A tank pressure control system and control method for low-pressure storage and transportation of liquid carbon dioxide.

Abstract

This invention discloses a tank pressure control system and method for low-pressure storage and transportation of liquid carbon dioxide. By monitoring the tank pressure in real time and automatically selecting to increase or decrease the tank pressure based on the tank pressure, automatic control of the tank pressure is achieved. This ensures that the tank pressure is always higher than the triple point pressure of the transported liquid carbon dioxide, effectively preventing the adverse phenomenon of liquid carbon dioxide transforming into a solid or gaseous state due to the tank pressure being lower than the triple point pressure, thus ensuring the safe transportation of liquid carbon dioxide. At the same time, it can effectively prevent the tank pressure from exceeding the tank design pressure, preventing the safety valve from tripping and releasing carbon dioxide, which would cause greenhouse gas pollution.

Description

Technical Field

[0001] This invention relates to the field of marine engineering equipment technology, and in particular to a tank pressure control system and control method for low-pressure storage and transportation of liquid carbon dioxide. Background Technology

[0002] As climate and environmental issues become increasingly prominent worldwide, countries are intensifying their efforts and measures to address global warming, resulting in a variety of carbon dioxide emission reduction schemes. Among these, carbon capture and storage (CCS) is an important and effective measure. How to transfer and transport captured carbon dioxide to storage facilities is a crucial aspect of advancing CCS projects.

[0003] Since the volume of liquid carbon dioxide is approximately 1 / 560th that of the same amount of gaseous carbon dioxide, using ships to transport liquid carbon dioxide can greatly improve transportation efficiency and reduce transportation costs.

[0004] Since carbon dioxide has a triple point pressure of 5.2 bar gauge pressure and a temperature of -56.6℃, it can only exist in two states—liquid and solid—when the pressure is below the triple point pressure, regardless of temperature changes. Therefore, to keep carbon dioxide in a liquid state during transportation, it is necessary to ensure that the liquid carbon dioxide is kept at a low temperature while the pressure inside the storage tank is always higher than the triple point pressure of 5.2 bar gauge pressure. However, excessively high design pressure for the storage tank would significantly increase its manufacturing cost. Considering all factors, selecting a low-pressure liquid carbon dioxide transportation scheme with a tank design pressure of 6–8 bar can further improve economic efficiency.

[0005] While low-pressure designs improve economic efficiency, they also carry the risk of operating pressure close to the triple point and having a small design margin. As shown in the "Carbon Dioxide Triple-Phase Diagram," if the internal pressure of the storage tank rises beyond its design pressure, the safety valve will activate, releasing carbon dioxide gas and causing secondary pollution. Conversely, if the tank pressure drops below the triple point, the liquid carbon dioxide cannot remain liquid and will transform into the other two phases (gas and solid) with temperature changes. The transformation of liquid carbon dioxide into dry ice or the generation of large amounts of gaseous carbon dioxide will both endanger the navigational safety of ships. Summary of the Invention

[0006] In view of this, the present invention provides a tank pressure control system and control method for low-pressure storage and transportation of liquid carbon dioxide. By monitoring the pressure inside the tank in real time, the system automatically selects the tank pressurization or depressurization process to achieve tank pressure control, so that the pressure inside the tank is always higher than the triple point pressure of the liquid carbon dioxide being transported.

[0007] A tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide includes a tank depressurization system and a tank pressurization system.

[0008] The tank depressurization system is used to depressurize and condense the vaporized carbon dioxide in the upper gas phase space of the tank into liquid carbon dioxide when the pressure inside the tank is close to the design pressure of the tank, and then transport the liquid carbon dioxide to the lower part of the tank. The tank depressurization system includes a gas compressor connected to a first pipeline at the inlet, a condensation and liquefaction device connected to the outlet of the gas compressor, a buffer tank connected to the condensation and liquefaction device, and a depressurization control component for automatically controlling the depressurization according to the pressure inside the tank. A refrigeration unit is connected to the condensation and liquefaction device. A second pipeline is provided at the bottom of the buffer tank. The end of the second pipeline extends downward to the lower part of the tank, and the end of the first pipeline extends to the upper gas phase space of the tank.

