KR100740686B1 - Bog reliquefaction apparatus - Google Patents

Abstract

The present invention is a problem of the prior art that may damage the blades of the expansion turbine by liquefying some of the nitrogen when the temperature of the nitrogen is reduced by using the expansion turbine in the BOG reliquefaction apparatus using the conventional reverse braton cycle of nitrogen In the present invention, only a portion of pressurized nitrogen is extracted and expanded using an expansion turbine, and the rest is lowered by lowering the temperature of nitrogen and then lowering the temperature through an expansion valve to lower the temperature of nitrogen without fear of droplet generation. There is an advantage. In addition, it is possible to vary the operation of the nitrogen cycle by changing the amount of extracted nitrogen can easily cope with the dynamic change of the BOG.

BOG, Reliquefaction, Coldbox, Module

Description

ＢＯ 액 reliquefaction unit {BOG RELIQUEFACTION APPARATUS}

1 shows a schematic diagram of a LNG reliquefaction apparatus according to the prior art.

2 shows a schematic diagram of an LNG reliquefaction apparatus according to an embodiment of the present invention.

3 shows a flowchart of a LNG reliquefaction method according to an embodiment of the present invention.

[Description of Symbols for Main Parts of Drawing]

10: storage tank 11: safety valve

20: recirculation valve 30: thermostat

40, 41: BOG compressor 50: BOG condenser

60, 61: first and second nitrogen heat exchangers 70, 71, 72: nitrogen compressor

80, 81, 82: intermediate cooler 90: expansion turbine

91: expansion valve 92: BOG non-condensing gas separator

93: valve 94: overheating mixer using a venturi nozzle

100: circulation pump 110: nitrogen buffer tank

200: coldbox

The present invention is an evaporation gas (BOG, boiled-off gas, hereinafter referred to as BOG) generated from a storage tank or a land storage tank of a carrier transporting cryogenic liquefied natural gas (LNG, Liquefied Natural Gas, LNG) It relates to a reliquefaction device of.

LNG is typically transported over long distances in a liquefied state. For example, an offshore tanker is used to transfer liquefied LNG to a second location where it is vaporized at the first location where LNG is liquefied and sent to a gas distribution system. Since LNG is liquefied at very low temperatures, that is, around -163 ° C at atmospheric pressure, LNG in storage tanks is continuously vaporized due to external heat transfer from the LNG carriers.When the pressure in the storage tanks exceeds the set safety pressure, BOG Is discharged to the outside through the safety valve. The discharged BOG is liquefied and returned to the storage tank or used as fuel for ships.

The conventional LNG carrier is a steam turbine propulsion method and was used as a propulsion fuel for ships by burning BOG in a boiler. Recently, as the generation of BOG is reduced due to the insulation technology of storage tanks, it is difficult to secure the amount of BOG required for the propulsion of steam turbine vessels and ship propulsion by more efficient diesel engines is preferred. Accordingly, new technologies are required for the treatment of BOG and securing the stability of storage tanks.

In a reliquefaction apparatus, a cooling cycle comprises the steps of compressing a working fluid in a plurality of compressors, cooling the compressed working fluid by indirect heat exchange, expanding the working fluid, and expanding the expanded working fluid. Heating by indirect heat exchange with the compressed working fluid, and returning the heated working fluid to one of the compressors. The LNG vapor after the compression step is at least partially condensed by indirect heat exchange with the working fluid to be heated. One example of an apparatus for performing this cooling method is described in US Pat. No. 3,857,245.

According to US Pat. No. 3,857,245, the working fluid is derived from the LNG itself and is therefore operated by an open cooling cycle. Expansion of the working fluid is carried out by a valve to obtain partially condensed LNG. Partially condensed LNG is separated into a gaseous phase mixed with natural gas to be sent to the liquid phase and combustion burners returned to the reservoir. The working fluid is heated and cooled in the same heat exchanger so only one heat exchanger is used. The heat exchanger is located on the first skid-mounted platform and the working fluid compressor is mounted on the second skid-mounted platform.

At present, it is preferred to use incombustible gases as working fluids. Furthermore, in order to reduce the compression of the working fluid supplied from the outside, an expansion turbine is preferred over a valve for expanding the working fluid.

