Return system for returning recirculated fuel gas and method and use related thereto

Abstract

To provide a return system for recovering recirculated petroleum gas, especially liquefied petroleum gas (LPG) for propulsion.SOLUTION: A recovery device (8) of a return system (2) for recovering petroleum gas is connected to a main engine (4) or an auxiliary engine (6) through supply lines (10a, 10b) by fluid conduction. The recovery device (8) includes a cooling device (26) for cooling recirculated flows (12a, 12b) supplied from the main engine (4) and / or the auxiliary engine (6). A gas supply system (14) for supplying liquefied petroleum gas (LPG) to the main engine (4) or the auxiliary engine (6) is connected by fluid conduction to the recovery device (8) through a recirculating line (16). The recirculating line (16) is provided with a control valve (18) for controlling a flow of the fluid and a recirculation tank (20). The tank is connected to the recovery device (8) through an inlet line (22) and connected to the gas supply system (14) through an outlet line (24).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a return system for returning recycled liquefied petroleum gas for propulsion, in particular recycled liquefied petroleum gas for propulsion of a ship's main and / or auxiliary engines. [Background technology]

[0002] The shipping industry is aiming to decarbonize shipping in line with climate change targets set by the International Maritime Organization (IMO) and other intergovernmental organizations. One way to achieve this goal is to use alternative, carbon-free fuels, such as ammonia. Internal combustion engines capable of running on both conventional fuels and ammonia, along with associated ammonia fuel injection systems, are currently under development. Such internal combustion engines are also known as dual-fuel engines. Many known or currently under development combustion engines are liquid-injected, and unused liquefied petroleum gas (also known as fuel gas) used for propulsion in these engines is generated as a so-called recirculation stream, which must be resupplied to the main or auxiliary engines as efficiently as possible. Summary of the Invention

[0003] A major challenge here is to avoid the occurrence of so-called flash gases in the associated recirculation systems or in the gas supply systems for the main or auxiliary engines. Furthermore, one of the key objectives is to prevent the solidification of oils that may be contained in the recirculation streams and gas supply systems, as well as to reduce the overall energy consumption of the associated return systems.

[0004] Against this background, the present invention is based on the object of further developing a return system of the type mentioned at the beginning and eliminating, as far as possible, the drawbacks of the prior art, in particular to provide an efficient return system which operates in an energy-saving manner and is intended to improve the utilization rate of ammonia liquefied petroleum gas for propulsion or alternative liquefied petroleum gas for propulsion.

[0005] According to the invention, the problems that return systems of the aforementioned type have on the outlet side are solved by providing a return system for recovering recycled liquefied petroleum gas for propulsion, in particular liquefied petroleum gas for propulsion, from a main and / or auxiliary engine of a ship, comprising a recovery device fluidly connected to the main or auxiliary engine via a supply line and including a cooling device configured to cool the recycled flow supplied by the main and / or auxiliary engine, a gas supply system configured to supply the main or auxiliary engine with liquefied petroleum gas for propulsion, the gas supply system fluidly connected to the recovery device via a recirculation line, the recirculation line being assigned a control valve for setting the fluid flow through the recirculation line, and a recirculation tank connected to the recovery device via an inlet line and to the gas supply system via an outlet line.

[0006] The present invention utilizes the knowledge that the recycle stream, which normally has a temperature above 60° C., can be cooled by using a chiller within the recovery unit. Cooling the recycle stream using a recovery unit and then feeding the cooled recycle stream to a gas supply system is an efficient way of reusing the recycle gas as liquefied petroleum gas for propulsion during normal operation of the main or auxiliary engines, for example when ammonia is used as liquefied petroleum gas for propulsion.