[0009] The tank pressurization system is used to pressurize and heat the liquid carbon dioxide in the lower part of the tank into vaporized carbon dioxide when the pressure inside the tank is close to the triple point pressure of carbon dioxide, and then transport the vaporized carbon dioxide to the gas phase space in the upper part of the tank. The tank pressurization system includes a delivery pump, a vaporization device connected to the delivery pump through a third pipeline, and a pressurization control component for automatically controlling the pressurization according to the pressure inside the tank. The outlet of the vaporization device is connected to a fourth pipeline, which is connected to the first pipeline.

[0010] Preferably, the pressure reduction control assembly includes a pressure sensor disposed on the top of the tank for monitoring the pressure inside the tank, a first pressure control valve disposed on the first pipeline for controlling its opening and closing based on the pressure value detected by the pressure sensor, a first temperature sensor for monitoring the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device, a pressure regulating valve disposed on the second pipeline, and a differential pressure control assembly for monitoring the pressure difference between the pressure of gaseous carbon dioxide exiting the tank and the pressure of liquid carbon dioxide returning to the tank, the differential pressure control assembly being disposed on the second pipeline.

[0011] Preferably, the differential pressure control component includes a differential pressure sensor and a PID controller. The high-pressure interface of the differential pressure sensor is connected to the second pipeline, and the low-pressure interface is connected to the first pipeline. The differential pressure value detected by the differential pressure sensor is transmitted to the PID controller. The PID controller controls the valve opening of the pressure regulating valve according to the received data so that the return pressure of liquid carbon dioxide always meets the design value.

[0012] Preferably, the design value of the return pressure of liquid carbon dioxide is the pressure after the pressure regulating valve. The pressure after the pressure regulating valve is 1 bar higher than the pressure of the gas phase space above the storage tank. This pressure difference setting value (1 bar) can be adjusted.

[0013] Preferably, the pressure boosting control assembly includes a second temperature sensor for monitoring the temperature of gaseous carbon dioxide at the outlet of the gasification unit, a temperature control valve for controlling the load of the gasification unit based on the temperature signal detected by the second temperature sensor, and a second pressure control valve for controlling its opening and closing based on the pressure inside the tank.

[0014] The second temperature sensor and the second pressure control valve are both located on the fourth pipeline, and the temperature control valve is located on the heating medium inlet pipeline of the gasification device.

[0015] Preferably, the delivery pump is located outside the storage tank or at the bottom of the tank body.

[0016] Preferably, the gas compressor is a variable frequency controlled screw or reciprocating compressor, the condensation and liquefaction device is a plate-and-shell heat exchanger, a shell-and-tube heat exchanger, or a coiled tube heat exchanger, the delivery pump is a centrifugal pump or a reciprocating pump, and the gasification device is a plate-and-shell heat exchanger, a shell-and-tube heat exchanger, or a coiled tube heat exchanger.

[0017] Preferably, the outer side of the buffer tank is covered with heat-insulating material.

[0018] A control method for a tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide specifically includes the following steps:

[0019] During the ship's loading and navigation process, pressure sensors monitor the pressure inside the tank in real time and transmit the detection data to the ship's control system.

[0020] When the pressure inside the tank approaches the design pressure, the ship's control system opens the first pressure control valve and starts the gas compressor. The gas compressor extracts excess vaporized carbon dioxide from the upper gas phase space of the tank and compresses it. The pressurized carbon dioxide is then transported to the condensation and liquefaction unit for heat exchange with the refrigeration unit and is re-condensed into liquid carbon dioxide. The first temperature sensor monitors the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction unit in real time. The ship's control system controls the load of the refrigeration unit based on the data detected by the first temperature sensor to ensure that the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction unit always meets the triple point temperature of carbon dioxide. The re-condensed liquid carbon dioxide is transported to the buffer tank. The return pressure of the liquid carbon dioxide is regulated by the differential pressure control component at the bottom of the buffer tank and then sent back to the bottom of the tank through the second pipeline.

