Page 143 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

Chapter 5
Principles for Gas Separation

With a good understanding of the properties of gaseous and particulate air pollutants,
this chapter starts with basic principles for the separation of unwanted gases from the
air. It is divided into two major sections as adsorption and absorption. Topics covered
in this chapter include adsorbate and adsorbent, adsorption affinity, adsorption iso-
therm, adsorption wave, absorption, absorption equilibrium, and chemical assisted
absorption. Condensation is not covered because it applies mainly to gases with very
high concentration, which do not happen often in air emission control.

5.1 Adsorption

5.1.1 General Consideration

Adsorption is a process by which gas molecules are attracted to the surfaces of a
solid or liquid and consequently separated from the main gas stream. The adsorbing
material is called adsorbent, and the gas molecules to be adsorbed is called
adsorbate.
Adsorption can be classified as physisorption and chemisorption. The differences
between these two types of adsorptions are summarized in Table 5.1. The former is
caused by van der Waals forces and the latter involves chemical reactions between
the adsorbent and the adsorbate. Thereby the mechanisms and models for chemical
adsorption are more complicated than physical adsorption.
Physioadsorption is a surface phenomenon where the adsorbate will stay on the
surface of the adsorbent. The attractive force normal to the surface tends to grab
adjacent molecules of adsorbate. For example, activated carbon has a tendency to
adsorb volatile organic compounds (VOCs) from an air stream. Chemisorption is
driven by a chemical reaction that takes place at the adsorbate-adsorbent interface.
As a result, a new chemical species is generated at the interface.
Both physisorption and chemisorption may occur simultaneously at the same
adsorption interface and physical adsorption can continue after the chemical
adsorption layer is completed. However, the bonding strength of chemisorption is

© Springer Science+Business Media Singapore 2014 117
Z. Tan, Air Pollution and Greenhouse Gases, Green Energy and Technology,
DOI 10.1007/978-981-287-212-8_5