Membrane separation of air is primarily a physical process. Based upon specific characteristics of each molecule, such as size and permeation rate. the molecules in air can be separated to form mostly pure forms of nitrogen, oxygen, or both.
In a membrane system, there is a hollow tube filled with thousands of very thin membrane fibers. Each membrane fiber is another hollow tube in which air flows. The walls of the membrane fiber are porous and are specially made so oxygen molecules can permeate through the wall at a faster rate than nitrogen, allowing a nitrogen-rich stream to flow out the other end of the fiber. Meanwhile, the air outside the fiber, in the hollow tube, is now oxygen-rich and can be collected somewhere else.
The purity of the nitrogen generated depends primarily on two factors: the flow rate and air pressure. If we have high air pressure, the oxygen molecules have greater incentive to permeate through the fiver wall. If our flow rate is slower, then oxygen has more time to permeate through the fiber wall. We can easily adjust both of these factors to allow a system operator to vary the amount and purity of the nitrogen generated in a very short amount of time.