An example of an apparatus which improves these two advantages is described in WO98 / 43029. Two heat exchangers are used in the apparatus of this document, one of which is to warm the working fluid in the heat exchanger while partially condensing the compressed natural gas vapor, and the other is to cool the compressed working fluid. Moreover, the working fluid is compressed in two different compressors, one connected to the expansion turbine. Although not disclosed in WO98 / 43029, such a conventional device is installed in a board-shaped vessel, and a compressor and heat exchanger connected to an expansion turbine are located in the ship's cargo machinery room and the other compressor is located in the engine compartment. WO2005 / 047761 has a similar structure and this application is characterized by the precooling of BOG.

Korean Patent Laid-Open Publication Nos. 2001-0088406 and 2001-0089142 relate to an apparatus for use in a board type vessel for reliquefaction of compressed steam, and the components are manufactured and used in a pre-assembly. Referring to FIG. 1, which shows this reliquefaction apparatus, reliquefaction is performed in a closed cycle, where the working fluid is compressed in at least one compressor 122, 124, and 126 and the first heat exchanger 140. Is cooled in and expanded in turbine 128 and warmed in a second heat exchanger, where the compressed steam is at least partially condensed. The apparatus comprises a first preassembly 172 comprising a second heat exchanger 140, and a second preheater on which the first heat exchanger 146, the compressors 122, 124 and 126, and the expansion turbine 128 are located. Assembly 182. Pre-assemblies 172 and 182 are located in respective platforms 170 and 180.

The reliquefaction apparatus as described above has been improved in terms of simplification of structure, ease of mounting on a ship, and reduction of heat loss, but there has been a need for improvement.

In particular, the conventional technology used a reverse Brayton cycle of nitrogen as a working fluid in a nitrogen cycle apparatus, which is a cold heat generating mechanism for BOG liquefaction, but has a disadvantage in that it cannot quickly cope with a change in the BOG treatment capacity according to the ship operating conditions. In addition, if the nitrogen temperature is lowered too much by the expansion turbine, droplets generated by reaching the liquefaction point of nitrogen may damage the turbine blades. Therefore, new technologies are needed to cope with BOG changes.

The present invention provides a BOG reliquefaction apparatus which improves the above-described prior arts in terms of simplification of structure, ease of mounting on a ship, reduction of heat loss, and the like. In particular, the present invention relates to a nitrogen cycle apparatus which is a cold heat generating mechanism in a reliquefaction system that reliquefies a BOG generated in a storage tank during operation in a LNG carrier driven by a diesel engine and discharged through a safety valve and then recovered into a storage tank. By lowering the nitrogen temperature to a lower temperature by using expansion valves as well as expansion turbines, the present invention provides a method and apparatus for ensuring LNG pressure and temperature loss in a stable state in any case.

In order to achieve the above object, the present invention, BOG compressor for compressing the evaporative vapor (BOG) generated in the storage tank of LNG; A BOG condenser for at least partially condensing the vaporized vapor compressed by the BOG compressor: a BOG condenser configured to supply cold heat to the BOG condenser to return the BOG re-liquefied by the BOG condenser to the tank. A reliquefaction apparatus, the nitrogen cycle apparatus comprising: nitrogen pressurized cooling means for pressurizing and cooling nitrogen; A first nitrogen heat exchanger configured to receive the high pressure nitrogen gas from the nitrogen pressurized cooling means and to cool it; And first expansion means for expanding the high pressure nitrogen gas from the first nitrogen heat exchanger to generate ultra low temperature nitrogen gas, wherein the ultra low temperature nitrogen gas expanded by the first expansion means receives cold heat from the BOG condenser. After being deprived, it is configured to return to the nitrogen pressurized cooling means via the first nitrogen heat exchanger, and a portion of the high pressure nitrogen gas flowing from the nitrogen pressurized cooling means to the first nitrogen heat exchanger is expanded by a second expansion means to form a low temperature low pressure. After cooling with nitrogen gas, the low pressure nitrogen gas from the BOG condenser is combined with the first nitrogen heat exchanger to return to the nitrogen pressure cooling means to provide a BOG reliquefaction apparatus.

