Description

LNG BOG RELIQUEFACTION APPARATUS

Technical Field

[1] The present invention relates to an apparatus for reliquefying boil-off gas

(hereinafter, referred to as "BOG") generated in a storage tank of a ship transporting cryogenic liquefied natural gas (hereinafter, referred to as "LNG"). Background Art

[2] In general, natural gas is transported in a liquefied state over long distance. For example, an LNG carrier is utilized for transporting LNG from a first location where natural gas is liquefied to a second location where LNG is vaporized and delivered to a gas distribution system. Since natural gas is liquefied at an extremely low temperature, i.e., a temperature of approximately -163 ⁰ C under normal pressure, external heat transferred to a storage tank of the LNG carrier continuously evaporates LNG in the storage tank. If pressure in the storage tank exceeds a preset safety pressure, boil-off gas from LNG is discharged to the outside via a safety valve. The discharged BOG is reliquefied and then returned to the storage tank or utilized as fuel for the ship.

[3] Since a conventional LNG carrier is propelled using a steam turbine, BOG is burned in a boiler to utilize BOG as fuel for propelling the ship. In recent, the quantity of BOG generated has been decreased due to the development of insulation technology for a storage tank, and thus, it is difficult to secure an amount of BOG required for a ship propelled by a steam turbine. Further, a ship propelled by a diesel engine with high efficiency has been preferred. Accordingly, there is a need for new technology for treating BOG and ensuring the stability of a storage tank.

[4] In a reliquef action apparatus, a refrigeration cycle is performed by the steps of compressing a working fluid by a plurality of compressors; cooling the compressed working fluid through indirect heat exchange; expanding the working fluid; heating the expanded working fluid through indirect heat exchange between the compressed working fluid and the expanded working fluid; and returning the heated working fluid to one of the compressors. After the compression step, LNG vapor is at least partially condensed through indirect heat exchange with the expanded working fluid. An example of an apparatus for carrying out such a cooling method is disclosed in U.S. Patent No. 3,857,245.

[5] According to U.S. Patent No. 3,857,245, a working fluid is induced from LNG itself, and thus, the apparatus is operated by an open refrigeration cycle. The fluid is expanded by a valve so that partially condensed LNG is obtained. The partially condensed LNG is divided into a liquid phase to be returned into a storage tank and a

vapor phase to be mixed with natural gas that is supplied to a combustion burner. Since the working fluid is heated and cooled in an identical heat exchanger, only one heat exchanger is employed. The heat exchanger is placed on a first skid-mounting platform and a compressor for compressing the working fluid is mounted to a second skid- mounting platform.

[6] In current, it is preferred that an incombustible gas be used as a working fluid.

Further, in order to alleviate compression of a working fluid supplied from the outside, an expansion turbine is more preferred to a valve for expanding a working fluid.

[7] One example of apparatuses that further improve these two advantages is disclosed in International Publication No. WO98/43029. In the apparatus disclosed in the publication, two heat exchangers are employed, wherein one of them partially condenses compressed natural gas vapor to heat a working fluid therein and the other cools a compressed working fluid. In addition, the working fluid is compressed by two different compressors, one of which is coupled with an expansion turbine. Although not disclosed in International Publication No. WO98/43029, such a conventional apparatus is installed in a board type ship such that the compressor connected to the expansion turbine and the heat exchanger are placed in a machinery room and the other compressor is placed in an engine room. An apparatus disclosed in International Publication No. WO2005/047761 also has a structure similar to that of the above apparatus and is characterized by pre-cooling of BOG.

[8] Reliquefaction methods of pre-cooling BOG are disclosed in U.S. Patent Nos.

4,843,829 and 4,843,862. Apparatuses for reliquefying BOG disclosed in these U.S. patents are constructed such that BOG supplied through a compressor passes through two heat exchangers to ultimately condense BOG. BOG is pre-cooled in the first heat exchanger and then ultimately condensed in the second heat exchanger.

[9] According to the apparatuses for reliquefying BOG disclosed in U.S. Patent Nos.

4,843,829 and 4,843,862, however, nitrogen compressed by passing through the compressor is divided into a first stream and a second stream. The first stream passes through an expansion valve and is used for reliquefaction of BOG and then returned to the compressor, whereas the second stream passes through an expansion turbine and is used for pre-cooling of BOG and then returned to the compressor. Accordingly, respective lines for circulating a plurality of nitrogen refrigerant streams should be constructed, and five or six paths should be provided in the heat exchanger. Due to this structure, there are problems in that the entire configuration of the reliquefaction apparatus is complicated and inefficient.

[10] Korean Patent Laid-Open Publication Nos. 2001-0088406 and 2001-0089142 relate to an apparatus for use in a board-type ship so as to reliquefy compressed vapor, wherein components of the apparatuses are manufactured into pre-assemblies.

Referring to Fig. 1 showing the reliquef action apparatus, a reliquef action process is carried out through a closed cycle. Here, a working fluid is compressed in one or more compressors 22, 24 and 25, cooled in a first heat exchanger 22, expanded in a turbine 28 and heated in a second heat exchanger 13. The compressed vapor is at least partially condensed. This apparatus comprises a first pre-assembly 10 including the second heat exchanger 13, and a second pre-assembly 20 including the first heat exchanger 22, the compressors 23, 24 and 25 and the expansion turbine 28. The pre-assemblies 10 and 20 are located on platforms 11 and 21, respectively.

[11] On the other hand, in connection with heat exchange between a working fluid and

BOG, Korean Patent Publication No. 1993-0008299, International Publication No. WO2005/71333 and the like are directed to stable condensation of BOG using three heat exchangers.

[12] Although these reliquefaction apparatuses have been improved in view of simplification of a structure, easy installation of an apparatus on a ship, reduction of heat loss and the like, there is still a need for improving such apparatuses.

[13] In particular, conventional technology utilizes a reverse Brayton cycle of nitrogen

(working fluid) in a nitrogen cycle device that is cold heat-generating equipment for reliquefaction of BOG. However, this conventional technology has a disadvantage in that if the quantity or temperature of BOG generated and the like are changed according to operating conditions of a ship, it is difficult to rapidly cope with the change of the conditions. Moreover, if the temperature of nitrogen is excessively lowered by an expansion turbine, the temperature of nitrogen reaches liquefaction temperature so that liquid droplets are generated and cause damage to blades of the turbine. Accordingly, there is a need for a new technique capable of sensitively coping with changes in the quantity or temperature of BOG generated. Disclosure of Invention Technical Problem

[14] Accordingly, the present invention is conceived to solve the problems in which conventional apparatuses cannot rapidly cope with changes in the quantity or temperature of BOG generated and the like according to operating conditions of a ship. An object of the present invention is to provide an apparatus for reliquefying LNG BOG, which can constantly maintain a temperature difference between BOG and nitrogen gas within a condenser in a preset range by pre-cooling BOG upstream of the condenser even though the quantity or temperature of BOG generated is changed, in a reliquefaction system for reliquefying BOG generated in a storage tank and flowing out through a safety valve and returning the liquefied BOG into the storage tank while an LNG carrier is plying.

[15] Another object of the present invention is to provide an apparatus for reliquefying

LNG BOG, which can stably maintain temperature and pressure in a storage tank and eliminate a loss of LNG under any conditions by freely lowering the temperature of nitrogen to a smaller value using an expansion valve as well as an expansion turbine, in a reliquefaction system for reliquefying BOG generated in the storage tank and flowing out through a safety valve and returning the liquefied BOG while an LNG carrier is plying.

[16] Moreover, a further object of the present invention is to provided an apparatus for reliquefying BOG, which is improved in view of simplification of a structure, easy installation thereof on a ship, reduction of heat loss, and the like. Technical Solution

[17] According to an aspect of the present invention for achieving the above objects, there is provided an apparatus for reliquefying boil-off gas (BOG) of liquefied natural gas (LNG), including a compressor for compressing BOG generated in a storage tank for LNG, a condenser for condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold heat to the compressor, so as to return BOG reliquefied by the condenser to the storage tank, wherein the nitrogen cycle device includes a nitrogen-pressurizing/cooling means for pressurizing and cooling nitrogen, a first nitrogen heat exchanger for cooling high-pressure nitrogen gas that has passed through the nitrogen-pressurizing/cooling means, and a first expansion means for expanding the high-pressure nitrogen gas discharged from the first nitrogen heat exchanger to generate cryogenic low-pressure nitrogen gas; wherein the cryogenic low-pressure nitrogen gas expanded in the first expansion means is deprived of cold heat in the condenser and is then returned to the nitrogen-pressurizing/cooling means via the first nitrogen heat exchanger; and wherein BOG compressed by the compressor is pre-cooled before it is condensed.

[18] According to another aspect of the present invention, there is provided an apparatus for reliquefying boil-off gas (BOG) of liquefied natural gas (LNG), including a compressor for compressing BOG generated in a storage tank for LNG, a condenser for condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold heat to the compressor, so as to return BOG reliquefied by the condenser to the storage tank, wherein the nitrogen cycle device includes a nitrogen- pressurizing/cooling means for pressurizing and cooling nitrogen, a first nitrogen heat exchanger for cooling high-pressure nitrogen gas that has passed through the nitrogen- pressurizing/cooling means, and a first expansion means for expanding the high- pressure nitrogen gas discharged from the first nitrogen heat exchanger to generate cryogenic low-pressure nitrogen gas; wherein the cryogenic low-pressure nitrogen gas

expanded in the first expansion means is deprived of cold heat in the condenser and is then returned to the nitrogen-pressurizing/cooling means via the first nitrogen heat exchanger; and wherein a portion of the high-pressure nitrogen gas supplied from the nitrogen-pressurizing/cooling means to the first nitrogen heat exchanger is expanded by a second expansion means and then cooled to be converted into low-temperature and low-pressure nitrogen gas, and the low-temperature and low-pressure nitrogen gas is added to low-pressure nitrogen gas from the condenser, introduced into the first nitrogen heat exchanger and then returned to the nitrogen-pressurizing/cooling means. [19] According to a further aspect of the present invention, there is provided An apparatus for reliquefying boil-off gas (BOG) of liquefied natural gas (LNG), including a compressor for compressing BOG generated in a storage tank for LNG, a condenser for condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold heat to the compressor, so as to return BOG reliquefied by the condenser to the storage tank, wherein the nitrogen cycle device includes a nitrogen-pressurizing/cooling means for pressurizing and cooling nitrogen, a nitrogen heat exchanger for cooling high-pressure nitrogen gas that has passed through the nitrogen-pressurizing/cooling means, and an expansion means for expanding the high- pressure nitrogen gas discharged from the nitrogen heat exchanger to generate cryogenic low-pressure nitrogen gas; and wherein the compressor, the nitrogen heat exchanger and the expansion means are formed into one module.

