JP2009501896A - LNGBOG reliquefaction equipment - Google Patents

Abstract

  The present invention relates to a reliquefaction device for evaporative gas (BOG) generated in a storage tank of a carrier ship carrying cryogenic liquefied natural gas (LNG). According to the present invention, a compressor for compressing BOG generated in an LNG storage tank, a condenser for at least partially condensing the vapor vapor compressed by the compressor, and nitrogen for supplying cold energy to the compressor In the BOG re-liquefaction device comprising the cycle device and returning the BOG re-liquefied by the condenser to the storage tank, the BOG compressed by the compressor is subjected to a pre-cooling process before the condenser, Even if the amount of BOG generated or the temperature changes, there is an advantage that the temperature of the BOG can be kept constant within a set range at the inlet of the condenser. In addition, according to the present invention, in the BOG reliquefaction apparatus, the unit element constituting the low temperature portion is manufactured by a heat-insulated cold box module, whereby the size of the LNG reliquefaction apparatus can be reduced. The low temperature region can be stably managed without loss of cold heat.

Description

  The present invention relates to a re-liquefaction apparatus for evaporative gas (Boil-Off Gas, hereinafter referred to as BOG) generated in a storage tank of a carrier ship that carries cryogenic liquefied natural gas (hereinafter referred to as LNG).

  LNG is usually transported over long distances in a liquefied state. For example, an LNG carrier is used to transfer the liquefied LNG to a second position where it is vaporized at a first position where the LNG is liquefied and sent to a gas distribution system. Since LNG liquefies at a cryogenic temperature, that is, a temperature around normal pressure-163 ° C., the storage tank of the LNG carrier receives external heat transfer, so that the LNG in the storage tank is continuously vaporized. When the pressure becomes equal to or higher than the set safety pressure, the BOG is discharged to the outside through the safety valve. The discharged BOG is re-liquefied and returned to the storage tank or used as fuel for the ship.

  Since the conventional LNG carrier is a steam turbine propulsion system, BOG is burned with a boiler and used as propulsion fuel for ships. In recent years, with the development of thermal insulation technology for storage tanks, the amount of BOG generated has decreased, making it difficult not only to secure the amount of BOG necessary for propulsion of steam turbine vessels, but also vessels with higher efficiency diesel engines. Promotion is preferred. As a result, new technologies for processing BOG and ensuring the stability of storage tanks are required.

  In the reliquefaction apparatus, the refrigeration cycle includes a step of compressing the working fluid with a plurality of compressors, a step of cooling the compressed working fluid by indirect heat exchange, a step of expanding the working fluid, and an expanded operation Heating the fluid 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 expanded working fluid. An example of an apparatus for performing such a cooling method is described in US Pat. No. 3,857,245.

  According to U.S. Pat. No. 3,857,245, the working fluid is derived from LNG itself and is therefore operated by an open refrigeration cycle. The working fluid is expanded by a valve to obtain a partially condensed LNG. The partially condensed LNG is separated into a liquid phase that is returned to the storage tank and a gas phase that is mixed with natural gas sent to the combustion banner. Since the working fluid is heated and cooled in the same heat exchanger, only one heat exchanger is used. The heat exchanger is located on the first skid flat foam and the working fluid compressor is located on the second skid flat foam.

  Currently, it is preferred to use a non-flammable gas as the working fluid. 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 that improves the above two advantages is described in International Patent Publication No. WO 98/43029. The apparatus of the above literature uses two heat exchangers, one of which heats the working fluid in the heat exchanger while partially condensing the compressed natural gas vapor, The other is to cool the compressed working fluid. Furthermore, the working fluid is compressed by two other compressors, one connected to the expansion turbine. Although not disclosed in International Patent Publication No. WO 98/43029, such a conventional apparatus is provided in a board-type ship, and a compressor and a heat exchanger connected to an expansion turbine are used in a ship cargo machine. The other compressor is located in the engine compartment. International Patent Publication No. WO2005 / 47761 also has a similar structure, and this application is characterized by BOG precooling.

  U.S. Pat. Nos. 4,843,829 and 4,843,862 disclose re-liquefaction methods for pre-cooling BOG, and the BOG re-liquefaction apparatus disclosed in these U.S. Patents is a compressor. In order to finally condense the BOG supplied via the first heat exchanger, the BOG is pre-cooled by the first heat exchanger and then the second heat exchanger. Finally it is condensed.

  However, according to the BOG reliquefaction apparatus disclosed in the aforementioned U.S. Pat. Nos. 4,843,829 and 4,843,862, the compressed nitrogen passing through the compressor is passed through the first stream and the second stream. The first stream is used for re-liquefaction of BOG via an expansion valve and then returned to the compressor. The second stream is used for pre-cooling of BOG via an expansion turbine and then returned to the compressor. Therefore, each line for circulating a plurality of nitrogen refrigerant jets must be constructed and 5 to 6 paths must be formed in the heat exchanger, whereby the reliquefaction device There is a problem that the overall configuration of the system is complicated and inefficient.

  Korean Patent Publication Nos. 2001-88406 and 2001-89142 relate to an apparatus for use in a board-type ship to reliquefy compressed steam, and the components are manufactured as a pre-assembly. I use it. Referring to FIG. 1 showing this reliquefaction device, reliquefaction takes place in a closed cycle, where the working fluid is compressed by at least one compressor 23, 24, 25, The steam that has been cooled in the heat exchanger 22, expanded in the turbine 28 and warmed in the second heat exchanger 13, where it has been compressed, is at least partially condensed. The apparatus includes a first pre-assembly 10 that includes a second heat exchanger 13 and a second pre-assembly in which a first heat exchanger 22, compressors 23, 24, and 25, and an expansion turbine 28 are located. 20. The pre-assemblies 10 and 20 are located on the respective flat foams 11 and 21.

  On the other hand, in relation to the heat exchange between the conventional working fluid and the BOG, Korean Patent Publication No. 1993-8299, International Patent Publication No. WO2005 / 71333, etc. use three heat exchangers to stabilize the BOG. Condensation is planned.

  Although such a reliquefaction apparatus has been improved in terms of simplification of structure, ease of mounting on a ship, reduction of heat loss, etc., there is still a need for improvement.

  In particular, the conventional technology uses a reverse Brayton cycle of nitrogen as a working fluid in a nitrogen cycle apparatus that is a cold heat generation mechanism for BOG liquefaction. However, there is a disadvantage that it is not dealt with promptly when it changes. Further, if the nitrogen temperature is excessively lowered by the expansion turbine, droplets generated by reaching the nitrogen liquefaction point cause damage to the turbine blade. Therefore, there is a need for a new technique that can cope with the amount of generated BOG and temperature changes.

  Therefore, the present invention is to solve the problem of not quickly responding when the amount of BOG generated, the temperature, etc. change depending on the operational status of the ship, and the object of the present invention is to provide an LNG carrier. In the reliquefaction system in which the BOG generated in the storage tank during operation and discharged from the safety valve is reliquefied and then collected in the storage tank, the generated BOG is subjected to a pre-cooling process before the condenser, An object of the present invention is to provide an LNG BOG reliquefaction device capable of maintaining a temperature difference between BOG and nitrogen gas constant within a set range within a condenser even when there is a change in the amount of BOG generated or temperature. .

  Another object of the present invention is not only an expansion turbine but also an expansion turbine in a reliquefaction system in which BOG generated in a storage tank during operation and discharged from a safety valve is reliquefied and recovered in a storage tank. In each case, the LNG BOG reliquefaction device that maintains the pressure and temperature in the storage tank in a stable state and eliminates LNG loss by freely lowering the nitrogen temperature to a lower temperature using a valve. It is to provide.

  Furthermore, still another object of the present invention is to provide a BOG reliquefaction apparatus in which the above-described prior art is improved in terms of simplification of structure, ease of mounting on a ship, reduction of heat loss, and the like. .

  In order to achieve the above technical problem, according to one embodiment of the present invention, a compressor for compressing vaporized vapor (BOG) generated in an LNG storage tank, and a BOG compressed by the compressor are condensed. In the LNG BOG reliquefaction device that returns the BOG reliquefied by the condenser to the storage tank, the nitrogen cycle device is configured to supply nitrogen. A nitrogen pressure cooling means for pressurizing and cooling, a first nitrogen heat exchanger for cooling the high pressure nitrogen gas that has passed through the nitrogen pressure cooling means, and a high pressure nitrogen gas from the first nitrogen heat exchanger. And a first expansion means for generating cryogenic and low pressure nitrogen gas, and the cryogenic and low pressure nitrogen gas expanded by the first expansion means is deprived of cold by the condenser. The first The LNG BOG reliquefaction apparatus is provided in which the BOG returned to the nitrogen pressure cooling means through the nitrogen heat exchanger and compressed by the compressor is precooled before being condensed.

  According to another embodiment of the present invention, a compressor for compressing evaporated vapor (BOG) generated in an LNG storage tank, a condenser for condensing BOG compressed by the compressor, and the compressor A LNG BOG reliquefaction device that returns the BOG reliquefied by the condenser to the storage tank, wherein the nitrogen cycle device pressurizes and cools nitrogen. A cooling means, a first nitrogen heat exchanger that cools the high-pressure nitrogen gas that has passed through the nitrogen pressure cooling means, and the high-pressure nitrogen gas from the first nitrogen heat exchanger is expanded to produce a cryogenic low-pressure nitrogen. First cryogenic expansion means for generating gas, and cryogenic and low-pressure nitrogen gas expanded by the first expansion means is deprived of cold heat by the condenser, and then the first nitrogen heat exchanger Through the nitrogen A part of the high-pressure nitrogen gas that is returned to the pressure cooling means and supplied from the nitrogen pressure cooling means to the first nitrogen heat exchanger is expanded by the second expansion means, and converted into low-temperature low-pressure nitrogen gas. Provided is an LNG BOG reliquefaction device which is cooled and then combined with the low-pressure nitrogen gas from the condenser, flows into the first nitrogen heat exchanger, and is returned to the nitrogen pressure cooling means. Is done.