As such, some of the pressure-cooled nitrogen passes through the second expansion means, and the temperature is lowered. The nitrogen is lowered to form internal heat exchange in the first nitrogen heat exchanger. Contribute to lowering. As such, the reason for obtaining a part of nitrogen into the internal heat exchanger by obtaining a low temperature nitrogen by using an expansion turbine is that the low pressure nitrogen is low in specific heat, so it is not enough to cool the high pressure nitrogen. It is advantageous to lower the temperature using and to help cool the high pressure nitrogen to the required temperature at the inlet of the expansion means such as an expansion valve.

A second nitrogen heat exchanger is further provided between the nitrogen pressurized cooling means and the first expansion means, and the high pressure nitrogen gas introduced into the second expansion means may be first cooled in the second nitrogen heat exchanger. As an example, as described in FIGS. 2 and 3. In the present invention, the first and second nitrogen heat exchangers serve as internal heat exchange between nitrogen gas. In other words, the high pressure nitrogen gas from the nitrogen pressurized cooling means is somewhat hot, but before expanding it to the cryogenic low pressure gas by the expansion means, the cold heat is deprived from the BOG condenser and the temperature rises slightly, but the cold nitrogen from the relatively low temperature BOG condenser is increased. Heat exchange with the gas serves to lower the temperature to the required temperature at the inlet of the expansion means.

An expansion valve or an expansion turbine may be used as the first or second nitrogen expansion means. Preferably, the first expansion means is an expansion valve and the second expansion means is an expansion turbine. In addition, by installing a generator in the expansion turbine, the power generated from the generator can be used as a power source such as a BOG compressor or nitrogen gas compressor.

In addition, when the BOG condenser, the first or second nitrogen heat exchanger, and the first and second expansion means are configured in one module and insulated, the BOG condenser, the first and second nitrogen heat exchangers, and the heat insulator may be stably operated without heat loss of the reliquefaction apparatus. If the module is manufactured in a preassembly it is easy to mount on the ship.

In addition, in order to reduce the load on the BOG compressor, the BOG condenser, and the like, and to stably reliquefy, it is preferable to perform temperature control by supplying (mixing) LNG from the reliquefied BOG or LNG tank at the previous stage or the later stage of the BOG compressor. . In addition, after the BOG partially condensed in the BOG condenser is separated by a non-condensing gas separator, the non-condensing gas is supplied (mixed) to the BOG compressed by the BOG compressor to supply the temperature of the BOG to the BOG condenser. It is also possible to allow cooling. In this case, the non-condensable gas is preferably supplied by a venturi-type nozzle.

According to another embodiment of the present invention, the BOG compression step of compressing the evaporative vapor (BOG) generated in the storage tank of LNG; A nitrogen gas pressurized cooling step of pressurizing and cooling a nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG: a nitrogen gas expansion step of expanding the pressurized nitrogen gas to generate cryogenic nitrogen gas; A condensation step of at least partially condensing the compressed BOG by heat exchange with the cryogenic nitrogen gas; In the reliquefaction method of BOG consisting of returning the BOG liquefied by the condensation to the tank, a portion of the pressure-cooled nitrogen gas is expanded to a low temperature gas by a path different from the expansion, the compressed Provided is a method for reliquefying BOG, which is combined with nitrogen gas whose temperature is increased by condensation of BOG and precooled by heat exchange before expansion of the pressurized nitrogen gas. At this time, by adjusting the extraction amount of the pressure-cooled nitrogen gas extracted for the pre-cooling it is possible to vary the operation of the nitrogen cycle can easily cope with the dynamic change of the BOG.

The BOG reliquefaction apparatus according to the present invention can be used as a BOG reliquefaction apparatus generated in LNG vessels or onshore LNG tanks.

Hereinafter, with reference to the accompanying Figures 2 and 3 will be described in detail the present invention. The following description is only one embodiment of the present invention, it is obvious that the present invention is not limited to this embodiment.

2 is a configuration of the LNG BOG reliquefaction apparatus according to the present invention. The apparatus may be composed of a BOG cycle apparatus, a nitrogen cycle apparatus, and a cold box apparatus for interfacing the two apparatuses, which will be described in terms of the above three configurations for convenience of description.

