JP2016169837A - Boil-off gas recovery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform an efficient recovery of boil-off gas at a liquefaction gas carrier and improve a degree of freedom in selection of a target ship speed.SOLUTION: BOG in a cargo tank 11 is sent as cooling side gas to a first heat exchanger 14 through a first piping 12 and further compressed at a multi-stage high pressure gas compressor 15. Middle pressure BOG with a pressure more than a critical point is extracted from a middle stage of the high pressure gas compressor 15, transported as cooled side gas to the first heat exchanger 14 through a second piping 16 and cooled. BOG is further cooled with a re-cooling device 24, expanded by a Joule-Thomson valve 17 and liquefied. The liquefied BOG and gas-liquid double phase flow of gas gasified through flash are transferred to a separator 18. Gas composition separated by the separator 18 is returned back an upstream side of the first heat exchanger 14 at the first piping 12 through a pressure adjustment valve 19, liquid composition is returned back to the cargo tank 11 through a transfer pump 21.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a system for collecting boil-off gas (BOG) generated in a cargo tank loaded with liquefied gas.

  Some liquefied natural gas carriers (LNG tankers) use boil-off gas (BOG) generated in cargo tanks as fuel for propulsion engines, power generation engines, and steam boilers. However, in recent years, the propulsion efficiency has been improved by adopting electric propulsion using a dual fuel-fired medium speed diesel power generation engine and dual fuel-fired low-speed diesel direct-propulsion. As a result, the amount of fuel gas consumed by the propulsion engine is suppressed, and the ship speed range in which all of the boil-off gas cannot be consumed as fuel is widening. Therefore, the surplus boil-off gas needs to be re-liquefied by a re-liquefaction device (Patent Documents 1 and 2) using a compressor and a refrigerant and collected in a cargo tank, or incinerated by a gas combustion device or a gas-fired boiler.

JP 2001-132899 A JP 2005-265170 A

  When a conventional reliquefaction apparatus using a compressor or a refrigerant is installed, the initial cost increases and the power consumption is large, so the operation cost is high. On the other hand, in an LNG tanker using a dual fuel-fired low-speed diesel direct drive, a high-pressure gas compressor is mounted to supply a high-pressure gas of about 30 MPa to a diesel engine. Therefore, in such a ship, it is conceivable to configure the BOG recovery device using a high-pressure gas compressor that supplies fuel gas to a dual fuel-fired low-speed diesel engine.

  An object of the present invention is to efficiently recover boil-off gas in a liquefied gas carrier ship and increase the degree of freedom in selecting a target ship speed.

  The boil-off gas recovery system of the present invention includes a high-pressure gas compressor that compresses boil-off gas from a cargo tank, boil-off gas that is sent to the high-pressure gas compressor, and boil-off gas that has been compressed to a pressure higher than a critical point in the high-pressure gas compressor. After depressurizing the boil-off gas compressed in the high-pressure gas compressor and cooled through the first heat exchanger to the predetermined pressure above the critical point or above the critical point. Further, recooling means for further cooling, boil-off gas cooled by the recooling means, liquefying means for liquefying at least a part thereof, and gas-liquid two-phase flow in which part of the boil-off gas is liquefied by the liquefaction means A separator that separates the gas component into a liquid component and a gas component separated by the separator. A inlet side of the high pressure gas compressor from data, causes reflux to the upstream side of the first heat exchanger is characterized by transferring the liquid component separated by the separator into the cargo tank.

  The liquefying means preferably comprises an expansion valve or a Joule Thomson valve. The boil-off gas recovery system preferably further includes a pressure adjusting valve in a path for returning the gas component separated by the separator to the upstream side of the first heat exchanger, and the pressure in the separator is reduced by the pressure adjusting valve. A predetermined pressure is maintained. The recooling means preferably includes a second heat exchanger that exchanges heat between the boil-off gas and the refrigerant. In addition, the boil-off gas recovery system may include a transfer pump that transfers the liquid component from the separator to the cargo tank.

