Design of a Device and System to Study the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG
2. Status of Research on Experimental Apparatus and System for Studying the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG under Low Temperature and High Pressure
3. Functions of Experimental Apparatus and System for Studying the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG under Low Temperature and High Pressure
3.1. Improved Design
3.1.1. Operate at High Pressures and Low Temperatures
3.1.3. Independent Temperature Control System
3.1.4. Continuous Sampling Is Possible
3.1.5. Variable Volume
3.2. Selection of Working Parameters
3.2.1. Operating Temperature
3.2.2. Design Pressure
4. Design of the Main Components of the Experimental Apparatus and System
4.1. Imaging System and Visualization Reactor
4.2. Vacuum Tank
4.3. Temperature Control System
4.4. Measurement System
5. Experimental Device and System Safety Design for Studying the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG under Low Temperature and High Pressure
5.1. Analysis of the Characteristics and Hazards of Experimental Devices and Experimental Materials
5.1.1. Characteristics of Each Part
- The vacuum tank has a low-temperature and high-pressure reactor with sapphire glass, liquid nitrogen and gas pipelines, measuring elements, etc. Therefore, for the vacuum tank, the main risk factors include high pressure and low temperature. When the leakage of the reactor leads to the severe gasification of the low-temperature liquid, the overpressure phenomenon occurs. If the vacuum box can play a certain buffering role and gain time for the discharge operation, the loss can be reduced as much as possible.
- The temperature control system includes the temperature controller, temperature sensor, and heater, which is responsible for the realization of two independent temperature control systems. The measurement system includes the temperature sensor, pressure sensor, paperless recorder, and gas–liquid sampling system. For the above measuring elements and heating elements, the line enters from the flange at the bottom of the vacuum tank into its interior. In order to ensure the pressure and sealing conditions of the vacuum box, as well as the normal operation of the temperature control system and the measurement system, special connection methods should be adopted. For the gas-phase sampling, because it is carried out at room temperature, it is only necessary to rely on the air pressure or pumping device in the kettle to make the gas enter the detection instrument or sampling bag; for the liquid-phase sampling, relying on the pressure and gravity in the kettle to flow into the buffer, the cryogenic liquid will rapidly vaporize and increase the pressure, so the buffer should also have a certain pressure-bearing capacity.
- The gas supply system includes high-pressure methane gas bottles, liquid carbon dioxide bottles, high-pressure nitrogen bottles, liquid nitrogen tanks, vacuum pumps, evacuation interfaces, nitrogen purging interfaces, nitrogen or experimental gas discharge piping, etc., whose role is being responsible for the supply of various gases of the experimental device, and their distribution. The gas supply system is hazardous because it involves high-pressure gas cylinders, liquid nitrogen tanks, and liquid carbon dioxide cylinders. It should be placed at a safe distance from the experimental personnel, and fixed and strictly managed.
5.1.2. Physical Properties and Hazard of Experimental Materials
- Nitrogen is a non-toxic, colorless, and odorless gas with a relative molecular mass of 28, which is very close to air. Nitrogen diffusion is difficult to detect, and a high nitrogen concentration easily causes suffocation accidents, so attention should be paid to laboratory ventilation. Liquid nitrogen is a low-temperature liquid at −196 °C under normal pressure, which is prone to low-temperature frostbite and endangers people‘s lives. The liquid nitrogen is stored in the liquid nitrogen tank at the experimental site. The discharge valve of the liquid nitrogen tank and the cold nitrogen outlet of the device may eject high-pressure and low-temperature gas. The corresponding protective measures should be taken to avoid danger.
- Methane, the most widely used experimental material in this device, is also a non-toxic, colorless, and odorless gas. Its relative molecular mass is 16, which is lower than air, and it easily gathers in the upper part of the space to cause suffocation accidents. More importantly, methane is a flammable and explosive gas. If it leaks into the environment and is not evacuated in time, it will mix with the air to form an explosive gas, and there is a risk of combustion and explosion when encountering heat sources and open fires. Therefore, safety measures should be designed to avoid the explosive mixture of methane and air.
- Carbon dioxide is a colorless and odorless gas at normal temperature and pressure. It is generally a non-toxic gas, but when the concentration of carbon dioxide in the air exceeds 5%, it will cause harm to the human body. For this device, the use of carbon dioxide is relatively small, and good ventilation can avoid poisoning events. Based on the characteristics of carbon dioxide, carbon steel cannot be used at low temperature. When the temperature is lower than −28.9 °C, austenitic stainless steel, aluminum, copper, and their alloys should be used. Carbon dioxide should be shipped in liquid or solid form, and the cylinder should not be overheated; otherwise, a violent explosion will occur.  Therefore, for this device, carbon dioxide mainly affects the material selection of the reactor and pipeline.
- In addition to liquid nitrogen, other low-temperature fluids appeared during the experiment, such as liquefied natural gas and low-temperature nitrogen. When leakage occurs, it can cause frostbite. When using liquid nitrogen and operating pipes and valves designed for cryogenic fluids, the appropriate safety measures are required to prevent risks such as frostbite.
5.2. Safety Measures for Devices and Systems
- Material selection: Since the device needs to work under the extreme conditions of low temperature and high pressure, the selection of equipment materials first considers its low-temperature performance and strength. For reactors and gas pipelines that have long-term contact with carbon dioxide at low temperatures, their corrosion resistance needs to be considered. Therefore, 304 stainless steel is selected as the material of the pipe valve. Titanium alloy has good tensile strength, compressive stress , and corrosion resistance at low temperature, so TC-4 titanium alloy is selected as the material of the reactor. On the one hand, the vacuum tank is required to maintain the vacuum for a long time; on the other hand, in order to achieve a certain buffer in the reactor overpressure, the vacuum box needs to have a certain pressure resistance. The vacuum box should have good corrosion resistance in order to prolong its service life due to its long exposure to the environment. Therefore, 2205 duplex stainless steel with good mechanical properties, corrosion resistance, and good weldability is selected as the manufacturing material of the vacuum tank.
