Introduction
Liquid ammonia, also known as anhydrous ammonia, is a colorless liquid with a strong irritating odor. As an important chemical raw material, ammonia is usually obtained as liquid ammonia by pressurizing or cooling the gaseous ammonia gas for convenient transportation and storage. Liquid ammonia is easily soluble in water, and when dissolved in water, it forms ammonium ions NH4+ and hydroxide ions OH-, which are alkaline solution. Liquid ammonia is stored in pressure-resistant 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 it has a high incidence of chemical accidents.
1. Product use
Liquid ammonia is mainly used in the production of nitric acid, urea and other chemical fertilizers, and also used as a raw material for medicine and pesticides, according to the 2013-2017 China Liquid Ammonia Industry Research and Investment Prospect Assessment Report.
In the defense industry, it is used to manufacture propellants for rockets and missiles. It can be used as the raw material for the ammoniation of organic chemical products, because liquid ammonia is transformed into gas ammonia after gasification, which can absorb a large amount of heat, and is known as “refrigerant”, while liquid ammonia has a certain bactericidal effect, so it is used for sterilization and cooling and refrigeration in the poultry breeding industry. Liquid ammonia can also be used for textile mercerization, NH3 molecule in the lone electron pair tends to and other molecules or ions to form coordination bonds, generating various forms of ammonia compounds.
Liquid ammonia is a good solvent, due to the nature of the molecule and the existence of hydrogen bonds, liquid ammonia in many physical properties with water is very similar. Some active metals can replace hydrogen in water and form hydroxides, which are not as easily replaced in liquid ammonia. However, liquid ammonia can dissolve metals to produce a blue solution. This liquid ammonia solution of metals conducts electricity and slowly decomposes to give off hydrogen gas, which is strongly reducing.
For example, sodium metal dissolved in liquid ammonia when it loses its valence electrons to generate positive ions: liquid ammonia heated to 800 ~ 850 ° C, under the action of nickel-based catalysts, ammonia decomposition, you can get a mixture of hydrogen and nitrogen containing 75% H2, 25% N2 gas. The gas obtained by this method is a good protective gas, which can be widely used in semiconductor industry, metallurgical industry, and other industries and scientific research that need protective atmosphere.
Ammonia and acetic acid gas are heated to 420℃, and acetonitrile is synthesized under the action of catalyst.
2. the synthesis of ammonia
The synthesis of ammonia is the process of ammonia production, and its task is to synthesize the refined hydrogen and nitrogen mixture into ammonia under the action of catalyst in a fast and economical way. For the synthesis system, liquid ammonia is its product.
Industrial ammonia synthesis of various processes are generally categorized by the level of pressure.
3. the synthesis of ammonia method
1, high-pressure method: operating pressure of 70 ~ 100MPa, the temperature is 550 ~ 650 ℃. The main advantage of this method is that the ammonia synthesis rate is high, the ammonia in the mixture is easy to be separated. Therefore, the process and equipment are more compact. However, because of the high synthesis rate, more heat is released, the catalyst temperature is high, easy to overheat and lose activity, so the service life of the catalyst is shorter. And because of the high temperature and high pressure operation, the equipment manufacturing, material requirements are higher, the investment cost is large. This method is seldom used in industry.
2, medium-pressure method: operating pressure of 20 to 60MPa, temperature 450 to 550 ℃, its advantages and disadvantages between the high-pressure method and the low-pressure method, this method is more mature technology, the economy is better. Because the determination of the synthesis pressure, no more than from the equipment investment and compression power consumption of these two aspects to consider. From the point of view of power consumption, the power consumption of the synthesis system accounts for the proportion of the total power consumption of the plant. Power consumption not only depends on the pressure term, but also depends on other process indicators and the arrangement of the process. In general, in the range of 15 ~ 30Pa, the difference in power consumption is not great, so the world uses this method a lot.
3, low-pressure method: operating pressure of about 10MPa, the temperature of 400 ~ 450 ℃. Because the operating pressure and temperature are relatively low, so the equipment requirements are low, 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 be poisoned, and the service life is short, so the purity of the raw gas refining requirements are strict. And because of the low operating pressure, the ammonia synthesis rate is low, the separation is more difficult, and the process is complicated. The actual industrial production of this method has not been used. Ammonia synthesis process can be roughly divided into: the preparation of raw gas; raw gas purification; gas compression and ammonia synthesis of four major parts.
4, other methods: aniline industrialized continuous production of diphenylamine (under the action of the catalyst) in the process of by-products of ammonia generation, the ammonia compressed and cooled down into liquid ammonia. In this process to carry out sewage, and impurities, so the liquid ammonia purity is lower, the use is limited.
