MX2010011500A

MX2010011500A - Dual nitrogen expansion process.

Info

Publication number: MX2010011500A
Application number: MX2010011500A
Authority: MX
Inventors: Bengt Olav Neeraas; Tor Erling Sandvik
Original assignee: Statoil Asa
Priority date: 2008-04-23
Filing date: 2009-04-23
Publication date: 2011-04-21
Also published as: US20110113825A1; CA2721494A1; GB2459484B; RU2010147631A; BRPI0911549A2; AU2009239763B2; WO2009130466A2; WO2009130466A3; AU2009239763A1; BRPI0911549A8; GB0807447D0; RU2496066C2; NO20101634A1; GB2459484A

Abstract

A method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams. The liquefaction can occur in three stages: in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas; in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas.

Description

DUAL NITROGEN EXPANSION PROCESS Description of the invention The present invention relates to a natural gas liquefaction process in which nitrogen is used as the main refining component. The process is particularly, but not exclusively, suitable for maritime use.

Natural gas can be obtained from the earth to form a natural gas feed that must be processed before it can be used commercially. The gas often liquefies before being transported to its point of use. This allows the gas volume to be reduced by approximately 600 times, which greatly reduces the costs associated with gas storage and transportation. Since natural gas is a mixture of gases, it liquefies in a range of temperatures. At atmospheric pressure, the normal temperature range at which liquefaction occurs is between -165 ° C and 155 ° C. Since the critical temperature of the natural gas is about -80 ° C to -90 ° C, the gas can not be liquefied only by compressing it. Therefore it is necessary to use cooling processes.

It is also known to cool the natural gas using heat exchangers in which a gaseous refrigerant is used. A known method comprises several Ref .: 214829 cooling circuits, typically three, in the form of a waterfall. In such cascades, cooling can be provided by methane, ethane and propane, or other hydrocarbons, with each cycle of the cascade operating at a lower temperature than the latter.

In each cycle cold and compressed refrigerant is expanded, causing additional cooling, and then it is fed to a heat exchanger where it is placed in direct contact with natural gas. The heat of the natural gas is heated and often vaporizes the refrigerant, thus cooling the natural gas. The heated coolant exits the heat exchanger and is then compressed and cooled, and then the cycle is repeated. Often the compressed refrigerant is cooled in the same heat exchanger as the natural gas, that is, the compressed refrigerant is cooled by the same refrigerant in an expanded form.

In a cascade system, in addition to cooling the natural gas, each cycle is also used to cool the refrigerants of the cooler subsequent refrigeration cycles. This cooling can take place in the same heat exchanger · as the cooling of the natural gas or in a separate heat exchanger.

A cascade arrangement using mixed refrigerant streams is described in WO 98/48227.

It will be appreciated that the use of hydrocarbons as refrigerants has a safety problem and this is particularly significant in the maritime environment, where it is highly desirable to have large inventories of liquid hydrocarbons in what is inevitably a confined space. ...

Several systems have been proposed in which carbon dioxide acts as a reflectant fluid. For example, US 6023942 describes a natural gas liquefaction process in which carbon dioxide can be used as a refrigerant. However, this process is not always suitable for large-scale or maritime applications, since it does not depend on a cascade arrangement but on an open circuit expansion process as the main means of cooling the LNG stream (liquefied natural gas). ). Expansion processes such as these do not allow sufficiently low temperatures to be obtained, and therefore the LNG has to be maintained at very high pressures to keep it in liquid form. From a safety and economic point of view, these high pressures are not suitable for the industrial production of LNG and particularly not for large scale or marine applications.

US 2003/0089125 describes the use of carbon dioxide in a closed loop cascade system for precooling natural gas. Although this pre-cooling circuit reduces the amount of hydrocarbon refrigerant required, hydrocarbons are still used in the subsequent liquefaction and subcooling cycles. This is because carbon dioxide can not be cooled to temperatures low enough to completely liquefy natural gas without solidification.

Another known alternative is to use a nitrogen refrigerant in a gas expansion process. Traditionally this has the disadvantage that the thermal efficiency of nitrogen is much lower than in a hydrocarbon-based system. In addition, because a gaseous refrigerant has a low heat transfer coefficient as compared to an evaporative refrigerant, a large heat transfer area is required to dissipate the heat expended from the process to a cooling medium.

