Expander-Based LNG Production Processes Enhanced With Liquid Nitrogen

Abstract

A method for producing liquefied natural gas (LNG). A natural gas stream is directed to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa). A liquid refrigerant subcooling unit is provided at a first location. Liquid refrigerant is produced at a second location that is geographically separate from the first location. The produced liquid refrigerant is transported to the first location. The pressurized LNG stream is subcooled in the liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one stream of the liquid refrigerant to thereby produce an LNG stream.

Claims

1. A method for producing liquefied natural gas (LNG), comprising: directing a natural gas stream to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa); providing a liquid refrigerant subcooling unit at a first location; producing liquid refrigerant at a second location that is geographically separate from the first location; transporting the produced liquid refrigerant to the first location; and subcooling the pressurized LNG stream in the liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one stream of the liquid refrigerant to thereby produce an LNG stream.

2. The method of claim 1, wherein the mechanical refrigeration unit includes an expander-based refrigeration process.

3. The method of claim 2, wherein the expander-based refrigeration process is one of an open loop feed gas expander-based process and a closed loop feed gas expander-based process.

4. The method of claim 2, wherein the expander-based refrigeration process is a feed gas expander-based process that comprises: discharging a first cooling stream from a warm-end expander; and discharging a two-phase stream from a cold-end expander; wherein a temperature of the first cooling stream is higher than a temperature of the two-phase stream.

5. The method of claim 4, wherein the pressurized LNG stream is a first pressurized LNG stream, and further comprising separating the two-phase stream into a second cooling stream and a second pressurized LNG stream.

6. The method of claim 2, wherein the expander-based refrigeration process is a feed gas expander-based process that comprises: discharging a first cooling stream from a warm-end expander; and location in the dual purpose carrier after the subcooled LNG stream has been offloaded from the dual-purpose carrier. discharging a second cooling stream from a cool-end expander; wherein a temperature of the first cooling stream is higher than a temperature of the second cooling stream.

7. The method of claim 5, wherein a pressure of the first cooling stream is the same or substantially the same as a pressure of the second cooling stream.

8. The method of claim 7, further comprising mixing the second pressurized LNG stream with the first pressurized LNG stream prior to directing the pressurized LNG stream to the liquid refrigerant subcooling unit.

9. The method of claim 1, wherein the liquid refrigerant subcooling unit comprises at least one heat exchanger, or at least one compressor and/or expander.

10. The method of claim 1, further comprising: using the vaporized liquid refrigerant stream to liquefy a second treated natural gas stream to produce an additional pressurized LNG stream; and mixing the additional pressurized LNG stream with the pressurized LNG stream prior to the subcooling of the pressurized LNG stream with the liquid refrigerant.

11. The method of claim 1, further comprising locating the mechanical refrigeration unit and the liquid refrigerant subcooling unit on a floating LNG facility.

12. The method of claim 1, further comprising re-liquefying LNG boil-off gas using the liquid refrigerant.

13. The method of claim 1, wherein the liquid refrigerant and/or a liquid refrigerant boil-off gas is used to keep the mechanical refrigeration unit and/or liquid refrigerant subcooling unit equipment cold during turndown and/or shutdown periods of the mechanical refrigeration unit.

14. The method of claim 1, wherein warm liquid refrigerant vapor is used to derime heat exchangers used to exchange heat.

15. The method of claim 1, further comprising: transporting the LNG stream from the first location to the second location in a dual-purpose carrier; and after the LNG stream has been offloaded from the dual-purpose carrier, transporting the liquid refrigerant from the second location to the first location in the dual purpose carrier.

16. The method of claim 1, wherein the mechanical refrigeration unit includes one of a single-mixed refrigerant process, a pure component cascade refrigerant process, or a dual-mixed refrigerant process.

17. The method of claim 1, wherein the pressurized LNG stream has a pressure greater than 100 psia (690 kPa) and less than 400 psia (2758 kPa).

18. The method of claim 1, wherein the pressurized LNG stream has a pressure greater than 200 psia (1379 kPa) and less than 300 psia (2068 kPa).

19. The method of claim 1, wherein the liquid refrigerant comprises liquid nitrogen (LIN), and further comprising producing the LIN by exchanging heat with LNG during LNG regasification.

20. The method of claim 1, wherein the liquid refrigerant comprises liquid nitrogen (LIN), and further comprising: pressurizing the LIN to a pressure greater than 400 psia (2758 kPa) to form a high pressure liquid nitrogen stream; exchanging heat between the high pressure liquid nitrogen stream and the pressurized LNG stream to form a warm nitrogen gas stream; and reducing, in the liquid refrigerant subcooling unit, a pressure of at least one warmed natural gas stream in at least one expander service, to reduce the pressure of the at least one warmed nitrogen gas stream, and to thereby produce at least one additionally cooled nitrogen gas stream.

21. The method of claim 20, wherein the at least one additionally cooled nitrogen gas stream exchanges heat with the pressurized LNG stream to form warmed nitrogen gas streams.

