JP2016128737A - Boil-off gas re-liquefaction facility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a heat exchange efficiency of a heat exchanger while reducing a flush flow rate to increase an efficiency without complicating a structure.SOLUTION: A boil-off gas re-liquefaction facility includes pressure setting means 60a for setting extraction pressure of a boil-off gas extracted to an extraction flow passage L2 to pressure that is lower than engine supply pressure and equal to or higher than critical pressure of the boil-off gas.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a storage tank that stores liquefied natural gas, a boil-off gas compression unit that compresses boil-off gas discharged from the storage tank, and a part of the liquefied boil-off gas compressed by the boil-off gas compression unit. A high-pressure injection type engine, an extraction passage for extracting and re-liquefying the other part of the boil-off gas compressed by the boil-off gas compressor, and a boil-off gas flowing through the extraction passage The present invention relates to a boil-off gas reliquefaction facility comprising a refrigeration cycle circuit that circulates a refrigerant that heat-exchanges and cools the boil-off gas on a water structure.

When transporting LNG (Liquid Natural Gas), it may be transported by an LNG ship equipped with a storage tank for storing the LNG. Although the storage tank is thermally insulated, LNG stored by external heat is gradually vaporized to generate boil-off gas (BOG). The generated boil-off gas is taken out of the storage tank to increase the pressure inside the storage tank, and supplied as fuel for a diesel engine that rotates at a medium speed as a main engine for propulsion of the ship.
In recent years, high-pressure injection engines (electronically controlled gas injection diesel engines for ships) in which fuel is supplied at a relatively high pressure have been developed as main engines for propulsion of the ships. Since the high-pressure injection engine is a high-pressure injection engine that injects fuel at a high pressure, when supplying boil-off gas as fuel to the high-pressure injection engine, it is necessary to increase the boil-off gas after increasing the pressure to about 30 MPaG. .

Since the high-pressure injection engine described above is highly efficient, when the same level of propulsive force as a conventional engine is exhibited, the amount of boil-off gas required as fuel is reduced, and surplus boil-off gas is increased. You need to handle them. Here, as a BOG reliquefaction facility on a ship that compresses the boil-off gas discharged from the storage tank to a high pressure of about 30 MPaG and supplies it to the high-pressure injection engine, the storage tank for storing LNG, and the discharge from the storage tank A boil-off gas compressing section that compresses the boil-off gas, a high-pressure injection engine to which the boil-off gas increased in pressure to the engine supply pressure to the high-pressure injection engine in the boil-off gas compression section, and the pressure increased to the engine supply pressure A branch flow path for branching a part of the boil-off gas, a heat exchanger for exchanging heat between the boil-off gas flowing through the branch flow path and the boil-off gas flowing from the storage tank to the boil-off gas compression unit, and a branch flow A decompression valve for decompressing the boil-off gas after passing through the heat exchanger in the road, and the decompression valve That a gas-liquid separator for gas-liquid separation of Iruofugasu is known (see Patent Document 1).
In the BOG reliquefaction facility on the ship, the boil-off gas compressed by the boil-off gas compression unit is cooled and decompressed in a form of heat exchange with the boil-off gas after being discharged from the storage tank by the heat exchanger. Thereafter, it is liquefied by the gas-liquid separator and returned to the storage tank. In the gas-liquid separator, a part of the gas is discharged as a gas flush without being liquefied, and the flush is mixed again with the boil-off gas flowing between the storage tank and the heat exchanger. .

On the other hand, as a reliquefaction facility for treating surplus boil-off gas, when considering the current prior art, a reliquefaction facility as shown in FIG. 5 is conceivable. In the reliquefaction facility, a refrigeration cycle circuit C that circulates a non-condensable refrigerant N2 (for example, nitrogen) made of nitrogen or the like is provided, and the refrigeration cycle circuit C compresses the refrigerant N2. And a cooler EX that cools the refrigerant N2 that has been compressed and heated by the refrigerant compressor CP, an expansion turbine EP that expands the refrigerant N2 that has been cooled by the cooler EX, and an expansion turbine EP that has been expanded by the expansion turbine EP. A first heat exchanger EX1 that supplies the cold heat of the refrigerant N2 to the boil-off gas as the medium to be cooled is provided.
Incidentally, in the refrigeration cycle circuit C shown in FIG. 5, the refrigerant N2 after being cooled by the cooler EX and the refrigerant that holds the cold after passing through the first heat exchanger EX1 as a condenser. A second heat exchanger EX2 for exchanging heat with N2 is provided.
Excess boil-off gas is cooled and liquefied in the form of heat exchange with the refrigerant N2 in the first heat exchanger EX1 of the refrigeration cycle circuit C, depressurized by the pressure reducing valve V, and then introduced to the gas-liquid separator 30. Will be.

Further, as another example of the reliquefaction equipment for processing the surplus boil-off gas, as described in Patent Document 2, a storage tank that stores LNG, and boil-off gas that has just been discharged from the storage tank are used as a cooling medium and heat. The first heat exchanger to be exchanged, the boil-off gas compression section (including a cooler) that compresses the boil-off gas after passing through the first heat exchanger, and the boil-off gas that has been compressed by the boil-off gas compression section is cooled. The thing provided with the 2nd heat exchanger cooled and liquefied by the form which heat-exchanges with a medium is known.
In the technology disclosed in Patent Document 2, as a refrigeration cycle circuit that circulates a non-condensable refrigerant composed of nitrogen or the like, a refrigerant compression compressor that compresses the refrigerant, and a refrigerant that has been compressed by the refrigerant compression compressor and that has been heated A cooler that cools the refrigerant, and a first refrigerant flow path that causes the refrigerant cooled by the cooler to flow as a cooling medium to the first heat exchanger and then flows as a cooling medium to the second heat exchanger; An expansion turbine that expands and lowers the temperature of the refrigerant that has flowed through the first refrigerant flow path, and a refrigerant that has been cooled down by the expansion turbine is allowed to flow to the second heat exchanger as a cooling medium to the refrigerant compression compressor. The thing provided with the 2nd refrigerant | coolant flow path to return is known.
In the refrigeration cycle circuit, in order to improve the heat exchange efficiency in the second heat exchanger, a part of the refrigerant flowing through the first refrigerant flow path is bypassed through the first heat exchanger. A bypass flow path for joining the refrigerant flowing through a part of the second heat exchanger to the refrigerant flowing through the upstream side of the second heat exchanger on the downstream side of the first heat exchanger in the first refrigerant flow path; Is provided.
That is, in the technique disclosed in Patent Document 2, in particular, in the second heat exchanger, three fluids of the boil-off gas, the refrigerant that has passed through the first heat exchanger, and the refrigerant that has bypassed the first heat exchanger. By adopting a configuration for exchanging heat, the temperature change rate of the refrigerant with respect to the exchange heat quantity is adjusted by adjusting the bypass quantity of the refrigerant, thereby improving the heat exchange efficiency.