Advantageous Effects

[20] According to the present invention, in an apparatus for reliquefying BOG of LNG, including a compressor for compressing BOG generated in a storage tank for LNG, a condenser for at least partially condensing BOG compressed by the compressor, and a nitrogen cycle device for supplying cold heat to the compressor, so as to return BOG reliquefied by the condenser to the storage tank, BOG compressed by the compressor is subjected to pre-cooling upstream of the condenser, and thus, there is an advantage in that the temperature of BOG can be constantly maintained in a preset range even though the quantity or temperature of BOG generated is changed.

[21] Further, according to the present invention, it is possible to reduce the size of the apparatus for reliquefying BOG and stably manage a cryogenic range of nitrogen gas without a loss of cold heat by incorporating unit elements constituting a low temperature section into a cold box device in the apparatus for reliquefying BOG. Brief Description of the Drawings

[22] Fig. 1 is a schematic view of a conventional apparatus for reliquefying LNG BOG.

[23] Fig. 2 is a schematic view of an apparatus for reliquefying LNG BOG according to a first embodiment of the present invention.

[24] Figs. 3 and 4 are flowcharts illustrating a method of reliquefying LNG BOG according to the first embodiment of the present invention.

[25] Fig. 5 is a schematic view of an apparatus for reliquefying LNG BOG according to a second embodiment of the present invention.

[26] Figs. 6 and 7 are flowcharts illustrating a method of reliquefying LNG BOG according to the second embodiment of the present invention.

[27] Fig. 8 is a schematic view of an apparatus for reliquefying LNG BOG according to a third embodiment of the present invention.

[28] Figs. 9 and 10 are flowcharts illustrating a method of reliquefying LNG BOG according to the third embodiment of the present invention.

[29] Fig. 11 is a schematic view of an apparatus for reliquefying LNG BOG according to a fourth embodiment of the present invention.

[30] Figs. 12 and 13 are flowcharts illustrating a method of reliquefying LNG BOG according to the fourth embodiment of the present invention. Best Mode for Carrying Out the Invention

[31] Hereinafter, the present invention will be described in detail with reference to Figs.

2 to 13. The following description will be merely made in connection with preferred embodiments of the present invention. Temperatures and pressures described herein are represented only by way of example, and the present invention is not limited to these numerical values.

[32]

[33] (First embodiment)

[34] Hereinafter, an apparatus and method for reliquefying LNG BOG according to a first embodiment of the present invention will be described in detail with reference to Figs. 2 to 4.

[35] Fig. 2 is a view showing a configuration of the apparatus for reliquefying LNG

BOG according to the first embodiment of the present invention. This apparatus comprises a BOG cycle device, a nitrogen cycle device and a cold box device for interfacing these two devices.

[36]

[37] BOG cycle device

[38] Natural gas in a gaseous phase is liquefied and stored in a storage tank 110 in a cryogenic state under atmospheric pressure (1.013 bars). However, during transportation of LNG, BOG is generated due to continuous heat transfer from the outside, thereby increasing pressure in the storage tank 110.

[39] Accordingly, in order to constantly maintain the pressure in the storage tank 110 at a level of the atmospheric pressure, a safety valve 111 is opened and BOG is

discharged outside of the storage tank 110 if the pressure in the storage tank 110 reaches about 1.03 bars, and the discharged BOG passes through two-stage bog compressor 115 and 116 and is then subjected to a reliquef action process.

[40] It is preferred that the temperature of hot BOG discharged from the storage tank

110 be sensed by a temperature sensor 112 and be controlled to a predetermined temperature through a temperature controller 114, and BOG then flow into the two- stage bog compressor 115 and 116. BOG that has passed through the temperature controller 114 is maintained in a superheated vapor state at a pressure of 1.03 bars and a temperature of -12O ⁰ C.

[41] The operation of the temperature controller 114 will be described in greater detail.

If the cryogenic BOG (LNG) that has been subjected to the reliquefaction process is re- circulated and mixed with hot BOG discharged from the storage tank 110, the temperature of BOG can be controlled. The amount of BOG re-circulated is controlled by adjusting the degree of opening of a recirculation valve 113. Further, it is possible to supply LNG in the storage tank 110 as well as the reliquefied BOG to the temperature controller 114 through a pump (not shown). Moreover, LNG in the storage tank 110 may be supplied to be mixed with BOG compressed by the BOG compressors 115 and 116, thereby lowering or controlling the temperature of BOG to be supplied into a BOG condenser 131. Typically, the temperature controller is operated only until the reliquefaction apparatus reaches a normal state, the recirculation valve 113 is then closed and the cryogenic reliquefied LNG is guided to the storage tank 110.

[42] When BOG that has passed through the temperature controller 114 passes through the two-stage bog compressor 115 and 116 driven by a motor M, BOG is discharged in a superheated vapor state at a pressure of 2.5 bars and a temperature of -73 ⁰ C.

[43] After BOG discharged from the two-stage bog compressor 115 and 116 passes through the BOG condenser 131, BOG is in a supercooled liquid state at a pressure of 2.3 bars and a temperature of -155°Cand then flows back into the storage tank 110 or is re-circulated to the temperature controller 114. The BOG condenser 131 will be described in detail below in connection with the cold box device.

[44] Although all BOG may be liquefied through the BOG condenser 131, about 70 to

99% of BOG is liquefied due to influence by a nitrogen component of which perfect liquefaction cannot be easily performed.

[45] Moreover, according to a modification of this embodiment, condensed BOG is supplied through spraying to pressurized BOG that has passed through the two-stage bog compressor 115 and 116, thereby controlling the temperature of the pressurized BOG to be supplied to the BOG condenser.

[46] The condensed BOG flows back into the storage tank 110 by a circulation pump

119. Methods of causing BOG to flow back into the storage tank 110 include a method

of spaying BOG through a spray head above the storage tank, a method of supplying BOG to the bottom of the storage tank, and the like. If BOG flows back to the bottom of the storage tank, a nitrogen component that is contained in non-condensed gas included in the condensed BOG is dissolved in LNG so that the ratio of nitrogen in the gaseous phase is maintained at a lower value. Since the liquefaction temperature of nitrogen is lower than that of methane that is a main component of LNG, an increase in the content of nitrogen in BOG can reduce a load applied to the two- stage bog compressor 115 and 116 or the BOG condenser 131.

[47]

[48] Nitrogen cycle device

[49] BOG is reliquefied in the BOG condenser 131 by means of heat exchange between

BOG and cryogenic nitrogen gas that is a working fluid. The following description is made in connection with a cycle device for obtaining cryogenic nitrogen gas required for the reliquef action of BOG.

[50] After nitrogen gas at a pressure of 10 bars and a temperature of 4O ⁰ C passes through three-stage compressor 121, 122 and 123 and intermediate coolers 125, 126 and 127, the pressure of the nitrogen gas is increased so that the nitrogen gas can be discharged at a pressure of 59 bars and a temperature of 43 ⁰ C. After the nitrogen gas is transferred to a cold box device, the nitrogen gas is subjected to inner heat exchange with nitrogen, which has been used for condensing BOG in the BOG condenser and has a slightly increased temperature, within a nitrogen heat exchanger 133, so that the nitrogen gas is pre-cooled to be in a gaseous state at a pressure of 59 bars and a temperature of -105 ⁰ C.

[51] The intermediate coolers 125, 126 and 127 are to cool the nitrogen refrigerant, which serves as the working fluid, in a compression step. Although conventional methods known from the prior arts disclosed in the "Background Art" herein may be used, it is desirable to use a cooling method using seawater as a refrigerant.

[52] The low-temperature and high-pressure nitrogen gas that has passed through the nitrogen heat exchanger 133 passes through an expansion turbine 136 to be converted into gas with an extremely low temperature of -167 ⁰ C and a low pressure of 10.5 bars. The converted nitrogen gas is moved to the BOG condenser 131 and performs a BOG reliquef action process so that it is in a gaseous state at a pressure of 10.3 bars and a temperature of -134 ⁰ C. Thereafter, the nitrogen gas is subjected to inner heat exchange with high-temperature nitrogen in the nitrogen heat exchanger 133 and reaches 10 bars and 4O ⁰ C, thereby completing the cycle.

[53] A nitrogen buffer tank 120 performs the function of adjusting a mass flow rate in the nitrogen cycle in response to changes in the reliquefied amount of BOG, i.e., changes in a cooling load of the nitrogen cycle. A source for supplying a sup-

plementary working fluid (nitrogen) may be further provided against a case where the amount of nitrogen is reduced.

[54]

[55] Cold box device

[56] The cold box device 130 comprises the BOG condenser 131 in which the reliq- uef action of BOG is performed, the nitrogen heat exchanger 133 in which inner heat exchange between a high temperature section and a lower temperature section of the nitrogen cycle is carried out, and the expansion turbine 136 in which cryogenic nitrogen gas is obtained.

[57] A generator G that is not an essential element of the present invention may be coupled to the expansion turbine 136 to generate electric power that in tune is used as an auxiliary power source for the BOG compressors 115 and 116, the nitrogen compressors 121, 122 and 123, or the like.

