U.S. patent application number 11/922623 was filed with the patent office on 2009-09-03 for natural gas liquefaction process for ling.
Invention is credited to Ronald R. Bowen, Linda J. Cote, Moses Minta, Kevin N. Stanley, John B. Stone.
Application Number | 20090217701 11/922623 |
Document ID | / |
Family ID | 37757866 |
Filed Date | 2009-09-03 |
United States Patent
Application |
20090217701 |
Kind Code |
A1 |
Minta; Moses ; et
al. |
September 3, 2009 |
Natural Gas Liquefaction Process for Ling
Abstract
Embodiments of this invention relate to a process for
liquefaction of natural gas and other methane-rich gas streams, and
more particularly to a process for producing liquefied natural gas
(LNG). In a first step of the process, a first fraction of the feed
gas is withdrawn, compressed to a pressure greater than or equal to
1500 psia, cooled and expanded to a lower pressure to cool the
withdrawn first fraction. The remaining fraction of the feed stream
is cooled by indirect heat exchange with the expanded first
fraction in a first heat exchange process. In a second step a
separate stream comprising flash vapor is compressed, cooled and
expanded to a lower pressure providing another cold stream. This
cold stream is used to cool the remaining feed gas stream in a
second indirect heat exchange process. The expanded stream exiting
from the second heat exchange process is used for supplemental
cooling in the first indirect heat exchange step. The remaining
feed gas is subsequently expanded to a lower pressure, thereby
partially liquefying this feed gas stream. The liquefied fraction
of this stream is withdrawn from the process as LNG having a
temperature corresponding to the bubble point pressure.
Inventors: |
Minta; Moses; (Missouri
City, TX) ; Stanley; Kevin N.; (Doha, QA) ;
Stone; John B.; (Kingwood, TX) ; Bowen; Ronald
R.; (Magnolia, TX) ; Cote; Linda J.; (Sugar
Land, TX) |
Correspondence
Address: |
Adam P. Brown;ExxonMobil Upstream Research Company
PO Box 2189, COPR-URC-SW337
Houston
TX
77252-2189
US
|
Family ID: |
37757866 |
Appl. No.: |
11/922623 |
Filed: |
May 24, 2006 |
PCT Filed: |
May 24, 2006 |
PCT NO: |
PCT/US2006/020121 |
371 Date: |
February 17, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60706798 |
Aug 9, 2005 |
|
|
|
60795101 |
Apr 26, 2006 |
|
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|
Current U.S.
Class: |
62/612 ;
62/613 |
Current CPC
Class: |
F25J 2230/08 20130101;
F25J 2205/04 20130101; F25J 2210/06 20130101; F25J 2230/30
20130101; F25J 1/0022 20130101; F25J 1/0035 20130101; F25J 2200/70
20130101; F25J 2220/62 20130101; F25J 1/005 20130101; F25J 3/0209
20130101; F25J 1/0045 20130101; F25J 1/0092 20130101; F25J 1/0215
20130101; F25J 3/0257 20130101; F25J 2270/06 20130101; F25J 1/0082
20130101; F25J 1/0037 20130101; F25J 1/0219 20130101; F25J 1/0294
20130101; F25J 1/0072 20130101; F25J 1/0214 20130101; F25J 1/0254
20130101; F25J 1/0042 20130101; F25J 1/025 20130101; F25J 1/0288
20130101; F25J 3/0233 20130101; F25J 1/004 20130101 |
Class at
Publication: |
62/612 ;
62/613 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Claims
1. A process for liquefying a gas stream rich in methane, said
process comprising: providing said gas stream at a pressure less
than 1,000 psia; providing a refrigerant at a pressure of less than
1,000 psia; compressing said refrigerant to a pressure greater than
or equal to 1500 psia to provide a compressed refrigerant; cooling
said compressed refrigerant by indirect heat exchange with a
cooling fluid; expanding said compressed refrigerant to further
cool said compressed refrigerant, thereby producing an expanded,
cooled refrigerant; passing said expanded, cooled refrigerant to a
heat exchange area; and passing said gas stream through said heat
exchange area to cool at least part of said gas stream by indirect
heat exchange with said expanded, cooled refrigerant, thereby
forming a cooled gas stream.
2. The process of claim 1 wherein providing said refrigerant at a
pressure of less than 1,000 psia comprises withdrawing a portion of
said gas stream for use as said refrigerant.
3. The process of claim 2 wherein said portion of said gas stream
is withdrawn before said gas stream is passed to said heat exchange
area.
4. The process of claim 2 wherein said portion of said gas stream
is withdrawn from said heat exchange area.
5. The process of claim 1 further comprising providing at least a
portion of the refrigeration duty for said heat exchange area using
a closed loop charged with a flash vapor produced in said process
for liquefying a gas stream rich in methane.
6. The process of claim 5 further comprising: expanding at least a
portion of said cooled gas stream to produce an expanded, cooled
gas stream; and further cooling said expanded, cooled gas stream by
indirect heat exchange with said closed loop charged with the flash
vapor.
7. The process of claim 1 further comprising: expanding at least a
portion of said cooled gas stream to produce an expanded, cooled
gas stream; and further cooling said expanded, cooled gas stream by
indirect heat exchange in one or more additional heat exchange
areas.
8. The process of claim 1 further comprising: cooling said gas
stream using a plurality of work expansion devices, each of said
work expansion devices expanding a portion of the feed gas stream
and thereby cooling said portion to form one or more expanded,
cooled side streams, wherein each of said portions of the feed gas
stream expanded in said work expansion devices is withdrawn from
said feed gas stream at a different stage of feed gas stream
cooling; and cooling said feed gas stream by indirect heat exchange
with said one or more expanded, cooled side streams.
