CN108369060B

CN108369060B - Expander-based LNG production process enhanced with liquid nitrogen

Info

Publication number: CN108369060B
Application number: CN201680069851.9A
Authority: CN
Inventors: F·小皮埃尔; M·W·迈尔斯
Original assignee: ExxonMobil Upstream Research Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2015-12-14
Filing date: 2016-11-10
Publication date: 2020-06-19
Anticipated expiration: 2036-11-10
Also published as: CA3006956C; EP3390939A1; EP3390939B1; JP2019505755A; KR102137939B1; US20170167785A1; CA3006956A1; CN108369060A; SG11201803523WA; AU2016372710B2; AU2016372710A1; WO2017105680A1; JP6772268B2; KR20180095870A

Abstract

A method of producing Liquefied Natural Gas (LNG). The 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 having a pressure greater than 50psia (345kPa) and less than 500psia (3445 kPa). A liquid refrigerant subcooling unit is provided at the first location. The liquid refrigerant is generated at a second location geographically separated from the first location. The resulting liquid refrigerant is delivered to a first location. Subcooling the pressurized LNG stream in a liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one liquid refrigerant stream to thereby produce an LNG stream.

Description

Expander-based LNG production process enhanced with liquid nitrogen

Cross Reference to Related Applications

The present application claims benefit of U.S. provisional patent application 62/266,979 entitled "EXPANDER-BASED LNG PRODUCTIONPROCESSES ENHANCED WITH LIQUID NITROGEN" filed on 12, 14/2015, the entire contents of which are incorporated herein by reference.

This application is related to the following patent applications: U.S. provisional patent application No.62/266,976 entitled "Method and System for separating Nitrogen from Liquefied Natural Gas Using Liquefied Nitrogen"; U.S. provisional patent application No.62/266,983 entitled "Method of Natural Gas chromatography LNG Carrier storing Liquid Nitrogen"; and U.S. provisional patent application No.62/622,985, entitled "Pre-coating of Natural Gas by High Pressure Compression and Expansion," having common inventors and assignee and filed on even date herewith, the disclosure of which is incorporated by reference herein in its entirety.

Background

FIELD OF THE DISCLOSURE

The present disclosure relates generally to the field of natural gas liquefaction to form Liquefied Natural Gas (LNG). More specifically, the present disclosure relates to the production and transfer of LNG from offshore and/or remote sources of natural gas.

Description of the Related Art

This section is intended to introduce various aspects of the art that may be related to the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Therefore, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

LNG is a rapidly growing means of supplying natural gas from sites with abundant supply of natural gas to remote sites with strong demand for natural gas. The conventional LNG cycle includes: a) initially treating natural gas resources to remove contaminants such as water, sulfur compounds and carbon dioxide; b) some heavier hydrocarbon gases, such as propane, butane, pentane, etc., are separated by various possible methods (including self-refrigeration, external refrigeration, lean oil, etc.); c) refrigerating natural gas substantially by external refrigeration to form liquefied natural gas at or near atmospheric pressure and at about-160 ℃; d) transporting the LNG product to a point of sale in a ship or tanker designed for this purpose; and e) repressurizing and regasifying the LNG at the regasification plant to form a pressurized natural gas stream that can be distributed to natural gas consumers. Step (c) of the conventional LNG cycle typically requires the use of large refrigeration compressors, typically powered by large gas turbine drives that discharge large amounts of carbon and other emissions. As part of a liquefaction plant, large capital investments in the billions of dollars and large-scale infrastructure are required. Step (e) of a conventional LNG cycle typically comprises re-pressurizing the LNG to the desired pressure using a cryogenic pump, and then re-vaporizing the LNG to form pressurized natural gas by exchanging heat with seawater via an intermediate fluid but ultimately or by combusting a portion of the natural gas to heat and vaporize the LNG. Typically, the available cryogenic LNG's useful energy is not utilized.

The relatively new LNG production technology is known as floating LNG (flng). The FLNG technology involves the construction of gas processing and liquefaction facilities on a floating structure (e.g., a barge or ship). FLNG is a technical solution for monetization of stranded gas offshore where it is not economically feasible to build a gas pipeline to shore. FLNG is also increasingly being considered for onshore and offshore natural gas fields located in remote, environmentally sensitive and/or politically challenging areas. This technology has certain advantages over conventional onshore LNG because it has a lower environmental footprint at the production site. The technology also allows projects to be delivered faster and at lower cost, since most LNG facilities are built at shipyards at lower labor rates and with reduced risk of implementation.

Despite the several advantages of FLNG over conventional onshore LNG, significant technical challenges remain in the application of this technology. For example, the FLNG structure must provide the same level of gas processing and liquefaction in an area that is typically less than one-fourth of the area available for an onshore LNG plant. Therefore, there is a need to develop techniques to reduce FLNG plant floor space while maintaining the capacity of the liquefaction facility to reduce overall project costs. One promising means of reducing floor space is to change the liquefaction technology used in FLNG plants. Known liquefaction technologies include Single Mixed Refrigerant (SMR) processes, Dual Mixed Refrigerant (DMR) processes, and expander-based (or expansion) processes. The expander-based process has several advantages, making it well suited for FLNG projects. The most significant advantage is that the technology provides liquefaction without the need for external hydrocarbon refrigerants. The removal of liquid hydrocarbon refrigerant inventory (e.g., propane storage) significantly reduces safety issues that are particularly serious for FLNG projects. An additional advantage of the expander-based process compared to the mixed refrigerant process is that the expander-based process is less sensitive to offshore movements, since the main refrigerant remains mostly in the gas phase.

