An ammonia BOG reliquefaction system and method for a liquid ammonia carrier, a liquid ammonia carrier

Abstract

The application discloses a kind of ammonia BOG reliquefaction system and method of liquid ammonia transport ship, liquid ammonia transport ship, ammonia BOG reliquefaction system includes liquid ammonia cargo hold, compressor unit, condenser, regenerator, throttle valve, gas-liquid separator, three-way valve and cold supply system, and the refrigerant of cold supply system is liquid ammonia;Liquid ammonia transport ship includes the ammonia BOG reliquefaction system described above;Ship ammonia BOG reliquefaction method, ship is the liquid ammonia transport ship described above.In the application, ammonia BOG is directly compressed, condensed and throttled, and then a low-temperature gas-liquid two-phase mixture is formed, and two-phase separation is carried out through a gas-liquid separator.The low-temperature gas phase ammonia separated by the gas-liquid separator exchanges heat with the liquid ammonia at the outlet of the condenser, further reducing the subcooling degree of the liquid ammonia before the throttle valve.The liquid ammonia separated by the gas-liquid separator, part of which returns to the liquid ammonia cargo hold, and the other part absorbs heat in the cold supply system as refrigerant, solving the technical problems of the prior art, such as complex ammonia BOG reliquefaction system, low efficiency, and unsatisfactory liquefaction effect.

Description

Technical Field

[0001] This invention relates to the field of marine ammonia BOG treatment technology, and more particularly to an ammonia BOG reliquefaction system and method for liquid ammonia transport ships, and a liquid ammonia transport ship. Background Technology

[0002] In the shipping industry, ammonia, as a zero-carbon fuel, has attracted widespread attention. As a fuel for ship engines, ammonia is usually stored in fuel tanks as liquid ammonia at -33°C. During storage, unavoidable environmental heat leakage will cause the liquid ammonia to absorb heat and evaporate. The accumulation of ammonia BOG (boil-off gas) will lead to an increase in pressure in the liquid ammonia storage tank. Therefore, it is necessary to treat the BOG in the liquid ammonia storage tank.

[0003] To reliquefy ammonia BOG, Chinese invention patent CN115468379A provides a marine ammonia evaporation gas reliquefaction system, including a main refrigeration system and a pre-cooling system. The pre-cooling system provides cooling capacity through a vapor compression refrigeration unit, requiring additional refrigerant to be supplied on board. However, it does not consider the energy consumption generated by the compressors of the main refrigeration system and the pre-cooling system, nor the complex auxiliary design issues of the pre-cooling system's refrigerant on the ship. Chinese invention patent CN115717679A provides an ammonia BOG reliquefaction system and method for ships, which connects to the atmosphere and / or a storage tank through one output end of a collection tank to remove non-condensable gases contained in the room-temperature liquid ammonia output from the condenser to form pure liquid ammonia. However, the process is complex, and the temperature of the liquefied liquid ammonia is too high. Even after passing through a reliquefaction regulating valve, it still contains ammonia gas when entering the cargo hold. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this application provides an ammonia BOG reliquefaction system and method for a liquid ammonia transport ship, as well as a liquid ammonia transport ship, to solve the technical problems of complex, inefficient, and unsatisfactory liquefaction effects of existing ammonia BOG reliquefaction systems.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] A liquid ammonia cargo ship BOG reliquefaction system includes a liquid ammonia cargo tank, a compressor unit, a condenser, a regenerator, a throttle valve, a gas-liquid separator, a three-way valve, and a cooling system. The refrigerant of the cooling system is liquid ammonia, and the cooling system supplies cooling to cold storage or air conditioning.

[0007] The outlet of the liquid ammonia cargo tank is connected to the inlet of the compressor unit; the outlet of the compressor unit is connected to the hot-side inlet of the condenser; the hot-side outlet of the condenser is connected to the hot-side inlet of the regenerator; the hot-side outlet of the regenerator is connected to the inlet of the throttle valve; the outlet of the throttle valve is connected to the inlet of the gas-liquid separator; the gas outlet of the gas-liquid separator is connected to the cold-side inlet of the regenerator; the cold-side outlet of the regenerator is connected to the inlet of the compressor unit; the liquid outlet of the gas-liquid separator is connected to the inlet of a three-way valve; one outlet of the three-way valve is connected to the liquid inlet of the liquid ammonia cargo tank; the other outlet of the three-way valve is connected to the refrigerant inlet of the cooling system; and the refrigerant outlet of the cooling system is connected to the inlet of the compressor unit.

