sending pure liquid nitrogen at the
top of the first tower to the supercooler for supercooling and to
the upper part of the second tower; optionally, a pipeline for
sending impure liquid nitrogen in the middle part of the first
tower to the supercooler for supercooling and to the second tower;
a pipeline for extracting impure nitrogen from the second tower and
sending the impure nitrogen to the supercooler for warming and to
the main heat exchanger for reheating; a pipeline for extracting
pure liquid oxygen from the main condensing evaporator and sending
the pure liquid oxygen to the liquid oxygen pump for pressurization
and then through the main heat exchanger; and a pipeline for
extracting pure nitrogen from the top of the lower tower and
sending the pure nitrogen to the main heat exchanger; and further
comprising a pipeline for sending a part of pure nitrogen, which
has been reheated, from the main heat exchanger to the nitrogen
expander, and returning the expanded pure nitrogen to the main heat
exchanger for reheating, wherein the nitrogen expander is braked by
means of a first expander booster, and the main air compressor, the
air booster and the first expander booster are sequentially
connected in series, and are connected to the main heat exchanger,
the liquid expander and then the lower tower via a pipeline.
[0011] In certain embodiments of the present invention, a nitrogen
product is only extracted from the top of the first tower, and the
pressure of this nitrogen product is generally a medium pressure of
5-6 bara. If a customer needs nitrogen with a higher pressure, the
reheated nitrogen can be further pressurized using a booster. If a
customer needs nitrogen with a lower pressure, instead of the
common practice of extracting low pressure nitrogen from a pure
nitrogen tower at the top of a low pressure tower, the present
invention uses the expansion of medium pressure nitrogen to obtain
desired low pressure nitrogen. Furthermore, the above-mentioned
nitrogen expander may be braked by means of an expander booster for
compressing air. It can be seen therefrom that the present
invention can flexibly produce nitrogen with different pressures,
while the energy consumption for producing pressurized air can be
reduced by utilizing the expansion work of nitrogen.
[0012] After medium pressure nitrogen is extracted at the top of
the first tower, the amount of pure liquid nitrogen that can be
used as a reflux liquid to the second tower will be reduced
accordingly. This will make greater use of the rectification
capacity of the second tower, and will also reduce the required
diameter of the second tower, making the transportation thereof
more convenient.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The drawings of the present disclosure are only intended to
illustrate the present invention for construing and explaining the
spirit of the present invention, but do not limit the present
invention in any respect.
[0014] FIG. 1 is an embodiment of the present invention, wherein a
first expander booster and a second expander booster are connected
in series.
[0015] FIG. 2 is another embodiment of the present invention,
wherein a first expander booster and a second expander booster are
connected in parallel.
[0016] FIG. 3 is a comparative solution for the present invention,
in which no nitrogen expander is comprised.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In the present disclosure, the term "air feed gas" refers to
a mixture mainly comprising oxygen and nitrogen.
[0018] The term "pure nitrogen" covers gaseous fluids with a
nitrogen content of not less than 99 mole percent, and the term
"impure nitrogen" covers gaseous fluids with a nitrogen content of
not less than 95 mole percent, with the content of nitrogen in the
"impure nitrogen" being less than that in the "pure nitrogen".
[0019] The term "oxygen-enriched liquid air" refers to a liquid
fluid with a molar percentage of oxygen greater than 30, and the
term "pure liquid oxygen" covers liquid fluids with a molar
percentage of oxygen greater than 99, with the content of oxygen in
the "pure liquid oxygen" being higher than that in the
"oxygen-enriched liquid air".
[0020] The term "pure liquid nitrogen" refers to a liquid fluid
with a molar percentage of nitrogen greater than 99, and the term
"impure liquid nitrogen" refers to a liquid fluid with a molar
percentage of nitrogen greater than 96, with the content of
nitrogen in the "impure liquid nitrogen" being less than that in
the "pure liquid nitrogen".