US 6446465 describes a liquefaction process using nitrogen in which two separate streams of nitrogen are used to liquefy natural gas. The two reflow currents are compressed and cooled so that one of these streams is fed through a heat exchanger where it is cooled, along with natural gas, which has already been precooled in a separate pre-cooling system. . The cooled nitrogen stream is then expanded to further lower its temperature and used in a second heat exchanger to further cool the gas. Meanwhile, the second nitrogen stream is expanded to the same pressure as the first nitrogen stream and combined with the first stream leaving the second heat exchanger. The first and second combined refrigerant streams are then introduced back to the first heat exchanger to provide cooling to the natural gas and to the first stream of compressed nitrogen. When the natural gas is pre-cooled before reaching the nitrogen refrigeration circuit, the energy requirements of this circuit are significantly reduced. In addition, feeding the second refrigerant stream directly to the expansion medium without passing through the first heat exchanger reduces the heat transfer area in the first heat exchanger.

US 2005/0056051 discloses an additional liquefaction system in which a nitrogen refrigeration circuit is used to cool at least partially liquefied natural gas. The natural gas is precooled and substantially liquefied by means of a separate circuit in which hydrocarbons are used as a refrigerant. This substantially liquefied natural gas is then fed to the nitrogen cooling system for further cooling. This document describes various configurations for the nitrogen refrigeration circuit, all of which involve a first heat exchanger, in which expanded low-pressure nitrogen cools the LNG, and a second heat exchanger in which the expanded nitrogen and heated from the first heat exchanger is used to cool the compressed high pressure nitrogen before expansion. In several of these embodiments, the first and second nitrogen streams expand at different pressures.

Therefore, there is currently no system that provides complete LNG liquefaction without the use of hydrocarbons. Furthermore, there is a need in the industry to provide a nitrogen refrigeration system that has reduced complexity, simple operation and greater efficiency. The system would provide great benefits, particularly in relation to maritime LNG production.

In accordance with one aspect of the present invention there is provided a natural gas liquefaction method comprising first and second streams of nitrogen refrigerant, each stream undergoing a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first intermediate pressure and the second nitrogen stream is expanded to a second lower pressure, and heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in a heat exchange relationship with natural gas, where, in at least one of the heat exchanger or heat exchangers, the first and second expanded nitrogen streams are in a heat exchange relationship with natural gas and with the first and second compressed nitrogen streams.

Viewed from another aspect the present invention provides a "natural gas liquefaction apparatus comprising one or more heat exchangers for placing natural gas in a heat exchange relationship with the first and second streams of nitrogen refrigerant; one or more compressors for compressing the first and second streams of nitrogen refrigerant, a first expander for expanding the first stream of nitrogen at a first pressure and a second expander for expanding the second stream of nitrogen to a second lower pressure; it is arranged such that, in at least one of the heat exchanger or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchange relationship with the natural gas and with the first and second nitrogen streams compressed.

Therefore, in the present invention nitrogen is expanded, at two different pressures. Compared to a single-expansion process, the use of two expanders reduces the volume of nitrogen which must be expanded to the lowest pressure and compressed from it and therefore reduces the required size and energy consumption of these components.

Both the first intermediate pressure nitrogen stream and the second lower pressure nitrogen stream are used to cool the natural gas as well as the compressed nitrogen streams before their expansion. Unlike systems of previous techniques, natural gas is cooled in each nitrogen heat exchanger. In addition, in at least one heat exchanger, the expanded nitrogen streams are also used to cool the compressed streams. This ensures that the maximum amount of heat exchange occurs between the refrigerant and the natural gas and. it allows this system to operate independently, that is, without the need for additional heat exchanger circuits to cool the natural gas.

Preferably the liquefaction method comprises three stages of cooling. These consist of a final cooling stage, in which the second stream of nitrogen expands to a low pressure and is arranged in a heat exchange relationship with natural gas, an intermediate stage, in which the first stream of nitrogen it expands to an intermediate pressure and is arranged in a heat exchange relationship with the natural gas to cool the natural gas before the final stage, and an initial cooling stage in which the first and second nitrogen streams, after undergoing heating in the final and / or intermediate stages, are arranged in a heat exchange relationship with the natural gas to cool it before the intermediate stage.

