Method for re-liquefying liquefied petroleum gas using centrifugal compressor

Abstract

Provided is a method for re-liquefying boil-off gas generated from liquefied petroleum gas accommodated in a liquefied petroleum gas storage tank. The method comprises: a discharge step of discharging boil-off gas from a liquefied petroleum gas storage tank; a pressurization step of increasing the pressure of the discharged boil-off gas; a liquefaction step of liquefying the compressed boil-off gas by making the compressed boil-off gas exchange heat with a refrigerant circulating in a separate refrigerant circulation system; a depressurization step of lowering the pressure of the liquefied boil-off gas; and a return step of returning the decompressed boil-off gas to the liquefied petroleum gas storage tank, wherein the boil-off gas is pressurized in the pressurization step by a centrifugal boil-off gas compressor.

Description

Method for re-liquefying liquefied petroleum gas using a centrifugal compressor

[0001] The present invention relates to a method for re-liquefying LPG evaporation gas generated in a refrigerated liquefied petroleum gas (LPG) storage tank, and more specifically, to a method for compressing evaporation gas generated in a refrigerated LPG storage tank using a centrifugal compressor, re-liquefying the evaporation gas through heat exchange with a separate refrigerant, and returning the evaporation gas to the refrigerated LPG storage tank.

[0002] Natural gas is a vital energy resource. While methane (CH4) is the most abundant component of natural gas, natural gas is also a complex of several hydrocarbons, including ethane (C2H6), propane (C3H8), and butane (C4H10). Pipeline natural gas and liquefied natural gas (LNG), commonly used, are primarily composed of relatively light hydrocarbons such as methane. Heavier hydrocarbons such as propane and butane are separated during LNG production or petroleum refining and used as liquefied petroleum gas (LPG). LPG is thus produced as a byproduct of the liquefied natural gas production or petroleum refining process. While LPG production is smaller than pipeline natural gas or LNG, its continued production at a certain rate makes it an important player in international energy trade.

[0003] LNG liquefies at approximately -161 degrees Celsius under ambient pressure, while propane (C3H8), a representative LPG, liquefies at approximately -42 degrees Celsius. Furthermore, propane can liquefy and maintain its liquid state without a separate refrigeration system at an absolute pressure of approximately 13.7 atm or higher, even at an ambient temperature of approximately 40 degrees Celsius. LPG can be stored pressurized at room temperature or frozen at ambient pressure.

[0004] Produced LPG is transported by land by truck, train, and other means. However, for large-scale international transactions, it is transported by LPG carriers. LPG carriers store and transport LPG in a pressurized, room-temperature state or refrigerated, room-pressure state.

[0005] When LPG is stored and transported under pressure and at room temperature, no separate LPG boil-off gas is generated, eliminating the need for complex LPG boil-off gas re-liquefaction systems. However, when LPG is stored and transported under refrigerated pressure, a certain amount of boil-off gas continues to be generated, even with thorough insulation, following the same principles as LNG. This boil-off gas must be properly handled, such as by re-liquefying and returning it to the storage tank.

[0006] Large LPG carriers are equipped with large, high-capacity storage tanks. However, manufacturing these large storage tanks as pressurized tanks is expensive and technically challenging. For this reason, large LPG carriers store LPG under refrigerated atmospheric pressure, and in this case, devices for re-liquefying the LPG vapor must be installed.

[0007] While propane is the most common cargo on LPG carriers, they must also be designed to store and transport butane, propylene, ammonia, and other chemicals. Reliquefaction equipment is also required to reliquefy these substances. For these diverse cargo types, even if the liquefaction temperatures for each cargo vary, the reliquefaction equipment itself is configured with a similar system.

[0008] Thus, the reliquefaction system in refrigerated LPG carriers requires significant power and consists of a complex system configuration. Furthermore, the efficient design and operation of the reliquefaction system are crucial factors in refrigerated LPG carriers.

[0009] Even if the LPG storage tanks in refrigerated LPG carriers are properly insulated, a certain amount of LPG boil-off gas (LPG) continues to be generated. If LPG boil-off gas is not treated through re-liquefaction or other methods, the pressure in the storage tank will rise, compromising its structural integrity.

[0010] Conventional refrigerated LPG carriers used reciprocating compressors for reliquefaction. However, these systems utilize reciprocating compressors, a key component of the reliquefaction system. The compressor itself is very heavy and bulky, making it difficult to design the mechanical layout onboard the vessel. This also leads to various issues, including vibration and lubrication system failure. Despite these challenges, the lack of a suitable alternative to reciprocating compressors has led to the long-term use of reciprocating compressor-based reliquefaction systems on LPG carriers.