Korean Published Patent No. 10-2013139150 (KR, A) Japanese Patent No. 5280351

However, in the BOG reliquefaction facility on the ship shown in the technique disclosed in Patent Document 1, the boil-off gas immediately after the cold heat source that cools the boil-off gas after exiting the boil-off gas compression section is discharged from the storage tank is used. Therefore, the supercooling of the boil-off gas after leaving the boil-off gas compression section becomes insufficient, and the flow rate of the flash flow generated after the pressure is reduced by the pressure reducing valve increases. Since the large amount of flush flow is mixed with the boil-off gas after leaving the storage tank, as the flush flow rate increases, the compression power of the boil-off gas compression compressor of the boil-off gas compression unit increases, leading to deterioration in efficiency.
In addition, when adopting a configuration in which the flush flow is not mixed with the boil-off gas after leaving the storage tank and discharged outside, a large amount of methane with high utility value is thrown away, so the economic efficiency deteriorates. To do.
On the other hand, when a simple reliquefaction facility as shown in FIG. 5 is employed to process surplus boil-off gas, in the first heat exchanger EX1 of the refrigeration cycle circuit C, the boil-off gas on the heat transfer side and the heat-reception side boil-off gas A TQ diagram showing a relationship between the temperature and the heat exchange amount with respect to the refrigerant N2 is as shown in FIG.
Here, in FIG. 6, the boil-off gas is a gas substantially composed of methane alone, so the TQ line (bold solid line in FIG. 6) of the boil-off gas on the heat transfer side is the temperature at which the temperature starts to decrease and condense. The change becomes discontinuous. And when it becomes a gas-liquid mixed state, it changes isothermally.
On the other hand, when non-condensable nitrogen N2 that is generally used as a refrigerant on the heat receiving side is used, the TQ line (thin solid line in FIG. 6) is shown in FIG. As shown in FIG. As a result, the temperature difference between the boil-off gas and the refrigerant N2 comes closest when the boil-off gas begins to condense (this point (the point indicated by P1 in FIG. 6) is called a pinch point). Due to this pinch point, the rate of change in temperature with respect to the heat exchange amount of the refrigerant N2 (inclination γ in FIG. 6) decreases according to the temperature change of the boil-off gas (that is, it is necessary to increase the flow rate of the refrigerant N2). For this reason, the compression power of the refrigerant compression compressor CP of the refrigeration cycle circuit C increases, leading to deterioration in efficiency.
Further, the difference between the temperature of the boil-off gas on the heat transfer side and the temperature of the refrigerant N2 on the heat reception side (ΔT2 and ΔT3 in FIG. 6) is large, which is the heat exchange efficiency in the first heat exchanger EX1. Indicates that it is bad.

Furthermore, in the technique disclosed in Patent Document 2, in the second heat exchanger, the boil-off gas, the refrigerant that has passed through the first heat exchanger, and the refrigerant fluid that bypasses the first heat exchanger exchange heat. Although the configuration is adopted, there is a possibility that the temperature change rate relative to the heat exchange amount of the boil-off gas cannot sufficiently follow the temperature change rate relative to the heat exchange amount of the refrigerant.
Furthermore, since the refrigeration cycle circuit has a complicated circuit configuration in which a bypass flow path and the like are provided in order to introduce three fluids having different temperatures to the second heat exchanger, the configuration of the entire system is complicated. In such a complicated configuration, in order to improve the heat exchange efficiency in the second heat exchanger, it is necessary to appropriately control the bypass flow rate and the like, and the flow rate control of the refrigerant becomes complicated. was there.

  The present invention has been made in view of the above-mentioned problems, and its object is to reduce the flow rate of the flash flow while improving the efficiency while avoiding the complexity of the configuration, and to perform heat exchange in the heat exchanger. An object of the present invention is to provide a boil-off gas reliquefaction facility capable of improving efficiency and further improving efficiency by reducing the compression power of the refrigeration cycle circuit.

In order to achieve the above object, the boil-off gas reliquefaction facility of the present invention comprises:
A storage tank that stores liquefied natural gas, a boil-off gas compression unit that compresses boil-off gas discharged from the storage tank, and a high-pressure injection that uses a part of the liquefied boil-off gas compressed by the boil-off gas compression unit as fuel Heat exchange with the mold engine, the extraction passage for extracting and re-liquefying the other portion of the boil-off gas compressed by the boil-off gas compression section, and returning it to the storage tank, and the boil-off gas flowing through the extraction passage And boil-off gas reliquefaction equipment comprising a refrigeration cycle circuit for circulating a refrigerant that cools the boil-off gas on the water structure,
The bleed flow passage bleeds the boil off gas before being boosted up to the engine supply pressure of the high pressure injection engine in the boil off gas compression unit,
The refrigeration cycle circuit flows through the refrigerant compression section that compresses the refrigerant, the expansion section that expands the refrigerant compressed by the refrigerant compression section, the refrigerant expanded by the expansion section, and the extraction channel. A first heat exchanger for exchanging heat with the boil-off gas,
A second heat exchanger that exchanges heat between the boil-off gas flowing from the storage tank to the boil-off gas compression unit and the refrigerant flowing from the refrigerant compression unit to the expansion unit in the refrigeration cycle circuit;
A pressure setting means is provided for setting the extraction pressure of the boil-off gas extracted into the extraction flow path to be lower than the engine supply pressure and higher than the critical pressure of the boil-off gas.

