KR20080081436A - Lng bog reliquefaction apparatus and method - Google Patents

Abstract

An LNG BOG(Liquefied Natural Gas Boil-Off Gas) reliquefaction apparatus and a method are provided to prevent thermal stress from being applied to a BOG compressor by compressing BOG of low temperature at high temperature and high pressure and mixing the BOG of low temperature with some of the compressed BOG by bypassing some of the compressed BOG through BOG bypass pipes. An LNG BOG reliquefaction apparatus is composed of a BOG cycle for compressing BOG generated from an LNG storing tank, by repeatedly pressurizing and heat-exchanging the BOG by a BOG compression unit having plural compressors(112a,112b,112c) and middle freezers(113a,113b,113c) and a nitrogen cycle for supplying cold heat to a condensing unit. BOG bypass pipes(115,116) make some of the BOG compressed in one of plural compressors bypass an inlet of the compression unit and mixes some of the compressed BOG with the BOG flowing to the BOG compression unit.

Description

LG BOG Reliquefaction Apparatus and Method {LNG BOG RELIQUEFACTION APPARATUS AND METHOD}

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

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

3 is a flowchart of a BOG of the LNG BOG reliquefaction method according to the first preferred embodiment of the present invention;

4 is a flow chart of nitrogen gas of the LNG BOG reliquefaction method according to the first embodiment of the present invention,

5 is a schematic diagram of an LNG BOG reliquefaction apparatus according to a second preferred embodiment of the present invention, and

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

<Description of Symbols for Main Parts of Drawings>

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

113a, 113b, 113c: Intermediate cooler 114: Self heat exchanger

115: 1st BOG bypass piping 116: 2nd BOG bypass piping

119: gas-liquid separator 120: condenser

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

133a, 133b, 133c: intermediate cooler 134: expansion turbine

135: first heat exchanger 136: second heat exchanger

137: cooler 139: nitrogen bypass piping

215: first control valve 216: second control valve

239: expansion valve

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

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

Since the liquefaction temperature of natural gas is in a cryogenic state around -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 to the LNG carrier, and BOG is generated. On the other hand, when the pressure of the storage tank exceeds the set safety pressure due to the generation of the BOG, the BOG is discharged to the outside of the storage tank through the safety valve, the discharged BOG is used as the fuel of the vessel or re-liquefied and stored again Must be returned to the 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, and expanding the expanded working fluid into the indirect heat exchange 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. In the apparatus disclosed in WO98 / 43029, two heat exchangers are used, one of which heats the working fluid in the heat exchanger while partially condensing the compressed natural gas vapor, and the other is a compressed working fluid. To cool. 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 connection with the heat exchange between the conventional working fluid and BOG, Korean Patent Publication No. 1993- 0008299, WO 2005/71333, etc., discloses a device 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 used a nitrogen cycle apparatus as a cold heat generating means for liquefying BOG, and the BOG was cooled and liquefied through heat exchange with the nitrogen gas.

However, since the temperature of the BOG flowing from the storage tank at a low temperature during the initial operation is -100 ° C., when the BOG flows into the BOG compression means (BOG compressor) as it is, thermal stress is applied to the apparatus, thereby causing a compressor blade. Is deformed or damaged, so it is preferable to heat and supply it to the BOG compression means at room temperature or higher. To this end, a separate heating device must be added to the BOG compression means inlet line, but in this case, additional energy must be supplied, which increases the driving force for operating the device, and it is difficult to install the heating device because BOG itself is highly flammable. .

Accordingly, the present invention is to solve the problems of the prior art, in the reliquefaction system for liquefying the BOG generated in the storage tank during operation in the LNG carrier and outflow through the safety valve, outflow from the storage tank during the initial operation To bypass the part of the compressed BOG in order to prevent the mechanical damage caused by thermal stress of the BOG compression means (BOG compressor) by heating the low temperature BOG flowing into the BOG compression means, An object of the present invention is to provide an LNG BOG reliquefaction apparatus and method in which a BOG bypass pipe is formed to be heated by mixing with BOG.

According to one embodiment of the present invention for achieving the above object, condensation after compressing the BOG generated in the LNG storage tank by repeatedly performing the pressurization and heat exchange process with a plurality of compressors and a plurality of intermediate coolers An LNG BOG reliquefaction apparatus comprising a BOG cycle condensed by means and a nitrogen cycle for supplying cold heat to the condensation means, the inlet and outlet of the BOG compression means bypassing a portion of the compressed BOG to the BOG compression means. LNG BOG reliquefaction apparatus is provided, characterized in that the BOG bypass pipe for mixing with the BOG introduced into the.

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 illustrate one example, and the present invention is not limited to these numerical values.

(First embodiment)

Hereinafter, an LNG BOG reliquefaction apparatus and a reliquefaction method according to a first preferred embodiment of the present invention will be described in detail with reference to FIGS. 2 to 4.

2 is a schematic configuration diagram of a LNG BOG reliquefaction apparatus according to a first 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 includes a plurality of BOG compressors 112a, 112b and 112c and intermediate coolers 113a, 113b and 113c such that the pressurization and cooling processes are repeated to pressurize and supply the BOG generated from the storage tank (not shown). The BOG compression unit 110 as a BOG compression unit 110, bypassing some of the compressed BOG while passing through the BOG compression unit 110 and mixed with the BOG flowing into the BOG compression unit 110 of the BOG compression unit 110 Cools the compressed BOG by heat-exchanging the BOG of normal temperature and high pressure BOG compressed by the BOG bypass pipes 115 and 116 and the BOG compression unit 110 connected to the inlet and the outlet. Self heat exchanger 114 as a cold heat recovery means formed in the BOG supply line, condensing means for condensing the BOG cooled in the self heat exchanger 114 through heat exchange with cryogenic working fluid (nitrogen gas) Condenser 120 as The LBOG separated by the gas condenser 119 and the gas-liquid separator 119 for separating BOG condensed in the gas 120 into non-condensed gas and condensed BOG (LBOG) are returned to the storage tank. It consists of a circulation pump (not shown) for the purpose.

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. .

Here, the BOG bypass pipes 115 and 116 are mainly the first BOG bypass pipe 115 used for initial startup or low temperature BOG inflow and the second BOG bypass pipe used for adjusting the flow rate of the BOG during operation. Are divided into 116.