22. The method of claim 20, further comprising: coupling the at least one expander service with at least one generator to generate electrical power, or at least one compressor that is used to compress warmed nitrogen gas streams.

23. The method of claim 1, further comprising: directing pressurized LNG streams from a plurality of mechanical refrigeration units to the liquid refrigerant subcooling unit to produce at least one LNG stream.

24. A system for producing liquefied natural gas (LNG), comprising: a mechanical refrigeration unit configured to liquefy a natural gas stream using a feed gas expander-based process and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa); a liquid nitrogen (LIN) subcooling unit located in a first location; a liquid nitrogen (LIN) stream produced at a second location that is geographically separate from the first location and transported to the LIN subcooling unit; wherein the LIN subcooling unit is configured to subcool the pressurized LNG stream by exchanging heat between the pressurized LNG stream and at least one stream of the LIN stream to thereby produce an LNG stream and at least one vaporized LIN stream.

25. The system of claim 24, wherein the mechanical refrigeration unit comprises: a warm-end expander configured to discharge a first cooling stream therefrom; and a cold-end expander configured to discharge a two-phase stream therefrom; wherein a temperature of the first cooling stream is higher than a temperature of the two-phase stream; wherein the pressurized LNG stream is a first pressurized LNG stream, and wherein the two-phase stream is configured to be split into a second cooling stream and a second pressurized LNG stream.

26. The system of claim 24, wherein the mechanical refrigeration unit comprises: A warm-end expander configured to discharge a first cooling stream therefrom; and a cool-end expander configured to discharge a second cooling stream therefrom; wherein a temperature of the first cooling stream is higher than a temperature of the second cooling stream.

27. The system of claim 24, wherein the mechanical refrigeration unit and the liquid refrigerant subcooling unit are located on a floating LNG facility.

28. The system of claim 27, further comprising: a dual-purpose carrier configured to transport the LNG stream from the first location to the second location and transport the liquid refrigerant from the second location to the first

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below.

[0020] FIG. 1 is a graph showing a temperature cooling curve for an expander-based heat exchanger process.

[0021] FIG. 2A is a simplified diagram of the value chain of known FLNG technology.

[0022] FIG. 2B is a simplified diagram of the value chain of the disclosed aspects.

[0023] FIG. 3 is a schematic diagram of a system according to a disclosed aspect.

[0024] FIG. 4 is a schematic diagram of a mechanical refrigeration unit according to disclosed aspects.

[0025] FIG. 5 is a schematic diagram of a liquid nitrogen (LIN) subcooling unit according to disclosed aspects.

[0026] FIG. 6 is a schematic diagram of a LIN subcooling unit according to disclosed aspects.

[0027] FIG. 7 is a flowchart showing a method according to disclosed aspects.

[0028] It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure.

DETAILED DESCRIPTION

[0029] To promote an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. For the sake clarity, some features not relevant to the present disclosure may not be shown in the drawings.

[0030] At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0031] As one of ordinary skill would appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name only. The figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. When referring to the figures described herein, the same reference numerals may be referenced in multiple figures for the sake of simplicity. In the following description and in the claims, the terms including and comprising are used in an open-ended fashion, and thus, should be interpreted to mean including, but not limited to.

[0032] The articles the, a and an are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements.

[0033] As used herein, the terms approximately, about, substantially, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure.

[0034] The term heat exchanger refers to a device designed to efficiently transfer or exchange heat from one matter to another. Exemplary heat exchanger types include a co-current or counter-current heat exchanger, an indirect heat exchanger (e.g. spiral wound heat exchanger, plate-fin heat exchanger such as a brazed aluminum plate fin type, shell-and-tube heat exchanger, etc.), direct contact heat exchanger, or some combination of these, and so on.

[0035] The term dual purpose carrier refers to a ship capable of (a) transporting LIN to an export terminal for natural gas and/or LNG and (b) transporting LNG to an LNG import terminal.

[0036] As previously described, the conventional LNG cycle includes: (a) initial treatments of the natural gas resource to remove contaminants such as water, sulfur compounds and carbon dioxide; (b) the separation of some heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a variety of possible methods including self-refrigeration, external refrigeration, lean oil, etc.; (c) refrigeration of the natural gas substantially by external refrigeration to form LNG at or near atmospheric pressure and about 160 C.; (d) transport of the LNG product in ships or tankers designed for this purpose to a market location; and (e) re-pressurization and regasification of the LNG at a regasification plant to form a pressurized natural gas stream that may distributed to natural gas consumers. The present disclosure generally involves liquefying natural gas using liquid nitrogen (LIN). In general, using LIN to produce LNG is a non-conventional LNG cycle in which step (c) above is replaced by a natural gas liquefaction process that uses a significant amount of LIN as an open loop source of refrigeration, and in which step (e) above may be modified to use the exergy of the cryogenic LNG to facilitate the liquefaction of nitrogen gas to form LIN that may then be transported to the resource location and used as a source of refrigeration for the production of LNG. The disclosed LIN-to-LNG concept may further include the transport of LNG in a ship or tanker from the resource location (export terminal) to the market location (import terminal) and the reverse transport of LIN from the market location to the resource location.

[0037] Aspects disclosed herein provide a method for enhancing a mechanical refrigeration process for the production of LNG using liquid refrigerant produced at a different location to subcool the liquefied natural gas coming from the mechanical refrigeration process. More specifically, a process is described in which treated natural gas may be directed to a mechanical refrigeration process. The natural gas may be completely liquefied within the mechanical refrigeration process to produce a pressurized LNG stream where the pressure of the pressurized LNG stream is greater than 50 psia (or 345 kPa) and less than 500 psia (or 3445 kPa), or more specifically greater than 100 psia (or 690 kPa) and less than 400 psia (or 2758 kPa), or more specifically greater than 200 psia (or 1379 kPa) and less than 300 psia (or 2068 kPa). The pressurized LNG stream may then be subcooled by exchanging heat with at least one liquid refrigerant stream to form an LNG stream. The liquid refrigerant stream is produced at a different geographic location than the location where the natural gas is liquefied, and may be 50 miles, or 100 miles, or 200 miles, or 500 miles, or 1,000 miles, or more than 1,000 miles from such location. The mechanical refrigeration process may be a single-mixed refrigerant process, a pure component cascade refrigerant process, a dual-mixed refrigerant process, an expander-based refrigeration process, or any other commonly known refrigeration process that can liquefy a natural gas stream to produce a pressurized LNG stream.

[0038] In an aspect, an expander-based process for the production of LNG may be enhanced by using LIN produced at a different location to subcool the pressurized LNG coming from the expander-based process. Natural gas may be treated to remove impurities, if present, such as water, heavy hydrocarbons, and sour gases, to make the natural gas suitable for liquefaction. The treated natural gas may be completely liquefied within the expander-based process to produce a pressurized LNG stream where the pressure of the pressurized LNG stream is greater than 50 psia (or 345 kPa) and less than 500 psia (or 3445 kPa), or more specifically greater than 100 psia (or 690 kPa) and less than 400 psia (or 2758 kPa), or more specifically greater than 200 psia (or 1379 kPa) and less than 300 psia (or 2068 kPa). The pressurized LNG stream may then be subcooled by exchanging heat with at least one LIN stream to form an LNG stream. The expander-based process may be a nitrogen gas expander-based process or may be a feed gas expander-based process.

[0039] FIG. 1 shows a typical temperature cooling curve 100 for an expander-based liquefaction process. The higher temperature curve 104 is the temperature curve for the natural gas stream. The lower temperature curve 102 is the composite temperature curve of a cold cooling stream and a warm cooling stream. As illustrated, the cooling curve is marked by three temperature pinch-points. The lowest temperature pinch-point 106 occurs where the colder of the two cooling streams, typically the cold cooling stream, enters the heat exchanger. The intermediate temperature pinch-point 108 occurs where the second cooling stream, typically the warm cooling stream, enters the heat exchanger. The warm temperature pinch-point 110 occurs where the cold and warm cooling streams exit the heat exchanger. The lowest temperature pinch-point 106 sets the required flow rate of the cold cooling stream. Since the cold cooling stream is first cooled by the warm cooling stream prior to being expanded to the low temperature, the flow rate of the cold cooling stream also impacts the required flow rate of the warm cooling stream. One way to increase the capacity of the expander-based process without significantly increasing equipment size and required power is to increase the temperature of lowest temperature pinch point. In such a case, to produce LNG additional refrigeration is needed to subcool the pressurized LNG coming from the expander-based process. It would not be advantageous nor efficient to subcool the pressurized LNG with another mechanical refrigeration cycle. For this reason, aspects described herein propose the use of a liquid refrigerant produced at a different location to subcool the pressurized LNG. The liquid refrigerant may be LIN.

[0040] Under certain circumstances, the liquid refrigerant can be produced with an amount of energy that makes the overall process of producing the pressurized LNG and liquefied refrigerant more thermodynamically efficient than a conventional LNG production process. For example, the refrigerant may be nitrogen produced from an air separation plant, where the nitrogen is liquefied using the cold available from the gasification of LNG. Typically during the gasification of LNG all the available exergy from gasifying the LNG is lost to the environment. Using this exergy can result in the production of LIN at a sufficiently low energy cost to make the overall energy requirement of the disclosed aspects comparable to or even less than the energy costs of a conventional LNG production process.

