2.2.2 Adsorption clearing of gases

Adsorption is the process by which residual molecular forces at the surface of solids attract molecules of gases and vapors. The adsorbing solid is called adsorbent, while the gas which is adsorbed is referred to as the adsorptive. Adsorption of gas molecules occurs at active sites of the solid surface. The active sites are called homogeneous when they all contain the same energy potential. With different energy potentials the sites are said to be heterogeneous. Gas molecules that are adsorbed are called adsorbate. The adsorbed phase consists of a thin gaseous layer, which includes the adsorbate, and a thin solid layer containing the active sites.

During adsorption molecules of gas are besieged on the surface of the firm body just as at condensation, and then are kept on it by physical strengths of an attraction or chemical forces (chemosorption) - depending on the chemical nature of a molecule and a surface. In some systems there can be both kinds of adsorption or intermediate conditions.

The most suitable for adsorption firm substances should possess the following properties: the high porosity, well advanced surface with the big effective area, hardness, propensity to caking at loading in a tower, mechanical durability, simplicity of regeneration after repeated use. These requirements are met by following materials, as coal, alumina, silica gel. They are applied as adsorbents.

Fig. 2. 29 – Illustration of porous structure of granular adsorbent

	d (nm) = 10-9 m
Macropores Mesopores Micropores Submicropores	50 ≤ d 2 ≤ d ≤ 50 1 ≤ d ≤ 2 d ≤ 1

Adsorbents are divided on three groups:

1) Non-polar firm substances where there is basically a physical adsorption;

2) Polar firm substances where there is a chemical adsorption without change of chemical structure of gas molecules and surface of adsorbent;

3) Surfaces with only chemical adsorption which desorb the gas molecules after chemical reaction – or catalytic reaction when the surface does not undergo changes, or non-catalytic reaction with atoms of adsorbent, and their replacement is required.

1 Group

Coal - unique non-polar adsorbent having industrial value. The big organic molecules are adsorbed very easy, fine inorganic molecules are adsorbed very bad. The activated coal is received at pyrogenic decomposition of suitable grades of coal or tree.

2 Group

Polar adsorbents are silica and other siliceous compounds or oxides of metals. Oxides of metals, the activated alumina or bauxite are used for removal of water vapours from gas streams. Siliceous synthetic zeolites (so-called molecular sieves) represent aluminosilicates of sodium or calcium, activated by heating at which crystalization water is removed. The activated synthetic zeolites can be used for drying of gases at high temperatures when silica gel and alumina lose the efficiency. Other field of application of these sorbents is selective adsorption of polar molecules of water, carbon dioxide, ammonia, etc., therefore they are used for clearing of inert and natural gases, for removal СО₂ and water from etholene before its polymerization.