9. The process of claim 1 further comprising: withdrawing one or
more portions of said gas stream; passing each of said one or more
portions of said gas stream to one or more work expansion devices
and expanding each of said one of more portions of said gas stream
to expand and cool said one or more portions, thereby forming one
or more expanded, cooled side streams; passing said one or more
expanded, cooled side streams to at least one heat exchange area;
passing said gas stream through said at least one heat exchange
area; and at least partially cooling said gas stream by indirect
heat exchange with said one or more expanded, cooled side
streams.
10. The process of claim 6, 7, 8, or 9 wherein said gas stream is
first compressed to a pressure above the gas supply pressure.
11. The process of claim 1 further comprising an expansion stage of
said cooled gas stream before a final heat exchange step and prior
to expansion to produce LNG.
12. The process of claim 1 further comprising: expanding at least a
portion of said cooled gas stream before a final heat exchange step
to produce an expanded, cooled gas stream; passing a portion of
said expanded, cooled gas stream to a work-producing expander and
further expanding said portion of said expanded, cooled gas stream
in said work-producing expander; and passing the stream emerging
from said work-producing expander to a heat exchange area to
further cool the balance of said expanded, cooled gas stream by
indirect heat exchange in said heat exchange area.
13. The process of claim 1 wherein said refrigerant is compressed
to a pressure greater than or equal to 3,000 psia to provide a
compressed refrigerant.
14. The process of claim 1 wherein said heat exchange area
comprises multiple heat exchange chambers.
15. The process of claim 1 further comprising: a sub-cooling heat
exchange area receiving said gas stream and cooled by expansion of
a second refrigerant to provide a sub-cooled gas stream; followed
by final expansion of said sub-cooled gas stream and recovery of
LNG.
16. The process of claim 15 wherein said second refrigerant is a
portion of said gas stream rich in methane.
17. The process of claim 15 wherein said second refrigerant is
sub-cooled in said sub-cooling heat exchange area prior to
expansion of said second refrigerant.
18. The process of claim 16 wherein said gas stream rich in methane
is re-pressurized before passing through said heat exchange area,
said cooled gas stream is expanded, and a portion of said expanded,
cooled gas stream is further expanded and used as said second
refrigerant in said sub-cooling heat exchange area.
19. The process of claim 15 wherein a portion of said sub-cooled
gas stream is expanded and a portion thereof is said second
refrigerant.
20. The process of claim 19 wherein said portion of said sub-cooled
gas stream is split into two partial streams, one of said partial
streams is further expanded, and both of said partial streams
comprise said second refrigerant.
21. The process of claim 1 further comprising rejecting nitrogen
with LNG recovery.
22. A process for liquefying a gas stream rich in methane, said
process comprising: providing said gas stream at a pressure less
than 1,000 psia; providing a refrigerant in a closed loop;
compressing said refrigerant to a pressure greater than or equal to
1500 psia to provide a compressed refrigerant; cooling said
compressed refrigerant by indirect heat exchange with a cooling
fluid; expanding said compressed refrigerant to further cool said
compressed refrigerant, thereby producing an expanded, cooled
refrigerant; passing said expanded, cooled refrigerant to a heat
exchange area; and passing said gas stream through said heat
exchange area to cool at least part of said gas stream by indirect
heat exchange with said expanded, cooled refrigerant.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/706,798, filed 9 Aug., 2005, and U.S.
Provisional Application No. 60/795,101, filed 26 Apr. 2006.
TECHNICAL FIELD
[0002] Embodiments of the invention relate to a process for
liquefaction of natural gas and other methane-rich gas streams, and
more particularly to a process for producing liquefied natural gas
(LNG).
BACKGROUND
[0003] 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, 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 (which is called "LNG")
for transport to market.
[0004] 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.
[0005] The refrigerants used may be 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 imposing logistics requirements. Alternatively,
some of the components of the refrigerant may be prepared,
typically by a distillation process integrated with the
liquefaction process.
[0006] The use of gas expanders to provide the feed gas cooling
thereby eliminating or reducing the logistical problems of
refrigerant handling has been of interest to process engineers. The
expander system operates on the principle that the feed 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. Supplemental refrigeration is typically
needed to fully liquefy the feed gas and this may be provided by a
refrigerant system. The power obtained from the expansion is
usually 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 6,895 kPa
(1,000 psia).
[0007] 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.
[0008] Because expander cycles result in a high recycle gas stream
flow rate and high inefficiency for the pre-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. Additional
cooling may also be required after the expander cooling and may be
provided by another external refrigerant system, such as nitrogen
or a cold mixed refrigerant.
[0009] Accordingly, there is still a need for an expander cycle
that eliminates the need for external refrigerants and has improved
efficiency, at least comparable to that of technologies currently
in use.
SUMMARY
[0010] Embodiments of the present invention 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. The term natural gas as used in this
specification, including the appended claims, means a gaseous feed
stock suitable for manufacturing LNG. The natural gas could
comprise gas obtained from a crude oil well (associated gas) or
from a gas well (non-associated gas). The composition of natural
gas can vary significantly. As used herein, natural gas is a
methane-rich gas containing methane (C.sub.1) as a major
component.
[0011] In one or more embodiments of the method for producing LNG
herein, a first step is carried out in which a first fraction of
the feed gas is withdrawn, compressed, cooled and expanded to a
lower pressure to cool the withdrawn first fraction. The remaining
fraction of the feed stream is cooled by indirect heat exchange
with the expanded first fraction in a first heat exchange process.