While expander-based processes have their advantages, the application of this technology to FLNG projects with LNG production in excess of 200 million tons per year (MTA) has proven to be less attractive than processes using mixed refrigerants. The capacity of known expander-based process trains (trains) is typically less than 1.5 MTA. In contrast, a mixed refrigerant process train (e.g., a propane pre-cooling process or a dual mixed refrigerant process) may have a train capacity greater than 5 MTA. The size of an expander-based process train is limited because its refrigerant remains mostly in the vapor state throughout the process and the refrigerant absorbs energy through its sensible heat. For these reasons, the refrigerant volumetric flow rate is large throughout the process, and the heat exchangers and piping sizes are proportionally larger than those used in the mixed refrigerant process. In addition, the limitations on the size of the compander horsepower results in parallel rotating machinery as the capacity of the expander-based process train increases. If multiple expander-based trains are allowed, the production rate of the FLNG project using the expander-based process can be made greater than 2 MTA. For example, for a 6MTA FLNG project, six or more parallel expander-based process trains may be sufficient to achieve the desired production. However, when multiple expander trains are used, the number of equipment, complexity and cost are increased. Furthermore, if the expander-based process requires multiple trains, and the mixed refrigerant process can achieve the desired production rate with one or two trains, the assumed process simplicity of the expander-based process compared to the mixed refrigerant process begins to be questioned. For these reasons, there is a need to develop FLNG liquefaction processes that have the advantages of expander-based processes while achieving high LNG production capacity. There is also a need to develop FLNG technology solutions that are better able to address the challenges that ship motion presents to gas treatment.

U.S. patent No.3,400,547 to Williams et al discloses a process within an LNG production facility in which liquid nitrogen (LIN) produced at a different location is used as a refrigerant to liquefy natural gas. The process used a propane chiller to cool the natural gas, which was then condensed by indirect heat exchange with vaporized LIN. GB patent No.1,596,330 to Thompson discloses a process within an LNG production facility in which LIN produced at a different location is used as a refrigerant to liquefy natural gas. The process uses propane and ethylene coolers in conjunction with LIN to vaporize natural gas to LNG. The processes disclosed in these two patents have the disadvantage of using a mechanical refrigeration system while still requiring a large amount of LIN to produce LNG. Both methods estimate that about one or more tons of LIN are required for each ton of LNG produced. In FLNG applications, the space for storing LIN on top of a floating structure or in the hull may be limited. LNG production technology using LIN on FLNG is advantageous because it will significantly reduce the headspace required for the liquefaction process. Furthermore, an LNG production technology that uses less than 1 ton LIN, or more preferably less than 0.75 ton LIN, or more preferably less than 0.5 ton LIN per ton of LNG produced would be advantageous.

U.S. patent No.6,412,302 to Foglietta describes a feed gas expander-based process in which two separate closed refrigeration loops are used to cool a feed gas to form LNG. The first closed refrigeration loop uses the feed gas or a component of the feed gas as the refrigerant. Nitrogen is used as the refrigerant for the second closed refrigeration loop. This technique has the advantage of requiring smaller equipment and head space compared to a dual loop nitrogen expander-based process. For example, the volumetric flow rate of refrigerant entering the low pressure compressor may be 20% to 50% less with this technique compared to a two-circuit nitrogen expander-based process. However, this technique is still limited to production capacities of less than 1.5 MTA.

U.S. patent No.8,616,012 to Minta describes a feed gas expander-based process in which the feed gas is used as a refrigerant in a closed refrigeration loop. In the closed refrigeration loop, the refrigerant is compressed to a pressure greater than or equal to 1500psia, or more preferably greater than 2500 psia. The refrigerant is then cooled and expanded to reach a cryogenic temperature. The cooled refrigerant is then used in a heat exchanger to cool the feed gas from a warm temperature to a cryogenic temperature. The feed gas is then further cooled using a subcooling refrigeration circuit to form LNG. In one embodiment, the subcooling refrigeration circuit is a closed circuit using flash gas as the refrigerant. This feed gas expander-based process has the advantage of not being limited to a range of unit capacities less than 1 MTA. A unit size of about 6MTA has been considered. However, this technique has the disadvantage of a high number of equipment and increased complexity, since it requires two separate refrigeration circuits and compression of the feed gas. Furthermore, high pressure operation also means that equipment and piping will be much heavier than other expander-based processes.

GB patent No.2,486,036 to mauder et al describes a feed gas expander-based process which is an open-loop refrigeration cycle comprising a pre-cooled expander loop and a liquefaction expander loop, wherein the expanded gas phase is used to liquefy natural gas. According to Maunder, the inclusion of a liquefaction expander in the process significantly reduces the recycle gas rate and overall required refrigeration power. This technique is simpler than the techniques described by Foglietta and mint, since only one type of refrigerant is used with a single compression string. However, this technology is still limited to production capacities of less than 1.5MTA and it requires the use of liquefaction expanders that are not standard equipment for LNG production. It has been shown that this technique is less efficient than the techniques described by Foglietta and Minta for lean natural gas liquefaction.

There remains a need to develop LNG production processes that have the advantages of expander-based processes while having high LNG production capacity and reduced facility footprint. There is also a need to develop LNG technology solutions that are better able to address the challenges that ship motion presents to gas processing. Such a high capacity expander-based liquefaction process would be particularly suitable for FLNG applications where the inherent safety and simplicity of the expander-based liquefaction process are highly valued.

