METHOD AND DEVICE FOR RELIQUIDING AND RETURNING BOG INTO AN LNG TANK

Description

[0001] The invention relates to the technical field of reliquefaction of exhaust gas (BOG) from a liquefied natural gas (LNG) tank.

[0002] Recently, the global consumption of liquefied gases, such as liquefied natural gas (LNG), has increased sharply. Liquefied gas, which is produced by cooling natural gas to extremely low temperatures, has a small volume and is therefore well-suited for storage and transport. Furthermore, liquefied gases like LNG are low in pollutants and thus more compliant with regulatory requirements than, for example, heavy fuel oil.

[0003] LNG is a colorless and transparent liquid produced by cooling natural gas, which consists mainly of methane, to approximately -163°C. However, because natural gas liquefies at an extremely low temperature of -163°C under normal pressure, LNG can easily evaporate with even a slight increase in temperature. Therefore, in an LNG storage tank, LNG continuously evaporates naturally, producing boil-off gas (BOG).

[0004] The formation of BOG (barrel overgas) means a loss of stored LNG and therefore reduces transport efficiency, for example on an LNG tanker. Furthermore, if BOG accumulates in a storage tank, there is a risk of pressure buildup and damage to the tank.

[0005] To address the problem, a method has been proposed in which BOG is reliquefied for return to an LNG storage tank, a method in which BOG is fed as an energy source to an internal combustion engine, such as a marine engine, and combinations thereof. For example, US 2019 / 0351988 proposes feeding BOG from an LNG tank to a DFDE engine, an X-DF engine, or an ME-GI marine engine. Simultaneously, it proposes using BOG as a refrigerant for the reliquefaction of compressed BOG in a partial reliquefaction system (PRS). KR 101 814 439 B1 discloses a generic reliquefaction system or process.

[0006] However, the aforementioned system has the disadvantage that nitrogen accumulates in the gas mixture during the reliquefaction cycles. Natural gas is a gas mixture consisting primarily of methane, but often also contains ethane, propane, butane, and other hydrocarbons. Other minor components can include hydrogen sulfide, nitrogen, and carbon dioxide. Nitrogen typically comprises approximately 1 to 15% of natural gas. Nitrogen has a boiling point of -196°C, significantly lower than the boiling point of methane, which transitions into the gaseous phase at -161°C. Since N₂ is difficult to reliquefy in conventional BOG reliquefaction systems, its proportion in the mixture increases over time. The quality of the natural gas decreases. Furthermore, a considerable portion of the compression unit's capacity is consumed by the increased N₂ content, and the system's efficiency steadily declines.

[0007] It is therefore the object of the present invention to overcome the disadvantages of the prior art. In particular, it is the object of the present invention to provide a method for the partial reliquefaction of BOG or a partial reliquefaction system (PRS) in which the enrichment of N₂ is reduced or prevented over consecutive cycles.

[0008] The problem is solved by a method having the features of claim 1, or by a device having the features of claim 7.

[0009] In particular, the problem is solved by a process for reliquefying and returning exhaust gas (BOG) to a liquefied natural gas (LNG) tank, comprising the steps: a) Extracting BOG (F2) from the headspace of an LNG tank; b) Compressing the BOG in a first compression stage to a first pressure p1 between 8 and 18 bara and diverting a first portion of this gas; c) Further compressing a second portion of the gas from step b) in a final compression stage to a second pressure p2 ≥ 120 bara, preferably 120 to 400 bara, particularly preferably 150 to 300 bara; d) Cooling at least a portion of the further compressed gas from step c) to a first temperature T1 between -20°C and -100°C; e) Expanding the gas from step d) to a third pressure p3 between 8 and 20 bara; f) Separating the gas from step e) into a liquid phase and a gaseous phase in order to f 1 ) combine the gaseous phase with the diverted first part of the gas from step c); and f 2 ) return the liquid phase to the LNG tank.