[0021] When the pressure inside the storage tank approaches the triple point pressure of carbon dioxide, the ship's control system opens the second pressure control valve and starts the delivery pump to pump the liquid carbon dioxide at the bottom of the tank to the vaporization device for vaporization and heating into gaseous carbon dioxide. The second temperature sensor monitors the temperature of the gaseous carbon dioxide at the outlet of the vaporization device in real time. Based on the data detected by the second temperature sensor, the ship's control system controls the amount of heating medium inside the vaporization device to ensure that the temperature of the gaseous carbon dioxide at the outlet of the vaporization device always meets the triple point temperature of carbon dioxide. The gaseous carbon dioxide is then sent back to the gas phase space at the top of the tank through the fourth pipeline and the first pipeline.

[0022] The beneficial effects of this invention are:

[0023] 1. This invention achieves automatic control of the tank pressure by real-time monitoring of the tank pressure and automatically selecting to increase or decrease the tank pressure based on the tank pressure. This ensures that the tank pressure is always higher than the triple point pressure of the transported liquid carbon dioxide, effectively preventing the adverse phenomenon of liquid carbon dioxide changing into a solid or gaseous state due to the tank pressure being higher or lower than the triple point pressure, thus ensuring the safe transportation of liquid carbon dioxide.

[0024] 2. This invention utilizes frequency conversion control equipment to the maximum extent possible, effectively reducing system energy consumption and improving economic efficiency while achieving tank pressure control.

[0025] 3. This invention uses high-precision pressure and temperature sensors to monitor the system pressure and temperature in real time, accurately controlling the tank pressure within a narrow range, which greatly improves the safety of carbon dioxide transport ship operation. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a system schematic diagram of the present invention.

[0028] Figure 2 This is a three-phase diagram of carbon dioxide.

[0029] The meanings of the labels in the diagram are as follows:

[0030] 1. Storage tank; 2. Gas compressor; 3. Condensation and liquefaction device; 4. Refrigeration unit; 5. Buffer tank; 6. First temperature sensor; 7-1. Differential pressure sensor; 7-2. PID controller; 8. Pressure regulating valve; 9. Transfer pump; 10. Vaporization device; 11. First pressure control valve; 12. Pressure sensor; 13. Second pressure control valve; 14. Second temperature sensor; 15. Temperature control valve; 16. Ship;

[0031] I. Tank pressure reduction system;

[0032] II. Tank pressurization system;

[0033] L1, First pipeline;

[0034] L2, second pipeline;

[0035] L3, third pipeline;

[0036] L4, the fourth pipeline. Detailed Implementation

[0037] 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.

[0038] 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.

[0039] 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."

[0040] 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.

[0041] This invention provides a tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide, including a tank depressurization system and a tank pressurization system.

[0042] The tank depressurization system is used to depressurize and condense the vaporized carbon dioxide in the upper gas phase space of the tank into liquid carbon dioxide when the pressure inside the tank is close to the design pressure of the tank, and then transport the liquid carbon dioxide to the lower part of the tank so that the pressure inside the tank is always higher than the triple point pressure of the transported liquid carbon dioxide.

[0043] The tank pressurization system is used to pressurize and heat the liquid carbon dioxide in the lower part of the tank 1 into vaporized carbon dioxide when the pressure inside the tank 1 is close to the triple point pressure of carbon dioxide, and then transport the vaporized carbon dioxide to the gas phase space in the upper part of the tank 1 so that the pressure inside the tank is always higher than the triple point pressure of the liquid carbon dioxide being transported.

[0044] Specifically, the storage tank 1 is installed inside the ship's structure or on the deck for storing liquid carbon dioxide, and the exterior of the storage tank 1 is covered with insulating material. The tank body of the storage tank 1 is designed to have a pressure greater than the triple point pressure of carbon dioxide (5.2 bar gauge pressure). The storage tank 1 is a Type C containment system as defined by the IMO, and there may be one or more of them. A pressure sensor 12 is installed on the top of the storage tank 1. The pressure sensor 12 monitors the pressure in the gas phase space inside the storage tank 1 to select and control the operation of either the tank depressurization system I or the tank pressurization system II.

[0045] The tank depressurization system includes a gas compressor 2, a condensation and liquefaction device 3 connected to the outlet of the gas compressor 2, a buffer tank 5 connected to the condensation and liquefaction device 3, and a depressurization control component for automatically controlling the depressurization according to the pressure inside the tank 1. The depressurization control component includes a pressure sensor 12, a first pressure control valve 11, a first temperature sensor 6, a pressure regulating valve 8, and a differential pressure control component.