According to the above characteristic configuration, the extraction flow path for extracting the boil-off gas from the boil-off gas compression unit extracts the boil-off gas before being boosted to the engine supply pressure of the high-pressure injection engine at the boil-off gas compression unit. The extraction pressure can be set to a low pressure lower than the engine supply pressure. For example, after the boil-off gas is liquefied, the pressure of the flash flow generated when gas-liquid separation is performed by reducing the pressure can be reduced.
Then, the boil-off gas flowing through the extraction channel (hereinafter sometimes abbreviated as “boil-off gas after pressurization”) is compressed by the refrigerant compression unit in the refrigeration cycle circuit in the first heat exchanger, and the second heat. It cools with the form which heat-exchanges with the refrigerant | coolant expanded by the expansion | swelling part, and having cooled temperature after collect | recovering the cold heat | fever of the boil-off gas before pressurization with an exchanger.
In particular, in the present invention, since the pressure setting means sets the extraction pressure to be lower than the engine supply pressure, as described above, for example, after the boosted boil-off gas is cooled by the first heat exchanger, It is possible to reduce the flow rate of the flash flow generated when gas-liquid separation is performed under reduced pressure.
Here, the power consumed in the process of liquefying the boil-off gas is the compression power in the boil-off gas compression section (specifically, the compression power of the boil-off gas compression compressor that compresses the boil-off gas in the boil-off gas compression section). And the compression power in the refrigeration cycle circuit (specifically, the compression power of the refrigerant compression compressor that compresses the refrigerant in the refrigeration cycle circuit), but also consider the heat exchange loss in the first heat exchanger Then, when the extraction pressure is sufficiently smaller than the extraction pressure set in the present invention, the relationship of “compression power in the refrigeration cycle circuit> compression power of the boil-off gas compression unit” is established per unit boil-off gas. .
Then, when the pressure setting means sets the extraction pressure, the lower limit pressure of the extraction pressure is set to be equal to or higher than the critical pressure of the boil-off gas. Therefore, the TQ diagram of the boil-off gas passing through the first heat exchanger in the extraction flow path. In the TQ line indicated by the boil-off gas passing through the first heat exchanger in the extraction channel, the width of the isotherm indicated by the wet saturated vapor that is a gas-liquid mixed state of the liquid and the gas (shown by A in FIG. 6) The extraction pressure can be set so as to reduce (or eliminate) the (width).
Thereby, the TQ line indicated by the boil-off gas passing through the first heat exchanger in the extraction flow path is, for example, as shown by the thick solid line in FIGS. 2 and 3 from the diagram as shown by the thick solid line in FIG. It becomes a simple diagram. As a result, the flow rate of the refrigerant can be reduced (the inclination of the refrigerant in the TQ diagram can be increased), and the compression power of the refrigerant compression compressor (specifically, the compression power of the refrigerant compression compressor that compresses the refrigerant in the refrigeration cycle circuit) ) Can be reduced, and the efficiency of the entire equipment can be improved. Further, in the entire heat exchange amount, the temperature difference between the heat transfer side TQ line and the heat reception side TQ line can be reduced, and the heat exchange efficiency can be improved.
Further, since the compression power in the refrigeration cycle circuit can be reduced, not only the liquefaction efficiency can be improved, but also a refrigerant compression compressor with a small capacity can be used, and the entire equipment can be made compact.
Further, according to the present invention, in the refrigeration cycle circuit, a simple configuration without providing a bypass flow path as in the technique disclosed in Patent Document 2 can be adopted, and the flow rate of the refrigerant flowing through the refrigeration cycle circuit Control can also be made simple.
As described above, it is possible to realize a boil-off gas reliquefaction facility that can improve the efficiency by reducing the compression power of the refrigeration cycle circuit while reducing the flash flow rate and improving the efficiency while avoiding the complicated configuration.

Further features of the boil-off gas reliquefaction facility of the present invention are:
A decompression valve that decompresses the boil-off gas that has passed through the first heat exchanger in the extraction flow path, and a gas-liquid separator that separates the boil-off gas decompressed by the decompression valve,
The pressure setting means is that the upper limit pressure of the extraction pressure is set to be less than the flash flow suppression pressure at which the flow rate of the flash flow discharged as gas from the gas-liquid separator is suppressed.

  According to the above characteristic configuration, the pressure setting means generates the extraction pressure from the gas-liquid separator because the extraction pressure is set to be less than the flash flow suppression pressure capable of suppressing the flow rate of the flash flow discharged as gas from the gas-liquid separator. It is possible to satisfactorily control the flush flow.

Further features of the boil-off gas reliquefaction facility of the present invention are:
The boil-off gas compression unit includes a plurality of boil-off gas compression compressors that compress boil-off gas,
The refrigerant compression unit includes a refrigerant compression compressor that compresses the refrigerant,
The pressure setting means includes a compression power related to the extraction pressure of the boil-off gas extracted from the compression power of the plurality of boil-off gas compression compressors in the process of re-liquefying the extracted boil-off gas, and the extraction flow path. The extraction pressure is set so that the total power with the compression power of the refrigerant compression compressor when cooling the boil-off gas passing through the first heat exchanger becomes small.

In the process in which the extracted boil-off gas is re-liquefied, if the boil-off gas extraction pressure is gradually increased, as shown in FIG. 4, the extraction power of the boil-off gas extracted from the compression power of the boil-off gas compression compressor is increased. The related compression power (power shown by the legend of ▲ in FIG. 4) gradually increases.
On the other hand, in the configuration of the present invention, when the boil-off gas extracted is re-liquefied, the first heat exchanger increases the compression power of the boil-off gas compression compressor until the extraction pressure reaches a predetermined pressure. The boil-off gas TQ line changes from a TQ line having a discontinuous temperature change as shown by a thick solid line in FIG. 6 to a smooth TQ line having a continuous temperature change as shown by a thick solid line in FIG. Thereby, the TQ line of the refrigerant in the second heat exchanger can increase the ratio of the temperature change with respect to the heat exchange amount in accordance with the temperature change of the TQ line of the boil-off gas (that is, the refrigerant flow rate can be reduced). The compression power of the refrigerant compression compressor (power indicated by the legend of ■ in FIG. 4) can be reduced.
On the other hand, when the extraction pressure exceeds a predetermined pressure and the compression power of the boil-off gas compression compressor is increased, the flow rate of the flash flow generated when the pressure is reduced by the pressure reducing valve after passing through the first heat exchanger is large. Therefore, in order to suppress the flow rate of the flash flow, it is necessary to increase the degree of supercooling of the boil-off gas in the first heat exchanger (in the example of FIG. 3, the degree of supercooling is increased by the temperature indicated by ΔT1. Therefore, the compression power of the refrigerant compression compressor increases.
From the above relationship, in the process of reliquefying the boil-off gas, the total power of the compression power related to the extraction pressure of the boil-off gas extracted from the compression power of the plurality of boil-off gas compression compressors and the compression power of the refrigerant compression compressor As shown in FIG. 4, the minimum is between a predetermined bleed pressure width (width indicated by ΔP in FIG. 4).
According to the above-described characteristic configuration, by setting the extraction pressure so that the total power becomes small, the efficiency of the entire equipment can be improved while appropriately performing the liquefaction of the boil-off gas while suppressing the generation of the flash flow. Can be improved.

