Recirculation system for recirculating liquefied propulsion gas, process and use

Abstract

The invention relates to a return system (2) for returning recirculated propulsion liquefied gas, in particular recirculated propulsion liquefied gas of a main engine (4) and / or auxiliary engine (6) of a ship (100). According to the invention, it is proposed that the recirculation system (2) comprises a recovery device (8), wherein the recovery device (8) is fluidically connected to a main engine (4) or an auxiliary engine (6) via a supply line (10a, 10b) and comprises a cooling device (26) which is designed to cool the recirculation flow (12a, 12b) supplied by the main engine (4) and / or the auxiliary engine (6), a gas supply system (14) which is designed to supply liquefied propulsion gas to the main engine (4) or the auxiliary engine (6), wherein the gas supply system (14) is fluidically connected to the recovery device (8) via a recirculation line (16), and wherein the recirculation line (16) is assigned a control valve (18) for adjusting a fluid flow through the recirculation line (16), and a recirculation container (20),which is connected to the recovery device (8) via an inlet line (22) and to the gas supply system (14) via an outlet line (24).

Description

[0001] The invention relates to a return system for returning recirculated propulsion liquefied gas, in particular recirculated propulsion liquefied gas of a main engine and / or auxiliary engine of a ship.

[0002] In the maritime industry, the climate targets set by the International Maritime Organization (IMO) and other intergovernmental organizations aim to decarbonize shipping. One way to achieve this goal is through the use of alternative fuels that do not contain carbon, such as ammonia. Combustion engines that can run on both conventional fuels and ammonia, as well as the associated ammonia fuel injection systems, are currently under development. Such combustion engines are also known as dual-fuel engines.

[0003] A large proportion of the combustion engines already known or currently under development have liquid injection, whereby liquid propulsion gas not used by the combustion engine, also known as fuel gas, occurs in the form of so-called recirculation streams and is to be fed back to the main engine or auxiliary engine as efficiently as possible.

[0004] A major challenge here is to avoid the occurrence of so-called flash gas within a recirculation system or a gas supply system for the main engine or the auxiliary engine.

[0005] In addition, a key objective is to prevent the build-up of potential oil components within the recirculation stream and in the gas delivery system and to reduce the overall energy consumption of the relevant recirculation systems.

[0006] Against this background, the invention was based on the object of developing a recirculation system of the type mentioned above in such a way that the disadvantages found in the prior art are eliminated as far as possible. In particular, an efficient recirculation system is to be provided that can be operated in an energy-saving manner and ensures improved utilization of the liquefied ammonia propulsion gas or alternative liquefied propulsion gases.

[0007] According to the invention, the object is achieved in a recirculation system of the type mentioned at the outset in that it has a recovery device, wherein the recovery device is fluidly connected to a main engine or an auxiliary engine via a supply line and has a cooling device which is designed to cool the recirculation flow supplied by the main engine and / or auxiliary engine, a gas supply system which is designed to provide liquefied petroleum gas to the main engine or the auxiliary engine, wherein the gas supply system is fluidly connected to the recovery device via a recirculation line and wherein the recirculation line is assigned a control valve for adjusting a fluid flow through the recirculation line and a recirculation container,which is connected to the recovery device via an inlet line and to the gas supply system via an outlet line.

[0008] The invention takes advantage of the discovery that the use of a cooling device in the recovery system allows the recirculation streams, which typically have a temperature greater than 60° Celsius, to be cooled. Cooling the recirculation streams with the aid of the recovery system and subsequently feeding the cooled recirculation streams to a gas supply system represents an efficient way to reuse the recirculated gas as propulsion liquefied gas during normal operation of the main engine or the auxiliary engine when using, for example, ammonia as propulsion liquefied gas.

