NATURAL GAS LIQUEFACTION SYSTEM INCLUDING AN INTEGRALLY-GEARED TURBO-COMPRESSOR

Abstract

According to one aspect of the present disclosure, a natural gas liquefaction system (100) is provided. The system comprises an integrally-geared turbo-compressor (150) with a plurality of compressor stages; a prime mover (160) for driving the compressor; a pre-cooling loop (110), through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages (151) of the plurality of compressor stages are adapted to pressurize the first refrigerant; a cooling loop (130), through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages (155) of the plurality of compressor stages are adapted to pressurize the second refrigerant; a first heat exchanger device (170) for transferring heat from a natural gas and/or from the second refrigerant to the first refrigerant; and a second heat exchanger device (180) for transferring heat from the natural gas to the second refrigerant. A further aspect relates to a compressor arrangement for a natural gas liquefaction system. A yet further aspect relates to a method of liquefying natural gas.

Description

TECHNICAL FIELD

The present disclosure relates to systems and methods for liquefying natural gas. More specifically, the present disclosure relates to a system for liquefying natural gas with an integrally-geared turbo-compressor as well as to a compressor arrangement including an integrally-geared turbo-compressor. Further, the present disclosure relates to a method of liquefying natural gas with an integrally-geared turbo-compressor.

BACKGROUND

Natural gas is becoming an increasingly important source of energy. In order to allow a cost-efficient transportation of the natural gas from the source of supply to the place of use, it is beneficial to reduce the volume of the gas. Cryogenic liquefaction has become a routinely practiced process for converting the natural gas into a liquid, which is more convenient, less expensive and safer to store and transport. Transportation by pipeline or ship vessels of liquefied natural gas (LNG) becomes possible at ambient pressure, by keeping the chilled and liquefied gas at a temperature lower than liquefaction temperature at ambient pressure.

In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled down to around −150 to −170° C., where the gas possesses a nearly atmospheric vapor pressure.

Several processes and systems are known for the liquefaction of natural gas, which provide for sequentially passing the natural gas at an elevated pressure through a plurality of cooling stages where the gas is cooled to successively lower temperatures by sequential refrigeration cycles until the liquefaction temperature is achieved.

Prior to passing the natural gas through the cooling stages, the natural gas is typically pretreated to remove impurities that can interfere the processing, damage the machinery or are undesired in the final product. Impurities include acid gases, sulfur compounds, carbon dioxide, mercaptans, water and mercury. The pre-treated gas from which impurities have been removed is then typically cooled by refrigerant streams to separate heavier hydrocarbons. The remaining gas mainly consists of methane and usually contains less than 0.1% hydrocarbons of higher molecular weight, such as propane or heavier hydrocarbons. The cleaned and purified natural gas is cooled down to the final temperature in a cryogenic section. The resulting LNG can be stored and transported at nearly atmospheric pressure.

Cryogenic liquefaction is usually performed by means of a multi-cycle process, i.e. a process using two or more refrigeration cycles. Depending upon the kind of process, each cycle can use a different refrigerant, or alternatively the same refrigerant can be used in two or more cycles. In a typical cryogenic liquefaction system, e.g. in the so-called APCI process, the natural gas is first cooled by a first refrigerant which circulates in a pre-cooling loop and is subsequently cooled by a second refrigerant which circulates in a cooling loop.

In the pre-cooling loop, the circulating first refrigerant may be compressed, condensed, and expanded, in order to subsequently remove heat from the natural gas. In the cooling loop, the circulating second refrigerant may be compressed and cooled, in order to subsequently remove heat from the natural gas. However, driving two cooling loops (pre-cooling loop and cooling loop) is energy-intensive, cost-intensive and space-consuming.

Accordingly, it would be beneficial to design and provide methods and systems for liquefying natural gas that provide a better energy efficiency and consume less space.

SUMMARY

In light of the above, a natural gas liquefaction system, a compressor arrangement as well as a method of liquefying natural gas are provided.

According to one aspect of the present disclosure, a natural gas liquefaction system is provided. The system includes: an integrally-geared turbo-compressor with a plurality of compressor stages; a prime mover for driving the compressor; a pre-cooling loop, through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages of the plurality of compressor stages are adapted to pressurize the first refrigerant; a cooling loop, through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages of the plurality of compressor stages are adapted to pressurize the second refrigerant; a first heat exchanger device for transferring heat from natural gas and/or from the second refrigerant to the first refrigerant; and a second heat exchanger device for transferring heat from the natural gas to the second refrigerant.

An integrally-geared turbo-compressor according to embodiments described herein includes at least one force transmission mechanism, particularly a gear, connected between two or more compressor stages of the plurality of compressor stages.

According to another aspect, a compressor arrangement for compressing a plurality of refrigerants is provided. The compressor arrangement includes: an integrally-geared turbo-compressor with a plurality of compressor stages; a first cooling loop, through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages of the plurality of compressor stages are adapted to pressurize the first refrigerant; and a second cooling loop, through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages of the plurality of compressor stages are adapted to pressurize the second refrigerant.

According to another aspect, a method of liquefying natural gas is provided. The method includes: providing an integrally-geared turbo compressor having a plurality of compressor stages; driving the compressor with a prime mover; circulating a first refrigerant through one or more first compressor stages of the plurality of compressor stages; circulating a second refrigerant through one or more second compressor stages of the plurality of compressor stages; cooling at least one of natural gas and the second refrigerant by heat exchange against the first refrigerant; and cooling the natural gas by heat exchange against the second refrigerant

Further aspects, advantages, and features of the present disclosure are apparent from the dependent claims, the description, and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the disclosure and are described in the following. Some embodiments are depicted in the drawings and are detailed in the description which follows.

FIG. 1 is a schematic diagram of a typical APCI process for liquefying natural gas;

FIG. 2 is a schematic diagram of a natural gas liquefaction system according to embodiments described herein;

FIG. 3 is a schematic diagram of a natural gas liquefaction system according to further embodiments described herein;

FIG. 4 is an enlarged schematic view of a compressor arrangement for a natural gas liquefaction system according to embodiments described herein;

FIG. 5 is a schematic diagram of a natural gas liquefaction system according to further embodiments described herein; and

FIG. 6 is a flow diagram illustrating a method of liquefying a natural gas according to embodiments described herein.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of the disclosure, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

Within the following description of the drawings, the same reference numbers refer to corresponding or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment applies to a corresponding part or aspect in another embodiment as well.

FIG. 1 shows a schematic diagram of a typical natural gas liquefaction system using the so-called APCI process. The shown process uses two refrigeration cycles. A pre-cooling cycle 12 uses a first refrigerant and a cooling cycle 2 uses a second refrigerant.