  The boil-off gas recovery system preferably further includes a flow rate control valve that controls the flow rate of the boil-off gas that is compressed in the high-pressure gas compressor and transferred to the first heat exchanger, and controls the liquefaction recovery amount of the boil-off gas recovery system. . The high-pressure gas compressor discharges a part of the compressed boil-off gas as fuel for a dual fuel-fired low-speed diesel engine. The high-pressure gas compressor is, for example, a multistage compressor, and boil-off gas extracted from the middle stage is supplied to the first heat exchanger. For example, the boil-off gas is decompressed from the discharge side of the high-pressure gas compressor and supplied to the first heat exchanger.

  The liquefied gas carrier of the present invention is characterized by including any of the above boil-off gas recovery systems.

  According to the present invention, a boil-off gas can be efficiently recovered and a degree of freedom in selecting a target ship speed can be increased in a liquefied gas carrier ship.

It is a block diagram which shows the structure of the boil off gas collection | recovery system of this embodiment of this invention. It is a figure which shows the thermodynamic path | route of the reliquefaction process of this embodiment in the pressure-specific enthalpy (ph) diagram of the methane which is a main component of boil-off gas. In the first heat exchanger, when (a) heat exchange is performed in a state where the gas to be cooled is under the critical point, and (b) heat exchange is performed in a state where the gas to be cooled is over the critical point. It is a graph which shows the amount of exchange heat inside a heat exchanger, and the change of temperature. It is a figure explaining the cooling effect of a 1st heat exchanger in the pressure-specific enthalpy (ph) diagram of methane which is the main ingredients of boil-off gas. It is a graph which shows the relationship between the ship speed at the time of liquefied gas loading in this embodiment, consumption gas fuel consumption, and boil-off gas generation amount. It is a block diagram which shows the structure of the boil off gas collection | recovery system of a modification.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a configuration of a boil-off gas recovery system according to an embodiment of the present invention. FIG. 2 shows a thermodynamic path in the reliquefaction process of this embodiment in the pressure-specific enthalpy (ph) diagram of methane, which is the main component of the boil-off gas.

  The boil-off gas recovery system 10 of the present embodiment is applied to a ship carrying a high-pressure gas compressor and carrying a liquefied gas such as natural gas, and in particular, a liquefied gas carrier ship using dual fuel-fired low-speed diesel (direct connection) propulsion. Application to is preferable.

  The liquefied gas (LNG in the present embodiment) is loaded on the cargo tank 11, and the boil-off gas (about −162 ° C.) generated in the cargo tank 11 is guided to the cargo machinery room 13 through the first pipe 12, and the first It is transferred to the high-pressure gas compressor 15 via the heat exchanger 14. The high-pressure gas compressor 15 is, for example, a multistage compressor, and a high-pressure gas of about 30 MPa is discharged from the discharge side, for example, a dual fuel-fired low-speed diesel engine (main engine) or a dual-fuel-fired diesel for power generation. It is supplied as gas fuel to engines and gas-fired boilers. In addition, when the high pressure gas compressor 15 is a multistage compressor and the required gas pressure of the power generating diesel engine or boiler is lower than the high pressure gas, the fuel gas to these is extracted and supplied from the middle stage of the compressor You can also.

  On the other hand, from the middle stage of the high-pressure gas compressor 15, surplus boil-off gas is extracted into the second pipe 16 as, for example, a medium-pressure gas (about 45 ° C.) of about 10 MPa, and the first heat is passed through the second pipe 16. It is transferred to the exchanger 14. In the first heat exchanger 14, the boil-off gas passing through the first pipe 12 is used as a cooling side fluid to cool the medium pressure boil-off gas (cooled side gas) in the second pipe 16, and the boil-off gas is, for example, about − It is cooled to around 100 ° C. (FIG. 2, path L2).

  The flow rate control valve 23 provided in the second pipe 16 controls the flow rate of the medium pressure gas transferred to the first heat exchanger 14 to adjust the liquefaction amount, thereby controlling the liquefaction recovery amount of the boil-off gas recovery system. can do. In FIG. 1, the flow control valve 23 is installed on the upstream side of the heat exchanger, but may be installed on the downstream side.

  Here, the pressure of the boil-off gas extracted from the middle stage of the high-pressure gas compressor 15 is a pressure equal to or higher than the critical point of the same gas (in this embodiment, the critical point pressure is about 4 MPa), and the first heat exchanger 14 The boil-off gas cooled in is in a supercritical state. In the high-pressure gas compressor 15, for example, an oil-free compressor is used for the stage until the excess boil-off gas is extracted, and an oil-feed compressor is used for the discharge-side high-pressure stage thereafter. Good. In addition, when using an oil supply type compressor for all, the filter (not shown) for removing the oil component carried over to the 2nd piping 16, for example is arrange | positioned.

