KR20080031609A - Lng bog reliquefaction apparatus and lng bog reliquefaction method - Google Patents

Abstract

An LNG BOG(Liquefied Natural Gas Boil-Off Gas) reliquefaction apparatus and a method are provided to compress nitrogen at low pressure and to reduce the power consumption by reducing the mass flow of nitrogen in a nitrogen cycle for reliquefying BOG by supplying cold heat to a condenser, by lowering the temperature of BOG supplied to the condenser. An LNG BOG reliquefaction apparatus is composed of: a self-heat exchanger(114) formed at a BOG supply line to cool down the compressed BOG by heat-exchanging high-pressure BOG compressed by a BOG compression unit(110) and cryogenic BOG generated from a storing tank; a first heat exchanger(135) installed at the lower side of a condenser(120) of a nitrogen cycle, to cool down BOG flowing into the condenser by heat-exchanging BOG and actuating fluid of low temperature passing through the condenser; and a cooler(137) installed between the self-heat exchanger and the first heat exchanger, to cool down the BOG flowing from the first heat exchanger by heat-exchanging BOG and actuating fluid of low temperature passing through the first heat exchanger.

Description

LG BOG RELIQUEFACTION APPARATUS AND LNG BOG RELIQUEFACTION METHOD

1 is a schematic diagram of a LNG BOG reliquefaction apparatus according to the prior art,

2 is a schematic diagram of a LNG BOG reliquefaction apparatus according to a preferred embodiment of the present invention,

3 is a flow chart of the BOG of the LNG BOG reliquefaction method according to a preferred embodiment of the present invention,

4 is a flow chart of nitrogen of the LNG BOG reliquefaction method according to a preferred embodiment of the present invention.

<Description of Symbols for Main Parts of Drawings>

110: BOG compression section 112a, 112b, 112c: BOG compressor

113a, 113b, 113c: intermediate cooler 114: self heat exchanger

119: gas-liquid separator 120: condenser

130: working fluid (nitrogen) compression unit 132a, 132b, 132c: working fluid compressor

133a, 133b, 133c: intermediate cooler 134: expansion means (expansion turbine)

135: first heat exchanger 136: second heat exchanger

137: cooler

The present invention relates to an apparatus for reliquefaction of boil-off gas (BOG) generated in a storage tank of an LNG carrier carrying a cryogenic liquefied natural gas (Liquefied Natural Gas, hereinafter referred to as LNG). .

Natural gas is usually liquefied and transported over long distances in LNG. At this time, an LNG carrier is used to transport LNG from a first position where the natural gas is liquefied to LNG to a second position where the LNG is vaporized and distributed to each use place.

Since the liquefaction temperature of natural gas is very low at about -163 ° C, the external heat is transferred to the LNG even if the LNG storage tank is insulated. As a result, LNG is continuously vaporized in the storage tank while the LNG is transported by the LNG carrier, and BOG is generated. When the pressure of the storage tank exceeds the set safety pressure due to the generation of BOG, the BOG is discharged to the outside of the storage tank through the safety valve. The BOG thus discharged must be used as fuel for the ship or liquefied and returned to the storage tank.

Usually, the reliquefaction apparatus of BOG has a refrigeration cycle, and it reliquefies by cooling BOG by this refrigeration cycle.

The refrigeration cycle comprises the steps of compressing the working fluid in a plurality of compressors, cooling the compressed working fluid by indirect heat exchange, expanding the working fluid, indirect heat exchange of the expanded working fluid with the compressed working fluid. By heating and returning the heated working fluid back to the plurality of compressors. On the other hand, the BOG is cooled by indirect heat exchange with the working fluid of the refrigeration cycle after the compression step and at least partially condensed.

An example of an apparatus for carrying out this method of reliquefaction of BOG is described in US Pat. No. 3,857,245. In the reliquefaction apparatus disclosed in U.S. Patent No. 3,857,245, the working fluid is derived from the LNG itself and is therefore operated by an open refrigeration cycle. Expansion of the working fluid is carried out by a valve to obtain partially condensed LNG. Partially condensed LNG is separated into a gaseous phase mixed with the liquid natural gas returned to the reservoir and the natural gas to be sent to the combustion burner. The working fluid is heated and cooled in the same heat exchanger so only one heat exchanger is used.

