Method for reliquefying BOG flow discharged from liquefied petroleum gas storage tanks, corresponding systems and vessels.
By evaporating and mixing liquid from the storage tank with BOG flow, then compressing and condensing it, the method effectively reduces volatile components, enabling efficient reliquefaction and safer operation by maintaining methane concentrations in fuel gas.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- TGE MARINE GAS ENG GMBH
- Filing Date
- 2025-11-14
- Publication Date
- 2026-06-02
Smart Images
Figure 2026090219000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method and system for the reliquefaction of a boil-off gas (BOG) stream discharged from a storage tank for liquefied petroleum gas.
Background Art
[0002] The long-distance transportation of liquefied gas is advantageously carried out by ship. For this purpose, it is reasonable to store the gas in a storage tank in a low-pressure cryogenic state in the liquid phase, which enables transportation at a higher storage density. The heat input from the environment heats the storage tank and thus also the corresponding cargo. Although measures such as thermal insulation are taken, such heat input cannot be completely avoided, so the cargo needs to be cooled.
[0003] In the prior art, for petrochemical gases such as LPG (liquefied petroleum gas), ethane, and ethylene, this is classically carried out by a one-stage or two-stage refrigeration process, where the heat supplied to the storage tank is dissipated via the evaporated gas, so-called boil-off gas (BOG), which is compressed and condensed against a cooling medium at a higher pressure. In practice, the loaded gas, such as LPG or ethane, constantly contains components that are very volatile compared to the main components. For example, in the case of ethane, this may contain up to 0.8% (mol) methane. One problem here is that when transporting in a nearly non-pressurized tank, the methane concentration in the BOG increases and can reach about 25% (mol).
[0004] In other words, due to the accumulation of volatile impurities in the BOG, even a small concentration in the liquid phase results in a significant concentration in the BOG.
[0005] In practice, it has been recognized that as a result, the mechanical limit values of the compressor or compressors can be exceeded, and in a two-stage compression process, it is no longer possible to completely liquefy the BOG in this way.
[0006] According to prior art, a three-stage compression process is routinely used for such loads. This allows for condensation at higher pressures and, at the same condensation temperature limited by the available refrigerant system, allows for a correspondingly higher content of volatile components.
[0007] To enable the liquefaction of more contaminated BOG while maintaining the mechanical limits of the system components used to date, particularly the compressor, DE 10 2013 010 414 B4 proposes an alternative: extracting liquid from the liquid phase of the storage tank and introducing it into the BOG flow to generate a mixed BOG flow. The mixed BOG flow is then compressed and condensed. It is envisioned that, before mixing with the liquid from the liquid phase of the storage tank, the BOG flow is compressed to a compression pressure lower than a predetermined final compression pressure to generate a partially compressed intermediate BOG flow, which is then mixed with the liquid. [Overview of the Initiative]
[0008] Such methods can be advantageously used in product fields where the BOG composition of the subsystem requires condensation temperatures that cannot be achieved with simple technical refrigerants within an achievable pressure range. However, in some applications, it has been found 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 backdrop, the objective of the present invention was to further improve the types of methods and systems described at the beginning and to eliminate as many of the disadvantages found in the prior art as possible. In particular, there was a need to identify a method and system for reliquefying BOG flow discharged from liquefied petroleum gas storage tanks that reduces the concentration of volatile components in the compressed gas.
[0010] According to the present invention, the problem is solved by the type of method mentioned at the beginning, which includes the following steps: a1) taking liquid from the liquid phase of a storage tank; a2) evaporating the liquid to form an evaporated liquid; b) introducing the evaporated liquid into a BOG flow to generate a mixed BOG flow; c) compressing the mixed BOG flow to a predetermined final compression pressure to generate a compressed mixed BOG flow; d) condensing the compressed mixed BOG flow to generate a condensed compressed mixed BOG flow.
[0011] By evaporating the liquid taken from the storage tank before mixing it with the BOG flow, the concentration of volatile components in the compressed gas supplied to the condenser is reduced by first evaporating the liquefied gas after it has been taken from the tank. Due to the lower pressure level, it becomes possible to liquefy components that cannot be condensed in the condenser using known processes. At the same time, the gas mixture of the mixed BOG flow changes, and the proportion of volatile components is reduced because the evaporated liquid mixed with the BOG flow has a liquid phase composition.