[0011] Furthermore, the compressor of an LPG re-liquefaction system is required to compress LPG vapor to an absolute pressure of approximately 15 atm or more. Despite their advantages in terms of size and retention, centrifugal compressors have struggled to achieve sufficient pressure for LPG re-liquefaction, preventing their practical application on ships. Specifically, achieving higher pressures in centrifugal compressors requires increasing the number of compression stages, which complicates the system and increases costs.

[0012] Furthermore, for a centrifugal compressor to function properly, the actual volume flow of the fluid must be sufficient. However, as the pressure increases, the density increases, and the actual volume flow at a given mass flow decreases, making it difficult to implement with a centrifugal compressor. For this reason, it is extremely difficult to compress to an absolute pressure of 15 atm or higher with a centrifugal compressor.

[0013] For these reasons, in order to use a centrifugal compressor, it is necessary to develop a system that can carry out the liquefaction process of LPG evaporation gas even at a relatively low pressure of 10 atm or less.

[0014] The present invention is intended to solve the above-mentioned problem, and provides a method of compressing evaporation gas generated in a refrigerated LPG storage tank to the above-mentioned low pressure using a centrifugal compressor, and then re-liquefying the evaporated gas through heat exchange with a separate refrigerant and returning it to the refrigerated LPG storage tank.

[0015] The present invention provides a method for liquefying compressed LPG vaporized gas through heat exchange with a separate refrigerant, rather than using the conventional method of directly liquefying (condensing) compressed LPG vaporized gas with seawater or the like. For this purpose, a separate refrigeration cycle is used.

[0016] According to one aspect of the present invention for achieving the above object, there is provided a method for re-liquefying boil-off gas generated from liquefied petroleum gas stored in a liquefied petroleum gas storage tank, comprising: a discharge step of discharging the boil-off gas from the liquefied petroleum gas storage tank; a pressurizing step of increasing the pressure of the discharged boil-off gas; a liquefaction step of liquefying the compressed boil-off gas by heat exchange with a refrigerant circulating in a separate refrigerant circulation system; a depressurizing step of decreasing the pressure of the liquefied boil-off gas; and a return step of returning the depressurized boil-off gas to the liquefied petroleum gas storage tank; wherein the re-liquefaction method is characterized in that in the pressurizing step, the boil-off gas is pressurized by a low-pressure centrifugal boil-off gas compressor.

[0017] According to one embodiment, the pressure of the evaporation gas compressed in the centrifugal evaporation gas compressor may have a value within a range of an absolute pressure of 5 atm or less, and a pressure value higher than or equal to a pressure value at which the evaporation gas can be processed by the re-liquefaction method to overcome the differential pressure and return to the liquefied petroleum gas storage tank.

[0018] According to one embodiment, the refrigerant circulation system includes a refrigerant compressor that compresses the refrigerant after cooling the evaporated gas by heat exchange, and the refrigerant compressor may be a centrifugal refrigerant compressor.

[0019] According to one embodiment, the pressure of the refrigerant compressed in the centrifugal refrigerant compressor may have a value of 10 atm or less in absolute pressure.

[0020] According to one embodiment, the refrigerant may be composed of a hydrocarbon component or a hydrocarbon mixture component that is heavier than the molecular weight of the liquefied petroleum gas contained in the liquefied petroleum gas storage tank.

[0021] According to the present invention, in an LPG carrier that stores and transports LPG in a frozen state, a re-liquefaction system for LPG evaporation gas can be implemented using a centrifugal compressor instead of applying a conventional reciprocating compressor having various disadvantages.

[0022] Furthermore, according to the present invention, when re-liquefying LPG evaporation gas, it is possible to liquefy LPG evaporation gas by compressing it at a low pressure of about 10 atm or less without the need to compress it at a high pressure of 15 atm or more as in conventional systems. This allows the number of compression stages of the centrifugal compressor to be reduced, simplifying the overall system and enabling more economical implementation.

[0023] Furthermore, according to the present invention, the liquefaction system and refrigerant system of LPG evaporation gas can be completely separated, thereby implementing a system with higher liquefaction efficiency. That is, a refrigerant having different components from the LPG cargo, high efficiency, and low condensation temperature, i.e., capable of liquefaction even at low pressures, can be used.