[0021] The low temperature rectification of the present disclosure
is a rectification method carried out at least in part at a
temperature of 150 K or less. "Tower" herein means a distillation
or fractionation tower or zone in which liquid and gas phases come
into countercurrent contact for effectively separating a fluid
mixture. The operating pressure of the "first tower" in the present
disclosure is generally 5 to 6.5 bara, which is higher than the
general operating pressure of the "second tower" by 1.1 to 1.5
bara. The second tower can be installed vertically at the top of
the first tower or the two towers are installed side by side. The
"first tower" is also generally referred to as a medium pressure
tower or a lower tower, and the "second tower" is also generally
referred to as a low pressure tower or an upper tower. The main
condensing evaporator is generally located at the top of the "first
tower", and it can make pure nitrogen produced at the top of the
first tower condense by means of heat exchange with pure liquid
oxygen produced at the bottom of the second tower to obtain pure
liquid nitrogen at the top of the first tower, while the liquid
oxygen is partially evaporated. Types for the main condensing
evaporator include a tube and shell type, a falling film type, an
immersion bath type, etc., and in the present invention, an
immersion bath type condensing evaporator may be used.
[0022] The air pre-cooling system in the present invention is used
for pre-cooling high temperature air (70-120.degree. C.) discharged
from the main air compressor to a temperature suitable for entering
the air purification system (generally 10-25.degree. C.).
High-temperature air generally comes into contact with ordinary
circulating cooling water and low-temperature water (generally
5-20.degree. C.) in an air cooling tower for heat exchange to
achieve the purpose of cooling. Low-temperature water can be
obtained by bringing ordinary circulating cooling water into
contact with gas products or by-products produced by the air
separation apparatus, such as contact with impure nitrogen for heat
exchange, or by means of a refrigerator.
[0023] The air purification system refers to a purification device
that removes dust, water vapor, CO.sub.2, hydrocarbons etc. from
the air. In the present invention, a pressure swing adsorption
method is generally used, wherein an adsorbent is involved which
may optionally be a molecular sieve plus alumina, or a molecular
sieve only.
[0024] In the main heat exchanger, the compressed, pre-cooled and
purified air feed gas undergoes non-contact heat exchange with gas
and/or liquid products produced by means of rectification, and is
cooled close or equal to the rectification temperature of the first
tower, generally less than 150 K. Common main heat exchangers
include split or integrated types, etc. The main heat exchangers
are divided into high pressure (>20 bara pressure) and low
pressure (<20 bara pressure) heat exchangers according to
suitable pressure ranges. In the present invention, both a high
pressure plate heat exchanger and a low pressure plate heat
exchanger, or an integral combined heat exchanger may be used.
[0025] In the present disclosure, the first pressure range is
consistent with the range of the working pressure of the first
tower or medium pressure tower, and is generally 5 to 6 bara, and
the air feed gas at atmospheric pressure can be compressed by the
main air compressor to reach this pressure range. The second
pressure range is a pressure range achieved by pressurizing the air
feed gas within the first pressure range by means of the air
booster, and is generally 40 to 60 bara. The third pressure range
is achieved by further pressurizing the air feed gas within the
second pressure range by means of the first expander booster and/or
the second expander booster, and is generally 60 to 75 bara. The
air feed gases within the second and third pressure ranges are
required to be capable of exchanging heat with pressurized liquid
oxygen in the main heat exchanger and causing same to evaporate and
vaporize, and therefore, the specific pressure thereof is
determined by the pressure of the liquid oxygen that needs to be
vaporized.
[0026] The first nitrogen product pressure refers to the pressure
of the pure nitrogen extracted from the top of the first tower or
medium pressure tower, and is generally 4 to 5 bara. According to
customer requirements, the pure nitrogen with the first nitrogen
product pressure can be expanded and decompressed to obtain the
second nitrogen product pressure, which is generally about 1.1
bara; alternatively, the pure nitrogen with the first nitrogen
product pressure can be pressurized by means of the nitrogen
booster to obtain the third nitrogen product pressure, which is
generally greater than 7 bara. The second and third nitrogen
product pressures can both be flexibly determined according to
customer requirements.