[0012] According to one embodiment, in step a2), the liquid is evaporated against a condensed, compressed, mixed BOG flow. In other words, the mixed BOG flow supplies the energy necessary to evaporate the liquid.
[0013] According to one embodiment, step a2) includes the following sub-steps: a2a) providing an E-condenser as a heat exchanger; a2b) introducing liquid from the liquid phase of a storage tank into the E-condenser; a2c) introducing a condensed compressed mixed BOG flow into the E-condenser; a2d) evaporating the liquid from the liquid phase of the storage tank in the E-condenser using the condensed compressed mixed BOG flow. An E-condenser refers to a heat exchanger that enables the evaporation of liquid from the liquid phase of a storage tank and utilizes a condensed compressed mixed BOG flow for this purpose.
[0014] According to one embodiment, in step b), the BOG flow is first compressed to a compression pressure lower than a predetermined final compression pressure to generate a partially compressed intermediate BOG flow, and the liquid removed from the liquid phase of the storage tank is introduced into this intermediate BOG flow after evaporation to generate a mixed BOG flow. Thus, in the case of reliquefaction in an advantageous embodiment, the BOG flow is first compressed to a compression pressure lower than a predetermined final compression pressure to generate a partially compressed intermediate BOG flow, the final compression pressure is preferably lower than the condensation pressure. The evaporated liquid from the liquid phase of the storage tank is introduced into this intermediate BOG flow to generate a mixed BOG flow, which is then compressed to the final compression pressure by two or more compression steps as needed. The finally compressed mixed BOG flow is then supplied to a condenser, where it is condensed.
[0015] According to one embodiment, step b) includes the following sub-steps: b1) providing an economizer as a mixing device; b2) introducing evaporated liquid into the economizer; and b3) introducing a partially compressed intermediate BOG flow into the economizer. An economizer is a device into which evaporated liquid and a partially compressed intermediate BOG flow are introduced and mixed internally.
[0016] According to one embodiment, this method further includes the following steps: e) returning the condensed compressed mixed BOG flow to a storage tank. The mixed BOG flow may be returned to the storage tank in whole or in part.
[0017] According to one embodiment, the method further includes the following steps: f) branching off a portion of the condensed compressed mixed BOG flow; g) mixing the branched portion with a fuel gas flow used to supply propulsion fuel gas to a consuming device, in particular a marine engine.
[0018] The objective here is to add the largest possible proportion of condensed compressed mixed BOG flow (so-called condensate) to the fuel gas flow to achieve the maximum proportion of methane in the fuel gas as technically defined by the design of the consuming equipment, such as the engine. Throughout the ship's voyage, this leads to a reduction in the overall methane content of the cargo, which in turn makes reliquefaction easier, allows reliquefaction to be performed at lower final pressures, and thus can be carried out in a more energy-efficient manner.
[0019] According to one embodiment, step g) of the method includes the following sub-steps: g1) a step of determining the pressure and temperature of the liquid in the storage tank; g2) a step of estimating the methane content of the BOG produced in the storage tank based on the pressure and temperature obtained in step g1); and g3) a step of measuring a partial flow relative to the fuel gas flow, particularly based on volumetric flow rate or mass flow rate, so that the methane concentration specified by the consuming equipment is maintained in the fuel gas flow.
[0020] In other words, in this method, the temperature of the liquid in a ship's cargo tank is clearly determined by the dominant pressure and the composition of the cargo. Here, the methane content is crucial in ethane-methane systems. Surprisingly, even considering technical measurement errors and the temperature distribution within the tank liquid phase that technically occurs, especially in large tanks, it is possible to adequately estimate the methane content of the BOG (Board of Grain) belonging to the liquid composition at pressure and temperature.
[0021] Following this process, particularly by evaporating the liquid from the storage tank using an e-condenser, it becomes possible to cool the BOG (boiling point) to a degree that cannot be achieved by pure evaporation alone at the evaporation temperature under medium pressure. This is because the liquid is drawn out at the tank temperature, and as a result, the condensate can be cooled to near its boiling point under tank conditions.