Specifically, the first BOG bypass pipe 115 is a high temperature compressed in order to prevent the BOG compressor from receiving a thermal stress by the low-temperature BOG flowing into the BOG compression means during the initial startup or when the low-temperature BOG inflow BOG and low temperature BOG serves to mix at the inlet of the BOG compression means. To this end, the first BOG bypass pipe may be arranged to connect the outlet of any one of the plurality of compressors provided in the BOG compression means with the inlet of the foremost BOG compressor, that is, the BOG compression means. As such, the outlet of the last BOG compressor and the inlet of the foremost BOG compressor are connected.

In addition, the second BOG bypass pipe 116 is the outlet and the front end of the intermediate cooler connected to the last BOG compressor to bypass the excess when a large amount of BOG is generated in the LNG storage tank during inflow exceeding the processing capacity during operation This pipe connects the inlet of BOG compressor.

Natural gas in gaseous state is liquefied to cryogenic temperature and stored and transported 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, causing BOG, which increases the pressure of the storage tank.

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

The BOG discharged from the storage tank passes through the BOG compression unit 110 consisting of three stages of BOG compressors 112a, 112b and 112c and three intermediate coolers 113a, 113b and 113c, and then the self-heat exchanger 114 ), The reliquefaction is performed while passing through the first heat exchanger 135 and the condenser 120.

In the BOG compression unit, -100 ° C and 1.05 bar BOG discharged from the storage tank are compressed and cooled while passing through three stages of the BOG compressors 112a, 112b and 112c and three intermediate coolers 113a, 113b and 113c. The process was repeated and compressed to 41 ° C., 6.945 bar.

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. The liquefaction efficiency can be improved.

On the other hand, at the initial startup of the LNG BOG reliquefaction apparatus according to the present invention, both the BOG cycle and the nitrogen cycle is in the normal temperature state, the BOG flowing from the storage tank is -100 ℃, the temperature difference with the BOG cycle greatly increases. Therefore, when the low-temperature BOG flows into the BOG compression unit 110 without being heated (preheated), thermal damage is likely to cause mechanical damage to the BOG compressors 112a, 112b, and 112c. The greatest thermal stress is applied to the first BOG compressor 112a, where BOG is first contacted.

The reason is that, even when the BOG in the BOG cycle and the low temperature BOG introduced from the storage tank during the initial start-up heat exchange in the self-heat exchanger 114 to heat the low-temperature BOG to a normal state, that is, near room temperature This is because the first BOG compressor 112a is in continuous contact with the low temperature BOG.

In order to solve this problem, the present invention reduces the inflow of BOG at the initial startup, and the high temperature and high pressure BOG compressed in the compressor, for example, the high temperature and high pressure compressed to 107 ° C. and 6.955 bar in the third BOG compressor 112c. Before cooling the BOG in the third intermediate cooler (113c), bypass the part and feed it to the front of the first BOG compressor (112a) and mix with the low-temperature BOG passed through the self-heat exchanger (114) to adopt the configuration of heating the BOG Doing.

In order to bypass the BOG, the third BOG compressor 112c and the first BOG compressor 112a inlet of the three stages of the BOG compressors 112a, 112b and 112c are connected to the first BOG bypass pipe 115. 1 The first control valve 215 is installed to control the temperature of the BOG by adjusting the opening degree of the BOG bypass pipe 115. Although the figure shows a first BOG bypass pipe 115 branching out of a third BOG compressor 112c outlet, the present invention is not so limited, and the first BOG bypass pipe is connected to the first or second BOG compressor. Branching is also possible connected to the front end of the first BOG compressor (112a).

In the method of controlling the temperature by the first control valve 215, the BOG may be introduced below the set temperature (about 36 ° C) by receiving a temperature signal from a temperature sensor (not shown) installed in front of the first BOG compressor 112a. In this case, a part of the BOG of the high temperature and high pressure compressed by opening the first control valve 215 may be bypassed to the first BOG bypass pipe 115. The BOG introduced through the first BOG bypass pipe 115 is mixed with the low temperature BOG to operate, and after a predetermined time, the temperature of the BOG introduced into the first BOG compressor 112a is in a steady state (about 36 ° C.). ), The first control valve 215 is closed. The first BOG bypass pipe 115 is mainly used at start-up, but a large amount of BOG flows from the LNG storage tank during operation, or the BOG is not heated enough by the self-heat exchanger 114, and the BOG is the first BOG in a low temperature state. Even when flowing into the compressor 112a, the first control valve 215 may be opened to bypass the compressed high temperature BOG to perform temperature control on the low temperature BOG.

On the other hand, when a large amount of BOG is generated in the LNG storage tank during the operation of the LNG carrier and flows in excess of the processing capacity, the excess must be circulated to circulate. For this purpose, the inlet and outlet of the BOG compression unit 110, that is, the first An inlet of the BOG compressor 112a and an outlet of the third intermediate cooler 113c connected to the third BOG compressor 112c are connected to the second BOG bypass pipe 116, and flow control is performed on the second BOG bypass pipe 116. The second control valve 216 for is installed. On the other hand, in order to control the opening and closing of the second control valve 216, it is preferable to install a flow rate sensor (not shown) in front of the first BOG compressor 112a so as to be connected to the second control valve 216. Do.

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 compression unit in a BOG supply line, that is, a self-heat exchanger 114 installed upstream of the BOG compression unit 110. Heat exchanged with the low temperature BOG supplied to 110) and cooled to -91.04 ℃, 6.845bar.

Here, the self (self-) of the self-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 low-temperature BOG before compression supplied from the storage tank.

The BOG immediately after being discharged from the LNG storage tank is in a low temperature state of about -100 ° C., and if this is supplied to the BOG compression unit 110 and compressed, the BOG compression unit 110 is mechanically damaged due to thermal stress due to a temperature difference. Not only do you wear it, but you also waste the cold heat (-100 ℃) that BOG has. However, when the low-temperature BOG flowing out of the storage tank passes through the self-heat exchanger 114, the cold heat of the BOG is supplied to the BOG compressed by the BOG compression unit 110 at room temperature and high pressure to be recovered into the system. You can do it.

As described above, the cold heat of the BOG immediately after flowing out of the storage tank is passed by the indirect heat exchange in the self-heat exchanger 114 before supplying the BOG compressed at room temperature and high pressure while passing through the BOG compression unit 110 to the condenser 120. Cool it down.

On the other hand, according to the present invention, the cold heat of the low-temperature BOG immediately after flowing out of the storage tank can be supplied to other high-temperature fluid, such as nitrogen gas before or after compression, for example.

