In everything from electronics to steel-making, environmentally-friendly soldering to heat treatment and fabrication to food production, not to mention environmental protection, nitrogen has much to offer. The reason: as a gas it is very inert, and with a boiling point of -196oC, as a liquid, nitrogen is very cold. It is also very abundant - nitrogen makes up 78 percent of the air we breathe - and is easily separated from air.
Nitrogen is used extensively in industrial processes that need to take place in an oxygen-free environment, for example heat treatment of metals, and in industries such as chemicals, glass and electronics. Because it is so cold, liquid nitrogen offers an ideal environmentally-friendly alternative to the use of hydrofluorocarbons (HFCs) and hydrocarbofluorocarbons (HCFCs) as a refrigerant in freezing and chilling applications. It also plays an essential role in applications involving high temperature superconductors.
Industry's flexible friend
Nitrogen gas has a wide application in a range of industrial processes where it is often used for displacing unwanted gases and vapours. In the chemicals, electronics and metals industries, nitrogen gas is used to push, or purge, unwanted gases or hydrocarbons out of process equipment. Nitrogen can also be used to displace other dissolved gases from solution in a process known as sparging. In nitrogen inerting, a blanket of nitrogen gas is used to exclude oxygen, water vapour or other chemical vapours from a storage vessel or reactor.
The chilling properties of liquid nitrogen are put to use in a number of industrial applications. For example, in cryogenic grinding, liquid nitrogen is used to chill or freeze materials down to their embrittlement stage (for example, plastics or rubber) so that they can be ground to a powder in high speed mills. Cryo-grinding can be used for recycling of waste plastic for use in products such as epoxy paint.
Liquid nitrogen also plays a key role in cooling high temperature superconductors. It is used to pre-cool the superconducting magnets used in magnetic resonance imaging scanners down to 77 K (-196oC), before final cooling to their operating temperature of 4.12 K (-296oC) with helium. It also plays a key role in the use of the superconducting wires used to boost the reliability of electricity distribution systems. Liquid nitrogen was used recently in a 350m long high temperature superconducting cable installed in the electricity distribution system in Albany, New York, US. The cable was designed to prevent major blackouts, such as the one which hit New York and Canada in 2003. In the cable liquid nitrogen flows through a vacuum-jacketed conduit where it bathes high temperature superconducting wire which is wrapped around a central copper coil.
Nitrogen's cooling properties are also used to cool and control exothermic (heat generating) chemical reactions. Chemical reaction cooling is an important process used by manufacturers of pharmaceuticals and fine chemicals.
Food for thought
In food processing nitrogen is widely used for chilling, freezing and modified atmospheric packaging. Liquid nitrogen is ideal for applications where quick chilling is an advantage. For some food freezing applications, such as individual quick frozen products (diced meat, sea foods etc.), liquid nitrogen is the only way. In other cases, liquid nitrogen provides a cost-effective alternative to carbon dioxide-based and mechanical refrigeration techniques.
On the food processing side, liquid nitrogen plays an important role in the cryo-grinding of spices such as nutmeg. In normal spice milling processes, the heat generated tends to drive off the aromatic and volatile oils and hence a lot of the aroma is lost. But by using liquid nitrogen to chill the spices while grinding, the oils are preserved, and the ground spices are of higher quality.
As an inert, odourless and tasteless gas, with a low solubility in water and fats, nitrogen also plays an important role in modified atmosphere packaging (MAP) and controlled atmosphere packing (CAP), where various percentages of nitrogen, argon, carbon dioxide and oxygen are used to form the atmospheres, or gas mixtures, to keep foods fresh for longer. In CAP these atmospheres are fed continuously into warehouses and containers to keep stored fruits and vegetables in top condition. The nitrogen in the gas mixture is chemically inert, odourless and tasteless and has a low solubility in water and fats. By excluding oxygen, it inhibits oxidation, bacteria, mould, yeast and insect activity, and prevents the oxidation of unsaturated fats.
MAP is now widely used to extend the shelf life of refrigerated foods ranging from salads to meats, fresh fruits and vegetables and baked goods. For food producers, MAP combines good product presentation with extended shelf life. For consumers, MAP is proving to be a convenient way to buy preservative-free prepared healthy foods - as the growing sales of those gas-filled bags of fresh salad greens demonstrate. And for junk food lovers, it is also used to preserve foods such as crisps, peanuts and other fat-rich snacks.
The cooling power of liquid nitrogen also plays an important role - stress screening, a standard procedure carried out by manufacturers of electronic components. In stress screening, components are placed in a special chamber than subjected to a number of heating and cooling cycles then tested to make sure they still work properly. In a typical stress screening, electronic components are heated to +40oC, the cooled rapidly down to -40oC using liquid nitrogen for 5 or 6 cycles. Components used in precision instruments or in critical applications, such as those used in computers in the nose cones of airplanes are subjected to a greater number of cycles. Those used in consumer items such as televisions typically undergo fewer cycles. But in all cases, the goal is the same - to make sure that electrical goods work first time, every time.
Nitrogen: how it's made
World wide over 1 million metric tons per day of nitrogen are produced by separating air into oxygen, nitrogen and argon. Small-scale users can receive gaseous nitrogen in cylinders or liquid nitrogen in tankers. For larger-scale users the nitrogen can be produced on-site using either cryogenic methods, which take place at very low temperatures (-196oC), or by means of non-cryogenic methods, such pressure swing adsorption (PSA) or membrane non-cryogenic generators, which are carried out at ambient temperatures.
The higher purities are produced using cryogenic methods. Cryogenic separation, generally carried out in air separation units (ASUs), relies on the fact that different gases in air have different boiling points and takes advantage of a fractional distillation process similar to that used to make products such as whisky and petrol or gasoline. Nitrogen is often manufactured as a co-product with oxygen in large ASUs located on customers' sites or delivered by pipeline. Gaseous nitrogen of various purities - including ultra-high purity for the semiconductor industry - can be manufactured in cryogenic nitrogen generators of varying size. These work like ASUs, but produce only nitrogen in a single distillation column. Many are specially designed with a small footprint to meet the needs of chip producers, where ultra high purity nitrogen is a necessity and floor space is at a premium.
In contrast, the non-cryogenic methods, such as pressure swing adsorption (PSA) and membrane separation take advantage of the fact that the different gases have different sizes of molecules which behave differently when exposed to various types of surface. The non-cryogenic methods are typically used to produce lower purity nitrogen with purities up to 99.9 percent.
This wide range of production options is essential because nitrogen finds uses in so many different industries, each with its own special requirements regarding purity, quantities, availability and cost.
How nitrogen is used
•For blow mould and blown film cooling
•For concrete cooling and ground freezing
•For environmental testing and stress screening
•Shrink fitting metals
•For stress screening for electrical components
•In chemical reaction cooling
•For pressure transfer and pressure testing
•In the control of volatile organic compounds (VOCs) and solvent recycling
•In enhanced oil recovery
•In controlled atmospheres used for annealing, galvanising, hardening and tempering
Processes in the metals industry
•In powder technologies such as atomising, brazing, thermal spraying and cryo-grinding
•In modified atmosphere packaging (MAP) to keep foods fresher longer and in controlled
•Atmosphere packaging (CAP) to store and transport fruit and vegetables.