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

An example of an improved device having both of these advantages is described in WO98 / 43029. The apparatus disclosed in WO98 / 43029 uses two heat exchangers, one of which heats the working fluid in the heat exchanger while partially condensing the compressed natural gas vapors, the other of which is compressed operation. To cool the fluid. Moreover, the working fluid is compressed in two different compressors, one connected to the expansion turbine.

International Patent Publication No. WO2005 / 047761 also discloses a device having a similar structure and characterized by precooling of BOG.

In addition, Korean Patent Laid-Open Nos. 2001-0088406 and 2001-0089142 disclose an apparatus for manufacturing a component as a pre-assembly as related to an apparatus for use in a board-type vessel for reliquefaction of compressed steam. It is.

Referring to Fig. 1, this conventional reliquefaction apparatus, reliquefaction is performed in a closed cycle. The working fluid nitrogen here is compressed in at least one compressor (23, 24, 25), cooled in the first heat exchanger (22), expanded in the turbine (28) and warmed in the second heat exchanger (13). After that, it returns to the compressor via the first heat exchanger (22). On the other hand, BOG is at least partially condensed in the second heat exchanger (13).

This conventional reliquefaction apparatus includes a first preassembly 10 including a second heat exchanger 13, a first heat exchanger 22, compressors 23, 24, 25, and an expansion turbine 28. And a second preassembly 20 comprising a.

On the other hand, in relation to the heat exchange between the conventional working fluid and BOG, Korean Patent Publication No. 1993-0008299, WO2005 / 71333, etc., discloses an apparatus for achieving stable condensation of BOG using three heat exchangers It is.

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

In particular, the prior art uses a nitrogen cycle apparatus as a cold heat generating means for liquefying BOG, and the BOG is cooled and liquefied through heat exchange with nitrogen. However, in order to further improve the liquefaction efficiency of the BOG, it is necessary to compress the BOG at a high pressure before heat exchange, and since the BOG compression temperature is also increased, in order to reduce the freezing load in the condenser, the temperature of the BOG is used by cooling means. There was a problem in that to cool to an appropriate temperature.

Accordingly, the present invention is to solve the problems of the prior art, in the reliquefaction system for recovering the storage tank after re-liquefying the BOG generated in the storage tank during operation in the LNG carrier through the safety valve, LNG BOG reliquefaction apparatus and method for reducing BOG temperature at low temperature and high pressure and supplying it to the condenser by exchanging BOG compressed to room temperature and high pressure by compressor before heat exchange with nitrogen The purpose is to provide.

According to an aspect of the present invention for achieving the above object, a BOG compression unit for compressing evaporated vapor (BOG) generated in the storage tank of LNG, a condenser for condensing the BOG compressed by the BOG compression unit, the In the LNG BOG reliquefaction apparatus which consists of a nitrogen cycle which supplies cold heat to a condenser, and returns BOG liquefied by the condenser to the said storage tank, BOG of the normal temperature and high pressure compressed by the BOG compression part, and the said storage tank LNG BOG reliquefaction apparatus is provided, comprising a self-heat exchanger formed in the BOG supply line for cooling the compressed BOG by heat-exchanging the cryogenic BOG generated from the.

In addition, according to another aspect of the present invention, the step of compressing the evaporated vapor (BOG) generated in the storage tank of LNG; Pressurizing and cooling the nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG, and expanding the pressurized cooled nitrogen gas to generate cryogenic nitrogen gas; Condensing at least a portion of the compressed BOG by heat exchange with the cryogenic nitrogen gas; And returning the BOG re-liquefied by the condensation to the storage tank, wherein the BOG of the normal temperature and high pressure compressed in the compression step of the BOG and the low-temperature BOG generated from the storage tank; An LNG BOG reliquefaction method is provided, comprising the steps of cooling the compressed BOG by heat exchange.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention. The temperature and pressure described below show one example, and the present invention is not limited to these numerical values.