In addition, when the BOG in the superheated vapor state compressed at room temperature and high pressure while passing through the three stages of the BOG compressors 112a, 112b and 112c is introduced into the condenser 120 as it is, the stream of BOG and nitrogen gas in the condenser 120 Since the temperature difference between the streams is increased and thermal stress may cause problems in durability of the heat exchanger, the BOG cooling process by the self-heat exchanger 114 may reduce the temperature difference between streams in the condenser 120. It has the advantage of reducing effectively.

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 after the condenser ( 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 is changed to a supercooled liquid state of -154.6 ° C and 6.742 bar while passing through the condenser 120, and then the pressure is reduced to 3 bar using a pressure control valve (not shown). Sent to the tank.

As described above, the BOG is pre-cooled by the low-temperature BOG in the self-heat exchanger 114 and cooled by the low-temperature working fluid (nitrogen gas) in the first heat exchanger 135, thereby reducing the amount of BOG generation or temperature change from the storage tank. Even if it is, there is an advantage that can maintain a constant temperature difference between the BOG and nitrogen gas in the condenser 120 within a set range.

Although the entire BOG may be liquefied through the condenser 120, nitrogen or the like contained in the BOG is not easily liquefied, so that approximately 70-99% of the BOG supplied to the condenser 120 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 it is supplied to the BOG compression unit 110 to perform a reliquefaction process by mixing with the BOG generated from the storage tank through a separate pipe not shown.

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, when the nitrogen content in the BOG is reduced, the load of the three stage BOG compressors 112a, 112b, 112c or the condenser 120 may be reduced.

Nitrogen cycle

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

The downstream side of the condenser 120 further cools 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.

In addition, between the first heat exchanger 135 and the working fluid compression unit 130 is compressed by 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 supplied working fluid at room temperature and high pressure.

Further, between the self heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, nitrogen introduced into the second heat exchanger 136 through heat exchange with the low temperature BOG that has passed through the self heat exchanger 114. A cooler 137 is formed to cool the gas, and the nitrogen gas cooled while passing through the cooler 137 is nitrogen gas compressed by the working fluid compression unit 130 in the second heat exchanger 136. Heat exchange with and further cools the working fluid flowing into the first heat exchanger 135 and the condenser 120.

The cooler 137 is formed on the nitrogen bypass pipe 139 branched from the working fluid circulation line of the outlet 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 nitrogen bypass pipe 139 having the cooler 137 is opened at an initial startup, and the opening of the valve (not shown) is respectively formed in the nitrogen bypass pipe 139 and the corresponding working fluid circulation line. By adjusting the working fluid flow rate, the total temperature of the cryogenic refrigeration cycle is gradually lowered.

Since the nitrogen gas temperature of the nitrogen cycle at the initial start is near room temperature, BOG reliquefaction (condensation) is impossible, but if the nitrogen cycle is repeatedly circulated while the nitrogen gas is cooled by the cooler 137, the temperature of the nitrogen gas gradually decreases. Reliquefaction of the BOG is possible.

According to the present invention, since the nitrogen gas is rapidly reached to a low temperature state by the cooler 137, the reliquefaction operation of the BOG can be performed quickly. That is, according to the present invention, the BOG flowing through the self-heat exchanger 114 and the nitrogen gas passing through the first heat exchanger 135 through the first heat exchanger 135 at the initial start-up heat exchange in the cooler 137 is relatively hot By cooling the nitrogen gas, the nitrogen cycle can be stabilized quickly.

An expansion turbine or an expansion valve may be used as the expansion means.

When using an expansion turbine as the expansion means 134, the generator is connected to the rotary shaft of the expansion turbine to use the electrical energy generated by the generator as the driving energy of the working fluid compression unit 130 or BOG compression unit 110. It is possible.

Hereinafter, in the embodiment of the present invention will be described for the cryogenic refrigeration cycle using the expansion turbine as the expansion means (134).

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 gas) of -68.97 ° C and 10.39 bar returned through the first heat exchanger 135 and the second heat exchanger 136, -59.27 ° C, Cool to 44.95 bar.

Subsequently, the first cooled -59.27 ° C and 44.95 bar nitrogen gas is heat exchanged in the first heat exchanger 135 with -134.1 ° C and 10.39 bar nitrogen gas returning through the condenser 120, and thus -109 ° C, Cool further to 44.95 bar.

Nitrogen gas cooled to -109 ° C and 44.95 bar by passing through the first and second heat exchangers 135 and 136 is lowered to -164.2 ° C and 10.69 bar while passing through the expansion turbine 134, thereby reducing BOG. 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 gas, in the first heat exchanger 135, precools the BOG sent to the condenser 120 and simultaneously cools the nitrogen gas sent to the expansion turbine 134, Becomes high and flows into the second heat exchanger 136 at -68.97 ° C and 10.39 bar.

After passing through the first heat exchanger 135, the nitrogen gas exchanges heat with nitrogen gas sent from the working fluid compression unit 130 to the first heat exchanger 135 while passing through the second heat exchanger 136. ℃, 10.09 bar.

Although not shown, the nitrogen cycle may include a nitrogen buffer tank (not shown), which adjusts the nitrogen gas flow rate of the nitrogen cycle in response to fluctuations in BOG generation, that is, fluctuations in the refrigeration load of the nitrogen cycle. Do this. In addition, the nitrogen buffer tank may be additionally installed for replenishing the working fluid (nitrogen gas) in case the amount of nitrogen gas in the nitrogen cycle is reduced.

Cold Box Unit

The cold box unit is connected to an outlet of the working fluid compression unit 130 to heat the compressed high temperature working fluid (nitrogen gas) by the low temperature working fluid passing through the condenser 120. Group 135; Condenser 120 is connected to the outlet of the first heat exchanger 135 to expand the cryogenic fluid to the expansion means 134, and to condense it through heat exchange with the BOG flowing from the BOG compression unit 110. ); And the working fluid compression unit 130 installed between the first heat exchanger 135 and the working fluid compression unit 130 through heat exchange with a low temperature working fluid that has passed through the first heat exchanger 135. It comprises a second heat exchanger 136 for cooling the working fluid of the room temperature and high pressure compressed by.

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 (nitrogen gas) is defined as a working fluid returned to the working fluid compression unit 130 after cryogenic expansion by the expansion turbine 134 and heat exchange with BOG in the condenser 120. .

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 connection pipe between the outlet of the expansion turbine 134 and the condenser 120 can be formed short, it is possible to minimize the temperature rise due to the nitrogen gas transfer.

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 first preferred embodiment of the present invention will be described with reference to FIGS.