In the boil-off gas reliquefaction facility of the present invention,
The pressure setting means preferably sets the extraction pressure to 10 MPaG or more and 13 MPaG or less.

The figure which shows schematic structure of the reliquefaction equipment of boil-off gas TQ diagram of boil-off gas on the heat transfer side and refrigerant on the heat reception side in the first heat exchanger when the extraction pressure is set to 7 MPaG and 10 MPaG TQ diagram of heat transfer side boil-off gas and heat reception side refrigerant in the first heat exchanger when the extraction pressure is set to 13, 30 MPaG The graph which shows the compression power of the boil-off gas compression compressor at the time of changing extraction air pressure, the compression power of a refrigerant compression compressor, and those total The figure which shows schematic structure of the reliquefaction equipment of the boil-off gas in a prior art FIG. 5 is a TQ diagram of the heat transfer side boil-off gas and the heat reception side refrigerant in the first heat exchanger in the configuration shown in FIG.

The boil-off gas reliquefaction facility according to the embodiment of the present invention improves the heat exchange efficiency in the heat exchanger while reducing the flow rate of the flash flow while improving the efficiency while avoiding the complexity of the structure, Furthermore, the efficiency can be improved by reducing the compression power of the refrigeration cycle circuit.
Hereinafter, a boil-off gas reliquefaction facility 100 according to an embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the boil-off gas reliquefaction facility 100 according to the embodiment of the present invention is provided on an LNG carrier 50 (an example of a floating structure) and stores a liquefied natural gas LNG. 10, a boil-off gas compression unit 20 that compresses the boil-off gas discharged from the storage tank 10, and a high-pressure injection engine that uses a part of the liquefied boil-off gas compressed and liquefied by the boil-off gas compression unit 20 as fuel 40, and the other part of the boil-off gas compressed by the boil-off gas compression unit 20 is extracted and re-liquefied to return to the storage tank 10, and heat exchange is performed with the boil-off gas flowing through the extraction flow path L2. And a refrigeration cycle circuit C for circulating a refrigerant N2 for cooling the boil-off gas.

The storage tank 10 employs a heat insulating structure that insulates from the external space, and is configured to store LNG at a relatively low temperature (for example, −163 ° C.) inside. Although the storage tank 10 is insulated from the outside, LNG is vaporized in a form in which warm heat is conducted from the outside, and boil-off gas containing methane as a main component is generated.
The storage tank 10 and the boil-off gas compression unit 20 are connected by a boil-off gas discharge path L1, and the boil-off gas generated in the storage tank 10 is sent to the boil-off gas compression unit 20 via the boil-off gas discharge path L1. Led.

The boil-off gas compression unit 20 includes a plurality of boil-off gas compression compressors CP1, CP2, CP3, CP4, and CP5 (five in the present embodiment) that compress the boil-off gas in the order described in the flow direction of the boil-off gas, Boil-off gas compression compressors CP1, CP2, CP3, CP4, and CP5 are cooled by a cooler EX1, EX2, EX3, EX4, and EX5 that cools the boil-off gas that has been heated and heated with another refrigerant. The boil-off gas in a state corresponding to each of the boil-off gas compression compressors CP1, CP2, CP3, CP4 and CP5 at the downstream outlet of each of the boil-off gas compression compressors CP1, CP2, CP3, CP4 and CP5 in the flow direction of Are arranged in the order of description in the flow direction.
In other words, the boil-off gas guided from the boil-off gas discharge path L1 to the boil-off gas compression unit 20 is compressed by the first boil-off gas compression compressor CP1, then cooled by the first cooler EX1, and then by the first cooler EX1. The cooled boil-off gas is compressed by the second boil-off gas compression compressor CP2, and then cooled by the second cooler EX2. The boil-off gas cooled by the second cooler EX2 is the third boil-off gas compression compressor. After being compressed by CP3, cooled by the third cooler EX3, the boil-off gas cooled by the third cooler EX3 is compressed by the fourth boil-off gas compression compressor CP4, and then is supplied to the fourth cooler EX4. The boil-off gas cooled by the fourth cooler EX4 is After compressed in Iruofugasu compression compressor CP5, is cooled by a fifth cooler EX5, for example, after being raised to more engine supply pressure 30MPaG, supplied to the high-pressure injection type engine 40. The high-pressure injection engine 40 employs an electronically controlled gas injection diesel engine for ships, and is a two-cycle engine that is directly connected to a propeller for propulsion of the LNG carrier 50 and rotates at a low speed.
The high-pressure injection engine 40 may be any engine that injects relatively high-pressure fuel, and is not limited to a two-cycle engine.

In the embodiment, in the compression process of the boil-off gas compression unit 20, a relatively low-pressure boil-off gas before being increased to the engine supply pressure is configured to be extracted.
To add a description, the boil-off gas compression unit 20 includes a first branch that separates and bleeds a part of the boil-off gas that has been compressed by the second boil-off gas compression compressor CP2 and then cooled by the second cooler EX2. A mechanism D1 is provided, and the boil-off gas branched by the first branch mechanism D1 is burned by various gas combustors.