The system, labeled 1 as a whole, includes the cooling cycle 2 including a line formed by a gas turbine 3, which drives a compressor train. The compressor train includes a first compressor 5 and a second compressor 7 in series for compressing the second refrigerant. An inter-stage cooler 9 may be provided to cool the second refrigerant delivered by the first compressor 5 to reduce the temperature and the volume of the second refrigerant before entering the second compressor 7. The compressed second refrigerant delivered by the second compressor 7 may be condensed against air or water in a second condenser 11. The second refrigerant is cooled and partly liquefied by a heat exchange against a first refrigerant which circulates in the pre-cooling cycle 12.

The pre-cooling cycle 12 includes a line including a gas turbine 13, which drives a compressor 15. The compressed first refrigerant delivered by the compressor 15 is condensed in a first condenser 17 against water or air. The condensed first refrigerant is used to pre-cool the natural gas down to −40° C. and to cool and partially liquefy the second refrigerant. The pre-cooling of the natural gas and the partial liquefaction of the second refrigerant are performed in a multi-pressure process, e.g. a four pressure process in the example shown in FIG. 1.

The stream of the condensed first refrigerant from the first condenser 17 is delivered to a first set of four, serially arranged auxiliary heat exchangers to cool and partly liquefy the second refrigerant, and to a second set of four, serially arranged, pre-cooling heat exchangers to pro-cool the natural gas. A first portion of the compressed first refrigerant streaming from the first condenser 17 is delivered through a pipe 19 to the first set of heat exchangers and is sequentially expanded in the serially arranged expanders 21, 23, 25 and 27 to four different, gradually decreasing pressure levels. Downstream from each expander, a portion of the expanded first refrigerant is diverted to a respective heat exchanger 29, 31, 33, and 35.

The compressed second refrigerant delivered from the second condenser 11 may flow in a pipe 37 toward a main cryogenic heat exchanger 38. The pipe 37 sequentially passes through the heat exchangers 29, 31, 33 and 35, such that the second refrigerant is gradually cooled and partly liquefied against the expanded first refrigerant.

A second fraction of the condensed first refrigerant from the first condenser 17 is delivered to a second pipe 39 and expanded sequentially in four serially arranged expanders 41, 43, 45, and 47. A portion of the first refrigerant expanded in each expander is diverted towards a corresponding pre-cooling heat exchanger 49, 51, 53 and 55, respectively. A main natural gas line 61 flows sequentially through said pre-cooling heat exchangers 49, 51, 53 and 55, such that the natural gas is pre-cooled before entering the main cryogenic heat exchanger 38. The heated first refrigerant exiting the pre-cooling heat exchangers 49, 51, 53 and 55 is collected with the first refrigerant exiting the heat exchangers 29, 31, 33 and 35, and is fed again to the compressor 15, which recovers the four evaporated streams of first refrigerant and re-compresses the vapor.

The system shown in FIG. 1 includes at least one compressor driven by a gas turbine 13 for compressing the first refrigerant, and at least one further compressor driven by a gas turbine 3 for compressing the second refrigerant. Accordingly, the energy-efficiency of the system shown in FIG. 1 is limited, and the two gas turbines 3, 13 consume a considerable amount of space.

A natural gas liquefaction system 100 in accordance with embodiments described herein is schematically shown in FIG. 2.

The natural gas liquefaction system 100 includes an integrally-geared turbo-compressor 150 (also simply referred to as compressor 150) with a plurality of compressor stages which is configured to be driven by a prime mover 160, particularly by a single prime mover such as an internal combustion engine or an electric motor. In other words, each compressor stage of the plurality of compressor stages of the compressor 150 may be driven directly or indirectly by the prime mover 160. A transmission mechanism 301, particularly a gear of the compressor including one or more gear wheels, and/or other transmission units such as pinions, pulleys, toothed wheels etc. may be connected between the plurality of compressor stages of the compressor 150, in order to drive the plurality of compressor stages into rotation. The driving force may be provided by the prime mover 160, e.g. via a main driving shaft connected to the integrally-geared turbo compressor.

By providing a compressor with an integral gear, the speed, the torque, and/or the direction of the rotational force provided by the prime mover can be changed as appropriate. For example, the rotational speed and/or the torque of the impellers of the plurality of compressor stages can be individually adjusted as appropriate. In some embodiments, the transmission mechanism can include a gear train or a transmission. The impellers of the compressor stages may be mounted on respective shafts which may be driven into rotation by one of the transmission elements of the gear. For example, the gear may include at least one gear wheel which may drive one or more shafts into rotation. A pinion may be mounted on each of the shafts which may mesh with at least one gear wheel. Further, one or two impellers of the plurality of compressor stages may be mounted on each of the shafts.

In an integrally-geared compressor, at least one or more transmission units such as one or more gearwheels are connected between at least some of the plurality of compressor stages, so that the respective impellers of the compressor stages can be rotated at different rotational speeds. When a gear or another force transmission mechanism is connected between at least some compressor stages, the compressor stages can be provided on different shafts, which may be adapted to rotate with different rotational speeds. For example, the impellers of the one or more first compressor stages may be rotated at different rotational speeds as the impellers of the one or more second compressor stages.

For example, in some embodiments, one or more force transmission elements such as one or more central gearwheels may be provided for driving the one or more first compressor stages and the one or more second compressor stages at different rotational speeds. The one or more first compressor stages may be provided on different shafts as the one or more second compressor stages.

In some embodiments, which may be combined with other embodiments described herein, a force transmission element such as one or more central gearwheels may be configured for driving two or more first compressor stages at different rotational speeds. In some embodiments, which may be combined with other embodiments described herein, a force transmission element such as one or more central gearwheels may be configured for driving two or more second compressor stages at different rotational speeds. In some embodiments, the integrally-geared compressor may include a plurality of force transmission elements such as a plurality of central gearwheels, in order to drive each stage of the plurality of compressor stages at a desired rotational speed.

As is further depicted in FIG. 2, the natural gas liquefaction system 100 includes a pre-cooling loop 110, through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages 151 of the plurality of compressor stages are adapted to pressurize the first refrigerant, and a cooling loop 130, through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages 155 of the plurality of compressor stages are adapted to pressurize the second refrigerant.

Each compressor stage of the plurality of first and second compressor stages may include a gas inlet, a gas outlet, and at least one impeller rotating on a respective shaft. The compressor stages may be axial or radial compressor stages.

The one or more first compressor stages 151 for pressurizing the first refrigerant may be directly or indirectly driven by the prime mover 160, e.g. via the transmission mechanism or gear of the compressor. The one or more second compressor stages 155 for pressurizing the second refrigerant may also be directly or indirectly driven by the prime mover 160, e.g. via the transmission mechanism or gear of the compressor 150.

