U.S. patent application number 14/853010 was filed with the patent office on 2016-03-17 for method and system for treating and liquefying natural gas.
The applicant listed for this patent is Propak Systems Ltd.. Invention is credited to W. Brent Mealey, Steven Shi.
Application Number | 20160076808 14/853010 |
Document ID | / |
Family ID | 55454399 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160076808 |
Kind Code |
A1 |
Mealey; W. Brent ; et
al. |
March 17, 2016 |
METHOD AND SYSTEM FOR TREATING AND LIQUEFYING NATURAL GAS
Abstract
A method for liquefying natural gas comprises: condensing at
least a portion of a natural gas feed stream to produce a partially
condensed stream; and separating the partially condensed stream
into a first liquid fraction and a first vapour fraction; wherein
the first liquid fraction comprises liquefied natural gas.
Inventors: |
Mealey; W. Brent; (Calgary,
CA) ; Shi; Steven; (Calgary, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Propak Systems Ltd. |
Airdrie |
|
CA |
|
|
Family ID: |
55454399 |
Appl. No.: |
14/853010 |
Filed: |
September 14, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62050447 |
Sep 15, 2014 |
|
|
|
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 1/004 20130101;
F25J 2230/30 20130101; F25J 1/0052 20130101; F25J 1/0215 20130101;
F25J 1/0022 20130101; F25J 1/0092 20130101; F25J 1/0278 20130101;
F25J 1/0288 20130101; F25J 1/005 20130101; F25J 3/0635 20130101;
F25J 1/0214 20130101; F25J 3/0645 20130101; F25J 1/025 20130101;
F25J 2215/04 20130101; F25J 1/0087 20130101; F25J 2220/64 20130101;
F25J 1/0082 20130101; F25J 3/061 20130101; F25J 2220/62 20130101;
F25J 3/066 20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Claims
1. A method for liquefying natural gas, the method comprising:
condensing at least a portion of a natural gas feed stream to
produce a partially condensed stream; and separating the partially
condensed stream into a first liquid fraction and a first vapour
fraction; wherein the first liquid fraction comprises liquefied
natural gas.
2. The method of claim 1, wherein separating the partially
condensed stream into the first liquid fraction and the first
vapour fraction comprises directing the partially condensed stream
into a first flash separator to concentrate nitrogen in the first
vapour fraction and reduce the concentration of nitrogen in the
first liquid fraction.
3. The method of claim 1, further comprising: condensing at least a
portion of the first vapour fraction to produce a partially
condensed first vapour fraction; and separating the partially
condensed first vapour fraction into a second liquid fraction and a
second vapour fraction; wherein the second liquid fraction
comprises liquefied natural gas.
4. The method of claim 3, wherein condensing at least a portion of
the first vapour fraction comprises heating the first vapour
fraction in a first indirect heat exchanger with the natural gas
feed stream, thereby providing a portion of the cooling and
condensing duty and reducing the refrigeration duty, compressing,
after-cooling in a second indirect heat exchanger, and then cooling
and partially condensing the first vapour fraction in the first
indirect heat exchanger.
5. The method of claim 3, wherein separating the partially
condensed first vapour fraction into the second liquid fraction and
the second vapour fraction comprises directing the partially
condensed first vapour fraction into a second flash separator to
concentrate nitrogen in the second vapour fraction and reduce the
concentration of nitrogen in the second liquid fraction, thereby
reducing methane component loss to the fuel gas product stream and
increasing the liquefied natural gas (LNG) product yield.
6. The method of claim 5, further comprising compressing,
after-cooling in a separate exchanger, and then cooling and
partially condensing the second vapour fraction to produce a
partially condensed second vapour fraction, directing the partially
condensed second vapour fraction into a third flash separator to
concentrate nitrogen in a third vapour fraction and reduce the
concentration of nitrogen in a third liquid fraction, thereby
further reducing methane component loss to the fuel gas product
stream and further increasing the liquefied natural gas (LNG)
product yield.
7. The method of claim 1, further comprising condensing heavy
hydrocarbon components in the natural gas feed stream and
separating the natural gas feed stream into a heavy hydrocarbon
liquid fraction and a residual vapour fraction, wherein at least a
portion of the residual vapour fraction is condensed and directed
to the partially condensed stream.
8. The method of claim 7, further comprising partially flashing the
heavy hydrocarbon liquid fraction and distilling heavy hydrocarbons
from the heavy hydrocarbon liquid fraction, leaving a residual
fraction, wherein the residual fraction is directed to the natural
gas feed stream for further processing.
9. The method of claim 1, further comprising directing at least a
portion of the first vapour fraction to one or more refrigerant
cycles for use as a refrigerant medium.
10. The method of claim 9, wherein the one or more refrigerant
cycles comprises a first methane refrigerant cycle that provides a
portion of the duty required to cool and condense at least one of:
the natural gas feed stream into the partially condensed stream;
the first vapour fraction into the partially condensed first vapour
fraction; the second vapour fraction into the partially condensed
second vapour fraction; and the heavy hydrocarbon components in the
natural gas feed stream; and/or to pre-cool the methane refrigerant
stream.
11. The method of claim 10, wherein the first methane refrigerant
cycle comprises the following steps: compressing at least a portion
of the first vapour fraction, which is a low pressure methane
refrigerant vapour stream, to produce an outlet stream; compressing
and pre-cooling the outlet stream to produce a pre-cooled high
pressure methane refrigerant stream; expanding the pre-cooled high
pressure methane refrigerant stream to produce expanded and cooled
methane refrigerant stream; heating the expanded and cooled methane
refrigerant stream to produce the low pressure methane refrigerant
vapour stream, thus completing the first methane refrigerant
cycle.
12. The method of claim 10, wherein the one or more refrigerant
cycles comprises a second methane refrigerant cycle that provides a
portion of the duty required to pre-cool the first vapour fraction
for use in the first methane refrigerant cycle.
13. The method of claim 12, wherein the second methane refrigerant
cycle further provides a portion of the duty required to cool and
condense at least one of: the natural gas feed stream into the
partially condensed stream; the first vapour fraction into the
partially condensed first vapour fraction; the second vapour
fraction into the partially condensed second vapour fraction; and
the heavy hydrocarbon components in the natural gas feed
stream.
14. The method of claim 12, wherein the second methane refrigerant
cycle comprises the following steps: compressing at least a portion
of the first vapour fraction, which is a low pressure pre-cooling
methane refrigerant vapour stream, to produce an outlet stream;
compressing the outlet stream to produce a pre-cooled methane
refrigerant stream; expanding the pre-cooled methane refrigerant
stream to produce expanded and cooled pre-cooling methane
refrigerant stream; heating the expanded and cooled pre--cooling
methane refrigerant stream to produce the low pressure pre-cooling
methane refrigerant vapour stream, thus completing the second
methane refrigerant cycle.
15. The method of claim 12, wherein the second methane refrigerant
cycle is used in place of a propane refrigeration cycle.
16. The method of claim 1, Wherein at least a portion of the duty
required to cool at least one of: the first vapour fraction for use
in the first methane refrigerant cycle; the natural gas feed stream
into the partially condensed stream; the first vapour fraction into
the partially condensed first vapour fraction; the second vapour
fraction into the partially condensed second vapour fraction; and
the heavy hydrocarbon components in the natural gas feed stream is
provided by a two-stage closed loop propane refrigeration
cycle.
17. The method of claim 16, wherein the propane refrigeration cycle
comprises a high pressure cycle and a low pressure cycle, linked
through a region of intermediate pressure.
