JP4142559B2 - Gas liquefaction apparatus and gas liquefaction method - Google Patents

Description

  The present invention relates to a gas liquefaction method and gas liquefaction apparatus using liquefied natural gas (LNG) cooling, and is particularly useful as a liquefaction technique for nitrogen gas produced by an air separation apparatus or the like.

  Natural gas (NG) is stored as liquefied natural gas (LNG) for convenience of transportation and storage, and after being vaporized, it is mainly used for thermal power generation and city gas. For this reason, a technique for effectively utilizing the coldness of LNG has been developed.

  As equipment for liquefying nitrogen gas etc. using the coldness of LNG, in general, nitrogen gas is compressed to a pressure that can be liquefied by heat exchange with LNG with a compressor, and then heat exchanged with LNG with a heat exchanger. A process of evaporating LNG at a temperature and liquefying nitrogen gas is used.

  However, there is a fluctuation in demand for thermal power generation and city gas, where demand is high during the day and demand is low at night. Therefore, since the amount of LNG supplied to the gas liquefaction process also fluctuates due to the fluctuation in demand, the amount of cold that can be used also fluctuates. For this reason, when the amount of LNG supplied is large, the amount of liquefied gas obtained by the gas liquefaction process can be increased. However, when the amount of LNG decreases, the amount of liquefied gas also decreases.

  On the other hand, the electric power for driving the compressor is set to be cheaper at night than the charge during the daytime, so the gas is efficiently liquefied taking into consideration the fluctuation in the supply amount of the LNG and the difference in the power charge. A gas liquefaction process has been proposed. For example, the liquefied natural gas is cooled by a liquefaction process including one or more gas compressors, one or more gas expansion turbines, and a heat exchanger that exchanges heat between the gas and liquefied natural gas. In the method of liquefying the gas, the expansion turbine is stopped or reduced when the supplied liquefied natural gas is increased, and the expansion turbine is operated or increased when the supplied liquefied natural gas is reduced. A gas liquefaction method using the chilling of liquefied natural gas, which is a feature, is known (see, for example, Patent Document 1).

  However, in the above liquefaction method, when the low-temperature gas having cold generated in the expansion turbine is introduced into the heat exchanger, the pressure becomes lower than that of liquefied natural gas (generally 1 MPa or more) into which the low-temperature gas is separately introduced. . For this reason, if a leak occurs in the heat exchanger, natural gas is mixed into the liquefied gas line and there is a risk of explosion. In addition, the gas component which passed through the heat exchanger and the pressure-reducing valve after being compressed by the gas compressor is also introduced into the heat exchanger at a lower pressure than liquefied natural gas.

  On the other hand, there is known a gas liquefaction method in which nitrogen gas is liquefied and cooled by LNG with a plurality of heat exchangers while being compressed in multiple stages (see, for example, Patent Document 2). However, even in this liquefaction method, the pressure of LNG introduced into the heat exchanger becomes higher than the pressure of the introduced gas, and the above-described explosion risk remains. In addition, since the low temperature gas after cooling is compressed by the heat exchanger, a low temperature compressor is necessary and is not suitable for intermittent operation. Therefore, the liquefaction process is intermittently adapted to the amount of LNG that fluctuates day and night. It was difficult to do. That is, when a low-temperature compressor is used, a preparation process for cooling and power costs are necessary, and intermittent operation increases process time and cost loss.

Furthermore, in the gas liquefaction method described in Patent Document 2, a method of returning only the vaporized components to the LNG heat exchanger by a gas-liquid separator after one-stage expansion of the low-temperature gas is employed. Driving is not possible.
Japanese Patent Laid-Open No. 5-45050 Japanese Patent Laid-Open No. 11-142054

  Accordingly, the object of the present invention is to avoid the danger of explosion due to natural gas leaks, and to perform intermittent operation suitably in response to the amount of LNG varying between day and night, and further to multistage compression. An object of the present invention is to provide a gas liquefaction method and a gas liquefaction apparatus capable of performing efficient gas liquefaction by expansion.

  The above object can be achieved by the present invention as described below.

