BACKGROUND
[0001] The present invention relates to a system and method for liquefaction of a gas stream,
and more specifically, to a system and method for liquefaction of a natural gas stream
in large capacity liquefaction plants.
[0002] Over the past few years, the liquid natural gas (LNG) industry has moved towards
using large capacity liquefaction plants to achieve favorable economics associated
with the large plants. Scale-up problems arise, however, when refrigerant mass and
volume flow rates are increased. For example, the design of compression equipment,
particularly the compression equipment associated with precooling, becomes problematic
because the increased flow rates require larger compressor impellers with higher tip
speeds, thicker and heavier casings, and higher inlet velocities to the impellers.
As the equipment is scaled up, the design of the compressor becomes more problematic
as fundamental aerodynamic limits are approached and, thus, the scale up may be limited
by these considerations. In addition these precooling compressors are large and often
contain multiple stages. Moreover, scale-up in many instances requires large, heavy
equipment that can be difficult and costly to manufacture and/or install.
[0003] U.S. Patent No. 6,962,060 (Petrowski et al.) assigned to the assignee of the present invention, discloses one alternative system
designed for liquefaction at large plants that includes a compressor system comprising
a first compressor having a first stage and a second stage wherein the first stage
of the first compressor is adapted to compress a first gas and the second stage of
the first compressor is adapted to compress a combination of a fourth gas and an intermediate
compressed gas from the first stage of the first compressor; and a second compressor
having a first stage and a second stage wherein the first stage of the second compressor
is adapted to compress a second gas and the second stage of the second compressor
is adapted to compress a combination of a third gas and an intermediate compressed
gas from the first stage of the second compressor.
[0004] There is a need for a method and system that provides stable operation at full rates
and during turndown for larger capacity liquefaction plants.
BRIEF SUMMARY
[0005] Embodiments of the present invention satisfy this need in the art by providing a
liquid natural gas liquefaction system and process that is stable and operational
at full rates and during turndown for larger capacity liquefaction plants.
[0006] According to a first aspect, the present invention provides a natural gas liquefaction
system, the system comprising: a first closed-loop precooling refrigeration system,
for precooling at least a natural gas feed stream via indirect heat transfer with
a precooling refrigerant circulating in said closed-loop of said first precooling
refrigeration system; a second closed-loop precooling refrigeration system, for precooling
at least a first refrigerant stream via indirect heat transfer with a precooling refrigerant
circulating in said closed-loop of said second precooling refrigeration system; and
a cryogenic heat exchanger in fluid connection with the first precooling refrigeration
system and the second precooling refrigeration system so as to accept the precooled
natural gas feed stream from the first precooling refrigeration system and the precooled
first refrigerant stream from the second precooling refrigeration system in order
to liquefy the natural gas feed stream; wherein the closed-loop of the second precooling
refrigeration system is separate from the closed-loop of the first precooling refrigeration
system, and wherein in operation no streams are precooled by both the second precooling
refrigeration system and the first precooling refrigeration system.
[0007] In some embodiments, the first refrigerant stream is a mixed refrigerant stream.
The first refrigerant stream may, for example, comprise nitrogen, methane, ethane,
and propane.
[0008] In some embodiments, the natural gas liquefaction system further comprises a subcooler
exchanger in fluid connection with the cryogenic heat exchanger, wherein the subcooler
exchanger accepts a second refrigerant stream from the cryogenic heat exchanger to
subcool the natural gas feed stream through indirect heat exchange.
[0009] In some embodiments, at least one of the first and second precooling refrigeration
systems comprises at least one heat exchanger that accepts at least two load streams.
[0010] In some embodiments, the first precooling refrigeration system and the second precooling
refrigeration system are CO
2 refrigeration systems.
[0011] In some embodiments, the first precooling refrigeration system and the second precooling
refrigeration system each comprise: at least one device for reducing the pressure
of or vaporizing at least a part of the precooling refrigerant; and a compressor in
fluid connection with the at least one pressure reducing or vaporizing device and
adapted to accept at least one precooling refrigerant stream.
[0012] In a preferred embodiment, where the precooling refrigerant is propane, the first
precooling refrigeration system and the second precooling refrigeration system each
comprise: at least one propane evaporator; and a propane compressor in fluid connection
with the at least one propane evaporator and adapted to accept at least one propane
vapor stream.
[0013] In a preferred embodiment, the natural gas liquefaction system further comprises
a first driver and a second driver: wherein the first driver drives the compressor
of the first precooling refrigeration system, the compressor of the second precooling
refrigeration system, and a first high pressure refrigerant compressor; and wherein
the second driver drives a first medium pressure refrigerant compressor and a first
low pressure refrigerant compressor.
[0014] In another preferred embodiment, the natural gas liquefaction system further comprises
a first driver and a second driver: wherein the first driver drives the compressor
of the first precooling refrigeration system and one of either a first low pressure
refrigerant compressor or a first high pressure refrigerant compressor; and wherein
the second driver drives the compressor of the second precooling refrigeration system
and the other of the first low pressure refrigerant compressor and the first high
pressure refrigerant compressor not driven by the first driver.
[0015] In another preferred embodiment, the natural gas liquefaction system further comprises
a first driver and a second driver: wherein the first driver drives the compressor
of the first precooling refrigeration system and the compressor of the second precooling
refrigeration system; and the second driver drives a first low pressure refrigerant
compressor and a first high pressure refrigerant compressor. The system may further
comprise a third driver, wherein the third driver drives a second low pressure refrigerant
compressor and a second high pressure refrigerant compressor.
[0016] In the preferred embodiments described above, the first driver, the second driver
and/or, if present, the third driver may, for example, be gas turbines.
[0017] In some embodiments, the cryogenic heat exchanger is a wound-coil heat exchanger.
[0018] According to a second aspect, the present invention provides a method for liquefying
natural gas, the method comprising the steps of: precooling in a first closed-loop
precooling refrigeration system, via indirect heat transfer with a precooling refrigerant
circulating in said closed loop of said first precooling refrigeration system, at
least a natural gas feed stream; precooling in a second closed-loop precooling refrigeration
system, via indirect heat transfer with a precooling refrigerant circulating in said
closed loop of said second precooling refrigeration system, at least a first refrigerant
stream; and vaporizing the precooled first refrigerant stream in a cryogenic heat
exchanger to cool the precooled natural gas feed stream through indirect heat exchange;
wherein the closed-loop of the second precooling refrigeration system is separate
from the closed-loop of the first precooling refrigeration system, and wherein the
second precooling refrigeration system precools only stream(s) having a composition
different from the stream(s) precooled by the first precooling refrigeration system.
