CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of United States Patent Application 63/109063, filed November 3, 2020, entitled NATURAL GAS LIQUEFACTION METHODS AND SYSTEMS FEATURING FEED COMPRESSION, EXPANSION AND RECYCLING, the entirety of which is incorporated by reference herein.

FIELD

[0002] The present disclosure relates to natural gas liquefaction processes.

BACKGROUND

[0003] Natural gas, a hydrocarbon resource comprising predominantly methane, has become an increasingly important energy source in recent years. Natural gas is often processed into liquefied natural gas (LNG) to facilitate transport from production fields to a locale having a strong commercial or consumer need for natural gas. Conventional LNG processing techniques may include: (a) initial treatments of the natural gas to remove contaminants such as water, sulfur compounds and carbon dioxide; (b) separation of heavier hydrocarbon gases (e.g., propane, butane, pentane, and the like), such as through self-refrigeration, external refrigeration, lean oil, and the like; (c) refrigeration of the natural gas, such as through external refrigeration to form LNG at near atmospheric pressure and about -160°C; (d) transportation of the LNG to a market location in specially designed ships or tankers; and (e) conversion of the LNG into pressurized natural gas at a site for processing or distribution to consumers. Refrigeration to produce LNG may employ large refrigeration compressors, often powered by gas turbine drivers that may emit substantial carbon and other emissions. Nitrogen or other cryogenic liquids may alternately be employed to promote formation of LNG, as described in U.S. Patents 3,878,689 and 5,139,547. Conversion of LNG into pressurized natural gas following delivery to a desired location may include repressurizing the LNG to a specified pressure using cryogenic pumps and then vaporizing the LNG through heat exchange with an intermediary fluid or combusting a portion of the natural gas to produce heat.

[0004] One approach for facilitating production of LNG is to expand the natural gas prior to liquefaction, thereby cooling the natural gas below its initial temperature and lessening the refrigeration burden for promoting liquefaction. Expansion follows processing steps a) and b) from above and initial compression thereafter to a pressure well above that obtained after such processing steps. Ambient cooling of the compressed natural gas places the natural gas in a lower entropic state better suited for undergoing subsequent liquefaction. U.S. Patent 6,412,302 describes an expander-based process in which two independent closed refrigeration loops are used to promote natural gas cooling during production of LNG. U.S. Patent 8,616,021 describes an expander-based process in which a natural gas feed is employed as a refrigerant in a closed refrigeration loop. Capacity constraints, as dictated by the maximum discharge pressures following natural gas feed compression and maximum liquefaction pressures post-expansion but prior to liquefaction, have heretofore limited LNG production processes employing expansion prior to liquefaction. In order to increase capacity further, significant equipment enhancements such as more robust metal grades and upgraded compressor designs may be required, which may add significant expense.

[0005] A related LNG production technique employs a compression-expansion cycle prior to liquefaction, as described in U.S. Patent Application Publication 2017/0167787. In addition to capacity constraints similar to those of expansion-promoted liquefaction techniques, compression-expansion liquefaction techniques may be restricted further by pressure limitations of the metal grades commonly used for piping, turbines, and other parts commonly employed during compression of a natural gas stream. These limitations may further constrain the turbine power that may be applied during compression, again limiting throughput. Although more robust metal grades may be utilized during compression-expansion techniques for producing LNG, doing so can significantly add to LNG production costs. Moreover, such LNG production techniques may be excessively influenced by outside environmental factors, particularly temperature, especially in terms of the turbine power needed to promote initial compression. In such conventional compression-expansion processes for producing LNG, the turbine power for promoting initial expansion may not be easily modified. Other factors that may not be easily accounted for in such conventional compression-expansion processes include, for example, pressure variation of an incoming natural gas stream, and variation in equipment operating conditions (e.g. , due to process upsets and sub-optimal operation resulting from unclean conditions). SUMMARY

[0006] In some aspects, the present disclosure provides pre-boost natural gas liquefaction methods comprising: providing a natural gas stream at a first temperature and a first pressure to a first compressor, optionally in series fluid communication with a second compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor and optionally the second compressor, thereby forming a compressed natural gas stream having a second temperature; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the first compressor and the second compressor, if present, thereby forming a cooled, compressed natural gas stream having a third pressure and a third temperature lower than the second temperature; performing a second heat exchange upon the cooled, compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fourth pressure and a fourth temperature lower than the third temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using a first expander from about the fourth pressure to a fifth pressure lower than the fourth pressure, thereby forming a chilled, expanded natural gas stream having a fifth temperature lower than the fourth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to the natural gas stream upstream from the first heat exchange location; wherein the second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream.

[0007] In some or other aspects, pre-boost natural gas liquefaction systems may comprise: a natural gas feed connected to a first compressor and a second compressor that are fluidly coupled in series; an optional first heat exchange location fluidly coupled to an output of the second compressor; a second heat exchange location downstream from the second compressor and fluidly coupled to an output of the first heat exchange location, if present; a first expander fluidly coupled to an output of the second heat exchange location; and an output line from the first expander that is subdivided into a main line and recycle line; wherein the recycle line is in thermal communication with the second heat exchange location, and the recycle line is in fluid communication with the natural gas feed.

[0008] In some aspects, the present disclosure provides post-boost natural gas liquefaction methods comprising: providing a natural gas stream at a first temperature and first pressure to a first compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor, thereby forming a compressed natural gas stream having a second temperature; optionally compressing the compressed natural gas stream to a third pressure higher than the second pressure using a second compressor in fluid communication with the first compressor, the compressed natural gas stream having a third temperature after being compressed to the third pressure; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the second compressor, thereby forming a cooled, compressed natural gas stream having a fourth pressure and a fourth temperature lower than the third temperature; performing a second heat exchange upon the compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fifth pressure and a fifth temperature lower than the fourth temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using an expander from about the fifth pressure to a sixth pressure lower than the fifth pressure, thereby forming a chilled, expanded natural gas stream having a sixth temperature lower than the fifth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to a location upstream from the first heat exchange location; wherein the second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream.

[0009] In some or other aspects, post-boost natural gas liquefaction systems may comprise: a natural gas feed connected to a first compressor; a second compressor fluidly coupled to an output of the first compressor; an optional first heat exchange location fluidly coupled to an output of the second compressor; a second heat exchange location downstream from the second compressor and fluidly coupled to an output of the first heat exchange location, if present; an expander fluidly coupled to an output of the second heat exchange location; and an output line from the expander that is subdivided into a main line and recycle line; wherein the recycle line is in thermal communication with the second heat exchange location, and the recycle line is in fluid communication with a location upstream from the first heat exchange location.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments.

[0011] FIG. 1 A is a diagram of aportion of afirst configuration of anatural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas. [0012] FIG. IB is a diagram of a portion of a second configuration of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas.

[0013] FIG. 2 A is a diagram of a portion of a first configuration of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using an expander.

[0014] FIG. 2B is a diagram of a portion of a second configuration of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using an expander, in which the natural gas feed compressor is driven by steam and integrated via a Heat Recovery Steam Generator (HRSG) with a gas turbine used to drive compressor(s) in a liquefaction process.

[0015] FIG. 3 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using a Joule-Thompson valve.

[0016] FIG. 4 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been formed and further expanded using a compander.

[0017] FIG. 5 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas, in which initial compression and expansion are conducted using a compressor-expander.

DETAILED DESCRIPTION

[0018] The present disclosure relates to natural gas liquefaction methods and systems and, more particularly, natural gas liquefaction methods and systems featuring compression, expansion, and recycling of a portion of a natural gas stream.