[0007] When it is necessary to discharge liquefied petroleum gas for propulsion from the return system, an inert gas such as nitrogen is usually used to flush the system. Conversely, after the flushing process, nitrogen remains in the line, which must be evacuated if operation is to resume with liquefied petroleum gas propulsion, such as ammonia. In this respect, the present invention makes use of the knowledge that it has proven preferable to supply the flow in question to a recirculation tank, which functions both as a collection tank and a liquefied petroleum gas separator. Preferably, the recirculation tank is operated at a lower pressure level than the recovery device and below the saturation pressure associated with the saturation temperature, so-called flash gas being generated in the recirculation tank. Furthermore, a large amount of nitrogen is also collected in the tank. Nitrogen is usually used to flush the return system and may be collected in the recirculation tank together with the liquefied petroleum gas for propulsion.

[0008] The combination of recovery device, gas supply system and recirculation tank according to the invention therefore makes it possible to maximize the retention of ammonia or fuel gas in the system in all operating scenarios, i.e., during normal operation, but also during flushing processes or when the system switches over to using liquefied petroleum gas, e.g., ammonia, for propulsion. By feeding the recirculation stream to the gas supply system and from there again to the main and / or auxiliary engines, the fuel gas resource is treated particularly efficiently. The amount of fuel gas pumped by the gas supply system is reduced, and therefore the specific energy consumption of the plant is also reduced.

[0009] According to one embodiment, the return system comprises a control device configured to open a control valve of the recirculation line in a first operating mode, in particular so that the recirculation flow is fully supplied to the gas supply system, and to close the control valve in a second operating mode, in order that the recirculated liquefied petroleum gas for propulsion is supplied to the recirculation tank by the recirculation device. This allows automatic switching between the respective operating modes, in particular between a normal operating mode in which the recirculation flow is fully supplied to the gas supply system and an operating mode in which the return system is filled with fuel gas or in which fuel gas is released. In this case, a mixture of nitrogen and fuel gas is first supplied to the recirculation tank.

[0010] According to one embodiment, a pressure sensor for detecting the fluid pressure in the recirculation line is assigned to the recirculation line, and the control device is configured in a first operating mode by controlling the control valve to set the back pressure in the supply line so that the recirculation flow remains in liquid phase. The flow in question is fed directly to the gas supply system, bypassing the recirculation tank, so to speak. The setting of the control valve and therefore the back pressure is preferably performed so as to avoid gas generation in the recirculation line and / or to minimize as much as possible the flash gas content that occurs during expansion through the control valve.

[0011] According to one embodiment, the recirculation tank is assigned a level sensor configured to detect the fluid level in the recirculation tank, the level sensor being connected in a data-transmitting manner to the control device, which is configured to release the outlet line when a predetermined level in the recirculation tank is exceeded, in other words, when liquid accumulates in the recirculation tank and exceeds a predetermined level, only liquid is supplied without supplying gas to the gas supply system, allowing the recirculation tank to be emptied as completely as possible in the direction of the gas supply system.

[0012] According to one embodiment, the return system comprises a disconnectable bypass line that connects the supply line directly to the inlet line in a fluid-conductive manner, and the recirculation tank is assigned a pressure sensor configured to sense the internal pressure in the recirculation tank, the pressure sensor being connected in a data-transmitting manner to a control device configured to direct the recirculation flow through the bypass line to the recirculation tank if the recirculation tank falls below a specified minimum pressure. This minimum pressure control aims to keep the pressure in the recirculation tank at a minimum value, allowing the discharge of potentially collected liquid. The pressure measurement is preferably performed on the tank using a pressure transmitter. When the pressure falls below the specified minimum pressure, warm recirculation gas is supplied to the recirculation tank through the bypass line without passing through the cooling device of the recovery device. This warm gas increases the pressure in the tank. When the specified minimum pressure is reached, the supply of warm gas through the bypass line can be stopped.

[0013] According to one embodiment, the internal pressure in the recirculation tank is controlled to be higher than the fluid pressure in the supply line to the high pressure pump of the gas supply system.

[0014] According to one embodiment, the system comprises a collection tank connected to the recirculation tank by a line including a discharge valve, and the control device is configured to open the discharge valve when a specified maximum pressure in the recirculation tank is exceeded, which releases excess gas and reduces the pressure in the recirculation tank again.

