Liquid ammonia, also known as anhydrous ammonia, is a colorless liquid with a strong pungent odor. Ammonia is an important chemical raw material. For the convenience of transportation and storage, gaseous ammonia is usually pressurized or cooled to obtain liquid ammonia. Liquid ammonia is easily soluble in water. After dissolving in water, it forms ammonium ions NH4+ and hydroxide ions OH-, which are alkaline alkaline solutions. Liquid ammonia is mostly stored in pressure-resistant steel cylinders or steel tanks, and cannot coexist with acetaldehyde, acrolein, boron and other substances. Liquid ammonia is widely used in industry. It is corrosive and easy to volatilize, so its chemical accident rate is very high.
1. Product Use
<2013-2017 Liquid Ammonia Industry Research and Investment Prospects Assessment Report> Liquid ammonia is mainly used to produce nitric acid, urea and other chemical fertilizers, and can also be used as a raw material for medicines and pesticides.
In the defense industry, it is used to make propellants for rockets and missiles. It can be used as an ammonia raw material for organic chemical products. Because liquid ammonia turns into gaseous ammonia after gasification, it can absorb a lot of heat and is known as a "refrigerant". At the same time, liquid ammonia has a certain bactericidal effect, so it is used for sterilization and cooling in poultry farming. Liquid ammonia can also be used for mercerization finishing of textiles. The lone electron pair in the NH3 molecule tends to form coordination bonds with other molecules or ions to form various forms of ammonia compounds.
Liquid ammonia is a very good solvent. Due to the molecular nature and the existence of hydrogen bonds, liquid ammonia is very similar to water in many physical properties. Some active metals can replace hydrogen from water and form hydroxides, but it is not so easy to replace hydrogen in liquid ammonia. But liquid ammonia can dissolve metals to form a blue solution. This metal liquid ammonia solution can conduct electricity, slowly decompose and release hydrogen, and has strong reducing properties.
For example, when sodium metal is dissolved in liquid ammonia, it loses its valence electrons to generate positive ions: when liquid ammonia is heated to 800-850℃, ammonia is decomposed under the action of a nickel-based catalyst to obtain a hydrogen-nitrogen mixed gas containing 75% H2 and 25% N2. The gas produced by this method is a good protective gas and can be widely used in the semiconductor industry, metallurgical industry, and other industries and scientific research that require a protective atmosphere.
Ammonia and acetic acid gas are heated to 420℃, and acetonitrile is synthesized under the action of a catalyst.
2. Synthetic Ammonia
The synthesis of ammonia is the first step in the production of synthetic ammonia. Its task is to synthesize ammonia from the refined hydrogen-nitrogen mixed gas under the action of a catalyst as quickly, efficiently and economically as possible. For the synthesis system, liquid ammonia is its product.
Industrial synthetic ammonia processes are generally classified by pressure.
Methods for synthesizing ammonia
1. High-pressure method:
The operating pressure is 70-100MPa and the temperature is 550-650℃. The main advantage of this method is that the ammonia synthesis rate is high and the ammonia in the mixed gas is easy to be separated. Therefore, the process and equipment are relatively compact. However, due to the high synthesis rate, the heat released is large, the catalyst temperature is high, and it is easy to overheat and lose activity, so the service life of the catalyst is short. Because it is a high-temperature and high-pressure operation, the requirements for equipment manufacturing and materials are high, and the investment cost is large. This method is rarely used in industry.
2. Medium-pressure method:
The operating pressure is 20-60MPa and the temperature is 450-550℃. Its advantages and disadvantages are between the high-pressure method and the low-pressure method. This method is more mature and economical. Because the determination of the synthesis pressure is nothing more than considering the two aspects of equipment investment and compression power consumption. From the perspective of power consumption, the power consumption of the synthesis system accounts for the proportion of the total power consumption of the plant. Power consumption depends not only on the pressure item, but also on other process indicators and the layout of the process. In general, within the range of 15 to 30Pa, the difference in power consumption is not large, so many people in the world use this method.
3. Low-pressure method:
The operating pressure is about 10MPa and the temperature is 400 to 450℃. Because the operating pressure and temperature are relatively low, the equipment requirements are low, it is easy to manage, and the catalyst activity is high, which is the advantage of this method. However, the catalyst used in this method is very sensitive to poisons, easy to poison, and has a short service life, so strict requirements are placed on the refining purity of the raw gas. Because of the low operating pressure, the synthesis rate of ammonia is low, separation is difficult, and the process is complicated. This method is no longer used in actual industrial production. The process flow of synthetic ammonia can be roughly divided into four parts: preparation of raw gas; purification of raw gas; gas compression and synthesis of ammonia.
4. Other methods:
In the process of industrial continuous production of diphenylamine (under the action of catalyst), ammonia gas is produced as a byproduct. The ammonia gas is compressed and cooled to become liquid ammonia. During this process, sewage is discharged, which brings in impurities. Therefore, the purity of the obtained liquid ammonia is low and its use is limited.
