BR112016004268B1 - NATURAL GAS LIQUEFACTION SYSTEM, AND, PRODUCTION METHOD OF A LIQUEFIED NATURAL GAS CURRENT - Google Patents

Abstract

natural gas liquefaction system, and method of producing a stream of liquefied natural gas. a nitrogen extractor uses a split distillation column that performs cryogenic distillation of multiple streams of liquefied natural gas that are drawn from multiple cryogenic heat exchangers. the split distillation column receives at least a portion of an expanded liquefied natural gas stream, and discharges a stream of liquefied natural gas through a bottom liquid outlet, and a tailing vapor through an overhead vapor outlet. the expanded liquefied natural gas stream is obtained by expanding a plurality of pressurized streams of liquefied natural gas discharged by cryogenic heat exchangers. the distillation column is connected to a reboiler system, which has a separate heat exchange core for each of the multiple liquefied natural gas streams that is dedicated to just one of these liquefied natural gas streams.

Description

[001] The present invention relates to a natural gas liquefaction system and a method of producing a stream of liquefied natural gas.

[002] Liquefied natural gas (LNG) forms an economically important example of such a cryogenic hydrocarbon stream.

[003] Natural gas is a useful fuel source as well as a source of various hydrocarbon compounds. It is generally desirable to liquefy natural gas in a liquefied natural gas plant in or near a natural gas stream for several reasons. As an example, natural gas can be stored and transported over great distances more readily as a liquid than in gaseous form as it occupies a smaller volume and does not need to be stored at high pressure.

[004] Natural gas liquefaction systems are described in US Patent 6,658,892. In one example, two remote liquefaction trains are provided, each having a cryogenic heat exchange system in which heat exchangers are provided to provide a cooled feed gas. Each remote liquefaction train comprises a flash valve and flash tank. The cooled feed gas exits each of the cryogenic heat exchange systems and flashes at the flash valve within a single remote liquefaction train before being drained to the flash tank belonging to the same liquefaction train. So in this example there are as many flash tanks as there are remote trains.

[005] Another example described in the same US Patent is based on dependent liquefaction trains. Each dependent liquefaction train is comprised of a cryogenic heat exchange system, similar to the two remote liquefaction trains of the first example. However, now a common flash valve or common hydraulic turbine and a common flash tank are used to handle the cooled feed gas from both cryogenic heat exchange systems. LNG flows from the bottom of the common flash tank.

[006] A problem arises where, instead of a simple common flash tank, a more sophisticated nitrogen separation unit that makes use of a reboiler system is selected in order to produce LNG within a desired content specification. of nitrogen.

[007] US Patent 6,014,869, for example, describes a so-called final flash unit in which the contents of low-boiling components, in particular nitrogen, can be reduced by more than 2 mol% to less. than 1 mol%. US Patent 5,893,274 describes another example. In both examples, a fractionation column is used to remove nitrogen and other low-boiling constituents from LNG. Heat from the pressurized stream of liquefied natural gas is used to provide heat to a reboiler belonging to the fractionating column, where the pressurized stream of liquefied natural gas itself is further cooled before being expanded in an expansion system.

[008] One problem that presents itself when subjecting LNG from multiple cryogenic heat exchange systems to a single fractionating column is how to deal with a potentially large opening that occurs if, for example, one of the two cryogenic heat exchange systems heat is taken off-line or otherwise not operational. The problem is even worse when the other heat exchanger is in open operation.

[009] According to a first aspect of the present invention, there is provided a natural gas liquefaction system comprising: - a first main cryogenic heat exchanger producing a first stream of pressurized liquefied natural gas in a first LNG discharge line, and a second cryogenic heat exchanger producing a second stream of pressurized liquefied natural gas in a second LNG discharge line, wherein said first main cryogenic heat exchanger is physically separate from said second main cryogenic heat exchanger and wherein the said first LNG offloading line is physically separated from said second LNG offloading line; - an expansion arrangement arranged in fluid communication with the first LNG discharge line and the second LNG discharge line, to receive the first stream of pressurized liquefied natural gas from the first LNG discharge line and the second stream of pressurized liquefied natural gas from the second LNG discharge line, and to produce at least one expanded liquefied natural gas stream comprising liquefied natural gas from the first pressurized liquefied natural gas stream from the first LNG discharge line and liquid natural gas from the second pressurized liquefied natural gas stream from the second LNG discharge line; and - a nitrogen extractor comprising a split distillation column provided with a gas/liquid contact section arranged within the split distillation column, wherein: - the split distillation column comprises a bottom liquid outlet for discharging a stream of liquefied natural gas, and a top steam outlet for discharging a tailings vapour, and at least one bottom feed stream inlet arranged gravitationally below the gas/liquid contact section, and one LNG inlet arranged gravitationally above at least one lower feed stream inlet and fluidly connected with the expansion arrangement to receive at least a portion of at least one expanded liquefied natural gas stream; - the nitrogen extractor further comprises a reboiler system comprising a first heat exchange core and, in addition, a second heat exchange core; - the first heat exchange core comprises a first hot side, and a first cold side in indirect heat exchange contact with the first hot side, and wherein the first cold side comprises a first lower supply current output in communication fluid with at least one of the at least one lower feed stream input and arranged to discharge a first lower feed stream from the first heat exchange core to at least one of the at least one lower feed stream input; - the second heat exchange core comprises a second hot side, and a second cold side in indirect heat exchange contact with the second hot side, and wherein the second cold side comprises a second lower supply current output in communication fluid with at least one of the at least one lower feed stream input and arranged to discharge a second lower feed stream from the second heat exchange core to at least one of the at least one lower feed stream input; wherein - the first hot side of the first heat exchanger core is arranged in the first LNG discharge line between the first main cryogenic heat exchanger and the expansion arrangement such that in operation in any single pass of the first natural gas stream pressurized liquefied from the first main cryogenic heat exchanger to the expansion arrangement the first pressurized liquefied natural gas stream passes through the first hot side; and - the second hot side of the second heat exchanger core is arranged in the second LNG discharge line between the second main cryogenic heat exchanger and the expansion arrangement such that in operation in any single pass of the second liquefied natural gas stream pressurized from the second main cryogenic heat exchanger to the expansion arrangement the second pressurized liquefied natural gas stream passes through the second hot side.

