What do stir fry and integrated circuits have in common? Both can be processed more effectively using nitrogen.

Liquid nitrogen freezing can be used to preserve the taste and texture of chocolate cakes.
Western diners are developing a growing appetite for dishes of the Far and Middle East. To meet the increasing demand for authentic Indian curries and oriental stir fry dishes, many producers of frozen ethnic food are turning to the use of liquid nitrogen. Cryogenic freezing with liquid nitrogen or carbon dioxide chills hot food more quickly than air blast freezers, which can aid in preserving taste and texture.

Nitrogen boils and becomes a gas at -320.8°F (-196°C). Because it is so cold, liquid nitrogen offers an environmentally friendly alternative to the use of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) as refrigerants in many freezing and chilling applications.

Stir fry cooking involves processing meats and vegetables at high temperatures, then cryogenic freezing them using liquid nitrogen.
Stir frying involves cooking sliced meats and vegetables quickly at high temperatures in a wok. To preserve the flavors and textures characteristic of this cuisine, the food product must not continue to cook during packaging. Cryogenic processing chills the food more quickly and prevents continued cooking.

In the cryogenic process, the cooked food empties directly from the wok into a mixer, where paddles toss it through a spray of liquid nitrogen. The food product is cooled to 41°F (5°C) within 10 to 12 min. The cooked ingredients then can be combined to make up different dishes and packaged for distribution. The quick chilling provided by the liquid nitrogen preserves the firm texture associated with stir fry meals.

Use of an inert nitrogen atmosphere during soldering allows PCB assemblers to employ CFC-free fluxes.

Out of Thin Air

Out of Thin Air Nitrogen is all around us - it makes up 78% of the air we breathe. At room temperature and pressure, nitrogen is a gas. But, unlike oxygen, the other major component of air, nitrogen is not very reactive. This inertness makes it useful for preventing unwanted chemical reactions that can cause an industrial process to fail.

Nitrogen plays an essential role in minimizing the environmental impact of the electronics assembly industry by helping reduce the use of ozone-damaging chlorofluorocarbons (CFCs). Also, it can enhance productivity and reduce defect levels.

The traditional wave and reflow soldering methods used to attach components such as chips, resistors and capacitors to printed circuit boards (PCBs) leave behind a residue of solids that must be removed. At one time, environmentally unfriendly CFCs provided the only satisfactory cleaning method. Now, use of an inert nitrogen atmosphere during soldering allows PCB assemblers to use fluxes (activated agents) to remove any oxide from the metal leads. These fluxes leave minimal residue behind, eliminating the need to clean the boards - and to use CFC cleaners - after soldering.

The nitrogen atmosphere also improves the surface properties of molten solder, making it easier to apply. This results in a lower percentage of defective connections. Additionally, nitrogen atmosphere reduces dross, the layer of oxidized lead that forms a slag on top of the molten solder and affects its flow.

Nitrogen plays an essential role in many industrial processes. Consider whether it may help yours.


How Nitrogen is Produced

Whether purchased in cylinders from a gas supplier or produced on site at your plant, nitrogen is farmed by separating it from the air. This is done either in cryogenic air separation units (ASUs) or at ambient temperatures using noncryogenic systems.

There are two main types of ASUs. A standalone ASU is located in an area where there is high demand for the unit's gaseous products. Gas is liquified for delivery to customers within a 200-mile radius. A piggyback ASU is located adjacent to a large customer, who receives a major portion of the unit's product via pipeline. The remaining product is liquified and distributed to other customers.

On-site production typically is through the use of a cryogenic generator, membrane noncryogenic generator, or pressure swing adsorption (PSA).

Cryogenic generator separation involves a compression of air, chilling and rapid expansion, followed by a step distillation of the different elements. The process represents over 90% of industrial gas production by volume.

In membrane generators, feed-air is passed through the bores of semipermeable hollow fiber membranes. Oxygen and other gases permeate through the membrane walls and are discharged as byproducts. Because the membrane walls are much less permeable to nitrogen, it flows along the fibers and is collected as a separate product stream.

PSA units exploit the difference in size between oxygen and nitrogen molecules to separate the gases by pushing air through a molecular sieve, a material which contains millions of narrow-necked pores.