[0024] In addition, according to the present invention, the LPG cargo and the refrigerant are completely separated, so that the refrigerant itself can be fundamentally prevented from mixing with the LPG cargo.

[0025] In addition, according to the present invention, since a centrifugal compressor is used, the weight of the entire equipment can be significantly reduced compared to an LPG re-liquefaction system implemented with a conventional reciprocating compressor, thereby drastically improving the machine layout design on a ship.

[0026] In addition, according to the present invention, by using a centrifugal compressor that uses an integrated high-speed motor that uses a foil bearing or a magnetic bearing, a lubricating oil system can be omitted and the size of the equipment can be significantly reduced.

[0027] In addition, according to the present invention, by applying a low-pressure centrifugal compressor, the actual volume flow at the compressor inlet can be increased, and the flow coefficient, which has a significant influence on the performance of the centrifugal compressor, can be increased, thereby improving the compression performance. In a centrifugal compressor, the flow coefficient must be sufficiently secured to obtain proper performance, but as the compression pressure increases, the actual flow decreases, lowering the compressor efficiency.

[0028] Furthermore, according to the present invention, in foil-type bearings or electromagnetic bearings where the stability of the thrust bearing is very important, a centrifugal compressor can be implemented at an absolute pressure of 10 atm or less, thereby reducing the thrust applied to both ends of the compressor and dramatically improving the safety and reliability of the bearing. On the other hand, when the absolute pressure is 15 atm or more, it becomes very difficult to ensure the safety of the thrust bearing of the foil-type bearing or electromagnetic bearing due to the high pressure difference between the input and output.

[0029] Figure 1 is a conceptual diagram illustrating a re-liquefaction system for a refrigerated LPG carrier using a low-pressure centrifugal compressor.

[0030] Figure 2 is a drawing for explaining a system for supercooling the liquefied LPG in Figure 1 before sending it to a storage tank.

[0031] Figure 3 is a conceptual diagram of a re-liquefaction system for a refrigerated LPG carrier using a conventional reciprocating compressor.

[0032] Hereinafter, a method for re-liquefying evaporated gas of an LPG carrier according to a preferred embodiment of the present invention will be described in detail with reference to the drawings.

[0033] The boil-off gas generated in the LPG storage tanks of refrigerated LPG carriers can be discharged from the LPG storage tanks, re-liquefied, and then returned to the tanks. This re-liquefaction process requires a compressor to compress the LPG boil-off gas. While reciprocating compressors have traditionally been used for re-liquefaction, centrifugal compressors are more desirable due to their smaller size and higher reliability.

[0034] FIG. 1 is a schematic conceptual diagram of an LPG evaporation gas re-liquefaction system using a centrifugal compressor according to a preferred embodiment of the present invention.

[0035] As illustrated in FIG. 1, the LPG boil-off gas re-liquefaction system includes an LPG storage tank (10) that contains both liquid liquefied petroleum gas (LPG, 10a) and gaseous boil-off gas (10b) evaporated from LPG, a centrifugal boil-off gas compressor (12) that compresses boil-off gas discharged from the LPG storage tank (10), a boil-off gas-refrigerant heat exchanger (20) that cools boil-off gas compressed in the centrifugal boil-off gas compressor (12) through heat exchange with a refrigerant, and a boil-off gas decompressor (16) that reduces the pressure of boil-off gas cooled in the boil-off gas-refrigerant heat exchanger (20).

[0036] The liquefied evaporation gas, which is depressurized as it passes through the evaporation gas pressure reducer (16), returns to the LPG storage tank (10). The evaporation gas pressure reducer (16) may be an expansion valve. In Fig. 1, the flow path of the gaseous evaporation gas (evaporation gas discharge line, L1) is indicated by a dotted line, and the flow path of the re-liquefied evaporation gas (evaporation gas return line, L2) is indicated by a solid line.

[0037] The evaporation gas (10b) discharged from the gas portion at the top of the LPG storage tank (10) is supplied to the centrifugal evaporation gas compressor (12) through the evaporation gas discharge line (L1) and compressed. According to the present invention, the LPG evaporation gas is not condensed through its own heat exchange, but is configured to condense through heat exchange with a refrigerant supplied from a separate refrigeration system. Therefore, the centrifugal evaporation gas compressor (12) used at this time only needs to be compressed to a pressure sufficient to overcome the differential pressure in the reliquefaction system and return to the LPG storage tank (10). To this end, the evaporation gas can be compressed to an absolute pressure of approximately 5 atm in the centrifugal evaporation gas compressor (12). The pressure compressed in the centrifugal evaporation gas compressor (12) can be set to a value of 5 atm or less as long as the evaporation gas can overcome the differential pressure in the reliquefaction system and return to the LPG storage tank (10).

[0038] The centrifugal evaporation gas compressor (12) may be configured as one or two stages. If the centrifugal evaporation gas compressor (12) is configured as two stages, the pressure of the evaporation gas after being compressed in the first stage is relatively low, so the temperature at the outlet of the first stage does not rise much, and therefore the installation of an intermediate cooler between the first and second stages can be omitted.

[0039] The evaporation gas compressed in the centrifugal evaporation gas compressor (12) can be cooled by a cooler (14) installed at the rear end of the centrifugal evaporation gas compressor (12). The cooler (14) can generally use separate cooling water supplied from the outside of the system, for example, fresh water or seawater. Since the cooler (15) is intended to simply cool the evaporation gas, the temperature of which has increased while being compressed, that is, it is not intended to change the phase of the evaporation gas from gas to liquid, its capacity does not need to be large.

[0040] The compressed and cooled LPG evaporation gas can be cooled in an evaporation gas-refrigerant heat exchanger (20). The refrigeration source used to cool and liquefy the evaporation gas in the evaporation gas-refrigerant heat exchanger (20) is not a typical cooling water such as chlorine or seawater, but a separate refrigerant. The refrigerant is described in more detail below, along with the refrigerant circulation system.

[0041] The LPG evaporation gas cooled in the evaporation gas-refrigerant heat exchanger (20) is depressurized in the evaporation gas pressure reducer (16) to become low-temperature liquefied LPG and is returned to the LPG storage tank (10) through the evaporation gas return line (L2).

[0042] The LPG evaporation gas re-liquefaction system further includes a refrigerant circulation system in which a refrigerant that cools the evaporation gas circulates in the evaporation gas-refrigerant heat exchanger (20). The refrigerant circulation system includes a centrifugal refrigerant compressor (22) that compresses the refrigerant that has been vaporized and has a high temperature in the evaporation gas-refrigerant heat exchanger (20), a refrigerant-external medium heat exchanger (24) that cools the refrigerant compressed in the centrifugal refrigerant compressor (22) through heat exchange with an external medium (e.g., an external cooling medium such as fresh water or seawater), and a refrigerant pressure reducer (26) that reduces the pressure of the refrigerant cooled in the refrigerant-external medium heat exchanger (24).

[0043] The refrigerant, which has been decompressed and has its temperature lowered further as it passes through the refrigerant pressure reducer (26), is supplied again to the evaporation gas-refrigerant heat exchanger (20). In this way, the refrigerant circulating within the refrigerant circulation system flows sequentially through the evaporation gas-refrigerant heat exchanger (20), the centrifugal refrigerant compressor (22), the refrigerant-external medium heat exchanger (24), and the refrigerant pressure reducer (26) via the refrigerant circulation line (L4). The refrigerant circulation system constitutes a closed circuit and is used in a completely separated state from the LPG and LPG evaporation gas contained in the LPG storage tank.

[0044] The refrigerant cooled and condensed in the refrigerant-external medium heat exchanger (24) is decompressed in the refrigerant pressure reducer (26) to lower its temperature. The refrigerant pressure reducer (26) may be configured as an expansion valve. The low-pressure, low-temperature refrigerant discharged from the refrigerant pressure reducer (26) can be used as a cold energy source for the evaporative gas-refrigerant heat exchanger (20).

[0045] As described above, the refrigerant that has undergone heat exchange in the evaporative gas-refrigerant heat exchanger (20) is again supplied to the suction side of the centrifugal refrigerant compressor (22). The refrigerant circulation system is configured as a closed circuit, and only heat exchange occurs between the main cargo, LPG, and the LPG cargo and refrigerant are not mixed with each other.

[0046] The refrigerant circulating in the refrigerant circulation system may be a heavier component than the LPG cargo, which mainly consists of propane. Therefore, the pressure at which the refrigerant can be liquefied may be significantly lower than that of LPG, and unlike conventional liquefaction systems, the re-liquefaction system of the present invention can liquefy LPG even at an absolute pressure of 10 atm or less. For example, based on the condensation temperature by cooling water of 50 degrees Celsius, propane is liquefied at an absolute pressure of approximately 17.1 atm, but butane is liquefied at an absolute pressure of approximately 5.0 atm, and a 20% propane and 80% butane mixed refrigerant is liquefied at an absolute pressure of approximately 7.1 atm. For example, the pressure of the refrigerant compressed in the centrifugal refrigerant compressor (22) may have a value within a range of an absolute pressure of 5 atm or more and an absolute pressure of 10 atm or less. The pressure of the refrigerant compressed in the centrifugal refrigerant compressor (22) can be determined differently depending on the type of refrigerant.

[0047] Since LPG carriers can transport not only propane but also butane, propylene, ammonia, etc., they must be configured to re-liquefy these vaporized gases. Although cargoes such as butane, propylene, and ammonia have different liquefaction temperatures compared to propane, the low-pressure centrifugal compressor of the present invention with an absolute pressure of 10 atm or less can be used based on the same principle. In this specification, a pressure exceeding 10 atm absolute is referred to as high pressure, and a pressure below 10 atm absolute is referred to as low pressure.

[0048] The refrigerant compressed in the centrifugal refrigerant compressor can be a hydrocarbon refrigerant such as propane, ammonia, and butane, which are the target cargoes of LPG carriers, or a non-flammable refrigerant other than the hydrocarbon series. In this way, if a non-flammable refrigerant other than the hydrocarbon refrigerant used in the cargo of LPG carriers is used, a general refrigeration system already in use in industries other than ships can be applied, which can bring about economic efficiency and simplification of the overall system. In the case of applying a separate refrigeration system commonly used in the industry, since there is no problem with thrust bearings due to high pressure, a general bearing system rather than a foil bearing or magnetic bearing can be used, and a high-pressure (absolute pressure of 10 atm or more) compressor can be used instead of a low-pressure one. Even when using a general refrigeration system for a non-flammable refrigerant, a low-pressure centrifugal evaporative gas compressor (12) is required to ensure the flow of LPG evaporative gas and its return to the storage tank.

[0049] The low-pressure centrifugal evaporative gas compressor (12) and the low-pressure centrifugal refrigerant compressor (22) may be configured as an integrated compressor in which the electric motor and the compressor impeller are built into the same casing, as described in Patent Registration No. 10-2014376. In this case, the rotor sealing device of the compressor can be omitted, and the leakage of hydrocarbon gas, which is harmful to the environment, can be fundamentally prevented. In addition, the low-pressure centrifugal evaporative gas compressor (12) and the low-pressure centrifugal refrigerant compressor (22) are equipped with foil-type bearings or electromagnetic bearings that do not require lubrication oil supply, which can significantly simplify the related peripheral systems and increase the overall reliability of the compressor.

[0050] As illustrated in FIG. 2, the LPG evaporation gas re-liquefaction system of the present invention may further include a subcooling heat exchanger (34) for subcooling a portion of the evaporation gas (hereinafter, also referred to as a “branch stream”) cooled and condensed in the evaporation gas-refrigerant heat exchanger (20). The subcooling heat exchanger (34) may be arranged between the evaporation gas-refrigerant heat exchanger (20) and the evaporation gas pressure reducer (16). A portion of the evaporation gas condensed in the evaporation gas-refrigerant heat exchanger (20) is configured to be branched from the evaporation gas return line (L2) through the branch line (L3), and then pressure-reduced in the subcooling pressure reducer (32) and supplied to the subcooling heat exchanger (34). The subcooling pressure reducer (32) may be configured as an expansion valve.

[0051] The liquefied evaporation gas, which is being transferred from the evaporation gas-refrigerant heat exchanger (20) to the evaporation gas pressure reducer (16) through the evaporation gas return line (L2), is sent to the evaporation gas pressure reducer (16) by exchanging heat with a branch stream whose temperature has been lowered by being decompressed in the subcooling pressure reducer (32) and is then further lowered into a supercooled liquefied evaporation gas (i.e., LPG in a supercooled state) by exchanging heat with the branch stream in the subcooling heat exchanger (34). By decompressing the liquefied evaporation gas in a supercooled state in the evaporation gas pressure reducer (16), the amount of flash gas can be reduced compared to the evaporation gas that has not been supercooled. Depending on the situation, the subcooling system illustrated in FIG. 2 may be omitted. The branch stream heated in the subcooling heat exchanger (34) may be joined with the evaporation gas supplied to the suction side of the centrifugal evaporation gas compressor (12).

[0052] In Fig. 3, for comparison, a schematic diagram is shown to explain a conventional re-liquefaction device using a reciprocating compressor.