Further, the boil-off gas compression unit 20 has a second branch mechanism D2 for branching and extracting a part of the boil-off gas that has been compressed by the fourth boil-off gas compression compressor CP4 and then cooled by the fourth cooler EX4. The boil-off gas extracted by the second branch mechanism D2 flows through the extraction flow path L2 connected to the second branch mechanism D2.
When the amount of boil-off gas discharged from the storage tank 10 exceeds the amount of boil-off gas required as fuel in the high-pressure injection engine 40, the excess boil-off is supplied to the extraction flow path L2. Gas will be led.

The pre-pressurized boil-off gas discharged from the storage tank 10 to the boil-off gas discharge path L1 gives cold heat to the refrigerant N2 by heat exchange with the refrigerant N2 circulating in the refrigeration cycle circuit C, and is extracted at the boil-off gas compression unit 20. In the process of flowing through the extraction flow path L2 after being compressed to the pressure, it is cooled and reliquefied by heat exchange with the refrigerant N2 circulating in the refrigeration cycle circuit C.
Here, first, the refrigeration cycle circuit C will be described. The refrigeration cycle circuit C circulates nitrogen as the non-condensable refrigerant N2, and cools and reliquefies the boil-off gas in the first heat exchanger EX10. It is the circuit provided in.
The refrigeration cycle circuit C includes a plurality of refrigerant compression compressors CP6, CP7, and CP8 (three in the present embodiment) that compress the refrigerant N2 in the order described in the flow direction of the refrigerant N2, as the refrigerant compression unit 70. Each of the refrigerant compressors EX6, EX7, EX8 that cools the boil-off gas after being compressed and heated by the refrigerant compressors CP6, CP7, CP8 in the form of heat exchange with other refrigerants is compressed in the flow direction of the refrigerant N2. At the downstream outlets of the compressors CP6, CP7, CP8, the refrigerant compression compressors CP6, CP7, CP8 are arranged one by one in the order described in the flow direction of the boil-off gas.
When the explanation is added, the refrigerant N2 is compressed by the sixth refrigerant compression compressor CP6 and then cooled by the sixth cooler EX6. The refrigerant N2 cooled by the sixth cooler EX6 is converted into the seventh refrigerant compression compressor. After being compressed by CP7, cooled by the seventh cooler EX7, the refrigerant N2 cooled by the seventh cooler EX7 is compressed by the eighth refrigerant compression compressor CP8, and then by the eighth cooler EX8. To be cooled.
Further, the refrigeration cycle circuit C includes a second heat exchanger EX9 that cools the refrigerant N2 cooled by the eighth cooler EX8 in a form of heat exchange with the boil-off gas flowing through the boil-off gas discharge path L1, and the second heat exchanger EX9. And an expander EP1 (an example of an expansion unit) that expands the refrigerant N2 after passing through the two heat exchangers EX9.
As a result, the refrigerant N2 circulating in the refrigeration cycle circuit C is cooled in the order described in the plurality of coolers EX6, EX7, EX8 while being compressed in the order described in the plurality of refrigerant compression compressors CP6, CP7, CP8. After further cooling in the heat exchanger EX9, the expander EP1 expands, and the boil-off gas is cooled to a temperature at which the boil-off gas can be supercooled (for example, a temperature of −170 ° C. or lower), and then the first heat exchanger EX10 is The refrigeration cycle circuit C is circulated in such a form that the boil-off gas passing through and passing through the extraction flow path L2 is cooled and reliquefied.

In the extraction flow path L2, the first heat exchanger EX10 that cools and reliquefies the boil-off gas flowing through the extraction flow path L2 by heat exchange with the refrigerant N2 circulating in the refrigeration cycle circuit C, and the first A pressure reducing valve V1 for reducing the pressure of the boil-off gas after passing through the heat exchanger EX10 and a gas-liquid separator 30 for separating the boil-off gas after being reduced by the pressure reducing valve V1 are provided.
The extraction flow path L2 connects the lower part of the gas-liquid separator 30 and the storage tank 10, and the liquefied boil-off gas (L) separated by the gas-liquid separator 30 is the extraction flow path. It is returned to the storage tank 10 via L2. On the other hand, the gas boil-off gas (G) that has been gas-liquid separated by the gas-liquid separator 30 is discharged from the gas-liquid separator 30 as a flush flow.

  In the boil-off gas reliquefaction facility 100 of the present embodiment, the control device 60 that functions as a pressure setting unit 60a (an example of a pressure setting unit) that can set the extraction pressure of the boil-off gas from the boil-off gas compression unit 20 is provided. The control device 60 is configured to be able to cooperate hardware composed of an integrated device such as an LSI and software composed of a plurality of programs.

Here, when the extraction pressure of the boil-off gas is a general extraction pressure (for example, a pressure of about 0.3 MPaG to 2.0 MPaG) in a shipboard boil-off gas reliquefaction system equipped with a conventional nitrogen refrigerant cycle, the first The TQ diagram showing the relationship between the heat exchange amount and the temperature of the boil-off gas to be heat exchanged in the heat exchanger EX10 is in a state as indicated by a thick solid line in FIG. That is, when the extraction pressure of the boil-off gas is low, the temperature change becomes discontinuous at the point where the boil-off gas starts to cool and condense. And when it becomes a gas-liquid mixed state, it changes isothermally.
Therefore, the temperature difference between the boil-off gas and the refrigerant N2 comes closest to the point where the boil-off gas begins to condense (this point (the point indicated by P1 in FIG. 6) is called a pinch point). Due to this pinch point, the rate of temperature change with respect to the heat exchange amount of the refrigerant N2 (inclination γ in FIG. 6) decreases with the temperature change of the boil-off gas. (In other words, it is necessary to increase the flow rate of the refrigerant N2.) For this reason, the compression power of the refrigerant compression compressors CP6, CP7, CP8 of the refrigeration cycle circuit C increases, leading to deterioration in efficiency.
In addition, the difference between the temperature of the boil-off gas on the heat transfer side and the temperature of the refrigerant N2 on the heat reception side (ΔT2 and ΔT3 in FIG. 6) is large, which is the heat exchange efficiency in the first heat exchanger EX10. Indicates that it is bad.