[0010] According to a second aspect of the present invention, there is provided a method of producing a stream of liquefied natural gas, comprising: - producing a first stream of pressurized liquefied natural gas in a first main cryogenic heat exchanger and discharging the said first stream of pressurized liquefied natural gas a first LNG discharge line, and producing a second stream of pressurized liquefied natural gas in a second main cryogenic heat exchanger and discharging said second stream of pressurized liquefied natural gas to a second discharge line of LNG, wherein said first main cryogenic heat exchanger is physically separate from said second main cryogenic heat exchanger and wherein said first LNG discharge line is physically separate from said second LNG discharge line; - passing the first pressurized liquefied natural gas stream from the first main cryogenic heat exchanger to an expansion arrangement through a first hot side of a first heat exchange core of a reboiler system, first heat exchange core which is arranged in the first LNG discharge line between the first main cryogenic heat exchanger and the expansion arrangement, and passing the second pressurized liquefied natural gas stream from the second main cryogenic heat exchanger to the expansion arrangement through a second hot side of a second heat exchange core of said reboiler system, second heat exchange core which is arranged in the second LNG discharge line between the second main cryogenic heat exchanger and the expansion arrangement; - transforming the first pressurized liquefied natural gas stream and the second pressurized liquefied natural gas stream to at least one expanded liquefied natural gas stream comprising liquefied natural gas from the first pressurized liquefied natural gas stream from the first discharge line of LNG and liquid natural gas from the second pressurized liquefied natural gas stream from the second LNG discharge line; and - passing at least a portion of at least one stream of expanded liquefied natural gas through an LNG inlet to a split distillation column of a nitrogen extractor, and discharging a stream of liquefied natural gas through a liquid outlet of bottom of the split distillation column, and discharging a tailings vapor through a top vapor outlet of the split distillation column; - discharging a first lower feed stream from a first cold side of the first heat exchange core, said first lower feed stream comprising heat obtained from the first pressurized liquefied natural gas stream as a result of the heat exchange in the first heat exchange core, and discharging a second lower feed stream from a second cold side of the second heat exchange core, said second lower feed stream comprising heat obtained from the second natural gas stream liquefied pressurized as a result of indirect heat exchange in the second heat exchange core; - passing the first bottom feed stream from the first heat exchange core to and into the split distillation column through at least one of at least one bottom feed stream inlet arranged gravitationally below the LNG inlet and gravitationally below of a gas/liquid contact section arranged within the split distillation column, and passing the second lower feed stream from the second heat exchange core to and within the split distillation column through at least one of the at least a said lower supply current input.

[0011] The invention will be further illustrated herein below by way of example only, and with reference to the non-limiting drawings in which; Fig. 1 schematically shows a process alignment representing a natural gas liquefaction system and method according to the general embodiment of the invention; Fig. 2 schematically shows a process alignment representing a natural gas liquefaction system and method according to a more detailed embodiment of the invention; Fig. 3 schematically shows an example of an expansion system suitable for use in the invention; Fig. 4 schematically shows a process alignment representing a natural gas liquefaction system and method according to another more detailed embodiment of the invention; Fig. 5 schematically shows a process alignment representing a natural gas liquefaction system and method in accordance with a further more detailed embodiment of the invention; Fig. 6 schematically shows a process alignment representing a natural gas liquefaction system and method according to a further detailed embodiment of the invention; and Fig. 7 schematically shows a process alignment representing a natural gas liquefaction system and method according to a further more detailed embodiment of the invention.

[0012] For the purpose of this description, a unique reference number will be assigned to a line as well as a current carried on that line. The same reference numbers refer to similar components. The person skilled in the art will readily understand that while the invention is illustrated with reference to one or more specific combinations of features and measures, many of these features and measures are functionally independent from other features and measures such that they can be equally or similarly applied independently in other modalities or combinations.

[0013] The currently proposed apparatus and method employ a nitrogen extractor based on a split distillation column that performs cryogenic distillation of multiple streams of liquefied natural gas that are extracted from multiple cryogenic heat exchangers. This split distillation column can be a single distillation column or a plurality of relatively smaller distillation columns configured in parallel operation thus effectively functioning as a single split column, split by each of multiple cryogenic heat exchangers. The split distillation column receives at least a portion of an expanded liquefied natural gas stream, and discharges a liquefied natural gas stream through a bottom liquid outlet, and a tailing vapor through an overhead vapor outlet. The expanded liquefied natural gas stream is obtained by expanding a plurality of pressurized liquefied natural gas streams in an expansion arrangement. Each pressurized stream of liquefied natural gas of the plurality of pressurized streams of liquefied natural gas is drawn from a different multiple cryogenic heat exchanger, each through its own LNG discharge line.

