JP6516430B2 - Boil-off gas reliquefaction plant - Google Patents

Description

The present invention includes a reservoir tank for storing the liquefied natural gas, the boil-off gas compressor unit for compressing the boil-off gas discharged from the storage tank, the fuel part of the boil-off gas compressed in the BOG compressor unit A high pressure injection type engine, a bleed passage for bleeding and reliquefying the other part of the boil off gas compressed by the boil off gas compressor, and returning it to the storage tank; boil off gas flowing through the bleed passage; The present invention relates to a boil-off gas reliquefaction facility provided with a refrigeration cycle circuit that circulates a refrigerant that exchanges heat and cools the boil-off gas on a floating structure.

When conveying LNG (Liquefied Natural Gas), it may be conveyed by the LNG ship equipped with the storage tank which stores the said LNG. Although the storage tank is adiabatically treated, external heat gradually vaporizes the stored LNG to generate boil-off gas. The generated boil-off gas is taken out from the storage tank to raise the pressure inside the storage tank, and is supplied as fuel for a medium-speed rotating diesel engine as a main engine for propulsion of the ship.
In recent years, a high pressure injection type engine (electronically controlled gas injection diesel engine for ships), in which fuel is supplied at a relatively high pressure, has been developed as a main engine for propulsion of the ship. The high-pressure injection engine is a high-pressure injection engine that injects fuel at high pressure. Therefore, when supplying boil-off gas as fuel to the high-pressure injection engine, the boil-off gas needs to be pressurized to about 30 MPaG and then supplied. .

The high-pressure injection type engine described above has high efficiency, and therefore, when the same level of propulsion as a conventional engine is exhibited, the amount of boil-off gas required as fuel is reduced and surplus boil-off gas is increased. You need to handle them. Here, a BOG reliquefaction facility on a ship that compresses boil-off gas discharged from a storage tank to a high pressure of about 30 MPaG and supplies it to a high pressure injection type engine such as a high pressure injection type engine includes a storage tank storing LNG. A boil-off gas compression unit for compressing boil-off gas discharged from the storage tank, and a high pressure injection engine for pumping the boil-off gas pressurized to the engine supply pressure to the high pressure injection engine by the boil off gas compression unit; A branch flow path for branching a part of the boil off gas boosted to the engine supply pressure, and heat for heat exchange between the boil off gas flowing through the branch flow path and the boil off gas flowing from the storage tank to the boil off gas compression section An exchanger, and a pressure reducing valve for reducing the pressure of boil-off gas after passing through the heat exchanger in the branch flow path, That a gas-liquid separator for gas-liquid separation of BOG is decompressed by pressure reducing valve has been known (see Patent Document 1).
In the BOG reliquefaction facility on the ship, the boil-off gas compressed in the boil-off gas compression section is cooled and decompressed in such a way as to exchange heat with the boil-off gas after being discharged from the storage tank in the heat exchanger. After that, it is liquefied in the gas-liquid separator and returned to the storage tank. In the gas-liquid separator, a part of the gas is discharged without being liquefied as a flash flow of gas, and the flash flow is again mixed with the boil-off gas flowing between the storage tank and the heat exchanger .

On the other hand, when considering the existing prior art as a reliquefaction facility that processes excess boil-off gas, a reliquefaction facility as shown in FIG. 5 can be considered. The reliquefaction facility includes a refrigeration cycle circuit C that circulates a noncondensable refrigerant N2 (for example, nitrogen) made of nitrogen or the like, and the refrigeration cycle circuit C includes a refrigerant compression compressor CP that compresses the refrigerant N2. A cooler EX for cooling the refrigerant N2 compressed and heated by the refrigerant compression compressor CP, an expansion turbine EP for expanding the refrigerant after temperature reduction by the cooler EX, and a refrigerant after expansion in the expansion turbine EP A first heat exchanger EX1 for supplying cold heat of N2 to a boil-off gas BOG as a medium to be cooled is provided.
Incidentally, in the refrigeration cycle circuit C shown in FIG. 5, the refrigerant N2 having cold energy after passing through the refrigerant after being cooled by the cooler EX and the first heat exchanger EX1 as a condenser And a second heat exchanger EX2 that exchanges heat with each other.
The excess boil-off gas BOG is cooled and liquefied in the form of heat exchange with the refrigerant N2 in the first heat exchanger EX1 of the refrigeration cycle circuit C, and the pressure is reduced by the pressure reducing valve V. It will be led.

As another example of a reliquefaction facility that processes excess boil off gas, as described in Patent Document 2, a storage tank storing LNG, boil off gas immediately after discharge from the storage tank, a medium to be cooled, and heat are disclosed. A first heat exchanger to be replaced, a boil-off gas compressor (including a cooler) for compressing boil-off gas after passing through the first heat exchanger, and boil-off gas after being compressed in the boil-off gas compressor It is known to have a second heat exchanger which cools and liquefies in the form of heat exchange with the medium.
In the technology disclosed in Patent Document 2, as a refrigeration cycle circuit C that circulates a non-condensable refrigerant made of nitrogen or the like, a refrigerant compression compressor that compresses the refrigerant N2, and compression by the refrigerant compression compressor and temperature increase And a first refrigerant flow path for passing the refrigerant cooled by the cooler as a cooling medium to the first heat exchanger as a cooling medium and then passing the cooling medium to the second heat exchanger And an expansion turbine for expanding and lowering the temperature of the refrigerant after flowing through the first refrigerant flow path, and a refrigerant after lowering the temperature by the expansion turbine is caused to flow through the second heat exchanger as a cooling medium. It is known to have a second refrigerant flow path back to the compressor.
In the refrigeration cycle circuit C, for the purpose of improving the heat exchange efficiency in the second heat exchanger, part of the refrigerant flowing through the first refrigerant flow path is bypassed to the first heat exchanger. And a bypass flow path for joining the refrigerant flowing through the upstream side of the second heat exchanger on the downstream side of the first heat exchanger in the first refrigerant flow path after flowing through a part of the second heat exchanger Is provided.
That is, in the technology disclosed in Patent Document 2, particularly, in the second heat exchanger, three fluids of the boil-off gas, the refrigerant that has passed through the first heat exchanger, and the refrigerant that bypasses the first heat exchanger By adopting a heat exchange configuration, the temperature change rate of the refrigerant with respect to the amount of heat exchange is adjusted by adjusting the bypass amount of the refrigerant, and the heat exchange efficiency is improved.

