Apparatus And Method For Liquefying Natural Gas By Refrigerating Single Mixed Working Medium

Abstract

A system and a method for liquefying natural gas using single mixed refrigerant as refrigeration medium are provided. The system comprises a two-stage mixed refrigerant compressor (1), coolers (21, 22), gas-liquid separators (31, 32), throttling devices (51, 52), a plate-fin heat exchanger group (8) and a LNG storage tank (9). The method of the present invention reduces the power consumption for gas compression by compressing and separating the mixed refrigerant stage by stage. The heat exchange curves of cold fluid and hot fluid in the total heat exchange process match with each other better by the aid of using multiple-stage heat exchange, which can reduce the flow of the mixed refrigerant. Further, the system of the present invention has a good adaptability to load-variable operation of the apparatus, and thus can effectively avoid abnormal liquid-flooding at the bottom of the cold box.

Description

FIELD OF THE INVENTION

The present invention relates to the liquefaction of a hydrocarbon-rich gas, and particularly to a method and a system for liquefying natural gas using single mixed refrigerant as refrigeration medium.

BACKGROUND OF THE INVENTION

Natural gas is becoming the best material to replace other fuels because of its environmental friendliness, and its fields of application has gradually expanded to power generation, automotive gas, industrial gas, domestic gas, chemical gas etc.

With the growth of natural gas consumption, the trade volume of liquefied natural gas as one of the most effective forms for supplying natural gas has become one of the fastest growth areas in the energy market. Continuous development of liquefied natural gas industry places stricter requirements for energy consumption, investment, efficiency etc. of the natural gas liquefaction method and apparatus.

Currently, the relatively well-established techniques for liquefying natural gas mainly comprise: cascade refrigeration technology, expansion refrigeration technology and mixed refrigerant refrigeration process. Among them, single mixed refrigerant refrigeration process is more preferable for medium-sized LNG plant.

In the existing process for liquefying natural gas by single mixed refrigerant refrigeration, the refrigerant compressor system comprises two compressor stages, and the liquefaction of natural gas is carried out by using one-stage heat exchange.

In the prior art, as shown in FIG. 1, the system comprises a two-stage mixed refrigerant compressor 1 driven by a motor, two coolers 21 and 22, two gas-liquid separators 31 and 32, two liquid pumps 4 and 4′, a plate-fin heat exchanger 8 and a LNG storage tank 9. The mixed refrigerant composed of C1˜C5 alkanes and N₂ in a reasonable proportion is fed into the inlet of the compressor, compressed to 0.6˜1 MPa by first stage compression, entered into the first stage cooler and cooled to 30˜40° C., and then introduced into the first stage gas-liquid separator for gas-liquid separation. The gas separated from the top of the first stage gas-liquid separator is fed to the inlet of the second stage compressor, compressed to 1.6˜2.5 MPa by the second stage compression; and the liquid obtained at the bottom of the first stage gas-liquid separator is pressurized by the first liquid pump, mixed with the gas from the outlet of the second stage compressor, further introduced to the second stage cooler and cooled to 30˜40° C., and then the mixed refrigerant after cooling is fed to the second stage gas-liquid separator for gas-liquid separation. The separated liquid is pressurized by the second liquid pump and mixed with the gas obtained at the top of the second stage gas-liquid separator. The resulting mixture is fed to the plate-fin heat exchanger, precooled to a determined temperature, throttled after exiting from the heat exchanger, and returned to the plate-fin heat exchanger again for providing cold energy for the overall heat exchanging process. The natural gas is supplied to the LNG storage tank after passing through the plate-fin heat exchanger.

In the above-said process, to ensure that the liquid and the gas is fed to the same passage of the plate-fin heat exchanger and participated in heat exchanging, the liquid obtained at the bottom of the final stage gas-liquid separator needs to be pressurized to overcome the liquid column pressure resulted from the height difference from the liquid outlet of the bottom of the gas-liquid separator to the refrigerant inlet at the top of the plate-fin heat exchanger, which is achieved by providing the final stage liquid pump. The heat exchange process between the refrigerant and natural gas in the plate-fin heat exchanger is a one-stage heat exchange, the optimization of the temperature difference for heat exchange between the streams is limited to some extent, the energy consumption of the apparatus is high, and abnormal liquid-flooding is easy to occur at the bottom of the cold box. Further, there is no good adaptability to load-variable operation of the apparatus.