[0009] If the return system of liquefied propulsion gas needs to be emptied, inert gases, such as nitrogen, are usually used to purge the system. Conversely, after a purging process, nitrogen remains in the lines and must be removed from them if operation is to be resumed with liquefied propulsion gas, such as ammonia. The invention takes advantage of the finding that it has proven advantageous to feed the relevant streams to a recirculation tank, which serves both as a collection tank and as a liquefied gas separator. The recirculation tank is preferably operated at a lower pressure level than the recovery device and below the saturation pressure associated with the saturation temperature, so that so-called flash gas is generated in the recirculation tank.In addition, larger quantities of nitrogen are also collected in the tank, whereby the nitrogen is typically used to purge the recirculation system and can be collected in the recirculation tank together with propulsion liquefied petroleum gas.

[0010] The combination of the inventive recovery device, gas supply system, and recirculation tank thus maximizes the retention of ammonia or fuel gas in the system in all operating scenarios, i.e., both during normal operation and during purging processes or when switching the system to the use of liquefied propulsion gas, such as ammonia. By supplying the recirculation streams to the gas supply system and from there back to the main engine and / or auxiliary engine, the fuel gas resource is used particularly efficiently. The specific energy consumption of the system is also reduced, as the amount of fuel gas to be pumped by the gas supply system is reduced.

[0011] According to one embodiment, the recirculation system comprises a control device configured to release the control valve of the recirculation line in a first operating mode, so that the recirculation flow is supplied, in particular, entirely to the gas supply system, and to shut off the control valve in a second operating mode, so that recirculated drive liquid gas is supplied to the recirculation tank from the recirculation device. The control device thus enables automated switching between the individual operating modes, i.e., in particular, between the normal operating mode, in which the recirculation flow is to be supplied entirely to the gas supply system, and that operating mode(s) in which the recirculation system is to be either filled with fuel gas or emptied of it.In this case, the mixture of nitrogen and fuel gas is first fed to the recirculation tank.

[0012] According to one embodiment, a pressure sensor for sensing a fluid pressure in the recirculation line is assigned to the recirculation line, wherein the control device is configured to adjust a backpressure in the supply line in the first operating mode by controlling the control valve such that the recirculation flow remains in the liquid phase. The respective flows are then fed directly to the gas supply system, bypassing the recirculation vessel. The control valve is preferably controlled and the backpressure is thus adjusted in such a way that no gas is generated in the recirculation line and / or the flash gas content generated during expansion via the control valve is minimized as much as possible.

[0013] According to one embodiment, a fill level sensor is assigned to the recirculation tank, which is configured to sense a fluid fill level in the recirculation tank. The fill level sensor is connected to the control device in a data-conducting manner, and the control device is configured to release the outlet line when a defined fill level in the recirculation tank is exceeded. In other words, when liquid accumulates in the recirculation tank and exceeds a defined fill level, the recirculation tank can be emptied as completely as possible toward the gas supply system without supplying gas to the gas supply system, but rather only liquid.

[0014] According to one embodiment, the recirculation system comprises a closable bypass line that fluidically connects the supply line directly to the inlet line. A pressure sensor configured to sense an internal pressure in the recirculation tank is assigned to the recirculation tank. The pressure sensor is connected in a data-conducting manner to a control device configured to supply the recirculation flow to the recirculation tank via the bypass line when the pressure in the recirculation tank falls below a defined minimum pressure. The objective of this minimum pressure control is to keep the pressure in the recirculation tank at a minimum value so that potentially collected fluid can be drained away. The pressure is preferably measured on the tank using a pressure transmitter.If the pressure drops below a defined minimum pressure, warm recirculated gas is fed to the recirculation tank via the bypass line. This warm gas does not pass through the recovery system's cooling system. This warm gas increases the pressure in the tank. Once the defined minimum pressure is reached, the supply of warm gas via the bypass line can be stopped.

[0015] According to one embodiment, the internal pressure in the recirculation tank is controlled such that it is greater than the fluid pressure in a supply line to a high-pressure pump of the gas supply system.