[0022] The condensed, compressed BOG flow, i.e., condensate, can be mixed with the fuel gas, i.e., the fuel. Preferably, a simple proportional mixing method based on volumetric flow rate measurement can be used. Since the maximum amount of methane in the condensed BOG is known by estimating the tank conditions based on force and temperature, the amount can be adjusted by volume ratio to ensure that the concentration specified by the consuming equipment in the fuel gas is maintained. This estimation is always conservative because the methane content in the condensate is further reduced by evaporated tank liquid. Another criterion limiting the maximum possible mixing amount when mixing condensate with the fuel gas is the degree of subcooling of the mixed flow technically required by the subsequent components. Therefore, the expected amount of condensate mixed into the fuel gas will be higher than known by conventional methods.
[0023] In currently used marine engines for ethane, the fuel must be supplied at approximately 350 bar. In this regard, only compression of liquid fuel by a piston pump is economically advantageous. Piston pumps available for fuel gas require the fuel to be supplied in a subcooled state. The suction pressure of the pump is typically on the order of the average pressure of the compressor. Therefore, in known methods, the condensate added from the fuel is close to saturation. Adding this to the fuel reduces the subcooling degree of the high-pressure pump suction flow, increasing the risk of cavitation. In the proposed method, the subcooling degree of the condensate relative to the high-pressure pump suction pressure is increased, resulting in improved operational safety.
[0024] As a result, the method according to the present invention has several advantages. On the one hand, the economics of reliquefaction are improved by using a two-stage compressor instead of a three-stage compressor while using the same refrigerant system. The condensate is burned by simply metering and mixing it with the fuel gas or fuel in a volume-proportional manner. This reduces the proportion of BOG generated during re-expansion into the tank, improving the efficiency of reliquefaction in the short term. Furthermore, a reduction in methane content is achieved in the medium term, thereby consistently lowering the final pressure required for liquefaction at a given condenser temperature level. In addition, the method according to the present invention simplifies supply by eliminating the need for cumbersome monitoring of the subcooling degree when supplying the fuel-condensate mixture to the high-pressure pump. Alternatively, metering based on mass flow rate can also be used. This simplifies calculations because it eliminates the need to calculate density as a function of temperature and composition. The decisive advantage is that analysis is unnecessary, as sensor technology for analysis is significantly complex and expensive.
[0025] According to one embodiment, the estimation of the maximum methane content in the BOG, and consequently the maximum methane content in the condensate and fuel gas, is performed 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 linked to the methane content of the liquid phase via the liquid's saturation temperature. Linear relationships exist in the pressure range of 0.4 to 0 bar under consideration. The parameters of the linear model can be expressed as linear functions of the tank pressure. This yields a simple linear model for estimating the methane content in the BOG as a function of tank pressure and measured liquid temperature.
[0026] According to one embodiment, the maximum methane content in the BOG, and thus the maximum methane content in the condensate and the fuel gas, is estimated by additionally considering the initial concentration of propane in the ethane. In this embodiment, it is proposed to perform a multivariable linear regression while fixing the propane content of the starting liquid. In other words, the cargo is modeled as a three-component mixture, and the methane content is estimated based on the input propane content and the measured pressure and temperature. The propane content is known from the cargo analysis, and the range of change during the voyage does not significantly affect the accuracy. By considering the initial concentration of propane, the estimation accuracy can be improved.
[0027] The present invention has been described above with regard to the method. From another perspective, the present invention relates to a system for the reliquefaction of a BOG stream discharged from a storage tank for liquefied petroleum gas, the system comprising an extraction device for taking out a liquid from the liquid phase of the storage tank, a mixing device for introducing the evaporated liquid into the BOG stream to generate a mixed BOG stream, a compression device comprising 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 fluid connections between the mixing device and the compression device, between the extraction device and the mixing device, and between the compression device and the condenser.
[0028] The present invention solves the problems mentioned at the beginning with respect to the system by arranging a heat exchanger, in particular an economizer, between the extraction device and the mixing device. This heat exchanger is configured such that the liquid from the liquid phase of the storage tank is evaporated before being introduced into the BOG stream, thereby generating the evaporated liquid.