In a second step, involving a sub-cooling loop, a separate stream
comprised of the flash vapor is compressed, cooled and expanded to
a lower pressure providing another cold stream. This cold stream is
used to cool the remaining feed gas stream in a second indirect
heat exchange process, which constitutes the sub-cooling heat
exchange process. The expanded stream exiting from the second heat
exchange process is used for supplemental cooling in the first
indirect heat exchange step. The remaining feed gas is subsequently
expanded to a lower pressure, thereby partially liquefying this
feed gas stream. The liquefied fraction of this stream is withdrawn
from the process as LNG having a temperature corresponding to the
bubble point pressure. The vapor fraction of this stream is
returned to supplement the cooling provided in the indirect heat
exchange steps. The warmed cooling gases from the various sources
are compressed and recycled.
[0012] In one or more other embodiments according to the present
invention, a process for liquefying a gas stream rich in methane is
provided, said process comprising providing a gas stream rich in
methane at a pressure less than 1,000 psia; providing a refrigerant
at a pressure of less than 1,000 psia; compressing said refrigerant
to a pressure greater than or equal to 1500 psia to provide a
compressed refrigerant; cooling said compressed refrigerant by
indirect heat exchange with a cooling fluid; expanding said
compressed refrigerant to further cool said compressed refrigerant,
thereby producing an expanded, cooled refrigerant; passing said
expanded, cooled refrigerant to a heat exchange area; and passing
said gas stream through said heat exchange area to cool at least
part of said gas stream by indirect heat exchange with said
expanded, cooled refrigerant, thereby forming a cooled gas stream.
In one or more other specific embodiments, providing the
refrigerant at a pressure of less than 1,000 psia comprises
withdrawing a portion of the gas for use as the refrigerant. In
other embodiments, the portion of the gas stream to be used as the
refrigerant is withdrawn from the gas stream before the gas stream
is passed to the heat exchange area. In still other embodiments,
the process according to the present invention further comprises
providing at least a portion of the refrigeration duty for the heat
exchange area using a closed loop charged with flash vapor produced
in the process for liquefying the gas stream rich in methane.
Additional embodiments according to the present invention will be
apparent to those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic flow diagram of one embodiment for
producing LNG in accordance with the process of this invention.
[0014] FIG. 2 is a schematic flow diagram of a second embodiment
for producing LNG that is similar to the process shown in FIG. 1,
except that the gaseous refrigerant in the compressed, cooled and
expanded loop is de-coupled from the feed gas and may therefore
have a different composition than the feed gas.
[0015] FIG. 3 is a schematic flow diagram of a third embodiment for
producing LNG in accordance with the process of this invention that
uses a plurality of work expansion steps for improved
efficiency.
[0016] FIG. 4 is a schematic flow diagram of a fourth embodiment
for producing LNG in accordance with the process of this invention
that uses a plurality of work expansion steps similar to FIG. 3,
but also incorporates an additional expansion step as well as
compression of the feed gas to improve performance of the expansion
steps.
[0017] FIG. 5 is a schematic flow diagram of a fifth embodiment for
producing LNG in accordance with the process of this invention that
is similar to the embodiment shown in FIG. 4, but utilizes an
additional side stream and expansion of process gas to provide
sub-cooling.
[0018] FIG. 6 is another embodiment similar to the embodiments
shown in FIG. 1 and FIG. 2 in which the refrigerant for the
sub-cooling loop is cooled in the sub-cooling heat exchanger prior
to expansion.
[0019] FIG. 7 is another embodiment in which the sub-cooling loop
is coupled to the feed gas.
[0020] FIG. 8 is another embodiment showing an alternative
arrangement for the sub-cooling loop.
[0021] FIG. 9 is a similar embodiment to that of FIG. 8 but using
split expanded streams through the sub-cooler wherein an expansion
valve, Joules-Thompson valve, or similar expansion device is used
for improved efficiency in the sub-cooler.
[0022] FIG. 10 is another embodiment in which a nitrogen rejection
stage has been integrated for situations in which nitrogen
rejection may be needed.
[0023] FIG. 11 is yet another embodiment in which the refrigerant
for the sub-cooling loop is derived from the flash vapor from the
nitrogen rejection unit and is therefore rich in nitrogen
content.
DETAILED DESCRIPTION
[0024] Embodiments of the present invention provide a process for
natural gas liquefaction using primarily gas expanders and
eliminating the need for external refrigerants. That is, in some
embodiments disclosed herein, the feed gas itself (e.g., natural
gas) is used as the refrigerant in all refrigeration cycles. Such
refrigeration cycles do not require supplemental cooling using
external refrigerants (i.e., refrigerants other than the feed gas
itself or gas that is produced at or near the LNG process plant) as
typical proposed gas expander cycles do, yet such refrigeration
cycles have a higher efficiency. In one or more embodiments,
cooling water or air are the only external sources of cooling
fluids and are used for compressor inter-stage or after
cooling.