Korean Published Patent No. 10-20130139150 (KR, A) Patent No. 5280351 gazette

However, in the BOG reliquefaction facility on a ship shown in the technique disclosed in Patent Document 1, a cold heat source for cooling the boil off gas after leaving the boil off gas compression section is immediately after being discharged from the storage tank. Since only cold heat (self-cool heat) is present, subcooling of the boil-off gas after leaving the boil-off gas compression section becomes insufficient, and the flow rate of the flush flow generated after pressure reduction by the pressure reducing valve increases. The large amount of flush flow is mixed with the boil-off gas after leaving the storage tank, so the compression power of the boil-off gas compression compressor of the boil-off gas compression section increases as the flush flow rate increases, resulting in the efficiency deterioration.
In the case where the flush flow is not mixed with the boil-off gas after leaving the storage tank and discharged to the outside, a large amount of methane having high utility value is discarded, which is economically disadvantageous. Do.
On the other hand, in the case of adopting a simple reliquefaction facility as shown in FIG. 5 in order to process excess boil off gas, the boil off gas BOG on the heat receiving side and the heat receiving side in the first heat exchanger EX1 of the refrigeration cycle circuit C. The TQ diagram showing the relationship between the temperature and the amount of heat exchange with respect to the refrigerant N2 of is as shown in FIG.
Here, in FIG. 6, since the boil-off gas is a gas consisting essentially of methane alone, the temperature change is discontinuous at the point where the temperature of the boil-off gas BOG on the heat receiving side is lowered and it begins to condense. And when it becomes a gas-liquid mixed state, it changes isothermally.
On the other hand, when using non-condensable nitrogen N2 generally used as the refrigerant on the heat receiving side, the TQ line becomes substantially linear as shown in FIG. 6 because the refrigerant N2 does not change its state . As a result, the temperature difference between the boil-off gas BOG and the refrigerant N2 comes closest to each other at the point where the boil-off gas BOG starts to condense (this point (point indicated by P1 in FIG. 6) is referred to as a pinch point). Because of this pinch point, the rate of temperature change with respect to the heat exchange amount of the refrigerant N2 (slope γ in FIG. 6) decreases with the temperature change of the boil-off gas BOG (that is, the flow rate of the refrigerant N2 needs to be increased). Therefore, the compression power of the refrigerant compression compressor CP of the refrigeration cycle circuit C is increased, which causes the efficiency deterioration.
In addition, the difference between the temperature of the heat-receiving boil-off gas BOG and the temperature of the heat-receiving refrigerant N2 (in FIG. 6, ΔT2 and ΔT3) is large. This is due to the heat exchange in the first heat exchanger EX1. It shows that the efficiency is bad.

Furthermore, in the technology disclosed in Patent Document 2, in the second heat exchanger, the boil-off gas, the refrigerant passing through the first heat exchanger, and the refrigerant fluid bypassing the first heat exchanger perform heat exchange. Although the configuration is adopted, there is a possibility that the temperature change rate with respect to the heat exchange amount of the boil-off gas can not sufficiently follow the temperature change rate with respect to the heat exchange amount of the refrigerant.
Further, in the configuration disclosed in Patent Document 2, the relationship “compression power in the refrigeration cycle circuit> compression power of the boil-off gas compression unit” per unit boil-off gas holds, but in the first heat exchanger, Since the cold heat of the boil-off gas is supplied to the inefficient refrigeration cycle circuit, the efficiency of the entire system is deteriorated.
Furthermore, since the refrigeration cycle circuit has a complicated circuit configuration in which a bypass flow path and the like are provided in order to introduce three fluids having different temperatures to the second heat exchanger, the configuration as a whole system is complicated. In such a complicated configuration, in order to improve the heat exchange efficiency in the second heat exchanger, it is necessary to appropriately control the bypass flow rate and the like, and the flow rate control of the refrigerant also becomes complicated. I was complaining.

  The present invention has been made in view of the above-described problems, and an object thereof is to reduce the flow rate of the flash flow while improving the efficiency while avoiding the complication of the configuration, and the heat exchange in the heat exchanger An object of the present invention is to provide a boil-off gas reliquefaction facility capable of improving efficiency and further improving efficiency by reducing compression power of a refrigeration cycle circuit.

In order to achieve the above object, the apparatus for reliquefying boil-off gas according to the present invention comprises a storage tank for storing liquefied natural gas, a boil-off gas compression unit for compressing boil-off gas discharged from the storage tank, and the boil-off gas compression. a high-pressure injection type engine to fuel a portion of the compressed boil-off gas at parts, bleed flow back to the storage tank and the extraction and re-liquefying the other portion of the boil-off gas compressed in the BOG compressor unit A re-liquefaction facility for a boil-off gas comprising a passage and a refrigeration cycle that circulates a refrigerant that exchanges heat with the boil-off gas flowing through the bleed passage and cools the boil-off gas, comprising: Feature configuration is
The bleed flow passage is for bleeding the boil-off gas before being boosted up to the engine supply pressure of the high-pressure injection engine by the boil-off gas compression unit.
A first heat exchanger configured to cool the boil-off gas flowing through the bleed passage in heat exchange with the boil-off gas flowing from the storage tank to the boil-off gas compression unit;
A second heat exchanger that cools the boil-off gas that has passed through the first heat exchanger in the bleed flow passage with a refrigerant flowing through the refrigeration cycle circuit;
And pressure setting means for setting the extraction pressure of the boil-off gas to be extracted into the extraction flow passage to be less than the engine supply pressure and equal to or higher than the critical pressure of the boil-off gas .