[0029] The system according to the present invention utilizes the same advantages and preferred embodiments as the method according to the present invention, and vice versa. To avoid repetition, reference is made to the above embodiments and their content is incorporated herein.
[0030] According to one embodiment, the heat exchanger is connected in liquid communication with the condenser, whereby 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 comprises a metering device connected in liquid communication with the heat exchanger. A partial stream of the condensed compressed mixed BOG stream passing through the heat exchanger and the propulsion fuel gas for the consumer device are supplied to the metering device, and the metering device is configured to meter the mixed BOG stream against the propulsion fuel gas, particularly based on the volume or mass flow rate.
[0032] Furthermore, the mixing device preferably comprises an economizer which is fluidly connected to a compression stage generating a partially compressed intermediate BOG stream and a heat exchanger on the inlet side, and on the outlet side is fluidly connected to a compression stage having a higher compression than the compression stage generating the partially compressed intermediate BOG stream. According to one embodiment, temperature sensors and pressure sensors are assigned to the storage tank.
[0033] According to one embodiment, the system comprises a control device which is configured and designed to perform the following steps. That is, a step of estimating the methane content of the BOG generated in the storage tank based on the pressure and temperature in the storage tank, and a step of metering the partial stream into the fuel gas stream by the metering device so that the methane concentration specified by the consumer device in the fuel gas stream is maintained.
[0034] From another aspect, the present invention relates to a ship comprising a system for reliquefying a BOG stream discharged from a storage tank for liquefied petroleum gas. The present invention solves the problems described at the beginning with respect to the ship by designing the system according to any of the above embodiments. This ship also utilizes the same preferred embodiments and advantages as the method and system according to the present invention, and vice versa. To avoid repetition, reference is made to the above embodiments and their content is incorporated herein.
[0035] According to one embodiment, the vessel is equipped with multiple systems for reliquefying BOG flow discharged from a liquefied petroleum gas storage tank, preferably two or three such systems, and more preferably designed according to any of the above embodiments.
[0036] The present invention will be described in further detail based on the following preferred embodiments with reference to the accompanying drawings, which are shown below. [Brief explanation of the drawing]
[0037] [Figure 1] Figure 1 is a block diagram showing a system for reliquefying BOG (Liquid Oil Gas) flow discharged from a liquefied petroleum gas storage tank. [Figure 2] Figure 2 is a block diagram illustrating a method for reliquefying BOG flow discharged from a liquefied petroleum gas storage tank according to the present invention. [Modes for carrying out the invention]
[0038] Figure 1 shows an exemplary vessel 2. Vessel 2 is equipped with three reliquefaction systems 4a to 4c for refractory BOG flow B discharged from a liquefied petroleum gas storage tank T. One of these systems, namely system 4a, is shown in detail in Figure 1.
[0039] System 4a includes a removal device 3 for extracting liquid F1 from the liquid phase of the storage tank T. Liquid F1 is sent to a heat exchanger 8, i.e., an economizer 8, via a liquid line 6. Liquid line 6 is connected to the economizer inlet 10, and liquid F1 then flows through the economizer 8 and reaches the economizer outlet 12. In the economizer 8, liquid F1 is evaporated, producing evaporated liquid F2. The evaporated liquid F2 leaves the economizer 8 at the outlet 12 and is sent to a mixing device 20, i.e., an economizer 20, via a connecting line 18. The evaporated liquid F2 is supplied to the mixing device 20 via a first inlet 24.
[0040] Furthermore, BOGB is taken from tank T via BOG supply lines 28a-28c and supplied to suction drum 32 via common BOG line 30. Figure 1 shows an example of tank T. The vessel 2 may also have multiple tanks T, in which case any of the BOG supply lines 28a-28c can be assigned to tank T. From suction drum 32, BOGB reaches first compression stage 36, where it is compressed. After compression, compressed BOGBV is obtained and supplied to mixer 20 from second inlet 26 via connecting line 38. The compressed BOGBV and evaporated liquid F2 are mixed in mixer 20, i.e., economizer 20.
