WO2001033145A1

WO2001033145A1 - A plant for extracting and a method for liquefying a gas

Info

Publication number: WO2001033145A1
Application number: PCT/SE2000/002159
Authority: WO
Inventors: Silvia Dahlund; Pan Min; Torbjörn RINGVALL; Per Skytt
Original assignee: Abb Ab
Priority date: 1999-11-02
Filing date: 2000-11-02
Publication date: 2001-05-10
Also published as: AU1185901A; NO995354L; NO995354D0; NO312262B1

Abstract

A plant for extracting natural gas, comprising means (1, 2, 3, 6, 9) for cooling and liquefying the extracted natural gas. The cooling and liquefying means (1, 2, 3, 6, 9) comprises a magnetic cooling device.

Description

A PLANT FOR EXTRACTING AND A METHOD FOR LIQUEFYING A GAS

FIELD OF THE INVENTION

The present invention relates to a plant for extracting natural gas, comprising means for cooling and liquefying the extracted natural gas. It also relates to a method of liquefying a natural gas, comprising the step of cooling the natural gas. The inven- tion also relates to a magnetic cooling device and a specific use thereof. Such a device could be used for the cooling and/or liquefying of any fluid, and is not necessarily restricted to natural gas.

The plant is primarily a plant for industrial extraction of natural gas, for example an offshore plant.

Also the method is adapted to the liquefying of an extracted natural gas on an industrial level.

The invention is particularly applicable in cases where it is difficult or impossible to provide a pipeline or the like for conducting the extracted natural gas directly, in a gaseous state, from the exploitation area to the consumer or consumers. In such cases, the gas is preferably cooled until it gets liquefied, and then transported in the liquid state from the exploitation area to the consumer or consumers.

PRIOR ART

According to prior art, natural gas liquids are recovered from natural gas using condensation processes, absorption processes employing hydrocarbon liquids similar to gasoline or kerosene as the absorber oil, or solid-bed adsorption processes us- ing adsorbents such as silica, molecular sieves, or activated charcoal. For condensation processes cooling can be provided by refrigeration units which frequently use vapour-compression cycles with propane as the refrigerant, or by using the Joule- Thompson expansion to lower the temperature of the feed gas, or through the use of expansion turbines which both reduce the temperature of the gas and derive work for use at other points in the recovery and separation process.

The condensation processes are generally favoured for recovering natural gas liquids. If the feed gas is very rich in liquids, plants based on simple refrigeration cycles may be used. When the liquid content of the feed gas is relatively low, use of the ex- pansion turbine may be favoured. For conditions providing a very high inlet pressure, the Joule-Thompson expansion may be more economical. Low inlet pressures generally favour an expanded plant or straight-refrigeration. Very low flow rates require a relatively simple process and may favour an automati- cally operated Joule-Thompson unit.

Absorber oil units offer the advantage that liquids can be removed at the expense of only a small pressure loss in the absorption column. If the feed gas is available at pipeline pres- sure, then little, if any, recompression is required to introduce the processed natural gas into the transmission system. However, the absorption and subsequent absorber oil regeneration process tends to be complex, favouring a simpler, more efficient expander plant.

However, such arrangements tend to get immensely large and are therefore not very suitable for offshore applications. Onshore, it is difficult to move such arrangements between different exploitation sites. Prior art compressor arrangements for liq- uefying the extracted natural gas are therefore not suitable for small gas fields where liquefying is required to make use of the gas.

OBJECT OF THE INVENTION

An object of the invention is to provide a plant for extracting natural gas which comprises means for cooling and liquefying the extracted natural gas and which can be made compact and therefore particularly suited for offshore applications, small natural gas exploitation fields onshore, and in all these applications where natural gas is extracted as a by-product, for exam- pie in connection to the extraction of oil in oil fields onshore as well as offshore. It is a particular object to reduce the size and cost of the means for cooling and liquefying the extracted natural gas in comparison to corresponding means according to prior art. The cooling and liquefying means should be of such a con- ceptual type that it can be easily transported and used on boats, trucks, etc., thereby being mobile between different exploitation sites.