SUMMARY

The present disclosure provides a process for producing Liquefied Natural Gas (LNG). The 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 having a pressure greater than 50psia (345kPa) and less than 500psia (3445 kPa). A liquid refrigerant subcooling unit is provided at the first location. The liquid refrigerant is produced at a second location geographically separated from the first location. The resulting liquid refrigerant is delivered to a first location. The pressurized LNG stream is subcooled in a liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one liquid refrigerant stream to thereby produce an LNG stream.

The present disclosure also provides a system for producing Liquefied Natural Gas (LNG). The mechanical refrigeration unit uses a process liquefied natural gas stream based on a feed gas expander and forms a pressurized Liquefied Natural Gas (LNG) stream having a pressure greater than 50psia (345kPa) and less than 500psia (3445 kPa). A liquid nitrogen (LIN) subcooling unit is provided at the first location. A stream of liquid nitrogen (LIN) is generated at a second location geographically separated from the first location. The LIN stream was sent to a LIN subcooling unit. A LIN subcooling unit subcools a pressurized LNG stream by exchanging heat between the pressurized LNG stream and at least one of the LIN streams to thereby produce an LNG stream and at least one vaporized LIN stream.

The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows may be better understood. Additional features will also be described herein.

Brief description of the drawings

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

Fig. 1 is a graph showing the temperature cooling curve of an expander-based heat exchanger process.

FIG. 2A is a simplified diagram of the value chain of a known FLNG technique.

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

FIG. 3 is a schematic diagram of a system in accordance with the disclosed aspects.

FIG. 4 is a schematic view of a mechanical refrigeration unit according to disclosed aspects.

Fig. 5 is a schematic diagram of a liquid nitrogen (LIN) subcooling unit according to disclosed aspects.

Fig. 6 is a schematic diagram of a LIN subcooling unit, according to disclosed aspects.

FIG. 7 is a flow chart illustrating a method in accordance with the disclosed aspects.

It should be noted that the drawings are merely examples and are not intended to limit the scope of the present disclosure thereby. Moreover, the drawings are not generally drawn to scale, but are drawn for convenience and clarity in order to clarify aspects of the disclosure.

Detailed description of the invention

For the purposes of promoting 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 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 clarity, some features not relevant to the present disclosure may not be shown in the drawings.

First, for ease of reference, certain terms used in the present application and their meanings 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. Furthermore, the technology is not limited by the use of the terms set forth below, as all equivalents, synonyms, new developments, and terms or techniques for the same or similar purpose are considered to be within the scope of the claims.

As one of ordinary skill in the art will 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 only in name. The drawings 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 drawings described herein, the same reference numbers may be referenced in multiple drawings for simplicity. In the following description and claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to.

The words "a", "an", and "the" are not necessarily limited to one meaning only, but are inclusive and open-ended, thereby optionally including multiple such elements.

As used herein, the terms "about," "substantially," and similar terms are intended to have a broad meaning consistent with the usual and acceptable usage by those of ordinary skill in the art to which the presently disclosed subject matter pertains. Those skilled in the art who review this disclosure will appreciate that these terms are intended to allow description of certain features described and claimed without limiting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be construed to represent insubstantial or inconsequential modifications or variations of the subject matter described, and are considered within the scope of the present disclosure.

The term "heat exchanger" refers to a device designed to efficiently transfer or "exchange" heat from one substance to another. Exemplary heat exchanger types include co-current or counter-current heat exchangers, indirect heat exchangers (e.g., spiral wound heat exchangers, plate fin heat exchangers such as brazed aluminum plate fin, shell and tube heat exchangers, etc.), direct contact heat exchangers, or some combination of these, and the like.

The term "dual purpose transport" refers to a vessel capable of (a) delivering LIN to an export terminal for natural gas and/or LNG and (b) delivering LNG to an LNG import terminal.

As previously mentioned, a conventional LNG cycle includes: (a) initially treating natural gas resources to remove contaminants such as water, sulfur compounds and carbon dioxide; (b) some heavier hydrocarbon gases, such as propane, butane, pentane, etc., are separated by various possible methods (including self-refrigeration, external refrigeration, lean oil, etc.); (c) refrigerating natural gas to form LNG at or near atmospheric pressure and about-160 ℃ substantially by external refrigeration; (d) delivering the LNG product to a point of sale in a ship or tanker designed for this purpose; and (e) repressurizing and regasifying the LNG at the regasification plant to form a pressurized natural gas stream that may be distributed to natural gas consumers. The present disclosure relates generally to the liquefaction of natural gas using liquid nitrogen (LIN). Typically, the use of LIN to produce LNG is an unconventional LNG cycle, wherein step (c) above is replaced by a natural gas liquefaction process using large amounts of LIN as an open-loop refrigeration source, and wherein step (e) above can be modified to use the available energy of cryogenic LNG to facilitate nitrogen liquefaction to form LIN, which can then be transported to a resource site and used as a refrigeration source for the production of LNG. The disclosed LIN to LNG concept may also include the transfer of LNG from a resource location (export terminal) to a sales location (import terminal) and the reverse transfer of LIN from a sales location to a resource location in a ship or tanker.