[0019] Liquefied natural gas (LNG) is a convenient form for transportation of natural gas from a production location to a distribution or processing location. Natural gas liquefaction processes usually utilize large-scale refrigeration and/or heat exchange with one or more cryogenic liquids to lower the natural gas below its boiling point. Since large volumes of natural gas are usually processed into LNG, it may be desirable to lessen the heat exchange burden during liquefaction to improve energy efficiency. Even incremental improvements in energy efficiency and throughput may afford significant cost savings for supplying LNG. Throughput may be improved by incorporating auxiliary power (e.g., from a gas or electric turbine, or through supplying waste heat from refrigeration) during initial compression of a natural gas stream, wherein the auxiliary power is separate from refrigerant power subsequently employed to promote liquefaction. To this end, natural gas liquefaction processes featuring compression or compression-expansion of a natural gas stream prior to liquefaction have been developed.

[0020] Despite the desirability of incorporating compression or compression-expansion into natural gas processing prior to liquefaction, there remain various difficulties associated with doing so. LNG production rates may be limited by material constraints for turbines, piping and other parts commonly employed during natural gas liquefaction. Common materials for turbines and piping employed during compression or compression-expansion processing of natural gas frequently have an approximate maximum use pressure of about 200 bar. This pressure ceiling therefore limits the amount of heat that may be rejected from an incoming natural gas stream at a specified temperature, since the incoming natural gas stream may not be pressurized further to reject more heat to a heat exchanger. Throughput is also limited due to the pressure ceiling. The main cryogenic heat exchanger (MCHX) for promoting liquefaction following expansion may have a maximum use pressure in the range of about 110 bar to about 150 bar, which also may limit throughput. These factors collectively decrease the amount of natural gas available to undergo liquefaction and lead to a potentially large temperature differential that must be overcome to promote liquefaction following compression and expansion (i.e., during a refrigerant cycle to form the LNG). Although higher pressures may be achieved by employing materials with higher pressure tolerances, doing so may significantly increase the cost of forming LNG. Moreover, natural gas liquefaction processes, including those employing compression or compression-expansion prior to liquefaction, may be heavily influenced by outside environmental factors (e.g, temperature, natural gas stream pressure, and the like), which may result in highly variable processing conditions depending on season. These factors may also impact the available auxiliary power that may be supplied during compression or compression-expansion processing of a natural gas stream. Although compression or compression-expansion liquefaction processes may afford some process flexibility to account for outside environmental factors, there are still constraints on the initial compression that may be limiting.

[0021] The present disclosure demonstrates that natural gas liquefaction processes employing compression-expansion prior to liquefaction may be significantly enhanced by recycling a portion of the natural gas stream following compression and expansion. In particular, a recycle stream of expanded natural gas may be returned upstream to promote indirect heat exchange of an incoming natural gas stream immediately prior to expansion and/or at one or more intermediate stages of expansion. In non-limiting examples, the recycle stream of expanded natural gas may be returned upstream from an expander, either being provided directly to an incoming natural gas stream or to one or more compressor stages in order to reject heat to the environment. Overall, recycling of a portion of an expanded natural gas stream, optionally with further expansion of the recycle stream being conducted, may promote further cooling of an incoming natural gas stream and lower downstream heat exchange burdens, particularly during liquefaction. By lowering the temperature of the incoming natural gas stream via recycling in the manner described herein, higher throughput may be achieved prior to reaching a pressure ceiling of materials in turbines and piping, thereby avoiding use of more expensive material grades. In addition, the disclosure herein allows greater and more efficient operational flexibility to be realized by redistributing power from expansion (refrigeration cycle) into the compression stages of natural gas processing. By controlling the temperature of the incoming natural gas stream, the amount of power applied during compression may allow the production rate of LNG to be altered dynamically. Greater operational flexibility during initial compression of the natural gas stream may be realized as a result of the disclosure herein. [0022] The recycle stream, having already been expanded, has a lower temperature than the compressed natural gas stream from which it was produced. In the disclosure herein, the recycle stream may undergo heat exchange with the compressed natural gas stream just prior to expansion to lower the temperature of the compressed natural gas stream. Expansion of the compressed natural gas stream from a lower starting temperature accordingly affords a lower temperature of the natural gas following expansion. The main stream of natural gas diverted toward forming LNG may then undergo liquefaction more readily, since it is at a lower entropic state than is the incoming natural gas stream. The lower entropic state of the main stream used for forming LNG may arise as a direct consequence of its rejection of entropy made possible by recycling of a portion of the expanded natural gas and the transfer of heat between the main stream and the recycle stream. To facilitate the foregoing heat and accompanying entropy transfer, the methods and systems described herein may further employ an optional secondary expansion of the recycle stream to allow for a greater temperature differential between the main stream and the recycle stream, thus promoting additional heat exchange with the compressed natural gas stream before being returned further upstream. Without being bound by theory or mechanism, the methods and systems described herein may convert an incoming natural gas stream into a lower entropic state capable of undergoing liquefaction more readily through input of auxiliary power according to the disclosure herein, specifically by providing, in an efficient manner, natural gas having a combination of a temperature that is as low as possible and a pressure that is as high as possible. As such, considerable flexibility may be realized for cooling the incoming natural gas stream, particularly to compensate for seasonal variation in environmental temperatures and other process variables. In non-limiting examples, the extent of cooling of the incoming natural gas stream may be regulated by changing the flow rate of the recycle stream and/or cooling the recycle stream through further expansion. Moreover, the temperature of the recycle stream may be further varied by the amount of heat exchange that takes place with the compressed natural gas stream (e.g., by varying the flow rate of the recycle stream), thereby facilitating additional temperature regulation of the incoming natural gas stream to improve processing throughput.

[0023] As used herein, the term "natural gas" refers to a multi-component gas obtained from a crude oil well (associated gas) or from a subterranean gas-bearing formation (nonassociated gas). The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane as a significant component, sometimes as a primary component. A natural gas stream may also contain ethane, higher molecular weight hydrocarbons (e.g., propane), and/or one or more acid gases. Minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, and crude oil, for example, may also be present. One or more of these components may be removed and/or lessened in concentration prior to processing a natural gas stream according to the disclosure herein.

[0024] As used herein, the term "compressor" refers to a machine, unit, device, or apparatus that increases the pressure of a gas stream by the application of work. Compressors may feature a single compression process or step, or compressors may feature multi-stage compressions or steps, more particularly multi-stage compressors located within a single casing or shell. Gas streams to be compressed may be provided to a compressor at different pressures. Some stages or steps of a cooling process may involve two or more compressors in parallel, series, or both. [0025] As used herein, the term "cooling" refers to lowering and/or dropping a temperature and/or internal energy of a substance by any suitable, desired, or required amount. Cooling may include a temperature drop of at least about 1°C, at least about 5°C, at least about 10°C, at least about 15°C, at least about 25°C, at least about 35°C, at least about 50°C, at least about 75°C, at least about 85°C, at least about 95°C, or at least about 100°C. The cooling may use any suitable heat sink, such as steam generation, hot water heating, cooling water, air, refrigerant, other process streams (integration), and combinations thereof. One or more sources of cooling or heat sinks may be combined and/or cascaded to reach a desired temperature. The cooling step may use a cooling unit with any suitable device and/or equipment. Cooling may include indirect heat exchange, such as with one or more heat exchangers. Alternately, cooling may use evaporative (heat of vaporization) cooling and/or direct heat exchange, such as a cooling liquid sprayed directly into a gas stream. More preferably, heat exchange may occur indirectly in the embodiments disclosed herein.

[0026] As used herein, the term "expander" refers to one or more devices suitable for reducing the pressure of a fluid in a line (e.g , a liquid stream, a gas stream, or a multiphase stream containing both liquid and gas). Unless a particular type of expansion device is specifically stated, an expander suitable for use in the disclosure herein may operate by (1) at least partially by isenthalpic means, or (2) at least partially by isentropic means, or (3) a combination of both isentropic means and isenthalpic means. Suitable devices for isenthalpic expansion of natural gas may include, but are not limited to, manually or automatically, actuated throttling devices such as, for example, valves, control valves, Joule-Thomson (J-T) valves, or Venturi devices. Suitable devices for isentropic expansion of natural gas may include equipment such as expanders or compressor-expanders, including turboexpanders, that extract or derive work from such expansion. Suitable devices for isentropic expansion of liquid streams may include equipment such as expanders, hydraulic expanders, liquid turbines, or compressor-expanders that extract or derive work from such expansion.

[0027] As used herein, the term "heat exchanger" refers to any device capable of transferring heat energy or cold energy from one medium to another medium, such as between at least two distinct fluids. Heat exchangers may include "direct heat exchangers" and "indirect heat exchangers." Thus, a heat exchanger may be of any suitable design, such as a co-current or counter-current heat exchanger, an indirect heat exchanger (e.g, a spiral wound heat exchanger or a plate-fin heat exchanger such as a brazed aluminum plate fin type), direct contact heat exchanger, shell-and-tube heat exchanger, spiral, hairpin, core, core-and-kettle, printed-circuit, double-pipe or any other type of heat exchanger. "Heat exchangers" may also refer to any column, tower, unit or other arrangement adapted to allow the passage of one or more streams for promoting direct or indirect heat exchange between one or more lines of refrigerant.

[0028] As used herein, the term “compressor-expander” refers to a machine in which an expander provides shaft power to drive a compressor. A “turboexpander” is a type of compressor-expander coupled together on a single (common) shaft. A “compander” is a type of compressor-expander in which the compressor and expander are coupled by separate gear- driven shafts.

[0029] Embodiments of the present disclosure will now be described with reference to the drawings. In FIGS. 1A, IB and 2-4, compression of a natural gas stream takes place in a compressor and compressor-expander placed sequentially in series prior to forming a recycle stream, thereby defining a pre-boost configuration, as explained in further detail below. In FIGS. 1A and IB, the recycle stream is not expanded further, whereas additional recycle stream expansion takes place in FIGS. 2-4. FIG. 5, in contrast, shows compression of a natural gas stream using a compressor-expander and a compressor placed sequentially in series prior to forming a recycle stream, thereby defining a post-boost configuration, as explained in further detail below. Post-boost compression-expansion configurations may present certain advantages over comparable pre-boost compression-expansion configurations.

[0030] FIG. 1A is a diagram of a first configuration of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas. No additional expansion of a recycle stream comprising the expanded natural gas takes place in FIG. 1A, nor in the related system and method configuration in FIG. IB.

[0031] As shown in FIG. 1A, system and method 100 provides natural gas stream 102 to first compressor 104 via feed line 101. Natural gas stream 102 is provided in feed line 101 at a first temperature (Tl) and a first pressure (Pl). In first compressor 104, natural gas stream 102 is pressurized to form a compressed natural gas stream. The first compressor 104 may be driven by a drive shaft comprising a motor drive, a gas turbine drive, and/or steam turbine drive, among others. Rejection of excess heat from the compressed natural gas stream then takes place in intermediate heat exchanger 108, which may be optionally omitted in some system and method configurations. The compressed natural gas stream is at an intermediate pressure and temperature state at this juncture, wherein the intermediate pressure state comprises a higher pressure than the first pressure (Pl). The compressed natural gas stream exiting intermediate heat exchanger 108 may then be further compressed. As shown, second compressor 110 and first heat exchanger 112 are fluidly coupled in series with intermediate heat exchanger 108. Second compressor 110 and/or first heat exchanger 112 may also be optionally omitted in some system and method configurations. Prior to entering first heat exchanger 112 following further compression with second compressor 110, the compressed natural gas stream may have a second temperature (T2) and a second pressure (P2) higher than the first pressure (Pl). After heat exchange has been performed at first heat exchanger 112, a cooled, compressed natural gas stream having a third pressure (P3) and a third temperature (T3) lower than the second temperature (T2) may be produced. Therefore, first compressor 104, second compressor 110, intermediate heat exchanger 108 and first heat exchanger 112 may collectively achieve the third pressure (P3) and the third temperature (T3) in the cooled, compressed natural gas stream. Intermediate heat exchanger 108 and first heat exchanger 112, if present, may function through indirect heat exchange, such as through air and/or water cooling of the compressed natural gas stream. Although intermediate heat exchanger 108 may be optionally omitted, intermediate cooling of natural gas stream 102 may be particularly desirable. Intermediate cooling may lower the power needed to cool the natural gas stream to the second pressure (P2), afford greater thermodynamic efficiency, and decrease volumetric flow. Decreased volumetric flow may allow a decreased casing size for second compressor 110 to be realized. Second compressor 110 may be omitted if first compressor 104 is sufficient to achieve satisfactory pressurization prior to expansion.

[0032] After exiting first heat exchanger 112, the cooled, compressed natural gas stream may be conveyed to first expander 116 after interacting with second heat exchanger 140, in a manner described in further detail hereinafter. The cooled, compressed natural gas stream may then have a fourth pressure (P4) and a fourth temperature (T4) lower than the third temperature (T3). Preferably, first expander 116 is operatively coupled to second compressor 110 by common drive shaft 117, thereby defining a compressor-expander, specifically a turboexpander-compressor. For embodiments in which second compressor 110 is uncoupled to first expander 116, drive shaft 117 attached to second compressor 110 may comprise a motor drive, a gas turbine drive, and/or steam turbine drive, among others. In addition, drive shaft 117 attached to first expander 116 may be replaced with an electric generator, oil break, or other commonly used power generating/dissipating drive or device.

[0033] Upon exiting first expander 116, the gas pressure decreases and thereby forms a chilled, expanded natural gas stream having a fifth pressure (P5) lower than the fourth pressure (P4) and a fifth temperature (T5) lower than the fourth temperature (T4). Preferably, the fifth pressure (P5) is not greater than about 140 bar. The 140 bar pressure limit may be chosen to protect downstream equipment and piping and may be increased if more robust materials are used. More preferably, the fifth pressure (P5) may be about 65 bar or above, or about 70 bar or above, and below about 140 bar, such that liquefaction takes place in a dense, single-phase system. Branch line 120 may be optionally present to divert at least a portion of the cooled, compressed natural gas stream from interacting with second heat exchanger 140. By having a portion of the cooled, compressed natural gas stream bypass second heat exchanger 140, process flexibility may be introduced to account for environmental factors, such as seasonal variation of ambient temperature, gas source variability, process and equipment variability, and the like. Additional approaches to account for seasonal temperature variation and other factors include, for example, increasing or decreasing the volumetric flow of recycle gas in recycle line 150, as discussed hereinafter.

[0034] The chilled, expanded natural gas stream exiting first expander 116 via outlet line 121 may then be split into a main stream in line 122 and a recycle stream in line 124, each of which is at the fifth pressure (P5) and the fifth temperature (T5). The main stream in line 122 may undergo liquefaction to produce LNG 130, such as through conventional refrigeration processes and/or interaction of the main stream with a cryogenic liquid. As such, the fifth pressure (P5) represents the liquefaction pressure.

[0035] The recycle stream in line 124 may be directly conveyed to second heat exchanger 140 or undergo further expansion, as described further herein in reference to FIGS. 2-4. Referring still to FIG. 1A, the recycle stream in line 124 is conveyed to second heat exchanger 140 at the fifth temperature (T5) and the fifth pressure (P5). Since the fifth temperature (T5) is lower than the fourth temperature (T4) and the fourth temperature (T4) is lower than the third temperature (T3), the recycle stream may be employed to promote further cooling of the cooled, compressed natural gas stream in second heat exchanger 140. In particular, the cooled, compressed natural gas stream and the recycle stream may undergo heat exchange within second heat exchanger 140 to afford the cooled, compressed natural gas stream at a lower fourth temperature (T4), thereby providing the chilled, expanded natural gas stream at a commensurately lower fifth temperature (T5) following expansion in first expander 116 (i.e., a fourth temperature (T4) of the cooled, compressed natural gas stream beyond that attainable with just intermediate heat exchanger 108 and first heat exchanger 112 alone) and decreasing the energy burden required when forming LNG 130 from the chilled, expanded natural gas stream. A more favorable entropic state may be attained as well. Second heat exchanger 140 may be a printed circuit heat exchanger (PCHE) in various embodiments of the present disclosure, but other heat exchanger configurations that bring the cooled, compressed natural gas stream and the recycle stream into close proximity to facilitate heat exchange may also be suitable.

[0036] After undergoing heat exchange at second heat exchanger 140, the recycle stream is returned to a location upstream from first heat exchanger 112 via recycle line 150. As depicted in FIG. 1A, recycle line 150 reconnects with feed line 101 prior to delivery of natural gas stream 102 to first compressor 104. Other locations upstream or downstream from first compressor 104 may also be suitable. For example, the recycle stream may be returned to the natural gas stream upstream from first compressor 104, downstream from first compressor 104 and upstream of second compressor 110 (either upstream or downstream from intermediate heat exchanger 108, if present), and/or downstream from second compressor 110 but upstream from first heat exchanger 112.

[0037] The recycle stream may decrease the temperature of natural gas stream 102 depending on the fifth temperature (T5) and the extent of heat exchange taking place at second heat exchanger 140, thereby lessening the energy input needed to afford the intermediate pressure state. By lowering the resulting temperature of the main stream in line 122 as a consequence of the foregoing, a greater amount of energy may be applied to first compressor 104 to increase throughput.

[0038] It is also to be appreciated that first compressor 104 may comprise multiple compressor stages and/or comprise multiple compressor casings and that are driven by separate drive shafts, and the recycle stream may be returned to any of these multiple compressor stages or at an interstage location of compression, as shown in an alternative configuration in FIG. IB. That is, certain system and method configurations may feature first compressor 104 being operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages.

[0039] FIG. IB is a diagram of a second configuration of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas. Identical reference characters are utilized in FIG. IB, as well as in the remaining FIGS., for in-common elements having similar features to those described above in FIG. 1 A. Accordingly, in the interest of brevity, features in the remaining FIGS, having similar operational characteristics to those described above in FIG. 1 A are not described again in detail. The configuration shown in FIG. IB differs from the configuration shown in FIG. 1A in that first compressor 104 is multi-stage in FIG. IB and the return location of recycle line 150 is at an interstage location downstream from first compressor 104.

[0040] As shown in FIG. IB, system and method 103 provides natural gas stream 102 via feed line 101 to first compressor 104, which provides multi-stage compression in the depicted configuration. Before exiting first compressor 104, the resulting compressed natural gas stream may undergo inter-stage cooling in inter-stage heat exchanger 106. After exiting first compressor 104, processing of the natural gas may then take place in a largely similar manner to that described above in reference to FIG. 1 A, with the exception of the location where recycle line 150 is returned. In the configuration shown in FIG. IB, recycle line 150 provides the recycle stream to the lines allowing inter-stage cooling to take place, preferably upon the return line connecting inter-stage heat exchanger 106 to first compressor 104. Alternately, recycle line 150 may provide the recycle stream to feed line 101, to the line fluidly connecting first compressor 104 and second compressor 110, either upstream or downstream of intermediate heat exchanger 108, if present, or even downstream from second compressor 110 and upstream from first heat exchanger 112. Thus, the recycle stream may be returned upstream from first compressor 104, downstream from first compressor 104 and upstream from second compressor 110, including at an inter-stage location, downstream from second compressor 110 and upstream from first heat exchanger 112, or any combination thereof. Again, second compressor 110 and first heat exchanger 112 may be optionally omitted in some system and method configurations. Inter-stage heat exchanger 106, intermediate heat exchanger 108, and first heat exchanger 112, if present, may function through indirect heat exchange, such as through air or water cooling of the compressed natural gas stream.

[0041] Additional expansion of the recycle stream in line 124 may also take place, as described hereinafter in reference to FIGS. 2-4. FIG. 2A and 2B are diagrams of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using an expander. FIG. 3 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using a Joule-Thompson valve. Other than incorporating additional expansion capabilities for the recycle stream, FIGS. 2A, 2B, and 3 are largely identical to FIG. 1A and may be better understood by reference thereto. Although not shown, it is to be appreciated that the configuration of FIG. IB may similarly incorporate additional expansion capabilities in the recycle line.

[0042] As shown in FIG. 2A, system and method 200 conveys the recycle stream in line 124 to second expander 131, whereupon the recycle stream exiting second expander 131 has a sixth pressure (P6) lower than the fifth pressure (P5) and a sixth temperature (T6) lower than the fifth temperature (T5). As such, the recycle stream exiting second expander 131 is again at a lower temperature than the cooled, compressed natural gas stream exiting first heat exchanger 112. Following expansion, the recycle stream may be utilized to promote further cooling of the cooled, compressed natural gas stream within second heat exchanger 140, thereby producing a more favorable entropic state for liquefaction. Because the sixth temperature (T6) is even lower than the fifth temperature (T5), more extensive cooling of the cooled, compressed natural gas stream may take place in second heat exchanger 140, as compared to that taking place when the recycle stream does not undergo further expansion (FIGS. 1A and IB).

[0043] The configuration depicted in FIG. 2B differs from the configuration shown in FIG. 2A in that the configuration of FIG. 2B employs integration of waste heat derived from the liquefaction of stream 122 to provide the power to drive first compressor 104. As shown in FIG. 2B, a system and method 201 liquefies stream 122 in a liquefaction process 132 using refrigerant streams 133, 134 to produce liquefied natural gas (LNG) stream 130. Liquefaction process 132 additionally uses refrigerant compressor(s) 135 to provide heating and cooling cycles of refrigerant streams 133, 134. The power source to drive refrigerant compressor(s) 135 is gas turbine 136. Exhaust heat 137 is recovered from gas turbine 136 via Heat Recovery Steam Generator (HRSG) 138. At least a portion of the resulting stream of pressurized steam generated by HRSG 138 is conveyed through line 139 to power first compressor 104, such as via a steam turbine (not shown). In additional aspects (not depicted by FIG. 2B), at least a portion of the stream of pressurized steam generated by HRSG 138 may be used to power second compressor 110, either in addition to or in lieu of being used to power first compressor 104. For example, for embodiments in which second compressor 110 is uncoupled to first expander 116, first compressor 104 and second compressor 110 may be operatively coupled to one another via a drive shaft driven by at least a portion of the stream of pressurized steam generated by HRSG 138. Additional heat 141 may optionally be supplied to HRSG 138 via supplementary firing. In the event that excess heat is supplied to HRSG 138, an additional steam stream 142 may optionally be withdrawn and employed in various aspects (not shown) of a facility in which system and method 201 is located, such as for process heating, generating electricity, among others.

[0044] Depending on the composition, temperature, pressure and flow rate of feed stream 102, a variable flow rate of the recycle stream in line 124 is returned (after passage through second expander 131 and second heat exchanger 140) to the inlet of first compressor 104. The variation in the flow rate of the returned recycle stream to first compressor 104 in turn results in a variation in the amount of power required to drive compressor 104. The heat integration afforded by the configuration depicted in FIG. 2B advantageously provides an additional degree of freedom in rebalancing the power distribution between first compressor 104 and refrigerant compressor(s) 135 via recycle stream 124 to optimize overall energy demand of the system and method 201.

[0045] FIG. 3 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been further expanded using a Joule-Thompson valve. Natural gas liquefaction system and method 300 depicted in FIG. 3 is substantially similar to that depicted in FIG. 2A, except for replacement of second expander 131 with Joule-Thompson valve 330. Suitable Joule-Thompson valves and their operation will be familiar to one having ordinary skill in the art. Although shown in FIG. 3, branch line 120 may be omitted when employing Joule- Thompson valve 330 to expand the recycle stream. Since Joule-Thompson valve 330 may provide less efficient cooling of the recycle stream than does second expander 131, there is less risk of overcooling the cooled, compressed natural gas stream exiting second heat exchanger 140 than when second expander 131 (FIG. 2A) is used.

[0046] In FIG. 3, Joule-Thompson valve 330 is the primary mechanism for promoting expansion within the recycle stream. In related system and method 200 in FIG. 2A, a bypass Joule-Thompson valve (not shown) may be present to bypass second expander 131. In nonlimiting embodiments, the bypass Joule-Thompson valve may allow natural gas processing to continue even when second expander 131 needs maintenance or otherwise becomes inoperable. [0047] FIG. 4 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas that has been formed and further expanded using a compander. Natural gas liquefaction system and method 400 in FIG. 4 differs that shown in FIG. 2A in that second compressor 110, first expander 116 and second expander 131 are incorporated within shared casing 350. Second compressor 110 and first expander 116 may again be operatively coupled to one another via drive shaft 352, and drive shaft 352 may be operatively coupled to drive shaft 354 of second expander 131 via bull gear 360. Collectively, shared casing 350, second compressor 110, first expander 116, second expander 131, shafts 352 and 354, and bull gear 360 define compander 370. By providing these components within a single casing, footprint and installation weight advantages may be realized. It is to be appreciated that other designs for compander 370 may also be suitable.

[0048] In systems and methods 100, 103, 200, 300 and 400, depicted in FIGS. 1A, IB, and 2-4, respectively, natural gas stream 102 is exposed sequentially to first compressor 104 and second compressor 110. Thus, in such configurations in which second compressor 110 comprises a portion of a compressor-expander, particularly a turboexpander, natural gas stream 102 is exposed to second compressor 110 of the compressor-expander following an initial compression of the natural gas stream with first compressor 104 to reach an intermediate pressure state. Such process configurations may be referred to a providing “pre-boost” compression, since natural gas stream 102 is compressed prior to being further compressed using second compressor 110 of the compressor-expander. Such pre-boost compression may be suitable for use in the disclosure herein. Because the compressor-expander is downstream from a main compressor in such configurations, the compressor-expander may need to be resistant to higher pressures than would otherwise be necessary than if the compressor- expander were to provide the initial compression of the natural gas stream. The intolerance to excessive pressures may lead to higher design costs and manufacturing challenges. In addition, lower compression efficiencies with the compressor-expander may occur in such configurations, and additional heat exchange between the first compressor and the compressorexpander may be highly desirable.

[0049] The foregoing difficulties may be at least partially alleviated by providing the natural gas stream to the compressor-expander (turboexpander) first, followed by additional compression thereafter, if needed, as described further below in reference to FIG. 5. Since such method and system configurations provide additional compression after initial compression with a compressor-expander, the processes may be referred to as providing “postboost” compression. Advantageously, the post-boost configuration of FIG. 5 may utilize similar parts to those utilized for the pre-boost configurations of FIGS. 1A, IB and 2-4. Moreover, such “post-boost” configurations may be implemented without conducting a secondary expansion of the recycle stream and may also allow the compressor side of the compressor-expander to be smaller than would otherwise be necessary. It is to be appreciated that secondary expansion of the recycle stream using an expander or Joule-Thompson valve may be incorporated in post-boost systems and methods as well, if needed.

[0050] FIG. 5 is a diagram of a portion of a natural gas liquefaction system and method employing compression-expansion of a natural gas stream and recycling of expanded natural gas, in which initial compression and expansion are conducted with a compressor-expander. As shown in FIG. 5, natural gas liquefaction system and method 500 provides natural gas stream 402 to first compressor 404 via feed line 401. Natural gas stream 402 is provided in feed line 401 at a first temperature (Tl) and a first pressure (Pl). First compressor 404 is operatively coupled to expander 430 via drive shaft 452, thereby defining a compressorexpander. In first compressor 404, natural gas stream 402 is pressurized to form a compressed natural gas stream having a second temperature (T2) and a second pressure (P2) higher than the first pressure (Pl). The compressed natural gas stream is then provided to second compressor 410, in which the compressed natural gas stream reaches a third temperature (T3) and a third temperature (P3) higher than the second pressure (P2). The natural gas stream may be provided directly from first compressor 404 to second compressor 410 without undergoing intermediate cooling in between. Optionally, an intermediate heat exchange may take place between first compressor 404 and second compressor 410 using an intermediate heat exchanger (not shown) to achieve an intermediate temperature and pressure state. Before exiting second compressor 410, the compressed natural gas stream may undergo inter-stage cooling in inter- stage heat exchanger 408. Upon exiting second compressor 410 at the third pressure (P3) and the third temperature (T3), the compressed natural gas stream may undergo additional heat exchange at first heat exchanger 412 to form a cooled, compressed natural gas stream having a fourth pressure (P4) and a fourth temperature (T4) lower than the third temperature (T3). Optionally, first heat exchanger 412 may be omitted. If present, inter-stage heat exchanger 408 and first heat exchanger 412 may function through indirect heat exchange, such as through air or water cooling of the compressed natural gas stream.

[0051] After exiting first heat exchanger 412, the cooled, compressed natural gas stream may be conveyed to expander 430 after interacting with second heat exchanger 440, in a manner described in further detail hereinafter. After interacting with second heat exchanger 440, the cooled, compressed natural gas stream may then have a fifth pressure (P5) and a fifth temperature (T5) lower than the fourth temperature (T4). Upon exiting expander 430 via outlet lines 421 , the gas pressure decreases and thereby forms a chilled, expanded natural gas stream having a sixth pressure (P6) lower than the fifth pressure (P5) and a sixth temperature (T6) lower than the fifth temperature (T5). Preferably, the sixth pressure (P6) is not greater than about 140 bar.

[0052] The chilled, expanded natural gas stream exiting expander 430 may then be split into a main stream in line 422 and a recycle stream in line 424, each at the sixth pressure (P6) and the sixth temperature (T6). The main stream in line 422 may undergo liquefaction to produce LNG 432, such as through conventional refrigeration processes and/or interaction of the main stream with a cryogenic liquid. Thus, the sixth pressure (P6) represents the liquefaction pressure.

[0053] The recycle stream in line 424 at the sixth pressure (P6) and the sixth temperature (T6) is at a lower temperature than the cooled, compressed natural gas stream exiting first heat exchanger 412. Like the pre-boost configurations shown in FIGS. 1A, IB and 2-4, the postboost configuration of FIG. 5 may similarly allow the recycle stream to promote further cooling of the cooled, compressed natural gas stream in second heat exchanger 440. In particular, the cooled, compressed natural gas stream and the recycle stream may undergo heat exchange within second heat exchanger 440 to afford the cooled, compressed natural gas stream at the fifth temperature (T5) and form a more favorable entropic state. Thus, the recycle stream and second heat exchanger 440 may decrease the temperature of the cooled, compressed natural gas stream beyond that attainable with first heat exchanger 412 alone, thereby decreasing the energy burden required when forming LNG 432 from the chilled, expanded natural gas stream obtained from expander 430. Second heat exchanger 440 may be a printed circuit heat exchanger (PCHE) in various embodiments of the present disclosure, but other heat exchanger configurations that bring the cooled, compressed natural gas stream and the recycle stream into close proximity to facilitate heat exchange may also be suitable. It is to be appreciated, however, additional expansion of the recycle stream may be conducted in some instances.

[0054] After facilitating heat exchange at second heat exchanger 440, the recycle stream is returned to a location upstream from first heat exchanger 412 via recycle line 450. In the configuration depicted in FIG. 5, recycle line 450 provides the recycle stream to the lines allowing inter-stage cooling to take place, preferably upon the return line to second compressor 410. Alternately, recycle line 450 may provide the recycle stream to feed line 401 or to the line fluidly connecting first compressor 404 and second compressor 410, preferably the line returning to first compressor 404. That is, the recycle stream may be returned to the natural gas stream upstream from first compressor 404, downstream from first compressor 404 and upstream from second compressor 410, downstream from second compressor 410 and upstream from first heat exchanger 412, or any combination thereof. The recycle stream may decrease the natural gas temperature at any of these locations, thereby allowing higher throughput to take place for forming LNG 432.

[0055] Accordingly, pre-boost methods for promoting liquefaction of a natural gas stream may comprise: providing a natural gas stream at a first temperature and a first pressure to a first compressor, optionally in series fluid communication with a second compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor and the second compressor, thereby forming a compressed natural gas stream having a second temperature; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the first compressor and the second compressor, if present, thereby forming a cooled, compressed natural gas stream having a third pressure and a third temperature lower than the second temperature; performing a second heat exchange upon the cooled, compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fourth pressure and a fourth temperature lower than the third temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using a first expander from about the fourth pressure to a fifth pressure lower than the fourth pressure, thereby forming a chilled, expanded natural gas stream having a fifth temperature lower than the fourth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to the natural gas stream upstream from the first heat exchange location. The second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream. The recycle stream may be returned to the natural gas stream after interacting with the cooled, compressed natural gas stream at the second heat exchange location.

[0056] The methods may further comprise: expanding the recycle stream to a sixth pressure lower than the fifth pressure, thereby cooling the recycle stream to a sixth temperature lower than the fifth temperature. After expansion of the recycle stream to the sixth pressure and sixth temperature, the recycle stream may undergo the second heat exchange with the cooled, compressed natural gas stream at the second heat exchange location. Expansion of the recycle stream may take place using a second expander or a Joule-Thompson valve. Optionally, expansion of the recycle stream may take place using a compander, in which the first expander, the second expander, and the second compressor are located within a shared casing.

[0057] The pressure of the natural gas stream may vary depending on its origin. In the preboost configurations disclosed herein, the natural gas stream may be provided to the first compressor at a first pressure of about 85 bar or less. Preferably, the first pressure (Pl) may be about 60 bar or less, or about 50 bar or less, more preferably a first pressure ranging from about 20 bar to about 85 bar, or about 40 bar to about 80 bar, or about 50 bar to about 85 bar, or about 20 bar to about 60 bar.

[0058] In the pre-boost configurations disclosed herein, the second pressure (P2) may be about 100 bar or more following compression with the first compressor and the second compressor. Preferably, the second pressure (P2) may be about 105 bar or more, about 110 bar or more, about 120 bar or more, or about 130 bar or more, more preferably a second pressure (P2) ranging from about 100 bar to about 200 bar.

[0059] In the pre-boost configurations disclosed herein, up to about 60% of the natural gas stream by volume may be returned upstream via the recycle stream, depending on particular cooling needs for the natural gas stream or the cooled, compressed natural gas stream. In more particular instances, about 1% to about 60% of the natural gas stream by volume may be returned via the recycle stream, or about 10% to about 50% of the natural gas stream by volume, or about 20% to about 40% of the natural gas stream by volume. Up to about 10% of the natural gas stream by volume may be returned upstream in other particular instances.

[0060] In the pre-boost configurations disclosed herein, the fifth pressure (P5) may be about 65 bar to about 130 bar or about 85 bar to about 140 bar, and the fifth temperature (T5) may range from about -30°C to about 10°C. The sixth pressure (P6) and sixth temperature (T6) may also reside within similar ranges, but remain at lower values than the corresponding fifth pressure (P5) and fifth temperature (T5).

[0061] The methods may further comprise performing one or more intermediate heat exchanges of the natural gas stream between the first compressor and the second compressor. The one or more intermediate heat exchanges may take place using air cooling, water cooling, or any combination thereof.

[0062] The first compressor may be operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages.

[0063] Likewise, systems for promoting liquefaction of a natural gas stream through preboost compression and expansion may comprise: a natural gas feed connected to a first compressor and an optional second compressor that are fluidly coupled in series; a first heat exchange location downstream from the second compressor and fluidly coupled to an output of the second compressor; a second heat exchange location fluidly coupled to an output of the first heat exchange location, if present; a first expander fluidly coupled to an output of the second heat exchange location; and an output line from the first expander that is subdivided into a main line and recycle line. The recycle line is in thermal communication with the second heat exchange, and the recycle line is in fluid communication with the natural gas feed.

[0064] The second compressor and the first expander may be operatively coupled by a common drive shaft, thereby defining a turboexpander.

[0065] The first heat exchange location, if present, may comprise a heat exchanger functioning by air cooling, water cooling, or any combination thereof. The systems may further comprise one or more intermediate heat exchange locations between the first compressor and the second compressor, wherein the one or more intermediate heat exchange locations comprise a heat exchanger functioning by air cooling, water cooling, or any combination thereof.

[0066] The first compressor may be operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages. The interstage cooling may take place by air cooling, water cooling, or any combination thereof.

[0067] The systems for promoting pre-boost compression and expansion may further comprise a second expander or a Joule-Thompson valve located within the recycle line. The recycle line may be in thermal communication with the second heat exchange location downstream from the second expander or the Joule-Thompson valve.

[0068] A second expander may be located within the recycle line. When a second expander is present, the first compressor, the first expander and the second expander may collectively define a compander. System configurations employing a second expander but not configured within a compander also reside within the scope of the present disclosure.

[0069] Similarly, post-boost configurations for promoting liquefaction of a natural gas stream may comprise: providing a natural gas stream at a first temperature and first pressure to a first compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor, thereby forming a compressed natural gas stream having a second temperature; compressing the compressed natural gas stream to a third pressure higher than the second pressure using a second compressor in fluid communication with the first compressor, the compressed natural gas stream having a third temperature after being compressed to the third pressure; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the second compressor, thereby forming a cooled, compressed natural gas stream having a fourth pressure and a fourth temperature lower than the third temperature; performing a second heat exchange upon the compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fifth pressure and a fifth temperature lower than the fourth temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using an expander from about the fifth pressure to a sixth pressure lower than the fifth pressure, preferably not greater than about 140 bar, thereby forming a chilled, expanded natural gas stream having a sixth temperature lower than the fifth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to a location upstream from the first heat exchange location. The second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream. The recycle stream may be returned to the natural gas stream after interacting with the cooled, compressed natural gas stream in the second heat exchange location.

[0070] The recycle stream may be returned to the natural gas stream upstream from the first compressor, downstream from the first compressor and upstream from the second compressor, downstream from the second compressor and upstream from the first heat exchange location, if present, or any combination thereof. The recycle stream may also be returned to a location of inter-stage cooling in certain system and method configurations.

[0071] The pressure of the natural gas stream may vary depending on its origin. In the post-boost configurations disclosed herein, the natural gas stream may be provided to the first compressor at a first pressure of about 85 bar or less. Preferably, the first pressure (Pl) may be about 60 bar or less, or about 50 bar or less, more preferably a first pressure ranging from about 20 bar to about 85 bar, or about 40 bar to about 80 bar, or about 50 bar to about 85 bar, or about 20 bar to about 60 bar.

[0072] In the post-boost configurations disclosed herein, the second pressure (P2) may be about 80 bar or more, about 90 bar or more, or about 100 bar or more, more preferably a second pressure (P2) ranging from about 80 bar to about 200 bar, or about 80 bar to about 140 bar, or about 80 bar to about 120 bar.

[0073] In the post-boost configurations disclosed herein, the third pressure (P3) may be about 100 bar or more following compression with the first compressor and the second compressor. Preferably, the third pressure (P3) may be about 105 bar or more, about 110 bar or more, about 120 bar or more, or about 130 bar or more, more preferably a third pressure (P3) ranging from about 100 bar to about 200 bar.

[0074] In the post-boost configurations disclosed herein, the sixth pressure (P6) may be about 65 bar to about 130 bar or about 85 bar to about 140 bar, and the sixth temperature (T6) may range from about -30°C to about 10°C. In the post-boost configurations disclosed herein, up to about 60% of the natural gas stream by volume may be returned upstream via the recycle stream, depending on particular cooling needs for the natural gas stream or the cooled, compressed natural gas stream. In more particular instances, about 1% to about 60% of the natural gas stream by volume may be returned via the recycle stream, or about 10% to about 50% of the natural gas stream by volume, or about 20% to about 40% of the natural gas stream by volume. Up to about 10% of the natural gas stream by volume may be returned upstream in other particular instances.

[0075] Compressing to the third pressure (P3) may take place in the second compressor in at least two stages, with inter-stage cooling taking place between the at least two stages. The inter-stage cooling may take place using air cooling, water cooling, or any combination thereof. [0076] Preferably, the first compressor and the expander are operatively coupled by a common drive shaft, thereby defining a turboexpander.

[0077] Likewise, systems for promoting liquefaction of a natural gas stream through postboost compression and expansion may comprise: a natural gas feed connected to a first compressor; a second compressor fluidly coupled to an output of the first compressor; an optional first heat exchange location fluidly coupled to an output of the second compressor; a second heat exchange location downstream from the second compressor and fluidly coupled to an output of the first heat exchange location; an expander fluidly coupled to an output of the second heat exchange location; and an output line from the expander that is subdivided into a main line and recycle line. The recycle line is in thermal communication with the second heat exchange location, and the recycle line is in fluid communication with a location upstream from the first heat exchange location.

[0078] The recycle line may be in fluid communication with the natural gas stream upstream from the first compressor, upstream from the first compressor and downstream from the second compressor, downstream from the second compressor and upstream from the first heat exchange location, if present, or any combination thereof.

[0079] Preferably, the first compressor and the expander are operatively coupled by a common drive shaft, thereby defining a turboexpander.

[0080] The second compressor may be operable to compress the natural gas feed in at least two stages, with inter-stage cooling takes place between the at least two stages. The inter-stage cooling may take place using air cooling, water, cooling, or any combination thereof.

[0081] Embodiments disclosed herein include:

[0082] A. Methods for forming LNG using pre-boost compression and expansion. The methods comprise: providing a natural gas stream at a first temperature and a first pressure to a first compressor, optionally in series fluid communication with a second compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor and optionally the second compressor, thereby forming a compressed natural gas stream having a second temperature; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the first compressor and the second compressor, if present, thereby forming a cooled, compressed natural gas stream having a third pressure and a third temperature lower than the second temperature; performing a second heat exchange upon the cooled, compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fourth pressure and a fourth temperature lower than the third temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using a first expander from about the fourth pressure to a fifth pressure lower than the fourth pressure, thereby forming a chilled, expanded natural gas stream having a fifth temperature lower than the fourth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to the natural gas stream upstream from the first heat exchange location; wherein the second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream. [0083] B. Systems for forming LNG using pre-boost compression and expansion. The systems comprise: a natural gas feed connected to a first compressor and a second compressor that are fluidly coupled in series; an optional first heat exchange location fluidly coupled to an output of the second compressor; a second heat exchange location downstream from the second compressor and fluidly coupled to an output of the first heat exchange location, if present; a first expander fluidly coupled to an output of the second heat exchange location; and an output line from the first expander that is subdivided into a main line and recycle line; wherein the recycle line is in thermal communication with the second heat exchange location, and the recycle line is in fluid communication with the natural gas feed.

[0084] C. Methods for forming LNG using post-boost compression and expansion. The methods comprise: providing a natural gas stream at a first temperature and first pressure to a first compressor; compressing the natural gas stream to a second pressure higher than the first pressure using the first compressor, thereby forming a compressed natural gas stream having a second temperature; compressing the compressed natural gas stream to a third pressure higher than the second pressure using a second compressor in fluid communication with the first compressor, the compressed natural gas stream having a third temperature after being compressed to the third pressure; performing an optional first heat exchange upon the compressed natural gas stream at a first heat exchange location downstream from the second compressor, thereby forming a cooled, compressed natural gas stream having a fourth pressure and a fourth temperature lower than the third temperature; performing a second heat exchange upon the compressed natural gas stream at a second heat exchange location downstream from the first heat exchange location, thereby affording a fifth pressure and a fifth temperature lower than the fourth temperature to the cooled, compressed natural gas stream; expanding the cooled, compressed natural gas stream using an expander from about the fifth pressure to a sixth pressure lower than the fifth pressure, thereby forming a chilled, expanded natural gas stream having a sixth temperature lower than the fifth temperature; splitting the chilled, expanded natural gas stream into a main stream and a recycle stream; liquefying the main stream; and returning the recycle stream to a location upstream from the first heat exchange location; wherein the second heat exchange of the cooled, compressed natural gas stream at the second heat exchange location is with the recycle stream.

[0085] D. Systems for forming LNG using post-boost compression and expansion. The systems comprise: a natural gas feed connected to a first compressor; a second compressor fluidly coupled to an output of the first compressor; an optional first heat exchange location fluidly coupled to an output of the second compressor; a second heat exchange location downstream from the second compressor and fluidly coupled to an output of the first heat exchange location, if present; an expander fluidly coupled to an output of the second heat exchange location; and an output line from the expander that is subdivided into a main line and recycle line; wherein the recycle line is in thermal communication with the second heat exchange location, and the recycle line is in fluid communication with a location upstream from the first heat exchange location.

[0086] Embodiments A and B may have one or more of the following additional elements in any combination:

[0087] Element 1 : wherein the recycle stream is returned to the natural gas stream upstream from the first compressor, downstream from the first compressor and upstream from the second compressor and one or more intermediate heat exchange locations, downstream from the second compressor, or any combination thereof.

[0088] Element 2: wherein the first pressure is about 85 bar or less.

[0089] Element 3: wherein the second pressure is about 100 bar or more.

[0090] Element 4: wherein the first heat exchange is performed and takes place by air cooling, water cooling, or any combination thereof.

[0091] Element 5: wherein the method further comprises performing one or more intermediate heat exchanges of the natural gas stream between the first compressor and the second compressor.

[0092] Element 6: wherein the one or more intermediate heat exchanges take place using air cooling, water cooling, or any combination thereof.

[0093] Element 7: wherein the first compressor is operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages.

[0094] Element 8: wherein the method further comprises expanding the recycle stream to a sixth pressure lower than the fifth pressure, thereby cooling the recycle stream to a sixth temperature lower than the fifth temperature before returning the recycle stream to the natural gas stream; wherein the second heat exchange with the recycle stream takes place after expanding to the sixth pressure and the sixth temperature.

[0095] Element 9: wherein expanding the recycle stream takes place using a Joule- Thompson valve or a second expander downstream from the first expander. [0096] Element 10: wherein expanding the recycle stream takes place using a second expander, and the second compressor, the first expander, and the second expander collectively define a compander.

[0097] Element 11: wherein the second compressor is present.

[0098] Element 12: wherein the second compressor and the first expander are operatively coupled by a common drive shaft.

[0099] Element 13: wherein up to about 60% of the natural gas stream by volume is returned via the recycle stream.

[0100] Element 14: wherein the first compressor is driven by a drive shaft comprising a steam turbine drive.

[0101] Element 15: further comprising: liquefying the main stream in a liquefaction process, the liquefaction process using a refrigerant compressor; recovering heat from a power source of the refrigerant compressor; generating a stream of pressurized steam from the recovered heat; and powering the first compressor using at least part of the stream of pressurized steam.