[0015] According to one embodiment, the gas supply system comprises a recondenser, and the recirculation line is connected to the recondenser in a fluid conducting manner, and in particular, the gas supply system also comprises a compressor connected to the recondenser in a fluid conducting manner, and the compressor is configured to transfer the recirculated propulsion liquefied petroleum gas and the propulsion liquefied petroleum gas taken from the liquefied petroleum gas tank to the inlet pressure of the main engine or the auxiliary engine.

[0016] The gas supply system allows the merging of the recirculation and fuel gas flows from the liquefied petroleum gas tanks and the appropriate treatment, in particular the pressurization of the propulsion liquefied petroleum gas to the inlet pressure of the main or auxiliary engines.

[0017] According to one embodiment, the recondenser is connected in a fluid-conducting manner to a liquefied petroleum gas tank via a liquefied petroleum gas line, which is assigned a heating device configured to heat the propulsion liquefied petroleum gas to a temperature above the freezing point of the machine oil contained in the recirculation flow. The recirculation flow may always be contaminated with machine oil. The provision of the heating device heats the fuel gas and prevents the oil from solidifying. Heating is preferably required only if the peak point (freezing point) of the oil contained in the recirculation flow is above the atmospheric saturation temperature of ammonia.

[0018] Preferably, a heating device is used to heat the oil. Alternatively, a bypass flow from the compressor can be added to the cold liquid flow to ensure that the mixture temperature is within the required range. Preferably, the pressure in the recirculation tank is maintained higher than the liquid inlet pressure to the recondenser, and the collected liquid is supplied to the recondenser by pressure difference. The selected pressure level also ensures that the liquid in the recirculation tank is preferentially returned to the process over the liquid pumped back from the liquefied petroleum gas tank. The pressure control in the recirculation tank is preferably selected so that the pressure is sufficient to supply a sufficient amount of liquid to the recondenser, while the amount of flash gas generated during expansion via the control valve or in the recirculation tank is kept as low as possible. According to one embodiment, once the return system has been emptied and flushed with an inert gas, in particular nitrogen, the valves of the individual recirculation lines are opened and a mixture of liquid and gas is supplied to the recirculation tank.

[0019] According to one embodiment, the recycle stream fed to the recondenser originates from a recycle tank or from the backpressure control of the recycle system. The stream in question preferably contains a gas component. The pressure level and amount of the liquid feed stream originating from the liquefied petroleum gas tank are preferably adjusted to generate sufficient subcooling for the complete absorption of the flash gas portion of the recycle stream. Preferably, the subcooling is selected to maintain residual subcooling in the subsequent operation of a compression device, in particular a high-pressure fuel gas pump. According to one embodiment, in addition to the actual recondensation, the recondenser ensures that the fed streams are mixed as uniformly as possible.

[0020] Overall, the return system according to the invention offers the following advantages: on the one hand, it maximizes the time that the product remains in the system in all operating scenarios; on the other hand, it ensures efficient fuel gas processing by supplying the recirculation stream to the outlet of the engine, i.e., the main engine and / or at least one auxiliary engine; furthermore, proper temperature control of the recirculation stream and the entire fuel gas stream prevents solidification of oil components; in addition, the amount of fluid delivered by the high-pressure pump is reduced, thereby reducing the unit energy consumption of the entire system. The process control also provides a cost-effective return system: on the one hand, the design pressure of the recirculation tank can be set lower than the design pressure of the entire return system; and on the other hand, the constant recirculation stream is guided around the recirculation tank.

[0021] The present invention has been described above with reference to a return system. In another aspect, the present invention relates to a ship, in particular a cargo ship, equipped with a liquefied petroleum gas propulsion system and a return system for returning liquefied petroleum gas for propulsion. Preferably, the liquefied petroleum gas propulsion system is designed as a dual-fuel propulsion system, i.e., configured to operate with ammonia in addition to conventional fuels, such as heavy fuel oil. The present invention solves the problem mentioned at the beginning with respect to the ship by designing the return system according to one of the claims. The ship enjoys the same advantages and preferred embodiments as the return system according to the present invention, and vice versa. In this regard, reference is made to the above-mentioned embodiments, the content of which is incorporated herein.

[0022] In a further aspect, the present invention relates to a method for returning recycled liquefied propulsion gas using a return system according to one of the aforementioned exemplary embodiments, the method comprising the steps of: supplying a recycle stream from the main and / or auxiliary engines to a recovery device, cooling the recycle stream in the recovery device, supplying the cooled recycle stream to a gas supply system, in particular to a recondenser of the gas supply system, in particular the back pressure of the recycle stream in the supply line is configured so that the recycle stream remains in liquid phase, or supplying the cooled recycle stream to a recycle tank and recovering the recycled liquefied propulsion gas in the recycle tank.

[0023] The method utilizes the knowledge that during normal operation of the main and / or auxiliary engines, the recycle stream undergoes additional treatment steps before being re-fed to the main and / or auxiliary engines, where the backpressure ensures that the recycle stream remains in the liquid phase. In scenarios such as starting an ammonia run or flushing the system after such run, the recycle stream generated under these circumstances, such as a gas used for flushing, e.g., nitrogen, can also be fed to and collected in a recycle tank.

[0024] The method is further developed through the following steps: Flushing the return system with inert gas, recovering the inert gas used for flushing in a recirculation tank.

[0025] The method is further developed by feeding uncooled recirculation stream to the recirculation tank via a bypass line when the pressure in the recirculation tank falls below a specified minimum pressure, and / or opening the discharge valve of the recirculation tank when the pressure in the recirculation tank exceeds a predetermined maximum pressure, so that the pressure in the recirculation tank can be controlled between the minimum and maximum pressures.

[0026] In one embodiment, the method further comprises the steps of: supplying propulsion liquefied petroleum gas from a liquefied petroleum gas tank to a recondenser of the gas supply system, wherein the propulsion liquefied petroleum gas supplied from the liquefied petroleum gas tank is heated to a temperature above at least one of the freezing points of the machinery oil contained in the recirculation stream, supplying the cooled recirculation stream to the recondenser, mixing the propulsion gas from the liquefied petroleum gas tank with the cooled recirculation stream, pressurizing the mixture and supplying it as propulsion gas to the main engine and / or auxiliary engine. This heating treatment prevents the machinery oil from freezing.

[0027] Furthermore, the present invention relates to the use of a return system according to any of the above embodiments for the return of recycled liquefied petroleum gas propulsion used to propel the main engine or one or more auxiliary engines of a ship, in particular the liquefied petroleum gas propulsion selected from the following list: LPG, ammonia, methanol.

[0028] The use of the return system according to the invention has proven to be particularly important in so-called dual-fuel engines, which use, on the one hand, conventional fuels such as heavy fuel oil and, on the other hand, the fuels mentioned above, in particular ammonia.

[0029] The present invention will be explained in more detail based on preferred embodiments with reference to the accompanying drawings. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a schematic diagram of a return system according to the present invention; [Figure 2] 1 is a schematic diagram of a vessel equipped with a return system according to the present invention; [Figure 3] FIG. 1 is a block diagram of a method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] FIG. 1 schematically illustrates a return system 2 for returning recirculated propulsion liquefied petroleum gas from the main engine 4 and the auxiliary engine 6 of a ship 100 (see FIG. 2 ). In the embodiment of FIG. 1 , one main engine 4 and one auxiliary engine 6 are illustrated. However, the return system 2 according to the present invention can also be operated for one or more main engines 4 and / or one or more auxiliary engines 6. A recirculation flow 12a is generated from the main engine 4, and a recirculation flow 12b is generated from the auxiliary engine 6, respectively. These two recirculation flows 12a, 12b are supplied to a recovery device 8 via supply lines 10a and 10b. The recovery device 8 includes cooling devices 26 corresponding to the recirculation flows 12a, 12b. The cooling devices 26 are configured to cool the recirculation flows 12a, 12b supplied from the main engine 4 and the auxiliary engine 6. Downstream of the cooling device 26, the latter is connected on the one hand via the inlet line 22 to the recirculation tank 20 and on the other hand via the intermediate control valve 18 and the recirculation line 16 to the gas supply system, in particular to the recondenser 46 of the gas supply system. Also connected in fluid communication with the inlet line 22 is a return tank 62. In the region of the discharge line, a valve 60 is provided.