Hereinafter, an LNG BOG reliquefaction apparatus and a reliquefaction method according to a preferred embodiment of the present invention will be described in detail with reference to FIGS. 2 and 3.

2 is a schematic diagram of a LNG BOG reliquefaction apparatus according to a preferred embodiment of the present invention. The LNG BOG reliquefaction apparatus of the present invention comprises a BOG cycle, a nitrogen cycle, and a cold box unit for reliquefying the BOG by performing heat exchange between the two cycles.

BOG cycle

The BOG cycle is a BOG connected with a plurality of BOG compressors 112a, 112b and 112c and intermediate coolers 113a, 113b and 113c such that pressurization and cooling processes are repeated to pressurize and supply BOG generated from a storage tank (not shown). Compression unit 110, a self-heat exchanger formed in the BOG supply line to cool the compressed BOG by heat-exchanging the high-temperature BOG compressed by the BOG compression unit 110 and the cryogenic BOG generated from the storage tank (Self heat exchanger) 114, the condenser 120 for condensing the BOG cooled in the self-heat exchanger 114 through the heat exchange with the cryogenic working fluid, fire against the BOG condensed in the condenser 120 It consists of a gas liquid separator (Separator) 119 for separating into condensed gas and condensed BOG (LBOG), and a pump (not shown) for returning the LBOG separated by the gas liquid separator 119 to a storage tank. It is.

In addition, a first heat exchanger 135 is formed at the front end of the condenser 120 to further cool the BOG flowing into the condenser 120 through heat exchange with a low temperature working fluid passing through the condenser 120. have.

Natural gas in gaseous state is liquefied to cryogenic temperature and stored in a storage tank (not shown) at atmospheric pressure (1.013 bar). However, LNG is vaporized due to continuous heat transfer from the outside during LNG transportation, resulting in BOG, which increases the pressure of the storage tank.

Therefore, in order to keep the storage tank constant at atmospheric pressure level, when the internal pressure of the storage tank reaches a predetermined value (about 1.03 to about 1.05 bar), a safety valve (not shown) is opened and the BOG is discharged out of the storage tank.

The -100 ° C, 1.05 bar BOG discharged from the storage tank passes through the BOG compression section 110 consisting of three stage BOG compressors 112a, 112b and 112c and three intermediate coolers 113a, 113b and 113c. Then, re-liquefaction is performed while passing through the self heat exchanger 114, the first heat exchanger 135, and the condenser 120.

According to the present invention, since the BOG is compressed in three stages by the three-stage compressor, the pressure of the BOG can be increased considerably compared with the conventional two-stage compressor, thereby improving the BOG reliquefaction efficiency.

The BOG compressed at room temperature and high pressure of 41 ° C. and 6.945 bar while passing through the BOG compression unit 110 is a BOG supply line, that is, the BOG in a self-heat exchanger 114 installed upstream of the BOG compression unit 110. Heat exchanged with the cryogenic BOG supplied to the compression unit 110 is cooled to -91.04 ℃, 6.845bar state.

Here, the self-self of the heat exchanger 114 is used to mean that the BOG compressed by the BOG compression unit 110 at room temperature and high pressure exchanges with the cryogenic BOG before compression supplied from the storage tank.

When the BOG in the superheated vapor state compressed to room temperature and high pressure while passing through the three stage BOG compressors 112a, 112b, and 113c flows into the condenser 120 as it is, the temperature difference between the streams in the condenser 120 is increased. Since thermal stress may cause problems in the durability of the heat exchanger, the BOG precooling process by the self heat exchanger 114 has an advantage of effectively reducing the temperature difference between the streams in the condenser 120.

In addition, since the self heat exchanger 114 increases the temperature of the BOG supplied to the compressor 112a from −100 ° C. to 36 ° C., the compression of the BOG is more easily performed.

