Compressed Nitrogen is nitrogen in a liquid state at an extremely low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colorless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.43. Liquid nitrogen is often referred to by the abbreviation, LN2 or "LIN" or "LN" and has the UN number 1977. At atmospheric pressure, liquid nitrogen boils at −196 °C (77 K; −321 °F) and is a cryogenic fluid which can cause rapid freezing on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks. |
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Compressed Nitrogen can easily be converted to a solid by placing it in a vacuum chamber pumped by a rotary vacuum pump. Liquid nitrogen freezes at 63 K (−210 °C; −346 °F). Despite its reputation, liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen rather than liquid nitrogen alone.
Production :-
Liquid nitrogen is produced commercially from the cryogenic distillation of liquified air. An air compressor is used to compress filtered air to high pressure; the high pressure gas is cooled back to ambient temperature, and allowed to expand to a low pressure. The expanding air cools greatly (the Joule–Thomson effect), and oxygen, nitrogen, and argon are separated by further stages of expansion and distillation. Liquid nitrogen may be produced for direct sale, or as a byproduct of manufacture of liquid oxygen used for industrial processes such as steelmaking. Liquid air plants producing on the order of tons per day of product started to be built in the 1930s but became very common after the Second World War; a large modern plant may produce 3000 tons/day of liquid air products.
Features :-
- Odourless and colourless
- Non-toxic
- Does not support combustion
- An asphyxiant (does not support life)
- Non-reactive except at very high temperatures
- Does not react with oxygen at low temperatures
- Inert, dry and slightly soluble
- Available in 300-bar MCPs
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Benefits :-
- Inert to nearly all substances at ambient temperatures and moderate conditions.
- Nitrogen keeps laser-cut faces oxide-free due to the chemical inertness to react with the atmosphere.
- At higher temperatures, nitrogen can react to form metal nitrides.
- Nitrogen gas is only slightly soluble in a wide range of liquids and is therefore used to flush unwanted gases from solutions.
- Reduces internal degradation of the rubber in tyres due to oxidisation.
- As a cryogenic liquid, nitrogen can be used to thermally contract metals.
Applications and Uses :-
- Used for blanketing and purging applications to displace or dilute all unwanted gas or vapour.
- Widely used for reasons of safety and the prevention of product degradation in the chemicals and petroleum industry :-
- Inerting reactors and storage tanks
- Purging vessels and pipelines
- Sparging
- Pressure-transfer of liquids
- Shielding or assist gas for plasma-cutting aluminium and stainless steel, and as an assist gas for laser-cutting stainless steel TIG-welding of tough-pitch copper.
- Heat-treatment processes to protect some of the most reactive metals.
- In electrochemical analysis, can be bubbled through to displace any oxygen.
- Pressure-transfer, where the movement of liquids or slurries from one piece of process plant to another is effected using gas pressure.
- Tyre inflation: helps to extend tyre life and prevent tyre bursts.
- Pressurising accumulators for batch-testing applications.
- Can be used for shrink-fitting: the intense cold of liquid nitrogen (-196°C), reduces the physical dimensions of a component placed in the liquid. Once cooled, the component is connected to a mating component before swelling to its original size as it reaches room temperature, thereby securing itself in place.