[0027] The Rahman's principle points out that when the upper tower
or the low pressure tower is used to produce pure oxygen, the
rectification capacity of the low pressure tower is not fully
utilized. In the present invention, one or more of the following
measures are used to improve this situation, in order to increase
the efficiency of the entire air separation system, reduce energy
consumption, and even reduce the volume of the tower. One of the
measures is to introduce part of the air feed gas directly into the
upper tower, i.e., the low pressure tower, so as to utilize the
excess rectification capacity of this tower; a second one of the
measures is to draw the pure nitrogen produced at the top of the
medium pressure tower as a nitrogen product, and correspondingly,
the amount of pure liquid nitrogen obtained after condensation by
the main condenser will be reduced, that is, the amount of the
reflux liquid sent to the low pressure tower will be reduced. On
the one hand, the reduction of the reflux liquid will make further
use of the rectification capacity of the low pressure tower; on the
other hand, the reduction of the reflux liquid requires a reduction
in the processing capacity of the low pressure tower, and the
diameter of the low pressure tower can be reduced accordingly,
causing easier transportation. Furthermore, compared with pure
nitrogen with a pressure of about 1 to 2 bara extracted from the
top of the low pressure tower as a nitrogen product, the pressure
of pure nitrogen drawn from the top of the medium pressure tower is
generally 4 to 5 bara. If the pressure of a nitrogen product
required by a customer is greater than 4 to 5 bara, such as 10
bara, the energy consumption for pressurizing the pure nitrogen
extracted from the medium pressure tower to 10 bara is greatly
reduced than that for pressurizing the pure nitrogen extracted from
the low pressure tower to 10 bara. If the pressure of a nitrogen
product required by a customer is less than 4 to 5 bara, such as 1
bara, the pure nitrogen extracted from the medium pressure tower
can be expanded to 1 bara, and the expansion work can be used for
power generation or a shaft-linked expander booster, thereby
reducing the energy consumption of the entire air separation
system.
[0028] As shown in FIG. 1, after air that has been pressurized to 6
bara in a main air compressor 21 is pre-cooled by means of a
pre-cooling system and purified by means of a purification system,
part 101 of the air enters a low pressure main heat exchanger 1 for
indirect heat exchange with medium pressure pure nitrogen 123
resulting from rectification and part of impure nitrogen 121 for
cooling to about -170.degree. C. and is then sent to the lower part
of a first tower 11 for rectification. The other part 102 thereof
is further pressurized to about 52 bara by means of an air booster
22, and then divided into two streams, wherein one stream 103
thereof is pressurized by means of a first expander booster 24 to
become a stream 105 of 58 bara, and all is sent to a second
expander booster 26 for further pressurization to become a stream
106 of 77 bara; and the other stream 104 of 102 is sent to a high
pressure main heat exchanger 2, partially cooled, then extracted
from the middle part, decompressed to 6 bara by means of an air
expander 25, and then also sent to the lower part of the first
tower 11 for rectification. Since the stream 105 that enters the
second expander compressor 26 all comes from the first expander
booster 24, the two form a series. The first expander booster 24
and the second expander booster 26 are respectively linked with a
nitrogen expander 23 and the air expander 25, and absorb the work
done by the expanders. The stream 106 pressurized to 77 bara enters
the high pressure main heat exchanger 2 for indirect heat exchange
with pure liquid oxygen 122, which has been pressurized to 88 bara,
and part of impure liquid nitrogen 121 for condensation into a
liquid while the high pressure pure liquid oxygen 122 evaporates
and vaporizes and is output as a high pressure oxygen product. The
condensed air feed gas is decompressed to 6 bara by means of the
liquid expander 28 and then separated into gas and liquid phases,
one of which is a gaseous stream 107 that is directly sent to the
lower part of the first tower 11, and the other one of which is a
part of liquid stream 108 that is supercooled by means of a
supercooler 3 and then sent to the middle part of the second tower
13.
