A process and system for replacing nitrogen in a carbon dioxide liquid cargo tank

Abstract

The present invention belongs to the field of liquefied gas carriers, and particularly relates to a process and system for replacing nitrogen in a carbon dioxide cargo tank. The process includes the following steps: drying the cargo tank; connecting the cargo tank pipelines and devices; standby of the dry air generator; starting the shipboard seawater pump; adjusting the seawater flow rate; carbon dioxide controlled-pressure evaporation replacement; conveying carbon dioxide vapor to the shore station; detecting the carbon dioxide content. The equipment includes: a dry air generator, a shore station, a cargo evaporator and a cargo tank; a bottom loading pipeline of the cargo tank and a top gas removal pipeline of the cargo tank are arranged in the cargo tank. The process and system of the present invention for replacing nitrogen in the cargo tank with superheated gaseous carbon dioxide realizes the forward movement of the carbon dioxide phase control point from the cargo tank to the cargo evaporator, effectively avoiding the risk of dry ice generation during the operation, which may cause harm to the cargo tank and the entire liquid cargo system, and has high safety and practicability.

Description

Technical Field

[0001] The present invention belongs to the field of liquefied gas carriers, and particularly relates to a process and system for replacing nitrogen in a carbon dioxide cargo tank. Background Art

[0002] The replacement of nitrogen in a carbon dioxide cargo tank is one of the important links in the operation of the cargo system. Its purpose is to use carbon dioxide vapor to expel the nitrogen in the cargo tank after drying the carbon dioxide cargo tank with nitrogen, so as to facilitate the subsequent loading of carbon dioxide cargo.

[0003] Common liquefied gases, such as LNGC, etc., are usually stored and transported in a fully refrigerated (low-temperature and normal-pressure) environment. There are only two media, gaseous and liquid, in its cargo system during operation. Therefore, when replacing nitrogen in a carbon dioxide cargo tank, only the pressure and temperature bearing capacity of the carbon dioxide cargo tank structure needs to be considered, without considering the influence of the medium itself on pressure and temperature.

[0004] Due to the triple point of carbon dioxide - the pressure and temperature when the three phases of solid, liquid, and gas coexist in equilibrium, its properties are special. In engineering, a semi-cooled and semi-pressurized method must be used to store and transport liquefied carbon dioxide. When replacing nitrogen in a carbon dioxide cargo tank, it is necessary to ensure that the temperature and pressure of carbon dioxide are above the triple point (5.18 bara / -56.6 °C) to avoid the formation of dry ice and endanger the safety of the carbon dioxide cargo system.

[0005] In the existing technology for the process and system of replacing nitrogen in a carbon dioxide cargo tank, the carbon dioxide phase control point is in the carbon dioxide cargo tank. Due to the lag and low control accuracy of the control, it is impossible to ensure that the temperature and pressure of carbon dioxide are above the triple point (5.18 bara / -56.6 °C). Therefore, dry ice will be formed, endangering the safety of the cargo system. Summary of the Invention

[0006] In order to solve the problems in the existing technology for the process and system of replacing nitrogen in a carbon dioxide cargo tank, where the carbon dioxide phase control point is in the carbon dioxide cargo tank, and due to the lag and low control accuracy of the control, it is impossible to ensure that the temperature and pressure of carbon dioxide are above the triple point, resulting in the formation of dry ice and endangering the safety of the cargo system, etc., the present invention provides a process for replacing nitrogen in a carbon dioxide cargo tank, including the following steps:

[0007] S1: Dry the carbon dioxide cargo tank;

[0008] S2: Connect the carbon dioxide cargo tank pipeline and device;

[0009] S3: Keep the dry air generator in standby;

[0010] S4: Start the shipborne seawater pump;

[0011] S5: Adjust the seawater flow rate;

[0012] Empty the air in the seawater pipe of the cargo evaporator; adjust the seawater outlet flow rate of the cargo evaporator;

[0013] S6: Control pressure evaporation replacement of carbon dioxide;

[0014] Receive liquefied carbon dioxide from the shore station, control the carbon dioxide outlet pressure of the cargo evaporator, and send the superheated gaseous carbon dioxide after evaporation into the carbon dioxide liquid cargo tank to displace the dry nitrogen therein;

[0015] S7: Transport carbon dioxide vapor to the shore station;

[0016] According to the inlet and outlet temperatures of carbon dioxide in the cargo evaporator, by controlling the carbon dioxide inlet pressure of the cargo evaporator, ensure that the carbon dioxide outlet of the cargo evaporator and the carbon dioxide liquid cargo tank maintain a certain pressure; at the same time, control the carbon dioxide outlet temperature of the cargo evaporator to ensure that the inlet and outlet temperatures of seawater in the cargo evaporator are above the water freezing point;

[0017] S8: Detect the carbon dioxide content;

[0018] Take samples of the gas in the carbon dioxide liquid cargo tank, confirm the carbon dioxide content in the gas, and stop the operation until the content meets the standard.