Aspects disclosed herein provide methods for enhancing a mechanical refrigeration process for producing LNG using liquid refrigerant produced at different locations to subcool liquefied natural gas from the mechanical refrigeration process. More specifically, processes are described in which treated natural gas may be directed to a mechanical refrigeration process. The natural gas may be fully liquefied in a mechanical refrigeration process to produce a pressurized LNG stream, wherein the pressure of the pressurized LNG stream is greater than 50psia (or 345kPa) and less than 500psia (or 3445kPa), or, more specifically, greater than 100psia (or 690kPa) and less than 400psia (or 2758kPa), or, more specifically, greater than 200psia (or 1379kPa) and less than 300psia (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 geographic location different from the location at which 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 1000 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 known refrigeration process that can liquefy a natural gas stream to produce a pressurized LNG stream.

In one aspect, an expander-based process for producing LNG can be enhanced by subcooling pressurized LNG from the expander-based process using LIN produced at a different location. The natural gas may be treated to remove impurities (such as water, heavy hydrocarbons and acid gases, if present) to make the natural gas suitable for liquefaction. The treated natural gas can be fully liquefied in an expander-based process to produce a pressurized LNG stream, wherein the pressure of the pressurized LNG stream is greater than 50psia (or 345kPa) and less than 500psia (or 3445kPa), or, more specifically, greater than 100psia (or 690kPa) and less than 400psia (or 2758kPa), or, more specifically, greater than 200psia (or 1379kPa) and less than 300psia (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 expander-based process or may be a feed gas expander-based process.

Fig. 1 shows a typical temperature cooling curve 100 for an expander-based liquefaction process. The higher temperature profile 104 is the temperature profile of the natural gas stream. The lower temperature profile 102 is a composite temperature profile of a cold cooling stream and a warm cooling stream. As shown, the cooling curve is marked by three temperature pinch points (ping-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 cooled stream (typically a warm cooled stream) enters the heat exchanger. The warm temperature pinch 110 occurs where the cold and warm cooling streams exit the heat exchanger. The minimum temperature pinch point 106 sets the flow rate required for the cold cooling stream. Since the cold cooling stream is first cooled by the warm cooling stream before being expanded to a lower temperature, the flow rate of the cold cooling stream also affects the flow rate required for the warm cooling stream. One way to increase the capacity of an expander-based process without significantly increasing the size of the equipment and the power required is to increase the temperature of the minimum temperature pinch point. In this case, additional refrigeration is required to subcool the pressurized LNG from the expander-based process in order to produce LNG. It would be neither advantageous nor efficient to subcool pressurized LNG with an additional mechanical refrigeration cycle. Accordingly, aspects described herein propose subcooling pressurized LNG using liquid refrigerant produced at different locations. The liquid refrigerant may be LIN.

In some cases, the liquid refrigerant may be produced with a certain amount of energy, which makes the overall process for producing pressurized LNG and liquid refrigerant thermodynamically more efficient than conventional LNG production processes. For example, the refrigerant may be nitrogen produced by air separation wherein the nitrogen is liquefied with cold available from LNG vaporization. Typically, during LNG vaporization, all of the available energy available from the LNG vaporization is lost to the environment. The use of such useful energy may result in the production of LIN at a significantly lower energy cost such that the overall energy requirements of the disclosed aspects are comparable to or even lower than the energy cost of conventional LNG production processes.

In accordance with the disclosed aspects, the expander-based process can be a feed gas expander-based process. The feed gas expander-based process may be an open-loop feed gas process, wherein the recycle loop comprises a warm-end expander loop and a cold-end expander loop. The warm end expander can discharge a first cooled stream and the cold end expander can discharge a second cooled stream. The temperature of the first cooled stream can be higher than the temperature of the second cooled stream. The pressure of the first cooled stream can be the same as or similar to the pressure of the second cooled stream. The cold-end expander can discharge a two-phase stream that is separated into a second cooled stream and a second pressurized LNG. The natural gas may be treated to remove impurities (such as water, heavy hydrocarbons and acid gases, if present) to make the natural gas suitable for liquefaction. The treated natural gas can be fully liquefied by indirectly exchanging 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 50psia (or 345kPa) and less than 500psia (or 3445kPa), or, more specifically, greater than 100psia (or 690kPa) and less than 400psia (or 2758kPa), or, more specifically, greater than 200psia (or 1379kPa) and less than 300psia (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 can include the use of at least one heat exchanger to allow indirect heat exchange between the vaporized LIN stream and the pressurized LNG stream. The subcooling process may additionally include other equipment, such as compressors, expanders, separators and/or other well known equipment to facilitate cooling of the pressurized LNG stream. After heat exchange with the pressurized LNG stream, the vaporized LIN stream can be used to liquefy the second treated natural gas stream to produce an additional pressurized LNG stream. An additional pressurized LNG stream may be mixed with the pressurized LNG stream prior to using LIN to subcool the pressurized LNG stream.

In one disclosed aspect, the produced LNG may be loaded onto an LNG carrier and/or a dual use carrier at an LNG production location and transported to an import terminal at a different location where the LNG is offloaded and regasified. The cold energy from the LNG vaporization can be used to liquefy the nitrogen, which is then loaded onto a LIN carrier and/or a dual purpose carrier and transported back to the LNG production location, where LIN is used to liquefy the processed natural gas.