[0029] The air feed gas introduced into the first tower 11 is
rectified in the first tower to produce oxygen-enriched liquid air
110 at the bottom of the tower and pure nitrogen at the top of the
tower. By means of indirect heat exchange in the main condensing
evaporator 12 with the liquid oxygen produced at the bottom of the
second tower, a part of pure nitrogen is condensed into pure liquid
nitrogen. The part of pure liquid nitrogen mentioned above is used
as a reflux liquid to the first tower. Optionally, a part is sent
to a storage tank as a liquid nitrogen product, and the other part
112 is supercooled and then input into the upper part of the second
tower 13 as a reflux liquid. Furthermore, gases that are
supercooled and input into the second tower also include the
oxygen-enriched liquid air 110, and optionally, the impure liquid
nitrogen 111 extracted from the middle part of the first tower 11
and the part of liquid air feed gas 108. The above-mentioned
streams are throttled and decompressed to about 1.3 to 1.4 bara and
then transported to the second tower 13, and participate in
rectification therein, and then, the impure nitrogen 121 with a
pressure of about 1.3 bara can be extracted from the upper part of
the second tower, while the pure liquid oxygen 122 with a pressure
of about 1.4 bara is obtained at the bottom of the second tower.
When a customer needs a high pressure oxygen product, the pure
liquid oxygen can be pressurized to about 88 bara by means of a
liquid oxygen pump 31, and then evaporated and vaporized by the
high pressure air feed gas 106, 104 in the high pressure main heat
exchanger 2 to obtain the high pressure oxygen product. When a
customer also needs a medium pressure oxygen product, a part of
high pressure liquid oxygen from the liquid oxygen pump can be
throttled and decompressed to obtain medium pressure liquid oxygen
with a pressure of about 30 bara, which is then similarly
evaporated and vaporized by the high pressure air feed gas 106, 104
in the high pressure main heat exchanger 2 to obtain the medium
pressure oxygen product.
[0030] In this embodiment, the only nitrogen product is the medium
pressure pure nitrogen 123 with a pressure of about 5.5 bara drawn
from the top of the first tower 11. When a customer needs both low
pressure and high pressure nitrogen products, the following
operations can be carried out. In the middle part of the low
pressure main heat exchanger 1, part 124 of the medium pressure
pure nitrogen 123, which has been partially reheated, is extracted,
decompressed to a desired pressure, which is referred to as a
second nitrogen product pressure, by means of the nitrogen expander
23, then returned to the main heat exchanger, and completely
reheated to obtain a second nitrogen product. The nitrogen expander
23 is braked by means of the first expander booster 24, thereby
converting the expansion work into energy required for the
compressed air feed gas. After the remaining part of the medium
pressure pure nitrogen 123 is completely reheated by means of the
low pressure main heat exchanger 1, this remaining part can be
output at the first nitrogen product pressure as a first nitrogen
product, or pressurized to a third nitrogen product pressure, as
required by a customer, by means of a nitrogen booster and output
as a third nitrogen product.
[0031] The main difference between the embodiments shown in FIG. 2
and FIG. 1 is the connection relationship between the first
expander compressor 24 and the second expander compressor 26. In
FIG. 2, the two are connected in parallel. Specifically, after air
feed gas that has been pressurized to 6 bara in a main air
compressor 21 is pre-cooled by means of a pre-cooling system and
purified by means of a purification system, part 101 of the air
feed gas enters a low pressure main heat exchanger 1 for indirect
heat exchange with medium pressure pure nitrogen 123 resulting from
rectification and part of impure nitrogen 121 for cooling to about
-170.degree. C. and is then sent to the lower part of a first tower
11 for rectification. The other part 102 thereof is further
pressurized to about 52 bara by means of an air booster 22, and
then divided into three streams, wherein one stream 115 thereof is
pressurized by means of the first expander booster 24 to become a
stream 116 of 76 bara; another stream 117 of 102 is sent to a high
pressure main heat exchanger 2, partially cooled, then extracted
from the middle part, decompressed to 6 bara by means of an air
expander 25, and then also sent to the lower part of the first
tower 11 for rectification; a third stream 118 of 102 is input into
the second expander booster 26 and then also pressurized to form a
stream 119 of 76 bara, which is mixed with the stream 116, then
enters the high pressure main heat exchanger 2 for indirect heat
exchange with pure liquid oxygen 122, which has been pressurized to
88 bara, and part of impure liquid nitrogen 121 for partial
condensation into a liquid while the high pressure pure liquid
oxygen 122 evaporates and vaporizes and is output as a high
pressure oxygen product. Since the respective streams that enter
the first and second expander compressors both come from a
pressurizing end of the air booster 22, the two form a parallel
connection. The condensed air feed gas 120 is decompressed to 6
bara by means of a liquid expander 28 and can be then separated
into two streams by means of a gas-liquid separator, one of which
is a gaseous stream 107 that is directly sent to the lower part of
the first tower 11, and the other one of which is a liquid stream
108 that is supercooled by means of a supercooler 3 and then sent
to the middle part of the second tower 13. The first expander
booster 24 and the second expander booster 26 are respectively
linked with a nitrogen expander 23 and the air expander 25, and
absorb the work done by the expanders.