[0053] As illustrated in FIG. 3, a conventional LPG boil-off gas re-liquefaction system includes an LPG storage tank (10) that contains both liquid liquefied petroleum gas (LPG, 10a) and gaseous boil-off gas (10b) evaporated from LPG, a reciprocating compressor (42) that compresses boil-off gas discharged from the LPG storage tank (10) to a high pressure of 10 atm or more, a heat exchanger (44) that cools the high pressure boil-off gas compressed in the reciprocating compressor (42) through heat exchange with an external cooling water such as fresh water or seawater, and a pressure reducing valve that reduces the pressure of the boil-off gas cooled and condensed in the heat exchanger (44).

[0054] The depressurized evaporation gas, passing through the pressure reducing valve, returns to the LPG storage tank (10). In Fig. 3, the flow path of the gaseous evaporation gas (evaporation gas discharge line, L11) is indicated by a dotted line, and the flow path of the re-liquefied evaporation gas (evaporation gas return line, L12) is indicated by a solid line.

[0055] In Fig. 3, a subcooling heat exchanger (48) is illustrated for subcooling a portion of the condensed evaporation gas (hereinafter also referred to as a “branch stream”) cooled in a heat exchanger (44). The subcooling heat exchanger (48) is arranged between the heat exchanger (44) and a pressure reducing valve, and a portion of the condensed evaporation gas in the heat exchanger (44) is branched from the evaporation gas return line (L12) through a branch line (L13), and then pressure-reduced in the subcooling pressure reducing valve (46) and supplied to the subcooling heat exchanger (48).

[0056] According to the conventional re-liquefaction system illustrated in Fig. 3, the LPG evaporation gas itself is compressed to high pressure and then cooled through heat exchange with a cooling water, thereby re-liquefying, without using a refrigeration system using a separate refrigerant. To condense the compressed LPG evaporation gas into the cooling water, the LPG evaporation gas must be sufficiently cooled and liquefied at the heat exchange temperature with the cooling water. Therefore, a high-pressure compressor capable of compressing at high pressure is required. The reciprocating compressor is configured in multiple stages and must be equipped with an intermediate cooler.

[0057] To achieve such a high compression ratio, a reciprocating compressor is suitable. However, centrifugal compressors are extremely difficult to achieve the required high pressure. For this reason, centrifugal compressors have not been used in conventional LPG carrier reliquefaction systems.

[0058] In contrast, the reliquefaction system of the present invention is configured to cool and condense the compressed evaporated gas using a separate refrigeration system, resulting in a relatively low pressure to be achieved by the compressor. For this reason, the reliquefaction system of the present invention can perform the reliquefaction process using a centrifugal compressor, which is simpler, cheaper, and smaller in volume than a reciprocating compressor.

[0059] While the detailed description of the present invention has described specific embodiments, it should be understood that various modifications are possible without departing from the technical spirit of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the claims set forth below but also by equivalents thereof.

Claims

1. A method for re-liquefying evaporated gas generated from liquefied petroleum gas stored in a liquefied petroleum gas storage tank, A discharge step for discharging the evaporated gas from the liquefied petroleum gas storage tank; A pressurizing step for increasing the pressure of the discharged evaporated gas; A liquefaction step of liquefying the compressed evaporated gas by heat exchange with a refrigerant circulating in a separate refrigerant circulation system; A depressurization step for lowering the pressure of the liquefied evaporated gas; A return step of returning the depressurized evaporated gas to the liquefied petroleum gas storage tank; Including, A re-liquefaction method characterized in that, in the pressurizing step, the evaporated gas is pressurized by a centrifugal evaporated gas compressor.

2. In claim 1, A re-liquefaction method, characterized in that the pressure of the evaporation gas compressed in the centrifugal evaporation gas compressor has a value within a range of an absolute pressure of 5 atm or less and a pressure value higher than a pressure value capable of overcoming the differential pressure and returning the liquefied petroleum gas to the liquefied petroleum gas storage tank while the evaporation gas is processed by the re-liquefaction method.

3. In claim 1, A re-liquefaction method characterized in that the above refrigerant circulation system includes a refrigerant compressor that compresses the refrigerant after cooling the evaporated gas by heat exchange.

4. In claim 3, A re-liquefaction method, characterized in that the pressure of the refrigerant compressed in the refrigerant compressor has an absolute pressure of 10 atm or less.

5. In claim 1, A re-liquefaction method, characterized in that the refrigerant is composed of a hydrocarbon component or a hydrocarbon mixture component having a molecular weight heavier than that of the liquefied petroleum gas stored in the liquefied petroleum gas storage tank.

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