SUMMARY OF THE INVENTION

The present invention provides a method and a system for liquefying natural gas using single mixed refrigerant as refrigeration medium. Natural gas is liquefied by single mixed refrigerant cooling in the present invention.

The method and system for liquefying natural gas using single mixed refrigerant as refrigeration medium in the present invention have a natural gas cycle and a mixed refrigerant refrigeration cycle. In the mixed refrigerant refrigeration cycle, the stage-by-stage compression of the mixed refrigerant is accompanied by stage-by-stage gas-liquid separation, and the liquid phase stream separated from the first stage compression does not participate in the subsequent compression process, which effectively reduces the power consumption of the subsequent gas compression. The gas phase and liquid phase mixed refrigerant streams obtained by compression are fed to different passages of the heat exchanger group respectively for throttling and heat exchanging, the final stage liquid pump is omitted compared to the existing process, and the heat exchange curves of cold fluid and hot fluid in the total heat exchange process match with each other better by the aid of using multiple-stage heat exchange. The gas phase from the final stage gas-liquid separator is throttled, formed as a backflow, reheated and entered into a refrigerant separator, which may effectively avoid abnormal liquid-flooding of the cold box.

The system for liquefying natural gas using single mixed refrigerant as refrigeration medium in the present invention comprises a mixed refrigerant compressor system and a cold box system, wherein the mixed refrigerant compressor system uses two-stage compression and comprises a two-stage mixed refrigerant compressor, two coolers, two gas-liquid separators, and a liquid pump, and the cold box system comprises a plate-fin heat exchanger group (two-stage heat exchange), two gas-liquid separator (comprising a heavy hydrocarbon separator and a refrigerant separator) and two throttling devices. The whole heat exchange of the mixed refrigerant and natural gas is conducted in the cold box system.

In the mixed refrigerant compressor system, the outlet of the first stage compressor is connected to the first cooler, and the latter is connected to the first gas-liquid separator. The gas phase port of the first gas-liquid separator is connected to the second stage compressor, the liquid phase port at the bottom of the first gas-liquid separator is connected to a liquid pump, and the output line of the liquid pump is converged with the output line of the second stage compressor and then connected to the second cooler. The second cooler is then connected to the second gas-liquid separator. The gas phase port at the top of the second gas-liquid separator is in fluid communication with the first heat exchange passage of the heat exchanger group, and the liquid phase port of the bottom of the second gas-liquid separator is in fluid communication with the second heat exchange passage of the heat exchanger group.

In the cold box system, the liquid phase port at the bottom of the second gas-liquid separator is connected to an end of the first throttling device via the second heat exchange passage of the heat exchanger group, and an another end of the first throttling device is connected to the first stage compressor via the third heat exchange passage of the heat exchanger group. The gas phase port at the top of the second gas-liquid separator is connected to the first heat exchange passage of the heat exchanger group for precooling, and then connected to an end of the second throttling device, and an another end of the second throttling device is connected to the refrigerant separator via the fourth heat exchange passage of the heat exchanger group. A natural gas line is connected to the fifth heat exchange passage of the heat exchanger group and then to the heavy hydrocarbon separator. The gas phase port at the top of the heavy hydrocarbon separator is connected to subsequent stages of the heat exchanger and then to the LNG storage tank, and the liquid phase obtained at the bottom of the heavy hydrocarbon separator is obtained as the liquefied petroleum gas (LPG) product.

In the mixed refrigerant compressor system, the gas exited from the first compressor stage is passed through the first cooler and cooled, and then entered into the first gas-liquid separator for gas-liquid separation; the gas phase after separation is passed through the second compressor stage, and the liquid phase after separation is pressurized by a liquid pump, converged with the hot gas obtained after the second stage compression, cooled by the second cooler, and introduced to the second gas-liquid separator for gas-liquid separation; and the gas phase obtained at the top of the second gas-liquid separator is passed through the first heat exchange passage (i.e. the gas phase passage) of the downstream heat exchanger, and the liquid phase obtained at the bottom of the second gas-liquid separator is passed through the second (liquid phase) heat exchange passage of the downstream heat exchanger. In the cold box system, the liquid refrigerant from the bottom of the second gas-liquid separator in the mixed refrigerant compressor system is passed through the heat exchanger group, precooled and then throttled by the first throttling device; the throttled stream is introduced to the middle part of the refrigerant separator; the gas refrigerant from the top of the second gas-liquid separator is passed through the heat exchanger group, precooled and then throttled by the second throttling device; the throttled gas stream is reversely passed through the heat exchanger group, reheated to a determined temperature, introduced into the middle part of the refrigerant separator, and converged with the precooled and throttled liquid refrigerant which is exited from the first throttling device and entered likewise into the above-mentioned refrigerant separator. The two refrigerants are separated into two phases, i.e., gas phase and liquid phase, by the refrigerant separator, and the two phases exited from the refrigerant separator are joined together and then returned to the heat exchanger group for providing cold energy. On the other hand, natural gas is firstly passed through the heat exchanger group, cooled to a given temperature and then entered to the heavy hydrocarbon separator for separation. The heavy hydrocarbon component is obtained at the bottom of the heavy hydrocarbon separator. The gas phase component obtained at the top of heavy hydrocarbon separator is further passed through the other stages of the heat exchanger group for heat exchanging, cooled to supercooled state, and thus obtained LNG is delivered to the LNG storage tank for storing.