[0008] In one embodiment, the compressor unit is a multi-stage cooling compressor unit, which includes a low-pressure compressor, a primary cooler, a high-pressure compressor, and an aftercooler in sequence according to the direction of ammonia flow. The outlet of the liquid ammonia cargo tank is connected to the inlet of the low-pressure compressor, the outlet of the low-pressure compressor is connected to the hot-side inlet of the primary cooler, the hot-side outlet of the primary cooler is connected to the inlet of the high-pressure compressor, the outlet of the high-pressure compressor is connected to the hot-side inlet of the aftercooler, and the hot-side outlet of the aftercooler is connected to the hot-side inlet of the condenser.

[0009] In one embodiment, the cooling medium for the primary cooler, aftercooler, and condenser is cooling water.

[0010] In one embodiment, a pressure sensor is installed inside the gas-liquid separator, and a pressure control valve is installed between the cold side outlet of the regenerator and the air inlet of the compressor unit. When the pressure sensor of the gas-liquid separator detects that the gas pressure in the gas-liquid separator is higher than a set value, the opening of the pressure control valve is increased to reduce the gas pressure inside the gas-liquid separator; when the pressure sensor of the gas-liquid separator detects that the gas pressure in the gas-liquid separator is lower than the set value, the opening of the pressure control valve is decreased to increase the pressure in the gas-liquid separator.

[0011] In one embodiment, a liquid level sensor is installed inside the gas-liquid separator, and a liquid level control valve is installed between the liquid outlet of the gas-liquid separator and the inlet of the three-way valve. When the liquid level sensor detects that the liquid level of the gas-liquid separator is higher than a set height, the opening of the liquid level control valve is increased to lower the liquid level of the gas-liquid separator; when the liquid level sensor detects that the liquid level of the gas-liquid separator is lower than a set height, the opening of the liquid level control valve is decreased to raise the liquid level of the gas-liquid separator.

[0012] In one embodiment, a liquid ammonia filter is installed between the outlet of the three-way valve and the inlet of the liquid ammonia cargo tank.

[0013] In one embodiment, a refrigerant inlet valve is provided between the other outlet of the three-way valve and the refrigerant inlet of the cooling system, and a refrigerant flow control valve is provided between the refrigerant outlet of the cooling system and the air inlet of the compressor unit.

[0014] This application also provides a method for reliquefying ammonia BOG on a liquid ammonia transport ship, comprising the following steps:

[0015] The ammonia BOG generated in the liquid ammonia cargo tank is compressed and cooled by the compressor cooler unit and then condensed into saturated or subcooled high-pressure liquid ammonia. The high-pressure liquid ammonia is cooled by the regenerator to further reduce the subcooling of the liquid ammonia before the throttle valve. It is then cooled further by the throttle valve to form a gas-liquid two-phase mixture that enters the gas-liquid separator. The low-temperature ammonia gas at the gas phase outlet of the gas-liquid separator is used by the regenerator to cool the saturated or subcooled high-pressure liquid ammonia at the condenser outlet and then enters the compressor cooler unit. A portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator enters the cooling system as a refrigerant. The ammonia gas at the outlet of the cooling system and the ammonia gas at the gas phase outlet of the liquid separator enter the compressor cooler unit together. The other portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator returns to the liquid ammonia cargo tank.

[0016] This application also provides a liquid ammonia transport vessel, including the aforementioned ammonia BOG reliquefaction system, wherein the liquid ammonia cargo tank is provided with a gas dome for discharging gaseous ammonia BOG and injecting liquid ammonia.

[0017] In one embodiment, the condenser and the regenerator are both plate heat exchangers, shell-and-tube heat exchangers, or coiled tube heat exchangers.