[0041] According to the disclosed aspects, the expander-based process may be a feed-gas expander-based process. The feed-gas expander-based process may be an open loop feed gas process where the recycling loop comprises a warm-end expander loop and a cold-end expander loop. The warm-end expander may discharge a first cooling stream and the cold-end expander may discharge the second cooling stream. The temperature of the first cooling stream may be higher than the temperature of the second cooling stream. The pressure of the first cooling stream may be the same or similar to the pressure of the second cooling stream. The cold-end expander may discharge a two-phase stream that is separated into a second cooling stream and a second pressurized LNG stream. Natural gas may be treated to remove impurities, if present, such as water, heavy hydrocarbons, and sour gases, to make the natural gas suitable for liquefaction. The treated natural gas may be completely liquefied by indirect exchange of heat with the first cooling stream and the second cooling stream to produce a first pressurized LNG stream. The first pressurized LNG stream may be mixed with the second pressurized LNG stream to form a pressurized LNG stream. The pressure of the pressurized LNG stream is greater than 50 psia (or 345 kPa) and less than 500 psia (or 3445 kPa), or more specifically greater than 100 psia (or 690 kPa) and less than 400 psia (or 2758 kPa), or more specifically greater than 200 psia (or 1379 kPa) and less than 300 psia (or 2068 kPa). The pressurized LNG stream may be subcooled by exchanging heat with at least one LIN stream to form an LNG stream. The subcooling process may include the use of at least one heat exchanger to allow for indirect heat exchange between the vaporizing LIN stream and the pressurized LNG stream. The subcooling process may additionally comprise other equipment such as compressors, expanders, separators and/or other commonly known equipment, to facilitate the cooling of the pressurized LNG stream. The vaporized LIN stream, after heat exchange with the pressurized LNG stream, may be used to liquefy a second stream of treated natural gas to produce an additional pressurized LNG stream. The additional pressurized LNG stream may be mixed with the pressurized LNG stream prior to the subcooling of the pressurized LNG stream with LIN.

[0042] In one disclosed aspect, the produced LNG may be loaded onto an LNG carrier and/or a dual-purpose carrier at the LNG production location and is transported to an import terminal at a different location where LNG is offloaded and regasified. The cold energy from the gasification of the LNG may be used to liquefy nitrogen that is then loaded onto a LIN carrier and/or a dual-purpose carrier and transported back to the LNG production location, where the LIN is used to liquefy the treated natural gas.

[0043] FIGS. 2A and 2B are simplified diagrams highlighting a difference between the value chain of the aspects disclosed herein and the value chain of conventional FLNG technology, where an FLNG facility contains all or virtually all equipment necessary to process and liquefy natural gas. As shown in FIG. 2A, an LNG cargo ship 200a transports LNG from an FLNG facility 202 to a land-based import terminal 204 where the LNG is offloaded and regasified. The LNG cargo ship 200b, now empty of cargo and ballast, returns to the FLNG facility 202 to be re-loaded with LNG. In contrast, the aspects disclosed herein and shown in FIG. 2B provide a floating processing unit (FPU) 206 having a much smaller footprint than the FLNG facility 202 (FIG. 2A). Referring to FIG. 2B, a LIN cargo ship or a dual purpose ship 208a, loaded with LIN at the import terminal 204, arrives at the FPU 206 and offloads its LIN cargo to storage tanks on and/or within the FPU 206. On the FPU 206 a mechanical refrigeration unit cools the natural gas into a pressurized LNG stream. The pressurized LNG stream is then subcooled within an LIN subcooling unit on the FPU 206 to produce LNG. The produced LNG is transported to the LNG cargo ship or the dual purpose ship 208b. The LNG cargo ship or dual purpose ship 208b, now loaded with LNG, sails to the import terminal 204, where the LNG may be offloaded and regasified. The cold energy from the regasification of the LNG is used to liquefy nitrogen at the import terminal 204. Nitrogen that is liquefied at the import terminal 204 may be produced at an air separation unit 210. The air separation unit 210 may be part of or within the import terminal 204, or a separate facility from the import terminal 204. The LIN may then be loaded into the LIN cargo ship or dual purpose ship, which returns to the FPU 206 to repeat the liquefaction process.

[0044] In another aspect, LIN may be used to liquefy LNG boil-off gas from the tanks during LNG production, transport and/or offloading. In another aspect, LIN and/or vaporized LIN from the subcooling process may be used to cool inlet air going into the gas turbines of the mechanical refrigeration process. In another aspect, LIN and/or LIN boil-off gas may be used to keep the liquefaction equipment cold during turndown or shutdown of the liquefaction process. In another aspect, nitrogen vapor may be used to derime the cryogenic heat exchangers during the periods between LNG production. The nitrogen vapor with contaminants may be vented to the atmosphere.