[0041] The mixture is supplied via line 40 to the final compression stage 42, where it is compressed, so that the mixed BOG flow BM is compressed and a compressed mixed BOG flow BMv is produced. This is led via line 44 to the condenser 46, entering the condenser 46 from inlet 48 and exiting the condenser 46 from outlet 50. In the condenser 46, the compressed mixed BOG flow BMv is condensed to produce a condensed compressed mixed BOG flow BMvk, i.e., condensate. The condenser 46 may comprise a series of heat exchangers having different temperature levels (not shown).
[0042] This is supplied to the condensate collection tank 54 via line 52, from where it is sent to the condensate discharge line 58 or back to the heat exchanger 8 via connecting line 56. Line 52 is also connected to vent 62. Connecting line 56 is connected to the condensate inlet 14 of the heat exchanger (condenser) 8. After passing through the heat exchanger (condenser) 8, the condensed compressed mixed BOG flow reaches the condensate outlet 16, from where it is discharged by the condensate discharge line 58. The condensate discharge line 58 is connected to line 59, which leads the condensate to tank T via intermediate valves 61, specifically the expansion valve 61. Valves 61 are configured to maintain the condensation pressure in system 4a and the liquid level in the condensate collection tank 54. Line 59 runs downstream of valves 61 in the flow direction.
[0043] The compressed and condensed mixed BOG flow BMvk is used to evaporate liquid F1 in a heat exchanger or condenser 8. Meanwhile, the condensate discharge line 58 is liquid-conductively connected to the condensate recirculation lines 60a-60c, from which it can be returned to at least one tank T.
[0044] The condensate discharge line 58 is also connected to the metering device D via line 63, which branches upstream of valve 61 in the flow direction. A partial flow S of the condensed compressed mixed BOG flow BMvk, which has passed through the heat exchanger 8, is supplied to the metering device D. By removing the condensate before valve 61, which can be configured as an expansion valve 61, the condensate has a higher pressure compared to when it is removed after valve 61. In addition to limiting the methane content in the propulsion fuel gas FG, the metering device D also serves the purpose of maintaining the pressure in line 58, particularly the condensation pressure, or the liquid level in the condensate collection tank 54.
[0045] Furthermore, a propulsion fuel gas FG for a consuming device M, such as a ship engine, is supplied to a metering device D. The metering device D is configured to meter the mixed BOG flow BMvk relative to the propulsion fuel gas FG, particularly based on volumetric or mass flow rate. The flow FG can be taken from the tank using the same device 3 as the flow F1. This yields a fuel gas flow FGM, which can be supplied to the consuming device M. A control device C is provided for metering. The control device C is configured and designed to determine the methane content of the BOGB produced in the storage tank T based on the pressure and temperature in the storage tank T. For measuring pressure and temperature, the storage tank T is provided with appropriate sensors, namely a pressure sensor p and a temperature sensor t. The metering of the partial flow S into the fuel gas flow FG by the metering device D is preferably carried out so that the methane concentration specified by the consuming device M is maintained in the fuel gas flow FGM.
[0046] Figure 2 shows one embodiment of method 100 according to the present invention for reliquefaction of BOG flow B discharged from a liquefied gas storage tank T shown in Figure 1. This method includes the following steps: a1) taking liquid F1 from the liquid phase of the storage tank T; a2) evaporating liquid F1 to form evaporated liquid F2; b) introducing evaporated liquid F2 into BOG flow B to generate mixed BOG flow BM; c) compressing mixed BOG flow BM to a specified final compression pressure to generate compressed mixed BOG flow BMv; d) condensing compressed mixed BOG flow BMv to generate condensed compressed mixed BOG flow BMvk, particularly using a condenser 46; e) returning the condensed compressed mixed BOG flow BMvk to the storage tank T; and f) branching a partial flow S of the condensed compressed mixed BOG flow BMvk and mixing the partial flow S with fuel gas FG, the fuel gas FG being for supplying propulsion fuel gas to consuming equipment M, particularly a ship engine M.