In the operation of the reliquefaction apparatus according to the present embodiment, the cold heat of the BOG discharged from the storage tank by heat-exchanging the cold heat of the vaporized vapor (BOG) immediately after flowing out of the LNG storage tank with the fluid BOG at room temperature. Recovering; Compressing the BOG by repeatedly performing the pressurization and heat exchange process of the BOG generated in the LNG storage tank with a BOG compression means including a plurality of compressors and a plurality of intermediate coolers; Bypassing a portion of the compressed BOG with a bypass pipe and supplying a portion of the BOG to be mixed with the BOG flowing into the BOG compression means to adjust the temperature or flow rate; Compressing nitrogen gas, which is a working fluid, by nitrogen gas compression means to condense the compressed BOG into cryogenic nitrogen gas in the condensation means; Expanding the compressed nitrogen gas by means of an expansion turbine to produce cryogenic nitrogen gas and supply it to the condensation means; Condensing at least a portion of the compressed BOG by heat-exchanging the cryogenic nitrogen gas and the compressed BOG in the condensing means; And returning the BOG re-liquefied by the condensation to the storage tank.

The cold heat recovery step is performed by heat-exchanging the low temperature BOG flowing out of the storage tank and the normal temperature BOG compressed by the BOG compression means in the cold heat recovery means installed in the BOG line between the storage tank and the BOG compression means. It is characterized by.

The bypass supplying of a part of the BOG may include an outlet of any one of the plurality of compressors, for example, in order to prevent the BOG compressor from being subjected to thermal stress due to the introduction of a low temperature BOG into the BOG compression means. For example, by bypassing a part of the compressed BOG through the first BOG bypass pipe connecting the outlet of the last BOG compressor and the inlet of the foremost BOG compressor, the compressed hot and cold BOG are at the inlet of the BOG compression means. It is characterized by mixing.

On the other hand, the step of bypass supplying a portion of the BOG, when a large amount of BOG is generated in the LNG storage tank and flows in excess of the processing capacity, the intermediate cooler connected to the last BOG compressor to bypass the excess to adjust the circulation flow rate Bypass the compressed excess BOG through the second BOG bypass pipe connecting the outlet and the inlet of the foremost BOG compressor, characterized in that the compressed normal temperature BOG and low temperature BOG is mixed at the inlet of the compression means.

In particular, according to the present invention, when the temperature of the nitrogen gas circulating in the nitrogen cycle is higher than the temperature of the BOG passing through the compression means, by cooling the relatively high temperature nitrogen gas by heat exchange with the BOG passed through the compression means It further comprises a nitrogen gas cooling step.

In addition, during the initial startup, between the BOG compression step and the BOG condensation step, a relatively high temperature BOG of the low temperature passed through the compression means by a cooler installed in the nitrogen bypass pipe branched from the working fluid circulation line passing through the condensation means. Further comprising a heat exchange step at start-up to cool the working fluid by supplying heat to the working fluid of the condensation means.

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.

The circulation of the BOG is a step in which the BOG of -100 ° C and 1.05 bar discharged from the storage tank flows into the self-heat exchanger 114 as a cold heat recovery means (ST 101), and the BOG passes through the self-heat exchanger 114. The temperature is increased to 36 ° C. and 1.02 bar (ST 102), and the BOG is compressed to 41 ° C. and 6.945 bar while passing through three stages of the BOG compressors 112a, 112b and 112c and the intermediate coolers 113a, 113b and 113c. When the low temperature BOG is suddenly introduced into the BOG compression means during the initial start-up or during operation, the BOG compressor generates a thermal stress due to the low temperature BOG. In order to prevent receiving, the outlet of any one of the plurality of compressors, for example, the outlet of the last BOG compressor (third BOG compressor 112c) and the inlet of the foremost BOG compressor (first BOG compressor 112a). Through the first BOG bypass pipe 115 to connect the Bypassing a portion of the compressed BOG at 107 ° C. and 6.955 bar to mix the compressed hot and cold BOG at the inlet of the BOG compression means (ST104), and the BOG exceeding the processing capacity from the storage tank is BOG compressed. Inlet of the BOG compression means by bypassing the excess of BOG compressed to 41 ° C. and 6.946 bar while passing through the three stage BOG compressors 112a, 112b and 112c and the intermediate coolers 113a, 113b and 113c when entering the means. Mixing with a low temperature BOG in step (ST105), the BOG recovers the cold heat while the temperature is lowered to -91.04 ℃, 6.845 bar through the self-heat exchanger 114 (ST 106), the BOG is three paths Heat exchanged with the cold nitrogen gas introduced from the condenser 120 while passing through the first heat exchanger 135 of the temperature lowering (cooling) to -118 ° C and 6.842 bar (ST 107), the cooled BOG Nitrogen gas introduced by cryogenic expansion by the expansion turbine 134 in the condenser 120 Heat-exchanging with and re-liquefying with a subcooled liquid at -154.6 ° C., 6.742 bar (ST 108), separating the non-condensable gas from the reliquefied BOG in gas-liquid separator 119 (ST 109), and re-liquefying The step of recovering and storing the BOG to the storage tank by the circulation pump (not shown) (ST 110).

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

At this time, as the cooling water for cooling BOG in each of the intermediate coolers 113a, 113b, and 113c, seawater, fresh water heat-exchanged with seawater, and the like can 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 saving can be saved as compared with using nitrogen gas cooled in a nitrogen cycle. Can be. In other words, in the intermediate coolers 113a, 113b, and 113c, since the hot BOG is cooled to room temperature by using sea water or fresh water, it is required to cool the hot BOG by being compressed and heated by the BOG compression unit 110. The refrigeration load of the nitrogen cycle is reduced, which can save energy.

As described above, since the BOG immediately after being discharged from the LNG storage tank is at a low temperature of about -100 ° C., the cold heat of the low temperature BOG is supplied to the BOG compressed by the BOG compression unit 110 at room temperature and high pressure. By doing so, the discarded cold heat can be recovered and used in the system. In addition, when a low temperature BOG suddenly flows into the BOG compression unit during initial startup or operation, the low temperature BOG is compressed by high temperature and high pressure, and some of them are bypassed and mixed with the low temperature BOG to be heated. It is possible to prevent the BOG compressor from undergoing thermal stress.

On the other hand, referring to Figure 4 looks at the circulation process of the nitrogen gas as a working fluid to provide cold heat for condensing the BOG as follows.