[0018] Compared with the prior art, this application has at least the following beneficial effects:

[0019] (1) This invention sets up a cooling system with liquid ammonia as the refrigerant in the ammonia BOG reliquefaction system. The ammonia BOG in the liquid ammonia cargo tank is directly compressed, condensed and throttled to form a low-temperature gas-liquid two-phase mixture. The mixture is then separated into two phases by a gas-liquid separator. The low-temperature gas phase ammonia separated by the gas-liquid separator exchanges heat with the liquid ammonia at the outlet of the condenser, further reducing the subcooling of the liquid ammonia before the throttle valve. Part of the liquid ammonia separated by the gas-liquid separator is returned to the liquid ammonia cargo tank, and the other part is used as refrigerant to absorb heat in the cooling system. Compared with adding an extra vapor compression refrigeration system, this invention is simpler and more efficient, and eliminates the need for an extra refrigerant auxiliary system.

[0020] (2) The present invention sets a three-way valve between the gas-liquid separator, the cooling system and the compressor unit. The liquid ammonia flow rate is matched with the cooling system demand through the three-way valve at the liquid outlet of the gas-liquid separator, so that the compressor unit does not need to be started or adjusted frequently. The ammonia BOG reliquefaction system can also operate stably when the ambient temperature changes greatly. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the ammonia BOG reliquefaction system in an embodiment of this application.

[0022] The specific explanations of the attached diagram labels are as follows: 1. Liquid ammonia cargo tank; 2. Gas dome; 3. Compressor unit; 3-1. Low-pressure compressor; 3-2. High-pressure compressor; 3-3. Primary cooler; 3-4. Aftercooler; 4. Condenser; 5. Regenerator; 6. Throttling valve; 7. Gas-liquid separator; 8. Gas pressure control valve; 9. Liquid level control valve; 10. Three-way valve; 11. Liquid ammonia filter; 12. Cryogenic ejector; 13. Refrigerant flow control valve; 14. Refrigerant inlet valve; 15. Cooling system. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0024] The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The singular forms “a,” “the,” and “the” as used in this disclosure and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0025] It should be understood that although the terms first, second, third, etc., may be used in this disclosure to describe various information, such information should not be limited to these terms and should not be construed as indicating or implying relative importance. These terms are used only to distinguish information of the same type from one another. For example, without departing from the scope of this disclosure, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0026] In the description of this invention, unless otherwise specified and limited, it should be noted that the terms "installation", "connection" and "linking" should be interpreted broadly. For example, they can refer to mechanical connection or internal connection between two components. They can be direct connection or indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0027] To better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0028] like Figure 1 As shown, this embodiment provides an ammonia BOG reliquefaction system for a liquid ammonia transport ship, including a liquid ammonia cargo tank 1, a compressor unit 3, a condenser 4, a regenerator 5, a throttle valve 6, a gas-liquid separator 7, a three-way valve 10, and a cooling system 15. The refrigerant of the cooling system 15 is liquid ammonia, and the cooling system supplies cooling to cold storage or air conditioning.

[0029] The outlet of the liquid ammonia cargo tank 1 is connected to the inlet of the compressor unit 3. The outlet of the compressor unit 3 is connected to the hot-side inlet of the condenser 4. The hot-side outlet of the condenser 4 is connected to the hot-side inlet of the regenerator 5. The hot-side outlet of the regenerator 5 is connected to the inlet of the throttle valve 6. The outlet of the throttle valve 6 is connected to the inlet of the gas-liquid separator 7. The gas outlet of the gas-liquid separator 7 is connected to the cold-side inlet of the regenerator 5. The cold-side outlet of the regenerator 5 is connected to the inlet of the compressor unit 3. The liquid outlet of the gas-liquid separator 7 is connected to the inlet of the three-way valve 10. One outlet of the three-way valve 10 is connected to the liquid inlet of the liquid ammonia cargo tank 1. The other outlet of the three-way valve 10 is connected to the refrigerant inlet of the cooling system 15. The refrigerant outlet of the cooling system 15 is connected to the inlet of the compressor unit 3.