[0016] According to one embodiment, the system comprises a collecting tank connected to the recirculation tank via a line having a drain valve, and wherein the control device is configured to open the drain valve when a defined maximum pressure in the recirculation tank is exceeded. Opening the drain valve releases excess gas, thereby reducing the pressure in the recirculation tank.

[0017] According to one embodiment, the gas supply system comprises a recondenser, wherein the recirculation line is fluidly connected to the recondenser, in particular, wherein the gas supply system further comprises a compression device fluidly connected to the recondenser, which is configured to pressurize the recirculated drive liquefied gas and the drive liquefied gas taken from a liquefied gas tank to an inlet pressure of the main engine or auxiliary engine.

[0018] The gas supply system enables the merging of recirculation flows and fuel gas flows from the LPG tank as well as appropriate treatment, in particular pressurization, of the propulsion LPG to an inlet pressure of the main engine or auxiliary engine.

[0019] According to one embodiment, the recondenser is fluidly connected to a liquefied gas tank via a liquefied gas line, wherein the liquefied gas line is assigned a heating device configured to heat the drive liquefied gas to a temperature above a pour point of an engine oil contained in the recirculation streams. Recirculation streams can, in principle, be contaminated with engine oil. By providing a heating device, the fuel gas can be heated, thus preventing the oil from clogging. Heating is preferably only necessary if the recirculation streams contain oil with a pour point above the atmospheric saturation temperature of ammonia. A heating device is preferably used for the purpose of heating the oil.Alternatively, bypass streams from the compression device can be mixed with a cold liquid stream so that the mixing temperature is within the required range.

[0020] Preferably, the pressure in the recirculation tank is kept above the pressure of the liquid flow to the recondenser so that the collected liquid can be fed to the recondenser via the pressure difference. The selected pressure level also ensures that the liquid in the recirculation tank is returned to the process preferentially over the liquid fed from the liquefied gas tank. The pressure control in the recirculation tank is preferably selected such that the pressure is sufficient to supply the liquid to the recondenser in sufficient quantity and, on the other hand, to keep the amount of flash gas generated during expansion via the control valve or into the recirculation tank as low as possible.According to one embodiment, in the event that the recirculation system is emptied and purged with inert gas, in particular nitrogen, valves of the individual recirculation lines are opened, which in turn supply a mixture of liquid and gas to the recirculation tank.

[0021] According to one embodiment, the recirculation streams fed to the recondenser originate either from the recirculation tank or from the backpressure control of the recirculation system. These streams preferably contain a gas component. The pressure level and the quantity of the liquid feed stream originating from the liquefied gas tank are preferably conditioned to ensure sufficient subcooling so that the flash gas component of the recirculation stream can be completely absorbed. Preferably, the subcooling is also selected to ensure residual subcooling for the subsequent operation of the compression device, in particular a high-pressure fuel gas pump. According to one embodiment, in addition to the actual recondensation, the recondenser ensures the most homogeneous mixing possible of the supplied streams.

[0022] Overall, the recirculation system according to the invention offers the following advantages: Firstly, the retention of the product in the system is maximized in all operating scenarios. Secondly, efficient fuel gas treatment is achieved by feeding the recirculation flows into the outlets to the engines, i.e., the main engine and / or at least one auxiliary engine. Furthermore, the stagnation of potential oil components is prevented by appropriate temperature management of the recirculation flows and the fuel gas flows in general. Furthermore, the specific energy consumption of the system is also reduced, as the amount of fluid to be pumped by the high-pressure pumps is reduced.The process control also provides a cost-effective recirculation system, since on the one hand the design pressure of the recirculation tank can be below the design pressure of the recirculation system and on the other hand the permanent recirculation flows are guided past the recirculation tank.