Nitrogen gas circulating process, the nitrogen gas of 40 ℃, 10.09 bar passes through the three-stage compressor (132a, 132b, 132c) and the intermediate coolers (133a, 133b, 133c), the pressure is increased to 41 ℃, 45.05bar Compressing step (ST 111), the high-pressure nitrogen gas is cooled to a low temperature state of -59.27 ℃, 44.95 bar through heat exchange with the low-temperature nitrogen gas in the second heat exchanger 136 (ST 112), the nitrogen gas Cooling to a low temperature state of -109 ℃, 44.95 bar through the heat exchange with the low-temperature nitrogen gas in the first heat exchanger 135 (ST 113), the high-pressure nitrogen gas passes through the expansion turbine 134 as expansion means While expanding to a low temperature low pressure gas of -164.2 ℃, 10.69 bar (ST 114), the low temperature low pressure nitrogen gas is re-liquefied BOG in the condenser 120 to -134.1 ℃, low temperature nitrogen gas of 10.39 bar Heating step (ST 115), while the low-temperature nitrogen gas passes through the first heat exchanger 135, -68.97 ° C, 1 The step of heating to a state of 0.39 bar (ST 116), the low-temperature nitrogen gas is passed through the second heat exchanger 136 is heated to a state of 40 ℃, 10.09 bar (ST 117).

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

At this time, as the cooling water for cooling the nitrogen gas in each of the intermediate coolers 133a, 133b, and 133c, seawater, fresh water heat-exchanged with seawater, and the like can be used.

In addition, as described above, between the self heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, the low-temperature BOG and the first heat exchanger (passed through the self heat exchanger 114) The heat exchanger of the relatively high temperature working fluid (nitrogen gas) passing through the coolant 135 may further cool the working fluid (nitrogen gas) flowing into the second heat exchanger 136 at the initial start-up of the reliquefaction apparatus. Cooler 137 is formed.

The cooler 137 is formed on the nitrogen bypass pipe 139 branched from the working fluid circulation line at the outlet side of the first heat exchanger 135 to form the working fluid compression unit 130 or the second heat exchanger 136. Some or all of the working fluid flowing in is passed through.

The nitrogen bypass pipe 139 in which the cooler 137 is formed is used during initial startup, and the opening of the valve (not shown) is formed in the nitrogen bypass pipe 139 and the corresponding working fluid circulation line. By adjusting the working fluid flow rate, the total temperature of the cryogenic refrigeration cycle is gradually lowered.

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.

(2nd Example)

Hereinafter, an LNG BOG reliquefaction apparatus and method according to a second preferred embodiment of the present invention will be described in detail with reference to FIG. 5. The LNG BOG reliquefaction apparatus and method of the second embodiment have a difference from the LNG BOG reliquefaction apparatus and method of the first embodiment in the nitrogen cycle, and only the differences will be described in detail for convenience.

5 is a schematic configuration diagram of a LNG BOG reliquefaction apparatus according to a second preferred embodiment of the present invention. The LNG BOG reliquefaction apparatus of this second embodiment is, as in the first embodiment, a cold box unit for re-liquefying the BOG by performing heat exchange between the BOG cycle, the nitrogen cycle, and the two cycles. It consists of. In addition, in order to prevent a low temperature BOG from flowing into the BOG cycle during the initial start-up or suddenly, the BOG cycle bypasses the compressed high-temperature BOG and mixes it with the low-temperature BOG to heat it. BOG bypass pipe 115 and the first control valve 215 is included. In addition, in the BOG cycle, the second BOG bypass pipe 116 and the second control to bypass the excess of the compressed BOG to join at the inlet of the BOG compression section in order to adjust the load of the BOG flowing in excess of the processing capacity of the BOG A valve 216 is included.

The BOG cycle and the cold box unit including the BOG compression unit 110 and the BOG bypass pipe are the same in the configuration of the first embodiment and the second embodiment, and thus the same components are designated by the same member numbers. Are omitted and only the nitrogen cycle will be described here.

Nitrogen cycle

The nitrogen cycle of the second embodiment, as in the first embodiment, is intermediate with the plurality of working fluid compressors 132a, 132b, and 132c such that the pressurizing and cooling processes are repeated to pressurize and supply the working fluid, for example nitrogen gas. The working fluid compression unit 130 to which the coolers 133a, 133b, and 133c are connected, and the expansion fluid for expanding the working fluid (nitrogen gas) at room temperature and high pressure compressed by the working fluid compression unit 130 to change to a cryogenic state. Means 134, 239, and a condenser 120 for condensing BOG by exchanging heat with the cryogenic working fluid expanded by the expansion means 239.

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.

Between the first heat exchanger 135 and the working fluid compression unit 130, in 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 a high temperature and high pressure supplied by compression.

Meanwhile, the nitrogen gas compressed by the working fluid compression unit 130 is branched after being cooled through the second heat exchanger 136 as described above, and part of the first heat exchanger is the same as in the first embodiment. The remainder is sent to 135 and the remainder is sent to the expansion turbine 134.

At this time, the nitrogen gas branched and sent to the first heat exchanger 135 is further cooled in the first heat exchanger 135 and then sent to the expansion valve 238 to be cryogenically expanded and then supplied to the condenser 120. In addition, the nitrogen gas branched and sent to the expansion turbine 134 is expanded at a low temperature while passing through the expansion turbine 134 and then joined with the nitrogen gas passing through the condenser 120 to be supplied to the first heat exchanger 135. The BOG supplied to the condenser 120 is cooled by performing heat exchange with the BOG supplied from the first heat exchanger 135 to the condenser 120.

Further, between the self heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, the second heat exchanger 136 through heat exchange with the low temperature BOG passing through the self heat exchanger 114. A cooler 137 is formed to cool the nitrogen gas flowing into the nitrogen gas, and the nitrogen gas cooled while passing through the cooler 137 is compressed in the working fluid 130 in the second heat exchanger 136. Heat exchanged with the compressed nitrogen gas by the) to further cool the working fluid flowing into the first heat exchanger 135 and the condenser 120.

The cooler 137 is formed on the nitrogen bypass pipe 139 branched from the working fluid circulation line at the outlet side of the first heat exchanger 135 to form the working fluid compression unit 130 or the second heat exchanger 136. Some or all of the working fluid flowing in is passed through.

The nitrogen bypass pipe 139 in which the cooler 137 is formed has an opening at an initial startup, and an opening of a valve (not shown) respectively formed in the nitrogen bypass pipe 139 and a corresponding working fluid circulation line. By regulating the flow rate of the working fluid, the total temperature of the cryogenic refrigeration cycle is gradually lowered.

