Page 144 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

118 5 Principles for Gas Separation

Table 5.1 Physisorption and chemisorption
Physical adsorption Chemical adsorption
Driving van der Waals force between mole- Chemical reactions between the
force cules. No change of properties of adsorbent and adsorbate. Forming
either the adsorbent or the adsorbate new adsorption products
Heat release Low, about 20 kJ/g mol High due to chemical reaction,
rate 20–400 kJ/g mol
Reversibility Can be easily reversed by reducing Difficult to reverse. Requires very
the pressure at the temperature at high temperature or positive ion
which the adsorption took place bombardment to remove the
adsorbate
Equilibrium Physical adsorption of a gas is related Chemisorption can take place at
pressure to liquefaction or condensation, it much lower pressures and much
only occurs at pressures and temper- higher temperatures than physical
atures close to those required for adsorption
liquefaction. Low-pressure adsorp-
tions take place mainly in fine porous
adsorbents by capillary effect
Thickness of A physical adsorption layer at A chemisorption layer can only be
reaction equilibrium can be several molecules one molecule thick, because the
layers thick newly formed compound layer
prevents the further reaction of the
adsorbent and the adsorbate

stronger than that of physisorption. A description of the chemisorption bond
requires a detailed understanding of molecules outside surfaces and the electronic
structure of atoms. The author of this book does not intend to extend the scope of
this book to interfacial chemistry; readers interested in this topic are directed to
specialized books devoted to chemisorption.
Common physioadsorbents include
• activated carbon,
• silica gel,
• activated alumina, and
• aluminosilicates (molecular sieves).

Activated carbon is a char-like material with a great surface area. Silica gel is a
hard, granular, and porous material made by precipitation from sodium silicate
solutions treated with an acid. Activated alumina is an aluminum oxide activated at
high temperature and used primarily for moisture adsorption. Aluminosilicates are
made of porous synthetic zeolites and are used primarily in gas separation processes.
Activated carbon is the most common adsorbent for air emission control. Acti-
vated carbon is made by the carbonization of carbon rich coal or biomass (wood,
fruit pits, or coconut shells) followed by activation with hot air or steam. It is
produced by a two-step process. First, pyrolysis of raw material with a high carbon
source such as coal, wood and nutshells results in charred highly carbonaceous solid
residue. Then activation of the charred residue by oxidation forms pores and passages