NATURAL GAS LIQUEFACTION SYSTEM AND METHOD OF PRODUCING A LIQUEFIED NATURAL GAS STREAM

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

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

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

Natural gas liquefaction systems are described in US

Patent 6,658,892. In one example, two stand-alone liquefaction trains are provided, each having a cryogenic heat exchange system wherein heat exchangers are provided to provide a cooled feed gas . Each stand-alone

liquefaction train comprises a flash valve and a flash tank. The cooled feed gas exits each of the cryogenic heat exchange systems and is flashed in the flash valve within one of the single stand alone liquefaction trains before it is flown into the flash tank that belongs to the same liquefaction train. Hence, in this example there are as many flash tanks as there are stand-alone trains .

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 stand-alone

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 of the cryogenic heat exchange systems. The

LNG flows from the bottom of common flash tank.

A problem presents itself where, instead of a simple common flash tank, a more sophisticated nitrogen

separation unit which makes use of a reboiler system is selected in order to produce LNG within a desired specification of nitrogen content .

US Patent 6,014,869, for example, describes a so- called end flash unit wherein the contents of low boiling point components, in particular nitrogen, can be reduced from 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 the LNG. Heat from the pressurized liquefied natural gas stream is used to provide heat to a reboiler belonging to the fractionation column, whereby the pressurized liquefied natural gas stream itself is further cooled prior to being expanded in an expansion system.

One problem that presents itself when subjecting LNG from multiple cryogenic heat exchange systems to a single fractionation column is how to handle a potentially large turndown which occurs if for instance one of two

cryogenic heat exchange systems is taken off-line or is otherwise not operational. The problem is even worse when the other heat exchange is in turndown operation.

In accordance with a first aspect of the present invention, there is provided a natural gas liquefaction system comprising: - a first main cryogenic heat exchanger producing a pressurized first liquefied natural gas stream in a first LNG discharge line, and a second cryogenic heat exchanger producing a pressurized second liquefied natural gas stream 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 said first LNG discharge line is physically separate from said second LNG discharge line;

- an expansion arrangement arranged in fluid

communication with the first LNG discharge line and the second LNG discharge line, to receive the pressurized first liquefied natural gas stream from the first LNG discharge line and the pressurized second liquefied natural gas stream from the second LNG discharge line, and to produce at least one expanded liquefied natural gas stream comprising liquefied natural gas from the pressurized first liquefied natural gas stream from the first LNG discharge line and liquid natural gas from the pressurized second liquefied natural gas stream from the second LNG discharge line; and

- a nitrogen stripper comprising a shared distillation column provided with a gas/liquid contacting section arranged inside the shared distillation column,

wherein:

- the shared distillation column comprises a bottom liquid outlet for discharging a liquefied natural gas stream, and an overhead vapour outlet for discharging a reject vapour, and at least one lower-feed stream inlet arranged gravitationally below the gas/liquid contacting section, and an LNG inlet arranged gravitationally above the at least one lower-feed stream inlet and fluidly connected to the expansion arrangement to receive at least a part of the at least one expanded liquefied natural gas stream;

- the nitrogen stripper 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 warm side, and a first cold side in indirect heat exchanging contact with the first warm side, and wherein the first cold side comprises a first lower-feed stream outlet in fluid connection with at least one of the at least one lower-feed stream inlet and arranged to discharge a first lower-feed stream from the first heat exchange core into at least one of the at least one lower-feed stream inlet;

- the second heat exchange core comprises a second warm side, and a second cold side in indirect heat exchanging contact with the second warm side, and wherein the second cold side comprises a second lower-feed stream outlet in fluid connection with at least one of the at least one lower-feed stream inlet and arranged to discharge a second lower-feed stream from the second heat exchange core into at least one of the at least one lower-feed stream inlet; wherein

- the first warm side of the first heat exchange 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 pressurized first liquefied natural gas stream from the first main cryogenic heat exchanger to the expansion arrangement the pressurized first liquefied natural gas stream passes through the first warm side; and

- the second warm side of the second heat exchange 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 pressurized second liquefied natural gas stream from the second main cryogenic heat exchanger to the expansion arrangement the pressurized second liquefied natural gas stream passes through the second warm side.