[0025] FIG. 1 illustrates one embodiment of the present invention
in which an expander loop 5 (i.e., an expander cycle) and a
sub-cooling loop 6 are used. For clarity, expander loop 5 and
sub-cooling loop 6 are shown with double-width lines in FIG. 1. In
this specification and the appended claims, the terms "loop" and
"cycle" are used interchangeably. In FIG. 1, feed gas stream 10
enters the liquefaction process at a pressure less than about 1200
psia, or less than about 1100 psia, or less than about 1000 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 10 will be about 800 psia. Feed gas
stream 10 generally comprises natural gas that has been treated to
remove contaminants using processes and equipment that are well
known in the art. Before it is passed to a heat exchanger, a
portion of feed gas stream 10 is withdrawn to form side stream 11,
thus providing, as will be apparent from the following discussion,
a refrigerant at a pressure corresponding to the pressure of feed
gas stream 10, namely any of the above pressures, including a
pressure of less than about 1000 psia. Thus, in the embodiment
shown in FIG. 1, a portion of the feed gas stream is used as the
refrigerant for expander loop 5. Although the embodiment shown in
FIG. 1 utilizes a side stream that is withdrawn from feed gas
stream 10 before feed gas stream 10 is passed to a heat exchanger,
the side stream of feed gas to be used as the refrigerant in
expander loop 5 may be withdrawn from the feed gas after the feed
gas has been passed to a heat exchange area. Thus, in one or more
embodiments, the present method is any of the other embodiments
herein described, wherein the portion of the feed gas stream to be
used as the refrigerant is withdrawn from the heat exchange area,
expanded, and passed back to the heat exchange area to provide at
least part of the refrigeration duty for the heat exchange
area.
[0026] Side stream 11 is passed to compression unit 20 where it is
compressed to a pressure greater than or equal to about 1500 psia,
thus providing compressed refrigerant stream 12. Alternatively,
side stream 11 is compressed to a pressure greater than or equal to
about 1600 psia, or greater than or equal to about 1700 psia, or
greater than or equal to about 1800 psia, or greater than or equal
to about 1900 psia, or greater than or equal to about 2000 psia, or
greater than or equal to about 2500 psia, or greater than or equal
to about 3000 psia, thus providing compressed refrigerant stream
12. As used in this specification, including the appended claims,
the term "compression unit" means 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" may
utilize one or more compression stages. Illustrative 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.
[0027] After exiting compression unit 20, compressed refrigerant
stream 12 is passed to cooler 30 where it is cooled by indirect
heat exchange with a suitable cooling fluid to provide a
compressed, cooled refrigerant. In one or more embodiments, cooler
30 is of the type that provides water or air as the cooling fluid,
although any type of cooler can be used. The temperature of
compressed refrigerant stream 12 as it emerges from cooler 30
depends on the ambient conditions and the cooling medium used and
is typically from about 35.degree. F. to about 105.degree. F.
Cooled compressed refrigerant stream 12 is then passed to expander
40 where it is expanded and consequently cooled to form expanded
refrigerant stream 13. In one or more embodiments, expander 40 is a
work-expansion device, such as gas expander producing work that may
be extracted and used for compression.
[0028] Expanded refrigerant stream 13 is passed to heat exchange
area 50 to provide at least part of the refrigeration duty for heat
exchange area 50. As used in this specification, including the
appended claims, the term "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.
[0029] Upon exiting heat exchange area 50, expanded refrigerant
stream 13 is fed to compression unit 60 for pressurization to form
stream 14, which is then joined with side stream 11. It will be
apparent that once expander loop 5 has been filled with feed gas
from side stream 11, only make-up feed gas to replace losses from
leaks is required, the majority of the gas entering compressor unit
20 generally being provided by stream 14. The portion of feed gas
stream 10 that is not withdrawn as side stream 11 is passed to heat
exchange area 50 where it is cooled, at least in part, by indirect
heat exchange with expanded refrigerant stream 13. After exiting
heat exchange area 50, feed gas stream 10 is passed to heat
exchange area 55. The principal function of heat exchange area 55
is to sub-cool the feed gas stream. Thus, in heat exchange area 55
feed gas stream 10 is sub-cooled by sub-cooling loop 6 (described
below) to produce sub-cooled stream 10a. Sub-cooled stream 10a is
then expanded to a lower pressure in expander 70, thereby partially
liquefying sub-cooled stream 10a to form a liquid fraction and a
remaining vapor fraction. Expander 70 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. Partially liquefied sub-cooled stream 10a is passed
to surge tank 80 where the liquefied fraction 15 is withdrawn from
the process as LNG having a temperature corresponding to the bubble
point pressure. The remaining vapor fraction (flash vapor) stream
16 is used as fuel to power the compressor units and/or as a
refrigerant in sub-cooling loop 6 as described below. Prior to
being used as fuel, all or a portion of flash vapor stream 16 may
optionally be passed from surge tank 80 to heat exchange areas 50
and 55 to supplement the cooling provided in such heat exchange
areas.
[0030] Referring again to FIG. 1, a portion of flash vapor 16 is
withdrawn through line 17 to fill sub-cooling loop 6. Thus, a
portion of the feed gas from feed gas stream 10 is withdrawn (in
the form of flash gas from flash gas stream 16) for use as the
refrigerant in sub-cooling loop 6. It will again be apparent that
once sub-cooling loop 6 is fully charged with flash gas, only
make-up gas (i.e., additional flash vapor from line 17) to replace
losses from leaks is required. In sub-cooling loop 6, expanded
stream 18 is discharged from expander 41 and drawn through heat
exchange areas 55 and 50. Expanded flash vapor stream 18 (the
sub-cooling refrigerant stream) is then returned to compression
unit 90 where it is re-compressed to a higher pressure and warmed.
After exiting compression unit 90, the re-compressed sub-cooling
refrigerant stream is cooled in cooler 31, which can be of the same
type as cooler 30, although any type of cooler may be used. After
cooling, the re-compressed sub-cooling refrigerant stream is passed
to heat exchange area 50 where it is further cooled by indirect
heat exchange with expanded refrigerant stream 13, sub-cooling
refrigerant stream 18, and, optionally, flash vapor stream 16.