[0014] The distillation column is connected with a reboiler system, which comprises a separate heat exchange core for each of multiple pressurized liquefied natural gas streams that are dedicated to only one of these liquefied natural gas streams. At each heat exchange core the pressurized stream of liquefied natural gas within a single LNG discharge line exchanges heat in an indirect manner, whereby heat is given off to a lower feed stream per heat exchange core. Thus, there are at least as many lower feed streams as there are LNG discharge lines and heat exchange cores. Each of the bottom feed streams is passed from its respective heat exchange core to and into the split distillation column through at least one of at least one bottom feed stream inlet.

[0015] The present invention is based on a knowledge that the heat exchanger in the reboiler system is the critical component that primarily determines the opening capacity of the nitrogen extractor. The provision of a separate heat exchange core for each of the multiple liquefied natural gas streams that is dedicated to only one of these liquefied natural gas streams has the advantage that when the LNG feed to the split distillation column is diminished as one of the associated heat exchange cores can be taken out of use if the cryogenic heat exchanger associated therewith is not producing a stream of pressurized liquefied natural gas in the LNG discharge line. Consequently, each of the other heat exchange cores can continue operation at a nominal flow rate of pressurized liquefied natural gas that is unaffected by a cryogenic heat exchanger that is not producing.

[0016] As there is a dedicated heat exchange core for each of a plurality of pressurized streams of liquefied natural gas, the operation of one of these heat exchange cores is decoupled from and not influenced by the operation of the other of these heat exchange cores. heat.

[0017] Furthermore, as there is a heat exchange core dedicated to each of the plurality of pressurized LNG streams, the invention can be realized by employing an expansion arrangement that for each of the LNG discharge lines has a system of separate expansion which is dedicated to that LNG discharge line. Thus, there is no need for a common expansion system such as a common flash valve or a common hydraulic turbine. An advantage of expanding each of the plurality of pressurized streams of liquefied natural gas separately (individually) is that expanders with a capacity extremely large enough to handle the plurality of pressurized streams of liquefied natural gas can be avoided, and instead the expansion arrangement can be provided with expanders where each has a smaller capacity.

[0018] The reboiler system can be based on any suitable type of heat exchanger, such as, for example, vessel heat exchangers or printed circuit heat exchangers. Suitably, each heat exchange core is a thermosyphon reboiler. An advantage of this type of reboiler is that the flow rate of the cold stream, which in this case corresponds to the lower feed stream, automatically adjusts to the amount of heat that is available from the hot stream (which in this case corresponds to the pressurized stream of liquefied natural gas within the associated LNG discharge line). If the hot stream is interrupted, the thermosyphon effect will end and the flow of the bottom feed stream will stop. If the reboiler system is not based on thermosiphon reboilers, the flow rate of the bottom feed stream from each heat exchanger core can be regulated by means of a valve and/or pump.

[0019] However, valves and/or pumps can also be used in combination with thermosyphon reboilers.

[0020] Preferably, the first heat exchange core and the second heat exchange core are respectively incorporated in the first and second reboiler heat exchangers which are physically and/or thermodynamically separated from each other. If the heat exchange cores can be thermodynamically coupled, the lack of pressurized LNG on one of the hot sides can potentially cause undesired effects on the heat flow pattern potentially leading to compression or malfunctions related to thermomechanical issues.

[0021] Separate reboiler heat exchangers also facilitate flexibility in maintenance, as one side can remain in operation while the other side can be taken out of service.

[0022] Suitably, the at least one lower feed stream input is gravitationally arranged below a gas/liquid contact section arranged within the split distillation column. Any vapor in the lower feed stream thus can travel upwards through the gas/liquid contact section arranged within the split distillation column, in counterflow against any liquid traveling downwards through the gas/liquid contact section. Such a gas/liquid contact section may take any suitable form, such as contact trays and/or packaging.

[0023] It will be understood that the invention may be embodied using any plurality of main cryogenic heat exchangers and heat exchanger cores alike. Much of the remainder of this description will refer to the first and second main cryogenic heat exchangers and the first and second heat exchanger cores, which represent the minimum required. However, more than two main cryogenic heat exchangers and likewise heat exchange cores are contemplated possibilities within the scope of the invention.

[0024] Figure 1 schematically illustrates a general embodiment of a natural gas liquefaction system according to the invention. A first main cryogenic heat exchanger 10 is arranged to produce a first stream of pressurized liquefied natural gas in a first LNG discharge line 14. A second cryogenic heat exchanger 20 is arranged to produce a second stream of pressurized liquefied natural gas in a second LNG discharge line 24. The first main cryogenic heat exchanger 10 is physically separated from the second main cryogenic heat exchanger 20, as is the first LNG discharge line 14 physically separated from the second discharge line of LNG 24. An expansion arrangement 110 is arranged in fluid communication with the first LNG discharge line 14 and the second LNG discharge line 24, to receive the first stream of pressurized liquefied natural gas from the first discharge line. of LNG 14 and the second stream of pressurized liquefied natural gas from the second LNG 24 discharge line. A variety of embodiments may suitably be used as the expansion arrangement 100, as will be further illustrated below. In general, the function of the expansion arrangement 100 is to produce at least one expanded liquefied natural gas stream 30 comprising liquefied natural gas from the first pressurized liquefied natural gas stream from the first LNG discharge line 14 and natural gas liquid from the second pressurized liquefied natural gas stream from the second LNG discharge line 24.

[0025] The process alignment of the system additionally comprises a nitrogen extractor. The nitrogen extractor consists of a number of functional parts, not all of which have been shown in the illustration of Figure 1. In the context of the present invention, the nitrogen extractor comprises at least a split distillation column 40 and a reboiler system cooperating with the split distillation column 40.