On the other hand, the high pressure BOG (-91.04 ℃, 6.845 bar) cooled by the self heat exchanger 114 is cooled to -118 ℃, 6.842 bar while passing through the first heat exchanger 135 on the nitrogen cycle condenser Supplied to 120. At this time, the cooling of the BOG in the first heat exchanger 135 having three paths is performed by heat exchange with nitrogen gas of low temperature and low pressure flowing from the BOG condenser 120 as described later.

The BOG cooled in the first heat exchanger 135 changes to a supercooled liquid state at -154.6 ° C and 6.742 bar while passing through the condenser 120, and then lowers the pressure to 3 bar using a pressure control valve (not shown). Sent to the storage tank.

Thus, BOG is first precooled by the cryogenic BOG in the self-heat exchanger 114 and cooled by the low-temperature working fluid (nitrogen) in the first heat exchanger 135, even though there is a BOG generation amount or temperature change. There is an advantage that the temperature difference between BOG and nitrogen gas within 120 can be kept constant within a set range.

Although it is possible to liquefy the entire BOG through the condenser 120, the nitrogen component and the like contained in the BOG is not easily liquefied, so that approximately 70-99% is liquefied.

The gas-liquid mixture of BOG is separated from gas and liquid in the gas-liquid separator 119 so that the liquid (condensed BOG) is re-introduced into the storage tank by a circulation pump (not shown), and the gas is generally discharged to the outside or The mixture is mixed with the BOG generated from the storage tank and supplied to the plurality of BOG compression units 110 to perform the reliquefaction process.

As a method of reflowing the condensed BOG into the storage tank, there is a method of spraying through the spray head from the top of the storage tank or feeding it to the bottom of the storage tank. When it enters the bottom of the storage tank, the nitrogen component of the uncondensed gas contained in the condensed BOG is dissolved in the LNG to keep the nitrogen ratio in the gas phase low. Since nitrogen has a lower liquefaction point than methane, which is the main component of LNG, increasing the nitrogen content in the BOG can reduce the load of the three stage BOG compressors 112a, 112b, 112c or the condenser 120.

Nitrogen cycle

The nitrogen cycle is a working fluid compression in which a plurality of working fluid compressors 132a, 132b, 132c and intermediate coolers 133a, 133b, 133c are connected such that the pressurizing and cooling processes are repeated to pressurize the working fluid, for example nitrogen. The expansion unit 134 for expanding the working fluid of normal temperature and high pressure compressed by the working fluid compression unit 130 to change to a cryogenic state, and the cryogenic expansion by the expansion means 134 It consists of a condenser 120 for condensing BOG by heat exchange with the working fluid of.

On the downstream side of the condenser 120, the working fluid compressed by the BOG and the working fluid compression unit 130 introduced into the condenser 120 through heat exchange with the low temperature working fluid passing through the condenser 120 The first heat exchanger 135 having three paths is formed to further cool the gas.

In addition, between the first heat exchanger 135 and the working fluid compression unit 130 , the working fluid compression unit 130 through heat exchange with the low temperature working fluid passing through the first heat exchanger 135. The second heat exchanger 136 is formed to cool the working fluid at room temperature and high pressure supplied by compression.

In addition, between the self heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, the first heat exchanger through heat exchange with a low temperature working fluid passing through the first heat exchanger 135. A cooler 137 is formed to further cool the BOG flowing into the 135.

The cooler 137 is formed on the bypass pipe branched from the outlet side working fluid circulation line of the first heat exchanger 135 and flows into the working fluid compression unit 130 or the second heat exchanger 136. Some or all of the working fluid is passed through.

The bypass pipe in which the cooler 137 is formed has an open flow path at initial startup, and the flow rate of the working fluid is controlled by opening degree adjustment of valves (not shown) respectively formed in the bypass pipe and the corresponding working fluid circulation line. Gradually lower the overall temperature of the cryogenic refrigeration cycle.

The expansion means is an expansion turbine or expansion valve.

A generator is connected to the rotary shaft of the expansion turbine to use the electrical energy generated by the expansion turbine as driving energy for the working fluid compression unit or the BOG compression unit.