After exiting heat exchange area 50, the re-compressed and cooled
sub-cooling refrigerant stream is expanded through expander 41 to
provide a cooled stream which is then passed through heat exchange
area 55 to sub-cool the portion of the feed gas stream to be
finally expanded to produce LNG. The expanded sub-cooling
refrigerant stream exiting from heat exchange area 55 is again
passed through heat exchange area 50 to provide supplemental
cooling before being re-compressed. In this manner the cycle in
sub-cooling loop 6 is continuously repeated. Thus, in one or more
embodiments, the present method is any of the other embodiments
disclosed herein further comprising providing cooling using a
closed loop (e.g., sub-cooling loop 6) charged with flash vapor
resulting from the LNG production (e.g., flash vapor 16).
[0031] It will be apparent that in the embodiment illustrated in
FIG. 1 (and in the other embodiments described herein) that as feed
gas stream 10 passes from one heat exchange area to another, the
temperature of feed gas stream 10 will be reduced until ultimately
a sub-cooled stream is produced. In addition, as side streams are
taken from feed gas stream 10, the mass flow rate of feed gas
stream 10 will be reduced. Other modifications, such as
compression, may also be made to feed gas stream 10. While each
such modification to feed gas stream 10 could be considered to
produce a new and different stream, for clarity and ease of
illustration, the feed gas stream will be referred to as feed gas
stream 10 unless otherwise indicated, with the understanding that
passage through heat exchange areas, the taking of side streams,
and other modifications will produce temperature, pressure, and/or
flow rate changes to feed gas stream 10.
[0032] FIG. 2 illustrates another embodiment of the present
invention that is similar to the embodiment shown in FIG. 1, except
that expander loop 5 has been replaced with expander loop 7. The
other items in FIG. 2 have been previously described above.
Expander loop 7 is shown with double-width lines in FIG. 2 for
clarity. Expander loop 7 utilizes substantially the same equipment
as expander loop 5 (for example, compressor 20, cooler 30, and
expander 40, all of which have been described above). The gaseous
refrigerant in expander loop 7 however, is de-coupled from the feed
gas and may therefore have a different composition than the feed
gas. That is, expander loop 7 is essentially a closed loop and is
not connected to feed gas stream 10. The refrigerant for expander
loop 7 is therefore not necessarily the feed gas, although it may
be. Expander loop 7 may be charged with any suitable refrigerant
gas that is produced at or near the LNG process plant in which
expander loop 7 is utilized. For example, the refrigerant gas used
to charge expander loop 7 could be a feed gas, such as natural gas,
that has only been partially treated to remove contaminants.
[0033] Like expander loop 5, expander loop 7 is a high pressure gas
loop. Stream 12a exits compression unit 20 at a pressure greater
than or equal to about 1500 psia, or greater than or equal to about
1600 psia, or greater than or equal to about 1700 psia, or greater
than or equal to about 1800 psia, or greater than or equal to about
1900 psia, or greater than or equal to about 2000 psia, or greater
than or equal to about 2500 psia, or greater than or equal to about
3000 psia. The temperature of compressed refrigerant stream 12a as
it emerges from cooler 30 depends on the ambient conditions and the
cooling medium used and is typically about from about 35.degree. F.
to about 105.degree. F. Cooled compressed refrigerant stream 12a is
then passed to expander 40 where it is expanded and further cooled
to form expanded refrigerant stream 13a. Expanded refrigerant
stream 13a is passed to heat exchange area 50 to provide at least
part of the refrigeration duty for heat exchange area 50, where
feed gas stream 10 is at least partially cooled by indirect heat
exchange with expanded refrigerant stream 13a. Upon exiting heat
exchange area 50, expanded refrigerant stream 13a is returned to
compression unit 20 for re-compression. In any of the embodiments
described herein, expander loops 5 and 7 may be used
interchangeably. For example, in an embodiment utilizing expander
loop 5, expander loop 7 may be substituted for expander loop 5.
[0034] FIG. 3 shows another embodiment for producing LNG in
accordance with the process of the invention. The process
illustrated in FIG. 3 utilizes a plurality of work expansion cycles
to provide supplemental cooling for the feed gas and other streams.
The use of such work expansion cycles results in overall improved
efficiency for the liquefaction process. Referring to FIG. 3, feed
gas stream 10 again enters the liquefaction process at the
pressures described above. In the particular embodiment shown in
FIG. 3, side stream 11 is fed to expander loop 5 in the manner
previously described, but it will be apparent that closed expander
loop 7 could be utilized in the place of expander loop 5, in which
case side stream 11 would not be necessary. Expander loop 5
operates in the same manner as described above for the embodiment
shown in FIG. 1, except that expanded refrigerant stream 13 is
passed through heat exchange area 56, described in detail below, to
provide at least a part of the refrigeration duty for heat exchange
area 56.
[0035] The portion of feed gas stream 10 that is not withdrawn as
side stream 11 is passed to heat exchange area 56 where it is
cooled, at least in part, by indirect heat exchange with expanded
refrigerant stream 13 and other streams described below. After
exiting heat exchange area 56, feed gas stream 10 is passed through
heat exchange areas 57 and 58 where it is further cooled by
indirect heat exchange with additional streams described below. In
the present embodiment, first and second work expansion cycles are
utilized for improved efficiency as follows: before feed gas stream
10 enters heat exchange area 57, side stream 11b is taken from feed
gas stream 10. After feed gas stream 10 exits heat exchange area
57, but before it enters heat exchange area 58, side stream 11c is
taken from feed gas stream 10. Thus, side streams 11b and 11c are
taken from feed gas stream 10 at different stages of feed gas
stream cooling. That is, each side stream is withdrawn from the
feed gas stream at a different point on the cooling curve of the
feed gas such that each successively withdrawn side stream has a
lower initial temperature than the previously withdrawn side
stream.
