U.S. patent application number 15/004210 was filed with the patent office on 2017-07-27 for method and system for providing auxiliary refrigeration to an air separation plant.
The applicant listed for this patent is Yang Luo, Zhengrong Xu. Invention is credited to Yang Luo, Zhengrong Xu.
Application Number | 20170211881 15/004210 |
Document ID | / |
Family ID | 56852442 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170211881 |
Kind Code |
A1 |
Xu; Zhengrong ; et
al. |
July 27, 2017 |
METHOD AND SYSTEM FOR PROVIDING AUXILIARY REFRIGERATION TO AN AIR
SEPARATION PLANT
Abstract
A method and system for cryogenic air separation that employs
both a primary refrigeration circuit and an auxiliary refrigeration
circuit is provided. The auxiliary refrigeration circuit is
configured in a manner that it can be easily tied-in or modified to
an existing air separation plant.
Inventors: |
Xu; Zhengrong; (East
Amherst, NY) ; Luo; Yang; (Amherst, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Xu; Zhengrong
Luo; Yang |
East Amherst
Amherst |
NY
NY |
US
US |
|
|
Family ID: |
56852442 |
Appl. No.: |
15/004210 |
Filed: |
January 22, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2200/20 20130101;
F25J 3/0429 20130101; F25J 3/04393 20130101; F25J 2245/40 20130101;
F25J 3/0409 20130101; F25J 3/04303 20130101; F25J 3/04412 20130101;
F25J 3/04812 20130101; F25J 2230/20 20130101; F25J 3/04296
20130101; F25J 3/04381 20130101; F25J 3/04678 20130101; F25J
3/04969 20130101; F25J 3/04278 20130101; F25J 3/04218 20130101 |
International
Class: |
F25J 3/04 20060101
F25J003/04 |
Claims
1. A method of separating air in an air separation unit comprising
a main heat exchanger configured to cool a compressed and purified
feed air stream to a temperature suitable for the rectification and
a distillation column system configured to rectify the compressed,
purified and cooled air stream to produce at least one liquid
product stream, the method comprising the steps of: compressing and
purifying a feed air stream to produce the compressed and purified
feed air stream; diverting a first portion of the compressed and
purified feed air stream to a first refrigeration circuit
configured to produce a first cooled refrigeration stream;
diverting a second portion of the compressed and purified feed air
stream to the main heat exchanger to cool the second portion of the
compressed and purified feed air stream and wherein the cooled
second portion of the compressed and purified feed air stream is
subsequently directed to the higher pressure column of the
distillation column system; diverting a third portion of the
compressed and purified feed air stream to a booster air
compression circuit configured to produce a further compressed feed
air stream and wherein part of the further compressed feed air
stream is directed to the main heat exchanger where the further
compressed feed air stream is cooled to produce a liquid air stream
that is directed to the distillation column system; diverting a
fraction of the further compressed feed air stream from the booster
air compression circuit to an auxiliary refrigeration circuit
configured to produce a second refrigeration stream, the auxiliary
refrigeration circuit comprising a second turbo-expander and an
auxiliary heat exchanger; diverting a fourth portion of the
compressed and purified feed air stream to the auxiliary heat
exchanger; diverting part of the first refrigeration stream from
the first refrigeration circuit to the auxiliary heat exchanger and
warming the diverted portion of the first refrigeration stream in
the auxiliary heat exchanger via indirect heat exchange with
diverted fourth portion of the compressed and purified feed air
stream; directing the fourth portion of the compressed and purified
feed air stream exiting the auxiliary heat exchanger to the
distillation column system; directing a remaining portion of the
first refrigeration stream to a lower pressure column of the
distillation column system to impart a first portion of the
refrigeration required by the distillation column system; and
directing the cooled second refrigeration stream to the higher
pressure column of the distillation column system to impart a
second portion of the refrigeration required by the distillation
column system.
2. The method of claim 1 further comprising the steps of: further
compressing the first portion of the compressed and purified feed
air stream within the first refrigeration circuit; cooling the
further compressed first portion of the compressed and purified
feed air stream; and expanding the further compressed first portion
of the compressed and purified feed air stream in a first
turbo-expander disposed within the first refrigeration circuit to
produce the first refrigeration stream.
3. The method of claim 2 wherein the step of cooling the further
compressed first portion of the compressed and purified feed air
stream further comprises cooling the further compressed first
portion of the compressed and purified feed air stream in an
aftercooler.
4. The method of claim 2 wherein the step of cooling the further
compressed first portion of the compressed and purified feed air
stream further comprises partially cooling the further compressed
first portion of the compressed and purified feed air stream in the
main heat exchanger.
5. The method of claim 1 wherein the step of directing the cooled
fourth portion of the compressed and purified feed air stream to
the distillation column system further comprises directing the
cooled fourth portion of the compressed and purified feed air
stream to the higher pressure column of the distillation column
system.
6. The method of claim 1 further comprising the steps of: diverting
a portion of the second refrigeration stream from the auxiliary
refrigeration circuit to the first refrigeration circuit; and
combining the diverted portion of the second refrigeration stream
with the first portion of the compressed and purified feed air
stream in the first refrigeration circuit.
7. The method of claim 6 further comprising the steps of: diverting
a portion of the second refrigeration stream that is partially
cooled from the auxiliary heat exchanger in the auxiliary
refrigeration circuit to the first refrigeration circuit; and
combining the diverted portion of the second refrigeration stream
with the first portion of the compressed and purified feed air
stream in the first refrigeration circuit upstream of the
turbo-expander.
8. The method of claim 1 wherein the step of diverting a fraction
of the further compressed feed air stream from the booster air
compression circuit to the auxiliary refrigeration circuit further
comprises: further compressing the third portion of the compressed
and purified feed air stream in a plurality of compression stages;
and diverting a first fraction of the third portion of the
compressed and purified feed air stream from an interstage location
of the plurality of compression stages to the auxiliary
refrigeration circuit.
9. The method of claim 1 wherein the step of diverting a fraction
of the further compressed feed air stream from the booster air
compression circuit to the auxiliary refrigeration circuit further
comprises: further compressing the third portion of the compressed
and purified feed air stream in a plurality of compression stages;
and diverting one or more fractions of the third portion of the
compressed and purified feed air stream from one or more interstage
locations of the plurality of compression stages to the auxiliary
refrigeration circuit; controlling the flow of the diverted one or
more fractions of the third portion of the compressed and purified
feed air stream with one or more flow control valves disposed
between the booster air compression circuit and the second
turbo-expander in the auxiliary refrigeration circuit; wherein the
inlet pressure to the second turbo-expander in the auxiliary
refrigeration circuit is controlled by adjusting the one or more
flow control valves which in turn controls the second portion of
the refrigeration required by the distillation column system.
10. The method of claim 1 wherein the auxiliary refrigeration
circuit further comprises an auxiliary compressor, the second
turbo-expander and the auxiliary heat exchanger and wherein the
method further comprises the steps of: diverting the fraction of
the further compressed feed air stream from the booster air
compression circuit to the auxiliary compressor; further
compressing the diverted fraction of the compressed feed air stream
from the booster air compression circuit; partially cooling the
further compressed diverted fraction in the auxiliary heat
exchanger via indirect heat exchange with the diverted portion of
the first refrigeration stream; expanding the partially cooled
further compressed diverted fraction in the second turbo-expander;
further cooling the expanded diverted fraction in the auxiliary
heat exchanger via indirect heat exchange with the diverted portion
of the first refrigeration stream to produce the cooled second
refrigeration stream; and directing the cooled second refrigeration
stream to the higher pressure column of the distillation column
system to impart the second portion of the refrigeration required
by the distillation column system.
11. An air separation unit configured to produce at least one
liquid product stream, the air separation unit comprising: an
incoming air compression and purification train configured to
produce a compressed and purified feed air stream; a primary
refrigeration circuit having a first turbo-expander, the primary
refrigeration circuit operatively coupled to the incoming air
compression and purification train and configured to receive a
first portion of the compressed and purified feed air stream and
expand the first portion of the compressed and purified feed air
stream in the first turbo-expander to produce a first cooled
refrigeration stream; a main heat exchanger operatively coupled to
the incoming air compression and purification train and configured
to receive a second portion of the compressed and purified feed air
stream and to cool the second portion of the compressed and
purified feed stream to a temperature suitable for the
rectification of the compressed and purified feed air stream; a
booster air compression circuit operatively coupled to the incoming
air compression and purification train and the main heat exchanger,
the booster air compression circuit configured to receive a third
portion of the compressed and purified feed air stream, further
compress the third portion and direct the further compressed third
portion to the main heat exchanger to produce a liquid air stream;
a second turbo-expander configured to receive a fraction of the
further compressed third portion and expand the fraction of the
further compressed third portion to produce a second refrigeration
stream; and an auxiliary heat exchanger operatively coupled to the
incoming air compression and purification train, the booster air
compression circuit and the primary refrigeration circuit, the
auxiliary heat exchanger configured to receive a fourth portion of
the compressed and purified feed air stream and cool the fourth
portion of the compressed and purified feed air stream via indirect
heat exchange with the second refrigeration stream and a diverted
portion of the first refrigeration stream; a distillation column
system operatively coupled to the primary refrigeration circuit,
the booster air compression circuit and the auxiliary heat
exchanger, the distillation column system configured to rectifying
some or all of the first refrigeration stream, the second
refrigeration stream, the liquid air stream, and the cooled second
portion of the compressed and purified feed air stream by a
cryogenic rectification process to produce the at least one liquid
product stream.
12. The air separation unit of claim 11 wherein the primary
refrigeration circuit further comprises a compressor configured for
further compressing the first portion of the compressed and
purified feed air stream within the primary refrigeration circuit;
and wherein the compressor is operatively coupled to the main heat
exchanger such that the further compressed the first portion of the
compressed and purified feed air stream is partially cooled in the
main heat exchanger.
13. The air separation unit of claim 11 wherein the cooled fourth
portion of the compressed and purified feed air stream exiting the
auxiliary heat exchanger is directed to a higher pressure column of
the distillation column system.
14. The air separation unit of claim 11 further comprising a
recycle circuit connecting the auxiliary heat exchanger with the
primary refrigeration circuit wherein a portion of the second
refrigeration stream is recycled to the first refrigeration
circuit.
15. The air separation unit of claim 14 wherein the portion of the
second refrigeration stream is recycled to the first refrigeration
circuit is partially cooled within the auxiliary heat exchanger and
is recycled to a location in the first refrigeration circuit
upstream of the first turbo-expander.
16. The air separation unit of claim 11 further comprising an
auxiliary refrigeration circuit that includes an auxiliary
compressor configured to receive the fraction of the further
compressed feed air stream diverted from the booster air
compression circuit, the second turbo-expander configured to
receive a compressed air stream from the auxiliary compressor and
expand the compressed air stream, and the auxiliary heat exchanger
configured to receive the expanded air stream from the second
turbo-expander.
17. The air separation unit of claim 11 wherein the booster air
compression circuit further comprises a plurality of compression
stages and a diversion circuit for diverting one or more fractions
of the further compressed feed air stream from one or more
interstage locations of the plurality of compression stages to the
auxiliary refrigeration circuit.
18. The air separation unit of claim 11 further comprising one or
more flow control valves disposed between the booster air
compression circuit and the second turbo-expander in the auxiliary
refrigeration circuit and configured for controlling the flow of
the diverted one or more fractions of the third portion of the
compressed and purified feed air stream.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and system for
cryogenic air separation involving production of liquid products by
using an integrated refrigeration system comprising a primary
refrigeration circuit and an auxiliary refrigeration circuit. More
particularly, the present invention relates to an auxiliary
refrigeration circuit that can be easily tied-in to an existing
cryogenic air separation plant and its existing refrigeration
system.
BACKGROUND
[0002] Oxygen, nitrogen and argon are separated from air through
cryogenic rectification in an air separation plant. Typically,
gaseous and/or liquid products are produced for on-site customers
or pipeline customers, with any excess products often converted to
merchant liquid products for nearby customers. For some cryogenic
air separation plants, the on-site or pipeline customer demand for
gaseous products, such as gaseous oxygen or gaseous nitrogen, may
decrease over time either on a long-term basis or perhaps on a more
temporary or mid-term basis. To satisfy the lower gaseous product
requirements, the cryogenic air separation plant may be operated so
as to vent some of the unneeded gaseous product which is
economically inefficient as such venting ultimately wastes the
power/energy costs used to produce the vented gaseous products.
Alternatively, the air separation plant may be operated in a
turn-down mode which produces less gaseous product but at less than
full plant capacity and separation efficiency. A third option is to
adjust the product slate of the cryogenic air separation plant to
produce more liquid products in lieu of the lowered gaseous product
requirement.
[0003] There have been numerous prior art cryogenic air separation
processes designed to address this third option of making
additional liquid products to offset decreased requirements of
gaseous products. See for example, U.S. Pat. Nos. 6,125,656;
6,666,048; 6,945,076; and 8,397,535; as well as United States
Patent Application Publication Nos. 2010-0058805; 2013-0192301;
2007-0101763; and European Patent Publication EP1544559 A1. As seen
in these prior art references, refrigeration must be supplied to
offset ambient heat leakage, warm end heat exchange losses and to
allow the extraction or production of the liquid products,
including liquid oxygen, liquid nitrogen, or liquid argon from one
or more air separation units. The conventional or main source of
refrigeration for a cryogenic rectification plant is typically
supplied by a turbine-based refrigeration system capable of
expanding part of the feed air stream or a waste stream to generate
a refrigeration stream that is then introduced into the main heat
exchanger or the distillation column system of the cryogenic air
separation plant. Supplemental refrigeration required to produce
additional liquid products may be supplied with an additional
turbine-based refrigeration source. Such additional turbine-based
refrigeration systems involve additional capital costs and are
often not optimized or fully integrated with the main source of
refrigeration for a cryogenic air separation plant.
[0004] What is needed, is an improvement to these prior art
supplemental liquid make solutions that allows the additional
liquid make system to be configured as an add-on feature to the air
separation plant that can be easily added to the cryogenic air
separation plant/unit after initial plant construction. Such add-on
supplemental liquid-make feature should be integrated with the main
source of refrigeration for the cryogenic air separation plant and
must also be both efficient and operationally flexible. In other
words, the supplemental or auxiliary refrigeration system should be
capable of and allow the plant to switch easily between a high
liquid make cycle and the original high gaseous product make cycle.
Finally, the add-on supplemental or auxiliary refrigeration system
should be portable, and preferably skid-mounted.
SUMMARY OF THE INVENTION
[0005] The present invention may be characterized as a method of
separating air in an air separation unit. The air separation unit
preferably comprises a main heat exchanger configured to cool a
compressed and purified feed air stream to a temperature suitable
for the rectification and a distillation column system configured
to rectify the compressed, purified and cooled air stream to
produce at least one liquid product stream. In such air separation
unit, the present method comprises the steps of: (a) compressing
and purifying a feed air stream to produce the compressed and
purified feed air stream; (b) diverting a first portion of the
compressed and purified feed air stream to a first refrigeration
circuit configured to produce a first cooled refrigeration stream;
(c) diverting a second portion of the compressed and purified feed
air stream to the main heat exchanger to cool the second portion of
the compressed and purified feed air stream and wherein the cooled
second portion of the compressed and purified feed air stream is
subsequently directed to the higher pressure column of the
distillation column system; (d) diverting a third portion of the
compressed and purified feed air stream to a booster air
compression circuit configured to produce a further compressed feed
air stream and wherein part of the further compressed feed air
stream is directed to the main heat exchanger where the further
compressed feed air stream is cooled to produce a liquid air stream
that is directed to the distillation column system; (e) diverting a
fraction of the further compressed feed air stream from the booster
air compression circuit to an auxiliary refrigeration circuit
configured to produce a second refrigeration stream, the auxiliary
refrigeration circuit comprising a second turbo-expander and an
auxiliary heat exchanger; (f) diverting a fourth portion of the
compressed and purified feed air stream to the auxiliary heat
exchanger; (g) diverting part of the first refrigeration stream
from the first refrigeration circuit to the auxiliary heat
exchanger and warming the diverted portion of the first
refrigeration stream in the auxiliary heat exchanger via indirect
heat exchange with diverted fourth portion of the compressed and
purified feed air stream; (h) directing the fourth portion of the
compressed and purified feed air stream exiting auxiliary heat
exchanger to distillation column system; (i) directing a remaining
portion of the first refrigeration stream to a lower pressure
column of the distillation column system to impart a first portion
of the refrigeration required by the distillation column system;
and (j) directing the cooled second refrigeration stream to the
higher pressure column of the distillation column system to impart
a second portion of the refrigeration required by the distillation
column system.
[0006] The present invention may also be characterized as an air
separation unit configured to produce at least one liquid product
stream. Characterized as such, the air separation unit comprises:
(i) an incoming air compression and purification train configured
to produce a compressed and purified feed air stream; (ii) a
primary refrigeration circuit having a first turbo-expander, the
primary refrigeration circuit operatively coupled to the incoming
air compression and purification train and configured to receive a
first portion of the compressed and purified feed air stream and
expand the first portion of the compressed and purified feed air
stream in the first turbo-expander to produce a first cooled
refrigeration stream; (iii) a main heat exchanger operatively
coupled to the incoming air compression and purification train and
configured to receive a second portion of the compressed and
purified feed air stream and to cool the second portion of the
compressed and purified feed stream to a temperature suitable for
the rectification of the compressed and purified feed air stream;
(iv) a booster air compression circuit operatively coupled to the
incoming air compression and purification train and the main heat
exchanger, the booster air compression circuit configured to
receive a third portion of the compressed and purified feed air
stream, further compress the third portion and direct the further
compressed third portion to the main heat exchanger to produce a
liquid air stream; (v) a second turbo-expander configured to
receive a fraction of the further compressed third portion and
expand the fraction of the further compressed third portion to
produce a second refrigeration stream; (vi) an auxiliary heat
exchanger operatively coupled to the incoming air compression and
purification train, the booster air compression circuit and the
primary refrigeration circuit, the auxiliary heat exchanger
configured to receive a fourth portion of the compressed and
purified feed air stream and cool the fourth portion of the
compressed and purified feed air stream via indirect heat exchange
with the second refrigeration stream and a diverted portion of the
first refrigeration stream; and (vii) a distillation column system
operatively coupled to the primary refrigeration circuit, the
booster air compression circuit and the auxiliary heat exchanger,
the distillation column system configured to rectifying some or all
of the first refrigeration stream, the second refrigeration stream,
the liquid air stream, and the cooled second portion of the
compressed and purified feed air stream by a cryogenic
rectification process to produce the at least one liquid product
stream.
[0007] In some embodiments, the first refrigeration circuit may
include a compressor for further compressing the first portion of
the compressed and purified feed air stream; a cooling means such
as an aftercooler and/or main heat exchanger configured to cool the
further compressed first portion of the compressed and purified
feed air stream; and a first turbo-expander disposed within the
first refrigeration circuit and configured to expand the further
compressed first portion of the compressed and purified feed air
stream to produce the first refrigeration stream. Similarly, the
auxiliary refrigeration circuit may also include an auxiliary
compressor and cooling means.
[0008] Other embodiments contemplate diverting a partially cooled
portion of the second refrigeration stream from the auxiliary
refrigeration circuit to the first refrigeration circuit and
combining the diverted portion with the first portion of the
compressed and purified feed air stream in the first refrigeration
circuit.
[0009] Finally, in some embodiments that employ a multi-stage
compression system within the booster air compression circuit, the
diversion of the fraction of the further compressed feed air stream
to the auxiliary refrigeration circuit preferably includes further
includes diverting one or more fractions of the third portion of
the compressed and purified feed air stream from one or more
interstage locations of the plurality of compression stages to the
auxiliary refrigeration circuit. One or more flow control valves
are disposed between the booster air compression circuit and the
second turbo-expander in the auxiliary refrigeration circuit to
control the flow of the diverted one or more fractions and the
inlet pressure to the second turbo-expander in the auxiliary
refrigeration circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] While the present invention concludes with claims distinctly
pointing out the subject matter that Applicants regard as their
invention, it is believed that the invention will be better
understood when taken in connection with the accompanying drawings
in which:
[0011] FIG. 1 is a schematic process flow diagram of a cryogenic
air separation plant integrated with an add-on supplemental or
auxiliary refrigeration circuit in accordance with the present
invention; and
[0012] FIG. 2 is a schematic process flow diagram of a cryogenic
air separation plant integrated with an alternate embodiment of the
add-on supplemental or auxiliary refrigeration circuit also in
accordance with the present invention.
DETAILED DESCRIPTION
[0013] In reference to FIGS. 1-3, an air separation unit 10
generally includes an incoming air compression and purification
train or circuit (not shown); a primary refrigeration circuit 20; a
booster air compression train or circuit 30; a main heat exchanger
40; and a distillation column system 50.
[0014] In the incoming air purification and compression train or
circuit, the incoming feed air is compressed in a multi-stage,
intercooled, main air compressor arrangement to a pressure that can
be between about 5 bar(a) and about 15 bar(a). This main air
compressor arrangement may be an integrally geared compressor or a
direct drive compressor arrangement. The compressed air feed is
then purified in a pre-purification unit to remove high boiling
contaminants from the incoming feed air. A pre-purification unit,
as is well known in the art, typically contains beds of alumina
and/or molecular sieve operating in accordance with a temperature
and/or pressure swing adsorption cycle in which moisture and other
impurities, such as carbon dioxide, water vapor and hydrocarbons,
are adsorbed.
[0015] As described in more detail below, the compressed and
purified feed air stream 12 is divided into a plurality of portions
which are further compressed and/or cooled. The different portions
of the compressed and purified air stream are then separated into
oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality
of distillation columns that comprise the distillation column
system 50. Preferably, the distillation column system 50 may
include thermally linked higher pressure column 54 and lower
pressure column 56, as well as an optional argon rectification
column 58.
[0016] Prior to such distillation however, portions of the
compressed and purified feed air stream 12 may be further
compressed in a booster air compression train or circuit 30 and/or
cooled to temperatures suitable for rectification within a primary
or main heat exchanger 40. The cooling is typically achieved using
refrigeration from the various oxygen, nitrogen and/or argon
streams produced by the air separation unit 10 as well as
refrigeration generated by one or more refrigeration circuits often
as a result of turbo-expansion of various air streams in an upper
column turbine (UCT) arrangement, a lower column turbine (LCT)
arrangement, and/or a warm recycle turbine (WRT) arrangement as
known to persons skilled in the art.
Air Separation Unit with Primary and Auxiliary Refrigeration
Circuits
[0017] Turning now to FIG. 1, an embodiment of the present
invention is illustrated that includes a plurality of divided
portions of the compressed and purified feed air stream. A first
portion 13 of the compressed and purified feed air stream,
resulting from the compression and pre-purification of the incoming
feed air, is diverted to a first or primary refrigeration circuit
20 shown as an upper column turbine (UCT) arrangement that is
configured to produce a first cooled refrigeration stream 22.
Preferably, within the first or primary refrigeration circuit 20,
the first portion 13 of the compressed and purified feed air stream
is further compressed in compressor 24 and cooled in an aftercooler
25 and/or main heat exchanger 40. The compressed and cooled (or
partially cooled) stream is then expanded in the first
turbo-expander 26 to produce the first refrigeration stream 22. A
portion of the first refrigeration stream is directed to the lower
pressure column while a second portion of the first refrigeration
stream is diverted to the auxiliary or second refrigeration circuit
60 as described in more detail below.
[0018] A second portion 15 of the compressed and purified feed air
stream is directed or diverted to the main heat exchanger 40 to
cool this portion of the compressed and purified feed air stream.
The resulting cooled second portion 42 of the compressed and
purified feed air stream is then directed to the higher pressure
column 54 of the distillation column system 50 as generally known
in the art and practiced in many cryogenic air separation
units.
[0019] In addition, a third portion 17 of the compressed and
purified feed air stream is diverted to a booster air compression
circuit 30 configured to produce a further compressed, high
pressure feed air stream 32. As illustrated, the booster air
compression circuit 30 employs a booster air compressor arrangement
33 having a plurality of compression stages with intercoolers and
aftercoolers 31 and forms a high pressure air stream 32 that is fed
to the main heat exchanger 40. The high pressure air stream forms a
liquid phase or a dense fluid if its pressure exceeds the critical
pressure after cooling in the main heat exchanger. This liquid air
stream 34 is then split into two portions 35, 36, with a first
portion 35 being directed through an expansion valve 37 and into
the higher pressure column 54 of the distillation column system 50
and a second portion 36 is expanded through another expansion valve
38 and introduced into the lower pressure column 56 of distillation
column system 50.
[0020] As seen in FIG. 1, a fraction 62A, 62B of the third portion
17 of the compressed and purified feed air stream is further
diverted from the booster air compression circuit 30 to an
auxiliary refrigeration circuit 60 configured to produce a second
refrigeration stream 66. The auxiliary refrigeration circuit 60
preferably includes an auxiliary compressor 63, a second
turbo-expander 64, and an auxiliary heat exchanger 65. This
fraction 62A, 62B of the further compressed feed air stream from
the booster air compression circuit 30 is diverted via one or more
flow control valves 67A, 67B, to the auxiliary compressor 63 where
the diverted fraction stream is further compressed (as stream 61),
optionally cooled or partially cooled and then expanded in a
turbo-expander 64. After expansion in the turbo-expander 64, the
diverted fraction stream is then cooled in the auxiliary heat
exchanger 65 via indirect heat exchange with one or more cooling
streams, preferably a diverted portion of the first refrigeration
stream 28, to produce the cooled second refrigeration stream 66
exiting the auxiliary heat exchanger 65 and a warmed stream 29. The
cooled second refrigeration stream 66 is then combined with the
cooled second portion 34 of the compressed and purified feed air
stream and the resulting combined stream 68 is then directed to the
higher pressure column 54 to impart another or second portion of
the refrigeration required by the distillation column system 50. As
briefly discussed above, part of the first refrigeration stream 22
is preferably diverted as a cooling stream 28 to the auxiliary heat
exchanger 65 where it cools the diverted fraction 62A, 62B of the
further compressed feed air stream in the auxiliary refrigeration
circuit 60. The remaining portion of the first refrigeration stream
22 is directed to the lower pressure column 56 to impart a portion
of the refrigeration required by the distillation column system 50.
In this arrangement the supplemental refrigeration created by the
expansion of the first portion 13 of the compressed and purified
air stream in the first or primary refrigeration circuit 20 is thus
imparted partly to the lower pressure column 56 and partly to the
auxiliary heat exchanger 65 thereby alleviating some of the cooling
duty of the primary heat exchanger 40.
[0021] The present embodiment also shows a fourth portion 19 of the
compressed and purified feed air stream that may also be diverted
from the incoming air purification and compression circuit (not
shown) as a carrier fluid to the auxiliary heat exchanger 65 where
it is cooled and subsequently directed to the higher pressure
column 54 of the distillation column system 50 so as to capture the
auxiliary refrigeration. As illustrated, this cooled fourth portion
69 of the compressed and purified feed air stream may be combined
with the warmed second refrigeration stream 66 and/or the cooled
second portion 42 of the compressed and purified feed air stream
exiting the main heat exchanger 40 with the resulting combined
stream 68 then directed to the higher pressure column 54.
[0022] In a preferred embodiment, the first portion of the
compressed and purified feed air stream directed to the primary
refrigeration circuit represents roughly 8% to 20% of the incoming
feed air stream. Of this first portion, 0% to 12% of the incoming
feed air stream is diverted as the second portion to the auxiliary
heat exchanger to balance the temperatures in the auxiliary heat
exchanger. Varying the amount of diverted air from the first
refrigeration circuit to the auxiliary refrigeration circuit
enables the air separation unit to readily switch between a high
gaseous product make cycle and a high liquid product make
cycle.
[0023] The third portion of the compressed and purified feed air
stream represents roughly 25% to 32% of the incoming feed air
stream with roughly 5% to 10% of the incoming feed air stream being
diverted to the auxiliary refrigeration circuit.
[0024] The second portion and fourth portion of the compressed and
purified feed air stream combined represents the remainder roughly
of the incoming feed air stream 48% to 67% of the incoming feed air
stream. The exact split between the second portion and fourth
portion of the compressed and purified feed air stream depends on
the heat exchange duties in the main heat exchanger and auxiliary
heat exchanger.
[0025] The main heat exchanger 40 and auxiliary heat exchanger 65
are preferably a brazed aluminum plate-fin type heat exchanger.
Such heat exchangers are advantageous due to their compact design,
high heat transfer rates and their ability to process multiple
streams. They are manufactured as fully brazed and welded pressure
vessels. The brazing operation involves stacking corrugated fins,
parting sheets and end bars to form a core matrix. The matrix is
placed in a vacuum brazing oven where it is heated and held at
brazing temperature in a clean vacuum environment. For small
plants, a heat exchanger comprising a single core may be
sufficient. For higher flows, a heat exchanger may be constructed
from several cores which may be connected in parallel or
series.
[0026] The turbo-expanders 26 and 64 are preferably linked with
booster air compressors 24 and 63 respectively, either directly or
by appropriate gearing. Although not shown, the turbo-expanders may
also to be connected or operatively coupled to a generator. Such
generator loaded turbo-expander arrangement allows the speed of the
turbo-expander to be maintained constant even at very high or low
loads. This arrangement is desirable in some applications because
the speed of the turbo-expander would remain generally constant at
the ideal efficiency across the entire operating envelope. In such
arrangements, the generator load may be connected to the
turbo-expander by means of a high speed generator. Alternatively,
the generator load may be connected to the turbo-expander by means
of a high speed coupling connected to an internal or external
gearbox and with a low speed coupling from the gearbox to the
generator.
[0027] The distillation column system 50 preferably includes a
thermally linked higher pressure column 54 and lower pressure
column 56 as well as an optional argon rectification column 58.
Within the columns, vapor and liquid are counter-currently
contacted in order to affect a gas/liquid mass-transfer based
separation of the respective feed streams. Such columns will
preferably employ structured packing or trays or combinations
thereof. The higher pressure column 54 typically operates in the
range from between about 20 bar(a) to about 60 bar(a) whereas the
lower pressure column 56 typically operates at pressures between
about 1.1 bar(a) to about 1.5 bar(a).
[0028] As indicated above, the higher pressure column 54 and the
lower pressure column 56 are linked in a heat transfer relationship
such that a nitrogen-rich vapor column overhead, extracted from the
top of higher pressure column as a stream 71, is condensed within a
main condenser-reboiler 55 located in the base of lower pressure
column 56 against boiling an oxygen-rich liquid column bottoms 72.
The boiling of oxygen-rich liquid column bottoms 72 initiates the
formation of an ascending vapor phase within lower pressure column
56. The condensation produces a liquid nitrogen containing stream
73 that is divided into streams 74 and 75 that reflux the higher
pressure column 54 and the lower pressure column 56, respectively
to initiate the formation of descending liquid phases in such
columns. If liquid nitrogen product is required, stream 76 may also
be recovered.
[0029] Streams 34, 66, and 69 are introduced into the higher
pressure column 54 along with the expanded liquid air stream 39 for
rectification by contacting an ascending vapor phase of such
mixture within a plurality of mass transfer contacting elements
with a descending liquid phase that is initiated by reflux stream
74. This produces a crude liquid oxygen column bottoms 77, also
known as kettle liquid and the nitrogen-rich column overhead 78. A
stream 79 representing a portion of the nitrogen-rich column
overhead 78 may be directed to the main heat exchanger 40 to
provide refrigeration to the feed air streams. In addition, a
stream 101 of the crude liquid oxygen column bottoms 77 may be
directed to the argon column 58 to as a reflux to aid in the
recovery of argon product 93. Alternatively, although not shown, a
stream of the crude liquid oxygen column bottoms may be expanded in
an expansion valve to the pressure at or near that of the lower
pressure column and introduced into the lower pressure column for
further rectification.
[0030] Lower pressure column 56 is also provided with a plurality
of mass transfer contacting elements that can be trays or
structured packing or random packing or other known elements in the
art of cryogenic air separation. As stated previously, the
separation produces an oxygen-rich liquid 80 and a nitrogen-rich
vapor column overhead 82 that is extracted as a nitrogen product
stream 84. Additionally, a waste stream 85 is also extracted to
control the purity of nitrogen product stream 84. Both nitrogen
product stream 84 and waste stream 85 are passed through a
subcooling unit 90 designed to subcool the reflux stream 75. A
portion of the reflux stream may optionally be taken as a liquid
product stream 76 and the remaining portion (shown as stream 75B)
may be introduced into lower pressure column 56 after passing
through expansion valve 99.
[0031] After passage through subcooling unit 90, nitrogen vapor
product stream 84 and waste stream 85 are fully warmed within main
heat exchanger 40 to produce a warmed nitrogen product stream 94
and a warmed waste stream 95. Although not shown, the warmed waste
stream 95 may be used to regenerate the adsorbents within
pre-purification unit. In addition, an oxygen-rich liquid stream 80
is extracted from the oxygen-rich liquid column bottoms 72 near the
bottom of the lower pressure column 56. Oxygen-rich liquid stream
80 can be pumped by a pump 83 to form a pumped product stream as
illustrated by pumped liquid oxygen stream 86. Part of the pumped
liquid oxygen stream 86 can optionally be taken directly as a
liquid oxygen product stream 88, with the remainder, namely stream
87, being directed to the main heat exchanger 40 where it is warmed
and vaporized to produce a pressurized oxygen product stream 97.
Although only one such stream is shown, there could be a plurality
of such streams that are fed into the main heat exchanger 40.
Pumped liquid oxygen stream 86 can be pressurized to above or below
the critical pressure so that oxygen product stream 97 when
discharged from main heat exchanger 40 will be a supercritical
fluid. Alternatively, the pressurization of pumped liquid oxygen
stream 86 could be lower to produce an oxygen product stream 97 in
a vapor form.
[0032] Turning now to the embodiment illustrated in FIG. 2, there
is shown an alternate embodiment of the add-on supplemental or
auxiliary refrigeration circuit 60. FIG. 2 differs from FIG. 1 in
that all or a portion of the partially warmed, expanded working
fluid 27 in the auxiliary refrigeration circuit 60 is recycled back
to the first refrigeration circuit 20 at a location upstream of the
first turbo-expander 26. In this manner, the working fluid 27
undergoes two stages of expansion in a serial arrangement. In other
words, the turbo-expander 64 of the auxiliary refrigeration circuit
60 is arranged in series with the turbo-expander 26 of the first
refrigeration circuit 20 with the resulted expanded working fluid
being directed to the lower pressure column 56 and/or the auxiliary
heat exchanger 65.
[0033] Another difference between the embodiment shown in FIG. 2
and that of FIG. 1 is found in the auxiliary refrigeration circuit
60. In the embodiment of FIG. 2, all or a portion of the diverted
fraction stream may optionally bypass the auxiliary compressor 63
and go directly to the second turbo-expander 64 and on to the
auxiliary heat exchanger 65. When flow control valve 67C is open
and flow control valve 67D is closed, the combined streams 62A and
62B are further compressed in auxiliary compressor 63, then
expanded in second turbo-expander 64 and warmed in auxiliary heat
exchanger 65. Conversely, when flow control valve 67C is closed and
flow control valve 67D is open, the combined working fluid streams
62A and 62B bypass the auxiliary compressor 63 and directed to the
second turbo-expander 64 and then warmed in auxiliary heat
exchanger 65. This arrangement allows for adjusting the pressure of
the working fluid in the auxiliary refrigeration circuit 60.
Integrating the Auxiliary Refrigeration Circuit with the Air
Separation Unit
[0034] As indicated above, air separation unit 10 is capable of
producing liquid products, namely, nitrogen-rich liquid stream 76
and liquid oxygen product stream 88. In order to increase the
production of such liquid products, additional refrigeration is
supplied by an add-on or auxiliary refrigeration circuit. In the
presently disclosed air separation unit or air separation plant,
the add-on refrigeration circuit is the auxiliary refrigeration
circuit 60 that is preferably configured to be added to or bolted
on the cryogenic air separation unit 10 after initial plant
construction. Thus, the design of the auxiliary refrigeration
circuit 60 is tailored for such late add-on or retrofit application
and the tie-in points to the cryogenic air separation unit 10 are
minimized.
[0035] In the illustrated embodiments, there are four or five key
tie-in points between the cryogenic air separation unit 1 and
auxiliary or second refrigeration circuit 60. The first tie-in
point 110 preferably occurs downstream of the main air compression
train or circuit where the fourth portion 19 of the compressed and
purified feed air stream 12 is diverted to the auxiliary or second
refrigeration circuit, and more particularly, to the auxiliary heat
exchanger 65. This first tie in point 110 is configured to provide
the carrier fluid (i.e. compressed and purified air) to which the
auxiliary refrigeration from the auxiliary refrigeration circuit 60
is provided.
[0036] The second tie-in point 120 is within the booster air
compression circuit 30 and is configured to divert a fraction of
the further compressed third portion of the compressed and purified
stream as compressed stream s 62A, 62B to the auxiliary
refrigeration circuit 60. This second tie in point 110 provides a
working fluid (i.e. boosted compressed air) that is to be expanded
to provide a portion of the auxiliary refrigeration from the
auxiliary refrigeration circuit 60.
[0037] The third tie-in point 130 is located within the
distillation column system 50 and is configured to return the
cooled carrier fluid 69 (i.e. compressed and purified air) as well
as the warmed working fluid 66 (i.e. fully warmed, expanded working
fluid) to the higher pressure column 54.
[0038] The fourth tie-in point 140 is located within the first
refrigeration circuit 20 and is configured to divert a portion 28
of the first refrigeration stream 22 to the auxiliary refrigeration
circuit 60 where it provides further cooling or refrigeration to
the carrier stream 19 via indirect heat exchange in the auxiliary
heat exchanger 65.
[0039] A fifth tie in point 150 is also required in the embodiment
shown in FIG. 2. This fifth tie-in point 150 is also located within
the first refrigeration circuit 20 and configured to return a
portion of the partially warmed, expanded working fluid 27 back to
the first refrigeration circuit 20 upstream of the first
turbo-expander 26.
[0040] Preferably, the supplemental or auxiliary refrigeration
system is configured and constructed as a portable, skid-mounted
refrigeration system that can be easily added to the cryogenic air
separation plant/unit after initial plant construction in a manner
that minimizes cold-box entry. The preferred skid-mounted
supplemental or auxiliary refrigeration system would include: (i)
one or more auxiliary compressors 63; (ii) the warm second
turbo-expander 64; (iii) the auxiliary heat exchanger 65; (iv)
associated piping to facilitate the above-identified four or five
tie-in points; and (v) one or more control valves 67A, 67B, 67C,
and 67D configured to control the air stream flows to the one or
more auxiliary compressors 63, second turbo-expander 64, and
auxiliary heat exchanger 65 as described above with reference to
FIGS. 1 and 2. In some embodiments, some of the flow control valves
67A, 67B, 67C, and 67D configured to control the air stream flows
to the one or more auxiliary compressors 63, second turbo-expander
64, and auxiliary heat exchanger 65 may be configured as part of
the cryogenic air separation plant and where the skid-mounted
supplemental or auxiliary refrigeration system is tied-in
downstream of such control valves.
[0041] By controlling the flow to the supplemental or auxiliary
refrigeration circuit via the one or more flow control valves, the
presently disclosed system can easily switch between a high gaseous
product cycle--when the flow control valves are closed and a high
liquid make cycle where the flow control valves are operated to
produce an increased amount of refrigeration and associated liquid
product make.
[0042] An advantage of the present system and method for providing
auxiliary refrigeration to a cryogenic air separation plant is the
ability to increase the amount of refrigeration and associated
liquid product make in a cost-effective manner. The amount of
refrigeration produced and amount of liquid make is adjusted by
varying the warm turbine inlet pressure and flow in the
supplemental or auxiliary refrigeration circuit. Adjustments to the
warm turbine inlet pressure and flow are effected by selectively
opening and/or closing the one or more flow control valves 67A,
67B, 67C, and 67D. The discharge flow from the warm second
turbo-expander is passed through the auxiliary heat exchanger and
then directed to the higher pressure column along with the main air
(i.e. cooled second portion of the of the compressed and purified
feed air stream) and the fourth portion of the of the compressed
and purified feed air stream exiting the auxiliary heat
exchanger.
[0043] An additional advantage presented by the present system and
method is that by diverting a portion of the first refrigeration
stream from the primary refrigeration circuit to the auxiliary
refrigeration circuit and thus bypassing the lower pressure column
separation, the gaseous oxygen product produced by the distillation
column system is reduced but the argon recovery within the
distillation column system can be maintained or possibly
enhanced.
[0044] Also, diverting a portion of the first refrigeration stream
to the auxiliary refrigeration circuit is preferably controlled to
balance the temperatures in auxiliary heat exchanger and preserve
recovery in the auxiliary booster-turbine arrangement. The flow and
pressure ratio within the primary refrigeration circuit is
maximized. In this fashion, the upper column turbine arrangement is
used more as a heat pump to improve liquid making capability of the
cryogenic air separation plant.
[0045] Although the present invention has been discussed with
reference to preferred embodiments, as would occur to those skilled
in the art that numerous changes and omissions can be made without
departing from the spirit and scope of the present inventions as
set forth in the appended claims.
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