According to embodiments described herein, a single integrally-geared multi-stage compressor driven by the prime mover 160 may be provided for pressurizing two or more refrigerants circulating in two or more cooling loops, e.g. in the pre-cooling loop 110 and in the cooling loop 130. In some embodiments, the whole LNG liquefaction system may include a single integrally-geared compressor configured for pressurizing the two or more refrigerants which are used for liquefying the natural gas.

The first compressor stages and the second compressor stages of the compressor may be housed in a single compressor casing, e.g. in a compact and space-saving way. For example, a wall of a compressor housing may enclose the first plurality of compressor stages, the second plurality of compressor stages, as well as the transmission elements of the gear of the compressor which connect the driving shafts of the compressor stages with each other.

By using an integrally-geared multi-stage compressor for pressurizing two, three or more refrigerants of the LNG liquefaction system, energy and space can be saved as compared to previously used systems which included one or more separate compressors. An adjustment of the rotational speeds of the compressor stages may still be possible, because the plurality of compressor stages are drivingly connected by the integral gear of the compressor.

As is further shown in FIG. 2, the natural gas liquefaction system 100 may further include a first heat exchanger device 170 configured for transferring heat from natural gas to the first refrigerant and/or from the second refrigerant to the first refrigerant, and a second heat exchanger device 180 for transferring heat from the natural gas to the second refrigerant.

In some embodiments, the natural gas is adapted to be sequentially cooled by the first refrigerant and by the second refrigerant. The natural gas may be guided through one or more first heat exchangers of the first heat exchanger device 170, where the natural gas may be pre-cooled by the first refrigerant, e.g. to a temperature below 0° C., particularly −40° C. or less. The natural gas may subsequently be guided through the second heat exchanger device 180, where the natural gas is cooled by the second refrigerant. The second heat exchanger device 180 may be the main cryogenic heat exchanger of the system which is configured to cool the natural gas down to the liquefaction temperature.

In the schematic diagram of FIG. 2, the second heat exchanger device 180 is depicted in a simplified way as a device which removes heat from the natural gas flowing through a main natural gas line 61 and transfers the heat to the second refrigerant flowing through the cooling loop 130.

The first refrigerant circulating in the pre-cooling loop 110 may be used for pre-cooling the natural gas at a position of the main natural gas line 61 upstream from the second heat exchanger device 180. Alternatively or additionally, the first refrigerant may be used for cooling the second refrigerant at a position of the cooling loop 130 upstream from the second heat exchanger device 180.

In the embodiment depicted in FIG. 2, the first heat exchanger device 170 includes a heat exchanger configured for pre-cooling the natural gas and a further heat exchanger configured for cooling the second refrigerant. The first refrigerant which leaves the first heat exchanger device 170 may be guided back to the compressor 150 to be re-compressed in the one or more first compressor stages 151 of the compressor.

In some embodiments, the second refrigerant which leaves the second heat exchanger device 180 may be guided back to the compressor 150 to be re-compressed in the one or more second compressor stages 155 of the compressor.

In some embodiments, which may be combined with other embodiments described herein, the pre-cooling loop 110 includes a first condenser 17 for removing heat from the first refrigerant after compression. The pre-cooling loop may further include at least one expansion element (not shown in FIG. 2) for expanding the first refrigerant upstream from the first heat exchanger device 170.

The cooling loop 130 may include a second condenser 11 for removing heat from the second refrigerant after compression.

In some embodiments, which may be combined with other embodiments described herein, the first refrigerant includes a gas with a molecular weight of 35 or more, particularly 40 or more, more particularly propane.

In some embodiments, which may be combined with other embodiments described herein, the second refrigerant is a mixed refrigerant, which may include a mixture including at least one or more of nitrogen, methane, ethane and propane.

In some embodiments, which may be combined with other embodiments described herein, at least one compressor stage of the plurality of compressor stages is provided with a movable inlet guide vane for autonomously regulating a flow entering in the at least one compressor stage. For example, each of the one or more first compressor stages 151 may be provided with a respective movable inlet guide vane.

As is schematically depicted in FIG. 2, a single prime mover may be provided for driving each of the one or more first and second compressor stages. In some embodiments, the prime mover 160 may be or include a gas turbine and/or a motor, e.g. an electric motor or an internal combustion engine. One or more gearbox elements of the integrally-geared turbo compressor may be connected between the prime mover, the one or more first compressor stages and/or the one or more second compressor stages. For example, at least some of the impellers of the one or more first compressor stages may rotate at a different rotational speed and be provided on different rotary shafts than at least some of the impellers of the one or more second compressor stages.

A natural gas liquefaction system 200 in accordance with embodiments described herein is schematically shown in FIG. 3. The basic setup of the natural gas liquefaction system 200 is similar to the system shown in FIG. 2 so that reference can be made to the above explanations which are not repeated here.

The natural gas liquefaction system 200 includes an integrally-geared turbo-compressor 150 with a plurality of compressor stages which is configured to be driven by a prime mover 160, particularly by a single prime mover such as a gas turbine or another internal combustion engine. In other words, each compressor stage of the plurality of compressor stages of the compressor 150 may be driven directly or indirectly by the prime mover 160. For example, a transmission mechanism, particularly a gear of the compressor, with a plurality of gear wheels and/or other transmission units such as pinions and/or pulleys may be connected between the prime mover 160 and the plurality of compressor stages of the compressor 150, in order to drive the plurality of compressor stages at appropriate rotational speeds.

In some embodiments, the compressor 150 includes a plurality of first compressor stages 151 configured for pressurizing the first refrigerant circulating in the pre-cooling loop 110. For example, four first compressor stages may be provided. In other embodiments, a different number of first compressor stages may be provided, e.g. two, three, or more than four first compressor stages.

The plurality of first compressor stages 151 may be sequentially arranged in the pre-cooling loop. For example, the first refrigerant which enters the compressor 150 at an initial first compressor stage, may be subsequently pressurized by said initial first compressor stage and by other first compressor stage(s) arranged downstream from the initial first compressor stage. The pressure of the first refrigerant may be increased in each of the sequentially arranged first compressor stages 151.

In some embodiments, which may be combined with other embodiments described herein, the compressor 150 may include a plurality of second compressor stages 155 configured for pressurizing the second refrigerant circulating in the cooling loop 130. For example, two, three, four or more second compressor stages 155 may be provided.

