Why does activated alumina have the ability of adsorption and drying? According to the different forces between activated alumina surface and adsorbate molecules, activated alumina adsorption can be divided into physical adsorption and chemical adsorption.
For the physical adsorption of activated alumina, the adsorbent and adsorbate attract each other through intermolecular force (also known as "van der Waals" force). In the chemical adsorption of activated alumina, the adsorbed molecules interact with the atoms on the surface of the adsorbent, and electron transfer, atomic rearrangement or the destruction and formation of chemical bonds will occur between activated alumina and adsorbate.
Physical adsorption occurs in the drying range of activated alumina adsorption by compressed air. Physical adsorption has the following characteristics:
1. The adsorption force is small, and the adsorbed gas molecules are easier to return to the gas phase - that is, it is easier to "desorb" (also known as "desorption");
2. Generally speaking, the physical adsorption process of activated alumina is reversible, almost no activation energy is required (even if it is required, it is very small), and the adsorption and desorption speed is very fast. It can be considered that once activated alumina contacts with adsorbate, adsorption will occur immediately;
3. All adsorption processes of activated alumina are exothermic reactions. The "adsorption heat" released by physical adsorption is much less than that of chemical adsorption, and its heat is close to the heat of vaporization in the state of adsorbate liquid or the heat of condensation in the state of gas;
4. Physical adsorption has no selectivity, that is, any solid can adsorb any gas, which only depends on the difference of adsorption capacity (the adsorption with too small adsorption capacity has no practical application value);
5. The physical adsorption of activated alumina is related to condensation, so it can only be carried out when it is lower than the boiling point of the adsorbed material.