According to the above feature configuration, the extraction flow path for extracting the boil-off gas from the boil-off gas compression section is for extracting the boil-off gas before the pressure is raised to the engine supply pressure of the high pressure injection type engine in the boil off gas compression section. The extraction pressure can be a pressure lower than the engine supply pressure, and, for example, after liquefying the boil-off gas, it is possible to reduce the flow rate of the flush flow generated when decompressing and gas-liquid separation.
Then, the boil-off gas (hereinafter sometimes abbreviated as boil-off gas after pressure rising) flowing through the extraction flow passage flows through the storage tank to the boil-off gas compression section in the first heat exchanger. (Hereafter, it may be abbreviated as boil-off gas before boosting.) After being cooled by the self-cooling of the boil-off gas before boosting, it flows through the refrigeration cycle circuit in the second heat exchanger. The refrigerant is cooled in the form of heat exchange with the refrigerant.
In particular, in the present invention, since the pressure setting means sets the bleed pressure to less than the engine supply pressure, as described above, for example, the first heat exchanger and the second heat exchange of the boil-off gas after pressure increase It is possible to reduce the flow rate of the flush flow which is generated at the time of pressure reduction and gas-liquid separation after cooling by the vessel.
Furthermore, the power consumed in the process of liquefying the boil off gas is the compression power in the boil off gas compression section (specifically, the compression of the boil off gas compression compressor that compresses the boil off gas in the boil off gas compression section) Power) and compression power in the refrigeration cycle circuit (specifically, compression power of the refrigerant compression compressor that compresses the refrigerant in the refrigeration cycle circuit), but the heat exchange loss and the like in the second heat exchanger Also in consideration of the above, when the extraction pressure is sufficiently smaller than the extraction pressure set in the present invention, the relationship of “compression power in refrigeration cycle circuit> compression power of boil-off gas compression section” per unit boil-off gas Is true.
Here, in the present invention, the pressure setting means sets the extraction pressure to a pressure higher than the pressure at which the boil-off gas flowing through the extraction flow path can be liquefied by the first heat exchanger. The exchanger can liquefy all of the boil-off gas flowing through the bleed flow path after pressure increase, and the second heat exchanger is only responsible for supercooling the boil-off gas that has been totally liquefied. Thereby, in the second heat exchanger, it is only necessary to execute the supercooling of the liquefied boil-off gas, so the compression power in the refrigeration cycle circuit (specifically, a refrigerant compression compressor that compresses the refrigerant in the refrigeration cycle circuit) Can be reduced, and the efficiency of the entire facility can be improved.
Furthermore, since the compression power in the refrigeration cycle circuit can be reduced, not only the liquefaction efficiency can be improved, but also a refrigerant compression compressor with a small capacity can be used, and downsizing of the entire equipment can be achieved.
From the above, it is possible to realize a boil-off gas reliquefaction facility which can realize the efficiency improvement by reducing the compression power of the refrigeration cycle circuit while reducing the flow rate of the flash flow and aiming to improve the efficiency while avoiding complication of the configuration.

According to the above configuration, the pressure setting unit sets the lower limit pressure of the extraction pressure to the critical pressure or more of the boil-off gas when setting the extraction pressure, so the boil-off that passes through the first heat exchanger in the extraction flow path In the TQ diagram of the gas, the width A of the isothermal line indicated by the wet saturated vapor, which is a gas-liquid mixed state of liquid and gas, among the TQ lines indicated by the boil-off gas passing through the first heat exchanger in the extraction flow passage. The bleed pressure can be set to reduce (or eliminate).

Thus, the TQ line indicated by the boil-off gas passing through the first heat exchanger in the extraction flow path is, for example, a line indicated by a thick solid line in FIG. 3 from a diagram as indicated by a thick solid line in FIG. It becomes a figure. As a result, the flow rate of the refrigerant N2 can be reduced (the inclination of the refrigerant N2 in the TQ chart can be increased), and the compression power of the refrigerant compression compressor can be reduced. Further, the temperature difference between the heat-receiving TQ line and the heat-receiving TQ line can be reduced over the entire heat exchange amount, and the heat exchange efficiency can be improved.

Further features of the reliquefaction plant of the present invention are:
The pressure reducing valve for reducing the pressure of the boil-off gas that has passed through the second heat exchanger in the bleed flow path, and a gas-liquid separator for separating the boil-off gas reduced by the pressure reducing valve.
The pressure setting means sets the upper limit pressure of the bleed pressure to less than the flush flow suppression pressure at which the flow rate of the flush flow discharged as gas from the gas-liquid separator is suppressed.

  According to the above feature configuration, the pressure setting means sets the extraction pressure to less than the flush flow suppression pressure that can suppress the flow rate of the flush flow discharged as gas from the gas-liquid separator. Can be well suppressed.

Further features of the reliquefaction plant of the present invention are:
The boil off gas compression unit includes a plurality of boil off gas compression compressors that compress boil off gas,
The refrigeration cycle circuit includes a refrigerant compression compressor that compresses a refrigerant.
The pressure setting means includes, in the process of reliquefying the boil-off gas to be extracted, compression power related to the extraction pressure of the boil-off gas to be extracted among the compression powers of the plurality of boil-off gas compression compressors; The bleed pressure is set so that the total power with the compression power of the refrigerant compression compressor when supercooling the boil-off gas passing through the second heat exchanger is reduced.

In the process of re-liquefying the boil-off gas to be extracted, when the extraction pressure of the boil-off gas is gradually increased, as shown in FIG. 4, the extraction pressure of the boil-off gas to be extracted out of the compression power of the boil-off gas compression compressor The associated compression power (power indicated by the legend of ▲ in FIG. 4) gradually increases.
On the other hand, in the configuration of the present invention, when the boil-off gas to be extracted is reliquefied, the first heat exchanger can be used to increase the compression power of the boil-off gas compression compressor until the extraction pressure reaches a predetermined pressure. Heat exchange efficiency is improved, and the amount of heat exchange in the first heat exchanger is increased. As a result, the amount of heat exchange of the second heat exchanger can be reduced (see FIG. 2), and hence the compression power of the refrigerant compression compressor (power shown by the legend of in FIG. 4) can be reduced.
On the other hand, if the extraction pressure exceeds a predetermined pressure, the amount of heat exchange in the second heat exchanger can not be reduced even if the compression power of the boil-off gas compression compressor is increased (see FIG. 3).
Adding the explanation for the reason, the pre-pressurized boil-off gas discharged from the storage tank is heated to about -120 ° C by the heat input of the pipe, etc. just before it flows into the first heat exchanger, With the boil-off gas of about -120 ° C, the boil-off gas after pressure increase can be cooled only to about -110 ° C at most. For this reason, even if the bleed pressure is increased and the heat exchange efficiency in the first heat exchanger is increased, the first heat exchanger can cool the boil-off gas after pressure increase only to -110 ° C. In other words, above a certain extraction pressure, the amount of heat exchange required in the second heat exchanger is the amount of heat that cools the boil-off gas after pressure increase to about -110 ° C to -160 ° C, and the heat in the second heat exchanger Since the amount of exchange hardly changes, the amount of heat exchange in the second heat exchanger can not be reduced even if the extraction pressure exceeds a predetermined pressure and the compression power of the boil-off gas compression compressor is increased. .
Further, when the extraction pressure exceeds a predetermined pressure, the flow rate of the flush flow at the time of gas-liquid separation of the boil-off gas becomes large, so the compression power of the refrigerant compression compressor becomes large.
From the above relationship, in the process of reliquefying the boil-off gas, the total power of the compression power related to the extraction pressure of the boil-off gas extracted among the compression power of the plurality of boil-off gas compression compressors and the compression power of the refrigerant compression compressor Is a minimum during a predetermined bleed pressure width (width indicated by .DELTA.P in FIG. 4), as shown in FIG.
According to the above feature configuration, by setting the extraction pressure so as to reduce the total power, the efficiency of the entire facility is suppressed while suppressing the generation of the flash flow while appropriately executing the reliquefaction of the boil-off gas. Can be improved.