[0019] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S1, before replacing nitrogen in the carbon dioxide liquid cargo tank, confirm that the carbon dioxide liquid cargo tank has completed the drying operation: the liquid cargo tank is filled with dry nitrogen at a pressure of 0.5 barg; the liquid phase pipe and the degassing pipe are filled with dry nitrogen at a pressure of 6.5 - 7.0 barg; the dew point temperature of the dry nitrogen ≤ -40°C.

[0020] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S2, connect the pipelines, valves, flanges and other devices of the replacement system and set the initial positions of the valves.

[0021] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S3, confirm that the dry air generator is in the standby mode and connect it to the gas phase pipe through an air hose. Immediately inject compressed air into the system once the liquid cargo system loses pressure to maintain the system pressure stability and avoid the generation of dry ice due to sudden pressure drop.

[0022] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S4, start the on - ship seawater pump to supply heating seawater to the cargo evaporator.

[0023] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S5, open the seawater inlet valve of the cargo evaporator to the fully open position; open the air vent valve of the seawater pipe of the cargo evaporator to evacuate the air in the seawater pipe of the cargo evaporator; adjust the seawater outlet valve of the cargo evaporator to adjust the seawater flow rate at the outlet of the seawater pipe of the cargo evaporator to ensure sufficient heat supply for the evaporation of liquefied carbon dioxide.

[0024] According to the process of replacing nitrogen in the carbon dioxide liquid cargo tank described above, in step S6, keep the carbon dioxide inlet valve and the carbon dioxide outlet valve of the cargo evaporator closed, open the shore connection valve of the first liquid phase pipe and the shore connection valve of the second liquid phase pipe to receive liquefied carbon dioxide from the shore station; monitor the pressure gauge of the liquid phase pipe. When the pressure exceeds the triple point and reaches 5.5 - 6.0 barg, slowly open the carbon dioxide inlet valve of the cargo evaporator to cool down the cargo evaporator to prevent damage to the cargo evaporator caused by sudden temperature drop; when the temperature of the cargo evaporator drops to the same as the medium in the liquid phase pipe and the pressure gauge of the carbon dioxide outlet of the cargo evaporator shows a pressure greater than 5.5 barg, slowly open the carbon dioxide outlet valve of the cargo evaporator to ensure that the reading of the pressure gauge at the carbon dioxide outlet of the cargo evaporator is above 5.5 barg.

[0025] According to the process of replacing nitrogen in the carbon dioxide cargo tank described above, in step S7, observe the readings of the carbon dioxide inlet thermometer of the cargo evaporator and the carbon dioxide outlet thermometer of the cargo evaporator, and slowly adjust the carbon dioxide inlet valve of the cargo evaporator until the carbon dioxide vapor state at the outlet of the cargo evaporator reaches the superheated state; keep the third carbon dioxide liquid phase pipe valve closed, and open the fourth carbon dioxide liquid phase pipe valve and the fifth carbon dioxide liquid phase pipe valve; open the first carbon dioxide liquid phase pipe valve and the second carbon dioxide liquid phase pipe valve, and inject the carbon dioxide vapor into the carbon dioxide cargo tank through the bottom loading pipeline of the carbon dioxide cargo tank until the reading of the carbon dioxide outlet pressure gauge of the cargo evaporator reaches 15.5 barg to ensure that the temperature will not be lower than the design temperature of the carbon dioxide cargo tank when the carbon dioxide gas diffuses into the carbon dioxide cargo tank; keep the first carbon dioxide gas phase pipe valve and the second carbon dioxide gas phase pipe valve closed, open the first gas phase shore connection valve and the second gas phase shore connection valve, and slowly open the third carbon dioxide gas phase pipe valve and the fourth carbon dioxide gas phase pipe valve to send the carbon dioxide vapor to the shore station through the top gas removal pipeline of the carbon dioxide cargo tank; during the above process, gradually adjust the first carbon dioxide liquid phase pipe valve, the second carbon dioxide liquid phase pipe valve, the third carbon dioxide gas phase pipe valve, and the fourth carbon dioxide gas phase pipe valve so that the reading of the carbon dioxide outlet pressure gauge of the cargo evaporator always remains above 11 barg to ensure that the temperature will not be lower than the design temperature of the carbon dioxide cargo tank when the carbon dioxide gas diffuses into the carbon dioxide cargo tank, and the reading of the cargo tank pressure sensor remains between 0.5 and 1.0 barg, so that the gas in the carbon dioxide cargo tank is smoothly replaced by the carbon dioxide vapor; control the carbon dioxide outlet temperature of the cargo evaporator by adjusting the carbon dioxide inlet valve of the cargo evaporator, and ensure that the readings of the seawater inlet thermometer of the cargo evaporator and the seawater outlet thermometer of the cargo evaporator are above +3 °C to prevent the seawater from freezing and blocking the cargo evaporator.

