JP2011157979A - Apparatus and method for re-liquefying boil-off gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a boil-off gas re-liquefaction apparatus which can completely liquefy a boil-off gas with a simple modification of facilities utilizing existing apparatuses and prevents dysfunction of re-liquefaction when the composition of an LPG changes or a sea water temperature rises. <P>SOLUTION: The boil-off gas re-liquefaction apparatus includes a low temperature reservoir tank 11 which stores a low-temperature liquefied gas, a first compressor 12 which takes out and compresses the boil-off gas generated in the low temperature reservoir tank 11, a first condenser 13 which cools and re-liquefies the boil-off gas compressed by the first compressor 12 by a first refrigerant, and a second condenser 21 which takes out a gas from a gas phase in the first condenser 13 and cools it by a second refrigerant for re-liquefaction. This device is structured in such a way that the liquefied gas re-liquefied by the first compressor 13 is expanded and returned to the low temperature reservoir tank 11 and the liquefied gas re-liquefied by the second condenser 21 is expanded and returned to the low temperature reservoir tank 11. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a boil-off gas re-liquefaction apparatus and method for re-liquefying boil-off gas vaporized by natural heat input from the outside when transporting and storing a low-temperature liquid such as LNG or LPG.

  Liquefied petroleum gas (LPG) is stored in a storage tank installed on land or on a transport ship. Since the internal temperature of such a storage tank is usually lower than the outside air temperature, the liquefied gas evaporates due to the intrusion heat from the outside and boil-off gas is generated, which causes an increase in the internal pressure. Therefore, in order to prevent abnormal pressure increase in the storage tank due to generation of boil-off gas, a re-liquefaction device is used which compresses and condenses (re-liquefies) the boil-off gas and returns it to the storage tank.

  FIG. 3 shows a general structure of the reliquefaction apparatus as described above.

  In this apparatus, the LPG low temperature storage tank 1 stores liquefied gas mainly composed of ethane and propane. The boil-off gas generated in the LPG cold storage tank 1 is compressed by the compressor 2 and introduced into the condenser 3. In the condenser 3, water or seawater is used as a refrigerant, and the boil-off gas is cooled and reliquefied. The reliquefied gas reliquefied by the condenser 3 is reduced in pressure through the expansion valve 4 and the temperature is lowered, and then returned to the LPG low temperature storage tank 1 again.

  In the case of such an apparatus, if the boil-off gas compressed by the compressor 2 is not completely liquefied by the condenser 3, ethane which is a lower boiling point component is not liquefied inside the condenser 3, but is vaporized. It will be concentrated and accumulated. When ethane that is not liquefied is accumulated in the condenser 3, the internal pressure of the condenser 3 gradually increases, causing a reduction in liquefaction capacity. Eventually, the re-liquefaction mechanism of the device will not function.

  Similarly, when water or seawater used as the refrigerant of the condenser 3 rises in temperature due to seasonal fluctuations or navigational areas, the boil-off gas is not completely liquefied in the condenser 3 and the reliquefaction mechanism does not function. Fall into.

  As apparatuses related to such a technique, apparatuses shown in Patent Documents 1 and 2 below are disclosed.

  In the reliquefaction apparatus for a low-temperature LPG ship described in Patent Document 1 below, in order to solve the problems caused by freezing seawater used as a refrigerant when the low-temperature LPG ship navigates a cold region, It has been proposed to use an antifreeze solution.

  In the reliquefaction device for a low temperature tank described in Patent Document 2 below, the boil-off gas liquefied by the condenser is further cooled by exchanging heat with LPG discharged for consumption, and the expansion valve It has been proposed to reduce the load on the reliquefaction device by suppressing the flash (vaporization) when the pressure is reduced.

  In these reliquefaction apparatuses, the boil-off gas is converted into the condenser 3 according to the composition of the boil-off gas (ethane and propane are the main components) and the cooling temperature in the condenser 3, that is, the temperature of the cold water or seawater to be used. The discharge pressure of the compressor 2 that can be completely liquefied is determined.

JP 09-2221098 A Japanese Utility Model Publication No. 05-006299

  However, once fossil fuels such as LPG have been used with a composition that is stable to some extent, due to the recent severe energy situation, the abundance ratio of low carbon components has increased in recent years compared to conventional LPG. For example, the same LPG with a composition different from the conventional one has begun to be used.