  According to another embodiment of the present invention, a compressor for compressing evaporated vapor (BOG) generated in an LNG storage tank, a condenser for condensing BOG compressed by the compressor, and the compression A LNG BOG re-liquefaction device that returns the BOG re-liquefied by the condenser to the storage tank, wherein the nitrogen cycle device is configured to add nitrogen to pressurize and cool the nitrogen. A pressure cooling means, a nitrogen heat exchanger that cools the high-pressure nitrogen gas that has passed through the nitrogen pressure cooling means, and a high-pressure nitrogen gas from the nitrogen heat exchanger is expanded to generate a cryogenic and low-pressure nitrogen gas. The LNG BOG reliquefaction apparatus is provided, wherein the compressor, the nitrogen heat exchanger, and the expansion means are configured as one module.

  According to the present invention, a compressor for compressing BOG generated in an LNG storage tank, a condenser for at least partially condensing the vapor vapor compressed by the compressor, and nitrogen for supplying cold energy to the compressor In the BOG re-liquefaction device comprising the cycle device and returning the BOG re-liquefied by the condenser to the storage tank, the BOG compressed by the compressor is subjected to a pre-cooling process before the condenser, Even if the amount of BOG generated or the temperature changes, there is an advantage that the temperature of the BOG can be kept constant within a set range at the inlet of the condenser.

  In addition, according to the present invention, in the BOG reliquefaction apparatus, the unit element constituting the low temperature portion is manufactured by a heat-insulated cold box module, whereby the size of the LNG reliquefaction apparatus can be reduced. The low temperature region can be stably managed without loss of cold heat.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. The following description is only an example of the present invention, and the temperature and pressure described below are merely examples, and the present invention is not limited to these numerical values.

(First embodiment)
Hereinafter, the LNG BOG reliquefaction apparatus and the reliquefaction method according to the first embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 2 is a configuration diagram of the LNG BOG reliquefaction apparatus according to the first embodiment of the present invention. This apparatus includes a BOG cycle apparatus, a nitrogen cycle apparatus, and a cold box apparatus that interfaces the two apparatuses.

BOG cycle apparatus The natural gas in the gaseous state is liquefied at a very low temperature and stored in the storage tank 110 at atmospheric pressure (1.013 bar). However, BOG is generated by the external heat transfer that is sustained during the transport of LNG, which acts as a pressure increase factor of the storage tank 110.

  Therefore, in order to keep the storage tank 110 constant at the atmospheric pressure level, when the pressure of the storage tank reaches about 1.03 bar, the safety valve 111 is opened, and the BOG is discharged to the outside of the storage tank 110, and the two stages. Through the BOG compressors 115 and 116, the reliquefaction process is started.

  The high temperature BOG discharged to the outside of the storage tank 110 is sensed by a temperature sensor 112, adjusted to a constant temperature via a temperature controller 114, and then flows into two-stage BOG compressors 115 and 116. It is preferable to do. The BOG that has passed through the temperature controller 114 is maintained in a superheated steam state of 1.03 bar and −120 ° C.

  The operation of the temperature controller 114 will be described in more detail. When the BOG (LNG) having undergone the reliquefaction process is recirculated and mixed with the high temperature BOG discharged from the storage tank 110, the temperature adjustment is performed. The recirculation amount is controlled by adjusting the opening degree of the recirculation valve 113. In addition, the temperature regulator 114 can supply not only the re-liquefied BOG but also LNG from the LNG storage tank 110 to a pump (not shown). It is also possible to supply the LNG in the LNG storage tank 110 so as to be mixed with the BOG compressed by the BOG compressors 115 and 116, and to cool or adjust the temperature of the BOG supplied to the BOG condenser 131. . The temperature adjustment is usually only activated until the reliquefaction device reaches a normal state, after which the recirculation valve 113 is closed and the cryogenic reliquefied LNG is directed to the storage tank 110.

  The BOG that has passed through the temperature controller 114 passes through the two-stage BOG compressors 115 and 116 driven by the motor M, and is discharged in a superheated steam state of 2.5 bar and −73 ° C.

  The BOG discharged from the two-stage BOG compressors 115 and 116 passes through the BOG condenser 131 and changes to a supercooled liquid state of 2.3 bar and −155 ° C., and then flows into the storage tank 110 again. Or recirculated to the temperature regulator 114. Details of the BOG condenser 131 will be described in the cold box device section described later.

  Although the entire BOG can be liquefied through the BOG condenser 131, about 70 to 99% is liquefied due to the influence of a nitrogen component or the like that is not easily liquefied.

  Further, according to the modification of the present embodiment, the condensed BOG is sprayed and supplied to the pressurized BOG through the two-stage BOG compressors 115 and 116, thereby being added to the BOG condenser. The temperature of the pressure BOG can also be adjusted.

  The condensed BOG flows again into the storage tank 110 by the circulation pump 119. As a method for re-injecting BOG into the storage tank 110, there is a method in which the BOG is distributed from the spray head at the top of the storage tank or supplied to the bottom of the storage tank. When flowing into the bottom of the storage tank, the nitrogen component in the uncondensed gas contained in the condensed BOG is dissolved inside the LNG, and the nitrogen ratio in the gas phase is kept 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 on the two-stage BOG compressors 115, 116 or the BOG condenser 131.

Nitrogen cycle device In the BOG condenser 131, BOG is reliquefied by heat exchange with a cryogenic nitrogen gas, which is a working fluid. The following explanation is based on the cryogenic nitrogen gas necessary for BOG reliquefaction. It is related with the cycle apparatus for obtaining.

  The nitrogen gas at 10 bar and 40 ° C. passes through the three-stage compressors 121, 122 and 123 and the intercoolers 125, 126 and 127, and then the pressure rises and is discharged as a gas at 59 bar and 43 ° C. The nitrogen gas is transferred to the cold box, and in the nitrogen heat exchanger 133, the internal temperature is reduced to 59 bar and −105 ° C. through internal heat exchange with the low temperature nitrogen whose temperature has slightly increased while condensing BOG with the BOG condenser. Precooled to gas state.

  The intercoolers 125, 126, 127 cool the nitrogen refrigerant acting as a working fluid in the pressurization stage, and use a conventional method known in the prior art described in the prior art part of the present invention. However, a method of cooling seawater as a refrigerant is preferable.

  The low-temperature and high-pressure nitrogen gas that has passed through the nitrogen heat exchanger 133 passes through the expansion turbine 136 and changes to a cryogenic and low-pressure gas of 10.5 bar and −167 ° C., moves to the BOG condenser 131, After the re-liquefaction process, the state is 10.3 bar, -134 ° C. Thereafter, the internal heat exchange with the high-temperature part nitrogen of the nitrogen heat exchanger 133 reaches 10 bar and 40 ° C. to complete the cycle.

  The nitrogen buffer tank 120 performs the function of adjusting the mass flow rate of the nitrogen cycle in accordance with the change in the BOG reliquefaction amount, that is, the change in the cooling load of the nitrogen cycle. A source of supplemental working fluid (nitrogen) may be further provided in case the amount of nitrogen is reduced.

Cold Box Device The cold box 130 contains a BOG condenser 131 that reliquefies BOG, a nitrogen heat exchanger 133 that performs internal heat exchange between the high temperature part and the low temperature part of the nitrogen cycle, and cryogenic nitrogen gas. Expansion turbine 136 to be obtained.

  Even if it is not an essential component of the present invention, a generator G is connected to the expansion turbine 136 to produce electric power, which is then used as an auxiliary to the BOG compressors 115 and 116 or the nitrogen compressors 121, 122, and 123. It can also be used as a power source.

  By including the apparatus in the above process as a single module in the cold box 130, the connecting pipe between the apparatuses can be shortened, which stably secures the cryogenic nitrogen necessary for BOG reliquefaction. To be able to. That is, since the length of the connecting pipe between the nitrogen heat exchanger 133 and the inlet of the expansion turbine 136 is shortened, the state of the inlet of the expansion turbine 136 and the cryogenic state of the outlet of the expansion turbine that is sensitive to temperature are stabilized. Can be done. Further, since the connecting pipe between the outlet of the expansion turbine and the BOG condenser is shortened, it is possible to expect a minimum increase in temperature due to the transfer of nitrogen gas.

  Further, in the present invention, in the configuration of the LNG BOG reliquefaction device, the BOG condenser 131, the nitrogen heat exchanger 133, and the expansion turbine 136, which are low-temperature devices, are configured by one cold box 130, It is preferable to insulate as a module. The heat insulation is performed using a generally known heat insulating material. With such a configuration, it is possible to stably manage the cryogenic temperature region of nitrogen gas. In addition, the cold box can be easily mounted on a ship by being manufactured as a preliminary assembly.

  As described above, the extremely low temperature portion of the nitrogen cycle is stably managed, so that the BOG reliquefaction temperature of −155 ° C. and the maximum temperature of −73 ° C. can be operated in the BOG cycle. The temperature difference can be reduced to 80 ° C. or lower, which is lower than usual. Thus, a simple BOG condenser design that does not require pre-cooling is possible.