[0045] FIG. 3 is a schematic diagram of a system 300 according to a disclosed aspect. Natural gas may be treated to remove impurities, if present, such as water, heavy hydrocarbons, and sour gases, to produce a treated natural gas stream 302 that is suitable for liquefaction. The treated natural gas stream 302 may be directed to a mechanical refrigeration unit 304 where the treated natural gas 302 is completely liquefied to produce a pressurized LNG stream 306. The pressure of the pressurized LNG stream 306 may be greater than 50 psia (or 345 kPa) and less than 500 psia (or 3445 kPa), or more specifically greater than 100 psia (or 690 kPa) and less than 400 psia (or 2758 kPa), or more specifically greater than 200 psia (or 1379 kPa) and less than 300 psia (or 2068 kPa). The mechanical refrigeration unit 304 may comprise a single-mixed refrigeration process, a pure component cascade refrigeration process, a dual-mixed refrigeration process, an expander-based refrigeration process, or any other commonly known refrigeration process that can liquefy the treated natural gas stream 302 to a pressurized LNG stream 306. The mechanical refrigeration unit 304 may comprise gas turbines that are used to provide the mechanical power to drive the compressors within the mechanical refrigeration unit 304. The pressurized LNG stream 306 may be directed to a liquid refrigerant subcooling unit 308 where the pressurized LNG stream 306 is subcooled by exchanging heat with a liquid refrigerant stream 310 to form an LNG stream 312. The liquid refrigerant stream 310 is produced at a different location than the location of the mechanical refrigeration unit 304 and the liquid refrigerant subcooling unit 308. The liquid refrigerant stream 310, after being vaporized and warmed within the liquid refrigerant subcooling unit 308 exits the liquid refrigerant subcooling unit 308 as a refrigerant gas vent 314. The liquid refrigerant subcooling unit 308 comprises at least one heat exchanger to allow for indirect heat exchange between the liquid refrigerant stream 310 and the pressurized LNG stream 306. The liquid refrigerant subcooling unit 308 may additionally comprise other equipment such as compressors, expanders, separators and/or other commonly known equipment, to facilitate the cooling of the pressurized LNG stream 306. The vaporized liquid refrigerant stream 310, after heat exchange with the pressurized LNG stream 306, may be used to liquefy a second stream of treated natural gas 316 to form an additional pressurized LNG stream. The additional pressurized LNG stream may be mixed with the pressurized LNG stream 306 prior to the subcooling of the pressurized LNG stream 306 with the liquid refrigerant stream 310 to form the LNG stream 312.

[0046] FIG. 4 is an illustration of a mechanical refrigeration unit 400 according to disclosed aspects. The mechanical refrigeration unit 400 includes a feed gas expander-based process. Natural gas to be liquefied by the mechanical refrigeration unit 400 may be treated to remove impurities, if present, such as water, heavy hydrocarbons, and sour gases, to produce a treated natural gas stream 402 that is suitable for liquefaction. The treated natural gas stream 402 is mixed with a recycled refrigerant stream 404 using a combining device 403. The combined natural gas stream 405 may then be separated by one or more manifolds, splitters, or other types of separators 406, 408, 409 to produce a second treated natural gas stream 410, a first refrigerant stream 412, a second refrigerant stream 414, and a small treated natural gas stream 415 to be liquefied using a liquid refrigerant, as will be explained herein. The first refrigerant stream 412 is expanded in a first expander 417 to produce a first cooling stream 416. The first cooling stream 416 enters at least one heat exchanger 418 where it exchanges heat with the second treated natural gas stream 410 and the second refrigerant stream 414 to cool these two streams. The first cooling stream 416, now heated, exits the at least one heat exchanger 418 as a first warm stream 420. The second refrigerant stream 414, after being cooled in the at least one heat exchanger 418, is expanded in a second expander 422 to produce a two-phase stream 424. The pressure of the two-phase stream 424 may be the same or near the same to the pressure of the first cooling stream 416. The two-phase stream 424 may be separated into its vapor component and its liquid component in a two-phase separator 426 to form a second cooling stream 428 and a second pressurized LNG stream 430. The temperature of the first cooling stream 416 may be higher than the temperature of the second cooling stream 428. The second pressurized LNG stream 430 may be pumped, using a pump 432, to a higher pressure after it has exited the two-phase separator 426. The second cooling stream 428 may enter the at least one heat exchanger 418 where it exchanges heat with the second treated natural gas stream 410 and the second refrigerant stream 414 to cool said streams. The heated second cooling stream exits the at least one heat exchanger 418 as a second warm stream 434. The second treated natural gas stream 410 may exchange heat with the first cooling stream 416 and the second cooling stream 428 to produce a first pressurized LNG stream 436. The first pressurized LNG stream 436 may be reduced in pressure in a hydraulic turbine 437 or other pressure-reducing device after the first pressurized LNG stream 436 has exited the at least one heat exchanger 418. The first pressurized LNG stream 436 may be mixed with the second pressurized LNG stream 430 to form a combined pressurized LNG stream 438. The pressure of the combined pressurized LNG stream 438 may be greater than 50 psia (or 345 kPa) and less than 500 psia (or 3445 kPa), or more specifically greater than 100 psia (or 690 kPa) and less than 400 psia (or 2758 kPa), or more specifically greater than 200 psia (or 1379 kPa) and less than 300 psia (or 2068 kPa). The pressurized LNG stream 438 may be directed to a LIN subcooling unit, as will be further described herein.