  In fact, in the conventional boil-off gas reliquefaction apparatus, the fact that the composition of LPG to be stored or transported has not been considered at all has not been taken into consideration. For example, Patent Documents 1 and 2 described above do not mention any countermeasures when the LPG composition changes. In addition, there is no mention of countermeasures when the temperature of seawater or the like as a refrigerant is increased.

  Therefore, when the composition of LPG changes and the ethane concentration, which is a low boiling point component in LPG, rises, the ethane concentration in the boil-off gas also rises, and the condenser 3 may not be completely liquefied. In addition, even when the temperature of seawater or the like as a refrigerant rises, the condenser 3 may not be completely liquefied. If the boil-off gas is not completely liquefied by the condenser, the reliquefaction mechanism will eventually fail.

  As described above, in order to cope with the fact that the boil-off gas is not completely liquefied in the condenser 3 and the reliquefaction mechanism becomes inoperative, it is necessary to increase the discharge pressure of the compressor 2. For this purpose, it has been necessary to significantly modify the apparatus itself, such as replacing the compressor 2 with a high-pressure apparatus or changing the condenser 3 to one with a pressure resistance.

  In this way, the user of the reliquefaction device hopes to continue to use it at low cost without making major equipment modifications and condition changes based on the existing equipment that has been used in the past. However, the actual situation is that a great burden is imposed on facilities, system operation, costs, and the like. Therefore, the development of an apparatus capable of dealing with the above problem with a simple facility change using an existing apparatus has been awaited.

  The present invention has been made in view of the above circumstances, and when the composition of LPG has changed or when the seawater temperature has risen, the boil-off gas can be completely liquefied by a simple equipment change using existing equipment. An object of the present invention is to provide a boil-off gas reliquefaction apparatus and method for preventing reliquefaction malfunction.

In order to achieve the above object, the boil-off gas reliquefaction device of the present invention includes a low-temperature storage tank that stores a low-temperature liquefied gas, a first compressor that takes out and compresses the boil-off gas generated in the low-temperature storage tank, The first condenser that cools the boil-off gas compressed by the first compressor with a first refrigerant and reliquefies the liquid, and the liquefied gas reliquefied by the first condenser is expanded and the low-temperature storage is performed. A boil-off gas reliquefaction device that returns to the tank,
The apparatus further comprises a second condenser that takes out the gas from the gas phase in the first condenser, cools it with a second refrigerant, and reliquefies, and expands the liquefied gas reliquefied by the second condenser. The gist is that it is configured to return to the low temperature storage tank.

In order to achieve the above object, the boil-off gas reliquefaction method of the present invention includes a first compression step of taking out and compressing the boil-off gas generated in the low-temperature storage tank storing the low-temperature liquefied gas, and the first A first condensing step in which the boil-off gas compressed in the compressing step is cooled with a first refrigerant and reliquefied, and the liquefied gas reliquefied in the first condensing step is expanded and returned to the low temperature storage tank. A boil-off gas reliquefaction device for performing a liquefaction step,
A second condensing step is further performed in which gas is extracted from the gas phase in the first condensing step, cooled by a second refrigerant, and reliquefied, and the liquefied gas reliquefied in the second condensing step is expanded. The gist is to return to the low temperature storage tank.

  That is, for example, even if there is a change in the composition of the liquefied gas to be stored or there is a temperature rise of the first refrigerant introduced into the first condenser, and the reliquefaction capacity in the first condenser is reduced, The gas extracted from the gas phase in the first condenser is introduced into a second condenser that is cooled and reliquefied by a second refrigerant, reliquefied, the liquefied gas is expanded, and the cold storage is performed. Can be returned to the tank. In this way, malfunction of reliquefaction can be prevented in advance. Moreover, if a second condenser is attached to an existing apparatus, the existing first compressor and the first condenser can be used without modification as they are. The boil-off gas can be completely liquefied.

  In the present invention, in the case of further comprising a cooling means for cooling the second refrigerant to a temperature lower than that of the first refrigerant and supplying the second refrigerant to the second condenser, the second refrigerant was not reliquefied by the first condenser. In the second condenser, the boil-off gas can be reliably liquefied with a refrigerant having a temperature lower than that of the first condenser, thereby preventing malfunction of the liquefaction.

  In the present invention, when controlling the cooling means so that the temperature of the second refrigerant becomes constant, the temperature control of the refrigerant used in the second condenser is made constant, so that the control is more complicated than necessary. In addition, the boil-off gas can be reliably liquefied in the second condenser by simple control, and malfunction of the liquefaction can be prevented beforehand.