Fig. 2A and 2B are simplified diagrams highlighting the differences between the value chain of aspects disclosed herein and that of conventional FLNG technology, where the FLNG facility contains all or nearly all of the equipment needed to process and liquefy natural gas. As shown in fig. 2A, an LNG carrier 200a transports LNG from an FLNG facility 202 to a land-based import terminal 204 where the LNG is offloaded and regasified. The LNG carrier 200b, now without cargo and ballast, returns to the FLNG facility 202 to reload with LNG. In contrast, the aspects disclosed herein and illustrated 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 or amphibian 208a (which is loaded with LIN at the import terminal 204) arrives at the FPU 206 and unloads 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 a LIN subcooling unit on the FPU 206 to produce LNG. The produced LNG is transferred to an LNG cargo or dual purpose vessel 208 b. The LNG cargo or amphibian 208b, now loaded with LNG, sails to the import terminal 204, where the LNG may be offloaded and regasified. Cold energy from the regasification of the LNG is used to liquefy nitrogen at the inlet terminal 204. The nitrogen liquefied at the inlet terminal 204 may be produced at an air separation unit 210. The air separation unit 210 may be part of or within the inlet terminal 204 or a separate facility from the inlet terminal 204. LIN may then be loaded to a LIN cargo or dual purpose vessel, which returns to the FPU 206 to repeat the liquefaction process.

In another aspect, LIN may be used to liquefy LNG boil-off gas from a storage tank during LNG production, transport, and/or offloading. In another aspect, LIN from a subcooling process and/or evaporated LIN can be used to cool inlet air to a gas turbine of a mechanical refrigeration process. On the other hand, LIN and/or LIN boil-off gas may be used to keep the liquefaction plant cool during turndown or shut down of the liquefaction process. In another aspect, the nitrogen vapor may be used to defrost (derime) the cryogenic heat exchanger during periods between LNG production. The nitrogen vapor containing contaminants may be vented to the atmosphere.

FIG. 3 is a schematic diagram of a system 300 in accordance with disclosed aspects. The natural gas may be treated to remove impurities (such as water, heavy hydrocarbons, and acid gases, if present) to produce a treated natural gas stream 302 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 fully liquefied to produce a pressurized LNG stream 306. The pressure of pressurized LNG stream 306 may be greater than 50psia (or 345kPa) and less than 500psia (or 3445kPa), or, more specifically, greater than 100psia (or 690kPa) and less than 400psia (or 2758kPa), or, more specifically, greater than 200psia (or 1379kPa) and less than 300psia (or 2068 kPa). The mechanical refrigeration unit 304 may include a single hybrid refrigeration process, a pure component cascade refrigeration process, a dual hybrid refrigeration process, an expander-based refrigeration process, or any other known refrigeration process that may liquefy the treated natural gas stream 302 to a pressurized LNG stream 306. The mechanical refrigeration unit 304 may include a gas turbine for providing mechanical power to drive a compressor within the mechanical refrigeration unit 304. Pressurized LNG stream 306 may be directed to liquid refrigerant subcooling unit 308, where pressurized LNG stream 306 is subcooled by exchanging heat with liquid refrigerant stream 310 to form LNG stream 312. The liquid refrigerant stream 310 is produced at a location different from the locations of the mechanical refrigeration unit 304 and the liquid refrigerant subcooling unit 308. The liquid refrigerant stream 310 exits the liquid refrigerant subcooling unit 308 as a refrigerant discharge 314 after being evaporated and warmed within the liquid refrigerant subcooling unit 308. The liquid refrigerant subcooling unit 308 comprises at least one heat exchanger to allow indirect heat exchange between the liquid refrigerant stream 310 and the pressurized LNG stream 306. The liquid refrigerant subcooling unit 308 can additionally include other equipment, such as compressors, expanders, separators and/or other known equipment to facilitate cooling of the pressurized LNG stream 306. After heat exchange with the pressurized LNG stream 306, the vaporized liquid refrigerant stream 310 may be used to liquefy a second treated natural gas stream 316 to form an additional pressurized LNG stream. Additional pressurized LNG stream may be mixed with pressurized LNG stream 306 prior to subcooling pressurized LNG stream 306 with liquid refrigerant stream 310 to form LNG stream 312.

Fig. 4 is an illustration of a mechanical refrigeration unit 400 in accordance with disclosed aspects. The mechanical refrigeration unit 400 includes a feed gas expander-based process. The natural gas to be liquefied by the mechanical refrigeration unit 400 can be treated to remove impurities (such as water, heavy hydrocarbons, and acid gases, if present) to produce a treated natural gas stream 402 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 cooled stream 416. The first cooled 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 both streams. The now heated first cooled stream 416 exits the at least one heat exchanger 418 as a first warm stream 420. The second refrigerant stream 414 is cooled in at least one heat exchanger 418 and expanded in a second expander 422 to produce a two-phase stream 424. The pressure of the two-phase stream 424 can be the same or nearly the same as the pressure of the first cooled stream 416. Two-phase stream 424 can be separated into its vapor components and its liquid components in two-phase separator 426 to form second cooled stream 428 and second pressurized LNG stream 430. The temperature of first cooled stream 416 can be higher than the temperature of second cooled stream 428. The second pressurized LNG stream 430 may be pumped to a higher pressure after exiting the two-phase separator 426 using a pump 432. The second cooled stream 428 can enter 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 the streams. The heated second cooled stream exits the at least one heat exchanger 418 as a second warmed stream 434. The second treated natural gas stream 410 can exchange heat with the first cooling stream 416 and the second cooling stream 428 to produce a first pressurized LNG stream 436. After the first pressurized LNG stream 436 has left the at least one heat exchanger 418, the first pressurized LNG stream 436 may be depressurized in a hydro turbine 437 or other pressure letdown device. First pressurized LNG stream 436 may be mixed with 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 50psia (or 345kPa) and less than 500psia (or 3445kPa), or, more specifically, greater than 100psia (or 690kPa) and less than 400psia (or 2758kPa), or, more specifically, greater than 200psia (or 1379kPa) and less than 300psia (or 2068 kPa). As will be described further herein, the pressurized LNG stream 438 may be directed to a LIN subcooling unit.