[58] The aforementioned devices may be incorporated into the cold box 130 as one module, so that pipes for connecting the devices can be shortened. This enables cryogenic nitrogen required for the reliquefaction of BOG to be secured stably. That is, since the length of a connecting pipe between the nitrogen heat exchanger 133 and an inlet of the expansion turbine 136 is shortened, a cryogenic condition of an outlet of the expansion turbine that may be sensitive to the condition or temperature of an inlet of the expansion turbine 136 can be stabilized. Further, since the length of a connecting pipe between the BOG condenser and the outlet of the expansion turbine is shortened, it is possible to expect that an increase in the temperature of the nitrogen gas caused by the transfer of the nitrogen gas is minimized.

[59] Moreover, in the configuration of the apparatus for reliquefying LNG BOG according to the present invention, it is preferred that the BOG condenser 131 which is at low temperature, the nitrogen heat exchanger 133 and the expansion turbine 136 be constructed into one cold box device 130 and these components be insulated as one module. The insulation is achieved by generally known insulating materials. Due to this configuration, a cryogenic range of the nitrogen gas can be stably managed. Further, the cold box device may be manufactured as a pre-assembly to facilitate mounting thereof on a ship.

[60] As described above, since the cryogenic section of the nitrogen cycle is stably managed, it is possible to operate the BOG cycle at a reliquefaction temperature of - 155 ⁰ C and up to a temperature of -73 ⁰ C and a temperature difference between an inlet and an outlet of the BOG condenser 131 can be reduced below 8O ⁰ C that is lower than that in a conventional case. Accordingly, it is possible to design a simple BOG condenser that does not require pre-cooling.

[61] Next, the operation of the reliquefaction apparatus according to the first

embodiment constructed as above will be described with reference to Figs. 3 and 4.

[62] In a method of reliquefying BOG generated in a cryogenic storage tank of an LNG carrier due to external heat transfer, BOG is circulated through a process comprising the steps of passing BOG, which is discharged after a valve is opened at a predetermined pressure, through the temperature controller and constantly maintaining BOG at a pressure of 1.03 bars and a temperature of -12O ⁰ C at the inlet of the two- stage bog compressor 115 and 116 (ST 111); compressing BOG in the two-stage compressor 115 and 116 to a pressure of 2.5 bars and a temperature of -12O ⁰ C so that BOG is in a high temperature and pressure state (ST 112); reliquefying BOG discharged from the BOG compressors 115 and 116 into supercooled liquid with a pressure of 2.3 bars and a temperature of -155°Cin the BOG condenser 131 (ST 113); pressurizing the reliquefied BOG by the circulation pump (ST 114); re-circulating a portion of the pressurized and reliquefied BOG through the temperature controller (ST 115); and recovering and storing the remainder of the pressurized and reliquefied BOG in the storage tank (ST 116). The circulation of nitrogen gas for supplying cold heat required for the reliquef action of BOG comprises the steps of passing nitrogen gas with a temperature of 4O ⁰ C and a pressure of 10 bars through the three-stage compressor 121, 122 and 123 and the intermediate coolers 125, 126 and 127 to increase the pressure of the nitrogen gas to a pressure of 59 bars at a temperature of 43 ⁰ C(ST 117); heat-exchanging the high-pressure nitrogen gas with nitrogen gas in the low-temperature section of the nitrogen heat exchanger to convert the high-pressure nitrogen gas into nitrogen gas with a low temperature of -105 ⁰ C and a pressure of 59 bars (ST 118); converting the high-pressure nitrogen gas with a low temperature of - 167 ⁰ C and a low pressure of 10.5 bars by passing the high-pressure nitrogen through the expansion turbine (ST 119); performing reliquefaction of BOG by the low- temperature and low-pressure nitrogen in the BOG condenser 131 so that the low- temperature and low-pressure nitrogen is converted into nitrogen gas with a pressure of 10.3 bars and a temperature of -134 ⁰ C (ST 120); and passing again the nitrogen gas through the nitrogen heat exchanger 133 so that the nitrogen gas is converted into nitrogen gas with a pressure of 10 bars and a high temperature of 4O ⁰ C (ST 121).

[63] Although the pressure, the temperature and the like are indicated by the specific numerals in the respective steps, it will be apparent that the pressure and temperature may be changed according to the amount of BOG, a control method and the like.

[64] According to the first embodiment of the present invention constructed as above, the following effects can be obtained.

[65] With the first embodiment of the present invention constructed as above, the pressure of a storage tank can be stably maintained without a loss of stored LNG during a voyage of an LNG carrier. In particular, the size of the apparatus for

reliquefying LNG BOG can be reduced by employing the simple cold box module and the cryogenic range of the nitrogen gas can be stably managed.

[66] Specifically, since the length of the connecting pipe between the nitrogen heat exchanger 133 and the inlet of the expansion turbine 136 is shortened, the cryogenic condition of the outlet of the expansion turbine which is sensitive to the condition or temperature of the inlet of the expansion turbine 136 can be stabilized. Further, since the length of the connecting pipe between the BOG condenser 131 and the outlet of the expansion turbine 136 is shortened, it is possible to minimize an increase in temperature produced upon delivery of the cryogenic nitrogen gas, and it is possible to prevent a loss of cold heat by incorporating the unit elements constituting the low temperature section into the cold box device in the apparatus for reliquefying LNG BOG and managing the cold box device.

[67]

[68] (Second embodiment)

[69] Hereinafter, an apparatus and method for reliquefying LNG BOG according to a second embodiment of the present invention will be described in detail with reference to Figs. 5 to 7.

[70] Fig. 5 is a view showing a configuration of the apparatus for reliquefying LNG

BOG according to the second embodiment of the present invention. This apparatus comprises a BOG cycle device, a nitrogen cycle device and a cold box device interfacing these two devices.

[71]

[72] BOG cycle device

[73] Natural gas in a gaseous phase is liquefied and stored in a storage tank 210 in a cryogenic state under atmospheric pressure (1.013 bars). However, during transportation of LNG, BOG is generated due to continuous heat transfer from the outside, thereby increasing pressure in the storage tank 210.

[74] Accordingly, in order to constantly maintain the pressure in the storage tank 210 at a level of the atmospheric pressure, a safety valve 211 is opened and BOG is discharged outside of the storage tank 210 if the pressure in the storage tank 210 reaches about 1.03 bars, and the discharged BOG passes through two-stage bog compressor 215 and 216 and is then subjected to a reliquef action process.

[75] It is preferred that the temperature of hot BOG discharged from the storage tank

210 be sensed by a temperature sensor 212 and be controlled to a predetermined temperature through a temperature controller 214, and BOG then flow into the two- stage bog compressor 215 and 216. BOG that has passed through the temperature controller 214 is maintained in a superheated vapor state at a pressure of 1.03 bars and a temperature of -12O ⁰ C.

[76] The operation of the temperature controller 214 will be described in greater detail.

If the cryogenic BOG (LNG) that has been subjected to the reliquefaction process is re- circulated and mixed with hot BOG discharged from the storage tank 210, the temperature of BOG can be controlled. The amount of BOG re-circulated is controlled by adjusting the degree of opening of a recirculation valve 213. Further, it is possible to supply LNG in the storage tank 210 as well as the reliquefied BOG to the temperature controller 214 through a pump (not shown). Moreover, LNG in the storage tank 210 may be supplied to be mixed with BOG compressed by the BOG compressors 215 and 216, thereby lowering or controlling the temperature of BOG to be supplied into a BOG condenser 231. Typically, the temperature controller is operated only until the reliquefaction apparatus reaches a normal state, the recirculation valve 213 is then closed and the cryogenic reliquefied LNG is guided to the storage tank 210.

[77] When BOG that has passed through the temperature controller 214 passes through the two-stage bog compressor 215 and 216 driven by a motor M, BOG is discharged in a superheated vapor state at a pressure of 2.5 bars and a temperature of -73 ⁰ C.

[78] It is preferred that BOG discharged from the two-stage bog compressor 215 and 216 be liquefied in the BOG condenser 231 and then mixed again with non-condensed components separated in a BOG non-condensed gas separator 218 so as to lower the temperature of superheated vapor.

[79] A conventional separator generally known in the prior arts disclosed in

"Background Art" herein may be utilized as the non-condensed gas separator 218. At this time, when a venture-shaped nozzle 239 is preferably employed, the flow speed of BOG is increased and a resulting pressure drop is used to introduce the non-condensed components from the BOG non-condensed gas separator 218.

[80] After BOG passes the BOG condenser 231 and is then converted to be in a supercooled liquid state at a pressure of 2.3 bars and a temperature of -155 ⁰ C, it flows back into the storage tank 210 or is re-circulated to the temperature controller 214. The BOG condenser 231 will be described in detail below in connection with the cold box device.

[81] Although all BOG may be liquefied through the BOG condenser 231 , about 70 to

99% of BOG is liquefied due to influence by a nitrogen component of which perfect liquefaction cannot be easily performed.

[82] The condensed BOG flows back into the storage tank 210 by a circulation pump

219. Methods of causing BOG to flow back into the storage tank 210 include a method of spaying BOG through a spray head above the storage tank, a method of supplying BOG to the bottom of the storage tank, and the like. If BOG flows back to the bottom of the storage tank, a nitrogen component that is contained in non-condensed gas included in the condensed BOG is dissolved in LNG so that the ratio of nitrogen in the

gaseous phase is maintained at a lower value. Since the liquefaction temperature of nitrogen is lower than that of methane that is a main component of LNG, an increase in the content of nitrogen in BOG can reduce a load applied to the two- stage bog compressor 215 and 216 or the BOG condenser 231.

[83]

[84] Nitrogen cycle device

[85] BOG is reliquefied in the BOG condenser 231 by means of heat exchange between

BOG and cryogenic nitrogen gas that is a working fluid. The following description is made in connection with a cycle device for obtaining cryogenic nitrogen gas required for the reliquef action of BOG.