[0023] The invention has been described above with reference to a return system. In a further aspect, the invention relates to a ship, in particular a cargo ship, with a liquefied gas drive and a return system for returning recirculated propulsion liquefied gas. The liquefied gas drive is preferably designed as a dual-fuel drive, i.e. it is configured to be operated with ammonia in addition to a conventional fuel, such as heavy fuel oil. The invention achieves the object described above with regard to the ship in that the return system is designed according to one of the preceding claims. The ship makes use of the same advantages and preferred embodiments as the return system according to the invention and vice versa. In this regard, reference is made to the above statements, the content of which is hereby incorporated.

[0024] In a further aspect, the invention relates to a method for recirculating recirculated propulsion liquefied gas using a recirculation system according to one of the preceding embodiments. The method comprises the steps of: supplying a recirculation stream from a main engine and / or auxiliary engine to a recovery device, cooling the recirculation stream in the recovery device, supplying the cooled recirculation stream to a gas supply system, in particular a recondenser of the gas supply system, wherein a backpressure of the recirculation stream, in particular in a supply line, is adjusted such that the recirculation stream remains in the liquid phase, or supplying the cooled recirculation stream to a recirculation tank and collecting the recirculated propulsion liquefied gas in the recirculation tank.

[0025] The process leverages the knowledge that during regular operation of the main engine and / or auxiliary engine, the recirculation stream is fed back to the main engine and / or auxiliary engine after further treatment steps. Adjusting the backpressure ensures that the recirculation stream remains in the liquid phase. In scenarios where, for example, ammonia operation is to be initiated or the system is to be purged after such operation, the recirculation streams, which in these scenarios also contain the gas used for purging, e.g., nitrogen, can be fed to a recirculation tank and collected there.

[0026] The process is further developed by the steps: purging the recirculation system with inert gas, collecting the inert gas used for purging in the recirculation tank.

[0027] The method is further developed by the following steps: supplying an uncooled recirculation stream to the recirculation tank via a bypass line when the pressure in the recirculation tank falls below a defined minimum, and / or opening a drain valve of the recirculation tank when the pressure in the recirculation tank exceeds a defined maximum. In this way, the pressure within the recirculation tank can be controlled between a minimum and a maximum pressure.

[0028] According to one embodiment, the method further comprises the steps of: providing propulsion liquefied gas from a liquefied gas tank to a recondenser of the gas supply system, wherein the propulsion liquefied gas from the liquefied gas tank is heated to a temperature above a pour point of the engine oil contained in the recirculation streams; providing the cooled recirculation stream to the recondenser; mixing the propulsion gas from the liquefied gas tank and the cooled recirculation stream, pressurizing the mixture, and providing the propulsion gas to a main engine and / or auxiliary engine. The proposed heating prevents the engine oil from clogging.

[0029] In a further aspect, the invention relates to the use of a recirculation system according to one of the preceding embodiments for recirculating recirculated propulsion liquefied petroleum gas intended for driving a main engine or one or more auxiliary engines of a ship. The propulsion liquefied petroleum gas is selected, in particular, from the list comprising: LPG, ammonia, methanol.

[0030] The use of a recirculation system according to the invention has proven particularly useful in the use of so-called dual-fuel engines, which are used with conventional fuels, e.g. heavy fuel oil, and also with one of the aforementioned fuels, in particular ammonia.

[0031] The invention is described in more detail below using preferred embodiments with reference to the attached figures.

[0032] Here we show: Fig. 1 shows a schematic representation of a return system according to the invention; Fig. 2 a ship according to the invention with a return system in a schematic representation; and Fig. 3 a block diagram of a method according to the invention.