[0032] Furthermore, a bypass line 34 is arranged upstream of the cooling device 26 and is configured to supply hot gas that has not yet been cooled by the cooling device 26, i.e., uncooled gas, to the recirculation tank 20. The bypass line 34 can be shut off by a valve. A control valve 18 can adjust the flow rate of fluid through the recirculation line 16. The recirculation tank 20 is connected via an inlet line 22 to the recovery device 8 and via an outlet line 24 to the gas supply system 14, in particular to a recondenser 46 of the gas supply system.

[0033] The return system 2 further comprises a control device 28. The control device 28 is configured to open the control valve 18 in the recirculation line 16 in a first operating mode, in particular to supply the recirculation flows 12a, 12b entirely to the gas supply system 14. In the second operating mode, the control valve 18 is shut off and the recovery device 8 supplies the recirculated liquefied petroleum gas for propulsion to the recirculation tank 20. A pressure sensor 30 is assigned to the recirculation line 16. The pressure sensor 30 is configured to detect the fluid pressure in the recirculation line 16. The control device 28 is further configured to control the control valve 18 in the first operating mode to set a back pressure in the supply lines 10a, 10b so that the recirculation flows 12a, 12b remain in a liquid phase. A liquid level sensor 32 is assigned to the recirculation tank 20. The liquid level sensor 32 is configured to detect the fluid level in the recirculation tank 20. The liquid level sensor 32 is connected to the control device 28 in a data-transmitting manner. The control device 28 is configured to open the valve 60 and discharge the liquid through the discharge line 24 when the liquid level in the recirculation tank 20 exceeds a predetermined level.

[0034] A pressure sensor 36 is assigned to the recirculation tank 20. The pressure sensor 36 is configured to detect the internal pressure in the recirculation tank 20. The pressure sensor 36 is connected to the control device 28 in a data-transmitting manner. The control device 28 is configured to supply the recirculation flows 12a, 12b to the recirculation tank 20 via the bypass line 34 if a predetermined minimum pressure in the recirculation tank 20 is not reached. In this case, uncooled recirculation fluid that has not passed through the cooling device 26 is supplied to the recirculation tank 20. The internal pressure in the recirculation tank 20 is controlled to be higher than the fluid pressure at which the liquefied petroleum gas is supplied to the recondenser 46. The pressure at the inlet of the recondenser 46 is higher than the pressure in the supply line 38.

[0035] The return system 2 further includes a recovery tank 40. The recovery tank 40 is connected to the recirculation tank 20 via a line 42, which is provided with a discharge valve 44. The control device 28 is configured to open the discharge valve 44 when the pressure in the recirculation tank 20 exceeds a predetermined maximum pressure. The recirculation line 16 and the discharge line 24 are fluidly connected to a recondenser 46. The gas supply system 14 includes a compressor 48 (also referred to as a high-pressure pump) fluidly connected to the recondenser 46. The compressor 48 is configured to apply inlet pressure to the recirculated propulsion liquefied petroleum gas and the propulsion liquefied petroleum gas extracted from the liquefied petroleum gas tank 52, to the main engine 4. A heat exchanger 58 is provided downstream of the compressor 48, and a filter 56 is further downstream thereof. The pressurized and filtered fuel gas is then transported to the main engine 4 or the auxiliary engine 6. The auxiliary engine 6 is also connected to the recondenser 46. The recondenser 46 is connected to the compressor 48 via the supply line 38. The compressed fluid passes through a heat exchanger 58 and a filter 56 before being supplied to the auxiliary engine 6. During operation, the recirculation tank 20 is supplied with either the recirculation streams 12a, 12b or with a fluid stream from the return tank 62, but preferably not both at the same time, which affects the dimensioning of the recirculation tank 20.