In order to better understand the present invention, the technical solution of the system of the present invention are summarized as follows.

The present invention provides a system for liquefying natural gas using single mixed refrigerant as refrigeration medium comprising a mixed refrigerant compressor system and a cold box system,

wherein:
the mixed refrigerant compressor system comprises:
a two-stage mixed refrigerant compressor;
a first cooler and a second cooler respectively connected to the first stage and the second stage of the two-stage mixed refrigerant compressor;
a first gas-liquid separator and a second gas-liquid separator respectively connected to the first cooler and the second cooler; and
a liquid pump connected to the first stage gas-liquid separator, and
the cold box system comprises:
a plate-fin heat exchanger group comprising at least six heat exchange passages, i.e., the first, second, third, fourth, fifth and sixth heat exchange passages, wherein the inlet ends of the first and second heat exchange passages are respectively connected to the gas phase port and liquid phase port of the second gas-liquid separator via two pipelines, and the outlet end of the third heat exchange passage are connected to the first stage compressor by pipeline;
a first throttling device connected to the outlet end of the second heat exchange passage of the plate-fin heat exchanger group;
a second throttling device connected to the outlet end of the first heat exchange passage and the inlet end of the fourth heat exchange passage of the plate-fin heat exchanger group;
a refrigerant separator connected to the inlet end of the third heat exchange passage, the outlet end of the fourth heat exchange passage of the plate-fin heat exchanger group and the first throttling device;
a heavy hydrocarbon separator connected to a separate heat exchange passage (i.e., the fifth heat exchange passage) of the plate-fin heat exchanger group,
the gas phase port of the first gas-liquid separator is connected to the second stage of the two-stage mixed refrigerant compressor,
the liquid phase discharge line of the first gas-liquid separator is converged via a liquid pump with the discharge line of the second compressor stage, and then connected to the second cooler,
the gas phase port and liquid phase port of the second gas-liquid separator are respectively connected to the inlet ends of two heat exchange passages, i.e., the first and second heat exchange passages of the plate-fin heat exchanger group,
wherein the first throttling device connected to the outlet end of the second heat exchange passage is additionally connected to the refrigerant separator,
the gas phase discharge line at the top of the refrigerant separator is converged with the liquid phase discharge line at the bottom of the refrigerant separator, and then connected to the inlet end of the third heat exchange passage, and the outlet end of the third heat exchange passage is connected to the first stage of the two-stage mixed refrigerant compressor,
the fourth heat exchange passage, which is connected on its inlet end to the second throttling device, is further connected on its outlet end to the refrigerant separator,
a natural gas line is connected to the heavy hydrocarbon separator via above-mentioned separate heat exchange passage, i.e., the fifth heat exchange passage of the plate-fin heat exchanger group, and
the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through a heat exchange passage, i.e., the sixth heat exchange passage of the plate-fin heat exchanger.

Optionally, the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through the sixth and further seventh heat exchange passages of the plate-fin heat exchanger successively. The “first compressor stage” and “first stage compressor” as described herein can be used interchangeably, and so on.