[0047] In step a2), liquid F1 is evaporated by heat exchange with the condensed compressed mixed BOG flow BMvk. In step b), the BOG flow B is first compressed to a compression pressure lower than the specified final compression pressure to produce a partially compressed intermediate BOG flow BV. Liquid F1 taken from the liquid phase of storage tank T is evaporated and then introduced into the intermediate BOG flow BV to produce a mixed BOG flow BM. Step g) includes determining the pressure p and temperature t of liquid F1 in storage tank T, estimating the methane content of the BOGB produced in storage tank T based on the pressure p and temperature t, and measuring the partial flow S relative to the fuel gas flow FG, particularly based on volumetric or mass flow rate, so that the methane concentration specified by the consuming device M in the fuel gas flow FGM is maintained. [Explanation of symbols]
[0048] 2 ships 3 Removal device 4a~4c BOG flow reliquefaction system 6. Suction gas line 8. Heat exchanger (condenser) 10. Condenser Inlet 12. E-condenser outlet 14. Condenser condensate inlet 16. Condenser condensate outlet 18 connection cables 20. Mixing device (economizer) 24 Economizer Entrance 1 26 Economizer Entrance 2 28a~28c BOG supply line 30 Common BOG Line 32 Suction Drum 34 Compressor supply line 36. First Compression Stage 38 Connection cable to the mixing device 40 Line to the final compression stage 42 Final Compression Stage 44 Line to the condenser 46 Condenser 48 Condenser inlet 50 Condenser outlet 52 Line to condensate collection tank 54 Condensate collection tank 56 Connection line between condensate collection tank and heat exchanger 58 Condensate discharge line 59 lines 60A~60C Condensate recirculation 61 valves 62 Bent 63 lines B BOG style BM mixed BOG flow BMv Compressed Mixed BOG Flow BMvk Condensed Compressed Mixed BOG Flow BV Compression BOG Flow C Control device D Weighing device d Pressure sensor F1 liquid F2 Evaporated liquid FG propulsion fuel gas FGM Fuel Gas Flow M Consumer equipment S partial flow T Storage Tank t temperature sensor 100 ways a1) Process of removing liquid from the liquid phase of a storage tank a2) Evaporation process of liquid b) Steps to introduce the evaporated liquid into the BOG flow. c) Compression process of mixed BOG flow d) Condensation process of compressed mixed BOG flow e) Return process of the condensed compressed mixed BOG flow f) Partial flow branching process g) A step of mixing the partial flow with the fuel gas flow.
Claims
1. A method (100) for reliquefying a BOG flow (B) discharged from a liquefied petroleum gas storage tank (T), a1) A step of removing liquid (F1) from the liquid phase of the storage tank (T), a2) A step of evaporating the liquid (F1) to produce the evaporated liquid (F2), b) A step of introducing the evaporated liquid (F2) into the BOG flow (B) to generate a mixed BOG flow (BM), c) A step of compressing the mixed BOG flow (BM) to a specified final compression pressure to generate a compressed mixed BOG flow (BMv), d) A method (100) comprising the steps of condensing the compressed mixed BOG flow (BMv) to produce a condensed compressed mixed BOG flow (BMvk), particularly using a condenser (46).
2. The method according to claim 1 (100), wherein in step a2), the liquid (F1) is evaporated relative to the condensed compressed mixed BOG flow (BMvk).
3. The method according to claim 1 or claim 2, wherein step a2) includes the following sub-steps (100): a2a) A step of providing an air condenser (8) as a heat exchanger, a2b) A step of introducing the liquid (F1) from the liquid phase of the storage tank (T) into the air condenser (8), a2c) A step of introducing the condensed compressed mixed BOG flow (BMvk) into the condenser (8), a2d) A step of evaporating the liquid (F1) from the liquid phase of the storage tank (T) in the condenser (8) relative to the condensed compressed mixed BOG flow (BMvk).
4. A method (100) according to any one of claims 1 to 3, wherein in step b), the BOG flow (B) is first compressed to a compression pressure lower than a specified final compression pressure to generate a partially compressed intermediate BOG flow (BV), and the liquid (F1) taken out from the liquid phase of the storage tank (T) is introduced into the intermediate BOG flow (BV) after evaporation to generate the mixed BOG flow (BM).
5. A method (100) according to any one of claims 1 to 4, wherein step b) includes the following sub-steps: b1) A step of providing an economizer (20) as a mixing device, b2) A step of introducing the evaporated liquid (F2) into the economizer (20), b3) A step of introducing the partially compressed intermediate BOG flow (BV) into the economizer (20).