- Selection of seals: The selection of seals generally needs to consider the working temperature, working pressure, sealing requirements, and other factors of the device. The seal of this device needs to work under low-temperature and high-pressure conditions, so it needs to have the characteristics of high-pressure and low-temperature resistance. For the general sealing part, the elastic energy storage ring with high pressure and low temperature is selected. For the special parts such as the seal under the sapphire glass of the reactor, we selected the combined pan-plug ring as the seal under the sapphire glass after research and many experiments.
- Gas purging: In order to avoid the combustible gas methane mixed with air to form a combustible gas, the system needs to be purged with inert gas before the experiment. After more than three times of the purge–vacuum cycle, there is no air inside the device. In order to reduce the experimental cost and ensure safety, we chose nitrogen as the purge gas, which is stable and cheap.
- Electrical connection: In order to ensure the pressure resistance and sealing performance of the vacuum box, and make the temperature control system and the measurement system work normally, we use the high-pressure sealing lead joint and the plug-in aviation joint to form a vacuum penetration on the bottom flange of the vacuum box to realize the electrical connection.
- Ventilation: Based on the danger of the experimental materials used in this device, good ventilation conditions are needed to reduce the possibility of forming an explosive mixture of methane and air in the closed area, and to avoid asphyxiation caused by the low-oxygen environment formed by nitrogen accumulation. Therefore, we chose the open area as the experimental site, while avoiding debris accumulation near the device, and we installed an explosion-proof exhaust fan to reduce the possibility of gas accumulation.
- Pressure relief: While enhancing the pressure resistance of the vacuum box to make it have a certain buffering effect, safety valves and bursting discs are also installed on the vacuum box. When the internal pressure of the vacuum box increases sharply, the pressure can be released in time to avoid greater harm.
- Fire and explosion proofing: Because of the flammable gas in the experimental materials, explosion-proof boxes and flame arresters should be used. The vacuum pump is used when vacuuming, but the vacuum pump generally has no explosion-proof measures, so it needs to be placed in the explosion-proof box to isolate the spark.
6. Experimental Verification
6.1. Discussion on the Advantages of This Device
- The experimental device designed by Shen et al. [11,12] and Xiong  has a pressure resistance of 6 MPa. In contrast, the experimental device designed in this paper has a design pressure of up to 20 MPa. On the one hand, it greatly broadens the scope of the work; on the other hand, when the liquid in the kettle is accidentally gasified, the pressure resistance is stronger and the safety is higher.
- The effective volume of the experimental device designed by Riva et al.  and Sampson et al.  was 43 mL and 3.5 mL, respectively. The former can only be used for visual observation, which results in inconvenient sampling; the volume of the latter is smaller, and the disturbance of the system is more obvious during sampling. In contrast, the maximum volume of the device designed in this paper is 300 mL, which can minimize the disturbance of the sampling to the balanced system.
- The experimental devices designed by Shen et al. and Xiong  and Riva et al.  are invisible closed containers. The device designed in this paper adds a visual function to observe the situation inside the reactor. The saturated solution can be ensured by observing the formation of a solid inside the reactor.
6.2. Experimental Validation Related to the Advantages of the Device
- Based on the existing device and the experimental requirements, we propose an improvement of the design of the device functions, such as increasing the pressure-bearing capacity, visualization, continuous sampling, variable volume, and stirring. The minimum working temperature of 77 K, the design pressure of 20 MPa, and the maximum working volume of 300 mL were selected based on the above requirements.
- The main components of the device, the imaging system and the visualization reactor, the vacuum tank, the temperature control system, and the measurement system, are specifically analyzed and designed. For example, the thickness of the sapphire window of the reactor was determined to be 15 mm based on the pressure-bearing capacity; and the inner diameter of the reactor with a known volume was determined to be 82 mm and the depth to be 19 mm based on the working distance of the microscope and the thickness of the window.
- The safety issues of the device and system are analyzed, and the corresponding safety measures are also proposed to ensure the smooth operation of the experiment. Overpressure and low-temperature damage are the two biggest safety problems of this device. Therefore, TC-4 with a strong pressure-bearing capacity was selected as the reactor material, and 2205 duplex stainless steel was selected as the vacuum box material. At the same time, in order to make the vacuum box have a certain buffering capacity, the pressure-bearing capacity is set to 5 MPa.
- Experimental verification of the superiority of the device was carried out. It was confirmed that the device can withstand pressure up to 20 MPa, that it can realize the variable volume function by means of a piston, and that the appearance of solids in the kettle can be clearly observed. At the same time, according to the data recorded by the paperless recorder, the device has a maximum temperature change of 1 °C and a maximum pressure change of 0.13 bar in about 13 min at a low temperature of about −155 °C, which basically maintains the stability of the temperature and pressure.
Data Availability Statement
Conflicts of Interest
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Zhu, J.; Li, Z.; Li, Y. Design of a Device and System to Study the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG. Energies 2023, 16, 3045. https://doi.org/10.3390/en16073045
Zhu J, Li Z, Li Y. Design of a Device and System to Study the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG. Energies. 2023; 16(7):3045. https://doi.org/10.3390/en16073045Chicago/Turabian Style
Zhu, Jianlu, Zihe Li, and Yuxing Li. 2023. "Design of a Device and System to Study the Liquid–Solid-Phase Equilibrium Experiment of CO2 in PLNG" Energies 16, no. 7: 3045. https://doi.org/10.3390/en16073045