Molecular sieve tetrahedrons consist of four oxygen atoms which surround a silicon or aluminium atom. Each oxygen atom has two negative charges, whereas every silicon atom possesses four positive charges. Owing to the trivalency of the aluminium, the tetrahedron of the aluminium oxide carries a negative charge. A positively charged ion (cation) is needed to compensate. This compensation can take place via potassium, sodium or other cations.
Figure 6.4.1.1
Molecular sieves or zeolites consist of blunt octahedrons, made up from tetrahedrons. These structural components are known as sodalite cages. If sodalite cages are built up in layers to form simple cubic shapes, a dice like network of hollow spaces is created, possessing a diameter of up to 15 Angström (1.5 nm). These voids are always of equal size and accessible through pores from six sides. In the case of sodium compounds, this crystalline structure is expressed by the following chemical formula : Na12(AlO2)12(SiO2)12] The water content fills the voids during crystallisation and is loosely linked. Heating causes the water content to be removed and the spaces previously filled with water have now become available as pore volumes for the adsorption of various gases.
Diagram 6.4.1.1
The number of water molecules in the structure for value x may amount statically to 27, water will then amount to 28.5% of the total weight of the zeolite in saturated form. However, a molecular sieve with a pore diameter of 10 Angström (1nm) is capable of taking up as much as 35% of its total weight in saturated form. A typical dynamic adsorption thermal for water with a molecular sieve is shown in Diagram 6.3.1.1. With this adsorption system, the adsorption capacity of drying medium molecular sieve rises quickly with increasing concentration to reach a high value followed by the saturation value. The relatively fast achievement of the saturation value at low concentrations, distinguishes molecular sieves from other drying media in common use. Diagram 6.4.1.2 shows the dependence of the H2O equilibrium capacity from relative humidity, in the case of the molecular sieve (MS), silica gel (SG).and aluminium gel (Al). The separation of differing molecules from a flow of air is influenced by factors of molecule diameter and pore diameter of the drying medium.
Diagram 6.4.1.2
The separation of water from a flow of air is dependent on whether the water molecule can pass through the pore opening of the drying medium and is stored within the internal structure of the zeolite. A water molecule has a critical diameter of 2.8 Angström (0.28 nm). Such a water molecule becomes stored in a drying medium with a pore size larger than 2.8 Angström. Occasionally, an unusual effect is observed at specific temperatures, i.e. the adsorption of molecules with a critical diameter larger than the effective diameter of the pore opening. This apparent contradiction can be explained by the elasticity of the adsorbed molecule and the vibrations within the crystal system of molecular sieves. Molecular sieves are manufactured in spherical form. Their active internal surface is of the order of size of 500 - 900 m2/g. Molecular sieves possess a macropore structure of high capacity, making possible rapid diffusion of the molecules to be adsorbed towards the internal surface of the drying medium sphere.