[0046] The gas compressor 2 is connected to a first pipeline L1 at its inlet. The end of the first pipeline L1 extends into the gas phase space at the top of the storage tank 1. The end port of the first pipeline L1 can extend downwards into the gas phase space or extend to the top of the tank and connect with the gas phase space. The gas compressor 2 can extract and compress gaseous carbon dioxide from the gas phase space of the storage tank 1 through the first pipeline L1. A first pressure control valve 11 is provided on the first pipeline L1. The opening and closing of the first pressure control valve 11 is controlled according to the pressure inside the tank detected by the pressure sensor 12. In this embodiment, the gas compressor 2 is a variable frequency controlled screw or piston compressor, which can adjust its speed and displacement according to the pressure of the storage tank 1 monitored by the pressure sensor 12.

[0047] The inlet of the condensation and liquefaction device 3 is connected to the outlet of the gas compressor 2 via a pipeline. The condensation and liquefaction device 3 is used to condense and liquefy compressed carbon dioxide into liquid carbon dioxide. A refrigeration unit 4 is connected to the condensation and liquefaction device 3. The refrigeration unit 4 consists of a refrigerant compressor, a refrigerant cooler, an expansion valve, and other equipment. The refrigerant compressor is frequency-controlled, and the opening of the expansion valve is adjustable. The subcooling unit compresses and cools the gaseous refrigerant into a high-pressure, liquid refrigerant. The high-pressure, liquid refrigerant is depressurized by the expansion valve and enters the condensation and liquefaction device 3 to exchange heat with the gaseous carbon dioxide inside the condensation and liquefaction device 3, thereby achieving heat absorption and evaporation, thus cooling and liquefying the gaseous carbon dioxide compressed by the gas compressor 2. The refrigerant can be propane or propylene. In this embodiment, the condensation and liquefaction device 3 is a plate-and-shell heat exchanger, a shell-and-tube heat exchanger, or a coiled-tube heat exchanger, and its cold source comes from the refrigeration unit 4.

[0048] A first temperature sensor 6 is installed at the outlet of the condensation and liquefaction device 3. This sensor monitors the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 in real time. Based on the temperature of the liquid carbon dioxide discharged from the condensation and liquefaction device 3, the load on the refrigerant compressor in the refrigeration unit 4 and the opening of the expansion valve are automatically controlled to ensure that the temperature of the condensed and liquefied liquid carbon dioxide remains within a set temperature range (this set temperature range depends on the purity and pressure of the transported liquid carbon dioxide), and that the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 always meets the triple point temperature of carbon dioxide. Taking high-purity carbon dioxide as an example, according to the carbon dioxide triple phase diagram, when the liquid carbon dioxide pressure is 10 bar, its set temperature range is -45℃ to -55℃. When the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 approaches -55℃, the load on the refrigeration unit 4 is reduced, thereby increasing the temperature of the liquefied carbon dioxide after condensation and liquefaction. When the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 approaches -45℃, the load on the refrigeration unit 4 is increased, thereby decreasing the temperature of the liquefied carbon dioxide after condensation and liquefaction.

[0049] The outlet of the condensation liquefaction device 3 is connected to the top inlet of the buffer tank 5 via a pipeline.

[0050] The buffer tank 5 is used to receive and temporarily store the liquid carbon dioxide after it has been reliquefied by the condensation and liquefaction device 3. The outside of the buffer tank 5 is covered with heat-insulating material. A second pipe L2 is provided at the bottom of the buffer tank 5, and the end of the second pipe L2 extends downward to the lower part of the storage tank 1 or near the bottom of the tank body.

[0051] The second pipeline L2 is fixed with a pressure regulating valve 8 and a differential pressure control component. The differential pressure control component is installed on the pipeline section after the pressure regulating valve 8.

[0052] The differential pressure control component is used to monitor the pressure difference between the gaseous carbon dioxide exit pressure and the liquid carbon dioxide return pressure. Based on the pressure difference detected by the differential pressure control component, the valve opening of the pressure regulating valve 8 can be controlled.

[0053] The differential pressure control component includes a differential pressure sensor 7-1 and a PID controller 7-2. The high-pressure interface of the differential pressure sensor 7-1 is connected to the second pipeline L2, and the low-pressure interface is connected to the first pipeline L1. The differential pressure value detected by the differential pressure sensor 7-1 is transmitted to the PID controller 7-2. The PID controller 7-2 controls the valve opening of the pressure regulating valve 8 according to the received data to ensure that the return pressure of liquid carbon dioxide always meets the set value, even if the return pressure of liquid carbon dioxide is lower than the triple point pressure of carbon dioxide. The set value of the return pressure of liquid carbon dioxide can be selected as the pressure after the pressure regulating valve 8 being 1 bar higher than the pressure of the gas phase space above the storage tank 1, that is, the set value of the differential pressure sensor 7-1 is 1 bar.

[0054] The tank pressurization system includes a delivery pump 9, a vaporization device 10 connected to the delivery pump 9 via a third pipeline L3, and a pressurization control component for automatically controlling the pressurization based on the pressure inside the tank 1. The pressurization control component includes a second temperature sensor 14, a temperature control valve 15, and a second pressure control valve 13.

[0055] The delivery pump 9 can be installed outside the storage tank 1 or at the bottom of the tank body. The delivery pump 9 is used to pressurize the liquid carbon dioxide at the bottom of the tank body and deliver it to the gasification device 10 through the third pipeline L3.

[0056] The vaporization device 10 is used to heat and evaporate liquid carbon dioxide to form gaseous carbon dioxide. A temperature control valve 15 is installed on the heating medium inlet pipe of the vaporization device 10. A fourth pipe L4 is connected to the outlet of the vaporization device 10. The fourth pipe L4 is connected to the first pipe L1, and the connection point between the fourth pipe L4 and the first pipe L1 is located before the first pressure control valve 11. A second temperature sensor 14 and a second pressure control valve 13 are installed on the fourth pipe L4.

[0057] The second temperature sensor 14 is used to monitor the temperature of the gaseous carbon dioxide at the outlet of the gasification device 10 in real time; the temperature control valve 15 is used to automatically control its valve opening according to the temperature signal detected by the second temperature sensor 14, thereby controlling the amount of heating medium in the gasification device 10 so that the temperature of the gaseous carbon dioxide flowing out of the gasification device 10 always meets the triple point temperature of carbon dioxide. The second pressure control valve 13 controls the opening and closing of the controller according to the tank pressure detected by the pressure sensor 12.

[0058] Gaseous carbon dioxide that meets the triple point temperature of carbon dioxide and is discharged from the outlet of the gasification device 10 returns to the storage tank 1 through the first pipeline L1 under the action of pressure difference.

[0059] In this embodiment, the delivery pump 9 is a centrifugal pump or a piston pump, the gasification device 10 is a plate-and-shell heat exchanger, a shell-and-tube heat exchanger or a coiled tube heat exchanger, and the heating medium in the gasification device 10 can be fresh water, seawater, steam, hot oil or ethylene glycol solution.

[0060] This invention also provides a control method for a tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide, specifically including the following steps:

[0061] During the ship's loading and navigation, pressure sensor 12 monitors the pressure inside tank 1 in real time and transmits the detection data to the ship's control system. If the pressure inside tank 1 approaches its design pressure and is not dealt with in time, allowing the pressure to continue to rise, the safety valve installed on tank 1 will activate when the design pressure is reached, thereby releasing excess carbon dioxide into the atmosphere, which will cause environmental damage. If the pressure inside tank 1 approaches the triple point pressure of carbon dioxide, and if the pressure drops further below the triple point pressure, the liquid carbon dioxide will not be able to remain in the liquid state, but will transform into a gaseous or solid state, forming dry ice or producing a large amount of carbon dioxide gas, which will have a serious impact on the ship's navigation safety and the environment.

[0062] Therefore, when the pressure inside tank 1 approaches the design pressure of the storage tank, the ship control system controls the opening of the first pressure control valve 11 and simultaneously starts the gas compressor 2. The gas compressor 2 extracts the excess vaporized carbon dioxide in the upper gas phase space of the storage tank 1 and compresses it. The pressurized carbon dioxide is then transported to the condensation and liquefaction device 3 and exchanged with the refrigeration unit 4 for heat exchange, and is re-condensed into liquid carbon dioxide. The first temperature sensor 5 monitors the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 in real time. The ship control system controls the load of the refrigeration unit 4 based on the data detected by the first temperature sensor 5 so that the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 always meets the triple point temperature of carbon dioxide. The re-condensed liquid carbon dioxide is transported to the buffer tank 5. The return pressure of the liquid carbon dioxide is adjusted by the differential pressure control component at the bottom of the buffer tank 5 and then sent back to the bottom of the tank 1 through the second pipeline L2.

[0063] When the pressure inside the storage tank 1 approaches the triple point pressure of carbon dioxide, the ship's control system opens the second pressure control valve 11 and starts the transfer pump 9 to pump the liquid carbon dioxide at the bottom of the tank 1 to the vaporization device 10 for vaporization and heating into gaseous carbon dioxide. The second temperature sensor 14 monitors the temperature of the gaseous carbon dioxide at the outlet of the vaporization device 10 in real time. Based on the data detected by the second temperature sensor 14, the ship's control system controls the amount of heating medium inside the vaporization device 10 to ensure that the temperature of the gaseous carbon dioxide at the outlet of the vaporization device 10 always meets the triple point temperature of carbon dioxide. The gaseous carbon dioxide is then sent back to the gas phase space above the tank 1 through the fourth pipeline L4 and the first pipeline L1.

[0064] The following example, using the tank pressure control of a 3000 cubic meter low-pressure liquid carbon dioxide transport ship, illustrates the control method of the tank pressure control system of the present invention.

[0065] The 3,000 cubic meter low-pressure transport vessel for liquid carbon dioxide is equipped with a Type C storage tank 1 with a design pressure of 8 bar and a design temperature of -60°C, for storing and transporting liquid carbon dioxide. The following description will be based on pure carbon dioxide (triple point pressure 5.2 bar gauge pressure, temperature -56.6°C).

[0066] During loading and navigation, pressure sensor 12 monitors the pressure inside tank 1 in real time. When the pressure inside tank 1 rises to 7.5 bar gauge pressure, the ship's control system will activate the tank depressurization system I, that is, control the opening of the first pressure control valve 11 to connect tank 1 and the inlet of gas compressor 2; at the same time, gas compressor 2 will start to extract excess carbon dioxide gas from tank 1 and compress it to about 10 bar gauge pressure. The load of gas compressor 2 is frequency-controlled according to the pressure of tank 1 monitored by pressure sensor 12. That is, if the pressure of tank 1 continues to rise after gas compressor 2 has started running, the speed of gas compressor 2 will be increased by the frequency converter to increase the displacement; when the pressure of tank 1 gradually drops to the normal design pressure after gas compressor 2 has started running, the speed of gas compressor 2 will be reduced by the frequency converter to reduce the displacement.

[0067] The pressurized carbon dioxide gas is transported to the condensation and liquefaction device 3, where it exchanges heat with the refrigerant from the refrigeration unit 4. The carbon dioxide gas is then re-condensed and liquefied into liquid carbon dioxide at a pressure of approximately 10 bar gauge pressure.

[0068] The first temperature sensor 6 monitors the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device 3 in real time. According to the carbon dioxide three-phase diagram, in this embodiment, the temperature of the first temperature sensor 6 is set to -52°C. The first temperature sensor 6 controls the working load of the refrigeration unit 4 to ensure that the pressure of the liquid carbon dioxide from the condensation and liquefaction device 3 is 10 bar gauge pressure and the temperature is -52°C. Its allowable temperature range can be -45°C to -55°C. When the outlet temperature of the condensation and liquefaction device 3 is close to -55°C, the load of the refrigeration unit 4 is reduced, thereby increasing the temperature of the liquefied carbon dioxide. When the temperature is close to -45°C, the load of the refrigeration unit 4 is increased, thereby decreasing the temperature of the liquefied carbon dioxide, so that the temperature of the liquid carbon dioxide discharged from the condensation and liquefaction device 3 is always maintained within the range of -45°C to -55°C.

[0069] The recondensed liquid carbon dioxide is transported to buffer tank 5 and then returns to storage tank 1 through the second pipeline L2 at the bottom of buffer tank 5. The pressure regulating valve 8 adjusts its valve opening in real time according to the current differential pressure value detected by the differential pressure sensor, so that the pressure of liquid carbon dioxide in the second pipeline L2 is always 1 bar lower than the current pressure value of the gas phase space in the chamber. Even if the pressure of liquid carbon dioxide in the second pipeline L2 is always 1 bar lower than the pressure value currently detected by pressure sensor 12, it ensures that liquid carbon dioxide can overcome the back pressure at the outlet of the second pipeline and return to the bottom of storage tank 1, while maintaining a certain margin in its physical properties from the triple point.

[0070] When the pressure inside storage tank 1 drops to 5.7 bar gauge pressure, the ship's control system will activate the storage tank pressurization system II, that is, control the opening of the second pressure control valve 13 to connect the fourth pipeline L4 with the first pipeline L1; at the same time, the transfer pump 9 will be activated to pressurize the liquid carbon dioxide at the bottom of storage tank 1 to 7.5 bar gauge pressure and deliver it to the vaporization unit 10. The load of the transfer pump 9 is controlled according to the pressure of storage tank 1 monitored by the pressure sensor 12.

[0071] The pressurized liquid carbon dioxide exchanges heat with the heating medium in the vaporization device 10. The opening of the temperature control valve 15 on the heating medium inlet pipeline of the vaporization device 10 is controlled by the second temperature sensor 14 at the outlet of the vaporization device 10. By adjusting the amount of heating medium, the temperature of the carbon dioxide gas vaporized by the vaporization device 10 is always at the set temperature.

[0072] The vaporized carbon dioxide returns to the gas phase space of storage tank 1 through the fourth pipeline L4 and the first pipeline L1, which can effectively increase the pressure inside storage tank 1, keep it away from the triple point, and ensure that the liquid carbon dioxide remains in a liquid state during transportation.

[0073] Since liquid carbon dioxide of different purities has different triple points, the shore station should provide the purity and triple point parameters of the liquid carbon dioxide to be loaded before each loading. The crew will adjust the settings of the relevant tank pressure control system, such as the pressure sensor, the first temperature sensor and the second temperature sensor, according to the triple point parameters provided by the shore station.

[0074] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide, characterized in that, This includes tank depressurization systems and tank pressurization systems. The tank depressurization system is used to depressurize and condense the vaporized carbon dioxide in the upper gas phase space of the tank (1) into liquid carbon dioxide when the pressure in the tank (1) is close to the design pressure of the tank, and to transport the liquid carbon dioxide to the lower part of the tank (1). The tank depressurization system includes a gas compressor (2) with a first pipeline (L1) connected to the inlet, a condensation and liquefaction device (3) connected to the outlet of the gas compressor (2), a buffer tank (5) connected to the condensation and liquefaction device (3), and a depressurization control component for automatically controlling the depressurization according to the pressure in the tank (1). A refrigeration unit (4) is connected to the condensation and liquefaction device (3). A second pipeline (L2) is provided at the bottom of the buffer tank (5). The end of the second pipeline (L2) extends downward to the lower part of the tank (1), and the end of the first pipeline (L1) extends to the upper gas phase space of the tank (1). The tank pressurization system is used to pressurize and heat the liquid carbon dioxide in the lower part of the tank (1) into vaporized carbon dioxide when the pressure in the tank (1) is close to the triple point pressure of carbon dioxide, and to transport the vaporized carbon dioxide to the gas phase space in the upper part of the tank (1). The tank pressurization system includes a delivery pump (9), a vaporization device (10) connected to the delivery pump (9) through a third pipeline (L3), and a pressurization control component for automatically controlling the pressurization according to the pressure in the tank (1). The outlet of the vaporization device (10) is connected to a fourth pipeline (L4), and the fourth pipeline (L4) is connected to the first pipeline (L1).

2. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 1, characterized in that, The pressure reduction control assembly includes a pressure sensor (12) installed on the top of the tank for monitoring the pressure inside the tank, a first pressure control valve (11) installed on the first pipeline (L1) for controlling its opening and closing based on the pressure value detected by the pressure sensor (12), a first temperature sensor (6) for monitoring the temperature of the liquid carbon dioxide at the outlet of the condensation liquefaction device (3), a pressure regulating valve (8) installed on the second pipeline (L2), and a differential pressure control assembly for monitoring the pressure difference between the gaseous carbon dioxide outlet pressure and the liquid carbon dioxide return pressure, the differential pressure control assembly being installed on the second pipeline (L2).

3. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 2, characterized in that, The differential pressure control component includes a differential pressure sensor (7-1) and a PID controller (7-2). The high-pressure interface of the differential pressure sensor (7-1) is connected to the second pipeline (L2), and the low-pressure interface is connected to the first pipeline (L1). The differential pressure value detected by the differential pressure sensor (7-1) is transmitted to the PID controller (7-2). The PID controller (7-2) controls the valve opening of the pressure regulating valve (8) according to the received data so that the return pressure of liquid carbon dioxide always meets the design value.

4. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 3, characterized in that, The design value of the return pressure of liquid carbon dioxide is the pressure after the pressure regulating valve (8), which is 1 bar higher than the pressure of the gas phase space above the storage tank (1).

5. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 1, characterized in that, The pressure boosting control assembly includes a second temperature sensor (14) for monitoring the temperature of gaseous carbon dioxide at the outlet of the gasification unit (10), a temperature control valve (15) for controlling the load of the gasification unit (10) based on the temperature signal detected by the second temperature sensor (14), and a second pressure control valve (13) for controlling its opening and closing based on the pressure inside the tank. The second temperature sensor (14) and the second pressure control valve (13) are both installed on the fourth pipeline (L4), and the temperature control valve (15) is installed on the heating medium inlet pipeline of the gasification device (10).

6. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 1, characterized in that, The delivery pump (9) is located outside the storage tank (1) or at the bottom of the tank body.

7. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 1, characterized in that, The gas compressor (2) is a variable frequency controlled screw or piston compressor, the condensation and liquefaction device (3) is a plate heat exchanger, a shell-and-tube heat exchanger, or a coiled tube heat exchanger, the delivery pump (9) is a centrifugal pump or a piston pump, and the gasification device (10) is a plate heat exchanger, a shell-and-tube heat exchanger, or a coiled tube heat exchanger.

8. The tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to claim 1, characterized in that, The outer side of the buffer tank (5) is covered with heat-insulating material.

9. A control method for a tank pressure control system for low-pressure storage and transportation of liquid carbon dioxide according to any one of claims 1-8, characterized in that, Specifically, it includes the following steps: During the ship's loading and navigation process, the pressure sensor (12) monitors the pressure inside the tank in real time and transmits the detection data to the ship's control system. When the pressure inside the tank approaches the design pressure of the storage tank, the ship control system controls the opening of the first pressure control valve (11) and starts the gas compressor (2) at the same time. The gas compressor (2) extracts the excess vaporized carbon dioxide in the upper gas phase space of the storage tank (1) and compresses it. The pressurized carbon dioxide is transported to the condensation and liquefaction device (3) and exchanges heat with the refrigeration unit (4) and is re-condensed into liquid carbon dioxide. The first temperature sensor (6) monitors the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device (3) in real time. The ship control system controls the load of the refrigeration unit (4) according to the data detected by the first temperature sensor (6) so that the temperature of the liquid carbon dioxide at the outlet of the condensation and liquefaction device (3) always meets the triple point temperature of carbon dioxide. The re-condensed liquid carbon dioxide is transported to the buffer tank (5). The return pressure of the liquid carbon dioxide is adjusted by the differential pressure control component at the bottom of the buffer tank (5) and then sent back to the bottom of the tank through the second pipeline (L2). When the pressure inside the storage tank (1) approaches the triple point pressure of carbon dioxide, the ship control system controls the opening of the second pressure control valve (13) and starts the delivery pump (9) to pump the liquid carbon dioxide at the bottom of the tank to the vaporization device (10) for vaporization and heating into gaseous carbon dioxide. The second temperature sensor (14) monitors the temperature of the gaseous carbon dioxide at the outlet of the vaporization device (10) in real time. The ship control system controls the amount of heating medium inside the vaporization device (10) according to the data detected by the second temperature sensor (14) so ​​that the temperature of the gaseous carbon dioxide at the outlet of the vaporization device (10) always meets the triple point temperature of carbon dioxide. The gaseous carbon dioxide is sent back to the gas phase space at the top of the tank through the fourth pipeline (L4) and the first pipeline (L1).

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