In accordance with a second aspect of the present invention, there is provided a method of producing a liquefied natural gas stream, comprising:

- producing a pressurized first liquefied natural gas stream in a first main cryogenic heat exchanger and discharging said pressurized first liquefied natural gas stream a first LNG discharge line, and producing a pressurized second liquefied natural gas stream in a second main cryogenic heat exchanger and discharging said pressurized second liquefied natural gas stream a second

LNG discharge line, 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 pressurized first liquefied natural gas stream from the first main cryogenic heat exchanger to an expansion arrangement through a first warm side of a first heat exchange core of a reboiler system, which first heat exchange core is arranged in the first LNG discharge line between the first main cryogenic heat exchanger and the expansion arrangement, and passing the pressurized second liquefied natural gas stream from the second main cryogenic heat exchanger to the expansion arrangement through a second warm side of a second heat exchange core of said reboiler system, which second heat exchange core is arranged in the second LNG discharge line between the second main cryogenic heat exchanger and the expansion arrangement;

- transforming the pressurized first liquefied natural gas stream and the pressurized second liquefied natural gas stream into at least one expanded liquefied natural gas stream comprising liquefied natural gas from the pressurized first liquefied natural gas stream from the first LNG discharge line and liquid natural gas from the pressurized second liquefied natural gas stream from the second LNG discharge line; and

- passing at least a part of the at least one expanded liquefied natural gas stream through an LNG inlet into a shared distillation column of a nitrogen stripper, and discharging a liquefied natural gas stream through a bottom liquid outlet of the shared distillation column, and discharging a reject vapour through an overhead vapour outlet of the shared 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 pressurized first liquefied natural gas stream as a result of indirect heat exchanging 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 pressurized second liquefied natural gas stream as a result of indirect heat exchanging in the second heat exchange core;

- passing the first lower-feed stream from the first heat exchange core to and into the shared distillation column through at least one of at least one lower-feed stream inlet arranged gravitationally below the LNG inlet and gravitationally below a gas/liquid contacting section arranged inside the shared distillation column, and passing the second lower-feed stream from the second heat exchange core to and into the shared distillation column through at least one of said at least one lower-feed stream inlet .

The invention will be further illustrated hereinafter by way of example only, and with reference to the non- limiting drawing in which;

Fig. 1 schematically shows a process line-up

representing a natural gas liquefaction system and method according to a general embodiment of the invention;

Fig. 2 schematically shows a process line-up

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 line-up

representing a natural gas liquefaction system and method according to another more detailed embodiment of the invention;

Fig. 5 schematically shows a process line-up

representing a natural gas liquefaction system and method according to yet another more detailed embodiment of the invention;

Fig. 6 schematically shows a process line-up

representing a natural gas liquefaction system and method according to yet another detailed embodiment of the invention; and

Fig. 7 schematically shows a process line-up

representing a natural gas liquefaction system and method according to still another more detailed embodiment of the invention. For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components. The person skilled in the art will readily understand that, while the invention is illustrated making reference to one or more a specific combinations of features and measures, many of those features and measures are functionally independent from other features and measures such that they can be equally or similarly applied independently in other embodiments or combinations.

The presently proposed apparatus and method employ a nitrogen stripper based on a shared distillation column that performs cryogenic distillation of multiple

liquefied natural gas streams that are sourced from multiple cryogenic heat exchangers. This shared

distillation column may be a single distillation column or a plurality of relatively smaller distillation columns configured in parallel operation thus effectively functioning as a single shared column, shared by each of the multiple cryogenic heat exchangers. The shared distillation column receives at least a part of an expanded liquefied natural gas stream, and discharges a liquefied natural gas stream through a bottom liquid outlet, and a reject vapour through an overhead vapour 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 liquefied natural gas stream of the plurality of pressurized liquefied natural gas streams is sourced from a different one of multiple cryogenic heat exchangers, each through its own LNG discharge line. The distillation column is connected to a reboiler system, which comprises a separate heat exchange core for each one of the multiple pressurized liquefied natural gas streams that is dedicated to only one of these liquefied natural gas streams. In each heat exchange core the pressurized liquefied natural gas stream within a single one of the LNG discharge lines is indirectly heat exchanged, whereby giving off heat to one lower-feed stream per heat exchange core. Thus, there are at least as many lower-feed stream as there are LNG discharge lines and heat exchange cores. Each of the lower-feed streams is passed from its respective heat exchange core to and into the shared distillation column, through at least one of at least one lower-feed stream inlet.