The second compressor stages 155 may be sequentially arranged in the cooling loop. In other words, the second refrigerant which enters the compressor 150 at an initial second compressor stage may be subsequently pressurized by said initial second compressor stage and by further second compressor stage(s) arranged downstream from the initial second compressor stage. The pressure of the second refrigerant may be increased by each of the sequentially arranged second compressor stages 155. The impellers of two or more second compressor stages may be mounted on different shafts and may be rotated at different rotational speeds in some embodiments.

For example, the compressor 150 may include four first compressor stages for pressurizing the first refrigerant and three (or alternatively four) second compressor stages for pressurizing the second refrigerant.

In some embodiments, the pre-cooling loop 110 may be configured to divide the first refrigerant into a plurality of precooling streams, which are guided to a respective one of said plurality of first compressor stages 151. The number of precooling streams may correspond to the number of first compressor stages. Each of the precooling streams may enter the compressor at an associated first compressor stage to be re-compressed by the associated first compressor stage and potentially by further first compressor stage(s) arranged downstream thereof, if any.

In some embodiments, a plurality of first expansion elements 241, 243, 245, 247 may be sequentially arranged in the pre-cooling loop 110 and configured for expanding the first refrigerant at a plurality of decreasing pressure levels. A plurality of first heat exchangers 249, 251, 253, 255 of the first heat exchanger device 270 may be provided for receiving respective precooling streams of said first refrigerant expanded through at least one of said plurality of first expansion elements 241, 243, 245, 247 and for transferring heat from the natural gas to the first refrigerant.

A plurality of return paths 261, 263, 265, 267 configured for returning said precooling streams of the first refrigerant from the plurality of first heat exchangers 249, 251, 253, 255 to a respective one of said plurality of first compressor stages 151 may be provided.

According to some embodiments, which may be combined with other embodiments described herein, at least one first auxiliary expansion element may be arranged in the precooling loop. Further, at least one first auxiliary heat exchanger may be provided for receiving at least a portion of said first refrigerant expanded through the at least one first auxiliary expansion element and for transferring heat from the second refrigerant to the first refrigerant.

According to some embodiments, which may be combined with other embodiments described herein, the system may include a plurality of first auxiliary expansion elements 221, 223, 225, 227 sequentially arranged in the pre-cooling loop 110 and configured for expanding the first refrigerant at a plurality of decreasing pressure levels. A plurality of first auxiliary heat exchangers 229, 231, 233, 235 of the first heat exchanger device 270 may be provided for receiving respective portions of said first refrigerant expanded through at least one of said plurality of first auxiliary expansion elements 221, 223, 225, 227 and for transferring heat from the second refrigerant to the first refrigerant.

The plurality of return paths 261, 263, 265, 267 may be configured for returning said portions of the first refrigerant from the plurality of first auxiliary heat exchangers 229, 231, 233, 235 and/or from the first heat exchangers 249, 251, 253, 255 to a respective one of said plurality of first compressor stages 151.

During operation of the natural gas liquefaction system 200, a flow of compressed first refrigerant may be delivered from a most downstream first compressor stage of the plurality of first compressor stages 151 to a first condenser 17. The flow of the first refrigerant delivered through the first condenser 17 may be cooled, e.g. against water or air, and condensed.

In some embodiments, the condensed first refrigerant is circulated in the pre-cooling loop 110 to pre-cool the natural gas in the plurality of first heat exchangers 249, 251, 253, 255, and/or to cool and optionally partially liquefy the second refrigerant circulating in the cooling loop 130 in the plurality of first auxiliary heat exchangers 229, 231, 233, 235.

In some embodiments, the pre-cooling loop 110 may be divided into a plurality of n pressure levels, e.g. four pressure levels. The number n of pressure levels may correspond to the number n of first compressor stages of the compressor 150 configured for compressing the first refrigerant. The flow of first refrigerant delivered through the first condenser 17 may be sequentially expanded at n progressively reducing pressure levels and be divided into n partial flows. Each partial flow of first refrigerant may be returned as a side flow to the compressor 150 at an inlet of a corresponding one of the plurality of first compressor stages 151.

A first delivery line 217 may deliver a first part of the condensed first refrigerant flow to the plurality of first expansion elements 241, 243, 245, 247. A second delivery line 218 branched off the first delivery line 217 may deliver a second part of the condensed first refrigerant flow to the plurality of first auxiliary expansion elements 221, 223, 225, 227.

The first part of the condensed first refrigerant from the first condenser 17 may be sequentially expanded in the plurality of first expansion elements 241, 243, 245, 247 at n different, gradually decreasing pressure levels. Downstream from each first expansion element, a portion of the flow of partly expanded first refrigerant may be diverted to a respective one of the plurality of first heat exchangers 249, 251, 253, 255. The remaining part of the partly expanded first refrigerant may be caused to flow through the next first expansion element and so on. The residual first refrigerant flowing through the most downstream one (247) of the plurality of first expansion elements 241, 243, 245, 247 may be delivered to a most downstream one (255) of the plurality of first heat exchangers 249, 251, 253, 255.

In each one of the plurality of first heat exchangers 249, 251, 253, 255, the first refrigerant may exchange heat against the natural gas flowing in the main natural gas line 61, thus pre-cooling and optionally partly liquefying the natural gas.

The second part of the condensed first refrigerant expanded in at least one of the plurality of first auxiliary expansion elements 221, 223, 225, 227 may be diverted towards a corresponding one of the plurality of first auxiliary heat exchangers 229, 231, 233, 235. The portion of first refrigerant delivered by each one of the plurality of first auxiliary expansion elements 221, 223, 225, 227 and which is not caused to flow through the respective first auxiliary heat exchanger is delivered through the subsequent one of the plurality of first auxiliary expansion elements 221, 223, 225, 227. The most downstream one (235) of said plurality of first auxiliary heat exchangers 229, 231, 233, 235 receives the residual fraction of first refrigerant expanded in the most downstream one (227) of the plurality of first auxiliary expansion elements 221, 223, 225, 227. In each first auxiliary heat exchanger, the first refrigerant exchanges heat against the second refrigerant which circulates in the cooling loop 130, so that at the delivery side of the most downstream one (235) of the plurality of first auxiliary heat exchangers 229, 231, 233, 235, the second refrigerant is cooled and optionally at least partly liquefied.

Heated first refrigerant exiting the plurality of first heat exchangers 249, 251, 253, 255 may be collected with the heated first refrigerant exiting the first auxiliary heat exchangers 229, 231, 233, 235 and may be fed again to the integrally-geared turbo-compressor 150 at the inlet of the respective first compressor stage.