[0032] The rest of the embodiment shown in FIG. 2 is the same as
that of the embodiment shown in FIG. 1. Both are examples for the
implementation of the present invention, but do not limit the
spirit and scope of the present invention in any way.
[0033] The following simulation calculations compare the operating
costs of the air separation apparatus. FIG. 2 shows a process flow
as an embodiment according to the present invention, and FIG. 3
shows a process flow as a comparative example. In FIG. 3, as
compared with FIG. 2, the upper part of a second tower is provided
with a pure nitrogen tower 14, and low pressure pure nitrogen 140
with a pressure of about 1.3 bara is directly extracted from the
top of the pure nitrogen tower. The low pressure pure nitrogen 140
is reheated by means of a supercooler 3 and a main heat exchanger 1
and then output as a second nitrogen product. In FIG. 3, medium
pressure pure nitrogen 123 with a pressure of about 5.5 bara is
still extracted from the top of a first tower; however, all of this
stream is reheated by means of the main heat exchanger 1 and then
used as a first nitrogen product, or alternatively, part of this
stream is further pressurized and then output as a third nitrogen
product. Correspondingly, the comparative example of FIG. 3 does
not have a nitrogen expander 23 and a first expander booster 24 in
linkage therewith. Specifically, after air feed gas that has been
pressurized to 6 bara in a main air compressor 21 is pre-cooled by
means of a pre-cooling system 301 and purified by means of a
purification system 302, part 101 of the air feed gas enters a low
pressure main heat exchanger 1 for indirect heat exchange with
medium pressure pure nitrogen 123 resulting from rectification and
part of impure nitrogen 121 and low pressure pure nitrogen 140 for
cooling to about -170.degree. C. and is then sent to the lower part
of a first tower 11 for rectification. The other part 102 thereof
is further pressurized to about 51 bara by means of an air booster
22, and then divided into two streams, wherein one stream 131 is
sent to a high pressure main heat exchanger 2, partially cooled,
then extracted from the middle part, and decompressed by means of
an air expander 25 to become a stream 132 of 6 bara, which is then
also sent to the lower part of the first tower 11 for
rectification; and the other stream 133 is pressurized to 76 bara
by means of a second expander booster 26, then enters the high
pressure main heat exchanger 2 for indirect heat exchange with pure
liquid oxygen 122, which has been pressurized to 88 bara, and part
of impure liquid nitrogen 121 for partial condensation into a
liquid while the high pressure pure liquid oxygen 122 evaporates
and vaporizes and is output as a high pressure oxygen product. The
condensed air feed gas is decompressed to 6 bara by means of a
liquid expander 28 and can be then separated into two streams by
means of a gas-liquid separator, one of which is a gaseous stream
107 that is directly sent to the lower part of the first tower 11,
and the other one of which is a liquid stream 108 that is
supercooled by means of a supercooler 3 and then sent to the middle
part of the second tower 13. The rest of this comparative example
is the same as that of the embodiment shown in FIG. 2.
[0034] The simulation calculations listed in the following table
are carried out using ASPEN software for an air separation system
with an oxygen output of 100,000 Nm.sup.3/h. In the air separation
system, a main heat exchanger is involved which is of an aluminum
plate-fin type, and a main air compressor (MAC) and an air booster
(BAC) are involved which are both steam turbines driven by high
pressure steam. The calculation of the operating cost is based on a
high pressure steam price of 100 RMB/ton and is evaluated based on
5 years of operation.