[0019] In some embodiments, the natural gas feed stream and the first refrigerant stream
are precooled to +60°F to -100°F (+16°C to -73°C).
[0020] In some embodiments, the method further comprises precooling a second refrigerant
stream in either the first precooling refrigeration system or the second precooling
refrigeration system, and vaporizing the second refrigerant stream to subcool the
natural gas feed stream.
[0021] In some embodiments, the first refrigerant stream and/or, if present, the second
refrigerant streams are mixed refrigerant streams.
[0022] In some embodiments, the precooling refrigerant is propane or CO
2.
[0023] In preferred embodiments, the method of the second aspect of the invention is carried
out in a natural gas liquefaction system according to the first or third aspects.
[0024] According to a third aspect, the present invention provides a natural gas liquefaction
system for large capacity liquefaction plants, the system comprising: a first closed-loop
precooling refrigeration system for precooling, via indirect heat transfer with a
precooling refrigerant circulating in said closed-loop of said first precooling refrigeration
system, one stream selected from the group consisting of a natural gas feed stream,
and an at least one refrigerant stream; a second closed-loop precooling refrigeration
system for precooling, via indirect heat transfer with a precooling refrigerant circulating
in said closed-loop of said second precooling refrigeration system, any remaining
stream(s) not precooled by the first precooling refrigeration system and from the
group consisting of the natural gas feed stream, and the at least one refrigerant
stream; and a cryogenic heat exchanger in fluid connection with the first precooling
refrigeration system and the second precooling refrigeration system and adapted to
accept the precooled natural gas feed stream and the precooled at least one refrigerant
stream from the first precooling refrigeration system and the second precooling refrigeration
system, wherein the at least one refrigerant stream is used to liquefy the natural
gas feed stream; wherein the closed-loop of the second precooling refrigeration system
is separate from the closed-loop of the first precooling refrigeration system, and
wherein in operation no streams are precooled by both the second precooling refrigeration
system and the first precooling refrigeration system.
[0025] In some embodiments, the at least one refrigerant stream is a mixed refrigerant stream.
[0026] In some embodiments, the at least one refrigerant stream comprises a first refrigerant
stream and a second refrigerant stream.
[0027] According to a fourth aspect, the present invention provides a natural gas liquefaction
system, the system comprising: a first precooling refrigeration system that accepts
at least a natural gas feed stream; a second precooling refrigeration system that
accepts at least a first refrigerant stream; and a cryogenic heat exchanger fluidly
connected to the first precooling refrigeration system and the second precooling refrigeration
system that accepts the natural gas feed stream from the first precooling refrigeration
system and the first refrigerant stream from the second precooling refrigeration system
to liquefy the natural gas feed stream, wherein the second precooling refrigeration
system accepts only stream(s) having a composition different from the stream(s) accepted
by the first precooling refrigeration system.
[0028] According to a fifth aspect, the present invention provides, a method for liquefying
natural gas, the method comprising the steps of: providing a natural gas feed stream;
providing a first refrigerant stream; precooling in a first precooling refrigeration
system at least the natural gas feed stream; precooling in a second precooling refrigeration
system at least the first refrigerant stream; and vaporizing the precooled first refrigerant
stream in a cryogenic heat exchanger to cool the precooled natural gas feed stream
through indirect heat exchange, wherein the second precooling refrigeration system
precools only stream(s) having a composition different from the stream(s) precooled
by the first precooling refrigeration system.
[0029] According to a sixth aspect, the present invention provides, a natural gas liquefaction
system for large capacity liquefaction plants is disclosed, the system comprising:
a first precooling refrigeration system that accepts one stream selected from the
group consisting of: a natural gas feed stream, and an at least one refrigerant stream;
a second precooling refrigeration system that accepts any remaining stream(s) not
accepted by the first precooling refrigeration system and from the group consisting
of: the natural gas feed stream, and the at least one refrigerant stream; and a cryogenic
heat exchanger fluidly connected to the first precooling refrigeration system and
the second precooling refrigeration system and adapted to accept the natural gas feed
stream and the at least one refrigerant stream from the first precooling refrigeration
system and the second precooling refrigeration system, wherein the at least one refrigerant
stream is used to liquefy the natural gas feed stream, wherein the second precooling
refrigeration system accepts only stream(s) having a composition different from the
stream(s) accepted by the first precooling refrigeration system.
[0030] Some exemplary and preferred embodiments of the fourth, fifth and sixth aspects of
the invention are as described above in relation to the first, second and third aspects.
BRIEF DESCRIPTION OF THE EXEMPLARY DRAWINGS
[0031] The foregoing brief summary, as well as the following detailed description of exemplary
embodiments, is better understood when read in conjunction with the appended drawings.
For the purpose of illustrating embodiments of the invention, there is shown in the
drawings exemplary embodiments of the invention; however, the invention is not limited
to the specific methods and instrumentalities disclosed. In the drawings:
Figure 1 is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 2A is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 2B is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 3 is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 4 is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 5 is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 6 is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 7A is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 7B is a flow chart illustrating an exemplary system and method involving aspects
of the present invention;
Figure 8A is a flow chart illustrating an exemplary system and method involving aspects
of the present invention; and
Figure 8B is a flow chart illustrating an exemplary system and method involving aspects
of the present invention.
DETAILED DESCRIPTION
[0032] Figure 1 illustrates an exemplary embodiment of the invention as applied to a pre-cooled
refrigerant system and process. In this exemplary system 100, propane is used to precool
both a natural gas feed stream 102 and a liquefaction refrigerant stream 104. The
natural gas feed stream 102 may be pretreated, for example. The liquefaction refrigerant
stream 104 may be a pure or a mixed refrigerant, for example. It should be noted that
while the exemplary embodiments described below may refer to the liquefaction refrigerant
stream as a mixed refrigerant stream, the liquefaction refrigerant stream described
below may also be a pure refrigerant stream, for example. Depending on refrigerant
availability in the local area and system requirements (
e.
g., adjusting the composition of the mixed refrigerant to match the cooling curve for
optimal cooling performance), the liquefaction refrigerant stream 104 may comprise
one or more of the following: nitrogen, methane, ethylene, ethane, propylene, propane,
iso-butane, n-butane, and iso-pentane, for example.