[86] After nitrogen gas at a pressure of 14 bars and a temperature of 35.4 ⁰ C passes through three-stage compressor 221, 222 and 223 and intermediate coolers 225, 226 and 227, the pressure of the nitrogen gas is increased so that the nitrogen gas can be discharged at a pressure of 58 bars and a temperature of 43 ⁰ C.

[87] The intermediate coolers 225, 226 and 227 are to cool the nitrogen refrigerant, which serves as the working fluid, in a compression step. Although conventional methods known from the prior arts disclosed in the "Background Art" herein may be used, it is desirable to use a cooling method using seawater as a refrigerant.

[88] After the discharged high-pressure nitrogen gas is transferred to the cold box device

230, the nitrogen gas is primarily cooled to a temperature of -83.5 ⁰ C through inner heat exchange with low-temperature nitrogen, which has passed through a first nitrogen heat exchanger 233 from the BOG condenser 231 and still has cold heat, in a second nitrogen heat exchanger 234.

[89] A portion of the high-pressure nitrogen gas that has passed through the second nitrogen heat exchanger 234 has reduced temperature while passing through an expansion turbine 236, and the low-pressure nitrogen gas of which the temperature is lowered by the expansion turbine 236 is subjected to inner heat exchange with the high-pressure nitrogen, which has passed through the second nitrogen heat exchanger 234, in the first heat exchanger 233. This contributes to reduction in the temperature of the high-pressure nitrogen to a preset temperature at an inlet of an expansion valve 237.

[90] The high-pressure nitrogen gas of which the temperature is lowered in the first nitrogen heat exchanger 233 passes through the expansion valve 237 and ultimately converted into cryogenic nitrogen gas with a temperature of -163 ⁰ C required for reliq- uefaction of BOG. This cryogenic nitrogen gas is heat-exchanged with BOG in the BOG condenser 231 to reliquefy BOG and accordingly has increased temperature.

[91] The reason why a portion of the high-pressure nitrogen gas is converted through the expansion turbine 236 into the low-temperature nitrogen gas which in turn is

introduced into the first nitrogen heat exchanger 233 is to assist in cooling the high- pressure nitrogen gas to a temperature required at the inlet of the expansion valve 237 using the portion of the nitrogen gas of which the temperature has been already reduced through the expansion turbine 236 since the low-pressure nitrogen gas returned from the BOG condenser 231 has low specific heat and thus is insufficient to cool the high-pressure nitrogen gas.

[92] Thereafter, remaining cold heat cools the high-pressure nitrogen gas in the second heat exchanger 234, and then, it is subjected to an increase in temperature and reaches 10 bars and 4O ⁰ C, thereby completing the cycle.

[93] A nitrogen buffer tank 230 performs the function of adjusting a mass flow rate in the nitrogen cycle in response to changes in the reliquefied amount of BOG, i.e., changes in a cooling load of the nitrogen cycle. A source for supplying a supplementary working fluid (nitrogen) may be further provided against a case where the amount of nitrogen is reduced.

[94]

[95] Cold box device.

[96] The cold box device 230 comprises the BOG condenser 231 in which the reliq- uefaction of BOG is performed, the first and second nitrogen heat exchangers 233 and 234 in which inner heat exchange between a high temperature section and a lower temperature section of the nitrogen cycle is carried out, the expansion valve 237 in which cryogenic nitrogen gas is obtained, and the expansion turbine 236 in which low- temperature nitrogen gas required for inner heat exchange is obtained.

[97] A generator G that is not an essential element of the present invention may be coupled to the expansion turbine 236 to generate electric power that in tune is used as an auxiliary power source for the BOG compressors 215 and 216, the nitrogen compressors 221, 222 and 223, or the like.

[98] The aforementioned devices may be incorporated into the cold box 230 as one module, so that pipes for connecting the devices can be shortened. This enables cryogenic nitrogen required for the reliquefaction of BOG to be secured stably. That is, since the length of a connecting pipe between the second nitrogen heat exchanger 234 and the inlet of the expansion turbine 236 or the length of a connecting pipe between the first nitrogen heat exchanger 233 and the inlet of the expansion valve 237 is shortened, a cryogenic condition of nitrogen at the outlet of the expansion turbine 236 or expansion valve 237 that may be sensitive to the condition or temperature of nitrogen at the inlet of the expansion turbine 236 or expansion valve 237 can be stabilized.

[99] Further, since the length of a connecting pipe between the BOG condenser 231 and the outlet of the expansion valve 237 is shortened, it is possible to expect that an

increase in the temperature of the nitrogen gas caused by the transfer of the nitrogen gas is minimized.

[100] Moreover, in the configuration of the apparatus for reliquefying LNG BOG according to the present invention, it is preferred that the BOG condenser 231 which is at low temperature, the nitrogen heat exchangers 233 and 234, and the expansion means 236 and 237 be constructed into one cold box device 230 and these components be insulated as one module. The insulation is achieved by generally known insulating materials.

[101] Due to this configuration, a cryogenic range of the nitrogen gas can be stably managed. Further, the cold box device may be manufactured as a pre-assembly to facilitate mounting thereof on a ship.

[102] As described above, since the cryogenic section of the nitrogen cycle is stably managed, it is possible to operate the BOG cycle at a reliquefaction temperature of - 155 ⁰ C and up to a temperature of -73 ⁰ C and a temperature difference between an inlet and an outlet of the BOG condenser 231 can be reduced below 8O ⁰ C that is lower than that in a conventional case. Accordingly, it is possible to design a simple BOG condenser that does not require pre-cooling.

[103]

[104] Next, the operation of the reliquefaction apparatus according to the second embodiment constructed as above will be described with reference to Figs. 6 and 7.

[105] The process of the reliquefaction apparatus according to the second embodiment comprises the steps of compressing BOG generated in the storage tank for LNG; pressurizing and cooling nitrogen gas which is a working fluid, so as to provide cold heat used for condensing the compressed BOG and expanding the pressurized and cooled nitrogen gas to generate cryogenic nitrogen gas; at least partially condensing the compressed BOG through heat exchange with the cryogenic nitrogen gas; and returning BOG reliquefied by means of the condensation to the storage tank. However, all the pressurized and cooled nitrogen gas is not expanded, but a portion of the pressurized and cooled nitrogen gas is extracted and expanded through a different path to generate low-temperature gas. Then, the low-temperature gas is added to nitrogen gas which has been heat-exchanged for condensing the compressed BOG and has slightly increased temperature, and is then heat-exchanged with the pressurized and cooled nitrogen gas prior to expansion thereof so that it can be pre-cooled.

[106] Referring to Figs. 6 and 7, in a method of reliquefying BOG generated in a cryogenic storage tank of an LNG carrier due to external heat transfer, BOG is circulated through a process comprising the steps of passing BOG, which is discharged after a valve 211 is opened at a predetermined pressure, through the temperature controller 214 and constantly maintaining BOG at a pressure of 1.03 bars and a

temperature of -12O ⁰ C at the inlet of the two-stage bog compressor 215 and 216 (ST 211); compressing BOG in the two-stage compressor 215 and 216 to a pressure of 2.5 bars and a temperature of -73 ⁰ C so that BOG is in a high temperature and pressure superheated state (ST 212); sucking gas separated in the BOG non-condensed gas separator 218 due to pressure difference in the venturi nozzle 239 so that the temperature of the gas is lowered (ST 213); reliquefying BOG discharged from the BOG compressors 215 and 216 into supercooled liquid with a pressure of 2.3 bars and a temperature of -155°Cin the BOG condenser 231 (ST 214); separating non- condensed gas contained in the reliquefied BOG (ST 215); pressurizing the reliquefied BOG by the circulation pump 219 (ST 216); re-circulating a portion of the pressurized and reliquefied BOG through the temperature controller 214 (ST 217); and recovering and storing the remainder of the pressurized and reliquefied BOG in the storage tank 210 (ST 218). Further, the circulation of nitrogen gas for supplying cold heat required for the reliquefaction of BOG comprises the steps of passing nitrogen gas with a pressure of 14 bars and a temperature of 35.4 ⁰ C through the three-stage compressor 221, 222 and 223 and the intermediate coolers 225, 226 and 227 to increase the pressure of the nitrogen gas to a pressure of 58 bars at a temperature of 43 ⁰ C (ST 219); internally heat-exchanging the high-pressure nitrogen gas with nitrogen gas in the low- temperature section of the second nitrogen heat exchanger 234 to convert the high- pressure nitrogen gas into nitrogen gas with a low temperature of -83.5 ⁰ C and a pressure of 57.3 bars (ST 220); converting a portion of the high-pressure nitrogen gas into low temperature and pressure gas with a low temperature of -14O ⁰ C and a low pressure of 14.5 bars by passing the portion of the high-pressure nitrogen through the expansion turbine 236 (ST 221); converting nitrogen gas, which has not been transferred to the expansion turbine 236, into nitrogen with a pressure of 57.5 bars and a temperature of -137.9 ⁰ C through the first nitrogen heat exchanger 233 (ST 222); converting the high-pressure nitrogen gas into gas with a reduced temperature of - 163°Cand a reduced pressure of 14.6 bars by passing the high-pressure nitrogen through the expansion valve 237 (ST 223); performing reliquefaction of BOG by the low-temperature and low-pressure nitrogen in the BOG condenser 231 so that the low- temperature and low-pressure nitrogen is converted into nitrogen gas with a pressure of 14.4 bars and a temperature of -138.9 ⁰ C (ST 224); adding the nitrogen gas to the nitrogen of which the temperature is lowered by the expansion turbine 236 and passing the nitrogen gas through the first nitrogen heat exchanger 233 to be converted into nitrogen gas with a pressure of 14.2 bars and a temperature of -106 ⁰ C (ST 225); and passing the nitrogen gas through the second nitrogen heat exchanger 234 to be converted into high-temperature nitrogen gas with a pressure of 14 bars and a temperature of 35.4 ⁰ C (ST 226).