18. The method of claim 17, wherein the propane refrigeration cycle
comprises the following steps: reducing the pressure on a high
pressure saturated liquid phase propane refrigerant stream to
produce an intermediate pressure stream; separating the
intermediate pressure stream into an intermediate pressure propane
refrigerant liquid fraction and an intermediate pressure propane
vapour fraction; heating a first portion of the intermediate
pressure propane refrigerant liquid fraction to produce a first
vapour-liquid stream; combining the first vapour-liquid stream with
the intermediate pressure stream; reducing the pressure on a second
portion of the intermediate pressure propane refrigerant liquid
fraction to produce a low pressure stream; separating the low
pressure stream into a low pressure propane refrigerant liquid
fraction and a low pressure propane refrigerant vapour fraction;
heating the low pressure propane refrigerant liquid fraction to
produce a second vapour-liquid stream; combining the second
vapour-liquid stream with the low pressure stream, compressing the
low pressure propane refrigerant vapour fraction to produce a
stream that is combined with the intermediate pressure propane
refrigerant vapour fraction to produce a combined intermediate
pressure propane refrigerant vapour fraction; compressing the
combined intermediate pressure propane refrigerant vapour fraction
to produce a high pressure propane refrigerant vapour stream; and
condensing the high pressure propane refrigerant vapour stream to
produce the high pressure saturated liquid phase propane
refrigerant stream, thus completing the propane refrigeration
cycle.
19. A system for liquefying natural gas, The system comprising: a
first condensing system for condensing at least a portion of a
natural gas feed stream to produce a partially condensed stream;
and a first flash separator for separating the partially condensed
stream into a first liquid fraction and a first vapour fraction;
wherein the first liquid fraction comprises liquefied natural
gas.
20. The system of claim 19, wherein the first flash separator
concentrates nitrogen in the first vapour fraction and reduces the
concentration of nitrogen in the first liquid fraction.
21. The system of claim 19, further comprising: a second condensing
system for condensing at least a portion of the first vapour
fraction to produce a partially condensed first vapour fraction;
and a second flash separator for separating the partially condensed
first vapour fraction into a second liquid fraction and a second
vapour fraction; wherein the second liquid fraction comprises
liquefied natural gas.
22. The system of claim 21, wherein the second flash separator
concentrates nitrogen in the second vapour fraction and reduces the
concentration of nitrogen in the second liquid fraction.
23. The system of claim 22, further comprising a third condensing
system and a third flash separator for concentrating nitrogen in a
third vapour fraction and reducing the concentration of nitrogen in
a third liquid fraction.
24. The system of claim 19, further comprising: a heavy hydrocarbon
condensing system for condensing heavy hydrocarbon components in
the natural gas feed stream; and a flash separator for separating
the natural gas feed stream into a heavy hydrocarbon liquid
fraction and a residual vapour fraction and directing at least a
portion of the residual vapour fraction to the partially condensed
stream.
25. The system of claim 24, wherein the heavy hydrocarbon
condensing system comprises at least one of an inlet gas
compressor, an inlet gas compressor after-cooler, and an indirect
heat exchanger.
26. The system of claim 24, further comprising a heavy hydrocarbon
distiller for distilling, heavy hydrocarbons from the heavy
hydrocarbon liquid fraction, leaving a residual fraction, and
directing the residual fraction to the natural gas feed stream for
further processing.
27. The system of claim 19, further comprising a valve for
directing at least a portion of the first vapour fraction to one or
more refrigerant cycles for use as a refrigerant medium.
28. The system of claim 27, wherein the one or more refrigerant
cycles comprises a first methane refrigerant cycle that provides a
portion of the duty required to cool and condense at least one of:
the natural gas feed stream into the partially condensed stream;
the first vapour fraction into the partially condensed first vapour
fraction; the second vapour fraction into the partially condensed
second vapour fraction; and the heavy hydrocarbon components in the
natural gas feed stream; and/or to pre-cool the methane refrigerant
stream, wherein the first methane refrigerant cycle comprises at
least one of an expander brake compressor, a first heat exchanger,
a multi-stage methane refrigerant compressor, a second heat
exchanger, an indirect heat exchanger, a methane refrigerant
expander, and an expander brake compressor.
29. The system of claim 27, wherein the one or more refrigerant
cycles comprises a second methane refrigerant cycle that provides a
portion of the duty required to pre-cool the first vapour fraction
for use in the first methane refrigerant cycle.
30. The system of claim 29, wherein the second methane refrigerant
cycle further provides a portion of the duty required to cool and
condense at least one of: the natural gas feed stream into the
partially condensed stream; the first vapour fraction into the
partially condensed first vapour fraction; the second vapour
fraction into the partially condensed second vapour fraction; and
the heavy hydrocarbon components in the natural gas feed stream,
wherein the second methane refrigerant cycle comprises at least one
of a pre cooling expander brake compressor, a first heat exchanger,
a multi-stage pre-cooling methane refrigerant compressor, a second
heat exchanger, a methane expander, and an indirect heat
exchanger.
31. The system of claim 27, wherein the second methane refrigerant
cycle is used in place of a propane refrigeration cycle.
32. The system of claim 19, wherein at least a portion of the duty
required to cool at least one of: the first vapour fraction for use
in the first methane refrigerant cycle; the natural gas feed stream
into the partially condensed stream; the first vapour fraction into
the partially condensed first vapour fraction; the second vapour
fraction into the partially condensed second vapour fraction; and
the heavy hydrocarbon components in the natural gas feed stream is
provided by a two-stage closed loop propane refrigeration
cycle.
33. The system of claim 32, wherein the propane refrigeration cycle
comprises a high pressure cycle and a low pressure cycle, linked
through a region of intermediate pressure.
34. A methane refrigerant system for use in liquefying natural gas,
the system comprising a methane gas source derived at least in part
from a vapour fraction of the natural gas feed stream.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 52/050,447, filed Sep. 15, 2014, entitled
"Method and System for Treating and Liquefying Natural Gas," the
entirety of which is hereby incorporated by reference for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates natural gas. More
specifically, the present invention relates to methods and systems
for liquefying natural gas.
BACKGROUND
[0003] Small-scale liquid natural gas (LNG) plants are known and
are often referred to as micro-LNG plants or peak shaving plants.
Large scale LNG plants provide a means to transport large
quantities of natural gas internationally and/or overseas, where
pipelines are not available or practical. In contrast, micro-LNG
plants are typically used to liquefy natural gas for the purpose of
storing the natural gas or for the purpose of transporting it on
and over shorter distances. Micro-LNG plants for natural gas
storage, typically called peak shaving plants, liquefy natural gas
and then store it in storage tanks during periods of low demand.
Micro-LNG plants can also be used to liquefy stranded natural gas
from the source, where no pipeline exists, so that it can then be
transported in a liquid state by a tanker truck. The tanker truck
can then deliver the LNG to another location. Furthermore,
micro-LNG plants can be used to liquefy natural gas so that it can
be stored in a vehicle fuel tank and used as a transportation
fuel.
[0004] The industry demands, particularly for micro-LNG plants,
high energy efficiency (low specific energy) relative to capital
cost, ease of operability, and safety. The energy efficiency for
methods of liquefying natural gas can be measured as the specific
energy required to liquefy the natural gas to produce one metric
ton of LNG (i.e., kW.d/ton) which can be compared relative to its
capital cost. Some known methods of liquefying natural gas have a
high specific energy requirement in the range of 25 to 40 kW.d/ton,
generally due to high operating costs associated with a high
refrigeration energy load, making the natural gas liquefaction
process less economical. Other known methods achieve a relatively
low specific energy requirement in the range of from 12.2 to 20.4
kW.d/ton, however have inherently higher capital costs, and/or have
complex operational and safety disadvantages. In addition, known
methods on not have the capability to integrally remove nitrogen
components, meaning that this component must be removed in separate
processing units adding cost to the LNG plant and thereby reducing
its economic efficiency.
[0005] There is a need for methods and systems for liquefying
natural gas that improve the efficiency of the liquefaction
process.
[0006] There is also a need for methods and systems for integrally
removing heavy hydrocarbon and/or nitrogen components from a
natural gas feed stream.
[0007] There is also a need for methods and systems for improving
the energy efficiency of refrigeration processes associated with
liquefying natural gas.
SUMMARY
[0008] According to an aspect, there is provided a method for
liquefying natural gas, the method comprising: [0009] condensing at
least a portion of a natural gas feed stream to produce a partially
condensed stream; and [0010] separating the partially condensed
stream into a first liquid fraction and a first vapour fraction;
[0011] wherein the first liquid fraction comprises liquefied
natural gas.
[0012] In an aspect, condensing at least a portion of the natural
gas feed stream comprises compressing and cooling the natural gas
feed stream.