1) BOG cycle device

Natural gas in a gaseous state is liquefied to cryogenic temperatures and stored in a storage tank 10 at atmospheric pressure (1.013 bar). However, BOG is generated due to heat transfer from the outside during LNG transportation, which acts as a pressure increase factor of the storage tank 10. Therefore, when the storage tank pressure reaches 1.03 bar to maintain the storage tank 10 at a constant atmospheric level, the safety valve 11 is opened and the BOG is stored in the storage tank 10 by the two-stage BOG compressors 40 and 41. ) Is discharged outside to undergo reliquefaction. In this case, the pump P may be used to pump the BOG.

The discharged high-temperature BOG is sensed by a temperature sensor 12, such as a thermometer, is adjusted to a constant temperature via the temperature controller 30 and then flows into the second stage BOG compressor 40, 41. BOG passed through the temperature controller 30 is maintained at 1.03 bar, -120 ℃ superheated steam.

Referring to the operation of the temperature controller 30, by recirculating the condensed BOG, that is, cryogenic LNG through the reliquefaction process to the hot BOG discharged from the storage tank, the temperature can be adjusted and the control of the recycle amount is a recirculation valve Perform by adjusting the opening degree of (20). In addition, the temperature controller 30 can supply not only re-liquefied BOG but also LNG from an LNG tank to a pump (not shown). Temperature control usually operates only until the reliquefaction apparatus reaches a steady state, after which the recirculation valve 20 is closed and the cryogenic LNG is led to the storage tank 10.

BOG passing through the temperature controller is discharged in a superheated steam state of 2.5 bar, -73 ℃ when passing through the two-stage BOG compressor 40, 41 driven by the motor (M).

The BOG discharged from the two-stage BOG compressors 40 and 41 is liquefied in the BOG condenser 50 and then remixed with the non-condensed components separated by the BOG non-condensing gas separator 92, thereby lowering the temperature of the superheated steam. It is desirable to lose. The non-condensable gas separator 92 may use a separator generally known in the prior art described in the prior art section. In this case, when the venturi-type nozzle 94 is used, the flow rate of the BOG may be increased, and the non-condensable gas component from the BOG non-condensable gas separator 92 may be introduced using the pressure drop generated therefrom.

Subsequently, the BOG is changed into a supercooled liquid state of 2.3 bar and −155 ° C. while passing through the BOG condenser 50 and then reintroduced into the storage tank 10 or recycled to the temperature controller 30. Details of the BOG condenser 50 are described in detail in the coldbox apparatus below.

Through the BOG condenser 50, the entire BOG can be liquefied, but 70-99% is liquefied due to the influence of nitrogen, which is not easily liquefied completely.

The condensed BOG is re-introduced into the storage tank 10 by the pumps P and 100. The reflowing method of BOG into the storage tank 10 is a method of spraying through the spray head from the top of the tank or feeding the tank to the bottom of the tank. When it enters the bottom of the tank, the nitrogen contained in the uncondensed gas contained in the condensed BOG is dissolved in the LNG to keep the nitrogen ratio in the gas phase low. Since nitrogen has a lower liquefaction point than methane, which is the main component of LNG, increasing the nitrogen content in the BOG can reduce the load of the two-stage BOG compressor 40 or 41 or the BOG condenser 50.

2) Nitrogen cycle device

In the BOG condenser 50, BOG reliquefaction is performed by cryogenic nitrogen gas as a working fluid. The following description relates to a cycle apparatus for obtaining cryogenic nitrogen gas required for BOG reliquefaction.

Nitrogen gas at 14 bar and 35.4 ° C passes through a three-stage nitrogen compressor (70) (71) (72) and an intermediate cooler (80) (81) (82), which are pressurized cooling means for nitrogen gas, and the pressure is increased to 58 bar, It is discharged with gas at 43 ° C. The intermediate coolers 80, 81 and 82 may use a conventional method known in the prior art described in the prior art portion of the present invention to cool nitrogen refrigerant acting as a working fluid in the pressurization step. The method of cooling using is preferable.