[107] Although the pressure, the temperature and the like are indicated by the specific numerals in the respective steps, it will be apparent that the pressure and temperature may be changed according to the amount of BOG, a control method and the like.

[108] According to the second embodiment of the present invention constructed as above, the following effects can be obtained.

[109] With the second embodiment of the present invention constructed as above, it is possible to solve a problem in a conventional BOG reliquefaction system utilizing a reverse Brayton cycle of nitrogen, in which if the temperature of nitrogen is lowered using an expansion turbine, a portion of nitrogen is liquefied, causing damage to blades of the expansion turbine. In the method of this embodiment, only a portion of pressurized nitrogen is extracted and expanded through the expansion turbine 236, the temperature of the remainder of nitrogen is lowered and is then further lowered through the expansion valve 237 so that the temperature of nitrogen can be lowered without a risk of generation of liquid droplets. Moreover, the operation of a nitrogen cycle can be changed variously by changing a flow rate of nitrogen supplied to the expansion valve 237, and thus, it is possible to easily and rapidly cope with dynamic changes of BOG, i.e., changes of load.

[110] With the second embodiment of the present invention constructed as above, the pressure of a storage tank can be stably maintained without a loss of stored LNG during a voyage of an LNG carrier. In particular, the size of the apparatus for reliquefying LNG BOG can be reduced by employing the simple cold box module and the cryogenic range of the nitrogen gas can be stably managed.

[I l l] Specifically, since the length of the connecting pipe between the nitrogen heat exchanger 233 and the inlet of the expansion valve 237 is shortened, the cryogenic condition of the outlet of the expansion valve which is sensitive to the condition or temperature of the inlet of the expansion valve 237 can be stabilized. Further, since the length of the connecting pipe between the BOG condenser 231 and the outlet of the expansion valve 237 is shortened, it is possible to minimize an increase in temperature produced upon delivery of the cryogenic nitrogen gas, and it is possible to prevent a loss of cold heat by incorporating the unit elements constituting the low temperature section into the cold box device 230 in the apparatus for reliquefying LNG BOG and managing the cold box device.

[112]

[113] (Third embodiment)

[114] Hereinafter, an apparatus and method for reliquefying LNG BOG according to a third embodiment of the present invention will be described in detail with reference to Figs. 8 to 10.

[115] Fig. 8 is a view showing a configuration of the apparatus for reliquefying LNG

BOG according to the third embodiment of the present invention. This apparatus comprises a BOG cycle device, a nitrogen cycle device and a cold box device interfacing these two devices.

[116]

[117] BOG cycle device

[118] Natural gas in a gaseous phase is cryogenically liquefied and stored in a storage tank 310 under an atmospheric pressure (1.013 bars). However, during transportation of LNG, BOG is generated due to continuous heat transfer from the outside, thereby increasing pressure in the storage tank 310.

[119] Accordingly, in order to constantly maintain the pressure in the storage tank 310 at a level of the atmospheric pressure, a safety valve 311 is opened and BOG is discharged outside of the storage tank 310 if the pressure in the storage tank 310 reaches about 1.03 bars, and the discharged BOG passes through two-stage bog compressor 315 and 316 and is then subjected to a reliquef action process.

[120] It is preferred that the temperature of hot BOG discharged from the storage tank

310 be sensed by a temperature sensor 312 and be controlled to a predetermined temperature through a temperature controller 314, and BOG then flow into the two- stage bog compressor 315 and 316. BOG that has passed through the temperature controller 314 is maintained in a superheated vapor state at a pressure of 1.03 bars and a temperature of -12O ⁰ C.

[121] The operation of the temperature controller 314 will be described in greater detail.

If the cryogenic BOG (LNG) that has been subjected to the reliquefaction process is re- circulated and mixed with hot BOG discharged from the storage tank 310, the temperature of BOG can be controlled. The amount of BOG re-circulated is controlled by adjusting the degree of opening of a recirculation valve 313. Further, it is possible to supply LNG in the storage tank as well as the reliquefied BOG to the temperature controller 314 through a pump (not shown). Moreover, LNG in the storage tank 310 may be supplied to be mixed with BOG compressed by the BOG compressors 315 and 316, thereby lowering or controlling the temperature of BOG to be supplied into a BOG condenser 331. Typically, the temperature controller is operated only until the reliquefaction apparatus reaches a normal state, the recirculation valve 313 is then closed and the cryogenic reliquefied LNG is guided to the storage tank 310.

[122] When BOG that has passed through the temperature controller 314 passes through the two-stage bog compressor 315 and 316, BOG is discharged in a superheated vapor state at a pressure of 3.6 bars and a temperature of -43 ⁰ C.

[123] BOG discharged from the two-stage bog compressor 315 and 316 is pre-cooled while passing through the first nitrogen heat exchanger 333 with three paths formed therein, so that BOG is in a superheated vapor state at a pressure of 3.3 bars and a

temperature of -134 ⁰ C. At this time, in the process of pre-cooling BOG, BOG is cooled in the first nitrogen heat exchanger 333 through heat exchange with nitrogen gas that is obtained by mixing low-pressure and low-temperature nitrogen gas discharged from the BOG condenser 331 to be described below with nitrogen gas which is in a cryogenic state by the expansion turbine 336.

[124] After the pre-cooled BOG passes through the BOG condenser 331 and is then converted to be in a supercooled liquid state at a pressure of 3.0 bars and a temperature of -154.7 ⁰ C, it flows back into the storage tank 310 or is re-circulated to the temperature controller 314. The BOG condenser 331 will be described in detail below in connection with the cold box device.

[125] Since BOG is pre-cooled by the low-temperature working fluid (nitrogen) as above, there is an advantage in that even though the quantity or temperature of BOG generated is changed, a temperature difference between BOG and nitrogen gas in the BOG condenser 331 can be constantly maintained in a pre-set range.

[126] Further, if BOG in the superheated vapor state discharged from the two-stage bog compressor 315 and 316 is introduced into the BOG condenser, a temperature difference between streams in the BOG condenser is increased, resulting in a problem with the durability of the heat exchanger due to thermal stress. Accordingly, the process of pre-cooling BOG is advantageous to reduction in a temperature difference between the streams in the BOG condenser.

[127] Although all BOG may be liquefied through the BOG condenser 331, about 70 to

99% of BOG is liquefied due to influence by a nitrogen component of which perfect liquefaction cannot be easily performed.

[128] Such a mixture of gas and liquid is divided into gas and liquid in a BOG non- condensed gas separator 318, so that the liquid (condensed BOG) flows back into the storage tank 310 or is re-circulated to the temperature controller 314 by a circulation pump 319 and gas is generally discharged to the outside.

[129] Methods of causing BOG to flow back into the storage tank 310 include a method of spaying BOG through a spray head above the storage tank, a method of supplying BOG to the bottom of the storage tank, and the like. If BOG flows back to the bottom of the storage tank, a nitrogen component that is contained in non-condensed gas included in the condensed BOG is dissolved in LNG so that the ratio of nitrogen in the gaseous phase is maintained at a lower value. Since the liquefaction temperature of nitrogen is lower than that of methane that is a main component of LNG, an increase in the content of nitrogen in BOG can reduce a load applied to the two- stage bog compressor 315 and 316 or the BOG condenser 331.

[130]

[131] Nitrogen cycle device

[132] BOG is reliquefied in the BOG condenser 331 by means of heat exchange between

BOG and cryogenic nitrogen gas. The following description is made in connection with a cycle device for obtaining cryogenic nitrogen gas required for the reliquefaction of BOG.

[133] After nitrogen gas at a pressure of 14.3 bars and a temperature of 36.08 ⁰ C passes through three-stage compressor 321, 322 and 323 and intermediate coolers 325, 326 and 327, the pressure of the nitrogen gas is increased so that the nitrogen gas can be discharged at a pressure of 58 bars and a temperature of 43 ⁰ C. After the discharged high-pressure nitrogen gas is transferred to the cold box device 330, the nitrogen gas is cooled to a temperature of -70.5 ⁰ C at a pressure 57.7 bars by means of inner heat exchange with low-pressure nitrogen in a low temperature section, which is returned while having passed through the BOG condenser 331 and a first nitrogen heat exchanger 333, in a second nitrogen heat exchanger 334.

[134] A portion of the cooled nitrogen gas that has passed through the second nitrogen heat exchanger 334 has reduced temperature while passing through an expansion turbine 336, and the nitrogen gas of which the temperature is lowered is mixed with nitrogen which is returned while having passed through the BOG condenser 331. The resulting nitrogen gas is introduced into the first nitrogen heat exchanger 333. At this time, it is preferred that the high-pressure nitrogen gas be further cooled through inner heat exchange with high-pressure nitrogen gas, which is partially cooled in the second nitrogen heat exchanger 334, in the first nitrogen heat exchanger 333. With such a cooling process, there is an advantage in that the temperature of the high-pressure nitrogen is lowered to a preset temperature at an inlet of an expansion valve 337.

[135] The high-pressure nitrogen gas of which the temperature is lowered in the first nitrogen heat exchanger 333 passes through the expansion valve 337 and is ultimately converted into cryogenic nitrogen gas with a temperature of 62.6 ⁰ C required for reliquefaction of BOG. This cryogenic nitrogen gas is heat-exchanged with BOG in the BOG condenser 331 to liquefy BOG and accordingly has increased temperature.