  In the present invention, in the case of further comprising a second compressor that further compresses the gas taken out from the gas phase in the first condenser into the second condenser, the first condenser By further compressing the boil-off gas that has not been reliquefied and increasing the pressure, the second condenser can be reliably reliquefied to prevent malfunction of reliquefaction.

In the present invention, when the control device further includes a control valve that detects the gas-phase pressure in the first condenser and takes out the gas when the pressure exceeds a predetermined pressure and sends the gas to the second condenser, When the reliquefaction capability of the condenser decreases and the internal pressure increases, the vapor boil-off gas can be automatically sent to the second condenser for reliquefaction. Therefore, it is possible to operate with the first condenser in normal times and reliquefy by using the second condenser together when the composition of the liquefied gas is changed or the temperature of the first refrigerant is increased. It becomes.

It is a schematic block diagram which shows the reliquefaction apparatus of the boil off gas of 1st Embodiment of this invention. It is a schematic block diagram which shows the reliquefaction apparatus of the boil off gas of 2nd Embodiment of this invention. It is a schematic block diagram which shows the conventional reliquefaction apparatus of boil off gas.

  Next, the best mode for carrying out the present invention will be described.

  FIG. 1 is a diagram showing an embodiment of a boil-off gas reliquefaction apparatus to which the present invention is applied.

  This apparatus is compressed by the low-temperature storage tank 11 that stores the low-temperature liquefied gas, the first compressor 12 that takes out and compresses the boil-off gas generated in the low-temperature storage tank 11, and the first compressor 12. And a first condenser 13 that cools the boil-off gas with a first refrigerant and reliquefies it.

  If it demonstrates in detail, the said low temperature storage tank 11 will store LPG, LNG, etc. as low temperature liquefied gas, for example.

  For example, in the case of LPG, a gas containing about 3 to 6% by volume of ethane gas based on propane gas can be applied.

  A heat insulating structure is employed on the outer wall of the low temperature storage tank 11 so as to block the internal low temperature liquefied gas from the outside temperature as much as possible. The internal upper space is filled with boil-off gas obtained by vaporizing liquefied gas stored by heat entering from the outside. Connected to the upper part of the low-temperature storage tank 11 is a first extraction path 15 for extracting boil-off gas in the upper space.

  The boil-off gas taken out in the first take-out passage 15 is compressed to a predetermined pressure or higher by the first compressor 12 and introduced into the first condenser 13. The first condenser 13 includes a first cooling coil 16 through which the first refrigerant passes, and cools and reliquefies the boil-off gas compressed by the first compressor 12.

  As said 1st refrigerant | coolant, if the said low temperature storage tank 11 is installed in the ground, normal cooling water, such as industrial water and a tap water, can be used, and the said low temperature storage tank 11 is a ship. Seawater can be used as long as it is installed in the plant, but is not limited thereto.

  At this time, the ultimate pressure of the boil-off gas in the first compressor 12 can be set to about 1.86 MPa when, for example, LPG is used as the liquefied gas. By setting the pressure to such a level, the boil-off gas can be re-liquefied into a liquefied gas of about 40 ° C. using normal temperature cooling water or seawater as the first refrigerant in the first condenser 13.

  In this case, the composition of the gas phase and the liquid phase in the first condenser 13 is maintained at, for example, a gas-liquid equilibrium composition at 1.86 MPa and 40 ° C. For example, even if there is a change in the composition of the stored LPG, this vapor-liquid equilibrium composition is maintained at the same composition while the first condenser 13 is reliquefied. In the above example of conditions, the liquid phase of the first condenser 13 is ethane 17.89% by volume + propane 82.11% by volume, and the gas phase is ethane 34.64% by volume + propane 65.36% by volume. .

  A first reflux path 17 for returning the reliquefied liquefied gas to the low temperature storage tank 11 is connected to the lower part of the first condenser 13. The first reflux path 17 is provided with a first expansion valve 14 that expands the liquefied gas taken out from the first condenser 13 and lowers the temperature. Thereby, the liquefied gas reliquefied by the first condenser 13 is expanded and returned to the low temperature storage tank 11.

  This is a reliquefaction apparatus in which the low temperature storage tank 11, the first compressor 12, the first condenser 13, the first expansion valve 14, and the like described so far are operated as existing facilities.

