Process and system for replacing nitrogen in carbon dioxide liquid cargo tank

Abstract

The invention belongs to the field of liquefied gas ships, and particularly relates to a process and system for replacing nitrogen in a carbon dioxide liquid cargo tank. Comprising the following steps: drying the liquid cargo tank; liquid cargo tank pipelines and devices are connected; the dry air generator is standby; starting a shipborne sea water pump; adjusting the seawater flow; performing carbon dioxide pressure-controlled evaporation replacement; conveying carbon dioxide steam to a shore station; the equipment comprises a dry air generator, a shore station, a cargo evaporator and a liquid cargo tank, a liquid cargo tank bottom loading pipeline and a liquid cargo tank top degassing pipeline are arranged in the liquid cargo tank. According to the process and system for replacing the nitrogen in the liquid cargo tank with the overheated gaseous carbon dioxide, the carbon dioxide phase state control point is moved forwards from the liquid cargo tank to the cargo evaporator, the risk that dry ice is generated in the operation process, and consequently the liquid cargo tank and the whole liquid cargo system are damaged is effectively avoided, and high safety and practicability are achieved.

Description

Technical Field

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

[0002] The replacement of nitrogen in the carbon dioxide liquid cargo tank is one of the important links in the operation of the liquid cargo system. Its purpose is to use nitrogen to dry the carbon dioxide liquid cargo tank, and then use carbon dioxide vapor to discharge the nitrogen in the liquid cargo tank to facilitate the subsequent loading of carbon dioxide liquid cargo.

[0003] Common liquefied gases, such as LNGC, are usually stored and transported in a fully refrigerated (low temperature and normal pressure) environment. During operation, there are only two media, gaseous and liquid, in the liquid cargo system. Therefore, when replacing nitrogen in a carbon dioxide liquid cargo tank, it is only necessary to consider the pressure and temperature tolerance of the carbon dioxide liquid cargo tank structure, without considering the effects of pressure and temperature on the medium itself.

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

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

[0006] The present invention is to solve the problems of the prior art process and system for replacing nitrogen in a carbon dioxide liquid cargo tank, in which the carbon dioxide phase control point is in the carbon dioxide liquid cargo tank. Due to the hysteresis of the control and the low control accuracy, it is impossible to ensure that the temperature and pressure of the carbon dioxide are above the triple point, so dry ice will be formed, endangering the safety of the liquid cargo system, etc. A process for replacing nitrogen in a carbon dioxide liquid cargo tank is provided, comprising the following steps: 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; Take samples of the gas in the carbon dioxide liquid cargo tank to confirm the carbon dioxide content in the gas, and stop the operation until the content reaches the standard.

[0007] According to the above-mentioned process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S1, before replacing nitrogen in the carbon dioxide liquid cargo tank, it is confirmed 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 is ≤-40℃.

[0008] According to the above-described process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S2, the pipes, valves, flanges and other devices of the replacement system are connected and the valve starting position is set.

[0009] According to the above-mentioned process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S3, it is confirmed that the dry air generator is in standby mode and is connected to the gas phase pipe through an air hose. Once the liquid cargo system loses pressure, compressed air is immediately injected into the system to maintain the system pressure stable and avoid dry ice generated by a sudden pressure drop.

[0010] According to the above-mentioned process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S4, the shipboard seawater pump is started to provide seawater for heating to the cargo evaporator.

[0011] According to the above-described process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S5, the cargo evaporator seawater inlet valve is opened to the fully open position; the cargo evaporator seawater pipe vent valve is opened to exhaust the air in the cargo evaporator seawater pipe; the cargo evaporator seawater outlet valve is adjusted to adjust the seawater flow rate at the cargo evaporator seawater pipe outlet to ensure that sufficient heat is provided for the evaporation of liquefied carbon dioxide.

[0012] According to the above-mentioned process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S6, the carbon dioxide inlet valve and the carbon dioxide outlet valve of the cargo evaporator are kept closed, and the first liquid phase pipe shore valve and the second liquid phase pipe shore valve are opened to receive liquefied carbon dioxide from the shore station; the liquid phase pipe pressure gauge is monitored, and when the pressure exceeds the triple point and reaches 5.5~6.0barg, the carbon dioxide inlet valve of the cargo evaporator is slowly opened to cool the cargo evaporator to prevent the sudden drop in temperature from causing damage to the cargo evaporator; when the temperature of the cargo evaporator drops to the same as that of the medium in the liquid phase pipe, and the carbon dioxide outlet pressure gauge of the cargo evaporator shows a pressure greater than 5.5barg, the carbon dioxide outlet valve of the cargo evaporator is slowly opened to ensure that the reading of the carbon dioxide outlet pressure gauge of the cargo evaporator is above 5.5barg.