Since the nitrogen gas temperature of the nitrogen cycle at the initial start is near room temperature, BOG can not be re-liquefied (condensation), but the nitrogen gas is gradually cooled if the nitrogen cycle is repeatedly circulated while the nitrogen gas is cooled by the cooler 137. This lowers the reliquefaction of the BOG.

According to the present invention, since the nitrogen gas is rapidly reached to a low temperature state by the cooler 137, the reliquefaction operation of the BOG can be performed quickly. That is, according to the present invention, the BOG flowing through the self heat exchanger 114 and the nitrogen gas passing through the first heat exchanger 135 at a low temperature during initial startup may be heat exchanged in the cooler 137 to be relatively hot. By cooling the phosphorus nitrogen gas, the nitrogen cycle can be stabilized in a short time.

An expansion turbine and an expansion valve may be used as the expansion means.

When an expansion turbine is used as the expansion means 134, a generator is connected to the rotary shaft of the expansion turbine 134 to transfer the electrical energy generated by the generator to the working fluid compression unit 130 or the BOG compression unit 110. It is also possible to use as driving energy.

Referring to the operating process of the nitrogen cycle, the nitrogen gas of 36 ℃, 10.43bar operating fluid compression unit 130 including three stage working fluid compressors (132a, 132b, 132c) and intermediate coolers (133a, 133b, 133c) After passing through), the pressure rises and is discharged as gas at 40 ℃ and 41.44bar.

The discharged high-pressure nitrogen gas is heat exchanged in the low-temperature working fluid (nitrogen gas) of -93.75 ° C and 10.68 bar returned through the first heat exchanger 135 and the second heat exchanger 136, and -82.2 ° C. Cool to 41.19 bar.

Nitrogen gas cooled while passing through the second heat exchanger 136 is branched and partly sent to the first heat exchanger 135 as in the first embodiment while the remainder is sent to the expansion turbine 134.

The nitrogen gas at -82.2 ° C and 41.19 bar branched and sent to the first heat exchanger 135 exchanges heat with the nitrogen gas at -141.2 ° C and 10.93 bar returned through the condenser 120 and the first heat exchanger 135. Cooled to -138.2 [deg.] C., 40.94 bar.

Nitrogen gas cooled to -138.2 ° C. and 40.94 bar through the first heat exchanger 135 has a temperature and pressure lowered to −167.7 ° C. and 11.18 bar while passing through the expansion valve 239, and thus cryogenic nitrogen required for BOG liquefaction. Converted to gas.

Cryogenic nitrogen gas, the temperature and pressure of which is lowered to -167.7 ° C and 11.18bar while passing through the expansion valve 239, is heated to -141.2 ° C and 10.93bar while heat-exchanging with BOG in the condenser 120 to liquefy BOG. do

On the other hand, the nitrogen gas at -82.2 ° C and 41.19 bar branched and sent to the expansion turbine 134 is lowered to -141.2 ° C and 10.93 bar while passing through the expansion turbine 134.

As such, the BOG is liquefied while passing through the expansion valve 239 and the condenser 120, and nitrogen gas of -141.2 ° C and 10.93bar and nitrogen of -141.2 ° C and 10.93bar while passing through the expansion turbine 134. The gas is merged on the downstream side of the condenser 120 to become -141.2 ° C. and 10.93 bar of cold nitrogen gas and flow into the first heat exchanger 135.

In the first heat exchanger 135, the nitrogen gas cools the BOG sent to the condenser 120 and at the same time the nitrogen gas sent to the expansion valve 238, and the temperature increases to -93.75 ° C and 10.68 bar. Into the second heat exchanger 136 in a state.

While passing through the second heat exchanger 136, the nitrogen gas is heated to 36 ° C. and 10.43 bar while being heat-exchanged with the nitrogen gas sent from the working fluid compression unit 130 to the first heat exchanger 135 or the expansion turbine 134. And is sent to the working fluid compression unit 130.

The operation of the reliquefaction apparatus according to the configuration of the second preferred embodiment of the present invention will now be described with reference to FIGS. 5 and 6.

In the operation of the reliquefaction apparatus according to the second embodiment, the BOG discharged from the storage tank by heat-exchanging the cold heat of the vaporized vapor (BOG) immediately after being discharged from the LNG storage tank with the normal temperature fluid (BOG) Recovering the cold heat having; Compressing the BOG by repeatedly performing the pressurization and heat exchange process of the BOG generated in the LNG storage tank with a BOG compression means including a plurality of compressors and a plurality of intermediate coolers; Bypassing a portion of the compressed BOG with a bypass pipe and supplying a portion of the BOG to be mixed with the BOG flowing into the BOG compression means to adjust the temperature or flow rate; Compressing nitrogen gas, which is a working fluid, by nitrogen gas compression means to condense the compressed BOG into cryogenic nitrogen gas in the condensation means; Branching the compressed nitrogen gas to expand a portion of the nitrogen gas by means of an expansion valve to produce cryogenic nitrogen gas and supply it to the condensation means; Condensing at least a portion of the compressed BOG by heat-exchanging the cryogenic nitrogen gas and the compressed BOG in the condensing means; The remaining nitrogen gas of the branched nitrogen gas is expanded by an expansion turbine and then joined with the nitrogen gas passing through the condensation means to perform heat exchange with the BOG supplied to the condensation means to cool the BOG supplied to the condensation means. Making a step; And returning the BOG re-liquefied by the condensation to the storage tank.

The cold heat recovery step is performed by heat-exchanging the low temperature BOG flowing out of the storage tank and the normal temperature BOG compressed by the BOG compression means in the cold heat recovery means installed in the BOG line between the storage tank and the BOG compression means. It is characterized by.

The bypass supplying of a part of the BOG may include an outlet of any one of the plurality of compressors, for example, in order to prevent the BOG compressor from being subjected to thermal stress due to the introduction of a low temperature BOG into the BOG compression means. For example, by bypassing a part of the compressed BOG through the first BOG bypass pipe connecting the outlet of the last BOG compressor and the inlet of the foremost BOG compressor, the compressed hot and cold BOG are at the inlet of the BOG compression means. It is characterized by mixing.

In the step of bypass supplying a portion of the BOG, when a large amount of BOG is generated in the LNG storage tank and flows in excess of the processing capacity, the outlet and the end of the intermediate cooler connected to the last BOG compressor to bypass the excess to control the circulation flow rate Bypassing the excess BOG compressed through the second BOG bypass pipe that connects the inlet of the shear BOG compressor, characterized in that the compressed normal temperature BOG and low temperature BOG is mixed at the inlet of the compression means.