That is, in the gas liquefaction method of the present invention, the main part of the gas compressed by the multi-stage compressor is introduced into the first heat exchanger to make a low temperature gas while exchanging heat with the liquefied natural gas. After expanding a part of the gas in a range of pressure higher than that of the liquefied natural gas, it is returned to the first heat exchanger and heated by heat exchange, and then an intermediate stage of an appropriate pressure of the compressor. And the remaining portion of the low-temperature gas derived from the first heat exchanger is expanded and further cooled to a low temperature and introduced into the first gas-liquid separator. The remaining portion of the gas compressed by the compressor is 2 It introduces into the heat exchanger, cools itself by heat exchange, expands, cools to a lower temperature, introduces it into the first gas-liquid separator, and the gas collected in the first gas-liquid separator is After returning to the second heat exchanger again and heating itself by heat exchange, While merging the suitable pressure intermediate stage or inhalation stage of the machine, said step of removing a is set liquid in the first gas-liquid separator seen including, and the liquefied natural gas introduced into the first heat exchanger A part is branched and discharged from the intermediate part of the first heat exchanger in the form of liquefied natural gas, and the remaining part is led out from the first heat exchanger in a gaseous state .

  According to the gas liquefaction method of the present invention, since the gas heated by heat exchange is joined to the inlet side of the compressor, there is no need to use a low-temperature compressor as the compressor, and intermittent operation is possible. It becomes possible. In addition, since the gas derived from the first heat exchanger is branched and a part thereof is expanded in a range of pressure higher than that of the liquefied natural gas, the first heat exchanger is introduced again into the first heat exchanger. Even if a leak occurs, the pressure of nitrogen or the like is higher, so that the danger of an explosion can be avoided without the natural gas leaking. Furthermore, the reason why gas can be efficiently liquefied by multi-stage compression / expansion is as follows.

  When using LNG cold for liquefaction of nitrogen gas etc., the temperature of LNG is around −155 ° C., while the atmospheric pressure boiling point of nitrogen is −196 ° C., so it is necessary to fill in the temperature level difference between them. . For this reason, a method is known in which the nitrogen gas is usually boosted to a critical pressure or higher (preferably 5 to 6 MPa) and then cooled to about −153 ° C. using the coldness of LNG, which is freely expanded to obtain a low temperature. ing. The lower the pressure after expansion, the lower the temperature is obtained, but the proportion of the gas component generated by expansion increases and the net liquefaction amount decreases. Further, in order to reuse the gaseous component, it is necessary to compress again, and the required power increases as the compression ratio increases. Therefore, the compression power required for liquefaction varies depending on how the expansion pressure is selected and in what stage the expansion is performed.

In addition, when the required amount of liquefaction is larger than the amount of LNG that can be used, cooling for liquefaction of nitrogen in a low temperature region is particularly insufficient. In order to solve this problem, it is effective to expand and return part of the low-temperature gas cooled by LNG. In this case, the compression ratio of compression is small again, and it can be limited to a slight increase in power, which is effective. For this reason, in this invention, efficient driving | operation is attained by combining multistage expansion | swelling. Here, the liquefied natural gas branched and discharged from the intermediate stage of the first heat exchanger can be increased or decreased according to the amount of the gas to be liquefied, and can be used for power generation after being evaporated. . Moreover, the natural gas derived | led-out in the gaseous state from the 1st heat exchanger can be used for an electric power generation. Therefore, the gas liquefaction operation using such liquefied natural gas as a cold can be performed according to the demand for power generation and city gas, and intermittent operation is suitably performed corresponding to the amount of LNG changing day and night. be able to.

  As a result of the above, the risk of explosion due to natural gas leaks can be avoided, and intermittent operation can be suitably performed in response to the amount of LNG changing day and night, and more efficient by multistage compression and expansion. It is possible to provide a gas liquefaction method capable of performing gas liquefaction.

  In the above, the liquid taken out from the first gas-liquid separator is further expanded at least one stage, further cooled to be introduced into the second gas-liquid separator, and the gas generated by the expansion is again supplied to the second heat exchanger. It is preferable that the method further includes a step of taking out the liquid of the second gas-liquid separator while joining itself to an intermediate stage or a suction stage of an appropriate pressure of the compressor after heating itself by heat exchange.

  In this case, the liquid taken out from the first gas-liquid separator is expanded and partially vaporized, whereby a liquefied gas close to atmospheric pressure can be obtained at a lower temperature due to the cooling effect during the expansion. In addition, the gas component is derived from the second gas-liquid separator, introduced into the second heat exchanger and heated, and then joined to the intermediate stage or the suction stage of an appropriate pressure of the compressor. By this gas component, the gas introduced into the second heat exchanger can be further cooled.