TABLE-US-00001 TABLE 1 Comparison of operating costs Comparative
Category Unit example Embodiment Recovery of O.sub.2 % 99.65%
99.00% Medium pressure pure Nm.sup.3/h 13000 40000 nitrogen
extraction Low pressure pure Nm.sup.3/h 27000 None nitrogen
extraction Flow rate of conversion of Nm.sup.3/h 0 27000 medium
pressure pure nitrogen to low pressure pure nitrogen Power of MAC
KW 40,339 40,587 Power of BAC KW 20,307 19,303 Total power of MAC +
KW 60,646 59,890 BAC .DELTA. (Total power of MAC + KW 0.0 -756 BAC)
High pressure steam Ton/hour 200.3 197.7 consumption .DELTA. (High
pressure steam Ton/hour 0.0 -2.569 consumption) Operating costs
(total of 5 Mrmb 801.2 790.9 years) .DELTA. (Operating costs (total
Mrmb 0.0 -10.3 of 5 years))
[0035] Due to the extraction of additional 27000 Nm.sup.3/h of
medium pressure pure nitrogen, the recovery of O.sub.2 obtained
according to the present invention is slightly lower than that of
the comparative example; however, the loss here is much less than
the overall energy saving achieved by the present invention. In the
above table, the "medium pressure pure nitrogen extraction" refers
to the flow rate of the medium pressure pure nitrogen extracted
from the top of the first tower, the "low pressure pure nitrogen
extraction" refers to the flow rate of the low pressure pure
nitrogen extracted from the top of the pure nitrogen tower, and the
"flow rate of conversion of medium pressure pure nitrogen to low
pressure pure nitrogen" refers to the flow rate of the part of
medium pressure pure nitrogen that is drawn from the middle part of
the main heat exchanger and sent to the nitrogen expander 23. In
the present invention, due to the utilization of the work done by
the nitrogen expander, the work required for producing the air feed
gas with substantially the same pressure and flow rate by the air
booster (BAC) is reduced, and the corresponding high pressure steam
consumed is also reduced. Based on a total of five years, an
operating cost of about 10 million can be saved.
[0036] While the invention has been described in conjunction with
specific embodiments thereof, it is evident that many alternatives,
modifications, and variations will be apparent to those skilled in
the art in light of the foregoing description. Accordingly, it is
intended to embrace all such alternatives, modifications, and
variations as fall within the spirit and broad scope of the
appended claims. The present invention may suitably comprise,
consist or consist essentially of the elements disclosed and may be
practiced in the absence of an element not disclosed. Furthermore,
if there is language referring to order, such as first and second,
it should be understood in an exemplary sense and not in a limiting
sense. For example, it can be recognized by those skilled in the
art that certain steps can be combined into a single step.
[0037] The singular forms "a", "an" and "the" include plural
referents, unless the context clearly dictates otherwise.
[0038] "Comprising" in a claim is an open transitional term which
means the subsequently identified claim elements are a nonexclusive
listing (i.e., anything else may be additionally included and
remain within the scope of "comprising"). "Comprising" as used
herein may be replaced by the more limited transitional terms
"consisting essentially of" and "consisting of" unless otherwise
indicated herein.
[0039] "Providing" in a claim is defined to mean furnishing,
supplying, making available, or preparing something. The step may
be performed by any actor in the absence of express language in the
claim to the contrary.
[0040] Optional or optionally means that the subsequently described
event or circumstances may or may not occur. The description
includes instances where the event or circumstance occurs and
instances where it does not occur.
[0041] Ranges may be expressed herein as from about one particular
value, and/or to about another particular value. When such a range
is expressed, it is to be understood that another embodiment is
from the one particular value and/or to the other particular value,
along with all combinations within said range.
[0042] All references identified herein are each hereby
incorporated by reference into this application in their
entireties, as well as for the specific information for which each
is cited.
* * * * *