SUMMARY OF THE INVENTION

The above object is achieved by means of the initially defined plant, which is characterised in that said cooling and liquefying means comprises a magnetic cooling device. Such a cooling device uses a refrigerant which upon application and removal of a magnetic field thereon presents a temperature decrease. The temperature decrease of the refrigerant is taken advantage of for the purpose of cooling the extracted natural gas. The principal function of magnetic cooling devices is known per se, but has never, as far as the applicant knows, been applied on plants for extracting natural gas, but only in different laboratory scale applications and in refrigerators. However, the applicant has realised the great advantages that can be obtained by using a magnetic cooling device for industrial scale exploitation of natural gas sources.

Accordingly, the magnetic cooling device comprises at least one cooling element which comprises a material that has an elevated magnetocaloric effect, said element being arranged to cool the natural gas upon application of and removal of a magnetic field thereon. Said element defines a refrigerant of the type mentioned above, where the material that has an elevated magneto- caloric effect preferably comprises an intermetallic alloy com- prising lanthanides, such as quaternary lanthanide, AINi inter- metallic alloys, or Gd-Ge-Si alloys. The cooling can be done directly through the action of the cooling element or indirectly via a separate heat exchanger.

The plant also comprises a means for generating a magnetic field adapted to affect said at least one cooling element for cooling the natural gas. Preferably, the means for generating the magnetic field is adapted to affect said at least one element cy- clically, thereby making it possible to cool the natural gas step- wise down to the temperature where it is finally liquefied.

According to the invention, the cooling and liquefying means comprises at least one heat exchanger and a conduit for con- ducting the natural gas to and through said at least one heat exchanger. Preferably, the cooling and liquefying means comprises a plurality of cooling elements and one or more heat exchangers, said one or more heat exchangers being connected to at least one cooling element. The cooling elements and heat ex- changers define a plurality of cooling stages arranged in series and through which heat from the natural gas is conducted step- wise via any cooling medium. Thereby, an efficient cooling of the gas down to its liquefied state is achieved, either directly by being cycled through said stages, or by exchanging heat with any of said stages via a heat exchanger. The cooling elements in the different stages may consist of different materials or be different in other ways in order to optimise the performance of the device. The cooling elements are preferably arranged in series circumferentially around the means for generating the mag- netic field, i.e. they define separate sectors of an annular wheel- shaped or barrel-shaped path that surrounds the means for generating the magnetic field. Preferably the material of each such sector, cooling element, is optimised with regard to its effect in the temperature region in which that specific cooling element is supposed to operate. According to one embodiment, at least one of said stages defines a path in which the natural gas is conducted through a cooling element and a heat exchanger. The liquefying and cooling means preferably comprises means for conducting the natu- ral gas that, after a final stage still has not been liquefied, through a heat exchanger of at least one of said stages in which there is a heat exchange between said non-liquefied gas and the gas passing through the path of that stage. Preferably, the flow of the gas is controlled in the individual stages by means of valve devices arranged in said stages.

Alternatively, or in combination with the embodiment described above, at least one of said stages defines a path in which a separate cooling medium, for example a separate gas, is con- ducted through a cooling element and a heat exchanger in which there is a heat exchange between the natural gas and said cooling medium. Accordingly, the cooling medium is preferably circulated in a series of cooling elements, at least one for each stage, and through a further heat exchanger in which it ex- changes heat with any other medium such as water or air. According to this embodiment, no natural gas is conducted through the cooling elements, but instead a pure cooling media with less contamination than in the extracted natural gas is passed through the cooling elements which are therefore subjected to less deterioration than if subjected directly to the natural gas.

Preferably, the cooling elements and the heat exchangers form a path around the means for generating the magnetic field, said means being arranged to rotate for the purpose of cyclically ap- plying a magnetic field on the cooling elements. Thereby, an advantageous, cylindrical arrangement of the cooling elements and heat exchangers around a rotating means for generating a magnetic field is provided for. It is important that, for a series of cooling elements, the magnetic field is applied sequentially to the cooling elements. This is accomplished since the cooling elements are arranged in series, defining different, individual sectors of the annular path.