[136] The reason why a portion of the high-pressure nitrogen gas is converted through the expansion turbine 336 into the low-temperature nitrogen gas which in turn is introduced into the first nitrogen heat exchanger 333 is to assist in cooling the high- pressure nitrogen gas to a temperature required at the inlet of the expansion valve 337 using the portion of the nitrogen gas of which the temperature has been already reduced through the expansion turbine 336 since the low-pressure nitrogen gas returned from the BOG condenser 331 has low specific heat and thus is insufficient to cool the high-pressure nitrogen gas. Thereafter, remaining cold heat cools the high- pressure nitrogen gas in the second heat exchanger 334, and then, it is subjected to an increase in temperature and reaches 14.3 bars and 36.08 ⁰ C, thereby completing the

nitrogen cycle.

[137] A nitrogen buffer tank 320 performs the function of adjusting a mass flow rate in the nitrogen cycle in response to changes in the reliquefied amount of BOG, i.e., changes in a cooling load of the nitrogen cycle. A source for supplying a supplementary working fluid (nitrogen) may be further provided against a case where the amount of nitrogen is reduced.

[138]

[139] Cold box device

[140] The cold box device 330 comprises the BOG condenser 331 in which the reliq- uefaction of BOG is performed, the nitrogen heat exchangers 333 and 334 in which inner heat exchange between a high temperature section and a lower temperature section of the nitrogen cycle is carried out and BOG is pre-cooled, the expansion valve 337 in which cryogenic nitrogen gas is obtained, and the expansion turbine 336 in which low-temperature nitrogen gas required for inner heat exchange is obtained.

[141] A generator G that is not an essential element of the present invention may be coupled to the expansion turbine 336 to generate electric power that in tune is used as an auxiliary power source for the BOG compressors 315 and 316, the nitrogen compressors 321, 322 and 323, or the like.

[142] The aforementioned devices may be incorporated into the cold box 330 as one module, so that pipes for connecting the devices can be shortened. This enables cryogenic nitrogen required for the reliquefaction of BOG to be secured stably. That is, since the length of a connecting pipe between the second nitrogen heat exchanger 334 and the inlet of the expansion turbine 336 or the length of a connecting pipe between the first nitrogen heat exchanger 333 and the inlet of the expansion valve 337 is shortened, a cryogenic condition of nitrogen at the outlet of the expansion turbine 336 or expansion valve 337 that may be sensitive to the condition or temperature of nitrogen at the inlet of the expansion turbine 336 or expansion valve 337 can be stabilized. Further, since the length of a connecting pipe between the BOG condenser 331 and the outlet of the expansion valve 337 is shortened, it is possible to expect that an increase in the temperature of the nitrogen gas caused by the transfer of the nitrogen gas is minimized.

[143] Moreover, in the configuration of the apparatus for reliquefying LNG BOG according to the present invention, it is preferred that the BOG condenser 331 which is at low temperature, the nitrogen heat exchangers 333 and 334, and the expansion means 336 and 337 be constructed into one cold box device 330 and these components be insulated as one module. The insulation is achieved by generally known insulating materials. Due to this configuration, a cryogenic range of the nitrogen gas can be stably managed. Further, the cold box device 330 may be manufactured as a pre-assembly to

facilitate mounting thereof on a ship.

[144]

[145] Next, the operation of the reliquefaction apparatus according to the third embodiment constructed as above will be described with reference to Figs. 9 and 10.

[146] The process of the reliquefaction apparatus according to the third embodiment comprises the steps of compressing BOG generated in the storage tank 310 for LNG; pressurizing and cooling nitrogen gas which is a working fluid, so as to provide cold heat used for condensing the compressed BOG and expanding the pressurized and cooled nitrogen gas to generate cryogenic nitrogen gas; at least partially condensing the compressed BOG through heat exchange with the cryogenic nitrogen gas; and returning BOG reliquefied by means of the condensation to the storage tank. However, by pre-cooling BOG generated in the storage tank for LNG in the first nitrogen heat exchanger located upstream of the BOG condenser, a temperature difference between BOG and nitrogen gas in the BOG condenser can be constantly maintained in a preset range even though the quantity or temperature of BOG generated is changed.

[147] Moreover, all the pressurized and cooled nitrogen gas is not expanded, but a portion of the pressurized and cooled nitrogen gas is extracted and expanded through a different path to generate low-temperature gas. Then, the low-temperature gas is added to nitrogen gas which has been heat-exchanged for condensing the compressed BOG and has slightly increased temperature, and is then heat-exchanged with the pressurized and cooled nitrogen gas prior to expansion thereof so that it can be pre-cooled.

[148] Referring to Figs. 9 and 10, in a method of reliquefying BOG generated in a cryogenic storage tank of an LNG carrier due to external heat transfer, BOG is circulated through a process comprising the steps of passing BOG, which is discharged after a valve 311 is opened at a predetermined pressure, through the temperature controller 314 and constantly maintaining BOG at a pressure of 1.03 bars and a temperature of -12O ⁰ C at the inlet of the two-stage bog compressor 315 and 316 (ST 311); compressing BOG in the two-stage compressor 315 and 316 to a pressure of 3.6 bars and a temperature of -43 ⁰ C so that BOG is in a high temperature and pressure superheated state (ST 312); passing BOG through the first nitrogen heat exchanger 333, which is a pre-cooling cold heat exchanger with three paths formed therein, to lower (pre-cool) the temperature of BOG to -134 ⁰ C at a pressure of 3.3 bars (ST 313); reliquefying the pre-cooled BOG into supercooled liquid with a pressure of 3.0 bars and a temperature of -154.7°Cin the BOG condenser 331 (ST 314); and recovering and storing the reliquefied BOG in the storage tank 310 (ST 318). Further, the circulation of nitrogen gas for supplying cold heat required for the reliquefaction of BOG comprises the steps of passing nitrogen gas with a pressure of 14.3 bars and a temperature of 36.08 ⁰ C through the three-stage compressor 321, 322 and 323 and the

intermediate coolers 325, 326 and 327 to increase the pressure of the nitrogen gas to a pressure of 58 bars at a temperature of 43 ⁰ C (ST 319); internally heat-exchanging the high-pressure nitrogen gas with nitrogen gas in the low-temperature section of the second nitrogen heat exchanger 334 to convert the high-pressure nitrogen gas into nitrogen gas with a low temperature of -7O ⁰ C and a pressure of 57.7 bars (ST 320); converting a portion of the high-pressure nitrogen gas into low temperature and pressure gas with a low temperature of -129.3 ⁰ C and a low pressure of 15.2 bars by passing the portion of the high-pressure nitrogen through the expansion turbine 336 (ST 321); converting nitrogen gas, which has not been transferred to the expansion turbine 336, into nitrogen with a pressure of 57.4 bars and a temperature of -132 ⁰ C through the first nitrogen heat exchanger 333 by means of inner heat exchange with low-temperature nitrogen mixed with nitrogen of which the temperature is reduced in the expansion turbine 336 (ST 322); converting the high-pressure nitrogen gas into gas with a reduced temperature of -162.6°Cand a reduced pressure of 15.2 bars by passing the high-pressure nitrogen through the expansion valve 337 (ST 323); performing reliquefaction of BOG by the low-temperature and low-pressure nitrogen in the BOG condenser 331 so that the low-temperature and low-pressure nitrogen is converted into low-temperature nitrogen gas with a pressure of 14.9 bars and a temperature of - 145.9 ⁰ C (ST 324); adding the low-temperature nitrogen gas to the nitrogen of which the temperature is lowered by the expansion turbine 336 and passing the nitrogen gas through the first nitrogen heat exchanger 333 to be converted into nitrogen gas with a pressure of 14.6 bars and a temperature of -86.42 ⁰ C (ST 325); and passing the nitrogen gas through the second nitrogen heat exchanger 334 to be converted into high- temperature nitrogen gas with a pressure of 14.3 bars and a temperature of 36.08 ⁰ C (ST 326).

[149] After the step of reliquefyng the pre-cooled BOG in the BOG condenser 331 (ST

314), the step of separating non-condensed gas contained in the reliquefied BOG (ST 315) may be further provided. The reliquefied BOG may be transferred to the storage tank 310 in the step of pressurizing the reliquefied BOG by the circulation pump 319 (ST 316).

[150] Moreover, a portion of the reliquefied BOG may be re-circulated while passing through the recirculation valve 313 and the temperature controller 314 (ST 317) and then supplied to a step before or after the step of compressing BOG (ST 312) so as to pre-cool BOG.

[151] Although the pressure, the temperature and the like are indicated by the specific numerals in the respective steps, it will be apparent that the pressure and temperature may be changed according to the amount of BOG, a control method and the like.

[152] According to the third embodiment of the present invention constructed as above,

the following effects can be obtained.

[153] With the third embodiment of the present invention constructed as above, there is an advantage in that the temperature of BOG at the inlet of the condenser can be constantly maintained in a preset range by pre-cooling BOG upstream of the BOG condenser even though a change occurs in the quantity or temperature of BOG generated.

[154] Further, according to the third embodiment of the present invention, it is possible to solve a problem in a conventional BOG reliquefaction system utilizing a reverse Brayton cycle of nitrogen, in which if the temperature of nitrogen is lowered using an expansion turbine, a portion of nitrogen is liquefied, causing damage to blades of the expansion turbine. In the method of this embodiment, only a portion of pressurized nitrogen is extracted and expanded through the expansion turbine 336, the temperature of the remainder of nitrogen is lowered and is then further lowered through the expansion valve 337 so that the temperature of nitrogen can be lowered without a risk of generation of liquid droplets. Moreover, the operation of a nitrogen cycle can be changed variously by changing a flow rate of nitrogen supplied to the expansion valve 337, and thus, it is possible to easily and rapidly cope with dynamic changes of BOG, i.e., changes of load.

[155] With the third embodiment of the present invention, the pressure of a storage tank can be stably maintained without a loss of stored LNG during a voyage of an LNG carrier. In particular, the size of the apparatus for reliquefying LNG BOG can be reduced by employing the simple cold box module and the cryogenic range of the nitrogen gas can be stably managed.