[0033] Fig. 1 shows a return system 2 for returning recirculated propulsion liquefied gas of a main engine 4 and an auxiliary engine 6 of a ship 100 (cf. Fig. 2). In the embodiment of the Fig. 1 shows, by way of example, precisely one main engine 4 and one auxiliary engine 6. However, it is equally possible to operate one or more main engines 4 and / or one or more auxiliary engines 6 with the corresponding recirculation system 2. A recirculation stream 12a originates from the main engine 4, and a recirculation stream 12b originates from the auxiliary engine 6. The two recirculation streams 12a, 12b are fed to a recovery device 8 via supply lines 10a and 10b. The recovery device 8 has a cooling device 26 for each recirculation stream 12a, 12b. The cooling device 26 is configured to cool the recirculation stream 12a, 12b supplied by the main engine 4 and the auxiliary engine 6.Downstream of the cooling device 26 in the flow direction, the cooling device 26 is connected, on the one hand, to a recirculation tank 20 via an inlet line 22 and, on the other hand, with the interposition of a control valve 18, via the recirculation line 16 to a gas supply system, in particular a recondenser 46 of the gas supply system. A return tank 62 is also fluidly connected to the inlet line 22. A valve 60 is provided in the area of ​​the outlet line.

[0034] Furthermore, bypass lines 34 are provided, which are arranged upstream of the cooling devices 26 and are designed to supply hot gas, i.e., gas that has not yet been cooled by the cooling devices 26, to the recirculation tank 20. The bypass line 34 can be shut off by valves. The control valve 18 enables adjustment of a fluid flow through the recirculation line 16. The recirculation tank 20 is connected to the recovery device 8 via the inlet line 22 and, via an outlet line 24, also to the gas supply system 14, in particular the recondenser 46.

[0035] The recirculation system 2 further comprises a control device 28. This is configured to open the control valve 18 of the recirculation line 16 in a first operating mode, so that the recirculation flow 12a, 12b is supplied, in particular, completely to the gas supply system 14, and to shut off the control valve 18 in a second operating mode, so that recirculated drive liquid gas from the recovery device 8 is supplied to the recirculation tank 20. A pressure sensor 30 is assigned to the recirculation line 16. The pressure sensor 30 is configured to sense the fluid pressure in the recirculation line 16. The control device 28 is further configured to adjust a backpressure in the supply line 10a, 10b in the first operating mode by controlling the control valve 18 such that the recirculation flow 12a, 12b remains in the liquid phase.A fill level sensor 32 is assigned to the recirculation tank 20. The fill level sensor 32 is configured to sense a fluid fill level in the recirculation tank 20. The fill level sensor 32 is connected to the control device 28 in a data-conducting manner. The control device 28 is configured to open the valve 60 and release the liquid through the outlet line 24 when a defined fill level in the recirculation tank 20 is exceeded.

[0036] A pressure sensor 36 is also assigned to the recirculation tank 20. The pressure sensor 36 is configured to sense an internal pressure in the recirculation tank 20. The pressure sensor 36 is connected to the control device 28 in a data-conducting manner. The control device 28 is configured to supply the recirculation flow 12a, 12b to the recirculation tank 20 via the bypass line 34 when a defined minimum pressure in the recirculation tank 20 is undershot. In this case, the recirculation tank 20 is supplied with uncooled recirculated fluid, which does not pass through the cooling devices 26. The internal pressure in the recirculation tank 20 is further controlled such that it is greater than the fluid pressure in the liquid gas supply to the recondenser 46. The pressure at the inlet of the recondenser 46 is higher than that in the supply lines 38.

[0037] The recirculation system 2 further comprises a collection tank 40. The collection tank 40 is connected to the recirculation tank 20 via a line 42, which has a drain valve 44. The control device 28 is configured to open the drain valve 44 when a defined maximum pressure in the recirculation tank 20 is exceeded. The recirculation line 16 and the outlet line 24 are fluidly connected to the recondenser 46. The gas supply system 14 has a compression device 48, which is also referred to as a high-pressure pump and is fluidly connected to the recondenser 46. The compression device 48 is configured to pressurize the recirculated propulsion liquefied gas and the propulsion liquefied gas taken from a liquefied gas tank 52 to an inlet pressure of the main engine 4. Downstream of the compression device 48, a heat exchanger 58 is also provided, as well as a filter 56 downstream of this.From there, the pressurized and filtered fuel gas reaches the main engine 4 or the auxiliary engine 6. The auxiliary engine 6 is also connected to the recondenser 46 in the same way. The recondenser 46 is connected to a compression device 48 via a supply line 38. From there, the compressed fluid passes through a heat exchanger 58 and a filter 56 and reaches the auxiliary engine 6. During operation, either the recirculation stream 12a, 12b is fed to the recirculation tank 20 or a fluid stream originating from the return tank 62, but preferably not both streams simultaneously, which preferably affects the dimensioning of the recirculation tank 20.