[0036] Side stream 11b, which is part of the first work expansion
cycle, is passed to expander 42 where it is expanded and
consequently cooled to form expanded stream 13b. Expanded stream
13b is passed through heat exchange areas 56 and 57 to provide at
least part of the refrigeration duty for heat exchange areas 56 and
57. Similarly, side stream 11c, which is part of the second work
expansion cycle, is passed to expander 43 where it is expanded and
consequently cooled to form expanded stream 13c. Expanded stream
13c is then passed through heat exchange areas 56, 57, and 58 to
provide at least part of the refrigeration duty for heat exchange
areas 56, 57, and 58. Accordingly, feed gas stream 10 is also
cooled in heat exchange areas 56 and 57 by indirect heat exchange
with expanded streams 13b and 13c. In heat exchange area 58 feed
gas stream 10 is also cooled by additional indirect heat exchange
with expanded stream 13c.
[0037] Upon exiting heat exchange area 56, expanded streams 13b and
13c are passed to compression units 61 and 62, respectively, where
they are re-compressed and combined to form stream 14a. Stream 14a
is cooled by cooler 32 prior to being re-combined with feed gas
stream 10. Cooler 32 can be the same type of cooler or cooler types
as coolers 30 and 31. Expanders 42 and 43 are work expansion
devices of the type well know to those of skill in the art.
Illustrative, non-limiting examples of suitable work expansion
devices include liquid expanders and hydraulic turbines. Thus, in
the embodiment shown in FIG. 3, the feed gas stream is further
cooled using a plurality of work expansion devices. It will be
apparent to those of ordinary skill in the art that additional work
expansion cycles can be added to the embodiment illustrated in FIG.
3, or that a single work expansion cycle could be employed.
Generally, therefore, one or more work expansion devices may be
employed in the manner described above. Each of the work expansion
devices expands a portion of the feed gas stream and thereby cools
such portion, wherein each of the portions of the feed gas stream
expanded in the work expansion devices is withdrawn from the feed
gas stream at a different stage of feed gas stream cooling (i.e.,
at a different feed gas stream temperature).
[0038] In one or more other embodiments according to the present
invention, the work expansion devices are utilized by withdrawing
one or more side streams from the feed gas stream; passing said one
or more side streams to one or more work expansion devices;
expanding said one of more side streams to expand and cool said one
or more side streams, thereby forming one or more expanded, cooled
side streams; passing said one or more expanded, cooled side
streams to at least one heat exchange area; passing said gas stream
through said at least one heat exchange area; and at least
partially cooling said gas stream by indirect heat exchange with
said one or more expanded, cooled side streams.
[0039] Referring again to FIG. 3, feed gas stream 10, after being
cooled in heat exchange areas 56, 57, and 58, is then passed to
heat exchange area 59 where it is further cooled to produce
sub-cooled stream 10a. The principal function of heat exchange area
59 is to sub-cool feed gas stream 10. Sub-cooled stream 10a is then
expanded to a lower pressure in expander 85, thereby partially
liquefying sub-cooled stream 10a to form a liquid fraction and a
remaining vapor fraction. Expander 85 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. Partially liquefied sub-cooled stream 10a is passed
to surge tank 80 where the liquefied fraction 15 is withdrawn from
the process as LNG having a temperature corresponding to the bubble
point pressure. The remaining vapor fraction (flash vapor) stream
16 is used as fuel to power the compressor units and/or as a
refrigerant in sub-cooling loop 8 in a manner substantially the
same as previously described for sub-cooling loop 6. As can be seen
from FIG. 3, sub-cooling loop 8 is similar to sub-cooling loop 6,
except that sub-cooling loop 8 supplies cooling to four heat
exchange areas (heat exchange areas 56, 57, 58, and 59).
[0040] FIG. 4 illustrates yet another embodiment of the present
invention. The embodiment shown in FIG. 4 is substantially the same
as the embodiment shown in FIG. 3, except that compression unit 25
and expander 35 have been added. Expander 35 may be any type of
liquid expander or hydraulic turbine. Expander 35 is placed between
heat exchange areas 58 and 59 such that feed gas stream 10 flows
from heat exchange area 58 into expander 35 where it is expanded,
and consequently cooled to produce expanded feed gas stream 10b.
Stream 10b then is passed to heat exchange area 59 where it is
sub-cooled to produce sub-cooled stream 10c. By expanding and
consequently cooling feed gas stream 10 in expander 35 to produce
stream 10b, the overall cooling load on sub-cooling loop 8 is
advantageously reduced. Thus, in one or more embodiments, the
present method is any of the other embodiments disclosed herein
further comprising expanding at least a portion of the cooled feed
gas stream to produce a cooled, expanded feed gas stream (e.g.,
stream 10b); and further cooling the cooled, expanded feed gas
stream by indirect heat exchange with a closed loop (e.g.,
sub-cooling loop 6 or 8) charged with flash vapor resulting from
the LNG production (e.g., flash vapor 16).