In some embodiments, the heated first refrigerant exiting one of the plurality of first auxiliary heat exchangers 229, 231, 233, 235 is at around the same pressure as the heated first refrigerant exiting a corresponding one of the plurality of first heat exchangers 249, 251, 253, 255. The first refrigerant collected at corresponding pressure levels may be delivered at the inlet of a corresponding stage of the plurality of first compressor stages of the compressor 150. A plurality of side streams of the first refrigerant is thus returned at gradually decreasing pressure levels at the inlets of the sequentially arranged first compressor stages 151.

In some embodiments, the plurality of return paths 261, 263, 265, 267 may be configured for delivering the side streams of expanded and exhausted first refrigerant from the plurality of first heat exchangers 249, 251, 253, 255 and/or from the plurality of first auxiliary heat exchangers 229, 231, 233, 235 to a corresponding stage of the plurality of first compressor stages 151.

In some embodiments, which may be combined with other embodiments described herein, the second refrigerant circulating in the cooling loop 130 may be compressed by the plurality of second compressor stages 155 which may be sequentially arranged in the cooling loop 130. The plurality of second compressor stages 155 are part of the same integrally-geared compressor as the plurality of first compressor stages 151.

In some embodiments, the integrally-geared compressor may include at least one multi-stage compressor unit with two or more compressor stages sequentially arranged on a single shaft, e.g. a multi-stage centrifugal compressor unit.

The prime mover 160 which drives the compressor may include an internal combustion engine or an electric motor. The prime mover 160 can be a gas turbine, e.g. an aeroderivative gas turbine.

In some embodiments, at least one first intercooler may be arranged between at least two sequentially arranged first compressor stages of the plurality of first compressor stages 151. In some embodiments, at least one second intercooler may be arranged between at least two sequentially arranged second compressor stages of the plurality of second compressor stages 155. The intercoolers may be configured to reduce the temperature and the volume of the respective refrigerant delivered by the respective compressor stage before entering the subsequent compressor stage or before leaving the compressor.

The second refrigerant delivered by the most downstream one of the plurality of second compressor stages 155 may be condensed by a second condenser 11. The second condenser 11 may be an air condenser of a water condenser, where the second refrigerant may be condensed by exchanging heat against air or water. The condensed second refrigerant may be subsequently delivered by a delivery line through the plurality of first auxiliary heat exchangers 229, 231, 233, 235, where the second refrigerant may be cooled and optionally liquefied by exchanging heat against the first refrigerant circulating in the pre-cooling loop 110, as described above.

The cooled second refrigerant delivered from the plurality of first auxiliary heat exchangers may be guided toward the second heat exchanger device 180, which may be a main cryogenic heat exchanger, where the second refrigerant may remove further heat from the pre-cooled natural gas, completing the liquefaction process. The heated second refrigerant may be returned through a return line 269 to an initial one of the plurality of second compressor stages 155 of the compressor 150.

In FIG. 3, the plurality of compressor stages of the integrally-geared turbo-compressor 150 is depicted in a schematic way only. The compressor 150 of an exemplary embodiment is illustrated in more detail in FIG. 4.

FIG. 4 is an enlarged schematic view of a compressor arrangement with an integrally-geared turbo-compressor 150 according to embodiments described herein. The compressor 150 may be driven by a prime mover 160 and may include a plurality of compressor stages which are directly or indirectly driven by the prime mover 160. The plurality of compressor stages includes one or more first compressor stages 151 for pressurizing the first refrigerant circulating in the pre-cooling loop 110, and one or more second compressor stages 155 for pressurizing the second refrigerant circulating in the cooling loop 130. More details of the pre-cooling loop 110 and of the cooling loop 130 are described above with reference to FIG. 2 and FIG. 3 and are not repeated here.

The compressor 150 may include a transmission mechanism 301, e.g. an integral gear, which may be arranged in a compressor housing 330 and configured to be driven by said prime mover 160. The compressor 150 may further include at least one first shaft 303 configured to be driven into rotation by said transmission mechanism 301 and configured for driving at least one of the plurality of first compressor stages 151. In other words, an impeller of at least one first compressor stage may be mounted on the at least one first shaft 303 such as to rotate together with the first shaft. Further, the compressor 150 may include at least one second shaft 305 configured to be driven into rotation by said transmission mechanism 301 and configured for driving at least one of the plurality of second compressor stages 155. Therein, an impeller of at least one second compressor stage may be mounted on the at least one second shaft 305 such as to rotate together with the second shaft.

In some embodiments, at least one first shaft 303 may drive two first compressor stages of the plurality of first compressor stages, e.g. two subsequent first compressor stages. Alternatively or additionally, at least one second shaft 305 may drive two second compressor stages of the plurality of second compressor stages, for example two subsequent second compressor stages.

In some embodiments, the at least one first shaft 303 may be provided with a pinion meshing with a gear wheel of the transmission mechanism 301, and/or the at least one second shaft 305 may be provided with a further pinion meshing with a gear wheel of the transmission mechanism 301. For example, in some embodiments, the transmission mechanism 301 may include a first gear wheel 307 configured for driving the at least one first shaft 303 and a second gear wheel 308 configured for driving the at least one second shaft 305.

Alternatively, e.g. in the embodiment schematically depicted in FIG. 5, the transmission mechanism 301 may include one center gear wheel 307 configured for driving the at least one first shaft 303 and for driving the at least one second shaft 305. For example, a single bull gear can be provided that is configured for (e.g., directly) driving each of the first and second compressor stages.

In other words, a first pinion with a first diameter may be connected to the at least one first shaft 303 and/or a second pinion with a second diameter may be connected to the at least one second shaft 305. The central gear wheel 307 of the gear may directly mesh with the first pinion and with the second pinion for driving the at least one first shaft and the at least one second shaft into rotation. In the embodiment depicted in FIG. 5, the central gear wheel 307 directly meshes with respective pinions connected to two or more first shafts 303 and to two or more second shafts 305. For example, the (single) central gear wheel may directly drive the shafts of three, four or more first compressor stages and of three, four or more second compressor stages.

The first diameter of the first pinion may correspond to the second diameter of the second pinion. Accordingly, the first shaft and the second shaft may rotate at corresponding rotational speeds. Alternatively, the first diameter and the second diameter may be different. Accordingly, the rotational speed of the first shaft and of the second shaft may be adjusted to differ as appropriate. For example, the rotational speeds of the first and second compressor stages may be adapted to the properties of the respective refrigerant guided therethrough.

In an alternative embodiment, two or more bull gears may be provided for driving the plurality of compressor stages. For example, a first bull gear may drive the one or more first compressor stages, and a second bull gear may drive the one or more second compressor stages.