[0033] The compression of the propane vapor resulting from the cooling of the natural gas
feed stream 102 may occur in one compressor 118 while the compression of the propane
vapor generated from cooling of liquefaction refrigerant stream 104 may occur in a
separate compressor 126.
[0034] Precooling of the natural gas feed stream 102 and the mixed refrigerant stream 104
may be accomplished by vaporizing a precooling refrigerant such as propane at four
different pressure levels in closed-loop precooling refrigeration system(s). The natural
gas feed stream 102 may be precooled because of equipment limitations and for efficiency
purposes. It should be noted that while propane may be used as the precooling refrigerant
for vaporizing at four different pressure levels (as illustrated in exemplary Figures
1-7A), carbon dioxide, methane, propane, butane, iso-butane, propylene, ethane, ethylene,
R22, HFC refrigerants, including, but not limited to, R410A, R134A, R507, R23, or
combinations thereof, may also be used, for example.
[0035] Cooling of the natural gas feed stream 102 is performed in unit 106. Unit 106 may
comprise a series of heat exchangers, valves, and separators as illustrated in Figure
2A. Natural gas feed stream 102 is cooled by indirect heat exchange against a precooling
refrigerant in a series of propane evaporators 202, 204, 206, 208 that may operate
at successively lower pressures (202 being the highest and 208 being the lowest, for
example) producing cooled successive streams 203, 205, 207, and 150. The evaporation
of propane at the four pressures results in four propane vapor streams 110, 112, 114,
116 that are then compressed in compressor 118. The resulting compressed stream 120
is then condensed in propane condenser 122, producing liquid stream 124 for reintroduction
into the series of propane evaporators 202, 204, 206, 208. Propane condensers used
in these types of methods and systems may include, for example, a propane de-superheater,
a condenser, an accumulator, and a propane subcooler. It should be noted that while
this exemplary embodiment illustrated in Figures 1, 2A, 2B, 3, 4, 5, 6, and 7A uses
a four stage pre-cooling system, the pre-cooling system may comprise a single-stage,
a two-stage, a three-stage, or systems with greater than four stages, for example,
where the series of propane evaporators may operate at successively lower pressures.
[0036] Cooling of the mixed refrigerant stream 104 is performed in unit 108. Unit 108 may
also comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2B. The mixed refrigerant stream 104 may also be cooled by indirect heat exchange
against the precooling refrigerant in a series of propane evaporators 222, 224, 226,
228 that may operate at successively lower pressures (222 being the highest and 228
being the lowest, for example) producing cooled successive streams 223, 225, 227,
and 138. The evaporation of propane at the four pressures results in four propane
vapor streams 130, 132, 134, 136 that are then compressed in compressor 126. The resulting
compressed stream 127 is then condensed in propane condenser 128, producing liquid
stream 129 for reintroduction into the series of propane evaporators 222, 224, 226,
228.
[0037] Cooled mixed refrigerant stream 138 is separated in phase separator 140 into a liquid
mixed refrigerant stream 142 and a vapor mixed refrigerant stream 144. Liquid mixed
refrigerant stream 142 is sub-cooled in the cryogenic heat exchanger (MCHE) 146 producing
stream 147. Stream 147 may then be reduced in pressure through isenthalpic valve 148
producing stream 149. Stream 149 may then be vaporized in the shell side of the MCHE
146 to provide cooling to tubeside streams 142, 144, 150.
[0038] Vapor mixed refrigerant steam 144 is condensed and sub-cooled in the MCHE 146 to
produce stream 151. Stream 151 may then be reduced in pressure through isenthalpic
valve 152 to produce stream 153. Stream 153 may then be vaporized in the shell side
of the MCHE 146 to provide cooling to tubeside streams 142, 144, 150.
[0039] The cooled natural gas feed stream 150 may enter the MCHE 146 where it is further
cooled producing product stream 166 that may be, for example, liquid natural gas (LNG).
[0040] Low pressure mixed refrigerant stream 145 exiting the MCHE 146 is compressed in the
low pressure mixed refrigerant compressor 154 to produce stream 155. It should be
noted that the refrigerant compressors of all of the exemplary embodiments may include
one or more intercoolers and compressor casings. For example, mixed refrigerant compressor
154 may include one or more intercoolers and at least one compressor casing. Intercoolers
and aftercoolers use an ambient heat sink (air or water) to reject compression heat
to the environment.
[0041] Stream 155 is cooled in intercooler 156 to produce stream 157. Stream 157 is further
compressed in the medium pressure mixed refrigerant compressor 158 to produce stream
159. Stream 159 is cooled in intercooler 160 to produce stream 161. Stream 161 is
further compressed in high pressure mixed refrigerant compressor 162 to produce stream
163. Stream 163 is cooled in aftercooler 164 to be recycled back as original mixed
refrigerant stream 104.
[0042] The exemplary embodiment illustrated in Figure 1 shows how the power supplied to
the refrigeration compressors 118, 126, 154, 158, 162 are provided by two equal sized
directly connected gas turbines 180, 182. For example, mixed refrigerant compressors
154, 158 are driven by gas turbine driver 180 while mixed refrigerant compressor 160
and the propane compressors 118, 126 are driven by gas turbine driver 182. In this
exemplary embodiment, the design pressure level between the mixed refrigerant compressors
158 and 162 may be chosen such that the work required by the two gas turbine drivers
180, 182 is essentially equal. The gas turbine drivers in all exemplary embodiments
may be single-shaft gas turbines or multi-shaft gas turbines, for example.
[0043] This exemplary embodiment is independent of the method used to power the refrigeration
compressors 118, 126, 154, 158 and 162. The refrigeration compressors 118, 126, 154,
158 and 162, and the refrigeration compressors of the other exemplary embodiments
may be driven by one or more gas turbines, electric motors, steam turbines, or a combination
of different drivers. As illustrated in Figure 1, the gas turbines 180, 182 may include
starter/helper electric motors 184, 186 respectively to assist in starting the gas
turbines 180, 182 and optimally, to provide additional power to assist the gas turbines
180, 182, or to generate power for exportation into the power grid when excess power
is available from the gas turbines. Moreover, for the exemplary embodiment illustrated
in Figure 1, and all other exemplary embodiments disclosed, the order of the compressor
bodies and the starter/helper electric motors coupled to each driver is not fixed
and may be manipulated/altered pursuant to any system requirements, maintenance requirements,
and/or plant design requirements. For example, starter/helper electric motor 186 in
Figure 1 may be positioned away from and not adjacent to driver 182 (i.e., at the
opposite end of the driver string). The positions of the compressor bodies 118, 126,
162 may also be exchanged.