[0041] Continuing to refer to FIG. 4, compression unit 25 is
utilized to increase the pressure of feed gas stream 10 prior to
entry into the liquefaction process. Thus, feed gas stream 10 is
passed to compression unit 25 where it is compressed to a pressure
above the feed gas supply pressure or, in one or more other
embodiments, to a pressure greater than about 1200 psia.
Alternatively, feed gas stream 10 is compressed to a pressure
greater than or equal to about 1300 psia, or greater than or equal
to about 1400 psia, or greater than or equal to about 1500 psia, or
greater than or equal to about 1600 psia, or greater than or equal
to about 1700 psia, or greater than or equal to about 1800 psia, or
greater than or equal to about 1900 psia, or greater than or equal
to about 2000 psia, or greater than or equal to about 2500 psia, or
greater than or equal to about 3000 psia. After compression, feed
gas stream 10 is passed to cooler 33 where it is cooled prior to
being passed to heat exchange area 56. It will be appreciated that
to the extent compression unit 25 is used to compress feed gas
stream 10 (and, hence, side stream 11) to a lower pressure than
that desired for compressed refrigerant stream 12, compression unit
20 may be used to boost the pressure.
[0042] The compression of feed gas stream 10 as described above
provides three benefits. First, by increasing the pressure of the
feed gas stream, the pressures of side streams 11b and 11c are also
increased, with the result that the cooling performance of work
expansion devices 42 and 43 is enhanced. Second, the heat transfer
coefficient in the heat exchange areas is improved. Thus, in one or
more embodiments, the process for producing LNG described herein is
carried out according to any of the other embodiments describe
herein wherein the feed gas is compressed to the pressures
described above prior to entry into a heat exchange area. In still
other embodiments, the present method comprises providing
supplemental cooling for the feed gas stream from a plurality of
work expansion devices, each of the work expansion devices
expanding a portion of the feed gas stream and thereby cooling the
portion to form one or more expanded, cooled side streams, wherein
each of the portions of the feed gas stream expanded in the work
expansion devices is withdrawn from the feed gas stream at a
different stage of feed gas stream cooling (i.e., at a different
feed gas stream temperature); and cooling said feed gas stream by
indirect heat exchange with said one or more expanded, cooled side
streams.
[0043] In still other embodiments, each of the above-described
portions of feed gas has a pressure, prior to expansion, greater
than about 1200 psia, or greater than or equal to about 1300 psia,
or greater than or equal to about 1400 psia, or greater than or
equal to about 1500 psia, or greater than or equal to about 1600
psia, or greater than or equal to about 1700 psia, or greater than
or equal to about 1800 psia, or greater than or equal to about 1900
psia, or greater than or equal to about 2000 psia, or greater than
or equal to about 2500 psia, or greater than or equal to about 3000
psia. In yet other embodiments, the present method is any of the
other embodiments described herein further comprising compressing
the feed gas stream to any of the pressures described above to
produce a pressurized feed gas stream; feeding the pressurized feed
gas stream to a work expansion device, or to a plurality of work
expansion devices; expanding the compressed feed gas stream through
the work expansion device, or through a plurality of work expansion
devices, to provide supplemental cooling for the feed gas
stream.
[0044] A third benefit obtained by compression the feed gas stream
as described above is that the cooling capacity of expander 35 is
improved, with the result that expander 35 is able to even further
reduce the cooling load on sub-cooling loop 8. It will be
appreciated that compression unit 25 and/or expander 35 could also
be advantageously added to other embodiments described herein to
provide similar reductions in the cooling load on the sub-cooling
loops utilized in those embodiments or other improvements in
cooling, and that compression unit 25 and expander 35 may be used
independently of each other in any embodiment herein. Moreover, it
will also be appreciated that the cooling capacity of expander 35
(or the work expansion devices 42 and 43) will be improved, even
without compression of the feed stream, to the extent the feed
stream is supplied at a pressure above the bubble point pressure of
the LNG. For example, if the feed gas is supplied at any of the
pressures described above resulting from compression of the feed
gas, the benefit of such pressure will obviously be obtainable
without additional compression. Therefore, in interpreting this
specification, including the appended claims, the use of work
expansion devices and/or expander 35 to expand streams having
pressures above about 1200 psia should not be construed as
requiring the use or presence of compression unit 25 or of any
other compressor or compression step.
[0045] FIG. 5 is a schematic flow diagram of a fifth embodiment for
producing LNG in accordance with the process of this invention that
is similar to the embodiment shown in FIG. 4, but utilizes yet
another expansion step to provide sub-cooling. Referring to FIG. 5,
it will be seen that sub-cooling loop 8 is not present in the
embodiment shown in FIG. 5. Instead, side stream 11d is taken from
stream 10b and passed to expansion device 105 where it is expanded
and consequently cooled to form expanded stream 13d. Expansion
device 105 is a work-producing expander, many types of which are
readily available. Illustrative, non-limiting examples of such
devices include liquid expanders and hydraulic turbines. Expanded
stream 13d is passed through heat exchange areas 59, 58, 57, and 56
to provide at least part of the refrigeration duty for those heat
exchange areas. As can be seen from FIG. 5, stream 10b is also
cooled by indirect heat exchange with expanded stream 13d, as well
as by the flash vapor stream 16. Thus, in one or more embodiments,
the inventive process further comprises expanding at least a
portion of the cooled gas stream (feed gas stream 10) in expander
35 before the final heat exchange step (for example, prior to heat
exchange area 59) to produce an expanded, cooled gas stream (for
example, stream 10b); passing a portion of said expanded, cooled
gas stream to a work-producing expander; further expanding said
expanded, cooled gas stream in said work-producing expander; and
passing the stream emerging from said work-producing expander (for
example, stream 13d) to a heat exchange area to further cool said
expanded, cooled gas stream by indirect heat exchange in said heat
exchange area.