[0036] The liquefied petroleum gas tank 52 is connected via a line 50 to a heating device 54. The fuel gas removed from the tank 52 passes through a filter 56 before being transferred to the recondenser 46. The heating device 54 is used to heat the propulsion liquefied petroleum gas to a temperature above the freezing point of the engine oil contained in the recirculation streams 12a, 12b.

[0037] In Fig. 2, a ship 100 is shown diagrammatically. The ship 100 is designed in particular as a cargo ship. The ship 100 is equipped with a liquefied petroleum gas propulsion system 102, which is preferably configured as a dual-fuel propulsion system. The ship 100 further comprises a return system 2, which is preferably designed in the configuration shown in Fig. 1. The return system 2 is fluidly connected to the main engine 4 or the auxiliary engine 6 via supply lines 10a, 10b.

[0038] A block diagram of a method 200 according to the present invention is shown in Figure 3. The method 200 includes the following steps: a step 202 of supplying the recirculation streams 12a, 12b from the main engines 4 and / or auxiliary engines 6 to the recovery device 8; a step 204 of cooling the recirculation streams 12a, 12b in the recovery device 8; a step 206 of supplying the cooled recirculation streams 12a, 12b to the gas supply system 14, in particular to the recondenser 46 of the gas supply system 14, wherein the back pressure in the supply lines 10a, 10b is set so that the recirculation streams 12a, 12b remain in a liquid phase; or a step 208 of supplying the cooled recirculation streams 12a, 12b to the recirculation tank 20 and recovering them in the recirculated propulsion liquefied petroleum gas recirculation tank 20; a step 210 of flushing the return system 2 with inert gas; a step 212 of recovering the inert gas used for flushing in the recirculation tank 20; 2. The method 200 includes a step 214 of supplying uncooled recirculation streams 12a, 12b to the recirculation tank 20 via the bypass line 34 if no uncooled recirculation streams 12a, 12b are present, and / or a step 216 of opening the discharge valve 44 if a predetermined maximum pressure in the recirculation tank 20 is exceeded, a step 220 of supplying propulsion liquefied petroleum gas from a liquefied petroleum gas tank 52 to a recondenser 46 of the gas supply system 14, wherein the propulsion liquefied petroleum gas from the liquefied petroleum gas tank 52 is heated to a temperature above the freezing point of the engine oil contained in the recirculation streams 12a, 12b, a step 222 of supplying the cooled recirculation streams 12a, 12b to the recondenser 46, a step 224 of mixing the propulsion gas from the liquefied petroleum gas tank 52 with the cooled recirculation streams 12a, 12b, and a step 226 of pressurizing the mixture and supplying it as propulsion gas to the main engines 4 and / or the auxiliary engines 6. However, in accordance with the present invention, the method may include only one of the illustrated steps or any combination thereof. [Explanation of symbols]