Therefore, when the lower limit pressure of the extraction pressure is set, the pressure setting unit 60a sets the lower limit pressure of the extraction pressure to the critical pressure of the boil-off gas (when the boil-off gas is a gas containing methane as a main component, the pressure exceeding 4.8 MPaG). When the boil-off gas is pure methane, the pressure exceeds 4.5 MPaG).
More specifically, the pressure setting unit 60a increases the extraction pressure of the boil-off gas higher than the general extraction pressure in the onboard boil-off gas reliquefaction system equipped with the conventional nitrogen refrigerant cycle, so that the thick solid line in FIG. In the TQ line of the boil-off gas, the width A indicating the isothermal change in the gas-liquid mixed state can be reduced. That is, the TQ line indicated by the thick solid line in FIG. 6 is changed to the TQ line indicated by the thick broken line in FIG. 2 (bleeding pressure 7 MPaG), and further the TQ line indicated by the thick solid line in FIG. As a result, the flow rate of the refrigerant N2 can be reduced (the inclination of the refrigerant N2 in the TQ diagram can be increased), and the compression power of the refrigerant compression compressors CP6, CP7, CP8 can be reduced.
The TQ diagram shown in FIG. 2 is a TQ diagram in the first heat exchanger EX10 when the extraction pressure of the boil-off gas is 7 MPaG and the extraction pressure of 10 MPaG. When the extraction pressure of the boil-off gas is 7 MPaG, saturated steam The difference in slope between the line (thick broken line near the center of the graph in FIG. 2) and the superheated steam line (thin broken line near the right side of the graph in FIG. 2) becomes larger compared to 10 MPaG (the swell of the graph increases). This is because when the extraction pressure is set to 7 MPaG, the rate of temperature change with respect to the heat exchange amount of the refrigerant N2 becomes smaller (that is, the N2 flow rate becomes larger) than when the extraction pressure is set to 10 MPaG. It shows that the compression power of the refrigerant compression compressor CP of the circuit C is increased and the efficiency is deteriorated. Further, the temperature difference between the TQ line indicated by the boil-off gas on the heat transfer side and the TQ line indicated by the refrigerant N2 on the heat receiving side is increased, indicating that the heat exchange efficiency is poor.
In other words, setting the extraction pressure to 10 MPaG can improve the heat exchange efficiency in the first heat exchanger EX10 than setting the extraction pressure to 7 MPaG, and the refrigerant N2 in the first heat exchanger EX10 can be improved. Since the loss of the amount of cold heat given from the refrigerant can be reduced, the compression power of the refrigerant compressor 70 (compression power of the refrigerant compression compressor) can be reduced.
Here, as the power consumed in the process of liquefying the boil-off gas, the compression power in the boil-off gas compression unit 20 (specifically, the boil-off gas compression compressor CP1 that compresses the boil-off gas in the boil-off gas compression unit 20, CP2, CP3, CP4) and the compression power in the refrigeration cycle circuit C (specifically, the compression power of refrigerant compression compressors CP6, CP7, CP8 that compresses the refrigerant N2 in the refrigeration cycle circuit C). However, considering the heat exchange loss in the first heat exchanger EX10 and the like, when the extraction pressure is sufficiently smaller than the extraction pressure set in the present invention, the unit is referred to as “refrigeration cycle” per unit boil-off gas. The relationship of “compression power in circuit C> compression power of boil-off gas compression unit 20” is established.
Here, the pressure setting unit 60a sets the extraction pressure to 10 MPaG or more, so that the temperature difference between the boosted boil-off gas passing through the first heat exchanger EX10 and the refrigerant N2 is changed to the first heat exchanger EX10. In the refrigeration cycle circuit C (specifically, the compression power of the refrigerant compression compressors CP6, CP7, CP8 for compressing the refrigerant N2 in the refrigeration cycle circuit C). It is possible to improve the efficiency of the entire equipment.
From the above, in the present embodiment, the pressure setting unit 60a sets the extraction pressure of the boil-off gas to 10 MPaG or more.
Incidentally, the calculation conditions of the TQ diagram when the extraction pressure of the boil-off gas is 10 MPaG are as follows [Table 1] in P1 to P6 shown in the boil-off gas discharge passage L1 and the extraction passage L2 in the schematic configuration diagram of FIG. The flow rate, temperature, and pressure shown in FIG. The underlined value is a substantially constant value regardless of the extraction pressure. The tank pressure is set to a pressure of about 0 MPaG to 0.035 MPaG.

  On the other hand, if the bleed pressure of the boil-off gas is excessively increased, the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 increases, resulting in a decrease in efficiency and the flash flow rate generated when the pressure is reduced by the pressure reducing valve V1. Increases, and the boil-off gas (G) that is discarded from the gas-liquid separator 30 increases.

FIG. 3 shows a TQ diagram when the extraction pressure of the boil-off gas is set to 13, 30 MPaG in the pressure setting unit 60a.
From the TQ diagram shown in FIG. 3, when the extraction pressure is increased from 13 MpaG to 30 MPaG, the compression power of the boil-off gas compression unit 20 is significantly increased, but the heat exchange efficiency in the first heat exchanger EX10 is further improved. To do. However, since it is necessary to suppress the increase in the flow rate of the flash flow accompanying the increase in the extraction pressure, it is necessary to increase the supercooling amount of the boil-off gas, so the extraction pressure is set to 13 MPaG as shown in FIG. Compared to the case, the refrigeration cycle circuit C needs to lower the temperature of the refrigerant N2 entering the first heat exchanger EX10 by a temperature indicated by ΔT1 in FIG. This means an increase in compression power in the refrigeration cycle circuit C, and therefore a deterioration in efficiency in the reliquefaction process.
On the other hand, when the extraction pressure is set to 13 MPaG, the temperature at which the refrigerant N2 enters the first heat exchanger EX10 in the refrigeration cycle circuit C can be set to substantially the same temperature as when the extraction pressure is set to 10 MPaG. This is because, when the extraction pressure is set to 13 MPaG, even when the temperature of the refrigerant N2 to the first heat exchanger EX10 is set to approximately the same temperature as when the extraction pressure is set to 10 MPaG in the refrigeration cycle circuit C, It shows that the flow rate of the flash flow can be sufficiently suppressed. Furthermore, when the extraction pressure is set to 13 MPaG, the TQ line indicated by the boosted boil-off gas on the heat transfer side is almost linear while maintaining the compression power in the refrigeration cycle circuit C constant. It can be changed to.
Therefore, in the present embodiment, the pressure setting unit 60a sets the extraction pressure of the boil-off gas to 13 MPaG or less.