  The downstream side of the first heat exchanger 14 of the second pipe 16 is led to a recooling device 24 including, for example, a second heat exchanger. In other words, the boil-off gas cooled by the first heat exchanger 14 exchanges heat with the low-temperature refrigerant cycle using the refrigerant pipe 25 in the second heat exchanger of the recooling device 24 and exceeds about −100 ° C. to −165 ° C. Further cooling is performed while maintaining the critical state (FIG. 2, path L4). Further, a Joule Thomson (JT) valve (or expansion valve) 17 is provided in the second pipe 16 on the downstream side of the recooling device 24. The boil-off gas is depressurized to a predetermined separator set pressure via the Joule-Thompson valve 17, and the boil-off gas becomes liquid from the supercritical state, and a part of the boil-off gas is flushed during expansion to become gas (path L5 in FIG. 2). As a result, the boil-off gas becomes a gas-liquid two-phase flow.

  The gas-liquid two-phase flow of the boil-off gas is then transferred to the separator 18 through the second pipe 16 and gas-liquid separation is performed. The boil-off gas separated by the separator 18 is refluxed to the upstream side of the first heat exchanger 14 of the first pipe 12 through the third pipe 20 provided with the pressure regulating valve 19. On the other hand, the separated liquefied gas (LNG) is transferred by the transfer pump 21 from the cargo machinery room 13 to the cargo tank 11 in the cargo section through the fourth pipe 22 and collected. If the reliquefied gas can be transferred to the cargo tank 11 by the separator pressure, the transfer pump 21 can be omitted. Further, the pressure in the separator 18 is maintained at a set pressure by the pressure adjusting valve 19.

  Next, the cooling effect in the first heat exchanger 14 will be described with reference to FIGS. 3 and 4. FIG. 3A shows the heat of the cooling side gas and the cooled side gas in the first heat exchanger 14 when the pressure of the boil-off gas (cooled side gas) extracted in the second pipe 16 is below the critical point. FIG. 3B is a graph showing changes in heat exchange and temperature inside the exchanger, and FIG. 3B corresponds to this embodiment, and the pressure of boil-off gas (cooled side gas) extracted to the second pipe 16 is critical. It is a graph corresponding to Drawing 3 (a) when it is more than a point. 3 (a) and 3 (b), the left end of the horizontal axis corresponds to the inlet of the cooling side gas and the outlet of the cooled side gas, and the right end corresponds to the outlet of the cooling side gas and the inlet of the cooled side gas. The axis is temperature (° C). FIG. 4 is a schematic pressure-specific enthalpy (ph) diagram of methane, which is the main component of the boil-off gas.

  As shown in FIG. 3 (a), when the pressure of the cooled side gas is below the critical point (for example, 3.5 MPa), the cooled side gas falls to the saturation temperature in the first heat exchanger 14, and thereafter Heat exchange is performed in a partially liquid state. At this time, since there is a heat exchange region where the heat exchange is at a low saturation temperature, the pinch point becomes severe during the heat exchange in the first heat exchanger 14 with the cooling side, and the amount of heat that can be exchanged is limited ( FIG. 4 path L1: cooling from 45 ° C. to −100 ° C. at a pressure below the critical point (3.5 MPa).

  On the other hand, as shown in FIG. 4, when the pressure of the cooled side gas is equal to or higher than the critical point (for example, 10 MPa), heat exchange is performed in a supercritical state, and the cooled side gas is liquefied in the first heat exchanger 14. There is nothing. That is, since there is no heat exchange in a saturated state (no phase change), the pinch point is relaxed (FIG. 3B), and a heat exchanger capable of sufficient heat exchange can be designed (path of FIG. 4). L2: cooled to 45 ° C. to −100 ° C. at a pressure (10 MPa) above the critical point).

  Next, the boil-off gas liquefaction amount adjustment control using the flow control valve 23 and the recooling device 24 will be described with reference to FIG.

  FIG. 5 is a graph showing the relationship between the operating vessel speed V and the used fuel gas consumption Q and the boil-off gas generation amount. In FIG. 5, the horizontal axis represents the ship speed V, and the vertical axis represents the fuel gas consumption Q. A curve S is a curve showing the relationship between the ship speed and the fuel gas consumption (unit time), and the fuel consumption Q is approximately proportional to the cube of the ship speed V. The straight line M1 (NATURAL BOG) is an amount per unit time at which the liquefied gas (natural gas) in the cargo tank 11 spontaneously evaporates and becomes boil-off gas. That is, in FIG. 5, when only the boil-off gas is used as fuel for the engine and boiler that consumes gas in the ship, a ship speed V1 corresponding to the intersection P1 of the curve S and the straight line M1 is obtained. On the other hand, on the lower speed side (V <V1 region) than the operating point P1, the difference between the straight line M1 and the curve S becomes surplus boil-off gas, and on the higher speed side (V> V1 region) than the operating point P1, the curve S and The difference between the straight lines M1 is the amount of gas fuel that needs to be added.

  When the flow control valve 23 is opened and the boil-off gas recovery device is operated while the recooling device 24 is stopped, the boil-off gas is partially liquefied by the first heat exchanger 14 and the Joule Thomson valve 17. As a result, the amount of boil-off gas generated per unit time is substantially reduced. Assuming that the amount of boil-off gas generated per unit time when the recovery by the liquefaction system using the first heat exchanger 14 and the Joule Thomson valve 17 is maximized is a straight line M2, the same liquefaction system allows the boil-off gas per unit time. Can be reduced from the straight line M1 to the straight line M2, and the intersection with the curve S indicating the operating point can be moved from P1 to P2. Therefore, it is possible to perform a decelerating operation that reduces the operation speed V from the ship speed V1 at the operating point P1 to the ship speed V2 at the operating point P2 while suppressing the generation of surplus boil-off gas.

  Further, when the operation speed V is operated between V1 and V2, the amount of liquefaction is controlled by controlling the flow rate of the cooled side gas transferred to the first heat exchanger 14 by the flow rate control valve 23. Therefore, the amount of boil-off gas can be optimally controlled between M1 and M2 in accordance with the gas fuel used, and the processing of surplus boil-off gas in this operation region is not necessary.

  Since M2 is a recovery limit of the liquefied gas of the liquefaction system using the first heat exchanger 14 and the Joule Thomson valve 17, in the present embodiment, the recooling device 24 is used for further deceleration operation. Thus, the amount of liquefaction of the boil-off gas is further increased, and the liquefied gas recovery capability of the boil-off gas recovery system 10 is increased. A straight line M3 in FIG. 5 per unit time when the first heat exchanger 14, the Joule Thomson valve 17 and the recooling device 24 are used and the liquefied gas recovery amount of the boil-off gas recovery system 10 of the present embodiment is maximized. The amount of boil-off gas generated. That is, the re-liquefaction amount during expansion through the Joule-Thomson valve 17 is adjusted by further lowering the temperature of the boil-off gas in the supercritical state before the Joule-Thomson valve 17 using the recooling device 24. Thereby, the boil-off gas generation amount per unit time can be adjusted between M2 and M3. In other words, it is possible to perform a reduced speed operation with the operating speed V lowered to the ship speed V3 of the operating point P3 that is the intersection of the straight line M3 and the curved line S without performing surplus boil-off gas processing (incineration disposal, etc.). .

  In the boil-off gas recovery system 10, the capacity of the high-pressure gas compressor 15 is determined from the amount of boil-off gas generated in the cargo tank 11 and the gas consumption of an engine or boiler that uses the gas as fuel. . The capacity of the high-pressure gas compressor is desirably a capacity obtained by further adding the amount of gas recirculated to the upstream side of the first heat exchanger 14 in order to increase the amount of liquefaction.

  Further, the cooling capacity (kW) in the recooling device 24 is liquefied using the first heat exchanger 14 and the Joule Thomson valve 17 from the separator set pressure and the boil-off gas generation amount when all the generated gas is reliquefied. When the partial reliquefaction is determined based on the gas amount obtained by subtracting the recovery limit amount by the system, liquefaction using the first heat exchanger 14 and the Joule Thomson valve 17 from the separator set pressure and the boil-off gas generation amount It is determined based on the recovery limit amount by the system and the gas amount obtained by subtracting the fuel gas consumption amount at the target ship speed.

