Page 17 - Adsorbents - fundamentals and applications

2 INTRODUCTORY REMARKS

Future applications of adsorption are limited by the availability of new and better
sorbents. Ideally, the sorbent should be tailored with specific attributes to meet
the needs of each specific application. Development of better sorbents can also
improve the performance of the current commercial processes. A good example
is the invention of the LiX (Si/Al = 1) zeolite (Chao, 1989). Air separation has
been performed by pressure swing adsorption, and the generic sorbents 13X (i.e.,
NaX) and 5A (i.e., CaA) zeolites were used prior to this invention. By switching
from NaX to LiX (Si/Al = 1), the productivity of oxygen increased instantly by
1.4–2.7 times and the power consumption reduced by 21–27% depending on the
operating conditions used (Leavitt, 1995).
The past two decades have shown an explosion in the developments of new
nanoporous materials. Tremendous advances have been made in our capabilities
to tailor the porosity and surface chemistry of oxide molecular sieves and new
forms of carbon (carbon molecular sieves, super-activated carbon, activated car-
bon fibers, carbon nanotubes, and graphite nanofibers). However, the potential use
of the adsorption properties of these new materials remains largely unexplored.

1.1. EQUILIBRIUM SEPARATION AND KINETIC SEPARATION

The adsorptive separation is achieved by one of three mechanisms: steric, kinetic,
or equilibrium effect. The steric effect derives from the molecular sieving proper-
ties of zeolites and molecular sieves. In this case only small and properly shaped
molecules can diffuse into the adsorbent, whereas other molecules are totally
excluded. Kinetic separation is achieved by virtue of the differences in diffu-
sion rates of different molecules. A large majority of processes operate through
the equilibrium adsorption of mixture and hence are called equilibrium separa-
tion processes.
Steric separation is unique with zeolites and molecular sieves because of the
uniform aperture size in the crystalline structure. The two largest applications
of steric separation are drying with 3A zeolite and the separation of normal
paraffins from iso-paraffins and cyclic hydrocarbons by using 5A zeolite (Yang,
1987). This type of separation is generally treated as equilibrium separation.
Although kinetic separation has had only limited applications, it holds high
potentials for many more. It is an option to consider when equilibrium separation
is not feasible. Air separation is a good example for which kinetic separation can
complement equilibrium separation. Air separation by PSA (i.e., pressure-swing
adsorption) using zeolite is based on the preferential adsorption of N 2 over O 2 .It
is hence used for the production of O 2 from air. N 2 constitutes about 78% of air.
If an O 2 -selective sorbent is used, air separation can be accomplished with about
1/4 of the work that is needed for the same separation by using zeolite. This is
particularly the case with nitrogen production form air. Oxygen diffuses about
30 times faster than nitrogen in carbon molecular sieve. Although the adsorption
capacity of carbon molecular sieve is only a fraction of that of zeolite, it is more
economical to use carbon molecular sieve for the production of nitrogen from air.
Separation of methane from CO 2 has also been performed by kinetic separation