[0026] The split distillation column 40 is provided with a gas/liquid contact section 42 which is arranged within the split distillation column 40. The gas/liquid contact section may be incorporated in the form of one or a plurality of of trays and/or packaging, which can be structured and/or unstructured. The split distillation column 40 further comprises a bottom liquid outlet 41 for discharging a stream of liquefied natural gas 90, and a top vapor outlet 42 for discharging a tailing vapor 80. The split distillation column 40 further comprises at least minus one bottom feed current input 48, shown here as two bottom feed current inputs shown at 48a and 48b. At least one lower supply current input 48 is gravitationally arranged below the gas/liquid contact section 42.

[0027] Additionally, the split distillation column 40 comprises an LNG inlet 46 gravitationally arranged above at least one lower feed stream inlet 48. The LNG inlet 46 is fluidly connected with the expansion arrangement 100 to receive at least a portion of at least one stream of expanded liquefied natural gas 30.

[0028] The reboiler system comprises a first heat exchange core 16 and, in addition, a second heat exchange core 26.

[0029] The first heat exchange core 16 is physically and/or thermodynamically separated from the second heat exchange core 26. Suitably, the first heat exchange core 16 and the second heat exchange core 26 are respectively incorporated in the first and second reboiler heat exchangers, which are physically and/or thermodynamically separated from each other.

[0030] The first heat exchange core comprises a first hot side 15, and a first cold side 17 in indirect heat exchange contact with the first hot side 15. The first cold side 17 comprises a first supply current output bottom 19 that is in fluid communication with at least one (48a) of at least one bottom supply current input 48. The second heat exchange core 26 comprises a second hot side 25, and a second cold side 27 in contact with each other. indirect heat exchange with the second hot side 25. The second cold side 27 comprises a second lower power current output 29 which is arranged in fluid communication with at least one (48b) of at least one lower power current input 48. The first hot side 15 of the first heat exchanger core 16 is arranged in the first LNG discharge line 14 between the first main cryogenic heat exchanger 10 and the expansion arrangement 100. of the hot 25 of the second heat exchanger core 26 is arranged in the second LNG discharge line 24 between the second main cryogenic heat exchanger 20 and the expansion arrangement 100.

[0031] In operation, in any single pass of the first pressurized liquefied natural gas stream from the first main cryogenic heat exchanger 10 to the expansion arrangement 100, the first pressurized liquefied natural gas stream passes through the first hot side 15 ; while at the same time the second stream of liquefied natural gas, in any single pass of the second stream of pressurized liquefied natural gas from the second main cryogenic heat exchanger 20 to the expansion arrangement 100, passes through the second hot side 25. first bottom feed stream 38a is discharged from the first cold side 17 of the first heat exchange core 16. As a result of indirect heat exchange in the first heat exchange core 16, this first bottom feed stream 38a comprises heat obtained from the first stream of pressurized liquefied natural gas. A second bottom feed stream 38b is discharged from the second cold side 27 of the second heat exchange core 26. This second bottom feed stream 38b comprises heat obtained from the second stream of pressurized liquefied natural gas as a result of the indirect heat exchange in the second heat exchange core 26. The first lower feed stream 38a is passed from the first heat exchange core 16 to and into the split distillation column 40 through at least one (48a) of at least one lower supply current input (48). Likewise, second lower feed stream 28b is passed from second heat exchange core 26 to and into split distillation column 40 through at least one (48b) of said at least one feed stream inlet. bottom 48.

[0032] The first pressurized liquefied natural gas stream is produced in the first main cryogenic heat exchanger 10, and discharged from the first main cryogenic heat exchanger 10 to the first LNG discharge line 14.

[0033] The second pressurized liquefied natural gas stream is produced in the second main cryogenic heat exchanger 20, and discharged from the second main cryogenic heat exchanger 20 to the second LNG discharge line 24. The first natural gas stream pressurized liquefied natural gas stream and the second pressurized liquefied natural gas stream are transformed in expansion arrangement 100 to at least one expanded liquefied natural gas stream 30. This expanded liquefied natural gas stream 30 comprises liquefied natural gas from the first pressurized gas stream liquefied natural gas from the first LNG discharge line 14 and liquefied natural gas from the second pressurized liquefied natural gas stream from the second LNG discharge line 24.

[0034] At least a portion of the at least one pressurized stream of liquefied natural gas 30 passes through the LNG inlet 46 to the split distillation column 40 of the nitrogen extractor. The liquefied natural gas stream 90 is discharged through bottom liquid outlet 41 of split distillation column 40, while tailings vapor is discharged from split distillation column 40 through overhead vapor outlet 42.

[0035] Figures 2 and 4 to 7 illustrate in a number of non-limiting detailed embodiments how the general embodiment of Figure 1 can be put into practice. Figure 2, for example, illustrates a contemplated source of first and second lower supply currents 38a and 38b. The split distillation column 40 in the embodiment of Figure 2 is provided with one or more reboiler current outlets 47a, 47b to establish fluid communication with respectively the first and second cold sides 17, 27 of the first and second cold cores. heat exchanger 16, 17. Here first and second reboiler streams 36a and 36b can be carried from the bottom of the split distillation column 40 through the first and second cold sides 17, 27 of the first and second heat exchange cores. 16, 17 respectively to the first lower feed stream output 19 and the second lower feed stream output 29. In the first and second cold heat exchange cores 16 and 26, the first and second reboiler streams 36a and 36b undergo indirect heat exchange with respectively the first and second pressurized streams of liquefied natural gas 14, 24.