6. A method (100) according to any one of claims 1 to 5, comprising the following steps: e) A step of returning the condensed compressed mixed BOG flow (BMvk) to the storage tank (T).
7. A method (100) according to any one of claims 1 to 6, comprising the following steps: f) A step of branching off a partial flow (S) of the condensed compressed mixed BOG flow (BMvk), g) A step of mixing the partial flow (S) with a fuel gas flow (FG), the fuel gas flow (FG) being for supplying propulsion fuel gas to a consuming device (M), particularly a ship engine (M).
8. A method (100) according to claim 7, wherein step g) includes the following sub-steps: g1) A step of determining the pressure (p) and temperature (t) of the liquid (F1) in the storage tank (T), g2) A step of estimating the methane content of the BOG (B) generated in the storage tank (T) based on the pressure (p) and temperature (t) obtained in step g1), g3) A step of measuring the partial flow (S) relative to the fuel gas flow (FG), particularly by measuring based on volumetric or mass flow rate, so that the methane concentration specified by the consuming device (M) is maintained within the fuel gas flow (FGM).
9. A system (4a to 4c) for reliquefying BOG flow (B) discharged from a liquefied petroleum gas storage tank (T), A removal device (3) for removing liquid (F1) from the liquid phase of the storage tank (T), A mixing device (20) for introducing evaporated liquid (F2) into a BOG flow (BV) to generate a mixed BOG flow (BM), A compression device (36, 42) comprising at least a first compression stage (36) and a final compression stage (42) for compressing the mixed BOG flow (BM) to the final compression pressure, A condenser (46) for condensing the compressed mixed BOG flow (BMv), The system includes fluid connections between the mixing device (20) and the compression devices (36, 42), between the removal device (3) and the mixing device (20), and between the compression devices (36, 42) and the condenser (46). Furthermore, a heat exchanger (8), particularly an air condenser (8), is positioned between the removal device (3) and the mixing device (20). The heat exchanger (8) is configured to evaporate the liquid (F1) from the liquid phase of the storage tank (T) and form the evaporated liquid (F2) before the liquid (F1) is introduced into the BOG flow (BM).
10. The system (4a to 4c) according to claim 9, wherein the heat exchanger (8) is connected to the condenser (46) via a liquid conductive connection, thereby allowing the liquid (F1) from the liquid phase of the storage tank (T) to evaporate in the heat exchanger (8) relative to the condensed compressed mixed BOG flow (BMvk).
11. A system (4a to 4c) according to claim 9 or claim 10, comprising a metering device (D), the metering device (D) being liquid-conductively connected to the heat exchanger (8), a partial flow (S) of the condensed compressed mixed BOG flow (BMvk) that has passed through the heat exchanger (8) and a propulsion fuel gas (FG) for a consuming device (M) being supplied to the metering device (D), the metering device (D) being configured to meter the mixed BOG flow (BMvk) relative to the propulsion fuel gas (FG) particularly on a volumetric or mass flow rate basis.
12. A system (4a to 4c) according to any one of claims 9 to 11, wherein the mixing device (20) comprises an economizer (20), the economizer (20) is fluidly connected at the inlet side to a compression stage (36) and a heat exchanger (8) that generate a partially compressed intermediate BOG flow, and at the outlet side to a compression stage (42) that is higher than the compression stage (36) that generates a partially compressed intermediate BOG flow.
13. A system according to any one of claims 9 to 12 (4a to 4c), wherein a storage tank (T) is assigned a temperature sensor (t) and a pressure sensor (p).
14. A system (4a to 4c) according to any one of claims 11 to 13, comprising a control device (C), wherein the control device (C) is configured and designed to perform the following steps: A step of 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), A step of measuring the partial flow (S) relative to the fuel gas flow (FG) via the metering device (D), thereby ensuring that the methane concentration specified by the consuming device (M) is maintained within the fuel gas flow (FGM).
15. A ship (2) equipped with a system (4a to 4c) for reliquefying a BOG flow (B) discharged from a liquefied petroleum gas storage tank (T), wherein the system (4a to 4c) is the system described in any one of claims 9 to 14.