The first warmed stream 420 can be combined with the second warmed stream 434 in a combining device 440 to form a combined warmed refrigerant stream 442. The combined warm refrigerant stream 442 can 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 generated by the first expander 417. The third compressor stage 448 may be driven solely by the shaft power generated by the second expander 422.

Coolers

450, 452, and 454 may cool combined warm refrigerant stream 442 after first, second, and third compressor stages 444, 446, 448, respectively.

Fig. 5 is a schematic diagram of a LIN subcooling unit 500, according to disclosed aspects. The LIN subcooling unit 500 can be used with the mechanical refrigeration unit 400 shown in fig. 4. LIN produced at a location different from the location of the LIN subcooling unit 500 is delivered to the location of the LIN subcooling unit 500 and is directed to at least one heat exchanger 502 as a LIN stream 504. The LIN stream 504 is vaporized in 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 second 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. Additional pressurized LNG stream 516 may be combined with pressurized LNG stream 506 in a combining unit 518 prior to entering at least one heat exchanger 502. The additional pressurized LNG stream 516 may be reduced in pressure in a water turbine 520 or other pressure reduction device prior to being combined with the pressurized LNG stream 506. The vaporized nitrogen gas stream 508 is heated in the second heat exchanger 512 by the treated natural gas stream 514 to form nitrogen vent gas 522, which nitrogen vent gas 522 may be vented to the atmosphere or used in other areas of the gas processing facility where the LIN subcooling unit 500 is located.

Fig. 6 is a schematic diagram of a LIN subcooling unit 600, according to disclosed aspects. The LIN subcooling unit 600 can be used with the mechanical refrigeration unit 400 shown in fig. 4. LIN produced at a location different from the location of the LIN subcooling unit 600 is delivered from a different location and is directed to the LIN subcooling unit 600 as a LIN stream 602. The pump 604 can pump the LIN stream 602 to a pressure greater than 400psi to form a high-pressure LIN stream 606. The high-pressure LIN stream 606 exchanges heat with the 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 stream 612. The first warmed nitrogen stream 612 can be expanded in a first expander 614 to produce a first additional cooled nitrogen stream 616. The first additionally cooled nitrogen stream 616 exchanges heat with the pressurized LNG stream 608 in the at least one heat exchanger 610 to form a second warmed nitrogen stream 618.

The second warmed nitrogen stream 618 may indirectly exchange heat with other process streams, such as in a second heat exchanger 619, before the second warmed nitrogen stream 618 is compressed in one or more compressor stages to form a compressed nitrogen stream 620. As shown in FIG. 6, the one or more compressor stages may include 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 generated by the first expander 614. The first compressor stage 622 may be driven solely by the shaft power generated by the second expander 626. After each compression stage, compressed nitrogen stream 620 may be cooled in

coolers

628, 630, respectively, by indirect heat exchange with ambient. The compressed nitrogen stream 620 can be expanded in a second expander 626 to produce a second additionally cooled nitrogen stream 632. The second additionally cooled nitrogen stream 632 exchanges heat with the pressurized LNG stream 608 in the at least one heat exchanger 610 to form a third warmed nitrogen stream 634. The pressurized LNG stream 608 is subcooled by exchanging heat with the high-pressure LIN stream 606, the first additional cooled nitrogen stream 616, and the second additional cooled nitrogen stream 632 to form an LNG stream 636. The third warmed nitrogen stream 634 can be directed to a third heat exchanger 638 to liquefy a treated natural gas stream 640 (which can be the same as the small treated natural gas stream 415 in fig. 4) to form an additional pressurized LNG stream 642. Additional pressurized LNG stream 642 may be combined with pressurized LNG stream 608 in a combining unit 644 before pressurized LNG stream 608 is subcooled in at least one heat exchanger 610. Additional pressurized LNG stream 642 may be reduced in pressure in water turbine 646 prior to being combined with pressurized LNG stream 608. The third warmed nitrogen stream 634 may be heated by the treated natural gas stream 640 to form a nitrogen vent gas 648, which nitrogen vent gas 648 may be vented to the atmosphere or used in other areas of the gas processing facility where the LIN subcooling unit 600 is located. The LIN subcooling unit 600 shown in fig. 6 reduces the LIN demand of the subcooled pressurized LNG stream by about 20% to 25% compared to the LIN subcooling unit 500 shown in fig. 5. However, the choice of subcooling unit may depend on criteria such as the cost of LIN and the available headspace for LIN storage and/or the LIN subcooling unit itself.

Fig. 7 is a flow diagram of a method 700 for producing Liquefied Natural Gas (LNG). At block 702, the 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 having a pressure greater than 50psia (345kPa) and less than 500psia (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 geographically separated from the first location. At block 708, the produced liquid refrigerant is delivered to a first location. At block 710, the pressurized LNG stream is subcooled in a liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one liquid refrigerant stream to thereby produce an LNG stream.

The steps depicted in fig. 7 are provided for illustrative purposes only, and specific steps may not be required to perform the disclosed methodology. Moreover, fig. 7 may not show all of the steps that may be performed. The claims, and only the claims, define the disclosed system and methodology.