[0036] Figure 3 illustrates one embodiment of an expansion system as it may be provided as part of the expansion arrangement 100. The expansion system as illustrated comprises a dynamic expander 102 followed by a passive expander 104. The dynamic expander 102 may be incorporated in the form of an expander turbine. The passive expander can be provided in the form of a Joule-Thomson valve. Optionally, an auxiliary expander 106 can be arranged in parallel with the dynamic expander 102, to be used in place of the dynamic expander 102 if desired or necessary. Expansion array 100 may comprise one or more such expansion systems. Each of the first and second expansion systems 110 and 120 may consist of or comprise one such expansion system as illustrated in Figure 3.

[0037] In one group of embodiments, the expansion arrangement may comprise a split expansion system arranged such that both the first pressurized liquefied natural gas stream and the second pressurized liquefied natural gas stream are mixed first and then passed together. through the split expansion system in which to transform the first pressurized liquefied natural gas stream and the second pressurized liquefied natural gas stream are joined to at least one expanded liquefied natural gas stream 30.

[0038] In the embodiments of Figures 2 and 4 to 7, on the other hand, the expansion arrangement 100 comprises a first expansion system 110 and a second expansion system 120 which are physically separated from each other. The first expansion system 110 is arranged in the first LNG discharge line 14, and during operation produces a first expanded stream of liquefied natural gas 18 by expanding the first stream of pressurized liquefied natural gas. The second expansion system 120 is arranged in the second LNG discharge line 24, and during operation produces a second expanded stream of liquefied natural gas 28. The first expanded stream of liquefied natural gas 18 and the second expanded stream of liquefied natural gas 28 are physically separated from each other.

[0039] In the embodiments that are illustrated in Figures 2 and 5 to 7, the first expanded stream of liquefied natural gas 18 is combined with the second expanded stream of liquefied natural gas 28 in an LNG stream combiner 31, thereby forming the at least one expanded liquefied natural gas stream 30. The LNG stream combiner 31 is arranged between the first expansion system 110 and the LNG inlet 46, and between the second expansion system 120 and the LNG inlet 46. combined LNG pipeline 30 is arranged between the LNG stream combiner 31 and the LNG inlet 46 such that the first expanded liquefied natural gas stream and the second expanded liquefied natural gas stream are combined in the combined LNG pipeline 30.

[0040] The embodiment of Figure 4, illustrates an alternative embodiment wherein the LNG inlet comprises a first LNG subinlet 46a and a second LNG subinlet 46b. The at least one expanded liquefied natural gas stream comprises the first expanded liquefied natural gas stream 18 and the second expanded liquefied natural gas stream 28. The first expanded liquefied natural gas stream 18 and the second expanded liquefied natural gas stream 28 are passed to the split distillation column 40 separately from each other through the first and second LNG sub-entries 46a and 46b, respectively. This alternative embodiment is illustrated as an alternative to the embodiment of Figure 2, but may be an alternative to any of the embodiments of the invention including those illustrated in Figures 5 to 7.

[0041] The embodiment of Figure 5 illustrates an alternative embodiment in which the one or more reboiler current outputs are incorporated in the form of a combined reboiler current output 47, which may be a single reboiler current output. A combined reboiler stream hereafter may be discharged into a single combined reboiler stream 36, which is passed from the bottom of the split distillation column 30 to a reboiler stream divider 37. Reboiler stream divider 37 divides the combined reboiler stream 36 in said first and said second reboiler streams 36a and 36b, wherein the first reboiler stream 36a has the same phase and the same composition as the second reboiler stream 36b. The reboiler current divider 37 is in fluid communication with respectively the first and second cold sides 17, 27 of the first and second heat exchange cores 16, 17, which operate as described hereinabove.

[0042] Another alternative option illustrated in Figure 5 is that the first and second lower feed streams 38a, 38b are combined in the lower feed stream combiner 39 with a single combined lower feed stream which is then passed from a bottom feed stream combiner 39 to the split distillation column 40 via a bottom feed stream inlet 48. This variant can be applied in combination with the variant involving the reboiler stream divider 37 as described in the previous paragraph or any other embodiment including those shown in Figures 1, 2, 4, and 7. Likewise, embodiments involving reboiler current divider 37 may be applied in combination with bottom feed stream combiner 37 or any other variant such as those shown in Figures 2 and 4.

[0043] The modalities as illustrated in Figures 6 and 7 differ from those illustrated in Figures 2, 3, and 5 in that instead of at the bottom the one or more reboiler current outlets are provided in the form of a side outlet 44 which is arranged to discharge liquids from a liquid withdrawal tray arranged in the split distillation column 40 gravitationally below the LNG inlet 46 and gravitationally above the gas/liquid contact section 42. From Figures 6 and 7, Figure 6 is a variant of the embodiment as shown in Figure 5 while Figure 7 is closer similar to the variant of Figure 2.

[0044] In any of the embodiments of the invention, each of the plurality of pressurized streams of liquefied natural gas may be produced in one or more cryogenic heat exchangers including at least the main cryogenic heat exchanger in which a stream of liquefied natural gas it is cooled and finally subcooled by indirect heat exchange against a refrigerant stream 12, 22. The refrigerant stream 12, 22 can be cycled in an open cycle, a half open cycle, or a closed or essentially closed cycle. Any suitable liquefaction process or system in which a cryogenic main heat exchanger is employed can be used in combination with the presently proposed invention.

[0045] Examples of suitable liquefaction systems may employ single refrigerant cycle processes (commonly single mixed refrigerant processes - SMR - such as PRICO described in the publication "LNG Production on floating platforms" by K R Johnsen and P Christiansen, presented in Gastech 1998 (Dubai), but a single component refrigerant is also possible such as for example the BHP-cLNG process also described in the aforementioned publication by Johnsen and Christiansen); dual refrigerant cycle processes (e.g. the widely applied Propane Blended Refrigerant process, commonly abbreviated C3MR, as described, for example, in U.S. Patent 4,404,008, or for example, dual mixed refrigerant - DMR processes - of which an example is described in US Patent 6,658,891, or for example two-cycle processes where each refrigerant cycle contains a single component refrigerant); and processes based on three or more compressor trains for three or more refrigeration cycles of which an example is described in US Patent 7,114,351.