[0044] Figure 3 illustrates another exemplary embodiment 300 where the propane compressors
318, 326 are powered by different drivers 380, 382 respectively. In this exemplary
embodiment, the power demand from the equivalent gas turbine drivers 380, 382 may
be balanced by adjustment of the discharge pressure of low pressure mixed refrigerant
compressor 354.
[0045] As illustrated in the exemplary embodiment 300 in Figure 3, cooling of the natural
gas feed stream 302 is performed in unit 306. Like unit 106 of Figure 1, unit 306
may comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2A. Natural gas feed stream 302 is cooled by indirect heat exchange to ultimately
produce cooled stream 350. The evaporation of propane at the four pressures results
in four propane vapor streams 310, 312, 314, 316 that may then be compressed in compressor
318. The resulting compressed stream 320 may then be condensed in propane condenser
322, producing liquid stream 324 for reintroduction into the series of propane evaporators
as shown in Figure 2A.
[0046] Cooling of the mixed refrigerant stream 304 is performed in unit 308. Unit 308 may
also comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2B. The mixed refrigerant stream 304 may also be cooled by indirect heat exchange
to ultimately produce cooled stream 338. The evaporation of propane at the four pressures
results in four propane vapor streams 330, 332, 334, 336 that may then be compressed
in compressor 326. The resulting compressed stream 327 may then be condensed in propane
condenser 328, producing liquid stream 329 for reintroduction into the series of propane
evaporators as shown in Figure 2B.
[0047] Again cooled mixed refrigerant stream 338 is separated in phase separator 340 into
a liquid mixed refrigerant stream 342 and a vapor mixed refrigerant stream 344. Liquid
mixed refrigerant stream 342 is sub-cooled in the cryogenic heat exchanger (MCHE)
346 producing stream 347. Stream 347 may then be reduced in pressure through isenthalpic
valve 348 producing stream 349. Stream 349 may then be vaporized in the shell side
of the MCHE 346 to provide cooling to tubeside streams 342, 344, 350.
[0048] Vapor mixed refrigerant steam 344 is condensed and sub-cooled in the MCHE 346 to
produce stream 351. Stream 351 may then be reduced in pressure through isenthalpic
valve 352 to produce stream 353. Stream 353 may then be vaporized in the shell side
of the MCHE 346 to provide cooling to tubeside streams 342, 344, 350.
[0049] The cooled natural gas feed stream 350 may enter the MCHE 346 where it is further
cooled producing product stream 366 that may be, for example, liquid natural gas (LNG).
[0050] Low pressure mixed refrigerant stream 345 exiting the MCHE 346 is compressed in the
low pressure refrigerant compressor 354 to produce stream 355. Stream 355 is cooled
in intercooler 356 to produce stream 357. Stream 357 is further compressed in the
high pressure refrigerant compressor 362 to produce stream 363. Stream 363 is cooled
in aftercooler 364 to be recycled back as original mixed refrigerant stream 304.
[0051] Power is supplied to the refrigeration compressors 318, 326, 354, 362 by two equal
sized directly connected gas turbines 380, 382. As illustrated in Figure 3, the gas
turbines 380, 382 may include starter/helper electric motors 384, 386 respectively
to assist in starting the gas turbines 380, 382 and optimally, to provide additional
power to assist the gas turbines 380, 382, or for exportation into the power grid
when excess power is available from the gas turbines.
[0052] Figure 4 illustrates another exemplary embodiment 400 where the position of compressors
418, 426 of Figure 3 may be swapped such that one of the drivers provides power to
the propane compressor 418 and the high pressure refrigerant compressor 462, while
the other driver provides power to the propane compressor 426 and the low pressure
refrigerant compressor 454.
[0053] As illustrated in the exemplary embodiment 400 in Figure 4, cooling of the natural
gas feed stream 402 is performed in unit 406. Like unit 106 of Figure 1, unit 406
may comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2A. Natural gas feed stream 402 is cooled by indirect heat exchange to ultimately
produce cooled stream 450. The evaporation of propane at the four pressures results
in four propane vapor streams 410, 412, 414, 416 that may then be compressed in compressor
418. The resulting compressed stream 420 may then be condensed in propane condenser
422, producing liquid stream 424 for reintroduction into the series of propane evaporators
as shown in Figure 2A.
[0054] Cooling of the mixed refrigerant stream 404 is performed in unit 408. Unit 408 may
also comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2B. The mixed refrigerant stream 404 may also be cooled by indirect heat exchange
to ultimately produce cooled stream 438. The evaporation of propane at the four pressures
results in four propane vapor streams 430, 432, 434, 436 that may then be compressed
in compressor 426. The resulting compressed stream 427 may then be condensed in propane
condenser 428, producing liquid stream 429 for reintroduction into the series of propane
evaporators as shown in Figure 2B.
[0055] Again cooled mixed refrigerant stream 438 is separated in phase separator 440 into
a liquid mixed refrigerant stream 442 and a vapor mixed refrigerant stream 444. Liquid
mixed refrigerant stream 442 is sub-cooled in the cryogenic heat exchanger (MCHE)
446 producing stream 447. Stream 447 may then be reduced in pressure through isenthalpic
valve 448 producing stream 449. Stream 449 may then be vaporized in the shell side
of the MCHE 446 to provide cooling to tubeside streams 442, 444, 450.
[0056] Vapor mixed refrigerant steam 444 is condensed and sub-cooled in the MCHE 446 to
produce stream 451. Stream 451 may then be reduced in pressure through isenthalpic
valve 452 to produce stream 453. Stream 453 may then be vaporized in the shell side
of the MCHE 446 to provide cooling to tubeside streams 442, 444, 450.
[0057] The cooled natural gas feed stream 450 may enter the MCHE 446 where it is further
cooled producing product stream 466 that may be, for example, liquid natural gas (LNG).
[0058] Low pressure mixed refrigerant stream 445 exiting the MCHE 446 is compressed in the
low pressure refrigerant compressor 454 to produce stream 455. Stream 455 is cooled
in intercooler 456 to produce stream 457. Stream 457 is further compressed in high
pressure refrigerant compressor 462 to produce stream 463. Stream 463 is cooled in
aftercooler 464 to be recycled back as original mixed refrigerant stream 404.