[0013] In an aspect, condensing the natural gas feed stream
comprises partially flashing and cooling the partially condensed
stream.
[0014] In an aspect, separating the partially condensed stream into
the first liquid fraction and the first vapour fraction comprises
directing the partially condensed stream into a first flash
separator to concentrate nitrogen in the first vapour fraction and
reduce the concentration of nitrogen in the first liquid
fraction.
[0015] In an aspect, the method further comprises: [0016]
condensing at least a portion of the first vapour fraction to
produce a partially condensed first vapour fraction, and [0017]
separating the partially condensed first vapour fraction into a
second liquid fraction and a second vapour fraction; [0018] wherein
the second liquid fraction comprises liquefied natural gas.
[0019] In an aspect, condensing at least a portion of the first
vapour fraction comprises heating, compressing, and cooling the
first vapour fraction.
[0020] In an aspect, condensing at least a portion of the first
vapour fraction comprises partially flashing and cooling the
partially condensed first vapour fraction.
[0021] In an aspect, separating the partially condensed first
vapour fraction into the second liquid fraction and the second
vapour fraction comprises directing the partially condensed first
vapour fraction into a second flash separator to concentrate
nitrogen in the second vapour fraction and reduce the concentration
of nitrogen in the second liquid fraction.
[0022] In an aspect, the method further comprises compressing,
cooling, and partially condensing the second vapour fraction to
produce a partially condensed second vapour fraction, directing the
partially condensed second vapour fraction into a third flash
separator to concentrate nitrogen in a third vapour fraction and
reduce the concentration of nitrogen in a third liquid
fraction.
[0023] In an aspect, the method comprises repeating the method
until a desired level of separation of nitrogen from the liquefied
natural gas is achieved.
[0024] In an aspect, the method comprises repeating the method
until the volume of vapour fraction is reduced to a desired
level.
[0025] In an aspect, the method further comprises condensing heavy
hydrocarbon components in the natural gas feed stream and
separating the natural gas feed stream into a heavy hydrocarbon
liquid fraction and a residual vapour fraction, wherein at least a
portion of the residual vapour fraction is condensed and directed
to the partially condensed stream.
[0026] In an aspect, the method further comprises partially
flashing the heavy hydrocarbon liquid fraction and distilling heavy
hydrocarbons from the heavy hydrocarbon liquid fraction, leaving a
residual fraction, wherein the residual fraction is directed to the
natural gas feed stream for further processing.
[0027] In an aspect, the method further comprises directing at
least a portion of the first vapour fraction to one or more
refrigerant cycles for use as a refrigerant medium.
[0028] In an aspect, the one or more refrigerant cycles comprises a
first methane refrigerant cycle that provides a portion of the duty
required to cool and condense at least one of: [0029] the natural
gas feed stream into the partially condensed stream; [0030] the
first vapour fraction into the partially condensed first vapour
fraction, [0031] the second vapour fraction into the partially
condensed second vapour fraction; and [0032] the heavy hydrocarbon
components in the natural gas feed stream.
[0033] In an aspect, the first methane refrigerant cycle comprises
the following steps: [0034] compressing at least a portion of the
first vapour fraction, which is a low pressure methane refrigerant
vapour stream, to produce an outlet stream; [0035] compressing and
pre-cooling the outlet stream to produces a pre-cooled high
pressure methane refrigerant stream; [0036] expanding the
pre-cooled high pressure methane refrigerant stream to produce
expanded and cooled methane refrigerant stream; [0037] heating the
expanded and cooled methane refrigerant stream to produce the low
pressure methane refrigerant vapour stream, thus completing the
first methane refrigerant cycle.
[0038] In an aspect, the one or more refrigerant cycles comprises a
second methane refrigerant cycle that provides a portion of the
duty required to pre-cool the first vapour fraction for use in the
first methane refrigerant cycle.
[0039] In an aspect, the second methane refrigerant cycle further
provides a portion of the duty required to cool and condense at
least one of: [0040] the natural gas feed stream into the partially
condensed stream; [0041] the first our fraction into the partially
condensed first vapour fraction; [0042] the second vapour fraction
into the partially condensed second vapour fraction; and [0043] the
heavy hydrocarbon components in the natural gas feed stream.
[0044] In an aspect, the second methane refrigerant cycle comprises
the following steps: [0045] compressing at least a portion of the
first vapour fraction, which is a low pressure pre-cooling methane
refrigerant vapour stream, to produce an outlet stream; [0046]
compressing the outlet stream to produce a pre-cooled methane
refrigerant stream; [0047] expanding the pre-pooled methane
refrigerant stream to produce expanded and cooled pre-cooling
methane refrigerant stream;
[0048] heating the expanded and cooled pre-cooling methane
refrigerant stream to produce the low pressure pre-cooling methane
refrigerant vapour stream, thus completing the second methane
refrigerant cycle.
[0049] In an aspect, the second methane refrigerant cycle is used
in place of propane refrigeration cycle.
[0050] In an aspect, at least a portion of the duty required to
cool at least one of: [0051] the first vapour fraction for use in
the first methane refrigerant cycle; [0052] the natural gas feed
stream into the partially condensed stream; [0053] the first vapour
fraction into the partially condensed first vapour fraction; [0054]
the second vapour fraction into the partially condensed second
vapour fraction; and [0055] the heavy hydrocarbon components in the
natural gas feed stream is provided by a two-stage closed loop
propane refrigeration cycle.
[0056] In an aspect, the propane refrigeration cycle comprises a
high pressure cycle and a low pressure cycle, linked through a
region of intermediate pressure.
[0057] In an aspect, the propane refrigeration cycle comprises the
following steps: [0058] reducing the pressure on a high pressure
saturated liquid phase propane refrigerant stream to produce an
intermediate pressure stream; [0059] separating the intermediate
pressure stream into an intermediate pressure propane refrigerant
liquid fraction and an intermediate pressure propane vapour
fraction; [0060] heating a first portion of the intermediate
pressure propane refrigerant liquid fraction to produce a first
vapour liquid stream; [0061] combining the first vapour-liquid
stream with the intermediate pressure stream; [0062] reducing the
pressure on a second portion of the intermediate pressure propane
refrigerant liquid fraction to produce a low pressure stream;
[0063] separating the low pressure stream into a low pressure
propane refrigerant liquid fraction and a low pressure propane
refrigerant vapour fraction; [0064] heating the low pressure
propane refrigerant liquid fraction to produce a second vapour
liquid stream; [0065] combining the second vapour-liquid stream
with the low pressure stream; [0066] compressing the low pressure
propane refrigerant vapour fraction to produce a stream that is
combined with the intermediate pressure propane refrigerant vapour
fraction to produce a combined intermediate pressure propane
refrigerant vapour fraction; [0067] compressing the combined
intermediate pressure propane refrigerant vapour fraction to
produce a high pressure propane refrigerant vapour stream; and
[0068] condensing the high pressure propane refrigerant vapour
stream to produce the high pressure saturated liquid phase propane
refrigerant stream, thus completing the propane refrigeration
cycle.
[0069] According to another aspect, there is provided a method for
producing a refrigerant medium from a natural gas feed stream for
use in cooling the natural as feed stream, the method comprising:
[0070] condensing at least a portion of the natural gas feed stream
to produce a partially condensed stream; [0071] separating the
partially condensed stream into a first liquid fraction and a first
vapour fraction; and [0072] directing at least a portion of the
first vapour fraction to a refrigerant cycle for use as a
refrigerant medium for cooling the natural gas.
[0073] According to another aspect, there is provided a method for
removing nitrogen from a nature; gas feed stream, The method
comprising: [0074] partially condensing at /east a portion of a
natural gas feed stream to produce a partially condensed stream;
and [0075] separating the partially condensed stream into a first
liquid fraction and a first vapour fraction in a flash separator to
concentrate nitrogen in the first vapour fraction and reduce the
concentration of nitrogen in the first liquid fraction.
[0076] In an aspect, the method further comprises: [0077] partially
condensing at least a portion of the first vapour fraction to
produce a partially condensed first vapour fraction; and [0078]
separating the partially condensed first vapour fraction into a
second liquid fraction and a second vapour fraction in a flash
separator to concentrate nitrogen in the second vapour fraction and
reduce the concentration of nitrogen in the second liquid
fraction.
[0079] In an aspect, the method further comprises: [0080] partially
condensing at least a portion of the second vapour fraction to
produce a partially condensed second vapour fraction; and [0081]
separating the partially condensed second vapour fraction into a
third liquid fraction and a third vapour fraction in a flash
separator to concentrate nitrogen in the third vapour fraction and
reduce the concentration of nitrogen in the third liquid
fraction.
[0082] According to another aspect, there is provided a system for
liquefying natural gas, the system comprising: [0083] a first
condensing system for condensing at least a portion of a natural
gas feed stream to produce a partially condensed stream; and [0084]
a first flash separator for separating the partially condensed
stream into a first liquid fraction and a first vapour fraction;
[0085] wherein the first liquid fraction comprises liquefied
natural gas.
[0086] In an aspect, the first condensing system comprises at least
one of an inlet gas compressor, an inlet gas compressor
after-cooler, an indirect heat exchanger, and a valve.
[0087] In an aspect, the first flash separator concentrates
nitrogen in the first vapour fraction and reduces the concentration
of nitrogen in the first liquid fraction.
[0088] In an aspect, the system further comprises: [0089] a second
condensing system for condensing at least a portion of the first
vapour fraction to produce a partially condensed first vapour
fraction; and [0090] a second flash separator for separating the
partially condensed first vapour fraction into a second liquid
fraction and a second vapour fraction; [0091] wherein the second
liquid fraction comprises liquefied natural gas.
[0092] In an aspect, the second condensing system comprises at east
one of an indirect heat exchanger, a flash gas compressor, a flash
gas compressor after-cooler, and a valve.
[0093] In an aspect, the second flash separator concentrates
nitrogen in the second vapour fraction and reduces the
concentration of nitrogen in the second liquid fraction.
[0094] In an aspect, the system further comprises a third
condensing system and a third flash separator for concentrating
nitrogen in a third vapour fraction and reducing the concentration
of nitrogen in a third liquid fraction. [0095] In an aspect, the
system further comprises: [0096] a heavy hydrocarbon condensing
system for condensing heavy hydrocarbon components in the natural
gas feed stream; and [0097] a flash separator for separating the
natural as feed stream into a heavy hydrocarbon liquid fraction and
a residual vapour fraction and directing at least a portion of the
residual vapour fraction to the partially condensed stream.
[0098] In an aspect, the heavy hydrocarbon condensing system
comprises at least one of an inlet gas compressor, an inlet gas
compressor after-cooler, and an indirect heat exchanger.
[0099] In an aspect, the system further comprises a heavy
hydrocarbon distiller for distilling heavy hydrocarbons from the
heavy hydrocarbon liquid fraction, leaving a residual fraction, and
directing the residual fraction to the natural gas feed stream for
further processing.
[0100] In an aspect, the system further comprises a valve for
directing at least a portion of the first vapour fraction to one or
more refrigerant cycles for use as a refrigerant medium.
[0101] In an aspect, the one or more refrigerant cycles comprises a
first methane refrigerant cycle that provides a portion of the duty
required to cool and condense at least one of: [0102] the natural
gas feed stream into the partially condensed stream; [0103] the
first vapour fraction into the partially condensed first vapour
fraction; [0104] the second vapour fraction into the partially
condensed second vapour fraction; and [0105] the heavy hydrocarbon
components in the natural gas feed stream, [0106] wherein the first
methane refrigerant, cycle comprises at least one of an expander
brake compressor, a first heat exchanger, a multi-stage methane
refrigerant compressor, a second heat exchanger, an indirect heat
exchanger, a methane refrigerant expander, and an expander brake
compressor.
[0107] In an aspect, the one or more refrigerant cycles comprises a
second methane refrigerant cycle that provides a portion of the
duty required to pre-cool the first vapour fraction for use in the
first methane refrigerant cycle.
[0108] In an aspect, the second methane refrigerant cycle further
provides a portion of the duty required to cool and condense at
least one of: [0109] the natural gas feed stream into the partially
condensed stream; [0110] the first vapour fraction into the
partially condensed first vapour fraction; [0111] the second vapour
fraction into the partially condensed second vapour fraction; and
[0112] the heavy hydrocarbon components in the. natural gas feed
stream, [0113] wherein the second methane refrigerant cycle
comprises at least one of a pre-cooling expander brake compressor,
a first heat exchanger, a multi-stage pre-cooling methane
refrigerant compressor, a second heat exchanger, a methane
expander, and an indirect heat exchanger.
[0114] In an aspect, the second methane refrigerant cycle is used
in place of a propane refrigeration cycle.
[0115] In an aspect, at least a portion of the duty required to
cool at least one of: [0116] the first vapour fraction for use in
the first methane refrigerant cycle; [0117] the natural as feed
stream into the partially condensed stream; [0118] the first vapour
fraction into the partially condensed first vapour fraction; [0119]
the second vapour fraction into the partially condensed second
vapour fraction; and [0120] the heavy hydrocarbon components in the
natural gas feed stream is provided by a two-stage closed loop
propane refrigeration cycle.
[0121] In an aspect, the propane refrigeration cycle comprises a
high pressure cycle and a low pressure cycle, linked through a
region of intermediate pressure.
[0122] In an aspect, the propane refrigeration cycle comprises at
least one of a first valve, an intermediate pressure propane
refrigerant drum, an indirect heat exchanger, a second valve, a low
pressure propane refrigerant drum, a low pressure compressor, a
high pressure compressor, and a heat exchanger.
[0123] According to another aspect, there is provided a system for
producing a refrigerant medium from a natural pas feed stream for
use in cooling the natural gas feed stream, the system comprising:
[0124] a first condensing system for condensing at least a portion
of a natural gas feed stream to produce a partially condensed
stream; and [0125] a first flash separator for separating the
partially condensed stream into a first liquid fraction and a first
vapour fraction and for directing at least a portion of the first
vapour fraction to a refrigerant cycle for use as a refrigerant
medium for cooling the natural gas.
[0126] According to another aspect, there is provided a system for
removing nitrogen from a natural gas feed stream, the system
comprising: [0127] a first condensing system for condensing at
least as portion of a natural gas feed stream to produce a
partially condensed stream; and [0128] a first flash separator for
separating the partially condensed stream into a first liquid
fraction and a first vapour fraction and for concentrating the
nitrogen in the first vapour fraction and reducing the
concentration of nitrogen in the first liquid fraction.
[0129] In an aspect, the system further comprises: [0130] a second
condensing system for condensing at least a portion of first vapour
fraction to produce a partially condensed first vapour fraction;
and [0131] a second flash separator for separating the partially
condensed first vapour fraction into a second liquid fraction and a
second vapour fraction and for concentrating the nitrogen in the
second vapour fraction and reducing the concentration of nitrogen
in the second liquid fraction.
[0132] According to another aspect, there is provided a system for
liquefying natural gas, wherein the system comprises a flash
separator for integrally removing nitrogen from the natural
gas.
[0133] According to another aspect, there is provided a system for
liquefying natural gas, wherein the system comprises a heavy
hydrocarbon separator for integrally removing heavy hydrocarbon
components from the natural gas.
[0134] According to another aspect, there is provided a system for
liquefying natural gas, wherein the system comprises at least one
methane refrigerant cycle, derived at least in part from a vapour
fraction of the natural gas feed stream.
[0135] In an aspect, the system further comprises a second methane
refrigerant cycle, derived at least in part from a vapour fraction
of the natural gas feed stream.
[0136] In an aspect, the system does not comprise a propane
refrigerant cycle.
[0137] In an aspect, the system has a specific energy requirement
of about 14.5 kW.d/ton or less.
[0138] In an aspect, the system has a specific energy requirement
of about 12.5 kW.d/ton or less.
[0139] According to another aspect, there is provided a methane
refrigerant system for use in liquefying natural gas, the system
comprising a methane gas source derived at least in part from a
vapour fraction of the natural gas feed stream.
[0140] According to another aspect, there is provided a micro-LNG
plant comprising the system described herein.
[0141] According to another aspect, there is provided a peak
shaving plant comprising the system of described herein.
[0142] Other features and advantages of the present invention will
become apparent from the following detailed description, it should
be understood, however, that the detailed description and the
specific examples, while indicating aspects of the invention, are
given by way of illustration only since various changes and
modifications within the spirit and scope of the invention will
become apparent to those skilled in the art from the detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0143] Aspects of the present invention will now be described, by
way of example only, with reference to the attached figures, in
which:
[0144] FIG. 1 shows a schematic diagram of a system for treating
and liquefying natural gas utilizing two flash stages with integral
nitrogen removal capability;
[0145] FIG. 2 shows a schematic diagram of a system for treating
and liquefying natural gas utilizing three flash stages with
integral nitrogen removal capability;
[0146] FIG. 3 shows a schematic diagram of an additional subsystem
for treating and liquefying natural gas that incorporates an
integral system for removing heavier hydrocarbons from a natural
gas stream for use in combination with the system of FIG. 1 or
2;
[0147] FIG. 4 shows a schematic diagram for a closed loop methane
refrigeration cycle for use in combination with the system of FIG.
1, 2, or 3;
[0148] FIG. 5 shows a schematic diagram for a pre-cooling dosed
loop two-stage propane refrigeration cycle for use in combination
with the system of FIG. 1, 2, 3, or 4 and
[0149] FIG. 6 shows a schematic diagram for a pre-cooling dosed
loop methane refrigeration cycle for use in combination with the
system of FIG. 1, 2, 3, or 4 as an alternate to the system of FIG.
5.
DETAILED DESCRIPTION
[0150] Natural gas supply in North America and worldwide has become
increasingly abundant, due to new shale gas discoveries and
associated recovery technologies. Due to a widening of the price
spread between crude oil and natural gas, fuel usage in these areas
is expected to switch from crude oil-derived fuels (gasoline &
diesel) to natural gas, thus increasing its demand. it is desirable
to convert the natural as to LNG, in order to make it more
transportable. Where natural gas pipelines do not exist, LNG
provides a means by which the natural gas fuel can reach natural
gas demands in remote areas. LNG can be used to supply more
economical natural gas fuel to remote towns, industries, mines,
etc. Furthermore, LNG can be stored and utilized as a fuel onboard
vehicles that are used in industrial, mining, and transportation
sectors; as a substitute to crude of based fuels or compressed
natural gas (CNG).
[0151] Using LNG as a natural gas fuel, in lieu to CNG, is
advantageous, since LNG is 1.7 times more dense than CNG. Thus,
more LNG can be stored on a vehicle relative to CNG. In addition,
LNG can be loaded in the vehicle's fuel tank faster than CNG, from
a practical stand point. Another advantage is LNG fuel engine
technology that achieves comparable engine specific powers close to
their diesel counterparts, which are 40% greater than CNG.
[0152] Described herein are methods and systems that relate to
various aspects of liquefying natural gas, cooling natural gas,
removing various components from a natural gas feed stream, and to
the storage and transportation of natural gas in the form of
liquefied natural gas (LNG); as well as the usage of natural gas as
LNG as a transportable fuel. Natural gas will convert to a liquid
state when coded to cryogenic temperatures. The method and system
described herein has the ability to cryogenically cod and convert
gaseous natural gas to LNG, and finds particular use in smaller
scale "micro-LNG plants" with a typical capacity in the range of
from about 20 to about 500 metric ton/D (about 7,300 to about
182,500 metric ton/annual).
[0153] A process and system to liquefy natural gas is described
herein, in particular for use in smaller scale micro-LNG plants,
with a capacity typically in the range of 20 to 500 metric ton/D.
in aspects, the process and system described herein realize
improved efficiency of natural gas liquefaction in relation to
associated capital cost and energy consumption, provide a design
that can effectively be packaged as a modular skid mounted
micro-LNG plant, improved operability, and/or improvements to the
operation safety. Further improvements, in aspects, include the
ability to integrally remove heavy hydrocarbon components (HHC;
some HHC can solidify in the cryogenic cooling process), and
integrally remove a portion of the nitrogen component (in order to
meet the LNG product specification limits for nitrogen content).
Further, the process in aspects can produce its own refrigerant
medium and the cooling duty is typically provided by two closed
loop refrigeration cycles.
[0154] As mentioned above, many conventional micro-LNG plants are
inefficient both in terms of energy consumption and economically.
Using the method and system described herein, as competitive
capital cost plant can be designed, having a specific energy
requirement of about 14.5 kW.d/ton or less (subject to inlet gas
and environment conditions). in another aspect, the method and
system described herein can be designed to have a specific energy
requirement of about 12.5 kW.d/ton or less (subject to inlet gas
and environment conditions), in aspects, the method and system
described herein improves the operability and safety, and/or energy
efficiency over comparable prior art. As will be understood, the
balance between a lower capital cost plant and higher energy
efficiency will be different for each particular application.
However, it will also be understood that the economics of the
method and system described herein are particularly advantageous in
the event nitrogen removal is required, since the nitrogen removal
does not require expensive additional equipment.
[0155] In an aspect, the system described herein, which
incorporates a main refrigerant medium as a single gaseous phase,
inherently has less hydrocarbon liquid inventory within the plant
than prior art mixed refrigerant type process technologies, which
have comparable high energy efficiency. Subsequently, the system
described herein is both safer and provides enhanced operability,
than prior art mixed refrigerant type process technologies.
[0156] The present design also provides a means to self-generate
the main refrigerant medium, used to cool and condense the natural
gas into LNG, from the natural gas supply. This advantageously does
not require the transporting, handling, and storage of an
externally sourced refrigerant medium. This reduces the capital
cost of the LNG plant, as well as improves its operability and
safety.
[0157] In another aspect, the system described herein provides a
means to self generate the main refrigerant as well as the
pre-cooling refrigerant medium; used for pre-cooling as well as for
use to cool and condense the natural gas into LNG. This further
enhances its operability and safety. This aspect would be
comparable in operability and safety to an open area nitrogen cycle
process technology, however the system described herein would have
an improvement in energy efficiency to the nitrogen cycle
process.
[0158] Additionally, the system and method described herein is
modular, which is advantageous because it greatly reduces the field
construction and overall cost of the facility.
[0159] The process and system described herein finds use in, for
example, peak shaving plants, such as in the case when natural gas
is being transported into a utility system via a pipeline. When
there is lower demand for the natural gas in the utility system.
the gas could be converted to LNG and then stored in a storage
tank. Then, during peak natural gas demand in the utility system,
the LNG is converted back to gaseous natural gas state, to meet a
demand that can be in excess of the pipeline capacity.
[0160] The process and system described herein also finds use in
capturing stranded natural gas from the source. One example of a
stranded natural gas source is associated natural gas produced with
oil production. If a natural gas pipeline does not exist at the
source where the oil is produced, typically the associated gas is
disposed of by means of flaring. The method and system described
herein provides a means to liquefy the natural gas and produce an
LNG product. that could then be transported in a liquid state to a
market. Liquefaction of stranded natural gas is typically in the
capacity range of micro-LNG plants, since larger quantities of
natural gas should warrant this installation of a natural gas
pipeline.
[0161] The process and system described herein finds particular use
in micro-LNG plants, to produce LNG for industrial, mining, and
transportation fuel usage. Applications for liquefaction of natural
gas, to produce LNG as a transportable fuel, are typically in the
capacity range of micro-LNG plants, since there is a practical
limit to the maximum distance that LNG can be transported over
land.
[0162] As will be explained, the method and system described herein
involve integration of flash separation and an open loop
refrigeration stream using the flash vapour with a base
refrigeration load method (a closed loop refrigeration method). The
combination of this effectively helps achieve the required
heating/cooling curves, provides part of the refrigeration duty,
and partially separates the nitrogen component out of the LNG
product.
[0163] Furthermore, in the process and system described herein, the
methane refrigeration cycle (loop) is typically the main
refrigeration load used for LNG liquefaction and is derived from
the inlet gas, in contrast, conventional processes and systems use
closed loop refrigeration utilizing ethane, ethylene, nitrogen, or
a mixed refrigerant.
[0164] In a typical aspect, the process and system described herein
for cooling and liquefying natural gas to produce LNG comprises a
main methane refrigeration cycle, with a supplemental propane
refrigeration cycle, and includes the integration of flash
separation and an open loop refrigeration using the flash vapour.
This complete arrangement provides, in aspects, a cost effective,
energy efficient process solution that has other features as noted,
such as operability, safety, integral nitrogen removal, etc.
[0165] "Natural gas" is a naturally occurring hydrocarbon gas
mixture consisting primarily of methane, The actual composition of
any given natural gas feed stream varies depending upon its source.
For example, the gas source may be a natural gas well, a natural
gas gathering system or pipeline transmission system, or flare gas,
A natural gas feed stream may include other hydrocarbons and gases
in various concentrations, including hydrogen, helium, carbon
dioxide, and nitrogen. Furthermore, the natural gas feed stream may
or may not contain contaminants such as hydrogen sulfide or
mercury. The natural gas feed stream may or may not contain
water.
[0166] Before natural gas can be used as a fuel, it typically
undergoes processing to clean the gas and remove impurities,
including water, to meet the specifications of marketable natural
gas and to protect equipment in the LNG plant, For example, water
is removed from the feed natural gas feed stream in order to reduce
hydrate formation and freezing in the LNG plant, and in order to
meet product specifications. Carbon dioxide is removed from the
natural gas feed stream in order to reduce solid formation and free
it in the plant, and also in order to meet product
specifications.
[0167] Additionally, processing of the natural gas, upstream of an
LNG plant, may include removal of heavier hydrocarbon components,
such as, for example, ethane, propane, butane, pentane, and higher
molecular weight hydrocarbons. Essentially all pentane and heavier
hydrocarbons are generally required to be removed from the gas
prior to entering the LNG plant in order to meet the LNG product
specifications as well as to prevent solidification of cyclo-hexane
components in the natural gas. The required extent of removal of
ethane, propane, and butane will depend on the LNG product
specifications as well as the LNG heating value specification
depending on its usage and fuel specifications. Thus, the
by-products of natural gas processing may include ethane, propane,
butanes, pentanes, and higher molecular weight hydrocarbons,
hydrogen sulfide, carbon dioxide, water vapour, helium, and
nitrogen.
[0168] "Liquefied natural gas (LNG)" is produced for the purpose of
storage and transportation of natural gas, because LNG has a volume
that is nearly 600 times smaller than that of natural gas. LNG is
becoming the preferred state for storage as fuel on a transport
vehicle, as opposed to compressed natural gas (CNG), since a
greater amount of fuel can be stored on board the vehicle.
Liquefied natural gas is one end-product of the method described
herein.
[0169] The insulation in storage containers, as efficient as it is,
will not keep LNG cold enough by itself. Inevitably, heat leakage
will warm and vaporise the LNG. Industry practice is to store LNG
as a boiling cryogen. That is, the liquid is stored at its boiling
point for the pressure at which it is stored (atmospheric
pressure). As the vapour boils off, heat for the phase change cools
the remaining liquid. Because the insulation is very efficient,
only a relatively small amount of boil off is necessary to maintain
temperature. This phenomenon is also called auto-refrigeration.
[0170] The term "cryogenic" relates to low temperatures. A
"cryogenic as" is a gas that has been cooled to a liquid state
below about 150 Kelvin.
[0171] "Heavy hydrocarbons (HHC)" are hydrocarbons that include
propane and any hydrocarbons heavier than propane, such as
cyclo-hexane.
[0172] Terms of degree such as "substantially", "about" and
"approximately" as used herein mean a reasonable amount of
deviation of the modified term such that the end result is not
significantly changed. These terms of degree should be construed as
including a deviation of at least .+-.5% of the modified term if
this deviation would not negate the meaning of the word it
modifies.
[0173] When introducing elements disclosed herein, the articles
"a", "an", "the", and "said" are intended to mean that there may be
one or more of the elements.
[0174] In understanding the scope of the present application, the
term "comprising" and its derivatives, as used herein, are intended
to he open ended terms that specify the presence of the stated
features, elements, components, groups, integers, and/or steps, but
do not exclude the presence of other unstated features, elements,
components, groups, integers end/or steps. The foregoing also
applies to words having similar meanings such as the terms,
"including", "having" and their derivatives. It will be understood
that any aspects described as "comprising" certain components may
also "consist of" or "consist essentially of" wherein "consisting
of" has a closed-ended or restrictive meaning and "consisting
essentially of" means including the components specified but
excluding other components except for materials present as
impurities, unavoidable materials present as a result of processes
used to provide the components, and components added for a purpose
other than achieving the technical effect of the invention.
Typically, a composition consisting essentially of a set of
components will comprise less than 5% by weight, typically less
than 3% by weight, more typically less than 1% by weight of
non-specified components.
[0175] It will be understood that any component or method step
defined herein as being included may be explicitly excluded from
the claimed invention by way of proviso or negative limitation.
[0176] A liquefied natural gas (LNG) plant 10 is shown
schematically in FIGS. 1 through 3. Although specific temperatures
and pressures will be referred to below for exemplary purposes, it
will be understood that the operating temperatures and pressures at
various locations within the process will depend upon the natural
gas feed steam composition, the plant inlet and outlet conditions
(including temperature and pressure), and whether or not nitrogen
and/or heavy hydrocarbon removal is required. Optimizing the
temperatures and pressures described below to meet the requirements
of any particular system requirement is within the ability of a
skilled person.
[0177] Referring to FIG. 1, an inlet natural has feed stream 12
enters the LNG plant 10, The inlet natural gas feed stream 12 has
been pre-treated and processed upstream of the LNG plant 10 to
remove water, carbon dioxide and heavier hydrocarbons and typically
has a composition similar to that shown in Table 1.
TABLE-US-00001 TABLE 1 Example feed stream composition. Component
Mole % Nitrogen 1.50 Carbon dioxide <50 ppm Methane 92.10 Ethane
5.15 Propane 1.25 Iso-butane Trace n-Butane Trace Iso-pentane Trace
n-Pentane Trace Hexane + <50 ppm Water <0.01 ppm
[0178] The inlet natural gas feed stream 12 is fed into an inlet
gas compressor 14 and an inlet gas compressor after-cooler 16,
producing feed stream 18. Inlet gas compressor 14 compresses the
inlet natural gas feed stream 12 followed by after-cooling the
compressed inlet gas in the inlet gas compressor after-cooler 16,
to a suitable pressure for the liquefaction process to take place
to produce feed stream 18. The gaseous state natural gas feed
stream 18 has, for example, a pressure of 7240 kPa absolute and a
temperature of 43.3.degree. C.
[0179] The feed stream 18 is fed into section 20 of an indirect
heat exchanger 22, where the feed stream 18 is cooled to
-131.7.degree. C., thereby condensing at least a portion of feed
stream 18 to produce partially condensed stream 24.
[0180] Partially condensed stream 24 is passed through valve 26 and
pressure is reduced. The drop in pressure across valve 26 causes
partially condensed stream 2$ to partially flash and cool further
due to the Joule-Thompson (JT) effect, producing stream 28. Stream
28 has a pressure of 152 kPa absolute and a temperature of
-156.3.degree. C. and contains both liquid and gas phases.
[0181] Stream 28 is fed into a first flash separator 30, which
separates stream 28 into a first liquid fraction 32 and a first,
vapour fraction 3$. The first liquid fraction 32 is fed out of the
LNG plant 10 and exits the LNG plant 10 as the first portion of the
LNG product. The nitrogen components in the gas tend to concentrate
in the first vapour fraction 34, thus lowering the nitrogen content
in the first liquid fraction 32 and the resulting LNG. product
[0182] A first portion of the first vapour fraction 34 is directed
to valve 115, where it is directed to stream 116 in FIG. 4
(described below), A second portion of the first vapour fraction 34
is directed to valve 191, where it is directed to stream 192 in
FIG. 6 (described below). A third portion of the first vapour
fraction 34 is directed to section 36 the indirect heat exchanger
22, where it is heated to 36.1.degree. C. to produce heated first
vapour fraction 38. The heated first vapour fraction 38 is
compressed to a pressure of 2137 kPa absolute by flash gas
compressor 40 to produce stream 42, which is then cooled by flash
gas compressor after-cooler 44, thereby producing compressed first
vapour fraction 46.