On the other hand, the gas liquefaction apparatus of the present invention includes a multi-stage compressor, a first heat exchanger that introduces a main part of the gas compressed by the compressor and exchanges heat with liquefied natural gas, and a first heat exchanger thereof. A first expansion valve that expands a part of the gas derived from one heat exchanger in a range of pressure higher than that of the liquefied natural gas, and a gas that has passed through the first expansion valve are introduced into the first heat exchanger to generate heat. A path for joining the intermediate stage of the compressor with an appropriate pressure after the exchange, a second expansion valve for expanding the remaining gas derived from the first heat exchanger and cooling it to a lower temperature, and the second expansion valve A first gas-liquid separator that introduces the fluid having passed through the second gas exchanger, a second heat exchanger that exchanges heat between the gas component derived from the first gas-liquid separator and the remainder of the gas compressed by the compressor, After the gas component derived from the first gas-liquid separator is introduced into the second heat exchanger, the compressor A path for joining an intermediate stage or a suction stage having an appropriate pressure, and the remainder of the gas compressed by the compressor are introduced into the second heat exchanger and then introduced into the first gas-liquid separator through a third expansion valve. And a path for taking out the liquefied liquid from the first gas-liquid separator , and a part of the liquefied natural gas introduced into the first heat exchanger from an intermediate part of the first heat exchanger It comprises a path for branching and discharging in the state of liquefied natural gas and a path for deriving the remainder from the first heat exchanger in a gaseous state .

According to the gas liquefaction apparatus of the present invention, since the gas heated by heat exchange is joined to the inlet side of the compressor, there is no need to use a low-temperature compressor as the compressor, and intermittent operation is possible. It becomes possible. In addition, in order to branch the gas derived from the first heat exchanger and expand a part thereof in a range of pressure higher than that of liquefied natural gas by the first expansion valve, the gas is again introduced into the first heat exchanger. Even if a leak occurs in the first heat exchanger, since the pressure of nitrogen or the like is higher, the risk of explosion can be avoided without the natural gas leaking. Furthermore, the liquefied natural gas branched and discharged from the intermediate stage of the first heat exchanger can be increased or decreased according to the amount of gas to be liquefied, and can be used for power generation or the like after being evaporated. Moreover, the natural gas derived | led-out in the gaseous state from the 1st heat exchanger can be used for an electric power generation. Furthermore, for the reasons described above, efficient gas liquefaction can be performed by multistage compression / expansion. As a result, the risk of explosion due to natural gas leaks can be avoided, and intermittent operation can be suitably performed corresponding to the amount of LNG changing day and night, and more efficient by multi-stage compression and expansion. It is possible to provide a gas liquefying apparatus capable of liquefying various gases.

  In the above, the fourth expansion valve that further expands the liquid taken out from the first gas-liquid separator at least one stage and cools it to a lower temperature, and the second gas-liquid separator that introduces the fluid that has passed through the fourth expansion valve And a path for deriving a gas component from the second gas-liquid separator, introducing it into the second heat exchanger and warming itself, and then joining the intermediate stage or the suction stage with an appropriate pressure of the compressor; It is preferable to further include a path for taking out the liquid of the second gas-liquid separator.

  In this case, the liquid taken out from the first gas-liquid separator is expanded by the fourth expansion valve to partially vaporize, thereby obtaining a liquefied gas at a lower temperature and close to atmospheric pressure due to the cooling effect during the expansion. be able to. In addition, the gas component is derived from the second gas-liquid separator, introduced into the second heat exchanger and heated, and then joined to the intermediate stage or the suction stage of an appropriate pressure of the compressor. By this gas component, the gas introduced into the second heat exchanger can be further cooled.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an example of a gas liquefying apparatus according to the present invention. In the present embodiment, the case of nitrogen gas is exemplified as the gas to be liquefied, but the present invention can be similarly applied to liquefaction of other gases such as air and argon. Moreover, conditions, such as temperature of each part, a pressure, and a flow volume, can be suitably changed according to other conditions, such as a kind and flow volume of gas.

  The gas liquefying apparatus of the present invention includes a compressor composed of a plurality of stages. In the present embodiment, as shown in FIG. 1, an example in which three compressors, that is, a low-pressure stage compressor C1, an intermediate-pressure stage compressor C2, and a high-pressure stage compressor C3 are provided from the upstream side.