  The reliquefaction apparatus of the present invention further includes a second condenser 21 that takes out the gas from the gas phase in the first condenser 13, cools it with a second refrigerant, and reliquefies the second condenser 21. The liquefied gas reliquefied by the condenser 21 is expanded and returned to the low temperature storage tank 11.

  More specifically, the first condenser 13 is filled with a non-condensed boil-off gas that could not be liquefied by heat exchange with the first refrigerant in the internal upper space. The second condenser 22 is connected to the upper part of the first condenser 13 to take out the boil-off gas in the upper space.

  The boil-off gas taken out through the second take-out path 22 is introduced into the second condenser 21 as it is. The second condenser 21 includes a second cooling coil 23 through which the second refrigerant passes. The introduced boil-off gas is cooled to a temperature lower than that of the first condenser 13 and reliquefied.

  As said 2nd refrigerant | coolant, if the said low temperature storage tank 11 is installed in the ground, normal cooling water, such as industrial water and a tap water, can be used, and the said low temperature storage tank 11 is a ship. Seawater can be used as long as it is installed in the plant, but is not limited thereto.

  In the second condenser 21, refrigeration as a cooling means for cooling the second refrigerant introduced into the second cooling coil 23 to a temperature lower than that of the first refrigerant and supplying the second refrigerant to the second condenser 21. A machine 24 is attached.

  The refrigerator 24 can perform temperature control so that the temperature of the second refrigerant is constant when being introduced into the second cooling coil 23 and supplied to the second condenser 21.

  That is, as described above, in the first condenser 13, even if the temperature of the first refrigerant fluctuates or the composition of the liquefied gas changes, the first condenser 13 re-liquefies. The amount of gas to be discharged (in other words, the amount of boil-off gas that is not re-liquefied here and is taken out by the second take-out passage 22) fluctuates, while a constant vapor-liquid equilibrium composition is maintained in the first condenser 13. ing. For this reason, the composition of the boil-off gas taken out in the second take-out path 22 is also a constant vapor-liquid equilibrium composition, and the boil-off gas is controlled by keeping the temperature of the second refrigerant introduced into the second cooling coil 23 constant. The gas can be reliably liquefied.

  Since the second refrigerant is cooled by the refrigerator 24 and supplied to the second condenser 21, the outlet temperature from the second condenser 21 is the supply temperature from the supply source of the second refrigerant. When it is lower than that, it is preferably used in a circulating manner. On the contrary, when the outlet temperature of the second refrigerant from the second condenser 21 is higher than the supply temperature from the supply source, the second refrigerant discharged from the second condenser 21 is not used for circulation. It is preferable to discard the refrigerant and supply a new second refrigerant to the refrigerator 24 for use.

  Further, the second take-out passage 22 is adjusted so that the gas-phase pressure in the first condenser 13 is detected and the boil-off gas is taken out and sent to the second condenser 21 when the pressure exceeds a predetermined pressure. A valve 25 is further provided.

  That is, if the recondensation in the first condenser 13 is not sufficiently performed and non-condensed components are accumulated in the gas phase, the internal pressure increases, and the control valve 25 detects this and automatically opens. And boil-off gas is removed from the first condenser 13. If reliquefaction is sufficiently performed in the first condenser 13, the internal pressure does not increase. Therefore, the control valve 25 is closed, and the boil-off gas is not taken out from the first condenser 13.

  In this way, the first condenser 13 is normally operated, and the second condenser 21 is used together when the composition of the liquefied gas is changed or the temperature of the first refrigerant is increased. Can be re-liquefied.

  At this time, the operation control of the refrigerator 24 can be performed in synchronization with the pressure fluctuation in the second condenser 21 accompanying the opening / closing of the control valve 25 or the like. That is, when the control valve 25 is opened and the pressure in the second condenser 21 is increased, the pressure detector 29 detects this and starts the operation of the refrigerator 24, and the second condenser 21. When the internal boil-off gas is liquefied again and the internal pressure is reduced, it can be detected and controlled to stop the operation of the refrigerator 24. As a result, the refrigerator 24 can be operated only when reliquefaction by the second condenser 21 is necessary, and the energy used for cooling the second refrigerant can be saved.

  Here, the degree of reliquefaction of the boil-off gas in the second condenser 21 can be detected by the pressure detector 29 and controlled to change the temperature of the second refrigerant by the refrigerator 24. That is, when the pressure of the pressure detector 29 is high, the boil-off gas is not sufficiently reliquefied in the second condenser 21, so that the temperature of the second refrigerant by the refrigerator 24 is controlled so as to decrease. Liquefaction can be promoted. On the contrary, when the pressure of the pressure detector 29 is low, the boil-off gas is sufficiently reliquefied in the second condenser 21, so that the temperature of the second refrigerant by the refrigerator 24 is controlled to be increased to save energy. Can be planned.

