Page 45 - Adsorbents fundamentals and applications

30 SORBENT SELECTION: CRITERIA

Langmuir isotherm:
K(T )P
q = (3.46)
1 + B(T )P
and T 0 is determined by:
C ps
K(T 0 ) = (3.47)
C pb
where q is the amount adsorbed; P is pressure; and C ps and C pb are, respec-
tively, the heat capacities for the solid-phase (sorbent plus adsorbate) and inert
carrier gas. The values for some systems are given in Table 3.1. As the tempera-
ture is increased beyond the characteristic temperature, however, the energy cost
also increases without a significant gain in desorption. Thus the characteristic
temperature is actually the optimal temperature for desorption.
Materials problems must also be considered in selecting the regeneration tem-
perature. In the presence of hydrocarbons, coke deposition occurs on zeolites as
◦
a result of the catalytic decomposition at temperatures as low as 100 C, depend-
ing on the hydrocarbon compound, its partial pressure, and other factors. Severe
◦
oxidation occurs in activated carbon at slightly above 100 C in the presence of
trace amounts of oxygen.

3.2.2. Pressure Swing Adsorption

A multitude of sophisticated PSA processes and designs have been implemented
for various separations. However, there are not many basic ideas on which these
designs are made. These ideas can be combined and synthesized, with minor
modifications, to accomplish the desired separations.
To facilitate the discussions that follow, some terminologies will first be
defined. The performance of the separation process is measured by three param-
eters: (1) product purity, (2) product recovery, and (3) adsorbent productivity.
If more than one of the constituents are to be recovered, (1) and (2) apply to
all products. The product purity is a volume-averaged quantity, as the effluent

Table 3.1. Characteristic temperatures, T 0 , for various
systems at 1 atm
◦
System T 0 , F
CO 2 /CH 4 /5A zeolite ≈230
H 2 O/air/5A zeolite >600
H 2 O/air/silica gel ≈250
H 2 S/CH 4 /5A zeolite ≈400
Acetone/air/activated carbon ≈300

Source: Basmadjian et al. (1975b). Reprinted with permission.
Values are accurate within ±10%, because of uncertainties in K,
C ps ,and C pb .