The present invention is based on an insight that the heat exchanger in the reboiler system is the critical component which in first instance determines the turn ^¬ down capability of the nitrogen stripper. Providing a separate heat exchange core, for each one of the multiple liquefied natural gas streams, which is dedicated to only one of these liquefied natural gas streams has the advantage that when the LNG feed to the shared

distillation column is decreased because one of the associated heat exchange cores can be taken out of use if the cryogenic heat exchanger associated therewith is not producing a pressurized liquefied natural gas stream in the LNG discharge line. Consequently, the other heat exchange core(s) can each continue operation with a nominal flow rate of pressurized liquefied natural gas that is not affected by the one cryogenic heat exchanger that is not producing.

As there is a heat exchange core dedicated to each of the plurality of pressurized liquefied natural gas streams, 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 .

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

capacity .

The reboiler system may be based on any suitable type of heat exchanger, such as for example kettle 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 warm stream (which in this case corresponds to the pressurized liquefied natural gas stream within the associated LNG discharge line) . If the warm stream is interrupted, the

thermosyphon effect will terminate and the flow of the lower-feed stream will stop. If the reboiler system is not based on thermosyphon reboilers, the flow rate of the lower-feed stream from each heat exchanger core may be regulated by means of a valve and/or pump.

Notwithstanding, valves and/or pumps may also be employed in combination with thermosyphon reboilers.

Preferably, the first heat exchange core and the second heat exchange core are respectively incorporated in first and second reboiler heat exchangers that are physically and/or thermodynamically separated from each other. If the heat exchange cores would be

thermodynamically coupled, the lack of pressurized LNG in one of the warm sides would potentially cause undesired effects in the heat flow pattern potentially leading to pinching or thermo-mechanical related malfunctions.

Separated reboiler heat exchangers also facilitates flexibility in maintenance, because one side can stay in operation while the other side can be taken off line to be serviced.

Suitably, the at least one lower-feed stream inlet is arranged gravitationally below a gas/liquid contacting section arranged inside the shared distillation column. Any vapour (s) in the lower-feed stream may thus travel upward through the gas/liquid contacting section arranged inside the shared distillation column, in counter flow against any liquid that travels downward through the gas/liquid contacting section. Such gas/liquid

contacting section may take any suitable form, such as contacting trays and/or packing.

It will be understood that the invention can be embodied using any plurality of main cryogenic heat exchangers and likewise heat exchange cores . Much of the remainder of the description will make reference to first and second main heat cryogenic exchangers and first and second heat exchange cores, which represents 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 .

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 pressurized first liquefied natural gas stream in a first LNG discharge line 14. A second cryogenic heat exchanger 20 is arranged to produce a pressurized second liquefied natural gas stream in a second LNG discharge line 24. The first main cryogenic heat exchanger 10 is physically separate from the second main cryogenic heat exchanger 20, as is the first LNG discharge line 14 physically separate from the second LNG discharge line 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 pressurized first liquefied natural gas stream from the first LNG discharge line 14 and the pressurized second liquefied natural gas stream from the second LNG discharge line 24. A large variety of embodiments can suitably by used as the expansion arrangement 100, as will be further illustrated below. Generally, 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 pressurized first liquefied natural gas stream from the first LNG discharge line 14 and liquid natural gas from the pressurized second liquefied natural gas stream from the second LNG discharge line 24.

The process line-up of the system further comprises a nitrogen stripper. The nitrogen stripper 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 stripper comprises at least a shared distillation column 40 and a reboiler system cooperating with the shared distillation column 40.