The means for generating the magnetic field could comprise one or more permanent or electromagnetic magnets, but preferably comprises at least one winding which comprises a superconducting material, said winding being adapted to conduct a current for the purpose of generating the magnetic field. Thereby, a strong and easily adaptable magnetic field can be generated. Preferably, said means has a rotational axis that is generally coaxial with the longitudinal centre axis of the cylinder or the like defined by the cooling elements and the heat exchangers around said means.

There is a further object of the invention to provide a method of liquefying a natural gas, comprising the step of cooling the natural gas. The method should be particularly suited for liquefying of natural gas in an industrial scale, preferably in connection to the exploitation of natural gas fields where small, compact and efficient arrangements for cooling and liquefying the natural gas is requested or required, for example in offshore applications.

This object is achieved by means of the initially defined method, which is characterised in that the cooling of the natural gas is executed by means of a magnetic cooling device. Thereby, a magnetic field is cyclically applied to and removed from an element which comprises a material that has an elevated magneto- caloric effect, for the purpose of generating a cooling effect on the natural gas. The natural gas is either conducted through at least one heat exchanger and one or more cooling elements and directly cooled thereby, or conducted through one or more heat exchangers connected to a respective cooling element in which a separate cooling medium is circulated.

According to one embodiment, the gas is conducted at least once through a plurality of cooling stages arranged in series for a stepwise cooling of the natural gas, said natural gas being subjected to a heat exchanging step at each cooling stage. Such a method corresponds to the idea of arranging the cooling elements and heat exchangers cylindrically around a means for generating the magnetic field as described above.

Another object of the invention is to provide a magnetic cooling device comprising a plurality of cooling elements arranged in series around a means for generating a magnetic field for af- fecting said elements. The cooling device should be suitable for liquefying of gases in general, such as ammonia, hydrogen, oxygen, natural gas, methane, propane, and butane, and particularly in an industrial scale. This objective is achieved by means of a magnetic cooling device which is characterised in that the means for generating a magnetic field is rotatably arranged for the purpose of cyclically applying a magnetic field on each of said cooling elements. Thereby, the cooling elements which preferably comprise a material that has an elevated mag- netocaloric effect and that is fragile can be held in a fixed posi- tion with a minimum of mechanical load thereon. Accordingly, the reliability and simplicity of the construction of the cooling device is promoted.

The invention also relates to the use of a magnetic cooling de- vice according to the invention for cooling and liquefying a gas, such as ammonia, hydrogen, oxygen, natural gas, methane, propane, and butane.

Further advantages and features of the invention will be defined in the appended claims and in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, preferred embodiments of the present invention will be described by way of example with reference to the annexed drawings, on which fig. 1 is a schematic perspective view showing a cross-section of a magnetic cooling device according to the invention, fig. 2 is a detailed view of a part of the cooling device of fig. 1 , fig. 3 is a schematic view showing the principles for stepwise cooling of natural gas according to a first embodiment, and fig. 4 is a schematic view showing the principles for stepwise cooling of gas according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows a schematic representation of an inventive magnetic cooling device according to the invention. The cooling de- vice is particularly adapted to be used in a plant for extraction of natural gas and for the purpose of cooling and finally liquefying the extracted gas. The cooling device comprises a plurality of cooling elements 1 , each of which comprises a material which has an elevated magnetocaloric effect and one or more chan- nels for conducting a medium to be cooled through said cooling element. When a magnetic field is applied to such a material, it will be heated, and when the magnetic field is removed, its temperature will go down to its initial temperature. This is taken advantage of in the present invention. The material preferably comprises a lanthanide. Here, a Gd-Ge-Si alloy is preferred.