[0026] According to the process of replacing nitrogen in the carbon dioxide cargo tank described above, in step S8, sample the gas in the cargo tank from 5 different positions of the carbon dioxide cargo tank respectively, and use a portable gas analyzer to confirm the carbon dioxide content in the gas until it reaches more than 95% and then stop the operation.

[0027] A system for implementing the process of replacing nitrogen in the carbon dioxide cargo tank described above includes: a dry air generator, a shore station, a cargo evaporator, and a carbon dioxide cargo tank; a bottom loading pipeline of the cargo tank and a top gas removal pipeline of the cargo tank are arranged in the carbon dioxide cargo tank;

[0028] The shore station is respectively connected to the bottom loading pipeline of the cargo tank and the carbon dioxide cargo tank; the dry air generator is connected to the top gas removal pipeline of the cargo tank;

[0029] Between the shore station and the loading pipeline at the bottom of the liquid cargo tank, the following are connected in sequence: the first liquid-phase pipe shore valve, the second liquid-phase pipe shore valve, the first carbon dioxide liquid-phase pipe valve,

[0030] the second carbon dioxide liquid-phase pipe valve, the fourth carbon dioxide liquid-phase pipe valve, the fifth carbon dioxide liquid-phase pipe valve; the outlet end of the second liquid-phase pipe shore valve is connected to the inlet end of the first carbon dioxide liquid-phase pipe valve with the inlet end of the third carbon dioxide liquid-phase pipe valve; the outlet end of the third carbon dioxide liquid-phase pipe valve is respectively connected to the inlet end of the fourth carbon dioxide liquid-phase pipe valve and the outlet end of the second carbon dioxide liquid-phase pipe valve;

[0031] Between the shore station and the carbon dioxide liquid cargo tank, the following are connected in sequence: the second gas-phase pipe shore valve, the first gas-phase pipe shore valve, the fourth carbon dioxide gas-phase pipe valve, the first carbon dioxide gas-phase pipe valve;

[0032] Between the drying air generator and the gas removal pipeline at the top of the liquid cargo tank, the following are connected in sequence: the gas removal pipe, the air hose, the second carbon dioxide gas-phase pipe valve, the third carbon dioxide gas-phase pipe valve; the outlet end of the second carbon dioxide gas-phase pipe valve is connected to the inlet end of the first carbon dioxide gas-phase pipe valve;

[0033] The shore station is connected to the inlet end of the cargo evaporator; between the shore station and the inlet end of the cargo evaporator, the following are connected in sequence: the first liquid-phase pipe shore valve, the second liquid-phase pipe shore valve, the liquid-phase pipe pressure gauge, the cargo evaporator carbon dioxide inlet valve, the cargo evaporator carbon dioxide inlet thermometer;

[0034] The outlet end of the cargo evaporator is connected to the inlet end of the first carbon dioxide liquid-phase pipe valve; between the outlet end of the cargo evaporator and the inlet end of the first carbon dioxide liquid-phase pipe valve, the following are connected in sequence: the cargo evaporator carbon dioxide outlet thermometer, the cargo evaporator carbon dioxide outlet pressure gauge, the cargo evaporator carbon dioxide outlet valve;

[0035] The seawater pipe outlet end of the cargo evaporator and the seawater pipe inlet end of the cargo evaporator are respectively connected to seawater; between the seawater pipe outlet end of the cargo evaporator and seawater, the following are connected in sequence: the cargo evaporator seawater pipe vent valve, the cargo evaporator seawater pipe flowmeter, the cargo evaporator seawater outlet thermometer, the cargo evaporator seawater outlet valve; between seawater and the seawater pipe inlet end of the cargo evaporator, the following are connected in sequence: the shipborne seawater pump, the cargo evaporator seawater inlet thermometer, the cargo evaporator seawater inlet valve;

[0036] A liquid cargo tank pressure sensor is provided on the carbon dioxide liquid cargo tank.

[0037] The beneficial effects of the present invention are described as follows:

[0038] 1. The present invention obtains the relationship between the carbon dioxide pressure and temperature at the outlet of the cargo evaporator and the carbon dioxide liquid cargo tank pressure through experiments, and formulates a comparison table of the relationship between the carbon dioxide pressure and temperature at the outlet of the cargo evaporator and the carbon dioxide liquid cargo tank pressure, which is used for the step of transporting carbon dioxide steam to the shore station in the process of replacing nitrogen in the carbon dioxide liquid cargo tank. By precisely controlling the carbon dioxide pressure and temperature at the outlet of the cargo evaporator, the carbon dioxide liquid cargo tank pressure is precisely controlled, enhancing the safety of the process of replacing nitrogen in the carbon dioxide liquid cargo tank.