In the reliquefaction plant of the present invention, the pressure setting unit preferably sets the extraction pressure to 7 MPaG or more and 10 MPaG or less.
In the reliquefaction plant of the present invention, the refrigerant flowing through the refrigeration cycle circuit is preferably nitrogen.

Diagram showing schematic configuration of boil-off gas reliquefaction plant TQ diagram of heat receiving side boil-off gas and heat receiving medium in the first and second heat exchangers when the extraction pressure is set to 5 MPaG and 7 MPaG TQ diagram of heat receiving side boil-off gas and heat receiving side medium in first heat exchanger and second heat exchanger when extraction pressure is set to 7, 10, 30MPa G Graph showing the compression power of the boil-off gas compression compressor and the compression power of the refrigerant compression compressor and their sum when the extraction pressure is changed A diagram showing a schematic configuration of a boil-off gas reliquefaction facility in the prior art TQ diagram of heat receiving side boil off gas and heat receiving side refrigerant in the first heat exchanger in the configuration shown in FIG. 5

The boil-off gas reliquefaction plant according to the embodiment of the present invention improves the heat exchange efficiency in the heat exchanger while reducing the flow rate of the flash flow to improve the efficiency while avoiding complication of the structure, Furthermore, the efficiency can be improved by reducing the compression power of the refrigeration cycle circuit.
Hereinafter, a boil-off gas reliquefaction facility 100 according to an embodiment of the present invention will be described based on the drawings.

  The boil-off gas reliquefaction facility 100 according to the embodiment of the present invention is provided on an LNG carrier 50 (an example of a floating structure) as shown in FIG. 1, and is a storage tank for storing liquefied natural gas LNG. 10, a boil-off gas compressor 20 for compressing boil-off gas BOG discharged from the storage tank 10, and high-pressure injection using as a fuel a part of liquefied boil-off gas BOG compressed and liquefied in the boil-off gas compressor 20 Engine 40, the other part of the boil-off gas BOG compressed by the boil-off gas compressor 20 is extracted and re-liquefied and returned to the storage tank 10, the boil-off gas BOG flowing through the extraction channel L2 And a refrigeration cycle circuit C for circulating a refrigerant N2 for cooling the boil off gas BOG by heat exchange.

The storage tank 10 employs a heat insulating structure that thermally insulates from the external space, and is configured to be able to store LNG having a relatively low temperature (for example, -163 ° C.) inside. In the storage tank 10, although thermally insulated from the outside, LNG is vaporized in the form of conduction of heat from the outside, and a boil-off gas BOG mainly composed of methane is generated.
The storage tank 10 and the boil-off gas compression unit 20 are connected by the boil-off gas discharge passage L1, and the boil-off gas BOG generated in the storage tank 10 is transmitted through the boil-off gas discharge passage L1. Led to

The boil-off gas compression unit 20 includes a plurality of boil-off gas compression compressors CP 1, CP 2, CP 3, CP 4 and CP 5 (five in this embodiment) for compressing the boil off gas BOG in the order described in the flow direction of the boil off gas BOG Together with coolers EX1, EX2, EX3, EX4 and EX5, which cool the boil off gas BOG after it has been compressed and heated by the boil off gas compression compressors CP1, CP2, CP3, CP4 and CP5 with the other refrigerant. In the flow direction of the boil-off gas BOG, the downstream side outlets of the boil-off gas compression compressors CP1, CP2, CP3, CP4, CP5 correspond to the boil-off gas compression compressors CP1, CP2, CP3, CP4, CP5, respectively. In the flow direction of the boil off gas BOG They are arranged in the order of mounting.
That is, the boil-off gas BOG guided from the boil-off gas discharge path L1 to the boil-off gas compression unit 20 is compressed by the first boil-off gas compression compressor CP1, and then cooled by the first cooler EX1 to the first cooler EX1. The cooled boil-off gas BOG is compressed by the second boil-off gas compression compressor CP2 and then cooled by the second cooler EX2, and the boil-off gas BOG cooled by the second cooler EX2 is the third boil-off After compressed by the gas compression compressor CP3, the boil-off gas BOG cooled by the third cooler EX3 and cooled by the third cooler EX3 is compressed by the fourth boil-off gas compression compressor CP4, then 4 Cooled by the cooler EX4 and cooled by the fourth cooler EX4 The boil-off gas BOG is compressed by the fifth boil-off gas compression compressor CP5, cooled by the fifth cooler EX5, boosted to an engine supply pressure of, for example, 30 MPaG or more, and supplied to the high pressure injection type engine 40 Ru. The high pressure injection type engine 40 employs a marine electronically controlled gas injection diesel engine, and is a two-stroke engine that is directly connected to a propeller for propulsion of the LNG carrier 50 and rotates at a low speed.
The high pressure injection type engine 40 of the present invention may be an engine that injects a relatively high pressure fuel, and is not limited to a two-stroke engine.

In this embodiment, in the compression process of the boil-off gas compression unit 20, the boil-off gas BOG having a relatively low pressure before being boosted to the engine supply pressure is configured to be capable of bleeding.
To add the explanation, in the boil-off gas compression unit 20, after being compressed by the second boil-off gas compression compressor CP2, a part of the boil-off gas BOG cooled by the second cooler EX2 is separated and extracted first A branching mechanism D1 is provided, and the boil-off gas BOG branched by the first branching mechanism D1 is burned by various gas combustors.

