Nitrogen is a chemical element with chemical symbol N and atomic number 7. The common elemental form of nitrogen is nitrogen. Under standard conditions, it is a colorless, tasteless and odorless diatomic gas, which is not easy to produce chemical reactions. Nitrogen is the most abundant gas in the earth's atmosphere, accounting for 78.09% of the total volume.
physical features
Nitrogen is a colorless and odorless gas under normal circumstances, and it is usually nontoxic. Nitrogen accounts for 78. 12% (volume fraction) of the total atmospheric volume, and its gas density is 1.25g/L under standard conditions. Nitrogen is almost insoluble in water, and only about 0.02 volume of nitrogen is soluble in 1 volume of water at normal temperature and pressure. Nitrogen is a gas that is difficult to liquefy. Nitrogen will liquefy into colorless liquid at extremely low temperature, and white crystalline solid will be formed when the temperature is further lowered. In production, nitrogen is usually filled in black steel cylinders. Other physical properties are shown in the following table:
chemical property
It can also be seen from the oxidation state-Gibbs free energy diagram of nitrogen that N2 molecule with oxidation number 0 is at the lowest point of the curve except NH4 ion, which indicates that N2 is thermodynamically stable compared with other nitrogen compounds with oxidation number. The numerical values of various nitrogen compounds with oxidation number between 0 and +5 are above the connecting line between HNO3 and N2 (dotted line in the figure), so these compounds are thermodynamically unstable and prone to disproportionation. The only molecule in the figure whose molecular value is lower than N2 is NH4+ ion. [2]
From the oxidation state-Gibbs free energy diagram of nitrogen and the structure of N2 molecule, it can be seen that N2 is inactive, and nitrogen can react with hydrogen to produce ammonia only under the conditions of high temperature, high pressure and catalyst.
chemical synthesis
Nitrogen is mainly used for synthetic ammonia, and the reaction formula is N2+3H2=2NH3 (under the condition of high pressure, high temperature and catalyst). The reaction is reversible) or an important raw material for synthetic fiber (nylon, acrylic fiber), synthetic resin, synthetic rubber, etc. Nitrogen is a nutrient element and can also be used to make fertilizer. For example: ammonium bicarbonate NH4HCO3, ammonium chloride NH4Cl, ammonium nitrate NH4NO3, etc.
food service industry
Food packaging can be kept fresh by filling nitrogen.
Other uses
Light bulbs are used as nitrogen fertilizer and explosives and as refrigerants (liquid nitrogen vaporizes and absorbs heat).
car tyre
1. Improve tire stability and comfort. [3]
Nitrogen is almost an inert diatomic gas, and its chemical properties are extremely inactive. The gas molecules are larger than oxygen molecules, so it is not easy to expand when heated and contract when cooled, and its deformation range is small. The seepage velocity of the tire wall is about 30-40% slower than that of air, which can maintain a stable tire pressure, improve the stability of tire driving and ensure driving comfort. The audio conductivity of nitrogen is low, which is equivalent to 1/5 of ordinary air. Using nitrogen can effectively reduce the noise of tires and improve the quietness of driving.
2. Prevent tires from bursting and overturning due to lack of air.
Flat tire is the number one killer of road traffic accidents. According to statistics, 46% of traffic accidents in expressway are caused by tire failures, of which puncture accounts for 70% of the total number of tire accidents. When a car is driving, the tire temperature will rise due to friction with the ground, especially when driving at high speed and braking suddenly, the temperature of gas in the tire will rise rapidly, and the tire pressure will suddenly rise, so there will be the possibility of puncture. High temperature leads to tire rubber aging, fatigue strength decline and serious tread wear, which is also an important factor that may cause tire puncture. Compared with ordinary high-pressure air, high-purity nitrogen has the characteristics of low thermal expansion coefficient, low thermal conductivity and slow temperature rise, which reduces the speed of tire heat collection, is nonflammable and does not support combustion, so it can greatly reduce the probability of tire puncture.
3. Extend the service life of tires
After using nitrogen, the stable volume of tire pressure changes little, which greatly reduces the possibility of irregular friction of tire such as crown wear, shoulder wear and eccentric wear, and improves the service life of tire. The aging of rubber is caused by the oxidation of oxygen molecules in the air. After aging, its strength and elasticity decrease, and cracks will appear, which is one of the reasons for shortening the service life of tires. Nitrogen separation device can remove impurities such as oxygen, sulfur, oil and water in the air to a great extent, effectively reduce the oxidation degree of tire liner and rubber corrosion, and will not corrode metal rims, prolong the service life of tires and greatly reduce the rust of rims.
4. Reduce oil consumption and protect the environment.
Insufficient tire pressure will increase the rolling resistance after heating, which will increase the fuel consumption of the car when driving; Nitrogen can not only keep the tire pressure stable and delay the tire pressure reduction, but also is dry, oil-free and waterless, with low thermal conductivity and slow temperature rise, which reduces the temperature rise when the tire is walking, improves the tire grip, has small deformation and reduces the rolling resistance, thus achieving the purpose of reducing fuel consumption.
Laboratory preparation method
The basic principle of preparing a small amount of nitrogen is to oxidize ammonia or ammonium salt with a suitable oxidant. The most commonly used methods are as follows:
(1) heating ammonium nitrite solution: (343k) NH4NO2 = = = = N2+2h2o.
⑵ Interaction between sodium nitrite and saturated solution of ammonium chloride: NH4Cl+Nano2 = = NaCl+2H2O+N2.
(3) passing ammonia through red-hot copper oxide: 2NH3+3CuO = = 3cu+3H2O+N2.
⑷ Reaction between ammonia water and bromine water: 8NH3+3Br2 (AQ) = = 6NH4Br+N2.
5. Thermal decomposition of ammonium dichromate: (NH4 NH4) 2cr2o7 = = = N2 =+Cr2O3+4h2o.
(6) Very pure nitrogen can be obtained by heating sodium azide for thermal decomposition.
Nitrogen production by low temperature air separation
It is a traditional air separation technology with a history of more than 90 years. It is characterized by large gas production and high nitrogen purity, and can be directly applied to magnetic materials without further purification. However, its technological process is complex, it occupies a large area, the capital construction cost is high, it needs special maintenance force, there are many operators, and the gas production is slow (18 ~ 24h). It is suitable for large-scale industrial nitrogen production, and the cost of nitrogen production is 0.7 yuan/ton.
Nitrogen production by pressure swing adsorption
Pressure swing adsorption gas separation technology is an important branch of non-cryogenic gas separation technology, which is the result of people's long-term efforts to find a simpler air separation method than cryogenic method. In 1970s, Essen Mining Company of West Germany successfully developed carbon molecular sieve, paving the way for the industrialization of nitrogen production by pressure swing adsorption air separation. In the past 30 years, this technology has developed rapidly and matured day by day, and has become a strong competitor of cryogenic air separation in small and medium-sized nitrogen production field.
Nitrogen production by pressure swing adsorption takes air as raw material and carbon molecular sieve as adsorbent. By using the characteristics of selective adsorption of oxygen and nitrogen in air by carbon molecular sieve and the principle of pressure swing adsorption (pressure adsorption, decompression desorption, molecular sieve regeneration), oxygen and nitrogen are separated at room temperature to produce nitrogen.
Nitrogen production by membrane separation and air separation
Nitrogen production by membrane separation takes air as raw material. Under a certain pressure, oxygen and nitrogen are separated to produce nitrogen by using the different permeation rates of oxygen and nitrogen in hollow fiber membranes. Compared with the above two nitrogen production methods, it has the characteristics of simpler equipment structure, smaller volume, no switching valve, simpler operation and maintenance, faster gas production (within 3 minutes) and more convenient capacity expansion. However, the hollow fiber membrane has stricter requirements on the cleanliness of compressed air, and the membrane is prone to aging and failure, difficult to repair and needs to be replaced. Nitrogen production by membrane separation is more suitable for small and medium-sized users with nitrogen purity ≤98%, and the effect is the best at this time. When the purity of nitrogen is required to be higher than 98%, its price is about 30% higher than that of the pressure swing adsorption nitrogen plant with the same specification. Therefore, when membrane separation nitrogen production and nitrogen purification device jointly produce high-purity nitrogen, the purity of ordinary nitrogen is generally 98%, which will increase the production cost and operation cost of purification device.
Nitrogen purification method
Hydrodeoxidation method
Under the action of catalyst, the residual oxygen in ordinary nitrogen reacts with the added hydrogen to generate water, and the reaction formula is 2H2+O2=2H2O, and then the water is removed by post-drying, thus obtaining high-purity nitrogen, the main components of which are N2≥99.999%, O2 ≤ 5× 10-6, and H2 ≤1500× 60.
O≤ 10.7× 10-6. The cost of nitrogen production is about 0.5 yuan /m3.
Method for remove oxygen and hydrogen by hydrogenation
The method is divided into three stages, the first stage is hydrodeoxygenation, the second stage is dehydrogenation, and the third stage is dehydration to obtain high-purity nitrogen, which consists of N2 ≥ 99.999%, O2 ≤ 5× 10-6, H2 ≤ 5× 10-6 and H2O ≤/kloc. The cost of nitrogen production is about 0.6 yuan /m3.
Carbon deoxidation method
Under the action of carbon-supported catalyst (at a certain temperature), the residual oxygen in nitrogen reacts with the carbon provided by the catalyst itself to generate CO2. Reaction formula: C+O2=CO2. Then, CO2 and H2O are removed to obtain high-purity nitrogen with the following composition: N2 ≥ 99.999%, O2 ≤ 5× 10-6, CO2 ≤ 5× 10-6, H2O ≤10.7×/kloc-0-6. The cost of nitrogen production is about 0.6 yuan /m3.