  The discharged high pressure nitrogen gas is transferred to the cold box and then, by the second nitrogen heat exchanger 61, through the internal heat exchange with the low temperature nitrogen which still has cold heat from the condenser passing through the first nitrogen heat exchanger 60. First cooled to -83.5 ° C.

Some of the high pressure nitrogen gas that has passed through the second nitrogen heat exchanger 61 passes through the expansion turbine 90, and the low pressure nitrogen gas having the low temperature has passed through the second nitrogen heat exchanger 61. Internal heat exchange in nitrogen and the first nitrogen heat exchanger (60) contributes to lowering the nitrogen temperature up to the set temperature at the inlet of the expansion valve. In the first nitrogen heat exchanger (60), the high-pressure nitrogen gas whose temperature is lowered passes through the expansion valve (91) and finally changes to cryogenic gas for BOG liquefaction of about -163 占 폚. The cryogenic nitrogen gas heats the BOG in the BOG condenser 50 to liquefy the BOG and the temperature rises. As such, a part of nitrogen is obtained by using an expansion turbine to obtain low temperature nitrogen into the internal heat exchanger because low pressure nitrogen is low in specific heat and thus insufficient to cool high pressure nitrogen. Use it to help cool the high pressure nitrogen to the required temperature at the expansion valve inlet. After the remaining cold heat to cool the high-pressure nitrogen in the second nitrogen heat exchanger 61 and the temperature rises to reach 14 bar, 35.4 ℃ to complete the cycle. The nitrogen buffer tank 110 performs a function of adjusting the mass flow rate of the nitrogen cycle in response to the variation of the BOG reliquefaction amount, that is, the variation of the cooling load of the nitrogen cycle. It may also further comprise a source of supplemental working fluid in case the amount of nitrogen is reduced.

3) Cold Box Device

Cold box 200 is a BOG condenser (50) process for performing the re-liquefaction, the first and second nitrogen heat exchanger (60), 61, and the cryogenic nitrogen gas is carried out the internal heat exchange between the hot and cold parts of the nitrogen cycle It consists of an expansion valve 91 to obtain the expansion turbine 90 for obtaining nitrogen at a low temperature required for internal heat exchange and a power production process using a generator therefrom. By including the devices of the above process in the cold box module 200, it is possible to shorten the connection pipe between the devices, which makes it possible to stably secure the cryogenic nitrogen required for BOG reliquefaction. That is, since the length of the connecting pipe between the second nitrogen heat exchanger 61 and the inlet of the expansion turbine is shortened, the cryogenic state of the inlet of the expansion turbine 90 or the outlet of the expansion turbine, which may be sensitive to temperature, is stable. Can be. In addition, since the connection pipe between the expansion valve outlet and the BOG condenser 50, the outlet of the first nitrogen heat exchanger 60 and the expansion valve 91 is shortened it can be expected to minimize the temperature increase due to the nitrogen gas transfer.

In the present invention, in the configuration of the BOG reliquefaction apparatus, the cold box BOG condenser 50, the first and second nitrogen heat exchanger (60, 61), and expansion means (90, 91) in one cold box It is preferable to comprise and to insulate them by one module. Insulation is generally performed using known insulators. By such a configuration, the cryogenic region of nitrogen gas can be stably managed, and the nitrogen heat exchanger can be easily designed and manufactured. In addition, the cold box may be manufactured in a preassembly to facilitate mounting on a ship.

As described above, since the cryogenic portion of the nitrogen cycle is stably managed, it is possible to operate the BOG reliquefaction temperature -155 ° C and the maximum temperature -72.6 ° C in the BOG cycle and lower the inlet / outlet temperature difference of the BOG condenser 50 to 80 It can be reduced below ℃. This allows the design of a simple BOG condenser without the need for pre-cooling.

In addition, in the present invention, the generator (G) can be installed in the expansion turbine, and the electric power generated from the generator (G) is used as a power source of the BOG compressors (40) (41) or the nitrogen gas compressors (70) (71) (72). Can be used.