The aspects described herein have several advantages over known techniques. For example, the described aspects can significantly increase the throughput of a conventional mechanical refrigeration process without significantly increasing the required power and footprint of the mechanical refrigeration process. For example, the feed gas expander-based processes described herein, along with LIN subcooling, can produce about 50% more LNG at comparable mechanical refrigeration power as compared to known feed gas expander-based processes. The required amount of LIN is about 0.26 tons LIN per ton of LNG produced. The reduced LIN amount makes this technique particularly suitable for FLNG applications. Using the disclosed aspects, the required volumetric flow rate of the low pressure compressor and the cryogenic heat exchanger load, respectively, are only increased by about 10% by 50% of the additional throughput of the feed gas expander-based process, as compared to known feed gas expander technologies.

The disclosed aspects can include any combination of the methods and systems shown in the following numbered paragraphs. This is not to be taken as a complete listing of all possible aspects, as any number of variations may be contemplated from the above description.

1. A method of producing Liquefied Natural Gas (LNG), comprising:

directing the natural gas stream to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized Liquefied Natural Gas (LNG) stream having a pressure greater than 50psia (345kPa) and less than 500psia (3445 kPa);

providing a liquid refrigerant subcooling unit at a first location;

generating a liquid refrigerant at a second location geographically separated from the first location;

delivering the produced liquid refrigerant to a first location; and

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

2. The method of paragraph 1, wherein the mechanical refrigeration unit comprises 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:

withdrawing a first cooled stream from the warm-end expander; and

discharging a two-phase stream from a cold-end expander;

wherein the temperature of the first cooled stream is greater than the temperature of the two-phase stream.

7. The process of paragraph 3, wherein the pressurized LNG stream is a first pressurized LNG stream, and the process further comprises separating the two-phase stream into a second chilled stream and a second pressurized LNG stream.

8. The method of paragraph 3, wherein the feed gas expander-based process comprises:

withdrawing a first cooled stream from the warm-end expander; and

withdrawing a second cooled stream from the cold side expander;

wherein the temperature of the first cooled stream is higher than the temperature of the second cooled stream.

9. The process of paragraph 7 or 8, wherein the pressure of the first cooled stream is the same or substantially the same as the pressure of the second cooled stream.

10. The process 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 process of any of paragraphs 1 to 12, wherein the vaporized liquid refrigerant stream is used to liquefy the second treated natural gas stream to produce an additional pressurized LNG stream.

14. The process of paragraph 13, wherein an additional pressurized LNG stream is mixed with the pressurized LNG stream prior to subcooling the pressurized LNG stream with a 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 the LNG boil-off gas with a liquid refrigerant.

17. The method of any of paragraphs 1-16, wherein the liquid refrigerant and/or liquid refrigerant boil-off gas is used to keep the mechanical refrigeration unit and/or liquid refrigerant subcooling unit device cool during conditioning and/or shutdown of the mechanical refrigeration unit.

18. The method of any of paragraphs 1-17, wherein warm liquid refrigerant vapor is used to defrost the heat exchanger used to exchange heat.

19. The method of any of paragraphs 1-18, further comprising:

transporting the LNG stream from the first location to a second location in a dual use transport; and

after the LNG stream has been offloaded from the dual use transport, the liquid refrigerant is transported in the dual use transport from the second location to the first location.

20. The method of any of paragraphs 1-19, wherein the mechanical refrigeration unit comprises one of a single mixed refrigerant process, a pure component cascade refrigerant process, or a dual mixed refrigerant process.

21. The process of any of paragraphs 1 to 20, wherein the pressurized LNG stream has a pressure greater than 100psia (690kPa) and less than 400psia (2758 kPa).

22. The process of any of paragraphs 1-21, wherein the pressurized LNG stream has a pressure greater than 200psia (1379kPa) and less than 300psia (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 generating LIN by exchanging heat with the LNG during the regasification of the LNG.

25. The method of paragraph 23, further comprising pressurizing LIN to a pressure greater than 400psia (2758kPa) to form the 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:

in the liquid refrigerant subcooling unit, the pressure of the at least one warmed natural gas stream is reduced in at least one expander means to reduce the pressure of the at least one warmed nitrogen stream and thereby produce at least one additional cooled nitrogen stream.

28. The process of paragraph 27, wherein the at least one additional cooled nitrogen stream exchanges heat with the pressurized LNG stream to form a warmed nitrogen stream.

29. The method of paragraph 27, further comprising:

connecting the at least one expander device with at least one generator for producing electrical power.

30. The method of paragraph 27, further comprising:

the at least one expander device is connected to at least one compressor for compressing the warmed nitrogen stream.

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

the pressurized LNG stream is directed from the plurality of mechanical refrigeration units to a liquid refrigerant subcooling unit to produce at least one LNG stream.

32. 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 having a pressure greater than 50psia (345kPa) and less than 500psia (3445 kPa);

a liquid nitrogen (LIN) subcooling unit disposed at a first location;

a liquid nitrogen (LIN) stream produced at a second location geographically separate from the first location and delivered to a 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 of the LIN streams, thereby producing an LNG stream and at least one vaporized LIN stream.

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

a warm end expander configured to discharge a first cooled stream therefrom; and

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

wherein the temperature of the first cooled stream is greater than the 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 divided into a second cooled stream and a second pressurized LNG stream.

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

withdrawing a second cooled stream from the cold side expander;

35. The system of paragraph 33 or 34, wherein the pressure of the first cooled stream is the same or substantially the same as the pressure of the second cooled 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 the 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:

a dual use transport configured to transport the LNG stream from the first location to the second location and to transport the liquid refrigerant from the second location to the first location in the dual use transport after the subcooled LNG stream has been offloaded from the dual use transport.