Further, the pressure setting unit 60a according to the embodiment includes the compression power related to the extraction of the boil-off gas compression compressors CP1, CP2, CP3, and CP4 and the refrigerant compression compressors CP6 and CP7 in the process of reliquefaction of the extracted boil-off gas. The extraction pressure is set so that the total power with the compression power of CP8 becomes small.
When the explanation is added based on FIG. 4, when the extraction pressure of the boil-off gas is gradually increased from 0 MPaG in the process of re-liquefying the extracted boil-off gas, the boil-off gas compression compressor CP1, as shown in FIG. Among the compression powers of CP2, CP3, and CP4, the compression power related to the extraction pressure of the boil-off gas extracted (power indicated by the legend in FIG. 4) gradually increases.
On the other hand, in the configuration according to this embodiment, when the boil-off gas extracted is reliquefied, the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 is reduced until the extraction pressure reaches a predetermined pressure. The larger the bleed pressure (the higher the bleed pressure), the boil-off gas TQ line in the first heat exchanger EX10 changes from a TQ line having a discontinuous temperature change as shown by a thick solid line in FIG. 6 to a thick solid line in FIG. The temperature change as shown changes to a continuous smooth TQ line. Thereby, the TQ line of the refrigerant N2 in the first heat exchanger EX10 can increase the ratio of the temperature change with respect to the heat exchange amount in accordance with the temperature change of the TQ line of the boil-off gas (that is, the flow rate of the refrigerant N2 can be reduced). Therefore, the compression power of the refrigerant compression compressors CP6, CP7, CP8 (power indicated by the legend of ■ in FIG. 4) can be reduced.
On the other hand, when the extraction pressure exceeds a predetermined pressure and the compression power of the boil-off gas compression compressors CP1, CP2, CP3, CP4 is increased, when the pressure is reduced by the pressure reducing valve V1 after passing through the first heat exchanger EX10. Therefore, it is necessary to increase the degree of supercooling of the boil-off gas in the first heat exchanger EX10 in order to suppress the flow rate of the flash flow (in the example of FIG. 3, Since it is necessary to increase the degree of supercooling by the temperature indicated by ΔT1, the compression power of the refrigerant compression compressors CP6, CP7, CP8 increases.
From these relationships, in the process of reliquefying the boil-off gas, the compression power related to the extraction pressure of the boil-off gas extracted from the compression power of the plurality of boil-off gas compression compressors CP1, CP2, CP3, CP4, and the refrigerant compression compressor As shown in FIG. 4, the total power including the compression power of CP6, CP7, and CP8 is the minimum when the extraction pressure is in the range of 10 MPaG to 13 MPaG (the width indicated by ΔP in FIG. 4).
Therefore, the pressure setting unit 60a according to the embodiment sets the extraction pressure to 10 MPaG or more and 13 MPaG or less in order to reduce the total power and improve the efficiency in the reliquefaction process.

  In addition, the concrete numerical value of the graph shown in FIG. 4 is shown in the following [Table 2]. Incidentally, the compression power of the boil-off gas compression compressor shown in [Table 2] is the compression power related to the extraction pressure of the boil-off gas extracted from the compression power of the plurality of boil-off gas compression compressors CP1, CP2, CP3, and CP4. However, it also includes compression power for compressing the boil-off gas introduced to the high-pressure injection engine 40. Refer to Table 1 for the calculation conditions of FIG. 4 and [Table 2].

In addition, in the boil-off gas reliquefaction equipment 100 according to the present embodiment, the technique disclosed in Patent Document 1, and the technique disclosed in Patent Document 2, the result of simulating the power consumption required for liquefaction per ton of boil-off gas is shown. It is shown in the following [Table 3]. Incidentally, [Table 3] shows the power consumption when the main engine load is 0%, and the main engine load indicates the load of the high-pressure injection engine. Here, although the technique disclosed in Patent Document 2 does not include a main engine (high-pressure injection engine), in the following simulation, a boil-off gas compression unit (including a cooler) disclosed in Patent Document 2 is used. ) On the downstream side of the second heat exchanger is connected to a flow path that branches a part of the boil-off gas, and the boil-off gas is supplied to the main engine (high pressure injection engine) through the flow path. It has been adopted.
In addition, as a simulation condition, boil-off is performed at the arrival temperature (for example, −120 ° C.) of each of the present embodiment, the technique disclosed in Patent Document 1, and the technique disclosed in Patent Document 2 at the respective liquefaction apparatuses. Assuming that 2883 kg / h of gas is generated and the main engine load is 30%, about 1400 kg / h of the generated boil-off gas is sent to the main engine, and the remaining part is sent to the generator. Was liquefied.
The electric power generated by the generator is used as power for the compressor for supplying power to the inboard power and the main engine and driving the refrigerant cycle. Further, since the liquefaction power is different between the present embodiment, the technique disclosed in Patent Document 1, and the technique disclosed in Patent Document 2, the amount of boil-off gas sent to the generator is different.

[Another embodiment]
(1) In the above embodiment, the boil-off gas reliquefaction facility 100 is provided in the LNG carrier 50 that transports LNG, but is not limited to this configuration.
As another example, after the mined LNG is refined at sea, it is directly liquefied and stored in the storage tank 10, and if necessary, the LNG stored in the storage tank 10 is transferred to the LNG carrier. It may be provided in an offshore plant such as LNG FPSO (Flowing Production Stage and Off-loading), which is a facility used to do this.

(2) In the above embodiment, the refrigerant N2 circulating in the refrigeration cycle circuit C has been described by taking nitrogen as a non-condensable refrigerant as an example, but the refrigerant is not limited to nitrogen. As an example of other refrigerants, a mixed refrigerant other than helium and hydrogen may be used.

(3) In the above embodiment, the extraction flow path L2 connects the gas-liquid separator 30 and the storage tank 10 and stores the liquefied boil-off gas (L) after the gas-liquid separator 30 separates the gas and liquid. A configuration example leading to the tank 10 is shown.
However, the boil-off gas recirculated with the boil-off gas flowing through the extraction flow path L2 does not necessarily need to be returned to the storage tank 10, and in some cases, the cold heat of the boil-off gas is used for air conditioning or cold power generation. A configuration may be adopted.