Referring to the operating process of the nitrogen cycle, 40 ℃, 10.09bar nitrogen gas working fluid compression unit 130 including three stage working fluid compressors (132a, 132b, 132c) and intermediate coolers (133a, 133b, 133c) After passing through), the pressure is increased and discharged as gas at 41 ℃ and 45.05bar.

The discharged high-pressure nitrogen gas is heat exchanged in the low temperature working fluid (nitrogen) of -68 ° C and 10.39 bar returned through the first heat exchanger 135 and the second heat exchanger 136 to be -59.27 ° C, 44.95. cooled to bar.

Subsequently, the first cooled -59.27 ° C. and 44.95 bar nitrogen was exchanged in the first heat exchanger 135 with -134.1 ° C. and 10.39 bar nitrogen returned through the condenser 120 to be -109 ° C. and 44.95 bar. Further cooled.

Nitrogen cooled to −109 ° C. and 44.95 bar through the first and second nitrogen heat exchangers 135 and 136 is lowered to −164.2 ° C. and 10.69 bar while passing through the expansion turbine 134. After being changed to cryogenic nitrogen gas for liquefaction, it is introduced into the condenser 120. The cryogenic nitrogen gas heats the BOG in the condenser 120 to liquefy the BOG and the temperature rises to -134.1 ° C and 10.39 bar.

After passing through the condenser 120, the nitrogen precools BOG sent to the condenser 120 at the first heat exchanger 135, and simultaneously cools nitrogen sent to the expansion turbine 134, and the temperature of the nitrogen is increased. Flows into the second heat exchanger 136 at −68.97 ° C. and 10.39 bar.

After passing through the first heat exchanger 135, nitrogen passes through the second heat exchanger 136 and exchanges heat with nitrogen sent from the working fluid compression unit 130 to the first heat exchanger 135 to obtain a temperature of 40 ° C. and 10.09. bar.

Although not shown, the nitrogen cycle may include a nitrogen buffer tank (not shown). The nitrogen buffer tank performs the nitrogen cycle nitrogen flow control function in response to a change in the amount of BOG generation, that is, a change in the refrigeration load of the nitrogen cycle. do. In addition, the nitrogen buffer tank may be additionally installed to replenish the working fluid (nitrogen) in case the amount of nitrogen in the nitrogen cycle is reduced.

Cold Box Unit

The cold box unit is connected to the outlet of the working fluid compression unit 130 and the first heat exchanger 135 for precooling the compressed working temperature fluid at room temperature and high pressure by the low temperature working fluid passing through the condenser 120. a condenser to give condensed through the first 1 BOG with the heat exchanger giving was connected to the exit of a cryogenic expansion of the working fluid to the expansion means 134, inflow them from the BOG compression portion 110 of the heat exchanger 135 ( 120) and the working fluid compression unit through heat exchange with a low temperature working fluid passed between the first heat exchanger 135 and the working fluid compression unit 130 and passed through the first heat exchanger 135. It comprises a second heat exchanger 136 for cooling the working fluid of room temperature and high pressure compressed by 130.

The cold box unit includes a first heat exchanger 135, a second heat exchanger 136, and a condenser 120 as one module.

Here, the low temperature part working fluid is defined as a working fluid that is cryogenically expanded by the expansion turbine 134 and heat exchanged with BOG in the condenser 120 and then returned to the working fluid compression part 130.

Although not an essential component of the present invention, by connecting a generator (not shown) to the expansion turbine 134 to produce electric power and using it as an auxiliary power source such as BOG compression unit 110 or nitrogen compression unit 130 It is also possible.

By including the devices as a module in the cold box, the connection pipe between the devices can be shortened, which makes it possible to stably secure the cryogenic nitrogen required for BOG reliquefaction. In addition, since the connecting pipe between the outlet of the expansion turbine 134 and the condenser 120 can be formed short, it can be expected to minimize the temperature increase due to the nitrogen gas transfer.

For example, the condenser 120, the first heat exchanger 135, and the second heat exchanger 135, 136, which are low-temperature devices among the LNG BOG reliquefaction apparatus, may be configured as one modular cold box. .

The cold box unit is preferably insulated into one module. Insulation is generally performed using known insulators. By such a structure, the cryogenic region of nitrogen gas can be made into stable management. In addition, the cold box may be manufactured in a preassembly to facilitate mounting on a ship.

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

Operation of the reliquefaction apparatus according to the configuration of the present embodiment, the step of compressing the evaporated vapor (BOG) generated in the storage tank of LNG; Pressurizing and cooling the nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG, and expanding the pressurized cooled nitrogen gas to generate cryogenic nitrogen gas; Condensing at least a portion of the compressed BOG by heat exchange with the cryogenic nitrogen gas; And returning the BOG reliquefied by the condensation to the tank.

In particular, according to the present invention, before supplying the low-temperature low-pressure BOG generated in the LNG storage tank to the BOG compression unit 110, the BOG and the self-heat exchanger 114 of room temperature and high pressure passed through the BOG compression unit 110 Cooling the compressed BOG by heat exchange)

In a reliquefaction method of LNG BOG generated by external heat transfer in a storage tank of a cryogenic LNG carrier, the circulation of the BOG will be described with reference to FIG. 3.

In the circulation of BOG, -100 ° C, 1.05 bar BOG discharged from the storage tank is introduced into the self-heat exchanger 114 (ST 101), and the BOG is 36 ° C, 1.02 via the self-heat exchanger 114. bar temperature is increased (ST 102), the BOG is compressed to 41 ℃, 6.945 bar while passing through the three-stage BOG compressor (112a, 112b, 112c) and the intermediate coolers (113a, 113b, 113c) at room temperature and high pressure Step to be in an overheated state (ST 103), the temperature is lowered to -91.04 ° C, 6.845 bar through the self heat exchanger 114 (ST 104), the BOG is a first heat exchanger 135 of three paths Further cooling (cooling) the temperature to -118 ° C., 6.842 bar (ST 105), and re-liquefying the cooled BOG to -154.6 ° C., 6.742 bar subcooled liquid in condenser 120. (ST 106), separating the non-condensing gas from the reliquefied BOG by the gas-liquid separator 119 (ST 107), and the reliquefied BOG by the circulation pump (not shown) Recovery to the storage tank and storing step (ST 108).

Although not shown, the state of the BOG changes as follows while passing through the three-stage BOG compressor and the intermediate cooler. That is, the BOG supplied through the self heat exchanger 114 to the BOG compression unit 110 in a state of 36 ° C. and 1.02 bar is compressed to 101.2 ° C. and 1.939 bar through the first BOG compressor 112a. Cooled to 41 ° C., 1.929 bar via an intermediate cooler 113a, compressed to 106.9 ° C., 3.668 bar via a second BOG compressor 112b, cooled to 41 ° C., 3.658 bar via a second intermediate cooler 113b, The third BOG compressor 112c is compressed to 107 ° C. and 6.955 bar, and the third intermediate cooler 113c is cooled to 41 ° C. and 6.945 bar to be sent back to the self-heat exchanger 114.

At this time, as the working fluid for cooling the BOG in each of the intermediate coolers 113a, 113b, and 113c, cooling water, seawater, fresh water heat exchanged with seawater, and the like may be used. As described above, according to the present invention, since the BOG is cooled to room temperature by using sea water or fresh water in the intermediate coolers 113a, 113b, and 113c, energy can be saved as compared with using nitrogen cooled in a nitrogen cycle. have.

On the other hand, referring to Figure 4 looks at the circulation of nitrogen as a working fluid that provides cooling heat for condensing BOG as follows.

In the nitrogen cycle, the nitrogen gas of 40.degree. C. and 10.09bar passed through the three-stage compressors 132a, 132b and 132c and the intermediate coolers 133a, 133b and 133c. In step ST 111, the high pressure nitrogen is changed to a low temperature state of −59.27 ° C. and 44.95 bar through heat exchange with the low temperature nitrogen in the second nitrogen heat exchanger 136 (ST 112), and the nitrogen is the first nitrogen. Step of changing to a low temperature state of −109 ° C. and 44.95 bar through heat exchange with the low temperature nitrogen in the heat exchanger 135 (ST 113), while the high pressure nitrogen passes through the expansion turbine 134 as an expansion means at −164.2 ° C., 10.69 changing to low temperature low pressure gas of bar (ST 114), the low temperature low pressure nitrogen is heated to low temperature nitrogen gas of -134.1 ° C. and 10.39 bar after performing reliquefaction of BOG in condenser 120, (ST 115), The low temperature nitrogen gas is passed through the first heat exchanger 135 to change to a state of -68.97 ℃, 10.39 bar ( ST 116), the low temperature nitrogen gas is passed through the second heat exchanger 136 to change to a state of 40 ℃, 10.09bar (ST 117).

Although not shown, the state of nitrogen changes as follows through the three stage working fluid (nitrogen) compressor and the intermediate cooler. Nitrogen supplied to the working fluid compression unit 130 in a state of 40 ° C. and 10.09 bar is compressed to 97.63 ° C. and 16.65 bar through the first working fluid compressor 132a, and 41 ° C. through the first intermediate cooler 133a. Cooled to 16.6 bar, compressed to 98.88 ° C., 27.38 bar via a second working fluid compressor 132b, cooled to 41 ° C., 27.33 bar via a second intermediate cooler 133b, and a third working fluid compressor 132 c It is compressed to 98.96 ° C., 45.1 bar, and cooled to 41 ° C., 45.05 bar via a third intermediate cooler 133c.

At this time, as the working fluid for cooling nitrogen in each of the intermediate coolers 133a, 133b, and 133c, cooling water, sea water, fresh water heat exchanged with sea water, and the like may be used.

In addition, as described above, between the self-heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, through the heat exchange with the low-temperature working fluid passing through the first heat exchanger 135 A cooler 137 is formed to further cool the BOG flowing into the first heat exchanger 135.

The cooler 137 is formed on the bypass pipe branched from the outlet side working fluid circulation line of the first heat exchanger 135 and flows into the working fluid compression unit 130 or the second heat exchanger 136. Some or all of the working fluid is passed through.

The bypass pipe in which the cooler 137 is formed is used during initial startup, and the flow rate of the working fluid is controlled by adjusting the opening degree of valves (not shown) respectively formed in the bypass pipe and a corresponding working fluid circulation line. Gradually lowering the total temperature of the cryogenic refrigeration cycle.

Although pressure, temperature, etc. in each step are described by a specific number, it can be changed according to the generation amount of BOG, a control method, etc.

According to the present invention, the BOG compressed at room temperature and high pressure while passing through the BOG compression unit 110 can be cooled by a self-heat exchanger 114 provided upstream of the BOG compression unit 110, thereby condenser. It is possible to improve the heat exchange efficiency at 120 and to suppress the occurrence of thermal stress inside the condenser 120.

Further, according to the present invention, since the temperature of the BOG supplied to the BOG compressor 112a by the self heat exchanger 114 can be increased, the compression in the compressor can be performed more easily.

In addition, according to the present invention, it is possible to stably manage the pressure of the storage tank without losing the LNG stored during operation of the LNG carrier. In particular, the introduction of a cold box module can reduce the size of the LNG re-liquefaction apparatus and can make stable management of the cryogenic region of nitrogen gas.

As described so far, according to the present invention, in a reliquefaction system in which a LNG tanker is liquefied in a storage tank during operation and outflowed through a safety valve, and then recovered into a storage tank, the compressor before heat exchange with nitrogen The BOG compressed to room temperature and high pressure by the heat exchange with the cryogenic BOG flowing out of the storage tank is provided an LNG BOG reliquefaction apparatus which can be supplied to the condenser by lowering the BOG temperature in the state of low temperature and high pressure.

Further, according to the present invention, since the temperature of the BOG supplied to the condenser is lower than in the related art, the mass flow rate of nitrogen can be reduced even in the nitrogen cycle for supplying cold heat to the condenser to liquefy the BOG. Accordingly, the compression of nitrogen can be made lower than in the prior art, and the effect of reducing power consumption can be obtained.

Claims (7)

It consists of a BOG compression unit for compressing evaporated steam (BOG) generated in the storage tank of LNG, a condenser for condensing the BOG compressed by the BOG compression unit, a nitrogen cycle for supplying cold heat to the condenser, by the condenser In the LNG BOG reliquefaction apparatus for returning the liquefied BOG to the storage tank, The LNG BOG reliquefaction apparatus, A self-heat exchanger formed in a BOG supply line to cool the compressed BOG by heat-exchanging the high pressure BOG compressed by the BOG compression unit and the cryogenic BOG generated from the storage tank; A first heat exchanger installed at the downstream side of the condenser in the nitrogen cycle to cool the BOG flowing into the condenser through heat exchange with a low temperature working fluid passing through the condenser; A cooler formed between the self-heat exchanger and the first heat exchanger to further cool the BOG flowing into the first heat exchanger through heat exchange with a low temperature working fluid passing through the first heat exchanger; LNG BOG reliquefaction apparatus comprising a. The method according to claim 1, The cooler is formed on the bypass pipe branched from the working fluid circulation line at the outlet side of the first heat exchanger, so that a part or all of the working fluid passing through the first heat exchanger passes. BOG Reliquefaction Unit. The method according to claim 1 or 2, The nitrogen cycle is expanded by a working fluid compression unit connected to a plurality of working fluid compressors and an intermediate cooler so as to repeat pressurizing and cooling processes to pressurize nitrogen, and expands the working fluid of normal temperature and high pressure compressed by the working fluid compression unit. LNG BOG reliquefaction apparatus comprising an expansion means for changing to a cryogenic state, and the condenser for condensing BOG by heat exchange with the cryogenic working fluid expanded by the expansion means. The method according to claim 1 or 2, Between the first heat exchanger and the working fluid compression unit, a second to cool the working fluid of the room temperature and high pressure supplied by the working fluid compression unit through heat exchange with the low temperature working fluid passing through the first heat exchanger LNG BOG reliquefaction apparatus, characterized in that the heat exchanger is formed. The method according to claim 3, The expansion means is LNG BOG reliquefaction apparatus, characterized in that the expansion valve or expansion turbine. The method according to claim 5, LNG BOG reliquefaction apparatus, characterized in that the expansion turbine is installed a generator. Compressing evaporated vapor (BOG) generated in a storage tank of LNG; Pressurizing and cooling the nitrogen gas which is a working fluid to provide cooling heat for condensing the compressed BOG, and expanding the pressurized cooled nitrogen gas to generate cryogenic nitrogen gas; Condensing at least a portion of the compressed BOG by heat exchange with the cryogenic nitrogen gas; And returning the BOG liquefied by the condensation to the storage tank, wherein the LNG BOG reliquefaction method includes: The LNG BOG reliquefaction method includes the step of cooling the compressed BOG by heat-exchanging the high-pressure BOG compressed in the compression step of the BOG with the low-temperature BOG generated from the storage tank, Generating the cryogenic nitrogen gas, the step of compressing the nitrogen gas to a high pressure, the nitrogen of the high pressure is cooled to a low temperature state by heat exchange with the low-temperature nitrogen in the second nitrogen heat exchanger, then the nitrogen is Further cooling to a low temperature state through heat exchange with low temperature nitrogen in a first nitrogen heat exchanger, converting the high pressure nitrogen into a cryogenic low pressure gas while passing through an expansion means, and the cryogenic low pressure nitrogen undergoes BOG reliquefaction in a condenser After the step of raising the temperature of the low temperature nitrogen gas, the step of raising the temperature of the low temperature nitrogen gas while passing through the first heat exchanger, the low temperature passed through the first heat exchanger to further cool the BOG flowing into the first heat exchanger Exchanging a part or all of nitrogen gas and BOG flowing into the first heat exchanger in a cooler; Subsequently, the low temperature nitrogen gas is further heated while passing through the second heat exchanger LNG bog reliquefaction method, characterized in that consisting of.

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