[0047] The first warm stream 420 may be combined with the second warm stream 434 in a combining apparatus 440 to form a combined warm refrigerant stream 442. The combined warm refrigerant stream 442 may be compressed in multiple compressor stages to form the recycled refrigerant stream 404. The compressor stages may include a first compressor stage 444, a second compressor stage 446, and a third compressor stage 448. The first compressor stage 444 may be driven by a gas turbine (not shown). The second compressor stage 446 may be driven solely by the shaft power produced by the first expander 417. The third compressor stage 448 may be driven solely by the shaft power produced by the second expander 422. Coolers 450, 452, and 454 may cool the combined warm refrigerant stream 442 after the first, second, and third compressor stages 444, 446, 448, respectively.

[0048] FIG. 5 is a schematic diagram of a LIN subcooling unit 500 according to disclosed aspects. The LIN subcooling unit 500 may be used with the mechanical refrigeration unit 400 depicted in FIG. 4. LIN produced at a different location than the location of the LIN subcooling unit 500 is transported to the location of the LIN subcooling unit 500 and directed to at least one heat exchanger 502 as a LIN stream 504. The LIN stream 504 is vaporized in the at least one heat exchanger 502 by subcooling a pressurized LNG stream 506 (which may be the same as the combined pressurized LNG stream 438 of FIG. 4) to produce a vaporized nitrogen stream 508 and an LNG stream 510. The vaporized nitrogen stream 508 may be directed to a secondary heat exchanger 512 to liquefy a treated natural gas stream 514, which may be the same as the small treated natural gas stream 415, to form an additional pressurized LNG stream 516. The additional pressurized LNG stream 516 may be combined with the pressurized LNG stream 506 in a combining apparatus 518 prior to entering the at least one heat exchanger 502. The additional pressurized LNG stream 516 may be reduced in pressure in a hydraulic turbine 520 or other pressure-reducing apparatus prior to being combined with the pressurized LNG stream 506. The vaporized nitrogen stream 508 is heated by the treated natural gas stream 514 in the secondary heat exchanger 512 to form a nitrogen vent gas 522 that may be vented to the atmosphere or used in other areas of the gas processing facility in which the LIN subcooling unit 500 is located.

[0049] FIG. 6 is a schematic diagram of a LIN subcooling unit 600 according to disclosed aspects. The LIN subcooling unit 600 may be used with the mechanical refrigeration unit 400 depicted in FIG. 4. LIN produced at a different location than the location of the LIN subcooling unit 600 is transported from the different location and directed to the LIN subcooling unit 600 as a LIN stream 602. A pump 604 may pump the LIN stream 602 to a pressure greater than 400 psi to form a high pressure LIN stream 606. The high pressure LIN stream 606 exchanges heat with a pressurized LNG stream 608 (which may be the same as the combined pressurized LNG stream 438 of FIG. 4) in at least one heat exchanger 610 to form a first warmed nitrogen gas stream 612. The first warmed nitrogen gas stream 612 may be expanded in a first expander 614 to produce a first additionally cooled nitrogen gas stream 616. The first additionally cooled nitrogen gas stream 616 exchanges heat with the pressurized LNG stream 608 in the at least one heat exchanger 610 to form a second warmed nitrogen gas stream 618.

[0050] The second warmed nitrogen gas stream 618 may indirectly exchange heat with other process streams, for example in a secondary heat exchanger 619, prior to the second warmed nitrogen gas stream 618 being compressed in one or more compressor stages to form a compressed nitrogen gas stream 620. As shown in FIG. 6, the one or more compressor stages may comprise two compressor stages, including a first compressor stage 622 and a second compressor stage 624. The second compressor stage 624 may be driven solely by the shaft power produced by the first expander 614. The first compressor stage 622 may be driven solely by the shaft power produced by a second expander 626. After each compression stage, the compressed nitrogen gas stream 620 may be cooled by indirect heat exchange with the environment in coolers 628, 630, respectively. The compressed nitrogen gas stream 620 may be expanded in the second expander 626 to produce a second additionally cooled nitrogen gas stream 632. The second additionally cooled nitrogen gas stream 632 exchanges heat with the pressurized LNG stream 608 in the at least one heat exchanger 610 to form a third warmed nitrogen gas stream 634. The pressurized LNG stream 608 is subcooled by exchanging heat with the high pressure LIN stream 606, the first additionally cooled nitrogen gas stream 616, and the second additionally cooled nitrogen gas stream 632 to form an LNG stream 636. The third warmed nitrogen gas stream 634 may be directed to a tertiary heat exchanger 638 to liquefy a treated natural gas stream 640, which may be the same as the small treated natural gas stream 415 in FIG. 4, to form an additional pressurized LNG stream 642. The additional pressurized LNG stream 642 may be combined with the pressurized LNG stream 608 in a combining apparatus 644 prior to the subcooling of the pressurized LNG stream 608 in the at least one heat exchanger 610. The additional pressurized LNG stream 642 may be reduced in pressure in a hydraulic turbine 646 prior to being combined with the pressurized LNG stream 608. The third warmed nitrogen gas stream 634 may be heated by the treated natural gas stream 640 to form a nitrogen vent gas 648 that may be vented to the atmosphere or used in other areas of the gas processing facility in which the LIN subcooling unit 600 is located. The LIN subcooling unit 600 illustrated in FIG. 6 reduces the LIN requirement for subcooling a pressurized LNG stream by approximately 20 to 25% compared to the LIN subcooling unit 500 illustrated in FIG. 5. However, the choice of subcooling units may depend on criteria such as cost of LIN and available topside space for LIN storage and/or the LIN subcooling unit itself

