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12.5 CO 2 Separation by Adsorption 363

12.5 CO 2 Separation by Adsorption

CO 2 can be separated from a gas stream by both physical adsorption and chemical
adsorption. The general differences between a physical and chemical adsorption has
been introduced in Chap. 5. Physical CO 2 adsorption operates at temperatures lower
than 100 °C; whereas the chemical counterpart operates at a high range of
400–600 °C.

12.5.1 Physical Adsorption

A good CO 2 physical adsorbent is expected to be characterized with high affinity
with CO 2 compared to other gases in the stream, high adsorption capacity, low heat
of adsorption, low adsorption hysteresis, and steep adsorption isotherm. These
features ensure the cost-effectiveness of the operation with high efficiency, low
energy consumption, and low material cost.
The best adsorbent is expected to be characterized with high CO 2 capacity at low
pressure, high selectivity for CO 2 , fast adsorption/desorption kinetics, good
mechanical properties, high hydrothermal and chemical stability, as well as low
costs of synthesis. Unfortunately, these criteria are too ideal for any single
adsorbent.
The most widely investigated low temperature CO 2 adsorbents are zeolites and
activated carbons. Compare to Zeolite-13X and natural zeolite, activated carbon
showed higher carbon adsorption capacity and steepest isotherm, but high hyster-
esis challenges the desorption process.
Metal–organic frameworks (MOFs) have recently attracted intense research
interest in CO 2 adsorption due to their large porous volume and surface areas
[42, 43]. In general, the CO 2 adsorption in MOFs varied with CO 2 pressure. At high
pressures, CO 2 adsorption capacities depend on surface areas and pore volumes of
the MOFs. Otherwise, the capacities depend on the heat of adsorption. In addition,
many MOFs have shown high CO 2 /N 2 and CO 2 /CH 4 selectivity. However, there
are two important challenges to MOFs into practical applications of CO 2 capture:
(1) the mass production of MOFs at low cost
(2) the stabilities of MOFs toward moisture, other acid gases, and heat for
regeneration.
Interested readers are encouraged to conduct a state-of-the-art literature review in
this area of research.
Continuous CO 2 adsorption can be implemented by a pressure swing adsorption
process. It is widely used for air drying using synthetic zeolites or activated alumina