It will be understood that numerical changes, modifications and substitutions may be made to the foregoing disclosure without departing from the scope of the present disclosure. Accordingly, the foregoing description is not intended to limit the scope of the present disclosure. Rather, the scope of the disclosure is to be determined solely by the appended claims and their equivalents. It is also contemplated that structures and features in the present embodiments may be changed, rearranged, substituted, deleted, duplicated, combined, or added to each other.

Claims

1. A method of producing Liquefied Natural Gas (LNG), comprising:

directing the natural gas stream to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized Liquefied Natural Gas (LNG) stream having a pressure greater than 50psia and less than 500 psia;

providing a liquid refrigerant subcooling unit at a first location, wherein the liquid refrigerant comprises liquid nitrogen (LIN);

delivering the produced liquid refrigerant to a first location; and

subcooling the pressurized LNG stream in a liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one liquid refrigerant stream to thereby produce an LNG stream, the process further comprising:

pressurizing LIN to a pressure greater than 400psia 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 stream; and

the pressure of the at least one warmed natural gas stream is reduced in at least one expander means in a liquid refrigerant subcooling unit to reduce the pressure of the at least one warmed nitrogen stream and thereby produce at least one additional cooled nitrogen stream.

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

3. The method as set forth in claim 2, wherein said 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 comprising:

withdrawing a first cooled stream from the warm-end expander; and

discharging a two-phase stream from a cold-end expander;

5. The process of claim 4, wherein the pressurized LNG stream is a first pressurized LNG stream, and the process further comprises separating the two-phase stream into a second cooled stream and a second pressurized LNG stream.

6. The method of claim 3 wherein the expander-based refrigeration process is a feed gas expander-based process comprising:

withdrawing a first cooled stream from the warm-end expander; and

withdrawing a second cooled stream from the cold side expander;

wherein the temperature of the first cooled stream is greater than the temperature of the second cooled stream.

7. The process of claim 5, wherein the pressure of the first cooled stream is the same or substantially the same as the pressure of the second cooled stream.

8. The process 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 a liquid refrigerant subcooling unit.

9. The method of any of claims 1-6, wherein the liquid refrigerant subcooling unit comprises:

at least one heat exchanger, or

At least one compressor and/or expander.

10. The method of any of claims 1-6, further comprising:

liquefying the second treated natural gas stream with the vaporized liquid refrigerant stream to produce an additional pressurized LNG stream; and

mixing the additional pressurized LNG stream with the pressurized LNG stream prior to subcooling the pressurized LNG stream with a liquid refrigerant.

11. The method of any of claims 1-6, further comprising locating the mechanical refrigeration unit and the liquid refrigerant subcooling unit on a floating LNG facility.

12. The method of any of claims 1 to 6, further comprising re-liquefying LNG boil-off gas using a liquid refrigerant.

13. The method of any of claims 1 to 6, wherein liquid refrigerant and/or liquid refrigerant boil-off gas is used to keep the mechanical refrigeration unit and/or liquid refrigerant subcooling unit equipment cool during conditioning and/or shutdown of the mechanical refrigeration unit.

14. The method as claimed in any one of claims 1-6, wherein warm liquid refrigerant vapour is used to defrost a heat exchanger for exchanging heat.

15. The method of any of claims 1-6, further comprising:

16. The method of any of claims 1-6, wherein the mechanical refrigeration unit comprises one of a single mixed refrigerant process, a pure component cascade refrigerant process, or a dual mixed refrigerant process.

17. The process of any one of claims 1-6, wherein the pressurized LNG stream has a pressure greater than 100psia and less than 400 psia.

18. The process of any one of claims 1-6, wherein the pressurized LNG stream has a pressure greater than 200psia and less than 300 psia.

19. The method of any one of claims 1-6, wherein the liquid refrigerant comprises liquid nitrogen (LIN), and the method further comprises generating the LIN by exchanging heat with LNG during the regasification of LNG.

20. The process of claim 1, wherein the at least one additional cooled nitrogen stream exchanges heat with the pressurized LNG stream to form a warmed nitrogen stream.

21. The method of claim 1, further comprising:

the at least one expander device is connected to at least one generator for producing electricity or at least one compressor for compressing a warm nitrogen stream.

22. The method of any of claims 1-6, further comprising:

23. 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 having a pressure greater than 50psia and less than 500 psia;

a liquid nitrogen (LIN) subcooling unit disposed at a first location;

wherein the LIN subcooling unit comprises:

a pump configured to pressurize LIN to a pressure greater than 400psia to form a high pressure liquid nitrogen stream;

a heat exchanger configured to subcool the pressurized LNG stream by exchanging heat between the pressurized LNG stream and the high-pressure LIN stream to thereby produce an LNG stream and a warm nitrogen stream;

at least one expander device configured to reduce the pressure of the warm natural gas stream and thereby produce at least one additional cooled nitrogen stream.

24. The system of claim 23, wherein the mechanical refrigeration unit comprises:

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

wherein the pressurized LNG stream is a first pressurized LNG stream, and

25. The system of claim 23, wherein the mechanical refrigeration unit comprises:

a cold-end expander configured to discharge a second cooled stream therefrom;

26. The system of any of claims 23-25, wherein the mechanical refrigeration unit and liquid refrigerant subcooling unit are disposed on a floating LNG facility.

27. The system of any one of claims 23-25, further comprising: