Liquefaction system and liquefaction method

Abstract

The present invention provides a liquefaction system and a liquefaction method that make it possible to suppress energy consumption and that are less likely to cause facility trouble.　According to the present invention, provided is a liquefaction system comprising a compression unit, a decompression unit, and a circulation unit, wherein: the compression unit is configured to use a plurality of stages of compressors to gradually increase the pressure of carbon dioxide to a target pressure for liquefaction of carbon dioxide; the decompression unit is configured to use a plurality of stages of expansion parts to gradually reduce the pressure of the carbon dioxide which has been liquefied by the compression unit to a target pressure; and the circulation unit is configured to circulate, to the compression unit via a plurality of paths which are independent of each other, flash gas that is generated at each stage of the plurality of stages of expansion parts.

Description

Liquefaction system and liquefaction method

[0001] The present invention relates to a liquefaction system and a liquefaction method for liquefying carbon dioxide.

[0002] In order to reduce carbon dioxide, which is said to be one of the causes of global warming, technology called CCS technology is sometimes used to capture carbon dioxide emitted from power plants, chemical plants, etc. and store and inject it deep underground. In CCS, when the carbon dioxide emission source is far from the storage site, a project has been launched to liquefy the carbon dioxide and transport it in a dedicated transport ship to a liquefied carbon dioxide receiving and storage facility near the storage and injection site.

[0003] Patent Document 1 discloses a liquid carbon dioxide production device that produces liquid or supercritical carbon dioxide at a desired temperature and pressure by subjecting the carbon dioxide to a multi-stage compression and cooling cycle.

[0004] Patent No. 4913733

[0005] In Patent Document 1, carbon dioxide is liquefied using only air or water cooling without using an external refrigerant such as propane or chlorofluorocarbon, and therefore the carbon dioxide is compressed to about 6.0 MPa, which requires a great deal of energy. In particular, almost all of the flash gas generated when decompressing the liquefied high-pressure carbon dioxide is compressed again using the same multiple-stage compression cycle, resulting in a great deal of wasted energy. In addition, the use of many multi-fluid heat exchangers makes equipment troubles due to thermal fatigue more likely to occur.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a liquefaction system and a liquefaction method that can reduce energy consumption and are less likely to cause equipment trouble.

[0007] According to the present invention, the following inventions are provided: [1] A liquefaction system comprising a compression section, a decompression section, and a circulation section, wherein the compression section is configured to compress carbon dioxide in stages to a target pressure for liquefying it using a multi-stage compressor, the decompression section is configured to decompress the carbon dioxide liquefied in the compression section to the target pressure in stages using a multi-stage expansion section, and the circulation section is configured to circulate flash gas produced in each stage of the multi-stage expansion section to the compression section via a plurality of paths that are independent of each other. [2] The liquefaction system according to [1], wherein the compression section has a dehydration section in a middle stage of the multi-stage compressor, and the circulation section circulates sub-zero flash gas, of the flash gas produced in each stage of the multi-stage expansion section, downstream of the dehydration section. [3] The liquefaction system according to [1] or [2], wherein the multi-stage expansion section has a first expansion section, a second expansion section, and a third expansion section, and wherein the target pressure of the compression section is 8.0 MPa or higher, the target pressure of the first expansion section is 4.0 to 6.0 MPa, the target pressure of the second expansion section is 1.5 to 3.5 MPa, and the target pressure of the third expansion section is 0.6 to 1.0 MPa. [4] The liquefaction system according to any one of [1] to [3], wherein the decompression section has an expansion turbine for power recovery. [5] The liquefaction system according to [4], wherein the circulation section has a compression turbine configured to compress the flash gas, and the expansion turbine is configured to drive the compression turbine. [6] A liquefaction system comprising a compression unit, a decompression unit, and a circulation unit, wherein the compression unit is configured to compress carbon dioxide in stages to a target pressure for liquefying it using a multi-stage compressor, the decompression unit is configured to decompress the liquefied carbon dioxide to the target pressure using an expansion turbine for power recovery, and the circulation unit is configured to circulate flash gas produced in the decompression unit to the compression unit.[7] A liquefaction method comprising a compression step, a depressurization step, and a circulation step, wherein in the compression step, carbon dioxide is compressed in stages to a target pressure for liquefying by a compression section having a multi-stage compressor, and in the depressurization step, the liquefied carbon dioxide is depressurized in stages to the target pressure by a multi-stage expansion section, and in the circulation step, flash gas produced in each stage of the multi-stage expansion section is circulated to the compression section via a plurality of paths that are independent of each other.