[0038] The liquefied gas tank 52 is connected to a heating device 54 via a line 50. The fuel gas extracted from the tank 52 passes through a filter 56 and then to the recondenser 46. The heating device 54 serves to heat the propulsion liquefied gas to a temperature above the pour point of an engine oil contained in the recirculation streams 12a, 12b.

[0039] Fig. Figure 2 shows a schematic representation of a ship 100. The ship 100 is designed, in particular, as a cargo ship. The ship 100 has a liquefied gas propulsion system 102, which is preferably designed as a dual-fuel propulsion system. The ship 100 further has a recirculation system 2, wherein the recirculation system 2 is preferably designed as shown in Fig. 1. The return system 2 is fluidly connected to the main engine 4 and the auxiliary engine 6 via the supply lines 10a, 10b.

[0040] Fig. 3 shows a block diagram of a method 200 according to the invention. The method 200 comprises the steps of: feeding 202 a recirculation stream 12a, 12b of a main machine 4 and / or an auxiliary machine 6 to a recovery device 8, cooling 204 the recirculation stream 12a, 12b in the recovery device 8, feeding 206 the cooled recirculation stream 12a, 12b to a gas supply system 14, in particular a recondenser 46 of the gas supply system 14, wherein a backpressure in the supply line 10a, 10b is adjusted such that the recirculation stream 12a, 12b remains in the liquid phase, or feeding 208 the cooled recirculation stream 12a, 12b to a recirculation tank 20 and collecting the recirculated propulsion liquid gas in the recirculation tank 20, purging 210 the recirculation system 2 with inert gas, collecting 212 the inert gas used for purging in the recirculation tank 20,Supplying 214 an uncooled recirculation stream 12a, 12b via a bypass line 34 to the recirculation tank 20 when a defined minimum pressure in the recirculation tank 20 is undershot, and / or opening 216 a drain valve 44 of the recirculation tank 20 when a defined maximum pressure in the recirculation tank 20 is exceeded, Providing 220 drive liquefied gas from a liquefied gas tank 52 to a recondenser 46 of the gas supply system 14, wherein the drive liquefied gas from the liquefied gas tank 52 is heated to a temperature above a pour point of the engine oil contained in the recirculation streams 12a, 12b, Providing 222 the cooled recirculation stream 12a, 12b to the recondenser 46, Mixing 224 of the drive gas from the liquid gas tank 52 and the cooled recirculation stream 12a, 12b,Pressurizing 226 the mixture and providing the drive gas to a main engine 4 and / or auxiliary engine 6. The process was described in , Fig. 3 are shown in a coherent manner. However, according to the invention, the method may also comprise only individual steps shown or a combination thereof. List of reference symbols 2 Return system 4 main engines 6 Auxiliary machine 8 Recovery facility 10a Main engine supply line 10b Auxiliary machine supply line 12a Recirculation current main engine 12b Recirculation flow auxiliary machine 14 Gas supply system 16 Recirculation line 18 Control valve for adjusting fluid flow through the recirculation line 20 recirculation tanks 22 Inlet line 24 Outlet line 26 Cooling device 28 Control device 30 Pressure sensor of the recirculation line and / or outlet line 32 Recirculation tank level sensor 34 Bypass line 36 Pressure sensor of the recirculation tank 38 High pressure pump supply line 40 collection containers 42 Line 44 Drain valve 46 recondensers 48 Compression device 50 LPG line from the LPG tank 52 LPG tank 54 Heating device 56 filters 58 heat exchangers 60 valve 62 return tanks 100 ships 102 LPG drive 200 procedures 202 Feeding a recirculation stream to a recovery device 204 Cooling of the recirculation stream in the recovery unit 206 Supplying the cooled recirculation stream to a gas supply system 208 Feeding the cooled recirculation stream to a recirculation tank 210 Purging the recirculation system with inert gas, 212 Collecting the inert gas used for purging in the recirculation tank 214 Supplying an uncooled recirculation stream via a bypass line 216 Opening a drain valve of the recirculation tank when a defined maximum pressure in the recirculation tank is exceeded 218 Supplying collected liquid from a return tank to the recirculation tank 220 Providing propulsion liquefied petroleum gas from a liquefied petroleum gas tank to a recondenser of the gas supply system 222 Providing the cooled recirculation stream to the recondenser 224 Mixing the drive gas from the LPG tank and the cooled recirculation stream 226 Pressurizing the mixture and supplying the propulsion gas to a main engine or auxiliary engine