The cooling device also comprises a means 2 for generating a magnetic field for the purpose of affecting the cooling elements 1. In order to obtain a cooling device which is as compact and efficient as possible, the cooling elements 1 are arranged in series, encircling the means 2 for generating the magnetic field, the latter being arranged rotatably for the purpose of cyclically and sequentially applying and removing the magnetic field to and from each cooling element 1 . The means 2 may comprise a permanent magnet, be arranged like a rotor in a generator, or, as is preferred here, comprise a core of a magnetisable material, such as iron, with windings or coils 3 of a super-conducting material at opposite ends of the core. The windings or coils are arranged to conduct an electric current, thereby defining the opposite poles of the means 2.

Each cooling element 1 is designed as a block or body which permits the natural gas or any other cooling medium to flow into and through it. The cooling elements 1 are generally arranged in series, and the cooling device comprises an inlet 4 guiding the natural gas to a first cooling element 1 , and an outlet 5 for ex- tracting liquefied natural gas from a last cooling element 1 in said series. Between the cooling elements 1 pumps, valves, and possibly heat exchangers, are arranged for making it possible to execute a stepwise cooling of the natural gas from the inlet 4 to the outlet 5 through said series of cooling elements 1. In fig. 1 , such valves, pumps, and heat exchangers are only schematically represented by blocks denoted 6. By means of such valves etc. the flow of the gas can be controlled in separate, individual cooling elements or stages.

As can be seen in fig. 1 , the cooling elements 1 are elongated and form part of a cylinder surrounding the means 2 for generating the magnetic field. Said means 2 has a length corresponding to the length of the cooling elements 1 in the lengthwise direction of the latter. Each winding or coil 3 also extends in said lengthwise direction. The rotational axis x of the means 2 is generally parallel to said lengthwise direction and preferably coaxial to a centre axis of the cylinder defined by the cooling elements 1 and the blocks denoted 6.

A hood or casing 17, preferably made of electrical steel, iron or iron alloy wires or another material with similar magnetic properties, is arranged around the unit comprised by the means 2 and the cooling elements 1 for the purpose of acting as a shield preventing human operators from being directly subjected to the strong magnetic field created by the means 2. For the liquefying of a natural gas in an industrial scale, typically in a plant for extracting natural gas, the diameter of the cylinder at least partly defined by the cooling elements 1 could be in the order of 5-10 metres, while the length of the cylinder may be in the order of 10 metres. It should be understood that the cooling device preferably is arranged in connection to and held in position by a frame or the like during operation. Nevertheless, it is obvious that a magnetic cooling device of such a size has the advantage of being mobile and very small and compact in comparison to compressor arrangements according to prior art for cooling and liquefying the extracted natural gas.

Fig. 2 is a somewhat more detailed view of a section of the cylindrical cooling device shown in fig. 1 , including an end of the means 2 for generating the magnetic field. In fig. 2, it can be seen that the cooling elements 1 comprises a radial inner space 7 and a radial outer space 8 that are generally free from the material 9 which has an elevated magnetocaloric effect. Said spaces 7 and 8 are adapted to act as conductors conducting the natural gas to and from said material 9, but could also be regarded as vessels for the gathering of gas in the system. The cooling material 9 is defined by a plurality of small bodies, possibly interconnected, or a powder, defining channels through which the gas is adapted to flow, preferably in a generally radial direction, for example as shown by the arrows in fig. 2. The blocks 6 define a very schematic representation of heat exchangers, and could also, or alternatively, include other elements arranged between each cooling element, such as pumps and valves.

The means 2 for generating the magnetic field comprises windings or coils of a super-conducting conductor at opposite ends thereof, one of which is shown in fig. 2. Said windings or coils 3 are enclosed in a space which is preferably filled with cooled media, for example liquid nitrogen. A channel 10 which comprises vacuum is arranged around said windings or coils 3 and a space in which those are arranged to avoid thermal conduction or convection.

Fig. 3 shows an embodiment in which the inventive magnetic cooling device comprises a plurality of cooling elements 1 , at least one heat exchanger 1 1 , and a plurality of vessels 12, all connected to each other via a system of conduits 14 for guiding a gas through the device. The cooling device also comprises a container 13 in which liquefied gas is finally gathered.