[0030] Compressor unit 3 is a multi-stage cooled compressor unit. In this embodiment, multi-stage cooled compressor unit 3 includes, in sequence according to the direction of ammonia flow, a low-pressure compressor 3-1, a primary cooler 3-3, a high-pressure compressor 3-2, and an aftercooler 3-4. The outlet of the liquid ammonia cargo tank 1 is connected to the inlet of the low-pressure compressor 3-1. The outlet of the low-pressure compressor 3-1 is connected to the hot-side inlet of the primary cooler 3-3. The hot-side outlet of the primary cooler 3-3 is connected to the inlet of the high-pressure compressor 3-2. The outlet of the high-pressure compressor 3-2 is connected to the hot-side inlet of the aftercooler 3-4. The hot-side outlet of the aftercooler 3-4 is connected to the hot-side inlet of the condenser 4. The outlet pressure of ammonia in the low-pressure compressor 3-1 is in the range of 3~5 bar abs, and the outlet pressure of ammonia in compressor unit 3 is in the range of 13.5~17 bar abs. The cooling medium for the primary cooler 3-3, the aftercooler 3-4, and the condenser 4 is cooling water.

[0031] To control the gas pressure of the gas-liquid separator 7, a gas pressure sensor (not shown in the attached diagram) is installed inside the gas-liquid separator 7. A gas pressure control valve 8 is installed between the cold side outlet of the regenerator 5 and the air inlet of the compressor unit 3. The gas pressure of the gas-liquid separator 7 is controlled within a set range by the gas pressure control valve 8. When the gas pressure sensor of the gas-liquid separator 7 detects that the gas pressure of the gas-liquid separator 7 is higher than the set value, the opening of the gas pressure control valve 8 is increased to reduce the gas pressure inside the gas-liquid separator 7. When the gas pressure sensor of the gas-liquid separator 7 detects that the gas pressure of the gas-liquid separator 7 is lower than the set value, the opening of the gas pressure control valve 8 is decreased to increase the pressure inside the gas-liquid separator 7.

[0032] Similarly, in order to control the liquid level of the gas-liquid separator 7, a liquid level sensor (not shown in the attached figure) is installed inside the gas-liquid separator 7, and a liquid level control valve 9 is installed between the liquid outlet of the gas-liquid separator 7 and the inlet of the three-way valve 10. The liquid level of the gas-liquid separator 7 is controlled within a set range by the liquid level control valve 9. When the liquid level sensor of the gas-liquid separator 7 detects that the liquid level of the gas-liquid separator 7 is higher than the set height, the opening of the liquid level control valve 9 is increased to reduce the liquid level of the gas-liquid separator 7; when the liquid level sensor of the gas-liquid separator 7 detects that the liquid level of the gas-liquid separator 7 is lower than the set height, the opening of the liquid level control valve 9 is decreased to increase the liquid level of the gas-liquid separator 7.

[0033] To reduce impurities entering the liquid ammonia cargo tank 1, a liquid ammonia filter 11 is installed between the outlet of the three-way valve 10 and the inlet of the liquid ammonia cargo tank 1.

[0034] A refrigerant inlet valve 14 is installed between the other outlet of the three-way valve 10 and the refrigerant inlet of the cooling system 15, and a refrigerant flow control valve 13 is installed between the refrigerant outlet of the cooling system 15 and the air inlet of the compressor unit 3, thereby avoiding frequent start-up or large-scale adjustment of the compressor unit 3, which would cause the system to be unstable.

[0035] The liquid ammonia cargo tank 1 inlet is connected to the cryogenic injector 12 inside the liquid ammonia cargo tank 1. Liquid ammonia is injected into the liquid ammonia cargo tank 1 through the cryogenic injector 12, ensuring a uniform temperature distribution inside the liquid ammonia cargo tank 1.