[0010] It has been found that this method allows nitrogen to be extracted from the system particularly efficiently, and the nitrogen-enriched gas can then be put to good use. In step f), the nitrogen will be almost entirely in the gaseous phase. When this gaseous phase is combined with the BOG compressed to p1 in the first compression stage (step c), it can be used to reliably operate a low-pressure gas injection engine.

[0011] The process also has the advantage that flash gas is generally extracted from the PRS reliquefaction system instead of being recompressed. This reduces the load on the first compression stage compared to conventional systems. It can be smaller and / or operated more efficiently. Overall, energy consumption is reduced.

[0012] Further compression of a second portion of the gas from step b) in a final compression stage to a second pressure p2, followed by cooling (step d)) and isenthalpic expansion (step e)), serves to efficiently reliquefy the gas, partly utilizing the Joule-Thomson effect. A relatively high pressure is targeted for the second pressure p2, so that after the usual water cooling to Tw, a highly compressed gas at approximately 35-45°C with a correspondingly low enthalpy is present. Through further cooling steps and isenthalpic expansion e) of the gas, the compressed gas can be brought down to a temperature first to T1 and then, through expansion to pressure p3, to even lower temperatures, thus reaching a state favorable for phase separation.

[0013] According to the invention, cooling in step d) is carried out at least partially by heat exchange, preferably indirect heat exchange, with cooling BOG (F2) from the headspace of the LNG tank. While one or more cooling steps of the process as described can also be carried out using a separate cooling circuit with a suitable refrigerant, typically N2, this is complex and energy-intensive. In contrast, at temperatures just above the boiling point of LNG, refrigerant is already present in the system as BOG.

[0014] In a preferred embodiment, in sub-step f2, the liquid phase is cooled to a temperature T2 between -140 and -161°C before being returned to the LNG tank. This reduces the re-formation of BOG. ​​Preferably, this cooling is achieved by heat exchange in counterflow to the BOG from the LNG tank. This ensures optimal use of the available cooling capacity. Before or during the introduction of the liquid phase into the LNG tank, the reliquefied gas is finally depressurized to ambient pressure of 1 bara.

[0015] If, as described above, cooling in both step f 2 ) and step d) is carried out by heat exchange, preferably indirect heat exchange, with cooling BOG from the headspace of the LNG tank, it is advantageous if the BOG taken directly from the LNG tank and particularly cold is used for cooling in sub-step f 2 ) and the BOG is then used for cooling in step d), at a slightly increased temperature.

[0016] In a particularly preferred embodiment, the cooling in step d) is carried out at least partially by heat exchange with the gaseous phase from step f). After phase separation, the gaseous phase has a pressure p3 and typically a temperature of around -80°C. Since the gaseous phase is intended for use in a low-pressure gas injection engine, such low temperatures and often such high pressures are not required. The gaseous phase can therefore be used as a refrigerant in a cooling process. By further expanding the gaseous phase from step f) before its use as a coolant, the temperature can be lowered further by utilizing the Joule-Thomson effect.

[0017] It is particularly preferred if the gaseous phase from step f) and the BOG from the headspace of the LNG tank are both used as coolants in step d), wherein the gaseous phase from step f) is used for precooling the warmer compressed gas, while the BOG from the LNG tank is used for cooling the already precooled compressed gas. In such an arrangement, the gas, compressed to p2 and typically present at temperatures well above 100°C, is first cooled to approximately 35–45°C by water cooling, then cooled to intermediate temperatures of approximately 25 to -15°C by heat exchange with the gaseous phase from step f), and further downstream cooled to a temperature T1 between -20 and -100°C by heat exchange with cooling BOG from the LNG tank.This sequence of heat exchange steps utilizes the existing cooling capacity of BOG and compressed gas in an optimized way for the cooling capacity available in the system.

[0018] It is preferred if, in step d), a portion of the further compressed gas from step c) is fed to a supply line for a high-pressure gas injection engine (2). In this embodiment, the highly compressed gas at pressure p2 can be used to drive a high-pressure gas injection engine or, alternatively, can be reliquefied. Particularly on a liquefied gas tanker, natural gas is the fuel of choice for limiting the emission of air pollutants to a relatively low level. The adjustability of the quantity supplied to the gas injection engine or to the PRS allows for flexible adaptation to climatic and meteorological conditions as well as the fuel requirements of the high-pressure gas injection engine.