  It is also possible to use a flow rate control valve instead of the control valve 25 so that the boil-off gas having a constant flow rate is taken out from the gas phase portion of the second condenser 21 through the second extraction path 22.

  A second reflux path 27 for returning the re-liquefied liquefied gas to the low temperature storage tank 11 is connected to the lower part of the second condenser 21. The second reflux path 27 is provided with a second expansion valve 26 that expands the liquefied gas taken out from the second condenser 21 and lowers the temperature.

  As a configuration in which the liquefied gas re-liquefied by the second condenser 21 is expanded and returned to the low temperature storage tank 11, the tip of the second reflux path 27A is connected to the first reflux as shown by the broken line in the drawing. It is also possible to join the upstream side of the first expansion valve 14 in the path 17. By doing so, the equipment of the second expansion valve 26 can be omitted.

  According to this embodiment, there exist the following effects.

  That is, for example, if there is a change in the composition of the liquefied gas to be stored, or there is a temperature rise of the first refrigerant introduced into the first condenser 13, and the reliquefaction capacity in the first condenser 13 is reduced. However, the gas taken out from the gas phase in the first condenser 13 is introduced into the second condenser 21 which is cooled and reliquefied by the second refrigerant, and reliquefied, and the liquefied gas is expanded. Can be returned to the low temperature storage tank 11. In this way, malfunction of reliquefaction can be prevented in advance. Moreover, if the 2nd condenser 21 is attached to it using an existing apparatus, the existing 1st compressor 12 and the 1st condenser 13 can be used as they are, without modification, and simple. The boil-off gas can be completely liquefied by changing the equipment.

  Further, since the second refrigerant is further provided with a refrigerator 24 as a cooling means for cooling the second refrigerant to a temperature lower than that of the first refrigerant and supplying the second refrigerant to the second condenser 21, the second refrigerant is reliquefied by the first condenser 13. The boil-off gas that has not been obtained can be reliably liquefied in the second condenser 21 with a refrigerant having a temperature lower than that of the first condenser 13, and malfunction of the liquefaction can be prevented beforehand.

  The refrigerator 24 as the cooling means controls the temperature of the second refrigerant 21 so that the temperature of the second refrigerant 21 is constant, so that the temperature of the second refrigerant 21 is constant. However, the boil-off gas can be reliably liquefied in the second condenser 21 by simple control, and malfunction of the liquefaction can be prevented in advance.

  In addition, since the pressure of the gas phase in the first condenser 13 is detected and the pressure exceeds a predetermined pressure, a control valve 25 is further provided to send the gas to the second condenser 21, so that the first condensation When the reliquefaction capability in the vessel 13 decreases and the internal pressure rises, the vapor boil-off gas can be automatically sent to the second condenser 21 for reliquefaction. Therefore, the first condenser 13 is normally operated, and when the composition of the liquefied gas is changed or the temperature of the first refrigerant is increased, the second condenser 21 is used together for reliquefaction. Is possible.

  FIG. 2 shows a second embodiment of the boil-off gas reliquefaction apparatus of the present invention.

  In this example, a second compressor 28 that further compresses the gas taken out from the gas phase in the first condenser 13 when the gas is introduced into the second condenser 21 is further provided.

  In other words, the second compressor 28 is provided on the downstream side of the control valve 25 in the second extraction path 22, and the boil-off gas that has not been reliquefied in the first condenser 13 is removed by the first compressor 12. It is compressed to a pressure higher than the compression pressure and introduced into the second condenser 21.

  Other than that, it is the same as that of 1st Embodiment, and attaches | subjects the same code | symbol to the same part.

  According to this embodiment, there exist the following effects.

  That is, since the second condenser 28 that further compresses the gas taken out from the gas phase in the first condenser 13 when the gas is introduced into the second condenser 21, the first condenser 13 is used. By further compressing the boil-off gas that has not been re-liquefied and increasing the pressure, the second condenser 21 can be reliably re-liquefied and malfunction of the re-liquefaction can be prevented.