  As described above, according to the boil-off gas recovery system of the present embodiment, the high-pressure gas compressor used for fuel gas supply of the dual fuel-fired low-speed diesel engine is used, and the boil-off gas before compression is used. Since the boil-off gas after compression is cooled and liquefied using the Joule Thomson valve (expansion valve), the boil-off gas can be re-liquefied and recovered very efficiently.

  It is also possible to arrange a recooling device (second heat exchanger) on the downstream side of the Joule Thompson valve. In this case, the recooling device (second heat exchanger) is cooled along the paths L2, L3, and L6 shown in FIG. Corresponds to the region cooled by the recooling device. Therefore, in the recooling device (second heat exchanger), heat exchange of the gas-liquid two-phase flow is performed. In this embodiment, since cooling is performed along the paths L2, L4, and L5 shown in FIG. 2 and the path L4 corresponds to the region cooled by the recooling device, the recooling device (second heat exchanger) is supercritical. Heat exchange is performed in the area. That is, in this embodiment, boil-off introduced into the recooling device (second heat exchanger) by disposing the recooling device (second heat exchanger) between the first heat exchanger and the Joule Thompson valve. The gas becomes a single phase in a supercritical state, the estimation of the physical properties of the boil-off gas is simplified, and the recooling device (second heat exchanger) can be easily designed. Further, the recooling device (second heat exchanger) is prevented from being eroded by droplets contained in the two-phase fluid.

  Further, when the recooling device (second heat exchanger) is disposed downstream of the Joule Thomson valve, the expanded boil-off gas is introduced into the recooling device (second heat exchanger), so that pressure loss is suppressed. Needs to increase the size of the recooling device (second heat exchanger). On the other hand, in this embodiment, since the boil-off gas before expansion is cooled, the reliquefaction device (second heat exchanger) can be reduced in size.

  Further, in this embodiment, since the gas component that has not been liquefied is recirculated to the cooling side of the first heat exchanger as a cooling side gas, a gas circulation amount in the boil-off gas recovery system can be secured at a certain amount or more, The cooling amount of the system can be increased by lowering the inlet temperature of the cooling side gas of the first heat exchanger.

  From the above, in the boil-off recovery system of the present embodiment, the boil-off gas can be recovered efficiently and the degree of freedom in selecting the target ship speed can be increased while reducing the initial cost and operating cost. It is done.

  Further, in this embodiment, the pressure in the separator is maintained at a set value by the pressure regulating valve, so that the inlet pressure of the transfer pump to the cargo tank is maintained at a constant value, and the effective suction head (available NPSH) of the transfer pump is Prevents falling below the required net suction head (required NPSH).

  In addition, as FIG. 6 shows, the 1st heat exchanger 14 and the 2nd heat exchanger of the recooling apparatus 24 may be integrated, and it is good also as the cooling apparatus 26, In this case, a collection | recovery system may be reduced more in size. it can.

  In the present embodiment, the cooled side gas is extracted from the middle stage of the high-pressure gas compressor and transferred to the heat exchanger, but it may be transferred from the discharge side to the heat exchanger through the decompressor. Good. In the present embodiment, only one recooling device is used. However, a plurality of small recooling devices may be arranged in parallel.

  Although the liquefied gas carrier ship which carries LNG which has methane as a main component was described in the above embodiment, it is applicable also to the liquefied gas carrier ship which carries cargoes other than LNG (for example, ethane etc.). Depending on the nature of the fuel gas used, the gas pressure at the engine inlet required by the low-speed diesel engine varies, and a higher pressure may be required than when LNG is used as fuel (for example, 40 MPa to 60 MPa). Since the pressure and temperature at the critical point of the liquefied gas fuel to be used and the temperature of the boil-off gas generated in the cargo tank differ depending on the properties of the liquefied gas being transported, the first heat exchanger outlet temperature and the second heat exchanger outlet temperature Need to be changed in accordance with the main component of the boil-off gas, but the same effect can be obtained with the same configuration as the embodiment using methane as the main component.