[0019] In a preferred embodiment, in step f), the pressure p3 is monitored and controlled so that it remains within a predetermined range. This can be achieved using a pressure sensor. The measured value allows the conditions in the gas-liquid separator to be optimized and, if necessary, the LNG delivery rate to be adjusted. Additionally or alternatively, in step f), a volume of the liquid phase can be monitored to control the return rate to the LNG tank depending on the value.

[0020] Another aspect of the invention relates to a device for reliquefying and returning exhaust gas (BOG) to a liquefied natural gas (LNG) tank comprising a first heat exchanger comprising a line for conveying cooling fluid, preferably BOG from an LNG tank, and a line for conveying compressed gas to be cooled, preferably in counterflow; a multi-stage compressor comprising at least a first compression stage and a last compression stage, wherein the first compression stage is configured to compress BOG (F2) from the LNG tank to a first pressure p1 between 8 and 18 bara, and wherein the last compression stage is configured to compress pre-compressed BOG to a second pressure p2 ≥ 120, preferably 120 to 400 bara, particularly preferably 150 to 300 bara; a branch line which is arranged to conduct fluid downstream of the first compression stage and which further downstream opens into a supply line for a low-pressure gas injection engine and / or a gas combustion unit; a return line;a first expansion unit configured to expand compressed gas from a second pressure p2 to a third pressure p3, wherein p3 is between 8 and 20 bara, preferably between 10 and 18 bara; a gas-liquid separator configured to separate a liquefied gas fraction at a pressure p3 for return to the LNG tank (3) and to direct a gaseous fraction into a bypass line, wherein the bypass line opens into the branch line; wherein the multi-stage compressor is connected upstream, carrying fluid, to the headspace of the LNG tank, preferably via the heat exchanger line for conveying cooling BOG, and wherein the multi-stage compressor is connected downstream, carrying fluid, via the return line to the line of the first heat exchanger for conveying compressed gas to be cooled, further downstream to the first expansion unit, and still further downstream to the gas-liquid separator; and wherein the first heat exchanger is in particular configured to transfer at least a portion of the gas to the second pressure p 2 further compressed BOG to a first temperature T 1 to cool between -20°C and -100°C.

[0021] Such a device is capable of carrying out a method according to the invention. The nitrogen present in the natural gas accumulates in the gas phase of the gas-liquid separator, is extracted from the system, and supplied to a useful purpose as a fuel mixture. Via the bypass line, the gaseous phase can be combined with the BOG compressed to p1 in the first compression stage, thereby ensuring a reliable fuel supply to the low-pressure gas injection engine. In general, the device according to the invention allows flash gas from the reliquefaction cycle to be extracted from the system instead of repeatedly compressing it, which reduces the load on the multi-stage compression arrangement, particularly the first compression stage, and enables a smaller design.

[0022] The first compression stage can comprise one or more piston compressors, each with a subsequent water cooling system. The same applies to each subsequent compression stage. It is preferred that the final compression stage also includes a water cooling system. This allows for the provision of highly compressed LNG at a pressure p2 and a temperature of approximately 35–45°C, which is well-suited for treatment in the PRS reliquefaction system.

[0023] The low-pressure gas injection engine, which is supplied with fuel via the supply line, typically uses gas at a pressure of approximately 6 to 18 bara, preferably at a pressure of around 6 bara. Since the pressure p2 in the branch line can be higher than this target pressure, a throttle valve can be provided between the branch line and the supply line to reduce the gas pressure.

[0024] The expansion unit can be an expansion valve or an expander. During expansion, the Joule-Thomson effect is used to further reduce the temperature of the gas to be reliquefied. Since the pressure p3 in the gas-liquid separator can be higher than the pressure p2 in the branch line, the bypass line can also have an expansion unit.

[0025] In a preferred embodiment, the device described above comprises a second heat exchanger, having a line for conveying cooling fluid, preferably BOG from an LNG tank, and a line for conveying compressed gas to be cooled, preferably in counterflow. In the second heat exchanger, the line for conveying compressed gas to be cooled is arranged fluid-carrying between the gas-liquid separator and the LNG tank, and preferably the line for conveying cooling fluid is arranged fluid-carrying between the headspace of the LNG tank and the first heat exchanger. With the aid of the second heat exchanger, the particularly cold BOG can be used immediately after escaping for cooling the reliquefied gas, while the already slightly warmed BOG is used as a coolant in a cooling stage downstream of the water cooling system but upstream of the gas-liquid separator.The targeted use of BOG as a coolant in different sections of the PRS increases the overall reliquefaction rate.

[0026] It is preferred that the device includes a third heat exchanger, the cooling line of which is part of the bypass line and the line to be cooled is part of the return line. This allows the gaseous phase isolated in the gas-liquid separator, which has a pressure p3 and a temperature of approximately -82°C, to be used first as a refrigerant and then downstream as fuel for a low-pressure gas injection engine. A further expansion unit can be arranged between the gas outlet of the gas-liquid separator and the third heat exchanger. By further expanding the gaseous phase from step f) before its use as a refrigerant, the temperature can be lowered further by utilizing the Joule-Thomson effect.

[0027] It is preferred if the device further comprises a second expansion unit which is configured to expand compressed gas from a third pressure p 3 to atmospheric pressure, wherein the second expansion unit is arranged fluid-carrying between the liquid outlet of the gas-liquid separator and the LNG tank, preferably between the line of the second heat exchanger for conveying compressed gas to be cooled and the LNG tank.

[0028] After the gas-liquid separator, the liquid phase is typically at a pressure p3 and a temperature of approximately -110°C. After cooling in the second heat exchanger to around -155°C, the reliquefied gas can be expanded again in a further expansion unit, for example to atmospheric pressure, and particularly low temperatures around the boiling point of natural gas can be achieved.

[0029] The device can be part of a fuel gas supply system for supplying a high-pressure gas injection engine with gas stored in the LNG tank, additionally comprising an outlet which is arranged to conduct fluid downstream of the second compression stage of the multi-stage compressor and further downstream leads into a supply line for a high-pressure gas injection engine, wherein the compressed gas, insofar as the quantity exceeds the fuel requirement of the high-pressure gas injection engine, can be fed to the return line, in particular from the outlet.

[0030] In this embodiment, the highly compressed gas at pressure p2 can either be used to drive a high-pressure gas injection engine or reliquefied in the PRS. Natural gas as a transported commodity can, for example, also be used to power the transport vehicle. Particularly on a liquefied gas tanker, natural gas is the fuel of choice to limit the emission of air pollutants to a relatively low level. The adjustable quantity supplied to the gas injection engine or to the PRS allows for flexible adaptation to climatic and meteorological conditions as well as the fuel requirements of the high-pressure gas injection engine.

[0031] One aspect of the invention relates to a device as described above, wherein the gas-liquid separator has a pressure sensor for measuring the pressure in the gas-liquid separator and a control unit for actuating a valve, arranged between a gas outlet of the gas-liquid separator and the bypass line, depending on the measured pressure. The device as described above can be configured such that the gas-liquid separator has a level sensor and a control unit for actuating a valve, arranged between a liquid outlet of the gas-liquid separator and the LNG tank, depending on the measured liquid level. By means of appropriate control, the pressure and the liquid level in the gas-liquid separator can be regulated via the valves.

[0032] The invention further relates to the use of a device as described above on a ship, in particular a ship powered by a high-pressure gas injection engine. Given the limited space available on the ship, it is particularly advantageous if the compression stages of the multi-stage compressor can be made smaller thanks to the increased efficiency and the continuous removal of the nitrogen from the LNG.

[0033] The invention is further explained using figures. The figures serve as illustrations and are not to be understood as restrictive.

[0034] They show: Figure 1 Schematic representation of a device according to the present invention; Figure 2 Schematic Mollier diagram to illustrate a method according to the present invention.