  That is, in this example, since the boil-off gas introduced into the second condenser 21 is compressed to a pressure higher than that of the first condenser 13, the second refrigerant supplied to the second condenser 21 is Even if it is the same temperature as 1 refrigerant | coolant, sufficient reliquefaction can be achieved.

  Other than that, there exists an effect similar to 1st Embodiment.

  Next, examples will be described together with comparative examples.

(Comparative example)
In the LPG low temperature storage tank 1 in which LPG of 3% by volume of ethane + 97% by volume of propane is stored at atmospheric pressure by the apparatus shown in FIG. 3, the temperature in the tank is about −45 ° C., and 17% by volume of ethane + 83% of propane % Boil-off gas is produced. When this boil-off gas is compressed to 1.9 MPa or more by the compressor 2 and cooled to 40 ° C. with seawater at 32 ° C., it completely liquefies. That is, in order to cool the boil-off gas having the above composition to 40 ° C. with seawater and completely liquefy it, it is necessary to design and manufacture the compressor at a discharge pressure of about 2 MPa.

  In this device, when the composition of the LPG gas is changed to 6% by volume of ethane + 94% by volume of propane, a discharge pressure of about 2.3 MPa is required to completely liquefy by compression and cooling in the same manner. Thus, this device cannot be completely liquefied. In addition, the compressor 2, the condenser 3, the expansion valve 4 and the like are designed and manufactured with a discharge pressure of 2 MPa, and it is very expensive to modify them.

(Example)
The apparatus shown in FIG. 1 was used for reliquefaction by storing 6% by volume of ethane + 94% by volume of propane. In the first condenser 13, only about 22 volume% of the volume of the boil-off gas is reliquefied, and the remaining 78 volume% is accumulated in the gas phase of the first condenser 13 and can cause a pressure increase. By extracting the boil-off gas of the non-condensed component from the gas phase of the first condenser 13 and introducing it into the second condenser 21, the liquid was completely liquefied by cooling to 27 ° C. with the second refrigerant. .

1: LPG cold storage tank 2: Compressor 3: Condenser 4: Expansion valve 11: Low temperature storage tank 12: First compressor 13: First condenser 14: First expansion valve 15: First extraction Path 16: first cooling coil 17: first reflux path 21: second condenser 22: second take-out path 23: second cooling coil 24: refrigerator 25: control valve 26: second expansion Valve 27: Second reflux path 27A: Second reflux path 28: Second compressor 29: Pressure detector

Claims (6)

A low-temperature storage tank for storing a low-temperature liquefied gas, a first compressor for taking out and compressing the boil-off gas generated in the low-temperature storage tank, and the boil-off gas compressed by the first compressor as a first refrigerant A first condenser that cools and reliquefies, and a boiloff gas reliquefaction device that expands the liquefied gas reliquefied by the first condenser and returns it to the low-temperature storage tank,
The apparatus further comprises a second condenser that takes out the gas from the gas phase in the first condenser, cools it with a second refrigerant, and reliquefies, and expands the liquefied gas reliquefied by the second condenser. The boil-off gas reliquefaction device is configured to return to the low temperature storage tank.   The boil-off gas reliquefaction device according to claim 1, further comprising cooling means for cooling the second refrigerant to a temperature lower than that of the first refrigerant and supplying the second refrigerant to the second condenser.   The boil-off gas reliquefaction apparatus according to claim 2, wherein the cooling means controls the temperature of the second refrigerant to be constant.   2. The boil-off gas reliquefaction apparatus according to claim 1, further comprising a second compressor that further compresses the gas extracted from the gas phase in the first condenser when the gas is introduced into the second condenser.   The control valve according to any one of claims 1 to 4, further comprising a control valve that detects a gas-phase pressure in the first condenser and extracts gas when the pressure exceeds a predetermined pressure and sends the gas to the second condenser. The boil-off gas re-liquefaction apparatus as described in 1. A first compression step for extracting and compressing the boil-off gas generated in the low-temperature storage tank that stores the low-temperature liquefied gas, and the boil-off gas compressed in the first compression step is cooled by the first refrigerant and reliquefied. A boil-off gas reliquefaction device that performs a first condensing step and a reliquefaction step of expanding the liquefied gas reliquefied in the first condensing step and returning it to the low-temperature storage tank,
A second condensing step is further performed in which gas is extracted from the gas phase in the first condensing step, cooled by a second refrigerant, and reliquefied, and the liquefied gas reliquefied in the second condensing step is expanded. A boil-off gas reliquefaction method comprising returning to the low temperature storage tank.

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