Nitrogen gas and its usage in Steel Plant


Nitrogen gas and its usage in Steel Plant

Nitrogen is a non-reactive component of the atmosphere which is not life supporting. The percentage of nitrogen in air is 78.06 % by volume or 77 % by weight of the air. The composition of air is shown in Fig 1.

Composition of air 1

Fig 1 Composition of air

The element nitrogen was discovered as a separable component of air, by Scottish physician Daniel Rutherford, in 1772. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air.

Nitrogen is produced in large quantities and at high purity as a gas or liquid through the liquefaction and distillation of ambient air at the cryogenic air separation plants. It is also produced on commercial scales as a lower purity gas by adsorption technologies (pressure swing adsorption, PSA), or diffusion separation processes (permeation through specially designed hollow fibers). Gaseous nitrogen is called in short as GAN while the liquid nitrogen is called in short as LIN.

Liquid nitrogen is a cryogenic liquid. Cryogenic liquids are liquefied gases that have a normal boiling point below – 150 deg C. Liquid nitrogen has a boiling point of -195.8 deg C. Because the temperature difference between the product and the surrounding environment is substantial, it is necessary to keep the liquid nitrogen insulated from the surrounding heat.

Nitrogen is often stored as a liquid, although it is used primarily as a gas. Liquid storage is less bulky and less costly than the equivalent capacity of high-pressure gaseous storage. A typical storage system consists of a cryogenic storage tank, one or more vaporizers and a pressure control system. The cryogenic tank is constructed, in principle, like a vacuum flask. There is an inner vessel surrounded by an outer vessel. Between the vessels is an annular space that contains an insulating medium from which all the air has been removed. This space keeps heat away from the liquid nitrogen held in the inner vessel. Vaporizers convert the liquid nitrogen into a gaseous state. A pressure control manifold then controls the gas pressure that is fed to the process or application. Vessels used in liquid nitrogen service are to be designed for the pressure and temperatures involved. Piping design is to follow the norms meant for such piping.

Uses of nitrogen

Nitrogen is generally liquefied so that it can be more effectively transported and stored in large volumes. However, most applications use nitrogen after it is vaporized to the gaseous form. Nitrogen is valued for its inertness. It is used to shield potentially reactive materials from contact with oxygen. Nitrogen is widely used in a steel plant. The major usage of nitrogen in the steel plant is given below.

  • Used in production of steel in the primary steel making processes (combined blowing in basic oxygen furnace and slag splashing) and in secondary steel making processes (AOD process )
  • Used in the blast furnace for cooling of the gear box of top charging equipment
  • Used for pulverized coal injection in the blast furnace
  • Used for dry quenching of hot coke pushed from the coke oven battery
  • Used in protective gas for the annealing of the cold rolled steel
  • Used for purging of pipelines, tanks and equipment
  • Liquid nitrogen cooling properties are used to separate shrink fit bearings from the shaft. Conversely liquid nitrogen is also used for shrink fitting. In shrink fitting, instead of heating the outer metal part, the inner part is cooled by liquid nitrogen so that the metal shrinks and can be inserted.  When the metal returns to its normal temperature, it expands to its original size, giving a very tight fit.
  • Nitrogen gas is used for blanketing purpose. Nitrogen blanketing is used to protect flammable or explosive solids and liquids from contact with air.
  • Nitrogen gas is used for pneumatic conveying of combustible materials
  • Nitrogen is used for heat treating of steels (nitriding process)
  • Nitrogen is used for laser cutting, welding and brazing purposes

Properties of nitrogen

The CAS number of nitrogen is 7727-37-9 while the UN number for gas is UN1066 and for liquid nitrogen is UN1977.

Nitrogen is an element with the chemical symbol N and atomic number 7. Under standard temperature and pressure conditions, two nitrogen atoms join to form nitrogen gas which is a colourless, odourless, and tasteless gas. Nitrogen is a di atomic gas. Its chemical or molecular formula is N2 which means that one molecule of nitrogen gas contains two atoms of nitrogen. Its atomic mass is 14 and molecular weight is 28 grams/mole. The melting point and boiling point of nitrogen is -210 deg C and -195.8 deg C respectively. Its density is 1.16 kg/cum at 21.1 deg C. The density of liquid nitrogen at atmospheric pressure and boiling point is 808.9 kg/cum. It is slightly lighter than air and its vapour density is 0.967 (air=1). Liquid to gas expansion ratio of nitrogen is 1 to 694 at 20 deg C. Because of high expansion ration, nitrogen gas has got high expanding properties.

The critical temperature and critical pressure of nitrogen are – 146.9 deg C and 34.59 kg/sq cm respectively.

Nitrogen is a non-reactive gas. It can join up with other elements. The bonds are very effective because nitrogen’s outermost electron shell has few electrons. That is the reason why it is sometimes used as a buffer gas.

Nitrogen condenses at its boiling point to a colourless liquid which is lighter than water.  It is slightly soluble in water and its solubility in water at 1 atmosphere pressure and 0 deg C is 0.023 volume by volume which is around 20 mg/litre.

Nitrogen is nonflammable and does not support combustion. Though nitrogen is an inert gas yet it is not truly inert.  It forms nitric oxide and nitrogen dioxide with oxygen, ammonia with hydrogen, and nitrogen sulfide with sulfur.  Nitrogen compounds are formed naturally through biological activity.  Compounds are also formed at high temperature or at moderate temperature with the aid of catalysts. At high temperatures, nitrogen will combine with active metals to form nitrides.  Nitrogen is necessary for various biological processes.

Nitrogen in the air is main cause of generation of oxides of nitrogen (NOx) in various furnaces during the combustion process. NOx is a green-house gas and contributes toward global warming.

Nitrogen is compatible with all common materials of construction. Pressure requirements are to be considered when selecting materials and designing systems.

Safety aspects related with the use of nitrogen

Since nitrogen is odourless, colourless, tasteless, and nonirritating, it has no warning properties. Humans possess no senses that can detect the presence of nitrogen. Contact with rapidly expanding nitrogen near the point of release can cause frostbite, with redness, skin colour change to gray or white, and blistering. Nitrogen gas can be dangerous since it can dissolve in the bloodstream and body fats. Liquid nitrogen can cause some burns in the body.

Nitrogen is sometimes mistakenly considered harmless because it is nontoxic and largely inert. However it can act as a simple asphyxiant by displacing the oxygen in the air to levels below which is required to support life. In addition nitrogen gas stored in pressurized containers and systems is stored energy which can cause serious injury if released in an uncontrolled manner.

Nitrogen can displace oxygen in the air, reducing the percentage of oxygen to below safe levels (below 19.5 %). At low oxygen concentrations, unconsciousness and death may occur in seconds and without warning.

No adverse ingestion effects are anticipated, but nitrogen is a simple asphyxiant. Effects of oxygen deficiency resulting from simple asphyxiates may include rapid breathing, diminished mental alertness, impaired muscular coordination, faulty judgment, depression of all sensations, emotional instability, and fatigue. As asphyxiation progresses, nausea, vomiting, prostration, and loss of consciousness may result, eventually leading to convulsions, and coma.

First aid measures are not required for gas. If frostbite is suspected, eyes are to be flushed with cool water for 15 minutes and immediate medical attention is to be obtained. For frostbite, skin is to be immersed in lukewarm water. No hot water is to be used.

In cases of inhalation and overexposure, prompt medical attention is required. Rescue personnel are to be equipped with self-contained breathing apparatus. Victims are to be assisted to an uncontaminated area so that they can inhale fresh air. Quick removal from the contaminated area is most important. Unconscious persons are to be moved to an uncontaminated area, and if breathing has stopped, artificial resuscitation and supplemental oxygen is to be given.

For preventing oxygen deficiency, areas where nitrogen is used need sufficient ventilation. At least four to six changes of fresh air per hour is to be provided, depending on room size, the quantity of nitrogen, and the presence of oxygen monitoring system. Design features should also include pressure relief devices to vent nitrogen to a safe area outside.

Oxygen detector is to be used in the area where nitrogen concentration is expected. Systems under pressure are to be regularly checked for leakages. Work permit system is to be used in the area for carrying out any work.

Nitrogen cylinders are to be used only in the ventilated area. They are to be handled carefully and not to be lifted by their valve protection cap. They are to be protected from physical damage and are not to be dragged, rolled, slided, or dropped. A purpose built trolley is to be used to move them. The cylinders are to be kept chained or clamped to prevent them from falling over. They are to be stored upright, with valve protection cap in place, and firmly secured to prevent falling or being knocked over in a well-ventilated storage area or compound. Nitrogen cylinder temperatures are not to exceed 52 deg C.

Containers containing liquid nitrogen are not to be sealed because of the danger of explosion. Any caps/lids must be vented, with a sufficient aperture to prevent blocking by ice.

Nitrogen is nonflammable. A nitrogen cylinder may vent rapidly or rupture violently from pressure when involved in a fire situation. Extinguishing agent suitable for the surrounding fire is to be used in case of a fire. Although most cylinders and containers are designed to vent contents when exposed to elevated temperatures, note that pressure in a container can build up due to heat and it may rupture if a pressure relief device should fail to function. Firefighters should wear respiratory protection (SCBA) and full turnout or Bunker gear. Fire-exposed containers are continued to be cooled until well after flames are extinguished.