Nitrogen, a simple substance formed by nitrogen element, has the chemical formula N ₂.It is a colorless and odorless gas under normal temperature and pressure. It can react with hydrogen to generate ammonia only under high temperature, high pressure and catalyst conditions, and can combine with oxygen to generate nitric oxide under discharge;Even active metals such as Ca, Mg, Sr and Ba can react with them only when heated.
This high chemical stability of nitrogen is related to its molecular structure. Two N atoms combine with three bonds to form a nitrogen molecule, which contains one σ bond and two π bonds. Because the π bond is the first one to be attacked in the chemical reaction, and the energy level of the π bond in the N ₂ molecule is lower than the σ bond, so it is difficult to open the π bond, which makes it difficult for N ₂ to participate in the chemical reaction.[1]
Nitrogen is the 30th abundant element on the earth.Considering that nitrogen accounts for 4/5 of the atmosphere, that is, more than 78% of the atmosphere, almost unlimited nitrogen can be used.Fractionation of liquid air is often used in industry to obtain a large amount of nitrogen[2]
Swedish chemist Carl Scheele and Scottish botanist Daniel Rutherford discovered nitrogen respectively in 1772.Reverends Cavendish and Lavoisier also independently obtained nitrogen at about the same time.Inspired by his teacher Joseph Black, Rutherford used KOH to remove CO when he studied the properties of the residual "air" left by carbon containing substances after burning in a limited amount of airtwo, thus obtaining nitrogen.He thought it was ordinary air that had absorbed phlogiston from the burned material.Some people disregard A50. Until 1840, Lavoisier's research results were still arguing about the basic properties of nitrogen.
The name Nitrogen was proposed by Jean Antoine ClaudeChaptal in 1970, based on the consideration that it is a component of nitric acid and nitrates (ό ό ό ό όόό, nitrates).Because of the asphyxiation of this gas, Lavoisier prefers to use the name azote (nitrogen) (in Greek ≤ ψ ν χη, lifeless), and this name is still used in grammar in the form of azo, diazao, azide, etc.The German name stickstoff refers to the same nature (stuck, suffocation or suffocation).[8]
Physical and chemical properties
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physical property
appearance
Colorless and odorless gas
Solubility
Slightly soluble in alcohol and water (2.4 ml nitrogen can be dissolved in 100 ml water at 273 K and 100 kPa)
volume fraction
Volume fraction in atmosphere: 78.1%[6]
melting point
-209.86℃[5]
boiling point
-196℃[1]
relative density
0.81 (- 196 ℃, water=1);Relative vapor density 0.97 (air=1)
A σ bond and two π bonds are formed between two nitrogen atoms in the nitrogen molecule.Similar to CO, CtwoHfourEquivalent molecular ratio, NtwoBonding molecular orbital σ2p(-15.59 eV) and π2p(-16.73 eV) energy is relatively low, anti bonding molecular orbital π*2p(8.17 eV) energy is relatively high, it is not only difficult to accept electrons, but also difficult to give electrons. It has strong stability. The dissociation energy is up to 945 kJ/mol, and it does not decompose even at 3273 K.[6]
Molar refractive index
three point eight seven
Molar volume (cmthree/mol）
thirty-one point nine
Isotonic specific volume (90.2 K)
seventy point five
Surface tension (dyne/cm)
twenty-three point six
Polarization (10-24cmthree）
one point five three[7]
chemical property
Nitrogen is an inert gas and generally does not react with other substances. However, under certain conditions, nitrogen can react with alkali metals or alkaline earth metals, which is equivalent to filling an electron on the anti bond molecular orbital of nitrogen molecules. The stronger the electron giving ability of metals, the easier the reaction will be.For example, lithium can react directly with nitrogen at room temperature, while calcium needs to be heated to certain conditions to react with nitrogen:
Some elements of Ⅲ A and Ⅳ A groups can react with nitrogen under heating conditions:
Under the conditions of high temperature, high pressure and the presence of catalyst, nitrogen can react with hydrogen to produce ammonia:
Nitrogen can react with oxygen to generate NO under discharge conditions:[6]
Nitrogen can also be oxidized to NO under catalysistwo：[12]
Preparation method
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Industrial preparation
Liquid air fractionation
Nitrogen is mainly produced by separation from the atmosphere or decomposition of nitrogen-containing compounds.More than 33 million tons of nitrogen is produced annually through liquefied air, and then nitrogen and other gases are produced in the atmosphere by fractionation.[1-2]
Cryogenic separation method
The cryogenic separation process has experienced more than 100 years of development, and has experienced a variety of different processes such as high pressure, high and low pressure, medium pressure and full low pressure processes.With the development of modern air separation process technology and equipment, high pressure, high pressure and low pressure air separation processes have been basically eliminated, and full low pressure processes with lower energy consumption and safer production have become large and medium-sized low-temperature processesAir separation unitPreferred by.