[0039] 2 Return System 4 Main engine 6. Auxiliary Bodies 8. Recovery Device 10a Supply line (for main engine) 10b Supply line (for auxiliary engines) 12a Recirculation flow (main engine origin) 12b Recirculation flow (from auxiliary engine) 14 Gas Supply System 16 Recirculation Line 18. Control valve for setting fluid flow through the recirculation line 20 Recirculation Tank 22 Inlet line 24 Discharge Line 26 Cooling device 28 Control Device 30 Pressure sensors (for recirculation and / or discharge lines) 32 Liquid level sensor (for recirculation tank) 34 Bypass Line 36 Pressure sensor (for recirculation tank) 38 High pressure pump supply line 40 Recovery Tank 42 lines 44 Discharge valve 46 Recondenser 48 Compression Device 50 LPG line from LPG tank 52 Liquefied petroleum gas tank 54 Heating device 56 filters 58 Heat exchanger 60 valves 62 Return tank 100 ships 102 Liquefied Petroleum Gas Propulsion System 200 ways 202 Supply of recirculation flow to recovery device 204 Cooling of recirculation stream in recovery unit 206 Supply of cooled recycle stream to gas supply system 208 Cooled recirculation flow supply to recirculation tank 210 Flushing of return systems with inert gas 212 Recovery of inert gas used for flushing into recirculation tank 214 Supply of uncooled recirculation flow via bypass line 216 Recirculation tank discharge valve opening when maximum pressure in the recirculation tank is exceeded 218 Supply of recovered liquid from return tank to recirculation tank 220 Supply of liquefied petroleum gas for propulsion from liquefied petroleum gas tank to recondenser 222 Cooled recirculation flow feed to recondenser 224 Mixing of propellant gas and refrigerated recirculation stream from liquefied petroleum gas tanks 226 Pressurizing the mixture and supplying it to the main / auxiliary engines

Claims

1. A return system (2) for recovering recirculated liquefied petroleum gas for propulsion, in particular recirculated liquefied petroleum gas for propulsion from a main engine (4) and / or auxiliary engine (6) of a ship (100), comprising: a recovery device (8), the recovery device (8) being fluidly connected to the main engine (4) or the auxiliary engine (6) via supply lines (10a, 10b), and including a cooling device (26) configured to cool a recirculation flow (12a, 12b) supplied from the main engine (4) and / or the auxiliary engine (6); a gas supply system (14) configured to supply liquefied petroleum gas for propulsion to the main engine (4) or the auxiliary engine (6); the gas supply system (14) is fluidly connected to the recovery device (8) via a recirculation line (16) assigned to the recirculation line (16) a control valve (18) for setting the flow of fluid through the recirculation line (16); A return system comprising a recirculation tank (20) connected to the recovery device (8) via an inlet line (22) and to the gas supply system (14) via an outlet line (24).

2. A control device (28) is provided, The control device (28) a) in a first operating mode, the control valve (18) of the recirculation line (16) is opened so that the recirculation flow (12a, 12b) is supplied, in particular completely, to the gas supply system (14); b) in a second operating mode, the control valve (18) is shut off and the propulsion liquefied petroleum gas recirculated by the recovery device (8) is supplied to the recirculation tank (20).

3. a pressure sensor (30) assigned to said recirculation line (16) for detecting the fluid pressure in said recirculation line (16); 3. The return system (2) of claim 1 or claim 2, wherein the control device (28) is configured, in the first operating mode, to set a back pressure in the supply line (10a, 10b) by controlling the control valve (18) so that the recirculation flow (12a, 12b) remains in a liquid phase.

4. The recirculation tank (20) is assigned a level sensor (32) configured to detect the fluid level in the recirculation tank (20), The level sensor (32) is connected to the control device (28) in a manner capable of transmitting data thereto; 4. The return system (2) of claim 1, wherein the control device (28) is configured to open the outlet line (24) when a defined level in the recirculation tank (20) is exceeded.

5. a cutoff bypass line (34) directly connecting the supply lines (10a, 10b) and the inlet line (22) in a fluid-conducting manner; The recirculation tank (20) is assigned a pressure sensor (36) configured to detect the internal pressure in the recirculation tank (20), The pressure sensor (36) is connected to the control device (28) in a manner capable of transmitting data thereto; the control device (28) is configured to supply the recirculation stream (12a, 12b) to the recirculation tank (20) via the bypass line (34) when the pressure in the recirculation tank (20) falls below a predetermined minimum pressure; 5. A return system (2) according to any one of claims 1 to 4, in particular, wherein the internal pressure in the recirculation tank (20) is controlled to be higher than the fluid pressure in the supply line (38) to the high-pressure pump (48) of the gas supply system (14).

