Page 146 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

120 5 Principles for Gas Separation

vapor from the air because it has an affinity for water. In air pollution control,
activated carbon is an ideal adsorbent for many VOCs because it has an affinity for
hydrocarbon gases. Both the surface area of the adsorbent and the affinity of the
adsorbent for the adsorbate affect adsorption efficiency. Adsorption efficiency
increases with the increase in surface area of adsorbent. Bulk sizes of the porous
adsorbent particles do not affect maximum adsorption capabilities, but they affect
the time to achieve equilibrium significantly.
Affinities can also be designed by adjusting the pore diameters of the adsorbent.
For example, a molecular sieve with a pore diameter of 30 Å can adsorb light gases,
such as NH 3 and H 2 O. However, when the diameter of the pores is increased to
40 Å, the sorbent is more effective on larger molecules, such as CO 2 and SO 2 .
Further increasing the pore diameters can enable the adsorption of large organic
molecules, such as benzene, phenol, and toluene.

5.1.3 Adsorption Isotherm

Physical and thermodynamic properties of different gases are particularly useful for
engineers who design adsorption systems, and analysis of isotherms can yield
important data. Adsorption equilibrium is reached when a stable relationship exists
between the concentration of the species in air and the amount of adsorbate
adsorbed per unit mass of adsorbent. The adsorption equilibrium is a function of
temperature.
An adsorption isotherm is a relationship of equilibrium adsorbent capability
versus the adsorbate concentration at a given temperature. Note that equilibrium
absorption is rarely achieved in real engineering operations. Factors that can reduce
the adsorption capacity include moisture in the air, heat waves, or residue moisture
in the adsorbent. This actual adsorption capacity is sometimes called working
adsorption capacity.

M eq ¼ f ðCÞ ð5:1Þ

M eq is the adsorbate to adsorbent mass ratio at equilibrium (kg adsorbate/kg
3
adsorbent), while C is adsorbate concentration in gas phase (kg/m , ppmv, etc.)
There are several models for this relationship described in Eq. (5.1), of which
Langmiur and Freundlich isotherms are widely used [11]. Both are introduced
below.

5.1.3.1 Langmuir Isotherm

Irving Langmuir was awarded the Nobel Prize in 1932 for his investigations con-
cerning surface chemistry. Langmuir model is an empirical model and it was
developed in the 1910s based on the following assumptions: