U.S. patent application number 16/144581 was filed with the patent office on 2019-04-25 for natural gas liquefaction by a high pressure expansion process using multiple turboexpander compressors.
The applicant listed for this patent is Fritz Pierre, JR., O. Angus Sites, William N. Yunker. Invention is credited to Fritz Pierre, JR., O. Angus Sites, William N. Yunker.
Application Number | 20190120548 16/144581 |
Document ID | / |
Family ID | 63963421 |
Filed Date | 2019-04-25 |
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United States Patent
Application |
20190120548 |
Kind Code |
A1 |
Pierre, JR.; Fritz ; et
al. |
April 25, 2019 |
Natural Gas Liquefaction by a High Pressure Expansion Process using
Multiple Turboexpander Compressors
Abstract
A method and system for liquefying a feed gas stream including
natural gas. The feed gas stream is provided at a pressure less
than 1,200 psia. A refrigerant stream having a pressure of at least
1,500 psia is cooled and then expanded in a first expander to an
intermediate pressure. The first expander is mechanically coupled
to a first coupled compressor to together form a first
turboexpander-compressor. The refrigerant stream is expanded in a
second expander, which is mechanically coupled to a second coupled
compressor to together form a second turboexpander-compressor. The
refrigerant stream cools the feed gas stream in one or more heat
exchangers. Using the second coupled compressor and a first driven
compressor, the refrigerant stream is compressed to a discharge
pressure within 300 psia of the intermediate pressure. The
refrigerant stream is compressed using the first coupled compressor
and is further compressed to provide the refrigerant stream.
Inventors: |
Pierre, JR.; Fritz; (Humble,
TX) ; Yunker; William N.; (Boling, IL) ;
Sites; O. Angus; (Spring, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pierre, JR.; Fritz
Yunker; William N.
Sites; O. Angus |
Humble
Boling
Spring |
TX
IL
TX |
US
US
US |
|
|
Family ID: |
63963421 |
Appl. No.: |
16/144581 |
Filed: |
September 27, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62576989 |
Oct 25, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2220/62 20130101;
F25J 2245/90 20130101; F25J 2210/06 20130101; F25J 1/0022 20130101;
F25J 1/004 20130101; F25J 2230/20 20130101; F25J 1/0205 20130101;
F25J 1/0208 20130101; F25J 1/0045 20130101; F25J 1/005 20130101;
F25J 1/0082 20130101; F25J 1/0288 20130101; F25J 1/0042 20130101;
F25J 1/0254 20130101; F25J 2230/30 20130101; F25J 2270/16 20130101;
F25J 1/0072 20130101; F25J 1/025 20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 1/02 20060101 F25J001/02 |
Claims
1. A method for liquefying a feed gas stream comprising natural
gas, the method comprising: providing the feed gas stream at a
pressure less than 1,200 psia; providing a compressed refrigerant
stream with a pressure greater than or equal to 1,500 psia; cooling
the compressed refrigerant stream by indirect heat exchange with a
cooling medium, thereby producing a compressed, cooled refrigerant
stream; expanding the compressed, cooled refrigerant stream in a
first expander to an intermediate pressure to further cool the
compressed, cooled refrigerant stream, thereby producing a first
expanded, cooled refrigerant stream, wherein the first expander is
mechanically coupled to a first coupled compressor to together form
a first turboexpander-compressor; expanding the first expanded,
cooled refrigerant stream in a second expander to further cool the
first expanded, cooled refrigerant stream, thereby producing a
second expanded, cooled refrigerant stream, wherein the second
expander is mechanically coupled to a second coupled compressor to
together form a second turboexpander-compressor; passing the second
expanded, cooled refrigerant stream through one or more heat
exchangers, thereby forming a warm refrigerant stream; passing the
feed gas stream through the one or more heat exchangers to cool at
least part of the feed gas stream by indirect heat exchange with
the second expanded, cooled refrigerant stream, thereby forming a
cool feed gas stream; using the second coupled compressor and a
first driven compressor, compressing the warm refrigerant stream to
a discharge pressure within 300 psia of the intermediate pressure,
thereby forming a first compressed refrigerant stream; compressing
the first compressed refrigerant stream using the first coupled
compressor, thereby forming a second compressed refrigerant stream;
and compressing the second compressed refrigerant stream to provide
the compressed refrigerant stream.
2. The method of claim 1, further comprising driving the first
driven compressor using at least one of a reciprocating engine, a
steam turbine, a gas turbine, and a motor.
3. The method of claim 1, wherein cooling the compressed
refrigerant stream comprises cooling the compressed refrigerant
stream via indirect heat exchange with a cooling medium.
4. The method of claim 1, wherein cooling the compressed
refrigerant stream comprises cooling the compressed refrigerant
stream by indirect heat exchange with a cooling medium having a
temperature lower than ambient conditions.
5. The method of claim 1, further comprising: cooling the warm
refrigerant stream by indirect heat exchange with a cooling medium
after being compressed in the second coupled compressor and prior
to being compressed in the first driven compressor.
6. The method of claim 1, further comprising: cooling the first
compressed refrigerant stream prior to being compressed in the
first coupled compressor.
7. The method of claim 1, further comprising: cooling the second
compressed refrigerant stream via indirect heat exchange with a
cooling medium prior to being compressed to provide the compressed
refrigerant stream.
8. The method of claim 1, wherein the compressed refrigerant stream
has a pressure of approximately 3,000 psia.
9. The method of claim 1, wherein the intermediate pressure is less
than 1,500 psia and greater than 1,000 psia.
10. The method of claim 1, wherein compressing the second
compressed refrigerant stream is accomplished using a second driven
compressor.
11. The method of claim 10, further comprising: driving the second
driven compressor using at least one of a reciprocating engine, a
steam turbine, a gas turbine, and a motor.
12. The method of claim 10, wherein the first driven compressor and
the second driven compressor share a common driver.
13. The method of claim 10, wherein the first driven compressor and
the second driven compressor are within a single compressor
casing.
14. The method of claim 1, further comprising: using a sub-cooling
loop, further cooling the cool feed gas stream to form a sub-cooled
feed gas stream.
15. The method of claim 14, further comprising: expanding the
sub-cooled feed gas stream to a pressure greater than or equal to
50 psia and less than or equal to 450 psia, to produce an expanded,
sub-cooled feed gas stream.
16. The method of claim 14, wherein the sub-cooled feed gas stream
is expanded within a hydraulic turbine.
17. The method of claim 14, wherein the sub-cooling loop is a
closed loop gas phase refrigeration cycle where nitrogen gas is the
refrigerant.
18. The method of claim 14, wherein the sub-cooling loop comprises:
withdrawing a portion not to exceed 50% of the expanded, sub-cooled
gas stream and reducing its pressure in a pressure reduction valve
to a range of about 30 to 300 psia to produce one or more reduced
pressure gas streams; and passing the one or more reduced pressure
gas streams through the one or more heat exchangers as the
sub-cooling refrigerant stream.
19. The method of claim 18, wherein the one or more reduced
pressure gas streams are at different pressures from each
other.
20. The method of claim 18, wherein the sub-cooling refrigerant
stream exiting the one or more heat exchangers is compressed to a
pressure approximate to that of the feed gas stream and is cooled
by indirect heat exchange with a cooling medium before mixing the
sub-cooling refrigerant stream with the feed gas stream.
21. The method of claim 15, wherein at least a portion of the
expanded, sub-cooled gas stream is further expanded and then
directed to a separation tank from which liquid natural gas is
withdrawn and remaining gaseous vapors are withdrawn as a flash gas
stream.
22. The method of claim 21, wherein the compressed refrigerant
stream comprises boil off gas of the liquid natural gas.
23. The method of claim 1, further comprising: adjusting one or
more of a discharge pressure of one or more of the compressors, and
an inlet pressure of one or more of the expanders, to thereby
maintain a fixed differential pressure between the discharge
pressure and the inlet pressure.
24. The method of claim 23, wherein the fixed differential pressure
is obtained through control algorithms using one or more of
compressor speed of one or more of the compressors, inlet guide
vanes of one ore more of the expanders, recycle valves of one or
more of the compressors, and bypass valves of one or more of the
expanders.
25. The method of claim 23, further comprising: using expander
thrust bearing temperature as a limit to protect thrust bearing
integrity while maximizing cycle efficiency.
26. A natural gas liquefaction system comprising: a first heat
exchanger configured to cool a compressed refrigerant stream by
indirect heat exchange with a cooling medium, thereby producing a
compressed, cooled refrigerant stream, wherein the compressed
refrigerant stream is provided to the first heat exchanger at a
pressure of at least 1,500 psia; a first expander configured to
expand the compressed, cooled refrigerant stream to an intermediate
pressure, to further cool the compressed, cooled refrigerant
stream, thereby producing a first expanded, cooled refrigerant
stream; a first coupled compressor mechanically coupled to the
first expander to together form a first turboexpander-compressor; a
second expander configured to expand the first expanded, cooled
refrigerant stream to further cool the first expanded, cooled
refrigerant stream, thereby producing a second expanded, cooled
refrigerant stream; a second coupled compressor mechanically
coupled to the second expander to together form a second
turboexpander-compressor; one or more heat exchangers arranged to
permit the second expanded, cooled refrigerant stream and a feed
gas stream to pass therethrough and exchange heat therein through
indirect heat exchange, thereby forming a warm refrigerant stream
and a cool feed gas stream, wherein the feed gas stream comprises
natural gas and is supplied to the one or more heat exchangers at a
pressure of less than 1,200 psia; a first driven compressor
configured to, along with the second coupled compressor, compress
the warm refrigerant stream to a discharge pressure within 300 psia
of the intermediate pressure, thereby forming a first compressed
refrigerant stream; wherein the first compressed refrigerant stream
is further compressed using the first coupled compressor, thereby
forming a second compressed refrigerant stream; and wherein the
second compressed refrigerant stream is compressed to provide the
compressed refrigerant stream.
27. The system of claim 26, further comprising a driving element
configured to drive the first driven compressor, wherein the
driving element comprises at least one of a reciprocating engine, a
steam turbine, a gas turbine, and a motor.
28. The system of claim 26, further comprising: a first cooler
configured to cool the compressed refrigerant stream via indirect
heat exchange with a cooling medium.
29. The system of claim 28, wherein the cooling medium has a
temperature lower than ambient conditions.
30. The system of claim 26, further comprising: a second cooler
configured to cool the warm refrigerant stream by indirect heat
exchange with a cooling medium after being compressed in the second
coupled compressor and prior to being compressed in the first
driven compressor; a third cooler configured to cool the first
compressed refrigerant stream prior to being compressed in the
first coupled compressor; and a fourth cooler configured to cool
the second compressed refrigerant stream via indirect heat exchange
with a cooling medium prior to being compressed, to thereby provide
the compressed refrigerant stream.
31. The system of claim 26, wherein the compressed refrigerant
stream has a pressure of approximately 3,000 psia.
32. The system of claim 26, wherein the intermediate pressure is
less than 1,500 psia and greater than 1,000 psia.
33. The system of claim 26, further comprising: a second driven
compressor configured to compress the second compressed refrigerant
stream.
34. The system of claim 33, further comprising: a driving element
configured to drive the second driven compressor, wherein the
driving element comprises at least one of a reciprocating engine, a
steam turbine, a gas turbine, and a motor.
35. The system of claim 33, wherein the first driven compressor and
the second driven compressor share a common driver.
36. The system of claim 32, wherein the first driven compressor and
the second driven compressor are within a single compressor
casing.
37. The system of claim 26, further comprising a sub-cooling loop
configured to further cool the cool feed gas stream to form a
sub-cooled feed gas stream.
38. The system of claim 37, further comprising: a hydraulic turbine
configured to expand the sub-cooled feed gas stream to a pressure
greater than or equal to 50 psia and less than or equal to 450
psia, to thereby produce an expanded, sub-cooled feed gas
stream.
39. The system of claim 37, wherein the sub-cooling loop is a
closed loop gas phase refrigeration cycle where nitrogen gas is the
refrigerant.
40. A method for liquefying a feed gas stream comprising natural
gas, the method comprising: providing the feed gas stream at a
pressure less than 1,200 psia; providing a compressed refrigerant
stream with a pressure greater than or equal to 1,500 psia; cooling
the compressed refrigerant stream by indirect heat exchange with a
first cooling medium, thereby producing a compressed, cooled
refrigerant stream; expanding the compressed, cooled refrigerant
stream in a first expander to an intermediate pressure to further
cool the compressed, cooled refrigerant stream, thereby producing a
first expanded, cooled refrigerant stream, wherein the first
expander is mechanically coupled to a first coupled compressor to
together form a first turboexpander-compressor; expanding the first
expanded, cooled refrigerant stream in a second expander to further
cool the first expanded, cooled refrigerant stream, thereby
producing a second expanded, cooled refrigerant stream, wherein the
second expander is mechanically coupled to a second coupled
compressor to together form a second turboexpander-compressor;
passing the second expanded, cooled refrigerant stream through one
or more heat exchangers, thereby forming a warm refrigerant stream;
passing the feed gas stream through the one or more heat exchangers
to cool at least part of the feed gas stream by indirect heat
exchange with the second expanded, cooled refrigerant stream,
thereby forming a cool feed gas stream; using a sub-cooling loop,
further cooling the cool feed gas stream to form a sub-cooled feed
gas stream having a liquid portion; using the second coupled
compressor and a first driven compressor, compressing the warm
refrigerant stream to a discharge pressure within 300 psia of the
intermediate pressure, thereby forming a first compressed
refrigerant stream; cooling the warm refrigerant stream by indirect
heat exchange with a second cooling medium after being compressed
in the second coupled compressor and prior to being compressed in
the first driven compressor; cooling the first compressed
refrigerant stream via heat exchange with a third cooling medium;
compressing the first compressed refrigerant stream using the first
coupled compressor, thereby forming a second compressed refrigerant
stream; cooling the second compressed refrigerant stream via heat
exchange with a fourth cooling medium; and compressing the second
compressed refrigerant stream to provide the compressed refrigerant
stream.
41. The method of claim 40, wherein at least one of the first
cooling medium, the second cooling medium, the third cooling
medium, and the fourth cooling medium comprises air or water.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit of U.S.
Provisional Application No. 62/576,989, filed Oct. 25, 2017
entitled "Natural Gas Liquefaction by a High Pressure Expansion
Process Using Multiple Turboexpander Compressors," the entirety of
which is incorporated herein.
BACKGROUND
Field of Disclosure
[0002] The disclosure relates generally to liquefied natural gas
(LNG) production. More specifically, the disclosure relates to LNG
production at high pressures.
Description of Related Art
[0003] This section is intended to introduce various aspects of the
art, which may be associated with the present disclosure. This
discussion is intended to provide a framework to facilitate a
better understanding of particular aspects of the present
disclosure. Accordingly, it should be understood that this section
should be read in this light, and not necessarily as an admission
of prior art.
[0004] Because of its clean burning qualities and convenience,
natural gas has become widely used in recent years. Many sources of
natural gas are located in remote areas, which are great distances
from any commercial markets for the gas. Sometimes a pipeline is
available for transporting produced natural gas to a commercial
market. When pipeline transportation is not feasible, produced
natural gas is often processed into liquefied natural gas (LNG) for
transport to market.
[0005] In the design of an LNG plant, one of the most important
considerations is the process for converting the natural gas feed
stream into LNG. Currently, the most common liquefaction processes
use some form of refrigeration system. Although many refrigeration
cycles have been used to liquefy natural gas, the three types most
commonly used in LNG plants today are: (1) the "cascade cycle,"
which uses multiple single component refrigerants in heat
exchangers arranged progressively to reduce the temperature of the
gas to a liquefaction temperature; (2) the "multi-component
refrigeration cycle," which uses a multi-component refrigerant in
specially designed exchangers; and (3) the "expander cycle," which
expands gas from feed gas pressure to a low pressure with a
corresponding reduction in temperature. Most natural gas
liquefaction cycles use variations or combinations of these three
basic types.
[0006] The refrigerants used in liquefaction processes may comprise
a mixture of components such as methane, ethane, propane, butane,
and nitrogen in multi-component refrigeration cycles. The
refrigerants may also be pure substances such as propane, ethylene,
or nitrogen in "cascade cycles." Substantial volumes of these
refrigerants with close control of composition are required.
Further, such refrigerants may have to be imported and stored,
which impose logistics requirements, especially for LNG production
in remote locations. Alternatively, some of the components of the
refrigerant may be prepared, typically by a distillation process
integrated with the liquefaction process.
[0007] The use of gas expanders to provide the feed gas cooling,
thereby eliminating or reducing the logistical problems of
refrigerant handling, is seen in some instances as having
advantages over refrigerant-based cooling. The expander system
operates on the principle that the refrigerant gas can be allowed
to expand through an expansion turbine, thereby performing work and
reducing the temperature of the gas. The low temperature gas is
then heat exchanged with the feed gas to provide the refrigeration
needed. The power obtained from cooling expansions in gas expanders
can be used to supply part of the main compression power used in
the refrigeration cycle. The typical expander cycle for making LNG
operates at the feed gas pressure, typically under about (1,000
psia). Supplemental cooling is typically needed to fully liquefy
the feed gas and this may be provided by additional refrigerant
systems, such as secondary cooling and/or sub-cooling loops. For
example, U.S. Pat. Nos. 6,412,302 and 5,916,260 present expander
cycles which describe the use of nitrogen as refrigerant in the
sub-cooling loop.
[0008] Previously proposed expander cycles have all been less
efficient thermodynamically, however, than the current natural gas
liquefaction cycles based on refrigerant systems. Expander cycles
have therefore not offered any installed cost advantage to date,
and liquefaction cycles involving refrigerants are still the
preferred option for natural gas liquefaction.
[0009] Because expander cycles result in a high recycle gas stream
flow rate and high inefficiency for the primary cooling (warm)
stage, gas expanders have typically been used to further cool feed
gas after it has been pre-cooled to temperatures well below
-20.degree. C. using an external refrigerant in a closed cycle, for
example. Thus, a common factor in most proposed expander cycles is
the requirement for a second, external refrigeration cycle to
pre-cool the gas before the gas enters the expander. Such a
combined external refrigeration cycle and expander cycle is
sometimes referred to as a "hybrid cycle." While such
refrigerant-based pre-cooling eliminates a major source of
inefficiency in the use of expanders, it significantly reduces the
benefits of the expander cycle, namely the elimination of external
refrigerants.
[0010] U. S. Patent Application US2009/0217701 introduced the
concept of using high pressure within the primary cooling loop to
eliminate the need for external refrigerant and improve efficiency,
at least comparable to that of refrigerant-based cycles currently
in use. The high pressure expander process (HPXP), disclosed in U.
S. Patent Application US2009/0217701, is an expander cycle which
uses high pressure expanders in a manner distinguishing from other
expander cycles. A portion of the feed gas stream may be extracted
and used as the refrigerant in either an open loop or closed loop
refrigeration cycle to cool the feed gas stream below its critical
temperature. Alternatively, a portion of LNG boil-off gas may be
extracted and used as the refrigerant in a closed loop
refrigeration cycle to cool the feed gas stream below its critical
temperature. This refrigeration cycle is referred to as the primary
cooling loop. The primary cooling loop is followed by a sub-cooling
loop which acts to further cool the feed gas. Within the primary
cooling loop, the refrigerant is compressed to a pressure greater
than 1,500 psia, or more preferably, to a pressure of approximately
3,000 psia. The refrigerant is then cooled against an ambient
cooling medium (air or water) prior to being near isentropically
expanded to provide the cold refrigerant needed to liquefy the feed
gas.
[0011] FIG. 1 schematically depicts an example of a known HPXP
liquefaction system 100, and is similar to one or more systems and
processes disclosed in U. S. Patent Application US2009/0217701. In
FIG. 1, an expander loop 102 (i.e., an expander cycle) and a
sub-cooling loop 104 are used. Feed gas stream 106 enters the HPXP
liquefaction process at a pressure less than about 1,200 psia, or
less than about 1,100 psia, or less than about 1,000 psia, or less
than about 900 psia, or less than about 800 psia, or less than
about 700 psia, or less than about 600 psia. Typically, the
pressure of feed gas stream 106 will be about 800 psia. Feed gas
stream 106 generally comprises natural gas that has been treated to
remove contaminants using processes and equipment that are well
known in the art.
[0012] In the expander loop 102, a compression unit 108 compresses
a refrigerant stream 109 (which may be a treated gas stream) to a
pressure greater than or equal to about 1,500 psia, thus providing
a compressed refrigerant stream 110. Alternatively, the refrigerant
stream 109 may be compressed to a pressure greater than or equal to
about 1,600 psia, or greater than or equal to about 1,700 psia, or
greater than or equal to about 1,800 psia, or greater than or equal
to about 1,900 psia, or greater than or equal to about 2,000 psia,
or greater than or equal to about 2,500 psia, or greater than or
equal to about 3,000 psia, thus providing compressed refrigerant
stream 110. After exiting compression unit 108, compressed
refrigerant stream 110 is passed to a cooler 112 where it is cooled
by indirect heat exchange with a suitable cooling medium to provide
a compressed, cooled refrigerant stream 114. Cooler 112 may be of
the type that provides water or air as the cooling medium, although
any type of cooler can be used. The temperature of the compressed,
cooled refrigerant stream 114 depends on the ambient conditions and
the cooling medium used, and is typically from about 35.degree. F.
to about 105.degree. F. Compressed, cooled refrigerant stream 114
is then passed to an expander 116 where it is expanded and
consequently cooled to form an expanded refrigerant stream 118.
Expander 116 is a work-expansion device, such as a gas expander,
which produces work that may be extracted and used for compression.
Expanded refrigerant stream 118 is passed to a first heat exchanger
120, and provides at least part of the refrigeration duty for first
heat exchanger 120. Upon exiting first heat exchanger 120, expanded
refrigerant stream 118 is fed to a compression unit 122 for
pressurization to form refrigerant stream 109.
[0013] Feed gas stream 106 flows through first heat exchanger 120
where it is cooled, at least in part, by indirect heat exchange
with expanded refrigerant stream 118. After exiting first heat
exchanger 120, the feed gas stream 106 is passed to a second heat
exchanger 124. The principal function of second heat exchanger 124
is to sub-cool the feed gas stream. Thus, in second heat exchanger
124 the feed gas stream 106 is sub-cooled by sub-cooling loop 104
(described below) to produce sub-cooled gas stream 126. Sub-cooled
gas stream 126 is then expanded to a lower pressure in an expander
128 to form a liquid fraction and a remaining vapor fraction.
Expander 128 may be any pressure reducing device, including, but
not limited to a valve, control valve, Joule Thompson valve,
Venturi device, liquid expander, hydraulic turbine, and the like.
The sub-cooled gas stream 126, which is now at a lower pressure and
partially liquefied, is passed to a surge tank 130 where the
liquefied fraction 132 is withdrawn from the process as an LNG
stream 134, which has a temperature corresponding to the bubble
point pressure. The remaining vapor fraction (flash vapor) stream
136 may be used as fuel to power the compressor units.
[0014] In sub-cooling loop 104, an expanded sub-cooling refrigerant
stream 138 (preferably comprising nitrogen) is discharged from an
expander 140 and drawn through second and first heat exchangers
124, 120. Expanded sub-cooling refrigerant stream 138 is then sent
to a compression unit 142 where it is re-compressed to a higher
pressure and warmed. After exiting compression unit 142, the
re-compressed sub-cooling refrigerant stream 144 is cooled in a
cooler 146, which can be of the same type as cooler 112, although
any type of cooler may be used. After cooling, the re-compressed
sub-cooling refrigerant stream is passed to first heat exchanger
120 where it is further cooled by indirect heat exchange with
expanded refrigerant stream 118 and expanded sub-cooling
refrigerant stream 138. After exiting first heat exchanger 120, the
re-compressed and cooled sub-cooling refrigerant stream is expanded
through expander 140 to provide a cooled stream which is then
passed through second heat exchanger 124 to sub-cool the portion of
the feed gas stream to be finally expanded to produce LNG.
[0015] FIG. 2 is a schematic diagram of another liquefaction system
200 according to known principles. Liquefaction system 200 is
similar to HPXP liquefaction system 100, and for the sake of
brevity similarly depicted or numbered components may not be
further described. Liquefaction system 200 includes a primary
cooling loop 202 and a sub-cooling loop 204. In liquefaction system
200, the sub-cooling loop 204 is an open refrigeration loop where a
portion 249 of the expanded, sub-cooled gas stream 248 is recycled
and used as the sub-cooling refrigerant stream. Specifically, the
portion 249 of the expanded, sub-cooled gas stream is directed
through the second heat exchanger 124 and first heat exchanger 120
as previously described before being compressed in a compressor
242, cooled in a cooler 246, and re-inserted into the feed gas
stream 206.
[0016] U. S. Patent Application US2010/0107684 disclosed an
improvement to the performance of the HPXP through the discovery
that adding external cooling to further cool the compressed
refrigerant to temperatures below ambient conditions provides
significant advantages which in certain situations justifies the
added equipment associated with external cooling. The HPXP
embodiments described in the aforementioned patent applications
perform comparably to alternative mixed external refrigerant LNG
production processes such as single mixed refrigerant processes.
However, there remains a need to further improve the efficiency of
the HPXP as well as overall train capacity. There remains a
particular need to improve the efficiency of the HPXP in cases
where the feed gas pressure is less than 1,200 psia.
[0017] U. S. Patent Application 2010/0186445 disclosed the
incorporation of feed compression up to 4,500 psia to the HPXP.
Compressing the feed gas prior to liquefying the gas in the HPXP's
primary cooling loop has the advantage of increasing the overall
process efficiency. For a given production rate, this also has the
advantage of significantly reducing the required flow rate of the
refrigerant within the primary cooling loop which enables the use
of compact equipment, which is particularly attractive for floating
LNG applications. Furthermore, feed compression provides a means of
increasing the LNG production of an HPXP train by more than 30% for
a fixed amount of power going to the primary cooling and
sub-cooling loops. This flexibility in production rate is again
particularly attractive for floating LNG applications where there
are more restrictions than land based applications in matching the
choice of refrigerant loop drivers with desired production
rates.
[0018] The advantages of the HPXP are made possible through the use
of turboexpanders capable of expanding the refrigerant gas from a
high pressure (1,500 to 3,500 psia) down to the low pressure
required to achieve the desired refrigerant temperature to liquefy
the feed gas.
[0019] For most applications of the HPXP, the required pressure
drop for the refrigerant is significantly greater than 1,000 psi.
For this reason, the expansion of the refrigerant may occur within
a train comprising at least two expanders operating in series. In
such a configuration, each expander is mechanically coupled to a
compressor such that the mechanical power recovered from the
expander is used to partially recompress the refrigerant back to
its maximum pressure. A compressor that is mechanically coupled to
an expander is hereto known as a turboexpander-compressor (TEC).
FIG. 3 illustrates a liquefaction system 300 that uses a
configuration of turboexpander-compressors (TEC) within its primary
cooling loop 302, according to known aspects. A compressed
refrigerant stream 308 is cooled in a cooler 310 against an ambient
cooling medium (air or water) to produce a compressed, cooled
refrigerant stream 312. The compressed, cooled refrigerant stream,
which may be at a pressure of, for example, 3,000 psia, is expanded
in a first expander 314 to an intermediate pressure of, for
example, 1,300 psia, to further cool the compressed, cooled
refrigerant stream, thereby producing a first expanded, cooled
refrigerant stream 316. The first expanded, cooled refrigerant
stream is additionally expanded in a second expander 318 to further
cool the first expanded, cooled refrigerant stream 316 and thereby
produce a second expanded, cooled refrigerant stream 320. The
second expanded, cooled refrigerant stream 320, which may be at a
pressure of, for example, 500 psia, is directed to one or more heat
exchangers 322 where it exchanges heat with the feed gas stream 306
to produce a warm refrigerant stream 324 and a liquefied gas stream
326. The liquefied gas stream 326 is further cooled in a
sub-cooling heat exchanger 328 using a sub-cooling loop 304, which
functions in a similar manner as previously described sub-cooling
loop 104 (FIG. 1). The warm refrigerant stream 324, which may be at
a pressure of, for example, 500 psia, is compressed by a first
compressor 330 (to a pressure of, for example, 700 psia), followed
by a second compressor 332 (to a pressure of, for example, 900
psia), and is then cooled in a cooler 334 against a cooling medium,
which may be an ambient cooling medium (air or water), to produce a
first compressed refrigerant stream 336. The first compressor 330
is coupled to the second expander 318 using a first mechanical
coupling 338 to form a low-pressure TEC. The second compressor 332
is coupled to the first expander 314 using a second mechanical
coupling 340 to form a high-pressure TEC. The first compressed
refrigerant stream 336 is further compressed in one or more
compressors 342, 344 to produce the compressed refrigerant stream
308. A cooler 343 may be disposed between the one or more
compressors 342, 344.
[0020] In liquefaction system 300, the low pressure TEC's operating
conditions are within the operating experience of commercially
available turboexpander-compressors. However, the high pressure
TEC's inlet pressure (3,000 psia), the first expander pressure drop
(3,000 psia-1,300 psia=1,700 psia), and the thrust differential
between the expander outlet side and compressor inlet side of the
high-pressure TEC (1,300 psia-700 psia=600 psia) are outside
industry operating experience. It is believed that the high inlet
pressure and high expander pressure drop can be handled with a
proper design of the high pressure TEC, which takes into account
these step out conditions. Managing the higher thrust, however, may
require significant TEC design changes. These changes may add cost
and complexity and decrease efficiency by 1 to 3%. For this reason,
there is a need to develop a new configuration for the TEC of HPXP
to eliminate the need for significant design changes to manage the
thrust.
SUMMARY
[0021] Aspects of the disclosure provide a method for liquefying a
feed gas stream comprising a natural gas feed gas stream is
provided at a pressure less than 1,200 psia. A compressed
refrigerant stream is provided with a pressure greater than or
equal to 1,500 psia. The compressed refrigerant stream is cooled by
indirect heat exchange with a cooling medium, thereby producing a
compressed, cooled refrigerant stream. The compressed, cooled
refrigerant stream is expanded in a first expander to an
intermediate pressure to further cool the compressed, cooled
refrigerant stream, thereby producing a first expanded, cooled
refrigerant stream. The first expander is mechanically coupled to a
first coupled compressor to together form a first
turboexpander-compressor. The first expanded, cooled refrigerant
stream is expanded in a second expander to further cool the first
expanded, cooled refrigerant stream, thereby producing a second
expanded, cooled refrigerant stream. The second expander is
mechanically coupled to a second coupled compressor to together
form a second turboexpander-compressor. The second expanded, cooled
refrigerant stream is passed through one or more heat exchangers,
thereby forming a warm refrigerant stream. The feed gas stream is
passed through the one or more heat exchangers to cool at least
part of the feed gas stream by indirect heat exchange with the
second expanded, cooled refrigerant stream, thereby forming a cool
feed gas stream. Using the second coupled compressor and a first
driven compressor, the warm refrigerant stream is compressed to a
discharge pressure within 300 psia of the intermediate pressure,
thereby forming a first compressed refrigerant stream. The first
compressed refrigerant stream is compressed using the first coupled
compressor, thereby forming a second compressed refrigerant stream.
The second compressed refrigerant stream is compressed to provide
the compressed refrigerant stream.
[0022] Aspects of the disclosure also provide a natural gas
liquefaction system. A first heat exchanger cools a compressed
refrigerant stream by indirect heat exchange with a cooling medium,
thereby producing a compressed, cooled refrigerant stream, wherein
the compressed refrigerant stream is provided to the first heat
exchanger at a pressure of at least 1,500 psia. A first expander
expands the compressed, cooled refrigerant stream to an
intermediate pressure, to further cool the compressed, cooled
refrigerant stream, thereby producing a first expanded, cooled
refrigerant stream. A first coupled compressor is mechanically
coupled to the first expander to together form a first
turboexpander-compressor. A second expander expands the first
expanded, cooled refrigerant stream to further cool the first
expanded, cooled refrigerant stream, thereby producing a second
expanded, cooled refrigerant stream. A second coupled compressor is
mechanically coupled to the second expander to together form a
second turboexpander-compressor. One or more heat exchangers
arranged to permit the second expanded, cooled refrigerant stream
and a feed gas stream to pass therethrough and exchange heat
therein through indirect heat exchange, thereby forming a warm
refrigerant stream and a cool feed gas stream. The feed gas stream
comprises natural gas and is supplied to the one or more heat
exchangers at a pressure of less than 1,200 psia. A first driven
compressor and the second coupled compressor compress the warm
refrigerant stream to a discharge pressure within 300 psia of the
intermediate pressure, thereby forming a first compressed
refrigerant stream. The first compressed refrigerant stream is
further compressed using the first coupled compressor, thereby
forming a second compressed refrigerant stream. The second
compressed refrigerant stream is compressed to provide the
compressed refrigerant stream.
[0023] Aspects of the disclosure also provide a method for
liquefying a feed gas stream comprising natural gas. The feed gas
stream is provided at a pressure less than 1,200 psia. A compressed
refrigerant stream is provided with a pressure greater than or
equal to 1,500 psia. The compressed refrigerant stream is cooled by
indirect heat exchange with a first cooling medium, thereby
producing a compressed, cooled refrigerant stream. The compressed,
cooled refrigerant stream is expanded in a first expander to an
intermediate pressure to further cool the compressed, cooled
refrigerant stream, thereby producing a first expanded, cooled
refrigerant stream. The first expander is mechanically coupled to a
first coupled compressor to together form a first
turboexpander-compressor. The first expanded, cooled refrigerant
stream is expanded in a second expander to further cool the first
expanded, cooled refrigerant stream, thereby producing a second
expanded, cooled refrigerant stream. The second expander is
mechanically coupled to a second coupled compressor to together
form a second turboexpander-compressor. The second expanded, cooled
refrigerant stream is passed through one or more heat exchangers,
thereby forming a warm refrigerant stream. The feed gas stream is
passed through the one or more heat exchangers to cool at least
part of the feed gas stream by indirect heat exchange with the
second expanded, cooled refrigerant stream, thereby forming a cool
feed gas stream. Using a sub-cooling loop, the cool feed gas stream
is further cooled to form a sub-cooled feed gas stream having a
liquid portion. Using the second coupled compressor and a first
driven compressor, the warm refrigerant stream is compressed to a
discharge pressure within 300 psia of the intermediate pressure,
thereby forming a first compressed refrigerant stream. The warm
refrigerant stream is cooled by indirect heat exchange with a
second cooling medium after being compressed in the second coupled
compressor and prior to being compressed in the first driven
compressor. The first compressed refrigerant stream is cooled via
heat exchange with a third cooling medium. The first compressed
refrigerant stream is compressed using the first coupled
compressor, thereby forming a second compressed refrigerant stream.
The second compressed refrigerant stream is cooled via heat
exchange with a fourth cooling medium. The second compressed
refrigerant stream is compressed to provide the compressed
refrigerant stream.
[0024] The foregoing has broadly outlined the features of the
present disclosure so that the detailed description that follows
may be better understood. Additional features will also be
described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] These and other features, aspects and advantages of the
disclosure will become apparent from the following description,
appending claims and the accompanying drawings, which are briefly
described below.
[0026] FIG. 1 is a schematic diagram of a system for LNG production
according to known principles.
[0027] FIG. 2 is a schematic diagram of a system for LNG production
according to known principles.
[0028] FIG. 3 is a schematic diagram of a system for LNG production
according to known principles.
[0029] FIG. 4 is a schematic diagram of a system for LNG production
according to disclosed aspects.
[0030] FIG. 5 is a schematic diagram of a system for LNG production
according to disclosed aspects.
[0031] FIG. 6 is a flowchart of a method according to aspects of
the disclosure.
[0032] FIG. 7 is a flowchart of a method according to aspects of
the disclosure.
[0033] It should be noted that the figures are merely examples and
no limitations on the scope of the present disclosure are intended
thereby. Further, the figures are generally not drawn to scale, but
are drafted for purposes of convenience and clarity in illustrating
various aspects of the disclosure.
DETAILED DESCRIPTION
[0034] To promote an understanding of the principles of the
disclosure, reference will now be made to the features illustrated
in the drawings and specific language will be used to describe the
same. It will nevertheless be understood that no limitation of the
scope of the disclosure is thereby intended. Any alterations and
further modifications, and any further applications of the
principles of the disclosure as described herein are contemplated
as would normally occur to one skilled in the art to which the
disclosure relates. For the sake of clarity, some features not
relevant to the present disclosure may not be shown in the
drawings.
[0035] At the outset, for ease of reference, certain terms used in
this application and their meanings as used in this context are set
forth. To the extent a term used herein is not defined below, it
should be given the broadest definition persons in the pertinent
art have given that term as reflected in at least one printed
publication or issued patent. Further, the present techniques are
not limited by the usage of the terms shown below, as all
equivalents, synonyms, new developments, and terms or techniques
that serve the same or a similar purpose are considered to be
within the scope of the present claims.
[0036] As one of ordinary skill would appreciate, different persons
may refer to the same feature or component by different names. This
document does not intend to distinguish between components or
features that differ in name only. The figures are not necessarily
to scale. Certain features and components herein may be shown
exaggerated in scale or in schematic form and some details of
conventional elements may not be shown in the interest of clarity
and conciseness. When referring to the figures described herein,
the same reference numerals may be referenced in multiple figures
for the sake of simplicity. In the following description and in the
claims, the terms "including" and "comprising" are used in an
open-ended fashion, and thus, should be interpreted to mean
"including, but not limited to."
[0037] The articles "the," "a" and "an" are not necessarily limited
to mean only one, but rather are inclusive and open ended so as to
include, optionally, multiple such elements.
[0038] As used herein, the terms "approximately," "about,"
"substantially," and similar terms are intended to have a broad
meaning in harmony with the common and accepted usage by those of
ordinary skill in the art to which the subject matter of this
disclosure pertains. It should be understood by those of skill in
the art who review this disclosure that these terms are intended to
allow a description of certain features described and claimed
without restricting the scope of these features to the precise
numeral ranges provided. Accordingly, these terms should be
interpreted as indicating that insubstantial or inconsequential
modifications or alterations of the subject matter described and
are considered to be within the scope of the disclosure. According
to disclosed aspects, these terms are intended to mean within 2%,
or within 5%, or within 10%, of a specified number or amount.
[0039] As used herein, the terms "compression unit" and
"compressor" mean any one type or combination of similar or
different types of compression equipment, and may include auxiliary
equipment, known in the art for compressing a substance or mixture
of substances. A compression unit or compressor may utilize one or
more compression stages. Illustrative compression units or
compressors may include, but are not limited to, positive
displacement types, such as reciprocating and rotary compressors
for example, and dynamic types, such as centrifugal and axial flow
compressors, for example.
[0040] As used herein, the term "cooling medium" means any type of
medium, whether in a solid, liquid, or gaseous state, that serves
to cool a fluid using indirect heat exchange therewith. A cooling
medium may be at ambient temperature, below ambient temperature, or
above ambient temperature, depending on the needed cooling and
available types of cooling media. As non-limiting examples, a
cooling medium may be water or air.
[0041] "Exemplary" is used exclusively herein to mean "serving as
an example, instance, or illustration." Any embodiment or aspect
described herein as "exemplary" is not to be construed as preferred
or advantageous over other embodiments.
[0042] The term "gas" is used interchangeably with "vapor," and is
defined as a substance or mixture of substances in the gaseous
state as distinguished from the liquid or solid state. Likewise,
the term "liquid" means a substance or mixture of substances in the
liquid state as distinguished from the gas or solid state.
[0043] As used herein, "heat exchange area" means any one type or
combination of similar or different types of equipment known in the
art for facilitating heat transfer. Thus, a "heat exchange area"
may be contained within a single piece of equipment, or it may
comprise areas contained in a plurality of equipment pieces.
Conversely, multiple heat exchange areas may be contained in a
single piece of equipment.
[0044] A "hydrocarbon" is an organic compound that primarily
includes the elements hydrogen and carbon, although nitrogen,
sulfur, oxygen, metals, or any number of other elements can be
present in small amounts. As used herein, hydrocarbons generally
refer to components found in natural gas, oil, or chemical
processing facilities.
[0045] As used herein, the terms "loop" and "cycle" are used
interchangeably.
[0046] As used herein, "natural gas" means a gaseous feedstock
suitable for manufacturing LNG, where the feedstock is a
methane-rich gas containing methane (CH.sub.4) as a major
component. Natural gas may include gas obtained from a crude oil
well (associated gas) or from a gas well (non-associated gas).
[0047] Aspects the disclosure provide a process for liquefying
natural gas and other methane-rich gas streams to produce liquefied
natural gas (LNG) and/or other liquefied methane-rich gases.
According to the disclosed aspect, the turboexpander compressors
and gas turbine (or motor) driven compressors of the primary
cooling loop are arranged to significantly reduce the TEC thrust
differential. Specifically, the turbo machinery within the primary
cooling loop are configured such that the absolute difference in
pressure between the TEC's compressor suction pressure and the high
pressure TEC's expander discharge pressure is less than 300 psi.
This configuration reduces the thrust differential to a more easily
managed level.
[0048] According to disclosed aspects, a method and system are
provided for liquefying a feed gas stream, particularly one rich in
methane. The method and system include: (a) providing the feed gas
stream at a pressure less than 1,200 psia; (b) providing a
compressed refrigerant stream with a pressure greater than or equal
to 1,500 psia; (c) cooling the compressed refrigerant stream by
indirect heat exchange with a cooling fluid, thereby producing a
compressed, cooled refrigerant stream; (d) expanding the
compressed, cooled refrigerant stream in a first expander to an
intermediate pressure to further cool the compressed, cooled
refrigerant stream, thereby producing a first expanded, cooled
refrigerant stream; (e) expanding the first expanded, cooled
refrigerant stream in a second expander to further cool the first
expanded, cooled refrigerant stream, thereby producing a second
expanded, cooled refrigerant stream; (f) passing the second
expanded, cooled refrigerant stream to heat exchanger areas,
thereby forming a warm refrigerant stream; (g) passing the feed gas
stream through the heat exchanger areas to cool at least part of
the feed gas stream by indirect heat exchange with the second
expanded, cooled refrigerant stream, thereby forming a cool gas
stream; (h) compressing the warm refrigerant stream to a discharge
pressure within 300 psi of the intermediate pressure, thereby
forming a first compressed refrigerant stream; (i) cooling the
first compressed refrigerant stream by indirect heat exchange with
a cooling medium, thereby producing a first compressed, cooled
refrigerant stream; (j) compressing the first compressed, cooled
refrigerant stream using a compressor mechanically coupled to the
first expander, thereby forming a second compressed refrigerant
stream; and (k) compressing the second compressed refrigerant
stream to provide the compressed refrigerant stream.
[0049] The first compressed refrigerant stream may be formed by
compressing the warm refrigerant stream in a compressor
mechanically coupled to the second expander and then further
compressing the warm refrigerant stream in a first compressor
driven by a reciprocating engine, a steam turbine, or a gas turbine
and/or motor. The warm refrigerant stream may be cooled by indirect
heat exchange with a cooling medium after being compressed in the
compressor mechanically coupled to the second expander and prior to
being compressed in the first compressor.
[0050] The TEC configuration according to disclosed aspects has the
advantage of reducing or eliminating the high thrust differential
of the high pressure TEC compared to the TEC arrangement of known
TEC arrangements. A pressure differential below approximately 200
psia should be manageable without any significant changes to the
conventional thrust balancing mechanism. The efficiency of the HPXP
may be increased by 1 to 3% using the disclosed TEC configurations.
It may be the case that the disclosed TEC configurations of the
present invention requires extra piping and controls, since the
high pressure and low pressure TECs may be located apart from the
gas turbine (or motor driven) driven compressor in a facility
layout. Additionally, the number of gas turbine or motor driven
compressor bodies may increase. It may also be desirable to add an
intercooler between the first and second compression stages in
order to improve efficiency and reduce volumetric flow into the
first compressor. Nevertheless, such additional costs are likely to
be outweighed by the efficiency increases to the HPXP process.
[0051] FIG. 4 illustrates a liquefaction system 400 according to
disclosed aspects. Liquefaction system includes a primary cooling
loop 402 and a sub-cooling loop 404. A compressed refrigerant
stream 408 is cooled in a first heat exchanger having one or more
coolers 410, which may use one or more cooling mediums to produce a
compressed, cooled refrigerant stream 412. The compressed, cooled
refrigerant stream 412 is expanded in a first expander 414 to an
intermediate pressure, and is thereby further cooled to produce a
first expanded, cooled refrigerant stream 416. The first expander
414 is coupled, using a first mechanical coupling 444, to a first
coupled compressor 438 to together form a high-pressure
turboexpander/compressor (TEC). The first expanded, cooled
refrigerant stream is additionally expanded in a second expander
418 to further cool the first expanded, cooled refrigerant stream
416 and thereby produce a second expanded, cooled refrigerant
stream 420. The second expander 414 is coupled, using a second
mechanical coupling 446, to a second coupled compressor 429 to
together form a low-pressure TEC. The second expanded, cooled
refrigerant stream 420 is directed to a main heat exchange area 422
comprising one or more heat exchangers, where the second expanded,
cooled refrigerant stream exchanges heat with the feed gas stream
406 to produce a warm refrigerant stream 424 and a liquefied gas
stream 426. The liquefied gas stream 426 is further cooled in a
sub-cooling heat exchanger 428 using the sub-cooling loop 404,
which will be further described. The warm refrigerant stream 424 is
compressed by second coupled compressor 429, cooled via indirect
heat exchange with a cooling medium in a second cooler 430, and is
further compressed by a first driven compressor 431 to produce a
first compressed refrigerant stream 432. The first driven
compressor may be driven by a gas turbine 431a, or alternatively
may be driven by a reciprocating engine, a steam turbine, or a
motor. The first compressed refrigerant stream 432 has a discharge
pressure what is within 300 psia, or within 250 psia, or within 200
psia, or within 150 psia, or within 100 psia, of the intermediate
pressure of the first expanded, cooled refrigerant stream 416. The
first compressed refrigerant stream is cooled in a third cooler 434
via indirect heat exchange with a cooling medium to form a first
compressed, cooled refrigerant stream 436, which is then compressed
in first coupled compressor 438 to form a second compressed
refrigerant stream 440. The second compressed refrigerant stream
may be cooled in a fourth cooler 441 via indirect heat exchange
with a cooling medium and then further compressed in a second
driven compressor 442 to produce the compressed refrigerant stream
408. The second driven compressor 442 may be driven by a gas
turbine 442a, or alternatively may be driven by a reciprocating
engine, a steam turbine, or a motor. The second driven compressor
442 may share a common driver with first driven compressor 431. The
first and second driven compressors 431, 442 may even be disposed
within a single compressor casing.
[0052] In sub-cooling loop 404, an expanded sub-cooling refrigerant
stream 448 (preferably comprising nitrogen) is discharged from a
sub-cooling expander 450 and drawn through the sub-cooling heat
exchanger 428 and the heat exchangers in the main heat exchange
area 422. Expanded sub-cooling refrigerant stream 448 is then sent
to a sub-cooling compression unit 452 where it is re-compressed to
a higher pressure and warmed, thereby forming a re-compressed
sub-cooling refrigerant stream 454. After exiting compression unit
442, the re-compressed sub-cooling refrigerant stream 454 is cooled
in a fifth heat exchanger comprising a cooler 456, which can be of
the same type as coolers 410 and/or 434, although any type of
cooler may be used. After cooling, the re-compressed sub-cooling
refrigerant stream 454 is passed through the heat exchangers in the
main heat exchange area 422 where it is further cooled by indirect
heat exchange with second expanded, cooled refrigerant stream 420
and expanded sub-cooling refrigerant stream 448. After exiting the
heat exchange area 422, the re-compressed and cooled sub-cooling
refrigerant stream is expanded through sub-cooling expander 450 to
provide the expanded, sub-cooled refrigerant stream 448, which is
then passed through sub-cooling heat exchanger 428 to sub-cool the
feed gas stream, and thereby produce a sub-cooled feed gas stream
458. Sub-cooled feed gas stream 458 is then expanded to a lower
pressure in an expander 460 to form an expanded, sub-cooled gas
stream 461 with a liquid fraction and a remaining vapor fraction.
In an aspect, the expanded, sub-cooled gas stream 461 may have a
pressure greater than or equal to 50 psia and less than or equal to
450 psia. Expander 460 may be any pressure reducing device,
including, but not limited to a valve, control valve, Joule
Thompson valve, Venturi device, liquid expander, hydraulic turbine,
and the like. The expanded sub-cooled gas stream 461, which is now
at a lower pressure and partially liquefied, is passed to a surge
tank 462 where the liquefied fraction 464 is withdrawn from the
process as an LNG stream 466. The LNG stream 466 has a temperature
corresponding to the bubble point pressure. The remaining vapor
fraction (flash vapor) stream 468 may be used as fuel to power the
compressor units.
[0053] The discharge pressure of one or more of the compressors
and/or expanders disclosed herein may be controlled by a control
system 401. Such a control system may include control logic to
optimize cycle efficiency by maximizing certain parameters within a
measured thrust bearing limit, such as temperature. The control
logic may also control one or more of, for example, the driven
compressor speed, driven compressor recycle valves, first coupled
compressor recycle valves, second coupled compressor recycle
valves, first expander bypass valves, second expander bypass
valves, first expander throttling valves, second expander
throttling valves, first expander inlet guide vanes, second
expander inlet guide vanes, or any combination of the foregoing
based on the suction pressure of the driven compressor, discharge
pressure of the driven compressor, discharge pressure of the first
couple compressor, discharge pressure of the second coupled
compressor, discharge pressure of the first expander, discharge
pressure of the second expander, and/or suction pressure of the
second expander, or any combination thereof. The control system may
adjust one or more of (a) a discharge pressure of one or more of
the compressors, and (b) an inlet pressure of one or more of the
expanders, to thereby maintain a fixed differential pressure
between the discharge pressure and the inlet pressure. The fixed
differential pressure may be obtained through control algorithms
using one or more of compressor speed of one or more of the
compressors, inlet guide vanes of one or more of the expanders,
recycle valves of one or more of the compressors, and bypass valves
of one or more of the expanders. Furthermore, cycle efficiency may
be optimized by maximizing certain parameters within a measured
thrust bearing limit, such as temperature. For the sake of clarity
in the Figures, necessary connections from control system 401 to
the various valves, compressors and expanders, are not shown.
[0054] FIG. 5 is a schematic diagram that illustrates a
liquefaction system 500 according to another aspect of the
disclosure. Liquefaction system 500 is similar to liquefaction
system 400 (FIG. 4), and for the sake of brevity similarly depicted
or numbered components may not be further described. Liquefaction
system 500 includes a primary cooling loop 502, which is
substantially identical to primary cooling loop 402, and a
sub-cooling loop 504. The sub-cooling loop 504 is an open
refrigeration loop where a portion 570 of the expanded, sub-cooled
feed gas stream 561 is recycled and used as the sub-cooling
refrigerant stream. Specifically, the portion 570 of the expanded,
sub-cooled gas stream 561 is directed through a sub-cooling heat
exchanger 526 and one or more heat exchangers in a main heat
exchange area 522 before being compressed in a compressor 572,
cooled in a cooler 574 via indirect heat exchange with a cooling
medium, and re-inserted into the feed gas stream 506. The feed gas
stream 506 passes through the heat exchangers in the heat exchange
area 522, where it is cooled by indirect heat exchange with the
portion of the expanded, sub-cooled gas stream 561 and the second
expanded, cooled refrigerant stream 520 of the primary cooling loop
502. After being expanded in an expander 560, the portion 575 of
the sub-cooled feed gas stream 561 not used as the sub-cooling
refrigerant stream is further expanded and cooled in an expander
576, which may be any pressure reducing device, including, but not
limited to a valve, control valve, Joule Thompson valve, Venturi
device, liquid expander, hydraulic turbine, and the like. The
sub-cooled gas stream, which is now at a lower pressure and
partially liquefied, is passed to a surge tank 580 where the
liquefied fraction 582 is withdrawn from the process as an LNG
stream 584. The LNG stream 584 has a temperature corresponding to
the bubble point pressure. The remaining vapor fraction (flash
vapor) stream 586 may be used as fuel to power the compressor
units. Liquefaction system 500 also includes a control system 501
having similar functionality as control system 401 (FIG. 4).
[0055] The sub-cooling refrigerant stream in FIG. 5 may be one
stream, as shown, or may comprise multiple streams at different
pressures: for example, a portion of the expanded, sub-cooling gas
stream--not to exceed 50% thereof--may be diverted and pass through
one or more pressure reduction valves to reduce its pressure to a
range of about 30 to 300 psia, to thereby produce one or more
reduced pressure gas streams. The reduced pressure gas streams may
then be passed through the first heat exchanger zone as the
sub-cooling refrigerant. Having multiple streams improves the
efficiency of the sub-cooling process. Alternatively, this
sub-cooling loop may be configured to be a closed refrigeration
loop.
[0056] Aspects of the disclosure illustrated in FIG. 5 demonstrate
that the primary refrigerant stream may comprise part of the feed
gas stream, which in a preferred aspect may be primarily or nearly
all methane. Indeed, it may be advantageous for the refrigerant in
the primary cooling loop of FIG. 4 to be comprised of at least 85%
methane, or at least 90% methane, or at least 95% methane, or
greater than 95% methane. This is because methane may be readily
available in various parts of the disclosed processes, and the use
of methane may eliminate the need to transport refrigerants to
remote LNG processing locations. As a non-limiting example, the
refrigerant in the primary cooling loop 402 in FIG. 4 may be taken
directly from feed gas stream 406 if the feed gas is high enough in
methane to meet the compositions as described above. Alternatively,
part or all of a boil-off gas stream 469a from an LNG storage tank
469 may be used to supply refrigerant for the primary cooling loop
402. Furthermore, if the feed gas stream is sufficiently low in
nitrogen, part or all of the end flash gas stream 468 (which would
then be low in nitrogen) may be used to supply refrigerant for the
primary cooling loop 402. Lastly, any combination of feed gas
stream 406, boil-off gas stream 469a, and end flash gas stream 468
may be used to provide or even occasionally replenish the
refrigerant in the primary cooling loop 402.
[0057] Aspects of the disclosure have shown how two TECs--i.e., a
high pressure TEC and a low-pressure TEC--can be disposed in series
in a high-pressure refrigerant stream for liquefying natural gas.