Operation of the reliquefaction apparatus according to the configuration of the present invention is basically similar to the method known in the prior art by the step of compressing evaporated steam (BOG) generated in the storage tank of LNG; Pressurizing and cooling the nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG, and expanding the pressurized nitrogen gas to generate cryogenic nitrogen gas; Heat exchange with the cryogenic nitrogen gas to at least partially condense the compressed BOG; Returning the BOG liquefied by the condensation to the tank, except that all of the pressurized nitrogen gas is not expanded, but a part thereof is extracted and expanded in a path different from the expansion to generate a low temperature gas. Thereafter, the step of pre-cooling by heat exchange prior to expansion of the pressure-cooled nitrogen gas combined with the nitrogen gas is slightly elevated in temperature by the condensation of the compressed BOG.

Referring to FIG. 3, in the BOG reliquefaction method generated by external heat transfer in a storage tank of a cryogenic LNG carrier, the BOG discharged after opening the valve at a set pressure passes through a temperature controller to a two-stage BOG. The compressor inlet state is constantly maintained at 1.03 bar, -120 ° C (ST 10), the BOG is a superheated state of high temperature and high pressure of 2.5 bar, -73 ° C in a two-stage BOG compressor (ST 20), gas liquid The gas separated in the separator is sucked by the pressure difference of the venturi nozzle (ST 30) to lower the temperature (ST 30), the BOG discharged from the BOG compressor is re-liquefied into 2.3bar, -155 ℃ subcooled liquid in the BOG condenser (ST 40), the step of separating the non-condensing gas contained in the re-liquefied BOG (ST 50), the step of pressurized by a pump (ST 60), a portion of the pressurized re-liquefaction BOG through a temperature control valve Recycle step (ST 70), the rest is low Recovering and storing in a tank (ST 80), after the nitrogen gas of 14 bar, 35.4 ° C passes through the three-stage compressor and the intermediate cooler to increase the pressure to 58 bar, 43 ° C (ST 90), the Step of changing the high pressure nitrogen to a low temperature state of 57.3 bar, -83,5 ° C through internal heat exchange with the low temperature nitrogen in the second nitrogen heat exchanger (ST 100), while a portion of the high pressure nitrogen passes through the expansion turbine 14.5 bar,- Changing to low temperature, low pressure gas at 140 ° C. (ST 110), nitrogen not sent to the expansion turbine is 57.5 bar, -137,9 ° C. through the first nitrogen heat exchanger (ST 120), nitrogen at high pressure is expanded Step of passing through the valve to -163 ℃, 14.6 bar state (ST 130), after the low temperature low pressure nitrogen to perform the BOG reliquefaction step to 14.4 bar, -138.9 ℃ state (ST 140), the nitrogen gas is removed 1 step of changing to the state of 14.2 bar, -106 ℃ while passing through a nitrogen heat exchanger (ST 150) Nitrogen, the temperature of which is lowered by the bin, is combined and the state is changed to a high temperature of 14 bar and 35.4 ° C. while passing through the second nitrogen heat exchanger (ST 160).

Pressure, temperature, etc. in each of the above steps is described by a specific number, but it is natural that it can be changed according to the amount of BOG, control method and the like.

The present invention having the configuration described above may cause a problem of damaging the blade of the expansion turbine by liquefying part of the nitrogen when the temperature of the nitrogen is lowered by using the expansion turbine in the BOG liquefaction system using the reverse braton cycle of nitrogen. As solving the problems of the prior art, the BOG reliquefaction apparatus of the present invention extracts only a portion of pressurized nitrogen and expands it using an expansion turbine, and lowers the temperature of the nitrogen through the expansion valve and then drops the droplets by lowering the temperature of nitrogen. There is an advantage that can lower the temperature of nitrogen without fear of occurrence. In addition, it is possible to vary the operation of the nitrogen cycle by changing the amount of extracted nitrogen can easily cope with the dynamic change of the BOG.

In addition, the present invention having the configuration as described above can stably manage the pressure of the storage tank without the loss of the stored LNG during operation of the LNG carrier. In particular, the introduction of a simple cold box module can reduce the size of the BOG reliquefaction apparatus, make stable management of the cryogenic region of nitrogen gas, and simplify the design and manufacture of the nitrogen heat exchanger. Specifically, since the connection pipe between the nitrogen heat exchanger 60 and the inlet of the expansion turbine is shortened, the cryogenic state of the inlet of the expansion turbine 90 or the outlet of the expansion turbine which is affected by temperature is stable. . In addition, since the connection pipe between the expansion turbine outlet and the BOG condenser is shortened, it is possible to minimize the temperature rise issued during the cryogenic nitrogen gas transfer, and by collecting and managing the unit elements constituting the low temperature part in the BOG reliquefaction unit in a cold box, Losses can also be prevented.