6. a recovery tank (40) connected to the recirculation tank (20) via a line (42); The line (42) is provided with a discharge valve (44), 6. The return system (2) of claim 1, wherein the control device (28) is configured to open the discharge valve (44) when a predetermined maximum pressure in the recirculation tank (20) is exceeded.

7. the gas supply system (14) comprises a recondenser (46); the recirculation line (16) is fluidly connected to the recondenser (46); In particular, the gas supply system (14) also comprises a compressor (48) fluidly connected to the recondenser (46); 7. The return system (2) according to any one of claims 1 to 6, wherein the compressor (48) is configured to pressurize the recycled propulsion liquefied petroleum gas and the propulsion liquefied petroleum gas taken out from the liquefied petroleum gas tank (52) to the inlet pressure of the main engine (4) or the auxiliary engine (6) and supply it.

8. the recondenser (46) is fluidly connected to the liquefied petroleum gas tank (52) via the liquefied petroleum gas line (50); A heating device (54) is assigned to the liquefied petroleum gas line (50), 8. The return system (2) according to any one of claims 1 to 7, wherein the heating device (54) is configured to heat the propulsion liquefied petroleum gas to a temperature above the freezing point of the machine oil contained in the recirculation stream (12a, 12b).

9. A ship (100), in particular a cargo ship, equipped with a liquefied petroleum gas propulsion system (102) and a return system (2) for the recirculation of recycled liquefied petroleum gas for propulsion, A vessel (100) wherein the return system (2) is a return system according to any one of claims 1 to 8.

10. A method (200) for the recirculation of recycled liquefied propulsion gas using a return system (2) according to any one of claims 1 to 9, comprising: Supplying (202) the recycle streams (12a, 12b) of the main engine (4) and / or auxiliary engine (6) to a recovery device (8); cooling (204) the recycle stream (12a, 12b) in a recovery system (8); and supplying (206) the cooled recycle stream (12a, 12b) to a gas supply system (14), in particular a recondenser (46) of the gas supply system (14), wherein a back pressure of the recycle stream (12a, 12b), in particular the recycle stream (12a, 12b) in the supply line (10a, 10b), is set (206) so as to maintain the recycle stream (12a, 12b) in a liquid phase; or supplying (208) the cooled recycle stream (12a, 12b) to a recycle tank (20) and recovering the recirculated liquefied petroleum gas for propulsion in the recycle tank (20).

11. flushing (210) the return system (2) with an inert gas; and recovering (212) the inert gas used in the flush in the recirculation tank (20).

12. feeding (214) the uncooled recirculation stream (12a, 12b) to the recirculation tank (20) via a bypass line (34) when the pressure in the recirculation tank (20) falls below a predetermined minimum pressure; and / or 12. The method (200) of claim 10 or claim 11, comprising the step (216) of opening a discharge valve (44) of the recirculation tank (20) when a predetermined maximum pressure in the recirculation tank (20) is exceeded.

13. supplying (220) propulsion liquefied petroleum gas from a liquefied petroleum gas tank (52) to a recondenser (46) of the gas supply system (14), the propulsion liquefied petroleum gas from the liquefied petroleum gas tank (52) being heated to a temperature above the freezing point of the machine oil contained in the recirculation stream (12a, 12b); feeding (222) the cooled recycle stream (12a, 12b) to the recondenser (46); mixing (224) the propellant gas from the liquefied petroleum gas tank (52) with the cooled recycle stream (12a, 12b) to obtain a mixture; and pressurizing (226) the mixture and supplying it as propulsion gas to a main engine (4) and / or an auxiliary engine (6).

14. Use of the return system according to any one of claims 1 to 8 for the recirculation of recycled liquefied propulsion gas used to propel the main engines (4) or auxiliary engines (6) of a ship (100), in particular, wherein the liquefied propulsion gas or propulsion fluid is selected from the group consisting of LPG, ammonia, methanol.

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