  Hereinafter, the operation of the reliquefaction apparatus according to the configuration of the first embodiment will be described with reference to FIGS.

  In the BOG reliquefaction method generated by external heat transfer in the storage tank of a cryogenic LNG carrier, first, the circulation of the BOG is performed after the valve is opened at the set pressure, and then the discharged BOG passes through the temperature controller, The step of maintaining the inlet state of the two-stage BOG compressors 115, 116 at 1.03 bar and -120 ° C. (ST111), and the BOG is 2.5 bar in the two-stage BOG compressors 115, 116. The BOG discharged from the BOG compressors 115 and 116 is compressed to −73 ° C. in a high temperature and high pressure state (ST112), and the supercooled liquid at 2.3 bar and −155 ° C. in the BOG condenser 131. Step (ST113) in which the liquefied BOG is pressurized by the circulation pump 119, and the pressurized liquefied liquid. A part of the BOG is recirculated through the temperature controller 114 (ST115), and the rest is recovered and stored in a storage tank (ST116). The cold heat for reliquefaction of the BOG The supply of nitrogen gas for supply is such that the nitrogen gas at 10 bar, 40 ° C. passes through the three-stage compressors 121, 122, 123 and the intercoolers 125, 126, 127, and then the pressure rises to 59 bar. Step (ST117) in which the pressure is increased to 43 ° C., and the high pressure nitrogen is changed to a low temperature state of 59 bar and −105 ° C. through internal heat exchange with the low temperature nitrogen in the nitrogen heat exchanger (ST118), The high-pressure nitrogen passes through the expansion turbine and changes to a low-temperature, low-pressure gas at 10.5 bar, −167 ° C. (ST119), and the low-temperature low-pressure nitrogen In the BOG condenser 131, after performing BOG reliquefaction, a step of becoming 10.3 bar and −134 ° C. (ST120), and while the nitrogen gas passes through the nitrogen heat exchanger 133 again, 10 bar, 40 And a step (ST121) of changing the state to a high temperature of ° C.

  Although the pressure, temperature, etc. in each step are described with specific numbers, it goes without saying that they can be changed according to the amount of BOG, the control method, and the like.

  According to the first embodiment of the present invention configured as described above, the following effects can be obtained.

  The present invention having the configuration of the first embodiment described above can stably manage the pressure of the storage tank without loss of LNG stored during operation of the LNG carrier. In particular, by introducing a simple cold box module, the size of the LNG BOG reliquefaction device can be reduced, and the cryogenic temperature region of nitrogen gas can be stably managed.

  Specifically, since the connecting pipe between the nitrogen heat exchanger 133 and the inlet of the expansion turbine 136 is shortened, the state of the inlet of the expansion turbine 136 and the cryogenic state at the outlet of the expansion turbine that are sensitive to temperature are affected. Can be stabilized. In addition, since the connecting pipe between the outlet of the expansion turbine 136 and the BOG condenser 131 is shortened, the temperature rise generated during the transfer of the cryogenic nitrogen gas can be minimized, and in the LNG BOG reliquefaction device By collecting and managing the unit elements constituting the low temperature part in a cold box, it is possible to prevent the loss of cold.

(Second embodiment)
Hereinafter, the LNG BOG reliquefaction apparatus and the reliquefaction method according to the second embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 5 is a block diagram of an LNG BOG reliquefaction apparatus according to a second embodiment of the present invention. This apparatus includes a BOG cycle apparatus, a nitrogen cycle apparatus, and a cold box apparatus that interfaces the two apparatuses.

BOG cycle apparatus The natural gas in the gaseous state is liquefied at a very low temperature and stored in the storage tank 210 at atmospheric pressure (1.013 bar). However, BOG is generated by the external heat transfer that is sustained during the transport of LNG, which acts as a pressure increase factor of the storage tank 210.

  Therefore, in order to keep the storage tank 210 constant at the atmospheric pressure level, when the pressure of the storage tank reaches about 1.03 bar, the safety valve 211 is opened, and the BOG is discharged to the outside of the storage tank 210. The BOG compressors 215 and 216 are used to undergo a reliquefaction process.

  The high-temperature BOG discharged to the outside of the storage tank 210 is sensed by a temperature sensor 212, adjusted to a constant temperature via a temperature controller 214, and then into a two-stage BOG compressor 215, 216. It is preferable to flow in. The BOG that has passed through the temperature controller 214 is maintained in a superheated steam state of 1.03 bar and −120 ° C.

  The operation of the temperature controller 214 will be described in more detail. When the BOG (LNG) having undergone the reliquefaction process is recirculated and mixed with the high-temperature BOG discharged from the storage tank 210, the temperature adjustment is performed. The recirculation amount is controlled by adjusting the opening degree of the recirculation valve 213. The temperature controller 214 can supply not only the re-liquefied BOG but also LNG from the LNG storage tank 210 to a pump (not shown). It is also possible to supply the LNG in the LNG storage tank 210 so as to be mixed with the BOG compressed by the BOG compressors 215 and 216, and to cool or adjust the temperature of the BOG supplied to the BOG condenser 231. . The temperature adjustment normally operates only until the reliquefaction device reaches a normal state, after which the recirculation valve 213 is closed and the cryogenic reliquefied LNG is directed to the storage tank 210.

  The BOG that has passed through the temperature controller 214 passes through the two-stage BOG compressors 215 and 216 driven by the motor M, and is discharged in a superheated steam state of 2.5 bar and −73 ° C.

  The BOG discharged from the two-stage BOG compressors 215 and 216 is liquefied by the BOG condenser 231 and then mixed again with the non-condensed components separated by the BOG non-condensable gas separator 218 while the temperature of the superheated steam. Is preferably further reduced.

  The non-condensable gas separator 218 may be a separator generally known in the prior art described in the prior art section above. At this time, when the venturi-type nozzle 239 is used, the flow rate of the BOG is increased, and it is preferable to flow in the non-condensable gas component from the BOG non-condensable gas separator 218 using the pressure drop generated from this.

  The BOG then passes through the BOG condenser 231 and changes to a 2.3 bar, −155 ° C. supercooled liquid state before re-entering the storage tank 210 or recirculated to the temperature regulator 214. . Details of the BOG condenser 131 will be described in the cold box device section described later.

  The entire BOG can be liquefied through the BOG condenser 231, but about 70 to 99% is liquefied due to the influence of a nitrogen component or the like that is not easily liquefied.

  The condensed BOG flows again into the storage tank 210 by the circulation pump 219. As a method for re-injecting BOG into the storage tank 210, there is a method in which the BOG is distributed from the spray head at the top of the storage tank or supplied to the bottom of the storage tank. When flowing into the bottom of the storage tank, the nitrogen component in the uncondensed gas contained in the condensed BOG is dissolved inside the LNG, and the nitrogen ratio in the gas phase is kept 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 on the two-stage BOG compressors 215, 216 or the BOG condenser 231.

Nitrogen cycle device In the BOG condenser 231, BOG is reliquefied by heat exchange with a cryogenic nitrogen gas that is a working fluid. The following explanation is based on the cryogenic nitrogen gas necessary for BOG reliquefaction. It is related with the cycle apparatus for obtaining.

  The nitrogen gas at 14 bar and 35.4 ° C. passes through the three-stage compressors 221, 222, and 223 and the intermediate coolers 225, 226, and 227, which are nitrogen gas pressurization and cooling means, and then the pressure rises to 58 bar. And 43 ° C. gas.

  The intercoolers 225, 226, and 227 cool the nitrogen refrigerant acting as a working fluid in the pressurization stage, and use a conventional method known in the prior art described in the prior art part of the present invention. However, a method of cooling seawater as a refrigerant is preferable.

  After the discharged high-pressure nitrogen gas is transferred to the cold box 230, the second nitrogen heat exchanger 234 still has the cold heat from the BOG condenser 231 passing through the first nitrogen heat exchanger 233. The primary cooling to −83.5 ° C. is performed through internal heat exchange with the low-pitched nitrogen.

  A part of the high-pressure nitrogen gas that has passed through the second nitrogen heat exchanger 234 is lowered in temperature through the expansion turbine 236, and the low-pressure nitrogen gas that has been lowered in temperature by the expansion turbine 236 is the first nitrogen gas. In the heat exchanger 233, internal heat exchange with the high-pressure nitrogen that has passed through the second nitrogen heat exchanger 234 is performed, and the high-pressure nitrogen contributes to lowering the nitrogen temperature to the set temperature at the inlet of the expansion valve 237. To do.

  The high-pressure nitrogen gas whose temperature has been lowered in the first nitrogen heat exchanger 233 is finally converted into a cryogenic gas necessary for BOG liquefaction at about −163 ° C. through the expansion valve 237. This cryogenic nitrogen gas exchanges heat with BOG in the BOG condenser 231 and the temperature rises while liquefying the BOG.

  In this way, a part of the high-pressure nitrogen gas is obtained by using the expansion turbine 236 to obtain low-temperature nitrogen and flow into the first nitrogen heat exchanger 233 because the low-pressure nitrogen returning from the BOG condenser 231 is Since the specific heat is low and it is not enough to cool the high-pressure nitrogen, the temperature of a part of the nitrogen is lowered by using the expansion turbine 236 in advance, and high-pressure nitrogen gas is required at the inlet of the expansion valve 237. Helps to cool down to the right temperature.

  Thereafter, the residual cooling heat cools high-pressure nitrogen in the second nitrogen heat exchanger 234, the temperature rises, reaches 14 bar, 35.4 ° C., and completes the cycle.

  The nitrogen buffer tank 230 performs a mass flow rate adjustment function of the nitrogen cycle in accordance with a change in the BOG reliquefaction amount, that is, a change in the cooling load of the nitrogen cycle. In addition, a supply source of the supplementary working fluid (nitrogen) may be further provided for the case where the amount of nitrogen is reduced.

Cold Box Device The cold box 230 includes a BOG condenser 231 that reliquefies BOG, and first and second nitrogen heat exchangers 233 and 234 that perform internal heat exchange between a high temperature portion and a low temperature portion of the nitrogen cycle. And an expansion valve 237 for obtaining cryogenic nitrogen gas and an expansion turbine 236 for obtaining low-temperature nitrogen necessary for internal heat exchange.

  Even if it is not an essential component of the present invention, a generator G is connected to the expansion turbine 236 to produce electric power, which is then used as an auxiliary for the BOG compressors 215 and 216 or the nitrogen compressors 221, 222 and 223 It can also be used as a power source.

  By including the apparatus in the above process as a module in the cold box 230, the connecting pipe between the apparatuses can be shortened, which stably secures the cryogenic nitrogen necessary for BOG reliquefaction. To be able to. That is, the connecting pipe between the second nitrogen heat exchanger 234 and the inlet of the expansion turbine 236 or the connecting pipe between the first nitrogen heat exchanger 233 and the inlet of the expansion valve 237 is shortened. The cryogenic state of nitrogen at the outlet of the expansion turbine 236 or expansion valve 237, which may be more sensitive to the temperature state of nitrogen at the inlet of the expansion turbine 236 or expansion valve 237, can be stably maintained.

  Further, since the connecting pipe between the outlet of the expansion valve 237 and the BOG condenser 231 is shortened, it can be expected that the temperature increase due to the transfer of nitrogen gas is minimized.

  In the present invention, in the configuration of the LNG BOG reliquefaction device, the BOG condenser 231, the nitrogen heat exchangers 233 and 234, and the expansion means 236 and 237, which are low-temperature devices, are configured by one cold box 230, It is preferable to insulate these as one module. The heat insulation is performed using a generally known heat insulating material.

  With such a configuration, it is possible to stably manage the cryogenic temperature region of nitrogen gas. In addition, the cold box can be easily mounted by manufacturing it as a preliminary assembly.

  As described above, the extremely low temperature portion of the nitrogen cycle is stably managed, so that the BOG reliquefaction temperature of −155 ° C. and the maximum temperature of −73 ° C. can be operated in the BOG cycle. The temperature difference can be reduced to 80 ° C. or lower, which is lower than usual. Thus, a simple BOG condenser design that does not require pre-cooling is possible.

  The operation of the reliquefaction apparatus according to the configuration of the second embodiment will be described below with reference to FIGS.

  The operation of the re-liquefaction apparatus according to the configuration of the second embodiment is operated to compress evaporative vapor (BOG) generated in the LNG storage tank and to provide cold heat for condensing the compressed BOG. Pressurizing and cooling nitrogen gas as a fluid, expanding the pressurized and cooled nitrogen gas to generate cryogenic nitrogen gas, exchanging heat with the cryogenic nitrogen gas, and at least compressing the compressed BOG A step of partially condensing, and a step of returning the BOG reliquefied by the condensation to the tank, provided that not all of the pressurized and cooled nitrogen gas is expanded, but a part thereof Is extracted and expanded in a path different from the expansion to generate a low-temperature gas, and then heat exchange is performed for the condensation of the compressed BOG to combine with the nitrogen gas whose temperature has risen slightly. Thereby, including the steps of pre-cooled by heat exchange prior to expansion of said pressurized cooling nitrogen gas.

  Referring to FIGS. 6 and 7 attached hereto, in the BOG reliquefaction method generated by external heat transfer in the storage tank of the cryogenic LNG carrier, first, the circulation of BOG is performed at the set pressure. After the valve 211 is opened, the discharged BOG passes through the temperature controller 214 and the state of the inlets of the two-stage BOG compressors 215 and 216 is kept constant at 1.03 bar and −120 ° C. (ST211) The BOG is compressed to 2.5 bar and −73 ° C. by the two-stage BOG compressors 215 and 216 to be in a superheated state at high temperature and high pressure (ST212), and separated by the BOG non-condensable gas separator 218. Gas is sucked by the pressure difference of the venturi nozzle 239 and the temperature is lowered (ST213), and the BOG discharged from the compressors 215 and 216 is condensed with BOG. In step 213, the liquid is reliquefied to a supercooled liquid of 2.3 bar and −155 ° C. (ST214), the step of separating the non-condensable gas in the reliquefied BOG (ST215), and the circulation pump 219 The step of being pressurized (ST216), the step of recirculating part of the pressurized reliquefied BOG through the temperature controller 114 (ST217), and the rest being recovered and stored in the storage tank 210. Step (ST218), and circulation of nitrogen gas for supplying cold heat for re-liquefaction of the BOG is performed at 14 bar and nitrogen gas at 35.4 ° C. with three-stage compressors 221, 222, 223 and intermediate cooling. After passing through the vessels 225, 226, 227, the pressure is increased and the pressure is increased to 58 bar, 43 ° C. (ST219), and the high-pressure nitrogen is In the elementary heat exchanger 234, a step of changing to a low temperature state of 57.3 bar and −83.5 ° C. through internal heat exchange with the low temperature part nitrogen (ST220), and a part of the high pressure nitrogen passes through the expansion turbine 236 The step of changing to a low temperature, low pressure gas of 14.5 bar, −140 ° C. (ST221), and the nitrogen not sent to the expansion turbine 236 is 57.5 bar, −137.9 ° C. through the first nitrogen heat exchanger 233. Step (ST222), a step in which high pressure nitrogen passes through the expansion valve 237 and is depressurized to a state of 14.6 bar and −163 ° C. (ST223), and the low temperature low pressure nitrogen is BOG regenerated in the BOG condenser 213. After the liquefaction, the step of heating to a state of 14.4 bar and -138.9 ° C. (ST224), and the nitrogen gas is supplied to the expansion turbine 2 The step of changing to the state of 14.2 bar and −106 ° C. while the nitrogen whose temperature is lowered by 36 is combined and passing through the first nitrogen heat exchanger 233 (ST225), and the second nitrogen heat exchanger 234 And a step of changing the state to a high temperature of 14 bar and 35.4 ° C. (ST226).

  The pressure, temperature, and the like at each stage are described with specific numbers, but it goes without saying that they can be changed depending on the amount of BOG, the control method, and the like.

  According to the second embodiment of the present invention configured as described above, the following effects can be obtained.

  In the present invention having the configuration of the second embodiment described above, when the existing BOG liquefaction system using the reverse Brayton cycle of nitrogen lowers the temperature of nitrogen using an expansion turbine, a part of nitrogen is liquefied. Therefore, the method of the present embodiment extracts only a part of the pressurized nitrogen, and the expansion turbine 236 The remaining portion has an advantage that the temperature of nitrogen can be lowered by lowering the temperature of the nitrogen through the expansion valve 237 and lowering the temperature through the expansion valve 237 without fear of droplets. Further, by changing the flow rate of nitrogen supplied to the expansion valve 237, the operation of the nitrogen cycle can be changed in various ways, and it is possible to easily and quickly respond to the dynamic change of the BOG, that is, the load change. it can.

  Further, the present invention having the configuration of the second embodiment described above can stably manage the pressure of the storage tank without loss of LNG stored during operation of the LNG carrier. In particular, by introducing a simple cold box module, the size of the LNG BOG reliquefaction device can be reduced, and the cryogenic temperature region of nitrogen gas can be stably managed.

  Specifically, since the connecting pipe between the nitrogen heat exchanger 233 and the inlet of the expansion valve 237 is shortened, the state of the inlet of the expansion valve 237 and the cryogenic state of the outlet of the expansion valve that is sensitive to temperature Can be stabilized. In addition, since the connecting pipe between the outlet of the expansion valve 237 and the BOG condenser 231 is shortened, the temperature rise generated when the cryogenic nitrogen gas is transferred can be minimized, and in the LNG BOG reliquefaction device By collecting and managing the unit elements constituting the low temperature part in the cold box 230, it is possible to prevent the loss of cold heat.

(Third embodiment)
Hereinafter, the LNG BOG reliquefaction apparatus and the reliquefaction method according to the third embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 8 is a configuration diagram of an LNG BOG reliquefaction apparatus according to a third embodiment of the present invention. This apparatus includes a BOG cycle apparatus, a nitrogen cycle apparatus, and a cold box apparatus that interfaces the two apparatuses.

BOG cycle apparatus The natural gas in the gaseous state is liquefied at a very low temperature and stored in the storage tank 310 at atmospheric pressure (1.013 bar). However, BOG is generated by the external heat transfer that is sustained during the transportation of LNG, which acts as a pressure increase factor of the storage tank 310.

  Therefore, in order to keep the storage tank 310 constant at the atmospheric pressure level, when the pressure of the storage tank reaches about 1.03 bar, the safety valve 311 is opened, and the BOG is discharged to the outside of the storage tank 310 and is The BOG compressors 315 and 316 are used to undergo a reliquefaction process.

  The high-temperature BOG discharged to the outside of the storage tank 310 is sensed by a temperature sensor 312, adjusted to a constant temperature via a temperature controller 314, and then supplied to two-stage BOG compressors 315 and 316. It is preferable to flow in. The BOG that has passed through the temperature controller 314 is maintained in a superheated steam state of 1.03 bar and −120 ° C.

  The operation of the temperature controller 314 will be described in more detail. When the BOG (LNG) having undergone the reliquefaction process is recirculated and mixed with the hot BOG discharged from the storage tank 310, the temperature control is performed. The recirculation amount is controlled by adjusting the opening of the recirculation valve 313. The temperature regulator 314 can be supplied with not only the re-liquefied BOG but also LNG from an LNG storage tank to a pump (not shown). It is also possible to supply the LNG in the LNG storage tank 310 so as to be mixed with the BOG compressed by the BOG compressors 315 and 316, and to cool or adjust the temperature of the BOG supplied to the BOG condenser 331. . The temperature adjustment normally operates only until the reliquefaction device reaches a normal state, after which the recirculation valve 313 is closed and the cryogenic reliquefied LNG is directed to the storage tank 310.

  The BOG that has passed through the temperature controller 314 passes through the two-stage BOG compressors 315 and 316 and is discharged in a superheated steam state of 3.6 bar and −43 ° C.

  The BOG discharged from the two-stage BOG compressors 315 and 316 is pre-cooled through a first nitrogen heat exchanger 333 having three paths, and is in a superheated steam state of 3.3 bar and −134 ° C. It becomes. At this time, the pre-cooling of the BOG is performed by mixing the low-temperature and low-temperature low-temperature portion nitrogen gas discharged from the BOG condenser 331, which will be described later, with the nitrogen gas cryogenized by the expansion turbine 336 in the first nitrogen heat exchanger 333. The heat is exchanged by the nitrogen gas and cooled.

  Then, the pre-cooled BOG passes through the BOG condenser 331 and changes to a supercooled liquid state of 3.0 bar and −154.7 ° C., and then re-enters the storage tank 310 or a temperature controller. 314 is recycled. Details of the BOG condenser 331 will be described in the cold box device section described later.

  As described above, the BOG is pre-cooled by the low-temperature working fluid (nitrogen), so that the temperature difference between the BOG and the nitrogen gas can be set within the set range in the BOG condenser 331 even if the BOG generation amount or the temperature changes. It has the advantage that it can be kept constant.

  Further, when the heated steam state BOG discharged from the two-stage BOG compressors 315 and 316 flows into the BOG condenser, the temperature difference between the streams in the BOG condenser is increased, and thermal stress causes the heat exchanger to The BOG pre-cooling process has the advantage of reducing the temperature difference between the streams in the BOG condenser, since durability can be problematic.

  Although the entire BOG can be liquefied through the BOG condenser 331, about 70 to 99% is liquefied due to the influence of a nitrogen component or the like that is not easily liquefied.

  Such a gas-liquid mixture is separated into a gas and a liquid by a BOG non-condensable gas separator 318, and the liquid (condensed BOG) is re-entered into the storage tank 310 by a circulation pump 319 or a temperature controller. Recirculated to 314 and the gas is generally discharged to the outside.

  As a method of re-injecting BOG into the storage tank 310, there is a method of distributing from the spray head at the top of the tank or supplying it to the bottom of the tank. When flowing into the bottom of the storage tank, the nitrogen component in the uncondensed gas contained in the condensed BOG is dissolved inside the LNG, and the nitrogen ratio in the gas phase is kept 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 on the two-stage BOG compressors 315, 316 or the BOG condenser 331.

Nitrogen cycle apparatus In the BOG condenser 331, BOG is reliquefied by heat exchange with cryogenic nitrogen gas. The following explanation is for obtaining cryogenic nitrogen gas necessary for BOG reliquefaction. The present invention relates to a cycle device.

  14.3 bar, 36.08 ° C. nitrogen gas passes through three stages of nitrogen compressors 321, 322, 323 and intercoolers 325, 326, 327, then the pressure rises to 58 bar, 43 ° C. gas Is discharged. The discharged high-pressure nitrogen gas is transferred to the cold box 330 and then returned to the low-pressure low-temperature nitrogen in the second nitrogen heat exchanger 334 via the BOG condenser 331 and the first nitrogen heat exchanger 333. It is cooled to 57.7 bar and -70 ° C through internal heat exchange with.

  At this time, a part of the nitrogen gas cooled by passing through the second nitrogen heat exchanger 334 decreases in temperature while passing through the expansion turbine 336, and then the low-temperature nitrogen is converted into the BOG condenser. It is combined with the nitrogen returned through 331 and flows into the first nitrogen heat exchanger 333. At this time, the first nitrogen heat exchanger 333 is partially cooled in the second nitrogen heat exchanger 334. It is preferable to further cool the high-pressure nitrogen gas through internal heat exchange with the high-pressure nitrogen gas. By such cooling, the high-pressure nitrogen has an effect of lowering the nitrogen temperature to a necessary set temperature at the inlet of the expansion valve 337.

  The high-pressure nitrogen gas whose temperature has been lowered in the first nitrogen heat exchanger 333 is finally changed into a cryogenic gas necessary for BOG liquefaction at −162.6 ° C. through the expansion valve 337. The cryogenic nitrogen gas exchanges heat with BOG in the BOG condenser 331, and the temperature rises while liquefying the BOG.

  The reason why a part of the high-pressure nitrogen gas is obtained by using the expansion turbine 336 to obtain low-temperature nitrogen and flow into the first nitrogen heat exchanger 333 is that low-pressure nitrogen returning from the BOG condenser 331 has low specific heat, Since it is not enough to cool the high-pressure nitrogen, the temperature of a part of the nitrogen is previously lowered by using the expansion turbine 336, and this is used to cool the high-pressure nitrogen gas to a temperature required at the inlet of the expansion valve 337. To be done. Thereafter, the residual cooling heat cools the high-pressure nitrogen in the second nitrogen heat exchanger 334, the temperature rises, reaches 14.3 bar, 36.08 ° C., and completes the nitrogen cycle.

  The nitrogen buffer tank 320 performs a mass flow rate adjustment function of the nitrogen cycle in accordance with a change in the BOG reliquefaction amount, that is, a change in the cooling load of the nitrogen cycle. In addition, a supply source of the supplementary working fluid (nitrogen) may be further provided for the case where the amount of nitrogen is reduced.

Cold box device A cold box 330 includes a BOG condenser 331 that performs reliquefaction of BOG, nitrogen heat exchangers 333 and 334 that perform internal heat exchange and BOG precooling between a high temperature part and a low temperature part of a nitrogen cycle, An expansion valve 337 for obtaining cryogenic nitrogen gas and an expansion turbine 336 for obtaining low-temperature nitrogen necessary for internal heat exchange.

  Even though it is not an essential component of the present invention, after the generator G is connected to the expansion turbine 336 to produce electric power, it is used as an auxiliary for the BOG compressors 315 and 316 or the nitrogen compressors 321, 322 and 323. It can also be used as a power source.

  By including the apparatus in the above process as a module in the cold box 330, the connecting pipe between the apparatuses can be shortened, which stably secures the cryogenic nitrogen necessary for BOG reliquefaction. To be able to. That is, the connection pipe between the second nitrogen heat exchanger 334 and the inlet of the expansion turbine 336 or the connection pipe between the first nitrogen heat exchanger 333 and the inlet of the expansion valve 337 is shortened. The cryogenic state of nitrogen at the outlet of the expansion turbine 336 or expansion valve 337, which may be more sensitive to the temperature state of nitrogen at the inlet of the expansion turbine 336 or expansion valve 337, can be stably maintained. Moreover, since the connection pipe between the outlet of the expansion valve 337 and the BOG condenser 331 is shortened, it can be expected that the temperature increase due to the transfer of nitrogen gas is minimized.

  In the present invention, in the configuration of the LNG BOG reliquefaction device, the BOG condenser 331, the nitrogen heat exchangers 333 and 334, and the expansion turbines 336 and 337, which are low-temperature devices, are configured by one cold box 330, It is preferable to insulate these as one module. The heat insulation is performed using a generally known heat insulating material. With such a configuration, it is possible to stably manage the cryogenic temperature region of nitrogen gas. Further, the cold box 330 can be easily mounted on a ship by being manufactured as a preliminary assembly.

  Hereinafter, the operation of the reliquefaction apparatus according to the configuration of the third embodiment will be described with reference to FIGS.

  The operation of the reliquefaction apparatus according to the configuration of the third embodiment is to compress the evaporated vapor (BOG) generated in the LNG storage tank 310 and to provide cold heat for condensing the compressed BOG. Pressurizing and cooling nitrogen gas as a working fluid, expanding the pressurized and cooled nitrogen gas, generating cryogenic nitrogen gas, exchanging heat with the cryogenic nitrogen gas, and compressing the compressed BOG A step of condensing at least partly and returning the BOG reliquefied by the condensation to the tank, provided that the BOG generated in the LNG storage tank is converted into a first nitrogen heat exchange preceding the BOG condenser. By passing the pre-cooling process in the condenser, the temperature difference between BOG and nitrogen gas is reduced inside the BOG condenser even if there is a BOG generation amount or temperature change. It can be maintained constant within the set range.

  In addition, not all of the pressurized and cooled nitrogen gas is expanded, but a part thereof is extracted and expanded in a path different from the expansion to generate a low temperature gas, and then compressed. It may include a step of performing heat exchange for the condensation of BOG, combining with nitrogen gas whose temperature has risen slightly, and pre-cooling by heat exchange before expansion of the pressurized and cooled nitrogen gas.

  Referring to FIGS. 9 and 10 attached hereto, in the BOG reliquefaction method generated by external heat transfer in the storage tank of the cryogenic LNG carrier, first, the circulation of BOG is performed at the set pressure. After the valve 311 is opened, the discharged BOG passes through the temperature controller 314 and the state of the inlets of the two-stage BOG compressors 315 and 316 is kept constant at 1.03 bar and −120 ° C. (ST311) The BOG is compressed to 3.6 bar and −43 ° C. by the two-stage BOG compressors 315 and 316 to be in a high temperature and high pressure superheat state (ST312), and the pre-cooling heat exchanger of the three paths 1 is passed through the nitrogen heat exchanger 333, and the temperature is lowered to -3.3 bar and -134 ° C (precooled) (ST313), the precooled BOG is converted into the BOG condenser 33. And re-liquefied to a supercooled liquid of 3.0 bar and -154.7 ° C. (ST314), and the re-liquefied BOG is recovered and stored in the storage tank 310 (ST318). The circulation of nitrogen gas for supplying cold heat for reliquefaction of the BOG is 14.3 bar, nitrogen gas at 36.08 ° C. is converted into three-stage compressors 321, 322, 323 and intermediate coolers 325, 326, After passing through 327, the pressure is increased and the pressure is increased to 58 bar and 43 ° C. (ST319), and the high-pressure nitrogen is exchanged with the low-temperature part nitrogen in the second nitrogen heat exchanger 334. Step of changing to a low temperature state of 57.7 bar, −70 ° C. (ST320), and a part of the high-pressure nitrogen passes through the expansion turbine 336, 15.2 bar, −129.3 ° C. The step of changing to low-temperature, low-pressure gas (ST321), and the nitrogen that has not been sent to the expansion turbine 336 is combined with the nitrogen that has been cooled in the expansion turbine 336 in the first nitrogen heat exchanger 333. The internal heat exchange is performed, and the step of 57.4 bar and −132 ° C. is performed (ST322), and the high pressure nitrogen passes through the expansion valve 337 and is cooled to 15.2 bar and −162.6 ° C. under reduced pressure. (ST323), and after the low-temperature low-pressure nitrogen is subjected to BOG reliquefaction in the BOG condenser 313, the low-temperature nitrogen gas is heated to a low-temperature nitrogen gas of 14.9 bar and −145 ° C. (ST324), 14.6 bar, −86 while the nitrogen whose temperature has been lowered by the expansion turbine 336 is combined and passes through the first nitrogen heat exchanger 333. A step of changing to a state of .42 ° C. (ST325) and a step of changing the state to a high temperature of 14.3 bar and 36.08 ° C. while passing through the second nitrogen heat exchanger 334 (ST326) It is characterized by that.

  After the step of reliquefying the pre-cooled BOG in the BOG condenser 331 (ST314), a step (ST315) of separating the non-condensable gas in the reliquefied BOG may be added. The liquefied BOG may be transferred to the tank 310 by the step of pressurization by the circulation pump 319 (ST316).

  Further, a part of the reliquefied BOG is recirculated through the temperature regulator 314 through the recirculation valve 313 (ST317), and the step before the BOG is compressed (ST312) or the step after the compression is performed. It may be supplied to pre-cool the BOG.

  The pressure, temperature, and the like at each stage are described with specific numbers, but it goes without saying that they can be changed depending on the amount of BOG, the control method, and the like.

  According to the third embodiment of the present invention configured as described above, the following effects can be obtained.

  In the present invention having the configuration of the third embodiment described above, the BOG is subjected to a pre-cooling process in the previous stage of the BOG condenser, so that the temperature of the BOG at the inlet of the condenser is changed even if the amount of BOG generated or the temperature changes. Can be maintained constant within a set range.

  In addition, in the third embodiment, in the BOG liquefaction system using the existing reverse Brayton cycle of nitrogen, when the temperature of the nitrogen is lowered using an expansion turbine, the nitrogen is partially liquefied, so that the expansion occurs. The problem of the prior art, which may cause a problem of damaging the blades of the turbine, is solved. The method of this third embodiment extracts only a part of the pressurized nitrogen and uses an expansion turbine 336. There is an advantage that the temperature of the nitrogen can be lowered by lowering the temperature of the nitrogen through the expansion valve 337 and lowering the temperature through the expansion valve 337 without fear of droplets. Also, by changing the flow rate of nitrogen supplied to the expansion valve 337, the operation of the nitrogen cycle can be changed in various ways, and it is possible to easily and quickly respond to the dynamic change of the BOG, that is, the load change. it can.

  Further, the invention of the third embodiment can stably manage the pressure of the storage tank without loss of LNG stored during operation of the LNG carrier. In particular, the introduction of a simple cold box module can reduce the size of the LNG reliquefaction device and can stably manage the cryogenic temperature region of nitrogen gas.

  Specifically, since the connecting pipe between the nitrogen heat exchanger 333 and the inlet of the expansion valve 337 is shortened, the cryogenic temperature at the outlet of the expansion valve 337 is sensitive to the state of the inlet of the expansion valve 337 and the temperature. The state can be stabilized. In addition, since the connecting pipe between the outlet of the expansion valve 337 and the BOG condenser 331 is shortened, the temperature rise generated when the cryogenic nitrogen gas is transferred can be minimized, and in the LNG BOG reliquefaction device By collecting and managing the unit elements constituting the low temperature part in the cold box 330, it is possible to prevent the loss of cold.

(Fourth embodiment)
Hereinafter, the LNG BOG reliquefaction apparatus and reliquefaction method according to the fourth embodiment of the present invention will be described in detail with reference to FIGS. 11 to 13.

  FIG. 11 is a configuration diagram of an LNG BOG reliquefaction apparatus according to a fourth embodiment of the present invention. This apparatus includes a BOG cycle apparatus, a nitrogen cycle apparatus, and a cold box apparatus that interfaces the two apparatuses.

BOG cycle apparatus Natural gas in a gaseous state is liquefied at a cryogenic temperature and stored in the storage tank 410 at atmospheric pressure (1.013 bar). However, BOG is generated by the external heat transfer that is sustained during the transport of LNG, and this acts as a pressure increase factor of the storage tank 410.

  Therefore, in order to keep the storage tank 410 constant at the atmospheric pressure level, when the pressure of the storage tank reaches about 1.03 bar, the safety valve 411 is opened, and the BOG is discharged to the outside of the storage tank 410 and is The BOG compressors 415 and 416 are used to undergo a reliquefaction process.

  The high temperature BOG discharged to the outside of the storage tank 410 is sensed by a temperature sensor 412, adjusted to a constant temperature via a temperature controller 414, and then supplied to a two-stage BOG compressor 415, 416. It is preferable to flow in. The BOG that has passed through the temperature controller 414 is maintained in a superheated steam state of 1.03 bar and −120 ° C.

  The operation of the temperature controller 414 will be described in more detail. When the BOG (LNG) having undergone the reliquefaction process is recirculated and mixed with the hot BOG discharged from the storage tank 410, the temperature control is performed. The recirculation amount is controlled by adjusting the opening of the recirculation valve 413. The temperature regulator 414 can be supplied with not only the re-liquefied BOG but also LNG from an LNG storage tank to a pump (not shown). It is also possible to supply the LNG in the LNG storage tank 410 so as to be mixed with the BOG compressed by the BOG compressors 415 and 416, and to cool or adjust the temperature of the BOG supplied to the BOG condenser. The temperature adjustment normally operates only until the reliquefaction device reaches a normal state, after which the recirculation valve 413 is closed and the cryogenic reliquefied LNG is directed to the storage tank 410.

  The BOG that has passed through the temperature controller 414 passes through the two-stage BOG compressors 415 and 416 and is discharged in a superheated steam state of 3.2 bar and −50.83 ° C.

  The BOG discharged from the two-stage BOG compressors 415 and 416 is pre-cooled through a first nitrogen heat exchanger 433 having three paths, and is in a superheated steam state of 3.1 bar and −130 ° C. It becomes. At this time, the pre-cooling of the BOG is performed by mixing, in the first nitrogen heat exchanger 433, the low-temperature and low-temperature nitrogen gas discharged from the BOG condenser 431 described later and the nitrogen gas cryogenized by the expansion turbine 436. The heat is exchanged by the nitrogen gas and cooled.

  Then, the pre-cooled BOG passes through the BOG condenser 431 and changes to a supercooled liquid state of 3.0 bar and −154.7 ° C., and then re-enters the storage tank 410 or a temperature controller. 414 is recirculated. Details of the BOG condenser 431 will be described in the cold box device section described later. As described above, the BOG is pre-cooled by the low-temperature working fluid (nitrogen), so that even if there is a BOG generation amount or a temperature change, the temperature difference between the BOG and the nitrogen gas is set within the set range in the BOG condenser 431. It has the advantage that it can be kept constant.

  Although the entire BOG can be liquefied through the BOG condenser 431, about 70 to 99% is liquefied due to the influence of a nitrogen component or the like that is not easily liquefied.

  Such a gas-liquid mixture is separated into a gas and a liquid by a BOG non-condensable gas separator 418, and the liquid (condensed BOG) is re-entered into the storage tank 410 by a circulation pump 419 or a temperature controller. Recirculated to 414, the gas is generally discharged to the outside.

  As a method for re-injecting BOG into the storage tank 410, there is a method in which the BOG is distributed from the spray head at the top of the storage tank or supplied to the bottom of the storage tank. When flowing into the bottom of the storage tank, the nitrogen component in the uncondensed gas contained in the condensed BOG is dissolved inside the LNG, and the nitrogen ratio in the gas phase is kept 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 on the two-stage BOG compressors 415, 416 or the BOG condenser 431.

Nitrogen cycle device In the BOG condenser 431, BOG is reliquefied by heat exchange with cryogenic nitrogen gas. The following explanation is for obtaining cryogenic nitrogen gas necessary for BOG reliquefaction. The present invention relates to a cycle device.

  14.2 bar, 35.46 ° C. nitrogen gas passes through three stages of nitrogen compressors 421, 422, 423 and intercoolers 425, 426, 427, and then the pressure rises to 58 bar, 43 ° C. gas. Is discharged.

  After the discharged high-pressure nitrogen gas is transferred to the cold box 430, the BOG condenser 431, the third nitrogen heat exchanger 435, and the first nitrogen heat exchange in the second nitrogen heat exchanger 434 are performed. It is cooled to 57.9 bar and -77 ° C through internal heat exchange with the low pressure cryogenic nitrogen returning through vessel 433.

  At this time, a part of the nitrogen gas cooled through the second nitrogen heat exchanger 434 passes through the expansion turbine 436 to lower the temperature, and then the low-temperature nitrogen is converted into the BOG condenser 431. And the nitrogen returned through the third nitrogen heat exchanger 435, and flows into the first nitrogen heat exchanger 433. At this time, in the first nitrogen heat exchanger 433, the second nitrogen Through the internal heat exchange with the high-pressure nitrogen gas partially cooled in the heat exchanger 434, the high-pressure nitrogen gas is further cooled and discharged as a gas at 57.8 bar and −134 ° C.

  The nitrogen gas that has passed through the first nitrogen heat exchanger 433 is heat-exchanged with the flow that has passed through the BOG condenser 431 in a third nitrogen heat exchanger 435, and is a gas at 57.7 bar and −137 ° C. Discharged. By such cooling, the high-pressure nitrogen has an effect of lowering the nitrogen temperature to a necessary set temperature at the inlet of the expansion valve 437.

  The high-pressure nitrogen gas whose temperature has been lowered in the third nitrogen heat exchanger 435 is finally cooled down to −163.3 ° C. through the expansion valve 437, and a two-phase flow at a very low temperature necessary for BOG liquefaction. To change. The cryogenic nitrogen gas exchanges heat with BOG in the BOG condenser 431, and the temperature rises while liquefying the BOG.

  Thus, by providing the third nitrogen heat exchanger 435, the temperature of nitrogen can be more stably increased at the inlet of the expansion valve 437 than when only the first nitrogen heat exchanger 433 is provided. In addition to being able to manage, the size of each nitrogen heat exchanger can be greatly reduced, and as a result, the size of the cold box 430 can be reduced.

  The reason why a part of the high-pressure nitrogen gas is obtained by using the expansion turbine 436 to obtain low-temperature nitrogen and flows into the first nitrogen heat exchanger 434 is that the BOG condenser 431 and the third nitrogen heat exchanger 435 are connected to each other. Since the low-pressure nitrogen that passes back has a low specific heat and is insufficient to cool the high-pressure nitrogen, the temperature of a part of the nitrogen is lowered by using the expansion turbine 436 in advance, and the high-pressure nitrogen gas is used. Helps to be cooled to the required temperature at the inlet of the expansion valve 437. Thereafter, the residual cooling heat cools the high-pressure nitrogen in the second nitrogen heat exchanger 434, the temperature rises, reaches 14.2 bar, 35.46 ° C., and completes the cycle.

  The nitrogen buffer tank 420 performs a mass flow rate adjustment function of the nitrogen cycle in accordance with a change in the BOG reliquefaction amount, that is, a change in the cooling load of the nitrogen cycle. In addition, a supply source of the supplementary working fluid (nitrogen) may be further provided for the case where the amount of nitrogen is reduced.

Cold Box Device The cold box 430 includes a BOG condenser 431 that reliquefies BOG, and nitrogen heat exchangers 433, 434, and 435 that perform internal heat exchange and BOG precooling between the high temperature portion and the low temperature portion of the nitrogen cycle. And an expansion valve 437 for obtaining cryogenic nitrogen gas and an expansion turbine 436 for obtaining low-temperature nitrogen necessary for internal heat exchange.

  Even though it is not an essential component of the present invention, after the generator G is connected to the expansion turbine 436 to produce electric power, it is used as an auxiliary to the BOG compressors 415 and 416 or the nitrogen compressors 421, 422, and 423. It can also be used as a power source.

  By including the apparatus in the above process as a module in the cold box 430, the connecting pipe between the apparatuses can be shortened, which stably secures the cryogenic nitrogen necessary for BOG reliquefaction. To be able to. That is, the connection pipes between the second nitrogen heat exchanger 434 and the inlet of the expansion turbine 436 and between the outlet of the third nitrogen heat exchanger 435 and the inlet of the expansion valve 437 are shortened, respectively. The cryogenic state of nitrogen at the outlet of the expansion turbine 436 or expansion valve 437, which can be more sensitive to the temperature state of nitrogen at the inlet of the expansion turbine 436 or expansion valve 437, can be stably maintained. Further, since the connection pipe between the outlet of the expansion valve 437 and the BOG condenser 431 is shortened, it is possible to expect a minimum increase in temperature due to the transfer of nitrogen gas.

  In the present invention, in the configuration of the BOG reliquefaction apparatus, the BOG condenser 431, the nitrogen heat exchangers 433, 434, and 435 and the expansion means 436 and 437, which are apparatuses in a low temperature state, are configured by one cold box 430. These are preferably insulated as a single module. The heat insulation is performed using a generally known heat insulating material. With such a configuration, it is possible to stably manage the cryogenic temperature region of nitrogen gas. Further, the cold box 430 can be easily mounted on a ship by being manufactured as a preliminary assembly.

  The operation of the reliquefaction apparatus according to the configuration of the fourth embodiment will be described below with reference to FIGS.

  The operation of the reliquefaction apparatus according to the configuration of the fourth embodiment is to compress the vaporized vapor (BOG) generated in the LNG storage tank 410 and to provide cold heat for condensing the compressed BOG. Pressurizing and cooling nitrogen gas as a working fluid, expanding the pressurized and cooled nitrogen gas, generating cryogenic nitrogen gas, exchanging heat with the cryogenic nitrogen gas, and compressing the compressed BOG The step of condensing, and the step of returning the BOG reliquefied by the condensation to the storage tank 410, provided that the BOG generated in the LNG storage tank 410 is converted into the first nitrogen heat exchange before the BOG condenser 431. By passing the pre-cooling process in the condenser 434, even if there is a BOG generation amount or a temperature change, BOG and nitrogen gas are generated inside the BOG condenser 431. The temperature difference between, can be maintained constant within the set range.

  In addition, not all of the pressurized and cooled nitrogen gas is expanded, but a part thereof is extracted and expanded in a path different from the expansion to generate a low temperature gas, and then compressed. Heat exchange for condensation of BOG, combined with nitrogen gas whose temperature has risen somewhat, and pressure cooling that flows into the expansion valve 437 through the first nitrogen heat exchanger 433 and the third nitrogen heat exchanger 435 A step of pre-cooling the nitrogen gas that has been subjected to heat exchange before expansion may be included.

  Referring to FIGS. 12 and 13 attached hereto, in the BOG reliquefaction method generated by external heat transfer in the storage tank 410 of the cryogenic LNG carrier, first, the circulation of the BOG is performed at a set pressure. In step ST411, after the valve 411 is opened, the discharged BOG passes through the temperature controller 414, and the state of the inlets of the two-stage BOG compressors 415 and 416 is kept constant at 1.03 bar and −120 ° C. ), The BOG is compressed to 3.2 bar and −50.83 ° C. by two-stage BOG compressors 415 and 416, and a high temperature and high pressure overheating state (ST412) and three paths are formed. Passing the first nitrogen heat exchanger 433 of the precooling heat exchanger to lower the temperature to 3.1 bar and −130 ° C. (precooling) (ST413), the precooled BO Is reliquefied into a supercooled liquid at 3.0 bar and −154.7 ° C. in the BOG condenser 431 (ST414), and the reliquefied BOG is recovered in the storage tank 410 and stored. (ST418) and the circulation of nitrogen gas for supplying cold heat for re-liquefaction of the BOG is performed by using nitrogen gas at 14.2 bar and 35.46 ° C. in three stages of nitrogen compressors 421, 422, and 423. And the intermediate coolers 425, 426, and 427, the pressure is increased and the pressure is increased to 58 bar and 43 ° C. (ST419), and the high-pressure nitrogen is supplied to the second nitrogen heat exchanger 434. A step of cooling to a low temperature state of 57.9 bar and −77 ° C. through internal heat exchange with the low temperature part nitrogen (ST420), and a part of the high pressure nitrogen passes through the expansion turbine 436. , 15.2 bar, −129.3 ° C., the step of changing to a low temperature, low pressure gas (ST421), and the nitrogen that has not been sent to the expansion turbine 436 is cooled by the expansion turbine 436 in the first nitrogen heat exchanger 433. The internal heat exchange is performed by the low temperature nitrogen combined with the converted nitrogen, and the step is cooled to 57.8 bar and −134 ° C. (ST422), and the BOG condenser 431 is passed through the third nitrogen heat exchanger 435. The internal temperature is exchanged with the low-temperature portion nitrogen, and is cooled to 57.7 bar and −137 ° C. (ST423), while the high-pressure nitrogen that has passed through the third nitrogen heat exchanger 435 passes through the expansion valve 437, A step (ST424) in which the pressure is reduced to 14.6 bar and −163.3 ° C. under reduced pressure, and the low-temperature low-pressure nitrogen is BOG reliquefied by the BOG condenser 431. After that, the step of heating to a low temperature nitrogen gas of 14.5 bar and −150.7 ° C. (ST425) and the low temperature nitrogen gas passing through the third nitrogen heat exchanger 435, 14.4 bar, − The step of heating to a temperature of 140 ° C. (ST426), and the low-temperature part nitrogen gas that has passed through the third nitrogen heat exchanger 435 are combined with the nitrogen gas whose temperature has been lowered by the expansion turbine 436, and the first While passing through the nitrogen heat exchanger 433, the step of heating to 14.3 bar, −98.88 ° C. (ST427) and passing through the second nitrogen heat exchanger 434, 14.2 bar, 35.46 ° C. And the step of heating (ST428).