The shared distillation column 40 is provided with a gas/liquid contacting section 42 that is arranged inside the shared distillation column 40. The gas/liquid contacting section may be embodied in the form of one or a plurality of trays and/or packing, which can be structured and/or unstructured. The shared distillation column 40 further comprises a bottom liquid outlet 41 for discharging a liquefied natural gas stream 90, and an overhead vapour outlet 42 for discharging a reject vapour 80. The shared distillation column 40 further comprises at least one lower-feed stream inlet 48, here shown as two lower-feed stream inlets shown at 48a and 48b. The least one lower-feed stream inlet 48 is arranged

gravitationally below the gas/liquid contacting section 42.

Furthermore, the shared distillation column 40 comprises an LNG inlet 46 arranged gravitationally above the at least one lower-feed stream inlet 48. The LNG inlet 46 is fluidly connected to the expansion

arrangement 100, to receive at least a part of the at least one expanded liquefied natural gas stream 30.

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

The first heat exchange core 16 is physically and/or thermodynamically separate 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 first and second reboiler heat exchangers, that are physically and/or thermodynamically separate from each other.

The first heat exchange core comprises a first warm side 15, and a first cold side 17 in indirect heat exchanging contact with the first warm side 15. The first cold side 17 comprises a first lower-feed stream outlet 19 that is in fluid connection with at least one (48a) of the at least one lower-feed stream inlet 48. The second heat exchange core 26 comprises a second warm side 25, and a second cold side 27 in indirect heat exchanging contact with the second warm side 25. The second cold side 27 comprises a second lower-feed stream outlet 29, which is arranged in fluid connection with at least one (48b) of the at least one lower-feed stream inlet 48. The first warm side 15 of the first heat exchange core 16 is arranged in the first LNG discharge line 14 between the first main cryogenic heat exchanger 10 and the expansion arrangement 100. The second warm side 25 of the second heat exchange core 26 is arranged in the second LNG discharge line 24 between the second main cryogenic heat exchanger 20 and the expansion arrangement 100.

In operation, in any single pass of the pressurized first liquefied natural gas stream from the first main cryogenic heat exchanger 10 to the expansion arrangement

100, the pressurized first liquefied natural gas stream passes through the first warm side 15; while at the same time the second liquefied natural gas stream, in any single pass of the pressurized second liquefied natural gas stream from the second main cryogenic heat exchanger

20 to the expansion arrangement 100, passes through the second warm side 25. A first lower-feed stream 38a is discharged from the first cold side 17 of the first heat exchange core 16. As a result of indirect heat

exchanging in the first heat exchange core 16, this first lower-feed stream 38a comprises heat obtained from the pressurized first liquefied natural gas stream. A second lower-feed stream 38b is discharged from the second cold side 27 of the second heat exchange core 26. This second lower-feed stream 38b comprises heat obtained from the pressurized second liquefied natural gas stream, as a result of indirect heat exchanging 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 shared distillation column 40 through at least one (48a) of the at least one lower-feed stream inlet (48) . Likewise, the second lower-feed stream 28b is passed from the second heat exchange core 26 to and into the shared distillation column 40 through at least one (48b) of said at least one lower-feed stream inlet 48.

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

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

liquefied natural gas stream from the first LNG discharge line 14 and liquid natural gas from the pressurized second liquefied natural gas stream from the second LNG discharge line 24.

At least a part of the at least one expanded

liquefied natural gas stream 30 passes through the LNG inlet 46 into the shared distillation column 40 of the nitrogen stripper. The liquefied natural gas stream 90 is discharged through the bottom liquid outlet 41 of the shared distillation column 40, while the reject vapour is discharged from the shared distillation column 40 through the overhead vapour outlet 42.

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

Figure 3 illustrates an embodiment of an expansion system as can 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 embodied in the form of an expander turbine. The passive expander may be provided in the form of a Joule-Thomson valve. Optionally, an auxiliary expander 106 may be arranged parallel to the dynamic expander 102, to be used instead of the dynamic expander 102 if desired or needed. The expansion arrangement 100 may comprises one or more of such expansion systems. Each of the first and second expansion systems 110 and 120 may consist of or comprise one of such expansion systems as illustrated in Figure 3.

In one group of embodiments, the expansion

arrangement may comprise a shared expansion system arranged such that both the pressurized first liquefied natural gas stream and the pressurized second liquefied natural gas stream are first mixed together and then passed jointly through the shared expansion system whereby transforming the pressurized first liquefied natural gas stream and the pressurized second liquefied natural gas stream are jointly into the at least one expanded liquefied natural gas stream 30.

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 separate from each other. The first expansion system 110 is arranged in the first LNG discharge line 14, and during operation it produces an expanded first liquefied natural gas stream 18 by expanding the pressurized first liquefied natural gas stream. The second expansion system 120 is arranged in the second LNG discharge line 24, and during operation it produces an expanded second liquefied natural gas stream 28. The expanded first liquefied natural gas stream 18 and the expanded second liquefied natural gas stream 28 are physically separate from each other.

In the embodiments that are illustrated in Figures 2 and 5-7, the expanded first liquefied natural gas stream 18 is combined with the expanded second liquefied natural gas stream 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. A combined LNG conduit 30 is arranged between the LNG stream combiner 31 and the LNG inlet 46, such that the expanded first liquefied natural gas stream and the expanded second liquefied natural gas stream are combined in the combined LNG conduit 30.

The embodiment of Figure 4, illustrates an

alternative embodiment wherein the LNG inlet comprises a first LNG sub inlet 46a and a second LNG sub inlet 46b. The at least one expanded liquefied natural gas stream comprises the expanded first liquefied natural gas stream

18 and the expanded second liquefied natural gas stream 28. The expanded first liquefied natural gas stream 18 and the expanded second liquefied natural gas stream 28 are passed into the shared distillation column 40 separately from each other through the first and second

LNG sub inlets 46a and 46b, respectively. This

alternative embodiment is illustrated as alternative to the embodiment of Figure 2, but it can be an alternative to any of the embodiments of the invention including those that are illustrated in Figures 5 to 7.

The embodiment of Figure 5 illustrates an alternative embodiment wherein the one or more reboil stream outlets are embodied in the form of a combined reboil stream outlet 47, which may be a single reboil stream outlet. A combined reboil stream can herewith be discharged into a single combined reboil stream 36, which is passed from the bottom of the shared distillation column 30 to a reboil stream splitter 37. The reboil stream splitter 37 divides the combined reboil stream 36 in said first and second reboil streams 36a and 36b, whereby the first reboil stream 36a has the same phase and composition as the second reboil stream 36b. The reboil stream splitter 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 .

Another alternative option illustrated in Figure 5 is that the first and second lower-feed streams 38a, 38b are combined in a lower-feed stream combiner 39 into a single combined lower-feed stream which is then passed from the lower-feed stream combiner 39 into the shared

distillation column 40 via one lower-feed stream inlet 48. This variant can be applied in combination with the variant involving the reboil stream splitter 37 as described in the previous paragraph or any other

embodiment including those shown in Figures 1, 2, 4, and 7. Likewise, the embodiments involving the reboil stream splitter 37 can be applied in combination with the lower- feed stream combiner 37 or any other variant such as those shown in Figures 2 and 4.

The embodiments as illustrated in Figures 6 and 7 differ from those illustrated in Figures 2, 3, and 5 in that instead of in the bottom the one or more reboil stream outlets are provided in the form of a side outlet 44 which is arranged to discharge liquids from a liquid draw off tray arranged in the shared distillation column 40 gravitationally below the LNG inlet 46 and

gravitationally above the gas/liquid contacting section 42. Of 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.

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

Examples of suitable liquefaction systems may employ single refrigerant cycle processes (usually single mixed refrigerant - SMR - processes, such as PRICO described in the paper "LNG Production on floating platforms" by K R Johnsen and P Christiansen, presented at Gastech 1998 (Dubai) , but also possible is a single component

refrigerant such as for instance the BHP-cLNG process also described in the afore-mentioned paper by Johnsen and Christiansen) ; double refrigerant cycle processes (for instance the much applied Propane-Mixed-Refrigerant process, often abbreviated C3MR, such as described in for instance US Patent 4,404,008, or for instance double mixed refrigerant - DMR - processes of which an example is described in US Patent 6,658,891, or for instance two- cycle processes wherein 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.