Furthermore, a second branching mechanism that branches off and extracts a portion of the boil-off gas BOG cooled by the fourth cooler EX4 after being compressed by the fourth boil-off gas compression compressor CP4 into the boil-off gas compression unit 20 D2 is provided, and the boil-off gas BOG extracted by the second branch mechanism D2 flows through the extraction flow path L2 connected to the second branch mechanism D2.
When the amount of boil-off gas BOG discharged from the storage tank 10 exceeds the amount of boil-off gas BOG required as fuel in the high pressure injection type engine 40 to the extraction flow path L2, the excess amount Boil off gas BOG is introduced.

The boil off gas BOG flowing through the bleed flow path L2 is cooled by heat exchange with the boil off gas BOG before pressure boosting flowing through the boil off gas discharge path L1, and then the heat with the refrigerant N2 circulating in the refrigeration cycle circuit C It is cooled by replacement and reliquefied.
To add the description, the boil-off gas BOG flowing through the bleed passage L2 flows through the boil-off gas discharge passage L1 connecting the storage tank 10 to the boil-off gas compressor 20 in the bleed passage L2. A second heat exchanger for cooling the first heat exchanger EX9 cooled by heat exchange with BOG and the boil-off gas BOG flowing through the extraction flow path L2 by heat exchange with the refrigerant N2 circulating in the refrigeration cycle circuit C EX10, a pressure reducing valve V1 for reducing the pressure of the boil off gas BOG after passing through the second heat exchanger EX10, and a gas liquid separator 30 for separating the gas and liquid of the boil off gas BOG after reducing the pressure by the pressure reducing valve V1. Is provided.
The extraction flow path L2 connects the lower part of the gas-liquid separator 30 to the storage tank 10, and the liquefied boil-off gas BOG (L) separated by the gas-liquid separator 30 is the bleed flow. It is returned to the storage tank 10 via the passage L2. On the other hand, the boil-off gas (G) of the gas separated in the gas-liquid separator 30 by the gas-liquid separation is discharged from the gas-liquid separator 30 as a flush flow.

The refrigeration cycle circuit C is a circuit provided to circulate nitrogen as the non-condensable refrigerant N2 and to mainly subcool the boil-off gas BOG in the second heat exchanger EX10.
The refrigeration cycle circuit C includes a plurality of (three in the present embodiment) refrigerant compressors CP6, CP7, CP8 for compressing the refrigerant N2 in the order described in the flow direction of the boil-off gas BOG, and the refrigerant compression compressor Coolers EX6, EX7, EX8 cool the boil off gas BOG after being compressed and heated by CP6, CP7, CP8 in the form of exchanging heat with other refrigerants. Each refrigerant compression compressor CP6, CP7 in the flow direction of the refrigerant N2 , And CP8 are arranged in the order described in the flow direction of the boil-off gas BOG in a state corresponding to each of the refrigerant compression compressors CP6, CP7, and CP8.
To add the description, the refrigerant N2 is compressed by the sixth refrigerant compression compressor CP6 and then cooled by the sixth cooler EX6, and the refrigerant N2 cooled by the sixth cooler EX6 is the seventh refrigerant compression compressor After being compressed by CP7, the refrigerant N2 cooled by the seventh cooler EX7 and cooled by the seventh cooler EX7 is compressed by the eighth refrigerant compression compressor CP8, and then compressed by the eighth cooler EX8 It is cooled.
Further, the refrigeration cycle circuit C further includes a third heat exchanger EX11 configured to cool the refrigerant N2 cooled by the eighth cooler EX8 in a form of heat exchange with the refrigerant N2 after passing through the second heat exchanger EX10; And an expander EP1 for expanding the refrigerant N2 after passing through the third heat exchanger EX11.
Thus, the refrigerant N2 circulating through the refrigeration cycle circuit C is cooled in the order described by the plurality of coolers EX6, EX7, EX8 while being compressed in the order described by the plurality of refrigerant compression compressors CP6, CP7, CP8, and the third After being further cooled in the heat exchanger EX11, the boil-off gas BOG is expanded in the expander EP1 and cooled to a temperature capable of supercooling the boil-off gas BOG (eg, a temperature of -170 ° C. or less), and then the second heat exchanger EX10 And circulate through the refrigeration cycle circuit C.

  As described above, in the reliquefaction facility 100 of the boil off gas BOG according to this embodiment, the boil off gas BOG discharged from the storage tank 10 is compressed to a predetermined bleed pressure by the boil off gas compression unit 20, It is cooled in the first heat exchanger EX9 and the second heat exchanger EX10 in the bleed flow path L2 and reliquefied, but the heat exchange efficiency of the first heat exchanger EX9 is improved and the second heat exchange From the viewpoint of reducing the compression power of the refrigerant compression compressors CP6, CP7, CP8 in the refrigeration cycle circuit C, the extraction pressure of the boil-off gas BOG is set to the first heat exchanger EX9 from the viewpoint of reducing the compression power of the refrigerant compression compressors CP6, CP7, CP8 It is preferable that the total amount be equal to or higher than the liquefiable liquefaction pressure.

  In the reliquefaction facility 100 of the boil off gas BOG according to the embodiment, a control device that functions as a pressure setting unit 60a (an example of a pressure setting unit) capable of setting the extraction pressure of the boil off gas BOG from the boil off gas compression unit 20. The control device 60 is configured to be able to cooperate hardware composed of an integrated device such as an LSI or the like and software composed of a plurality of program groups.

Here, when the extraction pressure of the boil-off gas BOG is a general extraction pressure (for example, a pressure of about 0.3 MPaG or more and 2.0 MPaG or less) in the ship boil-off gas BOG reliquefaction system equipped with a conventional nitrogen refrigerant cycle, The TQ diagram showing the relationship between the heat exchange amount of the boil-off gas BOG heat-exchanged by the first heat exchanger EX9 and the second heat exchanger EX10 and the temperature is as shown by the thick solid line in the schematic diagram 6 . That is, when the extraction pressure of the boil off gas BOG is low, the temperature change becomes discontinuous at the point where the temperature of the boil off gas BOG decreases and starts to condense. And when it becomes a gas-liquid mixed state, it changes isothermally.
Therefore, the temperature difference between the boil-off gas BOG and the refrigerant N2 approaches the closest point at which the boil-off gas BOG starts to condense (this point (point indicated by P1 in FIG. 6) is referred to as a pinch point). Because of this pinch point, the ratio of the temperature change to the heat exchange amount of the refrigerant N2 (slope γ in FIG. 6) becomes smaller in accordance with the temperature change of the boil-off gas BOG. (In other words, it is necessary to increase the flow rate of the refrigerant N2). As a result, the compression power of the refrigerant compression compressors CP6, CP7, CP8 of the refrigeration cycle circuit C is increased, leading to a deterioration in efficiency.
In addition, the difference between the temperature of the heat-receiving boil-off gas BOG and the temperature of the heat-receiving refrigerant N2 (ΔT2 and ΔT3 in FIG. 6) is large, which means that the first heat exchangers EX9 and 2 It shows that the heat exchange efficiency in the heat exchanger EX10 is poor.

