Page 33 - Adsorbents - fundamentals and applications

18 SORBENT SELECTION: CRITERIA

Consideration should also be given to other factors. Activated carbon is the
only major commercial sorbent used for wet gas stream processing. (A pre-dryer
is required for other sorbents.) Sorbent deactivation, primarily by coke deposi-
tion, is an important consideration in the processing of hydrocarbon containing
gases. Coke is formed catalytically, and zeolites are excellent catalysts for these
reactions due to their acidities. Pore-size distribution can play a role in the LUB,
but not as important as the equilibrium isotherm, since the commercial sorbent
pellets are designed to minimize the pore-diffusion resistance. Kinetic separa-
tion, that is, separation based on the difference between pore diffusivities of two
gases, has found only one major application: the production of nitrogen from
air by molecular sieve carbon. Dehydration of cracked gases with 3A zeolite
and the separation of normal and iso-paraffins with 5A zeolite are based on
selective molecular exclusion. All other commercial processes are based on the
equilibrium isotherms. Temperature for activation and regeneration of the sorbent
◦
should also be considered. A high temperature (in the 300 C range) is required
for zeolites, whereas activated carbon usually requires the lowest temperature for
regeneration.
The total void space in the bed, which varies with the sorbents, is also an
important factor. A low void space is desired for high product recoveries because
the gas mixture remaining in the void space of the saturated bed is usually not
recovered as a useful product. Silica gel and activated alumina have the lowest
void fractions, usually slightly below 70%. Activated carbon has the highest void
fraction, at nearly 80%.
Bulk separation refers to separation of a mixture that contains over approxi-
mately 10% in concentration for the component to be adsorbed. Sorbent selection
depends on the nature of the separation (i.e., bulk separation vs. purification) as
well as the process by which the separation will be accomplished (i.e., pressure
swing vs. temperature swing). As mentioned, the most important basis for sor-
bent selection is the equilibrium isotherm, while diffusivity is a secondary factor
for consideration. A brief summary of the equilibrium isotherms and diffusivities
will be given first. Because extensive reviews on isotherms and diffusion are
available elsewhere (Yang, 1987; Do, 1998), the summary here will only cover
those which are directly relevant to the discussion that follows.

3.1. EQUILIBRIUM ISOTHERMS AND DIFFUSION
3.1.1. Langmuir Isotherms for Single and Mixed Gases

The Langmuir-type isotherms remain to be the most widely used for practical
applications. The Langmuir isotherm for pure component adsorption is

q BP

θ = = (3.1)
q m 1 + BP
α Q/RT
B = e (3.1a)
β(2πmkT ) 1/2