[0036] The working process of the ammonia BOG reliquefaction system in this embodiment is as follows:

[0037] The ammonia BOG generated in the liquid ammonia cargo tank 1 is compressed and cooled by the compressor-cooler unit 3 and then enters the condenser 4 to be condensed into saturated or subcooled high-pressure liquid ammonia. The high-pressure liquid ammonia is cooled by the regenerator 5, which further reduces the subcooling of the liquid ammonia before the throttle valve 6. It is then further cooled by the throttle valve 6 to form a gas-liquid two-phase mixture that enters the gas-liquid separator 7. The low-temperature ammonia gas at the gas phase outlet of the gas-liquid separator 7 is used by the regenerator 5 to cool the saturated or subcooled high-pressure liquid ammonia at the outlet of the condenser 4, and then enters the compressor-cooler unit 3. A portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator 7 enters the cooling system 15 as a refrigerant. The ammonia gas at the outlet of the cooling system 15 and the ammonia gas at the outlet of the liquid phase 7 of the liquid separator enter the compressor-cooler unit 3 together. The other portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator 7 returns to the liquid ammonia cargo tank 1 through the liquid ammonia filter 11, thus forming a closed loop.

[0038] This embodiment also provides a liquid ammonia transport vessel, including the aforementioned ammonia BOG reliquefaction system, with a gas dome 2 installed on the liquid ammonia cargo tank for discharging gaseous ammonia BOG and injecting liquid ammonia. The cooling medium of the condenser 4 is seawater, using locally sourced materials to reduce operating costs.

[0039] To facilitate the installation of the ammonia BOG reliquefaction system on a ship with limited space, in this embodiment, the condenser 4 and the regenerator 5 are one of the following: plate heat exchanger, shell and tube heat exchanger, or coiled tube heat exchanger.

[0040] This embodiment also provides a method for reliquefying ammonia BOG on a liquid ammonia transport vessel, wherein the vessel is a liquid ammonia transport vessel as described above, and includes the following steps:

[0041] The ammonia BOG generated in the liquid ammonia cargo tank is compressed and cooled by the compressor-cooler unit 3 and then enters the condenser 4 to be condensed into saturated or subcooled high-pressure liquid ammonia. The high-pressure liquid ammonia is cooled by the regenerator 5 to further reduce the subcooling of the liquid ammonia before the throttle valve 6. It is then further cooled by the throttle valve 6 to form a gas-liquid two-phase mixture that enters the gas-liquid separator 7. The low-temperature ammonia gas at the gas phase outlet of the gas-liquid separator 7 is used by the regenerator 5 to cool the saturated or subcooled high-pressure liquid ammonia at the condenser outlet and then enters the compressor-cooler unit 3. A portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator 7 enters the cooling system 15 as a refrigerant. The ammonia gas at the outlet of the cooling system 15 and the ammonia gas at the outlet of the liquid phase 7 of the liquid separator enter the compressor-cooler unit 3 together. The other portion of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator 7 returns to the liquid ammonia cargo tank 1.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. An ammonia BOG reliquefaction system for a liquid ammonia carrier, characterized in that, The system comprises a liquid ammonia cargo tank, a compressor unit, a condenser, a regenerator, a throttle valve, a gas-liquid separator, a three-way valve and a cooling system, wherein the refrigerant of the cooling system is liquid ammonia, and the cooling system supplies cooling for a cold storage or an air conditioner. The gas outlet of the liquid ammonia cargo tank is connected with the gas inlet of the compressor unit, the outlet of the compressor unit is connected with the hot side inlet of the condenser, the hot side outlet of the condenser is connected with the hot side inlet of the regenerator, the hot side outlet of the regenerator is connected with the inlet of the throttle valve, the outlet of the throttle valve is connected with the inlet of the gas-liquid separator, the gas outlet of the gas-liquid separator is connected with the cold side inlet of the regenerator, the cold side outlet of the regenerator is connected with the gas inlet of the compressor unit, the liquid outlet of the gas-liquid separator is connected with the inlet of the three-way valve, one outlet of the three-way valve is connected with the liquid inlet of the liquid ammonia cargo tank, and the other outlet of the three-way valve is connected with the refrigerant inlet of the cooling system, and the refrigerant outlet of the cooling system is connected with the gas inlet of the compressor unit. A liquid ammonia filter is arranged between the outlet of the three-way valve and the liquid inlet of the liquid ammonia cargo tank, a refrigerant inlet valve is arranged between the other outlet of the three-way valve and the refrigerant inlet of the cooling system, a refrigerant flow control valve is arranged between the refrigerant outlet of the cooling system and the gas inlet of the compressor unit, and the cooling medium of the condenser is cooling water, and the compressor unit is a multi-stage cooling compressor unit.