The device also comprises a plurality of pumps 15 and valves 16 for executing a stepwise cooling of a batch of gas going through the system of cooling elements 1. Preferably, said pumps 15 and valves 16 are operatively connected to a process control means, preferably a computer comprising software for controlling the process of cooling and liquefying gas in the magnetic cooling device.

Here, only three cooling elements 1 a-c have been shown. How- ever, it should be understood that, in most cases, a larger number of cooling elements 1 might be required in order to obtain the cooling effect required for liquefying a gas.

According to the invention, a first batch of gas is introduced into the magnetic cooling device via an inlet, corresponding to the inlet 4 in fig. 1. In a first cooling stage, said batch of gas is circulated through a path going through a first cooling element 1 a and a heat exchanger 1 1 , while, simultaneously, the magnetic field is applied to the first cooling element 1 a. Heat dissipates from the cooling element 1 a to the gas and then dissipates via the heat exchanger 1 1 to some other medium, for example water, or air. The batch of gas is then gathered in a first vessel 12a connected to said path. Then, the magnetic field is removed from the first cooling element 1 a, and the batch of gas is con- ducted through the first cooling element 1 a, whereby heat from the gas dissipates to the cooling element 1 a due to the magne- tocaloric effect. Accordingly, the gas is cooled in a first cooling stage.

The magnetic field is now applied to a second cooling element 1 b, and the batch of gas is circulated through a path comprising the first cooling element 1 a and the second cooling element 1 b. Thereby, heat dissipates from the second cooling element 1 b to the batch of gas and to the first cooling element 1 a. The somewhat heated gas is gathered in a vessel 12b in said second path. Upon removal of the magnetic field on the second cooling element 1 b, the batch of gas is conducted through the second cooling element 1 b, whereby heat dissipates from the gas to the cooling element 1 b, resulting in a further cooling of the batch of gas. Now, the gas is conducted through a third path, defining a third cooling stage in the magnetic cooling device. Said third path comprises the second cooling element 1 b and a third cooling element 1 c. As the magnetic field is subsequently applied to the third cooling element 1 c, just after having been removed from the second cooling element 1 b, the batch of gas is circu- lated through said third path, whereby the gas and the second cooling element 1 b are somewhat heated due to heat dissipation from the third cooling element 1 c. The gas is gathered in a third vessel 12c. Finally, the third path is closed, and the gas gathered in the third vessel 12c is conducted through the third cool- ing element 1 c as the magnetic field is removed from that cooling element 1 c. After this final cooling stage, some of the gas is in a liquid state, and via an outlet corresponding to the outlet 5 of fig. 1 , such liquid gas is gathered and collected in the container 13. Gas that has not been liquefied may be recycled into the system, here into the third and final cooling stage, such cooled gas could, for example, be mixed with a subsequent batch of gas and contribute to the cooling of the latter.

Fig. 4 shows an alternative embodiment in which a separate cooling medium, for example a gas like H₂, N₂, He or Ar, is circulated through a plurality of cooling stages in an inventive magnetic cooling device. The cooling medium could be regarded as divided into a plurality of separate batches which are circulated in different, interconnected paths, and via which heat is dissipated from a first heat exchanger 1 1 a, in which heat is exchanged between a cooling medium and a gas to be liquefied, to a final, second heat exchanger 1 1 b, located at the opposite end of the series of cooling stages, and in which there is a heat exchange between the cooling medium and some other cooling medium, such as water, air, etc.

According to the invention, the first batch of cooling medium is circulated in a path comprising a first cooling element 1 a and said second heat exchanger 1 1 b and is then gathered or collected in a first vessel 12a during application of a magnetic field on the first cooling element 1 a. Thereby, heat dissipates from the cooling element 1 a to the cooling medium and is exchanged in the heat exchanger 1 1 b. As, subsequently, the magnetic field is removed from the first cooling element 1 a and applied to the second cooling element 1 b, the batch of cooling medium is cir- culated through a second path comprising the first and second cooling elements 1 a and 1 b. In this way, the cooling medium will be cooled stepwise until it reaches a final stage, in which it is circulated through a path comprising a final cooling element 1 c and the first heat exchanger 1 1 a, in which heat is exchanged between the cooling medium and a gas to be liquefied. Here, the cooling medium must have a temperature below the boiling temperature of the gas to be liquefied. The gas to be liquefied is guided or conducted to the container 13 via the heat exchanger 1 1 a. In the container 13, liquefied gas is gathered. Gas that has not been liquefied may be recycled and mixed with the gas which is introduced into the first heat exchanger 1 1 a, thereby contributing to a pre-cooling of the gas delivered to the heat exchanger.

The cooling elements 1 have the same function and are subjected to the same kind of magnetic field as has been described earlier in this description. The embodiment of fig. 4 could be regarded as a closed system, in which natural gas never goes through the cooling elements 1 , but merely exchanges heat therewith via at least one of the heat exchangers 1 1 . The ex- tracted natural gas is normally contaminated. The contaminants could possibly destroy and decrease the efficiency of the cooling elements 1 if the natural gas is passed through those elements 1. The embodiment according to fig. 4, however, has the advantage of subjecting the cooling elements 1 to less such con- taminants, as the cooling medium used in each cooling stage easily can be kept free from all kinds of contaminations or impurities that could damage the cooling elements 1 .

In fig. 4, which is a very schematic representation of the princi- pie described above, a plurality of pump means 15, at least one for each cooling stage and at least one for pumping the natural gas through the heat exchanger 1 1 a, have been indicated. However, it should be understood that further arrangements of pump means and valves 16 that might be required lie within the scope of the invention and only define measures that could be expected to be taken by a man skilled in the art for the purpose of controlling the process.

In figs. 3 and 4 there is also shown a so called expansion valve or flash valve 18. The valve 18 is arranged downstream the last cooling stage (Fig. 3) and it is arranged downstream the first heat exchanger 1 1 a (Fig. 4), at the outlet 5 of the respective device. By means of such a valve, for example a venturi-valve, the high pressure in the system can be reduced to atmospheric pressure. According to the Joule-Thompson effect an additional cooling effect of the gas/liquid leaving the system is achieved. This increases the overall efficiency of the device. The valve 18 may also be controlled with regard to the gas/liquid flow.

Of course, a plurality of alternative embodiments will be obvious for a man skilled in the art without leaving the scope of the in- vention, as defined in the appended claims supported by the description and the drawings.

For example, constructions that include combinations of the principles of the embodiments of fig. 3 and fig. 4 are well within the scope of the invention.

Preferably, the inventive magnetic cooling device comprises or is operatively connected to a computer means for controlling the cooling process of the device by controlling the pump devices 15 and valve devices 16 and other possible operative units that could be included in a control system for controlling the process. For example, sensor devices for measuring temperatures and pressures in one or more parts of the magnetic cooling device are preferably arranged as a part of such a control system.

Moreover, it should be understood that the inventive magnetic cooling device is preferably used for the cooling and liquefying of any medium, preferably all kinds of gases such as H₂, but most preferably extracted natural gas in a plant for extracting natural gas in an industrial scale for use in power applications.

Claims

1. A plant for extracting natural gas, comprising means (1 ,2,3,6,9) for cooling and liquefying the extracted natural gas, characterised in that said cooling and liquefying means (1 ,2,3,6,9) comprises a magnetic cooling device.

2. A plant according to claim 1 , characterised in that the magnetic cooling device comprises at least one cooling element (1 ) which comprises a material (9) that has an elevated magneto- caloric effect, said element being arranged to cool the natural gas upon application of and removal of a magnetic field thereon.

3. A plant according to claim 2, characterised in that it com- prises a means (2) for generating a magnetic field adapted to affect said at least one cooling element (1 ) for cooling the natural gas.

4. A plant according to claim 3, characterised in that the means (2) for generating a magnetic field is adapted to affect said at least one element cyclically.

5. A plant according to any one of claims 1 -4, characterised in that said cooling and liquefying means (2) comprises at least one heat exchanger (1 1 ), and a conduit (14) for conducting the natural gas to and through said at least one heat exchanger (11 ).

6. A plant according to claim 5, characterised in that the cool- ing and liquefying means (2) comprises a plurality of cooling elements and one or more heat exchangers (1 1 ), said one or more heat exchangers (1 1 ) being connected to at least one cooling element (1 ).

7. A plant according to any one of claims 1 -6, characterised in that the cooling elements (1 ) and heat exchangers (1 1 ) define a plurality of cooling stages arranged in series and through which the natural gas is conducted and cooled stepwise.

8. A plant according to claim 7, characterised in that at least one of said stages defines a path in which the natural gas is conducted through a cooling element (1 ) and a heat exchanger (1 1 ).

9. A plant according to claim 8, characterised in that it com- prises means (15, 16) for recirculating the natural gas that, after a final stage still has not been liquefied, to at least one of said stages in which there is a heat exchange between said non- liquefied gas and the gas passing through the path of that stage.

10. A plant according to any one of claims 7-9, characterised in that at least one of said stages defines a path in which a separate cooling medium is conducted through a cooling element (1 ) and a heat exchanger (1 1 ) in which there is a heat exchange between the natural gas and said cooling medium.

1 1 . A plant according to claim 3, characterised in that the cooling elements (1 ) form a path around the means (2) for generating a magnetic field and that the means (2) for generating the magnetic field is arranged to rotate for the purpose of cyclically applying a magnetic field on the cooling elements.

12. A plant according to claim 3 and any one of the preceding claims, characterised in that the means (2) for generating a magnetic field comprises at least one winding (3) comprising a super-conducting material, said winding (3) being adapted to conduct a current for the purpose of generating the magnetic field.

13. A method of liquefying a natural gas, comprising the step of cooling the natural gas, characterised in that the cooling of the natural gas is executed by means of a magnetic cooling device.

14. A method according to claim 13, characterised in, that a magnetic field is cyclically applied to and removed from an element (1 ) which comprises a material (9) which has an elevated magnetocaloric effect for the purpose of generating a cooling effect on the natural gas.

15. A method according to claim 14, characterised in that the natural gas which is cooled by said element is conducted through a heat exchanger (1 1 ).

16. A method according to any one of claims 14 or 15, characterised in that natural gas is conducted at least once through a plurality of cooling stages arranged in series for a stepwise cooling of the natural gas, said natural gas being subjected to a heat exchanging step at each cooling stage.

17. A method according to any one of claims 13-15, characterised in that one or more separate cooling media are circulated through a plurality of separate cooling stages, each of which comprises a cooling element (1 ) and that the natural gas is conducted through a heat exchanger (1 1 ) for the purpose of exchanging heat with said cooling medium or media.

18. A magnetic cooling device, comprising a plurality of cooling elements (1 ) arranged in series around a means (2) for generating a magnetic field for affecting said elements, characterised in that the means (2) for generating a magnetic field is rotatably arranged for the purpose of cyclically applying a magnetic field on each of said cooling elements (1 ).

19. A magnetic cooling device according to claim 18, characterised in that each cooling element (1 ) comprises a material (9) which has an elevated magnetocaloric effect and presents one or more channels for conducting a medium to be cooled through said cooling element (1 ).

20. A magnetic cooling device according to claim 18 or 19, characterised in that at least one heat exchanger (1 1 ) is connected to the cooling elements (1 ).

21. A magnetic cooling device according to any one of claims 19-20, characterised in that the means (2) for generating a magnetic field comprises at least one winding (3) of a conductor comprising a super-conducting material, said winding (3) being adapted to conduct an electric current for the purpose of creating a magnetic field that is applied cyclically to said elements.

22. Use of a magnetic cooling device according to any one of claims 18-21 for cooling and liquefying a gas.

23. Use according to claim 22, in which the gas comprises natural gas.