[0039] 2. The present invention realizes the forward movement of the carbon dioxide phase control point from the carbon dioxide liquid cargo tank to the cargo evaporator. By controlling the carbon dioxide pressure and temperature at the outlet of the cargo evaporator, the carbon dioxide liquid cargo tank pressure is controlled, which can effectively solve the problem of control lag and avoid in advance the risk of generating dry ice during the operation process, which may cause harm to the carbon dioxide liquid cargo tank and the entire liquid cargo system, and has high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 : Process flow diagram of a process for replacing nitrogen in a carbon dioxide liquid cargo tank of the present invention.

[0041] Figure 2 : System for implementing a process for replacing nitrogen in a carbon dioxide liquid cargo tank of the present invention.

[0042] In the figure: 1 - Dry air generator, 2 - Cargo evaporator, 3 - Seawater inlet valve of the cargo evaporator, 4 - Seawater outlet valve of the cargo evaporator, 5 - Seawater pipe vent valve of the cargo evaporator, 6 - Seawater pipe flowmeter of the cargo evaporator, 7 - Carbon dioxide inlet valve of the cargo evaporator, 8 - Carbon dioxide outlet valve of the cargo evaporator, 9 - First liquid phase pipe shore connection valve, 10 - Second liquid phase pipe shore connection valve, 11 - Liquid phase pipe pressure gauge, 12 - Carbon dioxide outlet pressure gauge of the cargo evaporator, 13 - Carbon dioxide inlet thermometer of the cargo evaporator, 14 - Carbon dioxide outlet thermometer of the cargo evaporator, 15 - First carbon dioxide liquid phase pipe valve, 16 - Second carbon dioxide liquid phase pipe valve, 17 - Third carbon dioxide liquid phase pipe valve, 18 - Fourth carbon dioxide liquid phase pipe valve, 19 - Fifth carbon dioxide liquid phase pipe valve, 20 - Bottom loading pipeline of the liquid cargo tank, 21 - Carbon dioxide liquid cargo tank, 22 - First carbon dioxide gas phase pipe valve, 23 - Second carbon dioxide gas phase pipe valve, 24 - Third carbon dioxide gas phase pipe valve, 25 - Fourth carbon dioxide gas phase pipe valve, 26 - First gas phase pipe shore connection valve, 27 - Second gas phase pipe shore connection valve, 28 - Top degassing pipeline of the liquid cargo tank, 29 - Seawater inlet thermometer of the cargo evaporator, 30 - Seawater outlet thermometer of the cargo evaporator, 31 - Liquid cargo tank pressure sensor, 32 - Gas phase pipe, 33 - Liquid phase pipe, 34 - Degassing pipe, 35 - Seawater, 36 - Shipborne seawater pump, 37 - Air hose, 38 - Shore station. Detailed implementation manners

[0043] Preferred implementation manners

[0044] A detailed description will be given below for an embodiment.

[0045] As Figure 1-2 shown: A system for replacing nitrogen in a carbon dioxide liquid cargo tank includes: a dry air generator 1, a shore station 38, a cargo evaporator 2, and a carbon dioxide liquid cargo tank 21; a liquid cargo tank bottom loading pipeline 20 and a liquid cargo tank top gas removal pipeline 28 are arranged in the carbon dioxide liquid cargo tank 21;

[0046] The shore station 38 is respectively connected to the liquid cargo tank bottom loading pipeline 20 and the carbon dioxide liquid cargo tank 21; the dry air generator 1 is connected to the liquid cargo tank top gas removal pipeline 28;

[0047] Between the shore station 38 and the liquid cargo tank bottom loading pipeline 20, the following are sequentially connected: a first liquid-phase pipe shore valve 9, a second liquid-phase pipe shore valve 10, a first carbon dioxide liquid-phase pipe valve 15,

[0048] a second carbon dioxide liquid-phase pipe valve 16, a fourth carbon dioxide liquid-phase pipe valve 18, and a fifth carbon dioxide liquid-phase pipe valve 19; the outlet end of the second liquid-phase pipe shore valve 10 is connected to the inlet end of the first carbon dioxide liquid-phase pipe valve 15 through the inlet end of a third carbon dioxide liquid-phase pipe valve 17; the outlet end of the third carbon dioxide liquid-phase pipe valve 17 is respectively connected to the inlet end of the fourth carbon dioxide liquid-phase pipe valve 18 and the outlet end of the second carbon dioxide liquid-phase pipe valve 16;

[0049] Between the shore station 38 and the carbon dioxide liquid cargo tank 21, the following are sequentially connected: a second gas-phase pipe shore valve 27, a first gas-phase pipe shore valve 26, a fourth carbon dioxide gas-phase pipe valve 25, and a first carbon dioxide gas-phase pipe valve 22;

[0050] Between the dry air generator 1 and the liquid cargo tank top gas removal pipeline 28, the following are sequentially connected: a gas removal pipe 34, an air hose 37, a second carbon dioxide gas-phase pipe valve 23, and a third carbon dioxide gas-phase pipe valve 24; the outlet end of the second carbon dioxide gas-phase pipe valve 23 is connected to the inlet end of the first carbon dioxide gas-phase pipe valve 22;

[0051] The shore station 38 is connected to the inlet end of the cargo evaporator 2; between the shore station 38 and the inlet end of the cargo evaporator 2, the following are sequentially connected: a first liquid-phase pipe shore valve 9, a second liquid-phase pipe shore valve 10, a liquid-phase pipe pressure gauge 11, a cargo evaporator carbon dioxide inlet valve 7, and a cargo evaporator carbon dioxide inlet thermometer 13;

[0052] The outlet end of the cargo evaporator 2 is connected to the inlet end of the first carbon dioxide liquid phase pipe valve 15; between the outlet end of the cargo evaporator 2 and the inlet end of the first carbon dioxide liquid phase pipe valve 15, the following are successively connected: a cargo evaporator carbon dioxide outlet thermometer 14, a cargo evaporator carbon dioxide outlet pressure gauge 12, and a cargo evaporator carbon dioxide outlet valve 8;

[0053] The seawater pipe outlet end and the seawater pipe inlet end of the cargo evaporator 2 are respectively connected to the seawater 35; between the seawater pipe outlet end of the cargo evaporator 2 and the seawater 35, the following are successively connected: a cargo evaporator seawater pipe vent valve 5, a cargo evaporator seawater pipe flowmeter 6, a cargo evaporator seawater outlet thermometer 30, and a cargo evaporator seawater outlet valve 4; between the seawater 35 and the seawater pipe inlet end of the cargo evaporator 2, the following are successively connected: a shipborne seawater pump 36, a cargo evaporator seawater inlet thermometer 29, and a cargo evaporator seawater inlet valve 3;

[0054] A liquid cargo tank pressure sensor 31 is provided on the carbon dioxide liquid cargo tank 21.

[0055] A process for replacing nitrogen in a carbon dioxide liquid cargo tank is carried out according to the following steps:

[0056] Step 1, before replacing nitrogen in the carbon dioxide liquid cargo tank, confirm that the carbon dioxide liquid cargo tank 21 has completed the drying operation: the carbon dioxide liquid cargo tank 21 is filled with dry nitrogen at a pressure of 0.5 barg; the liquid phase pipe and the degassing pipe are filled with dry nitrogen at a pressure of 6.5 - 7.0 barg; the dew point temperature of the dry nitrogen is ≤ -40°C.

[0057] Step 2, connect the pipelines, valves, flanges and other devices of the replacement system and set the initial positions of the valves;

[0058] Confirm that the short pipes and hoses connecting the nitrogen system in the replacement system have been removed, and the unused pipeline interfaces are sealed with blind flanges; confirm that the pipelines of the replacement system are correctly connected between the manifold area and the shore station, and the positions of all valves and spectacle flange devices are in the initial state; the spectacle flange is used to connect pipelines, and it is generally composed of 1 hollow flange and 1 solid flange connected together, playing a role similar to that of a valve: when it is necessary to connect the pipelines, the hollow flange can be connected to the pipeline and the medium can flow through; when it is necessary to disconnect the pipeline, the solid flange can be connected to the pipeline and the medium cannot pass through.

[0059] Step 3, confirm that the dry air generator 1 is in the standby mode and connect it to the gas phase pipe through the air hose 37. Once the liquid cargo system loses pressure, immediately inject compressed air into the system to maintain the system pressure stability and avoid the generation of dry ice due to a sudden pressure drop.

[0060] Step 4, start the shipborne seawater pump 36 to supply heating seawater to the cargo evaporator 2.

[0061] Step 5: Open the seawater inlet valve 3 of the cargo evaporator to the fully open position; open the air vent valve 5 of the seawater pipe of the cargo evaporator to evacuate the air in the seawater pipe of the cargo evaporator; adjust the seawater outlet valve 4 of the cargo evaporator to adjust the seawater flow rate at the outlet of the seawater pipe of the cargo evaporator until the seawater flowmeter 6 of the seawater pipe of the cargo evaporator shows that the seawater flow rate reaches 800 m 3 / h to ensure sufficient heat supply for the evaporation of liquefied carbon dioxide.

[0062] Table 1 Comparison Table of the Relationship between the Carbon Dioxide Pressure and Temperature at the Outlet of the Cargo Evaporator and the Pressure of the Carbon Dioxide Liquid Cargo Tank

[0063]

[0064] Step 6: Keep the carbon dioxide inlet valve 7 and the carbon dioxide outlet valve 8 of the cargo evaporator closed, open the shore connection valve 9 of the first liquid phase pipe and the shore connection valve 10 of the second liquid phase pipe to receive liquefied carbon dioxide from the shore station 38; monitor the liquid phase pipe pressure gauge 11. When the pressure exceeds the triple point and reaches 5.5 - 6.0 barg, slowly open the carbon dioxide inlet valve 7 of the cargo evaporator to cool down the cargo evaporator 2 to prevent damage to the cargo evaporator 2 caused by a sudden temperature drop; when the temperature of the cargo evaporator 2 drops to the same as the medium in the liquid phase pipe and the carbon dioxide outlet pressure gauge 12 of the cargo evaporator shows that the pressure is greater than 5.5 barg, slowly open the carbon dioxide outlet valve 8 of the cargo evaporator to ensure that the reading of the carbon dioxide outlet pressure gauge 12 of the cargo evaporator is above 5.5 barg. To prevent dry ice from being generated due to the pressure of liquefied carbon dioxide dropping below the triple point during vaporization. Observe the readings of the carbon dioxide inlet thermometer 13 and the carbon dioxide outlet thermometer 14 of the cargo evaporator, and slowly adjust the carbon dioxide inlet valve 7 of the cargo evaporator until the carbon dioxide vapor state at the outlet of the cargo evaporator 2 reaches the superheated state - the carbon dioxide vapor temperature is higher than the saturated vapor temperature under the same pressure conditions. Through experiments, the comparison table of the relationship between the carbon dioxide pressure and temperature at the outlet of the cargo evaporator and the pressure of the carbon dioxide liquid cargo tank in Table 1 is obtained. For example: According to Table 1, the carbon dioxide vapor state at the outlet of the cargo evaporator 2 is 15 barg, -9 °C, which is 17 °C higher than the saturated state temperature; keep the third carbon dioxide liquid phase pipe valve 17 closed, and open the fourth carbon dioxide liquid phase pipe valve 18 and the fifth carbon dioxide liquid phase pipe valve 19; slowly open the first carbon dioxide liquid phase pipe valve 15 and the second carbon dioxide liquid phase pipe valve 16, and inject the carbon dioxide vapor into the carbon dioxide liquid cargo tank 21 through the loading pipeline 20 at the bottom of the liquid cargo tank until the reading of the carbon dioxide outlet pressure gauge 12 of the cargo evaporator reaches 15.5 barg to ensure that the temperature will not be lower than the design temperature of the carbon dioxide liquid cargo tank 21 when the carbon dioxide gas diffuses into the carbon dioxide liquid cargo tank 21.

[0065] Step 7: Keep the first carbon dioxide gas-phase pipe valve 22 and the second carbon dioxide gas-phase pipe valve 23 closed, open the first gas-phase pipe onshore valve 26 and the second gas-phase pipe onshore valve 27, and slowly open the third carbon dioxide gas-phase pipe valve 24 and the fourth carbon dioxide gas-phase pipe valve 25 to send the carbon dioxide vapor to the onshore station through the degassing pipeline 28 at the top of the cargo tank; during the above process, gradually adjust the first carbon dioxide liquid-phase pipe valve 15, the second carbon dioxide liquid-phase pipe valve 16, the third carbon dioxide gas-phase pipe valve 24, and the fourth carbon dioxide gas-phase pipe valve 25 so that the reading of the carbon dioxide outlet pressure gauge 12 of the cargo evaporator always remains above 11 barg to ensure that the temperature of the carbon dioxide gas diffusing into the carbon dioxide cargo tank 21 will not be lower than the design temperature of the carbon dioxide cargo tank 21, and the reading of the cargo tank pressure sensor 31 remains between 0.5 and 1.0 barg, so that the gas in the carbon dioxide cargo tank 21 is smoothly displaced by the carbon dioxide vapor; control the carbon dioxide outlet temperature of the cargo evaporator 2 by adjusting the carbon dioxide inlet valve 7 of the cargo evaporator, and ensure that the readings of the seawater inlet thermometer 29 and the seawater outlet thermometer 30 of the cargo evaporator are above +3 °C to prevent seawater freezing from causing blockage of the cargo evaporator.

[0066] Step 8: Take gas samples from 5 different positions in the carbon dioxide cargo tank 21 respectively, and use a portable gas analyzer to confirm the carbon dioxide content in the gas. Stop the operation until it reaches more than 95%. Thus, the operation of replacing nitrogen in the carbon dioxide cargo tank with superheated gaseous carbon dioxide ends.

[0067] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered within the protection scope of the present invention.

Claims

1. A system for replacing nitrogen in a carbon dioxide liquid cargo tank, characterized in that: include: A dry air generator (1), a shore station (38), a cargo evaporator (2) and a carbon dioxide liquid cargo tank (21); the carbon dioxide liquid cargo tank (21) is provided with a loading pipeline (20) at the bottom of the liquid cargo tank and a degassing pipeline (28) at the top of the liquid cargo tank; The shore station (38) is respectively connected to the loading pipeline (20) at the bottom of the liquid cargo tank and the carbon dioxide liquid cargo tank (21); the dry air generator (1) is connected to the degassing pipeline (28) at the top of the liquid cargo tank; The shore station (38) and the loading pipeline (20) at the bottom of the liquid cargo tank are connected in sequence with: a first liquid phase pipe to shore valve (9), a second liquid phase pipe to shore valve (10), a first carbon dioxide liquid phase pipe valve (15), a second carbon dioxide liquid phase pipe valve (16), a fourth carbon dioxide liquid phase pipe valve (18), and a fifth carbon dioxide liquid phase pipe valve (19); the outlet end of the second liquid phase pipe to shore valve (10) and the inlet end of the first carbon dioxide liquid phase pipe valve (15) are connected to the inlet end of the third carbon dioxide liquid phase pipe valve (17); the outlet end of the third carbon dioxide liquid phase pipe valve (17) is connected to the inlet end of the fourth carbon dioxide liquid phase pipe valve (18) and the outlet end of the second carbon dioxide liquid phase pipe valve (16) respectively; The shore station (38) and the carbon dioxide liquid cargo tank (21) are connected in sequence with: a second gas phase pipe shore valve (27), a first gas phase pipe shore valve (26), a fourth carbon dioxide gas phase pipe valve (25), and a first carbon dioxide gas phase pipe valve (22); The dry air generator (1) and the degassing pipeline (28) at the top of the cargo tank are connected in sequence: a degassing pipe (34), an air hose (37), a second carbon dioxide gas phase pipe valve (23), and a third carbon dioxide gas phase pipe valve (24); the outlet end of the second carbon dioxide gas phase pipe valve (23) is connected to the inlet end of the first carbon dioxide gas phase pipe valve (22); The shore station (38) is connected to the inlet end of the cargo evaporator (2); the shore station (38) and the inlet end of the cargo evaporator (2) are connected in sequence with: a first liquid phase pipe shore valve (9), a second liquid phase pipe shore valve (10), a liquid phase pipe pressure gauge (11), a cargo evaporator carbon dioxide inlet valve (7), and a cargo evaporator carbon dioxide inlet thermometer (13); The outlet end of the cargo evaporator (2) is connected to the inlet end of the first carbon dioxide liquid phase pipe valve (15); the cargo evaporator carbon dioxide outlet thermometer (14), the cargo evaporator carbon dioxide outlet pressure gauge (12), and the cargo evaporator carbon dioxide outlet valve (8) are sequentially connected between the outlet end of the cargo evaporator (2) and the inlet end of the first carbon dioxide liquid phase pipe valve (15); The seawater pipe outlet end of the cargo evaporator (2) and the seawater pipe inlet end of the cargo evaporator (2) are respectively connected to the seawater (35); the cargo evaporator seawater pipe vent valve (5), the cargo evaporator seawater pipe flow meter (6), the cargo evaporator seawater outlet thermometer (30), and the cargo evaporator seawater outlet valve (4) are connected in sequence between the seawater (35) and the seawater pipe inlet end of the cargo evaporator (2); the shipboard seawater pump (36), the cargo evaporator seawater inlet thermometer (29), and the cargo evaporator seawater inlet valve (3) are connected in sequence between the seawater (35) and the cargo evaporator (2) seawater pipe inlet end; and a cargo tank pressure sensor (31) is provided on the carbon dioxide cargo tank.

2. A process for replacing nitrogen in a carbon dioxide liquid cargo tank by a system for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 1, characterized in that: The following steps are involved: S1: Dry carbon dioxide cargo tank; S2: Connecting the CO2 cargo tank pipeline and equipment; S3: dry air generator standby; S4: Start the onboard seawater pump; S5: Regulate seawater flow; Empty the air in the cargo evaporator seawater pipe; adjust the cargo evaporator seawater outlet flow; S6: carbon dioxide pressure controlled evaporation replacement; Receive liquefied carbon dioxide from the shore station, control the carbon dioxide outlet pressure of the cargo evaporator, and send the evaporated superheated gaseous carbon dioxide into the carbon dioxide liquid cargo tank to replace the dry nitrogen therein; S7: transporting carbon dioxide vapor to the shore station; According to the inlet and outlet temperatures of the cargo evaporator CO2, the inlet pressure of the cargo evaporator CO2 is controlled so that the outlet of the cargo evaporator CO2 and the CO2 tank are kept at a certain pressure; at the same time, the outlet temperature of the cargo evaporator CO2 is controlled to ensure that the inlet and outlet temperatures of the cargo evaporator seawater are above the freezing point of water; S8: Detect carbon dioxide content; The gas in the carbon dioxide cargo tank is sampled to confirm the carbon dioxide content in the gas, and the operation is stopped until the content reaches the standard; in step S1, before the nitrogen replacement in the carbon dioxide cargo tank is performed, it is confirmed that the carbon dioxide cargo tank has completed the drying operation: The carbon dioxide cargo tank is filled with dry nitrogen at a pressure of 0.5 barg; the liquid phase pipe and the degassing pipe are filled with dry nitrogen at a pressure of 6.5~7.0 barg; the dew point temperature of the dry nitrogen is ≤-40°C; in step S2, the pipelines, valves and flanges of the system for replacing the nitrogen in the carbon dioxide cargo tank are connected and the valve starting position is set; in step S3, it is confirmed that the dry air generator (1) is in standby mode and is connected to the gas phase pipe through an air hose (37). Once the system for replacing the nitrogen in the carbon dioxide cargo tank loses pressure, compressed air is immediately injected into the system for replacing the nitrogen in the carbon dioxide cargo tank to maintain the system pressure of the nitrogen in the carbon dioxide cargo tank stable to avoid a sudden drop in pressure and the generation of dry ice; in step S4, the shipboard seawater pump (36) is started to provide seawater for heating to the cargo evaporator (2); in step S5, the cargo evaporator seawater inlet valve (3) of the cargo evaporator (2) is opened to the fully open position; the cargo evaporator seawater pipe vent valve (5) is opened to exhaust the air in the cargo evaporator seawater pipe; The cargo evaporator seawater outlet valve (4) is adjusted to adjust the seawater flow rate at the cargo evaporator seawater pipe outlet to ensure that sufficient heat is provided for evaporation of liquefied carbon dioxide. In step S6, the cargo evaporator carbon dioxide inlet valve (7) and the cargo evaporator carbon dioxide outlet valve (8) are kept closed, and the first liquid phase pipe shore valve (9) and the second liquid phase pipe shore valve (10) are opened to receive liquefied carbon dioxide from the shore station (38). The liquid phase pipe pressure gauge (11) is monitored, and when the pressure exceeds the triple point and reaches 5.5-6.0 barg, the cargo evaporator carbon dioxide inlet valve (7) is slowly opened to cool the cargo evaporator (2) to prevent the sudden drop in temperature from causing damage to the cargo evaporator (2); when the temperature of the cargo evaporator (2) drops to the same as that of the medium in the liquid phase pipe, and the cargo evaporator carbon dioxide outlet pressure gauge (12) shows a pressure greater than 5.5 barg, the cargo evaporator carbon dioxide outlet valve (8) is slowly opened to ensure that the reading of the cargo evaporator carbon dioxide outlet pressure gauge (12) is above 5.5 barg.

3. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 2, characterized in that: In step S7, the readings of the cargo evaporator carbon dioxide inlet thermometer (13) and the cargo evaporator carbon dioxide outlet thermometer (14) are observed, and the cargo evaporator carbon dioxide inlet valve (7) is slowly adjusted until the carbon dioxide vapor at the cargo evaporator (2) outlet reaches a superheated state; the third carbon dioxide liquid phase pipe valve (17) is kept closed, and the fourth carbon dioxide liquid phase pipe valve (18) and the fifth carbon dioxide liquid phase pipe valve (19) are opened; the first carbon dioxide liquid phase pipe valve (15) and the second carbon dioxide liquid phase pipe valve (16) are opened, and the carbon dioxide vapor is injected into the carbon dioxide liquid cargo tank (21) through the loading pipeline (20) at the bottom of the liquid cargo tank until the reading of the cargo evaporator carbon dioxide outlet pressure gauge (12) reaches 15.5 barg, so as to ensure that the temperature of the carbon dioxide gas when it diffuses into the carbon dioxide liquid cargo tank (21) will not be lower than the design temperature of the carbon dioxide liquid cargo tank (21); ... The carbon dioxide gas phase pipe valve (22) and the second carbon dioxide gas phase pipe valve (23) are closed, the first gas phase pipe shore valve (26) and the second gas phase pipe shore valve (27) are opened, and the third carbon dioxide gas phase pipe valve (24) and the fourth carbon dioxide gas phase pipe valve (25) are slowly opened to send carbon dioxide vapor to the shore station through the degassing pipeline (28) on the top of the liquid cargo tank. During the above process, the first carbon dioxide liquid phase pipe valve (15), the second carbon dioxide liquid phase pipe valve (16), the third carbon dioxide gas phase pipe valve (24) and the fourth carbon dioxide gas phase pipe valve (25) should be gradually adjusted so that the reading of the carbon dioxide outlet pressure gauge (12) of the cargo evaporator is always maintained above 11 barg, so as to ensure that the temperature of the carbon dioxide gas will not be lower than the design temperature of the carbon dioxide liquid cargo tank (21) when it diffuses into the carbon dioxide liquid cargo tank (21), and the reading of the cargo tank pressure sensor (31) is maintained at 0.5~1.0 barg. , so that the gas in the carbon dioxide liquid cargo tank (21) is smoothly replaced by carbon dioxide vapor; the carbon dioxide outlet temperature of the cargo evaporator (2) is controlled by adjusting the cargo evaporator carbon dioxide inlet valve (7), and it is ensured that the cargo evaporator seawater inlet thermometer (29) and the cargo evaporator seawater outlet thermometer (30) indicate above +3°C to prevent the cargo evaporator from being blocked by seawater freezing.

4. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 3, characterized in that: In step S8, samples are taken from different positions of the carbon dioxide liquid cargo tank (21) to confirm the carbon dioxide content in the gas using a portable gas analyzer. The operation is stopped when the carbon dioxide content reaches 95% or more.

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