In the present invention, a gas to be liquefied (for example, nitrogen gas (GN ₂ )) is introduced into the low-pressure stage compressor C1. This gas may be at a temperature lower than room temperature (25 ° C.), but a temperature near room temperature is preferable because the use of a low-temperature compressor can be avoided. Moreover, in this invention, since the gas heated by heat exchange joins to the inlet side of compressor C1-C3, any compressor C1-C3 can avoid use of a low-temperature compressor. In the present embodiment, for example, nitrogen gas at normal temperature and near atmospheric pressure is introduced into the compressor C1 (34000 Nm ³ / h) and compressed to about 0.4 MPa (gauge pressure, the same applies hereinafter), and then the compressor C2 performs about Compress to 3 MPa, then compress to about 5 MPa with compressor C3. In this invention, it is preferable to finally compress to 5-6 MPa with the compressor C3.

  The main part of this compressed gas is introduced into the first heat exchanger E1, and is cooled by exchanging heat with liquefied natural gas (LNG) introduced into the counterflow. In this embodiment, the 1st heat exchanger E1 shows the example comprised by two heat exchanger E1a and heat exchanger E1b. The liquefied natural gas is introduced into the heat exchanger E1b at, for example, about 1.9 MPa, −155 ° C. and 45 ton / h, and after being led out from the heat exchanger E1b, a part thereof is in the state of liquefied natural gas as excessive cold. It is branched and discharged, and the remainder is introduced into the heat exchanger E1a.

Natural gas derived in a gas state by the heat exchanger E1a is about 1.9 MPa and is near room temperature, and can be used for power generation and the like. Further, the liquefied natural gas branched and discharged can be used in the same manner after evaporating. In general, thermal power generation and city gas have a high demand at daytime during the day and a low demand at night. Therefore, in the present invention, the gas liquefaction method is performed only during the daytime period to perform intermittent operation. be able to. The branching / exhaust of liquefied natural gas can be increased or decreased according to the amount of gas to be liquefied.

The compressed gas is heat-exchanged with liquefied natural gas (LNG) in the first heat exchanger E1 and cooled to about −153 ° C. Part of the low temperature gas (about 8000 Nm ³ / h) derived from the first heat exchanger E1 (heat exchanger E1b) is introduced into the first expansion valve V1, and is higher than the liquefied natural gas introduced into the heat exchanger E1b. The pressure is expanded to a range (for example, about 3 MPa). This expansion corresponds to free expansion and generates cold.

  The gas that has passed through the first expansion valve V1 is introduced into the first heat exchanger E1 for heat exchange, and then merged with an intermediate stage having an appropriate pressure in a plurality of stages of compressors. In the present embodiment, an example is shown in which the high-pressure stage compressor C3 is joined to the inlet side. At this time, the gas introduced into the first heat exchanger E1 is heated to the normal temperature addition and cools the gas from the compressor C3.

On the other hand, the remainder of the gas derived from the first heat exchanger E1 is introduced into the second expansion valve V2 (about 26000 Nm ³ / h), and is expanded to cool to a lower temperature. This expansion corresponds to free expansion. In addition to generating cold, part of the gas can be liquefied. The pressure after expansion is 0.4 MPa in this example.

The fluid after expansion is introduced into the first gas-liquid separator B1, and gas-liquid separation is performed. The liquefied liquid is taken out from the path connected to the bottom of the first gas-liquid separator B1. This liquid content is about 17400 Nm ³ / h.

The gas component derived from the first gas-liquid separator B1 is introduced into the second heat exchanger E2 (approximately 8600 Nm ³ / h), branched from the outlet side of the high-pressure stage compressor C3, and introduced in a countercurrent manner. Heat exchange with the remaining gas (about 4400 Nm ³ / h). As a result, the remaining gas from the compressor C3 is cooled to -175 ° C.

  The gas component derived from the first gas-liquid separator B1 is introduced into the second heat exchanger E2, and then merged into an intermediate stage or an intake stage having an appropriate pressure of a plurality of compressors. In the present embodiment, an example is shown in which the medium is joined to the inlet side of the compressor C2 in the intermediate pressure stage. Further, the gas branched from the outlet side of the high-pressure stage compressor C3 is introduced into the second heat exchanger E2, and then introduced into the first gas-liquid separator B1 through the third expansion valve V3.

In the third expansion valve V3, the gas introduced at about 5 MPa is expanded (free expansion) to about 0.4 MPa, further cooled to a low temperature, and thereby partially liquefied in a saturated state. The gas component (200 Nm ³ / h) and the liquid component (4200 Nm ³ / h) are mixed with the fluid that has passed through the second expansion valve V2, and are respectively derived from the first gas-liquid separator B1 as a mixture.

In this embodiment, as shown in FIG. 2, the liquid taken out from 1st gas-liquid separator B1 is expanded further, and the example cooled further to low temperature is shown. That is, the liquid taken out from the first gas-liquid separator B1 is introduced into the fourth expansion valve V4 and expanded (free expansion) to 0.03 MPa, and a part thereof is vaporized. At this time, a liquid content of 18000 Nm ³ / h and a gas content of 3600 Nm ³ / h are generated, and these are introduced into the second gas-liquid separator B2.

  A gas component is derived from the second gas-liquid separator B2 and introduced into the second heat exchanger E2. At this time, the introduced gas is heated by cooling the gas from the outlet side of the high-pressure compressor C3. The gas components derived from the second heat exchanger E2 are joined to the intermediate stage or the suction stage having an appropriate pressure of a plurality of stages of compressors. In this example, they are joined to the inlet side of the low-pressure stage compressor C1. .

From the bottom of the second gas-liquid separator B2, liquefied liquid (LN ₂ ) is taken out at 18000 Nm ³ / h, about 0.03 MPa, and about −194 ° C.

  In the gas liquefaction method of the present invention, as described above, the main part of the gas compressed by the multistage compressors C1 to C3 is introduced into the first heat exchanger E1 to exchange heat with the liquefied natural gas. After a part of the derived low-temperature gas is expanded in a range of pressure higher than that of the liquefied natural gas, it is returned to the first heat exchanger E1 and heated by heat exchange, and then compressed. And the remaining low-temperature gas derived from the first heat exchanger E1 is expanded and cooled to a lower temperature and introduced into the first gas-liquid separator B1. The remaining part of the gas compressed by the compressors C1 to C3 is introduced into the second heat exchanger E2, cooled by itself through heat exchange, expanded and then cooled to a lower temperature, and then the first gas-liquid separator. Gas introduced into B1 and collected in the first gas-liquid separator B1 After returning to the second heat exchanger E2 and heating itself by heat exchange, the first gas-liquid separator B1 is joined to the intermediate stage or the suction stage of the compressors C1 to C3 at an appropriate pressure. And a step of taking out the liquid collected in step (b).

  In the present embodiment, the liquid taken out from the first gas-liquid separator B1 is further expanded at least one stage to be further cooled to be introduced into the second gas-liquid separator B2, and the gas component generated by the expansion is again supplied to the first gas-liquid separator B1. 2 After returning to the heat exchanger E2 and heating itself by heat exchange, the liquid in the second gas-liquid separator B2 is added to the intermediate stage or the suction stage at an appropriate pressure of the compressors C1 to C3. The example further including the process of taking out was shown.

According to the operating conditions shown in this example, the liquefaction unit for obtaining saturated liquid nitrogen of about 0.03 MPa is 0.283 kWh / Nm ³ . This value is a significant reduction in power as compared with the liquefaction unit 0.6 to 0.7 kWh / Nm ^{3 of the} conventional liquefaction method using an expansion turbine (without LNG).

[Other Embodiments]
(1) In the above-described embodiment, an example in which a three-stage compressor is provided and a circulation path including compression, heat exchange, and expansion according to three types of expansion pressures is provided. A multistage compressor may be provided, and a circulation path composed of compression, heat exchange, and expansion may be provided according to the multistage expansion pressure.

  In that case, a fifth expansion valve that expands the liquid taken out from the second gas-liquid separator to partially vaporize, a third gas-liquid separator that introduces the fluid that has passed through the fifth expansion valve, and a third thereof A gas component derived from the gas-liquid separator, introduced into the third heat exchanger and heated, and then joined to the inlet side of the n-3rd stage compressor; and the third gas-liquid separation The gas liquefying apparatus includes a path for taking out the liquefied liquid from the vessel.

  (2) In the above-described embodiment, an example in which each of the three-stage compressors is provided has been described. However, the compression may be performed in a plurality of stages by a single compression device, or may be further performed in multiple stages. . In addition, the return gas is joined to the middle of each of the three-stage compressors, but the return gas may be joined to the intermediate stage having an appropriate pressure of the compressor.

The schematic block diagram which shows an example of the liquefying apparatus of the gas of this invention The schematic block diagram which shows the other example of the gas liquefying apparatus of this invention

Explanation of symbols

E1 1st heat exchanger E2 2nd heat exchanger C1 Compressor (low pressure stage)
C2 compressor (medium pressure stage)
C3 compressor (high pressure stage)
V1 1st expansion valve V2 2nd expansion valve V3 3rd expansion valve V4 4th expansion valve B1 1st gas-liquid separator B2 2nd gas-liquid separator

Claims (4)

The main part of the gas compressed by the multi-stage compressor is introduced into the first heat exchanger to exchange heat with the liquefied natural gas to form a low temperature gas, and a part of the derived low temperature gas is higher than the liquefied natural gas. After expanding in the range of pressure, it is returned to the first heat exchanger again and heated by heat exchange, and then merged with an intermediate stage of an appropriate pressure of the compressor, and the first heat exchanger The remainder of the low temperature gas derived from is expanded and cooled to a low temperature and introduced into the first gas-liquid separator, and the remainder of the gas compressed by the compressor is introduced into the second heat exchanger to perform heat exchange. It cools itself, expands, cools to a lower temperature, and introduces it into the first gas-liquid separator. The gas gathered in the first gas-liquid separator is returned to the second heat exchanger and heated again. After heating itself by exchange, the intermediate stage or suction at the appropriate pressure of the compressor While merging, the saw including a step of removing a liquid that has been set by the first gas-liquid separator, and a portion of the liquefied natural gas introduced into the first heat exchanger, the first heat exchanger A gas liquefaction method comprising: branching and discharging from an intermediate part in a state of liquefied natural gas; and discharging the remaining part from a first heat exchanger in a gaseous state . The liquid taken out from the first gas-liquid separator is further expanded at least one stage to be further cooled to be introduced into the second gas-liquid separator, and the gas generated by the expansion is returned to the second heat exchanger again. The gas of claim 1, further comprising the step of taking out the liquid in the second gas-liquid separator while warming itself by heat exchange and then joining it to an intermediate stage or suction stage of an appropriate pressure of the compressor. Liquefaction method. A compressor composed of a plurality of stages, a first heat exchanger that introduces a main part of the gas compressed by the compressor and exchanges heat with liquefied natural gas, and a part of the gas derived from the first heat exchanger A first expansion valve that expands the liquefied natural gas in a range of pressure higher than that of the liquefied natural gas, and a gas that has passed through the first expansion valve is introduced into the first heat exchanger, and after heat exchange, an intermediate intermediate pressure of the compressor A path for joining the stages, a second expansion valve for expanding the remainder of the gas derived from the first heat exchanger and cooling it to a lower temperature, and a first gas-liquid separation for introducing fluid through the second expansion valve , A second heat exchanger for exchanging heat between the gas component derived from the first gas-liquid separator and the remainder of the gas compressed by the compressor, and a gas component derived from the first gas-liquid separator Is introduced into the second heat exchanger and then into the intermediate stage or suction stage of the compressor with an appropriate pressure. A flow path, a path for introducing the remainder of the gas compressed by the compressor into the second heat exchanger and then introducing the remaining gas into the first gas-liquid separator through a third expansion valve, and the first gas-liquid separation A path for taking out the liquefied liquid from the vessel , and a part of the liquefied natural gas introduced into the first heat exchanger is branched and discharged from the intermediate portion of the first heat exchanger in the state of liquefied natural gas. A gas liquefaction apparatus comprising: a path; and a path for deriving the remainder from the first heat exchanger in a gaseous state . A fourth expansion valve for further expanding the liquid taken out from the first gas-liquid separator at least one stage and further cooling to a low temperature; a second gas-liquid separator for introducing the fluid that has passed through the fourth expansion valve; A path for deriving a gas component from the second gas-liquid separator, introducing it into the second heat exchanger, heating it, and then joining it to an intermediate stage or suction stage of an appropriate pressure of the compressor; The gas liquefying apparatus according to claim 3, further comprising a path for taking out the liquid of the gas-liquid separator.

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