Therefore, when setting the lower limit pressure of the extraction pressure, the pressure setting unit 60a sets the lower limit pressure of the extraction pressure to the critical pressure of the boil off gas BOG (if the boil off gas BOG is a gas mainly composed of methane, 4.8 MPaG When the pressure is higher than boil-off gas BOG is pure methane, the pressure is set to 4.5 MPaG or higher.
That is, the pressure setting unit 60a increases the bleed pressure of the boil off gas BOG more than the general bleed pressure in the ship boil off gas BOG reliquefaction system equipped with the conventional nitrogen refrigerant cycle, thereby making the thick solid line boil off in FIG. In the TQ line of gas BOG, the width A showing the isothermal change in the gas-liquid mixed state can be reduced. That is, the TQ line indicated by the thick solid line in FIG. 6 is changed to the TQ line indicated by the thick broken line in FIG. 2 (extraction pressure 5 MPaG), and further to the TQ line indicated by the thick solid line in FIG. 2 (extraction pressure 7 MPaG). Thus, the flow rate of the refrigerant N2 can be reduced (the inclination of the refrigerant N2 in the TQ diagram can be increased), and the compression power of the refrigerant compression compressors CP6, CP7, CP8 can be reduced.
The TQ diagram shown in FIG. 2 is a TQ diagram for the first heat exchanger EX9 and the second heat exchanger EX10 when the extraction pressure of the boil off gas BOG is 5 MPaG and the extraction pressure 7 MPaG, but the boil off gas BOG When the extraction pressure is 5 MPaG, the difference in slope between the saturated vapor line (thick broken line near the center of the graph in FIG. 2) and the superheated vapor line (thin broken line near the right of the graph in FIG. 2) is larger than 7 MPaG. (The swell of the graph gets bigger). This is because the TQ line indicated by the boil-off gas BOG on the heat receiving side and the heat medium on the heat receiving side (the first heat exchanger EX9) show a better setting when the extraction pressure is set to 5 MPaG than when the extraction pressure is set to 7 MPaG. In the graph, the temperature difference between the boil off gas BOG before boosting and the TQ line indicated by the refrigerant N2 in the second heat exchanger EX10 is large, indicating that the heat exchange efficiency is poor.
As a result, as shown in FIG. 2, when the extraction pressure is set to 5 MPaG, the boil-off gas BOG after pressure increase is boosted before the pressure increase in the first heat exchanger EX9 as compared to the case where the extraction pressure is set to 7 MPaG. The amount of cold heat received from the gas BOG decreases. Thereby, in the second heat exchanger EX10, it is necessary to increase the amount of cold heat (supercooling amount) received from the refrigerant N2 by ΔE1, and the compression power of the refrigerant compression compressors CP6, CP7, CP8 in the refrigeration cycle circuit C is increased. .
That is, when the extraction pressure is 7 MPaG, the ratio of the amount of cooling by the boil-off gas BOG before boosting is higher among the cold heat necessary for liquefying the boil-off gas BOG on the heat receiving side than when the extraction pressure is lower The ratio of the amount of cooling by the refrigeration cycle circuit C is low. This indicates that the cold heat of the boil-off gas BOG before boosting can be used effectively, and the power of the refrigerant compression compressors CP6, CP7, CP8 in the refrigeration cycle circuit C can be reduced, and the efficiency as a liquefaction system is improved I mean to do.
From the above, in the present embodiment, the pressure setting unit 60a is set to 7 MPaG or more of the extraction pressure of the boil-off gas BOG.
Incidentally, the calculation conditions of the TQ diagram when the extraction pressure of the boil-off gas BOG is 7 MPaG are as follows [Table 1] in P1 to P6 shown in the boil-off gas discharge passage L1 and the extraction passage L2 in the schematic configuration diagram of FIG. The flow rate, temperature, and pressure shown in 1] shall be indicated. The underlined values are substantially constant values regardless of the bleed pressure. In addition, the tank pressure is set to a pressure of about 0 MPaG to about 0.035 MPaG.

  On the other hand, if the extraction pressure of the boil-off gas BOG is too high, the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 becomes large and the efficiency decreases, and the flash generated when the pressure is reduced by the pressure reducing valve V1. Since the flow rate increases and the boil-off gas BOG (G) of the gas discarded from the gas-liquid separator 30 increases, it is not preferable.

FIG. 3 shows a TQ diagram when the extraction pressure of the boil-off gas BOG is set to 7, 10, and 30 MPaG in the pressure setting unit 60a.
From the TQ diagram shown in FIG. 3, when the extraction pressure is increased from 7 MpaG to 30 MPaG, the compression power of the boil-off gas compression unit 20 is increased as when the extraction pressure is increased in the range less than 7 MpaG. 2) The amount of cooling (the amount of heat exchange) in the heat exchanger EX10 can not be reduced, and the compression power in the refrigeration cycle circuit C can not be reduced. Furthermore, since it is necessary to suppress the increase in the flow rate of the flush flow accompanying the increase in the extraction pressure, it is necessary to increase the subcooling amount of the boil-off gas BOG. Therefore, as shown in FIG. Compared to the setting, it is necessary to make the temperature at which the refrigerant N2 enters the second heat exchanger EX10 in the refrigeration cycle circuit C lower by a temperature indicated by ΔT1 in FIG. Since this means an increase in compression power in the refrigeration cycle circuit C, it means a deterioration in efficiency in the reliquefaction process.
On the other hand, when the extraction pressure is set to 10 MPaG, the inlet temperature of the refrigerant N2 to the second heat exchanger EX10 in the refrigeration cycle circuit C can be set to substantially the same temperature as the case where the extraction pressure is set to 7 MPaG. This is because even when the extraction temperature is set to 10 MPaG, the temperature at which the refrigerant N2 enters the second heat exchanger EX10 in the refrigeration cycle circuit C is set to substantially the same temperature as when the extraction pressure is set to 7 MPaG. It shows that the flow rate of the flush flow can be sufficiently suppressed. Furthermore, when the extraction pressure is set to 10 MPaG, while maintaining the compression power in the refrigeration cycle circuit C constant, the TQ line indicated by the boil-off gas BOG after pressure increase on the heat receiving side is nearly linear. It can be changed to the state.
Therefore, in the present embodiment, the pressure setting unit 60a sets the extraction pressure of the boil-off gas BOG to 10 MPaG or less.

Furthermore, in the process of reliquefying the boil-off gas BOG to be extracted, the pressure setting unit 60a according to this embodiment includes the compression power related to the extraction of the boil-off gas compression compressors CP1, CP2, CP3, and CP4, the refrigerant compression compressor CP6, The bleed pressure is set so that the total power with the CP7 and CP8 compression power is reduced.
To add the description based on FIG. 4, when the extraction pressure of the boil-off gas BOG is gradually increased from 0 MPaG in the process of reliquefying the boil-off gas BOG to be extracted, as shown in FIG. Of the compression powers of CP1, CP2, CP3 and CP4, the compression power (power indicated by the ▲ legend in FIG. 4) related to the extraction pressure of the boil-off gas BOG extracted is gradually increased.
On the other hand, in the configuration according to the embodiment, when the boil-off gas BOG to be extracted is reliquefied, the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 until the extraction pressure is a predetermined pressure. The heat exchange efficiency in the first heat exchanger EX9 is improved and the amount of heat exchange in the first heat exchanger EX9 is increased as the extraction pressure is increased (the extraction pressure is increased). As a result, the amount of heat exchange in the second heat exchanger EX10 can be reduced (see FIG. 2), so the compression power of the refrigerant compression compressors CP6, CP7, CP8 (power shown by the ▪ in FIG. 4) is reduced. can do. On the other hand, if the extraction pressure exceeds a predetermined pressure, the amount of heat exchange in the second heat exchanger EX10 can not be reduced even if the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 is increased ( See Figure 3).
To add the explanation for the reason, the pre-pressurized boil-off gas BOG discharged from the storage tank 10 is heated to about -120 ° C. by the heat input of the pipe and the like immediately before it flows into the first heat exchanger EX9. The boil-off gas BOG of about -120.degree. C. can cool the boil-off gas BOG after pressure increase to about -110.degree. C. at most. For this reason, even if the extraction pressure is increased and the heat exchange efficiency in the first heat exchanger EX9 is increased, the first heat exchanger EX9 can cool the boil-off gas BOG after pressure increase only to -110 ° C. In other words, above a certain extraction pressure, the amount of heat exchange required in the second heat exchanger EX10 is the amount of heat that cools the boil-off gas BOG after pressure increase from about −110 ° C. to about −160 ° C., and the second heat exchanger EX10 Because the amount of heat exchange in the case hardly changes, even if the extraction pressure exceeds the predetermined pressure and the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 is increased, the second heat exchanger EX10 The amount of heat exchange can not be reduced.
In addition, since the flow rate of the flash flow at the time of gas-liquid separation of the boil-off gas BOG is increased, the cooling amount (supercooling amount) by the refrigerant N2 in the second heat exchanger EX10 is set to suppress the flow rate of the flash flow. Since it is necessary to increase, the compression power of the refrigerant compression compressors CP6, CP7, CP8 becomes large.
From these relationships, in the process of reliquefying the boil-off gas BOG, the compression power related to the extraction pressure of the boil-off gas BOG extracted among the compression powers of the plurality of boil-off gas compression compressors CP1, CP2, CP3, CP4 and refrigerant The total power with the compression power of the compression compressors CP6, CP7, CP8 is, as shown in FIG. 4, minimum between the range of 7 MPaG to 10 MPaG (the width shown by .DELTA.P in FIG. 4). Become.
Therefore, the pressure setting unit 60a according to the embodiment sets the extraction pressure to 7 MPaG or more and 10 MPaG or less in order to improve the efficiency in the reliquefaction process by reducing the total power.

  The specific numerical values of the graph shown in FIG. 4 are shown in the following [Table 2]. Incidentally, the compression power of the boil-off gas compression compressor shown in [Table 2] is the compression power related to the bleed pressure of the boil-off gas BOG extracted among the compression powers of the plurality of boil-off gas compression compressors CP1, CP2, CP3, CP4. However, it also includes the compression power for compressing the boil-off gas BOG led to the high pressure injection type engine 40. For the calculation conditions of FIG. 4 and Table 2, refer to Table 1.

In addition, in the reliquefaction facility 100 of the boil off gas according to the present embodiment, the technology disclosed in Patent Document 1, and the technology disclosed in Patent Document 2, simulation results of power consumption required for liquefaction per boil off gas BOG are shown. , [Table 3] below. Incidentally, [Table 3] shows the power consumption when the main engine load is 0% and 30%, and the main engine load indicates the load of the high pressure injection type engine. Here, the technology disclosed in Patent Document 2 does not include the main engine (high-pressure injection engine), but in the following simulation, the boil-off gas compression section (including cooler) of the technology disclosed in Patent Document 2 A flow path for branching a part of the boil-off gas BOG is connected on the upstream side of the second heat exchanger on the downstream side of b), and the boil-off gas is supplied to the main engine (high-pressure injection engine) by the flow path Was adopted.
In addition, as conditions of simulation, in the temperature (for example, -120 degreeC) at the time of arrival to each liquefier of this embodiment, the technique of indication to patent document 1, and the technique of indication to patent document 2, it is boil off. Assuming that 2883 kg / h of gas BOG is generated and the main engine load is 30%, about 1400 kg / h of generated boil-off gas BOG is sent to the main engine, and the remaining part is sent to the generator. The condition was to liquefy the remainder.
The electric power generated by the generator is used as the power for supplying the fuel to the ship and the main engine or the like, and as the compressor motive power for driving the refrigerant cycle. Further, since the liquefaction power differs between this embodiment, the technology disclosed in Patent Document 1, and the technology disclosed in Patent Document 2, the amounts of boil-off gas BOG to be sent to the generator are respectively different.

[Another embodiment]
(1) Although the reliquefaction installation 100 of the boil-off gas has been described as an example provided in the LNG carrier 50 for transporting the LNG in the above embodiment, it is not limited to the configuration separately.
As another example, after the mined LNG is refined at sea, it is directly liquefied and stored in the storage tank 10, and if necessary, the LNG stored in the storage tank 10 is transferred to the LNG carrier It may be provided in an offshore plant such as LNG FPSO (Floating Production Storage and Off-loading), which is a facility used to

(2) In the above-mentioned embodiment, although nitrogen which is a non-condensable refrigerant was explained as an example as refrigerant N2 which circulates refrigeration cycle circuit C, the refrigerant concerned is not limited to nitrogen. As another example of the refrigerant, other than helium, hydrogen and the like, a mixed refrigerant may be used.