[0102] Element 16: wherein the recycle line is in fluid communication with the natural gas feed upstream from the first compressor, downstream from the first compressor and upstream from the second compressor and one or more intermediate heat exchange locations, downstream from the second compressor, or any combination thereof.

[0103] Element 17: wherein the first heat exchange location is present and comprises a heat exchanger functioning by air cooling, water cooling, or any combination thereof.

[0104] Element 18: wherein the system further comprises one or more intermediate heat exchange locations between the first compressor and the second compressor, the one or more intermediate heat exchange locations comprising a heat exchanger functioning by air cooling, water cooling, or any combination thereof.

[0105] Element 19: wherein the second compressor and the first expander are operatively coupled by a common drive shaft.

[0106] Element 20: wherein the first compressor is operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages. [0107] Element 21 : wherein the system further comprises a second expander or a Joule- Thompson valve located within the recycle line; wherein the recycle line is in thermal communication with the second heat exchange location downstream from the second expander or the Joule-Thompson valve.

[0108] Element 22: wherein a second expander is located within the recycle line.

[0109] Element 23: wherein the second compressor, the first expander, and the second expander collectively define a compander.

[0110] Embodiments C and D may have one or more of the following additional elements in any combination:

[0111] Element 1 ’: wherein the recycle stream is returned to the natural gas stream upstream from the first compressor, downstream from the first compressor and upstream from the second compressor, downstream from the second compressor and upstream from the first heat exchange location, or any combination thereof.

[0112] Element 2’: wherein the first pressure is about 85 bar or less.

[0113] Element 3’: wherein the third pressure is about 100 bar or more.

[0114] Element 4’: wherein the first heat exchange is performed and takes place by air cooling, water cooling, or any combination thereof.

[0115] Element 5’: wherein the sixth pressure is not greater than about 140 bar.

[0116] Element 6’: wherein compressing to the third pressure in the second compressor takes place in at least two stages, with inter-stage cooling taking place between the at least two stages.

[0117] Element 7’: wherein the inter-stage cooling takes place by air cooling, water cooling, or any combination thereof.

[0118] Element 8’: wherein the first compressor and the expander are operatively coupled by a common drive shaft.

[0119] Element 9’: wherein up to about 60% of the natural gas stream by volume is returned via the recycle stream.

[0120] Element 10’: wherein the recycle line is returned to the natural gas stream upstream from the first compressor, downstream from the first compressor and upstream from the second compressor, downstream from the second compressor and upstream from the first heat exchange location, or any combination thereof.

[0121] Element 11’: wherein the first heat exchange location is present and comprises a heat exchanger functioning by air cooling, water cooling, or any combination thereof.

[0122] Element 12’: wherein the second compressor is operable to compress the natural gas feed in at least two stages, with inter-stage cooling taking place between the at least two stages.

[0123] Element 13’: wherein the inter-stage cooling takes place using air cooling, water cooling, or any combination thereof.

[0124] Element 14’ : wherein the first compressor and the expander are operatively coupled by a common drive shaft.

[0125] Illustrative combinations applicable to A and B may include, but are not limited to, 1 and 2; 1 and 3; 1 and 4; 1 and 5; 1, 5 and 6; 1 and 7; 1 and 8; 1, 8 and 9; 1, 8 and 10; 1 and 11; 1, 11 and 12; 1 and 13; 2 and 3; 2 and 4; 2 and 5; 2, 5 and 6; 2 and 7; 2 and 8; 2, 8 and 9; 2, 8 and 10; 2 and 11; 2, 11 and 12; 2 and 13; 3 and 4; 3 and 5; 3, 5 and 6; 3 and 7; 3 and 8; 3, 8 and 9; 3, 8 and 10; 3 and 11; 3, 11 and 12; 3 and 13; 4 and 5; 4, 5 and 6; 4 and 7; 4 and 8; 4, 8 and 9; 4, 8 and 10; 4 and 11; 4, 11 and 12; 4 and 13; 5 and 7; 5 and 8; 5, 8 and 9; 5, 8 and 10; 5 and 11; 5, 11 and 12; 5 and 13; 7 and 8; 7, 8 and 9; 7, 8 and 10; 7 and 11; 7, 11 and 12; 7 and 13; 8 and 9; 5, 8, 9 and 10; 8 and 11; 8, 11 and 12; 8 and 13; 11 and 12; 11 and 13; 12 and 13; 16 and 17; 16 and 18; 16 and 19; 16 and 20; 16, 21 and 22; 16, 22 and 23; 17 and 18; 17 and 19; 17 and 20; 17, 21 and 29; 17, 22 and 23; 18 and 19; 18 and 20; 18, 21 and 22; 18, 22 and 23; 18 and 20; 18, 21 and 22; and 18, 22 and 23.

[0126] Illustrative combinations applicable to C and D may include, but are not limited to,l’ and 2’; 1 ’ and 3’; 1’ and 4’; 1’ and 5’; 1’ and 6’; 1’ and 7’; 1 ’ and 8’; 1’ and 9’; 2’ and 3’; 2’ and 4’; 2’ and 5’; 2’ and 6’; 2’ and 7’; 2’ and 8’; 2’ and 9’; 3’ and 4’; 3’ and 5’; 3’ and 6’;

3’ and 7’; 3’ and 8’; 3’ and 9’; 4’ and 5’; 4’ and 6’; 4’ and 7’; 4’ and 8’; 4’ and 9’; 5’ and 6’;

5’ and 7’; 5’ and 8’; 5’ and 9’; 6’ and 7’; 6’ and 8’; 6’ and 9’; 6’ and 8’; 6’ and 9’; 7’ and 8’;

7’ and 9’; 8’ and 9’; 10’ and 11’; 10’ and 12’; 10’ and 13’; 10’ and 14’; 11’ and 12’; l l ’and

13’; 11’ and 14’; 12’ and 13’; 12’ and 14’; and 13’ and 14’.

[0127] To facilitate a better understanding of the disclosure herein, the following examples of various representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the present disclosure. EXAMPLES

[0128] Three simulated systems were modeled based on a total gas turbine power of 124 MW (two refrigerant gas turbines having available power of 49 MW each, and a feed gas turbine having an available power of 25.6 MW). Systems corresponding to Figures 1 A, 2 and 3 were modelled under these conditions. A comparative system with the same total power but no recycling of expanded natural gas was also modelled as well. Results are shown in Table 1.

Table 1 As shown in Table 1, expansion in the recycle stream produced more efficient operation relative to the comparative system.

[0129] All documents described herein are incorporated by reference herein for purposes of all jurisdictions where such practice is allowed, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the disclosure have been illustrated and described, various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited thereby. For example, the compositions described herein may be free of any component, or composition not expressly recited or disclosed herein. Any method may lack any step not recited or disclosed herein. Likewise, the term “comprising” is considered synonymous with the term “including.” Whenever a method, composition, element or group of elements is preceded with the transitional phrase “comprising,” it is understood that we also contemplate the same composition or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “selected from the group of consisting of,” or “is” preceding the recitation of the composition, element, or elements and vice versa.

[0130] One or more illustrative incarnations incorporating one or more invention elements are presented herein. Not all features of a physical implementation are described or shown in this application for the sake of clarity. It is understood that in the development of a physical embodiment incorporating one or more elements of the present invention, numerous implementation-specific decisions must be made to achieve the developer's goals, such as compliance with system-related, business-related, government-related and other constraints, which vary by implementation and from time to time. While a developer's efforts might be time-consuming, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in the art and having benefit of this disclosure.

[0131] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the present specification and associated claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claim, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0132] Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed, including the lower limit and upper limit. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. [0133] Therefore, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to one having ordinary skill in the art and having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present disclosure. The embodiments illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein.