Method for reliquefying a BOG stream discharged from a storage tank for liquefied gas, corresponding system and vessel

Abstract

The invention relates to a method (100) for reliquefying a BOG stream (B) drawn from a storage tank (T) for liquefied gas, comprising the steps: a1) extracting liquid (F1) from a liquid phase of the storage tank (T), a2) evaporating the liquid (F1) to produce an evaporated liquid (F2), b) introducing the evaporated liquid (F2) into the BOG stream (B) to generate a mixed BOG stream (BM), c) compressing the mixed BOG stream (BM) to a predetermined final compression pressure to produce a compressed mixed BOG stream (BMv), d) condensing the compressed mixed BOG stream (BMv) to produce a condensed compressed mixed BOG stream (BMvk), in particular with a condenser (46).

Description

[0001] The invention relates to a method and a system for reliquefying a boil-off gas (BOG) stream discharged from a storage tank for liquefied gas.

[0002] The transport of liquefied gases over long distances is most efficiently carried out by ship. For this purpose, it is advantageous to store the gases in cryogenically cooled liquid form at low pressures in storage tanks, as this allows for transport at higher storage densities. Heat from the surrounding environment warms the storage tanks and consequently the cargo. Although measures such as thermal insulation are taken, this heat input cannot be completely avoided, and therefore cooling the cargo is necessary.

[0003] In the current state of the art, this is classically achieved for LPG (liquefied petroleum gas), ethane, and petrochemical gases such as ethylene via one- or two-stage refrigeration processes. In these processes, the heat supplied to the storage tank is dissipated via evaporating gas, the so-called boil-off gas (BOG). This gas is then compressed and condensed at higher pressure against a cooling medium. In practice, the gases to be stored, such as LPG and ethane, regularly contain components that are more volatile than the main component. For ethane, this can be up to 0.8% (mol) of methane, for example. One challenge here is that transport in nearly pressureless tanks leads to increased methane concentrations in the BOG, which can reach approximately 25% (mol).

[0004] In other words, volatile impurities in the cargo accumulate in the BOG, so that even slight concentrations in the liquid phase result in significant concentrations in the BOG.

[0005] In practice, it has been recognized that the mechanical limits of the compressors can be exceeded as a result, and that the BOG can no longer be completely liquefied using two-stage compression processes.

[0006] According to current technology, a three-stage compression process is regularly used for such charges. This allows condensation at a higher pressure and, consequently, a higher concentration of volatile components, while maintaining the same condensation temperature, which is limited by the available refrigerant systems.

[0007] To enable the liquefaction of more heavily contaminated BOG while maintaining the mechanical limits of the system components used so far, particularly the compressors, DE 10 2013 010 414 B4 proposes an alternative method: liquid is drawn from the liquid phase of a storage tank and introduced into a BOG stream to create a mixed BOG stream. This mixed BOG stream is then compressed and condensed. The method involves compressing the BOG stream to a lower compression pressure than the specified final compression pressure before mixing it with the liquid from the storage tank's liquid phase. This creates a partially compressed intermediate BOG stream, which is then mixed with the liquid.

[0008] Such a process can be profitably used for a product area where the composition of the BOG of a subsystem requires condensation temperatures within the achievable pressure range that are not attainable with simple technical refrigerants. However, for some applications, it has proven desirable to further reduce the concentration of volatile components in the compressed gas supplied to the condenser compared to solutions known from the prior art.

[0009] Against this background, the invention was based on the objective of further developing a method and a system of the type described above in such a way as to eliminate the disadvantages found in the prior art as far as possible. In particular, a method and a system for the reliquefaction of a BOG stream discharged from a storage tank for liquefied gas was to be provided, in which the concentration of volatile components in the compressed gas is reduced.

[0010] According to the invention, the problem is solved in a method of the type mentioned at the outset by comprising the following steps: a1) Extracting liquid from a liquid phase of the storage tank, a2) Evaporating the liquid so that an evaporated liquid is produced, b) Introducing the evaporated liquid into the BOG stream to generate a mixed BOG stream, c) Compressing the mixed BOG stream to a predetermined final compression pressure so that a compressed mixed BOG stream is produced, d) Condensing the compressed mixed BOG stream so that a condensed, compressed mixed BOG stream is produced.

[0011] By evaporating the liquid extracted from the storage tank before mixing it with the BOG stream, the concentration of volatile components in the compressed gas fed to the condenser is reduced. This is achieved by first evaporating the liquefied gas after it is extracted from the tank. Due to the lower pressure level, it is thus possible to liquefy the components that cannot be condensed in the condenser using the known process. Simultaneously, the gas mixture of the mixed BOG stream changes, and the proportion of volatile components is reduced because the evaporated liquid mixed with the BOG stream has the same composition as the liquid phase.

[0012] According to one embodiment, in step a2) the liquid is evaporated against the condensed, compressed mixed BOG stream. In other words, the mixed BOG stream provides the energy required to evaporate the liquid.

[0013] According to one embodiment, step a2) comprises the following sub-steps: a2a) providing an e-condenser as a heat exchanger, a2b) introducing the liquid from the liquid phase of the storage tank into the e-condenser, a2c) introducing the condensed, compressed mixed BOG stream into the e-condenser, a2d) evaporating the liquid from the liquid phase of the storage tank in the e-condenser against the condensed, compressed mixed BOG stream. The e-condenser is a heat exchanger that enables the evaporation of the liquid from the liquid phase of the storage tank and utilizes the condensed, compressed mixed BOG stream for this purpose.

[0014] According to one embodiment, in step b), the BOG stream is first compressed to a lower compression pressure than the predetermined final compression pressure to generate a partially compressed intermediate BOG stream. The liquid extracted from the liquid phase of the storage tank is then introduced into the intermediate BOG stream after evaporation to create the mixed BOG stream. Thus, in an advantageous embodiment, during reliquefaction, the BOG stream is first compressed to a lower compression pressure than the predetermined final compression pressure to generate a partially compressed intermediate BOG stream, the final compression pressure preferably being below the liquefaction pressure. The evaporated liquid from the liquid phase of the storage tank is introduced into this intermediate BOG stream to generate the mixed BOG stream, which is then compressed to the final compression pressure, optionally in two or more compression stages.The final compressed mixed BOG stream is then directed to a capacitor and condensed there.

[0015] According to one embodiment, step b) comprises the following sub-steps: b1) providing an economizer as a mixing device, b2) introducing the evaporated liquid into the economizer, b3) introducing the partially compressed intermediate BOG stream into the economizer. An economizer is defined as a device in which the evaporated liquid and the partially compressed intermediate BOG stream are introduced and mixed.

[0016] According to one embodiment, the method further comprises the step: e) Returning the condensed, compressed mixed BOG stream to the storage tank. The return of the mixed BOG stream to the storage tank can be complete or partial.

[0017] According to one embodiment, the method further comprises the steps: f) diverting a partial stream of the condensed, compressed mixed BOG stream, g) mixing the partial stream with a fuel gas stream that serves to provide a propulsion fuel gas for a consumer, in particular a ship engine.

[0018] The aim here is to add as large a proportion as possible of the condensed, compressed mixed BOG stream, also known as condensate, to the fuel gas stream, thereby achieving the maximum methane content in the fuel gas that is technically determined by the design of the consumer, for example, the engine. Over the course of a voyage, this leads to a reduction in the methane content of the entire cargo and thus facilitates reliquefaction, which can then take place at lower final pressures and is therefore more energy-efficient.

[0019] According to one embodiment, step g) of the method comprises the following sub-steps: g1) Determining the pressure and temperature of the liquid in the storage tank, g2) Estimating the methane content of the BOG generated in the storage tank based on the pressure and temperature from step g1), g3) Metering, in particular volumetric or mass flow-based metering, of the partial flow to the fuel gas stream such that a methane concentration specified by the consumer is maintained in the fuel gas stream.

[0020] In other words, this method uses the prevailing pressure and cargo composition to precisely determine the temperature of the liquid in the ship's cargo tank. For the ethane-methane system, the methane content is the determining factor. Despite technical measurement inaccuracies and temperature variations in the liquid phase of the tank, which are particularly common in large tanks, it is surprisingly possible to accurately estimate the methane content of the BOG (Bottle of Gas) within the liquid composition at a given pressure and temperature.

[0021] By evaporating the liquid from the storage tank according to the established procedure, particularly using the condenser, it is possible to cool the BOG more effectively than would be possible with simple evaporation at the evaporation temperature under medium pressure. This is possible because the liquid is drawn off at tank temperature, allowing the condensate to be cooled to near its boiling point under tank conditions.

[0022] The condensed, compressed mixed BOG stream, i.e., the condensate, can now be blended with the fuel gas, i.e., the fuel. Preferably, a simple proportional blending method based on volumetric flow rate measurements can be used. Since the maximum methane content of the condensed BOG is known by estimating it from the tank's condition based on pressure and temperature, the amount can be adjusted via a volume fraction to ensure that the concentrations specified by the user are reliably maintained in the fuel gas. This estimation is always conservative due to the additional reduction in the methane content of the condensate resulting from the evaporation of the tank liquid. Another criterion limiting the maximum possible blending of condensate with the fuel gas is the subcooling of the mixed stream required by the subsequent components.Due to the further subcooling of the condensate relative to the tank liquid compared to a cargo economizer known in the art, the limitation caused by this aspect shifts towards a higher possible condensate admixture. Thus, the expected condensate admixture to the fuel gas is higher than in the known process.

[0023] For use in currently available marine engines running on ethane, the fuel must be supplied at approximately 350 bar. In this respect, only the compression of liquid fuel using a piston pump is economically advantageous. The available piston pumps for fuel gases require the fuel to be supplied in a supercooled form. The suction pressure of the pumps is typically on the same order of magnitude as the mean effective pressure of the compressors. Therefore, in known processes, the condensate added from the fuel is close to its saturation state. Adding this condensate to the fuel reduces the supercooling of the high-pressure pump intake flow and increases the risk of cavitation. The proposed process increases this supercooling of the condensate relative to the suction pressure of the high-pressure pump, thereby improving operational reliability.

[0024] As a result, the process according to the invention offers several advantages. Firstly, the economic efficiency of reliquefaction is improved by using two-stage instead of three-stage compressors with the same refrigerant system. The simple volumetric dosing of condensate into the fuel gas or fuel results in condensate combustion. This increases the efficiency of reliquefaction in the short term by reducing the proportion of BOG (Bottled Oxide Gas) produced during the expansion back to the tank. Secondly, a medium-term reduction in the methane content is achieved, thereby permanently reducing the final pressure required for condensation at a given condenser temperature. Furthermore, the process according to the invention also simplifies the feeding of the fuel-condensate mixture to the high-pressure pump, as complex monitoring of the subcooling is no longer necessary.Alternatively, mass flow-based dosing can be used. This simplifies the calculation, as it eliminates the need to calculate density as a function of temperature and composition. A key advantage is that no analytical methods are required, the sensors for which would be considerably more complex and expensive.

[0025] According to one embodiment, the maximum methane content in the BOG (boiled gas liquid) and thus also in the condensate and fuel gas is estimated using a linearized model of the methane content in the BOG as a function of tank pressure and measured temperature. For a given tank pressure, the methane concentration in the BOG is physically related to the methane content of the liquid phase via the liquid's saturation temperature. A linear relationship results within the considered pressure range of 0.4 to 0 barg. The parameters of the linear models can each be represented as a linear function of the tank pressure. This yields a simple linear model for estimating the methane content in the BOG as a function of the tank pressure and the measured liquid temperature.

[0026] According to one embodiment, the estimation of the maximum methane content in the BOG (Bottle Oil Gas) and thus also in the condensate and fuel gas is performed by additionally considering the initial propane concentration in the ethane. This embodiment proposes performing a multiple linear regression where the propane content of the initial liquid is fixed. In other words, the cargo is modeled as a tri-component mixture, and then the methane content is estimated based on the input propane content and the measured pressures and temperatures. The propane content is known from the loading analysis and changes over the voyage only within a range that does not significantly affect the accuracy. Considering the initial propane concentration improves the estimation accuracy.

[0027] The invention has been described above with reference to a method. In a further aspect, the invention relates to a system for reliquefying a BOG stream drawn from a storage tank for liquefied gas, comprising a withdrawal device for extracting liquid from the liquid phase of the storage tank, a mixing device for introducing a vaporized liquid into the BOG stream to generate a mixed BOG stream, a compression device with at least a first compression stage and a final compression stage for compressing the mixed BOG stream to a final compression pressure, a condenser for condensing the compressed mixed BOG stream, and flow connections between the mixing device and the compression device, between the withdrawal device and the mixing device, and between the compression device and the condenser.

[0028] The invention solves the problem described above with regard to the system by arranging a heat exchanger, in particular an condenser, between the extraction device and the mixing device, which is designed to evaporate the liquid from the liquid phase of the storage tank before it is introduced into the BOG stream, so that the evaporated liquid is produced.

[0029] The system utilizes the same advantages and preferred embodiments as the method according to the invention, and vice versa. To avoid repetition, reference is made to the above statements, and their content is incorporated herein.

[0030] According to one embodiment, the heat exchanger is fluidly connected to the condenser, so that the liquid from the liquid phase of the storage tank can be evaporated in the heat exchanger against the condensed, compressed mixed BOG stream.

[0031] According to one embodiment, the system has a metering device which is fluidly connected to the heat exchanger. A partial flow of the condensed, compressed mixed BOG flow passing through the heat exchanger and a drive fuel gas for a consumer are supplied to the metering device, wherein the metering device is configured to meter the mixed BOG flow to the drive fuel gas, in particular volumetrically or on a mass flow basis.

[0032] Furthermore, the mixing device preferably includes an economizer which, on the inlet side, is in flow communication with the compression stage generating the partially compressed intermediate BOG stream and with the heat exchanger, and on the outlet side is in flow communication with a higher compression stage than the one generating the partially compressed intermediate BOG stream. According to one embodiment, a temperature sensor and a pressure sensor are associated with the storage tank.

[0033] According to one embodiment, the system has a control device which is set up and configured to perform the following steps: estimating the methane content of the BOG generated in the storage tank based on the pressure and temperature in the storage tank, and metering the partial flow to the fuel gas stream by means of the metering device in such a way that a methane concentration specified by the consumer is maintained in the fuel gas stream.

[0034] In another aspect, the invention relates to a ship with a system for reliquefying a BOG stream discharged from a liquefied gas storage tank. The invention solves the aforementioned problem with respect to the ship by designing the system according to one of the preceding embodiments. The ship also benefits from the same preferred embodiments and advantages as the inventive method and system, and vice versa. To avoid repetition, reference is made to the above statements, and their content is incorporated herein.

[0035] According to one embodiment, the ship has several systems for reliquefying a BOG stream discharged from a storage tank for liquefied gas, preferably two or three such systems, which are preferably designed according to one of the preceding embodiments.

[0036] The invention is described in more detail below with reference to preferred embodiments and the accompanying figures. These figures show: Fig. 1. A system for the reliquefaction of a BOG stream discharged from a liquefied gas storage tank, shown in a block diagram view; and Fig. 2 a method according to the invention for reliquefying a BOG stream discharged from a storage tank for liquefied gas in a representation as a block diagram.

[0037] Fig. Figure 1 shows an exemplary representation of ship 2. Ship 2 has three systems 4a-c for reliquefying a BOG stream B discharged from a storage tank T for liquefied gas. One of the systems, namely system 4a, is in Fig. 1 in detail.

[0038] System 4a includes a withdrawal device 3 for extracting liquid F1 from the liquid phase of storage tank T. The liquid F1 flows via a liquid line 6 to a heat exchanger 8, also referred to as an evaporator 8. The liquid line 6 is connected to an evaporator inlet 10, and the liquid F1 then flows through the evaporator 8 to an evaporator outlet 12. In the evaporator 8, the liquid F1 is evaporated, producing evaporated liquid F2. The evaporated liquid F2 exits the evaporator 8 at outlet 12 and flows via a connecting line 18 to a mixing device 20, also referred to as an economizer 20. The evaporated liquid F2 is fed to the mixing device 20 via a first inlet 24.

[0039] Furthermore, BOG B is drawn from tank T via BOG supply lines 28a-c and fed to a suction drum 32 via a common BOG line 30. Fig. Figure 1 shows an example of a tank T. The ship 2 can also have more than one tank T, in which case each of the BOG supply lines 28a-c can be assigned to a tank T. From the suction drum 32, the BOG B reaches a first compression stage 36 and is compressed there. After compression, compressed BOG BV is present, which is fed via a connecting line 38 to the mixing unit 20 via a second inlet 26. The compressed BOG BV and the vaporized liquid F2 are mixed in the mixing unit 20 or in the economizer 20.

[0040] The mixture is fed via line 40 to a compression stage 42 and compressed there, so that the mixed BOG stream BM is compressed to produce a compressed mixed BOG stream BMv. This is conveyed via line 44 to a condenser 46, entering it via an inlet 48 and exiting it via an outlet 50. In the condenser 46, the compressed mixed BOG stream BMv is condensed to produce a condensed, compressed mixed BOG stream BMvk, which is also referred to as condensate. The condenser 46 can have a series of heat exchangers with different temperature levels (not shown).

[0041] This is fed via line 52 to a condensate collection tank 54 and from there either to a condensate discharge line 58 or, via a connecting line 56, back to the heat exchanger 8. Line 52 is also connected to a vent 62. The connecting line 56 is connected to a condensate inlet 14 of the heat exchanger (condenser) 8. After passing through the heat exchanger (condenser) 8, the condensed, compressed mixed BOG stream reaches a condensate outlet 16 and is discharged from there via the condensate discharge line 58. The condensate discharge line 58 is connected, via a valve 61, in particular a pressure relief valve 61, to a line 59, which carries the condensate to the tank T. The valve 61 is designed to maintain the condensation pressure in system 4a and the fill level in the condensate collection tank 54. Line 59 branches off downstream of valve 61 in the direction of flow.

[0042] The compressed, condensed mixed BOG stream BMvk serves to evaporate the liquid F1 in the heat exchanger or condenser 8. The condensate discharge line 58 is fluid-conductingly connected to condensate return lines 60a-c and can be returned from there to at least one tank T.

[0043] The condensate drain line 58 is also connected via a line 63 to a metering device D, with the line 63 branching off upstream of the valve 61 in the direction of flow. A partial flow S of the condensed, compressed mixed BOG flow BMvk, which passes through the heat exchanger 8, is supplied to the metering device D. Because the condensate is drawn upstream of the valve 61, which can be designed as a pressure relief valve 61, the condensate has a higher pressure compared to a draw-off downstream of the valve 61. In addition to limiting the methane content in the propulsion fuel gas FG, the metering device D also serves to maintain the pressure in line 58, in particular a condensation pressure, or the fill level in the condensate collection tank 54.

[0044] Furthermore, a propulsion fuel gas FG for a consumer M, for example a ship's engine, is supplied to the metering device D. The metering device D is configured to meter the mixed BOG stream BMvk to the propulsion fuel gas FG, in particular volumetrically or on a mass flow basis. The stream FG can be drawn from the tank using the same device 3 as the stream F1. This produces a fuel gas stream FGM, which can then be supplied to the consumer M. A control device C is provided for the purpose of metering. This device is configured and designed to determine the methane content of the BOG B produced in the storage tank T based on the pressure and temperature in the storage tank T. Appropriate sensors are provided in the storage tank T to determine the pressure and temperature, namely, in particular, a pressure sensor p and a temperature sensor t.The metering of the partial flow S to the fuel gas flow FG by means of the metering device D preferably takes place in such a way that a methane concentration specified by the consumer M is maintained in the fuel gas flow FGM.

[0045] Fig. Figure 2 shows an embodiment of a method 100 according to the invention for reliquefying a liquid gas from the storage tank T, which is located in Fig.Figure 1 shows the BOG stream B being drawn out. The method comprises the following steps: a1) Extracting liquid F1 from a liquid phase of the storage tank T, a2) Evaporating liquid F1 to produce evaporated liquid F2, b) Introducing the evaporated liquid F2 into the BOG stream B to generate a mixed BOG stream BM, c) Compressing the mixed BOG stream BM to a predetermined final compression pressure to produce a compressed mixed BOG stream BMv, d) Condensing the compressed mixed BOG stream BMv to produce a condensed, compressed mixed BOG stream BMvk, in particular using a condenser 46, e) Returning the condensed, compressed mixed BOG stream BMvk to the storage tank T, f) Diverting a partial stream S of the condensed, compressed mixed BOG stream BMvk. Mixing the partial flow S with a fuel gas FG, which serves to provide a propulsion fuel gas for a consumer M, in particular for a ship engine M.

[0046] In step a2), the liquid F1 is evaporated against the condensed, compressed mixed BOG stream BMvk. In step b), the BOG stream B is first compressed to a lower compression pressure than the specified final compression pressure to generate a partially compressed intermediate BOG stream BV. The liquid F1 taken from the liquid phase of the storage tank T is introduced into the intermediate BOG stream BV after evaporation to generate the mixed BOG stream BM. Step g) includes determining the pressure p and temperature t of the liquid F1 in the storage tank T, estimating the methane content of the BOG B produced in the storage tank T based on the pressure p and temperature t, and dosing, in particular volumetric or mass flow-based dosing, a partial flow S to the fuel gas flow FG such that a methane concentration specified by the consumer M is maintained in the fuel gas flow FGM. Reference symbol list 2 ships 3. Sampling device 4a-c System for reliquefaction of a BOG stream 6 Suction gas line 8 heat exchangers (condensers) 10 Econdenser input 12 Econdenser output 14 Ecodenser condensate inlet 16 Econdenser Condensate outlet 18 Connecting line 20 Mixing device (economizer) 24 first entry of the Economizer 26 Second entrance of the Economizer 28a-c BOG supply lines 30 joint BOG management 32 Suction drum 34 Compressor supply line 36 first compaction stage 38 Connecting line to the mixing unit 40 Line to the compression stage 42 Compression stage 44 Line to capacitor 46 Capacitor 48 Capacitor inlet 50 Capacitor outlet 52 Pipe to the condensate collection tank 54 Condensate collection containers 56 Connecting pipe between condensate collection tank and heat exchanger 58 Condensate drain line 59 Management 60a-c Condensate return 61 Valve 62 Ventilation 63 Management B BOG power BM Mixed-BOG-Electricity BMv compressed mixed-BOG electricity BMvk condensed, compressed mixed-BOG electricity BV compressed BOG electricity C Control unit D Dosing device d pressure sensor F1 Liquid F2 evaporated liquid FG drive fuel gas FGM Fuelgas-Strom M Consumer S partial current T Storage tank t temperature sensor 100 procedures a1) Extraction of liquid from a liquid phase of the storage tank a2) Evaporation of the liquid b) Introducing the evaporated liquid into a BOG stream c) Compacting the mixed BOG flow d) Condensing the compressed mixed BOG stream e) Return of the condensed, compressed mixed BOG flow f) Branching off a partial flow g) Mixing the partial stream with a fuel gas stream QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2013 010 414 B4

[0007]

Claims

Method (100) for reliquefying a BOG stream (B) drawn from a storage tank (T) for liquefied gas, comprising the steps: a1) extracting liquid (F1) from a liquid phase of the storage tank (T), a2) evaporating the liquid (F1) to produce an evaporated liquid (F2), b) introducing the evaporated liquid (F2) into the BOG stream (B) to generate a mixed BOG stream (BM), c) compressing the mixed BOG stream (BM) to a predetermined final compression pressure to produce a compressed mixed BOG stream (BMv), d) condensing the compressed mixed BOG stream (BMv) to produce a condensed compressed mixed BOG stream (BMvk), in particular using a condenser (46). Method (100) according to claim 1, wherein the liquid (F1) is evaporated in step a2) against the condensed, compressed mixed BOG stream (BMvk). Method (100) according to claim 1 or 2, wherein step a2) comprises the following sub-steps: a2a) providing an e-condenser (8) as a heat exchanger, a2b) introducing the liquid (F1) from the liquid phase of the storage tank (T) into the e-condenser (8), a2c) introducing the condensed, compressed mixed BOG stream (BMvk) into the e-condenser (8), a2d) evaporating the liquid (F1) from the liquid phase of the storage tank (T) in the e-condenser (8) against the condensed, compressed mixed BOG stream (BMvk). Method (100) according to one of the preceding claims, wherein in step b) the BOG stream (B) is first compressed to a lower compression pressure than the predetermined final compression pressure in order to generate a partially compressed intermediate BOG stream (BV), wherein the liquid (F1) taken from the liquid phase of the storage tank (T) is introduced into the intermediate BOG stream (BV) after evaporation in order to generate the mixed BOG stream (BM). Method (100) according to one of the preceding claims, wherein step b) comprises the following sub-steps: b1) providing an economizer (20) as a mixing device, b2) introducing the evaporated liquid (F2) into the economizer (20), b3) introducing the partially compressed intermediate BOG stream (BV) into the economizer (20). Method (100) according to one of the preceding claims, comprising step: e) returning the condensed, compressed mixed BOG stream (BMvk) to the storage tank (T). Method (100) according to one of the preceding claims, comprising the step: f) diverting a partial stream (S) of the condensed, compressed mixed BOG stream (BMvk), g) mixing the partial stream (S) with a fuel gas stream (FG) which serves to provide a propulsion fuel gas for a consumer (M), in particular for a ship engine (M). Method (100) according to claim 7, wherein step g) comprises the following sub-steps: g1) Determining the pressure (p) and temperature (t) of the liquid (F1) in the storage tank (T), g2) Estimating the methane content of the BOG (B) generated in the storage tank (T) based on the pressure (p) and temperature (t) from step g1), g3) Metering, in particular volumetric or mass flow-based metering, of the partial flow (S) to the fuel gas flow (FG) such that a methane concentration specified by the consumer (M) is maintained in the fuel gas flow (FGM). System (4a-c) for reliquefying a BOG stream (B) drawn from a storage tank (T) for liquefied gas, comprising: - a withdrawal device (3) for extracting liquid (F1) from the liquid phase of the storage tank (T), - a mixing device (20) for introducing a vaporized liquid (F2) into the BOG stream (BV) to generate a mixed BOG stream (BM), - a compression device (36, 42) with at least a first compression stage (36) and with a final compression stage (42) for compressing the mixed BOG stream (BM) to a final compression pressure, - a condenser (46) for condensing the compressed mixed BOG stream (BMv), and - flow connections between the mixing device (20) and the compression device (36, 42), between the withdrawal device (3) and the mixing device (20), and between the compression device (36, 42). and condenser (46), characterized in that a heat exchanger (8) is located between the extraction device (3) and the mixing device (20),in particular an condenser (8) is arranged which is configured to evaporate the liquid (F1) from the liquid phase of the storage tank (T) before introducing it into the BOG stream (BM), so that the evaporated liquid (F2) is produced. System (4a-c) according to claim 9, wherein the heat exchanger (8) is fluidly connected to the condenser (46) so that the liquid (F1) from the liquid phase of the storage tank (T) can be evaporated in the heat exchanger (8) against the condensed, compressed mixed BOG stream (BMvk). System (4a-c) according to claim 9 or 10, comprising a metering device (D) which is fluidly connected to the heat exchanger (8), wherein a partial flow (S) of the condensed, compressed mixed BOG flow (BMvk) and a drive fuel gas (FG) for a consumer (M) is supplied to the metering device (D), wherein the metering device (D) is configured to meter the mixed BOG flow (BMvk) to the drive fuel gas (FG), in particular volumetrically or on a mass flow basis. System (4a-c) according to one of claims 9 to 11, wherein the mixing device (20) has an economizer (20) which is in flow communication on the inlet side with the compression stage (36) generating the partially compressed intermediate BOG stream and with the heat exchanger (8) and is in flow communication on the outlet side with a higher compression stage (42) than the compression stage (36) generating the partially compressed intermediate BOG stream. System (4a-c) according to one of claims 9 to 12, wherein a temperature sensor (t) and a pressure sensor (p) are associated with the storage tank (T). System (4a-c) according to one of claims 11 to 13, comprising a control device (C) which is configured and designed to perform the following steps: - Estimating the methane content of the BOG (B) generated in the storage tank (T) based on the pressure (p) and temperature (t) in the storage tank (T), - Metering the partial flow (S) to the fuel gas flow (FG) by means of the metering device (D) such that a methane concentration specified by the consumer (M) is maintained in the fuel gas flow (FGM). Ship (2) with a system (4a-c) for reliquefying a BOG stream (B) discharged from a storage tank (T) for liquefied gas, wherein the system (4a-c) is configured according to any one of claims 9 to 14.

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