(3) In the above embodiment, the extraction flow path L2 connects the gas-liquid separator 30 and the storage tank 10, and the liquefied boil-off gas BOG (L) after being separated in the gas-liquid separator 30 by the gas-liquid separator 30 is The example of composition led to storage tank 10 was shown.
However, the boil-off gas BOG flowing through the extraction flow path L2 does not have to be returned to the storage tank 10, and in some cases, the cold heat of the boil-off gas BOG is used for air conditioning and cold heat generation. You may adopt a configuration that

(4) In the above embodiment, the number of boil off gas compression compressors is not particularly limited to that shown in the above embodiment, and the amount of boil off gas BOG (or the capacity of the storage tank 10) or the engine It can be suitably changed by the supply pressure. Furthermore, the number of boil-off gas compression compressors provided on the upstream side of the bleed flow path L2 in the flow direction of the boil-off gas BOG before pressure boosting is not limited to four, and can be changed as appropriate.
Further, the number of refrigerant compression compressors is not particularly limited, and can be appropriately changed according to the temperature and the amount of cold heat required for the second heat exchanger EX10.
And the number of coolers can be suitably changed in the state corresponding to the number of boil-off gas compression compressors and refrigerant compression compressors.

(5) In the above embodiment, various configurations can be considered as a configuration for supplying cold heat to the cooler in the boil-off gas compression unit 20 and the refrigeration cycle circuit C, but for example, the following configuration may be adopted. .
That is, an absorption type refrigerator (not shown) using the exhaust heat of the high pressure injection type engine 40 as a main engine as a heat source, and cold energy generated by the absorption type refrigerator is compressed and boil-off gas compression is performed. You may employ | adopt the structure provided with the heat-medium circulation circuit (not shown) which can circulate through the heat medium supplied by the cooler in the part 20 and the refrigerating cycle circuit C.
The heat medium circulation circuit may be disposed to supply cold to all of the coolers in the boil-off gas compressor 20 and the refrigeration cycle circuit C, or may be disposed to supply cold to a part thereof. It does not matter.

  The configurations disclosed in the above embodiment (including the other embodiments, the same applies hereinafter) can be applied in combination with the configurations disclosed in the other embodiments as long as no contradiction arises. The embodiment disclosed in the present specification is an exemplification, and the embodiment of the present invention is not limited thereto, and can be appropriately modified without departing from the object of the present invention.

  The boil-off gas reliquefaction plant of the present invention improves the heat exchange efficiency in the heat exchanger while reducing the flow rate of the flash flow to improve the efficiency while avoiding complication of the configuration, and further, the refrigeration cycle The present invention can be effectively used as a boil-off gas reliquefaction facility that can realize an improvement in efficiency by reducing the compression power of a circuit.

10: Storage tank 20: Boil off gas compression unit 30: Gas-liquid separator 40: High pressure injection type engine 60a: Pressure setting unit 100: Reliquefaction facility BOG: Boil off gas C: Refrigeration cycle circuit CP1: 1st boil off gas compression compressor CP2 : 2nd boil off gas compression compressor CP3: 3rd boil off gas compression compressor CP4: 4th boil off gas compression compressor CP5: 5th boil off gas compression compressor CP6: 6th refrigerant compression compressor CP7: 7th refrigerant compression compressor CP8: 8th refrigerant Compression compressor EX10: second heat exchanger EX9: first heat exchanger L2: extraction flow path LNG: liquefied natural gas N2: refrigerant V1: pressure reducing valve

Claims (5)

A storage tank for storing liquefied natural gas, a boil-off gas compression unit for compressing boil-off gas discharged from the storage tank, and a high-pressure injection type using as a fuel a part of the boil-off gas compressed in the boil- off gas compression unit Heat is exchanged between the engine and the extraction flow path for extracting and reliquefying the other part of the boil off gas compressed by the boil off gas compression section and returning it to the storage tank, and the boil off gas flowing through the extraction flow path And a refrigeration cycle circuit for circulating a refrigerant that cools the boil-off gas, the boil-off gas reliquefaction facility comprising:
The bleed flow passage is for bleeding the boil-off gas before being boosted up to the engine supply pressure of the high-pressure injection engine by the boil-off gas compression unit.
A first heat exchanger configured to cool the boil-off gas flowing through the bleed passage in heat exchange with the boil-off gas flowing from the storage tank to the boil-off gas compression unit;
A second heat exchanger that cools the boil-off gas that has passed through the first heat exchanger in the bleed flow passage with a refrigerant flowing through the refrigeration cycle circuit;
And a pressure setting unit configured to set the extraction pressure of the boil-off gas to be extracted into the extraction flow channel at a pressure lower than the engine supply pressure and equal to or higher than a critical pressure of the boil-off gas. The pressure reducing valve for reducing the pressure of the boil-off gas that has passed through the second heat exchanger in the bleed flow path, and a gas-liquid separator for separating the boil-off gas reduced by the pressure reducing valve.
Said pressure setting means, the upper limit pressure of the bleed air pressure, the BOG of claim 1, the flow rate of the flash stream is discharged as gas from the gas-liquid separator is set to less than the flash flow suppressing pressure is suppressed Reliquefaction facility. The boil off gas compression unit includes a plurality of boil off gas compression compressors that compress boil off gas,
The refrigeration cycle circuit includes a refrigerant compression compressor that compresses a refrigerant.
The pressure setting means includes, in the process of reliquefying the boil-off gas to be extracted, compression power related to the extraction pressure of the boil-off gas to be extracted among the compression powers of the plurality of boil-off gas compression compressors; boil-off in according BOG passing through the second heat exchanger such that the sum power of the compression power of the refrigerant compressor compressor when the subcooling is reduced, to claim 1 or 2 to set the bleed pressure Gas reliquefaction facility. The said pressure setting means sets the said extraction pressure to 7 MpaG or more and 10 MpaG or less, The reliquefaction installation of the boil off gas as described in any one of Claims 1-3 .   The refrigerant which flows through the said refrigerating cycle circuit is nitrogen, The reliquefaction installation of the boil off gas as described in any one of Claims 1-4.

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