Claims

[1] Return system (2) for returning recirculated propulsion liquefied gas, in particular recirculated propulsion liquefied gas of a main engine (4) and / or auxiliary engine (6) of a ship (100), with - a recovery device (8), wherein the recovery device (8) is fluidly connected to the main machine (4) or the auxiliary machine (6) via a supply line (10a, 10b) and has a cooling device (26) which is designed to cool the recirculation flow (12a, 12b) supplied by the main machine (4) and / or the auxiliary machine (6), - a gas supply system (14) which is designed to supply propulsion liquefied gas to the main engine (4) or the auxiliary engine (6), wherein the gas supply system (14) is fluidly connected to the recovery device (8) via a recirculation line (16) and wherein the recirculation line (16) is assigned a control valve (18) for adjusting a fluid flow through the recirculation line (16), - and a recirculation tank (20) which is connected to the recovery device (8) via an inlet line (22) and to the gas supply system (14) via an outlet line (24), characterized by that the gas supply system (14) has a recondenser (46) and wherein the recirculation line (16) is fluidly connected to the recondenser (46). [2] Feedback system (2) according to claim 1, comprising a control device (28) which is arranged to a) in a first operating mode, to release the control valve (18) of the recirculation line (16) so that the recirculation flow (12a, 12b) is supplied in particular completely to the gas supply system (14), and b) to shut off the control valve (18) in a second operating mode so that recirculated drive liquid gas is supplied from the recovery device (8) to the recirculation tank (20). [3] Recirculation system (2) according to one of the preceding claims, wherein the recirculation line (16) is assigned a pressure sensor (30) for sensing a fluid pressure in the recirculation line (16), wherein the control device (28) is configured to set a back pressure in the supply line (10a, 10b) in the first operating mode by controlling the control valve (18) such that the recirculation flow (12a, 12b) remains in the liquid phase. [4] Recirculation system (2) according to one of the preceding claims, wherein the recirculation tank (20) is assigned a fill level sensor (32) which is configured to sense a fluid fill level in the recirculation tank (20), wherein the fill level sensor (32) is connected to the control device (28) in a data-conducting manner and the control device (28) is configured to release the outlet line (24) when a defined fill level in the recirculation tank (20) is exceeded. [5] Return system (2) according to one of the preceding claims, with a lockable bypass line (34) which connects the supply line (10a, 10b) directly to the inlet line (22) in a fluid-conducting manner, wherein the recirculation tank (20) is assigned a pressure sensor (36) which is designed to sense an internal pressure in the recirculation tank (20), and wherein the pressure sensor (36) is connected in a data-conducting manner to the control device (28), which is designed to supply the recirculation flow (12a, 12b) to the recirculation tank (20) via the bypass line (34) when a defined minimum pressure in the recirculation tank (20) is undershot, in particular wherein the internal pressure in the recirculation tank (20) is controlled such that it is greater than the fluid pressure in a supply line (38) to a high-pressure pump (48) of the gas supply system (14). [6] Recirculation system (2) according to one of the preceding claims, wherein the recirculation system (2) comprises a collecting tank (40) which is connected to the recirculation tank (20) via a line (42) which has a drain valve (44), and wherein the control device (28) is configured