Each membrane separator contains thousands of asymmetric hollow fibers that act as a molecular filter. When high-pressure gas mixtures enter the separator, the gas components divide as a result of selective permeation. Fast gases (such as oxygen) permeate easily through the membrane wall and exit the side port. Slow gases (such as nitrogen) have a difficult time passing through the membrane wall and travel the bore of the fiber exiting through the port at the end of the shell.