2. The liquid ammonia carrier ammonia BOG reliquefaction system of claim 1, wherein, The multi-stage cooling compressor unit comprises, in sequence according to the ammonia flow direction, a low-pressure compressor, a primary cooler, a high-pressure compressor and a post-cooler, the gas outlet of the liquid ammonia cargo tank is connected with the inlet of the low-pressure compressor, the outlet of the low-pressure compressor is connected with the hot side inlet of the primary cooler, the hot side outlet of the primary cooler is connected with the inlet of the high-pressure compressor, the outlet of the high-pressure compressor is connected with the hot side inlet of the post-cooler, and the hot side outlet of the post-cooler is connected with the hot side inlet of the condenser.

3. The liquid ammonia carrier ammonia BOG reliquefaction system of claim 2, wherein, The cooling medium of the primary cooler and the post-cooler is cooling water.

4. The liquid ammonia carrier ammonia BOG reliquefaction system of claim 1, wherein, A gas pressure sensor is arranged in the gas-liquid separator, and a gas pressure control valve is arranged between the cold side outlet of the regenerator and the gas inlet of the compressor unit, when the gas pressure sensor of the gas-liquid separator monitors that the gas pressure of the gas-liquid separator is higher than a set value, the opening degree of the gas pressure control valve is increased to reduce the gas pressure in the gas-liquid separator, and when the gas pressure sensor of the gas-liquid separator monitors that the gas pressure of the gas-liquid separator is lower than the set value, the opening degree of the gas pressure control valve is reduced to increase the pressure of the gas-liquid separator.

5. The liquid ammonia carrier ammonia BOG reliquefaction system of claim 1, wherein, A liquid level sensor is arranged in the gas-liquid separator, and a liquid level control valve is arranged between the liquid outlet of the gas-liquid separator and the inlet of the three-way valve, when the liquid level sensor of the gas-liquid separator monitors that the liquid level of the gas-liquid separator is higher than a set height, the opening degree of the liquid level control valve is increased to reduce the liquid level of the gas-liquid separator, and when the liquid level sensor of the gas-liquid separator monitors that the liquid level of the gas-liquid separator is lower than the set height, the opening degree of the liquid level control valve is reduced to increase the liquid level of the gas-liquid separator.

6. A method of ammonia BOG reliquefaction for a liquid ammonia carrier, characterized by, The liquid ammonia transport ship ammonia BOG reliquefaction system is used in the following steps: The ammonia BOG generated in the liquid ammonia cargo tank is compressed and cooled by the compressor unit, and then enters the condenser to be condensed into saturated or supercooled high-pressure liquid ammonia. The high-pressure liquid ammonia is cooled by the regenerator to further reduce the supercooling degree of the liquid ammonia before the throttling valve. The liquid ammonia is further cooled by the throttling valve to form a gas-liquid two-phase mixture, which enters the gas-liquid separator. The low-temperature ammonia gas at the gas phase outlet of the gas-liquid separator is used to cool the saturated or supercooled high-pressure liquid ammonia at the outlet of the condenser through the regenerator, and then enters the compressor unit. A part of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator enters the cooling system as a refrigerant. The ammonia gas at the outlet of the cooling system enters the compressor unit together with the ammonia gas at the gas phase outlet of the gas-liquid separator. Another part of the low-temperature liquid ammonia at the liquid phase outlet of the gas-liquid separator returns to the liquid ammonia cargo tank.

7. A liquid ammonia carrier, characterized by The ammonia BOG reliquefaction system of the liquid ammonia carrier according to any one of claims 1-5, wherein a dome is arranged on the liquid ammonia cargo tank for discharging gas ammonia BOG and injecting liquid ammonia.

8. The liquid ammonia carrier of claim 7, wherein The condenser and the regenerator are one of a plate heat exchanger, a shell-and-tube heat exchanger, or a coiled tube heat exchanger.

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