[0051] FIG. 7 is a flowchart of a method 700 for producing liquefied natural gas (LNG). At block 702 a natural gas stream is directed to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa). At block 704 a liquid refrigerant subcooling unit is provided at a first location. At block 706 liquid refrigerant is produced at a second location that is geographically separate from the first location. At block 708 the produced liquid refrigerant is transported to the first location. At block 710 the pressurized LNG stream is subcooled in the liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one stream of the liquid refrigerant to thereby produce an LNG stream.

[0052] The steps depicted in FIG. 7 are provided for illustrative purposes only and a particular step may not be required to perform the disclosed methodology. Moreover, FIG. 7 may not illustrate all the steps that may be performed. The claims, and only the claims, define the disclosed system and methodology.

[0053] The aspects described herein have several advantages over known technologies. For example, the described aspects may significantly increase the capacity of a conventional mechanical refrigeration process without significantly increasing required power and footprint of the mechanical refrigeration process. For example, compared to known feed gas expander-based processes, the feed gas expander-based process coupled with LIN subcooling described herein can produced approximately 50% more LNG at an equivalent mechanical refrigeration power. The amount of LIN needed is approximately 0.26 ton of LIN for every ton of LNG produced. The reduced amount of LIN makes this technology particularly suitable for FLNG applications. Using the disclosed aspects, the 50% extra throughput through the feed gas expander-based process only increases the required volumetric flow to the low pressure compressor and the cryogenic heat exchanger load by approximately 10% respectively compared to known feed gas expander technologies.

[0054] Disclosed aspects may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible aspects, as any number of variations can be envisioned from the description above.

1. A method for producing liquefied natural gas (LNG), comprising:

[0055] directing a natural gas stream to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa);

[0056] providing a liquid refrigerant subcooling unit at a first location;

[0057] producing liquid refrigerant at a second location that is geographically separate from the first location;

[0058] transporting the produced liquid refrigerant to the first location; and

[0059] subcooling the pressurized LNG stream in the liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one stream of the liquid refrigerant to thereby produce an LNG stream.

2. The method of paragraph 1, wherein the mechanical refrigeration unit includes an expander-based refrigeration process.
3. The method of paragraph 2, wherein the expander-based refrigeration process is a feed gas expander-based process.
4. The method of paragraph 3, wherein the feed gas expander-based process is an open loop feed gas expander-based process.
5. The method of paragraph 3, wherein the feed gas expander-based process is a closed loop feed gas expander-based process.
6. The method of paragraph 3, wherein the feed gas expander-based process comprises:

[0060] discharging a first cooling stream from a warm-end expander; and

[0061] discharging a two-phase stream from a cold-end expander;

[0062] wherein a temperature of the first cooling stream is higher than a temperature of the two-phase stream.

7. The method of paragraph 3, wherein the pressurized LNG stream is a first pressurized LNG stream, and further comprising separating the two-phase stream into a second cooling stream and a second pressurized LNG stream.
8. The method of paragraph 3, wherein the feed gas expander-based process comprises:

[0063] discharging a first cooling stream from a warm-end expander; and

[0064] discharging a second cooling stream from a cool-end expander;

[0065] wherein a temperature of the first cooling stream is higher than a temperature of the second cooling stream.

9. The method of paragraphs 7 or 8, wherein a pressure of the first cooling stream is the same or substantially the same as a pressure of the second cooling stream.
10. The method of paragraph 9, further comprising mixing the second pressurized LNG stream with the first pressurized LNG stream prior to directing the pressurized LNG stream to the liquid refrigerant subcooling unit.
11. The method of any of paragraphs 1-10, wherein the liquid refrigerant subcooling unit comprises at least one heat exchanger.
12. The method of any of paragraphs 1-11, wherein the liquid refrigerant subcooling unit comprises at least one compressor and/or expander.
13. The method of any of paragraphs 1-12, wherein the vaporized liquid refrigerant stream is used to liquefy a second treated natural gas stream to produce an additional pressurized LNG stream.
14. The method of paragraph 13, wherein the additional pressurized LNG stream is mixed with the pressurized LNG stream prior to the subcooling of the pressurized LNG stream with the liquid refrigerant.
15. The method of any of paragraphs 1-14, further comprising locating the mechanical refrigeration unit and the liquid refrigerant subcooling unit on a floating LNG facility.
16. The method of any of paragraphs 1-15, further comprising re-liquefying LNG boil-off gas using the liquid refrigerant.
17. The method of any of paragraphs 1-16, wherein the liquid refrigerant and/or a liquid refrigerant boil-off gas is used to keep the mechanical refrigeration unit and/or liquid refrigerant subcooling unit equipment cold during turndown and/or shutdown periods of the mechanical refrigeration unit.
18. The method of any of paragraphs 1-17, wherein warm liquid refrigerant vapor is used to derime heat exchangers used to exchange heat.
19. The method of any of paragraphs 1-18, further comprising:

[0066] transporting the LNG stream from the first location to the second location in a dual-purpose carrier; and

[0067] after the LNG stream has been offloaded from the dual-purpose carrier, transporting the liquid refrigerant from the second location to the first location in the dual purpose carrier.