[0008] In the liquefaction system and method of the present invention, flash gas generated in each stage of a multi-stage expansion section can be circulated to the high-pressure side of the multi-stage compressor via multiple, mutually independent paths. This also reduces the number of heat exchangers required, eliminating the need for a multi-fluid heat exchanger. This reduces energy consumption and reduces the likelihood of equipment problems.

[0009] FIG. 4A is a diagram showing a schematic configuration of a liquefaction system 1 according to a first embodiment. FIG. 5A is a diagram showing a schematic configuration of a liquefaction system 1A according to a second embodiment. FIG. 6A is a diagram showing a schematic configuration of a liquefaction system 1B according to a third embodiment. FIG. 6A is a diagram showing a schematic configuration of a liquefaction system 1C, and FIG. 6B is a diagram showing a schematic configuration of a liquefaction system 1D.

[0010] The following describes embodiments of the present invention. The various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently.

[0011] 1, a liquefaction system 1 includes a compression unit 2, a pressure reduction unit 3, and a circulation unit 4. The compression unit 2 is configured to compress carbon dioxide in stages to a target pressure for liquefying the carbon dioxide using a multi-stage compressor 20. The compression unit 2 includes first to fourth compressors 20A to 20D, a dehydration unit 21, and first to fourth coolers 22A to 22D.

[0012] When carbon dioxide is liquefied using only air or water cooling without using an external refrigerant such as propane or chlorofluorocarbon, the target pressure of the compression section 2 is preferably 8.0 MPa or higher. In the present invention, the pressure is an absolute pressure (MPaA). This target pressure is, for example, 8.0 to 15.0 MPa, specifically, for example, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, or 15.0 MPa, and may be in a range between any two of the values ​​exemplified here. In this embodiment, the ambient temperature is assumed to be approximately 30 to 40°C, and the target pressure of the compression section 2 is set to 10.0 MPa. The compression unit 2 uses four compressor stages, first compressor 20A to fourth compressor 20D, to stepwise increase the pressure of carbon dioxide gas from atmospheric pressure (approximately 0.1 MPa) to 10.0 MPa, for example. The first to fourth coolers 22A to 22D are configured to cool the carbon dioxide, which has been heated by compression (pressurization) by the first to fourth compressors 20A to 20D, to approximately 30 to 40°C by air or water cooling. Depending on the environment, such as the temperature of the land where the liquefaction system 1 is installed, the first to fourth coolers 22A to 22D may further cool the carbon dioxide to a lower temperature (for example, approximately 0 to 20°C).

[0013] The compression unit 2 includes a dehydration unit 21 in the middle stage of a multi-stage compressor 20. The middle stage of the multi-stage compressor 20 refers to the position downstream of the most upstream compressor and upstream of the most downstream compressor. Here, the dehydration unit 21 is located between the second compressor 20B in the second stage and the third compressor 20C in the third stage, but this is not limiting. In this embodiment, the dehydration process is performed when the pressure is increased to approximately 1.5 MPa to 2.5 MPa, taking into consideration the appropriate operating pressure of the dehydration unit 21. If the operating pressure of the dehydration unit 21 is lower than 1.5 MPa, the saturated water vapor content of the carbon dioxide gas increases, resulting in increased moisture retention in the carbon dioxide gas, and therefore requiring a large amount of desiccant. On the other hand, increasing the operating pressure of the dehydration unit 21 more than necessary is undesirable because it increases the pressure-temperature rating of the piping and equipment, requiring more expensive piping and equipment. In this embodiment, the operating pressure of the dehydration section 21 is set to 2.0 MPa.

[0014] The dehydration unit 21 includes a pre-cooler 21A and a dehydrator 21B. The pre-cooler 21A is configured to cool carbon dioxide from about 30 to 40°C to about 20°C. The cooling in the pre-cooler 21A preliminarily removes condensed moisture from the carbon dioxide. The pre-cooler 21A preferably uses, as a refrigerant, liquefied carbon dioxide at a temperature of about 15°C generated in the pressure reduction unit 3, for example, but is not limited to this. The dehydrator 21B is configured to remove water contained in the carbon dioxide by adsorbing the moisture with a desiccant or the like.