to open the drain valve (44) when a defined maximum pressure in the recirculation tank (20) is exceeded. [7] Recirculation system (2) according to one of the preceding claims, wherein the gas supply system (14) further comprises a compression device (48) fluidly connected to the recondenser (46) and configured to pressurize the recirculated drive liquefied gas and the drive liquefied gas taken from a liquefied gas tank (52) to an inlet pressure of the main engine (4) or auxiliary engine (6). [8] Recirculation system (2) according to one of the preceding claims, wherein the recondenser (46) is fluidly connected to the liquefied gas tank (52) via a liquefied gas line (50) and wherein the liquefied gas line (50) is assigned a heating device (54) which is designed to heat the drive liquefied gas to a temperature above a pour point of an engine oil contained in the recirculation streams (12a, 12b). [9] Ship (100), in particular a cargo ship, with a liquefied gas drive (102) and a return system (2) for returning recirculated drive liquefied gas, wherein the return system (2) is designed according to one of the preceding claims. [10] Method (200) for recirculating recirculated propulsion liquid gas with a recirculation system (2) according to one of the preceding claims, wherein the method (200) comprises the steps: - feeding (202) a recirculation flow (12a, 12b) of a main machine (4) and / or auxiliary machine (6) to a recovery device (8), - cooling (204) the recirculation stream (12a, 12b) in the recovery device (8), - feeding (206) the cooled recirculation stream (12a, 12b) to a gas supply system (14), in particular a recondenser (46) of the gas supply system (14), wherein a backpressure of the recirculation stream (12a, 12b), in particular in a supply line (10a, 10b), is adjusted such that the recirculation stream (12a, 12b) remains in the liquid phase, or - feeding (208) the cooled recirculation stream (12a, 12b) to a recirculation tank (20) and collecting the recirculated drive liquid gas in the recirculation tank (20). [11] Method (200) according to claim 10, comprising the steps: - purging (210) the recirculation system (2) with inert gas, - collecting (212) the inert gas used for purging in the recirculation tank (20). [12] Method (200) according to one of claims 10 or 11, comprising the steps: - supplying (214) an uncooled recirculation stream (12a, 12b) via a bypass line (34) to the recirculation tank (20) when a defined minimum pressure in the recirculation tank (20) is undershot, and / or - Opening (216) a drain valve (44) of the recirculation tank (20) when a defined maximum pressure in the recirculation tank (20) is exceeded. [13] Method (200) according to one of claims 10 to 12, comprising the steps: - Providing (220) drive liquefied gas from a liquefied gas tank (52) to the recondenser (46) of the gas supply system (14), wherein the drive liquefied gas from the liquefied gas tank (52) is heated to a temperature above a pour point of the engine oil contained in the recirculation streams (12a, 12b), - Providing (222) the cooled recirculation stream (12a, 12b) to the recondenser (46), - mixing (224) the drive gas from the liquid gas tank (52) and the cooled recirculation stream (12a, 12b), - Pressurizing (226) the mixture and providing the drive gas to a main engine (4) and / or auxiliary engine (6). [14] Use of a return system (2) according to one of the preceding claims for returning recirculated propulsion liquefied gas, which is provided for driving a main engine (4) or auxiliary engines (6) of a ship (100), in particular wherein the propulsion liquefied gas or the propulsion liquid is selected from the list comprising: - LPG, - ammonia, - Methanol.

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