[0046] Upon exiting heat exchange area 56, expanded stream 13d is
passed to compression unit 95 where it is re-compressed and
combined with the streams emerging from compression units 61 and 62
to form part of stream 14a, which is cooled and then re-cycled to
feed stream 10 as before.
[0047] A further embodiment shown in FIG. 6 is similar to the
embodiment shown in FIG. 1 and described above, except that
sub-cooling loop 6 has been modified such that after exiting heat
exchange area 50, the re-compressed and cooled sub-cooling
refrigerant stream is further cooled in heat exchange area 55 prior
to being expanded through expander 41. This embodiment is favorable
where a cooling fluid is used that does not present much
condensation after expander 41.
[0048] FIG. 7 depicts another embodiment in which sub-cooling loop
6a uses a portion of feed gas 10. The portion of feed gas 10 is
re-pressurized in compressor 25 and cooled in cooler 33 from 201,
in the same fashion as in FIG. 4.
[0049] FIG. 8 is another embodiment similar to FIG. 7 showing an
alternative arrangement for the sub-cooling loop 6. Depending on
the composition of feed gas 10, an additional compressor (not
shown) may be used to prevent condensation in the sub-cooling loop
or to ensure adequate line pressures.
[0050] FIG. 9 depicts an embodiment for use with certain feed gas
10 compositions and/or pressures. To better match the cooling curve
of the feed gas 10 being cooled for LNG collection, to the cooling
curve of that portion of feed gas 10 being used for cooling in
sub-cooling heat exchange area 55, it may be necessary to further
expand a split of the portion of the refrigerant gas going to the
sub-cooling loop 6. This is accomplished using an expansion valve
82 or other expander (e.g., a Joules-Thompson valve) to provide
supplemental cooling in sub-cooling loop 6.
[0051] FIG. 10 represents another embodiment showing the
integration of a nitrogen rejection stage using distillation column
81 or equivalent device, for the case where nitrogen rejection is
needed, based on feed gas 10 composition. This may be needed to
meet the nitrogen specification of product LNG for transmission and
end use.
[0052] FIG. 11 represents another embodiment showing the
integration of a nitrogen rejection unit, where the flash vapor
from the nitrogen rejection unit is used as refrigerant for the
sub-cooling loop. The resulting refrigerant is therefore rich in
nitrogen.
EXAMPLE
[0053] A hypothetical mass and energy balance was carried out to
illustrate the embodiment shown in FIG. 4, and the results are
shown in the Table below. The data were obtained using a
commercially available process simulation program called HYSYS.TM.
(available from Hyprotech Ltd. of Calgary, Canada); however, other
commercially available process simulation programs can be used to
develop the data, including for example HYSIM.TM., PROII.TM., and
ASPEN PLUS.TM., which are familiar to those of ordinary skill in
the art. This example assumed that feed gas stream 10 had the
following composition in mole percent: C.sub.1: 90.25%; C.sub.2:
5.70%; C.sub.3: 0.01%; N.sub.2: 4.0%; He: 0.04%. The data presented
in the Table are offered to provide a better understanding of the
embodiment shown in FIG. 4, but the invention is not to be
construed as unnecessarily limited thereto. The temperatures,
pressures, and flow rates can have many variations in view of the
teachings herein. The specific temperature, pressure, and flow rate
calculated for state points 201 through 214 (at the locations shown
in FIG. 4) are set forth in the Table.
[0054] In one embodiment of the inventive method, by controlling
the temperature of the stream emerging from the final heat exchange
area, the volume of flash vapor stream 16 is controlled to match
the fuel requirements of the compression units and other equipment.
For example, referring to FIG. 4, the temperature at state point
207 can be controlled to produce more or less flash vapor (stream
16) depending on the fuel requirements. Higher temperatures at
state point 207 will result in the production of more flash vapor
(and hence more available fuel), and vice-versa. Alternatively, the
temperature may be adjusted such that the flash vapor flow rate is
higher than the fuel requirement, in which case the excess flow
above the fuel flow requirement may be recycled after compression
and cooling.
TABLE-US-00001 TABLE Temperature Pressure Flow State Point (deg.
F.) (psia) (lb-mole/hr) 201 262 985 3.35 .times. 10.sup.5 202 100
1500 1.08 .times. 10.sup.6 203 -36 1480 4.85 .times. 10.sup.5 204
-130 1470 3.35 .times. 10.sup.5 205 -213 1460 3.35 .times. 10.sup.5
206 -229 48 3.35 .times. 10.sup.5 207 -236 42 3.35 .times. 10.sup.5
208 -254 18 3.35 .times. 10.sup.5 209 -217 71 3.12 .times. 10.sup.5
210 -140 420 2.29 .times. 10.sup.4 211 100 126 2.57 .times.
10.sup.4 212 -240 44 2.57 .times. 10.sup.4 213 100 3000 8.57
.times. 10.sup.5 214 -40 895 8.57 .times. 10.sup.5
[0055] A person skilled in the art, particularly one having the
benefit of the teachings herein, will recognize many modifications
and variations to the specific embodiments disclosed above. For
example, features shown in one embodiment may be added to other
embodiments to form additional embodiments. Thus, the specifically
disclosed embodiments and example should not be used to limit or
restrict the scope of the invention, which is to be determined by
the claims that follow.
* * * * *