Liquid ammonia, a chemical widely used in industry, has long been a key topic of discussion for its unique cryogenic properties and high-pressure storage methods, encompassing a wide range of industries, including chemical, refrigeration, and energy. This article will delve into the cryogenic properties of liquid ammonia, the necessity, technical requirements, and safety measures for high-pressure storage, to provide a comprehensive understanding of this "cold" knowledge.
I. Cryogenic Properties of Liquid Ammonia
(I) Physical Properties of Liquid Ammonia: Liquid ammonia (chemical formula: NH₃) is a colorless liquid with a pungent odor. At room temperature and pressure, ammonia is a gas, but when the temperature drops to -33.34°C, it liquefies. Liquid ammonia has a density lower than that of water, approximately 0.617 g/cm³ (at -33°C), and is highly volatile. These physical properties of liquid ammonia make it valuable for applications in refrigeration, chemical, and agricultural fields.
(II) The Importance of Cryogenic Properties: Liquid ammonia's cryogenic properties are one of its key advantages in industrial applications. First, liquid ammonia has a low liquefaction temperature, which means it can be stored and transported at lower temperatures, reducing the need for high-temperature, high-pressure equipment. Second, liquid ammonia absorbs a significant amount of heat during the liquefaction process, making it an ideal refrigerant. In refrigeration systems, liquid ammonia absorbs heat through evaporation, rapidly lowering the ambient temperature. It is widely used in cold storage, food processing, and chemical cooling. Furthermore, liquid ammonia's low-temperature properties give it potential applications in the energy sector. For example, in the production of liquefied natural gas (LNG), liquid ammonia can be used as a refrigerant to liquefy the gas and reduce its volume, making it easier to transport and store.
II. The Necessity of High-Pressure Storage
(I) Principles of Liquefaction and Storage: The high-pressure storage of liquid ammonia is primarily based on its liquefaction properties. At room temperature, ammonia requires a relatively high pressure to liquefy. Generally, ammonia can liquefy at around 0°C at a pressure of approximately 1.013 MPa (approximately 10 atmospheres). As the temperature rises, the pressure required for liquefaction also increases. For example, at 20°C, the pressure required to liquefy ammonia is approximately 1.5 MPa. Therefore, to store liquid ammonia at room temperature, it must be placed in a high-pressure container.
(II) Advantages of High-Pressure Storage: High-pressure storage of liquid ammonia offers many benefits. First, high-pressure storage can significantly reduce the volume of liquid ammonia, improving storage efficiency. Under high pressure, the density of liquid ammonia increases, and the storage capacity per unit volume also increases accordingly. This is particularly important for industrial sites that require large-scale storage of liquid ammonia, such as fertilizer plants and refrigeration plants, as it can reduce storage space and equipment costs. Second, high-pressure storage ensures the stability of liquid ammonia. Under high pressure, the volatility of liquid ammonia decreases, reducing the risk of leakage. Furthermore, the design of high-pressure containers can withstand certain pressure fluctuations, ensuring the safety of liquid ammonia during storage and transportation. Finally, high-pressure storage facilitates the transportation and use of liquid ammonia. Liquid ammonia can be transported via pipelines or high-pressure tank trucks, eliminating the need for additional refrigeration equipment during transportation and reducing transportation costs. During use, liquid ammonia can be directly converted to gaseous ammonia through a pressure reduction device for use in chemical reactions or refrigeration systems.
III. Technical Requirements for High-Pressure Storage of Liquid Ammonia
(I) High-Pressure Container Design: High-pressure storage of liquid ammonia requires the use of specially designed high-pressure containers. These containers must possess high strength, corrosion resistance, and excellent sealing properties. Liquid ammonia high-pressure containers are typically made of carbon steel or stainless steel and undergo rigorous pressure and durability testing. The design pressure of the container is generally higher than the maximum pressure required for liquefaction of the liquid ammonia to ensure safe operation even under harsh conditions. Furthermore, liquid ammonia high-pressure containers are typically equipped with safety devices such as safety valves, pressure gauges, and liquid level gauges. The safety valve automatically releases pressure in the event of excessive pressure, preventing explosion. The pressure gauge and liquid level gauge monitor the pressure within the container and the amount of liquid ammonia stored in real time, ensuring that operators can promptly detect any abnormalities.
(II) Temperature Control: Although liquid ammonia can be stored at room temperature under high pressure, temperature control remains a critical technical requirement. The liquefaction temperature of liquid ammonia varies with pressure. Excessively high temperatures increase its volatility, increasing the risk of leakage. Therefore, liquid ammonia high-pressure containers typically require a cooling system to ensure the temperature within the container remains within a suitable range. The cooling system can maintain the temperature within the container between -30°C and 30°C through air or water cooling.
(III) Leak Detection and Prevention: Liquid ammonia has a strong, pungent odor and is harmful to the human body at high concentrations. Therefore, liquid ammonia high-pressure storage systems must be equipped with leak detection devices, such as ammonia sensors and alarm systems. Upon detecting an ammonia leak, the alarm system will immediately sound an alarm, alerting operators to take action. Furthermore, liquid ammonia storage areas should be equipped with protective equipment, such as gas masks, protective clothing, and eyewash stations, to ensure personnel safety.
IV. Safety Measures for High-Pressure Storage of Liquid Ammonia
(I) Operating Regulations: The high-pressure storage and operation of liquid ammonia must strictly adhere to relevant safety regulations. Operators must undergo professional training and be familiar with the physical and chemical properties of liquid ammonia and the operating procedures. During operations, they must wear appropriate protective equipment and strictly follow operating procedures. For example, when filling liquid ammonia, the pressure and temperature of the container must be within safe ranges and the process must be slow to avoid sudden pressure fluctuations.
(II) Emergency Measures: Despite implementing various safety measures, accidents such as liquid ammonia leaks may still occur. Therefore, liquid ammonia storage sites must have a comprehensive emergency plan. Emergency plans should include leak detection, personnel evacuation, leak handling, and accident reporting. If a leak occurs, the emergency plan should be immediately activated, with measures taken to control the source, evacuate nearby personnel, and notify relevant departments for handling.
(III) Regular Maintenance and Inspection: Regular maintenance and inspection of liquid ammonia high-pressure containers and storage systems are critical to ensuring safety. Regular inspections should include visual inspections of the containers, pressure testing, functional checks of safety devices, and pipeline leak checks. Through regular maintenance and inspections, potential safety hazards can be promptly identified and the safe operation of the liquid ammonia storage system ensured.
Liquid ammonia's low-temperature properties and high-pressure storage methods are crucial foundations for its industrial applications. Its low-temperature properties make it an ideal refrigerant and an effective chemical raw material, while high-pressure storage ensures its safe and efficient storage and transportation. Through the rational design of high-pressure containers, strict temperature control, the installation of leak detection devices, and comprehensive safety measures, the safety of liquid ammonia during storage and use can be effectively guaranteed. With the continuous advancement of technology, it is expected that the application areas of liquid ammonia will continue to expand.