It is within the scope of the disclosed aspects to employ three or
more TECs in series in the high-pressure refrigerant stream for
liquefying natural gas.
[0058] The coolers disclosed in FIGS. 4 and 5 may use any type of
cooling media, including air or water. For purposes of clarity, the
various disclosed cooling media may be denoted as a `first cooling
medium,` `second cooling medium,` etc., it being understood that
one or more of the cooling media used herein have similar or
identical composition, regardless of how they have been
denoted.
[0059] FIG. 6 is a flowchart of a method 600 for liquefying a feed
gas stream comprising natural gas. At block 602 the feed gas stream
is provided at a pressure less than 1,200 psia. At block 604 a
compressed refrigerant stream is provided with a pressure greater
than or equal to 1,500 psia. At block 606 the compressed
refrigerant stream is cooled by indirect heat exchange with a
cooling medium, thereby producing a compressed, cooled refrigerant
stream. At block 608 the compressed, cooled refrigerant stream is
expanded in a first expander to an intermediate pressure to further
cool the compressed, cooled refrigerant stream, thereby producing a
first expanded, cooled refrigerant stream. The first expander is
mechanically coupled to a first coupled compressor to together form
a first turboexpander-compressor. At block 610 the first expanded,
cooled refrigerant stream is expanded in a second expander to
further cool the first expanded, cooled refrigerant stream, thereby
producing a second expanded, cooled refrigerant stream. The second
expander is mechanically coupled to a second coupled compressor to
together form a second turboexpander-compressor. At block 612 the
second expanded, cooled refrigerant stream is passed through one or
more heat exchangers, thereby forming a warm refrigerant stream. At
block 614 the feed gas stream is passed through the one or more
heat exchangers to cool at least part of the feed gas stream by
indirect heat exchange with the second expanded, cooled refrigerant
stream, thereby forming a cool feed gas stream. At block 616, using
the second coupled compressor and a first driven compressor, the
warm refrigerant stream is compressed to a discharge pressure
within 300 psia of the intermediate pressure, thereby forming a
first compressed refrigerant stream. At block 618 the first
compressed refrigerant stream is compressed using the first coupled
compressor, thereby forming a second compressed refrigerant stream.
At block 620 the second compressed refrigerant stream is compressed
to provide the compressed refrigerant stream.
[0060] FIG. 7 is a flowchart of a method 700 for liquefying a feed
gas stream comprising natural gas. At block 702 the feed gas stream
is provided at a pressure less than 1,200 psia. At block 704 a
compressed refrigerant stream is provided with a pressure greater
than or equal to 1,500 psia. At block 706 the compressed
refrigerant stream is cooled by indirect heat exchange with a first
cooling medium, thereby producing a compressed, cooled refrigerant
stream. At block 708 the compressed, cooled refrigerant stream is
expanded in a first expander to an intermediate pressure to further
cool the compressed, cooled refrigerant stream, thereby producing a
first expanded, cooled refrigerant stream. The first expander is
mechanically coupled to a first coupled compressor to together form
a first turboexpander-compressor. At block 710 the first expanded,
cooled refrigerant stream is expanded in a second expander to
further cool the first expanded, cooled refrigerant stream, thereby
producing a second expanded, cooled refrigerant stream. The second
expander is mechanically coupled to a second coupled compressor to
together form a second turboexpander-compressor. At block 712 the
second expanded, cooled refrigerant stream is passed through one or
more heat exchangers, thereby forming a warm refrigerant stream. At
block 714 the feed gas stream is passed through the one or more
heat exchangers to cool at least part of the feed gas stream by
indirect heat exchange with the second expanded, cooled refrigerant
stream, thereby forming a cool feed gas stream. At block 716, using
a sub-cooling loop, the cool feed gas stream is further cooled to
form a sub-cooled feed gas stream having a liquid portion. At block
718, using the second coupled compressor and a first driven
compressor, the warm refrigerant stream is compressed to a
discharge pressure within 300 psia of the intermediate pressure,
thereby forming a first compressed refrigerant stream. At block 720
the warm refrigerant stream is cooled by indirect heat exchange
with a second cooling medium after being compressed in the second
coupled compressor and prior to being compressed in the first
driven compressor. At block 722 the first compressed refrigerant
stream is cooled via heat exchange with a third cooling medium. At
block 724 the first compressed refrigerant stream is compressed
using the first coupled compressor, thereby forming a second
compressed refrigerant stream. At block 726 the second compressed
refrigerant stream is cooled via heat exchange with a fourth
cooling medium. At block 728 the second compressed refrigerant
stream is compressed to provide the compressed refrigerant
stream.
[0061] The steps depicted in FIGS. 6 and 7 are provided for
illustrative purposes only and a particular step may not be
required to perform the disclosed methodology. Moreover, FIGS. 6-7
may not illustrate all the steps that may be performed. The claims,
and only the claims, define the disclosed system and
methodology.
[0062] The aspects described herein have several advantages over
known technologies. For example, the described technology may
greatly reduce the size and cost of systems that treat sour natural
gas.
[0063] Aspects of the disclosure may include any combinations of
the methods and systems shown in the following numbered paragraphs.
This is not to be considered a complete listing of all possible
aspects, as any number of variations can be envisioned from the
description above.
1. A method for liquefying a feed gas stream comprising natural
gas, the method comprising:
[0064] providing the feed gas stream at a pressure less than 1,200
psia;
[0065] providing a compressed refrigerant stream with a pressure
greater than or equal to 1,500 psia;
[0066] cooling the compressed refrigerant stream by indirect heat
exchange with a cooling medium, thereby producing a compressed,
cooled refrigerant stream;
[0067] expanding the compressed, cooled refrigerant stream in a
first expander to an intermediate pressure to further cool the
compressed, cooled refrigerant stream, thereby producing a first
expanded, cooled refrigerant stream, wherein the first expander is
mechanically coupled to a first coupled compressor to together form
a first turboexpander-compressor;
[0068] expanding the first expanded, cooled refrigerant stream in a
second expander to further cool the first expanded, cooled
refrigerant stream, thereby producing a second expanded, cooled
refrigerant stream, wherein the second expander is mechanically
coupled to a second coupled compressor to together form a second
turboexpander-compressor;
[0069] passing the second expanded, cooled refrigerant stream to
one or more heat exchangers, thereby forming a warm refrigerant
stream;
[0070] passing the feed gas stream through the one or more heat
exchangers to cool at least part of the feed gas stream by indirect
heat exchange with the second expanded, cooled refrigerant stream,
thereby forming a cool feed gas stream;
[0071] using the second coupled compressor and a first driven
compressor, compressing the warm refrigerant stream to a discharge
pressure within 300 psia of the intermediate pressure, thereby
forming a first compressed refrigerant stream;
[0072] compressing the first compressed refrigerant stream using
the first coupled compressor, thereby forming a second compressed
refrigerant stream; and
[0073] compressing the second compressed refrigerant stream to
provide the compressed refrigerant stream.
2. The method of paragraph 1, further comprising driving the first
driven compressor using at least one of a reciprocating engine, a
steam turbine, a gas turbine, and a motor. 3. The method of
paragraph 1 or paragraph 2, wherein cooling the compressed
refrigerant stream comprises cooling the compressed refrigerant
stream via indirect heat exchange with a cooling medium. 4. The
method of any one of paragraphs 1-3, wherein cooling the compressed
refrigerant stream comprises cooling the compressed refrigerant
stream by indirect heat exchange with a cooling medium having a
temperature lower than ambient conditions. 5. The method of any one
of paragraphs 1-4, further comprising:
[0074] cooling the warm refrigerant stream by indirect heat
exchange with a cooling medium after being compressed in the second
coupled compressor and prior to being compressed in the first
driven compressor.
6. The method of any one of paragraphs 1-5, further comprising:
[0075] cooling the first compressed refrigerant stream prior to
being compressed in the first coupled compressor.
7. The method of any one of paragraphs 1-6, further comprising:
[0076] cooling the second compressed refrigerant stream via
indirect heat exchange with a cooling medium prior to being
compressed to provide the compressed refrigerant stream.
8. The method of any one of paragraphs 1-7, wherein the compressed
refrigerant stream has a pressure of approximately 3,000 psia. 9.
The method of any one of paragraphs 1-8, wherein the intermediate
pressure is less than 1,500 psia and greater than 1,000 psia. 10.
The method of any one of paragraphs 1-9, wherein compressing the
second compressed refrigerant stream is accomplished using a second
driven compressor. 11. The method of paragraph 10, further
comprising:
[0077] driving the second driven compressor using at least one of a
reciprocating engine, a steam turbine, a gas turbine, and a
motor.
12. The method of paragraph 10 or paragraph 11, wherein the first
driven compressor and the second driven compressor share a common
driver. 13. The method of any one of paragraphs 10-12, wherein the
first driven compressor and the second driven compressor are within
a single compressor casing. 14. The method of any one of paragraphs
1-13, further comprising:
[0078] using a sub-cooling loop, further cooling the cool feed gas
stream to form a sub-cooled feed gas stream.
15. The method of paragraph 14, further comprising:
[0079] expanding the sub-cooled feed gas stream to a pressure
greater than or equal to 50 psia and less than or equal to 450
psia, to produce an expanded, sub-cooled feed gas stream.