[156] Specifically, since the length of the connecting pipe between the nitrogen heat exchanger 333 and the inlet of the expansion valve 337 is shortened, the cryogenic condition of the outlet of the expansion valve 337 which is sensitive to the condition or temperature of the inlet of the expansion valve 337 can be stabilized. Further, since the length of the connecting pipe between the BOG condenser 331 and the outlet of the expansion valve 337 is shortened, it is possible to minimize an increase in temperature produced upon delivery of the cryogenic nitrogen gas, and it is possible to prevent a loss of cold heat by incorporating the unit elements constituting the low temperature section into the cold box device 330 in the apparatus for reliquefying LNG BOG and managing the cold box device.

[157]

[158] (Fourth embodiment)

[159] Hereinafter, an apparatus and method for reliquefying LNG BOG according to a fourth embodiment of the present invention will be described in detail with reference to Figs. 11 to 13.

[160] Fig. 11 is a view showing a configuration of the apparatus for reliquefying LNG

BOG according to the fourth embodiment of the present invention. This apparatus comprises a BOG cycle device, a nitrogen cycle device and a cold box device interfacing these two devices.

[161]

[162] BOG cycle device

[163] Natural gas in a gaseous phase is cryogenically liquefied and stored in a storage tank 410 under an atmospheric pressure (1.013 bars). However, during transportation of LNG, BOG is generated due to continuous heat transfer from the outside, thereby increasing pressure in the storage tank 410.

[164] Accordingly, in order to constantly maintain the pressure in the storage tank 410 at a level of the atmospheric pressure, a safety valve 411 is opened and BOG is discharged outside of the storage tank 410 if the pressure in the storage tank 410 reaches about 1.03 bars, and the discharged BOG passes through two-stage bog compressor 415 and 416 and is then subjected to a reliquef action process.

[165] It is preferred that the temperature of hot BOG discharged from the storage tank

410 be sensed by a temperature sensor 412 and be controlled to a predetermined temperature through a temperature controller 414, and BOG then flow into the two- stage bog compressor 415 and 416. BOG that has passed through the temperature controller 414 is maintained in a superheated vapor state at a pressure of 1.03 bars and a temperature of -12O ⁰ C.

[166] The operation of the temperature controller 414 will be described in greater detail.

If the cryogenic BOG (LNG) that has been subjected to the reliquefaction process is re- circulated and mixed with hot BOG discharged from the storage tank 410, the temperature of BOG can be controlled. The amount of BOG re-circulated is controlled by adjusting the degree of opening of a recirculation valve 413. Further, it is possible to supply LNG in the storage tank as well as the reliquefied BOG to the temperature controller 414 through a pump (not shown). Moreover, LNG in the storage tank 410 may be supplied to be mixed with BOG compressed by the BOG compressors 415 and 416, thereby lowering or controlling the temperature of BOG to be supplied into a BOG condenser. Typically, the temperature controller is operated only until the reliquefaction apparatus reaches a normal state, the recirculation valve 413 is then closed and the cryogenic reliquefied LNG is guided to the storage tank 410.

[167] When BOG that has passed through the temperature controller 414 passes through the two-stage bog compressor 415 and 416, BOG is discharged in a superheated vapor state at a pressure of 3.2 bars and a temperature of -50.83 ⁰ C.

[168] BOG discharged from the two-stage bog compressor 415 and 416 is pre-cooled while passing through a first nitrogen heat exchanger 433 with three paths formed

therein, so that BOG is in a superheated vapor state at a pressure of 3.1 bars and a temperature of -13O ⁰ C. At this time, in the process of pre-cooling BOG, BOG is cooled in the first nitrogen heat exchanger 433 through heat exchange with nitrogen gas that is obtained by mixing low-pressure and low-temperature nitrogen gas discharged from the BOG condenser 431 to be described below with nitrogen gas which is in a cryogenic state by the expansion turbine 436.

[169] After the pre-cooled BOG passes through the BOG condenser 431 and is then converted to be in a supercooled liquid state at a pressure of 3.0 bars and a temperature of -154.7 ⁰ C, it flows back into the storage tank 410 or is re-circulated to the temperature controller 414. The BOG condenser 431 will be described in detail below in connection with the cold box device. Since BOG is pre-cooled by the low- temperature working fluid (nitrogen) as above, there is an advantage in that even though the quantity or temperature of BOG generated is changed, a temperature difference between BOG and nitrogen gas in the BOG condenser 431 can be constantly maintained in a pre-set range.

[170] Although all BOG may be liquefied through the BOG condenser 431 , about 70 to

99% of BOG is liquefied due to influence by a nitrogen component of which perfect liquefaction cannot be easily performed.

[171] Such a mixture of gas and liquid is divided into gas and liquid in a BOG non- condensed gas separator 418, so that the liquid (condensed BOG) flows back into the storage tank 410 or is re-circulated to the temperature controller 414 by a circulation pump 419 and gas is generally discharged to the outside.

[172] Methods of causing BOG to flow back into the storage tank 410 include a method of spaying BOG through a spray head above the storage tank, a method of supplying BOG to the bottom of the storage tank, and the like. If BOG flows back to the bottom of the storage tank, a nitrogen component that is contained in non-condensed gas included in the condensed BOG is dissolved in LNG so that the ratio of nitrogen in the gaseous phase is maintained at a lower value. Since the liquefaction temperature of nitrogen is lower than that of methane that is a main component of LNG, an increase in the content of nitrogen in BOG can reduce a load applied to the two- stage bog compressor 415 and 416 or the BOG condenser 431.

[173]

[174] Nitrogen cycle device

[175] BOG is reliquefied in the BOG condenser 431 by means of heat exchange between

BOG and cryogenic nitrogen gas. The following description is made in connection with a cycle device for obtaining cryogenic nitrogen gas required for the reliquefaction of BOG.

[176] After nitrogen gas at a pressure of 14.2 bars and a temperature of 35.46 ⁰ C passes

through three-stage compressor 421, 422 and 423 and intermediate coolers 425, 426 and 427, the pressure of the nitrogen gas is increased so that the nitrogen gas can be discharged at a pressure of 58 bars and a temperature of 43 ⁰ C.

[177] After the discharged high-pressure nitrogen gas is transferred to the cold box device

430, the nitrogen gas is cooled to a temperature of -77 ⁰ C at a pressure 57.9 bars by means of inner heat exchange with low-pressure nitrogen in a low temperature section, which is returned while having passed through the BOG condenser 431 and a third nitrogen heat exchanger 435 and through a first nitrogen heat exchanger 433, in a second nitrogen heat exchanger 434.

[178] At this time, a portion of the cooled nitrogen gas that has passed through the second nitrogen heat exchanger 434 has reduced temperature while passing through an expansion turbine 436, and the nitrogen gas of which the temperature is lowered is mixed with nitrogen which is returned while having passed through the BOG condenser 331 and the third nitrogen heat exchanger 435. The resulting nitrogen gas is introduced into the first nitrogen heat exchanger 433. At this time, the high-pressure nitrogen gas is further cooled through inner heat exchange with high-pressure nitrogen gas, which is partially cooled in the second nitrogen heat exchanger 434, in the first nitrogen heat exchanger 433. Thus, nitrogen gas with a pressure of 57.8 bars and a temperature of -134 ⁰ C is discharged.

[179] The nitrogen gas, which has passed through the first nitrogen heat exchanger 433, is heat-exchanged with the stream of nitrogen gas, which has passed through the BOG condenser 431, in the third nitrogen heat exchanger 435 and then discharged as gas with a pressure of 57.7 bars and a temperature of -137 ⁰ C. Such a cooling process is advantageous to reduction in the temperature of the high-pressure nitrogen gas to a preset temperature required at an inlet of the expansion valve 437.

[180] The high-pressure nitrogen gas of which the temperature is lowered in the third nitrogen heat exchanger 435 passes through the expansion valve 437 and is cooed ultimately to a temperature of -163.3 ⁰ C to be converted into a cryogenic ideal stream required for reliquefaction of BOG. The cryogenic nitrogen gas is heat-exchanged with BOG in the BOG condenser 431 to reliquefy BOG and has increased temperature.

[181] As compared with a case where only the first nitrogen heat exchanger 433 is provided, the provision of the third nitrogen heat exchanger 435 allows the temperature of nitrogen at the inlet of the expansion valve 437 to be more stably managed and enables great reduction in the size of each of the nitrogen heat exchangers. Consequently, there is an advantage in that the size of the cold box 430 can be reduced.

[182] The reason why a portion of the high-pressure nitrogen gas is converted through the expansion turbine 436 into the low-temperature nitrogen gas which in turn is introduced into the first nitrogen heat exchanger 433 is to assist in cooling the high-

pressure nitrogen gas to a temperature required at the inlet of the expansion valve 437 using the portion of the nitrogen gas of which the temperature has been already reduced through the expansion turbine 436 since the low-pressure nitrogen gas returned through the BOG condenser 431 and the third nitrogen heat exchanger 435 has low specific heat and thus is insufficient to cool the high-pressure nitrogen gas. Thereafter, remaining cold heat cools the high-pressure nitrogen gas in the second heat exchanger 434, and then, it is subjected to an increase in temperature and reaches 14.2 bars and 35.46 ⁰ C, thereby completing the nitrogen cycle.

[183] A nitrogen buffer tank 420 performs the function of adjusting a mass flow rate in the nitrogen cycle in response to changes in the reliquefied amount of BOG, i.e., changes in a cooling load of the nitrogen cycle. A source for supplying a supplementary working fluid (nitrogen) may be further provided against a case where the amount of nitrogen is reduced.

[184]

[185] Cold box device

[186] The cold box device 430 comprises the BOG condenser 431 in which the reliq- uefaction of BOG is performed, the nitrogen heat exchangers 433, 434 and 435 in which inner heat exchange between a high temperature section and a lower temperature section of the nitrogen cycle is carried out and BOG is pre-cooled, the expansion valve 437 in which cryogenic nitrogen gas is obtained, and the expansion turbine 436 in which low-temperature nitrogen gas required for inner heat exchange is obtained.

[187] A generator G that is not an essential element of the present invention may be coupled to the expansion turbine 436 to generate electric power that in tune is used as an auxiliary power source for the BOG compressors 415 and 416, the nitrogen compressors 421, 422 and 423, or the like.

[188] The aforementioned devices may be incorporated into the cold box 430 as one module, so that pipes for connecting the devices can be shortened. This enables cryogenic nitrogen required for the reliquefaction of BOG to be secured stably. That is, since the length of a connecting pipe between the second nitrogen heat exchanger 434 and the inlet of the expansion turbine 436 and the length of a connecting pipe between an outlet of the third nitrogen heat exchanger 435 and the inlet of the expansion valve 437 are shortened, a cryogenic condition of nitrogen at the outlet of the expansion turbine 436 or expansion valve 437 that may be sensitive to the condition or temperature of nitrogen at the inlet of the expansion turbine 436 or expansion valve 437 can be stabilized. Further, since the length of a connecting pipe between the BOG condenser 431 and the outlet of the expansion valve 437 is shortened, it is possible to expect that an increase in the temperature of the nitrogen gas caused by the transfer of

the nitrogen gas is minimized.

[189] Moreover, in the configuration of the apparatus for reliquefying LNG BOG according to the present invention, it is preferred that the BOG condenser 431 which is at low temperature, the nitrogen heat exchangers 433, 434 and 435, and the expansion means 436 and 437 be constructed into one cold box device 430 and these components be insulated as one module. The insulation is achieved by generally known insulating materials. Due to this configuration, a cryogenic range of the nitrogen gas can be stably managed. Further, the cold box device 430 may be manufactured as a pre-assembly to facilitate mounting thereof on a ship.

[190]

[191] Next, the operation of the reliquefaction apparatus according to the fourth embodiment constructed as above will be described with reference to Figs. 12 and 13.

[192] The process of the reliquefaction apparatus according to the fourth embodiment comprises the steps of compressing BOG generated in the storage tank 410 for LNG; pressurizing and cooling nitrogen gas which is a working fluid, so as to provide cold heat used for condensing the compressed BOG and expanding the pressurized and cooled nitrogen gas to generate cryogenic nitrogen gas; condensing the compressed BOG through heat exchange with the cryogenic nitrogen gas; and returning BOG reliquefied by means of the condensation to the storage tank 410. However, by pre- cooling BOG generated in the storage tank 410 for LNG in the first nitrogen heat exchanger 433 located upstream of the BOG condenser 431, a temperature difference between BOG and nitrogen gas in the BOG condenser 431 can be constantly maintained in a preset range even though the quantity or temperature of BOG generated is changed.

[193] Moreover, all the pressurized and cooled nitrogen gas is not expanded, but a portion of the pressurized and cooled nitrogen gas is extracted and expanded through a different path to generate low-temperature gas. Then, the low-temperature gas is added to nitrogen gas which has been heat-exchanged for condensing the compressed BOG and has slightly increased temperature, and is then heat-exchanged with the pressurized and cooled nitrogen gas, which is introduced into the expansion valve 437 via the first and third nitrogen heat exchangers 433 and 435, prior to expansion thereof so that it can be pre-cooled.

[194] Referring to Figs. 12 and 13, in a method of reliquefying BOG generated in a cryogenic storage tank 410 of an LNG carrier due to external heat transfer, BOG is circulated through a process comprising the steps of passing BOG, which is discharged after a valve 411 is opened at a predetermined pressure, through the temperature controller 414 and constantly maintaining BOG at a pressure of 1.03 bars and a temperature of -12O ⁰ C at the inlet of the two-stage bog compressor 415 and 416 (ST

411); compressing BOG in the two-stage compressor 415 and 416 to a pressure of 3.2 bars and a temperature of -50.83 ⁰ C so that BOG is in a high temperature and pressure superheated state (ST 412); passing BOG through the first nitrogen heat exchanger 433, which is a pre-cooling cold heat exchanger with three paths formed therein, to lower (pre-cool) the temperature of BOG to -13O ⁰ C at a pressure of 3.1 bars (ST 413); reliquefying the pre-cooled BOG into supercooled liquid with a pressure of 3.0 bars and a temperature of -154.7°Cin the BOG condenser 431 (ST 414); and recovering and storing the reliquefied BOG in the storage tank 410 (ST 418). Further, the circulation of nitrogen gas for supplying cold heat required for the reliquef action of BOG comprises the steps of passing nitrogen gas with a pressure of 14.2 bars and a temperature of 35.46 ⁰ C through the three-stage nitrogen compressors 421, 422 and 423 and the intermediate coolers 425, 426 and 427 to increase the pressure of the nitrogen gas to a pressure of 58 bars at a temperature of 43 ⁰ C (ST 419); internally heat- exchanging the high-pressure nitrogen gas with nitrogen gas in the low-temperature section of the second nitrogen heat exchanger 434 to convert the high-pressure nit rogen gas into nitrogen gas with a low temperature of -77 ⁰ C and a pressure of 57.9 bars (ST 420); converting a portion of the high-pressure nitrogen gas into low temperature and pressure gas with a low temperature of -129.3 ⁰ C and a low pressure of 15.2 bars by passing the portion of the high-pressure nitrogen through the expansion turbine 436 (ST 421); converting nitrogen gas, which has not been transferred to the expansion turbine 436, into nitrogen with a pressure of 57.8 bars and a temperature of -134 ⁰ C through the first nitrogen heat exchanger 433 by means of inner heat exchange with low-temperature nitrogen mixed with nitrogen of which the temperature is reduced in the expansion turbine 436 (ST 422); performing inner heat exchange with low- temperature nitrogen gas, which has passed through the BOG condenser 431, in the third nitrogen heat exchanger 435 to be cooled to a temperature of -137 ⁰ C ata pressure of 57.7 bars (ST 423); converting the high-pressure nitrogen gas, which has passed through the third nitrogen heat exchanger 435, into gas with a reduced temperature of - 163.3°Cand a reduced pressure of 14.6 bars by passing the high-pressure nitrogen through the expansion valve 437 (ST 424); performing reliquef action of BOG by the low-temperature and low-pressure nitrogen in the BOG condenser 431 so that the low- temperature and low-pressure nitrogen is converted into low-temperature nitrogen gas with a pressure of 14.5 bars and a temperature of -150.7 ⁰ C (ST 425); passing the low- temperature nitrogen gas through the third nitrogen heat exchanger 435 to be heated to a temperature of -14O ⁰ C at a pressure of 14.4 bars (ST 426); adding the low- temperature nitrogen gas, which has passed through the third nitrogen heat exchanger 435, to the nitrogen of which the temperature is lowered by the expansion turbine 436 and passing the nitrogen gas through the first nitrogen heat exchanger 433 to be

converted into nitrogen gas with a pressure of 14.3 bars and a temperature of -98.88 ⁰ C (ST 427); and passing the nitrogen gas through the second nitrogen heat exchanger 434 to be heated to a temperature of 35.46 ⁰ C at a pressure of 14.2 bars (ST 428).

[195] After the step of reliquefyng the pre-cooled BOG in the BOG condenser 431 (ST

414), the step of separating non-condensed gas contained in the reliquefied BOG (ST 415) may be further provided. The reliquefied BOG may be transferred to the storage tank 410 in the step of pressurizing the reliquefied BOG by the circulation pump 419 (ST 416). Moreover, a portion of the reliquefied BOG may be re-circulated while passing through the recirculation valve 413 and the temperature controller 414 (ST 417) and then supplied to a step before or after the step of compressing BOG (ST 412) so as to pre-cool BOG.

[196] Although the pressure, the temperature and the like are indicated by the specific numerals in the respective steps, it will be apparent that the pressure and temperature may be changed according to the amount of BOG, a control method and the like.

[197] According to the fourth embodiment of the present invention constructed as above, the following effects can be obtained.

[198] With the fourth embodiment of the present invention constructed as above, there is an advantage in that the temperature of BOG at the inlet of the condenser can be constantly maintained in a preset range by pre-cooling BOG upstream of the BOG condenser even though a change occurs in the quantity or temperature of BOG generated.

[199] Further, according to the fourth embodiment of the present invention, it is possible to solve a problem in a conventional BOG reliquefaction system utilizing a reverse Brayton cycle of nitrogen, in which if the temperature of nitrogen is lowered using an expansion turbine, a portion of nitrogen is liquefied, causing damage to blades of the expansion turbine. In the method of the fourth embodiment, only a portion of pressurized nitrogen is extracted and expanded through the expansion turbine 436, the temperature of the remainder of nitrogen is lowered and is then further lowered through the expansion valve 437 so that the temperature of nitrogen can be lowered without a risk of generation of liquid droplets. Moreover, the operation of a nitrogen cycle can be changed variously by changing a flow rate of nitrogen supplied to the expansion valve 437, and thus, it is possible to easily and rapidly cope with dynamic changes of BOG, i.e., changes of load.

[200] With the fourth embodiment of the present invention, the pressure of a storage tank can be stably maintained without a loss of stored LNG during a voyage of an LNG carrier. In particular, the size of the apparatus for reliquefying LNG BOG can be reduced by employing the simple cold box module and the cryogenic range of the nitrogen gas can be stably managed.

Specifically, since the length of the connecting pipe between the nitrogen heat exchanger 435 and the inlet of the expansion valve 437 is shortened, the cryogenic condition of the outlet of the expansion valve 437 which is sensitive to the condition or temperature of the inlet of the expansion valve 437 can be stabilized. Further, since the length of the connecting pipe between the BOG condenser 431 and the outlet of the expansion valve 437 is shortened, it is possible to minimize an increase in temperature produced upon delivery of the cryogenic nitrogen gas, and it is possible to prevent a loss of cold heat by incorporating the unit elements constituting the low temperature section into the cold box device 430 in the apparatus for reliquefying LNG BOG and managing the cold box device.