[0046] Other examples of suitable liquefaction systems are described in: US Patent 5,832,745 (Shell SMR); US Patent 6,295,833; US Patent 5,657,643 (both are variants of Black and Veatch SMR); Pat. US 6,370,910 (Shell DMR). Another suitable example of DMR is the so-called Axens LIQUEFIN process, as described for example in the publication entitled "LIQUEFIN: AN INNOVATIVE PROCESS FOR REDUCING LNG COSTS" by P-Y Martin et al, presented at the 22nd World Gas Conference in Tokyo, Japan (2003). Other suitable three-cycle processes include, for example, U.S. Pat. US 6,962,060; US 2011/185767 ; Pat. US 7,127,914; AU4349385; Pat. US 5,669,234 (commercially known as optimized waterfall process); Pat. US 6,253,574 (commercially known as a mixed fluid cascade process); Pat. US 6,308,531; US Application Publication 2008/0141711; Mark J. Roberts et al "Large capacity single train AP-X(TM) Hybrid LNG Process", Gastech 2002, Doha, Qatar (13-16 October 2002).

[0047] Parallel mixed refrigerant processes are particularly suitable, as described for example in US Patent 6,389,844 (Shell PMR process), US Patent Application Publication No. 2005/005635, 2008/156036, 2008/156037, or Pek et al in "LARGE CAPACITY LNG PLANT DEVELOPMENT" 14th International Liquefied Natural Gas Conference, Doha, Qatar (March 21-24, 2004); or complete dependent or independent liquefaction trains such as described, for example, in US Patent 6,658,892; or single trains comprising multiple parallel main cryogenic heat exchangers as described, for example, in US Patent 6,789,394, US Patent Pre-Grant Publication No. 2007/193303, or by Paradowski et al in "An LNG train capacity of 1 BSCFD is a realistic objective", Presented at GPA European Chapter Annual Meeting, Barcelona, Spain (27-29 September 2000).

[0048] These suggestions are provided to demonstrate broad applicability of the invention, and are not intended to be exclusive and/or exhaustive of the list of possibilities. Not all of the examples listed above employ gas turbines (aeroderivatives) as primary refrigerant compressor drivers. It will be clear that any driver other than gas turbines may be substituted for a gas turbine to enjoy certain preferred benefits of the present invention.

[0049] The temperature of each of the plurality of pressurized streams of liquefied natural gas is generally less than -120°C, preferably between -165°C and -120°C, more preferably between -165°C and -135°C. °C, even more preferably between -160 °C and -140 °C. Each of the plurality of pressurized streams of liquefied natural gas preferably is at a pressure of at least 1.5 MPa abs. (15 bar absolute (bara)), more preferably at a pressure between 1.5 MPa abs. (15 bar absolute (bara)) and 12 MPa abs. (120 bara), even more preferably at a pressure between 0.5 MPa abs. (50 bara) and 12 MPa abs. (120 bar). Each of the plurality of pressurized streams of liquefied natural gas comprises methane at least 85% by mol, CO2 up to 150 ppm (mol) (preferably up to 50 ppm (mol) CO2) and nitrogen between 1% by mol and 10% by mol . The equilibrium typically consists of one or more of ethane, propane, butane and traces of inerts such as helium.

[0050] The hydrocarbon streams from which the multiple streams of liquefied natural gas in the examples disclosed herein are produced can be obtained from natural gas or oil reservoirs or coal beds. These hydrocarbon streams can also be obtained from another source, including as an example a synthetic source such as a Fischer-Tropsch process, or from a mixture of different sources. Preferably the hydrocarbon streams comprise at least 50 mol% methane, more preferably at least 80 mol% methane.

[0051] Depending on their source, one or more of the hydrocarbon streams may contain varying amounts of components other than methane and nitrogen, including one or more non-hydrocarbon components other than water, such as CO2, Hg, H2S and other compounds. of sulfur; and one or more hydrocarbons heavier than methane such as in particular ethane, propane and butanes, and possibly smaller amounts of pentanes and aromatic hydrocarbons.

[0052] If desired, the hydrocarbon streams may have been pre-treated to reduce and/or remove one or more of the unwanted components such as CO2 and H2S, or have gone through other steps such as pre-pressurization or the like. Such steps are well known to the person skilled in the art, and their mechanisms are not discussed further here. the final composition of the hydrocarbon streams thus varies depending on the type and location of the gas and the pre-treatments applied.

[0053] The tailings vapor preferably has a composition having a relatively greater amount of nitrogen than the at least part of at least one expanded liquefied natural gas stream that is fed to the split distillation column through the LNG inlet. The liquefied natural gas stream being discharged from the distillation column split through the bottom outlet has a relatively lower amount of nitrogen than at least part of at least one expanded liquefied natural gas stream that is fed to the distillation column. divided through the LNG inlet. The relative amount of methane in the liquefied natural gas stream being discharged from the distillation column split through the bottom outlet is greater than the relative amount of methane in at least part of at least one expanded liquefied natural gas stream that is fed to the split distillation column through the LNG inlet. Typically, the liquefied natural gas stream being discharged from the distillation column split through the bottom outlet contains less than 1.1 mol% nitrogen and optionally more than 90 mol% methane.

[0054] The various modalities of how the split distillation column 40 is incorporated into the alignment as described above are not limiting on the invention.

[0055] Alternatives such as, for example, described in US Patents 5,421,165; 5,893,274; 6,014,869, each modified in accordance with the present specification which incorporates the use of multiple reboilers, are contemplated and incorporated herein by reference. For example, the one or more reboiler streams may be drawn from one or more expansion systems, as described in US Patent 6,014,869, rather than from the split distillation column 40.

[0056] The invention and embodiments described herein can be applied to a natural gas liquefaction plant located onshore, or one located off-shore such as a floating natural gas liquefaction plant.

[0057] The person skilled in the art will understand that the present invention can be carried out in various ways without departing from the scope of the appended claims.

Claims (12)

1. A natural gas liquefaction system, comprising: - a first main cryogenic heat exchanger (10) producing a first stream of pressurized liquefied natural gas in a first LNG discharge line (14), and a second cryogenic heat exchanger (20) producing a second stream of pressurized liquefied natural gas in a second LNG discharge line (24), characterized in that: - first main cryogenic heat exchanger (10) is physically separated from the second cryogenic heat exchanger ( 20) and wherein the first LNG offloading line (14) is physically separated from the second LNG offloading line (24); - an expansion arrangement (110) arranged in fluid communication with the first LNG discharge line (14) and the second LNG discharge line (24), for receiving the first pressurized liquefied natural gas stream from the first line LNG discharge line (14) and the second pressurized liquefied natural gas stream from the second LNG discharge line (24), and to produce at least one expanded liquefied natural gas stream (30) comprising liquefied natural gas from the first pressurized liquefied natural gas stream from the first LNG discharge line (14) and liquefied natural gas from the second pressurized liquefied natural gas stream from the second LNG discharge line (24); and - a nitrogen extractor comprising a split distillation column (40) provided with a gas/liquid contact section (42) arranged within the split distillation column (40), wherein: - the split distillation column (40) ) comprises a bottom liquid outlet (41) for discharging a stream of liquefied natural gas (90), and a top vapor outlet (42) for discharging a tailings vapor (80), and at least one stream inlet bottom feed (48) arranged gravitationally below the gas/liquid contact section (42), and an LNG inlet (46) arranged gravitationally above at least one bottom feed stream inlet (48) and fluidly connected with the expansion arrangement (110) for receiving at least a portion of at least one expanded liquefied natural gas stream (30); - the nitrogen extractor further comprises a reboiler system comprising a first heat exchange core (16) and, in addition, a second heat exchange core (26); - the first heat exchange core (16) comprises a first hot side (15), and a first cold side (17) in indirect heat exchange contact with the first hot side (15), and wherein the first side cold (17) comprises a first lower feed stream output (19) in fluid communication with at least one of at least one lower feed stream input (48) and arranged to discharge a first lower feed stream (38a) to from the first heat exchange core (16) to at least one of the at least one lower supply current input (48); - the second heat exchange core (26) comprises a second hot side (25), and a second cold side (27) in indirect heat exchange contact with the second hot side (25), and wherein the second side cold (27) comprises a second lower feed stream output (29) in fluid communication with at least one of the at least one lower feed stream input (48) and arranged to discharge a second lower feed stream (38b) to from the second heat exchange core (26) to at least one of the at least one lower supply current input (48); wherein - the first hot side (15) of the first heat exchanger core (16) is arranged in the first LNG discharge line (14) between the first main cryogenic heat exchanger (10) and the expansion arrangement (110) ) such that in operation in any single pass of the first pressurized liquefied natural gas stream from the first main cryogenic heat exchanger (10) to the expansion arrangement (110) the first pressurized liquefied natural gas stream passes through the first side hot (15); and - the second hot side (25) of the second heat exchanger core (26) is arranged in the second LNG discharge line (24) between the second main cryogenic heat exchanger (20) and the expansion arrangement (110) such that in operation in any single pass of the second pressurized liquefied natural gas stream from the second main cryogenic heat exchanger (20) to the expansion arrangement (110) the second pressurized liquefied natural gas stream passes through the second hot side (25). 2. Natural gas liquefaction system according to claim 1, characterized in that the first heat exchange core (16) is physically and/or thermodynamically separated from the second heat exchange core (26). 3. Natural gas liquefaction system according to any one of claims 1 or 2, characterized in that the first heat exchange core (16) and the second heat exchange core (26) are respectively incorporated into the first and in the second reboiler heat exchangers which are physically and/or thermodynamically separated from each other. 4. Natural gas liquefaction system according to claim 3, characterized in that the first and second reboiler heat exchangers are thermosiphon heat exchangers. 5. Natural gas liquefaction system according to any one of claims 1 to 4, characterized in that the expansion arrangement (110) comprises: - a first expansion system (110) arranged in the first LNG discharge line (14) to produce a first expanded stream of liquefied natural gas (18); and - a second expansion system (120) arranged in the second LNG discharge line (24) to produce a second expanded stream of liquefied natural gas (28), - wherein the at least one stream of expanded liquefied natural gas (30) ) comprises the first expanded stream of liquefied natural gas (18) and the second expanded stream of liquefied natural gas (28) physically separated from the first expanded stream of liquefied natural gas (18), and wherein at least the first expansion system ( 110) is physically separate from the second expansion system (120). 6. Natural gas liquefaction system according to claim 5, characterized in that it additionally comprises an LNG stream combiner (31) arranged between the first expansion system (110) and the LNG inlet (46), and between the second expansion system (120) and the LNG inlet (46), and a combined LNG pipeline arranged between the LNG stream combiner (31) and the LNG inlet (46) such that the first expanded stream of liquefied natural gas (18) and the second expanded stream of liquefied natural gas (28) are combined in the combined LNG pipeline. 