The full low-pressure air separation process can be divided into external compression process and internal compression process according to the different compression links of oxygen and nitrogen products.The full low pressure external compression process produces low pressure oxygen or nitrogen, and then the product gas is compressed by an external compressor to the required pressure for users.The full low pressure internal compression process will pressurize the liquid oxygen or liquid nitrogen generated by distillation in the cold box to the pressure required by the user through the liquid pump, and then vaporize it, and supply it to the user after reheating in the main heat exchanger.The main process includes raw air filtration, compression, cooling, purification, pressurization, expansion, rectification, separation, reheating and external supply.[3]
Cryogenic separation process
Membrane separation method
Membrane separation technology is based on the characteristics of selective permeation and diffusion of gas components in the membrane to achieve the purpose of gas separation and purification.The speed of various components in the gas passing through the membrane is different, and the speed of each component passing through the membrane is related to the nature of the gas, the characteristics of the membrane, and the differential pressure between the two sides of the membrane.The gas component penetrating the membrane cannot reach 100% purity.Gas separation membranes can generally be divided into porous materials and non porous materials, which are inorganic（Porous glass, ceramics, metals, electronic conductive solids and palladium alloys) or organic polymers (microporous polyethylene, porous acetate fiber, homogeneous acetate fiber, polysiloxane rubber and polycarbonate).[4]
The purified compressed air passes through the buffer tank and the combined filter and enters from one end of the membrane group. Under the pressure, the gas molecules first contact the high-pressure side of the membrane.The mixed gas dissolves in the membrane at different solubility on the high pressure side surface of the membrane, and then the molecules of the mixed gas diffuse to the low pressure side of the membrane at different speeds under the pressure difference between the two sides of the membrane.After the selection of two processes of dissolution and diffusion, the mixed gas is finally separated into various components.For example, the permeability of air and oxygen is faster than that of nitrogen. After membrane separation, the gas left at the high-pressure side is rich in nitrogen, while the gas passing through is rich in oxygen.[4]
Technological process of membrane separation
PSA method
This method uses compressed air as raw material and molecular sieve as adsorbent. Under a certain pressure, oxygen is enriched in the adsorption phase and nitrogen is enriched in the gas phase within a certain time by making use of the difference in the adsorption amount of oxygen and nitrogen molecules on the surface of different molecular sieves;After pressure relief, molecular sieve adsorbent is regenerated and recycled.[3]Besides molecular sieve, adsorbent can also be usedActivated alumina, silica gel, etc.[4]
At present, pressure swing adsorption is commonly usedNitrogen production unitIt is made of compressed air,Carbon molecular sieveAs an adsorbent, oxygen and nitrogen can be separated by making use of the differences in adsorption capacity, adsorption rate, adsorption capacity, etc. of oxygen and nitrogen on carbon molecular sieves and the characteristics that molecular sieves have different adsorption capacities for oxygen and nitrogen with different pressures.First, oxygen in the air is preferentially adsorbed by carbon molecular sieves, thus enriching nitrogen in the gas phase.In order to obtain nitrogen continuously, two adsorption towers need to work alternately.[4]
PSA process
Ammonia decomposition method
Under high temperature and nickel catalysis, ammonia gradually decomposes into nitrogen and hydrogen:
Then, the mixture is burned in the combustion chamber and the air proportion is controlled, so that hydrogen is incompletely burned. The combustion products can be deoxidized and dried to obtain protective gases with different nitrogen hydrogen mixture ratios (the hydrogen content can be controlled between 1% and 25%), which are used for bright annealing of copper (this method is only applicable when the nitrogen and hydrogen sources are difficult and the ammonia price is low).[9]
Laboratory preparation
Ammonium nitrite decomposition method
The laboratory usually heats the mixture of saturated ammonium chloride solution and solid sodium nitrite to prepare nitrogen:[6]
Ammonium dichromate decomposition method
Ammonium dichromate can release nitrogen after thermal decomposition:[8]
Ammonia production of nitrogen
Ammonia and bromine water or copper oxide can generate nitrogen at high temperature:[8]
Sodium azide decomposition method
At 300 ℃, sodium azide decomposes under careful control:
The purity of nitrogen produced by this method is high.[8]
application area
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1. The chemical property of nitrogen is very stable, and it generally does not react with other substances.This inert quality makes it widely used in many anaerobic environments. For example, nitrogen is used to displace the air in a specific container, which plays the role of isolation, flame retardancy, explosion-proof and anti-corrosion. This technology has been applied in industrial and civil fields such as light hydrocarbon unit maintenance, LPG engineering, gas transmission pipeline and liquefied gas pipeline network purging[11]。Nitrogen can alsoIt is used as cover gas in the packaging of processed food and medicine, sealing cables, telephone lines, and pressurizing expandable rubber tires.As a preservative, nitrogen is often displaced with the well to slow down the corrosion caused by the contact between the string and the formation fluid.[8]
2. High purity nitrogen is used to refine the metal melt in the metal melting and casting process to improve the quality of the casting billet.Charcoal furnace gas based on nitrogen (composition: 64.1% Ntwo，34.7%CO，1.2%HtwoAnd a small amount of COtwo）As a protective gas during copper melting and casting, the copper melt is free from oxidation on the casting surface, thus ensuring the product quality.[9]
liquid nitrogen
3. About 10% of the nitrogen produced is used as refrigerant, mainly including: coagulation and grinding of usually soft or rubber like substances, low-temperature processing of rubber, cold shrink fitting and installation of engineering technical parts, biological specimens, such as the preservation of blood, refrigeration during transportation, etc.[8]
4. Nitrogen can be used to synthesize nitric oxide or nitrogen dioxide to produce nitric acid, which has high purity and low price.In addition, nitrogen can also be used for synthetic ammonia and metal nitrides.[2]
Safety measures
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Health hazards
The nitrogen content in the air is too high, which will reduce the oxygen partial pressure of the inhaled gas and cause hypoxia and asphyxia.When the concentration of inhaled nitrogen is not too high, the patient initially feels chest tightness, shortness of breath, and weakness;Then there are restlessness, extreme excitement, running, shouting, absent-minded, unsteady gait, called "nitrogen intoxication", which can lead to lethargy or coma.Inhalation of high concentration can lead to rapid coma and death due to respiratory and cardiac arrest.
When divers dive deeply, they may have the anesthetic effect of nitrogen;If you change from a high-pressure environment to a normal pressure environment too quickly, nitrogen bubbles will form in your body, compressing nerves and blood vessels or causingcapillaryBlocking“Decompression sickness”。[10]
Hazard prevention
Emergency treatment method: quickly evacuate the personnel in the leakage pollution area to the windward place, isolate them, and strictly restrict access.It is recommended that emergency personnel wear self-contained positive pressure respirators and general work clothes.Cut off the leakage source as much as possible.Reasonable ventilation to accelerate diffusion.Leaking containers shall be properly treated, repaired and inspected before reuse.
Operation method: closed operation, providing good natural ventilation conditions, operators must be specially trained and strictly abide by the operating procedures.Prevent gas leakage into the air of the workplace.Loading and unloading shall be carried out with care to prevent the cylinder and accessories from being damaged, and leakage emergency treatment equipment shall be provided.
If excessive nitrogen is inhaled, leave the site quickly to a place with fresh air to keep the respiratory tract unobstructed.If breathing is difficult, give oxygen.When breathing and heartbeat stop, perform artificial respiration and external chest compression immediately, and seek medical advice in time.[10]
Storage and transportation
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Storage method
Store in a cool and ventilated warehouse.Keep away from kindling and heat sources.The warehouse temperature should not exceed 30 ℃.The storage area shall be equipped with leakage emergency treatment equipment[10]。
Transport method
When steel cylinders are used for transportation, safety helmets on steel cylinders must be worn.Cylinders are generally placed horizontally, and the mouth of the cylinder should be in the same direction, not crossed;The height shall not exceed the protective fence of the vehicle, and shall be firmly clamped with triangular wood pad to prevent rolling.It is strictly prohibited to load and transport with combustibles or combustibles.Transport in the morning and evening in summer to prevent sunlight exposure.It is forbidden to slip during railway transportation.[10]
