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The air that we breathe is 78.09% N2, 20.95% Oxygen and the remainder is Argon, CO2, Water vapor and other trace gases. Separating air has been done cryogenically for over a hundred years. The cryogenic process involves fractional condensation of the gases into their separate components. Oxygen liquefies at -297F (-183C) whereas Nitrogen liquefies at -320F (-195C), so by maintaining air at -297F, all the oxygen will liquefy out of the mixture, then by holding the remaining gas at -320F, the nitrogen is separated as a liquid. The nitrogen liquid is then pumped into insulated trucks for delivery to the point of use. Needless to say, this 100+ year old technology is very energy intensive.


There are two methods of separating gases non-cryogenically — hollow fiber membranes and pressure swing adsorption technology.

Hollow fiber membranes are very interesting fibers. They are extruded through a spinneret like many fibers are, but a stream of dry nitrogen fills and creates an inner annulus to make a fiber with a center bore, much like a drinking straw, hence the name hollow fiber. The outer surface of the Parker fiber is non-porous, but extremely permeable as its thickness is less than 40 nanometers. Compressed air travels down the center bore while oxygen permeates through the permeable membrane wall at a rate about 3-4 times as fast as nitrogen, so by passing compressed air through the hollow fiber, oxygen permeates and the nitrogen does not. Thus by the time the air reaches the end of the fiber, most of the oxygen has been depleted. Bundle hundreds or thousands of fibers together and arrange them like a shell and tube heat exchanger and you can create a product that can handle thousands of cubic feet per hour.

Pressure swing adsorption (PSA) works much differently. A special material, called carbon molecular sieve (CMS), is about the size of a grain of rice. The CMS is able to hold on to (adsorb) oxygen under high-pressure, then release it (desorb) under low pressure. This sieve is loaded into cylindrical towers using a process called “snow storm filling”. One tower operates to remove the oxygen under pressure, while the other tower is held at low pressure where the oxygen is released. Then, on a predetermined cycle, the towers switch so that the one that was at high pressure holding oxygen is desorbed and re-generated at low-pressure while the other tower takes on the job of capturing the oxygen under pressure. This cycle progresses back and forth and is completely reversible and systems can operate for many years without the need for a reload of sieve. Our pre-filtration system keeps the CMS beds clean so the regeneration is not contaminated.

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