7. A method of producing a stream of liquefied natural gas (90), comprising: - producing a first stream of pressurized liquefied natural gas in a first main cryogenic heat exchanger (10) and discharging the first stream of pressurized liquefied natural gas a first LNG discharge line (14), and producing a second stream of pressurized liquefied natural gas in a second main cryogenic heat exchanger (20) and discharging the second pressurized liquefied natural gas stream to a second LNG discharge line (24). ), characterized by the fact that: the first main cryogenic heat exchanger (10) is physically separated from the second main cryogenic heat exchanger (20) and wherein the first LNG discharge line (14) is physically separated from the second line LNG discharge (24); - passing the first stream of pressurized liquefied natural gas from the first main cryogenic heat exchanger (10) to an expansion arrangement (110) through a first hot side (15) of a first heat exchanger core (16) of a reboiler system, first heat exchanger core (16) which is arranged in the first LNG discharge line (14) between the first main cryogenic heat exchanger (10) and the expansion arrangement (110), and passing the second stream of pressurized liquefied natural gas from the second main cryogenic heat exchanger (20) to the expansion arrangement (110) through a second hot side (25) of a second heat exchanger core (26) of the reboiler system, second heat exchanger core (26) which is arranged in the second LNG discharge line (24) between the second main cryogenic heat exchanger (20) and the expansion arrangement (110); - transforming the first pressurized liquefied natural gas stream and the second pressurized liquefied natural gas stream to at least one expanded liquefied natural gas stream (30) comprising liquefied natural gas from the first pressurized liquefied natural gas stream from the first discharge line for LNG (14) and liquid natural gas from the second pressurized liquefied natural gas stream from the second LNG discharge line (24); and - passing at least a portion of at least one stream of expanded liquefied natural gas (30) through an LNG inlet (46) to a split distillation column (40) of a nitrogen extractor, and discharging a stream of gas natural liquid (90) through a bottom liquid outlet (41) of the split distillation column (40), and discharging a tailing vapor (80) through a top vapor outlet (42) of the split distillation column (40); - discharging a first lower feed stream (38a) from a first cold side (17) of the first heat exchange core (16), the first lower feed stream (38a) comprising heat obtained from the first lower feed stream (38a) pressurized liquefied natural gas as a result of indirect heat exchange in the first heat exchange core (16), and discharging a second lower feed stream (38b) from a second cold side (27) of the second heat exchange core (16). heat (26), the second lower feed stream (38b) comprising heat obtained from the second stream of pressurized liquefied natural gas as a result of indirect heat exchange in the second heat exchange core (26); - passing the first bottom feed stream (38a) from the first heat exchange core (16) to and into the split distillation column (40) through at least one of at least one bottom feed stream input ( 48) arranged gravitationally below the LNG inlet (46) and gravitationally below a gas/liquid contact section (42) arranged within the split distillation column (40), and passing the second lower feed stream (38b) to from the second heat exchange core (26) to and into the split distillation column (40) through at least one of the at least one lower feed stream inlet (48). 8. Method according to claim 7, characterized in that the first heat exchange core (16) is physically and/or thermodynamically separated from the second heat exchange core (26). Method according to either of claims 7 or 8, characterized in that passing the first stream of pressurized liquefied natural gas from the first main cryogenic heat exchanger (10) through the first hot side (15) of the first heat exchange core (16) comprises passing the first stream of pressurized liquefied natural gas through a first reboiler heat exchanger; and wherein passing the second stream of pressurized liquefied natural gas from the second main cryogenic heat exchanger (20) through the second hot side (25) of the second heat exchange core (26) comprises passing the second stream of natural gas pressurized liquid through a second reboiler heat exchanger, wherein the second reboiler heat exchanger is kept physically and/or thermodynamically separate from the first reboiler heat exchanger. 10. Method according to claim 9, characterized in that the first and second reboiler heat exchangers are thermosiphon heat exchangers in which the flows of the first and second lower feed streams are driven by a thermosyphon effect heat driven from the first and second pressurized streams of liquefied natural gas, respectively. 11. Method according to any one of claims 7 to 10, characterized in that transforming the first stream of pressurized liquefied natural gas and the second stream of pressurized liquefied natural gas to at least one stream of expanded liquefied natural gas (30) comprises: - keeping the second pressurized liquefied natural gas stream physically separate from the first pressurized liquefied natural gas stream; - expanding the first pressurized liquefied natural gas stream in a first expansion system (110) arranged in the first LNG discharge line (14) in this way transforming the first pressurized liquefied natural gas stream into a first expanded liquefied natural gas stream (18), and expanding the second pressurized liquefied natural gas stream in a second expansion system (120) arranged in the second LNG discharge line (24) thereby transforming the second pressurized liquefied natural gas stream into a second expanded stream. of liquefied natural gas (28), wherein the at least one expanded liquefied natural gas stream (30) comprises the first expanded liquefied natural gas stream (18) and the second expanded liquefied natural gas stream (28) physically separate from the first expanded stream of liquefied natural gas (18). 12. Method according to claim 11, characterized in that it additionally comprises combining the first expanded stream of liquefied natural gas (18) and the second expanded stream of liquefied natural gas (28) in which to form a stream of liquefied natural gas foam (30) comprising a mixture of the first expanded stream of liquefied natural gas (18) and the second expanded stream of liquefied natural gas (28); and passing at least a portion of an expanded liquefied natural gas stream (30) through the LNG inlet (46) to the split distillation column (40).

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