[0059] Power is supplied to the refrigeration compressors 418, 426, 454, 462 by two equal
sized directly connected gas turbines 480, 482. As illustrated in Figure 4, the gas
turbines 480, 482 may include starter/helper electric motors 484, 486 respectively
to assist in starting the gas turbines 480, 482 and optimally, to provide additional
power to assist the gas turbines 480, 482, or for exportation into the power grid
when excess power is available from the gas turbines.
[0060] Figure 5 illustrates yet another exemplary embodiment 500 as applied to a three loop
refrigeration system. In this exemplary embodiment 500, unit 506 precools a third
refrigerant stream 503 in addition to the natural gas feed stream 502. Like unit 106
of Figure 1, unit 506 may comprise a series of heat exchangers, valves, and separators
as illustrated in Figure 2A. Natural gas feed stream 502 is cooled by indirect heat
exchange to ultimately produce cooled stream 550. The evaporation of propane at the
four pressures results in four propane vapor streams 510, 512, 514, 516 that may then
be compressed in compressor 518. The resulting compressed stream 520 may then be condensed
in propane condenser 522, producing liquid stream 524 for reintroduction into the
series of propane evaporators as shown in Figure 2A.
[0061] Cooling of the mixed refrigerant stream 504 is performed in unit 508. Unit 508 may
also comprise a series of heat exchangers, valves, and separators as illustrated in
Figure 2B. The mixed refrigerant stream 504 may also be cooled by indirect heat exchange
to ultimately produce cooled stream 538. The evaporation of propane at the four pressures
results in four propane vapor streams 530, 532, 534, 536 that may then be compressed
in compressor 526. The resulting compressed stream 527 may then be condensed in propane
condenser 528, producing liquid stream 529 for reintroduction into the series of propane
evaporators as shown in Figure 2B.
[0062] Cooled mixed refrigerant stream 538 is subcooled in the cryogenic heat exchanger
(MCHE) 546 producing stream 547. Stream 547 may then be reduced in pressure through
isenthalpic valve 548 producing stream 549. Stream 549 may then be vaporized in the
shell side of the MCHE 546 to provide cooling to tubeside streams 505, 538, and 550.
[0063] Cooled mixed refrigerant stream 505 may also be subcooled and liquefied in MCHE 546
producing stream 569 then subcooled in exchanger 568 producing stream 551. Exchanger
568 may be a wound coil type exchanger, for example. The resulting stream 551 may
then be reduced in pressure through isenthalpic valve 552 to produce stream 553. Stream
553 may then be vaporized in exchanger 568 to provide refrigeration for subcooling
both the feed gas stream (entering as stream 567 and exiting as 566) and the third
refrigerant stream 569. After vaporization and warming, third refrigerant stream 553
exits exchanger 568 as stream 593 and is then compressed by compressor 594 to produce
stream 595. Stream 595 is then cooled in the mixed refrigerant intercooler 596 to
produce stream 597. Stream 597 is compressed in compressor 598 to produce stream 599.
Stream 599 is then cooled in mixed refrigerant aftercooler 501 to be recycled back
as original stream 503.
[0064] The cooled natural gas feed stream 550 may enter the MCHE 546 where it is further
cooled producing stream 567. Stream 567 may then be subcooled in exchanger 568 to
produce product stream 566 that may be, for example, liquid natural gas (LNG).
[0065] Low pressure mixed refrigerant stream 545 exiting the MCHE 546 is compressed in the
low pressure refrigerant compressor 554 to produce stream 555. Stream 555 is cooled
in intercooler 556 to produce stream 557. Stream 557 is further compressed in high
pressure refrigerant compressor 558 to produce stream 559. Stream 559 is cooled in
aftercooler 564 to be recycled back as original mixed refrigerant stream 504.
[0066] Power is supplied to the refrigeration compressors 518, 526, 554, 558, 594, 598 by
three equal sized directly connected gas turbines 580, 582, 592. As illustrated in
Figures 1, 3, and 4, the gas turbines may include starter/helper electric motors (not
shown in this embodiment) to assist in starting the gas turbines and optimally, to
provide additional power to assist the gas turbines, or for exportation into the power
grid when excess power is available from the gas turbines.
[0067] Figure 6 illustrates yet another exemplary embodiment 600 as applied to another three
loop refrigeration system. In this exemplary embodiment 600, unit 606 precools the
natural gas feed stream 602 only. Like unit 106 of Figure 1, unit 606 may comprise
a series of heat exchangers, valves, and separators as illustrated in Figure 2A. Natural
gas feed stream 602 is cooled by indirect heat exchange to ultimately produce cooled
stream 650. The evaporation of propane at the four pressures results in four propane
vapor streams 610, 612, 614, 616 that may then be compressed in compressor 618. The
resulting compressed stream 620 may then be condensed in propane condenser 622, producing
liquid stream 624 for reintroduction into the series of propane evaporators as shown
in Figure 2A.
[0068] In this exemplary embodiment, both mixed refrigerant streams 603, 604 are cooled
in unit 608. Unit 608 may also comprise a series of heat exchangers, valves, and separators
as illustrated in Figure 2B. The mixed refrigerant streams 603, 604 may also be cooled
by indirect heat exchange to ultimately produce cooled streams 605, 638. The evaporation
of propane at the four pressures results in four propane vapor streams 630, 632, 634,
636 that may then be compressed in compressor 626. The resulting compressed stream
627 may then be condensed in propane condenser 628, producing liquid stream 629 for
reintroduction into the series of propane evaporators as shown in Figure 2B.
[0069] Cooled mixed refrigerant stream 638 is subcooled in the cryogenic heat exchanger
(MCHE) 646 producing stream 647. Stream 647 may then be reduced in pressure through
isenthalpic valve 648 producing stream 649. Stream 649 may then be vaporized in the
shell side of the MCHE 646 to provide cooling to tubeside streams 605, 638, and 650.
[0070] Cooled mixed refrigerant stream 605 may also be subcooled and liquefied in MCHE 646
producing stream 669 then subcooled in exchanger 668 producing stream 651. Exchanger
668 may be a wound coil type exchanger, for example. The resulting stream 651 may
then be reduced in pressure through isenthalpic valve 652 to produce stream 653. Stream
653 may then be vaporized in exchanger 668 to provide refrigeration for subcooling
both the feed gas stream (entering as stream 667 and exiting as 666) and the third
refrigerant stream 669. After vaporization and warming, third refrigerant stream 653
exits exchanger 668 as stream 693 and is then compressed by compressor 694 to produce
stream 695. Stream 695 is then cooled in the mixed refrigerant intercooler 696 to
produce stream 697. Stream 697 is compressed in compressor 698 to produce stream 699.
Stream 699 is then cooled in mixed refrigerant aftercooler 601 to be recycled back
as original stream 603.
[0071] The cooled natural gas feed stream 650 may enter the MCHE 646 where it is further
cooled producing stream 667. Stream 667 may then be subcooled in exchanger 668 to
produce product stream 666 that may be, for example, liquid natural gas (LNG).
[0072] Low pressure mixed refrigerant stream 645 exiting the MCHE 646 is compressed in the
low pressure refrigerant compressor 654 to produce stream 655. Stream 655 is cooled
in intercooler 656 to produce stream 657. Stream 657 is further compressed in the
high pressure refrigerant compressor 658 to produce stream 659. Stream 659 is cooled
in aftercooler 664 to be recycled back as original mixed refrigerant stream 604.
[0073] Power is supplied to the refrigeration compressors 618, 626, 654, 658, 694, 698 by
three equal sized directly connected gas turbines 680, 682, 692. As illustrated in
Figures 1, 3, and 4, the gas turbines may include starter/helper electric motors (not
shown in this embodiment) to assist in starting the gas turbines and optimally, to
provide additional power to assist the gas turbines, or for exportation into the power
grid when excess power is available from the gas turbines.
[0074] Figure 7A illustrates another exemplary embodiment 700A as applied to yet another
three loop refrigeration system. In this exemplary embodiment 700A, unit 706 precools
the natural gas feed stream 702 and the mixed refrigerant stream 704. Like unit 106
of Figure 1, unit 706 may comprise a series of heat exchangers, valves, and separators
as illustrated in Figure 2A. Natural gas feed stream 702 and mixed refrigerant stream
704 is cooled by indirect heat exchange to ultimately produce cooled streams 750,
738. The evaporation of propane at the four pressures results in four propane vapor
streams 710, 712, 714, 716 that may then be compressed in compressor 718. The resulting
compressed stream 720 may then be condensed in propane condenser 722, producing liquid
stream 724 for reintroduction into the series of propane evaporators as shown in Figure
2A.
[0075] In this exemplary embodiment, only mixed refrigerant stream 703 is cooled in unit
708. Unit 708 may also comprise a series of heat exchangers, valves, and separators
as illustrated in Figure 2B. The mixed refrigerant stream 703 is cooled by indirect
heat exchange to ultimately produce cooled streams 705. The evaporation of propane
at the four pressures results in four propane vapor streams 730, 732, 734, 736 that
may then be compressed in compressor 726. The resulting compressed stream 727 may
then be condensed in propane condenser 728, producing liquid stream 729 for reintroduction
into the series of propane evaporators as shown in Figure 2B.
[0076] Cooled mixed refrigerant stream 738 is subcooled in the cryogenic heat exchanger
(MCHE) 746 producing stream 747. Stream 747 may then be reduced in pressure through
isenthalpic valve 748 producing stream 749. Stream 749 may then be vaporized in the
shell side of the MCHE 746 to provide cooling to tubeside streams 705, 738, and 750.
[0077] Cooled mixed refrigerant stream 705 may also be subcooled and liquefied in MCHE 746
producing stream 769 then subcooled in exchanger 768 producing stream 751. Exchanger
768 may be a wound coil type exchanger, for example. The resulting stream 751 may
then be reduced in pressure through isenthalpic valve 752 to produce stream 753. Stream
753 may then be vaporized in exchanger 768 to provide refrigeration for subcooling
both the feed gas stream (entering as stream 767 and exiting as 766) and the third
refrigerant stream 769. After vaporization and warming, third refrigerant stream 753
exits exchanger 768 as stream 793 and is then compressed by compressor 794 to produce
stream 795. Stream 795 is then cooled in the mixed refrigerant intercooler 796 to
produce stream 797. Stream 797 is compressed in compressor 798 to produce stream 799.
Stream 799 is then cooled in mixed refrigerant aftercooler 701 to be recycled back
as original stream 703.
[0078] The cooled natural gas feed stream 750 may enter the MCHE 746 where it is further
cooled producing stream 767. Stream 767 may then be subcooled in exchanger 768 to
produce product stream 766 that may be, for example, liquid natural gas (LNG).
[0079] Low pressure mixed refrigerant stream 745 exiting the MCHE 746 is compressed in the
low pressure refrigerant compressor 754 to produce stream 755. Stream 755 is cooled
in intercooler 756 to produce stream 757. Stream 757 is further compressed in the
high pressure refrigerant compressor 758 to produce stream 759. Stream 759 is cooled
in aftercooler 764 to be recycled back as original mixed refrigerant stream 704.
[0080] Power is supplied to the refrigeration compressors 718, 726, 754, 758, 794, 798 by
three equal sized directly connected gas turbines 780, 782, 792. As illustrated in
Figures 1, 3, and 4, the gas turbines may include starter/helper electric motors (not
shown in this embodiment) to assist in starting the gas turbines and optimally, to
provide additional power to assist the gas turbines, or for exportation into the power
grid when excess power is available from the gas turbines.
[0081] Figure 7B illustrates yet another exemplary embodiment 700B similar to 700A, however,
in this exemplary embodiment 700B, unit 706 precools the natural gas feed stream 702
and the mixed refrigerant stream 704 through indirect heat exchange with a mixed refrigerant
stream in a two-stage mixed refrigerant precooling system. While Figure 7B discloses
use of a two-stage mixed refrigerant precooling system, the precooling may be performed
using a single-stage mixed refrigerant precooling system, or mixed refrigerant precooling
systems with greater than two stages, for example. Additionally, a mixed refrigerant
precooling system may be interchanged with the propane precooling systems disclosed
in any of the exemplary embodiments.
[0082] Figures 8A and 8B illustrate exemplary units 706 and 708 shown in Figure 7B. Unit
706 may comprise two heat exchangers 810, 812 where streams 702, 704, and at least
a portion of stream 724 are cooled through indirect heat exchange against stream 713
in heat exchanger 810. Stream 724 enters heat exchanger 810 and is cooled producing
stream 830. Stream 830 is split into two streams 831, 832 where stream 831 is further
cooled in heat exchanger 812 while stream 832 is let down in pressure across isenthalpic
valve 814 to produce stream 833. Stream 833 then enters heat exchanger 810 to provide
cooling to streams 702, 704, 724 and exits the heat exchanger 810 as stream 713.
[0083] After stream 831 is cooled in heat exchanger 812 to produce stream 834 and let down
in pressure across isenthalpic valve 816, the resulting stream 835 is introduced into
heat exchanger 812 to provide further cooling for resultant streams 738, 750, 834.
[0084] Unit 708 may comprise two heat exchangers 818, 820 where streams 703, 729 are cooled
through indirect heat exchange against stream 733 in heat exchanger 818. Stream 729
enters heat exchanger 818 and is cooled producing stream 840. Stream 840 is split
into two streams 841, 842 where stream 841 is further cooled in heat exchanger 820
while stream 842 is let down in pressure across isenthalpic valve 822 to produce stream
843. Stream 843 then enters heat exchanger 818 to provide cooling to streams 703,
729 and exits the heat exchanger 818 as stream 733.
[0085] After stream 841 is cooled in heat exchanger 820 to produce stream 844 and let down
in pressure across isenthalpic valve 824, the resulting stream 845 is introduced into
heat exchanger 820 to provide further cooling for resultant streams 705, 844.
[0086] Heat exchangers 810, 812, 818, 820 may be wound-coil heat exchangers, plate-and-fin
brazed aluminum (core) type heat exchangers, or shell and tube heat exchangers, for
example. Heat exchangers 810, 812 may be combined into a single heat exchanger, for
example. Heat exchangers 818, 820 may also be combined into a single heat exchanger,
for example. Finally, heat exchangers 810, 812, 818, 820 may be combined into a single
heat exchanger, for example. Heat exchangers 810, 812, 818, 820 may accept two or
more load streams, for example.
[0087] Pre-cooling in units 106, 108 may provide, for example, enough cooling to feed stream
102 and liquefaction refrigerant stream 104 such that the temperatures of streams
150 and 138 may reach +60°F (+16°C) to as low as -100°F (-73°C) before further cooling
in the MCHE 146. The same cooling ranges may be achieved in Figures 3-7B. In one embodiment,
for example, propane may be used as the pre-cooling refrigerant to reach the temperature
range of +20°F to -40°F (-7°C to -40°C).
[0088] The isenthalpic valves 148, 152 (and the corresponding isenthalpic valves in Figures
3-7B) may optionally be replaced by work extracting liquid turbines, for example,
to improve efficiency. Additionally, propane condensers 122, 128 (and the corresponding
propane condensers in Figures 3-7A) may be ambient heat sink coolers used to condense,
desuperheat, and/or optimally subcool precooling refrigerant, for example.
EXAMPLE
[0089] The following example is based on a computer simulation of Figures 1, 2A, and 2B
as applied to a propane precooled mixed refrigerant process. As in Figure 1, the natural
gas feed stream 102 entered unit 106 after pretreatment, including the removal of
moisture (H
2O), carbon dioxide (CO
2), sulfur dioxide (SO
2), mercury, and other heavy components, including, but not limited to, benzene, ethylbenzene,
and toluene, if they exist in the natural gas feed stream 102 in concentrations that
would lead to freezing in the MCHE 146. The pretreated natural gas feed stream 102
was at 35°C and 40 bar absolute and had a flow rate of 12,260 kg-mole/hr. Natural
gas feed stream 102 was cooled by indirect heat exchange in a series of propane evaporators
202, 204, 206, 208 (illustrated in Figure 2A) that operate at successively lower pressures
of 7.16 bar, 4.25 bar, 2.54 bar and 1.47 bar, where propane evaporator 202 is at the
highest pressure and propane evaporator 208 is at the lowest pressure. The evaporation
of propane at the four pressures resulted in four propane vapor streams 110, 112,
114, 116 that were then compressed in compressor 118. Resulting stream 120 (at 16.2
bar, and 10,930 kgmole/hr) was then condensed in propane condenser 122 using an ambient
heat sink (air or water), producing liquid stream 124.
[0090] The natural gas feed stream 102 was precooled by the propane to -22.5 °C. Resulting
cooled stream 150 was then cooled and liquefied in MCHE 146 by vaporizing mixed refrigerant
producing liquid natural gas (LNG) stream 166 at -163.3°C.
[0091] The mixed refrigerant stream 104 had a molar composition as follows:
Table I
Component |
Mole Composition (%) |
Nitrogen |
12 |
Methane |
38 |
Ethane |
42 |
Propane |
8 |
[0092] The mixed refrigerant stream 104 was at 35 °C and 62 bar absolute and had a flow
rate of 50,250 kg-mole/hr. The mixed refrigerant stream 104 was cooled by indirect
heat exchange in a series of propane evaporators 222, 224, 226, 228 (illustrated in
Figure 2B) that operate at successively lower pressures of 7.16 bar absolute, 4.25
bar, 2.54 bar and 1.47 bar where propane evaporator 203 is the highest and propane
evaporator 209 is the lowest. The evaporation of propane at the four pressures results
in four propane vapor streams 130, 132, 134, 138 which are then compressed in compressor
126. Resulting stream 127 (at 16.2 bar absolute and 31,600 kgmole/hr) is condensed
in propane condenser 128 using an ambient heat sink (air or water), producing liquid
stream 129.
[0093] The precooled mixed refrigerant stream 138 is then separated into liquid stream 142
and vapor stream 144 in phase separator 140. Liquid stream 142 is then subcooled to
-125°C, flashed isenthalpically through valve 148, and then vaporized in the shell
side of exchanger 146 to provide cooling to the tubeside streams 142, 144, 150. Vapor
stream 144 is liquefied, subcooled to a temperature of -163°C, flashed isenthalpically
through valve 152, and then vaporized and warmed in the shell side of exchanger 146
to provide cooling to the tubeside streams 142, 144, 150. After vaporization and warming,
the combined mixed refrigerant stream 145 exits the MCHE 146 at a temperature of -
32.7°C and a pressure of 4.14 bar absolute. The combined mixed refrigerant stream
154 is then compressed in three stages of compressors 156, 158, 160 back to a pressure
of 62 bar absolute, completing the loop.