It is noted that, in some embodiments, the at least one first shaft and/or the at least one second shaft may drive two compressor stages which may be arranged on opposite ends of the respective shaft. In FIG. 3 and in FIG. 5, two compressor stages provided on a single shaft are schematically illustrated by two arrowheads directed in opposite directions which are connected by a connection line illustrating the common shaft. For example, a first impeller of one compressor stage may be mounted in a first portion of a common shaft, and a second impeller of a further compressor stage may be mounted in a second portion of the common shaft.

Referring back to FIG. 3 and to FIG. 4, in some embodiments, which may be combined with other embodiments described herein, the first gear wheel 307 may drive the plurality of first compressor stages 151, and the second gear wheel 308 may drive the plurality of second compressor stages 155. The gear wheels may be toothed wheels which are driven in rotation directly or indirectly by the prime mover 160, respectively. The first and second shafts may each comprise a pinion mounted thereon and meshing with a respective toothed wheel. The first and second shafts and the impeller(s) mounted on the shafts can therefore rotate at different rotational speeds.

The diameter of the second gear wheel 308 may be smaller than the diameter of the first gear wheel 307. When the first gear wheel 307 directly meshes with the second gear wheel 308, the second gear wheel 308 may rotate at a higher rotational speed than the first gear wheel 307. Accordingly, the at least one second shaft 305 driven by the second gear wheel 308 may be rotated at a higher rotational speed than the at least one first shaft 303 driven by the first gear wheel 307. Thus, the impeller(s) of the first compressor stage(s) which are mounted on the at least one first shaft 303 may be rotated at a higher rotational speed than the impeller(s) of the second compressor stage(s) which are mounted on the at least one second shaft 305.

In some embodiments, which may be combined with other embodiments described herein, the compressor may include two or more first shafts which drive the plurality of first compressor stages 151, wherein the two or more first shafts may be driven by the first gear wheel 307. At least one first shaft may be configured to drive two sequentially arranged first compressor stages. Alternatively or additionally, at least one first shaft may be configured to drive a single first compressor stage. In the latter case, the impeller of a single first compressor stage may be mounted on the first shaft.

In some embodiments, which may be combined with other embodiments described herein, the compressor may include two or more second shafts for driving the plurality of second compressor stages 155, wherein the two or more second shafts may be driven by the second gear wheel 308. At least one second shaft may be configured to drive two sequentially arranged second compressor stages. Alternatively or additionally, at least one second shaft may be configured to drive a single one of the plurality of second compressor stages.

Each compressor stage of the plurality of compressor stages may include a gas inlet, a gas outlet, and at least one impeller mounted on a respective shaft. Each impeller can be a radial impeller, with an axial inlet and a radial outlet. The fluid processed through the impeller may be collected in a respective volute of the compressor stage. The impellers can be paired, wherein a pair of impellers (e.g. belonging to two subsequent compressor stages) may be mounted on a common rotary shaft.

In some embodiments, the plurality of first compressor stages 151 may be configured to compress the first refrigerant so that the pressurized first refrigerant is delivered from the most downstream first compressor stage 312 of the plurality of first compressor stages 151 at a pressure ranging from 10 bar to 40 bar absolute, particularly from 20 bar to 30 bar absolute, more particularly from 22 bar to 24 bar absolute. The pressure of the first refrigerant at the inlet of the most upstream first compressor stage 315 may be between 1 bar absolute and 2 bar absolute in some embodiments.

Alternatively or additionally, the plurality of first compressor stages 151 may be configured to compress the first refrigerant so that the pressurized first refrigerant is delivered from the most downstream first compressor stage 312 of the plurality of first compressor stages 151 at a temperature ranging from 60° C. to 100° C., particularly from 75° C. to 85° C. For example, no inter-cooling stage may be provided between the two first compressor stages.

In some embodiments, the at least one first shaft 303 on which one or more impellers of one or more first compressor stages 151 are mounted may be configured to rotate at a rotational speed from 3.000 rpm (rotations per minute) to 7.000 rpm, particularly from about 4.000 rpm to about 5.500 rpm. In some embodiments, two or more first shafts may be provided, on which the impellers of all of the first compressor stages are mounted. Each first shaft may be configured to rotate at a rotational speed from 3000 rpm to about 7000 rpm. The shaft of the most upstream first compressor stage may rotate at a lower speed than the shaft of the most downstream first compressor stage.

The plurality of first compressor stages 151 may deliver the compressed first refrigerant at a flow rate ranging from about 10,000 actual m³/h to about 70,000 actual m³/h.

The plurality of first compressor stages 151 may absorb a power ranging from about 10 MW to about 40 MW, particularly ranging from about 25 MW to about 35 MW. Alternatively or additionally, the plurality of second compressor stages 155 may absorb a power ranging from about 10 MW to about 40 MW, particularly ranging from about 25 MW to about 35 MW. Accordingly, in some embodiments, the prime mover 160 may provide a power ranging from 20 MW to 80 MW, particularly from 50 MW to 70 MW.

In some embodiments, the plurality of second compressor stages 155 may be configured to compress the second refrigerant so that the pressurized second refrigerant is delivered from the most downstream second compressor stage 316 of the plurality of second compressor stages 155 at a pressure ranging from 50 bar to 100 bar absolute, particularly from 55 bar to 65 bar absolute. The pressure of the second refrigerant at the inlet of the most upstream second compressor stage 319 may be below 10 bar absolute in some embodiments.

In some embodiments, the plurality of second compressor stages 155 may be configured to compress the second refrigerant so that the pressurized second refrigerant is delivered from the most downstream second compressor stage 316 of the plurality of second compressor stages 155 at a temperature ranging from 60° C. to 120° C., particularly from 80° C. to 100° C. For example, one, two or more inter-cooling stages 320 may be provided between at least two subsequent second compressor stages. Thus, the exit temperature of the second refrigerant can be reduced.

The at least one second shaft 305 on which the impeller(s) of one or more second compressor stages is mounted may be configured to rotate at a rotational speed from 7.000 rpm to 20.000 rpm, particularly from 8.000 rpm to about 15.000 rpm. In some embodiments, two or more second shafts may be provided for driving the impellers of all second compressor stages 155. The shaft of the most upstream second compressor stage 319 may rotate at a lower speed (e.g. between 9.000 rpm and 11.000 rpm) than the shaft of the most downstream second compressor stage 316 (e.g. at a speed between 14.000 rpm and 16.000 rpm).

FIG. 4 shows an exemplary embodiment, in which the plurality of first compressor stages 151 includes a total of four subsequently arranged first compressor stages. The upstream pair of first compressor stages is driven by a rotary shaft, and the downstream pair of first compressor stages is driven by a further rotary shaft, wherein both rotary shafts are driven by the first gear wheel 307. In other words, the impellers of the upstream pair of first compressor stages are mounted on a common rotary shaft, and the impellers of the downstream pair of first compressor stages are mounted on a further common rotary shaft. Alternatively, only the upstream pair of first compressor stages may be driven by a common rotary shaft, whereas the two downstream first compressor stages may be driven by a separate rotary shaft, respectively, or vice versa.

In the exemplary embodiments of FIG. 4, the plurality of second compressor stages 155 includes a total of four subsequently arranged second compressor stages. The upstream pair of second compressor stages is driven by a rotary shaft, and the downstream pair of second compressor stages is driven by a further rotary shaft, wherein both rotary shafts are driven by the second gear wheel 308. In other words, the impellers of the upstream pair of second compressor stages are mounted on a common rotary shaft, and the impellers of the downstream pair of second compressor stages are mounted on a further common rotary shaft. Alternatively, only three subsequently arranged second compressor stages may be provided, wherein the upstream pair of second compressor stages may be driven by a common rotary shaft, and the downstream second compressor stage may be driven by a separate rotary shaft, or vice versa.

Other possible arrangements and numbers of first and second compressor stages on respective rotary shafts driven into rotation by the transmission mechanism or gear of the compressor will be apparent to the skilled person.

According to a further aspect, a compressor arrangement for compressing a plurality of refrigerants is provided. The compressor arrangement includes an integrally-geared turbo-compressor 150 with a plurality of compressor stages which may have some or all of the features of the above described compressors.

The compressor arrangement may include a first cooling line, e.g. being part of the pre-cooling loop, through which a first refrigerant is adapted to flow, wherein one or more first compressor stages of the plurality of compressor stages are adapted to pressurize the first refrigerant streaming through the first cooling line. The compressor arrangement may further include a second cooling line, e.g. being part of the cooling loop, through which a second refrigerant is adapted to flow, wherein one or more second compressor stages of the plurality of compressor stages are adapted to pressurize the second refrigerant streaming through the second cooling line.

The compressor arrangement may be used in a natural gas liquefaction system according to any of the embodiments described above.

The compressor arrangement may include a transmission mechanism or gear with some or all of the features of the embodiments described above. Further, all compressor stages may be included in a single housing in some embodiments.

According to a further aspect described herein, a method of liquefying a natural gas is provided. A flow diagram of a method according to embodiments described herein is schematically depicted in FIG. 6.

In box 710, an integrally-geared turbo compressor having a plurality of compressor stages is provided. In box 720, the compressor is driven with a prime mover. In box 730, a first refrigerant is circulated through one or more first compressor stages of the plurality of compressor stages, and a second refrigerant is circulated through one or more second compressor stages of the plurality of compressor stages. In box 740, at least one of natural gas and the second refrigerant is cooled by heat exchange against the first refrigerant. In box 750, the natural gas is cooled by heat exchange against the second refrigerant.

In some embodiments, the compressed first refrigerant and/or the compressed second refrigerant may be condensed. The condensed first refrigerant may be expanded, e.g. in a plurality of sequentially arranged first expansion elements.

In some embodiments, the first refrigerant may be divided in a plurality of partial flows.

In some embodiments, at least a part of the first refrigerant may be sequentially compressed by a plurality of first compressor stages, e.g. three, four or more first compressor stages, and/or the second refrigerant may be sequentially compressed by a plurality of second compressor stages, e.g. three, four or more second compressor stages.

Movable inlet guide vanes may be provided at inlets of at least one of the plurality of first compressor stages. The movable inlet guide vanes may be individually controlled to regulate partial flows at the suction side of the plurality of first compressor stages, particularly as a function of flow conditions of the partial flows.

In some embodiments, the method may further include: expanding the first refrigerant through a plurality of sequentially arranged first expansion elements at a plurality of decreasing pressure levels; circulating portions of the expanded first refrigerant from the first expansion elements through a plurality of first heat exchangers to remove heat from the natural gas; and returning the portions of expanded first refrigerant from the plurality of first heat exchangers to respective ones of the one or more first compressor stages.

In some embodiments, the method may further include: expanding the first refrigerant through a plurality of sequentially arranged first auxiliary expansion elements at a plurality of decreasing pressure levels; circulating portions of the expanded first refrigerant through a plurality of first auxiliary heat exchangers to remove heat from the second refrigerant; and returning the portions of first refrigerant from the plurality of first auxiliary heat exchangers to a respective one of the one or more first compressor stages.

The prime mover may drive a transmission mechanism, e.g. an internal gear, of the compressor, wherein the transmission mechanism may drive at least one first shaft and at least one second shaft into rotation.

The at least one first shaft may be driven into rotation by said transmission mechanism at a rotation speed of 3.000 rpm or more and 7.000 rpm or less. The impellers of one or two first compressor stages may be mounted on the at least one first shaft and may rotate at the rotational speed of the at least one shaft.

The at least one second shaft may be driven into rotation by said transmission mechanism at a rotation speed of 8.000 rpm or more and 15.000 rpm or less and may drive at least one of the second compressor stages. In other words, an impeller of at least one second compressor stage may be mounted on the at least one second shaft.

In some embodiments, the first refrigerant may be sequentially circulated through three, four or more first compressor stages of the compressor and compressed to an exit pressure ranging from 10 bar to 40 bar absolute, particularly from 20 bar to 30 bar absolute.

In some embodiments, the second refrigerant may be sequentially circulated through three, four or more second compressor stages of the compressor and compressed to an exit pressure ranging from 40 bar to 100 bar absolute, particularly from 50 bar to 80 bar absolute.

The use of an integrally-geared turbo-compressor for pressurizing two or more different refrigerants circulating in two or more cooling loops may result in an enhanced efficiency of the natural gas liquefaction system and thus reduced power consumption, and may further result in considerable cost savings when compared to systems with two or more separate compressors and compressor driving units. Further, the gear of the compressor may be adjusted such that each compressor stage may rotate at an appropriate rotation speed. Using a single compressor unit in a natural gas liquefaction system is an advantage in terms of cost, footprint and flexibility.

The number of first and second compressor stages as well as details of the internal gear of the compressor (e.g. details of the transmission mechanism) may depend on the properties of the refrigerants to be compressed. Further, a larger or a modified transmission mechanism may be provided, if three, four or more refrigerants are to be compressed by the integrally-geared compressor.

While the foregoing is directed to embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A natural gas liquefaction system (100), comprising:

an integrally-geared turbo-compressor (150) with a plurality of compressor stages;

a single prime mover (160) for driving the compressor (150);

a pre-cooling loop (110), through which a first refrigerant is adapted to circulate, wherein one or more first compressor stages (151) of the plurality of compressor stages are adapted to pressurize the first refrigerant;

a cooling loop (130), through which a second refrigerant is adapted to circulate, wherein one or more second compressor stages (155) of the plurality of compressor stages are adapted to pressurize the second refrigerant;

a first heat exchanger device (170) for transferring heat from a natural gas and/or from the second refrigerant to the first refrigerant; and

a second heat exchanger device (180) for transferring heat from the natural gas to the second refrigerant;

wherein said single prime mover (160) drives each of the one or more first compressor stages (151) and second compressor stages (155).

2. The system of claim 1, wherein the compressor (150) comprises a plurality of first compressor stages (151), particularly four sequentially arranged first compressor stages, for pressurizing the first refrigerant, and/or a plurality of second compressor stages (155), particularly three or four sequentially arranged second compressor stages, for pressurizing the second refrigerant.

3. The system of claim 1, wherein the compressor (150) comprises:

a transmission mechanism (301), particularly including a gear, configured to be driven into rotation by said prime mover;

at least one first shaft (303) configured to be driven into rotation by said transmission mechanism (301) and configured for driving at least one of the first compressor stages; and

at least one second shaft (305) configured to be driven into rotation by said transmission mechanism (301) and configured for driving at least one of the second compressor stages.

4. The system of claim 3, wherein the transmission mechanism (301) comprises a first gear wheel (307) meshing at least one first pinion connected to the at least one first shaft (303) for driving the at least one first compressor stage.

5. The system of claim 3, wherein the first gear wheel (307) further meshes at least one second pinion connected to the at least one second shaft (305) for driving the at least one second compressor stage.

6. The system of claim 3, wherein the transmission mechanism (301) comprises a first gear wheel (307) configured for driving the at least one first shaft (303), and a second gear wheel (308) configured for driving the at least one second shaft (305), particularly wherein the diameter of the second gear wheel (308) is smaller than the diameter of the first gear wheel (307) and/or wherein the first gear wheel and the second gear wheel are directly meshing gear wheels.

7. The system of claim 3, wherein at least one of the at least one first shaft and the at least one second shaft drives two compressor stages arranged on opposite ends of the respective shaft.

8. The system of claim 1, wherein the compressor (150) comprises a plurality of first compressor stages (151), and wherein the pre-cooling loop (110) is configured to divide the first refrigerant into a plurality of precooling streams, which are guided to a respective one of said plurality of first compressor stages (151).

9. The system of claim 8, comprising:

a plurality of first expansion elements (241, 243, 245, 247) sequentially arranged in the pre-cooling loop (110) and configured for expanding the first refrigerant at a plurality of decreasing pressure levels;

a plurality of first heat exchangers (249, 251, 253, 255) of the first heat exchanger device (170, 270) for receiving respective precooling streams of the first refrigerant expanded through at least one of said plurality of first expansion elements (241, 243, 245, 247) and for transferring heat from the natural gas to the first refrigerant; and

a plurality of return paths (261, 263, 265, 267) configured for returning said precooling streams of the first refrigerant from the plurality of first heat exchangers (249, 251, 253, 255) to a respective one of the plurality of first compressor stages (151).

10. The system of claim 1, comprising at least one first auxiliary expansion element arranged in the pre-cooling loop (110) and at least one first auxiliary heat exchanger of the first heat exchanger device (170, 270) configured for receiving a portion of said first refrigerant expanded through the at least one first auxiliary expansion element and for transferring heat from the second refrigerant to the first refrigerant.

11. The system of claim 10, comprising:

a plurality of first auxiliary expansion elements (221, 223, 225, 227) sequentially arranged in the pre-cooling loop (110) and configured for expanding the first refrigerant at a plurality of decreasing pressure levels;

a plurality of first auxiliary heat exchangers (229, 231, 233, 235) of the first heat exchanger device (170, 270) configured for receiving respective portions of said first refrigerant expanded through at least one of said plurality of first auxiliary expansion elements (221, 223, 225, 227) and for transferring heat from the second refrigerant to the first refrigerant; and

a plurality of return paths (261, 263, 265, 267) configured for returning said portions of the first refrigerant from the plurality of first auxiliary heat exchangers (229, 231, 233, 235) to a respective one of said plurality of first compressor stages (151).

12. The system of claim 1, wherein the first refrigerant comprises a gas with a molecular weight of 40 or more, particularly propane, and/or wherein the second refrigerant is a mixed refrigerant, particularly a mixture comprising methane, ethane, propane and/or nitrogen.

13. The system claim 1, wherein said prime mover (160) comprises an electric motor or an internal combustion engine, particularly a gas turbine.

14. A method of liquefying natural gas, comprising:

providing an integrally-geared turbo-compressor (150) having a plurality of compressor stages;

driving the compressor (150) with a single prime mover (160);

circulating a first refrigerant through one or more first compressor stages (151) of the plurality of compressor stages, each of the one or more first compressor stages (151) being driven by said single prime mover (160);

circulating a second refrigerant through one or more second compressor stages (155) of the plurality of compressor stages, each of the one or more second compressor stages (155) being driven by said single prime mover (160);

cooling at least one of natural gas and the second refrigerant by heat exchange against the first refrigerant; and

cooling the natural gas by heat exchange against the second refrigerant.

15. The method of claim 14, further comprising:

expanding the first refrigerant through a plurality of sequentially arranged first expansion elements (241, 243, 245, 247) at a plurality of decreasing pressure levels;

circulating portions of the first refrigerant from the plurality of sequentially arranged first expansion elements through a plurality of first heat exchangers (249, 251, 253, 255) to remove heat from the natural gas; and

returning the portions of the first refrigerant from the plurality of first heat exchangers to respective ones of said one or more first compressor stages.

16. The method of claim 14, wherein a transmission mechanism (301) of the compressor is driven by the prime mover (160),

at least one first shaft (303) is driven into rotation by said transmission mechanism (301) at a rotation speed of 3.000 rpm or more and 7.000 rpm or less and drives at least one of the first compressor stages; and

at least one second shaft (305) is driven into rotation by said transmission mechanism (301) at a rotation speed of 8.000 rpm or more and 20.000 rpm or less and drives at least one of the second compressor stages.

17. The method of claim 14, wherein the first refrigerant is sequentially circulated through three, four or more first compressor stages and compressed to an exit pressure ranging from 10 bar to 40 bar absolute, and/or wherein the second refrigerant is sequentially circulated through three, four or more second compressor stages and compressed to an exit pressure ranging from 50 bar to 100 bar absolute.

18. The method of claim 14, further comprising controlling independently movable inlet guide vanes to regulate partial flows at a suction side of the one or more first compressor stages, particularly as a function of flow conditions of respective partial flows.