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); US Pat. 6,370,910 (Shell DMR) . Another suitable example of DMR is the so-called Axens LIQUEFIN process, such as described in for instance the paper entitled "LIQUEFIN: AN INNOVATIVE PROCESS TO REDUCE

LNG COSTS" by P-Y Martin et al, presented at the 22 ^nd World Gas Conference in Tokyo, Japan (2003) . Other suitable three-cycle processes include for example US Pat. 6,962,060; US 2011/185767; US Pat. 7,127,914;

AU4349385; US Pat. 5,669,234 (commercially known as optimized cascade process); US Pat. 6,253,574

(commercially known as mixed fluid cascade process); US Pat. 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) .

Particularly suitable are parallel mixed refrigerant processes, such as described for instance in US Patent 6,389,844 (Shell PMR process), US Patent application publication Nos . 2005/005635, 2008/156036, 2008/156037, or Pek et al in "LARGE CAPACITY LNG PLANT DEVELOPMENT" 14th International Conference on Liquefied Natural Gas, Doha, Qatar (21-24 March 2004); or full dependent or independent liquefaction trains such as described in for instance US Patent 6,658,892; or single trains comprising multiple parallel main cryogenic heat exchangers such as described in for instance 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) .

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

The temperature of each of the plurality of

pressurized liquefied natural gas streams is generally lower than -120 °C, preferably between -165 °C and

-120 °C, more preferably between -165 °C and -135 °C, most preferably between -160 °C and -140 °C. Each of the plurality of pressurized liquefied natural gas streams is preferably at a pressure of at least 15 bar absolute (bara) , more preferably at a pressure of between 15 bara and 120 bara, most preferably at a pressure of between 50 bara and 120 bara. Each of the plurality of pressurized liquefied natural gas streams comprises methane for at least 85 mol%, CO2 up to 150 ppm (mol) (preferably up to

50 ppm (mol) C02) and nitrogen between 1 mol% and

10 mol%. The balance typically consists of one or more of ethane, propane, butane and traces of inerts such as helium .

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

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 C0 ₂ , Hg, H ₂ S and other sulphur compounds; and one or more hydrocarbons heavier than methane such as in particular ethane, propane and butanes, and, possibly lesser amounts of pentanes and aromatic hydrocarbons.

If desired, the hydrocarbon streams may have been pre-treated to reduce and/or remove one or more of undesired components such as C0 ₂ and H ₂ S, or have

undergone other steps such as pre-pressurizing or the like. Such steps are well known to the person skilled in the art, and their mechanisms are not further discussed here. The ultimate composition of the hydrocarbon streams thus varies depending upon the type and location of the gas and the applied pre-treatment (s) .

The reject vapour preferably has a composition having relatively a higher amount of nitrogen than the at least part of the at least one expanded liquefied natural gas stream that is fed into the shared distillation column through the LNG inlet. The liquefied natural gas stream being discharged from the shared distillation column through the bottom outlet has relatively a lower amount of nitrogen than the at least part of the at least one expanded liquefied natural gas stream that is fed into the shared distillation column through the LNG inlet. The relative amount of methane in the liquefied natural gas stream being discharged from the shared distillation column through the bottom outlet is higher than the relative amount of methane in the at least part of the at least one expanded liquefied natural gas stream that is fed into the shared distillation column through the LNG inlet. Typically, the liquefied natural gas stream being discharged from the shared distillation column through the bottom outlet contains less than 1.1 mol% of nitrogen and, optionally, more than 90 mol% of methane.

The various embodiments of how the shared

distillation column 40 is incorporated in the line-up as described above are not limiting on the invention.

Alternatives such as for example described in US Patents 5,421,165; 5,893,274; 6,014,869, each modified according to the present specification whereby incorporating the use of multiple reboilers, are contemplated and herewith incorporated by reference. For instance, the one or more reboil streams may be drawn from the one or more

expansion systems, such as is described in US Patent 6,014,869, instead of from the shared distillation column 40.

The invention and the embodiments described herein can be applied in an on-shore located natural gas liquefaction plant, or an off-shore located one such as a floating natural gas liquefaction plant.

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