Preferably the second nitrogen stream is also used in the intermediate stage, after heating in the final stage, to provide additional cooling to the natural gas. Preferably the intermediate stage also provides cooling for the second stream of compressed nitrogen, before its expansion and use in the final stage.

The use of three nitrogen stages to cool natural gas is considered inventive by itself and therefore, seen from a further aspect of the present invention provides a natural gas liquefaction method in which the natural gas is cooled by means of a first and second streams of nitrogen refrigerant, each stream undergoes a cycle of compression, cooling, expansion and heating, the method comprises three stages in which: in an initial stage the first heated and expanded nitrogen stream and the second stream of heated and expanded nitrogen are used to cool the natural gas, - at an intermediate stage the first stream of compressed nitrogen is expanded to an intermediate pressure and used to cool the natural gas; Y in a final stage the second stream of compressed nitrogen is expanded to a low pressure and used to cool the natural gas.

Preferably the second expanded nitrogen stream is also used to cool the natural gas in the intermediate stage. Preferably the second stream of compressed nitrogen is cooled in the intermediate stage.

By reusing the expanded nitrogen currents (low pressure) and heated to contribute to previous cooling stages this system can operate independently, without need for a circuit of pre-cooling or additional liquefaction. However, in some embodiments, the system can also be used with a precooler which applies external precooling (preferably in the range of 0o to -60 ° C) of the natural gas, and preferably also the nitrogen streams. Although the use of a pre-cooler increases the complexity of the system, it reduces the energy consumption of the system.

Since only one refining circuit is required, the liquefaction apparatus is quite simplified.

Although they are described as independent currents, the first and second currents do not always need to be separated. For example, it is possible for the first and second compressed streams to be combined during the first stage and during compression. It is only necessary that the currents are transported separately through all those stages of the cycle in which the currents are at different pressures.

Preferably, in the initial stage the first expanded nitrogen stream and the second expanded nitrogen stream are used to cool the first and second expanded nitrogen streams as well as the natural gas.

This increases the efficiency of the process.

Preferably, the first expanded nitrogen stream is compressed from the intermediate pressure after cooling the natural gas in the initial and intermediate stages.

This reduces the energy required in the process compression stage, since only the second expanded nitrogen stream must be compressed from the lowest pressure to a higher pressure. This is an improvement to existing processes, in which all the refrigerant must be compressed from the lowest pressure.

By intermediate pressure is meant any pressure less than the compressed nitrogen stream but greater than that of the second expanded refrigerant stream.

Preferably the intermediate pressure is in the range of 15-25 bar. Low pressure means any pressure lower than the intermediate pressure. Preferably the low pressure is in the range of 5-20 bar.

The steps described by the present invention can each occur in a single heat exchanger or in multiple heat exchangers. Alternatively, one or more stages may be combined in a single heat exchanger and it is possible for all three stages to occur in a single heat exchanger. Therefore the cooling stages are not defined by heat exchangers but by the nitrogen streams that are used to provide cooling.

Preferably the first nitrogen stream, which expands at a first intermediate pressure, comprises a larger volume of nitrogen than the second nitrogen stream. This significantly reduces the volume of nitrogen that expands at low pressure and therefore reduces the power requirements of the low pressure expander. This also reduces the energy requirements of the low pressure compressor, or the first stage of a multi-stage compressor, which is used to compress the second stream of expanded refrigerant.

In a preferred embodiment, the natural gas liquefaction method provides complete cooling of the natural gas. That is, the natural gas does not need to undergo any pre-cooling or partial liquefaction before being cooled by means of the present invention. Instead, the nitrogen refrigerant streams provide all the necessary cooling to the natural gas from the ambient temperature to the storage temperature. This simplifies the liquefaction system.

Pre-cooling refers to cooling the flow of natural gas to a temperature at which the liquefaction of C3 components begins to occur. This allows these heavier components to separate from the natural gas stream before further cooling. This is advantageous because otherwise these heavier components can "freeze" during liquefaction and impede the flow of natural gas. Typically, the pre-cooling phase of a natural gas liquefaction process cools the gas to a temperature of about -50 ° C.

Subcooling means cooling the condensed liquefied gas below the bubble point temperature.