20. The method of any of paragraphs 1-19, wherein the mechanical refrigeration unit includes one of a single-mixed refrigerant process, a pure component cascade refrigerant process, or a dual-mixed refrigerant process.
21. The method of any of paragraphs 1-20, wherein the pressurized LNG stream has a pressure greater than 100 psia (690 kPa) and less than 400 psia (2758 kPa).
22. The method of any of paragraphs 1-21, wherein the pressurized LNG stream has a pressure greater than 200 psia (1379 kPa) and less than 300 psia (2068 kPa).
23. The method of any of paragraphs 1-22, wherein the liquid refrigerant comprises liquid nitrogen (LIN).
24. The method of paragraph 23, further comprising producing the LIN by exchanging heat with LNG during LNG regasification.
25. The method of paragraph 23, further comprising pressurizing the LIN to a pressure greater than 400 psia (2758 kPa) to form a high pressure liquid nitrogen stream.
26. The method of paragraph 25, further comprising exchanging heat between the high pressure liquid nitrogen stream and the pressurized LNG stream to form a warm nitrogen gas stream.
27. The method of paragraph 23, further comprising:

[0068] reducing, in the liquid refrigerant subcooling unit, a pressure of at least one warmed natural gas stream in at least one expander service, to reduce the pressure of the at least one warmed nitrogen gas stream, and to thereby produce at least one additionally cooled nitrogen gas stream.

28. The method of paragraph 27, wherein the at least one additionally cooled nitrogen gas stream exchanges heat with the pressurized LNG stream to form warmed nitrogen gas streams.
29. The method of paragraph 27, further comprising:

[0069] coupling the at least one expander service with at least one generator to generate electrical power.

30. The method of paragraph 27, further comprising:

[0070] coupling the at least one expander service with at least one compressor that is used to compress warmed nitrogen gas streams.

31. The method of any of paragraphs 1-30, further comprising:

[0071] directing pressurized LNG streams from a plurality of mechanical refrigeration units to the liquid refrigerant subcooling unit to produce at least one LNG stream.

32. A system for producing liquefied natural gas (LNG), comprising:

[0072] a mechanical refrigeration unit configured to liquefy a natural gas stream using a feed gas expander-based process and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa);

[0073] a liquid nitrogen (LIN) subcooling unit located in a first location;

[0074] a liquid nitrogen (LIN) stream produced at a second location that is geographically separate from the first location and transported to the LIN subcooling unit;

[0075] wherein the LIN subcooling unit is configured to subcool the pressurized LNG stream by exchanging heat between the pressurized LNG stream and at least one stream of the LIN stream to thereby produce an LNG stream and at least one vaporized LIN stream.

33. The system of paragraph 32, wherein the mechanical refrigeration unit comprises:

[0076] a warm-end expander configured to discharge a first cooling stream therefrom; and

[0077] a cold-end expander configured to discharge a two-phase stream therefrom;

[0078] wherein a temperature of the first cooling stream is higher than a temperature of the two-phase stream;

[0079] wherein the pressurized LNG stream is a first pressurized LNG stream, and

[0080] wherein the two-phase stream is configured to be split into a second cooling stream and a second pressurized LNG stream.

34. The system of paragraph 32, wherein the mechanical refrigeration unit comprises:

[0081] A warm-end expander configured to discharge a first cooling stream therefrom; and

[0082] discharging a second cooling stream from a cool-end expander;

[0083] wherein a temperature of the first cooling stream is higher than a temperature of the second cooling stream.

35. The system of paragraphs 33 or 34, wherein a pressure of the first cooling stream is the same or substantially the same as a pressure of the second cooling stream.
36. The system of paragraph 35, wherein the second pressurized LNG stream is mixed with the first pressurized LNG stream prior to directing the pressurized LNG stream to the LIN subcooling unit.
37. The system of any of paragraphs 32-35, wherein the at least one vaporized liquid refrigerant stream is used to liquefy a second treated natural gas stream to produce an additional pressurized LNG stream.
38. The system of any of paragraphs 32-36, wherein the mechanical refrigeration unit and the liquid refrigerant subcooling unit are located on a floating LNG facility.
39. The system of any of paragraphs 32-37, further comprising:

[0084] A dual-purpose carrier configured to transport the LNG stream from the first location to the second location and transport the liquid refrigerant from the second location to the first location in the dual purpose carrier after the subcooled LNG stream has been offloaded from the dual-purpose carrier.

[0085] It should be understood that the numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.