In particular, according to the present invention, when the temperature of the nitrogen gas circulating in the nitrogen cycle is higher than the temperature of the BOG passing through the compression means, by cooling the relatively high temperature nitrogen gas by heat exchange with the BOG passed through the compression means It further comprises a nitrogen gas cooling step.

In addition, during the initial startup, between the BOG compression step and the BOG condensation step, a relatively high temperature BOG of the low temperature passed through the compression means by a cooler installed in the nitrogen bypass pipe branched from the working fluid circulation line passing through the condensation means. Further comprising a heat exchange step at start-up to cool the working fluid by supplying heat to the working fluid of the condensation means.

In the reliquefaction method of LNG BOG generated by external heat transfer in the storage tank of the cryogenic LNG carrier, since the BOG circulation process is the same as the first and second embodiments, the Looking only at the circulation process is as follows.

Nitrogen gas circulating process, the nitrogen gas of 36 ℃, 10.43bar after passing through the three-stage compressor (132a, 132b, 132c) and the intermediate coolers (133a, 133b, 133c) to increase the pressure to 40 ℃, 41.44bar Compressing step (ST 211), the high-pressure nitrogen gas is cooled to a low temperature state of -82.2 ℃, 41.19 bar through heat exchange with the low-temperature nitrogen gas in the second heat exchanger 136 (ST 212), the second heat exchange Nitrogen gas passed through the unit 136 is branched, so that some nitrogen gas is cooled to a low temperature and high pressure state of -138.2 ° C and 40.94 bar through heat exchange with the low temperature nitrogen gas in the first heat exchanger 135 (ST 213, the low-temperature, high-pressure nitrogen gas is expanded while passing through the expansion valve 239 as expansion means to cryogenic expansion of the cryogenic low-pressure gas of -167.7 ℃, 11.18bar (ST 214), passing through the expansion valve 239 One cryogenic low pressure nitrogen gas was subjected to reliquefaction of BOG in the condenser 120 and then -141.2 ° C, 10.93bar The step of heating to the low temperature nitrogen gas of (ST 215), the remaining nitrogen gas is not branched after passing through the second heat exchanger 136 and sent to the first heat exchanger 135 is the expansion turbine 134 as expansion means The low temperature nitrogen gas and the expansion turbine after liquefying the BOG while passing through the expansion valve 239 and the condenser 120 in order to expand at low temperature to -141.2 ℃, low temperature nitrogen gas of 10.93bar (ST 216) The low temperature nitrogen gas passed through 134 is merged on the downstream side of the condenser 120 to become -141.2 ° C and 10.93 bar of low temperature nitrogen gas while passing through the first heat exchanger 135 at -93.75 ° C and 10.68bar. The heating step (ST 217), the low temperature nitrogen gas is passed through the second heat exchanger 136, the step of heating to a state of 36 ℃, 10.43 bar (ST 218).

Although not shown, the state of nitrogen gas changes as follows while passing through the three-stage working fluid (nitrogen gas) compressors 132a, 132b, and 132c and the intermediate coolers 133a, 133b, and 133c. Nitrogen gas supplied to the working fluid compression unit 130 at a temperature of 36 ° C. and 10.43 bar is compressed to 91.19 ° C. and 16.69 bar through the first working fluid compressor 132a, and then through the first intermediate cooler 133a. 40 ° C., 16.44 bar, second working fluid compressor 132b to 95.94 ° C., 26.3 bar, second intermediate cooler 133b to 40 ° C., 26.05 bar, third working fluid The compressor is compressed to 96.05 ° C. and 41.69 bar via the compressor 132c and cooled to 40 ° C. and 41.44 bar via the third intermediate cooler 133c.

At this time, as the cooling water for cooling the nitrogen gas in each of the intermediate coolers 133a, 133b, and 133c, seawater, fresh water heat-exchanged with seawater, and the like can be used.

In addition, as described above, between the self heat exchanger 114 and the first heat exchanger 135 of the BOG cycle, the low-temperature BOG and the first heat exchanger (passed through the self heat exchanger 114) The heat exchanger of the relatively high temperature working fluid (nitrogen gas) passing through the coolant 135 may further cool the working fluid (nitrogen gas) flowing into the second heat exchanger 136 at the initial start-up of the reliquefaction apparatus. Cooler 137 is formed.

The cooler 137 is formed on the nitrogen bypass pipe 139 branched from the working fluid circulation line at the outlet side of the first heat exchanger 135 to form the working fluid compression unit 130 or the second heat exchanger 136. Some or all of the working fluid flowing in is passed through.

The nitrogen bypass pipe 139 in which the cooler 137 is formed is used during initial startup, and the opening of the valve (not shown) is formed in the nitrogen bypass pipe 139 and the corresponding working fluid circulation line. By adjusting the working fluid flow rate, the total temperature of the cryogenic refrigeration cycle is gradually lowered.

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, when a low temperature BOG is suddenly introduced into the BOG compression unit 110 during initial startup or operation, the low temperature BOG is compressed to high temperature and high pressure, and some of them are bypassed to be mixed with the low temperature BOG and heated. This prevents the BOG compressor from undergoing thermal stress due to low temperature BOG.

In addition, the nitrogen cycle can be stabilized in a short time by rapidly cooling the working fluid (nitrogen gas) in a relatively high temperature state at the initial start-up by the cooler 137.

Further, according to the present invention, the storage tank in the self-heat exchanger 114 as a cold heat recovery means provided on the upstream side of the BOG compression unit 110, the BOG compressed at room temperature and high pressure while passing through the BOG compression unit 110. It can be cooled by BOG immediately after flowing out, and the heat exchange (reliquefaction) efficiency in the condenser 120 can be improved, and the generation of the thermal stress in the condenser 120 is suppressed.

In addition, 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 raised to room temperature, the compression in the compressor can be performed more easily.

In addition, by using the LNG BOG reliquefaction apparatus 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 BOG reliquefaction apparatus, and it is possible to stably manage the cryogenic region of nitrogen gas.

As described above, the LNG BOG reliquefaction apparatus and method according to the present invention have been described as utilizing specific nitrogen cycles with reference to the illustrated drawings, but these specific nitrogen cycles do not limit the present invention, Of course, if it can be condensed, any cryogenic refrigeration cycle using any working fluid can be utilized.

In addition, the present invention is not limited to the embodiments and drawings described above, it is a matter of course that various modifications and variations can be made by those skilled in the art within the scope of the claims. .