[0035] Figure 1Figure 2 is a schematic representation of a device for reliquefying and returning exhaust gas (BOG) to a liquefied natural gas (LNG) tank. The BOG F2 is extracted from the LNG tank 3 at approximately -161°C and initially fed to the second heat exchanger 21, where it is carried as a cooling fluid through line 5 in counterflow to the reliquefied gas to be cooled. Further downstream, the BOG is fed to a first heat exchanger 20, specifically a line for carrying cooling fluid, in counterflow to the compressed gas to be cooled. The BOG, thus heated to temperatures of approximately 30°C, is then fed to a multi-stage compressor 10 and compressed in a first compression stage 70a. Preferably, the first compression stage comprises one or two piston compressors 71, 72 connected in parallel or in series, each with subsequent water cooling. The first compression stage 70a is set up to compress the BOG to a first pressure p 1 of, for example, 12 bara.After the first compression stage 70a, a branch line 6 is arranged downstream, conducting fluid and leading further downstream into a supply line for a low-pressure gas injection engine 4. The gas can be expanded to the pressure required by the gas injection engine 4, for example 6 bara, by means of a valve arranged on the branch line 6. The figure shows that the multi-stage compressor arrangement 10 has a second compression stage, which is also the final compression stage 70b. The final compression stage 70b is configured to compress pre-compressed BOG to a second pressure p2 of approximately 300 bara. This is achieved by means of three piston compressors 73, 74, 75, each with subsequent water cooling. However, a different number or type of compressor can also be used, and they can be connected in parallel or in series.

[0036] Downstream of the multi-stage compressor assembly 10, a fluid-carrying outlet 7 is arranged on one side, which leads into a supply line for a high-pressure gas injection engine 2. On the other side, a return line 8 is arranged, the contents of which are indirectly cooled further downstream—among other things—by means of the first heat exchanger 20 to a temperature of, for example, approximately -70°C. This allows the compressed gas, insofar as the quantity exceeds the fuel requirement of the high-pressure gas injection engine 2, to be fed into the return line 8. Further downstream of the first heat exchanger 20, a first expansion unit 30 is connected, which is configured to expand the compressed and cooled gas from pressure p2 to a third pressure p3 of approximately 15 bar using an isenthalpic process, thereby further reducing the temperature to approximately -110°C.

[0037] A gas-liquid separator 40 is connected downstream of the expansion unit 30, carrying the fluid. This separator is designed to separate a liquefied gas fraction at pressure p 3 for refeeding into the LNG tank 3 and to direct a gaseous fraction into a bypass line 9, which connects to the branch line 6. Figure 1It is evident that the exemplary device comprises a third heat exchanger 22, the cooling line of which is part of the bypass line 9 and the line to be cooled of which is part of the return line 8, the latter part corresponding to a section of the return line upstream of the first heat exchanger 20. According to the embodiment shown, the highly compressed, water-cooled BOG from the last compression stage 70b is first pre-cooled in indirect heat exchange with the gaseous phase from the gas-liquid separator and then further cooled in indirect heat exchange with the BOG from the LNG tank before expansion and phase separation take place.

[0038] The liquid phase leaves the gas-liquid separator 40 and is indirectly cooled further in the second heat exchanger 21 to a temperature T2 of only about -155°C using BOG, which is taken directly from the LNG tank and fed into a cooling fluid line 5. Finally, the liquid is expanded to atmospheric pressure in the expansion unit 31 and returned to the LNG tank.

[0039] Valves 80 and 31, respectively, and 50, are used to control the pressure and liquid level in the gas-liquid separator 40. They can be actuated depending on the pressure and / or fill level measured in the gas-liquid separator. Optionally, a valve can also be installed between the outlet of the gas-liquid separator 40 and the line of the second heat exchanger 21 for the flow of fluid to be cooled, in order to control the fill level in the gas-liquid separator.

[0040] Figure 2Figure 1 schematically shows a Mollier diagram to illustrate a process according to the present invention. The x-axis represents the enthalpy of the system, and the y-axis represents the pressure of the gas. Certain temperatures are shown as isothermal lines Tw, T1, and – dotted – T2; the boiling line and dew line are also shown. The process steps, each associated with a change in enthalpy, temperature, and / or pressure, are shown with dashed lines.

[0041] In step a), the BOG is extracted from the headspace of the LNG tank and heated to a temperature Tw by ambient temperature and also by its use as a coolant in one or more heat exchangers. In step b), the BOG is compressed in a first compression stage, here comprising two compression cycles followed by water cooling, to a first pressure p1 between 8 and 18 bara, and a first portion of this gas is diverted (not shown). In step c), the gas is further compressed in a final compression stage, here consisting of three compression cycles, each followed by water cooling, to a high pressure p2. In step d), at least a portion of the further compressed gas from step c) is cooled, first by water cooling to Tw and then to a first temperature T1 between -20°C and -100°C. In step e), isenthalpic expansion to a third pressure p3 between 8 and 20 bara follows.Step f) follows, with the separation of the gas into a liquid and a gaseous phase, in order to combine the gaseous phase with the diverted first part of the gas from step b) (sub-step f 1 ) and to return the liquid phase to the LNG tank 3 (sub-step f 2 ).

[0042] From the Figure 2 It becomes apparent that in sub-step f 2 ) the liquid phase is cooled again before being returned to the LNG tank at ambient pressure to a temperature T 2 which is only slightly above the boiling point of natural gas, which corresponds to the cooling in the second heat exchanger 21. Figure 1 This corresponds to sub-step f 1 ). In addition, it can be seen in sub-step f 1 ) that the gaseous phase can be further expanded and / or heated again before being combined with the pre-compressed BOG from the first compression stage b), for example in indirect heat exchange with compressed gas to be cooled, which corresponds to its use as a refrigerant in heat exchanger 22 of the Figure 1corresponds.

Claims

1. A method for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank, comprising the steps of: a) Withdrawing BOG (F2) from the headspace of an LNG tank (3); b) Compressing the BOG in a first compression stage (70a) to a first pressure p1 between 8 and 18 bara and tapping of a first portion of this gas; c) Further compressing a second portion of the gas from step b) in a final compression stage (70b) to a second pressure p2 ≥ 120 bara, preferably 120 to 400 bara, particularly preferably 150 to 300 bara; d) Cooling at least part of the further compressed gas from step c) to a first temperature T1 between -20 °C and -100 °C; e) Expanding the gas from step d) to a third pressure p3 between 8 and 20 bara; f) Separating the gas from step e) into a liquid phase and a gaseous phase to f1) combine the gaseous phase with the tapped first portion of the gas from step b); and f2) return the liquid phase to the LNG tank (3), characterized in that the liquid phase before being returned to the LNG tank (3) is cooled to a temperature T2 between -140 and -161 °C by heat exchange with cooling BOG (F2) from the headspace of the LNG tank (3).

2. Method according to claim 1, wherein the cooling in step d) is carried out at least partly by heat exchange with cooling BOG (F2) from the headspace of the LNG tank (3).

3. Method according to any one of the preceding claims, wherein the cooling in step d) is carried out at least partly by heat exchange with the gaseous phase from step f).

4. Method according to one of the preceding claims, wherein in step d) a portion of the further compressed gas from step c) is fed to a supply line for a high-pressure gas injection engine (2).

5. Method according to one of the preceding claims, wherein in step f) the pressure p3 is monitored and controlled so that it has a value within a predetermined range.

6. Method according to one of the preceding claims, wherein in step f) a volume of the liquid phase is monitored in order to regulate the return quantity into the LNG tank as a function of the value.

7. An apparatus for reliquefying and returning boil-off gas (BOG) into a liquefied natural gas (LNG) tank comprising - a first heat exchanger (20) comprising a line for passing through cooling fluid, preferably BOG from an LNG tank (3), and a line for passing through compressed gas to be cooled to a first temperature T1 between -20°C and -100 °C, preferably in counterflow; - a multi-stage compressor (10) comprising at least a first compression stage (70a) and a final compression stage (70b), the first compression stage (70a) being configured to compress BOG (F2) from the LNG tank (3) to a first pressure p1 between 8 and 18 bara and wherein the final compression stage (70b) is configured to compress precompressed BOG to a second pressure p2 ≥ 120 bara, preferably 120 to 400 bara, particularly preferably 150 to 300 bara; - a branch line (6) which is arranged downstream of the first compression stage (70a) in a fluid-conducting manner and which opens further downstream into a supply line for a low-pressure gas injection engine (4) and / or a gas combustion unit; - a return line (8) which is arranged downstream of the final compression stage (70b); - a first expansion unit (30) configured to expand compressed gas from a second pressure p2 to a third pressure p3, wherein p3 is between 8 and 20 bara, preferably between 10 and 18 bara; - a gas-liquid separator (40) configured to separate a liquefied gas portion for feeding back into the LNG tank (3) at a pressure p3 and to feed a gaseous portion into a bypass line (9), the bypass line (9) opening into the branch line (6); wherein the multi-stage compressor (10) is connected upstream in a fluid-carrying manner to the headspace of the LNG tank (3), preferably via the line of the heat exchanger (20) for passing through cooling BOG, and wherein the multi-stage compressor is connected downstream in a fluid-carrying manner via the return line (8) to the line of the first heat exchanger (20) for passing through compressed gas to be cooled, is connected further downstream to the first expansion unit (30), and is connected still further downstream to the gas-liquid separator (40), characterized in that the apparatus further comprising a second heat exchanger (21), having a line (5) for passing through BOG from an LNG tank (3), and a line for passing through compressed gas to be cooled, preferably in counterflow, wherein in the second heat exchanger (21) the line for passing through compressed gas to be cooled is arranged in a fluid-conducting manner between the liquid outlet of the gas-liquid separator (40) and the LNG tank (3).

8. Apparatus according to claim 7, characterized in that the line (5) for passing through BOG fluid is arranged in a fluid-conducting manner between the headspace of the LNG tank (3) and the line of the first heat exchanger (20) for passing through cooling fluid.

9. Apparatus according to one of claims 7 or 8, further comprising - a third heat exchanger (22), the cooling line of which is part of the bypass line (9) and the line to be cooled of which is part of the return line (8).

10. Apparatus according to any one of claims 7 to 9, further comprising - a second expansion unit (31) configured to expand compressed gas from a third pressure p3 to atmospheric pressure, wherein the second expansion unit (31) is configured to conduct fluid between the liquid outlet of the gas-liquid separator (40) and the LNG tank (3), preferably between the line (5) of the second heat exchanger (21) for passing through compressed gas to be cooled and the LNG tank (3).

11. Apparatus according to any one of claims 7 to 10, wherein the apparatus is part of a fuel gas supply system for supplying a high-pressure gas injection engine (2) with gas stored in the LNG tank (3), additionally comprising - an outlet (7) which is arranged downstream of the second compression stage (70a) of the multi-stage compressor (10) in a fluid-conducting manner and opens further downstream into a supply line for a high-pressure gas injection engine (2), wherein the compressed gas, insofar as the quantity exceeds the fuel requirement of the high-pressure gas injection engine (2), can be fed to the return line (8).

12. Apparatus according to any one of claims 7 to 11, wherein the gas-liquid separator (40) comprises a pressure sensor to measure the pressure in the gas-liquid separator, and a controller to actuate a valve (80) arranged between a gas outlet of the gas-liquid separator (40) and the bypass line (9) as a function of the measured pressure.

13. Apparatus according to any one of claims 7 to 12, wherein the gas-liquid separator (40) comprises a level sensor and a controller to actuate a valve (80) arranged between a liquid outlet of the gas-liquid separator (40) and the LNG tank (3) as a function of the measured level.

14. Use of an apparatus according to any one of claims 7 to 13 on a vessel, in particular a vessel propelled by means of a high-pressure gas injection engine (2).