The method for liquefying natural gas by using the above-described system comprises:

Natural Gas Cycle:

Purified natural gas is firstly passed through the plate-fin heat exchanger group, precooled to −30° C.˜−80° C. and then entered to the heavy hydrocarbon separator for gas-liquid separation,
The gas phase stream separated from the top of heavy hydrocarbon separator is further passed through the other stages of the heat exchanger group for heat exchanging, cooled to −130° C.˜−166° C., and thus obtained LNG is delivered to the LNG storage tank for storing;

Mixed Refrigerant Cycle:

The mixed refrigerant composed of C1˜C5 alkanes and N₂ (usually four or five or six components selected from C1, C2, C3, C4, C5 alkanes and N₂, and these components are mixed in any volume ratio or in substantially equal ratio) is fed into the inlet of the compressor, compressed to 0.6˜1.8 MPa by first stage compression, entered into the first cooler and cooled to 30˜40° C., and then introduced into the first gas-liquid separator for gas-liquid separation;
the gas separated from the top of the first stage gas-liquid separator is fed to the inlet of the second stage compressor, compressed to 1.2˜5.4 MPa by the second stage compression;
the liquid separated from the liquid phase port at the bottom of the first stage gas-liquid separator is pressurized to 1.2˜5.4 MPaA by the liquid pump, mixed with the hot gas from the outlet of the second stage compressor, further introduced to the second cooler and cooled to 30˜40° C., and then the mixed refrigerant after cooling is fed to the second gas-liquid separator for gas-liquid separation;
the gas obtained at the top of the second stage gas-liquid separator is passed through the first heat exchange passage of the main heat exchanger group for heat exchanging, and the liquid separated from the bottom of the second stage gas-liquid separator is passed through the second heat exchange passage of the main heat exchanger group for heat exchanging;
the liquid separated from the bottom of the second gas-liquid separator is precooled to about −30° C.˜−80° C. in the second heat exchange passage of the heat exchanger group, throttled to 0.2˜0.8 MPaA by the first throttling device, and then introduced to the middle part of the refrigerant separator;
the gas phase stream of the mixed refrigerant separated from the top of the second gas-liquid separator is passed through the gas phase passage of the heat exchanger group, cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaA by the second throttling device; the throttled gas stream is reversely passed through the heat exchanger group for providing cold energy and reheated to −30° C.˜−80° C., and then introduced into the middle part of the refrigerant separator after exiting from the heat exchanger group, and converged, in the refrigerant separator, with the cooled and throttled liquid refrigerant (exited from the first throttling device and entered likewise into the above-mentioned refrigerant separator). The converged two refrigerants are separated into two phases, i.e., gas phase and liquid phase, by the refrigerant separator, and the two phases exited from the refrigerant separator are joined together, returned to the heat exchanger group for providing cold energy and then introduced into the first compressor stage as the mixed refrigerant.

The pressure unit “MPaA” as described herein refers to Megapaskal, absolute pressure.

The method of the present invention and the system used in the method have been described as above sufficiently.

The advantages of the present invention are as follows:

1. The system of the present invention has a good adaptability to load-variable operation of the apparatus, and the gas phase from the final stage gas-liquid separator is throttled, formed as a backflow, reheated and entered into a refrigerant separator, which can effectively avoid abnormal liquid-flooding at the bottom of the cold box, thus ensuring energy consumption at low load conditions is close to energy consumption at normal operating conditions.
2. The method of the present invention uses a two-stage mixed refrigerant compressor to compress and separate the mixed refrigerant stage by stage, which reduces the power consumption for gas compression.
3. The liquid stream obtained at the bottom of the first gas-liquid separator does not participate in the subsequent stream compression process, which to some extent reduces the influence of the fluctuation of the mixed refrigerant ratio on the compressor running conditions, making the system easier to operate.
4. The heat exchange curves of cold fluid and hot fluid in the whole heat exchange process match with each other better by the aid of using two-stage heat exchange, which can reduce the flow of the mixed refrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system for liquefying natural gas in the prior art.

FIG. 2 shows the system for liquefying natural gas using mixed refrigerant of the present invention.

• 1 two-stage mixed refrigerant compressor
• 4, 4′ liquid pump
• 6 heavy hydrocarbon separator
• 7 refrigerant separator
• 8 plate-fin heat exchanger group
• 9 LNG storage tank
• 21, 22 cooler
• 31, 32 gas-liquid separator
• 51, 52 throttling device

THE MODE OF CARRYING OUT THE INVENTION

The embodiments of the present invention are further described with reference to the Drawings.

Natural Gas Cycle:

As shown in FIG. 2, purified natural gas as raw material is firstly passed through the fifth heat exchange passage of the plate-fin heat exchanger group 8, cooled to −30° C.˜−80 ° C. and then entered to the heavy hydrocarbon separator 6 for gas-liquid separation.
The gas phase stream separated from the top of heavy hydrocarbon separator 6 is further passed through the other stages of the main heat exchanger group 8 (i.e., the sixth heat exchange passage) for heat exchanging, cooled to −130° C.˜−166° C., and thus obtained liquefied natural gas (LNG) is delivered to the LNG storage tank 9 for storing. Liquefied petroleum gas (LPG) is obtained at the bottom of the heavy hydrocarbon separator 6.

Mixed Refrigerant Cycle:

The mixed refrigerant composed of C1˜C5 alkanes and N₂ (i.e., four or five or six components selected from C1, C2, C3, C4, C5 alkanes and N₂, and these components are mixed in any volume ratio or in substantially equal ratio) is fed into the inlet of the compressor 1, compressed to 0.6˜1.8 MPa by first stage compression, entered into the first cooler 21 and cooled to 30˜40° C., and then introduced into the first gas-liquid separator 31 for gas-liquid separation;
the gas separated from the top of the first gas-liquid separator 31 is fed to the inlet of the second stage compressor, compressed to 1.2˜5.4 MPa by the second stage compression;
the liquid separated from the liquid port at the bottom of the first gas-liquid separator 31 is pressurized to 1.2˜5.4 MPaA by the liquid pump 4, mixed with the hot gas from the outlet of the second stage compressor, further introduced to the second cooler 22 and cooled to 30˜40° C., and then the mixed refrigerant after cooling is fed to the second gas-liquid separator 32 for gas-liquid separation;
the gas obtained at the top of the second gas-liquid separator 32 is passed through the first heat exchange passage of the main heat exchanger group 8 for heat exchanging, and the liquid separated from the bottom of the second gas-liquid separator 32 is passed through the second heat exchange passage of the main heat exchanger group 8 for heat exchanging.
the liquid separated from the bottom of the second stage gas-liquid separator 32 is precooled to about −30° C.˜−80° C. in the second heat exchange passage of the heat exchanger group, throttled to 0.2˜0.8 MPaA by the first throttling device 51, and then introduced to the refrigerant separator 7;
the gas phase stream of the mixed refrigerant separated from the top of the second gas-liquid separator 32 is passed through the gas phase passage (i.e., the first heat exchange passage) of the heat exchanger group 8, cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaA by the second throttling device 52; the throttled gas stream is reversely passed through the fourth heat exchange passage of the heat exchanger group 8 for providing cold energy and reheated to −30° C.˜−80° C., and then introduced into the middle part of the refrigerant separator 7 after exiting from the heat exchanger group, and converged, in the refrigerant separator, with the cooled and throttled liquid refrigerant stream (exited from the first throttling device and entered likewise into the above-mentioned refrigerant separator) which is introduced from the second stage gas-liquid separator 32. The converged two refrigerants are separated into two phases, i.e., gas phase and liquid phase, by the refrigerant separator, and the two phases exited from the refrigerant separator are joined together, returned to the third heat exchange passage of the heat exchanger group for providing cold energy and then introduced into the first compressor stage as the mixed refrigerant.

Claims

1. A system for liquefying natural gas using single mixed refrigerant as refrigeration medium comprising a mixed refrigerant compressor system and a cold box system, wherein:

the mixed refrigerant compressor system comprises:

a two-stage mixed refrigerant compressor;

a first cooler and a second cooler respectively connected to the first stage and the second stage of the two-stage mixed refrigerant compressor;

a first gas-liquid separator and a second gas-liquid separator respectively connected to the first cooler and the second cooler; and

a liquid pump connected to the first stage gas-liquid separator,

and the cold box system comprises:

a plate-fin heat exchanger group comprising at least six heat exchange passages, i.e., the first, second, third, fourth, fifth and sixth heat exchange passages, wherein the inlet ends of the first and second heat exchange passages are respectively connected to the gas phase port and liquid phase port of the second gas-liquid separator via two pipelines, and the outlet end of the third heat exchange passage are connected to the first stage compressor by pipeline;

a first throttling device connected to the outlet end of the second heat exchange passage of the plate-fin heat exchanger group;

a second throttling device connected to the outlet end of the first heat exchange passage and the inlet end of the fourth heat exchange passage of the plate-fin heat exchanger group;

a refrigerant separator connected to the inlet end of the third heat exchange passage, the outlet end of the fourth heat exchange passage of the plate-fin heat exchanger group and the first throttling device;

a heavy hydrocarbon separator connected to a separate heat exchange passage, i.e., the fifth heat exchange passage of the plate-fin heat exchanger group,

the gas phase port of the first gas-liquid separator is connected to the second stage of the two-stage mixed refrigerant compressor,

the liquid phase discharge line of the first gas-liquid separator is converged via a liquid pump with the discharge line of the second compressor stage, and then connected to the second cooler,

the gas phase port and liquid phase port of the second gas-liquid separator are respectively connected to the inlet ends of two heat exchange passages, i.e., the first and second heat exchange passages of the plate-fin heat exchanger group,

wherein the first throttling device connected to the outlet end of the second heat exchange passage is additionally connected to the refrigerant separator,

the gas phase discharge line at the top of the refrigerant separator is converged with the liquid phase discharge line at the bottom of the refrigerant separator, and then connected to the inlet end of the third heat exchange passage, and the outlet end of the third heat exchange passage is connected to the first stage of the two-stage mixed refrigerant compressor,

the fourth heat exchange passage, which is connected on its inlet end to the second throttling device, is further connected on its outlet end to the refrigerant separator,

a natural gas line is connected to the heavy hydrocarbon separator via said separate heat exchange passage, i.e., the fifth heat exchange passage of the plate-fin heat exchanger group, and

the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through a heat exchange passage, i.e., the sixth heat exchange passage of the plate-fin heat exchanger.

2. The system according to claim 1, characterized in that the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through the sixth and further seventh heat exchange passages of the plate-fin heat exchanger successively.

3. A method for liquefying natural gas using single mixed refrigerant as refrigeration medium comprises:

a natural gas cycle wherein:

Purified natural gas is firstly passed through a plate-fin heat exchanger group, precooled to −30° C.˜−80° C. and then entered to a heavy hydrocarbon separator for gas-liquid separation,

The gas phase stream separated from the top of the heavy hydrocarbon separator is further passed through the other stages of the heat exchanger group for heat exchanging, cooled to −130° C.˜−166° C., and thus obtained LNG is delivered to a LNG storage tank for storing;

a mixed refrigerant cycle wherein:

The mixed refrigerant composed of C1˜C5 alkanes and N2 is fed into the inlet of a two-stage mixed refrigerant compressor, compressed to 0.6˜1.8 MPa by first stage compression, entered into a first stage cooler and cooled to 30˜40° C., and then introduced into a first stage gas-liquid separator for gas-liquid separation;

the gas separated from the top of the first stage gas-liquid separator is fed to the inlet of the second stage compressor, compressed to 1.2˜5.4 MPa by the second stage compression;

the liquid separated from the liquid phase port at the bottom of the first stage gas-liquid separator is pressurized to 1.2˜5.4 MPaA by a liquid pump, mixed with the hot gas from the outlet of the second stage compressor, further introduced to a second cooler and cooled to 30˜40° C., and then the mixed refrigerant after cooling is fed to a second stage gas-liquid separator for gas-liquid separation;

the gas obtained at the top of the second stage gas-liquid separator is passed through the first heat exchange passage of the main heat exchanger group for heat exchanging, and the liquid separated from the bottom of the second stage gas-liquid separator is passed through the second heat exchange passage of the main heat exchanger group for heat exchanging;

the liquid separated from the bottom of the second stage gas-liquid separator is precooled to about −30° C.˜−80° C. in the second heat exchange passage of the heat exchanger group, throttled to 0.2˜0.8 MPaA by a first throttling device, and then introduced to the middle part of a refrigerant separator;

the gas phase stream of the mixed refrigerant separated from the top of the second gas-liquid separator is passed through the gas phase passage, i.e., the first heat exchange passage of the heat exchanger group, cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaA by a second throttling device; the throttled gas stream is reversely passed through the fourth heat exchange passage of the heat exchanger group for providing cold energy and reheated to −30° C.˜−80° C., and then introduced into the middle part of the refrigerant separator after exiting from the heat exchanger group, and converged, in the refrigerant separator, with the cooled and throttled liquid refrigerant which is discharged from the first throttling device and entered likewise into the above-mentioned refrigerant separator,

the converged two refrigerants are separated into two phases, i.e., gas phase and liquid phase, by the refrigerant separator, and the two phases exited from the refrigerant separator are joined together, returned to the third heat exchange passage of the heat exchanger group for providing cold energy and then introduced into the first compressor stage as the mixed refrigerant.

4. The method according to claim 3, characterized in that the mixed refrigerant comprises four or five or six components selected from C1, C2, C3, C4, C5 alkanes and N2, and these components are mixed in any volume ratio or in substantially equal ratio.