Comparison with U.S. Patent No. 6,962,060
[0094] Computer simulations of the exemplary embodiment illustrated in Figure 1 were performed
on the same basis as the simulation of a propane precooled mixed refrigerant process
utilizing the precooling arrangement of
U.S. Patent No. 6,962,060.
[0095] Results for the simulations are listed in Table II below. For both simulations, the
same propane low pressure suction pressure was assumed and two compressor casings
were required. For both simulations, preliminary sizing calculations for the compressors
were performed. In the case of the exemplary embodiment illustrated in Figure 1, the
compressor casings 118 and 126 were smaller in diameter and had lower volumetric flow
rates translating into lower cost. In addition, depending on the vendor and the scale
of the plant, construction of large diameter impellers and casings may not have been
feasible, thus, the solution utilizing the prior art may have been more limited in
scale-up potential.
[0096] As illustrated in Table II, the exemplary embodiment of Figure 1 allows more optimal
and feasible compressor designs than the system disclosed in
U.S. Patent No. 6,962,061 using the same number of compressor casings and providing the same pre-cooling service.
This is achieved by segregating the heat loads requiring pre-cooling refrigeration
into two independent systems.
Table II
|
U.S. Patent No. 6,962,060 |
Exemplary Embodiment in Figure 1 |
|
Precooling Temperature (°C) |
-30.2 |
-30.2 |
|
Liquid Natural Gas Production (kg/h) |
490,000 |
490,000 |
Compressor 1 |
Identifier |
Compressor 43 |
Compressor 126 |
|
Maximum Impeller Diameter (inches) |
55 |
50 |
|
Maximum Volume Flow Rate (m3/hr) |
149,000 |
119,000 |
Compressor 2 |
Identifier |
Compressor 49 |
Compressor 118 |
|
Maximum Impeller Diameter (inches) |
52 |
51 |
|
Maximum Volume Flow Rate (m3/hr) |
78,000 |
57,000 |
1. A natural gas liquefaction system, the system comprising:
a first closed-loop precooling refrigeration system, for precooling at least a natural
gas feed stream via indirect heat transfer with a precooling refrigerant circulating
in said closed-loop of said first precooling refrigeration system;
a second closed-loop precooling refrigeration system, for precooling at least a first
refrigerant stream via indirect heat transfer with a precooling refrigerant circulating
in said closed-loop of said second precooling refrigeration system; and
a cryogenic heat exchanger in fluid connection with the first precooling refrigeration
system and the second precooling refrigeration system so as to accept the precooled
natural gas feed stream from the first precooling refrigeration system and the precooled
first refrigerant stream from the second precooling refrigeration system in order
to liquefy the natural gas feed stream;
wherein the closed-loop of the second precooling refrigeration system is separate
from the closed-loop of the first precooling refrigeration system, and wherein in
operation no streams are precooled by both the second precooling refrigeration system
and the first precooling refrigeration system.
2. The system of claim 1, further comprising a subcooler exchanger in fluid connection
with the cryogenic heat exchanger, wherein the subcooler exchanger accepts a second
refrigerant stream from the cryogenic heat exchanger to subcool the natural gas feed
stream through indirect heat exchange.
3. The system of any preceding claim, wherein at least one of the first and second precooling
refrigeration systems comprises at least one heat exchanger that accepts at least
two load streams.
4. The system of any preceding claim, wherein the first precooling refrigeration system
and the second precooling refrigeration system each comprise:
at least one device for reducing the pressure of or vaporizing at least a part of
the precooling refrigerant; and
a compressor in fluid connection with the at least one pressure reducing or vaporizing
device and adapted to accept at least one precooling refrigerant stream.
5. The system of claim 4, further comprising a first driver and a second driver:
wherein the first driver drives the compressor of the first precooling refrigeration
system, the compressor of the second precooling refrigeration system, and a first
high pressure refrigerant compressor; and wherein the second driver drives a first
medium pressure refrigerant compressor and a first low pressure refrigerant compressor.
6. The system of claim 4, further comprising a first driver and a second driver:
wherein the first driver drives the compressor of the first precooling refrigeration
system and one of either a first low pressure refrigerant compressor or a first high
pressure refrigerant compressor; and wherein the second driver drives the compressor
of the second precooling refrigeration system and the other of the first low pressure
refrigerant compressor and the first high pressure refrigerant compressor not driven
by the first driver.
7. The system of claim 4, further comprising a first driver and a second driver:
wherein the first driver drives the compressor of the first precooling refrigeration
system and the compressor of the second precooling refrigeration system; and the second
driver drives a first low pressure refrigerant compressor and a first high pressure
refrigerant compressor.
8. The system of claim 7, further comprising a third driver, wherein the third driver
drives a second low pressure refrigerant compressor and a second high pressure refrigerant
compressor.
9. The system of any one of claims 5 to 8, wherein the first driver, the second driver
and/or, if present, the third driver are gas turbines.
10. The system of any preceding claim, wherein the cryogenic heat exchanger is a wound-coil
heat exchanger.
11. A method for liquefying natural gas, the method comprising the steps of:
precooling in a first closed-loop precooling refrigeration system, via indirect heat
transfer with a precooling refrigerant circulating in said closed loop of said first
precooling refrigeration system, at least a natural gas feed stream;
precooling in a second closed-loop precooling refrigeration system, via indirect heat
transfer with a precooling refrigerant circulating in said closed loop of said second
precooling refrigeration system, at least a first refrigerant stream; and
vaporizing the precooled first refrigerant stream in a cryogenic heat exchanger to
cool the precooled natural gas feed stream through indirect heat exchange;
wherein the closed-loop of the second precooling refrigeration system is separate
from the closed-loop of the first precooling refrigeration system, and wherein the
second precooling refrigeration system precools only stream(s) having a composition
different from the stream(s) precooled by the first precooling refrigeration system.
12. The method of claim 11, wherein the natural gas feed stream and the first refrigerant
stream are precooled to +16°C to -73°C (+60°F to -100°F).
13. The method of claim 11 or 12, the method further comprising precooling a second refrigerant
stream in either the first precooling refrigeration system or the second precooling
refrigeration system, and vaporizing the second refrigerant stream to subcool the
natural gas feed stream.
14. The method of any one of claims 11 to 13, wherein the first refrigerant stream and/or,
if present, the second refrigerant streams are mixed refrigerant streams.
15. The method of any one of claims 11 to 14, wherein the precooling refrigerant is propane
or CO2.