ABSTRACT
Just after the termination of World War II, the CEO of one of the largest industrial gas
companies advised his engineering department not to be too complacent in regard to the
expanding sales of industrial gases produced by cryogenic systems. Some day an enterprising
engineer would develop a machine in which air will be fed through one end, and its compon-
ents will come out separately from the other. Little did he realize at the time that his off-
the-cuff statement was prophetic. About five years later, a young engineer delivered a
technical paper at an AIChE meeting in New York City, demonstrating that it was possible
to separate air into oxygen and nitrogen by passing compressed air through a series of porous
membranes. Admittedly, the purity of the oxygen was not up to the desired standards, but it
could be improved by adding more membranes in series. The audience at the meeting, though
mildly interested, opined that the proposed process could not compete with the cryogenic
distillation processes already in use. They were wrong. Although the new idea has not
displaced the cryogenic processes completely, noncryogenic separation and purification of
gases has become a serious player in the industrial gas industry over the last several decades.
Permeation was the first process considered for noncryogenic separation of industrial gases.
Permeation is the diffusion of a substance in solution through a barrier. Permeability, on the
other hand, is the capacity of a porous material for transmitting a fluid. The standard unit
of permeability is the darcy, equivalent to the passage of 1 cm3 of fluid (having a viscosity of
1 cP) per second through a sample of 1 cm2 cross-sectional area, under a pressure
of 1.013 barA=cm of thickness. In the gas industry, the feed gas is labeled the permeator, and the selected component to be separated is known as the permeate. The membranes used
are thin, dense, and continuous films formed from cellulose acetate or polymers. The separ-
ation of a component in a gas mixture is carried out in three steps: the component must
dissolve in the membrane wall, diffuse through the membrane material, and be desorbed on
the opposite side of the membrane wall. This procedure may be defined by Henry’s law of
solubility-The solubility of a gas in a liquid is proportional to the partial pressure of the
gas-and by Fick’s law of diffusivity as expressed with the following equations.