Claims (13)

BOG compressor for compressing evaporated steam (BOG) generated in the storage tank of LNG; A BOG condenser for at least partially condensing the vaporized vapor compressed by the BOG compressor: a BOG condenser configured to supply cold heat to the BOG condenser to return the BOG re-liquefied by the BOG condenser to the tank. A reliquefaction apparatus, the nitrogen cycle apparatus comprising: nitrogen pressurized cooling means for pressurizing and cooling nitrogen; A first nitrogen heat exchanger configured to receive the high pressure nitrogen gas from the nitrogen pressurized cooling means and to cool it; And first expansion means for expanding the high pressure nitrogen gas from the first nitrogen heat exchanger to generate ultra low temperature nitrogen gas, wherein the ultra low temperature nitrogen gas expanded by the first expansion means receives cold heat from the BOG condenser. After being deprived, it is configured to return to the nitrogen pressurized cooling means via the first nitrogen heat exchanger, and a portion of the high pressure nitrogen gas flowing from the nitrogen pressurized cooling means to the first nitrogen heat exchanger is expanded by a second expansion means to form a low temperature low pressure. And after cooling with nitrogen gas, the low pressure nitrogen gas from the BOG condenser is combined with the first nitrogen heat exchanger to return to the nitrogen pressurized cooling means. The method of claim 1, wherein a second nitrogen heat exchanger is further provided between the nitrogen pressurized cooling means and the first expansion means, and the high pressure nitrogen gas flowing into the second expansion means is primarily cooled in the second nitrogen heat exchanger. BOG reliquefaction apparatus. The apparatus of claim 1, wherein the first or second expansion means is an expansion valve or an expansion turbine. The apparatus of claim 3, wherein a generator is installed in the expansion turbine. The apparatus for reliquefying BOG according to any one of claims 1 to 4, wherein the BOG condenser, the first and second nitrogen heat exchangers, and the first and second expansion means are configured as one module. The apparatus of claim 5 wherein the module is insulated. The apparatus of claim 5 wherein the module is made of a preassembly. The method according to claim 1, wherein the BOG supplied to the BOG compressor is supplied with LNG from the reliquefied BOG or LNG tank to cool the temperature of the BOG supplied to the BOG condenser. BOG reliquefaction unit. The method according to any one of claims 1 to 4, characterized in that to supply the LNG from the reliquefied BOG or LNG tank to the BOG compressed by the BOG compressor to cool the temperature of the BOG supplied to the BOG condenser. BOG reliquefaction apparatus. The BOG condenser of claim 1, wherein the partially condensed BOG in the BOG condenser is separated by a non-condensing gas separator, and then the non-condensing gas is supplied to the BOG compressed by the BOG compressor. BOG reliquefaction apparatus, characterized in that for cooling the temperature of the BOG supplied to. The apparatus of claim 10, wherein the non-condensable gas is supplied by a venturi-type nozzle. BOG compression step of compressing evaporated steam (BOG) generated in the storage tank of LNG; A nitrogen gas pressurized cooling step of pressurizing and cooling a nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG: a nitrogen gas expansion step of expanding the pressurized nitrogen gas to generate cryogenic nitrogen gas; A condensation step of at least partially condensing the compressed BOG by heat exchange with the cryogenic nitrogen gas; In the reliquefaction method of BOG consisting of returning the BOG liquefied by the condensation to the tank, A portion of the pressure-cooled nitrogen gas is expanded to a low temperature gas by being expanded in a path different from the expansion, and is combined with the nitrogen gas whose temperature is increased by the condensation of the compressed BOG, and the nitrogen gas before the expansion step of the pressure-cooled nitrogen gas. A method of reliquefaction of BOG, characterized in that precooling by heat exchange between the liver.  The method of claim 12, wherein the extraction amount of the pressurized nitrogen gas extracted for the pre-cooling can be controlled.

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