16. The method of paragraph 14 or paragraph 15, wherein the
sub-cooled feed gas stream is expanded within a hydraulic turbine.
17. The method of any one of paragraphs 14-16, wherein the
sub-cooling loop is a closed loop gas phase refrigeration cycle
where nitrogen gas is the refrigerant. 18. The method of paragraph
14, wherein the sub-cooling loop comprises:
[0080] withdrawing a portion not to exceed 50% of the expanded,
sub-cooled gas stream and reducing its pressure in a pressure
reduction valve to a range of about 30 to 300 psia to produce one
or more reduced pressure gas streams; and
[0081] passing the one or more reduced pressure gas streams through
the one or more heat exchangers as the sub-cooling refrigerant
stream.
19. The method of paragraph 18, wherein the one or more reduced
pressure gas streams are at different pressures from each other.
20. The method of paragraph 18 or paragraph 19, wherein the
sub-cooling refrigerant stream exiting the one or more heat
exchangers is compressed to a pressure approximate to that of the
feed gas stream and is cooled by indirect heat exchange with a
cooling medium before mixing the sub-cooling refrigerant stream
with the feed gas stream. 21. The method of paragraph 15, wherein
at least a portion of the expanded, sub-cooled gas stream is
further expanded and then directed to a separation tank from which
liquid natural gas is withdrawn and remaining gaseous vapors are
withdrawn as a flash gas stream. 22. The method of paragraph 21,
wherein the compressed refrigerant stream comprises boil off gas of
the liquid natural gas. 23. The method of any one of paragraphs
1-22, further comprising:
[0082] adjusting one or more of [0083] a discharge pressure of one
or more of the compressors, and [0084] an inlet pressure of one or
more of the expanders, to thereby maintain a fixed differential
pressure between the discharge pressure and the inlet pressure. 24.
The method of paragraph 23, wherein the fixed differential pressure
is obtained through control algorithms using one or more of
compressor speed of one or more of the compressors, inlet guide
vanes of one ore more of the expanders, recycle valves of one or
more of the compressors, and bypass valves of one or more of the
expanders. 25. The method of paragraph 23 or paragraph 24, further
comprising:
[0085] using expander thrust bearing temperature as a limit to
protect thrust bearing integrity while maximizing cycle
efficiency.
26. A natural gas liquefaction system comprising:
[0086] a first heat exchanger configured to cool a compressed
refrigerant stream by indirect heat exchange with a cooling medium,
thereby producing a compressed, cooled refrigerant stream, wherein
the compressed refrigerant stream is provided to the first heat
exchanger at a pressure of at least 1,500 psia;
[0087] a first expander configured to expand the compressed, cooled
refrigerant stream to an intermediate pressure, to further cool the
compressed, cooled refrigerant stream, thereby producing a first
expanded, cooled refrigerant stream;
[0088] a first coupled compressor mechanically coupled to the first
expander to together form a first turboexpander-compressor;
[0089] a second expander configured to expand the first expanded,
cooled refrigerant stream to further cool the first expanded,
cooled refrigerant stream, thereby producing a second expanded,
cooled refrigerant stream;
[0090] a second coupled compressor mechanically coupled to the
second expander to together form a second
turboexpander-compressor;
[0091] one or more heat exchangers arranged to permit the second
expanded, cooled refrigerant stream and a feed gas stream to pass
therethrough and exchange heat therein through indirect heat
exchange, thereby forming a warm refrigerant stream and a cool feed
gas stream, wherein the feed gas stream comprises natural gas and
is supplied to the one or more heat exchangers at a pressure of
less than 1,200 psia;
[0092] a first driven compressor configured to, along with the
second coupled compressor, compress the warm refrigerant stream to
a discharge pressure within 300 psia of the intermediate pressure,
thereby forming a first compressed refrigerant stream;
[0093] wherein the first compressed refrigerant stream is further
compressed using the first coupled compressor, thereby forming a
second compressed refrigerant stream; and
[0094] wherein the second compressed refrigerant stream is
compressed to provide the compressed refrigerant stream.
27. The system of paragraph 26, further comprising a driving
element configured to drive the first driven compressor, wherein
the driving element comprises at least one of a reciprocating
engine, a steam turbine, a gas turbine, and a motor. 28. The system
of paragraph 26 or paragraph 27, further comprising:
[0095] a first cooler configured to cool the compressed refrigerant
stream via indirect heat exchange with a cooling medium.
29. The system of paragraph 28, wherein the cooling medium has a
temperature lower than ambient conditions. 30. The system of any
one of paragraphs 26-29, further comprising:
[0096] a second cooler configured to cool the warm refrigerant
stream by indirect heat exchange with a cooling medium after being
compressed in the second coupled compressor and prior to being
compressed in the first driven compressor;
[0097] a third cooler configured to cool the first compressed
refrigerant stream prior to being compressed in the first coupled
compressor; and
[0098] a fourth cooler configured to cool the second compressed
refrigerant stream via indirect heat exchange with a cooling medium
prior to being compressed, to thereby provide the compressed
refrigerant stream.
31. The system of any one of paragraphs 26-30, wherein the
compressed refrigerant stream has a pressure of approximately 3,000
psia. 32. The system of any one of paragraphs 26-31, wherein the
intermediate pressure is less than 1,500 psia and greater than
1,000 psia. 33. The system of any one of paragraphs 26-32, further
comprising:
[0099] a second driven compressor configured to compress the second
compressed refrigerant stream.
34. The system of paragraph 33, further comprising:
[0100] a driving element configured to drive the second driven
compressor, wherein the driving element comprises at least one of a
reciprocating engine, a steam turbine, a gas turbine, and a
motor.
35. The system of paragraph 33 or paragraph 34, wherein the first
driven compressor and the second driven compressor share a common
driver. 36. The system of any one of paragraphs 32-34, wherein the
first driven compressor and the second driven compressor are within
a single compressor casing. 37. The system of any one of paragraphs
26-33, further comprising a sub-cooling loop configured to further
cool the cool feed gas stream to form a sub-cooled feed gas stream.
38. The system of paragraph 37, further comprising:
[0101] a hydraulic turbine configured to expand the sub-cooled feed
gas stream to a pressure greater than or equal to 50 psia and less
than or equal to 450 psia, to thereby produce an expanded,
sub-cooled feed gas stream.
39. The system of paragraph 37 or paragraph 38, wherein the
sub-cooling loop is a closed loop gas phase refrigeration cycle
where nitrogen gas is the refrigerant. 40. A method for liquefying
a feed gas stream comprising natural gas, the method
comprising:
[0102] providing the feed gas stream at a pressure less than 1,200
psia;
[0103] providing a compressed refrigerant stream with a pressure
greater than or equal to 1,500 psia;
[0104] cooling the compressed refrigerant stream by indirect heat
exchange with a first cooling medium, thereby producing a
compressed, cooled refrigerant stream;
[0105] expanding the compressed, cooled refrigerant stream in a
first expander to an intermediate pressure to further cool the
compressed, cooled refrigerant stream, thereby producing a first
expanded, cooled refrigerant stream, wherein the first expander is
mechanically coupled to a first coupled compressor to together form
a first turboexpander-compressor;
[0106] expanding the first expanded, cooled refrigerant stream in a
second expander to further cool the first expanded, cooled
refrigerant stream, thereby producing a second expanded, cooled
refrigerant stream, wherein the second expander is mechanically
coupled to a second coupled compressor to together form a second
turboexpander-compressor;
[0107] passing the second expanded, cooled refrigerant stream to
one or more heat exchangers, thereby forming a warm refrigerant
stream;
[0108] passing the feed gas stream through the one or more heat
exchangers to cool at least part of the feed gas stream by indirect
heat exchange with the second expanded, cooled refrigerant stream,
thereby forming a cool feed gas stream;
[0109] using a sub-cooling loop, further cooling the cool feed gas
stream to form a sub-cooled feed gas stream having a liquid
portion;
[0110] using the second coupled compressor and a first driven
compressor, compressing the warm refrigerant stream to a discharge
pressure within 300 psia of the intermediate pressure, thereby
forming a first compressed refrigerant stream;
[0111] cooling the warm refrigerant stream by indirect heat
exchange with a second cooling medium after being compressed in the
second coupled compressor and prior to being compressed in the
first driven compressor;
[0112] cooling the first compressed refrigerant stream via heat
exchange with a third cooling medium;
[0113] compressing the first compressed refrigerant stream using
the first coupled compressor, thereby forming a second compressed
refrigerant stream;
[0114] cooling the second compressed refrigerant stream via heat
exchange with a fourth cooling medium; and
[0115] compressing the second compressed refrigerant stream to
provide the compressed refrigerant stream.
41. The method of paragraph 40, wherein at least one of the first
cooling medium, the second cooling medium, the third cooling
medium, and the fourth cooling medium comprises air or water.
[0116] It should be understood that the numerous changes,
modifications, and alternatives to the preceding disclosure can be
made without departing from the scope of the disclosure. The
preceding description, therefore, is not meant to limit the scope
of the disclosure. Rather, the scope of the disclosure is to be
determined only by the appended claims and their equivalents. It is
also contemplated that structures and features in the present
examples can be altered, rearranged, substituted, deleted,
duplicated, combined, or added to each other.
* * * * *