[0183] Compressed first vapour fraction 46, at a pressure of 2117
kPa absolute and a temperature of 38.degree. C., is fed into
section 48 of the indirect heat exchanger 22. This cools compressed
first vapour fraction 46 to a temperature of -142.6.degree. C.,
thereby condensing at least a portion of compressed first vapour
fraction 46 to produce partially condensed first vapour fraction
50.
[0184] Partially condensed first vapour fraction 50 is passed
through valve 52 and pressure is reduced. The drop in pressure
across valve 52 causes partially condensed first vapour fraction 50
to partially flash and cool further due to the Joule-Thompson (JT)
effect, producing stream 54. Stream 54 has a pressure of 152 kPa
absolute and a temperature of -161.2.degree. C. and contains both
liquid and gas phases.
[0185] Stream 54 is fed into a second, flash separator 56, which
separates stream 54 into a second liquid fraction 58 and a second
vapour fraction 60. The second liquid fraction 58 is fed out of the
LNG plant 10 and exits the LNG plant 10 as the second portion of
the LNG product. The nitrogen components in the gas tend to
concentrate in the second vapour fraction 60, thus lowering the
nitrogen content in the second liquid fraction 58 and the resulting
LNG product. Second vapour fraction 60 is heated in exchanger 62
and is then used as fuel gas 64 for utility consumption.
[0186] Turning now to FIG. 2, an additional LNG flash separation
stage is incorporated into the LNG plant 10 in order to increase
separation of the nitrogen from the LNG product and thereby
decrease the amount of methane gas in the separated nitrogen
product stream. As well, this method improves the energy efficiency
by means of additional indirect heat exchange in section 66 of the
indirect heat exchanger 22, between he flash as and cooling and
condensing inlet gas, thus reducing the refrigerant duty. This
provides a second flash separation step that reduces the methane
component loss to stream 94 (as an alternate to stream 64), as will
be described below, thus increasing the liquefied natural gas (LNG)
product yield.
[0187] In this additional LNG flash separation stage, instead of
being directed to exchanger 62 for use as fuel as 64, the second
vapour fraction 60 is directed to section 66 of the indirect heat
exchanger 22. In section 66 of the indirect heat exchanger 22, the
second vapour fraction 60 is heated from -162.5.degree. C. to
36.1.degree. C. to produce a heated second vapour fraction 68. The
heated second vapour fraction 68 is compressed to a pressure of
2068 kPa absolute by flash gas compressor 70 to produce stream 72,
which a then cooled by flash gas compressor after-cooler 74,
thereby producing compressed second vapour fraction 76.
[0188] Compressed second vapour fraction 76, at a pressure of 2034
kPa absolute and a temperature of 38.degree. C., is fed into
section 78 of the indirect heat exchanger 22. This cools compressed
second vapour fraction 76 to a temperature of -156.7.degree. C.,
thereby condensing at least a portion of compressed second vapour
fraction 76 to produce partially condensed second vapour fraction
80.
[0189] Partially condensed second vapour fraction 80 is passed
through valve 82 and pressure is reduced. The drop in pressure
across valve 82 causes partially condensed second vapour fraction
80 to partially flash and cool further due to the, Joule-Thompson
(ST) effect, producing stream 84. Stream 84 has a pressure of 138
kPa absolute and a temperature of -179.0.degree. C. and contains
both liquid and gas phases.
[0190] Stream 84 is fed into a third flash separator 86, which
separates stream 84 into a third liquid fraction 88 and a third
vapour fraction 90. The third liquid fraction 88 is fed out of the
LNG plant 10 and exits the LNG plant 10 as the third portion of the
LNG product. The nitrogen components in the gas tend to concentrate
in the third vapour fraction 90, thus lowering the nitrogen content
in the third liquid fraction 88 and the resulting LNG product.
Third vapour fraction 80 is heated in exchanger 92 and is then used
as fuel gas 94 for utility consumption.
[0191] Turning now to FIG. 3. if the natural gas feed stream 12
contains propane and heavier hydrocarbons, including cyclohexanes,
removal of these hydrocarbons may be required to either meet INC
product specifications or to mitigate their solidification during;
the cryogenic LNG process. In this case, the natural gas feed
stream 12 enters the LNG plant 10 ands fed into the inlet gas
compressor 14 and the inlet gas compressor after-cooler 16,
producing feed stream 18, which is at a pressure of 7240 kPa
absolute and a temperature of 43.3.degree. C.
[0192] The feed stream 18 is fed into section 96 of the indirect
heat exchanger 22, where the feed stream 18 is cooled to
-31.degree. C., thereby condensing the propane and heavier
hydrocarbons in the feed stream 18 to produce partially condensed
stream 98.
[0193] Partially condensed stream 98 is fed into heavy hydrocarbon
(HHC) separator 100, which separates partially condensed stream 98
into a heavy hydrocarbon liquid fraction 101 and a residual vapour
fraction 104. The heavy hydrocarbon liquid fraction 101 is dropped
in pressure across valve 102 to produce a partially flashed heavy
hydrocarbon liquid fraction 103. The partially flashed heavy
hydrocarbon liquid fraction 103 is then fed to a heavy hydrocarbon
distiller 106, which distils heavy hydrocarbons 108 from the
partially flashed heavy hydrocarbon liquid fraction 103 and
recycles the residual stream 110 to the natural gas teed stream 12
for further processing.
[0194] Residual vapour fraction 104 is fed into section 112 of the
indirect heat exchanger 22, where the residual vapour fraction 104
is cooled to -131.7.degree. C., thereby condensing at least a
portion of vapour fraction 104 to produce partially condensed
stream 24. Partially condensed stream 24 is then processed as
described above.
[0195] Turning now to FIG. 4, a methane closed loop refrigeration
cycle 114 is shown. The closed loop methane refrigeration cycle 114
provides a portion of the duty required to cool and condense the
natural gas feed stream 18 in sections 20 (or 96 and 112 in the
case of FIG. 3) and the compressed first vapour fraction 46 in
section 48 of the indirect heat exchanger 22 (and the compressed
second vapour fraction 76 in section 78 of the indirect heat
exchanger 22, in the case of FIG. 2).
[0196] A source of low pressure methane refrigerant vapour stream
116, at 36.1.degree. C. and 641 kPa absolute, is compressed by an
expander brake compressor 118, producing discharge stream 120. The
discharge stream 120 has a temperature of 94.degree. C., and a
pressure of 1189 kPa absolute and is after-cooled in heat exchanger
122 to produce outlet stream 124.
[0197] Outlet stream 124 has a temperature of 43.degree. C. and is
fed into a multi-stage methane refrigerant compressor 126,
producing stream 128. Stream 128 is after-cooled heat exchanger
130, producing stream 132. Stream 132 has a pressure of 5516 kPa
absolute and a temperature of 43.degree. C. Stream 132 is fed into
section 134 of the indirect heat exchanger 22, where it is
pre-cooled and exits as pre-cooled high pressure methane
refrigerant stream 136.
[0198] Pre-cooled high pressure methane refrigerant stream 136 has
a temperature of -34.4.degree. C. and a pressure of 5500 kPa
absolute and is expanded to a discharge pressure of 655 kPa
absolute in a methane refrigerant expander 138. Methane expander
138 extracts work, which is used to mechanically drives the
expander brake compressor 118. This lowers the enthalpy of the
pre-cooled high pressure methane refrigerant stream 136, so that it
exits the methane expander 138 as expanded and cooled methane
refrigerant stream 140, having a temperature of -133.degree. C.
[0199] Expanded and cooled methane refrigerant stream 140 is fed
into section 142 of the indirect heat exchanger 22, where it
provides cooling duty to the system and is therefore heated from
-133.degree. C. to 36.1.degree. C. Expanded and cooed methane
refrigerant stream 140 exits section 142 of the indirect heat
exchanger 22 as low pressure methane refrigerant vapour stream 116,
thereby completing the methane refrigeration cycle 114.
[0200] Turning now to FIG. 5, a two-stage closed-loop propane
refrigeration cycle 144 shown. The propane refrigeration cycle 144
provides a portion of the pre-cooling duty required to cool and
condense the natural gas feed stream 18 in sections 20 (or 96 and
112 in the case of FIG. 3) and the compressed first vapour fraction
46 in section 48 of the indirect heat exchanger 22 (and the
compressed second vapour fraction 76 in section 78 of the indirect
heat exchanger 22, in the case of FIG. 2), and also pre-cools the
pre-cooled high pressure methane refrigerant stream 136 in section
134 of the indirect heat exchanger 22.
[0201] The propane refrigeration cycle 144 contains discrete
regions; a high pressure HP cycle and low pressure LP cycle,
indicated by the dashed lines. The high pressure HP cycle and low
pressure LP cycle are linked through a region of intermediate
pressure.
[0202] A high pressure saturated liquid phase propane refrigerant
stream 146 at a temperature of 43.degree. C. and a pressure of 1500
kPa absolute is passed through valve 148 and pressure is reduced.
The drop in pressure across valve 148 causes the high pressure
saturated liquid phase propane refrigerant stream 146 to partially
flash and cool further due to the Joule-Thompson (JT) effect,
producing intermediate pressure stream 150. Intermediate pressure
stream 150 has a pressure of 386 kPa absolute and a temperature of
-6.8.degree. C. and contains both liquid and gas phases.
intermediate pressure stream 150 is fed into intermediate pressure
propane refrigerant drum 162, which separates intermediate pressure
stream 150 into an intermediate pressure propane refrigerant liquid
fraction 154 and an intermediate pressure propane refrigerant
vapour fraction 156.
[0203] A first portion 158 of the intermediate pressure propane
refrigerant liquid fraction 154 is thermo-syphoned into section 160
of the indirect heat exchanger 22, where it is heated and vaporized
at a temperature of -6.8.degree. C. to produce vapour-liquid stream
162. Vapour-liquid stream 162 is fed back into intermediate
pressure propane refrigerant drum 152, where it combines with
intermediate pressure stream 150.
[0204] A second portion 164 of the intermediate pressure propane
refrigerant fraction 154 is passed through valve 166 and pressure
is reduced. The drop in pressure across valve 166 causes the second
portion 164 of the intermediate pressure propane refrigerant liquid
fraction 154 to partially flash and cool further due to the
Joule-Thompson (JT) effect, producing low pressure stream 168. Low
pressure stream 168 has a pressure of 107 kPa absolute and a
temperature of -40.degree. C. and contains both liquid and gas
phases. Low pressure stream 168 is fed into a low pressure propane
refrigerant drum 170, which separates low pressure stream 168 into
a low pressure propane refrigerant liquid fraction 72 and a low
pressure propane refrigerant vapour fraction 174.
[0205] The low pressure propane refrigerant liquid fraction 172 is
thermo-syphoned into section 176 of the indirect heat exchanger 22,
where it is heated and vaporized at a temperature of -40.degree. C.
to produce vapour-liquid stream 178, Vapour liquid stream 178 is
fed back into low pressure propane refrigerant drum 170, where it
combines with low pressure stream 168.
[0206] Low pressure propane refrigerant vapour fraction 174 is
compressed to a pressure of 386 kPa absolute in a low pressure
compressor 180 and exits as stream 182. Stream 182 combines with
intermediate propane refrigerant vapour fraction 156 resulting in a
combined intermediate pressure propane refrigerant vapour fraction
157.
[0207] The combined intermediate pressure propane refrigerant
vapour fraction 157, having a pressure of 386 kPa absolute, is then
fed into a high pressure compressor 184 where it is compressed to
high pressure propane refrigerant vapour stream 186, which has a
pressure of 1530 kPa absolute. High pressure propane refrigerant
vapour stream 186 is then fed into heat exchanger 188, where it is
cooled and condensed to produce the high pressure saturated liquid
phase propane refrigerant stream 146, which has a temperature of
43.degree. C. and a pressure of 1500 kPa absolute, thus completing
the two-stage closed-loop propane refrigeration cycle 114.
[0208] Turning now to FIG. 6, an alternate pre-cooling methane
refrigeration cycle 190 is shown. The pre-cooling closed loop
methane refrigeration cycle 190 is an alternative to the
pre-cooling propane refrigeration cycle 144 that is suitable for
use in areas where fire hazard risks are a particular concern, such
as in off-shore applications. For example, the pre-cooling methane
refrigeration cycle 190 finds use on an off-shore platform, a
floating production storage and offloading (FPSO) vessel, or an LNG
transport ship. Fire hazards risks are reduced through use of the
pre cooling methane refrigeration cycle 190 in place of the propane
refrigeration cycle 144, as the pre-cooling methane refrigeration
cycle 190 uses refrigerants that do not require liquid inventory
within the LNG plant 10.
[0209] Like the pre-cooling propane refrigeration cycle 144, the
alternate pre-cooling closed loop methane refrigeration cycle
provides a portion of the pre-cooling duty required to cool and
condense the natural gas feed stream 18 in sections 20 (or 96 and
112 in the case of FIG. 3) and the compressed first vapour traction
46 in section 48 of the indirect heat exchanger 22 (and the
compressed second vapour fraction 76 in section 78 of the indirect
heat exchanger 22, in the case of FIG. 2), and also pre-cools the
pre-cooled high pressure methane refrigerant stream 136 in section
134 of the indirect heat exchanger 22.
[0210] A source of low pressure pre-cooling methane refrigerant
vapour stream 192, at 21.degree. C. and 600 kPa absolute, is
compressed by a pre-cooling expander brake compressor 194,
producing discharge stream 196. The discharge stream 196 has a
temperature of 107.4.degree. C. and a pressure of 1543 kPa absolute
and is after-cooled in heat exchanger 198 to produce outlet stream
200.
[0211] Outlet stream 200 has a temperature of 43.degree. C. and is
fed into a multi-stage pre-coding methane refrigerant compressor
202, producing stream 204. Stream 204 is after-cooled in heat
exchanger 206, producing stream 208. Stream 208 has a pressure of
4482 kPa absolute and a temperature of 43.degree. C.
[0212] Stream 208 is expanded to a discharge pressure of 620 kPa
absolute in a pre cooling methane expander 210, Methane expander
210 extracts work, which is used to mechanically drives
there-cooling expander brake compressor 194. This lowers the
enthalpy of stream 208, so that it exits the pre-cooling methane
expander 210 as expanded and cooled pre-cooling methane refrigerant
stream 212, having a temperature of -70.degree. C.
[0213] Expanded and cooled pre-cooling methane refrigerant stream
212 is fed into section 214 of the indirect heat exchanger 22,
where it provides cooling duty to the system and is therefore
heated from -70.degree. C. to 21.degree. C. Expanded and cooled
pre-cooling methane refrigerant stream 212 exits section 214 of the
indirect heat exchanger 22 as low pressure pre-cooling methane
refrigerant vapour stream 192, thereby completing the pre-cooling
methane refrigeration cycle 190.
[0214] The refrigerant medium for the closed loop methane
refrigeration cycle 114 (shown in FIG. 4) and the pre-cooling
closed loop methane refrigeration cycle 190 (shown in FIG. 6), is
sourced from the first vapour fraction 34. As required for make-up,
a portion of the first vapour fraction 34 is directed via valves
115 and 191 to stream 116 (shown in FIG. 4) and stream 192 (shown
in FIG. 6), respectively.
[0215] The above disclosure generally describes the present
invention. Changes in form and substitution of equivalents are
contemplated as circumstances may suggest or render expedient.
Although specific terms have been employed herein, such terms are
intended in a descriptive sense and not for purposes of
limitation.
[0216] All publications, patents and patent applications are herein
incorporated by reference in their entirety to the same extent as
if each individual publication, patent or patent application was
specifically and individually indicated to be incorporated by
reference in its entirety.
[0217] Although exemplary aspects of the invention have been
described herein in detail, it will be understood by those skilled
in the art that variations may be made thereto without departing
from the spirit of the invention or the scope of the appended
claims.
* * * * *