  After the step of reliquefying the precooled BOG in the BOG condenser 431 (ST414), a step (ST415) of separating the non-condensable gas in the reliquefied BOG may be added. The liquefied BOG may be transferred to the storage tank 410 by the step of pressurization by the circulation pump 419 (ST416). Further, a part of the reliquefied BOG is recirculated through the temperature regulator 414 through the recirculation valve 413 (ST417), and the step before the BOG is compressed (ST412) or after being compressed. BOG may be supplied to the step to pre-cool the BOG.

  The pressure, temperature, and the like at each stage are described with specific numbers, but it goes without saying that they can be changed depending on the amount of BOG, the control method, and the like.

  According to the fourth embodiment of the present invention configured as described above, the following effects can be obtained.

  In the present invention having the configuration of the fourth embodiment described above, the BOG is subjected to a pre-cooling process in the preceding stage of the BOG condenser, so that the temperature of the BOG at the inlet of the condenser is changed even if the amount of BOG generated or the temperature changes. Can be maintained constant within a set range.

  In addition, when the temperature of the nitrogen is lowered by using the expansion turbine of the BOG liquefaction system using the existing reverse Brayton cycle of nitrogen, the invention of the fourth embodiment is caused by a part of the nitrogen being liquefied. The method of this fourth embodiment extracts only a portion of the pressurized nitrogen and uses an expansion turbine 436 to solve the problems of the prior art that can cause problems that could damage the blades of the turbine. The remaining portion is expanded, and the temperature of the nitrogen is further lowered, and then the temperature is lowered through the expansion valve 437. Thus, there is an advantage that the temperature of the nitrogen can be lowered without generating a droplet. Further, by changing the flow rate of nitrogen supplied to the expansion valve 437, it is possible to change the operation of the nitrogen cycle in various ways, and to easily and quickly respond to the dynamic change of the BOG, that is, the load change. it can.

  Further, the invention of the fourth embodiment can stably manage the pressure of the storage tank without loss of LNG stored during operation of the LNG carrier. In particular, by introducing a simple cold box module, the size of the LNG BOG reliquefaction device can be reduced, and the cryogenic temperature region of nitrogen gas can be stably managed.

  Specifically, since the connecting pipe between the nitrogen heat exchanger 435 and the inlet of the expansion valve 437 is shortened, the cryogenic temperature at the outlet of the expansion valve 437 that is sensitive to the state and temperature of the inlet of the expansion valve 437. The state can be stabilized. Further, since the connecting pipe between the outlet of the expansion valve 437 and the BOG condenser 431 is shortened, the temperature rise generated when the cryogenic nitrogen gas is transferred can be minimized. In the LNG BOG reliquefaction apparatus, By collecting and managing the unit elements constituting the low temperature part in the cold box 430, it is possible to prevent the loss of cold.

It is the schematic which shows the LNG reliquefaction apparatus by a prior art. 1 is a schematic diagram illustrating an LNG BOG reliquefaction apparatus according to a first embodiment of the present invention. 2 is a flowchart illustrating a method for reliquefying LNG BOG according to a first embodiment of the present invention. 2 is a flowchart illustrating a method for reliquefying LNG BOG according to a first embodiment of the present invention. It is the schematic which shows the LNG BOG reliquefaction apparatus by 2nd Example of this invention. 3 is a flowchart illustrating a method for reliquefying LNG BOG according to a second embodiment of the present invention. 3 is a flowchart illustrating a method for reliquefying LNG BOG according to a second embodiment of the present invention. It is the schematic which shows the LNG BOG reliquefaction apparatus by 3rd Example of this invention. 6 is a flowchart illustrating a method for reliquefying LNG BOG according to a third embodiment of the present invention. 6 is a flowchart illustrating a method for reliquefying LNG BOG according to a third embodiment of the present invention. It is the schematic which shows the LNG BOG reliquefaction apparatus by 4th Example of this invention. 6 is a flowchart illustrating a LNG BOG reliquefaction method according to a fourth embodiment of the present invention. 6 is a flowchart illustrating a LNG BOG reliquefaction method according to a fourth embodiment of the present invention.

Claims (15)

A compressor for compressing vaporized vapor (BOG) generated in an LNG storage tank, a condenser for condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold to the compressor, In the LNG BOG reliquefaction device for returning the BOG reliquefied by the condenser to the storage tank,
The nitrogen cycle apparatus includes a nitrogen pressure cooling means for pressurizing and cooling nitrogen, a first nitrogen heat exchanger for cooling high-pressure nitrogen gas that has passed through the nitrogen pressure cooling means, and the first nitrogen heat. A first expansion means for expanding the high-pressure nitrogen gas from the exchanger to generate a cryogenic low-pressure low-pressure nitrogen gas,
The cryogenic and low-pressure nitrogen gas expanded by the first expansion means is deprived of cold heat by the condenser, and then returned to the nitrogen pressure cooling means via the first nitrogen heat exchanger,
A part of the high-pressure nitrogen gas supplied from the nitrogen pressure cooling means to the first nitrogen heat exchanger is expanded by the second expansion means and cooled to a low-temperature low-pressure nitrogen gas, and then the condenser. The LNG BOG re-liquefaction apparatus, wherein the LNG BOG re-liquefaction apparatus is combined with the low-pressure nitrogen gas from the gas, flows into the first nitrogen heat exchanger, and returns to the nitrogen pressure cooling means.   The LNG BOG reliquefaction apparatus according to claim 1, wherein the BOG compressed by the compressor is pre-cooled before being condensed.   A second nitrogen heat exchanger is further provided between the nitrogen pressure cooling means and the first nitrogen heat exchanger, and the high-pressure nitrogen gas flowing into the second expansion means is the second nitrogen The LNG BOG reliquefaction device according to claim 1, wherein the LNG BOG reliquefaction device is primarily cooled by a heat exchanger.   After the non-condensable gas partially contained in the BOG condensed by the condenser is separated by the non-condensable gas separator, the non-condensable gas is compressed by the compressor and supplied to the condenser. 2. The LNG BOG reliquefaction device according to claim 1, wherein the temperature of the BOG supplied to the condenser is cooled by being mixed with the BOG.   The LNG BOG reliquefaction apparatus according to claim 4, wherein the non-condensable gas is mixed with BOG supplied to the condenser by a venturi-type nozzle.   2. The LNG BOG reliquefaction apparatus according to claim 1, wherein the BOG compressed by the compressor is heat-exchanged by the first nitrogen heat exchanger and precooled before being condensed.   A third nitrogen heat exchanger for further cooling the high-pressure nitrogen gas that has passed through the first nitrogen heat exchanger is further provided between the first nitrogen heat exchanger and the first expansion means. The LNG BOG reliquefaction apparatus according to claim 1, wherein:   The LNG BOG reliquefaction apparatus according to claim 1, wherein the first or second nitrogen expansion means is an expansion valve or an expansion turbine.   The LNG BOG reliquefaction apparatus according to claim 8, wherein the expansion turbine is provided with a generator.   The BOG supplied to the compressor is supplied with the liquefied BOG or LNG from the LNG storage tank, and the temperature of the BOG supplied to the condenser is cooled. LNG BOG reliquefaction device.   The BOG compressed by the compressor is supplied with the liquefied BOG or LNG from the LNG storage tank, and the temperature of the BOG supplied to the condenser is cooled. LNG BOG reliquefaction device.   The LNG BOG reliquefaction apparatus according to any one of claims 1 to 11, wherein the compressor, the nitrogen heat exchanger, and the expansion means are configured as one module.   The LNG BOG reliquefaction device according to claim 12, wherein the module is thermally insulated.   The LNG BOG reliquefaction apparatus according to claim 12, wherein the module is manufactured as a preliminary assembly. A compressor for compressing vaporized vapor (BOG) generated in an LNG storage tank, a condenser for condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold to the compressor, In the LNG BOG reliquefaction device for returning the BOG reliquefied by the condenser to the storage tank,
The nitrogen cycle apparatus includes a nitrogen pressure cooling unit that pressurizes and cools nitrogen, a nitrogen heat exchanger that cools high-pressure nitrogen gas that has passed through the nitrogen pressure cooling unit, and high-pressure nitrogen from the nitrogen heat exchanger. Expansion means for expanding the gas and generating nitrogen gas at a low temperature and low pressure,
The LNG BOG reliquefaction apparatus, wherein the compressor, the nitrogen heat exchanger, and the expansion means are configured as one module.

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