Page 18 - Adsorbents - fundamentals and applications

COMMERCIAL SORBENTS AND APPLICATIONS 3

with carbon molecular sieve. The feasibility for propane/propylene separations
by using AlPO 4 -14 has been demonstrated (see Chapter 10). The upgrading of
natural gas by removal of nitrogen from methane is a large potential application
for kinetic separation. This subject will also be discussed in Chapter 10.
For equilibrium separation, the starting point for sorbent design/selection is to
examine the fundamental properties of the targeted molecule that is to be adsorbed
(compared with the other molecules in the mixture): polarizability, magnetic
susceptibility, permanent dipole moment, and quadrupole moment. If the targeted
molecule has high polarizability and magnetic susceptibility, but no polarity,
carbon with a high surface area would be a good candidate. Sorbents with highly
polar surfaces (e.g., activated alumina, silica gel, and zeolites) would be desirable
for a targeted molecule that has a high dipole moment (and high polarizability). If
the targeted molecule has a high quadrupole moment, sorbents with surfaces that
have high electric field gradients are needed. Zeolites are the only such sorbents,
as the cations are dispersed above the negatively charged oxides on their surfaces.
Cations with high valences (i.e., charges) and small ionic radii would result in
strong interactions. The methodology for calculating these interactions is given
in Chapter 2 (for all sorbents) and Chapter 7 (for zeolites). The above discussion
applies only to the bonding between the targeted molecule and the adsorption site.
The targeted molecule also interacts with other atoms on the surfaces of the pore.
These interactions are secondary but are also important. Monte Carlo simulation
includes pairwise additivity and integrates the interactions over all sites. Sorbent
design/selection is a complex problem, because the process for which the sorbent
is used needs to be considered at the same time. For purification, particularly
ultrapurification, strong adsorption bonds are needed. Strong bond yields high
Henry’s constant, which leads to ultrahigh product purity. Sorbents that form
weak chemical bonds with the targeted molecule can be particularly useful. For
this type of sorbents, molecular orbital theory is the most powerful tool for
sorbent design, and is discussed in Chapter 8.
For kinetic separation, the pore size needs to be precisely tailored to lie
between the kinetic diameters of the two molecules that are to be separated.
Many microporous molecular sieves with various pore dimensions have been
synthesized (Hartman and Kevan, 1999), which have yet to be used as sorbents.

1.2. COMMERCIAL SORBENTS AND APPLICATIONS
Only four types of generic sorbents have dominated the commercial use of adsorp-
tion: activated carbon, zeolites, silica gel, and activated alumina. Estimates of
worldwide sales of these sorbents are (Humphrey and Keller, 1997)

Activated carbon $1 billion
Zeolites $100 million
Silica gel $27 million
Activated alumina $26 million