In such "full cooling" modes the outlet temperature of the natural gas from the initial cooling stage is typically between -10 ° C and -30 ° C. After passage through the intermediate stage, in which the first nitrogen stream is expanded, the outlet temperature is typically between -70 ° C and -90 ° C.

After the final stage, in which the second nitrogen stream is expanded, the outlet temperature of the natural gas is typically in the range of -140 ° C to -160 ° C. This allows natural gas to be stored and transported in a liquid state without the requirement that it has to be pressurized. However, the methods and apparatus according to the present invention can produce liquid natural gas at elevated pressure (1 to 20 bar) with a corresponding temperature of -100 ° C to 165 ° C.

Preferably the method additionally includes the step of removing C5 + hydrocarbons from natural gas. More preferably C3 + hydrocarbons are removed. This prevents these heavier hydrocarbons from "freezing" during liquefaction. This separation stage occurs between the pre-cooling and liquefaction phases. Most commonly this point takes place in the intermediate cooling stage, however in some embodiments the separation may take place between the initial and intermediate stages.

The separation step is carried out by means of a hydrocarbon removal column. Such columns are well known in the art. As stated above, the exact location of the heavy hydrocarbon column (HHC) will depend on the temperature of the natural gas at different points in the process.

Preferably the first and second nitrogen streams are compressed in a three-stage compression process. These can be supplied by individual compressors or by means of a multi-stage compressor. In the initial compression stage the second expanded nitrogen stream is compressed at the intermediate pressure of the first expanded nitrogen stream and then cooled, preferably by means of seawater or air. The second compression stage compresses the second stream of partially compressed nitrogen and the first expanded nitrogen stream. The final compression stage compresses both the first and the second nitrogen streams. Any of these stages can be supplied by means of two or more compressors in parallel. The arrangement allows the first and second expanders to drive the third decompression stage, which makes the system more efficient. Alternatively, a two-stage compression process can be envisaged.

Seen from a further aspect the present invention provides a natural gas liquefaction apparatus arranged to carry out the method of the present invention.

Preferred embodiments of the present invention will now be described by way of example only, with reference to the accompanying figures in which: Figure 1 shows a natural gas liquefaction process comprising two nitrogen refrigerant circuits in accordance with the present invention; Figure 2 shows a further embodiment of the present invention in which a column of heavy hydrocarbons is used.

Figure 1 shows a natural gas liquefaction system 100. The feed gas enters system a. through line 1. This gas could be at room temperature or pre-cool by heat exchange with air or water. The feed gas is cooled by means of several heat exchangers 101, 102, 103 in such a way that the subcooled liquefied natural gas leaves the last heat exchanger 103 through line 4. This LNG is expanded at atmospheric pressure by means of of the expansion valve 109 and fed via line 5 to the separation column 110. The LNG exits this column 110 via a bottom 6 stream while removing any remaining gaseous elements via line 7 for further cooling.

The natural gas is cooled in the heat exchangers 101, 102, 103 by means of the first and second nitrogen streams 121, 122. These streams 121, 122 are cooled together by means of a compression system 108, which will be described later. The combined streams leave the compression system via line 22 and then divide before entering the first heat exchanger 101. Both streams of nitrogen 121, 122 are cooled in this heat exchanger 101. The first nitrogen stream 121 exits the heat exchanger via line 24 and expands to an intermediate pressure via the expander 111. The first expanded nitrogen stream 121 is then fed. via line 25 to second heat exchanger 102 as a cooling fluid. The first expanded nitrogen stream 121 cools the natural gas and the second nitrogen stream 122 in this heat exchanger 102. Upon leaving the second heat exchanger 102 the first expanded and heated nitrogen stream 121 is reintroduced to the first heat exchanger. heat 101 through line 26. Here again it acts to cool the natural gas as well as the first and second compressed nitrogen streams 121, 122. The expanded and heated nitrogen stream 121 is then directed through the line 27 to the compression system 108.

After cooling in the first heat exchanger 101 the second stream of compressed nitrogen 122 is fed through line 29 to the second heat exchanger 102 for additional cooling. After leaving the second heat exchanger 102, the second stream of compressed nitrogen 122 is fed through line 30 to the second expander 112 to expand at a pressure less than the intermediate pressure provided by the expander 111. The second stream of nitrogen cooled and expanded 122 is then fed through line 31 to third heat exchanger 103 to exchange heat with natural gas. Upon exiting the third heat exchanger 103 the second expanded and heated nitrogen stream is fed to the second heat exchanger 102 via the line 32 to aid in the cooling of the natural gas and the second stream of compressed nitrogen 122 before finally feeding into the first heat exchanger 101 by means of line 33 to aid in the cooling in the initial stage of the natural gas and the first and second compressed nitrogen streams 121, 122. After leaving the first heat exchanger 101 the second stream of expanded nitrogen and heated 122 returns to compression system 108 by means of line 10.

In the first embodiment, the compression system 108 comprises three stages of compression. The first compression stage comprises a single compressor 113 which compresses the second stream of heated low pressure nitrogen 122, supplied via line 10 from the first heat exchanger 101. The second stream of partially compressed nitrogen is then combined with the first intermediate pressure heated nitrogen stream 121 supplied by line 27. The first and second combined nitrogen streams 121, 122 are further compressed in the second compression step 114. The last compression step comprises two compressors 115a, 115b, which operate in parallel and are driven by the expanders 111, 112. The first and second combined nitrogen streams 121, 122 they are divided into lines 16, 19 and compressed in the compressors of the third stage 115a, 115b. The division of the combined nitrogen streams 121, 122 at this point does not necessarily result in the entire first stream of nitrogen passing through a third stage compressor while the second stream of nitrogen passes through the other. Instead, compressed currents can be divided at any point at this point. Between each compression stage the nitrogen is cooled by means of the heat exchangers 116, 117, 118a and 118b.After compression in the final stage the combined nitrogen streams are returned to line 22 for separation and reintroduction to the first 101 heat exchanger.

In the embodiment of the present invention described above, nitrogen streams 121 and 122 provide all the necessary cooling to natural gas. The first heat exchanger 101 cools the natural gas between -10 ° C and -30 ° C, the second heat exchanger 102 between -70 ° C and -90 ° C while the final heat exchanger 103 cools the natural gas between -140 ° C to 160 ° C. The expander 111 typically expands the first nitrogen stream 121 to a pressure of 15-20 bar, while the expander 112 typically expands the second nitrogen stream 122 to a pressure of 5-20 bar. The first and second nitrogen streams 121, 122 do not contain the same volume of nitrogen. Instead the larger flow is in the first nitrogen stream 121. This reduces the energy requirements of the low pressure expander 112 and the first stage compressor 113.

The use of expanded and heated nitrogen streams 121, 122 to continue to provide cooling in the above heat exchangers ensures that the system is efficient and allows the nitrogen to provide complete cooling without relying on any other cooling medium to provide liquefaction, partial liquefaction or pre-cooling.

Figure 2 shows a refining system 200 very similar to that of Figure 1. Identical components have been indicated by the use of the same reference numerals. Again the first and second nitrogen streams 121, 122 are cooled together by means of the compression system 108 and further cooled in the heat exchanger 101. The first stream of refrigerant 121 is then sent through line 24 to the expander 111 and it expands to an intermediate pressure. The expanded nitrogen is then fed via line 25 to the intermediate or second cooling stage. Unlike the system of Figure 1, in this system 200 the intermediate cooling stage takes place in two separate heat exchangers 202a, 202b. In both of these heat exchangers 202a, 202b the first expanded refrigerant stream 121 and the second heated and expanded refrigerant stream 122 exiting the heat exchanger 103 are used to provide cooling to the natural gas and second compressed nitrogen stream 122 - Between the heat exchangers 202a, 202b the natural gas is fed via line 2a to a Heavy Hydrocarbons Column (HHC) 219. This separates the heavier components, such as C3 +, from the natural gas stream, These components Heavier are removed via line 8 while the natural gas stream is fed via line 2b to heat exchanger 202b to continue the cooling process. Natural gas is diverted and fed through the HHC 219 at a stage in the cooling process in which pre-cooling has occurred but before liquefaction. The removal of heavier hydrocarbons at this stage prevents them from "freezing" during later parts of the cooling process.

The second nitrogen stream 122, which is cooled in the intermediate stage, is not fed through the HHC 219 but is transferred directly from the heat exchanger 202 to the heat exchanger 202b through the line 29a. Similarly, the first expanded and heated refrigerant stream and the second expanded and heated nitrogen stream are transported through the lines 25a and 32a respectively directly between the heat exchangers 202a, 202b.

The remainder of the process 200 is identical to the process 100.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A method of. natural gas liquefaction characterized in that it comprises a first and second streams of nitrogen refrigerant, each stream undergoes a cycle of compression, cooling, expansion and heating, during which the first stream of nitrogen expands at a first intermediate pressure and the second. Nitrogen stream is expanded to a second lower pressure, and heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in a heat exchange relationship with natural gas, where , in at least one of the heat exchanger or heat exchangers, the first and second expanded nitrogen streams are in a heat exchange relationship with the natural gas and with the first and second compressed nitrogen streams.

2. A method according to claim 1, characterized in that it comprises three stages of cooling; a final cooling stage, in which the second stream of nitrogen expands to a low pressure and is arranged in a heat exchange relationship with natural gas, an intermediate stage, in which the first stream of nitrogen is expanded to an intermediate pressure and is arranged in a heat exchange relationship with the natural gas to cool the natural gas before the final stage, and an initial cooling stage in which the first and second nitrogen streams, after undergoing heating in the final and / or intermediate stages, they are arranged in a heat exchange relationship with the natural gas to cool it before the intermediate stage.

3. A method according to claim 2, characterized in that the second nitrogen stream, after heating in the final stage, is used in the intermediate stage, to provide additional cooling to the natural gas.

4. A method according to claim 2 or 3, characterized in that the intermediate stage provides cooling for the second stream of compressed nitrogen, before its expansion and use in the final stage.

5. A method of natural gas liquefaction in which the natural gas is cooled by means of a first and second streams of nitrogen refrigerant, each stream undergoes a cycle of compression, cooling, expansion and heating, characterized in that it comprises three stages in which: in an initial stage the first stream of heated and expanded nitrogen and the second stream of heated and expanded nitrogen are used to cool the natural gas; in an intermediate stage the first stream of compressed nitrogen expands to an intermediate pressure and is used to cool the natural gas; Y in a final stage the second stream of compressed nitrogen is expanded to a low pressure and used to cool the natural gas.

6. A natural gas liquefaction method according to claim 5, characterized in that in the intermediate stage the first stream of compressed nitrogen is expanded to an intermediate pressure and used, together with the second stream of expanded and heated nitrogen, to cool the natural gas and the second stream of compressed nitrogen.

7. A natural gas liquefaction method according to claim 5 or 6, characterized in that the first and second compressed streams are combined during the initial stage and during compression.

8. A natural gas liquefaction method according to claim 5, 6 or 7, characterized in that in the initial stage the first expanded nitrogen stream and the second expanded nitrogen stream are used to cool the first and second streams of compressed nitrogen as well like natural gas.

9. A natural gas liquefaction method according to any of claims 5 to 8, characterized in that the first expanded nitrogen stream is compressed from the intermediate pressure after cooling the natural gas in the initial and intermediate stages.

10. A method according to any of the preceding claims, characterized in that the first stream of nitrogen comprises a greater volume of nitrogen than the second stream of nitrogen.

11. A method according to any of the preceding claims, characterized in that the first and second nitrogen streams are compressed in a three-stage compression process.

12. A method according to any of the claims. above, characterized in that it additionally comprises the step of removing C3 + hydrocarbons from natural gas after pre-cooling.

13. A method according to any of the preceding claims, characterized in that it provides a complete cooling of the natural gas.

14. A natural gas liquefaction apparatus, characterized in that it comprises: one or more heat exchangers for placing the natural gas in a heat exchange relationship with the first and second streams of nitrogen refrigerant, one or more compressors for compressing the first and second streams of nitrogen refrigerant; a first expander to expand the first stream of nitrogen to a first pressure and a second expander for expanding the second stream of nitrogen to a second lower pressure; wherein the apparatus is arranged in such a way that, in at least one heat exchanger of one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchange relationship with natural gas and with the first and second streams of compressed nitrogen.

15. A natural gas liquefaction apparatus characterized in that it is arranged to carry out the method according to any of claims 1 to 13.

16. A natural gas liquefaction apparatus according to claim 14 or 15, characterized in that it is arranged to provide complete cooling of the natural gas.