[0015] <Decompression Section 3 - Decompression Step> The decompression section 3 is configured to gradually decompress the carbon dioxide liquefied in the compression section 2 to a target pressure using a multi-stage expansion section. In this embodiment, the carbon dioxide is gradually decompressed using three stages: the first expansion valve 30A, the second expansion valve 30B, and the third expansion valve 30C. The target pressure of the first expansion valve 30A (first expansion section) is preferably 4.0 to 6.0 MPa. Specifically, the target pressure is, for example, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, or 6.0 MPa, and may be within a range between any two of the values ​​exemplified here. Here, the target pressure is set to approximately 5.0 MPa. The target pressure of the second expansion valve 30B (second expansion section) is preferably 1.5 to 3.5 MPa. Specifically, the target pressure is, for example, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, or 3.5 MPa, or may be in a range between any two of the values ​​exemplified here. Here, it is set to about 2.5 MPa. The target pressure of the third expansion valve 30C (third expansion section) is preferably 0.6 to 1.0 MPa. Specifically, for example, the pressure is 0.60, 0.62, 0.64, 0.66, 0.68, 0.70, 0.72, 0.74, 0.76, 0.78, 0.80, 0.82, 0.84, 0.86, 0.88, 0.90, 0.92, 0.94, 0.96, 0.98, or 1.00 MPa, or may be in a range between any two of the values ​​exemplified here. Here, the pressure is set to about 0.8 MPa.

[0016] A first separator 31A to a third separator 31C are disposed downstream of the first expansion valve 30A to the third expansion valve 30C, respectively. In the first separator 31A to the third separator 31C, the gas-liquid two-phase flow generated in the first expansion valve 30A to the third expansion valve 30C is separated into a gas phase (flash gas) and a liquid phase (liquid carbon dioxide). In addition, a distillation column 32 is provided downstream of the second separator 31B. The distillation column 32 is a column for removing volatiles and is configured to remove volatiles by distillative separation.

[0017] <Circulation section 4 - Circulation step> The circulation section 4 is configured to circulate flash gas generated during pressure reduction at each stage of the first expansion valve 30A to the third expansion valve 30C to the compression section 2 via a plurality of paths that are independent of each other. In this embodiment, the circulation section 4 includes a first circulation path 40A to a third circulation path 40C. The first circulation path 40A to the third circulation path 40C are provided with a first pressure reducing valve 41A to a third pressure reducing valve 41C, respectively.

[0018] The first circulation path 40A circulates the flash gas separated in the first separator 31A downstream of the third compressor 20C (to the inlet side of the fourth compressor 20D). The flash gas separated in the first separator 31A has a pressure of about 5.0 MPa and therefore suitably merges with the fluid downstream of the third compressor 20C. The first pressure reducing valve 41A prevents flash gas with an excessively high pressure from circulating through the first circulation path 40A.

[0019] The second circulation path 40B circulates the flash gas separated in the second separator 31B downstream of the second compressor 20B (to the inlet side of the third compressor 20C). The flash gas separated in the second separator 31B has a pressure of about 2.5 MPa and therefore suitably merges with the fluid downstream of the second compressor 20B. The second pressure reducing valve 41B prevents flash gas with an excessively high pressure from circulating through the second circulation path 40B.

[0020] The flash gas flowing through the first circulation path 40A described above normally has a temperature of about 15°C, but the flash gas flowing through the second circulation path 40B may be below freezing. For this reason, the second circulation path 40B is connected downstream of the dehydration section 21. Because moisture is removed from the carbon dioxide gas downstream of the dehydration section 21, equipment problems that may occur due to freezing of carbon dioxide gas containing moisture (damage to the compressor due to ice fragments, freezing and damage to piping, etc.) are prevented.

[0021] The third circulation path 40C circulates the flash gas separated in the third separator 31C downstream of the first compressor 20A (to the inlet side of the second compressor 20B). The flash gas separated in the third separator 31C has a pressure of approximately 0.8 MPa and therefore preferably merges with the fluid downstream of the first compressor 20A. The third pressure reducing valve 41C prevents excessively high-pressure flash gas from circulating through the third circulation path 40C. By providing pressure reducing valves in the multiple, mutually independent paths of the circulation section 4, it is not necessary to match the pressures of each stage of the compression section 2 and each stage of the pressure reducing section 3. In other words, the pressures of each stage of the compression section 2 and each stage of the pressure reducing section 3 can be adjusted within a predetermined range, allowing for sufficient operational adjustment, facilitating a highly flexible and suitable facility design.

[0022] The flash gas flowing through the third circulation path 40C has been cooled to approximately −45°C through multiple expansions. For this reason, a heat exchanger 42 is provided in the third circulation path 40C. The heat exchanger 42 is configured to exchange heat between the carbon dioxide liquefied in the compression section 2, which has a temperature of approximately 30 to 40°C, and the flash gas flowing through the third circulation path 40C. The heat exchanger 42 warms the flash gas, which has a temperature of approximately −45°C, to 10 to 20°C. Therefore, even if the third circulation path 40C is connected upstream of the dehydration section 21, the problem of freezing of the moisture-containing carbon dioxide gas does not occur. In the circulation section 4, the flash gas generated in the pressure reduction section 3 that is below freezing is circulated downstream of the dehydration section 21, and is appropriately heated when circulated upstream of the dehydration section 21.

[0023] <Multi-stage pressure reduction by pressure reduction section 3> As described above, pressure reduction section 3 is configured to reduce the pressure of the carbon dioxide liquefied in compression section 2 to a target pressure in stages using first expansion valve 30A to third expansion valve 30C. Furthermore, because the target pressure at each stage is appropriately set, a reduction in energy consumption is achieved.

[0024] As described above, carbon dioxide compressed to approximately 10.0 MPa and liquefied is first decompressed to approximately 5.0 MPa in the first expansion valve 30A. Of the gas-liquid two-phase flow that passes through the first expansion valve 30A, the gas phase is introduced into the first circulation path 40A as described above. Meanwhile, the liquid phase is sent downstream to the second expansion valve 30B through a conduit. The temperature of the liquid carbon dioxide at this time is approximately 15°C, which is optimal as a refrigerant for the precooler 21A of the dehydration unit 21. Therefore, a portion of the liquid carbon dioxide separated in the first separator 31A is sent to the precooler 21A via the conduit 40D. The carbon dioxide that passes through the precooler 21A and vaporizes by heat exchange with carbon dioxide gas passes through the fourth pressure reducing valve 41D and then merges with the first circulation path 40A. This configuration optimizes the dehydration process in the dehydration unit 21 and contributes to reducing energy consumption in the dehydration unit 21.

[0025] Next, in the second expansion valve 30B, the liquid carbon dioxide is decompressed from approximately 5.0 MPa to approximately 2.5 MPa. Thereafter, the liquid carbon dioxide separated in the second separator 31B enters the distillation column 32 through a conduit while maintaining a pressure of approximately 2.5 MPa. This pressure is optimal for re-condensing the volatiles distilled and separated in the distillation column 32. Because the pressure is sufficiently reduced by the two-stage expansion, there is no risk of the separation efficiency decreasing due to high pressure when separating impurities from the liquid carbon dioxide. Furthermore, there is no risk of the operating pressure of the distillation column 32 increasing more than necessary, which would result in an increase in the pressure-temperature rating class of the piping and equipment and therefore an increase in facility costs.

[0026] In the third expansion valve 30C, which is the most downstream, the liquid carbon dioxide is decompressed from approximately 2.5 MPa to approximately 0.8 MPa. This pressure is optimal for recovering the liquid carbon dioxide in a pressure vessel for transportation. In this embodiment, Joule-Thomson valves are used as the first expansion valve 30A to the third expansion valve 30C, but the applicable type of expansion valve is not limited to this. When controlling the first expansion valve 30A to the third expansion valve 30C, a known control method can be used in which valve control is performed based on the detection results of a pressure sensor to adjust the regulated pressure within a desired range. Note that although the temperature can be estimated from the pressure of the circulating fluid, it is preferable to install thermometers at various locations in the conduits.

[0027] As described above, by employing multi-stage decompression processing in decompression section 3, energy consumption can be reduced by approximately 30% compared to simply decompressing liquid carbon dioxide from approximately 10.0 MPa to approximately 0.8 MPa in one stage. In particular, the amount of circulating gas (flash gas) that must be processed by relatively low-pressure compressors such as first compressor 20A and second compressor 20B of the multi-stage compressors 20 is reduced, which can contribute to reducing the power required for the multi-stage compressors 20.

[0028] The additional equipment required for performing the multi-stage depressurization treatment in the depressurization section 3 is relatively inexpensive, such as an expansion valve or a flash drum (vessel). On the other hand, the number of heat exchangers required can be reduced, and a multi-fluid heat exchanger can be dispensed with, thereby reducing the equipment costs of the liquefaction system 1. Although the number of heat exchangers for recovering cold energy has been reduced, the cooled flash gas is joined with the carbon dioxide gas at an optimal position as needed, so the cold energy to be recovered is not wasted.

[0029] 2. Second Embodiment Next, a liquefaction system 1A according to a second embodiment will be described with reference to FIG. 2. The basic configuration of the liquefaction system 1A is the same as that of the liquefaction system 1 described above. However, the liquefaction system 1A employs an expansion turbine 33 for power recovery instead of the first expansion valve 30A. The expansion turbine 33 is connected to a generator 34. The expansion turbine 33 expands liquid carbon dioxide at approximately 10.0 MPa and reduces the pressure to approximately 5.0 MPa. As the liquid carbon dioxide passes through the expansion turbine 33, power is generated by the generator 34. Considering the power obtained by the generator 34, energy consumption can be reduced by approximately 5% compared to the liquefaction system 1 according to the first embodiment.

[0030] 3. Third Embodiment Next, a liquefaction system 1B according to a third embodiment will be described with reference to FIG. 3 . The basic configuration of the liquefaction system 1B is the same as that of the above-described liquefaction system 1A. However, in the liquefaction system 1B, a compression turbine 43 is provided midway along the third circulation path 40C in the circulation section 4. The compression turbine 43 is configured to compress the flash gas circulating through the third circulation path 40C. The compression turbine 43 is also coaxially connected to the expansion turbine 33. Therefore, when liquid carbon dioxide passes through, the expansion turbine 33 rotates and drives the compression turbine 43. The flash gas circulating through the third circulation path 40C is pressurized from approximately 0.8 MPa to approximately 2.5 MPa by passing through the compression turbine 43. As a result, the flash gas circulating through the third circulation path 40C can be merged with the second circulation path 40B midway and circulated to the compression section 2. The compression turbine connected coaxially with the expansion turbine 33 can reduce the power required to recirculate the flash gas of 31C, so that the energy consumption can be reduced by about 10% more than in the liquefaction system 1.

[0031] As described above, the second circulation path 40B is connected downstream of the dehydration section 21, and moisture is removed from the carbon dioxide gas downstream of the dehydration section 21. This makes it possible to merge the sub-zero flash gas introduced into the third circulation path 40C with the carbon dioxide gas in the compression section 2 without heating it. As a result, the heat exchanger 42 and the third pressure reducing valve 41C arranged in the third circulation path 40C become unnecessary.

[0032] 4. Other embodiments

[0033] FIG. 4A shows a schematic configuration of the liquefaction system 1C. In the liquefaction system 1C, liquid carbon dioxide at approximately 10.0 MPa is expanded and depressurized to approximately 2.5 MPa by a power recovery expansion turbine 33. As the liquid carbon dioxide passes through the expansion turbine 33, power is generated by a generator 34. Compared to a conventional configuration in which pressure is reduced using an expansion valve, energy consumption can be reduced by the amount of power obtained by the generator 34. The gas-liquid two-phase flow expanded by the expansion turbine 33 is separated into a gas phase and a liquid phase by a fourth separator 31D. The gas-phase flash gas is returned to the compression section 2 via a second circulation path 40B. Volatiles are removed from the liquid-phase liquid carbon dioxide in a distillation column 32. The liquid carbon dioxide is then depressurized to approximately 0.8 MPa by the fourth expansion valve 30D and separated into a gas phase and a liquid phase by a fifth separator 31E. The gaseous flash gas is returned to the compression section 2 via the third circulation path 40C, and the liquid carbon dioxide is recovered in a pressure vessel for transportation.

[0034] 4B shows a schematic configuration of a liquefaction system 1D. In the liquefaction system 1D, a compressor turbine 43 is added to the configuration of the liquefaction system 1C. The flash gas separated in the fifth separator 31E is pressurized by the compressor turbine 43. The flash gas pressurized by the compressor turbine 43 is merged into the second circulation path 40B. This makes it possible to reduce the amount of flash gas that must be processed on the relatively low-pressure side, such as the first compressor 20A and the second compressor 20B, and thus reduce the power required for the multiple compressor stages 20. Furthermore, a heat exchanger can be eliminated.

[0035] In the above-described embodiment, an example was described in which liquefied carbon dioxide pressurized to approximately 10.0 MPa was reduced in pressure to approximately 0.8 MPa in three stages, but the pressure may be reduced to the target pressure in two stages, or in four or more stages.

[0036] 1: Liquefaction system, 1A: Liquefaction system, 1B: Liquefaction system, 1C: Liquefaction system, 1D: Liquefaction system, 2: Compression section, 3: Decompression section, 4: Circulation section, 20: Compressor, 20A: First compressor, 20B: Second compressor, 20C: Third compressor, 20D: Fourth compressor, 21: Dehydration section, 21A: Precooler, 21B: Dehydrator, 22A: First cooler, 22B: Second cooler, 22C: Third cooler, 22D: fourth cooler, 30A: first expansion valve, 30B: second expansion valve, 30C: third expansion valve, 31A: first separator, 31B: second separator, 31C: third separator, 31D: separator, 32: distillation column, 33: expansion turbine, 34: generator, 40A: first circulation path, 40B: second circulation path, 40C: third circulation path, 41A: first pressure reducing valve, 41B: second pressure reducing valve, 41C: third pressure reducing valve, 42: heat exchanger, 43: compression turbine

Claims

1. A liquefaction system comprising a compression section, a decompression section, and a circulation section, wherein the compression section is configured to compress carbon dioxide in stages to a target pressure for liquefying it using a multi-stage compressor, the decompression section is configured to decompress the carbon dioxide liquefied in the compression section in stages to the target pressure using a multi-stage expansion section, and the circulation section is configured to circulate flash gas produced in each stage of the multi-stage expansion section to the compression section via multiple paths that are independent of each other.

2. A liquefaction system as claimed in claim 1, wherein the compression section has a dehydration section in the middle stage of the multi-stage compressor, and the circulation section circulates sub-freezing flash gas from among the flash gases generated in each stage of the multi-stage expansion section downstream of the dehydration section.

3. A liquefaction system as set forth in claim 1 or claim 2, wherein the multi-stage expansion section has a first expansion section, a second expansion section, and a third expansion section, the target pressure of the compression section is 8.0 MPa or higher, the target pressure of the first expansion section is 4.0 to 6.0 MPa, the target pressure of the second expansion section is 1.5 to 3.5 MPa, and the target pressure of the third expansion section is 0.6 to 1.0 MPa.

4. A liquefaction system according to claim 1 or 2, wherein the pressure reducing section has an expansion turbine for power recovery.

5. A liquefaction system according to claim 4, wherein the circulation section has a compression turbine configured to compress the flash gas, and the expansion turbine is configured to drive the compression turbine.

6. A liquefaction system comprising a compression section, a decompression section, and a circulation section, wherein the compression section is configured to compress carbon dioxide in stages to a target pressure for liquefying it using a multi-stage compressor, the decompression section is configured to decompress the liquefied carbon dioxide to the target pressure using an expansion turbine for power recovery, and the circulation section is configured to circulate flash gas produced in the decompression section to the compression section.

7. A liquefaction method comprising a compression step, a depressurization step, and a circulation step, wherein in the compression step, carbon dioxide is compressed in stages to a target pressure for liquefaction by a compression section having multiple compressor stages, in the depressurization step, the liquefied carbon dioxide is depressurized in stages to the target pressure by a multiple expansion section, and in the circulation step, flash gas produced in each stage of the multiple expansion sections is circulated to the compression section via multiple paths that are independent of each other.

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