  In the embodiment, only one Joule-Thomson valve (or an expansion valve) is installed, and the recooling device (second heat exchanger) is arranged on the upstream side of the Joule-Thomson valve. If the inlet pressure of the recooling device (second heat exchanger) needs to be lower than the outlet pressure of the first heat exchanger due to the design pressure resistance of the heat exchanger), the first heat exchanger and the recooling A first-stage first Joule-Thompson valve is provided between the apparatus (second heat exchanger) and the primary cooling (Fig. 2, path L3 ') is performed, and then the recooling apparatus (second heat exchanger) The boil-off gas is introduced into the pipe and cooled (FIG. 2, path L4 ′), and a second-stage second Joule-Thompson valve is provided on the outlet side of the recooling device (second heat exchanger) to reduce the pressure to the set pressure of the separator (FIG. 2, path L5 ′). In this case, the inlet gas pressure of the recooling device (second heat exchanger) needs to be a pressure higher than the critical pressure.

10, 10 'Boil-off gas recovery system 11 Cargo tank 12 First piping 13 Cargo machinery room 14 First heat exchanger 15 High-pressure gas compressor (multistage compressor)
16 Second piping 17 Joule Thomson valve (expansion valve)
18 Separator 19 Pressure adjustment valve 20 3rd piping 21 Transfer pump 22 4th piping 23 Flow control valve 24 Recooling device (2nd heat exchanger)
25 Refrigerant piping 26 Cooling device

Claims (11)

A high-pressure gas compressor that compresses boil-off gas from the cargo tank;
A first heat exchanger for exchanging heat between the boil-off gas sent to the high-pressure gas compressor and the boil-off gas compressed to a pressure above a critical point in the high-pressure gas compressor;
Re-cooling means for further cooling after the pressure of the boil-off gas compressed in the high-pressure gas compressor and cooled via the first heat exchanger is reduced to a predetermined pressure equal to or higher than the critical point;
Liquefying means for expanding the boil-off gas cooled by the recooling means and liquefying at least a part thereof;
A separator that separates a gas-liquid two-phase flow in which a part of the boil-off gas is liquefied by the liquefaction means into a gas component and a liquid component;
The gas component separated by the separator is refluxed from the separator to the upstream side of the first heat exchanger on the inlet side of the high-pressure gas compressor, and the liquid component separated by the separator is A boil-off gas recovery system for liquefied gas, which is transferred to a cargo tank.   The boil-off gas recovery system according to claim 1, wherein the liquefying means includes an expansion valve or a Joule Thomson valve.   A pressure adjusting valve is further provided in a path for returning the gas component separated by the separator to the upstream side of the first heat exchanger, and the pressure in the separator is maintained at a predetermined pressure by the pressure adjusting valve. The boil-off gas recovery system according to any one of claims 1 and 2.   The boil-off gas recovery system according to any one of claims 1 to 3, wherein the recooling unit includes a second heat exchanger that exchanges heat between the boil-off gas and a refrigerant.   The boil-off gas recovery system according to any one of claims 1 to 4, further comprising a transfer pump that transfers the liquid component from the separator to the cargo tank.   The liquefied recovery amount of the boil-off gas recovery system is controlled by further comprising a flow rate control valve that controls the flow rate of the boil-off gas that is compressed in the high-pressure gas compressor and transferred to the first heat exchanger. Item 6. The boil-off gas recovery system according to any one of Items 1 to 5.   The boil-off gas recovery system according to any one of claims 1 to 6, wherein the high-pressure gas compressor discharges a part of the compressed boil-off gas as fuel for a dual fuel-fired low-speed diesel engine.   The boil-off according to any one of claims 1 to 7, wherein the high-pressure gas compressor is a multi-stage compressor, and the boil-off gas extracted from an intermediate stage is supplied to the first heat exchanger. Gas recovery system.   The boil-off gas recovery system according to any one of claims 1 to 7, wherein the boil-off gas is supplied to the first heat exchanger by reducing pressure from a discharge side of the high-pressure gas compressor.   The boil-off gas recovery system according to claim 4, wherein the first heat exchanger and the second heat exchanger of the recooling means are integrated.   A liquefied gas carrier ship comprising the boil-off gas recovery system according to any one of claims 1 to 10.

Images