(4) In the above embodiment, the number of boil-off gas compression compressors is not particularly limited to the one shown in the above-described embodiment, but the amount of boil-off gas (or the capacity of the storage tank 10), the engine supply It can be appropriately changed depending on the pressure. Furthermore, the number of boil-off gas compression compressors provided on the upstream side of the extraction flow path L2 in the flow direction of the boil-off gas before pressurization is not limited to four, and can be changed as appropriate.
Further, the number of refrigerant compression compressors is not particularly limited, and can be appropriately changed according to the temperature and the amount of cold heat required in the first heat exchanger EX10.
And the number of coolers can also be changed suitably in the state corresponding to the number of boil-off gas compression compressors and refrigerant compression compressors.

(5) In the above-described embodiment, various configurations are conceivable as a configuration for supplying cold heat to the coolers in the boil-off gas compression unit 20 and the refrigeration cycle circuit C. For example, the following configurations may be adopted. .
That is, an absorption refrigeration machine (not shown) that uses the exhaust heat of the high-pressure injection engine 40 as the main engine as a heat source, and recovers cold heat generated in the absorption refrigeration machine and boil-off gas compresses the recovered cold heat You may employ | adopt the structure provided with the heat medium circulation circuit (not shown) which can circulate the heat medium supplied with the cooler in the part 20 and the refrigerating cycle circuit C. FIG.
The heat medium circulation circuit may be arranged to supply cold heat to all of the coolers in the boil-off gas compression unit 20 and the refrigeration cycle circuit C, or may be arranged to supply cold heat to a part thereof. It doesn't matter.

  When the main engine is operating at a high load and the boil-off gas amount is less than the fuel amount required by the main engine, the LNG stored in the tank is discharged by a pump and forced to vaporize into the main engine. Although the case where it supplies may be considered, you may employ | adopt the structure which uses the coolers EX1-EX5 of the boil-off gas compression part 20 as a vaporization heat source at this time.

  The configuration disclosed in the above embodiment (including another embodiment, the same shall apply hereinafter) can be applied in combination with the configuration disclosed in the other embodiment, as long as no contradiction occurs. The embodiment disclosed in this specification is an exemplification, and the embodiment of the present invention is not limited to this. The embodiment can be appropriately modified without departing from the object of the present invention.

  The boil-off gas reliquefaction facility according to the present invention improves the heat exchange efficiency in the heat exchanger while reducing the flow rate of the flash flow while improving the efficiency while avoiding the complexity of the configuration. It can be effectively used as a boil-off gas reliquefaction facility that can improve efficiency by reducing the compression power of the circuit.

DESCRIPTION OF SYMBOLS 10: Storage tank 20: Boil-off gas compression part 30: Gas-liquid separator 40: High pressure injection type engine 60a: Pressure setting part 70: Refrigerant compression part 100: Reliquefaction equipment BOG: Boil-off gas C: Refrigeration cycle circuit CP1: First Boil-off gas compression compressor CP2: Second boil-off gas compression compressor CP3: Third boil-off gas compression compressor CP4: Fourth boil-off gas compression compressor CP5: Fifth boil-off gas compression compressor CP6: Sixth refrigerant compression compressor CP7: Seventh refrigerant compression compressor CP8: 8th refrigerant compression compressor EP1: Expander EX10: 1st heat exchanger EX9: 2nd heat exchanger L2: Extraction flow path LNG: Liquefied natural gas N2: Refrigerant V1: Pressure reducing valve

Claims (4)

A storage tank that stores liquefied natural gas, a boil-off gas compression unit that compresses boil-off gas discharged from the storage tank, and a high-pressure injection that uses a part of the liquefied boil-off gas compressed by the boil-off gas compression unit as fuel Heat exchange with the mold engine, the extraction passage for extracting and re-liquefying the other portion of the boil-off gas compressed by the boil-off gas compression section, and returning it to the storage tank, and the boil-off gas flowing through the extraction passage A boil-off gas reliquefaction facility comprising a refrigeration cycle circuit for circulating a refrigerant for cooling the boil-off gas on the water structure,
The bleed flow passage bleeds the boil off gas before being boosted up to the engine supply pressure of the high pressure injection engine in the boil off gas compression unit,
The refrigeration cycle circuit flows through the refrigerant compression section that compresses the refrigerant, the expansion section that expands the refrigerant compressed by the refrigerant compression section, the refrigerant expanded by the expansion section, and the extraction channel. A first heat exchanger for exchanging heat with the boil-off gas,
A second heat exchanger that exchanges heat between the boil-off gas flowing from the storage tank to the boil-off gas compression unit and the refrigerant flowing from the refrigerant compression unit to the expansion unit in the refrigeration cycle circuit;
A boil-off gas reliquefaction facility comprising pressure setting means for setting a bleed pressure of boil-off gas to be bleed into the bleed flow path to be lower than the engine supply pressure and higher than a critical pressure of the boil-off gas. A decompression valve that decompresses the boil-off gas that has passed through the first heat exchanger in the extraction flow path, and a gas-liquid separator that separates the boil-off gas decompressed by the decompression valve,
2. The boil-off gas according to claim 1, wherein the pressure setting means sets the upper limit pressure of the extraction pressure to be less than a flash flow suppression pressure at which a flow rate of a flash flow discharged as a gas from the gas-liquid separator is suppressed. Reliquefaction equipment. The boil-off gas compression unit includes a plurality of boil-off gas compression compressors that compress boil-off gas,
The refrigerant compression unit includes a refrigerant compression compressor that compresses the refrigerant,
The pressure setting means includes a compression power related to the extraction pressure of the boil-off gas extracted from the compression power of the plurality of boil-off gas compression compressors in the process of re-liquefying the extracted boil-off gas, and the extraction flow path. 3. The boil-off gas according to claim 1, wherein the extraction pressure is set so that a total power with a compression power of the refrigerant compression compressor when cooling the boil-off gas passing through the first heat exchanger is reduced. Reliquefaction equipment.   The boil-off gas reliquefaction facility according to any one of claims 1 to 3, wherein the pressure setting means sets the extraction pressure to 10 MPaG or more and 13 MPaG or less.

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