As described above, according to the present invention, when a low temperature BOG is suddenly introduced into the BOG compression unit during initial startup or during operation, a BOG bypass pipe is formed at the inlet and the outlet of the BOG compression unit to produce a low temperature BOG at a high temperature and high pressure. The present invention provides a LNG BOG reliquefaction apparatus and method for preventing a BOG compressor from being subjected to thermal stress due to a low temperature BOG by compressing the same, bypassing a portion thereof, and mixing and heating the low temperature BOG.

Moreover, according to this invention, the cold heat which the low temperature BOG immediately after flowing out from a storage tank can collect | recover into a system using the cold heat recovery means (self heat exchanger) provided upstream of BOG compression part.

In addition, 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 a 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.

In addition, the nitrogen cycle can be stabilized in a short time by rapidly cooling the working fluid (nitrogen gas) in a relatively high temperature state at the initial start-up by the cooler.

Claims (12)

BOG cycle for compressing the BOG generated in the storage tank of LNG by repeated compression and heat exchange process by BOG compression means having a plurality of compressors and intermediate cooler and then condensing by condensation means; An LNG BOG reliquefaction apparatus comprising a nitrogen cycle for supplying cold heat to the means, LNG BOG reliquefaction apparatus comprising a BOG bypass pipe for bypassing a portion of the BOG compressed by any one of the plurality of compressors to the inlet of the compression means and mixed with the BOG flowing into the BOG compression means . The method of claim 1, Between the storage tank and the BOG compression means further comprises means for recovering the cold heat of the BOG flowed out of the storage tank by heat-exchanging the low-temperature BOG discharged from the storage tank with the compressed normal temperature BOG LNG BOG Reliquefaction Unit. The method of claim 1, The nitrogen cycle includes a nitrogen gas compression means for compressing nitrogen gas (working fluid) and expansion means for expanding the compressed nitrogen gas, The expansion means expands the compressed nitrogen gas and generates cryogenic nitrogen gas and supplies it to the condensation means, and performs heat exchange with the compressed BOG supplied to the condensation means to condense the BOG. Reliquefaction apparatus. The method of claim 1, The nitrogen cycle includes a nitrogen gas compression means for compressing nitrogen gas (working fluid) and expansion means for expanding the compressed nitrogen gas, The expansion means is composed of an expansion valve and an expansion turbine to which the compressed nitrogen gas is branched and supplied respectively, and the expansion valve generates cryogenic nitrogen gas by expanding some nitrogen gas of the branched nitrogen gas to the condensation means. The expansion turbine expands the remaining nitrogen gas branched and then merges with the nitrogen gas passing through the condensation means to cool the BOG supplied to the condensation means by performing heat exchange with the BOG supplied to the condensation means. LNG BOG reliquefaction apparatus, characterized in that. The method of claim 1, The BOG bypass pipe, In order to prevent cold BOG from entering the BOG compression means during initial startup or to prevent the BOG compressor from undergoing thermal stress, the hot end of the BOG and the low temperature BOG are mixed at the inlet of the BOG compression means. LNG BOG re-liquefaction apparatus comprising a first BOG bypass pipe connecting the outlet of the BOG compressor and the inlet of the foremost BOG compressor. The method of claim 1, The BOG bypass pipe, The second BOG bypass pipe connects the outlet of the middle cooler connected to the last BOG compressor and the inlet of the foremost BOG compressor to bypass the excess when a large amount of BOG is generated in the LNG storage tank and flows in excess of the processing capacity. LNG BOG reliquefaction apparatus comprising a. The method according to claim 5 or 6, LNG BOG reliquefaction apparatus, characterized in that the first and second BOG bypass pipe is provided with a valve for adjusting the flow rate. Recovering, by means of cold heat recovery means, the cold heat of the BOG discharged from the storage tank by heat-exchanging the cold heat of the vaporized vapor (BOG) immediately after flowing out of the LNG storage tank with a fluid at room temperature; Compressing the BOG by repeatedly performing the pressurization and heat exchange process on the BOG generated in the storage tank by the BOG compression means including a plurality of compressors and a plurality of intermediate coolers; Bypassing a portion of the compressed BOG with a bypass pipe and bypassing a portion of the BOG so as to adjust the temperature or flow rate by mixing with the BOG introduced into the BOG compression means; Compressing nitrogen gas, which is a working fluid, by nitrogen gas compression means to condense the compressed BOG into cryogenic nitrogen gas in the condensation means; Expanding the compressed nitrogen gas to produce cryogenic nitrogen gas and supplying it to the condensation means; Condensing at least a portion of the compressed BOG by heat-exchanging the cryogenic nitrogen gas and the compressed BOG in the condensing means; LNG BOG reliquefaction method comprising the. The method of claim 8, Supplying cryogenic nitrogen gas to the condensation means, LNG BOG reliquefaction method, characterized in that for expanding the compressed nitrogen gas by an expansion turbine to produce cryogenic nitrogen gas to the condensing means. The method of claim 8, Supplying cryogenic nitrogen gas to the condensation means, Branching the compressed nitrogen gas to expand a portion of the nitrogen gas by means of an expansion valve to produce cryogenic nitrogen gas and supply it to the condensing means; After condensing at least a portion of the compressed BOG, the remaining nitrogen gas of the branched nitrogen gas is expanded by an expansion turbine and then joined with the nitrogen gas passed through the condensation means to be supplied to the condensation means. And cooling the BOG supplied to the condensation means by performing heat exchange with the LNG BOG reliquefaction method. The method of claim 8, Bypass supplying a portion of the BOG, A first BOG bypass pipe connecting the outlet of the last BOG compressor and the inlet of the foremost BOG compressor, Hot BOG compressed by bypassing part of the compressed BOG through the first BOG bypass to prevent cold BOG from entering the BOG compression means during initial start-up or into the BOG compression means. LNG BOG reliquefaction method, characterized in that for mixing the low-temperature BOG at the inlet of the BOG compression means. The method of claim 8, Bypass supplying a portion of the BOG, A second BOG bypass pipe connecting the outlet of the intermediate cooler connected to the last BOG compressor and the inlet of the foremost BOG compressor, When a large amount of BOG is generated in the LNG storage tank and flows in excess of the processing capacity, it bypasses the excess and adjusts the circulating flow rate so as to bypass the excess BOG compressed through the second BOG bypass pipe and the compressed BOG at low temperature and low temperature. LNG BOG reliquefaction apparatus characterized in that for mixing the BOG at the inlet of the compression means.

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