[0013] According to the above-mentioned process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S7, the readings of the cargo evaporator carbon dioxide inlet thermometer and the cargo evaporator carbon dioxide outlet thermometer are observed, and the cargo evaporator carbon dioxide inlet valve is slowly adjusted until the carbon dioxide vapor state at the cargo evaporator outlet reaches a superheated state; the third carbon dioxide liquid phase pipe valve is kept closed, and the fourth carbon dioxide liquid phase pipe valve and the fifth carbon dioxide liquid phase pipe valve are opened; the first carbon dioxide liquid phase pipe valve and the second carbon dioxide liquid phase pipe valve are opened, and the carbon dioxide vapor is injected into the carbon dioxide liquid cargo tank through the loading pipeline at the bottom of the carbon dioxide liquid cargo tank until the cargo evaporator carbon dioxide outlet pressure reading reaches 15.5 barg, so as to ensure that the temperature of the carbon dioxide gas will not be lower than the carbon dioxide liquid when it diffuses into the carbon dioxide liquid cargo tank. The design temperature of the cargo hold; keep the first CO2 gas phase pipe valve and the second CO2 gas phase pipe valve closed, the first gas phase pipe shore valve and the second gas phase pipe shore valve open, slowly open the third CO2 gas phase pipe valve and the fourth CO2 gas phase pipe valve, and send the CO2 vapor to the shore station through the degassing pipeline on the top of the CO2 liquid cargo tank; during the above process, the first CO2 liquid phase pipe valve, the second CO2 liquid phase pipe valve, the third CO2 gas phase pipe valve and the fourth CO2 gas phase pipe valve should be gradually adjusted to ensure that the CO2 outlet pressure reading of the cargo evaporator is always kept above 11 barg to ensure that the temperature of the CO2 gas will not be lower than the design temperature of the CO2 liquid cargo tank when it diffuses into the CO2 liquid cargo tank, and the reading of the liquid cargo tank pressure sensor is kept at 0.5~1.0 barg, so that the gas in the CO2 liquid cargo tank is smoothly replaced by the CO2 vapor; the CO2 outlet temperature of the cargo evaporator is controlled by adjusting the CO2 inlet valve of the cargo evaporator, and ensure that the readings of the cargo evaporator seawater inlet thermometer and the cargo evaporator seawater outlet thermometer are above +3℃ to prevent the cargo evaporator from being blocked by seawater freezing.

[0014] According to the above-described process for replacing nitrogen in a carbon dioxide liquid cargo tank, in step S8, samples are taken from five different positions of the carbon dioxide liquid cargo tank, and a portable gas analyzer is used to confirm the carbon dioxide content in the gas, and the operation is stopped when it reaches more than 95%.

[0015] A system for realizing the above-mentioned process of replacing nitrogen in a carbon dioxide liquid cargo tank comprises: a dry air generator, a shore station, a cargo evaporator and a carbon dioxide liquid cargo tank; the carbon dioxide liquid cargo tank is provided with a loading pipeline at the bottom of the liquid cargo tank and a degassing pipeline at the top of the liquid cargo tank; The shore station is connected to the loading pipeline at the bottom of the liquid cargo tank and the carbon dioxide liquid cargo tank respectively; the dry air generator is connected to the degassing pipeline at the top of the liquid cargo tank; The shore station and the loading pipeline at the bottom of the liquid cargo tank are connected in sequence: the first liquid phase pipe to shore valve, the second liquid phase pipe to shore valve, the first carbon dioxide liquid phase pipe valve, The second carbon dioxide liquid phase pipe valve, the fourth carbon dioxide liquid phase pipe valve, and the fifth carbon dioxide liquid phase pipe valve; the outlet end of the second liquid phase pipe valve and the inlet end of the first carbon dioxide liquid phase pipe valve are connected to 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; The shore station and the carbon dioxide liquid cargo tank 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, and the first carbon dioxide gas phase pipe valve; The dry air generator and the degassing pipeline on the top of the liquid cargo tank are connected in sequence: a degassing pipe, an air hose, a second carbon dioxide gas phase pipe valve, and a 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; The shore station is connected to the inlet end of the cargo evaporator; the shore station and the inlet end of the cargo evaporator are connected in sequence with: a first liquid phase pipe shore valve, a second liquid phase pipe shore valve, a liquid phase pipe pressure gauge, a cargo evaporator carbon dioxide inlet valve, and a cargo evaporator carbon dioxide inlet thermometer; The outlet end of the cargo evaporator is connected to the inlet end of the first carbon dioxide liquid phase pipe valve; the outlet end of the cargo evaporator and the inlet end of the first carbon dioxide liquid phase pipe valve are connected in sequence: a cargo evaporator carbon dioxide outlet thermometer, a cargo evaporator carbon dioxide outlet pressure gauge, and a cargo evaporator carbon dioxide outlet valve; The seawater pipe outlet end of the cargo evaporator and the seawater pipe inlet end of the cargo evaporator are respectively connected to seawater; the seawater pipe outlet end of the cargo evaporator and the seawater are connected in sequence: the cargo evaporator seawater pipe vent valve, the cargo evaporator seawater pipe flow meter, the cargo evaporator seawater outlet thermometer, and the cargo evaporator seawater outlet valve; the shipboard seawater pump, the cargo evaporator seawater inlet thermometer, and the cargo evaporator seawater inlet valve are connected in sequence between the seawater and the seawater pipe inlet end of the cargo evaporator; A cargo tank pressure sensor is provided on the carbon dioxide cargo tank.

[0016] The beneficial effects of the present invention are as follows: 1. The present invention obtains the relationship between the pressure temperature of carbon dioxide at the cargo evaporator outlet and the pressure of the carbon dioxide liquid cargo tank through experiments, and formulates a comparison table of the relationship between the pressure temperature of carbon dioxide at the cargo evaporator outlet and the pressure of the carbon dioxide liquid cargo tank, which is used for the step of conveying carbon dioxide vapor to the shore station in the process of replacing nitrogen in the carbon dioxide liquid cargo tank. By accurately controlling the pressure temperature of carbon dioxide at the cargo evaporator outlet, the pressure of the carbon dioxide liquid cargo tank is accurately controlled, thereby enhancing the safety of the process of replacing nitrogen in the carbon dioxide liquid cargo tank.

[0017] 2. The present invention realizes moving the carbon dioxide phase control point from the carbon dioxide liquid cargo tank to the cargo evaporator, and controls the carbon dioxide pressure temperature at the cargo evaporator outlet, thereby controlling the carbon dioxide liquid cargo tank pressure, which can effectively solve the problem of control lag, avoid the risk of dry ice generation during operation and thus causing harm to the carbon dioxide liquid cargo tank and the entire liquid cargo system, and has high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : A process flow chart of replacing nitrogen in a carbon dioxide liquid cargo tank according to the present invention.

[0019] Figure 2 : A system of the present invention realizes the process of replacing nitrogen in a carbon dioxide liquid cargo tank.

[0020] In the figure: 1-dry air generator, 2-cargo evaporator, 3-seawater inlet valve of cargo evaporator, 4-seawater outlet valve of cargo evaporator, 5-vent valve of seawater pipe of cargo evaporator, 6-seawater pipe flow meter of cargo evaporator, 7-carbon dioxide inlet valve of cargo evaporator, 8-carbon dioxide outlet valve of cargo evaporator, 9-shore valve of first liquid phase pipe, 10-shore valve of second liquid phase pipe, 11-pressure gauge of liquid phase pipe, 12-pressure gauge of carbon dioxide outlet of cargo evaporator, 13-carbon dioxide inlet thermometer of cargo evaporator, 14-carbon dioxide outlet thermometer of 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 liquid cargo tank, 21-carbon dioxide liquid cargo tank, 22-the first carbon dioxide gas phase pipe valve, 23-the second carbon dioxide gas phase pipe valve, 24-the third carbon dioxide gas phase pipe valve, 25-the fourth carbon dioxide gas phase pipe valve, 26-the first gas phase pipe shore valve, 27-the second gas phase pipe shore valve, 28-the degassing pipeline on the top of the liquid cargo tank, 29-the seawater inlet thermometer of the cargo evaporator, 30-the seawater outlet thermometer of the cargo evaporator, 31-the liquid cargo tank pressure sensor, 32-the gas phase pipe, 33-the liquid phase pipe, 34-the degassing pipe, 35-the seawater, 36-the shipboard seawater pump, 37-the air hose, 38-the shore station. DETAILED DESCRIPTION

[0021] Preferred Embodiments A possible embodiment is described in detail below.

[0022] like Figure 1-2 As shown: a system for replacing nitrogen in a carbon dioxide liquid cargo tank, comprising: 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 connected to the loading pipeline 20 at the bottom of the liquid cargo tank and the carbon dioxide liquid cargo tank 21 respectively; 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: 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, The second carbon dioxide liquid phase pipe valve 16, the fourth carbon dioxide liquid phase pipe valve 18, and the fifth carbon dioxide liquid phase pipe valve 19; the outlet end of the second liquid phase pipe 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 to shore valve 27, a first gas phase pipe to 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: 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 outlet end of the cargo evaporator 2 and the inlet end of the first carbon dioxide liquid phase pipe valve 15 are connected in sequence: 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; 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 seawater pipe outlet end of the cargo evaporator 2 and the seawater 35 are connected in sequence: 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; 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 seawater pipe inlet end of the cargo evaporator 2; The carbon dioxide liquid cargo tank 21 is provided with a liquid cargo tank pressure sensor 31 .

[0023] A process for replacing nitrogen in a carbon dioxide liquid cargo tank is carried out according to the following steps: Step 1: Before replacing the nitrogen in the carbon dioxide cargo tank, confirm that the carbon dioxide cargo tank 21 has completed the drying operation: the carbon dioxide 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℃.

[0024] Step 2: Connect the pipes, valves, flanges and other devices of the replacement system and set the valve starting position; Confirm that the short pipes and hoses connecting the nitrogen system in the replacement system have been removed, and the unused pipe interfaces are sealed with blind flanges; confirm that the pipelines of the replacement system, including the connection between the manifold area and the shore station, are correctly connected, and the positions of all valves and spectacles flange devices are in the initial state; the spectacles flange is used to connect the pipeline, it is generally composed of a hollow flange and a solid flange connected together, and plays a role similar to a valve: when the pipeline needs to be connected, the hollow flange can be connected to the pipeline, and the medium can flow through; when the pipeline needs to be disconnected, the solid flange can be connected to the pipeline, and the medium cannot pass through.

[0025] Step 3: confirm that the dry air generator 1 is in standby mode and is connected 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 stable and avoid dry ice generation due to a sudden drop in pressure.

[0026] Step 4: Start the onboard seawater pump 36 to provide the cargo evaporator 2 with seawater for heating.

[0027] Step 5: Open the cargo evaporator seawater inlet valve 3 to the fully open position; open the cargo evaporator seawater pipe vent valve 5 to exhaust the air in the cargo evaporator seawater pipe; adjust the cargo evaporator seawater outlet valve 4 to adjust the seawater flow rate at the cargo evaporator seawater pipe outlet until the cargo evaporator seawater pipe flow meter 6 shows that the seawater flow rate reaches 800m 3 / h to ensure sufficient heat for the evaporation of liquefied carbon dioxide.

[0028] Table 1 Comparison of the relationship between the carbon dioxide pressure and temperature at the cargo evaporator outlet and the carbon dioxide liquid cargo tank pressure Step 6: Keep the cargo evaporator carbon dioxide inlet valve 7 and cargo evaporator carbon dioxide outlet valve 8 closed, open the first liquid phase pipe shore valve 9 and the second liquid phase pipe shore valve 10, and receive liquefied carbon dioxide from the shore station 38; monitor the liquid phase pipe pressure gauge 11, and when the pressure exceeds the triple point and reaches 5.5~6.0barg, slowly open the cargo evaporator carbon dioxide inlet valve 7 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 the medium in the liquid phase pipe, and the cargo evaporator carbon dioxide outlet pressure gauge 12 shows a pressure greater than 5.5barg, slowly open the cargo evaporator carbon dioxide outlet valve 8 to ensure that the reading of the cargo evaporator carbon dioxide outlet pressure gauge 12 is above 5.5barg. This is to prevent the liquefied carbon dioxide from producing dry ice due to the pressure dropping below the triple point during the vaporization process. Observe the readings of the cargo evaporator carbon dioxide inlet thermometer 13 and cargo evaporator carbon dioxide outlet thermometer 14, and slowly adjust the cargo evaporator carbon dioxide inlet valve 7 until the carbon dioxide vapor state at the cargo evaporator 2 outlet reaches the superheated state - the carbon dioxide vapor temperature is higher than the saturated vapor temperature under the same pressure conditions. Through the test, the cargo evaporator outlet carbon dioxide pressure temperature and carbon dioxide liquid cargo tank pressure comparison table 1 is obtained, such as: According to Table 1, the carbon dioxide vapor state at the cargo evaporator 2 outlet is 15barg, -9℃, 17℃ higher than the saturated temperature; keep the third carbon dioxide liquid phase pipe valve 17 closed, the fourth carbon dioxide liquid phase pipe valve 18 and the fifth carbon dioxide liquid phase pipe valve 19 open; 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 cargo tank bottom loading pipeline 20 until the cargo evaporator carbon dioxide outlet pressure gauge 12 reading reaches 15.5barg, 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.

[0029] Step 7: Keep the first CO2 gas phase pipe valve 22 and the second CO2 gas phase pipe valve 23 closed, the first gas phase pipe shore valve 26 and the second gas phase pipe shore valve 27 open, slowly open the third CO2 gas phase pipe valve 24 and the fourth CO2 gas phase pipe valve 25, and send the CO2 vapor to the shore station through the degassing pipeline 28 on the top of the liquid cargo tank; during the above process, the first CO2 liquid phase pipe valve 15, the second CO2 liquid phase pipe valve 16, the third CO2 gas phase pipe valve 24 and the fourth CO2 gas phase pipe valve 25 should be gradually adjusted so that the CO2 outlet pressure gauge 12 of the cargo evaporator always keeps the reading above 11 barg, so as to ensure that the temperature of the CO2 gas will not be lower than the design temperature of the CO2 liquid cargo tank 21 when it diffuses into the CO2 liquid cargo tank 21, and the reading of the liquid cargo tank pressure sensor 31 is kept 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 show values ​​above +3°C to prevent the cargo evaporator from being blocked by seawater freezing.

[0030] Step 8: Sample the gas in the carbon dioxide cargo tank 21 from 5 different locations, and use a portable gas analyzer to confirm the carbon dioxide content in the gas until it reaches more than 95%, then stop the operation. At this point, the operation of replacing nitrogen in the carbon dioxide cargo tank with superheated gaseous carbon dioxide is completed.

[0031] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A process for replacing nitrogen in a carbon dioxide liquid cargo tank, 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; Take samples of the gas in the carbon dioxide liquid cargo tank to confirm the carbon dioxide content in the gas, and stop the operation until the content reaches the standard.

2. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 1, characterized in that: In step S1, before performing nitrogen replacement in the carbon dioxide cargo tank, confirm 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 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℃.

3. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 2, characterized in that: In step S2, the pipes, valves, flanges and other devices of the replacement system are connected and the valve starting position is set.

4. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 3, characterized in that: In step S3, it is confirmed that the dry air generator (1) is in standby mode and is connected to the gas phase pipe via an air hose (37). Once the liquid cargo system loses pressure, compressed air is immediately injected into the system to maintain the system pressure stable and avoid dry ice generation due to a sudden pressure drop.

5. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 4, characterized in that: In step S4, the onboard seawater pump (36) is started to provide seawater for heating to the cargo evaporator (2).

6. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 5, characterized in that: In step S5, the cargo evaporator seawater inlet valve (3) of the cargo evaporator (2) is opened to a fully open position; the cargo evaporator seawater pipe vent valve (5) is opened to exhaust the air in the cargo evaporator seawater pipe; and 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 the evaporation of liquefied carbon dioxide.

7. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 6, characterized in that: 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.

8. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 7, 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.

9. A process for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 8, 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.

10. A system for replacing nitrogen in a carbon dioxide liquid cargo tank according to claim 9, 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: 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 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.

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